Colloids and Surfaces A: Physicochem. Eng. Aspects 352 (2009) 103–112 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa Plasma activation induced changes in surface chemistry of pigment coating components Maiju Pykönen a,∗ , Hanna Silvaani a , Janet Preston b , Pedro Fardim c , Martti Toivakka a a Laboratory of Paper Coating and Converting and Center for Functional Materials, Åbo Akademi University, Porthaninkatu 3, FI-20500 Turku, Finland b Imerys Minerals Ltd., Par Moor Centre, Par Moor Road, Par, Cornwall, PL 24 2SQ, United Kingdom c Laboratory of Fibre and Cellulose Technology, Åbo Akademi University, Porthaninkatu 3, FI-20500 Turku, Finland article info Article history: Received 31 July 2009 Received in revised form 1 October 2009 Accepted 3 October 2009 Available online 12 October 2009 Keywords: Atmospheric plasma activation Surface chemistry Pigment coating Sheet-fed offset printing 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 print- ing 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Atmospheric plasma activation in a form of industrial corona treatment has been utilized already decades as surface treatment for low surface energy plastic films and other polymer mate- rials to improve adhesion in various applications, for instance between paper and extrusion coated polymer, electrophotographic toner and polymers and water-based inks and polymers. When the substrate passes through the plasma state, various chemical reactions occur due to exposure of the substrate to the reactive particles, which increase the surface energy and change the chemi- cal structure of the surface molecules [1]. Typical industrial corona equipment consists of two metal electrodes, and at least one is coated with an insulating dielectric layer. Plasma state is generated between the electrodes in air, and the gap is few millimeters. The other electrode is usually a grounded metal roll, and the substrate ∗ Corresponding author. Tel.: +358 40 7050981. E-mail addresses: maiju.pykonen@abo.fi (M. Pykönen), hanna.silvaani@abo.fi (H. Silvaani), janet.preston@imerys.com (J. Preston), pfardim@abo.fi (P. Fardim), martti.toivakka@abo.fi (M. Toivakka). moves between the electrodes. Since the atmospheric dielectric barrier discharges (DBD) are often marketed as corona [1–4], the industrial corona treatment should not be confused with the true corona source. Definition for dielectric barrier discharge plasma is that plasma is created in the space between the two electrodes, in which at least one is covered by insulating dielectric material. A true corona is generated in a strong electric field near sharp points or fine wires [1]. Atmospheric DBD typically exhibits in a filamen- tary mode, where the plasma is generated through succession of microdischarges rather than homogenously in the volume between the electrodes [2,4,5]. At atmospheric pressure, high voltages are required for gas breakdown to ignite the plasma. Therefore stream- ers, thin ionized channels between electrodes, are easily formed and the current may be significantly increased to form a spark [4]. One major drawback of the industrial corona treatment is the non- uniformity due to filamentary character of the DBD plasma and easily occurring streamers. It has also been shown that the stream- ers may create pin holes in the wax-coatings lowering the barrier properties [6]. Furthermore, the back side treatment of the sub- strate is not desirable in every application. Additional drawback of the industrial corona treatment is aging of the modified surface [2]. Due to these drawbacks, new types of atmospheric DBD’s have 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.10.008