Polyurethane is a family of materials built by reacting an isocyanate with a polyol. The isocyanate you choose determines reactivity, the foam's cell structure, the final mechanical properties and how the material must be handled. TDI and MDI dominate the market — here's how they compare and when to use each.
The polyurethane reaction in brief
An isocyanate carries reactive –NCO groups that react with the –OH groups of a polyol to form the urethane linkage. Add water and the isocyanate also reacts to generate CO₂, which blows the foam. Catalysts, surfactants and other additives tune the speed and the cell structure. The ratio of isocyanate to polyol — the index — is a key formulation lever.
TDI — toluene diisocyanate
TDI is the classic choice for flexible foam. It exists as two isomers, 2,4- and 2,6-TDI, and is supplied in blends — most commonly 80/20 (80% 2,4-) and 65/35 — that tune reactivity and load-bearing. TDI is highly reactive and produces soft, resilient, open-cell foams ideal for mattresses, furniture cushioning and automotive seating. Its drawback is volatility: TDI has a relatively high vapour pressure, so it demands careful handling, ventilation and exposure control.
MDI — methylene diphenyl diisocyanate
MDI is lower in volatility than TDI and comes in several forms:
Pure (monomeric) MDI
Used for high-performance elastomers, thermoplastic polyurethane (TPU), spandex fibres, and cast prepolymers for wheels, rollers and mechanical goods.
Polymeric MDI (PMDI)
The workhorse for rigid foam — insulation panels, cold-chain, appliances and boardstock — as well as semi-rigid foams, binders and composite wood boards.
Modified MDI & prepolymers
Tailored blends and prepolymers formulated for specific systems, including flexible moulded foams and CASE (coatings, adhesives, sealants, elastomers).
TDI vs MDI — side by side
| Factor | TDI | MDI / PMDI |
|---|---|---|
| Typical foam | Flexible, open-cell | Rigid, closed-cell |
| Key end uses | Mattresses, furniture, seating | Insulation, panels, CASE, elastomers |
| Reactivity | High | Moderate, more controllable |
| Volatility / vapour pressure | Higher (more care needed) | Lower |
| Product character | Soft, resilient | Tough, high-performance |
| Forms | 80/20, 65/35 isomer blends | Pure MDI, PMDI, modified/prepolymer |
Choosing by product
- Flexible slabstock & moulded foam (mattresses, furniture, seating) — TDI, or MDI/TDI blends for moulded parts.
- Rigid insulation foam (panels, cold-chain, appliances) — PMDI.
- Spray polyurethane foam (SPF) — MDI-based systems.
- CASE (coatings, adhesives, sealants, elastomers) — MDI systems; aliphatic isocyanates for light-stable coatings.
- TPU, spandex, cast elastomers — pure MDI.
When you'd blend the two
Formulators sometimes combine TDI and MDI in moulded flexible foam to balance the softness and processing of TDI with the load-bearing and firmness of MDI. The right split depends on the part and the moulding process.
Other isocyanates worth knowing
Beyond the two workhorses: aliphatic isocyanates such as HDI and IPDI are used in light-stable, non-yellowing coatings; PPDI and NDI serve high-performance elastomers. These are specialty products for demanding niches rather than commodity foam.
Handling & safety
All isocyanates are respiratory sensitisers and must be handled with proper ventilation, personal protective equipment and exposure controls — this is more critical for volatile TDI than for MDI. Follow the supplier's Safety Data Sheet, store away from moisture (isocyanates react with water), and train operators. Good handling is part of good formulation.
The role of polyols & additives
The isocyanate is only half the system. The polyol (its molecular weight, functionality and hydroxyl number) and the additive package — catalysts, silicone surfactants, blowing agents, flame retardants and colorants — determine whether you get a soft cushion or a rigid insulation board. Ambizent supplies the complete system: MDI, TDI, PMDI, polyols and additives.
Storage, shelf life & moisture sensitivity
Isocyanates react readily with water, so storage discipline directly affects yield and quality. Keep TDI and MDI in sealed containers under a dry inert atmosphere (nitrogen blanket for bulk), protected from humidity — moisture ingress generates CO₂ and insoluble ureas that cloud the product and can pressurise drums. MDI in particular has a limited shelf life and can partially solidify or form dimer if stored cold or too long; it's often stored warm and re-melted with controlled heating rather than direct flame. Polyols absorb moisture too and are usually dried before use. Rotate stock first-in-first-out, record batch numbers against your finished goods, and confirm remaining shelf life on delivery — a drum that has aged or taken up water will throw off your index and your foam.
For a wider look at the polymers behind manufacturing, see our PVC, PP, PE & plasticizers guide.
Frequently asked questions
Can TDI and MDI be mixed?
Which isocyanate is used for mattress foam?
Which isocyanate is used for insulation?
Is MDI safer to handle than TDI?
What is PMDI?
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