A method for preparing a medium-temperature curing flame-retardant lignin-based curing agent

Abstract

This invention provides a method for preparing a medium-temperature curing flame-retardant lignin-based curing agent. First, enzymatically hydrolyzed lignin is thermally degraded to increase the content of active hydroxyl groups on the lignin. Then, phosphorus and silicon are introduced onto the lignin molecules through a reaction between chlorophosphosilane ester and lignin. Finally, a primary amine is introduced onto the lignin molecules through a Mannich reaction between lignin phenolic hydroxyl groups and formaldehyde and a diamine, resulting in a flame-retardant lignin-based curing agent that can cure epoxy resin at medium temperatures. Carbon fiber composites prepared with this curing agent exhibit excellent mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of epoxy curing agent technology, specifically relating to a method for preparing a medium-temperature curing flame-retardant lignin-based curing agent. Background Technology

[0002] With the scarcity of petroleum resources and the increasing severity of environmental pollution, the preparation of "environmentally friendly" green products and green chemical processes using natural renewable resources as raw materials has become a hot topic and frontier in current chemistry, chemical engineering, and their interdisciplinary research. Lignin is an important component of plants, accounting for approximately 20-30 wt% of plant weight. It is the second largest renewable resource after cellulose and is hailed as one of the richest green resources available to humankind in the 20th century.

[0003] Enzymatic hydrolysis of lignin is lignin extracted from the residue of microbial enzymatic hydrolysis of corn stalks to produce energy ethanol. Enzymatic hydrolysis of corn stalks to produce energy ethanol not only saves grain but also makes full use of resources. If the byproduct enzymatic hydrolysis of lignin is fully utilized, it will further improve the economic value of corn stalks.

[0004] Researchers have conducted extensive studies on the reuse of enzymatically hydrolyzed lignin. Because the hydroxyl groups in lignin molecules can participate in the curing of epoxy resins, and the benzene ring structure can increase the heat resistance of epoxy resins, it has the potential to be used as an epoxy resin curing agent. For example, in Guan Lizhu et al.'s "Synthesis of Enzymatically Hydrolyzed Lignin Amine and its Curing of Epoxy Resin," lignin amine was prepared by using corn straw enzymatically hydrolyzed lignin as raw material and diethylenetriamine through a Mannich reaction. In Liu Tianqin et al.'s "Choline Chloride / Urea Eutectic Solvent Modified Lignin Epoxy Resin Curing System," eutectic solvent (ChCl / U) was prepared using inexpensive and widely available choline chloride (ChCl) and urea (U) as raw materials. After modifying enzymatically hydrolyzed lignin (CEL) with this solvent, it was used directly as a curing agent for epoxy resins without separation.

[0005] The above studies have all successfully prepared epoxy resin curing agents using enzymatic hydrolysis of lignin as raw material. However, epoxy resin is flammable, and most applications require flame retardancy. Therefore, flame retardancy needs to be imparted to this curing agent during the transformation process of enzymatic hydrolysis of lignin into epoxy resin curing agent.

[0006] The applicant's prior patent application 2023113811282 discloses a method for preparing a flame-retardant lignin-based epoxy resin. First, the active hydroxyl content of lignin is increased by thermally degrading enzymatically hydrolyzed lignin. Then, nitrogen is introduced onto the lignin molecules through the Mannich reaction between lignin phenolic hydroxyl groups and formaldehyde and secondary amines. Next, phosphorus and silicon are introduced onto the lignin molecules through the reaction between chlorophosphosilane esters and lignin. Finally, a flame-retardant lignin-based epoxy resin is obtained by reacting the hydroxyl groups on the lignin with epichlorohydrin. However, this epoxy resin has a high melting point, and heating and melting it to make carbon fiber prepreg consumes a lot of energy. Further improvements based on this flame-retardant technology to prepare a flame-retardant curing agent would be of great significance. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a method for preparing a medium-temperature curing flame-retardant lignin-based curing agent. First, enzymatically hydrolyzed lignin is thermally degraded to increase the content of active hydroxyl groups on the lignin. Then, phosphorus and silicon are introduced onto the lignin molecules through a reaction between chlorophosphosilane ester and lignin. Finally, a primary amine is introduced onto the lignin molecules through a Mannich reaction between lignin phenolic hydroxyl groups and formaldehyde and a diamine, resulting in a flame-retardant lignin-based curing agent that can cure epoxy resin at medium temperatures.

