WO2018109262A1

WO2018109262A1 - Method for producing a binder composition

Info

Publication number: WO2018109262A1
Application number: PCT/FI2017/050784
Authority: WO
Inventors: Suvi Pietarinen; Christian Hübsch
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2016-12-15
Filing date: 2017-11-14
Publication date: 2018-06-21
Anticipated expiration: 2019-06-15
Also published as: FI20165971A7; FI127750B; FI20165971L

Abstract

The invention relates to a method for producing a binder composition, wherein the method comprises: a) preparing a prepolymer of resorcinol and glyoxal by allowing resorcinol to react with glyoxal in an aqueous composition; and b) mixing the prepared prepolymer and alkalated lignin, wherein the alkalated lignin has an alkalinity of 1-10 weight-%, at a temperature of 15-30°C to allow the prepared prepolymer to react with the alkalated lignin until a binder composition with a predetermined viscosity value is formed, wherein the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is at least 60 %.

Description

METHOD FOR PRODUCING A BINDER COMPOSITION

FIELD OF THE INVENTION

The invention relates to a method for producing a binder composition. Further, the invention relates to a binder composition, to an adhesive composition and to a layered composite structure.

BACKGROUND OF THE INVENTION

Lignin is a natural polymer, which can be extracted from e.g. wood. As lignin is a natural biopolymer its use as a component in glues instead of synthetic materials has been investigated in order to come up with a more environmentally friendly adhesive composition. Lignin has previously been used for replacing phenol during the production of lignin- phenol-formaldehyde resin. However, also formaldehyde is a component used in resins for e.g. plywood production that exhibits a health risk both in the plywood mill during the production process as well as for the plywood users, e.g. inhabitants in houses and homes as well as in offices. The inventors have recognized a need for a method to produce a binder composition wherein the use of e.g. phenol and formaldehyde is decreased.

PURPOSE OF THE INVENTION

The purpose of the invention is to provide a new type of method for producing a binder composition. Further, the purpose of the invention is to provide a new type of binder composition and an adhesive coating as well as a layered composite structure. SUMMARY

The method according to the present invention is characterized by what is presented in claim 1.

The binder composition according to the present invention is characterized by what is presented in claim 10.

The adhesive composition according to the present invention is characterized by what is presented in claim 11.

The layered structure composite according to the present invention is characterized by what is presented in claim 12.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing a binder composition, wherein the method comprises :

a) preparing a prepolymer of resorcinol and glyoxal by allowing resorcinol to react with glyoxal in an aqueous composition; and

b) mixing the prepared prepolymer and alkalated lignin, wherein the alkalated lignin has an alkalinity of 1 - 10 weight-%, at a temperature of 15 - 30 °C to allow the prepared prepolymer to react with the alkalated lignin until a binder composition with a predetermined viscosity value is formed,

wherein the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is at least 60 %.

In one embodiment, the method for producing a binder composition consists of: a) preparing a prepolymer of resorcinol and glyoxal by allowing resorcinol to react with glyoxal in an aqueous composition; and

The amount of alkalated lignin used in the method is determined based on the amount of resorcinol used for the preparation of the prepolymer of resorcinol and glyoxal. During the preparation of the prepolymer at least part of the used resorcinol is reacted with glyoxal but some part of the resorcinol may remain as unreacted monomers in the aqueous composition. The amount of resorcinol used for the preparation of the prepolymer should, however, be understood as the total amount of resorcinol that has been added during the preparation of the prepolymer. The inventors surprisingly found out that, as a result of the new method, the amount of resorcinol used in the production of the binder composition could be replaced to a high extent by the biobased alkalated lignin .

In one embodiment, the method comprises: a) preparing a prepolymer consisting of resorcinol and glyoxal. In one embodiment, the method comprises: a) preparing a prepolymer consisting of the reaction product of resorcinol and glyoxal. In this specification, unless otherwise stated, the expression "prepolymer" should be understood as a polymer or an oligomer the molecules of which are capable of entering, through reactive groups, into further polymerization.

The inventors surprisingly found out that the prepared prepolymer of resorcinol and glyoxal remains reactive and enables a higher amount of the synthetic components in a binder composition to be replaced with the biobased lignin, and especially with alkalated lignin, during the production of the binder composition. The inventors surprisingly found out that the reactivity of the prepared prepolymer of resorcinol and glyoxal remained on a much higher level than e.g. when allowing phenol and formaldehyde to react with each other.

