TPU Hydrolysis Testing: How to Build an Aging Plan for Humidity, Water Immersion, and Real Service Conditions

A TPU grade described as “hydrolysis resistant” may still behave differently when temperature, water exposure, wall thickness, stress, additives, and processing history change. A useful aging plan therefore does more than soak a plaque and inspect it. It compares controlled baseline specimens with aged specimens, measures retained properties, and includes a finished-part test that represents the real service condition.

Quick answer

Build TPU hydrolysis validation in three layers:

1. Controlled humidity aging to study hot, moist air without condensation.
2. Water immersion to study continuous liquid contact at defined temperatures.
3. Real-service testing on production-intent parts with the actual load, assembly, water quality, cleaners, and duty cycle.

At each interval, record mass and dimensions, inspect the surface, and measure the properties that determine failure—often tensile strength, elongation, modulus or stress at a selected strain, tear resistance, and part function. Test some specimens promptly in the wet state and another set after controlled reconditioning. This helps distinguish reversible water uptake or plasticization from damage that does not recover, although confirming polymer-chain degradation may require molecular-weight or chemical analysis.

TPU pellets. The image is contextual and does not identify a specific GOAFLEX grade or hydrolysis result. Photo: Luigi Chiesa, Wikimedia Commons, CC BY-SA 3.0.

What hydrolysis changes in TPU

TPU is a segmented polymer. Its hydrolysis response depends partly on the chemistry of its soft segment, but chemistry names are only a starting point.

In susceptible polyester-based TPU, water can cleave ester bonds. As molecular weight falls, mechanical properties can decline. Higher temperature and sustained moisture generally make this risk more severe. BASF describes irreversible breakdown of polyester chains after lengthy exposure to hot water, water vapor, or tropical climates; Covestro similarly notes that high temperature and high humidity can reduce molecular weight and mechanical properties in flexible polyester TPU.

Polyether TPU is often the first family considered for prolonged wet or humid service because it generally has better hydrolysis resistance. That is not a universal pass. Hardness, formulation, stabilization, pigment or additive package, processing damage, residual stress, geometry, and the actual medium can all change the result. Compare specific grades, not only “polyether” versus “polyester.” For a broader material-selection discussion, see Polyether vs. Polyester TPU for Tubing and Hose.

Hydrolysis after molding is also different from moisture-driven degradation during melt processing. Wet resin can lose molecular weight in the extruder or molding machine before a service-aging test even begins. Control resin drying, transfer, residence time, and melt history so the aging study does not compare specimens with different starting damage. See TPU Drying and Moisture Control.

Separate water uptake from irreversible damage

A specimen can gain water, swell, soften, or change dimensions without permanent chain scission. Conversely, a part can lose critical tensile or fatigue performance while showing little visible change.

Use paired measurements to avoid confusing these effects:

• As-removed or wet-state results show how the part behaves while conditioned by water. Define blotting, transfer time, and test delay.
• Reconditioned results show how much performance returns after a controlled dry or standard-atmosphere step.
• Unaged controls show normal lot and test variation over the same calendar period.
• Optional molecular analysis—such as size-exclusion chromatography, solution viscosity, or another method agreed with the resin supplier or laboratory—can support a chain-scission diagnosis.

Recovery after reconditioning suggests that reversible water uptake contributed to the change. Failure to recover can indicate irreversible degradation, extraction, morphology change, or another mechanism; it is not, by itself, proof of hydrolysis.

Start with the service map, not a favorite chamber setting

Before selecting temperature and duration, write down how water actually reaches the part.

Define the exposure

• Is the part exposed to humid air, splash, condensation, rain, washdown, or continuous immersion?
• What are the normal, maximum, and transient temperatures?
• Is the water deionized, tap, chlorinated, salt-containing, acidic, alkaline, or mixed with detergent, oil, disinfectant, or metal ions?
• Is exposure continuous or cyclic? Does the part dry between events?
• Is the TPU under strain, compression, internal pressure, bending, abrasion, or electrical stress while wet?
• Are cut edges, welds, bonds, inserts, or thin sections exposed?
• What service duration and failure mode matter to the customer?

Water immersion in clean laboratory water is not a substitute for a cleaner solution, seawater, hot chlorinated water, or repeated wet–dry cycling. If the medium is uncertain, obtain the customer’s formulation, safety data, concentration range, pH, and replacement schedule before freezing the test plan.

Environmental chamber shown as a contextual example. Verify that the selected chamber can control and log the required temperature and relative humidity without unintended condensation. Photo: Cjp24, Wikimedia Commons, CC BY-SA 3.0.

Build a three-layer aging matrix

The conditions below are a planning framework, not universal acceptance requirements. Final severities, specimen counts, intervals, and pass/fail limits should come from the application risk, customer specification, resin-supplier guidance, and the laboratory’s controlled procedures.