[0008] To achieve the above objectives, the following specific technical solutions are adopted:

[0009] A method for preparing a medium-temperature curing flame-retardant lignin-based curing agent includes the following steps:

[0010] 1) Add enzymatic hydrolyzed lignin, solvent, and alkali to the reaction vessel and mix evenly. Heat and keep the temperature constant for the reaction. After the reaction is completed, cool and filter. Distill the filtrate to remove the solvent and obtain the degraded enzymatic hydrolyzed lignin DL.

[0011] 2) Dissolve phosphoryl dichloride in organic solvent 1, control the temperature and keep it constant, add hydroxyalkylsilane dropwise, and carry out the reaction under stirring after the addition is completed. After the reaction is completed, distill to obtain chlorophosphosilane ester.

[0012] 3) Mix chlorophosphosilane ester, degraded enzymatically hydrolyzed lignin DL, organic amine catalyst, and organic solvent 2 evenly, control the temperature and keep it constant during the reaction. After the reaction is completed, add water until no precipitate is formed, filter, wash the filter residue and dry to obtain chlorophosphosilane esterified lignin.

[0013] 4) Mix chlorophosphorylated lignin and diamine evenly, heat and keep the temperature constant, add formaldehyde solution dropwise, and heat to reflux under stirring after the addition is complete. After the reaction is complete, cool down and distill under reduced pressure to obtain the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent.

[0014] Step 1) The enzymatically hydrolyzed lignin is derived from the process of preparing ethanol by microbial enzymatic hydrolysis of corn stalks. The weight-average molecular weight of the enzymatically hydrolyzed lignin is 3000-6000 g / mol. The solvent is a mixed solvent of ethanol and water at a volume ratio of 1:1-2. The alkali is selected from one or a combination of two of sodium hydroxide and potassium hydroxide. The amount of alkali used is 1-3 wt% of the enzymatically hydrolyzed lignin. The temperature is raised to 200-300℃. The reaction time is 30-120 min. The weight-volume ratio (g / mL) of the enzymatically hydrolyzed lignin to the solvent is 1:5-10.

[0015] Step 2) The molar ratio of phosphoric acid dichloride to hydroxyalkylsilane is 1:0.95-1; the phosphoric acid dichloride is selected from one or a combination of two of ethyl dichlorophosphate, methyl dichlorophosphate, 1-propylphosphoric acid dichloride, and tert-butyl dichlorophosphate; the organic solvent 1 is selected from one or a combination of two or more of DMF, chloroform, and dioxane; the temperature control is to maintain the temperature between 0-25℃; and the hydroxyalkylsilane is selected from trimethylsilylmethanol, triethylsilylmethanol, and 1-( The hydroxyalkylsilane solution contains one or more of the following: trimethylsilyl)ethanol, 3-(trimethylsilyl)-1-propanol, 3-triethylsilylprop-1-ol, 2-trimethylsilylprop-1-ol, 1-trimethylsilyl-1-propanol, and 2-(triethylsilyl)ethanol. The concentration of the hydroxyalkylsilane solution is 30-40 wt%, and the solvent is the same as the organic solvent 1 mentioned above. The hydroxyalkylsilane solution is added dropwise over 30-60 minutes, and the reaction time is 1-3 hours.

[0016] In step 2), the molar ratio of phosphoryl dichloride to hydroxyalkylsilane is controlled to be close to 1:1, and the hydroxyalkylsilane is added slowly so that phosphoryl dichloride is always in excess relative to hydroxyalkylsilane in the system. Finally, the product of the reaction of one chlorine of phosphoryl dichloride and hydroxyalkylsilane is obtained, and one chlorine is reserved for the reaction with the hydroxyl groups on the enzymatically hydrolyzed lignin DL in step 3).

[0017] In step 3), the weight ratio of chlorophosphate silane ester to degraded enzymatically hydrolyzed lignin DL is 0.3-0.35:1. The temperature is controlled at 20-40℃. The reaction time is 3-8h. The organic amine catalyst is selected from one or a combination of two of triethylamine and triethylenediamine. The amount of catalyst used is 1-3wt% of the degraded enzymatically hydrolyzed lignin DL. The organic solvent 2 is the same as organic solvent 1.

