Patent Publication Number: US-2007122616-A1

Title: Panel containing bamboo and cedar

Description:
BACKGROUND OF THE INVENTION  
      Wood is a common material that has been used to construct a variety of different objects of different sizes and for different functions. It remains in wide use to this present day as one of the most widely-used structural materials, even after the development of several new species of composite materials, because of its excellent strength and stiffness, pleasing aesthetics, good insulation properties and easy workability.  
      However, in recent years the cost of solid timber wood has increased dramatically as its supply shrinks due to the gradual depletion of old-growth and virgin forests. Such wood is particularly expensive to manufacture products from, because typically less than half of harvested timber wood is converted to natural solid wood lumber, the remainder being discarded as scrap.  
      Accordingly, because of both the cost of high-grade timber wood as well as a heightened emphasis on conserving natural resources, wood-based and lignocellulosic-based alternatives to natural solid wood lumber have been developed that make more efficient use of harvested wood and reduce the amount of wood discarded as scrap. Plywood, particle board and oriented strand board (“OSB”) are examples of engineered, composite alternatives to natural solid wood lumber that have replaced natural solid wood lumber in many structural applications in the last seventy-five years.  
      These wood-based composites not only use the available supply of timber wood more efficiently, but they can also be formed from lower-grade wood species that are less commonly used.  
      Bamboo is an example of one such less commonly used material. Bamboo is a lignocellulosic material widely used throughout Asia as a building material because of its high strength, durability and excellent dimensional stability, as well as its ready supply and rapid replenishment—bamboo grows very rapidly, reaching full maturity within 2 to 6 years, while even the fastest growing wood tree species take as long as 15 to 30 years to grow to full maturity.  
      However, while bamboo has these advantages, it also shares some of these disadvantages of other lignocellulosic and wood products. Notably it is susceptible to attack by insects such as moths and termites as well as molds and fungus.  
      Termites are a cause of significant damage to residential and commercial buildings in the United States. The vast majority of this damage being caused by subterranean termites, which typically enter a building or structure from the surrounding soil to feed on wood in the building. Subterranean termites are extremely difficult to detect, among only a very few clues that they may be active is the presence of streamers or foragers. In order to prevent termite infestations into buildings and structures, builders may install (or in some jurisdictions are required to install) termite shields or spray termiticide in and around the soil where the building enters into the ground. However, such measures often fail, because, e.g., the termite shield may not be properly installed, the termiticide misapplied, or after several years, the termiticide simply loses its effectiveness. Thus, given the difficulty in detecting termites and preventing termite infestations it would be desirable to have a wood or lignocellulosic material that is resistant to termites.  
      Although less viable as a subject for a Hollywood horror film, the cumulative damage that can be caused by fungus may greatly exceed that presented by termites. Fungus, the most widely-known examples of which are white rot and the brown rot, actively decompose lignocellulosic material utilizing the natural components of wood as carbon and energy sources.  
      A variety of techniques have been developed to address the issue of fungus and rot in lignocellulosic materials. For example, pressure-treating with arsenic containing compounds, borates, and halogenated compounds have met with some, limited success. Unfortunately, these compounds are either extremely poisonous or are not suitable for use in manufacturing wood-based and lignocellulosic-based composites.  
      Given the foregoing, there is a need in the art for a strand composite panel or board that has the excellent strength performance of bamboo, but is also resistant to insect and fungus infestations without the use of toxic or potentially harmful pressure-treating chemicals.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention relates to a strand composite board comprising: lignocellulosic strands containing about 25 wt % to about 75 wt % bamboo strands, about 25 wt % to about 75 wt % cedar strands, and an isocyante binder resin. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      All parts, percentages, and ratios used herein are expressed by weight unless otherwise specified. All documents cited herein are incorporated by reference.  
      As used herein, “lignocellulosic material” is intended to mean a cellular structure, having cell walls composed of cellulose and hemicellulose fibers bonded together by lignin polymer. Wood is a species of lignocellulosic material.  
      By “strand composite material” it is meant a composite material that comprises lignocellulosic material and one or more other additives, such as adhesives or waxes. Non-limiting examples of wood composite materials include oriented strand board (“OSB”), such as, waferboard, particle board, chipboard, medium-density fiberboard, plywood, and boards that are a composite of strands and ply veneers. As used herein, “flakes”, “strands”, and “wafers” are considered equivalent to one another and are used interchangeably. A non-exclusive description of composite materials may be found in the Supplement Volume to the Kirk-Othmer Encyclopedia of Chemical Technology, pp 765-810, 6 th  Edition, which is hereby incorporated by reference.  
