WO2011042610A1

WO2011042610A1 - Bio-adhesive and wood board

Info

Publication number: WO2011042610A1
Application number: PCT/FI2010/050784
Authority: WO
Inventors: Samantha Kiljunen; Gatja Tiusanen; Esa Lappalainen; Sanna Lehtinen; Ulf Hotanen
Original assignee: UPM Kymmene Wood Oy
Current assignee: UPM Plywood Oy
Priority date: 2009-10-08
Filing date: 2010-10-08
Publication date: 2011-04-14
Anticipated expiration: 2012-04-08
Also published as: FI20096036A0

Abstract

The invention relates to a bio-adhesive. In accordance with the invention, the bio-adhesive is formed by reaction, selected from the group of esterification, condensation and their combinations, of natural based binder material containing active groups with a reactant having reactive groups capable of forming bonds with -OH and/or -NH₂ groups of the binder material. Further, the invention relates to a wood board in which veneers are glued by the bio-adhesive.

Description

BIO-ADHESIVE AND WOOD BOARD

FIELD OF THE INVENTION

The invention relates to a bio-adhesive as defined in the preamble of claim 1 and a wood board as defined in the preamble of claim 14.

BACKGROUND OF THE INVENTION

Known from prior art are various wood boards, e.g. plywoods, veneer boards or the like. Known from prior art is the gluing of veneers for providing a wood board.

Known from prior art are different types of chemicals and adhesive compositions, e.g. a polyurethane or phenolic glue, to be used in connection with the wood boards. Further, known from prior art are different coatings for the wood boards.

EPO752027 discloses esterfication of lignin. The lignin is utilised from kraft black liquor. CN101157833 discloses a starch-based adhesive polymerised with polyisocyanate . WO

2007149589 discloses an adhesive of tannin-furfuryl alcohol catalyzed by acid.

Further, a cross-linking of chitosan and lignin with polycarboxylic acids is known. Also, known is the depolymerisation step to reduce the molecules molecular weight.

Problem of plywood is that flat plywood is very difficult to maintain with changing humidity. Also, the use of plywood for outdoor applications is limited owing to swelling and shrinking of the wood. OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new type of a bio-adhesive for gluing veneers of a wood board. Further, the objective of the invention is to disclose a new type of a wood board wherein the veneers are glued together with the bio- adhesive .

SUMMARY OF THE INVENTION

A bio-adhesive and a wood board according to the invention are characterized by what is presented in the claims.

The invention is based on a bio-adhesive for gluing veneers of a wood board. In accordance with the invention the bio-adhesive is formed by reaction, selected from the group of esterification, condensation and their combinations, of natural based binder material containing active groups, typically hydroxyl or amine groups, with a reactant having suitable reactive groups capable of forming bonds with -OH and/or -NH₂ groups of the binder material. In one embodiment the bio-adhesive is formed by esterification reaction of natural based binder material containing active -OH and/or -NH₂ groups with a reactant having reactive groups capable of forming bonds, e.g. ester bonds, with -OH and/or -NH₂ groups of the binder material. The bio-adhesive can also be formed by condensation reaction of natural based binder material containing active -OH and/or -NH₂ groups with a reactant having reactive groups capable of forming bonds, e.g. ether, amine or carbon-carbon bonds, with -OH and/or -NH₂ groups of the binder material. In one embodiment the reactive groups of the reactant can form ester, ether, amine, amide and/or carbon-carbon bonds with -OH groups of the natural based binder material and the wood of the veneer. In one embodiment the reactive groups of the reactant can form ester, ether, amine, amide and/or carbon-carbon bonds with -NH₂ groups of the natural based binder material and the wood of the veneer.

In one embodiment the binder material can be treated or modified, and/or different reactants can be added to the binder material.

In a preferred embodiment the bio-adhesive consists mainly of a binder material and a reactant which is able to transform a liquid or soft binder to cured material. Preferably the binder material is formed of natural based bio-material. In this context, binder material can be also binder polymer.

