POLYISOCYANATE ADHESIVE FOR PLYWOOD

Abstract: Use of a polyisocyanate composition as an adhesive in lignocellulosic-based structural lamination manufacture characterised in that said polyisocyanate composition comprises an isocyanate-containing prepolymer composition having an NCO content of from 5 to 25 % by weight as obtainable by reacting an isocyanate-reactive polymer composition having an average nominal functionality of from 2 to 4 and an average equivalent weight of from 500 to 3000 with a stoichiometric excess of a diphenylmethane diisocyanate compositon.

Description: Polyisocyanate Adhesive for Plywood DESCRIPTION This invention relates to an adhesive composition comprising isocyanates useful for manufacturing plywoods. Various forms of plywood have found utility as structural or load-bearing components in the construction industry. Plywood is a glued wood panel comprised of relatively thin layers or plies. Generally, in the production of plywood, a wet log is taken out of water, and then sawed to a desired length and cut by means of a rotary lathe to wet veneers of 0.5 to 7 mm in thickness, which are then dried, coated with an adhesive composition, laminated, and bonded by hot-pressing to obtain plywood. For the production of plywood, adhesives consisting mainly of thermosetting resins such as phenolic resins or urea resins have previously been used, but these adhesives suffer from various defects. At present phenol-formaldehyde (PF) resins dominate the worldwide market for the production of exterior and marine-grade plywood. However, PF resins present a problem in that a variety of formulations are employed due to the very different types of wood used for veneers. Furthermore, PF resins have some serious limitations. For instance, they are not able (i) to consistently bond veneers of higher moisture content (8 to 12 %), (ii) to bond equally well with the same formulation medium density veneers and high density veneers (o.6 to 0.8 kg/m3) into structural plywood and to accomplish all the above with lower adhesive spread levels. A resin of any nature or composition which is capable of overcoming all the defects of PF resins listed above, while still maintaining their excellent performance, would indeed be a great step forward in adhesives for exterior and marine-grade plywood. Diisocyanates (in particular, polymeric MDI) have been known in the wood industry for almost 20 years. These materials can produce excellent results when used as adhesives for paniculate products but have been recognised as unsuitable for plywood. It is an object of the present invention to provide a process for producing plywood having excellent bond strength, excellent water-proofhess, and excellent ease of bending using an isocyanate adhesive. The present invention provides a composition comprising an isocyanate-containing prepolymer composition having an NCO content of from 5 to 25 % by weight, preferably 8 to 20 % by weight, most preferably about 15 % by weight as obtainable by reacting an isocyanate-reactive polymer composition having an average nominal functionality of from 2 to 4 and an average equivalent weight of from 500 to 3000, preferably of from 1000 to 2000 with a stoichiometric excess of a diphenylmethane diisocyanate composition for use as an adhesive in structural lamination manufacture (for example, plywood). The term "nominal functionality" refers to the functionality, with respect to isocyanates, that an isocyanate-reactive polymer would be expected to have with regard to its monomeric components. Thus, for a polyether polyol, the average nominal functionality is the average functionality (number of active hydrogen atoms) of the initiator or initiators used in its preparation. The diphenylmethane diisocyanate composition used to make the prepolymers can contain so-called polymeric MDI which is a mixture of diphenylmethane diisocyanates and higher functional oligomers thereof. Thus mixtures can be used containing pure MDI and so-called polymeric MDI. The average isocyanate functionality of the MDI composition is preferably from 2 to 2.3 although higher functionality MDI compositions can be used as well. Preferably the MDI composition contains at least 60 %, preferably 100 % by weight of difunctional MDI's: 2,4'-, 2,2'- and 4,4'-isomers of diphenylmethane diisocyanate and mixtures thereof. Preferably the MDI composition contains at least 60 %, preferably at least 80 % by weight of diisocyanate components of 4,4'-MDI and not more than 40 % by weight, preferably less than 20 % by weight of the 2,4'-isomers and 2,2'-isomers of MDI. Unmodified forms of MDI can be used as well as modified forms that is to say MDI modified in known manner by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues. Still