Hydrolysis Resistance in TPU and PU Coatings

Why PU Coatings Degrade in Storage

A polyurethane is built by reacting a diisocyanate with a long-chain polyol, and it is the polyol that determines how the material ages. The repeating links in a conventional polyester-based polyol are ester groups. Water attacks ester groups, splitting the chain into shorter fragments, and one of the products of that reaction is a carboxylic acid. That acid catalyses further cleavage, so the reaction is autocatalytic: it accelerates itself once it begins, which is why the damage curve looks flat for a long time and then falls away abruptly.

What the User Eventually Sees

The sequence is predictable. First the coating loses molecular weight and softens, and users describe it as sticky or tacky to the touch. Next it loses tensile strength and elongation, so ordinary flexing produces cracks. Finally it delaminates from the base cloth and comes away in flakes, taking any printed branding with it. None of those stages requires abrasion, sunlight or misuse. A stack of rolls eight months past its coating date in a hot warehouse can reach the sticky stage before a single bag is cut.

• Stage one, invisible: molecular weight falls while the coating still looks and tests perfectly.
• Stage two, tactile: the surface becomes tacky and prints a mark when pressed with a thumb.
• Stage three, mechanical: elongation collapses, and the coating cracks along folds and stitch lines.
• Stage four, visible: delamination and flaking expose the base cloth and the barrier fails.
• Nothing in this sequence needs sunlight, dirt or misuse; time, heat and humidity are sufficient.

Distinguish carefully between the two material families while specifying. Polyurethane coating is normally applied to a woven substrate as a liquid that dries into a film, and it is the cheapest way to make a fabric waterproof. Thermoplastic polyurethane is an extruded or calendered film, either laminated to the substrate or used as a monolithic sheet. TPU generally carries better hydrolysis resistance and better abrasion resistance, but only when someone actually chose the right soft segment, which is not written on the datasheet by default.

This is why the storage conversation belongs in procurement rather than logistics. A bag that fails at eighteen months may have been sound when it left the dock, because the clock started at coating date, not at retail. Require lot dating, require a maximum age at assembly, and verify hydrostatic performance on any material held beyond that limit before releasing it to cutting.

Polyester vs Polyether TPU

Ask any two mills for a quote on TPU-coated fabric and you may receive two chemically different products with similar headline specifications. The soft segment is the difference, and there are three families worth knowing. Each trades mechanical performance against hydrolysis resistance, and the exchange rate between them is the central purchasing decision in this article.

The trade-off is genuinely a trade-off rather than a free upgrade. Polyester-based TPU wins on tensile strength, tear resistance and often abrasion resistance, and it costs less, so it is the rational choice for a bag sold into a temperate market with a fast distribution chain. Polyether-based gives up a little mechanical performance to buy years of additional chemical life. If you specify polyether for a product that never sees humidity, you have paid for capability nobody will notice.

Reading Between the Datasheet Lines

Datasheets usually report thickness, weight, hydrostatic head and sometimes abrasion cycles. They rarely name the soft segment, which is the single most predictive line in the document. Ask for it directly, in writing, alongside the coating date. A supplier unwilling to state the soft-segment family either does not know their material or has made a choice they would rather not discuss, and either answer is informative during supplier qualification.

There is a second question worth asking, particularly for products that live outdoors: whether the resin is aromatic or aliphatic. Aromatic grades cost less and yellow under ultraviolet exposure, while aliphatic grades hold color far better. Hydrolysis and ultraviolet behaviors are separate mechanisms and are usually improved by different decisions, so do not assume that a hydrolysis-resistant resin also solves UV. Pair this reading with our broader comparison of TPU versus PVC shell materials.

Test Methods for Hydrolysis Resistance

No buyer can wait years for a qualification result, so the industry screens with accelerated humidity ageing and then interprets the results relatively. The value of a screen is comparative rather than absolute: place candidates side by side with a control fabric whose real-world behavior is already known, and the ranking is reliable even though the absolute numbers are not.

1. Expose conditioned specimens to elevated temperature and humidity, commonly around 70 degrees C and 95 percent relative humidity.
2. Run for a defined duration, typically seven, fourteen or twenty-eight days depending on how demanding the programme is.
3. Measure retained tensile strength and elongation, often referencing coated fabric tensile methods such as ISO 1421.
4. Measure retained hydrostatic head per ISO 811 as the functional criterion that actually predicts leaks.
5. Compare against an unaged control from the same lot and against a reference fabric with known field performance.
6. Repeat at every fabric lot renewal so you can trend results instead of resetting the baseline each season.

