Surface Analysis of Pigments and Stressed Polymer Films Using X-Ray Photoelectron Spectroscopy

Abstract

XPS (X-ray Photoelectron Spectroscopy) is a technique well-suited for the compositional analysis of material surfaces and for their associated variations in surface conditions or surface history. This paper reports on XPS investigations of some commercial pigments and polymer films used in the printing and packaging areas. It also reports on a mechanistic pathway by which species such as siloxanes, trapped within a matrix of an otherwise acceptable film, may be induced to migrate (segregate or diffuse) to the surface due to film stress, thereby altering surface physical and chemical characteristics. The present work also discusses some of the factors involved in causing non-sealability of films. In particular, it provides data to substantiate the correlation of thermal sealing inhibition with the presence of surface siloxane. Siloxanes of this general type (such as PDMS-polydimethylsiloxane) are common additives in both printing inks and some commercial pigments to facilitate properties such as pigment dispersal, slip, mar resistance and gloss of films. Not all film containing such additives fails to seal or print satisfactorily. This paper proposes a mechanism whereby a safe or less than critical surface silicone level may be exceeded as a result of stress applied to film. Such conditions may arise in printing and rewind processes as well as in stored rolls, resulting in film which possesses unsatisfactory sealing properties.