Treatment with Ar–O2 low-pressure plasma of vulcanized rubber sole containing noticeable amount of processing oils for improving adhesion to upper in shoe industry

Abstract

Vulcanized rubber (L3 rubber) containing intentionally noticeable excess of processing oils in its formulation was treated with Argon–Oxygen (Ar–O2) (2:1, vol/vol) low-pressure (LP) plasma for achieving a satisfactory level of adhesion to waterborne polyurethane adhesive. The effectiveness of the Ar–O2 LP plasma treatment of L3 rubber depended on both the configuration of the plasma chamber shelves and the treatment time. Surface modifications were assessed by attenuated total reflectance-IR and X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy. Ar–O2 LP plasma treatment in direct configuration provided the most effective surface modification of the L3 rubber, and the increase in the treatment time improved the extent of the surface modifications. However, even important surface modifications were produced by Ar–O2 LP plasma treatment, adhesion of treated L3 rubber was not improved due to the creation of weak boundary layer at the polyurethane–rubber interface after joint formation. Heating at 80 °C for 12 h of the as-received L3 rubber prior to Ar–O2 LP plasma treatment enhanced the extent of the surface modifications, and improved adhesion was obtained for Ar–O2 LP plasma treatment times higher than 600 s.