PBT vs PET heat resistance is a decision that shows up in connectors, appliance housings and under-hood parts, where the gap between a 130 degree continuous rating and a 115 degree rating decides whether a part survives warranty. Both are thermoplastic polyesters built from similar chemistry, but PBT crystallizes faster, tolerates moisture better and holds stiffness at higher temperatures in glass-filled grades. PET stays the cheaper, clearer choice whenever service temperatures remain moderate and transparency or food-contact packaging approvals are on the list.
PBT vs PET Heat Resistance: What the Numbers Say
| Property | PBT | PET |
|---|---|---|
| HDT unfilled, 1.8 MPa | ~55-60 C | ~70 C amorphous |
| HDT with 30% glass | ~205 C | ~183-220 C |
| Continuous use, filled grades | 130-140 C | 110-115 C |
| Moisture absorption | 0.08-0.15% | 0.1-0.3% |
| Cycle time, unfilled | 20-45 s | 45-90 s |
| Melt temperature | 240-270 C | 260-290 C |
| Light transmission | Opaque | 85-90% |
The pattern is consistent: neither polyester is a high-heat resin unfilled, but reinforcement lifts both into engineering territory, and PBT reaches the higher continuous rating in the grades buyers actually quote.
How Glass Reinforcement Changes the Picture
- 30% glass raises modulus to 8,000-14,000 MPa in both resins
- Filled PBT reaches HDT near 205 C; filled PET spans 183-220 C depending on crystallinity
- Reinforcement cuts mold shrinkage from 1.5-2.3% down to 0.3-0.7%
- Flame-retardant versions of both reach UL94 V-0, with a slightly higher oxygen index on PET
Moisture, Drying and Processing Windows
Both polyesters hydrolyze at melt temperature, so drying below 0.02 percent moisture is mandatory, typically four to six hours at 120-160 C. The pbt continuous use temperature advantage pairs with lower absorption, 0.08-0.15 percent against 0.1-0.3 percent for PET, which gives a wider safety margin on the shop floor and tighter tolerances in humid seasons. PET needs 10-30 degrees higher melt temperature, raising energy use and screw shear, while the pet heat deflection temperature stays modest unfilled. PBT's fast crystallization shortens cycles and frees press capacity, which is why converters quote it for connector and switch programs.
Cost, Supply and Sourcing Considerations
PBT pricing typically runs 10-20 percent above PET, and supply is concentrated among a smaller set of engineering-resin producers, so lead times can stretch during automotive peaks. Both resins ship in 25 kg bags and 500-1000 kg octabins, and both accept masterbatch coloring, though PBT's fast crystallization makes color uniformity easier to hold. When sourcing, ask for lot-specific datasheets, drying instructions and regrind policy, since regrind content affects both heat aging and color match on finished parts.
Where Each Polyester Wins
PBT leads in connectors, relays, fuse boxes, sensor housings and appliance parts where dimensional stability and heat resistance decide the design. PET wins in bottles, trays, films and any application needing clarity or packaging approvals. Toy and consumer-electronics programs often blend the two, using PBT for structural clips and PET for transparent windows in the same assembly. A PBT part typically carries a 10-20 percent resin premium over PET, so the switch pays for itself only when heat, cycle time or moisture stability is the binding constraint.
How Buyers Verify Heat Ratings
Ask which standard the rating follows: HDT is reported at a stated load, 0.45 or 1.8 MPa under ISO 75, and continuous-use temperature comes from long-term thermal aging rather than a single HDT number. A datasheet that lists HDT without the load is incomplete, and long-term ratings should reference the exact grade and filler level quoted, not a generic resin family.
Choosing a Polyester for Hot Service
For sustained service above 110-115 degrees C, glass-filled PBT is the safer default; for moderate temperatures and transparent parts, PET remains hard to beat on cost. Browse the plastic materials hub for grades, compare PBT resin sourcing criteria and PPS vs PBT high-temperature options, or visit the company directory to request samples.
