Process for preparing lignin-phenol-formaldehyde resins of controlled viscosity

Abstract

The present invention relates to a method for dissolving and controlling the molecular weight of a lignin material to prevent a potential reduction in its reactivity and thereby to control the viscosity of both the lignin material and the resulting condensation product. Thus, the method comprises dissolving lignin with a first portion of phenol under acidic conditions, followed by hydroxymethylation under alkaline conditions, and condensation and cross-linking to produce a final resin product.

Description

Technical Field

[0001] The present invention relates to a method for producing a lignin-phenol-formaldehyde resin having controlled viscosity. Furthermore, the present invention relates to the resin produced thereby. Background Art

[0002] Phenol-formaldehyde resins (or PF resins) are a widely used group of adhesives derived from petroleum and natural gas for a variety of reconstructed wood products such as plywood, laminated veneer lumber, fiberboard and particleboard as well as decorative laminates and molding compounds. There is widespread interest in replacing some of the phenols in resin formulations in order to provide more sustainable alternatives.

[0003] Because lignin is a natural compound based on phenol, the use of lignin has become a preferred choice to replace phenol in such resins, thereby producing lignin-phenol-formaldehyde resins (LPF resins). Typically, before being used in resin production, lignin is dissolved and possibly further activated, such as by phenolization or depolymerization. Choosing the right conditions for these steps is crucial because this will affect the reactivity of lignin with the other components of the resin.

[0004] Lignin solubility can be improved in a variety of ways. For example, lignin can be dissolved in an alkali or an organic solvent such as methanol. In US20190048192 A1, lignin is dissolved in methanol under alkaline conditions. However, methanol should be avoided in this industrial process, one of the reasons being its flammability.

[0005] In the past, lignin was activated to formaldehyde by phenolization. WO 2015079106 A1 describes a method for reacting lignin with phenol under neutral or alkaline conditions. The option of mixing the reactants at a pH level <8 is discussed, but the pH does not reach acidic levels due to the addition of a base. However, neutral or alkaline conditions are not suitable for all lignin types and may result in resins with high viscosity due to, for example, lignin condensation and / or formation of poorly soluble aggregates, resulting in poor reactivity of the lignin to formaldehyde and high resin viscosity.

[0006] The present inventors surprisingly found that by changing the conditions for dissolving lignin, the reactivity of lignin in the subsequent reaction with formaldehyde is not weakened by harmful self-condensation reactions, resulting in controlled viscosity of the resin, thereby significantly improving the properties of the resin and providing a wide variety of suitable lignin types. Summary of the invention

[0007] The invention is defined by the features of the independent claim. Certain specific embodiments are defined in the dependent claims.

[0008] According to a first aspect of the present invention, there is provided a method for dissolving a lignin material and preventing a potential reduction in its reactivity, thereby maintaining its suitability for use in resin manufacturing.

[0009] According to a second aspect of the present invention, there is provided a method for producing a lignin-phenol-formaldehyde resin using the dissolved lignin material.

[0010] According to a third aspect of the present invention, there is provided a method for producing a lignin-phenol-formaldehyde resin using a wide range of suitable lignin starting materials from different types of biomass and fractionation processes.

[0011] According to a further aspect of the present invention, there is provided a novel lignin-phenol-formaldehyde resin having advantageous properties.

[0012] The present invention is therefore based on providing a dissolved lignin material and using it as a partial replacement for phenol in phenol-formaldehyde resins. Typically, the goal is to replace 30-50% of the phenol in such resins.

[0013] The purpose of the dissolution is to prevent self-condensation of the lignin, which would increase its molecular weight, reduce its reactivity toward formaldehyde, and increase the viscosity of the resin to undesirable levels.

[0014] Therefore, the present invention includes using phenol to dissolve lignin under acidic conditions to prevent lignin from condensing into molecules with higher molecular weight or insoluble aggregates, which can occur when using some lignin materials. The dissolution of the present method will produce lignin with sufficient reactivity to formaldehyde, regardless of the type of lignin used. At least kraft lignin, soda pulping lignin, lignin sulfonate, organosolv lignin and ethanol process lignin are suitable for the method of the present invention.

