When working with polypropylene (PP), small adjustments to chemistry can shift thermal behavior dramatically. Engineers and formulators need to know exactly how ethylene comonomer influences glass transition temperature (Tg) and melting point (Tm). Yet rapid, reliable measurement has traditionally been difficult.
- DSC (ASTM D7426) reported a flat Tg of –35.1 °C for all three PP grades, masking real differences
- Premier ESR with Sub-Zero Technology resolved distinct Tg and Tm values for homopolymer, block and random copolymer
- Ethylene segments dropped Tg by over 20 °C from homopolymer to random copolymer; block copolymer showed the highest Tm
- One closed-cavity ESR test delivers these insights in under 30 minutes
What Happens When You Add Ethylene to Polypropylene?
Polypropylene is a semi-crystalline thermoplastic known for stiffness, thermal stability, and chemical resistance. When applications demand low-temperature toughness, or more impact absorption, manufacturers often turn to copolymers, typically by incorporating ethylene into the molecular backbone.
Ethylene disrupts crystallinity and increases chain flexibility, lowering Tg so the material stays softer and more ductile at lower temperatures, ideal for cold-chain packaging, flexible automotive parts, and outdoor products.
Quantifying exactly how much the Tg and Tm change with varying ethylene levels is the challenge.
Why It’s Been Hard to Measure
Many thermoplastics demand precise thermal-history control, and traditional methods like ASTM D7426 often produce faint transitions, which makes pinpointing Tg quite difficult. In our study, this method:
Missed weak transitions
Was insufficiently sensitive to weak glass transitions.
Identical results
Produced nearly identical Tg values across PP grades.
Couldn’t tell grades apart
Showed a limited ability to distinguish homopolymers from copolymers.
Our recent study illustrates the issue: DSC reported a flat Tg of –35.1 °C for all three PP grades, regardless of backbone structure – masking differences that directly influence performance. This result ignores real-world mechanical differences that engineers rely on to choose materials.
A More Precise Technique: Rheology
Using Alpha Technologies’ Premier™ ESR with Sub-Zero Technology, we performed closed-cavity oscillatory shear tests on three PP grades: PP Homopolymer (HP), PP Block Copolymer (BcP) and PP Random Copolymer (RcP).
We ramped the temperature from +190 °C to –25 °C while monitoring loss modulus (G″), capturing clear molecular transitions in less than 60 minutes, including sample preparation.
Key findings: Tg by Sub-Zero rheometry (G″ peak)
PP Homopolymer
HP
PP Block Copolymer
BcP
PP Random Copolymer
RcP
For comparison, DSC reported a flat −35.1 °C Tg for all three grades.
| Grade | Tg (G″ peak) | Tm (G″ rise) | DSC Tg / Tm |
|---|---|---|---|
| PP HP | 8.36 °C | 127.95 °C | –35.1 °C / 165.05 °C |
| PP BcP | –4.75 °C | 139.71 °C | –35.1 °C / 148.67 °C |
| PP RcP | –14.68 °C | 125.22 °C | –35.1 °C / 165.97 °C |
Ethylene segments increase chain mobility, dropping Tg by over 20 °C between homopolymer and random copolymer. Block copolymer crystallinity shifts Tm higher, as phase-segregated blocks organize more efficiently. BcP showed the highest Tm due to phase separation and associated crystallinity.
DSC could not resolve these transitions for these specific PP grades using the standard ASTM testing method; ESR rheology revealed them immediately.
Lower Formulation Risk, Shorter Development Cycles
If you’re comparing new copolymer grades, blending recycled content with virgin PP, evaluating the thermal performance of masterbatch additives, or running QA on incoming raw materials, accurate Tg and Tm data that reflect real-world behavior reduce formulation risk, shorten development cycles, and prevent downstream failures. An ESR test delivers those insights in under 30 minutes.
Grade comparison
Resolve Tg/Tm shifts that DSC masks, enabling precise copolymer selection.
Recycling blends
Verify true thermal limits when mixing recycled and virgin PP.
Additive screening
Assess masterbatch effects on Tg/Tm in a single run.
Incoming-lot QA
Deploy an under 60-minute ESR profile instead of separate DSC/DMA tests.
See the Full Data Yourself
Download “Pinpointing Polypropylene Glass Transition Temperatures Using Sub-Zero Rheology” for detailed methods, side-by-side DSC vs. ESR comparisons, and implementation guidance.