The short answer
Below about 60°C (140°F) most plastics cope with water. Above that, the polyesters (PBT, PET), polycarbonate and standard nylon start to break down. For hot water pipe, use PEX, CPVC or PP-R. For molded parts, PPS, polysulfone, PPSU and PEEK are the safest choices, acetal copolymer works at moderate temperatures, and hydrolysis-stabilized glass-filled PA66 or PPA handles coolant. For repeated steam sterilization, PPSU lasts longest.
Hot water attacks plastics in three ways, and a resin can pass one and fail another. Every limit below depends on the grade, the stress in the part, the water chemistry and how long it runs, so treat the numbers as a starting point and test parts under real conditions.
Why some plastics fail in hot water
- Hydrolysis. Water splits the chains of polyesters (PBT, PET), polycarbonate and polyamides. It speeds up with temperature and with acids or bases, and it leaves no mark until the part turns brittle.
- Water uptake. Nylon absorbs 8–10% water when saturated, swells and loses much of its stiffness, even before any chemical damage.
- Oxidation by disinfectants. Chlorine and chloramine in tap water attack some resins, and the attack is faster when the water is hot.
- Creep. Every plastic is weaker hot. Pipe and pressurized housings are rated by long-term hydrostatic strength at temperature, not by a short-term heat number.
Plastics to avoid or use with care
PBT and PET: producer literature puts standard grades as stable in water to about 60°C (140°F). Above that, unstabilized grades lose impact strength and elongation. Hydrolysis-resistant grades extend the range, but polyesters are rarely the first choice for hot water. See glass-filled nylon vs. PBT.
Polycarbonate: brief contact with hot water is fine, but continuous use above about 60°C (140°F) is not recommended, and repeated autoclaving crazes it.
Standard nylon 6 and 66: they absorb water, lose stiffness and slowly hydrolyze. Hydrolysis-stabilized glass-filled grades are a different story (see below).
Acetal homopolymer and ABS: homopolymer is weaker than copolymer in hot water, and ABS is limited to about 60–80°C (140–176°F) even in air.
Good choices for hot water and steam
| Resin | Continuous hot water limit (approx.) | Steam / autoclave | Hydrolysis risk | Typical uses |
|---|---|---|---|---|
| PP homopolymer, random copolymer (PP-R) | PP-R pipe ~70°C (158°F) sustained, peaks to ~95°C (203°F); unstressed parts often ~90–100°C (194–212°F) | 121°C (250°F) cycles for unstressed labware; not for loaded parts | None | PP-R hot and cold water pipe, dishwasher and washing machine parts, labware |
| PEX (crosslinked PE) | 82°C (180°F) at 100 psi; 93°C (200°F) at 80 psi (typical ASTM F876 ratings) | No | None; chlorine resistance is rated separately | Potable hot and cold water pipe, radiant heating |
| CPVC | 82°C (180°F) at 100 psi (ASTM D2846 tube) | No | None | Hot and cold water pipe, fire sprinkler pipe, industrial lines |
| Acetal copolymer | ~80–85°C (176–185°F); homopolymer lower, about 60°C (140°F) in some producer data | Not for repeated autoclaving | Low for copolymer; higher for homopolymer | Faucet cartridges, pump and valve parts, coffee machine parts |
| Glass-filled PA66 or PPA, hydrolysis-stabilized | Water-glycol coolant at cooling-system temperatures; unstabilized grades lose strength | Limited | Moderate; lower with stabilizers and in PPA | Radiator end tanks, thermostat housings, coolant pipes, water pump parts |
| PVDF | ~120°C (248°F) unpressurized; lower in pressure pipe, per the system rating | Tolerates steam sanitizing; check the grade | Very low | High-purity and pharmaceutical water, hot chemical lines, pump and valve linings |
| Polysulfone (PSU) | Good at 82°C (180°F) and above; check grade data near the boil | Repeated cycles; fewer than PPSU | Very low | Plumbing manifolds, hot water fittings, dairy and coffee equipment |
| PPSU | Good at 82°C (180°F) and above; the most chlorine-resistant sulfone | Best: 1,000+ cycles at 132–134°C (270–273°F) in producer tests | Very low | PEX fittings, manifolds, sterilization cases and trays |
| PPS, glass-filled | Good well above 100°C (212°F) | Yes | Very low | Pump impellers and housings, water meter parts, valve components |
| PEEK | Hot water and pressurized steam above 134°C (273°F) | Yes; 1,000+ cycles in producer tests | Very low | Medical and dental instruments, valve seats, pump parts |
Approximate values for typical grades, deliberately conservative. Real limits depend on grade, wall thickness, stress, pressure, water chemistry (chlorine, pH) and service life, and pipe ratings apply only to the rated product. Confirm with the data sheet and long-term data for your grade.
