Meaning
Mass transfer through a solid polymer involves the movement of penetrant molecules through the intermolecular spaces of the polymer chains. Analyzing diffusion kinetics provides the rate at which moisture, gases, or additive molecules migrate through a plastic film or moulded part over time. This analysis governs the barrier performance of packaging materials and the migration of plasticizers from the polymer bulk to the surface.
It does not apply to porous membranes where convective flow dominates the transport process.
Barrier Performance
Packaging films rely on low permeation rates to protect sensitive foods or pharmaceuticals. Semicrystalline polymers like polyethylene terephthalate exhibit slow transport because crystalline regions act as impermeable barriers. High crystalline fractions prolong the shelf life of packaged goods.
Migration Rate
Additives such as antioxidants or slip agents must move to the surface at a controlled speed to remain effective. This rate is determined by the concentration gradient and the molecular size of the migrant. Large molecules migrate slowly through dense polymer networks.
Mechanical Impact
Swelling caused by solvent ingress alters the mechanical integrity of the plastic part. Plasticization of the polymer matrix reduces tensile strength and increases elongation at break. This structural change can lead to premature mechanical failure in pressurized polymer pipes.
