High-temperature-resistant polyamide acid sizing agent for carbon fibers, and preparation method and application thereof

Abstract

The present application relates to carbon fiber sizing agent and composite material technical field, specifically disclose the high temperature resistant polyamide acid sizing agent for carbon fiber and its preparation method and application, in order to solve the problem of insufficient thermal stability of existing sizing agent, and the processing temperature of thermoplastic polyimide (TPI) matrix does not match, the interface is weak, the present application adopts 3, 3', 4, 4'-benzophenone tetracarboxylic dianhydride and 2, 2'-bis (trifluoromethyl) diphenyl diamine with molar ratio 1.02:1 by low temperature polycondensation reaction to synthesize polyamide acid sizing agent. The polyimide coating formed by the sizing agent after thermal imidization 5% heat weight loss temperature reaches 565 DEG C, can form a uniform continuous coating on the surface of carbon fiber. The interlaminar shear strength of the obtained CF / TPI composite material reaches 80.6 MPa, which is 48.7% higher than that of commercial carbon fiber composite material, and is suitable for the requirement of high temperature resistant high performance composite material in aerospace field.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber sizing agents and composite materials, specifically to a high-temperature resistant polyamic acid sizing agent for carbon fiber, its preparation method, and its application. Background Technology

[0002] Carbon fiber reinforced thermoplastic composites possess advantages such as lightweight, high strength, recyclability, and short molding cycles, leading to their widespread application in aerospace, automotive, and other fields. Among them, thermoplastic polyimide (TPI) resin, due to its excellent high-temperature resistance, mechanical properties, and dielectric properties, has become an ideal material for manufacturing high-temperature structural components for advanced aircraft and space probe assemblies. However, the contradiction between the chemical inertness of carbon fiber surfaces and the high melt viscosity and low reactivity of TPI resin often results in weak interfacial bonding in the composite material, limiting the full realization of its mechanical properties.

[0003] Sizing is the most widely used surface treatment method for carbon fibers in industry. By coating the fiber surface with a polymer film, fiber protection, improved bundle cohesion, and enhanced interfacial bonding can be achieved simultaneously. Currently, the mainstream commercial sizing agents on the market are mainly epoxy resin systems, whose thermal decomposition temperature is usually around 250 ℃, far lower than the processing and molding temperature of TPI resin at 420 ℃. During the hot pressing process of CF / TPI composites, the epoxy sizing layer undergoes thermal decomposition, producing small molecule volatiles that form pores and defects in the interfacial region, compromising interfacial integrity. In addition, the significant differences in molecular structure between epoxy resin and TPI result in poor interfacial compatibility, making it difficult to form effective molecular chain diffusion and entanglement, further weakening the interfacial bonding strength.

[0004] Polyamic acid (PAA), as a precursor to polyimide (PI), can be converted into a PI sizing layer with a chemical structure similar to that of the TPI matrix after thermal imidization, theoretically achieving good interfacial compatibility and excellent thermal stability. However, existing polyimide sizing agents often suffer from a lack of diverse molecular structure design, failing to simultaneously ensure both uniform spreadability of the sizing layer on the carbon fiber surface and interfacial bonding strength. In particular, the synergistic effect of the polar enhancement effect of ketone carbonyl groups and the steric hindrance effect of trifluoromethyl groups in the interface formation process has not been fully utilized or systematically studied.

[0005] Currently, most mainstream industrial carbon fiber sizing agents use epoxy resin and polyurethane as the matrix, and their initial thermal decomposition temperature is generally below 300℃. When applied to high-temperature resistant systems such as polyimide and polyetheretherketone (PEEK) with processing temperatures exceeding 350℃, the sizing layer undergoes severe thermal decomposition and interface defects during hot pressing, ultimately leading to a significant decrease in the room-temperature mechanical properties of the composite material. To address the insufficient thermal stability of traditional sizing agents, researchers both domestically and internationally have conducted a series of research and development efforts on high-temperature resistant sizing agents. Chinese Patent Publication No. CN107022901A discloses a composite water-based carbon fiber sizing agent and its preparation and application methods. The sizing agent described contains epoxy resin components accounting for up to 50% of the main sizing material. This component begins to undergo significant thermal decomposition below 200℃. During high-temperature processing of resins such as polyimide and PEEK above 380℃, the sizing layer undergoes large-scale thermal degradation, making the generation of interface defects unavoidable.

