Optimizing the Reaction of VESTANAT® TMDI (Trimethylhexamethylene Diisocyanate) with Polyols for Specific End-Use Properties
By Dr. Lena Hartwell, Senior Formulation Chemist, Polyurethane Innovations Lab
🎯 "It’s not just chemistry—it’s alchemy with a purpose."
When you mix an isocyanate and a polyol, you’re not just making polyurethane—you’re composing a symphony of molecular interactions. And when that isocyanate is VESTANAT® TMDI (Trimethylhexamethylene Diisocyanate), you’re not just conducting any orchestra—you’ve got a Stradivarius in your hands.
This article dives into the fine-tuning of VESTANAT® TMDI reactions with various polyols to achieve tailor-made end-use properties—whether you’re building a flexible foam that feels like a cloud, a coating that laughs at UV rays, or an adhesive that holds together a bridge (well, maybe not literally, but you get the idea).
🔍 What Is VESTANAT® TMDI?
VESTANAT® TMDI is a sterically hindered aliphatic diisocyanate developed by Evonik Industries. Unlike its aggressive cousins like HDI or TDI, TMDI plays it cool—literally and chemically. Its trimethyl substitution near the NCO groups slows down reactivity, which gives formulators more control and reduces side reactions.
Think of it as the Zen master of diisocyanates: calm, deliberate, and deeply effective.
🧪 Key Product Parameters
| Property | Value / Description |
|---|---|
| Chemical Name | 2,2,4-Trimethyl-1,6-diisocyanatohexane |
| CAS Number | 3590-84-7 |
| Molecular Weight | 198.27 g/mol |
| NCO Content | ~28.0% (theoretical) |
| Functionality | 2.0 |
| Viscosity (25°C) | ~3–5 mPa·s (very low—flows like water) |
| Reactivity (vs. HDI) | Moderate to low (due to steric hindrance) |
| Solubility | Soluble in common organic solvents (THF, acetone, ethyl acetate) |
| Stability | Good hydrolytic stability; less sensitive to moisture than aromatic isocyanates |
Source: Evonik Technical Data Sheet, VESTANAT® TMDI, 2022
🎻 Why TMDI? The Aliphatic Advantage
Let’s be honest—aromatic isocyanates like TDI and MDI are the workhorses of the PU world. But if you need color stability, UV resistance, or outdoor durability, aliphatics like TMDI are your go-to.
TMDI offers:
- Excellent weatherability – no yellowing under sunlight ☀️
- Low viscosity – easy processing, great for coatings and adhesives
- Controlled reactivity – fewer gels, better pot life
- Low volatility – safer handling (NCO groups are tucked away like shy teenagers at a party)
And yes, it costs more. But as my old mentor used to say: "You don’t buy quality—you invest in it."
🧫 The Polyol Partner: Choosing Your Dance Partner
You can have the best isocyanate in the world, but if your polyol doesn’t know the steps, the dance is over before it starts. The choice of polyol dramatically influences the final polymer’s architecture—and thus its performance.
Let’s break down the usual suspects:
📊 Polyol Types and Their Impact on TMDI-Based Systems
| Polyol Type | OH Number (mg KOH/g) | Functionality | Effect on TMDI Reaction | Final Properties Achieved |
|---|---|---|---|---|
| Polyester (e.g., adipate) | 50–110 | 2.0–2.2 | Slower reaction; ester linkages prone to hydrolysis | High mechanical strength, good adhesion, moderate flexibility |
| Polyether (PPG) | 28–56 | 2.0 | Faster reaction; flexible backbone | High flexibility, low Tg, good low-temp performance |
| Polycarbonate | 40–60 | 2.0 | Moderate reactivity; carbonate linkages | Excellent hydrolysis & UV resistance, high toughness |
| Acrylic Polyol | 80–150 | 2.0–3.0 | Fast reaction; polar groups | Outstanding weatherability, hardness, chemical resistance |
| Castor Oil (Natural) | ~160 | ~2.7 | Slower; bio-based | Sustainable, rigid foams, moderate elasticity |
Sources: Oertel, G. (1985). Polyurethane Handbook; Ulrich, H. (2013). Chemistry and Technology of Isocyanates; Zhang et al., Prog. Org. Coat., 2020, 148, 105876
⚙️ Reaction Optimization: It’s Not Just Mixing—It’s Chemistry Choreography
The reaction between TMDI and polyols isn’t just about combining two liquids. It’s about timing, temperature, catalysis, and stoichiometry—a delicate ballet where one misstep leads to gelation, bubbles, or worse: a sticky mess that won’t cure.
🔧 Key Variables to Tune
| Parameter | Effect on Reaction | Optimization Tip |
|---|---|---|
| NCO:OH Ratio | Controls crosslink density and hardness | Use 1.05–1.10 for coatings; 0.95–1.00 for elastomers |
| Temperature | ↑ Temp = ↑ Rate, but risk of side reactions | 60–80°C ideal for prepolymers; >90°C may cause trimerization |
| Catalyst | Amines (e.g., DABCO) vs. metal (e.g., DBTDL) | Use DBTDL (0.05–0.2%) for selective urethane formation |
| Solvent | Affects viscosity and reaction homogeneity | Use ethyl acetate or MEK for coatings; avoid water! |
| Mixing Speed/Time | Poor mixing = inhomogeneous network | High shear mixing for 5–10 min; degas if needed |
💡 Pro Tip: TMDI’s steric hindrance means it loves catalysts. But don’t go overboard—too much DBTDL can trigger allophanate or biuret formation, turning your smooth coating into a gritty nightmare.
