Preparation method and application of water-based polyamide acid sizing agent

Abstract

The invention discloses a preparation method and application of a water-based polyamide acid sizing agent, and belongs to the technical field of sizing agents. The method comprises the following steps: synthesizing polyamic acid in an organic solvent by adopting dianhydride and diamine in a polymerization manner, after the reaction is finished, cooling a polyamic acid solution to room temperature, transferring into a dropping funnel, dropwise adding into a solvent of which the volume is 10-25 times that of the solution, separating, completely dropwise adding, collecting a precipitate, and drying to obtain solid polyamic acid; and adding a low-grade ester or low-grade ketone solvent into the deionized water, adding the polyamide acid powder into the mixed solvent according to the solid content of 1-20wt.%, stirring at room temperature until the solid disappears, and standing for liquid separation to obtain a lower layer which is the polyamide acid water-based sizing agent. The sizing agent disclosed by the invention shows excellent dispersity and long-term stability. After sizing, polyamide acid on the surface of the carbon fiber is converted into polyimide through a thermal imidization reaction, so that the carbon fiber has high thermal stability and excellent compatibility.

Description

Technical Field

[0001] The invention belongs to the technical field of sizing agents, and particularly relates to a preparation method and application of a water-based polyamic acid sizing agent. Background Art

[0002] Carbon fiber-reinforced thermoplastic composites (CFRTP) are currently attracting significant attention in the engineering field. Compared to thermoset composites, they offer performance advantages such as impact resistance, rapid molding, and easy recycling. However, the development of compatible carbon fiber sizing agents has lagged, limiting the full potential of CFRTP. This is primarily due to drawbacks such as insufficient heat resistance and poor interface compatibility. Sizing, a key process in carbon fiber (CF) production, protects the carbon fibers and reduces damage caused by friction and static electricity. Sizing agents also improve the interfacial compatibility between the carbon fibers and the resin, which in turn impacts the composite's performance. Compared to other surface treatment methods (such as oxidation, plasma treatment, chemical grafting, radiation, and microwave treatment), sizing agents are the most widely used in industrial applications. The most widely used epoxy resin sizing agents have poor compatibility with thermoplastics, making it difficult for matrix resins such as polyetheretherketone (PEEK), polyphenylene sulfide (PPS), and polyetherimide (PEI) to fully impregnate the carbon fiber surface. What’s more disadvantageous is that the thermal decomposition temperature of epoxy resin usually does not exceed 300°C, which makes it difficult to withstand the processing temperature of high-performance CFRTP. The violent decomposition of the sizing agent during the molding process significantly destroys the integrity of the composite material interface, and small molecule products can also form defects in the interface or matrix.

[0003] Sizing agents suitable for CFRTP are mainly prepared with high-performance thermoplastic resins as raw materials, such as polyimide, polyetheretherketone, polyphenylene sulfide, etc. The solubility of such resins is limited, making it difficult to prepare them into sizing agents. Polyamic acid, as a precursor of polyimide, is often dissolved by N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, etc. due to its good solubility, so as to be sizing treated on carbon fibers. Its imidization process is simple. The sizing agent can be first prepared in the form of a polyamic acid aqueous dispersion, and then high-temperature thermal imidization is performed on the carbon fiber surface to convert the polyamic acid into a polyimide sizing agent layer with excellent thermal stability and compatibility. However, these solvents have high boiling points and high toxicity, resulting in increased environmental hazards and rising costs. Therefore, researchers are committed to completely eliminating organic solvents or replacing them with more environmentally friendly alternatives.

[0004] Polyamic acid has excellent water-dispersibility. Currently, in addition to direct dissolution in organic solvents, water solubility can be enhanced by ionizing the polyamic acid with the addition of triethylamine. While this significantly improves the water solubility of polyamic acid, triethylamine is highly toxic and volatile, and is released during the thermal imidization process of the polyamic acid triethylamine salt, posing a significant risk to human health and the environment.

