WO2025056117A1 - Two-component-adhesive-system component, and two-component adhesive system, and use - Google Patents

Abstract

The invention relates to a two-component-adhesive-system component A and is characterised in that the two-component-adhesive-system component A comprises bio-based conjugated dicarbonyls as reactive components, wherein the bio-based conjugated dicarbonyls are an oxidation product of lignin or highly oxidised lignin and/or tannins, and the bio-based conjugated dicarbonyls are in the form of reactive quinone groups. The invention also relates to a two-component adhesive system having the two-component-adhesive-system component A and a two-component-adhesive-system component B, and to an associated use.

Description

TWO-COMPONENT ADHESIVE SYSTEM COMPONENT AND TWO-COMPONENT

ADHESIVE SYSTEM AND USE

The invention relates to a two-component adhesive system component. Furthermore, the invention relates to a two-component adhesive system and a use of the two-component adhesive system.

Technical bonding processes are often greatly hampered by the presence of large amounts of water, sometimes with high salt concentrations. This is one of the reasons why marine invertebrates living underwater, such as mussels and their "mussel adhesive ^," have become the subject of research.

The state of the art shows various approaches for the production of adhesives with mussel-inspired thiol-catechol-based adhesive functionality or adhesives using lignin or structurally related polyphenols.

Raw material streams from renewable resources containing bio-based carbon with many phenolic structural elements are interesting as starting materials for sustainable materials. For example, the tannins and lignins classes contain many phenolic elements, some of which form the backbone of the tannin/lignin structure and are linked via various C-O and C-C bonds, making these materials an interesting candidate for applications in fields such as medicine, electrochemistry, concrete additives, structural additives, plastic fillers, and adhesives. Depending on the source, the three types of hydroxycinnamic alcohol subunits or monolignols, namely p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), are linked together in varying ratios to form a biopolymeric "nano-/microgel." Typically, lignin or tannins isolated from wood pellets, plant fibers, or plant residues are processed to create structures with lower molecular weight or higher purity that are soluble in alkaline solutions, organic solvents, or aqueous media. Upon removal of impurities such as cellulose and hemicellulose compounds, the chemical network of lignin is restructured to create an artificial lignin structure. Under suitable conditions, it is possible to completely depolymerize lignin. Soluble polymeric lignin, known as kraft or organosolv lignin, or soluble tannic acids such as tannic acid and tannins, contain inherent polyphenolic units that are intensively utilized and, usually partially or completely converted (consumed) during material production to produce innovative materials.

The publication DE 10 2020 108 099 B4 shows a polymeric adhesive with at least two thiocatechol connectivities (TCC) functionalities per repeating unit, wherein the polymeric adhesive is formed from monomers of the type “bisphenol” oxidized to quinones and at least difunctional thiols after an A _n B _n linkage.

In “Thermo-sensitive, injectable, and tissue adhesive sol-gel transition hyaluronic acid/pluronic composite hydrogels prepared from bio-inspired catechol-thiol reaction,” Soft Matter, 2010, 6, 977-983, Lee et al. describe TCC-functionalized polymers for adhesion, which were generated via a Michael-analogous reaction between quinone and thiol.

In addition, the publication CN 102250363 discloses a modified lignin mixed with mercaptans and thiols as an adhesive.

Document CN 114149785 discloses a mussel-inspired biometric two-component adhesive, one component of which is an aqueous, alkaline solution of lignin. The second component, a modified polyaspergin, exhibits catechol functions.

WO 2016/009054 A1 describes a binder in which a lignosulfonate is first reacted with a polymer containing several phenolic groups, including catechol. The resulting product is then mixed with a polyamine.

In addition, the document DE 102014 107 826 A1 discloses an adhesive based on fiber-containing materials which have activatable adhesive components, characterized in that the adhesive is a wettable and/or meltable textile sheet material made of fibers with fiber diameters of 5 pm or less and with one or more fiber types, that it is a fiber material which can be activated by adding liquid, thermally and/or by pressure, in each case with the formation of covalent bonds, coordinate bonds and/or ionic bonds, and that the fiber or fibers contain at least two mutually reactive adhesive components (in particular A and B) which can be activated during the activation in the matrix of at least one fiber type, as well as their production and use.

The technology of “Catechol Mussel-Inspired polymers” is currently a very focused research topic.

The problems in the state of the art are essentially that for the production of most of the “Catechol Mussel-Inspired polymers” known to date, the monomer L-3,4-dihydroxyphenylalanine (L-Dopa) or its derivatives are used, which also leads to the peptide bonding of these adhesive polymers.

The high prices of L-Dopa and its derivatives for large-scale applications, as well as the petrochemical origin of these substances and the lack of large-scale availability of the Tyrosinase for the in situ generation of catechols from phenols (e.g. Börner et al. "Mussel-glue derived, enzyme activated anti-fouling coatings" ACS Macro Lett. 2012, 1, 871-875) limit the use of these adhesive polymers to high-cost applications in biomedical and cosmetic applications.

Bisphenols such as those used in the publication DE 102020 108 099 B4 are inexpensive and readily available on an industrial scale, but they are produced synthetically. Due to the consumption of fossil raw materials in the synthetic production of bisphenols and the hormonal activity of bisphenol A, alternatives to adhesive systems containing bisphenol A are being sought.

The present invention is based on several objects.

An object of the invention is to provide a first component A of a two-component adhesive system which is in particular sustainable and bio-based and consists of at least a portion of renewable carbon.

One task is to provide a sustainable adhesive system. The adhesive system should be available in variants for dry, wet, and even seawater applications.

The aim is to provide an adhesive system based on a renewable raw material that has polymeric / polymer-bound / phenolic structural elements.

A manufacturing process should be simple, inexpensive and scalable.

The adhesive system should be applicable to different substrates under various conditions and, in particular, tolerate salt water. Furthermore, the adhesive system should be particularly compatible with the marine biological environment.

These objects are achieved with a two-component adhesive system component A according to the main claim and with a two-component adhesive system according to the independent claim. Furthermore, the solution is also achieved by a use according to the invention according to the use claim.

The two-component adhesive system component A is characterized in that the two-component adhesive system component A comprises bio-based conjugated dicarbonyls as reactive component, wherein

- the bio-based conjugated dicarbonyls are an oxidation product of lignin or highly oxidized lignin and/or tannins and

- the bio-based conjugated dicarbonyls are formed as reactive quinone groups.

In addition, the bio-based share of renewable or bio-based carbon in the

Two-component adhesive system component A in particular greater than 30% or greater than 50% or greater than 90% or 100% (unless otherwise stated, % figures refer to weight %)

In a particularly preferred embodiment, the two-component adhesive system component A can have 0.3 to 5 mmol/g conjugated dicarbonyl groups or 0.8 to 3.8 mmol/g conjugated dicarbonyl groups or 1.3 to 3.0 mmol/g conjugated dicarbonyl groups.

The two-component adhesive system with the two-component adhesive system component A and a two-component adhesive system component B is characterized in that

- the two-component adhesive system component B comprises nucleophilic hardeners as a reactive component and

- the two-component adhesive system component A and the two-component adhesive system component B react with each other and form covalent bonds and/or a covalently bridged network.

The nucleophilic hardeners of the two-component adhesive system component B can comprise polythiols and/or polyamines.

Furthermore, the nucleophilic hardeners can also contain mixtures with monothiols or monoamines (reactive diluents).

In particular, the two-component adhesive system component B of the two-component adhesive system can comprise a branched or unbranched molecule or polymer or any mixtures of these substances with at least two, particularly three to four nucleophilic functionalities, but preferably thiols, also with reactive group distributions and partly monofunctional impurities.

The two-component adhesive system component B can

- be an aliphatic molecule or be predominantly composed of aliphatic polyester or polyether structures and/or

- have molecular weights of 140-1400 g/mol and/or

- Polarities with calculated c(logP) coefficients between 1.84-12.43.

Component B can be calculated, in particular, as substances whose thiol functionalities are formally present in the form of ethyl thioether to more clearly describe the polarity of the crosslinker core structure. For this purpose, the calculation program Chem3D (PerkinElmer version 20.1.1) can be used, which calculates the c(logP) coefficients based on MOSES (by Molecular Networks GmbH) [Polykovskiy, D. et al. Frontiers in pharmacology 2020, 11, 565644]. Examples of difunctional thiols (1,6-dithiohexane) cLogP=4.33. Trifunctional polymeric crosslinkers include ethoxylated trimethylpropane tri(3-mercaptopropionate) (ETTMP) with DPn,PEO=9 (arm length x=y=z=3; c(logP)=3.48), or DPn,PEO=18 (arm length x=y=z=6; c(logP)=1.84), or tetrafunctional crosslinkers such as polycaprolactone tetra(3-mercaptopropionate) (PCL4MP) with DPn=8 (arm length w=x=y=z=2; c(logP)=12.43).

