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.
Tie-Layer Migration
Meaning
Transport phenomenon known as tie-layer migration occurs when the chemical components of an adhesive resin move from their intended position into adjacent layers of a multi-layer structure. Tie-layers are used to bond incompatible polymers like polyethylene and evoh, and they contain functionalized molecules like maleic anhydride. If these molecules migrate, they can change the properties of the barrier layer or even reach the surface of the part.
This movement is driven by temperature and the concentration gradient between the different material layers.
Interfacial Stability
Bond strength of a coextruded part depends on the concentration of the adhesive species at the interface. When tie-layer migration happens during the moulding process, the bond can be weakened, leading to delamination under mechanical stress. This is particularly problematic in flexible packaging where the layers are thin and the surface area is high.
Engineers must select tie-layer resins with a molecular weight and chemistry that minimize this unwanted movement while maintaining a strong bond.
Safety Implications
Components of the adhesive system can sometimes move all the way through the polymer wall to contact the food. This makes tie-layer migration a subject for safety assessments, especially when using new or highly reactive adhesive chemistries. If the migrating molecules are not approved for food contact, they can render the entire package non-compliant.
Testing involves simulating the shelf life of the product and analyzing each layer to see where the adhesive components have ended up.
Processing Control
Thermal history of the melt during coextrusion significantly influences the rate and extent of this molecular movement. High temperatures and long residence times in the feedblock or die increase the mobility of the tie-layer components. Moulders use careful temperature profiling to keep the resins within their stable range and prevent excessive migration.
Regular testing for bond strength and layer thickness ensures that the tie-layer is performing its role without compromising the integrity of the rest of the structure.