Activation of the Internal Surfaces of Polymer Tubes with Argon Plasma Through Hollow Cathode Effect

The focus of this study is on generating hollow cathode discharge (HCD) plasma inside three types of polymer tubes, resulting in the activation of the tubes' inner surfaces. To achieve this goal, both theoretical calculations and experiments were carried out to prove that the adopted methodology was feasible. By applying optical emission spectroscopy, with line ratio and modified Boltzmann's plot methods, the plasma sheath could be estimated and applied to the prediction of HCD generation. To verify whether HCD prevails in the tube, the various power inputs (70~260 W), tube diameters (3~7 mm), and working pressure (0.1 and 0.4 torr) were selected. Accordingly, it was confirmed that the tube diameter must be greater than twice the plasma sheath in order to form HCD, which would then activate the surface. Ar gas was used with a capacitive-coupled RF plasma system. PET, PU, and PTFE tubes were used in this study. The tubes' inner surfaces were characterized with XPS and water-contact-angle (WCA) analysis. The results also show that once HCD is generated inside the tubes, these tubes' inner surfaces could become hydrophilic, and the values of WCA might decrease by 40°, 45°, and 70° respectively for PET, PU, and PTFE, due to the combined effects of the enhanced ion bombardment and radical oxidation. XPS studies show that, for PET and PU, C-O and, particularly, C=O bonds increase significantly. For PTFE, the C-F bonds decrease, while carbon-oxygen bonds increase. Overall, the surface activation is thought to be enhanced by ion bombardment attributable eventually to the hollow cathode effect (HCE).