WO2014184446A1

WO2014184446A1 - Method for producing a curing agent

Info

Publication number: WO2014184446A1
Application number: PCT/FI2014/050371
Authority: WO
Inventors: Sanna VALKONEN; Markus BIESALSKI; Roland Klein; Sabrina MEHLHASE; Matthias BAASKE; Michael Duetsch; Okko RINGENA; Matthias Rehahn
Original assignee: Upm-Kymmene Corporation
Priority date: 2013-05-17
Filing date: 2014-05-15
Publication date: 2014-11-20
Also published as: UY35570A; FI20135527L

Abstract

The invention relates to a method for producing a curing agent, wherein the method comprises the following steps: a) mixing a polyamine compound with an alkaline solution, wherein lignin having an average molecular weight of 3000 -15000 g/mol is dissolved;b) decreasing the pH of the solution formed in step a) by at least 0.5 pH units, with the proviso that the pH is decreased at least to the value of 12,and mixing the solution with a carbonyl compound; and c) heating the solution formed in step b) for forming aminated lignin. The method further relates to a curing agent and the uses thereof.

Description

METHOD FOR PRODUCING A CURING AGENT

FIELD OF THE INVENTION

The invention relates to a method for produc- ing a curing agent. Further, the invention relates to a curing agent obtainable by the method according to the present invention and to the uses thereof.

BACKGROUND OF THE INVENTION

Polyamine curing agents are traditionally used for the production of composites with epoxy res^¬ ins. A composite refers to a material consisting of two or more individual constituents. A reinforcing constituent, such as fiber, is embedded in a matrix to form the composite. Commonly used polyamine compounds for epoxy resins are diethylenetriamine (DETA) , tri- ethylenetetramine (TETA) , tetraethylenepentamine (TE- PA) , ethylenediamine, aminoethylpiperazine (AEP) , di- cyanamide (Dcy) , diethyl toluene diamine (DETDA) , di- propenediamine (DPDA) , diethyleneaminopropylamine (DEAPA) , hexamethylenediamine, N-aminoethylpiperazine (N-AEP) , menthane diamine (MDA) , isophoronediamine (IPDA), m-xylenediamine (m-XDA) and metaphenylene dia^¬ mine (MPDA) . There is, however, a need for more sus- tainable curing agents that can be used to replace conventional polyamine curing agents.

Lignin is a complex chemical compound most commonly derived from wood. Lignin is abundantly de^¬ rived as a by-product from pulping industry. The use of lignin as binder or filler agent mixed with epoxy resin in composites is described in the prior art. However, in these composites a conventional polyamine curing agent is required. Prior art recognizes amina- tion of lignin in general. However, the amination re- actions of lignin described in prior art are not bene^¬ ficial for producing a curing agent. The inventors of the present invention have therefore recognized the need for a method for produc^¬ ing a more environmentally friendly curing agent. PURPOSE OF THE INVENTION

The purpose of the invention is to provide a new type of method for producing a curing agent. Espe^¬ cially the purpose is to produce a more environmental^¬ ly friendly curing agent to be used e.g. for the pro- duction of epoxy resin matrix and fiber reinforced composite .

SUMMARY

The method according to the present invention is characterized by what is presented in claim 1.

The curing agent according to the present in^¬ vention is characterized by what is presented in claim 10.

The uses according to the present invention are characterized by what is presented in claims 12 and 13.

The fiber reinforced composite according to the present invention is characterized by what is pre^¬ sented in claim 14.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is included to provide a further understanding of the invention and constitutes a part of this specification, illus- trates one embodiment of the invention and together with the description helps to explain the principles of the invention. In the drawing:

Fig. 1 is a flow chart illustration of a method according to one embodiment of the present in- vention. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing a curing agent comprising the following steps:

a) mixing a polyamine compound with an alka^¬ line solution, wherein lignin having an average molecular weight of 3000 - 15000 g/mol is dissolved;

b) decreasing the pH of the solution formed in step a) by at least 0.5 pH units, with the proviso that the pH is decreased at least to the value of 12, and mixing the solution with a carbonyl compound; and c) heating the solution formed in step b) for forming aminated lignin.

In one embodiment of the present invention the average molecular weight of lignin is preferably 3500 - 15000 g/mol, more preferably 4000 - 10000 g/mol, and even more preferably 5000 - 8000 g/mol.

The expression "average molecular weight" should be understood in this specification, unless otherwise stated, as weight average molecular weight.

The inventors surprisingly found out that the method according to the present invention enables ef^¬ ficient production of aminated lignin that can be used as a curing agent for epoxy resins. Especially it was noted that the curing agent produced according to the present invention could be used with epoxy resin for fabricating fiber reinforced composites. The purity and functionality of the produced aminated lignin make it possible to use the aminated lignin as curing agent. Furthermore, the amino group content of the aminated lignin is advantageous for curing purposes.