In one embodiment, the molar ratio of the glyoxal to the resorcinol used for the preparation of the prepolymer is 2:1 - 5:1, or 2:1 - 4:1, or 3:1 - 4:1. In one embodiment, the molar ratio of the glyoxal to the resorcinol used for the preparation of the prepolymer is 2.5:1 - 3.5:1, or about 3:1.

In one embodiment, the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal in alkaline conditions. In one embodiment, the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal while keeping the pH of the aqueous composition at 7.5 - 9.5, or at 8 - 9. In one embodiment, the prepolymer of resorcinol and glyoxal is prepared in the presence of an alkali. In one embodiment, the aqueous composition comprises alkali. In one embodiment, the alkali comprises a hydroxide of an alkali metal. In one embodiment, the alkali is selected from a group consisting of sodium hydroxide, potassium hydroxide, and any combination thereof. In one embodiment, the alkali is sodium hydroxide. In one embodiment, the pH of the aqueous composition is adjusted while allowing the resorcinol to react with the glyoxal. In one embodiment, the amount of water in the aqueous composition is 60 - 90 weight-%, or 65 - 85 weight-%, or 70 - 80 weight-%.

In one embodiment, the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal at a temperature of 20 - 50 °C, or at a temperature of 30 - 45 °C, or at a temperature of about 40 °C . These temperature ranges have the added utility of providing the prepolymer a suitable level of reactivity for further polymerization process with alkalated lignin. The fact that heating may be reduced or omitted from the process for preparing the prepolymer may also reduce production costs.

In one embodiment, the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal for 15 - 40 minutes, or for 20 - 30 minutes.

The lignin used in the method for producing the binder composition is alkalated lignin. In this specification, unless otherwise stated, the expression "lignin" should be understood as lignin originating from any suitable lignin source. The lignin may include essentially pure lignin. By the expression "essentially pure lignin" should be understood as at least 90 % pure lignin, preferably at least 95 % pure lignin. In one embodiment, the essentially pure lignin comprises at most 10 %, preferably at most 5 %, of other components. Extractives and carbohydrates such as hemicelluloses can be mentioned as examples of such other components. In one embodiment, the lignin contains less than 10 weight-%, preferably less than 6 weight-%, and more preferably less than 4 weight-% of carbohydrates. The amount of carbohydrates present in lignin can be measured by high performance anion exchange chromatography with pulsed amperometric detector (HPAE-PAD) in accordance with standard SCAN- CM 71.

In one embodiment, the lignin is selected from a group consisting of kraft lignin, steam explosion lignin, biorefinery lignin, supercritical separation lignin, hydrolysis lignin, flash precipitated lignin, biomass originating lignin, lignin from alkaline pulping process, lignin from soda process, lignin from organosolv pulping, and combinations thereof. In one embodiment, the lignin is wood based lignin. The lignin can originate from softwood, hardwood, annual plants or from any combination thereof.

In one embodiment, the lignin is Kraft lignin. By "kraft lignin" is to be understood in this specification, unless otherwise stated, lignin that originates from kraft black liquor. Black liquor is an alkaline aqueous solution of lignin residues, hemicellulose, and inorganic chemicals used in a kraft pulping process. The black liquor from the pulping process comprises components originating from different softwood and hardwood species in various proportions. Lignin can be separated from the black liquor by different, techniques including e.g. precipitation and filtration. Lignin usually begins precipitating at pH values below 11 - 12. Different pH values can be used in order to precipitate lignin fractions with different properties. These lignin fractions differ from each other by molecular weight distribution, e.g. Mw and Mn, polydispersity, hemicellulose and extractive contents. The molar mass of lignin precipitated at a higher pH value is higher than the molar mass of lignin precipitated at a lower pH value. Further, the molecular weight distribution of lignin fraction precipitated at a lower pH value is wider than of lignin fraction precipitated at a higher pH value. The precipitated lignin can be purified from inorganic impurities, hemicellulose and wood extractives using acidic washing steps. Further purification can be achieved by filtration.

