EP3612586A1

EP3612586A1 - Phenol-free impregnation resin

Info

Publication number: EP3612586A1
Application number: EP18728202.5A
Authority: EP
Inventors: Tiina PESONEN; Peter LINGENFELTER; Harri LEPISTÖ; Marika JÄNIS
Original assignee: Prefere Resins Finland Oy
Current assignee: Prefere Resins Finland Oy
Priority date: 2017-04-21
Filing date: 2018-04-23
Publication date: 2020-02-26
Also published as: WO2018193166A1; FI129275B; FI20175363A1

Abstract

The present invention comprises a process for the production of essentially phenol-free impregnation resin and the composition and end-uses of the resin produced by this process or an otherwise similar resin. In the process, as the material comprising phenolic OH groups, essentially only lignin is used which is condensed with formaldehyde in the presence of catalyst at an elevated temperature.

Description

Phenol-free impregnation resin

Background of the invention Field of the invention

The present invention relates to a process for the production of essentially phenol-free impregnation resin, the resin produced by this process or otherwise having a similar composition, as well as end-uses of the resin.

Descriptionof Related Art

The resins commonly used in impregnation are either phenol resins or melamine resins, in each case depending on the application of the impregnated paper. While phenol resins are a viable option in a number of applications, melamine resins are used when aiming at improved water resistance and similar properties. Certain applications require the properties of both resins, in which case a mixture of melamine and phenol resins is being used. Melamine resins are more expensive than phenol resins in their raw materials, but they are more safe, because they do not contain free phenol, which is toxic.

The structure and properties of finished resin are dependent on the reaction conditions employed in production, such as e.g. condensation temperature, condensation time, catalyst system, solvent system and the formaldehyde/phenol molar ratio. The choice of reaction conditions depends on the end-use for which the resin is being made.

The availability of petrochemical raw materials such as phenol is limited and therefore it would be important to replace them with renewable natural raw materials such as lignin, which is obtained from wood and possesses a polyphenolic structure. At the same time, the adverse environmental impacts caused by phenol would be avoided.

The substitution of phenol-formaldehyde resins with renewable natural raw materials is already known. In published application WO2014080033 a process for the production of a phenol- formaldehyde impregnation resin is disclosed, wherein phenol is replaced with lignin to an extent of 60 to 100%. According to the publication, the use of mixtures of water and tetrahydrofuran, water and ammonia and water and ethyl acetate is mentioned for the dissolution of lignin.

In patent specification US5177169 an organic solvent, which is typically polar and at least partially immiscible with water, such as ethyl acetate, is added to an aqueous solution containing lignin. The mixture is oxidized in order to achieve an aqueous layer and an organic solvent layer containing the demethylated lignin. In order to produce the resin, the demethylated lignin is dissolved into a basic solution to which an aldehyde source has been added. As the aldehyde source, formaldehyde in the solution stage can be used, for example. Proceeding according to prior art solutions involves a risk of uncontrollable temperature rise of the reaction mixture, which affects the quality of the final product. At the same time, consumption of formaldehyde increases significantly.

Summary of the invention

It is an object of the present invention to eliminate at least part of the problems related to prior art and achieve an improved process for the production of phenol-free impregnation resin. The present invention comprises a process for the production of essentially phenol-free impregnation resin by using essentially only lignin as the material comprising phenolic OH groups, and a resin produced by this process or otherwise having a similar composition. In the process, formaldehyde is condensed with lignin in the presence of a catalyst at an elevated temperature.

The invention is based on the idea that lignin is first brought into the liquid phase by dissolving it into a solvent or solvent mixture, after which formaldehyde is added into the solution thus obtained in the presence of an alkaline catalyst to bring about a condensation reaction between formaldehyde and lignin. Alkali is being fed gradually into the liquid stage, whereby the reaction of formaldehyde with the phenolic groups can be taken into completion. According to the invention, an essentially phenol-free impregnation resin is accomplished, which resin comprises a condensation product of formaldehyde and lignin, wherein the concentration of free formaldehyde is not more than 5 wt.-%, especially not more than 1 wt.-%, for example 0.1 to 1.0 wt.-%. The finished impregnation resin is most preferably in the form of an aqueous mixture in which the dry matter content with respect to the resin is higher than about 20 wt.-% and usually not more than about 70 wt.-%, typically the dry matter content of the resin is between 30 and 40 wt.-%.

