A method for evaluating the wettability of large-tow carbon fibers based on a capillary model

Abstract

The application discloses a kind of based on capillary model big tow carbon fiber wettability evaluation method, including pre-evaluation resin on big tow carbon fiber wettability, then based on pre-evaluation result, the K number of carbon fiber is as part of calculation formula, by directly measuring the width of carbon fiber bundle Capillary model is constructed, the wettability change of different K number carbon fiber bundle is evaluated, the process condition applied in the processing of carbon fiber is adjusted to improve the wettability of carbon fiber, to prepare higher performance composite material.The benefit of the present application is that the steps of wettability evaluation are greatly simplified by two-step wettability evaluation method, efficient evaluation of resin wettability on big tow carbon fiber is realized, the wettability of resin on big tow carbon fiber is improved, which lays a foundation for efficient performance of big tow carbon fiber composite material, and helps to popularize and apply big tow carbon fiber.

Description

Technical Field

[0001] This invention relates to a method for evaluating the wettability of large-tow carbon fibers based on a capillary model, belonging to the field of composite material molding technology. Background Technology

[0002] Carbon fiber possesses excellent properties such as lightweight, high strength, good corrosion resistance, and high specific modulus, making it a consistently hot research topic as a novel carbon material. High-performance resin-based composites made from carbon fiber are widely used in aerospace, defense, wind power, and automotive industries. However, the high cost and price of small-tow carbon fiber have limited its development in the civilian sector. Driven by the demand for large-scale use of carbon fiber in wind turbine blades and hydrogen storage cylinders, large-tow carbon fiber, with its low cost, high cost-effectiveness, and high molding efficiency, has received widespread attention and rapid development in recent years.

[0003] However, large-tow carbon fibers present a challenge in efficient resin impregnation. Compared to small-tow fibers, large-tow carbon fibers have twice the number of filaments, which are stacked and difficult to spread. During the manufacturing process, the stacking of filaments makes it difficult for the resin to fully impregnate the interior of the filaments, resulting in insufficient resin at the composite material interface, forming voids, inducing cracks, and reducing the performance of the composite material. To address the problem of reduced composite material strength caused by poor impregnation during the preparation of large-tow carbon fibers, many new processes and methods have been developed. For example, the patent "An Automatic Spreading Device for Continuous Fibers for Prepreg Preparation, CN218256139 U" developed a continuous fiber spreading device that uses multi-stage heated rollers to evenly spread the carbon fibers, preparing a thinner and flatter fiber layer and achieving an ideal impregnation effect. The patent "A Processing Method for Spreading Carbon Fibers, CN108035030 A" developed a method of heating during the carbon fiber spreading process and impregnating the spread fiber bundles with a sizing agent, which greatly reduces voids at the interface caused by poor impregnation when preparing composite materials. These methods provide a technical approach to improve the performance of carbon fiber composites through the yarn spreading process, but they fail to achieve quantitative characterization of the changes in wettability before and after yarn spreading, and cannot predict the wetting effect of carbon fiber bundles. This results in large fluctuations in the performance of composite materials, as well as high time and cost, which restricts the widespread application of large-tow carbon fiber composites.

[0004] To address the aforementioned issues, this invention provides a method for evaluating the wettability of large-tow carbon fibers based on a capillary model. This method can more intuitively and accurately evaluate the changes in wettability of large-tow carbon fibers before and after yarn spreading, improve the performance stability of large-tow carbon fiber composites, fully utilize the mechanical properties of composite materials, and facilitate the expansion of applications of large-tow carbon fibers. Summary of the Invention

[0005] This invention aims to fill the gap in the evaluation method of wettability of large-tow carbon fiber, and can intuitively and accurately characterize the changes in the overall wettability of large-tow carbon fiber, simplify the evaluation steps of compatibility between large-tow carbon fiber and resin, improve the mechanical properties of large-tow carbon fiber composite materials, and promote their further development and application.

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

[0007] A method for evaluating the wettability of resin on large-tow carbon fibers based on a capillary model includes the following steps: ① Carbon fiber pretreatment: A 10-20cm section of a carbon fiber bundle is cut along its length and placed on a flat plate to ensure a smooth and compact surface, eliminating gaps within the bundle. ② Tension clamping: The carbon fiber bundle is fixed using a tension clamping device, with tension applied to both ends to straighten it. The bundle width is altered using the device to obtain a test sample. ③ Cross-sectional model construction: Based on the K-number and actual width and thickness of the carbon fiber bundle, a two-dimensional rectangular model is constructed to represent the cross-section of the bundle. The number of adaptable layers is determined based on different widths and K-numbers, and an in-layer coefficient is established based on the number of adaptable layers and K-number to determine the space occupied by each carbon fiber layer. ④ Capillary area pre-evaluation: The actual width variation of the carbon fiber is measured. In the capillary model, a triangular area enclosed by three carbon fibers is considered a capillary channel. The hexagonal area occupied by one carbon fiber as the center and surrounding carbon fibers is used as the basis for calculating the capillary channel area ratio. The capillary channel area of ​​the carbon fiber bundle under different widths is calculated based on the in-layer coefficient and the capillary model formula. ⑤ Mechanical property testing: For large-tow carbon fibers with different widths, resin is uniformly coated on the carbon fiber bundles to prepare composite materials, and the interlaminar shear strength of the composite materials after curing is tested.

