RU2841666C2 - Aqueous adhesive composition for lignocellulosic materials such as wood and process for preparation thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to an aqueous adhesive composition for lignocellulosic materials such as wood. Aqueous adhesive composition for lignocellulosic materials contains: component (i) in the form of oxidised lignin, component (ii) in the form of a cross-linking agent, component (iii) in the form of a plasticiser, wherein said adhesive composition does not contain formaldehyde. Component (i) has a carboxyl group content of 0.05–10 mmol/g with respect to the dry weight of component (i), component (ii) is present in amount of 1–40 wt. % with respect to the dry weight of component (i), and component (iii) is present in amount of 0.5 wt. % to 50 wt. % with respect to the dry weight of component (i). Component (ii) in the form of a crosslinker is selected from crosslinkers based on β-hydroxyalkylamides and/or oxazoline-based cross-linkers selected from the group consisting of multifunctional organic amines such as alkanolamine, diamines, such as hexamethylenediamine, triamines, and is an epoxidised oil based on a fatty acid triglyceride or an elastic oligomer or polymer, such as an acrylic polymer, such as a vinyl polymer, such as polyether, which contains reactive functional groups such as carbodiimide groups, such as anhydride groups, such as oxazoline groups, such as amino groups, such as epoxy groups, or molecule having 3 or more epoxy groups. Component (iii) in the form of a plasticiser is selected from the group consisting of polyethylene glycols, polyethylene glycol ethers, polyethers, hydrogenated sugars and/or acids, such as adipic acid, vanillic acid, lactic acid and/or ferullic acid, acrylic polymers, polyvinyl alcohol, polyurethane dispersions, ethylene carbonate, propylene carbonate, lactons, lactams, lactides, acrylic polymers with free carboxy groups and/or polyurethane dispersions with free carboxy groups. Invention also relates to a method of producing a glued lignocellulose product and to a glued lignocellulose product.

EFFECT: obtaining an adhesive which is particularly suitable for gluing lignocellulosic materials such as wood, in which renewable materials are used as starting materials, in which corrosive and/or hazardous materials are in reduced quantities or are eliminated, and which is relatively cheap to produce.

17 cl, 5 dwg, 8 tbl, 10 ex

Description

Field of technology to which the invention relates

The present invention relates to an aqueous adhesive composition for lignocellulosic materials, such as wood, and a method for producing a glued lignocellulosic product, which includes the step of bringing one or more lignocellulosic materials into contact with the adhesive and curing said composition.

Prerequisites for the creation of an invention

The list of adhesive types for lignocellulosic materials such as wood is long and includes adhesives such as aliphatic resin, animal glue, casein, contact cement, epoxy glue, hot melt glue, phenol formaldehyde and urea formaldehyde adhesives.

Each of these adhesives has its pros and cons. Some are strong but use harmful materials or are expensive, such as isocyanate-based adhesives; others are cheap but have poor resistance to heat and/or moisture, such as polyvinyl acetate-based adhesives; and others use cheap, renewable materials but have poor bond strength.

With this oxidized lignin based adhesive, the authors have a product that has good adhesive properties, uses cheap and renewable resources, and the adhesive is stable under various conditions such as heat and humidity.

Lignin is used mainly to generate thermal energy by burning it, used as animal feed or simply disposed of as waste. A small part of lignin is used in various processes, such as as a dispersant, such as in cement and asphalt, or as a starting material for some aromatic compounds, such as vanillin.

One possible use of lignins is in adhesives, such as wood glues.

A number of adhesive or adhesive-type lignocellulosic products can be found in the prior art. JP2014065779 describes a resin composition comprising organic solvent-soluble lignin in combination with compounds containing several oxazoline rings. WO18138450 describes a lignin-based fibrous board that is used for carbon fibers obtained from bio-based materials, wherein non-chemically modified lignin, a plasticizer and a cross-linking agent are melt-extruded and then carbonized to form said fibers. US2009169867 describes a composite material comprising a support material and a cured/dried adhesive composition of polymer particles, defatted soy flour, any mixture of the following crosslinking agents: glycerol, glycerol derivatives, diethanolamine, triethanolamine, pentaerythritol, hydroxyalkyl urea, urea, oxazoline, polyvinyl alcohol, zirconium or zinc metal ions. This patent also concerns the addition of lignin or lignin sulfonate, which in this case can act as a dry diluent or filler.

US2010069533 describes a wood composite bonded with an adhesive composition for wood comprising any mixture of soy protein isolate, soy flour, flax meal, flax meal, hemp meal, and cereal meal; a component containing one or more oxazoline functional groups, and optionally lignin or lignosulfonate and/or an acid generating compound. It is important to note that US2010069533 does not describe any plasticizers; it does not describe any chemical treatment of lignin or lignosulfonate, such as oxidation, and it does not use lignin or lignosulfonate as a major component.

US2011159768 describes an aqueous adhesive composition comprising a component of one or more lignins or lignosulfonates grafted with ethylenically unsaturated carboxylic acids, and a component of polymers or resins containing one or more oxazoline functional groups. It is important to note that US2011159768 describes an aqueous adhesive composition comprising lignins or lignosulfonates chemically treated in a completely different manner than the present invention, and does not describe the use of plasticizers or oxidation.

There are several important characteristics of lignin in relation to adhesives. Lignin is an aromatic polymer with a high glass transition temperature ( _Tg ). Lignin thermally decomposes over a wide temperature range because the different oxygen-containing fragments have different stabilities, and the reactions that occur can be sequential but also competitive due to the hindered structure of the lignin polymer. The surface chemical properties of lignin (as well as the surface tension components) are similar to those of cured phenol-formaldehyde (PF) adhesives. This situation makes it reasonable to assume that the adhesive properties of lignin may be at the same level as those of adhesives long used in wood products, etc. However, lignin is a naturally heterogeneous material, and in addition, the properties and structure of lignin vary depending on the different methods used to extract lignin from biomass, as well as the biomass itself. The differences lie in the structure, the nature of the binding of aromatic units of lignin, molecular weight, functional groups, etc.

The reactive functional group present in large quantities in typical lignin is the hydroxyl group, which is a phenolic or aliphatic hydroxyl group. The presence of the phenolic hydroxyl group also activates the aromatic ring for reactions with aldehydes. Overall, it can be said that the structure of lignin limits the choice of crosslinkers to those reagents that most often involve environmental compromises, and therefore limits the ability to use lignin as a feedstock in processes that involve chemical reactions.

In order to use lignins as starting materials for various applications, chemical derivatization of lignins is proposed. One of the proposed ways of derivatization of lignins is oxidation. Oxidation of lignin is usually carried out using strong oxidizing agents in the presence of alkali metal hydroxides.

The essence of the invention

Accordingly, the object of the present invention is to provide an adhesive which is particularly suitable for gluing lignocellulosic materials such as wood, in which renewable materials are used as starting materials, in which corrosive and/or harmful materials are present in reduced quantities or eliminated, and which is comparatively inexpensive to produce.

Another object of the present invention is a product bonded with such an adhesive.

According to a first aspect of the present invention, there is provided an aqueous adhesive composition for lignocellulosic materials such as wood, comprising

- component (i) in the form of one or more oxidized lignins;

- component (ii) in the form of one or more crosslinkers;

- component (iii) in the form of one or more plasticizers.

According to a second aspect of the present invention, a method for producing a lignocellulosic product is provided, which comprises the step of contacting one or more lignocellulosic materials with the aqueous adhesive composition described above.

According to a third aspect of the present invention, there is provided a lignocellulosic product comprising one or more lignocellulosic materials in contact with a cured adhesive composition according to the present invention.

The inventors of the present invention have unexpectedly found that it is possible to obtain a bonded lignocellulosic product as a result of curing an adhesive whose main component is oxidized lignin as a main component, obtained from inexpensive renewable materials, which does not contain or contains to a small extent any corrosive or harmful agents.

