Bio-based adhesive composition based on lignin

Abstract

The present invention relates to an adhesive composition based on a lignin structure, wherein the lignin structure is a lignin functionalised by reaction with dimethylformamide and acrylic acid.

Description

[0001] Bio-based adhesive composition based on lignin

[0002] The present invention relates to a bio-based adhesive composition based on lignin. The present invention further relates to a process for producing such an adhesive composition.

[0003] Phenol-formaldehyde resins are used for a wide range of applications, including bonding metals, wood, plastics, paper, and rubber at reasonable cost and with reasonable mechanical properties. Therefore, formaldehyde is also used in some other resins, such as urea-formaldehyde and melamine-formaldehyde resins, which are produced in large quantities and used for wood adhesives or mineral fiber binders.

[0004] However, both phenol and formaldehyde are considered environmentally harmful chemicals and are associated with various health problems.

[0005] There is therefore interest in finding alternative substances to replace phenol and / or formaldehyde in such applications.

[0006] In this regard, emerging biorefinery technologies are gaining increasing interest to meet market demand by producing renewable and sustainable chemicals, materials, fuels, and polymers. Lignin from biorefineries and the pulp industry remains a solid waste stream that is currently burned for energy generation, except for the partial recovery of certain chemicals.

[0007] In recent decades, lignin has been used as a partial or complete replacement for phenol in phenol-formaldehyde wood adhesives. Partial replacement or substitution of formaldehyde with less toxic, less volatile, or bio-based compounds can provide further benefits over the health risks to workers and environmental problems associated with chronic exposure to formaldehyde during the resin manufacturing process.

[0008] For example, Kalami, S.; Arefmanesh, M.; Master, E.; Nejad, M., “Replacing 100% of phenol in phenolic adhesive formulations with Lignin”, Journal of Applied Polymer Science 2017, 134 (30), 45124, describe the replacement of phenol with lignin in adhesive compositions.

[0009] The solutions known from the prior art may therefore still have potential for improvement, in particular with regard to providing improved adhesive compositions.

[0010] It is therefore the object of the present invention to provide a measure by which at least one disadvantage of the prior art is at least partially overcome. In particular, it is an object of the present invention to provide a solution by means of which adhesive compositions can be improved, particularly with regard to their environmental impact or toxicity.

[0011] The object is achieved according to the invention by an adhesive composition having the features of claim 1. The object is further achieved according to the invention by a method having the features of claim 5, an adhesive composition having the features of claim 9 and a use having the features of claim 10. Preferred embodiments of the invention are disclosed in the subclaims, in the description, the example and in the figures, wherein further features described or shown in the subclaims or in the description or the figures or the example can represent an object of the invention individually or in any combination, unless the context clearly indicates the opposite.

[0012] The present invention relates to a bio-based adhesive composition comprising a lignin structure, wherein the lignin structure is a lignin functionalized by reaction with dimethylformamide and acrylic acid.

[0013] Such an adhesive composition enables a strong and secure bond and good weather resistance in a wide range of applications. Furthermore, ecological and safety aspects can be improved compared to state-of-the-art solutions.

[0014] The present invention thus relates to a bio-based adhesive composition. The term "bio-based" in the context of the invention means that the adhesive composition is not or not entirely synthetically produced, but that at least one component, preferably the main component, is a natural polymer.

[0015] In the context of the present invention, it has been shown that lignin, in particular untreated lignin or raw lignin, is particularly suitable as a starting point.

[0016] In this regard, it should be noted that lignin is a phenolic heteropolymer with a variety of bonds and functional groups and is a component of lignocellulosic biomass. Lignocellulosic biomass consists primarily of three biopolymer units: cellulose, hemicellulose, and lignin. Lignin can account for a significant proportion of 15-25% of dried lignocellulosic biomass. It is produced, for example, as a byproduct of biorefineries and is commonly used as boiler fuel. The structure of lignin consists primarily of three alcohol units: coumaryl, coniferyl, and sinapyl alcohols.

[0017] Lignin is used in the production of various high-value products that are marketed in small quantities, including carbon fibers and lignin-based adhesives. In recent decades, lignin has been used as a partial or complete replacement for phenol in PF wood adhesives. However, these compositions usually still contain formaldehyde.

[0018] However, according to the invention it was shown that formaldehyde can be dispensed with even further if the lignin structure is specifically functionalized. Starting with lignin it was shown in detail that specific functionalization can lead to particularly advantageous properties for an adhesive. It was noted that the lignin structure can be functionalized in particular in two ways, namely via the hydroxyl groups (i.e. primary (1°) hydroxyl, secondary (2°) hydroxyl or phenolic (Ph-OH) hydroxyl groups) and via the double bonds of the aromatic rings. It is therefore particularly preferred according to the invention if the lignin is functionalized at at least one position selected from a side chain, an aryl ring and / or a hydroxyl group. With regard to the hydroxyl groups, for example only the groups of the side chains can be functionalized since, as indicated above, these are primary hydroxyl groups and are therefore more reactive.

[0019] This was exploited in the invention by using lignin as the lignin structure, which has been functionalized by reaction with dimethylformamide and acrylic acid. Such functionalization, in particular, can offer significant advantages over prior art solutions, particularly the provision of lignin in an adhesive composition, as already indicated above.

[0020] It has been shown that an adhesive composition generally exhibits a very strong bonding effect, regardless of the type of joining partners used. This allows for a very stable bond, which enables a wide range of applications.