[0008] The tension clamping device described in step ② maintains the arrangement of the carbon fiber bundle by applying tension to both sides of the carbon fiber simultaneously. The tension can be set in the range of 0 to 600 N.

[0009] The tension clamping device described in step ② changes the width range of the carbon fiber bundle to 6–180 mm.

[0010] The large-tow carbon fiber mentioned in step ③ has a K number of 24 to 300K, a single carbon fiber diameter of 4 to 8 μm, an adaptation layer number of 20 to 50, and a corresponding in-layer coefficient of 800 to 8000.

[0011] The number of adaptive layers mentioned in step ③ is the actual number of carbon fiber bundles determined based on the minimum theoretical number of stacked layers. The in-layer coefficient represents the maximum number of single carbon fibers that a single layer of carbon fiber can accommodate under measured broadening, and the stacking coefficient represents the spatial compression caused by the stacking of carbon fibers.

[0012] The capillary model construction described in step ③ includes the following steps: a) The model length is constructed based on the actual widening, using the measured values ​​of different carbon fiber bundles to construct the model length; b) A single capillary channel is constructed using a triangular region formed by three carbon fibers; c) The overall capillary channel area is calculated using a regular hexagonal region centered on a single carbon fiber.

[0013] The capillary channel construction described in step ④ is as follows Figure 1 As shown, its capillary model formula is as follows:

[0014]

[0015] S = w / q

[0016] q=α*K / L

[0017] Where f f is the area fraction of the capillary channel, S is the distance between the center points of two carbon fibers, r is the radius of a single carbon fiber, w is the broadening of the carbon fiber bundle, q is the in-layer coefficient, K is the K number of the carbon fiber, and L is the number of adaptation layers.

[0018] The capillary model formula described in step ④ uses a packing factor α = 1 for carbon fiber bundles ranging from 24 to 100K (inclusive). For carbon fiber bundles ranging from 100 to 200K (excluding 100K but inclusive), due to severe packing compression of space, the in-layer factor needs to be reduced, so a packing factor α = 0.9 is used. For carbon fiber bundles ranging from 200 to 300K (excluding 200K but inclusive), due to further packing compression of space, the in-layer factor needs to be reduced, so a packing factor α = 0.8 is used.

[0019] The capillary channel area is calculated using the capillary model formula from step ④. The changes in capillary area for carbon fibers with different K numbers are directly compared with the changes in the broadening of the carbon fiber bundle and the in-layer coefficient. A larger capillary channel area indicates better wettability. The percentage change in capillary channel area (C%) represents the degree of improvement in wettability, and the percentage is graded according to the following rules:

[0020] Increase in capillary channel area (C%) Immersion level C％≤5 Range 5＜C％＜10 generally 10≤C％＜15 good C％≥15 Excellent

[0021] The resin mentioned in step ⑤ is a liquid mixture of pure resin and curing agent, and has flowability after mixing.

[0022] The beneficial technical effects of this invention are:

[0023] ① The wettability evaluation method for large-tow carbon fiber based on a capillary model described in this invention utilizes the arrangement of individual filaments within the large-tow carbon fiber bundle to form numerous capillary channels, generating a strong capillary wicking effect that drives the resin to continuously wet the entire carbon fiber bundle along the capillary channels; ② The larger the area of ​​the capillary channels, the stronger the driving force for resin penetration per unit time, and the better the wettability of the carbon fiber bundle; ③ Based on the capillary model, the change in capillary channel area after widening can be quantitatively characterized, thereby evaluating the resin's wettability of the carbon fiber bundle. Improved wettability helps the resin to adhere more tightly to individual carbon fibers, thus enhancing the interlaminar shear strength of the large-tow carbon fiber composite material.

