Fundamentals of 

Composite 

Processing 

Proceedings of a Workshop 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

General 

Technical 

Report 

FPL−GTR−149 

Presented by the U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, 

Wisconsin in cooperation with the Wood-Based Composites Center, Virginia Tech, Blacksburg, Virginia 
Abstract 

The USDA Forest Service, Forest Products Laboratory 

(Madison, Wisconsin) and the Wood-Based Composites 

Center of Virginia Tech (Blacksburg, Virginia) co-sponsored 

a conference, held November 5ñ6, 2003, in Madison, Wisconsin, on the fundamentals of composite processing. The 

goals were to assess what we know, define what we need to 

know, and then establish the state of the art in hot-pressing 

of wood-based, particulate composites. Academic and industrial professionals from around North America and Europe 

were invited to participate because of their expertise and 

interest in this area of research. The workshop covered four 

critical topics associated with hot-pressing of composites: 

resin curing and bonding, press control, physics of hotpressing, and computer simulations of the pressing process. 

This report is the official record of the presentations and 

discussions that occurred during this workshop. 

Keywords: composites, processing, engineered wood 

June 2004 

Winandy, Jerrold E.; Kamke, Frederick A., Eds. 2004. Fundamentals of 

composite processing. Proceedings of a workshop; November 5ñ6, 2003; 

Madison, WI. Gen. Tech. Rep. FPL-GTR-149. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 118 p. 

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726ñ2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds 

of libraries in the United States and elsewhere. 

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the U.S. Department of Agriculture of 

any product or service. 

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national 

origin, sex, religion, age, disability, political beliefs, sexual orientation, or 

marital or familial status. (Not all prohibited bases apply to all programs.) 

Persons with disabilities who require alternative means for communication 

of program information (Braille, large print, audiotape, etc.) should contact 

the USDAís TARGET Center at (202) 720ñ2600 (voice and TDD). To file a 

complaint of discrimination, write USDA, Director, Office of Civil Rights, 

Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, DC 20250ñ9410, or call (202) 720ñ5964 (voice and TDD). USDA is 

an equal opportunity provider and employer. 

Contents 

 Page 

Preface ................................................................................iii 

Overview of Composites Processing ..................................iv 

General Processing Considerations .................................iv 

Oriented Strandboard ......................................................iv 

Fiberboard ........................................................................ v 

Fundamentals of Composite Processing..........................vi 

Workshop Introduction ........................................................ 1 

Physics of Hot-Pressing ....................................................... 3 

Discussion ...................................................................... 19 

Monitoring Resin Cure in the Mat for 

Hot-Compression Modeling............................................... 26 

Adhesive Interactions With Wood..................................... 29 

Discussion ...................................................................... 39 

Press Control for Optimized Wood Composite 

Processing and PropertiesóPart 1: Pressing 

Variables and Sensors........................................................ 54 

Press Control for Optimized Wood Composite 

Processing and PropertiesóPart 2: Properties 

and Press Control Strategies .............................................. 65 

Discussion ...................................................................... 83 

Simulation of the Pressing Process .................................... 90 

Discussion .................................................................... 102 

Summary and Wrap-up .................................................... 113 

AppendixóParticipant List ............................................. 116 
3

Fundamentals of Composite Processing 

Proceedings of a Workshop 

Presented by the 

U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory, 

Madison, Wisconsin 

in cooperation with the 

Wood-Based Composites Center 

Virginia Tech, Blacksburg, Virginia 

November 5ñ6, 2003 

Madison, Wisconsin 

Jerrold E. Winandy, Workshop Co-Chair and Editor 

U.S. Department of Agriculture 

Forest Service 

Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, Wisconsin 53726ñ2398 

Frederick A. Kamke, Workshop Co-Chair 

Wood Science and Forest Products 

1650 Ramble Road 

Blacksburg, VA 24061 
4 
iii

Preface 

The goals and objectives of this November 2003 workshop were related to those 

of an earlier workshop on composite consolidation held at Virginia Tech, Blacksburg, 

Virginia, in September 1990. The consensus from that first workshop was that process 

engineering required knowledge of the levels and rates of heat and mass (that is, moisture) transfer within a consolidating mat, considering that it could consist of a diverse 

array of materials. That knowledge was then used to explain why and how much those 

thermodynamic factors influenced both the chemistries and physical states of wood materials and resins within a consolidating composite mat. The use of models to interpolate 

between known conditions was endorsed. But, participants also reached consensus that 

models should not be black boxes spouting conditions. Rather, they should facilitate our 

understanding of what is going on and when events occur within a mat. 

The goal of this second workshop is to assess what have we learned since 1990, identify 

what it is that we still need to know, and then re-establish the state of the art in hotpressing of wood-based, particulate composites. Academic and industrial professionals 

from around North America and Europe were invited to participate because of their 

expertise and interest in this area of research. This workshop was hosted by the U.S. 

Department of Agriculture, Forest Service, Forest Products Laboratory, in cooperation 

with the Wood-Based Composites Center at Virginia Tech in Blacksburg, Virginia. 

It was chaired by Jerrold E. Winandy (FPL), with formal presentations offered by 

Chunping Dai (ForintekñCanada Ltd) 

Charles Frazier (WBC Center, Virginia Tech) 

Charles Frihart (FPL) 

Frederick A. Kamke (WBC Center, Virginia Tech) 

Heiko Thoemen (University of Hamburg) 

Siqun Wang (University of Tennessee) 

More than 30 participants also attended the workshop at their own expense. Those participants are listed in the Appendix. The organizers of this workshop would like to thank 

the presenters and all the workshop attendees for their valuable contributions to this reappraisal of the state of the art and the fundamentals of composite processing. 

Jerrold E. Winandy 

Workshop Chairman 

Madison, Wisconsin
iv 

Overview of Composites 

Processing 

Before we begin this review of the presentations and discussion that occurred during our workshop, a brief, general 

overview may be appropriate to help some readers understand the current state of the art in composites processing. 

Today, many wood-based composite materials are normal 

materials in our day-to-day lives. These composites are 

usually available in panel form and are widely used in housing and furniture. Wood composites are typically made with 

a heat-curing adhesive that holds the wood fibers or woodparticle components together. Each type of wood-based fiber 

or wood-particle panel composites uses different wood 

materials and resins and has different physical and mechanical properties and surface characteristics. Thus, their manufacturing processes and intended uses vary accordingly. 

General Processing Considerations 

Still, most products in the family of particle and fiber composite materials are processed in similar ways. Raw materials for oriented strandboard (OSB), waferboard, and fiberboard are obtained by flaking or chipping roundwood. For 

fiberboard, chips are reduced to wood fiber using refiners 

that usually use steam to soften the wood. The comminuted 

wood flakes or fibers are then dried, a thermosetting adhesive is applied, and a mat of wood particles, fibers, or strands 

is loosely formed. That mat is then pressed in a platen-type 

press under heat and pressure until the adhesive is cured. 

The bonded product is allowed to cool and is further processed into specified width, length, and surface qualities. Two 

important wood composite products manufactured in North 

America are OSB and medium-density fiberboard (MDF). 

While the two end products are intended for different uses, 

many of the manufacturing and processing parameters used 

to make these two products are similar. 

Oriented Strandboard 

Oriented strandboard is an engineered structural-use panel 

manufactured from thin wood strands bonded together with 

waterproof resin under heat and pressure. It is used extensively for roof, wall, and floor sheathing in residential and 

commercial construction. Orientation of wood strands with a 

typical aspect ratio (that is, strand length divided by width) 

of at least 3 can produce a panel product with greater bending strength and stiffness in the oriented or aligned direction. 

Aspen was used as the raw material for the original waferboard product, which was made from square wafers. As this 

industry expanded and OSB became the predominant product manufactured, other species such as Southern Pine, white 

birch, red maple, sweetgum, and yellow-poplar were found 

to be suitable raw materials as well. Small amounts of other 

low- to medium-density hardwoods are also commonly used 

for OSB. 

In the general manufacturing process for OSB, debarked 

logs are often heated in soaking ponds and then sliced into 

thin wood elements. The strands are dried, blended with 

resin and wax, and formed into thick, loosely consolidated 

mats that are pressed under heat and pressure into large 

panels. Figure 1 shows an overview of the OSB manufacturing process. Oriented strandboard is made from long, narrow 

strands, with the strands of each layer aligned parallel to one 

another but perpendicular to strands in adjacent layers, like 

the cross-laminated veneers of plywood. It is this perpendicular orientation of different layers of aligned strands that 

gives OSB its unique characteristics and allows it to be 

engineered to suit different uses. 

Typically, logs are debarked and then sent to a soaking pond 

or directly to the stranding process. Long log disk or ring 

stranders are commonly used to produce wood strands typically measuring 114 to 152 mm (4.5 to 6 in.) long, 12.7 mm 

(0.5 in.) wide, and 0.6 to 0.7 mm (0.023 to 0.027 in.) thick. 

Green strands are stored in wet bins and then dried in a 

traditional triple-pass dryer, a single-pass dryer, a combination triple-pass/single-pass dryer, or a three-section conveyor 

dryer. A relatively recent development is a continuous chain 

dryer, in which the strands are laid on a chain mat that is 

mated with an upper chain mat and the strands are held in 

place as they move through the dryer. The introduction of 

new drying techniques allows the use of longer strands, 

reduces surface inactivation of strands, and lowers dryer 

outfeed temperatures. Dried strands are screened and sent to 

dry bins. 

The blending of strands with adhesive and wax is a highly 

controlled operation, with separate rotating blenders used for 

face and core strands. Typically, different resin formulations 

are used for face and core layers. Face resins may be liquid 

or powdered phenolics, whereas core resins may be phenolics or isocyanates. Several different resin application systems are used; spinning disk resin applicators are frequently 

used. 

Mat formers take on a number of configurations, ranging 

from electrostatic equipment to mechanical devices containing spinning disks to align strands along the panelís length 

and star-type cross-orienters to position strands across the 

panelís width. All formers use the long and narrow characteristic of the strand to place it between the spinning disks or 

troughs before it is ejected onto a moving screen or conveyor belt below the forming heads. Oriented layers of 

strands within the mat (face, core, face, for example) are 

dropped sequentially, each by a different forming head. 

Modern mat formers either use wire screens laid over a 

moving conveyor belt to carry the mat into the press or 

screenless systems in which the mat lies directly on the 

conveyor belt. 
v

In hot-pressing, the loose layered mat of oriented strands is 

compressed under heat and pressure to cure the resin. As 

many as sixteen 3.7- by 7.3-m (12- by 24-ft) panels may be 

formed simultaneously in a multiple-opening press. A more 

recent development is the continuous press for OSB. The 

press compacts and consolidates the oriented and layered 

mat of strands and heats it to between 177°C and 204°C 

(350°F and 400°F) to cure the resin in 3 to 5 min. 

Fiberboard 

The term fiberboard includes hardboard, MDF, and insulation board. Several things differentiate fiberboard from OSB 

and particleboard. Most notably, the physical configuration 

of the comminuted material is that of wood fiber, not a chip, 

flake, or strand of wood fibers. Because wood is fibrous by 

nature, fiberboard exploits the inherent strength of wood to a 

greater extent than does particleboard. 

To make fibers for composites, bonds between the wood 

fibers must be broken. In its simplest form, this is accomplished by attrition milling. Attrition milling is an age-old 

concept whereby material is fed between two disks, one 

rotating and the other stationary. As the material is forced 

through the preset gap between the disks, it is sheared, cut, 

and abraded into fibers and fiber bundles. Grain has been 

ground in this way for centuries. Attrition milling, or refining as it is commonly called, can be augmented by water 

soaking, steam cooking, or chemical treatments. Steaming 

the lignocellulosic material weakens the lignin bonds between the cellulosic fibers. As a result, the fibers are more 

readily separated and usually are less damaged than fibers 

processed by dry processing methods. Chemical treatments, 

usually alkali, are also used to weaken the lignin bonds. All 

of these treatments help increase fiber quality and reduce 

energy requirements, but they may reduce yield as well. 

Refiners are available with single- or double-rotating disks, 

as well as steam-pressurized and unpressurized configurations. For MDF, steam-pressurized refining is typical. 

Fiberboard is normally classified by density and can be made 

by either a dry- or wet-forming process. Dry processes are 

applicable to MDF. The following discussions briefly describe the manufacturing of medium-density dry-process 

fiberboard. Suchsland and Woodson (1986) and Maloney 

(1993) provide more detailed information. 

 Figure 1óOSB manufacturing process (Courtesy of Structural Board Association, 

 Willowdale, Ontario, Canada.) 
vi 

Dry-process fiberboard is made in a similar fashion to OSB, 

except wood fibers instead of wood flakes are formed into a 

mat. Urea-formaldehyde (UF) or phenol-formaldehyde (PF) 

resin and other additives may be applied to the fibers by 

spraying in short-retention blenders or a blow-line blenderñ

dryer. The adhesive-coated fibers are then air-laid into a mat 

for subsequent pressing, much the same as mat formation for 

OSB without the elaborate layering or strand orientation. 

Pressing procedures for dry-process fiberboard differ somewhat from particleboard procedures. After the fiber mat is 

formed, it is typically pre-pressed in a band press, the densified mat is then trimmed by disk cutters and transferred 

directly to a multi-opening hot press. MDF is usually pressed 

at approximately 140°C to 165°C (284°F to 329°F) for UFbonded products and just greater than 190°C (410°F) for PFbonded products. Continuous pressing using large, highpressure band presses is also gaining in popularity in mills 

built since 1997, especially those in Europe. Board density is 

the basis for classification and properties and is an indicator 

of board quality. Since density is greatly influenced by 

moisture content, this is constantly monitored by moisture 

sensors. 

Fundamentals of Composite 

Processing 

Many aspects involved in the hot-pressing of wood composites and the chemistries and physical processes occurring 

within those wood mats during high-temperature consolidation have commonalities. To address these commonalities, 

this workshop involved four critical topics associated with 

hot-pressing of composites: 

• Physics of hot-pressing, 

• Resin curing and bonding, 

• Press control strategies, and 

• Computer simulations of the pressing process. 

One or two formal presentations on each topic were presented by recognized authorities in those fields, those presentations were then followed by open discussion intended to 

identify what we now need to investigate and to re-establish 

the state of the art in hot-pressing of wood-based, particle 

composites. This workshop proceedings is a record of the 

presentations and discussions that took place between the 

workshop participants. It is made available to the general 

public as an FPL-produced General Technical Report (GTR) 

to help everyone involved in composite processing understand the fundamental chemistry and physics of composite 

processing. We also want to acknowledge and thank our 

workshop co-sponsor, the Wood-Based Composite Center of 

Virginia Tech, Blacksburg, Virginia. 
Workshop Introduction 

Jerry Winandy, Supv. Research Wood Scientist 

Chris Risbrudt, Director 

Forest Products Laboratory, Madison, Wisconsin 

Jerry Winandy 

I want to welcome you all and just want to say a couple of 

brief things before we have a word of introduction from our 

Director Chris Risbrudt. First of all, I would like to thank 

you all for attending. We appreciate the efforts and costs you 

have undertaken to help us generate the revised state of the 

art in the fundamentals of composite processing. 

I also want you to be aware that everything will be recorded 

today, both video and audio. Weíre going to make a transcription of this and send it out to all members so people can 

make sure that what you said was what you said. There will 

also be a published proceedings and each of you will be sent 

a copy of that as soon as we get it published and it will be 

publicly available. With that, Iíd like to introduce Chris 

Risbrudt, the Director of the Forest Products Laboratory. 

Chris Risbrudt 

Okay, thanks Jerry. Jerry and I share the first 15 min of the 

agenda I saw and I will try and get through my remarks 

relatively quickly because I know our project leaders can 

hardly get warmed up in 10 min so I will try and get through 

mine in 5 so we will stay on schedule. As always, it is my 

responsibility and my pleasure to welcome participants in 

meeting to the Forest Products Laboratory and in Madison 

so welcome to you all that are from outside of town and I 

hope you will enjoy your stay here at the FPL and in the fine 

city of Madison. I am sure during the course of this if you 

havenít been to the lab before we can make arrangements to 

show you the various parts of the laboratory you may have 

not seen before. We want you to leave here with solid 

knowledge of the kind of work that is going on here at the 

lab that you might be interested in. I also want to thank you 

all for participating in this workshop on the fundamentals of 

composite processing. You are the academic and industrial 

professionals from around North America and Europe who 

are the experts in this area and you have been invited here to 

participate because of your expertise and your recognized 

interest in this area of research. I also want to thank our 

workshop cooperator, the Wood Based Composite Center of 

Virginia Tech University in Blacksburg. So Fred, thank you 

and the other people that you had at Virginia Tech helping 

you make arrangements for this. 

I know you all know that the use of composites is growing 

because they are efficient both in cost and environmental 

impact, and they give us an ability to combine the desirable 

characteristics of a variety of materials, and they allow us to 

tailor material properties to end use performance requirements, which I think is an important aspect of the research in 

this area today. Now I want to talk particularly about Forest 

Service perspective on composites and I am sure these facts 

also apply to many of your own organizations. We strongly 

feel that composites are an important tool in helping us 

achieve sustainable forest management. I think it is the hot 

area of research in forest products utilization that has a major 

potential to change or to provide more options for forest 

management. Composites give us the ability to meet user 

needs for wood products while still using small-diameter 

trees from fire-prone forests. I know you are all aware of the 

critical fires now raging just outside San Diego in southern 

California, which follows on the heels of major fires 

throughout the west. In fact, of all the forest service leaderships meetings I have been at throughout the past year, 

virtually all of them have featured a tour of one mega fire or 

another. You know, when I started my career in the Forest 

Service, we thought a 5,000-acre fire was a monster. And 

now we talk about 1-h slopovers that reach 5,000 acres. And 

some of those fires, one last year in Colorado burned 

16 miles in one 8-h period, and the fires that I visited in 

Arizona a few months ago, at the peak of the burning period 

on the hottest day of the fire, I think I have got this right, it 

was burning 2 square miles an hour of forest. Just tremendous fire behavior that we have never imagined before, let 

alone seen. And so we are faced with trying to manage about 

200 million acres for United States forest that are in unhealthy condition and they are unhealthy because they have 

too many small trees. What can we do with small trees? 

One good alternative is turning them into products through 

composites. And then also using residues from sawmilling 

operations and then using recovered post-consumer materials. So composites constitute a very important area of research for not just the forest products industry, but for forest 

management in general to provide those managers cost 

effective options for doing something with that material. For 

the Forest Service, because of the way we do business and 

the cost of doing business under the rules of Federal forest 

management, our costs range between $500 and $1,000 an 

acre for thinning these forests, and we have 73 million acres 

of national forest that need treatment. 
2 

So we need to figure out ways that we can reduce that cost 

by turning some of that material into useful products, and so 

this is an important area of research. But for all our advantages, we still need to improve the performance of these 

wood-based composites if they are to continue to grow into 

new end-use applications and to prevent substitution by 

alternative materials. What we learn in this workshop will 

help us make improvements in composite durability, dimensional stability, create performance, decay resistance, and 

termite resistance. The last workshop of this type was held 

13 years ago at Virginia Tech. A lot has been learned in that 

time, and I am looking forward to the results of this workshop. We expect that these results will help many organizations identify and prioritize the needed course and direction 

for composites research for the next number of years until 

we decide to hold another one of these kinds of meetings. I 

want to thank you for helping us synthesize this state of the 

art in hot-pressing wood-based composites, and I will turn it 

back to you Jerry. 

Jerry Winandy 

Now, I think you each have a copy of the agenda. Weíve 

broken down the two-day workshop into a series of discussions into four different areas. These areas will be physics of 

hot pressing, resin curing and bonding, press control, and 

computer simulations. We are going to start with physics of 

hot pressing, but in each discussion we will have a presentation by a group leader followed by a group discussion that I 

encourage you all to participate in. I want to encourage you 

all to participate because the knowledge base we put together in this workshop will identify the critical needs that 

are important to the users of composites, which is our public 

at the Forest Products Laboratory, the industry that produces 

these materials, and the engineering community that designs 

with these materials. The knowledge base that we put together is only as good as what you and the rest of us put into 

it. So my challenge to each of you is that I would like to ask 

everyone sitting at these tables to add some value to each 

discussion. Your comments are critical to getting the best 

information out there and helping us synthesize this knowledge base. 

Now, the goal of this workshop is to assess what exactly it is 

we know and where exactly we should go in the future 

research of hot-pressing wood-based particulate composites. 

We ask that you work with us to define what is happening 

with respect to wood and resin chemistry to consider the 

physics of mat consolidation, consider how do we control 

these events to get the performance and properties we want, 

and to learn to use this knowledge to improve current composites, and to develop the next generation of wood-based 

particulate composites. As I mentioned before, there will be 

a published report, which will be published as a USDA 

Forest Service General Technical Report. My goal is to have 

that report published within 6 months of this event. My goal 

is to get it transcribed and to you in draft form within 6-8 

weeks from now. So I would then ask you to return your 

comments back to me within 30 days after receiving that and 

I will send out a note afterwards detailing all this information. Each one of the speakers will give us a written paper on 

their presentations and those speakers will be Fred Kamke, 

the director from the Wood-Based Composite Center at 

Virginia Tech University, Charles Frazier from the WoodBased Composite Center at VA Tech and Charles Frihart, 

the project leader of the Forest Products Labís Wood Adhesive Science and Technology group. They will then be followed by Chunping Dai, the head of the Composites group 

from Forintek, Siqun Wang at the University of Tennessee, 

and the simulation of processing Heiko Thoemen from 

University of Hamburg. So with that, I would like to start 

our talk by introducing Fred Kamke. Fred is a professor in 

the forest products and wood science technology, he earned 

his Ph.D. at Oregon State University and is the founder and 

director of the Wood-Based Composite Center. Fredís research specialization is heat and mass transfer in wood-based 

composites.
3

Physics of Hot Pressing 

Frederick A. Kamke, T.M. Brooks Professor and Director 

Wood-Based Composites Center, College of Natural Resources, Virginia Tech, Blacksburg, Virginia 

Introduction 

Wood is a complex natural material. It may be characterized 

as capillary-porous, cellular, anisotropic, and viscoelastic. In 

addition, the structure and properties of wood are widely 

variable in comparison to concrete, metals, and synthetic 

polymers. These characteristics present a formidable challenge for the design and manufacture of wood-based composites. Whether the composite is made of fibers, particles, 

strands, or veneer, the natural characteristics of wood play 

an important role in the manufacture and performance of the 

resulting composite. Current wood-based composite manufacturing technology relies on consolidation of a mat, or 

billet, of wood elements using thermal energy and mechanical compression. The thermal energy is used to cure the 

thermosetting adhesive, and the mechanical compression is 

needed to mate the surfaces of the wood elements for the 

creation of an adhesive bond. This paper will address the 

physics of consolidation of a particulate, wood-based composite. The focus will be on heat and mass transfer and the 

mechanics of mat compression. Temperature and gas pressure data will be used to illustrate the active heat and mass 

transfer mechanisms. The term particle will be used in a 

generic sense to include fibers and strands, as would be 

found in fiberboard and OSB products. 

Temperature, wood moisture content, compression stress, 

and time are the primary factors, along with the intrinsic 

wood properties, that create the properties of the resulting 

composite. A forming machine establishes the mat structure 

by controlling particle orientation and separation into layers 

by particle size and moisture content. The hot-press provides 

the thermal energy and mechanical force of compression to 

consolidate the mat. Hot-presses are either batch-type with 

heated platens, or continuous-type with moving, heated belts 

(Maloney 1993). Most presses rely on conduction heat transfer from the platens, or belts, to the mat surface. Some 

presses employ steam-injection through the platens to accelerate heat transfer (Geimer 1982). A few presses use high 

frequency electric fields to augment heat transfer. Regardless 

of the press design, the mechanisms of heat and mass transfer in the mat are the same but vary by degree of importance 

and direction of flows. For simplicity, the present discussion 

will consider batch-type hot-presses. 

Internal Mat Conditions 

The study of the fundamental physics of mat consolidation 

began with experimentation of hot-pressing parameters and 

their influence on final properties. Kelly (1977) provided an 

excellent review of prior research relating manufacturing 

parameters to composite properties. The rate of temperature 

rise has an obvious impact on the rate of adhesive cure, and 

therefore, received early attention. Using thermocouples, 

Maku and others (1959) and Strickler (1959) monitored the 

rate of temperature change in the core of particleboard mats 

during hot-pressing. The influence of initial mat moisture 

content was clearly demonstrated, which indicated heat 

transfer and mass transfer are interdependent and that latent 

heat of vaporization significantly impacts the temperature 

rise (Fig. 1). It was also demonstrated that particle geometry 

affects water vapor flow, and consequently temperature rise, 

in the mat. The authors recognized that pressure gradients 

result in steam flow from the mat surface to the core early in 

the press cycle and then laterally from the mat core to the 

edges. Farni (1954) saw the benefit of steam flow for rapid 

heat transfer during hot-pressing and proposed the ìsteamshockî effect, whereby higher moisture content wood particles are used in the surface layers of the mat. 

Because the bulk flow of steam in the mat was an important 

means of heat transfer and excessive steam pressure at the 

end of a press cycle could cause delamination, a technique to 

measure gas pressure in the mat was developed and demonstrated (Denisov and Sosnin 1967, Kavvouras 1977, Kamke 

and Casey 1988). The technique utilizes a small-diameter 

(approx. 0.25-mm inside diameter) rigid tube, with the open 

end inserted inside the mat and the other end connected to a 

sensitive pressure sensing device. The tube may be filled 

with a liquid, such as a low vapor pressure silicon oil, to 

eliminate condensation of water vapor inside the tube. This 

method can only measure total gas pressure and canít differentiate partial pressures. For example, the manufacture of 

OSB using polymeric-diphenylmethane-diisocyanate 

(pMDI) adhesive produces significant amounts of CO2 as a 

product of polymerization, which would falsely indicate high 

steam pressure during hot-pressing (Geimer and others 

1991). This technique is a point measurement. Mats with a 

low permeability, such as high-density strand mats made 

from low-density wood, may have a high gas pressure gradient through the thickness and laterally from center to edge. 
4 

The measurement of temperature and gas pressure inside a 

mat during hot-pressing has now become common practice 

in the industry with the introduction of PressMANÆ mat 

monitoring system in 1993 (Alberta Research Council 2003). 

The PressMANÆ system includes specially designed temperatureñpressure probes for measuring internal mat temperature and gas pressure during pressing. Systems for single 

and multi-daylight presses and systems for continuous 

presses are currently in use at more than 80 manufacturing 

facilities and research laboratories in 20 countries. Direct 

measurements of internal mat conditions are also necessary 

for the verification of hot-pressing simulation models (Zombori and others 2003). These measurements have also been 

adapted to continuous presses (Steffan and others 1999). 

Heat and Mass Transfer 

Suchsland 1967) reviewed some of the fundamental aspects 

of mat structure, heat transfer, and mat compression. Bolton 

and Humphrey (1989) provided a more recent review of the 

physics of hot-pressing, which they implemented into a hotpress simulation model. Simulation modeling of the hotpressing process relies on a thorough understanding of the 

fundamentals of heat and mass transfer, mat rheology and 

the physical properties of wood. Several modeling studies 

have provided original data, or adapted the data of others, 

for describing the physics of hot-pressing (Humphrey and 

Bolton 1989; Bolton and others 1989a,b; Dai and others 

2000; Carvalho and others 2001; Garcia and others 2002, 

Humphrey and Thˆmen 2003, Zombori and others 2003). 

During the hot-pressing cycle, the internal conditions of the 

mat change rapidly. Heat is transported by conduction from 

the hot platens to the mat surface. The abrupt increase of 

temperature vaporizes the moisture within the particles at the 

surface. The air (which also contains water vapor) that resides in the voids of the mat also increases in temperature 

and thus increases in pressure in proportion to the temperature rise. The addition of evaporated water at the surface of 

the mat, plus the heated air, causes a pressure differential 

that drives the heated gases by hydrodynamic flow toward 

the center of the mat. If the water vapor gives up enough 

heat to the surrounding mat during its journey to the center, 

it will condense. The amount of condensation depends on the 

rate at which it is able to reach equilibrium with the wood at 

the prevailing temperature. Some of the condensate will 

become bound water and some will become liquid water. 

Therefore, a vertical water vapor flow from the surface of 

the board towards the center can be observed at the beginning of the press cycle. 

The initial rise of surface temperature is often followed by a 

temperature plateau. The plateau in temperature is due to the 

latent heat of vaporization and the vapor pressureñmoisture 

contentñtemperature equilibrium. The more moisture present 

in the surface particles, the more pronounced is the temperature plateau. A low moisture content will not produce a 

plateau. When present, the temperature plateau is approximately 100°C, which corresponds to the boiling point of 

water at 1 atmosphere of pressure. The presence of the hygroscopic wood substance tends to reduce the equilibrium 

vapor pressure and suppress the temperature. However, 

restrictions to gas flow cause an elevation in vapor pressure, 

which may elevate the plateau temperature slightly above 

100°C. As the rate of moisture evaporation at the surface 

declines, the surface temperature continues its rise to reach 

the platen temperature. The rate of temperature rise at this 

stage is dependent on the resistance to conduction heat transfer from the platens and the loss of heat toward the center of 

the mat. 

At the beginning of the press cycle, the increase of temperature in the center of the mat is delayed due to the low thermal conductivity of the porous mat and time required for the 

steam to migrate from the surface. Convection heat transfer 

from the steam to the particles is small, since the heat capacity of water vapor is small. However, when the steam condenses, the latent heat of vaporization is recaptured as sensible heat energy, and the temperature rise in the center is then 

rapid. The gas pressure in the center increases as steam 

continues to arrive from the direction of the surface. Furthermore, conduction heat transfer, that now has accelerated 

due to the compaction of the mat and corresponding increase 

of thermal conductivity, helps to increase the temperature of 

the gas phase, and thus increases gas pressure. As temperature continues to increase, the condensed moisture will 

change back to vapor and consume latent heat energy, thus 

inhibiting further temperature rise. The build-up of gas 

pressure in the center creates a hydrodynamic potential for 

flow toward the edges of the mat. Gas flow occurs laterally 

and escapes to the surrounding environment. Eventually, the 

vertical flow of steam to the center will be less than the sum 

of lateral flow and steam loss due to condensation. At this 

time, the gas pressure in the center will have reached a 

maximum and begin to decline. 

The temperature rise in the center of the mat is a delayed 

response until significant steam flow arrives from the surface. A plateau in temperature is pronounced and at a value 

that corresponds to an equilibrium with vapor pressure. The 

greater the vapor pressure, the greater the plateau temperature. The center temperature may never rise above the plateau if there is sufficient condensed water in the center. 

Indeed, the temperature may actually decline when venting 

occurs or if the lateral steam flow is excessive. 

The process of venting the mat prior to press opening serves 

to reduce the internal gas pressure and minimize the potential for panel delamination. Venting is caused by a slow and 

controlled rate of mat expansion in the vertical direction as 

the press platens separate. The wood particles exhibit elastic 

expansion and minute voids between the particles open, thus 

increasing mat permeability and gas flow. The venting gas 

may move to the edges and toward the surfaces of the mat. 
5

The placement of a screen caul on one surface of the mat 

reduces the resistance to mass flow and accelerates venting. 

The presence of the screen caul will also reduce the rate of 

steam flow to the center of the mat early in the press cycle, 

because the steam will have an alternative pathway for flow. 

The rate of vertical and horizontal mass transfer is influenced by the porous structure of the mat. During press closure, the voids between the particles are reduced, but not 

entirely eliminated. Particle geometry, wood density, and 

compression of the wood particles all affect mat porosity and 

permeability. Wide and thin particles, such as wafers and 

strands, are able to align themselves in a planer orientation, 

thus creating strong resistance to gas flow. Thick particles 

promote gaps at their edges where particles overlap. Lowdensity wood requires more particles to produce a panel of 

fixed density compared with high-density wood. The lowdensity wood particles must then be compressed to a greater 

degree, thus eliminating more void space. The compression 

of the wood particles is not uniform from the surface to the 

center of the mat. The viscoelastic behavior of the wood, and 

dependence on temperature and moisture content, creates a 

vertical density profile. Thus, the porous structure of the 

mat, and consequently permeability, varies from surface to 

center. 

Heat Transfer Mechanisms 

The mechanisms of heat transfer during conventional hotpressing include 

• Conduction from platen to mat surface 

• Conduction within the mat 

• Convection between gas and particles in the mat 

• Convection at the mat edges 

• Bulk flow of gas in the mat 

• Bulk flow of gas at the boundaries 

Conduction heat transfer at the surface is absolutely essential 

to introduce energy into the mat from the heated platens. The 

degree of contact between the mat surface and the platen will 

influence the amount of resistance to conduction at this 

boundary. The insertion of a caul plate or caul screen will 

introduce more resistance. Instead of accounting for surface 

resistances between platen and caul, conduction through the 

caul, and caul to mat surface, an overall surface conduction 

heat transfer coefficient, U, may be defined and measured 

experimentally. The rate of heat transfer is then directly 

proportional to the difference between the platen temperature and mat surface (Eq. (1)). When the mat is under compaction pressure, the resistance at the mat surface is likely to 

be small. 

If only conduction is considered, the steady-state heat conduction through the mat is represented by Eq. (2), where 

flow is in the thickness direction. 

( ) qz = U TPlaten −Ts

•

 (1) 

z

T

qz z ∂

∂ = − •

k (2) 

where qz

•

 = heat flux in thickness direction, J/s/m2

U = surface conduction heat transfer 

 coefficient, J/s/°C/m2

 T = temperature, °C (s refers to the 

 mat surface) 

kz = thermal conductivity of the mat 

 in thickness direction, J/s/°C/m 

z = distance in Cartesian coordinates, m. 

The thermal conductivity may be dependent on the direction 

of heat flow. This is the case with oriented strand mats since 

the thermal conductivity of solid wood is approximately 2.5 

times that in the radial or tangential directions (Siau 1995). 

In any of the flat-pressed particulate composites, where the 

particle aspect ratio corresponds with the grain, there will be 

a difference between thermal conductivity in the thickness 

direction and lateral directions. This occurs because the 

particles tend to lay in a plane parallel to the lateral plane of 

the mat. Furthermore, the thermal conductivity of the mat is 

dependent on the combined thermal conductivity of the 

particles, moisture, and voids between the particles. Thermal 

conductivity increases with moisture content and density 

(Siau 1995; Kamke and Zylkowski 1989). Empirical equations have been derived for estimating the thermal conductivity of wood particle mats and panels (von Haas 1998, 

Kamke and Zylkowski 1989). Zombori and others (2003) 

estimated thermal conductivity of wood strand mats during 

hot-pressing based on the thermal conductivity of the mat 

components: wood cell wall substance, moisture, and air 

(adhesive and wax were ignored). With a thermal conductance analogy after Siau (1995), the thermal conductivity of 

the strand mat in the thickness direction was given as 

a ( ) m T m

a T z k v k v

k k k − + = 1 (2a) 

( ) T G cw w m a vd k = k + k + k (2b) 

where ka = thermal conductivity of air, J/s/m/°C 

kT = thermal conductivity of wood in 

 transverse direction, J/s/m/°C 

kcw = thermal conductivity of cell wall 

 substance, J/s/m/°C 

kw = thermal conductivity of water, J/s/m/°C 
6 

vm = inter-strand fractional void volume of mat 

vd = fractional void volume of lumens in wood 

m = moisture content fraction of wood 

G = specific gravity of wood 

The thermal conductivity of the mat in the lateral directions 

was estimated in a similar manner, with consideration for the 

strand orientation and the degree of alignment, and assuming 

the thermal conductivity of wood in the longitudinal direction is 2.5 times greater than the transverse directions. 

The specific heat of the mat is the property that determines 

the amount of energy necessary per unit of temperature 

change, per unit of mass. It may be defined for the combined 

wood, moisture, and void system, using the method of mixtures. Liquid water or bound water has a greater heat capacity than wood substance, which has a greater heat capacity 

than water vapor or air. Higher moisture content means a 

higher specific heat. 

The flow of gas through the mat is accompanied by the 

energy content of the moving gas, and thus energy is transported by bulk flow. This flow is initiated by a total pressure 

gradient, limited by the permeability of the mat, with the 

energy content of the gas represented by its enthalpy. The 

steady-state, bulk flow component of heat transfer is given 

by Eq. (3), with flow in the thickness direction. There is also 

a phase change associated with the bulk flow, which impacts 

the temperature change. The evaporation of bound water 

consumes energy equal to the sum of the latent heat of vaporization and differential heat of sorption. During vaporization, the sensible heat energy is reduced. Depending on the 

rate of vaporization, the temperature raises slowly, remains 

constant, or the temperature may decline. A declining temperature occurs when there is a drop in water vapor pressure 

coincident with rapid vaporization, such as occurs during 

venting. When water vapor condenses, the change in phase 

to a lower energy state releases energy and a rapid rise of 

temperature occurs. 

Figure 2 illustrates the significance of the latent heat effect 

on energy exchange. A purely convection heat transfer 

would transfer energy from water vapor to the wood particles without condensing the vapor. The amount of energy 

exchanged per unit volume would equal the change in enthalpy that is associated with the loss of temperature, multiplied by the density of water vapor. If the temperature of the 

vapor dropped from 200°C to 100°C, the enthalpy change is 

only 200 kJ/kg. If the water vapor density were 0.46 kg/m3

, 

this would be an exchange of energy of 92 kJ/m3

 (200 kJ/kg 

× 0.46 kg/m3). Alternatively, if the water vapor condenses to 

liquid water at 100°C, the energy released is 1.0 × 103

 kJ/m3

(2.2 × 103

 kJ/kg × 0.46 kg/m3), which is more than 10 times 

the energy exchange due to convection. 

g

g

z bulk h z

P

RT

K M P

E 















∂

∂ = −

•

η

 (3) 

where: 

•

Ebulk = heat flux due to bulk flow, J/m2/s 

Kz = gas permeability in thickness 

 direction, m2

P = total gas pressure, Pa 

 hg = enthalpy of the gas, J/kg 

 M = molecular weight, kg/mol 

ηg = viscosity of gas, Pa·s 

 R = gas constant, J/mol/K 

Mass Transfer Mechanisms 

The mechanisms for mass transfer during hot-pressing include hydrodynamic flow and diffusion. Hydrodynamic 

flow, or bulk flow, occurs as a result of a total pressure 

gradient. This mass transfer mechanism is coupled with heat 

transfer, as discussed previously. The amount of moisture 

present in the mat will have a direct influence on the maximum pressure experienced in the mat during hot-pressing. 

However, other factors, such as mat permeability, the type of 

caul system, mat size, and the rate and amount of heat transfer into the mat, will also influence the gas pressure. 

Permeability is a property of the mat that will be dependent 

on the shape of the wood particles, size of the particles, 

density of the wood, and degree of compression of the mat. 

Large cube-like particles will not pack together closely, and 

this creates many gaps for gas flow. Wide and thin particles, 

such as OSB strands, may pack together to form small gaps 

that are more widely spaced apart. As the mat is compressed, 

the gaps are diminished but not completely eliminated. This 

means that during press closing the mat permeability is high. 

After closure, the permeability is considerably smaller 

(Fig. 3). Permeability will not be uniform from surface to 

center of the mat. Since the particulate composites form a 

density gradient, the permeability will be greater in the lower 

density regions. Permeability is also a directional property. 

Von Haas (1998) measured the greatest permeability in the 

direction parallel to the grain of the strands in OSB, and the 

lowest permeability in the thickness direction. Near the end 

of the press cycle, when venting begins, the press opens very 

slowly. With the restraining force reduced, the wood particles are able to expand slightly, which opens up additional 

gaps between particles, and increases mat permeability. The 

resistance to hydrodynamic flow is then reduced and rapid 

mass transfer occurs. The pathway for flow is tortuous, with 

gas expanding around particles rather than through particles. 
7

The ratio of mat density to wood density, called compaction 

ratio, will impact mat permeability. The greater the compaction ratio, the greater the compression strain of the particles, 

and thus the smaller the gaps between the particles. Highdensity wood species would tend to have higher mat permeability than low-density wood species when pressed to the 

same panel density. 

At the mat surface and edge, convective mass transfer, diffusion, and bulk flow may occur. If a solid caul plate is present, with sufficient compression stress, no mass transfer 

will occur at that surface boundary. A caul screen would 

permit bulk flow and gas phase diffusion. At the edges of the 

mat, all three mechanisms may occur. Convection is promoted by rapid air flow of the surrounding environment and 

driven by a concentration difference between the mat edge 

and the moving air stream. Bulk flow occurs at the boundaries due to a total pressure difference. At the edges, the 

resistance to bulk flow is essentially zero. Diffusion is driven 

by a concentration difference between the mat edge and the 

surrounding environment. 

n c x = c ∆ •k (4) 

kc = convective mass transfer coefficient, m/s 

∆c = Concentration difference at boundary, 

 kg/m3

Diffusion inside the mat during hot-pressing consists of gas 

phase diffusion and bound water diffusion. It occurs as a 

result of a chemical potential gradient, which usually coincides with a concentration gradient. Gas phase diffusion 

occurs in the gaps between the particles. Random molecular 

motion will result in a high concentration of a gas dispersing 

in a direction of a lower concentration, until equilibrium is 

reached. Similarly, bound water moves through wood as a 

result of random motion of the water molecules. In solid 

wood, gas phase diffusion and bound water diffusion are 

usually lumped together, and a single diffusion coefficient is 

defined (Eq. (5)). Gas phase diffusion is much more rapid 

than bound water diffusion. Therefore, low-density wood, 

which has a large proportion of void volume, has a greater 

diffusion potential (large diffusion coefficient). 

z

c

n Dz z ∂

∂ = − •

(5) 

where Dz = combined diffusivity of water vapor 

 and bound water in thickness direction, m2

/s

 c = moisture concentration, kg/m3

Compared with other mass transfer mechanisms, diffusion is 

a slow process, particularly for bound water in the wood, 

and therefore, has a negligible influence on total mass flow 

during hot-pressing. Zombori and others (2003) studied the 

relative significance of several heat and mass transfer 

mechanisms using a simulation model. Diffusion was found 

to be negligible during the short time associated with hotpressing. After hot-pressing, when total pressure gradients 

have diminished, diffusion will become a significant factor 

for moisture movement. 

Mat Compression 

The compression behavior of the mat is influenced by its 

structure and the mechanical properties of the wood. The 

mat structure is comprised of particles and voids. The voids 

are large and dominate the mat early in the press cycle. 

Particles with a large aspect ratio are able to bridge gaps and 

contribute to compression resistance as a result of bending. 

Cube-like particles experience essentially no resistance to 

compression strain until continuous columns of wood are 

formed from bottom to top (Fig. 4). Bending or sliding of the 

wood particles contributes to nonlinear compression behavior. At the beginning of press closure, the mat compresses in 

a linear elastic manner, as shown in Figure 4A. The compression modulus of the mat is so low that the stress is barely 

measurable. During this time, the particles are largely unrestrained and slide past one another. A yield point (Fig. 4B) is 

reached when particle to particle contact is made from bottom to top of the mat and wide spread particle bending occurs. This region is marked by large strain with little or no 

stress. Period C begins when individual particles begin to 

compress and lumen volume starts to diminish. Period D 

begins when the majority of the cell lumens have been 

eliminated and cell wall substance is being compressed. 

Figure 5 illustrates the void structure in an OSB mat during 

compression. At 0% strain, large voids are evident, which 

are readily accessible for bulk flow. As the strain is increased, the voids become smaller. Many voids are completely eliminated, as shown by the arrow that follows the 

progression of one void from zero strain to 75% strain. 

Figure 5 also demonstrates that the randomized mat structure 

causes some vertical regions to be completely filled with 

particles, and thus under stress, while other vertical regions 

still have voids and no compression stress. This nonuniform 

void structure is more prevalent in mats with large particles. 

The wood particles have their own cellular structure, with 

the cell lumens comprising the voids. While under compression perpendicular to the grain, the cell walls will bend and 

subsequently buckle. With excessive strain, or large strain 

rate, the cell walls may fracture. The collapse of the cell wall 

is the primary reason for the nonlinear compression behavior 

of wood (Gibson and Ashby 1988). Wolcott and others 

(1994) demonstrated that wood under transverse compression behaves similarly to other cellular materials and that 

the nonlinear behavior is adequately attributed to cell 

wall collapse. 

When placed under a compression stress, wood responds 

with elastic strain, delayed elastic strain, and viscous strain 
8 

(Fig. 6). The elastic strain is recoverable immediately after 

removal of the stress. The delayed elastic strain is also recoverable but not immediately. The viscous strain is not 

recovered upon removal of the stress. This behavior is called 

viscoelasticity and is a characteristic of the natural polymers 

that comprise the cell wall. Temperature, moisture content, 

and time influence the viscoelastic behavior. 

It is the amorphous structure of lignin, hemicelluloses, and 

portions of the cellulose that cause the viscoelastic nature of 

wood. These polymers appear to be glass-like (stiff and 

brittle) over a short time domain, low temperature, or low 

bound water content. At long time, high temperature, and 

high bound water content, the cell wall exhibits rubbery 

behavior (ductile and soft). Between these two distinct regions is a transition zone. Considering a constant moisture 

content, constant time domain, and varying temperature, this 

transition phase is called the glass transition temperature 

(Tg). Increasing the time domain, or increasing bound water 

content, will reduce the Tg (Fig. 7). The master curve shown 

in Figure 7 explains the change in modulus of an amorphous 

polymer across a broad domain of time and temperature. In 

effect, time and temperature have an equivalent influence on 

the modulus. Wolcott and others (1994) showed that moisture content also has an equivalent influence on the modulus 

of wood, as demonstrated by the master curve shown in 

Figure 8. The tensile modulus data was shifted for temperature and moisture content. 

A wood particle mat during hot-pressing behaves like a 

cellular, viscoelastic material. There are two levels of cellularity, the lumens in the particles and the voids between the 

particles. The collapse of the voids, and subsequently the 

lumens, leads to nonlinear compression behavior. Furthermore, the transient temperature and moisture content straddles the Tg at various times and locations throughout the mat. 

Once a region in the mat reaches the Tg, there is a sudden 

reduction in modulus and densification occurs. Even after 

the press has reached the target thickness, stress is still evident. As certain regions in the mat reach Tg, the compression 

modulus will be reduced and stress will be relieved. Since 

another region in the mat may have a greater modulus, because it is below the Tg, that region may recover some of the 

elastic and delayed elastic strain. This means the vertical 

density profile continues to develop after the target thickness 

is reached. 

The viscous strain component is particularly important in 

hot-pressing. With sufficient temperature, moisture, and 

time, the nonrecoverable viscous strain is increased. This 

happens because of the enhanced ability for the amorphous 

polymer segments in the cell wall to reorient themselves in a 

new configuration, rather than merely stretching the chains. 

The increased viscous strain is realized as reduced springback out of the press and a lower potential for thickness 

swell when the composite is exposed to moisture. In 

addition, thermal decomposition may reduce the 

hygroscopicity of the cell wall, thus reducing water adsorption potential. 

Another phenomenon that probably occurs during hotpressing is mechanosorption. Mechanosorption (MS) is 

characterized by a sudden stress relaxation or increase in 

strain rate coincident with a rapidly changing moisture content. It occurs with both adsorption and desorption of bound 

water. Wu and Milota (1995) demonstrated that MS strain is 

the dominate factor in creep of small wood specimens during 

desorption. They also found that MS in compression is 

approximately three times MS in tension perpendicular to 

the grain. Several theories have been proposed as to the 

cause of MS. One theory suggests that the movement of 

water molecules to or from a sorption site in the polymer 

network requires more intermolecular space and thus a lesser 

degree of polymer entanglement and fewer, or less energetic, 

secondary bonds between polar molecular groups. Since hotpressing produces a sudden loss of bound water in the surface regions and a sudden bound water gain in the center, 

mechanosorptive strain is highly probable. 

The viscoelastic behavior of amorphous polymers may be 

modeled using springs and dashpots. The springs represent 

the elastic component, and the dashpots represent the viscous component. A spring and a dashpot in parallel (Fig. 9) 

simulates the delayed elastic response. Figure 9 shows one 

such spring and dashpot arrangement. A changing temperature and moisture content will change the value of the spring 

constant and viscosity of the dashpot. 

This discussion of the compression behavior of particulate 

mats has neglected the contribution of the adhesive. The 

degree of cure, penetration of adhesive into the wood particles, and mechanical properties of the cured adhesive, will 

surely influence stress and strain behavior of the mat 

throughout the press cycle. This topic is beyond the scope of 

this paper. 

Effect of Mat Structure and 

Press Cycle on Internal 

Mat Conditions 

Examples of measured internal mat conditions are given in 

Figures 10 through 15. These data were collected on laboratory OSB mats of final dimension 19 by 610 by 610 mm 

(0.75 by 24 by 24 in.) (Zombori and others 2003). Gas temperature and pressure were monitored at six locations in each 

mat, with reference to a vertical mid-plane. Platen temperature, target density, and mat moisture content were parameters in the study. Figures 10 and 11 illustrate the influence of 

platen temperature on mat temperature. The rate of temperature rise is much greater with the higher platen temperature 

seen in Figure 10 than Figure 11. Plateau temperatures at 

the center horizontal plane are evident at both temperatures. 

Position 6 in Figure 10 actually shows a declining 
9

temperature after the peak gas pressure is reached and begins 

to decline (Fig. 12). The 200°C platen temperature produced 

the higher internal gas pressures, with the highest at location 

5 (center-surface) and the lowest at location 2 (center-edge). 

The lower density at the center and the proximity to the 

edge, provided the least resistance to gas flow, which led to 

the low pressure. The gas pressure increase was more gradual with the lower platen temperature, as a result of less 

intense vaporization early in the press cycle and lower gas 

pressure at the surface (Fig. 13). In both cases, gas pressure 

was greater near the platen and lower near the edge, which 

supports the idea of bulk flow from the surface toward the 

center and from the center toward the edge. 

Figures 14 and 15 depict the influence of target density on 

heat and mass transfer. The medium-density mat had the 

greatest rate of temperature rise in the center, while the 

highest density mat had the slowest rate. Mat density has 

contrasting effects on bulk flow and thermal conduction. A 

high-density mat is less permeable and experiences less heat 

transfer by bulk flow. However, a high-density mat is a 

better thermal conductor. Since heat transfer by bulk flow is 

the dominant heat transfer mechanism, the high-density mat 

has the lowest rate of temperature rise (Fig. 14). In addition, 

a higher density mat has a higher thermal mass, and therefore, requires more energy per degree change in temperature. 

The gas pressure in the center of the low-density mat has the 

lowest peak value (Fig. 15). This occurs because it has a 

higher permeability and gas can more easily escape to the 

edges. The low-density mat has the lowest plateau temperature, due to its inability to hold water vapor pressure by more 

than 0.4 bar (gage pressure). 