[0018] In step 4), the weight ratio of chlorophosphophosphated lignin, diamine, and formaldehyde is 1:0.23-0.26:0.15-0.20. The diamine is an aliphatic diamine with 2-6 carbon atoms, selected from one or more combinations of ethylenediamine, 1,2-propanediamine, 1,3-propanediamine, 1,4-butanediamine, neopentanediamine, 1,3-pentanediamine, 1,2-pentanediamine, 1,5-pentanediamine, 1,4-pentanediamine, 1,6-hexanediamine, and 2-methyl-1,5-pentanediamine. The temperature is raised to 30-60°C, the formaldehyde solution concentration is 30-40 wt%, the formaldehyde solution is added dropwise over 30-60 minutes, and the reaction time under reflux is 3-5 hours.

[0019] This invention first degrades enzymatically hydrolyzed lignin to increase the content of reactive hydroxyl groups. Then, a chlorophosphate silane ester prepared from phosphoryl dichloride and hydroxyalkylsilane reacts with the degraded enzymatically hydrolyzed lignin (DL), introducing phosphorus (P) and silicon (Si) elements onto the degraded lignin macromolecules. Finally, the lignin is chlorophosphate esterified, and a Mannich reaction occurs between the diamine and formaldehyde, yielding a medium-temperature curing flame-retardant lignin-based curing agent. The relative amount of chlorophosphate silane ester to the degraded enzymatically hydrolyzed lignin (DL) and the number of carbon atoms in the diamine not only affect the flame-retardant properties but also the "exposure" of the reactive amino groups in the curing agent macromolecules, thus affecting the curing reaction time. Therefore, the proportions of components in each step are crucial.

[0020] The present invention also provides a carbon fiber composite material, comprising the following raw materials: epoxy resin, carbon fiber material, diluent, and the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent.

[0021] Furthermore, the carbon fiber composite material comprises the following raw materials in parts by weight: 100 parts epoxy resin, 250-280 parts carbon fiber material, 20-30 parts diluent, and 15-20 parts of the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent.

[0022] The epoxy resin is a bisphenol A type epoxy resin with an epoxy value of 0.41-0.54 mol / 100g.

[0023] The diluent is selected from one or a combination of two or more of the following: n-butyl glycidyl ether, 2-ethylhexyl glycidyl ether, phenyl glycidyl ether, cresol glycidyl ether, and diglycidyl ether.

[0024] The carbon fiber material is selected from one of carbon fiber cloth, carbon fiber yarn, and carbon fiber felt.

[0025] The present invention also provides a method for preparing the carbon fiber composite material, comprising the following steps:

[0026] S1. Mix epoxy resin, diluent, and the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent evenly and vacuum to obtain epoxy resin adhesive for later use.

[0027] S2 involves impregnating the carbon fiber material with the aforementioned epoxy resin adhesive, then laying, sealing, vacuuming, curing, and demolding it on a mold to obtain the carbon fiber composite material.

[0028] The curing temperature in step S2 is 50-100℃, and the curing time is 3-8h.

[0029] The present invention also provides an application of carbon fiber composite materials in the structural reinforcement of electronic devices, medical devices, aerospace, transportation vehicles, and sports equipment.

[0030] Compared with the prior art, the beneficial effects of this invention are:

[0031] This invention first thermally degrades enzymatically hydrolyzed lignin to increase the content of active hydroxyl groups on the lignin. Then, phosphorus and silicon are introduced onto the lignin molecules through the reaction between chlorophosphosilane ester and lignin. Finally, primary amines are introduced onto the lignin molecules through the Mannich reaction between lignin phenolic hydroxyl groups and formaldehyde and diamines to obtain a flame-retardant lignin-based curing agent that can cure epoxy resin at medium temperature. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments, but is not limited to the contents of the specification. Unless otherwise specified, all "parts" mentioned in the embodiments of the present invention are parts by weight. All reagents used are commercially available in the art.

[0033] Enzymatically hydrolyzed lignin was purchased from Shanxi Biomass New Materials Industry Research Institute Co., Ltd., with a weight-average molecular weight of 5970 g / mol.

[0034] The carbon fiber cloth was purchased from Jiangsu Tianniao High-Tech Co., Ltd., brand name C-1107.

[0035] Preparation of medium-temperature curing flame-retardant lignin-based curing agent

[0036] Preparation Example 1

[0037] 1) Add 100g of enzymatically hydrolyzed lignin, 1000mL of a mixed solvent of ethanol and water in a 1:1 volume ratio, and 3g of sodium hydroxide to a reaction vessel and mix thoroughly. Heat to 250℃ and maintain the temperature for 30 minutes. After the reaction is complete, cool and filter. Distill the filtrate to remove the solvent, obtaining the degraded enzymatically hydrolyzed lignin DL (phenolic hydroxyl content 2.25mmol / g).

