Question: We need a fast-curing sealant with water resistance and high flexibility for some construction applications. Should we consider silicones or polyurethanes?

Answer: You can use fast-curing versions of conventional sealants like silicones or two-component polyurethane sealants. Polyurethanes have become very popular in this sort of application, and are also widely used for the production of many items, including coatings and foams. They are made by reacting a polyol with an isocyanate.

However, it is fair to say that there is resistance in some industries and localities to use them because, without proper protection, isocyanates can cause health problems such as coughing, tightness of the chest, shortness of breath, nausea, vomiting, and eye and skin irritations. Recently, so-called “non-isocyanate” polyurethanes have become available, but they have not achieved widespread use.

I would suggest that you consider a two-part polyurea. Polyureas are similar to polyurethanes, but are based on reacting a polyisocyanate with a multifunctional amine rather than with a polyol. This chemical reaction is extremely fast without added catalysts, and can give good seals within a few minutes (although full curing may take several hours). Polyureas tend to be a little more expensive than polyurethanes, but their very fast cure and outstanding properties often outweigh this extra cost.

You do have to be careful with surface preparation because the polyureas cure so quickly that complete wetting of the surface is sometimes difficult, leading to reduced adhesion. Thorough cleaning and the use of primers when necessary should solve this problem.