Low-pressure plasma chlorination of styrene–butadiene block copolymer for improved adhesion to polyurethane adhesives

Studies concerning plasma-surface chlorination of a styrene–butadiene block copolymer (SBS), improving its adhesion to polyurethane adhesives (PU), are presented in this paper. The plasma was generated by an RF discharge (13.56 MHz, plate electrode reactor) in CCl4 under low pressure. The 180°-peel test, contact angle measurements and XPS spectroscopy were utilized to investigate the SBS surface. A drastic increase in the adhesion (the peel strength 5–7 times higher than that for the non-treated surface) was observed after only a few seconds of the plasma treatment. It was shown that CCl, COH and >CO are the most important functional groups formed as a result of the plasma treatment and they play the crucial role in the chemical bonding between the SBS surface and the adhesive. H2O molecules strongly attached to the SBS surface were also found. It was determined, however, that they reduce the gluing power. A very good correlation between the concentration of the functional groups and the peel strength was established. On the other hand, no correlation between the peel strength and the surface free energy (estimated from the contact angle measurements) was observed. It indicates that the thermodynamic adhesion is unimportant in this case and confirms the dominant role of the chemical adhesion.