further diphenylmethane diisocyanate compositions which may be used in preparing the prepolymer include mixtures of the above described MDI types and up to 20 % by weight of another polyisocyanate or mixture of polyisocyanates. Other polyisocyanates which may be used in admixture with the MDI include aliphatic, cycloaliphatic and araliphatic polyisocyanates, especially diisocyanates, for example, hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane-l,4-diisocyanate, 4,4'-dicyclohexylmethane diisocyanates and m- and p-tetramethylxylene diisocyanates and, especially, aromatic polyisocyanates such as tolylene diisocyanates and phenylene diisocyanates. The average nominal functionality of the isocyanate-reactive composition is from 2 to 4, preferably 2 to 3. Preferred average equivalent weights lie in the range from 1000 to 2000. Mixtures of two or more isocyanate-reactive polymers varying in functionality, equivalent weight and/or chemical constitution (end groups or backbone) may be used provided such mixtures conform to the average functionality and average equivalent weight criteria specified herein. Isocyanate-reactive groups which may be present in the isocyanate-reactive polymer include primary amine, secondary amine, thiol, carboxy, enamino and, especially, hydroxyl groups. Particularly important isocyanate-reactive polymers include polymeric polyols. Suitable polyols and methods for their preparation have been fully described in the prior art and, as examples of such polyols, there may be mentioned polyesters, polyesteramides, polythioethers, polycarbonates, polyacetals, polyolefins, polysiloxanes and, especially, polyethers. Polyether polyols which may be used include products obtained by the polymerisation of a cyclic oxide, for example, ethylene oxide, propylene oxide, butylene oxide or tetrahydrofuran in the presence, where necessary, of polyfunctional initiators. Suitable initiator compounds contain a plurality of active hydrogen atoms and include water and polyols, for example, ethylene glycol, propylene glycol, diethylene glycol, cyclohexane dimethanol, resorcinol, bisphenol A, glycerol, trimethylolpropane, 1,2,6-hexanetriol or pentaerythritol. Mixtures of initiators and/or cyclic oxides may be used. Especially useful polyether polyols include polyoxypropylene diols and triols and poly(oxyethylene-oxypropylene) diols and triols obtained by the simultaneous or sequential addition of ethylene and propylene oxides to di- or trifunctional initiators as fully described in the prior art. Random copolymers having oxyethylene contents of 10 to 80 %, block copolymers having oxyethylene contents of from 2 to 30 %, preferably from 5 to 25 % and random/block copolymers having oxyethylene contents of up to 50 %, based on the total weight of oxyalkylene units may be mentioned. Mixtures of the said diols and triols can be particularly useful. Other particularly useful polyether polyols include polytetramethylene glycols obtained by the polymerisation of tetrahydrofuran. Particularly useful are also mixtures of polypropylene-polyethylene oxide polyols with up to 5 % of another polyol, for example a polyalkylene oxide, a polyester polyol, a polycarbonate polyol, a polyacetal polyol or a polytetramethylene glycol. A particularly interesting isocyanate-reactive composition is one consisting of polyether polyols having an average oxyethylene content of from 15 to 60% by weight of total oxyalkylene residues due to the presence therein of at least one polyoxyalkylene polyol containing oxyethylene (ethylene oxide) residues. Preferred polyol components comprise at least one poly(oxyethylene-oxypropylene) polyol each having an oxyethylene content in the range from 5 to 90 % on a weight basis of total oxyalkylene residues. Other useful polyol components in this category contain a mixture of polyols including polyols, for example poly(oxyethylene-oxypropylene) polyols, polyoxypropylene polyols and/or polyoxyethylene polyols, having oxyethylene contents outside the 5 to 90 % range provided the overall oxyethylene content of the component is within the specified range. Such mixtures may optionally contain one or more poly(oxyethylene-oxypropylene) polyol having an oxyethylene content in the 5 to 90 % range. In addition to the possibility of using mixtures of polyols varying in oxyethylene content, mixtures of two or more polyols varying in functionality, equivalent weight and/or polymer backbone may be used provided such mixtures conform to the average functionality and average equivalent weight criteria specified herein. A preferred isocyanate-reactive composition for use in the present invention comprises two poly(oxyethylene-oxypropylene) polyols of functionality 2 to 4 and equivalent molecular