[0021] This is further enhanced by the high stability of the resulting adhesive layer. This resulted in very strong mechanical stability. Furthermore, excellent weathering resistance and thus pronounced long-term stability were demonstrated.

[0022] Furthermore, the starting materials for producing the adhesive composition are far less problematic, especially compared to prior art solutions involving phenol-formaldehyde resins. Lignin, acrylic acid, and dimethylformamide are environmentally friendly and cause fewer and / or no health problems, especially compared to phenol and formaldehyde. The present invention demonstrates the complete replacement of phenol with lignin and formaldehyde with acrylic acid, together with dimethylformamide as the solvent or functionalizing reagent. The toxicity of the adhesive can thus be significantly reduced. This significantly simplifies production and application.

[0023] Lignin, as an essential component of lignocellulosic biomass, is a highly sustainable and renewable solution that can be processed to completely replace petroleum-derived fuels, chemicals, materials, and polymers. Particularly preferably, the adhesive composition can be free of formaldehyde. Particularly in this embodiment, where formaldehyde is completely eliminated, the adhesive composition can be improved in terms of its ecological properties. This can significantly reduce the risk to living beings and the environment, especially compared to conventional phenol-formaldehyde resins. The possibility of completely eliminating formaldehyde can be achieved, in particular, through the special type of functionalization of the lignin structure through a reaction with dimethylformamide and acrylic acid.This makes it possible to achieve the properties described above, particularly with regard to adhesive strength and durability, even without the use of formaldehyde. This is not possible with prior art solutions, which often still rely on formaldehyde as a component of the adhesive composition.

[0024] The same applies if the use of phenol is omitted, meaning the adhesive composition is completely free of phenol.

[0025] A further advantage of the adhesive composition described here is its improved sustainability during production. This is because unused or unreacted chemicals, such as acrylic acid (boiling point 141 °C) and dimethylformamide (boiling point 153 °C), can be recycled through distillation and reused. Any sodium hydroxide used to create a basic medium can also be separated, for example, from a washing solution for washing or cleaning the manufactured adhesive, and returned to the adhesive production process. The improved sustainability is also reflected in the fact that no waste is generated during the recycling of lignin. Rather, the adhesive composition can be used immediately after production without further significant processing steps, such as fractionation or depolymerization.The adhesive composition described may further consist of functionalized lignin, i.e., comprise approximately 100% by weight of functionalized lignin. However, it is also within the scope of the present invention if the adhesive composition also comprises used and non-functionalized lignin. For example, the adhesive composition may comprise functionalized lignin in an amount of > 80% by weight, for example > 90% by weight, preferably > 95% by weight, approximately > 99% by weight. The remainder may, as indicated above, comprise non-functionalized lignin or other traces resulting from the functionalization reaction, since isolation of the functionalized lignin is not necessary. The aforementioned proportions relate in particular to the dry content of the adhesive composition and in particular not to any water content resulting from a washing process or another solvent.

[0026] It may further be preferred for the lignin structure to be cross-linked. In this embodiment, an adhesive layer to be formed can exhibit a particularly strong adhesive effect and also be particularly durable. For example, the mechanical stability of the adhesive layer to be formed can be improved.

[0027] Cross-linking can occur particularly during functionalization, for example by creating cross-linking points via side chains or by creating an aryl network.

[0028] Lignin has two active sites: the aromatic ring and the functionalities present throughout the ring and side chains. These side chains contain various types of groups, including ester, carboxyl, carbonyl, hydroxyl, methoxy, etc. These functional groups can be activated as needed, significantly altering the chemical, physical, and biological properties of modified lignin and increasing its value. These processes create a new macromolecule that can be more reactive and effective for further tuning the modified lignin via the hydroxyl groups (primary, secondary, or phenolic hydroxyl group) or the active ring site.

[0029] For further advantages and technical features of the adhesive composition, reference is made to the description of the process, the use, the figures and the description of the figures.

[0030] A process for producing an adhesive composition is further described, characterized in that the process comprises the following steps: a) providing lignin; b) reacting the lignin with dimethylformamide in a basic medium; c) reacting the reaction solution formed in process step b) with acrylic acid; d) optionally at least partially drying the resulting reaction product; and e) optionally washing the reaction product.

[0031] The method described here thus describes the production or production of an adhesive composition. This is particularly configured as described above and thus comprises a lignin structure, wherein the lignin structure is a lignin functionalized by reaction with dimethylformamide and acrylic acid.

[0032] To achieve this, the process first involves the provision of lignin according to process step a). For example, lignin can be provided as a byproduct of biorefineries or can also be extracted from plants, since approximately 20 to 30% of the dry mass of woody plants consists of lignin.

[0033] In principle, any lignin can be provided and used as a starting point for subsequent modification or functionalization. However, in process step a), an untreated lignin can preferably be provided. For the purposes of the invention, an untreated lignin can be understood as a raw lignin that has been isolated directly from a plant, for example. Complex processing steps for the lignin are not required.

[0034] The provided lignin can then react with dimethylformamide in a basic medium according to process step b). Accordingly, it can be advantageous to use dimethylformamide as a solvent to reduce the substances used and also to reduce unreacted residual products. Sodium hydroxide can be added to establish a basic medium. An exemplary pH value can be in a range from > 7, preferably > 8, approximately in a range from > 11 to < 12. Furthermore, this reaction can proceed at room temperature (20°C). In principle, room temperature or slightly elevated temperatures are suitable, up to slightly elevated temperatures, for example in a range from 20°C to 30°C, and further reaction times in a range from 30 minutes to two hours, approximately one hour.

