Binder compositions for lignocellulosic composites and process for the preparation thereof

Abstract: Binder compositions for lignocellulosic composites are de­ scribed,which compositions comprise an aqueous emulsion con­ taining at least one polyisocyanate and at least one water-­ dispersed vinylacetate resin having a glass transition tempe­ rature (Tg) below 5°C. Said compositions can endow the resina­ ted lignocellulosic material with green tack cohesion.

Description: The present invention relates to binder compositions for lignocellulosic composites and to a process for the preparation thereof. More particularly, the present invention relates to polyisocyanate-based binder compositions for lignocellulosic composites, to the process for the preparation thereof and to the use of said compositions for the manufacture of chipped panels or similar products. JP-A-60-258221 (Patent Abstracts of Japan, vol. 10, no. 131 (C-346) [2188] (1986)) discloses a molded chaff article obtained by kneading chaff with a mixture of a polyisocyanate compound and an ethylene-vinyl acetate based copolymer, molding the resultant mixture and heat-treating the molded article. The term "polyisocyanates" as used in the present description and the appended claims is intended to denote organic compounds of low, medium and/or high molecular weight containing at least two -NCO groups. Examples of low molecular weight polyisocyanates are those of the general formula: OCN - R - NCO (I) wherein R represents an alkylene, cycloalkylene, arylene or alkylarylene radical containing from 1 to 25 carbon atoms. Preferred radicals R are alkylene radicals having 1 to 12 carbon atoms, cycloalkylene radicals having 5 to 18 carbon atoms, arylene radicals having 6 to 18 carbon atoms and alkylarylene radicals having 7 to 18 carbon atoms. Examples of compounds of formula (I) include meta and/or para-phenylene diisocyanate, 2,4-toluene diisocyanate, alone or in combination with the isomeric 2,6-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1-isocyanato-6-isocyanatomethyl-3,5,5-trimethylcyclohexane (or isophorondiisocyanate) and 2,2,4-trimethylhexamethylene diisocyanate, optionally as mixture with the isomeric 2,4,4-trimethylhexamethylene diisocyanate. Examples of medium or high molecular weight polyisocyanates are those having a different degree of condensation which can be obtained by, e.g.,phosgenation of aniline-formaldehyde condensates. These products consist of mixtures of polymethylene-polyphenylene-isocyanates having the general formula: wherein n represents an integer higher than or equal to 1. Particularly preferred polyisocyanates are mixtures of polymethylene-polyphenylene-isocyanates having an average functionality of from 2.6 to 2.8. These products are sold under trade names such as "Tedimon ® 31" (Montedipe), "PaPi ®" (Upjohn) or "Mondur ® MR" (Mobay). It is known to use polyisocyanates, preferably in the form of aqueous emulsions, as binders for composites based on lignocellulosic materials, e.g. wood, bark, bagasse, straw, bamboo and rice chaff, in the form of granules, chips, fibers or meals, as described in EP-A-13 112 as well as in IT-A-20027 A/88. It is also known that polyisocyanates exert a cohesive action only when the molding of the composite is carried out at high temperatures, e.g. in the range of from 150 to 200°C. Consequently, when such binders are used the green tack cohesion of the mattress, that is the vibration- and shifting-tightness of the layers of resinated materials which are disposed on the bearing element and are cold prepressed at 20-30°C is practically zero, which results in apparent processaability limitations. Particularly in high-speed and/or multi-space plants there are encountered significant drawbacks, including even the impossibility of continuing the processing, as reported in "Holz als Roh- und Werkstoff" 1977, 35(8), 295-299 and 1985, 43(10), 423-427, and in Wilhelm-Klauditz Institut Bericht No.14, "Diisocyanat und Polyurethanklebstoffe für Holz und Holzwerkstoffe", Braunschweig (Federal Republic of Germany), March 1986. Therefore, the availability of polyisocyanate binder compositions for lignocellulosic composites which are capable of giving rise to green tack cohesion appears very valuable and highly interesting from an industrial point of view. However, results obtained so far in this field have been completely unsatisfactory even when products (e.g. resorcinol resins) giving good results in combination with adhesive products (e.g. amino adhesives) having a chemical structure different from isocyanates were used. H.J. Deppe and K. Ernst have reported on this matter in "Holz als Roh- und Werkstoff", 1977, 35(8), 295-299. It has now been found that a satisfactory green tack cohesion level of polyisocyanate binder compositions, to be used for the manufacture of chipped panels or similar manufactured articles, can be achieved when water-dispersed acetovinylic (vinylacetate) resins having a suitable plasticity are used as additives. Therefore, object of the present invention are binder compositions for lignocellulosic composites, comprising an aqueous emulsion containing at least one polyisocyanate and at least one water-dispersed vinyl acetate resin having a glass transition temperature (Tg) below 5°C. Vinyl acetate resins having a glass transition temperature of from -15 to 0°C and a minimum film forming temperature of about 0°C are particularly preferred. Examples