A modified epoxy resin emulsion, its preparation method and application

Abstract

This invention relates to a modified epoxy resin emulsion, its preparation method, and its application, and pertains to the field of carbon fiber surface modification technology. The preparation method of the modified epoxy resin emulsion includes the following steps: 1) prepolymerizing epoxidized hydroxyl-terminated polybutadiene with an aliphatic polyisocyanate to obtain a prepolymer reaction product; 2) adding a chain extender, an organic solvent, and a catalyst to the prepolymer reaction product to carry out chain extension and crosslinking reactions to obtain a chain extension and crosslinking reaction product; 3) adding epoxy resin, a diluent, an emulsifier, and an organic solvent to the chain extension and crosslinking reaction product to graft and modify the epoxy resin to obtain a modified reaction product; 4) post-treating the modified reaction product to obtain a modified epoxy resin emulsion. This invention is mainly used to prepare a modified epoxy resin emulsion, which, as a sizing agent for carbon fibers, can effectively improve the flexibility of carbon fibers without reducing the original strength of the carbon fiber tow.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber surface modification technology, and in particular to a modified epoxy resin emulsion, its preparation method, and its application. Background Technology

[0002] Carbon fiber is widely used due to its excellent properties such as high specific strength and high specific modulus. However, due to its poor toughness, single filaments often break during the winding or weaving process due to mechanical friction, which greatly reduces the strength utilization rate of carbon fiber.

[0003] In the carbon fiber production process, sizing can effectively solve the above problems. Sizing agents can enhance the interfacial properties between carbon fiber and resin, thus sizing has become a crucial step in fiber production.

[0004] However, with the continuous development of the carbon fiber composite materials industry, the performance requirements for sizing agents are also constantly increasing, and sizing agents are shifting from general-purpose to functional types. The main component of existing carbon fiber sizing agents is epoxy resin. When this type of emulsion is applied to the surface of carbon fiber, the benzene rings on the epoxy resin backbone cannot rotate, resulting in poor flexibility of the carbon fiber itself.

[0005] Therefore, there is an urgent need for a sizing agent that can further improve the flexibility of carbon fibers without reducing the strength of the carbon fiber tow. Summary of the Invention

[0006] In view of this, the present invention provides a modified epoxy resin emulsion, its preparation method and application. The main purpose is to prepare a modified epoxy resin emulsion, which, as a sizing agent for carbon fibers, can effectively improve the flexibility of carbon fibers without reducing the original strength of the carbon fiber bundles.

[0007] To achieve the above objectives, the present invention mainly provides the following technical solutions:

[0008] On one hand, embodiments of the present invention provide a method for preparing a modified epoxy resin emulsion, comprising the following steps:

[0009] 1) Prepolymerize epoxidized hydroxyl-terminated polybutadiene with aliphatic polyisocyanate to obtain the prepolymer reaction product;

[0010] 2) Add chain extender, organic solvent and catalyst to the prepolymer reaction product to carry out chain extension and cross-linking reaction to obtain chain extension and cross-linking reaction product;

[0011] 3) Add epoxy resin, diluent, emulsifier and organic solvent to the chain extension and crosslinking reaction product to graft and modify the epoxy resin to obtain the modified reaction product.

[0012] 4) The modified reaction product is post-treated to obtain a modified epoxy resin emulsion.

[0013] Preferably, the raw materials used are in the following proportions by weight:

[0014] Epoxidized hydroxyl-terminated polybutadiene: 44.54-48.30 parts by weight;

[0015] Aliphatic polyisocyanates: 3.84-5.04 parts by weight;

[0016] Epoxy resin: 42.22-48.08 parts by weight;

[0017] Chain extender: 0.10-0.25 parts by weight;

[0018] Catalyst: 0.08-0.12 parts by weight;

[0019] Diluent: 1.0-4.50 parts by weight;

[0020] Emulsifier: 2.50-4.00 parts by weight.

