Patent Publication Number: US-2005138891-A1

Title: Monolithic hurricane resistant structural panels made from low density composites

Description:
RELATED APPLICATIONS  
      This application claims the benefit of provisional application Ser. No. 60/512,546 filed Oct. 17, 2003, the contents of which are fully incorporated herein by reference. 
    
    
     U.S. GOVERNMENT RIGHTS  
      This invention was made under a project entitled Affordable Composites from Renewable Sources sponsored by the National Science Foundation (NSF) and the Department of Energy (DoE). 
    
    
     FIELD OF THE INVENTION  
      This invention relates to a lightweight composite structure and more particularly to a lightweight monolithic composite structure and the method for manufacturing such structure for use as a structural element including a roof in a hurricane resistant building.  
     BACKGROUND OF THE INVENTION  
      Over the past fifteen years, the United States has experienced three major hurricanes, Hugo (1989), Andrew (1992), and Iniki (1992), causing over $27.5 billion dollars cumulatively in damage to insured property. Typically, when houses are exposed to hurricane forces, the roofs are most susceptible to damage, followed by walls, and then foundations. The most common roof damage is the loss of cladding or sheathing (tiles, shingles, etc.) resulting from the very high suction pressures that develop at the roof/wall interface. Once the sheathing is lost, the roof no longer acts as a diaphragm, and the lateral load carrying ability of the structure is compromised. In addition, the loss of sheathing can result in costly water damage, as particularly noticed with hurricane Andrew.  
      Efforts to develop materials of construction that will provide the necessary resistance to hurricane force winds include hurricane resistant shingles of the type disclosed in U.S. Pat. No. 5,822,943 issued Oct. 20, 1998 to Frankoski et al. and assigned to Tamko Roofing Products Inc. The proposed solution involves the use of composite shingles that include a substrate including a scrim bonded to a mat coated with filled asphalt and granules.  
      An alternate approach is to use large precast composite panels as disclosed in U.S. Pat. No. 6,668,507 issued Dec. 30, 2003 to Blanchet on an application first published Jun. 13, 2002. This patent discloses a precast composite building system usable for walls, roofs, and floors of buildings, comprising a concrete composite panel element having embedded steel, I-beams, wire mesh, embedded plates, and steel tension reinforcement bars interconnected vertically, horizontally, and angularly by columnar elements rigidly fixed to the supporting foundation, embedded into the panel elements affixed to a transverse steel beam so as to form a perimeter tie-beam connection structure to which additional floor, roof, and wall elements are attached, forming a unitary, superior, load-bearing structure.  
      Such methods of construction are time consuming and require the use of numerous crews of highly skilled tradesmen to complete each segment of the project. In order to render such construction more economical, and to allow a home to be completed in a shorter time frame, various alternative construction methods have been developed. For example, so-called modular homes have been constructed which use pre-fabricated sections, e.g. roof trusses, walls, and sometimes entire rooms, which sections are interconnected on-sight so as to form the finished structure. However, such structures require numerous modifications in order to make them storm or hurricane resistant.  
      U.S. Pat. No. 6,185,891 issued Feb. 13, 2001 to Moore, attempts to resolve the strength and cost issues by a new method of building construction which eliminates traditional framed wall and trussed roof construction methods.  
      The disclosed method of construction utilizes a polymer bonded foam-concrete structural composite building material formed from a styrene foam having a fiber reinforced, ethylene-vinyl acetate containing concrete emulsion integrally cured thereto, resulting in enhanced impact resistance and enhanced ability to withstand tensile load. The resultant structure has enhanced thermal insulation properties. The disclosed invention is further directed to a foam panel interface construction which renders the resultant structure impervious to wind damage at velocities in the range of about 155-310 mph. However the disclosure teaches using foam panels comprising reinforcing channels which contain steel rods in a concrete slurry. Such structure again requires on-site labor and therefore increased costs. Furthermore, the concrete and steel rods add substantial weight to the structure requiring substantial supports.  
      There is thus still a need for a structural material for use inter alia, in a roof structure that will maintain its integrity and survive normal loads which are conservatively equivalent to those with winds up to 150+ MPH, which is a Category-5 worst-case hurricane, such as Andrew. There is further need for such material that is lightweight, inexpensive, and preferably employing renewable resources.  
