Controlled Rheology Visbreaking Degradation Risks in Food Contact Packaging Resins
Visbroken polypropylene risks organoleptic contamination and thermal degradation; verify residual peroxide, NIAS limits, and OIT before food contact use.
Chemical Degradation
Meaning
Irreversible reduction in polymer molecular weight occurs when polymer chains are broken by exposure to heat or chemical additives. This process of chemical degradation is frequently utilized by compounders to adjust the flow properties of polypropylene resins. Extrusion processes often employ this reaction to convert thick extrusion grades into high-flow injection molding grades.
Reaction Mechanism
Radical-initiated cleavage represents the primary pathway for breaking the carbon-carbon bonds. Hydrogen abstraction creates unstable polymer radicals that undergo rapid beta-scission. This pathway reduces the average chain length without generating excessive branch structures.
Degradation Indicator
Melt flow rate remains the primary measurement for tracking the extent of polymer breakdown. Processors monitor this value during production to ensure that chemical degradation has proceeded to the desired endpoint. A higher melt flow rate indicates that the average chain length has decreased, resulting in a material with lower shear viscosity.
This measurement allows compounders to verify that the modified resin will perform consistently during the subsequent moulding cycle.
Melt Defect
Uncontrolled degradation causes severe losses in the mechanical properties of the moulded part. The reduction in molecular weight diminishes both impact strength and tensile elongation. Moulders often observe flashing or part brittleness when the resin has undergone excessive chemical degradation.