Thermoplastic carbon fiber is an important direction for the future development of the carbon fiber industry. Because of its high mechanical properties and reshapeable processing characteristics, it can play a greater role in the processing and reuse of thermoplastic carbon fiber products. Thermoplastic carbon fiber is suitable for high-end fields such as aerospace and aviation. It has better high-temperature resistance and more comprehensive application scenarios. At this stage, there are not many types of thermoplastic carbon fibers that can be prepared in batches, including CF/PEEK, CF/PPS, CF/PA, etc.
1. Toughening agent blending and toughening: In composite material modification, physical blending modification is the simplest and most commonly used modification method. The properties of thermoplastic carbon fiber composites are closely related to the interface bonding state between carbon fiber and thermoplastic resin matrix. In order to increase the toughness of the composite material, a certain toughening agent can be added to improve the interface bonding effect of the materials and enhance the toughness of the composite material.
Maleic anhydride (MAH) can react with polyamide (PA) to compatibilize it. The two have good compatibility. Due to the good compatibility of MAH and PA, the toughening agent can be quickly dispersed into the PA matrix. Research shows that for PA6/CF composite materials, the addition of toughening agent POE‑g‑MAH can significantly improve the impact properties of the composite materials. The impact strength is increased from 6.2kJ/m2 to 9.0kJ/m2, and the toughening effect is obvious.
2. Reinforcement hybrid toughening: In addition to carbon fiber and thermoplastic resin, other reinforcement materials, such as glass fiber (GF), aramid fiber (AF) and carbon nanotubes, are added to thermoplastic carbon fiber composite materials to improve the thermoplastic carbon fiber The toughness of composite materials can also achieve the toughening effect.
Experiments show that CF, GF and POE‑g‑MAH will be added to PA66 to prepare hybrid fiber/POE‑g‑MAH composite reinforced PA66 materials. When the glass fiber addition amount is 15%, the toughening effect is the best, which is better than CF The impact strength of filling alone increased by 34.02%, and the improvement effect was obvious. When preparing CF/PET composite materials, aramid fiber is coated in it for modification. The impact strength of the composite material is significantly improved. The impact strength of the composite material is increased by 65.8% with one layer of coating and 45.6% with two layers of coating. The toughening effect is significantly improved.
Some studies have found that the advantages of two reinforcing materials, CF and halloysite nanotubes (HNTs), were combined to study the effect of synergistic toughening and reinforcement of HNTs and CF on PA6. Mechanical property test results show that PA6/30%CF/10%HNTs has a maximum impact strength of 8.9kJ/m2, HNTs has a toughening effect on PA6/CF composite materials, and HNTs and CF have a synergistic effect in toughening.
3. Carbon fiber surface treatment: Unmodified carbon fiber is brittle, has a strong surface inertness, and lacks active groups, resulting in poor compatibility between the carbon fiber and the thermoplastic resin matrix, and the interface structure and performance are affected. By surface treatment of carbon fiber, its surface chemical activity, surface free energy or surface roughness can be increased, which can help improve the degree of wetting between carbon fiber and thermoplastic matrix, thereby improving the overall performance of the composite material, including its own toughness. There are many ways to treat the surface of carbon fiber, including sizing agent treatment, surface physical modification and surface chemical modification.
The improvement of the interface properties of composite materials by sizing agents can be achieved through physical effects such as infiltration and adhesion, as well as chemical effects through the combination of a large number of active groups on the carbon fiber surface with the matrix to produce covalent bonds. Experimental data shows that raw materials such as adipic acid are melt-condensed to form thermoplastic copolyamide, which is formulated into a sizing agent (co-PA) to modify the PA6/CF composite material. At the optimal sizing agent content of 4%, the interfacial shear strength (IFSS) of the composite material reaches 37.6MPa, which is 43.76% higher than that of unsized PA6/CF.
Other physical modification methods for carbon fiber surface include ultrasonic dispersion, surfactant treatment and electrochemical deposition. Experimental data shows that polydiallyldimethylammonium chloride (PDDA) was used to modify graphene oxide (P‑SG) and mixed with CF for ultrasonic treatment, so that P‑SG was successfully attached to the CF surface to obtain PA6/C ‑SG composites. The conclusion is that the impact strength of modified composite materials increases significantly with the increase of carbon fiber content. When the carbon fiber content is 13%, the impact strength is 36.52kJ/m2, and the impact performance is increased by 113.17%.
Chemical modification of carbon fiber surface is to put carbon fiber into a solution environment and selectively modify the surface of the material to give more active groups and increase the interface bonding force; or use other solvents to improve the roughness of the CF surface to control the surface chemical properties, among which the coupling agent method is one of the more commonly used chemical modification methods. Experimental data shows that when the surface of carbon fiber is chemically modified with silane coupling agent (KH550) to prepare PA6/CF composite materials, the unnotched impact test results show that when the modified carbon fiber content is 20%, the unnotched impact strength of PA6/CF reaches the maximum value (18.5±0.6) kJ/m2, which is 52% higher than the corresponding untreated content.
4. Processing and molding process control: The molding and processing of thermoplastic carbon fiber composite sheets and the material component connection technology are also important factors affecting the final material properties. By controlling the molding temperature, molding pressure, etc. during the material molding process, the interface bonding situation of the composite material can be controlled and the interface performance can be changed.
In addition, different molding methods also have a significant impact on the final mechanics and other properties of the material. Experimental data show that ABS/CF composite materials using extrusion/injection process and long fiber thermoplastic (LFT)/injection molding process were compared, and the effects of the two processes on the fiber length distribution, tensile, impact and other properties of the material were compared. Influence. The results show that the minimum CF length in ABS/L‑CF composites is approximately 3 times the maximum fiber length in ABS/E‑CF composites. The impact strength of ABS/L‑CF is about 105%~155% higher than that of ABS/ECF.
Research on thermoplastic carbon fiber composites at home and abroad has never stopped, and increasing toughness is only one of the research directions. As a cutting-edge new composite material, thermoplastic carbon fiber has huge potential, but it is equally difficult to transform it into industrial assistance. If the properties of thermoplastic carbon fiber are to be put into application in a more complete and mature manner, research in directions such as toughening is indispensable. Only if the foundation is solid enough can the development of the carbon fiber industry be solid enough.