Question: We are looking at replacing mechanical fasteners with adhesives for our metal assembly operations. Should we be using epoxies, acrylics or polyurethanes?

Answer: Some 20-30 years ago, we would be looking only at epoxies, which were the acknowledged high-strength industrial adhesives at the time. However, reactive acrylics and polyurethanes are certainly viable alternatives today.

We cannot totally generalize, but it is true that epoxies (which can be two-part mixable systems or one-part heat-curable systems) give very high strengths, but are somewhat slow to cure, rigid, and brittle. They have been well-accepted in industries such as aerospace, where the somewhat long cure cycles are not a major issue. Reactive acrylic adhesives came around in about 1980 and gave the major advantages of very fast cure times (fixture in minutes), more flexibility—particularly in impact strength—and the ability to bond both metals and plastics, including unprepared or even oily surfaces. However, they do not generally have the unique very high shear strengths or high temperature resistance of epoxies. Polyurethanes, usually two-part mixable systems, have the unique performance characteristic of extreme flexibility and are well-suited for applications where a lot of movement is required.

You need to evaluate your productivity and ultimate performance requirements and test each system. 

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