Meaning
Physical measurement quantifies the increase in volume or mass of a resin sample when it is exposed to a specific solvent. The polymer swelling index is a key indicator of the chemical resistance of a material and its suitability for use in seals, gaskets, or fluid-handling parts. A high index value suggests that the solvent is penetrating the polymer chains and weakening the intermolecular forces.
Absorption Mechanism
Liquid molecules move into the free space between the polymer chains, causing the matrix to expand. In the calculation of the polymer swelling index, the sample is weighed before and after immersion until it reaches a stable state. This process is highly dependent on the solubility parameter of both the polymer and the solvent, with the greatest swelling occurring when the two values match.
Mechanical Impact
Swelling often leads to a significant reduction in the hardness and tensile strength of the moulded part. As the polymer swelling index increases, the part may become too soft for its intended function or may fail to fit within its assembly due to dimensional changes. Designers must account for this expansion when specifying tolerances for parts that will be in constant contact with oils or fuels.
Index Variation
Cross-linking in elastomers and some high-performance plastics restricts the ability of the chains to move apart.
