PBT is an engineering polyester that is stiffer, stronger and more resistant to fuels and oils than polypropylene, while polypropylene is the lighter and cheaper material and the better one for alkalis, hot water and living hinges. PBT is the usual choice for connectors and other electrical parts that must pass flame tests at thin walls; neat PP needs no drying before moulding and, in its impact-copolymer grades, is the tougher of the two.
Unfilled PBT and PP homopolymer deflect at about the same temperature under a 1.80 MPa load and have similar notched impact strength. PBT’s heat advantage shows under light load, at the melt and in glass-filled grades.
What Is PBT
PBT, polybutylene terephthalate, is a semi-crystalline thermoplastic polyester made from 1,4-butanediol and terephthalic acid or its dimethyl ester. Its closest equivalent is PET, from the same polyester family.
PBT is sold unfilled and glass-filled, as in PBT-GF30. The comparison below uses unfilled PBT.
What Is Polypropylene
Polypropylene is the polyolefin made by polymerising propylene. In the parts where it competes with PBT, it is used as an injection-moulding homopolymer or impact copolymer, or as a glass-filled compound.
PP’s flow is rated as melt flow rate at 230 °C and 2.16 kg, in g/10 min. PBT datasheets give a melt volume-flow rate at 250 °C and 2.16 kg, in cm³/10 min, so a PP MFR and a PBT MVR are different quantities at different temperatures and cannot be compared, even when the numbers look close.
The Difference Between PBT and Polypropylene
Stiffness, density, chemical resistance, hydrolysis, processing and price separate the two polymers clearly; on heat, impact and shrinkage, the grade of each matters as much as the polymer. Unless a heading says otherwise, the PP values below are for injection-moulding homopolymer, with impact copolymer added where it changes the answer, and all property values are typical values from resin producers’ datasheets.
Strength and Stiffness
Unfilled PBT is both stronger and stiffer than unfilled PP. Its yield stress, 53–60 MPa, is 1.4 to 1.9 times that of injection-moulding PP homopolymer at 32–37 MPa.
Tensile modulus to ISO 527, as the producers print it, is highest for unfilled PBT, lower for PP homopolymer and lowest for PP impact copolymer.
Heat Resistance
PBT holds its shape to a higher temperature only under light load and near its melting point: on producer datasheets, unfilled PBT and PP homopolymer deflect at about the same temperature under 1.80 MPa, and PBT deflects far higher under 0.45 MPa.
The PP values at 1.80 MPa come from nucleated homopolymers, and nucleation raises a homopolymer’s deflection temperature. With 30 % glass, PBT deflects at 205 °C under the same load.
Unfilled PBT melts at 223–225 °C and PP homopolymer at 162–165 °C, although the producers measure melting point to different standards.
For long-term heat, an unfilled PBT grade is rated at a UL 746B relative temperature index of 125–140 °C. PP producer datasheets do not usually print such an index; PP’s long-term heat limit is set by oxidation and depends on how the grade is stabilised.
Density
Unfilled PBT is about 44 % denser than polypropylene: 1,300–1,310 kg/m³ against 900–905 kg/m³, both to ISO 1183.
Impact Resistance
At room temperature, unfilled PBT and PP homopolymer are about equally tough once notched, and PP impact copolymer is tougher than both. In the producers’ notched Charpy tests at 23 °C, the impact copolymers score well above the other two.
In the cold, the impact copolymers still test 4–7.5 kJ/m² notched at −20 °C, while unfilled PBT tests 3–4 kJ/m² at −30 °C. The producers test at different temperatures, so the two sets of figures are close but not a direct match. PP homopolymer turns brittle near freezing as it approaches its glass transition, so it is not the grade for parts that take impact in the cold.
Unnotched, PBT does not break at 23 °C and absorbs 150–180 kJ/m² at −30 °C, but a notch brings it down to the level of PP homopolymer, so for a PBT part with sharp internal corners the notched figure is the one that applies.
Chemical Resistance
Neither is more chemically resistant overall: PP resists alkalis and most non-oxidising acids, and PBT resists fuels, oils and many common solvents.
- Aqueous alkalis: PP. It is rated unaffected by 50 % sodium hydroxide at 20 and 60 °C. BASF states that its PBT is not resistant to aqueous alkalis.
- Acids and oxidisers: PP, for dilute and non-oxidising acids. Strong oxidisers attack it, including 98 % sulfuric acid, concentrated nitric acid and hypochlorite bleach. PBT has only limited resistance to dilute acids.
- Fuels, oils and solvents: PBT. It is highly resistant to fuels, brake fluid, transformer oil, alcohols, esters and aliphatic hydrocarbons, while PP absorbs aromatic solvents such as toluene and xylene, and gasoline, more so when hot.
- Water and salt solutions at room temperature: both.
Hot Water and Hydrolysis
PP is the material for continuous hot water and steam. PBT tolerates brief contact with hot water, but long-term hot, wet service requires a hydrolysis-stabilised grade.
