Recyclable Semi-EV Sulphur Cured Natural Rubber Elastomer Composites

Abstract

Natural Rubber (NR) has been widely vulcanised for most practical engineering applications using sulphur for the last 180 years, however, the crosslinked network significantly inhibits recyclability. Minimising reduction in performance after recycling is essential in developing a more circular economy within the rubber industry. Previously, this research group exploited controlled inhibition of the disulphide metathesis, through addition of copper (II) methacrylate (CuMA) to the compound, enabling significant recyclability of rubber waste in conventionally vulcanised (CV) NR networks. CV networks predominantly contain poly-and di-sulphide crosslinks, however, in practice, a greater proportion of rubber products are made exploiting the more thermally stable efficient vulcanising (EV) or semi-efficient vulcanising (SEV) sulphur crosslinking systems. This work demonstrates that by optimising the CuMA concentration, inhibition of the disulphide metathesis works well with the most widely exploited SEV system. A CuMA concentration of 0.62 phr enabled excellent recyclability, providing excellent mechanical properties in materials post recycling up to very large recycled rubber content. It is demonstrated the system also achieves high recyclability using a model SEV tyre compound including 50 phr carbon black filler. This discovery opens the opportunity to recycle the 15 million tonnes of elastomer materials within tyre waste produced globally each year.