1. An enzymatic hydrolysis lignin-based curing accelerator is characterized by having the following structural formula:

in the formula, Lignin represents enzymolysis Lignin, R₁、R₂Independently is a C1-C4 hydrocarbyl group, R₃Is H or a hydrocarbon radical of C1-C4, R₄Is H or CH₃。

2. The curing accelerator as claimed in claim 1, wherein the curing accelerator is a product obtained by performing Mannich reaction on abundant phenolic hydroxyl groups on enzymatic hydrolysis lignin, formaldehyde and secondary amine to obtain tertiary amine modified enzymatic hydrolysis lignin, and performing Wlliamson reaction on the tertiary amine enzymatic hydrolysis lignin to perform etherification on phenolic hydroxyl groups on the tertiary amine enzymatic hydrolysis lignin with alkyl halide.

3. The curing accelerator according to claim 2, wherein the weight ratio of the formaldehyde to the secondary amine is 5-7:14-20, and the weight of the formaldehyde is 1.5-1.8 times of the weight of the enzymatic hydrolysis lignin; the alkyl halide is 1.2 to 1.5 times of the weight of the tertiary amination enzymolysis lignin.

4. The cure accelerator according to claim 2, wherein the secondary amine is at least one selected from the group consisting of diethylamine, dipropylamine, N-ethylpropylamine, and N-methylpropylamine; the alkyl halide is at least one selected from methyl chloride, methyl bromide, ethyl chloride, propyl chloride, methyl iodide, ethyl bromide, propyl bromide, ethyl iodide and 1-iodopropane.

5. The curing accelerator according to claim 2, wherein the enzymatic lignin extraction is solution extraction.

6. A process for producing the curing accelerator as claimed in any one of claims 1 to 5, which comprises the steps of:

1) tertiary amination of enzymatic lignin: adding alkali liquor into a reaction kettle containing enzymatic hydrolysis lignin, adjusting pH, stirring, heating, keeping constant temperature, adding secondary amine, stirring uniformly, dropwise adding a formaldehyde solution, carrying out reflux reaction under a stirring state, cooling to room temperature after the reaction is finished, diluting, carrying out acid precipitation, filtering, washing filter residues to be neutral, and obtaining tertiary amination enzymatic hydrolysis lignin for later use;

2) etherification of tertiary amination enzymatic lignin: adding the tertiary amination enzymatic hydrolysis lignin obtained in the step 1) into a mixed solution of alkali, n-butanol and dioxane, stirring until the tertiary amination enzymatic hydrolysis lignin is completely dissolved, adding alkyl halide, stirring uniformly, keeping stirring and heating to a reflux state, reacting at a constant temperature, naturally cooling to room temperature after the reaction is finished, then pouring ice water, filtering when no precipitate is separated out, washing with the ice water, and drying in vacuum to obtain brown viscous liquid.

7. An epoxy resin prepreg comprises the following raw materials in parts by weight: 70-100 parts of bisphenol A epoxy resin, 120-160 parts of carbon fiber, 3-10 parts of dicyandiamide and 0.5-2 parts of the curing accelerator based on enzymatic hydrolysis lignin as claimed in any one of claims 1-5.

8. The prepreg of claim 7, further comprising 1 to 3 parts of a substituted urea promoter having a hydroxyl group on the benzene ring.

9. The prepreg of claim 8, wherein the substituted urea having a hydroxyl group on the benzene ring is at least one selected from the group consisting of N- (2-hydroxyphenyl) -N ', N' -dimethylurea, N- (5-chloro-2-hydroxyphenyl) -N ', N' -dimethylurea, N- (2-hydroxy-4-nitrophenyl) -N ', N' -dimethylurea, N- (2-hydroxy-5-nitrophenyl) -N ', N' -dimethylurea.

