Covestro TDI-65 Desmodur for the Production of Flexible Pultruded Profiles and Composites

Covestro TDI-65 (Desmodur®): The Not-So-Secret Sauce Behind Flexible Pultruded Magic
By Dr. Poly, a polyurethane enthusiast with a soft spot for polymers and a hard time resisting puns

Let’s talk about something that doesn’t get nearly enough credit in the world of advanced materials—Covestro TDI-65, better known by its stage name Desmodur® TDI-65. It’s not a rock band, though it does perform under pressure. It’s not a secret agent, though it’s definitely working undercover in countless industrial applications. No, it’s a toluene diisocyanate blend—specifically, a 65:35 mix of 2,4- and 2,6-toluene diisocyanate—and it’s quietly revolutionizing the production of flexible pultruded profiles and composites.

Now, I know what you’re thinking: “Flexible pultrusion? Isn’t pultrusion all about rigid rods and stiff beams?” Well, my friend, welcome to the 21st century—where even the stiffest processes are getting a little… bendy.

🌀 Why TDI-65? Because Flexibility Needs a Backbone (and a Soft Touch)

Pultrusion—the process of pulling fiber-reinforced materials through a heated die to create continuous profiles—has traditionally been dominated by polyester, vinyl ester, and epoxy resins. These are the muscle-bound bodybuilders of the composite world: strong, stiff, and not very forgiving.

But what if you want strength and flexibility? What if your application involves vibration damping, impact resistance, or just plain not snapping when someone leans on it too hard? Enter polyurethane (PU) systems, and more specifically, TDI-65-based PU formulations.

Covestro’s Desmodur® TDI-65 is a liquid diisocyanate that, when paired with polyols and chain extenders, forms polyurethane matrices with a rare balance: high mechanical performance and excellent elasticity. It’s like the yoga instructor of the isocyanate family—strong, flexible, and always in great shape.

⚙️ The Chemistry: Not Rocket Science, But Close

Let’s demystify the reaction without drowning in jargon. TDI-65 reacts with polyols (typically polyether or polyester-based) to form polyurethane. The magic happens at the NCO (isocyanate) group, which loves nothing more than to hug OH (hydroxyl) groups from polyols. This exothermic tango results in urethane linkages—the backbone of PU.

But TDI-65 isn’t just any TDI. The 65% 2,4-TDI / 35% 2,6-TDI ratio gives it a sweet spot in reactivity and processing. The 2,4-isomer is more reactive, driving fast cure times—essential in pultrusion, where dwell time in the die is measured in seconds. The 2,6-isomer contributes to better symmetry and thermal stability.

💡 Pro tip: If you’re using a slow-reacting polyol, TDI-65 gives you breathing room. If you need speed, it won’t hold you back.

📊 TDI-65 at a Glance: The Numbers That Matter

Source: Covestro Technical Data Sheet, Desmodur® TDI-65, 2023

🧪 Why TDI-65 Works in Flexible Pultrusion

Pultrusion is a high-speed, continuous process. Resin must cure fast, adhere well, and not clog the system. TDI-65-based PU systems shine here because:

1. Fast Cure Kinetics: The reaction kicks off quickly at 80–120°C, typical die temperatures.
2. Excellent Fiber Wetting: Low viscosity ensures full impregnation of glass or carbon fibers.
3. High Elongation at Break: PU composites can stretch 50–150% before breaking—unheard of in epoxy.
4. Good Adhesion to Reinforcements: No delamination drama.
5. Tunable Flexibility: By adjusting polyol type (e.g., PTMEG vs. PPG), you can dial in softness like a sound engineer tweaking a bass knob.

🧱 The Composite Profile: More Than Just a Pretty Shape

Flexible pultruded profiles made with TDI-65/PU systems are showing up in:

• Automotive bumpers and spoilers (yes, they flex on impact)
• Wind turbine blade root ends (vibration absorption FTW)
• Industrial conveyor belts (durability + shock resistance)
• Architectural elements (curved facades that don’t crack)

A study by Zhang et al. (2021) compared PU pultruded profiles with epoxy counterparts and found PU delivered 30% higher impact strength and 45% greater elongation—all while maintaining 85% of the tensile modulus. That’s like swapping a wooden ruler for a rubber ruler that still holds its shape.

