Method for enhancing interface and mechanical strength of thermoplastic composite material

Abstract

The invention relates to a method for enhancing interface and mechanical strength of a thermoplastic composite material, which comprises the following steps: dissolving hexafluorodianhydride and 2, 4-diaminobenzene sulfonic acid in DMF (Dimethyl Formamide) to react, and protecting by flowing nitrogen or argon in the whole reaction process; precipitating the product, and carrying out imidization reaction to obtain S-PI; the method comprises the following steps: dissolving S-PI with an organic solvent, and diluting with deionized water to obtain an S-PI sizing agent with the concentration of 0.5-2.0 wt%; carrying out desizing treatment on carbon fibers, soaking and then drying; the obtained carbon fibers and matrix resin powder are alternately laid and then transferred into a press vulcanizer to be formed, and the thickness is controlled to be 2.0 + / -0.1 mm. A sulfonic acid group is introduced into a polyimide structure, so that the hydrophilicity and solubility of polyimide are remarkably improved, and the polyimide can be uniformly dispersed in water under the condition that a small amount of organic solvent is added; and the sizing agent can be subjected to chemical reaction with OA-POSS grafted carbon fibers, so that the performance of the composite material is obviously enhanced. The method is superior to the prior art in environmental protection and interface enhancement effects.

Description

Technical Field

[0001] The invention belongs to the technical field of advanced composite materials, and specifically relates to a preparation method of a high-temperature resistant water-based polyimide sizing agent, an octaaminopropyl POSS grafted carbon fiber and a thermoplastic composite material. Background Art

[0002] Carbon fiber reinforced thermoplastic composites (CFRTP) are gradually replacing the mainstream position of epoxy resin-based composites due to their advantages such as high impact resistance, high heat resistance and rapid molding. The performance of carbon fiber composites is significantly affected by the quality of the fiber / matrix interface. The surface modification of CF by oxidation, plasma, chemical grafting, high-energy radiation and sizing treatment can effectively improve the interfacial bonding force. Sizing plays a vital role in protecting CF and improving processing performance, and is therefore an indispensable part of the CF production process. The sizing process plays a key role in determining the adhesion between carbon fiber and thermoplastic matrix, thereby affecting the overall mechanical properties of the composite. In addition, sizing agents reduce the damage to fibers during production, transportation and weaving by enhancing fiber bundling and wear resistance. At present, commercial carbon fiber sizing agents are mainly epoxy resin-based sizing agents, which usually degrade at around 250°C and are difficult to apply to high-performance thermoplastic composites with molding temperatures exceeding 300°C. The decomposition of epoxy sizing agents introduces a large number of defects in the interface layer, significantly reducing the performance of the composite. At the same time, the epoxy sizing agent has poor compatibility with the thermoplastic resin matrix, resulting in poor interface bonding and also affecting the performance of the composite material.

[0003] The interface layer between fiber and resin is the key to the performance of composite materials. With the change of matrix resin, the interface construction between carbon fiber and resin should also change accordingly. At present, there are two main sizing agents used for thermoplastic composites. One is solvent-based sizing agent, which directly dissolves the main slurry polymer in organic solvent. This process generates huge costs and environmental hazards, and most of the high-performance engineering plastics are difficult to dissolve in organic solvents, which makes solvent-based sizing agents unable to be industrially applied. The second is emulsion sizing agent, which dissolves polymers similar to the matrix material in a small amount of solvent and disperses them in water to form an emulsion by adding a large amount of emulsifier. However, the low heat resistance of the emulsifier will cause defects in the composite molding process, resulting in performance degradation. These problems limit the application scope of thermoplastic composites and significantly increase the processing and use costs. At present, there is an urgent need for a water-based sizing agent that combines thermal stability and green environmental protection. Compared with other engineering plastics, aromatic polyimide has the potential to be used as a water-based sizing agent due to its rich monomer selection and two-stage synthesis method while meeting high heat resistance. Thermoplastic matrices such as polyetheretherketone (PEEK) and polyetherimide (PEI) are able to form a physically tight bond through a “like dissolves like” process. However, due to the lack of reactive functional groups, it is unable to form a strong chemical bond with the CF surface. One way to form a chemical bond between the fiber and the sizing agent is to introduce functionalized nanoparticles such as polyhedral oligomeric silsesquioxane (POSS), carbon nanotubes (CNTs), and silica (SiO 2 )wait.

