The origin of surface tension in a liquid is the forces of attraction between the molecules that make up the liquid. In the absence of other forces, this mutual attraction of the molecules caused the liquid to coalesce to form spherical droplets. This can be seen, for example, when rain falls on a freshly waxed car body.
As a general rule, the greater the proportion of polar groups (e.g O-H groups) in a molecule the stronger the attractive forces between them. Strong attractive forces give rise to a high surface tension and a tendency to form discreet droplets on a surface rather than wet it evenly. The large proportion of O-H groups in water are responsible for its high surface tension. Alcohols, with their smaller proportion of O-H groups, have lower surface tensions.
Clearly, all things being equal, the lower the surface tension of a liquid coating, the easier it will be to form a satisfactory wet film from it.
Surface tension can be thought of as the force that holds a liquid together. In the depths of a volume of liquid, each molecule is surrounded on all sides by other molecules; the forces between them balance out and the entire mass is in equilibrium. The situation is different at the surface of a liquid. At a liquid-air interface for example, the molecules at the surface are being attracted by the surrounding liquid but not by the air. The forces are imbalanced and consequently the liquid behaves as if had a stretched skin.
Surface tension can therefore be quantified in terms of the forces acting on a unit length at the liquid-air interface. The units are dynes per centimetre (dyn/cm) or newtons per metre (Nm-1). 1 dyn/cm is equal to 0.001 Nm-1 or 1 mNm-1.
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 wooden desk top 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 desk top, 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 (see above).
The water droplets that land on the un-polished wood 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 desktop and more specifically from the forces that hold the molecules of the desktop material together.
A desktop 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 un-polished wood 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.
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.
There are several methods of quantifying the adhesion of a coating to a substrate and these are described on our Mechanical Properties page. 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.
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