Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles
Reexamination Certificate
1999-12-16
2002-07-09
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
C264S122000
Reexamination Certificate
active
06416696
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for the production of wood composite materials. This process consists of combining wood particles with a composition, and molding or compressing the combination formed. Suitable compositions comprise a blocked polyisocyanate and an aqueous solution of phenolic resin.
Composite materials such as oriented stand board, particle board and flake board are generally produced by blending or spraying lignocellulose materials such as wood flakes, wood fibers, wood particles, wood wafers, strips or strands, pieces of wood or other comminuted 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 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.
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.
Phenolformaldehyde and urea-formaldehyde binders are relatively inexpensive and provide lignocellulosic composites with properties suitable for many application areas. They can be formulated so that emissions of formaldehyde are negligible. Phenol-formaldehyde resins and urea-formaldehyde resins are typically supplied in aqueous media giving them ease of application. They typically have to be used with lignocellulosic materials having lower moisture contents. Otherwise, they demand high temperature and long pressing times to provide cured lignocellulosic composites. They provide boards with high structural strength but may have high water absorption limiting the areas where they can be used.
Isocyanate binders are commercially desirable because they have flexibility in formulation, versatility with respect to cure temperature and rate, the ability to bond with lignocellulosic materials having high water contents, low water absorption, high adhesive and cohesive strength, excellent structural properties, 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 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.
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.
The large scale industrial manufacture of composite materials which are bonded exclusively with polyisocyanates has previously been limited. The use of some of the polyisocyanates, particularly the better performing isocyanates, such as polymethylene diisocyanate has been limited by their cost. Because of the cost constraints, the level of use of these expensive isocyanates is kept low for a given composite material. One approach to the use of levels of these isocyanates has involved chain extending the isocyanate with inexpensive extenders. In most cases, the extenders must be substantially water-free.
U.S. Pat. No. 4,944,823 describes a composition for bonding solid lignocellulosic materials. Suitable binder formulations are based on the reactive mixture of an isocyanate and a carbohydrate material. These are both effective and inexpensive, and eliminate health hazards associated with the use of formaldehyde. Carbohydrate materials include, for example, sugars and starches, in the presence or absence of other active materials. These carbohydrates are mixed with a liquid diisocyanate and applied to the wood, which is then pressed to form a composite product.
Binder compositions comprising phenolic resins and polyisocyanates are known and described in, for example, U.S. Pat. Nos
Miller Todd R.
Rosthauser James W.
Bayer Corporation
Brown N. Denise
Gil Joseph C.
Vargot Mathieu D.
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