Atmospheric-pressure plasma treatments of NBR for the improvement of adhesion in footwear applications

Abstract

Strong adhesion between the sole and the upper part of shoes is necessary to maintain their integrity and protect the foot from external conditions. Vulcanized rubbers are commonly used for sole manufacturing but, because of their nonpolar nature, they need to be conditioned before bonding. In this work, atmospheric-pressure air plasma treatments on flat and mechanically roughened nitrile butadiene rubber (NBR) were studied as environmentally friendly alternatives to the usual halogenation process to improve the adhesion between NBR and leather parts. Surface activation and coatings by plasma-polymerization of (3-aminopropyl) triethoxysilane were used, and the evolution of their effectiveness during the following 30 days was studied. Both treatment types achieved bond strengths that were greater than those of the untreated NBR. Coating treatments caused an initial increase in the surface polar carbon-oxygen species, which was kept for at least the following 6 days and allowed for an improvement of the bond strength when the coatings were bonded with the leather within that period. However, the presence of these species on the coatings decreased during storage, resulting in weaker adhesion than that of the untreated NBR before the end of the aging study. The most durable effectiveness (30 days) was shown by plasma-activated, mechanically roughened NBR. Chemical and morphological characterizations suggested that this durable adhesion improvement was due to ablation by the plasma irradiation, which had a roughening effect at a microscopic scale, additionally to the macroscopic effects of mechanical roughening.