This invention relates to a process for producing wood composite material by combining wood particles with a thixotropic binder composition. Suitable thixotropic binder compositions comprising: 1)(a) a polyisocyanate, preferably a polymethylene poly(phenylisocyanate) or 1)(b) a semi-prepolymer of a polyisocyanate, preferably a semi-prepolymer of polymethylene poly(phenylisocyanate); and 2) a thixotropic agent which is selected from the group consisting of: (a) a mineral species, (b) a rheological species, (c) an adduct of (i) a polyisocyanate or a semi-prepolymer thereof, with (ii) a substituted aromatic amine and/or (iii) a fatty amine, and (d) mixtures thereof.
Composite materials such as oriented stand board, particle board and flake board are generally produced by blending or spraying comminuted 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 comminuted materials are blended by tumbling or agitating them 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.degree. C. in the presence of varying amounts of steam, generated by libration 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.
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) (hereinafter "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 (MDI) 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.multidot.s at 25.degree. 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.
U.S. Pat. No. 5,002,713 discloses a method for compression molding articles from lignocellulosic materials having moisture contents of at least 15%, generally from 15 to 40%. In this disclosed method, a catalyst is applied to the lignocellulosic material. A water resistant binder is then applied to the lignocellulose with catalyst and the coated materials are then compression shaped at a temperature of less than 400.degree. F. to form the desired composite article. The catalyst is a tertiary amine, an organometallic catalyst or a mixture thereof. The binder may be a hydrophobic isocyanate such as any of the polymeric diphenylmethane diisocyanates, m- and p-phenylene diisocyanates, chlorophenylene diisocyanates, toluene diisocyanates, toluene triisocyanates, triphenyl-methane triisocyanates, diphenylether-2,4,4'-triisocyanate and polyphenol polyisocyanates. The catalyst is included to ensure that the isocyanatel/water reaction is not slowed to such an extent that the pressing time necessary to produce the molded product is significantly increased.
Pressing of wafer board, oriented strand board, and parallel strand lumber using steam injection and a conventional binder such as a urea-formaldehyde resin or a polymeric diphenylmethane diisocyanate (PMDI) is known. Examples of such known pressing processes are disclosed in U.S. Pat. Nos. 4,684,489; 4,393,019; 4,850,849; and 4,517,147. These processes yield a product having satisfactory physical properties if the binder is completely cured.
The completeness of binder cure may, of course, be determined by destructive testing of samples which have been permitted to cure for varying amounts of time under the process conditions. The cure time to be used during the production process is determined on the basis of the sample which had completely cured in the least amount of time. The disadvantages of this method are readily apparent. Valuable product is destroyed in the testing. Further, any variation in wood composition, extent of binder dispersion on the wood particles, etc. or processing conditions which would affect the rate of binder cure are not taken into consideration in the above-described method.
Binding compositions comprising urea extended polyisocyanates derived from a combination of a polyisocyanate and urea which is in solution with water, and the process for preparing the binding compositions is disclosed in U.S. Pat. No. 5,128,407. This reference also describes a process for preparing a composite material from comminuted particles or veneers of a lignocellulose material comprising coating the particles or veneers with these binding compositions.
A process for producing compression molded articles of lignocellulose type materials by use of an organic polyisocyanate compound as a binder is disclosed by U.S. Pat. No. 5,744,079. The binders comprise (A) an organic polyisocyanate such as, for example, MDI or PMDI, (B) an aqueous emulsion of a wax having a melting point ranging from 50.degree. C. to 160.degree. C., (C) an organic phosphate ester derivative, and (D) optionally, water.
The large scale industrial manufacture of composite materials which are bonded exclusively with polyisocyanates have previously been limited. The use of some of the polyisocyanates, particularly the better performing isocyanates, such as polymeric MDI, 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 material. One approach to the use of levels of these isocyanates has involved chain extending the isocyanate with inexpensive extenders.
It has been known that organic polyisocyanate resins have excellent adhesion properties and workability as the adhesive for thermo-compression molded articles such as particle boards and medium-quality fiber boards produced from a lignocellulose type material such as wood chips, wood fibers, and the articles exhibit excellent physical properties. However, the excellent adhesiveness of the organic polyisocyanate resins causes disadvantage in that the compression molded article adheres firmly to the contacting metal surface of the heating plate in a continuous or batch thermo-compression process.
To solve the disadvantages of the undesired adhesion to the heating plate, it is required that a releasing agent is preliminarily sprayed onto the heating plate surface to form a releasing layer. Japanese Patent Publication No. 3-21321 discloses a method different from the external releasing agent spray, in which a mixture of an organic polyisocyanate and a mineral wax is sprayed onto the lignocellulose type material prior to thermo-compression molding. Japanese Patent laid open application No. 4-232004 discloses a method of thermo-compression molding of a lignocellulose type material by addition of a neutral ortho-phosphate ester as a compatibilizing agent, the wax and the polyisocyanate.
Another property that low viscosity organic polyisocyanate resins have is their tendency to quickly penetrate into the surface of the wood particles. Penetrated resin is not as effective as resins that sit on the surface of the wood particles towards promoting adhesion of the particles to each other. Higher viscosity resins penetrate less quickly than low viscosity ones. A simple and common method used in the coatings and adhesives industries to control viscosity is to add a thixotrope to the resin compositions.
A process for the preparation of air-drying, thixotropic binders containing free isocyanate groups is described in U.S. Pat. No. 4,719,278. These are prepared by reacting selected prepolymers or semi-prepolymers containing free isocyanate groups with selected aromatic diamines. Suitable prepolymers or semi-prepolymers are those containing free isocyanate groups and having an isocyanate content of about 0.5 to 20% by weight. Suitable aromatic diamines are those compounds which have at least one alkyl substituent in the ortho position to each amino group. The NCO:NH.sub.2 equivalent ratio is about 0.5:1 to 50:1 based on the isocyanate groups of the isocyanate component and the amino groups of the aromatic diamine, provided that the equivalent ratio of isocyanate groups to amino groups is at least 10:1.
U.S. Pat. No. 4,692,479 describes the use of thixotropic additives to polymeric MDI for the purpose of improving mixing of the low viscosity polyisocyanate with much higher viscosity phenolic resins. These compositions have very high viscosities of about 30,000 to 40,000 cps and are subsequently used as binders for foundry cores but are too viscous for use as wood binders.
U.S. Pat. Nos. 5,459,185 and 5,668,222 both describe the usefulness of thixotropes in moisture curing polyurethane adhesives. Here, the principle is to use gelling agents or incompatible mixtures to provide thixotropic properties to the polyurethane compositions. Again, very high viscosities of greater than 30,000 cps are required for the adhesives.
It is the purpose of this invention to utilize small amounts of thixotropes to reduce penetration of the PMDI resin into the wood particles prior to forming wood composites. The viscosity must remain low enough so that the resin can be spray applied. Also, the addition of these materials must not detract from the structural properties of the formed composites. Reducing the amount of penetration of resin into the wood particles reduces the amount of water that comes into contact with the reactive isocyanate groups prior to pressing. Thus, the workable open time of the resin coated particles can be increased even in the presence of higher moisture content in the wood particles.