Meaning
Polymer degradation modeling uses mathematical rate equations to predict the decrease in molecular weight of a resin during processing. This mechanism, known as chain scission kinetics, describes how thermal and mechanical energy break the polymer chains. Processors apply these kinetics to predict changes in melt viscosity during compounding.
Cleavage Rate
Rate equations for chain cleavage dictate the speed of peroxide-induced degradation. This reaction follows first-order kinetics with respect to the initiator concentration. Polymer molecular weight decreases exponentially as peroxide concentration increases.
Shear Influence
Mechanical shear in the extruder barrel accelerates the breaking of chemical bonds in high molecular weight chains. High shear stresses stretch and weaken the polymer backbones, making them more susceptible to scission at the centre of the chains. When chain scission kinetics are influenced by both thermal and mechanical inputs, the molecular weight distribution narrows significantly.
Moulders benefit from this shear-assisted degradation because it creates a more uniform melt that exhibits lower die swell during extrusion.
Quality Control
Inconsistent residence time in the extrusion barrel disrupts the degradation process and leads to product variations. Melt flow rate drift occurs when the peroxide reacts unevenly due to temperature fluctuations. Compounding lines use online rheometers to track these kinetics and adjust feed rates in real time.
