Adhesives and sealants troubleshooting tips from Dr. Dave Dunn.

Question:I want to use an epoxy adhesive for a high-temperature situation, but I don’t want to use a long heat cure. Can I use a fast-curing, two-part epoxy cured at room temperature?

Answer:In general, most two-part epoxies that are cured at room temperatures have a heat distortion temperature in the 60-80°C range, which means that they will lose rigidity and tend to creep in a bond-line that is under stress. However, highly filled fast-curing epoxies have been used successfully for many years in high-temperature applications such as under-the-hood auto repairs. By using metallic or metal oxide fillers, these so-called “cold welding compounds” perform very well in semi-structural bonds and repairing cracks and holes, and can withstand high temperatures for long periods of time.

That being said, if you really need structural strength at high temperatures, it is necessary to get maximum crosslinking of the epoxy resin. You will definitely need to heat cure at temperatures above your anticipated in-service temperature. High temperature resistance can also be achieved by replacing part of the commonly used bisphenol A-based epoxy resin with epoxy phenol Novolac resins (EPN) or epoxy cresol Novolac resins (ECN).

Question:I need to use a surface primer with a cyanoacrylate adhesive, but I don’t want to have flammable or toxic solvents in my production area. What can I do?

Answer:One option is to pre-apply the primer off-line in a suitable area, but you have to be careful that the primer has a suitable on-part life before it loses its activity. This on-part life can vary from minutes up to several hours; consult with your adhesive supplier for the best recommendations to suit your particular situation.