Meaning
The direct cleavage of polymer backbone bonds driven by excessive thermal energy during extrusion or moulding leads to a rapid reduction in molecular weight and a loss of physical properties. Thermal scission occurs when the temperature of the polymer melt exceeds the dissociation energy of the carbon-carbon or carbon-heteroatom bonds in the polymer chain. This process generates highly reactive macroradicals that can initiate further chain degradation or cross-linking.
In injection moulding, it is often triggered by excessive residence times in the barrel or high shear heats generated in narrow gates.
Molecular Mechanism
Heat energy breaks the longest polymer chains first, because they are subjected to the highest shear stresses in the flow channel. During thermal scission, the average molecular weight drops, and the molecular weight distribution narrows. This change decreases the melt viscosity, causing the resin to become too fluid to mold properly.
In polycarbonates and polyesters, this hydrolysis and scission are accelerated by any trace moisture in the resin.
Material Degradation
Degraded parts exhibit reduced impact strength, ductility, and environmental stress crack resistance. Because thermal scission breaks the structural backbone of the polymer, the material becomes brittle and prone to catastrophic failure under low loads. This defect is particularly problematic in load-bearing components where part toughness is a primary design requirement.
The moulder will observe a rise in flash, short shots, and part cracking.
Process Optimization
Preventing chain breakdown requires strict control of the melt temperature and barrel residence time. Moulders can minimize thermal scission by optimizing screw speed, reducing back pressure, and selecting the smallest barrel size that meets the shot weight requirement. These adjustments ensure that the polymer is exposed to high temperatures for the shortest possible duration.
