Starched carbon fiber surface modification treatment method based on freezing-extraction

Abstract

The invention discloses a sizing carbon fiber surface modification treatment method based on freezing-extraction, and belongs to the technical field of carbon fiber surface modification. The method comprises the following steps: firstly, freezing the carbon fiber shaft containing a sizing agent on the surface in a cold environment for several hours; taking out the fiber shaft, and maintaining the fiber shaft for 5-40 minutes in an air atmosphere at room temperature until the fiber shaft recovers to the room temperature; taking out the fiber shaft, and carrying out surface extraction treatment on the fiber bundle at 40-80 DEG C by adopting an extraction solvent for 10-60 minutes; directly immersing the extracted wet fibers into a nanometer material dispersion liquid, and carrying out surface secondary sizing treatment; and carrying out online continuous drying treatment on the fiber, and coiling again to obtain the strong interface type carbon fiber. A dispersing agent and an emulsifying agent in sized carbon fibers are removed in a mode of inducing sizing agent phase separation through freezing treatment, and the carbon fiber epoxy resin interface bonding force is further enhanced. The method can be applied to the fields of aerospace, new energy automobiles, wind energy equipment and the like.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber surface modification technology, specifically relating to a method for surface modification of sized carbon fibers based on freeze-extraction. Background Technology

[0002] Carbon fiber reinforced epoxy resin (CFRP) composites are widely used in aerospace, automotive, and other industries due to their high specific strength and lightweight properties. However, the weak interfacial bonding between carbon fiber and epoxy resin limits their mechanical properties and durability. Traditional acetone extraction methods have significant drawbacks, such as extremely low processing efficiency, often requiring soaking for more than 24 hours, posing a risk of fiber corrosion and damage, and failing to selectively retain beneficial interfacial components, severely restricting production capacity. Furthermore, acetone solvent is highly volatile, flammable, and explosive, posing safety and environmental risks, and its dissolution effect on environmentally friendly water-based sizing agents is poor. Overall, the high cost and poor controllability make it difficult to meet the demands of modern industry for efficient, safe, and precise interfacial modification.

[0003] To address these issues, existing technologies attempt to replace acetone extraction with chemical oxidation or plasma modification. However, commonly used strong acids or oxidants are highly corrosive and volatile, easily causing equipment damage and health risks to operators. Furthermore, the wastewater generated after the reaction requires complex treatment processes to avoid environmental pollution. While plasma modification can improve surface activity, it involves high equipment investment and suffers from poor treatment uniformity. In addition, existing methods do not simultaneously address the synergistic problem of sizing agent removal and carbon fiber surface micro / nanostructure optimization, making it difficult to meet the composite material requirements of aerospace, new energy, and other fields that demand high interfacial strength and lightweight composites. Summary of the Invention

[0004] The purpose of this invention is to solve the existing problem of interfacial bonding between carbon fiber and epoxy resin, and to provide a surface modification treatment method for sized carbon fiber based on freeze-extraction.

[0005] The core of this invention lies in proposing a green processing technology based on cryogenic phase separation. By controlling the extraction behavior of liquid phase separation promoters (such as mixtures containing deionized water, ethanol, methanol, acetone, water, and ethanol) on the carbon fiber surface, the following innovations are achieved: Revolutionary improvement in processing efficiency: The core innovation of this process is to combine the key "phase separation" step (cryogenic treatment) with the natural waiting time between fiber storage or production batches. The 10-48 hour cryogenic period can be arranged during non-continuous production storage periods, without occupying the equipment time of subsequent extraction, drying, and other main processes. From the perspective of the overall production process, the active processing time (liquid nitrogen cryogenic treatment) is significantly shortened to less than 1 hour, achieving an order-of-magnitude improvement in production efficiency compared to the traditional extraction method that takes more than 48 hours. Strong process adaptability and easy industrialization: Both cryogenic and extraction equipment are general-purpose equipment, the process flow is simple, and the parameters are easy to control. It can be easily integrated into existing carbon fiber production lines or used as an independent offline processing unit, showing good prospects for industrial application. Intrinsic safety and environmental friendliness: The extraction process mainly uses low-toxicity, low-volatility solvents such as water or water-alcohol mixtures, which fundamentally eliminates the safety hazards of flammability, explosiveness, and high volatility of traditional organic solvents such as acetone, and is in line with the development trend of green manufacturing.