weight 1000 to 2000 in a weight ratio of 30:70 to 70:30, preferably about 50:50, one of which is a block copolymer containing 5 to 25 %, preferably 8 to 20 %, more preferably about 15 % by weight of ethylene oxide units (EO-tipped), the other a random copolymer containing 30 to 90 %, preferably 50 to 80 %, more preferably about 75 % ethylene oxide units. Polyester polyols which may be used include hydroxyl-terminated reaction products of polyhydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, 1 ,4-butanediol, neopentyl glycol, 1 ,6hexanediol, cyclohexane dimethanol, bis(hydroxylethyl) terephthalate, glycerol, trimethylolpropane, pentaerythritol or polyether polyols or mixtures of such polyhydric alcohols, and polycarboxylic acids, especially dicarboxylic acids or their ester-forming derivatives, for example succinic, glutaric and adipic acids or their dimethyl esters, sebacic acids, phthalic anhydride, tetrachlorophthalic anhydride or dimethyl terephthalate or mixtures thereof. Polyesteramides may be obtained by the inclusion of aminoalcohols such as ethanolamine in polyesterification mixtures. Polyesters obtained by the polymerisation of lactones, for example caprolactone, in conjunction with a polyol, or of hydroxy carboxylic acids such as hydroxy caproic acid, may also be used. Polythioether polyols which may be used include products obtained by condensing thioglycol either alone or with other glycols, alkylene oxides, dicarboxylic acids, formaldehyde, amino-alcohols or aminocarboxylic acids. Polycarbonate polyols which may be used include products obtained by reacting diols such as 1,3-propanediol, 1,4-butanediol, 1 ,6-hexanediol, diethylene glycol or tetraethylene glycol with diaryl carbonates, for example diphenyl carbonate, or with phosgene. Polyacetal polyols which may be used include those prepared by reacting glycols such as diethylene glycol, triethylene glycol or hexanediol with formaldehyde. Suitable polyacetals may also be prepared by polymerising cyclic acetals. Suitable polyolefin polyols include hydroxy-terminated butadiene homo- and copolymers and suitable polysiloxane polyols include polydimethylsiloxane diols and triols. Other useful isocyanate-reactive polymers for preparing the prepolymers include polymeric polyamines, especially diamines and triamines, corresponding to the above described polymeric polyols. Suitable polyamines of polyether polyols as described, for example, in US-P-3654370 or may be obtained by cyanoethylation of polyols followed by hydrogenation. Polyoxypropylene diamines and triamines and mixtures thereof are preferred. Also useful are polymers containing both amino and hydroxyl groups obtained by the partial amination of polyols. Further isocyanate-reactive polymers which may be used in preparing the prepolymers include imino-functional polymers. Such polymers have been described in US-P-4794129 together with methods for their preparation and include polymers terminating in imine, oxazoline, imidazoline, N-alkyl imidazoline, oxazine, diazine, imino-ester, amidine, imidine, isourea and guanidine groups. The preferred imino-functional polymers are imine-terminated polyethers such as may be obtained, for example by reacting a polyether polyamine, especially a polyoxypropylene diamine or triamine, with an aldehyde or ketone. Enamine functional polymers may be prepared either from secondary amine terminated resins (i.e. polyethers) by reaction with ketones/aldehydes having one or more alpha hydrogens, or by reacting ketone/aldehyde terminated resins (bearing alpha hydrogens) with secondary amines, providing for removal of the water formed in the reactions. Secondary amine terminated resins can be obtained, for example by catalytic hydrogenation of the imino-functional polymers described hereinabove. Ketone/aldehyde terminated resins may be obtained, in general, by oxidation of the corresponding secondary or primary hydroxyl terminated resin. More highly enamine functional polymers can be prepared by oxidising a primary hydroxy functional resin to the corresponding polycarboxylic acid, conversion of the said groups to orthoesters, end treatment of the latter, with an excess of a secondary amine. Each orthoester must contain at least one alpha hydrogen atom. The isocyanate-terminated prepolymer may be prepared by reacting the diphenylmethane diisocyanate composition with the isocyanate-reactive polymer under conditions that have been fully described in the prior art for the preparation of prepolymers. The viscosity of the prepolymer for use in the present process is preferably between 500 and 2000 MPas at 25C, preferably between 800 and 1100 MPas. The polyisocyanate composition further may comprise conventional additives like flame retardants, lignocellulosic