PHYSICOCHEMICAL PROPERTIES OF SURFACE-MODIFIED POLYMERS

Contact angle, electrokinetic, and X-ray photoelectron spectroscopic ( XPS) measurements have been used to study the surface properties of fl ame- and oxygen plasma-pretreated polypropylene/ethylenepropylene-dien e monomer rubber (PP-EPDM) blends and of ethylene vinyl acetate (EVA) copolymers grafted with carboxyl group-containing monomers. The contac t angles of pure test liquids (water, methylene iodide, and ethylene g lycol) were used to calculate the dispersive and polar components of t he surface free energy according to Owens and Wendt, and the acid-base parameters according to Van Oss and coworkers. In addition, the acid- base properties of the differently pretreated polymers could be evalua ted quantitatively by measuring the zeta potential vs. the pH in a 10( -3) mol/l KCl solution. The zeta potential measurements show that oxyg en plasma-treated PP-EPDM and grafted EVA indicate an acidic surface c haracter, whereas the flame-treated PP-EPDM blends possess both acidic and basic surface groups. The basic surface character of flame-treate d PP-EPDM injection-moulded sheets could be enhanced by the presence o f sterically hindered amine light stabilizers in the blend. This incre ase in the basic surface character was not only proved by zeta potenti al measurements, but also by the contact angle method according to Van Oss and co-workers. These results correlate with an increase of the o xygen content in the surface region and the occurrence of nitrogen-con taining functional groups detected by XPS. The plasma-treated surface region of PP-EPDM blends contained an increased amount of carboxyl gro up-containing species (O=C-O). Flame-treated surfaces with additional light stabilizers in the blend indicated an increased concentration C- OH groups together with protonated nitrogen in the surface region. It was found that the adhesion strength of water-based primers was higher at these surfaces. A general interrelation between the acidic and bas ic parameters determined by zeta potential measurements, on the one ha nd, and the acidic and basic parameters determined by contact angle me asurements, on the other hand, could not be found. A direct correlatio n was found between the increasing acidic character of EVA grafted wit h different amounts of carboxyl group-containing monomers and the decr ease in the receding contact angle.