In one embodiment, the lignin is flash precipitated lignin. The term "flash precipitated lignin" should be understood in this specification as lignin that has been precipitated from black liquor in a continuous process by decreasing the pH of a black liquor flow, under the influence of an over pressure of 200 - 1000 kPa, down to the precipitation level of lignin using a carbon dioxide based acidifying agent, preferably carbon dioxide, and by suddenly releasing the pressure for precipitating lignin. The method for producing flash precipitated lignin is disclosed in patent application FI 20106073. The residence time in the above method is under 300 s. The flash precipitated lignin particles, having a particle diameter of less than 2 ym, form agglomerates, which can be separated from black liquor using e.g. filtration. The advantage of the flash precipitated lignin is its higher reactivity compared to normal kraft lignin. The flash precipitated lignin can be purified and/or activated if needed for the further processing.

In one embodiment, the lignin is separated from pure biomass. The separation process can begin with liquidizing the biomass with strong alkali followed by a neutralization process. After the alkali treatment the lignin can be precipitated in a similar manner as presented above. In one embodiment, the separation of lignin from biomass comprises a step of enzyme treatment. The enzyme treatment modifies the lignin to be extracted from biomass. Lignin separated from pure biomass is sulphur-free.

In one embodiment, the lignin is steam explosion lignin. Steam explosion is a pulping and extraction technique that can be applied to wood and other fibrous organic material.

By "biorefinery lignin" is to be understood in this specification, unless otherwise stated, lignin that can be recovered from a refining facility or process where biomass is converted into fuel, chemicals and other materials.

By "supercritical separation lignin" is to be understood in this specification, unless otherwise stated, lignin that can be recovered from biomass using supercritical fluid separation or extraction technique. Supercritical conditions correspond to the temperature and pressure above the critical point for a given substance. In supercritical conditions, distinct liquid and gas phases do not exist. Supercritical water or liquid extraction is a method of decomposing and converting biomass into cellulosic sugar by employing water or liquid under supercritical conditions. The water or liquid, acting as a solvent, extracts sugars from cellulose plant matter and lignin remains as a solid particle.

In one embodiment, the lignin is hydrolysis lignin. Hydrolysed lignin can be recovered from paper- pulp or wood-chemical processes.

In one embodiment, the lignin originates from an organosolv process. Organosolv is a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose.

In this specification, unless otherwise stated, the expression "alkalated lignin" should be understood as lignin that has been subjected to a process for treating lignin with alkali in alkaline conditions. Thus, alkalated lignin should be considered e.g. as an alkaline dispersion of lignin. In this specification, unless otherwise stated, the expression "alkaline conditions" should be understood as conditions where the pH value is 7 or higher. In one embodiment, alkalated lignin is prepared before the preparation of the binder composition. In one embodiment, alkalated lignin is prepared using the following method:

i) forming, under heating at a temperature of

30 - 98 °C, an aqueous dispersion comprising alkali and lignin, wherein the alkali comprises a hydroxide of an alkali metal; and

ii) heating the formed dispersion at a temperature of 50 - 95 °C .

In one embodiment, the alkali is selected from a group consisting of sodium hydroxide, potassium hydroxide and any combination thereof. In one embodiment, the alkali is sodium hydroxide. In one embodiment, the concentration of alkali is 5 - 50 weight-%, or 10 - 25 weight-%, based on the total weight of the dispersion in step i) . In one embodiment, the concentration of lignin in step i) is 10 - 50 weight-%, or 20 - 50 weight-%, or 20 - 45 weight-%, based on the total weight of the dispersion in step i) . In one embodiment, step i) is carried out at a temperature of 30 - 80 °C, or at a temperature of 30 - 70 °C, or at a temperature of 50 - 65 °C . In one embodiment, step i) is carried out at a temperature of 71 - 94 °C . In one embodiment, step i) is carried out at a temperature of 76 - 94 °C, or at a temperature of 71 - 90 °C, or at a temperature of 76 - 90 °C . In one embodiment, step i) is carried out for 5 minutes - 12 hours, or for no longer than 5 hours, or for 0.5 - 2.5 hours. In one embodiment, step ii) is carried out at a temperature of 60 - 85 °C . In one embodiment, step ii) is carried out for 5 minutes - 24 hours, or for no longer than 5 hours, or for 0.5 - 1.5 hours.

