Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles
Reexamination Certificate
1999-08-17
2002-03-05
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
C264S122000, C156S062200
Reexamination Certificate
active
06352661
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for producing lignocellulose composite materials by combining lignocellulose particles with a binder composition comprising a polymethylene poly(phenylisocyanate) component and a liquid hydrophobic diluent having a flash point above about 250° F., followed by molding or compressing the combined lignocellulose particles and the binder composition.
Composite materials such as oriented strand board, particle board and flake board are generally produced by blending or spraying lignocellulose materials with a binder composition, while the materials are tumbled or agitated in a blender or like apparatus. After blending sufficiently to form a uniform mixture, the materials are formed into a loose mat, which is compressed between heated platens or plates to set the binder and bond the flakes, strands, strips, pieces, etc., together in densified form. Conventional processes are generally carried out at temperatures of from about 120 to 225° C. in the presence of varying amounts of steam, either purposefully injected into or generated by liberation of entrained moisture from the wood or lignocellulose materials. These processes also generally require that the moisture content of the lignocellulose material be between about 2 and about 20% by weight, before it is blended with the binder.
Lignocellulose, according to the present invention used to prepare the lignocellulosic-containing composite articles include wood chips, wood fibers, wood flakes, wood particles, wood wafers, wood shavings, wood flour, sawdust, wood wool, cork, bark, pieces of wood or other comminuted or stranded wood products, and other like products for the wood-working industry. Fibers, particles, etc. from other natural products which are lignocellulosic such as straw, flax residues, bamboo, esparto, dried weeds and grasses, nut shells, sisal fibers, wool, soy, bagasse straw (sugar cane waste), hulls from cereal crops such as corn, rice and oats and the like may be used. These materials may be used in the form of granulates, shavings or chips, fibers, strands, spheres, or powder. In addition, the lignocellulosic materials may be mixed with inorganic flakes or fibrous material such as glass fibers or wool, mica and asbestos, as well as with rubber and plastic materials in particulate form. The lignocellulose may contain a moisture (water) content of up to about 40% by weight, preferably less than 25% by weight, but most preferably contains between 4 and 12% by weight.
Plywood production is accomplished by roll coating, knife coating, curtain coating, or spraying a binder composition onto veneer surfaces. A plurality of veneers are then laid-up to form sheets of required thickness. The mats or sheets are then placed in a heated press and compressed to effect consolidation and curing of the materials into a board.
Binder compositions which have been used in making such composite wood products include phenol formaldehyde resins, urea formaldehyde resins and isocyanates. See, for example, James B. Wilson's paper entitled, “Isocyanate Adhesives as Binders for Composition Board” which was presented at the symposium “Wood Adhesives—Research, Applications and Needs” held in Madison, Wis. on Sep. 23-25, 1980, in which the advantages and disadvantages of each of these different types of binders are discussed.
Isocyanate binders are commercially desirable because they have low water absorption, high adhesive and cohesive strength, flexibility in formulation, versatility with respect to cure temperature and rate, excellent structural properties, the ability to bond with lignocellulosic materials having high water contents, and no formaldehyde emissions. The disadvantages of isocyanates are difficulty in processing due to their high reactivity, adhesion to platens, lack of cold tack, high cost and the need for special storage. U.S. Pat. No. 3,870,665 and German Offenlegungsschrift No. 2,109,686 disclose the use of polyisocyanates (and catalysts therefor) in the manufacture of plywood, fiberboard, compression molded articles, as well as various technical advantages when used as binders.
It is known to treat cellulosic materials with polymethylene poly(phenyl isocyanates) (“polymeric MDI”) to improve the strength of the product. Typically, such treatment involves applying the isocyanate to the material and allowing the isocyanate to cure, either by application of heat and pressure (see, e.g., U.S. Pat. Nos. 3,666,593, 5,008,359, 5,140,086, 5,143,768, and 5,204,176) or at room temperature (see, e.g., U.S. Pat. Nos. 4,617,223 and 5,332,458). While it is possible to allow the polymeric MDI to cure under ambient conditions, residual isocyanate groups remain on the treated products for weeks or even months in some instances. It is also known, but generally less acceptable from an environmental standpoint, to utilize toluylene diisocyanate for such purposes.
Isocyanate prepolymers are among the preferred isocyanate materials which have been used in binder compositions to solve various processing problems, particularly adhesion to press platens and high reactivity. U.S. Pat. No. 4,100,328, for example, discloses isocyanate-terminated prepolymers which improve product release from a mold. U.S. Pat. No. 4,609,513 also discloses a process in which an isocyanate-terminated prepolymer binder is used to improve product release. A binder composition in which a particular type of isocyanate prepolymer is used to improve adhesiveness at room temperature is disclosed in U.S. Pat. No. 5,179,143.
A major processing difficulty encountered with isocyanate binders is the rapid reaction of the isocyanate with water present in the lignocellulosic material and any water present in the binder composition itself. One method for minimizing this difficulty is to use only lignocellulosic materials having a low moisture content (i.e., a moisture content of from about 3 to about 8%). This low moisture content is generally achieved by drying the cellulosic raw material to reduce the moisture content. Such drying is, however, expensive and has a significant effect upon the economics of the process. Use of materials having low moisture contents is also disadvantageous because panels made from the dried composite material tend to absorb moisture and swell when used in humid environments.
The problems of the rapid reaction of the isocyanate with water can be aggravated by adding diluents that are hydrophilic or hydroscopic to the isocyanate binder. Addition of these materials to the binder can draw entrained moisture in the wood or in the manufacturing environment to come into more intimate contact with the isocyanate resulting in pre-cure of the resin prior to densification of the mat in the press.
Another approach to resolving the moisture and isocyanate reactivity problem is disclosed in U.S. Pat. No. 4,546,039. In this disclosed process, lignocellulose-containing raw materials having a moisture content of up to 20% are coated with a prepolymer based on a diphenylmethane diisocyanate mixture. This prepolymer has a free isocyanate group content of about 15 to about 33.6% by weight and a viscosity of from 120 to 1000 mPa·s at 25° C. This prepolymer is prepared by reacting (1) about 0.05 to about 0.5 hydroxyl equivalents of a polyol having a functionality of from 2 to 8 and a molecular weight of from about 62 to about 2000 with (2) one equivalent of a polyisocyanate mixture containing (a) from 0 to about 50% by weight of polyphenyl polymethylene polyisocyanate and (b) about 50 to about 100% by weight isomer mixture of diphenylmethane diisocyanate containing 10 to 75% by weight of 2,4′-isomer and 25 to 90% by weight of 4,4′-isomer.
CA Patent Application 2,143,883 discloses polyisocyanate binders for lignocellulose-containing raw materials. These isocyanate binders are low viscosity, isocyanate terminated prepolymers made from a mixture of monomeric and polymeric MDI and an isocyanate-reactive material having at least one hydroxyl group and a molecular weight of from about 62 to about 6,000
Markusch Peter H.
Rosthauser James W.
Sarpeshkar Ashok M.
Thompson James A.
Bayer Corporation
Brown N. Denise
Gil Joseph C.
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