Ask a flexible-foam producer which raw material decides whether today's batch comes out right, and most will point to the isocyanate or the polyol. In practice, it is often the catalyst package quietly doing the deciding. Amine Catalyst A33 and Stannous Octoate T9 are the two catalysts behind the vast majority of flexible polyurethane foam made anywhere in the world, and understanding how they divide the work between them is the difference between a foam that forms cleanly and one that collapses, splits or shrinks on the line.
Two reactions racing each other
Every polyurethane foam is the product of two chemical reactions happening at the same time, in a race that has to finish in balance:
- The gel reaction — isocyanate reacting with polyol — builds the polymer network that gives the foam its strength and structure.
- The blow reaction — isocyanate reacting with water — generates the CO2 gas that expands the foam.
If the network builds faster than the gas expands, the foam is tight, dense and prone to splitting. If the gas expands faster than the network can support it, the foam rises impressively and then collapses back on itself, or shrinks as it cools. Catalysts are how a formulator controls the relative speed of these two reactions — and that is exactly why two different catalyst types exist, each doing a different half of the job.
Amine Catalyst A33: the dual-reaction accelerator
Amine Catalyst A33 is a solution of 33% triethylenediamine (TEDA) in 67% dipropylene glycol. Triethylenediamine is a strongly active tertiary amine that accelerates both the gel and blow reactions — it does not strongly favour one over the other. That makes A33 a broad, effective driver of overall reactivity, useful across flexible, rigid and CASE systems, but it does not on its own give a formulator fine control over the gel/blow balance. For that, a second, more selective catalyst is needed.
Stannous Octoate T9: the gel specialist
Stannous Octoate — sold as T9 or T-9 — is tin(II) 2-ethylhexanoate, an organotin catalyst that is strongly gel-selective. It accelerates the isocyanate-polyol reaction far more than the isocyanate-water reaction, which means adding more T9 firms up and speeds up network formation without proportionally speeding up gas generation. This selectivity is precisely what makes it useful: paired with A33's broader activity, T9 gives the formulator a second dial that moves gelling relative to blowing, rather than moving both together.
Why the two are almost always used together
Picture the catalyst package as two dials rather than one. A33 sets the overall pace of reaction. T9 shifts the balance toward gelling. Using both together lets a formulator hit a specific target — a certain rise time, a certain cell structure, a certain demould or cure speed — that neither catalyst alone can reliably deliver. This is why almost every commercial flexible slabstock and moulded foam formulation contains both an amine and a tin catalyst, at levels tuned to that specific system's line speed, ambient conditions and target foam properties.
Reading the defect back to the catalyst balance
When a foam defect appears, the catalyst package is one of the first places to look, because so many common faults are really the gel/blow balance showing itself physically:
- Collapse (the bun rises then sinks) — often too little gel strength relative to blow. Consider more tin catalyst, or check that amine levels have not been reduced without compensating elsewhere.
- Splitting or a tight, hard-to-process foam — often the reverse: gel running ahead of blow. Trimming tin catalyst or adjusting the amine level can help.
- Shrinkage after cooling — can relate to catalyst balance affecting cell opening, though surfactant and index are usually the first things to check alongside it.
Our companion guide on troubleshooting flexible foam defects covers the full diagnostic picture, of which catalyst balance is one important piece. The discipline that matters most: change one variable at a time, and confirm every adjustment with a controlled trial rather than a live production run.
Handling and storage: protecting the catalysts you have
Both catalysts are sensitive to moisture and require careful handling:
- Amine Catalyst A33 can absorb atmospheric moisture and carbon dioxide over time if left open, which affects its reactivity. Keep containers sealed, stored cool and dry, away from direct sunlight.
- Stannous Octoate T9 hydrolyses on contact with water, and hydrolysis reduces its catalytic activity. It should not be pre-mixed into a water-containing polyol premix for extended storage — dose it close to the point of use, and keep containers tightly sealed between uses.
A catalyst that has degraded in storage produces the same symptoms as a catalyst dosed incorrectly — inconsistent reactivity, unpredictable rise, defects that seem to appear "for no reason." Good storage discipline removes this as a variable before you ever start troubleshooting a formulation.
Sourcing catalysts as part of a system, not in isolation
Because A33 and T9 work as a pair, and because their correct dosage depends on the rest of the formulation — the isocyanate index, the polyol, the surfactant — buying them well means thinking in systems rather than single commodities. A supplier who understands how the catalyst package interacts with your TDI or MDI and your polyol blend can help you troubleshoot a defect or scale a formulation, rather than simply shipping drums against a purchase order.
How Ambizent supplies PU catalysts
Ambizent supplies Amine Catalyst A33 and Stannous Octoate T9 to flexible-foam manufacturers worldwide, alongside the TDI, MDI and polyols that complete the system, with a Certificate of Analysis on every shipment. If you are dialing in a new formulation or troubleshooting a catalyst-balance issue, our team can help you think through the gel/blow dynamics before you commit to a production trial. Explore our polyurethane catalysts range, read our companion guide on TDI vs MDI for flexible foam, and contact us to source A33, T9 and your isocyanate/polyol as a matched system.