[0038] 2) Dissolve 1 mol of ethyl dichlorophosphate in 250 mL of LDM, keep the temperature at 5 °C and maintain the temperature, add 0.95 mol of trimethylsilylmethanol dropwise, and after the addition is completed in 60 min, carry out the reaction under stirring for 3 h. After the reaction is completed, distill to remove the solvent to obtain chlorophosphosilane ester.

[0039] 3) Mix 35g of chlorophosphosilane ester, 100g of degraded enzymatically hydrolyzed lignin DL, 3g of triethylamine, and 200mL of solvent composed of DMF and dioxane in a volume ratio of 5:1. Control the temperature at 25℃ and react at a constant temperature for 5h. After the reaction is completed, add water until no precipitate is formed. Filter, wash the filter residue with water 3 times, and dry at 100℃ to constant weight to obtain chlorophosphosilane esterified lignin.

[0040] 4) Take 100g of chlorophospho-phosphated lignin and 26g of 1,6-hexanediamine, mix them evenly, heat to 60℃ and keep the temperature constant, add dropwise a 37wt% formaldehyde solution containing 20g of formaldehyde, and after the dropwise addition is completed in 30min, heat to reflux and react for 5h under stirring. After the reaction is completed, cool down and remove unreacted raw materials and solvents by vacuum distillation to obtain the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent.

[0041] Preparation Example 2

[0042] The rest is the same as in Preparation Example 1, except that the amount of 1,6-hexanediamine used in step 4) is 23g.

[0043] Preparation Example 3

[0044] The rest is the same as in Preparation Example 1, except that 1,6-hexanediamine is replaced with an equal amount of ethylenediamine.

[0045] Preparation Example 4

[0046] The rest is the same as in Preparation Example 1, except that the amount of chlorophosphosilane ester used is 30g.

[0047] Preparation Example 5

[0048] The rest is the same as in Preparation Example 1, except that the amount of chlorophosphosilane ester used is 40g.

[0049] Preparation Example 6

[0050] The rest is the same as in Preparation Example 1, except that the amount of chlorophosphosilane ester used is 25g.

[0051] Comparative Preparation Example 1

[0052] The rest is the same as in Preparation Example 1, except that step 2) is omitted, and an equal amount of dimethyl chlorophosphate is used instead of silane chlorophosphate in step 3).

[0053] Comparative Preparation Example 2

[0054] The rest is the same as in Preparation Example 1, except that steps 2) and 3) are omitted.

[0055] 1) Add 100g of enzymatically hydrolyzed lignin, 1000mL of a mixed solvent of ethanol and water in a 1:1 volume ratio, and 3g of sodium hydroxide to a reaction vessel and mix thoroughly. Heat to 250℃ and maintain the temperature for 30 minutes. After the reaction is complete, cool and filter. Distill the filtrate to remove the solvent, obtaining the degraded enzymatically hydrolyzed lignin DL (phenolic hydroxyl content 2.25mmol / g).

[0056] 2) Take 100g of degraded enzymatic hydrolyzed lignin DL and 26g of 1,6-hexanediamine, mix them evenly, heat to 60℃ and keep the temperature constant, add dropwise a 37wt% formaldehyde solution containing 20g of formaldehyde, and after the addition is completed in 30min, heat to reflux under stirring and react for 5h. After the reaction is completed, cool down and remove unreacted raw materials and solvent by vacuum distillation to obtain lignin-based curing agent.

[0057] Comparative preparation example 3

[0058] The rest is the same as in Preparation Example 1, except that the lignin is not degraded by enzymatic hydrolysis.

[0059] 1) Dissolve 1 mol of ethyl dichlorophosphate in 250 mL of LDM, keep the temperature at 5 °C and maintain the temperature, add 0.95 mol of trimethylsilylmethanol dropwise, and after the addition is completed in 60 min, carry out the reaction under stirring for 3 h. After the reaction is completed, distill to remove the solvent to obtain chlorophosphosilane ester.

[0060] 2) Mix 35g of chlorophosphosilane ester, 100g of enzymatically hydrolyzed lignin, 3g of triethylamine, and 200mL of solvent composed of DMF and dioxane in a volume ratio of 2:1. The mixture is stirred at 25℃ for 5 hours. After the reaction is complete, water is added until no precipitate is formed. The mixture is filtered, the residue is washed three times with water, and dried at 100℃ to constant weight to obtain chlorophosphosilane esterified lignin.

