Polypropylene Homopolymer and Copolymer Impact Performance Differences
Polypropylene impact copolymers sacrifice flexural modulus to gain sub-zero toughness through dispersed ethylene-propylene rubber domains in an isotactic matrix.
Reprocessing Scission
Meaning
Irreversible breakage of chemical bonds within the backbone of a polymer occurs when the material is subjected to repeated heat and shear cycles. This phenomenon is a primary concern for moulders using regrind or recycled content. Reprocessing scission shortens the average length of the molecular chains and alters the physical characteristics of the resin.
Molecular Reduction
Heat and mechanical stress provide the energy needed to sever the covalent bonds of the polymer. Long chains that once provided strength are reduced to smaller pieces through the mechanism of reprocessing scission. The extent of this damage depends on the temperature of the melt and the residence time inside the machine.
Viscosity Change
Shorter chains slide past one another more easily, which causes a measurable increase in the melt flow rate. This shift makes the material easier to pump but harder to control during the cooling and solidification stages of moulding. Monitoring the melt flow index is the standard way to detect the onset of reprocessing scission in a batch of material.
Performance Loss
Reduction in molecular weight leads to lower impact strength and decreased resistance to environmental stress cracking. Parts made with material that has undergone extensive reprocessing scission are more likely to fail prematurely under load. Success in using recycled resins requires careful blending to offset the effects of this degradation on the final part quality.