How Hydrolysis Ages Polyester and TPU in Waterproof Bags, and How to Specify Against It

What hydrolysis is: water cutting the ester bond

An ester linkage is the bond formed when an acid reacts with an alcohol, and it is the structural repeating unit in polyethylene terephthalate and in the polyester polyol soft segments of a polyurethane. Water attacks that bond and splits it, producing a carboxylic acid at one end and a hydroxyl at the other. Each scission event shortens a polymer chain, and the mechanical properties of a polymer depend directly on chain length. Enough scission events and the material loses tensile strength, tear strength, elongation and eventually cohesion, turning soft, tacky and weak in a way that no cleaning or repair reverses.

Two conditions govern the rate and both must be present. Water must be available, which means elevated relative humidity or direct contact. Thermal energy must be available, because the reaction has an activation barrier and its rate rises steeply with temperature. The practical consequence is the one buyers find surprising: the combination is what matters, and hydrolysis resistance cannot be inferred from a material that has only ever been tested in one of the two conditions. Waterproof bag storage ageing failures are overwhelmingly cases where both were present for long enough, usually in a container, a warehouse or a cupboard rather than in use.

The third factor is chemistry of the environment rather than of the polymer. Hydrolysis is catalysed by both acids and bases, so a slightly acidic or slightly alkaline residue on the surface changes the rate materially. Salt, sunscreen, insect repellent and some cleaning agents all shift the local pH, and a bag used in the sea and then stored damp carries both the water and the catalytic residue. This is why two nominally identical bags in the same warehouse can age at different rates.

• Water alone at ambient temperature is slow, because the reaction rate is thermally limited.
• Heat alone without moisture does nothing, because water is a reactant, not just an accelerant.
• Heat and humidity together is the dangerous combination, and it is exactly what a sealed shipping container provides.
• Acid or alkali residues catalyse the reaction, which is why contaminated bags age faster than clean ones.

Polyester versus polyether TPU: the most expensive knowledge gap here

Thermoplastic polyurethane is a block copolymer built from hard segments and soft segments, and the soft segment is a long-chain polyol of one of two families. If that polyol is a polyester, the material is a polyester-based TPU. If it is a polyether, it is a polyether-based TPU. This single choice, which is almost never stated on a quotation, determines hydrolysis resistance, and the trade is direct and unavoidable: polyester-based grades are stronger, more abrasion resistant, more oil resistant, generally clearer and cheaper, and they hydrolyse. Polyether-based grades resist hydrolysis and microbial attack far better and stay flexible in the cold, and they are softer, weaker in abrasion, more expensive, and they tend to haze.

The consequences in this product category are significant and predictable. A polyester-based TPU dry bag stored damp and warm in a monsoon climate can lose a substantial part of its tensile strength within a couple of years and can go tacky at the surface well before that. The same construction in polyether will typically still be serviceable at the same point. The difference in material cost is real but modest compared with the cost of the failure, and the difference is invisible at sampling because a fresh polyester TPU is the better material on nearly every measured property.

The table is the reason this is called the most expensive knowledge gap in the category: a quotation that says only "TPU" will be supplied as polyester-based, because it is cheaper and better on every property a buyer normally tests, and it will be the wrong choice for any programme that ships to a hot humid market and then sits in a warehouse. Our comparison of TPU and PVC waterproof materials covers the wider selection, and the gauge decision is treated in our TPU film thickness guide.

There is a middle path worth knowing about. A polycaprolactone-based polyester soft segment hydrolyses more slowly than a standard adipate-based one, and a polyester grade with a carbodiimide stabiliser package closes much of the gap. Neither matches a polyether, but both are cheaper than switching and either may be sufficient for a moderate duty and a moderate climate.

Autocatalysis: why the damage accelerates instead of levelling off

Most degradation mechanisms proceed at a roughly constant or slowly declining rate. Hydrolysis does the opposite, and understanding why changes how a buyer should think about early detection. Each scission of an ester bond produces a carboxylic acid end group. That acid group catalyses the hydrolysis of neighbouring ester bonds, so every cleavage event makes the next one faster locally. The acid number of the polymer rises as degradation proceeds, and the rate rises with it.