The two-component adhesive system can preferably be characterized in that the two-component adhesive system component B is designed as

- branched or unbranched polymers with at least two thiol groups or as

- hydrophilic, thiol-group-bearing polyethylene oxide (ETTMP) or as

- hydrophobic thiol group-bearing poly(E-caprolactone) (PCL4MP) or as

- chemically reduced thiol group-bearing proteins.

In addition, the two-component adhesive system component A and/or the two-component adhesive system component B of the two-component adhesive system may comprise/comprise further individual components or combinations of components selected from:

- fillers;

- Flame retardant;

- impact modifiers;

- Silane-modified polymers as additional co-crosslinkers;

- Epoxies as additional crosslinkers;

- Methacrylates as additional co-crosslinkers;

- Acrylates as additional cover crosslinkers;

- Vinyl ethers as additional crosslinkers;

- Styrene derivatives as additional co-crosslinkers;

- reactive diluents;

- Curing catalysts

- Rheology aids / rheology-modifying fillers; rheology additives;

- Exothermic-regulating fillers;

- Hydrolysis stability-improving fillers;

- wetting agents;

- Dyes.

When combining with a co-binder, care is taken to ensure that there are no obvious stability problems. In the two-component adhesive system, in particular the conjugated dicarbonyl groups of the two-component adhesive system component A can react with thiol groups of the two-component adhesive system component B when the two components are combined, forming thiol-catechol linkages or also partially forming disulfide linkages.

The two-component adhesive system can be used as:

- Underwater adhesive;

- shell glue;

- Coral glue;

- Adhesives for ceramic components;

- Adhesive for concrete;

- Adhesive for stone;

- Glue for glass;

- Adhesives for metal and/or metal substrates;

- Adhesives for technical polymers;

- Adhesives for PET and/or PP and/or PE and/or PU and/or polyamides;

- Coatings and/or coating;

- Sealant.

In particular, applications of the two-component adhesive system can be seen in the bonding / coating of aluminum, steel, iron, titanium, glass, stone and concrete but also plastic materials consisting of PP, PE, PA, PS, PU, PET.

In addition, the adhesive system can tolerate salt water in particular and is compatible with the marine biological environment.

The tannin class includes chemically closely related derivatives of the phenolic groups found in lignin, allowing the numerous naturally occurring tannins to be sustainably obtained as biobased oligo-/polyhydroxyphenols as plant extracts. Polyphenol derivatives of condensed or hydrolyzable tannins, such as tannic acid in particular, can be chemically converted into conjugated carbonyls through oxidation. The resulting conjugated carbonyls exhibited pronounced reactivity with various thiols. On this basis, two-component adhesive mixtures of oxidatively activated tannin and commercially available multithiols could be produced and bonded to various substrates.

Further explanations of the invention are given below, although these do not necessarily limit the generality of the invention: In one embodiment, the two-component adhesive system can be formed comprising a first two-component adhesive system component A and a second two-component adhesive system component B, wherein these react with each other and form covalent bonds and/or a covalently bridged network. The two-component adhesive system component A comprises bio-based conjugated dicarbonyls as a reactive component, wherein the bio-based conjugated dicarbonyl is an oxidation product of lignin or highly oxidized lignin and/or tannins, and the conjugated dicarbonyls are formed as reactive quinone groups. And the two-component adhesive system component B comprises nucleophilic hardeners as a reactive component.

The bio-based share of renewable or bio-based carbon in two-component adhesive system component A can be greater than 30% or greater than 50% or greater than 90% or 100%.

This means that component A can be formed in particular on the basis of renewable raw materials with phenolic structural elements and further preferably with a renewable carbon content of >30% or preferably >50%, particularly preferably >90% or even 100%, whereby mixtures with conventional bio-based / non-bio-based reactive resins are also possible.

In particular, the two-component adhesive system component A can have 0.3 to 5 mmol/g conjugated dicarbonyl groups or 0.8 to 3.8 mmol/g conjugated dicarbonyl groups or 1.3 to 3.0 mmol/g conjugated dicarbonyl groups.

The nucleophilic hardeners of the two-component adhesive system component B can then comprise or be polythiols and/or polyamines.

In particular, the two-component adhesive system component B can be a branched or unbranched molecule or polymer or any mixtures of these substances with at least two, especially three to four nucleophilic functionalities, but preferably thiols, also with reactive group distributions and partly monofunctional impurities.

Mixtures of substances with at least two to ten, especially three to four functionalities such as amines, but preferably thiols, also with reactive group distributions and partly monofunctional impurities are possible.

The two-component adhesive system component B can be an aliphatic molecule or predominantly composed of aliphatic polyester or polyether structures and/or have molecular weights of 140-1400 g/mol and/or have polarities with calculated c(logP) coefficients between 1.84-12.43. For the calculation, crosslinker component B was calculated as a substance whose thiol functionalities were formally present in the form of ethyl thioether to more clearly describe the polarity of the crosslinker core structure. The calculation program Chem3D (PerkinElmer version 20.1.1) was used, which calculates the c(logP) coefficients based on MOSES (MOSES, by Molecular Networks GmbH. Polykovskiy, D. et al. Frontiers in pharmacology 2020, 11, 565644).

In addition, the two-component adhesive system component B can be formed as branched or unbranched polymers with at least two thiol groups or as hydrophilic, thiol group-bearing polyethylene oxide (ETTMP) or as hydrophobic thiol group-bearing poly(E-caprolactone) (PCL4MP) or as chemically reduced thiol group-bearing proteins.

The two-component adhesive system component A and/or the two-component adhesive system component B of the two-component adhesive system can comprise further individual components or combinations of components selected from: fillers, flame retardants, impact modifiers, silane-modified polymers as further co-crosslinkers, epoxies as further co-crosslinkers, methacrylates as further co-crosslinkers, acrylates as further co-crosslinkers, vinyl ethers as further co-crosslinkers, styrene derivatives as further co-crosslinkers, reactive diluents, curing catalysts, rheology aids/rheology-modifying fillers, rheology additives, exothermic-controlling fillers, hydrolysis stability-improving fillers, wetting agents, dyes.

The conjugated dicarbonyl groups of component A can also preferentially react with thiol groups of component B when the two components are combined to form thiol-catechol linkages or even partially to form disulfide linkages.

The formation of thiol-catechol connectivity (TCC) linkages or even partial disulfide linkages is achieved when combined via a 1,4-Michael addition.

During the formation of linkages such as thiol-catechol, phenolic hydroxyl groups are generated from the dicarbonyl groups by the reaction, which remain available (not consumed) in the lignin/tannin for adhesive interactions.

Components A and B can be combined to form the two-component adhesive system in such a way that 30 to 60 percent by weight (not including additives and fillers) of component A is present in the mixture. Component A itself may also contain additional fillers or additives. On average, this means that up to 70 percent by weight of fillers or supplementary materials are likely to be present in the overall adhesive system. In corresponding embodiments, the conjugated dicarbonyl groups and/or reactive quinone groups of the first component A can also be made detectable by means of IR spectroscopy by a vibration at A = 1660 cm ¹ .

The lignin used can be primarily but not exclusively softwood lignin.

A possible process according to the invention for producing the illustrated two-component adhesive system with lignin (or alternatively tannin can be used instead of lignin) comprises the following steps using a concrete example (not necessarily limiting the entirety of the invention): A Production of component A;

A 1 Lignin preparation process as direct oxidation (nDoxL process):

A 1.1 Provision of lignin;

A 1.2 Fractionation of lignin in acetone;

A 1.3 Oxidation of fractionated lignin with sodium periodate;

A 1.4 Processing of activated lignin; or

A 2 Lignin preparation process as oxidation with prior demethylation (DoxL process):

A 2.1 Provision of lignin;

A 2.2 Fractionation of lignin in acetone;

A 2.3 Demethylation of the fractionated lignin with reagents that cleave methyl ether bonds of the aromatic subunits;

A 2.4 Oxidation of fractionated, demethylated lignin with sodium periodate;

A 2.5 Processing of activated lignin;

B Provision of component B;

C Combination of component A and component B, whereby the conjugated dicarbonyl groups of component A react with the thiol groups of component B when the two components are combined to form thiol-catechol linkages.

