A fully bio-based schiff base type epoxy curing agent, and a preparation method and application thereof

Abstract

The present application relates to a kind of full biological base Schiff base type epoxy curing agent and its preparation method and application, and the chemical structural formula of the epoxy curing agent is as follows: Wherein, R1 Group is -H or -O-CH3;R2 Group is any one of the following structures: Compared with prior art, the raw material of the present application can be converted from biomass resources, and the preparation method is simple, and the performance of epoxy resin after curing is excellent.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of curing agents, and relates to a full-bio-based Schiff base type epoxy curing agent and a preparation method and application thereof. BACKGROUND

[0002] There are various types of epoxy curing agents, mainly including amine, carboxylic acid, polyphenol, acid anhydride and the like. Most curing agents rely on the existence of active hydrogen in the structure to make the epoxy group in the epoxy group open ring and then complete curing. In the past few years, the global curing agent market has grown rapidly, and the global market size has reached 25.3 billion US dollars by 2023, of which the curing agent market share in China is as high as 45%. It is the existence of various epoxy curing agents that endows epoxy resin with many excellent properties, and then is widely used in composite materials, electronic packaging and many other fields. It can be said that the epoxy curing agent plays a decisive role in the application of the epoxy resin.

[0003] However, the traditional epoxy curing agent is a fossil resource, and the raw material is not renewable. At the same time, the serious environmental challenges brought by the process of oil product mining and processing also greatly limit the development of epoxy resin based on fossil raw materials. Therefore, developing and utilizing bio-based renewable resources to prepare new full-bio-based epoxy curing agents that can replace traditional petroleum-based epoxy curing agents is an effective way to solve the above problems. The present application is also proposed based on this. SUMMARY

[0004] The purpose of the present application is to provide a full-bio-based Schiff base type epoxy curing agent and a preparation method and application thereof, which can be fully converted from biomass resources, and the preparation method is simple, and the performance of the cured epoxy resin is excellent.

[0005] The purpose of the present application can be achieved by the following technical solutions:

[0006] In a first aspect, a full-bio-based Schiff base type epoxy curing agent has a chemical structural formula as follows:

[0007]

[0008] In which, R1 group is - H or - OCH3; R2 group is any one of the following structures:

[0009]

[0010] In a second aspect, the present application provides a preparation method of a full-bio-based Schiff base type epoxy curing agent, which contains or does not contain amino groups, and when the full-bio-based Schiff base type epoxy curing agent contains amino groups, the preparation method comprises the following steps:

[0011] (1) dissolving the first bio-based amino compound into anhydrous ethanol, adding an ethanol solution of bio-based aldehyde-based compound, heating and warming, and reacting;

[0012] (2) after the reaction is completed, cooling to room temperature, washing, and drying to obtain a full bio-based Schiff base type epoxy curing agent containing an amino group;

[0013] or when the full bio-based Schiff base type epoxy curing agent does not contain an amino group, the preparation method comprises the following steps:

[0014] S1, dissolving the second bio-based amino compound and inorganic base into an ethanol solution, adding an ethanol solution of bio-based aldehyde-based compound, heating and warming, and reacting;

[0015] S2, after the reaction is completed, cooling to room temperature, filtering once, washing once, re-dispersing into deionized water, adjusting the pH value to be acidic, precipitating yellow powder, filtering twice, washing twice, and drying to obtain a full bio-based Schiff base type epoxy curing agent not containing an amino group.

[0016] Further, the first bio-based amino compound is selected from one of tryptophan, histidine, lysine, glutamine, asparagine or arginine. Preferably, asparagine, lysine, glutamine, arginine.

[0017] Further, the second bio-based amino compound is selected from one of glycine, alanine, valine, leucine, isoleucine, phenylalanine, tyrosine, aspartic acid, glutamic acid, methionine, serine, threonine, cysteine or levodopa. Preferably, glycine, tyrosine, aspartic acid, glutamic acid, levodopa.

[0018] Further, the molar ratio of the first bio-based amino compound, bio-based aldehyde-based compound is 1:1-4.

[0019] Further, the molar ratio of the second bio-based amino compound, inorganic base, bio-based aldehyde-based compound is 1:(1-2):(1-4).

[0020] Further, the inorganic base is one of lithium hydroxide, sodium hydroxide or potassium hydroxide.

[0021] Further, the bio-based aldehyde-based compound is one of vanillin or syringaldehyde.