More specifically, the solution according to the invention is mainly characterized by what is disclosed in the independent claims.

Considerable benefits are achieved with the present invention. In the invention, phenol in the phenol-formaldehyde impregnation resin is essentially completely replaced with lignin, alternatively allowing a complete avoidance of the use of the toxic phenol. The invention allows the control and adjustment of the reactivity of the condensation reaction between lignin and formaldehyde. In the process, the resin can be condensed so that virtually all free formaldehyde is brought into reaction. Low levels of free

formaldehyde can be achieved by the cooking system according to the invention, whereby the solution can be used in many applications in which low formaldehyde levels are required. At the same time, it is possible to render the viscosity of the resin to a level allowing its use in paper impregnation applications. Such impregnated fiber sheets can be used for the production of composite veneer products, for example.

The resin produced by the process of the invention or a resin having a corresponding composition is thus suitable for general use as a substitute for conventional impregnation resin e.g. in film and core papers. In addition to impregnation resins, possible applications include various fiberboards, for example. Brief description of the drawings

Shown in Figure 1 are photographs of finished copacted pieces of lignin impregnates as well as a picture of the reference sample and

shown in Figure 2 is a schematic exploded view of the structure of the compact laminate.

Embodiments In the present context, percentages refer to percentages by weight, unless otherwise stated.

"Resin" refers to a reaction product obtained via a polymerization reaction of a starting material comprising phenolic OH groups, and formaldehyde. In one embodiment the phenol-free resin is produced by using essentially only lignin as the material comprising phenolic OH groups. In practice, at least 90 weight-%, preferably at least 95 weight-%, most preferably at least 98 weight-%, or even 100 weight-% of the raw material comprising phenolic OH groups is lignin. In one application, another biomass, such as tannin, is used as the material comprising phenolic OH groups in addition to lignin.

The use of minor amounts (10 weight-%, most suitably not more than 5 weight-%, especially not more than 2 weight-%) of phenol, cresol or resorcinol or a mixture thereof is also possible in the process.

Lignin used in the present invention is especially lignin obtained from a biomass, such as wood or annual or perennial plants or, correspondingly, lignin obtained from

lignocellulose. In particular, material isolated from spent liquor obtained from cooking of biomass is used as the lignin starting material.

As examples of lignin starting material to be used, mention may be made of lignin isolated from biomass by an alkaline cooking process, such as kraft lignin (i.e. lignin from sulphate process) or soda lignin (i.e. lignin from soda pulping). The use of organosolv lignin (i.e. lignin obtained from organosolv pulping) is also possible in the process. As lignin starting materials that can be used, mention can also be made of pyrolytic lignin, steamed lignin, diluted acid lignin and alkaline oxidative lignin. Mixtures of the above-mentioned lignin starting materials may also be used as the lignin starting material.

In addition to lignin, the lignin starting material may also contain other materials, such as extracts or carbohydrates, such as cellulose or hemicellulose or degradation products thereof. In general, the lignin starting material comprises at least 90 weight- , most suitably at least 95 weight-%, especially at least 98 weight-% lignin.

The lignin starting material may be in the form of a solid, such as powder. This type of lignin starting materials is represented by commercially marketed lignin products. It is also possible to use liquid stage lignin, as will be described in more detail below.

Production takes place by dissolving the lignin starting material into a solvent comprising an alkaline substance as catalyst. Formaldehyde, especially an aqueous solution of formaldehyde, i.e. formalin, is added to the solution. The lignin and formaldehyde are subsequently condensed at an elevated temperature until virtually all formaldehyde, i.e. at least 90 weight-%, more preferably at least 95 weight-%, most suitably at least 99 weight- % of the formaldehyde has reacted. The resin thus formed in collected.

