Influence of atmospheric pressure plasma jet processing parameters on the wettability and surface chemistry of polypropylene: relevance for adhesion phenomena

Abstract

Atmospheric pressure plasma jet technology has been applied to surface treatment of polypropylene prior to its adhesive bonding to aluminium using structural adhesives. Surface modifications of polypropylene induced duringplasma treatments were investigated using surface free energy measurements, attenuated total reflectance infrared spectroscopy (ATR-IR) analyses, and mechanical evaluation of epoxy and urethane bonded aluminium-polypropylene joints. On the basis of the surface free energy criterion, the influence of parameters describing plasma treatments (i.e. primary-to-secondary gas ratios, output power source, treatment speed, plasma-to-sample distance) was determined for each gas combination employed to generate the plasma (He with O\u2082, N\u2082, CO\u2082, N\u2082O or air). By submitting polypropylene to the optimised plasma conditions defined for each gas combination, it was found using ATR-IR analyses that a complex mixture of carbonyl functionalities are induced on the surface of processed materials. Using a fitting procedure of Gaussian bands, ATR-IR spectra were resolved into single C=O stretching vibrations. It was then found that regardless of the gas mixture injected in the plasma generator, different extents of amide and COO-based chemical functions (carboxylic acids and/or esters) were introduced onto polypropylene surfaces. From the mechanical evaluation of joint strengths of adhesively bonded hybrid aluminiumpolypropylene assemblies, it was generally observed that the surface chemistry induced by the plasma plays a more important role in adhesion than the surface free energy parameter. Finally, using correlations established between Owens et al. and LWAB surface free energy theories, plasma modified polypropylene surfaces were found to be basic in nature.Peer reviewed: YesNRC publication: Ye