The practical shape of this is a curve that looks flat for a long time and then falls away. A bag can lose ten per cent of its strength over two years and then lose half of what remains in the following six months. That shape has two consequences worth writing down. First, early testing is misleading in the reassuring direction: a sample that has lost very little at six months tells you almost nothing about month thirty. Second, intervention has a deadline: once the acid number has climbed, the process is self-driving and drying the bag out will slow it but not stop it.

• Ignore results that only cover the early flat part of the curve. Insist on a duration long enough to reach the falling region.
• Treat any measurable surface tackiness as late-stage rather than early-stage, because tackiness means substantial chain scission has already occurred.
• Dry storage slows the reaction but does not reverse it: the catalytic acid groups are already in the polymer.
• A stabiliser works by neutralising those acid groups, which is why it must be present from the start rather than added later.

This is also the reason a hydrolysis stabiliser is a formulation decision rather than an additive that can be topped up. Carbodiimide-type stabilisers react with the carboxylic acid groups as they form, converting them and interrupting the catalytic cycle. They are consumed in doing so, which means a stabilised grade has a finite capacity, and the capacity is what you are buying. Ask for it as a duration at a stated condition rather than as a percentage.

Temperature and humidity together: how to read an accelerated condition

Accelerated hydrolysis testing exposes material to elevated temperature and elevated relative humidity simultaneously and reports property retention against time. The conventional harsh condition uses a temperature around seventy degrees Celsius with relative humidity at or near ninety-five per cent, and milder conditions step down to around forty or fifty degrees with similar humidity. The result is reported as a percentage of the original tensile strength, elongation or hardness retained after a stated number of days.

The single most important thing to understand about these tests is that the acceleration factor is not a fixed number and cannot be converted into years by dividing. The relationship between an accelerated condition and real service depends on the polymer, the stabiliser package, the thickness and the actual service climate, and any supplier who offers a simple multiplier is guessing. What a test is legitimately good for is ranking: if two materials are exposed to the same condition and one retains seventy per cent while the other retains thirty per cent, the first is meaningfully better for hydrolysis resistance in that construction.

The last row deserves attention because it reproduces a real service pattern that steady-state testing misses. When a damp bag dries out, dissolved contaminants and catalytic species concentrate rather than dispersing, and the next wetting cycle starts from a worse position. A cyclic protocol is slower to run and harder to specify, but it correlates with field behaviour better than a steady soak for products that are used intermittently.

The protocols and their honest interpretation are covered in our article on accelerated ageing tests for durability prediction, and the chamber capability and conditioning practice are described in our piece on environmental testing of waterproof bags.

Monsoon storage versus a dry warehouse: a concrete comparison

The clearest way to make this subject concrete is to compare two storage histories for the same bag. The first is a container or non-climatised warehouse in a tropical port city during the wet season: ambient temperatures routinely in the low to mid thirties, relative humidity frequently above eighty per cent for weeks at a time, and internal container temperatures that can exceed fifty degrees when the sun is on the roof. The second is a climatised warehouse in a temperate climate: twenty degrees and fifty per cent relative humidity year round.

For a polyester-based TPU or a polyester fabric with a polyurethane coating, the difference between those two histories is not incremental. Roughly speaking, each ten degree rise in temperature multiplies the hydrolysis rate by a factor of two to three when moisture is not limiting, and the difference between fifty and eighty per cent relative humidity changes the amount of water actually available at the polymer. Combining the two, a year in the first warehouse can be equivalent to several years in the second for the same material. A bag that would have lasted five years in the temperate warehouse can be visibly degraded in under two in the tropical one.

• Container interiors are the worst common environment: solar gain drives temperature while cargo moisture drives humidity, and both peak together.
• The wet season is the risk window. A bag that arrives in the dry season and ships out within weeks may be unaffected.
• Desiccant and vapour-barrier packaging pay for themselves on long tropical transits, and they cost cents per unit.
• Inventory age matters as much as inventory conditions: a slow-moving line in a humid warehouse accumulates risk every month.

