Mapping physicochemical surface modifications of flame-treated polypropylene

Abstract

The aim of this work was to investigate how the surface morphology of polypropylene (PP) is influenced by the surface activation mediated by a flame obtained using a mixture of air and propane under fuel-lean (equivalence ratio φ = 0.98) conditions. Morphological changes observed on flamed samples with smooth (S), medium (M), and high (H) degree of surface roughness were attributed to the combined effect of a chemical mechanism (agglomeration and ordering of partially oxidized intermediate-molecular-weight material) with a physical mechanism (flattening of the original roughness by the flame’s high temperature). After two treatments, the different behavior of the samples in terms of wettability was totally reset, which made an impressive surface energy of ~43 mJ•m–2 possible, which is typical of more hydrophilic polymers (e.g., polyethylene terephthalate – PET). In particular, the polar component was increased from 1.21, 0.08, and 0.32 mJ•m–2 (untreated samples) to 10.95, 11.20, and 11.17 mJ•m–2 for the flamed samples S, M, and H, respectively, an increase attributed to the insertion of polar functional groups (hydroxyl and carbonyl) on the C–C backbone, as demonstrated by the X-ray photoelectron spectroscopy results