Structural Adhesives FAQs

How cured is the adhesive?
If the adhesive has just reached handling strength or is still somewhat liquid, temperature exposure will do something different than if it’s fully cured three weeks or six months after you assembled it.

What is the absolute magnitude of temperature that will be seen in the application?
How high is the high and how low is the low? This helps understand whether there will be any thermal degradation issues due to the adhesive reaching those extremes.

How long does the assembly see those temperature extremes and every point in between?
If a part sees an absolute high of 150°C, it makes a difference whether it sees that high for five minutes or five weeks, so you have to think about total temperature exposure and any degradation effects based on that. Frequency is also relevant: how often does the part move to the temperature extremes? An outdoor application in the desert that cycles every 24 hours between 40°F at night and 115°F during the day is very different from something that sees the same extremes but for months at a time with a year-long cycle.

What is the actual load being applied to the adhesive while it’s exposed to the temperature?
This last question may be the most important. Even if the adhesive doesn’t suffer from thermal degradation, it’s still a polymer and will undergo physical changes. Specifically, as temperature increases past a certain point (the Glass Transition Temperature) it will go from a glassy rigid state through a transition to a softer, rubbery state. Physical properties of the adhesive will change as it warms and cools through the transition stage, including rigidity, thermal expansion coefficient and heat capacity, among others, and this can affect the load-bearing ability of the adhesive.

That said, there are four broad categories of substrates, and even within those there are different adhesive chemistries that bond to each one.

Metals have very high surface energy, so if the surface is clean and dry the adhesive should readily wet out, but all metals are not the same. Take aluminum versus copper. Aluminum is a passivated (inactive) metal and fairly inert, whereas copper is an active metal that will continue to corrode, so even with surface prep considerations you need to consider whether there will be degradation over time from the corrosion.

Traditional materials are things like glass, wood, leather and concrete. They have a middle range of surface energies, but each usually has some unique factor that must be considered. Roughness is one example. Another is natural leather that contains oils from the tanning process – over time these can leach into the adhesive, plasticize it and degrade the bond. Hydrolysis of glass means that moisture penetration is sensitive when you’re bonding glass to ensure it doesn’t degrade.

Engineered plastics are higher surface energy performance plastics like acrylic, polycarbonate, ABS and epoxy-resin composites. These materials are really unique because bonding isn’t just about surface energy – the adhesive may wet out across the surface, but it’s ability to bond will also be dictated by the crystallinity and polarity of the plastic. A material like nylon has a fairly high surface energy, but it’s very crystalline and not very polar. When you look at some of the mechanisms of adhesion, many of the adhesives may bond initially, but then over time the adhesive will fail unless you do more rigorous surface preparation.

Low-Surface-Energy plastics (LSE plastics) are commodity-type plastics like polypropylene and polyethylene, and also things with really low surface energies like fluorinated plastics and silicones. Polyolefins and LSE plastics are kind of a category in themselves because you will need to use primer or some kind of corona treatment, or use a specialty adhesive that’s designed to kind of penetrate into that plastic and create an entangled bond with the polymer of the substrate itself.

All those variables show why there is no easy answer and you typically still need to do testing and prototyping to make sure an adhesive works in your process. A good first place to look at substrate information is the Bonding and Assembly pages of 3M.com, which have more extensive background on these topics.

A second suggestion is to review the technical data pages of adhesives you’re looking at because they show adhesion to a lot of different substrates. These pages typically report two things: a number showing strength under stress in either psi or megaPascals (for overlap shear) or pounds per inch (for peel), and also a failure mode. A failure mode of cohesive failure means that adhesive tested under the conditions listed remained bonded to both substrates after it pulled to failure: the adhesive itself failed rather than the bond. Adhesive failure indicates the adhesive pulled away from one of the substrates. This can provide a rough guide as to whether an adhesive might be suitable and should remain in your consideration group.

The third option, if you have a specific substrate in mind or a question about an additive that might be migrating into the adhesive, is to reach out to 3M. Our technical team can look at what might be happening and do a technical service request to try to help you understand which adhesives over the duration of the part might be a better option.