[0022] Further, in steps (1) and S1, the temperature of the reaction is 60-120°C, and the time is 5h-12h.

[0023] In a third aspect, the present application provides a full bio-based Schiff base type epoxy curing agent for preparing a cured epoxy resin.

[0024] Further, in the preparation of the cured epoxy resin, the curing agent is first dissolved in a solvent, and mixed with the epoxy monomer and the curing accelerator, and after vacuum removal of the solvent, the obtained mixed system is poured into a mold for curing and forming to obtain the cured epoxy resin.

[0025] Further, the epoxy monomer is one or a mixture of several of bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenolic epoxy resin, tetraphenyl ethane epoxy resin, resorcinol formaldehyde epoxy resin, phthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, terephthalic acid diglycidyl ester, isophthalic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, methyl tetrahydrophthalic acid diglycidyl ester, end methylene tetrahydrophthalic acid diglycidyl ester, adipic acid diglycidyl ester, pentaerythritol epoxy resin, melamine epoxy resin, aniline epoxy resin.

[0026] Further, the curing accelerator is one or a mixture of several of acetylacetone metal complex, imidazole, 1,2-dimethylimidazole, 1,3-dimethyl-2-imidazolidinone, dimethylbenzylamine and tetrabutylphosphonium bromide.

[0027] Further, the addition ratio of the epoxy monomer and the curing agent satisfies that the equivalent ratio of the epoxy groups in the epoxy monomer to the active hydrogen from the carboxyl, phenolic hydroxyl or amino group in the curing agent is 1:0.7-1.

[0028] Further, the total mass fraction of the curing accelerator in the mixed system after removal of the solvent is 0.5-3%.

[0029] Further, the curing and forming process is: first, incubate at 50-100℃ for 1-4h, then incubate at 100-150℃ for 1-4h, and finally incubate at 150-200℃ for 1-4h.

[0030] Further, in step S2, the first washing is washing with anhydrous methanol for 3-6 times, and the second washing is washing the filter cake with 30mL of deionized water for 3-6 times; the pH adjustment process is adding 5% HCl solution dropwise until the solution is weakly acidic, and the yellow powder slowly precipitates.

[0031] Compared with the prior art, the present application has the following advantages:

[0032] (1) The main industrial production method of the amino compound required for the preparation of the full-bio-based Schiff base type epoxy curing agent provided by the present application is microbial fermentation, and the aldehyde compound has also been industrialized from lignin extraction, which is a new type of epoxy curing agent, can solve the problem of excessive dependence on petroleum resources of existing epoxy curing agents, and realizes green and sustainable development.

[0033] (2) Based on the different amino acids introduced during the preparation process of the curing agent as the bio-based amino compound, different chain or aromatic ring structures contained therein can meet the multiple requirements of different application scenarios for the toughness and rigidity of the cured resin, and the high cross-linking density caused by the multiple active sites can effectively improve the thermal stability of the cured epoxy resin, so it can be applied to high-tech fields such as aerospace and electronic products as a high-performance epoxy resin. Finally, the preparation process is simple, efficient, stable and controllable, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The nuclear magnetic resonance hydrogen spectrum of the full-bio-based Schiff base type epoxy curing agent VLYS.

[0035] Figure 2 The nuclear magnetic resonance hydrogen spectrum of the full-bio-based Schiff base type epoxy curing agent VLEV.

[0036] Figure 3 The nuclear magnetic resonance hydrogen spectrum of the full-bio-based Schiff base type epoxy curing agent VCYS.

[0037] Figure 4 The nuclear magnetic resonance hydrogen spectrum of the full-bio-based Schiff base type epoxy curing agent VGLU.

[0038] Figure 5 The tensile property diagram of the epoxy resin cured by the full-bio-based Schiff base type epoxy curing agent prepared by the present application and the comparative curing agent.

[0039] Figure 6 The tan delta-temperature curve diagram of the epoxy resin cured by the full-bio-based Schiff base type epoxy curing agent prepared by the present application and the comparative curing agent, and the temperature corresponding to the tan delta peak is the glass transition temperature of the prepared resin. DETAILED DESCRIPTION

[0040] The present application will be described in detail below in combination with the drawings and specific examples. The present embodiment is implemented on the premise of the technical solution of the present application, and detailed implementation and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0041] In the following examples, the bisphenol A type epoxy resin used is purchased from Hangzhou Wuhuihang Adhesive Co., Ltd., and the model number is E51.