The pH scale is based on a logarithmic scale, meaning that an increase or decrease of an integer value or pH unit changes the concentration by a ten^¬ fold. In this specification, unless otherwise stat^¬ ed, the expression "polyamine" should be understood as an organic compound comprising two or more primary amino groups -NH₂.

The expression "amination", "amination reaction" or any other corresponding expression should be understood in this specification, unless otherwise stated, as referring to the process by which at least one amino group is introduced into an organic mole- cule.

In this specification, unless otherwise stat^¬ ed, the expression "lignin" should be understood as lignin originating from any suitable lignin source. The lignin used can be essentially pure lignin. By the expression "essentially pure lignin" should be under^¬ stood as at least 90% pure lignin, preferably at least 95 % pure lignin. In one embodiment of the present in^¬ vention the essentially pure lignin comprises at most 10 %, preferably at most 5 %, of other components. Ex- tractives and carbohydrates such as hemicelluloses can be mentioned as examples of such other components.

In one embodiment of the present invention the lignin is selected from a group consisting of kraft lignin, sulfonated lignin, lignosulfonate, sul- fomethylated lignin, steam explosion lignin, biorefin- ery lignin, supercritical separation lignin, hydrolysis lignin, flash precipitated lignin, biomass originating lignin, lignin from alkaline pulping process, lignin from soda process, lignin from organosolv pulp- ing and combinations thereof. In one embodiment of the present invention the lignin is wood based lignin. The lignin can originate from softwood, hardwood, an^¬ nual plants or from a combination thereof.

Different lignin components may have differ- ent properties, e.g. molecular weight, molar mass, polydispersity, hemicellulose and extractive contents and compositions. In one embodiment of the present in^¬ vention the lignin includes water but no solvent.

By "kraft lignin" is to be understood in this specification, unless otherwise stated, lignin that originates from kraft black liquor. Black liquor is an alkaline aqueous solution of lignin residues, hemicel- lulose, and inorganic chemicals used in a kraft pulp^¬ ing process. The black liquor from the pulping process comprises components originating from different soft- wood and hardwood species in various proportions. Lig^¬ nin can be separated from the black liquor by different techniques including e.g. precipitation and filtration. Lignin usually begins precipitating at pH values below 11 - 12. Different pH values can be used in order to precipitate lignin fractions with differ^¬ ent properties. These lignin fractions differ from each other by molecular weight distribution, e.g. Mw and Mn, polydispersity, hemicellulose and extractive contents. The molar mass of lignin precipitated at a higher pH value is higher than the molar mass of lignin precipitated at a lower pH value. Further, the mo^¬ lecular weight distribution of lignin fraction precipitated at a lower pH value is wider than of lignin fraction precipitated at a higher pH value. Thus the properties of the lignin can be varied depending on the end use.

The precipitated lignin can be purified from inorganic impurities, hemicellulose and wood extrac^¬ tives using acidic washing steps. Further purification can be achieved by filtration.

In one embodiment of the present invention the lignin is flash precipitated lignin. The term "flash precipitated lignin" should be understood in this specification as lignin that has been precipitat- ed from black liquor in a continuous process by de^¬ creasing the pH of a black liquor flow, under the influence of an over pressure of 200 - 1000 kPa, down to the precipitation level of lignin using a carbon dioxide based acidifying agent, preferably carbon dioxide, and by suddenly releasing the pressure for precipitat^¬ ing lignin. The method for producing flash precipitat- ed lignin is disclosed in patent application FI 20106073. The residence time in the above method is under 300 s. The flash precipitated lignin particles, having a particle diameter of less than 2 μπι, form agglomerates, which can be separated from black liquor using e.g. filtration. The advantage of the flash pre^¬ cipitated lignin is its higher reactivity compared to normal kraft lignin. The flash precipitated lignin can be purified and/or activated if needed for the further processing .

In one embodiment, the flash precipitated lignin is un-dried. Water can be removed from the flash precipitated lignin e.g. by pressing or using air. In one embodiment of the present invention water is removed from the flash precipitated lignin by using pressing and air blowing. In one embodiment of the present invention the temperature of the air blowing is less than or equal to the temperature of the lignin at said process stage. In one embodiment of the pre^¬ sent invention the temperature of the air blowing is room temperature. In one embodiment of the present in^¬ vention the air is fed through the lignin filtrate for less than 1 minute, preferably less than 30 seconds, in order to remove water. The advantage of this kind of lignin is its higher activity and the possibility to re-dissolve the lignin. Further, this kind of un- dried lignin is easier to handle in further processing as a result of the lignin being less dusty.