[0004] In summary, the sizing agents currently used in the field of thermoplastic composites have almost no interaction with the fiber surface, and only play the role of enhancing resin impregnation, and it is difficult to form a strong interface bond. In addition, thermoplastic sizing agents require a large amount of organic solvents to dissolve, which also brings serious environmental hazards and usage costs. Summary of the invention

[0005] The purpose of the present invention is to solve the problems of poor heat resistance of sizing agents, environmental pollution, poor interface bonding, etc. in the field of thermoplastic composite materials, and to provide a method for enhancing the interface and mechanical strength of thermoplastic composite materials.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for enhancing the interface and mechanical strength of a thermoplastic composite material, the method comprising:

[0008] Step 1: Synthesis of self-emulsifying polyimide (S-PI): Dissolve hexafluorodianhydride and 2,4-diaminobenzenesulfonic acid in DMF for reaction, and protect the reaction with flowing nitrogen or argon throughout the reaction; after the reaction is completed, add the solution dropwise into excess ether, perform high-speed magnetic stirring during the addition process, and stand for 30 minutes after complete addition to obtain a precipitate; dry the precipitate to obtain S-PAA; transfer the S-PAA to an oven for imidization reaction to obtain S-PI;

[0009] Step 2: Preparation of polyimide sizing agent: dissolving S-PI in an organic solvent to obtain a concentrated solution with a concentration of 20 wt%; then, diluting the concentrated solution with deionized water and magnetically stirring for 10 to 30 minutes to obtain an S-PI sizing agent with a concentration of 0.5 to 2.0 wt%;

[0010] Step 3: Surface grafting and sizing of carbon fiber:

[0011] The carbon fiber fabric is cut into small pieces, and acetone is heated to reflux to remove the sizing agent on the surface of the commercial carbon fiber to obtain a desized carbon fiber; the desized carbon fiber is soaked in an S-PI sizing agent and then dried;

[0012] Step 4: Preparation of composite materials: The carbon fibers and matrix resin powder obtained in step 3 are laid alternately, and then transferred to a flat vulcanizer for molding. The thickness of the composite material is controlled at 2.0 ± 0.1 mm by using gaskets and fiber fabric layers. The final product is a bidirectional reinforced laminate with a carbon fiber mass fraction of 59 to 62 wt%.

[0013] Furthermore, in step 1, the ratio of hexafluorodianhydride to 2,4-diaminobenzenesulfonic acid is 1:1-1.1 mol, the solid content of the solution is 5-15 wt %, the reaction temperature is 80-110° C., and the reaction time is 4-24 hours.

[0014] Furthermore, in step 1, the drying is specifically as follows: the precipitate is initially dried by suction filtration, and during the process, it is repeatedly washed with ether three times, and then the precipitate is transferred to a vacuum oven for drying to obtain light yellow S-PAA.

[0015] Furthermore, in step 1, the temperature of the imidization reaction is 250° C. to 300° C., and the time is 1 to 4 hours.

[0016] Furthermore, in step 2, the organic solvent is one of DMF, DMAc or DMSO.

[0017] Furthermore, in step three, the soaking time is 3 to 10 minutes.

[0018] Furthermore, in step 4, the molding temperature is 350-370° C., the pressure is 5-10 MPa, and the time is 1-2 hours.

[0019] Furthermore, in step 4, the matrix resin is PEEK or PEI.

[0020] Furthermore, in step 3, after the carbon fiber is desizing, the following operations are also included:

[0021] performing oxidation treatment on the desized carbon fiber;

[0022] The oxidized carbon fiber was rinsed in acetone and vacuum dried, and then 100 g of the dried carbon fiber was placed in 1 L of thionyl chloride, 0.01 to 0.1 mL of DMF was added, and the mixture was reacted at 50 to 80 ° C for 12 to 24 hours to convert the carboxyl groups on the fiber surface into acyl chloride;

[0023] After further rinsing with acetone and drying, 0.1-1 g of octa(aminopropyl)polyhedral oligomeric silsesquioxane was dispersed in 1 L DMF, 100 g of carbon fiber fabric was added, and the mixture was reacted at 50-80 ° C for 12-24 hours, and then the carbon fiber was dried under vacuum to obtain POSS-grafted carbon fiber.

