TECHNICAL FIELD This invention is directed to methods for forming polyurea-cellulose composites by curing substantially delignified cellulosic material impregnated with an isocyanate. More particularly, this invention is directed to forming such composites by application of heat and pressure.

BACKGROUND OF THE INVENTION

Polyisocyanate-cellulose composites have been described in previous

U.S. patent applications, including U.S. Patent Application Serial Nos. 07/275,824 and 07/618,723, both entitled "Isocyanate Modified Cellulose Products and Method for Their Manufacture." Both of these applications, which are commonly owned with the present disclosure, are incorporated herein by reference.

The '824 application disclosed methods for making a modified polyisocyanate-cellulose composite comprising impregnating a cellulosic fibrous material with a "loading level" of about 8% to about 20% of a substantially uncatalyzed liquid polyisocyanate binder. The polyisocyanate binder was cured by applying a densifying pressure of 400 psig to about 1200 psig at a temperature of about 221 °F to about 473°F for a period of time not exceeding five minutes. The methods disclosed in the '824 application utilized lower amounts of polyisocyanate binder to produce composites having mechanical properties similar to or better than analogous prior-art products, including products made using substantially more polyisocyanate.

An example of a prior-art product used for comparison purposes was described in U.S. Patent No. 3,666,593 to Lee. The Lee patent required isocyanate loading levels of at least 30% up to about 150% to produce a composite having acceptable mechanical properties.

The '723 application disclosed a method for forming a polyurea- cellulose composite by impregnating a cellulosic fibrous material with a liquid

polyisocyanate resin comprising about 20% of a miscible organic solvent. The solvent, when used in amounts of about 5% to about 20%, conferred increased strength to the polyurea-cellulose composite compared to similar products made without the solvent. Curing of the resin was effected by application of heat and pressure.

Unfortunately, polyurea-cellulose composites made according to the methods described above are limited to a maximal thickness of about 0.10 inch.

This is because curing a liquid polyisocyanate resin to form polyurea generates carbon dioxide gas. In addition, when the curing pressure is released, water within the composite (pre-existent in the cellulose) "flashes" and is released as steam.

Thicker composites tend to entrap the gas within the thickness dimension of the composite during curing. Lowering of temperature and pressure after completely curing a thick composite can cause the gas to remain entrapped for a time sufficient to cause delamination of the composite. Delamination may be particularly likely upon a rapid decrease in pressure with the composite at an elevated temperature. Such entrapment of high-pressure gas can cause the differential pressure between the interior of the composite and the atmosphere to exceed the composite's tensile strength in the thickness dimension, thereby causing delamination.

Apparatuses thatallow steam to escape during hot-pressing procedures, or methods whereby a hot-pressed material is allowed to cool, are known in the art. For instance, U.S. Patent No. 4,162,877 to Nyberg disclosed a press wherein particleboard is "consolidated" by injection of pressurized steam. After an initial compression of the board, the board is subjected to a vacuum to exhaust air, then pressurized with steam which permeates the entire board. The board is then further compressed to the desired thickness. Afterward, the steam is slowly exhausted while maintaining compression of the board.

U.S. Patent No. 3,699,202 to Verbestel disclosed application of heat and pressure to a lignocellulosic board containing a phenolic binder to effect densification of the board. Afterward, the pressure was gradually lowered while the board cooled in the press.

Neither of the above-cited patents is directed to the distinctive problems associated with curing of a liquid polyisocyanate binder or of a liquid resin comprising a polyisocyanate and an organic solvent, such as press times, temperatures, or pressures. Neither reference provides any teaching of how to make thick composites of cellulose and polyurea. Neither reference teaches how to make cellulose polyurea composites having comparatively low amounts of polyurea relative to the cellulose. Also, neither reference discloses or suggests how to produce polyurea-cellulose composites that do not delaminate after curing.

Accordingly, there is a need for a method for forming a polyurea- cellulose composite, using loading levels of polyisocyanate binder no greater than 30% w/w relative to the cellulose, including such composites having a thickness greater than 0.10 inch.

There is also a need for such a method that employs heat and pressure for curing the polyisocyanate but does not cause the composites thus formed, particularly thick composites, to delaminate when the curing pressure is released.

There is also a need for a polyurea-cellulose composite having the combination of a thickness greater than 0.01 inch and a polyurea loading level of no greater than 30 percent w/w.