[0024] Invention Effects

[0025] This invention provides a method for evaluating the wettability of large-tow carbon fibers based on a capillary model. Compared with traditional methods, it has the following advantages: (1) It establishes corresponding in-layer coefficients for large-tow carbon fibers with different K numbers, calculates the area of ​​capillary channels in the carbon fiber bundle by constructing a capillary model, and quantitatively characterizes the change in the wettability of the resin on the large-tow carbon fibers after the broadening change, which has high evaluation efficiency; (2) By directly measuring the broadening of the carbon fiber bundle and substituting the measured broadening value into the model formula, it can evaluate the wettability of carbon fiber bundles with different K numbers and different broadenings, which has strong universality; (3) It verifies the accuracy of the evaluation model by testing the interlaminar shear strength of the composite material, which has high reliability. Attached Figure Description

[0026] Figure 1 A diagram illustrating a capillary model construction method provided by this invention. Detailed Implementation

[0027] The embodiments of the present invention are further illustrated below, but the present invention is not limited to these embodiments.

[0028] Example 1:

[0029] Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fibers were used as test samples, and the prepared resin was in liquid state. The wettability evaluation method included the following steps: 1) Cut the three types of carbon fibers to 10-20 cm in length, ensuring a smooth surface without internal gaps, and fix them on a tension clamping device. 2) Without applying tension, maintain the natural flatness of the carbon fiber bundles, and measure the overall width of the Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fiber bundles to be 7.01, 39.2, and 83.5 mm, respectively; the number of layers were 28, 26, and 30, respectively; and the in-layer coefficients were 857, 4153, and 8000, respectively. Calculate the increase in capillary channel area. 3) Uniformly coat the resin onto the carbon fiber bundles with different widths to prepare composite materials for interlaminar shear strength testing. All data were collected, and the test results are shown in Table 1.

[0030] Example 2:

[0031] Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fibers were used as test samples, and the prepared resin was in liquid state. The wettability evaluation method included the following steps: 1) Cut the three types of carbon fibers to 10-20 cm in length, ensuring a smooth surface without internal gaps, and fix them on a tension clamping device. 2) Apply the same 50 N tension evenly to both ends to keep the carbon fiber bundles taut and maintain their alignment. Measure the overall width of the Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fiber bundles, which were 7.39, 40.25, and 84.8 mm, respectively; the number of layers were 28, 26, and 30, respectively; and the in-layer coefficients were 857, 4153, and 8000, respectively. Calculate the increase in capillary channel area. 3) Coat the resin evenly onto the carbon fiber bundles with different widths to prepare composite materials for interlaminar shear strength testing. All data were collected, and the test results are shown in Table 1.

[0032] Example 3:

[0033] Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fibers were used as test samples, and the prepared resin was in liquid state. The wettability evaluation method included the following steps: 1) Cut the three types of carbon fibers to 10-20 cm in length, ensuring a smooth surface without internal gaps, and fix them on a tension clamping device. 2) Apply the same 100N tension evenly to both ends to keep the carbon fiber bundles taut and maintain their alignment. Measure the overall width of the Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fiber bundles, which were 7.88, 42.85, and 86.5 mm, respectively; the number of layers were 28, 26, and 30, respectively; and the in-layer coefficients were 857, 4153, and 8000, respectively. Calculate the increase in capillary channel area. 3) Coat the resin evenly onto the carbon fiber bundles with different widths to prepare composite materials for interlaminar shear strength testing. All data were collected, and the test results are shown in Table 1.

[0034] Example 4:

[0035] Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fibers were used as test samples, and the prepared resin was in liquid state. The wettability evaluation method included the following steps: 1) Cut the three types of carbon fibers to 10-20 cm in length, ensuring a smooth surface without internal gaps, and fix them on a tension clamping device. 2) Apply the same 200 N tension evenly to both ends to keep the carbon fiber bundles taut and maintain their alignment. Measure the overall width of the Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fiber bundles, which were 7.94, 43.68, and 89.7 mm, respectively; the number of layers were 28, 26, and 30, respectively; and the in-layer coefficients were 857, 4153, and 8000, respectively. Calculate the increase in capillary channel area. 3) Coat the resin evenly onto the carbon fiber bundles with different widths to prepare composite materials for interlaminar shear strength testing. All data were collected, and the test results are shown in Table 1.

[0036] Example 5:

[0037] Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fibers were used as test samples, and the prepared resin was in liquid state. The wettability evaluation method included the following steps: 1) Cut the three types of carbon fibers to 10-20 cm in length, ensuring a smooth surface without internal gaps, and fix them on a tension clamping device. 2) Apply the same 500 N tension evenly to both ends to keep the carbon fiber bundles taut and maintain their alignment. Measure the overall width of the Toray T700SC-24K, Mitsubishi 2*P330-120 K, and Mitsubishi 5*P330-300 K carbon fiber bundles, which were 8.87, 45.96, and 93.3 mm, respectively; the number of layers were 28, 26, and 30, respectively; and the in-layer coefficients were 857, 4153, and 8000, respectively. Calculate the increase in capillary channel area. 3) Coat the resin evenly onto the carbon fiber bundles with different widths to prepare composite materials for interlaminar shear strength testing. All data were collected, and the test results are shown in Table 1.