Summary 

The hot-pressing of particulate wood composites involves 

simultaneous heat, mass, and momentum transfer. Although 

not discussed here, chemical reactions, such as thermal 

decomposition or adhesive polymerization, also are active 

during hot-pressing. The particle mat is a complex structure 

on multiple levels, including the cellular structure of the 

wood and the pseudo cellular structure of the mat. The 

transport mechanisms are interdependent. Many characteristics of wood are manifested in the mat behavior, such as 

thermal conductivity, specific heat, and compression 

modulus. However, the mat structure reorients the orthogonal nature of the individual wood particles, which leads to 

some unique characteristics. Permeability is heavily influenced by the mat structure, and plays a critical role in both 

heat and mass transfer. Direct measurement of temperature, 

gas pressure, and compaction pressure during hot-pressing 

opens a window onto the manifestations of the mechanisms 

discussed in this paper. However, a better understanding of 

the implications of the observed behavior will require a 

mathematical model, which will have the ability to investigate the interdependence of the transport mechanisms. 

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Wolcott, M.P.; Kamke, F.A.; Dillard, D.A. 1994. Fundamental aspect of wood deformation pertaining to manufacture of wood-based composites. Wood and Fiber Science. 

26(4): 496ñ511. 

Wu, Q.; Milota, M.R. 1995. Rheological behavior of Douglas-fir perpendicular to the grain at elevated temperatures. 

Wood and Fiber Science. 27(3): 285ñ295. 

Zombori, G.B.; Kamke, F.A.; Wastson, L.T. 2003. Simulation of the internal conditions during the hot-pressing 

process. Wood and Fiber Science. 35(1): 2ñ23. 
11

Figure 1óCore temperature of particleboard during hot-pressing as influenced by 

initial mat moisture content (Maku and others 1959). Platen temperature is 135°C. 

Figure 2óEnthalpy of liquid and vapor water as a function of temperature. 

0 

20 

40 

60 

80 

100 

120 

140 

160 

0 10 20 30 40 50 60 70 80 

Time (min)

Temperature (°C)

 1

11

14

22

30

Mat MC (%)

Heat of 

Vaporization

0.0E+00 

1.0E+06 

2.0E+06 

3.0E+06 

4.0E+06 

0 50 100 150 200 250 

Temperature (°C) 

Enthalpy (J/kg)

Water vapor 

Liquid water 

Heat of 

Vaporization

Heat of 

vaporization 
12 

Figure 3óOriented strandboard mat permeability as a function of direction and mat density for 

resin content of 5% (von Haas 1998). 

Figure 4óStress and strain diagram for wood particle mat under compression. 

1.0E-16

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

200 300 400 500 600 700 800 900 1000 1100 1200

Parallel

-

-

-

-

-

-

-

-

-

-

Density (kg/m3)

Permeability (m2

)

Parallel

Perpendicular

Transverse

Strain

Stress

A B 

C 

D 
13

Figure 5óOriented strandboard mat undergoing compression from zero to 75% strain. Arrows 

indicate the progression of one void (Lenth and Kamke 1996a,b). 

Figure 6óStress and strain behavior for a linear viscoelastic material. 

Time

Stress 

Elastic 

strain 

Delayed elastic & 

viscous strain 

Strain 

Delayed elastic 

recovery 

Viscous 

strain 
14 

Figure 7óTypical relaxation modulus for an amorphous polymer with time, 

temperature, and diluent concentration. 

Figure 8óMaster curve for relaxation modulus of yellow poplar shifted for 

temperature and moisture content. Reference conditions are 3% MC and 60°C. 

Log time (sec) 

Relaxation modulus (GPa)

Log time 

Relaxation modulus 
15

Figure 9óA Burger element of springs and dashpots representing the elastic, 

delayed elastic, and viscous components of a viscoelastic material. 

Figure 10óExample of internal temperature at six locations during hot-pressing of a 

laboratory oriented strandboard mat. Platen temperature is 200°C. 

Dashpot

Spring

0 

20 

40 

60 

80 

100

120 

140 

160 

0 100 200 300 400 500 600 700

Time (s)

1

2

3

4

5

6

Panel density = 609 kg/m3

Moisture content = 12%

Press closing time = 40 s

Platen temperature = 200oC

Temperature (°C) 

5 3 1

6 4 2

Side view of 24-inch by 24-inch mat

Probe locations

Side view of 24-inch by 24-inch mat

Probe locations
16 

Figure 11óExample of internal temperature at six locations during hot-pressing of 

a laboratory oriented standboard mat. Platen temperature is 150°C. 

Figure 12óExample of internal gas pressure at six locations during hot-pressing of a 

laboratory oriented strandboard mat. Platen temperature is 200°C.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 100 200 300 400 500 600 700

Time (s)

Total pressure (bar)

1

2

3

4

5

6

Panel density = 609 kg/m3

Moisture content = 12%

Press closing time = 40 s

Platen temperature = 200°C

5 3 1

6 4 2

Side view of 24-inch by 24-inch mat

Probe locations

Side view of 24-inch by 24-inch mat

Probe locations

0

20

40

60

80

100

120

140

160

0 100 200 300 400 500 600 700

Time (s)

1

2

3

4

5

6

Panel density = 609 kg/m 3

Moisture content = 12%

Press closing time = 40 s

Platen temperature = 150 oC

Temperature (°C)

Side view of 24-inch by 24-inch mat

Probe locations 6 4 2

5 3 1
17

Figure 13óExample of internal gas pressure at six locations during hot-pressing of a 

laboratory oriented strandboard mat. Platen temperature is 150°C. 

Figure 14óCenter mat temperature during hot-pressing of oriented strandboard 

for three target densities. 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 100 200 300 400 500 600 700

Time (s)

Total pressure (bar)

1

2

3

4

5

6

Panel density = 609 kg/m3

Moisture content = 12%

Press closing time = 40 s

Platen temperature = 150°C

5 3 1

6 4 2

Side view of 24-inch by 24-inch mat

Probe locations 5 3 1

0

20

40

60

80

100

120

140

160

0 100 200 300 400 500 600

Time (s)

Temperature (°C)

673 kg m3

641 kg m3

609 kg m3

Location 6 (center -core)

Moisture content = 12%

Press closing time = 40 s

Platen temperature = 200°C
18 

Figure 15óCenter mat gas pressure during hot-pressing of oriented strandboard 

for three target densities. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400 500 600

Time (s)

Total pressure (bar)

673 kg/m3

641 kg/m3

609 kg/m3

Location 6 (center-core)

Moisture content = 12%

Press closing time = 40 s

Platen temperature = 200°C
19

DiscussionóPhysics of 

Hot Pressing 

Fred Kamke 

Pablo Garcia: For the glass transition temperature of wood, 

do you know what is the significance of 200°Celsius, both 

hemicellulose and lignin seem to have the same glass 

transition temperature? 

Kamke: Yes, the data that you are referring to came from 

one particular study (Kelley and others 1987 Journal of 

Material Science 22:617ñ624). You have to realize that 

when you bring wood up to 200°C, you are going to start 

getting some thermal decomposition happening. Some of the 

polymers are going to start to decompose, and when that 

happens, you are actually changing the structure of the 

polymer. So what is the significance of 200°C here, for this 

particular experiment, I think that they have reached the 

limit at which they are able to collect data. How reliable is 

that data at 200°C? I would say it is questionable. The lines 

that you see here are just a fit of the equation to their data 

points and then that fit converges at 200°C. But is that some 

hard and fast number for dry wood? I would guess that if 

you look in the literature you will find some other results for 

the glass transition temperature of dry wood. 

David Harper: Referring to this particular study, can you 

recall what form the lignin and hemicellulose is in, or is this 

just estimated from a DMA result of whole wood? 

Kamke: Good question. This particular study used whole 

wood in a DMA. If you are not familiar with DMA, it is just 

a bending experiment, but in a dynamic way. You have some 

frequency and amplitude in which you are bending these 

small specimens. These were solid pieces of wood. They 

were only about 1-1/2 in. long and about a millimeter thick. 

You have to make some assumptions. Youíve got a mixture 

of polymers in this case that are contributing to the behavior 

and you have to then make a leap of faith and say well, that 

little peak that I see is due to hemicellulose and that peak is 

due to lignin. That is exactly what they did. They didnít just 

make wild guesses obviously, but they used results of others, 

who have been able to isolate lignin. I donít know if they 

isolated hemicellulose. Perhaps they were able to isolate 

hemicellulose and test it in another manner and then assume 

that those peaks could be attributed to hemicellulose or to 

lignin. 

Heiko Thoemen: The question about the density profile 

development is a very interesting question. We cannot look 

into the mat, so it is good there are a lot of unknowns. So far, 

what I always assumed at least for MDF, is that most of the 

density profile develops before the final thickness has been 

reached. I think I remember these graphs from Siqun Wang 

and Paul Winistorferís work. So I am interested in your 

opinion about the difference between OSB and MDF. Are 

there differences and do you agree that for OSB most of the 

density profile develops after the final position of the platens 

has been reached? 

Siqun Wang: You are right, a good question. Tomorrow I 

have one slide to show the difference between OSB and 

MDF. My opinion is MDF is very soft, and is very easy to 

compress. Another difference is MDF is pressed using a very 

long press schedule. Specifically, we can say the closing 

time for 3/4 in. MDF was probably 200-s to close to the final 

position, but for OSB we probably used a 50-s cycle to close 

press to final position. The resistance from both mats of 

course, is different. OSB mats are higher resistance, so that 

means we need to use the higher ram pressure to close the 

press. MDF is much lower. Because the mats are different, 

we press the panels in different ways. Because we are closing the press so slow for the MDF, I agree the density profile 

probably is mainly formed during the closing period time. Of 

course after closing, they still change, but maybe is a slight 

change compared to OSB. 

Thoemen: I donít really agree about the slow closing time 

for MDF, at least in a continuous press MDF is compressed 

relatively fast. But maybe one reason for this difference is 

that when the environmental conditions that reach one layer 

in the OSB mat that does not mean that the temperature and 

the moisture content of the strand changes immediately. This 

is much slower than in MDF, because in MDF, if the temperature and the moisture reach one position, then you immediately have a change in the stiffness of this fiber. Probably in OSB it is not like that. It takes some time until the 

strand picks up the temperature and the moisture content, so 

maybe that is one important difference between OSB and 

MDF. Maybe that is one of the reasons that you have delay 

in the density profile development in OSB. 

Chunping Dai: Sounds like a very serious talk. On this 

topic of vertical density profile, I sort of agree with Heiko on 

the timing of the formation. It is a very interesting subject, 

when is it formed and whether there is a continuous formation after the platen reaches to the target position. Most of us 

probably know the traditional belief is that density is formed 

during platen closing. We did some work that sort of helped 

to explain that. When you do a loading and unloading test of 

a mat, it is very easy to do, you compare the strain and stress 

relationship. During loading you have this very slow pressure build up in the beginning, and then suddenly, the pressure goes up. Then when you unload, that is the stress relaxation, that is sort of an unloading situation. When you 

unload, if you plot the load against the strain, you are going 

to see a dramatic difference in the paths. When you unload a 

lot of the compression wouldnít recover, so it is a form of 

plasticity. When you densify a mat, it stays there pretty much 

although you talk about a springback and the elasticity of it. 

Yes, there is some springback. I wonder if that springback is 

really negligible compared to what has to permanently stay 
20 

after you densify it. So in that sense, once you reach the 

platen target position, that is your loading period, and you 

densify it. The surface is densified more and core is less, 

because of the temperature and moisture difference there. 

And then you go to this stress unloading, basically, relaxation. How much of the core can recover and the surface 

continue to densify. I think there might be some experimentation to be conducted, namely this loading and unloading 

test, that is my opinion. I have another point about the effect 

of density on temperature. Maybe you can comment on this 

before I go into the next question. 

Kamke: Well I guess my comment is to both Heiko and 

Chunping in regards to fiber mats as opposed to strand mats. 

I have to admit that most of my observations are related to 

stand mats, since this is the work that I have been primarily 

involved with. So that just means that I donít have experience with fiber mats and whether or not this actually occurs 

before of after press closure with fiber mats. There may be a 

fundamental difference between the two types of mats. I still 

would stick by my earlier statement regardless whether we 

are talking about particles or strands or fibers. It is really the 

press closing time that establishes the conditions that will 

later influence the developments of the vertical density 

profile. So, for example, those of us that have fooled around 

with press schedules and press closing times know that we 

can change the shape of the vertical density profile by 

changing the press-closing rate. Siqun has shown some fairly 

intricate press closing strategies that can move the density 

profile around based on how he manipulates the press closing time and various stepwise procedures and so forth. But 

my point is that by changing the press closing time you are 

going to change the dynamics of temperature, gas pressure 

and moisture content within the mat, which will subsequently have a big influence on the development of the 

vertical density profile. As to the comment you made about 

recovery of the mats after compression, in addition to some 

viscous response that takes place in the wood, of course that 

is a response that is in the wood, you probably also have 

some reorientation of particles or fibers as a result of compression. As you compress them, the fibers may slide past 

one another and reach a more favorable conformation, so 

that when you take the load off, they have been packed 

together a little more orderly and there is not going to be as 

much recovery. I am not sure that is a real strong indication 

of viscous flow or viscous strain. And now I am going to 

finish my comment by just saying that since there are so 

many things that are happening simultaneously, I am positive that I canít work it out in my head which way it should 

go. That is why I think having these models is a way for us 

to play around with some of these relationships. Are these 

models 100% accurate? Well, no I donít believe they are 

100% accurate, but they certainly do better than what I can 

work out in my head. 

Rick Rammon: I just wanted to comment and add another 

factor into all of the discussion, I think you have to combine 

the fact that there is an adhesive curing in the system at the 

same time. That is obviously going to have some impact, not 

only on the development of the stress and strain as you close, 

but also as you release the load particularly when you have 

face and core resin systems. We can do an awful lot to impact the density profile by the rate of the cure of the resins 

and the adhesives that we are using. This throws another 

complication into the whole mix I think. 

Dai: Fred, can you go back to your experimental test on the 

effect of mat density or core temperature. I guess you mentioned the fact that density affects the temperature variation 

in terms of the conductivity and the permeability. I think 

there might be another factor, that is the specific heat. The 

heavier mat has a higher specific heat, so in a way it takes 

more energy to increase temperature by 1 degree. 

Kamke: I am glad you brought that up because I agree with 

you. The specific heat stays the same because specific heat is 

based on mass of material present. There is more material 

present in a higher density mat isnít there? Therefore, you 

have a higher heat sink. That is a good point. 

John Hunt: My question has to do with property development, where you had the 24-in. square mat, and you measured temperature at the core and 12 in., 6 in., and 3 in. from 

the edge. You had talked about a study where they looked at 

the relationship of the 24 × 24 in. mat versus a full size mat. 

I donít have a good feel for the effect of property change as 

you get closer to the edges. Is it a 1% loss as you go toward 

the edge, is it a 5% loss or 10% loss? That leads me into the 

second question. If it is significant, and it is due to steam 

pressure, is there an effect that we can have by pinching off 

around the perimeter so we control the amount of steam that 

goes off the edge, and therefore, have a more uniform 

board? So I guess my first question is, is there anything in 

the literature that you know of that has property differences. 

Kamke: First of all, I could go back to the study we did with 

the ARC (Alberta Research Council) in 1991. I probably 

could find the numbers that would tell you what percentage 

differences there were, in IB strength for example, from the 

edge to the center, but I think that difference would only be 

relevant for that particular panel type that was made with 

that schedule. The schedules were exactly the same, the only 

thing different was the size of the mat. With some of these 

properties you know there is an upper limit, if they were just 

related to degree of adhesive cure. Near the edge maybe we 

werenít getting the degree of cure that we wanted. But if you 

have just a longer press schedule the edges would catch up 

and then the end result is that you may not see a significant 

difference between the edge and the center of the mat. So, I 

donít think I can answer your question about a percentage 

difference in certain properties from edge to center. There 

may not be any if you have a conservative enough press 

schedule. Is there data out there that has been published 

about this effect? I seem to recall that there was and it was 
21

done here at the FPL, and it was Dobbin McNatt, but my 

memory escapes me. Maybe somebody here can recall. 

Thoemen: I am not sure about the literature, but someone 

from the industry might tell you there are differences between the center and the edges. From continuous presses I 

know that the density profile is pretty different between the 

cores or maybe really different between the centerline and 

edges. And I think your last question was what can we do 

about it. At least with continuous presses you have flexibility. You can change the pressing pressure over the width of 

the mat, so there are some possibilities to play around with 

and to work against these differences. 

Kamke: I am going to throw out a question here to John 

Noffsinger about this, since he seems to be one of the few 

people here today who is involved on a daily basis with hot 

pressing in the mill. Do companies also perhaps build up the 

mat thickness on the edges to counteract this variation from 

center to edge? 

John Noffsinger: Not intentionally, no we do not. If it 

happens, itís only because our foremen are not laying it 

down like they should be. 

Chuck Frihart: Obviously one of the things that youíve 

looked at is resin penetration. Does the resin penetrate on the 

chips when the adhesive is applied or does it go in during the 

pressing? Obviously if you're starting to distort cells you will 

greatly affect the penetration, so I was wondering if you 

have any feel for that. 

Kamke: I donít have a feel for that because I have never 

measured resin penetration on strands coming out of the hot 

press, just resin distribution. This sounds like a topic for this 

afternoon. Maybe we can talk about it more. 

Alain Coutier: First of all I would like to thank you for this 

presentation, I think you have covered it pretty well, it was 

excellent. I want to come back on the heat and mass transfer 

model. I have been working on lumber drying in the past and 

was starting to work on that kind of topic and I am still in the 

learning curve about that. Marsha here has been working on 

the heat and mass transfer model over the last year. When 

we look at the different models and the literature it seems 

there is kind of an agreement on how those models should be 

laid out, or developed. However, when we programmed this,

what we found is that some physical parameters or characteristics of the mat are tremendously important, and greatly 

impact the results we get. For instance, gas permeability is 

one of them. Also absorption in terms of high temperatures 

seems to be very important in the results. My question is 

what do you think are gaps in the knowledge that we should 

be looking at in terms of characterizing the mat to get those 

results?

Kamke: I think that is a very good observation and excellent 

question. I think this is a topic that we will be able to cover 

much more effectively when Heiko is up here tomorrow 

afternoon, because I know that is going to be part of his 

presentation. I will just make one brief comment now because you asked about what knowledge gaps we may have. I 

think you hit it on the head that the physical properties of 

these mats are not real well known. There is some data for 

certain types of mats over limited ranges of conditions and 

that is it. That is all we have. Obtaining that data is not 

simple. 

Dai: Fred, you mentioned about trying to increase the density around the mat edge for industrial panels. Actually some 

of the mills intentionally do that. The reason is to hopefully 

seal the steam and contain it, so that when you press you 

increase the effect of thermal softening, and the heat treatment, of the strands. Therefore, you actually have better 

properties and particularly dimensional stability. As a matter 

of fact, I have done some laboratory work to compare data in 

terms of dimensional stability or thickness swell with industrial panels. You find that your laboratory made panel is a lot 

worse than an industrial panel. One of the big reasons is that 

the industrial panel is so big that steam or gas pressure is 

well sealed during pressing. That creates so much heat and 

steam treatment that you have better properties. And also in 

going back to John Huntís question about whether there has 

been study on steam sealing. I only saw one actually. I think 

it is in German and published in one of those European 

journals. This fellow looked at a study where he actually

purposely sealed the edge and tried to contain the steam 

inside so he compared that to a case without sealing the 

edge. He measured the temperature difference and the gas 

pressure difference and also he compared the properties. 

And if you can give me your name card I can probably 

forward that to you. I donít know if that is in German or 

English, but if it is German we probably had it translated 

somehow. 

Wang: Just following your comment of the core and edge, 

based on my experience, the vertical density profile would 

be much flatter on the edge compared to the core. So what 

we do at the lab is cut one straight strip that goes from one 

edge to the other edge and then compare the density profiles. 

A single specimen doesnít represent the whole panel. I think 

an industry panel would be another story. Another question I 

want to ask, we know conductivity may not be a player to 

transfer heat from the platen into the core area, and probably 

condensation would be the major key. The question here is 

what is the percentage between how much heat is coming 

from conduction and how much heat is coming from the 

steam condensation? Another related question, if we press 

the panel, is the steam condensation higher in a specific 

layer? When we measure the temperature inside a mat, we 

know that somehow a layer would start to increase the temperature really quick. Does that mean the water vapor will 

go there and condense to raise the temperature? This is the 

case if we use the conventional press. The steam is transferred layer by layer to the core. Another question is related 

more to the afternoon session. We know steam is a transfer 
22 

mechanism. Do we have tests to know what the resin does if 

we use the different types of resin systems? Is there any 

research related to this, or the case if you used steaminjection? So that means we also have a condensation problem. Can you comment on that? 

Kamke: I would agree with the comments that you made. 

My simple explanations up here were kind of a gross separation between face and core. I think one of the points that you 

were making is that its really a phenomenon that is happening throughout the thickness of the mat as this vapor is 

making its travels from the surface areas towards the core. 

We get condensation along the way and itís a continuum of 

condensation from surface to core rather than just a discreet 

event that happens just in the core. I agree with you, its hard 

for me to think in those terms, it is easier to talk about it just 

from discreet layers of surface and core. In reality it actually 

happens in continuous fashion from surface to core, which 

adds to the complexity of the whole process. 

Jerry Winandy: I have a comment. If you could go to your 

glass transition slide showing the difference between lignin 

and hemicellulose. Weíve been studying the effect of heat on 

the chemical composition of wood and on the properties of 

wood for a number of years at the Forest Products Laboratory. We have developed kinetic models that talk about 

properties of solid wood and how they degrade over time at 

temperature and at moisture content and we have also developed preliminary chemical composition models as a function 

of thermal degrade. What I want to point out is that we know 

that glass transition temperature is a very critical function of 

a difference between lignin and hemicellulose. What we are 

finding out is that the various hemicelluloses each have a 

very distinct degradation rate. We havenít even begun to 

look at the glass transition temperature of those five basic 

components of the hemicellulose and I think that before we 

get into a very important assumption of any kind of modeling of stress build up and stress relaxation in pressing, we 

have to have a much better definition of glass transition 

temperature of hemicellulose, of the various components of 

the hemicellulose, and how those temperatures actually 

change as the components change. We see from some of the 

early data that, certain components, for example, arabinose, 

just from preparation of the flakes and of the drying of the 

flakes, that you have already seriously degraded the arabinose and you may only have 50% of the virgin arabinose left 

in the material just from initial processing. And we can go 

through each of the components and we can see that they 

degrade, or they have the potential to deteriorate, as a function of their temperature environment. We have to look at 

the temperature environment of the whole process, from the 

time we harvest a tree to the time we actually put that flake 

or the fiber material in the press, and we need to define 

those characteristics. 

Al Christiansen: One thing building on what Jerry just said, 

is that when Bob Geimer and I were doing some experiments 

on steam-injection pressing, he measured the pH of water 

coming off the press at about 3. This is with an alkaline 

phenolic resin, but you are getting de-acetylation of the 

wood. You may be changing the resin curing kinetics by the 

wood itself having an effect on the resin. 

Kamke: Al, I remember back when that was done, I canít 

remember the date, but I remember having conversations 

with Bob Geimer about making those measurements. Was 

that ever published? 

Christiansen: I donít think we said that specifically in a 

paper. I think there was a summary paper that we wrote 

together (Geimer & Christiansen 1996) where we mentioned 

it, but I donít think we did a study of it of any sort. 

Kamke: And so the data was never reported, you just commented on it? 

Christiansen: I think so. 

Kamke: By the way, Al Christiansen just reminds me Jerry 

had talked about a similar workshop that we had back in 

1990 in Blacksburg, VA. As I look around the room I think 

the only other person there, besides myself, was Al. Correct 

me if I am wrong. 

Dave Marr: I have a question on the permeability graph you 

have there, I was wondering why the perpendicular permeability wasnít closer to the transverse permeability? 

Kamke: Well, I think I am going pass this question onto 

Heiko because I suspect that Heko might have given this 

more consideration than I have. 

Thoemen: I am not really so familiar with these measurements, but from what I understand, just by introduction I 

would say that the transverse permeability must be different 

from the vertical permeability. Okay, it has to do with the 

mat factors I am not really so deep into it so that I can explain exactly why it would be bigger in the one direction, but 

I am not surprised to see this big difference. 

Kamke: I will follow up on that and point out that the data 

shown here on this graph is for an oriented stand mat. So 

with an oriented strand mat youíve got strands, I donít know 

what the size was here, but perhaps they are 4 in. long and 

maybe only 1/2 or 1 in. wide. The pathways represented by 

the gaps are quite long along the length of the strand as 

opposed to the perpendicular direction. That is just how I 

would visualize it, but I canít really say with certainty. 

Marr: Thank you Fred. It just seemed to me that since it was 

an oriented strand mat I can definitely see the pathways 

parallel to the orientation being the higher permeability. It 

just seemed like you had potential for the permeability to 

drop perpendicular because of the orientation. 

Kamke: Okay, I think I missed your question to begin 

with. You were actually asking the question between the 
23

perpendicular direction and the parallel direction in relation 

to strand orientation? 

Marr: Correct. 

Kamke: In this case again, keep it in mind this is a strand 

mat, we have relatively wide particles and the gas is going to 

move around the particles, not through them, they will follow the path of least resistance and so in the transverse 

direction these OSB strands tend to lie flat in the plane of the 

panel. The pathway around the strands is somewhat a torturous path. It has to move around the strand as it moves in the 

vertical direction. Whereas in the transverse direction, or in 

the lateral plane, of the mat it can follow edges of strands in 

that direction rather than having to go around strands. Perhaps parallel to the strand orientation the pathway is less 

torturous. 

Ted Laufenburg: I want to make a comment on a different 

subject about compaction ratios. Chunping talked a little bit 

about this. In the past we selected species based on their 

density characteristics because we thought we knew we had 

to get these particles in close proximity to each other but 

since then we have been moving more and more toward 

more dense species. Eddie Priceís work with high-density 

hardwoods was an eye opener for us because he mixed the 

two and we didnít see as much plasticization of the hardwoods as we did from the lower density softwoods. But it 

brings us to that point about how much plastic deformation 

do we have in our strands and how much of it is elastic? Our 

compaction ratios used to be targeted something like 20%, 

so you are choosing a panel density that is approximately 

20% higher then the native wood density in order to achieve 

a good quality panel. Sort of a rule of thumb, we may be 

closer to the actual density of the base wood material now 

because of the higher density species we are using. There are 

some trade-offs to be made there and I just wanted to comment. In order to get to a compaction ratio of 20% in the 

past, we probably induced between 30 and 40% plastic 

deformation in the transverse direction on these strands, and 

now we may be much less than that. At the same time that 

elastic recovery has not changed. That percentage should 

stay approximately the same and I am sure someone wants to 

comment on that. 

Kamke: I donít know if I can comment intelligently or not. I 

think the point you bring up is an important one. Upon 

changing raw material mix and the need to use higher density species, that may impact how we have to modify the 

press schedule to accommodate the higher densities. The 

scenario you just described, for example, would mean that 

we would have mats with higher permeabilities because we 

are not going to compress those higher density strands as 

much are we? We are going to have more gaps between the 

high-density particles and therefore, the higher permeabilities, which might make it more difficult to build up that 

steam pressure that Chunping was talking about, and getting 

those temperatures in the core up higher. It is an interesting 

comment in that it has implications beyond just compaction 

ratios and how much force is required in bonding, but also 

the internal mat conditions are going to be influenced. 

Noffsinger: I am not sure how to answer that question or 

even comment, but we run over 21 different species in our 

mats we make at the mill in West Virginia. In order to get 

decent properties to compare with aspen board we run densities considerably higher than what Eddie Price referred to or 

any others. We compensate for the denser species. In fact 

we are now running oak in our board, as much as 10%, and 

it requires that you run some high panel densities to get the 

properties. 

Dai: It is a very interesting subject talking about that compaction ratio to achieve close contact between strands. I did 

some work with Paul Steiner on what degree one has to 

densify the mat to achieve 80% of contact. The relationship, 

if you plot the compaction ratio on the x-axis, and you look 

at your bonding area between strands, you are going see this 

very distinct ìsî shape. In the beginning it is a little slow, but 

after initial slow starting period, it is going to go up linearly, 

very quickly and then after that, it is going to level off. This 

is based on a model that we developed. With this mat structure model you can predict the strand to strand contact. 

Knowing the strand size and the mat density variation you 

should be able to calculate that the level off point is around 

1.55 to 1.6. You have to densify the mat 50 to 60% higher 

than the original wood density. If you are making OSB using 

a wood density like 0.4, and you densify lets say 50%, your 

end density would be 0.6 and at that point you pretty much 

reach that plateau area which is giving you something like 

90% contact between strands. Now that relationship changes 

with two very important factors. One is species; with denser 

species you have to densify more to achieve that 90% contact. The other factor is strand thickness, lets say you use 

thinner strands, you can densify less. If you use very thick 

strands, you have to densify more. I will talk a little bit about 

that tomorrow, but we have sort of an equation or model in 

order to calculate that or calculate the effect of species and 

strand thickness. 

Noffsinger: What strand thickness are you talking about? 

Dai: Strand thickness in the model can be anything. 

It can be, lets say 0.4 to 1.5 mm, thatís input and you can use 

anything you want. Species, as long as you know the density, 

can be input into the equation and you can calculate that. It

is species and density related. 

Garcia: That reminds me of something else, the quality of 

the strand. Up in Canada you have aspen and you get very

large, flat strands. Down in the South you tend to look at 

them more as splinters. I donít know if it is just a species 

thing or also something that up in Canada they controlled. I 

donít know if anyone has any comment on that? 
24 

Noffsinger: Log conditioning will have an influence on 

strand quality. Not much conditioning happens in the Southern U.S., so you will have a lot more fines content coming 

through. I am just wondering how the fines affect mat consolidation and even heat transfer and everything else that is 

going on inside that board, because we will have a lot more 

fines in some of our boards at different times of the year. We 

will have to worry about resin content to compensate for 

those fines so that our bond durabilities are there and so on. 

Kamke: How does the content of fines affect mat permeability and perhaps some other properties of the mat? I donít 

think I can answer that with any degree of confidence. I can 

make some guesses and my guess would be that in regards to 

an OSB product that the inclusion of fines is going to increase permeability. I would say that simply because they 

may be smaller voids but you are going to have more voids. 

And also a big wide strand presents more of a barrier to gas 

flow certainly in the thickness direction than fines would. 

That would be my guess. We actually are doing a study right 

now on permeability of strand mats. But unfortunately we 

are not using fines content as one of the variables. We are 

using fairly nice looking, big strands, so we wonít be able to 

answer that question even after this study is completed. And 

I know that fines were not included in that work presented 

by Von Haas (1998 dissertation, University of Hamburg; 

also see Von Hass and others 1998, Holz als Roh- und 

Werkstoff 56(6):386ñ392). Maybe someone else could 

comment on that. 

Hunt: I donít have anything to comment on that last one, but 

I do have a question though. If you were to create your 

ideal mat, how would you describe that mat if you were to

optimize your physics and what would you like to see in an 

ideal world? How would you like that process to go?

Kamke: Well, that is really going to depend on what the 

product is that you are trying to produce and what other 

properties you would like to have in this product. I should 

probably say that this development of the vertical density 

profile is actually quite advantageous for sheathing products 

in a bending mode. You put the high strength and stiffness 

densified material in the outside layers of the panel and that 

works out pretty nicely for that application, but maybe not so 

good for others. It really depends on the end application that 

you are interested in. I would say that overall mat uniformity 

is important. If you can produce a uniform mat, with particles that are well defined the way that you would like them 

to be, that would be my ideal mat. And then after that, it 

really depends on how you want to manipulate the vertical 

density profile or perhaps manipulate the curing of the adhesive. But if you start with a more uniform mat structure, that 

is well defined, then your chances of achieving the latter 

conditions are going to be greatly improved. 

Jim Wescott: Fred, going back to the physics of the hot 

pressing, and this is probably something that most of the 

people in this room already figured out, but I have looked at 

the slides for 2 h and canít figure it out so I will ask the 

question anyway. You are showing on your estimated moisture content slide that there is an increase in moisture content 

for both the face and the core. I am assuming that you have 

attributed this to the resin curing or something to give off the 

water. I guess I have two questions on this. Are you actually

attributing this to resin curing condensation to increase 

moisture content? The second question is why so much more 

in the core? Why are you not seeing that in your actual vapor 

pressure of the mat?

Kamke: This is a calculation as you pointed out, and it is 

actually a calculation of the equilibrium moisture content. 

This calculation is the equilibrium moisture content condition. Then I have assumed that the moisture content at a 

specific location is equal to that equilibrium moisture content. So our assumption is that we have local thermodynamic 

equilibrium. Of the water that is moving throughout the mat, 

some of it is in the vapor phase, some of it is in the bound 

water phase and some may be in the liquid phase. It is only 

the water that is in the bound water phase that is going to be 

part of the equilibrium moisture content. That is going to be 

the only component that has an influence on the glass transition temperature, for example, so that is kind of the build up 

to answer your question. We have got moisture that is

migrating to the core and we also have change of temperature of the core. In our calculation we used a sorption equilibrium relationship which is just data that is fit to an equation that we use to estimate the equilibrium moisture content. 

It includes both temperature and the surrounding water 

vapor pressure. That is what that equation depends on, temperature and water vapor pressure. Initially in the core, we 

have the temperature increasing but the water vapor pressure 

hasnít increased a great degree, and so the equilibrium moisture content declines. Unfortunately I donít have the pressure plotted on here, but when that water vapor pressure 

increases, then the EMC increases along with it. You can 

kind of see this shape right here, if I had the water vapor 

pressure plotted on here it would follow this same pathway. 

Right here is where venting begins, water vapor pressure 

decreases, temperature stays high and so the equilibrium 

moisture content drops off dramatically as well. So the result 

of the shape of the EMC curve is due to changing temperature and water vapor pressure in the core. Out in the surface 

region the trend is more of just a general reduction in the 

water vapor pressure with a dramatic increase in temperature. So it just drops off. You see a little bit of movement up 

and down here but again that is only because of pressure 

differential. 

Hunt: In your heat of vaporization slides, the slides that 

show the enthalpy change, is there any data that shows the 

energy requirement to release water from the saturated fiber 

to a vapor stage? There is an additional energy required to 

release that? Is there anything in the literature?
25

Kamke: Yes, the graph you are referring to is just showing 

the heat of vaporization, so it is the change from pure liquid 

water to the water vapor state. If you have bound water 

instead of liquid water there is a little extra energy required 

to break that hydrogen bond with the wood substance. The 

term used for that is ìdifferential heat of sorption,î and it is a 

relatively small effect in comparison to the heat of vaporization. But is it a real effect. It is there, but you can ignore it, I 

think, in most practical cases. 

Wang: Fred, today you didnít talk about relative humidity 

related to adhesive cure? I am not really sure, is that an 

important issue or maybe it is not very important. I know 

they did research a couple years ago to see what the effect 

relative humidity had on resin bonding. 

Kamke: I think that sounds like a good question for this 

afternoon session. We talked about the gas pressures and 

some of the vast differences we have in gas pressure from 

core to edge and core to surface. Most of that gas pressure 

increase is water vapor and weíve got adhesive systems that 

interact with water vapor. Weíve got these aqueous systems 

like the PFís and UFís, and the weíve got the PMDI that 

actually reacts with water to polymerize. Does this change in 

water vapor pressure actually affect the polymerization, not 

only the rate but perhaps even the structure of the final 

polymer? If someone would like to comment on that now, I 

think it might be a good use of our time. Otherwise, I think 

we might defer it until later for an in-depth discussion. 

Christiansen: That is not directly what I was going to comment on actually. I was thinking about pure water too. When 

youíve got hot water in wood, you are not going to have a 

lot of pure water, at least in the condensed phase. Even in the 

vapor phase you are probably going to get a mixture of 

things happening in there. Especially in pine, so you have a 

very complicated system. 

Kamke: You always have to throw things in there that are

going mess things up, donít you? In these saturated water 

vapor pressure relationships that we use in these calculations 

we do assume you have pure water. If you do put some 

impurities in that water, then, yes, things are going to 

change. There are other gaseous components that contribute 

to these total gas pressure measurements that are made with 

these probes. Southern pine has a lot of pinene in it. It is 

quite volatile. I actually did a calculation one time on how 

much pinene could contribute to a total gas pressure just 

based on some information that was available on vapor 

pressure of pinene. It was pretty small, at the temperatures 

that were involved, and so I just assumed it was insignificant, but it is there, you cannot deny it is there, it has got to

be there. 

Marr: I have another question. Speaking of gas pressure, 

one of the things that I have noticed on some of your graphs, 

and some of the pressing that we have done since we have 

been monitoring gas pressures toward the end of the press 

cycle, before we go to the degas or venting stage, the gas 

pressure suddenly dropped. I am not sure exactly what that 

is? 

Kamke: I assume you eliminated any hiccup in the press 

control system. Actually, we have seen that also. Sometimes 

we have observed a pressure drop that occurs at some location where we happen to put that gas pressure probe. Do you 

know how I explain that? It is actually in a publication from 

quite some years ago (Kamke and Wolcott 1990. Fundamentals of flakeboard manufacture: wood-moisture relationships. 

Wood Sci. and Tech. 25(1):57ñ71). It had to do with this 

delayed elastic response and the continued formation of the 

density profile. I think that could happen. I donít know if that

is what happened in your case, but it could happen. The 

region where you had the tip of your probe perhaps was able 

to expand a little bit at the expense of some compression 

somewhere else in the mat at that stage of the press cycle. It 

could happen, I donít know if it actually happens. 

Pablo: Some of the drops are quite dramatic. I always put it 

down to maybe the mat shifting a little bit. The pressure is 

high enough to blow a fine out and suddenly give a venting 

path to the side. It could suddenly blow it out because some 

of the drops are quite immediate. 

Wang: During the venting period, we know the press will 

open a little bit. I canít find a good term, maybe we can use 

springback. With overpressing, that means we first press the 

panel a little bit thinner, then during the venting period time 

it opens a little bit and the panel goes back a little bit. Depending on the panel thickness, that could be 20 thousandth 

or 30 thousandth inch difference in the thickness. One of the 

research studies I did a couple years ago found that springback was not uniform during the venting period. The core 

springback is more than the face springback. 

Kamke: Our time is up. I think we really had a great discussion and I really appreciate everyone stepping forward and 

making their comments known. I enjoyed this session and 

I hope you continue on throughout the rest of today and 

tomorrow. 

Jerry: First of all, I want to thank you Fred and letís give 

him a round of applause. Fantastic lead in for the workshop 

and in leading of this discussion. I also want to thank everyone in attendance for jumping right in and facilitating the 

discussions. 
26 

Monitoring Resin Cure in the Mat 

for Hot-Compression Modeling 

Charles E. Frazier, Associate Professor, Wood-Based Composites Center 

Department of Wood Science and Forest Products, Virginia Tech, Blacksburg, Virginia 

Introduction 

The hot-compression and cure of wood-based composites is 

a process for which accurate modeling would be extremely 

useful. The many variables and their interactions are enormously complex, too complex to understand without advanced modeling. Appropriately, many contemporary research and development activities are directed towards this 

challenging goal. Success will require the convergence of 

several disciplines and the collaboration of many people in 

industrial, federal, and academic laboratories. 

Much of the recent activity has focused on heat and mass 

transfer since these are the critical variables that control 

wood cell wall properties and resin cure. Lagging far behind 

is the associated modeling of resin cure. Why is this? By 

necessity, accurate modeling is preceded by reliable measurements under realistic conditions. Herein lies the problem. 

The conditions within the particulate wood mat are as hostile 

as they are dynamic. Particularly troubling are the extremes 

in pressure and moisture; when coupled with resin levels of 

only 2% to 3%, one wonders if accurate cure monitoring is 

even possible. These difficulties have so far prevented substantial progress towards in situ cure analysis. 

What techniques are available, and which strategies should 

we devise? Under idealized laboratory conditions, we currently exploit many physical and chemical phenomena to 

evaluate thermoset cure; a few of those are 

• volumetric changes as in thermomechanical analysis and 

dilatometry, 

• rheological changes as in dynamic mechanical analysis, 

acoustics and viscometry, 

• capacitance and conductivity changes as in dielectric 

analysis, 

• chemical changes as in nuclear magnetic resonance, 

infrared, Raman, and ultraviolet spectroscopies, and 

• heat capacity changes and heat flow as in differential 

scanning calorimetry. 

There are other methods of course, and perhaps their omission will spark discussion in this forum. Depending on oneís 

perspective, many methods listed above are immediately 

excluded. Naturally, methods such as dynamic mechanical 

analysis and differential scanning calorimetry are not amenable to continuous in situ cure analysis. We must therefore 

consider separately our desires for modeling on the one hand 

and in-line process control on the other. Continuous in situ 

cure analysis would be ideal in all cases, but the practical 

barriers currently leave continuous analysis wanting for 

important details. The combination of continuous and 

discontinuous methods may be essential for building 

accurate models. 

Continuous In Situ Resin 

Cure Analysis 

Presently, it would seem that perhaps only three methods 

could be suitable for continuous in situ resin cure analysis: 

Infrared spectroscopy using optical fiber, acoustics, and also 

dielectric spectroscopy. 

Infrared Spectroscopy 

Infrared (IR) spectroscopy could be an extremely fruitful 

technique since it provides direct chemical structure information. Obviously, there are several complications within 

the wood particle mat that challenge IR analysis. One is that 

wood and resin spectral details often overlap, an obstacle 

which may render IR useless since wood signals will be so 

strong. In this respect, in situ analysis of isocyanate resin 

cure may be very rewarding. The isocyanate IR absorption is 

well resolved from wood signals, and monitoring isocyanate 

consumption should be possible. This will require optical 

sensors that are not only optimized for signal transmission 

but also for mechanical strength, size, and geometry. The 

author is unaware of optical fiber technologies that can 

withstand the bending forces present in the mat. However, it 

seems probable that the material science of optical fiber will 

meet this challenge. Considering the expected improvements 

in fiber technology, perhaps two other obstacles are more 

serious: (1) assuring resinñprobe contact, and (2) localized 

pressure sinks near the sensor. It would be difficult to ensure 

that the optical sensor will actually contact an isocyanate 

resin droplet. Consequently, IR analysis will probably require sensor doping. While sensor doping will provide signal, it may mask woodñresin interactions that could modify 
27

cure kinetics. Likewise, small voids will form near the sensor tip because of inadequate compaction. Such voids will be 

steam pressure sinks that could alter the signal. Are very 

small, and/or flat optical fibers possible? Small round fibers 

are feasible, but flattened fibers may not be. 

Acoustical Analysis 

There has been a great deal of research in acoustical analysis 

of high performance composites and also wood-based materials. The speed of sound waves traveling through a thermosetting resin may be correlated to viscosity and cure state. 

Efforts from the University of California, Berkeley, reveal 

that the acoustic monitoring of wood-based composite hotcompression is feasible. However, the potential of this 

method has not been fully developed. Recent improvements 

in hardware and signal processing have yet to be focused on 

wood mat hot-compression. This methodology deserves 

further study because it will provide novel information that 

will nicely complement other in situ methods such as IR and 

dielectric analysis. 

Dielectric Analysis 

Presently, dielectric analysis (DEA) may be the most promising for in situ cure analysis. Several researchers within the 

wood research community have been active with this 

method. Consequently, many woodñresin DEA fundamentals are understood. Furthermore, DEA hardware can be 

mechanically robust and simply integrated into hot-press 

platens. The outstanding obstacle is that press platen DEA 

sensors will observe all dipolar species, which includes 

wood, resin, and most importantly moisture. The resulting 

dielectric spectra are complex, and in this regard, dielectric 

and acoustic methods suffer a similar flaw. Many of the 

observable relaxations are convolutions of molecular response, complex summations from interacting effects. Fortunately for DEA, various dipoles display different frequency responses. The tremendous range of useable 

dielectric frequencies may offer an avenue to deconvolute 

the varied responses. As an aside, we should also recognize 

that DEA should be useful for monitoring nonresin responses, such as moisture movement and wood cell wall 

relaxations. 

Currently, it would seem that DEA may offer the greatest 

potential for in situ cure analysis. However, the great complexity of DEA relaxations in the wood mat will require 

other complementary methods. We must improve our 

knowledge of the events which correlate with DEA relaxations. For example, DEA may help us observe cure levels, 

but it cannot reveal other important details that would be 

desirable for hot-compression modeling; details such as 

mechanical strength development, chemical structural 

changes, and cell wall resin penetration. A concerted effort 

is required to develop correlations to continuous analysis. 

This is why discontinuous methods will be important to 

achieve better models of resin cure. 

Discontinuous Resin 

Cure Analysis 

Even as continuous in situ methods improve, we will need to 

correlate continuous data streams with more traditional resin 

analyses. This requires the insertion of specialized samples 

into strategic locations within the mat. The mat is hotcompressed for discrete time periods, and samples are retrieved for analyses in the laboratory. This discontinuous 

approach will be challenging and labor intensive. Furthermore, special methods will be required to quench cool samples so that accurate correlations are developed with continuous methods. The need for sample quenching presents a 

considerable challenge. On the one hand, it would seem that 

very small mats (less than 0.6 by 0.6 m (2 by 2 ft)) would be 

most amenable to quench cooling in liquid nitrogen. However, such small wood mats may not accurately reflect industrial conditions. On the other hand, larger mats may be difficult to handle, particularly if the test sample must be quenchcooled after removing the panel from the hot-press. Perhaps 

rapid sample removal and quenching requires the use of 

nonresinated wood mats. This would provide a realistic mat 

environment and would also allow for the rapid removal of a 

sample. 

Aside from an effective sample quenching strategy, other 

aspects of sample preparation must also be considered. 

Obviously, the sample must not be damaged during retrieval. 

Consequently, techniques must be developed to prevent 

adhesion of the sample to the surrounding wood particles. 

Also and depending upon the subsequent analysis, extreme 

bending forces must be prevented from acting on the sample. 

Finally, the sample form must not influence the localized 

heat and mass transfer. It is likely that careful thought and 

ingenuity could solve these challenges. What methods will 

be useful for the subsequent analysis of these retrieved 

samples? 

Differential Scanning Calorimetry 

Differential scanning calorimetry (DSC) could be very useful because this will provide a direct measure of residual 

heat of thermoset cure. However, it is unlikely that DSC 

could detect the very low resin loadings that are typical for 

particulate wood-based composites. The only way around 

this problem is to prepare samples that have unusually high 

resin loadings. Naturally, we must understand if high resin 

loadings influence the very observations we wish to make. 
28 

Rheological Analysis 

DSC should be useful. However, it does not reveal gelation 

and so cannot tell us about mechanical strength development. Could dynamic mechanical analysis be an effective 

discontinuous analytical method? Perhaps, but this would 

require a great deal of method development. As with DSC, 

higher than normal resin loadings might be required. 

Solid-State Nuclear 

Magnetic Resonance 

Solid-state nuclear magnetic resonance (NMR) could be very 

helpful to correlate continuous data streams with chemical 

changes in the resin. It can also track morphological information through relaxation measurements. The associated 

complication with this approach is that the resins must be 

synthesized with magnetic isotopes to provide detection 

sensitivity. In the case of formaldehyde resins such as phenol- or urea-formaldehyde, this is not a difficult task. Carbon-13 labeled formaldehyde is readily available for direct 

use. However, it would be best if an industrial manufacturer 

could prepare these labeled formaldehyde resins so that the 

properties better match industrial preparations. 

Labeled isocyanate resins are quite a bit more difficult to 

prepare, but fortunately, an experienced chemist can closely 

match (but not exactly match) the properties of industrial 

isocyanate resins. 

In summary, the industrial development of wood-based 

composites will be improved with accurate modeling of the 

hot-compression process. So far, most modeling efforts have 

correctly focused on heat and mass transfer. However, we 

should strive to incorporate resin curing in these models. 

The most desirable methods would provide continuous in 

situ cure analysis, as in the case of infrared, acoustic, and 

dielectric spectroscopy. However, none of these methods 

will provide all of the information that we will desire. Consequently, there will also be a need to correlate these continuous methods with discontinuous techniques that involve 

sample retrieval and subsequent analysis. Presently, a great 

deal of work is required to establish valid correlations 

between continuous and discontinuous methods. 
29

Adhesive Interactions With Wood 

Charles R. Frihart, Project Leader 

Forest Products Laboratory, Madison, Wisconsin 

Summary 

While the chemistry for the polymerization of wood adhesives has been studied systematically and extensively, the 

critical aspects of the interaction of adhesives with wood are 

less clearly understood. General theories of bond formation 

need to be modified to take into account the porosity of 

wood and the ability of chemicals to be absorbed into the 

cell wall. Sufficient penetration is important for good bond 

formation, but it is not clear whether the penetration into 

lumens or the cell wall is more critical. For the bond forming 

process, thermodynamic wetting and viscosity are critical, 

but viscoelastic energy dissipation is important for the 

debonding process. The interphase formed during bond 

formation should influence the bonded assemblyís ability to 

dissipate the internal and external stress during use, but the 

exact relationship is not clear. Studying of the bonding and 

debonding processes can be broken down into examination 

on the millimeter, micrometer, and nanometer spatial scales. 

The millimeter and larger scale is what is used for typical 

evaluations. However, the micrometer scale plays an important role because this scale relates to the cross-sectional size 

of cells that are the basic structural unit of wood. In this 

study, models are proposed for the adhesiveñwood interphase that would reduce the stress concentration between the 

wood and the adhesive as the dimensions of the wood vary 

with changes in the wood moisture content. The nanometer 

scale is important because it relates to the interaction of the 

adhesive with the wood on the scale of the cellulose fibrils 

and the hemicellulose and lignin domains. The internal and 

external forces on the adhesive wood bond vary considerably for different products, such as laminated lumber, plywood, oriented strandboard (OSB), and fiberboard. A better 

understanding of the critical aspects of the adhesiveñwood 

interaction should lead to a more efficient development of 

improved adhesives. 

Introduction 

Adhesion and adhesive strength are often confused. Adhesion is the interaction of the adhesive with the substrate 

surface and is mainly influenced by thermodynamic wetting 

and rheological properties. Adhesion is critical to forming a 

bond, but the woodñadhesive interface is not the only location where energy is dissipated in adhesive strength testing. 

Adhesive strength is the ability of the bonded assembly to 

remain intact under some set of conditions that test its ability 

to viscoelastically dissipate energy and withstand chemical 

degradation. For most processes, it is important to consider 

the differences between wood and other substrates, which 

will alter the models for bonding and debonding. 

An adhesive goes through three stages. In the first stage, the 

adhesive is a liquid that needs to flow to wet the surface of 

the substrate. To wet a surface well, the adhesive needs a 

lower surface energy than the substrate; the difference in 

surface energies between the substrate and the adhesive is 

determined by the contact angle. A low contact angle indicates that the adhesive will wet the substrate surface given 

enough time. However, a highly viscous adhesive may not 

sufficiently wet the surface especially those with substantial 

microroughness (Pocius 1997). Thus, the rheolological 

properties in the adhesive are also very important. Without 

the adhesive coming into contact with the substrate on a 

molecular level, it is not possible to form a bond. 