There is also a need for such a polyurea-cellulose composite that can be formed of multiple plies, wherein each ply comprises a substantially delignified cellulose material impregnated with a polyurea binder at a loading level of no greater than about 30% w/w, and wherein the distribution of polyurea is substantially uniform throughout the multiple-ply composite.

SUMMARY OF THE INVENTION

The present invention is directed to polyurea-cellulose composites produced from substantially delignified cellulosic material. The cellulosic material is impregnated with a liquid polyisocyanate binder comprising at least one polyisocyanate compound. In the liquid binder, the polyisocyanate compound is typically, but not necessarily, diluted with an organic solvent that is miscible with the polyisocyanate to form a "resin". A preferred solvent is propylene carbonate.

The maximal amount of solvent is about 20% w/w relative to the mass of the resin.

The amount of resin used to impregnate the cellulosic material is about 8-30% w/w relative to the mass of cellulosic material (referred to herein as the "loading level").

In a method according to the present invention, the polyisocyanate- impregnated cellulosic material is cured (or "densified") at a pressure of about 400 psig to about 1200 psig for a period of time sufficient to obtain a temperature, midway through the thickness dimension of the material (i.e., a "core temperature"), within a range of about 275°F to about 350°F. After the desired core temperature is obtained, the stated pressure is maintained while the press is allowed to cool until the core temperature drops to 100°F to less than 212°F. The pressure is then lowered to atmospheric.

In a first embodiment of the method, the polyisocyanate-impregnated cellulose is densified by employing a preheated press to apply the requisite heat and pressure. In a second embodiment, the heating of actual press is begun only after the polyisocyanate-impregnated cellulose is loaded into the press, thereby effecting a slower increase of temperature up to the curing temperature. Either embodiment can include at least one "breathing cycle" step during densification of the material but before curing is complete. During a breathing cycle, the pressure in the press is decreased slowly and momentarily released to atmospheric for about four to about ten seconds, followed by a resumption of the curing pressure. Since complete curing of the polyisocyanate typically requires several minutes at the stated pressure and temperature, it will be appreciated that each such breathing cycle is only momentary (about 4 to about 10 seconds).

Single or multiple plies of the polyurea-cellulose composite can be produced according to the present invention. For example, multiple-ply planar composites are made by hot-pressing laminarily arranged sheets of polyisocyanate- impregnated cellulose in a manner similar to methods according to the present invention used for making single-ply polyurea-cellulose composites.

The liquid polyisocyanate binder can be any of various lower aliphatic, alicyclic, and aromatic polyisocyanates or mixtures thereof. The preferred polyisocyanate is poly(diphenylmethane diisocyanate), known in the art as "PMDI".

The organic solvent can be any of various organic solvents miscible with the polyisocyanate, such as, but not limited to, alkylene carbonates, aromatics, halogenated aromatics, ethers, ketones and alkyl acetates. The preferred solvent is propylene carbonate. The substantially delignified cellulosic material is a fibrous material capable of absorbing the polyisocyanate. The cellulosic material can be, but is not limited to, pulp cellulose fibers, chemical pulps, thermomechanical pulps, bleached and unbleached paper and paper-like materials, non-woven mats, sheets, and felts.

A primary object of the present invention is to provide a method for producing a polyurea-cellulose composite, wherein the composite comprises about

8% to about 30% w/w polyurea relative to the mass of the cellulose and has structural characteristics comparable or superior to those found in analogous prior-art materials.

Another object is to provide a method for forming such a polyurea- cellulose composite having a thickness that can be greater than about 0.1 inch.

Another object is to provide a method for forming such a polyurea- cellulose composite employing application of heat and pressure for curing but wherein the composite, particularly such a composite having a thickness greater than 0.1 inch, does not experience delamination after curing is complete. Another object is to provide such a composite comprised of multiple plies of polyurea-impregnated cellulose, the composite having a thickness that can be greater than about 0.1 inch, and having bond strength and tensile strength properties that are superior to analogous prior-art materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows plots of water absorption versus soak time for polyurea- cellulose composite boards made according to the present invention, compared with analogous prior-art boards.

FIG. 2 shows plots of the increases in thickness resulting from soaking the boards of FIG. 1 in water for various periods of time.

FIG. 3 shows plots of the linear expansion (in the machine direction) of the various boards referenced in FIG. 1 after soaking in water.