The fourth bullet connects to a commercial decision that is rarely made with hydrolysis in mind. Slow-moving stock is ageing stock. A programme that buys a full year of inventory upfront and holds it in a tropical warehouse is making a materials decision as much as a logistics one, and our article on warehousing and inventory turnover sets out the stock-side view. Seasonal demand patterns in humid markets are discussed in our piece on monsoon season travel.

Packaging is the cheapest intervention by a wide margin. A sealed polyethylene liner with desiccant inside the carton, and a carton that is not itself a source of moisture, removes most of the humidity term for the cost of a few cents. Compared with switching the entire material specification to a polyether grade, it is the first thing to try.

Storage is the largest controllable factor, and nobody manages it

Of everything that determines whether a bag survives hydrolytically, storage is both the largest and the least specified. Material selection is fixed at sampling, climate is fixed by the market, and use pattern is fixed by the customer. Storage is the one the buyer controls, and it is almost always left to whoever happens to be holding the goods. Four rules cover it, and all four are cheap.

• Keep goods dry. Below roughly sixty per cent relative humidity the reaction slows dramatically, and a dehumidified room is a trivial cost next to a write-off.
• Keep goods cool. Every ten degrees removed roughly halves or thirds the rate, and ventilation alone achieves several degrees.
• Never pack or store a damp bag. A bag returned from use, a bag washed in a pre-production clean, or a bag caught in rain during loading should be dried before it is sealed into anything.
• Rotate stock and record age. Hydrolysis is a function of time as well as conditions, and an unlabelled pallet has no age and therefore no risk assessment.

The third rule is the one most often broken in factories, and it is worth being specific about why. Goods are frequently packed at the end of a line after a wash-down, or loaded into a container in the rain, or packed while the fabric still carries moisture from a coating process that has not fully dried. The moisture is then sealed in with the product, the container heats up, and the conditions for hydrolysis are created by the packing operation itself. A single rule, that nothing is sealed while damp, removes the majority of this.

There is also a simple diagnostic available to any buyer. A hygrometer in the warehouse and a data logger in one container per shipment cost very little and produce the two numbers that determine whether a hydrolysis failure was a material problem or a storage problem. Without them, every such failure becomes an argument with no evidence on either side.

Polyester fabric and coatings fail by the same chemistry, differently

The discussion so far has focused on TPU, but the base fabric and the coating are frequently the weaker link. Polyethylene terephthalate fabric hydrolyses by the same ester cleavage mechanism, and because a fabric is thin and has enormous surface area relative to its mass, it can degrade faster in practice than a thick film. Polyurethane coatings on polyester cloth contain ester linkages as well, and a coated polyester shell therefore has two hydrolysing components in the same stack-up.

The symptom differs from film failure and is worth learning to recognise. A hydrolysing film goes soft, tacky and weak. A hydrolysing fabric loses strength and abrades away: the surface becomes powdery, the weave loses cohesion, and the material tears at loads it previously carried easily. A coated fabric additionally loses adhesion between coating and cloth, because the degradation is happening at the interface where both materials meet, and that produces flaking or peeling that looks like a bonding defect.

The comparison between nylon and polyester base cloth, which is usually argued on abrasion and UV grounds, also has a hydrolysis dimension that favours neither cleanly: polyamide hydrolyses too, but more slowly than polyester under neutral conditions and faster under acid conditions. Our article on nylon versus polyester waterproof fabrics covers the full comparison.

The bond line row is the one to watch, because it interacts directly with the adhesive work covered in our article on adhesive bonding chemistry and curing. Humidity ageing attacks adhesive interfaces preferentially, so a bonded joint that survives dry heat and fails humid heat is showing interfacial weakness that no dry test would have found.

