Almost every polyurethane product on earth starts from one of two isocyanates: TDI or MDI. They sit on the same side of the polyurethane reaction — both react with polyols to form the urethane bond — but they behave differently enough that choosing the wrong one can make a formulation impossible. Here is how they compare and how to decide.
What is TDI?
TDI, or Toluene Diisocyanate, is an aromatic isocyanate usually supplied as TDI 80/20 — a blend of 80% 2,4-TDI and 20% 2,6-TDI. It is highly reactive with polyether polyols and is the long-standing standard for flexible foam.
What is MDI?
MDI, or Diphenylmethane Diisocyanate, comes in monomeric, polymeric and prepolymer forms. It has higher functionality and much lower vapour pressure than TDI, which makes it safer to handle and better suited to rigid foam and many CASE applications.
The core differences
- Volatility: TDI is more volatile and a respiratory sensitiser, demanding strict ventilation and PPE. MDI’s lower vapour pressure makes handling easier.
- Functionality: Polymeric MDI has higher functionality, building the dense crosslinked networks that rigid foam needs. TDI suits the more open networks of flexible foam.
- End product: TDI leans toward soft, flexible foam; MDI toward rigid foam, insulation and durable elastomers.
When to choose TDI
Choose TDI for flexible slabstock and moulded foam — mattresses, furniture cushioning, automotive seating and packaging — where its reactivity with polyether polyols gives consistent, high-volume production.
When to choose MDI
Choose MDI for rigid foam and insulation (polymeric MDI), for high-performance elastomers and fibres (monomeric MDI), and for adhesives, sealants and footwear (prepolymer MDI). Its lower volatility is also a meaningful workplace-safety advantage.
Can you use both?
Yes — many plants run TDI and MDI side by side because they make different product lines, and some specialty foams use TDI/MDI blends to balance properties. The choice is rarely ideological; it follows the product you are making and the polyol it is paired with.
The bottom line
Think flexible, soft and high-volume? Start with TDI. Think rigid, insulating or durable CASE? Start with MDI. Then tune the system with the right catalysts and surfactants. If you are unsure which fits your line, our team can talk it through against your target properties.
Creston Chem supplies polyols, TDI, MDI and polyurethane additives to manufacturers across India. For grade-specific guidance, talk to our desk.