The invention is specifically based on the adhesive system comprising a natural based binder material, e.g. tannin, lignin, cellulose, hemicellulose, chitosan or the like, possessing active hydroxyl or amine groups and a polycarboxylic acid or other material capable of forming bonds with the natural based material. The invention also is based on the method of producing plywood or wood board using the said adhesive. The esterification for the forming a binder material out of the biomass creates a bio- adhesive with unique properties.

In this context, a veneer refers to any veneer of a wood board. The wood board can be a wood panel product, plywood product, composite product, beam, pressed panel product or the like, formed of a number of layers, preferably veneer layers, and principally of wood-based materials, in which the layers are laid one upon the other and glued together. The veneer can be formed of any material, e.g. wood-based material, fiber material, composite material or the like. In this context, the veneer refers to any layer of the wood board. Typically the veneer is a thin layer of the wood board. The thicknesses of the veneer layers can vary.

In one embodiment to the bio-adhesive it can be added additional reactants.

In one embodiment of the invention the reactant is selected from the group: polycarboxylic acids and anhydrides, e.g. propionic anhydride, butyric anhydride and acetic anhydride, amino acids, hydroxycarboxylic acids, isocyanates, e.g. di- and poly-isocyanates and thio-isocyanates, aldehydes with nitric acids, fatty acids, their derivates and their combinations. The polycarboxylic acids or polymeric carboxylic acids possess acid groups. Anhydrides comprise acid groups with water already removed. Isocyanates form a nitrogen containing esters.

In one embodiment of the invention the reactant is a cross-linking agent and is able to combine chemically fragments of the binder material and the cross-linking agent forms cross-linking bonds with -OH and/or -N¾ groups. Typically the cross- linking agent forms ester, ether or carbon-carbon cross-linking bonds. In one embodiment the cross- linking agent has carboxyl groups and the carboxyl groups of the cross-linking agent being reactive with -OH and/or -N¾ groups of the binder material and the wood of the veneers. Preferably, the carboxyl groups are reactive via a two step process, where there is, in step one a formation of a cyclic anhydride from the acid. This intermediate then reacts by ring opening and forming ester bonds with -OH and/or -NH₂ groups. In one embodiment condensation reactions between the hydroxyl groups of the cross-linking agent and the binder material occur during the curing and/or cross- linking.

In one embodiment of the invention the cross- linking agent is polycarboxylic acid. Preferably, polycarboxylic acid has three or more reactive groups where at least two of them are carboxylic acids in order to cross-link by reaction with hydroxyl groups of the binder material.

In one embodiment of the invention the reactant is selected from the group: 1,2,3,4- butanetetracarboxylic acid, citric acid, maleic acid, succinic acid, itaconic acid, trans-aconitic acid, cis-aconitic acid, tricarballylic acid, talloid fatty acid and suberin fatty acid and their oligomers, glucosamine, polyethylene glycol, polypropylene glycol, polyglycols, proteins, sugars, polyvinyl alcohol, polyalkylene oxides, polyalkylene alcohols, oligomers or polymers of glycerol, glyoxal, furfuryl alcohol and aldehyde, pentaerythritol , phloroglucinol , eugenol, resorcinols, 1, 2-benzenedicarboxylic acid and anhydride, 1 , 3-benzenedicarboxylic acid, 1,4- benzenedicarboxylic acid, 1 , 2 , 3-benzenetricarboxylic acid, 1, 2, 4-benzenetricarboxylic acid, 1,2,3,4- cyclobutanetetracarboxylic acid, tetrahydrofuran- 2 , 3 , 4 , 5-tetracarboxylic acid, 1,2,4,5- benzenecarboxylic acid, succinic anhydride, maleic anhydride, poly-maleic acid and their anhydrides and their combinations. The reactants containing only two acid groups are able to assist in the polymer growth but unable to form side chains by grafting.. In addition to ester and amide bonds also ether and carbon-carbon bonds can be formed but for these alternative cross-linking agents and often a catalyst is required. Alternative reactants can be aldehydes, e.g. acetaldehyde and benzaldehyde, with a nitric acid or furfuryl aldehydes, furfuryl alcohol, monolignols, p-coumaryl alcohol, coniferyl alcohol, sinapyl alcohol, dimethylamine ethanal, suberin fatty acids, ε-caprolactam, glycerol, glyoxal, derivatives of lignin and products of pyrolysis and degradation/depolymerization of lignin. The above materials can be used as reactants, cross-linking agents or polymerisation chemicals instead of or in combination with ester bond forming chemicals, such as polycarboxylic acids and anhydrides.