Alkalated lignin has an increased reactivity as compared to untreated lignin or so-called raw lignin. Without limiting the invention to any specific theory about why step i) and step ii) results in a more reactive lignin being formed, it is to be considered that these steps result in the macromolecular structure of lignin being opened whereby the steric hindrances that usually disable reactive groups in lignin structures are removed. These steps, or a so-called alkalation process, may also add charged groups to the lignin macromolecule . The advantage of using alkalated lignin e.g. for producing a binder composition is that the compatibility and reaction behavior is much better than in a normal case, where non-treated lignin has been used in the cooking or polymerizing stage.

In one embodiment, the alkalinity of the alkalated lignin is 1 - 10 weight-%. In one embodiment, the alkalinity of the alkalated lignin is 1.5 - 8 weight-%, or 2 - 8 weight-%, or 3 - 7 weight- %, or 4 - 5 weight-%. When lignin is separated or isolated from e.g. biomass it is commonly rather acidic. When then subjected to a process for alkalating it, part of the alkali used in the process will be used for neutralizing the lignin dispersion and part of the alkali used in the process will be used for alkalating the lignin structure. The amount of alkali used for producing alkalated lignin may thus vary depending on the type of lignin used. The choice of the amount of alkali needed is within the knowledge of the skilled person based on this specification.

In this specification, unless otherwise stated, the expression "alkalinity of the alkalated lignin" or any similar expression should be understood as the amount, in weight-%, of residual alkali, such as NaOH and/or KOH, found in a lignin dispersion. The alkalinity of alkalated lignin can be determined following the procedure of standard SCAN-N 33:94 (Scandinavian pulp, paper and board, testing committee, "Residual alkali (hydroxide ion content) ") . In this specification, unless otherwise stated, the expression "alkalinity" refers to the alkalinity of the alkalated lignin. I.e. it should not be taken as the alkalinity of e.g. the composition formed when mixing the prepared prepolymer and the alkalated lignin.

Providing alkalated lignin having an alkalinity of 1 - 10 weight-% has the added utility of the lignin having a suitable level of reactivity. The inventors found out that the residual free alkali, such as NaOH, catalyzes the reaction between the prepared prepolymer and the alkalated lignin. In case an excess amount of free alkali is present, the reaction may be conducted at a too early stage or too vigorously whereby the reactivity of the formed binder composition is inhibited resulting in a poor gluing quality during e.g. the plywood production. The inventors found out that it was possible to inhibit the reaction between the prepolymer and the alkalated lignin to be too vigorous by providing alkalated lignin, the alkalinity of which is 1 - 10 weight-%.

In one embodiment, the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is at least 70 %, or at least 80 %, or at least 90 %, or at least 95 %, or at least 99 %. In one embodiment, the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is at most 80 %, or at most 90 %, or at most 95 %, or at most 99 %. In one embodiment, the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is 60 - 99 %, or 70 - 99 %, or 80 - 99 %, or 90 - 99 %, or 95 - 99. In one embodiment, the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is 60 - 95 %, or 70 - 95 %, or 80 - 95 %, or 90 - 95 %. In one embodiment, the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is 60 - 90 %, or 70 - 90 %, or 80 - 90 %.

In one embodiment, the amount of the alkalated lignin to be mixed with the prepolymer is 60

- 90 mol-%, or 70 - 90 mol-%, or 80 - 90 mol-%, or 60

- 95 mol-%, or 70 - 95 mol-%, or 80 - 95 mol-%, or 90

95 mol-%, based on the total amount of all the different components used for producing the binder composition .

In one embodiment, mixing of the prepared prepolymer and the alkalated lignin is continued for 0.5 - 20 minutes, or for 1 - 15 minutes.

In one embodiment, mixing of the prepared prepolymer and the alkalated lignin is carried out at a temperature of 15 - 30 °C, or at a temperature of 20

- 25 °C, or at a temperature of 18 - 22 °C . In one embodiment, mixing of the prepared prepolymer and the alkalated lignin is carried out at room temperature.

In one embodiment, mixing of the prepared prepolymer and the alkalated lignin is carried out in the presence of an alkali. In one embodiment, the alkali comprises a salt or a hydroxide of an alkali metal. In one embodiment, the alkali is selected from a group consisting of sodium hydroxide, potassium hydroxide, and any combination thereof. In one embodiment, the alkali is sodium hydroxide.