[0035] Furthermore, at the start of process step b), lignin can be initially introduced in dimethylformamide in such an amount that a solid to liquid ratio of > 100 g / l to < 200 g / l (g [lignin] / l [dimethylformamide]) is present.

[0036] Furthermore, according to process step c), the reaction solution obtained in process step b) is reacted with acrylic acid. Thus, in this step, after a suitable reaction phase in process step b), a suitable amount of acrylic acid can be added to react the lignin, which may already have been functionalized with dimethylformamide.

[0037] Process step c) can take place at a temperature which, compared to

[0038] Process step b) is increased. Suitable temperatures range from approximately > 50 °C to < 200 °C, for example from > 90 °C to < 125 °C, approximately from > 100 °C to < 110 °C. Furthermore, reaction times in a range from > 12 hours to < 48 hours, approximately from > 21 hours to < 27 hours, approximately from < 24 hours are advantageous. The total time for process steps b) and c) can be in a range from < 30 hours to approximately < 25 hours.

[0039] According to process step d), the resulting reaction product can then be dried, or according to process step e), the reaction product can be washed. Washing can serve to remove unreacted reactants, such as DMF, acrylic acid, lignin and / or base impurities, and drying can remove the washing liquid, such as water. This can be done by active drying, for example by adding heat to the reaction product, in particular using a heating unit. Alternatively, passive drying can also be carried out, which can take place without any active steps under ambient conditions, for example at room temperature (approximately 22°C). Suitable drying times can then range from several hours to a few days, for example two days, for example from 23 to 25 hours, or approximately 24 hours.

[0040] The reaction product dried, for example, to a moisture content, in particular based on water, of 80% by weight can then be used directly as an adhesive, whereby, apart from the washing described above, which can be carried out before drying, further processing steps can preferably be dispensed with.

[0041] Accordingly, the process for producing an adhesive composition comprises a process for producing an adhesive. Preferably, process steps b) and c) can be carried out at least partially simultaneously in a common reaction mixture by increasing the temperature in process step c) compared to process step b). This allows the functionalization of the lignin to proceed in the desired manner using both functionalization reagents, DMF and acrylic acid.

[0042] However, it is preferred that lignin and dimethylformamide are first mixed for a certain period of time, and only then is acrylic acid added to the mixture according to process step c) to react with lignin or, if appropriate, the product already reacted with dimethylformamide. For example, step b) can be carried out over a period of 10 minutes to 120 minutes, for example, 45 minutes to 70 minutes, before acrylic acid is added. In this embodiment, heating can, for example, only take place once the mixture is fully formed.

[0043] The advantage of initially reacting dimethylformamide with lignin and subsequently adding acrylic acid to the lignin-DMF reaction is associated with several advantages, such as maximum solubilization of lignin and under a basic medium. Furthermore, lignin or dimethylformamide can be activated for a spontaneous reaction with acrylic acid.

[0044] With regard to the ratio of the reaction educts used, it may also be advantageous that lignin and acrylic acid are used relative to each other in a ratio of

[0045] Lignin:acrylic acid ratios of greater than 0.5:1, preferably greater than 0.7:1, for example up to 2:1, approximately up to 3:1, expressed as weight percent. These values ​​refer to the amounts originally introduced into the reaction mixture. It has surprisingly been shown that, particularly in this composition, a particularly high yield of the formed adhesive is possible. This allows recycling steps for reusing the reactants to be further reduced, and applicability can be improved accordingly.

[0046] The development of lignin-based adhesive resins with high yields, approximately up to 90% based on the dried lignin used, enables efficient production and offers a particularly advantageous alternative to PF resins in today's bio-based adhesives. Furthermore, purification, fractionation, or other additional steps are not required to synthesize a lignin-based adhesive, further enhancing its application.

[0047] Thus, the described method offers a possibility for producing an adhesive composition or an adhesive which is characterized by simple and sustainable production and enables an adhesive with excellent adhesive properties for a wide variety of applications.

[0048] An adhesive composition produced by a process as described above is therefore also described.

[0049] For further advantages and technical features of the process, reference is made to the description, the adhesive composition, the use, the figures and the description of the figures.

[0050] The use of lignin functionalized with acrylic acid and dimethylformamide as an adhesive, in particular for paper, plastics, glass, wood and metals, is also described.

[0051] While the adhesive described here generally offers very good properties for bonding all materials, it has been found that particularly good adhesive properties are achieved when used for bonding paper, plastics, glass, wood, and metals, at least as one joining partner, preferably as both joining partners. For example, bonding with at least one bonding partner made of a wide variety of paper types, such as kraft paper, glass, aluminum, stainless steel, polycarbonate, polyvinyl chloride, and wood, may be preferred.

[0052] For further advantages and technical features of the use, reference is made to the description, the adhesive composition, the process, the figures and the description of the figures.

[0053] The invention is explained below by way of example with reference to the attached drawings, wherein the features shown below can represent an aspect of the invention both individually and in combination, and wherein the invention is not limited to the following drawing, the following description and the following embodiment.