Description of Preferred Embodiments

An aqueous adhesive composition for lignocellulosic materials such as wood according to the present invention comprises

- component (i) in the form of one or more oxidized lignins;

- component (ii) in the form of one or more crosslinkers;

- component (iii) in the form of one or more plasticizers.

In a preferred embodiment, the adhesives of the present invention do not contain formaldehyde.

For the purposes of this application, the term "formaldehyde-free" means that no formaldehyde is added during the preparation of the adhesive composition.

Component (i)

Component (i) is in the form of one or more oxidized lignins.

Lignin, cellulose and hemicellulose are the three main organic components in the cell wall of plants. Lignin can be thought of as the glue that holds cellulose fibres together. Lignin contains both hydrophilic and hydrophobic groups. It is the second most abundant natural polymer in the world, second only to cellulose, and is estimated to account for 20-30% of all carbon contained in biomass, which is over 1 billion tonnes worldwide.

Fig. 1 shows a cross-section of a possible structure of lignin.

There are at least four groups of technical lignins available on the market. These four groups are shown in Fig. 3. A possible fifth group, Biorefinery lignin, is slightly different as it is not described by an extraction process but by a process of origin, such as biorefining, and so it may be similar or different from the other groups mentioned. Each group is different from every other group, and each is suitable for different applications. Lignin is a complex heterogeneous material formed by up to three different phenylpropane monomers, depending on the source. Softwood lignins consist primarily of coniferyl alcohol units, see Fig. 2, and as a result they are more homogeneous than hardwood lignins, which have a higher content of syringyl alcohol units, see Fig. 2. The appearance and consistency of lignin is quite variable and depends heavily on the technological process.

A brief overview of the properties of these technical lignins is given in Fig. 4.

Lignosulfonate from the sulfite pulping process remains the largest commercially available source of lignin with a capacity of 1.4 million tons. But putting that aside, the kraft process is now the most widely used pulping process and is slowly replacing the sulfite process. An estimated 78 million tons per year of lignin is generated worldwide by kraft pulping, but most of it is burned to produce steam and energy. Kraft recovery is currently estimated at 160,000 tons, but sources indicate that recovery is currently only about 75,000 tons. Kraft lignin is produced from black liquor, the spent liquor of the kraft or kraft process. There are currently 3 well-known processes used to produce kraft lignin: LignoBoost, LignoForce and SLRP. These three processes are similar in that they involve the addition of _CO2 to lower the pH to 9-10, followed by acidification to further lower the pH to around 2. The final step involves some combination of washing, leaching and filtration to remove ash and other impurities. These three processes are in various stages of commercialization worldwide.

The Kraft process introduces thiol groups, stilbene, while some carbohydrates remain. Also sodium sulfate is present as an impurity due to precipitation of lignin from the liquor by sulfuric acid, but this can potentially be avoided by changing the lignin extraction route. The Kraft process leads to a large number of phenolic hydroxyl groups, and this lignin dissolves in water when these groups are ionized (above pH ~10).

Commercial kraft lignin is usually purer than lignosulfonates. The molecular weight is 1000-3000 g/mol.

Soda lignin is derived from sodium hydroxide pulping processes, which are mainly used for wheat straw, bagasse and flax. The properties of soda lignin are similar to kraft lignins in terms of solubility and _Tg . No sulfur is used in this process and there is no covalently bound sulfur. Ash levels are very low. Soda lignin has low solubility in neutral and acidic media, but is completely soluble at pH 12 and above.

The lignosulfonate process introduces a large number of sulfonate groups, making the lignin soluble in water but also in acidified aqueous solutions. Lignosulfonates contain up to 8% sulfur as sulfonate, while kraft lignin contains 1-2% sulfur, mainly bound to the lignin. The molecular weight of lignosulfonate is 15,000-50,000 g/mol. This lignin contains more residual carbohydrates than other types and has a higher average molecular weight. The typical hydrophobic core of lignin, together with the large number of ionized sulfate groups, makes this lignin attractive as a surfactant, and it often finds application in cement dispersion, etc.

Another group of lignins is becoming available - lignins produced by biorefining processes, in which the carbohydrates are separated from the lignin by chemical or biochemical processes to form a carbohydrate-rich fraction. This remaining lignin is called biorefined lignin. Biorefineries focus on energy production and the production of substitutes for fossil fuel and petrochemical-derived products as well as lignin. The lignin from this process is considered a low-value product or even a waste product, used primarily for thermal incineration or used as low-grade feed or otherwise disposed of.

The availability of organosolv lignin is still being considered on a semi-industrial scale. The process involves the extraction of lignin using water together with various organic solvents (most commonly ethanol) and some organic acids. The advantage of this process is the higher purity of the resulting lignin, but at a higher cost compared to other technical lignins and solubility in organic solvents rather than water.

Previous attempts to use lignin as a base compound for adhesive compositions for lignocellulosic materials have failed because it is difficult to find suitable crosslinkers that provide the desired mechanical properties of the cured mineral wool product while avoiding hazardous and/or corrosive components. Lignin is currently used to replace petroleum-derived chemicals such as phenol in phenolic resins in adhesive or bitumen applications. It is also used as an additive to cement and concrete and in some aspects as a dispersant.

A polymer crosslinker should generally provide improved properties such as mechanical, chemical and thermal stability, etc. Lignin is particularly rich in phenolic and aliphatic hydroxyl groups, which can interact to form a crosslinked lignin structure. Different lignins will also have other functional groups available that can potentially be used. The existence of these other groups depends largely on the way in which the lignin was separated from the cellulose and hemicellulose (thiol groups in kraft lignin, sulfonate in lignosulfonate, etc.), depending on the source.

The crosslinking potential of hydroxyl groups is relatively limited. Lignin is of course very reactive with isocyanates and can form polyurethanes. However, polyurethanes are of less interest because of the toxicity of isocyanates. Similarly, phenolic hydroxyls can react to open rings with epoxides and participate in the standard amine curing of epoxy resins, but again epoxides are of less interest because of toxicity. Phenolic hydroxyls activate aromatic rings for reaction in standard phenolic resins using aldehydes as crosslinkers, but again this is of less interest because of the toxicity of aldehydes. For example, the crosslinking of lignins with aldehydes is well known, see EP3299421 A1, which discloses an adhesive comprising lignin, glutaraldehyde, ammonia, glucose and lysine. Several examples of lignin-containing adhesives including formaldehyde are also known, for example from WO14080033 and US3227667.

The inventors of the present invention have unexpectedly discovered that by using oxidized lignins, it is possible to obtain adhesive compositions for lignocellulosic materials that provide excellent properties for the glued lignocellulosic product obtained with their help, and at the same time, the adhesive compositions do not require the inclusion of hazardous and/or corrosive components.

In one embodiment, component (i) is in the form of one or more oxidized kraft lignins.

In one embodiment, component (i) is in the form of one or more oxidized sodium lignins.

In one embodiment, component (i) is in the form of one or more lignins oxidized with ammonia. For the purposes of the present invention, the term "lignins oxidized with ammonia" shall be understood as lignin that has been oxidized by an oxidizing agent in the presence of ammonia. The term "lignin oxidized with ammonia" is abbreviated as AOL.

In another embodiment, ammonia is partially or completely replaced by an alkali metal hydroxide, in particular sodium hydroxide and/or potassium hydroxide.

A typical oxidizing agent used to produce oxidized lignins is hydrogen peroxide.

In one embodiment, the ammonia-oxidized lignin comprises one or more compounds selected from the group of ammonia, amines, hydroxides, or any salts thereof.