[0061] 3) Take 100g of chlorophospho-phosphated lignin and 26g of 1,6-hexanediamine, mix them evenly, heat to 60℃ and keep the temperature constant, add dropwise a 37wt% formaldehyde solution containing 20g of formaldehyde, and after the dropwise addition is completed in 30min, heat to reflux and react for 5h under stirring. After the reaction is completed, cool down and remove unreacted raw materials and solvents by vacuum distillation to obtain the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent.

[0062] Preparation of carbon fiber composite materials

[0063] Example 1

[0064] S1 mixes 100 parts of epoxy resin E51, 30 parts of n-butyl glycidyl ether, and 20 parts of medium-temperature curing flame-retardant lignin-based curing agent of Preparation Example 1 evenly and vacuums the mixture to obtain epoxy resin adhesive for later use.

[0065] S2 takes 280 parts of 16-layer carbon fiber cloth, immerses them in the above-mentioned epoxy resin adhesive for impregnation, and then lays them layer by layer on the mold, seals them, vacuums them, cures them at 80°C for 8 hours, and demolds them to obtain carbon fiber composite material.

[0066] Examples 2-6

[0067] The rest is the same as in Example 1, except that the medium-temperature curing flame-retardant lignin-based curing agent was prepared in Examples 2-6.

[0068] Example 7

[0069] The rest is the same as in Example 1, except that the amount of medium-temperature curing flame-retardant lignin-based curing agent used is 15 parts.

[0070] Comparative Examples 1-3

[0071] The rest is the same as in Example 1, except that the medium-temperature curing flame-retardant lignin-based curing agent was prepared in Comparative Preparation Examples 1-3; and the curing time of Comparative Example 3 was 20 hours.

[0072] Comparative Example 4

[0073] The rest is the same as in Example 1, except that an equal mass of low molecular weight polyamide (amine value 200 mg KOH / g) is used to replace the medium-temperature curing flame-retardant lignin-based curing agent in Example 1.

[0074] The epoxy resin binder in the carbon fiber composites prepared in the above examples and comparative examples was subjected to the following performance tests:

[0075] UL-94 Vertical Burning Test: Tested in accordance with standard GB / T2408-2008.

[0076] Curing temperature and time: The isothermal DSC method was used in a nitrogen atmosphere to test the adhesives at a curing temperature of 80℃ for different durations to obtain their respective curing times.

[0077] The carbon fiber composite materials prepared in the above examples and comparative examples were subjected to the following performance tests:

[0078] Shear properties: The interlaminar shear strength was tested according to the standard JC / T 773-2010 fiber-reinforced plastics short beam method.

[0079] Table 1

[0080] project UL-94 Curing temperature ℃ Curing time h Shear strength (MPa) Example 1 V0 80 5.6 45.22 Example 2 V0 80 6.0 44.94 Example 3 V0 80 7.7 47.16 Example 4 V0 80 5.5 46.47 Example 5 V0 80 5.8 42.15 Example 6 V1 80 5.4 47.28 Example 7 V0 80 7.2 40.53 Comparative Example 1 V2 80 5.5 45.41 Comparative Example 2 V2 80 3.9 36.75 Comparative Example 3 V2 80 17.3 48.62 Comparative Example 4 V2 80 4.1 38.96

[0081] As can be seen from Table 1, the medium-temperature curing flame-retardant lignin-based curing agent prepared by the present invention has excellent flame-retardant properties. The carbon fiber composite material containing this curing agent exhibits good mechanical properties. It can be rapidly cured at 80℃ within 5-8 hours and has excellent mechanical properties.

[0082] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a medium-temperature curing flame-retardant lignin-based curing agent, characterized in that, Includes the following steps: 1) Add enzymatic hydrolyzed lignin, solvent, and alkali to the reaction vessel and mix evenly. Heat and maintain the temperature for reaction. After the reaction is completed, cool and filter. Distill the filtrate to remove the solvent and obtain the degraded enzymatic hydrolyzed lignin DL. 2) Dissolve phosphoryl dichloride in organic solvent 1, control the temperature and keep it constant, add hydroxyalkylsilane dropwise, and carry out the reaction under stirring after the addition is completed. After the reaction is completed, distill to obtain chlorophosphosilane ester. 3) Mix chlorophosphosilane ester, degraded enzymatically hydrolyzed lignin DL, organic amine catalyst, and organic solvent 2 evenly, control the temperature and keep it constant during the reaction. After the reaction is completed, add water until no precipitate is formed, filter, wash the filter residue and dry to obtain chlorophosphosilane esterified lignin. 4) Mix chlorophosphophosphate lignin and diamine evenly, heat and keep at a constant temperature, add formaldehyde solution dropwise, and after the addition is complete, heat to reflux under stirring. After the reaction is complete, cool down and distill under reduced pressure to obtain the above-mentioned medium-temperature curing flame-retardant lignin-based curing agent; the weight ratio of chlorophosphosilane ester and degraded enzymatic hydrolyzed lignin DL in step 3) is 0.3-0.35:

