Organic chemical devulcanization of rubber vulcanizates in supercritical carbon dioxide and associated less eco-unfriendly approaches: a review

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https://doi.org/10.1177/0734242X211008515
Waste Management & Research
1 –14
© The Author(s) 2021
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DOI: 10.1177/0734242X211008515
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Introduction
Background
The global production of natural and synthetic rubber products, 
such as automotive tyres, mechanical seals, conveyer belts and 
hoses, continue to grow every year and this is due to the ever-
rising demand of these products. According to Markl and Lackner 
(2020), 70% of the natural and synthetic rubbers are consumed 
by the tyre manufacturing industry, with relatively larger amount 
of natural rubber (NR) being used in truck and off the road (OTR) 
tyres than in passenger car tyres, as can be seen in Table 1. It has 
been estimated that as of 2017, over one billion passenger vehi-
cles in the world are in use daily, and this is expected to double 
by the year 2035 (Becker, 2017). With this large number of vehi-
cles on the road, the amount of waste tyres will continue to rise 
exponentially every year and consequently have a negative 
impact on the natural environment because it is difficult for these 
materials to return to the ecological environment via biological 
degradation, hydrolysis or decomposition processes due to their 
structural characteristics (Joseph et al., 2015; Zhang et al., 2018).
Rubber recycling
Finding a viable solution for the environmental pollution created 
by waste tyres is a challenge for the tyre industry worldwide. This 
problem is exacerbated when waste tyres are not re-used, recycled 
or properly disposed (Adhikari et al., 2000; Bell and Cave, 2010; 
Institute of Waste Management of Southern Africa (IWMSA), 
2017; Markl and Lackner, 2020). Waste tyres can be re-used after 
regrooving or retreading processes, and those that do not qualify 
to undergo these processes are normally recycled, where the steel 
and fibre materials are removed from the tyre to get the rubber