[0005] In summary, using polyimide as a sizing agent can significantly improve the interface and mechanical properties of composite materials, but dissolving polyimide requires dispersion in highly toxic and high-cost polar organic solvents, which brings obvious harm to human body and environment. Summary of the Invention

[0006] The present invention aims to address the environmental hazards associated with the large amounts of organic solvents used in thermoplastic carbon fiber sizing agents. It provides a method for preparing and applying a water-based polyamic acid sizing agent. This method utilizes a green solvent to regulate the conformation of polyamic acid, thereby enhancing its solvation capacity. This method produces an environmentally friendly water-based sizing agent with excellent dispersibility and stability.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing a water-based polyamic acid sizing agent, comprising:

[0009] Step 1: Synthesis and separation of polyamic acid

[0010] The polyamic acid is synthesized by polymerization of dianhydride and diamine in an organic solvent. After the reaction is completed, the polyamic acid solution is cooled to room temperature, transferred to a dropping funnel, and added dropwise to a solvent with a volume 10-25 times that of the solution for separation. After complete dropwise addition, the precipitate is collected and dried to obtain solid polyamic acid.

[0011] Step 2: Preparation of sizing agent

[0012] Add 5-10 wt% of a low-grade ester or low-grade ketone solvent to deionized water, add polyamic acid powder to the mixed solvent at a solid content of 1-20 wt.%, stir at room temperature until the solid disappears, let it stand for 2-4 hours to separate the liquid, and collect the lower layer of yellow transparent liquid, which is the polyamic acid water-based sizing agent.

[0013] Furthermore, in step 1, the dianhydride is one or more of bisphenol A diether dianhydride, 4,4'-diphenyl ether dianhydride or 3,3,4,4-benzophenonetetracarboxylic dianhydride; the diamine is 2,4-diaminobenzenesulfonic acid; the organic solvent is one or more of DMF, DMAc, DMSO or NMP; and the solvent is one of acetone, butanone and diethyl ether.

[0014] Furthermore, in step 1, the monomer molar ratio of the dianhydride to the diamine is 1.0-1.1:1, and the solid content of the two in the organic solvent is 5-15 wt.%.

[0015] Furthermore, in step 1, the polymerization temperature is 80-100° C., and the time is 4-24 hours.

[0016] Furthermore, in step 2, the lower ester solvent is one of ethyl acetate, ethyl formate, ethyl propionate, ethyl butyrate, propyl acetate, and butyl acetate; and the lower ketone solvent is acetone or butanone.

[0017] An application of the water-based polyamic acid sizing agent prepared by the above preparation method, wherein the application is:

[0018] Step 1: Surface treatment and sizing process of carbon fiber

[0019] The desized carbon fibers were immersed in 0.25-2.0 wt% sizing agent and then transferred to an oven at 300 °C for 10 min for imidization treatment.

[0020] Step 2: Preparation of composite materials

[0021] Carbon fiber fabrics (20 × 20 cm, ten layers) and matrix resin (80 g) were alternately stacked, and the stacked materials were transferred to a flat vulcanizer for molding to obtain a biaxially laminated composite material with a thickness of 2.0 ± 0.1 mm.

[0022] Furthermore, in step 1, the commercial carbon fiber (CCF) is refluxed with acetone at 60-85° C. for 24-48 hours to obtain desized carbon fiber (DCF).

[0023] Furthermore, in step 2, the matrix resin is PEEK or PEI.

[0024] Furthermore, in step 2, CF / PEEK is molded at 350-390° C. and 5-15 MPa for 60-120 minutes, and CF / PEI is molded at 350-370° C. and 5-15 MPa for 80-200 minutes.

[0025] Compared with the prior art, the present invention has the following advantages: it eliminates highly toxic components in the thermoplastic sizing agent, uses water as the main solvent, and uses a small amount of green volatile solvent to assist in dissolution, resulting in a small particle size of the sizing agent and a long storage period.

[0026] The present invention has jumped out of the limitation of traditional methods. First, the specially designed polyamic acid structure carries a strongly hydrophilic sulfonic acid group and a carboxyl structure that can be partially ionized. The ionization of this part of the polar structure or the hydrogen bonding formed with water molecules serves as the main solvation driving force. However, due to the presence of a large number of benzene rings in the polyamic acid structure, the molecules easily accumulate, hindering the movement of water molecules to the inside of the polyamic acid molecules. A small amount of esters or ketone compounds, by intervening in the interaction between water molecules and polyamic acid molecules, changes the molecular conformation of polyamic acid, weakens the intermolecular force of polyamic acid, thereby enabling it to form an aqueous solution quickly, stably and uniformly. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the infrared spectrum of polyamic acid and polyimide;

[0028] Figure 2 is the thermogravimetric curve of polyimide;

[0029] Figure 3 This is a physical picture of the sizing agent after it is newly prepared and stored;

