Validation of Methods for the Optical Characterisation of the Wettability of Polymeric Films for Food Packaging

A methodology is described for the theoretical-experimental evaluation of the measurement uncertainty of the polar and dispersive components of the surface free energy (SFE) in polypropylene films; these parameters are related to the film wettability of adhesives and inks. The proposed method is based on the measurement by means of a vision system of the contact angles of liquid drops deposited on the film itself, which allows for obtaining, through mathematical models drawn from the literature, the physical quantities of interest. The effect of the principal influence parameters has been experimentally evaluated, and testing has allowed the defining of the technical procedures readily transferable in the industry. The uncertainty assessment is interesting not only to correctly evaluate experimental data but also to characterise the reproducibility of the effects of techniques for improving the wettability of films, such as surface treatments. 1. Introduction The possibility of measuring the wettability of polymeric films, particularly of polypropylene materials for food packaging applications, is becoming increasingly important from an industrial and economic standpoint because it determines the possibility of using new materials for bonding and new inks for printing. These technological innovations, provided they ensure a satisfactory and stable surface adhesion, allow for creating packaging that is increasingly pleasing from an aesthetic point of view and for geometries of packaging that are more and more particular, as the food market requires. If, instead, the characteristics of wettability of adhesives are taken into consideration, their better characterisation allows for reducing the area of ？？overlap for the gluing and consequently to reduce the film surface required by more than 10%, as experimentally verified. The wettability is the technological characteristic that takes into account the capacity of adhesion of a liquid on a solid film; the physical quantities to evaluate it are the polar and dispersive components of the surface free energy (SFE), usually expressed in millinewtons per meter, for which there are no methods for direct measurement. The polar SFE, constituting together with the dispersive component the overall SFE, has greater effect on the wettability of polypropylene films; the methods currently in use, as defined in ASTM D 2578 [1], that provide the overall SFE are, thus, only partially exhaustive. They, in fact, are not always able to provide the information necessary to support the experimental investigations, which are