[0054] They show:

[0055] Fig. 1 is a diagram showing the yield of an adhesive according to the invention at different reactant ratios;

[0056] Fig. 2 is a diagram showing an FT-IR analysis of lignin and a lignin-based adhesive according to the invention in a first spectral range;

[0057] Fig. 3 shows the FT-IR analysis from Figure 2 in a wider spectral range;

[0058] Fig. 4 DSC curves of lignin and a lignin-based adhesive according to the present invention;

[0059] Fig. 5 shows a thermogravimetric analysis of lignin and a lignin-based adhesive according to the present invention;

[0060] Fig. 6 shows a 13C NMR of lignin; Fig. 7 shows a 13C NMR of a lignin-based adhesive according to the present invention;

[0061] Fig. 8 13C-1H correlation HSQC NMR of lignin and a lignin-based adhesive together with a representative structure of various bonds in lignin and modified lignin;

[0062] Fig. 9 Storage modulus and tan delta data of an adhesive according to the invention;

[0063] Fig. 10 Loss modulus data of an adhesive according to the invention and a

[0064] Vergl ei ch skle eb Stoff s ;

[0065] Fig. 11 mechanical data of an adhesive according to the invention; and

[0066] Fig. 12 is a schematic representation of an arrangement for testing the adhesive properties of an adhesive composition according to the invention

[0067] The figures show various measurements for characterizing and investigating an adhesive, wherein the adhesive is a component of an adhesive composition, wherein the adhesive composition is a bio-based adhesive composition comprising a lignin structure, wherein the lignin structure is a lignin functionalized by reaction with dimethylformamide and acrylic acid.

[0068] As described in more detail below, such an adhesive can basically be produced by a process comprising the following steps: a) providing lignin; b) reacting the lignin with dimethylformamide in a basic medium; c) reacting the reaction solution resulting in process step b) with acrylic acid; d) optionally at least partially drying the resulting reaction product; and e) optionally washing the reaction product. An example is described below which describes the production of an adhesive composition or an adhesive in one embodiment.

[0069] Materials used

[0070] UPM BioPivaTM 100 lignin was obtained from UPM Biochemicals, Helsinki, Finland, and air-dried for 24 hours at room temperature prior to use. Sodium hydroxide was obtained from Sigma-Aldrich, Darmstadt, Germany.

[0071] Dimethylformamide (99.8%), methanol (99.8%), ethanol (99.9%), and tetrahydrofuran (THF, 99.9%) were purchased from Th. Geyer GmbH. Dimethyl sulfoxide-d6 was purchased from Carl Roth GmbH. Acrylic acid (AA), stabilized with 200 ppm of 4-methoxyphenol, was purchased from Alfa Aesar, Germany. Chromium(III) acetylacetonate was purchased from Sigma-Aldrich, USA. Anhydrous dimethylformamide (99.8%) and anhydrous pyridine (99.8%) were purchased from Sigma Aldrich, Germany. All chemicals were used as received, without the need for any purification steps.

[0072] Synthesis of lignin-based adhesive

[0073] Lignin (1.0 g, based on dried lignin) was placed in a glass pressure tube (35 mL, or

[0074] 120 mL for a 3.0 g sample was added to FengTecEx GmbH, Darmstadt, and 10 or 30 mL of DMF and 100 or 300 mg of NaOH were added, respectively. The reaction mixture was stirred with a stir bar (500 rpm) at room temperature (RT) for 1 h. Afterwards, acrylic acid (1.0 g or 3.0 g) was added to the same mixture, and stirring was continued at room temperature for 24 h (total reaction time: 25 h). The resulting reaction mixture was processed for lignin functionalization for 45 h in a preheated oven at 105 ± 2 °C. The stirred mixture was then cooled to room temperature (cooling time: ~ 20 min). DI water (50 mL or 150 mL) was added to the stirred mixture, and the mixture was centrifuged for 10 minutes at 4000 rpm (Universal 320, Hettich, Germany).The obtained semi-solid lignin-based adhesive was dried at room temperature for 24 hours, and the yield was calculated using loaded lignin as the denominator after the adhesive was dried at 105±2 °C for 4 hours.

[0075] In addition, adhesive (lignin:acrylic acid 1.0:1.0 w / w) was synthesized on a large scale (3.01 kg) for further applications as wood particle boards, wood fiber boards and wood-related materials.

[0076] The adhesive was also produced in different reactant ratios. Lignin:acrylic acid, 1.0:0.5 and 0.5:1.0 w / w, was prepared using the same procedure as the lignin:acrylic acid, 1.0:1.0 w / w, adhesive, except for the different amounts of acrylic acid and lignin. Acrylic acid (0.5 g) and lignin (0.5 g) were used for the synthesis of adhesive (lignin:AA, 1.0:0.5) and adhesive (lignin:AA, 0.5:1.0), respectively.

[0077] The following methods were used to characterize and analyze the produced adhesive.

[0078] Fourier transform infrared (FT-IR)

[0079] The functional groups in both the lignin and the adhesive were characterized using the ATR technique (ALPHA Bruker) and the OPUS 7.5 software. The spectra were recorded with a resolution of 4 cm' 1 and 32 phase resolutions. The sample scan time was 24 scans, and the data were acquired from 4000-400 cm' 1 Before analysis, the samples were dried for 4 hours at 105±2 °C and stored in a glass vial with a PTFE lid. Thermogravimetric analysis (TGA)

[0080] The thermal decomposition of the samples was measured at the TG 209 Fl, NETZSCH, Germany. Approximately 10 mg of sample was used for the thermal degradation measurement at a heating rate of 20 K / min up to 800 °C in a N2 atmosphere.

[0081] Differential scanning calorimetry (DSC)

[0082] The DSC analysis of the samples was measured using a DSC 200 F3, NETZSCH, Germany. Approximately 10 mg of the sample was heated to 300 °C in a N2 atmosphere at a heating rate of 10 K / min.