[0015] Phenolation has been described in the past as a method for activating lignin, but the present invention is based on an alternative procedure in which phenol is used to provide solubilization of the lignin and prevent a potentially detrimental increase in the molecular weight of the lignin before it reacts with formaldehyde.

[0016] The inventors have found that when the dissolution of lignin is carried out under phenol-induced acidic conditions, significant advantages are obtained using the present invention. In addition to the components used in conventional resin manufacturing, no additional acid is required in the dissolution step. Under such phenol-induced acidic conditions, the reactivity of lignin will be prevented from being reduced, for example, by self-condensation or aggregate formation, which has been shown to occur in some lignin raw materials under neutral or alkaline conditions. Therefore, the undesirable increase of the final resin viscosity caused by the lignin of too high molar mass will also be prevented. Without wishing to be bound by a specific theory, we assume that the phenol molecules, in addition to acting as a solvent, can also act as a physical barrier between the lignin molecules, otherwise the lignin molecules will condense into a higher molecular weight lignin structure with reduced reactivity to formaldehyde, and cause a higher resin viscosity.

[0017] Based on studies of isolated lignin, we further hypothesize, again without wishing to be bound by a particular theory, that acidic conditions may also lead to beneficial changes in the lignin structure, such as the breaking of alkyl-aryl ether bonds between lignin units, which can enhance its reactivity towards formaldehyde by generating new phenolic hydroxyl groups and reducing its molar mass, while some non-essential elements of lignin, such as ester groups in organosolv lignin, are hydrolyzed and removed, leaving behind lignin with a higher phenolic unit content on a mass basis.

[0018] The formaldehyde will further react efficiently, leaving a final resin formulation with a low level of unreacted formaldehyde, making it suitable for any type of application, including those applications where there are regulatory restrictions on formaldehyde emissions.

[0019] The obtained resin formulation will have a controlled viscosity, which is achieved without the use of a separately added viscosity reducing solvent (such as methanol, which is preferably avoided in industrial processes). Thus, the amount of reagents and solvents added to the mixture during the resin preparation is minimized, and the amount of reagents and solvents that need to be separated from the mixture after the reaction is completed is also minimized. In contrast, phenol can be used to dissolve lignin, which is a component of the final resin and is therefore always added to the reaction mixture in at least one step of the resin preparation process.

[0020] In particular, the acidic solution enables better control of the viscosity of the final lignin-phenol-formaldehyde mixture and the final resin product.These advantages can be further emphasized by using preferred conditions, such as temperature and pressure, in the steps of the process.

[0021] Because the viscosity can be controlled, it is also possible to incorporate higher amounts of lignin in the final resin formulation without causing a reduction in reactivity or a reduction in resin properties. In addition, an increased dry matter content can be achieved for the final resin formulation.

[0022] These advantages are particularly prominent in applications requiring low viscosity, such as in impregnating paper. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 illustrates a process configuration in accordance with at least some embodiments of the present invention. DETAILED DESCRIPTION

[0024] The present invention relates to a method for producing a lignin-phenol-formaldehyde resin, comprising:

[0025] - dissolving the lignin material in the first portion of phenol at elevated temperature at a pH of 0.5-3.9 to obtain dissolved acidic lignin, this step being achieved without a potentially detrimental increase in the molar mass of the lignin,

[0026] - the pH of the obtained lignin solution is increased to >9, and a first portion of formaldehyde is added to provide a lignin-phenol-formaldehyde mixture and produce a hydroxymethylation reaction, so the obtained mixture is also called a hydroxymethylated lignin-phenol mixture,

[0027] forming a lignin-phenol-formaldehyde condensation mixture from the hydroxymethylated lignin-phenol mixture by adjusting the pH to ≥ 9.5 and stirring the mixture, optionally including the substeps of adding a second portion of phenol, ensuring that the pH is ≥ 9.5, stirring the mixture, and adding a second portion of formaldehyde, and

[0028] - cross-linking the components of the condensation mixture to produce a lignin-phenol-formaldehyde resin

[0029] wherein the first portion of phenol comprises 25-100 w-% of the total amount of phenol used in the process, and wherein the first portion of formaldehyde comprises 50-100 w-% of the total amount of formaldehyde used in the process.

