Meaning
Mass transfer of low molecular weight substances from a polymer matrix into contact fluids defines the thermodynamic transport mechanism in food-contact plastics. The rate of chemical migration depends on temperature, contact duration, polymer free volume, and initial additive concentration within the container wall. Testing compares laboratory extraction against published specific migration limits to verify regulatory compliance before resin selection.
Matrix Diffusion
Fickian diffusion equations govern how residual oligomers, plasticizers, processing aids, and thermal stabilizers pass through amorphous inter-chain regions. High processing temperatures during injection molding degrade polymer chains, generating light degradation products that accelerate chemical migration during room-temperature storage. When regrind loading increases, volatile thermal breakdown products accumulate within the melt, raising the overall mass flux across the contact interface.
Long thermal exposure in extrusion barrels expands local free volume, allowing volatile additives to travel rapidly toward the part surface.
Partitioning Limit
Thermodynamic equilibrium constants govern the relative concentration between polymer and contact fluid. High partition coefficients reduce total chemical migration even when initial additive loading in virgin material remains high.
Regulatory Barrier
Standardized compliance protocols evaluate total mass transfer using food simulants under controlled time and temperature regimes. Exceeding specific chemical migration thresholds renders a resin batch unusable for food or pharmaceutical packaging, forcing moulders to purge contaminated tooling and source certified virgin grades.
