Comparing the performance of three commercial atmospheric plasma jets for the activation of PET

摘要

Plasma treatments are widely used to activate polymer surfaces prior to adhesive bonding. The objective of this study is to compare the performance of three commercial atmospheric plasma jet systems for the activation of polyethylene terapthalate (PET). The jets investigate were manufactured by PlasmaTreat (Openair), SurFx (Atmflo 400) and DowCorning (PlasmaStream). The latter two systems normally form a helium discharge, while the PlasmaTreat system forms an air plasma. Both the PlasmaTreat and PlasmaStream sources operate at approx. 20 kHz, while the SurFx source operates at 27 MHz. Each of the jets was mounted on a CNC system to control both the substrate to jet orifice distance and speed of the treatment. The level of polymer surface activation was evaluated based on the change in water contact angle after plasma treatment. A key issue is to maximise the level of plasma activation (generally associated with the greatest reduction in water contact angle), while preventing thermal damage to the polymer substrate. Optimised processing conditions were obtained for all three systems, which reduced the contact angle from 90° to less than 25° without thermally damaging the PET polymer surface. The polymer surface properties were also monitored using AFM and XPS measurements. The heating effect of the plasma was monitored using both infrared thermographic camera and thermocouple measurements. The latter was used to measure the gas temperature within the flowing afterglow at 15 mm from the jet nozzles and 1 mm above the substrate surface. The active species in the PlasmaStream, PlasmaTreat and SurFx systems were compared using optical emission spectroscopy. From this study it was concluded that each of these plasma jet systems has its advantages and some limitations. The PlasmaTreat has the advantage of processing with air and thus there is a significant cost saving with respect to the use of helium. This plasma however operates at a higher temperature t- an the helium plasmas and thus the exposure of the jet to the polymer substrate has to be minimized. A further issue is the deposition of trace amounts of metal (i.e. Cu, W) from the jet orifice onto the substrate being treated. The SurFx system uses much higher flow rates of He (30 L/min) compared with the PlasmaStream system (10 L/min). The treatment rate however is also three times higher at approx. 0.09 m²/min compared with that obtained for the PlasmaStream system.