In particular, the lignin and formaldehyde are condensed at a temperature of about 50 to 90 °C until all formaldehyde has reacted, whereafter the resin is collected.

The lignin and formaldehyde are most preferably condensed in the liquid stage formed by the lignin solvent or a mixture thereof. According to a preferred embodiment, lignin is in the production process of the resin dissolved at the beginning of resin cooking into a solvent mixture comprising the solvent, water and the alkaline catalyst. The addition of solvent and water at the beginning of resin cooking makes controlling of viscosity possible during condensation so that viscosity does not rise too high at any reaction stage.

The catalyst can also de dosed in a number of portions, which also facilitates the control of steady proceeding of the condensation reactions. Owing to dosing of the solvent, water and the catalyst as well as the ways of dosing them, it will be possible to condense the resin to a point where free formaldehyde has reacted virtually completely.

In a preferred embodiment kraft lignin is used to replace phenol. This type of lignin, also available commercially, is usually supplied as a powder, whereby the dry matter content of lignin is over 90 weight-%, e.g. about 93 weight-%. The process also allows the use of an aqueous slurry of lignin having a dry matter content of about 50 weight-% or higher. The dry matter content can thus be at least 60 weight-%, for example at least 70 weight-%. In one embodiment the dry matter content is about 75 weight-%.

The solvent for lignin is typically a polar liquid which is miscible with water.

As examples of the solvent, mention may be made of aliphatic and aromatic alcohols such as methanol, and aliphatic ketones such as acetone. The solvent is used in an amount sufficient to maintain a suitable dry matter content in the mixture, i.e. between about 20 to 70 weight-%, in particular 30 to 70 weight-%, considering the further reaction.

In one application, the lignin is dissolved into a mixture of the actual solvent and water. Methanol or mixtures thereof, such as aqueous solution of methanol, are usually used as the solvent.

According to one embodiment the concentration of solvent in water is 0.1 to 30 weight-%, preferably 1.0 to 20 weight-%, in each case depending on the solvent used.

For example, in the case of methanol, the concentration of methanol in water is usually about 1.5 to 15 weight-%, such as 1.6 to 12 weight-%. The presence of solvent in the cooking stage allows condensation of the resin for a longer time without a too rapid increase in viscosity, and a low level of formaldehyde is achieved. When lignin is dissolved in a water/methanol mixture, the resin remains soluble for a longer time and enables the control of viscosity during the condensation reaction. This in turn makes it possible to bring virtually all free formaldehyde into reaction, whereby the concentration of free formaldehyde after the reaction is not more than 5 weight-% based on the weight of the resin, in particular not more than 1 weight-%, for example 0.1 to 1,0 weight-%. According to one embodiment, sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH₃), in particular as ammonium hydroxide, or a mixed catalyst system thereof, such as e.g. a mixture of sodium hydroxide and ammonia, or a mixture of sodium hydroxide and potassium hydroxide can be used as the catalyst in the reaction. Other nitrogen bases, such as organic amines can also be used as the catalyst. The catalysts are preferably added to the mixture as aqueous solutions having a concentration that varies slightly according to the catalyst. According to one embodiment, the concentrations of sodium and potassium hydroxide, for example, are between 40 to 60 weight-%, for sodium hydroxide preferably about 50 weight-% and for potassium hydroxide preferably about 46 weight-%.

Below, the aqueous solution of sodium hydroxide is also termed "lye".

The formation of a polymerization chain during the reaction can be influenced through the catalysts by accelerating the condensation reaction between formaldehyde and lignin. The catalysts allow for the reacting of virtually all free formaldehyde in a reasonable time, alternatively either at normal pressure or elevated pressure.