10. A process for the preparation of a prepreg according to any one of claims 7 to 9, comprising the steps of: 1) uniformly mixing bisphenol A type epoxy resin, dicyandiamide and the curing accelerator, adding the mixture into a reaction kettle, and heating and melting the mixture;

2) adding the molten mixture obtained in the step 1) into a glue tank of a hot-melt glue spreader, preparing a film, cooling and rolling to obtain a glue film;

3) placing the glue film obtained in the step 2) in a compound machine, synchronously feeding the carbon fibers and the glue film into a compound roller of the compound machine, heating and compounding, and then cooling and rolling to obtain the carbon fiber resin prepreg.

Background

The latent curing agent is a compound which is not reacted after being compounded with epoxy resin and is placed at room temperature for a long time, and can rapidly crosslink and cure the epoxy resin under the conditions of heating, illumination, moisture, pressurization and the like. Among them, dicyandiamide is the most used heat-activated latent curing agent, also known as dicyandiamide, cyanoguanidine. Dicyandiamide is usually dispersed in epoxy resin in powder form, the storage period at normal temperature can reach more than half a year, but the curing temperature (curing condition: 160-180 ℃ for 20-60min) is higher than the temperature which can be borne by a plurality of products or production processes, and the application range is limited, so how to reduce the curing temperature of dicyandiamide without excessively damaging the storage period and the application performance of dicyandiamide becomes a research hotspot of dicyandiamide as an epoxy resin curing agent.

Patent CN201510951638.8 discloses an epoxy resin composition for prepreg, a preparation method thereof and the prepreg, wherein the composition comprises a component A and a component B, the component A contains solid epoxy resin and a toughening agent, and the component B contains liquid epoxy resin, dicyandiamide curing agents, imidazole curing agents and urea accelerators; the weight ratio of the dicyandiamide curing agent to the imidazole curing agent to the urea promoter is 1:2:1.5-1:7.5: 1.5. The epoxy resin composition for the prepreg effectively improves the minimum viscosity retention time, improves the gelation time, and greatly prolongs the process time window. Patent CN201610703684.0 discloses a low-temperature curing epoxy resin adhesive and a preparation method thereof, which comprises the following components in percentage by weight: 25-40% of bisphenol F epoxy resin, 15-25% of bisphenol A epoxy resin, 10-20% of amine curing agent, 10-20% of polythiol accelerator, 4-10% of thixotropic agent, 10-20% of calcium carbonate powder and 0.1-1% of toner. Wherein the amine curing agent is Dicyandiamide (DICY), the polythiol accelerator is 3-mercaptopropionate (TMPMP), and the curing temperature of the epoxy resin adhesive is obviously reduced, and the curing time is greatly shortened. The above patents all mention epoxy/dicyandiamide curing systems, which are all seen to be relatively high in curing temperature, and even with the use of accelerators, there is no significant reduction in curing temperature, or even a reduction in curing temperature but a concomitant reduction in pot life.

Therefore, there is a need for an accelerator for epoxy/dicyandiamide curing systems that is effective in lowering the curing temperature without causing excessive damage to pot life and performance.

Disclosure of Invention

Aiming at the technical problems, the invention provides an accelerator of a curing agent and a preparation method thereof, wherein the accelerator is prepared by sequentially carrying out tertiary amination and etherification two-step modification on enzymatic hydrolysis lignin, and the obtained accelerator not only can reduce the curing temperature of an epoxy/dicyandiamide system, but also has the effect of improving the toughness of epoxy resin.

An enzymatic hydrolysis lignin-based curing accelerator is characterized by having the following structural formula:

in the formula, Lignin represents enzymolysis Lignin, R₁、R₂Independently is a C1-C4 hydrocarbyl group, R₃Is H or a hydrocarbon radical of C1-C4, R₄Is H or CH₃。

The curing accelerator is a product obtained by performing Mannich reaction on phenolic hydroxyl groups rich in enzymatic hydrolysis lignin, formaldehyde and secondary amine to obtain tertiary amine modified enzymatic hydrolysis lignin, and performing Wlliamson reaction on the tertiary amine modified enzymatic hydrolysis lignin and alkyl halide to etherify the phenolic hydroxyl groups on the tertiary amine enzymatic hydrolysis lignin.

The enzymatic hydrolysis lignin is solvent type enzymatic hydrolysis lignin.

