How TPU Film Hydrolysis Resistance Decides Service Life in Humid Climates
Why Polyether and Polyester Grades Fail at Different Rates, and What to Specify Before You Buy
TPU film hydrolysis resistance is the single property that separates a film that lasts a decade outdoors from one that turns tacky in three summers. Polyester-based grades lose molecular weight in hot, humid conditions far faster than polyether-based grades, and no amount of thickness compensates for the wrong soft segment. The practical decision is therefore a climate decision, not a strength comparison.
Everything below explains the mechanism, the conditions that accelerate it, the test data worth asking for, and how to write a specification that survives contact with a real installation.
What Hydrolysis Actually Does to a TPU Film
Hydrolysis breaks the chemical bonds that hold the polymer chain together, and the damage is irreversible. Water molecules attack the ester linkages in the soft segment, splitting the chain and permanently reducing molecular weight. Once that happens, no drying cycle or surface treatment restores the original film.
The visible sequence is consistent. The surface first dulls and develops a chalky haze, then feels slightly tacky under finger pressure, then shows fine cracks along fold lines, and finally tears at a fraction of its original elongation. Mechanical tests detect the decline long before any of this is visible.
This is why a film can pass incoming inspection at full tensile strength and still fail in service within two seasons. The property that matters is retained elongation after humid ageing, not the number printed on the initial datasheet.
Why TPU Film Hydrolysis Resistance Differs Between Polyether and Polyester Grades
The soft segment decides the outcome: polyether-based TPU resists hydrolysis well, polyester-based TPU resists oil and abrasion well, and very few grades do both at the highest level. The choice is a trade between water stability and mechanical durability, and the application has to pick a side.
Polyether soft segments use ether linkages, which water does not readily attack. That gives polyether grades their reputation in roofing membranes, outdoor covers, cold-chain equipment and any application exposed to condensation for years. The trade-off is slightly lower oil resistance and, in some formulations, a softer feel under sustained load.
Polyester soft segments use ester linkages, which give higher tensile strength, better abrasion resistance and good resistance to fuels and lubricants. They are the right choice for conveyor belting, hydraulic seals, tooling surfaces and dry industrial environments. Put the same grade in a warm damp basement and the ester bonds will hydrolyse.
Polycaprolactone-based grades sit between the two. They hydrolyse more slowly than conventional polyester types but are not equivalent to polyether, and a supplier claiming universal resistance should be asked for the ageing data rather than the marketing line.
| Property | Polyether-based TPU | Polyester-based TPU |
|---|---|---|
| Hydrolysis resistance | High | Low to moderate |
| Abrasion resistance | Moderate | High |
| Oil and fuel resistance | Moderate | High |
| Low temperature flexibility | High | Moderate |
| Microbial attack in soil | Resistant | Susceptible |
The Conditions That Accelerate Hydrolysis
Temperature and sustained humidity do most of the damage, and acid or alkali makes it dramatically worse. A film that survives years of dry heat can fail within months when the same heat arrives with condensation, and a mildly acidic contact medium can cut the service window again.
Reaction rate rises steeply with temperature, roughly doubling for every ten degrees Celsius in the range where TPU is normally used. This is why accelerated ageing uses 70°C or 85°C at high relative humidity: it compresses years of ambient exposure into weeks, but it also magnifies any weakness in the grade.
Trapped moisture is more damaging than ambient humidity. Water that sits between a film and its substrate, or inside a folded edge, keeps the interface saturated and cannot evaporate. Details that allow drainage and ventilation matter as much as the polymer choice.
- Residual moisture in the resin before extrusion, which is why drying control matters at the film line
- Acidic or alkaline cleaning agents left on the surface after installation or maintenance
- Metal ions and residues that catalyse the breakdown reaction
- Continuous tensile or flexural stress, which opens micro-cracks for water to enter
How Hydrolysis Resistance Is Tested
Ask for retained tensile strength and retained elongation after a defined humid ageing cycle, not a pass or fail stamp. The useful data set is a curve at several time points, because hydrolysis is progressive and a single endpoint hides how fast the film is degrading.
Typical laboratory conditions are 70°C at 95% relative humidity, or 85°C at 85% relative humidity for harsher screening. Samples are pulled at 250, 500, 1000 and 2000 hours, and the change in mechanical properties is plotted against time. The shape of that curve tells you more than any single number.
A specification should state the test condition, the exposure time and the acceptance threshold together, for example a minimum retained elongation after 1000 hours at 70°C and 95% relative humidity. Without all three, the claim is not comparable between suppliers.
