Lignin-based epoxy resin curing agent as well as preparation method and application thereof

Abstract

The invention provides a lignin-based epoxy resin curing agent and a preparation method and application thereof, and the preparation method comprises the following steps: mixing lignin, an alkaline solution and an amine reagent to obtain a mixed solution; the mixed solution is heated to 60-90 DEG C, formaldehyde is added for a Mannich reaction, the reaction time is 2.5-4 h, and the lignin-based epoxy resin curing agent is obtained; the mass ratio of the lignin to the amine reagent is 1: (0.2-0.35), and the mass ratio of the lignin to the formaldehyde is 1: (0.6-1.2). The preparation method provided by the invention is simple to operate, the prepared lignin-based epoxy resin curing agent and epoxy resin can be rapidly cured at low temperature, the cured product has good performance, and the lignin-based epoxy resin curing agent has a wider application range.

Description

Technical Field

[0001] The invention relates to the field of epoxy resin curing agents, and in particular to a lignin-based epoxy resin curing agent and a preparation method and application thereof. Background Art

[0002] Epoxy resin is an oligomer that can form a thermosetting resin through the reaction of epoxy groups. Because of its excellent properties such as good adhesion, low shrinkage, electrical insulation, chemical stability, wear resistance, high and low temperature resistance, it is widely used in the fields of construction, machinery, electronics and electrical, aerospace, etc. However, epoxy resin itself is a linear structure, and it must be cross-linked by a curing agent to show its excellent performance. Therefore, curing agents occupy a very important position in the application of epoxy resins. Traditional curing agents, such as amine curing agents such as ethylenediamine and diethylenetriamine, have the disadvantages of high volatility at room temperature, low toughness, low strength and toxicity. Curing agents that can solve these shortcomings, such as polyetheramines, are expensive and their application and promotion are limited. Therefore, it is of great practical value to develop new epoxy resin curing agents with good process performance, non-toxicity and low price.

[0003] Lignin is a natural polymer material and a biomass resource second only to cellulose in terms of its reserves in nature. Its macromolecular structure contains many active hydrogens that can participate in the curing of epoxy resins, and its benzene ring structure can increase the heat resistance of epoxy resins. Chemically modifying lignin to increase its reaction activity and preparing it into an epoxy resin curing agent can not only reduce people's dependence on petrochemical products, reduce the environmental and safety risks brought by traditional epoxy resin curing agents, but also reduce the pollution caused by the use of thermosetting epoxy resins to the environment, which is of great benefit to the development of a low-carbon economy. The existing lignin-based epoxy resin curing agent preparation method is complex, the curing temperature is high, and the performance of the cured product is unstable, resulting in its weak market competitiveness and limiting its wide application.

[0004] Therefore, it is of great significance to reduce the preparation difficulty and curing temperature of lignin-based epoxy resin curing agents and provide stability of the cured products. Summary of the invention

[0005] The main purpose of the present invention is to provide a lignin-based epoxy resin curing agent and a preparation method and application thereof. The technical problem to be solved is how to provide a lignin-based epoxy resin curing agent with a simple preparation method, low curing temperature and good curing performance, so as to be more suitable for practical use.

[0006] The purpose of the present invention and the technical problem to be solved are achieved by adopting the following technical solutions. According to the preparation method of a lignin-based epoxy resin curing agent proposed in the present invention, it comprises the following steps:

[0007] mixing lignin, an alkaline solution and an amine reagent to obtain a mixed solution;

[0008] The mixed solution is heated to 60-90° C., and formaldehyde is added to carry out a Mannich reaction for 2.5-4 hours to obtain a lignin-based epoxy resin curing agent;

[0009] The mass ratio of the aforementioned lignin to the aforementioned amine reagent is 1:0.2-0.35, and the mass ratio of the aforementioned lignin to formaldehyde is 1:0.6-1.2.

[0010] The purpose of the present invention and the solution to its technical problems can be further achieved by adopting the following technical measures.

[0011] In some embodiments, according to the aforementioned method for preparing the lignin-based epoxy resin curing agent, the mass ratio of the aforementioned lignin to the amine reagent is 1:0.3-0.35.

[0012] In some embodiments, according to the aforementioned method for preparing the lignin-based epoxy resin curing agent, the mass ratio of the aforementioned lignin to formaldehyde is 1:0.8-1.2.