FIG. 4 shows plots of the linear expansion (in the cross direction) of the various boards referenced in FIG. 1 after soaking in water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a substantially delignified cellulosic fibrous material is impregnated with a "neat" (undiluted) liquid polyisocyanate binder or with a liquid resin comprising a mixture of a polyisocyanate compound and an organic solvent miscible with the polyisocyanate.

The cellulosic materials contemplated by the present invention comprise substantially delignified cellulose fibers including, but not limited to, pulped fibers,

"chemical pulps," "thermomechanical pulps," recycled pulp fiber, bleached and unbleached paper and paper-like materials, non-woven mats, sheets or felts, and the like, including mixtures thereof. "Delignified" as used herein refers to cellulosic materials that have had most to substantially all of any indigenous lignin and analogous naturally-occurring binders removed. Delignification has been found to increase the opportunity for reaction between isocyanate groups on binder molecules and the hydroxyl groups present on cellulose molecules. A preferred cellulosic material is "kraft linerboard," which is a paper-like board made according to the kraft (sodium sulfate) method of papermaking. Kraft linerboards are manufactured in a number of basis-weight grades, ranging from about 26 lb basis weight (26 lb/1000 ft ² ) to about 90 lb basis weight (90 lb/1000 ft ² ).

The cellulosic fibrous material should have a residual moisture level of about 3 to 15% w/w (relative to the dry mass of the cellulose) to ensure the presence of sufficient water to participate in polymerization reactions whereby polyisocyanate becomes polyurea.

The delignified cellulosic material is typically laid in substantially two- dimensional structures such as sheets. Sheets are generally easier to impregnate with the liquid polyisocyanate binder using conventional machinery. However, three- dimensional cellulosic structures may also be impregnated.

The binder used in the present invention is a polyisocyanate compound having at least two isocyanate (-NCO) groups per molecule. Such bi- or polyfunctional polyisocyanate molecules are capable of reacting with cellulosic hydroxyl groups to form cross-linked composite structures. Candidate isocyanates include lower (C _j - C ₁₂ ) aϋphatic, alicyclic, and aromatic polyisocyanates. These polyisocyanates may be used as single compounds or mixtures of two or more different polyisocyanate compounds. The preferred isocyanate compound is a poly(diphenylmethane diisocyanate), referred to herein as "PMDI."

Suitable solvents according to the present invention must be miscible with the polyisocyanate. Candidate solvents include, but are not limited to, aromatics such as benzene, halogenated benzenes, nitrobenzenes, alkyl benzenes such as toluene and xylenes, halogenated lower aliphatics, ethers, ketones, alkyl acetates, other alkylene carbonates, and mixtures thereof.

The preferred organic solvent is propylene carbonate, principally because it is substantially odorless and colorless, has low viscosity, low toxicity, low vapor pressure at room temperature, and low flammability. Also, the high boiling point (240°C) of propylene carbonate is advantageous because it either (1) has a lower solvent vapor pressure than other candidate solvents or (2) is not vaporized under the temperature and pressure conditions associated with polyisocyanate curing. An amount of solvent in the resin, up to about 20% w/w relative to the polyisocyanate, confers surprisingly improved mechanical properties to the polyurea- cellulose composite made according to the present invention, compared to similar composites made using "neat" (undiluted) polyisocyanate. Improved properties become apparent when the amount of solvent is at least about 5 % . Using an amount of solvent greater than about 20% w/w may yield improved mechanical properties but such improved mechanical properties are usually counterbalanced by an unacceptably high degree of water-absorption. Hence, about 20% w/w solvent is regarded as the practical upper limit and about 5% w/w is regarded as the practical lower limit for the solvent relative to the polyisocyanate. The solvent dilutes the polyisocyanate binder and therefore decreases the cost of the resin. The solvent may also reduce the viscosity of the relatively

viscous polyisocyanate. Reduced resin viscosity improves the ability of the resin to penetrate the physical matrix of the cellulosic material. The solvent may also enhance the reactivity of the polyisocyanate with the cellulose molecules through solvation effects. Also, propylene carbonate may serve as a copolymerizable reactant with the polyisocyanate. Propylene carbonate has been reported to react in the presence of a catalyst with organic polyisocyanates to form isocyanurates under certain conditions. ____, Tzuzuki et al., "New Reactions of Organic Isocyanates. I. Reaction With Alkylene Carbonates," J. Org. Chem. 25:1009 (1960). The resin is formed by combining the polyisocyanate and the solvent in the desired w/w ratio either by a batch or a continuous process using equipment known in the art.

