Effect of humid air exposure between successive helium plasma treatments on PET foils

It is well known that helium plasmas can successfully alter the surface properties of polyethylene terephthalate (PET). However, the influence of humid (atmospheric) air exposure between successive helium plasma treatment steps has not been previously examined. In this work, PET foils are treated in successive short treatment steps of 5 s with a DBD operating in helium at medium pressure (5.0 kPa). Between every plasma treatment step, the samples are either kept in the plasma chamber at 5.0 kPa under helium atmosphere or are exposed to humid air at atmospheric pressure. The plasma-treated samples are examined using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Results clearly show that exposure to humid air between successive helium plasma treatment steps leads to a larger decrease in water contact angle and a more pronounced oxygen incorporation. XPS results have shown that during each helium plasma treatment, O-C═O groups are introduced on the PET surfaces. However, when the samples are exposed to humid air between plasma treatments, also C-O groups are created on the PET surfaces. These C-O groups are mainly introduced during exposure to humid air: every helium plasma treatment creates surface radicals, which can in turn react with oxygen species present in the atmosphere during subsequent humid air exposure, leading to the incorporation of C-O groups.