[0013] In some embodiments, according to the aforementioned method for preparing the lignin-based epoxy resin curing agent, the reaction temperature is 70-90° C., and the reaction time is 3-4 hours.

[0014] In some embodiments, according to the aforementioned method for preparing the lignin-based epoxy resin curing agent, the mass of the aforementioned mixed solution is 100%, and the rate of adding formaldehyde is 5% to 25% / min.

[0015] In some embodiments, according to the aforementioned method for preparing the lignin-based epoxy resin curing agent, the aforementioned amine reagent is at least one of diethylenetriamine, triethylenetetramine and tetraethylenepentamine.

[0016] The purpose of the present invention and the technical problem solved by the present invention are also achieved by the following technical solutions. According to a lignin-based epoxy resin curing agent proposed by the present invention, the structural formula of the aforementioned lignin-based epoxy resin curing agent is as follows:

[0017] Wherein R is -(CH2)2-(NH-(CH2)2)n-, and n=1~3.

[0018] The purpose of the present invention and the solution to its technical problems can be further achieved by adopting the following technical measures.

[0019] In some embodiments, according to the aforementioned lignin-based epoxy resin curing agent, the gel content of the aforementioned lignin-based epoxy resin curing agent after curing with epoxy resin is greater than 80%; the curing method is pre-curing at 60°C for 45 minutes and then post-curing at 105°C for 1 hour.

[0020] The purpose of the present invention and the technical problems solved therein are also achieved by the following technical solutions: The present invention provides a method for using any of the aforementioned lignin-based epoxy resin curing agents, wherein epoxy resin and the aforementioned lignin-based epoxy resin curing agent are mixed in a mass ratio of 1:0.1-0.6 and then cured.

[0021] The purpose of the present invention and the technical problems solved therein are also achieved by adopting the following technical solutions: The present invention proposes the application of any of the aforementioned lignin-based epoxy resin curing agents in the field of curing agents.

[0022] Through the above technical scheme, the lignin-based epoxy resin curing agent and its preparation method and application of the present invention have at least the following advantages:

[0023] The preparation method of the lignin-based epoxy resin curing agent proposed by the present invention strictly controls the mass ratio of lignin to amine reagents to be 1:0.2-0.35 and the mass ratio of lignin to formaldehyde to be 1:0.6-1.2 under alkaline conditions, and introduces a large amount of active amines to lignin through the Mannich reaction, so that the lignin-based epoxy resin curing agent obtained after amination can achieve low-temperature curing, and the performance of the cured product after curing is stable. The lignin-based epoxy resin curing agent prepared by the present invention can be quickly cured at low temperatures, the gel content of the cured product can be as high as 98.80%, and the bonding strength of the cured product is high. At the same time, the preparation method of the lignin-based epoxy resin curing agent proposed by the present invention is simple to operate, low in cost, and more suitable for industrial production.

[0024] The lignin-based epoxy resin curing agent proposed by the present invention uses green and renewable natural polymer lignin as the main component of epoxy resin, which not only improves the thermal properties and mechanical properties of epoxy resin, but also reduces the degradation cycle of epoxy resin. In addition, lignin-based epoxy resin can reduce the emission of volatile organic compounds and enhance the sustainability of wood adhesives.

[0025] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a two-dimensional NMR spectrum of the lignin fat region in some embodiments of the present invention;

[0027] Figure 2 is a two-dimensional NMR spectrum of the fat region of the lignin-based epoxy resin curing agent in some embodiments of the present invention;

[0028] Figure 3 It is a two-dimensional nuclear magnetic spectrum of the aromatic region of industrial lignin disclosed in some embodiments of the present invention;

[0029] Figure 4 It is a two-dimensional nuclear magnetic spectrum of the aromatic region of the lignin-based epoxy resin curing agent disclosed in some embodiments of the present invention. DETAILED DESCRIPTION

[0030] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following is a detailed description of a lignin-based epoxy resin curing agent and its preparation method and application according to the present invention, its specific implementation method, structure, characteristics and effects, in combination with the accompanying drawings and preferred embodiments. In the following description, different "one embodiment" or "embodiment" does not necessarily refer to the same embodiment. In addition, specific features, structures or characteristics in one or more embodiments may be combined in any suitable form.

