Study On The Surface Modification Of Carbon Fibers And Carbon Fiber Reinforced Polyamide 12 Composites

Carbon fiber (CF) reinforced polymer composites have been widely used in many fields such as aerospace, automotive industries and sports equipment owing to their excellent properties, such as high specific strength, high modulus and low density. However, the mechanical properties of these composites are limited by the poor interfacial bonding between carbon fiber and polymer matrix because of the excessive smoothness, chemical inertness and less reactive functional groups of CF, and surface treatment of CF is an effective way to improve the interfacial adhesion for stress transfer at the interface.In this paper, polyamide 12 (PA12) and carbon fibers are used respectively as the matrix and reinforcing material to prepare carbon fiber/polyamide 12 composite. Firstly, the effect of carbon fiber length on the tensile properties of composite was studied, and then three methods were used to modify the surface of CF, i.e. liquid-phase oxidation, coupling agent treatment and polymer grafting, and liquid-phase oxidation was the basis of the other two methods. Effects of liquid-phase oxidation temperature and time, types and amount of coupling agent on the surface conditions of CF and tensile properties of composites were studied. For polyaniline grafted carbon fibers, the grafting amount was controlled by changing the amount of aniline, and for polyamide 6 grafted carbon fibers, the grafting amount was controlled by changing initiator dosage and reaction time. The effects of polymer grafting on the tensile properties of composites were discussed. The main conclusions are described as follows:(1) As the carbon fiber length increases, tensile strengths of carbon fiber/polyamide 12 composite increase. The composite powder composed of carbon fibers with length between 150-250?m(60-100 meshes) and polyamide 12 powder in a 3:7 weight ratio showed a good molding performance and compared to the pure polyamide 12, the tensile strength of the composite was increased by 43.35%, from 37.18MPa to 53.30MPa.(2) After HNO3 oxidation, the carbon fibers were cleaner and grooves on the fiber surface got more or deeper. The O element content and functional groups on the fiber surface were increased. After oxidation in HNO3 at 80? for 2h, the hydroxyl group content on the CF surface was the highest and after treated at 80? for 4h, the carboxyl group content was the highest. The corresponding tensile strengths of these two composites were 60.70MPa and 61.32MPa, respectively. Oxidation temperature and time had less effect on the tensile strength.(3) Carbon fibers without oxidation were non-surface active, so the effect of coupling agent treatment on the original CF was not obvious. It can't induce aniline or caprolactam to polymerization and graft on the fiber surface either. So the tensile strengths of these three composites were nearly the same as the original CF/PA12.(4) The tensile strengths of composites reinforced by carbon fibers treated by coupling agent after oxidation can be higher than the oxidized CF/PA12 composte, and KH550, KH560 and HY201 all showed good effects. With the increasing of dosage of coupling agent, the tensile strength of composite increased firstly and then decreased. The optimized dosage of KH55O, KH560 and HY201 was 0.3%,0.3% and 0.4%, respectively and the corresponding tensile strength of the composite was 66.34MPa,66.82MPa and 68.81MPa, respectively.(5) There were carboxyl groups on the CF surfaces after HNO3 oxidation, so the polymerization of aniline or caprolactam and grafting was successful. For the polyaniline grafted carbon fibers, the grafting amount decreased with the decreasion of aniline addition ratio and meanwhile the distribution of grafted polyaniline was more uniform. For the polyamide 6 grafted carbon fibers, the grafting amount was very low when the initiator dosage was low, and later increased as the initiator dosage increased, but there was an Optimal value. The grafting amount also increased as the reaction time got longer. The tensile strengths of polyaniline grafted oxidized CF/polyamide 12 composites were all lower than the oxidized CF/PA12 composite, indicating that polyaniline was not an appropriate grafting polymer, while the tensile strength of polyamide 6 grafted oxidized CF/polyamide 12 was 64.14MPa,4.60% higher than the oxidized CF/PA12 composite.