[0083] 13C and 13C-lH correlation 2D-HSQC-NMR

[0084] The heteronuclear 13C and 13C-1H single-quantum correlation (HSQC) NMR spectra of lignin and lignin-based adhesive samples were measured using Brücker 500 MHz and 600 MHz instruments, respectively. 100 mg of (dried) sample and chromium(III) acetylacetonate (~2.0 mg) were placed in a glass vial. DMSO-d6 (0.6 mL) was added to the same vial and sonicated for 10 minutes to facilitate solubilization. The completely dissolved sample was transferred to an NMR tube and processed for NMR spectra. The 13C NMR spectra were recorded from -31.21 ppm to 230.30 ppm with an acquisition time of 996.15 ms (AQ) and a receiver gain of 203 μs. The 13C NMR tube was processed for 13C-1H correlation 2D HSQC NMR spectra on a 600 MHz instrument, and the analysis was performed at 600.25 MHz and 150.93 MHz for 1H and 13C nuclei, respectively. The current pulse program hsqcedetgpsisp2.3 was used.The 1H and 13C spectra were recorded from 0 to 15.2 ppm and 0 to 220.0 ppm with AQs of 112.64 and 7.71 ms, respectively. The semiquantitative relative abundance of bonds, including ß-O-4, ß-ß, and ß-5, was measured using Bruker TopSpin 4.0.7 software. 15N-1H correlation 2D-HMBC NMR.

[0085] A similar solution to that used for the 13C NMR spectra was prepared for the 15N-1 El correlation, 2D heteronuclear multiple bond correlation (HMBC). The HMBC analysis was performed at 500.25 MHz and 50.71 MHz for 1H and 15N nuclei, respectively. The hmbcgpndqf current pulse program was applied with an AQ of 204.80 ms and 67.32 ms for 1H and 1SN nuclei, respectively. The applied receiver gain was 203.

[0086] DMTA measurement

[0087] The DMTA measurement was performed using a DMA GABO EPLEXOR system (NETZSCH GABO Instruments GmbH) with a 50.0 N force sensor. The applied static and dynamic force loads were 10.0 N (0.1% limit) and 5.0 N (0.05% limit), respectively. The applied contact force was 0.5 N. The soaking time for all samples was 300 s. The temperature sweep test (10–80 °C) was performed at 30% relative humidity (RH) with a frequency of 10.0 Hz and a temperature ramp rate of 2 K / min. The frequency sweep test (0.1–100.0 Hz) was performed at 25 °C and 30% relative humidity with a frequency ramp rate of 8.0 steps per decimal place. The RH (10%-80%) sweep test was conducted at 25 °C with a frequency of 10.0 Hz. The time sweep test (0-5 h) was conducted at 25 °C and 30% RH with a frequency of 10.0 Hz. Before analysis, the adhesive samples were bonded between two glass slides and dried at 105±2 °C for 2 h.The bonded samples with different dimensions of 10.0 mm x 0.005 mm for adhesive (lignin: AA, 1.0:1.0 w / w) and 9.0 mm x 0.001 mm for adhesive (lignin: AA, 1.0:0.5 w / w) were used for DMTA. Furthermore, the dimensional correction factor was applied for the different samples. The DMTA testing of the adhesive (lignin: AA, 1.0:1.0 w / w) was performed in replicates, and the average of the replicates (temperature, time, frequency, and humidity sweep tests) is shown in Figures 9 and 10. Contact angle and surface free energy.

[0088] The contact angle (CA) and surface free energy (SFE, polar and dispersive energy) of samples with and without adhesive were determined using OWRK, an SFE model. The water droplet on the samples was recorded with a video camera. The contact angle was measured using image analysis software. To measure the SFE, water and diiodomethane were used as liquids with different polarities. To determine the CA and SFE, at least 3 to 6 surface points per sample were taken, with 30 to 60 data sets, of which the average is presented. The flat surfaces of glass (G), polyvinyl chloride (PVC), stainless steel (SS), polycarbonate (PC), and aluminum (Al), as well as their bonded samples, were used to determine CA and SFE. Before analysis, very fine adhesive layers (adhesive thickness 0.031 ± 0.007 mm) were applied to the materials and dried at 105 ± 2 °C for 2 h.

[0089] Physical-mechanical testing

[0090] Vertical or horizontal physical-mechanical tests of the adhesive were conducted on various materials, including G, PVC, PC, and pinewood (approximately 20 years old). Approximately 10 mg (dried) was applied to laminate the materials (except wood - 100 mg, dried base, due to porosity) between two layers, and the laminated samples were dried at 105±2 °C for 2 h.

[0091] Weathering test

[0092] The PC and PVC-bonded samples were secured with a cable tie over a wire mesh and tilted at a 45-degree angle. The mesh height above the ground was approximately 1.2 meters. The samples were subjected to weathering testing at the Georg August University of Göttingen's test field from September 13, 2021, to November 15, 2021. Internal bond strength testing

[0093] The internal bond strength (IBS) of the adhesives to G, SS, PC, and Al was measured according to DIN EN 1607 (2013). Prior to the IBS measurement, the adhesive samples were dried for 2 h at 105 ±2 °C. The various G, SS, PC, and Al IBS samples (50 mm x 50 mm, except for Al) were bonded between two Al yokes (for Al IBS, the sample was used directly without a yoke). The bonded samples were tested using a universal testing machine (Zwick Roell, Ulm, Germany).