[0031] The present invention provides these embodiments to make the present invention thorough and complete, and fully express the scope of the present invention to those skilled in the art. It should be noted that unless otherwise specifically stated, the relative arrangement of the parts and steps, the composition of materials, numerical expressions and numerical values ​​set forth in these embodiments should be interpreted as being merely exemplary, rather than as limiting.

[0032] The present invention provides a method for preparing a lignin-based epoxy resin curing agent, which comprises the following steps:

[0033] mixing lignin, an alkaline solution and an amine reagent to obtain a mixed solution;

[0034] The mixed solution is heated to 60-90° C., and formaldehyde is added to carry out a Mannich reaction. The mixture is stirred during the reaction. The reaction time is 2.5-4 hours to obtain a lignin-based epoxy resin curing agent.

[0035] The mass ratio of the aforementioned lignin to the aforementioned amine reagent is 1:0.2-0.35, and the mass ratio of the aforementioned lignin to formaldehyde is 1:0.6-1.2.

[0036] Specifically, the lignin uses soluble lignin, common industrial lignin can be used, for example, the lignin is at least one of hydrolyzate lignin, alkali lignin and sulfate lignin. The alkaline solution can be a sodium hydroxide solution or a potassium hydroxide solution. The amine reagent uses a primary or secondary fatty amine or its hydrochloride, for example, the amine reagent can be at least one of diethylenetriamine, triethylenetetramine and tetraethylenepentamine.

[0037] The Mannich reaction of lignin is a chemical modification method for introducing amine functional groups, which can improve the hydrophilicity of lignin or increase its adsorption capacity for metal ions. The Mannich reaction of lignin under alkaline conditions has the following advantages:

[0038] Catalyst effect: Under alkaline conditions, alkaline substances such as sodium hydroxide can effectively promote the Mannich reaction as a catalyst, reduce the activation energy of the reaction, and thus increase the reaction rate.

[0039] Improve reaction activity: Under alkaline conditions, the hydroxyl groups in the lignin molecules are more easily replaced by amine groups, thereby improving the activity and efficiency of the reaction.

[0040] Optimization of reaction conditions: Alkaline conditions are not only conducive to the performance of the catalyst, but also can further improve the yield and efficiency of the reaction by reasonably selecting conditions such as reaction time and temperature.

[0041] Introduction of amine groups: Under alkaline conditions, the phenolic hydroxyl groups in the lignin structure can react with amine groups to introduce amine functional groups, thereby increasing the hydrophilicity and functionality of lignin.

[0042] Structural changes: The Mannich reaction under alkaline conditions can lead to an increase in the electron density on the phenolic benzene ring at the C-5 position in the lignin structure, which is conducive to the introduction of amine groups.

[0043] In summary, the hydroxyl groups in the lignin molecules are more easily replaced by amine groups under alkaline conditions in the lignin Mannich reaction, which improves the yield and efficiency of the reaction. Lignin containing more amine groups is used as an epoxy resin curing agent, so that it can be cured quickly at low temperatures. Taking diethylenetriamine as an amine reagent as an example, the chemical reaction process of the present invention is as follows:

[0044]

[0045] When mixing lignin, alkaline solution and amine reagent, lignin can be first dissolved in the alkaline solution, and then the amine reagent is added and stirred evenly to obtain a mixed solution.

[0046] Formaldehyde can be added in the form of an aqueous solution, for example, a formaldehyde solution with a mass concentration of 35% to 40% can be used. To reduce the occurrence of side reactions, the formaldehyde solution should be added slowly to the reaction system and fully stirred during the addition, for example, when the mass of the mixed solution is 100%, the addition rate of formaldehyde is 5% to 25% / min.

[0047] The mass ratio of lignin to amine reagents is 1:0.2 to 0.35. If the mass ratio of lignin to amine reagents is greater than 1:0.2, too little amine reagent is added, the degree of lignin amination is low, the prepared lignin-based curing agent and epoxy resin are not completely cured, and the degree of cross-linking is too low; if the mass ratio of lignin to amine reagents is less than 1:0.35, too much amine reagent is added, more by-products are generated, and the performance of the curing agent is affected; therefore, the mass ratio of lignin to amine reagents is controlled at 1:0.2 to 0.35. More preferably, the mass ratio of lignin to amine reagents is 1:0.3 to 0.35.

