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 CO2 and H2O. 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 CO2. 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 CO2 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-C3 oxidized species (O/C ≈ 50-100%) and finally to CO2 (and H2O). 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.