[0030] When the first portion of phenol and formaldehyde comprises 100% of the total amount of phenol and formaldehyde used in the method, the above-mentioned addition of the second portion of phenol and the second portion of formaldehyde is optional. If the first portion of phenol comprises less than 100% of the total amount of phenol, the condensation mixture is formed in a step comprising adding the second portion of phenol. Similarly, if the first portion of formaldehyde comprises less than 100% of the total amount of formaldehyde, the condensation mixture is formed in a step comprising adding the second portion of formaldehyde.

[0031] Figure 1 shows the main stages of the above process, where dissolution 1 is the acid dissolution described above, dissolution 2 comprises raising the pH to a level > 9, hydroxymethylation comprises the subsequent addition of formaldehyde to the lignin-phenol solution, and condensation comprises the final step of the above process.

[0032] The lignin material can be selected from any lignin, such as organosolv lignin, kraft lignin, soda pulp lignin, lignin sulfonate or enzymatic hydrolysis lignin. Preferably, it is selected from technical softwood, hardwood or non-wood lignin obtained from lignin-containing biomass such as wood sources and non-wood agricultural residues or perennial plants, and more preferably selected from organosolv lignin, ethanol lignin or kraft lignin extracted from these lignin-containing biomasses.

[0033] In an embodiment, the lignin material is selected from hardwood lignin. In another embodiment, the lignin material is selected from organic solvent lignin. The lignin of these embodiments has the most favorable initial molecular weight and is therefore particularly suitable for preparing resins of low molecular weight and favorable viscosity. Therefore, the lignin material is preferably selected from the lignin with an initial molecular weight (before being used in the dissolution step) <6000g / mol, or even <5000g / mol, more preferably less than 4500g / mol, and most suitably less than 4300g / mol.

[0034] Sources of lignin include, for example, common softwood species such as spruce and pine, common hardwood species such as eucalyptus and birch, and common non-wood species (usually in the form of grass-type agricultural residues or perennial plants) such as bagasse, bamboo, wheat straw, rye straw, barley straw, rice straw, and reed canary grass.

[0035] The phenol used in the process is typically unsubstituted phenol. Other phenolic compounds such as cresols and tannins are also suitable, but due to the availability of unsubstituted phenol, its use is most convenient.

[0036] As mentioned above, the first portion of phenol accounts for 25-100 w-% of the total amount of phenol used in the process. Preferably, the first portion of phenol accounts for about 50 w-% of the total amount of phenol used in the process. Similarly, the first portion of formaldehyde accounts for 50-100 w-%, preferably about 75 w-% of the total amount of formaldehyde used in the process.

[0037] In an embodiment of the invention, the dissolution of the lignin material using the first portion of phenol is carried out in an aqueous solution, preferably having a water content of >30 wt-%, more preferably 40-60 w-%, thereby avoiding the addition of an organic solvent that needs to be separated from the final resin mixture. The water in the solution will prevent phenolation of the lignin during the dissolution step, which may lead to an undesirable increase in the viscosity of the material. Preferably, the lignin used is present in an amount of 20-85 wt-%, more preferably 25-80 wt-%, or 25-60 wt-%, and most suitably 30-60 wt-% of the combined weight of lignin and phenol in the final resin (thus describing the total amount of phenol replacement in the resin).

[0038] Typically, dissolution is carried out at a total dry matter content of 40-60 w-%.

[0039] During the phenol-based dissolution stage, the pH of the reaction mixture is usually adjusted using a phenol dosage, which will bring the pH of the solution to a desired level. Optionally, a small amount of additional acid, preferably an inorganic acid such as sulfuric acid, may be used. However, phenol alone generally provides sufficient acidity.

[0040] A suitable phenol dosage may be, for example, a dosage such that the lignin to phenol mass ratio is >1.0, preferably 1.8-2.5, and more preferably about 2.0, during the acidic lignin solubilization stage.

[0041] As mentioned above, the pH during acidic dissolution is 0.5-3.9, the preferred pH range is 0.5-3.5, and the more preferred range is 2.0-3.0. When the pH stabilizes after the addition of phenol, it is usually at a level of 2.5-2.8.

