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Surface Energy

Asked by November 13, 2018

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.

Aluminum Bonding

Asked by September 4, 2018

Aluminum is the most abundant metal on the face of the planet. Its high strength-to-weight ratio, corrosion resistance, and range of alloy properties has made it one of the most useful engineering materials. Aluminum alloys can be found in industries ranging from electronics to automotive to aerospace and beyond. Although many of its properties are attractive, there are unique challenges with attaching it to other parts in an application.

Common attachment methods include screws, rivets, and bolts. These work in many applications, but there are a few downsides to using traditional fasteners. One is that point fasteners create stress concentrations which can lead to material failure under high load scenarios. Another is that these create a hole in the material which may be undesirable for strength or water intrusion issues. In certain applications, aesthetics is a primary concern and the appearance of a screw head may be unwanted. In the case of aluminum welding, highly skilled workers are needed to get a good weld.

In many applications, aluminum is being attached to different materials which can be challenging if they have different thermal expansion rates (Coefficient of Thermal Expansion or CTE). Aluminum’s CTE is twice as high as glass while some plastics are five times higher than aluminum. This can lead to buckling or cracking depending on the rigidity of the joined materials. The stress concentration created with point fasteners is exacerbated when attaching to plastics that are not as strong as aluminum. Galvanic corrosion needs to be managed if the secondary substrate is another metal.

A common solution that addresses these concerns is adhesive bonding. Adhesives spread stress over a larger area which reduces stress concentrations. They can also be designed to either transfer or isolate loads depending on adhesive selection. Isolating loads can make it easy to design with CTE differences while maintaining a flexible and strong attachment.

Water intrusion is easier to manage when you have fewer holes (water intrusion points) to start with. In addition to keeping water out, adhesive joints can also reduce galvanic corrosion by separating dissimilar metals. Gaskets, tapes, and liquid adhesives are all used to separate mechanically joined metals, but removing the contact altogether with a high strength bonding tape or adhesive can improve the corrosion resistance even more.

Perhaps one of the most important benefits with using tapes or adhesives is the speed and ease of creating a bond. Even with low-skilled labor, a durable bond can be formed quickly and easily.

Aluminum has excellent compatibility with most adhesives. It has a very high surface energy which allows adhesives to flow onto its surface, and the surface of aluminum is receptive to many types of bonding. That is the reason that aluminum is rarely found as a pure metal in the environment – it is almost exclusively found alloyed with other materials. It is this reactivity that gives aluminum alloys a protective oxide layer. Aluminum forms this microscopic oxide layer very quickly when exposed to air.

The oxygen in air reacts with the active surface of aluminum creating a passive layer of aluminum oxide that prevents additional corrosion (called passivation). This is similar to steel rusting, but unlike rust which flakes off, aluminum oxide stays put to protect the surface.

Structural epoxies will adhere well to the oxide layer, but it is so strong that when put to the test, the oxide layer fails first. The oxide layer can also attract moisture which further undercuts the bond strength. This weak interfacial layer limits the holding power of structural adhesives and should be removed prior to bonding. Typical surface preparation includes cleaning with acetone to remove top-layer dirt and oil, abrading with a fine abrasive (3M™ Scotch-Brite 7447 pads work well), then finally cleaning with acetone. The final cleaning step should be repeated until all residue is removed. At this point, bare aluminum is exposed and ready for the adhesive.

Apply adhesive shortly after cleaning to ensure the oxide layer does not reform. Once formed, a structural epoxy bond is extremely strong and can transfer loads efficiently in high stress applications. Structural adhesives are used extensively in the transportation, aerospace, and electronics markets to create rigid aluminum bonds.

Foam tapes, like 3M™ VHB™ Tapes, also require a clean surface free of dirt and oil, but removal of the oxide layer is optional. The viscoelastic properties of VHB tape gives it the ability to isolate stresses. When a force acts on the bond, the tape directs the force into its core. The core absorbs the stress which reduces the stress on the interface between the tape and the substrate. The isolating nature of VHB tape also makes it useful for absorbing vibrations and bonding materials with differential CTEs. A common application for VHB tape is bonding glass windows to aluminum frames in industrial structural glazing which requires a strong bond with careful isolation of substrate movement due to thermal cycling.

Post a question to the forum if you have specific aluminum bonding questions. We look forward to hearing from you.