[0021] Preferably, the molecular weight M of the epoxidized hydroxyl-terminated polybutadiene EHTPB is... W The value is 3000-3500; and / or the aliphatic polyisocyanate is one or a mixture of several of isoflurane diisocyanate (IPDI), hydrogenated phenylmethane diisocyanate (H-MDI), and hexamethylene diisocyanate (HDI); and / or the epoxy resin is a bisphenol A type epoxy resin; more preferably, one or a mixture of two of E-44 and E-51; and / or the diluent is one or a mixture of two of butyl glycidyl ether (BGE) and dodecyl to tetradecyl glycidyl ether (AGE); and / or the chain extender is one or a mixture of two of ethylene glycol (EG) and propylene glycol (PG); and / or the catalyst is one or a mixture of two of dibutyltin dilaurate and triphenylbismuth; and / or the emulsifier is N-methyldiethanolamine; and / or the organic solvent is one or a mixture of several of acetone, butanone, and N-methylpyrrolidone.

[0022] Preferably, step 1) includes:

[0023] 11) Vacuum dehydration treatment was performed on epoxidized hydroxyl-terminated polybutadiene to obtain vacuum dehydrated epoxidized hydroxyl-terminated polybutadiene.

[0024] 12) At a temperature of 65-75°C, the epoxidized hydroxyl-terminated polybutadiene after vacuum dehydration is mixed with the aliphatic polyisocyanate, and a prepolymerization reaction is carried out under nitrogen protection and reflux conditions to obtain the prepolymer reaction product.

[0025] Preferably, in step 11), the temperature of the vacuum dehydration treatment is 100-110°C and the time of the vacuum dehydration treatment is 60-90 min;

[0026] Preferably, in step 12), the prepolymerization reaction takes 2-4 hours, preferably 3 hours;

[0027] Preferably, in the prepolymer reaction product: the isocyanate index R in the prepolymer is 1.01-1.40; wherein, the isocyanate index R = n(NCO) / n(OH)); n(NCO) is the molar amount of aliphatic polyisocyanate-NCO groups, and n(OH) is the molar amount of epoxidized hydroxyl-terminated polybutadiene-OH groups.

[0028] Preferably, step 2) includes: adding a chain extender and an organic solvent to the prepolymer reaction product at a temperature of 65-75°C, then adding a catalyst dropwise, and after the addition is complete, performing a chain extension and crosslinking reaction for 2-4 hours, preferably 2 hours, to obtain the chain extension and crosslinking reaction product; preferably, the amount of the organic solvent added is 3.5-6.5 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, and chain extender, preferably 5 wt%.

[0029] Preferably, step 3) includes: adding epoxy resin, diluent, emulsifier, and organic solvent to the chain extension and crosslinking reaction product at a temperature of 65-75°C, and then reacting for 2-4 hours to obtain the modified reaction product; preferably, the amount of organic solvent added is 3.5-6.5 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin, preferably 5 wt%.

[0030] Preferably, step 4) includes: lowering the temperature of the modified reaction product to 40-45°C, adding an organic solvent, further lowering the temperature to 10-15°C, adding deionized water for dispersion and emulsification, and finally removing the organic solvent by vacuum distillation to obtain a modified epoxy resin emulsion; preferably, the amount of organic solvent added is 50-60 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin.

[0031] On the other hand, embodiments of the present invention provide a modified epoxy resin emulsion, wherein the main component of the modified epoxy resin emulsion is a cross-linked network structure prepolymer modified epoxy resin; wherein the cross-linked network structure prepolymer is obtained by prepolymerization, chain extension and cross-linking reaction of epoxidized hydroxyl-terminated polybutadiene and aliphatic polyisocyanate; preferably, the modified epoxy resin emulsion is prepared by the preparation method of the modified epoxy resin emulsion described in any one of the above embodiments; preferably, the solid content of the modified epoxy resin emulsion is 25-35 wt%.

[0032] On the other hand, the above-mentioned modified epoxy resin emulsion is used as or in the preparation of carbon fiber sizing agents; preferably, the modified epoxy resin emulsion is used as or in the preparation of polyacrylonitrile-based carbon fiber sizing agents.

