4,4′-Methylene diphenyl diisocyanate
The aromatic diisocyanate at the heart of most spray polyurea and many polyurethane systems: fast, tough and economical, but not UV-stable.
4,4′-Methylene diphenyl diisocyanate (4,4′-MDI) consists of two benzene rings joined by a methylene bridge, each carrying one isocyanate group in the para position. Pure 4,4′-MDI is a white to pale-yellow solid that melts at around 40 °C. It is produced alongside its 2,4′ isomer and polymeric MDI from aniline, formaldehyde and phosgene.
Because pure MDI is solid at room temperature and tends to slowly form dimers in storage, coatings rarely use it as-is. Instead it appears as:
With a molecular weight of 250.25 g/mol and two NCO groups, pure 4,4′-MDI is 33.6% NCO, giving an equivalent weight of about 125 g/eq. Its aromatic NCO groups react quickly with amines (forming urea) and, with catalysis, with polyols (forming urethane). The aromatic rings that make it reactive also absorb UV light, so MDI-based films yellow and chalk outdoors unless topcoated.
MDI is a respiratory and skin sensitizer. Its vapor pressure is low at room temperature, but heating and especially spraying create inhalable aerosols. Keep containers sealed and dry, since moisture forms insoluble ureas and CO₂. Suppliers specify storage temperatures for pure MDI to limit dimer formation.
Pure (monomeric) 4,4′-MDI is a single molecule with two NCO groups and is solid at room temperature. Polymeric MDI is a dark liquid blend of MDI with higher oligomers, with an average functionality above two.
The aromatic rings in MDI absorb UV light and undergo photo-oxidation, forming colored products. Aliphatic topcoats such as polyaspartics are used where color stability matters.
