Study On Laminated-Toughened CFRPs Based On Thermoset/Thermoplastic Diphase Systems

Advanced polymer matrix composites (laminated graphite composites) exhibit low damage resistance against low-velocity impact, resulting usually in delamination. The low toughness of interlaminar resin makes it mostly responsible. The idea of interleaf was developed to overcome this problem and showed certain effectiveness in improving the impact toughness of laminates. However, the introduction of extra toughening layers reduces the fiber volume fraction, and reduced specific strength and stiffness are the price one has to pay.For advanced polymer composites, thermoplastics (TP) are often used to toughen intrinsically brittle thermoset (TS) resins by formation of TS/TP diphase system. By studying the diffusion and migration behavior of TS/TP component in adjacent contact based on the Ex-Situ concept developed by Prof. Yi Xiaosu, it was found that both components formed a certain concentration gradient in their counter part's matrices. The phase decomposition and phase inversion induced by the following curing resulted in a special interphase structure, which comprised of interlaced TS continuous phases and TS/TP co-continuous phases. Inspired by this behavior a novel toughening concept, namely laminated-toughening, was suggested for the first time. The basic idea was introduction of thin TP into the graphite plies. During the curing cycle, the TP layer could be dissolved or swollen by TS monomers, generating a TS/TP co-continuous phase structure in the interlaminar regions, while the general composition of the graphite plies stayed unaffected.Laminated-toughened TS matrix laminates showed remarkable enhancement in impact damage resistance, while their static mechanic properties were well kept. There were no separate toughening layers in the cured system so that the fiber volume content was still high. Graphite prepregs treated in this way possessed desirable drapability and stickiness, which is especially convenient for hand laying,Observation on delamination initiation and development during impact loading revealed that the interlaminar damage could be effectively surpressed by laminated-toughened structure. Satisfying impact resistance could be attained when only a few of the most damage-prone interlayers were toughened. This fact may help designing highly-effective toughening structure and the selective toughening.