[0030] Figure 4 The particle size distribution diagram of the sizing agent after fresh preparation and storage;

[0031] Figure 5 This is a transmission electron micrograph of the sizing agent;

[0032] Figure 6 This is the X-ray photoelectron spectrum of carbon fiber;

[0033] Figure 7 is the carbon fiber tow drape value diagram;

[0034] Figure 8 This is a comparison chart of carbon fiber surface energy;

[0035] Figure 9 This is a comparison chart of the mechanical properties of CF / PEEK composite materials;

[0036] Figure 10 This is a comparison chart of the mechanical properties of CF / PEI composite materials;

[0037] Figure 11 This is the fracture morphology of CF / PEEK composite product;

[0038] Figure 12 This is the fracture morphology of CF / PEI composite material product. DETAILED DESCRIPTION

[0039] The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments described herein are only partial examples of the present invention and are not all possible implementation methods. Based on the technical content disclosed in the present invention, other implementation methods that can be derived by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0040] This invention designs and synthesizes a water-based polyamic acid polymer containing sulfonic acid groups. Based on its structural characteristics, a solution is designed to dissolve the polymer in a small amount of a green solvent, ethyl acetate and water, to prepare an environmentally friendly water-based sizing agent. The sizing agent exhibits excellent dispersibility and long-term stability. After sizing, a thermal imidization reaction converts the polyamic acid on the carbon fiber surface into polyimide, resulting in high thermal stability and excellent compatibility. The sized carbon fibers exhibit excellent mechanical properties when reinforced with polyetheretherketone or polyetherimide.

[0041] Example 1:

[0042] 1. Synthesis and separation of polyamic acid

[0043] Polyamic acid was synthesized by polymerizing dianhydride and diamine in an organic solvent. The monomer molar ratio of dianhydride to diamine was 1.0:1.0; the organic solvent was DMF; the dianhydride monomer was 4,4'-diphenyl ether dianhydride; the diamine monomer was 2,4-diaminobenzenesulfonic acid; the reaction temperature was between 90°C and the reaction time was 12 hours.

[0044] The polyamic acid solution after the reaction was cooled to room temperature, transferred to a dropping funnel, and added dropwise to ether 20 times the volume of the solution for separation. After the dropwise addition was complete, the precipitate was collected and dried to obtain solid polyamic acid.

[0045] 2. Preparation of sizing agent

[0046] Add 10 wt% ethyl acetate to deionized water. Add polyamic acid powder to the deionized water at a solids content of 1-20 wt%. Stir at room temperature until the solids disappear. Allow to stand for 2 hours to separate the liquids. Collect the lower layer of yellow, transparent liquid, which is the polyamic acid water-based sizing agent.

[0047] 3. Surface treatment and sizing process of carbon fiber

[0048] Commercial carbon fiber (CCF) was refluxed with acetone at 85°C for 48 hours to obtain desized carbon fiber (DCF). The DCF was immersed in different concentrations of sizing agent (0.25 wt.%, 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.%) and then transferred to a 300°C oven for 10 minutes of imidization treatment. The resulting products were named CF0.25, CF0.5, CF1, CF1.5, and CF2, respectively.

[0049] 4. Preparation of composite materials

[0050] Ten layers of carbon fiber fabric (20 × 20 cm) were alternately stacked with a matrix resin (PEEK or PEI, 80 g). The stacked materials were transferred to a flat-bed vulcanizer for molding. CF / PEEK was molded at 370°C and 10 MPa for 60 minutes, while CF / PEI was molded at 350°C and 10 MPa for 80 minutes. The final product was a biaxial laminate composite with a thickness of 2.0 ± 0.1 mm.

[0051] Figure 1 The Fourier transform infrared (FT-IR) spectra of polyamic acid and polyimide are shown. The bending vibration peak of the C-NH structure in polyamic acid appears at 1542 cm -1However, this peak disappears in the spectrum of polyimide, indicating that the secondary amine groups in the polyamic acid structure have reacted. In the spectrum of polyimide, the absorption peak corresponding to the C=O symmetric stretching vibration appears at 1670 cm -1 This indicates the formation of an imide ring with a symmetrical C=O structure. In addition, at 2500-3000 cm -1 The broad peak in the range attributed to the stretching vibration of OH in the carboxyl group is almost absent in the spectrum of polyimide.