According to one embodiment, formaldehyde is used in the reaction as an aqueous solution having a formaldehyde concentration of 30 to 60 weight-%, typically 50 to 60 weight-%.

According to one application, technical grade formaldehyde is used. Technical grade formaldehyde starting material may among other things contain metallic impurities as well as some, typically not more than 12 weight-% methanol. According to a preferred application, the formaldehyde content of the technical grade formaldehyde used is most suitably at least 95 weight- %.

According to one embodiment the process according to the invention for the production of resin involves several stages, comprising 2 to 10 stages. The process typically involves 2 to 3 stages, preferably 3 stages, in which case the addition of alkaline catalyst takes place gradually during the process in several stages. The gradual addition of alkaline catalyst enables the solids content of the mixture to be kept low during the reaction, allowing for an effective control of the viscosity. This in turn enables the viscosity of the reaction mixture to be kept low enough in order to be able to effectively carry out the condensation reaction.

"Gradual addition" means that a longer time is used for the addition than would take if a predetermined amount is added at once (i.e. a "one-off addition" is made) without any interruption. The duration of addition is usually at least 2 times longer than would be required for a one-off addition, most suitably 5 to 100 times longer, e.g. 10 to 50 times longer.

The catalyst is usually added in two, three or more portions, in which case the reaction is allowed to proceed after the addition of one catalyst portion before the addition of the next portion.

In one application, 5 to 95 mol-%, for example 10 to 80 mol-% or 25 to75 mol-% of the total amount of catalyst are added in the first portion. In a preferred embodiment, in the first step of the process according to the invention, lignin is dissolved into a mixture composed of a solvent, especially water and solvent, and the catalyst, especially a water-soluble alkaline catalyst (optionally the first portion).

Dissolving takes place by stirring the mixture at room temperature (about 20°C) or at an elevated temperature. The temperature is usually below 70°C, most suitably below 65°C, for example not higher than 60°C. In one embodiment operation takes place at 30 to 50°C, usually at 30 to 35°C. Dissolution time is influenced by the chosen reaction conditions and materials, such as the lignin, solvent/solvent mixture, catalyst and the dissolution temperature being used. The dissolution time varies between 0.5 and 2 hours, typically being about 1 hour. When lignin has completely, i.e. at least to 90%, preferably at least to 95%, dissolved into the mixture, the temperature of the mixture is raised typically above 60°C, for example about 60 to 100°C, preferably about 65°C, and water is added. The calculated amount of formaldehyde is then added gradually while stirring the mixture typically for about 0.1 to 2 hours, preferably for about 15 to 60 minutes. After the addition of formaldehyde the temperature is raised above 80°C, for example about 80 to 100°C, preferably about 85°C. The condensation reactions between lignin and formaldehyde take place at this

temperature. The reaction is continued for a period of 0.1 to 2 hours, typically for about 15 to 45 min. In a multi-stage embodiment, the second portion of the alkaline catalyst is added, the mixture is cooled to 80°C or below, e.g. to a temperature range of 20 to 80°C, and the condensation reaction is continued. In this way, the condensation reaction can be taken to completion, but if necessary, the second portion of the catalyst can be dosed even in more stages, for example 2 to 5, typically two stages.

After the addition of the second catalyst portion the mixture is being condensed.

Condensation takes place for a period of about 0,1 to 2 hours, e.g. about 1 hour. After this, the third portion of catalyst is optionally added and condensation is again carried out for about 0.1 to 2 hours, e.g. about 1 hour. Operation is continued in a corresponding manner, if the catalyst has been further divided into multiple proportions. After all catalyst has been added, the condensation reaction has been taken into completion.

Typically, after completion of the condensation reaction, the resin being formed is cooled to room temperature, i.e. about 20°C, which is its typical storage temperature.

According to one embodiment the condensation reaction is continued until virtually all formaldehyde has reacted. As noted above, in a preferred application this means that at least 95 weight-% of the formaldehyde required for the reaction has reacted, most suitably at least 99 weight-%, for example 99.0 to 99.9 weight- of the formaldehyde required for the reaction has reacted.

