Water absorption characteristics of some unfilled resins.

The water absorption characteristics of resins, upon which many composite filling materials are based, have been studied. In general, the equilibrium uptake is less than that predicted from the water absorption of composite filling materials, if it is assumed that the polymer alone absorbs the water. Also, whereas there are very large differences between sorption and desorption diffusion coefficients in composites, there is very little difference in the case of unfilled resins. Furthermore, sorption in the case of composites is generally very much slower near equilibrium than predicted. These phenomena are all explicable because composite filling materials contain water soluble impurities which enhance uptake, which is ultimately determined by a balance between osmotic and elastic pressures. This implies the existence of internal strains in composites.