Full-bio-based Schiff base type epoxy curing agent as well as preparation method and application thereof

Abstract

The invention relates to a full bio-based Schiff base type epoxy curing agent as well as a preparation method and application thereof, the chemical structural formula of the epoxy curing agent is # imgabs0 #, in the formula, R1 is-H or-O-CH3; compared with the prior art, the epoxy resin has the advantages that the raw materials can be completely obtained by converting biomass resources, the preparation method is simple, and the cured epoxy resin is excellent in performance.

Description

Technical Field

[0001] The invention belongs to the technical field of curing agents and relates to a fully bio-based Schiff base epoxy curing agent and a preparation method and application thereof. Background Art

[0002] There are various types of epoxy curing agents, mainly amines, carboxylic acids, polyphenols, anhydrides, etc. Most curing agents rely on the presence of active hydrogen in the structure to open the epoxy group in the epoxy group and complete the curing. In the past few years, the global curing agent market has grown rapidly. By 2023, the global market size has reached US$2.53 billion, of which my country's curing agent market accounts for as much as 45%. It is the existence of various epoxy curing agents that gives epoxy resins many excellent properties, and they are widely used in composite materials, electronic packaging and many other fields. It can be said that epoxy curing agents play a decisive role in the application of epoxy resins.

[0003] However, traditional epoxy curing agents are all fossil resources, and the raw materials are non-renewable. At the same time, the severe environmental challenges brought about by the exploitation and processing of petroleum products have greatly restricted the development of epoxy resins based on fossil raw materials. Therefore, developing and utilizing bio-based renewable resources to prepare new all-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 invention is also proposed based on this. Summary of the invention

[0004] The purpose of the present invention is to provide a fully bio-based Schiff base epoxy curing agent and a preparation method and application thereof. The raw materials can be obtained entirely from biomass resources, and the preparation method is simple and the cured epoxy resin has excellent performance.

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

[0006] In the first aspect, a fully bio-based Schiff base epoxy curing agent has a chemical structural formula of:

[0007]

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

[0009]

[0010] In a second aspect, the present invention provides a method for preparing a fully bio-based Schiff base epoxy curing agent, wherein the fully bio-based Schiff base epoxy curing agent contains an amino group or does not contain an amino group, and when the fully bio-based Schiff base epoxy curing agent contains an amino group, the preparation method comprises the following steps:

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

[0012] (2) After the reaction is completed, the mixture is cooled to room temperature, washed, and dried to obtain an amino-containing all-biobased Schiff base epoxy curing agent;

[0013] Or when the all-biobased Schiff base 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 the inorganic base into an ethanol solution, then adding the ethanol solution of the bio-based aldehyde compound, heating and reacting;

[0015] S2. After the reaction is completed, cool to room temperature, filter once, wash once, and then redisperse in deionized water, adjust the pH value to be acidic, precipitate yellow powder, filter twice, wash twice, and dry to obtain a fully bio-based Schiff base epoxy curing agent that does not contain amino groups.

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

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

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

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

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

[0022] Furthermore, in step (1) and step S1, the reaction temperature is 60 to 120° C. and the reaction time is 5 to 12 hours.

[0023] In a third aspect, the present invention provides a use of a fully bio-based Schiff base epoxy curing agent in the preparation of cured epoxy resin.

[0024] Furthermore, when preparing the cured epoxy resin, the curing agent is first dissolved in a solvent and mixed evenly with the epoxy monomer and the curing accelerator. After the solvent is removed in vacuo, the resulting mixed system is poured into a mold for curing and forming to obtain the cured epoxy resin.