[0048] The mass ratio of lignin to formaldehyde is 1:0.6-1.2. If the mass ratio of lignin to formaldehyde is greater than 1:0.6, the amount of formaldehyde added is too little, resulting in incomplete reaction, reduced reaction selectivity, which is not conducive to lignin amination, and the performance of the lignin-based epoxy resin curing agent product is reduced; if the mass ratio of lignin to formaldehyde is less than 1:1.2, the amount of formaldehyde added is too much, resulting in excessive cross-linking and difficulty in reaction control. Secondly, excessive formaldehyde may release unreacted formaldehyde during the curing process, which is not only harmful to the environment and human health, but also affects the safety of the product; therefore, the mass ratio of lignin to formaldehyde is controlled to be 1:0.6-1.2. More preferably, the mass ratio of lignin to formaldehyde is 1:0.8-1.2.

[0049] The reaction temperature is 60-90°C, and the reaction time is 2.5-4h. When the reaction temperature is lower than 60°C, the Mannich reaction rate is slow and the yield decreases; when the reaction temperature is higher than 90°C, thermal decomposition of the reactants or intermediates may occur, making the reaction control difficult and affecting the performance of the curing agent; therefore, the reaction temperature is controlled at 60-90°C. When the reaction time is less than 2.5h, the lignin amination is not sufficient, and the low-temperature curing effect of the lignin-based epoxy resin curing agent is poor; when the reaction time is longer than 4h, the reaction is complete, resulting in energy waste. More preferably, the reaction temperature is 70-90°C, and the reaction time is 3-4h.

[0050] The present invention provides a lignin-based epoxy resin curing agent, the structural formula of the aforementioned lignin-based epoxy resin curing agent is as follows:

[0051] Wherein R is -(CH2)2-(NH-(CH2)2)n-, and n=1~3.

[0052] In some embodiments, the gel content of the aforementioned lignin-based epoxy resin curing agent after curing with the epoxy resin is greater than 80%; the curing method is pre-curing at 60° C. for 45 minutes and then post-curing at 105° C. for 1 hour.

[0053] The present invention provides a method for using any of the aforementioned lignin-based epoxy resin curing agents, wherein epoxy resin is mixed with the aforementioned lignin-based epoxy resin curing agent in a mass ratio of 1:0.1-0.6 and then cured.

[0054] The present invention proposes the use of any of the aforementioned lignin-based epoxy resin curing agents in the field of curing agents.

[0055] The present invention will be further described below in conjunction with specific embodiments, but this should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by technicians in this field based on the above-mentioned contents of the present invention still fall within the scope of protection of the present invention.

[0056] Unless otherwise specified, the materials, reagents, etc. involved below are all commercially available products well known to those skilled in the art; unless otherwise specified, the methods described are all methods well known in the art. Unless otherwise defined, the technical terms or scientific terms used should have the common meanings understood by ordinary technicians in the field to which the present invention belongs.

[0057] In the following Examples 1 to 13, Example 21 and Comparative Examples 1 to 3, the curing method used before the gel content test is as follows: After epoxy resin E51 and the corresponding curing agent in each example and comparative example are mixed in a certain proportion, 1% DMP-30 is added as an epoxy accelerator, and the mixture is placed in a vacuum drying oven for vacuum degassing. The degassed epoxy resin and curing agent system are injected into a mold, placed in an oven, pre-cured at 60° C. for 45 minutes, and then post-cured at 105° C. for 1 hour. The size of the mold is 30 mm long, 10 mm wide, and 2 mm high.

[0058] Example 1

[0059] This embodiment provides a lignin-based epoxy resin curing agent and a preparation method and application thereof. The lignin in this embodiment uses hydrolyzed lignin, and the amine reagent uses diethylenetriamine.