In the process, the resin can be cooked in a non-pressurized reaction vessel, i.e. in an ordinary reactor which is operated under normal pressure, or alternatively in a pressure cooker (pressure vessel).

In both cases the condensation reactor is most preferably equipped with devices for heating and cooling the reactor, as well as temperature sensors and temperature control. The non- pressurized reaction vessel further most preferably comprises a condenser, such as a vertical condenser, which enables the condensation of the possibly evaporating solvent and its recycling to the reaction vessel. This also ensures that solvent for the resin is always available during the condensation reaction. The catalyst system and dry matter influence the rate of development of the viscosity of the resin. By carrying out the condensation reaction in the manner described above, in which solvent for the lignin, such as methanol, is present during the reaction and the catalyst is added gradually to the reaction mixture, the dry matter content can be raised without increasing the viscosity too much, i.e. above 10,000 cP.

According to what is stated above, the dry matter content of the reaction mixture with respect to the resin is typically about 20 to 70 weight-%. As required, it is possible to evaporate solvent, such as methanol, ethanol or acetone, away from the finished resin. In this way the resin becomes better suitable for various end-uses in which the presence of solvent is not allowed.

The concentration of solvent in the finished resin is typically 0 to 15 weight-%. The resin can thus also be recovered in a completely solvent-free form. The reaction is continued until all formaldehyde has reacted. The reaction is typically continued until a predetermined viscosity is reached. The resin formed is collected. The resin is typically formulated for further use, as will be described below. "Collecting" comprises also the option that the resin is carried on for further use without isolation or without formulation. In one application the Brookfield viscosity of the resin is 20 to 10,000 cP, especially 20 to 1,000 cP, e.g. 20 to 300 cP or 50 to 300 cP.

According to a preferred embodiment the viscosity of the produced resin is on the same level as that of normal impregnation resins. A resin having a Brookfield viscosity of 20 to 50 cP is especially suited for impregnation of core paper. A resin having a Brookfield viscosity of 100 to 300 cP is in turn suited for impregnation of film paper.

As required, solvents can also after evaporation be added back to the resin, or the evaporated solvent, such as methanol, ethanol or acetone, can be totally or partly replaced e.g. with water in applications that do not allow the presence of said solvent at all or above a certain limit, such as e.g. over 5 weight-% based on the total weight of the resin.

The resin produced according to the process is in itself already thermosetting, i.e. addition of additives is not necessary in order to have it formulated into an adhesive. The composition of the resin can, however, be further modified, for example by mixing it with extenders and crosslinkers.

Compounds that are known per se can be used as extenders, such as amide or amine compounds, such as urea, or monomeric, oligomeric or polymeric carbohydrates, such as sugars.

Compounds that are known per se can be used as crosslinkers, such as amine compounds, such as hexamethylene tetramine, or vinyl compounds, such as divinyl benzene. The finished resin can be impregnated into the paper as such or formulated with additives and then compressed into a laminate under the influence of heat and pressure.

A paper impregnated with the resin produced by the process according to the invention or with a resin having a corresponding composition can be compacted into a laminate. For compressing, a multiple opening press can be used, in which compacting takes place under a pressure over 70 bar at about 140 to 150°C. Alternatively, use can be made of a short cycle press, continuous press or some other method commonly used in the industry. The short cycle press used typically comprises 1 to 3 openings and its processing temperature is between 170 and 200°C.

When using a continuous press, typical process parameters include a pressure of 20 to 50 bar and a temperature of 170 to 180°C.

In the present invention, the lignin starting material, such as kraft lignin, is typically utilized as such and the reaction is conducted with formaldehyde, whereby the product of the reaction consists of the reaction product between lignin and formaldehyde.

The following non-limiting examples represent applications of the present technology. Examples The resin was cooked in an ordinary reactor under normal pressure. The reaction involved a vertical condenser enabling the condensation of the possibly evaporating solvent back to the reaction mixture. In the first stage water I, methanol and catalyst, i.e. the first portion of the catalyst, comprising mere sodium hydroxide or sodium hydroxide in combination with either ammonia or potassium hydroxide, was dosed into the reactor. At this stage, pH of the reaction was 7 to 8.5. To this mixture was added the kraft lignin, whereafter the mixture was stirred for about 1 hour, keeping the temperature below 50°C, generally between 30 and 35°C. After stirring, the temperature of the mixture was raised to about 65 °C and water II added. Next, formalin was added into the mixture at a steady rate over a period of about 40 minutes. After the addition, the temperature of the mixture was raised to about 85°C and the mixture was condensed for about 30 minutes. The mixture was subsequently dosed with catalyst II, i.e. the second portion of the catalyst, and the mixture was slightly cooled, to about 80°C. The mixture was condensed at this temperature for about 1 hour. Catalyst III was added, i.e. the third portion of the catalyst, and the mixture condensed for another hour. The resin was finally cooled.

The following examples describe cookings conducted by the above-described process:

Cookin example 1

Kraft lignin (75%) 25.16

Formalin (54%) 12.17

Lye (50%) 13.52

Water I 22.19

Water II 20.29

Methanol 6.68

Yield 100.00

Cooking example 2

Kraft lignin (75%) 24.16

Formalin (54%) 11.69

Ammonia (25%) 5.19

Lye (50%) 5.19

Water I 21.29

Water II 19.48

Methanol 12.99

Yield 100.00

Cooking example 3

Kraft lignin (75%) 31.26

Formalin (54%) 15.97

Lye (50%) 15.13

Water I 27.56

Water II 8.4

Methanol 1.68

Yield 100.00 Cooking example 4

Working examples

The present process can be used for the production of essentially phenol-free impregnation resin, and the resin produced by this process or having a similar composition generally as a substitute for conventional impregnation resin. The finished final product can be used as such for impregnating or as a mixture with normal impregnation resins. The finished resin can be impregnated into film/core paper (60 to 200 g/m²). The amount of resin in the paper varies between 20 to 50 weight- %, in each case depending on the end-use. The amount of volatile substances in the impregnate is between 5 and 10 weight- %.

Shown in Figure 1, right side, are samples (6 pieces) of finished pressings of lignin impregnates by using different catalyst systems. A reference sample produced from phenol-formaldehyde resin is shown on the left. The present process is especially suited for the production of impregnation resin of core paper and for its use in a compact laminate (Figure 2) as an adhesive.

In addition to impregnation resins, mention can in particular be made of the following applications: production of plywood, LVL, chipboard, MDF (medium-density fiberboard), HDF (high-density fiberboard), i.e. various fiberboards in general. In addition to these, production of sandpaper, beam glue as well as wool resins, especially mineral wool resin, such as glass wool and rock wool resins deserve to be mentioned.

List of reference publications

Patent literature:

WO 2014/080033

US 5177169

Claims

Claims:

1. Process for the production of phenol-free impregnation resin, wherein

- formaldehyde is condensed with a material containing phenolic OH groups in the presence of alkaline catalyst in order to form a resin,

characterized in that

- lignin is used as the material containing phenolic OH groups,

- the lignin is dissolved in order to form a solution,

- part of the required alkaline catalyst is included in the lignin solution,

- formaldehyde is added to the lignin solution, and

- lignin and formaldehyde are condensed in the liquid stage until virtually all

formaldehyde has reacted, whereafter

- the resin thus formed is collected.

2. The process according to claim 1, characterized in that Kraft lignin is used as the lignin, either as a powder or an aqueous slurry having a lignin dry matter content which is most suitably over 50 weight-%, especially at least 60 weight-%, preferably at least about 70 weight-%, e.g. about 75 weight-%.

3. The process according to claim 1 or 2, characterized in that the lignin is dissolved by using a solvent, which is a polar liquid which is miscible with water, such as an aliphatic or aromatic alcohol, such as methanol or ethanol, or an aliphatic ketone, such as acetone.