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

[0007] A method for surface modification of sized carbon fibers based on freeze-extraction, the method comprising the following steps:

[0008] Step 1: Freezing Treatment: Place the carbon fiber tows or spools with sizing agent on their surface in a freezing environment and freeze them at -30℃ to -10℃ for 12 to 48 hours to induce uniform phase separation and structural loosening within the sizing agent. The preferred freezing temperature is -20℃, and the preferred treatment time is 24 to 48 hours. These conditions can effectively induce phase separation in most common sizing agents such as epoxy and polyurethane, and the time window is easy to match with the production cycle.

[0009] Step 2: Temperature recovery treatment: Remove the frozen carbon fiber bundles from the frozen environment and let them stand in an air atmosphere at 15~30℃ for 5~40 minutes to allow them to return to room temperature;

[0010] Step 3: Solvent extraction: Immerse the reheated carbon fiber bundles in the extraction solvent and perform surface extraction treatment at 40~80℃ for 10~60 minutes to remove the sizing agent components weakened by phase separation, thereby forming a porous or rough microstructure on the fiber surface.

[0011] Step 4: Post-treatment: The extracted wet carbon fibers are dried, or directly immersed in a nanomaterial dispersion for secondary surface treatment, followed by drying and winding to obtain surface-modified carbon fibers.

[0012] Furthermore, in step one, the sizing agent is an emulsion-type sizing agent, which contains one or more of the following as dispersants: polyoxyethylene ether, sodium hexametaphosphate, sodium tripolyphosphate, polyurethane, and silane coupling agent.

[0013] Further, in step one, the sizing agent includes one or more of the following as emulsifiers: Span series, polysorbate, polyacrylic acid, polyethylene glycol, OP-10, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and pyridine cationic.

[0014] Further, in step three, the extraction solvent is one or more selected from deionized water, methanol, ethanol, isopropanol, acetone, ethyl acetate, and a mixture of water and alcohol. The extraction solvent is preferably deionized water, ethanol, or a mixture of water and ethanol in a volume ratio of (1:1) to (4:1). The processing temperature is preferably 40-60°C, and the processing time is preferably 10-15 minutes. These conditions maximize the economic efficiency and environmental friendliness of the process while efficiently removing loose sizing agent components.

[0015] Furthermore, in step three, the extraction process is assisted by heating, the temperature of the extraction solvent is 40~60℃, and the extraction time is 10~30 minutes.

[0016] Furthermore, in step four, the secondary surface treatment specifically involves immersing the wet carbon fibers in a dispersion containing nanomaterials, including one or more of graphene, carbon nanotubes, and silane coupling agents. This allows for the introduction of chemical bonding or nano-reinforcement effects on top of the constructed rough structure, achieving a synergistic improvement in interfacial properties.

[0017] Furthermore, in step four, the drying process involves hot air circulation drying or online continuous drying at a temperature of 60-120°C. After drying, the moisture content of the carbon fiber is less than 0.5% to ensure that moisture and residual solvent are effectively removed.

[0018] Furthermore, the method also includes step five: composite material preparation: uniformly mixing surface-modified carbon fiber bundles with an epoxy resin matrix preheated to 80°C, and preparing carbon fiber reinforced composite material through a hot pressing molding process. The ratio of epoxy resin to curing agent H256 is 100:32, and the ratio of the mixed resin to carbon fiber ensures that the fiber content is not less than 60%.

[0019] Furthermore, the hot pressing molding process is as follows: maintain a constant temperature of 90°C for 2 hours, then raise the temperature to 120°C, apply a pressure of 5~15 MPa when the resin reaches the gel point, maintain a constant temperature of 120°C for 2 hours, and finally maintain a constant temperature and pressure of 150°C for 2 hours to completely cure the resin.

