Thermosetting adhesive composition based on condensed tannins and use thereof in the wood industry

Abstract: A thermosetting adhesive composition based on condensed tannins comprises a hardening agent selected from the group which consists of: A) products of the reaction of a nitroparaffin with formaldehyde in an alkaline medium, of the general formula: in which R 1 and R 2 are selected, independently of one another, from C 1 -C 4 alkyl and C 1 -C 4 hydroxyalkyl groups; B) water-miscible urea-formaldehyde and melamine-formaldehyde condensation products with degrees of polymerization no greater than 4; C) products of the reaction of formaldehyde, in a basic medium, with urea-carbohydrate adducts, the adducts being products of the reaction of urea with a carbohydrate in an anhydrous medium, in the fused state.

Description: The present invention relates to a thermosetting adhesive composition based on condensed tannins which is particularly suitable for use as a glue for the production of wood-based materials in the timber industry. The use of natural tannins as bases for glues for the timber industry has been known for some time and is reported in the literature (J. Macromol. Sci. Chem. A16 (7), 1243-1250 [1981]). Tannin glues are used in competition with phenol-formaldehyde (PF), urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF) and polyurethane (PMDI) resins. Natural tannins are divided into two large classes: hydrolysable tannins and condensed tannins. Only condensed tannins are suitable for use as glues. Within the scope of the present invention, tannins coming from four different plant species are taken into consideration: mimosa ( acacia mearnsii, acacia mollissima ), quebracho ( schinopsis lorentzii, schinopsis balancae ), pine ( pinus radiata, pinus Douglas Fir and other species), and pecan ( carya illinoensis ); the invention is not, however, intended to be limited to tannins coming from the above-mentioned plant species. For the most part (70-80%), these plant extracts are constituted by prepolymers the basic unit of which is flavan-3-ol. The various species mentioned above are distinguished according to the positions and numbers of OH groups present on the resorcinol A ring and on the catechol B ring. These prepolymers have average molecular weights of between 1000 and 3000 and are stable in concentrated aqueous solution by virtue of colloidal phenomena. The remaining 20-30% is constituted by hexose and pentose sugars resulting from partial hydrolysis of hemicellulose. Extensive teaching and documentation relating to wood adhesives based on fanning is given in the texts Advanced Wood Adhesives Technology, Vol. 1 (1983) and Vol. 2 (1989) by Antonio Pizzi, published by Marcel Dekker, Inc., New York. Conventional tannin glue technology is based on rapid polymerization of the natural tannin in an alkaline medium by means of formaldehyde in the form of solid paraformaldehyde suspended in the solution, of formalin (aqueous solution of formaldehyde), or of hexamethylene-tetramine (formaldehyde-ammonia adducts). These systems are two-component systems since the hardener (such as those mentioned above) is added at the time of use or is mixed separately with the wood particles. The use of paraformaldehyde or of formaldehyde in solution produces highly water-resistant products with formaldehyde emission in line with European standards (E1 type emission). The main problem is the difficulty in handling the formaldehyde in the aforementioned forms since it is a hazardous substance with consequent safety problems for users. EP-A-0 648 807 describes tannin-based adhesive agents which use compounds with weak acid reactions such as silica and boric acid as hardeners. These adhesive compositions are characterized by a low formaldehyde emission value. US-A-4 174 980 discloses compositions including vegetable tannins and melamine - for maldehyde resins. Monomeric, dimeric and higher order resins are suitable. There is, however, a need to provide new tannin-based adhesive compositions which, as well as having low formaldehyde emission values, also have improved mechanical properties and bulking in water and stability characteristics. The invention therefore proposes the provision of novel glue compositions based on condensed tannins which use alternative and innovative hardening systems. This object is achieved by virtue of the presence, as hardeners, of methylolated compounds alone or, preferably, in combination with Lewis acids. In particular, a subject of the invention is a thermosetting adhesive composition based on condensed tannins, characterized in that it comprises one or more hardening agents