The second stage is for the liquid adhesive to turn into a 

solid. The three main processes for converting the liquid to a 

solid adhesive are polymerization, loss of solvent, and solidification from the melt. Polymerization is the most common 

for making composite wood products. A number of these 

adhesives also contain solvent, mainly water, which is lost in 

the process. It is important for the adhesive to wet the substrate before the solidification process takes place because 

during polymerization or drying, the viscosity of the adhesive increases, which reduces wetting. 

The third stage is the ability of the bonded assembly to stay 

intact under the desired end-use conditions. Durability is a 

very important issue in bonded wood assemblies since most 

adhesives give strong bonds to wood initially. How durability is determined depends upon the product and how it is 

used. Because bonded wood products have a long lifespan, 

accelerated tests are usually used to determine durability. It 

needs to be understood that integrity is dependent upon the 

viscoelastic energy dissipation in all portions of the product 

and not just the adhesiveñwood interface. In addition, the 

internal stresses from the dimensional changes in the wood 

need to be considered as well as the applied forces. 

Why is it important to understand the interaction of the 

adhesive with the wood? Currently, there are no good ways 

to screen new or improved adhesives by measuring their 

physical and chemical properties. The only accepted way to 

measure the performance of the adhesive is to bond the 

selected wood or fiber into a desired product and test it 

under accepted standards. Thus, an extensive amount of 
30 

work is needed to test each formulation. If we understood 

how the adhesive interacts with the wood, we could possibly 

evaluate new adhesives through other properties and accelerate development of new or improved adhesives. 

Performance of Bonded 

Assemblies 

Because there are many different domains in which a bonded 

assembly can come apart, it is important to understand where 

in the bonded assembly the failure has occurred and why. If 

the failure occurs within the wood, then the adhesive has 

acceptable performance. In this instance, improving the 

adhesive performance does not add to the strength of the 

overall assembly. Cohesive failure in the bulk of the adhesive indicates a need to improve the cohesive strength of the 

adhesive. These first two types of failure are generally easy 

to observe. However, failure in the interphase region is much 

harder to analyze. The interphase region is defined as the 

point where the local properties begin to change from that of 

the bulk adherend to the point where the local properties 

reflect that of the bulk adhesive (ASTM 2003). Part of this 

region is the true interface where the bulk material changes 

from adhesive to the wood. The adhesion process reflects 

mainly what takes place at the true interface. If the adhesive 

does not sufficiently wet the wood, the interface is the most 

likely place for failure to occur. It is important to consider 

that failure often occurs in the adhesive and the wood interphase regions. 

There has been investigation on the strength of the wood 

interphase in bonded assemblies. The preparation of the 

wood surface for bonding has a significant effect on the 

strength of the wood surface (River 1994). The most common occurrence is the collapse of the wood cell structure to 

form a mechanically weak interphase (River and Minutti 

1975, Stehr and Johansson 2000). If the adhesive does not 

penetrate and strengthen this region, then it will serve as the 

weak link in the bonded assembly. The preparation of wood 

surfaces can also induce fracture in the cell walls that can 

lead to cleavage of the cell wall layers. Another aspect is a 

chemically weak interphase (Stehr and Johansson 2000); this 

usually occurs when there is a layer of low molecular weight 

compounds, such as extractives, on the surface. Examples of 

this problem are bonding to an oily wood such as teak and 

bonding to creosote-treated wood. Fracture within the adhesive interphase has not been discussed much in wood bonding literature, but will be covered later in this paper. 

Determining where this failure occurs is much more difficult 

with wood than it is with most other adherends. One difficulty is the relatively rough surface of the wood that makes it 

hard to use light microscopy, scanning electron microscopy 

(SEM), and atomic force microscopy (AFM). A second 

difficulty is the chemical complexity and inhomogeneity of 

the surface that makes it hard to use many surface 

compositional analysis techniques, such as x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). Light microscopy is somewhat easier for dark 

colored adhesives, but it is hard with uncolored adhesives. A 

main problem with the literature is that it only gives percentage wood failure (ASTM 1999), if it indicates at all where 

the failure occurs. When the failure occurs in a bondline, 

there is no indication if the failure occurs in the bulk adhesive or in which part of the interphase. 

The examination of where and why bondline failure occurs 

is under investigation. Many epoxy adhesives give a high 

percentage wood failure when dry, but usually all give low 

wood failure when wet (Vick and others 1996). The failure 

of the epoxy is unexpected given the fact that epoxies give 

durable bonds to most substrates. Given the polarity of the 

epoxy, it should be able to form strong secondary bonds to 

the wood, and therefore interfacial failure seemed unlikely. 

In trying to understand where in the bondline the failure was 

occurring when epoxy bonds were subject to water and 

drying but no external loads, we have relied upon a number 

of techniques. The first was to use light at an obtuse angle to 

look at the glossiness of the surface because adhesives give a 

smoother surface that tends to increase the gloss compared 

with wood. Another method was to use stain of the surface, 

which reacts with the amine groups of an epoxy, such as pdimethylaminocinnaminaldehyde (Frihart 2003a). These 

techniques show that the fracture surface ran along the plane 

of the wood with morphology very similar to the wood, but 

containing a significant amount of adhesive residue on the 

surface (Fig. 1). In addition to the light microscopy studies, 

scanning electron microscopy gave support to our observations. The SEM images showed adhesives filling the lumens 

with evidence of viscoelastic failure of the adhesive. Thus, 

failure in the adhesive interphase has to be considered as 

well as in the wood interphase, interfacial, and adhesive bulk 

when there is bondline failure. 

Spatial Level of Examination 

Wood bonding surfaces and failed bondlines need to be 

examined on three different spatial scales (millimeter, micrometer, and nanometer). The millimeter or larger scale is 

most often used in examining wood bonding (Frihart 2003b). 

In preparing wood surfaces, one looks for the crushing of 

cells and roughness of the surface usually using crosssectional analysis at this spatial scale. This scale is also used 

for measuring wetting of the surface, although wetting can 

be affected by factors in smaller dimensions. In addition, the 

percentage wood failure is determined at this level. Even 

though it is important to look at fracture bondlines at this 

level for the initial evaluation, it is also important to understand what is happening at finer levels. 

Understanding what is happening at the micrometer level in 

both bond formation and bond breakage is essential to 
31

understanding the adhesive performance. Because cells are 

in tens of micrometers in diameter, this level indicates the 

interaction of the adhesive with the basic unit of construction 

of the wood. Given the variety of wood cell types, it is unwise to consider the adhesiveñwood interaction being of a 

single type. For example, in hardwoods, there should be 

large differences between the flow into the vessels compared 

with the fiber cells. The vessels should be easy to fill given 

their large diameters compared with the fiber cells and rays. 

The trachid cells of softwoods are in between these two. 

During bond fracture, it is important to consider the stress 

concentration as the dimension of the cells changes during 

swelling, as will be discussed in more detail later. 

The nanometer scale has a profound effect on the bonding 

process that contributes to bond strength, but it is the hardest 

of the three spatial scales to examine. For bonds to form, the 

adhesive and wood have to interact on the nanometer scale. 

This is independent of whether this interaction involves 

mechanical interlock or primary or secondary bonds. The 

literature indicates that cell fibrils and hemicellulose and 

lignin domains are in the order of ten nanometers (Fengel 

and Wegener 1984). Thus, the interaction of the adhesive 

with the wood components in the cell wall is taking place at 

this level. Fracture of the bonded substrate requires the 

breaking of primary and secondary bonds, which are nanometer scale events. 

As evidence that one needs to consider what is taking place 

in all three scales, we will examine events during bond 

formation. First, the adhesive needs to wet the surface of the 

wood; this process depends upon the thermodynamics because the surface energy of the adhesive needs to be less 

than that of the wood. For this event to take place in a reasonable time frame, the viscosity of the adhesive needs to be 

low enough. General observation of the wetting process 

takes place at the millimeter scale. However, penetration into 

the lumens that has been shown to have a significant effect 

on bond strength is on the micrometer scale. The adhesive 

flow into the cell lumens may be different from the flow of 

the adhesive itself due to the filtration of the fillers from the 

adhesive. In going to the nanometer scale, a completely 

different series of factors become critical. For penetration 

into this cell wall, the molecular size and shape of the cell 

and the solubility parameter become critical. 

Cellular Level Processes 

Because wood is made up of a variety of cells and the cells 

are made up of different cell layers, it is a mistake to think of 

wood as a single type of surface. Consideration needs to be 

given to what types of surfaces are available for bonding for 

each product class. For lamination, OSB, and particleboard 

binding, it is desirable for the adhesive to wet the exposed 

lumen walls to give greater area for bonding. Longitudinal 

transwall fracture should make the lumen walls available for 

bonding (Fig. 2) (River 1994). Depending upon species and 

cell type, the lumen wall may be the S 3 layer that has high 

cellulose content or it may be the warty layer that has high 

lignin content. For some earlywood cells and vessels, the 

lumen walls can be up to 80 percent of the bonding surface. 

When the walls split, it is not known whether it is through 

the lignin, hemicellulose, or cellulose domains. If the types 

of molecules on the surface are not known, the type and 

strength of bonds between the adhesive in the wood are a 

matter of conjecture. However, under some conditions, such 

as high cleavage temperatures of latewood cells, the cells 

may be split along the middle lamella giving a surface that is 

high in lignin content. The chemical composition of the 

surface can play a major role in determining the type of 

surface interaction with the adhesive and therefore the interfacial bond strength. When using a water-borne adhesive, a 

cellulosic surface should be easier to wet and should more 

readily form polar bonds and potentially covalent bonds 

with the adhesive. On the other hand, a lignin surface will 

be harder to wet with a water-borne adhesive and will show 

preferential interaction with only a limited number of 

adhesives. 

In the bonding process, sufficient penetration is important 

for forming strong bonds. This is generally considered to be 

the flow into the lumens of the cells that have an opening on 

the wood surface. The tendency is to fill the large diameter 

cells like vessels first and then the smaller cells for those that 

have a portion of their cell wall removed on the surface. It 

has not been demonstrated that the adhesive flows from one 

cell to another through the pits. The flow into lumens depends upon the viscosity of the adhesive, applied pressure, 

temperature, and time. On the other hand, some of the adhesive components can flow into the cell wall, as will be 

discussed in a later section. Thus, it is not clear whether 

penetration into the lumens or into the cell wall is the 

most critical. 

As in the bonding process, the cellular structure of wood is 

important in evaluating the durability of bonds. For example, 

the adhesive could bond strongly to the surface of the wood, 

but that surface layer could peel away from other layers of 

the cell (Fig. 3). This has been shown in at least one case 

with epoxy adhesive (Saiki 1984). In some cases, the cell 

walls may be weakened by cracks that developed during the 

drying process or the preparation of the surface by cutting or 

planing. Even if the cell is not damaged, we still need to 

know how the adhesive responds as the cell expands and 

shrinks upon absorption and desorption of moisture. Although drying of the wood should cause compressive force 

on the adhesives, the tensile forces upon moisture pickup 

generally are more critical because many adhesives are 

weaker in tension than in compression. 

Adhesion Models 

The various models for the interaction of the adhesive and 

the substrate can be classified in a number of ways. For this 
32 

discussion on wood bonding, the four general categories are 

mechanical interlocking, diffusion, interface interactions, 

and electrostatic interactions. Given the porosity of wood, 

mechanical interlocks through the filling of the lumens was 

the original concept (McBain and Hopkins 1925). It certainly has to play a role because it allows the stress to be 

distributed over a greater volume. Stress concentration is bad 

because it causes the breakage of chemical bonds leading to 

fracture of the bonded assembly. For mechanical interlock to 

occur, the adhesive has to wet the surface of the wood and 

be low enough in viscosity to flow into the pores. Although 

wood is a relatively polar substrate, water-borne adhesives 

usually contain additives or solvents to aid in wetting of the 

wood. Diffusion can be thought of as a molecular level 

mechanical interlock. With most adhesive bonds, diffusion 

does not play a significant role in bond formation. It is clear 

that few adhesives have the ability to penetrate metals, and 

most adhesives are not compatible enough with organic 

polymers to flow into them. On the other hand, it has been 

shown that wood will swell by absorbing small molecules 

into the cell wall. Thus, diffusion may play a more important 

role for wood bonding than in most other adhesive applications. Interface interactions were proposed to be important to 

wood bond strength more than 70 years ago using the terminology specific adhesion (Browne and Brouse 1929). The 

interface interactions are generally classified into primary 

and secondary bond categories. The primary bonds are 

covalent bonds whose existence in wood adhesion has been 

an area of disagreement due to the complexity of wood that 

makes it difficult to carry out definitive experiments. However, recent work supports the formation of primary bonds 

with polymeric diphenylmethane diisocyanate (Frazier 

2003). The secondary bonds fall into several categories, 

ranging from weak to strong interactions. The weakest force 

is the van der Waals forces that exist between all molecules 

in close proximity. For wood adhesives, stronger interaction 

forces are available because both the wood and the adhesives 

possess polar groups. Dipolar interactions should be plentiful between any of the main components of wood and the 

common wood adhesives. Because hydroxyl groups are 

present in the cellulose, hemicellulose, and lignin fractions, 

hydrogen bonding to the wood adhesives should also occur. 

The other type of bond that has been proposed is electrostatic interaction, but this type of mechanism is more likely 

to occur in debonding rather than during bond formation. 

An interesting issue in wood bonding is the penetration of 

the adhesives into the cell walls. The migration of phenolformaldehyde resins into the cell walls has been shown using 

fluorescence microscopy (Saiki 1984), audioradiography 

(Smith 1971), transmission electron microscopy (Nearn 

1965), scanning electron microscopy with x-ray dispersive 

emissions (Smith and CotÈ 1971), dynamic mechanical 

analysis (Laborie and others 2002), and antishrink efficiency 

(Stamm and Seborg 1936). For polymeric adhesives (methylene diphenyl diisocyanate), the presence of adhesives in 

the cell walls has been shown by x-ray micrography, nuclear 

magnetic resonance spectroscopy (Macinko and others 

2001). Some of the same techniques and other techniques 

have been used to show the presence of urea-formaldehyde, 

melamine-formaldehyde, and epoxy resins in the wall layers 

(Bolton and others 1985, 1988; Furuno and Goto 1975, 

Furuno and Saiki 1988), as well as UV microscopy (Gindl 

and others 2002) and nano-indentation (Gindl and Gupta 

2002). The question is whether the presence of the adhesives 

in the cell wall is just a byproduct of the process or actually 

contributes to the strength of the bond. Most certainly, the 

presence of sufficient adhesive material in the cell wall 

should change its properties, especially altering the swelling 

and shrinking properties. Four different models are proposed 

here to explain the possible interactions of the adhesives and 

the walls. The simplest model is that the adhesives migrate 

into the expanded wall and occupy the free volume reducing 

the shrinkage and re-expansion of the wall during the loss 

and gain of moisture. The second model is that fingers of the 

adhesive penetrate into the wall to form a molecular interdigitation; this can be thought of as nanometer scale mechanical interlock (Fig. 4) (Berg 2002). The third model 

involves the adhesive forming a separate network in the wall 

as either a cross-linked adlayer or a fully interpenetrating 

polymer network. The fourth model involves the adhesive 

components reacting with the cell wall components to crosslink them. All of these models would involve limiting the 

ability of cell walls to expand and contract. 

Reducing the volume change of the wood would also reduce 

the stress concentration between the wood and the adhesive. 

Swelling of the wood causes a large stress gradient at the 

adhesiveñwood interface if the adhesive expands in a similar 

fashion. Some work indicates that phenol-resorcinolformaldehyde adhesives expand in water like wood, but not 

as much as wood in the tangential direction (Muszyński and 

others 2002). Because wood expands greatly when wet in 

the radial and tangential directions, the adhesive at the interface is under a great deal of tensile stress in those directions 

but is under less stress in the longitudinal direction. For 

adhesives with limited ability to elongate, such as epoxy 

adhesives, fracture can be initiated at the adhesive surface. 

Because there is limited swelling in the longitudinal direction, the fracture pattern should have similar directionality 

and morphology to the wood. Adhesives that are more ductile are more likely to resist fracture. 

The importance of considering the cellular structure of wood 

in relation to bond strength developed from trying to understand why epoxies do not give durable wood bonds. Some of 

this work was discussed in an earlier section. Additional 

work has involved the examination of bonding to acetylated 

wood, which involves converting the exposed hydroxyl 

groups to acetates. If the interface interactions were the 

critical link in this adhesion, then acetylation of the wood 

should reduce the adhesion due to decreased strength of the 

polar interactions. Surprisingly, the acetylated wood gave a 
33

more durable bond with the epoxy than did the unmodified 

wood. The much greater percentage wood failure in the wet 

shear block test of the acetylated wood compared with the 

unacetylated wood is consistent with reduced swelling forces 

at the interface of the acetylated wood. In support of this 

theory, planing of the acetylated wood gave less durable 

bonds probably because of the opening of unacetylated sites 

on the surface of the wood by the planing process that led to 

increased swelling forces (Frihart and others 2004) (Fig. 5). 

Effect of Composite Type 

on Adhesive Performance 

Each type of composite places different demands on the 

adhesive for acceptable performance. All applications require sufficient shear strengthódry strength for interior 

applications and both dry and wet strength for exposure to 

high moisture levels and exterior applications. For most 

applications, the adhesive needs to be cross-linked to have 

sufficient creep resistance because there are generally applied forces in addition to the internal forces. However, both 

the applied and internal forces vary considerably for the 

different types of composites. 

For laminated materials, usually an entire film of adhesive 

exists between the pieces, allowing the stresses to be distributed across a large surface area. Laminated lumber has significant applied loads in the longitudinal direction that the 

adhesive bond has to withstand while the wood swells and 

shrinks with variations in moisture levels in the cross direction. In contrast, plywood bonds need to withstand high 

internal stress levels rather than high strain levels because 

cross plies restrict swelling of the wood. In addition, plywood adhesives are sometimes formulated to give better 

hold, but the surface roughness can limit the ability to obtain 

close contact in some areas of the surface. Some plywoods 

are rated for exterior applications; thus, they have to withstand greater swelling and shrinking internal forces as well 

as not being hydrolyzed by the water. 

For products made of smaller pieces of wood, the adhesive 

does not cover the entire wood surface. This is a significant 

difference in that the adhesive is applied as droplets instead 

of a continuous film; thus, it is like the comparison of a spot 

weld to a continuous weld. With a spot weld there is a higher 

stress concentration in the bond than there is with a continuous weld. At high moisture conditions, the bonds feel not 

only the normal swelling stresses but also the stress from the 

wood wanting to spring back to its normal shape from the 

distorted shape it obtained during the compression step. 

Composites made from chips, like OSB, have very different 

performance requirements than do laminated assemblies. 

Most laminated products are subjected to shear tests; OSB is 

tested using an internal bond test that exerts a normal force 

on the bond. A complication of this test is that most adhesives are weaker in the normal direction than they are in a 

shear direction, and full cure is more likely to be important 

in this test. It is well known for bonding OSB that there are 

large temperature and moisture gradients that alter the adhesive cure rates. This becomes critical when the internal steam 

pressure exceeds breaking stress of the adhesive, leading to a 

blowout if the adhesive is not highly cured. The stress at 

break is low for the uncured resins, but increases as the 

polymerization and cross-linking take place. 

There are dramatic differences when talking about making 

fiberboard. In making fiber, the normal mode for fracturing 

the wood structure is to separate the cells in the middle 

lamella as opposed to a planing operation that aims to split 

the cells open. Consequently, the aim is to bond to the exterior part of the cell rather than the interior part. The adhesive 

is more likely to experience a very different bonding surface 

with fiberboard than it does with other applications. Given 

the differences in the wood surface between fiberboard and 

other applications and the ability to bring fibers closer together in fiberboard, self-adhesion is a more significant 

factor with fiberboard than with other applications, especially in the case of high-density fiberboard. 

Nanometer Scale Interactions 

For adhesion to occur, the adhesive has to encounter the 

wood on the nanometer scale level. This is the level about 

which we know very little, and there are many limitations in 

advancing our understanding on this spatial level. The complexity of the wood on both the morphological and chemical 

aspects makes it difficult to understand exactly what makes 

up the bonding surface. For example, earlywood cells split 

by longitudinal transwall fracture have a large ratio of lumen 

walls to other cell wall surfaces leading to a large difference 

in bonding area compared with latewood cells split in the 

middle lamella. The lumen wall can be composed of a high 

lignin content warty layer or high cellulose content S 3 wall. 

The cleavage of cell wall layers can yield both lignin and 

cellulosic compounds on the surface. The different cell types 

can be cleaved in different locations due to their ability to 

bear the applied load. In addition, the wood is comprised of 

more than the normal tracheid and fiber cells that offer 

different bonding surfaces. The fiber that goes into fiberboard probably has a quite different surface from the wood 

produced by cleavage for plywood or OSB. A surface that 

has high lignin content would have different bonding interaction with the adhesive than a surface that is high in cellulose or hemicellulose. Thus, without a clear idea what components are at the surface, it is hard to predict how the 

adhesive is going to interact on a molecular basis with 

the wood. 

Diffusion of the adhesive into the cell wall is also important. 

It has been shown that some adhesive components will 

migrate into the cell wall, but it has not been demonstrated 

that these components play a role in bond durability or how 
34 

these components interact with the cell wall components. 

The voids between the polymeric components in the cell 

walls must be small, and the migration of the adhesive components is likely to be determined by their molecular sizes 

and shapes and their solubility parameters. This is also a 

nanometer scale process. When wood swells, water most 

likely inserts itself between the components that are hydrogen bonded together, such as the cellulose and hemicellulose. The adhesives might play a somewhat similar role of 

migrating between the polymer chains. However, only certain adhesive components can enter the cell wall. Generally, 

the cutoff is about three thousand molecular weight for 

linear polymers (Tarkow and others 1966). Penetration is 

also dependent upon the solubility parameter of the adhesive 

relative to the wood components. Bulkier molecules are less 

likely to fit in-between the other polymers and may not 

penetrate as well. These adhesive components can then 

polymerize to form a reinforcing network within the cell 

wall (Fig. 6). However, some of the less polar adhesives 

might prefer swelling the lignin rather than the cellulosic 

components. 

Summary 

Understanding the role of wood adhesives is difficult because of the complexity of wood, which allows the proposal 

of many models. In trying to understand wood bonding, it is 

important to realize that the factors related to bond formation 

are probably not identical to those related to the debonding 

process. In general, the bond formation process is highly 

dependent upon thermodynamic wetting and the rheology of 

the adhesive. The chemical composition of the wood surface 

also plays a large role in the bonding process; the portion of 

the cellular structure that is exposed controls the interaction 

with the adhesive. Lignin on the surface would lead to 

poorer wetting by the adhesive and less hydrogen bonding 

between the adhesive and the wood surface. On the other 

hand, more cellulose or hemicellulose on the surface would 

have the opposite effect. From the models of bond formation, it is likely that the bond formation would involve more 

than interfacial interactions. It is known that many adhesives 

will migrate into the cell wall. These adhesive molecules 

could form nanometer scale mechanical interlocks, an interpenetrating polymer network, or crosslink cell wall components. Understanding whether these adhesive components 

stabilize the cell wall could lead to the design of improved 

adhesives. 

Bond strength is dependent upon the ability of the assembly 

to distribute both the internal and applied stresses. The internal stresses can be large due to swelling and shrinking of the 

cells as the moisture changes. For example, penetration of 

and reaction of the adhesive components in the cell wall may 

play a role in bond durability by reducing the volume 

changes during the moisture cycling and therefore the stress 

concentration at the wood adhesive interface. The type of 

assembly also plays a large role in the type of forces to 

which the bond is subjected. For laminated products, the 

laminated lumber needs more ductile adhesives to cope with 

the swelling, while the plywood requires adhesives that have 

high stress at break because the cross plies limit the elongation. For other composites, the normal bond properties are as 

important as the shear properties, because these products 

need to pass internal bond tests. 

For both the bonding and debonding processes, the understanding has to be done on three very different spatial scales. 

The normal consideration is on the millimeter or larger scale 

because this is the easiest to observe. Given that wood is 

composed of cells, the micrometer scale is important for 

understanding the more detailed interaction of the adhesive. 

Finally, all the processes are actually taking place on the 

nanometer level. 

Literature Cited 

ASTM. 2003. Standard terminology for adhesives. ASTM D 

907ñ03. West Conshohocken, PA: American Society for 

Testing and Materials. 

ASTM. 1999. Standard practice for estimating the percentage of wood failure in adhesive bond joints. ASTM D5266ñ

99. West Conshohocken, PA: American Society for Testing 

and Materials. 

Berg, J.C. 2002. Semi-empirical strategies for predicting 

adhesion. In: Chaudhury, M; Pocius, A.V., eds. Adhesion 

science and engineeringó2: Surfaces, chemistry and 

applications. Amsterdam: Elsevier. 

Bolton, A.J.; Dinwoodie, J.M.; Beele, P.M. 1985. The 

microdistribution of UF resins in particleboard. In: Proceedings, IUFRO symposium Forest Products Research International: Achievements and the Future. Pretoria, S. Africa: 

6: 17.12ñ1 to 17.12ñ19. 

Bolton, A.J.; Dinwoodie, J.M.; Davies, D.A. 1988. The 

validity of the use of SEM/EDAX as a tool for the detection 

the UF resin penetration into wood cell walls in particleboard. Wood Science and Technology. 22: 345ñ356. 

Browne, F.L.; Brouse, D. 1929. Nature of adhesion between glue and wood. Industrial and Engineering Chemistry. 

21(1): 80ñ84. 

Fengel, D.; Wegener, G. 1984. Wood: Chemistry, ultrastructure, reactions. Berlin: Walter de Gruyter. 

Frazier, C. 2003. Isocyanate wood binders. In: Pizzi, A.; 

Mittal, K.L., eds. Handbook of adhesive technology. 2nd ed. 

New York: Marcel Dekker: 681ñ694. 
35

Frihart, C.R. 2003a. Durable wood bonding with epoxy 

adhesives. In: Proceedings, 26th annual meeting; 2003 February 23ñ26; Myrtle Beach, SC: Blacksburg, VA: Adhesion 

Society, Inc.: 476ñ478. 

Frihart, C.R. 2003b. Wood structure and adhesive strength. 

Characterization of the cellulosic cell wall; 2003, August 

25ñ27; Grand Lodge, CO. Madison, WI: Society of Wood 

Science and Technology. 

Frihart, C.R.; Brandon, R.; Ibach, R.E. 2004. Selectivity 

of bonding for modified wood. In: 27th Annual Meeting of 

the Adhesion Society, Inc.; 2004 February 15ñ18; Wilmington, NC: The Adhesion Society, Inc.: 329ñ330. 

Furuno, T.; Goto, T. 1975. Structure of the interface between wood and synthetic polymer. VII: Fluorescence microscopic observation of glue line of wood glued with epoxy 

resin adhesive. Mokuzai Gakkaishi. 21(5): 265ñ334. 

Furuno, T.; Saiki, H. 1988. Comparative observations with 

fluorescence and scanning microscopy of cell walls adhering 

to the glue on fractured surfaces of wood-glue joints. 

Mokuzai Gakkaishi. 34 (5): 409ñ416. 

Gindl, W.; Gupta, H.S. 2002. Cell wall hardness and 

Youngís modulus of melamine-modified spruce wood by 

nano-indentation. Composites: Pt. A. 33: 1141ñ1145. 

Gindl, W.; Dessipri, E.; Wimmer, R. 2002. Using 

UV-microscopy to study diffusion of melamine-ureaformaldehyde resin in cell walls of spruce wood. 

Holzforschung. 56 (1): 103ñ107. 

Laborie M.P.; SalmÈn, L.; Frazier, C.E. 2002. The cooperativity analysis of segmental motion in the wood adhesive 

interphase: A probe of nanoscale morphology. In: Proceedings, 6th Pacific-rim bio-based composites symposium & 

workshop on the chemical modification of cellulosics; 2002, 

November 10ñ13; Portland, OR: Oregon State University, 

Wood Science and Engineering Department: 18ñ25. 

Macinko, J.; Phanopoulos, C.; Teachey, P. 2001. Why 

does chewing gum stick to hair and what does this have to 

do with lignocellulosic structural composite adhesion. In: 

Wood Adhesives 2000; 2000 June 22ñ23; South Lake Tahoe, NE. Madison, WI: Forest Products Society: 111ñ121. 

McBain, J.W.; and Hopkins, D.G. 1925. On adhesives and 

adhesive interaction. Journal of Physical Chemistry. 29: 

188ñ204. 

Muszyński, L.; Wang, F.; Shaler, S.M. 2002. Short-term 

creep tests on phenol-resorcinol-formaldehyde (PRF) resin 

undergoing moisture content changes. Wood and Fiber 

Science. 34(4): 612ñ624. 

Nearn, W.T. 1965. Woodñadhesive interface relations. 

Official Digest. Federation of Paint and Varnish Production 

Clubs. 37 (485):720ñ733. 

Pocius, A.A. 1997. Adhesion and adhesives technology: An 

introduction. Munich, Germany: Hanser Publishers. 

River, B.H. 1994. Fracture of adhesively bonded wood 

joints. In: Pizzi, A.; Mittal, K.L., eds. Handbook of Adhesive 

Technology. New York: Marcel Dekker: 151ñ177. 

River, B.H.; Minutti, V.P. 1975. Surface damage before 

gluingóweak joints. Wood and Wood Products. 80 (7): 35ñ

36, 38. 

Saiki, H. 1984. The effect of the penetration of adhesives 

into cell walls on the failure of wood bonding. Mokuzai 

Gakkaishi. 30 (1): 88ñ92. 

Smith, L.A. 1971. Resin penetration of wood cell wallsó

implications for adhesion of polymers to wood. Syracuse, 

NY: Syracuse University. 193 p. Ph.D. thesis. 

Smith, L.A.; CotÈ, W.A. 1971. Studies of penetration of 

phenol-formaldehyde resin into wood cell walls with the 

SEM and energy-dispersive x-ray analyzer. Wood Fiber. 

3 (1): 56ñ57. 

Stamm, A.J.; Seborg, R.M. 1936. Minimizing wood 

shrinkage and swelling. Treating with synthetic resinforming materials. Industrial and Engineering Chemistry. 

28 (10): 1164ñ1169. 

Stehr, M.; Johansson, I. 2000. Weak boundary layers on 

wood surfaces. Journal of Adhesion Science and Technology. 14:1211ñ1224. 

Tarkow, H.; Feist, W.C.; Southerland, C. 1966. Interaction of wood with polymeric materials, II. Penetration versus 

molecular size. Forest Products Journal. 16(10): 61ñ65. 

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. 
36 

Figure 1óTangential surface of bondline failure of epoxy wood, using ASTM D 2559. 

Figure 2óPossible bonding surfaces for the tangential or radial plane of wood. 

Cell wall 

cracks 

Adhesive 

S2 layer 

S1 layer 

S3 layer 

Warty layer 

Primary 

cell wall 

Middle 

lamella 

S2 is highest in cellulose 

ML is highest in lignin 
37

Figure 3óFailure in the interphase region of adhesively bonded wood. 

Figure 4óModel adhesive interactions with the cell walls.

Middle 

lamella 

crack 

Wall layer failure 

Adhesive cracking 

Adhesive 

failure 

Adhesive failure 

Cell wall 

failure 

Cell crack 

Adhesive 

Cell wall 

Adhesive 

Cell wall 

Adhesive 

Cell wall 

Adhesive 

Cell wall 

Adhesive 

Interfacial adhesion

Interdigitation Adlayer 

Interpenetrating 

polymer network 
38 

Figure 5óModels for adhesiveñwood interactions. 

Figure 6óModel for adhesive forming an interpenetrating polymer network with the wood. Adhesive may form a crosslinked network mainly through the hemicellulose and lignin portions that can limit the ability of the wood to swell. 

Hemicellulose

Cellulose

Lignin

Adhesive 

Wood

Wood

Adhesive 

Adhesive 

• Low cell wall penetration 

ñ Greater swelling causes 

large force at cell edges 

ñ Force exceeds tensile 

elongation of epoxy 

• High cell wall penetration 

ñ Less swelling causes 

less force at cell edges 

ñ Force does not exceed 

tensile elongation of 

epoxy 
39

DiscussionóAdhesion and 

Controlling Resin Cure For 

Optimized Wood Composites 

Charles E. Frazier and Charles R. Frihart 

Siqun Wang: To Frihart: You have one slide showing the 

rate of penetration, can you tell me how much resin is loaded 

on the fiber? 

Frihart: Do you mean the PF or the pMDI? 

Wang: Both. 

Frihart: Do you have some idea Fred, since I borrowed the 

slides from you? 

Fred Kamke: The question was how much resin was applied? I have no idea off the top of my head. The images that 

were shown by Frihart were both prepared with same 

amount of adhesive solids added but I do not recall how 

much that was on a square centimeter basis. 

Frazier: I believe it was 20 grams per square meter. I just 

was writing a publication for this but they were continuous 

bond lines, not sprayed. 

Kamke: This is a question for Chip. You were talking about 

this dielectric technique for monitoring adhesive cure and 

you talked about the technique in general but I know that are 

different types of dielectric techniques, one using a microprobe and I know you have a lot of experience using a microprobe, and then you also have that system that does the 

parallel plate arrangement. You indicated that there is a 

possibility for a lot of influence by the moisture that may be 

present around that adhesive and interfering with the signal. 

Can you comment on the possibilities of the parallel plate 

arrangement as opposed to the microprobe arrangement? 

Frazier: Not really. I am not so familiar. I think that you 

have complications in both cases. Iím not sufficiently familiar to be able to distinguish between the two; perhaps there is 

less of a problem with the moisture in the case of the parallel 

plate measurement, particularly if the surface material is 

dried out like we think it is. So maybe the parallel plate 

measurement is not as complicated by electrode polarization 

in particular, which is one of the things that happens. So 

maybe it is not as bad but in terms of the overall complexity 

of the signal, I donít know there is a great deal of difference 

and it is still complex signal. I donít think I can really comment on that with any authority. 

David Harper: I was curious about using the dielectric 

analysis, especially the micro dielectric system. It could be 

used as a way of monitoring moisture permeability through 

the panel, at specific locations, which would be the permeability you described in your talk. 

Frazier: Some of the data that was supplied to me by Signature Control Systems, their results indicated just that. That it 

is an effective method for tracking moisture movement. So 

yes, that is one thing that can be done readily with the dielectric approach. 

Pablo Garcia: Dielectric analysis has strong correlation 

with moisture content; they really donít like that because 

they want it to be a cure predictor. Moisture meter and I 

think with working with cure, we might be able to look at 

moisture content effect on the cure, whether it is pMDI, 

maybe it has some moisture absorption a bit or prepolarization of phenol formaldehyde resins. Both the bulk 

parallel plate signal and the local signals are very much 

affected by moisture content. The bulk signal would be 

affected by more of the average of the shines right through 

from the top to the bottom, the micro dielectric probe will 

give you more of a localized reading from wherever you 

place it. 

Frazier: That is why I am really interested in the acoustic 

method. It is not as complicated by moisture as dielectrics so 

perhaps acoustics is something that we really need to get 

back into. 

Kamke: I wanted to comment on the dielectric influence by 

moisture content. Isnít the dielectric properties also highly 

influenced by temperature and trying to monitor the migration of moisture through a mat during the hot pressing process, you would have to somehow account for the change in 

temperature as well and correct for that. 

Frazier: Yes. Temperature is going to have a big effect. 

Jong Lee: To Frihart: As a modeler trying to predict engineering properties of the OSB, I want to see if there is an 

applicable method, to quantify the bond line strength or bond 

line performance because when I deal with the elasticity or 

mechanical properties of wood, we have to assume that there 

is 100% bonding in interface. 

Frihart: The problem with being able to do the modeling is 

that you could probably get an estimation of your contact 

area between your segments by looking at your resin distribution. You can know the mechanical properties of the 

wood, you can get it for the bulk adhesive, but I am not sure 

how you can get the mechanical properties in the interphase 

region. If you are not having failure there, maybe it doesnít 

matter and you can just discount that area. But obviously, if 

it is an area that doesnít have enough strength to hold up, 

then that becomes really difficult without some data for that 

region. I donít know if you can do it by applying extremely 

thin films, doing extremely thin wood and measuring each 

layers mechanical properties or not. But that is certainly a 

real problem, it is very hard to model across adhesive bond 

lines just because you have so many different zones and you 

donít know the mechanical properties of those zones. 
40 

Garcia: Getting back to the comments of dielectric probes. 

The dielectric probes impart an alternating electric field on a 

polar material. This material then tries to align with the field. 

Moisture is the smallest, freest dipole, so it will dominate the 

signal. The effect of temperature I think is especially with 

respect to bound water, is that what we look at is I think is in 

the thermodynamic state of that bound water has a big effect 

because temperature has an effect on the signal. We increase 

the temperature where essentially the bound water becomes 

less bound and starts moving toward the free water. It is 

essentially still bound but still has a bit more energy so I 

think we might be able to account for the temperature effect 

somehow thermodynamically through energy of activation, 

calculations or something like that. I started to look at that a 

bit and I presented some of that work at the last Forest Products Society meeting, I have done a little bit more work on 

moisture content effects now and I am getting that all together now and hope to have some papers out on that soon. 

Frazier: I think that those points are well taken. When you 

want to talk about monitoring resin cure, you can see that 

there is so much more information than just about the resin, 

it is difficult but obviously it is still a valuable method for all 

those other things particularly moisture movement. It is 

challenging to learn about the resin. 

Kamke: I will keep the discussion going about dielectrics 

again. What about using multiple frequencies in this dielectric monitor. Each of these polar components that are found 

in wood whether it is water or the hydroxyl sites or some 

other polar groups in there, they each have a different time 

response to an alternating electric field. If you change the 

frequency and measure the response and change the frequency and measure the response again, some of those polar 

components are going to change their response and others 

will not. If you do enough multiple frequencies perhaps you 

can use some type of multi-varied analysis or whatever and 

break that down and isolate the influence of just the adhesive 

polymer and the water then would be a separate component. 

Frazier: That is exactly right. You have different frequency 

response and also the different materials, the different dielectrics are going to have a greater or lesser capacitants or 

conductivity response so it is really two other dimensions 

that you could work on. So it is a lot of work there but yes, 

there are going to be different frequency responses and the 

moisture should give you a greater capacitant or loss response so I think there is still some hope that you could 

deconvolute some of those things. 

Chunping Dai: To Frihart: I have a question, you show that 

the effect of water soaking or the moisture absorption on 

bonding with epoxy glue in wood, did you ever look at the 

MOE or modulus of elasticity for epoxy glue itself? I am 

asking that because I remember way back there was a paper 

published by Mark Irle and James Bolton (Irle and Bolton 

1988) of durability of urea formaldehyde versus phenol 

formaldehyde. Their study not only looked at the assembly

between wood and glue, but also the cured glue itself. When 

glue cured in the case of urea formaldehyde, urea becomes 

very brittle, like glass. Whereas, PF resin is more elastic or 

more forgiving for deformation. I donít know if that is the 

case here with epoxy glue.

Frihart: In the urea formaldehyde, there was work at FPL 

done a number of years ago in which they looked at the 

assemblies and what they found was that there were cracks 

in the UF, shrinkage cracks which then when you put it in 

water, propagated into the wood (Ebewele and others 1994). 

So part of the solution was to make UF more flexible and 

that seemed to solve the problem. They used one approach.

There has been a recent paper that used another approach, for 

the UF to try to do that. Epoxies are generally tough enough 

that they donít form shrinkage cracks upon cure. On the 

other hand, they do have very limited tensile strain to failure. 

I donít know if people in industry have good tensile data on 

PF resins, we found it hard to make a film and really measure strain to failure. One of the things that Chip and I have 

talked about is better understanding mechanical properties 

and we probably have to develop some methods to try and 

figure out how to get those properties. Along this line, there 

is a recent paper out at the University of Maine where their 

answer was the PRF was more durable because when you 

water soaked it, it would tend to elongate similar to the wood 

and so you didnít build up the stress factor. And that certainly might be a mechanism of doing it. I think whatever 

you do you have to really distribute the stress, which is the 

way most people go about making better adhesives is prevent stress concentration. So in some cases, a flexible 

adhesive is valid and there are some people who flexibilize 

epoxies. We are doing some work to see if that really does 

relate to some of the performance properties. So it is a 

good question. 

Harper: To Chip: I know he worked with Marie Laborie 

(Laborie 2001) along these lines with the interaction of 

resins with wood. I was wondering if you could comment on 

the change that pMDI would impart on the glass transition of 

wood as opposed to the PF and UF.

Frazier: Well, I wish I could. We havenít looked yet at that 

system. Just to give a little background, Marie did look at 

two different phenolic systems, one of a high molecular 

weight, one of them a very low molecular weight, which 

was mostly monomers and dimers, and her work demonstrated 

that those small molecules enter the cell wall and influence 

the relaxation, the distribution of relaxations for whatever 

the major softening was, lignin or whatever you want to call 

it. There is no change in the TG, but there was just a change 

in the mechanism of that relaxation. And interestingly, the 

high molecular weight resin did not influence the distribution of relaxations but it did actually reduce the TG and so 

those are two very different and subtle effects. Now how 

isocyanate influences that, we donít know, although some 
41

work out of Huntsman Polyurethanes by Joe Marsinko 

demonstrated that the isocyanate does I believe depress the 

glass transition temperature or the, I struggle with the term 

glass transition temperature in wood or the softening relaxation because that seems a more generic term, perhaps a safer 

term, but I think he demonstrated that there is a reduction in 

that softening transition. And it is pretty well established 

although not all the data has been published, but also work 

by Marsinko from Huntsman, established that the isocyanate

does get into the cell wall and plasticize those polymers. So 

there is an intimate association there, so you would expect 

that it would have some influence on the relaxations. But 

the other thing is the phenolics do it as well, if you have just 

the right system. I donít know this from experience, but 

I think it is about the issue of molecular size more than 

anything else. 

Frihart: Along those lines, if the adhesive forms a separate 

network within the wood, it is not going to change the Tg 

because you are not changing that domain. On the other 

hand, if it is being dissolved within the material, then it 

would affect the Tg so that may tell you something about 

where the adhesive is going in your system. As Chip points 

out, these things that are somewhat elusive for really knowing exactly what you are measuring. 

Frazier: In Laborieís work, she demonstrated that for a low 

molecular weight resin, the distribution of relaxations associated with the softening suggests strongly that you are 

impeding the reorganization. Therefore, something must be 

in there or itís at least consistent with this concept of interpenetrating network. In the case where the softening temperature is reduced by the high molecular weight resin, the 

hypothesis is, even in the high molecular weight resin, there 

is a tail or some component of the resin, which is monomeric. There is always going to be some of that. And so that 

stuff gets in there and plasticizes or creates free volume,

which could reduce the TG, but it does not have connectivity 

to the bulk adhesive layer and so it doesnít have the same 

influence. But it is curious, if you are depressing the TG, that 

is a whole other influence perhaps in terms of changing the 

relaxations and the stress relaxations and who knows how 

that works. So it is complicated. 

Kamke: No, I donít want to comment on that because I 

donít know about that. I was hoping that Chuck could pull 

up his slide there that showed his list of adhesive mechanisms and I want to talk about durability of the adhesive 

bonds and in particular resistance of adhesive bonds to the 

intrusion of moisture. I guess resisting the shrinking and 

swelling that you have been talking about. 

Frihart: This is the one of how molecules interact or the 

cells? 

Kamke: You had a list of bonding mechanisms slide. 

Frihart: This one? 

Kamke: Yes, the adhesion models. My question is, which 

one of those mechanisms would play the most important role 

in resistance to moisture?

Frihart: Certainly if you get to where it is determined 

strictly by interface interactions, the covalent bond would 

resist moisture more. With the hydrogen bond, water can 

always intersperse itself, same way with other polar interaction such as dipole interactions or Van der Waals bonds. It 

really depends upon contact and if water intersperses itself. 

The mechanisms for epoxies de-adhering from metal is that 

water preferentially absorbs on metal to displace the adhesives. So if water gets into that interface, it will want to 

intersperse itself. On the other hand if you get into where the 

adhesive is actually penetrating the wall, that has got to be 

more durable than covalent bonds because covalent bonds 

you are only going to get a few of those and so you are 

really depending upon that bond strength of how many of 

those you form. So I think that a good adhesive is one that 

can get in the wall and do some effects on it and tie back into 

your network. I donít know. That is just my feeling. I donít

know if Chip has a different feeling on it. 

Frazier: Well, I would just add that when you pose a question on what the effects of moisture are, my first question is,

what do you mean exactly? Do you mean about the adhesion 

mechanism perhaps or do you mean about swell? There are all

kinds of things of course. In terms of swell, this concept of 

the interpenetrating network morphology could be important 

just by removing those absorption sites.

Kamke: Well my question was really relating to the strength 

of that adhesive bond as it may be affected by the intrusion 

of moisture. From what I hear from both of you, if you were 

going to design an adhesive system for wood to make it 

better in terms of resistance to moisture you would go after 

this interpenetrating network. 

Frazier: I would go after something that requires cleavage 

of primary bonds. And so that would be covalent bonding or 

if you have a truly interpenetrating network, then to cleave 

that you would also have to break primary bonds so either 

way, yes, covalent bonding or some type of molecular level 

interpenetration. 

Kamke: And so if I could add on to my question, what 

about just plain old creep? 

Frazier: Just plain old creep, good question. I would presume then that if, I guess than the presumption might be that 

the interface would be sufficiently sizable to influence creep. 

I kind of see as a bulk effect and the interface being such a 

small component, I am not certain that you could influence 

it. That is a great question, but if you look at that I would 

probably go with an IPN type of a morphology because that 

would likely inhibit any type of reorganization, whereas 
42 

as covalent bonding you inhibit relaxation to a point but 

perhaps not like you would with molecular level 

interpenetration. 

Frihart: If I can interject along this point, a lot of this discussion assumes that you have a nice strong wood surface 

and from your slides of showing a lot of change of the wood 

structure and you donít know how much the walls are fracturing. If you are introducing a lot of fracture in the walls at 

that process, you can bond as strongly as you want to the 

initial wall but it is going to serve as a weak point. For years, 

one of the areas that we worked in was to make adhesives 

that were used for bags and people would come back and say 

your adhesive is no good. Well, we found out the adhesive 

stuck very well to the ink but the ink wasnít stuck strong 

enough to the paper. It was coming off. So what you have to 

look at is where your failure is really occurring. Because it 

may be that a good adhesive, like a PF, just goes in there and 

reinforces weak cells so that your whole interphase region 

which goes from your bulk adhesive to your bulk wood is 

strong enough. This is the part that we really donít know and 

obviously are areas we want to understand in the future. 

Garcia: Do you know of any work or published work that 

deals with fiber optic probes into mat during hot pressing. 

Frazier: I do and I know that Bayer has done some of that 

work; I have forgotten the name of the gentlemen who did 

that work. Yes, and so I think that was the most recent use of 

fiber optics and I know that Fred also has dabbled in that 

area but I think he had some difficulty with the mechanical 

integrity of the probe. Yes, Rosthauser and others (1997) 

was the other name on that publication. That was a publication from the Washington State University Particleboard 

Symposium. 

Dai: I have a comment and question regarding the question 

you raised, Fred, about the intrusion of moisture on quality 

and stuff like that. I just remembered one of the interesting 

publications and subsequent modifications on the property 

on the molecular weight distribution of resin on MDF board 

property. Professor Kowiee and his research team from the 

University of Kyoto did some work while he was working 

on agricultural fiber and he had to find a way to reduce 

thickness swell of this product. One of the work he did was 

that he had to use low molecular weight resin to, like we are 

talking about curing and you try to use the high molecular 

weight more advanced so you cure faster and stuff, however, 

in this case you have to use lower molecular weight and it is 

going to cure slower, however, that helps to penetrate into 

the wood fiber better therefore actually substantially reduce 

the thickness swell of the product. We actually heard his 

presentation while he was visiting Forintek, and we actually 

did some mill trial helping one of our members and we 

found that you can just do that to improve dimensional 

stability of MDF. I donít know if you guys can comment on 

the mechanism of that. I also have a question about the 

curing of MDI versus PF: Why is it that MDI is so much 

stronger than PF? We are talking about the bonding, is there 

a high degree of covalent bonding going on there? Also, is it 

true that MDI can cure at lower temperature and at a faster 

rate? Can you comment on that?

Frazier: Well, let me go back to the question about the low 

molecular weight material and improving the shrinkage 

properties. I think it is well established that if you are able to 

get something into the wood cell wall, then you are going to 

bind up an absorption site for water and it relates to this 

anti-shrink efficiency. 

Frihart: And it may also be changing your properties if you 

have another network in here, which is basically serving as a 

framework for preventing a lot of the swelling and shrinkage 

as indicated in this drawing, that even though there may be a 

tendency, it is restricted from doing that. Epoxy adhesives 

for example, like to absorb moisture, but they are so tightly 

cross linked that they canít absorb moisture. On the other 

hand, if you have a crack in epoxy and you put it in water 

the crack will tend to expand because it wants to absorb 

moisture and it has a way to expand, but in a regular network 

it canít. 

Frazier: I guess that the fact the shrinkage and swelling has 

changed, really is proof that the molecules are going into the 

cell wall. If you go back to the work by Alfred Stamm 

(1964) many years ago here at the FPL, that is how he demonstrated whether or not certain molecules enter the cell 

wall. If after exposure to the small molecules it can no 

longer shrink and swell under the effects of water it means 

that something is in there occupying that site. The mere fact 

you do change the shrinkage and swelling properties is very 

strong evidence that the small molecules are in there. I am 

sorry you will have to repeat your specific questions about 

the isocyanate and the PF. 

Dai: It is just I asked that why is MDI so much stronger 

compared to PF at the same amount of the dosage? The other 

factor is it is so much more expensive I guess. And the other 

aspect is that is it true that MDI can cure faster at the lower 

temperature? 

Frazier: I believe that it can cure at lower temperatures, I 

think that other folks here who can comment at that with 

some authority. I believe it does, but then again, I think you 

can tweak the other systems to meet certain cure rates. Why 

itís stronger? I donít know, that is very difficult, in fact I 

need to applaud Chuck for his early distinction between 

adhesion and adhesive strength and I might have to ask you 

what you mean by a stronger bond. It is so difficult to know. 

You have a measurement, but then what does that actually 

reveal about adhesion? Because it is a very subtle difference 

there but a very importance difference. I am not certain that, 

I donít know that there are great differences, I do know that 

there are differences in the shrinkage effects for example, 
43

thatís likely a strong relationship to the low molecular 

weight of isocyanate and the other thing of course that when 

the isocyanate cures it forms a polyurea network and if you 

had to design a hydrogen bonding material, that would be it. 

