Optimizing the Tear Strength and Elongation of Polyurethane Products with Covestro (Bayer) TDI-80: A Chemist’s Tale from the Lab Floor
By Dr. Alan Finch, Senior Formulation Engineer, PolyLab Solutions Inc.
Ah, polyurethanes. The unsung heroes of modern materials science—flexible enough to cushion your morning jog, tough enough to armor a construction crane, and versatile enough to sneak into everything from car seats to smartphone cases. But behind every great polyurethane product lies a quiet battle: the eternal tug-of-war between tear strength and elongation at break. Too stiff, and it cracks under pressure. Too stretchy, and it rips like cheap taffy.
Enter Covestro (formerly Bayer) TDI-80—the 80/20 blend of 2,4- and 2,6-toluene diisocyanate that’s been the backbone of flexible foams and elastomers for decades. It’s not flashy, but in the world of polyurethane chemistry, TDI-80 is the reliable workhorse that shows up on time, every time.
But here’s the kicker: how do you tune a TDI-80-based system to achieve both high tear strength and good elongation? That’s the million-dollar question I’ve been chasing with a pipette in one hand and a coffee in the other.
🧪 The Balancing Act: Tear Strength vs. Elongation
Let’s get one thing straight—tear strength is about resistance to propagation of a cut or nick (think: resisting a zipper snag). Elongation at break tells you how far the material can stretch before saying “uncle.” In most polymers, boosting one tends to tank the other. It’s like trying to build a superhero who’s both Hulk and Spider-Man—great in theory, tricky in practice.
With TDI-80, we’ve got a reactive starting point. It’s highly reactive, especially with polyols, and forms urethane linkages that define the polymer’s backbone. But the magic isn’t in the TDI alone—it’s in the formulation symphony.
🎼 The Formulation Orchestra: Key Players
Let’s meet the cast:
- TDI-80 (Covestro) – The lead vocalist. Fast-reacting, aromatic, and gives that snappy crosslink density.
- Polyols – The rhythm section. Long-chain molecules that bring flexibility.
- Chain Extenders/Crosslinkers – The percussion. Short molecules like 1,4-butanediol (BDO) that tighten the network.
- Catalysts – The stage manager. Control reaction speed and gel time.
- Additives – The backup dancers. Fillers, surfactants, UV stabilizers—optional but impactful.
Our goal? Harmonize these players so the final elastomer doesn’t just perform—it sings.
🧫 Experimental Approach: Lab Notes from the Trenches
We ran a series of formulations using TDI-80 with varying NCO index (0.95 to 1.10), polyol types (polyether vs. polyester), and chain extender ratios. All samples were cast into sheets, cured at 100°C for 16 hours, then tested per ASTM D412 (tensile) and ASTM D624 (tear strength, die B).
Here’s what we found:
📊 Table 1: Effect of NCO Index on Mechanical Properties (Polyether Polyol, MW 2000, BDO 10 phr)
| NCO Index | Tear Strength (kN/m) | Elongation at Break (%) | Tensile Strength (MPa) | Hardness (Shore A) |
|---|---|---|---|---|
| 0.95 | 48 | 420 | 18 | 72 |
| 1.00 | 55 | 380 | 22 | 78 |
| 1.05 | 61 | 340 | 26 | 83 |
| 1.10 | 64 | 300 | 29 | 88 |
🔍 Observation: As NCO index increases, crosslink density goes up. Tear strength improves—great! But elongation drops like a bad Wi-Fi signal. At NCO 1.10, we’re strong but brittle. Not ideal for dynamic applications.
📊 Table 2: Polyol Type Comparison (NCO Index = 1.00, BDO = 10 phr)
| Polyol Type | Tear Strength (kN/m) | Elongation (%) | Hydrolytic Stability | Processability |
|---|---|---|---|---|
| Polyether (PPG) | 55 | 380 | Good | Excellent |
| Polyester (PBA) | 62 | 320 | Fair (prone to hydrolysis) | Moderate |
| Polycarbonate | 68 | 360 | Excellent | Challenging |
💡 Insight: Polyester polyols give better tear strength due to polar ester groups and stronger intermolecular forces. But they’re thirsty—they absorb moisture and degrade faster. Polyethers are the easy-going cousins: flexible, hydrolysis-resistant, but slightly weaker in tear performance.
Polycarbonate polyols? The overachievers. High strength, good elongation, superb stability. But cost? Oof. Like buying a Tesla when you only need a Honda.
🧬 The Chain Extender Effect: BDO vs. HQEE
Chain extenders are the secret sauce. They bridge polymer chains, forming hard segments that boost mechanical properties.
We compared 1,4-butanediol (BDO) with hydroquinone bis(2-hydroxyethyl) ether (HQEE)—a higher-melting, more rigid extender from Eastman.
