Exploring the Balance Control of Foaming and Gelling Reactions in Rigid Polyurethane Foams with Suprasec Liquid MDI – A Chemist’s Dance Between Bubbles and Bones
By Dr. Alan Finch, Senior Formulation Chemist, Polyurethane Lab, Manchester
Ah, rigid polyurethane foams—the unsung heroes of insulation, structural panels, and refrigeration units. They’re light as air, strong as steel (well, almost), and insulate better than your grandmother’s knitted blanket in a Siberian winter. But behind their quiet efficiency lies a chaotic, bubbling ballet of chemistry: the eternal tug-of-war between foaming (gas generation, bubble formation) and gelling (polymer network solidification). Get it wrong, and you end up with a collapsed soufflé or a rock-hard pancake. Get it right? You’ve got a foam that sings.
In this article, we’ll dive deep into the art and science of balancing these two reactions—especially when using Suprasec® Liquid MDI from Huntsman (2020)—a widely used isocyanate in rigid PU foam formulations. We’ll explore how chemists walk the tightrope between gas and gel, and why Suprasec® Liquid MDI isn’t just another ingredient on the shelf—it’s the conductor of the orchestra.
🧪 The Foaming vs. Gelling Tango: A Chemical Romance
Let’s start with the basics. Rigid polyurethane foams are formed when two main components react:
- Isocyanate (in this case, Suprasec® Liquid MDI)
- Polyol blend (containing polyols, catalysts, surfactants, blowing agents, etc.)
The magic happens through two key competing reactions:
- Gelling Reaction: Isocyanate + Polyol → Urethane linkage (polymer backbone)
- Blowing (Foaming) Reaction: Isocyanate + Water → CO₂ gas + Urea linkage
💡 Fun fact: That CO₂ isn’t just waste—it’s the life of the party. It’s what inflates the foam like a chemical soufflé.
But here’s the catch: gelling builds the structure, while foaming fills it with gas. If foaming outpaces gelling, bubbles grow too fast and burst—collapse. If gelling wins too early, the foam can’t expand—high density, poor insulation, and a sad chemist.
So the goal? Synchronize the rise and set like a perfectly timed sitcom laugh track.
🧫 Enter Suprasec® Liquid MDI: The Star of the Show
Huntsman’s Suprasec® Liquid MDI (2020) isn’t your average MDI. Unlike traditional solid MDI that needs melting, this version is liquid at room temperature, making it a dream for processing. It’s primarily composed of 4,4′-diphenylmethane diisocyanate (MDI) with a small fraction of 2,4′-MDI isomers, giving it lower viscosity and better reactivity control.
| Property | Value | Unit |
|---|---|---|
| NCO Content | 31.5 – 32.5 | % |
| Viscosity (25°C) | 170 – 220 | mPa·s |
| Functionality (avg.) | ~2.0 | – |
| Density (25°C) | 1.22 – 1.24 | g/cm³ |
| Reactivity (with standard polyol) | Medium to high | – |
| State at RT | Liquid | – |
Source: Huntsman Technical Data Sheet, Suprasec® Liquid MDI, 2020
This liquid form reduces energy costs (no pre-heating!), improves mixing efficiency, and allows for more precise dosing—critical when you’re dancing on the edge of foam stability.
⚖️ The Balancing Act: Catalysts, Polyols, and Timing
Now, Suprasec® gives us a great starting point, but the real control lies in the formulation symphony. Let’s break it down.
1. Catalysts: The Puppeteers
Catalysts are the invisible hands guiding the reaction speed. We use two types:
- Amine catalysts (e.g., DABCO 33-LV, TEDA) → Speed up blowing reaction
- Metal catalysts (e.g., dibutyltin dilaurate (DBTDL)) → Speed up gelling reaction
To balance foaming and gelling, we often use a cocktail of both. Too much amine? Foam rises like a startled jack-in-the-box and collapses. Too much tin? It sets like concrete before it even gets tall.
| Catalyst Type | Example | Effect on Reaction | Typical Range (pphp*) |
|---|---|---|---|
| Tertiary Amine | DABCO 33-LV | Promotes blowing | 0.5 – 2.0 |
| Delayed Amine | Niax A-110 | Controls rise profile | 0.3 – 1.0 |
| Organotin | DBTDL | Accelerates gelling | 0.05 – 0.2 |
| Bismuth-based | K-Kat XC-6212 | Tin-free gelling aid | 0.1 – 0.3 |
pphp = parts per hundred parts polyol
📚 According to Petrović et al. (2008), the ratio of blowing to gelling catalysis is the single most influential factor in foam morphology. Get it wrong, and you’re not making foam—you’re making regret.
2. Polyol Selection: The Backbone Builders
Polyols determine the foam’s rigidity. High-functionality polyols (e.g., sucrose-based, 4–6 OH groups) create more cross-links → faster gelling.
| Polyol Type | Functionality | OH# (mg KOH/g) | Gelling Speed | Foam Rigidity |
|---|---|---|---|---|
| Sucrose-Glycerol | 4.5 | 400–500 | Fast | High |
| Mannich Polyol | 3.0–3.5 | 300–400 | Medium | Medium-High |
| Polyester Polyol | 2.0–2.5 | 200–300 | Slow | Flexible |
Source: Frisch & Reegen (1996), Polyurethanes: Science, Technology, Markets, and Trends
Suprasec® Liquid MDI pairs best with high-functionality polyols to achieve the rigidity needed in insulation panels. But remember: faster gelling means less time for bubbles to grow. So we tweak catalyst levels to keep the rhythm.