1.

2. The preparation method of the medium-temperature curing flame-retardant lignin-based curing agent according to claim 1, characterized in that, In step 4), the weight ratio of chlorophosphophosphated lignin, diamine, and formaldehyde is 1:0.23-0.26:0.15-0.20; the diamine is an aliphatic diamine with 2-6 carbon atoms, selected from one or more combinations of ethylenediamine, 1,2-propanediamine, 1,3-propanediamine, 1,4-butanediamine, neopentanediamine, 1,3-pentanediamine, 1,2-pentanediamine, 1,5-pentanediamine, 1,4-pentanediamine, 1,6-hexanediamine, and 2-methyl-1,5-pentanediamine.

3. The preparation method of the medium-temperature curing flame-retardant lignin-based curing agent according to claim 1, characterized in that, Step 4) The temperature is raised to 30-60℃, the concentration of the formaldehyde solution is 30-40wt%, the formaldehyde solution is added dropwise over 30-60 minutes, and the reaction time under reflux is 3-5 hours.

4. The preparation method of the medium-temperature curing flame-retardant lignin-based curing agent according to claim 1, characterized in that, Step 2) The molar ratio of phosphoryl dichloride to hydroxyalkylsilane is 1:0.95-1; the phosphoryl dichloride is selected from one or a combination of two of ethyl dichlorophosphate, methyl dichlorophosphate, 1-propylphosphorus dichloride, and tert-butyl dichlorophosphate; the hydroxyalkylsilane is selected from one or a combination of two or more of trimethylsilylmethanol, triethylsilylmethanol, 1-(trimethylsilyl)ethanol, 3-(trimethylsilyl)-1-propanol, 3-triethylsilylprop-1-ol, 2-trimethylsilylprop-1-ol, 1-trimethylsilyl-1-propanol, and 2-(triethylsilyl)ethanol.

5. The preparation method of the medium-temperature curing flame-retardant lignin-based curing agent according to claim 1, characterized in that, The organic solvent 1 and organic solvent 2 are independently selected from one or more combinations of DMF, chloroform, and dioxane. The temperature is controlled at 0-25℃. The hydroxyl silane is added dropwise over 30-60 minutes, and the reaction time is 1-3 hours.

6. A carbon fiber composite material, characterized in that, It includes the following raw materials: epoxy resin, carbon fiber material, diluent, and a medium-temperature curing flame-retardant lignin-based curing agent prepared by the preparation method according to any one of claims 1-5.

7. The carbon fiber composite material according to claim 6, characterized in that, The carbon fiber composite material comprises the following raw materials in parts by weight: 100 parts epoxy resin, 250-280 parts carbon fiber material, 20-30 parts diluent, and 15-20 parts of the medium-temperature curing flame-retardant lignin-based curing agent.

8. The carbon fiber composite material according to claim 6, characterized in that, The epoxy resin is a bisphenol A type epoxy resin with an epoxy value of 0.41-0.54 mol / 100g; the diluent is selected from one or more combinations of n-butyl glycidyl ether, 2-ethylhexyl glycidyl ether, phenyl glycidyl ether, cresol glycidyl ether, and diglycidyl ether; the carbon fiber material is selected from one of carbon fiber cloth, carbon fiber yarn, and carbon fiber felt.

9. A method for preparing the carbon fiber composite material according to any one of claims 6-8, characterized in that, Includes the following steps: S1. Mix epoxy resin, diluent, and medium-temperature curing flame-retardant lignin-based curing agent evenly and vacuum to obtain epoxy resin adhesive for later use. S2 involves impregnating the carbon fiber material with the aforementioned epoxy resin adhesive, then laying, sealing, vacuuming, curing, and demolding it on a mold to obtain the carbon fiber composite material.