The oxidation in step A 1.3 or A 2.4 can preferably be carried out in the presence of 30% by weight of sodium periodate.

Reagents that cleave methyl ether bonds of the aromatic subunits include hydroiodic acid (Hl), hydrogen bromide (H Br) or, in particular, iodocyclohexane (ICH).

The activated lignins react under moderate conditions with multithiols under

Consumption of quinones and thiols and cure to TCC adhesives. The adhesives can then be applied under various conditions, tolerate salt water and bond different substrates such as aluminum, steel, wood and/or stone with sometimes surprising adhesive strengths.

The resulting adhesives are sustainable and can be used, for example, for coral reef reconstitution.

The lignin preparation process can be conducted as a direct oxidation or as a multi-stage process with prior demethylation. Direct oxidation requires inexpensive chemicals and allows for scale-up.

Lignin preparation process as oxidation with prior demethylation (DoxL process)

The following are examples of the production of the two-component adhesive system using the lignin preparation process as oxidation with prior demethylation. These examples are intended to illustrate the invention and are not to be construed as limiting.

Material synthesis

Fractionation of lignin

In the first step, a lignin, in this example, softwood lignin, is fractionated. The softwood lignin is dissolved in acetone (0.1 g/mL). After stirring for 2 hours at room temperature, the lignin is filtered, and the acetone is removed under reduced pressure. The resulting crystalline solid is dried under high vacuum (10 ^-3 mbar) for final drying. This allows a yield of 80% of the acetone extract to be achieved.

By extraction from acetone, insoluble parts such as ash, residual saccharides and cross-linked lignins can be removed, thus enabling better processing of the lignin in subsequent modification.

Demethylation of fractionated lignin

The demethylation of fractionated lignin is carried out according to the modified method of Sawamura et al. Fractionated lignin (10 g) is weighed into a two-necked round-bottom flask, dissolved in DMF (150 mL), and stirred at room temperature until homogeneous. Iodocyclohexane is passed through a small column of alumina to remove the inhibitor before use and then added dropwise to the lignin dispersion. A reflux condenser and a degasser containing aqueous ammonia are set up to quench the evolved gas and by-products. The reaction mixture is heated in an oil bath at 140 °C for 6 hours and then cooled to room temperature. The reaction mixture is washed with n-hexane and poured into 1 L of water. The resulting precipitate is The product was then collected by filtration, washed several times with water, and freeze-dried to obtain the demethylated product. The yield of the obtained mass was 85%.

Oxidation of fractionated and demethylated lignin

Dried, ethylated lignin (5 g) is dissolved in DMF (30 mL) and sodium acetate buffer (3 mL, pH 5, 0.1 M) is added. NaCl (1.5 g, 30 wt%) is added, and the homogeneous mixture is stirred at room temperature for 2 h.

Processing of activated lignin (demethylated oxidized lignin-DoxL)

The reaction mixture is then poured into 1 L of water. The resulting precipitate is filtered off, washed several times with water, and freeze-dried to obtain the oxidized product. The yield of the obtained mass is 81%.

A scale-up of the activated lignin preparation to up to 50 g per batch is easily possible. The acetone fractionation of lignin is already feasible on a sufficiently large scale of 50 g per batch. For demethylation, fractionated lignin (50 g) is weighed into a two-necked round-bottom flask, dissolved in DMF (750 mL), and stirred at room temperature until homogeneous conditions are achieved. Iodocyclohexane (ICH; 0.3 mol, 315.5 g, 194 mL) is passed through a small column of alumina to remove the inhibitor prior to use and then added dropwise to the lignin dispersion. A reflux condenser and a reflux chamber with aqueous ammonia are set to quench the evolved gases and by-products. The reaction mixture is then heated in an oil bath at 140 °C for 6 hours and then cooled to room temperature. The reaction mixture is washed with n-hexane and poured into 5 L of water. The resulting precipitate is separated by filtration, washed several times with water, and freeze-dried to obtain the demethylated product. The demethylation reaction is linearly scalable. The recovered mass yield is approximately 82%.

Oxidation in larger quantities of more than 10 g of fractionated and demethylated lignin leads to a noticeable thickening of the extraction mixture, which can be circumvented by diluting the mixture. Dried demethylated lignin (50 g) is dissolved in DMF (500 mL), and sodium acetate buffer (30 mL, pH 5, 0.1 M) is added. NaI (15 g, 30 wt%) is added, and the homogeneous mixture is stirred for 2 hours at room temperature. The reaction mixture is poured into 10 L of water. The resulting precipitate is then collected by centrifugation, washed several times with water, and freeze-dried to obtain the oxidized product. The recovered mass yield is approximately 79%. Dried, ethylated lignin (50 g) was dissolved in DMF (500 mL), and sodium acetate buffer (30 mL, pH 5, 0.1 M) was added. NaI (15 g, 30 wt%) was added, and the homogeneous mixture was stirred at room temperature for 2 hours. The reaction mixture was poured into 10 L of water.

Initially, the acetone fractionation of lignin was already performed on a sufficiently large scale of 50 g per batch. For demethylation, fractionated lignin (50 g) was weighed into a two-necked round-bottom flask, dissolved in DMF (750 mL), and stirred at room temperature until homogeneous conditions were achieved. Iodocyclohexane (ICH; 0.3 mol, 315.5 g, 194 mL) was passed through a small column of alumina to remove the inhibitor before use and then added dropwise to the lignin dispersion. A reflux condenser and a reflux chamber with aqueous ammonia were set up to quench the evolved gases and byproducts. The reaction mixture was heated to 140 °C in an oil bath for 6 hours and then cooled to room temperature. The reaction mixture was washed with n-hexane and poured into 5 L of water. The resulting precipitate was separated by filtration, washed several times with water, and freeze-dried to obtain the demethylated product. The demethylation reaction is linearly scalable. The recovered mass yield is 82%.

Oxidation in larger quantities of more than 10 g leads to a noticeable thickening of the extraction mixture, which can be avoided by diluting the mixture. Scale-up to up to 50 g per batch is possible without any problems.

Lignin preparation process as direct oxidation (nDoxL process)

In order to avoid the costly ICH used during demethylation and thus also to avoid the production of toxic methyl iodide as a by-product, the activation of the lignin was reduced to a modification step, the direct oxidation.

The ortho-methoxyphenols of lignin are to be oxidized to quinones.

The following are examples of the preparation of the two-component adhesive using the lignin preparation process as direct oxidation. These examples are intended to illustrate the invention and are not to be construed as limiting.

Material synthesis

Fractionation of lignin

In the first step, a lignin, in this example a softwood lignin, is fractionated. The softwood lignin is dissolved in acetone (0.1 g/mL). After 2 hours of stirring at At room temperature, the lignin is filtered, and the acetone is removed under reduced pressure. The resulting crystalline solid is dried under high vacuum (10' ³ mbar) for final drying. This allows a yield of 80% of the acetone extract to be achieved.

By extraction from acetone, insoluble parts such as ash, residual saccharides and cross-linked lignins can be removed, thus enabling better processing of the lignin in subsequent modification.

Oxidation of fractionated lignin

The fractionated lignin is pre-dissolved in dimethylformamide (DMF) (0.17 g/mL) for oxidation. Sodium periodate (30 wt%) is suspended in acetate buffer (0.1 M, pH 5) and DMF. The lignin solution is added to the periodate suspension and stirred for 2 hours at a temperature between 0 °C and room temperature, preferably 0 °C. Oxidation at 0 °C results in fewer crosslinks in the lignin and appears to be a more gentle method for the substrate.

Processing of activated lignin (non-demethylated oxidized lignin-nDoxL)

The reaction mixture is then precipitated in ultrapure water (1:20 ratio of solvent to water) and filtered. For purification, the solid is repeatedly washed with ultrapure water until the excess periodate is removed. The conductivity of the filtrate can be measured for confirmation. Conductivity measurements have shown that the conductivity of the reaction mixture can be reduced from approximately 700 pS/cm to approximately 100 pS/cm by washing once, and that excessive washing achieves a conductivity below 2 pS/cm (cf. distilled water 1-2 pS/cm). Instead of filtration, it is also possible to separate the resulting precipitate by centrifugation and wash it several times with ultrapure water. The oxidized product is then freeze-dried.

