Which plastic survives heat? Tg, HDT and service temperature

At a glance: the temperature numbers our profiles print

What is the difference between glass transition, heat deflection and service temperature?

They answer three different questions. Glass transition temperature (Tg) is where the amorphous part of the plastic goes from hard to rubbery. Heat deflection temperature (HDT) is where a test bar bends by a set amount under a set load. Maximum service temperature is a limit a supplier or standard gives for running hot over a stated time. This is general engineering knowledge, not in our data; the numbers that follow are.

Tg is not the ceiling. Polyvinylidene fluoride has a Tg of -40 to -38 °C and deflects at 125 to 140 °C at 0.45 MPa. Perfluoroalkoxy alkane (PFA) deflects at only 91 °C at 0.45 MPa, yet its Vicat softening temperature is 287 °C and it is rated to 260 °C. The Polytetrafluoroethylene (PTFE) profile lists -103 °C, but that is one low-temperature transition quoted from a paper, and other references put PTFE transitions higher (general engineering knowledge, not in our data), so we do not lean on it.

Amorphous plastics tested at 1.8 MPa deflect below Tg. ULTEM 1000 resin deflects at 201 °C with a Tg of 217 °C; Polysulfone (PSU) deflects at 174 °C against a general Tg range of 190 to 230 °C; PC deflects at 123 °C against 142 to 144 °C. Tg is an upper bound, not a use limit. Polybenzimidazole (PBI) breaks the pattern: its 435 °C sits above its Tg of 415 to 425 °C, and it comes from a film listing, so we do not know whether it is a standard bar test.

Tg also depends on how it is read: the Polyetheretherketone (PEEK) profile gives 150 °C as a DSC midpoint and 143 °C for the onset. Service temperatures are the least comparable of the three. The Polytetrafluoroethylene (PTFE) sheet gives 260 °C for 20,000 h, PBT has 200 °C for short-cycle operation only, and PA 66 has a temperature index of 118 °C for 5000 h and 101 °C for 20000 h at 50 % tensile loss.

The melting point is the ceiling for a semicrystalline plastic, not the limit. Polyetheretherketone (PEEK) melts at 343 °C and decomposes above 450 °C, but our profile gives no deflection or service temperature. PA 66 melts at 255 to 262 °C yet deflects at 70 to 75 °C at 1.8 MPa. Perfluoroalkoxy alkane (PFA) melts at 302 to 310 °C, PBT at 223 to 230 °C and PET at 254 to 265 °C. For ETFE two documents disagree (265 °C on the datasheet, 210 to 230 °C on the safety sheet).

A note on the at-a-glance table: a blank cell means the value is not in the profile, not that nothing is known. Heat deflection is left out of the table because it depends on the load; the 0.45 MPa and 1.8 MPa values, each with its load, are in the text.

Which high-temperature plastic has the highest heat deflection temperature?

Only within one load. At 0.45 MPa our data ranks Polybenzimidazole (PBI) at 435 °C, then ULTEM 1000 resin at 210 °C, PBT up to 165 °C, Polyvinylidene fluoride at 125 to 140 °C and Perfluoroalkoxy alkane (PFA) at 91 °C. The PBI figure is a film listing, not a confirmed bar test. The 0.45 MPa and 1.8 MPa figures cannot be compared, so the chart shows 1.8 MPa only.

At about 1.8 MPa the leaders are Polyetherimide (PEI) (173 to 201 °C) and Polysulfone (PSU) (174 °C for Udel P-1700). The 173 °C is the Ultem 9085 printing grade, a printed-part datasheet; 9085 is a PEI and polycarbonate blend (general engineering knowledge, not in our data), so check the 173 °C against the maker's sheet. Only ULTEM 1000 resin beats PSU on deflection. Deflection is not a use limit: the PEI profile lists continuous ratings of 170 °C for PEI and 160 °C for PSU, with no load stated.

Below that: PC 123 °C, Polyoxymethylene (acetal), POM 95 to 101 °C, PA 66 70 to 75 °C. PBT needs care: for one grade (Ultradur B 4520) it shows 165 °C at 0.45 MPa and 55 °C at 1.8 MPa; a 150 °C short-period figure comes from another source and grade. The Polyamide 6 (nylon 6) (PA 6) profile prints 200 °C at 1.8 MPa for one unfilled grade, which is implausible next to nylon 66 (general engineering knowledge, not in our data), so it is treated as a probable data error and not drawn. The Polyetheretherketone (PEEK) profile has no deflection temperature, and we do not guess one.

Which plastics can run continuously above 200 °C?

