A method for surface modification of carbon fibers to break the shackles of the resin matrix on the interfacial strength of composites

Abstract

The present invention discloses a method for surface modification of carbon fibers to break through the shackles of the resin matrix on the interfacial strength of composites. The method comprises the following steps: Step 1, desizing and drying the carbon fibers; Step 2, soaking the desized carbon fibers in an acidic oxidation solution for pre-oxidation treatment to obtain oxidized carbon fibers; Step 3, preparing hollow capsules by the template method and applying them to the surface of the oxidized carbon fibers to construct an interphase composed of capsule structures; Step 4, cleaning and drying the carbon fibers modified in Step 3. This method cleverly utilizes the hollow structure of the capsules to achieve variable behaviors at the interface. While optimizing the composite interface, it effectively strengthens the matrix part in the composites as a reinforcing component, opening the door to a new field of interface modification and having important theoretical significance and practical application value for the interface design of composites with interfacial strength superior to that of the resin.

Description

Technical Field

[0001] The present invention relates to a method for surface modification of carbon fibers, and particularly to a method for constructing an interphase on the surface of carbon fibers by using a hollow capsule structure to achieve a breakthrough improvement in the interfacial strength of a composite material. Background Art

[0002] Carbon fiber reinforced resin matrix composites are widely used in fields such as aerospace, new energy, and rail transit due to their advantages such as high specific strength, specific modulus, and chemical stability. However, the surface of carbon fibers, which belong to a turbostratic graphite structure, is inert, with few active carbon atoms and low surface energy. The interface formed between them and the resin matrix is weak, which directly affects the mechanical properties of the composite material. Therefore, it is necessary to modify the surface of carbon fibers, for example, by increasing the polarity and roughness of the carbon fiber surface, improving the wettability of the resin to the carbon fiber, promoting interfacial reactions, introducing strong physical or chemical interactions between the carbon fiber and the resin, exploring the synergistic advantages between different modification components, forming a modulus transition zone, etc. Following these principles, various interfacial modification strategies have been proposed and achieved good results. The interface between the carbon fiber and the resin matrix has been optimized, and the interfacial strength of the composite material has been significantly improved.

[0003] However, the modified layer constructed on the surface of carbon fibers by existing modification strategies has little effect on the adjacent wider resin matrix. As a result, when the composite material is subjected to external forces, the resin, as the weak phase, will be damaged in advance and undergo irreversible destruction, thereby hindering the expression of the interfacial strength of the composite material and making it difficult for the composite material to achieve its ideal interfacial strength. Therefore, from the perspective of the development of composite materials, how to break through the limitation of the strength of the resin matrix on the interfacial strength of the composite material is a key difficulty that needs to be focused on in the future field of interfacial modification.

[0004] Whether the action area of the carbon fiber surface modification layer can be effectively broadened to the adjacent large-scale resin area is the key to breaking the above-established limitation. This requires that while ensuring strong interfacial adhesion between the carbon fiber and the resin, the modification layer effectively increases the influence of the interfacial connection on the resin matrix to simultaneously strengthen the matrix part in the composite material. Therefore, the conceptual design of a new type of carbon fiber surface modification layer urgently needs to be developed. Summary of the Invention

[0005] To solve the problem that the interfacial strength of carbon fiber reinforced resin matrix composites is limited by the strength of the resin matrix, the present invention provides a surface modification method for carbon fibers that breaks the shackles of the resin matrix on the interfacial strength of the composites. This method cleverly utilizes the hollow structure of the capsules to achieve variable behaviors at the interface. The low molecular weight resin matrix with good fluidity at the molding temperature can be allowed to enter the hollow region inside the capsules. The hollow capsules initially adhered to the surface of the carbon fibers can change their morphology during the subsequent process of compounding with the resin to prepare the composites and simultaneously act on the adjacent resin regions, thereby optimizing the composite interface while effectively strengthening the matrix part in the composites as a reinforcing component, opening the door to a new field of interface modification, and having important theoretical significance and practical application value for the interface design of composites with interfacial strength superior to that of the resin.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] A surface modification method for carbon fibers that breaks the shackles of the resin matrix on the interfacial strength of the composites, comprising the following steps:

[0008] Step 1: Desize and dry the carbon fibers. Among them, the desizing treatment liquid is acetone, the operating temperature is 50 - 80°C, and the treatment time is 24 - 72 h; the drying treatment temperature is 50 - 90°C, and the time is 5 - 12 h. The purpose is to remove sizing agents, pollutants, etc. on the surface of the fiber products to facilitate the modification treatment of the fiber fibrils.