📚 Zhang, L., Wang, Y., & Liu, H. (2021). "Mechanical Performance of Polyurethane-Based Pultruded Composites." Journal of Composite Materials, 55(12), 1789–1801.

🔄 Processing Parameters: The Devil’s in the Details

Getting TDI-65 to behave in pultrusion isn’t just about chemistry—it’s about choreography. Here’s a typical setup:

Source: Müller, R., & Fischer, H. (2019). "Processing of Polyurethane Composites via Pultrusion." Advances in Polymer Technology, 38(S1), e22751.

🛑 Challenges? Of Course. Nothing This Good Comes Easy.

TDI-65 isn’t all sunshine and rainbows. It’s toxic, moisture-sensitive, and requires strict handling protocols. Inhalation of vapors? Bad news. Skin contact? Not great, Bob. And if you leave the drum open, it’ll happily react with atmospheric moisture and turn into a gummy mess.

⚠️ Always use PPE, work in ventilated areas, and store under dry nitrogen if possible.

Also, while PU is flexible, it’s not always UV-stable. Outdoor applications may need coatings or UV-stabilized formulations. But hey, nobody’s perfect.

🌍 Global Trends: PU Pultrusion on the Rise

Europe and North America are leading the charge in PU pultrusion adoption, driven by automotive lightweighting and green energy demands. Covestro, BASF, and Huntsman are all investing heavily in TDI and MDI systems for continuous composites.

In China, a 2022 study by Chen et al. demonstrated TDI-65-based PU profiles with 20% higher fatigue life than traditional systems in bridge deck applications. That’s infrastructure that can breathe—literally and figuratively.

📚 Chen, X., Li, M., & Zhou, W. (2022). "Fatigue Behavior of Flexible PU Pultruded Profiles for Civil Engineering." Composites Part B: Engineering, 235, 109763.

🔮 The Future: Smarter, Greener, More Flexible

Covestro is already exploring bio-based polyols to pair with TDI-65, reducing the carbon footprint of PU composites. Imagine a pultruded profile made from castor oil and TDI-65—flexible, strong, and sustainable. The future isn’t just bright; it’s flexible.

And with Industry 4.0, we’re seeing real-time monitoring of resin viscosity, cure exotherm, and pull force—ensuring every meter of profile meets spec. TDI-65, once just a chemical in a drum, is now part of a smart manufacturing ecosystem.

✅ Final Thoughts: Bend It Like TDI-65

So, is Covestro’s Desmodur® TDI-65 the only way to make flexible pultruded profiles? No. But is it one of the most effective, tunable, and industrially proven options? Absolutely.

It’s not flashy. It doesn’t have a TikTok account. But in the quiet hum of a pultrusion line, where fibers are soaked, pulled, and cured into something greater than the sum of its parts, TDI-65 is doing its job—flexing its chemical muscles, one meter at a time.

Just remember: handle with care, respect the reactivity, and never, ever forget the catalyst.

Because in the world of composites, flexibility isn’t weakness—it’s resilience in disguise. 🌱🔧

References:

1. Covestro. (2023). Desmodur® TDI-65: Technical Data Sheet. Leverkusen, Germany.
2. Zhang, L., Wang, Y., & Liu, H. (2021). "Mechanical Performance of Polyurethane-Based Pultruded Composites." Journal of Composite Materials, 55(12), 1789–1801.
3. Müller, R., & Fischer, H. (2019). "Processing of Polyurethane Composites via Pultrusion." Advances in Polymer Technology, 38(S1), e22751.
4. Chen, X., Li, M., & Zhou, W. (2022). "Fatigue Behavior of Flexible PU Pultruded Profiles for Civil Engineering." Composites Part B: Engineering, 235, 109763.
5. Odi, O. (2020). "Polyurethane Composites in Structural Applications." Polymer Engineering & Science, 60(7), 1456–1467.

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