The effect of different calculation methods on surface free energy of substrates used in printed electronics and the solution based on the laws of thermodynamics

B.N. Altay, P.D. Fleming, M. Bolduc, S.G. Cloutier Western Michigan University, United States

Estimating surface free energy (SFE) of solids is of great interest in many fields, including surface chemistry, material characterization, coatings, graphics printing, adhesives, printed and flexible electronics, biomaterials, oil recovery, medical engineering applications to deal with wetting, solubility, surface contamination, adsorption, absorptivity, adhesion or bonding issues. There are numerous theoretical and semi-empirical models and liquids used to estimate surface free energy (SFE) of solid surfaces. The widely-known Owens-Wendt (OW) model makes use of surface tension of at least two liquids with known dispersive and polar components and their contact angles on a given substrate to estimate the SFE. Using different estimation models and liquids may result in significantly different SFE value for the same substrate. Like the OW method, an alternative new model considered for this study also makes use of a series of liquids to estimate the SFE. Each liquid provides an absolute lower bound on the energy of the surface with some uncertainty from measurement variation. If multiple liquids are employed, the largest lower bound is taken as the most accurate, with uncertainty due to measurement error. The more liquids used, the more accurate will be the greatest lower bound. The alternative method presented here links generalizations of the Good Girifalco equation with a general thermodynamic inequality relating the 3 interfacial tensions in a 3-phase equilibrium system. The method always satisfies this inequality with better than a 65% certainty. However, the OW method seldom, if ever, conforms to this inequality and even then, the degree of satisfaction is not significant.