Covestro TDI-65 Desmodur: A Technical Guide for the Synthesis of Thermoplastic Polyurethane (TPU) Elastomers

Date：2025-08-30 Categories：News

Covestro TDI-65 Desmodur: A Technical Guide for the Synthesis of Thermoplastic Polyurethane (TPU) Elastomers
By Dr. Ethan R. Vale — Polymer Chemist & Coffee Enthusiast

☕ “Polyurethane is like a good cup of coffee — the magic lies in the blend.”
And when it comes to crafting thermoplastic polyurethane (TPU) elastomers, the choice of isocyanate is the espresso shot in your morning brew. Enter Covestro TDI-65 Desmodur, the dark, pungent liquid that’s been whispering sweet no-reaction secrets to polymer chemists since the 1950s. Let’s roll up our lab coats and dive into the nitty-gritty of how this aromatic isocyanate shapes the backbone of flexible, resilient, and downright sassy TPUs.

🔬 What Exactly Is TDI-65?

TDI-65, formally known as Toluene Diisocyanate 65/35, is a mixture of two isomers:

2,4-TDI (65%)
2,6-TDI (35%)

It’s not a pure compound — more of a well-balanced cocktail of reactivity and processability. Covestro (formerly Bayer MaterialScience) markets it under the Desmodur brand, and TDI-65 is one of the most widely used aromatic diisocyanates in flexible foams and elastomers.

Unlike its stiffer cousin MDI, TDI-65 brings a certain lightness to the polymer chain — literally and figuratively. Its lower molecular weight and higher functionality per unit mass make it ideal for soft segments in TPU, especially when you’re chasing that bouncy, huggable feel.

🧪 Why TDI-65 for TPU?

You might ask: “Why not just use MDI or HDI?” Fair question. But TDI-65 offers a unique combo:

Faster reaction kinetics — thanks to the electron-withdrawing methyl group on the benzene ring, the NCO groups are more electrophilic.
Lower viscosity — easier processing, especially in prepolymer routes.
Cost-effective — let’s be real, budgets matter in R&D.

However, there’s a trade-off: aromatic rings degrade under UV light, so outdoor applications? Maybe not your best bet. But for indoor cables, shoe soles, or medical tubing? TDI-65 sings like a tenor in a cathedral.

🧱 The Chemistry: Building TPU from the Ground Up

TPU is a block copolymer — a copolymère à blocs, if you will — made of alternating hard segments (from isocyanate + chain extender) and soft segments (from long-chain polyols). Think of it like a molecular tango: the hard segments stick together like best friends at a party, forming physical crosslinks, while the soft segments sway in the breeze, giving elasticity.

With TDI-65, the reaction typically follows a two-step prepolymer method:

Prepolymer Formation:
TDI-65 + Polyol (e.g., PTMG, PPG) → NCO-terminated prepolymer
(Reaction temp: 70–85°C, under nitrogen, no water — we’re not making bubbles here!)
Chain Extension:
Prepolymer + Chain extender (e.g., 1,4-butanediol) → TPU
(Melt process at 180–210°C, extrusion or casting)

⚠️ Pro tip: Moisture is the arch-nemesis of isocyanates. One drop of water, and you’ll spend the afternoon scraping urea gunk off your reactor walls. Not fun.

📊 Key Parameters of Covestro TDI-65 Desmodur

Let’s get technical — but not boring technical.

Property	Value	Notes
Chemical Name	Toluene-2,4 and 2,6-diisocyanate	65% 2,4, 35% 2,6
Molecular Weight (avg)	~174 g/mol	Lighter than MDI (~250)
NCO Content	48.5–49.5%	High reactivity
Viscosity (25°C)	6–9 mPa·s	Syrupy, but flows better than honey
Specific Gravity (25°C)	~1.22	Heavier than water — sinks, literally and emotionally
Reactivity with OH	High	Faster than aliphatic isocyanates
Flash Point	~121°C	Flammable — keep away from sparks and bad decisions
Storage	Dry, <30°C, N₂ blanket	Moisture-sensitive — treat like a vampire

Source: Covestro Technical Data Sheet, Desmodur TDI-65 (2022)

🧰 Choosing the Right Partners: Polyols & Chain Extenders

You can’t make TPU with just TDI-65 and good intentions. It needs dance partners.

1. Polyols (Soft Segment Builders)

Polyol Type	Example	Effect on TPU
PTMG (PolyTHF)	Terathane® 1000–2000	High resilience, low hysteresis — great for wheels
PPG	Polypropylene glycol	Lower cost, but weaker mechanicals — budget elastomers
Polycaprolactone	CAPA® series	Hydrolysis-resistant, biocompatible — medical grade

💡 Fun fact: PTMG-based TPUs are what make Segways glide so smoothly. TDI-65 + PTMG = robotic charisma.