The prepared prepolymer and the alkalated lignin are allowed to react until a binder composition with a predetermined viscosity value is formed. In one embodiment, the predetermined viscosity value of the binder composition is at least 40 cP, or at least 50 cP, or at least 80 cP. In one embodiment, the predetermined viscosity value of the binder composition is at least 40 but not more than 250 cP, or at least 50 cP but not more than 150 cP, or at least 80 but not more than 120 cP. In one embodiment, the predetermined viscosity value of the binder composition is at least 250 cP, or at least 300 cP, or at least 500 cP. In one embodiment, the predetermined viscosity value of the binder composition is at least 250 cP but not more than 1500 cP, or at least 300 cP but not more than 1200 cP, or at least 500 but not more than 1000 cP. The viscosity is measured at 25 °C using a rotary viscometer. The predetermined viscosity value of the binder composition may vary depending on the specific application where the binder composition is to be used.

The precise amount of the components used for producing the binder composition may vary and the choice of the amounts of the different components is within the knowledge of the skilled person based on this specification. The temperature can be controlled during the production of the binder composition by cooling and/or heating the composition.

The present invention further relates to a binder composition obtainable by the method as defined in the current application.

The present invention further relates to an adhesive composition comprising the binder composition as defined in the current application. The adhesive composition can further comprise one or more adhesive components selected from a group consisting of other binders, extenders, additives, catalysts and fillers. A binder is a substance, which is mainly responsible for creating the growing and cross-linking of polymer and thus assists in the curing of polymer systems. An extender is a substance, which assists the binder by adjusting physical properties for example by binding moisture. The additive can be a polymer or an inorganic compound, which assists in properties like filling, softening, reducing costs, adjusting moisture, increasing stiffness and increasing flexibility. The catalyst is a substance, which usually boosts and adjusts the curing speed. By "substance" is herein to be understood as including a compound or a composition. The binder composition of the present invention may serve as a binder, an extender, an additive, a catalyst and/or a filler in the adhesive composition.

The present invention further relates to a layered composite structure formed of two or more layers including at least one wood veneer layer, wherein the layers are arranged the one above the other and glued together with the binder composition as defined in the current application and/or with the adhesive composition as defined in the current application. In this specification, unless otherwise stated, the term "wood veneer" is used to address a veneer, which can be formed of any material, e.g. wood- based material, fiber material, composite material or the like. In this context, the thickness of the wood veneer can be varied. Typically the thickness of wood veneer is below 3 mm.

In one embodiment, the layered composite structure is selected from a group consisting of a wood panel product, a plywood product, a composite product, and a pressed panel product. The layered composite structure can be formed of a number of layers, preferably wood veneer layers, in which the layers are laid one upon the other and glued together.

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. A method, a binder composition, an adhesive composition or a layered composite structure, to which the invention is related, may comprise at least one of the embodiments of the invention described hereinbefore .

The method described in the current application has the added utility of providing a binder composition that does not contain the harmful synthetic components of phenol and formaldehyde and still exhibits properties suitable for its use e.g. in gluing together veneers to produce plywood.

The method described in the current application has the added utility of being less harmful for people at the production process as well as for the end users of e.g. the plywood.

The method described in the current application has the added utility of resulting in a more environmentally friendly binder composition since in the binder production method the natural polymer lignin, which is a phenolic polymer, has replaced a great part of the of the synthetic substances usually used in the production of phenolic compositions such as phenol formaldehyde resin. The binder composition described in the current application has the added utility of containing a high level of the biobased lignin . EXAMPLES

Reference will now be made in detail to the embodiments of the present invention.

The description below discloses some embodiments of the invention in such a detail that a person skilled in the art is able to utilize the invention based on the disclosure. Not all steps of the embodiments are discussed in detail, as many of the steps will be obvious for the person skilled in the art based on this specification.

EXAMPLE 1 - Alkalating lignin

In this example kraft lignin was alkalated before being used for the production of a binder composition .

The following components and their amounts were used:

Firstly, water and the sodium hydroxide were loaded into a reactor and heated up to a temperature of 70 °C . Then lignin was added thereto in batches. When all of the lignin has been added and dispersed, the reaction mixture was allowed to react at a temperature of about 70 - 90 °C for about 15 to 30 minutes such that examples a and b were heated at about 70 °C for about 30 minutes and example c was heated at 90 °C for about 15 minutes.