In one embodiment, component (i) has a carboxyl group content of 0.05-10 mmol/g, such as 0.1-5 mmol/g, such as 0.20-1.5 mmol/g, such as 0.40-1.2 mmol/g, such as 0.45-1.0 mmol/g relative to the dry weight of component (i).

In one embodiment, component (i) has an average carboxyl group content of greater than 1.5 groups per macromolecule of component (i), such as greater than 2 groups, such as greater than 2.5 groups.

Without wishing to be bound by any particular theory, the inventors believe that the carboxyl group content of the oxidized lignins plays an important role in the unexpected advantages of the aqueous adhesive compositions for lignocellulosic materials according to the present invention. In particular, the inventors believe that the carboxyl group in the oxidized lignins improves the crosslinking properties and, therefore, provides better mechanical properties of the cured adhesive lignocellulosic products.

Component (ii)

Component (ii) is in the form of one or more crosslinkers.

Component (ii) in one embodiment is in the form of one or more crosslinkers selected from β-hydroxyalkylamide based crosslinkers and/or oxazoline based crosslinkers.

β-hydroxyalkylamide crosslinkers are curing agents for acid-functional macromolecules. They provide a hard, tough, corrosion-resistant, and solvent-resistant crosslinked polymer network. β-hydroxyalkylamide crosslinkers are believed to cure the polymer via an esterification reaction to form multiple ester linkages. The hydroxyl functionality of β-hydroxyalkylamide crosslinkers should average at least 2, preferably greater than 2, and more preferably 2-4, to obtain an optimal curing reaction.

Crosslinkers containing oxazoline groups are polymers containing one or more oxazoline groups in each molecule, and, in general, oxazoline-containing crosslinkers can be easily prepared by polymerizing oxazoline derivatives. Patent US6818699 B2 discloses such a process.

In one embodiment, component (ii) is one or more crosslinkers selected from the group consisting of multifunctional organic amines such as alkanolamine, diamines such as hexamethylenediamine, triamines.

In one embodiment, component (ii) is an epoxidized fatty acid triglyceride oil.

It is noted that epoxidized oils based on fatty acid triglycerides are not considered hazardous, and therefore the use of these compounds in the adhesive compositions according to the present invention does not make these compositions unsafe for work.

In one embodiment, component (ii) is a molecule having 3 or more epoxy groups.

In one embodiment, component (ii) is one or more elastic oligomers or polymers, for example a low _Tg acrylic polymer, for example a low _Tg vinyl polymer, for example a low _Tg polyether, which contains reactive functional groups, for example carbodiimide groups, for example anhydride groups, for example oxazoline groups, for example amino groups, for example epoxy groups.

In one embodiment, component (ii) is selected from the group comprising crosslinkers participating in the curing reaction, such as a hydroxyalkylamide, an alkanolamine, a reaction product of an alkanolamine and a polycarboxylic acid. A reaction product of an alkanolamine and a polycarboxylic acid can be found in US6706853B1.

Without wishing to be bound by any particular theory, the inventors of the present invention believe that the very advantageous properties of the aqueous adhesive compositions according to the present invention are due to the interaction of the oxidized lignins used as component (i) and the crosslinkers mentioned above. It is believed that the presence of carboxyl groups in the oxidized lignins makes it possible to crosslink the oxidized lignins very effectively. Another advantage is that the crosslinkers based on β-hydroxyalkyl amides and the crosslinkers based on oxazolines, which are preferably used as crosslinkers in the aqueous adhesive composition according to the present invention, are non-hazardous, in particular non-toxic and non-corrosive. These crosslinkers react very effectively with oxidized lignins containing increased amounts of carboxyl groups, thereby making it possible to have advantageous mechanical properties of the glued lignocellulosic products according to the present invention.

In one embodiment, the adhesive composition according to the present invention comprises component (ii) in an amount of 1-40 wt.%, such as 4-20 wt.%, such as 6-12 wt.%, relative to the dry weight of component (i).

Component (iii)

Component (iii) is in the form of one or more plasticizers.

It has been unexpectedly found that the inclusion of plasticizers in the aqueous adhesive compositions according to the present invention significantly improves the mechanical properties of the bonded lignocellulosic products according to the present invention.

The term "plasticizer" refers to a substance that is added to a material to make the material softer, more flexible (by lowering the glass transition temperature T _g ), and easier to process.

In one embodiment, component (iii) is in the form of one or more plasticizers selected from the group consisting of polyethylene glycols, polyethylene glycol ethers, polyethers, hydrogenated sugars, phthalates and/or acids such as adipic acid, vanillic acid, lactic acid and/or ferulic acid, acrylic polymers, polyvinyl alcohol, polyurethane dispersions, ethylene carbonate, propylene carbonate, lactones, lactams, lactides, acrylic polymers with free carboxyl groups and/or polyurethane dispersions with free carboxyl groups.

In one embodiment, component (iii) is in the form of one or more plasticizers selected from the group consisting of carbonates such as ethylene carbonate, propylene carbonate, lactones, lactams, lactides, compounds with a structure similar to lignin, like vanillin, acetosyringone, solvents used as coalescing agents, like alcohol ethers, polyvinyl alcohol.

In one embodiment, component (iii) is in the form of one or more non-reactive plasticizers selected from the group consisting of polyethylene glycols, polyethylene glycol ethers, polyethers, hydrogenated sugars, phthalates and/or other ethers, solvents used as coalescing agents like alcohol ethers, acrylic polymers, polyvinyl alcohol.

In one embodiment, component (iii) is one or more reactive plasticizers selected from the group consisting of carbonates such as ethylene carbonate, propylene carbonate, lactones, lactams, lactides, di- or tricarboxylic acids such as adipic acid or lactic acid and/or vanillic acid and/or ferulic acid, polyurethane dispersions, acrylic polymers with free carboxyl groups, compounds with a structure similar to lignin, like vanillin, acetosyringone.

In one embodiment, component (iii) is in the form of one or more plasticizers selected from the group consisting of polyethylene glycols, polyethylene glycol ethers.

Another separate unexpected aspect of the present invention is that the use of plasticizers like polyethylene glycols and polyethylene glycol ethers having a boiling point above 100°C, in particular 140-250°C, significantly improves the mechanical properties of the bonded lignocellulosic products according to the present invention, although, given their boiling point, it is likely that these plasticizers will at least partially evaporate during the curing of aqueous adhesives in contact with lignocellulosic materials.

In one embodiment, component (iii) is in the form of one or more plasticizers having a boiling point above 100°C, such as 110-280°C, preferably 120-260°C, preferably 140-250°C.

Without wishing to be bound by any particular theory, the inventors believe that the effectiveness of these plasticizers in the aqueous adhesive composition according to the invention is due to the effect of enhancing the mobility of the oxidized lignins during the curing process, whereby at the same time they evaporate during this curing process. It is believed that the enhanced mobility of the lignins or oxidized lignins during the curing process facilitates effective crosslinking. Another advantage of this aspect is that the plasticizer is almost absent in the cured adhesive lignocellulosic product, so that its side effects are absent; for example, in the cured adhesive lignocellulosic product, absorption of water or a change in mechanical properties occurs during the aging process.

In one embodiment, component (iii) is in the form of one or more polyethylene glycols having an average molecular weight of 150-50000 g/mol, in particular 150-4000 g/mol, more often 150-1000 g/mol, preferably 150-500 g/mol, more preferably 200-400 g/mol.

In one embodiment, component (iii) is in the form of one or more polyethylene glycols having an average molecular weight of 4000-25000 g/mol, in particular 4000-15000 g/mol, more often 8000-12000 g/mol.

In one embodiment, component (iii) is capable of forming covalent bonds with component (i) and/or component (ii) during the curing process. Such component will not evaporate and remains as part of the composition, but must be effectively modified so as not to cause undesirable side effects, such as absorption of water by the cured product. Non-limiting examples of such a component are caprolactone and acrylic polymers with free carboxyl groups.