The liquid polyisocyanate binder is preferably prepared and cured without an added catalyst. However, a catalyst can be used without departing from die principles of the present invention. But, certain problems may occur with the use of a catalyst, such as resin "pre-cure." Pre-cure is a too-rapid onset of the polymerization reaction. Pre-cure is characterized by excessive polyurea formation after the cellulosic material is impregnated with the binder but before heat and pressure are deliberately applied to cure the liquid binder. Pre-cure can cause inconsistent quality of the composite and/or general degradation of physical properties of the composite such as internal bond strength. If pre-cure can be controlled, a suitable catalyst can be selected from a variety of catalysts used in the art for isocyanate polymerization, including aliphatic tertiary amines such as 1,4- diazobicyclo-(2,2,2)-octane ("Dabco"), aliphatic metal complexes such as dibutyltin dilaurate or tin octoate, and acetyl acetonates.

The liquid polyisocyanate binder is applied to the cellulosic material in any manner capable of achieving satisfactory cellulosic fiber impregnation. Binder loading levels according to the present invention are within a range of about 8% to about 30% w/w polyisocyanate binder relative to the mass of the cellulose. Binder loading levels within this range yield polyurea-cellulose composites exhibiting excellent end-use properties at reasonable manufacturing costs. -Binder loading levels greater than about 22 % w/w, however, generally require that curing of the composite include a "breathing cycle" as discussed in detail hereinbelow.

Where the cellulosic material is in sheet form, the binder can be applied on one or both sides of each sheet by spraying, dipping, rolling or other suitable process known in the art. Impregnation can occur using a batch or a continuous process, each employing different types of machinery. A continuous process is especially suitable when the cellulosic material is provided as a sheet in the form of a continuous roll such as with various papers and linerboards. A two- roller applicator is an effective means for continuously applying binder to the cellulosic material in such a form. Accordingly, a sheetlike cellulosic material can be fed through a continuous bead of resin lying in the nip zone of two press rollers. The press rollers force the liquid binder into the fiber matrix of the cellulosic material. A gravure coater or a dip-bath containing the binder through which the cellulosic material is passed can also be effective applicator means. However the binder is applied, it is important that the binder substantially uniformly penetrate through the "Z" or thickness dimension of the cellulosic material. The polyisocyanate binder impregnating the cellulosic material is cured

(or "densified") by application of heat and pressure. Heat can be applied by any suitable method including, but not limited to, conduction, microwave-heating, RF heating, or other electromagnetic radiation means. Pressure can be applied using any of a number of suitable pressurizing means known in the art including, but not limited to, a platen press, a continuous double-belt press, a "Mende" press (belt-drum), a membrane press, or autoclave.

Two alternative methods are discussed hereinbelow as representative specific examples, not intended to be limiting, of methods for curing the binder according to the present invention. A first curing method comprises loading a polyisocyanate-impregnated cellulosic material into a preheated press such as a platen press or other suitable press. Before loading, the press is preheated to a temperature within the range of about 325°F to about 400°F. A preferred press temperature is 375°F.

When employing a preheated press to effect curing, the preferred loading level is about 15% w/w. The thickness of the polyisocyanate-impregnated cellulose is limited principally by shortcomings of available heating and pressurizing

apparatuses. With presses utilizing conduction heating, the thickness is preferably about 0.01 inch to about 1.25 inches.

Li the preheated press, the polyisocyanate-impregnated cellulose is subjected to a curing pressure within a range of about 300 psig to about 1200 psig. A preferred pressure is 800 psig. Thus, the polyisocyanate-impregnated cellulose loaded into the press is "hot pressed" until a "core" temperature within the range of about 275°F to 350°F is reached. The "core" as used herein is located midway through the thickness dimension of the impregnated cellulosic material. A preferred core temperature is about 300°F, and a typical core temperature heating rate in the press is about 5 to 300° F per minute. It will be appreciated that heating rates of the core are highly dependent upon the thickness of the composite.