Polyurea would be about the strongest hydrogen bonder that 

we know of. On the flip side, phenolic, also phenolic hydroxyl groups or aphenolates are extremely effective for 

hydrogen bonding. But perhaps there is not as many of 

those. I am not sure I can comment and itís because again do 

we really know that there is a difference in the nature of 

adhesion or that we have these measured differences in 

whatever we feel is adhesive strength? And it is a big difference, it is a subtle difference and I am not certain that often 

times the measurement are influenced by the measurements 

themselves that is the mechanics of the system so I am not 

certain one is really better than the other but they are vastly 

different, just the fact that isocyanate has such a low molecular weight, the other thing that is fascinating about it of 

course is that is it essentially a two part adhesive and the 

second part being moisture is everywhere around in there. 

When you talk about the phenolic needing to be, is a great 

analogy that I heard earlier from Jim, was that with the 

phenolic adhesive is like field position in a football game, if 

you start at the ten yard line and you are going to the goal 

line, to the end zone, you're closer and so if you start with the 

higher molecular weight you are getting there more readily. 

And the question would be, what is the deal with isocyanate, 

how come you donít have a similar effect there? It is just an 

entirely different cure and the fact that moisture is the other 

integral component and that it is everywhere in the substrate, 

that you essentially do have a network an existing network 

of water and isocyanate molecules, all they are just waiting 

to do is just kick over instantly, so in that regard I think it is 

faster but I think you can manipulate the phenolic system to 

be as fast perhaps. 

Ted Laufenberg: I had to follow up on the question. I believe that when Fred asked about creep, we are really talking 

about the number of binding sites that are available for 

movement within a composite and the equilibrium moisture 

content seems to be strongly affected by molecular weights. 

That is what I found when I did my creep studies at least, 

that EMC was a much better predictor of creep capability 

and that was strongly influenced by the adhesive type. At the 

time, I am not a chemist, but at the time, I didnít go into it 

any further but perhaps you can comment on that. How 

EMC affects creep, dimensional stability, all these other 

volumetric properties strength aside. 

Frazier: I donít know that I can comment, well, if the EMC 

is higher, then I would expect a greater plastic response, but 

how the adhesive influences creep, a big big issue there is 

that we compare apples to apples and so if there is some 

concern about differences in creep response caused by resins, my first question would be were those systems consolidated in hot press under identical conditions or are folks 

imposing the contemporary hot press conditions that are 

specific to those adhesives? In which case the thermal treatment of the wood cell wall is different. So I have heard 

people talk about creep with different resins and I think that 

unless they are compared under identical temperature, time, 

moisture conditions, it is difficult to say that okay the adhesive really has a contribution to this creep effect. It could, 

but if you are adapting the hot press system to two different 

resins that may have two different cure requirements then of 

course the thermal treatment of the wood cell wall is different. And so I think that it is critical when you talk about how 

creep is influenced by the resin that you recognize that by 

using different resins and process conditions, you are not 

using the same thermal treatment. I donít know the answer to 

that, but I think it is an important distinction. 

Wang: Iíd like to follow up the comment on the durability. 

When we compare durability, we often either boil the sample 

or soak a sample in the water. If we look for the micro-scale 

or cellular-scale, we can imagine that on the interface we 

have a lot of internal stress. I mean that these swelling 

stresses can easily destroy the bonds. In the research that I 

did with Tony Pizzi a few years ago, we tried to use another 

way to see if maybe the bond lines were possibly too strong 

and not flexible enough. We thought that if we could improve the bond line, for example by putting other molecules 

inside and making those molecules more flexible, then the 

bond lines would be more flexible and, maybe this could 

improve their durability. The research that we did, we broke 

down long nylon fiber to small nylon fiber and added it to a 

UF resin system and made plywood. Then we tested durability to see if we could improve the durability. The concept 

worked, in that it significantly improved the durability. So 

that meant, that when we added the nylon molecule, which 

was more flexible compared than UF, the nylon fiber was 

connected at one end to UF and the other end was connected 

to another UF molecule. This made the bonding line more 

flexible so that during the boiling test, bonding lines within 

the mat were not as easy to destroy when they were undergoing swelling or shrinkage. 

Jim Wescott: I have a couple of comments to add on the 

urethane discussion, as well as on the durability. Urethanes, 

isocyanates obviously are very unique. The things about 

them I have observed over the last couple years of reading 

on this and Chuck showed a couple of excellent slides. A 

couple of things separates them from the phenolics; one is

that the fact there is no water present vastly affects how they 

spread throughout the material, their surface tension is so 

different. I think that they are greatly more efficient because 

in my opinion the phenol formaldehyde that is filling the 

lumen is inefficient because basically it is phenol formaldehyde bonding to itself within inside the lumen. I think that is 

primarily the reason why isocyanates give you comparable 

performance at a lower load, it because of how they wet. In 

particular how they wet inside the lumens themselves. The 
44 

rate of the reaction is certainly vastly greater then PF and to 

what Chip stated, we really canít increase the rate of the 

reaction of phenol formaldehyde, all we can do is start the 

ball closer to the goal line, and that is really all we have been 

doing over the years is going with more aggressive higher 

molecular weight systems trying to counteract the high 

molecular weight with the addition of the urea and other 

modifiers to decrease the viscosity so we donít decrease the 

overall flow. You donít have that problem obviously with 

the isocyanates and you are not trying to do a condensation 

of a manic type reaction, which for those of you have done 

DSC see that that typically is a 130 to 150 degree exotherm. 

Isocyanates are tremendously much faster once they are 

catalyzed with a small amount of water so there really is no 

comparison to the rate of the reaction. Chip is exactly correct 

as part of the reason why they are also better is if you were 

to actually try to build the highest molecule with the greatest 

amount of hydrogen bonding, you would build a polyurea, 

which is what you have here with this system so it is phenomenal hydrogen bonding and of course there is a big 

debate whether or not there is any reaction or substantial 

reaction of the isocyanate with the actual wood itself. I think 

it is generally considered that this is minimal at best, but 

probably the majority of the actual bonding is taking place of 

hydrogen bonding as a result of the very strong urea linkage 

in there. Hope that helped a little bit there anyway. Back on 

the water swell. A couple of questions on that. I like what 

you mentioned about trying to flexiblize the adhesive and 

some of the things that we try to look at. I think most people 

really struggle with thickness swell and then really overall 

durability, not only how much does the wood swell, but after 

the wood swells, what types of properties do you still retain? 

And certainly I would think that if you are getting tremendous failure of the glue line, as a result of the swelling, than 

that should basically be a nonreversible exchange for your 

adhesive and when you try to do an internal bond by redrying the wet sample, you should see substantial loss, if the 

resin is more flexible and gives with the water absorption 

then I think that you would lose less of your internal bond 

upon redrying. Now I havenít seen any data to support that, 

but I would be curious on cases where you try to make the 

adhesive more flexible, and even making the adhesive more 

flexible it doesnít really matter how flexible it is if you are 

still in a condition where you are below the Tg, it is still 

going to fail when you test it. So I mean you can try to make 

it more flexible but unless you have a couple different domains where you are basically have a soft and hard domain if 

the whole molecule itself still has a high enough Tg it is still 

going to have a very high modulus under those conditions 

where you are applying the stress. So I am not sure if it 

would work if you didnít do it in the right manner. One last 

comment on the use of phenol formaldehyde today, weíve 

looked at a lot of commercial phenol formaldehydes and 

curing them up and doing extractions, I would think that one 

of the problems with durability today is as we try to fight 

press times by going with higher molecular weight PF resins 

and throwing in the urea type modifiers to them, Iíve seen 

PF resins that when I cure them and do a water extraction, I 

can pull out 35% of the cured resin. So obviously, if your 

adhesive bond itself is still a third water-soluble you are 

probably going to have a difficult time of having good durability. So obviously we are trying to win one battle at the 

same point and time most of the time, at least in the United

States, not worrying about boil we mask a lot of the water 

problems with wax, which is really a mask because ultimately it is not going to do its job in the future. That is just a 

couple comments and I donít know if anybody really wants 

to add to that. 

Frazier: The whole issue, without going too far down this 

path, in terms of covalent bonding with isocyanated wood 

we have studied this for a long time and have come down on 

both sides in our results and we have used labeled compounds and solid state NMR and based on most recent results, we feel that we have a tentative identification of urethane linkage using the double label, N15 and C13 label 

isocyanate molecule. The problem with this, we see a signal 

that we attribute to the urethane carbonyl. There is still some 

question about the identification of that signal because with 

solids NMR you know you can get things moving around the 

chemical shifts as you know change not only because of the 

environment but how they, well because the environment, 

because of the interactions it may not be just chemical identity, but the interactions with the environment. But what I am 

trying to get to is that weíve made a tentative identification 

of a urethane linkage and it is not rock solid. It is not completely proven but nevertheless I feel more confident now 

that we do have the identification, I canít say that we have it 

for certain, I think we do. The point I want to make is, IF we 

do, there is a lot of urethane formation. If we have identified 

what I think we have, there is quite a bit. We are still working on that. 

Wescott: If you had the wood extremely dry, then it is not 

able to have as much water to catalyze the isocyanate to 

form the urea. Then you should see a greater amount of your 

urethane system in a dry wood environment. Would you 

agree with that? 

Frazier: Yes, well what we have done is using different 

moisture contents we have seen a change in the signal that 

we attribute to the urethane carbonyl. We have not though 

with this double-labeled resin, which is the system that we 

think is useful now; we have not driven that with a completely dry wood sample. We have done that before just with 

nitrogen 15 labeling just to get the model urethane chemical 

shift. But no, we havenít done that experiment, we have only 

looked a difference in moisture content from about 4 or 5 up 

to about 12 and see a difference in the peaks that we attribute 

to urea but the urethane still, the peak that we identify as 

urethane is still there. No, the answer is no. We have not 

done that, an absolutely dry wood system with this 
45

double-labeled resin, which is the one that we think is so 

useful for this analysis. 

John Noffsinger: Can you have too much moisture? Could I 

leave my strand too green or too wet and affect the pMDI 

reaction? 

Frazier: I donít believe so. I think the issue there is the one 

that Fred touched on earlier is more about gas pressure, I 

believe. There is such a huge excess of water anyway that I 

am not certain that the isocyanate sees that as a difference. 

Al Christiansen: Yes, I think it would affect the surface 

energy of the wood so as Jim points out that would affect 

how it wets and penetrates. 

Frazier: Well, based on just a theoretical analysis, if you 

increase the moisture content typically that is attributed to 

increasing the surface energy. And that would enhance 

wetting and enhance penetration. 

Christiansen: Two things. One of which is youíve got a lot 

of sodium hydroxide in your resin and that probably doesnít 

stay with the resin entirely, because a lot of it goes into the 

lignin, which is also phenolic, so you are probably weakening your wood that way. And the second thing is, you probably donít get great cure in the center of the board, maybe 

they get hot enough, but for phenolic resin, the higher temperature you take it to, the better the cure you get. With the 

isocyanate you are probably getting almost complete cure, 

with the phenolics you are probably not curing them fully in 

commercial processes and often not in our lab pressings. 

Frazier: What I donít have an appreciation for is, what is

overcure? I think we do know what overcure is for a phenolic, that it can become brittled. We donít know if there is a 

similar type of overcure for isocyanates. I am not aware of it. 

I would suspect that there would be. We do know that there 

is a lot more urea formation and if urethane formation is 

occurring, if, there is a more than urea in urethane formation, 

there is a lot of other cross linking occurring, and so we 

know that the cross linking those other side reactions for 

biurete formation are occurring and what we donít know,

what I donít know, is if that has a negative influence on the 

toughness of the network and if over cure is a possibility for 

those materials. 

Kamke: Question for Al Christiansen. Can you explain the 

influence of sodium hydroxide on the lignin and why you 

think that would weaken the wood? 

Christiansen: My thought would be that sodium hydroxide 

strongly attracts water and so it will probably selectively 

absorb maybe even more water than you would normally see 

in wood but in those regions especially. And so if the sodium 

hydroxide goes to lignin it may disrupt the cell wall to some 

extent by absorbing more water where sodium is associated 

with the lignin molecules. Itís probably not going to absorb 

as much into the hemicellulose and cellulose because it just 

isnít attracted as much as it would be to the phenolic hydroxyl. And so once it gets there, the sodium is probably 

going to take more water into that location and free things up 

a bit. 

Frazier: And also you have to remember that it is not present as sodium hydroxide, it is present as sodium phenolate 

or sodium lignolite or something like that. Not that that 

changes the issue but if you still have those salts in there, 

those are certainly hydrophilic. The negative influence on 

the chemical structure, there is some indication that you can 

harm the carbohydrate, but again, since it is in the form of 

phenolate in one sense or the other, I donít have a feel for 

how much chemical degradation it causes but the hydrofalicity is certainly unquestionable. 

Wang: Can anybody comment on how bonding would be 

affected by wood structure? When we look at the surface of 

the wood, what we are really looking at, is it really hemicellulose or cellulose. But they are separate components. Of 

course when we cut wood into flakes or fiber, the three 

components are exposed to the surface differently. So different resin system working with three components I guess 

would be different and who can comment to which kind of 

bonding would be stronger. For example, PF is more likely 

bonded to lignin or bonded to hemicellulose. 

Frazier: Yes, thanks for that question. Actually that reminds 

me of something that is interesting. I sure donít know in the 

case of the isocyanate what it prefers to bond to. I suspect 

also lignin and Jim, I want to make sure you hear this comment because it relates to this issue of urethane formation; 

we have some indirect evidence that perhaps the isocyanate 

has a preferential reaction with the lignin. Because one of 

the things about the urethane linkage is itís thermally unstable. Depending upon the temperature and depending upon 

the hydroxyl group that reacts to the isocyanate, if you have 

a urethane linkage between an aromatic isocyanate and a 

phenolic compound, the thermal decomposition of that will 

occur on the order of about 100°C to 120°C, and so we do 

see evidence of the thermal decomposition of the peak that 

we identify as a urethane and it occurs at this lower temperature. Based on just pure theory alone, that would suggest that 

it is a preferential reaction of the lignin. But, that could be 

extremely naÔve; I mean that would be ignoring the influence 

of environment on the thermal stability of other urethanes, 

aliphatic urethanes, that is if you had hydroxyl groups from, 

aliphatic hydroxyl groups from cellulose or hemicellulose. 

They could perhaps also be more thermally liable because of 

the environment but based just on what we know of model 

compounds and that we see that the peak that we identify is 

urea thane thermally decomposes at around 120°C, that 

suggests based on theory alone that it is a reaction with a 

phenolic compound and so that perhaps it is preferential 

reaction with lignin. But that is conjecture. I mean that is just 

consistent with theory. 
46 

Kamke: I am going to keep asking questions until someone 

shuts me up. This is a question for either Charles the first or 

Charles the second, not quite sure who, is who but the question is, is there any influence on the molecular morphology 

of these cured adhesive systems in the presence of steam? In 

other words, pre-polymerization in the presence of steam,

and perhaps we are talking about a saturated steam environment. 

Frihart: I would guess yes, but I am not aware of any data. 

Frazier: Yes, I would say the same thing. The problem is 

that we really donít have any really good measurement of 

that. We had a project a few years ago working on that and 

we couldnít find a strong indicator. Based on just our freshman chemical expectations you would expect to change the 

kinetics and that you might promote one chemical pathway 

over another. And that would influence the morphology but I 

donít think that anyone has shown that conclusively. All that 

I recall from our work on that was that the steam enhances 

heat transfer and so the resin kicks over more rapidly, but at

the same time if you have moisture present, particularly in 

the condensation reaction, you know our freshman chemistry 

tell us that it should change it, it should change the bonding 

pattern and then how that would influence the morphology, 

morphology defined as this wood resin material in the interface, and so it should change it, but I donít know that anyone 

has a good idea about how it does that. It should, thatís too 

bad though, and thatís the problem with it. We have a theoretical expectation but very little evidence one way of the 

other how that should go. 

Kamke: The project that you were referring to was Aldo 

Ballerini Ph.D. project (Ballerini 1994) from years ago. I 

cannot recall all the results but one of the results I do recall 

is that when the phenol formaldehyde, it was a liquid phenol 

formaldehyde system that we were looking at being polymerized in a saturated steam environment, was subjected to 

solvent swelling the amount of swelling in the adhesive 

system cured in the saturated steam environment was much 

greater than the amount of swelling of the same adhesive 

system that was cured in a non-saturated environment. 

Frazier: Yes, well that would be a very clear indication then 

of the change in the network properties. The other thing also 

is what does it do to the formation of bubbles; you know the 

small microvoids that I think are typical for phenolic resins. 

Those are I believe toughening mechanisms for phenolics, 

you often have small nano or micro voids from gas from 

water vapor because of the polymerization and what does 

saturated steam do to that. And that is sense; it is purely a 

mechanical influence and not a chemical effect. But donít 

know the answer there but so in that particular case what you 

refer to Fred is the clear effect on the network properties, if 

the swelling is changed. 

Dai: I have a comment, going back to what Jim was saying 

about the difference between the MDI and phenolic resin 

being mainly the penetration to the cell wall rather than into 

the lumen. I think that is a very interesting observation and 

this reminds me of the need for using MDI for bonding 

residue like straw. You know that straw is very difficult to 

penetrate if you use urea other adhesive you wonít penetrate 

because of the waxy surface and inorganic compound on the 

surface. Whereas, if you use MDI it works very well and you 

almost have to use MDI, otherwise you have to treat the 

straw surface, enabling it to absorb or penetrate better. I 

guess that is something that I like to reinforce what Jim was 

saying. I have a question probably for you two and also 

mainly for Al Christiansen. You and Geimer did some work 

on mechanism curing, I remember the paper talking about 

the process of chemical curing versus mechanical curing 

(Geimer and Christiansen 1996). At that time I didnít quite 

understand what that was and I still donít understand, maybe 

you can shed some light on that. 

Christiansen: It means that we didnít write the paper very 

well I guess. Basically, a lot of the cure that goes on in phenolic adhesives is just building up molecular weight and 

maybe branching. You only get your mechanical properties 

at the end of that process. But when you look at cure by 

DSC, you are looking at the whole process. And so you 

think everything is building up mechanical properties. Well 

you are not yet, not until you get to the point where you are 

actually getting gelation and other things. So basically, the 

cure you detect depends on what part of the curing process 

you are in. Early on, you are not building up mechanical 

properties, later on you are. The trouble is we tend to confuse the one with the other. 

Frazier: Yes, that is exactly right. The question is, where 

does gelation occur and more importantly when does vitrification occur after that? So those are the two critical aspects 

of mechanical strength development. Just as Al indicates, 

there are a lot of methods that show us chemical changes, but

unless we can pick off specifically gelation vitrification, we 

cannot have an appreciation of when we can open the press 

for example and when there is enough mechanical integrity 

to open the press. 

Christiansen: And I might add to that that really building up 

molecular weight does not necessarily mean that you have to 

build up a lot of molecular weight. For cross-linking and that 

sort of thing, a little goes a long way. If you see a gelation 

diagram for a PF adhesive, it goes up really fast. 

Heiko Thoemen: I have a question about the thermodynamics, not so much about the strength development. We know if 

we talk about reaction rates that some water gets free and 

that the thermal reaction alters some energy. How much does 

it contribute to the temperature rise and how much water 

actually is deliberated? 
47

Frazier: That is a great question. I donít know the answer 

but it would be very easy to calculate how much heat and 

how much temperature would be, easy to calculate in a DSC 

you could easily get the heat of reaction for the neat resin, 

the question would be, what is the heat of reaction in the 

presence of wood and then you could still even get that 

information. So that would be easy to measure and calculate. 

The amount of moisture produced in terms of the phenolic 

cure is not as clear to me, although still you should be able to 

calculate that based on what you expect from the known 

mechanisms. 

Thoemen: Okay, when I prepared my presentation for tomorrow, I asked one of my colleagues from the chemistry department actually the same question and we came to the 

conclusion that between 0.5% and 1.5% moisture content 

added and was thought to be pretty much. 

Frazier: That sounds like a lot. 

Thoemen: I just want some comments on whether it is 

realistic or not. We calculated that between 2°C and 5°C of 

the temperature may be added due to the extra thermal reaction of the adhesive. Sounds pretty much to me as well. 

Frihart: I have a little problem with the amount of water 

because it depends on how much adhesive you are using 

butÖ. 

Thoemen: Okay, we talked about UF resin and we talked 

about like 10% adhesive content. 

Frihart: Yes, if youíve got enough resin, if you are doing 

something like particleboard it is probably more significant

than if you are talking about 3% resin on oriented strandboard, it's probably a lot less. 

Rick Rammon: How many moles of water you generate 

depends on how far that reaction goes, so I donít think it is

going to go completely. 

Thoemen: Yes, we already had this in our mind. I am curious whether anybody in this room maybe has some ideas 

about literature or did such calculations. 

Ted Frick: This doesnít apply to the UF question but there 

is a lot of literature about that for MDIs and the different 

heats of reaction for isocyanate urethane reaction or isocyanate urea reaction and the order of magnitude similar, I 

think, to what youíve calculated a little bit less on the amount 

of temperature that you would have contributed by that 

exothermic reaction because the binder amounts are lower, 

so well less than 5°C contributed, and then on the moisture 

side of course because of the stoikeometry even 5% MDI 

would only consume about 0.3% moisture, so again a very, 

very minimal effect there. But there is literature if you are 

interested in heats of reaction for isocyanates. 

Garcia: Iíve done some experiments looking at internal 

bond strength versus hot pressing time. If I were to redo 

them, have you got any recommendations on quenching 

method, I mean what worked for you if youíve done some 

in the past? 

Frazier: No, I donít have any recommendations, well liquid 

nitrogen is good. The problem is getting, you know, whatís the 

form of your sample? We have done some quenching where 

we had small tiny mats where you can just readily grab it 

and throw it into the liquid nitrogen. But with the larger mat, 

I think that is a more significant problem and I was talking to 

Fred actually yesterday about the concept of injecting liquid 

nitrogen into a pilot scale system, but it really doesnít seem

feasible when you consider the drop in permeability that 

Fred discussed earlier. You know, it could be feasible at the 

early stages perhaps. The other thing also it might be interesting to simple press a mat with no resin, but with resonated 

samples in there and if that would allow you to open the 

press and inject liquid nitrogen you know to open up the 

permeability, but no, we havenít done any work in that 

regard and that would be a significant obstacle to overcome. 

Garcia: Do you think freezing the wood might have an 

effect on the properties Iím trying to measure, trying to 

measure internal bond strength, the strength of the wood? 

Frazier: Yes, yes. 

Frihart: I wonder on the method, if you have a lot of water 

around and you are freezing the water, does it cause fracture?

Frazier: Yes, yes. If you are talking about pulling a sample 

out to observe the properties of the resin, you will not influence residual heat, you know, you will freeze that, if you are 

able to devise some type of mechanical analysis you may 

cause some fractures and that could be a problem. So yeah, 

yeah, but if you have an analysis where you are looking at

residual heat or chemical structure, it shouldnít be a problem, it should be okay to freeze it, but any type of mechanical analysis and Iíd be in particular, I think it would have an 

effect, so again the concept of the observation affecting the 

sample is a problem there. 

Harper: I have a question: What size is your sample?

Garcia: 18 by 18 [cannot hear]

Harper: Okay, because what I did previously for like lap 

shear specimens, I just put it between two steel plates at 

room temperature and that was enough of a quenching effect 

for a mechanical test. 

Frazier: Yes, for a lap shear specimen I could see that, but 

for a mat it will take quite awhile. But whatís the thickness?

Garcia: 3/4 in. 
48 

Frazier: That will take quite a bit of time for that to cool 

down. 

Kamke: I would like to throw up a question to the group 

here and get some comments back on it. In regards to adhesive cure rates and press times, it seems like the trend has 

been for many years, if you come with faster and faster 

adhesive systems and reduce press time so people can make 

more product in a shorter period of time, it seems to me that 

certain properties such as moisture resistance and thickness 

swell are not going to react very well. What I am getting at is 

the process of hot pressing is also a thermal treatment of the 

wood and making it less hygroscopic and therefore reducing 

its potential to absorb water and swell. By reducing press 

time, what the industry is actually doing is making their 

problem even worse in regards to thickness swells and resistance to water. I would like to get some comments on that. 

Are we going in the wrong direction in terms of trying to 

improve process efficiency and fighting a losing battle? 

Rammon: Yes, I tend to agree with what Fred said. Iíve 

seen that as we go quicker, not only do you lose the effect of 

the thermal treatment, I think you lock in some more residual 

stresses into that panel that is like allowing spring then when 

you do get moisture back into it, it has got a lot more stress 

built in there that is just ready to burst. I remember years 

ago, I think it was here at FPL, that they had done some work 

on an UF system, the inherent effect of the work really 

significantly slowed down the cure of the resin. It was very 

slow curing resin but ultimately because it cured slowly, it 

ended up being very stable to moisture and we just assumed 

it was because it cured so slow, residual stresses were dissipated and werenít locked into the system. So I think there is 

a whole lot to go that we may be avoided or problems that 

we are inducing because of this desire for speed. 

Winandy: Weíve been considering the same thing for a 

couple of years and we have been trying to work with some 

of the industrial folks to find out why people are doing this. 

The primary reason is production speed. Everyone recognizes that there could be benefits to longer press time and 

you could inpart higher properties. The down side is in 

general they sell these products in a commodity market and 

standards do not differentiate. So everybody makes an OSB 

that meets this minimum level and only this minimum level. 

There is no way to grade out their product so that they can 

command a premium price for a premium product. People 

have tried that privately and some companies have been very 

successful in it, but until the standards allow the people to 

grade out their products, there are no incentives from the 

manufacturing community. That is the opinion that weíve 

come to, there is no incentive. 

Kamke: The question to our moderators then is, can we 

achieve both, and can you design an adhesive system that 

retains its rate of cure but also performs some of the same 

functions as an extended press time? I mean can you 

enhance stress relaxation, for example, or impart some type of 

moisture resistance because you tie up some of these hydroxyl sites?

Frazier: I think you could, but you might be running up 

against a similar barrier and the added expense having to, 

and not just meaning adding more resin, you could add other 

components as well, but I think you could, you can plasticize 

the cell wall with the adhesive depending upon how you do 

it, so that could be a component of the relaxation that you 

may get aside from the heat and mass transfer that you get in 

the press anyway. So, yes you could, I believe, but you just 

have the similar problem of the added expense. So perhaps 

its more about the issue that Jerry brings up because I think 

Jerry is right on the money. Itís a matter of whoever it is that 

controls the whole situation or that everyone gets together 

and talks about changing that standard. Beause it could, in my

opinion and I am very naÔve on this and forgive me if I am 

out of line, we could be going down the wrong path in terms 

of the consumer perception of the product and if we keep 

going this way and we end up harming the material and in 

the long run harm ourselves because folks get the negative 

perception of the product. 

Noffsinger: That standard is changing, Huber is come up 

with a board that they call Advantek, LPís got a board that 

they call Home Advantage, Weyerhaeuserís got a product 

that they call Performance Plus Panel. So those are all supposed to have less thickness swell, but there is still the pressure to get more volume out of the middle because youíve 

got millions of dollars tied up in that capital asset, so how do 

I get more through. Fred, I like your question to the two 

speakers, how can we have both, eat my cake and the whole 

nine yards. Time is precious in a plant, but at the same time 

we found out if you pull it out too soon, you will have thickness swell problems. The other question comes in too though 

is something that Mike Wolcott in 2000 WSU Composites 

symposium talked about raising MDI levels up to I think he 

extracted up in his regression model up to 10% or 12% MDI 

to try and reduce the thickness swell, and he proposed that

when you do swell that board you are actually breaking 

bonds and losing strength. So the question comes to my 

mind, if I get a customer complaint because the floor has 

been rained on for 4 months while he hadnít gotten the roof 

over it, whatís the strength now in that floor? That is kind of 

a scary question to ask. 

Thoemen: Maybe to answer your question, I am not a chemist but I remember that Phil Humphrey and Mohammed 

Chowdhurry, they did some work on using ammonia to 

elasticized the material so that may be one interesting way to

go into this direction. And in another track there have been 

projects at the WKI in Germany, I think they didnít publish 

it so far, but they overpressed their mat and then they relax

the mat a little bit and after relaxation they hardly had 

any residual stresses, so that may be another way to think 

about it. 
49

Frick: Just a general comment Fred on that. We talked a lot 

this morning about the physics of the pressing process and 

the thermal history of the wood when you run a shorter cure, 

you are really only chopping the last x number of seconds 

off of that curing cycle and the ideas you presented about the 

vertical density profile, one that develops, thatís all set in the 

board to begin with I think so we probably have much more 

flexibility to change things up to that point before you open 

the press, irregardless of the resin but controlling things 

about how you close the press, how you set up your press 

program, that will affect the thickness swell a lot more than, 

is that resin 99% cured or 99.9% cured? That would be my 

guess that you could make the resin faster and to be able to 

have the strength to withstand opening the press and still 

through your moisture grading and through your pressing 

cycle, control the other variables that influence that thickness 

swell even more than how cured the resin is. 

Frazier: But the point there is that the longer time and the 

heat exposure that you are talking about imparts chemical 

changes to the cell wall. That doesnít relate to what the 

adhesive is doing so that might be a separate issue. 

Karl Englund: I kind of agree with Heiko in a sense that 

maybe we need to stop looking, or not stop looking at the 

resin and chemistry, but maybe start looking at new process 

development, and start looking at what can we do to the mat

before it enters the press. Can we do some type of post treatment to it after it exits the press? Maybe we need to start 

looking at the dinosaur of the continuous and batch pressing 

procedures that we have and start changing some of those 

things. 

Frihart: One way of compensating for shorter times is 

higher temperature. Obviously there is a trade off in that too 

high of a temperature will speed up your cure and certainly 

some of the other processes, such as some dehydration to get 

more of a case hardened material. However, now that it is 

bonded, the dehydration may not interfere with the bonding. 

This could reduce some moisture absorption of your product 

so it might be a way of looking at temperature of your 

platens or doing some programming temperature of the 

platens instead of constant temperature. 

Frazier: Perhaps. Because if you think back to what Fred 

taught us this morning, why we need modeling, the platen 

temperature may not necessarily influence the core temperature for example depending upon the details of the heat and 

mass transfer so its perhaps. 

Noffsinger: Can I carry your comment one step further,

Chuck, and that is, what is high temperature?

Frihart: I donít know what they normally run at but that 

is obviously one of the variables. I mean most industries, if 

they try to speed up, they have to end up increasing 

temperature and sometimes you have to change the rest of 

the system to compensate for that. 

Noffsinger: I mean, is there a point when you get too much 

temperature and we start to do something negative to the 

wood itself and in effect we start breaking it down? 210°C? 

What is that in Fahrenheit? 

I guess we are hearing there are some folks out there that are 

running it up to 425°F on the press, one concern you have to 

worry about are press fire. Big capital investment out there 

and you burn it down. 

Wescott: I have pressed some boards at around 215°C, and 

the surfaces of my boards are noticeably darker and they 

bond well too. 

Wang: Regarding the continuous press, I know that the 

industry uses a much higher temperature in early stage because the press is controllable in different stages at different 

temperatures. I think it is around 450°F or 460°F. But you 

know, when they reach that certain severe condition and then 

go to the following the stage, their temperature goes down so 

it only spends a short period at the higher temperatures. 

Wescott: Chip, you can probably answer this question better 

than most people for sure. The temperature at which can start 

to degrade the wood, I assume it is a function of how much 

moisture is in there because you know you can press a board 

very quickly with high temperatures and be okay. I assume 

the reason why it starts to get dark at the 415 or the 210°C to 

215°C is simply too much time and too dry in the surface. 

Frazier: Yes. I think that you get degradation at much lower 

temperatures, but then it is about time. So it is a time-temperature thing. Getting your oven dry moisture content is a 

degradative process. 

Christiansen: I donít know if people have been doing anything, but is there any way of increasing the mat temperature 

slightly before even going into the press somehow? Does that

help? 

Harper: I have a question based on pMDI from Chipís 

work: Between 100-160 degrees there is a wide range of 

reaction products with pMDI, or there might be a shift in 

products as you change the different temperature and cure 

rate. In my own work, I saw different lap shear strengths at

different press temperatures. 

Frazier: Yes, yeah, there are shifts in the cure chemistry of 

isocyanate as a function of time and temperature and also to 

a degree of the moisture content, which just goes to show that

through the thickness if you have all isocyanate material, 

that the cure chemistry differs and what we donít know is 

how that translates into performance and gross density and 

those kinds of things. So yeah, there are changes, we know 
50 

that, but we donít really know what the changes mean exactly in terms of performance. 

Winandy: We did not do it in a press, we did it in a kiln-like 

environment on solid wood particles. For a number of years, 

we have been studying the desequencing in the chemical 

composition of wood because of problems with wood degrading when used in high temperature environments. Currently, we donít believe that there is a threshold. The bottom 

line, every piece of data that we have says that wood is 

constantly degrading at any temperature. It is just a matter of 

time-temperature (i.e., kinetics). We have gone out to 6 

months at 50°C, which is about 130°F, and we have measured no reduction in density or change in property, but at 

150°F, which is 65°C, in as little as a month you can have up 

to a percent of mass loss and you can have a measurable 

probably 4% to 5%, strength loss, and that process continues 

on relatively a pure kinetic basis (Lebow and Winandy

1999). You had talked about discoloration in the press at 

some of these high temperatures and you were talking minutes at 200°C. Our work would show that we can create the 

same kind of degradation by degrading the arabinose, which 

is a pentose-structured ring which degrades into furfural. 

Furfural is a dark chocolate brown color, which is normally

what you see as the first degradation product of the carbohydrate as we have seen that discoloration in just a few months 

at 65°C, and basically for every 18°F, you have a doubling 

of the reaction rate every 10°C, that is not exactly true but it 

is a good rule of thumb, so you can see that going from 

200°C to 220°C in a press would have not a doubling but a 

quadrupling in your reaction rate in the degradation of these 

carbohydrates. They would be accelerated by steam because 

steam would enhance the hydrolysis which the main degradation method that these things degrade. There are papers by 

Patti Lebow and myself in Wood and Fiber Science (1996, 

1999) modeling kinetic degradation rates. 

Christiansen: Jerry, does that mean that boards put on our 

roofs which get very hot are likely to become less tough and 

strong as time goes on? 

Winandy: With our work we talk of energy absorption 

rather than toughness because toughness is a just one particular test, whereas energy absorption describes a category 

of properties (including toughness) dealing with how much 

energy is required to cause some kind of failure criteria in 

the material. Our work has shown that you do get a decrease 

in energy absorption in materials as soon as you start to 

degrade some of these hemicelluloses. If youíll allow me to 

use an analogy that we often use in our talks on this subject, 

we think that the secondary bonding of polymers in wood is 

similar to how we might think of Velcro. As you begin to 

degrade these side chains off the hemicellulose, you get less 

and less stress transfer and strain transfer from one polymer 

to another similar to if you start to cut the little ringlets off 

your piece of Velcro and it can carry progressively less and 

less stress. Thus, you get less stress transfer from one 

polymeric unit to the next. Eventually you start to see strain 

energy build up which then causes the fracture and thatís 

why you get big reductions in energy absorption. Itís related 

to how quickly you can dissipate a high concentration of 

stress in a very specific location away from that area. As you 

lose these little Velcro hooks you end up with bigger strain. 

Eventually you get to the point where you create fracture 

because you get enough fracture energy that you get a catastrophic failure. This is why we believe that heat treated or 

thermally degraded wood almost always fails in a brash 

manner rather than the ductile manner that we often see with 

clear wood material. 

John Hunt: I have just a question related with Karl 

(Englund)ís questioning difference processes and I am not 

sure. I am not in this area of research necessarily, but the 

question is, if there are multiple resin systems where, for 

example, you put MDI in the core and phenolic in the faces, 

then does the MDI still catalyze as fast in that core? How 

about the PF on the surfaces? Maybe someone has experience or any comments on that.

Noffsinger: In the industry, MDI in the core will definitely 

speed up your process so you could do that, so thatís standard. The real question is there something else we could add, 

nobodyís used in the past, and I think somebody called it 

gorilla juice. Is there some gorilla juice out there someplace 

that we can make this board go faster? 

Frazier: Yes, it costs $5 a pound. Thatís the problem. 

Winandy: I just wanted to reiterate a point that I made 

earlier is that the way we sell composites in this country, you 

had mentioned that there were private industrial entities that 

are selling those products, but they are basically selfstandardizing it too. Yes, it meets PS-2, the OSB sheathing 

standard, but it way exceeds that. The problem is that there is 

nothing in ANSI PS-2 that allows you to say that ìmy product exceeds level three or level four of PS-2.î PS-2 only has 

level one. 

Noffsinger: Actually, what is happening is we are finding 

two companies come out with design value products, evaluation products through the code agencies. Huberís already has 

theirs, and Weyerhaeuser is going to have one very shortly so 

that takes care of that part of the equation. 

Winandy: I guess that is my point, they are doing it privately through the codes. We have now seen this in treated 

products, fire retardant treated products, and preservative 

treated products, that the standards wonít differentiate these 

products. So the people go directly to the code and they 

differentiate their products that way. But the bottom line is 

that you could do it through the standards if you were to 

pursue commodity differentiation. If there was a consensus 

in the industry that I can add value, then I can afford to keep 

something in the press for 30 s longer, while slowing my 
51

process by 25%. We can slow up by 25% if we can add 30% 

value, but until you put those gradations in these standards, I 

donít think small companies can get that kind of payback on 

value added. 

Noffsinger: That is correct, you would be able to slow the 

press down by 25% if you could add a value of 30%, but that

doesnít happen in the marketplace. The issue that you run 

into is how do I reduce thickness swell and make sure that 

the customer realizes that that is a value to them. One of the 

problems is if you build a house and get it covered quickly 

before it rains on it, you donít need to have these high end 

boards, in fact I was with a builder who builds over a 1,000 

homes in the Baltimore, Washington, DC, area, and he says 

he has maybe 30 homes where he actually has to sand the 

edges where they have swollen too much. So why would he 

pay as much as $10 more a pound for some high-end board. 

So you get the pushback from users on why do I need this 

high-end board when I normally can get by with standard 

PS-2 board. And oh, by the way, wait until the hurricanes 

come through or the fires in California hit, you will see the 

market shoot up because everybody runs out and buys it all 

anyway. I believe that is why, right now, OSB is priced as 

high as it is. It is nice to be there because we can afford to 

take 5 s longer to press with the prices we are getting. 

Wang: I have a question probably for everybody. We know 

when we create adhesion between two surfaces we need a 

good surface, so that means we have wetability or maybe 

scientific term is a good surface energy. In that sense does it 

play the key the rule what we consider bonding or maybe we 

forget about this part, because we know the resin companies 

focus more trying to decrease the pressing time and to let the 

resin cure faster. For panel industry maybe they do the same 

thing, but I am not sure we have anybody looking at what 

kind of process could make better boards. Of course, before 

we apply the resin on the surface, surface energy is consistent. But when we press it then, inside the press conditions 

change. Assume surface energy would be changed. I donít 

know any research related to this area or not. 

Frihart: Well certainly as far as wetting is concerned, one 

of the factors is pressure. I mean if you put enough pressure, 

you will tend to get spreading across the surface. We measure things under atmospheric pressure, but really do not 

know how does this relate to actually wetting in the press 

where you would have high moisture contents and high 

pressures, I am not sure how you would do that relationship, 

so I donít know the answer. One of the things, which we 

tend to do in most studies, is use one species of wood. I 

guess one question is, how much of a problem is it out there 

in industry when your dealing with multiple species of wood 

which have different penetration rates, and how big of an 

issue is that for industry to deal with? 

Noffsinger: I try to ignore as much as I can because like I 

said earlier, we run over 20 different species in our products,

so if I knew all that you all know about 1 species, with 20, I 

probably would go home because I would be scared to think 

about what I am doing. Iíve got a question: We start off with

pressing, and we start off with blending, but what about the 

effect that I have on the outlet temperature of my dryer on 

strand quality and surface quality and its effect on bonding?

We have got a Canadian mill that runs at 1200-1400°F, on 

their outlet temperature. We have other mills that are 

running down lower than that. Is it better to run a lower 

temperature than a higher temperature, or has anybody

looked at that? 

Frazier: Well, I think the critical question there is what is 

the surface temperature on the particle? The high temperatures at the end of the dryer may not necessarily mean that 

the surface temperature of the wood is extreme. But coupled 

with that is the distribution of moisture contents, and as you 

know, you get a tail of that distribution that is low in the MC 

and then you start to get excessively high temperatures and 

then the negative impact is the reduction of surface energy. 

So it is definitely a potentially important issue and so perhaps the question is what is the distribution of moisture, how 

much of that low-end MC tail is getting harmed or how 

much of that low MC material is present which is the stuff 

that is going to be susceptible to thermal deactivation. And 

getting back to that, it is certainly important you know the 

surface chemistry, but I think I have no idea but based on a 

theoretical argument, I think that we donít see the effect 

unless it is a severely deactivated surface, then we know we 

canít bond it, like if we had bad veneer, but many times it 

may deactivated but not bad enough that it prevents you 

from getting a good mechanical bond in the plant, and so 

there is no notion that there is a problem. But the real issue 

with surface energy of the wood is a durability issue. If you 

donít maintain a good high wood surface energy, down the 

road is where it may end up becoming more apparent in the 

durability in the fact that if you have a low wood surface 

energy, not so low that you canít wet it all and canít bond it 

there, but low enough that you can get a mechanical bond, 

and you can ship it out but still it is low enough that you 

donít appreciate that at that surface you still have an energy 

potential there, that moisture wants to go there to reduce 

energy and so that would be a durability issue. 

Frick: I think that brings up a general point we are talking 

about: Are we making a higher grade of composite and do we 

have the right standard or code to quantify that? But really

what are we testing also, and are we testing the right thing? 

I mean, we can design a composite that can meet whatever 

tests we say that composite has to pass but if we have the 

wrong test we are going to come up with the wrong 

composite?

Frazier: Yes, the whole concept of the test is so complicated 

and again getting back to the very important point that 

Chuck made earlier, the difference between adhesion strength 

and adhesion. I mean there is a difference there, and we can 
52 

talk about it, but you would ask appropriately, well okay, 

letís talk, tell me what the test is? That is not a trivial issue, 

I donít know the answer, what the test is, and it is very 

difficult. 

Lee: I have a comment on the test, well actually we were 

doing some OSB creep testing for about 3 months and after 

that we wanted to see some change in the interface by doing 

the IB test. I had two different faculty members have different ideas. One wanted to test IB, the other one didnít want to 

do that because it was unknown what relationship IB had 

with other types of the testing. Currently many people depend on the IB test, but there are lots of different variables 

with IB testing. If you have a whole lot of the variabilities

with the IB specimens, you cannot determine what you want 

to from that testing. 

Frihart: Yes, that certainly goes to the point that I made 

before on different materials, if you are doing IB, you are 

doing a normal force type of test on the material. But if you 

are doing creep, you are doing a shear type of test. They are 

not really the same thing; so trying to compare one to the 

other without understanding the mechanics of the bonded 

assembly is not a good idea. 

Frazier: Yeah, I just donít know what to say about testing, it

is such a complicated issue, I mean we have to live with 

what weíve got obviously, we are never going to replace IB, 

we can complain about it, and I know that some of us do, I 

do, but it all we have got, I donít know the answer. It is a 

very difficult question, difficult problem. Maybe down the 

road as we develop these statistical models and correlations 

maybe in the end it is not going to be a mechanical property 

that we test, maybe it is going to be some 4th order of the 

response to x-rays or something like this that we donít really 

have a feel for, kind of like, you know, the new theories in

physics, we canít understand them but mathematically they 

make sense and perhaps that is going to be the way to go, but 

not in the near term. 

Garcia: It seems to be, I think, the state currently is the dielectric cure sensing. At the moment like you said there is a 

lot of things to incorporate a lot of temperature effects, 

moisture effects, I know this, I think you guys did some work 

on glass transition temperature detection with the dielectric 

system.

Frazier: Of the wood cell wall? 

Garcia: Yes, so all of that gets jumbled up into one signal. 

The current state is maybe we canít say what the signal 

actually means. We can roughly say a little bit safer there is 

a greater temperature, or very much higher, there is good 

moisture content but from a process control prospective 

maybe you donít need to know exactly what that signal is.

Last time we pressed a good panel we got that sort of signal 

and signal was changing. 

Frazier: I couldnít agree more, I think there is a big difference between modeling and process, and the dielectric is 

going to be extremely useful for in-line process control,

particularly coupled with statistical analysis and I think 

acoustics could be as well. I think there is some huge potential for process control and I think that is great, you know,

there is a lot of work to be done there. 

Garcia: The acoustics stuff is quite interesting, while I was 

working with a bunch of ex-navy guys on this dielectric stuff 

and I was telling them about the acoustic things and they go 

ìoh yeah, you ping the boardî and we are all thinking about

the sonar. The noise stuff that seems to hold a lot of promise 

but it is not exactly the quietest environment and inside a 

mill and so that has a lot of limitations on it. 

Frazier: Yes, I have no knowledge of the hardware but 

certainly there must be some signal isolation issues there, but

perhaps if you get the right frequency, maybe it is not a 

problem, but that is interesting. And the other question, back 

to modeling, I thought to put this in my presentation, but 

maybe we donít have to model resin cure at all, perhaps we 

just need to perfect our models of heat and mass transfer and 

then simply devise special apparatus for exposing our resins 

to these conditions and measuring what happens to the resins 

in an indirect fashion and then back that information into a 

model. I think that Rick Rammonís comment is the one that 

puts that whole approach into question is that if the resin is 

influencing the relaxation and the heat and mass transfer 

effects then we are not going to be able to back that information up correctly, so I think that is a good observation, a good 

comment that Rick made earlier, important one. Because 

these methods of pulling samples out of mats and analyzing 

them, it can be done, it is just extremely labor intensive. That 

is why I think the more important point is that we have to 

have larger, collaborative efforts, we have to get together 

and agree that that is the thing that we want to do. Maybe it 

is not the thing that we want to do presently because it seems 

like there is still quite a bit that we need to learn about heat 

and mass transfer modeling, but those are huge projects so 

we have to agree that that is important, get together and 

work together on that and remain persistent because that is 

not even a four year project. 

Frihart: A lot of questions were raised; I know they are not 

going to be solved within the next couple years so that they 

can be discussed at the Wood Adhesives 2005, which will be 

November in San Diego so everybody can work out the 

solutions by then. 

Someone in crowd: Are they going to have the fires put out 

by then? 

Frazier: Not only will the fires be out, but there will be 

new products coming out of the forest from small diameter 

timber, right? 
53

Winandy: Does anybody have any other questions? 

Lee: Any specific idea about the benefit we can have by 

using the nanoscale technology to the adhesion research? 

Frihart: The first problem that you run into doing nanoscale 

work is that with wood you have a huge number of surfaces. 

So what surface are you actually measuring? It works nice if 

you have got single crystals, monolayers, etc., where you 

can know what you are studying. However, if you try to 

apply the same thing like atomic force microscopy, normally 

what happens is you fall into the lumens and you can look at 

the pits nicely. But how do you really look at all the surfaces 

on the wood? So I think that maybe we can eventually work 

our way there but I think we really have to try and understand what is happening at the cellular level first before we 

go to a smaller scale. At least that is my assessment of the 

surface analysis issue. 

Frazier: Of course there are always those carbonados. Haha. 

I think that we can but perhaps not for improving the properties but maybe as sensors. And maybe nano technology is a 

new type of sensor and not necessarily an electronic sensor. 

Perhaps it could be something that could store information 

electronically but perhaps it is something that just is when it 

is in a particular environment exposed to a temperature or 

pressure it undergoes its own morphological change, which 

is very well characterized. Then we could retrieve that material and essentially observe its thermal history and then 

deduce what the environment was. So I think nano technology may be valuable for us, not in terms of material science 

of our material, but in a part of this modeling and learning 

about what the local environment is really about, maybe. It 

seems feasible, question is when would it be economically 

feasible and of course for it to be economically feasible for 

you and I. Jong is different, you know, because in research 

you can afford to put more into sensors and so forth, so it 

may not be that far off actually. I hope. 

Lee: Some of the literature review that I made with the resin 

impression cutting was that in Europe or some Japanese 

colleagues worked on that and they used the nanoindentation techniques for the AFM to see if there is a significant changing material properties by the wetting impregnation and mostly they did with melamine formaldehyde, but 

in the U.S., we use phenol, so is it worth it to work with this 

kind of technique to see if there is any feasibility using the 

low molecular weight PF resin as a reinforcement material. 

Frihart: I think certainly that understanding how chemical 

treatment may alter the physical properties of the surface of 

the wood is important because that is obviously where the 

adhesives are interacting. If you are studying how the resins 

are interacting with that surface, I think that can provide 

some additional information. And the thing is that you have 

to then correlate that to the adhesive performance test you 

are doing. I think that there are some things that can be done 

and we have gotten some other ideas from that in some of 

the work we are doing. 

Frazier: I would also add regarding the impregnation of 

phenolics into the cell wall, you know much of that has been 

done of course way back long time ago, I forget the name is 

it impreg or compreg, I mean much of the fundamental of 

understanding of low molecular weight phenolics and how 

those interact with the wood cell wall and how they change 

the anti-shrink efficiency and all of that is well established,

so perhaps we have a lot of answers already in the old 

literature. 

Winandy: Well it looks like we are at a point where the 

conversation is starting to slow down quite a bit. For a number of questions, we actually had 3 or 4 seconds between 

questions, so it must be time to stop. 

It has been a very exciting day, both the morning and afternoon session. I congratulate each of you on making it 

through almost 1-1/2 hours without anyone taking a breath; I 

think that we ought to thank our two speakers for leading 

this wild group of people. 
54 

Press Control for Optimized Wood 

Composite Processing and Properties 

Part 1: Pressing Variables and Sensors 

Chunping Dai, Senior Scientist 

Wood Composites, Forintek Canada Corporation, Vancouver, BC 

Siqun Wang, Assistant Professor 

Tennessee Forest Products Center, University of Tennessee, Knoxville, Tennessee 

Introduction 

Hot-pressing is a key operation in wood-based composite 

manufacture. During such an operation, mats of resinated 

wood fibers, particles, or flakes are consolidated under heat 

and pressure to create close contact and form bonds between the wood constituents. Due to limited amount of 

resin usage, effective bonds rely on a high degree of mat 

densification. Increasing mat density, on the other hand, 

causes negative effects such as heavier products, more 

wood consumption and more importantly excessive thickness swell in service when the product is subjected to high 

humidity conditions (Kelly 1977). The necessity and detrimental effects of mat densification suggest the importance of process optimization. 