[0052] The thermal stability of SA-polyimide was studied by thermogravimetric analysis (TGA). Figure 2 As shown in Figure 2, T1% (1% mass loss temperature) occurs at 390.8°C, while T5% (5% mass loss temperature) occurs at 477.5°C. High-performance engineering plastics are often processed at temperatures up to 390°C, requiring fillers with excellent thermal stability. The developed filler meets these requirements, maintaining its stability during the CFRTP manufacturing and molding process.

[0053] The sizing agent exhibits uniform dispersion in the aqueous phase and has long-term stability. Figure 3 The photo of the sizing agent is shown. After 12 months, no significant changes were observed. The liquid remained completely homogeneous and transparent with no precipitation. In addition, the particle diameter distribution was basically the same as that of the freshly prepared solution, with an average particle size of about 20 nanometers. Figure 4 Transmission electron microscope (TEM) image ( Figure 5 ) showed that the particle size of the sizing agent was between 5 and 10 nanometers and had good dispersion uniformity.

[0054] The elemental composition of carbon fiber surface changes significantly after size treatment. Figure 6 As shown in Figure 1, XPS spectrum analysis of the carbon fiber surface shows two significant features: the appearance of sulfur element signals and a significant increase in oxygen content. The specific quantitative data are shown in Table 1. After sizing, the contents of oxygen, nitrogen, and sulfur elements on the fiber surface gradually increase with the increase of sizing agent concentration.

[0055] Table 1 Elemental composition of carbon fiber surface

[0056]

[0057] Too high a sizing agent concentration will increase the rigidity of the fiber bundle, making it difficult to handle during weaving or prepreg preparation, thus causing processing difficulties and affecting the quality of the final product. In order to quantitatively analyze this effect, the present invention measured the drape value of the carbon fiber, such as Figure 7As shown in Figure 2, with increasing sizing agent concentration, the rigidity of the fiber bundle gradually increases, and CF2 even completely loses its elasticity. Compared with CCF, CF0.5 still retains a certain degree of flexibility, making it a more suitable candidate material for practical applications.

[0058] With the increase of sizing agent concentration, the surface energy of the fiber shows a gradually increasing trend, e.g. Figure 8 As shown in Figure 2, the sulfonic acid and imine groups in the sizing agent possess strong polarity, while the synergistic effect of C=O and N atoms further enhances surface polarity. The polyimide backbone typically contains numerous benzene rings or biphenyl structures, whose π electron cloud strengthens van der Waals forces with the carbon fiber graphite crystallite surface. Furthermore, the rigid aromatic ring structure promotes close packing of the molecular chains, thereby increasing surface dispersion energy.

[0059] The interlaminar shear strength (ILSS), impact strength and flexural strength of carbon fiber / polyetheretherketone (CF / PEEK) composites were systematically analyzed. Figure 9 As shown. Among all the samples, the CF0.5-based composites showed the most significant performance improvement. For the CF0.5 / PEEK composite, the ILSS reached 87.7 MPa, which was an increase of 32.1% and 40.5% compared with DCF / PEEK (66.4 MPa) and CCF / PEEK (62.4 MPa), respectively. Similarly, the impact strength increased to 42.7 kJ / m², which was an increase of 26.3% and 35.5% compared with DCF / PEEK (33.8 kJ / m²) and CCF / PEEK (31.5 kJ / m²), respectively. The flexural strength was also significantly improved, from 766.2 MPa for DCF / PEEK and 728.8 MPa for CCF / PEEK to 973.7 MPa, equivalent to an increase of 27.1% and 33.6%, respectively. The mechanical properties of carbon fiber / polyetherimide (CF / PEI) composites are shown in Figure 2. Figure 10 shown.

[0060] The DCF and CCF values ​​in the performance section were obtained through commercial or conventional processing methods for comparison purposes, demonstrating the effectiveness of the sizing agent in this invention. CF0.25, CF0.5, and other concentrations correspond to different sizing agent concentrations in this invention, with a total of five concentrations compared.

