Meaning
Chemical breakdown of polymer chains caused by excessive heat or prolonged exposure to high processing temperatures alters material molecular structure. Thermal degradation results in chain scission, cross-linking or volatile generation that compromises mechanical properties and optical clarity of plastic products. The term stops applying to intentional thermal pyrolysis or chemical recycling processes conducted under controlled reactor conditions.
Chain Scission
Elevated melt temperatures break covalent bonds within polymer backbones, lowering average molecular weight. Reduced molecular weight lowers melt viscosity, leading to web instability and reduced impact strength in finished parts. Excessive barrel temperatures in extruders accelerate chain degradation and generate discolored streaks in extruded products.
Melt Temperature
Thermocouples mounted along extruder barrels monitor temperature profiles to keep polymer melt within safe operating windows. Shearing action from high screw speed generates internal viscous heating that can drive melt temperatures above setpoint targets. Proper screw design minimizes localized overheating and prevents thermal damage to temperature sensitive polymers.
Residence Time
Long residence times inside heated extruder barrels exacerbate thermal damage, even at normal processing setpoints. Stagnant melt pockets in dead zones along die manifolds undergo severe thermal breakdown, forming carbonized gel particles that ruin product surface finish. Lab trials with short barrel residence times fail to expose degradation problems that occur when commercial lines experience unexpected line pauses.
Material supplier temperature guidelines represent safe maximums under continuous flow conditions rather than extended hold times. Operating processing machinery without auditing actual melt temperature and residence time distribution risks batch contamination from degraded polymer gels.