Polypropylene and acetal copolymer
Polypropylene has no bonds that water can split, so its limit is softening and creep, not hydrolysis. Random copolymer PP (PP-R) is the grade used for hot and cold water pipe. See polypropylene.
Acetal copolymer is a common choice for faucet cartridges, pump impellers and valve parts at moderate temperatures. It resists hot water and alkalis better than homopolymer. Both are attacked over time by chlorinated water, more so at higher temperatures. Acetal insert fittings in older polybutylene plumbing systems are a well-known example of failures in chlorinated water, although stress and installation also played a part.
Steam and autoclave cycles
Common sterilization cycles are 121°C (250°F) and 134°C (273°F) saturated steam. PPSU keeps its properties through more than 1,000 cycles in producer tests and is the usual choice for reusable trays and instrument parts. Polysulfone handles repeated cycles, but fewer than PPSU. PEEK and PPS take repeated steam as well. PEI also takes repeated cycles, though cycle life varies widely by grade. Cleaning chemicals and steam additives shorten life, so test with your real cycle. See polysulfone and PPSU, PEEK and PPS.
Chlorinated water
Ask for chlorinated-water data at your temperature and chlorine level, not just hot water data. PEX pipe is tested for oxidative resistance to hot chlorinated water under ASTM F2023, and pipe standards assign it a chlorine resistance rating. In one independent lab comparison of sulfones, PPSU was the most resistant and PSU the least, though all did well. PVDF and PPS also have good hot chlorinated water records. See chemical-resistant plastics and PVDF.
Potable water: NSF/ANSI/CAN 61
NSF/ANSI/CAN 61 sets health limits on what pipes, fittings, valves and other drinking water components leach into the water. Listings are for a specific formulation and product at a stated temperature: cold 23°C (73°F), domestic hot 60°C (140°F) or commercial hot 82°C (180°F). A cold listing does not cover hot water. The component maker holds the listing for the finished part, so a listed resin helps but does not transfer automatically. Lead content is covered separately under NSF/ANSI/CAN 372.
Coolant and automotive
Water-glycol coolant is hot water with extra chemistry. Hydrolysis-stabilized glass-filled PA66 is the standard for radiator end tanks and coolant housings, and PPA takes over where it is hotter or wetter. Glass-filled PBT and PET lose strength in hot glycol, so they are not used for coolant contact. See PPA vs. nylon 66.
What to send us for a quote
- The grade you run now, or the part and its function
- Water temperature, pressure and duty cycle, or the steam cycle and number of cycles
- Water chemistry: chlorine or chloramine level, glycol, cleaners
- Listings needed (NSF/ANSI/CAN 61, FDA, other)
- Quantity for this order and per year, and ship-to city
Common questions
What plastic is best for hot water?
It depends on the part. For pipe, PEX, CPVC and PP-R are the usual choices. For molded parts, PPS, polysulfone, PPSU and PEEK hold up best, hydrolysis-stabilized glass-filled PA66 or PPA suits coolant, and acetal copolymer works at moderate temperatures.
Can polypropylene handle boiling water?
Short exposure, yes. PP-R pressure pipe is typically rated for about 70°C (158°F) sustained with peaks to about 95°C (203°F), and unstressed parts often run to about 90–100°C (194–212°F). Long service near the boil under load is beyond most grades.
Which plastics can be steam autoclaved?
PPSU and PEEK are the best for repeated cycles, followed by polysulfone and PPS. PEI also takes repeated cycles, though cycle life varies widely by grade. Unstressed polypropylene labware takes 121°C (250°F) cycles. Polyethylene, nylon and acrylic should not be autoclaved, and polycarbonate degrades with repeated cycles.
Why does PBT fail in hot water?
PBT is a polyester, and hot water splits its ester bonds (hydrolysis). Producer literature puts standard PBT as stable in water to about 60°C (140°F); above that, unstabilized grades lose impact strength and elongation. Hydrolysis-resistant grades extend the limit.
Is acetal OK in hot water?
Acetal copolymer handles hot water to roughly 80–85°C (176–185°F) and is better than homopolymer. Both are attacked over time by chlorinated water, more so as it gets hotter, so ask for chlorinated-water data at your conditions.
What does NSF/ANSI/CAN 61 cover?
It sets health limits on what drinking water components leach into the water. Listings are for a specific formulation and product at a stated temperature: cold 23°C (73°F), domestic hot 60°C (140°F) or commercial hot 82°C (180°F).
Is nylon good for hot water?
Standard nylon absorbs 8–10% water when saturated, loses much of its stiffness and slowly hydrolyzes in hot water. Hydrolysis-stabilized glass-filled PA66 is the usual choice for engine coolant, and PPA for hotter, wetter service.
Tell us the part and the water it sees
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province