[0094] Evaluation of the investigations

[0095] The lignin-based universal adhesive was synthesized using lignin and acrylic acid in dimethylformamide as the base medium, varying the reactant ratios as described above. The results are shown in Figure 1, where the x-axis represents the ratio of lignin to acrylic acid in weight percent and the y-axis represents the yield.

[0096] Three different ratios of lignin and acrylic acid (1.0:1.0, 1.0:0.5 and 0.5:1.0 w / w) with constant solid-liquid ratios (10 wt%, lignin w / v DMF, except as explicitly noted later (20 wt%)) were used and almost similar yields of 84% and 86% were obtained using lignin and acrylic acid at ratios of 1.0:1.0 and 1.0:0.5, respectively. The yield at a lignin:acrylic acid ratio (0.5:1.0) is comparatively low, which is attributed to the high solubility of lignin or modified lignin in DMF, which was washed out after centrifugation.

[0097] The functionalization of lignin was further demonstrated as follows.

[0098] The presence of nitrogen in a lignin-based adhesive confirms the

[0099] Lignin functionalization involving dimethylformamide, which is also confirmed by the appearance of new peaks at 1654 cm' 1 for amide and 1385 cm' 1 for -C(CH3)2 groups is confirmed in a corresponding IR spectrum. This can be shown by the FT-IR spectrum of lignin and the produced adhesive. The corresponding spectra are shown in Figures 2 and 3, where the X-axis represents the wavenumber in [cm' 1 ], and the Y-axis the corresponding absorption. Furthermore, curve A corresponds to the starting product, lignin, and curve B corresponds to the functionalized lignin, i.e., the adhesive.

[0100] The appearance of new peaks in the lignin-based adhesive represents the change in the lignin structure due to functionalization with dimethylformamide or acrylic acid, which can also be demonstrated as a characterization of the lower glass transition temperature of the adhesive compared to lignin. This is evident in the corresponding DSC spectra as shown in Figure 4. In Figure 4, curve A corresponds to the starting product lignin, and curve B corresponds to the functionalized lignin, i.e., the adhesive. Furthermore, the x-axis shows the temperature in [°C], and the y-axis shows the heat flow in [w / g].

[0101] The lower glass transition temperature of the lignin-based adhesive further indicates the crosslinking of the lignin structure. The functionalization or crosslinking of lignin to form a lignin-based adhesive is confirmed by thermogravimetric analysis, as shown in Figure 5, which displays spectra from corresponding TGA measurements. In this regard, it should be noted that the crosslinked functionalities of the adhesive degrade at lower temperatures compared to lignin, which leads to the lower glass transition temperature. In Figure 5, curve A corresponds to the starting product lignin, and curve B corresponds to the functionalized lignin, i.e., the adhesive. Furthermore, the x-axis represents the temperature in [°C], and the y-axis represents the weight loss in [%].To support the FT-IR, DSC, and TGA characterization studies shown in Figures 2 to 5, and thus to confirm the functionalization of the lignin, 13C NMR spectra of both lignin and lignin-based adhesive were recorded. These are shown in Figures 6 and 7, with Figure 6 being a 13 C-NMR of the starting product lignin and where Figure 7 shows a 13 C-NMR of the functionalized lignin, i.e. the adhesive.

[0102] A significant number of new 13C peaks are observed in the lignin-based adhesive. The chemical shift for the -N(CH3)2 peak is observed at 43.21 ppm, and this peak in the lignin-based adhesive can be confirmed by the solvent used to solubilize and functionalize the lignin. Similarly, the peaks at 55.57 and 162.30 ppm for the Ar-CFL and R-(CO)-N(CH3)2 groups can also be explained by solvent-induced lignin functionalization.

[0103] In addition, some additional peaks at 130.40 and 167.40 ppm are characterized for the RO(CO)CH-CH- and RO(CO)CH-CH-carbons, and it can be explained that these carbons in the lignin-based adhesive are generated by the acrylic acid that functionalized the lignin.

[0104] To confirm the functionalization of lignin by DMF and acrylic acid in the lignin-based adhesive, 13C-1H correlation 2D HSQC NMR spectra were also recorded, with Figure 8 showing the corresponding 2D HSQC NMR images. The left images (I) refer to lignin, and the right images (II) to the molded adhesive. New signals in aliphatic and aromatic regions are assigned to the introduced RN(CH3)2 and RO(CO)CH-CH groups when comparing the spectra of lignin and adhesive. The lower part of Figure 8 also shows characteristic regions of lignin (left) and functionalized lignin, respectively. (A) shows ß-aryl ether (ß-O-4'), (B) shows phenylcoumarin (ß-5, αO-4), (C) shows functionalized lignin, and (D) shows guaiacyl.

[0105] Mechanical properties of the adhesive

[0106] The mechanical properties, such as storage modulus G' in [GPa], loss modulus G“ in [GPa] and tan delta, of lignin-based adhesives with different lignin / acrylic acid ratios were investigated using dynamic mechanical thermal analysis (DMTA) to investigate the synergistic crosslinking of lignin with dimethylformamide and acrylic acid.

[0107] The DMTA analysis of the samples was investigated over time (t), temperature (T), frequency (f) and relative humidity (RH), as shown in Figures 9 and 10.

[0108] More specifically, Figure 9 shows DMTA data for the storage modulus (G') of the adhesive (lignin:acrylic acid (AA), 1.0:1.0, and 1.0:0.5 wt / wt). (A) Time sweep at 25 °C and 30% RH with a frequency of 10 Hz, (B) the frequency sweep at 25 °C and 30% RH, (C) the RH sweep at 25 °C, with a frequency of 10 Hz, and (D) the temperature sweep at 30% RH at a frequency of 10 Hz and a temperature ramp rate of 2 K / min.

