Development of high performance carbon fiber based prepregs for honeycomb structure

Abstract

This research is aimed to develop a highly processable carbon fiber prepregs based on ternary mixture matrices of benzoxazine, epoxy, and phenolic resins. Prepregs is referred to a continuous fiber pre-impregnation with matrix resins. They are supplied in roll or sheet form, ready for immediate use at a composite manufacturing facility, and are widely used in the aerospace and other industries for high performance structural applications as well as honeycomb core materials. Polybenzoxazine (BA-a), a polymer based on phenolic resin, possesses some outstanding properties such as ease of synthesis, low A-stage viscosity, near-zero shrinkage, low water absorption, and high thermal stability. Alloying with epoxy and phenolic novolac to form a ternary resin mixture can greatly broaden the properties of the polymer. The epoxy acts as a reactive diluent and crosslink enhancer of the ternary system while the phenolic resin can function as a polymerization initiator for the polybenzoxazine and/or a hardener for the epoxy resin. The effects of the resin mixture composition on processability, thermal, mechanical, and some physical properties for the composites such as cure temperature, flexural strength, and thermal stability are characterized. The resin composition was varied using different resin mass ratios i.e. benzoxazine/epoxy/phenolic (BEP) equals 361, 362, 363 and 364. It is observed that the viscosity of the matrices decreases with increasing epoxy mass fractions. In addition, the increase in phenolic novolac mass fraction effectively resulted in the decrease in curing temperature of the ternary systems. These results can be seen from the shift of an exothermic peak in the DSC thermograms to lower temperature. Processability and gel point of the matrices were investigated by using Fourier Transform Mechanical Spectroscopy (FTMS). It is found that the ternary system can provide a relatively wide range of processing window from 50 to 200 degree celsius and can maintain low melt viscosity for a long period time. In addition, BEP362 show the highest processability for prepregging process due to low melt viscosity i.e. 1.4 Pa.s at 100 degree celsius and can gel in less than 5 min at 180 degree celsius. The glass transition temperatures of the above four ternary systems are relatively similar in values of approximately 150-165OC with BEP362 showing the highest Tg of 165 degree celsius. Furthermore, BEP362 can be kept at room temperature (~33 degree celsius) with minimal effect on its processability i.e. a little change in Tg of the resin (less than 25% conversion of BEP362 up to 270 days). Therefore, the optimum composition of the ternary systems for making high processable carbon fiber prepregs is BEP362. The flexural modulus and flexural strength of the carbon fiber-reinforced BEP362 at approximately 55% volume of the fiber in cross-ply orientation and without any surface treatment render relatively high values of about 68 GPa and 1,156 MPa respectively. The honeycomb structure from carbon fiber-BEP362 prepregs is found to be relatively easy to fabricate as a results of the above mentioned characteristics.