Devulcanization of natural rubber industry waste in supercritical carbon dioxide combined with diphenyl disulfide

An overview on waste rubber recycling by microwave devulcanization

This review deals with waste rubber recycling by devulcanization treatment using microwave method. In fact, vulcanized rubbers have been extensively used in various fields due to their superior performances. Subsequently, the massive use of such materials, especially in the automotive industry, has generated a substantial amount of wastes which are not easily to be degraded due to the three-dimensional network formed by the vulcanization process. One of the optimal solutions for the successful recycling of rubber is devulcanization, i.e., the process in which the sulfur bonds in the vulcanized material are selectively broken. Currently, to achieve rubber devulcanization, the microwave treatment has been proposed as a promising alternative process due to its precise manipulation of process variables. Furthermore, the microwave process is easily to be coupled with effects of other elements such as chemical and swelling agents. In this work, different microwave devulcanization methods are reviewed, the utilization of the corresponding devulcanized materials has also been discussed. The reviewed contents are believed to be of great interest to academics and industries since they represent a great challenge from scientific, economic and environmental points of view.

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

In this study, our focus was on developing and investigating rubber recipes that are suitable for devulcanized ground tire rubber (dGTR). Devulcanized rubber has a powdery or sticky uncured rubber-like appearance depending on the extent of main-chain degradation that occurs with selective crosslinking scission. Still, it has a significantly shorter scorch time than a new rubber compound. Therefore, our primary goal was to slow down the vulcanization process of dGTR and improve its mechanical properties via recipe development. We formulated several recipes (sulfur-, peroxide-, and phenolic resin-based) and studied the vulcanization process and the main properties of the revulcanized rubber sheets. We observed that the vulcanization process could be altered with different vulcanization methods: using peroxide and vulcanizing resin extended the process significantly. Peroxide vulcanization also provided enhanced elongation compared to sulfuric systems. With a balance of properties in mind, we selected a semi-efficient sulfur-based recipe and studied the characteristics of natural rubber/dGTR mixtures with the help of plasticizer oils. We successfully replaced a notable portion of natural rubber with dGTR, maintaining its properties without much compromise.

Phosphate adsorption from water and wastewater using non-devulcanised and devulcanised tyre rubber

The secondary use of tyre rubber is a potentially sustainable environmental solution. However, the sorption properties of used-tyre rubber have not yet been fully investigated. In this study, the rubber type (vulcanised or devulcanised part-worn tyre rubber) that can sorb phosphate phosphorus from aqueous solutions or wastewater more effectively is determined. The capacity of granules (0.3-1.0 mm in diameter) of non-devulcanised ground tyre rubber and uniquely chemically devulcanised rubber to adsorb phosphorus is evaluated under laboratory conditions. The results show that under the filtration of an aqueous solution or biologically treated wastewater at a flow rate of 0.75 m/h (1.2 L/h), 1 g of the devulcanised rubber medium accumulates 5.16 mg of phosphorus, which is five times more than that accumulated by the non-devulcanised rubber medium. The surface structure of the non-devulcanised rubber medium is more suitable for the sorption of devulcanised rubber granules. The sorption capacity and effectiveness of non-devulcanised rubber for phosphorus removal are more favourable compared with those of the tested natural and waste-prepared sorbents. Further research into this material as a medium for filter layers and for accumulating drainage should be conducted. The findings of this study are important for addressing issues associated with the secondary use of tyre rubber.

Characterization of VOCs during Nonheating and Heating Periods in the Typical Suburban Area of Beijing, China: Sources and Health Assessment