[0042] Example 1

[0043] In a 500 mL round-bottom flask, 5 g of lysine was added and dissolved in 100 mL of anhydrous ethanol solvent. Under vigorous stirring, 100 mL of anhydrous ethanol solution containing 5.2 g of vanillin was added dropwise into the round-bottom flask. After the dropwise addition was completed, nitrogen was introduced into the round-bottom flask, and the system was heated to 60°C for 4 h. Yellow powder gradually formed in the system. After the reaction was completed, the filter cake was washed with 30 mL of anhydrous ethanol, and then vacuum dried to remove the solvent to obtain a full-bio-based Schiff base type epoxy curing agent, denoted as VLYS Figure 1 ).

[0044] Example 2

[0045] In a 500 mL round-bottom flask, 5 g of levodopa and 1.01 g of NaOH were added and dissolved in 100 mL of 95% ethanol. Under vigorous stirring, 100 mL of 95% ethanol solution containing 3.95 g of vanillin was added dropwise into the round-bottom flask. After the dropwise addition was completed, nitrogen was introduced into the round-bottom flask, and the system was heated to 80°C for 4 h. Yellow powder gradually formed in the system. After the reaction was completed, the filter cake was washed with 30 mL of anhydrous ethanol three times. The filter cake was dissolved in 30 mL of deionized water, and a 5% HCl solution was added dropwise to adjust the pH to 5. Yellow powder slowly precipitated, and the filter cake was washed with 30 mL of deionized water and vacuum dried to remove the solvent to obtain a full-bio-based Schiff base type epoxy curing agent, denoted as VLEV Figure 2 ).

[0046] Example 3

[0047] The difference from Example 2 is that 5 g of levodopa is replaced by 4.1 g of cysteine, and the obtained curing agent is denoted as VCYS Figure 3 In addition to the difference in molecular weight from VLEV, the curing agent VCYS synthesized using cysteine introduces a sulfur element in the side chain.

[0048] Example 4

[0049] The difference from Example 2 is that 5 g of levodopa is replaced by 5 g of glutamic acid, and the obtained curing agent is denoted as VGLU Figure 4 In addition to the difference in molecular weight from VLEV, the curing agent synthesized using glutamic acid reduces the content of benzene ring and introduces more carboxyl groups in the molecular structure. Example 5

[0050] Firstly, 7 g VLYS was dissolved in 10 mL anhydrous methanol. Then, 0.17 g imidazole, 0.17 g 1,2-dimethylimidazole and 28 g bisphenol A type epoxy resin were added to ensure the molar ratio of epoxy groups in the resin to VLYS was 4:1. After stirring at room temperature for 10 min, the solvent was fully evaporated by vacuum distillation. The obtained viscous liquid was poured into a metal mold with corresponding size, and then degassed at 50 °C for 0.5 h. Finally, the resin was cured using the following program: 80 °C for 2 h, 130 °C for 2 h, 150 °C for 2 h, to obtain the cured epoxy resin VLYS-E. After complete curing, it was cooled at room temperature for 4 h to eliminate internal stress, and its tensile properties were measured using a universal tensile machine, the stress-strain curve is shown in Figure 5 , the glass transition temperature of VLYS-E was obtained using DMA Figure 6 , and the data are shown in Table 1.

[0051] Example 6

[0052] Firstly, 10.4 g VLEV was dissolved in 10 mL anhydrous methanol. Then, 0.26 g imidazole, 0.09 g 1,2-dimethylimidazole and 24.6 g bisphenol A type epoxy resin were added to ensure the molar ratio of epoxy groups in the resin to VLVE was 4:1. After stirring at room temperature for 10 min, the solvent was fully evaporated by vacuum distillation. The obtained viscous liquid was poured into a metal mold with corresponding size, and then degassed at 50 °C for 0.5 h. Finally, the resin was cured using the following program: 90 °C for 2 h, 150 °C for 2 h, 190 °C for 2 h, to obtain the cured epoxy resin VLYS-E. After complete curing, it was cooled at room temperature for 4 h to eliminate internal stress, and its tensile properties were measured using a universal tensile machine, the stress-strain curve is shown in Figure 5 , the glass transition temperature of VLEV-E was obtained using DMA Figure 6 , and the data are shown in Table 1.