      The following describes preferred embodiments of the present invention, which provides an engineered material comprising a mixture of cedar wood and bamboo, lingo-cellulosic composite components and one or more bamboo layers. The presence of cedar acts as both an insect and fungus repellant. As used herein, “fungus” refers to a large and diverse group of eucaryotic microorganisms whose cells contain a nucleus, vacuoles, and mitochondria. Fungi include algae, molds, yeasts, mushrooms, and slime molds. See, Biology of Microorganisms, T. Brock and M. Madigan, 6.sup.th Ed., 1991, Prentice Hill (Englewood Cliffs, N.J.). Exemplary fungi include Ascomycetes (e.g., Neurospora, Saccharomyces, Morchella), Basidiomycetes (e.g., Amanita, Agaricus), Zygomycetes (e.g., Mucor, Rhizopus), Oomycetes (e.g., Allomyces), and Deuteromycetes (e.g., Penicillium, Aspergillus).  
      One exemplary mold is the Basidiomycetes mold commonly referred to as white rot and brown rot. Most fungi, especially members of the Basidiomycetes decompose lignocellulosic material including wood, paper, cloth, and other products derived from natural sources, utilizing the cellulose or lignin as carbon and energy sources. The decomposition of lignin in nature occurs almost exclusively through the agency of these wood-rotting fungi. Brown rot attacks and decomposes the cellulose and the lignin is left unchanged. White rot attacks and decomposes both cellulose and lignin. See, Biology of Microorganisms, T. Brock and M. Madigan, 6.sup.th Ed., 1991, Prentice Hill (Englewood Cliffs, N.J.).  
      Thus, the combination of bamboo and cedar strands allows for a strand composite product or more specifically, a panel that has the strength properties characteristic of bamboo, while at the same time having the insect- and fungus-repellant properties of cedar.  
      Bamboo strands  
      Like other lignocellulosic materials, bamboo&#39;s basic components are cellulose fibers bonded together by lignin polymer, but bamboo differs from other wood materials in the organization and morphology of its constituent cells. Generally, most strength characteristics of bamboo (tensile strength, flexural strength and rigidity) are greatest in the longitudinal direction of the bamboo and the bamboo fibers. This is due to the relatively small micro-fibrillar angle of the cellulose fibers in the longitudinal direction. The hardness of the bamboo culm itself is dependent on the density of bamboo fibers bundles and their manner of separation. The percentage of fibers is not consistent either in the longitudinal direction of the bamboo culm or in a cross section of the culm. In the longitudinal direction, the density of fibers increases from the bottom of the culm to its top, while the density of fibers in the bamboo culm cross-section is highest closer to the outer surface and decreases going deeper into the core of the material. Moreover, the strength and hardness of the outer portion of the bamboo culm is increased by the presence of a silica-deposited, cutinized layer coated with wax, which covers the surface of the outer part of the culm. Thus, the bamboo on or near the outer surface of the culm has superior strength characteristics, and in most processes for using bamboo. Unlike previous techniques for using bamboo wood in which the cutinized layer is stripped off and thus the strongest part of the culm discarded, in the present invention the cutinized layer is used and thus the high strength properties of the bamboo are maintained.  
      Overall, the cellulose fibers in bamboo are stiffer and stronger than the fibers of most wood species, so that boards incorporating bamboo could have a much higher strength to weight ratio than boards made from other types of wood fibers. In the first step according to the present invention, bamboo trunks are split either along (1) the entire length of the bamboo trunk (a distance typically between 4 to 40 feet) or alternatively (2) into shorter pieces. For improved compatibility and adhesion with the conventional wood strands, the bamboo strands are preferably cut into thicknesses of less than about 0.2 inch, such as less than 0.15 inches, such as in the range of about 0.01 inches to about 0.15 inches; and cut into widths of preferably greater than about 0.1 inches, such as more than about 0.15 inches, such as more than about 0.5 inches.  
      This cutting may be done either manually or with mechanized clipping equipment. For purposes of improved strength the bamboo strands should be cut along the longitudinal axis into strands preferably longer than about 2 inches, such as about 3 inches, such as about 5 inches. While not intending to be limited by theory, it is believed that the longer strip length will result in more closely aligned strands when the strands are oriented using a disk strand orienter, and without being limited by theory, it is believed that more closely aligned strands will result in a final wood composite board product that has an improved modulus of elasticity along the longitudinal axis.  
      After being cut, the bamboo strands are dried in an oven and then mixed with cedar strands. The strands are mixed together in a proportion of about 25 wt % to about 75% cedar and about 25 wt % to about 75% bamboo (all of these weight fractions are based on the dry weight of the wood strands alone, without additional additives).  
      The bamboo and cedar strands are then both coated with isocyanate resins (as described below). The isocyante resin and the other various additives that are applied to the wood materials are referred to herein as a coating, even though the isocyanate resin and additives may be in the form of small particles, such as atomized particles or solid particles, which do not form a continuous coating upon the wood material. Conventionally, the isocyanate resin, wax and any other additives are applied to the wood materials by one or more spraying, blending or mixing techniques, a preferred technique is to spray the wax, resin and other additives upon the wood strands as the strands are tumbled in a drum blender.  
