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) when working with PP.
Yet rapid, reliable measurement has traditionally been difficult.
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 was:
• Insufficiently sensitive to weak glass transitions
• Produced nearly identical Tg values across PP grades
• 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 – masking differences that directly influence performance., regardless of backbone structure. 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)
• 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
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.
What This Means for You
If you’re…
• Comparing new copolymer grades
• Blending recycled content with virgin PP
• Evaluating the thermal performance of masterbatch additives
• 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.
Some additional benefits:
• 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 the white paper “Pinpointing Polypropylene Glass Transition Temperatures Using Sub-Zero Rheology” for detailed methods, side-by-side DSC vs. ESR comparisons, and implementation guidance.