The weight ratio of the formaldehyde to the secondary amine is 5-7:14-20, and the weight of the formaldehyde is 1.5-1.8 times of that of the enzymatic hydrolysis lignin.

The alkyl halide is 1.2 to 1.5 times of the weight of the tertiary amination enzymolysis lignin.

A preparation method of a modified enzymatic hydrolysis lignin epoxy/dicyandiamide curing accelerator comprises the following steps:

1) tertiary amination of enzymatic lignin: adding alkali liquor into a reaction kettle containing enzymatic hydrolysis lignin, adjusting pH, stirring, heating, keeping constant temperature, adding secondary amine, stirring uniformly, dropwise adding a formaldehyde solution, carrying out reflux reaction under a stirring state, cooling to room temperature after the reaction is finished, diluting, carrying out acid precipitation, filtering, washing filter residues to be neutral, and obtaining tertiary amination enzymatic hydrolysis lignin for later use;

2) etherification of tertiary amination enzymatic lignin: adding the tertiary amination enzymatic hydrolysis lignin obtained in the step 1) into a mixed solution of alkali, n-butanol and dioxane, stirring until the tertiary amination enzymatic hydrolysis lignin is completely dissolved, adding alkyl halide, stirring uniformly, keeping stirring and heating to a reflux state, reacting at a constant temperature, naturally cooling to room temperature after the reaction is finished, then pouring ice water, filtering when no precipitate is separated out, washing with the ice water, and drying in vacuum to obtain brown viscous liquid.

The secondary amine in the step 1) is at least one of diethylamine, dipropylamine, N-ethylpropylamine and N-methylpropylamine; the alkali liquor is not particularly limited, sodium hydroxide or potassium hydroxide solution commonly used in the field can be used, the concentration of the alkali liquor is 20-40 wt%, the pH is adjusted to 10.5-12, the temperature is raised to 30-60 ℃, the dropping time of the formaldehyde solution is controlled to 20-50min, the concentration of the formaldehyde solution is 30-50 wt%, the constant temperature reaction time is 3-5h, the dilution is water dilution, and the acid precipitation is hydrochloric acid solution with the concentration of 20-40 wt%, so that precipitate is precipitated.

Step 1) is a Mannich reaction, and tertiary amination enzymolysis lignin can improve the electron cloud density around nitrogen atoms so as to improve the alkalinity of the lignin and improve the promotion effect.

The alkali in the step 2) is sodium hydroxide or potassium hydroxide commonly used in the art, the concentration of the alkali in the mixed solution of the alkali, the n-butanol and the dioxane is 5-10 wt%, and the volume ratio of the n-butanol to the dioxane is 1-3: 10; the alkyl halide is at least one selected from methyl chloride, methyl bromide, ethyl chloride, chloropropane, methyl iodide, ethyl bromide, propyl bromide, ethyl iodide and 1-iodopropane, and the constant-temperature reaction time is 1-3 h.

And step 2) is Wlliamson reaction, the power supply effect of the ether is greater than that of the phenolic hydroxyl group, further etherification treatment is favorable for improving the alkalinity of the accelerator, and the influence of the acidity of the phenol on the enzymatic hydrolysis lignin on the accelerating effect is eliminated.

The enzymatic hydrolysis lignin is a natural high molecular polymer, is a biomass resource with second content stored globally, and is extracted from enzymatic hydrolysis residues of chemicals prepared from plant straws, corncobs and the like. The enzymatic hydrolysis lignin is chemically treated to be used as a curing accelerator of an epoxy/dicyandiamide system, so that the reutilization of resources is facilitated.

The enzymatic hydrolysis lignin is extracted from fermentation residues of plant straws, corncobs and the like, and is extracted by a solvent, and the process does not comprise the process steps of high temperature and high pressure, strong acid and strong alkali and the like, so that various active groups of the lignin are well reserved. The water content of the enzymatic hydrolysis lignin is 5-7%.

An epoxy resin prepreg comprises the following raw materials in parts by weight: 70-100 parts of bisphenol A epoxy resin, 120-160 parts of carbon fiber, 3-10 parts of dicyandiamide and 0.5-2 parts of the curing accelerator.

