Shrinkage Control in Fibre Reinforced Polymers II: Epoxy Resins with Expanding Monomers

Abstract

Experiments have been carried out with dinorbornene spiro ortho carbonate (DNSOC) to determine the extent to which this expanding monomer can be used to reduce the shrinkage stress in a moderately high Tg epoxy resin. Although the copolymers had good mechanical properties and past experience has shown that they can be used to make excellent fibre composites, the effect of the expanding monomer on shrinkage stress was small. Only about 7% of the monomer could normally be dissolved in the epoxy at 100°C, and this contributed only a very small shrinkage stress reduction. At the same time the Tg was reduced. The lack of useful effect is because high Tg polymers can normally only be produced by curing at high temperatures. During cure, most of the beneficial effect of the expansion disappears due to stress relaxation at the curing temperature. Hence, expanding monomers are not recommended for high performance composites for aerospace use.