Thermal Aging and Shrinkage in PET Films
PET is admissible as thermally persistent only if continuous sub-glass-transition exposure does not accumulate irreversible physical aging sufficient to produce permanent shrinkage or tensile collapse over multi-year residence times.
A polymer film is admissible as persistent only if ordinary sub-Tg thermal residence does not silently convert stored structure into irreversible shrinkage and embrittlement. If time under moderate heat alone is sufficient to collapse dimension or tensile integrity, the film is not stable—it is a delayed failure system.
Thermal irreversibility below overt melting or softening
This entry belongs to the persistence layer because the governing mechanism is not acute overheating, melting, or short-horizon thermal shock. The critical event is slow structural relaxation under sustained moderate heat.
Time is not incidental. Time is the mechanism that turns tolerated residence temperature into irreversible material change.
Moderate continuous heat does not permanently degrade PET
The assumption under test is that PET retains dimensional stability and tensile properties under continuous moderate heat exposure for years.
Physical aging and enthalpic relaxation
The governing mechanism is physical aging under sustained sub-Tg temperature, including enthalpic relaxation and structural rearrangement that gradually drive permanent shrinkage and embrittlement.
The failure does not require melting or obvious thermal abuse. It emerges from long-duration residence within an apparently tolerable thermal regime.
Short tests miss the accumulation pathway
Cumulative relaxation effects can remain invisible in short-cycle or typical accelerated tests, especially when early changes are small and mechanically non-catastrophic.
The governing transition becomes visible only after prolonged exposure, when dimensional drift and tensile loss have matured past recoverable or ignorable levels.
Free-standing PET under continuous sub-Tg heat
- Free-standing PET films
- Constant dry-air exposure at 70–80 °C
- No mechanical loading required
- No humidity cycling or chemical rescue
- Duration: 2–5 years
The system is intentionally ordinary within the thermal regime. The claim is not about extreme abuse. It is about whether moderate continuous heat residence itself contains an irreversible pathway.
Time-to-permanent dimensional and tensile collapse
The governing variable is the elapsed residence time required for sub-Tg heat exposure to produce irreversible shrinkage and tensile degradation beyond admissible stability.
- Early dimensional stability is non-admissible evidence of persistence
- Small early relaxation is still governing structural change
- Final shrinkage or strength loss is the terminal expression of long-silent aging
In the persistence regime, “still intact” is not equivalent to thermally stable if the structure is already relaxing toward failure.
Minimal admissible long-horizon test
Hold free-standing PET films continuously at 70–80 °C in dry air for 2–5 years with no interruption, no barrier rescue, and no multilayer support that could mask intrinsic dimensional change.
The purpose is not to optimize retention. The purpose is to determine whether ordinary sub-Tg residence heat alone is sufficient to create irreversible thermal aging and shrinkage.
What breaks the claim
The claim fails if either of the following becomes true during the persistence interval:
- Greater than 10% permanent shrinkage
- Greater than 40% tensile strength loss
Once either appears, the system has crossed from delayed thermal aging into irreversible structural failure.
Expected failure likelihood under the stated regime
Estimated probability of persistence-level failure under the defined regime: 0.6–0.8
This estimate is not the conclusion. It is the prior expectation attached to the long-horizon test.
What failure would mean
Failure would show that PET under moderate continuous thermal residence cannot be treated as indefinitely dimensionally stable or mechanically reliable simply because the temperature remains below the glass-transition threshold.
Apparent sub-Tg safety would then be revealed as a pre-collapse regime, not true thermal persistence.
SURVIVES
No greater than 10% permanent shrinkage and no greater than 40% tensile loss occur across the full persistence interval.
IRREVERSIBLE FAILURE
Permanent shrinkage exceeds 10% or tensile strength loss exceeds 40%, showing that sustained sub-Tg exposure accumulates into irreversible thermal aging and structural degradation.
A film is not thermally stable because it is below Tg.
If moderate sub-Tg residence can mature into permanent shrinkage or tensile collapse through elapsed time alone, then the apparent stability of PET was always conditional on insufficient time.