Hydrolysis stabilisers: what they buy and what they cost

A carbodiimide hydrolysis stabiliser works by reacting with the carboxylic acid groups that ester cleavage produces, converting them to stable products and thereby removing the catalyst that makes the reaction autocatalytic. It is consumed in the process, so its effect is a capacity rather than a permanent property, and the capacity is what a buyer should be asking about. Two mechanisms are in commercial use: monomeric carbodiimides, which are effective and relatively cheap but can migrate and be extracted, and polymeric carbodiimides, which stay in place and last longer at higher cost.

• Ask for stabiliser performance as a duration at a stated temperature and humidity, not as an additive percentage.
• Ask whether the stabiliser is monomeric or polymeric. Polymeric grades resist extraction by water and survive washing.
• Check that the stabiliser package does not conflict with the required compliance declarations for the target market.
• Remember that a stabiliser extends the flat part of the curve; it does not make a polyester behave like a polyether.

The fourth bullet is the honest limit and it is worth stating plainly, because stabiliser claims are sometimes presented as removing the trade-off. They do not. A well-stabilised polyester TPU is meaningfully better than an unstabilised one and still not as good as a polyether in a hot humid climate. Where the duty genuinely requires it, the polyether grade is the answer; where the duty is moderate, a stabilised polyester is usually better value.

Compliance is a real constraint here, because stabiliser packages are part of the chemical declaration for the product. The obligations are summarised in our article on REACH and CPSIA compliance testing, and the declaration practice is discussed in our piece on chemical transparency in manufacturing.

How to read a hydrolysis test report without being misled

Hydrolysis reports are easy to produce and easy to read wrongly, and four conventions cause most of the misunderstanding. Knowing them takes a few minutes and prevents most bad decisions.

• Retention is reported against the unaged value of the same material. Ninety per cent retention of a weak material is still weak; ask for absolute values as well as percentages.
• The property chosen changes the picture. Hardness often holds while elongation collapses, and elongation is usually the sensitive one.
• The duration matters more than the condition. A fourteen-day result at severe conditions may sit entirely in the flat part of the curve.
• Conditioning before testing is not neutral. A sample dried out and tested at ambient can partly recover and flatter the result.

The second bullet is worth expanding, because it is the most common way a poor result is presented as acceptable. As hydrolysis proceeds, the polymer first loses elongation at break, then tensile strength, then hardness and cohesion. A report quoting only hardness retention will look fine well into serious degradation. Insist on elongation and tensile retention together, and treat a large gap between them as a warning rather than as noise.

Independent verification is inexpensive and worth doing on any programme with humid-market exposure. Laboratories such as SATRA run humidity ageing routinely, and the conditioning procedures are standardised by ISO with parallel methods published by ASTM International. Ask for the method, the condition, the duration, the properties reported and the absolute values, and you will have something that can be compared between suppliers.

Telling hydrolysis apart from UV and oxidation on a returned bag

Three degradation mechanisms produce superficially similar damage on a waterproof bag, and they have completely different fixes, so misdiagnosis is expensive. Hydrolysis needs heat and moisture, attacks the interior of the material as well as the surface, and produces softening, tackiness and loss of strength. Photo-oxidation needs light, affects only the depth ultraviolet penetrates, and produces chalking, fading and surface crazing. Thermal-oxidative degradation needs heat without much moisture, and produces embrittlement and discolouration.

The first row is the signature pattern and it is diagnosable without instruments: if the material is degraded through its full thickness rather than at the surface, water was involved, because light cannot penetrate and heat alone produces brittleness rather than tackiness. A buyer holding a returned bag can make this call in a minute with a knife and a fingernail.

Ultraviolet behaviour is covered in our article on UV resistance in outdoor bag materials, and the thermal picture in our piece on cold and heat resistance. In practice the two often act together on an outdoor bag, and the honest framing is that hydrolysis is the mechanism that dominates whenever the product is also stored damp.

How the other bag materials compare on hydrolysis

Material selection for a humid market is not only a TPU question, and the comparison is useful because the ranking does not match the ranking on any other property. Polyolefins have no hydrolysable linkage at all and are effectively immune. Polyamide hydrolyses slowly. Polyester and polyester-based polyurethane are the vulnerable ones. Polyvinyl chloride does not hydrolyse but loses plasticiser, which is a different failure that produces superficially similar brittleness.