[0006] Chinese patent publication number CN100999867A discloses a heat-resistant carbon fiber emulsion sizing agent, its preparation method, and its application. Although the described sizing agent incorporates a thermoplastic polyimide component, its molecular structure has poor compatibility with high-performance thermoplastic resins such as polyimide and polyetheretherketone (PEEK). The degree of molecular chain diffusion and entanglement at the interface is limited, and it lacks active functional groups capable of forming strong non-covalent interactions, making it difficult to further improve the interfacial bonding strength. Therefore, developing a high-temperature resistant sizing agent that combines high thermal stability, good film-forming properties, and excellent interfacial reinforcement is of significant theoretical and practical value for improving the overall performance of CF / TPI composite materials. Summary of the Invention

[0007] The purpose of this invention is to overcome the problems of insufficient thermal stability of existing commercial epoxy sizing agents, mismatch with TPI matrix processing temperature, and poor interfacial compatibility; existing polyimide sizing agents fail to simultaneously achieve uniform spreading of the sizing layer and strong interfacial bonding, resulting in weak interfacial bonding and limited mechanical properties of CF / TPI composite materials. This invention provides a high-temperature resistant polyamic acid sizing agent for carbon fibers, its preparation method, and its application.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a high-temperature resistant polyamic acid sizing agent for carbon fibers, wherein the sizing agent is polyamic acid, synthesized by low-temperature polycondensation reaction of 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and 2,2'-bis(trifluoromethyl)biphenyl diamine (TFMB); the molar ratio of BTDA to TFMB is 1.02:1, and the sizing agent is a solution of N,N-dimethylacetamide (DMAc) with a solid content of 18 wt%.

[0009] As a preferred embodiment of the present invention, the polyamic acid has an anhydride-terminated structure, and the molecular weight of the polyamic acid is controlled at 1.5 × 10⁻⁶. 4 ~2.5×10 4 Within the range.

[0010] A method for preparing a high-temperature resistant polyamic acid sizing agent for carbon fibers as described above is carried out under anhydrous and oxygen-free, ice bath, and argon protection conditions, and includes the following steps: Step 1: Monomer pretreatment: BTDA was vacuum dried at 150~180 ℃ for 6~8 h, TFMB was vacuum dried at 80~110 ℃ for 4~6 h, and DMAc was dried by molecular sieve to remove water; Step 2: Low-temperature polycondensation: Dissolve TFMB in DMAc and stir at 320 r / min until completely dissolved; add BTDA in 3-5 batches, with an interval of 6-8 min between each batch; after the addition is complete, adjust the speed to 360 r / min and continue the reaction for 90 min. After the system no longer shows obvious exothermic reaction, adjust the speed to 200 r / min and continue the reaction until the viscosity of the system stabilizes. Step 3: Defoaming by standing: After the reaction is complete, seal and let stand for 24 hours to obtain a transparent, viscous, light yellow polyamic acid sizing agent.

[0011] A method for sizing carbon fibers using the sizing agent described above includes the following steps: S1: Carbon fiber pretreatment: Commercial carbon fiber ultrasonically cleaned with acetone. back The carbon fiber was washed with anhydrous ethanol and deionized water several times in sequence, and then dried under vacuum at 120 °C to obtain desized carbon fiber (DCF). S2: Dilute the polyamic acid sizing agent with DMAc to 1~3 wt%, immerse DCF for 3 min, and squeeze out the excess solution with a roller; S3: Gradient thermal imidization: Dry at 150 ℃ for 30 min to remove solvent; Gradiently increase temperature to 210 ℃ and hold for 30 min, 260 ℃ and hold for 30 min, 310 ℃ and hold for 10 min, then cool naturally to room temperature to obtain PI-KetoneF sized carbon fiber (PI-KetoneF@DCF).

[0012] As a preferred embodiment of the present invention, the commercial carbon fiber is a carbon fiber fabric.