The dry solids content of the examples a, b, and c were determined by allowing samples thereof to dry at a temperature of 105 °C for about 2 hours. The alkalinity of the samples was determined following the procedure of standard SCAN-N 33:94 and the pH values of the samples were determined from a 10 % solution at room temperature. The results are indicated in the below table:

EXAMPLE 2 - Producing a binder composition In this example a binder composition was produced .

Firstly a prepolymer of resorcinol and glyoxal was prepared. The following components and their amounts were used:

Firstly, water and sodium hydroxide were mixed in a round shaped flask. Thereafter glyoxal was added while keeping the temperature of the mixture at about 40 °C . Thereafter the resorcinol was slowly added to the mixture and the resorcinol was allowed to react with glyoxal for about 30 minutes. After the prepolymer of resorcinol and glyoxal was formed, it was mixed with 150 g of alkalated lignin from example 1, sample c (the proportion of dry lignin in alkalated lignin was 51.75 g (n = 0.528)) by slowly adding the mixture to the alkalated lignin.

The resulting binder composition was liquid and was found to be reactive when tested with wooden spatulas . EXAMPLE 3 - Producing a binder composition

In this example a binder composition was produced .

The resulting binder composition was liquid and was found to be reactive when tested with wooden spatulas . EXAMPLE 4 - Preparing an adhesive composition

In this example the binder composition produced in Example 2 was used for the production of an adhesive composition. The binder composition was mixed with a commercially available hardener that is suitable to be used for plywood production and water, thus forming the adhesive composition. The resulting adhesive composition has a FC6 viscosity of about 11 s .

EXAMPLE 5 - Applying the binder composition for producing a plywood product

Wood veneers having the thickness of below 3 mm were glued together with the binder composition produced in Example 2 for producing a 7-plywood. Results showed that the gluing effect was sufficiently good for gluing wood veneers.

EXAMPLE 6 - Applying the adhesive composition for producing a plywood product In this example the adhesive composition of

Example 4 was applied onto wood veneers. The wood veneers were joined together by the adhesive composition for forming a plywood. The dry matter content of the adhesive composition was about 26 %. The wood veneers with the adhesive composition at an amount of 155 g/m² were firstly cold pressed followed by hot-pressing technique at a temperature between 120 - 170 °C . The adhesive composition was simultaneously cured. The adhesive composition was found suitable for gluing wood veneers together and thus for manufacturing plywood. It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

Claims

1. A method for producing a binder composition, wherein the method comprises:

2. The method of claim 1, wherein the molar ratio of the glyoxal to the resorcinol used for the preparation of the prepolymer is 2:1 - 5:1, or 2:1 - 4:1, or 3:1 - 4:1.

3. The method of any one of claims 1 - 2, wherein the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal in alkaline conditions.

4. The method of any one of claims 1 - 3, wherein the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal at a temperature of 20 - 50 °C, or at a temperature of 30 - 45 °C .

5. The method of any one of claims 1 - 4, wherein the prepolymer of resorcinol and glyoxal is prepared by allowing the resorcinol to react with the glyoxal for 15 - 40 minutes, or for 20 - 30 minutes.

6. The method of any one of claims 1 - 5, wherein the alkalinity of the alkalated lignin is 1.5 - 8 weight-%, or 2 - 8 weight-%, or 3 - 7 weight-%, or 4 - 5 weight-%.

7. The method of any one of claims 1 - 6, wherein the molar percentage of the amount of alkalated lignin to be mixed with the prepared prepolymer to the total amount of alkalated lignin to be mixed with the prepared prepolymer and of resorcinol used for the preparation of the prepolymer is at least 70 %, or at least 80 %, or at least 90 %, or at least 95 %, or at least 99 %.

8. The method of any one of claims 1 - 7, wherein the mixing of the prepared prepolymer and the alkalated lignin is continued for 0.5 - 20 minutes, or for 1 - 15 minutes.

9. The method of any one of claims 1 - 8, wherein the mixing of the prepared prepolymer and the alkalated lignin is carried out at a temperature of 15

- 30 °C, or at a temperature of 20 - 25 °C, or at a temperature of 18 - 22 °C .

10. A binder composition obtainable by the method of any one of claims 1 - 9.

11. An adhesive composition comprising the binder composition of claim 10.

12. A layered composite structure formed of two or more layers including at least one wood veneer layer, wherein the layers are arranged the one above the other and glued together with the binder composition of claim 10 and/or the adhesive composition of claim 11.