selected from the group which consists of: A) products of the reaction of a nitroparaffin with formaldehyde in an alkaline medium, of the general formula: in which R 1 and R 2 are selected, independently of one another, from C 1 -C 4 alkyl and C 1 -C 4 hydroxyalkyl groups, and mixtures of these compounds; B) water-miscible products obtained from condensation of urea-formaldehyde and melamine-formaldehyde in the presence of a C 1 -C 4 alkanolamine with degrees of polymerization no greater than 4; C) products of the reaction of formaldehyde, in a basic medium, with urea-carbohydrate adducts, the adducts being products of the reaction of urea with a carbohydrate in an anhydrous medium, in the fused state; and D) mixtures of the aforesaid agents A), B) and/or C). The methylolated compounds of type A) preferably comprise tri(hydroxymethyl)nitromethane (identified as TN below) which is the product of the reaction of nitromethane with three molecules of formaldehyde in an alkaline medium (electrophilic substitution of hydrogen by formaldehyde). The reaction between the nitroparaffin and formaldehyde which leads to the production of the methylolated compounds of formula (I) takes place at ambient temperature with an alkaline pH. For this reason, the compounds of formula (I) can also be formed in situ by mixing of the tannin glue mixture comprising the corresponding nitroparaffin with a source of formaldehyde, preferably formaldehyde in aqueous solution. The scope of the invention therefore includes adhesive compositions based on condensed tannins including a nitroparaffin or mixtures of nitroparaffins which are intended to be mixed with a source of formaldehyde at the time of use to form methylolated compounds of formula (I) in situ. The condensation products B) preferably comprise dimethylolurea and trimethylolmelamine and their oligomers; naturally mixtures comprising the aforesaid monomers and their oligomers may be used. The degree of polymerization is preferably limited to the dimer and to the trimer which may be used mixed with the monomer. The characteristics of stability and reactivity of the aforementioned condensation products are modified by the introduction, at the condensation stage, of an alkanolamine having from 1 to 4 carbon atoms, preferably ethanolamine, in percentages of up to 40% by weight relative to the urea or relative to the melamine. Condensation in the presence of alkanolamine brings about the formation of oxazolidine and dimethylolurea/oxazolidine which are characterized by greater stability and reactivity with respect to tannin. These products may be further modified by the addition of bisulphite. Bisulphitation confers greater stability in an acid medium but does not per se improve reactivity with respect to tannin. Unlike commercial UF and MUF resins, the type B precondensates described herein, which are based on urea, melamine, formaldehyde and possibly alkanolamine, are fully miscible with water and are relatively stable even in neutral and acid media and their compatibility with tannin is unlimited. Although some conventional techniques for the use of tannin provide for the introduction of UF resin into the adhesive mixture, these are in any case small percentages (5-10% by weight) which are added to the tannin/paraformaldehyde system in order to strengthen the polymer. Conventional UF and MUF resins cannot be used as hardeners for tannin glues for three basic reasons: they are not sufficiently reactive in an alkaline medium; they are barely soluble in an aqueous medium; quantities greater than 20-30% cause precipitation of complexes with tannin. The type B condensation products are preferably used in quantities of from 15 to 30% by weight or, preferably, between 20 and 30% by weight relative to the anhydrous tannin. These compounds are advantageously used as hardeners in combination with type A compounds, particularly with TN; in this case, the preferred concentrations are from 10 to 15% w/w of compound B and from 5 to 10% w/w of compound A, relative to the anhydrous tannin. The type C reaction products are formed by the preliminary reaction of urea with carbohydrates, preferably selected from glucose, xylose and molasses, or even complexes, in an anhydrous medium at a temperature of 130-150°C, for a few hours, in the fused state. Partial dehydration of the carbohydrate takes place with the formation of precursors of furan products which react with the urea by amide-type bonds. This water-soluble and still barely reactive urea-carbohydrate adduct is then methylolated by reaction with formaldehyde in a basic medium. A preferred type C compound is dimethyloldiglycosylurea. The