Low coefficient of thermal expansion of thermoset composite materials

Abstract

Selection of the correct resin – filler combination is important in achieving materials which have a low coefficient of thermal expansion (CTE). A study of nanosilica-modified resin, in combination with silica fillers, allows low levels of CTE to be achieved. In this study, a novel curing agent, ytterbium triflate, is reported. This curing agent provides a stable catalyst system which can be used to create the viscous composite mixtures but has the facility of effective cure over a relatively narrow temperature range from 70 °C to 100 °C. A series of formulations were examined based on incorporation of fillers with nanoscale silica particles into either the pure resin or a resin which contained nanoscale functionalized silica particles. The filler incorporation leads to a significant increase in the glass transition temperature as determined by dynamic mechanical thermal analysis. The CTE was observed to be lower below Tg than above it. Changing the nanosilica particle size and distribution produced significant changes in the values. However, the CTE scaled according to the total silica content; and the values were in general lower than those calculated theoretically. The use of a highly functional o-cresol epoxy novolac demonstrated how increasing functionality raised the Tg and lowered the CTE, but the use of too high a post-cure temperature reversed this trend. Very good results were achieved using 3,4-epoxycyclohexylmethyl, 3,4 epoxycyclohexanecarboxylate, which has a low viscosity and allowed high levels of silica to be readily achieved. This article indicates how the adjustment of the epoxy selected to be used as the base material and the type of silica particles used allows the values of Tg and CTE to be modified in a composite material.