What is surface energy?
Surface energy describes the degree of attraction or repulsion force from a substrate to another material. It can be thought of in terms of a car that has not been waxed for some time. When water contacts the surface, it spreads in large puddles; exhibiting high surface energy (HSE) – the molecular attraction allows the water to flow. In comparison, when a car has recently been waxed, water beads up into small spheres, exhibiting low surface energy (LSE) – the liquid does not flow out.
How does surface energy influence adhesion?
Surface energy affects an adhesive’s ability to ‘wet-out’ or spread over the substrate. Substrates with a low surface energy can be difficult to bond to because the surface resists adhesive wet-out, leaving poor surface contact and low adhesion. The opposite is seen with high surface energy substrates; adhesive can flow/wet-out easily, which assures greater surface contact and a stronger bond.
Bonding to high surface energy substrates
High surface energy surfaces include metals and some plastics, for example, copper, aluminium and stainless steel metals and Kapton®, Nylon and Polyester plastics. Before specifying a bonding tape, consideration must be given to any surface treatments or coatings to the substrate, for example a metal will have a HSE but powder coating will turn this into a LSE substrate. For bonding two different substrates, double sided differential tapes are available. These have different adhesives on each side.
Bonding to low surface energy substrates
Low surface energy substrates include plastics such as PVA, Polyethylene, Polypropylene and PTFE. It is possible to bond to LSE substrates through surface modification techniques which can change the chemical composition to increase the surface energy. These techniques include flame, corona or plasma treatment, acid etching or use of solvent based adhesion promoters that contain higher surface energy resins which entangle with the low surface energy substrate when the solvent swells the surface.
Specially formulated adhesives are also available for bonding to LSE substrates; these contain modified acrylic and synthetic adhesives with better flow characteristics. Rubber based adhesives offer better adhesion to LSE substrates as they are soft and flow well.
Ensuring an optimum bond
Whether you’re bonding to a HSE or LSE substrate, there are a number of things you can do to ensure you’re achieving an optimum bond. We’ve put together some helpful guides on using adhesive materials, including the correct surface preparation of your substrates. The key things to consider are summarised below: