Composite Panels Made 

With Biofiber or Office 

Wastepaper Bonded With 

Thermoplastic and/or 

Thermosetting Resin 

James H. Muehl 

Andrzej M. Krzysik 

Poo Chow 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Note 

FPL−RN−0294 
Abstract 

The purpose of this study was to evaluate two groups of 

composite panels made from two types of underutilized 

natural fiber sources, kenaf bast fiber and office wastepaper, 

for their suitability in composite panels. All panels were 

made with 5% thermosetting phenol-formaldehyde (PF) 

resin and 1.5% wax. Also, an additional 10% polypropylene 

(PP) thermoplastic resin was used in half the panels. The PP 

was added to determine the effect it had on the mechanical 

and physical properties of the panels. The resin and wax 

were applied with a pneumatic spray gun while tumbling the 

fiber in a rotary blender. All fibers were formed into nonwoven air-laid mats and compression molded into 356- by 

356-mm (14- by 14-in.) composite panels, cut into test 

specimens, and tested for mechanical and physical properties. All mechanical property values were below the minimum standard established by the American Hardboard Association for medium-density fiberboard and particleboard 

(AHA 1995). However, composites with PP had more shortterm moisture resistance than did composites without PP. 

The poor mechanical test results were probably due to the PF 

resin being absorbed into the fiber rather than remaining on 

the fiber surface, insufficient cure of PF, and interference of 

the PP when used with the PF resin. 

Keywords: kenaf, office wastepaper, polypropylene, 

phenolic 

November 2004 

Muehl, James H.; Krzysik, Andrzej M.; Chow, Poo. 2004. Composite panels 

made with biofiber or office wastepaper bonded with thermoplastic and/or 

thermosetting resin. Res. Note FPL-RN-0294. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 6 p. 

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

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an equal opportunity provider and employer. 
Composite Panels Made With Biofiber 

or Office Wastepaper Bonded With 

Thermoplastic and/or Thermosetting Resin 

James H. Muehl, Forest Products Technologist

Andrzej M. Krzysik, Visiting Scientist 

Forest Products Laboratory, Madison, Wisconsin 

Poo Chow, Professor 

University of Illinois, Urbana, Illinois 

Introduction 

Kenaf is an agricultural plant consisting of a long durable 

bast outer fiber with a relatively short, low-density core fiber 

(pith). The desirable properties of the bast fiber make it 

useful for many products, although paper is the primary use. 

The core fiber has very different properties that are not well 

suited to paper products, and thus it must be separated from 

the bast fiber. However, kenaf, even though it is an underutilized fiber source, has many desirable qualities that would 

make it suitable as a fiber source for composite panels. Some 

of these qualities are that it has low density, is nonabrasive, 

has high specific properties, is easily recyclable, can be 

formed over sharp contours, is biodegradable, generates 

rural jobs, and is low cost (Sanadi and others 1997). 

Kenaf has a relatively short growing period, especially compared with trees. Plant maturity can be reached in only 5 to 

6 months. Plants generally grow from 2 to 5 m high. Given 

the right growing conditions, kenaf can reach up to 6 m high, 

with the outer bast fibers making up to 20% to 25% of the 

stalk on a weight basis (LaMahieu and others 1991). Yield 

ranges from 6 to 10 tons per acre per year as far north as 

Illinois, with higher production rates farther south where the 

growing season is longer. Once mature, however, the entire 

crop must be harvested and stored, requiring extensive storage capacity. The rapid growth rate of kenaf provides farmers with potential income if sufficient markets can be developed. Trees, on the other hand, have a minimum rotation 

period of 7 to 20 years before harvesting. However, the 

advantage of trees is that they can be harvested year-round, 

thus requiring a relatively short storage period, and they do 

not require covered storage. 

Kenaf bast fiber can be used alone, or it can be used as the 

primary component for a product and mixed with a shorter 

fiber material, such as wood fiber. This combination of 

materials is often used to make erosion control mats. In this 

type of product, kenaf bast fiber acts as a matrix fiber to 

impart strength and integrity (that is, resistance to tearing) to 

the mat. This integrity is also useful when making threedimensional compression molded panels. Kenaf fiber can be 

compression molded into composite panels when combined 

with a thermoplastic that acts as the binder (English and 

others 1997). In a study involving fiberboard made with 

kenaf, Youngquist and others (1993) reported that properties could be achieved that were nearly equal or equal to the 

minimum standards set by the American National Standard 

Institute of the American Hardboard Association for basic 

hardboards. 

