Polymeric MDI (polymethylene polyphenyl isocyanate)
Polymeric MDI is the dark-brown, multifunctional workhorse behind rigid foam and many aromatic binders and coatings.
Polymeric MDI (pMDI), also called polymethylene polyphenyl isocyanate, is a liquid mixture of monomeric MDI with three-ring and larger oligomers. The blend gives an average functionality around 2.7 NCO groups per molecule and a typical NCO content of about 31–32%.
pMDI’s higher functionality builds tightly crosslinked networks. It is the standard isocyanate for rigid polyurethane and polyisocyanurate foam, including spray foam insulation, and for wood-composite binders. In coatings it appears in some aromatic polyurea and polyurethane A-sides, primers and adhesives where cost and crosslink density matter more than color.
At about 31–32% NCO, pMDI has an equivalent weight of roughly 131–135 g/eq. Compared with pure MDI it is liquid at room temperature, easier to handle and less prone to crystallization, but its color is dark and its aromatic structure is not UV-stable.
Like all diisocyanates, pMDI is a respiratory and skin sensitizer. It reacts with moisture to release CO₂, which can pressurize sealed containers, so drums must be kept closed and dry. Use spray-appropriate respiratory protection and full skin protection.
Some aromatic polyurea and hybrid A-sides use pMDI or blends containing it, but most spray polyureas use MDI-based quasi-prepolymers for better flexibility and a convenient 1:1 ratio.
It is a crude, multi-component mixture straight from the manufacturing process, which includes colored by-products. That color does not affect its use in foam and many industrial products.
