Hygroscopic expansion of resin based composites during 6 months of water storage.

Water absorption of composite materials causes an expansion (hygroscopic) which may result in an outward force being generated against the cavity walls. The purpose of this work was to investigate the relationship between this force and the water absorption of the material over a 6 month period of water storage. This investigation was performed by measuring the force required to displace specimens of the test materials from near cylindrical cavities. An outward force developed over a period of a few days of water storage for all test materials. The magnitude of the force was greatest for a microfilled composite, which also had the greatest water absorption. The smallest forces were observed for a hybrid composite. The magnitude of the force was independent of the Young's modulus of the test material. It would appear that expansion due to water absorption is able to rapidly compensate the effects of polymerisation shrinkage. In some cases the forces produced against the cavity walls give rise to some concern for the integrity of the restored tooth.