Process for producing wood particleboard

Details Process for producing wood particleboard Process for producing wood particleboard

2001-10-31

2003-07-29

Lechert, Jr., Stephen J. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming articles by uniting randomly associated particles

With subsequent cutting, grooving, breaking, or comminuting

C264S109000, C264S123000

Reexamination Certificate

active

06599455

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing wood particleboard by bonding together with the wood particles by means of a crosslinkable binder.
2. Background Art
To produce wood particleboard, the wood particles, for example wood fibers or wood chips, are glued together by means of an organic adhesive under pressure and at elevated temperature. The most important binders for chipboard production are urea-formaldehyde resins (UF resins). To produce moisture-resistant wood chipboard, phenol-formaldehyde resins (PF resins) are of great importance. Melamine-formaldehyde resins (MF resins) or mixtures of all three resin types mentioned are also used for improving the moisture resistance of wood particleboard. A disadvantage of these adhesives is that formaldehyde is emitted both during production of the particleboard and during use of the finished, pressed particleboard. Organic isocyanates have become established as formaldehyde-free binders for wood particleboard, in which case diphenylmethane 4,4′-diisocyanate (MDI) is generally used. However, handling MDI also requires special safety precautions during production of the wood particleboard because of the very reactive isocyanate groups and the associated irritation of skin and bronchial passages. A further disadvantage of these reactive resins becomes apparent in the production of MD and HD fiberboard: here, the isocyanate groups react in the blow-line during the direct addition of binder because of the elevated temperature, which leads to a loss of up to 25% of the binding potential.
Thermoplastic adhesives, for example those based on polyvinyl acetate, are likewise used for producing wood particleboard, as disclosed in U.S. Pat. No. 4,940,741. A problem with polyvinyl acetate binders is dimensional stability over the relatively high temperature range used in particleboard production, for example during removal from the mold. For this reason thermoplastic adhesives are frequently used in combination with the abovementioned formaldehyde condensate resins, with the abovementioned disadvantages. A low-VOC adhesive for bonding wood particleboard is described in DE-A 19941904. This adhesive is an aqueous mixture of high-viscosity polyvinyl alcohol, carboxyl-functional polyvinyl acetate and a water-soluble metal salt of a monobasic acid.
Formaldehyde-free, thermally curable, aqueous binders for producing wood particleboard are known, for example, from WO-A 97/31059. In this publication, a mixture of carboxyl-functional copolymer and an alkanolamine having at least two hydroxy groups is used. Aqueous adhesive compositions comprising polycarboxylic acid and hydroxyalkyl-substituted aminoaliphatics are described in WO-A 97/45461. WO-A 99/02591 discloses compositions comprising a carboxyl-functional copolymer and long-chain amines. A disadvantage of these systems, which crosslink via an esterification reaction, is that crosslinking occurs only in the water-free state, on drying.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a process for producing wood particleboard in which the emission of pollutants such as formaldehyde is avoided but high-quality bonding is nevertheless obtained, and premature reaction of functional groups is largely prevented. These and other objects are achieved by the binder systems of the subject invention, which comprise a copolymer containing carboxyl, hydroxy, or NH-functional monomers, and an epoxy, N-methylol, isocyanate, or ethylene carbonate-functional crosslinker reactive with the functional groups of the copolymer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The invention provides a process for producing wood particleboard, in which wood particles, if appropriate after a preceding comminution step, are treated with a crosslinkable binder and are subsequently pressed together under pressure and, if desired, elevated temperature to produce particleboard, wherein the binder comprises two constituents A) and B), where A) comprises a copolymer comprising one or more comonomer units selected from the group consisting of vinyl esters of unbranched or branched alkylcarboxylic acids having from 1 to 18 carbon atoms, acrylic esters or methacrylic esters of branched or unbranched alcohols having from 1 to 15 carbon atoms, dienes, olefins, vinylaromatics and vinyl halides, and from 0.1 to 50% by weight, based on the total weight of the copolymer, of one or more units comprising carboxyl, hydroxy or NH groups, and B) comprises at least one compound having at least two reactive groups which can react with the carboxyl, hydroxy or NH groups specified under A) and are selected from the group comprising epoxy, N-methylol, isocyanate and ethylene carbonate groups.
Suitable vinyl esters are vinyl esters of unbranched or branched carboxylic acids having from 1 to 18 carbon atoms. Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of &agr;-branched monocarboxylic acids having from 5 to 11 carbon atoms, for example VeoVa9
R
or VeoVa10
R
vinyl esters available from Shell. Particular preference is given to vinyl acetate.
Suitable monomers from the group consisting of esters of acrylic acid or methacrylic acid are esters of unbranched or branched alcohols having from 1 to 15 carbon atoms. Preferred methacrylic esters or acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate and norbornyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate and norbornyl acrylate.
Suitable dienes are 1,3-butadiene and isoprene. Examples of copolymerizable olefins are ethene and propene. Vinylaromatics which can be copolymerized include styrene and vinyltoluene. From the group of vinyl halides, use is usually made of vinyl chloride.
Suitable carboxyl-functional comonomers for copolymer A) are ethylenically unsaturated monocarboxylic and dicarboxylic acids, preferably acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid and maleic acid. The carboxyl function can also be introduced into the copolymer A) by copolymerization of maleic anhydride. Suitable hydroxy-functional comonomers are hydroxyalkyl acrylates and hydroxyalkyl methacrylates having a C
1
-C
8
-alkyl radical, preferably hydroxyethyl acrylate and methacrylate, hydroxypropyl acrylate and methacrylate, and hydroxybutyl acrylate and methacrylate. Suitable NH-functional comonomers are (meth)acrylamide, diacetoneacrylamide, maleimide, amides of monoalkyl maleates and fumarates, diamides of maleic and fumaric acids, amides of monovinyl glutarates and succinates, and amides of monoallyl glutarates and succinates. The NH-functional units can also be introduced into the copolymer A) as aminofunctional oligomers containing primary or secondary NH groups, preferably those containing primary NH groups such as Jeffamine® amines. The proportion of functional units in copolymer A) is preferably from 1 to 30% by weight, more preferably from 5 to 20% by weight, in each case based on the total weight of the copolymer.
Preference is given to the following base polymer compositions for the copolymer A) which in addition to the base monomers, further comprise the abovementioned functional groups: vinyl acetate polymers; vinyl ester-ethylene copolymers such as vinyl acetate-ethylene copolymers; vinyl ester-ethylene-vinyl chloride copolymers in which the vinyl esters present are preferably vinyl acetate and/or vinyl propionate and/or one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, vinyl esters of alpha-branched carboxylic acids having from 5 to 11 carbon atoms, in particular vinyl esters of Versatic acid, i.e. VeoVa9
R
and/or VeoVa10
R
; vinyl acetate copolymers with one or more

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