[0042] Elevated temperatures are typically used, meaning temperatures above room temperature (25° C.), preferably temperatures are 50-90° C., more preferably 70-80° C. Likewise, a suitable residence time is selected to allow the reaction to complete, such as a residence time of 30-120 min, preferably 30-60 min.

[0043] Throughout the process, when not otherwise stated, the preferred pressure is ambient pressure.

[0044] In an embodiment of the present invention, the next step of the method comprises raising the pH of the dissolved acidic lignin solution to>9, particularly to a pH of 9.5-10, preferably using an alkali, which can be, for example, sodium hydroxide. After the addition of the alkali, the formaldehyde of the first portion is added, preferably slowly, or over a period of 45-75min or about 60min, and preferably the temperature is increased to promote the desired hydroxymethylation reaction. A suitable temperature can be a temperature of 60-75°C, preferably a temperature of about 70°C. The elevated temperature is maintained under continuous stirring until the reaction is complete, such as for up to 30min, although it is generally sufficient to maintain the temperature for 10-15min.

[0045] A condensation mixture is prepared from a hydroxymethylated lignin-phenol mixture using process steps comprising at least adjusting the pH and stirring the mixture.

[0046] In an embodiment of the present invention, the condensation mixture is prepared by first adding a second portion of phenol to the hydroxymethylated lignin-phenol mixture while stirring and ensuring that the pH of the mixture is ≥ 9.5, preferably 9.5-10. Subsequently, a second portion of formaldehyde is added, typically slowly, or over a period of 45-70 min or about 60 min, while stirring and maintaining the mixture at a temperature of, for example, 60-75° C.

[0047] Preferably, for the total amount of phenol and formaldehyde used in the process, a formaldehyde to phenol molar ratio of 1 to 2, preferably 1.5 to 1.7 is used, the amount of formaldehyde depending on the content of formaldehyde reactive sites on the lignin used.

[0048] The final crosslinking step of forming the resin from the condensation mixture can be achieved by raising the temperature of the reaction mixture, preferably to 75-95°C, more preferably 85-95°C, and maintaining the elevated temperature for 15-35 min, preferably 20-30 min.

[0049] In an exemplary embodiment, the resin is prepared using organic solvent lignin from wheat straw. In the first step, lignin, water and phenol (a portion accounting for 50% of the total phenol content in the resulting resin) are combined. Without pH adjustment, the pH is stabilized at about 2.5. The dissolved mixture is then mixed at 70°C for about 1h. In the next stage, a base is added to adjust the pH to 10, and the resulting base mixture is mixed again at 70°C for about 1h. In the next hydroxymethylation stage, formaldehyde (50% of the total formaldehyde content in the final resin) is added at a temperature of 70°C over a period of about 1h. The mixture is then cooled and mixed for another 10min. The final condensation stage consists of the following consecutive steps: 1) adding the remaining phenol, 2) adjusting the pH to 9.5-10, 3) at 70°C, adding the remaining formaldehyde over a period of about 1h, 4) raising the temperature to 90°C and maintaining the temperature for about 1h, and 5) cooling the resulting product to room temperature.

[0050] The steps of the method for making lignin-phenol-formaldehyde resin described herein, particularly the use of acid solubilization of lignin and the sequence of steps described, allow for efficient formaldehyde reaction and viscosity control of the condensation mixture or the resin, and typically both.

[0051] Therefore, the present invention also relates to lignin-phenol-formaldehyde resins made using the methods described herein. Such resins typically have a viscosity of <550 cP, preferably <350 cP, and most suitably <200 cP when measured at 20°C.

[0052] It should be understood that the embodiments of the present invention disclosed are not limited to the specific structures, process steps or materials disclosed herein, but are extended to their equivalents as will be recognized by those skilled in the relevant art. It should also be understood that the terms used herein are only used for the purpose of describing specific embodiments and are not intended to be limiting.

