Resolving Saturated Polyolefin Hydrocarbon Oligomer Interference in Mineral Oil Migration Dossiers
Polyolefin oligomer co-elution requires epoxidation clean-up and GCxGC deconvolution to prevent false mineral oil failures in migration dossiers.
Beta Scission Degradation
Meaning
Chain-breaking reactions occurring in the backbone of polymer molecules during thermal or mechanical stress lead to a permanent reduction in molecular weight. Beta scission degradation involves the cleavage of a carbon-carbon bond at the position relative to a radical site on the polymer chain. It is particularly prevalent in polypropylene during extrusion where high shear and heat generate macroradicals.
The process results in a significant increase in the melt flow index of the resin.
Melt Rheology
Shorter chains produced by the reaction decrease the viscosity of the molten plastic. While beta scission degradation might seem helpful for filling thin-wall molds, it drastically reduces the impact strength of the solidified part. Moulders often monitor the difference between virgin resin flow and regrind flow to quantify this damage.
Radical Chemistry
Primary radicals formed by heat or peroxide initiators abstract hydrogen atoms from the polymer backbone to start the sequence. Once a tertiary carbon radical forms, beta scission degradation splits the chain to create a terminal double bond and a new radical. This self-propagating cycle continues until an antioxidant intercepts the reactive species or two radicals terminate.
Stabilizers like hindered phenols are added to prevent this breakdown during the first heat history.
Physical Property
Tensile strength and elongation at break drop as the average chain length decreases. Parts produced from material that has undergone extensive beta scission degradation often exhibit brittle failure under load.