Binder comprising modified lignin

Abstract

The present invention is directed to a process for preparing a binder, wherein lignin is subjected to demethylation whereby methoxy groups of lignin are converted to hydroxy groups. The demethylated lignin is mixed with a cross-linker and heated. The invention is also directed a binder obtained according to the process and to the use thereof.

Description

[0001] BINDER COMPRISING MODIFIED LIGNIN

[0002] Field of the invention

[0003] The present invention is directed to a process for preparing a binder, wherein lignin is subjected to demethylation whereby methoxy groups of lignin are converted to hydroxy groups. The demethylated lignin is mixed with a crosslinker and heated. The invention is also directed a binder obtained according to the process and to the use thereof.

[0004] Background

[0005] Lignin, an aromatic polymer is a major constituent in e.g. wood, being the most abundant carbon source on Earth second only to cellulose. In recent years, with development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry.

[0006] Lignin has been extensively investigated as suitable substitute for phenol during production of phenol-formaldehyde binders for production of panel boards such as plywood, hard board, medium density fiberboard or particle boards.

[0007] Lignin is an amorphous and highly branched polymer. Lignin comprises three different units: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The units are connected to each other by carbon-carbon or ether bonds to a three- dimensional heterogenous structure. The relatively low reactivity of lignin is a major obstacle towards implementing higher substitution levels.

[0008] Modification of lignin is known in the art. For example, depolymerization, pyrolysis and solvolysis has been described, all of which lead to depolymerization of lignin. Oxidation of lignin has also been evaluated, to obtain various phenolic derivatives. Functionalization of hydroxyl groups on lignin has also been explored, such as by etherification, esterification, oxidation, phenolation, silylation and esterification.

[0009] Summary of the invention

[0010] The present invention provides a solution to one more of the problems of the prior art. A particular advantage of the process according to the present invention is that reactivity of the lignin can be increased, which is particularly desirable for certain areas of application.

[0011] The present invention is directed to a process for preparing a binder, wherein lignin is subjected to a demethylation process whereby methoxy groups of lignin are converted to hydroxy groups. The modified lignin is then used in the production of a binder. The modified lignin can also be mixed with lignin which has not been modified, before the preparation of the binder. By mixing modified and unmodified lignin a mixture having an adapted degree of reactivity can be obtained.

[0012] Thus, the present invention is directed to a process for production of a binder, comprising the steps of a) providing lignin generated in the Kraft process, wherein the lignin contains less than 10% impurities; b) subjecting the lignin from step a) to a demethylation reaction whereby methoxy groups of lignin are converted to hydroxy groups, wherein the demethylation reaction is carried out until at least 10% of the methoxy groups of lignin have been converted to hydroxy groups; c) mixing the modified lignin obtained in step b) with a crosslinking agent and optionally additives and heating the mixture to a temperature in the range of from 30 to 220°C to react the modified lignin and the crosslinking agent to obtain a binder.

[0013] The present invention is also directed to a binder obtained according to the process of the invention. The present invention is also directed to the use of the binder in the production of engineered wood products such as plywood, hard board, medium density fiberboard or particle boards.

[0014] Description of the figures

[0015] Figure 1 shows shear strength as a function of time from starting to mix a 20% lignin solution with Glycerol diglycidyl ether until the press in an Automated Bonding Evaluation System is closed.

[0016] Detailed description

[0017] It is intended throughout the present description that the expression "lignin" embraces lignin originating from any type of plants, e.g. lignin originated from hardwood, softwood or annual plants. The lignin is an alkaline lignin generated in the Kraft process. Preferably, the lignin has been purified or isolated before being used in the process according to the present invention. The lignin may be isolated from black liquor and may optionally be further purified before being used in the process according to the present invention. The purification is typically such that the purity of the lignin is at least 90%, preferably at least 95%. Thus, the lignin used according to the process of the present invention preferably contains less than 10%, preferably less than 5% impurities. The lignin may then be separated from the black liquor by using the process disclosed in W02006031175. The lignin may be provided in the form of particles, such as particles having an average particle size of from 20 micrometers to 5 mm. Preferably, the lignin used according to the present invention is not modified chemically after its extraction from wood and isolation before being used according to the present invention.

[0018] The lignin to be provided for use in the production of modified lignin according to the present invention is preferably provided in the form of a solution or dispersion. The solution or dispersion is prepared by dissolving or dispersing lignin in an aqueous medium. The pH of the aqueous medium may be adjusted by addition of acid or base. The medium in which the lignin is dispersed or dissolved may also contain additives, such as solvents, surfactants, dispersing agents, surfactants, coupling agents, plasticizers and fillers.

