Myth #3: “I Tried One so I’ve Tried Them All”

If an adhesive failed to work, even on an entirely different project, the decision to trial another adhesive can be difficult. This is especially true if the engineer has historically had success with traditional fastening methods. Before giving up structural adhesives, it’s important to investigate why the previous option failed to meet expectations. For instance, there may have been process issues such as surface preparation, dispensability or cure time, or even end-use issues such as a poor match between the properties of the adhesive and the mechanical properties (modulus, elongation, etc.) or environmental resistance required. Because there are so many different structural adhesive chemistries and 3M has many options within each family, knowing the reason for failure can help guide the choice of a different solution.

For instance, there are three main families of structural adhesives: acrylic, epoxy and polyurethane, and while they’re all strong, each has different benefits. Acrylics build strength the fastest, epoxies have higher overall strength and superior environmental resistance, and polyurethane-based products provide energy absorption and vibration damping. Processing characteristics such as shelf life, rate of strength build and the amount of surface preparation required can also affect how well a structural adhesive works for a given assembly.

Viscosity affects how an adhesive stays in place when applied. Low-viscosity self-levelling adhesives can pool to fill gaps but will run on non-horizontal surfaces. High-viscosity non-sag adhesives will stay in place even ‘underneath’ the substrate but may need to be spread at application to achieve coverage. 3M makes adhesive at both extremes and all the way between to meet different production needs.

1,000 psi overlap shear strength is the minimum for an adhesive to be considered structural, but many applications require significantly stronger adhesives. Knowing how much strength you need can help you test and select the right adhesive without risking bond failure or overengineering your bonding solution.

Instant adhesion allows zero work time, so parts must be applied correctly the first time but are then strong enough to move to the next processing step. Longer work times allow greater opportunity to adjust part placement while the adhesive is still workable. Whereas instant adhesives (cyanoacrylates) are true to their name, the work times of certain other adhesive chemistries can be adjusted by slight variations in the formulation.

Elongation measures how much an adhesive bond will stretch or flex before failing, and of course different applications have vastly different requirements. Very stiff bonds will transfer nearly all applied energy, which is good for something like the head of a golf club where it is bonded to the shaft. Other applications such as panels on a trailer may be better with a more flexible bond that can absorb energy to reduce noise and vibrations, or bonds between joints on a building that have to flex with normal building movement but still maintain integrity.