In one embodiment, component (iii) is present in an amount of 0.5-50, preferably 2.5-25, more preferably 3-15 wt.% relative to the dry weight of component (i).

Aqueous adhesive composition for lignocellulosic materials comprising components (i) and (iia)

In one embodiment, the present invention relates to an aqueous adhesive composition for lignocellulosic materials comprising

- component (i) in the form of one or more oxidized lignins;

- component (iia) in the form of one or more modifiers.

The inventors of the present invention have found that excellent adhesive properties can also be obtained using a two-component system which comprises component (i) in the form of one or more oxidized lignins and component (iia) in the form of one or more modifiers and, optionally, any of the other components mentioned above and below.

In a preferred embodiment, component (iia) is a modifier in the form of one or more compounds selected from the group consisting of epoxidized oils based on fatty acid triglycerides.

In one embodiment, component (iia) is a modifier in the form of one or more compounds selected from molecules having 3 or more epoxy groups.

In one embodiment, component (iia) is a modifier in the form of one or more elastic oligomers or polymers, for example a low _Tg acrylic polymer, for example a low _Tg vinyl polymer, for example a low _Tg polyether, which contains reactive functional groups, for example carbodiimide groups, for example anhydride groups, for example oxazoline groups, for example amino groups, for example epoxy groups.

Without wishing to be bound by any particular theory, the inventors of the present invention believe that the excellent adhesive properties achieved by the adhesive composition for lignocellulosic materials comprising components (i) and (iia) and, optionally, other components, are at least partly due to the effect that the modifiers used as component (iia) at least partly act as a plasticizer and a crosslinker.

In one embodiment, the aqueous adhesive composition comprises component (iia) in an amount of 1-40 wt.%, such as 4-20 wt.%, such as 6-12 wt.%, relative to the dry weight of component (i).

Other components

In some embodiments, the aqueous adhesive composition of the present invention includes additional components.

In one embodiment, the aqueous adhesive composition according to the present invention comprises a catalyst selected from inorganic acids, such as sulfuric acid, sulfamic acid, nitric acid, boric acid, hypophosphorous acid and/or phosphoric acid, and/or their salts, such as sodium hypophosphite, and/or ammonium salts, such as ammonium salts of sulfuric acid, sulfamic acid, nitric acid, boric acid, hypophosphorous acid and/or phosphoric acid. The presence of such a catalyst can improve the curing properties of the aqueous adhesive compositions according to the present invention.

In one embodiment, the aqueous adhesive composition according to the present invention comprises a catalyst selected from Lewis acids that can accept an electron pair from a donor compound to form a Lewis adduct, such as ZnCl ₂ , Mg(ClO ₄ ) ₂ , Sn[N(SO ₂ -nC ₈ F ₁₇ ) ₂ ] ₄ .

In one embodiment, the aqueous adhesive composition according to the present invention comprises a catalyst selected from metal chlorides such as KCl, MgCl ₂ , ZnCl ₂ , FeCl ₃ and SnCl ₂ .

In one embodiment, the aqueous adhesive composition according to the present invention comprises a catalyst selected from organometallic compounds, such as titanate-based catalysts and tin-based catalysts.

In one embodiment, the aqueous adhesive composition according to the present invention comprises a catalyst selected from chelating agents, for example transition metals, for example iron ions, chromium ions, manganese ions, copper ions. In one embodiment, the aqueous adhesive composition according to the present invention comprises a component in the form of one or more components selected from the group of ammonia, amines or any salts thereof.

The present inventors have found that the inclusion of ammonia, amines or any salts thereof as an additional component may be useful, in particular when oxidized lignins are used in component (i), where the oxidized lignin has not been oxidized in the presence of ammonia.

In one embodiment, the aqueous adhesive composition according to the present invention also comprises an additional component in the form of urea, in particular in an amount of 5-40 wt.%, such as 10-30 wt.%, 15-25 wt.% relative to the dry weight of component (i).

In one embodiment, the aqueous adhesive composition according to the present invention also comprises an additional component in the form of one or more carbohydrates selected from the group consisting of sucrose, reducing sugars, in particular dextrose, polycarbohydrates and mixtures thereof, preferably dextrins and maltodextrins, more preferably glucose syrup, and more preferably glucose syrup with a dextrose equivalent value DE = 30 to less than 100, for example DE = 60 to less than 100, for example DE = 60-99, for example DE = 85-99, for example DE = 95-99.

In one embodiment, the aqueous adhesive composition according to the present invention also comprises an additional component in the form of one or more carbohydrates selected from the group consisting of sucrose and reducing sugars, in an amount of 5-50 wt.%, such as from 5 to less than 50 wt.%, such as 10-40 wt.%, such as 15-30 wt.%, relative to the dry weight of component (i).

In the context of the present invention, an adhesive composition with a sugar content of 50% by weight or more relative to the total dry weight of the adhesive components is considered to be a sugar-based adhesive. In the context of the present invention, an adhesive composition with a sugar content of less than 50% by weight relative to the total dry weight of the adhesive components is considered to be a non-sugar-based adhesive (non-sugar base).

In one embodiment, the aqueous adhesive composition according to the present invention also comprises an additional component in the form of one or more surfactants which are in the form of non-ionic and/or ionic emulsifiers, for example polyoxyethylene (4) lauryl ether, for example soya lecithin, for example sodium dodecyl sulfate.

In one embodiment, the aqueous adhesive composition of the present invention comprises

- component (i) in the form of one or more lignins oxidized with ammonia having a carboxyl group content of 0.05-10 mmol/g, for example 0.1-5 mmol/g, for example 0.20-1.5 mmol/g, for example 0.40-1.2 mmol/g, for example 0.45-1.0 mmol/g relative to the dry weight of component (i);

- component (ii) in the form of one or more crosslinkers selected from crosslinkers based on β-hydroxyalkylamides and/or crosslinkers based on oxazolines, and/or is one or more crosslinkers selected from the group comprising multifunctional organic amines such as alkanolamine, diamines such as hexamethyldiamine, triamines;

- component (iii) in the form of one or more polyethyleneglycols having an average molecular weight of 150-50,000 g/mol, in particular 150-4,000 g/mol, more often 150-1,000 g/mol, preferably 150-500 g/mol, more preferably 150-300 g/mol, or one or more polyethyleneglycols having an average molecular weight of 4,000-25,000 g/mol, in particular 4,000-15,000 g/mol, more often 8,000-12,000 g/mol; wherein preferably the aqueous adhesive composition comprises component (ii) in an amount of 1-40 wt.%, for example 4-20 wt.%, 6-12 wt.% based on the dry weight of component (i), and (iii) is present in an amount of 0.5-50, preferably 2.5-25, more preferably 3-15 wt.% based on the dry weight of component (i).

In one embodiment, the aqueous adhesive composition of the present invention comprises

- component (i) in the form of one or more lignins oxidized with ammonia having a carboxyl group content of 0.05-10 mmol/g, for example 0.1-5 mmol/g, for example 0.20-1.5 mmol/g, for example 0.40-1.2 mmol/g, for example 0.45-1.0 mmol/g relative to the dry weight of component (i);

- component (iia) in the form of one or more modifiers selected from epoxidized oils based on fatty acid triglycerides.