The pressing of a wood composite mat involves physical, 

mechanical, and chemical interactions. Once the press 

closes, moisture and heat transfer takes place between the 

hot platens and the mat. During the course of pressing, 

temperature and moisture content inside the mat are both 

spatially and temporally dependent (Humphrey and Bolton 

1989). At the same time, the platens exert compressive 

forces onto the mat causing reduction of voids and compression deformation in wood constituents (Dai and Steiner 

1993, Lang and Wolcott 1996). The mat deformation is 

usually not uniform across the mat thickness due to the 

variations of heat and moisture content from the surface to 

the core layers. Besides softening the mat, elevated temperature also accelerates resin polymerization, which combines with mat deformation to form permanent bonds 

between wood constituents. With little springback, the glue 

bonds also freeze the overall and layered mat deformation 

upon press opening. This leads to formation of the wellknown vertical density profile (VDP) in pressed wood 

composite panels, which in turn has a significant effect on 

the physical and mechanical properties of the final products 

(Suchsland 1959; Kelly 1977; Harless and others 1987; 

Wolcott and others 1990; Winistorfor and others 2000, 

Wong and others 1999, Dai and others 2000). 

Press control is important not only because pressing affects 

the board properties but also it is closely associated with 

the production costs. Regardless of product types, board 

dimensions always need to be controlled within specifications. In this regard, pressing plays a critical role in maintaining the final board thickness or minimizing the sanding 

waste. While heat is necessary to soften the mat and accelerate the resin curing, the combination of high temperature, 

moisture content, and time can sometimes lead to development of excessive gas pressure inside the mat and therefore cause blows upon press opening. Thus, certain press 

opening procedures are needed to vent the gas, which often 

means extra pressing time. The press opening time and the 

total pressing time should be minimized from the productivity standpoint. Less pressing time normally means higher 

productivity and lower manufacturing costs. The manufacturing costs can be further reduced by the control of the 

hot-pressing operation because it also represents one of the 

biggest energy consumption items in wood composites 

manufacture. 

The general objective of the paper is to provide a state-ofthe-art analysis of press control for wood composites. The 

specific objectives for Part 1 of this publication series are 

• to introduce the basic concepts and variables of press 

control, 

• to evaluate the responses of press variables during hotpressing, and 

• to review the sensing and control technologies for hotpressing. 

Basics of Press Control 

Automatic Control Process 

Process control is widely used in our daily life and manufacturing operations. Control of ambient temperature in a 

room and driving speed of a car are common examples. 

Figure 1 identifies the elements of a typical process control 
55

system: process, measurement, controller, and control 

element (Johnson 2000). In general, a process can consist 

of a complex assembly of phenomena which relate to some 

manufacturing sequence. The process may involve many 

variables. Therefore one must define the control variable 

(c) first. To control a variable in a process, information 

about the variable must be gathered usually through measurements using a sensor or sensors. The measurements are 

usually in the form of electrical signals (e.g., 4-20 mA) 

which can then be further converted to real values (b) 

meaningful to the controlled variable. The measured values 

are compared with the reference points (r). If an error is 

detected between the real value and the reference value, the 

controller will send a signal (p) to the control element for 

necessary adjustments to the process (u) until the controlled variable reaches the reference value. Since the 

variable is controlled automatically in real time, such a 

control is often referred to as automatic control. 

The automatic control process is applicable to control some 

hot-pressing variables such as platen temperature, pressure, 

and position. Here, the pressing process involves changes 

in thickness and pressure of resinated wood furnish mats. 

The mat pressure and position are measured using pressure 

probes and LVDT sensors, respectively. The measurement 

and the error detection along with the controller are all 

enclosed in a programmable logic controller (PLC) in 

which input (I) and output (O) signals are linked from the 

sensors and to the control element–servo valve (Fig. 1). 

The hydraulic valve can regulate the pressure and position 

based on the output signals from the PLC. 

Program Control and 

Press Control Variables 

When it comes to control hot-pressing, not all the variables 

can be controlled automatically or in real time. Probably 

due to lack of effective monitoring sensors, most of the 

pressing variables are currently controlled by predetermined programs or often known as pressing schedules. The 

pressing schedules control such important variables as mat 

core temperature and gas pressure, vertical density profile, 

degree of resin cure, and minimum pressing time. The key 

to the program control of hot-pressing is solid understanding and accurate modeling of the responses of these control 

variables during hot-pressing and their relationships with 

all other variables before and after pressing. The pre-press 

variables include species, furnish types and dimensions, 

moisture content, resin content, mat density, and mat structure. The post-press variables are product types and dimensions as well as board properties. Variations of these variables would require adjustments of the pressing schedules 

to achieve maximum product performance and minimum 

production costs. 

Responses of Press Variables 

During Hot-Pressing 

Significant progress has been made in understanding hotpressing processes over the past 15 years both through 

modeling and experimentation. Bolton and Humphrey 

(1988) were among the first to provide a comprehensive 

investigation of the heat and mass transfer process in hotpressing. An excellent review on hot-pressing was first 

reported and then followed by the development of a model 

for heat and mass transfer (Humphrey and Bolton 1989). 

Their stimulating work identified primary physical mechanisms of the hot-pressing process and at the same time 

revealed the complexity and interactive nature of the subject. Three main aspects of hot-pressing: heat and mass 

transfer, mat consolidation, and resin curing will now be 

discussed with respect to press control. 

Heat and Mass Transfer 

The variations of mat internal environmental conditions 

have been extensively documented in many studies both 

through experimentation (Maku 1954, Strickler 1959, 

Kamke and Casey 1988) and modeling (Humphrey and 

Bolton 1989, Zombori and others 2003, Dai and Yu, unpublished data). Figure 2 shows typical variations of mat 

core temperature and core gas pressure for a 11-mm-thick, 

0.9- by 0.9-m (7/16-in.-thick, 3- by 3-ft) laboratory oriented strandboard (OSB) panel. Due to the nature of the 

heat and mass transfer process, the core temperature 

change is highly nonlinear. Particularly, the core temperature does not experience significant changes until 40 s after 

pressing. Then, it quickly rises probably due to convective 

flow and condensation of steam generated from moisture 

evaporation in the surface layers. As the core temperature 

rises, the steam accumulates and gas pressure starts to build 

up. The increased core gas pressure forces the trapped 

steam to escape from the edges, which leads to energy loss. 

As a result, the core temperature starts to level off and the 

core gas pressure usually reaches a maximum before it 

starts to drop. At the end of pressing, the press starts to 

open allowing the internal gas to release its pressure by 

flowing from the core to the surface and to the atmosphere, 

which also leads to a drop in the core temperature. 

The fore-mentioned core temperature and gas pressure 

variations are typical of small panels. In the cases of commercial production, both the core temperature and the gas 

pressure usually experience some noticeable changes. First, 

the use of a screen caul plate can slow down the heat transfer from the platen to the mat surface, leading to slower 

core temperature rise (Fig. 3). Second, larger panel size 

reduces the energy loss from the edges, resulting in higher 

maximum core temperature and much higher core gas 

pressure. Particularly, the core gas pressure tends to always 

increase with pressing time again due to the reduced edge 
56 

effect. Because of this, a prolonged degas time is usually 

needed to allow for gas pressure release before press opening. Finally, the increased panel size can magnify the effects of furnish moisture content, furnish geometry, and 

mat density on the core temperature and gas pressure. 

Mat Consolidation 

Nonlinear Compression 

StressñStrain Relationship 

A simple compression test of a composite mat can reveal a 

highly nonlinear stressñstrain curve, which is characterized 

by an extended low-stress plateau at the beginning and 

steep stress rise towards the end. Such a relationship is 

similar to pure transverse compression behavior of solid 

wood except that solid wood clearly exhibits a linear elastic 

behavior at the start. Figure 4 compares the compression 

stressñstrain relationships between solid wood (or stack of 

strands) and a randomly formed strand mat. The similarity 

in both compression trends is probably due to the fact that 

wood substances are under compression in both cases 

whereas the difference can probably be attributed to random mat structure. This phenomenon was first identified 

by Suchsland (1959) and then later modeled by Dai and 

Steiner (1993). The model was based on the Poisson distribution of random strand overlaps (Dai and Steiner 1994a, 

b) and a modified Hookeís law for transverse wood compression (Wolcott and others 1994). The strand overlap 

variation results in uneven stress distribution; only higher 

density areas contribute to load bearing at the beginning 

and lower density areas do not bear significant load until 

larger deformation occurs. The nonlinear compression 

behavior of mats was later reported with presentation of 

similar models by Lang and Wolcott (1996) and Lenth and 

Kamke (1996). 

Viscoelasticity and Elastoplasticity 

Since wood is a viscoelastic material (Wolcott and others 

1990), mats of wood elements exhibit qualitatively similar 

but quantitatively different time-dependent behavior, 

namely strain creep under constant load and stress relaxation under fixed deformation. The difference is also attributed to the random mat structural formation. The knowledge of mat structure allows for the derivation of 

viscoelasticity relationships between mats and wood 

elements (Dai 2001). 

The combination of high pressure and heat also results in 

both permanent mat deformation and springback upon and 

after pressing. Such mat behavior can be best described by 

the concept of elastoplasticity (Dai and others 2000). Elastoplasticity states that the mat compression stressñstrain 

curves go through different paths during loading and 

unloading, and the mat deformation only recovers to a 

limited extent upon load removal. This implies that the 

mat properties can be manipulated by both pressing closing 

and opening. 

Mat Pressure and Thickness Relationship

The hot-pressing process is mechanically controlled by mat 

thickness and pressure through controlling the total deformation and stresses equally applied to each layer. At any 

given time in pressing, the overall mat deformation is 

controlled either by position or pressure. Thus, one is the 

resultant of the other. 

Figure 5 is a predicted relationship between overall mat 

pressure and thickness for OSB hot-pressing (Dai and 

others 2000). During the first stage of press closing, the 

press platens are usually controlled by position. Mat pressure increases as mat thickness decreases. The variation of 

mat pressure and thickness is governed by the mat stressñ

strain relationships, which vary from surface layers to core 

layers due to the variations of temperature and moisture 

content. During the second stage, the mat reaches maximum pressure and the press is switched to pressure control. 

Mat thickness is determined by the layer stressñstrain 

relationships and creep properties at the maximum pressure. Each layer behaves differently again due to the difference in temperature and moisture content. During the third 

stage, the mat reaches the target thickness and maintains it 

for resin curing. In the same time, mat pressure sharply 

drops and then gradually levels off. The decrease in mat 

pressure is a coupled response of mat softening (decrease 

of compression modulus) and viscoelastic stress relaxation 

(Fig. 5). During the last stage of pressing, the press is 

switched back to pressure control during which the mat 

pressure linearly drops to zero. This allows the internal gas 

pressure to release before the press opens. The removal of 

mat pressure also leads to a certain degree of mat thickness 

springback. The springback is characteristic of elastoplasticity which is affected by degree of resin cure, mat densification, temperature, and moisture content. The springback 

varies from surface layers to core layers and always results 

in the out-of-press panel thickness being greater than the 

press opening (Wang and Winistorfer 2002). 

Vertical Density Profile 

At any given time, the stresses applied to each layer are the 

same and equal the mat pressure. While the total mat deformation is controlled by the press opening, the deformation in each layer is distributed according to its viscoelastic 

and elastoplastic properties, which are functions of temperature, moisture content, and degree of resin cure. Figure 6 shows predicted density profiles at various stages of 

pressing (Dai and others 2000). Before pressing, the mat 

density is assumed to be uniform in all layers. During press 

closing (Fig. 5), the layer deformations are calculated 

based on their stressñstrain relationships. The layer and 

overall mat densities in this stage incur the biggest increases. Particularly, the surface densities increase at 
57

remarkably higher rates than core densities because of their 

lower compression modulus due to higher temperature. The 

density differences are much less noticeable in surface 

layers. This stems from the fact that mat softening due to 

high temperature in the surface layers adjacent to the 

heated platens is masked by the hardening effect of rapid 

moisture loss and thus softening effect of increased moisture content in the subsurface layers. Such a dry-out effect 

becomes more pronounced as the mat continues to be 

consolidated. By the time the mat reaches the target thickness, a slightly lower density is formed in the outer layer, 

leaving the density to peak in the inner surface layers. At 

this point in pressing, most of the density profile is formed. 

In other words, VDP is substantially formed during press 

closing. 

After the mat reaches the target thickness, VDP seems to 

experience only minor changes (Fig. 6). As mentioned 

earlier, mat pressure decreases rapidly during the early 

stage of the period of holding target thickness (Fig. 5). The 

elastoplastic nature of mat deformation implies that a certain degree of springback can occur in the low temperature 

core layers, leading to further deformation in the softer 

surface layers. Such a give-and-take phenomenon of mat 

deformation may or may not lead to density changes because of the concurrent mass movement due to moisture 

flow between layers. As shown in Figure 6, neither the 

core density nor the surface density changes appear to be 

noticeable. This is because the core layers experience 

significant increases in moisture content, while the opposite is true in the surface layers at this time of pressing. The 

opposite mass movement seems to counter out the mat 

deformation exchanges, leaving the layered density more 

or less unchanged in this case (Wang and Winistorfer 

2000). 

In Figure 7, the mat exhibits some immediate (and delayed) 

springback upon press opening. It is believed that springback occurs mostly on the surface layers immediately 

adjacent to the platens due to so-called resin pre-cure (resin 

is exposed to premature heating and dry-out before the 

surface layers reach their maximum densities). In fact, 

springback due to pre-cure can lead to significantly lower 

density surfaces. In some cases, the surface density is even 

lower than the core. 

Resin Curing and Bond 

Strength Development 

One of the fundamental objectives of hot-pressing is to 

accelerate resin curing and develop bond strength between 

wood elements. Pressing operations can be best controlled 

if the conditions for optimum curing and bonding are 

known. Although the chemistry of resin curing for pure 

adhesives is well documented (Pizzi 1994), the curing and 

bond strength development of resin–wood mix seems much 

less understood probably due to several general reasons. 

First, the resin amounts for most of the wood composite 

products are very low (usually much less than 15% by 

ovendry weight of wood) and yet the resin distribution and 

penetration are not well defined. The amount of resin 

added to OSB furnish is low and is typically about 2% 

(wt.) resin solids based on ovendry weight of wood. These 

factors can significantly affect the accuracy and variability 

of resin analyses. Second, the common chemical analytical 

methods can no longer be applied to analyze resin curing 

when resin is mixed with wood. Third, the physical, 

chemical, and mechanical properties of wood can change 

dramatically when wood is subjected to compression 

stresses and elevated temperatures. These changes, which 

should have a significant effect on resin curing and bond 

strength development, are not well defined. Finally, the 

interactions between resin and wood also need to be taken 

into account when resin is analyzed. Nevertheless, several 

studies have been performed to understand the curing 

mechanisms (Troughton and Chow 1968, Chow and 

Steiner 1969, Humphrey 1997, Geimer and Christiansen 

1996). Some of the details were discussed in the previous 

papers in this proceedings by Frazier and Frihart. 

It is generally understood that resins accelerate curing 

under elevated temperature and curing needs time to complete. Therefore, most of the pressing schedules are developed to achieve a certain core temperature and maintain the 

temperature for a certain period of time. Table 1 lists 

curing conditions for some typical composite products. 

Besides product types, resin types and board thickness also 

have defining effects on pressing schedules. For example, 

methylene diphenyl diisocyanate (MDI) resins seem to 

accelerate curing at a lower temperature than phenol formaldehyde (PF) resins. Thicker boards create a greater 

heat-sink effect after pressing and therefore usually require 

lower core temperatures. 

Table 1—Curing conditions for some typical 

composite productsa

Product 

type 

Thickness 

(in.)b Resin 

type

Target core 

temp (°C) 

Holding 

time (s) 

Plywood 3/8 PF 110–115 

LVL 1.5 PF 100–105 

OSB 7/16 PF >100 

130 

60–90 

10 

OSB 7/16 MDI 100–110 30 

MDF/PB 3/4 UF 95 100 

a

LVL, laminated veneer lumber; OSB, oriented strand 

 board; MDF, medium-density fiberboard; PB, particleboard;

 PF, phenol formaldehyde; MDI, methylene diphenyl 

 diisocyanate; UF, urea formaldehyde. 

b

1 in. = 25.4 mm. 
58 

Another important factor concerning resin curing is that the 

curing should normally take place after platens reach the 

target position. As an example, many composite boards 

experience low surface density due to pre-curing, which 

occurs before the surface layers are consolidated. More 

precisely, resin curing in each layer should take place after 

the layer density is formed. 

Figure 7 depicts a pressing schedule for OSB. During the 

press closing, platens come to the target position to form 

the desired board thickness and vertical density profile. 

The governing variables in this stage include maximum 

allowable mat pressure and closing rate. During the second 

stage, the mat thickness is maintained to allow the heat to 

transfer into the core and cure the resin. Core temperature 

and holding time need to be controlled to achieve a certain 

degree of resin curing before the press starts to open. Mat 

pressure must gradually drop during a certain period of 

time to allow internal gas pressure to release to avoid 

blows. 

Challenges and Knowledge Gaps 

Hot-pressing of a resinated wood furnish mat presents a 

very complex subject. Most pressing variables except 

platen pressure, position, and temperature are program 

controlled and subjected to real time process variations. 

Program control requires quantitative understandings of 

responses of pressing variables. Despite the efforts by 

many researchers, mechanisms of hot-pressing have yet to 

be fully understood. The data on physical and mechanical 

properties of mats during pressing is particularly lacking. 

Sensor and Monitoring 

Technologies 

The current sensing and monitoring technologies concerning press control may be classified into three categories: 

automatic control sensors, on-line post-press sensors, and 

off-line/laboratory sensors. 

Automatic Control Sensors 

Automatic control sensors are usually an integral part of 

press equipment. Standard pressure and position sensors 

can be used to monitor and control platen (mat) pressures 

and openings. Thermocouples are also available to monitor 

and control platen temperatures. 

On-line Post-Press Sensors 

Commercial systems are available to measure on-line board 

thickness using LVDT and detect blisters in pressed boards 

using ultrasonic methods. Figure 8 also shows an X-ray 

system that has been developed and used for on-line 

monitoring of vertical density profile of pressed boards 

(Dueholm 1996). The technology is of significant 

importance in improving press control, although the 

measurements are carried out after pressing with a speed of 

20 to 40 s per scan. 

Off-line/Laboratory Senors 

Several sensing technologies have been investigated or 

developed to monitor in-situ behavior of wood composites 

during pressing. A system called PressMAN has been 

developed by Alberta Research Council (Edmonton, Alberta, Canada) and widely used for monitoring and controlling laboratory presses (Wellwood and others 1999). It uses 

special probes to measure mat gas pressure and temperature (Fig. 9). The system has also been used for trouble 

shooting in many commercial productions. 

Winistorfer and others (2000) developed a unique laboratory radiation system to monitor in-situ density development during pressing. This technique is a step forward 

compared with the traditional post-press X-ray methods, 

and the data provide valuable insight into vertical density 

profiles at various stage of pressing (Wang and Winistorfer 

2000). 

Several researchers also used dielectric or ultrasonic sensors to monitor resin curing in laboratories (Wolcott and 

Rials 1995, Magill and Sauter 1999, Chen and Beall 2000, 

Wang and Winistorfer 2003). Although promising results 

were reported, none of the methods seemed to gain 

wide use. 

One other useful method is that of Humphrey (1999). The 

automatic bond evaluation system (ABES) unit essentially 

consists of a mini-press and a lap-shear testing device. The 

bonding strength of two overlapped strands can be pressed 

under controlled temperature and pulled at any given time 

immediately after pressing. Such a system was used by 

researchers to evaluate resin performance. 

Knowledge and Technology Gaps 

Most sensors are limited to laboratory or off-line applications. It is not clear whether and how on-line sensors can 

be developed due to the complexity of hot-pressing. Methods for monitoring resin curing in composite boards during 

pressing are particularly lacking. 

Summary 

Hot-pressing processes are controlled by two methods: 

automatic control and program control. While the platen 

pressure, opening, and temperature are automatically controlled, mat internal environmental conditions, vertical 

density profile, and curing are controlled by predefined 

pressing schedules, which are subjected to process 

variations. Optimum control requires more quantitative 

understanding of variable responses during hot-pressing. 
59

More accurate sensors are needed for lab experimentation 

and on-line monitoringñcontrol.

Literature Cited 

Bolton, A.J.; Humphrey, P.E. 1988. The hot-pressing of 

dry-formed wood-based composites. Pt I. A review of the 

literature, identifying the primary physical process and the 

nature of their interaction. Holzforshung. 42(6): 403ñ406. 

Chen, L.; Beall, F.C. 2000. Monitoring bond strength 

development in particleboard during pressing using 

acousto-ultrasonics. Wood and Fiber Science. 

32(4): 466ñ477. 

Chow, S.Z.; Steiner, P.R. 1969. A kinetic study of the 

polymerization of pheno-formaldehyde resin in the presence of cellulose materials. Wood Science. 1(4): 215ñ221. 

Dai, C. 2001. Viscoelastic behaviour of wood composite 

mats during consolidation. Wood and Fiber Science. 

33(3): 353ñ363. 

Dai, C.; Steiner, P.R. 1993. Compression behaviour of 

randomly-formed wood flake mats. Wood and Fiber 

Science. 25(4): 349ñ358. 

Dai, C.; Steiner, P.R. 1994a. Spatial structure of wood 

composites in relation to processing and performance 

characteristics. Pt. II. Modelling and simulation of a 

randomly-formed flake layer network. Wood Science 

and Technology. 28(2): 135ñ146. 

Dai, C.; Steiner, P.R. 1994b. Spatial structure of wood 

composites in relation to processing and performance 

characteristics. Pt. III. Modelling and simulation of a 

random multi-layered flake mat. Wood Science and 

Technology. 28(3): 229ñ239. 

Dai, C.; Yu, C.; Hubert, P. 2000. Modeling vertical 

density in wood composites during hot pressing. In: 

Proceedings, 5th Pacific Rim bio-based composites symposium; 2000 December 10ñ13; Canberra Australia. 

Dueholm, S. 1996. Determination of density profiles of 

wood-based panels: from traditional off-line laboratory 

techniques to continuous-in-line real time monitoring. In: 

Maloney, T.M., ed. Proceedings, 30th international particleboard/composites materials symposium; 1996 April 15ñ

18; Washington State University, Pullman. WA: 45ñ57. 

Geimer, R.L.; Christiansen, A.W. 1996. Critical variables in the rapid cure and bonding of phenolic resins. 

Forest Products Journal. 46: 67ñ72. 

Harless, P.E.,; Wagner, F.G.; Short, P.H. [and others]. 

1987. A model to predict the density profile of particleboard. Wood and Fiber Science 19(1): 81ñ92. 

Humphrey, P.E. 1999. The bonding speed of adhesives: 

an automatic evaluation system. In: Proceedings, 33d 

international particleboard/composites materials symposium; 1999 April 13ñ15; Washington State University, 

Pullman. WA: 139ñ146. 

Humphrey, P.E. 1997. Thermoplastic characteristics of 

partially cured thermosetting-to-wood bonds. In: Proceedings, 3d Pacific Rim bio-based composites symposium, 

Kyoto, Japan. P 366ñ373. 

Humphrey, P.E.; Bolton, A.J. 1989. The hot pressing of 

dry-formed wood-based composites. Pt. II A simulation 

model for heat and moisture transfer, and typical results. 

Holzforschung. 43(3): 199ñ206. 

Johnson, C.D. 2000. Process control instrumentation 

technology. 6th ed. Upper Saddle River, NJ: Prentice-Hall, 

Inc. 678 p. 

Kamke, F.A.; Casey, L.J. 1988. Fundamental of flakeboard manufacture: Internal mat conditions. Forest 

Products Journal. 38(6): 38ñ44. 

Kelly, M. 1977. Critical review of relationships between 

processing parameters and physical properties of particleboard. Gen. Tech. Rep. FPLñ10. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 

Lang, E.M.; Wolcott, M.P. 1996. A model for viscoelastic 

consolidation of wood strand mats: Pt. II: Static stress-stain 

behavior of the mat. Wood and Fiber Science. 

28(3): 369ñ379. 

Lenth, C.A.; Kamke, F.A. 1996. Investigations of flakeboard mat consolidation, Pt. II. Modelling mat consolidation using theories of cellular materials. Wood and Fiber 

Science. 28(3): 309ñ319. 

Magill, R.; Sauter, S.L. 1999. In-situ impedance sensors 

provide real-time monitoring and intelligent control for the 

curing of engineered wood products. In: Proceedings, 33d 

international particleboard/composites materials symposium; 1999 April 13ñ15; Washington State Univ., Pullman. 

WA: 181. 

Maku, T. 1954. Studies on the heat conduction in wood. 

Wood Research (Kyoto) 13: 1ñ80. 

Pizzi, A. 1994. Advanced wood adhesives technology. 

New York, NY: Marcel Dekker Inc 289 p. 

Strickler, M.D. 1959. The effect of press cycles and moisture content on properties of Douglas fir flakeboard. Forest 

Products Journal. 9(7): 203ñ207. 

Suchsland, O. 1959. An analysis of the particle board 

process. Michigan Quarterly Bulletin. 42(2): 350ñ372. 
60 

Troughton, G.E.; Chow, S. 1968. Evidence for covalent 

bonding between melamine formaldehyde glue and wood. 

Pt. 1óBond degradation. Journal of the Institute of Wood 

Science. 21(9): 29ñ34. 

Wang, S.; Winistorfer, P.M. 2000. Fundamentals of 

vertical density profile formation in wood composites. 

Pt. 2. Methodology of vertical density formation under 

dynamic condition. Wood and Fiber Science 32(2): 220ñ

238. 

Wang, S.; Winistorfer, P.M. 2002. Monitoring in-situ

density change for in-process measurement and control of 

hot pressing. Forest Products Journal .52(7/8):77ñ82. 

Wang, S.; Winistorfer, P.M. 2003. Monitoring resin cure 

during particleboard manufacture using a dielectric system. 

Wood and Fiber Science. 37(4):532ñ539. 

Wellwood, R.; Kirincic, S.; Mallas, B. 1999. Techniques 

for measuring internal panel environments in the laboratory 

and plant. In: Proceedings, 33d international particleboard/composites materials symposium; 1999 April 13ñ15; 

Washington State University, Pullman. WA. 

Winistorfer, P.M.; Moschler, W.W.; Wang, S. [and 

others]. 2000. Fundamentals of vertical density profile 

formation in wood composites. Pt. 1. In-situ density measurement of the consolidation process. Wood and Fiber 

Science. 32(2): 209ñ219. 

Wolcott, M.P.; Rials, T.G. 1995. In-situ cure monitoring 

of isocyanate adhesives using microdieletric analysis. 

Forest Products Journal. 45(2):72ñ77. 

Wolcott, M.P.; Kamke, F.A.; Dillard, D.A. 1990. Fundamentals of flakeboard manufacture: viscoelastic behavior 

of the wood component. Wood and Fiber Science. 

22(4): 345ñ361. 

Wolcott, M.P.; Kamke, F.A.; Dillard, D.A. 1994. Fundamental aspect of wood deformation pertaining to manufacture of wood-based composites. Wood and Fiber Science. 26(4): 496ñ511. 

Wong, E.D.; Zhang, M.; Wang, Q.; Kawai, S. 1999. 

Formation of the density profile and its effects on the 

properties of particleboard. Wood Science and Technology. 33: 327ñ340. 

Zombori, G.B.; Kamke, F.A.; Wastson, L.T. 2003. 

Simulation of the internal conditions during the hotpressing process. Wood and Fiber Science. 35(1): 2ñ23. 

Figure 1óTypical process control system. 

Servo Valve

r 

- 

e = r - b 

Controller

p Control element 

Process

u 

Measurement 

c 

b 

Su m ming 

 point 

PLC

O

I

Press

 + 

Mat

Pressure Probe

VDT
61

Figure 2óMat core temperature and gas pressure for a 11-mm-thick, 0.9- by 0.9-m 

(7/16-in.-thick, 3- by 3-ft) laboratory oriented strandboard (OSB) panel 

(1 lb/in2

 = 6.9 kPa; °F = 1.8(°C) + 32). 

Figure 3óEffect of screen caul plate on heat and mass transfer in a large panel (1 lb/in2

 = 6.9 kPa). 

1 31 61 91 121 151 

10

15

20

60

Core temperature

! Slower rising rate

! Higher maximum

!

!

! Screen ⇒ Slower heat transfer

! Large panel size ⇒ Little edge effects

7/16-in. board thickness

0 

50 

100 

150 

200 

250

Pressing time (s)

0

5

25

30

35

40

45

50

55

Gas pressure (lb/in2) 

!

!

!

!

Pressure (bars) and temperature (°C)

Increase with time

Much higher maximum

⇒ Core gas pressure

⇒

Screen 

Large panel size 

Slower heat transfer

Little edge effects

!
62 

Figure 4óComparison of compression stressñstrain relationships between solid wood 

(or stack of strands) and a randomly formed strand mat. 

Figure 5óPredicted relationship between mat pressure and mat thickness. 

0 

1 

2 

3 

4 

5 

6

7 

8 

0 50 100 150 200 250 300 350 400 

Pressing time (s)

0 

5 

10 

15 

20 

25 

30 

35 

40 

Mat pressure (MPa)

Mat thickness (mm)

1. Thermal softening

2. Viscoelasticity 

Stress relaxation:

Thickness

Pressure

0 1

1

3

0.2 0.4 0.6 0.8

0 

2 

4 

5

Strain

Stress (x1000 lb/in2

)

Stress (MPa) 

6.9

13.8

20.7

27.6

34.5

0 

Strands

Mat
63

Figure 6óPredicted density profiles at various stages of pressing. 

Figure 7óTypical pressing schedule for oriented strandboard (OSB) 

(1 lb/in2

 = 6.9 kPa; °F = 1.8(°C) + 32; 1 in. = 25.4 mm). 

200 

250 

300 

350 

400 

450 

500 

550 

600 

650 

700 

750

800

-15 -12 -9 -6 -3 0 3 6 9 12 15

Mat thickness (mm)

Final

Mat density (kg/m3

)

Target Thickness (50 s)

Press Opening (300 s)

Final

Max. Pressure (30 s)

Before Pressing (0 s)
64 

Figure 8óX-ray monitoring system for on-line measurement of vertical density profile. 

Figure 9óPressMAN probes (Alberta Research Council, Edmonton, Alberta, Canada). 
65

Press Control for Optimized Wood 

Composite Processing and Properties 

Part 2: Properties and Press Control Strategies 

Siqun Wang, Assistant Professor 

Tennessee Forest Products Center, University of Tennessee, Knoxville, Tennessee 

Chunping Dai, Senior Scientist 

Wood Composites, Forintek Canada Corporation, Vancouver, BC 

Introduction 

The hot-press is a key piece of equipment in composite panel 

production. It determines the performance of products and 

the processing efficiency of the whole production line. There 

is a philosophy that says, in simple terms, when the press 

closes, 90% of the cost of production has been incurred, and 

96% of the oriented strandboard (OSB) panels must be good 

panels because quality cannot be added in the warehouse 

(Lin 1996). However, the press cannot be regarded as a 

quality control cure-all. Any problem that already existed 

before pressing, including furnish preparation, resin blending, and mat formation, may be only partially compensated 

for in the pressing operations. 

The previous paper in this series discussed the importance 

and complexity of hot-pressing in wood composite manufacturing, material behavior during hot pressing and sensing, 

and control of hot-pressing process. The purpose of this 

paper is to discuss the optimized panel properties and press 

control strategies. 

Manipulated Board Properties 

During Pressing Process 

Manipulating panel properties can be done through the 

whole panel process including furnish preparation, drying, 

blending, forming, pressing, and post-treatment. For a specific formed mat, manipulating panel properties has to be 

done during the pressing. Correct pressing parameters can 

result in an improved vertical density profile and consequently alter product performance. 

There are several ways to control and change the vertical 

density profile of panels to alter product performance. Closing rate, mat MC, and distribution, steam-injection pressing, 

pressing temperature, wood species, and species distribution 

have been effectively used by the board industry and by 

laboratory researchers to manipulate the vertical density 

profile. Because of the strong relationship between panel 

density, compaction characteristics, and subsequent panel 

properties such as bending strength, dimensional stability, 

surface quality, edge machining, and fastener performance, 

understanding the nature of the density gradient and 

through-the-thickness properties in wood composite panels 

is of critical importance to manufacturers and researchers. 

Basic Property Requirements 

Based on furnish geometry and preparation methods, woodbased particulate composites can be divided into three categories: fiber-based products including medium- and highdensity fiberboard; particle-based products including chipboard, particleboard, and flakeboard; and strand-based products including OSB, rim board, and laminated strand lumber. 

The clear line of demarcation among the three categories has 

been blurred in the past decade. To compete with thin fiberboard, thin particleboard is made of very fine particles that 

are similar to wood fibers. 

Medium- and high-density fiberboard thickness ranges 

between 1.5 and 80 mm. Table 1 shows the individual qualities for fiberboards available in the market. Medium- and 

high-density fiberboard are widely used in the manufacture 

of furniture, kitchen cabinets, door parts, moldings, millwork, and laminated flooring. Each type of panel is 

manufactured in a variety of sizes, average densities, and 

density profile to provide the end product with the desired 

properties. 

Through-the-Thickness Properties 

Vertical Density Profile 

Previous researchers have examined various aspects of 

through-the-thickness properties of wood composite panels. 

Perhaps the most commonly obtained through-the-thickness 

panel property is the density, or the vertical density profile 

(VDP) (Strickler 1959) The VDP is typically reflected by the 

presence of high-density face layers and low-density core 

layers within the panel. 
66 

When the density distribution through the panel thickness is 

plotted on an xñy axis, the resulting plot is frequently referred to as the ìshapeî of the density profile. Shape is a 

qualitative descriptive term used to refer to the relationship 

between face layer density, core layer density, and panel 

thickness. The density profile is usually symmetrical in 

shape when viewed about a midpoint that is the centerline 

representing total panel thickness. Historically, the density 

profile has been measured using a gravimetric approach, but 

in the last decade, nondestructive nuclear and x-ray instruments have become the standard means of analysis (Wang 

1986; Haag 1992; Quintek Measurement Systems, Inc. 

1999). A drill resistance technique and an air-coupled acoustic emission sensor technique were also used to measure 

density profiles in wood-based products (Winistorfer and 

others 1995; Lemaster and Green 1992). Europe has developed an on-line system for full-scale production monitoring 

of the density profile immediately after pressing (Dueholm 

1996). 

The shape of a density profile is commonly described as the 

letter M or U (Fig. 1). Comparing and modeling the VDP is 

a necessary step to investigate the relationship between the 

VDP and other panel properties. There are two basic methods to analyze the VDP quantitatively. The first method is 

using some characteristics to describe the shape of VDP. 

Wang (1987) calculated the following characteristic data of 

oriented strand composite plywood: maximum density in the 

surface layers and its relative positions, minimum density, 

and the difference between maximum and minimum densities. Schulte and Fr ̧hwald (1996a,b) calculated the following characteristic data of medium-density fiberboard (MDF) 

and particleboard: mean density, minimum density, maximum density in the surface layers, their relative positions, 

the average density in a window of ±1 mm around the minimum density, the slope from the maxima to the inner part, 

and the angle between the outer maxima and the first inner 

maxima of the profile. Winistorfer and others (1996) calculated mean density, the average density in a window of ±1 

mm around the minimum density, and the average density in 

a window of ±1 mm around the maximum density to describe the VDP of commercial MDF and particleboard. 

Another method is to describe the VDP quantitatively by 

fitting a mathematical expression to the relationship of density and panel thickness (Winistorfer and others 1996). Xu 

Table 1óProperties and market for fiberboarda

Board type Properties Field of application 

Board thickness 

2-4 mm 

• Smooth and paintable on one side 

• Surface layer density more than 

1,000 kg/m3

 on the painted side 

Drawer bottoms, rear panels of furniture, door surfaces formed or smooth, rear panels of posters 

 

• Smooth and paintable on both side 

• Thickness swelling bellow 20% 

• Good internal bond 

• Thickness tolerance + 0.1 mm 

Building cars and RV, interior trim of railway 

compartments, picture frames, packing materials, 

printed circuit board, surface layers for OSB, 

underlay for, e.g., drills, formwork 

Board thickness 

4-8 mm • Light MDF with densities below 650 g/m3 Wall facing, furniture 

 • MDF with densities from 650 to 850 g/m3 Partition walls, furniture, conversion of attics 

 • HDF with densities from 850 to 1,050 kg/m3

• Quality depending on 24-h thickness swelling Flooring board with lamination 

Board thickness 

8-80 mm 

• high surface and edge-machining 

• Uniform density throughout the panel 

Kitchen cabinets, molding, furniture, picture framing 

and photo mounting, window components 

 • Good surface properties 

• high bending properties Furniture, kitchen cabinets, door core, counter tops 

a

Part of information from the Dieffenbacherís brochure; MDF, medium-density fiberboard; HDF, high-density fiberboard; 

 OSB, oriented strandboard. 
67

and Winistorfer (1996a) fitted an equation to VDP data 

using Fourier analysis. This was achieved first by performing the Fourier transform of the density profile data and then 

by synthesizing the mathematical equation in the form of 

cosines based on the transform output. Two sets of density 

profile data were used in this analysis, and a good fit of the 

VDP was obtained for each case. This technique and the 

mathematical expression of VDP are recommended for 

studies investigating the VDPñproperty relationship. 

The actual density profiles are not smooth lines, especially in 

the case of OSB. Recent studies on the commercial OSB 

(Gu and others 2003; Wang and others 2003) showed that 

VDP varied from one OSB manufacture to another, especially for the thicker aspen made panels (Fig. 2). 

Other Physical Properties 

Xu and Winistorfer (1995a,b) developed two techniques to 

determine thickness swell (TS) characteristics of individual 

layers within a wood composite panel. They developed an 

intact specimen algorithm to determine TS distribution 

across the board thickness based on vertical density distribution changes measured before and after the water exposure 

treatment. The procedure uses a linear relationship between 

adjacent density data points in the VDP and assumes a constant weight of an individual horizontal layer after swell. An 

algorithm was developed to make the estimation. A thin, 

horizontal layer within a sample has a known volume and 

mass from radiation measurement. After water exposure, the 

same known volume expands some unknown amount. As the 

thin layer expands due to swelling stresses, the mass of the 

original volume decreases as material moves out of the 

original volume (expands) due to swelling pressures. For TS, 

the algorithm uses the original mass of the known layer 

thickness and solves for the new unknown volume of the 

expanded layer. For water absorption, the algorithm uses 

radiation absorption principles involving two elements to 

separate the wood mass from the water mass. The second 

technique is a layer slicing procedure in which thin horizontal layers are sectioned from composite samples and subsequently tested for TS after a 24-h water soak. Both techniques show that discrete layer TS is positively correlated to 

the layer density and suggest that efforts to improve TS 

should focus on treatments or processes that impact the more 

dense surface layers of composite panels. 

Both techniques have some limitations that may restrict their 

widespread use as a standard protocol. The intact algorithm 

method requires the nondestructive measurement of layer 

density through the sample thickness. The commercial densitometers for the panelboard industry utilize the standard 

50- by 50-mm internal bond (IB) sample for density profile 

measurement. The same limitation applies to the layer slicing 

technique in that only 50- by 50-mm samples can be prepared with the technique, and the standard TS sample is 150 

by 150 mm. Other limitations to the layer slicing technique 

include the removal of saw kerf material as an additional 

source of measurement error, and that individual layers once 

removed from the intact specimen more easily absorb water 

and swell more or less than the intact, whole sample without 

the influence of internal stress. 

Wang and Winistorfer (2002a, 2003a) developed a nondestructive optical technique to determine TS of discrete layers 

within intact samples of wood composites. They considered 

the nondestructive optical technique an improvement over 

the first two techniques. Based on this technique, strong 

correlations between the actual TS and layer density were 

built for lab-made MDF (Wang and others 2001b), lab-made 

OSB (Wang and Winistorfer 2000a), and commercial OSB 

(Gu and others 2004; Wang and others 2003). 

Wang and Winistorfer (2001) conducted research to evaluate 

compression and swelling characteristics of individual furnish elements sampled through the thickness of lab panels 

pressed without resin. The results of this work showed that 

flakes from surface layers exhibited compression of 25% to 

37%, about double that of flakes in core layers. As expected, 

flakes from surface layers showed much greater TS than 

core flakes and the response was accentuated with higher 

equilibrium moisture content (EMC) conditions. The correlation between the flake TS and compression ratio was at the 

1% significance level, except for the correlation between the 

flake TS and compression ratio for relative humidity (RH) 

35% to 65%, which was at the 10% significance level. The 

individual layer TS of the pine panel through the panel 

thickness direction is very similar to flake TS. Face layers 

swelled much more than core layers due to high face flake 

compression. The layer sawing technique was also employed 

to obtain specimens for water absorption (WA) tests (Xu and 

Winistorfer 1996b). These tests revealed that WA was positively correlated to layer density and layer TS. 

Mechanical Properties 

There has been a large number of research reports published 

that describe the correlation between VDP and panel mechanical properties. Some of the mechanical properties that 

are influenced by density include torsion shear strength of 

particleboard (Shen and Carroll 1970), tensile strength of 

particleboard (Plath and Schnitzler 1974), tensile and compression strength through OSB thickness (Steidl and others 

2003), modulus of elasticity (MOE) and modulus of rupture 

(MOR) (Rice and Carey 1978; Hse 1975; Wong and others 

1998), IB (Schulte and Fr ̧hwald 1996a,b), and shear 

modulus (Schulte and Fr ̧hwald 1996a). 

Xu and Winistorfer (1995a) sawed an OSB specimen into 

nine layers, and measured the IB of each layer. Although 

there was a positive correlation between IB and density, the 

degree of correlation was small (r

2

 between 0.20 and 0.25). 

The lowest IB did not always occur in the low-density core 
68 

layer, and the highest IB did not necessarily occur in the 

high-density face layers. 

To determine the variation in strength and stiffness through 

the thickness of the panel, Steidl and others (2003) sawed a 

commercial OSB into 15 layers to obtain thin layer specimens for tension and compression testing. Specimens were 

obtained both parallel and perpendicular to the length of the 

panel. The specimens were tested in tension using straightsided specimens and unbonded tabs. For specimens parallel 

to the length of the panel, the face layers had a tensile 

strength approximately an order of magnitude greater than 

the core. Greater face tensile strength was due to a combination of strand orientation and density. An apparatus was 

designed to test the thin specimens in compression. The 

average compression strength was significantly higher than 

average tension strength. However, the average compression 

MOE was significantly lower than average tension MOE. 

These layer tension and compression properties were related 

to the VDP with high r

2

 values (>0.75), thus indicating a 

strong linear relationship exists. The layer properties were 

used to predict the panel properties. 

A study completed by Andrews (1998) examined the relationship of the formation of the VDP in OSB as influenced 

by furnish moisture content (MC) and press closure rate. 

Results showed that there was a negative correlation 

(r = ñ0.65) between the location of maximum density in the 

tension face layer and the bending MOE of the panel As the 

maximum density location moved closer to the panel surface, the stiffness of the panel increased. The location of the 

maximum density influenced the MOE more than the density 

value itself. The same relationship was true for MOR, but 

the correlation was lower (r = ñ0.33). 

Geimer and others (1975) examined the effects of layer 

characteristics on three-layer particleboards. In one series of 

tests, face and core layers were separated and tested for 

stiffness in tension parallel to the board surface. They suggested that there was a nonlinear relationship between the 

MOE of the face layer and the density of the face layer, 

while there was a linear relationship between MOE of the 

core and density of the core layer. Laminate theory was used 

to predict board properties from layer properties. The predicted stiffness averaged 78% of the measured stiffness in 

two-point loading tests and 87% of the measured stiffness in 

single-point loading tests. Using the density gradient in 

successive 0.8-mm (1/32-in.) increments along with the 

developed nonlinear modulusñdensity relationships resulted 

in improved predictions, 92% for the two-point loading and 

100% for the single-point loading test. 

Geimer (1979) measured tension, compression, and bending 

MOE and failure stress of full thickness flakeboards made 

with uniform densities throughout their thickness and different degrees of flake alignment. Logarithmic relationships 

between stiffness (or strength) and specific gravity and wave 

speed were developed. Several important behavioral aspects 

were determined from this work. The failure stress (or 

MOR) was highly correlated with the stiffness. The stiffness 

of boards with a density gradient could be predicted to 

within ±20% using the stiffnessñdensity relationship from 

uniform density boards. The bending MOR was almost 

double that of the tension failure stress for the same level of 

stiffness. 

Carll and Link (1988) studied the layer behavior of 12.7-

mm- (0.5-in.-) thick aspen, and Douglas-fir flakeboard panels 3.2-mm- (1/8-in.-) thick face layers and 6.35-mm- (1/4-

in.-) thick core layers were tested in tension and compression. A logarithmic relationship was developed between the 

tensile or compressive MOE and the specific gravity and 

wave speed. These relationships were used with the density 

measured in six layers through the thickness (10%, 15%, 

25%, 25%, 15%, and 10% of the thickness) to predict the 

bending MOE. The predicted MOE was consistently 10% to 

15% higher than the measured MOE. 

Xu (1999) used laminate theory and simulated linear layer 

MOE-layer density relationships to examine the effect of 

different VDPs on the panel MOE. The analysis showed that 

the MOE benefits from the high-density surface layer and 

increases linearly with an increase in peak density, but the 

maximum MOE does not occur when the peak density is 

located at the extreme board surface. 

After measuring shear modulus, IB, and density profile for 

five different types of commercial MDF, a correlation between the failure position and parameters derived from the 

VDP could not be established (Schulte and Fr ̧hwald 

1996a). Sixty-nine percent of the failures were found in the 

outer layers of the specimens (15%ñ35% and 56%ñ85% of 

thickness, respectively), even if the lowest density was found 

in the core layer. Only if the minimum density is more than 

30% lower than the mean density is the failure position near 

the point of minimum density. There is a high correlation 

between minimum density and IB. There are high correlations between interlaminar shear modulus, shear strength, 

and IB as well as between mean density and IB, shear 

modulus, and shear strength. 

Schulte and Fr ̧hwald (1996b) also determined the relation 

between IB, parameters derived from the density profile, and 

the respective failure position for five different thicknesses 

of furniture grade particleboard. Independent of panel thickness, the failure position was found to be in a range of the 

25% to 75% of panel thickness. A correlation between the 

failure position and parameters derived from the density 

profile could not be found. There appears to be three predetermined failure lines (35%, 50%, and 65% of panel thickness). The correlation between mean density and IB is high 

for the total sample (r = 0.81). The correlation between 

minimum density and IB is lower, especially with regard to 

results relating to a single panel thickness. Humphrey (1991) 
69

predicted the variation of inter-particle bond strength across 

the thickness of a single-layer spruce particleboard. Based 

on his simulation model, he indicated that bond strengths 

seem to be at their lowest at some distance away from the 

core layer of the panel. This may help to explain the tendency of boards to delaminate at this location upon premature press opening. 

Wang and Winistorfer (2000b) report on the nature of the 

unsteady contacting phase that exists during hot-pressing 

and explains some phenomena relating to manufacturing 

processes of wood composites and end-product panel properties. The process of bonding wood is a complex interaction 

of physical and chemical variables. The bonding of wood 

elements in a hot-pressed composite panel is a recognized 

complex phenomenon that includes polymerization of the 

resin, chemical reaction, and mechanical interlocking of the 

resin with the wood substrate. The bonding phenomena in 

hot-pressed composites occur under varying environmental 

conditions that include dynamic temperature, RH, MC, 

pressing pressure, and steam pressure. During pressing, the 

wood furnish elements are not in a steady state of contact, 

due to significant consolidation changes that occur throughout the mat during the entire press cycle, even after the press 

reaches final position. Another study (Wang and Winistorfer 

2000d) showed that the VDP of wood composites is formed 

from a combination of actions that occurs both during consolidation and also after the press has reached final position. 

Recognition of the consolidation changes during pressing 

implies that there is not a steady state for resin bonding 

during pressing. The more severe the unsteady phase during 

hot-pressing, the poorer the quality of the bond formation. 

Any method that could reduce the unsteady contacting phase 

during pressing should result in better bond formation and 

consequently better end-product panel properties. Knowledge of the unsteady contacting phase during pressing enables an adequate explanation of the phenomena why most 

of the failures were found in the outer layers of the specimens, even if the lowest density was found in the core layer. 

Horizontal Density Distribution 

Density variations in the plane of the panel are referred to as 

the horizontal density distribution (HDD) while density 

variation through the thickness of a panel is described by the 

VDP. The HDD is mainly dependent on furnish geometry 

and forming. As strands are formed into a mat, some areas in 

the panel will have more strands overlapping than other 

areas. As the mat is pressed to a constant thickness, these 

areas are densified to a greater degree than the areas with 

fewer overlapping strands. On the other hand, void (Sugimori and Lam 1999) or low-density areas may also exist in 

the panel. Suchsland (1962, 1973) described the nonuniform 

HDD as undesirable because differential thickness swelling 

between areas of varying density could cause damaging 

stresses in a panel. Suchsland and Xu (1989, 1991) 

simulated the HDD in flakeboard by crossing narrow strips 

of veneer in perpendicular layers to form a mat. Variations in 

the HDD could be controlled by the number of strips in each 

layer. This density variation has a direct effect on both the 

IB and the TS of the model and an indirect effect as damaging swelling stresses develop during water exposure. 

Xu (1993) measured the HDD by removing finite specimens 

and showed that apparent HDD in wood composites is dependent on the size of the specimens used to measure the 

density variations. Smaller specimen sizes will have more 

variability in density than larger specimen sizes. The results 

showed that MOR, MOE, IB, and TS of particleboard were 

shown to be greatly controlled by nonuniformity of board 

structure. All these properties were improved as structure 

uniformity improved. 

Lu and Lam (1999) determined the overlaps and HDD in a 

robot-formed wood flakeboard mat using x-ray scanning. 

The HDD for a full 1.2- by 2.4-m (4- by 8-ft) panel can be 

determined using x-ray scanning equipment developed by 

the Alberta Research Council (Linville and Wolcott 2001). 

The equipment has a resolution of 4 pixels per inch and can 

calculate discrete horizontal density variations based on a 

grid size as small as 12.7 mm (0.5 in.). Linville (2000) developed a model to predict failure in an OSB panel due to 

the HDD. The model predicts that increases in average panel 

density or horizontal density variations will increase damage 

in the panel due to differential swelling. Dai and Steiner 

(1997) developed a mathematical model for characterizing 

horizontal density variation of a short-fiber wood composite 

panel by point density variance, point-to-point autocorrelation function, and variance of local average. Oudjehane and 

Lam (1998) showed that during the manufacture of wood 

composite panels, the random layout of flakes has a strong 

influence on the density variation within the panel. Van 

Houts and others (2003) analyzed mat structure made from 

industrially manufactured strands using simulation modeling. Information provided by the model includes the number 

and geometrical details of voids and strand overlap. 

Winistorfer and others (1998) procured commercial OSB 

and plywood products for comparison of whole panel (1.2 

by 2.4 m (4 by 8 ft)) water absorption, TS, and MC. The 

results showed that a significant nonuniform distribution of 

water absorption, TS, and MC existed across the panel. The 

nonuniform water absorption has also been found in small 

samples (van Houts and others 2003, unpublished data). 

