Comparison Between the Thermal and Microwave Curing of Bismaleimide Resin Ismail Zainol, 1 Richard Day, 2 Frank Heatley 3 1 Advanced Materials Research Centre, SIRIM Berhad, Lot 34, Jalan Hi-Tech 2/3, Kulim Hi-Tech Park, 09000 Kulim, Kedah, Malaysia 2 Manchester Materials Science Centre, UMIST, Grosvenor Street, Manchester M1 7HS, United Kingdom 3 Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom Received 16 July 2002; accepted 2 April 2003 ABSTRACT: Microwave energy was used to cure Matrimid 5292A bismaleimide resin. The degree of cure at different cure times was determined with differential scanning calorimetry. A comparison was made with the same resin cured in a conventional oven. The resin cured faster in the microwave oven than in the conventional oven. The glass-transition temperature (T g ) depended on the degree of cure. The samples cured with a conventional oven showed slightly higher T g ’s than the microwave- cured samples at higher degrees of cure. At low degrees of cure, the two heating methods yielded materials with similar T g ’s. Fourier transform infrared spectroscopy and solid-state cross-polarity/magic-angle-spinning 13 C NMR spectroscopy were used to investigate the cured struc- tures. There was no difference in the chemical reactions taking place during the microwave cure and the thermal cure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2764 –2774, 2003 INTRODUCTION The microwave processing of thermosetting materials offers significant advantages over conventional heat- ing, such as reduced cure cycles, more uniform curing, and improved physical and mechanical properties. 1 Microwave heating takes place through dipolar inter- actions between molecules and an alternating electric field rather than through thermal conduction or con- vection as in conventional heating. Microwave radia- tion penetrates materials, and the heat is generated internally. Studies on epoxy resin systems have shown that microwave processing significantly reduces the cure cycle without sacrificing the mechanical proper- ties. 2,3 Bismaleimide (BMI) resins are presently the best candidates to replace epoxy resins in high-perfor- mance composites because of their high temperature stability and resistance to hot and wet environments. 4 BMI resins contain reactive double bonds (CAC) in the maleimide groups that can polymerize with or without the presence of a catalyst. Various curing reactions for BMI resin formulations have been sug- gested. 5 The main reaction proposed is homopolymer- ization via a free-radical mechanism. 6 Conventional curing processes involve heating in an oven, auto- clave, or compression mold, in which heat transfer to the material is accomplished through conduction or convection. Just as for other thermosetting materials, the low thermal conductivity of these resins leads to thermal gradients inside the resin, and the curing cycle becomes longer. Initial studies of the microwave processing of BMI systems have been reported. 7,8 Researchers 8 found that the reaction kinetics in conventional and micro- wave ovens were identical under isothermal condi- tions. They concluded that no microwave effect could be observed. The term microwave effect implies the possibility of altering the rate and path of a chemical reaction to produce a different morphology and dif- ferent physical and mechanical properties. The afore- mentioned researchers used in situ fiber-optic Fourier transform near-infrared spectroscopy to measure the extent of the reaction. Liptak et al. 7 however, found more rapid curing when microwave heating was used than when a conventional oven was used. They used extraction methods to determine the gel content in the cured resin. The apparent contradiction between these two reports is probably due to the different techniques used to cure and characterize the samples. Neither shows in detail the cure chemistry in microwave and conventional ovens. It is important to study the curing path because the final structure and mechanical prop- erties of a network depend on the reaction path. The objective of this study was to compare the cure chemistry of a BMI resin when conventional and mi- crowave heating were used. The cure reactions were followed with spectroscopic techniques. The effect of Correspondence to: I. Zainol (ismail@sirim.my). Contract grant sponsor: SIRIM Berhad. Journal of Applied Polymer Science, Vol. 90, 2764 –2774 (2003) © 2003 Wiley Periodicals, Inc.