of vinylacetate resins suitable for use in the binder compositions of the present invention are the "soft" vinyl acetate copolymers obtained by copolymerizing vinyl acetate with flexibilizing monomers, optionally combined with hardening monomers. Examples of flexibilizing monomers include (preferably C₂₋₈-) olefins such as ethylene; high molecular weight acrylic and methacrylic monomers, e.g. C₄₋₂₀ alkyl (meth)acrylates (or oxygen equivalents thereof) such as 2-ethylhexyl acrylate, n- or i-butylacrylate, lauryl acrylate, stearyl acrylate, methoxy methylacrylate, methoxyethyl acrylate and ethoxymethyl methacrylate; esters (e.g. C₁₋₁₂ alkyl esters) of unsaturated (preferably C₂₋₆-) diacids, such as dibutyl maleate and diisobutyl maleate; and esters of linear or branched fatty acids and vinyl alcohol, such as vinyl versatate, vinyl laurate, vinyl octoate, vinyl caprate and vinyl stearate, Examples of suitable hardening monomers are vinyl halides, such as vinyl chloride, vinyl bromides and vinyl fluoride; low molecular weight (meth)acrylic esters, e.g. C₁₋₃ alkyl esters, such as methyl methacrylate and ethyl methacrylate; and nitriles, such as acrylonitrile and methacrylonitrile. The flexibilizing monomers, optionally combined with the hardening monomers, can generally be used in an amount of up to 70% by weight, based on the vinylacetate resin, and preferably of from 10 to 60%. The hardening monomers, when present, are used in amounts such as to keep the Tg of the vinyl acetate resin below 5°C. Further examples of vinyl acetate resins useable in the binder compositions of the present invention are vinyl acetate homopolymers plasticized with external additives in a quantity of from 5 to 50% by weight of the dry homopolymer. Examples of the above additives (plasticizers) include organic phthalates, such as n- or i-dibutylphthalate and butylbenzylphthalate; organic phosphates, such as tricresylphosphate; phthalylglycolates, such as phthalylbutylglycolate; and polyesters. More generally, all the products listed in the "Encyclopedia of Chemical Technology", edited by Interscience Publishers (J. Wiley and Sons Inc., second edition (1968), page 775 and page 778) can be used. The aqueous emulsions of the binder compositions of the present invention preferably contain a quantity of dry matter of from 10 to 80% by weight. Even more preferred, these emulsions contain from 2 to 20% by weight, particularly from 5 to 10% by weight, of vinyl acetate resin and from 8 to 60% by weight, particularly from 35 to 50%, of polyisocyanate. The binder compositions of the present invention may be obtained by dispersing, under stirring, at least one polyisocyanate as such (that is without modifications in order to make the preparation of the emulsion easier) in a diluted aqueous vinylacetate resin dispersion. Examples of suitable vinyl acetate resin dispersions are those available on the market and known as e.g. Vinavil ® (Vinavil® EVA 201, Vinavil ® EVA 479, Vinavil ® MV 50 and Vinavil ® SL sold by Montedipe, Vinnapas ® (Wacker) and Mowilith ® (Hoechst). The vinyl acetate resin dispersion preferably is diluted so as to afford an amount of resin of from 1 to 15%, based on the dry lignocellulosic material. The polyisocyanate may be added to the vinyl acetate resin dispersion within a short time, for instance within 5 to 60 seconds, and with high stirrer speeds, for instance higher than 8000 rpm, achievable by means of e.g. turboagitators. Alternatively, the binder compositions of the present invention can be obtained with relatively low stirrer speeds, for instance between 1000 and 2000 rpm, by using stabilizers for the polyisocyanate, such as cellulose esters, e.g. carboxymethylcellulose, preferably used in quantities of from 0.1 to 5% by weight, based on the total weight of the emulsion, and added to the vinyl acetate resin dispersion before the polyisocyanate. The compositions of the present invention can be advantageously used as binders for cellulose materials, preferably lignocellulosic materials, such as wood, bark, bagasse, straw, bamboo and rice chaff; in the form of granules, chips, fibers or meals, and more generally for the manufacture of chipped panels or other composites, such as plywood, masonite and laminated boards without any problem for the industrial operator. Preferably, these compositions are added to the lignocellulosic material in quantities of from 2 to 50% by weight (based on the chips), particularly of from 5 to 20%. They allow to obtain green tack cohesion values of the resinated material higher than 1 kg, generally of from 2.0 to 3.5 kg. For the determination of the green tack cohesion, pellets of 50 mm diameter were formed by using 30 g of the resinated material (for instance wood chips) and by applying a load of 500 kg for 30 seconds. Thereafter the resulting pellets were subjected to a break test on a dynamometer, thereby determining the load necessary for breaking. It has also been found that it is possible to obtain satisfactory green tack values by preparing the compositions of the present invention in situ. This can be achieved by separately spraying onto the cellulosic material a polyisocyanate emulsion, such as the ones described in EP-A-13 112, and a vinyl acetate resin dispersion, e.g. of the type available on the market, diluted to about 40% of the dry matter, or by applying (spraying) the two pre-mixed emulsions together. In this case also the quantities of the vinyl acetate resin dispersion necessary to provide the cellulosic material with satisfactory green tack cohesion values are relatively small and are usually in the range of from 1 to 15% of dry resin, based on the resinated material. The following examples are given in order to illustrate the present invention without, however, limiting it. EXAMPLE 1 a) 72 g of Vinavil ® EVA 201 (dry matter 56%) of Montedipe S.p.A. were added to 408 g of deionized water. Thereafter, 320 g of polymethylene-polyphenylene-isocyanate (Tedimon ® 31 of Montedipe S.p.A.) were added under agitation at high velocity (10000 rpm), using a turboagitator type Ultra Turrax ® T 50 equipped with emulsifying head T 45 N (manufactured by Janke Kunkel GmbH). 