From elastomer microstructure to thermomechanically damaged nanocomposites: an insight into the parameters influencing the biodegradability of rubbers by natural microbial consortia

Tire and road wear particles (TRWP) have raised questions about their environmental fate, and there is a particular lack of knowledge regarding their biodegradability. Recent studies suggest their persistence beyond 2 years in soil or water. Conversely, several microbial strains have been shown to be able to metabolize poly( cis -1,4-isoprene), largely used in tire tread compositions and other rubber goods. To understand how microbial degradation evolves when transitioning from pure poly( cis -1,4-isoprene) to TRWP, the mineralization of various elastomers and model materials was evaluated over several weeks using standardized biodegradation tests in soil, compost, and aqueous environments. CO ₂ release monitoring showed that pure poly( cis -1,4-isoprene) reached 50% (%ThCO ₂ ) mineralization in 1 year in soil, versus approximately 50 days in sludge-inoculated water and compost. No difference was observed between natural (NR) and synthetic poly( cis -1,4-isoprene) (IR). However, poly( cis -1,4-butadiene) (BR) and styrene-butadiene rubber (SBR) were more recalcitrant over the investigated period. Similarly, non-stereospecific polyisoprene showed limited biodegradability compared to the stereospecific cis grade. The mineralization of an IR/BR mix was commensurate with the proportion of IR, suggesting the absence of cometabolic mineralization of BR in the presence of IR. The mineralization of a non-crosslinked NR filled with 5%vol carbon black was unaffected, whereas a two-fold reduction was observed after 45 days with 20%vol carbon black or silica. Thermomechanical wearing of a carbon black filled vulcanized IR was shown to increase the mineralization level of the composite, likely due to chain scission and devulcanization.