[0053] Example 7

[0054] Preparation of epoxy resin VCYS-E based on VCYS: The difference from Example 6 is that 10.4 g VLEV is replaced by 8.0 g curing agent VCYS obtained from Example 3. The tensile properties of VCYS-E were measured using a universal tensile machine, the stress-strain curve is shown in Figure 5 , the glass transition temperature of VCYS-E was obtained using DMA Figure 6 , and the data are shown in Table 1.

[0055] Example 8

[0056] Preparation of epoxy resin VGLU-E based on VGLU: The difference from Example 6 is that 10.4 g of VLEV is replaced by 8.8 g of curing agent VGLU obtained from Example 4. The tensile properties of VCYS-E are measured using a universal tensile machine, and the stress-strain curve is shown in Figure 5 The glass transition temperature of VGLU-E is obtained using DMA ( Figure 6 ), and the data are shown in Table 1.

[0057] Comparative Example 1

[0058] (1) Preparation of epoxy curing agent VLEVNa for comparison. The difference from Example 3 is that the yellow powder obtained by reaction is filtered, washed with 30 mL of anhydrous ethanol for 3 times, and then directly vacuum dried to obtain a curing agent containing sodium carboxylate salt, which is denoted as VLEVNa.

[0059] (2) Preparation of epoxy resin VLEVNa-E for comparison. The difference from Example 7 is that VLEVNa is used instead of VLEV for the curing of the epoxy resin, and the cured epoxy resin is denoted as VLEVNa-E. The mechanical properties thereof are measured using a universal tensile machine, and the stress-strain curve is shown in Figure 5 , and the data are shown in Table 1.

[0060] Comparative Example 2

[0061] Preparation of epoxy resin DDM-E for comparison. A commercially available epoxy curing agent (DDM) is used for the curing of bisphenol A type epoxy resin. The difference from Example 7 is that N,N-diamino diphenyl methane (DDM) is used as a curing agent instead of VLEV for the curing of the epoxy resin, and the cured epoxy resin is denoted as DDM-E. The mechanical properties thereof are measured using a universal tensile machine, and the stress-strain curve is shown in Figure 5 The glass transition temperature of VLEV-E is obtained using DMA ( Figure 6 ), and the data are shown in Table 1.

[0062] As can be seen from Table 1, the tensile strength and glass transition temperature of the epoxy resin prepared based on the full-bio-based epoxy curing agent synthesized according to the present application are higher than those of the commercially available curing agent DDM-E. It is indicated that the full-bio-based epoxy curing agent prepared according to the present application not only has excellent mechanical properties and thermodynamic properties, but also can reduce the consumption of petroleum-based compounds and the environmental pollution problems caused in the mining process, meet the demand of green and sustainable development of chemical products, and has great potential in replacing traditional petroleum-based curing agents in the fields of aerospace, electronic chemicals and the like.

[0063] Comparative Example 5-8, the introduction of different kinds of amino acids gives the resin different performance, which shows that the appropriate kind of curing agent can be selected according to the performance requirements of different application scenarios. VLYS-E and VLEV-E have the most crosslinking sites (primary amino, carboxyl and phenolic hydroxyl), the resin is more rigid and the elongation at break is relatively low; VGLU-E contains the most carboxyl structure, which can react with the epoxy groups of epoxy resin to form ester bond, which is beneficial to improve the toughness of the resin, and the elongation at break is relatively high; according to the mechanical property data of VCYS-E, it is found that the introduction of sulfur element does not cause serious adverse effects on the performance of the resin.

[0064] Comparative Example 6 and Comparative Example 1, it is found that for the full bio-based epoxy curing agent prepared from the second bio-based amino compound and not containing amine group, the hydrochloric acid solution washing step is necessary. The mechanical properties of the resin without acid washing will be seriously deteriorated, and compared with VLEV-E, the tensile strength of VLEVNa-E is reduced by nearly half.

[0065] In summary, the epoxy resin obtained based on the curing agent of the present application has excellent mechanical and thermal properties, and the raw materials used for the preparation of the curing agent are renewable, which has special advantages in high-end application fields such as aerospace, electronic chemicals, etc.