In one embodiment a reactant is also formed of bio-material .

In one embodiment of the invention the binder material, is selected from the group: tannin, chitosan, starch, cellulose, lignin, hemicellulose, alginic acid, pectins, hyaluronic acid, chitin, glucosamine copolymers, polyglycols, proteins, sugars, e.g. sorbitol, xylitol, sucrose, glucose or fructose, polyvinyl alcohol, hydroxyl or amine containing polymers, polyalkylene oxides, polyalkylene alcohols, fatty acid oligomers and polymers, oligomers or polymers of glycerol, and their derivates and their combinations. Basically, any binder material containing hydroxyl or amine groups capable of reacting with the reactant to form an ester bond are suitable. In one embodiment the binder material is selected from lignin, tannin and their combinations.

In one embodiment polyethylene glycol, polyglycols, polyvinyl alcohol, polyalkylene oxides, polyalkylene alcohols and/or oligomers or polymers of glycerol can be used as reactants to modify the binder material .

Chitosan is a naturally occurring polysaccharide and is cationic in nature composed of mainly (1,4) linked 2-amino-2-deoxy- p-D-glucan and soluble in acidic solutions but insoluble in alkaline solutions. Both the amino groups in the chitosan molecule are pH sensitive. Chitosan is a derivative from shells and possesses a primary amine group on its polysaccharide ring which may be grafted onto wood by incorporation of a bi-functional cross-linking agent.

Cellulose is also a polysaccharide and will therefore form cross-links in similar manner as chitosan .

In one embodiment the bio-adhesive is impregnated into the wood cell wall of the veneer so that the reactive groups or the carboxyl groups of the reactant form, via a two step process, ester bonds with -OH groups of the wood and the binder material in the wood cell wall for improving dimensional stability of the wood board.

In the case where the binder material also possesses an amine group, e.g. chitosan, there can also be ionic bridging between the binder material and the reactant.

In one embodiment of the invention the binder material is depolymerized for reducing the molecular weight of the material to monomers and oligomers and at the same time activating or increasing the reactivity of the binder material prior to the reaction, e.g. polymerization, cross-linking, esterification or condensation or their combination. In one embodiment the formed monomers and oligomers are polymerized again. The activating of the binder material can occur prior to or during the polymerization of the oligomers. In one embodiment the binder material is too big molecularly to create a bio-adhesive that both penetrates into the wood and also creates the glue-line; therefore the binder material has to be depolymerized. In the case of the small molecules the binder material can be impregnated to wood without further processing but larger molecules need to be depolymerized first. When the binder material is reduced in molecular weight they are more soluble in water and they have improved biological activities.

In one embodiment the binder material is depolymerized by oxidation. In one embodiment the oxidation is carried out with hydrogen peroxide, hydrogen peroxide and sodium nitrite (NaN0₂) , hydrogen peroxide and sodium nitrate (NaN0₃) or other hydrogen peroxide combinations. In one alternatively embodiment the depolymerization method is selected from the group; acid hydrolysis, alkaline hydrolysis, enzymatic treatment and physical methods. Physical methods can include radiation and pyrolysis. The depolymerization techniques can be combined with the isolation techniques.

In one embodiment of the invention the binder material is isolated prior to the reaction, e.g. esterification or condensation or their combination. In one embodiment the binder material is isolated by precipitation. In one embodiment the binder material is isolated by precipitation and ultrafiltration in the case of lignin. The invention is not limited to these techniques only.

Chitosan is the N-deactylated form of chitin and is derived from chitin by deacetylation . Lignin can be isolated from black liquor as described below. However, the final isolated product may still contain hemicellulose and other materials.

