Question: We have a number of unsaturated polyester parts (fiberglass) that we need to bond for indoor applications. Some of them need very fast fixturing and short cure times, while others require a lot of time for assembly before the adhesive cures. What do you recommend?
Answer: Historically, epoxy adhesives would have always been used for this sort of application. They offer good bond strengths on thermoset plastics like unsaturated polyesters. However, in recent years, reactive acrylics have often been the choice for this sort of application. These adhesives do not have the extreme heat resistance or fluid resistance as epoxies, but are more than adequate for most applications. They are also tough adhesives with good impact resistance.
Two types of reactive acrylics are available: surface-activated adhesives, where an activator is applied to one side of the bond and the adhesive to the other side; and two-part mixable adhesives, similar to the way that epoxies are mixed and applied. These are supplied in syringes or cartridges, with typically a 1:1 or 4:1 mix ratio.
Reactive acrylics can be formulated with very fast cure times, or can be modified so that no cure occurs for many minutes, followed by a rapid cure. For example, a major application of these adhesives is the assembly of fiberglass boats, where it is often necessary to apply the adhesive some 30-60 min before the parts can be positioned correctly. While I am a strong proponent of epoxy adhesives for many applications, I am not aware of them displaying these unique curing characteristics.