The production of cross-linked rubber goods such as vehicle tyres, latex gloves etc., is a global industry responsible for the consumption of millions of tons of rubbery polymers such as polybutadiene (PBd), polyisoprene (PI) and natural rubber every year. All of these materials comprise long chain hydrocarbon molecules with unsaturated carbons in their backbone. While cross-linking, or vulcanisation is essential to provide the necessary mechanical properties, this process also renders the tyre material insoluble; therefore making it extremely difficult to recycle or reprocess tyre components at the end of their life. Traditionally, rubber is reprocessed either by ambient temperature milling or cryofracturing, both of which are energy intensive processes, leading to a crumb product, which can be mixed with a fresh elastomer to produce a new material, typically with somewhat diminished properties.

The loss of properties has significant implications for the reuse of rubber reclaimed in this way, and it is generally restricted to low performance materials. Applications requiring high performance such as vehicle tyres typically do not incorporate more than a small fraction of reclaimed rubber.

… Here, we demonstrate for the first time the effectiveness of CM [cross metathesis] reactions to break down rubbery polymers into viscous liquids, which then have the potential to be reprocessed.

—Smith et al.

The team used first and second generation (G1 and G2) Grubbs’ ruthenium catalysts to break down polybutadiene (PBd) networks at their double bonds via cross-metathesis (CM) reactions to produce readily soluble molecules. As the chains fragment, the material disintegrates into rubber crumb at room temperature.

Increasing the reaction time and temperature increased the extent of the breakdown, resulting in an oil comprising low molecular weight oligomers.

CM reactions could be used as an efficient way of making rubber crumb since only a very small amount of catalyst is added and the extent to which it is broken down can be determined by the reaction time. These reactions for linear PBd and SBR show that the higher the crosslinking, the longer the reaction time needed. Nevertheless, our results show that CM is an effective method for the breakdown of PBd and this reaction has potential to be used in the breakdown of rubber crumb and hence might be developed as a method for chemically reprocessing waste vehicle tyres. We anticipate that other polymers which contain carbon double bonds (e.g. polyisoprenes, natural rubber) in the chain backbone could also be broken down using this method. If the full breakdown of tyres via a CM reaction can be achieved it could redefine the way rubber is recycled.

—Smith et al.

Resources

• R. F. Smith, S. C. Boothroyd, R. L. Thompson and E. Khosravi (2016) “A facile route for rubber breakdown via cross metathesis reactions” Green Chem. doi: 10.1039/C5GC03075G