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Feb 12, 2014
Surface tension can be considered in various ways, but perhaps best as a result of the forces of attraction existing between the molecules of a liquid. It is measured by the force per unit length acting in the surface at right angles to an element of any line drawn in the surface (mN/m).
Wetting is the effect of displacing one fluid from the substrate surface by another liquid. In general, the liquid must have the same or a lower surface tension than the substrate; otherwise, no wetting will be achieved. This will produce a positive spreading coefficient and lead to a good coating. This can be best expressed in equation (1). By addition of the correct additive, the surface tension of the coating can be modified to improve substrate wetting. The additive acts by lowering the surface tension of the coating, enabling it to wet-out over the substrate.
Levelling is the process of eliminating the surface irregularities of a continuous liquid film under the influence of the liquid’s surface tension. Levelling is an important step in obtaining a smooth and uniform film. Concentration and surface tension gradients in coatings are the main causes of poor levelling, which develop as the coating dries, due to variations in drying rates. Addition of the appropriate additive will modify the surface tension. For instance, a silicone additive will migrate to the liquid/air interface, giving an even surface tension across the film during the drying process, thus reducing surface tension gradients.
Wetting and levelling are therefore totally different processes. Though the surface tension of the coating plays an important role in both processes, the effects of surface tension on spreading is opposite to that observed in levelling. Substrate wetting can be subdivided into two distinct categories: “spreading wetting” and “adhesion wetting.” Spreading wetting is necessary for even coverage of applied films, in particular, for the low-surface-energy substrates of many plastics.