Layer	Purpose	Example variables to define	Recommended outputs
Baseline and controls	Establish starting performance and test variation	Lot, molding or extrusion settings, conditioning, specimen orientation, control storage	Initial dimensions, mass, hardness, tensile/tear data, appearance, part function
Controlled humidity	Screen hot, humid air exposure without liquid immersion	Temperature, RH, condensation control, airflow, duration, loaded or unloaded condition	Wet-state and reconditioned retention, dimensional change, visual change
Water immersion	Evaluate continuous contact and temperature sensitivity	Water composition, temperature, renewal or circulation, specimen spacing, time	Mass change, dimensions, surface condition, tensile/tear retention, extract or water observations
Service simulation	Test the real failure mode	Finished geometry, mating parts, preload, pressure, flexing, cleaning cycle, contaminants, wet–dry cycle	Leakage, force, fatigue cycles, adhesion, abrasion, electrical or other functional result
Field correlation	Check whether accelerated ranking predicts service	Retained samples, field-return observations, exposure log	Agreement or disagreement between laboratory and actual failure mode

Choose temperatures that answer different questions

A practical program commonly includes a service-representative condition and an elevated-temperature screen. The elevated condition can rank candidates sooner, but it should not be presented as a direct number of years in service unless a validated acceleration model exists.

Temperature can alter water diffusion, polymer morphology, additives, stress relaxation, and the dominant degradation mechanism. Avoid conditions that deform the part or create a failure mode that cannot occur in use. If several temperatures are used to model aging, define the response endpoint and verify that the same mechanism operates across the range before applying Arrhenius-type extrapolation.

Use multiple intervals, not one end point

A single end-of-test result cannot show whether performance changed early, reached equilibrium, or began accelerating. A screening study might use logarithmically spaced withdrawals—for example, initial, 1, 3, 7, 14, and 28 days—then extend or shorten the program based on the trend. These are planning examples, not required durations.

Use independent specimens at each destructive test interval. Include enough replicates to understand test scatter, and define the statistical treatment before testing. Keep retained controls from the same material and production lot.

Control specimen history

Record at minimum:

• resin grade, lot, color, additives, and regrind content;
• drying and closed-transfer conditions;
• process, melt-temperature profile, residence time, and key pressure or output data;
• specimen source, flow or extrusion direction, dimensions, and surface condition;
• annealing or post-conditioning;
• baseline storage and time between production and testing;
• chamber or bath temperature history, RH or medium composition, and interruptions.

Finished-part geometry matters because diffusion distance, cut edges, knit lines, welds, bonded interfaces, residual stress, and local wall thickness can change the result. Standard specimens are useful for grade comparison; production-intent parts are necessary for application decisions.

Circulating water bath shown as a contextual example; it does not show TPU specimens or a prescribed hydrolysis method. Photo: Athikhun.suw, Wikimedia Commons, CC BY-SA 4.0.

Decide what to measure before aging starts

“Pass” should be tied to function. A cable jacket, seal, tube, belt, film, and overmold do not fail in the same way.

Core material measurements

Depending on the part, select from:

• mass and dimensional change;
• visual appearance, cracking, tack, haze, blistering, discoloration, or surface deposits;
• Shore hardness, reported with test conditions and conditioning state;
• tensile strength and elongation at break;
• modulus or stress at application-relevant strain;
• tear resistance;
• compression set or stress relaxation;
• abrasion, flex fatigue, rebound, or adhesion to a substrate;
• melt-flow, viscosity, molecular weight, or chemical analysis when investigating degradation.

Report retained property as well as the absolute value:

Property retention (%) = aged value ÷ initial value × 100

Do not hide a poor starting value behind a high retention percentage. Review initial performance, retained performance, and variability together.

ASTM D570 and ISO 62 provide established frameworks for water absorption of plastics. ASTM states that water absorption can be related to mechanical properties, dimensions, appearance, and electrical behavior, and notes that specimen thickness strongly influences time to saturation. These methods are useful for controlled uptake comparisons, but mass gain alone does not establish hydrolysis resistance.

For tensile testing, use a method appropriate to the specimen and material, such as ASTM D412 for thermoplastic elastomers or an agreed ISO method. Keep geometry, conditioning, temperature, humidity, and test speed consistent. ASTM D412 specifically cautions that tensile results depend on these conditions and that tensile data alone may not represent end-use performance.

Tensile testing machine shown as a contextual example. The photo does not depict TPU or the exact fixture and specimen geometry required for this program. Photo: Cdang, Wikimedia Commons, public domain.

Add an application-level functional test

Material coupons answer “what happened to this specimen?” A functional test answers “does the component still work?” Examples include:

• pressure cycling and burst testing for tubing;
• leakage and retained sealing force for a gasket;
• repeated flexing under wet conditions for a cable jacket;
• peel or adhesion retention for an overmold or bonded film;
• abrasion under the actual counterface, load, speed, and wet contaminant;
• dimensional fit and insertion or extraction force after conditioning.

