Methylene diphenyl diisocyanates (MDIs) are essential building blocks in the production of polyurethanes, urethane-prepolymers, and polyisocyanurate polymers in many industries. Because of their reactivity and versatility of combining with an wide array of curatives and chain extenders, MDIs are used in countless diverse applications. End-uses for these quite versatile diisocyanates include foam products, cast elastomers, thermoplastic polyurethanes (TPUs), fibers, and CASE (coatings, adhesives, sealants & elastomers) products. MDIs are the most-produced diisocyanate on a global basis.
There are many structural variants of Mdi, from pure monomer Mdi (MDI) to multifunctional polymerized Mdi (PMDI). The performance provided by various media is very different in different MDI levels. Formulators mainly use monomer Mdi to produce prepomers for casting polyurethane applications, such as wheels and rollers and mechanical products; TPU elastomers and spandex fibers. The core markets of polymer MDI manufacturers are hard foams for electrical insulation, packaging and construction applications; and semi-rigid and flexible foams for furniture, cushioning, transportation, vibration isolation, soles and synthetic leather; and polyisocyanurate and thermosetting foams.
Other widely used diisocyanates include toluene diisocyanate (TDI), terephthalene diisocyanate (PPDI), naphthalene diisocyanate (NDI), and aliphatic diisocyanates such as hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI). Each type of diisocyanate has different advantages and properties. MDI systems are the most reactive and usually react with polyols and chain extenders or curing agents (such as diols or triols). When 4,4'-MDI and its prepolders are chain-extended with low molecular weight diols (such as 1,4-butane diol) or special diols (such as hydroquinone bis (2-hydroxyethyl) ether (HQEE)), the linearity and symmetry of the hard block largely lead to the fact that MDI-based polyurethane exhibits excellent mechanical properties and shows advantages in dynamic properties and wear resistance.
Polymeric MDIs
Polymeric MDIs are generally used in manufacturing rigid and semi-rigid polyurethane foam for construction panels, spray-on insulation, refrigeration insulation, and automotive interior panels.