QUESTION: I need to assemble some sheet metal parts and must choose between a tough acrylic adhesive and an epoxy. Which would you recommend?

ANSWER: Both types of adhesives are very versatile; the choice depends on your operating environments and manufacturing situation. Invented in the 1940s, epoxies are the oldest and most tested adhesive for this application. A wide range of epoxy adhesives is available, from heat-cured, one-component systems (normally requiring several hours to fully cure) to two-component, ambient-cured systems. The general rule of thumb is that if your adhesive must resist very high temperatures and aggressive environments, then you need to use a heat-cured system. Higher temperatures give better molecular mobility to a curing adhesive system and promote more chemical crosslinking. 

Two-component, ambient-cured systems (epoxy component plus hardener) are definitely the most widely used epoxies. They range from the 1:1 mix ratio—the so-called “five-minute” adhesives, which are generally sold for consumer and light industrial applications—to quite high-performance systems with varying mix ratios. Epoxies tend to give high tensile and shear strengths, but are somewhat brittle.

On the other hand, tough acrylic adhesives—often called reactive acrylics or methacrylate adhesives—are relatively newer systems (from about 1980) and comprise an acrylic monomer system cured by either a surface activator or two components mixed together like an epoxy. Their major attributes include the ability to cure to high strengths very quickly, good flexibility and the ability to bond to dirty or oily surfaces without extensive surface preparation. In general, they do not have the same high-temperature or liquid resistance as most epoxies. My recommendation is to evaluate the specific requirement of your application in terms of heat resistance, environmental resistance and manufacturing productivity required so you can make a calculated decision.