When the part stays loaded in service, include loaded aging or apply the load immediately after exposure. Unloaded plaques can miss cracking, creep, interface failure, or stress-assisted damage.

How standards fit into the plan

Standards improve repeatability, but no single standard reproduces every TPU service condition.

• ASTM D570-22 / ISO 62:2008: controlled water-absorption measurement for plastics. Use for uptake and dimensional context, not as a stand-alone lifetime claim.
• ISO 175:2010: methods for determining the effects of liquid chemicals on plastics. It is relevant when the “water” includes cleaners, salt, process fluids, or other constituents.
• IEC 60068-2-78:2025: steady-state damp-heat testing for components or equipment at constant temperature without condensation. It can help frame chamber control and reporting, although it is not a TPU-specific hydrolysis standard.
• ASTM D412-16(2021): tensile properties of vulcanized rubber and thermoplastic elastomers. Use the current controlled procedure and report the specimen and test conditions.

Purchase and follow the full current standard where contractual compliance is required. A standard designation in a report is not sufficient unless the specimen, procedure, conditioning, deviations, and equipment controls conform.

Common mistakes that weaken hydrolysis testing

Calling every wet-state change “hydrolysis”

Water uptake, plasticization, swelling, extraction, morphology change, and chain scission can overlap. Pair wet-state and reconditioned measurements, then use analytical work when the mechanism matters.

Using one severe condition and converting hours into years

An aggressive screen can rank candidates, but it does not automatically predict service life. The mechanism must remain representative, and any acceleration model needs evidence.

Comparing grades processed under different conditions

Moisture or excessive residence time during processing can lower the starting molecular weight. Make production history part of the test record.

Measuring only hardness or appearance

A specimen may look normal while tensile, tear, fatigue, adhesion, or sealing performance declines. Select endpoints from the real failure mode.

Ignoring water chemistry and test maintenance

Water quality, pH, additives, evaporation, microbial growth, and bath renewal can change exposure. Define and log the medium rather than writing only “water.”

Testing only a standard plaque

Plaques help compare materials, but they do not reproduce weld lines, residual stress, thin walls, bonded interfaces, or installed strain. Confirm the final choice in production-intent parts.

Hydrolysis-aging plan checklist

Before releasing the protocol, confirm that it includes:

• [ ] a written service map and target failure mode;
• [ ] specific TPU grades, lots, colors, additives, and process histories;
• [ ] baseline conditioning and unaged controls;
• [ ] defined humidity, immersion, and real-service layers where relevant;
• [ ] temperature, RH, water chemistry, circulation or renewal, and exposure intervals;
• [ ] specimen geometry, orientation, edge condition, and installed stress;
• [ ] wet-state handling and test-delay rules;
• [ ] reconditioning rules and separate specimen sets;
• [ ] mechanical, dimensional, visual, and functional endpoints;
• [ ] pre-agreed acceptance limits and statistical treatment;
• [ ] chamber or bath logs, deviations, and retained samples;
• [ ] a field-correlation or service-monitoring step for critical applications.

What to send GOAFLEX for a material and sample review

Send the following to hello@goaflex.com:

• application and drawing, including wall thickness and critical dimensions;
• target hardness, color, transparency, and processing method;
• continuous, peak, and cycling temperatures;
• humidity pattern, immersion or splash condition, and wet–dry cycle;
• water source and any salt, chlorine, cleaner, disinfectant, oil, or pH range;
• load, strain, pressure, flexing, abrasion, and mating materials;
• required lifetime, current failure mode, and acceptance criteria;
• applicable customer or industry standards;
• current resin, process settings, drying history, and test data;
• quantity and form needed for a trial.

GOAFLEX can help narrow the candidate TPU family and prepare samples for a controlled comparison. Final qualification should use the customer’s production process, finished geometry, and service-representative validation plan. For help interpreting supplier data before testing, read How to Read a TPU Technical Data Sheet.

References and further reading

1. BASF, Elastollan® — Thermoplastic Polyurethane Elastomers (TPU): Material Properties; sections on chemical structure, conditioning, chemical properties, and hydrolysis resistance. PDF
2. Covestro, “Material selection and their chemical properties,” including chemical resistance and hydrolysis resistance of TPU. Source
3. ASTM International, ASTM D570-22, Standard Test Method for Water Absorption of Plastics. Official page
4. ISO, ISO 62:2008, Plastics — Determination of water absorption. Official page
5. ISO, ISO 175:2010, Plastics — Methods of test for the determination of the effects of immersion in liquid chemicals. Official page
6. IEC, IEC 60068-2-78:2025, Environmental testing — Part 2-78: Tests — Test Cab: Damp heat, steady state. Official page
7. ASTM International, ASTM D412-16(2021), Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension. Official page

Contextual images do not prove GOAFLEX ownership, endorsement, exact TPU chemistry, test compliance, or grade performance.