Figure 3 is a nuclear magnetic resonance (NMR) image after 

12 h of water soak in the XY plane through the center of a 

sample with industrial sealant applied on the upper and right 

edge (van Houts and others, unpublished data). The effectiveness of the industrial sealant in preventing water from 

entering the edges is quite apparent. However, in one location, a significant amount of water has entered the right edge 

even though there is sealant present. This location corresponds to a low-density zone with a large number of voids. 
70 

This example shows how a low-density zone, even when the 

edge is sealed with industrial sealant, is susceptible to rapid 

water penetration. This would also affect the TS of the 

sample. 

Macro-voids exist in strand-based wood composites (Sugimori and Lam 1999; van Houts and others 2003). The voids 

contribute to the rate and directional characteristics of moisture movement through a panel. Macro-voids, low-density 

spots, and high-density spots should be considered as defects 

in strand-based wood composites. These defects will cause 

potentially damaging internal stresses to develop in the 

wood panel. 

In a study on the density and property relationship of wood 

strand composites, Dai and others (2002) analyzed the strand 

overlaps and the influence of horizontal density variation. 

Increasing density can increase strand to strand contact 

which can lead to better bonding. However, internal stresses 

increase with densification and even wood cell wall damage 

may occur at excessive densification. These contradicting 

effects suggest the existence of optimum densification for 

bonding. The optimum IB strength can be clearly detected in 

elements with no or little horizontal density variation. With 

horizontal density variation, the density and IB relationships 

of strand boards become monotonically linear. Density 

variation also causes a constraining effect on TS, which may 

lead to lower overall TS. While it does not appear to have an 

effect on MOE, horizontal density variation causes a negative effect on MOR. Further tests on real strand boards 

indicate that the density property relationship is also governed by pressing conditions, which may be linked to internal stress development and resin cure. The information 

generated from this study is useful for developing models for 

predicting board properties and low-density strand products. 

There is little literature regarding influence of hot-pressing 

on HDD formation. Significant in-plane mass flow is not 

expected to exist in a strand mat during hot-pressing because 

the size of the large strands does not allow the strand to 

move. However, in-plane mass flow could exist to some 

extent in a fiber mat during pressing. Wang and Winistorfer 

(2000c) used a high-resolution x-ray computed tomography 

(CT) system to generate spatial density variation of MDF 

panels. The CT scanner was set at 420 kV, 3 mA, 0.5-mm 

slice thickness, and 0.166-mm pixel size. The images included the in-plane images taken at the midplane of each 

sample, and the transverse images taken at three heights. The 

CT in-plane images (Fig. 4) showed the horizontal density 

variation in MDF panels. The enlargement of a small region 

of the slice of MDF panel showed there is a ±45 degree 

weave of fibers as well. 

Control Strategies 

for Hot-Pressing 

In pressing, a number of factors are involved, including 

the resin type and level, wax type and level, pressing 

temperature, wood species and furnish geometry, mat density and distribution, mat moisture level and distribution, 

press closure rate, and pressure. As is usual in the board 

manufacturing process, all of these factors interact. 

In the previous section, we have discussed that there are 

different panel property requirements for different applications, and panel properties are strongly influenced by density 

distribution. In this section, control strategies for hotpressing are discussed. 

Key Criteria and Methods 

of Press Control 

Hot-pressing is a process of pressing a mat between hot 

platens or hot rollers of a press to compact and set the mat 

structure by simultaneous application of heat and pressure. 

The major goal of hot-pressing is to achieve a designed 

panel physical and mechanical properties within a limited 

pressing cycle. There are two basic requirements that must 

be met and considered when making wood-based, particulate 

composites: (1) the properties must be optimized to impart 

the designed properties to the panel mainly through sufficient resin cure and correct vertical density distribution, and 

(2) the process must be optimized to achieve minimum 

pressing time without blows and blisters. 

Degree of Resin Cure 

Most wood-based composites processing done today is 

controlled by a fixed ìtime and temperature scheduleî based 

on empirical observation of the process variables and resin 

chemistry. The thermal-setting adhesives in commercial uses 

require temperatures in excess of 100°C to accelerate their 

polymerization. Measurements of temperature inside woodbased panels during hot-pressing are commonly done with 

thermocouples and used to estimate pressing cycle in all 

kinds of hot-presses. The minimum pressing cycle is commonly estimated based on the time for core temperature to 

reach 100°C and the time for resin to cure above 100°C. The 

internal temperature hardly exceeds 110°C at the end of the 

pressing process for a thick panel. However, internal temperatures easily exceed 130°C to 140°C for a thin panel. 

The processing of wood-based composites is further complicated by lot-to-lot variations associated with adhesive systems, variations in wood species, moisture content, pH, and 

seasonal plant environmental conditions. In the panel industry, trial-and-error methods are now used to determine the 

appropriate press cycle for a particular condition. Because 

there is no real-time feedback of panel property information 

for the press operator, pressing control is generally not 
71

optimized (Wang and Winistorfer 2002b). There is a need 

for an on-line sensor system that can determine the state of 

adhesive cure within the composite panel during pressing, 

without compromising the integrity of the finished products 

due to invasive methods. To provide real-time monitoring 

and intelligent control for the curing of wood composites 

during hot-pressing, several techniques are currently being 

developed. Wang and Winistorfer (2002b) recommended the 

in-process measurement and control of hot pressing through 

monitoring the internal density change during press opening. 

Chen and Beall (2000) investigated bond strength development in particleboard during pressing using acoustoultrasonics. 

For control purposes, dielectric analysis (DEA) appears to be 

the most promising and popular method for monitoring the 

resin state in composites (Kim and Lee 1993; Maffezzoli and 

others 1994; Magill and van Doren 2000; Rubitshun 1981; 

Wang and Winistorfer 2000b, 2003b; Wolcott and Rials 

1995). DEA has been used to provide large amounts of data 

about basic phenomena in resin-based systems. 

Vertical Density Profile 

To impart the designed properties to the panel, the targeted 

vertical density distribution should be achieved at the same 

time sufficient resin cure is being achieved. A density gradient through the panel thickness is typically reflected by the 

presence of high-density face layers and low-density core 

layers within the panel but may take on many forms depending on manufacturing conditions and desired end-product 

attributes The density gradient has been referred to by many 

names including vertical density gradient, vertical density 

profile, density profile, and vertical density distribution. The 

density profile will commonly be nearly symmetrical in 

shape when viewed about a midpoint that is the centerline 

representing total panel thickness (Fig. 1). 

A superior MDF panel for laminating, gluing, and finishing 

should have a deep density profile, in which the face density 

is considerably higher than the core density (Fig. 1). Painting, grain printing, and overlaying the new generation of 

lightweight papers is also enhanced by a high-density panel 

surface. The homogeneous core of MDF makes it especially 

suitable for embossing, molding, and general machining. 

Most MDF panels are usually sanded to remove precured 

surfaces. 

Recent research on commercial OSB (Gu and others 2004; 

Wang and others 2003) showed that VDP varied from one 

OSB manufacturer to another, especially for the thicker 

aspen panel. The 11-mm (7/16-in.) pine OSB panels had 

smooth profiles with a wide face density, which is located 

far from the panel surface. The highest peak density is located on the top surface for most of the panels (Wang and 

others 2003). Density profiles of three aspen panels (Fig. 2) 

demonstrated three significantly different processing 

techniques when making the panels (Gu and others 2004). 

Panels 19 and 21 had steep density profiles and wide lowdensity core regions. The difference between the two profiles was that panel 21 had two relative flat shoulders at the 

surface areas instead of two narrow peak densities as panel 

19. Panel 20 had a less steep density profile with high core 

density. Two peak densities near the surfaces were pushed 

inside. The mechanical and TS performance of the three 

aspen OSB panels were expected to be affected by their 

different density profiles. It should be noticed that 11-mm- 

(7/16-in.-) thick sheathing OSB are usually not sanded and 

18.3-mm- (23/32-in.-) thick flooring products are usually 

sanded before entering the market. Panel 21 was sanded. 

Medium-density fiberboard is one of the most rapidly growing composite products available in the marketplace. MDF is 

seeing increased application in many product areas such as 

furniture, kitchen cabinets, and ready-to-assemble furniture. 

Thin MDF is mainly used for laminating flooring substrate, 

door skins, and furniture parts. Traditional press schedules 

for MDF manufacture include a creep closing until the press 

reaches final board thickness or position. It is easy for heat 

to transfer from the face to the core while thin MDF is 

pressed. Consequently, there is a smaller temperature gradient in the thin panel than in thick MDF. As a result, there is a 

more uniform VDP in thin panels than in thick panels. A 

uniform VDP will result in better IB but lower bending 

strength and bending stiffness. 

To improve surface quality of thin MDF, Dieffenbacher has 

developed a few techniques used in their thin MDF lines, 

including decompression joint, surface preheating, and water 

spraying. Decompression joint in the continuous press (CP) 

helps quick reduction of the specific pressure which permits 

production of MDF boards with a big density difference 

between the surface layer and the core layer on very short 

presses. The spraying of the formed mat with a waterñ

chemical mix enhances heat transfer through the mat during 

pressing and results in surface hardening by increased density. This provides optimal precondition for subsequent 

lacquering of the board which minimizes paint consumption 

(Jager 2002). An over-pressing technique was developed to 

modify the vertical density profile by Wang and others. 

(2001a). With the over-pressing schedule, the mat is first 

compressed thinner than the final panel thickness and then 

press position is relaxed to the final position. The overpressing schedules significantly changed the traditional 

shape of the VDP of the laboratory-made thin MDF panels. 

The over-pressing schedules resulted in a deep VDPs with a 

low core density. 

Blow and Blister Control 

From the process control point of view, the pressing process 

should be accomplished without blows or blisters using a 

minimum press time. Blows are caused by excessive internal 

gas pressure, which is stronger than the IB of the panel at the 
72 

moment the press opens. Excessive MC, high press temperature, high density, high face density, or incorrect press decompression are common reasons to lead to high internal gas 

or steam pressures in the mat and consequently to cause 

blows. If the internal vapor pressure in the OSB mat is 

higher than 20.7 to 34.5 kPa (3 to 5 lb/in2

) at the time the 

press opens when there is no mat contact, the panel will 

normally blow if the IB is approximately 275.8 kPa 

(40 lb/in2

) or less (Alexopoulos 1999a,b). 

When blows resulted from process variation, plant personnel 

initiated a number of changes to eliminate the blows. These 

include changing the closing strategy, increasing the decompression time, and possibly press temperature reduction. 

Gunnells (1999) studied the effect of closing strategy 

changes and press temperatures on the gas pressure. The 

closing strategy changes, which were the removal of one or 

more C-pumps from a particular close sequence, had very 

little effect on the gas pressure. Changes in press temperature were more effective at reducing the gas pressure. 

Delaminations or blisters are a result of high MC furnish, 

which sometimes can lead to under cured resin. Pressing 

time, mat MC, press temperature, or any other factors that 

influence resin cure, are potential causes of delaminations. 

Press mechanical problems, for example, bent platens, 

simultaneous closing alignment, across the width alignment, 

and steam flow problems can cause blow or delamination 

as well. 

Very recently, presses have been built with a special cooling 

zone in approximately the last third of the length of the 

press. Experience with the Kusters press at the MDF Hallein 

factory shows that with press cooling, the gas pressure in the 

board at the press outlet is reduced as the cooling temperature decreases (Barbu and others 2002). Without increased 

production capacity, the IB of the boards will be 5% to 10% 

higher and production will become more reliable concerning 

variations of production parameters (lower risk of blisters 

and delamination and wider operational window). The initial 

MC of the fiber mat can be increased; this accelerates the 

heat transfer into the fiber mat during hot-pressing and 

increases the MC of the finished boards. In combination with 

the higher initial mat MC, the production capacity of the 

press can be increased by 10% to 20%. 

Typical Pressing Procedures 

Oriented Strandboard 

Different pressing strategies are being used to manipulate the 

VDP of wood composites and consequently improve product 

performance. Figure 5 shows schematic diagrams of platen 

position during hot-pressing. Schedule B in the Figure 5 

represents a typical pressing schedule used by many researchers. Schedule B is an ideal hot-pressing schedule, 

which is typically reflected by a continual closing period 

under a constant closing speed followed by a period of the 

final press position. However, industrial pressing schedules 

are more complicated than schedule B. The position of the 

moving platen is generally determined by the computerbased position-control system or pressure-control system. 

The pressure-control system is commonly used in plywood 

production. Schedule C in Figure 5 is a typical pressing 

schedule used in OSB production. Industrial pressing using 

the position-control system does not always mimic laboratory press closing due to hydraulic system limitation. The 

press quickly closes to a position near final panel thickness 

and then closing speed slows as the press reaches maximum 

pressure. While the press maintains maximum pressure, 

press platen movement is dependent on wood plasticization 

and further densification. To manipulate the end-product 

density profile attributes, other pressing schedules have been 

or will be used in the panel industry. For instance, schedule 

A includes a creep closing for MDF pressing (Park and 

others 1999), and schedule D is a step-closing schedule for 

particleboard, OSB, and MDF pressing (Wang and others 

2000, 2001b,c). 

Research results showed that the step closure schedules 

significantly changed the traditional shape of the VDP of the 

laboratory-made OSB panels (Wang and others 2000). The 

step schedules resulted in density profiles with multiple 

surface densification peaks, flat core density, and main 

density peaks with good symmetry about the panel central 

axis (Fig. 6). Step pressing schedules are one processing 

method that can be used to alter the densification process 

in the panel and subsequently influence panel physical properties. 

Medium-Density Fiberboard 

Medium-density fiberboard is one of the most rapidly growing composite board products available in the marketplace. 

MDF is experiencing increased application in many product 

areas such as furniture, kitchen cabinets, and ready-toassemble furniture. A key product attribute of MDF is the 

density profile through the panel thickness. A superior MDF 

panel for laminating, gluing, and finishing should have a 

deep density profile, in which the face density is considerably higher than the core density. Painting, grain printing, and 

overlaying the new generation of lightweight papers is also 

enhanced by a high-density panel surface. The homogeneous 

core of MDF makes it especially suitable for embossing, 

molding, and general machining. A uniform density through 

the panel thickness results in better fastening properties. 

Fiber mats are easy to compress, consequently resulting in 

steep M shape VDP if a similar pressing strategy as OSB 

pressing is used. Wang and others (2001b) compared differences in the VDP of OSB and MDF pressed under the same 

step-closure schedules. Figure 7 shows the VDP of MDF 

panels pressed under similar OSB step-closure schedules. A 

20-s closing rate in schedule A produced the control panel 

with a steep density profile consisting of a high face density 

and low core density. The 80-s closing rate in schedule B 
73

resulted in a higher core density and lower face density than 

the control panel. There were two shoulders in the density 

profile of panel B, which obviously resulted from the second 

closing step. The highest core density was in panel F, 

which was pressed under a total 135-s closing rate. The 

overall shape of the VDPs of the above three panels were 

very similar. 

Figure 8 shows the VDPs of OSB panels pressed under the 

same MDF step-closure schedules. Step-closure schedules C, 

I, and Control used in OSB (Wang and others 2000) were 

similar to MDF schedules B, F, and A, respectively. The 80-

s two-step closure schedule for panel C resulted in two 

internal minor peaks. The 140-s closing rate (three-step) 

produced panel I with a flat core density. 

There were larger differences between the face and core 

density in the MDF samples than in the OSB samples. For 

instance, a 20-s closing rate produced MDF with 1.13 g/cm3

maximum face density and 0.633 g/cm3

 minimum core 

density. Surface density was 78.6% higher than core density. 

The maximum and minimum densities of OSB were 0.885 

and 0.662 g/cm3

, respectively. The surface density was 

33.6% higher than the core density. 

For the MDF and OSB panels described, it should be mentioned that there were some differences in the pressing parameters. The pressing temperature of the OSB mats was 

200°C (Wang and others 2000), which was much higher 

than the 160°C MDF pressing temperature. Target thickness 

of the MDF was 15.9 mm, which was slightly thicker than 

the 12.7-mm OSB. Target average density of the MDF was 

0.760 g/cm3

, which also was higher than the 0.672 g/cm3 for 

OSB. However, pressing pressure data recorded showed that 

compression resistance of the MDF mat was significantly 

less than pressure for the OSB mat. MDF mats composed of 

fibers were more homogeneous and compacted more easily 

than heterogeneous OSB mats composed of large size flakes. 

Maloney (1993) stated that the contribution of Owen 

Haylock at Canterbery Timber Products in Canterbury, New 

Zealand, should be noted in the development of the MDF 

industry. Haylock and his colleagues developed a pressing 

technique in their multiple-opening press resulting in MDF 

boards with relatively high surface densities and uniform 

core densities. The pressing strategy uses a step system, 

rapidly bringing the mat after insertion into the press to a 

thickness about 30% greater than the final targeted panel 

thickness. This produces a panel with a high-density surface. 

Subsequent steps are then taken to bring the panel to final 

caliper. This technique eliminates much of the soft-surface 

face found in the early production lines using platen heating 

only for pressing. Most of the earliest MDF plants used a 

combination of platen heating and high-frequency curing. 

This pressing strategy, often referred as creep closure technique, is widely used in MDF production now. For a typical 

15.9-mm- (5/8-in.-) thick MDF pressing schedule using 

creep closure, the total press closure time for the 15.9-mm- 

(5/8-in.-) thick MDF is about 120 s, which is much longer 

than OSB production. Cooking time is only 15 s. 

Wang and others (2001b) investigated step-closure schedules and their impact on the formation of the VDP of labmade MDF panels. Results showed that the effect of stepclosure pressing schedules on the density profile formation 

occurred as a function of the intermediate step position 

(percentage of final panel thickness), time at intermediate 

position, and rate of closing speed. An intermediate position 

of 120% or greater significantly changed the shape of laboratory MDF pressed with a conventional single closing step. 

Coreline temperature was also clearly affected by the intermediate position, subsequently influencing compaction 

pressure and resulting density profile shape. A longer intermediate position resulted in a minor density peak near the 

panel center. A fast closing speed in the first closing step 

influenced the location of the surface peak density. A fast 

closure speed during the second and third closing periods 

resulted in significant internal density peaks. A slow closure 

speed during the second and third closing period minimized 

or eliminated internal density peaks. The results of MDF 

research also showed that step-closure schedules resulted in 

improved IB strength of the tested specimens (Wang and 

others 2001c). 

Minimizing Pressing Time 

and Preheating Methods 

Shortening Closure Time and 

Short Initial Closing Time 

The research conducted by Heebink and Lehmann (1972) 

showed that increasing platen temperature and rate of press 

closure shortens the warm-up period by increasing the rate 

of heat transfer. Both of these variables, however, are limited 

by other desired board characteristics such as the extent of 

surface layer resin precure, wood damage, and the panel 

VDP. The platen temperature can be up to 238°C for OSB 

production. 

Press closing time is mainly adjusted based on the VDP 

requirement. Alexopoulos (1999a) investigated 11-mm- 

(7/16-in.-) thick strandboard panels produced in the lab with 

liquid, powder, and powderñliquid combination resin, using 

a gradual consistent closing and an exaggerated fast initial 

closing with creep closing to thickness. The fast closing 

reached 105% of the target thickness in 10 s from the start of 

mat pressure build-up, and the slower close took approximately 30 s to reach the same thickness. Total closing time 

to thickness was approximately 42 s for all conditions. The 

results showed that the faster closing resulted in faster heat 

transfer to the core and in higher temperature at thickness. 

The faster initial closing also resulted in IB values as much 

as 35% higher with the powderñliquid system It appears that 

the higher temperature at thickness during high compaction 

allows the powder to soften and flow for greater bonding. 
74 

Radio Frequency or Microwave 

Most of the earliest MDF plants used a combination of 

platen heating and high-frequency curing. Radio frequency 

(RF) or high-frequency preheating units are placed at the 

entry of continuous presses making thicker panels instead of 

directly heating mats inside a press. This reduces press times 

up to 40%. A uniform density through the panel thickness is 

another advantage of using preheating units. The highfrequency preheating units have been used in thin fiberboard 

line as well. 

Testa (2002) recently made a good overview on application 

of microwave and RF in the engineered wood industry. The 

first use of RF in the wood products industry was for plywood, immediately following World War II. Now, major 

applications of RF are LVL, glulam, and I-joist manufacture 

primarily for preheating. Microwave is much newer than RF. 

MacMillan Bloedel (Vancouver, BC, Canada) began experimenting with the technology in the 1970s and began 

using it commercially for its trademarked parallel strand 

lumber product and Parallam in the 1980s. Both RF and 

microwave ensure a direct and rapid curing of glue, despite 

their very different methods of application. Microwave 

preheating is also being used for LVL manufacture. They 

both have particular application to the thicker, longer products in today’s market, especially since most raw stock is too 

small to create these products and must be laminated or 

fingerjointed to do so. 

Steam Shock and Steam Injection 

Heat is transferred from the heated platens to the mat surfaces and toward the interior. Heat transfer mechanisms can 

be summarized as conduction, convection, and the diffusion 

of water vapor and air. Heat conduction is generally considered as a major heat transfer mode during hot-pressing. The 

warming up of the core layers is based on the evaporation of 

water in the face layer and the transportation of the heat via 

the steam shock; in the cold core layer of the mat, the vapor 

condenses and warms up the core layer. The higher the 

amount of vapor and the steeper the vapor pressure gradient, 

the quicker the warming up of the core layer, enabling 

shorter press times. Using high MC of face furnishes has 

became a standard process for particleboard and OSB productions that require separate drying and blending lines for 

the face and core furnish. The water spraying technique was 

mainly developed for MDF and high-density fiberboard 

having uniform mat structure. To eliminate mold, press 

fouling, and formation of deposits on the product, release 

agent needs to be added in the spray water (Godber and 

others 2002). Other spray additives can be used to improve 

panel surface properties. For example, reactive surface 

sealer is used to reduce paint consumption for doorskin 

manufacture. 

Alternatives to limited steam shock process include direct 

injection of steam and preheating. The steam-injection 

pressing is a relatively new technology, which utilizes perforated platens to inject steam directly into the board and 

permits the transfer of heat into the core of a board much 

faster than in conventional pressing. The reduction of press 

time using steam-injection pressing is the major goal. Steaminjection pressing reduced laminated veneer lumber and 7-

ply plywood pressing time by 30% (Troughton and Lum 

2000). In addition to reduced press time, an additional advantage of steam-injection pressing is adjustment of VDP. 

The density of the steam-injection pressing boards appeared 

to be uniform throughout the panel (Kwon and Geimer 

1998). 

Preheating 

A system was developed to preheat the strand mat before it 

enters the press to further reduce pressing times and condition the mat (Wöstheinrich and Meier 2001). A controlled 

mixture of steam and air is injected into the mat, thus the 

entire mat is heated quickly and homogeneously to the desired temperature. In the preheating system, steam–air flow 

is directed symmetrically from the top and the bottom face 

into the mat with the surplus escaping through the mat 

edges. Through condensation of the steam, high specific 

energy is released and transferred to the strand mat. As long 

as the temperature of the mat is below the dew point, the 

steam condenses. The dew point and final mat temperature 

depend on the steam volume contained in the air–steam mix. 

It is not possible to exceed the set mat temperature because 

steam does not continue to condense beyond its dew point. 

Through heating by steam condensation, the MC of the mat 

is raised by 0.7% per 10°C temperature rise. Preheating can 

offer the following advantages: (1) reduced pressing time by 

25% when preheating the mat to 85°C to 90°C, (2) reduced 

specific pressure required in the press with increasing moisture and temperature, (3) improved swelling properties. This 

particular preheating system was developed for use with 

continuous press process and is not suitable for multidaylight presses because of the different mat retention times 

in the loader between preheating and press closure. Additionally, the resin is likely to pre-cure differently before the 

mats enter the press. 

Metso Panelboard (Helsinki, Finland) introduced a preheating unit to MDF industry called Coreheater. As the fiber mat 

approaches the hot press on the conveyor belt, it is sawn in 

half laterally by an integrated beltsaw. The lower part of the 

mat remains on the conveyor belt, while the upper section 

flows over the wedge-shaped steam bar. Steam is injected 

into the fiber mat through nozzles arranged crosswise on the 

upper and lower sides of the steam bar, heating the core to 

approximately 40°C to 60°C. The company claims that 

production increases of up to 20% can be achieved for board 

thicknesses between 12 and 38 mm. 
75

Other Pressing 

Method Developments 

Investigations were conducted at a few laboratories to extend 

the technology of steam injection pressing to the injection of 

other gases. Geimer and others (1994) investigated the introduction of three different gases into wood composites. The 

gases were methyl borate (MeB) as a fire retardant, methyl 

formate (MeF) as a resin catalyst, and carbon dioxide (CO2) 

as an accelerator for cement-bonded wood composites. 

Results indicate that the introduction of MeB can improve 

the fire resistance of phenolic- and isocyanate-bonded panels 

but only at the expense of lowering strength properties. 

Experiments with MeF indicated that this gas has the potential to decrease press times for wood composites bonded 

with phenolic resin. Ho and others (1992) tested the efficiency of injecting ammonia into strandboards at the final 

stage of the steam pressing operation, with the intention of 

reducing formaldehyde emission from the boards. 

Chowdhury (1999) designed a cylindrical sealed pressing 

system in which preformed wood fiber networks were consolidated under a controlled thermodynamic and chemical 

environment. His work emphasized the selection of treatment chemicals. Kim (2002) consolidated beams in a specially developed rectangular gas injection pressing system 

using ammonia as a softening agent and an ester as a lowtemperature adhesive catalyst. Structural beams were made 

from oriented hemp and random wood fiber networks with 

phenol formaldehyde adhesive. It was found that density 

gradients could be created within beams by injecting ammonia from one platen. Methyl formate was found to hasten the 

rate of strength development and final network strength by 

51% and 22%, respectively. 

To increase OSB properties, OSB can be overlaid with other 

thin materials, such as wood veneer and perforated aluminum foil, during OSB pressing. These materials will affect 

heat and mass transfer and consequently make a vertical 

density formation contribution, especially with relatively 

thick wood veneers (Wang 1987). 

Summary 

The hot-press is a key piece of equipment in composite panel 

production. It determines the performance of products and 

the processing efficiency of the whole production line. 

Based on furnish geometry and preparation methods, woodbased particulate composites can be divided into three categories: fiber-based products including medium- and highdensity fiberboard; particle-based products including chipboard, particleboard, and flakeboard; and strand-based products including OSB, rim board, and laminated strand lumber. 

Each type of panel is manufactured in a variety of sizes, 

average densities, and density profiles to provide the end 

product with the specific properties needed. 

Through-the-thickness properties of wood composite include 

the VDP, layer TS, IB, tension, compression strength, and 

other mechanical properties There have been a large number 

of research reports published that describe the correlation 

between VDP and panel physical and mechanical properties. 

The major goal of hot-pressing is to achieve designed panel 

physical and mechanical properties within a limited pressing 

cycle. To impart the designed properties to the panel, correct 

vertical density distribution should be achieved at the same 

time sufficient resin cure is being achieved. Different pressing strategies are being used to manipulate the VDP of wood 

composites and consequently improve product performance. 

There have been significant technology improvements in 

minimizing pressing time and developing new pressing 

methods during the past decade. There is still a need for 

process control and new pressing technology developments. 

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79

Figure 1óTypical vertical density profiles of oriented 

strandboard (OSB) and medium-density fiberboard (MDF) 

(Wang and Winistorfer 2003a) (1 lb/ft3

 = 16 kg/m3). 

Figure 2óVertical density profiles for three aspen oriented strandboard 18.3-mm- (23/32-in.-) 

thick panels (Gu and others 2004) (1 lb/ft3

 = 16 kg/m3). 

10

15

20

25

30

35

40

45

50

55

0 5 10 15 20

Board thickness (mm)

Density (lb/ft3

)

No.19

No.20

No.21

No.19 average

No.20 average

No.21 average

From top to bottom face of panel

0 

10 

20 

30 

40 

50 

60 

70 

0 2 4 6 8 10 12 14 16 18

Panel thickness (mm)

Density (lb/ft3

)

OSB

MDF 
80 

Figure 3óNuclear magnetic resonance image of OSB taken through the XZ plane after 

a total of 12 h of watersoak of a specimen with industrially applied sealant on two 

edges. Also shown is a photograph of one of the sealed edges (YZ plane) with a 

low-density zone indicated (van Houts and others 2003b). 

Figure 4óX-ray computed tomography (CAT) of MDF, 228.6 by 228.6 mm (9 by 9 in.), 0.5-mm slice 

thickness, and 0.16-mm pixel size 
81

Figure 5óPlaten position during hot-pressing using 

different pressing schedules. Schedules shown are 

(A) creep closing schedule for medium-density fiberboard pressing, (B) theoretical one-step pressing 

schedule, (C) typical oriented strandboard industrial 

pressing schedule, and (D) three-step closing schedule. 

Figure 6óVertical density profiles of schedule C 

(two-step closing, 80-s closure time), H (three-step 

closing, 140-s closure time), and control (one-step 

closing, 20-s closure time) boards (Wang and 

others 2000) (1 lb/ft3

 = 16 kg/m3; 1 in. = 25.4 mm). 

Position (in.)

20

25

30

35

40

45

50

55

60

0 0.1 0.2 0.3 0.4 0.5

Density (lb/ft3

)

Schedule C

Schedule H

Control
82 

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 2 4 6 8 10 12 14 16

Position (mm)

Density (g/cm3

)

MDF F

MDF B

MDF A

Figure 7óVertical density profiles of medium-density 

fiberboard (MDF) (Wang and others 2001a). 

Figure 8óVertical density profiles of oriented 

strandboard (OSB) (Wang and others 2000). 

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 2 4 6 8 10 12 14

Position (mm)

Density (g/cm3

)

Schedule C

Schedule I

Control
83

DiscussionóPress Control for 

Optimized Wood Composite 

Processing and Properties 

Siqun Wang and Chunping Dai 

Winandy: Once again, I would like to apologize to both 

speakers. I wish I could have had an hour for each one of the 

talks. They were exactly what we asked for, but we just tried

to cram a whole bunch of critical information into too short a 

time period. Heiko Thoemen is going to be prepared to do 

his talk at 11:05 so that gives us about a 1 hr and 5 min to do 

1-1/2 h of discussion. Thus, we want to keep on pace with 

this cramming as much into as little bit of time as possible. 

We will have both of the speakers standing here, ready and 

able to answer your questions, I would again suggest that if 

you have a particular slide that you want to see or you want 

to bring up and re-discuss, lets try to do that in the first few 

minutes of the conversation and then we will get back and 

get the conversation flowing. Once again, I remind you that 

the conversation should be from table to table, rather than 

from table to the speakers. It should go back and forth; this 

is a roundtable, not a lecture at this point. So if the speakers 

are ready, I guess that I would like to start off with a 

question. 

We had a discussion over coffee during the break. We talked 

about vertical density profile and how controlling it could 

give you different kinds of properties and that optimizing 

one property affected another. I was wondering if you could 

just review what might be known in that area. For example, 

we talked about nail holding, is it better to have a big vertical density profile at the same average density or is it better 

to have a uniform density in the product for nail holding? 

Wang: The correlation between vertical density profile and 

board property in some part is quite clear. For example, to 

optimize for MOE we create a steep density profile to get 

better bending strength and stiffness. But at same time in 

doing this, we produce a steep density profile and may 

achieve lower internal bond. I showed a slide in my talk that 

in an internal bond test the board may not break in the lowest 

density area, but there still is a strong correlation between 

lower internal bond strength and lower density. For example,

the screw holding strength is based on how we test. If we 

know it is MDF and particleboard we test in two directions. 

One is perpendicular to the panel plane and the other is on 

edge. I cannot recall which kind of correlation when tested 

normal to panel plane because either you have a very steep 

density profile that means that you have high screw holding 

on the surface area, or you have less screw holding in the 

core. Either way I am not sure which way is better. I canít 

recall but I recall that somebody did that research. When we 

test screw holding on the edge, which means that the screw 

holding stress occurs in a lower density area, which means 

that when we have lower core density, we have lower screw 

holding properties. 

Dai: On the vertical density profile optimization, remember 

the industry really tries to minimize the average density. 

Letís say we try to optimize the bending strength by 

densifying the surface, but we have only so much to give. At 

a given average density, we might run into problems of 

meeting the IB standard and I guess IB for OSB is low compared to other products and I guess the other thing about IB 

is that it affects the degas and pressing time. If we donít 

have enough IB, we canít pull the panel out without blows 

and so I think it is pretty much a balance for OSB mills, 

when they try to reduce the overall density of the product. 

And as far as nail holding, I donít know. I agree with Siqun, 

you sort of increase the surface holding by densifying the 

surface, but then the core is suspect. ARC has done some 

work and I donít really know if they are looking at density 

profile as a parameter. 

Jong Lee: Well I guess about nail withholding strengths you 

have to have overall high density board rather than vertical 

density profile or the uniform density because density is the 

key factor controlling the withholding strengths of the nail. 

So if you have aspen OSB or the southern pine OSB, I guess 

southern pine OSB performs better with withholding 

strengths. But if you deal with the screw holding strengths it 

is a little bit different, if you deal with nail withholding 

strength you have got to have a high surface area and a high 

density surface area to provide surface hardness. We are 

getting more low density wood from the low density hardwood species so if you think about the nail withholding 

strengths and some other structure performance rather than 

just a simple bending or a shear, you got to have some other 

strategies to control the final properties. 

Jim Wescott: As far as the vertical density profiles goes and 

how that affects the thickness swell, I think that you showed 

an excellent slide on how obviously the higher density surfaces do swell substantially more than the lower density 

core. I guess I have a couple questions on that. Does the 

overall total thickness of the board increase as a result of an 

extreme vertical density profile and second of all, I guess it 

is kind of a similar question, has anyone looked at normalizing that data for like a swell per gram of dry wood and trying 

to compare different resins and perhaps even comparing a 

24 h swell to boil, just to try to get a better feel for how the 

resin is actually either contributing of impeding thickness 

swell? 

Wang: Yes, thickness swell is a very difficult topic because 

we know the thickness swell is affected by many things. 

Density profile is one of them. If we say we want to have 

lowest thickness swell and in my mind I am still not very 

clear which kind of density profile would be best because we 

still have a good question related to the bonding and the 

durability and especially when we are talking about water 
84 

soak and boiling at the test that that would be a different 

situation. I remember that last year I tested a very thin fiberboard. The fiberboard had a coating surface on top because 

we wanted to have a good surface exposed to the outside. 

Thus, we used a special chemical treatment on that top surface. When we tested layer thickness swell, under water 

soaking at room temperature, the top surface layer swelled 

more than the bottom surface layer. But when we boiled the 

specimen, the swelling was the opposite. So that means that 

when we boil a sample we cannot merely look at the thickness swell. It's also effects of the durability on the bonding. 

Dai: On that I would like to also add that when we talked 

about the thickness swell, it really depends on what sort of 

test we use. If we are looking at the maximum potential, the 

thickness swell should be tested in boiled water. But for 

most OSB the normal exposure is more like a temporary test. 

The Japanese standard calls for a 72-h water shower test, but 

for most of the tests it is really a 24-h water soak under room 

temperature conditions. If we densify the product we usually 

achieve lower thickness swell. But then the opposite can 

happen if we do a boil test because there are two things 

happening there. If we densify the board the moisture 

doesnít get in. So if I am in a mill and I want to try something, I would try to densify the surface to suppress the 

moisture from getting in. But I am cheating a bit in that I am 

just delaying the swell. Somebody who is familiar with 

construction can tell us whether we need a boil test for OSB, 

but it is a different story as far as the density effect goes. 

Fred Kamke: I want to make a brief comment first about 

the nail holding resistance of OSB and refer you to Alex 

Salenikovich, now a professor at University of Laval. Alex, 

when he was at Virginia Tech as a graduate student, was 

working on this very topic with OSB and its nail holding 

capabilities in regards to the rigidity of wall structures particularly in earthquake situations and he has got quite a bit of 

data on that. Alex then went to Mississippi State where he 

continued some of that research. 

My other question had to do with I think it was Siqunís slide 

number 34. It was labeled ìTypical Pressing Procedures for 

OSB.î It had to do with the influence of the two step closing 

process on the development of the vertical density profile. It 

is easier to see the vertical density profiles on the printout 

than it is on your slide. However, you do have quite distinct

differences in the peak density and then you show a result of 

what I think was a 24 h thickness swell test. Visually they 

look dramatically different, the one on the right with conventional pressing looks much worse than the one on the left, 

why is that? 

Wang: Maybe we need to explain that two closing strategies. In our conventional pressing. I used just constant speed 

closing so that the press starts at the beginning position and 

it goes to the final position in 60 s. Because they were slow to 

close, there is more time for the surface to heat up and the 

resin on the surface to cure. So after a 24-h water soak, you 

can see that flakes peeled off from the surface. That performance issue is probably related to press procedures. After 

60 s the resin may not be fully cured and that results in some

kind of effect. Did that answer your question? 

Kamke: Maybe I misunderstood the press schedule here. So 

the conventional schedule was just a constant rate closure 

over a period of 60 s and the two step closing had a more 

rapid press closing rate initially and then it slowed down for 

a total closing time of 60 s. Is that correct? 

Wang: Yes, total closing time is the same; it is a 60-s close 

to final position. In the other pressing method, the step closing strategy, initially we very quickly closed to 110% of 

final thickness and held there for a while, then closed to 

target thickness over the final few seconds. The quick close 

results in a denser surface. By holding there, we let the core 

warm up and when we close from 110% of target thickness 

to the final thickness, we now densify the warmer core. 

Cloutier: I would just like to point out that when we are 

talking about optimizing the density profile we must not 

forget about dimensional stability of the panel in service. I 

have been working with Stefan Ganev from Forintek East 

over the last two three years on those problems of warping 

and it is clear that the linear extension is the driving force for 

warping of panel in service. We have also found working 

with CPA that a steeper density profile produced a panel that

is more prone to warping due to the earlier expansion 

in the plane, this parameter increases with wood density so I 

think that this is quite important and should be considered.

For those interested in that, there is a report available from

CPA (Ganev and Cloutier 2003) on those results that we 

obtained. But the density profile clearly has an impact on 

that. 

Wang: I think that is a good point. Another thing I want to 

explain is how that applies when running an OSB mill, where 

we may want good quality with a limited density range. 

So the challenge for the OSB mill usually is maximize bending properties, such as MOE, when using low density species like aspen. So usually what they do is try to first achieve 

adequate bending properties, then consider other issues, such 

as density, and then adjust accordingly. If after such adjustments, everything meets the base standard they stay there. If 

they are much higher, they just try to retain the benefit while 

they decrease overall the density. 

Ted Frick: One of the things that you will see in practice in 

a mill of course is fluctuating moisture content in your surface and core layer. Have you done any studies within the 

normal range where moisture content can fluctuate how that 

affects vertical density profile when you stick to one standard press schedule? 

Wang: No, I have not done anything specifically looking at 

the influence of different moisture distributions. 
85

Al Christiansen: Is it possible that as we are releasing 

pressure we are breaking bonds that have been formed already and is it a problem related to the differences between 

the faces and the cores? I mean if you get your resin cure too 

soon before it is relaxed you are going to be breaking a 

number of bonds. 

Wang: Yes, I think that is what happened. Otherwise we 

cannot explain why the high-density area on the outer layer 

might have lower shear stress or internal bond strengths 

compared to the lower-density core area. So that is the reason why the resin companies now supply different face 

resins and core resins. They want to slow down the face cure 

and speed up the core cure. This use of two different resins 

changes the cure history. That means that as we build up the 

surface we need to make sure resin is not completely cured. 

Otherwise, if we already have resin cure and we try to 

further build up the density profile, we break down the 

bonding. 

Dai: Like I was saying Al, in a mill where they have low IB, 

normally what they see is more blister or blows. So they use 

IB as a control on the blow too. And the other factor relating 

to what Siqun was saying about the weakest link not being at 

the very center has to do with the fact that sometimes in 

pressing the intermediate layer accumulates higher moisture 

than the core layer. Phil Humphrey has done some work 

showing that in some pressing strategies we can have a 

situation like that, especially when we create a high humidity 

in intermediate layers due to fast closing or a fast pressing 

schedule. The curing wonít be as good because there is so 

much moisture there in those layers. Then the intermediate 

layers can become the weak link. 

Katherine Friedrich: I believe that one of you had a slide 

in which you showed cure temperatures for different types of 

curing. It was a chart on which you had a table and I was 

wondering, how do you determine and what type of test did 

you do to determine whether it had cured or not. 

Dai: Yes, that is my slide. What type of test did we do? Well, 

for bonding, we usually test for IB just to check whether itís 

cured. The rule of thumb is such that we need to keep certain 

temperature and to hold it for a certain period of time, and 

for different thickness or different resin, the holding time 

might be different, particularly for MDI, because the target 

temperature can be a little bit lower. Now the holding time is 

very much depending on the thickness of the panel we press. 

I mean if we press a thicker panel, we have a bigger heat 

sink so the core temperature can be lower than when pressing a thinner panel. Also, the residual heat there helps cure 

to continue after pressing. Looking at the table, the plywood 

and the LVL actually donít list the holding time, the main 

target is just to achieve a certain core temperature because in 

plywood or LVL pressing it takes awhile for it to decompress, usually stepwise decompress helps provide enough 

time to cure the resin and also in LVL plywood situations 

the compression force is so much lower compared to OSB 

and other particle composites. We talked about this compression issue yesterday. We had to densify as much as 50%ñ

60% to achieve a certain intimate contact. But the byproduct 

of that is residual stress. In plywood and LVL we donít have 

that and so we are able to press lower and use less time. 

Freidrich: So it was a specific level of strength that you 

were looking for in the internal bond test then, to determine 

whether it had cured? 

Dai: Yes, itís the curing but resin companies they do a lot of 

instrumental analyses, I mean like what degree resins start to 

cure and accelerate. Maybe Chip and the adhesive guys can 

add on that. I mean each resin has a temperature in which 

curing starts to accelerate and then you still need a little bit 

more time to achieve full cure. But then when we are applying the conditions to real wood samples, we have to deal 

with them situation by situation. One rule of thumb is that 

we try to achieve, letís say over 100°C for a certain period of 

time and then at the end we try to reach a maximum temperature, say 150°C, but in that process I have achieved 

the 100°C core temperature for say 3 min or 2 min or even 1 

min, so that is the sort of criteria to apply to determine the curing

conditions. 

Pablo Garcia: Something I know that I have seen in use that 

panel MSR system where they flex the panel online after the 

hot press. Do you know how well they are doing or whether 

they have seen an increase of popularity in that use for process control? 

Dai: When I put these slides together about the testing

methods and stuff like that I realized I missed the end 

panel MSR and there are two types, one is developed by 

ARC. CAE is the commercial arm for that. Then there is 

another type out there by TECO, there is some moderate 

success, I wouldnít call it an even success by either product, 

the ARC system is good and it is a beautiful machine, they 

have two units installed, one in BC and the other is in a LP 

mill, which I think it is in Dawson Creek. So they have two 

units sold and its basic concept is to shoot a panel through a 

set of rolls depending on the deflection, oh no, it is the same 

deflection but it depends on the load cell and it can measure 

the stiffness of the panel. And it is very fast, itís online so it 

can monitor the strengths of the product being made, 

panel by panel. But I think there are two things that are 

causing, it not going further. First thing is this can cost

over a half million dollars for hardware, and then plus installation it is a million dollar equipment. The second thing is that 

it is very useful but they havenít been able to teach or work 

with the mill to come up with some sort of a process control, 

methodology that can be used to mills profit. You know, 

they just canít link that to the profit, I mean it is good data 

but mills say so what, I canít use it, and what do I adjust? So 

then it becomes a question of knowing what sort of products 

they are making now, what should they be adjusting. Is it 
86 

density, is it resin content, is there anything else, closing 

rate? And that has to be established, the software side hasnít 

been well established and I think those are the two reasons 

that that is not taking off as well as it is should be. And I 

think it is still a good technique and as far as the TECO unit, 

I hear that, no mill has actually used it so I donít want to 

comment further on that. 

Karl Englund: One question I wanted to ask, we talked 

about determining our MOE and MOR, has anybody looked 

at changing the span to depth ratio on these things? As far as 

if we are developing this large gradient in the vertical density profile, if we change our span to depth ratio in testing, 

we would get a different type of failure. I would think if we 

have a very low core density we would start to see a little bit 

more shear failure in our bending. It goes along the lines 

with online processing control, do they change the span to 

depth in those rollers at all and take into account those 

things? 

Dai: There has been some work done by Forintek and ARC 

looking at shear failure and even looking at the effect of fines 

on shear failure and that kind of stuff. As far as changing the 

load and depth ratio, I havenít seen any mill try to test it 

differently, they follow the standard right? I mean are you 

talking about changing the codes or standard when changing 

the span ratio for different products? 

Englund: No, I am not talking about changing the codes; I 

wouldnít want to do that. With these increased press run 

times on the presses, we are starting to see these density 

profiles really kind of affecting properties, thickness swell, 

etc. I think not everything out in the world is a 16 to 1 span 

ratio. There are going to be scenarios where you are going to 

have a shorter span where you might start seeing shear 

failures. I was just wondering if anybody has taken it into 

consideration. 

Dai: I havenít seen much work done, I think that will be a 

question put to the codes and standards people actually 

because whatever the mills do, they follow the standard, 

they try the minimum test to pass the standard and then you 

know to generate the maximum profit. Ted you want to add 

anything? 

Ted Laufenberg: The panel performance codes embody a 

separate test for that type of function for shear capability and 

concentrated load testing is that, whereas the flexural test is 

strictly for that long span bending assuming we donít have 

shear failure. I wanted to make a comment earlier though 

and this is a good time to do that. Having to do with thickness swell and what we see is not always just a function of 

vertical density profile, it is also a function of horizontal 

density profile. You have got a lot of variability in your 

panel, in your forming; your constituents perhaps may cause 

some of that. If you have only got 30 constituents per thickness versus 3000 as you have with MDF versus OSB, then 

you are going to have a lot more horizontal density variation 

and that results in property changes and thickness swell is 

one of those. If we could talk more about fasteners and the 

variability there, but there is also an opportunity, if we have 

a horizontal density profile that we have control of, one 

situation might be racking strength, where you are looking 

for enhanced fastener capability at the edges. Somebody 

smart enough could make their panel denser at the edges to 

make that a marketable product and really benefit the industry. I donít know where else to go with that but shear capacity is highly influenced by density yes, but we have separate 

testing in the codes. 

Dai: I think I can sort of comment on that Ted. It is true that 

vertical density profile is always there everybody knows, 

horizontal density profile sort of like I mentioned in the 

presentation, Otto Suchsland sort of started, you know he is 

the one who first mentioned it and then we actually did quite 

a bit of work in modeling that. The relationship between the 

horizontal density variation and the strand geometry and 

even species and the fines content, and also based on that. 

ARC has established a laboratory testing scanner, itís a x-ray 

scanner you can run the panels through you will know the 

density and variation and I know that some of that is already 

being used by Truss Joist, looking at their timber strand and 

I know it is very important. Yes, there are two things as far 

as I am concerned about the horizontal density variation, the 

effect of that on property. One thing is, we all know, horizontal density variation contributing to the graphing or 

uneven thickness swell the other thing is, I didnít have time 

to mention in my presentation, Iíd like to make this analogy, 

I donít know if it is correct, if you look at a plywood and 

compare to OSB, their properties are comparable but then if 

you look at plywood average density which is 0.4 or 0.45 

okay so what I am saying is the densification of that product 

might be like 5% to 10% right? Then if you look at OSB, we 

talked about it yesterday, it is 50% or 60%. Because of this 

inherent density variation, in order to create intimate contact 

between strands, you have to densify the product that much, 

the byproduct of using discrete elements or constituents like 

strands. In the case of LVL/plywood you have more 

uniform and continuous veneer sheets and you have less 

variation so you can just press a little bit and its done, OSB, 

no, you canít do that. But that is just the inherent part of it that

is governed by the geometry of the strand, the fines content 

and species. However, there is a move in the industry right

now, people are more looking at the forming issue, like how 

well the forming machines do? I mean like you have the 

inherent variation but the machine can also induce variation 

so that is why it is important to monitor what sort of a variation you have with your OSB mat, so once you see it exceeds your target limit and you have to do something about 

it, especially for a commercial product where the width of 

line is expanding, what is the widest production now, is it 

16 ft or 24 ft? And so over that span, I mean your mass 

variation, itís very difficult to control so it is becoming very 

critical and talking about the importance of making low 
87

density product, unless you have a good control on your 

density variation you canít go too far with lowering your 

overall density of the board. 

Wang: Yes, I agree that the horizontal density distribution is 

definitely a big player for thickness swell and Mike Wolcott 

had a student who researched this a couple years ago. They 

had a publication on their Pullman proceeding. The point is 

here, we know the horizontal density distribution is very 

important but manufacturers donít want to do a lot about it. 

For example, a typical OSB strand right now is around 

28-thousandth-in. thickness. If we use the thinner flakes 

making the same product, the horizontal density distribution 

will be less than when we use the thick flakes and the property will be increased. The problem is if we use the thinner 

furnish that means we have more surface to bond so we 

have to use more resin. Maybe resin companies will be 

happy about that, but if you are making a 3/8-in. thick panel 

and we use the 28-thousandth-in. flakes, there are not many

flakes on that layer. I mean whole thickness probably includes only 10 to 20 layers of furnish creating tremendous 

internal stress over that thickness. 

Dai: And on the strand geometry, I would like to add to 

Wangís comments about the thickness effect, yes, we found 

that the thickness has the biggest effect on horizontal density 

variation followed by width and fines content has a huge 

effect too. But if you try to manipulate thickness and there is 

another factor in the production, that is, you try to make it 

thinner, you have more bulk volume to deal with and for 

some of the forming and you know the production it is designed for certain volume so you have more volume and you 

canít handle that and so that is one thing. Another thing is 

about thickness effect which is really intriguing, if you use 

too thin strands, I donít know if it is much to do with the 

overall consumption of the resin, thinner strands create less 

micro voids, especially on the surface, you will have a hard 

time to vent, you will have blow. So there are a lot of factors 

to consider when it comes to pressing. 

Lee: You can find some articles on the linear expansion and 

the thickness swell in OSB, done by Quinglin Wn at LSU as 

an extended work of Otto Suchland and we did some modeling to predict the thickness swelling through the thickness of 

OSB and we found that there is a higher relationship between density and the thickness swelling but here the density, the localized density, so if you have averaging, I mean 

two specimens with the same average density you cannot 

predict that they would have the same thickness swelling. In 

case you have a vertical density profile, the highest density 

creates problems, so uniform density profile will make you 

less thickness swelling overall. In the same concept, if you 

have a very high horizontal density variation, any spot with 

very high density will cause the problem so in order to reduce the horizontal density variation, you have got to have a 

low density strand rather than thinner strand because there is 

a certain limitation that you can make the same strand 

because strand properties determine the OSB properties. If 

we have lower density strands you can control much easier 

the overall horizontal density distribution. 

