Functionalisation of polyolefins and its effects on surface chemistry and energetics
The surface functionalisation of polyethylene and polypropylene by industrial and laboratory scale corona treatments and by laboratory flame treatment was studied. The surface sensitive techniques of X-ray photoelectron spectroscopy (XPS), attenuated total reflection infra-red spectroscopy (FTIR-ATR), contact angle measurement and electron microscopy (SEM and TEM) were employed. Corona and flame treatments resulted in incorporation of oxygen only into the surfaces of both polyethylene and polypropylene, resulting in improved surface wettabilities. A variety of oxygen functional groups were introduced by the two treatments. The industrial and laboratory scale treatments of both polymers were found to be similar in terms of the oxygen concentrations incorporated and surface wettabilities achieved. The presence of significant amounts of chain scission products were indicated on corona treated surfaces, while only minimal quantities were indicted on flame treated surfaces. This was attributed to their volatilisation during flame treatment. Introduction of sulfur dioxide into the flame and corona regions during treatment resulted in significant improvements in surface wettability. Incorporation of sulfur and nitrogen resulted from the presence of sulfur dioxide. A possible mechanism involving the formation of sulfonic acid groups and ammonium sulfonate groups was suggested. An oxidation depth model developed for use with variable take-off angle XPS showed that significantly deeper oxidation occurred in the presence of sulfur dioxide. Corona treatment was more effective in improving surface wettabilities than flame treatment, this being attributed to heat induced functional group reorientation during flame . treatment for polyethylene and to differences in surface chemistry resulting from the two treatments in the case of polypropylene. The surface wettability of poly ethylene was more readily improved than the surface wettability of polypropylene after all the treatments investigated. A method for estimating functional group concentrations using chemical derivatisation and contact angle measurement was developed. Functional group estimates for flame treated polyethylene were found to be in good agreement with chemical derivatisation used in conjunction with XPS measurements.