[0025] Furthermore, the epoxy monomer is one or a mixture of bisphenol A epoxy resin, bisphenol F epoxy resin, phenolic epoxy resin, tetraphenol ethane epoxy resin, resorcinol formaldehyde epoxy resin, diglycidyl phthalate, diglycidyl hexahydrophthalate, diglycidyl terephthalate, diglycidyl isophthalate, diglycidyl tetrahydrophthalate, diglycidyl methyltetrahydrophthalate, diglycidyl endomethyltetrahydrophthalate, diglycidyl adipic acid, pentaerythritol epoxy resin, melamine epoxy resin, and aniline epoxy resin.

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

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

[0028] Furthermore, the total mass fraction of the curing accelerator accounts for 0.5-3% of the mixed system after the solvent is removed.

[0029] Furthermore, the curing molding process is: first keep warm at 50℃~100℃ for 1h~4h, then keep warm at 100℃~150℃ for 1h~4h, and finally keep warm at 150℃~200℃ for 1h~4h.

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

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

[0032] (1) The main industrial production method for preparing the amino compounds required for the all-biobased Schiff base epoxy curing agent provided by the present invention is to obtain them through microbial fermentation. There are also industrial production cases of aldehyde compounds extracted from lignin. It is a new type of epoxy curing agent that can solve the problem of excessive dependence of existing epoxy curing agents on petroleum resources and achieve green and sustainable development.

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

[0034] Figure 1 This is the hydrogen nuclear magnetic resonance spectrum of the all-biobased Schiff base epoxy curing agent VLYS.

[0035] Figure 2 This is the hydrogen nuclear magnetic resonance spectrum of the all-biobased Schiff base epoxy curing agent VLEV.

[0036] Figure 3 This is the hydrogen nuclear magnetic resonance spectrum of the all-biobased Schiff base epoxy curing agent VCYS.

[0037] Figure 4 This is the hydrogen nuclear magnetic resonance spectrum of the all-biobased Schiff base epoxy curing agent VGLU.

[0038] Figure 5 The graph is a tensile performance graph of epoxy resin cured by the all-biobased Schiff base epoxy curing agent prepared by the present invention and a comparative curing agent.

[0039] Figure 6 The graph is a temperature-dependent curve of tan δ of epoxy resins cured with the all-biobased Schiff base epoxy curing agent prepared by the present invention and the comparative curing agent. The temperature corresponding to the tan δ peak is the glass transition temperature of the prepared resin. DETAILED DESCRIPTION

[0040] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0041] In the following examples, the bisphenol A epoxy resin used was purchased from Hangzhou Wuhuigang Adhesive Co., Ltd., with a brand name of E51.

[0042] Example 1

[0043] 5g of lysine was added to a 500mL round-bottom flask, and 100mL of anhydrous ethanol solvent was poured in to dissolve. Under vigorous stirring, 100mL of anhydrous ethanol solution containing 5.2g of vanillin was dripped into the round-bottom flask. After the addition was completed, nitrogen was introduced into the round-bottom flask, and the mixture was heated to 60°C and kept warm for 4h. A yellow powder was gradually generated in the system. After the reaction was complete, the filter cake was washed with 30mL of anhydrous ethanol after filtration, and then the solvent was removed by vacuum drying to obtain a fully bio-based Schiff base epoxy curing agent, which was recorded as VLYS ( Figure 1 ).

[0044] Example 2

[0045] Add 5g of levodopa and 1.01g of NaOH to a 500mL round-bottom flask, pour in 100mL of 95% ethanol to dissolve, and under vigorous stirring, drip 100mL of 95% ethanol solution containing 3.95g of vanillin into the round-bottom flask. After the addition is complete, nitrogen is passed into the round-bottom flask, heated to 80°C and kept warm for 4h, and a yellow powder is gradually generated in the system. After filtration, the filter cake is washed 3 times with 30mL of anhydrous ethanol. Dissolve the filter cake in 30mL of deionized water, drip 5% HCl solution to adjust the pH to 5, and the yellow powder slowly precipitates. After filtration, wash the filter cake with 30mL of deionized water, and vacuum dry to remove the solvent to obtain a fully bio-based Schiff base epoxy curing agent, recorded 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 resulting curing agent is recorded as VCYS ( Figure 3 ). In addition to the difference in molecular weight from VLEV, the curing agent VCYS synthesized using cysteine ​​introduces sulfur elements on the side chain.

