Water sorption/desorption in polyacid-modified composite resins for dentistry

Abstract

The water sorption and desorption behaviour of three commercial polyacid-modified composite resins used in clinical dentistry have been studied in detail. Cured specimens of each material were subjected to two successive water uptake cycles in an atmosphere of 93% relative humidity, with one intervening desorption cycle in a desiccating atmosphere over concentrated sulfuric acid. Specimens were found to absorb and desorb water according Fick’s law until M_t/M_∞ values of approximately 0.5. Diffusion rates for uptake varied between cycles, ranging from 2.37–4.53 × 10⁻⁹ cm² s⁻¹ for 1st cycle to 0.85–2.72 × 10⁻⁸ cm² s⁻¹ for 2nd cycle. Desorption rates were similar to those for 2nd cycle sorption, and ranged from 0.86 to 5.47 × 10⁻⁸ cm² s⁻¹. Equilibration times for 1st cycle water uptake were greater than for 2nd cycle sorption and for desorption and overall the behaviour of polyacid-modified composites in a high humidity atmosphere was similar to that of conventional composites in water. It is concluded that the hydrophilic components of the former do not bring about an enhanced rate of water transport.