The bisphenol A epoxy resin is selected from at least one of E-44, E-51 and E-54.

The particle size of the dicyandiamide curing agent is 20-50 mu m.

The prepreg may further include 1 to 3 parts of a substituted urea accelerator having a hydroxyl group on the benzene ring.

The substituted urea containing hydroxyl on the benzene ring is at least one selected from N- (2-hydroxyphenyl) -N ', N' -dimethyl urea, N- (5-chloro-2-hydroxyphenyl) -N ', N' -dimethyl urea, N- (2-hydroxy-4-nitrobenzene) -N ', N' -dimethyl urea and N- (2-hydroxy-5-nitrobenzene) -N ', N' -dimethyl urea.

The invention also provides a preparation method of the epoxy resin prepreg, which comprises the following steps:

1) uniformly mixing bisphenol A type epoxy resin, dicyandiamide and the curing accelerator, adding the mixture into a reaction kettle, and heating and melting the mixture;

2) adding the molten mixture obtained in the step 1) into a glue tank of a hot-melt glue spreader, preparing a film, cooling and rolling to obtain a glue film;

3) placing the glue film obtained in the step 2) in a compound machine, synchronously feeding the carbon fibers and the glue film into a compound roller of the compound machine, heating and compounding, and then cooling and rolling to obtain the carbon fiber resin prepreg.

Compared with the prior art, the invention has the beneficial effects that:

the curing accelerator capable of reducing the curing temperature of an epoxy/dicyandiamide system and improving the toughness of epoxy resin is prepared by carrying out tertiary amination and etherification modification on enzymatic hydrolysis lignin sequentially.

The invention unexpectedly discovers that the accelerator prepared by the invention has the effect of synergistically reducing the gel temperature and the gel time with the substituted urea containing hydroxyl on a benzene ring.

The curing accelerator prepared by the invention is found not to have adverse effects on the mechanical properties of the prepreg after curing, and particularly the impact toughness of the prepreg is also obviously improved.

Detailed Description

The present invention will be further described with reference to the following examples, but the present invention is not limited to the descriptions in the following. Unless otherwise specified, "parts" in the examples of the present invention are parts by weight. All reagents used are commercially available in the art.

Preparation of modified enzymatic hydrolysis lignin epoxy/dicyandiamide curing accelerator

Preparation example 1

1) Adding 30 wt% sodium hydroxide solution into a reaction kettle containing 100 parts of enzymatic hydrolysis lignin, adjusting the pH value to 12, stirring, heating to 50 ℃, keeping the temperature constant, adding 165 parts of diethylamine, stirring uniformly, dripping 445.9 parts of 37 wt% formaldehyde aqueous solution at a constant speed within 30min, refluxing and reacting for 3h under a stirring state, cooling to room temperature after the reaction is finished, adding 60 parts of deionized water for dilution, dripping 30 wt% hydrochloric acid for acid precipitation until no precipitate is precipitated, filtering, washing filter residues to be neutral, and obtaining tertiary amine enzymatic hydrolysis lignin for later use;

2) adding 100 parts of tertiary amination enzymolysis lignin obtained in the step 1) into a mixed solution of 1000 parts of sodium hydroxide, n-butanol and dioxane, wherein the volume ratio of the sodium hydroxide to the n-butanol to the dioxane is 3:10, stirring until the tertiary amination enzymolysis lignin is completely dissolved, adding 150 parts of methyl iodide, stirring uniformly, keeping stirring and heating to a reflux state, reacting at a constant temperature for 3 hours, naturally cooling to room temperature after the reaction is finished, then pouring ice water, filtering when no precipitate is separated out, washing with the ice water, and drying in vacuum to obtain brown viscous liquid.

Preparation example 2

The procedure was repeated, except that 180 parts of formaldehyde and 198 parts of diethylamine were used.

Preparation example 3

The procedure was repeated, except that 135 parts of diethylamine was used.