[0013] As a preferred embodiment of the present invention, in step S2, the polyamic acid sizing agent is diluted with DMAc to 2 wt%. The application of the high-temperature resistant polyamic acid sizing agent for carbon fibers described above in the preparation of carbon fiber reinforced thermoplastic polyimide (CF / TPI) composite materials.

[0014] As a preferred embodiment of the present invention, the sizing agent is used as an interface modification layer for carbon fibers.

[0015] A carbon fiber reinforced thermoplastic polyimide composite material, wherein the composite material is obtained by sizing carbon fiber with a sizing agent and then combining the sized carbon fiber with thermoplastic polyimide resin.

[0016] Compared with the prior art, the beneficial effects of the present invention are: (1) Excellent thermal stability: The 5% thermogravimetric temperature of PI-KetoneF obtained by thermal imidization of PAA-KetoneF is ( T The temperature reaches 565 ℃ (5%), which is much higher than the 420 ℃ hot pressing temperature of CF / TPI composite material. There is no obvious thermal decomposition during high-temperature processing, thus avoiding the generation of interface defects.

[0017] (2) Good film-forming properties: The trifluoromethyl side group introduced by TFMB effectively inhibits the close packing of PI molecular chains through steric hindrance, so that the sizing agent forms a uniform and continuous film layer on the carbon fiber surface without particulate deposition.

[0018] (3) Excellent interfacial reinforcement effect: At the optimal sizing concentration of 2 wt%, the interlaminar shear strength of PI-KetoneF@DCF / TPI composite material reached 80.6 MPa, which is 48.7% higher than that of commercial carbon fiber composite material; the flexural strength reached 523.7 MPa, which is 24.7% higher than that of commercial carbon fiber composite material. Attached Figure Description

[0019] Figure 1 These are scanning electron microscope (SEM) images of carbon fibers before and after treatment with the sizing agent of this invention at different magnifications. A1, A2: Untreated commercial CF; B1, B2: Desizing DCF; C1, C2: PI-KetoneF@DCF. Detailed Implementation

[0020] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0021] Example 1 The preparation method of 1-PI-KetoneF@DCF / TPI composite laminate includes the following steps: First, dry 3.222 g BTDA, 3.202 g TFMB, and 47.02 g DMAc. Under ice bath and argon protection, dissolve TFMB in approximately half the volume of DMAc and disperse BTDA in approximately one-third of the volume of DMAc. Add the TFMB solution to the BTDA dispersion in three batches, rinsing the vessel walls with the remaining DMAc. After the addition is complete, react at 360 r / min for 80 min. Once the system no longer shows significant exothermic reaction, continue the reaction at 200 r / min for 5 h. Seal and allow to stand. back The resulting PAA-KetoneF sizing agent stock solution has a solid content of approximately 18 wt%.

[0022] Take 2.78 g of the sizing agent stock solution, add 47.22 g of DMAc to dilute to a solid content of 1 wt%, stir to dissolve, and let stand to remove bubbles to obtain 1 wt% sizing agent.

[0023] Carbon fiber desizing and composite material molding: T300-3K plain weave carbon fiber fabric was desizing with acetone and then dried to obtain desizing carbon fiber (DCF). It was then impregnated in 1 wt% sizing agent for 3 min, the excess liquid was extruded by rollers, and then dried. After gradient heating and imidization, it was naturally cooled to obtain PI-KetoneF sized carbon fiber. Twelve layers of sized carbon fiber and eleven layers of TPI powder (PD450) were alternately laminated (carbon fiber to TPI mass ratio 3:2), placed in a mold, and a release cloth was laid on top. The laminate was then hot-pressed in a pneumatic flat vulcanizing apparatus to obtain a modified composite laminate, named 1-PI-KetoneF@DCF / TPI.

[0024] Bending performance test: The bending strength and bending modulus of the 1-PI-KetoneF@DCF / TPI composite material were tested according to ASTM D7264 standard; its bending strength was 458.1 MPa and its bending modulus was 36.7 GPa.

[0025] Example 2 Preparation of 2-PI-KetoneF@DCF / TPI composite laminate: Sizing agent dilution: Take 5.56 g of the stock solution, add 44.44 g of DMAc, and prepare a 2 wt% sizing agent in the same manner as in Example 1. The remaining carbon fiber treatment, imidization, and hot pressing steps are the same as in Example 1. The composite material is named 2-PI-KetoneF@DCF / TPI.

