Question: We are looking for a compatible duct sealant that is resistant to concentrated nitric acid.  The installation will use 316 stainless steel to vent activities in a laboratory fume hood.

Answer: Nitric acid is one of the most difficult acids to seal against. Several sealants will resist it, but if you need the absolute best solution you should consider using a fluoroelastomer. These sealants are most commonly used as pre-formed gaskets or O rings, but they are also available as liquid sealants. They usually come in a two-component form where you mix the sealant with a curative.

Question: We supply RTV silicones to the home do-it-yourself market for use as bathtub sealants, etc. Is there a way to prevent fungal growth and the yellowing of these products on a long-term basis?

Answer: The basic structure of the cured silicones, called polydimethylsiloxane, does not support the growth of fungus. However, in a home environment exposed to organic contaminants such as dirt, detergents, etc., it is possible to get fungal growth. While it is normal to add fungicides to these sealants, they are often fairly toxic chemicals, which means there are limits to the amount you can add to a consumer product. They can also be washed out of the sealant over a long period of time.

The yellowing of the silicone is a more complicated subject that is often related to some of the catalysts that are used in its preparation. Some companies claim that new formulations can overcome this problem. The bottom line on silicone sealants is that they offer outstanding long-term performance versus alternatives, but they only have a limited lifetime from an aesthetic point of view. I once asked a senior scientist from a major silicone manufacturer for advice on the best product to use as a bathroom sealant, and he replied, “Use one that can be easily removed in a few years because it will eventually discolor.”