• Polyethylene and polypropylene: no ester linkage, essentially immune to hydrolysis. Limited by other properties, especially weldability and feel.
• Polyamide: hydrolyses, but more slowly than polyester at neutral pH and faster under acidic conditions.
• Polyethylene terephthalate: vulnerable, and more so in thin fabric than in thick section.
• Polyester-based TPU: the most vulnerable common bag film material.
• Polyether-based TPU: resistant, and the default answer for humid duty.
• PVC: does not hydrolyse, but plasticiser loss produces brittleness that is easily mistaken for it.

The last row matters because misdiagnosis is common here. A PVC bag that has gone stiff and cracked after years in a hot climate has lost plasticiser, not chain-scissioned, and the two have completely different prevention strategies. Our article on plasticiser migration and PVC ageing covers that mechanism in detail.

The recycled and bio-based routes are worth a note because they are increasingly specified. Recycled TPU and bio-based TPU follow the same chemistry as their virgin equivalents, which means a recycled polyester-based grade carries the same hydrolysis risk, and sometimes more, because prior thermal history can have consumed part of the stabiliser package. Our pieces on recycled TPU performance and bio-based TPU cover the wider trade-offs.

Specifying hydrolysis resistance: conditions, duration and pass criteria

A hydrolysis requirement is only meaningful with four elements: the condition, the duration, the property and the threshold. Written without any one of them, it is a slogan. The structure below works and can be quoted against directly.

• Name the material chemistry explicitly, including whether the TPU soft segment is polyester or polyether. This is the single most important line.
• State the exposure condition in temperature and relative humidity, chosen to match the real storage and use climate rather than copied from a datasheet.
• State the duration, long enough to reach the falling part of the curve rather than the flat part.
• State the properties to be reported: elongation at break and tensile strength at minimum, hardness as supporting evidence only.
• State the threshold as a percentage retention and, better, as an absolute minimum value the material must still meet.
• State the conditioning before testing, and require that samples not be dried out first unless that matches service.
• Require a stabiliser declaration: type, whether monomeric or polymeric, and the claimed capacity in hours at a stated condition.
• Require the same test on the finished construction, not only on the raw film, because a weld and a bond line are part of the system.

The eighth line is the one that catches the failures that actually happen. A raw film can pass a humidity ageing test comfortably while the finished bag fails, because the weakest element is the adhesive interface, the coating-to-fabric bond, or the heat-affected zone beside a weld. Testing the construction captures all three and costs very little more.

The final commercial point is about matching the requirement to the market rather than maximising it. A programme shipped to Northern Europe and sold within a season does not need a polyether grade, and specifying one wastes money. A programme shipped to a tropical market, held in a non-climatised warehouse and sold over two or three years does. The specification should follow the actual distribution chain, which means the buyer has to know what that chain is.

If you want a material selected against a specific distribution climate, send the destination, the expected storage conditions and the intended shelf life, and the chemistry can be chosen against those rather than against a datasheet. You can see how a programme moves from first enquiry through sampling into bulk production, and every style we produce starts at 500 pieces minimum, with samples in 6–10 working days and bulk in 35–50 days, quoted FOB Xiamen.

Frequently Asked Questions

Q1. What is hydrolysis in a waterproof bag?

The chemical cleavage of ester bonds by water, which shortens polymer chains and reduces strength. It needs heat and moisture together, and it produces softening, tackiness and loss of tensile and tear strength.

Q2. What is the difference between polyester-based and polyether-based TPU?

Polyester-based grades are stronger, more abrasion and oil resistant, clearer and cheaper, but they hydrolyse. Polyether-based grades resist hydrolysis and microbial attack and stay flexible in cold, but they are softer, weaker and cost more.

Q3. Why does a bag fail hydrolysis when it has never been used?

Because storage is the dominant exposure. A sealed container in a hot humid climate provides heat and moisture together for weeks or months, which is exactly the condition the mechanism needs.

