Effect of temperature on the extent of migration of low molecular weight moieties to rubber surface

Vulcanized rubbers contain different low molecular weight additives in their formulation, including antiozonants, plasticizers, oils, etc. These moieties – mostly paraffin wax – often migrate to the surface causing a weak boundary layer of non-rubber contaminants which is deleterious for adhesion of rubber to adhesives (such as polyurethane and polychloroprene adhesives). One of the key steps in the manufacturing of rubber/adhesive joints is the reactivation, i.e. sudden heating of the thin adhesive layers on the substrates to be joined under infrared (IR) radiation to 80–90 °C for a few seconds to allow diffusion of the polymeric chains under pressure. This reactivation may cause the migration of low molecular weight additives to the rubber surface causing a lack of adhesion. The main aim of this study was to .identify the influence of the reactivation temperature (40 to 170 °C) on the surface properties of sulphur vulcanized styrene–butadiene rubber and determine the extent of the diffusion of paraffin wax and zinc stearate to the rubber surface. The changes produced on the rubber surface were measured immediately after reactivation treatment by ethylene glycol contact angle measurements, attenuated total reflectance infrared spectroscopy (ATR-IR) and scanning electron microscopy (SEM). Additionally, the weight loss of the rubber after reactivation at different temperatures was recorded.The reactivation of the rubber at different temperatures produced changes in the morphology and thickness of the paraffin wax layer on the surface. By heating at temperature close to that of the paraffin wax melting point, the paraffin wax migration was favoured and at the same time the crystals of paraffin wax on the rubber surface were melted. As a consequence a thicker and smoother film of melting paraffin wax was formed. By increasing the reactivation temperature, a partial removal of paraffin wax was produced and the thickness of the paraffin wax film on the rubber surface was reduced. For reactivation temperatures below 90 °C, the higher the temperature, the lower the weight loss of the rubber, because the increase in the surface area of the melted paraffin wax layer that prevented migration from the rubber bulk. However, for reactivation temperature higher than 90 °C, the weight loss of the rubber increased with the reactivation temperature and this was likely due to sublimation of the paraffin wax on the rubber surface. Besides, even after reactivation at 170 °C, a thin film of paraffin wax always remained on the rubber surface as was evidenced by contact angle measurements. On the other hand, a critical reactivation temperature at 90–100 °C existed at which the migration of zinc stearate to the paraffin wax layer on the rubber surface was favoured.