Air-laid composites can be made from a variety of agricultural fibers, wastepaper, and waste plastic materials in the 

postconsumer waste stream. These composites exhibit a 

wide range of properties that should make them useful in 

numerous commercial applications and value-added products. Examples of potential products that can be made with 

the air-laid process include storage bins; furniture components; automobile and truck components; paneling for interior wall sections, partitions, and door systems; floor, wall 

and roof systems for light-frame construction; and packaging 

materials including containers, cartons, and pallets (Krzysik 

and Youngquist, 1991). 

Office wastepaper is an abundant postconsumer waste product, and paper that is not repulped is normally sent to landfills. Any further use that can be developed for wastepaper 

could add value to it and would reduce the demand on landfills. Paper can be broken down into a variety of forms, 

which allows it to be suitable for numerous applications 

including composite panels made through the mediumdensity fiberboard (MDF) process or the air-laid nonwoven 

mat process. 
2 

Objectives 

The objective of this study was to establish an understanding 

of processing and pressing techniques needed to make composites from kenaf fiber or recycled office paper. We will 

then use this understanding to further the development of the 

technology for combining waste plastic with agricultural 

kenaf fiber and shredded office paper to make durable composite panel products that are recyclable and environmentally friendly. Successful results will help broaden alternative markets for both underutilized biofiber and recycled 

office paper materials. Using wastepaper and plastic for 

value-added products decreases the amount of these materials entering the municipal solid waste stream. Increasing the 

potential uses for kenaf provides farmers with an alternative 

crop which would help increase farm income. 

Experimental Design 

This research concentrated on testing compression molded 

panels made from nonwoven mats composed of kenaf stalk 

bast fiber and office wastepapers bonded with thermoplastic 

and thermosetting resins. The experiment consisted of four 

formulations: two with agricultural and synthetic fiber and 

two with paper and synthetic fiber. The weights were based 

on ovendry fiber weight. Panels containing different formulations were compared. The details of the experimental 

design are shown in Table 1. 

Flat panels with a specific gravity of 1.0 and target thickness 

of 6 mm were produced for each experimental condition. 

Each of the four formulations was evaluated using five 

replicate panels. In total, 20 panels were made for this study. 

Materials 

The kenaf was grown on University of Illinois Experimental 

Station test plots on the ChampaignñUrbana campus. The 

kenaf was planted in early June and harvested in late 

October after the first frost. By this time, the kenaf had 

reached a height of 4 to 5 m, and stalk diameter ranged 

between 25 and 35 mm, typical for a 175- to 185-day 

growing period. The harvest averaged a dry weight of 

6 tons per acre. 

The office wastepaper was collected from the USDA Forest 

Service, Forest Products Laboratory (FPL), paper recycling 

program. This provided a clean uniform source of paper 

consisting of discarded copy paper. Nearly all the paper had 

text on it from either a printer or a copier. 

Two types of polymer fibers were used that were in the form 

of textile fibers: virgin polypropylene (PP) and recycled 

polyethylene terephthalate (PET) fiber. A virgin PP fiber 

was used for the nonwoven mat because a recycled PP fiber 

could not be located with the required specifications. The PP 

was provided by Hercules, Inc. (Norcross, Georgia), and had 

the following characteristics: 2.2 denier, 37 mm long, 

crimped, a density of 0.91 g/cm3

, and a melting point of 

162°C. The PET fiber was provided by Wellman Co., Inc. 

(Johnsonville, South Carolina). It was spun from recycled 

soft drink containers and had the following characteristics: 

6.00 denier, 51 mm long, and crimped. The PET fiber was 

not considered a binder in the office wastepaper (OW)ñPET 

panels because its melting temperature was above the temperature used for hot pressing. The purpose of the PET was 

to act as a matrix fiber to help hold the office wastepaper 

fibers together within the formed mat. 