In one embodiment of the present invention the dry matter content of the lignin, e.g. the flash precipitated lignin, is below 70 %, preferably 40 - 70 %, and more preferably 50 - 60 %. In one embodiment of the present invention the lignin is separated from pure biomass. The separa^¬ tion process can begin with liquidizing the biomass with strong alkali or strong acid followed by a neu- tralization process. After the alkali treatment the lignin can be precipitated in a similar manner as presented above. In one embodiment of the present inven^¬ tion the separation of lignin from biomass comprises a step of enzyme treatment. The enzyme treatment modi- fies the lignin to be extracted from biomass. Lignin separated from pure biomass is sulphur-free and thus valuable in further processing.

By "lignosulfonate" is to be understood in this specification, unless otherwise stated, lignin that can be received as a by-product from the produc^¬ tion of wood pulp using sulfite pulping. In one embod^¬ iment of the present invention the lignin is steam ex^¬ plosion lignin. Steam explosion is a pulping and extraction technique that can be applied to wood and other fibrous organic material.

By "biorefinery lignin" is to be understood in this specification, unless otherwise stated, lignin that can be recovered from a refining facility or pro^¬ cess where biomass is converted into fuel, chemicals and other materials.

By "supercritical separation lignin" is to be understood in this specification, unless otherwise stated, lignin that can be recovered from biomass us^¬ ing supercritical fluid separation or extraction tech- nique. Supercritical conditions correspond to the tem^¬ perature and pressure above the critical point for a given substance. In supercritical conditions, distinct liquid and gas phases do not exist. Supercritical wa^¬ ter or liquid extraction is a method of decomposing and converting biomass into cellulosic sugar by em^¬ ploying water or liquid under supercritical condi^¬ tions. The water or liquid, acting as a solvent, ex- tracts sugars from cellulose plant matter and lignin remains as a solid particle.

In one embodiment of the present invention the lignin is hydrolysis lignin. Hydrolysed lignin can be recovered from paper-pulp or wood-chemical process^¬ es .

In one embodiment of the present invention the lignin originates from an organosolv process. Or- ganosolv is a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose .

In one embodiment of the present invention, the selected lignin is fractionated or purified by di^¬ alysis, ultrafiltration, nanofiltration, solvent extraction or any combination thereof. The purification step removes low molecular weight phenolic compounds, carbohydrates, sugars, wood extractives and part of salts and inorganic compounds, which reduces side re^¬ actions. Therefore, the curing agent produced accord^¬ ing to the method of the present invention is pure and has a higher substitution level.

In one embodiment of the present invention, the selected lignin is purified to remove at least 70 % of low molecular weight lignin. By the expression "low molecular weight lignin" should be understood as lignin having an average molecular weight of 500

3000 g/mol, and preferably 1000 - 2000 g/mol. The av^¬ erage molecular weight of lignin can be measured using high pressure size-exclusion chromatography (HP-SEC) . In one embodiment of the present invention purified lignin is used in the method for producing the curing agent. The removal of low molecular weight lignin makes the method according to the present invention more efficient by reducing unwanted side reactions. Low molecular weight lignin is more reactive than high molecular weight lignin and thus more easily causes side reactions. Furthermore, the method according to the present invention is easier to control in the ab^¬ sence of low molecular weight lignin.

In one embodiment of the present invention the average molecular weight of the purified lignin is preferably 3500 - 15000 g/mol, more preferably 4000 - 10000 g/mol, and even more preferably 5000 - 8000 g/mol .

In one embodiment of the present invention the polydispersity index (PDI) of the purified lignin is 1.5 - 5, preferably 3 - 4, as determined by size- exclusion high-performance liquid chromatography (SEC- HPLC) . The polydispersity index (PDI) is a measure of the distribution of molecular mass in a given polymer sample. The PDI is calculated as the weight average molecular weight divided by the number average molecu^¬ lar weight. PDI indicates the distribution of individ^¬ ual molecular masses in a batch of polymers.

In one embodiment of the present invention the glass transition temperature (T_g) of the purified lignin is 120 - 180 °C, and preferably 140 - 160 °C. Glass transition temperature is measured by differen^¬ tial scanning calorimeter (DSC) , which defines glass transition as a change in heat capacity during the transition of the polymer matrix from glass state to rubber state.

In one embodiment of the present invention the ash percentage of the purified lignin is 1.5 weight-% or less. The ash content can be determined by carbonifying and quickly burning a lignin sample so that alkali salts are not melted before the organic matter has been burned (e.g. 20-200°C for 30 minutes, after which temperature is adjusted to 200-600°C for 1 hour, and thereafter adjusting the temperature to 600- 700°C for 1 hour), and finally the lignin sample is ignited at 700°C for 1 hour. Ash content of a lignin sample refers to the mass that remains of the sample after burning and ignition, and it is presented as per cent of the sample's dry content.