[0033] In another aspect, embodiments of the present invention provide a carbon fiber sizing agent, wherein the carbon fiber sizing agent is the aforementioned modified epoxy resin emulsion; preferably, the method of using the carbon fiber sizing agent is as follows: first, the aforementioned modified epoxy resin emulsion is diluted to obtain a sizing solution; then, the carbon fiber tow is surface-impregnated with the sizing solution; finally, the sized carbon fiber is dried; more preferably, the aforementioned modified epoxy resin emulsion is diluted to a mass concentration of 2-4 wt% to obtain a sizing solution; more preferably, the impregnation time of the carbon fiber tow is 5-30 s, preferably 10 s; more preferably... Preferably, the carbon fiber tow is surface-impregnated with a sizing solution at a temperature of 20-30℃; more preferably, the sized carbon fiber is dried in a hot air drying oven; preferably, the hot air temperature is 190-220℃ and the drying time is 2-3 minutes; more preferably, the carbon fiber is polyacrylonitrile-based carbon fiber; more preferably, after sizing the 6-24K polyacrylonitrile-based carbon fiber, the polyacrylonitrile-based carbon fiber has a stiffness of 12-14 cm, a tensile strength of 4.50-5.50 GPa, a tensile modulus of 235-255 GPa, and a bulk density of 1.78-1.82 g / cm³. 3 .

[0034] Compared with the prior art, the modified epoxy resin emulsion, its preparation method, and its application of the present invention have at least the following beneficial effects:

[0035] This invention provides a modified epoxy resin emulsion, its preparation method, and its application. First, epoxidized hydroxyl-terminated polybutadiene undergoes a prepolymerization reaction with an aliphatic polyisocyanate to obtain a prepolymer reaction product. Then, a chain extender, organic solvent, and catalyst are added to the prepolymer reaction product to carry out chain extension and crosslinking reactions, yielding a chain-extended and crosslinked reaction product. Next, epoxy resin, diluent, emulsifier, and organic solvent are added to the chain-extended and crosslinked reaction product to graft and modify the epoxy resin, obtaining a modified reaction product. Finally, the modified reaction product undergoes post-treatment to obtain a modified epoxy resin emulsion. It should be noted that epoxidized hydroxyl-terminated polybutadiene is a linear molecule containing both epoxy and hydroxyl groups in its molecular chain. Adding epoxidized hydroxyl-terminated polybutadiene and its related reactants to the epoxy resin can increase the flexibility of the epoxy resin, thereby improving the flexibility of the sizing solution and enhancing subsequent processing performance. Therefore, the modified epoxy resin emulsion prepared through the above steps has good stability when used as a carbon fiber sizing agent. While ensuring the strength of the carbon fiber, it can significantly improve the flexibility of the carbon fiber and endow the carbon fiber with good subsequent processing performance, thus solving problems such as loosening, fuzzing and yarn breakage during carbon fiber processing.

[0036] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below. Detailed Implementation

[0037] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the following detailed description, in conjunction with preferred embodiments, provides a detailed explanation of the specific implementation methods, structures, features, and effects according to the present invention. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0038] To overcome the shortcomings and defects of the prior art, this invention provides a modified epoxy resin emulsion and its preparation method. The modified epoxy resin emulsion can be used as a sizing agent for carbon fibers. Without reducing the original fiber strength, it can effectively improve the flexibility of carbon fibers, thereby enhancing the application of carbon fibers in the field of composite materials.

[0039] The specific solution of the present invention is as follows:

[0040] On one hand, embodiments of the present invention provide a method for preparing a modified epoxy resin emulsion, comprising the following steps:

[0041] 1) The metered epoxidized hydroxyl-terminated polybutadiene is subjected to vacuum stirring and dehydration treatment at a temperature of 100-110℃ (preferably 110℃) for 60-90 min; then, after the temperature of the above liquid is reduced to 65-75℃, aliphatic polyisocyanate is added to it, and the reaction is carried out under nitrogen protection and refluxed for 2-4 h, preferably 3 h, to obtain the prepolymer reaction product.

[0042] 2) At a temperature of 65-75℃ (this temperature range is mainly determined based on the degree of reaction at the temperature. If the temperature is too high, the reaction is too vigorous and difficult to control; if the temperature is too low, the reaction time is too long), add a chain extender to the prepolymer reaction product and add an organic solvent for dilution. Finally, add the catalyst dropwise to the reaction system. After the addition is complete, react for 2-4 hours, preferably 2 hours, to obtain the chain-extended and cross-linked reaction product.

[0043] In this step, the amount of organic solvent added is 3.5-6.5 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, and chain extender, preferably 5 wt%.

[0044] 3) At a temperature of 65-75℃, epoxy resin, diluent, and emulsifier are added to the chain extension and crosslinking reaction product, followed by the addition of organic solvent. After the addition is complete, the reaction is carried out for 2-4 hours to obtain the modified reaction product.