Garcia: I was really interested about the aspect of densifying 

the edges of the mat; I think there are sort of several advantages in that. I know, well yesterday it was pointed out that 

some mills will densify their edges, I also heard that some 

mills will somehow drop more furnish in the middle and that 

has the advantage for racking strength because your mat is 

cut down the middle and you cut into 4x8 sheets and that is 

where you are sticking the nails through. But from a heat and 

mass transfer side, yesterday we heard the lateral permeability has a very important effect and so I started looking at

densifying the edges in the heat and mass transfer model and 

what I seem to get is, well what you are essentially doing is 

sealing the gas pressure in the mat, you are sealing the moisture in, there is less moisture escaping out the edges, less heat 

escapes out the edges, so you move from essentially getting 

toward the edges, you move from a more rounded distribution, more dryer edges, more cooler edges, to a more tabletop distribution with very uniform internal conditions and 

towards core. You would expect that gas pressures instead of 

sealing everything in, you are probably more at risk of blows 

and stuff because now you have not got high gas pressures 

inside, but I didnít test this on big mats, this is pure modeling, but since the mat is so big now, 24 ft by 12 ft master 

panels, in the middle of the mat where you expect your 

highest gas pressures, the increase is barely noticeable because you are so far away from the edge anyway. So all 

these effects are going to undoubtedly have an effect on 

vertical density profile and uniformity of vertical density 

profile from the middle of the mat towards the edges. If 

uniformity is a sign of quality, densifying the edges could be 

a way of getting more uniformity in your mat from that side. 

Kamke: Something dawned on me, Chunping, when you 

were talking about the amount of hydraulic pressure required 

to close the press and you showed a slide where the hydraulic pressure reached a maximum and then was held constant 

for some period of time prior to reaching the target thickness 

of the mat. You had to rely on creep strain to achieve your 

final target thickness. I guess my first question is, is that 

maximum in hydraulic pressure, because of the maximum 

pressure available in the press or was that actually intentional to use the maximum and allow it to reach the target 

thickness just based on creep? 

Dai: Basically, you are talking about pressure reaching to 

the max some sort of a plateau there and there being held 

more or less let the thickness to creep to the target. 

Kamke: Yes. 
88 

Dai: Yes, it is very common in OSB pressing and yes, it 

pretty much depends on the maximum pressure of the press. 

It is something limited by the equipment and you know the 

higher the pressure you get, the more you have to pay I 

guess. So in a way it is limited by the maximum pressure 

you can apply in practice. I know in a lab it can go higher 

than that. 

Kamke: Well, what dawned on me is I wonder if this might 

be an advantage, maybe not intentional but an advantage in 

regards to the final strength of this product. My point is that 

if you donít have such a high mat compaction pressure, 

perhaps you do not cause compression failures in the wood 

strands. Particularly if you think about the horizontal density 

distribution in some of those areas within the mat you are 

going to have extremely high density. Your probability of

causing some compression failure in those strands might be 

quite high. In fact it is probably guaranteed that you cause 

compression failures and the cell walls fracture. Well, if that 

happens then I would expect that you are not going to have 

very good strength properties and it is not due to a poor 

adhesive bond. It is due to the fact that you have got wood 

that has been fractured and it has virtually no resistance to 

bending stresses and shear forces. 

Dai: Fred, that is a very good observation and I would completely agree with that and we actually did some tests in the 

lab where we just looked at a solid wood, we compressed it

into different compression ratios, lets say 5%ñ10% representing plywood and then 25% even to 60%. After that we 

wanted to see what was the internal bond strength after 

pressing. We found that the IB was reduced. If IB is the test 

of the integrity of the product or the bonding or whatever, in 

the wood, then these results indicate that the wood is ruptured due to pressing. And of course high densification 

helped to create more intimate contact for bonding for OSB 

if you just keep the compression relatively low and you 

donít have much high pressure built in. So in this case, yes, 

it could be an advantage. I believe this sort of pressing strategy is going to be used for OSB as they try to reduce the 

overall density. You want to create a minimum density with 

maximum bonding. How do you do that? You do it by 

achieving minimum wood failure and minimal stress. 

Heiko Thoemen: I would like to introduce another aspect. 

We basically are just talking about batch presses here. My 

own estimation is that in Northern America approximately 

80% of the wood based panels are produced on batch presses 

so that is the reason why continuous presses are not of much 

interest here. In Europe, it is the other way around. About 

80% of the wood based panels are produced on continuous 

presses so there was a development in the last ten years. 

There are definitely differences between batch and continuous presses. Chunping, you mentioned that there is not much 

underlying money to run control and probably that is right 

for batch presses. But I think for continuous presses there is 

a lot of underlying money for process control already. So my 

question is, where is North America heading? Will North 

American companies move toward continuous pressing 

within the next ten or twenty years or will they stay with 

batch presses? 

Dai: Okay, my first comment is that I guess continuous presses

are very good and nice, but the initial capital investment is 

large. I think probably the trend, like in your new European 

mills now, is that they are moving towards the continuous 

press because it is gives you so much production. The total 

volume production and also the potential for manipulating 

the process to achieve real-time control, instead of the programmed control is vast. So I think that is where the future 

of North America pressing is going. But then again, it is cost 

driven. Anybody from the industry can add on that? 

Wang: In a continuous press, what we are gaining is more 

flexibility to control it when compared to multiple opening 

or single opening presses. For example, in continuous pressing we often try to use different temperatures at different 

stages. We might add in a cooling stage before the mat 

enters into the continuous press or they might have a preheating unit to heat up the mat first. I think that in the future, 

the trend in industry will be to use the continuous press 

rather than the multiple opening or single opening. But it 

depends on the application. I think we agree that when we 

press thin panels using a continual press will have a lot of 

advantages. Often they donít even need to sand the surface. 

They are ready for finish and painting. Often, a batch press 

will not achieve this same surface quality. 

Kamke: I have a follow up question directed at the PressMAN monitoring and control systems. This question goes to 

both of the speakers. How are these companies using this 

data on the internal mat conditions? Is it just a curiosity or is 

it actually being implemented and improving the process? 

Dai: Some mills use the data offline. Whenever they have 

some problems, they study and use it to check out and correct things. But, steam pressure and temperature probes are 

also sometimes used in mills. They are used as a tool for 

looking at internal mat condition, and then they use that data 

to modify their process. 

Kamke: I have used the gas pressure probes and temperature probes a lot in a lab press and witnessed their use in 

commercial presses. In the lab press where we have a much 

smaller mat to work with and a lot more time to insert 

probes, it is still quite a challenge to get it positioned where 

we want it to be. If you want to have a center core measurement, it is difficult to find that center core measurement 

particularly with a strand mat. I wonder about an industrial 

process, where you have got things moving down a line and 

you donít have much opportunity to stop things and get 

things aligned, how can they really use that data if they donít 

know exactly where that probe is located? 
89

Dai: They sort of look at, it is not just one board though, 

they have to look at a bunch of boards and develop some 

understanding on their conditions and they use more than 

one sensor usually for one board, too, so you can look at the 

average and remind you itís the temperature that is causing 

most of the problem if there is variation, the gas pressure as 

you and I know, has little variation between the surface and 

the core as the pressure is built up. So the positioning of the 

probe isnít going to cause any big problem to monitoring the 

gas pressure. Yes, I agree with you about the temperature 

measurement. I have seen they use just thermocouples and 

stuff like that to monitor temperature. 

Kamke: So, if I understand you correctly, do you believe 

that the industry, at least that you are familiar with, in their 

applications of the PressMAN monitoring system, that they 

are interpreting that data in a reasonable way and being able 

to use it to their benefit? 

Dai: I think they have benefit, yes. Otherwise how can you 

measure mat conditions? PressMan is a valuable tool for 

measuring gas pressure.

Kamke: I know that GP Resins has been distributing a 

system that they call ìPress Monitor.î I thought there was 

somebody else too that had put together their own little 

system. PressMAN might be the only one that is actually 

being marketed and sold. But the concern Iíve had since that 

technology has been adopted is that there is a lot of data 

collected and the people in the mills are overwhelmed with 

data and donít necessarily know what to do with it other than 

in its very simple form. 

Dai: Oh, I agree, it is like this thing about the MSR and once 

you get all this data collected in the mill, you have got to 

make it easy for them to use. And there is no easy way, no 

established way, that they can follow like A, B, C stepwise 

to develop their recipe. They still have to interpret it themselves and the other challenge like I said, if you stick a probe 

in the mat and if you donít know what kind of a moisture 

and species or fines content you have, your measurement is 

only very general because it varies with the kind of material 

you are dealing with and so that is another challenge in the 

mills. 

Wang: Another thing that we can use is the core temperature 

signal. We know we need to get to a certain temperature 

above the boiling point and for resin cure. But how long 

should we stay there after reaching 100 degrees? Thinner 

panels require a shorter time after close to final position to 

get the core temperature up to 100 degrees. But when we 

press a very thick panel, we take a few minutes, maybe even 

10 min longer to get to that point. My point here is that the 

resin on the thick panel is starting to cure even before we 

reach the 100 degrees. Based on a typical core temperature 

curve, we can look at how much energy is being transferred 

into the mat and how long it is above 100 degrees. 

Winandy: I would like to thank our two session moderators 

here. Please give them a round of applause. 
90 

Simulation of the Pressing Process 

Heiko Thoemen 

Department of Wood Science, University of Hamburg, Hamburg, Germany 

Abstract 

There has been a rapid growth of computer simulation techniques in almost all engineering disciplines for the last two 

decades. Correspondingly, an increasing number of research 

groups are working on hot-pressing simulation models, and 

such models are already used for a wide range of applications. An overview of some of the existing models is given 

in this paper, and typical applications are described. While 

all modeling approaches for heat and mass transfer are relatively similar, different approaches for rheology and adhesion are used. Main challenges for the modeling community 

are the reduction of execution time and, even more important, the improvement of the quality of model predictions. 

Both the refinement of existing modeling approaches and the 

determination of material property data are critical for improving the model predictions. Although some repetitions 

are fruitful, redundancies should be reduced to improve the 

efficiency of research in modeling the hot-pressing process. 

Introduction 

Simulation techniques have entered almost all disciplines of 

science and technology. One of the most prominent examples is the flight simulator. And the design of modern cars 

would not be thinkable without simulating and optimizing 

the air flow patterns in a wind tunnel (Figure 1). The advantages of using simulation techniques are evident. Compared 

with trial and error procedures conducted in the real system, 

simulations are relatively inexpensive, they can be executed 

with hardly any risks, and they are fast, because usually only 

little equipment setup is necessary. Furthermore, simulations 

can help us understand events that are not measurable, such 

as events in history (e.g., continental drift), in future (e.g., 

ecological systems), or processes that are inaccessible 

(e.g., in space). 

On the other hand, there is the danger of oversimplification 

when applying simulation techniques. When the German car 

manufacturer MercedesñBenz developed the A-Class (socalled Baby-Benz) during the 1990s, simulations were used 

to secure the curve stability of the car. However, it turned 

out in real tests that the vehicle had the tendency to overturn 

when going too fast through a narrow curve. To solve this 

problem, each A-Class Mercedes was provided with an 

electronic stabilization system (ESS) for about 900 US$. The 

improper usage of simulation can have costly consequences, 

as demonstrated in this example. 

Nevertheless, usually the advantages prevail, and consequently, there has been a boom of simulation in recent years. 

A large number of software packages have been developed 

as simulation tools, entire companies have specialized in 

numerical simulations, and there is a huge and still growing 

scientific community dealing with numerical simulation. 

When simulating the pressing process of wood furnish mats, 

the subject of heat and mass transfer through a porous hygroscopic material has to be addressed. The same subject is 

of importance in numerous other disciplines, as well. Examples include food drying, paper manufacture, convective and 

diffusive transport processes in textiles, and heat and mass 

transfer phenomena investigated in geoscience. In the future, 

the different disciplines working on similar topics may 

profit from each other much more than has been the case in 

the past. 

Simulation may be defined as a research or teaching technique in industry, science, or education that reproduces 

actual events and processes under test conditions (Encyclopedia Britannica 2003). This is a very general definition and 

includes computer simulations, the subject of this paper. 

The typical sequence of computer simulation is displayed in 

Figure 2. The real system to be simulated is described by a 

model that typically consists of a set of equations. Once the 

model has been implemented into a programming code, the 

program can be executed to compute the model predictions. 

The model predictions have to be validated by comparing 

them with the real system, and a new iteration may be necessary if predictions do not match the real world data. 

To do the execution of the computer program, the use of 

appropriate material property data is of essence (e.g., thermal 

conductivity or permeability of the mat). In simulating the 

pressing process of wood-furnish mats, these data are highly 

dependent on the local mat conditions, such as mat temperature, moisture content, or density. Knowing these dependencies is critical to achieve accurate model predictions. Because the mat conditions vary considerably within the mat 

and throughout the process (e.g., local densities may vary 

between 50 and 1,200 kg/m3 for medium-density fiberboard 

(MDF)), determining material property data for the entire 

range of material conditions is time consuming and may 

require elaborate testing equipment. The lack of appropriate 

material property data is still one of the limits in simulating 

the pressing process of wood-furnish mats. Some references 
91

of material property data presented in the scientific literature 

are included in this paper. 

State of the Art 

An overview of those physical mechanisms most relevant 

during hot-pressing is given in Figure 3. The mechanisms 

may be allocated to three different groups of mechanisms, 

which are heat and moisture transfer (or heat and mass transfer, respectively, if air flow through the void system is included), rheology, and adhesion. 

The individual mechanisms listed in Figure 3 are highly 

dependent on each other and, therefore, cannot be looked at 

in isolation. Consequently, for modeling the process, an 

integrated approach is required that accounts for the interdependencies of the mechanisms. However, for this state-ofthe-art report, a separation of the groups of mechanisms is 

helpful. The three groups will be described and discussed 

individually here, followed by a brief overview of numerical 

approaches used by different research groups and of the 

different press types simulated so far. 

Heat and Mass Transfer 

Contrary to the other two groups of mechanisms, all researchers working on modeling the pressing process use a 

similar approach to describe heat and mass transfer. This 

approach was introduced for the first time by Humphrey 

(1982) and Humphrey and Bolton (1989) for flake mats and 

has been adopted for MDF and oriented strandboard (OSB) 

mats, respectively. Since then, it has been adopted by Carvalho and Costa (1998), Hubert and Dai (1999), Thoemen 

(2000), Zombori (2001), and Garcia (2002). The following 

description is an enhancement of the approach presented by 

Humphrey and Bolton. 

Even in its highly compacted stage, the wood furnish mat is 

a capillary porous material that consists of a matrix of wood 

material and a system of inter- and intra-particle voids. The 

voids are filled with a gas mixture consisting of air and 

water vapor as its main components. 

Three different mass transfer mechanisms that may be relevant during hot-pressing are illustrated in Figure 4. While 

gas convection is the main mass transfer mechanism, it is 

generally agreed that gas diffusion and bound water diffusion only play a minor role. Gas convection is the bulk flow 

of the gas mixture along a total gas pressure gradient. 

Such gradients develop both perpendicular to the mat surface 

and within the mat plane. If the convective gas flow through 

a porous material is laminar, it can be described by 

Darcyís law. 

Both heat conduction and heat convection are important 

mechanisms during hot-pressing (Figure 5). Heat conduction 

may be described by Fourierís first law; the flux is proportional to the temperature gradient. On a microscopic level, 

the main flow path of conductive heat transfer is the wood 

material. Contrary, convective heat transfer is associated 

with convective gas flow through the void system of the mat, 

in combination with phase change of water. Radiation as a 

third heat transfer mechanism is believed to be insignificant 

in this type of process. 

Two important equilibrium assumptions are made by all 

research groups that have published their models, so far. The 

first assumption is the assumption of instantaneous sorption 

equilibrium. It says that there is always local sorption equilibrium between the atmosphere in the voids and the moisture content in the adjacent wood material and that there are 

no moisture content gradients within single particles and no 

partial pressure gradients within single voids. Second, it is 

assumed that there are no temperature gradients on a microscopic scale. These two assumptions are more realistic the 

smaller the wood particles are. They may, however, be critical for strands or other types of large-size particles. 

The most important material properties needed to describe 

heat and mass transfer are thermal conductivity, permeability, and hygroscopicity. A comprehensive literature review 

on data for these properties is given by Thoemen (2000). 

Only a brief excerpt from this review and some newer work 

is presented in this paper. 

Thermal conductivity is a function of density, moisture 

content, flow direction, and to a lesser degree of temperature. Measurements of thermal conductivity have been conducted, among others, by Shao (1989) using unresinated 

fiber mats, and by von Haas (2000) using MDF, particleboard, and OSB panels, as well as unresinated mats. Both 

workers covered a wide range of material conditions. A 

different approach was used by Zombori (2001), who calculated the thermal conductivity of the mat, instead of measuring it, based on the thermal conductivity of solid wood, the 

thermal conductivity of air, and the structure of the mat. 

Permeability of the wood furnish mat depends strongly on 

the mat density and the flow direction, while mat moisture 

content and temperature has only a little, if any, effect on 

this property. Unlike thermal conductivity, the direct derivation of the permeability from the mat structure appears to be 

difficult; so far, only measurements have been reported in 

the literature. The most comprehensive set of permeability 

data for MDF, particleboard, and OSB panels of different 

density levels has been presented by von Haas and others 

(1998). A method designed for a quick data acquisition has 

been introduced by Haselein (1998). The wood furnish 

material samples are densified step by step during a single 

sequence of measurements, so that permeability data for a 

given material type can be attained without having to manufacture several panels of different densities. 

Hygroscopicity, i.e., the affinity of the wood material towards water, is independent of the mat structure but is a 

function of relative humidity in the pore spaces and of 
92 

temperature. So far, only few data are available for high 

temperatures, and there are considerable discrepancies 

between those data, particularly for high relative humidities. 

Both theoretical models and measurements have been presented in the past. 

The most frequently used theoretical model is the twohydrate Hailwood and Horrobin (1946) model. However, for 

temperatures above 150°C, the model gives somewhat erratic values that deviate considerably from experimental 

results at all relative humidity levels. Another set of hygroscopicity data often used is that presented by Kauman (1956) 

for temperatures up to 180°C and high relative humidities; 

they are extrapolations from measurements done at atmospheric pressure. 

Among the most comprehensive hygroscopicity data for 

elevated temperatures and relative humidities available today 

are those presented in the Wood Handbookís (FPL 1999) for 

temperatures up to 130°C and those measured by Engelhardt 

(1979) for temperatures up to 170°C (Fig. 6). 

Those heat and mass transfer models developed in recent 

years typically predict the development of mat temperature, 

moisture content, water vapor pressure, and air pressure, 

among others, over space and time. Examples of predictions 

obtained with three different models are displayed in 

Figures 7 and 8. 

Rheology 

Taking the term rheology in its narrower sense, it describes 

only phenomena such as viscous or delayed-elastic deformation. Stress relaxation and creep processes are consequences 

of such types of material behavior. However, in this paper, 

the term rheology will be used in a broader sense, following 

the convention facilitated by Ren (1991). According to this 

definition, the rheological behavior of a material includes 

time-dependent as well as instantaneous deformation processes. Such usage of the term rheology reflects the fact that 

both aspects of the material behavior are interactive with 

each other. 

As already stated above, some of the material properties 

needed to compute heat and mass transfer phenomena are 

highly dependent on the local density of the material. Consequently, a heat and mass transfer model will only provide 

reliable model predictions if the density distribution in the 

mat is accounted for; a procedure to compute the densification of the material is required. 

Two different approaches to model the rheological material 

behavior during hot-pressing are summarized here. The first 

one goes back on work done by Ren (1991) working together with Humphrey; it uses a rheological model, consisting of three springs and two dashpots (Figure 9a). Each of 

the five rheological coefficients is a function of temperature, 

moisture content, and density. Before the model can be 

applied for a specific material type, the coefficients have to 

be determined experimentally for the entire range of conditions. A set of rheological coefficients for MDF have been 

presented by Thoemen (2000), covering the entire range of 

internal mat conditions relevant during hot-pressing. These 

data have been derived from experimental work done by von 

Haas (1998), who also included two different flake furnish 

materials and one OSB material in his study. 

The second approach has been applied by Dai and Steiner 

(1993), Lang and Wolcott (1996), and Lenth and Kamke 

(1996). The models of these three research groups have in 

common that the compaction behavior of the mat is assumed 

to be governed by two independent factors: the geometry of 

mat and wood constituents and the properties of solid cell 

wall substance. If the influence of environmental variables 

such as temperature and moisture content is restricted to the 

cell wall substance, as proposed by Wolcott (1989), this 

approach provides a technique to compute the stressñstrain 

relationship under different environmental conditions relatively easily, once the relevant geometrical parameters are 

known. So far, research based on probabilistic and geometrical theory to model mat consolidation has been restricted to 

mats made of strands or small pieces of veneer. 

By combining a rheological model with a heat and mass 

transfer model, the development of the cross-sectional density profile, as well as the horizontal density distribution, can 

be simulated. Figure 9b displays the cross-sectional density 

profile development simulated by Thoemen (2000) for an 

MDF mat. 

Adhesion 

Adhesive cure has an impact on the thermodynamics of the 

system on the one hand and on the bond strength development on the other hand. The first of these two effects is due 

to the exothermal cure reaction of the adhesive, which generates heat, and to the condensation reaction, which adds 

water to the system. 

Clearly, the localized bond strength development has a direct 

impact on the final panel properties but also on the rheological behavior of the mat inside the press. Particularly, modeling the impact on the panel properties is of high economical 

significance and can, therefore, be expected to be one of the 

great challenges of the next decades. Important work towards this goal has been done by Humphrey (1994), who 

developed the Automated Bonding and Evaluation System 

(ABES, Figure 10a), and by Heinemann (2003), who presented, only lately, the Integrated Pressing and Testing 

System (IPATES, Figure 10b). While the ABES can be used 

to test the bond strength development on small veneer strips 

in shear mode, the build-up of the internal bond strength 

perpendicular to the plane of fiber or flake samples can be 

observed with the IPATES. By using the data obtained by 

IPATES, in combination with a heat and mass transfer 
93

model and a rheological model, the development of the 

internal bond strength can be simulated (Figure 11). 

Numerical Approaches 

It is not intended at this point to give a full description of the 

different numerical approaches used for simulating the hotpressing process. However, it seems to be worthwhile to 

mention the large variety of numerical methods used by 

different research groups. Hubert and Dai (1999) use the 

finite-element method, Humphrey (1982) and Thoemen 

(2000) describe an algebraic approach, Zombori (2001) and 

Carvalho and Costa (1998) apply the finite-difference 

method, and finally Garcia (2002) describes his approach as 

a finite-volume method. Obviously, all the approaches mentioned here are appropriate to solve the set of equations 

describing the hot-pressing process. 

Different grid schemes have been used in the past. Humphrey and Bolton (1989) started with a cylindrical coordinate 

system. Today, two-dimensional (Zombori 2001) or threedimensional (Thoemen 2000) grids are used. Hubert and Dai 

(1999) describe their grid scheme as a one-dimensional grid, 

but they account for vapor escape through the edges of the 

mat by an approximation of the in-plane vapor flow. 

Simulation of different press types 

The press most frequently simulated is the conventional 

batch press of either one- or multi-opening type. A model to 

simulate the processes inside a steam injection press has 

been presented by Hata and others (1990). Simulation models of the pressing process have also been extended to include pre-heating of the wood-furnish material (Pereira and 

others 2001). 

Because the continuous pressing technology has gained 

considerable importance during the last two decades and 

because the number of continuous presses will certainly 

continue to grow, Thoemen and Humphrey (2003) presented 

a model that can be used to simulate both batch and continuous presses. Important differences between these two press 

types are the gas flow patterns at the press entry and outlet 

and the fact that the boundary conditions vary along the 

length of the press in production direction. Figure 12 displays the total gas pressure distribution and the horizontal 

gas flow pattern within the mat while passing through the infeed section of a continuous press. 

Applications, Potential, 

and Limits 

Typical applications of existing hot-press models are education and training, process optimization, equipment development, product design and development, and research. Some 

examples of such applications are discussed here. 

When using a model as a tool for education and training, the 

learning goals may vary considerably, according to the target 

group addressed. While clarifying and visualizing the interactions between the different process parameters and mechanisms may be more applicable for students on the college or 

university level, the technologist and plant operator is 

probably more interested in the specific effects of the pressing parameters on the panel properties. In any case, statements should be presented as clearly and straightforward as 

possible. Particularly for training operators, the user interface of the model should resemble the input variables and 

logic known from the control screens of the real process as 

close as possible. 

A typical process optimization problem can be described by 

means of the following example. Differences in the vertical 

density profile across the width of the panel are a well 

known phenomenon. When addressing such problem, important questions are, what are the reasons for this phenomenon, 

and what can be done about it? A hot-pressing model that is 

able to predict the development of the vertical density profile 

is certainly a helpful tool to investigate these questions. In 

case the model predictions are rather qualitative than quantitative in nature, vital insights valuable for solving the problem can still be expected from simulations. 

Hot-pressing models already play an important role in 

equipment development. A good example for this is the 

design of a pre-heating system. Typical questions that have 

been addressed are how does the design of the system affect 

the efficiency of pre-heating and how does the pre-heating 

affect the density profile and the pressing time. Expensive 

and time consuming experiments cannot be completely 

substituted by using a simulation model, but experience 

shows that they can be significantly reduced. 

To stimulate the use of simulation models by the woodbased composites industry, the modeling community has to 

understand the demands of the industry. First, models have 

to be easy to use. Second, the industry has to become familiar with the potential of simulations. In other words, appropriate promotion is required. And third, the limits of the 

models have to be stated clearly. 

These are important prerequisites for widening the use of 

models. Above, of course, great challenges are the reduction 

of the execution time of the models and the improvement of 

the quality of model predictions. To meet the first of these 

challenges, it may be necessary to further optimize the numerical solution schemes or to use parallel computing techniques or super computers. However, it is reasonable to 

assume that the hardware industry will continue to rapidly 

improve the power of personal computers. Therefore, working out concepts to reduce the execution time of the model is 

not necessarily the highest priority for the model developers. 

The improvement of the quality of model predictions clearly 

is the greater challenge for the modeling community. 
94 

Although some of the existing models have reached a stage 

where they can be used for a wide range of applications, a 

further enhancement of the simulation results is still desired. 

Such objective requires work on material property data, as 

well as significant model refinements itself. 

Reducing the execution time and improving the quality of 

model predictions are also prerequisites for applications that 

may become important in the future, such as solving optimization problems or online process control. An example of an 

optimization problem is to specify a desired density profile 

and to approach this profile by an iterative procedure. Such 

iterations are typically very time consuming. An online 

process control application could be a situation where a 

sensor detects a change in moisture content of the mat. The 

model then evaluates the new situation and decides to adjust 

the pressing time. Neither work on optimization problems 

nor online process control applications have been reported 

for hot-pressing models so far. 

One important feature of a model that has not been mentioned yet is its availability to the potential user. Unfortunately, not all hot-pressing models that have been developed 

are available to the industry. 

The simulation software MatPress (Hubert and Dai 1999, 

Dai and others 2000) of Forintek Canada Corporation is 

accessible only to Forintek members. The hot-pressing 

model developed by Zombori (2001) is part of a larger simulation environment, the WBC Simulator (Figure 13). The 

software cannot be purchased, but it runs on the server of the 

WBC Center (Virginia Tech, Blacksburg, Virginia) and can 

be accessed by the internet. 

The simulation software Virtual Hot Press (VHP) can be 

purchased from the University of Hamburg (Germany) and 

installed on a standard PC. It uses the model developed by 

Thoemen and Humphrey (2003) as a core module. Batch as 

well as continuous presses can be simulated, and material 

property data are available for MDF, particleboard, and 

OSB. The VHP output platform (Figure 14) visualizes the 

simulation results for mat thickness, pressing pressure, internal gas pressure, temperature, moisture content, and density 

profile. Above, additional output variables are stored in data 

files. 

Conclusions 

For several years, an increasing number of research groups 

have been working on hot-pressing simulation models, and 

such models are already used for a wide range of applications. However, the potential of todayís models are by far 

not exploited yet. The reasons are many fold, and the situation is comparable with other engineering disciplines, as 

pointed out by Gibson and others (2003): ìGrowth in the use 

of simulation technology has been less than expected. Today 

there are still many applications that could benefit from 

simulation, but that do not use it.î 

Challenges for the next years are the reduction of execution 

time and, more important, the improvement of the quality of 

model predictions. Both appropriate model refinements and 

the determination of material property data are critical for 

elevating the predictive power of the models. Concerning the 

material property data, in the long term, it is not enough to 

characterize only few material types. The material properties 

of the wood-furnish material strongly depend on the wood 

species mix, as well as on the size distribution and shape of 

the wood particles. Both the species mix and the geometry of 

the particles vary considerably between materials from 

different manufacturers, and new material types will be 

added in the future. Consequently, methods should be developed to easily determine the characteristics of any new 

material. 

Although some repetitions are fruitful, the modeling community should avoid excessive repetitions to gain maximum 

benefits from research done in the field of modeling and 

computer simulation. It is a waste of resources if everybody 

who wants to contribute to the improvement of todayís 

models starts from scratch. Bilateral cooperations between 

researchers could be a first step towards a reduction of redundancies. To stimulate the information exchange within 

the modeling community, and also between the modeling 

community and other researchers or industry, a web-based 

database would be helpful. Such a database could include 

descriptions as well as material property data and would 

contribute to transparency and stimulate communication 

among the parties. 

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Thesis. 

Thoemen, H.; Humphrey, P.E. 2003. Modeling the continuous pressing process for wood-based composites. Wood 

and Fiber Science. 35(3): 456ñ468. 

von Haas, G. 1998. Untersuchungen zur Heiflpressung von 

Holzwerkstoffmatten unter besonderer Ber ̧cksichtigung des 

Verdichtungsverhaltens, der Permeabilit‰t, der Temperaturleitf‰higkeit und der Sorptionsgeschwindigkeit. Dissertation, 

Hamburg, Germany: University of Hamburg. 264 p. 

von Haas, G. 2000. Untersuchungen zur W‰rmeleitf‰higkeit 

von Faser- und Spanmatten und Platten. Holzforschung und 

Holzverwertung. 52(5): 102ñ104. 

von Haas, G.; Steffen, A.; Fr ̧hwald, A. 1998. Untersuchungen zur Permeabilit‰t von Faser-, Span- und OSBMatten f ̧r Gase. Holz als Roh-und Werkstoff. 

56(6): 386ñ392. 

Wolcott, M.P. 1989. Modeling viscoelastic cellular materials for the pressing of wood composites. Petersburg, VA: 

Virginia State University. Ph.D. Thesis. 

Zombori, B.G. 2001. Modeling the transient effects during 

the hot-compression of wood-based composites. Petersburg, 

VA: Virginia State University. Ph.D. Thesis. 
96 

Figure 1óSimulation examples: (a) flight simulator. 

(b) wind tunnel. 

Real System

Model Computer

Modeling Execution

Prediction

Implemen

tation

Validation

Material

Property Data

Real System

Model Computer

Modeling Execution

Prediction

Implemen

tation

Validation

Material

Property Data

Figure 2óTypical sequence of computer simulation. 

Figure 3óMechanisms relevant during hot-pressing 

of wood-furnish mats (Humphrey 1994). 
97

Gas convection

(bulk flow)

Gas diffusion

i

abs

i

eff

d

i p RT

M j = −D ∇

Fick's first law

p p RT

k M j i

abs

c p i

i = − ∇

η

Darcy's law

Bound water

diffusion

v

p

b

T

b

b

D p

j D T

+ ∇

= − ∇

 

(Dominant mechanism)

Gradient

Gas convection

(bulk flow)

Gas diffusion

i

abs

i

eff

d

i p RT

M j = −D ∇

Fick's first law

Gas diffusion

i

abs

i

eff

d

i p RT

M j = −D ∇

Fick's first law

i

abs

i

eff

d

i p RT

M j = −D ∇

Fick's first law

p p RT

k M j i

abs

c p i

i = − ∇

η

Darcy's law

p p RT

k M j i

abs

c p i

i = − ∇

η

Darcy's law

Bound water

diffusion

v

p

b

T

b

b

D p

j D T

+ ∇

= − ∇

 

(Dominant mechanism)

Bound water

diffusion

v

p

b

T

b

b

D p

j D T

+ ∇

= − ∇

 

(Dominant mechanism)

Gradient Gradient Figure 4óMass transfer mechanisms. 

Conduction Conduction Convection

Associated with

mass transfer

and phase change

q = −kt∇T

Fourier's first law

Radiation

(Dominant mechanisms) (Insignificant)

Gradient

Conduction Conduction Convection

Associated with

mass transfer

and phase change

q = −kt∇T

Fourier's first law

Radiation

(Dominant mechanisms) (Insignificant)

Gradient Gradient Figure 5óHeat transfer mechanisms. 

Figure 6óSorption isopsychrens for beech (Thoemen 2000, refined after Engelhardt 1979). 

0

5

10

15

20

25

30

φ = 100 %

90

80

70

60

50

40

30

20

10

measured extrapolated

20 40 60 80 100 120 140 160 180 200 220

0

5

10

15

20

25

30

Temperature (°C)

φ

90

80

70

60

50

40

30

20

10

Measured Extrapolated

20 40 60 80 100 120 140 160 180 200 220

10

20

25

Equilibrium moisture content (%)
98 

Figure 7óSimulation results presented by Garcia (2002) for mat temperature (a) and Zombori (2001) 

for mat moisture content (b). Both simulations refer to OSB mats. 

800 1000

(a) (b)

0 200 400 600

Elapsed time [s]

180

160

140

120

100

Gas pressure [kPa]

0 200 400 600 800 1000

Elapsed time [s]

trial 1

trial 2

trial 3

trial 1

trial 2

trial 3

Measurement Simulation

180

160

140

120

100

180

160

140

120

100

180

160

140

120

100

Gas pressure [kPa] 180

160

140

120

100

180

160

140

120

100

800 1000

(a) (b)

0 200 400 600

Elapsed time [s]

180

160

140

120

100

Gas pressure [kPa]

0 200 400 600 800 1000

Elapsed time [s]

trial 1

trial 2

trial 3

trial 1

trial 2

trial 3

Measurement Simulation

180

160

140

120

100

180

160

140

120

100

180

160

140

120

100

Gas pressure [kPa] 180

160

140

120

100

180

160

140

120

100

Figure 8óComparison of measured (a) and simulated (b) total gas pressure in the center of an MDF 

mat (Thoemen 2000). 

0 s

60 s

240 s

480 s

0 s

60 s

240 s

480 s

Experimental and Modelled 

Temperature

Core

Face

20

100

180

0 120 240 360 480 600

Pressing-time (s)

Temperature (°C) 

Experimental and Modelled 

Temperature

Core

Face

(a) (b)

0 s

60 s

240 s

480 s

0 s

60 s

240 s

480 s

20

100

180

0 120 240 360 480 600

Pressing-time (s)

Experimental and Modelled 

Temperature

Core

Face

20

100

180

0 120 240 360 480 600

Pressing-time (s)

Experimental and Modelled 

Temperature

Core

Face

(a) (b)
99

Eel

Vvi

Ede Vde

Epmf

Eel

Vvi

Ede Vde

Epmf

(a) (b)

0

200

400

600

800

1000

0

10

20

0 30 20 40 60 80 Cross-sectional position [mm]

Distance fro

m

press entry [m]

Density [kg m/ 3]

Second densification step

Press outlet

0

200

400

600

800

1000

0

10

20

0 30 20 40 60 80 Cross-sectional position [mm]

Distance fro

m

press entry [m]

Density [kg m/ 3]

Second densification step

Press outlet

Eel

Vvi

Ede Vde

Epmf

Eel

Vvi

Ede Vde

Epmf

(a) (b)

0

200

400

600

800

1000

0

10

20

0 30 20 40 60 80 Cross-sectional position [mm]

Distance fro

m

press entry [m]

Density [kg m/ 3]

Second densification step

Press outlet

0

200

400

600

800

1000

0

10

20

0 30 20 40 60 80 Cross-sectional position [mm]

Distance fro

m

press entry [m]

Density [kg m/ 3]

Second densification step

Press outlet

Figure 9ó(a) BurgersñHumphrey model (refined after Ren 1991). (b) Simulated density profile 

development in a continuous MDF press (Thoemen 2000). 

Figure 10ó(a) Automated Bonding and Evaluation System (ABES, Humphrey 1994). (b) Integrated Pressing and 

Testing System (IPATES, Heinemann 2003). 
100 

Figure 11óSimulated development of the local internal bond strength 

in an MDF mat. Internal bond model input data from Heinemann (2003). 

0123456

0.0

0.5

1.0

1.5

2.0

2.5

100 120 140 160 180

Gas pressure [kPa]

Distance from press entry [m]

Width [m]

Reference arrow:

0.1 m/s

0123456

0.0

0.5

1.0

1.5

2.0

2.5

100 120 140 160 180

Gas pressure [kPa]

Distance from press entry [m]

Width [m]

Reference arrow:

0.1 m/s

Figure 12óSimulated total gas pressure distribution and gas 

velocities (arrows) for the first 6 m within the central layer of 

the mat (Thoemen and Humphrey 2003). 
101

Figure 13óWeb-based WBC simulator of Virginia Tech. 

Figure 14óOutput platform of the simulation software VHP (University of Hamburg). 
102 

DiscussionóSimulation of 

the Pressing Process 

Heiko Thoemen 

Fred Kamke: Actually, I just want to make a comment 

about the WBCSIM website. You gave us an old URL that 

has been now updated. It was correct on your other slide. If 

anyone wants the correct URL, just call Jong or I. 

Thoemen: Okay, it is http://kansa.cs.vt.edu./~wbcsim/

Kamke: I have a question. I wanted to refer to the information you had about the ABES system and the other system 

you are developing at Hamburg. In the graphs you showed 

the ABES system, for the shear strength versus time, the 

regression equations were straight lines. For the IBs versus 

time, from the EPOTIS system the lines were nonlinear and 

neither one goes to the origin. I wanted to ask why, first of 

all, this one is a straight line and the other one is nonlinear 

and then why neither one of them goes through the origin.

Thoemen: Okay, maybe I start with the second question, 

why it doesnít go through the origin. There are basically two 

reasons, one reason is it takes some time to get the temperature to the glue bond. There is probably a second reason and 

Professor Frazier might better comment on it, because there 

is no immediate start of the reaction. That is what I suppose. 

What was the first part of your question?

Kamke: This relationship from ABES is a straight line and 

the MDF-IB strength is a nonlinear relationship? 

Thoemen: Okay, probably this data from the ABES they 

would get similar curves so they would deviate from this 

straight line if you would continue the measurement for 

higher strength values, but what I was told that at high 

strength values you get kind of a deviation from the plane so 

you get other effects so that is one of the reasons why it 

makes sense to stop the lines here and we didnít have this 

type of problem here so that is one of the reasons. The other 

reason was the way Phil Humphrey was using his data. He 

was looking for a straight line (i.e., the slope) so he could 

estimate the bonding rate and the rate of bond strength development at any one specific temperature. With this MDF 

data, we used a different modeling approach where we didnít 

need a constant rate for one temperature. So does it answer 

your question?

Kamke: It answers my question as far as why you did it that 

way; I am trying to think of how I would apply this data to a 

modeling situation?

Thoemen: Okay, the easier way is to use this type of data, 

for any temperature you have a defined bond strength development rate and so you can calculate it step by step so for 

any time for any position, you know the rate and you can 

calculate the gain in bond strength.

Jerry Winandy: So if I understand what you are saying, 

you are basically just going to use that and develop a kinetic 

rate constant and then you can do it over a whole continuum 

of temperatures once you develop that kinetic rate constant.

Thoemen: Yes, that is right. So the concept behind it is that 

the bond strength development rate at any given temperature 

is a constant, so whether it is early during the process or late 

in the process, that is the assumption that is made here. 

Probably this assumption is debatable but it is the first try to 

get some simulations of the bond strength development.

Kamke: I guess the problem I have with this data is trying 

to understand how I would implement it in a model. This is a 

bond strength or this is a shear strength you are showing us 

and I am thinking in terms of a reaction, a polymerization. 

The reaction rate to that polymerization would follow a 

much different type of relationship mathematically, compared to what you are showing right here.

Thoemen: Okay, one point is that, we already had this

discussion I think, adhesive cure is the chemical reaction to 

something different to the strength development, so there are 

two different paths that I followed and I agree that this data 

probably you have to start like here at zero and then it goes 

up and then it reaches an end level at sometime, that would 

be probably what you are referring to. But the method is not 

exact enough so that there would be speculation to anything 

to fit these curves to any data down here so that is why I do 

not start at the zero.

Chuck Frihart: On your previous data, were those tested at 

those temperatures? On the other slide, the shear strength, 

those measured values were at that temperature?

Thoemen: Yes, that is correct.

Frihart: So, there is certainly a rheological factor in there in 

that at the lower temperature your material is going to be 

much more viscous and is going to have a certain shear 

resistance. You are probably not going to see that in an 

internal bond strength because you are already starting with 

some type of polymer chain when you put the adhesive on. 

As the water disappears you have a base strength value 

which exists even with no reaction. At the higher temperature because your material naturally thins out, it is going to 

have less strength.

Thoemen: Probably you are right, I personally did not go 

into this adhesive strength development too much and I am 

not a chemist. I am aware that there are many mechanisms 

happening that affect this strength development and probably it is more complex to talk about these first regions. Basically what we did here was we just ignored all the chemical 

development. I donít know whether that makes sense so we 

just looked at the strength development, we did not care so 

much about what are the chemical effects.
103

Frihart: I would think though that if you're concerned about

blowout resistance that your IB values are much more indicative of resistance to that force than your shear strength. I 

know it doesnít model as well but you might want to look at 

really putting your IB values in there as being resistance to 

blowout because I think that more relates to that.

Felisa Chan: I think there is a difference between that graph 

and the other one because one is on shear strength, probably 

plywood, while the other is maybe MDF and or something?

Thoemen: Okay, the difference is this one is determined on 

smaller strengths, some smaller veneer strips, so you have 

kind of two strips, you glue them together inside this apparatus, you press them, you have a defined pressure and a defined temperature and then after a defined time you pull the 

two strips apart and then you measure the shear strength of 

this bond and if you go to this machine, it is a completely 

different setting. Here we have fiber or flake materials, 

particles that could be particles or fibers.

Chan: And another one is do you consider like the first 

graph can you use that for different types of resin, for example formaldehyde or phenol formaldehyde?

Thoemen: You can use both approaches to investigate the 

different resins and I donít want to talk about the ABES too 

much. Phil (Humphrey) should do that. The concept is that if 

you want to develop a method where you can measure the 

strength development as a function of temperature and as a 

function of the different resin type. Phil wanted a method 

where he used both components, the wood and the adhesive. 

I think that is the charming thing about this approach that 

you have a way to measure the strength development, you 

have not just the adhesive as it is in many other methods but 

you have the combination of wood and adhesive.

Winandy: On the ABES system, if I understand it, you 

basically take two flakes so you have a very thin profile 

which is not going to retain latent heat very long so it cools 

and that might explain why that is relatively linear. With the 

EPOTIS system you have a thicker profile and so it is going 

to generate more latent heat and it is going to have more post 

heating curing involved which would then give you that 

second order effect?

Thoemen: That is one possibility, but I think that the main 

thing at the moment if we wanted we could fit linear lines 

through this curve and if we would cut this graph let's

say here, then it would be easier to fit linear curves to it as 

an approximation so probably this is the more realistic 

curve. Now if you look here, if you would continue your 

measurements towards higher shear strength values then you 

would probably get something like a leveling off as well. 

The problem is, as I mentioned before, we donít get this 

leveling off because before we get there, we get kind of a 

buckling of our sample so that is the reason we stopped here 

and we donít continue our measurements.

Alain Cloutier: I want to change the subject a little bit, first 

I would like to thank you for this excellent presentation. I 

totally agree with you when you mention that the physical 

parameters are key in the modeling processes. You mentioned the thermal conductivity, permeability, and the absorption isotherm as key parameters. I also think that you are 

right, this is what weíve seen, so that those parameters are 

very important. But I would like to concentrate on this absorption isotherm. You showed us a slide where you extrapolated the data from Englehardt. My questions are how comfortable are you with the curves you extrapolate in there and 

do you think that there should be more investigation in that 

part of the absorption isotherm? 

Thoemen: That is a very good question, how confident are 

we with this data in this range here. I did not see any data for 

this range here. I am not sure whether Chris (Lenth) went to 

that range, so I totally agree that it would be helpful if we 

have measurements. Whenever we extrapolate anything, the 

better way would be to measure it. Just from what I read and 

I think Chris had the same experience, it is extremely difficult to obtain this data and that is probably the reason why 

there is so little data available so far.

Kamke: You are referring to Chris Lenth (Lenth 1999, 

Lenth and Kamke 2001), who worked on this as a graduate 

student as part of his Ph.D. work about 3 or 4 years ago. He 

did measure equilibrium moisture contents of a couple of 

different species up to 180°C. It did require some very specialized equipment to do that, and he had to account for 

thermal decomposition and loss of other materials from the 

wood in addition to the water. As a consequence, he was 

only able to do that for a few temperatures. I do want to 

make one comment however, and it is something that Chris 

saw in his data and then was able to collaborate it with some 

work that was done by Strickler. Strickler (1968) also collected some data at high temperatures. He observed this 

phenomena and made a comment without going into it in 

any more detail, which he thought that something was happening in the wood cell wall that disrupted this behavior and 

caused some stepwise change in absorption characteristics. 

What Chris believes happens is that when you pass through 

the glass transition temperature, the wood cell wall

changes in its ability to absorb moisture and you actually 

would get higher equilibrium moisture contents than you 

would expect by simple extrapolation of low temperature 

data.

David Harper: I am going to switch gears again and pose 

this question to all of the modelers here. Why did you 

choose not to release your source code? It seems like what 

essentially you have done is provide a framework for inputting separate models into different parts of your program. 

There are a lot of different institutions working on specific 
104 

problems associated with the hot press, maybe not the holistic view that this model is taking, but it would be nice to 

have the code (i.e. framework). Then users would be able to 

insert their research into that framework and see how it 

affects the overall process which you have put together.

Thoemen: It is a question about the open source approach, I 

agree with you that I see the advantage of having such an 

open source approach in which anybody who wants to can 

make improvements. But there are some restrictions to it, 

which I mentioned. There are reasons why we donít see an 

open source approach at the moment in our field, but I want 

to mention some other problems. If anybody would use my 

code and would try to implement aspects or change aspects, 

it would probably take them several weeks to step into it and 

really understand my program. So it is probably the easier 

way to contact me or whomever and talk about it and try to 

do it together. And actually about the open source approach, 

the concept is that you put the source code into the internet, 

there are hundreds of people interested in the subject that 

like to sit down to work on it to find a bug and to improve 

the program and that may be true in other disciplines or in 

other fields. But I donít really see these hundreds of people 

in our field to sit down and look at the source code and come 

up with suggestions.

Harper: Well, you have at least identified seven different 

groups or so that are interested in this type so there is an 

interest here, that is why we are here. And there are as far as 

making commercially available another way to go about 

licensing it, you are not just limited to an open source type 

model, you are thinking about the GPL or something like 

that, you could go to a Berkeley type model which is you 

release the source code, anybody can take it, and commercialize it but they donít necessarily have to release it which 

is how Windows NT got started. 

Thoemen: I think part of it, just say it again.

Harper: Well, there are different approaches you can take as 

far as this but I am personally of the opinion, especially as 

public institutions, putting up this research, that you should 

submit it and let the community look at it and try to improve 

upon it. Yes, the first place to start would be to contact those 

who made the model in the first place and then start working 

on it together. But I think that it would advantageous for us 

all to be able to see the code. Transparency will foster 

innovation.

Thoemen: I agree partly, if there is the big interest in the 

source code it is possible to approach Fred or me or Chunping. I am sure there will be ways that you can look at the 

source code and improve it. I just donít see that if I put my 

source code into the net, that there would be such a big 

group of people that look at it, I just see the danger if I do 

this, then at the moment there would be no commercial value 

of the model anymore. And I just can speak for myself, I am 

interested in the other opinion at the moment. It is just not

feasible to do that.

Kamke: This question of distribution of the software for 

these models and making it available to anybody is something that we have done at Virginia Tech for a long time. 

One name that wasnít discussed up here in addition to 

Balazs Zombori and I, is Layne Watson. Layne Watson is a 

computer scientist at Virginia Tech, he has absolutely no 

interest in wood or wood composites, but is very much 

interested in modeling, he is also very much interested in 

getting sophisticated simulation models out to people who 

need to use them and that is the reason why we went in the 

direction of having a web-based simulation model. We came 

to the conclusion that this is not a money maker. The source 

code is not a money maker; the program is not a money 

maker. However, if we want to continue to make it available 

to people and to improve upon it, we have to have some way 

of financing those things, particularly web-based. You canít 

put something on the web and leave it there, without maintaining it. And so what we are struggling with is where we 

can find a way to make this really available to people and yet 

still be able to cover our expenses of maintaining a high end 

computing system as the host for this model and how we can 

continue to make changes to it as new information comes 

available. That is our big challenge. How do we do that? 

What is the market for these types of software codes if you 

were to sell it? Well, very limited, not very many people are 

going to want to buy it. So what are you going to charge, 

$10,000? Well that is a drop in the bucket in what

it cost to develop it. Of course, most of that money came 

from the Federal government; the rest of it came from the 

state government. In your case I think there was some private industry funds involved with it as well, Heiko. But that 

is money already spent, itís gone, you are not going to recover that. No one is going to get rich off these software 

codes. I like the idea of an open source code approach and I 

think some value is still there in regards to the interface. You 

have the source code but the interface really makes it usable 

and that is basically our conclusion now with the WBCSim 

website. Perhaps we should just make the code available to 

others, how much we could support that is a different question. I donít know about you Heiko, but I am not a programmer by training. Balazs Zombori was certainly not a 

programmer by training. We have all learned this by the seat

of our pants as we went along and the end result is a code 

that is perhaps not the most efficient in the world, but it does 

work. 

Harper: That gets back to my point. Most of us here are not 

programmers. That is why I feel it is important that we make 

source code available. So if someone has a model, and the 

code is available, they can work with their own computer 

scientist/programmer and improve it. 

Pablo Garcia: Heiko, I have a question about your modeling. In that simulation that you showed, you have a very
105

interesting, and maybe you can explain theoretically, because it is a theoretical model, why weíve got a kink. I noticed that for the certain time when you were tracking three 

time through the pressing time, you have got a change in an 

inflection point in your vertical density profile. Yes, that 

one.

Thoemen: Okay, so which point were you talking about?