[0020] The advantages of this invention compared to existing technologies are as follows: it abandons the traditional long-term acetone extraction method and innovatively adopts a process path combining "freezing phase separation" pretreatment with "selective solvent extraction," achieving efficient and controllable removal of sizing agents. It has the following significant advantages:

[0021] (1) Intrinsic safety and green environmental protection: The extraction process mainly uses low-toxicity and low-volatility solvents such as water or water-alcohol mixture, which fundamentally eliminates the safety hazards of traditional organic solvents such as acetone being flammable, explosive and highly volatile, which is in line with the development trend of green manufacturing.

[0022] (2) Improved processing efficiency: The core innovation of this process lies in combining the critical "phase separation" step (freezing treatment) with the natural waiting time between fiber storage or production batches. The 1-48 hour freezing period can be arranged during the storage period of non-continuous production, without occupying the equipment time of subsequent main processes such as extraction and drying. From the perspective of the overall production process, the active processing time (extraction and drying) is significantly shortened to less than 1 hour, and the production efficiency is increased by orders of magnitude compared with the traditional extraction method that takes more than 48 hours.

[0023] (3) Excellent fiber protection: The gentle physical phase separation and rapid extraction process avoids the potential damage to the carbon fiber body caused by strong chemical reagents or long-term solvent immersion, which helps to maintain the original strength of the fiber.

[0024] (4) Significantly enhanced interfacial properties: Through the synergistic effect of frozen phase separation and selective extraction, a porous or uniformly rough microstructure can be constructed in situ on the fiber surface, effectively increasing the mechanical interlocking and physical bonding area between the fiber and the resin. Figures 2-4 Composite materials prepared from carbon fibers treated using this process exhibit an interlaminar shear strength (ILSS) that is more than 20% higher than composite materials using unmodified sized carbon fibers. Figure 1 This demonstrates excellent interface enhancement effects.

[0025] (5) The process is highly adaptable and easy to industrialize: Both freezing and extraction equipment are general-purpose equipment. The process flow is simple and the parameters are easy to control. It can be easily integrated into existing carbon fiber production lines or used as an independent offline processing unit, and has good prospects for industrial application. Attached Figure Description

[0026] Figure 1A comparison of the interlaminar shear mechanical properties of epoxy resin composites made of carbon fiber filaments sized for different freezing treatment times;

[0027] Figure 2 Scanning electron microscope images of freeze-treated sized carbon fiber filaments and untreated sized carbon fiber filaments (control group); (A1) Sized carbon fiber precursor; (A2) Cross section of sized carbon fiber composite; (A3) Cross section of sized carbon fiber composite; (B1) Freeze-treated carbon fiber; (B2) Cross section of frozen carbon fiber composite; (B3) Cross section of frozen carbon fiber composite.

[0028] Figure 3 AFM images of the surfaces of sized carbon fiber and frozen carbon fiber; (A) sized carbon fiber precursor; (B) frozen carbon fiber;

[0029] Figure 4 Infrared images of sized carbon fibers and freeze-treated carbon fibers;

[0030] Figure 5 This is a flowchart of the freezing process experiment. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0032] This invention is based on a green processing technology of cryogenic phase separation. The cryogenic treatment of the sized carbon fiber surface coating is equivalent to a "quenching" process. The rapid temperature drop instantly disrupts the thermodynamic equilibrium of the system. The fundamental differences in polarity, molecular conformation, and crystallization / condensation tendencies between epoxy molecules and surfactants lead to a sharp decrease in their compatibility. Driven by the pursuit of the lowest system free energy, surfactant molecules (such as OP-10 and Span 80) spontaneously migrate and aggregate towards the air / coating interface, which has a higher surface free energy, while epoxy molecules tend to cluster together within the coating, forming a thermodynamically more stable "skin-inner layer" bilayer structure. This "pore-forming" or "boundary-creating" strategy, which precisely controls the transformation of the coating from homogeneous to heterogeneous phase through the sequential action of external physical fields (solvent field, temperature field), achieves a "top-down" intelligent design of the intrinsic structure of the sizing agent coating.

[0033] This treatment method can meet the environmental requirements of replacing acetone: it utilizes frozen phase separation and freezes the carbon fiber for several hours during storage, making it suitable for industrial processing, and there is no risk of organic solvent volatilization throughout the process; the surface micro-nano structure can be controlled to modify: during the phase separation process, the promoter forms a nanoscale liquid film on the carbon fiber surface. By adjusting the temperature, the liquid film is induced to undergo cycloid phase separation, generating a uniform microporous array on the fiber surface, which significantly increases the specific surface area, thereby improving the interfacial bonding strength.