In BASF’s storage tests, a standard glass-filled PBT showed no hydrolytic degradation in 60 °C water but degraded at 85 °C and 85 % relative humidity. PP is steam-sterilised at 121 °C as standard and withstands 134 °C fast autoclave cycles.
PBT also takes up more water, 0.5 % at equilibrium in 23 °C water for unfilled grades against 0.03 % for an unfilled PP compound.
Mould Shrinkage and Warpage
Unfilled, the two shrink by similar amounts, about 1–2 %, and warpage is mainly a concern in their glass-filled grades.
Unfilled PBT shrinks almost the same along and across the flow, 1.6 % and 1.9 % for one grade by ISO 294-4. PP producers give 1–2 % for unfilled homopolymer, depending on wall thickness and moulding conditions, but not to ISO 294-4.
Warpage follows the difference between shrinkage along and across the flow, and glass fibre widens it: PBT with 30 % glass shrinks 0.3 % along the flow and 1.1 % across it.
Electrical Properties
PP is the lower-loss insulator: nonpolar PP has a far lower dissipation factor than PBT, whose figure at 1 MHz is 0.02.
Unfilled grades of both reach CTI 600, the top tracking class; the producers’ figures show glass fibre and flame retardant lowering PBT’s tracking index, while flame-retardant PP holds 600.
Unfilled grades of both are rated HB, so a V-0 part needs a flame-retardant grade of either. Flame-retardant PBT reaches V-0 at 0.4 mm, against 0.8 mm for the best-rated flame-retardant PP compared. An FR in the grade name is not a rating, so the UL 94 class has to be read at the part’s wall thickness.
Fatigue and Living Hinges
PP survives repeated flexing. It is the standard material for integral hinges. PBT is too stiff and too notch-sensitive for a moulded-in hinge, so PBT parts that must open and close are designed with a pinned hinge instead.
Wear Resistance
PBT resists wear and sliding friction better. PP is seldom used in sliding contact without a lubricating additive.
Drying and Processing
PBT has to be dried before moulding and runs hotter in both melt and mould, while neat PP is moulded as delivered; a PP line needs a dryer and closed conveying before it can run PBT.
The producers’ moisture limits differ, so the limit for a given grade comes from its own processing sheet:
| Resin | Drying before moulding | Moisture limit | Melt temperature | Mould temperature |
|---|---|---|---|---|
| Unfilled PBT, BASF Ultradur B 4520 | 4 h at 80–120 °C, dry-air dryer | ≤ 0.04 % | 250–275 °C | 40–70 °C |
| Unfilled PBT, Celanese Celanex 2002-2 | 4 h at 121 °C | ≤ 0.02 % | 235–260 °C | 65–93 °C |
| Neat PP, ExxonMobil grades | Not required | – | 199–239 °C | 16–54 °C |
| PP with 30 % long glass, Borealis Fibremod GB303HP | About 2 h at 80 °C | Not stated | 220–260 °C | 40–80 °C |
Cost
PBT costs more both per kilogram and per part. In January 2026, modelled spot assessments for Northwest Europe put PBT at about 1,980 USD per tonne and fibre-grade PP at 1,130, so PBT cost 1.75 times as much per kilogram.
A PBT part weighs 1.44 times as much as the same part in PP, so at those prices its material cost about 2.5 times as much.
Treat those ratios as indicative, because the two assessments are on different terms (PBT ex-works, fibre-grade PP delivered) and prices for standard thermoplastics, PP among them, surged in April 2026. A PP part that needs thicker walls or ribs to match PBT’s stiffness also gives back part of the per-part saving.
How to Choose the Right One
Choose PP unless the part has to meet one of the conditions below that calls for PBT, since PP gives the lighter and cheaper part:
- Fuels, oils, brake fluid or solvents: PBT.
- Aqueous alkalis: PP.
- Continuous hot water or repeated steam sterilisation: PP.
- Load at elevated temperature: glass-filled PBT.
- V-0 at thin walls in an electrical part: flame-retardant PBT.
- Sliding contact: PBT.
- A living hinge: PP.
- Impact below freezing: PP impact copolymer.
- Lowest weight and part cost: PP.
Typical PBT parts are connectors, sensors, control-unit housings and automotive parts such as wiper arms and headlamp structures. Typical PP homopolymer parts are caps, closures, lids and thin-wall containers, and glass-filled PP goes into front-end and door-module carriers.
Both are sold with 30 % glass fibre, and in that form PBT still deflects at a higher temperature: 220 °C under 0.45 MPa for PBT-GF30 against 165 °C for a 30 % long-glass PP, while the PP compound stays lighter at 1,120 kg/m³ against 1,530.
Outdoors, PP needs UV stabilisers. PBT is the less UV-sensitive of the two, though BASF advises black-coloured material for outdoor parts.
PP is difficult to paint or bond without surface activation such as flame or plasma treatment, while PBT bonds after the usual preparation of cleaning, degreasing, roughening and drying.