[0060] Weigh lignin in a three-necked flask, add 0.4 mol / L NaOH aqueous solution, mix well; slowly add amine reagent, stir well to obtain a mixed solution. Slowly drop a 37% formaldehyde solution into the mixed solution, with the mass of the mixed solution being 100% and the formaldehyde addition rate being 5% to 25% / min; react at 90°C for 3h, stirring during the reaction, to obtain a lignin-based epoxy resin curing agent. Among them, the mass ratio of lignin to NaOH aqueous solution is 1:5, the mass ratio of lignin to amine reagent is 1:0.2, and the mass ratio of lignin to formaldehyde is 1:1.

[0061] After curing the epoxy resin E51 and the lignin-based epoxy resin curing agent, the gel content was measured to be 90.19%. The epoxy resin E51 and the lignin-based epoxy resin curing agent were mixed in a mass ratio of 1:0.4.

[0062] Embodiments 2 to 13

[0063] The difference between Examples 2 to 13 and Example 1 is that at least one of the type of lignin, the mass ratio of lignin to amine reagent, the mass ratio of lignin to formaldehyde, the reaction temperature and the reaction time is different from that of Example 1. The specific experimental conditions and the gel content after curing with epoxy resin E51 are shown in Table 1.

[0064] The lignin and lignin-based epoxy resin curing agent in Example 3 were subjected to nuclear magnetic resonance detection, and the two-dimensional nuclear magnetic spectra of lignin before and after amination modification were shown in Figure 1-4 . Figure 1 and Figure 3 These are the 2D NMR spectra of the aliphatic and aromatic regions of lignin, respectively; Figure 2 and Figure 4 These are the two-dimensional NMR spectra of the aliphatic region and aromatic region of the lignin-based epoxy resin curing agent. Figure 1 and Figure 2 As shown, compared with the unmodified lignin, the lignin linking bond β-O-4 was significantly cleaved during the modification process, and a large number of new signals appeared in the side chain region of the lignin derivative after the amination modification, indicating that the amination reagent was successfully grafted onto the lignin through the Mannich reaction. The Mannich reaction mainly occurs at the ortho and para positions of the phenolic hydroxyl group. In the aromatic region, there is a huge difference between lignin and lignin-based epoxy resin curing agent. It was found that the peaks related to the G and H lignin units almost completely disappeared, while the signal of the S lignin unit was relatively stable. It can be inferred that these amine groups are introduced into the G and H positions of lignin through the Mannich reaction. The chemical reaction process in Example 3 is as follows:

[0065]

[0066] Table 1 Summary of experimental conditions and gel content after curing in Examples 2 to 13

[0067]

[0068] Embodiment 14

[0069] This embodiment provides an application of a lignin-based epoxy resin curing agent.

[0070] Epoxy resin E51 was mixed with the lignin-based epoxy resin curing agent in Example 3 at a mass ratio of 1:0.1, and 1% DMP-30 was added as an epoxy accelerator to obtain a lignin adhesive. The lignin adhesive was applied to the beech board, and two beech boards were bonded and fixed with a clamp. The dimensions of the two beech boards were 20 mm in width, 80 mm in length, and 5 mm in thickness; the area of ​​the lignin adhesive coating was 20 mm × 12 mm. The two bonded and fixed beech boards were placed in an oven, pre-cured at 60°C for 45 minutes, and then post-cured at 105°C for 1 hour, and then a universal tensile tester was used to perform a tensile shear test, and the bonding strength reached 10.32 MPa.

[0071] Embodiments 15 to 19

[0072] The difference between Examples 15 to 19 and Example 14 is that, when mixed, the mass ratio of epoxy resin E51 to the lignin-based epoxy resin curing agent in Example 3 is different. The specific values ​​and the bonding strength measured by the tensile shear test are shown in Table 2.

[0073] Table 2 Experimental conditions in Examples 15 to 19 and bonding strength measured by tensile shear test

[0074] E51: Curing agent Bonding strength / Mpa Embodiment 14 1:0.1 10.32 Embodiment 15 1:0.2 9.13 Example 16 1:0.3 10.69 Embodiment 17 1:0.4 12.18 Embodiment 18 1:0.5 8.83 Embodiment 19 1:0.6 7.79

[0075] Embodiment 20

[0076] The difference between Example 20 and Example 14 is that the beech wood board is replaced by an aluminum board; the measured bonding strength is 10.82 MPa.

[0077] Embodiment 21

[0078] This embodiment provides a lignin-based epoxy resin curing agent and a preparation method and application thereof.