In one embodiment, the aqueous adhesive composition of the present invention comprises

- component (i) in the form of one or more lignins oxidized with ammonia, having an average content of carboxyl groups greater than 1.5 groups per macromolecule of component (i), for example greater than 2 groups, for example greater than 2.5 groups;

- component (ii) in the form of one or more crosslinkers selected from crosslinkers based on β-hydroxyalkylamides and/or crosslinkers based on oxazolines, and/or is one or more crosslinkers selected from the group comprising multifunctional organic amines such as alkanolamine, diamines such as hexamethyldiamine, triamines;

- component (iii) in the form of one or more polyethyleneglycols having an average molecular weight of 150-50,000 g/mol, in particular 150-4,000 g/mol, more often 150-1,000 g/mol, preferably 150-500 g/mol, more preferably 150-300 g/mol, or one or more polyethyleneglycols having an average molecular weight of 4,000-25,000 g/mol, in particular 4,000-15,000 g/mol, more often 8,000-12,000 g/mol; wherein preferably the aqueous adhesive composition comprises component (ii) in an amount of 1-40 wt.%, for example 4-20 wt.%, 6-12 wt.% based on the dry weight of component (i), and (iii) is present in an amount of 0.5-50, preferably 2.5-25, more preferably 3-15 wt.% based on the dry weight of component (i).

In one embodiment, the aqueous adhesive composition of the present invention comprises

- component (i) in the form of one or more lignins oxidized with ammonia, having an average content of carboxyl groups greater than 1.5 groups per macromolecule of component (i), for example greater than 2 groups, for example greater than 2.5 groups;

- component (iia) in the form of one or more modifiers selected from epoxidized oils based on fatty acid triglycerides.

In one embodiment, the aqueous adhesive composition of the present invention consists essentially of

- component (i) in the form of one or more oxidized lignins;

- component (ii) in the form of one or more crosslinkers;

- component (iii) in the form of one or more plasticizers;

- optionally, a component in the form of one or more compounds selected from the group of ammonia, amines or any salts thereof;

- optionally, a component in the form of urea;

- optionally, a component in the form of more reactive or non-reactive silicones;

- optionally, hydrocarbon oil;

- optionally, one or more surfactants;

- water.

In one embodiment, the aqueous adhesive composition of the present invention consists essentially of

- component (i) in the form of one or more oxidized lignins;

- component (iia) in the form of one or more modifiers selected from epoxidized oils based on fatty acid triglycerides;

- optionally, a component in the form of one or more compounds selected from the group of ammonia, amines or any salts thereof;

- optionally, a component in the form of urea;

- optionally, a component in the form of more reactive or non-reactive silicones;

- optionally, hydrocarbon oil;

- optionally, one or more surfactants;

- water.

Method for producing glued lignocellulosic product

The present invention also relates to a method for producing a glued lignocellulosic product by gluing one or more lignocellulosic materials with an adhesive composition.

Accordingly, the present invention also relates to a method for producing an adhesive lignocellulosic product, which comprises the step of contacting one or more lignocellulosic materials with an adhesive composition comprising

- component (i) in the form of one or more oxidized lignins;

- component (ii) in the form of one or more crosslinkers;

- component (iii) in the form of one or more plasticizers.

Examples of lignocellulosic products include, but are not limited to, medium density fiberboard, oriented strand board, particle board, laminated veneer lumber, and plywood.

The method includes the steps of contacting lignocellulosic materials with an adhesive composition comprising

- component (i) in the form of one or more oxidized lignins;

- component (ii) in the form of one or more crosslinkers;

- component (iii) in the form of one or more plasticizers.

Examples of lignocellulosic materials include, but are not limited to, hardwood, wood fiber, sawdust, paper, straw.

In the method of the present invention, the components are thoroughly mixed together before the mixture is brought into contact with the lignocellulosic materials.

The adhesive composition of the present invention can be brought into contact with the lignocellulosic material in various ways, such as by spreading, such as by spraying, such as by mixing, etc.

After the contact is made, the lignocellulosic material is pressed for a certain time and under a certain pressure. In one embodiment, the level of pressure applied is from 0.1 to 10 MPa, such as from 0.5 to 6 MPa. In one embodiment, the pressing is carried out for a time from 30 seconds to 20 minutes, such as 2-10 minutes. The pressing is usually carried out in a hot pressing press at a temperature above room temperature - from 100 to 250 °C, such as from 140 to 180 °C.

Bonded lignocellulosic product according to the present invention

The present invention also relates to a bonded lignocellulosic product comprising one or more lignocellulosic materials in contact with the cured adhesive composition described above.

Oxidized lignins that can be used as component (i) in an aqueous adhesive composition for lignocellulosic materials according to the present invention, and a method for producing such oxidized lignins

The following describes the oxidized lignins that can be used as component (i) and their preparation.

The oxidized lignins that can be used as component (i) for the adhesives according to the present invention can be obtained by a process comprising bringing into contact

- component (a), including one or more lignins,

- component (b), comprising ammonia, one or more amino components and/or any salt thereof,

- component (c), including one or more oxidizing agents.

Component(s)

Component (a) comprises one or more lignins.

In one embodiment of the method according to the present invention, component (a) comprises one or more kraft lignins, one or more soda lignins, one or more lignosulfonate lignins, one or more organosolv lignins, one or more lignins from biorefining processes of lignocellulosic feedstocks, or any mixture thereof.

In one embodiment, component (a) comprises one or more kraft lignins.

Component (b)

In one embodiment according to the present invention, component (b) comprises ammonia, one or more amino components and/or any salts thereof. Without wishing to be bound by any particular theory, the inventors of the present invention believe that the replacement of alkali metal hydroxides used in previously known lignin oxidation processes with ammonia, one or more amino components and/or any salts thereof plays an important role in improving the properties of the oxidized lignins obtained according to the process of the present invention.

The inventors of the present invention have unexpectedly found that lignins oxidized with an oxidizing agent in the presence of ammonia or amines contain significant amounts of nitrogen as part of the structure of the oxidized lignins. Without wishing to be bound by any particular theory, the inventors of the present invention believe that the improved flame retardancy properties of the oxidized lignins when used in products in which they are incorporated in an adhesive composition, said oxidized lignins obtained by the method according to the present invention, are exhibited at least in part due to the nitrogen content in the structure of the oxidized lignins.

In one embodiment, component (b) comprises ammonia and/or any salt thereof.

Without wishing to be bound by any particular theory, the inventors believe that the improved stability properties of the derivatized lignins produced according to the present invention are at least partly due to the fact that ammonia is a volatile compound and therefore evaporates from the final product or can be easily removed and reused. In contrast, it is difficult to remove residual amounts of alkalis used in the previously known oxidation process.

In any event, in the process according to the present invention it may be advantageous when component (b), in addition to ammonia, one or more amino components and/or any salts thereof, also comprises a comparatively small amount of an alkali and/or alkaline earth metal hydroxide, such as sodium hydroxide and/or potassium hydroxide.

In embodiments in which component (b), in addition to ammonia, one or more amino components and/or any salts thereof, comprises as a component alkali and/or alkaline earth metal hydroxides, such as sodium hydroxide and/or potassium hydroxide, the amount of alkali and/or alkaline earth metal hydroxides is typically small, such as 5-70 parts by weight, such as 10-20 parts by weight of alkali and/or alkaline earth metal hydroxide relative to ammonia.

Component (c)

In the method according to the present invention, component (c) comprises one or more oxidizing agents.

In one embodiment, component (c) comprises one or more oxidizing agents in the form of hydrogen peroxide, organic or inorganic peroxides, molecular oxygen, ozone, halogenated oxidizing agents, or any mixture thereof.

In the early stages of oxidation, active radicals from the oxidant will typically abstract a proton from the phenolic group, as this bond in lignin has the lowest dissociation energy. Due to the ability of lignin to stabilize radicals via mesomerism, several pathways are open for the reaction to proceed (but also to terminate), and various intermediates and end products are obtained. The average molecular weight can either increase or decrease due to this complexity (and the conditions chosen), and in these experiments we typically saw a modest increase in average molecular weight of about 30%.

In one embodiment, component (c) comprises hydrogen peroxide.

