Surface Modifaction of Ultra High Molecular Weight Polyethylene by Low Energy DC Plasma Discharge

Surface effects produced by plasma processing Ultra High Molecular Weight polyethylene (UHMW) were examined. The goal was to enhance adhesion of UHMW to a variety of polar polymers. This research focussed on enhancing adhesion to polyurethane. UHMW samples were immersed in a DC discharge and subjected to various processing conditions. The background gases experimented with were oxygen, nitrogen, and argon, and the pressure was varied between 6-67 Pa. The processing time, discharge current and discharge potential were also varied. A polar polymer (polyurethane) was subsequently applied to the processed UHMW samples and the tensile strength of the bonds was measured. Standard dogbone specimens were fabricated, and the polymer interfaces placed in the middle of the gage section and oriented normal to the tensile axis. The specimens were tested to failure in displacement control at an approximate strain rate of 0.2% per sec. In general, significant enhancement in yield strength was observed over unprocessed samples, where essentially no bonding occurred. Samples subjected to plasma processing in oxygen showed the strongest adhesion, while those in argon were almost as good, followed by nitrogen.The contact angle of water on the processed UHMW samples ranged from about 45° to less than 2°, while unprocessed samples had a contact angle of about 70°. These data correlated with the yield tests. XPS was used to examine the composition of the UHMW surfaces before and after plasma processing with oxygen. It is believed that the plasma etched off surface layers, creating unsaturated carbon bonds that can chemically react with polar polymer groups. Precast, processed UItMW surfaces were found to be reactive for periods of up to one week. Optimum conditions of plasma processing were identified, and will be discussed in light of the chemistry occurring at the interface.