Thermoplastic Elastomers (TPE, TPU, TPV): Microphase Separation, Properties & Processing
Microphase separation morphology, hard domain physical crosslinks, soft segment elasticity, dynamic vulcanization (EPDM/PP TPV), and 2K overmoulding.
Thermoplastic Elastomers (TPE, TPU, TPV): Microphase Separation, Properties & Processing
1. Why This Topic Matters
Thermoplastic elastomers (TPEs) bridge rubber and thermoplastics — rubber-like at service temp, processable like thermoplastics. They eliminate vulcanisation, reduce scrap, and enable recycling. Kraton India, Polyman (Chennai), and PolyOne (Avient, Mumbai) supply TPE/TPU for automotive seals, medical devices, footwear, and consumer goods. Bata India, Hero MotoCorp, and Maruti Suzuki are major end-users.
2. Learning Objectives
- Explain microphase separation and its role in TPE elastomeric behaviour.
- Distinguish SBS/SEBS, TPU, TPV, and TPO by structure and performance.
- Calculate compression set from ASTM D395 data.
- Select TPE grade for a given application (temperature, oil resistance, compression set).
- Identify ISO 37 (tensile), ASTM D2240 (hardness), ASTM D395 (compression set).
3. Core Theory
3.1 Microphase Separation
TPEs have hard segments (crystalline/glassy — physical crosslinks) and soft segments (amorphous rubbery — elasticity). Nanoscale phase separation (10-30 nm) creates the dual network. Heating dissolves hard domains → thermoplastic flow; cooling reforms them → rubber elasticity returns.
3.2 TPE Families
| Family | Hard Segment | Soft Segment | Key Property | Limitation |
|---|---|---|---|---|
| SBS | Polystyrene | Polybutadiene | Low cost, clear | Poor UV; max 60°C |
| SEBS | Polystyrene | Hydrogenated PB | UV stable | Higher cost |
| TPU | Hard urethane | Polyether/ester | Abrasion resistance | Moisture sensitive |
| TPV | PP crystallites | Vulcanised EPDM | Compression set, heat | Limited elongation |
| TPO | PP crystallites | EPDM blend | Low cost | Poor compression set |
3.3 Compression Set — ASTM D395
\text{CS (%)} = \frac{t_0 - t_r}{t_0 - t_s} \times 100 Where: t₀=original, t_r=recovered, t_s=compressed thickness.
| Material | CS (70h/70°C) |
|---|---|
| NR vulcanised | 10-25% |
| TPV (Santoprene) | 25-45% |
| SBS | 60-80% |
4. Worked Example
A TPV seal: t₀=4.0mm, t_s=3.0mm, t_r=3.5mm after 70h/70°C. CS = \frac{4.0-3.5}{4.0-3.0}\times100 = 50% Result: CS=50% exceeds typical automotive spec of <40%. Reformulate with higher vulcanisation degree.
5. Indian Industry Context
Polyman Composites (Chennai) compounds SEBS for Bata India footwear achieving UV stability >500h (ISO 4892-2) and CS<35%. Minda Industries (Noida) uses SEBS overmoulding on PP for Maruti Suzuki gear lever boots, replacing moulded PVC.
6. Key Takeaways & Glossary
- Microphase separation: Hard/soft nanoscale domains; reversible thermoplastic/rubber duality.
- TPV: PP + dynamically crosslinked EPDM; best compression set among polyolefin TPEs.
- SEBS: Hydrogenated SBS — UV/ozone stable; automotive exterior applications.
- Compression set: Lower = better sealing; thermoset rubber < TPV < SBS.
7. Standards
- ISO 37 — Tensile stress-strain of rubber; 2. ASTM D2240 — Shore A/D hardness; 3. ASTM D395 — Compression set; 4. ASTM D412 — Tensile of vulcanised rubber
8. Practice Questions
- Calculate CS for: t₀=6mm, t_s=4.5mm, t_r=5.0mm. 2. Why does SEBS resist UV degradation but SBS does not? 3. Compare TPV vs TPO for automotive weatherseal.
9. Quiz
Q1. Microphase separation: B) Hard/soft segments thermodynamically incompatible Q2. Best compression set: C) TPV Q3. SEBS vs SBS UV stability: B) Hydrogenated backbone Q4. Shore A used for: A) Soft rubbers/elastomers Q5. Compression set measures: C) Permanent deformation after sustained compression
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