[0038] Table 1. Evaluation results of the wettability of large-tow carbon fibers using capillary models at various expansion levels.

[0039]

[0040] For carbon fiber bundles with different K numbers, the graded changes in wettability of carbon fiber bundles were successfully evaluated by combining the formulas of broadening and capillary model. The results were verified by mechanical property testing and showed a trend consistent with the changes in wettability evaluation results.

Claims

1. A method for evaluating the wettability of large-tow carbon fibers based on a capillary model, characterized in that, Includes the following steps: ① Carbon fiber pretreatment: Cut a bundle of carbon fiber 10-20cm along its length, place it on a flat plate to keep the surface of the carbon fiber flat and compact, so that there are no gaps inside the carbon fiber bundle. ②Tension fastening treatment: The carbon fiber bundle is fixed by a tension clamping device, a certain tension is applied to both ends to make it straight, and the width of the carbon fiber bundle is changed by the device to obtain the test sample; ③ Cross-sectional model construction: Based on the K number of the carbon fiber bundle and the actual width and thickness, a two-dimensional planar rectangular model is constructed to represent the cross-section of the carbon fiber bundle. The number of adaptive layers is determined based on different widths and K numbers. The in-layer coefficient is established based on the number of adaptive layers and K number to determine the space occupied by each layer of carbon fiber. ④ Capillary area pre-evaluation: Measure the actual width change of carbon fibers. In the capillary model, the triangular area enclosed by three carbon fibers is taken as a capillary channel. The hexagonal area occupied by one carbon fiber as the center and the surrounding carbon fibers is taken as the basis for calculating the proportion of capillary channel area. Calculate the capillary channel area of ​​carbon fiber bundles under different widths according to the in-layer coefficient and capillary model formula. ⑤ Wetting performance evaluation: The changes in capillary area of ​​carbon fibers with different K numbers are directly compared by the changes in the broadening of the carbon fiber bundle and the in-layer coefficient. The greater the increase in capillary channel area, the better the wettability. The capillary model construction includes the following steps: a) The model length is constructed based on the actual broadening, using the measured values ​​of different carbon fiber bundle broadenings as a benchmark; b) A single capillary channel is constructed using a triangular region formed by three carbon fibers; c) The overall capillary channel area is calculated using a regular hexagonal region centered on a single carbon fiber; d) The capillary model formula is calculated as follows: S = w / q q=α*K / L Where f f is the area fraction of the capillary channel, S is the distance between the center points of two carbon fibers, r is the radius of a single carbon fiber, w is the broadening of the carbon fiber bundle, q is the in-layer coefficient, α is the packing coefficient, K is the K number of the carbon fiber, and L is the number of adaptive layers.

2. The method for evaluating the wettability of large-tow carbon fibers based on a capillary model as described in claim 1, characterized in that, The tension clamping device maintains the arrangement of the carbon fiber bundle by applying tension to both sides of the carbon fiber simultaneously, and the tension can be set in the range of 0 to 600 N.

3. The method for evaluating the wettability of large-tow carbon fibers based on a capillary model as described in claim 1, characterized in that, The tension clamping device changes the width range of the carbon fiber bundle to 6–180 mm.

4. The method for evaluating the wettability of large-tow carbon fibers based on a capillary model as described in claim 1, characterized in that, The large-tow carbon fiber has a K number of 24 to 300K, a single carbon fiber diameter of 4 to 8 μm, an adaptation layer number of 20 to 50, and a corresponding in-layer coefficient of 800 to 8000.

5. The method for evaluating the wettability of large-tow carbon fibers based on a capillary model as described in claim 1, characterized in that, The number of adaptive layers is the actual number of carbon fiber bundles determined based on the minimum theoretical number of stacked layers. The in-layer coefficient represents the maximum number of single carbon fibers that a single layer of carbon fiber can accommodate under measured broadening, and the packing coefficient represents the spatial compression caused by the stacking of carbon fibers.

6. The method for evaluating the wettability of large-tow carbon fibers based on a capillary model as described in claim 1, characterized in that, For carbon fibers with a packing factor of α = 1 (24–100K, including 100K), the packing factor of α = 0.9 (100–200K, excluding 100K but including 200K), and the packing factor of α = 0.8 (200–300K, excluding 200K but including 300K).

7. The method for evaluating the wettability of large-tow carbon fibers based on a capillary model as described in claim 1, characterized in that, The resin is a liquid mixture of pure resin and curing agent, and it is flowable after mixing.

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