Hydrolysis does not announce itself. A paper-mill roller cracks at the surface after two years of damp service, a hydraulic seal turns chalky after one hot-water washdown season, and a batch of polyurethane soles loses tear strength long before hardness changes enough to trigger a durometer alarm. By the time the failure is visible, ester or urethane bonds have already split at the molecular level. The practical question is not whether hydrolysis can happen; it is which formulation and processing levers push it far enough out to match the intended service life.
Where Hydrolysis Starts
The first question is which bond the water attacks. Ester groups are typically the most vulnerable link. When an ester linkage in a polyester soft segment hydrolyzes, it breaks into a carboxylic acid and an alcohol; the acid by-product can autocatalyze further ester hydrolysis, so a polyester-based system can enter a self-accelerating loop [1]. Urethane and urea linkages are comparatively more resistant, which is why backbone chemistry — not just cure completeness — dominates wet-service life.
Hardness is the wrong early warning. Hydrolytic damage begins as chain scission and a slow drop in tensile retention, not as a large hardness shift. A part can still read within specification on a durometer while substantial molecular weight has already been lost.
Soft Segment Choice: Polyether vs. Polyester
The largest formulation decision is the polyol backbone. Polyether soft segments (PTMEG and PPG) are far less susceptible to hydrolysis than polyester soft segments, because the ether linkage is intrinsically more water-stable than the ester [2]. The tradeoff is mechanical and chemical: polyester systems typically deliver better oil resistance, abrasion resistance, and load-bearing, which is why they remain common in hydraulic seals and dry-wear rollers.
For applications that combine wet contact with high oil exposure, polycarbonate diols occupy the premium end: they give among the strongest hydrolysis and oxidation resistance of the common backbones, at a meaningfully higher materials cost [2].
| Backbone | Hydrolysis resistance | Oil resistance | Typical wet-service trade-off |
|---|---|---|---|
| Polyether (PTMEG/PPG) | Good | Moderate | Marine, water immersion, food contact |
| Polyester (adipate) | Low–moderate | High | Hydraulic seals, dry-wear rollers |
| Polycarbonate | Very good | High | High-cost sealing and wet-wear parts |
The selection is not a ranking exercise; it is a cost-performance decision. A polyester part that costs half as much as a polyether alternative may still be the right choice if the wet exposure is intermittent and the specification allows shorter life.
Hard Segment Chemistry in Wet Service
In high-moisture service, hard segment choice is as important as the polyol. MDI–BDO hard segments typically retain properties longer in hot water than TDI–MOCA systems at comparable hard segment content, because the regular, highly packed MDI–BDO domains present a more ordered barrier to water penetration [2]. For the most demanding wet applications, PPDI-based elastomers combine high hardness retention with markedly lower water uptake, at a correspondingly higher raw-material cost.
Hydrolysis failures are most visible in cast elastomers such as rollers and seals that run continuously in wet contact. <How Polyurethane Rollers Are Made From Casting To Cure> covers the casting-to-cure sequence where BDO-cured systems are chosen for exactly this kind of damp service.
Anti-Hydrolysis Additives
For an existing polyester-based formula that already passes oil-resistance and abrasion tests, changing the backbone can be a project. The most common retrofit is the addition of a carbodiimide-class stabilizer at roughly 1–3 wt%. Carbodiimides react with the carboxylic acid groups generated during ester hydrolysis, quenching the autocatalytic loop before it accelerates further chain scission [1].
The dosage is not a fixed number. It depends on the polyester acid value, service temperature, and how much tensile retention the specification demands after aging. Start from the stabilizer supplier’s guidance, then confirm with the same hot-water aging test used for the finished part.
Processing Levers That Protect the Part
Formulation matters, but processing determines whether the chosen chemistry reaches the mold intact. Three levers have an outsized effect on wet-service life.
First, dry the polyol side. Residual moisture in polyols reacts with isocyanate during casting, consuming stoichiometry and generating carbon dioxide. Unreacted water-reactive sites left in a poorly controlled mix can become ingress points later. Vacuum degassing below about −0.095 MPa, combined with consistent tank temperature control, keeps the polyol side within a repeatable moisture window.
Second, hold the index. A one-percentage-point deviation in the component ratio can shift the concentration of unreacted isocyanate or unreacted hydroxyl groups; both can leave softer, more water-accessible regions in the finished elastomer. In one elastomer casting line, dynamic metering within ±0.3% kept the index window narrow enough to avoid under-cured, water-accessible soft segments.
Third, complete the cure. A short post-cure cycle after demold builds the final hard segment order and reduces residual stress. An under-cured part may pass an initial hardness check and still lose tensile retention faster in wet service.