[0024] The beneficial effects of the present invention relative to the prior art are as follows: the present invention significantly improves the hydrophilicity and solubility of polyimide by introducing sulfonic acid groups into the polyimide structure, so that it can be uniformly dispersed in water under the addition of a small amount of organic solvent; more importantly, the multiple reactivity of the sulfonic acid group brings potential for the combination of fiber and sizing agent. The present invention first treats the carbon fiber, grafts octaaminopropyl POSS, and the carbon fiber can react chemically with the sizing agent during sizing to construct a strong interface bond, which significantly enhances the interface and mechanical properties of the thermoplastic composite material. Therefore, the present invention is superior to the prior art in terms of environmental protection and interface enhancement effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the change and reaction diagram of CF surface;

[0026] Figure 2 FT-IR spectra of S-PAA and S-PI;

[0027] Figure 3 is the XPS spectrum of S-PI;

[0028] Figure 4 is the TGA curve of S-PI;

[0029] Figure 5 This is the actual picture of S-PI sizing agent;

[0030] Figure 6is the particle size distribution diagram of S-PI sizing agent;

[0031] Figure 7 This is the test diagram of carbon fiber drape value;

[0032] Figure 8 Schematic diagram of carbon fiber surface energy;

[0033] Fig. 9 SEM and AFM images of carbon fibers (a) DCF; (b) CCF; (c) SCF; (d) SPCF;

[0034] Fig.10 is the tensile strength diagram of carbon fiber monofilament;

[0035] Fig.11 ILSS, impact strength and flexural strength diagram of CF / PEEK;

[0036] Fig.12 ILSS, impact strength and flexural strength diagram of CF / PEI. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0038] Embodiment 1:

[0039] 1. Synthesis of self-emulsifying polyimide (S-PI)

[0040] First, add 0.0102 mol of hexafluorodianhydride (6FDA) into a three-necked flask, add 100 ml of dimethylformamide (DMF), and stir at 300 rpm at room temperature until 6FDA is completely dissolved. Then add 0.01 mol of 2,4-diaminobenzenesulfonic acid into the three-necked flask and react at 80-100°C for 20-24 hours. The whole reaction is carried out with flowing N 2 After the reaction, the solution was added dropwise to an excess of ether, with high-speed magnetic stirring during the addition. After complete addition, the solution was allowed to stand for 30 minutes to obtain a precipitate. The precipitate was initially dried by filtration, and was repeatedly washed with ether three times during the process. The precipitate was then transferred to a vacuum oven for drying to obtain light yellow S-PAA. The reaction was carried out at 300°C for 2 hours to allow S-PAA to undergo imidization reaction to obtain brown S-PI.

[0041] 2. Preparation of polyimide sizing agent

[0042] 20 g of S-PI was dissolved in 80 g of DMF to obtain a concentrated solution with a concentration of 20 wt%. Then, deionized water was quickly poured into the solution and magnetically stirred for 10 minutes to obtain a sizing agent with an S-PI concentration of 0.5 wt%.

[0043] 3. Surface grafting and sizing of carbon fiber

[0044] The carbon fiber fabric was cut into 20 × 20 cm pieces and refluxed with acetone at 75 °C for 48 h to remove the sizing agent on the surface of commercial carbon fiber to obtain desized carbon fiber (DCF). The carbon fiber was then oxidized by a mild method to remove AgNO 3 (0.01mol) and K 2 S 2 O 8 (0.1 mol) was dissolved in 1L deionized water, 100g DCF was added to the solution, and then stirred at 70°C until the solution was clear. The oxidized carbon fibers were rinsed in acetone and dried in vacuum, and then the dried carbon fibers were placed in 1L thionyl chloride, 0.05mL DMF catalyst was added, and the reaction was carried out at 76°C for 24 hours to convert the carboxyl groups on the fiber surface into acid chlorides. After further rinsing with acetone and drying, the carbon fibers were reacted with octa(aminopropyl)polyhedral oligomeric silsesquioxane (OA-POSS) in DMF solvent at 70°C for 24 hours, and then the carbon fibers were dried under vacuum to obtain POSS-grafted carbon fibers (PCF). DCF and PCF were soaked in S-PI sizing agent for 3 minutes and then dried at 160°C, and were named SCF and SPCF, respectively. The changes and reactions on the CF surface are shown in Figure 2. Figure 1 shown.

[0045] 4. Preparation of composite materials

[0046] The CF plain fabric and matrix resin (PEEK or PEI) powder are laid alternately and then transferred to a flat vulcanizer for molding. The thickness of the CFRTP is controlled at 2.0±0.1mm by using gaskets and fiber fabric layers. The final product is a bidirectional reinforced laminate with a fiber mass fraction of 59-62wt%.

[0047] Figure 2 The Fourier transform infrared (FT-IR) spectra of S-PAA and S-PI are shown. Both spectra show a -1 -HSO 3 The stretching vibration of -1 and 1145cm -1 SO and S=O, and the -1CS stretching vibration. This confirms that the sulfonic acid group structure on S-PI is retained during the imidization process. -1 The absence of a peak at 1670 cm-1 in the S-PI spectrum corresponds to the bending vibration of C-NH in the S-PAA structure, proving the depletion of secondary amine groups in the PAA structure. -1 A new absorption peak appeared at , indicating the symmetrical stretching of C=O, indicating the formation of an imide ring with a symmetrical C=O structure. In general, the high-temperature imidization process resulted in the complete conversion of S-PAA to S-PI, and the hydrophilic sulfonic acid group was retained in the S-PI structure. The X-ray photoelectron spectroscopy (XPS) spectrum of S-PI ( Figure 3 ) clearly shows peaks corresponding to the characteristic elements F, N and S. Combined with the results of infrared spectroscopy, it can be concluded that the polymer has successfully introduced the expected structural features.

[0048] The thermal stability of S-PI was investigated by thermogravimetric analysis (TGA). Figure 4 As shown, T1% (temperature at 1% weight loss) was observed to be 400.5°C and T5% (temperature at 5% weight loss) reached 433.3°C. The processing temperature of high-performance engineering plastics is usually up to 390°C, and S-PI sizing agents can withstand the thermal demands encountered during the manufacturing and processing of CFRTP materials.

[0049] The sizing agent configured is as Figure 5 After six months of storage, the state was consistent with the newly prepared sizing agent, remaining uniform, transparent, and without precipitation or agglomeration. In order to study the stability of the sizing agent, a comparative analysis was conducted between the newly prepared formulation and the sample stored for six months. The particle size distribution of the sizing agent is shown in Figure 6 The average particle size is 85 nm, and the peak value and distribution of the particle size are basically consistent after storage for six months.

[0050] Carbon fiber tow drape test Figure 7 As shown. SCF and SPCF show similar bundling and drape values ​​as CCF, and are able to maintain a stable bundling state while maintaining appropriate softness. In contrast, DCF cannot be bundled, but disperses and entangles during processing, complicating the subsequent CFRTP material preparation. This also leads to a significant reduction in the performance of CFRTP materials and a significant increase in processing difficulty.

[0051] The polar groups on the CF surface increased significantly after treatment, including sulfonic acid and amino groups, which enhanced the surface energy of CF. This change improved the wettability of the resin on the fiber and promoted the formation of a tight interface. The surface energy of CF was calculated from the contact angle results ( Figure 8), which reflects the wettability between the fiber surface and the liquid. Compared with DCF, the surface energy of SCF is increased from 34.29mN·m -1 Increased to 47.77 mN·m -1 This is due to the high polarity of the acyl and imine groups in the polyimide main chain. At the same time, the sulfonic acid groups in the side chains further increase the surface polarity. 2 The polarity of SPCF is 50% higher than that of DCF.

[0052] Significant changes in surface morphology were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), such as Fig. 9 As shown. The surface of DCF is generally flat with many small grooves. In contrast, the surface of SPCF is almost completely covered with sizing agent. This sizing agent layer protects the fiber, prevents the generation of static electricity, and reduces damage. In addition, the sizing agent partially fills the defects on the CF surface, thereby improving the tensile strength of the monofilament, although this improvement is limited. Fig.10 As shown, the Weibull distribution of the single-filament tensile strength indicates that the fiber has a slightly improved strength compared to DCF, which is consistent with the findings of other researchers.