For oxidation, it is possible to vary both the batch size and the periodate concentration. Production of up to 50 g of activated lignin per batch is feasible. It should be noted that fractionation with acetone is typically performed on a 50 g scale and can be scaled up linearly. For larger oxidation batches of over 10 g, the same batch concentration leads more quickly to crosslinking reactions, which manifest themselves in the form of gelation. Such gelation can be avoided by reducing the initial concentration.

An established method for analyzing lignins is derivatization via acetylation of the OH groups, so that aliphatic and phenolic acetates can be distinguished by ¹ H NMR spectroscopy. Furthermore, by using an internal standard and by standardizing the weighed mass, the content of Methoxy groups, aliphatic and phenolic OAc functionalities can be determined. This allows the oxidation stoichiometry and the number of periodate equivalents to be used to be calculated. Approximately 60 mg of lignin are dissolved in 1 mL of pyridine and 1 mL of acetic anhydride is added. The mixture is stirred overnight, precipitated in 40 mL of ultrapure water, centrifuged, and lyophilized. The residue is completely dissolved in 0.5 mL of CDCl and subjected to ¹ H and HSQC NMR spectroscopy. The integral ranges used are δ=4.2–3.6 (OMe), 25–2.17 (Ph-OAc), and 2.17–1.8 (Al-OAc) ppm.

Below is a further description of a simplified activation process:

Catecholation of (technical or fractionated) lignin

The catecholation of, for example, fractionated lignin is carried out using a modified method by Zhao et al. For this, see S. Zhao, X. Huang, A. J. Whelton, M. M. Abu-Omar, ACS Sustain. Chem. Eng. 2018, 6, 10628-10636. For this, lignin (20 g) and catechol (40 g) are mixed in a round-bottom flask and melted at 115 °C in a laboratory experiment under inert gas. Sulfuric acid (98%, 2.1 mL) is added to the liquid mixture, after which the apparatus, with the drying tube attached, is heated in an oil bath at 115 °C for 2 hours with stirring. After cooling to room temperature, the viscous reaction mixture is dissolved in 200 mL of acetone and precipitated in 2 L of an aqueous solution of sodium chloride (60 g, 3 wt%) and sodium ascorbate (40 g, 2 wt%). The resulting precipitate is then filtered off, washed several times with water, and dried to obtain the catecholated product. The yield of the obtained mass is typically greater than 95%, with a phenolic OH value of 6.1–6.6 mmol/g being obtained. The reaction is based on simple, inexpensive basic chemicals; the melt viscosity remains within an unproblematic range during the reaction, and the process steps are technically standard, so that scale-up in the laboratory to 250 g is possible and further industrial scaling appears unproblematic.

Oxidation of catecholated lignin

Catecholated lignin (30 g) is dissolved in DMF (180 mL) and potassium phosphate buffer (18 mL, pH 6.5, 0.1 M) is added. NaI (45 g, 150 wt%) is added, and the homogeneous mixture is stirred for 1 h at 0 °C.

Processing of activated lignin (catecholated oxidized lignin-CoxL, lignin with conjugated biscarbonyls)

The oxidation product is precipitated in water. The resulting precipitate is separated (filtration/centrifugation), washed with water and dried to obtain the oxidized product. The yield of the obtained mass is between 82-91%, with a typical content of quinone groups of 1.0-1.5 mmol/g and, in particular, up to 5 mmol/g.

Bonding experiments

Bonding experiments in the form of tensile shear tests demonstrated the suitability of the activated lignin-containing component A as a component for the two-component adhesive according to the invention. For this purpose, aluminum substrates were bonded with the two-component adhesive. Two different liquid multithiols bearing either hydrophilic polyethylene glycol (ETTMP) or hydrophobic poly(Σ-caprolactone) segments (PCL4MP) were used as component B for the exemplary and non-limiting bonding experiments. The ETTMP used is a trivalent thiol, more precisely an ethoxylated trimethylpropane tri(3-mercaptopropionate) with a molecular weight of M _n =1300 g/mol, and the PCL4MP used is a tetravalent thiol, more precisely a polycaprolactone pentaerythritol tetra(3-mercaptopropionate) with a molecular weight of M _n =1350 g/mol.

Thiol groups are required for the Michael-type polyaddition with quinone functionalities to form TCCs, but can also react with quinones to form disulfides, which also contribute to network formation. The adhesives according to the invention exhibit a mixed network of two different types of linkages. On the one hand, there are covalent thioether bonds, which are formed via a Michael addition between thiol and ortho-quinone, and on the other hand, primarily dynamic disulfide bridges, which are formed oxidatively.

Since the reaction between the components involves a solvent-free polymerization, complete homogenization of the powdered lignin with the liquid component multithiol is essential, and thorough mixing is required. The resulting paste can be easily applied and spread on pretreated substrates (degreased and cleaned, for example, with acetone and UV/ozone).

In the experiments, the substrates were clamped together after application of the paste with an overlap area of 40 to 70 mm ² and cured overnight at 60 °C and a change in the physical properties was observed, which led to cured materials with rubber-elastic components.

The physical properties are examined using shear-tensile tests. The bond strength is calculated by normalizing the maximum measured force (N) to the respective bond area (mm). At room temperature, only an increase in viscosity occurred, but no curing occurred.

The hydrophobic PCL4MP can stabilize the resin network in the two-component adhesive system, making it particularly advantageous for underwater applications. Tests of this system with curing under marine conditions have shown that comparable tensile shear strengths can be achieved as with dry bonding and dry curing, making the material platform suitable and applicable for underwater bonding.

Although the hydrophilic ETTMP can improve the overall wettability of the adhesive, water resistance in this system is compromised by a lower swelling capacity.

Resistance to organic solvents

Using so-called "leaching experiments," it is possible to gain indirect insight into the network structure of two-component adhesive systems, which are otherwise difficult to elucidate spectroscopically. For this purpose, the cured two-component adhesive systems are examined for their swelling behavior in both organic solvents and water, both with and without a reducing agent, which can irreversibly cleave disulfides.

The experiments were conducted using two adhesive systems with a mass ratio of lignin 4o/thiol 6o. PCL 4MP and ETTMP were selected as the thiol components. The adhesive systems were mixed, fully cured, and frozen using liquid nitrogen. They were ground into small pieces and transferred to acetone or tetrahydrofuran (THF). Tributylphosphine was added to some of the samples, as this can irreversibly reduce disulfides to thiols. The samples were extracted for 2 weeks at room temperature, and the supernatant was separated from the remaining solid.

The exemplary two-component adhesive systems used show overall moderate resistance to the organic solvents tested.

In ¹ H-NMR studies of the extracts, it can also be shown that predominantly the multithiol, which is readily soluble in acetone and THF, is dissolved from the adhesive system.

The invention is described below with reference to the accompanying figures in the

The description of the figures is intended to illustrate the invention and is not to be considered limiting. They show: Figure 1 shows an exemplary schematic representation of the activation process of softwood lignin to non-demethylated, oxidized lignin (nDoxL) and the subsequent reaction with two different polar multithiols as two-component adhesive systems;

Figure 2 shows an exemplary schematic representation of the bio-inspired concept (Fig. 2a) by combining the toughening mechanisms of wood through lignin (left) with key adhesive functions from the bio-adhesion apparatus of marine mussels (right) in a lignin-based two-component adhesive system and an exemplary schematic representation of the activation process of softwood lignin to demethylated, oxidized lignin (DoxL) as component A and two different polar multithiols as component B (Fig. 2b) in the lignin-based two-component adhesive system;

Figure 3 shows a schematic representation of the possible oxidative reactions of fractionated lignin with sodium periodate in the nDoxL process;

Figure 4 shows a comparison of the HSQC spectra of lignin before and after oxidation with sodium periodate in the nDoxL process;

Figure 5 shows a summary of the results for the quantitative determination of methoxy groups, aliphatic and phenolic acetates based on ¹ H NMR measurements after acetylation of the hydroxy groups in the nDoxL process;

Figure 6 shows the results of the FTIR analysis of different oxidation sets in the nDoxL process

Figure 7 shows the superposition of the apparent mass distributions of different nDoxL batches in DMSO+0.075 mM NaNOs;

Figure 8 shows an overview of the proportions of periodate used to optimize the oxidation reaction of lignin in the nDoxL process in direct relation to the reaction with monothiols;

Figure 9 shows an exemplary photo series to illustrate the processing steps after oxidation in the nDoxL process;

Figure 10 DSC diagrams of uncured nDoxL ₄ o/ETTMP ₆ o samples (Fig. 10a) and nDoxL4o/PCL4MPeo samples (Fig. 10b);

