Modification of Hydrophilic Property of Polypropylene Films by a Parallel-Plate Nitrogen-Based Dielectric Barrier Discharge Jet

Modification of the hydrophilic properties of polypropylene (PP) films has been investigated using the postdischarge region of a pulsed nitrogen-based dielectric barrier discharge under atmospheric-pressure conditions. Results show that, for the stationary PP films, the contact angle (CA) decreases dramatically from 103 $^{\circ}$ (untreated) to less than 30 $^{\circ}$ (treated) with a wide range of $\hbox{O}_{2}/\hbox{N}_{2}$ ratios ( $<$1%) and treating distances ($<$10 mm). In addition, the CA can still be maintained at $\sim\!\! 40^{\circ}$ after 24 h of the aging test. For the nonstationary PP films, a highly hydrophilic surface can only be obtained when the PP film is placed near the jet exit with $\hbox{O}_{2}/ \hbox{N}_{2}$ ratios of 0.06%–0.2%. Additionally, the CA can only be maintained at $\sim \!\!80^{\circ}{-}90^{\circ}$ when the moving speed is $\sim$1 cm/s after 24 h of the aging test. These observations are explained through measured optical emission spectra and ozone concentration data, in which the metastable nitrogen plays a key role in breaking the surface chemical bounds and the UV emission (200–300) participates in the process of converting the ozone into oxygen radical. Furthermore, X-ray photoelectron spectroscopy analysis showed that the O/C ratio increases dramatically for improved hydrophilic surface and the incorporation of functional groups containing oxygen (e.g., C–O and $\hbox{C} = \hbox{O}$ ) is critical in increasing the aging time after plasma treatment.