BACKGROUND OF THE INVENTIONWood is a common structural material that has been used for thousands of years for building purposes. Even today, after the development of several new types of composite materials, wood remains one of the most widely used structural 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 natural solid timber wood material. This has had a concomitant effect on users of structural wood material to look for alternative materials. For example, an “I joist” has three parts: two flange members with an interconnecting webstock member; each flange member has a groove into which the webstock member is inserted. At one time flanges were made from solid wood lumber, in particular from visually and/or machine stress rated dense softwood lumbers such as from the black spruce tree which grows in Canada's northern boreal forests. However, recently because of both the cost of high-grade timber wood as well as a heightened emphasis on conserving natural resources, flanges have become increasingly more likely to be made from alternatives to solid wood lumber, such as engineered wood composite materials, including laminated veneer lumber (“LVL”), laminated strand lumber (“LSL”), and parallel strand lumber (“PSL”) to meet the higher stiffness and strength requirements, especially for the bottom flanges. Similarly, other structural building components such as beams and headers are also more likely to be constructed from these wood composite materials.
While the aforementioned wood composites have been used successfully in replacing natural solid wood lumber in structural applications, other wood composite materials like plywood and oriented strand board (“OSB”) have met with less success in being incorporated in structural applications demanding higher stiffness and strength, such as beams, headers, and wood I-joist flanges. This is because the wood composite materials often have insufficient strength and stiffness for certain applications. For instance, LVL typically has a modulus of elasticity (“MOE”) of 1.6 to 2.6 mmpsi, LSL a MOE of between and 1.3 to 1.9 mmpi, and PSL a MOE of 1.6 to 1.9 mmpi. By contrast, for OSB and plywood, the MOE values are much lower: OSB, from 0.45 to 1.15 mmpsi when measured along the major panel axis, and from 0.08 to 0.49 mmpsi when measured across the major panel axis. Thus, OSB and plywood are considerably less stiff and strong than LVL, LSL, or PSL. This limitation on the use of OSB and plywood is unfortunate because these materials are highly efficient uses of the available supply of timber wood and can even be produced from lower-grade wood species, as well as wood wastes.
Accordingly, there is a need in the art for a wood panel, especially an OSB or plywood panel, of sufficient stiffness and strength to be used in a wide variety of structural applications.
The present invention relates to a reinforced wood panel comprising: a first lignocellulosic layer, and a prepreg layer.
The present invention further relates to a process for manufacturing a reinforced wood panel comprising laminating a first lignocellulosic layer, and a prepreg layer at a temperature of about 260° F. to about 350° F., and a pressure of less than 800 psi.
The present invention further relates to a reinforced wood panel according to claim1, wherein the prepreg layer has a sufficiently high MOE value so that when incorporated into the reinforced wood panel, the reinforced wood panel has an MOE of about 1.3 to about 2.5 mmpsi.
The present invention further relates to a reinforced wood panel comprising: a first lignocellulosic layer, a first prepreg layer, and a second prepreg layer, wherein the first lignocellulosic layer forms a core layer disposed between an upper surface layer formed by the first prepreg layer and a lower surface layer formed by the second prepreg layer.
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, “wood” is intended to mean a cellular structure, having cell walls composed of cellulose and hemicellulose fibers bonded together by lignin polymer. It should further be noted that the term “wood” encompasses lignocellulosic material generally.
By “wood composite material” it is meant a composite material that comprises wood and one or more wood composite additives, such as adhesives or waxes. The wood is typically in the form of veneers, flakes, strands, wafers, particles, and chips. Non-limiting examples of wood composite materials include oriented strand board (“OSB”), waferboard, particle board, chipboard, medium-density fiberboard, plywood, parallel strand lumber, LVL, PSL, OSL LSL, and structural composite lumber (“SCL”). Common characteristic of the wood composite materials are that they are composite materials comprised of strands and ply veneers binded with polymeric resin and other special additives. As used herein, “flakes”, “strands”, “chips”, “particles”, and “wafers” are considered equivalent to one another and are used interchangeably. A non-exclusive description of wood composite materials may be found in the Supplement Volume to the Kirk-Rothmer Encyclopedia of Chemical Technology, pp 765-810, 6thEdition.
The present invention is directed to a reinforced wood panel comprising at least one prepreg reinforcement layer attached to at least one lignocellulosic substrate. By combining the prepreg reinforcement layer with the lignocellulosic substrate, a wood panel is produced which has a strength and stiffness significantly greater than if the lignocellulosic layer was used by itself without a prepreg reinforcement layer. Additionally, the wood panel may also be constructed to handle increased compression loads and forces. The strength and stiffness/and or the enhanced compression load performance of the reinforced wood panel makes the panel suitable for a wide variety of structural applications.
The components of the present invention, the lignocellulosic layer and the prepreg reinforcement layer, will now be described in greater detail.
A prepreg is a resin-containing, fiber-composite material. The prepregs can contain nonwoven, randomly oriented fibers loosely held together by a polymer resin. Or the fibers can be woven into two-dimensional or even three-dimensional fabrics, and these fabrics can then be impregnated with a polymer resin, which is typically selected from thermosetting polymers. In the present invention the fibers are selected from materials including synthetic materials such as glass fibers, carbon fibers, armaric fibers, as well as natural fibers such as wood fibers and bamboo fibers.