Black liquor from the Kraft pulping process is good source for lignin. Black liquor is an alkaline aqueous solution of lignin residues, hemicellulose, and inorganic chemicals used in the process. In the pulping process where the black liquor components originate both from softwood and hardwood different wood species can be used in various proportions. For the lignin isolation the black liquor can be taken from several points of the process cycle. Dry content of black liquor at evaporation plant used for lignin precipitation is normally 25 - 30 wt% and the weak black liquor approximately 15 wt% . Lignin can be isolated from black liquor by precipitation. Precipitation of lignin is a generally well-known technique. Lignin starts usually to precipitate when pH declines under 11 - 12. Precipitation of lignin can be performed using for example sodium bisulphite, sulphuric acid or CC>2-gas. Lignin fractions with different properties are achieved by precipitating to different pH levels or to several sequential pH levels. These lignin fractions differ from each other by molecular weight distribution, e.g. Mw and Mn, polydispersity, hemicellulose and extractive contents. In general the molar mass of the lignin precipitated in the high pH is higher that the lignin precipitated at the low pH. Also the molecular weight distribution of the lignin fraction precipitated at a low pH level is wider than at a high pH level. Therefore each of the separated lignin fractions has properties that make it optimal for specific gluing applications. The purity of the lignin fractions vary according to the number and efficiency of the acidic washing stages and filtration steps used after precipitation. An acidic washing step purifies the lignin from inorganic impurities, hemicelluloses and wood extractives.

At low pH amines are protonated and therefore can only form a salt linkage with other groups. Such bonds are weak and will not form a strong adhesive. However, at high pH this is not the case and will work very well with lignin.

Ultrafiltration using membrane or several membranes with different cut-off values is effective technique to isolate and fractionate lignin straight from black liquor or to fractionate already precipitated lignin. The membrane material, filtration temperature and pressure, and diafiltration steps are critical factors to achieve satisfactory flow level and concentration during the ultrafiltration. The most critical factor in obtaining the lignin fractions with optimal molar mass and molecular weight distribution in perspective of adhesive applications is the cut-off values of used membranes. The molar mass of lignin fractionated with ultrafiltration is also connected to activity and reactivity of lignin. The lignin fraction with small molar mass is more reactive than the lignin fraction with high molar mass and with a more polymeric structure. Both of these fractions have its purpose in the bio-adhesive applications.

The lignin isolation process is selected and performed according to properties wanted for the lignin fraction or fractions. The combination of the both techniques described above can also be used to achieve the optimal lignin fraction or fractions for further modification and bio-adhesive applications.

In the case of isolated lignin from Kraft liquor the de-polymerization process can remove the brown colour. This is advantageous from the point of view of using the material as an adhesive.

The binder material is reacted and/or polymerized/cross-linked with the reactant with or without a catalyst. In one embodiment the reactant contains a catalyst or catalysts. In one embodiment the bio-adhesive contains a catalyst or catalysts. In one embodiment the bio-adhesive is typically containing a binder material, a reactant and catalyst or catalysts which aids in activating, modifying or curing of the binder material.

Catalysts can be used to treat binder material chemically, i.e. to manipulate it by polymerization, depolymerization, activation, modification etc.

In one embodiment of the invention the catalyst is selected from the group: sodium hypophosphite monohydrate (SHP) , sodium hypophosphite (NaH₂PC>2) , sodium phosphate (NaH₂P0₄) , sodium phosphinate monohydrate (NaH₂PC>2 H₂0) , titanium dioxide, triethylamine , acid catalysts, e.g. citric acid, and other neutral catalysts and their combinations. In one embodiment the amount of the catalyst is 2 - 5 %.

In one embodiment the catalysts used are heat sensitive and therefore for example cross-linking will not occur until a certain temperature is reached. Therefore at room temperature there will not be any cross-linking so that diffusion into the cell wall of the chemicals is not hindered. In one embodiment the cross-linking occurs at temperatures 120 - 130 °C, preferably time is from 12 minutes to 12 hours, which are typical temperatures and times used during the gluing process in the production of wood board. However, it is important to mention that there will be some cross-linking at lower temperatures and this is important with regards to the temperature applied for diffusion. High temperatures are desirable for increasing the rate of diffusion but it is important that the temperature does not activate the cross- linking or the chemicals will cross-link before diffusing into the cell wall.