     SUMMARY OF THE INVENTION  
      There is, therefore, provided in accordance with the present invention a composite panel structure having a three dimensional structure with thickness less than about 22.5 cm, overall density less than 0.2 g/cm., and exhibiting a global modulus greater than 1.2 GPa and stiffness greater than 15 kN-m 2 . The panel structure comprises at least two fiber-reinforcing mats interconnected in a spaced substantially parallel configuration by a skeletal web. A closed cell foam fills the spaces between the panels. The closed cell foam may be natural, synthetic, or combinations thereof. The mats comprise a plurality of fibers in a resin matrix. The resin is a thermosetting low viscosity resin, and may also be natural, synthetic, or a combination of the two.  
      Preferably, the composite panel composition comprises resin in an amount of between about 30 and 40 wt %, fiber mats in an amount of between about 20 and 40 wt %, and foam in an amount of between about 30 and 40 wt % of the total composite weight. The structures exhibit a high thermal resistance R-value due to their high content of foam insulation.  
      These structures may be used as roofs or walls and are manufactured by VARTM or similar processes as a three-dimensional monolithic planar, or curved shape, designed to resist removal by wind forces, depending on their intended function as walls or roofs. They can also be produced in other planar or curved designs for other applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective schematic representation of a sectioned portion of a panel showing the internal supporting beam structure of the panel.  
       FIG. 2  is a schematic view representing a cross section of a preferred composite panel prepared in accordance with the present invention.  
       FIG. 3  is a schematic view representing a vacuum assisted resin transfer molding (VARTM) process and resin flow into a fiber bed in accordance with the present invention.  
       FIG. 4  is a schematic view representing a monolithic house roof using a composite panel in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The invention will next be described with reference to the figures wherein same numerals are used to identify same elements in all figures. The figures illustrate the invention and are not intended to act as engineering or construction drawings, therefore they are not to scale and do not include all elements that may be included in such drawings, as inclusion of such elements would unduly clutter the drawings.  
      Referring next to  FIG. 1  there is shown a three dimensional cross sectional view of a panel  10  constructed in accordance with this invention. The panel comprises at least two fiber-reinforcing mats  12  formed from a plurality of fibers embedded in a resin matrix. The composite structure  10  further includes a skeletal web structure  18  connecting the fiber-reinforcing mats  12  to one another in a substantially parallel configuration. Closed-cell foam core  20  is sandwiched between the fiber-reinforcing mats  12 .  
      Many natural fibers, including, but not limited to, flax, cellulose, chicken feathers and hemp, are suitable, in addition to synthetic fibers, including, but not limited to, glass, carbon and Kevlar® fibers. Low-cost recycled newspaper and cardboard are good sources of reinforcing cellulose fiber.  
      In one embodiment, the fiber pre-form bed was made from a stack of papers and was successfully wetted and infused with a compatible resin, thereby forming a composite of more than 50 weight % paper. The resins suited to this application typically consist of both natural and synthetic co-monomers, with optimal blends selected for their reactivity, compatibility with the fibers, cure properties and appropriate viscosity suited to the infusion process. The tensile and flexural modulus and the tensile and flexural strength of these composite materials increased to about 5 times the neat resin modulus or strength. The strength of these structures typically exceeds 25 kN, and the composite skin modulus exceeds 5 GPa.  
      Plant oil-based resins (soy, corn, linseed, sunflower, genetically engineered high oleic, etc.) are preferred. A number of natural oil based resins are known. See for example, U.S. Pat. No. 6,121,398, and/or Can E. Kusefoglu S, Wool RP. “Rigid thermosetting liquid molding resins from renewable resources (“2) copolymers of soyoil monoglycerides maleates with neopentyl glycol and bisphenol-A maleates.” J. Appl Polym Sci 2002; 83: 972.  
      A preferred resin is commercially available Acrylated Epoxidized Soybean Oil (AESO) known as Ebecryl 860 from UCB Chemicals. The AESO resin is mixed with styrene in an optimized 2:1 weight ratio and the resin mixture mixed with 3 weight % initiator, cumul peroxide, commercially available as Trigonox 239A from Akzo Nobel, and 0.8 weight % catalyst, Cobalt Naphthenate (Mahogany Co.). The styrene content at 33.3 wt %, or a 2:1 AESO to styrene ratio, gave maximum properties at minimal styrene content. At 33.3 wt % styrene content, the storage modulus was 1.1 GPa with a loss modulus of 193 MPa and a glass transition temperature of 66° C. The neat resin properties can, however, be further improved with higher levels of acrylation and addition of maleic anhydride to the hydroxyl groups on the triglycerides.  