[0045] In this step, the amount of organic solvent added is 3.5-6.5 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin, preferably 5 wt%.

[0046] 4) Reduce the temperature of the modified reaction product to 40-45℃, add an organic solvent (the amount of organic solvent added is 50-60 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin), then cool it to below 10-15℃ in an ice-water bath, add deionized water, disperse and emulsify using a high-speed shear mill, and finally remove the organic solvent by vacuum distillation to obtain the modified epoxy resin emulsion.

[0047] Here, the solid content of the modified epoxy resin emulsion is 25-35 wt%.

[0048] Preferably, in the above steps, the raw materials used, by weight, are as follows:

[0049] Epoxidized hydroxyl-terminated polybutadiene: 44.54-48.30 parts by weight;

[0050] Aliphatic polyisocyanates: 3.84-5.04 parts by weight;

[0051] Epoxy resin: 42.22-48.08 parts by weight;

[0052] Chain extender: 0.10-0.25 parts by weight;

[0053] Catalyst: 0.08-0.12 parts by weight;

[0054] Diluent: 1.0-4.50 parts by weight;

[0055] Emulsifier: 2.50-4.00 parts by weight.

[0056] Here, the isocyanate index R in the prepolymer is controlled to be 1.01-1.40 by controlling the mass ratio of epoxidized hydroxyl-terminated polybutadiene to polyisocyanate.

[0057] Wherein, the isocyanate index R = n(NCO) / n(OH)); n(NCO) is the molar amount of aliphatic polyisocyanate-NCO groups, and n(OH) is the molar amount of epoxidized hydroxyl-terminated polybutadiene-OH groups.

[0058] When the R value is small, the prepolymer has a relatively large molecular weight and no residual -NCO groups. It cannot react chemically with the chain extender, making the prepolymer difficult to disperse and resulting in a large emulsion particle size and poor emulsion static stability. When the R value is too large, the amount of residual -NCO increases. -NCO will react with water to form urea bonds. The increase in residual -NCO groups leads to an increase in the number of urea bonds formed. The urea bond segments are highly hydrophobic, forming insoluble polyurea, which leads to a larger latex particle size and reduced stability.

[0059] Preferably, the molecular weight (MW) of the epoxidized hydroxyl-terminated polybutadiene (EHTPB) is 3000-3500. Preferably, the aliphatic polyisocyanate is one or a mixture of several selected from isophorone diisocyanate (IPDI), hydrogenated phenylmethane diisocyanate (H-MDI), and hexamethylene diisocyanate (HDI). Preferably, the epoxy resin is a bisphenol A type epoxy resin; more preferably, it is one or a mixture of two selected from E-44 and E-51. Preferably, the diluent is one or a mixture of two selected from butyl glycidyl ether (BGE) and dodecyl to tetradecyl glycidyl ether (AGE). Preferably, the chain extender is one or a mixture of two selected from ethylene glycol (EG) and propylene glycol (PG). Preferably, the catalyst is one or a mixture of two selected from dibutyltin dilaurate and triphenylbismuth. Preferably, the emulsifier is N-methyldiethanolamine. Preferably, the organic solvent is one or a mixture of several of acetone, butanone, and N-methylpyrrolidone.

[0060] The present invention will be further illustrated below through specific experimental examples:

[0061] Examples 1-5

[0062] Examples 1-5 provide a preparation scheme for a modified epoxy resin emulsion. Specifically, each example follows the steps below, using the raw materials (types and weight parts) listed in Table 1:

[0063] 1) The measured epoxidized hydroxyl-terminated polybutadiene (EHTPB) was vacuum stirred and dehydrated at 110°C for 60-90 min (60 min for Example 1, 75 min for Examples 2-3, and 90 min for Examples 4-5) to obtain vacuum-dehydrated epoxidized hydroxyl-terminated polybutadiene (EHTPB) with a molecular weight of approximately 3000-3500.

[0064] After the temperature of the vacuum-dehydrated epoxidized hydroxyl-terminated polybutadiene (EHTPB) was reduced to 65-75°C (the temperature of Examples 1-2 was reduced to 65°C, the temperature of Examples 3-4 was reduced to 70°C, and the temperature of Example 5 was reduced to 75°C), an aliphatic polyisocyanate was added, and the reaction was carried out under nitrogen protection and refluxed for 3 hours to obtain the prepolymer reaction product.

