Meaning
The transport of low molecular weight, fat-soluble additives from a polymer matrix into adjacent fatty or oily media represents a major degradation and contamination pathway. This lipophilic migration occurs when plasticizers, stabilizers, or residual monomers dissolve in contact lipids, driven by chemical potential gradients. The phenomenon is most pronounced in flexible polyvinyl chloride and polyolefins where the polymer chains have high mobility at room temperature.
It establishes the limit for food-contact approvals and medical device compliance because the migrating species can alter the properties of both the polymer and the contact medium.
Diffusive Mechanism
Chemical diffusion rate is governed by the molecular size of the migrant and the free volume of the host polymer. In lipophilic migration, the rate increases with higher temperatures and longer exposure times, particularly when the contact medium is a liquid oil. Low molecular weight plasticizers move rapidly through the polymer matrix to enter the lipid phase.
Choosing high molecular weight or polymeric plasticizers reduces this diffusion rate by anchoring the additives within the entangled polymer chains.
Barrier Performance
Protective surface coatings or multi-layer coextrusions create a high-density path that resists additive movement. When lipophilic migration must be prevented in food packaging, a thin barrier layer of ethylene vinyl alcohol or polyamide is coextruded onto the polyolefin substrate. These polar materials have low free volume and resist penetration by non-polar fat molecules.
Consequently, the additives are locked within the bulk polymer and cannot leach into the packaged product.
Chemical Safety
Regulatory compliance demands extraction testing using food simulant solvents like vegetable oils. If lipophilic migration exceeds established limits, the polymer loses its compliance status. Moulders must then select alternative resin grades to ensure safety.
