Ageing effect in atmospheric plasma activation of paper substrates

This work concerns the ageing effect of the atmospheric plasma and corona treatments when used to treat paper substrates. Pigment coated and surface sized papers were modified using two types of atmospheric plasma equipment; one at the pilot scale and one at the laboratory scale. In addition, the plasma treatments were compared to conventional corona treatment. Surface energy was estimated by contact angle measurements and surface chemistry by X-ray photoelectron spectroscopy (XPS) as a function of the time during three months. The treatments increased surface energy and oxidation level of surface for both papers. The ageing effect could be detected only in the surface energy, whereas the oxidation level remained stable during the twelve weeks. The decay in surface energy was faster during the first weeks of storage and subsequently leveled off leading to a permanent change. The permanent change was explained as a contribution of oxygen containing polar molecular groups, which were detected by XPS. The ageing effect was suggested to originate from already existing polar molecular groups, which have rotated on the surface by plasma-related process and then rotate back into the material in time. A part of the decay was also explained by the plasma cleaning model, in which the ageing effect occurred through re-contamination. Paper is a multicomponent system, where the constituents that have the lowest surface energy were suggested to migrate to paper surfaces.