The increase in weight, as a percent of initial weight, after a material is immersed in water. Standardized tests for plastics are done according to ASTM D570 or ISO 62, while other standard tests (such as ASTM C1403 for masonry mortars) exist for non-polymeric materials.
The effects of water absorption on plastics tend to be consistent in type, but not necessarily magnitude. Dimensions increase as the material swells. This is an important consideration for precisely mated parts. In addition, the absorbed water has a plasticising effect, leading to an increase in toughness and ductility, at the cost of a reduction in stiffness. These effects are reversible if the material is dried.
Irreversible hydrolysis may also occur, causing degradation of polymer chains. This is mainly a concern for plastics processing, as the reaction happens very slowly at room temperature.
In whitewares, there is a substantial difference in water absorption between vitrified and non-vitrified forms of a material, and that can be used to assess the degree of vitrification of a sample. Whiteware materials with water absorption above 10% are considered non-vitrified.