PLASMA TREATMENT OF POLYPROPYLENE SURFACES - CHARACTERIZATION BY CONTACT-ANGLE MEASUREMENTS

Air plasma treatment has been used to introduce polar functional groups onto polypropylene surfaces to enhance the adhesion of coatings and paints. Plasma exposure times as well as the reaction medium (ambient, technical, and moistured technical air) have been varied. Time-dependent changes subsequent to the plasma treatment are monitored by contact-angle measurements. After short plasma exposures, the polar component of the surface energy is observed to decrease, to reach a minimum after almost-equal-to 40 min. This behavior is interpreted in terms of a rotation of the introduced surface hydroperoxide groups into the interior of the polymer. This undesired loss in surface functionalization can be avoided by (a) increasing the treatment time in dry air, (b) use of moistured air as the reaction medium, or (c) posttreatment of the plasma-treated parts by water immersion. A plasma exposure of 120 s in dry air, followed by posttreatment in an aqueous medium at room temperature for 2-5 min, results in reproducible values of the surface energy sigma, with polarities (sigma(polar)/sigma(total)) close to the desired value of almost-equal-to 50%. (C) 1993 John Wiley & Sons, Inc.