Polyolefin Hydrocarbon Oligomer Migration Risk Screening
Polyolefin oligomer screening couples GC-FID envelope quantitation with diffusion modeling to clear POSH fractions against the 1.8 mg/kg food threshold.
Amorphous Phase Diffusion
Meaning
Molecular transport mechanism identified as amorphous phase diffusion describes the movement of small molecules through the disordered regions of a semi-crystalline polymer. It is the primary pathway for gas and liquid permeation in materials like polyethylene and nylon. Because crystalline regions are too dense for molecules to pass through, the rate of migration depends entirely on the volume and tortuosity of the non-crystalline areas.
This process determines the shelf life of packaged goods and the chemical resistance of moulded parts.
Morphology Influence
Polymer crystallinity acts as a physical barrier that restricts the available space for molecular transit. While the crystals are impermeable, amorphous phase diffusion occurs in the loose chains that connect these ordered structures. Increasing the degree of crystallinity through controlled cooling or secondary annealing reduces the diffusion rate by narrowing the available paths.
Moulders adjust cooling cycles specifically to manage the ratio between these two phases.
Temperature Dependency
Activation energy required for molecules to hop between free volume sites increases as the polymer approaches its glass transition temperature. Above this point, amorphous phase diffusion accelerates significantly due to increased segmental mobility of the polymer chains. This relationship is critical for products stored in hot environments or those subjected to sterilization cycles.
Designers must account for these fluctuations when selecting resin grades for barrier applications.
Barrier Performance
Selection of high-barrier resins involves choosing materials with low free volume and high molecular orientation. In multi-layer structures, amorphous phase diffusion is slowed by thin layers of specialized polymers like evoh or pvdc. The effectiveness of these layers depends on the continuity of the amorphous regions across the interface.
Monitoring the permeability coefficient during production ensures that the final assembly meets the protection requirements for the contents.