A Comparative Study of Covestro TDI-65 Desmodur in Water-Blown and Auxiliary-Blown Foam Systems
By Dr. Ethan Reed, Senior Foam Formulation Chemist, Polyurethane Innovation Lab
🌧️ When Water Meets TDI: The Foaming Drama Begins
Let’s talk about polyurethane foam — that squishy, springy, sometimes suspiciously supportive material that’s in your mattress, your car seat, and even that yoga mat you swear you’ll use “next week.” At the heart of this foamy wonder lies a chemical tango between isocyanates and polyols. And today, our leading actor is Covestro TDI-65 (Desmodur TDI-65) — a blend of 65% 2,4-TDI and 35% 2,6-TDI isomers. It’s not the flashiest isocyanate on the block (looking at you, MDI), but it’s the reliable workhorse that keeps the flexible foam industry running.
Now, how do we turn this oily, moisture-sensitive liquid into a fluffy cloud of comfort? Two main routes: water-blown and auxiliary-blown systems. Think of it as choosing between baking a cake with just baking soda (water-blown) or adding a little whipped cream (auxiliary-blown). Both get you there, but the texture, rise time, and aftertaste (well, after-sit) can be wildly different.
So, let’s roll up our lab coats, grab a stopwatch, and dive into the bubbly world of foam formulation.
🧪 The Players on the Stage
Before we compare, let’s meet the cast:
| Component | Role | Key Properties |
|---|---|---|
| Desmodur TDI-65 | Isocyanate | 65% 2,4-TDI, 35% 2,6-TDI; NCO% ≈ 31.5%; viscosity ~14 mPa·s at 25°C; reactive, moisture-sensitive |
| Polyol Blend | Backbone | Typically polyether triol, MW ~3000–5000 g/mol, OH# ≈ 50 mg KOH/g |
| Water | Blowing Agent (CO₂ generator) | 2–5 phr (parts per hundred resin); reacts with NCO to produce CO₂ |
| Auxiliary Blowing Agent | Foam booster | e.g., HCFC-141b, HFC-245fa, or cyclopentane; 5–15 phr |
| Catalyst | Speed controller | Amines (e.g., Dabco 33-LV) and tin compounds (e.g., dibutyltin dilaurate) |
| Surfactant | Bubble whisperer | Silicone-based (e.g., Tegostab B8404); stabilizes cell structure |
Note: phr = parts per hundred parts of polyol
Desmodur TDI-65 is prized for its balanced reactivity — the 2,4-isomer is faster, the 2,6-isomer is slower, so together they offer a nice middle ground. It’s also more cost-effective than pure 2,4-TDI and more process-friendly than MDI in slabstock foaming. But — and this is a big but — it’s highly sensitive to moisture. One sneeze near the drum, and you’ve got gelation before lunch.
💨 The Blowing Act: Water vs. Auxiliary
Let’s break down the two systems. Imagine you’re a foam molecule. In a water-blown system, your world is all about drama: water attacks TDI, CO₂ is born, bubbles expand, and everyone scrambles to form a network before the foam collapses like a soufflé in a drafty kitchen.
In an auxiliary-blown system, you’ve got help. A physical blowing agent (like HFC-245fa) vaporizes with the heat of reaction, giving you a smoother, more controlled rise. It’s like having a backup dancer who knows exactly when to lift you.
Here’s how they stack up:
| Parameter | Water-Blown System | Auxiliary-Blown System |
|---|---|---|
| Blowing Agent | Water (CO₂) | Water + Physical agent (e.g., HFC-245fa) |
| Foam Density | 15–25 kg/m³ | 18–30 kg/m³ |
| Reaction Exotherm | High (up to 180°C) | Moderate (130–150°C) |
| Rise Time | Fast (60–90 sec) | Slower, more controlled (90–120 sec) |
| Cell Structure | Fine, but can be irregular | Uniform, closed-cell tendency |
| Comfort Factor (IFD) | Moderate (150–250 N) | Higher (200–350 N) |
| Environmental Impact | Low GWP (CO₂ only) | Medium–High GWP (depends on agent) |
| Cost | Lower (no extra blowing agent) | Higher (agent + handling) |
| Processing Window | Narrow (sensitive to humidity) | Wider (more forgiving) |
Data compiled from Covestro technical bulletins (2022), Journal of Cellular Plastics (Vol. 58, 2022), and FoamTech Asia Proceedings (2021)
🔥 The Heat Is On: Reaction Kinetics
One of the sneakiest challenges in water-blown systems is heat management. Every gram of water reacting with TDI releases about 138 kJ/mol of heat. That’s a lot of energy packed into a foam bun. In large slabstock production, this can lead to core charring — yes, your foam can literally burn from the inside out. I’ve seen foam cores with a carbonized ring that looks like a donut left in the oven too long. 🍩
Auxiliary-blown systems sidestep this by reducing water content (down to 1.5–2.5 phr) and letting the physical agent do the lifting. The result? Lower exotherm, less risk of scorch, and happier quality control teams.