The adhesive, Cascophen OS 707, obtained from Borden 

Chemical, Inc. (Sheboygan, Wisconsin), was a watersoluble, liquid phenol formaldehyde resin. It had a solids 

content of 56%, viscosity of 140 cps, pH of 9.5 to 11.5, and 

specific gravity of 1.233. The wax, Cascowax EW 58S, also 

obtained from Borden Chemical, Inc., had a solid content of 

58%, viscosity of 131 cps, pH of 8.25 to 9.25, and specific 

gravity of 1.0. Both resin and wax contents were based on 

solids content of total board weight. 

Process 

Fibers 

Following harvesting, the kenaf was cut into nominal 

51-mm lengths and air-dried to approximately 15% moisture 

content (MC) to prevent it from rotting while being stored. 

After all the kenaf was harvested, the bark had to be separated from the core. This is most easily done when the kenaf 

is in the green state, but because it had been dried to prevent 

decomposition during storage, moisture had to be reintroduced. This was done by exposing the kenaf to a vacuumñ

pressureñsoak cycle. The vacuum was for 15 min at 84.7 kPa 

(25 in. of mercury), and the water soak was for 30 min at 

689 kPa. The kenaf was removed from the soak tank, the 

bark was removed from the core by hand, and then the kenaf 

was air-dried again, reducing the MC to 8%. 

Table 1óPanel compositiona

Natural fiber 

content,% 

Synthetic fiber 

content, % 

Panel Kenaf 

Office 

wastepaper PP PET 

Phenolic 

resinb

(%) 

Waxb

(%) 

K 93.5 0 0 0 5 1.5 

KñPP 83.5 0 10 0 5 1.5 

OWñPET 0 83.5 0 10 5 1.5 

OWñPP 0 83.5 10 0 5 1.5 

a

PP, polypropylene; PET, polyester; K, kenaf; OW, 

 office wastepaper. 

b

Phenolic resin and wax are based on solids content. 
3

The office wastepaper was exclusively white copy paper 

obtained from the FPL paper recycling program. The paper 

had to be reduced to particle and fiber bundle form to be 

processed into a nonwoven mat. This was accomplished by 

hammermilling the paper with a screen opening of 9.6 mm. 

Following the hammer mill operation, the paper had an MC 

of 4%. 

Wax and Adhesive 

The wax and resin were sprayed onto kenaf or office wastepaper fibers in a drum-type blender using a single pneumatic 

spray gun applicator. The resin and wax were applied separately with the wax applied first. Enough fiber was blended 

in one batch so that all the panels for each board formulation 

could be formed from the one blending operation. During 

resin application, some balling of the kenaf fibers occurred 

as a result of the tackiness of the resin and the tumbling 

action of the rotating drum. One level of phenolic resin (5%) 

and wax (1.5%) was used in this study. 

Mat Formation 

The air-laid nonwoven mats used for this study were made 

on a 305-mm-wide, lab-scale, Rando-Webber (Macedon, 

New York) web forming machine. This was done in three 

stages. First, the kenaf or paper fibers were passed through a 

pin drum opener. This broke up any kenaf clumps or balls 

made during the resin blending operation and provided an 

initial blend with either the PP or PET plastic fiber. A second pass through the pin drum provided a more thorough, 

uniform blending of the kenaf or paper and plastic fiber. In 

the second stage, the blended fibers were elevated to feed 

rolls, transferred to a lickerin, and formed into a nonwoven 

mat on a fiber condenser belt. At this point, the nonwoven 

web had very low integrity and was easily damaged if handled. The third stage improved the density and integrity of 

the mat by passing it through a needler, which provided a 

vertical intertwining of the plastic fiber with the kenaf or 

paper fiber. It was then in a condition to be cut into strips 

and laid up into a mat for pressing. The mat made with 

recycled office wastepaper, but without PP fiber, had 10% 

PET fiber added to it. The PET was necessary to provide the 

mat with enough integrity to prevent it from falling apart 

when being handled, formed, and pressed. 