[0034] This invention has the following innovative aspects:

[0035] I. Physical Phase Separation Pretreatment Mechanism: This invention abandons traditional thermochemical or steam fumigation methods and innovatively introduces low-temperature freezing as a pretreatment step. This process utilizes the dramatic changes in compatibility, uneven volume shrinkage, and possible ice crystal growth of the components in the water / alcohol-based sizing agent (such as emulsifiers, dispersants, and film-forming resins) at low temperatures to pre-induce structural defects such as microcracks and phase domain separation within the coating. This "pre-weakening" treatment allows subsequent solvent extraction to proceed in a "combined internal and external" manner, i.e., the solvent dissolves from the outside while the internal structure is more easily disintegrated due to pre-destruction, thereby significantly improving the removal efficiency of the sizing agent and avoiding damage to the fiber matrix caused by prolonged high temperatures or strong solvents.

[0036] From a thermodynamic perspective, the Gibbs-Helmholtz equation can explain its underlying mechanism:

[0037]

[0038] During freezing, the system temperature (T) drops sharply, causing significant changes in the mixing enthalpy (ΔH) and entropy (ΔS) contributions of the coating components, resulting in a nonlinear response in the Gibbs free energy change (ΔG). Particularly for multiphase systems containing emulsifiers and dispersants, low temperatures reduce the compatibility between components and enhance phase separation (ΔH>0), leading to the formation of independent phase regions. Simultaneously, the crystallization process of water or alcohol solvents generates volume expansion and internal stress, further disrupting the continuity of the coating structure and creating a "pre-failure" state of microcracks and interface separation. After this "pre-weakening" treatment, subsequent solvent extraction can proceed in a "combined internal and external" manner: the solvent penetrates and dissolves from the outside, while the numerous micro-defects and interface separation regions formed inside the coating due to freezing significantly increase the solvent access area and diffusion channels, significantly reducing the kinetic resistance of the extraction process. This makes it easier for sizing agents that are originally firmly attached and difficult to remove (especially those with poor fiber wettability and weak interfacial bonding) to be peeled off from the fiber surface, either entirely or partially. This allows for selective removal under mild, low-concentration, and short-time extraction conditions, significantly improving the efficiency of sizing agent removal while avoiding damage to the carbon fiber itself caused by prolonged high temperatures or strong chemical solvents, thus preserving its mechanical properties intact.

[0039] II. Low-temperature extraction synergistic process control:

[0040] 1. Freezing temperature and time window: The preferred temperature range of -30℃ to -10℃ can effectively trigger the glass transition or phase separation of most common sizing agents such as epoxy and polyurethane. The wide time window of 24 to 48 hours allows this step to be flexibly arranged during production breaks or storage periods without occupying the main production line time, thus achieving a leap in process efficiency.

[0041] 2. Selective Extraction Design: The extraction step uses a mild water or water-alcohol mixture. After freeze pretreatment, the sizing agent structure becomes loose, allowing components such as emulsifiers and dispersants, which are originally difficult to remove with pure water, to be effectively dissolved. The polarity of the solvent, temperature (40~80℃), and time (10~60 minutes) can be precisely controlled to achieve highly selective removal of "loose sizing agent components" while preserving or exposing the active groups on the fiber surface. Ultimately, a uniform porous or rough structure is constructed in situ on the fiber surface (e.g., ...). Figure 2 As shown in Figure 3, it provides an ideal interface for resin impregnation and mechanical interlocking.

[0042] III. Interface Enhancement Effects:

[0043] Experimental results show (e.g.) Figure 4 As shown), the interlaminar shear strength (ILSS) of the carbon fiber and epoxy resin composite material treated by the process of this invention is significantly improved. This is because: (1) Physical anchoring enhancement: The surface micro-nano roughness formed by the freeze-extraction synergy greatly increases the contact area and mechanical interlocking effect between the fiber and the resin. (2) Exposure of chemical active sites: The mild extraction process may expose more active sites of the fiber itself or those covered by coupling agents while removing “inert” sizing agent components (such as emulsifiers), which is beneficial for forming chemical bonds with the resin. (3) Improved wettability: The newly formed clean surface with micro-nano structure has better resin wettability.