- Record the grade, hardness and thickness of every sample, because all three shift the result
- Test the finished laminate where relevant, not only the base film, since adhesives and edges fail first
- Keep unaged control samples from the same roll so the comparison is like for like
- Repeat the test on a production roll, because a laboratory plaque and a blown film are not identical
Writing a Specification That Holds Up
A hydrolysis specification is only useful if it describes the environment, not just the material. The same grade can be correct on a ventilated facade and wrong inside a sealed panel, so the environment has to be written down before the grade is chosen.
The table below lists the fields that remove most of the ambiguity. If a supplier cannot answer them with data, the conversation is not yet technical enough to place an order.
| Specification field | What to write |
|---|---|
| Soft segment type | Polyether, polyester or polycaprolactone, stated explicitly |
| Service environment | Temperature range, relative humidity, contact media, drainage |
| Ageing test | Condition, duration and the property measured |
| Acceptance threshold | Minimum retained elongation or strength at that duration |
| Laminate details | Adhesive type, edge sealing and substrate preparation |
Where the Property Matters Most
Roofing, footwear, automotive interiors and washable textiles are the four applications where hydrolysis decides whether the product survives its warranty period. Each one exposes the film to a different combination of heat, water and chemistry, so each one stresses a different part of the specification.
Roof membranes face years of rain, UV and ponding water. A polyester grade may look excellent on day one and then embrittle at seams where moisture is trapped. Polyether grades are the conventional choice, and the lamination detail matters as much as the film, which is why the base film used in roofing and building laminates is usually specified together with the bonding process.
Footwear sees sweat, detergents and repeated flexing. Hydrolysis shows up as midsole cracking and upper delamination, and it often appears first in stock that has been stored in a warm warehouse rather than in the field. Automotive interiors combine heat build-up with plasticiser and cleaning chemical exposure.
Washable textiles are a special case because the film is thin, laminated and mechanically worked in every cycle. A grade that performs well as a standalone sheet can fail quickly once it is bonded to fabric and washed repeatedly.
How Manufacturing Control Supports the Result
Resin drying, melt temperature and line speed decide whether the chemistry that leaves the extruder matches the chemistry that was ordered. Moisture in the feed resin is the most common self-inflicted cause of poor hydrolysis performance, and it is entirely a process control issue.
Huanlong runs blown film, extrusion and foaming across 44 production lines from three bases, with two R&D centres and more than 30 engineers supporting grade development and process tuning. That structure allows a hydrolysis requirement to be handled as a formulation and drying question rather than a post-sale complaint.
Zhejiang Huanlong New Material Science Technology Co.,Ltd operates under ISO 9001:2015, ISO 14001 and ISO 45001, and holds OEKO-TEX® 100, Bluesign® and GRS certifications. Those systems govern batch traceability and process consistency, which is what makes an ageing result reproducible from one order to the next.
The short version: choose the soft segment by climate and contact medium, then prove it with retained elongation after humid ageing on a production roll. Strength data alone will not tell you how long the film lasts.
Ask for the ageing curve, the test condition and the acceptance threshold in the same document.
Frequently Asked Questions About TPU Film Hydrolysis Resistance
Is TPU film waterproof or only water resistant?
A non-porous TPU film blocks liquid water and is used as a waterproof layer, but it is not a barrier to water vapour in every grade. Waterproofing and hydrolysis resistance are different questions: the first is about penetration, the second about chemical stability over time.
Which TPU grade is best for hot and humid climates?
A polyether-based grade is the normal starting point. Where oil or abrasion also matters, a polycaprolactone grade or a blended formulation can be considered, but it should be supported by ageing data rather than assumed.
Can hydrolysis damage be reversed?
No. Chain scission is permanent, so drying a degraded film may temporarily reduce surface tack but will not restore elongation or strength. Replacement is the only remedy once the mechanical properties have fallen below the service requirement.
Does a thicker film last longer in humid conditions?
Thickness buys mechanical margin, not chemical resistance. A thick polyester film will still hydrolyse at the same rate per unit of material, and the surface will degrade on the same schedule. Thickness should be chosen for mechanical duty, not as a substitute for the right grade.
What should be requested from a supplier before ordering?
Ask for the soft segment type, an ageing curve at a stated condition, retained elongation at the intended service duration, and the laminate details if the film will be bonded. A datasheet without those four items does not answer the hydrolysis question.
Send the service environment, the contact media and the expected life, and the grade can be matched to the application instead of guessed. Huanlong can review the requirement against the available TPU film range and propose a construction for testing.
Review the TPU Film Range Discuss TPU Film Hydrolysis Resistance