Figure 11 shows a tabular overview of the proportions of periodate used to investigate the optimal degree of activation of lignin via the nDoxL process in direct relation to the tensile shear strength; Figure 12 shows an exemplary comparison of the force-displacement diagrams of lignin adhesives with 30 wt% NalO4 oxidation and 5 wt% NalO4 oxidation and ETTMP (n DoxL process);

Figure 13 shows an exemplary overview of the tensile shear strengths of two-component adhesive systems according to the invention (nDoxL process) with ETTMP and PCL4MP in comparison;

Figure 14 shows an exemplary tabular overview of the tensile shear strengths of the two-component adhesive systems with nDoxL (oxidized with 30 wt% _NaI04 ) and multithiols in different lignin/thiol ratios;

Figure 15 shows thermogravimetry (TGA) results of the exemplary

Systems nDoxL ₄ o/ETTMP ₆ o and nDoxL ₄ o/PCL4MP6o;

Figure 16 shows a comparison of the gravimetric analysis of exemplary cured two-component adhesive systems after contact with different organic solvents by showing the polymer content for each two-component adhesive system;

Figure 17 shows a schematic pictorial representation of the two-component adhesive system manufacturing process (Fig. 17a) and a schematic pictorial representation of the procedure for shear tensile experiments with the two-component adhesive system according to the invention (Fig. 17b);

Figure 18 shows a representation of the chemical transformation steps for lignin activation on a technically available softwood kraft lignin via the DoxL process (Fig. 18a), a representation of the results of the hydroxyl pattern analysis of fractionated and demethylated lignin using ³¹ P-NMR (Fig. 18b), a representation of the results of the ¹ H-NMR analysis comparing the fractionated lignin before and after demethylation (Fig. 18c), a representation of the results of a GPC analysis showing the molecular weight distribution of the different lignin products (Fig. 18d) and a representation of FTIR spectra confirming the chemical transformations in the heterogeneous lignin network that lead to quinone structures after oxidation (Fig. 18e);

Figure 19 shows representative FTIR spectra comparing activated lignin (DoxL process) in exemplary two-component adhesive systems in the uncured state (nc) and after curing DoxL ₅ o/ETTMP ₅ o (Fig. 19a) and DoxL ₅ o/PCL4MP ₅ o (Fig. 19b); Figure 20 shows the dry mechanical bonding properties of the two-component lignin-based adhesive systems (DoxL process); overlap shear tests as force-resistance curves with a representative example for DoxL/ETTMP (Fig. 20a) and DoxL/PCL4MP (Fig. 20b) at different weight ratios of DoxL/Thiol and images of the sample seam of the shear bond tests via adhesive and cohesive mixture for D0XL55/ETTMP45 (Fig. 20c) and DoxL ₅ 5/PCL4MP ₄ 5 (Fig. 20d);

Figure 21 shows a tabular overview of the dry adhesion properties of two-component lignin-based adhesive systems (DoxL process);

Figure 22 shows a representation of underwater curing results of the exemplary two-component adhesive system with lignin (DoxL) and PLC4MP with results for overlapping shear tests (Fig. 22 a-c) and an image of the lignin/PCL4MP seam after underwater application and curing for the ratios 40/60, 50/50 and 60/40 wt% lignin/PCL4MP (Fig. 22d).

Figure 23 various exemplary representations of the properties of the two-component adhesive system according to the invention (activated lignin via DoxL process) after application;

Figure 24 is a schematic representation of the leaching process as a means of determining the solvent resistance of the adhesive systems according to the invention and

Figure 25 shows a representation of polarity.

Figure 1 shows an exemplary schematic representation of the activation process of softwood lignin to non-demethylated, oxidized lignin (nDoxL) and its subsequent reaction with two different polar multithiols as two-component adhesive systems. An oxidation reaction functionalizes ortho-methoxyphenols to active quinones (nDoxL), which lead to thiol-catechol bond formation (TCC) via 1,4-Michael addition between thiol (component B) and quinone (component A). Through interaction of the TCC adhesive groups with polar surfaces and combined with the intrinsic cohesion of the lignin, the TCC polymers form excellent adhesives.

In the illustration, the components ETTMP and PCL4MP are used as examples of multithiols.

Fig. 2 shows an exemplary schematic representation of the bio-inspired concept (Fig. 2a) by combining the toughening mechanisms of wood by lignin (left) with adhesive key functions from the bio-adhesion apparatus of marine mussels (right) in a lignin-based two-component adhesive system and an exemplary schematic representation of the activation process of softwood lignin to demethylated, oxidized lignin (DoxL) as component A and two different polar multithiols as component B (Fig. 2b) in the lignin-based two-component adhesive system.

In the illustration, the components ETTMP and PCL4MP are used as examples of multithiols.

In the experiments shown in the following figures, the softwood lignin BioPivs 300 from UPM was used as an example. The use of this softwood lignin is to be considered exemplary and not limiting.

Figure 3 shows a representation of the possible oxidative reactions of fractionated lignin with sodium periodate. Not only are ortho-methoxyphenols converted to quinones, but certain ether linkages are also oxidized by sodium periodate. Hydroxy groups can be highly oxidized to aldehydes and ketones, respectively. ß-O-4 and ß-O-5 bridges can be oxidatively cleaved.

Figure 4 shows a comparison of the HSQC spectra of lignin before and after oxidation with sodium periodate, measured in DMSO-dß at 500 MHz. It can be seen that after oxidation, some characteristic resonances of the ether bridges, such as ß-O-5' linkages and ß-O-5 linkages, which were present in the pre-fractionated lignin, have disappeared. The cleavage of these ether bonds can open the lignin network, which can favor the reaction with multi-thiols.

Figure 5 shows a summary of the results for the quantitative determination of methoxy groups, aliphatic, and phenolic acetates based on ¹ H NMR measurements after acetylation of the hydroxy groups. Figure 5a shows the methoxy content, Figure 5b the ratio of methoxy content to total hydroxy group content, and Figure 5c the ratio of phenolic and aliphatic hydroxy groups as a function of periodate equivalents. Fractionated and demethylated lignin are shown as references.

The consumption of methoxy groups can be quantified using ¹ H NMR measurements. Figure 5a clearly shows that the total OMe consumption of the respective nDoxL batches (directly oxidized with 0.3 to 2 equivalents) is significantly higher than the consumption during demethylation in the DoxL route. After oxidation with 0.3 equivalents of periodate, depending on the reaction temperature at 0 °C or room temperature, a reduction in the OMe content of 90% can be determined. For batches with higher periodate equivalents, a consumption of even 95% could be determined in experiments. However, it is important to note that the content of phenolic and aliphatic hydroxy groups is significantly reduced (Fig. 5b, 5c). This becomes particularly evident when the content of total OH groups is compared with the MeOH groups. The OMe/OH total ratios of the oxidized samples are of a similar magnitude, ranging from 0.42 to 0.73 (compare before demethylation OMe/OH total = 0.67 and after demethylation OMe/OH _total = 0.29). The Ph-OH/Ali-OH ratios in the oxidized samples show similar contents of phenolic and aliphatic OH groups and indicate no significant differences with regard to the periodate equivalents used. In comparison, an excess of phenolic OH can be observed in demethylation reactions.

In addition, Fig. 6 shows the results of the FTIR analysis of different oxidation sets. Fig. 6a shows the FTIR spectra of fractionated lignin and lignins oxidized at room temperature and 0 °C with 0.3 equivalents of periodate per o-methoxyphenol. Fig. 6b shows the superposition of the FTIR spectra of various oxidized lignins (0.3 to 2 equivalents per o-methoxyphenol at 0 °C) measured via ATR. The increase in the intensity of the vibrational band at v=1660 cm ^-1 indicates the presence of conjugated ketones (condition: 66.6 M fractionated lignin in DMF/aqueous sodium acetate buffer (0.1 M, pH 5), room temperature or 0 °C, 2 h).

Fig. 6b shows that increasing the amount of periodate also significantly increases the intensity of the C=O vibrational band at v=1660 cm ^-1 . Sodium periodate itself has a prominent band at v=840 cm ^-1 and is no longer visible in all four spectra, indicating proper processing of the reaction mixture. CH out-of-plane deformation vibrations of aromatics appear as weak bands at v=853 cm ^-1 and v=815 cm ^-1 , which are already present in the fractionated lignin. It is striking that oxidation reactions at 0 °C lead to an increase in several vibrational bands v=1740 cm ^-1 , v=1367 cm ^-1 and v=1215 cm ^-1 (Figs. 6a and 6b).