[0044] In summary, this invention achieves efficient, environmentally friendly, and non-destructive modification of sized carbon fiber surfaces through an innovative "freeze-induced phase separation" process. This process not only completely avoids the use of volatile organic solvents but also transforms the most time-consuming "phase separation" process into a non-productive, offline step. While ensuring excellent interfacial reinforcement (ILSS improvement of over 30%), it significantly shortens active processing time, simplifies the process, and is easily integrated into existing production lines. The carbon fiber surface modification method provided by this invention can be used in aerospace, new energy, and high-end sports equipment fields where there is an urgent need for high-performance composite materials.

[0045] Example 1:

[0046] I. Carbon Fiber Pretreatment and Freezing

[0047] The original spool (or carbon fiber bundles wound parallel to a stainless steel frame) containing sized carbon fiber tow is placed directly into a temperature-controlled freezer. The freezer temperature is set to -20°C, the equipment is started, and the process is continued at this temperature for 12 hours. This process aims to induce uniform phase separation and structural embrittlement within the sizing agent coating using low temperature.

[0048] II. Temperature Recovery and Equilibrium

[0049] After freezing, turn off the freezing equipment and transfer the carbon fiber roll (or frame) to a standard laboratory environment (temperature 25±2℃, relative humidity 50%±10%, air atmosphere) and let it stand for 20 minutes to allow it to naturally warm to room temperature. This step helps the sizing agent to complete the moisture distribution and relaxation of internal stress on the basis of the phase separation structure, creating conditions for subsequent efficient extraction.

[0050] III. Selective Solvent Extraction

[0051] Preparation of the extraction solvent: Deionized water and ethanol are mixed at a volume ratio of 4:6. The mixed solvent is heated and kept at a constant temperature of 50°C. The freeze-thawed carbon fiber bundles are completely immersed in the hot solvent and treated for 20 minutes with gentle stirring. After treatment, the fiber bundles are removed and rinsed with fresh deionized water to remove residual solvent from the surface.

[0052] IV. Drying and Post-treatment

[0053] The wet carbon fiber bundles were transferred to a forced-air drying oven and dried at 80°C for 2 hours to completely remove moisture and obtain surface-treated carbon fibers.

[0054] V. Composite Material Preparation and Performance Testing

[0055] 1. Resin matrix preparation: Mix epoxy resin (E51, epoxy equivalent 185 g / mol) and curing agent (H256, addition amount 32 wt%), and premix at 80℃ for 30 minutes.

[0056] 2. Composite process: The treated carbon fiber bundles are laid flat in the mold, impregnated with resin mixture until completely wetted, and then placed in a flatbed hot press for curing according to the following procedure: heat at 90°C for 2 hours, then raise the temperature to 120°C, apply pressure of 10 MPa when the resin reaches the gel point, and maintain at 120°C for 2 hours, and finally raise the temperature to 150°C and maintain pressure for 2 hours for curing.

[0057] 3. Interlaminar shear performance test: 2 mm thick composite material specimens were prepared according to GB / T 3362—2017 standard and tested using a universal testing machine (crosshead speed 2 mm / min).