[0026] Bending performance test: The bending strength and bending modulus of the 2-PI-KetoneF@DCF / TPI composite material were tested according to ASTM D7264 standard; its bending strength was 523.7 MPa and its bending modulus was 41.8 GPa.

[0027] Example 3 Preparation of 3-PI-KetoneF@DCF / TPI composite laminate: Sizing agent dilution: Take 8.33 g of the stock solution, add 41.67 g of DMAc, and prepare a 3 wt% sizing agent in the same manner as in Example 1. The remaining carbon fiber treatment, imidization, and hot pressing steps are the same as in Example 1. The composite material is named 3-PI-KetoneF@DCF / TPI.

[0028] Bending performance test: The bending strength and bending modulus of the 3-PI-KetoneF@DCF / TPI composite material were tested according to ASTM D7264 standard; its bending strength was 483.2 MPa and its bending modulus was 38.5 GPa.

[0029] Comparative Example 1 Preparation of CF / TPI composite laminate: Commercial T300-3K plain weave carbon fiber fabric was ultrasonically cleaned with acetone, then rinsed three times with anhydrous ethanol and deionized water, and vacuum dried at 120 °C for 4 h. Twelve layers of carbon fiber and eleven layers of TPI powder (PD450) were alternately laminated (mass ratio 3:2), and molded under the same hot-pressing conditions as in Example 1. The laminate was then naturally cooled and depressurized to obtain the CF / TPI composite laminate.

[0030] Bending performance test: The bending strength and bending modulus of the CF / TPI composite material were tested according to ASTM D7264 standard; its bending strength was 369.4 MPa and its bending modulus was 29.6 GPa.

[0031] Performance test results: The flexural properties of the 1-PI-KetoneF@DCF / TPI, 2-PI-KetoneF@DCF / TPI, and 3-PI-KetoneF@DCF / TPI composite laminates prepared in Examples 1, 2, and 3, as well as the CF / TPI composite laminate prepared in Comparative Example 1, were tested according to ASTM D7264. The results are shown in Table 1.

[0032] Table 1: Flexural properties of CF / TPI composites modified with different concentrations of PAA-KetoneF sizing agent

[0033] As shown in Table 1, the composite material exhibits the best mechanical properties when the sizing agent concentration is 2 wt%. According to the ASTM D7264 standard, its flexural strength reaches 523.7 MPa and its flexural modulus reaches 41.8 GPa. Compared with commercial carbon fiber composites (CF / TPI), the flexural strength and flexural modulus are increased by 41.8% and 41.2%, respectively.

[0034] The interlaminar shear strength at this concentration was tested according to ASTM D2344 for the 2-PI KetoneF@DCF / TPI composite laminate prepared in Example 2 and the CF / TPI composite laminate prepared in the comparative example (Example 4). The results are shown in Table 2.

[0035] Table 2: Interlaminar Shear Strength of CF / TPI Composites Modified with 2 wt% PAA-KetoneF Sizing Agent

[0036] As shown in Table 2, the interlaminar shear strength at this concentration, tested according to ASTM D2344 standard, is 80.6 MPa, which is 48.7% higher than that of commercial carbon fiber composites.

[0037] The sizing agents PAA-KetoneF prepared in Examples 1, 2, and 3 were subjected to thermal imidization to form PI-KetoneF. Their thermal stability was then tested using a thermogravimetric analyzer (TGA) in a nitrogen atmosphere at a heating rate of 10 °C / min, within a temperature range of 25–875 °C. The thermal stability of the sizing agents and their applicability to high-temperature molding processes for composite materials were evaluated by analyzing the thermogravimetric curves of PAA and PI. The results are shown in Table 3.

[0038] Table 3: PI-KetoneF Temperature at which 5% thermal weight loss occurs

[0039] Table 3 shows that the results indicate the 5% thermogravimetric temperature of PI-KetoneF ( T The 5% temperature is 565 ℃, which is much higher than the 420 ℃ hot pressing temperature of CF / TPI composite material, and there is no obvious thermal decomposition during the processing.