Based on the literature, the intense band at v=1740 cm ^-1 can be identified as a carbonyl absorption, the band at v=1215 cm- ¹ as a CO single bond of a phenol or an ether, and the band at v=1367 cm-1 as an OH deformation vibration. The band at v=1740 cm ^-1 is also a C=O resonance, which occurs in aldehydes. The results show that a kinetically controlled product is preferentially formed at 0 °C, which, based on the assigned vibrational bands, is aldehydes and ketones. Upon increasing the periodate concentration, a simultaneous reduction of the three band intensities mentioned can be observed, confirming the hypothesis that this is a kinetic product and that it is only formed at low temperatures and low periodate concentrations. The reaction to form quinones therefore appears to be thermodynamically controlled. Oxidation at room temperature shows the named bands are not present. In Fig. 6a, this is evident from a much more pronounced band at v=1660 cm/ ^s , which the product exhibits at room temperature with the same amount of periodate as the product at 0 °C.

Fig. 7 shows the overlay of the apparent mass distributions of different nDoxL batches in DMSO+0.075 mM NaNO3 with a listing of the apparent, number-average, and weight-average molecular weights as well as the polydispersities of the lignin samples fractionated with acetone and activated with 1:0.3 eq. NaIC, 1:1 eq. NaICU, 1:1.5 eq. NaICU, and 1:2 eq. NaICU at 0 °C (Fig. 7a). Fig. 7b shows the overlay of the apparent, number-average, and weight-average molecular weights as well as the polydispersities of lignin samples activated with 1:0.3 eq. NaICU at room temperature and 0 °C.

Figure 7 shows that with lowering the temperature and increasing sodium periodate content, the peak shoulder decreases in intensity. Presumably, an increased periodate concentration shifts the equilibrium strongly toward oxidation, without the quinones themselves reverting to catechols after being "consumed" by oxidation. A lower amount of catechols presumably also reduces cross-linking, which would subsequently lead to a bimodal distribution. Oxidation at 0 °C appears to be a gentler method, which also results in fewer cross-links, which is reflected in the overall slightly narrower mass distribution.

Fig. 8 provides an overview of the proportions of periodate used to optimize the oxidation reaction of lignin in the nDoxL process in direct relation to the reaction with monothiols, here ethanethiol, under the following reaction conditions: 66.6 mM fractionated lignin in DMF/aqueous sodium acetate buffer (0.1 M, pH 5), room temperature or 0 °C, 2 h and 0.8 M activated lignin in N-methyl-2-pyrrolidone (NMP), 40 °C or 80 °C, 2 h or 16 h).

The model reaction is carried out to confirm the formation of TCC groups via Michael addition with ethanethiol. The choice of ethanethiol is justified by the fact that the peak of the methyl group in the ¹ H NMR spectrum stands out from the other lignin peaks and is easily integrated. The aforementioned methyl peak broadens considerably after the reaction with the activated lignin. Due to the poor solubility of the activated lignin, the reactions take place in NMP and require a significant excess of thiols per ortho-quinone unit. The Michael-type addition of thiols with ortho-quinones has already been described in numerous publications and proceeds at low molecular weight at room temperature in a few minutes.

Here, the thiol addition takes place with a lignin macropolymer, which is present as a nanogel. To ensure that the accessibility to the active ortho-quinone groups is not hindered, the The temperature is increased, the reaction time is extended, and excess thiols are added. The lignin-TCC adduct is obtained by initial precipitation in ether and then in ultrapure water. Any ethanethiol remaining in the system can be removed under high vacuum due to the high vapor pressure. Using an internal standard, the TCC content can be quantitatively determined by both ¹ H NMR and ³¹ P NMR.

It can be seen that at the standardized 30 wt% periodate, which stoichiometrically corresponds to 0.3 equivalents per MeO unit of the lignin, and under the optimized conditions of 0 °C for 2 hours, a maximum of 4 thiol linkages per molecule are formed. Increasing the periodate equivalents to 2 equivalents per oxidizable unit increases the number of thiol linkages to approximately e per lignin molecule. The TCO calculations refer to mean values averaged over the M _w of the various oxidized lignins. Important here is the broad mass distribution of the lignin, whereby molecular weights higher than M _w tend to form more TCC groups, but statistically account for the smaller proportion.

Fig. 9 shows an example of a series of photos illustrating the processing steps after oxidation in the nDoxL process. A 10-liter bucket and 10 liters of water were used to precipitate 50 g of lignin, and the mixture was then distributed among six centrifuge cups (Fig. 9a). After taring the cups (Fig. 9b), they can be placed in a centrifuge (Fig. 9c). A further 10 liters of water are used for washing. Since the oxidized lignin is a finely powdered solid, centrifugation was used instead of conventional filtration to separate it. Centrifuges with a capacity of 36 liters can thus process 20 g of oxidation batches within 3 to 4 hours.

Additionally, Figure 10 shows dynamic difference thermal analysis (DSC) diagrams of uncured nDoxL ₄ o/ETTMP ₆ o samples (Fig. 10a) and nDoxL ₄ o/PCL4MP ₆ o samples (Fig. 10b). The onset temperature shown is the extrapolated reaction start, which is 28 °C for nDoxL ₄₀ /ETTMP ₆ o and 50 °C for nDoxL ₄₀ /PCL4MP ₆ o. The curing temperature was set to 60 °C accordingly.

Figure 11 shows a tabular overview of the periodate concentrations used to investigate the optimal degree of lignin activation via the nDoxL process (condition: 66.6 mM fractionated lignin in DMF/aqueous sodium acetate buffer (0.1 M, pH 5), room temperature, 2 h). Different sodium periodate concentrations ranging from 5 to 50 wt. % were used under the same reaction conditions. After oxidation, the lignins were purified by extraction with ultrapure water, and excess periodate was removed. The produced lignin materials were investigated in a two-component bonding with both ETTMP and PCL4MP and the tensile shear strengths were determined.

Fig. 12 shows an exemplary comparison of the force-displacement diagrams of lignin adhesives with 30 wt% NalC oxidation (left) and 5 wt% (right) NalO4 oxidation and ETTMP (conditions: nDoxL process; mass ratio lignin/thiol - 1/1; curing at 60 °C, 16 h).

A clear difference in the curve shape up to fracture is evident. In the sample oxidized with 30 wt% sodium periodate, an almost linear curve progression and a sharp-edged break at maximum force are evident, which can be attributed to complete curing of the material. The curve progression of the sample oxidized with 5 wt% periodate, on the other hand, indicates an incompletely cured substance. Examination of the interfaces also shows that the material in the sample oxidized with 5 wt% periodate is still sticky and lubricating.

Tensile shear tests showed that oxidation with 30 wt% sodium periodate leads to the highest mean tensile shear strength, both with the ETTMP used as an example and with PCL4MP.

Fig. 13 shows an exemplary overview of the tensile shear strengths of two-component adhesive systems according to the invention (nDoxL process) with ETTMP (left) and PCL4MP (right) in comparison. Various proportions of oxidizing agent were used to activate the lignin via oxidation with NaI in the nDoxL process. The purified lignin component was tested via two-component bonding with a 1:1 lignin/thiol ratio using tensile shear experiments. It can be seen that the optimum bond strength is at 30 wt.% NaI for the direct oxidation of fractionated lignin. A direct comparison of the bond strengths achieved in the figure shows that the adhesive systems with PCL4MP exhibit significantly poorer tensile shear strengths than those with ETTMP. Maintaining the initial 1/1 mass ratio between lignin and thiol, it has been shown that lignin is significantly less miscible with higher-viscosity PCL4MP. Under constant oxidation conditions of two hours reaction time at room temperature, a minimum addition of 20 wt.% NaI4 is necessary to achieve good bond strengths of approximately 10 MPa in the lignin/ETTMP adhesive system.

In addition, Fig. 14 provides an exemplary tabular overview of the tensile shear strengths of the two-component adhesive systems with nDoxL (oxidized with 30 wt.% NaI4) and multithiols in different lignin/thiol ratios. The tear force is standardized to an adhesive surface of 40 to 70 mm ² . It has been found that complete homogenization of the two components in the adhesive system is necessary and that this is no longer possible in the lignin/PCL4MP system if the lignin content is too high. Nevertheless, the two-component adhesive systems according to the invention possess a high degree of modularity and tunability, as they are resin-like bulk adhesives. By varying the lignin/thiol ratio, different tensile shear strengths can be achieved. When using the polar ETTMP, a mixing window between 40 and 60 wt.% activated lignin results; below this threshold, the viscosity of the mixture is too low to be applied. Above 60 wt.%, the homogeneity of the two-component formulation mixture has proven to be no longer achievable. It has also been shown that the higher viscosity and non-polar PCL4MP is less compatible with nDoxL, resulting in a narrower blend range of 30 to 50 wt.% activated lignin. The best applicability and good tensile shear strengths are achieved at a lignin mass content of 40 wt.%.