      Closed cell foam  20  may be any one of a number of commercially available closed cell foams. It is selected primarily to provide high insulation value (thermal or acoustic, depending on the intended application for the composite panel) and to impart three dimensional form and rigidity to the panel. A preferred foam is Elfoam, T300, manufactured by Elliott Company, Indiana. T300 foam has a density of 44.85 kg/m 3  (2.8 lb/ft 3 ) and operating temperature range from −297 to +300° C. It is a closed cell polyisocyanurate foam that is chemically similar to, but higher performing than, polyurethane foam. Outstanding thermal and moisture resistance coupled with light weight make this foam a highly effective insulating material.  
       FIG. 2  shows an alternate composite structure. The composite panel  10 ′ comprises two substantially parallel fiber reinforcing mats  12 ′ in a resin matrix  16 ′. The mats separated by a skeletal structure  18 ′ and the space between the mats is again filled with a closed cell foam core  20 ′. Between the skeletal structure  18 ′ and the mats  12 ′ as well as between adjacent sections of the skeletal structure there is shown a high porosity fluffy fibrous layer  38  whose function will be explained below. Otherwise the structure is substantially the same as the one described hereinabove in connection with  FIG. 1 .  
      Preferably, the composite panels are produced using vacuum assisted resin transfer molding (VARTM) process, which is a variant of vacuum-infusion RTM (Resin Transfer Molding) in which one of the solid tool faces is replaced by a flexible polymeric film or vacuum bag  24 , as represented in  FIG. 3 . As shown in  FIG. 3 , the process draws resin  16  into a dry reinforcement on a vacuum bagged tool, using only the partial vacuum V to drive the resin  16 . The process increases the composite  10  mechanical properties and fiber  14  content by reducing void percentage, when compared to other large-part manufacturing processes, such as hand lay-up. As one of the tool faces is flexible, the molded laminate thickness depends in part on the compressibility of the fiber-resin  14 - 16  composite before curing and the negative pressure of the vacuum V.  
      Composite panels  12  were manufactured from AESO and various natural fiber  14  reinforcements using the VARTM process. The composite panel  12  specimens were manufactured with the dimensions 30.5×30.5×0.635 cm (12×12×0.25 in). The preform  12  is vacuum bagged  24  on a one-sided mold  26  as shown in  FIG. 3  and the resin  16  is drawn into the preform  12  (as represented by flow arrows F) under the negative pressure created by the vacuum V. Breather cloth  28  and peel ply  30  are part of the molding device, and tool plate  32  is typically a table surface. A bucket  34  contains excess resin  16 .  
      The resin  16  is cured at room temperature and gels after approximately 3 to 5 hours. However, the panel specimens  12  were left under vacuum V overnight to improve consolidation and then demolded. Full vacuum V, if reached, is equivalent to atmospheric pressure (14.7 psi or 101.3 kPa). This level of pressure may seem to be low, but it is very significant when parts with large surface areas (such as a house roof) are molded compared with the same pressure to be achieved mechanically using a body force. Bagging a large part like a structural panel  12  and applying vacuum V at one side gives a uniform negative pressure over the whole part  12  equivalent to 101.325 kPa. Simple calculation shows that exerting this pressure over a 7×10 meter roof panel would require a body force of 709,275 kg.  
      In order to successfully manufacture the large composite panels according to this invention using VARTM, the resin must be able to permeate into all parts of the composite structure. Thus permeability becomes an important factor. Permeability is defined as the volume of a fluid of unit viscosity passing through a unit cross section of the medium in unit time under the action of a unit pressure gradient. It is a constant determined by the structure of the medium. The natural fiber mats  12  used in this work can have random or oriented fibers  14 , with or without binders; they can be processed using an air laid or wet laid process and the fiber length can be varied. The mat permeability plays a key role in determining the fiber content of the resulting composite. To overcome flow problems, other porous fibers  38  may be used in small quantities along with the main reinforcement fibers  14  as illustrated in  FIG. 2  to form the composite panel.  
