Tin-free catalysts for the production of aliphatic thermoplastic polyurethanes

Polyurethanes based on aliphatic isocyanates are of great interest for industrial applications that require UV-stability. To overcome the low reactivity of (cyclo)aliphatic diisocyanates a catalyst is usually added to accelerate urethane formation. In most applications, organotin-based compounds are used; however, due to growing concerns about the toxicity of organotin compounds, their use is being restricted and the need for alternative catalysts is growing. This paper investigates the catalytic activity of several non-tin catalysts in the formation of (poly)urethanes based on cycloaliphatic diisocyanates. Initial screening in a small-scale solvent-free model system under mild conditions shows that several Lewis acid catalysts remain highly active at very low concentrations. Additional screening in a 40 g polymer system under harsher conditions demonstrates that Fe(III)-based compounds, in particular FeCl3 may serve as viable alternatives to the currently used organotin compounds.