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 degrees(untreated) to less than 30 degrees (treated) with a wide range of O-2/N-2 ratios (<1%) and treating distances (<10 mm). In addition, the CA can still be maintained at similar to 40 degrees 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 O-2/N-2 ratios of 0.06%-0.2%. Additionally, the CA can only be maintained at similar to 80 degrees-90 degrees when the moving speed is similar to 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 C = O) is critical in increasing the aging time after plasma treatment.