methylolated hardeners A), B) or C) are generally used in quantities of from 10 to 50% by weight relative to the dry tannin. These methylolated compounds can give polycondensation reactions with condensed tannins in an alkaline medium. It is thought that the methylol groups react in positions 6 and 8 of the A ring of the monomeric unit of tannin. It has been found that the thermosetting effect with the formation of a strong three-dimensional lattice finds a synergic effect if it is associated with the autocondensation promoted by Lewis acids. If used alone, on the other hand, the autocondensation mechanism catalyzed by the Lewis acids gives poor-quality results. If the same mechanism is associated with polycondensation, however, a polymer with a large molecular weight, capable of producing products with optimal water-resistance is obtained. The Lewis acids used are preferably silica in the form of silicates (for example, tetrasodium silicate) and boric acid in the form of borates (for example, trisodium borate). The synergic effect of silica and other Lewis acids with trihydroxymethylnitromethane (and the other methylolated compounds) can be explained by the simultaneous polycondensation reactions (which affect the A ring) and autocondensation reactions (which affect the C ring). The two mechanisms acting simultaneously at different active sites give rise to a lattice which is highly cross-linked at the three-dimensional level and is completely insoluble. The good water-resistance and optimal adhesive capacity is explained precisely by the very high molecular weight. Unlike the carbon-nitrogen bonds present in amino-plastic resins, the carbon-carbon bonds produced by these reactions are not susceptible to hydrolysis in an aqueous medium. In the case of tannin-based glues, however, better or poorer water resistance depends on the solubility of the thermosetting polymer which is a function of the molecular weight and of the three-dimensionality of the lattice. Autocondensation alone cannot produce sufficiently large molecular weights. Synergic effects are not observed in paraformaldehyde/silica systems since highly electrophilic and reactive products such as formaldehyde (in its various forms) almost completely repress autocondensation. The above-mentioned Lewis acid compounds are normally added to the adhesive composition in quantities of from 2 to 10% by weight, preferably from 2 to 8% by weight relative to the anhydrous tannin. The adhesive compositions of the invention can be used for a large range of products in the timber industry, particularly for the production of chipboard panels, fibreboard panels (MDF), plywood, mouldings, and laminates. The main advantages achieved by the invention in comparison with more conventional systems, both from the point of view of industrial application and from the point of view of the finished product, are as follows: the use of resins with a low environmental impact at an industrial level of production (press fumes, water discharge, storage of products); the production of panels with formaldehyde emission equal to that of natural wood; greater stability of the tannin/activator mixtures at various pH values and, by virtue of the improved stability, the possibility, in the case of TN, of formulating single-component tannin and hardener compositions; complete compatibility and miscibility with precondensed methylolates, in contrast with commercial UR and MUF resins. Chipboard and fibreboard panels produced by the technology of the invention are characterized by optimal water resistance (type V100 panels - DIN-EN conditioning) low values for bulking in water and an absence of free formaldehyde. Table 1 below gives typical values of formaldehyde emission from panels in accordance with the ISO-EN 120 standard (perforator method) according to the type of glue used. TYPE OF RESIN TYPICAL EMISSION VALUE mg HCHO/100 g panel urea-formaldehyde 6-10 melamine-urea-formaldehyde 8-12 phenol-formaldehyde 3-6 tannin-formaldehyde 3-6 tannin-TN-silica 0.3-0.5 tannin-dimethylolurea-silica 0.4-0.6 wood alone 0.3-0.5 Tables 2a and 2b give the stability values for various hardener systems for quebracho tannin and for mimosa tannin, respectively, at various pH values and for various quantities of hardener relative to the dry tannin. The same tables give comparative values relating to compositions comprising conventional hardeners constituted by paraformaldehyde or formaldehyde. The stability value given should be understood as the time for which a solution containing 40% by weight of dry tannin, activated by the hardener, maintains a viscosity of less than 1000 cP (Brookfield method - LV2 Probe, 30 rpm) without thixotropic or rheopectic effects. The stability values given confirm that it is possible to use single-component formulations, particularly in the case of the TN hardener, with acid pH values (pH3-6). Stability of various hardener systems for quebracho tannin Type of hardener Quantity (% relative to dry tannin) pH Stability at 25°C 12 4 60 days TN 12 6 40 days TN 12 10 24 hours dimethylolurea/ethanolamine 25 4 22 hours dimethylolurea/ethanolamine 25 6 48 hours dimethylolurea/ethanolamine 25 10 16 hours paraformaldehyde 8 4 8 hours paraformaldehyde 8 6 6 hours paraformaldehyde 8 10 40 mins. formaldehyde 8 4 6 hours formaldehyde 8 6 3 hours formaldehyde 8 10 25 mins. Stability of various hardener systems for mimosa tannin Type of hardener Quantity (% relative to dry tannin) pH Stability at 25°C(*) TN 12 4 40 days TN 12 6 30 days TN 12 10 24 hours dimethylolurea/ethanolamine 25 4 22 hours dimethylolurea/ethanolamine 25 6 40 hours dimethylolurea/ethanolamine 25 10 12 hours paraformaldehyde 8 4 6 hours paraformaldehyde 8 6 4 hours paraformaldehyde 8 10 32 mins. formaldehyde 8 4 5 hours formaldehyde 8 6 2.5 hours formaldehyde 8 10 20 mins. The following examples relate to laboratory tests of use, in which the glue compositions according to the invention and commercial glues based on PF, UF, MUF and based on tannin were used for gluing plywood and chipboard panels. a - Production of chipboard panels General operative conditions Thickness: 10 (± 0.5) mm Density: 700 (± 20) Quantity of glue: 12% by weight of dry glue on dry wood Temperature of press surfaces: 170°C Maximum pressure: 30 kg/cm 2 The characteristics of chipboard panels produced by conventional systems, by way of reference data, and of the chipboard panels produced by the gluing systems according to the invention, are given in Tables 3 and 4 below. The percentages are expressed by weight unless indicated otherwise. The percentages of hardener are always expressed as percentages by weight relative to the anhydrous tannin. b - Production of plywood General operative conditions Veneers: 30x30 cm of poplar and of beech, thickness 2.0 mm Number of layers: 5 Temperature of press surfaces: 120°C Maximum pressure: 8 kg/cm 2 Quantity of glue: 120 g/m 2 per individual glue line Filler: coconut flour, 10% relative to the dry glue. The plywood produced with various types of tannin glue according to the invention were examined in accordance with ISO EN 314 test, Class 3 and the breaking load results are given in Table 5 below. Characteristics of chipboard panels produced by conventional systems (reference data) Gluing system Press time (s/mm) Vertical tensile stress DIN-EN (N/mm 2 ) Vertical tensile stress 2 hours boiling DIN-EN (N/mm 2 ) Bulking in water 24 hr. -20°C DIN-EN (%) Emission of formaldehyde DIN-EN 120 (mg HCHO/100g panel) commercial PF resin ( ) 18 0.9 0.30 7 3.5 commercial UF resin ( ) 12 0.8 0 16 8.4 commercial MUF resin ( ) 12 0.8 0.22 9 9.5 quebracho tannin/paraform aldehyde ( ) 15 0.8 0.20 8 3.0 tannin/paraform aldehyde ( ) 12 0.8 0.20 8 2.5 quebracho tannin/silica ( ) 15 0.5 0 28 0.3 quebracho tannin/paraform aldehyde/silica ( ) 15 0.8 0.20 8 3.0 Characteristics of chipboard panels produced by innovative systems Gluing system Press time (s/mm) Vertical tensile stress DIN-EN (N/mm 2 ) Vertical tensile stress 2 hours boiling DIN-EN (N/mm 2 ) Bulking in water 24 hr. - 20°C DIN-EN (%) Emission of formaldehyde DIN-EN 120 (mg HCHO/100g panel) quebracho/TN/silica 15 0.8 0.23 8 0.3 quebracho/TN/silica 15 0.7 0.14 10 0.3 quebracho/TN 15 0.7 0 15 0.3 mimosa/TN/silica 14 0.8 0.21 7 0.3 quebracho/dimethylolu rea/ethanolamine/sili ca 15 0.8 0.12 10 0.4 quebracho/dimethylolu rea/ ethanolamine 15 0.6 0 16 0.4 quebracho/diglycosyl dimethylolurea/silica 15 0.8 0.12 11 0.4 quebracho/dimethylolu rea/ethanolamine/sili ca/PF resin 15 0.9 0.32 6 0.6 quebracho/trimethylol melamine/silica 15 0.8 0.15 7 0.5 quebracho/TN/dimethyl olurea 15 0.8 0.19 8 0.5 Gluing system Conditioned marine ply ISO EN 314 Class 3 breaking load (N/mm 2 ) k quebracho /10% TN 2 l quebracho /8% TN 1.7 m quebracho /6% TN/15% 1.9 n quebracho /6% TN 0.9 p quebracho /15% dimethylolurea 0.8 The scope of the invention includes tannin-based glue compositions in single-component form in which the hardener, particularly the TN, is mixed with the condensed tannin, and two-component compositions, in which the hardener is kept separate from the tannin in order to be mixed at the time of use. A method of gluing woody materials and particularly for producing woody panels with the use of the glue compositions described above also falls within the scope of the invention.