Coated-fabric ageing standards exist to structure this kind of exposure; ISO 1419 provides accelerated ageing methods for rubber- or plastics-coated fabrics including high-humidity procedures, and many programmes adapt its principles. Where a buyer wants a broader environmental simulation, MIL-STD-810 also contains humidity exposure methods widely referenced in professional equipment qualification. Neither turns hours into years on its own, and any claim that X days equals Y years of service is a correlation somebody has to prove with field data.

Two practical cautions. First, set your acceptance threshold on retained properties rather than on appearance, because a coating can look untouched while its elongation has halved. A common commercial threshold is retention of around 70 to 80 percent of initial tensile or hydrostatic performance after the agreed exposure. Second, test the full assembly rather than only flat cloth, since seam tape, welding heat history and adhesives each have their own chemical vulnerability and often fail before the coating does.

Whatever protocol you choose, keep it identical across the life of the programme. Consistency turns the screen into a trend line that warns you two seasons early; changing methods every year produces numbers nobody can compare and hides slow deterioration until it becomes a recall. Our outgoing lots are released against AQL 2.5 (ISO 2859-1) with retained-property data attached to the inspection record for every waterproof bag line we build.

Storage and Container Heat Reality

The most damaging environment a waterproof bag experiences is frequently not the river. It is the inside of a steel shipping container. A box standing on a quay in direct sun can reach internal temperatures far above ambient, and if cartons were loaded on a humid morning the moisture stays inside for the entire voyage. Twenty to forty days of that treatment can consume more coating life than several years of ordinary use.

Read the relative rates as an order of magnitude rather than a precision instrument. The underlying rule is that degradation roughly doubles for every 10 degrees C rise, and humidity multiplies whatever the temperature has already done. Those two inputs together explain why identical products sold into Scandinavia and into Southeast Asia generate completely different warranty profiles even when both were made from the same roll.

Controls That Actually Reduce the Risk

Three controls carry most of the benefit and none of them are expensive. First, never load damp cartons; finishing, cleaning and even plain atmospheric humidity inside a carton supply the water the reaction needs. Second, specify desiccant by quantity per container rather than as an optional extra, and count cartons rather than trusting that someone remembered. Third, avoid the hottest shipping windows for tropical routes where your programme has scheduling flexibility, since a few degrees of average interior temperature shifts the outcome measurably.

On the receiving side, add two cheap habits. Record lot and arrival dates for every receipt so inventory rotation is a fact rather than a claim, and set a maximum age from coating to assembly with a retest requirement beyond it. Then apply the same discipline downstream: finished goods should not sit folded under compression in a hot warehouse, because folds concentrate stress at exactly the point where a chemically aged coating gives way first.

Formulation Additives That Help

Chemistry choice is not the only lever. A number of additives delay hydrolytic attack meaningfully, and they are often the most cost-effective upgrade available when switching resin family would break a cost target. Understanding what each one does also stops buyers from accepting marketing language in place of an actual formulation change.

• Carbodiimide-based stabilisers react with the carboxylic acid produced by cleavage, interrupting the autocatalytic cycle that makes hydrolysis accelerate over time.
• Acid scavengers and buffering agents neutralise acidic residues left from polymerisation that would otherwise catalyse attack from inside the film.
• Antioxidants and heat stabilisers protect against oxidative degradation that runs alongside hydrolysis at elevated temperature.
• Ultraviolet absorbers and hindered amine light stabilisers address the separate UV mechanism, which often drives surface chalking and discoloration.
• Biocides and antifungal agents matter for polyether-based systems stored damp, since they resist fungal growth less well than some alternatives.

Use additives honestly and you will not be disappointed. A carbodiimide stabiliser is consumed as it works, so it delays rather than prevents degradation, and a polyester-based coating with a good stabiliser package still loses to a plain polyether-based coating over enough time. The practical pattern is to combine them: specify polyether-based resin where the budget allows, and insist on stabilisation wherever polyester-based resin stays in the programme for cost or mechanical reasons.

One caution about specification language. Asking simply for a stabilised coating invites the lowest-cost interpretation. Ask instead for the additive family, an indicative loading level, and evidence that the supplier actually used it: either a mill declaration naming the additive or your own retained-property data from an ageing screen. Since the additive is invisible in the finished film, evidence is the only way to verify that you received what you negotiated.