[0053] References to one embodiment or embodiments throughout the specification mean that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the invention. Thus, the phrases "in one embodiment" or "in an embodiment" appearing throughout this specification do not necessarily all refer to the same embodiment. Where terms such as, for example, about or substantially are used to reference values, the exact value is also disclosed.

[0054] As used herein, for convenience, multiple items, structural elements, constituent elements and / or materials may be presented in a public list. However, these lists should be interpreted as if each member in the list is individually identified as a separate and unique member. In addition, various embodiments and embodiments of the present invention may be mentioned herein together with alternatives to its various components. It should be understood that such embodiments, embodiments and alternatives should not be interpreted as equivalents in fact to each other, but should be regarded as separate and autonomous representations of the present invention.

[0055] In addition, in one or more embodiments, the described features, structures or characteristics may be combined in any suitable manner. In this description, many specific details are provided to provide a thorough understanding of embodiments of the present invention. However, those skilled in the relevant art will recognize that the present invention may be practiced without one or more specific details.

[0056] Although the foregoing examples illustrate the principles of the present invention in one or more specific applications, it is obvious to those skilled in the art that various modifications can be made in form, usage and implementation details without exercising creativity and without departing from the principles and concepts of the present invention. Therefore, the present invention is not intended to be limited except as set forth below.

[0057] The following non-limiting examples are intended merely to illustrate the advantages obtained by embodiments of the present invention.

[0058] Example

[0059] Example 1 - Preparation of Resins of the Invention

[0060] Lignin-phenol-formaldehyde resins (LPF resins) were prepared using organosolv lignin from wheat straw according to four different recipes (LPF 1-LPF 4), wherein LPF 1 and LPF 2 were prepared according to the present invention, the first step of the preparation being dissolution under acidic conditions, and LPF 3 and LPF 4 were reference resins prepared by performing dissolution 1 under alkaline conditions.

[0061] LPF 1 differs from LPF 2, in particular in terms of the pH level used for the acidic dissolution, LPF 1 being prepared with the addition of an inorganic acid, thus providing an even lower pH after dissolution 1 than LPF 2 with the addition of phenol alone. LPF 3 differs from LPF 4, in particular in that LPF 4 was prepared without the addition of phenol during the dissolution stage. The main steps of the formulation are shown in FIG1 , and further details are shown in

[0062] Table 1.

[0063]

[0064]

[0065] Example 2 - Evaluation of Molecular Weight and Viscosity of Reaction Mixtures and Resins

[0066] The dissolved mixture obtained after stage 2 and the finished resin obtained after stage 4 of Table 1 above were further analyzed. In particular, the molecular weight of the dissolved mixture and the viscosity of the resin were determined. The results are shown in Table 2 below.

[0067] Table 2. Effect of dissolution conditions on the molecular weight of dissolved lignin and the viscosity of LPF resin

[0068] Solvents in Stage 1 Solvents in stage 2 <![CDATA[Viscosity, cP i > <![CDATA[M w ,g / mol ii ]]> Lignin before dissolution -- -- -- 3660 LPF resin 1 Phenol at pH 0.7 Base at pH 10 138 3108 LPF resin 2 Phenol at pH 2.5 Base at pH 10 131 4637 LPF resin 3 Phenol at pH 10 Base at pH 10 301 5160 LPF resin 4 Base at pH 10 Base at pH 10 851 8182

[0069] i Viscosity after stage 4; ii Molar mass of lignin after stage 2

[0070] The results showed that both the pH level of solution 1 and the presence of phenol in the same solution 1 affected the molecular weight of the dissolved lignin and the viscosity of the final resin.

[0071] As mentioned above, we hypothesize that these results are caused by the phenol molecules, which, in addition to acting as a solvent, also act as a physical barrier between lignin molecules that would otherwise condense into higher molecular weight lignin structures that are less reactive toward formaldehyde and produce higher resin viscosities.

[0072] Furthermore, acidic conditions can lead to beneficial changes in the lignin structure, such as the cleavage of alkyl-aryl ether bonds between lignin units, which reduces the molar mass and increases the content of phenolic hydroxyl groups, thereby increasing the reactivity of lignin towards formaldehyde.

