| With the rapid development of low cost large-tow carbon fibers, carbon fiber reinforced unsaturated polyester(CF/UP) composites has a wide application prospect in automobile industry because of their advantages of low weight, high strength and stiffness. Compared with epoxy resin, unsaturated polyester has advantages of short cure time, low raw material cost, forming grade A surface materials, etc. The high volume shrinkage of unsaturated polyester lead to poor interfacial adhesion strength between carbon fiber and matrix, which confine the application of carbon fiber reinforced unsaturated polyester composites in automotive field.In this thesis, based on carbon fiber reinforced unsaturated polyester composites, functionalized CNTs and flexible molecular chain with double bonds were introduced into the interphase of composites through coating and chemical grafting methods respectively. Chemical bonds between carbon fiber and matrix were formed and the surface roughness of carbon fiber was increased. Meanwhile, CNTs and flexible chain can restrain the volume shrinkage of unsaturated polyester. The interfacial strength and impact toughness of carbon fiber reinforced unsaturated polyester composites were enhanced simultaneously.The top priority for coating methods is to screen the appropriate main polymers of sizing agent. There are four kinds of main polymers as candidates. The effect of molecular structure of main polymers on the physical and chemical properties of carbon fiber surface were characterized. The experimental results show that the sizing agent with epoxy and vinyl resin as main polymers can obtain a uniform film on carbon fiber surface than MR13006 unsaturated polyester sizing agent. The surface energy of carbon fiber surface has at least 20% improvement more than that of untreated carbon fibers. The sizing agent with MR13006 unsaturated polyester as main polymer cannot obtained a uniform film on carbon fiber surface. Uniformity of the coating is attributed to hydroxyl groups on carbon fiber surface,which were introduced by vinyl resin and epoxy sizing agent treatment. Hydroxyl group is beneficial for improving wettability of carbon fiber surface. After treated with MR13006 sizing agent, the carbon fiber surface has the highest oxygen content. The O-C=O groups content are increased significantly. The hydroxyl groups, which are beneficial for improving wettability, are relatively less. The interface bonding strength and impact toughness testing show that the sizing agent with M7270 and R806 as main polymers are more effective on improving the interface bonding strength of composites. The sizing agent with epoxy and MR13006 unsaturated polyester as main polymers has no obviously improvement effect for interface bonding strength. However, all of four sizing agents has no obviously improvement effect for impact toughness.Next, we designed interface chemistry reactions based on free radical polymerization and stepwise polymerization. M7270 and R806 vinyl resins were chosen as main polymers to prepare sizing agent containing functionalized CNTs. Research on the effect of interphase containing CNTs on the impact toughness of carbon fiber reinforced unsaturated polyester composites was conducted. The experiment results showed that functionalized CNTs were dispersed well on the surface of carbon fibers. After treated with sizing agent containing CNTs, the surface roughness and surface energy of carbon fibers were improved, which are beneficial for improving wettability of unsaturated polyester on fiber surface and interlocking effect between matrix and fibers. The interface bonding strength and impact toughness testing show that, compared with vinyl sizing agent with out CNTs, the vinyl sizing agent containing CNTs can enhance the interfacial strength and impact toughness of composites simultaneously. The mechanical interlocking effect and restricting of the crack propagation caused by CNTs were key factors for improving the interfacial properties of composites.Multi-scale reinforcement were prepared through grafting CNTs and undecylenyl alcohol with flexible segments onto the carbon fiber surface using poly(acryloyl chloride) as the linkage. Research on the effect of multi-scale reinforcement on the interfacial properties of carbon fiber reinforced unsaturated polyester composites was conducted. The results showed that multi-scale reinforcement can restrain the volume shrinkage of unsaturated polyester and improve the impact toughness. One end of the flexible chain composed of poly(acryloyl chloride) and undecylenyl alcohol was chemical connected with carbon fiber surface, while the other end can react with unsaturated polyester. The chemical bonds between carbon fiber and matrix is beneficial for improving interfacial adhension. Meanwhile, the polar functional groups on multi-scale reinforcement could improve the oxygen content of fiber surface, then increase surface energy and wettability of fiber surface, which could maximize the degree of the molecular contact and avoid forming defect. Nanoscale rough surface with CNTs could increase the ability of mechanical interlocking between fibers and matrix, consequently restrict the mobility of the resin molecules in the interphase region. The movement ability of flexible chain can restrain the volume shrinkage of unsaturated polyester in the interface region and improve the impact toughness of carbon fiber reinforced unsaturated polyester composites. When carbon fiber reinforced unsaturated polyester composites were fractured, the CNTs could efficiently restrict the crack propagation, disperse stress, augment fracture area and absorb impact energy. The CNTs and the chemical bonds between fibers and matrix improved the modulus of interphase area, and formed the transition region with gradient relatively modulus. This transition region is not only beneficial for transferring and dispersing exterior stress, but also beneficial for eliminating curing interior stress caused by different thermal expansion coefficient between carbon fibers and polymers. |
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