In one embodiment the cross-linking is carried out in connection with wood board manufacturing. In one embodiment the cross-linking is carried out after the wood board manufacturing, e.g. at the end of the production line. In one embodiment the curing of the adhesive and the cross-linking in the cell wall is performed by radiation treatment, e.g. by E-beam, microwave or X-ray treatment.

In one embodiment of the invention the bio- adhesive contains at least one additive selected from the group: extenders, fillers, property modification agents, reactivity increasing agents, viscosity enhancers, surfactants, other additives or their combinations. In one embodiment the bio-adhesive contains dyes, pigments, biocides, fire retardants and/or fluorescent particles to create functional and pigmented adhesives.

In one embodiment the bio-adhesive contains extenders. In one embodiment of the invention the bio- adhesive contains at least one or more of the following: wheat flour, rye flour, barley flour, oatmeal, wood flour, such as hard wood, soft wood and mixtures of them, different types of native and modified starches based on rice, potato, maize and tapioca, proteins, chitosan, such as feather flour, shrimp flour, clamshell flour and bone meal, casein, gelatine, carbohydrates and their combinations. In one embodiment the bio-adhesive contains at least one or more of the following additives and/or catalysts: different types of thickening agents, ethanol, methanol and other alcohols C₁-C5, a₂C0₃, K₂C0₃, CaCC>3 , NaOH, KOH, NH₄OH, amines, amides, anilines, melamine, urea, thiourea, acetone, formic acid, amino acids, silanes, Na₂S0₄, Na₂S₂0₇, NH₄C1 and other ammonium salts, sulphate-ammonium ions, and carbonate ions, and their derivates and their combinations. The above mentioned catalysts can also act like additives and tackifiers .

In one embodiment the bio-adhesive contains phenol formaldehyde resin partially substituted with bio-phenolic derivates, e.g. from bio-oil, or the bioresin containing aliphatic or aromatic polymers as the major component. Resin curing is possible by heat, E-beam and X-ray treatment for deeper penetration.

In one embodiment of the invention the binder material is dissolved in a polar solvent prior to the reaction, e.g. esterification, condensation or their combination .

In one embodiment the binder material is polymerized with cross-linking agents and catalyst or with catalyst to the reactive form. In one embodiment there are alkaline process conditions during the reaction selected from condensation, esterification and their combination.

In one embodiment the bio-adhesive contains 10 - 80 wt% binder material, 5 - 50 wt% solvent and 5 - 50 wt% other chemicals like reactants and additives.

Further, the invention is based on a wood board which is formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue. In accordance with the invention, the veneers are glued by the bio-adhesive which is formed by reaction, selected from the group of esterification, condensation and their combinations, of natural based binder material containing active groups with a reactant having reactive groups capable of forming ester, ether, amine, amide or carbon-carbon bonds with -OH and/or - Ν¾ groups of the binder material and the veneers.

In one embodiment of the invention the bio- adhesive is applied between the veneers in a liquid form, a film form or a solid form and the veneers are joined together by means of pressing and heat.

In one embodiment of the invention the bio- adhesive is applied between the veneers and the veneers are joined together by a cold press at room temperature .

In one embodiment of the invention the veneers of the wood board are impregnated with the bio- adhesive by a hot-cold thermal process. Each veneer can be impregnated wholly or partly.

In one embodiment the veneer can be pre- treated prior to the manufacturing wood board. The veneer can be treated by drying, heat-treatment, rewetting, corona treatment, UV/IR treatment, microwave treatment and/or by means of coupling agent or primer or by their combinations. In an alternatively embodiment the veneer is in green state.

In one embodiment the whole wood board can be treated with the chemicals to create a dimensionally stable wood board.

The bio-adhesive can be dosed to the veneers by the following methods: roller application, stripe application, spray application, foam extrusion, curtain application, dipping or their combination. Spreading amounts of the bio-adhesive is from 80 g/m² to 270 g/m² depending on process parameters for example the application method, wood species and thickness and quality of the veneers and structure of the wood panel. The adhesive can be applied in the form of powder, film, dispersion, colloid, liquid., aerosol or foam.