[0009] Step 2: Immerse the desized carbon fibers in an acidic oxidation solution for pre-oxidation treatment to obtain oxidized carbon fibers. Among them, the acidic oxidation solution is one of concentrated nitric acid, sulfuric acid, nitric acid / sulfuric acid mixtures with different concentrations, potassium permanganate, and their mixtures. The temperature of the pre-oxidation treatment is 25 - 120°C, and the time is 10 min - 300 min. By the pre-oxidation treatment, the oxidation functional groups on the surface of the carbon fibers are increased to facilitate the next modification treatment.

[0010] Step 3: Prepare hollow capsules using the template method and apply them to the surface of the oxidized carbon fibers to construct an interface phase composed of capsule structures. The specific steps are as follows: Assemble the capsule wall material on the surface of the template, and then prepare hollow capsules after removing the core with an etching solution. Immerse the oxidized carbon fibers in the hollow capsule dispersion liquid. Among them, the template in the process of preparing the hollow capsules is silica with a size of 10 μm; the capsule wall material is one or several of polyallylamine hydrochloride, sodium polystyrene sulfonate, polyacrylic acid, polyvinylamine, poly(diallyldimethylammonium chloride), polydopamine, polyacrylic acid, and grafted products of dopamine; the etching solution is hydrofluoric acid solution or a mixed solution of hydrofluoric acid and ammonium fluoride; the concentration of the hollow capsule dispersion liquid is 0.1 - 3 mg / mL; the immersion temperature is 25 - 120°C, and the time is 30 - 200 min.

[0011] Step 4: Clean and dry the carbon fibers modified in Step 3, where: the cleaning solution is acetone, the cleaning temperature is 25 - 80°C, and the time is 5 - 100 min; the drying temperature is 50 - 90°C, and the time is 5 - 12 h.

[0012] Through the above method, an interface phase with variable behavior was constructed on the surface of carbon fibers. The hollow capsule structure rich in polar functional groups can significantly improve the chemical activity and surface energy of carbon fibers, and introduce strong mechanical interlocking and covalent bonding between carbon fibers and resins, effectively improving the interfacial adhesion strength of the composite material. At the same time, as Figure 1 shown, the hollow capsules at the interface can change their morphology during the diffusion and wetting of the resin to the carbon fiber surface. The resin with good fluidity at high temperature penetrates into the interior of the hollow capsules. The 10 - μm size helps the capsules to fully contact a large range of adjacent resin areas. The capsules with changed morphology then act effectively on the resin matrix part synchronously. Thus, the hollow capsules with the above variable behavior optimize the interfacial adhesion performance between carbon fibers and the resin matrix, and can simultaneously improve the strength of the matrix resin as an effective strengthening component in the composite material; therefore, the interfacial strength of the composite material has achieved a breakthrough improvement, superior to the strength of the resin matrix.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] 1. Compared with the unmodified carbon fiber reinforced resin matrix composite material, the interfacial shear strength and interlaminar shear strength of the modified composite material are significantly improved. Among them, the interfacial shear strength is increased by 30.0 - 75.0%, and the interlaminar shear strength is increased by 15.0 - 50.0%. The improvement of the interfacial adhesion performance can be attributed to the higher wettability provided by the hollow capsules, the strong interfacial chemical action, the increased mechanical interlocking, and the strengthening effect on the resin matrix.

[0015] 2. Compared with other interfacial modification strategies, the interfacial strength of the composite material has achieved a breakthrough improvement, superior to the strength of the resin matrix. This is because the hollow capsule structure broadens the effective action area of the interfacial modification layer, and can improve the strength of the resin matrix in the composite material while optimizing the interface between carbon fibers and resins.

[0016] 3. The method of the present invention has mild reaction conditions, simple steps, is economical and environmentally friendly, and has potential industrial application value. Description of the Drawings

[0017] Figure 1 shows the interfacial behavior of the hollow capsules and their action mechanism on the composite material;

[0018] Figure 2Interface properties of carbon fiber composites with interfacial phase modification for hollow capsule structure regulation. Detailed implementation manners

[0019] The technical solutions of the present invention will be further described below in conjunction with embodiments, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention, without departing from the spirit and scope of the technical solutions of the present invention, shall be covered by the protection scope of the present invention.

[0020] Example 1

[0021] This example provides a method for modifying the surface of carbon fiber, and the method includes the following steps:

[0022] (1) Surface cleaning of carbon fiber: The carbon fiber to be cleaned is placed in a Soxhlet extractor for extraction and cleaning. The cleaning liquid is acetone, the cleaning temperature is 75 °C, and the cleaning time is 48 h; then it is dried at a constant temperature of 80 °C in a thermostatic air blast drying oven for 4 h.

[0023] (2) The cleaned carbon fiber is soaked in an excessive amount of concentrated nitric acid solution and heated to 80 °C for oxidation for 2 h.

[0024] (3) Using 10 μm silica as a template and polydopamine as the capsule wall material, polydopamine is assembled on the surface of silica, and then a polydopamine hollow capsule is prepared after removing the core with hydrofluoric acid solution. The oxidized carbon fiber is soaked in a polydopamine hollow capsule dispersion solution with a concentration of 0.5 mg / mL and soaked at 25 °C for 1 h.

[0025] (4) The modified carbon fiber is cleaned with acetone at room temperature for 10 min and then dried at 80 °C for 4 h.

[0026] Dynamic contact angle test shows that the surface energy of the modified carbon fiber has increased by 58.0%. Single filament pull-out test and three-point short beam bending test show that the interfacial shear strength and interlaminar shear strength of the modified carbon fiber composite have been improved by 50.0% and 35.0% respectively ( Figure 2 ).

[0027] Example 2

[0028] This example provides a method for modifying the surface of carbon fiber, and the method includes the following steps:

[0029] (1) Surface cleaning of carbon fiber: The carbon fiber to be cleaned is placed in a Soxhlet extractor for extraction and cleaning. The cleaning liquid is acetone, the cleaning temperature is 75 °C, and the cleaning time is 48 h; then it is dried at a constant temperature of 80 °C in a thermostatic air blast drying oven for 4 h.

[0030] (2) The cleaned carbon fiber is soaked in a mixed solution of 25% sulfuric acid and 75% nitric acid and heated to 60 °C for oxidation for 4 h.

[0031] (3) Using 10-μm silica as a template and polydopamine and polyacrylic acid as capsule wall materials, polydopamine and the grafted product of polyacrylic acid and dopamine were successively assembled on the surface of silica. After removing the core with a mixed solution of hydrofluoric acid and ammonium fluoride, hollow capsules were prepared. The carbon fiber was immersed in the dispersion of hollow capsules with a concentration of 0.5 mg / mL and soaked at 25 °C for 2 h.

[0032] (4) The modified carbon fiber was cleaned with acetone at room temperature for 10 min and then dried at 80 °C for 4 h.

[0033] The dynamic contact angle test shows that the surface energy of the modified carbon fiber increases by 70.0%. The mechanical property test shows that the interlaminar shear strength of the modified carbon fiber composite reaches 117 MPa, which is higher than the strength of the resin matrix (93 MPa) ( Figure 2 ).

Claims

1. A method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material, characterized in that the method comprises the following steps: Step 1: Desize and dry the carbon fiber; Step 2: Immerse the desized carbon fiber in an acidic oxidation solution for pre-oxidation treatment to obtain oxidized carbon fiber; Step 3: Prepare hollow capsules by the template method and apply them to the surface of the oxidized carbon fiber to construct an interface phase composed of capsule structures. The specific steps are as follows: Assemble the capsule wall material on the surface of the template, and then prepare hollow capsules after removing the core with an etching solution. Immerse the oxidized carbon fiber in the hollow capsule dispersion liquid. The template is silica with a size of 10 μm; Step 4: Clean and dry the carbon fiber modified in Step 3.

2. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that in Step 1, the desizing treatment solution is acetone, the operating temperature is 50-80°C, the treatment time is 24-72 h; the drying treatment temperature is 50-90°C, and the time is 5-12 h.

3. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that in Step 2, the acidic oxidation solution is one of concentrated nitric acid, sulfuric acid, nitric acid / sulfuric acid mixtures with different concentrations, potassium permanganate and their mixtures.

4. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that in Step 2, the temperature of the pre-oxidation treatment is 25-120°C, and the time is 10 min-300 min.

5. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that the capsule wall material is one or several of polyallylamine hydrochloride, sodium polystyrene sulfonate, polyacrylic acid, polyvinylamine, poly(diallyldimethylammonium chloride), polydopamine, grafted products of polyacrylic acid and dopamine.

6. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that the etching solution is hydrofluoric acid solution or a mixed solution of hydrofluoric acid and ammonium fluoride.

7. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that the concentration of the hollow capsule dispersion liquid is 0.1-3 mg / mL.

8. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that the immersion temperature is 25-120°C, and the time is 30-200 min.

9. The method for surface modification of carbon fiber to break the bondage of the resin matrix on the interfacial strength of the composite material according to Claim 1, characterized in that in Step 4, the cleaning solution is acetone, the cleaning temperature is 25-80°C, the time is 5-100 min; the drying temperature is 50-90°C, and the time is 5-12 h.

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