Plasma Oxidation of Polyolefins - Course of O/C Ratio from Unmodified Bulk to Surface and Finally to CO2 in the Gas Phase: A Critical Review

The polyolefin oxidation on exposure to oxygen plasma starts with introduction of O-functional groups by attachment of O-species from the plasma and sputtering of atoms and fragments of the polymer material and then continues by auto-oxidation initiated by reaction of plasma-UV-produced radicals with neutral oxygen molecules. The oxidation continues to the formation of Low-Molecular Weight Oxidized Material (LMWOM) and finishes with gasification of its fragments, formation of highly oxidized alcohols, aldehydes, ketones and carboxylic acids, and their ultimate conversion to CO<inf>2</inf> and H<inf>2</inf>O. This succession is reviewed here by evaluating the literature, in analogy to chemical oxidation of paraffins, waxes and polyolefins. In this context, the course of the O/C ratio from polyolefin bulk to the surface is of interest up to the final formation of CO<inf>2</inf>. The maximum measured O/C ratio was about 30% at polyolefin surface. After isolation of dusty LMWOM from the topmost polymer layer by washing the polymer with water, now 56% O/C was measured. However, the jump from 56% to 200% O/C in CO<inf>2</inf> was not clear as yet. It can be assumed that LMWOM is further fragmented, then gasified and further oxidized in the gas phase to C1-C3oxidized species (O/C=50-100%) and finally to CO<inf>2</inf> (and H<inf>2</inf>O). Furthermore, the oxidation depth in the polymer is of interest although it has been already investigated. It depends on the transmittance of the plasma-emitted vacuum UV radiation.