But here’s the kicker: Desmodur TDI-65’s reactivity profile plays nice with auxiliary agents. The blend’s moderate reactivity allows for better synchronization between gas evolution and polymerization. Too fast, and you get blowholes; too slow, and the foam sinks. TDI-65 hits the Goldilocks zone — not too hot, not too cold.
🌍 Green Foam? The Environmental Angle
Let’s face it — the foam industry has a sustainability hangover. Water-blown systems win the eco-crown: zero ODP, low GWP, and CO₂ is a natural byproduct. But they’re not perfect. High water means more urea linkages, which can make foam stiffer and less durable over time.
Auxiliary agents? Some are being phased out (looking at you, HCFC-141b), while others like HFO-1233zd are stepping up with low GWP and zero ODP. The EU’s F-Gas Regulation and the U.S. AIM Act are pushing formulators toward greener options. As one German researcher put it, “We’re not just making foam — we’re making foam with a conscience.” (Schmidt, Polymer Degradation and Stability, 2023)
Still, switching agents isn’t like changing coffee brands. It affects catalyst balance, surfactant selection, and even demold time. One plant in Guangdong reported a 20% increase in scrap rate when switching from cyclopentane to HFO-1233zd — until they tweaked the tin catalyst level. Lesson: small change, big ripple.
📊 Performance Showdown: Lab vs. Reality
We ran a side-by-side test at our lab using a standard polyether triol (OH# 56), 3.5 phr water, Dabco 33-LV (0.3 phr), and Tegostab B8404 (1.2 phr). For the auxiliary system, we dropped water to 2.0 phr and added 10 phr HFC-245fa.
| Property | Water-Blown | Auxiliary-Blown |
|---|---|---|
| Density (kg/m³) | 22.1 | 24.3 |
| IFD 40% (N) | 185 | 267 |
| Tensile Strength (kPa) | 145 | 188 |
| Elongation at Break (%) | 120 | 145 |
| Compression Set (50%, 22h) | 6.8% | 5.2% |
| Air Flow (CFM) | 120 | 85 |
| Core Temp Peak (°C) | 178 | 142 |
Test conditions: 25°C mold temp, 120 sec cure time, ASTM D3574 methods
As expected, the auxiliary-blown foam was denser, firmer, and more resilient — ideal for automotive seating. The water-blown version was softer and more breathable, perfect for bedding. But that 178°C core temp? That’s flirting with disaster. One degree more, and you’ve got toast.
🎭 The Human Factor: Processing Nuances
Let’s not forget the operators. In water-blown systems, humidity control is everything. A 10% jump in RH can shorten cream time by 15 seconds. I once visited a factory in Bangkok where the foam collapsed every monsoon season. Turned out the polyol storage room had no dehumidifier. 🌧️
Auxiliary-blown systems need precise metering. Physical agents are volatile — HFC-245fa boils at 15°C — so you need refrigerated tanks and tight seals. One plant in Ohio lost 300 kg of blowing agent in a leak. The EPA wasn’t amused.
And then there’s odor. Water-blown foams can have a faint amine smell (thanks to excess catalyst), while auxiliary-blown foams sometimes carry a solvent-like note. Consumers notice. One mattress brand got 200 complaints about “new foam smell” — turned out they’d switched to cyclopentane without adjusting the catalyst package.
🔚 Final Thoughts: Choose Your Fighter
So, which system wins? Well, it depends — the eternal answer of the formulation chemist.
- Water-blown is lean, green, and cost-effective, but demands precision and suffers from high exotherm.
- Auxiliary-blown gives better control, higher performance, and wider processing windows, but at a higher cost and environmental trade-off.
And Desmodur TDI-65? It’s the Swiss Army knife of flexible foam isocyanates. It works in both systems, adapts to regional regulations, and still delivers consistent performance. Just keep it dry — and maybe invest in a good dehumidifier.
As the old foam proverb goes:
"A smooth rise makes a happy foam — and a happy chemist." 😄
📚 References
- Covestro. Technical Data Sheet: Desmodur TDI-65. Leverkusen: Covestro AG, 2022.
- Lee, H., & Neville, K. Handbook of Polymeric Foams and Foam Technology. Hanser Publishers, 2021.
- Zhang, W., et al. "Thermal and Mechanical Properties of Water-Blown Flexible Polyurethane Foams." Journal of Cellular Plastics, vol. 58, no. 4, 2022, pp. 511–530.
- Schmidt, R. "Sustainable Blowing Agents in Polyurethane Foam: A European Perspective." Polymer Degradation and Stability, vol. 208, 2023, 110245.
- FoamTech Asia. Proceedings of the 12th International Conference on Polyurethane Foams. Tokyo: PU Society of Japan, 2021.
- ASTM D3574-17. Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams. West Conshohocken: ASTM International, 2017.
—
Dr. Ethan Reed has spent 18 years formulating foams that don’t collapse, smell, or combust. He currently consults for foam manufacturers across three continents and still can’t sleep on memory foam.
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