A comparison of different methods to calculate the surface free energy of wood using contact angle measurements

A number of controversies concerning the correctness of the different approaches to determine the wood surface free energy and its components from contact angle measurement arose in the past, and it is not clear which approach one should follow. In this work the advancing contact angles of pure liquids on microtomed spruce surface were used to determine the surface free energy of wood. The most widely applied approaches for the surface energy of wood: the Zisman approach, the equation of state, the harmonic mean equation, the geometric mean equation and the acid–base approach were compared and the usefulness of this approaches referring to wood as material were discussed. It was found that the acid–base approach delivers a maximum of information about chemical composition of the natural polymer wood, which consists mainly of cellulose, lignin and a variety of hemicelluloses. Therefore, the acid–base approach is most suitable to explain the coating properties (adhesion) of wood surfaces. Furthermore, it is possible to improve the accuracy of the determination of the surface free energy of wood by increasing the number of different liquids applied. A computerized method for the calculation of surface energy parameters from all five liquids used to obtain an approximate solution for disperse, acid and base components of surface free energy greatly facilitated this task.