Surface Chemistry Stabilization by Esterification of Polypropylene Treated by Direct Fluorination

Abstract

Direct fluorination, based on a gas/solid reaction with elemental fluorine F₂, is a surface treatment that is able to modify the superficial properties of polymers, like surface energy or tribological behavior. Nevertheless, side reactions occur with oxygen either present in the reactive molecular fluorine gas (F₂) or during a post-reaction when the sample is exposed to air atmosphere. A systematic study of the fluorination of polypropylene at different temperatures identifies the formation of acyl fluoride COF groups, coexisting with the fluorinated groups. COF groups evolve into carboxylic acid (COOH) groups during the first few hours after fluorination. Despite the presence of fluorine, the polar component of the surface energy increased. By introducing an esterification post-treatment of the COF groups with an alcohol in the gas phase after the fluorination step, it is possible to stabilize the surface chemistry and thus prevent its hydrolysis. Surfaces post-treated by this singular approach are less hydrophilic than their counterparts only treated by direct fluorination.

Keywords: direct fluorination; esterification; gas phase; polypropylene; surface functionalization.