In one embodiment of the present invention the purified lignin contains less than 5 weight-%, preferably less than 1.5 weight-%, and more preferably less than 1 weight-% of carbohydrates. The amount of carbohydrates present in purified lignin can be meas^¬ ured by high performance anion exchange chromatography with pulsed amperometric detector (HPAE-PAD) in ac- cordance with standard SCAN-CM 71.

Carbohydrates are a group of organic com^¬ pounds that consist of carbon, hydrogen, and oxygen. Examples of carbohydrates are sugars, cellulose, and hemicellulose . Lignin can contain carbohydrates bonded or linked to the lignin molecules or as a free impuri^¬ ty.

In one embodiment the purified lignin con^¬ tains essentially no fiber and woody material.

In one embodiment of the present invention, the purified lignin is dried. In one embodiment of the present invention, the purified lignin is dried by vacuum or pressure filtration, air drying, spray drying, vacuum drying, centrifugation or freeze-drying . In one embodiment of the present invention the puri- fied and dried lignin is ground to dry powder. Dried and ground lignin powder is easily handled in the sub^¬ sequent amination reaction.

In one embodiment of the present invention the polyamine compound is selected from a group con- sisting of diethylenetriamine (DETA) , triethylenetet- ramine (TETA) , tetraethylenepentamine (TEPA) , eth- ylenediamine, aminoethylpiperazine (AEP) , dicyanamide (Dcy) , diethyl toluene diamine (DETDA) , dipropenedia- mine (DPDA) , diethyleneaminopropylamine (DEAPA) , hexa- methylenediamine, N-aminoethylpiperazine (N-AEP) , men- thane diamine (MDA) , isophoronediamine (IPDA), m- xylenediamine (m-XDA) , metaphenylene diamine (MPDA) , dipropylenetriamine, 3- (2-Aminoethyl) amino-propylamine (N3-Amine) , N, ' -Bis ( 3-aminopropyl ) -ethylenediamine

(N4-Amine) , neopentanediamine, 1 , 3-diaminopropane, and dimethylaminopropylamine .

In one embodiment of the present invention the polyamine is added gradually in the alkaline solu^¬ tion comprising lignin dissolved therein. In one embodiment of the present invention the polyamine is added dropwise in the alkaline solution comprising lignin dissolved therein.

In one embodiment of the present invention the alkaline solution comprises a hydroxide of an al^¬ kali metal, such as sodium hydroxide (NaOH) and potas^¬ sium hydroxide (KOH) .

In one embodiment of the present invention the pH is adjusted in step b) by using a protic acid, such as hydrochloric acid (HC1) , sulphuric acid (H₂S0₄) , nitric acid (HN0₃) , or phosphoric acid (H₃P0₄) .

The expression "carbonyl compound" should be understood in this specification as a compound com^¬ prising a carbonyl group. A carbonyl group contains a carbon-oxygen double bond. Aldehydes and ketones are carbonyl compounds .

In one embodiment of the present invention the carbonyl compound is an aldehyde.

In one embodiment of the present invention the aldehyde is gradually added to the solution in step b) .

In one embodiment of the present invention the aldehyde is selected from a group consisting of paraformaldehyde, formaldehyde and glyoxal.

In one embodiment of the present invention the pH of the alkaline solution in step a) is above 12, preferably 12.5 - 14, more preferably 12.8 - 14, even more preferably 12.5 - 13.5, and most preferably about 13. This pH range allows advantageous substitu^¬ tion level of lignin by allowing beneficial dissolu- tion of lignin. Furthermore, the pH of the solution has an effect on lignin solubility. In addition to pH, the solubility of lignin is also affected by the origin of lignin and by the methods used for lignin purification. pH range 12.5 - 13.5 has been found to enhance the solubility of lignin.

In one embodiment of the present invention step b) comprises decreasing the pH of the solution formed in step a) preferably to 10.5 - 11.5. In one embodiment of the present invention the pH of the al^¬ kaline solution is adjusted in step b) before adding the carbonyl compound to the solution. In one embodi^¬ ment of the present invention the pH of the alkaline solution is adjusted and the carbonyl compound is add- ed at the same time in step b) . In one embodiment of the present invention the carbonyl compound is gradu^¬ ally mixed with the solution formed in step a) . In one embodiment of the present invention the carbonyl com^¬ pound is mixed with the solution formed in step a) in a stepwise manner. Adjusting the pH in step b) has an advantageous effect on amination reaction efficiency and substitution level. The inventors of the present invention found out that the amount of unwanted side reactions can be decreased in this pH range. Also, the possibility of lignin precipitation can be decreased in this pH range.