In one embodiment the adhesive film can be formed by extrusion of the binder material and cross- linking agent. The film is then applied to the veneers and the adhesion is initiated by heat with or without a catalyst.

In one embodiment the cross-linking is carried out in connection with the wood board manufacturing, e.g. in a gluing of the veneers and/or in a hot- pressing. In one embodiment the hot-pressing can be carried out at temperatures 120 - 170 °C for curing the glue and cross-linking in cell wall.

Curing of the bio-adhesive can be done by cold press, hot press, IR or UV light depending on application method, the form of adhesive, the formulation of adhesive and the structure of the panel. Curing process can also include some or all of curing methods. Curing time can be from 3 min to several hours depending on process parameters and the structure of the wood panel. Typically curing is done by hot pressing at 100 - 170 °C and in cold press at room temperature. Curing time depends on the thickness of the panel, typically 5 - 15 min by hot press and several hours by cold press.

The manufacturing wood board can be carried out using apparatuses known per se. Laying the veneers one upon the other, joining them together by glue and other typical steps can be performed in any manner known per se in the art.

The wood board according to the invention may be used in different embodiments.

The bio-adhesive of the invention is easy to prepare. Thanks to the invention, it is possible to provide many different veneer and wood board modifications. This invention produces the wood board with dimensional stability. The bio-adhesive is nontoxic and therefore creates a non toxic glue-line in a plywood or a wood board production.

The bio-adhesive and the wood board in accordance with the invention are suitable for various applications .

LIST OF FIGURES

In the following, the invention is described by means of detailed embodiment examples with reference to accompanying figures 1 - 3, in which

Fig. 1 shows a mechanism of a formation of ester bonds in polymerization and cross-linking,

Fig. 2 shows a mechanism of a succinylation or maleation of chitosan, and

Fig. 3 shows a mechanism of a condensation of polyglycols .

DETAILED DESCRIPTION OF THE INVENTION

Example 1

In this example, various bio-adhesive compositions according to the invention and plywoods comprising the bio-adhesive as glue were prepared.

First, binder material was selected and isolated. The binder material was lignin. The binder material was de-polymerized by oxidation with H2O2. A cross-linking agent and a catalyst were selected. The cross-linking agent was polycarboxylic acid and the catalyst was SHP (sodium hypophosphite monohydrate) . Said adhesive components were combined together.

Then the formed bio-adhesive was applied onto the veneers and the veneers were joined together by the bio-adhesive for forming a plywood. The veneers were pressed together by a hot-pressing at temperature between 120 - 170 °C or by a cold-pressing at room temperature. At the same time the bio-adhesive was cured by cross-linking for forming ester bonds.

The bio-adhesive compositions contained 10 - 80 wt% lignin, 5 - 50 wt% solvent and 5 - 50 wt% polycarboxylic acid and catalyst.

From the tests it was discovered that the bio-adhesive compositions of the invention are suitable glues to be used for gluing the veneers together and for the manufacturing of plywood.

Example 2

In this example, bio-binder according to the invention was formed by the ester polymerization of chitosan .

The bio-binder was prepared in a 250 ml flask reactor with mechanical stirring. 20 g of citric acid was dissolved in 150 g of water. 20 g of chitosan or chitosan oligosaccharide was added with stirring. The temperature was increased from 20 to 100 °C in 30 min. The mixture was refluxed for 60 minutes. Excess water was evaporated in rotavapor. The yield was a water solution and a gel like polymeric material.

In FTIR analysis the polymeric material showed the formed ester in peaks 1709, 1616, 1567, 1381 cm^"1, of which none was coming from the initial materials .

For the plywood gluing the glue mixture was made by blending polymeric material (65 parts per wt) , water (20 parts per wt) and a starch powder, e.g. potato starch Emsol, (15 parts per wt) . Birch plywood panels with light brown glue seams were produced by using trivial industrial production parameters. The prepress tack was good. The wood failure value of 80 % was measured under dry conditions.

Example 3

In this example, bio-binder according to the invention was formed by the ester polymerization of polyglycol .

