Meaning
Degradation pathways in thermal processing of polyethylene terephthalate yield volatile organic compounds through ester scission. Aldehyde side reactions generate acetaldehyde and related low molecular weight compounds during resin melting inside extrusion barrels. Packaging extruders monitor these chemical pathways because residual volatile species alter sensory performance in food contact bottles and containers.
The boundary where these chemical mechanisms stop governing resin behavior is the solid state below thermal degradation thresholds.
Thermal Cleavage
Molten polyester chains undergo beta-scission when thermal stress exceeds chemical bond stability inside processing equipment. Vinyl ester intermediate groups form rapidly and break down into acetaldehyde under melt temperatures above two hundred eighty degrees Celsius. Screw residence time and local shear heating accelerate chain cleavage rates, increasing volatile concentrations in the output stream.
Sensory Drift
Off-taste generation in packaged water correlates directly with free acetaldehyde concentration.
Melt Mitigation
Thermal stabilizers and scavenger additives reduce volatile aldehyde generation during injection moulding. Phosphite antioxidants suppress peroxide radical formation, which reduces downstream ester scission. Scavenger compounds chemically bind free acetaldehyde inside the molten phase before pelletization or preform moulding.
High moisture levels in processing resin accelerate hydrolytic chain degradation, increasing intermediate species that undergo thermal breakdown. Controlled drying reduces raw material moisture below fifty parts per million, limiting degradation yields during moulding runs.