In one embodiment of the present invention the solution is heated in step c) at a temperature of 50 - 100 °C, and preferably at a temperature of 70 - 95 °C . Increasing the temperature reduces reaction time and improves the yield of aminated lignin.

In one embodiment of the present invention step c) is carried out for 1 - 24 hours, preferably 2 - 16 hours, and more preferably 3 - 6 hours.

In one embodiment the aminated lignin pro^¬ duced according to the method of the present invention is purified by dialysis, ultrafiltration, nanofiltra- tion, precipitation or any combination thereof. In one embodiment, the aminated lignin produced according to the method of the present invention is precipitated using ethanol, isopropanol or methanol precipitation or any combination thereof.

In one embodiment of the present invention, the purified aminated lignin is centrifuged. In one embodiment the purified aminated lignin is dried. In one embodiment of the present invention, the purified aminated lignin is dried by vacuum or pressure filtra^¬ tion, air drying, spray drying, vacuum drying, cen- trifugation or freeze-drying . In one embodiment of the present invention the purified and dried aminated lig^¬ nin is ground to dry powder. Dried and ground lignin powder is easily handled in the subsequent curing re^¬ action .

The present invention further relates to a curing agent obtainable by the method according to the present invention. The inventors of the present inven- tion surprisingly found that the method according to the present invention allows the production of aminat^¬ ed lignin, which is suitable for use as a curing agent for epoxy resin. On the contrary, unaminated lignin is not suitable for use as a curing agent.

In one embodiment the present invention fur^¬ ther relates to the curing agent wherein the substitu^¬ tion level of lignin is 60 - 95 %. In one embodiment the present invention further relates to the curing agent wherein the substitution level of reactive, ter- minal positions of lignin is 80 - 100 %. The reacted or substituted sites of aminated lignin can be meas^¬ ured by e.g. potentiometric titration or elemental analysis .

In one embodiment the present invention fur- ther relates to the curing agent wherein the amount of bound amino groups in mmol per gram of lignin is 3 - 10 (mmol/g), as determined by potentiometric titra- tion. The amino groups are bound to lignin used in the present invention. In one embodiment of the present invention, the aminated lignin used in the present in^¬ vention contains essentially no free polyamine com- pounds. In one embodiment of the present invention the curing agent contains at least 5 weight-% of bound ni^¬ trogen, and preferably 5 - 20 weight-% of bound nitro^¬ gen, as determined by elemental analysis. In one em^¬ bodiment of the present invention the polydispersity index (PDI) of the curing agent is 3 - 9 as determined by size-exclusion high-performance liquid chromatog^¬ raphy (SEC-HPLC) .

In one embodiment of the present invention the ash percentage of the curing agent is 1.5 weight-% or less. The ash content can be determined by carboni- fying and quickly burning an aminated lignin sample so that alkali salts are not melted before the organic matter has been burned (e.g. 20-200°C for 30 minutes, after which temperature is adjusted to 200-600°C for 1 hour, and thereafter adjusting the temperature to 600- 700°C for 1 hour), and finally the aminated lignin sample is ignited at 700°C for 1 hour. Ash content of an aminated lignin sample refers to the mass that re^¬ mains of the sample after burning and ignition, and it is presented as per cent of the sample's dry content.

In one embodiment of the present invention the curing agent contains less than 5 weight-%, pref^¬ erably less than 1.5 weight-%, and more preferably less than 1 weight-% of carbohydrates. The amount of carbohydrates present in the curing agent can be meas^¬ ured by high performance anion exchange chromatography with pulsed amperometric detector (HPAE-PAD) in accordance with standard SCAN-CM 71.

The present invention further relates to the use of the curing agent for the production of an epoxy resin matrix. In one embodiment of the present invention the weight ratio of the curing agent to the resin is 1:5 to 2:1. In one embodiment of the present invention the curing agent of the present invention is used in combination with conventional polyamine curing agents.

In one embodiment of the present invention, the resin is a cold curing resin. In one embodiment of the present invention the resin is a hot curing resin.

The present invention further relates to the use of the curing agent for the production of fiber reinforced composite. The present invention further relates to the use of the curing agent in applications such as coating, paints, tooling, casting and adhe- sives. A fiber reinforced composite comprises a rein- forcing constituent of fibers embedded in a resin ma^¬ trix. A composite refers to a material consisting of two or more individual constituents. A reinforcing constituent of fibers is embedded in a matrix to form the composite. Common composites are composed of glass or carbon fiber in a plastic resin. Natural fibers can also be used. Resins can be of the form of thermoset or thermoplastic materials which each have their own unique properties. Epoxy resins are traditionally used for fiber reinforced composites. However, the epoxy resins require the use of a curing agent.

