The pp vs pbt properties comparison is one of the most common material decisions in moulded components, and it is rarely close: PBT is a semi-crystalline polyester that buys temperature, stiffness and solvent resistance with density and price, while PP is a polyolefin that buys cost, hot-water tolerance and low-temperature toughness with a much lower thermal ceiling. This guide sorts the property differences that show up on a datasheet, then maps which parts belong on which side of the line.
How the Polymer Families Differ
PP is a polyolefin — nonpolar, semi-crystalline, and the lightest of the common thermoplastics at 0.90-0.91 g/cm3. PBT is a polyester, roughly 44% denser at 1.30-1.31 g/cm3, with ester groups in the backbone that raise stiffness, improve solvent resistance and enable vivid dyeing and gloss. The polyolefin vs polyester split drives everything below: polarity decides chemical behaviour, crystallisation speed decides cycle time, and backbone chemistry decides how each resin ages in heat and humidity.
PP vs PBT Properties at a Glance
| Property | PP (unfilled) | PBT (unfilled) |
|---|---|---|
| Density | 0.90-0.91 g/cm3 | 1.30-1.31 g/cm3 |
| Tensile strength | 25-35 MPa | 50-60 MPa |
| Notched impact, 23 C | 3-6 kJ/m2 homo; 8-25 copolymer | 2.8-5 kJ/m2 |
| Continuous service temperature | 80-100 C | 120-140 C |
| Moisture uptake | Under 0.01% conditioned | Under 0.1% conditioned; about 0.5% in 23 C water |
| Shrinkage | 1-2%, varies with wall | 1.6% along and 1.9% across flow |
| Alkalis and dilute acids | Resistant | Alkalis attack PBT |
| Fuels, oils, solvents | Absorbs aromatics | Resistant |
Two rows decide most projects. The pbt continuous use temperature advantage — 120-140 C against 80-100 C for standard PP — opens engine-bay, appliance and hot-fill applications that simply soften a polyolefin. The moisture row runs the other way: unfilled PBT reaches roughly 0.5% uptake at equilibrium in 23 C water against under 0.03% for PP, which is why long-term immersion and steam duty still belong to PP even though PBT is the engineering resin on paper.
Heat, Hydrolysis and Hot Water
PP's thermal limit is set by oxidation and depends on how the grade is stabilised; standard grades hold 80-100 C in continuous service, and PP is steam-sterilised at 121 C as standard and withstands 134 C fast autoclave cycles. PBT carries a higher ceiling but a different failure mode: hydrolysis. In producer storage tests a standard glass-filled PBT showed no degradation in 60 C water but degraded at 85 C and 85% relative humidity — so any PBT part in sustained hot, wet service needs a hydrolysis-stabilised grade named on the drawing, not just the letters PBT.
Cold, Impact and Living Hinges
PP homopolymer turns brittle near freezing as it approaches its glass transition, which is why winter-duty clips and housings should specify impact copolymer: notched Charpy figures jump from 3-6 to 8-25 kJ/m2 at 23 C and hold 4-7.5 kJ/m2 at -20 C. Unfilled PBT tests 2.8-5 kJ/m2 notched at 23 C but keeps 3-4 kJ/m2 at -30 C, and unnotched it does not break at all, absorbing 150-180 kJ/m2 — so notch sensitivity matters: sharp internal corners put PBT back at PP homopolymer level. For living hinges and snap-fits, PP's fatigue resistance is the reason it keeps winning closures, lids and flip-top dairy tubs decade after decade.
Chemistry, Electricity and What to Specify
Neither resin is simply more chemical resistant: PP is rated unaffected by 50% sodium hydroxide at 20 and 60 C and handles dilute non-oxidising acids, while PBT resists fuels, brake fluid, transformer oil, alcohols, esters and aliphatic hydrocarbons. PP absorbs aromatic solvents such as toluene and xylene, and gasoline worse when hot; PBT has only limited resistance to dilute acids and is attacked by aqueous alkalis. Electrically, PP is the lower-loss insulator and both families reach CTI 600 unfilled, though glass fibre and flame retardants lower PBT's tracking index. Where the project sits between the two, our PBT resin sourcing guide covers grade selection, the PBT vs PET heat resistance comparison covers the adjacent polyester choice, and verified suppliers in both families can be shortlisted through our plastic materials category and company directory.
