Method of tempering fiberboard doorskin using liquid thermosets

Details Method of tempering fiberboard doorskin using liquid thermosets Method of tempering fiberboard doorskin using liquid thermosets

2001-07-27

2002-09-17

Cameron, Erma (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Heating or drying

Reexamination Certificate

active

06451384

ABSTRACT:

FIELD OF THE INVENTION
The invention is directed to a method of tempering fiberboard with a substantially solvent-free liquid tempering composition. In particular, the invention is directed to a method of tempering fiberboard with a substantially solvent-free liquid tempering composition comprising at least one thermoset material.
BACKGROUND OF THE INVENTION
Fiberboard is typically manufactured from cellulosic fibers such as wood fibers. The principal processes for the manufacture of fiberboard include “wet” processes, “dry” processes and “wet-dry processes”, wherein the cellulosic fibers based on wood products e.g., wood fibers, chips, flakes, dust, etc. undergo several steps to form an integral consolidated structure, e.g., a board or a panel, under heat and pressure. A post-treatment of the basic board to further bind and reinforce the surface fiber, known as tempering, is often carried out using various drying or tempering oils followed by heat curing or baking in an oven at about 250° F. or greater for several hours to cure the tempering oil. To date, the most commonly used tempering material or tempering oil is linseed oil due to its low viscosity and low cost. The use of linseed oil, however, requires an additional heating step to cure, which inevitably increases both the cost and the time associated with producing the fiberboard. Because linseed oil is a mixture of several components having different molecular weights, it also tends to give off volatile organic component (VOC), e.g., low molecular weight organic components during the heat curing, which is environmentally undesirable.
SUMMARY OF THE INVENTION
In one aspect, the invention features a method of tempering fiberboard. The method includes (a) applying a substantially solvent-free, liquid tempering composition on at least one surface of the fiberboard, and (b) curing the tempering composition. The tempering composition includes at least one thermoset material and has a viscosity at application temperature sufficiently low to allow the composition to wet out the surface and penetrate into the fiber structure of the fiberboard. The tempered fiberboard exhibits door skin cleavage strength of at least about 8 lb.
In one embodiment, the tempering composition is applied to both the face surface and the back surface of the fiberboard.
In some embodiments, the tempering composition comprises one-part moisture cure polyurethane prepolymer that can be cured at ambient conditions, e.g., ambient temperature and relative humidity, such that the heat-curing step required by the conventional tempering processes is not needed.
In another aspect, the invention features a tempered fiberboard manufactured by an above-described method.
In some embodiments, the fiberboard is tempered by a tempering composition including one-part moisture cure polyurethane prepolymer such that, upon cure, the tempered fiberboard exhibits either comparable or stronger surface and internal fiber bonds relative to the fiberboard tempered with linseed oil. In one embodiment, the tempered fiberboard exhibits door skin cleavage strength of at least about 8 lb. In another embodiment, the tempered fiberboard exhibits door skin cleavage strength of at least about 9 lb. In other embodiments, the tempered fiberboard exhibits door skin cleavage strength of at least about 10 lb. The tempered fiberboard also exhibits a tape pull value of at least about 22 lb.; a cross hatch adhesion rating of at least 3, preferably, at least 4; and percentage substrate failure (% substrate failure) of from about 33% to about 100%.
The tempering composition of the invention contains a very low content of volatile organic component, preferably, no greater than about 5.0 g/l. Hence, the method of the invention can proceed without emitting volatile organic components (VOC) that are associated with the use of linseed oil in the conventional tempering processes.
The method of invention can also proceed without the need to heat or bake the tempering composition to achieve cure, thereby saving the energy cost and the time associated with manufacturing the tempered fiberboard. More surprisingly, by using the tempering composition of the invention, the tempered fiberboard, even cured at lower temperature, e.g., at ambient temperature, exhibits stronger surface and internal fiber bond strengths relative to the linseed oil tempered fiberboard.
DETAILED DESCRIPTION OF THE INVENTION
The tempering composition includes at least one thermoset material and is substantially solvent free. That is, the tempering composition does not contain water and does not contain any substantial amount, e.g., greater than about 10 wt % and in some cases, greater than about 5 wt %, based on the total weight of the tempering composition, of organic solvent(s). Preferably the tempering composition is a 100% solids liquid. The tempering composition is formulated such that at application temperature it wets out the surface(s) of the fiberboard, and preferably penetrates into the fiber structure of the fiberboard. When the tempering composition is applied on at least one surface of the fiberboard, the desirable average penetration depth into the fiberboard is preferably from about 0.01 inch to about 0.02 inch, more preferably about 0.015 inch.
The tempering composition can be formulated to have a viscosity suitable for the type of application being used. Preferably, the viscosity of the tempering composition is such that the tempering composition does not form an appreciable coating film on the surface(s) of the fiberboard. Preferably, the tempering composition is formulated to have a viscosity of no greater than about 10,000 cps, more preferably no greater than about 3,000 cps, most preferably no greater than about 500 cps at 77° F.
Useful thermoset materials include, e.g., two-part epoxy resins, two-part polyurethane, two-part acrylates, moisture cure thermoset resins and UV curable resins.
Moisture cure thermoset resins are thermoset resins that can be cured by contacting with atmospheric moisture at ambient temperature. Moisture cure thermoset resins preferably are 100% solids liquid at ambient temperature. Useful moisture cure thermoset resins include, e.g., alkoxy, acetoxy, oxyamino silane terminated polyethers and polyether urethanes; alkyl siloxane polymers crosslinked with alkoxy, acetoxy, and oxyamino organo functional silanes, or combination thereof; moisture curable isocyanate functional polyoxyalkaline polymers and polyalkaline polymers; thiol functional polymers and oligomers such as polyethers, polyesters, polyether urethanes and polythioethers; hydroxy-terminated polybutadiene; hydroxy-terminated acrylic polyols; and amine-terminated polyethers. Preferred moisture cure thermoset resins include one-part moisture cure polyurethane.
Preferred one-part moisture cure polyurethane includes an isocyanate terminated prepolymer that includes the reaction product of at least one isocyanate compound and at least one dihydroxy polyol. Examples of useful dihydroxy polyols include polyester polyols, polyether polyols, polyalkylene polyols, and mixtures thereof.
The one-part moisture cure polyurethane preferably has a viscosity of no greater than about 15,000 cps at 77° F. and a weight average molecular weight (Mw) of no greater than about 3,000.
The tempering composition may further include at least one viscosity modifier to reduce the viscosity of the composition. Suitable viscosity modifiers exhibit low viscosity and a flash point of greater than about 100° F., more preferably greater than about 250° F. Suitable viscosity modifiers are preferably liquid at ambient temperature and do not contain any functional groups including e.g., hydroxyl, amine, carboxyl, or thiol groups, which would react with the isocyanate groups in the polyurethane chain. The viscosity modifier preferably has a viscosity of no greater than about 200 cps, more preferably from about 10 cps to about 200 cps at 77° F.
Examples of useful viscosity modifiers include mineral oils; vegetable oils; hydrocarbon oils that are low in aromatic content

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