Claims: A thermosetting adhesive composition based on condensed tannins, characterized in that it comprises a hardening agent selected from the group which consists of: A) products of the reaction of a nitroparaffin with formaldehyde in an alkaline medium, of the general formula: in which R 1 and R 2 are selected, independently of one another, from C 1 -C 4 alkyl and C 1 -C 4 hydroxyalkyl groups, and mixtures thereof; B) water-miscible products obtained from condensation of urea-formaldehyde or melamine-formaldehyde in the presence of a C 1 -C 4 alkanolamine with degrees of polymerization no greater than 4; C) products of the reaction of formaldehyde, in a basic medium, with urea-carbohydrate adducts, the adducts being products of the reaction of urea with a carbohydrate in an anhydrous medium, in the fused state, and D) mixtures of the aforesaid agents A), B) and/or C). A composition according to Claim 1, in which the hardener is of type A and comprises tri(hydroxymethyl)nitromethane. A composition according to Claim 1, including a type B hardening agent in which the alkanolamine is added in quantities of up to 40% by weight relative to the urea or to the melamine. A composition according to Claim 3, in which the alkanolamine is ethanolamine. A composition according to Claim 3 or Claim 4, in which the condensation product comprises oxazolidine and/or dimethylolurea/oxazolidine. A composition according to any one of Claims 3, 4 and 5, in which the condensation product is modified by the addition of bisulphite at the condensation stage. A composition according to Claim 1, in which the hardening agent is of type C and in which the urea-carbohydrate adduct is the product of the reaction of urea with a carbohydrate selected from the group which consists of glucose, xylose, molasses and mixtures thereof. A composition according to Claims 1 to 6, comprising a hardening agent of type A) in combination with an agent of type B. A composition according to Claim 8 comprising from 5 to 10% by weight of compound A and from 10 to 15% by weight of compound B, relative to the anhydrous tannin. A composition according to any one of the preceding claims, comprising a Lewis acid compound. A composition according to Claim 10, in which the Lewis acid is selected from silicates and borates. A composition according to any one of the preceding claims, in which the quantity of hardening agent A), B) or C) added to the tannin is from 10 to 30% by weight relative to the dry tannin. A composition according to Claim 10 or Claim 11 in which the quantity of Lewis acid compound added is from 2 to 10% relative to the anhydrous tannin. A composition according to any one of the preceding claims, in the form of a two-component composition. A composition according to Claim 1 in which the hardening agent is of type A, in the form of a single-component composition. An adhesive composition based on condensed tannins comprising products of the reaction of a nitroparaffin with formaldehyde in an alkaline medium, of the general formula: in which R 1 and R 2 , independently of one another, are C 1 -C 4 alkyl groups. A method of gluing wood-based materials, particularly for producing panels, characterized in that a composition according to any one of the preceding claims is used as a gluing composition.

Inventors: TROSA ALESSANDRO, TROSA, ALESSANDRO
Applicants: SILVA SRL, SILVA S.R.L.
Application year: 1997
Main classification: C09J161
IPC codes: C08L61, C09J161