Meaning
Polymer degradation models for polyolefins describe the rate at which carbon-carbon bonds in the polymer backbone break under thermal and mechanical stress. These olefin scission kinetics govern the reduction in molecular weight that occurs during melt processing of polypropylene and polyethylene. Understanding this reaction rate helps compounders anticipate changes in melt flow and physical properties.
Degradation Mechanism
High melt temperatures and intense shear forces inside the extruder barrel generate free radicals along the polymer chains. These radical intermediates undergo beta-scission, which splits the long molecules into shorter fragments. In polypropylene, this scission occurs rapidly at elevated temperatures and results in a narrower molecular weight distribution and lower melt viscosity.
Molecular Weight Change
The rate of chain scission determines the drop in viscosity and the loss of melt strength during recycling and moulding. This change in molecular weight causes a reduction in the impact strength and tensile modulus of the finished part. To counteract these changes, additive packages containing primary and secondary antioxidants are added to the resin to scavenge free radicals before they can propagate the scission reactions.
This stabilization preserves the polymer chains and maintains mechanical integrity.
Process Adaptation
Adjusting the melt temperature and minimizing the screw speed helps limit the reaction rate of chain scission. When using regrind material, the kinetics of degradation must be accounted for by blending the recycled flakes with virgin resin to maintain the required viscosity. Processors monitor the melt flow index of the incoming resin to adjust the moulding parameters and ensure consistent fill rates.
