Robotised atmospheric plasma treatment to improve the adhesion of vulcanised and thermoplastic rubber materials for a more sustainable footwear

Abstract

Surface treatment is essential to ensure the resistance and durability of adhesive upper-to-sole joints for footwear. This study investigates robotised atmospheric pressure plasma technology to increase adhesion on styrene-butadiene vulcanised (SBR) and styrene-butadiene-styrene thermoplastic (SBS) rubbers, which constitutes a more sustainable surface treatment as an alternative to solvent-based chemical treatments (i.e., halogenation). In this work, the different parameters such as application speed and distance of the nozzle of the atmospheric plasma system have been optimised and the effects of the plasma treatment on the physicochemical properties in the materials’ surface, roughness, wettability, as well as the adhesion properties have been evaluated by means of different experimental techniques: Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron (FESEM), Contact Angles (CA) and Surface Free Energy (SFE) measurements and T-peel strength tests. The results show that the values obtained for the T-peel strength of the reactive hot melt polyurethane adhesive (HMPUR) joints studied exceed the minimum quality requirements for footwear according to standardised tests. In consequence, the optimisation and validation of operating plasma conditions and adhesion properties of rubbers could consolidate atmospheric plasma as a solvent-free alternative for the preparation of footwear soling materials. The great advantage of this technology is its introduction into the production line by fully automating the bonding process with the use of robots.