Panel Pressing 

Each 5-kg batch of blended kenaf, or wastepaper, was made 

into a nonwoven web 30 cm wide and approximately 15 m 

long and was then cut into 30- by 30-cm webs. Each web 

was weighed and put into like piles of webs that had a 

weight range of 10 g. The next step was to form a mat that 

would be pressed into a panel. A panel basis weight was 

calculated, and webs were removed from the piles and 

stacked to form a mat equal the basis weight of the pressed 

panel. 

The theory behind the air-laid nonwoven mat forming technology is that the fibers are laid up into a continuous web in 

a completely random orientation. In reality, however, during 

web formation, the forming equipment provided slight 

alignment of the fiber in the machine direction, that is, parallel to the direction of movement of the web. Thus, when the 

webs were laid up into the mattress for pressing, they were 

placed so that the machine direction of each web was perpendicular to the machine direction of the previous web. 

This minimized any effect machine direction and cross 

machine direction might impart to the panel properties. Mat 

lay-up was also confounded by the wide variation in web 

density requiring from as few as two to as many as six webs 

to achieve the basis weight to make up a mat. 

The mats were pressed in a manually controlled, steamheated press with platen temperature held at 197°C. Mats 

were held under pressure for 3 min at a maximum pressure 

of 12.4 MPa for kenaf panels and 8.1 MPa for panels containing office wastepaper. This was the amount of time 

necessary to reach the melting temperature of the PP in the 

core of the panels. During this time, mat core temperature 

reached 100°C within 60 to 90 s, allowing more than sufficient time to cure the phenolic resin, that is, core temperature 

above 100°C for more than 60 s. A steel frame, 356 by 356 

by 6 mm, was used to contain the composite panel during 

heating and to prevent over-pressing (that is, the frame also 

acted as press stops). One to two de-gassing cycles were 

made during the pressing cycle, depending on mat moisture 

content and amount of steam released during each degassing cycle. During the heating stage, the mat flowed to 

fill the steel frame. Nevertheless, the edges of the panel had 

lower density than the rest of the panel. Thus, the edges were 

trimmed off, and the final panel size was 305 by 305 mm. 

This allowed for less variability in density (hence properties) 

within each panel. 

Tests 

Mechanical and physical property tests were made on specimens cut from the experimental panels. Each panel was 

weighed and measured, and the specific gravity was calculated. All tests were done in conformance with the American 

Society for Testing and Materials (ASTM 1994, part B basic 

hardboard specifications) according to the specific test being 

done. 

Prior to mechanical and physical property testing, the specimens were conditioned at 50% relative humidity and 20°C. 

Three-point static bending tests to get modulus of elasticity 

(MOE) and modulus of rupture (MOR), tension tests to get 

tensile strength, and internal bond (IB) tests were performed 

on a Tinius Olsen (Horsham, Pennsylvania) model 290 

testing machine. Twenty-four hour thickness swell (TS) and 

water absorption (WA) measurements were at ambient temperature. Differential linear expansion (LE) tests were made 
4 

after equilibrating each specimen at 50% relative humidity 

(RH), 27°C, and then again at 90% RH, 27°C. 

Results and Discussion 

Mechanical and physical property data are presented in 

Tables 2 and 3. The results were statistically analyzed and 

are reported as the mean and coefficient of variation. 

Kenaf Panels 

We anticipated that the PP would help improve the mechanical and physical property values of the panels by assisting 

the PF resin in bonding the kenaf fiber together. However, 

this did not appear to be the case. The test results from the 

panels (Tables 2 and 3) were analyzed within their specific 

formulations. The mechanical properties of the kenaf only 

(K) panels were consistently higher than those of the kenaf 

and PP (KñPP) panels. The K panel MOR was 35% higher 

than the KñPP panel MOR. Similarly, MOE, TS, and IB 

values were 31%, 9%, and 35% higher, respectively, for K 

panels than for KñPP panels. This indicates that the addition 

of PP probably resulted in the reduction of mechanical properties. It may also suggest that the phenolic resin bonded 

