Elastomeric Compounds Released from Rubber While in Aqueous Solution Leads to a Promising Future for Rubber Recycling

Styrene butadiene rubber (SBR), which is commonly found in tires, has been thought to have limited recycling capabilities. This is largely due to the changes in SBR’s chemical makeup during the vulcanization process. SBR, as well as other types of vulcanized rubber, cannot be recycled in the same way as other polymers due to the extensive network of polymer chains and sulfur cross-links that come from vulcanization. A study conducted by the American Water Works Association (AWWA) in 2007, however, refuted this claim as it showed that minimal concentrations of monochloramine in water led to the degradation of SBR. The effect of monochloramine on SBR noted in the AWWA study not only suggested but encouraged further analysis into the possible opportunities to recycle SBR.

In order to test the potential recycling capabilities of SBR, monochloramine was developed in a reaction of sodium hypochlorite in solution and ammonium hydroxide following the ASTM D 6284 method. The SBR was then degraded with elevated temperatures and concentrations of monochloramine. From this experimentation, a rate of decomposition was able to be calculated for this reaction with a rate constant of approximately 0.0096 min^-1. Once the rate of decomposition was determined, tests continued over a 30-day period with monochloramine addition to rubber. After the period of reaction time, degraded monochloramine solution was removed, and fresh monochloramine was added to the solution to maintain a consistent monochloramine concentration throughout the experimental timeframe. Acetone was used to extract the SBR from the aqueous solution.

After an elastomer sample was recovered, it was characterized using FTIR, Carbon NMR, and Proton NMR techniques. These techniques showed notable concentrations of alkanes and alkenes, including cis/trans and vinyl carbon double bonds, and aromatic rings associated with styrene. This confirmed the existence of SBR elastomer molecules with residual carbon-carbon double bond sites, which would be available for re-vulcanization with sulfur to form cross links. These conclusions provide promise in the future of recycling SBR and call for further analysis and experimentation into its’ capabilities.