preserving agents, fungicides, waxes, sizing agents, thickeners, fillers, catalysts, surfactants and other binders like formaldehyde condensate adhesive resins. In the process of manufacturing plywood the present adhesive in used in an amount of between 30 and 100, preferably between 40 and 60 g/m2, most preferably about 50 g/m2. The adhesive composition may be applied by conventional roller coating, both direct and indirect, spray application, dip application, or any application technique that is necessary, desirable or convenient. Plys of wood can be laminated in conventional fashion under pressures typically used in wood laminate or plywood production. The pressed laminates may be held at ambient indoor temperature for curing of the adhesive or may be heated. Structural laminates of excellent quality result by using the adhesive composition of the present invention. The following advantages are obtained over existing PF resins: the moisture content of the veneers can be higher (up to 25 %) which gives savings on production costs since the need for drying the veneers is smaller, easier handling of the veneers and less damage to the veneers due to the higher moisture content; shrinkage of the veneers is less (3 to 4 % compared to 7-8 % for PF-resins); shorter presstimes (about 40 s/mm compared to 50-60 s/mm for PF resins); reduced amounts of adhesive (about 50 g/m2 compared to about 180 g/m2 for PF resins) which leads to lighter boards. Very good mechanical properties are obtained and also with some compositions colorless joints are obtained. If desired, other conventional ahesives, such as formaldehyde condensate adhesive resins, may be used in conjunction with the polyisocyanate composition. More detailed descriptions of methods of manufacturing plywood and similar laminated wooden materials are available in the prior art. The techniques and equipment conventionally used can be adapted for use according to the present invention. The plywood produced according to the present invention has excellent mechanical properties and may be used in any of the situations where such articles are customarily used. The invention is illustrated but not limited by the following examples. SUPRASEC is a trademark of Imperial Chemical Industries. EXAMPLE Logs of wood were peeled and cut into pieces. The veneers were dried to a moisture content of 10 to 12 %. The adhesive, SUPRASEC 1024, available from Imperial Chemical Industries, was sprayed at a loading of 40 to 60 g/m2 onto the dried veneers using an airless spraying system. The glued veneers were stacked and hotpressed. The presstime was about 40 s/mm, the pressure 1.8 N/mm2. 12 mm thick birchplywood of 400 x 400 mm was obtained using veneers of thickness 1.5 mm. The bonding quality of the plywood was tested according to standards EN 314-1 and -2 and fulfilled the demands of the tests.

Claims: CLAIMS 1. Use of a polyisocyanate composition as an adhesive in lignocellulosic-based structural lamination manufacture characterised in that said polyisocyanate composition comprises an isocyanate-containing prepolymer composition having an NCO content of from 5 to 25 % by weight as obtainable by reacting an isocyanate-reactive polymer composition having an average nominal functionality of from 2 to 4 and an average equivalent weight of from 500 to 3000 with a stoichiometric excess of a diphenylmethane diisocyanate composition. 2 Use according to claim 1 wherein the NCO content of the prepolymer composition is from 8 to 20 % by weight. 3. Use according to claim 1 or 2 wherein the average equivalent weight of the isocyanate-reactive composition is from 1000 to 2000. 4. Use according to any one of the preceding claims wherein the diphenylmethane diisocyanate composition contains at least 60 % by weight of the 4,4'-isomer and less than 40 % by weight of 2,4'- and 2,2'-isomer. 5. Use according to any one of the preceding claims wherein the isocyanate-reactive composition comprises at least one poly(oxyethylene-oxypropylene) polyol. 6. Use according to claim 5 wherein the oxyethylene content of the isocyanate-reactive composition is from 15 to 60 % by weight of total oxyalkylene residues. 7. Use according to any one of the preceding claims wherein the polyisocyanate composition is used in an amount of between 30 and 100 g/m2. 8. Use of a polyisocyanate composition as an adhesive in plywood manufacture characterised in that the polyisocyanate composition is as defined in any one of the preceding claims.

Inventors: BOLANGIER LUC, SEPPALA RISTO, BOLANGIER, LUC, SEPPALA, RISTO
Applicants: BOLANGIER LUC, ICI PLC, SEPPALA RISTO, BOLANGIER, LUC, IMPERIAL CHEMICAL INDUSTRIES PLC, SEPPALA, RISTO
Application year: 1998
Main classification: B32B21
IPC codes: C08G18, C09J175