Plasma activation induced changes in surface chemistry of pigment coating components

Abstract

The influence of plasma activation on pigment coating components was investigated. Four model pigment-coated papers were treated with industrial corona, experimental pilot scale argon plasma, laboratory scale nitrogen plasma equipment, and finally printed in a pilot scale sheet-fed offset printing press. Surface characterization was performed by contact angle measurements, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Gallium focused ion beam (FIB) and optical microscope imaging were used to investigate the reason for the decreased print density caused by intensive laboratory scale plasma. According to the contact angle and XPS results, plasma activation increased surface wettability and oxygen/carbon (O/C) ratio. The changes seemed to occur especially in respect to high molecular weight dispersion chemicals of both pigment and latex particles. The used pigment coating components responded differently to the treatments. Water contact angles of kaolin containing papers decreased more than papers containing calcium carbonate or talc. Ground calcium carbonate (GCC) containing paper had the smallest change in O/C ratio. When compared to corona treatment, pilot scale argon plasma provided higher polarity on the surface and smaller O/C ratio on the reverse side on the paper. The intensive laboratory scale plasma treatment led a decrease in surface strength and print density. FIB and optical microscope images showed that micro-picking was occurring in the surface layers of the coating leaving some areas unprinted.