[0109] Furthermore, Figure 10 shows DMTA data of adhesive (lignin:acrylic acid (AA), 1.0:1.0 and 1.0:0.5 w / w), where (A) shows the time sweep at 25 °C and 30% RH with a frequency of 10 Hz, (B) the frequency sweep at 25 °C and 30% RH, (C) the RH sweep at 25 °C with a frequency of 10 Hz, and (D) the temperature sweep at 30% RH with a frequency of 10 Hz and a temperature rise rate of 2 K / min. The storage modulus and tan delta values ​​are stable for more than 4 hours at 25 °C, 30% RH, and a frequency of 10 Hz, which evokes the local relaxation and dynamic response states of lignin-based adhesives. However, the DMTA data of lignin-based adhesives (lignin:AA ratio, 1.0:0.5 wt / wt) show the high storage modulus (35.4 GPa) compared to lignin:AA ratio (1.0:1.0 wt / wt)., 21.2 GPa), which can be plausibly explained by the minimal functionalization (i.e., side chains) of the lignin-based adhesive (lignin:AA ratio (1.0:0.5 wt / wt)) (Figure 9). The good mechanical stability of the lignin-based adhesive under the applied static and dynamic loads represents the stiffness and robustness of the material without any fatigue over a long period of time.

[0110] The storage modulus of both lignin-based adhesives increases significantly across the tested frequency (0.1 to 100 Hz) at 25 °C and 30% relative humidity. The significant increase in storage and loss moduli indicates crosslinking and densification of the lignin-based adhesive at the tested frequency, which further strengthens the adhesive's internal and external bonds.

[0111] Lignin binds to both the hydrophobic (aryl moiety) and hydrophilic (alkyl alcohol side chains) groups, resulting in the formation of cross-linked chains or aryl networks. Figure 9C shows the influence of relative humidity on the mechanical properties of lignin-based adhesives. The storage modulus of the adhesive (lignin:AA ratio, 1.0:0.5 wt / wt) increased slightly from 21.1 GPa to 26.7 GPa as the relative humidity increased from 19.8% to 79.6%.

[0112] The physiological properties, such as the storage modulus of adhesives, generally change with the applied temperature; at low temperatures, the materials mostly remain in a glassy / crystalline state and transform into a rubbery / molten state at higher temperatures. The storage modulus and tan delta of the two lignin-based adhesive samples are high and stable at low temperatures (-16.0 °C), but decrease significantly up to 48.8 °C (-75%) and then decrease slightly up to the tested temperature (-24.6%, 78.7 °C) (Fig. 9D). This can be plausibly explained by the formation of the polymer gel at a higher temperature, which tends to ensure maximum mobility of the polymer chains or reduce the strength of intermolecular interactions.

[0113] Figure 11 further shows under (A) the internal bond strength (IBS) in [kPa] of the adhesive, shown on the Y-axis, with a reference across different materials on the X-axis, namely glass (G), stainless steel (SS), aluminum (Al), and polycarbonate (PC). Furthermore, Figure 11 shows under (B) the cumulative surface energy (SFE) polar (p) and dispersive (d), each in [mN / m] on the Y1-axis, and the contact angle CA, [°] on the Y2-axis, where S for the materials describes adhesions with adhesive. Figure 11 further shows under (C) a correlation between the cumulative surface energy (SFE) in [mN / m] polar on the Y1-axis and internal bond strength in [kPa] on the Y2-axis. Unless otherwise stated, the measurements were carried out with a lignin / acrylic acid ratio of 1.0:1.0 (weight / weight).

[0114] The adhesion properties of the lignin-based adhesive depend on the internal bonding chains between the adherends and the adhesive. PC bonded strongly to the adhesive compared to the other tested adhesives. Adhesive synthesized from both the lignin:AA ratio (1.0:1.0 and 1.0:0.5 wt / wt) characterized for adhesion properties on Al and the lignin:AA ratio (1.0:1.0 wt / wt) showed higher internal bond strength (-35%) than the lignin:AA ratio (1.0:0.5 wt / wt) derived adhesive.

[0115] The disperse surface energy (hydrophilicity) of the adhesives (GS, Al-S, SS-S, and PC-S) increases with the exception of PVC-S, while the polar surface energy (hydrophobicity) also increases across the board (except for GS). The significant difference in the polar surface energy of PC (with and without adhesive) is measured (Fig. 10B), which can confirm the higher IBS of PC-S.

[0116] However, according to the acid-base theory, the polar interaction of acids and bases played a significant role in adhesion. The hydrophilicity and hydrophobicity of lignin-based adhesives compared to the different adhesive types was determined by measuring the contact angle (CA) (Fig. 11 B). The contact angle of GS and PVC-S samples increased compared to the control samples, while a negative trend was measured for Al-S, SS-S, and PC-S. This can be plausibly attributed to the orientation of the chemical components, e.g., functionalized phenol moieties and side chains of the lignin-based adhesive.

[0117] Furthermore, the GS, Al-S, and PC-S samples showed a strong correlation between the SFE pole and the IBS (Fig. 11 C). An excellent correlation fits the polar SFE and the IBS of G, Al, and PC adhesions with lignin-based adhesive, plotted with R2 of 0.98 and 0.96, respectively. This correlation indicates strong interactions between the G, Al, and PC adhesions and the lignin-based adhesive.

