In last month’s column we covered urethane chemistry. This month we will continue with urethanes and look at 100%-solids systems. Since these systems can be based on aromatic or aliphatic isocyanates, we will discuss aromatic systems. The main aromatic isocyanates are methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), and their structures are shown in Figure 1.
So why would we want to make 100%-active aromatic urethane systems? There are numerous reasons. They have very good physical- and chemical-resistance properties. They contain nearly zero volatile organic compounds (VOCs) and they are low viscosity (well under 1,000 centipoise). Since the urethane system is 100% solids, you can build thick coatings (up to inches thick), with little shrinkage, something that cannot be done with solvent- or water-containing systems. This makes them very useful in self-leveling floor coatings and elastomeric roof coatings, amongst other applications.