Last updated: 2026-09-02 · 11 min read

Which Resins Survive Hot Water — and What Is Hydrolysis Resistance?

Hydrolysis resistance is a resin’s ability to keep its molecular chain length — and therefore its strength and toughness — when water attacks the polymer’s chemical bonds at temperature, and the resins that survive hot water service are led by PP, HDPE, PPS, PEEK and properly stabilized PBT, while unstabilized PC, PET and aliphatic nylons are the ones that degrade fastest. The failure is silent and cumulative: a part can look perfect while its molecular weight falls, until one day it cracks at a bolt boss after months of exposure that no short-term test flagged.

Key Facts

  • Hydrolysis is chemistry, not washing: at elevated temperature, water molecules cut the ester or amide bonds in the polymer chain. Every cut shortens the chain, and shorter chains mean lower impact strength and embrittlement — the properties fall while the part still looks intact.
  • The susceptibility ranking follows the bonds: polyesters (PC, PET, PBT) and polyamides (PA) carry hydrolyzable backbones; polyolefins (PP, HDPE) and many high-performance resins (PPS, PEEK) do not, which is why they dominate hot-water and steam service.
  • Temperature is the accelerator: hydrolysis that takes decades at room temperature can take months near the boil, and each grade’s data sheet reflects tests at specified conditions — not a universal rating that transfers between applications.
  • Hydrolysis-resistant grades exist for vulnerable resins: polyester and polycarbonate grades with carbodiimide-based stabilizers extend hot-water life substantially, and specifying the stabilized grade — by exact designation, on the drawing — is the single most effective material decision for hot-water parts.
  • Verification is retention-based: exposed parts are compared to unexposed controls for melt flow (chain length), impact strength and dimensions over thousands of hours, because hydrolysis proves itself in retained properties, not in appearance.

    • Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

    What Hydrolysis Actually Does to a Part

    Water alone is nearly harmless to most plastics at room temperature. Heat changes the math: hydrolysis is a chemical reaction whose rate roughly doubles with every ten degrees, so a polymer connection rated for years in warm tap water can be consumed in months in a ninety-degree loop. Pressure and steam accelerate the attack further, which is why washer and sterilization duties are their own category of risk.
    The attack happens at the molecular level — water inserts itself into the ester or amide linkages that hold the chain together and splits them. Nothing is visible at first: the part keeps its color, its gloss, its dimensions. What changes is the molecular weight distribution, and the mechanical property that falls fastest is impact resistance.
    The classic field failure is a housing that survived two years of hot-water cycling and then cracked during a routine service procedure — the embrittlement had been accumulating the whole time, and the final load was modest.

    The compounds most at risk are exactly the ones most specified in consumer and industrial products: polycarbonate lens covers and water-meter windows, PET strapping and housings, PBT connectors near hot surfaces, and nylon fittings that live in continuous wet heat. That does not retire these resins; it means the application owes them either a stabilized grade or a different resin — and that decision belongs in material selection, with the exposure temperature and duty cycle written down, not discovered after the first winter of service.

    The Resin Ranking for Hot-Water Service

    Resin Family Hot-Water Behavior Typical Service
    PP / HDPE Non-hydrolyzable backbone; effectively immune to chain-scission hydrolysis, limited instead by temperature rating Water tanks, plumbing fittings, appliance tubs
    PPS / PEEK Excellent hydrolysis resistance across the hot range; retain properties in steam and continuous hot water Pump internals, sterilizable components, downhole parts
    PBT (stabilized) Hydrolyzable ester backbone, but hydrolysis-resistant grades with carbodiimide stabilizers hold up well in continuous hot water Water-heater connectors, pump housings, sensor bodies
    PA 6 / PA 66 Absorb water and plasticize; amide bonds hydrolyze at temperature, though grades with heat stabilizers manage moderate duty Fittings and clips in intermittent wet heat
    PC (stabilized) Notorious for hot-water hydrolysis; hydrolysis-resistant grades extend life but continuous near-boil service remains risky Warm-water covers and sight glasses with duty limits
    PET / PBT (unstabilized) Fastest embrittlement of the common engineering resins in hot wet service Not recommended for continuous hot water

    Why the Same Resin Name Performs Differently

    Two parts molded from “the same PBT” can age completely differently in hot water, because the hydrolysis story lives in the grade, not the family name.
    Hydrolysis-resistant polyester and polycarbonate grades carry carbodiimide stabilizers that scavenge the carboxyl end groups — the very species that catalyze further chain cutting — and the difference between a stabilized and an unstabilized grade at eighty-five degrees can be a factor of several in retained impact strength. Molecular weight adds a second layer: a higher-intrinsic-viscosity grade starts with longer chains and simply has more length to lose before embrittlement matters.
    Fillers and additives complete the picture — some glass coupling chemistries age poorly in wet heat, and certain flame-retardant packages accelerate degradation, which means the full compound, not just the base resin, is what gets specified and verified.

    This is why a hot-water program’s material decision is written as an exact grade designation on the drawing, with the supplier’s data sheet for that grade attached to the specification. “PBT, hydrolysis resistant” is an intention; the grade number is a contract the supplier’s document actually supports — and the molding program holds the process to that grade, with lot traceability, so a quiet substitution cannot reset the aging clock.

    How Hydrolysis Resistance Is Verified

    1

    Define the duty cycle honestly

    Continuous versus intermittent exposure, the real water temperature at the part — not the boiler setpoint — plus chemistry: additives in the water, detergents, glycols. The test conditions come from this record, and an application that cycles daily between cold and hot tests under cycling, because thermal cycling and hydrolysis compound each other.