The present invention further relates to a fiber reinforced composite comprising epoxy resin crosslinked with the curing agent according to the present invention.

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to^¬ gether to form a further embodiment of the invention. A method, a curing agent, uses and a fiber reinforced composite, to which the invention is related, may com^¬ prise at least one of the embodiments of the invention described hereinbefore. An advantage of the method according to the present invention is that a more environmentally friendly curing agent is obtained. There is a need to invent new ways to utilize lignin produced as a by- product of pulping processes. The aminated lignin pro^¬ duced with the method according to the present inven^¬ tion can be used to replace conventional polyamine curing agents. Also, lignin is a readily available and inexpensive source material.

The inventors of the present invention sur^¬ prisingly found a method with reaction conditions ena^¬ bling the production of aminated lignin suitable to be used as a curing agent. The aminated lignin produced according to the method of the present invention is readily soluble or dispersing in epoxy resins. Also, the substitution level of the aminated lignin produced according to the method of the present invention is high and the purity of aminated lignin is beneficial. Because of these properties the use of the aminated lignin as a curing agent results in efficient curing. An advantage of the method according to the present invention is that the yield of the curing agent pro^¬ duced is good. Total curing of the epoxy resin can be achieved with the aminated lignin hardener.

EXAMPLES

Reference will now be made in detail to the embodiments of the present invention, an example of which is illustrated in the accompanying drawing.

The description below discloses some embodi^¬ ments of the invention in such a detail that a person skilled in the art is able to utilize the invention based on the disclosure. Not all steps of the embodi^¬ ments are discussed in detail, as many of the steps will be obvious for the person skilled in the art based on this specification. Figure 1 illustrates a method according to one embodiment of the present invention for producing a curing agent.

Before forming an alkaline solution compris- ing lignin, the source of components used in the meth^¬ od of the present invention, and especially the source of lignin, is chosen. As presented above, lignin can be selected from kraft lignin, flash precipitated lig^¬ nin, biomass originating lignin, lignin from alkaline pulping process, lignin from soda process and combina^¬ tions thereof. The selected lignin is firstly purified e.g. by dialysis, solvent extraction or ultrafiltra^¬ tion, whereby small phenolic components and salts are being removed. Also the other components and their amounts to be used are selected.

Following the various preparations and pre- treatments, in the embodiment of the present invention shown in Fig. 1, step a) is carried out. The selected lignin is dissolved in an alkaline solution. Then a polyamine compound is added thereto.

After step a), step b) is carried out, i.e. the pH of the solution is decreased by at least 0.5 pH units to e.g. 10 - 12 and a carbonyl compound is mixed therein. The formed solution is heated in step c) at a temperature of 50 - 100 °C for 1 - 24 hours for allow^¬ ing the lignin to react and aminated lignin to form.

EXAMPLE 1 : Determination of the molecular weight of lignin

The molecular weight of lignin can be determined using a high-performance size-exclusion chroma^¬ tography (SEC) . Detection of lignin is made using UV- detector PDA-100 at the wavelength of 280 nm. This UV- detector is sensitive for components originating from aromatic lignin based material. Solid lignin sample is dried overnight in an oven at a temperature of 105 °C . 10 g of dried lignin is weighed and transferred into a 10 ml volumetric flask. The lignin sample is fully dissolved in 0.1 M (1 mg/ml) sodium hydroxide (NaOH) . Before performing the chromatography, the lignin solution is filtrated through a 0.45 μΜ filter.

The molecular weight of lignin is determined using a high-performance size-exclusion chromatography as follows. Two parallel measurements are carried out. 0.1 M NaOH is used as an eluent. The calibration is done using Na-polystyrene sulfonate standards having a molecular weight of 1100 - 73900 g/mol. For quality control, standard quality kraft lignin and PSS molecu- lar weight standard are used. The columns used are PSS MCX precolumns, 1000 A and 100 000 A separation col^¬ umns filled with sulfonated styrene-divinylbenzene co^¬ polymer matrix. Isocratic run program is used. The run time is 45 minutes. The injection volume is 50 μΐ. The flux is 0.5 ml per minute. The temperature is 25 °C .

As a result of the chromatography, number average molecular weight M_n, weight average molecular weight M_w, peak molecular weight M_p and polydispersity index PDI values can be reported.

EXAMPLE 2 : Preparing a curing agent

In this example lignin was aminated according to the embodiment of the present invention shown in Fig. 1.

Before performing the amination reaction the lignin used in this example, Standard Kraft Lignin (SKL) was purified following the below procedure: 5 g SKL was dissolved in 50 ml of 0.1 M NaOH solution and purified via dialysis (dialysis-tube: 3500 nominal mo^¬ lecular weight limit (NMWL) ) in 1.5 1 of water, which was changed three times every eight hours for 24 hours. Thereafter, the SKL was freeze-dried to give a fluffy brown powder of purified SKL (pSKL) (yield: 4.1 g, 82 %) .

