Thermal Stability of PET Bottles: Hot-Fill Applications
Hot-fill PET bottles must withstand filling temperatures up to 85°C without deformation. This article examines the thermal performance requirements and design considerations.
Hot-Fill Process Parameters
- Filling Temperature: 82-88°C (typical for juices, teas)
- Hold Time: 30-60 seconds at temperature
- Cooling Time: 2-3 minutes to 40°C
- Vacuum Formation: 0.3-0.6 bar internal vacuum
Material Modifications
- Crystallization: Heat-set process increases crystallinity to 25-35%
- Glass Transition (Tg): Increased from 75°C to 85-90°C
- Thermal Shrinkage: ≤2% at 85°C for 30 minutes
Design Features
- Vacuum panels for controlled deformation
- Reinforced base for hot-fill stability
- Crystallized neck finish (non-shrinking)
- Optimized wall thickness distribution
Performance Testing
✓ Thermal stability: 30 min at 85°C, ≤3% deformation
✓ Top load after hot-fill: ≥150N
✓ Vacuum resistance: no panel inversion at 0.5 bar
✓ Drop test after hot-fill: 1.2m height
Anwendungen
Fruit juices, iced teas, sports drinks, vitamin waters, ready-to-drink beverages with pH <4.5 and Brix <15.
PET or PP for a hot-fill project?
Heat-set PET is not the only answer to a hot-fill requirement, and PP is worth comparing against it before the resin is specified. PP does not need a heat-set step: its melting point sits far above the fill temperature, so a standard blow-molded PP bottle tolerates the fill without the vacuum-panel and reinforced-base geometry a PET bottle needs. The trade is optical — PP is hazy where PET is glass clear — and for a juice or iced tea where the consumer expects to see the product, that trade is usually unacceptable.
PET therefore remains the default wherever clarity matters. The cost of choosing it is a longer process: the heat-set step that raises crystallinity and glass transition, the panel design that absorbs the vacuum formed on cooling, and a base reinforced for the same reason. None of those are optional once the fill temperature rises above the glass transition of unoriented PET, around 75°C; below that line a standard PET bottle is usually sufficient.
When comparing the two, judge the whole package rather than the resin alone. The closure and liner see the same temperature as the contents, so a cap that softens at 85°C undoes either choice. Our comparison of PET, HDPE and PP resins sets out the property differences behind that decision.
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