[0019] Examples of fillers and / or hardeners include limestone, cellulose, sodium carbonate, and starch. Coupling agents are for example silane-based coupling agents.

[0020] If the lignin is provided in the form of an aqueous solution of lignin, the solution may be obtained by dissolving lignin in water and alkali. Alternatively, the lignin may dissolved in a solution comprising water and ammonia and / or an organic base, which can be prepared by methods known in the art, such as by mixing lignin and ammonia and / or organic base with water. The pH of the aqueous solution of lignin is preferably in the range of from 10 to 14. Examples of organic bases include amines, such as primary, secondary and tertiary amines and mixtures thereof. Preferably, the organic base is selected from the group consisting of methylamine, ethylamine, propylamine, butylamine, ethylenediamine, methanolamine, ethanolamine, aniline, cyclohexylamine, benzylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, dimethanolamine, diethanolamine, diphenylamine, phenylmethylamine, phenylethylamine, dicyclohexylamine, piperazine, imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2- isopropylimidazole, 2- phenylimidazole, 2-methylimidazoline, 2- phenylimidazoline, trimethylamine, triethylamine, dimethylhexylamine, N- methylpiperazine, dimethylbenzylamine, aminomethyl propanol, tris(dimethylaminomethyl)phenol and dimethylaniline or mixtures thereof. The total amount of ammonia and / or organic base in the aqueous solution is preferably in the range of from 0.1 wt-% to 20 wt-%, preferably 0.1 wt-% to 10 wt-%, of the total weight of the aqueous solution comprising water, lignin and ammonia and / or an organic base. The amount of lignin in the aqueous solution of lignin comprising ammonia and / or an organic base is preferably from 1 wt-% to 60 wt-% of the solution, such as from 10 wt-% to 30 wt-% of the solution.

[0021] The demethylation step b) is carried out using methods known in the art, such as by chemical demethylation or by enzymatic demethylation. Enzymatic demethylation can for example be done by laccase. Chemical demethylation can for example be done by hydrogen iodide, hydrogen chloride.

[0022] In one embodiment, the demethylation is carried out by reacting the lignin with sodium sulfite and / or sodium bisulfite. In one embodiment, the demethylation is carried out by reacting the lignin with elemental sulfur or sulfur dioxide. The demethylated lignin can be isolated as the modified lignin. The demethylation may be carried out during 1 second to several hours, such as up to 10 hours. In one embodiment, heating is carried out at a temperature of for example 30-95°C such as 70-85°C for 1-8 hours such as 1-4 hours.

[0023] In the demethylation step, at least a portion of the methoxy groups is converted into hydroxyl groups. As a consequence of said conversion, the number of free ortho and para sites relative to the hydroxy group on the phenolic ring increases.

[0024] The degree of demethylation can be observed by quantitative13C-NMR by dissolving a known amount of lignin together with a known amount of trioxane as internal standard. The degree of methylation is expressed as the intensity of the peak representing methoxy carbon before and after methylation (Balakshin and Capanema, RSC Adv., 2015,5, 87187-87199).

[0025] The modified lignin is used in the preparation of a binder. The modified lignin is used in combination with a crosslinker, such as an epoxy-based crosslinker or used in combination with for example phenol and an aldehyde such as formaldehyde, to prepare a binder. Alternatively, the crosslinking agent may be an amine. Such binders may be used for example in the production of insulation or engineered wood products, such as plywood, oriented strandboard (OSB), laminated veneer lumber (LVL), medium density fiberboard (MDF), high density fiberboard (HDF), parquet flooring, curved plywood, veneered particleboard, veneered MDF or particle board.

[0026] It has surprisingly been found that the modified lignin produced according to the present invention is particularly useful in the preparation of a binder for use in the production of fiberboard, particularly when an epoxy-based crosslinker is used.

[0027] For the preparation of a binder, the modified lignin can be mixed with nonmodified lignin to provide a mixture of modified and non-modified lignin. Such a mixture may comprise 1-99 wt-% of modified lignin and 1-99 wt-% nonmodified lignin, the combined amount thereof being 100 wt-%.

[0028] There are several methods for preparing binders, wherein a crosslinking agent is mixed with lignin. For example, binders can be prepared by adding phenol and formaldehyde to a mixture, such as an aqueous solution, comprising the modified lignin. Typically, the amount of phenol added is approximately the same as the amount of lignin, but it is appreciated that more or less phenol can be added depending on what type of binder that is desired. According to the present invention, the mixture of modified lignin and crosslinking agent is preferably heated to a temperature in the range of from 30 to 220°C to react the modified lignin and the crosslinking agent to obtain a binder. The reaction time is preferably short to allow high production efficiency. Alternatively, a binder can be prepared by dissolving the modified lignin in an aqueous solution, preferably having a pH in the range of from 10 to 14, and mixing with an epoxy-based crosslinking agent according to methods known in the art, followed by heating.

[0029] Epoxy-based crosslinking agent is an agent which functions as a crosslinker and wherein the crosslinking takes place by reaction involving the epoxy group. Examples of epoxy-based crosslinkers include glycerol diglycidyl ether, polyglycerol diglycidyl ether, polyglycerol polyglycidyl ether, glycerol triglycidyl ether, sorbitol polyglycidyl ether, alkoxylated glycerol polyglycidyl ether, trimethylolpropane triglycidyl ether, trimethylolpropane diglycidyl ether, polyoxypropylene glycol diglycidylether, polyoxypropylene glycol triglycidyl ether, diglycidylether of cyclohexane dimethanol, resorcinol diglycidyl ether, isosorbide diglycidyl ether, pentaerythritol tetraglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether having 2-9 ethylene glycol units, propylene glycol diglycidyl ether having 1-5 propylene glycol units, diglycidyl-, triglycidyl- or polyglycidyl- ether of a carbohydrate, diglycidyl-, triglycidyl- or polyglycidyl-ester of a carbohydrate, diglycidyl-ether or diglycidyl ester of salicylic acid, vanillic acid, or 4-hydroxybenzoic acid, an epoxidized or glycidyl substituted plant-based phenolic compound (such as tannin, cardanol, cardol, anacardic acid) or epoxidized plant-based oil (such as rapeseed oil, linseed oil, soy bean oil), tris(4-hydroxyphenyl) methane triglycidyl ether, N,N-bis(2,3-epoxypropyl)aniline, p-(2,3-epoxypropoxy-N,N- bis(2,3-epoxypropyl)aniline, diglycidyl ether of bis-hydroxymethylfuran, and / or diglycidyl ether of terminal diol having a linear carbon chain of 3-6 carbon atoms, and a crosslinker having functional groups selected from glycidyl amine, diglycidyl amine, triglycidyl amine, polyglycidyl amine, glycidyl amide, diglycidyl amide, triglycidyl amide, polyglycidyl amide, glycidyl ester, diglycidyl ester, triglycidyl ester, polyglycidyl ester, glycidyl azide, diglycidyl azide, triglycidyl azide, polyglycidyl azide, glycidyl methacrylate, diglycidyl methacrylate, triglycidyl methacrylate, or polyglycidyl methacrylate. Glycidyl ethers with more functional epoxide groups are further examples, such as glycerol diglycidyl ether, glycerol triglycidyl ether and sorbitol polyglycidyl ether. Other glycidyl ethers having two to nine alkylene glycol groups (such as 2-4 alkylene glycol groups or 2-6 alkylene glycol groups) are further examples, such as diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether and tripropylene diglycidyl ether. Other epoxy-based crosslinkers include crosslinkers having functional groups selected from glycidyl amine, diglycidyl amine, triglycidyl amine, polyglycidyl amine, glycidyl amide, diglycidyl amide, triglycidyl amide, polyglycidyl amide, glycidyl ester, diglycidyl ester, triglycidyl ester, polyglycidyl ester, glycidyl azide, diglycidyl azide, triglycidyl azide, polyglycidyl azide, glycidyl methacrylate, diglycidyl methacrylate, triglycidyl methacrylate and polyglycidyl methacrylate.

[0030] When preparing the binder, the amount of lignin in the binder is preferably from 1 wt-% to 45 wt-%, calculated as the dry weight of lignin and the total weight of the binder. More preferably, the amount of lignin in the bonding resin is from 5 wt-% to 30 wt-%, calculated as the dry weight of lignin and the total weight of the binder.

[0031] The binder used when producing engineered wood is typically an aqueous solution containing from 1% to 80% by weight of binder. In one embodiment of the present invention, the resin solution contains from 10% to 40% of bonding resin, but also higher concentrations can be used, such as an aqueous solution containing 40-80% by weight of bonding resin. The pH of the bonding resin solution is generally above pH 9, preferably above pH 10, more preferably in the range of from pH 10 to pH 13. The bonding resin solution may optionally contain additional components such as pigments, surfactants, curing agents, pH stabilizers and fire retardants.

[0032] The quantity of binder applied on a surface when producing engineered wood products can be for example 10-400 g / m2, advantageously 10-150 g / m2, preferably 30-100 g / m2and most advantageously about 50 g / m2, when calculated as hardened binder compared to dry product produced. Examples

[0033] Sodium sulfite was purchased from Merck, and the NaOH (reagent grade) and the Glycerol diglycidyl ether (GDE, technical grade) were purchased from Sigma Aldrich.

