On the work of adhesion and peel strength between pressure sensitive adhesives and the polymeric films used in LCD devices

Abstract

Peel strength, a convenient measure of bond strength in adhesive/adherend systems, is known to be a function of various factors such as the thermodynamic work of adhesion, rate of measurement, thermal history, and temperature. Generally, it is believed that the work of adhesion is primarily involved in the first stage of adhesion through wetting phenomenon and beyond that its role diminishes in that the portion of thermodynamic contribution to actual bond strength is insignificant. In practice, however, we often observe that a suitable surface treatment increases the surface energy of the substrate, which further enhances the bond strength. One practical example is the surface treatment carried out in LCD industry to obtain sufficient bond strength between pressure sensitive adhesives and polymeric films. To further our understanding of the effect of surface treatment, we attempted to establish a possible correlation, if any, between the thermodynamic work of adhesion and peel strength. For this, we carefully measured the contact angles of water and diiodomethane against various polymeric films, and calculated the surface energy and the thermodynamic work of adhesion using the two widely used approaches: Young-Fowkes-Girifalco-Good, and Wu methods. Before establishing a correlation, some general aspects of the above two methods are discussed. The values of the work of adhesion obtained were compared with the measured peel strength. Indeed, we observed a clear correlation between the two quantities: the increase of the work of adhesion led to the increase of peel strength. As a reason for this correlation, we proposed that the increase of surface energy might be associated with the increase of various surface functional groups, which, in turn, contributed to the formation of chemical bonding with the PSA leading to the increase of peel strength.