Garcia: There, you see at about 15/16 or something and 

through the thickness you have got the slide formation, you 

have got the first peak in the vertical density. Yes. That one.

Thoemen: Okay the question is where do these bumps come 

from? Okay we can just go further back with our simulation 

and if you look at this moment so it is like after 8 meters, 

there is quite a high pressure on the mat and that is the 

moment where these layers get the temperature and the 

moisture condition, so they are softened and so that this 

model load is enough to further densify it so it is basically 

far after the main pressure had been applied.

Gracia: Whatís your wood rheology model based upon 

at that point? Is it a function of temperature or moisture 

content?

Thoemen: Which rheology model is the function of moisture content, temperature and density? No, okay, maybe we 

can go back. Okay, as I said we have to know the coefficients and to determine these coefficients it was very time 

consuming work that was done by Funhaus. He utilized a 

miniature press, where he could inject steam or climatized 

air so he could adjust the moisture content and the temperature within this little press. Then he did all kinds of different 

pressing schedules so he determined these different coefficients. He determined elastic response of the mat to pressure, 

he determined viscous behavior and so on. And he did this 

for the whole range of temperature and moisture contents. 

And this data is the basis for our simulations.

Chunping Dai: I guess I am one of the three modelers in 

this room and maybe I should comment a little bit about the 

open source or closed source issue. My answer to that I donít

know. But for those of you who are familiar with Forintek, 

we are sort of a membership based company jointly sponsored by the government and also by our industry. So we are 

obliged to keep some projects only available to members. 

But on the general subject though, I believe that some of the 

models created, such as the hot pressing model, are so complex. You make it open source or you make it available to 

any company and you still have to, like Heiko was saying, 

train them to be able to run it, understand it, and use it more 

efficiently. In that sense, I donít know if open source is the 

best idea or the best way to go. In Forintek, we agree that it 

is not going to be a money maker, but we try to use it as a 

tool for us to provide our service for our members. For 

example, if someone has a question regarding pressing or 

doing a new product and we actually use it for ourselves so 

that we educate ourselves. Therefore, we are becoming more 

efficient in delivering our service to our members and so it is 

something that, it is a program that we developed ourselves, 

we are very familiar with, and we find that is the most efficient way for that to be utilized. But anyway, I do have a 

question about the other, go back to that tester, not the 

ABES, the other one, and yes the IB one. Can you sort of 

explain in more detail to us, how do you prepare a 

sample? Is it a closed system or is it open because it is so 

small, as people talk about? Can you contain the steam, you

know, or all that kind of stuff. Can you elaborate on that? And 

for me, I am particularly interested in OSB, and did you do any

work on OSB? Or the person who has developed this, has any

work been done on OSB?

Thoemen: Okay, good question. Last question first, we have 

a diameter of 10 centimeters, 100 millimeters, so it is designed for MDF and flakes, it is not designed for OSB, and 

we knew this from the beginning on. The way the system is 

operated is you first form your mat in this big tube with this 

diameter and you precompress it so that it fits into this press. 

Then you put the cylinder into the press and position the 

platen exactly to fit into the cylinder. Originally it was intended that this whole system would be sealed so that you 

could create higher gas pressures in there for the beginning.

We ignored this option and we just used different moisture 

contents (i.e. mat MC) and different temperatures of our mat 

and of course different heating platen temperatures. The idea 

would be if we would have an instantaneous temperature rise 

in the core so what we actually have is kind of a slower 

increase of the temperature but what we like to have is one 

defined temperature. That is the reason we make it relatively 

thin, so it is like 4 millimeters thick so have relatively little 

time delay. My assistant, Christian, did this work some 

weeks ago to test whether he could use it as a sealed system. 

At this moment, we still have some problems due to wear 

from extensive use of our equipment. It is not sealed anymore so we have to replace the cover and the inside of it. So 

that would be the next step but so far we just will get atmospheric pressure. Does that kind of give you an idea of how 

this apparatus is operated?

Dai: Did he try to measure the gas pressure?

Thoemen: Yes, Christian did and he told me just two weeks 

ago that at the moment he does not get an increase in gas 

pressure.

Siqun Wang: Can you go back to visual hot press slide, the 

simulation, the interface, yes that one? You use the pressure 

control for simulation and I wonder how do you control the 

thickness of the final product or maybe you can give me 

more background about continuous press. What is the general

way to control.

Thoemen: Okay, it is good question. The question is 

whether it is realistic to assume a pressure control or load 

control and the answer is actually in the in-feed section you 
106 

kind of have thickness control because the in-feed rate dictates the thickness of your mat. So early on, it is not pressure 

controlled but thickness controlled. Then usually in industry 

you have a period where the load is controlled and you donít 

care about the thickness; you just care about the load. Then, 

at the later stage, where you kind of calibrate the thickness 

of your panel, you again have thickness control. So far, we 

can switch between 100% load control and 100% thickness 

control but it is already implemented. That is one of the next 

steps that we are doing here, it is already implemented, that 

we can switch between these two so that we can specify a 

minimum thickness and a load so if the mat touches the 

minimum thickness, then it is not further compressed so we 

just have to change the user interface to incorporate that but 

it's already incorporated in the model.

Frihart: In your models, do you assume that all the flakes 

are oriented the same direction or do you do it like in OSB 

where you have a middle section that is cross oriented?

Thoemen: Okay, the main differences between the different 

orientations is the permeability so the permeability is dependent on the direction and that is incorporated. If you look 

here you can choose, lets say we want to simulate a three 

layered OSB mat and you can click on the three layered and 

you can specify the core layer, lets say that across OSB 

strands with perpendicular alignment and you have to specify the surface layers with parallel OSB strands and then the 

permeability data they are available for both directions.

Frihart: Do you consider that thereís different adhesives in 

the middle layer and therefore has different properties as far 

as cure rates?

Thoemen: Okay, good question. In this version of our 

model we donít have the adhesion included so thatís the next 

step, that is what Christian and I are working on and that is 

the next step to incorporate it into the model. So to answer 

your question, no, so far we cannot consider different resins. 

We just donít have the data for different adhesives at least 

for OSB, we have this data for UF and PF resin for flakes 

but not for OSB. I like to refer to talking about the heat and 

mass transfer, that work started more then 20 years ago;

talking about rheology, that worked out for us about 8 years 

ago; and talking about the adhesion, that started like 3 years 

ago and thatís just in a preliminary stage. So cure is not in 

our version of our virtual hot press. That is not incorporated.

Garcia: I was thinking about Davidís question. I developed 

a heat and transfer mass model for my Ph.D. I have no interest in commercializing. I suppose it's open source 

code, it's ugly looking because I am not a programmer, but 

when making my program, it was most useful to me. I would 

have liked to simplify the code, but what was more interesting was algorithms, mathematical equations, and stuff. I 

developed mine off wood drying models because it is similar 

drying of hydroscopic materials, which more specifically I 

developed it off of a drying model by Patrick Perry and Ian 

Tuner. I took their equations and modified them a little bit. I 

think scientifically that it might be a little more valuable to 

just sort of share that stuff because Balaz Zombori had a 

really good deravation in the thesis and all of us, last time we 

met, were talking about that absorption isotherm history. 

That is one of the brick walls that we all came across and we 

all worked our different models in slightly different ways 

and there is where you can increase your efficiency by just 

changing your algorithm a little bit. For example, Chuckís 

question there, the way it would work my algorithms in 

terms of resin cure and stuff like that, I didnít dabble too 

much into it but I have made allowances for it in the equation system basically as its some black box in the function of 

time. What links from there out of resin cure is that you have 

got mass source, if it's a condensation reaction where all of a 

sudden got a source of vapor, that will vary with time 

depending on the kinetics. To me that is the black box, so I 

just leave that as another arbitrary function of science that 

can be thought of in the future. Different resin distributions 

would mean different initial conditions and different 

functions that would now be a function of time as well as 

space because what you are talking about is that the core 

would have different resin curing kinetics so this function 

now becomes some arbitrary function that you could take it 

or someone real cleared up on adhesives could take this stuff 

and see it as a function of x, y and z and time and those are 

the ways that I think algorithm could be very useful.

Cloutier: I would just like to come back to the open source 

point that we have. At Lavel we do have a research center on 

numerical methods called and this group, Marcell and myself, we are members of that group. We had an analyzed 

discussion on the open source, should we go open source or 

should we keep the code in house and so on. The code we 

use is still in house made and the policy we came up with is 

that the code is made available to anybody who wants to use 

it for research purposes and this way we know who it is 

distributed to and the condition to use it is to send back the 

developments that people make with that code to the center 

and that is the way that works fine for us so far. To put the 

code in open source, I see some difficulties, first the support 

that you must give to the users that want to use it as you 

mentioned, they want to use it, you can have a lot of questions to answer. We donít have time to do that and we donít 

have resources to do that. Also for users to download such a 

code there is a lot of work to do to understand the code and I 

donít think it would be so useful for someone outside so my 

suggestion or my point is that I think it could be interesting 

to have such a group interested in that topic to share and this 

way itís a common code and you can download on your 

computer, do your own development with it and maybe send 

it back for the comment of the community of the people who 

are working with it. Although I was very interested by the 

parameters, the idea you mentioned about the common 

database, I think would be very helpful also.
107

Thoemen: Maybe just one example that comes into my 

mind talking about open source, I think all of our works are 

based on the work of Phil Humprey. He put his source code 

at the end in the appendix to his Ph.D. thesis. So it is available to everybody and I am almost sure that nobody used 

this code. You did? Okay, then I was wrong. I can say from 

my own perspective that when I started to work with Phil, I 

decided to start from scratch, I got the concepts in endless discussion with him but his source code was written in 

Fortran. I wanted to do it in C++, so his source code was not 

a help for me, it was much more the discussions and talking 

about the concepts and so I agree with Pablo that exchanging 

algorithms or black boxes was more helpful probably.

Jong Lee: In case you need the webpage address, I have one 

here so I am going to pass around later and the presentation 

you made on the rheology, so far I have been dealing with 

the rheology and the simulation of vertical density profile 

about six months and I have kind of a problem to deal with 

determining the questions involving the rheology because if 

we use the single layer mat, which has less variation in the 

properties in the mat, we donít have any problem running 

the three layer hot pressing simulation model but in case you 

deal with the three layer mat structure we come with more 

variation at the different positions of the mat then we end up 

crashing the simulation model, so I cannot run the 

model again completely so somehow I just need the equations for the rheology model so if you can translate some

technology in your Van Haas German dissertation they 

would really appreciate it.

Thoemen: Okay, the rheology model is no secret, we are on 

the way to publish it and it is written in my Ph.D. thesis as 

well. I think that there are two different things you are talking about, one is the numerical stability and the other one is 

the rheological model. I am pretty much aware of these nasty 

crashes of the program and that you have to work some 

weeks to find a way to get around it, but probably in this 

case it doesnít have too much to do with the rheological 

model itself.

Lee: Well, I have been running with the old equations that 

you provided in your dissertation at the end in the appendix, 

all different moisture contents and temperature conditions 

and you determined equations but that didnít work with the 

OSB mat. When you deal with you know quite a high variation of the mat structure.

Thoemen: Okay, in my Ph.D. thesis, I just provided the data 

for MDF so there are no OSB data in there so far.

Lee: Right, so it didnít work for the OSB mat.

Thoemen: So which data are you talking about?

Lee: You mentioned on the presentation that material MDF, 

particleboard and OSB rheology data is available? So, I want 

to see if I can get some OSB values.

Thoemen: Okay, I derived this data from the data from Van 

Haas. I sense that is one of the big problems that his publications are mainly in German. I think there is one in English 

but most of them are in German, and his Ph.D. is in German 

as well. So that is one of the problems, we can talk afterwards and of course I can give you the data.

John Hunt: I would like to talk about your modeling as it 

goes. We expressed the difference of the U.S., which is primarily a batch system, solid platens, you know, constant

heat, you know, steam heat or oil heat, whereas your continuous press is able to have different sections and zones of 

heating and pressure and the cooling, it seems, at the end. I 

have a question in regard to when you go in the laboratory to 

model this process, what kind of press do you use? You have 

this small sample, are there any batch wise systems that 

simulate the continuous process?

Thoemen: Okay, that is a good question whether we can 

simulate a continuous press on a small batch lab press. In a 

way you can change, you can edit the pressing schedule 

from a continuous press so that the pressure profile or the 

thickness profile, what you cannot do is you cannot change 

the temperatures. From what I know is, for example the 

cooling, it is difficult to simulate a continued cooling press 

in a small batch press. I think that Metsu has one lab press in 

Sweden where they are relatively flexible but it really 

doesnít match the real situation and then of course the other 

problem is the size of the press so you have much higher gas 

pressures due to the bigger size of your mat in a continuous 

press but I donít consider this as a big problem because if we 

simulate it and we realize our simulation matches the situation in the batch press then we can run the model for a continuous press or for a bigger press and we can kind of be 

somewhat confident that we get the right predictions for the 

continuous press.

Hunt: You mentioned the one that goes, in Sweden you 

said, where was the one that you said has a batch small one?

Thoemen: It is Metso I am talking about, it's in Sweden. 

Actually I havenít been there but I just heard about that and 

they have a little continuous press. I am not really sure 

whether they use the little continuous press for their modeling or whether they also have a batch press where they can 

just insert let's say cold water through the platens to cool 

it on.

Hunt: How do they cool? I am curious, in the continuous 

press do they cool it with water in this thin belt?

Thoemen: Youíre talking about the real press.

Hunt: The real press, the continuous press, is that water 

cooled?

Thoemen: There was a nice picture in, was it Chunpingís or 

Siqunís presentation, basically what they do is they have the 

steel bed running through the whole press but they have the 
108 

roller carpets and that does not run through the whole press 

but it runs through lets say 2/3 of your press and then it takes 

off and there is a separated circuit for the cooling zone. So 

you have your steel bed going through but the roller carpets 

that are much heavier so you couldnít cool it down so fast so 

that is they reason they separate it, and so then in the end of 

the press in the last say 1/3 of the press they have separated 

heating or cooling systems so they can heat it or cool it and 

they can do it either with water or with oil. Maybe one last 

thing, if we talk about cooling we donít really talk about 

temperatures like in a refrigerator, we talk about heating 

circuit temperatures of about 80ñ85 degrees.

Hongmei Gu: I have a question about modeling, I guess we 

have been talking the whole day today about the theoretical 

modeling. In mathematical and analytical modeling, there 

are two ways to go. One is regression (i.e., statististical) 

modeling from the experimental data where, for example, 

you perform regression analysis. The data is usually from the 

real tests or from industry. The other way to go is mathematical modeling. So I would like to hear you comment on 

how these two approaches right now in the wood composite 

area go and I am more curious about how the mathematical 

model predicts the results.

Thoemen: Okay, that is a very good question about different 

modeling approaches. We just talked so far about analytical 

or theoretical model and what you refer to are statistical 

models. I am not sure whether you actually talk about like 

SPOC that is a system from Sympocom from ATR, that 

statistical model for process control and Metso has a similar 

approach. Actually, I had two slides about it in my presentation and I removed them last night to avoid over-running my 

time because I was scared of Jerry and his 5 min warnings. 

Briefly, these are two completely different approaches, so 

what a statistical process control model does, it takes the 

input parameters, it takes the output parameters, and it kind 

of treats the process itself as a kind of black box. Input 

parameters could be temperatures, pressing conditions, mat 

moisture content, material and pressing conditions. Output 

parameters could be the IB or any property of the panel. 

They use statistical methods to get a link between the input 

parameters and the output parameters. And they have established such a model or such a link and then they can run 

their model, they can input the input conditions and then 

they can predict lets say the IB, the bending strength. If you 

look at our approach, an analytical model, it is completely 

the other way around. We focus on the process itself and we 

use input parameters later, we donít need the input parameters for our model development but we need them if we want 

to make simulations so it is kind of a completely different 

way of doing it. And you are asking about the reliability of 

your predictions or the quality of your predictions. Of course 

there are discrepancies between what you simulate and what 

you measure, actually I think I showed some of the graphs, 

they are not too bad. Like for the temperature development 

you can get relatively good estimates of the real temperature 

situation in your press. If you come towards simulating lets 

say the IB of course we are still relatively off, okay talking 

about the statistical modeling of course they match much 

better, the situations so if they have a good model then it 

should be possible to match the IB for example within what 

they say within some percent.

Karl Englund: I was wondering has anybody done a sensitivity analysis on either the statistical model or the analytical 

model to see which variables are most important for your 

response variables?

Thoemen: Okay, maybe someone else wants to answer this?

Ted Laufenberg: I guess earlier I was eluding to that. I had 

used Phil Humphreyís fortran model in conjunction with 

Phil, he didnít have it up at OSU yet, but we ran a sensitivity 

analysis of all the parameter, vertical permeability, horizontal permeability, looking at really mundane data, and wondering whether the assumptions he made, whether we could 

improve on them by developing better data, better parametric 

data for these physical constants. We found some that were 

really in need of improvement, we based a proposal to the 

USDA on that analysis. The sensitivity analysis showed that 

horizontal permeability was the one we were most in need 

of. Prediction of the density profile, I am pragmatic about 

this, I stuck in the final density profile on a number of cases 

to see whether or not going immediately to the final density 

profile and just squishing the entire mat made a difference in 

the behavior and it didnít. I guess some of that could be 

derived from the vertical permeability not being a factor and 

thermal conductivity was not highly influenced by that 

density profile either. We did do the sensitivity analysis in 

conclusion and found that the curves were most needed. The 

permeability data was lacking and the change in thermal 

conductivity as a function of relative humidity and of pressure. Moisture content was also needed to be supplemented, 

but we never produced the results from that because I was 

moved to a different project right after it got funded and I 

handed it off to another researcher.

Thoemen: What you are asking is would anybody change the 

material properties and how much does it affect the outcome. 

I think all of us of course did it, some of us published it. I 

think Balaz did write some of his findings in his thesis, I 

know that Alain talked about it already, that you did tests 

and I completely agree about the permeability that horizontal 

permeability is important and that vertical permeability is not 

important. What I found was that thermal conductivity is 

very important so if you change the thermal conductivity by 

a factor of 0.1 then you get a strong impact on your results 

and maybe just one small thing here we have one button 

advance and you can do exactly as that what you are saying 

so you can shift up the thermal conductivity by 10% or 30%, 

whatever you want, and so any of the material properties you 

can change by a certain number and look how does it affect 

the simulations. 
109

Kamke: Just to follow up on that comment, Balazs Zombori 

did include a sensitivity analysis in his dissertation. That is 

also going to be included in a publication and I am told it is 

going to be out in the April 2004 issue of Wood and Fiber 

Science. 

David Marr: A question for Fred I have wanted to ask. 

Does the Virginia Tech program allow you to shift between 

force and displacement control?

Kamke: The program that we have is only a position control 

program at this time.

Chip Frazier: I would like to talk more and learn more 

about the rheological modeling and ask what types of viscoelastic functions or viscoelastic models that you have used 

or other folks are using?

Thoemen: Okay, I can answer for my part and maybe Chunping or Fred can answer for their approach. What we basically use is Burgerís model, that is this part of the 

rheological model and we added this one component to it so 

basically we account for elastic deformation so its instantaneously recoverable, we account for viscous deformation that

is not instantaneous, not recoverable, we account for delayed 

elastic deformation that is recoverable not instantaneous and 

by adding this last element we also account for instantaneous 

but not recoverable deformation.

Frazier: Now I understand. At first I didnít, but once you 

mentioned that, I remembered. So no one is using more 

models such as KaulareñWilliamsñWatts equation approach 

or the other common viscoelastic examples. You are just 

using these simple spring and dashpot models not the phenomenalogical approach where folks make the measurements and just fit them to various equations.

Thoemen: I am not sure about the second approach but we 

just use this relatively simple model. It is complex enough 

for us at the moment but maybe if we move on towards more 

reliable predictions maybe we need some more elements. 

Maybe we need another approach, that is another option of 

course.

Frazier: Another question along this line, I just have kind 

of a new interest in rheology of the wood cell wall and one 

of the big issues is whether or not you are working in a 

linear region or in a nonlinear region and I guess where a 

nonlinear response is that a big issue or does that not matter 

in how you have to model your material?

Thoemen: Of course we are in the nonlinear region and itís 

probably our approach that helps us get along with it so we 

model the response of our material for each or we calculate 

the response of our material for each time, so step by step. 

Maybe Fred can comment on it, probably if you first derive a 

differential equation, then solve this differential equation, 

itís much more complex to account for a nonlinear material.

Hunt: In your model or any of the other models we talked 

about the industry concerns, they have 20 different varieties 

and flavors and flakes that they put in their board. You know 

from oak to aspen. When you put your model here do you 

tweak something when going from a fast grown low density 

aspen compared to a slow growth suppressed something or 

other? Do you go into your model and tweak it that way or 

how do you change your parameters based on your furnish?

Thoemen: Okay, itís a question about the effect of the problem that you have different material and the material has 

different properties and basically as I mentioned toward the 

end of my presentation that the ideal solution would be that 

we would have a range of a set of different measurement 

techniques that we can relatively easily determine these 

material properties so that if you come with a new material, 

just give it to me and it is measured within a short period of 

time. We donít have that at the moment so we have to live 

with it. Basically we assume a standard material. At the 

moment we have a standard MDF, standard core layer flake 

material, surface layer material, and OSB material. If we 

know, for example, that the stiffness of the material that we 

have is higher then we can adjust the coefficients but that is 

a very rough way of doing it at the moment. It works, it is 

not the optimum so maybe the approach that was proposed 

by Steiner and Chunping, that could help so from, if I understand that approach correctly, then you donít have to care 

too much about the structure you just have to determine the 

property of your cellular wall material for the temperature 

and moisture content range and that is probably much easier 

than to determine these five coefficients for the whole temperature, density, and moisture range.

Gu: In one of your slides for the future research on the 

optimization problems, you mentioned how to achieve a 

specified density profile, you talked about the other way 

around, to model the problem which is a known specified 

density profile which is good for properties or performance 

of the product and then you go back the way to see how to 

simulate this processing order to achieve this profile. Can

you comment a little bit more on this thought? How do you 

do that?

Thoemen: Okay, so far we cannot go this way around. I 

think Chunping mentioned the independent and the dependent variables and so far we can use independent variables as 

input parameters and we simulate the dependent variable. It 

would be nice if we could do it the other way around so if 

we could input the dependent variables like the density 

profile and could simulate the independent variables but that 

requires an iterative procedure and that requires a lot of 

calculations you have to run your model again and again. 

One difficulty is also that you have such a wide range of 

different possibilities so you have to tell your models somehow in which direction to go so at the moment it is not possible to simulate that way around in order to solve the optimization problems.
110 

Kamke: A question about optimization, the program that 

Balazs Zambori developed for hot pressing process was not 

designed to optimize but simply to take a collection of inputs, run it through the code and then come up with the 

collection of outputs. However, there is software publicly 

available optimization routines that will work with independent codes. At this website, WBCSsim, we have an 

optimization program, a public domain optimization program, that we have linked with some other simulation models. We have a rotary drying model, for example, that was 

never designed to be an optimizing code, but it is when it is 

linked with this other optimizing code called DOT. We 

simply linked them together. The way DOT works is it is an 

iterative approach, but its an intelligent interactive approach. 

It runs your simulation over the range of conditions that your 

simulation is defined for and then based on that it makes 

intelligent decisions as to what should be the next guess that 

it makes. It still takes a long time, but you donít use that 

much time in trying to come up with the optimum solution. 

We havenít yet tried to link it with the hot pressing model 

simply because the hot pressing model does require a lot of 

computation time and would probably tie up our workstation 

for hours, if not days if you try to run the optimization on it.

Gu: Right, Iím really interested in that. I know that for 

process control techniques, there is another technique called 

Expert system. You probably heard about it and that is similar to what you are talking about the optimization software. 

If you have a set of simulations run in a range of condition, 

and then next time the system will give you the prediction of 

what to do next step through this Expert system, like an 

expert predicts. That is what I am curious about. Before I 

am giving up, I have a question for you maybe also for 

Chunping and Heiko about this modeling. You guys have 

been doing this modeling for quite a long time. I know there 

are some limits, you have mentioned a couple of, one is that 

Heiko mentioned about the computer limits that we really 

canít do anything about that. We will leave that up to the 

computer person, but in our field, wood science field, what 

do you think about the future for the modeling and somebody mentioned about the properties which I really think 

that is a big issue, material properties. But other than that

what do you think the limits right now for our modeling?

Kamke: I agree with Heiko in the limitations that he mentioned to modeling, I do think that the very important limitation is our knowledge about some of the physical properties, 

and we just donít have enough data for that. That is an area 

that we are concentrating our efforts on at Virginia Tech 

right now. We are not going to be able to do it all. There is

just too much to do. We are concentrating on strand products 

right now and that is not going to be very helpful for people 

that want to work with particles and fibers. The limitations 

based on hardware as Heiko says, will take care of themselves and really it may not be a limitation in certain cases 

right now depending on how sophisticated you want your 

model to be. If you donít try to simulate everything under 

the sun, all possibilities, all these mechanisms, if you are 

willing to accept some simplifying assumptions so you donít 

have to make so many computations, then we have plenty of 

computing power right now. If you donít think you need a 

3-dimensional simulation, you cut your computing time 

down by a power of ten. Heiko gave us a demonstration 

right here on a laptop computer. It depends on how much 

you are willing to limit your model and what assumptions 

you are trying to make. That is not such an important limitation in my mind. I think the more difficult limitation is our 

lack of data on physical properties.

Thoemen: I agree that the computing time, in the long term, 

will not be the limitation. The lack of property data will be. 

There are actually two ways we have to go. One way is we 

have to measure those data if they are not available. But the 

first way should be to assemble those data that are available. 

I realize that in the German literature, there is some data 

available that are just not recognized. I am almost certain 

that in the Russian literature there is a lot of that none of us 

is aware just because of the language problem. So measuring 

this property data is one thing, finding them is the other 

thing, finding the existing one. And if I think about the heat 

and mass transfer, I am relatively confident with our model 

predictions. Talking about density profile development and 

talking about development of the mechanical properties, I 

think we definitely need a lot of work regarding the model 

development itself.

Hunt: Iíd like to just make a comment about this, optimization problems. As we try to go towards a performance based 

and in this statement here, I know it is an example, but in the 

statement it says that density drives all performance and I 

guess I would like to see that we say that density may be a 

function to get you there but it is really a performance based 

and what is it that I need, you know percent of glue, density 

is just one component of my performance you know it would 

relate to percent, all the variables that would be necessary to 

describe the performance parameters that youíre looking for 

and that would be the optimization that you are really looking for and not just the density profile. Although right now 

we use it because itís the best predictor of properties. 

Thoemen: So what you are saying is that basically we want 

one performance. Letís say we want one specific bending 

strength of our panel and we can achieve it through the 

density profile or through choosing the right adhesion content. Maybe there is some other parameters and if we think 

about that then these optimization problems really become 

difficult.

Hunt: Right, it expands it but I realize this is just an example and we use density as a predictor but the next step would 

be, at least I would propose it, how do we define performance based on things other than just density and try to see 

how that drives our process.
111

Thoemen: I am sure that the mechanical properties will be 

an issue or a challenge within the next years concerning 

modeling.

Kamke: This question of Johnís about what do we really 

need to predict here with these simulation models, I agree 

with you 100%, we havenít taken these far enough yet. I 

mean we are still working on these and we donít quite have 

these right yet. All that we are able to do up to this point is 

predict density profiles and maybe predict some degree of 

cure or degree of bond strength development. But what does 

that mean in terms of the actual engineering properties of the 

final composite? I believe that is as big a challenge as it has 

been just getting to this point. We are about three years into 

a project just with strand composites to try to do a small part 

of that. Elena Kultikova (Ph.D. student at Virginia Tech) is 

trying to predict some mechanical properties, some bending 

properties and some shear properties. A lot of that is empirical based and that means that when she is done it's still going 

to be somewhat limited to that range of conditions she was 

able to include in her experimental work. I would like to see 

some effort done that could be more fundamental based. In

other words, if you know the physical and mechanical properties of the raw materials going in and then know something 

about how those properties may change as a result of the 

manufacturing process, you should be able to predict the 

mechanical properties of the final composite. But that is a 

huge task.

Dai: If nobody talks, Iíll talk. Fred and I have been fighting 

for the microphone, but can I have a little bit of time now, 

thank you. Haha. Just kidding. But yes, about this whole 

issue of dealing with the property side you know looking 

into the effect of species and the strand geometry other than 

density and also Chip you are talking about the rheological 

side of things, like you are doing tests on glue and ideally 

you would like to play that with two strands or you donít 

want to play with a bunch of strands, you want two strands 

so you can isolate the problem because its easier to observe 

things and eliminate a variation of other unpredictables. That 

has sort of been the driving force for at least my approach in 

a way that we are studying. The program was developing a 

model for mat structure so itís a model where you can actually monitor each strand as it is being laid up, I mean their 

positioning, the orientation, and you can define the property 

of each strand and then you can input you know with a 

percentage of dimensions or species and then you can observe how thatís deformed and how that structure changes 

and you know effect your permeability and rheology and 

that kind of stuff. So we have achieved some success with 

that and I presented some of that this morning and Heiko 

mentioned that. The first thing that we were able to achieve 

that yes, we have such a structure, sort of a geometric computer simulation program now and we can link that to the 

compression behavior overall mat to individual strands, if 

you just test individual strands like Fred (Kamke) and Wolcott did, compression tests of individual strands, you give 

me the property and I can plug into the mat formation model

and I can tell you what kind of a deformation and strand 

stress relationship for mat. Then also because the mat is not 

being uniform I can also tell to what extent localized stress 

varies within a mat and there was some mention about the 

horizontal density variation, the effect of that on dimensional 

stability. I know it is a long way to go to predict that but I 

think my idea is that we have got to start with trying to 

understand the structure of the material first and you are 

talking about strands, basically, that define the length, 

width and thickness and how they might vary, and thatís it. 

Computer might take a long time to calculate but I think the 

idea is there. We are able to also link the viscoelastic property of a mat to the viscoelasticity of single strands and also 

we are working on how the mat structure changes, how that 

relates to permeability and we have achieved some of that 

already and we are going to publish some of that pretty soon. 

From my standpoint I find it is really useful to start with 

modeling structure of a mat but it is going to take a longer 

time but it's really starting to sort of link everything together 

now, the sort of global approach and the structure approach 

and then link that to various other individual tests on strands, 

on glowing and that kind of stuff so anyway I found that 

would be sort of a direction that I would be pursuing to 

understand the structure of the material.

Thoemen: If I can just comment on it, it's just a discussion at

the moment about the two different approaches, a more 

fundamental approach and a more empirical approach and I 

think probably we have to go both directions. It just depends on 

what we want. The more fundamental approach I agree that 

is in the long term what we need to understand the structure 

and maybe we also need it so that we can easily account for 

changes in the raw material so that we donít have to do this 

whole range of measurements but the drawback of this more 

fundamental approach is that you are kind of at one level of 

uncertainty in your predictions and we have to be aware of 

that and so if you, for example, want to run your model as a 

process control to it, maybe that the more empirical approach in the shorter term is the more realistic approach to it.

Dai: We have a similar type of a model like yours and now 

we are also working on the structure model. One benefit I 

found is that understanding the structure you are able to 

predict the average response of the mat or average let's say

permeability of a mat because you have to understand the 

local structure and then you can predict the voids and that 

kind of stuff.

Gu: I just want to keep comment on that structure model 

and, I feel like I completely agree with that, because I am 

interested in structure models. Start with the fundamental 

structure of wood to build up the model and Fred probably 

knows that I did a little bit for my Ph.D., and I am still doing 

that here. We are right now dealing with the solid wood, not 

into the composite yet but I really found promising to start 

with the fundamental structure of wood but I am not sure 
112 

about the structure of wood start from this point of the solid 

wood compared to the, you were talking about, the structure of the composite. Do you think we should start from the 

basic fundamental structure of the wood itself or the structure of the composite, the fiber or strand?

Dai: Yes, going back when I did my Ph.D. and studied in the 

early 90s with Paul Steiner, we sort of studied with modeling 

the formation of a mat, you know the structure model of a 

mat so in a way it is structure of the composite before itís 

been densified. The structure aspect you talk about is the 

structure of a wood, I mean whatís good about understanding the structure of the composite is that you can sort of 

break it down. Let's take example of OSB. You can look a 

localized structure it's really you are dealing with two 

overlapping strands and then you can isolate that, you can do 

tests on just the two strands, overlapping strands. And if you 

magnify that, you can look at compression or the property of 

the solid wood itself and there has been a lot of work done 

by the wood physics, just look at solid wood itself and then 

you look at the bonding and that so that is the unique thing 

about that is that you are able, if you understand the structure, you are able to link all these things together. And if I 

am wood physics I would start with wood structure, that 

would be information not only useful for understanding 

wood itself but useful as an input for the model or for wood 

composite products.

Winandy: Well, I think that we reached that time where we 

need to take a break for about 8 to 10 min and try to come 

back just after 3:00 with some more questions and answers 

and a summary and we will try to end. Many of you are 

probably ready to leave.
113

Summary and Wrap-up 

Jerry Winandy: Alright, are we ready to rock and roll? I 

want everyone to stand up; we are going to do the chicken 

dance. Haha. First of all, while I am thinking about it, I want 

to ask everybody that was here that referred to published or 

unpublished work that either you yourself did or that you 

knew of from a co-worker or a collaborator, if you mentioned a report, please send me the citation or if you can, the 

whole paper so I can get it right.

Fred Kamke: Can you wait to do that until after we receive 

the transcripts?

Winandy: I am sure there will be plenty of cases for that. 

Also feel free to, in the proceedings, when you see the written transcript, if a stroke of genius hits you, oh I remember 

Smith did this in 1903, you know, you can refer to that and 

that makes you look really smart in the proceedings so I give 

you the authority to improve on that. I would like to go over 

some of my notes on what I saw and heard. Maybe that will 

spur some questions. I was scribbling like crazy when I 

heard someone in their presentations or in their conversations that went on that identified what I thought were real 

research needs. Something that anyone of us might have the 

possibility of going back to our shops and working on either 

individually or collaboratively and if we could just answer 

one or two questions in a collaborative type environment this 

conference I think would be major success and I know we 

will get there. They are in no particular order and I apologize 

if I cannot read my own writing. 

Research needs identified at workshop: 

Basic questions remain on vertical density profile, whether it 

occurs at closure or after closure, and some of the fundamental characteristics that drive it need to be defined. 

Need to determine the effects of fiber versus strands on 

vertical density profile formation. 

Need to define the recovery and the rate of that recovery of 

the mat after press opening. 

Need to define wood based composite properties as a function of location in the press and correlating lab presses to 

industrial presses. 

Need to define vertical density profile as a function of location in the press and correlating lab presses to industrial 

presses. 

Need to define how elastic recovery, i.e., relaxation is a 

function of compaction ratio and wood density. 

Need to identify how fines interact with mat consolidation 

and mat permeability, which seems to be unknown but 

seems to be a general assumption on the part of most folks 

that the more fines, the more permeability. But do we have 

specific documentation on that? 

From the session on adhesives and resins, we need to define 

what the relationship is between resin cure and mechanical 

property development. 

Need to develop better understanding of how and how much 

resin cure is a function of time, temperature, and distribution 

on the flakes, chips, or fibers. 

Need to address how we monitor and measure the development of that resin cure? 

Need to address questions on how creep is influenced by 

resin type, different types of resin (PF, pMDI, etc.). 

Agreed that fundamental work is needed to define creep at 

identical process times, temperatures and times, rather than 

at traditional press schedules for one resin or another resin 

where we say this creep is more than that, what happens at 

identical process conditions. 

Need to define the effect of controlling resin flexibility on 

the eventual bond strength and how that affects the properties of the board. 

Need a definition of which properties are best evaluated by 

one test methodology versus some other test methodology, 

for example, is creep more a function of flexural MOE and 

not a function of IB, that was said as a statement that we 

know that, but do we know that. 

Need to identify how environmental factors interact with that 

property evaluation as a function of test method. Maybe 

creep is more influenced in one environment by one property 

and in a different environment by some other property. Do 

we know that? 

When we are dealing with press strategies, we need to learn 

how to control the press to optimize resin cure and strength 

rather than using some predetermined recipe. 

Need to develop alternatives to pressing, or at least the fundamental science to know if they have any real applicability. 

Need fundamental data on how strand size through the thickness of the mat affect permeability and the problems it might 

create with blows. 

Need to define resin cure as a function of heat energy as 

opposed to defining it as a function of temperature. 

On the session on simulation, simulation of mechanical 

properties will proceed over the next decade as has simulation of temperature and pressure. Can we work together to 

accomplish that in a more efficient manner? 
114 

Main challenges are improvements in quality of model predictions and the reduction in processing execution time. 

There is an on-going concern about the benefits and the 

drawbacks of open source code versus prioritized based 

codes. This concern should be discussed in an open forum; 

consideration should be given to intellectual property rights, 

both individual and corporate versus the potentials for 

improvements in simulation efficiencies through open 

source codes. 

A series of simulation methods have been developed, each 

model has many very reliable prediction capabilities, and 

each may have some less reliable functions. Either as a 

group of developers or a group of users or a combination of 

the two, we probably need to define what works in each one 

of the systems and what doesnít work so that we can 

improve the efficiency and reliability of those modeling 

simulations. 

Need to learn how to reverse engineer simulation models to 

allow process optimization in the future, especially as computer tools become more available to us and processing 

enables that. 

Simulation models are limited by our lack of understanding 

of fundamental physical properties maybe itís the time to 

develop a consensus across the whole industry that we need 

to return to a certain amount of basic fundamental research 

on wood physics. 

Now I guess I would like to open it up, if anybody had any 

comments. 

Al Christiansen: One of the things that we were talking 

about before, there seems to be a lot of work being written 

up in foreign languages. Maybe we have to work at trying to 

find way of translating that material and getting it into general circulation somehow, we could save ourselves a lot of 

time.

Winandy: I have one comment on that. We had a conversation among a small group of us yesterday in that for example, the laboratory has a whole storehouse of Russian literature that was translated by a man who worked here for 

20 years who did a great job and its all in our library. But I 

would almost contend that very few people know it exists, 

that the availability of that knowledge exists. We have a 

policy at the laboratory of putting everything we do, since 

about probably 1995 or 1996, on our Internet, on our webpage. And we are trying to put the critical papers from the 

past on the webpage too but we only have one or two people 

doing this and they can only do so much so we need to 

prioritize what needs to be on the web. One thing I will say 

to you, if any of you ever know of an old FPL report that 

you think is critical to your work, let us know. We do have a 

policy that priority in putting old papers on the web is given 

to people who ask. So if you need Stammís paper from 

1938, we will put it on the web or at least it will be a higher 

priority paper.

Kamke: Those Russian articles you are talking about, is that 

searchable in a database somewhere or how do we know 

what is contained in those articles?

Winandy: My understanding is that most of those articles 

are in our bibliography and we have our FPL library catalog 

available on the internet at 

http://www.cybertoolsforlibraries.com/asp/usdafs.html. Then 

click on ìSearch the catalog.î The FPL library catalog is 

under construction and not everything is there yet, so ask for 

assistance. Our librarian might be able to identify what else 

we have. If you just call our main FPL switchboard (608-

231-9200) and ask for the library, they will help you get the 

answer. Anybody else have any questions?

Ted Laufenberg: There are a lot of ideas for new projects to 

undertake and I am afraid that you all have brought in way 

too many ideas so we all need to get ready for homework.

Winandy: I think 60 h a week of our working will be plenty. 

If no one has anything else then I would like to move to 

closing. First of all, I would like to thank Professor Fred 

Kamke, pioneer of the Wood Based Composites Center at 

Virginia Tech for helping us organize this workshop. It has 

been terribly informative for me; I hope it has been informative for you. I want to thank the presenters whose hard work 

in their research and in preparing their talks, it sets the stage 

for the success of this workshop. As Ted just alluded to, we 

have got dozens and I mean dozens of ideas that we can each 

take back and work on. I think that it shows that if we get 

together and we freely throw out ideas, these ideas grow into 

something that is really viable. I am sure that in reading the 

literature in 5 or 6 years, there are going to be a number of 

things that come out of this workshop. I think that many of 

us will point our finger at something and say ìI remember 

when we talked about that.î But for now, thank you to the 

presenters, but now for the hook. I have one paper, I am 

expecting six, so please get your papers to me as soon as 

possible. That gives me time to prepare the audio transcriptions and try to turn this thing around before next spring and 

you are all out fishing or something. Number two, I would 

like to thank Steve Schmeiding, the FPL AV specialist for all 

his efforts in AV recording and all the general technical 

assistance in getting all these talks transferred from Virginia 

Tech, the University of Tennessee, Forintek, and the University of Hamburg. You know as they were coming in, one 

person said ìyour FTP site works wonderfully.î That was 

exciting to hear and it is all because of Steve. So thank you 

Steve. I would also like to thank Viththal Talati who in the 

back there has been instrumental in keeping our audio online 

so that you can hear it. Special thanks to Dave Marr and 

Katherine Freidrich for running the hand microphones during the discussions. Thanks to Sue Paulson, Nancy Keen, 

Sandy Morgan and George Couch for running the 
115

registration desk, organizing the lunches, breakfasts, and 

Wednesdayís social event at Captain Billís. Especially want 

to thank them for how well they enabled us to keep modifying the schedule on the fly. They were just wonderful to 

work with in that way. Thanks also to Linda Caudill at Virginia Tech for all her preliminary work in inviting each of us 

and for just organizing this meeting and helping with the 

hotel arrangements and all that. And finally, and I think most 

importantly, thanks to each of you for attending this and 

helping us synthesize what I think will become a state of the 

art, that I think that many of us will show to our students or 

to the junior engineers that work for you or scientists that 

work for you and say go forth and do this good work. So 

thanks to each of you, have a safe trip home, thanks for 

coming to Madison.

Kamke: One more comment, I think you forgot to mention 

yourself. We all should give Jerry a hand for coming up with 

the idea of having such a conference and doing such a great 

job in leading us the last few days and keeping us all on 

course. That was a big help, it wouldnít have happened 

without you Jerry, thanks a lot. 

Winandy: Class dismissed. Thank you all, and now go forth 

and do great things related to the fundamentals of composite 

processing.

Literature Cited 

Ballerini, A.A. 1994. Cure of aqueous phenol-formaldehyde 

resin at elevated water vapor pressures. Ph.D. thesis. 

Blacksburg, VA: Virginia Tech. 

Ebewele, R.O.; River, B.H.; Myers, G.E. 1994. Behavior 

of amine-modified urea-formaldehyde-bonded wood joints 

at low formaldehyde/urea molar ratios. Journal of Applied 

Polymer Science. 52: 689ñ700. 

Ganev, S., Cloutier, A. 2003. Determination of the diffusion coefficients and modeling of the hygro-mechanical 

warping of particleboard. Final Rep. Gaithersburg, MD: 

Composite Panel Association. 

Geimer, R.L., Christiansen, A.W. 1996. Critical variables 

in the rapid cure and bonding of phenolic resins. Forest 

Products Journal. 46(11/12):67ñ72 (see page 70). 

Irle, M.A.; Bolton, A.J. 1988. Physical aspects of wood 

adhesive bond formation with formaldehyde based adhesives, Part 2: Binder physical properties and particleboard 

durability. Holzforschung 42: 53ñ58.

Laborie, M. 2001. Investigation of the wood/phenolformaldehyde adhesive interphase morphology. Ph.D. thesis. 

Blacksburg, VA: Virginia Tech. 

Lenth, C.A. 1999. Wood material behavior on severe environments. Ph.D. thesis. Blacksburg, VA: Virginia Tech. 

Lenth, C.A.; Kamke, F.A. 2001. Equilibrium moisture 

content of wood in high-temperature pressurized environments. Wood and Fiber Science 33(1): 104ñ118). 

Rasthauser, J.W.; Haider, K.W.; Hunt, R.N.; Gustavich, 

W.S. 1997. Chemistry of PMDE wood binders: Model 

studies. In: M. Wolcott (ed.) Proceedings of the 31st International Particleboard/Composite Materials Symposium. Pullman, WA: Washinton State University Press. p. 161ñ176. 

Stamm, A. 1964. Wood and cellulose science. New York, 

NY: Roland Press Company. 

Strickler, M.D. 1968. High temperature relations of grand 

fir. Forest Products Journal. 18(4): 69ñ75). 
116 

AppendixóParticipant List 

Bill Augustine 

Research Project Leader 

Trus JoistñA Weyerhaeuser Business 

TJ Technology Center 

P.O. Box 8449 

Boise, ID 83707-2449 

Phone: (208) 332-4211 

Fax: (208) 332-4220 

Email: augustb@trusjoist.com 

Marcia Vidal Bastias 

Ph.D. Candidate 

Laval University 

Department of Wood and Forest Sciences 

Ste-Foy, Quebec 

G1K 7P4 CANADA 

Phone: (418) 656-5851 

Fax: (418) 656-5262 

Email: mavib2@agora.ulaval.ca 

Felisa Chan 

Research Scientist 

University of New Brunswick 

Fsc of Forestry & Environmental Management 

P. O. Box 44555 

Fredericton, NB 

E3B 6C2 CANADA 

Phone (506) 452-6156 

Fax (506) 453-3538 

Email: fchan@unb.ca 

Al Christiansen 

Chemical Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9289 

Fax: (608) 231-9592 

Email: achristiansen@fs.fed.us 

Craig Clemons 

Research General Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9396 

Fax: (608) 231-9852 

Email: cclemons@fs.fed.us 

Alain Cloutier 

Professor 

Laval University 

Department of Wood and Forest Sciences 

Ste-Foy, Quebec 

G1K 7P4 CANADA 

Phone: (418) 656-5851 

Fax: (418) 656-5262 

Email: alain.cloutier@sbf.ulaval.ca 

Chunping Dai (Presenter) 

Senior Scientist-Wood Composites 

Forintek Canada Corporation 

2665 East Mall 

Vancouver, BC 

V6T 1W5 CANADA 

Phone: (604) 222-5736 

Fax: (604) 222-5690 

Email: dai@van.forintek.ca 

Karl Englund 

Research Engineer 

Wood Materials and Engineering Lab 

P.O. Box 1806 

Washington State University 

Pullman, WA 99164-1806 

Phone: (509) 335-6259 

Email: englundk@wsu.edu 

Ted Frick, Technical Manager 

Bayer Polyurethanes 

Wood and Foundry Products 

100 Bayer Road, Bldg. 1 

Pittsburgh, PA 15205 

Phone: (412) 777-2944 

Fax: (412) 777-7864 

Email: ted.frick.b@bayer.com 

Katherine Friedrich 

General Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9398 

Fax: (608) 231-9852 

Email: kfriedrich@fs.fed.us 
117

Charles Frihart (Presenter) 

Project Leader 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9208 

Fax: (608) 231-9592 

Email: cfrihart@fs.fed.us 

Pablo Garcia 

Post-Doctoral Research Associate 

University of Tennessee 

Forest Products Center 

2506 Jacob Drive 

Knoxville, TN 37996-4570 

Phone: (865) 946-1878 

Fax: (865) 946-1109 

Email: pgarcia@utk.edu 

Hongmei Gu 

Post-Doctoral Fellow 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9574 

Fax: (608) 231-9852 

Email: hgu@fs.fed.us 

David Harper 

Post-Doctoral Fellow 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9398 

Fax: (608) 231-9852 

Email: dpharper@fs.fed.us 

Shane Hartzke 

Research Technician 

Grant Forest Products, Inc. 

P.O. Box 160, Hwy 11N 

Earlton, Ontario P0J 1E0 

CANADA 

Phone: (705) 563-2460 

Fax: (705) 563-3213 

Email: shartzke@gfp-inc.com 

John Hunt 

Research General Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9433 

Fax: (608) 231-9852 

Email: jfhunt@fs.fed.us 

Fred Kamke (Presenter) 

Professor and Director 

Wood-Based Composites Center 

Sustainable Engineered Materials Institute 

Virginia Tech 

Brooks Center (0503) 

1650 Ramble Road 

Blacksburg, VA 24061 

Phone: (540) 231-7100 

Fax: (540) 231-8868 

Email: kamkefa@vt.edu 

Ted Laufenberg 

Research General Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9433 

Fax: (608) 231-9852 

Email: tlaufenberg@fs.fed.us 

Jong Lee 

Research Scientist 

Sustainable Engineered Materials Institute 

Virginia Tech 

1650 Ramble Road 

Blacksburg, VA 24061 

(540) 231-5219 

Email: jolee6@vt.edu 

Zipporah Magee 

Technical Service Representative 

National Casein Company 

601 W. 80th Street 

Chicago, IL 60620 

Phone: (773) 846-7300 

Email: cgolab@nationalcasein.com 

David Marr 

General Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9586 

Fax: (608) 231-9852 

Email: dmarr@fs.fed.us 

John Noffsinger 

Technical Manager 

Weyerhaeuser Company 

P. O. Box 487 

Sutton, WV 26601 

Phone: (304) 765-4220 

Fax: (304) 765-4285 

Email: john.noffsinger@weyerhaeuser.com 
118 

Nian Ou 

R&D Manager 

J. M. Huber Corporation 

P.O. Box 670 

Commerce, GA 30529 

Phone: (706) 336-3123 

Fax: (706) 336-3164 

Email: wpnho@huber.com 

Rick Rammon 

R&D Manager, Wood Adhesives 

Georgia-Pacific Resins, Inc. 

2883 Miller Road 

Decatur, GA 30035-4088 

Phone: (770) 593-5921 

Fax: (770) 322-9973 

Email: rmrammon@gapac.com 

David Ruth 

Product Development Specialist 

Boise Cascade Corporation 

P.O. Box 2400 

White City, OR 97503 

Phone: (541) 826-0225 

Fax: (541) 826-0219 

Email: DaveRuth@boisebuilding.com 

Nicole Stark 

Chemical Engineer 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9392 

Fax: (608) 231-9852 

Email: nstark@fs.fed.us 

Heiko Thoemen (Presenter) 

Federal Research Center for Forestry and Forest Products 

University of Hamburg 

Leuschnerstr. 91 

21031 Hamburg 

GERMANY 

+49-40-73962-603 

Email: thoemen@holz.uni-hamburg.de 

Siqun Wang (Presenter) 

Assistant Professor 

University of Tennessee 

Department of Forestry, Wildlife, and Fisheries 

Knoxville, TN 37901-1071 

Phone: (865) 946-1120 

Fax: (865) 946-1109 

Email: swang@utk.edu 

Jim Wescott 

Director of Research 

Heartland Resource Technology 

c/o USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 469-0256 

Fax: (608) 231-9592 

Email: jmwme@aol.com 

Jerry Winandy (Host, General Chair) 

Project Leader ñ Performance Engineered Composites 

USDA Forest Products Laboratory 

One Gifford Pinchot Drive 

Madison, WI 53705-2398 

Phone: (608) 231-9316 

Fax: (608) 231-9852 

Email: jwinandy@fs.fed.us