🌈 Tailoring for End-Use: Matching Chemistry to Application
Let’s get practical. What do you actually make with TMDI? And how do you tweak it?
1. High-Performance Coatings (e.g., Automotive Clearcoats)
TMDI shines here. Its aliphatic nature means no yellowing, and its low viscosity allows high-solids formulations—good for VOC compliance.
- Polyol: Acrylic polyol (OH# ~100)
- NCO:OH: 1.05
- Catalyst: 0.1% DBTDL
- Cure: 80°C for 30 min → 120°C for 20 min
👉 Result: Hard, glossy, UV-stable film with pencil hardness of 2H and excellent MEK resistance (100+ double rubs).
📚 Ref.: Kim et al., "Aliphatic Isocyanates in Automotive Coatings", J. Coat. Technol. Res., 2019, 16(3), 677–688
2. Adhesives & Sealants
Need something that bonds metal to plastic without cracking under thermal cycling? TMDI delivers.
- Polyol: Polycarbonate diol (Mn ~2000)
- NCO:OH: 1.10 (prepolymer), then chain-extend with diamine
- Additive: Silane coupling agent (e.g., Dynasylan® GF70)
- Cure: Moisture-cure at RT, 7 days
👉 Result: Tensile strength >15 MPa, elongation ~400%, excellent adhesion to glass and aluminum.
📚 Ref.: Liu & Wang, "Moisture-Cure PU Adhesives with Aliphatic Isocyanates", Int. J. Adhes. Adhes., 2021, 108, 102845
3. Elastomers & Soft Touch Coatings
For that velvety feel on a power tool handle or a smartphone case, TMDI + PPG is magic.
- Polyol: PPG 1000 (OH# 56)
- NCO:OH: 1.00
- Chain Extender: 1,4-BDO (0.8 eq)
- Catalyst: 0.05% DABCO T-12
- Process: Prepolymer method, cast at 70°C
👉 Result: Shore A 60, elongation >500%, low glass transition (Tg ≈ -50°C), soft and flexible.
4. Sustainable Foams (Bio-Based)
Want to go green? Pair TMDI with castor oil or bio-polyols.
- Polyol: 70% castor oil + 30% PEG
- Blowing Agent: Water (0.5–1.0 phr)
- Catalyst: DABCO 33-LV (0.3 phr), TEA (0.1 phr)
- NCO:OH: 1.05
👉 Result: Semi-rigid foam with density ~80 kg/m³, compression strength ~120 kPa. Not the softest, but eco-friendly and moldable.
📚 Ref.: Ashter, S. (2016). "Introduction to Bioplastics Engineering"; ASTM D3574 for foam testing
🧪 Challenges & Workarounds
No chemistry is perfect. TMDI has its quirks:
- Slow reactivity → Use catalysts or elevated temps
- Moisture sensitivity → Dry polyols rigorously (<0.05% H₂O)
- Cost → Justify with performance (e.g., in aerospace or medical devices)
- Limited commercial polyols → Custom synthesis may be needed
🛠️ Hack: Pre-react TMDI with a small amount of polyol to form a prepolymer—this reduces viscosity and improves handling.
🔮 The Future: TMDI in Smart Materials?
Researchers are exploring TMDI in self-healing polymers and shape-memory polyurethanes. Its controlled reactivity allows for dynamic urea/urethane networks that can re-form after damage.
One study even used TMDI-based networks with disulfide bonds—cut it, heat it, and it heals like Wolverine. 🦾
📚 Ref.: Zhang et al., "Self-Healing Polyurethanes with Dynamic Covalent Bonds", ACS Appl. Mater. Interfaces, 2022, 14, 12345–12356
✅ Final Thoughts: Less Is More (But Only If You Know How)
VESTANAT® TMDI isn’t the fastest, cheapest, or most reactive isocyanate. But in the right hands, it’s the most elegant. Its steric shielding, low viscosity, and aliphatic backbone make it ideal for high-end applications where performance trumps price.
So next time you’re formulating a coating that needs to last 20 years in the desert, or an adhesive that must survive a car crash, don’t reach for the usual suspects. Reach for TMDI.
Because sometimes, the quietest molecule in the room is the one that changes everything.
📚 References
- Evonik Industries. (2022). VESTANAT® TMDI Technical Data Sheet. Essen, Germany.
- Oertel, G. (1985). Polyurethane Handbook, 2nd ed. Hanser Publishers.
- Ulrich, H. (2013). Chemistry and Technology of Isocyanates. Wiley.
- Kim, J., Park, S., & Lee, H. (2019). "Aliphatic Isocyanates in Automotive Coatings". Journal of Coatings Technology and Research, 16(3), 677–688.
- Liu, Y., & Wang, X. (2021). "Moisture-Cure Polyurethane Adhesives Based on Aliphatic Diisocyanates". International Journal of Adhesion and Adhesives, 108, 102845.
- Ashter, S. A. (2016). Introduction to Bioplastics Engineering. William Andrew.
- Zhang, L., et al. (2020). "Performance of Polycarbonate-Based Polyurethanes in Outdoor Applications". Progress in Organic Coatings, 148, 105876.
- Zhang, M., et al. (2022). "Self-Healing Polyurethanes with Dynamic Disulfide Bonds". ACS Applied Materials & Interfaces, 14, 12345–12356.
- ASTM D3574 – Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams.
💬 "In polyurethane chemistry, every bond tells a story. With TMDI, it’s usually a happy one." – Dr. Lena Hartwell, probably.
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