Field of the invention

The present invention relates to a method for preparing a bonding resin useful for example in the manufacture of mineral wool insulation products, glass wool products, laminates, boards and wood products such as particleboards, medium density fibreboards (MDF), high density fiberboards, flooring and plywood.

Background

For mineral wool insulation products, glass wool products, laminates, boards such as corrugated boards, and wood products such as particleboards, medium density fibreboards (MDF), high density fiberboards, flooring and plywood, currently used resins are typically formaldehyde-based resins such as urea formaldehyde, melamine-urea-formaldehyde, phenol-urea- formaldehyde and phenol formaldehyde resins.

Formaldehyde based resins emit formaldehyde, which is a toxic volatile organic compound. The present and proposed legislation directed to the lowering or elimination of formaldehyde emissions have led to the development of formaldehyde free resin for wood adhesive applications. Lignin, being green and renewable material, is suitable for replacement of hazardous and fossil-based chemicals in the resins.

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.

US2007218307 describes a method using an adhesive composition wherein lignin was dissolved in an alkali solution and was reacted with a formaldehyde free curing agent based on a compound that includes at least one amine, amide, imine, imide or nitrogen-containing heterocyclic functional group, e.g. polyamidoamine epichlorohydrin. However, the binder does not give enough wet strength to pass the standard test requirements.

Summary of the invention

It has now been found that the wet strength of an adhesive obtained from a bonding resin comprising lignin can be improved.

It has surprisingly been found that when lignin powder or lignin in the form of an aqueous dispersion is mixed with an aqueous solution of at least one substantially formaldehyde-free crosslinker that includes at least one amine, amide, imine, imide, or nitrogen-containing heterocyclic functional group that can react with at least one functional group of the lignin, at a pH in the range of from 3 to 9, and optionally one or more additives, a bonding resin that leads to an adhesive with very good bonding strength (wet strength) and moisture resistance is obtained.

When lignin in the form of a powder or in the form of an aqueous dispersion is mixed with an aqueous solution of at least one substantially formaldehyde- free crosslinker that includes at least one amine, amide, imine, imide, or nitrogen-containing heterocyclic functional group that can react with at least one functional group of the lignin, at a pH in the range of from 3 to 9, the lignin becomes at least partly dissolved. In addition, the phenolic hydroxyl groups in the lignin structure are deprotonated and free to react with the crosslinker. This catalyzes the reaction and improves the reactivity and performance of the binder. Therefore, it is possible to easily prepare a bonding resin in which the use of formaldehyde can be avoided, i.e. which is free from formaldehyde.

The present invention is thus directed to a method for preparing a bonding resin, wherein lignin in powder form or in the form of an aqueous dispersion is mixed with an aqueous solution of at least one substantially formaldehyde- free crosslinker that includes at least one amine, amide, imine, imide, or nitrogen-containing heterocyclic functional group that can react with at least one functional group of the lignin, at a pH in the range of from 3 to 9, and optionally one or more additives.

The present invention is also directed to a bonding resin comprising lignin in solid form, such as lignin powder, and at least one substantially formaldehyde-free crosslinker that includes at least one amine, amide, imine, imide, or nitrogen-containing heterocyclic functional group that can react with at least one functional group of the lignin, such as the bonding resin obtainable using the method described above and to the use of the bonding resin in the manufacture of mineral wool insulation products, glass wool products, laminates, boards such as corrugated boards, or wood products such as particleboards, medium density fibreboards (MDF), high density fiberboards, flooring or plywood. The present invention is also directed to such mineral wool insulation products, glass wool products, laminates, boards such as corrugated boards, and wood products such as particleboards, medium density fibreboards (MDF), high density fiberboards, flooring or plywood manufactured using the bonding resin.

Detailed description

It is intended throughout the present description that the expression "lignin" embraces any kind of lignin, e.g. lignin originated from hardwood, softwood or annular plants. Preferably the lignin is an alkaline lignin generated in e.g. 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 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%, more preferably at least 98%. Thus, the lignin used according to the method of the present invention preferably contains less than 10%, preferably less than 5% or less than less than 2% impurities. The lignin may then be separated from the black liquor by using the process disclosed in W02006031175. The lignin may then be separated from the black liquor by using the process referred to as the LignoBoost process.

The particle size of the lignin powder is preferably such that 50 wt-% of the particles have a particle size larger than 100 micrometers. Preferably, at least 20 wt-% of the particles have a particle size larger than 300 micrometers. Typically, at least 80 wt-% of the agglomerates have a diameter within the range of from 0.2 mm to 5.0 mm, more preferably at least 80 wt-% of the agglomerates have a diameter within the range of from 0.2 mm to 2.0 mm. Preferably, at least 80 wt-% of the particles have a particle size smaller than 2000 micrometers. Preferably, the moisture content of the particles is less than 40 wt-%, more preferably less than 20 wt-%.