If you are trying to hold a hydrolysis-prone formula inside a tight index window on an existing casting line, the fastest first step is a metering audit. Contact our application engineers at Info@chinahaifeng.com or on WhatsApp at 86 13566296633 with your current ratio deviation, material temperature, and wet-aging results; we can help you decide whether the fix belongs to the formula, the drying system, or the machine.
Testing Hydrolysis Resistance Before You Commit
Run the wet-aging test on the same geometry and hardness as the production part. The standard approach is hot-water immersion, commonly 70–80 °C for 7–14 days, followed by tensile strength and elongation measurement [4]. A frequently used acceptance threshold is at least 70% tensile retention after the aging period, but the exact value must come from the application specification. Water absorption can be measured separately per ISO 62 to compare how much water a given backbone takes up before mechanical testing [3].
Durometer is usually the first check a QC lab runs after aging, but it can mask hydrolysis damage: the surface often reads close to the original value while tensile retention has already dropped. <Shore A Vs Shore D Hardness In Polyurethane> covers what the two scales tell you when comparing aged and unaged parts.
Decision Checklist for Wet-Service Formulas
- Confirm the failure mode before changing chemistry: run a hot-water tensile-retention test, not just a hardness check.
- Match the backbone to the full service environment, including oil, temperature, and cost constraints.
- For polyester systems, evaluate a 1–3 wt% carbodiimide stabilizer first.
- Specify the hard segment (MDI–BDO, PPDI, or TDI–MOCA) for wet contact, not only for hardness.
- Verify polyol dryness, index control, and post-cure completeness on the existing line before blaming the formula.
Talk to a Process Engineer Before the Next Wet-Service Failure
Haifeng Polyurethane Machinery has spent 40 years on R&D for metering precision and turnkey polyurethane production — from low-pressure casting units to high-pressure foaming lines across footwear, automotive, and rail transit. As the sole drafter of China’s PU pouring machine industry standard, we treat hydrolysis failures as a process problem, not a brochure problem.
If you are specifying a new wet-service part or chasing a field failure, send us:
- the current backbone and index,
- service temperature and immersion time,
- current tensile retention versus the specification,
- and your existing machine’s metering accuracy and degassing setup.
Reach us at Info@chinahaifeng.com or on WhatsApp at 86 13566296633, and we will help you separate the formulation issue from the machine issue before you commit to a costly material change.
Frequently Asked Questions About Hydrolysis Resistance
Does polyether polyurethane always outperform polyester in wet service?
Not in every case. Polyether backbones are generally more hydrolysis-resistant, but polyester systems often provide better oil resistance, abrasion, and load-bearing. Polycarbonate offers the strongest wet-service performance of the common backbones but at a higher cost. The right choice depends on the full service environment, not on hydrolysis alone.
How much carbodiimide stabilizer should a formulation use?
A common starting range is 1–3 wt%, but the optimum depends on the polyester acid value, service temperature, and the tensile-retention target. Confirm the dose with a hot-water aging test on the production part rather than accepting a fixed recipe.
What test catches hydrolysis early?
Tensile retention after hot-water immersion is the most revealing early indicator. Hardness checks can miss early chain scission because the surface may remain within specification while tensile strength has already fallen. Water absorption per ISO 62 is useful for comparing backbones before mechanical aging.
Can post-curing improve hydrolysis resistance?
It helps, but only within limits. A complete post-cure builds final hard segment order and reduces residual stress, which supports wet-service property retention. It cannot compensate for a polyester backbone that is fundamentally wrong for continuous immersion.
Does hardness affect hydrolysis resistance?
Hardness alone is a weak predictor. Hard segment content and network order influence water uptake and retention, but two formulas can share the same Shore hardness and behave very differently in hot water. Specify the chemistry and run the aging test instead of relying on hardness.
References
[1] G. Oertel, Ed., Polyurethane Handbook: Chemistry – Raw Materials – Processing – Application – Properties, 2nd ed., Munich, Germany: Hanser Publishers, 1994.
[2] C. Hepburn, Polyurethane Elastomers, 2nd ed., London, U.K.: Elsevier Applied Science, 1992.
[3] ISO 62:2008, Plastics — Determination of water absorption, International Organization for Standardization, 2008.
[4] ASTM D3137, Standard Test Method for Rubber Property—Hydrolytic Stability, ASTM International, West Conshohocken, PA, USA.
If you’re interested, check out these related articles:
Causes Of Shrinkage In Polyurethane Foam How To Control It
How To Calculate Shot Weight In Polyurethane
The Role Of Rise Time In Polyurethane Foaming