The bio-binder was prepared in a 250 ml flask reactor with magnetic stirring. 15 g of citric acid was added and dissolved in 150 g of water. 10 g of polyglykol (PEG 200) was added. The mixture was refluxed for 90 minutes. The temperature was cooled down and excess water was removed by rotavapor. The yield was a viscous colorless sticky liquid which can be used as a cold press adhesive material as such or by blending a similar glue mixture than in Example 2.

In FTIR analysis indicated new ester in peaks 2936, 2874, 2614, 1713s, 1190, 1113 and 1057 cm^"1, of which none was coming from the initial materials.

Example 4

In this example, bio-binder according to the invention was formed by lignin activation.

The bio-binder was prepared in a 250 ml flask reactor with magnetic stirring. 10 g of Kraft lignin was dissolved in 100 g of water solution. The slurry solution was adjusted to acidic by sulphuric acid and was stirred at room temperature for 15 min. 0.2 g of iron sulphate (FeS0₄) and a surfactant were added to decrease the surface tension if needed. The mixture was stirred at RT for 15 min. 3 ml of hydrogen peroxide (H₂C>2 , 30 % solution) was added at room temperature in 200 microliters portions and the mixture was stirred for 5 minutes. 1.0 g of polyglycol (1, 1, 1-tris (hydroxymethyl) propane, THMP) was added and the temperature was raised to 100 °C with stirring. Then the temperature was cooled down after 1 h stirring . The glue mixture was made by blending 70 parts per wt of the binder solution and 30 parts per wt of a suitable starch extender. Plywood gluing with the glue mix indicated a sufficient penetration into the veneer and a satisfactory wood failure.

Example 5

The bio-binder was prepared in a 250 ml flask reactor with magnetic stirring. 10 g of Kraft lignin was dissolved in 100 g of water solution. The slurry solution was adjusted to medium alkalic by sodium hydroxide solution and was stirred at room temperature for 15 min. 2 ml of hydrogen peroxide (H₂0₂, 30 % solution) was added at room temperature in 200 microliters portions and the mixture was stirred for 5 minutes. 1.0 g of polyglycol (1,1,1- tris (hydroxymethyl) propane, THMP) was added and the temperature was raised to 100 °C with stirring. Then the temperature was cooled down after 1 h cooking.

The glue mixture was made by blending 80 parts per wt of the binder solution and 20 parts per wt of a suitable starch extender. Plywood gluing with the glue mix indicated a sufficient penetration into the veneer and a satisfactory wood failure.

A bio-adhesive and a wood board according to the invention are suitable in its different embodiments for different types of applications.

The embodiments of the invention are not limited to the examples presented rather many variations are possible within the scope of the accompanying claims .

Claims

1. A bio-adhesive, ch a r a c t e r i z e d in that the bio-adhesive is formed by reaction, selected from the group of esterification, condensation and their combinations, of natural based binder material containing active groups with a reactant having reac^¬ tive groups capable of forming bonds with -OH and/or - NH₂ groups of the binder material.

2. The bio-adhesive according to claim 1, c h a r a c t e r i z e d in that the reactant is selected from the group: polycarboxylic acid, anhydrides, aldehydes with nitric acids, fatty acids, their derivates and their combinations.

3. The bio-adhesive according to claim 1 or 2, c h a r a c t e r i z e d in that the reactant is a cross-linking agent and the cross-linking agent forms cross-linking bonds with -OH and/or -NH₂ groups of the binder material.