The average molecular weight of the purified lignin was determined in accordance with example 1. The analysis showed that the average molecular weight of purified lignin was about 6000 g/mol.

The purified lignin was then treated in the following manner:

1.6 g of purified SKL was dissolved in 100 ml of 0.5 M NaOH-solution in a 250 ml 3-necked-flask with dimroth-cooler and dropping funnel (pH = 13.3, adapting temperature (aT) = 22.9 °C) . Then 3.3 ml (3.17 g, 30.7 mmol) of diethylenetriamine (DETA) was added dropwise (pH = 13.3) . Under constant stirring the pH value was decreased to pH = 11 - 11.2 by adding con^¬ centrated HC1 solution (aT = 29.1 °C) . After five minutes of stirring, 0.8 ml (3.46 g, 29.2 mmol) of CH₂0-solution (37 % in water) was added dropwise over a period of 15 minutes and then the solution was heat^¬ ed to 90 °C and stirred under constant temperature for 16 hours for preparing aminated lignin.

After cooling down to room temperature, the 100 ml solution was dialysed (dialysis tube: 3500 NMWL) in 1.5 1 of water, which was changed three times every eight hours for 24 hours and freeze-dried to yield a strong hygroscopic fluffy, light brown powder (yield: 1.90 g) .

The average molecular weight of the aminated lignin was determined in accordance with example 1. The analysis showed that the average molecular weight of aminated lignin produced in line with example 2 was about 9000 g/mol. EXAMPLE 3: Purification of lignin using solvent extraction and the use thereof for preparing a curing agent In this example lignin was purified using solvent extraction and the purified lignin was aminat- ed according to the method of the present invention.

Before performing the amination reaction, sulphur-free lignin was purified according to the fol- lowing process. 1000 g of sulfur-free lignin was ex^¬ tracted with ethylacetate, wherein the ratio of lignin to ethylacetate was 1:10. Extraction was done at room temperature for 3 hours. After the extraction, lignin was separated by filtration, washed with ethylacetate and dried.

The average molecular weight of purified lig^¬ nin was determined in accordance with example 1. The analysis showed that the average molecular weight of purified lignin was about 5000 g/mol.

The purified lignin was then treated in the following manner:

800g of purified lignin was dissolved in 3000 ml of 0.5 M NaOH solution (pH 12.8). Then, 400 ml of triethylenetetramine (TETA) was added into the solu- tion. Under constant stirring the pH value of the so^¬ lution was decreased to pH = 11.5 by adding H₂S0₄. 125 ml of formaldehyde-solution (CH₂0) (37 % in water) was added dropwise over a period of 30 minutes. The reac^¬ tion mixture was then heated to 90 °C and stirred un- der constant temperature for 6 hours for preparing aminated lignin.

After cooling down to room temperature, the aminated lignin was precipitated out with isopropanol and filtrated. The final product was washed and dried.

The average molecular weight of the aminated lignin was determined in accordance with example 1. The analysis showed that the average molecular weight of aminated lignin produced in line with example 3 was about 9000 g/mol.

EXAMPLE 4: Use of aminated lignin as a curing agent for epoxy resin

In this example the use of aminated lignin produced in accordance with example 2 as a curing agent for epoxy resin was tested.

15 g of aminated lignin from example 2 was dispersed in 17 g of Epilox® L285 epoxy resin (Epilox® A19-00 + Epilox® P 13-20) in a snap-on lid glass and cured for 48 hours at 125 °C. After cooling down to room temperature a black-brown solid was formed.

The formation of crosslinking was determined by immersing the cured epoxy resin formed in accord^¬ ance with example 4 into THF solvent for different time periods. 5 g of cured epoxy resin was immersed into THF solvent for 2, 6 and 24 hours. After said pe- riod, the solid residue was extracted and weighed. The results are presented in table 1. According to the re^¬ sults, the material formed was insoluble in THF (after one day) and did not swell in solution. From the test results it was noted that the use of aminated lignin resulted in crosslinks being formed between epoxy res^¬ in and aminated lignin, i.e. aminated lignin was able to cure the epoxy resin.

Table 1. Extraction test in THF for epoxy resin cured in accordance with example 4

Extraction time in THF

0 h 2 h 6 h 24 h

Weight of 5.0 g 5.0 g 5.0 g 4.8 g epoxy resin

cured with

aminated

COMPARATIVE EXAMPLE 5 : Use of SKL lignin with epoxy resin A comparative example using unaminated Stand^¬ ard Kraft Lignin (SKL) was performed. In a similar manner as in example 4, 15 g of SKL was dispersed in 17 g of Epilox® L285 epoxy resin (Epilox® A19-00 + Epilox® P 13-20) in a snap-on lid glass and cured for 48 hours at 125 °C. After cooling down to room temperature a brown solid was formed.