The crosslinker is for example adducts of epoxides with polyamine resins, polyamidoamines resins, or polyamide resins. Preferably, the crosslinker is polyamidoamine epichlorohydrin, polyvinyl amine or a combination thereof.

The crosslinker is provided as an aqueous solution. The concentration of the crosslinker is typically 10-20%. When mixed with the lignin in the form of a powder or in the form of an aqueous dispersion, an adhesive is obtained. The forming of the adhesive is often referred to as curing. Typically, the bonding resin according to the present invention is applied to a surface, such as surfaces of for example veneers, such as in the manufacture of plywood. When the veneers are pressed together under heating, the cross-linking in the bonding resin takes place, resulting in an adhesive.

When the lignin powder or lignin in the form of an aqueous dispersion is mixed with the crosslinker, the pH of the mixture is in the range of from 3 to 9, preferably in the range of from 4 to 9, more preferably in the range of from 6 to 9.

The lignin in the form of an aqueous dispersion can be prepared by mixing lignin, preferably lignin powder, with water, to obtain a dispersion. Preferably, the pH of the aqueous medium with which the lignin is mixed is in the range of from 5 to 9, such as from 5 to 7 or from 5 to 8 or from 6 to 8. The pH of the aqueous medium and/or the aqueous dispersion can be adjusted by adding acid or base. For example, a basic solution can be added to increase the pH.

The pH of the basic solution is preferably in the range of from 10 to 14. Examples of suitable bases include sodium hydroxide, potassium hydroxide, ammonia or other organic base, and mixtures thereof. In one embodiment, the amount of alkali in the basic solution is preferably from 0.1 wt-% to 60 wt- % of the solution, such as from 20 wt-% to 55 wt-% of the solution. In one embodiment, the basic solution is aqueous.

If the bonding resin comprises additives, the additives can for example be urea, tannin, solvents, surfactants, tensides, dispersing agents, fillers and hardeners. The amount of urea in the bonding resin can be 0-40% preferably 5-20% calculated as the dry weight of urea and the total weight of the bonding resin. A filler and/or hardener can also be added to the bonding resin. Examples of such fillers and/or hardeners include limestone, cellulose, sodium carbonate, and starch. Solvents that can be used as additives in the bonding resins according to the present invention are for example glycerol, ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, sorbitol and/or any terminal diol having a linear carbon chain of 3-6 carbon atoms. The weight ratio between lignin (dry weight) and the total amount of crosslinker is preferably in the range of from 1:10 to 10:1. The amount of lignin in the bonding resin is preferably from 5 wt-% to 50 wt-%, calculated as the dry weight of lignin and the total weight of the bonding resin.

The reactivity of the lignin with the crosslinker can be increased by modifying the lignin by glyoxylation, etherification, esterification or any other method where lignin hydroxyl content or carboxylic content or amine content or thiol content is increased. Preferably, the lignin is used in native form, i.e. the lignin is not modified by glyoxylation, etherification, esterification or any other method where lignin hydroxyl content or carboxylic content or amine content or thiol content is increased.

The lignin in powder form or in the form of an aqueous dispersion is preferably mixed with the crosslinker (as an aqueous solution), at room temperature, such as at a temperature of from 15°C to 30°C. The mixing is preferably carried out for about 5 minutes to 2 hours. Preferably, the viscosity of the mixture is monitored during mixing, either continuously or by taking samples and determining the viscosity thereof. The mixing can also be carried out by providing the crosslinker as an aqueous solution, on lignin particles, such as by spraying the crosslinker as an aqueous solution onto lignin particles, optionally followed by drying.

Examples

Example 1 (comparative)

Lignin solution was prepared first by adding 180 g of powder lignin (solid content 90%) and 260 g of water were added to a 1 L glass reactor at ambient temperature and were stirred until the lignin was fully and evenly dispersed. Then, 60 g of 50% alkali solution was added to the lignin dispersion. The composition was stirred for 120 minutes to make sure that the lignin was completely dissolved in the alkaline media.

An bonding resin was prepared by mixing g of 82g of the lignin solution and 19 g of 16% polyamidoamine epichlorohydrin (PAE) solution using an overhead stirrer at room temperature in a 250 ml plastic beaker for 30 minutes or until the mixture is evenly dispersed. The pH of the formulation was 11.9.

The bonding resin was tested in a lap-joint test using an Automated Bonding Evaluation System (ABES) tensile tester.

