Effects of the chromic acid etching on propylene polymer surfaces

The chromic acid etching of three propylene polymers has been investigated by contact angle measurement, X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared (ATR-IR), scanning electron microscopy (SEM), and an adhesion test. It has been shown that etching with such an oxidative acid has a much more significant effect on the receding contact angle than on the advancing contact angle of a test liquid. Significant water and diiodomethane contact angle hystereses were found and were attributed mainly to surface chemical heterogeneity caused by the acid etching. Surface roughness resulting from the etching may also account for the contact angle hystereses in some cases. The acid etching significantly increased the surface oxygen concentration of all three polymers. Chemical derivatisation with trifluoroacetic anhydride (TFAA) and hydrazine have indicated that the acid etching introduced C=O/COOH but there is no evidence of -OH groups on the polymer surfaces. The adhesion level of the polymer surfaces with an epoxy adhesive was remarkably increased by the chromic acid etching.