After the desired core temperature is reached, at which curing is deemed completed, the curing pressure in the press is maintained while the press is allowed to actively or passively cool. (Tn the interest of expediency, active cooling is preferred.) The press is cooled until the core temperature drops to within a range of about 100°F to less than 212° F. A preferred cooled core temperature is about 150°F, and a typical cooling rate is about °5 to 50° F per minute. Afterward, the pressure in the press is allowed to drop to atmospheric so that the resulting polyurea-cellulose composite can be removed therefrom. An alternative curing method according to the present invention comprises curing ("densifying") the polyisocyanate-impregnated cellulose material in a press without first preheating the press. In other words, the press at time of loading the impregnated material therein is at or nearly at ambient ("room") temperature. For curing, the press applies a curing pressure within the range of about 300 psig to about 1200 psig. After the polyisocyanate-impregnated material is loaded into the press, the press is heated until a target core temperature of about 275°F to about 350°F is reached. A typical heating rate is about 5 to 300° F per minute. After the desired core temperature is reached, the curing pressure in the press is maintained while the press is actively or passively cooled at a rate of about 5 to 50°F per minute. After the core temperature drops to within a range of about 100°F to less than 212°F, the pressure in the press is lowered to atmospheric. The

preferred core temperature at time of lowering the pressure to atmospheric is about 150°F.

It has been found that densifying the polyisocyanate-impregnated cellulose using a non-preheated press can help to prevent premature curing of the polyisocyanate. Also, a relatively cool press at the time the polyisocyanate- impregnated cellulose is loaded in the press minimizes toxicity risks associated with volatilized polyisocyanate vapors.

In any curing method according to the present invention, a "breathing cycle" can be employed. A "breathing cycle" is a temporary (about four to ten seconds) lowering of curing pressure to atmospheric while the polyisocyanate- impregnated cellulose is being cured under heat and pressure. A breathing cycle allows higher resin loading levels to be used, up to about 30% w/w, without necessarily causing delamination of the composite when curing is completed. This is because higher polyisocyanate loading levels cause more CO ₂ to be generated during curing. The breathing cycle allows a portion of the entrapped gases (particularly CO ₂ and water) to be released from inside the hot impregnated cellulose material before the material has become fully cured.

To illustrate a "breathing cycle" and not intended to be limiting, sheets of delignified cellulose impregnated with 10-30% w/w PMDI are inserted into a press preheated to a temperature of about 325 to about 400°F. The press then applies about 300-1200 psig curing pressure to the polyisocyanate-impregnated- cellulose material. (The curing pressure serves to promote heat transfer into the product rather than to consolidate the structure by curing the polymer.) After the core temperature reaches about 225 °F, before the polyisocyanate completely cures, the pressure is gradually lowered and temporarily released to atmospheric for a period of about 4 to about 10 seconds to allow vaporized moisture and carbon dioxide, as well as other gases, to escape from the material. The curing pressure is then reapplied until the target core temperature rises to within the range of about 275-350°F. If necessary or desired, several such "breathing cycles" can be employed while curing a polyisocyanate-impregnated cellulosic material according

to the present invention. Multiple breathing cycles are particularly advantageous when curing unusually thick composites. For example, a breathing cycle can be initiated each time a progressively "deeper" location (toward the core) in the material reaches about 225 ^C F. One ply of a polyurea-cellulose composite can be produced according to the present invention by curing only a single "lamina" or ply of polyisocyanate- impregnated cellulosic material. Multiple plies can also be made according to the present invention by curing superposed laminae of polyisocyanate-impregnated cellulosic material. A higher curing pressure for multiple plies ensures good interply contact and adhesion during curing. In addition, multiple breathing cycles may be particularly advantageous when curing multiple-ply composites.

The press temperature required to cure the polyisocyanate is greater than the boiling points of water at low pressure (at or near atmospheric), present as residual moisture associated with the cellulosic materials. Carbon dioxide is in a gaseous state during application of heat and pressure to a polyisocyanate-impregnated cellulosic material. The water can be present either in the liquid or gaseous state, depending upon local temperature and pressure conditions within the material. However, if the pressure of a hot composite is abruptly lowered after full curing of the polyisocyanate resin, entrapped high-pressure vapor in the interior of the hot composite will seek immediate release to the atmosphere. In addition, high-pressure water present in the liquid phase will "flash" to steam and seek immediate release to the atmosphere. Such entrapped vapor will apply considerable expansive forces to a composite. If such forces exceed the composite's internal-bond strength, delamination of the composite can result. However, if the fully cured composite is cooled with sustained application of pressure according to the present invention, such as by cooling in a press until the core temperature lowers to within a range of about 100°F to less than 212°F, the vapor pressures of water and carbon dioxide can be reduced sufficiently to alleviate a large portion of the expansive force exerted by entrapped vapor after completion of curing. In particular, such cooling converts water vapor back to liquid

water, thereby substantially eliminating any explosive force that would otherwise be generated by water.

