Meaning
The volumetric expansion of a polymer matrix occurs when the material absorbs solvent molecules from the surrounding environment. This physical phenomenon, termed polymer swelling, is driven by the thermodynamic compatibility between the polymer chains and the solvent. It leads to a decrease in the mechanical strength and stiffness of the plastic part.
The extent of this expansion depends on the crosslink density of the resin and the similarity of their solubility parameters.
Solvent Penetration
When an organic solvent penetrates a semi-crystalline polymer, it accumulates primarily in the amorphous regions. This absorption pushes the polymer chains apart, causing localized polymer swelling. The structural changes increase chain mobility, which accelerates the migration of additives out of the polymer.
For this reason, swelling must be minimized in chemical storage containers to maintain structural integrity.
Mechanical Consequence
Swollen polymers experience a measurable reduction in their yield strength and elastic modulus. This degradation can lead to mechanical failure in load-bearing components exposed to oils or solvents. Part designers must select resins that are chemically resistant to the expected environment.
For example, nylon components in automotive engines must resist swelling from fuels and lubricants to ensure reliable operation.
Dimensional Distortion
Swelling alters the dimensions of high-tolerance parts. Moulded gears can jam if they swell. This distortion causes system failure.
Material selection is the primary solution.