The formation of crosslinking was determined by immersing the mixture produced in accordance with comparative example 5 into THF solvent for different time periods. 5 g of formed mixture was immersed into THF solvent for 2, 6 and 24 hours. After said period, the solid residue was extracted and weighed. The re^¬ sults are presented in table 1. The material swelled in solution and was soluble in THF (after one day) and resulted in a brown solution. The test results indi^¬ cated that no crosslinking had occurred.

Table 2. Extraction test in THF for mixture produced in accordance with comparative example 5

Extraction time in THF

0 h 2 h 6 h 24 h

Weight of 5.0 g 4.5 g 3.9 g 2.4 g epoxy resin

cured with

unaminated

Standard

kraft lig^¬ nin From the above results it can be seen that unaminated lignin is not suitable for use as a curing agent and the use of traditional curing agents are needed for the epoxy resin. Contrary to unaminated lignin, the lignin aminated with the method according to the present invention could be used as a curing agent for epoxy resin.

EXAMPLE 6: Preparing a laminate using pre-impregnated glass fiber fabrics dicyanodiamide (DICY) 2 g aminated lignin 15 g

DGEBA-based epoxy prepolymer 100 g imidazole (accelerator) 0.1 g acetone or propyl alcohol 40 g glass fiber matts 5 laye

The aminated lignin was produced in accord- ance with example 2. The aminated lignin produced in line with example 2 contained less than 5 weight-% of carbohydrates and it had an average molecular weight of about 9000 g/mol. The properties of the aminated lignin produced enabled it to be used as a curing agent in the present example.

15 g of aminated lignin, 2 g of DICY, 100 g of epoxy resin, 0.1 g of imidazole and 40 g of acetone or propyl alcohol were mixed thoroughly. The formed resin matrix was used to impregnate glass fiber cloths to yield a prepreg with suitable properties for lami^¬ nation. The epoxy resin, aminated lignin and DICY con^¬ stituted about 45 weight-% of the formed prepreg, and glass fibers (55 weight-%) , constructed in layers, served as the reinforcements.

Laminates were produced using five layers of the prepreg. The constructions were pressed at a pres^¬ sure of 12 bar for 70 minutes at a temperature of 175 °C during which crosslinks were formed between epoxy resin and aminated lignin.

From test results it was noted that aminated lignin efficiently cured the epoxy resin. From test results it was also noted that the use of aminated lignin allowed the replacement of conventional petro^¬ leum based curing agent, e.g. DICY, without affecting the properties of the final composite. It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

Claims

1. A method for producing a curing agent, c h a r a c t e r i z e d in that the method comprises the following steps:

2. The method of claim 1, wherein the average molecular weight of lignin is preferably 3500 - 15000 g/mol, more preferably 4000 - 10000 g/mol, and even more preferably 5000 - 8000 g/mol.

3. The method of any one of claims 1 - 2, wherein the polyamine compound is selected from a group consisting of diethylenetriamine (DETA) , trieth- ylenetetramine (TETA) , tetraethylenepentamine (TEPA) and ethyleneamine .

4. The method of any one of claims 1 - 3, wherein the carbonyl compound is an aldehyde.

5. The method of claim 4, wherein the alde^¬ hyde is selected from a group consisting of paraformaldehyde, formaldehyde and glyoxal.

6. The method of any one of claims 1 - 5, wherein the pH of the alkaline solution in step a) is above 12, preferably 12.5 - 14, more preferably 12.8 - 14, even more preferably 12.5 - 13.5, and most pref^¬ erably about 13.

7. The method of any one of claims 1 - 6, wherein step b) comprises decreasing the pH of the so^¬ lution formed in step a) preferably to 10.5 - 11.5.

8. The method of any one of claims 1 - 7, wherein the solution is heated in step c) at a temperature of 50 - 100 °C, and preferably at a temperature of 70 - 95 °C.

9. The method of any one of claims 1 - 8, wherein step c) is carried out for 1 - 24 hours, pref^¬ erably 2 - 16 hours, and more preferably 3 - 6 hours.

10. A curing agent obtainable by the method as defined in any one of claims 1 - 9.

11. The curing agent of claim 10, wherein the substitution level of lignin is 60 - 95 %.

12. Use of the curing agent of any one of claims 10 - 11 for the production of an epoxy resin matrix .

13. Use of the curing agent of any one of claims 10 - 11 for the production of fiber reinforced composite .

14. A fiber reinforced composite comprising epoxy resin crosslinked with curing agent of claim 10.