Only Polytetrafluoroethylene (PTFE) and Perfluoroalkoxy alkane (PFA) have a rating above 200 °C with a stated basis, 260 °C: PTFE names 20,000 h on one sheet and PFA is continuous service in air. FEP is also above 200 °C at 204 °C, from a single encyclopedia infobox. Polybenzimidazole (PBI) is printed as 260 to 400 °C on a film listing and 310 °C long term on a data sheet, with no hours given, so we do not call it a continuous rating.

Kapton HN polyimide film needs a caution. The maker states 400 °C only as the highest temperature of successful use for the bare film, not a continuous rating, and the safety sheet lists temperature above 400 °C as a condition to avoid.

Lower entries are about the clock. PBT reaches 200 °C only in short-cycle operation and has a temperature index of 135 °C for 20000 h. PET is rated 150 °C as uncoated film in air, loses about 10 % of its tensile strength after 30 min at 220 °C and shatters after under a minute at 235 °C. ETFE is rated 150 to 160 °C. For a part that must last years, ask for the hours behind the temperature.

What should I ask for at each temperature band?

Below 150 °C and dry: PC, Polyoxymethylene (acetal), POM, PA 66, PBT and PET are the grades our data covers. Ask for heat deflection at your load, the moisture state for nylon, and hot-water exposure for the polyesters.

From 150 to 200 °C: Polyetherimide (PEI) and Polysulfone (PSU), with continuous ratings of 170 °C and 160 °C, and ETFE at 150 to 160 °C. Ask for a creep or stress-against-temperature curve. Polyetheretherketone (PEEK) has a Tg of 150 °C and melts at 343 °C, but our data gives no deflection or service temperature, so ask the supplier for both.

Above 200 °C: Polytetrafluoroethylene (PTFE) and Perfluoroalkoxy alkane (PFA) for light loads or harsh chemicals, Polybenzimidazole (PBI) for stiffness, and Kapton HN polyimide film as film. PBI stock is sintered or compression-moulded rather than injection-moulded (general engineering knowledge, not in our data), and our profiles hold no cost or availability data for it.

What can our data say about strength when the plastic is hot?

Little, and we say so rather than fill the gap. The one profile with a direct hot-against-cold pair is Kapton HN polyimide film: tensile strength 231 MPa at 23 °C and 139 MPa at 200 °C, tensile modulus 2.5 GPa and 2.0 GPa, yield point 69 MPa and 41 MPa. Our data stops at 200 °C.

For the others the data gives room-temperature values and a rating, not a curve. Yield stress and tensile strength are different measures, so do not rank them together. Polyetheretherketone (PEEK): 4,000 MPa modulus and 98 MPa yield stress. Polyetherimide (PEI) (ULTEM 1000 resin): 3.2 GPa and 110 MPa yield strength. Polysulfone (PSU) (Udel): 2.48 GPa and 70.3 MPa tensile strength. Polybenzimidazole (PBI) (film listing): 5.9 GPa and 160 MPa tensile strength at 3 % elongation.

Time and water also take strength away. PA 66 has a modulus of 3 GPa dry (Q601) and 1.2 GPa conditioned (Q603); a conditioned entry shows 2.5 to 3.1 % water at 50 % RH, and saturation of the dry grade is 8.5 %. A 50 % tensile loss comes at 101 °C after 20000 h for PA 66 and 135 °C for PBT. A Polyoxymethylene (acetal), POM datasheet curve shows a tensile strength above 55 MPa for about 5 years at 60 °C.

Which high-temperature plastic is the stiffest?

Polybenzimidazole (PBI) is the stiffest drawn at 5.9 GPa, which is 12.3 times the modulus of Perfluoroalkoxy alkane (PFA) (0.48 GPa); both are film values. Polyetheretherketone (PEEK) has a tensile modulus of 4,000 MPa under another property name and is not placed in the chart.

The moulded grades sit between: Polyetherimide (PEI) 3.2 GPa, Polysulfone (PSU) 2.48 GPa, PC 2.35 to 2.4 GPa and PBT 2.4 to 2.6 GPa. Polyoxymethylene (acetal), POM has a tensile modulus of 2,600 to 3,100 MPa. The fluoropolymers are the soft end: Perfluoroalkoxy alkane (PFA) has a flexural modulus of 0.54 to 0.59 GPa and FEP 0.52 GPa, and their strength is stretch, not stiffness (Perfluoroalkoxy alkane (PFA) shows 300 % elongation).

Stiff does not mean tough. Polybenzimidazole (PBI) prints 3 % elongation at break, against 50 to 100 % for Polysulfone (PSU) and 50 % for ULTEM 1000 resin. A part that takes impact or flexing is a poor match for PBI.

Terms used

Glass transition temperature (Tg)

the temperature range where the amorphous part of a plastic changes from hard and glassy to soft and rubbery. The value depends on the method (DSC or DMA) and on where it is read (onset or midpoint).