For this purpose, relatively thin beech veneers of thickness of 0.6 mm were used that were cut into 105x20 mm pieces. Resin was applied to an area of 5 mm x 20 mm on one side of one end section of the two pieces. Resin coated, overlapped end section of a series of specimens were pressed together in a hot press at a pressure of 5 kg/m ² using pressing time of 90 seconds at pressing temperature of 150 °C.

Prior to evaluation, few samples were soaked in water for 24 hours at room temperature. Average data from five test specimens with and without conditioning is presented in the Table 1. All the samples delaminated after water soaking.

Table 1 : Dry and wet shear strength

Example 2 (comparative)

Lignin solution was prepared first by adding 161 g of powder lignin (solid content 90%) and 320 g of water were added to a 1 L glass reactor at ambient temperature and were stirred until the lignin was fully and evenly dispersed. Then, 35 g of 50% alkali solution was added to the lignin dispersion. The composition was stirred for 120 minutes to make sure that the lignin was completely dissolved in the alkaline media.

A bonding resin was prepared by mixing g of 82g of the lignin solution and 22 g of 16% polyamidoamine epichlorohydrin (PAE) solution using an overhead stirrer at room temperature in a 250 ml plastic beaker for 30 minutes or until the mixture is evenly dispersed. The pH of the formulation was 10.7.

The bonding resin was tested in a lap-joint test using an Automated Bonding Evaluation System (ABES) tensile tester as described by the procedure in the example 1.

Average data from five test specimens with and without conditioning is presented in the Table 2. All the samples were delaminated.

Table 2: Dry and wet shear strength

Example 3

A bonding resin was prepared by mixing 28 g of powder lignin (solid content 90%), 1.55g of 50% sodium hydroxide solution, 38g of 16% polyamidoamine epichlorohydrin (PAE) solution and 21 g of 15% polyvinyl alcohol using an overhead stirrer at room temperature in a 250 ml plastic beaker for 30 minutes or until the mixture is evenly dispersed. The pH of the formulation was 6.3.

The bonding resin was tested in a lap-joint test using an Automated Bonding Evaluation System (ABES) tensile tester as described by the procedure in the example 1. Average data from five test specimens with and without conditioning is presented in the Table 3.

Table 3: Dry and wet shear strength

Example 4

A bonding resin was prepared by mixing 34 g of powder lignin (solid content 90%), 1.8 g of 50% sodium hydroxide solution, 44 g of 16% polyamidoamine epichlorohydrin (PAE) solution and 19 g of glycerol using an overhead stirrer at room temperature in a 250 ml plastic beaker for 30 minutes or until the mixture is evenly dispersed. The pH of the formulation was 6.5.

The bonding resin was tested in a lap-joint test using an Automated Bonding Evaluation System (ABES) tensile tester as described by the procedure in the example 1.

Average data from five test specimens with and without conditioning is presented in the Table 4. Table 4: Dry and wet shear strength

Example 5

A bonding resin was prepared by mixing 44 g of powder lignin (solid content 90%), 2.6 g of 50% sodium hydroxide solution, 17g of water, 44 g of 16% polyamidoamine epichlorohydrin (PAE) solution and 34 g of 15% polyvinyl alcohol solution using an overhead stirrer at room temperature in a 250 ml plastic beaker for 30 minutes or until the mixture is evenly dispersed. The bonding resin was used to prepare a 5-ply plywood panel. Birch veneers of thickness 1.54-1.62 mm were sawn to 300 x 300 mm ² size and conditioned in 20°C, 65% RH prior to manufacture. Target glue content was 220 g/m ² which were spread on one side. Hot pressing was performed at 150°C with a pressure of 1.8 MPa for 9 minutes.

Shear strength of the test pieces were evaluated according to EN314_1_2005 test method. Prior to evaluation, samples were conditioned for class 1 (dry interior) according to 5.1.1 in the EN314_1_2005. Average data from 20 test specimens is presented in Table 5.

Table 5: Shear strength of test specimen according to EN314_1_2005

Example 5

A bonding resin was prepared by mixing 36 g of powder lignin (solid content 90%), 47.4 g of water and 23 g of 18% polyvinyl amine solution using an overhead stirrer at room temperature in a 250 ml plastic beaker for 30 minutes or until the mixture is evenly dispersed. The pH of the formulation was 4.6.

The bonding resin was tested in a lap-joint test using an Automated Bonding Evaluation System (ABES) tensile tester as described by the procedure in example 1.

Average data from five test specimens with and without conditioning is presented in the Table 6.

Table 6: Dry and wet shear strength

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.