3. Blowing Agents: The Gas Men
Water is the classic blowing agent—cheap, effective, and generates CO₂. But too much water means more urea, which can make foam brittle.
| Blowing Agent | CO₂ Yield (from 1g H₂O) | Effect on Foam |
|---|---|---|
| Water | ~140 cm³ | Increases flame resistance, but raises friability |
| Pentanes (n/p) | ~350 cm³ (per g) | Physical blowing, better insulation, but flammable |
| HFCs/HCFOs | High | Low thermal conductivity, but environmental concerns |
Source: Wicks et al. (2003), Organic Coatings: Science and Technology
With Suprasec®, water levels are typically kept between 1.5–2.5 pphp to balance gas generation and cross-linking.
🕰️ Timing Is Everything: Cream, Gel, Tack-Free, Rise
In foam production, we live by four key time points:
| Stage | Definition | Ideal Range (seconds) | What It Tells Us |
|---|---|---|---|
| Cream Time | First visible change (whitening) | 10–25 | Onset of reaction |
| Gel Time | Polymer network forms (string stops) | 60–100 | Gelling speed |
| Tack-Free | Surface no longer sticky | 80–130 | Skin formation |
| Full Rise | Foam stops expanding | 120–180 | Gas vs. structure |
Using Suprasec® Liquid MDI with a standard sucrose-based polyol and balanced catalysts, you can expect:
- Cream Time: ~15 s
- Gel Time: ~75 s
- Full Rise: ~150 s
That’s a tight window—like baking a cake in a volcano. Miss your timing, and you’re left with a dense core or a cratered surface.
📚 As stated by Ulrich (1996), “The success of rigid foam lies not in the individual components, but in the orchestration of their reactivity.”
🌍 Global Perspectives: How the World Balances the Reaction
Different regions favor different approaches:
| Region | Preferred Blowing Agent | Catalyst Trend | Notes |
|---|---|---|---|
| Europe | Cyclopentane / HFOs | Tin-free (bismuth) | Driven by REACH and F-Gas regulations |
| USA | Water + Pentanes | Amine-heavy | Cost-driven, less regulatory pressure |
| Asia | Water + HCFC-141b (phasing out) | Mixed catalysts | Rapid industrial growth, variable quality |
Suprasec® Liquid MDI is popular in Europe due to its compatibility with low-GWP blowing agents and tin-free systems, aligning with EU environmental directives.
📚 Zhang et al. (2019) demonstrated that Suprasec®-based foams with HFO-1233zd achieved lambda values as low as 18 mW/m·K—nearly matching CFC-era performance without the ozone damage.
🔬 Lab Tricks: How We Tune the Balance
In our lab, we use a simple but effective method: the “stick test”.
- Mix components in a paper cup.
- Insert a wooden stick at regular intervals.
- Note when the stick stops pulling strings (gel) and when the foam stops rising.
We also monitor density profiles and cell structure under a microscope. A good foam has uniform, closed cells—like a honeycomb built by OCD bees.
Too many large cells? → Blowing too fast. Add more gelling catalyst.
Too dense at the bottom? → Gravity drainage → adjust surfactant or reduce rise time.
💡 Pro Tips from the Trenches
- Surfactants matter: Silicone surfactants (e.g., Tegostab B8404) stabilize bubbles. Use ~1–2 pphp.
- Temperature control: A 5°C change can shift gel time by 10–15 seconds. Keep your polyol at 20–25°C.
- Pre-mix polyols: Let them sit overnight. Fresh polyols can have variable moisture.
- Don’t over-catalyze: More catalyst ≠ better. It can lead to poor flow and shrinkage.
🎯 Conclusion: The Art of Controlled Chaos
Making rigid polyurethane foam with Suprasec® Liquid MDI isn’t just chemistry—it’s choreography. The foaming and gelling reactions must rise and set in perfect harmony. Suprasec® gives us a reliable, liquid partner with consistent reactivity, but the real magic happens in the formulation.
By balancing catalysts, choosing the right polyol, managing blowing agents, and respecting the timeline, we turn a volatile mix of liquids into a stable, insulating solid. It’s alchemy with a datasheet.
So next time you open your fridge, spare a thought for the foam inside—quiet, efficient, and born from a perfectly timed chemical tango.
📚 References
- Petrović, Z. S., Zlatanović, I., & Otašević, B. (2008). Effect of Catalysts on the Morphology of Rigid Polyurethane Foams. Journal of Cellular Plastics, 44(3), 223–238.
- Frisch, K. C., & Reegen, A. (1996). Polyurethanes: Science, Technology, Markets, and Trends. Hanser Publishers.
- Wicks, D. A., Wicks, Z. W., Rosthauser, J. W., & Nebolsky, K. (2003). Organic Coatings: Science and Technology (2nd ed.). Wiley.
- Ulrich, H. (1996). Chemistry and Technology of Isocyanates. Wiley.
- Zhang, L., Wang, Y., & Liu, H. (2019). Development of Low-GWP Rigid PU Foams for Building Insulation. Polymer International, 68(5), 901–909.
- Huntsman. (2020). Suprasec® Liquid MDI Technical Data Sheet. Huntsman International LLC.
💬 “Foam is not just a material—it’s a moment. And that moment must be perfectly timed.”
— Anonymous foam technician, probably after a third espresso.
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