Meaning
The irreversible breakdown of polymer chains during melt processing or end-use service is caused by exposure to heat, shear, or oxygen. This chemical decomposition, known as resin degradation, reduces the molecular weight of the polymer, compromising its mechanical and aesthetic properties. It leads to yellowing, embrittlement, and a measurable increase in the melt flow index.
Processors must control temperature and residence time to prevent this structural damage.
Processing Cause
High temperatures and high shear rates within the extruder barrel are the primary drivers of polymer breakdown. When the melt resides in the barrel for too long during an injection moulding cycle, thermal oxidation occurs. This reaction generates free radicals that attack the polymer backbone, causing chain scission or crosslinking.
To minimize resin degradation, moulders use screw designs that promote gentle mixing and avoid hot spots.
Mechanical Failure
The loss of molecular weight during processing severely degrades the impact strength and tensile properties of the finished part. Degraded parts are highly susceptible to premature failure under mechanical stress. This issue is particularly problematic when using regrind material, which has already undergone multiple thermal histories.
Moulders must limit the regrind percentage to maintain acceptable part performance.
Additive Protection
Formulators add heat stabilizers and antioxidants to prevent resin degradation. These compounds scavenge free radicals during processing. This protects the polymer chains.
Stabilization ensures consistent properties.