Heat deflection temperature (HDT)

the temperature at which a standard test bar bends by a set amount under a set bending load. It is a short test, not a use limit, and it changes with the load: our profiles show 0.45 MPa and 1.8 MPa. Vicat softening is a similar needle test.

Maximum service temperature

a limit set by a supplier or standard for how hot the plastic may run for a stated time. It is a rating, not a measurement, and its basis (continuous, short-cycle, air or inert gas) differs between sources.

Temperature index

a rating from long-term heat ageing (IEC 60216): the temperature at which tensile strength falls to half its starting value after a stated number of hours.

Semicrystalline and amorphous

semicrystalline plastics (PEEK, PTFE, PBT, nylons) have ordered crystal regions and a melting point. Amorphous plastics (PEI, PSU, polycarbonate) soften over a range. This is general engineering knowledge, not in our data.

Hydrolysis

breakdown of a polymer chain by water, faster when hot. Polyesters such as PBT and PET are sensitive to it.

Elastic modulus

stiffness: stress carried per unit of stretch in the elastic range. Tensile and flexural moduli differ for the same grade.

Creep

slow, permanent stretching under a steady load, worse when hot. Our profiles hold no creep data. This is general engineering knowledge, not in our data.

Abbreviations

PBI polybenzimidazole; PEI polyetherimide (Ultem); PSU polysulfone; PEEK polyetheretherketone; PTFE polytetrafluoroethylene; PFA perfluoroalkoxy; FEP fluorinated ethylene propylene; ETFE ethylene tetrafluoroethylene; PVDF polyvinylidene fluoride; PBT and PET polybutylene and polyethylene terephthalate; POM polyoxymethylene (acetal); PA polyamide (nylon); PC polycarbonate.

What to check before you order

1. Ask which load and standard the heat deflection temperature used. Write the request as a spec: HDT per ISO 75-2 Method A at 1.8 MPa, annealed or not; the continuous temperature with the hours behind it; the exact grade and filler; and modulus at your working temperature.
2. Ask for the PEI grade. The range spans the Ultem 9085 printing grade and ULTEM 1000 resin, and glass-filled grades have different numbers.
3. For nylons, state the moisture condition (dry or conditioned). For PBT, PET and POM, check hot water and steam exposure.
4. For PBI, ask for moisture data: the profile shows 0.4 % water uptake over 24 hours and 7.5 % at saturation. For PEI, the profile lists stress cracking in chlorinated solvents. For PTFE and PFA, ask about cold flow and creep (general engineering knowledge, not in our data).
5. Plan ventilation for heating above 250 °C (FEP), 300 °C (PTFE) or 350 °C (PFA): these give off hydrogen fluoride or fume. Check the supplier's safety sheet.
6. If the part flies in low Earth orbit, compare the atomic-oxygen erosion yields. Polytetrafluoroethylene (PTFE) (1.4e-25 cm3/atom), Perfluoroalkoxy alkane (PFA) (1.7e-25 cm3/atom) and FEP (2.0e-25 cm3/atom) erode least. Polyetheretherketone (PEEK) (3.0e-24 cm3/atom), Polysulfone (PSU) (2.9e-24 cm3/atom), Kapton HN polyimide film (2.8e-24 cm3/atom), Polybenzimidazole (PBI) (at least 2.2e-24 cm3/atom) and Polyetherimide (PEI) (at least 3.3e-24 cm3/atom) sit close together, and PEI's is a lower bound, at least 23.3 times PTFE's rate. ETFE (9.6e-25 cm3/atom) and Polyvinylidene fluoride (1.3e-24 cm3/atom) lie between. PBT and Polyoxymethylene (acetal), POM are higher in NASA's Table I, though the PBT profile notes an unresolved second value.

What do we not know yet?

• Polyetheretherketone (PEEK) has a Tg and a melting point but no heat deflection temperature, service temperature or modulus-against-temperature data, so it is missing from most charts. PPS, PAI, PPSU, LCP and filled grades are not covered.
• Only Kapton HN polyimide film has paired room-temperature and 200 °C strength and stiffness. We hold no modulus-against-temperature curves for the other fifteen plastics, and no creep or fatigue data.
• The Polyamide 6 (nylon 6) (PA 6) heat deflection value is excluded as a probable data error, and we hold no conditioned heat deflection data for either nylon. The Ultem 9085 value of 173 °C comes from a printed-part datasheet on a reseller page and should be checked against the maker's sheet.
• Several glass-transition values are not clean grade data: the PSU range is general, the PTFE value is quoted from a paper in kelvin, the PET value is a bottle-grade copolymer and the PBT value is approximate.
• Service temperatures come from product pages and one encyclopedia infobox (FEP), on different bases, so no ranking of service limits is a test result. Temperature indexes exist for only PA 66 and PBT.

Questions people ask