[0073] Example 3 - Evaluation of Molecular Weight and Viscosity of Reaction Mixtures and Resins Prepared from Various Raw Materials

[0074] Lignin-phenol-formaldehyde resin (LPF resin) was prepared using lignin feedstock according to the LPF2 recipe described in Table 1 of Example 1, with stage 1 (S1) pH between 2.5-3.7 for all lignins.

[0075] The molecular weight of both the starting lignin and the stage 4 product was determined. Likewise, the viscosity of the stage 4 (S4) product was determined. The results are shown in Table 3 below.

[0076] Table 3. Molecular weight of lignin and LPF resin, and resin viscosity

[0077]

[0078] As shown in the results, the method for producing LPF resin of the present invention is well suited for various lignins and can provide low viscosity resins while controlling the molecular weight of the lignin components and condensation products.

[0079] In order to provide a low viscosity lignin-phenol-formaldehyde resin, it has been found that it is advantageous to use reagents (lignin, phenol and formaldehyde) with low molecular weight and also react to form condensation products with low molecular weight in the process. This has been achieved using the reaction conditions of the above LPF2 formulation.

[0080] Using the above reagents and reaction conditions of the present invention further tends to produce LPF resins containing a phenol component, 10-25% by mass of which consists of phenol (MW 94 Da) and low MW hydroxymethylation and condensation products of phenol with formaldehyde (MW < 400 Da).

[0081] These low Mw hydroxymethylation and condensation products are generally composed of one or more of the following: 2-hydroxymethylphenol, 4-hydroxymethylphenol; 2,4,6-tris(hydroxymethyl)phenol; 2,6-bis(hydroxymethyl)phenol; 2,4-bis(hydroxymethyl)phenol; 4,4′-bis(hydroxyphenyl)methane; 2-(hydroxymethyl)phenol hemiformal; 4-(hydroxymethyl)phenol hemiformal; 3,3′-bis(hydroxymethyl)-4,4′-bis(hydroxyphenyl)methane; 3,5-bis(hydroxymethyl)-4,4′-bis(hydroxyphenyl)methane; 3,5′-bis(hydroxymethyl)-2,2′-bis(hydroxyphenyl)methane; 3,5-bis(hydroxymethyl)-2,2′-bis(hydroxyphenyl)methane; 3-hydroxymethyl-2,2′-bis(hydroxyphenyl)methane Methane; 3-hydroxymethyl-2,2′-bis(hydroxyphenyl)methane; 3-hydroxymethyl-4,4′-bis(hydroxyphenyl)methane; 2,6-bis(hydroxymethyl)phenol (mono) hemiformal; 2,4-bis(hydroxymethyl)phenol (mono) hemiformal; 2,4-bis(hydroxymethyl)phenol (di) hemiformal; 2,4,6-tris(hydroxymethyl)phenol (mono) hemiformal; 2,4,6-tris(hydroxymethyl)phenol (di) hemiformal; 2,4,6-tris(hydroxymethyl)phenol (tri) hemiformal; 3,3′,5,5′-tetrakis(hydroxymethyl)-4,4′-bis(hydroxyphenyl)methane; 3,3′,5,5′-tetrakis(hydroxymethyl)-2,2′-bis(hydroxyphenyl)methane; tris(hydroxymethyl)-bis(hydroxyphenyl)methane.

[0082] By looking at Table 3 we can see that over 50% of the molecules in this resin product mixture therefore have a molecular weight below 2500 Da.

[0083] Industrial Applicability

[0084] The method of the present invention for making lignin-phenol-formaldehyde resins will provide a resin with controlled viscosity, in which a portion of the phenol has been replaced by lignin to provide a more sustainable alternative. Thus, using the method of the present invention, higher amounts of lignin can be introduced into the final resin formulation without resulting in reduced reactivity or reduced resin performance compared to existing resins on the market.

[0085] In particular, the formaldehyde component of the lignin-phenol-formaldehyde resin will react efficiently with the dissolved mixture, leaving no free formaldehyde in the final resin formulation. Such resins with low levels of unreacted formaldehyde are particularly suitable for use in any application where there are regulatory restrictions on formaldehyde emissions.

[0086] Because of the advantageous viscosities achieved, resins made as described herein are particularly suitable for applications such as resins for saturating paper where low viscosity is desired.

[0087] Citation List

[0088] US20190048192 A1

[0089] WO 2015079106 A1。

Claims

1. A method for producing a lignin-phenol-formaldehyde resin, the method comprising - dissolving the lignin material in the first portion of phenol at an elevated temperature at a pH of 0.5-3.9 to obtain dissolved acidic lignin, - raising the pH of the obtained lignin solution to >9 and incorporating a first portion of formaldehyde to provide a lignin-formaldehyde mixture and effecting a hydroxymethylation reaction, - forming a lignin-phenol-formaldehyde condensation mixture from the hydroxymethylated lignin-phenol mixture by adjusting the pH to ≥ 9.5 and stirring the mixture, the step optionally comprising: Add the second portion of phenol, ensuring pH ≥ 9.5, stir, and add the second portion of formaldehyde, and - cross-linking the components of the condensation mixture to produce a lignin-phenol-formaldehyde resin wherein the first portion of phenol comprises 25-100 w-% of the total amount of phenol used in the process, and wherein the first portion of formaldehyde comprises 50-100 w-% of the total amount of formaldehyde used in the process.

2. The method according to claim 1, wherein: The lignin material is selected from technical softwood, hardwood or non-wood lignin obtained from lignin-containing biomass such as wood sources, agricultural residues, eg straw or perennial plants, preferably from kraft lignin, ethanol lignin or organosolv lignin.

3. The method according to claim 1 or 2, wherein: The lignin material is dissolved in the first portion of phenol in an aqueous solution, and the lignin content used is 20-85 wt-%, preferably 25-80 wt-%, or 25-60 wt-%, and most suitably 30-60 wt-%.

4. A method according to any one of the preceding claims, wherein: The first portion of phenol comprises about 50 w-% of the total amount of phenol used in the process.

5. A method according to any one of the preceding claims, wherein: The first portion of formaldehyde represents about 75 w-% of the total amount of formaldehyde used in the process.

6. A method according to any one of the preceding claims, wherein: The lignin material is dissolved in a dosage of phenol that will bring the pH of the solution to the desired level, preferably a lignin to phenol mass ratio of >1.5, more preferably 1.8-2.5, and most suitably about 2.

0.

7. A method according to any one of the preceding claims, wherein: The step of dissolving the lignin material in phenol is performed at a pH of 2-3.

8. A method according to any one of the preceding claims, wherein: The step of dissolving the lignin material in phenol is carried out at a temperature of 50-90°C, more preferably 70-80°C, typically using a residence time of 30-120 min, preferably 30-60 min.

9. A method according to any one of the preceding claims, wherein: The pH of the dissolved acidic lignin solution is raised using a base, preferably sodium hydroxide, to a pH > 9, most suitably to a pH of 9.5-10.

10. A method according to any one of the preceding claims, wherein: The step of reacting the alkaline lignin solution with the first portion of formaldehyde is performed by adding said formaldehyde to a solution having a pH>9 and subsequently increasing the temperature, preferably to a temperature of 60-75°C, and maintaining said temperature for a maximum of 30 min, preferably 10-15 min.

11. A method according to any one of the preceding claims, wherein: For the total amount of phenol and formaldehyde used in the process, a formaldehyde to phenol molar ratio of 1 to 2, preferably 1.5 to 1.7, and more preferably about 1.6 is used.

12. A method according to any one of the preceding claims, wherein: The condensation mixture is obtained by first adding the second portion of phenol to the lignin-formaldehyde mixture, ensuring that the pH of the mixture is ≥ 9.5, preferably 9.5-10, and then adding the second portion of formaldehyde, typically while maintaining the temperature of the mixture at 60-75°C.

13. A method according to any one of the preceding claims, wherein: The cross-linking is achieved by increasing the temperature of the reaction mixture, preferably to a temperature of 75-95°C, and maintaining the elevated temperature for 15-35 min, preferably 20-30 min.

14. A lignin-phenol-formaldehyde resin produced using the method of any one of claims 1 to 13.

15. The resin of claim 14 having a viscosity of <550 cP when measured at 20°C.

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