Molten polymers are non-Newtonian fluids with high viscosities, and the interaction between their thermal and mechanical degradation can be complex. At low temperatures, the polymer-melt is more viscous and more prone to mechanical degradation via shear stress. At higher temperatures, the viscosity is reduced, but thermal degradation is increased. Friction at points of high sheer can also cause localised heating, leading to additional thermal degradation.
Mechanical degradation can be reduced by the addition of lubricants, also referred to as processing aids or flow aids. These can reduce friction against the processing machinery but also between polymer chains, resulting in a decrease in melt-viscosity. Common agents are high-molecular-weight waxes (paraffin wax, wax esters, etc.) or metal stearates (i.e.zinc stearate).