Hydrogen peroxide is perhaps the most commonly used oxidant due to its combination of low cost, good efficiency, and relatively low environmental impact. When hydrogen peroxide is used in the absence of catalysts, alkaline conditions and temperature are important due to subsequent reactions leading to radical formation.

The inventors of the present invention have found that the derivatized lignins produced by the process of the present invention contain increased amounts of carboxyl groups as a result of the oxidation process. Without wishing to be bound by any particular theory, the inventors of the present invention believe that the carboxyl group content of the oxidized lignins produced by the process of the present invention plays an important role in the desirable reactivity properties of the derivatized lignins produced by the process of the present invention.

Another advantage of the oxidation process is that the oxidized lignin is more hydrophilic. Higher hydrophilicity can enhance water solubility and facilitate adhesion to polar substrates such as lignocellulosic materials.

Other components

In one embodiment, the method according to the present invention comprises additional components, in particular component (d) in the form of an oxidation catalyst such as one or more transition metal catalysts such as iron sulfate, as catalysts containing manganese, palladium, selenium, tungsten.

Such oxidation catalysts can increase the reaction rate, thereby improving the properties of the oxidized lignins obtained by the method according to the present invention.

Mass ratios of components

Those skilled in the art will use components (a), (b) and (c) in relative amounts that achieve the desired degree of oxidation of the lignins.

In one incarnation

- component (a) includes one or more lignins,

- component (b) includes ammonia,

- component (c) includes one or more oxidizing agents in the form of hydrogen peroxide,

wherein the mass ratios of lignin, ammonia and hydrogen peroxide are such that the amount of ammonia is 0.01-0.5 parts by mass, for example 0.1-0.3, for example 0.15-0.25 parts by mass of ammonia relative to the dry mass of lignin, and wherein the amount of hydrogen peroxide is 0.025-1.0 parts by mass, for example 0.05-0.2 parts by mass, for example 0.075-0.125 parts by mass of hydrogen peroxide relative to the dry mass of lignin.

Way

There are several possibilities to bring components (a), (b) and (c) into contact to achieve the desired oxidation reaction.

In one embodiment, the method comprises the following steps:

- a step of providing component (a) in the form of an aqueous solution and/or dispersion of one or more lignins, wherein the lignin content in the aqueous solution is 1-50 wt.%, for example 5-25 wt.%, for example 15-22 wt.%, for example 18-20 wt.% relative to the total weight of the aqueous solution;

- a pH adjustment step by adding component (b) comprising an aqueous ammonia solution, one or more amino components and/or any salts thereof;

- an oxidation stage by adding component (c) containing an oxidizing agent.

In one embodiment, the pH adjustment step is performed such that the resulting aqueous solution and/or dispersion has a pH ≥ 9, such as ≥ 10, such as ≥ 10.5.

In one embodiment, the pH adjustment step is performed such that the resulting aqueous solution and/or dispersion has a pH in the range of 10.5-12.

In one embodiment, the pH adjustment step is performed such that the temperature is allowed to rise to ≥ 25°C and then adjusted to a range of 25-50°C, such as 30-45°C, such as 35-40°C.

In one embodiment, during the oxidation step, the temperature is allowed to rise to ≥ 35°C and then controlled in the range of 35-150°C, such as 40-90°C, such as 45-80°C.

In one embodiment, the oxidation step is performed for a time from 1 second to 48 hours, such as 10 seconds - 36 hours, such as 1 minute - 24 hours, such as 2-5 hours.

Reaction product

The present invention also relates to oxidized lignins obtained by the method according to the present invention.

The inventors of the present invention have surprisingly found that the oxidized lignins obtained according to the method of the present invention have very favorable reactivity properties and at the same time exhibit improved fire retardancy properties when used in products in which they are incorporated in the adhesive composition and improved long-term stability compared to previously known oxidized lignins.

Oxidized lignin also exhibits improved hydrophilicity.

An important parameter for the reactivity of the oxidized lignins obtained by the method according to the present invention is the content of carboxyl groups in the oxidized lignins.

In one embodiment, the oxidized lignin obtained according to the present invention has a carboxyl group content of 0.05-10 mmol/g, such as 0.1-5 mmol/g, such as 0.20-1.5 mmol/g, such as 0.40-1.2 mmol/g, such as 0.45-1.0 mmol/g, relative to the dry weight of component (a).

Another way to describe the carboxyl content is to use the average carboxyl content per lignin macromolecule according to the following formula:

.

In one embodiment, the oxidized lignin obtained according to the present invention has an average carboxyl group content of greater than 1.5 groups per macromolecule of component (a), such as greater than 2 groups, such as greater than 2.5 groups.

The following examples are intended to further illustrate the invention without limiting its scope.

Examples

In the following examples, several adhesives are prepared and compared with commercially available adhesives to further illustrate the invention without limiting its scope.

Kraft lignin is supplied by UPM as LignoBoost™ as a powder, oxidized lignin (AOL) is obtained from Aarhus University as a solution in aqueous ammonia with 18.3 wt% dry matter and a carboxyl content of 0.80 mmol/g, number average molecular weight Mn 2503 g/mol, and weight average molecular weight Mw 34503 g/mol. Primid XL552 is supplied by EMS-CHEMIE AG, 100% dry matter. Polyethylene glycol 300 and polyvinyl alcohol, Mw 9000-10000 g/mol, 80% hydrolyzed, are supplied by Sigma-Aldrich and are considered anhydrous for simplicity.

SikaBond 540 and 545, obtained from Sika, are used as examples for comparison.

Shear strength test of adhesive lap joints

The mechanical properties of the adhesives are quantified using a simple lap shear test, which is a well-known method for comparing the shear strength of adhesives and there are standards for the different substrates to be bonded. The method used is in accordance with EN205, with modifications to suit the available laboratory setup. The preparation of the specimens involves applying the adhesive to the substrate, placing another piece of the designated overlap area on it, applying pressure to the overlap area and curing the adhesive under specified conditions. The size of the wood specimens is 75 x 5 x 1.3 mm. The overlap area is 10 x 5 mm. Ten specimens are obtained for each composition.

Water resistance test

Some of the bonded specimens prepared for the lap joint shear strength test are also exposed to water at 70°C for three hours. Twelve specimens are obtained for each composition. After removing the specimens from the water, the residual strength of the specimens is tested while the specimens are still wet.

Example for comparison A

SikaBond 540 is a polyvinyl acetate based wood adhesive with approximately 46% solids by weight, corresponding to resistance class D3 according to EN 204/205. The lap shear test is carried out under the following conditions suggested by the manufacturer (technical data sheet plus product label). The adhesive application rate is 100 g/ ^m2 and the specimens are pressed at 22±1°C and 1.5 MPa for 30 minutes.

Example for comparison B

SikaBond 545 is a methylene diphenyl diisocyanate (MDI) based wood adhesive with approximately 100 wt% solids, moisture curing and corresponds to resistance class D4 according to EN 204/205. The lap shear tests are carried out under the following conditions suggested by the manufacturer (technical data sheet plus product label). The adhesive application rate is 100 g/ ^m2 and the specimens are pressed at 21±1°C and 1.5 MPa for 7 hours. The relative humidity in the laboratory is approximately 41% and the moisture content of the wood specimens is approximately 9 wt%.

Examples of adhesives based on oxidized dignin

All samples for testing mechanical properties are obtained with an adhesive load of 100 g/ ^m2 , and the samples are pressed at a pressure of 1.5 MPa for 5 minutes. The curing temperature is changed for different samples, as shown in Tables 1.1-1.2.

Glue example 1

To a mixture of 18.3% aqueous AOL (100 g, accordingly 18.3 g AOL is effectively added) with stirring at room temperature 2.5 g Primid XL552.