4. The process according to any of the preceding claims, characterized in that the solvent is used in an amount sufficient to maintain the dry matter content of the mixture suitable for the subsequent reaction.

5. The process according to any of the preceding claims, characterized in that methanol is used as the solvent and the concentration of methanol in water is about 1.6 to 12 weight-%.

6. The process according to any of the preceding claims, characterized in that NaOH or KOH or ammonia or an organic amine, or a mixture of two or more substances is used as the catalyst.

7. The process according to any of the preceding claims, characterized in that formaldehyde is used as an aqueous solution having a formaldehyde concentration of about 30 to 60 weight-%.

8. The process according to any of the preceding claims, characterized in that lignin and formaldehyde are condensed at a temperature of about 50 to 90°C until all

formaldehyde has reacted, whereafter the resin is collected.

9. The process according to any of the preceding claims, characterized in that lignin and formaldehyde are condensed in the liquid stage formed by the lignin solvent or mixture thereof.

10. The process according to any of the preceding claims, characterized in that production of the resin is a multi-stage process, wherein addition of the alkaline catalyst takes place during the process in two or more stages.

11. The process according to claim 10, characterized in that addition of the alkaline catalyst takes place in several stages to keep the viscosity of the reaction mixture low enough in order to be able to effectively carry out the condensation reaction.

12. The process according to any of the preceding claims, characterized in that the lignin is first dissolved into the mixture composed of water and solvent and the water- soluble alkaline catalyst (first portion), stirring the mixture at room temperature or at a slightly elevated temperature which is below 70°C.

13. The process according to any of the preceding claims, characterized in that after the lignin has completely dissolved, the temperature of the solution is raised above 60°C, for example about 65°C, and water is added, whereafter the calculated amount of formaldehyde is gradually added while stirring.

14. The process according to any of the preceding claims, characterized in that after addition of formaldehyde, the temperature is raised above 80°C, for example to about 85°C, in which temperature the condensation reaction between lignin and formaldehyde takes place.

15. The process according to any of the preceding claims, characterized in that the second portion of alkaline catalyst is added, the mixture is cooled to 80°C or below and the condensation reaction is continued.

16. The process according to any of the preceding claims, characterized in that the condensation reaction is continued until the formaldehyde content of the resin is not more than 5%, especially not more than 1 weight-%, e.g.0.1 to 1.0 %, and its Brookfield viscosity is at least 20 cP and not more than 10,000 cP.

17. The process according to any of the preceding claims, characterized in that the condensation reaction is conducted in the presence of the lignin solvent in order to maintain the viscosity of the reaction mixture sufficiently low.

18. The process according to claim 17, characterized in that the condensation reaction is conducted in a non-pressurized reaction vessel or in an pressure cooker, wherein the reactor is most suitably equipped with devices for heating and cooling the reactor, as well as temperature sensors and temperature control.

19. The process according to any of the preceding claims, characterized in that the resin is collected as solvent- free or containing not more than 15 weight-% of solvent.

20. Resin produced by the process according to any of the preceding claims.

21. Phenol-free impregnation resin comprising a condensation product of formaldehyde and lignin, having a concentration of free formaldehyde of not more than 5 weight-%.

22. The impregnation resin according to claim 20 or 21, c h a r a c t e r i z e d in that it is the form of an aqueous mixture having a dry matter content of about 30 to 70% with respect to the resin.

23. The impregnation resin according to any of claims 20 to 22, which is obtained by evaporating off the solvent residues included in the resin in a separate evaporation step.

24. The use of impregnation resin according to any of claims 20 to 23 for impregnation of core paper, impregnation of film paper, production of plywood, LVL, chipboard or production of fiberboards, such as MDF (medium-density fiberboard) or HDF- (high- density fiberboard) boards, production of sandpaper, as beam glue, in wool resins, especially in mineral wool, such as glass wool or rock wool.