Specifying for Humid Markets

Turning all of this into a purchasing decision means mapping destination climate and logistics to a minimum chemistry. The useful output is not one global specification but a tiering rule that lets you spend money where it matters and save it where it does not, without asking anyone on the production floor to make a judgment call.

Whatever tier you land on, four specification lines make it enforceable: name the soft-segment family, state the maximum permitted coating age at assembly, state the retained-property threshold after humidity ageing, and require the same evidence at every lot renewal. Add your inspection level, normally AQL 2.5 (ISO 2859-1) at MOQ 500 pcs per style and colorway, and you have converted a chemistry preference into a contractual requirement.

Do not forget that transportation terms shape who manages the risk. Most export programmes run FOB Xiamen, which means the buyer controls the ocean leg where most of this damage happens. If your route runs through a long tropical transit, write the storage expectation into the shipping instructions rather than leaving it implicit, and treat that clause as part of the material specification for your custom dry bag programme.

Cost of Higher-Grade Formulations

Buyers usually overestimate this cost and underestimate the alternative. Understanding how the premium actually behaves makes the decision much easier, because the incremental cost sits in a narrow part of the bill of materials while the alternative cost arrives years later in a form nobody planned for.

• The premium applies to the fabric and film line rather than to the whole unit, so even a double-digit percentage increase translates into a small share of the finished cost.
• Polyether-based over polyester-based is usually a modest step up, often described by mills in the low double-digit percentage range on the material line.
• Polycarbonate-based sits clearly higher again and is best reserved for premium tiers where the retail price can carry it.
• Stabiliser packages add little per unit and are often the cheapest single improvement available on a polyester-based construction.
• Volume leverage works: standardising one approved chemistry across a programme reduces the premium compared with running several small lots.

Set that against the actual cost of failure. A field failure costs several times the unit cost once credit, freight, replacement, retail goodwill and administrative handling are counted, and it consumes attention disproportionate to the money involved. Even a return rate difference of one or two percent typically dwarfs the resin premium. This arithmetic is why we push buyers toward the better chemistry at the specification stage rather than offering it later as an upgrade.

There is a straightforward way to make the decision with evidence rather than instinct. Quote both versions on one representative style, compare the landed cost difference against your historical return rate for that product family, and let the numbers decide. Ask your supplier to build both samples in the same aesthetics so nothing but the chemistry differs, then use the 6 to 10 working day sampling window to test both before committing the 35 to 50 day bulk run. Our production teams build both variants in the same aesthetics for exactly this purpose.

Warranty Implications

Warranty language should follow from the material decision, not precede it. If you sell into a tropical market on polyester-based coating, a long warranty term is a liability you have priced incorrectly, whereas the same term on polyether-based TPU is defensible. Writing a policy that matches your actual material choice protects both your margin and your credibility.

1. Match warranty duration to the chemistry specified, and review the term whenever you downgrade material for cost reasons.
2. Define degradation caused by storage, cleaning or misuse as excluded, but describe it concretely so claims handling is consistent.
3. Require lot numbers on every claim you process, since lot-level data is what reveals whether a failure is material or environmental.
4. Track claims by destination climate as well as by product, which often reveals the storage route rather than the material as the culprit.
5. Review your terms annually against field data rather than against whatever the marketing team last published.

The administrative discipline matters as much as the wording. Hydrolysis failures arrive in clusters, typically two to three years after the batch was made, so a claim pattern that looks sudden usually reflects a storage event eighteen months earlier. If you cannot connect claims back to a lot and a shipping route, you will spend that season blaming the material when the answer was a hot container, and you will change the wrong thing.

Close the loop with your supplier in writing. Send the lot number, photographs showing the failure mode, storage history and a retained sample, and ask for a root cause statement rather than a replacement offer. Suppliers who maintain their own incoming lot records and ageing data can usually answer in days, and that response quality is one of the better predictors of whether they will still be supporting your programme five years from now. Our team maintains retained samples and lot records for every shipment, and buyers can request hydrolysis screening data directly through the quality documentation request route.

Frequently Asked Questions

Q1: What is the difference between hydrolysis resistance in TPU and PU?

A: Both suffer the same chemical mechanism, because both contain ester links when built on polyester soft segments. The practical difference is that TPU is normally supplied as an extruded or calendered film with better inherent toughness and usually better resistance, while PU coating is applied as liquid that dries to a film. Chemistry still dominates, so always ask which soft segment the specific resin uses.

Q2: Why do bags fail with sticky coating straight out of the box?