[0048] Example 4

[0049] The difference from Example 2 is that 5g of levodopa is replaced with 5g of glutamic acid, and the obtained curing agent is recorded as VGLU ( Figure 4 ). In addition to the difference in molecular weight from VLEV, the curing agent synthesized using glutamic acid has a reduced benzene ring content and more carboxyl groups are introduced into the molecular structure. Example 5

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

[0051] Example 6

[0052] First, 10.4 g of VLEV was dissolved in 10 mL of anhydrous methanol. Then 0.26 g of imidazole, 0.09 g of 1,2-dimethylimidazole and 24.6 g of bisphenol A epoxy resin were added to ensure that the molar ratio of epoxy groups to VLVE in the resin 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 of corresponding size and then degassed at 50 °C for 0.5 hours. Finally, the resin was cured using the following procedure: 90 °C for 2 hours, 150 °C for 2 hours, and 190 °C for 2 hours to obtain the cured epoxy resin VLYS-E. After complete curing, it was cooled at room temperature for 4 hours to eliminate internal stress. Its tensile properties were measured using a universal tensile machine. The stress-strain curve is shown in Figure 5 DMA was used to obtain the glass transition temperature of VLEV-E ( Figure 6 ), 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 of VLEV is replaced by 8.0 g of curing agent VCYS obtained in Example 3. The tensile properties of VCYS-E are measured using a universal tensile machine, and the stress-strain curve is shown in Figure 5 , DMA was used to obtain the glass transition temperature of VCYS-E ( Figure 6 ), 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 DMA was used to obtain the glass transition temperature of VGLU-E ( Figure 6 ), the data are shown in Table 1.

[0057] Comparative Example 1

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

[0059] (2) Preparation of epoxy resin VLEVNa-E for comparison, which differs from Example 7 in that VLEVNa was used to replace VLEV for curing epoxy resin, and the cured epoxy resin was recorded as VLEVNa-E. The mechanical properties were measured using a universal tensile machine, and the stress-strain curve is shown in Figure 5 , data see Table 1.

[0060] Comparative Example 2

[0061] Preparation of epoxy resin DDM-E for comparison, using commercial epoxy curing agent (DDM) for curing bisphenol A epoxy resin, the difference from Example 7 is: using N, N-diaminodiphenylmethane (DDM) as curing agent instead of VLEV for curing epoxy resin, the cured epoxy resin is recorded as DDM-E. The mechanical properties are measured using a universal tensile machine, and the stress-strain curve is shown in Figure 5 DMA was used to obtain the glass transition temperature of VLEV-E ( Figure 6 ), 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 all-biobased epoxy curing agent synthesized in the present invention are higher than those of the epoxy resin DDM-E obtained by the commercially available curing agent. This indicates that the all-biobased epoxy curing agent prepared in the present invention not only generally has excellent mechanical and thermodynamic properties, but also can reduce the consumption of petroleum-based compounds and the environmental pollution problems caused during the mining process, meet the demand for 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, etc.

[0063] Comparative Examples 5-8 show that the introduction of different types of amino acids gives the resin different performance, indicating that the appropriate type of curing agent can be selected according to the performance requirements of different application scenarios. VLYS-E and VLEV-E have the most cross-linking sites (primary amino groups, carboxyl groups and phenolic hydroxyl groups), so the resin has stronger rigidity and relatively low elongation at break; VGLU-E contains the most carboxyl structures, which can react with the epoxy groups of epoxy resin to form ester bonds, which is beneficial to improve the toughness of the resin and has a relatively high elongation at break; according to the mechanical properties data of VCYS-E, it was found that the introduction of sulfur did not have a serious adverse effect on the performance of the resin.