Preparation example 4

The procedure was as in preparation 1, except that the diethylamine used was replaced with the same amount of dipropylamine.

Preparation example 5

The procedure was as in preparation 1, except that the diethylamine used was replaced with the same amount of dibutylamine.

Preparation example 6

The procedure was repeated, except that 120 parts of methyl iodide was used.

Preparation example 7

The procedure was as in production example 1 except that methyl iodide was replaced with an equal part by weight of chloropropane.

Preparation of epoxy resin prepreg

Example 1

1) Uniformly mixing 100 parts of E-44, 10 parts of dicyandiamide with the average particle size of 30 mu m and 2 parts of the accelerator prepared in preparation example 1, adding the mixture into a reaction kettle, and heating the mixture to a molten state;

2) adding the molten mixture obtained in the step 1) into a glue tank of a hot-melt glue spreader, preparing a film, cooling and rolling to obtain a glue film;

3) placing the adhesive film obtained in the step 2) in a compound machine, synchronously feeding 160 parts of carbon fibers and the adhesive film into a compound roller of the compound machine, heating and compounding twice, cooling and rolling to obtain the carbon fiber resin prepreg with the weight of 150 per square gram.

Example 2

The same as example 1 except that E-44 was used in an amount of 70 parts, dicyandiamide having an average particle diameter of 30 μm was used in an amount of 3 parts, the accelerator prepared in preparation example 1 was used in an amount of 0.5 part, and the carbon fiber was used in an amount of 120 parts.

Examples 3 to 8

The procedure is as in example 1, except that the accelerators used are those prepared in preparation examples 2 to 7.

Example 9

The process was the same as in example 1, except that 0.3 part of N- (2-hydroxyphenyl) -N ', N' -dimethylurea was further included in the step 1).

Example 10

The process was the same as in example 1 except that 0.1 part of N- (2-hydroxyphenyl) -N ', N' -dimethylurea was further included in step 1).

Comparative example 1

The procedure is as in example 1, except that 2 parts of N- (2-hydroxyphenyl) -N ', N' -dimethylurea are used as the accelerator in step 1).

Application example

And (3) coating a release agent on a mould, paving 15 layers of the prepreg prepared in the embodiment on the mould, wherein the thickness is 2mm, and carrying out hot pressing at 140 ℃ and 0.8MPa for 12min to obtain the cured prepreg.

The prepregs prepared in the above examples were subjected to the following performance tests:

before curing:

gel time-temperature, samples were heated on a constant temperature (90 ℃, 100 ℃, 110 ℃, 120 ℃, 130 ℃, 140 ℃) hot plate, and gel time was tested by wire drawing.

After curing:

impact properties: and (3) testing according to the standard GB/T1451, fixing the test sample strips on a simply supported beam impact testing machine, carrying out 5 test sample strips with the size of 10 multiplied by 80mm, and finally taking an average value.

TABLE 1

As can be seen from Table 1, the epoxy/dicyandiamide curing system added with the accelerator prepared by the invention has low gel temperature and gel time, and the gel time at 90 ℃ is more than 30min, which indicates that the curing reaction is very slow at 90 ℃, and the epoxy/dicyandiamide curing system has certain storage property at room temperature, and the storage property at room temperature is proved to be at least 6 months by experiments. Within the medium temperature range of 100 ℃ and 140 ℃, the gel time is rapidly reduced, which indicates that the medium temperature can be rapidly cured and is suitable for rapid molding. After the substituted urea accelerator containing hydroxyl on a benzene ring is added in the formula, the substituted urea accelerator is found to be compounded with the accelerator obtained based on enzymatic hydrolysis lignin modification for use, and has the effects of synergistically reducing the gel temperature and the gel time.

Through testing the impact performance, the curing accelerator prepared by the invention is found to not have adverse effects on the mechanical properties of the prepreg after curing, and particularly the impact toughness of the prepreg is also obviously improved.

The above detailed description is specific to one possible embodiment of the present invention, and the embodiment is not intended to limit the scope of the present invention, and all equivalent implementations or modifications without departing from the scope of the present invention should be included in the technical scope of the present invention.