Meaning
Thermal breakdown reactions inside molten polyethylene terephthalate yield volatile organic compounds that alter the organoleptic profile of packaged liquids. This specific chemical degradation, known as acetaldehyde generation, occurs when polymer chains cleave under high heat and shear force during injection moulding or extrusion. Processors monitor this reaction to protect delicate beverages like mineral water from off-flavors.
The formation threshold limits maximum allowable barrel temperatures and residence times during preform production.
Degradation Kinetic
Chain scission along the polyester backbone creates vinyl ester end groups that subsequently re-align into volatile aldehydes. Excess residence time in the injection unit accelerates acetaldehyde generation through sustained exposure to high thermal energy. Thermally degraded virgin resin releases higher quantities of acetaldehyde than properly dried polymer processed at minimal melt temperatures.
Controlling screw speed limits frictional heating during plasticization.
Processing Boundary
Temperature limits within the injection cylinder determine how rapidly polymer chains break down into unwanted side products. Melt temperatures exceeding two hundred and eighty degrees Celsius cause rapid increases in acetaldehyde generation across standard preform cycles. Lowering cylinder temperatures reduces thermal stress, though higher hydraulic pressure becomes necessary to fill multi-cavity moulds.
Melt pumps maintain steady flow without adding shear energy.
Organoleptic Threshold
Sensory contamination becomes noticeable in bottled water when volatile compounds migrate from container walls at concentrations above ten parts per billion. Measuring acetaldehyde generation requires headspace gas chromatography performed on freshly moulded bottle preforms. Datasheet values from resin suppliers reflect ideal laboratory drying conditions rather than real shop floor preform moulding operations.
High regrind content exacerbates off-flavor migration due to cumulative thermal exposure.
