Polyisocyanates and Their Prepolymers

Abstract

Polyisocyanates, a group of low-molecular-weight cross-linking agents, are the most commonly identified cause of occupational asthma (OA) worldwide (1–3). Polyisocyanates are generally synthesized by the reaction of amines or their hydrochlorides with phosgene, and are unique in their ability to catalyze the production of polyurethane— a product of great cultural and commercial importance. The common feature of all polyisocyanates is the presence of more than one N1⁄4C1⁄4O group. Diisocyanate monomers (Fig. 1), the forerunners of all polyisocyanates, were first discovered in the late 1840s (4). One prototype, toluene diisocyanate (TDI) was developed by Farben in Germany during World War II for the manufacture of polyurethane, while another, hexamethylene diisocyanate (HDI), was developed by Reinke to create a melt spinnable fiber that would circumvent DuPont’s nylon patents (2,4). A number of related compounds have subsequently been developed and utilized commercially, with methylene diphenyl diisocyanate (MDI), naphthalene diisocyanate (NDI), and isophorone diisocyanate (IPDI) being the other major diisocyanates. Diisocyanate dimers, trimers, and polymers based on HDI and MDI are now increasingly being used (Fig. 2). Although these polyisocyanates have inherently lower vapor pressures and volatility in comparison with their corresponding diisocyanate, they can still cause diisocyanate asthma (DA). Polyisocyanates are used worldwide in a number of important industries. Aliphatic isocyanates such as HDI polymers are used primarily in external coatings and paints (3,5–8). Aromatic isocyanates such as MDI and TDI are used to produce a number of products such as flexible and rigid foams, adhesives, and sealants. For example, they are used extensively in the automobile industry for production of foam rubber cushions,