      Composite panels were made out of the 14 different fiber mats listed in Table 1.  
               TABLE 1                          Natural Fiber Reinforcements used in AESO Composites                     Chart Reference   Description               Flax/PET 40/40   40% Flax 40% PET 20% Starch Binder, supplied by           Cargill Ltd.       Flax Mat 60/40   60% Flax 40% Binder, supplied by Cargill Ltd.       Flax Mat 85/15 20 oz.   85% Flax 15% Binder 20 oz., supplied by Cargill Ltd.       Cellulose 200 g/m 2     Air Laid 200 g/m 2  84% cellulose 16% binder, supplied           by Concert Fabrication, Canada       CTMP Pulp   Chemical Thermal Mechanical Pulp, supplied by M&amp;J           Fibretech a/s, Denmark       Fluff Pulp   Wet Laid Fluff Pulp Low Density Mat 100% Cellulose 640 g/m 2 ,           supplied by Rayonier       Chemically Treated   Chemically Treated Pulp Contains a Hydrophobic       Pulp   Debonder, supplied by Rayonier       Caustic Treated Pulp   Caustic Treated Pulp 100% Cellulose High Porosity Mat           used for Filtration Products, supplied by Rayonier       Cellulose 150 g/m 2     Air Laid 150 g/m 2  82% cellulose 18% binder, supplied           by Concert Fabrication, Canada       Flaxcraft Hemp   550 g/m 2  non-woven Hemp, supplied by Flaxcraft       Flaxtech Flax   Flax-Distribution of flax fibers with varying lengths and           low binder content, supplied by Flaxtech       Newspaper   Newspaper       Recycled Paper   110 g/m 2  Recycled Paper from Cardboard Boxes,           supplied by Interstate Resources, PA, USA       Chicken Feathers Mat   97% chicken feathers ground to about 6 μm diameter           and 8 mm long fibers with 3% low molecular weight           polymeric binder, solid feather density is 0.8 g/cm 3  [40]                  
 
 The storage modulus, E′, the loss modulus, E″, and the glass transition temperature, T g , were measured at a temperature range of 35.0-150.0° C. for the various room-temperature cured Acrylated Epoxidized Soybean Oil (AESO) natural fiber composites. The glass transition temperature was obtained from the maximum point of the tan δ curve. The storage modulus E′ of the neat resin was 1.1 GPa and with natural fiber reinforcements, E′ increased up to more than 5 GPa at approximately 50 wt % fiber. The highest E′ values were obtained for cellulose derived from newspaper or recycled paper. Significantly, the recycled paper was the cheapest of all the natural fibers examined in this work and is therefore an excellent candidate for use in high-volume large structures, such as houses. Values of the loss modulus were observed at two different temperatures corresponding to a temperature of 37° C. and also at the temperature at which the loss modulus achieves its maximum value. Similar behavior was observed for the loss modulus; the neat resin had a loss modulus of ˜190 MPa, and with fiber reinforcement it increased to about 430 MPa. 
 
      The structural or material damping of a composite material may also be analyzed using DMA testing. Tan δ is the ratio of the loss modulus to storage modulus or the ratio of the energy lost to the energy retained during a loading cycle. Tan δ values were measured at 37° C. and also at its maximum value (the glass transition temperature). The most significant result was obtained from the cellulose composites with a maximum tan δ of approximately 0.3. This result indicates that natural fiber (cellulose-based) reinforced composites have good structural damping properties. Due to their reduced weight, environmental survivability, and noise suppression, several automotive applications are possible.  
      Preferably, low-cost products are used to make composite materials. Recycled paper from corrugated cardboard boxes is a cheap source for cellulose fiber. Old newspaper was initially considered and tested and, despite the flow problem associated with making large parts using newspaper, the resulting composites exhibited very good mechanical properties at the lab scale. The positive results with recycled newspaper led to a more porous recycled (cardboard) paper which showed no flow problems, and the resin perfectly infiltrated and bonded into the paper bed. Mechanical testing showed that these natural composite materials are suitable for housing applications such as roofs, walls, and construction lumber.  
