Surface energy and surface tension play important roles in adhesive bonding. Surface energy can be used to estimate a substrate’s receptivity to adhesives while surface tension can be used to estimate an adhesive’s ability to flow over a substrate. This flowing is also called “wetting” or “wet-out” and is an important part of choosing the best adhesive for a substrate.
Higher surface energy materials tend to be easier to stick to and lower surface energy materials tend to be harder to stick to. For example, an aluminum frying pan tends to be easier to stick to than a PTFE coated non-stick pan because aluminum has a much higher surface energy than PTFE. The general rule of thumb is that if the surface has a higher surface energy than the liquid’s surface tension, it will flow and wet-out the surface. That’s why water (surface tension = 72 dyn/cm) will wet-out an aluminum pan (surface energy = 840 dyn/cm), but it will bead-up on a PTFE non-stick pan (surface energy = 18 dyn/cm).
This occurs because molecules prefer to be in lower energy states whenever possible. Surfaces are less stable than the bulk material, so when a liquid contacts the material’s surface, the material tries to pull that liquid over the surface to make more stable bonds and lower their energy. The liquid prefers to be formed in a ball – the lowest surface area to volume ratio and hence the lowest surface energy it can get by itself (usually called “surface tension” for liquids). The material tries to pull the liquid over its surface, and the liquid tries to resist and ball up on the surface. The net interaction needs to result in lower energy for the system of the two. If the surface energy of the material is greater than the surface tension of the liquid, the material will pull the liquid over itself. If the surface tension is higher than the surface energy, the liquid will resist flowing onto the surface.
The same thing happens when you dispense a liquid adhesive onto a substrate, so considering the surface energy of substrates is a common way to estimate whether an adhesive will bond to a substrate. If it doesn’t wet-out, it doesn’t bond. Pressure sensitive adhesives (PSA or “tape”) also rely on good wet-out to form a bond. Instead of curing on a surface, PSAs flow into the microstructure of the surface and form bonds through intermolecular forces. Higher surface energy substrates tend to pull PSAs better and hold on to them better than lower surface energy substrates which tend to prevent this close contact. This is also the reason why pressure is used to activate a PSA – by applying pressure you are helping the wet-out process begin. After the initial pressure, the PSA will continue to wet-out the substrate.
Knowing the surface energy of your substrates can help inform adhesive selection. While there are exceptions due to chemical reasons (covalent bonding, diffusion, etc), the following chart should be helpful for estimation purposes. The broad categories are High Surface Energy (HSE), Medium Surface Energy (MSE), and Low Surface Energy (LSE).
Most adhesives will bond most HSE substrates after cleaning the substrate. Rubber adhesives/PSAs tend to bond most MSE substrates. Acrylics will bond to many MSE substrates but may benefit from using a primer to increase adhesion. Epoxy adhesives will struggle with most MSE substrates. LSE substrates can be very challenging to bond and often require specially designed adhesives and/or surface treatments and primers to get a good bond. 3M™ 300LSE Low Surface Energy Acrylic Tapes and 3M™ Scotch-Weld™ Structural Plastic Adhesive DP8010 Blue are two examples of adhesives designed to bond to some LSE substrates without a primer.
Surface energy is just one component of adhesion. Adhesion can also be affected by surface roughness, contamination, surface treatments, diffusion, temperature, CTE, active chemistry, and other factors. Surface Energy can be helpful to make initial selections for testing, but it cannot replace real-world testing to determine whether a specific adhesive is appropriate for a specific application.
Post a question below if you have specific bonding questions. We look forward to hearing from you.