Ask Dr. Dave
Question: I am looking for an adhesive that can be applied to rivets inserted into thin-walled, stainless steel sheets (a patch) that will be exposed to exhaust fumes from a nitric chemical vat. What can you recommend?
Answer: Normally, a high-strength anaerobic retaining compound would be most convenient for this type of “wet riveting” because these materials are both relatively low-viscosity liquids and single-component adhesives. However, there may be an issue with the long-term resistance of the cured adhesive to the nitric fumes, and some epoxy adhesives will perform better under these conditions (with the disadvantage of being two-component mixable systems).
If you require maximum protection against the acid, you should consider using a fluorocarbon rubber. Although these rubbers are usually supplied in a pre-formed state, two-component liquid curable versions are available. I always recommend testing any product in the lab before using it.
Question: We are manufacturing chlorinated polyvinyl chloride (CPVC) solvent cement for CPVC pipes. Our final product’s viscosity increases with storage. What is the solution to stabilize the viscosity?
Answer: Although I do not know all the details of your formula, I can suggest that loss of chlorine from PVC in your product (often called dehydrochlorination) may be contributing to the problem. The first thing to check is that you have totally dissolved the PVC in your solvents without gel formation. You also need to ensure that the solvents in your products are dry; the addition of a drying agent may help here.
You should also look at adding stabilizers to your product. The common stabilizers for PVC are usually organotin compounds and phosphites. If you don’t stop this loss of chlorine, then, in addition to changes in viscosity, you might also see corrosion of metallic packages due to the generation of HCl in storage, particularly if storing at high temperatures.