[0064] Comparing the mechanical property data of Example 6 and Comparative Example 1, it is found that the hydrochloric acid solution washing step is necessary for the all-bio-based epoxy curing agent without amine group prepared from the second bio-based amino compound. The mechanical properties of the resin without acid washing will be seriously deteriorated, and the tensile strength of VLEVNa-E is reduced by nearly half compared with VLEV-E.

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

[0066] Table 1 Mechanical and thermodynamic properties test results of epoxy resins obtained in the examples and comparative examples

[0067] Tensile strength(MPa) Elongation at break (%) Glass transition temperature(℃) 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 embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A fully bio-based Schiff base epoxy curing agent, characterized in that: Its chemical structure is: Wherein, the R1 group is - H or - OCH3; R2 group is any of the following structures:

2. The method for preparing a fully bio-based Schiff base epoxy curing agent according to claim 1, characterized in that: The all-bio-based Schiff base epoxy curing agent contains amino groups or does not contain amino groups. When the all-bio-based Schiff base epoxy curing agent contains amino groups, the preparation method comprises the following steps: (1) dissolving a first bio-based amino compound in anhydrous ethanol, then adding an ethanol solution of a bio-based aldehyde compound, heating and reacting; (2) After the reaction is completed, the mixture is cooled to room temperature, washed, and dried to obtain an amino-containing all-biobased Schiff base epoxy curing agent; Or when the all-biobased Schiff base epoxy curing agent does not contain an amino group, the preparation method comprises the following steps: S1, dissolving the second bio-based amino compound and the inorganic base into an ethanol solution, then adding the ethanol solution of the bio-based aldehyde compound, heating and reacting; S2. After the reaction is completed, cool to room temperature, filter once, wash once, and then redisperse in deionized water, adjust the pH value to be acidic, precipitate yellow powder, filter twice, wash twice, and dry to obtain a fully bio-based Schiff base epoxy curing agent that does not contain amino groups.

3. The method for preparing a fully bio-based Schiff base epoxy curing agent according to claim 2, characterized in that: The first bio-based amino compound is selected from one of tryptophan, histidine, lysine, glutamine, asparagine or arginine; 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.

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 first bio-based amino compound to the bio-based aldehyde compound is 1:1 to 4; The molar ratio of the second bio-based amino compound, the inorganic base, and the 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 step (1) and step S1, the reaction temperature is 60 to 120° C. and the reaction time is 5 to 12 hours.

7. Use of the all-biobased Schiff base epoxy curing agent as claimed in claim 1 in the preparation of cured epoxy resin.

8. The use of the all-biobased Schiff base epoxy curing agent according to claim 7, characterized in that: When preparing the cured epoxy resin, the curing agent is first dissolved in a solvent and mixed evenly with the epoxy monomer and the curing accelerator. After the solvent is removed in vacuo, the resulting mixed system is poured into a mold for curing and forming to obtain the cured epoxy resin.

9. The use of the all-biobased Schiff base epoxy curing agent according to claim 8, characterized in that: The epoxy monomer is one or a mixture of bisphenol A epoxy resin, bisphenol F epoxy resin, novolac epoxy resin, tetraphenol ethane epoxy resin, resorcinol formaldehyde epoxy resin, diglycidyl phthalate, diglycidyl hexahydrophthalate, diglycidyl terephthalate, diglycidyl isophthalate, diglycidyl tetrahydrophthalate, diglycidyl methyltetrahydrophthalate, diglycidyl endomethyltetrahydrophthalate, diglycidyl adipic acid, pentaerythritol epoxy resin, melamine epoxy resin, and aniline epoxy resin; The curing accelerator is one or a mixture of acetylacetone metal complex, imidazole, 1,2-dimethylimidazole, 1,3-dimethyl-2-imidazolidinone, dimethylbenzylamine and tetrabutylphosphonium bromide.

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

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