well to the kenaf fiber but when 10% PP was added, there 

may have been some interference between the resin and PP 

or the PP may have absorbed latent heat energy in the mat 

that impeded PF resin cure. Since the PP is hydrophobic, the 

PF resin may not have bonded to the PP, preventing a good 

bond between the PP and kenaf thus causing weak areas in 

the panels at the PPñkenaf contact points. Furthermore, 

because kenaf is hydrophilic, the two materials have some 

difficulty bonding together in the first place. With only 10% 

PP fiber, there was probably insufficient PP to provide PPñ

PP bonding. Thus, only the PP and kenaf bonded. If a higher 

percentage of PP had been used allowing PPñPP bonding to 

occur, perhaps 20% to 30%, higher mechanical properties 

could possibly have been achieved compared with using 

10% PP or none at all. The addition of PP reduced the values 

of each of the mechanical properties, even causing the MOR 

to drop below the American National Standards Institute 

(ANSI) standard (NPA 1994). 

Conversely, the addition of PP enhanced some of the physical properties. The KñPP panels had 12% less TS and 2% 

less WA but 28% higher LE than the K panels. This may 

indicate that PP had some positive effect for improving 

short-term water resistance to the wood fiberñPP composite 

reducing its short term TS and WA. For LE though, the 

specimens were allowed to reach equilibrium. In these 

long-term tests, the moisture had time to fully penetrate all 

the kenaf fibers, ultimately causing higher LE in the KñPP 

panels. This indicates that PP impedes moisture sorption but 

does not prevent it. An additional factor possibly contributing to the reason the KñPP panels have better physical properties than the K panels is that the KñPP panels have less 

kenaf in them, having 10% replaced by the PP. Because the 

PP doesnít absorb moisture and swell, the KñPP panels will 

probably swell about 10% less. 

The processing procedure could also have added to both the 

poor mechanical and physical property values. When the PF 

resin was applied to the kenaf fiber, sufficient fiberñresin 

was blended at one time to make all five panels in each 

board formulation to try to minimize board to board inconsistency. Following resin application, the fiber was processed into a continuous nonwoven mat requiring it to pass 

through various transfer steps involving high air flows. The 

continuous mat also had to be cut into 30- by 30-cm squares 

and sorted before pressing. Each step tends to dry the resin 

causing it to lose its tack, thus reducing its effectiveness as 

a binder. 

Table 2óMechanical properties of air-laid panels made with phenolic resina

Propertiesb

Static bending, N/mm2

Board formulationc Specific gravity MOR MOE 

Tensile strength 

(N/mm2

) 

Internal bond 

(N/mm2) 

K 0.88 (6.1) 31.6 (35.3) 4,094.9 (33.9) 21.4 (52.1) 0.086 (45.8) 

KñPP 0.83 (5.8) 20.4 (42.7) 2,829.0 (38.8) 19.5 (10.7) 0.056 (59.4) 

OWñPET 0.91 (5.9) 10.1 (28.4) 508.0 (30.3) 10.2 (5.5) 0.036 (38.1) 

OWñPP 0.87 (13.0) 11.2 (53.8) 673.1 (78.2) 8.9 (23.7) 0.057 (52.0) 

ANSI 208.2-1994 MDF standard 0.64ñ0.80 24 2,400 0.60 

a

K, kenaf; PP, polypropylene; PET, polyester; OW, office wastepaper; ANSI, American National Standards Institute; 

 MDF, medium-density fiberboard; MOR, modulus of rupture; MOE, modulus of elasticity. b

Values in parentheses are coefficients of variation (%). c

All boards were formulated with 5% resin and 1.5% wax. 
5

Office Wastepaper Panels 

Office wastepaper panels had substantially lower mechanical 

values than the kenaf panels although no direct comparison 

was made between the two formulations. As with the kenaf 

panels, we expected the addition of PP to improve both the 

mechanical and physical properties of the office wastepaper 

panels. With the PP melting point being below the pressing 

temperature, we thought the PP would aid in the bond 

strength of the OWñPP panels compared with that of the 

OWñPET panels. Because the melting point of the PET was 

above the pressing temperature, we didnít expect that it 

would provide a bonding advantage. Although the mechanical values for both office wastepaper formulations were low, 

the panels with PP had higher values for MOR, MOE, and 

IB than the panels with PET. The values were 10%, 25%, 

and 37%, respectively. This was not the case for TS, however, where the panels with PET had 13% higher strength 

than the panels with PP (Table 2). 