[0065] 2) Keeping the temperature constant (i.e., the temperature of Examples 1-2 is kept at 65°C, the temperature of Examples 3-4 is kept at 70°C, and the temperature of Example 5 is kept at 75°C), a chain extender is added to the prepolymer reaction product, and an organic solvent is added to dilute it (the amount of organic solvent added is 5 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, and chain extender). Then, the catalyst (the catalyst used in Examples 1-2 is dibutyltin dilaurate, the catalyst used in Example 3 is triphenylbismuth, and the catalyst used in Examples 4-5 is dibutyltin dilaurate and triphenylbismuth in a weight ratio of 1:1) is added dropwise to the reaction system. After the addition is complete, the reaction is carried out for 2 hours to obtain the chain extension and crosslinking reaction product. 3) Keep the temperature constant (i.e., the temperature of Examples 1-2 is kept at 65°C, the temperature of Examples 3-4 is kept at 70°C, and the temperature of Example 5 is kept at 75°C), add epoxy resin, diluent, and emulsifier to the chain extension and crosslinking reaction product, and then add organic solvent (the amount of organic solvent added is 5 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin). After the addition is complete, react for 2 hours to obtain the modified reaction product.

[0066] 4) The temperature of the modified reaction product is lowered to 40-45℃ (40℃ for Examples 1-2, 43℃ for Examples 3-4, and 45℃ for Example 5), and an organic solvent is added (the amount of organic solvent added is 50-60 wt% of the sum of the amounts of epoxidized hydroxyl-terminated polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin). Then, the mixture is cooled to 15℃ in an ice-water bath, dispersed and emulsified with deionized water using a high-speed shearing machine, and finally the organic solvent is removed by vacuum distillation to obtain a modified epoxy resin emulsion with a solid content of 25-35 wt%.

[0067] Comparative Example 1

[0068] Comparative Example 1 is a commercially available epoxy resin emulsion sizing agent (i.e., unmodified epoxy resin emulsion).

[0069] Table 1 shows the specific formulations of Examples 1-5 and Comparative Example 1 (unmodified sizing solution).

[0070] The appearance, stability, and particle size of the emulsions prepared in Examples 1-5 and Comparative Example 1 were tested, and the data are shown in Table 2.

[0071] Table 2 shows the emulsion test results of Examples 1-5 and Comparative Example 1.

[0072]

[0073] Note: The emulsions in Examples 4 and 5 showed sedimentation because the resulting molecular chains were too large after the reaction, and the emulsion particle size was too large, causing the sizing agent to settle.

[0074] As can be seen from the comparison between the examples and comparative examples in Table 1-2, the modified epoxy resin emulsion for carbon fiber of the present invention has good stability and does not demulsify or separate after more than 5 weeks, while the emulsion of Comparative Example 1 demulsifies and separates after 3 weeks.

[0075] The modified epoxy resin emulsions prepared in Examples 1-5 and the unmodified epoxy resin emulsion of Comparative Example 1 were used as sizing agents and applied to polyacrylonitrile-based carbon fibers:

[0076] The emulsion was diluted to a mass concentration of 2-4 wt% to obtain a sizing solution. The sizing solution was applied to the sizing tank of the polyacrylonitrile-based carbon fiber production line to impregnate the surface of the polyacrylonitrile-based carbon fiber in operation. The temperature of the emulsion was 25℃, and the impregnation time was 20 seconds. After impregnation, the polyacrylonitrile-based carbon fiber was dried in a hot air drying oven at 190-220℃ for 2-3 minutes. Then, the application evaluation was carried out, and the bundle properties, abrasion resistance, sizing amount and other indicators of the polyacrylonitrile-based carbon fiber were compared and evaluated. The evaluation results are shown in Table 3.

[0077] Table 3 shows the evaluation results of the application of the emulsions prepared in Examples 1-5 and Comparative Examples 1-2 as sizing agents.

[0078]

[0079] The abrasion resistance test method in Table 2 is as follows: the carbon fiber bundle is brought into contact with a chrome-plated stainless steel rod at a contact angle of 120° under a unwinding tension of 200g, and then rubbed through it; after friction, the carbon fiber bundle is passed through two polyurethane sponges (friction load of 250g) for a certain length (50m) at a certain speed (15m / min), and the weight of the fuzz attached to the polyurethane sponge is measured, which is the amount of carbon fiber fuzzing due to friction.

