The effect of pulse voltage rise rate on the polypropylene surface hydrophilic modification by ns pulsed nitrogen DBD

The nanosecond (ns) pulsed nitrogen dielectric barrier discharge (DBD) is employed to enhance the hydrophilicity of polypropylene (PP) surface and improve its application effect. The discharge characteristics of the ns pulsed nitrogen DBD with different pulse rise times (from 50 to 500 ns) are investigated by electrical and optical diagnostic methods and the discharge uniformity is quantitatively analyzed by image processing method. To characterize the surface hydrophilicity, the water contact angle (WCA) is measured, and the physical morphology and chemical composition of PP before and after modification are analyzed to explore the effect of plasma on PP surface. It is found that with increasing pulse rise time from 50 to 500 ns, DBD uniformity becomes worse, energy efficiency decreases from 20% to 10.8%, and electron density decrease from 6.6 × 10¹¹ to 5.5 × 10¹¹ cm⁻³. The tendency of electron temperature is characterized with the intensity ratio of N₂/N₂⁺ emission spectrum, which decreases from 17.4 to 15.9 indicating the decreasing of T_e with increasing pulse rise time from 50 to 500 ns. The PP surface treated with 50 ns pulse rise time DBD has a lower WCA (~47°), while the WCA of PP treated with 100 to 500 ns pulse rise time DBD expands gradually (~50°‒57°). According to the study of the fixed-point WCA values, the DBD-treated PP surface has superior uniformity under 50 ns pulse rise time (3° variation) than under 300 ns pulse rise time (8° variation). After DBD treatment, the increased surface roughness from 2.0 to 9.8 nm and hydrophilic oxygen-containing groups on the surface, i.e. hydroxyl (−OH) and carbonyl (C=O) have played the significant role to improve the sample's surface hydrophilicity. The short pulse voltage rise time enhances the reduced electric field strength (E/n) in the discharge space and improves the discharge uniformity, which makes relatively sufficient physical and chemical reactions have taken place on the PP surface, resulting in better treatment uniformity.