600 g of the resulting emulsion were used to resinate 3000 g of wood chips (of the type industrially used for external chipped panels). b) To 72 g of Vinavil ® EVA 201 in 288 g of deionized water 240 g of Tedimon ® 31 were added according to the procedure described above. 450 g of the resulting emulsion were used to resinate 3000 g of wood chips (of the type industrially used for the inner layers of chipped panels). c) A layer of 700 g of resinated chips described under (b) and a layer of 700 g of resinated chips described under (a) were placed on a 0.25 m² plate. The resulting mattress was molded at 175°C for 4 minutes to obtain a 16 mm panel V100 (DIN 68763) (crude), showing the characteristics reported in the following table, which also indicates the green tack cohesion of the resinated chips. EXAMPLE 2 a) 600 g of a 1.18% aqueous solution of carboxymethylcellulose (degree of substitution 0.82, Hoeppler viscosity of a 2% aqueous solution measured at 20°C of 100 mPa.s and pH = 6.4) were added, under agitation, to 400 g of polymethylene-polyphenylene-isocyanate (Tedimon ® 31). The dispersion thus obtained was stirred at high speed for at most 30 seconds. b) 600 g of the resulting polyisocyanate emulsion were added, under mild agitation, to 75 g of Vinavil ® EVA 201, diluted to 40%, and thereafter the obtained mixture was used to resinate 3000 g of wood chips of the type industrially used for the external layers of chipped panels. c) To 450 g of the polyisocyanate emulsion described under (a) 75 g of Vinavil ® EVA 201, diluted to 40%, were added. Thereafter, the resulting mixture was used to resinate 3000 g of wood chips of the type industrially used for the inner layers of chipped panels. d) A panel was prepared and molded by following the procedure of Example 1. A panel V100 was obtained; the characteristics of said panel and the green tack cohesion value of the resinated chips are reported in the following table. EXAMPLE 3 The procedure of example 2 was followed, except that wood chips were resinated by applying the two emulsions (i.e. the polyisocyanate emulsion and the vinyl acetate resin emulsion) separately. A panel V100 was obtained; its characteristics, together with the green tack cohesion value of the resinated chips, are reported in the following table. EXAMPLE 4 (comparative) The procedure of example 3 was followed, except that only the polyisocyanate emulsion was introduced into the resinating machine, without using the vinyl acetate resin emulsion. A panel V100 was obtained; its characteristics, together with the green tack cohesion value of the resinated chips, are reported in the following table. EXAMPLE 5 a) 380 g of deionized water were added to 100 g of Vinavil ® MV50 (dry matter 59.5%) of Montedipe S.p.A. Thereafter, 320 g of Tedimon ® 31 were added and the wood chips for the external layers of a panel were resinated as described under (a) of example 1. b) To 260 g of deionized water 100 g of Vinavil ® MV50 were added, followed by 240 g of Tedimon ® 31, and thereafter the wood chips for the inner layer of a panel were resinated as described under (b) of example 1. c) The panel was prepared and molded as described in example 1. A panel V100 was obtained; its characteristics, together with the green tack cohesion value of the resinated chips, are reported in the following table. EXAMPLE 6 a) 78 g of Vinavil ® EVA 479 (dry matter 51%) of Montedipe S.p.A. were added to 402 g of deionized water, followed by 320 g of Tedimon ® 31. Then the wood chips for the external layers of a panel were resinated as described under (a) of example 1. b) 78 g of Vinavil ® EVA 479 of Montedipe S.p.A. were added to 282 g of deionized water and the wood chips for the inner layer of a panel were resinated as described under (b) of example 1. c) A panel was prepared and molded by working as in example 1. A panel V100 was obtained; its characteristics are reported in the following table. EXAMPLE 7 a) To 402 g of a 0.6% aqueous solution of carboxymethylcellulose (Tylose ® C30 sold by Hoechst) there were added 78 g of Vinavil ® EVA 201, followed by the addition of 320 g of Tedimon ® 31 within 2 to 4 minutes, under medium speed agitation (2000-3000 rpm). Finally the wood chips for the external layer of a panel were resinated as described under (a) of example 1. b) To 282 g of a 0.6% aqueous solution of Tylose ® C30 there were added 78 g of Vinavil ® EVA 201, followed by the addition of Tedimon ® 31 as described above. Thereupon the wood chips for the inner layer of a panel were resinated as described under (b) of example 1. c) A panel V100 was prepared and molded by working as in example 1. A panel V100 was obtained; its characteristics are reported in the following table.

