Effects of air-based nonequilibrium atmospheric pressure plasma jet treatment on characteristics of polypropylene film surfaces

[Display omitted] •Air plasma jet adequately enhanced the hydrophilicity of PP surface in 1 min.•The enhanced hydrophilicity was due to 60% O-based FPGs grafted on the surface.•The grafted FPGs facilitated the formation of nanopore structures on the surface.•The FPG grafting was due mainly to oxygen radical ions impinging from air plasma jet.•This can also be due secondarily to heat-induced oxidation from the air plasma jet. Polypropylene (PP) film surfaces were treated using air-based nonequilibrium atmospheric pressure plasma jets generated with a twisted wires-cylindrical electrode configuration. For comparison, PP samples were also processed with Ar plasma jets. The flux of charged particles imparted to the polymer surface by the air plasma jet greatly increased with decreases in both the gas flow rate and nozzle-to-sample distance, which was not the case for the Ar plasma jet. Reducing the gas flow rate and the nozzle-to-sample distance greatly enhanced the extent to which the surface was rendered hydrophilic by the air plasma within a short treatment time of 1 min. This enhanced effect is believed to originate from a high concentration of oxygen-based functional polar groups (FPGs) containing CO/COH and CO/COOH bonds grafted onto the surface. The hydrophilic surfaces resulting from this process also exhibited nanopore structures. The large number of oxygen-based FPGs produced by the air plasma can be attributed primarily to oxygen radical ions impinging from the air plasma on the surface. This can further be attributed secondarily to heat-induced oxidation rather than the sticking of oxygen radicals and UV-induced oxidation from the plasma. The nanoporous structure can also be ascribed to oxidation from UV photogenerated holes.