4. The bio-adhesive according to any one of claims 1 to 3, c h a r a c t e r i z e d in that the reactant is selected from the group: 1,2,3,4- butanetetracarboxylic acid, citric acid, maleic acid, succinic acid, itaconic acid, trans-aconitic acid, cis-aconitic acid, tricarballylic acid, talloid fatty acid and suberin fatty acid and their oligomers, glucosamine, polyethylene glycol, polypropylene glycol, polyglycols, proteins, sugars, polyvinyl alcohol, polyalkylene oxides, polyalkylene alcohols, oligomers or polymers of glycerol, glyoxal, furfuryl alcohol and aldehyde, pentaerythritol , phloroglucinol, eugenol, resorcinols, 1 , 2-benzenedicarboxylic acid and anhy^¬ dride, 1 , 3-benzenedicarboxylic acid, 1,4- benzenedicarboxylic acid, 1 , 2 , 3-benzenetricarboxylic acid, 1 , 2 , 4-benzenetricarboxylic acid, 1,2,3,4- cyclobutanetetracarboxylic acid, tetrahydrofuran- 2 , 3 , 4 , 5-tetracarboxylic acid, 1,2,4,5- benzenecarboxylic acid, succinic anhydride, maleic anhydride, poly-maleic acid and their anhydrides and their combinations.

5. The bio-adhesive according to any one of claims 1 to 4, c h a r a c t e r i z e d in that the binder material is selected from the group: tannin, starch, cellulose, lignin, hemicellulose, chitosan, alginic acid, pectins, hyaluronic acid, chitin, glucosamine, polyglycols, proteins, sugars, polyvinyl alco- hoi, polyalkylene oxides, polyalkylene alcohols, fatty acid oligomers and polymers, oligomers or polymers of glycerol and their derxvates and their combinations.

6. The bio-adhesive according to any one of claims 1 to 5, c h a r a c t e r i z e d in that the binder material is selected from lignin, tannin and their combinations.

7. The bio-adhesive according to any one of claims 1 to 6, c h a r a c t e r i z e d in that the binder material is depolymerized for reducing the mo- lecular weight of the binder material and activating the binder material prior to the reaction.

8. The bio-adhesive according to any one of claims 1 to 7, c h a r a c t e r i z e d in that the binder material is isolated prior to the reaction.

9. The bio-adhesive according to any one of claims 1 to 8, c h a r a c t e r i z e d in that the bio-adhesive contains binder material, reactant and catalyst .

10. The bio-adhesive according to any one of claims 1 to 9, c h a r a c t e r i z e d in that the catalyst is selected from the group: sodium hypo- phosphite monohydrate, sodium hypophosphite, sodium phosphate, sodium phosphinate monohydrate, titanium oxide, triethylamine and their combinations.

11. The bio-adhesive according to any one of claims 1 to 10, c h a r a c t e r i z e d in that the bio-adhesive contains at least one additive selected from the group: extenders, fillers, property modification agents, reactivity increasing agents, viscosity enhancers, surfactants, other additives and their combinations .

12. The bio-adhesive according to any one of claims 1 to 11, c h a r a c t e r i z e d in that the bio-adhesive contains at least one additive selected from the group: wheat flour, rye flour, barley flour, oatmeal, wood flour, different types of native and modified starches, proteins, chitosan, casein, gelatine, carbohydrates, different types of thickening agents, ethanol, methanol and other alcohols C1-C5, Na₂C0₃, K2CO3, CaC0₃, NaOH, KOH, NH₄OH, amines, amides, anilines, melamine, urea, tiourea, acetone, formic acid, amino acids, Al-salts, silanes, Na₂S0₄, a2S₂0₇, NH₄CI and other ammonium salts, sulphate-ammonium ions and carbonate ions, and their combinations.

13. The bio-adhesive according to any one of claims 1 to 12, c h a r a c t e r i z e d in that the binder material is dissolved in a polar solvent prior to the reaction.

14. A wood board, which is formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue, c h a r a c t e r i z e d in that the veneers are glued by the bio-adhesive which is formed by reaction selected from the group of esterification, condensation and their combinations, of natural based binder material containing active groups with a reactant hav- ing reactive groups capable of forming bonds with -OH and/or -NH₂ groups of the binder material and the veneers .

15. The wood board according to claim 14, c h a r a c t e r i z e d in that the bio-adhesive is applied between the veneers and the veneers are joined together by means of pressing and heat.

16. The wood board according to claim 14 or 15, c h a r a c t e r i z e d in that the bio-adhesive is applied between the veneers and the veneers are joined together by a cold press at room temperature.

17. The wood board according to any one of claims 14 to 16, c h a r a c t e r i z e d in that the veneers of the wood board are impregnated by a hot- cold thermal process.