Thermal characterization of a bis‐maleimide/bis‐cyanate/epoxy thermosetting resin for composites

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Abstract
The thermosetting resin investigated here was a mixture of bis‐maleimide and bis‐cyanate, frequently referred to as BT (bis‐maleimide triazine). Triazine is the reaction product of the cyclotrimerization of bis‐cyanate during curing. For circuit board applications, a brominated epoxy resin was blended with BT to impact flame resistance. Resin cure was extensively investigated using a combination of thermoanalytical techniques (thermal analysis, heated cell infrared spectroscopy, and dynamic mechanical analysis). The ultimate glass transition temperature was found to be 240°C, which could only be obtained using cure temperatures above 225°C. At lower temperatures, the reaction does not reach full conversion, since the glass transition temperature of the curing network equals or slightly exceeds the cure temperature. Differential scanning calorimetry (DSC) indicates a minimum of two separate reactions. Fourier transform infrared spectroscopy provided more detailed information on the crosslinking reactions during cure. The onset of cyclotrimerization was found to start at 150°C, correlating with one of the peaks in the DSC. At higher temperatures, the epoxide reacts with the cyanate functionality forming oxazoline ring structures. It was not possible to unambiguously assign the origins of the high temperature peaks in the DSC. These high temperature peaks may be attributed to several reactions, including epoxy homopolymerization and polymerization of bis‐maleimide. The high temperature reaction mechanisms warrant further investigation.

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