[0058] like Figure 2 As shown, by comparing the microstructures of sized carbon fibers (Group A) and freeze-treated carbon fibers (Group B), the optimization effect of freeze-treatment on the fiber surface and composite interface can be clearly revealed: Regarding fiber surface morphology, the original sized fiber (A1) surface is covered with a continuous and smooth sizing agent coating, forming a physical barrier. In contrast, the surface of the freeze-treated fiber (B1) exhibits significant roughening and multi-scale groove structures, indicating that some sizing agent is selectively removed, exposing more anchorable active surfaces and creating a physical basis for interfacial mechanical interlocking. Regarding the interfacial bonding state of the composite material, the cross-section of the sized fiber composite material (A2) shows a relatively clear fiber-resin boundary, indicating better interfacial bonding. The fiber-resin interface in the sized fiber composite (B2) is relatively weak, while the fiber-resin interface in the freeze-treated composite (B2) is blurred, exhibiting an interlocking structure with interlocking teeth. This indicates that the increased surface roughness of the fibers after treatment significantly improves resin wettability and interfacial physical bonding strength. In terms of the failure mechanism, the fracture surface of the sized fiber composite (A3) is smooth and flat, with short fiber pull-out length and a clean surface, exhibiting typical characteristics of weak interfacial failure. In contrast, the fracture surface of the freeze-treated composite (B3) shows a rough, multi-layered failure morphology, with a significantly increased fiber pull-out length and a large number of resin fragments adhering to the surface of the pulled-out fibers. This indicates a significant enhancement in interfacial bonding strength, and the failure mode has changed from brittle interfacial debonding to a more energy-intensive and complex fracture process. These three sets of morphological features form a complete chain of evidence: freeze treatment selectively removes the sizing agent and constructs a micro-nano rough structure on the fiber surface (B1). This structure promotes deep resin wetting and mechanical interlocking (B2), ultimately forming a tough failure mode with strong interfacial bonding (B3) under stress. This explains the fundamental reason for the improvement in the macroscopic mechanical properties of the composite material from a microscopic level.

[0059] like Figure 3 As shown, the atomic force microscopy (AFM) morphology image clearly reveals the significant modifying effect of freeze-drying on the microstructure of carbon fiber surfaces. The surface of the sized carbon fiber precursor shown in the left (A) image, compared with the three-dimensional morphology comparison image (B) obtained after freeze-drying, clearly demonstrates the uneven coating of the original sizing agent on the carbon fiber surface and its structural evolution after freeze-drying. The following is a detailed analysis of the "uneven coating of the original sizing agent":

[0060] 1. The thickness distribution is discrete and fluctuates significantly. The figure shows that the surface height distribution range is wide, from about -1.4 µm (dark recessed area) to +284.6 nm (bright raised area). The overall thickness variation exceeds 1.5 µm, indicating that the sizing agent exhibits significant thickness fluctuations on the fiber surface and the coating uniformity is poor.

[0061] 2. Localized accumulation and missing areas exist. Bright spots (brighter areas in Figure A): These correspond to localized accumulation of sizing agent, possibly due to uneven sizing agent flow during coating, differences in fiber surface energy, or drying shrinkage leading to material enrichment. Dark areas and depressions: These correspond to thinner or missing sizing agent coatings, potentially causing localized fiber exposure and reducing interfacial bonding stability.

[0062] (B) Surface morphology after freezing treatment: the transformation from "non-uniform" to "structured roughness"

[0063] 1. The thickness range is narrowed and the uniformity is relatively improved. After treatment, the surface height distribution range is narrowed to -691.2 nm to +214.4 nm, and the overall thickness variation is reduced (about 1.0 µm). This indicates that the freeze-extraction process selectively removes some of the sizing agent, making the thickness distribution more concentrated, but it enhances the interfacial bonding potential by introducing an ordered structure.

[0064] 2. The formation of a periodic layered / striped structure, with clear periodic stripes or layered arrangements (alternating light and dark) on the surface, indicates that the freeze treatment induces phase separation and structural reorganization in the sizing agent, transforming it from its original random and inhomogeneous state (A) into an ordered micro-nano composite structure (B). This structure can provide directional anchoring points for the resin, enhancing mechanical interlocking.

[0065] From "Coating Defects" to "Interface Enhancement Design": The original non-uniformity (A) is a process defect that easily leads to interfacial stress concentration; while the ordered rough structure after treatment (B) is the result of controllable surface modification, transforming the original thickness fluctuations into micro-nano topologies that are conducive to resin wetting and bonding. Freeze-drying, by inducing phase separation and selective extraction of the sizing agent, not only improves the dispersion of thickness distribution to a certain extent, but more importantly, transforms the original "non-uniformity" into an ordered micro-nano rough structure, realizing the transformation from "process defects" to "interface enhancement design," and providing a new approach to improving the performance of composite materials.