[0079] The lignin in this embodiment uses hydrolyzed lignin, and the amine reagent uses triethylenetetramine.

[0080] Weigh lignin in a three-necked flask, add 0.4 mol / L NaOH aqueous solution, mix well; slowly add amine reagent, stir well to obtain a mixed solution. Slowly drop a 37% formaldehyde solution into the mixed solution, with the mass of the mixed solution being 100% and the formaldehyde addition rate being 15% / min; react at 90°C for 3h, stirring during the reaction, to obtain a lignin-based epoxy resin curing agent. Among them, the mass ratio of lignin to NaOH aqueous solution is 1:5, the mass ratio of lignin to amine reagent is 1:0.3, and the mass ratio of lignin to formaldehyde is 1:1.

[0081] After curing the epoxy resin E51 and the lignin-based epoxy resin curing agent, the gel content was measured to be 98.24%. The epoxy resin E51 and the lignin-based epoxy resin curing agent were mixed in a mass ratio of 1:0.4.

[0082] Epoxy resin E51 and lignin-based epoxy resin curing agent were mixed in a mass ratio of 1:0.4, and 1% DMP-30 was added as an epoxy accelerator to obtain a lignin adhesive. The lignin adhesive was applied to the PVC board, and the two PVC boards were bonded and fixed with a clamp. The dimensions of the two PVC boards were 20 mm in width, 80 mm in length, and 5 mm in thickness; the area of ​​the lignin adhesive coating was 20 mm × 12 mm. The two bonded and fixed PVC boards were placed in an oven, pre-cured at 60 ° C for 45 minutes, and then post-cured at 105 ° C for 1 hour, and then a universal tensile tester was used to perform a tensile shear test, and the bonding strength reached 3.52 MPa.

[0083] Comparative Example 1

[0084] Weigh the hydrolyzed lignin in a three-necked flask, add 0.4 mol / L NaOH aqueous solution, mix evenly; slowly add amine reagent, stir evenly to obtain a mixed solution. Slowly drop a 37% formaldehyde solution into the mixed solution, with the mass of the mixed solution being 100% and the formaldehyde addition rate being 15% / min; react at 90°C for 3h, stir during the reaction, and obtain an epoxy resin curing agent. Among them, the mass ratio of hydrolyzed lignin to NaOH aqueous solution is 1:5, the mass ratio of hydrolyzed lignin to amine reagent is 1:0.1, and the mass ratio of hydrolyzed lignin to formaldehyde is 1:1.

[0085] After curing the epoxy resin E51 and the epoxy resin curing agent in this comparative example, the gel content was measured to be 69.37%. The epoxy resin E51 and the epoxy resin curing agent were mixed in a mass ratio of 1:0.4.

[0086] Epoxy resin E51 and epoxy resin curing agent obtained in comparative example 1 were mixed in a mass ratio of 1:0.4, and 1% DMP-30 was added as an epoxy accelerator to obtain a lignin adhesive. The lignin adhesive was applied to the beech board, and two beech boards were bonded and fixed with a clamp. The dimensions of the two beech boards were 20 mm in width, 80 mm in length, and 5 mm in thickness; the area of ​​the lignin adhesive coating was 20 mm × 12 mm. The two bonded and fixed beech boards were placed in an oven, pre-cured at 60°C for 45 minutes, and then post-cured at 105°C for 1 hour, and then a universal tensile machine was used to perform a tensile shear test, and the bonding strength was 2.89 MPa.

[0087] Comparative Example 2

[0088] Weigh the hydrolyzed lignin in a three-necked flask, add 0.4 mol / L NaOH aqueous solution, mix evenly; slowly add amine reagent, stir evenly to obtain a mixed solution. Slowly drop a 37% formaldehyde solution into the mixed solution, with the mass of the mixed solution being 100% and the formaldehyde addition rate being 15% / min; react at 90°C for 3h, stir during the reaction, and obtain an epoxy resin curing agent. Among them, the mass ratio of hydrolyzed lignin to NaOH aqueous solution is 1:5, the mass ratio of hydrolyzed lignin to amine reagent is 1:0.3, and the mass ratio of hydrolyzed lignin to formaldehyde is 1:0.4.