[0080] Conclusion and Analysis: Table 3 shows that, compared with Comparative Example 1, the modified epoxy resin emulsion prepared in this embodiment of the invention, when used to prepare a sizing solution with a mass concentration of 2-4 wt%, results in a higher sizing amount for polyacrylonitrile-based carbon fibers. Furthermore, the modified epoxy resin emulsion prepared in this embodiment of the invention, when used as a sizing agent, significantly improves the abrasion resistance and bundle properties of the fibers. Crucially, the modified epoxy resin emulsion prepared in this embodiment of the invention, when used as a sizing agent, has a significant characteristic: it can improve the flexibility of polyacrylonitrile-based carbon fibers (see stiffness data; the lower the stiffness, the better the flexibility). The principle is that since epoxidized hydroxyl-terminated polybutadiene is a linear molecule, and its molecular chain contains both epoxy and hydroxyl groups, adding epoxidized hydroxyl-terminated polybutadiene and its related reactants to the epoxy resin can increase the flexibility of the epoxy resin, thereby improving the flexibility of the sizing solution and enhancing subsequent processing performance (e.g., weaving performance).

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a carbon fiber sizing agent, characterized in that, The carbon fiber sizing agent is a modified epoxy resin emulsion; wherein, the preparation method of the modified epoxy resin emulsion includes the following steps: 1) Prepolymerizing epoxidized hydroxyl-terminated polybutadiene with aliphatic polyisocyanate to obtain a prepolymer product; step 1) includes: 11) Vacuum dehydration treatment was performed on epoxidized hydroxyl-terminated polybutadiene to obtain vacuum dehydrated epoxidized hydroxyl-terminated polybutadiene; 12) At a temperature of 65-75℃, the epoxidized hydroxyl-terminated polybutadiene after vacuum dehydration is mixed with the aliphatic polyisocyanate, and a prepolymerization reaction is carried out for 2-4 hours under nitrogen protection and reflux to obtain the prepolymer reaction product. 2) Add chain extender, organic solvent and catalyst to the prepolymer reaction product to carry out chain extension and crosslinking reaction to obtain chain extension and crosslinking reaction product; wherein, step 2) includes: at a temperature of 65-75°C, first add chain extender and organic solvent to the prepolymer reaction product, then add catalyst dropwise, and after the dropwise addition is completed, carry out chain extension and crosslinking reaction for 2-4 hours to obtain chain extension and crosslinking reaction product; 3) Add epoxy resin, diluent, emulsifier, and organic solvent to the chain extension and crosslinking reaction product to graft and modify the epoxy resin, thereby obtaining a modified reaction product; wherein, step 3) includes: adding epoxy resin, diluent, emulsifier, and organic solvent to the chain extension and crosslinking reaction product at a temperature of 65-75°C, and then reacting for 2-4 hours to obtain the modified reaction product; 4) The modified reaction product is post-treated to obtain a modified epoxy resin emulsion; The raw materials used, by weight, are as follows: Epoxidized hydroxyl-terminated polybutadiene: 44.54-48.30 parts by weight; Aliphatic polyisocyanates: 3.84-5.04 parts by weight; Epoxy resin: 42.22-48.08 parts by weight; Chain extender: 0.10-0.25 parts by weight; Catalyst: 0.08-0.12 parts by weight; Diluent: 1.0-4.50 parts by weight; Emulsifier: 2.50-4.00 parts by weight; The molecular weight (MW) of the epoxidized hydroxyl-terminated polybutadiene is 3000-3500. The aliphatic polyisocyanate is isoflurone diisocyanate (IPDI).

2. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, The diluent is one or a mixture of two of butyl glycidyl ether (BGE) and dodecyl to tetradecyl glycidyl ether (AGE); and / or The chain extender is one or a mixture of two of ethylene glycol (EG) and propylene glycol (PG); and / or The catalyst is one or a mixture of two of dibutyltin dilaurate and triphenylbismuth; and / or; The emulsifier is N-methyldiethanolamine; and / or The organic solvent is one or a mixture of several of acetone, butanone, and N-methylpyrrolidone.

3. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, The epoxy resin is a bisphenol A type epoxy resin.