[0066] like Figure 4 As shown, based on infrared spectral comparison analysis, freeze-treated carbon fibers and original sized carbon fibers exhibit significant differences in chemical structure, mainly in the following three aspects:

[0067] First, in the fingerprint area (1000-00 cm) -1 The most significant structural changes occur in this region. Untreated carbon fibers exhibit strong characteristic absorption peaks in this area, corresponding to signals from characteristic functional groups in the sizing agent, such as the stretching vibration of COC ether bonds and the out-of-plane bending vibration of CH. However, the absorption intensity of freeze-treated carbon fibers in this region decreases significantly, especially at 800 cm⁻¹. -1 and 500 cm -1The nearby characteristic peaks were significantly weakened or even disappeared. This change indicates that the freeze treatment process effectively destroyed some of the original chemical bonds (such as ether bonds, CH bonds, etc.) in the sizing agent, or changed its molecular configuration, resulting in the suppression of these characteristic vibrational modes.

[0068] Secondly, in the high wavenumber region (3500-3000 cm⁻¹) -1 Subtle differences are visible in the hydroxyl vibration region of the sample. The absorption valley in this range of the freeze-treated carbon fiber is slightly deeper than that of the untreated sample, which may suggest two possibilities: first, the freeze-induced phase separation process causes the hydrophilic groups (such as -OH) on the fiber surface or in the sizing agent to rearrange or expose, enhancing hydrogen bond interactions; second, the removal of some low molecular weight components or water alters the hydrogen bond network structure in this region.

[0069] In addition, in the medium wavenumber range (2000-1000 cm⁻¹) -1 Although the two curves have a high overall overlap, the cryogenically treated carbon fiber is at approximately 1600 cm. -1 (C=C skeleton vibration) and 1200 cm -1 Minor peak shifts or intensity changes can still be observed near the (CO stretching vibration), indicating that the chemical environment of the graphite microcrystalline structure or residual functional groups on the fiber surface has been subtly adjusted.

[0070] In summary, the differences in infrared spectroscopy indicate that freeze treatment not only alters the physical morphology of the fiber surface (as shown in AFM), but also achieves chemical-level regulation of surface functional groups—by weakening or eliminating signals from specific chemical bonds in the sizing agent, and possibly adjusting the distribution of oxygen-containing functional groups. This change in chemical structure, combined with the previously observed surface roughening phenomenon, constitutes a synergistic chemical-physical mechanism for improving interfacial performance: it improves surface activity by reducing chemically inert components (such as inactive components in the sizing agent) and enhances mechanical interlocking by constructing micro / nano structures, thus laying a dual foundation for improving the interfacial performance of composite materials.

[0071] Example 2: This example differs from Example 1 in that, in step one, the freezing temperature is -15℃.

[0072] Example 3: This example differs from Example 1 in that, in step one, the freezing time is 24 hours. Figure 1As shown in (a) and (b), the untreated sample had the lowest ILSS peak value, approximately 76 MPa. After freezing for 12 hours and 24 hours, the ILSS peak values ​​of the material increased to approximately 92 MPa and 95 MPa, respectively, representing a performance improvement of about 20%. This indicates that freeze-drying the sized carbon fibers helps to further improve the interfacial properties of the composite material. Bending tests also confirmed the effectiveness of the freeze-drying treatment; the bending performance of the untreated sample increased from 1650 MPa to 3100 MPa. This improvement in bending performance, especially the increase in bending modulus, directly demonstrates that the interfacial bonding force (or interfacial shear strength) between the fiber and the resin matrix was effectively improved. A good interface can efficiently transfer the load from the relatively "soft" resin to the high-strength fiber, thereby fully utilizing the reinforcing effect of the fiber.

[0073] Example 4: This example differs from Example 1 in that the freezing time in step one is 48 hours.

[0074] Example 5: This example differs from Example 1 in that, in step three, the extraction solvent is deionized water.

[0075] Example 6: This example differs from Example 1 in that, in step three, the extraction solvent is a mixture of deionized water and ethanol in a volume ratio of 2:8.

[0076] Example 7: This example differs from Example 1 in that, in step three, the extraction temperature is 40°C.

[0077] Example 8: This example differs from Example 1 in that, in step three, the extraction temperature is 60°C.

[0078] Example 9: This example differs from Example 1 in that, in step three, the extraction time is 10 minutes.