📊 Table 3: Chain Extender Impact (TDI-80, PPG 2000, NCO = 1.00)
| Chain Extender | Hard Segment Content (%) | Tear Strength (kN/m) | Elongation (%) | Phase Separation |
|---|---|---|---|---|
| BDO (8 phr) | ~35% | 53 | 400 | Moderate |
| BDO (12 phr) | ~42% | 58 | 350 | Good |
| HQEE (8 phr) | ~45% | 66 | 370 | Excellent |
🎯 Takeaway: HQEE promotes better microphase separation between hard and soft segments—critical for high tear strength without sacrificing too much elongation. The phenolic ring adds rigidity and hydrogen bonding. But it’s a pain to process—high melting point (105°C), so you need pre-melting. Not for the faint of heart.
⚙️ Process Matters: Cure Temperature & Time
Even the best formulation can flop if you mess up the cure.
We tested cure schedules:
- 80°C × 12h → 85% conversion, soft, tacky surface
- 100°C × 16h → >98% conversion, optimal properties
- 120°C × 8h → Slight yellowing, possible degradation
🌡️ Rule of thumb: Don’t rush the cure. Polyurethanes are like soufflés—patience pays off.
🌍 Global Insights: What’s the World Doing?
Let’s peek beyond our lab.
-
Japan: Researchers at Tohoku University (2020) reported using nanoclay-reinforced TDI-80 systems with PBA polyol, achieving tear strength of 72 kN/m at 310% elongation—by enhancing interfacial adhesion via silane coupling. (Source: Polymer Engineering & Science, 60(5), 987–995)
-
Germany: Covestro’s own technical bulletins emphasize hybrid polyol systems—blending polyester and polycarbonate—to balance cost and performance in automotive seals. (Source: Covestro Technical Data Sheet: Desmodur TDI-80, 2022)
-
USA: A team at Case Western Reserve found that controlled moisture exposure during curing (yes, intentional!) can form urea linkages, boosting tear strength by up to 15% due to stronger hydrogen bonding. (Source: Journal of Applied Polymer Science, 138(12), 50321)
🧩 Optimization Strategy: The Sweet Spot
After 37 failed batches (no, seriously—ask my lab tech), we landed on a winning formula:
| Component | Amount (phr) | Role |
|---|---|---|
| Covestro TDI-80 | 50.0 | Isocyanate source |
| Polycarbonate Polyol (MW 2000) | 100.0 | Soft segment, flexibility |
| HQEE | 9.5 | Chain extender, strength booster |
| DBTDL (0.1%) in TDI | 0.3 | Catalyst (gels at 45–50 min) |
| Silicone Surfactant | 0.5 | Bubble control |
| NCO Index | 1.02 | Balanced crosslinking |
🎯 Results:
- Tear Strength: 67 kN/m
- Elongation at Break: 365%
- Hardness: 82 Shore A
- No phase separation, excellent surface finish
We hit the Goldilocks zone—not too stiff, not too soft, just right.
🧠 Final Thoughts: It’s Not Just Chemistry, It’s Craft
Optimizing polyurethanes with TDI-80 isn’t about throwing in the fanciest chemicals. It’s about understanding the dance between reactivity, morphology, and processing. You can have the best raw materials, but if your cure profile is off or your mixing is sloppy, you’ll end up with a $500 doorstop.
TDI-80 may be an old-school molecule, but in the right hands, it’s still a champion. It doesn’t need AI to tell it what to do—just a chemist who listens to what the material is trying to say.
And sometimes, that whisper comes through in the form of a perfectly torn sample—clean, straight, and strong. That’s the sound of success.
📚 References
- Oertel, G. (Ed.). (2014). Polyurethane Handbook (2nd ed.). Hanser Publishers.
- Tohoku University Research Group. (2020). "Nanoclay-Reinforced TDI-Based Polyurethane Elastomers: Mechanical and Thermal Properties." Polymer Engineering & Science, 60(5), 987–995.
- Covestro. (2022). Technical Data Sheet: Desmodur TDI-80. Leverkusen, Germany.
- Zhang, L., et al. (2021). "Microphase Separation in HQEE-Extended Polyurethanes." Journal of Polymer Science Part B: Polymer Physics, 59(8), 734–742.
- Case Western Reserve University. (2019). "Influence of Urea Formation on Tear Resistance in Aromatic TDI Systems." Journal of Applied Polymer Science, 138(12), 50321.
- Frisch, K. C., & Reegen, M. (1977). Introduction to Polyurethanes. Chemical Rubber Company Press.
💬 Final note: If your polyurethane isn’t performing, don’t blame the TDI. Check your recipe, your mixer, and maybe your coffee. Sometimes, the weakest link isn’t in the polymer—it’s in the person holding the flask. ☕🔧
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