Carbonylation Synthesis Process of MDI - Nitrobenzene Carbonylation Process

MDI and liquefied MDI exhibit higher reactivity than TDI, a more regular structure than TDI, and very low volatile toxicity, making them suitable for high-strength foam plastics, such as microcellular polyurethane elastomers. Overall, pure MDI and its modifications are used in lesser amounts in foams.

Diphenylmethane diisocyanate (MDI) has a molecular structure containing two benzene rings, with a relative molecular mass of 250.26, larger than that of TDI. It has a very low vapor pressure and low volatility, making it relatively less toxic to humans. MDI is an important isocyanate used in the synthesis of polyurethane resins. It is primarily used in the manufacture of various polyurethane elastomers but is also employed in the production of microcellular polyurethane elastic materials (such as shoe soles, solid tires, self-skinning foams, automobile bumpers, interior parts, etc.).

Generally, pure MDI refers to 4,4’-MDI, containing more than 99% of 4,4’-diphenylmethane diisocyanate, also known as MDI-100. At room temperature, it is a white to light yellow solid, which melts into a colorless to light yellow liquid. When heated, it has a pungent odor, with an open flash point of 202°C. It is soluble in acetone, carbon tetrachloride, benzene, chlorobenzene, nitrobenzene, and dioxane. MDI mainly consists of 4,4’-MDI and also contains small amounts of 2,4’-MDI and 2,2’-MDI isomers, with the 2,2’-MDI content being very low.

MDI products with high 2,4’-MDI content have lower reactivity and melting points compared to 4,4’-MDI. During MDI production, using catalysts such as solid acidic aluminum silicate during the condensation of aniline and formaldehyde can increase the mass fraction of the 2,4’-MDA isomer to 94%, with only 6% being the 4,4’-MDA isomer. After phosgenation, this becomes liquid MDI. Typically, when the 2,4’-isomer constitutes 25% (mass fraction) of MDI, it is in a liquid state at room temperature.

MDI-50, which consists of 2,4’-MDI and 4,4’-MDI, is a colorless to light yellow transparent liquid at room temperature. Due to the reactivity difference between 2,4’- and 4,4’-MDI, MDI-50 offers better flow properties for molding applications and can be used as a TDI substitute in the production of flexible polyurethane foam, reducing environmental pollution and improving working conditions.

MDI has a lower vapor pressure than TDI and is irritating to the respiratory system. Animal tests have shown that MDI is less toxic than TDI. The permissible concentration in the air is 0.02x10-6. MDI easily forms insoluble dimers at room temperature, requiring storage below 15°C, preferably below 5°C, and should be used as soon as possible.