[0034] Demethylation of lignin - Sample 1

[0035] Water (465 g) was added to Kraft lignin (311.7 g, S.C. 96.26%, purity approximately 95%) and sodium sulfite (6 g) in a glass reactor equipped with a condensor and a mechanical overhead stirrer while stirring at 200 RPM.

[0036] A NaOH solution (120 g, 50 wt%) was added while stirring. The temperature of the reaction was increased to 90 °C and was then maintained at that temperature for 2 h before cooling down to ambient temperature.

[0037] The mixture was then diluted to a lignin concentration of 20% and NaOH concentration of 5% by diluting with a 3% NaOH solution.

[0038] Demethylation of lignin - Sample 2

[0039] Water (465 g) was added to Kraft lignin (311.7 g, S.C. 96.26%, purity approximately 95%) and sodium sulfite (15 g) in a glass reactor equipped with a condensor and a mechanical overhead stirrer while stirring at 200 RPM.

[0040] A NaOH solution (120 g, 50 wt%) was added while stirring. The temperature of the reaction was increased to 90 °C and was then maintained at that temperature for 2 h before cooling down to ambient temperature.

[0041] The mixture was then diluted to a lignin concentration of 20% and NaOH concentration of 5% by diluting with a 3% NaOH solution. Characterization of demethylated lignin by quantitative13C-NMR

[0042] The demethylated lignin was initially extracted by means of precipitation by reduction of pH. The precipitated lignin was thoroughly washed and freeze- dried. Prior to analysis, the dried lignin was dissolved in DMSO-d6 and subjected to quantitative 13C-NMR analysis as described elsewhere, most notably by Balakshin and Capanema (RSC Adv., 2015,5, 87187-87199). Trioxane was used as internal standard. Values are tabulated as mmol / g in Table 1

[0043] Table 1. Methoxy group content.

[0044] Evaluation of demethylated lignin

[0045] The reactivity was evaluated using an Automated Bonding Evaluation System (ABES) provided by Adhesive Evaluation Systems.

[0046] A sample of the demethylated lignin solution prepared in Example 1 - Sample 1 (5 g) was mixed with GDE (0.75 g) for 1 minute. The binder mixture was then tested according to the following method:

[0047] Sliced beech veneers (cut with the pneumatic cutter to a size of 117 mm long, 20 mm wide and 0.6 mm thick) conditioned at 23 °C and 50% R.H were used. The binder mixture was applied on the outer section of 5 x 20 mm (100 mm2) of a veneer using a pipette. The binder amount was then adjusted using a wire bar applicator (150 microns, from neurtek) to create an even amount of applied binder.

[0048] The veneer was pressed against a second veneer with a press temperature of 150 °C, for 90 seconds followed by an air-cooling step of 3 seconds and a pulling step of 4 seconds.

[0049] Figure 1 shows the shear strength as a function of time from starting to mix the 20% lignin solution with GDE until the press in ABES is closed.

[0050] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims1. A process for production of a binder, comprising the steps of a) providing lignin generated in the Kraft process, wherein the lignin contains less than 10% impurities; b) subjecting the lignin from step a) to a demethylation reaction whereby methoxy groups of lignin are converted to hydroxy groups, wherein the demethylation reaction is carried out until at least 10% of the methoxy groups of the lignin have been converted to hydroxy groups; c) mixing the modified lignin obtained in step b) with a crosslinking agent and optionally additives and heating the mixture to a temperature in the range of from 30 to 220°C to react the modified lignin and the crosslinking agent to obtain a binder.

2. The process according to claim 1 , characterized in that the demethylation reaction in step b) is carried out until at least 15% of the methoxy groups of the lignin have been converted to hydroxy groups.

3. The process according to claim 1 or 2, wherein the lignin is provided in the form of an aqueous solution.

4. The process according to claim 3, wherein the aqueous solution has a pH in the range of from 10 to 13.

5. A process according to any one of claims 1-4, wherein the crosslinking agent is an epoxy-based crosslinker.

6. A process according to any one of claims 1-4, wherein the crosslinking agent is an aldehyde and wherein phenol is also added in step c).

7. A process according to any one of claims 1-4, wherein the crosslinking agent is an amine.

8. A process according to any one of claims 1-8, wherein the modified lignin is mixed with lignin which has not been modified before step c).

9. Binder obtainable according to the process of any one of claims 1 to 8.

10. Engineered wood product produced using a binder according to claim 9.

11. Engineered wood product according to claim 10, wherein said engineered wood product is plywood, oriented strandboard (OSB), laminated veneer lumber (LVL), medium density fiberboard (MDF), high density fiberboard (HDF), parquet flooring, curved plywood, veneered particleboard, veneered MDF or particle board.

12. Insulation product produced using a binder according to claim 9.

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