Polyurea-cellulose composites made according to the present invention exhibit excellent dimensional stability and resist swelling when exposed to moisture or high humidity. Also, an unexpected benefit is an increased composite density of about 6 to 17% relative to other composite materials having similar amounts of impregnant. It was initially believed that any increase in density achieved by a process according to the present invention would be negated by "spring back": a post-curing rebound of composite thickness to the thickness dimension of the composite prior to curing. Surprisingly, however, most of the increased density imparted during curing according to the present invention is retained after curing. Associated with this increased density are increases in material stiffness and strength, as discussed in further detail below.

Polyurea-impregnated composites according to the present invention also warp and twist less than analogous materials made according to prior-art processes. The reasons for the reduced warp and twist are not entirely known. It may be that residual water in the cellulosic fibrous material participates in the polyisocyanate curing reaction and becomes unavailable for causing subsequent warping phenomena. It may also be that some of the residual water in a composite according to the present invention becomes water of hydration in the polyurea matrix, rendering the matrix less able to absorb substantially more water. Also, because composites according to the present invention are remarkably uniform in composition, the possibility of warp and twist is also reduced. In addition, composites according to the present invention are released from the press (or other pressurizing device) at higher moisture contents than composites made by other known methods. Therefore, composites according to the present invention absorb less water while re-equilibrating to normal atmospheric conditions. Non-uniform absorption of water vapor by cellulosic composites is a known cause of twist and warp. FIGS. 1-4 summarize various tests performed with a 64-ply (0.66-inch thick) polyurea-cellulose composite board made according to the present invention.

The polyisocyanate-impregnated cellulose used to make the board had a resin loading level of 20-22% w/w. The impregnated cellulose was cured to a core temperature of 350° F and 800 psig in a press. No breathing cycles were employed during curing. Similar tests were also performed with boards of birch and maple plywood, and boards made from PERMAPLEX 1 and 2 manufactured by EHV Weidmann of St. Johnsburg, Vermont.

FIG. 1 comprises plots of water absorption by the various boards as a function of time. To perform the test, boards measuring 4 inches by 4 inches by 0.66 inches thick were soaked in water at 70°F for various lengths of time. FIG. 1 shows that the 64-ply composite made according to the present invention has virtually zero water absorption, even after 250 hours. The maple and birch plywood boards show a water absorption of about 38-40% after 50 hours.

FIG. 2 shows a plot of the thickness swell of the boards of FIG. 1 as a function of time. The 64-ply board made according to the present invention experienced a thickness swell of virtually zero, even after 270 hours. This was the lowest thickness swell of any of the boards tested.

FIG. 3 shows the machine direction (MD) linear expansion of the six boards of FIG. 1 as a function of soak time. The 64-ply board made according to the present invention exhibited zero linear expansion after 120 hours of soak time. The MD linear expansion of the maple and birch plywood boards ranged from about +0.4 to +0.5%, whereas the PERMAPLEX 1 and 2 boards actually shrank about 0.2 to about 0.3% after 100 hours of soak time.

FIG. 4 shows the cross-direction (CD) linear expansion of the various boards of FIG. 1 as a function of soak time. The 64-ply board made according to the present invention exhibited virtually no CD linear expansion after about 150 hours soak time. The PERMAPLEX boards actually shrank about -0.3 % to about - 0.4 % . The plywood boards exhibited a CD linear expansion of about 0.4 % to about 0.5%.

FIGS. 1-4 indicate that polyurea-cellulose composites made according to the present invention have water absorption, linear expansion, and thickness swell properties that are superior to analogous products known in the art.

Table I presents additional information comparing a 64-ply polyurea- cellulose composite board made according to the present invention to the birch and maple plywood boards, and to the PERMAPLEX 1 and 2 boards. Table I indicates that the 64-ply board typically has physical parameters that are either comparable to or superior to the other boards tested. In particular, the 64-ply board, although it has a lower density than the PERMAPLEX boards, has much higher tensile and internal bond strengths than the PERMAPLEX boards.

The present invention has been described with reference to preferred embodiments. However, it should be understood that the invention can be modified without departing from such principles. We claim as our invention all such modifications as fall within the scope of the following claims.