Flame Treatment of Polypropylene: A Study by Electron and Ion Spectroscopies

The effects of flame treatment on the surface characteristics of four injection moulded, automotive grade, polypropylene samples, pigmented with carbon black, have been studied. The changes in wettability have been monitored by water contact angle and Dyne inks, whilst XPS has been used to establish the changes in oxygen surface concentration as a function of flame treatment. As expected carbon pigmented and carbon plus talc filled samples showed a significant increase in oxygen concentration and surface wettability with increasing flame treatment. For the glass filled sample this effect was not so pronounced. Inspection of the XPS valence band shows initial attack in the flame treatment process to be at the pendant methyl group of the poly(propylene) molecular architecture. XPS in conjunction with cluster ion bombardment shows the depth of surface treatment to range from ca. 7 nm at one pass of flame treatment to some 15 nm following seven passes. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) shows the segregation of characteristic additives during the injection moulding process which are subsequently greatly reduced during the flame treatment. As treatment level increases oxygen increases from monoatomic to diatomic attachment. This work extends the understanding of the flame treatment of moulded polyolefines and establishes that the beneficial properties conferred are the result of the conjoint effect of the oxygenation of the bulk polymer along with the removal of surface segregated processing aids.