Polypropylene’s glass transition sits close to 0 °C: read as the tan δ peak in a DMA run at 1 Hz and 3 °C/min, the isotactic matrix relaxes at about 1 °C (33.8 °F). ISO 11357-2 defines Tg as a characteristic value of a temperature range, and states that the assigned value varies with the property measured and with the conditions chosen to measure it.
Tg Values for Homopolymer, Random Copolymer and Impact Copolymer PP
The isotactic homopolymer matrix gives the value near 1 °C. An impact copolymer shows two transitions, its ethylene–propylene rubber phase relaxing far below the matrix, and for a random copolymer grade no published value with a method behind it exists.
| Grade family | Tg | Method and signal |
|---|---|---|
| Isotactic homopolymer (matrix) | about 1 °C | DMA, tan δ peak, 1 Hz, 3 °C/min |
| Impact copolymer | about −45 °C (rubber phase) and 1 °C (matrix) | DMA, tan δ peaks, 1 Hz, 3 °C/min |
| Random copolymer | lower than the homopolymer; no measured value published | — |
| Atactic PP (amorphous reference) | −12 °C | adiabatic specific-heat calorimetry |
| Atactic PP (second sample) | −29.5 °C | dilatometry at very slow equilibrium |
Against the other commodity resins, PP is unusual in landing near ambient: polyethylene’s transition lies far below it, polystyrene’s and PVC’s well above.
Why Quoted Tg Values for PP Disagree
Quoted values disagree because PP’s glass transition is spread over a range of temperatures, and because the material and the instrument both decide where inside that range a number gets assigned.
Tacticity, Crystallinity and Ethylene Content
Three properties of the material itself move the number: how regular the chain is, how much of it crystallised, and what comonomer sits in it.
Isotactic PP has its transition above atactic PP’s. Commercial grades are isotactic above 95 % with a degree of crystallinity that reaches about 60 % in industrial product.
Crystallinity has no single direction, so the comparison has to be named every time. Against a fully amorphous sample, the transition of a semicrystalline polymer’s mobile amorphous fraction sits higher and is much broader. Within isotactic PP, raising the crystallinity lowers that transition.
Random ethylene comonomer lowers the matrix transition; the direction is measured, the slope per unit of ethylene is not published.
DSC vs DMA: The Test Method Moves the Number
DSC and DMA do not measure the same property, and DMA alone yields several temperatures from one scan, so a figure is comparable only when the technique and the signal come with it.
DSC reads the transition as a step in heat capacity, and ISO 11357-2 notes that for partially crystalline polymers the step height is proportional to the amorphous content.
DMA gives four defined temperatures: the storage-modulus inflection, the onset, the loss-modulus peak and the tan δ peak. In a published DMA comparison of commercial PP at 10 Hz those came out at −7 °C, +4 °C and +10 °C — a 17 °C spread on one specimen in one run.
So ask for the signal, the frequency or heating rate, and which heating the value came from. ISO 11357-2 requires the type of DSC determination in the test report and prefers the second heating; ISO 6721-11 defines Tg(0), the value extrapolated to a zero heating rate, as the one used for specification and contract requirements.
Tg vs Melting Point: Why PP Is Used Above Its Tg
PP works above its Tg because the crystalline phase keeps the part stiff once the amorphous phase has gone rubbery. Tg and the melting point are the two ends of the usable window: below Tg the amorphous phase is glassy and the part breaks rather than bends, above it the part stays serviceable until the crystals melt — 165 °C for a conventional Ziegler–Natta homopolymer by DSC at 10 K/min, and lower for a random copolymer.
The softening point is a third quantity and is not the glass transition. Vicat softening temperature is a load-deformation temperature near the top of the window.
What Tg Means for Cold Service and Grade Selection
You will not select a PP grade on Tg, because the paperwork does not carry it. Across 46 producer datasheets covering all three families, none states a glass transition temperature and none states a brittleness temperature. The line to read for cold service is notched impact at a stated sub-zero temperature, and it is largely an impact-copolymer line: 20 of 22 impact copolymer sheets publish one, against 1 of 16 homopolymers and none of 8 random copolymers.
Homopolymer PP1074KNE1 gives 19 J/m notched Izod at −20 °C by ASTM D256A, where impact copolymer PP7032KN gives 80 J/m at −18 °C by the same method. Grades built for cold still lose most of their toughness getting there — PPC 4660 falls from 18 to 5.5 kJ/m² by ISO 180 between 23 °C and −20 °C — so an ambient-only figure says nothing about the fall.
“No Break” printed at 23 °C is not a standard result under ASTM D256, and cannot be compared with a figure from a specimen that broke. A brittleness temperature by ASTM D746 or ISO 974 is a 50 %-failure temperature under one prescribed impact geometry, and neither standard treats it as the lowest temperature at which the material can be used.
Match the family to the duty:
- Cold-chain crates and tote boxes: an impact copolymer.
- Clear cold-stored packaging: a random copolymer holds the clarity; request a sub-zero impact figure before the grade is approved.
- Outdoor woven bags and FIBCs in winter: there is no cold acceptance test for them. 49 CFR 178.603 conditions rigid plastic packagings to −18 °C before the drop test but excepts plastic bags, and 178.810 gives flexible IBCs no cold conditioning at all, so the winter criterion has to come from your own specification.