Furthermore, Fig. 15 shows the thermogravimetry (TGA) results of the exemplary systems nDoxL ₄ o/ETTMP ₆ o and nDoxL ₄ o/PCL4MPeo. A plot of temperature versus mass is shown. The lignin materials with different multithiol components show no clear differences in thermal decomposition behavior, even though either polyether or polyester segments are present as the backbone of the multithiols. At a temperature of 310 to 320 °C, a material loss of 5% is observed for the nDoxL materials. The maximum decomposition rate for both systems is approximately 370 °C. The main weight loss in the cured nDoxL ₄ o/ETTMP ₆ o and nDoxL ₄ o/PCL4MP ₆ o systems occurs as a two-stage degradation with a small weight loss of 15% at 300 °C, followed by a weight loss of 40% at 390 to 420 °C. The degradation temperatures of the lignin samples are well above the expected curing temperatures of 60 to 90 °C for the two-component adhesive systems according to the invention.

Fig. 16 shows a comparison of the gravimetric analysis of exemplary cured two-component adhesive systems after contact with various organic solvents (acetone, tetrahydrofuran (THF)) by depicting the polymer content for each two-component adhesive system after conducting leaching experiments with and without reducing agent in acetone and THF. A schematic representation of the leaching experiments is shown in Fig. 24. Fig. 15 shows that both systems exhibit a certain resistance to organic solvents and cannot be completely dissolved. It is important to note that both the nDoxL and the multithiols are very soluble in the solvents. The mass percentages shown refer to the dissolved polymer content. A moderate resistance of the ETTMP-containing system to acetone (31 wt.%) and THF (37 wt.%) is obtained. The addition of tributylphosphine increases the mass fraction to 44 wt.% in acetone and 46 wt.% in THF. In the PCL4MP-containing system, 58 wt.% dissolved in acetone and 69 wt.% in THF, and after the addition of tributylphosphine, even 75 wt.% in acetone and 85 wt.% in THF. In THF, a stronger swelling behavior of the hydrophobic lignin glue can be observed. However, since the reducing agent also causes an increase in the dissolved polymer content and a similarly high degree of disulfide cleavage (9 to 17 wt.%) is achieved regardless of the solvent or multithiol, the network structure does not appear to be primarily dominated by disulfides.

Additionally, Fig. 17 shows a schematic representation of the two-component adhesive system manufacturing process (Fig. 17a) and a schematic representation of the procedure for tensile shear experiments with the inventive two-component adhesive system (Fig. 17b). In Fig. 17a, oxidized lignin and multithiol are combined to form a two-component adhesive system, and the mixture is left to stand for 3 hours. The viscosity changes during these 3 hours depending on the lignin content of the sample. In Fig. 17b, the prepared test specimens for tensile shear tests are bonded together with the two-component adhesive system and cured overnight at 60 °C. The tensile shear experiments are then carried out. The use of a texture analyzer is particularly recommended for this purpose.

Figure 18 shows the chemical conversion steps for lignin activation on a commercially available softwood kraft lignin using the DoxL process (Fig. 18a). These steps include fractionation, demethylation of G units, and oxidation to quinone units. Fig. 18b shows the results of the hydroxy pattern analysis of fractionated and demethylated lignin using ³¹ P NMR, and Fig. 18c shows the results of the ¹ H NMR analysis comparing the fractionated lignin before and after demethylation. Fig. 18d shows the results of a GPC analysis showing the molecular weight distribution of the various lignin products. Fig. 18e shows FTIR spectra confirming the chemical transformations in the heterogeneous lignin network leading to quinone structures after oxidation (conditions: 16.6 mM commercial lignin in acetone; 11.1 mM fractionated lignin with 0.3 mol ICH, reflux, 6 h; 66.6 mM demethylated lignin in DMF/acetate buffer (0.1 M, pH 5) with 7 mmol NaOH, room temperature, 2 h; ³¹ P-NMR: lignin derivatized with CDP using Cr(acac) ₃ and PhsPO as internal standard in pyridine: CDCh-1.6/1 v/v%, room temperature, 15 min; ¹ H-NMR: lignin derivatized with acetate anhydride in pyridine, room temperature, 15 min; GPC: c=1 g/L in DMSO + 0.076 M NaNOs). The increase in the intensity of the vibrational band at v=1660 cm' ¹ indicates conjugated ketones.

Figure 19 shows representative FTIR spectra showing significant changes in functionality when comparing activated lignin with the uncured 2K mixtures (nc) and those after curing, for example for DoxLso/ETTMPso (Figure 19a) and D0XL50/PCL4MP50 (Figure 19b).

It has been shown that a remarkably broad mixing window is available between 40 and 60 wt% DoxL, extending up to a lignin weight fraction of 60 wt%. Above this composition, homogeneous binary blends are not possible without the addition of further additives, and below this threshold, the viscosity of the blends is too low to be applicable. Within the lignin content window of 40-60 wt%, both multithiols with DoxL lead to homogeneous and spreadable pastes at every tested composition.

FTIR spectroscopy analysis allows comparison of pure DoxL with the two mixtures of the two different two-component adhesive systems before and after curing. The FTIR spectra of representative mixtures with DoxL contents of 50 wt.% provide insight into the curing chemistry. The stretching vibration for conjugated C=O is present in the activated lignin at 1660 cm' ^-1 , and this band, characteristic of the structural elements of quinones, disappeared in both two-component mixtures after curing. Although TCC formation is difficult to directly detect, the loss of the bands typical of quinones indicates the formation of TCC Michael addition products. The consumption kinetics of the quinones appear to depend on the local thiol concentration and/or the conformational dynamics of the polymer multithiols in the bulk reaction mixtures. However, for both multithiols, the quinone bands in DoxL do not reduce, or at least not completely, prior to curing, suggesting that the reaction proceeds slowly in the viscous bulk at room temperature. Furthermore, the characteristic ETTMP ether and ester bands, v _c =o at 1090 cm' ¹ and v _c =o at 1703 cm' ¹ , typical for PCL4MP, were found not to decrease in intensity in the DoxL mixtures during curing, suggesting that no severe degradation occurs.

Figure 20 shows a representation of the dry mechanical bonding properties of the two-component lignin-based adhesive systems (DoxL process). In Figure 20a, the results of overlap shear tests are presented as force-resistance curves, with a representative example for DoxL/ETTMP; in Figure 20b, for DoxL/PCL4MP at different DoxL/Thiol weight ratios. Figures 20c and 20d show images of the sample seam from the shear bond tests using the adhesive and cohesive blends for D0XL55/ETTMP45 (Figure 20c) and DoxL ₅₅ /PCL4MP ₄₅ (Figure 20d). Figure 21 provides a tabular overview of the dry adhesion properties of two-component lignin-based adhesive systems (DoxL process). The mixing ratio of DoxL and multithiols influences the achieved bond strength, and a maximum of approximately 15 MPa is achieved at 55 wt% DoxL, regardless of the use of hydrophilic or hydrophobic multithiols. Considering the quinone number from the ¹ H NMR demethylation analysis, the optimal quinone/thiol ratio is practically met for D0XL55/PLC4MP45, achieving the ideal stoichiometry within the error limits of the analysis (Q/T = 1.00/0.97). In comparison, the D0XL55/ETTMP45 adhesive using ETTMP tri-thiol with a Q/T ratio of 1.00/0.76 deviates from the ideal stoichiometry. It should be noted that the curing process appears to be remarkably robust with respect to the equivalents of the reactive functionalities. This advantage may be due to the nature of the lignin polyquinone. The "nano-gel" type appears to be able to introduce a certain buffering capacity, where the internal quinone functionalities do not necessarily have to react. Furthermore, the excess of thiols also appears to be balanced. For example, a 5 wt.% reduction in the DoxL content leads to a slightly reduced bond strength of 14 MPa for both exemplary two-component adhesive systems. A more dramatic change in properties is observed in formulations containing 60 wt% DoxL, where DoxLeo/PCL4MP4o approaches the limit of homogeneous mixing, thus achieving 55% lower bond strengths. It should be noted that the two-component adhesive systems studied are minimal two-component blends, and advanced formulation strategies could potentially increase the DoxL content and performance through the use of viscosity modifiers, plasticizers, primers, or wetting agents.

