THE EFFECT OF FLUORINATION ON THE SURFACE CHARACTERISTICS AND ADHESIVE PROPERTIES OF POLYETHYLENE AND POLYPROPYLENE

Polyethylene and polypropylene have been treated with fluorine/nitrogen or fluorine/oxygen/nitrogen mixtures at atmospheric pressure. Changes in surface chemistry and topography as well as depth of fluorination have been examined using Fourier transform infra-red analysis, X-ray photoelectron spectroscopy (X.p.s.), atomic force microscopy (AFM) and ellipsometry. Even very short exposure times caused a large substitution of the hydrogen atoms by fluorine. No change in surface topography was noticed at magnifications of up to 5000 times with the scanning electron microscope (SEM), but AFM showed that fluorination led to an increase of microroughness. The influence of fluorine or fluorine/oxygen concentration, as well as time of treatment and time of storage before adhesive bonding, on adhesion of polypropylene to steel was investigated with a bending peel test. Significant improvement in peel strength was already achieved with minor fluorination intensity. Increase of fluorination intensity did not lead to further improvement in peel strength. Analysis of the fracture surfaces was carried out with the SEM and by energy dispersive X-ray spectroscopy and X.p.s. The findings showed that the samples failed cohesively in the polymer or directly beneath the fluorinated layer. A model to describe the formation of specific interactions between substrate and adhesive is suggested.