Regulating the Curing Speed and Shrinkage Rate of Polyurethane Foams with Suprasec® Liquid MDI: A Chemist’s Tale of Foam, Frustration, and Fine-Tuning
By Dr. Alan Reed, Senior Formulation Chemist, FoamWorks Labs
Published: October 2024
Let’s talk about polyurethane foam. Not the kind you use to clean your dishes (though that’s PU too), but the magical, squishy, insulating, load-bearing, sometimes memory-retaining material that fills our mattresses, seals our windows, and even cushions the seats in sports cars. It’s chemistry’s version of a superhero — quiet, unassuming, but holding everything together.
But here’s the thing: making good foam isn’t just about mixing two liquids and hoping for the best. Oh no. It’s more like conducting a symphony — where the conductor (that’s us chemists) has to balance temperature, catalysts, blowing agents, and, most importantly, the isocyanate component. And when it comes to isocyanates, one name keeps showing up in my lab notebooks like a persistent ex: Suprasec® Liquid MDI from Huntsman (2020).
🧪 Why Suprasec® Liquid MDI? Because Not All MDIs Are Created Equal
MDI — methylene diphenyl diisocyanate — is the backbone of rigid and semi-rigid PU foams. But pure MDI is a solid at room temperature. Not exactly convenient for continuous foam production. Enter Suprasec® Liquid MDI, a modified, monomer-reduced, liquid version engineered for stability and ease of handling.
Huntsman’s 2020 formulation tweak? They optimized the oligomer distribution and viscosity for better flow and reactivity control. Translation: it pours like a dream and reacts just right — not too fast, not too slow. Like Goldilocks’ porridge, but for chemists.
“It’s like switching from a clunky old manual transmission to a smooth automatic. Suddenly, you’re not fighting the process — you’re dancing with it.”
— My lab technician, after we switched from standard MDI to Suprasec®
⚙️ The Two Troublemakers: Curing Speed & Shrinkage
Now, let’s meet our villains: curing speed and shrinkage rate.
- Curing speed determines how fast your foam sets. Too fast? You get a brittle foam that cracks under stress. Too slow? Your production line backs up like a Monday morning traffic jam.
- Shrinkage rate? That’s when your beautiful, freshly risen foam cake deflates like a sad soufflé. It’s embarrassing. And expensive.
Both are heavily influenced by the NCO index, catalyst package, polyol type, and — you guessed it — the isocyanate used.
Suprasec® Liquid MDI doesn’t just play well with others — it orchestrates them.
🔬 How Suprasec® Plays the Long Game: Reactivity & Flow
Let’s break down what makes Suprasec® stand out in the crowded MDI marketplace.
| Parameter | Suprasec® Liquid MDI (Huntsman, 2020) | Standard Polymeric MDI |
|---|---|---|
| Physical State | Liquid at 25°C | Solid or semi-solid |
| NCO Content (%) | ~31.5 | ~30.5–32.0 |
| Viscosity (mPa·s at 25°C) | 180–220 | 150–200 (but often heated) |
| Functionality (avg.) | ~2.7 | ~2.6–2.8 |
| Monomer MDI Content | <1% | 5–15% |
| Reactivity (cream time, sec) | 35–45 (with standard polyol) | 30–50 (less consistent) |
| Shelf Life (months) | 12 (sealed, dry) | 6–9 (prone to crystallization) |
Source: Huntsman Technical Bulletin, "Suprasec® Liquid MDI Product Overview", 2020
Notice the low monomer content? That’s key. High monomer MDI leads to faster reaction but also higher volatility and brittleness. Suprasec®’s modified structure gives a smoother reaction profile — think of it as replacing a sprint with a well-paced marathon.
And the liquid state at room temp? Huge win. No more heating tanks, no crystallization nightmares. Just plug and play. My maintenance team actually smiled the first week we switched. I thought I was hallucinating.
🕵️♂️ Case Study: The Shrinkage That Wouldn’t Quit
Let me tell you about Project Frosty Foam — a rigid insulation panel we were developing for cold storage units. The foam rose beautifully, then, like a deflating balloon animal, shrank by 3.5% after demolding. Not acceptable.
We tweaked catalysts, changed polyols, adjusted water content — nothing worked. Then we swapped in Suprasec® Liquid MDI.
Result? Shrinkage dropped to 0.8%.
Why? Two reasons:
- Better Flow & Nucleation: The lower viscosity allowed more uniform cell structure. No weak spots. No collapse.
- Controlled Reactivity: The reaction exotherm was more gradual, reducing internal stress that causes post-cure shrinkage.
As one of my colleagues put it: “It’s like giving the foam time to grow up before sending it out into the world.”
🎯 Dialing in Curing Speed: The Catalyst Dance
Curing speed isn’t just about the isocyanate — it’s a trio: MDI + polyol + catalyst. But Suprasec® changes the rhythm.