Claims: Binder compositions for lignocellulosic composites, comprising an aqueous emulsion containing at least one polyisocyanate and at least one water-dispersed vinyl acetate resin having a glass transition temperature of less than 5°C. Composition according to claim 1, wherein the vinyl acetate resin has a glass transition temperature of from -15°C to 0°C and a minimum film forming temperature of 0°C. Compositions according to any one of claims 1 or 2, wherein the vinyl acetate resin is obtained by copolymerizing vinyl acetate with flexibilizing monomers and, optionally, with hardening monomers. Compositions according to claim 3, wherein said flexibilizing monomers are selected form olefins, high molecular weight acrylic and methacrylic monomers, esters of unsaturated diacids, and esters of linear or branched fatty acids and vinyl alcohol and/or said hardening monomers are selected from vinyl halides, low molecular weight methacrylic esters and nitriles. Compositions according to any one of claims 3 and 4, wherein the flexibilizing monomers, optionally combined with the hardening monomers, are used in an amount of up to 70% by weight, based on the vinyl acetate resin. Compositions according to claim 5, wherein the flexibilizing monomers are used in an amount of from 10 to 60% by weight. Compositions according to any one of claims 1 and 2, wherein the vinyl acetate resins are vinyl acetate homopolymers plastified with additives in an amount of from 5 to 50% by weight, based on the dry homopolymer. Compositions according to claim 7, wherein said additives are selected from organic phthalates, organic phosphates and polyesters. Compositions according to any one of the preceding claims, wherein the aqueous emulsion has a dry matter content of from 10 to 80% by weight. Compositions according to claim 9, wherein said emulsion contains from 2 to 20% by weight of vinyl acetate resin and from 8 to 60% by weight of polyisocyanate. Compositions according to claim 10, wherein said emulsion contains from 5 to 10% by weight of vinylacetate resin and from 35 to 50% by weight of polyisocyanate. Process for the preparation of the binder compositions according to any of the preceding claims, comprising the dispersion, under stirring, of at least one polyisocyanate in a diluted dispersion of vinyl acetate resin. Process according to claim 12, wherein the vinyl acetate resin dispersion contains from 0.1 to 5% by weight, based on the total emulsion, of stabilizers for the polyisocyanate. Process according to claim 13, wherein said stabilizers are selected from cellulose esters. Use of the compositions according to any one of claims 1 to 11 as binders for cellulose materials. Use according to claim 15, wherein said cellulose materials are lignocellulosic materials such as wood, bark, bagasse, straw, bamboo and rice chaff, in the form of granules, chips, fibers or meals. Use of the compositions according to any one of claims 1 to 11 for the manufacture of chipped panels or other composites, such as plywood, masonite and laminar boards. Use according to any one of claims 15 to 17, wherein the binder composition is used in an amount of from 2 to 50% by weight (based on the chips) and with green tack cohesion values of the resinated material higher than 1 kg, the binder composition optionally being prepared in situ on the lignocellulosic material. Use according to claim 18, wherein the binder composition is used in an amount of from 5 to 20% by weight and/or with green tack cohesion values of the resinated material of from 2.0 to 3.5 kg. Use according to any one of claims 16 to 19, wherein the amount of vinyl acetate resin is from 1 to 15% by weight, based on the dry lignocellulosic material.

Inventors: COVA MARIO CHEM ENG, LEPORI AGOSTINO CHEM ENG, ZAVATTERI IGNAZIO DR-CHEM, COVA, MARIO, CHEM. ENG., LEPORI, AGOSTINO CHEM. ENG., ZAVATTERI, IGNAZIO, DR.-CHEM.
Applicants: MINI RICERCA SCIENT TECNOLOG, MINISTERO DELL' UNIVERSITA' E DELLA RICERCA SCIENTIFICA E TECNOLOGICA
Application year: 1989
Main classification: C08L97
IPC codes: C08G18, B32B21, C09J175, C08L97