      The isocyanate resins are selected from the diphenylmethane-p,p′-diisocyanate group of polymers, which have NCO— functional groups that can react with other organic groups to form polymer groups such as polyurea, —NCON—, and polyurethane, —NCOON—; a binder with about 50 wt % 4,4-diphenyl-methane diisocyanate (“MDI”) or in a mixture with other isocyanate oligomers (“pMDI”) is preferred. A suitable commercial pMDI product is Rubinate 1840 available from Huntsman, Salt Lake City, Utah, and Mondur 541 available from Bayer Corporation, North America, of Pittsburgh, Pa. Other suitable resins useful as adhesive bnidner separately or in combination with pMDI are the formaldehyde-based liquid PF, powder PF, UP MUF binders, and combinations of these. Commercial MUF binders are the LS 2358 and LS 2250 products from the Dynea Corporation. In a preferred formulation, the isocyantes are applied at a concentration of about 2 wt % to about 12 wt % (based on the total weight of the lignocellulosic strands).  
      After being coated with isocyanates, the coated bamboo and cedar strands are used to form a multi-layered mat, preferably a three layered mat which is then pressed to form a composite wood component. This layering may be done in the following fashion. The coated flakes are spread on a conveyor belt to provide a first ply or layer having flakes oriented substantially in line, or parallel, to the conveyor belt, then a second ply is deposited on the first ply, with the flakes of the second ply oriented substantially perpendicular to the conveyor belt. Finally, a third ply having flakes oriented substantially in line with the conveyor belt, similar to the first ply, is deposited on the second ply such that plies built-up in this manner have flakes oriented generally perpendicular to a neighboring ply. Alternatively, but less preferably, all plies can have strands oriented in random directions. The multiple plies or layers can be deposited using generally known multi-pass techniques and strand orienter equipment. In the case of a three ply or three layered mat, the first and third plys are surface layers, while the second ply is a core layer. The surface layers each have an exterior face.  
      After the multi-layered mats are formed according to the process discussed above, they are compressed under a hot press machine that fuses and binds together the wood materials, binder, and other additives to form consolidated OSB panels of various thickness and sizes. The high temperature also acts to cure the binder material. Preferably, the panels of the invention are pressed for 2-15 minutes at a temperature of about 175° C. to about 240° C. The resulting composite panels will have a density in the range of about 35 lbs/ft 3  to about 55 lbs/ft 3  (as measured by ASTM standard D1037-98). The thickness of the OSB panels will be from about 0.6 cm (about ¼″) to about 5 cm (about 2″), such as about 1.25 cm to about 6 cm, such as about 2.8 cm to about 3.8 cm.  
      The invention will now be described in more detail with respect to the following, specific, non-limiting examples.  
     EXAMPLES  
      A strand composite board containing a mixture of bamboo and cedar strands was prepared as follows. Strands were cut from bamboo logs (in some cases the bamboo was soaked in water for 24 hours before stranding) in average dimensions of 0.025 inch thick×2 inch wide, by 6 inch long. The bamboo strands were mixed in three separate ratios (see Table I, below) with cedar strands. The cedar strands had average dimensions of 0.032 inch thick×3 inch wide, by 5 inch long. MDI resin was applied at a concentration of about 5 wt %, with an application of about 1.5 wt % slack wax.  
      Strand composite panels were prepared with 60% of the strands lying in the surface layers, and 40% of strands in the core layer, wherein the strands in the core and the strands in the surface layers have substantially perpendicular orientations. Three sets of panels were made with three different blends of cedar and bamboo strands as set forth in Table I, below. The panels were prepared by pressing the strands at a temperature of 400° F. for 175 seconds at a pressure of 200 psi or greater to a ¾ inch target thickness with a target density of 44 pcf.  
      For each made, the MOE and MOR was tested and measured (according to ASTM standard D1037-98), and the results averaged for each individual set. The results are set forth below in Table I, below.  
                               TABLE I                                   75% Cedar   50% Cedar   25% Cedar           25% Bamboo   50% Bamboo   75% Bamboo                                                            MOE (psi)   530,000   840,000   935,000           MOR (psi)   2,400   5,700   4,800                      
 
      The tests results show that the panels with the higher fraction of strands taken from bamboo had higher strength than those with a higher cedar strand fraction. Nonetheless, all of the panels showed at least satisfactory performance for use in structural applications.  
      Resistance to fungal attack was measured according to NWWDA TM1 test method. The weight loss of ¾″ cubes due to fungal attack is measured over a 16 week test period. For this particular study, the brown rot fungus  Gleophyllum trabeum  was used to attack the samples, as this fungus is known to aggressively attack bamboo. Samples of 100% wood weight bamboo OSB showed a weight loss average of 21% over the 16 week test. Samples of 100% cedar showed a weight loss average of 8.7%, which represents the maximum benefit achievable when using cedar. When 25 to 75% cedar by wood weight was added to OSB containing bamboo, the weight loss was between 13% and 18%, which gives a benefit of 25 to 65 % of the maximum benefit.  
      It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.