What is the difference of MDI Prepolymer vs TDI Prepolymer?
Polyurethane prepolymers are important raw materials used in cast elastomers, rollers, wheels, screens, seals, industrial parts, adhesives and coatings. Among various polyurethane systems, MDI prepolymers and TDI prepolymers are the two most widely used types.
Although both are polyurethane prepolymers containing isocyanate groups, they exhibit significant differences in structure, processing behavior, mechanical performance and final applications. Understanding these differences can help manufacturers select the most suitable material for their production requirements.
TDI prepolymers are produced using toluene diisocyanate (TDI) and polyols. The TDI molecule contains a single benzene ring with two isocyanate groups. Because of its relatively simple structure, TDI exhibits high reactivity and produces polyurethane materials with excellent flexibility.
MDI prepolymers are synthesized from methylene diphenyl diisocyanate (MDI) and polyols. MDI contains two benzene rings connected by a methylene bridge, resulting in a more symmetrical molecular structure. This structure contributes to higher hardness, strength and thermal stability in the final polyurethane products.
One of the most noticeable differences between MDI and TDI prepolymers is their NCO content and viscosity.
Typical TDI prepolymers generally contain 8% to 15% NCO and maintain relatively low viscosity. Their excellent flowability makes them suitable for casting and processing applications where easy handling is required.
MDI prepolymers commonly contain 15% to 30% NCO. Due to the higher hard segment content, they usually exhibit higher viscosity than TDI-based systems. However, they provide superior mechanical properties after curing.
TDI prepolymers typically show higher reaction activity and shorter gel times during processing. Their low viscosity also improves mixing and casting performance.
However, TDI-based polyurethane elastomers often require a longer post-curing period to achieve their optimal physical properties.
MDI prepolymers offer better reaction control and faster strength development. In many cast elastomer applications, MDI systems can reach most of their final mechanical strength within a relatively short period, helping manufacturers reduce production cycles and improve efficiency.