A: Because hydrolysis began during storage rather than use. Ester bonds have already started cleaving, autocatalysed by the acid the reaction produces, and the coating has lost molecular weight before anyone opened the carton. Look at the coating date, the storage temperature and the shipping route. This is a chemistry and logistics problem, and no change to design or sewing will prevent it recurring.

Q3: Is polyether TPU always better than polyester TPU?

A: Not always. Polyether wins decisively on hydrolysis resistance and usually on low-temperature behavior, but polyester grades commonly offer higher tensile strength, better abrasion resistance and lower cost. For a temperate market with fast distribution, polyester is often the rational choice. For humid climates, long sea routes or rental fleets, polyether earns its premium several times over in avoided claims.

Q4: How do you test hydrolysis resistance properly?

A: Run an accelerated humidity ageing screen, typically around 70 degrees C and 95 percent relative humidity for seven to twenty-eight days, following the spirit of coated fabric ageing methods such as ISO 1419. Afterwards measure retained tensile strength and retained hydrostatic head per ISO 811, then compare against an unaged control and a reference fabric with known field behavior. Always interpret results comparatively, never as a prediction of service years.

Q5: What temperatures damage PU coating in a shipping container?

A: Interior temperatures above 60 degrees C are realistic for a container standing in direct sun, and the damage comes from combining that heat with trapped moisture. Since degradation roughly doubles for every 10 degrees C rise, a few weeks in those conditions can consume more coating life than several years of ordinary use. Specify desiccant by quantity and never load damp cartons.

Q6: Can additives stop hydrolysis completely?

A: No. Carbodiimide-based stabilisers and acid scavengers interrupt the autocatalytic cycle and buy meaningful additional life, but they are consumed as they work. A stabilised polyester-based coating still loses to an unstabilised polyether-based coating over enough time. Use additives to extend acceptable options where cost rules out a resin change, not as a permanent substitute for specifying the right chemistry.

Q7: How old is too old for PU-coated fabric?

A: There is no universal number, but a working maximum of twelve months from coating date to assembly is common for polyether-based constructions under reasonable conditions, and shorter for tropical routes. Always pair the limit with storage conditions and require a retained-property retest before releasing material beyond it. Lot dating is essential, because without it no age rule can be enforced.

Q8: Does hydrolysis affect the hydrostatic rating before failure?

A: Yes, though not always visibly. Loss of molecular weight shows up first as reduced elongation and tensile strength, sometimes before hydrostatic head measured per ISO 811 changes much. That is why retained-property screening measures mechanical performance rather than relying on appearance, and why a coating can look perfectly sound while having already lost half of its useful life.

Q9: Is TPU coating worth the extra cost over PU coating?

A: For most export programmes, yes, particularly where there is a long ocean leg or a humid destination. The premium applies to the material line rather than the whole unit, so its impact on landed cost is smaller than buyers expect, while the cost of a single round of field failures is larger. Quote both versions on one style and compare against your own returns data.

Q10: What soft segment should I specify for Southeast Asia?

A: Specify polyether-based as the minimum for bag programmes sold into high year-round humidity, and consider polycarbonate-based for premium lines where retail price can carry it. Pair that choice with a maximum coating age, desiccant per container and cool storage at destination. Those controls together matter more than any single upgrade, because chemistry cannot overcome months of heat and trapped moisture in transit.

Q11: Does pH or cleaning chemistry accelerate hydrolysis?

A: Yes. Both strongly acidic residues from manufacture and alkaline residues from some industrial cleaning agents accelerate chain scission. This is why professional laundries can destroy coating faster than the field ever would. If your bags go through rental channels, ask specifically which detergent programme is used and require a compatibility check rather than assuming the material will tolerate whatever cleaning process the operator prefers.

Q12: How does a buyer know a supplier really changed the resin?

A: Ask for written confirmation of the soft-segment family on the mill declaration for every lot, then verify with your own retained-property screen rather than relying on the certificate alone. Additives are invisible in the finished film, so request an indicative loading level too. Suppliers with genuine lot records and ageing data answer this question in days; anyone vague about their own material is telling you something important.

Q13: What warranty term is realistic for a polyether TPU bag?

A: It depends more on the market and storage than on the product, which is honest but unsatisfying, so use your own claims data. Programmes selling polyether-based constructions into temperate regions routinely offer multi-year coverage, while the same product sold through a tropical bonded warehouse earns a shorter term. Review hardware durability separately, because closures often fail before coating does and should be excluded or limited explicitly.

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