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Summary:
A new method is described for determining the surface free energy (SFE) of a film or flat solid sample in terms of its polar and non-polar (dispersive) energy components. The method is based on the static contact angle technique, but offers an alternative to the use of methylene iodide or bromonaphthalene for determining the non-polar component. Instead, contact angles of two polar liquids (such as ethylene glycol and dipropylene glycol) which are chemically similar but which differ in degree of polarity are taken. It is not necessary to know the polar and non-polar components of these probe liquids since only the total surface tension for each is used in the calculations. The cosines of these two contact angles are extrapolated to determine the non-polar component of the surface free energy of the solid. The polar component is then calculated from the contact angle of water on the solid. Advantages of using this method include greater availability of the probe liquids, lower solvency power of the probe liquids, and greater reliability of results as compared with methods using methylene iodide or bromonaphthalene.