| Carbon fiber Preregs are prepared by impregnating continuous fibers or fabric with B-staged resin matrix and used as intermediate-product of composites. Epoxy resin as the common resin systems of moderate-temperature curing prepregs required favourable toughness to ensure effective stress transfer of fibers. At the same time, the viscosity of resin matrix must be adjusted to control prepreg stacky and resin flow of prepreg needs to be lowered to meet the process technology of composites. In this paper, Intrinsic Flexible Epoxy Resin (IFEP) were synthesized by grafting polyurethane prepolymer (PPU) in epoxy molecular and applied in toughened resin matrix and moderate temperature curing prepregs.1. PPU 1000, PPU2000 and PPU3000 were synthesized from 4, 4'-diphenylmethane diisocyanate (MDI) and Poly propylene glycol (PPG) (Mn=1000,2000,3000). IFEP1000, IFEP2000 and IFEP3000 were further prepared through the reaction between OH group of epoxy resin and NCO group of PPU. According to NCO content test, The NCO content of PPU reached the theoretical value after 4h. The structure of PPU and IFEP were confirmed by FTIR and NMR spectra. GPC remits showed that the molecular weights of PPU and IFEP were consistent with the theoretical values.2. Various IFEP matrices were prepared by mixing IFEP 1000, IFEP2000 and IFEP3000 with epoxy resin systems and the viscoelasticity, curing kinetics, fracture toughness, thermal properties and toughening mechanism were studied. Rheological tests showed that the relaxation time of IFEP/epoxy matrices was longer as the PPG molecular weight increases, whereas the gelation time became shorter. IFEP toughened epoxy resin exhibited excellent toughness, and IFEP2000/epoxy system particularly performed the best fracture toughness (KiC) impact strength. SEM results showed that microphase separation of PPU took place in the epoxy resin matrices due to the solubility parameters gap between PPU and epoxy component. Medium size and distribution of microphase domain were observed in IFEP2000/epoxy system. It can be concluded that IFEP2000/epoxy system exhibited the best comprehensive properties.3. The influence of the content of IFEP on the mechanical properties, curing characteristics, morphology, et al. of epoxy resin systems was studied using IFEP2000 with different mass fraction. Mechanical tests showed that the impact strength increased as IFEP content increased and 60wt% IFEP2000 system exhibited the best toughening effect. DSC results showed that the heat released during curing reaction was decreased in IFEP toughened systems. DMA results showed that as the IFEP2000 content rose, the glass transition temperature (Tg) firstly descended then ascended and reached 166? for 60wt%IFEP2000 system. Microphase separation was observed in SEM pictures in which the biggest degree of separation with the domain size of 0.148?0.497 ?m existed in IFEP2000 system.4. Prepregs were prepared with IFEP2000 matrix with the mass fraction of 60wt% and T300 carbon fiber based CFRP was prepared using the two step hot-melt impregnating method further. The rheological properties and the resin flow were analyzed and the mechanical properties and morphology of CFRP laminates were characterized. Wider process window of IFEP toughened prepregs was observed and its resin flow was reduced compared to unmodified prepregs. The general mechanical properties, such as interlaminar fracture toughness (Gic), interlaminar shear strength and compression strength after impact (CAI) were greatly improved. SEM pictures showed that there was close bonding between carbon fibers and epoxy resin matrix in IFEP toughened CFRP laminates where the microphase seperation of polyurethane component was also observed. |
PDF Full Text Request