2. Chain Extenders (Hard Segment Glue)

Extender	Role	Notes
1,4-Butanediol (BDO)	Most common	Crystalline hard domains, high tensile strength
Hydroquinone bis(2-hydroxyethyl) ether (HQEE)	High-performance	Better heat resistance, used in oil/gas seals
Ethanediol	Faster cure	But can lead to brittleness — like over-brewed coffee

🏭 Processing Tips: From Lab Bench to Factory Floor

Making TPU with TDI-65 isn’t just chemistry — it’s craftsmanship. Here’s how to avoid turning your reactor into a science fair volcano.

🔹 Prepolymer Method (Lab Scale)

Use dried polyol (water < 0.05%).
React at 80°C for 2–3 hours under N₂.
Monitor NCO% by titration (ASTM D2572).
Chain extend at 100–110°C with BDO (stoichiometry: NCO:OH ≈ 1.05–1.10).

🔹 Melt Processing (Industrial)

Extrusion at 180–210°C.
Avoid residence time > 10 min — yellowing starts, and nobody likes a yellow TPU.
Pelletize quickly — we want granules, not caramel.

🌡️ Thermal degradation begins around 220°C. Push beyond that, and your TPU starts smelling like burnt almonds — not in a good way.

📈 Performance Characteristics of TDI-65-Based TPU

Let’s see how this aromatic magic translates into real-world performance.

Property	Typical Range	Test Method
Shore A Hardness	70–95	ASTM D2240
Tensile Strength	30–50 MPa	ASTM D412
Elongation at Break	400–700%	ASTM D412
Tear Strength	80–120 kN/m	ASTM D624
Compression Set (22h, 70°C)	15–25%	ASTM D395
Heat Resistance (HDT)	~80–100°C	ASTM D648

Note: Values depend on polyol type, NCO index, and processing.

Compared to MDI-based TPUs, TDI-65 versions are generally softer, more flexible, and faster curing, but less thermally stable. It’s the difference between a yoga instructor and a powerlifter — both impressive, just different specialties.

🌍 Sustainability & Safety: The Elephant in the Lab

Let’s not ignore the pachyderm — TDI-65 is toxic and regulated.

TLV (Threshold Limit Value): 0.005 ppm (8-hour TWA) — yes, parts per billion.
Symptoms of exposure: Coughing, asthma-like reactions, and regret.
PPE Required: Full-face respirator, nitrile gloves, and a healthy respect for fume hoods.

Covestro has been working on closed-loop systems and safer handling protocols. But honestly, if you’re working with TDI, you should treat it like a sleeping dragon — don’t wake it, don’t provoke it, and definitely don’t spill it.

🌱 On the green front: TDI-65 isn’t biodegradable, but TPUs made from it are recyclable via reprocessing. Some companies (like Lubrizol and BASF) are blending bio-based polyols with TDI to reduce carbon footprint.

📚 Literature & References (No URLs, Just Brains)

Oertel, G. (1985). Polyurethane Handbook. Hanser Publishers.
— The bible of polyurethanes. Dusty, but gold.
Kricheldorf, H. R. (2004). Polymers from Renewable Resources. Wiley-VCH.
— For those dreaming of greener TPUs.
Frisch, K. C., & Reegen, A. (1977). "Kinetics of TDI-Polyol Reactions." Journal of Applied Polymer Science, 21(5), 1355–1367.
— Old but gold. Explains why TDI reacts faster than MDI.
Wicks, D. A., et al. (2003). Organic Coatings: Science and Technology. Wiley.
— Covers isocyanate chemistry in depth.
Covestro. (2022). Technical Data Sheet: Desmodur TDI-65. Leverkusen, Germany.
— The official word. Print it, laminate it, keep it in your lab coat.
Salamone, J. C. (Ed.). (1996). Concise Polymeric Materials Encyclopedia. CRC Press.
— Great for quick lookups on TPU properties.

🎯 Final Thoughts: Is TDI-65 Still Relevant?

In an era of aliphatic isocyanates, bio-TPUs, and UV-stable polymers, you might wonder: Is TDI-65 outdated?

No. It’s like vinyl records or manual typewriters — classic, reliable, and still loved by those who know their craft. For applications where cost, flexibility, and fast processing matter, TDI-65 remains a workhorse.

But — and this is a big but — it’s not for every application. Outdoor use? Think twice. High heat? Look elsewhere. But for shoe midsoles, cable jackets, or even inflatable rafts? TDI-65 is still kicking butt and chewing gum — and it’s all out of gum.

🔬 So next time you lace up your running shoes or plug in a high-flex cable, remember: somewhere, a molecule of TDI-65 did its job quietly, efficiently, and without asking for credit.

And that, my friends, is the beauty of polymer chemistry.

— Ethan ✍️
Lab notes, coffee stains, and all.

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