Fig. 22 shows the underwater curing results of the two-component adhesive system containing lignin (DoxL) and PLC4MP. Fig. 22a shows the results of overlapping shear tests during dry application and curing followed by immersion in salt water (599 mM salt concentration) for 7 days. At a DoxL/PCL4MP mass ratio of 40/60, the highest adhesion force of 3.7±0.9 mPa is obtained. Increasing the amount of DoxL in the mixture leads to a drastic decrease in the adhesion force. This could be due to the fact that increasing the DoxL content decreases the PCL4MP content, which reduces the hydrophobicity of the adhesive and makes it more susceptible to absorbing ions and water, which could act as plasticizers. Fig. 22b shows the results of overlap shear tests after dry application and subsequent dry curing (60 °C for 12 hours) and immersion in salt water for 3 days at 40 °C. At a mass ratio of DoxL/PCL4MP of 40/60, the highest adhesion force is achieved with 2.8±0.2 mPa was obtained. Considering that the transfer of the entire curing chemistry into saltwater represents a drastic change in conditions and the curing temperature was set much lower than ideal, the achieved bond strength in marine environments is within a suitable window and meets the same ranges as epoxy putty adhesives optimized for aquaculture reef building applications. Fig. 22c shows the results of lap shear tests after an underwater application followed by curing and 3 days of immersion in saltwater at 40 °C. At a DoxL/PCL4MP mass ratio of 40/60, the highest adhesion force of 2.7±0.5 mPa is obtained. It should be noted that undried substrates and a salinity of 599 mM pose a significant challenge for some adhesives, as the adhesive interface is difficult to install effectively. However, the proposed bonding system is less affected by the harsh conditions, as the same bond strength is achieved during dry application/water curing as during both underwater steps. Analysis of the invoice side allows the failure of the aluminum-bonded mixture to be categorized as a cohesive mechanism, regardless of the saltwater application protocol used. Figure 22d shows images of the lignin/PCL4MP joint after underwater application and curing for the 40/60, 50/50, and 60/40 wt% lignin/PCL4MP ratios.

Increasing the lignin content shifts the fracture mechanism toward adhesive failure. Analysis of the aluminum substrates after 7 days of immersion in salt water revealed significant deterioration of the substrate surfaces under the harsh salt concentrations. It is hypothesized that the lower viscosity of blends with higher PCL4MP contents wets the substrate surface more homogeneously. This could prevent or reduce the formation of microchannels into which salt water could penetrate and compromise the adhesive layer. Therefore, the 40/60 wt% lignin/PCL4MP blend appears to offer the best balance between cohesive strength and adhesive strength due to its improved wettability.

In addition, Fig. 23 shows various exemplary representations of the properties of the inventive two-component adhesive system (activated lignin via the DoxL process) after application. Fig. 23a shows the recovered solids content of the adhesive system after leaching in water or salt water. Fig. 23b shows the results of ecotoxicity tests on farmed fish eggs in DIN water mixed with cured and uncured adhesive starches for 7 days. The percentages of surviving eggs correlate with either no dilution or 1/1 dilution with untreated water. Fig. 23c shows images of the dry application of the DoxL4o/PCL4MPeo system to various rough surfaces and underwater curing under saline conditions at 23 °C. Fig. 23d, in turn, shows a schematic representation of shear tests with A dead coral as a substrate for adhesion to two aluminum substrates using a commercially available epoxy adhesive on the left and Doxl_4o/PCL4MP6o on the other substrate. Fig. 23e shows the results of the lap shear tests on the Al-coral-Al substrates without immersion in saline solution. Finally, Fig. 23f shows the results of the lap shear test on the bonded Al-coral-Al substrates after immersion of the system in saline solution at 40 °C for 7 days.

Figure 24 schematically illustrates the leaching process used to determine the solvent resistance of the adhesive systems according to the invention. First, the components of the adhesive system are mixed in a 2/3 lignin/thiol ratio, cured until complete, and then frozen and ground. The ground solid is added to the respective solvents to be tested. The samples are shaken for 7 days, and then the solid is separated from the supernatant.

The process according to the invention for producing a two-component adhesive system converts readily available lignin, which in the past was mainly used as a material for heat generation, into a promising material platform by simple modification either via a DoxL (demethylated oxidized lignin) process or an nDoxL (non-demethylated oxidized lignin) process.

The two-component adhesive system is very robust and allows both dry and underwater application and is applicable under saltwater conditions.

In addition, the system is compatible with marine biological environments, as shown in Fig. 23 through ecotoxicological studies.

The two-component adhesive system combines the low-cost, highly complex nature of lignin, which can be converted with thiols and the formation of TCCs to create an extremely robust and widely usable adhesive system.

Claims

1. Two-component adhesive system component A, characterized in that the two-component adhesive system component A comprises bio-based conjugated dicarbonyls as reactive component, wherein - the bio-based conjugated dicarbonyls are an oxidation product of lignin or highly oxidized lignin and/or tannins and - the bio-based conjugated dicarbonyls are formed as reactive quinone groups. 2. Two-component adhesive system component A according to claim 1, characterized in that the bio-based proportion of renewable or bio-based carbon is greater than 30% or greater than 50% or greater than 90% or 100%. 3. Two-component adhesive system component A according to claim 1 or 2, characterized in that component A - 0.3 to 5 mmol/g conjugated dicarbonyl groups or - 0.8 to 3.8 mmol/g conjugated dicarbonyl groups or - 1.3 to 3.0 mmol/g of conjugated dicarbonyl groups. 4. Two-component adhesive system with the two-component adhesive system component A according to one of the preceding claims and a two-component adhesive system component B, characterized in that - the two-component adhesive system component B comprises nucleophilic hardeners as a reactive component and - the two-component adhesive system component A and the two-component adhesive system component B react with each other and form covalent bonds and/or a covalently bridged network. 5. Two-component adhesive system according to the preceding claim, characterized in that the nucleophilic hardeners of the two-component adhesive system component B comprise or are polythiols and/or polyamines. 6. Two-component adhesive system according to one of the two preceding claims, characterized in that the two-component adhesive system component B comprises a branched or unbranched molecule or polymer or any mixtures of these substances with at least two, particularly three to four nucleophilic functionalities, but preferably thiols, also with reactive group distributions and partly monofunctional impurities. 7. Two-component adhesive system according to one of the three preceding claims, characterized in that the two-component adhesive system component B - is an aliphatic molecule or is predominantly composed of aliphatic polyester or polyether structures and/or - has molecular weights of 140-1400 g/mol and/or - polarities with calculated c(logP) coefficients between 1.84-12.43. 8. Two-component adhesive system according to one of the four preceding claims, characterized in that the two-component adhesive system component B is designed as - branched or unbranched polymers with at least two thiol groups or as - hydrophilic, thiol-group-bearing polyethylene oxide (ETTMP) or as - hydrophobic thiol group-bearing poly(E-caprolactone) (PCL4MP) or as - chemically reduced thiol group-bearing proteins. 9. Two-component adhesive system according to one of the five preceding claims, characterized in that the two-component adhesive system component A and/or the two-component adhesive system component B comprise/comprises further individual components or combinations of components selected from: - fillers; - Flame retardant; - impact modifiers; - Silane-modified polymers as additional co-crosslinkers; - Epoxies as additional crosslinkers; - Methacrylates as additional co-crosslinkers; - Acrylates as additional cover crosslinkers; - Vinyl ethers as additional crosslinkers; - Styrene derivatives as additional co-crosslinkers; - reactive diluents; - Curing catalysts - Rheology aids / rheology-modifying fillers; rheology additives; - Exothermic-regulating fillers; - Hydrolysis stability-improving fillers; - wetting agents; ■ Dyes. 10. Two-component adhesive system according to one of the six preceding claims, characterized in that the conjugated dicarbonyl groups of the two-component adhesive system component A react with thiol groups of the two-component adhesive system component B when the two components are combined to form thiol-catechol linkages or also partly to form disulfide linkages. 11. Use of the two-component adhesive system according to one of the seven preceding claims as: - Underwater adhesive; - Shell glue; - Coral glue; - Adhesives for ceramic components; - Adhesive for concrete; - Adhesive for stone; - Adhesives for glass; - Adhesives for metal and/or metal substrates; - Adhesives for technical polymers; - Adhesives for PET and/or PP and/or PE and/or PU and/or polyamides; - Coatings and/or coating; - Sealant.