The diisocyanates used to produce polyurethane adhesives can be toxic, and so it's important to reduce the amount that users are exposed to. Polyurethane prepolymers can help achieve this, but usually at the cost of increases in viscosity. However, now acylurea prepolymers have proven able to keep residual both diisocyanate monomer levels and viscosity down, while also allowing a high isocyanate content in the prepolymer matrix. This can prove especially useful for reactive hot-melt polyurethanes, and also adhesives used in laminating flexible packaging.
The isocyanates used in adhesives, such as isophorone diisocyanate (IPDI) or toluene diisocyanate (TDI), are not gases like methyl isocyanate at room temperature, and are therefore less toxic. However, their vapour pressure can still particularly affect anyone applying PUR adhesives to large areas in confined spaces. This is especially problematic in spray adhesives, and hot-melt or lamination adhesives that reach temperatures where diisocyanates become volatile.