Answer: If you are currently using a flexible polyvinyl chloride (PVC), it may be best to use either a polyurethane adhesive or a butadiene-acrylonitrile. If a more rigid PVC is the substrate, then you might want to try using a polyester or epoxy adhesive.

-ROGER J. LOHMAN

Question: I have always thought that epoxy adhesives offered the ultimate in strength, but lately I’ve seen many other so-called structural adhesives touting ultrahigh strength. There seems to be a vast array of choices. What are the differences between these adhesives, and how do I select an adhesive for the use I might have in mind?

Answer: Epoxies were certainly among the first structural or engineering adhesives (developed in the 1940s), but today there are numerous types of so-called structural adhesives available. The choice of adhesive depends upon how you are planning to apply the adhesive, as well as the final bond properties you are looking to achieve. Perhaps the best place to begin is with the materials you are trying to adhere together.

Epoxy adhesives work with most metals and are available in formulations offering cure times from a few minutes to several hours. As you noted, these adhesives are very versatile, but most require careful mixing of the two components, usually in equal amounts. Epoxy adhesives must also be applied within their particular handling time. Clamping is usually recommended until the adhesive can develop suitable handling strength. In many applications, the two-component nature of most epoxy adhesives and their need to be mixed in precise ratios make automated application difficult without specially designed application equipment. You will need to determine if your application warrants such an investment.

If you are trying to bond plastics, you may wish to also consider either structural acrylic adhesives or polyurethanes. Structural acrylic adhesives are also two-component materials, but some versions are more forgiving in mix ratios. These acrylic adhesives offer the potential to bond oily and other difficult surfaces, a weakness of epoxy adhesives. Once again, application equipment is available to automate the mixing and placement of the adhesive. Small, hand-held applicators are also commonly used for lower-volume applications.

Polyurethane adhesives are available in plural- and single-component varieties. Generally, the two-part systems offer higher ultimate bond strength, and faster fixture and ultimate cure, but some of the newer single-component systems offer significantly simpler application. Single-component polyurethane adhesives are most often moisture-curing. Water vapor in the atmosphere reacts with the formulated adhesive to crosslink or cure the material. This can take several hours to several days, depending upon ambient conditions, but handling strength can often be achieved relatively quickly. There are even some newer hot melt polyurethane adhesives that cure in this fashion, although they do require specialized application equipment. In any event, the single-part, moisture-cure urethane systems do need to be kept dry until they are applied.

It may seem complicated to choose the appropriate structural adhesive, but the effort is worth it to achieve the optimum application and performance characteristics you desire.

-Bob Smith