[0040] The surface morphology of carbon fibers before and after sizing treatment was observed using scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown.

[0041] exist Figure 1 As can be seen, A1 and A2 are untreated commercial carbon fiber (CF), B1 and B2 are desized DCF, and C1 and C2 are PI-KetoneF@DCF. The untreated carbon fiber surface is smooth and contains a small amount of residual sizing agent; after desizing, fine etched grooves appear on the carbon fiber surface, exposing active sites; after treatment with the sizing agent of this invention, a uniform and continuous PI coating is formed on the carbon fiber surface, without particulate deposition or agglomeration, proving that the sizing agent has good film-forming and spreading properties.

[0042] The above embodiments merely illustrate implementation methods of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A high-temperature resistant polyamic acid sizing agent for carbon fiber, characterized in that, The sizing agent is polyamic acid, synthesized from 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and 2,2'-bis(trifluoromethyl)benzyl diamine (TFMB) via a low-temperature polycondensation reaction; the molar ratio of BTDA to TFMB is 1.02:1, and the sizing agent is a solution of N,N-dimethylacetamide (DMAc) with a solid content of 18 wt%.

2. The high-temperature resistant polyamic acid sizing agent for carbon fiber according to claim 1, characterized in that, The polyamic acid has an anhydride-terminated structure, and the molecular weight of the polyamic acid is controlled at 1.5 × 10⁻⁶. 4 ~2.5×10 4 Within the range.

3. A method for preparing a high-temperature resistant polyamic acid sizing agent for carbon fiber as described in any one of claims 1 to 2, characterized in that, The procedure is carried out under anhydrous and oxygen-free conditions, in an ice bath, and under argon protection, and includes the following steps: Step 1: Monomer pretreatment: BTDA was vacuum dried at 150~180 ℃ for 6~8 h, TFMB was vacuum dried at 80~110 ℃ for 4~6 h, and DMAc was dried by molecular sieve to remove water; Step 2: Low-temperature polycondensation: Dissolve TFMB in DMAc and stir at 320 r / min until completely dissolved; add BTDA in 3-5 batches, with an interval of 6-8 min between each batch; after the addition is complete, adjust the speed to 360 r / min and continue the reaction for 90 min. After the system no longer shows obvious exothermic reaction, adjust the speed to 200 r / min and continue the reaction until the viscosity of the system stabilizes. Step 3: Defoaming by standing: After the reaction is complete, seal and let stand for 24 hours to obtain a transparent, viscous, light yellow polyamic acid sizing agent.

4. A method for sizing carbon fibers using the sizing agent according to claim 1 or 2, characterized in that, Includes the following steps: S1: Carbon fiber pretreatment: Commercial carbon fiber ultrasonically cleaned with acetone. back The carbon fiber was washed with anhydrous ethanol and deionized water several times in sequence, and then dried under vacuum at 120 °C to obtain desized carbon fiber (DCF). S2: Dilute the polyamic acid sizing agent with DMAc to 1~3 wt%, immerse DCF for 3 min, and squeeze out the excess solution with a roller; S3: Gradient thermal imidization: Dry at 150 ℃ for 30 min to remove solvent; Gradiently increase temperature to 210 ℃ and hold for 30 min, 260 ℃ and hold for 30 min, 310 ℃ and hold for 10 min, then cool naturally to room temperature to obtain PI-KetoneF sized carbon fiber (PI-KetoneF@DCF).

5. The method according to claim 4, characterized in that, The commercial carbon fiber is a carbon fiber fabric.

6. The method according to claim 4, characterized in that, In step S2, the polyamic acid sizing agent is diluted to 2 wt% with DMAc.

7. The application of the high-temperature resistant polyamic acid sizing agent for carbon fibers as described in claim 1 or 2 in the preparation of carbon fiber reinforced thermoplastic polyimide (CF / TPI) composite materials.

8. The application according to claim 7, characterized in that, The sizing agent is used as an interface modification layer for carbon fibers.

9. A carbon fiber reinforced thermoplastic polyimide composite material, characterized in that, The composite material is made by combining sized carbon fibers obtained by the method described in any one of claims 4 to 6 with thermoplastic polyimide resin.

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