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

Abstract

There is a growing need to recover raw materials from waste due to increasing environmental concerns and the widely adopted transition to circular economy. For waste tyres, it is necessary to continuously develop methods and processes that can devulcanize rubber vulcanizates into rubber products with qualities and properties that can closely match those of the virgin rubber. Currently, the most common, due to its efficiency and perceived eco-friendliness in recovering raw rubber from waste rubbers, such as tyres, is devulcanization in supercritical carbon dioxide (scCO2) using commercial and typical devulcanizing agents. The scCO2 has been generally accepted as an attractive alternative to the traditional liquid-based devulcanization media because of the resultant devulcanized rubber has relatively better quality than other processes. For instance, when scCO2 is employed to recover rubber from waste tyres (e.g. truck tyres) and the recovered rubber is blended with virgin natural rubber (NR) in various compositions, the curing and mechanical properties of the blends closely match those of virgin NR. The atmospheric toxicity and cost of the commonly used devulcanization materials like chemical agents, oils and solvents have enabled a shift towards utilization of greener (mainly organic) and readily available devulcanization chemical components. This literature review paper discusses the approaches, which have less negative impact on the environment, in chemical devulcanization of rubber vulcanizates. A special focus has been on thermo-chemical devulcanization of waste tyres in scCO2 using common organic devulcanizing agents.

Thermochemistry of Sulfur-Based Vulcanization and of Devulcanized and Recycled Natural Rubber Compounds

“…To avoid the possible recombination of the just mentioned Scentered radicals, chemical additives are used during the devulcanization process in order to stabilize provisory the devulcanized network. This specific aspect is treated in various reviews [25][26][27][28][29][30][31][32][33][34][35]. In Scheme 2 we show that the sulfur radicals derived by the mechanochemically induced homolysis are saturated by hydrogen atoms.…”

“…In contrast, a really devulcanized rubber compound should be conceived as a formerly cured and used rubber compound which has undergone a selective mechanochemical processing targeted exclusively to break the sulfur-based crosslinks without causing the undesired main rubber chain scission as well as avoiding generalized mechanochemical oxidative degradation. These concepts were perhaps first expressed more than twenty years ago [20][21][22], were expanded by Myhre and Khait in their contribution to the book "Rubber Recycling" [23,24] and then discussed and expanded in a clearer and distinct way in a series of excellent review works specifically dedicated to devulcanization [24][25][26][27][28][29][30][31][32][33][34][35]. In the present work, we will start with the principle that vulcanization is an exothermal reaction process, we will study such exothermal reaction in a carbon black filled NR-based compound at different sulfur loadings and we will examine in a simplified model the molecular reasons linked to such exothermicity.…”

Rubber Compounds from Devulcanized Ground Tire Rubber: Recipe Formulation and Characterization

“…Devulcanization achieves this goal by selective crosslink scission; reclamation is accompanied by chain scission [ 13 ]. There are several ways of devulcanization; the most widespread methods are thermomechanical [ 14 , 15 , 16 ], thermochemical [ 17 , 18 ] and microwave devulcanization [ 19 , 20 ].…”

Enhancing interface adhesion between waterjet‐produced rubber powders and natural rubber by wet‐mixing method

“…Tires are the main products of rubber materials, accounting for over 70% of rubber products. 1 Nowadays the global volume of scrap tires exceeds 1 billion each year. 2 The accumulation of scrap tires is a hotbed for bacteria to breed, and it also brings serious safety risks.…”