TDI and MDI: How to Select and Procure the Two Core Polyurethane (PU) Raw Materials

Polyurethane (PU) is a polymer formed by the reaction of isocyanates with polyols, and its properties can be tuned continuously across flexible foam, rigid foam, elastomers, coatings and adhesives. The two main players on the isocyanate side are TDI (toluene diisocyanate) and MDI (diphenylmethane diisocyanate).

TDI: The Fast-Reaction Choice for Flexible Foam and Elastomers

TDI is highly reactive and foams fast, making it the mainstream choice for flexible polyurethane foam (mattresses, sofas, car seats), elastomers and in-situ foaming. The ratio of its 2,4- and 2,6-isomers affects cure rate and product feel, so the grade and isomer ratio must be specified at procurement.

MDI: The Backbone for Rigid Foam, CASE and Structural Materials

MDI (pure MDI and polymeric MDI/PAPI) reacts more gently and requires a catalyst, so it is used for rigid foam (fridges, cold stores, building insulation) and CASE (coatings, adhesives, sealants, elastomers). Polymeric MDI has high viscosity and functionality, making it suitable for insulation panels and cast parts.

Procurement and Compliance Points

Watch purity, NCO content, isomer ratio, hydrolyzable chlorine (which affects storage stability) and colour. Both TDI and MDI are hazardous chemicals: they are moisture-sensitive and polymerise easily, so they must be stored cool, dry, sealed and protected from light, and transported under hazardous chemical regulations. Procurement should verify hazardous chemicals business qualifications and MSDS. For application pairings in polyurethane adhesives, see Adhesive Raw Material Selection. Haiyu Zhonglong holds a hazardous chemicals business licence and can provide compliant TDI/MDI supply and selection support. Contact us via Online Inquiry or Contact Us; product overview in the Product Centre.