      To study the possibility of manufacturing the roof represented in  FIG. 4 , beams of the proposed material were first manufactured and tested for strength requirements. Lengths of composite panels similar to the composite panel illustrated in  FIG. 1  were made in the form of beams to test the suitability of this composite for use in roofs. The overall dimensions of the beam  10  are 1067×89×203 mm (42×3.5×8 in); the face sheets  12  as well as the webs  18  have a nominal thickness of 6.4 mm (0.25 in). The foam core  20  is required for the manufacture of the beam  10  and is integral to it, but while it contributes significantly to thermal and sound insulation, it is not expected to contribute significantly to the strength and stiffness of the beam. The beam was designed as a flexural member to carry loads transverse to its longitudinal axis.  
      Four-point bending tests were done on each of the structural beams  10 , which were loaded to failure. This testing gives load versus deflection results and strain measurements. All data were obtained using a data acquisition system. The specimen  10  was first loaded reversibly three times in the elastic region, and then was taken to failure. The different beams were tested to failure and Table 2 shows three beams made of recycled paper from old cardboard boxes and three different interlaminar or integral distribution media in a form of chicken feathers mat, one-ply corrugated paper, or one ply of woven e-glass fiber. Table 2 also presents a comparison between the beams and the three most common wood structures used in building construction, and shows that the newly developed material properties matched or superseded that of the wood structures. Using woven E-glass fiber ply as an interlaminar integral distribution media provided ductility and prevented the undesired brittle failure.  
               TABLE 2                          Comparison of Composite Beam Properties versus Typical Wood       Section Properties                                     Flexural               Beam   Rigidity - EI (kN-m 2 )   Strength (kN)                             Composite Beams                                 Recycled   12.4   24.2           paper/Chicken           Feathers           Recycled   14.8   25.8           paper/Corrugated           Paper           Recycled   19.9   25.6           paper/E-Glass Fiber                 Wood Beams                                 Douglas Fir   18.0-30.3   15.4-29.7           Spruce   16.0-25.0   10.7-24.5           Cedar   10.0-26.4    9.5-28.8                      
 
       FIG. 4  shows the use of a panel constructed in accordance with this invention as a hurricane resistant roof for a dwelling H. As shown in the figure the VARTM manufacturing process permits the production of large and if desired shaped composite panels. The roof is a monolithic panel made according to the present invention comprising a composite panel structure having a three dimensional structure with thickness less than about 22.5 cm, overall density less than 0.2 g/cm, and exhibiting a global modulus greater than 1.2 GPa and stiffness greater than 15 kN-m 2 . The panel structure comprises at least two fiber-reinforcing mats  12  interconnected in a spaced substantially parallel configuration by a skeletal web  18 . A closed cell foam  20  fills the spaces between the panels. The closed cell foam may be natural, synthetic, or combinations thereof. The mats comprise a plurality of fibers in a resin matrix. Preferably the fibers comprise a renewable resource such as cellulose and/or chicken feathers. The skeletal web  18  may also, but does not have to, be constructed by the same materials as the mats  12 . The resin is a thermosetting low viscosity resin, and may also be natural, synthetic, or a combination of the two. An optional integral weather-protection layer  22  is also shown.  
      Preferably, the composite panel composition comprises resin in an amount of between about 30 and 40 wt %, fiber mats in an amount of between about 20 and 40 wt %, and foam in an amount of between about 30 and 40 wt % of the total composite weight.  
      The composites according to the present invention may also be used as a substitute for Stay-In-Place (SIP) bridge forms, the corrugated sheets of material that span the distance between bridge girders. SIP forms are formwork for the concrete bridge deck and are designed to carry the dead load of the deck while the concrete cures. Before SIP forms were invented, wooden formwork was used for the same purpose, however, wooden forms are labor-intensive, requiring scaffolding to be built from ground-up and then requiring removal after the concrete has cured.  
      Corrugated SIP forms are widely used today. SIP forms are made of light gauge steel, and are approximately two feet (0.610 meters) wide by four (1.22 m) to ten (3.05 m) feet long. They are screwed into angles that are welded onto bridge stringers.  
      SIP forms manufactured using narrow strips of the composite structure of the present invention can replace the currently used steel forms. The composite beam forms allow the form to “breathe,” and the water to pass through and away from the concrete deck, therefore reducing the risk of corrosion. The form is biodegradable, and will break down naturally to allow bridge inspectors to examine the bottom of the deck. Finally, the forms are lightweight compared to their steel counterparts, and allow for faster installation and lower labor costs.  
      While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.