[0089] After curing the epoxy resin E51 and the epoxy resin curing agent in this comparative example, the gel content was measured to be 75.49%. The epoxy resin E51 and the epoxy resin curing agent were mixed in a mass ratio of 1:0.4.

[0090] Epoxy resin E51 and epoxy resin curing agent obtained in comparative example 2 were mixed in a mass ratio of 1:0.4, and 1% DMP-30 was added as an epoxy accelerator to obtain a lignin adhesive. The lignin adhesive was applied to the beech board, and two beech boards were bonded and fixed with a clamp. The dimensions of the two beech boards were 20 mm in width, 80 mm in length, and 5 mm in thickness; the area of ​​the lignin adhesive coating was 20 mm × 12 mm. The two bonded and fixed beech boards were placed in an oven, pre-cured at 60°C for 45 minutes, and then post-cured at 105°C for 1 hour, and then a universal tensile machine was used to perform a tensile shear test, and the bonding strength was 3.41 MPa.

[0091] Comparative Example 3

[0092] The commercial epoxy resin curing agent in this comparative example is diethylenetriamine with a mass concentration of ≥99%.

[0093] After curing the epoxy resin E51 and the commercial epoxy resin curing agent, the gel content was measured to be 97.78%, wherein the epoxy resin E51 and the commercial epoxy resin curing agent were mixed in a mass ratio of 1:0.2.

[0094] Epoxy resin E51 and commercial epoxy resin curing agent were mixed in a mass ratio of 1:0.2, and 1% DMP-30 was added as an epoxy accelerator to obtain lignin adhesive. The lignin adhesive was applied to the beech board, and two beech boards were bonded and fixed with a clamp. The dimensions of the two beech boards were 20 mm in width, 80 mm in length, and 5 mm in thickness; the area of ​​the lignin adhesive coating was 20 mm × 12 mm. The two bonded and fixed beech boards were placed in an oven, pre-cured at 60 ° C for 45 min, and then post-cured at 105 ° C for 1 h. The tensile shear test was performed using a universal tensile machine, and the bonding strength was 11.88 MPa.

[0095] The technical features in the claims and / or the specification of the present invention may be combined, and the combination is not limited to the combination obtained by reference in the claims. The technical solution obtained by combining the technical features in the claims and / or the specification is also within the protection scope of the present invention.

[0096] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a lignin-based epoxy resin curing agent, characterized in that: It includes the following steps: mixing lignin, an alkaline solution and an amine reagent to obtain a mixed solution; The mixed solution is heated to 60-90° C., and formaldehyde is added to carry out a Mannich reaction for 2.5-4 hours to obtain a lignin-based epoxy resin curing agent; The mass ratio of the lignin to the amine reagent is 1:0.2-0.35, and the mass ratio of the lignin to formaldehyde is 1:0.6-1.

2.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the lignin to the amine reagent is 1:0.3-0.

35.

3. The preparation method according to claim 1, characterized in that: The mass ratio of the lignin to formaldehyde is 1:0.8-1.

2.

4. The preparation method according to claim 1, characterized in that: The reaction temperature is 70-90°C and the reaction time is 3-4h.

5. The preparation method according to claim 1, characterized in that: Taking the mass of the mixed liquid as 100%, the adding rate of formaldehyde is 5% to 25% / min.

6. The preparation method according to claim 1, characterized in that: The amine reagent is at least one of diethylenetriamine, triethylenetetramine and tetraethylenepentamine.

7. A lignin-based epoxy resin curing agent, characterized in that: The structural formula of the lignin-based epoxy resin curing agent is as follows: Wherein R is -(CH2)2-(NH-(CH2)2)n-, and n=1~3.

8. The lignin-based epoxy resin curing agent according to claim 7, characterized in that: The gel content of the lignin-based epoxy resin curing agent after curing with the epoxy resin is greater than 80%. The curing method is pre-curing at 60° C. for 45 minutes and then post-curing at 105° C. for 1 hour.

9. A method for using the lignin-based epoxy resin curing agent according to any one of claims 7 to 8, characterized in that: The epoxy resin and the lignin-based epoxy resin curing agent are mixed in a mass ratio of 1:0.1-0.6 and then cured.

10. Use of the lignin-based epoxy resin curing agent according to any one of claims 7 to 8 in the field of curing agents.

Images