Glue example 2

Polyvinyl alcohol, Mw 9000-10000 g/mol, hydrolyzed to 80%, is dissolved in water at 80°C and cooled to room temperature as a 7 wt.% solution. 40 g of polyvinyl alcohol, Mw 9000-10000 g/mol, hydrolyzed to 80% (7 wt.% solution, respectively effectively 2.8 g of polyvinyl alcohol) are added to a mixture of 18.3% aq. AOL (100 g, respectively effectively 18.3 g of AOL) with stirring at room temperature.

Example of glue 3

To a mixture of 18.3% aqueous AOL (100 g, accordingly 18.3 g AOL is effective) 5.0 g polyethyleneglycol 300 and 2.5 g Primid XL552 are added with stirring at room temperature.

Further observations and conclusions can be drawn from Tables 1.1 and 1.2.

It is clear that oxidized lignin without modifiers does not provide bonding strength. Only after adding one or more modifiers can this adhesive bond lignocellulosic materials such as wood. Different modifiers can provide different final bonding strengths.

It can also be seen that by combining different modifiers, even higher mechanical properties can be achieved compared to using such modifiers separately.

Table 1.2 also shows the effect of curing temperature on the crosslinker Primid XL552. A higher temperature, while other conditions (such as pressing time, for example) remain unchanged, leads to higher mechanical properties.

The specific compositions shown in the tables also show comparable strength to commercially available adhesives when tested by the same methods (composition 5 vs. comp. A; composition 7 vs. comp. A and comp. B). One of these compositions (composition 7) is also tested for water resistance and again shows comparable residual strength properties to the commercial comparison product (compar. 8, a moisture-curing MDI adhesive).

Overall, this means that it is possible to produce an adhesive composition with a high content of renewable lignin-based material that has mechanical properties comparable to those of standard systems.

Table 1.1 Adhesive composition Compar. A Comp. B 1 2 3 4 5 Oxidized Kraft Lignin (g) - - 100 100 100 100 100 Primid XL552 (g) - - - 2.5 - - 2.5 PEG 300 (g) - - - - 5 - 5 Polyvinyl alcohol, Mw 9000-10000 g/mol,
hydrolyzed by 80% (g) - - - - - 40 - Adhesive properties Shear strength (MPa) 4.4 6.5 0 2.9 3.5 2.7 4.3 Curing temperature (°C) 22 21 175 175 175 175 175

Table 1.2 Adhesive composition Compar. A Comp. B 6 5 7 Oxidized Kraft Lignin (g) - - 100 100 100 Primid XL552 (g) - - 2.5 2.5 2.5 PEG 300 (g) - - 5 5 5 Polyvinyl alcohol, Mw 9000-10000 g/mol,
hydrolyzed by 80% (g) - - - - - Adhesive properties Shear strength (MPa) 4.4 6.5 3.2 4.3 5.6 Residual strength after water resistance test (MPa) - 3.5 - - 3.9 Curing temperature (°C) 22 21 155 175 195

The following examples relate to the production of oxidized lignin which can be used as component (i) in the aqueous adhesive composition according to the present invention.

Examples

Example A. Oxidation of lignin with hydrogen peroxide in an aqueous solution of ammonia

The quantities of ingredients used according to Example A are given in Tables A 1.1 and A 1.2.

During the development of the method according to the present invention, the inventors started with laboratory scale experiments, which were carried out on a scale of approximately 1 liter.

Although kraft lignin dissolves in water at a relatively high pH, it is known that at a certain weight percent the viscosity of the solution will increase dramatically. It is generally believed that the reason for the increase in viscosity lies in a combination of strong hydrogen bonding and the interaction of the π-electrons of the numerous aromatic rings present in the lignin. In the case of kraft lignin, a sharp increase in viscosity in water is observed at about 21-22 wt.%, and in the example presented, 19 wt.% of kraft lignin is used.

Aqueous ammonia solution is used as a base in the pH adjustment stage. The amount is fixed at 4 wt.% relative to the total mass of the reaction mixture. The pH after the adjustment stage and the pH at the beginning of oxidation are 10.7.

Table A 2 shows the results of elemental analysis of CHNS before and after oxidation of kraft lignin. Before analysis, the samples were heated at 160°C to remove absorbed ammonia. The analysis shows that during the oxidation process, some nitrogen becomes part of the structure of the oxidized lignin.

During testing in batch experiments, it was determined that the oxidation was favored by adding the entire amount of hydrogen peroxide at one time during a short period of time, as opposed to adding the peroxide in small portions over a long period of time. In this example, 2.0 wt.% _H2O2 was used relative to the total _weight of the reaction mixture.

Oxidation is an exothermic reaction and a temperature increase is observed after the addition of peroxide. In this example, the reaction is maintained at 60°C for three hours.

After oxidation, the amount of functional groups in lignin per gram of sample increases as determined by ^31P NMR and aqueous titration. Sample preparation for ^31P NMR is performed using 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) as the phosphitylation agent and cholesterol as the internal standard. The NMR spectra of kraft lignin before and after oxidation are shown in Fig. 4, and the results are summarized in Table A 3.

Fig. 5 shows the ³¹ P NMR of kraft lignin and ammonia-oxidized kraft lignin (AOL). The graph shows the various hydroxyl groups as well as the internal standard, where S, G and H refer to syringyl, guaiacyl and coumaryl (hydroxyphenyl), respectively. The inset shows the signals from the carboxyl groups without cross-linking. The change in COOH groups is also determined by aqueous titration and using the following formula

,

where V _2s and V _1s are the final volumes of the sample, while V _2b and V _1b are the void volumes. C _acid in this case is 0.1 M HCl, and m _s is the mass of the sample. The volumes obtained in the aqueous titration before and after oxidation are shown in Table A 4.

Average COOH functionality can also be quantified by the saponification number, which is the number of mg KOH required to saponify 1 g lignin. This method can be found in AOCS Official Method Cd 3-25.

Also, before and after oxidation, the average molecular weight was determined using a PSS PolarSil column (eluent 9:1 (v/v) dimethyl sulfoxide/water with 0.05 M LiBr) and a UV detector at 280 nm. The combination of COOH concentration and average molecular weight also allows the calculation of the average content of carboxyl groups per lignin macromolecule, and these results are given in Table A 5.

Example B. Scaling up the oxidation of lignin with hydrogen peroxide in ammonia to a semi-industrial scale

The oxidation of lignin with hydrogen peroxide is an exothermic process, and even at laboratory scale a significant increase in temperature is seen with the addition of peroxide. This is a natural problem when scaling up chemical processes, since the amount of heat produced is related to the size to the 3rd power (volume), whereas cooling typically only increases with the square of the size (area). In addition, due to the high viscosity of the sticky intermediates, the process equipment must be carefully selected or designed. Therefore, scaling up is carefully designed and carried out in several steps.

The first step of scale-up is carried out from 1 L (lab scale) to 9 L using a professional stainless steel mixer with very efficient mechanical stirring. Scale-up results in only slightly higher final temperatures than observed at lab scale, which is explained by the efficient air cooling of the reactor and the slow addition of hydrogen peroxide.

The next scale-up step is carried out in a closed 200 L reactor using an efficient water jacket and an efficient propeller stirrer. The scale is now 180 L and the hydrogen peroxide is added in two portions with approx. 30 minute separation. This scale-up goes relatively well, although a problem is the fairly strong foaming, partly due to the high filling degree of the reactor. To control the foaming, a small amount of food grade antifoam is sprayed on the foam. Most importantly, a controlled temperature and a final temperature below 70°C is obtained using external water cooling.

The reactions on a semi-industrial scale are carried out in an 800-L reactor with a water jacket for cooling and a two-bladed propeller stirrer. 158 kg of lignin (UPM LignoBoost TM BioPiva 100) with a dry matter content of 67 wt.% are ground and suspended in 224 kg of water and mixed to form a homogeneous suspension. While continuing to stir, 103 kg of a 25% ammonia solution in water are pumped into the reactor and mixed for another 2 hours to form a dark viscous lignin solution.