[0118] Physical-mechanical properties are important for characterizing adhesive strength, long-term durability, environmental compatibility, and stability in various media for a wide range of potential applications. To demonstrate the interfacial adhesion and weathering test of lignin-based adhesives to G, PC, PVC, paper, and wood, a rectangular surface was bonded with adhesive in an arrangement 10 and subjected to a vertical and horizontal physical-mechanical weight test (see Fig. 12). The surface resulted in a contact surface or adhesive surface 16 between two substrates 12, 14. The samples were then dried for 2 hours at 105±2 °C. In summary, the following tests were performed.

[0119] A vertical and horizontal physical-mechanical test was carried out using a weight of 1,500 g of the adhesive on PVC with adhesive surfaces of 2.5 cm each2 or 5.0 cm 2 and PC with an adhesive surface of 5.0 cm 2 at different pH values ​​and days (d).

[0120] The same bonded samples were immersed in DI water for 30 minutes at room temperature, and their vertical mechanical strength was measured. The samples were able to support a metal weight of 1500 g without failure.

[0121] Furthermore, the samples previously treated with DI water were immersed in acidic water (pH 3.08, using 72% H2SO4) for 30 minutes at room temperature, and their mechanical strength was retested. The samples were successfully able to support a metal weight of 1500 g without failure.

[0122] The acidified samples were then immersed in a basic solution (pH 10.22, NaOH) for 30 minutes at room temperature, and their mechanical strength was tested by placing a 1500 g metal weight on the same day and after 30 days (storage at room temperature). The vertical physical-mechanical weight test of the PC and PVC bonded samples confirmed that the lignin-based adhesive is highly stable under neutral, acidic, and basic media over 30 days.

[0123] The samples immersed in basic solution were then processed for the weathering test over 64 days (from September 13, 2021, to November 15, 2021, Göttingen, Germany). The samples demonstrated similar metal weight holding performance (1500 g) without failure. The lignin-based adhesive thus demonstrated excellent physico-mechanical properties after immersing the adhesive samples in neutral, acidic (pH 3.08), and basic (pH 10.22) media. The lignin-based adhesive also demonstrated very high bond strength and excellent stability during the weathering test (> 64 days) and was fundamentally capable of holding more than 2,000 g of metal weight without failure, even after 94 days. A strong correlation was found between surface energy (SFE) and polar and internal bond strength with R2 values ​​of 0.98 and 0.96 for different substrates.

[0124] A solubility study (0.1% wt / v) of the lignin-based adhesive was conducted in DI water and organic solvents, including methanol, ethanol, tetrahydrofuran (THF), and dimethylformamide (DMF). The qualitative data showed that this adhesive is insoluble in DI water and ethanol, moderately soluble in methanol and THF, and slightly more soluble in DMF. These data also demonstrate the stability of the adhesive in common environments.

[0125] Effective bonding was also demonstrated when bonding paper. A tensile test showed that the bond held up and the paper tore. This was even confirmed when using kraft paper, which has exceptionally high tensile strength. Accordingly, exceptionally high bonding performance was also demonstrated with paper.

[0126] In summary, a green, sustainable, and bio-based universal adhesive was produced using lignin, dimethylformamide (DMF), and acrylic acid (AA) as the base medium. The internal bond strength (IBS) of the lignin-based adhesive was measured for different types of adhesives (G, Al, SS, and PC), and the promising IBS was validated for the PC-bonded sample. The universal bio-based adhesive is environmentally safe (>64 d) and chemically stable to acidic (pH 3.08), neutral, and basic (pH 10.22) media. Excellent dynamic and physico-mechanical properties were measured for lignin-based adhesives. Thus, a synergistic, simple synthesis method for a lignin-based universal adhesive was developed to replace the commercially widely used formaldehyde-based resins and offer the possibility of a potential solution to environmental and health problems.

Claims

Patent claims 1. Bio-based adhesive composition comprising a lignin structure, characterized in that the lignin structure is a lignin functionalized by reaction with dimethylformamide and acrylic acid.

2. Bio-based adhesive composition according to claim 1, characterized in that the lignin is functionalized at at least one position selected from a side chain, an aryl ring, and a hydroxyl group.

3. Bio-based adhesive composition according to claim 1 or 2, characterized in that the adhesive composition is free of formaldehyde.

4. Bio-based adhesive composition according to one of claims 1 to 3, characterized in that the adhesive composition is free of phenol.

5. Bio-based adhesive composition according to one of claims 1 to 4, characterized in that the lignin structure is cross-linked.

6. A method for producing an adhesive composition, characterized in that the method comprises the following steps: a) providing lignin; b) carrying out a reaction of the lignin with dimethylformamide in a basic medium; c) carrying out a reaction of the reaction solution obtained in process step b) with acrylic acid; d) optionally at least partially drying the reaction product obtained; and e) optionally washing the reaction product.

7. Method according to claim 6, characterized in that process steps b) and c) are carried out at least partially simultaneously in a common reaction mixture.

8. Method according to claim 6 or 7, characterized in that dimethylformamide is used as a solvent.

9. Method according to one of claims 6 to 8, characterized in that lignin and acrylic acid are present relative to each other in a lignin:acrylic acid ratio of greater than 0.5:1, preferably greater than 0.7:1, expressed in weight percent.

10. Method according to one of claims 6 to 9, characterized in that in process step a) an untreated lignin is provided.

11. Adhesive composition, produced according to a method according to any one of claims 6 to 10.

12. Use of lignin functionalized with acrylic acid and dimethylformamide as an adhesive, especially for paper, plastics, glass, wood and metals.

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