[0066] Table 1 Mechanical and thermal properties of epoxy resins obtained from examples and comparative examples

[0067] Tensile strength (MPa) Elongation at break (%) Glass transition temperature (°C) VLYS-E 61.5 5.93 141 VLEV-E 82.0 4.37 181 VCYS-E 64.8 4.78 132 VGLU-E 73.7 7.02 134 VLEVNa-E 44.5 3.18 - DDM-E 52.5 5.53 120

[0068] The above description of the examples is for the convenience of the ordinary skilled person in the art to understand and use the invention. Those skilled in the art can easily make various modifications to these examples, and apply the general principles described herein to other examples without the need for creative labor. Therefore, the present application is not limited to the above examples, and the improvements and modifications made by those skilled in the art without departing from the scope of the present application should be within the scope of protection of the present application.

Claims

1. A fully bio-based Schiff base epoxy curing agent, characterized in that, Its chemical structural formula is: ， Wherein, R1 group is -H or -O-CH3; R2 group is any of the following structures: 。 2. The preparation method of a fully bio-based Schiff base epoxy curing agent as described in claim 1, characterized in that, Includes the following steps: S1. Dissolve the second bio-based amino compound and the inorganic base in an ethanol solution, then add an ethanol solution of the bio-based aldehyde compound, heat to increase the temperature, and react. S2. After the reaction is complete, cool to room temperature, filter once, wash once, and then redisperse in deionized water. Adjust the pH to acidic, and precipitate a yellow powder. Filter again, wash twice, and dry to obtain an amino-free, fully bio-based Schiff base epoxy curing agent.

3. The method for preparing a fully bio-based Schiff base epoxy curing agent according to claim 2, characterized in that, The second bio-based amino compound is selected from one of glutamic acid, cysteine, or levodopa.

4. The method for preparing a fully bio-based Schiff base epoxy curing agent according to claim 2, characterized in that, The molar ratio of the second bio-based amino compound, inorganic base, and bio-based aldehyde compound is 1:(1~2):(1~4).

5. The method for preparing a fully bio-based Schiff base epoxy curing agent according to claim 2, characterized in that, The inorganic base is one of lithium hydroxide, sodium hydroxide, or potassium hydroxide; The bio-based aldehyde compound is one of vanillin or syringaldehyde.

6. The method for preparing a fully bio-based Schiff base epoxy curing agent according to claim 2, characterized in that, In steps (1) and S1, the reaction temperature is 60~120℃ and the time is 5 h~12 h.

7. The application of the fully bio-based Schiff base epoxy curing agent as described in claim 1 in the preparation of cured epoxy resin.

8. The application of the fully bio-based Schiff base epoxy curing agent according to claim 7, characterized in that, In preparing cured epoxy resin, the curing agent is first dissolved in a solvent and mixed evenly with epoxy monomers and curing accelerators. After removing the solvent under vacuum, the resulting mixture is poured into a mold and cured to obtain cured epoxy resin.

9. The application of the fully bio-based Schiff base epoxy curing agent according to claim 8, characterized in that, The epoxy monomer is one or a mixture of several of the following: bisphenol A epoxy resin, bisphenol F epoxy resin, phenolic epoxy resin, tetraphenol ethane epoxy resin, resorcinol formaldehyde epoxy resin, diglycidyl phthalate, hexahydrophthalic acid diglycidyl phthalate, diglycidyl terephthalate, diglycidyl isophthalate, tetrahydrophthalic acid diglycidyl phthalate, methyltetrahydrophthalic acid diglycidyl phthalate, methylenetetrahydrophthalic acid diglycidyl phthalate, diglycidyl adipate, pentaerythritol epoxy resin, melamine epoxy resin, and aniline epoxy resin. The curing accelerator is one or a mixture of several of the following: acetylacetone metal complex, imidazole, 1,2-dimethylimidazolium, 1,3-dimethyl-2-imidazolinone, dimethylbenzylamine, and tetrabutylphosphine bromide.

10. The application of the fully bio-based Schiff base epoxy curing agent according to claim 8, characterized in that, The addition ratio of the epoxy monomer to the curing agent satisfies the following: the equivalent ratio of the epoxy group in the epoxy monomer to the active hydrogen from the carboxyl, phenolic hydroxyl or amino group in the curing agent is 1:0.7~1. The total mass fraction of the curing accelerator is 0.5-3% of the mixed system after solvent removal; The curing process is as follows: first, keep at 50℃~100℃ for 1 h~4 h, then keep at 100℃~150℃ for 1 h~4 h, and finally keep at 150℃~200℃ for 1 h~4 h.

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