Matrix Swelling

Physical Expansion

Increase in the volume of the part can lead to dimensional changes and a loss of mechanical strength. Matrix swelling is particularly common in semi-crystalline polymers like polyethylene when they are exposed to non-polar fats or oils. This change in size can cause lids to fit poorly or seals to leak during transport.

Solvent Interaction

Degree of expansion depends on the chemical similarity between the plastic and the substance it touches. Matrix swelling is more pronounced when the solubility parameters of the two materials are closely matched. This interaction also increases the rate of migration, as the loosened structure allows additives to move more freely.

Dimensional Stability

Retention of the original shape is a critical requirement for precision-moulded components. Matrix swelling can be minimized by choosing resins with a higher crystallinity or by adding reinforcing fillers. Engineers must account for this expansion when designing parts for use in environments where they will be in constant contact with liquids.

The phenomenon is a major factor in the environmental stress cracking of plastic containers.