The physical properties for the office wastepaper panels 

were also considerably worse than the properties for the 

kenaf panels. The addition of PP to office wastepaper did 

reduce the TS and WA by 21% and 6%, respectively, but the 

OWñPET panels had 23% less LE than the OWñPP panels 

(Table 3). 

Some of the poor performance of both the mechanical and 

physical properties of the office wastepaper panels could be 

attributed to the large flakes made by the hammer mill and 

used as furnish for the panels. These large flakes may not 

have been an optimum form to use. Another possible cause 

could have been that the resin may have soaked into the 

paper causing resin-starved bonding sites rather than remaining on the office wastepaper fiber surface where it would 

have been more effective. A third factor may be inequities in 

mat formation during pressing causing a difference in curing 

of the PF resin. 

Conclusions 

This study has helped set the baseline for future work on 

hybrid composites made from kenaf and recycled office 

wastepaper and using both thermoplastic and thermosetting 

resin. Two types of nonwoven mats were made with the airlaid process using kenaf for the main component in one set 

of panels and recycled office wastepaper in a second set. All 

panels had 5% thermosetting phenolic resin as the binder 

and 1.5% wax. Half the panels had 10% thermoplastic PP 

textile fiber added to them. In the case of the kenaf panels, 

the PP appeared to be detrimental to, or possibly interfere 

with, the bonding of the fibers because all the mechanical 

properties of the panels containing PP were lower than those 

without PP. For physical property tests, neither the PF resin 

nor the PP bonded the fibers well enough to prevent excessive swelling or expansion. 

Composite panels made from recycled office wastepaper had 

lower mechanical properties when made with 5% phenolic 

resin and 10% PP compared with panels made from kenaf. 

However, these panels had enhanced short-term moisture 

resistance compared with kenaf panels. 

The poor bonding performance of both the phenolic resin 

and PP was probably due to a number of reasons. The first is 

that the absorbent characteristics of both the kenaf fiber and 

office wastepaper drew the resin deeper into the fibers and 

away from the fiber surface. This was most likely the case 

more with the office wastepaper than with the kenaf. To 

obtain strong bonding characteristics, the resin must remain 

on the surface of the fiber and yet have a strong bond to that 

fiber. One way to prevent the resin from being absorbed into 

the fiber is to treat the fiber to keep the resin on the surface. 

Uniform resin blending is also critical for making strong 

fiber to fiber bonds. Fiber balling during blending starves the 

fiber in the center of the ball for resin and concentrates the 

resin on the fiber on the outside of the fiber balls, thus creating uneven resin distribution. Even though the fiber balls are 

broken up during mat forming, fiber with insufficient resin 

can cause low mechanical and physical property values. To 

achieve improved mechanical and physical property values, 

increased levels of PF resin and/or PP would be required as 

well as an improved method for blending the fiber and resin 

to prevent fiber balling. Further study should focus on the 

effects of pressing temperature and pressing duration on the 

process of mat consolidation and on the resin cure. 

Table 3óPhysical properties of air-laid panels made 

with phenolic resina

24-h water soak 

Board 

formulationb

Specific 

gravity 

Thickness 

swell (%) 

Water 

absorption (%) 

Linear 

expansion 

(%) 

K 

0.88 

(6.1) 

172.8 

(23.1) 

172.2 

(26.6) 

0.39 

(23.9) 

KñPP 

0.83 

(5.8) 

150.9 

(15.7) 

168.3 

(14.4) 

0.54 

(15.1) 

OWñPET 

0.91 

(5.9) 

65.2 

(7.0) 

95.7 

(4.5) 

0.33 

(29.7) 

OWñPP 

0.87 

(13.0) 

51.4 

(15.7) 

90.4 

(18.4) 

0.43 

(8.8) 

a

K, kenaf; PP, polypropylene; PET, polyester; OW, office 

 wastepaper. Values in parentheses are coefficients of 

 variation (%). 

b

All boards were formulated with 5% resin and 1.5% wax. 
6 

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