[0053] Fig.11 and Fig.12 The interlaminar shear strength (ILSS), impact strength and flexural strength of the composite materials were demonstrated. Compared with DCF / PEEK and CCF / PEEK, the mechanical properties of SCF / PEEK were improved by about 30%, while SPCF / PEEK further improved the above mechanical properties. The ILSS of SPCF / PEEK increased from 66.30MPa (DCF / PEEK) and 68.86MPa (CCF / PEEK) to 99.70MPa, an increase of 50.4% and 44.8%, respectively; the impact strength of SPCF / PEEK increased from 35.30kJ / m 2 (DCF / PEEK) and 35.46 kJ / m 2 (CCF / PEEK) increased to 57.02 kJ / m 2 , which are 61.5% and 60.8% higher than DCF / PEEK and CCF / PEEK respectively. The flexural strength of SPCF / PEEK increased from 858.80MPa (DCF / PEEK) and 824.89MPa (CCF / PEEK) to 1180.16MPa, an increase of 37.2% and 43.1% respectively; compared with CCF / PEI, the ILSS, impact strength and flexural strength of SPCF / PEI were also significantly improved, by 55.83%, 50.2% and 41.8% respectively.

Claims

1. A method for enhancing the interface and mechanical strength of a thermoplastic composite material, characterized in that: The method is: Step 1: Synthesis of self-emulsifying polyimide (S-PI): Dissolve hexafluorodianhydride and 2,4-diaminobenzenesulfonic acid in DMF for reaction, and protect the reaction with flowing nitrogen or argon throughout the reaction; after the reaction is completed, add the solution dropwise into excess ether, perform high-speed magnetic stirring during the addition process, and stand for 30 minutes after complete addition to obtain a precipitate; dry the precipitate to obtain S-PAA; transfer the S-PAA to an oven for imidization reaction to obtain S-PI; Step 2: Preparation of polyimide sizing agent: dissolving S-PI in an organic solvent to obtain a concentrated solution with a concentration of 20 wt%; then, diluting the concentrated solution with deionized water and magnetically stirring for 10 to 30 minutes to obtain an S-PI sizing agent with a concentration of 0.5 to 2.0 wt%; Step 3: Surface grafting and sizing of carbon fiber: The carbon fiber fabric is cut into small pieces, and acetone is heated to reflux to remove the sizing agent on the surface of the commercial carbon fiber to obtain a desized carbon fiber; the desized carbon fiber is soaked in an S-PI sizing agent and then dried; Step 4: Preparation of composite materials: The carbon fibers and matrix resin powder obtained in step 3 are laid alternately, and then transferred to a flat vulcanizer for molding. The thickness of the composite material is controlled at 2.0 ± 0.1 mm by using gaskets and fiber fabric layers, and the mass fraction of carbon fibers is 59-62 wt%.

2. A method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step 1, the ratio of hexafluorodianhydride to 2,4-diaminobenzenesulfonic acid is 1:1-1.1 mol, the solid content of the solution is 5-15 wt %, the reaction temperature is 80-110° C., and the reaction time is 4-24 hours.

3. The method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step 1, the drying is specifically as follows: the precipitate is initially dried by suction filtration, and during the process, it is repeatedly washed with ether three times, and then the precipitate is transferred to a vacuum oven for drying to obtain light yellow S-PAA.

4. The method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step 1, the temperature of the imidization reaction is 250° C. to 300° C., and the time is 1 to 4 hours.

5. The method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step 2, the organic solvent is one of DMF, DMAc or DMSO.

6. The method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step three, the soaking time is 3 to 10 minutes.

7. The method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step 4, the molding temperature is 350-370° C., the pressure is 5-10 MPa, and the time is 1-2 hours.

8. The method for enhancing the interface and mechanical strength of thermoplastic composite materials according to claim 1, characterized in that: In step 4, the matrix resin is PEEK or PEI.

9. A method for enhancing the interface and mechanical strength of a thermoplastic composite material according to any one of claims 1 to 8, characterized in that: In step 3, after the carbon fiber is desizing, the following operations are also included: performing oxidation treatment on the desized carbon fiber; The oxidized carbon fiber was rinsed in acetone and vacuum dried, and then 100 g of the dried carbon fiber was placed in 1 L of thionyl chloride, 0.01 to 0.1 mL of DMF was added, and the mixture was reacted at 50 to 80 ° C for 12 to 24 hours to convert the carboxyl groups on the fiber surface into acyl chloride; After further rinsing with acetone and drying, 0.1-1 g of octa(aminopropyl)polyhedral oligomeric silsesquioxane was dispersed in 1 L DMF, 100 g of carbon fiber fabric was added, and the mixture was reacted at 50-80 ° C for 12-24 hours, and then the carbon fiber was dried under vacuum to obtain POSS-grafted carbon fiber.

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