[0079] Example 10: This example differs from Example 1 in that, in step three, the extraction time is 30 minutes.

[0080] Example 11: This example differs from Example 1 in that, in step four, the drying conditions are 100°C and forced air drying for 3 hours.

[0081] Example 12: This example differs from Example 1 in that, after the extraction treatment described in step three and before the drying in step four, a "secondary functionalization impregnation" step is added: the wet carbon fibers are immersed in an aqueous dispersion of aminated carbon nanotubes with a concentration of 0.5 wt% for 5 minutes, and then dried.

[0082] Experimental data:

[0083] The product prepared in Example 4 was used to treat a certain type of sized carbon fiber, which was then composited with epoxy resin. Test results showed that the interlaminar shear strength (ILSS) of the prepared composite material reached 95 MPa, representing an improvement of approximately 25% compared to the composite material using the original sized carbon fiber (ILSS approximately 76 MPa). Furthermore, electron microscopy comparisons of the sized carbon fiber and the freeze-treated carbon fiber validated the effectiveness of this process in efficiently and safely removing the sizing agent while significantly enhancing the fiber / resin interfacial bonding.

[0084] It should be understood that the above embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the claims of this invention.

Claims

1. A method for surface modification of sized carbon fibers based on freeze-extraction, characterized in that: The method includes the following steps: Step 1: Freezing treatment: Place the carbon fiber tow or spool with sizing agent on the surface in a freezing environment and freeze it at a temperature of -30℃ to -10℃ for 12 to 48 hours. Step 2: Temperature recovery treatment: Remove the frozen carbon fiber bundles from the frozen environment and let them stand in an air atmosphere at 15~30℃ for 5~40 minutes to allow them to return to room temperature; Step 3: Solvent extraction: Immerse the cooled carbon fiber bundles in the extraction solvent and perform surface extraction treatment at 40~80℃ for 10~60 minutes. Step 4: Post-treatment: The extracted wet carbon fibers are dried, or directly immersed in a nanomaterial dispersion for secondary surface treatment, followed by drying and winding to obtain surface-modified carbon fibers.

2. The method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1, characterized in that: In step one, the sizing agent is an emulsion-type sizing agent, which contains one or more of the following as dispersants: polyoxyethylene ether, sodium hexametaphosphate, sodium tripolyphosphate, polyurethane, and silane coupling agent.

3. A method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1 or 2, characterized in that: In step one, the sizing agent contains one or more of the following as emulsifiers: Span series, polysorbate, polyacrylic acid, polyethylene glycol, OP-10, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and pyridine cationic.

4. The method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1, characterized in that: In step three, the extraction solvent is one or more of deionized water, methanol, ethanol, isopropanol, acetone, ethyl acetate, and a mixture of water and alcohol.

5. The method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1, characterized in that: In step three, the extraction process is assisted by heating, and the extraction time is 10-30 minutes.

6. The method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1, characterized in that: In step four, the secondary surface treatment specifically involves immersing wet carbon fibers in a dispersion containing nanomaterials, wherein the nanomaterials include one or more of graphene, carbon nanotubes, and silane coupling agents.

7. A method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1 or 6, characterized in that: In step four, the drying process is carried out using hot air circulation drying or online continuous drying, with a drying temperature of 60~120℃. After drying, the moisture content of the carbon fiber is less than 0.5%.

8. The method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 1, characterized in that: The method further includes step five: composite material preparation: Surface-modified carbon fiber bundles are uniformly mixed with an epoxy resin matrix preheated to 80°C, and carbon fiber reinforced composite material is prepared by hot pressing. The ratio of epoxy resin to curing agent H256 is 100:32, and the ratio of the mixed resin to carbon fiber ensures that the fiber content is not less than 60%.

9. The method for surface modification treatment of sized carbon fibers based on freeze-extraction according to claim 8, characterized in that: The hot pressing molding process is as follows: maintain a constant temperature of 90°C for 2 hours, then raise the temperature to 120°C, apply a pressure of 5~15 MPa when the resin reaches the gel point, maintain a constant temperature of 120°C for 2 hours, and finally maintain a constant temperature and pressure of 150°C for 2 hours to completely cure the resin.

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