Q4. Does humidity alone cause hydrolysis?

Very slowly at ambient temperature. The rate depends strongly on temperature, so humidity without heat is a much smaller risk than the two together.

Q5. How much does temperature affect the rate?

As a working approximation, each ten degree rise roughly doubles to triples the rate when moisture is not limiting. That is why a tropical container is so much more aggressive than a temperate warehouse.

Q6. What does autocatalytic mean here?

Each bond cleavage produces a carboxylic acid group that catalyses further cleavage. The reaction therefore accelerates, so the damage curve stays flat for a long time and then falls away sharply.

Q7. What are the first visible signs of hydrolysis?

Loss of elongation, then softening and surface tackiness, then loss of strength and cohesion. Tackiness is a late sign, which means substantial degradation has already occurred.

Q8. Can a bag be saved once hydrolysis has started?

No. Drying slows the reaction but does not remove the acid groups already formed. The process is self-driving once the acid number has climbed.

Q9. What does a carbodiimide stabiliser do?

It reacts with the carboxylic acid groups produced by cleavage, removing the catalyst and interrupting the autocatalytic cycle. It is consumed as it works, so it provides a finite capacity rather than permanent immunity.

Q10. Does a stabiliser make polyester TPU as good as polyether?

No. It closes much of the gap and is often the best value for moderate duty, but a polyether grade remains the right answer where humid exposure is severe or the shelf life is long.

Q11. How should I read a hydrolysis test report?

Look for absolute values as well as percentage retention, insist on elongation and tensile rather than hardness alone, check the duration reaches the falling part of the curve, and confirm the conditioning before testing.

Q12. How do I tell hydrolysis from UV damage?

Hydrolysis degrades the full thickness and produces softening and tackiness. UV damage is shallow, chalky and faded. Cut a section: if the core is as weak as the surface, it was water.

Q13. Does PVC suffer from hydrolysis?

No. PVC degrades by losing plasticiser, which produces stiffness and cracking that looks similar. The prevention strategies are completely different, so the distinction matters.

Q14. Does polyester fabric hydrolyse as well as TPU film?

Yes, and in practice often faster, because a thin fabric has a large surface area relative to its mass. It loses strength and becomes powdery rather than going tacky like a film.

Q15. What is the cheapest way to reduce hydrolysis risk?

Control storage. Keep goods dry and cool, never seal a damp bag into packaging, use desiccant in long tropical transits, and rotate stock with recorded ages.

Q16. Should I specify polyether TPU for every humid market?

Only where the duty justifies it. A programme shipped to a temperate market and sold within a season is better served by a cheaper polyester grade and good storage, and the specification should follow the real distribution chain.

Q17. What should a hydrolysis specification contain?

The chemistry named as polyester or polyether, the exposure condition, the duration, the properties to report, the retention threshold and an absolute minimum value, the conditioning, a stabiliser declaration, and testing on the finished construction rather than the raw film.

People Also Ask

What is hydrolysis in bag materials?

Water splitting the ester bonds in polyester and polyester-based polyurethane. It needs heat and moisture together and causes softening, tackiness and loss of strength.

Which TPU resists hydrolysis better?

Polyether-based TPU, by a wide margin. Polyester-based TPU is stronger and cheaper but vulnerable, and the difference is rarely stated on quotations.

Can hydrolysis happen in storage?

Yes, and storage is the most common route. A hot humid container or warehouse provides exactly the combination of heat and moisture the reaction needs.

How do you slow hydrolysis down?

Keep goods dry and cool, never pack damp, use desiccant in tropical shipping, and specify a polyether or stabilised grade for humid markets.

What are the signs of hydrolytic damage?

Loss of stretch, then surface tackiness, then softening and loss of strength through the full thickness. Tackiness means the damage is already advanced.

Is hydrolysis the same as UV damage?

No. Hydrolysis affects the whole thickness and softens the material. UV damage is surface-only, chalky and faded, and needs light rather than moisture.