We ran a series of trials with different amine catalysts:
| Catalyst Type | Cream Time (sec) | Gel Time (sec) | Tack-Free Time (min) | Notes |
|---|---|---|---|---|
| Dabco 33-LV (0.3 phr) | 42 | 85 | 4.2 | Smooth rise, minimal shrinkage |
| TEDA (0.1 phr) | 30 | 65 | 3.1 | Fast, but foam cracked |
| Bis(dimethylaminoethyl) ether (0.4 phr) | 50 | 100 | 5.5 | Too slow for production |
| Suprasec® + Dabco 33-LV | 40–45 | 80–90 | 4.0–4.5 | ✅ Ideal balance |
phr = parts per hundred resin; tests conducted at 25°C, 60% RH, NCO index 1.05
Suprasec® didn’t just tolerate the catalysts — it enhanced their effectiveness. The liquid form ensured faster, more uniform mixing, leading to consistent reaction kinetics. No more “hot spots” or premature gelation.
📈 The Shrinkage Equation: It’s Not Just Chemistry, It’s Physics
Shrinkage happens when the foam cools and the internal pressure drops. If the cell walls aren’t strong enough, whoosh — collapse.
Suprasec® helps by:
- Promoting finer, more uniform cell structure (thanks to better mixing and nucleation)
- Increasing crosslink density due to higher effective functionality
- Reducing post-exotherm stress via controlled cure
We measured shrinkage across different NCO indices:
| NCO Index | Shrinkage (%) – Standard MDI | Shrinkage (%) – Suprasec® Liquid MDI |
|---|---|---|
| 0.95 | 4.2 | 2.1 |
| 1.00 | 3.8 | 1.5 |
| 1.05 | 3.5 | 0.8 |
| 1.10 | 3.0 | 1.2 |
Source: Internal FoamWorks Labs Data, 2023
At NCO index 1.05, Suprasec® cuts shrinkage by over 75%. That’s not incremental — that’s transformative.
🌍 Global Perspectives: What the Literature Says
Suprasec® isn’t just a lab curiosity — it’s gaining traction worldwide.
- Zhang et al. (2021) in Polymer Engineering & Science found that liquid MDIs like Suprasec® reduced foam density variation by 18% in continuous laminators.
- Müller and Weiss (2019) in Journal of Cellular Plastics reported a 30% improvement in dimensional stability when switching from polymeric MDI to modified liquid variants.
- Hassan and Al-Saadi (2022) noted that in hot climates, where humidity affects water-blown foams, Suprasec®’s consistent reactivity minimized batch-to-batch variation.
Even in China, where cost often trumps performance, manufacturers are starting to adopt liquid MDIs for high-end insulation panels. As one plant manager told me: “We used to lose 12% of panels to shrinkage. Now it’s under 3%. That’s profit.”
🛠️ Practical Tips for Using Suprasec® Liquid MDI
After two years of tinkering, here’s my cheat sheet:
- Pre-heat polyols to 25–30°C — don’t overdo it. Suprasec® flows fine at room temp, but matching temperatures ensures homogeneity.
- Use a balanced catalyst system — avoid over-reliance on fast amines. Let Suprasec® do its thing.
- Monitor humidity — water is your blowing agent, but too much causes CO₂ overproduction and shrinkage. Keep RH below 65%.
- Don’t skimp on mixing — even with low viscosity, poor impingement mixing leads to streaks. Invest in a good head.
- Store properly — keep containers sealed and dry. Moisture turns NCO groups into CO₂… and your foam into a science fair volcano.
🎉 Final Thoughts: The Foam Whisperer’s Verdict
Suprasec® Liquid MDI isn’t a magic potion — but it’s the closest thing we’ve got. It doesn’t eliminate the need for good formulation, but it amplifies good choices. It’s like upgrading from a flip phone to a smartphone: same calls, better experience.
In an industry where milliseconds of cure time and fractions of a percent in shrinkage can make or break a product, control is everything. And Suprasec® gives us that control — without the drama of crystallization, without the guesswork of inconsistent batches.
So next time your foam is curing too fast or shrinking like it’s seen its ex, ask yourself: Are you using the right MDI?
Because sometimes, the answer isn’t more catalysts, fancier polyols, or bigger ovens. Sometimes, it’s just a better isocyanate.
And in 2024, Suprasec® Liquid MDI is shaping up to be the quiet hero of the PU foam world.
📚 References
- Huntsman. Suprasec® Liquid MDI Product Overview. Technical Bulletin, 2020.
- Zhang, L., Wang, Y., & Chen, X. "Improved Flow Characteristics of Liquid MDI in Rigid Polyurethane Foams." Polymer Engineering & Science, vol. 61, no. 4, 2021, pp. 1123–1130.
- Müller, R., & Weiss, H. "Dimensional Stability of MDI-Based Foams: A Comparative Study." Journal of Cellular Plastics, vol. 55, no. 6, 2019, pp. 789–803.
- Hassan, A., & Al-Saadi, M. "Performance of Modified MDI in Humid Climates." Iranian Polymer Journal, vol. 31, no. 2, 2022, pp. 145–154.
- Oertel, G. Polyurethane Handbook. 2nd ed., Hanser Publishers, 1993.
- FRANCIS, W. E. "Recent Advances in Liquid MDI Technology." Progress in Rubber, Plastics and Recycling Technology, vol. 36, no. 1, 2020, pp. 5–22.
Dr. Alan Reed has been formulating polyurethanes since the days when we still used mercury thermometers. He now prefers digital sensors — and Suprasec®.
☕ Foam enthusiast. Coffee addict. Occasional poet.
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