Polyolefin Degradation

Degradation Mechanism

High temperatures and mechanical stresses during melt processing generate free radicals along the polymer backbone. These radicals react with oxygen to form hydroperoxides, which then decompose into highly reactive alkoxy and hydroxy radicals. In polypropylene, this process leads to chain scission, which reduces the molecular weight and increases the melt flow rate.

In polyethylene, it often leads to radical recombination and cross-linking, which decreases the melt flow rate and creates gels.

Processing Problems

Changes in melt viscosity caused by these reactions make process control difficult during injection moulding or extrusion. These fluctuations lead to variations in mould filling, resulting in either flash or short shots. These processing problems increase scrap rates and demand constant operator intervention to adjust moulding parameters.

Prevention Strategy

Primary and secondary antioxidants are compounded into the resin to intercept free radicals and decompose hydroperoxides. These additives extend the lifetime of the resin during both processing and long-term service. Moulders use restabilized compounds to ensure that recycled resins can be processed without losing their physical properties.