The industrial applications of thermoplastic polymers are often limited by their poor adhesion properties. In the case of polyolefins, surface oxyfluorination was shown as an interesting pretreatment to modify polymer surfaces and increase their adhesion.
In this work the effect of surface oxyfluorination on the adhesion properties was investigated for polyethylene (PE), polyoxymethylene (POM), polybutylene terephthalate (PBT) and polyamide 6 (PA6). This enables to determine a possible influence of the initial polymer structure and broaden the knowledge on oxyfluorination for adhesion purposes.
The adhesive joint strength was quantified using lap-shear test and the adhesion of coatings using pull-off test. These results were correlated with the changes in the surface chemistry, determined by X-ray photoelectron spectroscopy (XPS), in the surface free energy, measured using the contact angle method, and in the topography, using white-light confocal microscopy.
The adhesive strength is strongly improved for all four polymers. Even though the highest strength was observed with PA6, the strongest increase relatively to untreated state was achieved with PE, with an up to ten time enhanced adhesion. The effect of oxyfluorination on the adhesive strength was found to decrease with increasing initial strength. These results were confirmed by the investigation of coatings adhesion.
The surface free energy shows a similar trend. For all four polymers a high surface free energy of over 50 mN/m was observed after oxyfluorination, whereby the polar component was strongly predominant. The strongest increase was observed with PE, similarly to adhesive strength. However, POM showed a limited increase and a relatively small polar component. Therefore, the increased surface free energy or the more polar surface cannot completely explain the improved adhesion.
Surface topography measurements show no significant increase of the surface roughness which could explain the increased adhesion. So the effect of oxyfluorination results primarily of changes in surface chemistry. XPS measurements confirm the integration of oxygen groups in the polymer chain, which correlates with the increased polarity. The detailed influence of these groups remains to determine. |