The absorption of water by acrylic resins and other materials

Abstract

The absorption and desorption of water by acrylic resin obeys the mathematical laws of diffusion. This means that there are two physical parameters necessary to define the water absorption or desorption process, namely the diffusion coefficient and the equilibrium concentration. The former parameter only governs the time necessary to saturate or “dry-out” an appliance, so is clearly a physical property of great importance. Temperature has a marked effect on the diffusion coefficient (it increased by a factor of two to three in the temperature range 22.5 °C. to 37.4 °C.), but has no effect on the equilibrium water content. It follows, therefore, that the time required to saturate or dry-out an appliance will depend markedly on temperature. As the water absorption and desorption processes obey the mathematical laws of diffusion, the time required to saturate or dry-out appliances (and hence the time for accompanying dimensional changes to occur) can be predicted. Such times appear to be much larger than those allowed for in practice; for example, the usual forty-eight hours immersion of a crown before insertion is inadequate, especially if cold water is used. Also, the critical dependence of these times on appliance thickness does not appear to be generally realized.