Microwave irradiation of nadic-end-capped polyimide resin (RP-46) and glass-graphite-RP-46 composites: cure and process studies

Abstract

ABSTRACT:Microwave energy was investigated to cure nadic-end-capped polyimide precursors (RP-46 resin) using a Cober Electronics Model LBM 1.2A/7703 microwave oven at a frequency of 2.45 GHz. Both neat resin samples and glass cloth and hybrid glass cloth-graphite cloth-RP-46 resin composites were studied. For the resin studies, the effect of various parameters, such as power level, sample size, processing temperature, time, and graphite fiber absorber, were investigated. The variables investigated with the composite study were the power level, mold material, vacuum, and low pressure. The results showed that microwave energy was effective in curing both neat resin samples and composite specimens. The presence of a small quantity of absorber (chopped carbon fiber) accelerates the cure dramatically. Moreover, soapstone mold material was found to be an efficient absorber for glass and glass-graphite composite processing, causing an effective cure in less than 1 h. Glass and glass-graphite hybrid composites with flexural strengths of 372-588 MPa (54 -85 ksi) and moduli of 28.7-31.7 GPa (4.2-4.6 Msi) have been fabricated. This is equivalent to 50 to 80% of the properties of composites fabricated by conventional means.

Processing of Composites Using Variable and Fixed Frequency Microwave Facilities

“…The mechanical properties were also much better. It can be argued that the ceramic plates absorbed a large amount of microwaves and converted into heat which was then conducted to the last 4 specimens [22].…”

“…The HAZ (heat affected zone) seemed to be confined to the lapped area, which could be bent by hand easily and outside which the warming and heating effect of microwave energy could not be felt. The tensile tests revealed that sixty percent of the failures were due to the adhesive and took place at the joints [11,[20][21][22].…”

“…Four types of composites in terms of lay-up, material and state of resin were placed in the three types of moulds for curing. They are processed in different microwave conditions in terms of power input and duration of exposure to microwave irradiation [22].…”

“…The one with least energy input exhibited a flexural strength of 386 N/mm 2 and modulus value of 19.4 kN/mm 2 respectively and failed between piles. The one with most energy input showed the best mechanical properties [22].…”

“…Two samples of glass cloth and graphite cloth prepregs were fully cured in 50 minutes and their densities were higher than the previous ten samples; this implied that their void content was low. Sample with the best mechanical properties exhibited a flexural strength of 588 N/mm 2 and modulus of 31.4 kN/mm 2 respectively [22].…”