    2

    Expose parts, not plaques alone

    Standard plaques rank materials; actual parts carry the weld lines, wall variations and residual stress that decide where degradation shows first. A credible program runs both, with parts molded from the production tool at the production process.

    3

    Measure retention, not survival

    At fixed intervals — commonly thousands of hours — exposed samples are pulled and compared against unexposed controls: melt flow rate as the chain-length proxy, impact strength as the embrittlement signal, plus mass and dimensional checks. A part that “still works” after exposure is not the finding; the retained percentage against control is.

    4

    Tie the verdict to the grade and the lot

    The report names the exact compound, the lot, the process conditions and the aging data together. That binding is what lets a program claim hot-water life with evidence — and what makes a later grade substitution detectable instead of silent.

    Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. See also related guide.

    Design Decisions That Extend Hot-Water Life

  • Remove stress from the wet surface: residual mold-in stress at bosses and corners accelerates environmental cracking; generous radii, balanced packing and annealing where the data supports it all buy margin.
  • Watch the weld lines: a weld line is where molecular chains meet imperfectly, and it is consistently the first place hydrolysis-driven embrittlement opens a crack — fill analysis and gate design keep weld lines off load-bearing wet surfaces.
  • Mind the wall and the gradient: thick sections hold heat longer after shutdown and age faster near the hot side; uniform walls reduce the internal gradient that drives differential aging.
  • Specify the stabilized grade explicitly: the drawing’s material callout carries the full grade designation, and the DFM confirms its supplier document covers hot-water exposure at the stated temperature.
  • Plan inspection around the failure mode: since hydrolysis hides in retained properties, the maintenance plan checks the embrittlement signals — impact-indicating features, torque retention at screws — not just appearance.
  • Respect the temperature ceiling: every stabilized grade still has a duty limit; the specification states the maximum continuous water temperature the design will expose it to, and the resin supplier’s document supports that number.

    • Frequently Asked Questions

    Is hydrolysis the same thing as water absorption?

    No — absorption is water entering the polymer and plasticizing it, which is usually reversible on drying; hydrolysis is water cutting the polymer’s own bonds, which is permanent. Nylon demonstrates the difference: it absorbs a lot of water and recovers, but chain-cut hydrolysis in sustained heat accumulates without recovery. Both matter in hot water; only one is irreversible.

    Can a hydrolysis-resistant grade make PC or PBT safe in boiling water?

    Stabilized grades extend life substantially at moderate hot-water temperatures, but continuous near-boil exposure remains beyond what most polycarbonate grades can promise regardless of stabilization. The honest route is to match the grade’s documented test conditions to the application’s duty — and when the duty exceeds the evidence, move to PPS, PEEK or a polyolefin rather than hope the stabilizer stretches.

    Does glass filling help or hurt hydrolysis resistance?

    The glass itself is inert; the concern is the interface. Some fiber coupling chemistries degrade in wet heat, letting water attack along the fiber-matrix boundary and weakening the compound faster than the neat resin would age. Hydrolysis-resistant filled grades exist with coupling systems qualified for wet heat — the supplier’s hot-water data for that exact grade is the answer, not the generic glass-filled datasheet.

    How long do tests need to run before results mean something?

    Long enough for the melt-flow trend to establish a direction — usually several thousand hours at the application temperature, with intermediate pulls so the degradation curve, not just an endpoint, is on record. Accelerated tests at higher temperature help rank candidates, but they extrapolate by rule of thumb only; the release decision rests on data at or near the real duty temperature.

    Do additives in the water change the picture?

    Significantly: detergents, glycols, chlorides and water-treatment chemicals each shift the chemistry, and some combinations attack resins that pure water would leave alone. The aging program tests in the fluid the part will actually see — or documents plainly that the data covers pure water only, so nobody overclaims.

    What is the most common specification mistake on hot-water parts?

    Naming the resin family without the grade: a drawing that says PBT or PC lets any grade through the door, including ones with no hydrolysis package at all. The fix costs one line — the full grade designation with its supplier document referenced — and it converts hot-water life from a hope into a controlled property.

    Key Takeaways

    1. Hydrolysis is heat-accelerated chain cutting: parts embrittle invisibly as molecular weight falls, so hot-water life is verified through retained properties, never appearance.
    2. Polyolefins, PPS and PEEK carry non-hydrolyzable backbones and dominate continuous hot-water service; polyesters and nylons need stabilized grades and documented duty limits.
    3. The grade, not the family name, decides aging — specify the exact hydrolysis-resistant designation on the drawing, with lot traceability holding substitutions out.
    4. Verification runs real parts from the production tool in the real fluid, measuring melt flow and impact retention against unexposed controls over thousands of hours.
    5. Design multiplies material life: stress-free wet surfaces, weld lines kept off load paths and uniform walls all extend the time before embrittlement reaches a failure threshold.

    Bring aCatMould the water temperature, the duty cycle and the fluid chemistry, and the material recommendation names the exact hydrolysis-resistant grade with its supplier document — then the trial report pairs it with the aging and retention plan, so the part’s hot-water life is a tested property the program can defend, not a datasheet number borrowed from a different application.
    Where the duty outruns the evidence for a candidate grade, the recommendation says so plainly and offers the resin that does carry the data — a quiet answer written down beats a loud failure in the field.

    Sources
    Plastics Technology — hydrolysis-resistant polyester and PC grades
    SPE — polymer degradation and hot-water aging
    MoldMaking Technology — material selection and aging verification

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet