Monday, 22 June 2015

Surface Energy

Surface energy [: defined as the excess energy at the surface of a material compared to the bulk or the work per unit area done by the force that creates the new surface]

How can a surface have energy?

At first sight this is not an unreasonable question. Energy is defined as the capacity to do work and if we take the example of the average automobile panel, it is difficult to find evidence that it is engaged in any form of work.

The situation becomes clearer when we spray water droplets on a cars roof, part of which has been wax polished. The droplets that land on the polished areas will form discrete near-spherical droplets. This is due to the surface tension of the water

The water droplets that land on the unpolished panel behave differently. They tend not to form droplets but to spread out to form a thin film. In other words the surface tension forces that hold the water droplets together have been overcome. It takes energy to overcome the surfaces tension forces and this energy has to come from somewhere. In fact it comes from the surface of the panel and more specifically from the forces that hold the molecules of the panel’s material together.

A panel which has been polished using a hydrocarbon wax will have a surface rich in hydrocarbon molecules. The forces that hold hydrocarbons together are much weaker that the forces that act between water molecules and consequently water on a hydrocarbon surface remains in the droplet form.

An unpolished surface will have at its surface a complex mixture of molecules made from carbon hydrogen and oxygen and (unlike hydrocarbons) there will be a significant proportion of polar groups (e.g. O-H) present. The forces of attraction between polar molecules are stronger than those between non-polar hydrocarbon molecules and in this example they are sufficiently strong to overcome the surface tension forces of water and cause the droplets to spread out and form a film.

It is common in the coatings industry to refer to low energy and high energy surfaces. Polyethylene and polypropylene are examples of low energy surfaces. The forces between the hydrocarbon molecules that make up the polymers are weak and consequently polar liquids tend to form droplets on the surface rather than spread out.

It is difficult to coat low energy surfaces but fortunately there are numerous ways of converting low energy into high energy surfaces. All the methods aim to form oxygen containing species at the surface and this oxidation can be achieved by exposure to ultraviolet radiation, plasma or corona discharge or by flame or acid treatment.

Surface energy is quantified in terms of the forces acting on a unit length at the solid-air or the solid-liquid interface. The units of measurement are exactly the same as for surface tension.

Contact Angle

The definitions of surface tension and surface energy have involved consideration of the behaviour of liquids in contact with solids and the formation of droplets or thin films. One convenient way of quantifying this behaviour is to measure the angle θ formed by the liquid-solid and the liquid-liquid interfaces:-

If θ is greater than 90° the liquid tends to form droplets on the surface. If θ is less than 90° the liquid tends to spread out over the surface and when the liquid forms a thin film, θ tends to zero.

Surface energy + Adhesion + Contact angle 


Adhesion [: is the tendency of dissimilar particles or surfaces to cling to one another (cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another)]

There are several methods of quantifying the adhesion of a coating to a substrate and these are described above. Although none of these methods requires a fundamental understanding of the mechanism of adhesion, it is appropriate to mention it here because surface tension, surface energy and adhesion are all interrelated.

The numerical difference between the surface tension of a coating and the surface energy of a substrate has a profound effect on the way in which the liquid coating flows out over the substrate and on the strength of the adhesive bond between the substrate and the dry film.

If the surface tension of the coating is greater than the surface energy of the substrate then the coating will not spread out and form a film. As we increase the surface energy of the substrate, we can reach a stage where the coating will spread out and form a film but, when dry, has poor adhesion. Further increases in the surface energy of the substrate will result in easier wet-film formation and better dry-film adhesion.

It is important to emphasise that surface energy is only one aspect governing the complex phenomenon that we refer to as adhesion. Adhesive testing involves the application of force to remove the coating from the substrate. The intention is to measure the force needed to overcome the forces of adhesion between coating and substrate. In practice however, the cohesive strength of the coating and of the substrate both have an effect on how easy it is remove the coating. In fact there is a supportable case for saying that there is no such thing as a true adhesive failure since, at the molecular level, all failures are cohesive failures of the coating or the substrate.


1. PRA Coatings Technology Centre Publications

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This was a tough subject to explain, so I hope this makes sense and helps you. As always if you have questions, I’ll do my best to answer; bear in mind the only stupid questions is the one that was unasked. Questions and/ or constructive comments are always appreciated

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