Plasma surface modification of polyethylene: short-term vs. long-term plasma treatment

A remote plasma reactor, with air as the plasma gas, has been used for in-line surface modification of linear low-density polyethylene tape (LLDPE) passing 10 cm below the main plasma zone. Line speeds to carbon (O C) ratios averaging 0.11 were observed on a reproducible basis. The reactor was also used for static plasma treatment under similar experimental conditions. This allowed a comparative study of short-term (milliseconds) vs. long-term (several seconds) plasma treatment. High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the treated polymer surface suggested the formation of hydroxyl (C-OH), carbonyl (C-O) and carboxyl (O-C=O) groups, even after short plasma treatment. The intensities of these components were seen to increase in approximately equal quantities with increasing O-C ratio. Water washing of polyethylene surfaces with high O-C ratios showed a loss of oxygen, apparent as a decrease in O-C=O groups in the CIs spectra. A smaller loss in oxygen was observed when washing samples that had been plasma-treated for milliseconds. A surface ageing study revealed that polyethylene surfaces that had been plasma-treated for short time periods showed only a negligible loss of oxygen on prolonged exposure to air. Surfaces treated for longer time periods showed a loss of up to 50% of the total oxygen on the surface within a few days of treatment. Static secondary ion mass spectrometry has provided some supporting evidence for surface damage of the treated films.