[0061] The fracture morphology of the composite material after failure was observed by scanning electron microscopy (SEM). Figure 11 、 Figure 12Composites prepared using DCF and CCF exhibited poor interfacial adhesion, with failure occurring at the weak interface. Numerous debonded fibers and residual grooves were visible on the fracture surface, indicating insufficient bonding between the fibers and the resin. In contrast, composites prepared using CF0.25 showed significant improvement; however, the interfacial strength remained lower than that of the matrix due to incomplete coverage by the low-concentration coating agent. When CF0.5 was used, the uniform coating layer significantly improved the interfacial adhesion between the resin and the fibers, leading to a transition from interfacial failure to cohesive failure within the matrix resin, resulting in a relatively smooth fracture surface morphology. However, with further increases in the sizing agent concentration, the thicker and more uneven sizing layer hindered the effective penetration of the resin into the fiber bundle, resulting in suboptimal interfacial bonding. Under stress, some interfacial failure still occurred, but the overall performance remained superior to that of DCF and CCF. There was little interaction between DCF and the thermoplastic resin matrix. In contrast, the application of the coating agent enriched the fiber surface with functional groups, acting as an interfacial layer to enhance the adhesion between the fiber and the resin matrix. The functionalized fiber surface promotes hydrogen bonding and π-π stacking interactions with the carbon fibers, while mechanical interlocking and hydrogen bonding contribute to a strong bond with the resin matrix. This interfacial reinforcement mechanism effectively improves the overall performance of the composite, as evidenced by the improved mechanical properties observed in this study.

Claims

1. A method for preparing a water-based polyamic acid sizing agent, characterized in that: The method is: Step 1: Synthesis and separation of polyamic acid The polyamic acid is synthesized by polymerization of dianhydride and diamine in an organic solvent. After the reaction is completed, the polyamic acid solution is cooled to room temperature, transferred to a dropping funnel, and added dropwise to a solvent with a volume 10-25 times that of the solution for separation. After complete dropwise addition, the precipitate is collected and dried to obtain solid polyamic acid. Step 2: Preparation of sizing agent Add 5-10 wt% of a low-grade ester or low-grade ketone solvent to deionized water, add polyamic acid powder to the mixed solvent at a solid content of 1-20 wt.%, stir at room temperature until the solid disappears, let it stand for 2-4 hours to separate the liquid, and collect the lower layer of yellow transparent liquid, which is the polyamic acid water-based sizing agent.

2. The method for preparing a water-based polyamic acid sizing agent according to claim 1, wherein: In step 1, the dianhydride is one or more of bisphenol A diether dianhydride, 4,4'-diphenyl ether dianhydride or 3,3,4,4-benzophenone tetracarboxylic dianhydride; the diamine is 2,4-diaminobenzenesulfonic acid; the organic solvent is one or more of DMF, DMAc, DMSO or NMP; and the solvent is one of acetone, butanone and diethyl ether.

3. The method for preparing a water-based polyamic acid sizing agent according to claim 1 or 2, wherein: In step 1, the monomer molar ratio of the dianhydride to the diamine is 1.0-1.1:1, and the solid content of the two in the organic solvent is 5-15 wt.%.

4. The method for preparing a water-based polyamic acid sizing agent according to claim 1, wherein: In step 1, the polymerization temperature is 80-100° C. and the time is 4-24 hours.

5. The method for preparing a water-based polyamic acid sizing agent according to claim 1, wherein: In step 2, the lower ester solvent is one of ethyl acetate, ethyl formate, ethyl propionate, ethyl butyrate, propyl acetate, and butyl acetate; and the lower ketone solvent is acetone or butanone.

6. Use of an aqueous polyamic acid sizing agent prepared by the preparation method according to any one of claims 1 to 5, characterized in that: The applications are: Step 1: Surface treatment and sizing process of carbon fiber The desized carbon fibers were immersed in 0.25-2.0 wt% sizing agent and then transferred to an oven at 300 °C for 10 min for imidization treatment. Step 2: Preparation of composite materials The carbon fiber fabrics and matrix resin were alternately stacked, and the stacked materials were transferred to a flat vulcanizer for molding to obtain a biaxially laminated composite material with a thickness of 2.0 ± 0.1 mm.

7. The use according to claim 6, characterized in that: In step 1, commercial carbon fiber (CCF) is refluxed with acetone at 60-85°C for 24-48 hours to obtain desized carbon fiber (DCF).

8. The use according to claim 6, characterized in that: In step 2, the matrix resin is PEEK or PEI.

9. The use according to claim 7, characterized in that: In step 2, CF / PEEK is molded at 350-390°C and 5-15 MPa for 60-120 minutes, and CF / PEI is molded at 350-370°C and 5-15 MPa for 80-200 minutes.

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