In addition to the above requirements, the prepreg reinforcement layer must also be able to be incorporated into the reinforced wood panel secondary assembly process and so accordingly must be able to withstand processing conditions in such an assembly process, such as pressures of up to 800 psi, and temperatures of up to 460° F. Additionally, the failure strain of the prepreg reinforcement layer must be less or equal to the failure strain of the lignocellulosic substrates. The prepreg reinforcement layer has a thickness of between 0.010 inch to 0.250 inch, preferably, 0.040 inch to 0.150 inch, more preferably 0.050 inch to 0.100 inch. The prepreg reinforcement layer has an MOE value of 1.7 to 50 mmpsi, preferably in the range of 2.1 to 20 mmpsi, more preferably in the range of 2.6 to 17 mmpsi.
The lignocellulosic layer according to the present invention may incorporate strands from several different lignocellulosic species, including naturally occurring hard or soft woods species, to form lignocellulosic layers (see the more detailed description above with respect to “wood composite materials”). The lignocellulosic layer may be made into one of the forms of wood composite material discussed above, of which OSB and plywood are particularly envisioned. Methods for making the lignocellulosic layer as well as wood composite materials are well known in the art. Preferably, the lignocellulosic substrates have MOE values of 0.5 to 1.2 mmpsi, preferably in the range of 0.8 to 1.15 mmpsi. Additionally, the lignocellulosic substrates preferably have a thickness ranging from ¼″ to 2.5″, more preferably in the range of ⅜″ to 1.75″, and even more preferably in the range of 0.7″ to 1.5″.
The present invention includes one or more prepreg layers that may each have different compositions or may have the same composition; for example, the one or more prepreg layers may comprise a first prepreg layer, and optionally a second prepreg layer, and optionally a third prepreg layer, and optionally a fourth prepreg layer, each of which may have the same or different composition. In a similar manner, the present invention includes one or more lignocellulosic layers which may include a first lignocellulosic layer, and optionally a second lignocellulosic layer, and optionally a third lignocellulosic layer, and optionally a fourth lignocellulosic layer, each of which may have the same or different composition.
The reinforced wood panel contains at least two surface layers (i.e., there will be only two layers when there are only two total layers present between the lignocellulosic layers and the prepreg layers) and optionally one or more core layers (when there are three or more total layers). Depending on the end-use application, the lignocellulosic layers and prepreg layers may be distributed in any manner between core and surface layers.
Finally, the reinforced wood panel is manufactured in a secondary laminating process in which one or more prepreg layers are laminated to the one or more lignocellulosic layers or the lignocellulosic substrate in a press to form the reinforced wood panel. In this secondary assembly process, the prepreg layers and the lignocellulosic substrate are manufactured separately and then attached or laminated to one another between press platens at an elevated temperature (up to 460° F., preferably 260° F. to 350° F.) and pressure (up to 800 psi). If necessary an adhesive layer may be used; however, adhesion may also be provided by the resin in the prepreg.
(The reinforced wood panel may also be manufactured in a less-preferred primary process in which resin-coated wood strands are spread on a conveyor belt in one or more layers along with one or more prepreg layers and then compressed under a hot press machine that fuses and binds together the prepreg layers, the wood materials, binder, and other additives to form the reinforced wood panel. In the primary process, the curing temperature of the resin in the prepreg must be less than the temperature of the composite press cycle so that the resin is sufficiently cured at the end of the process)
The reinforced wood panels of the present invention may be used in a variety of functional and structural applications where a wood composite material or panel is needed that has high strength and stiffness, and/or which can handle a heavy compression load. These include, inter alia, light weight sub-flooring, fire-proof web stock and sheathing, blast-resistant sheathing, and earthquake sheathing. Of particular interest is that the present material can be used in constructing I joists. As mentioned above, I joists typically include three sections: a top flange (which is exposed primarily to compression forces) and a bottom flange (which is exposed primarily to tension forces) that are interconnected by a webstock member. Typically the cross-sections of the flange are rectangular and have a pair of wider (or major) faces of between one inch and two inches, and a dimension along the other pair of faces (or minor faces) of between one inch and three inches. Because the flange components are typically exposed to high forces whether in tension or compression, they are often made from solid wood lumber or premium grade wood composite materials such as OSL or LVL. However, in the present invention OSB can be used in structural applications, because it is incorporated into a reinforced wood panel that also comprises a prepreg layer, and the prepreg layer sufficiently boosts the strength and stiffness of the reinforced wood panel so that the panel can be used in structural applications, for example the flange components in an I joist. A flange made from the reinforced wood panel as disclosed in the present invention will preferably have an MOE of about 1.3 to about 2.5 mmpsi.
The invention will now be described in more detail with respect to the following specific, non-limiting examples.
Reinforced wood panels were prepared according to the present invention by laminating two surface layers of commercial fiber glass preperg L-526PG-1825-50″ onto a core 1.75″ thick TruSpec panel. The fiber glass preperg L-526PG-1825-50″ is available from the J.D. Lincoln Inc., Costa Mesa, Calif.; the TruSpec panel is available from Huber Engineered Woods LLC, Charlotte, N.C. L310 PF adhesive films supplied also by J.D. Lincoln Inc. were used as the adhesive. The lamination process took place under 600 psi pressure and at 350° F. for 60 minutes. The thickness of the finished laminated composites was about 1.459 to 1.464″. The final MOE for the composites was 951,114 psi (parallel) and 468,278 psi (perpendicular). This compares with MOE values of 723,129 psi (parallel) and 252,379 psi (perpendicular) measured for the TruSpec without the prepreg layer. Thus, the increase in strength and stiffness is significant in the wood composite materials prepared according to the present invention.
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.