Meaning
Chemical degradation pathways during thermal processing of polyester resins produce volatile organic compounds that alter polymer properties. An aldehyde side reaction occurs when polyalkylene terephthalates undergo thermal breakdown at elevated temperatures inside extrusion barrels or injection moulding cylinders. Chain scission releases acetaldehyde or related low molecular weight compounds, causing organoleptic contamination in beverage containers and reducing intrinsic viscosity.
Processing parameters such as melt temperature and residence time control the rate of degradation. The reaction boundary ends where thermal stabilizers effectively passivate ester linkages or where melt temperatures remain below the activation threshold.
Polymer Degradation
Thermal energy breaks ester bonds along the polymer backbone during high temperature processing steps. The aldehyde side reaction accelerates rapidly when melt temperatures exceed processing targets, creating vinyl ester intermediates that cleave into acetaldehyde. Degradation reduces melt strength, causing wall thickness variation during stretch blow moulding operations.
Processing recycled resin exacerbates these reactions due to residual catalyst species and moisture traces.
Volatile Generation
Gaseous degradation products migrate into container cavities and contaminate sensitive liquid contents. Quantifying an aldehyde side reaction requires gas chromatography combined with head-space sampling to measure volatile concentrations in preforms and blown bottles. Off-gas accumulation alters taste profiles even when physical mechanical properties appear intact.
Lowering barrel temperature profiles limits volatile generation without sacrificing plasticating efficiency.
Target Boundary
Quality specifications set strict upper limits for volatile generation in food contact grade resins. A high aldehyde side reaction rate forces moulders to reduce cycle speeds or reject production lots. Chemical scavengers added to virgin or recycled resins bind generated compounds before migration occurs.
Effective stabilization prevents off-taste issues while preserving the intrinsic viscosity needed for structural performance.
