A protocol for determining the surface free energy of dental materials

Objective. The purpose of this study was to develop a standard methodology for measuring the surface free energy (SFE), and its component parts, of dental biomaterials. Methods. The contact angle of each of four samples of two materials—low density polyethylene and poly(methyl methacrylate)—was measured three times in each of six liquids (1-bromonaphthalene, diiodomethane, ethylene glycol, formamide, glycerol and distilled water). Critical surface tension estimates were obtained from Zisman plots. Data were then analyzed by the least-squares method to estimate the components of SFE. Estimates were also made for each of 12 liquid triplets, and by maximum likelihood and Bayesian analyses. Results. The use of liquid triplets could yield misleading estimates of the components of SFE. A testing protocol is suggested in which multiple test liquids are used, and multiple methods of statistical analyses employed. Significance. SFE is important, in that high SFE is desirable when adhesion is required, but undesirable if plaque resistance is needed. Methodology that avoids some of the limitations of existing studies has been proposed.