4. The method for preparing the carbon fiber sizing agent according to claim 3, characterized in that, The epoxy resin is one or a mixture of two of E-44 and E-51.

5. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, In step 11): The temperature of the vacuum dehydration treatment is 100 - 110°C, and the time of the vacuum dehydration treatment is 60 - 90 min.

6. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, In step 12): The time of the prepolymerization reaction is 3 h.

7. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, In the prepolymer reaction product: The isocyanate group index R in the prepolymer is 1.01 - 1.40; where the isocyanate group index R = n(NCO) / n(OH); n(NCO) is the molar amount of the -NCO group of the aliphatic polyisocyanate, and n(OH) is the molar amount of the -OH group of the epoxy-terminated hydroxyl polybutadiene.

8. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, In step 2): The addition amount of the organic solvent is 3.5 - 6.5 wt% of the sum of the amounts of the epoxy-terminated hydroxyl polybutadiene, aliphatic polyisocyanate, and chain extender.

9. The method for preparing the carbon fiber sizing agent according to claim 8, characterized in that, In step 2): The addition amount of the organic solvent is 5 wt% of the sum of the amounts of the epoxy-terminated hydroxyl polybutadiene, aliphatic polyisocyanate, and chain extender.

10. The method for preparing the carbon fiber sizing agent according to claim 8, characterized in that, In step 3): The addition amount of the organic solvent is 3.5 - 6.5 wt% of the sum of the amounts of the epoxy-terminated hydroxyl polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin.

11. The method for preparing the carbon fiber sizing agent according to claim 10, characterized in that, In step 3): The addition amount of the organic solvent is 5 wt% of the sum of the amounts of the epoxy-terminated hydroxyl polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin.

12. The method for preparing the carbon fiber sizing agent according to claim 1, characterized in that, Step 4) includes: Lower the temperature of the modified reaction product to 40 - 45°C, add an organic solvent thereto, then further lower the temperature to 10 - 15°C, add deionized water thereto, perform shear emulsification, and finally remove the organic solvent by vacuum distillation to obtain a modified epoxy resin emulsion.

13. The method for preparing the carbon fiber sizing agent according to claim 12, characterized in that, In step 4): The addition amount of the organic solvent is 50 - 60 wt% of the sum of the amounts of the epoxy-terminated hydroxyl polybutadiene, aliphatic polyisocyanate, chain extender, and epoxy resin.

14. A carbon fiber sizing agent, characterized in that, The carbon fiber sizing agent is prepared by the preparation method of the carbon fiber sizing agent according to any one of claims 1 - 13.

15. The carbon fiber sizing agent according to claim 14, characterized in that, The solid content of the modified epoxy resin emulsion is 25 - 35 wt%.

16. The carbon fiber sizing agent according to claim 14, characterized in that, The usage method of the carbon fiber sizing agent is as follows: First, dilute the modified epoxy resin emulsion to obtain a sizing solution; then, use the sizing solution to surface wet the carbon fiber tow; finally, dry the sized carbon fiber.

17. The carbon fiber sizing agent according to claim 14, characterized in that, Dilute the modified epoxy resin emulsion to a mass concentration of 2 - 4 wt% to obtain a sizing solution.

18. The carbon fiber sizing agent according to claim 16, characterized in that, The time for wetting the carbon fiber tow is 5 - 30 s.

19. The carbon fiber sizing agent according to claim 18, characterized in that, The time for wetting the carbon fiber tow is 10 s.

20. The carbon fiber sizing agent according to claim 16, characterized in that, Use the sizing solution with a temperature of 20 - 30°C to surface wet the carbon fiber tow.

21. The carbon fiber sizing agent according to claim 16, characterized in that, Dry the sized carbon fiber in a hot air drying oven; where the hot air temperature is 190 - 220°C and the drying time is 2 - 3 min.

22. The carbon fiber sizing agent according to claim 16, characterized in that, The carbon fiber is a polyacrylonitrile-based carbon fiber.

23. The carbon fiber sizing agent according to claim 22, characterized in that, After sizing treatment, 6-24K polyacrylonitrile-based carbon fibers exhibit the following properties: stiffness of 12-14 cm, tensile strength of 4.50-5.50 GPa, tensile modulus of 235-255 GPa, and bulk density of 1.78-1.82 g / cm³. 3 .

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