To the lignin solution, 149 kg of 7.5 wt.% hydrogen peroxide are added over 15 minutes with stirring at 20-25°C. During and after the addition of hydrogen peroxide, the temperature and foam level are carefully monitored, and cooling water is added to the cooling jacket to maintain an acceptable foam level and a temperature rise of less than 4°C per minute, as well as a final temperature below 70°C. After the temperature rise has ceased, the cooling is turned off and the product mixture is stirred for a further 2 hours before transferring to a shipping container.

Based on the large-scale runs, it can be concluded that although the reactions are exothermic, most of the heat of reaction is actually balanced by the heat capacity of water going from room temperature to about 60°C, and only the last part has to be removed by cooling. It should be noted that because of this and the short reaction time, this process would be ideal for scale-up and intensification using continuous reactors such as in-line mixers, tubular reactors or CSTRs. This would provide good temperature control and a more precise reaction process.

Testing of semi-industrial batches shows that the resulting oxidized lignin has properties consistent with those of laboratory-produced batches.

Table A 1-1
Quantities of materials used in the supplied form Material wt.-% UPM BioPiva 100, kraft lignin 28 H ₂ O ₂ , 30 wt.% solution in water 6.6 NH3 _, 25 wt.% aqueous solution 16 Water 49.4

Table A 1.2
Quantities of active material used Material wt.% kraft lignin 19 _{H2O2 ​} 2 _NH3 4 Water 75

Table A 2
Elemental analysis of kraft lignin before and after oxidation Sample N (wt.%) C (wt.%) H (wt.%) S (wt.%) kraft lignin 0,1 64.9 5.8 1.7 Kraft lignin oxidized with ammonia 1.6 65.5 5.7 1.6

Table A 3
Functional group distribution of kraft lignin before and after oxidation obtained using ³¹ P-NMR Concentration (mmol/g) Sample Aliphatic OH Phenolic OH OH acids kraft lignin 1.60 3.20 0.46 Kraft lignin oxidized with ammonia 2.11 3.60 0.80

Table A 4
Content of COOH groups in mmol/g determined by aqueous titration sample COOH groups (mmol/g) kraft lignin 0.5 oxidized kraft lignin 0.9

Table A 5
Number average (Mn) and weight average (Mw) molecular weights determined by size exclusion chromatography, expressed in g/mol, together with the average content of carboxyl groups per lignin macromolecule before and after oxidation Sample Mn, g/mol Mw, g/mol average COOH-functional kraft lignin 1968 21105 0.9 Kraft lignin oxidized with ammonia 2503 34503 2.0

Claims (39)

1. An aqueous adhesive composition for lignocellulosic materials such as wood, comprising - component (i) in the form of oxidized lignin; - component (ii) in the form of a crosslinker; - component (iii) in the form of a plasticizer, and the said adhesive composition does not contain formaldehyde, wherein component (i) has a carboxyl group content of 0.05-10 mmol/g relative to the dry weight of component (i), component (ii) is present in an amount of 1-40% by weight relative to the dry weight of component (i), and component (iii) is present in an amount of 0.5-50 wt.% relative to the dry weight of component (i), wherein component (ii) in the form of a crosslinker is selected from - crosslinkers based on β-hydroxyalkylamides and/or crosslinkers based on oxazolines, - a group consisting of multifunctional organic amines such as alkanolamine, diamines such as hexamethylenediamine, triamines, - an epoxidized oil based on a fatty acid triglyceride or an elastic oligomer or polymer such as an acrylic polymer, such as a vinyl polymer, such as a polyether, which contains reactive functional groups such as carbodiimide groups, such as anhydride groups, such as oxazoline groups, such as amino groups, such as epoxy groups, or - a molecule having 3 or more epoxy groups, and wherein component (iii) in the form of a plasticizer is selected from the group consisting of polyethylene glycols, polyethylene glycol ethers, polyethers, hydrogenated sugars and/or acids such as adipic acid, vanillic acid, lactic acid and/or ferulic acid, acrylic polymers, polyvinyl alcohol, polyurethane dispersions, ethylene carbonate, propylene carbonate, lactones, lactams, lactides, acrylic polymers with free carboxyl groups and/or polyurethane dispersions with free carboxyl groups. 2. An aqueous adhesive composition according to claim 1, wherein component (i) is in the form of oxidized lignin, said lignin being selected from the group of kraft lignins, soda lignins, lignosulfonate lignins, organosolv lignins, lignins from biorefining processes of lignocellulosic raw materials, or any mixture thereof. 3. An aqueous adhesive composition according to claim 1 or 2, wherein component (i) is in the form of oxidized kraft lignin. 4. An aqueous adhesive composition according to claim 1 or 2, wherein component (i) is in the form of oxidized sodium lignin. 5. An aqueous adhesive composition according to any one of the preceding claims, wherein component (i) is in the form of ammonia-oxidized lignin (AOL). 6. An aqueous adhesive composition according to claim 5, wherein the lignin oxidized with ammonia comprises one or more compounds selected from the group of ammonia, amines or any salts thereof. 7. An aqueous adhesive composition according to any one of the preceding claims, wherein component (i) has a carboxyl group content of 0.1-5 mmol/g, such as 0.20-1.5 mmol/g, such as 0.40-1.2 mmol/g, such as 0.45-1.0 mmol/g relative to the dry weight of component (i). 8. An aqueous adhesive composition according to any one of the preceding claims, wherein component (i) has an average carboxyl group content of greater than 1.5 groups per macromolecule of component (i), such as greater than 2 groups, such as greater than 2.5 groups. 9. An aqueous adhesive composition according to any one of the preceding claims, comprising component (ii) in an amount of 4-20% by weight, such as 6-12% by weight, relative to the dry weight of component (i). 10. An aqueous adhesive composition according to any one of the preceding claims, wherein component (iii) is in the form of a plasticizer having a boiling point of 100-280°C, preferably 120-260°C, more preferably 140-250°C. 11. An aqueous adhesive composition according to any one of the preceding claims, wherein component (iii) is in the form of polyethyleneglycol having an average molecular weight of 150-50000 g/mol, in particular 150-4000 g/mol, in particular 150-1000 g/mol, preferably 150-500 g/mol, more preferably 200-400 g/mol. 12. An aqueous adhesive composition according to any one of the preceding claims, in which component (iii) is present in an amount of 2.5-25, preferably 3-15% by weight relative to the dry weight of component (i). 13. An aqueous adhesive composition according to any of the preceding claims, also comprising a component in the form of one or more components selected from the group of ammonia, amines or any salts thereof. 14. An aqueous adhesive composition according to any one of the preceding claims, comprising an additional component in the form of urea, in particular in an amount of 5-40 wt.%, such as 10-30 wt.%, such as 15-25 wt.%, relative to the dry weight of component (i). 15. An aqueous adhesive composition according to any one of the preceding claims, essentially consisting of - component (i) in the form of oxidized lignin; - component (ii) in the form of a crosslinker; - component (iii) in the form of a plasticizer; - optionally, a component in the form of one or more compounds selected from the group of ammonia, amines or any salts thereof; - optionally, a component in the form of urea; - optionally, a component in the form of reactive or non-reactive silicones; - optionally, hydrocarbon oil; - optionally, one or more surfactants; - water. 16. A method for producing a glued lignocellulosic product, comprising the step of contacting one or more lignocellulosic materials with an adhesive composition according to any one of paragraphs 1-15 and curing the adhesive composition. 17. A bonded lignocellulosic product comprising one or more lignocellulosic materials in contact with a cured adhesive resulting from the curing of an adhesive composition according to any one of claims 1 to 15.