Surface energy is the relative measurement of energy found at the surface (surprisingly) of a solid. It is usually only used to describe solids – surface tension is most commonly used for the same phenomenon of liquids. Whilst the atoms in the ‘bulk’ of the material are relatively stable, with a balanced set of bonds (or interactions), the atoms found on the surface have an incomplete, unbalanced set of bonds, complete with unrealised bonding energy.
This ‘free’ energy at the surface is part of the way a solid interacts with the world – a low surface energy material, such as Teflon is great for use on frying pans because it does not interact with the world significantly. Wood has a significantly higher surface energy and is easy to paint and coat. In the manufacturing world, we most often deal with common plastics such as Polypropylene (PP), Polyethylene (PE), Nylon (PAx) or Polycarbonate (PC) which generally have mediocre surface energy in the 30mN/m to 40mN/m range. Low surface energy (normally considered less than 40mN/m) react with the world less and less as the energy gets lower.
The other side of the coin is the surface tension of a liquid, which works in reverse. A low surface tension liquid will react with the world readily – wetting and flowing over a surface with ease. A high surface tension liquid will try to minimise its surface area and not react with the world, this normally looks like a sphere – think water when seen in space images.
How is it measured?
A high surface energy is vital for wetting, which is the first step in getting an intimate covering of a coating, adhesive or ink on to a surface. Surface treatments are one of the common methods for increasing surface energy. There are numerous types of treatments including (but not limited to), Plasma and Corona treatments, UV, Ozone and Flame treatments.
A safe, readily available and cheap solvent is water – but with a surface tension of 72mN/m, which is very high, it prefers to keep to itself and not react with the world. To overcome the liquid being a sphere where it is energetically stable, the solid it is going on to must have a higher surface energy than the surface tension of the liquid.
Using our example of a water-containing liquid, such as printing ink, the water-based ink will have a high surface tension and prefer to be a sphere. Unless the solid has a high surface tension, it will not attract the ink toward it and the liquid will sit like water on a waxed car bonnet. With a high surface energy, the solid will pull the liquid closer, leading to increased wetting, flow and chance of interaction and bonding compared to a low surface energy.
In the past, rather than increasing surface energy, people have lowered surface tension. This makes scientific sense, but the problem is that these low surface tension liquids become expensive and dangerous. MEK, xylene and trichloroethylene based products were once very normal, but these have been linked to serious health risks and cannot be used in the same way in the modern factory.
If we measure both surface energy and surface tension, it gives us a picture of the molecular world so we can predict the way a material will react, and can be used as a quality tool to confirm your process will work.
Plasma treatment
Plasma treatments are often used to increase surface energy, as well as cleaning surfaces. Air is ionised with high voltage and the energised air is blown out of a nozzle, or contained within a chamber, creating a reactive, potential free plasma. Plasma interacts with the surface in several ways (via ablation, bombardment with electrons and ions or reactions with species in the plasma itself) and the exact impact of plasma treatment itself depends on the gas used. When used as a nozzle, the treatment comes out powerful for fast treatment of specific areas – perfect for extrusions such as cable and pipe printing, or coupled with robotics as a pre-treatment prior to adhesive dispensing. Chamber systems are designed to create an environment which is entirely plasma, giving the ability to treat complex shapes or components without the need for tooling or programming.
Corona treatment
Whilst similar to plasma treatments, corona treatments use high voltage, but unlike plasma, you use the voltage as part of the treatment and the component goes through this voltage. This technique involves the creation of ionised gas by discharging high frequency voltage from an electrode, over a grounded surface. The surface energy in this case is raised as solid phase is passed under the electrode, treating the surface. Manipulating high voltage in this way allows for wide treatment or specific area treatment, making it perfect for high-speed specific applications such as medical device treatment, or for web, foam or board treatment.
For more information on how Tantec UK & Ireland can help you with surface energy measuring, head to https://www.tantec-uk.com
