Meaning
Molecular migration through semi-crystalline polymers like polyethylene and polypropylene controls both the permeability of packaging and the rate of additive leaching. The polyolefin diffusion process involves the random thermal movement of penetrant molecules through the amorphous regions of the polymer. It determines how quickly gases, flavor compounds or contaminants move through the container wall.
Understanding this rate is essential for evaluating the shelf life of packaged goods and the progress of polymer recycling.
Crystalline Barrier
Structural density within the polymer matrix restricts the pathways available for migrating molecules. In semi-crystalline resins, polyolefin diffusion occurs almost exclusively within the disordered amorphous regions, as the tightly packed crystalline lamellae are impermeable to most penetrant molecules. High-density polyethylene, which has a higher crystallinity than low-density polyethylene, therefore exhibits lower migration rates.
This difference makes high-density grades more suitable for applications requiring high barrier properties against moisture or gases.
Thermal Effect
Energy inputs during processing and storage accelerate the rate of mass transport within the polymer. Rising temperatures increase the kinetic energy of both the polymer chains and the diffusing molecules, leading to a higher rate of polyolefin diffusion. This temperature dependence is described by an Arrhenius relationship, where minor temperature shifts can lead to large changes in migration speed.
This behavior determines the performance of containers that must withstand hot-filling or sterilization cycles without releasing additives into the food or absorbing flavors from it.
Contaminant Removal
Recycled plastics undergo thermal processing to extract volatile organic compounds. This decontamination step relies on polyolefin diffusion to drive the unwanted substances to the surface where they can be swept away. This extraction is necessary for restoring the material to food-grade purity.
