Adhesives and bonding tapes are incredibly versatile materials: They enable thin metal surfaces bonding, eliminate unappealing screw or rivet heads, eliminate holes in structures, and can provide damping properties. They can be used to bond dissimilar substrates and offer significant aesthetic benefits.

Despite millions of successful examples of these benefits, and despite many articles, much study, and even a few "expert systems," selecting an appropriate adhesive or bonding tape is often difficult. Typical difficulties encountered in adhesive and bonding tape selection tend to fall in one or more of the following categories described below.

No one single measure of strength. You are getting failures in your tests of a moisture-curing urethane adhesive with a 1,500 psi overlap shear rating. So you call your adhesive representative and ask for something stronger. He recommends a "3000 psi" rigid epoxy adhesive. You try this new "stronger" adhesive and get even more failures. What happened?

"What happened" was that a single dimension of strength was used to infer performance in other dimensions of strength. The most common measure of adhesive strength is overlap shear. But in the real world, of course, adhesive bonds are subject to a wide range of temperatures and environmental conditions; a wide variety of peel, impact, tensile, vibration and other forces; and multiple surface preparation methods. Not only are higher overlap shear ratings poor predictors of higher ratings in these other dimensions, they are often diametrically opposed.

For instance, high-strength double-sided foam pressure-sensitive bonding tapes typically provide about 150 psi in overlap shear at room temperature. Typical rigid, two-part epoxies provide 3,000 psi in overlap shear at room temperature. Yet high-strength, double-sided bonding tapes provide up to six times as much peel strength as rigid, two-part epoxies. Moreover, the viscoelastic properties of the lower shear-strength, double-sided foam tape provide greatly improved vibrational damping characteristics and often greater impact strength than most high-strength liquid epoxy adhesives.

Spec more than usage stress.

Good specifications depend on good market research, and good market research depends on the development of a comprehensive profile of the stresses to which end users will subject a product. Right?

Partly right.

Think, for example, about a window. Other than an occasional interaction with a golf ball, most windows see very low impact stresses once installed. Yet windows can be dropped during manufacture, vibrated during shipping, and shoved into place during installation. Clearly, an adhesive specification based solely on the low impact stresses of an installed window could fail in these other parts of the product life cycle.

To help users objectively evaluate adhesives and bonding tapes, most manufacturers test according to standards such as ASTM D 1002-94 "Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (metal to metal.)" These standards usually involve tests on well prepared/etched aluminum panels at room temperature in laboratory settings, and generally provide data on the maximum possible shear strength of the adhesive.

Most users of adhesives and bonding tape, however, find it difficult to mimic these perfect conditions in their manufacturing facilities. Most materials require some form of surface preparation prior to application. Consider, for example, the dramatic effect that different methods of surface preparation can have on the shear strength of a liquid adhesive. The table above shows that overlap shear values of an epoxy adhesive can vary from 444 to 3,091 psi when subjected to different methods of surface preparation.

One might argue that these tests of maximum performance at least provide a relative measure of adhesive and bonding tape performance. While this may be generally true at typical room temperature conditions when considering materials from similar chemical families (e.g. an epoxy compared to an epoxy), it is generally not true when comparing materials from different chemical families (e.g. a silicone compared to an epoxy). The chart above, right, shows the shear strength of two-part, room-temperature curing epoxy adhesive compared to a one-part, moisture-curing silicone adhesive. At room temperature, the epoxy adhesive is about four times as strong as the silicone. But at higher temperatures, the silicone is stronger.

Plan for best, select for worst.

You may be wondering how one can possibly select an adhesive or bonding tape by considering all stresses in all parts of the product life cycle under all types of surface preparation variables in the assembly environment. Imagine for instance, that you wanted to specify shear, impact, peel, and fatigue strength for your adhesive at three temperatures under three levels of humidity, subject to two types of solvents and three methods of surface preparation.

Because most manufacturers don't provide this level of detailed data on their adhesives and bonding tapes (with the notable exception of aerospace adhesives), you may be tempted to conduct these tests yourself. Assuming three repetitions of each test, a designed experiment would require at least 51 trials for each measure of strength or 204 trials altogether. Do you really have the time, staff, or inclination to do this many trials?

A simple and practical approach to selecting adhesives and bonding tapes is selection for the worst case. If an adhesive or bonding tape will survive the worst possible situation, it will usually survive milder situations. Worst case tests can either be performed by you, or, if resources are limited, by your supplier. By testing only for worst-case conditions, the number of variables under consideration will be dramatically reduced and the selection of an appropriate adhesive or bonding tape can be easy and quick.

For most adhesives and bonding tapes, minimal expected surface preparation is a good worst case condition to test. While there is not necessarily a need to test all worst case conditions, other conditions are listed in the table on page 72.

Once you have narrowed down your list through a consideration of performance under worst-case conditions, it is time to test your final candidates. In testing, it is important to be both tough and realistic. You can be tough by creating an "ironman test," which links several worst-case conditions together, for example maximum UV exposure followed by maximum impact strength. But also be realistic. Don't test impact strength of your window frame at -150F if the window will never get below -30F.

Remember, too, that the purpose of your testing is not only to achieve statistical confidence but also "causal confidence." You want to understand not only when your adhesive or bonding tape will fail, but also why it fails. In many cases, the mode and conditions of failure will provide important clues regarding alternate materials, alternate methods of surface prep, better joint design, and so on. To achieve causal confidence, talk frequently with your supplier's technical service department, ask for an interpretation of your test results, and repeat as many times as necessary until you have gained confidence that you thoroughly understand the behavior of your final choices.

Select by application. Many suppliers provide descriptions of applications of adhesives and bonding tapes with statements such as "general purpose bonding of metals, ceramics, glass, and most hard plastics." While these types of statements are effective at generating calls to the sales office, they provide little help in selection. Additionally, these general statements provide little insight into the suitability of the material for a particular type of manufacturing process.

In such cases, look for a supplier with application experience in your industry or with the ability to test to the conditions your industry requires. Your first question to an adhesive or bonding tape manufacturer should be "What adhesives or tapes do you supply that have been used for X (your application)?" If they can't name a material for your application, you should ask what adhesives or bonding tapes are available for applications analogous to the worst case conditions your design is likely to see.

Your job will be greatly simplified in those cases when an adhesive or bonding tape has been specifically designed for, and proven in, an application. Behind every successful application is an extraordinary amount of previous laboratory testing and trial and error in real world conditions-including manufacturing conditions.

This is not to suggest that your own testing can be eliminated. No two applications are ever exactly the same, and you must thoroughly test any adhesive or bonding tape you plan to use. However, the initial adhesive and bonding tape selection process can be greatly simplified if a manufacturer can provide evidence that others have successfully used the adhesive or bonding tape in an application identical or similar to yours.