Meaning
Polymer chain cleavage is the scission of covalent bonds within the backbone of a macromolecule that reduces the average molecular weight. When energy input from thermal, mechanical, or oxidative sources overcomes the bond dissociation energy of the polymer chain, the material experiences a permanent reduction in its physical properties. This reduction directly alters melt flow behavior by lowering the viscosity of the resin.
Molecular Degradation
During high-shear processing, such as injection moulding or extrusion, long chains often break into shorter fragments due to mechanical stress. This process results in an increased melt flow rate, which might cause flash or part brittleness if the degradation exceeds the stability limits of the resin grade. Moulders must monitor melt index variations closely because regrind material frequently shows higher rates of chain cleavage than virgin pellets.
Processing Impact
Thermal exposure inside the barrel of a machine accelerates the rate at which individual polymer chains fracture into smaller units. Excessive residence time causes the material to reach a state where the structural integrity of the final part is no longer guaranteed by the initial resin specifications. The consequence of these uncontrolled scissions is a measurable decrease in mechanical performance, particularly regarding impact strength and elongation at break.
Analytical Variance
Datasheet values provide a baseline for molecular stability, yet the actual performance of a resin depends on the specific shear and temperature profile applied during production. A resin might meet its certificate of analysis while in the hopper but fail to retain its structural properties after passing through a high-compression screw. Control over temperature zones remains the primary method to prevent excessive chain cleavage during the moulding cycle.