In recent years, the “coal to electricity” project (CTEP) using clean energy instead of coal for heating has been implemented by Beijing government to cope with air pollution. However, VOC pollution after CTEP was rarely studied in suburbs of Beijing. To fill this exigency, 116 volatile organic compounds (VOCs) were observed during nonheating (P1) and heating (P2) periods in suburban Beijing. The results showed that the total of VOCs (TVOCs) was positively correlated with PM2.5, PM10, NO2, CO, and SO2 but negatively correlated with O3 and wind speed. The average TVOCs concentration was 19.43 ± 12.41 ppbv in P1 and 16.25 ± 8.01 ppbv in P2. Aromatics and oxygenated VOCs (OVOCs) were the main contributors to ozone formation potential (OFP). Seven sources of VOCs identified by the positive matrix factorization (PMF) model were industrial source, coal combustion, fuel evaporation, gasoline vehicle exhaust, diesel vehicle exhaust, background and biogenic sources, and solvent usage. The contribution of coal combustion to VOCs increased significantly during P2, whereas industrial sources, fuel evaporation, and solvent usage exhibited opposite trends. The potential source contribution function (PSCF) and concentration weighted trajectory (CWT) were used to analyze the source distributions. The results showed that VOC pollution was caused mainly by air mass from southern Hebei during P1 but by local emissions during P2. Therefore, although the contribution of coal combustion after heating increased, TVOCs concentration during P2 was lower than that during P1. Chronic noncarcinogenic risks of all selected VOC species were below the safe level, while the carcinogenic risks of most selected VOC species were above the acceptable risk level, especially for tetrachloromethane and 1,2-dichloroethane. The cancer risks posed by gasoline vehicle emissions, industrial enterprises, and coal combustion should be paid more attention.

Recycling NR/SBR waste using probe sonication as a new devulcanizing method; study on influencing parameters

In this work, a vulcanized blend of natural rubber (NR) and styrene butadiene rubber (SBR) (i.e. at weight ratio of 50 : 50) as a model for tire rubber was devulcanized using probe sonication. The effect of processing parameters such as sonication media, power, temperature and time on sol/gel contents and devulcanization percent of rubbery samples was investigated. Moreover, the influence of pre-immersion of vulcanized NR/SBR samples in different liquids (i.e. water, oil and toluene) was assessed for different sonication times (i.e. 10 to 60 min) and powers (i.e. 30 to 60 W). It was found that pre-immersion of rubber particles in oil significantly increased the devulcanization percent. The optimum conditions for devulcanization of the NR/SBR blend via probe sonication were found to be 60 Watts, 20 min, oil and 24 h for sonication power, sonication time, pre-immersion/sonication media and pre-immersion time, respectively. The highest obtained devulcanization percent in this step was about 40%. The effects of two devulcanizing chemical agents (i.e. diphenyl disulfide and VitaX) on devulcanization performances of the samples were also studied. Results showed that higher devulcanization percent (i.e. about 52%) was obtained by using VitaX. It was also observed that VitaX significantly improved re-vulcanization speed (i.e. cure rate index) and decreased scorch time. It was also found that lower content of VitaX (i.e. 0.6 phr) caused better curing properties but lower mechanical properties compared to the higher content (i.e. 1.2 phr).

In this work, a vulcanized blend of natural rubber (NR) and styrene butadiene rubber (SBR) (i.e. at weight ratio of 50 : 50) as a model for tire rubber was devulcanized using probe sonicationP.

Low-Temperature Mechano-Chemical Rubber Reclamation Using Terpinene as a Swelling Agent to Enhance Bond-Breaking Selectivity

Common swelling agents used in the mechano-chemical rubber devulcanization process usually require high temperatures to achieve satisfactory swelling effects, which results in severe production of pollutants and reduces the selectivity of bond scissions. This work presents an environmentally friendly swelling agent, terpinene, which can swell the rubber crosslink structures at low temperatures. Both a rubber swelling experiment and a rubber reclaiming experiment with a mechano-chemical devulcanization method are conducted to explore the swelling effects of terpinene. After soaking in terpinene at 60 °C for 90 min, the length elongation of the rubber sample reaches 1.55, which is much higher than that in naphthenic oil and is comparable to that in toluene. When adding 3 phr of terpinene for every 100 phr of waste rubber during the reclaiming process, the bond scissions exhibit high selectivity. After revulcanization, the reclaimed rubbers have a tensile strength of 17 MPa and a breaking elongation of 400%. Consequently, the application of terpinene as the swelling agent in the LTMD method can greatly improve the properties of reclaimed rubbers, thereby enhancing the dual value for the economy and environment.

Sustainable mobility: The route of tires through the circular economy model

Until nowadays, the concept of the 3Rs (Reduce, Reuse, Recycle) has tried to develop responsible consumption habits. Nonetheless, the rise of ecological thinking has generated the appearance of four new Rs in addition to these basic 3Rs; the currently 7Rs (Reduce, Reuse, Recycle, Redesign, Renew, Repair and Recover) which refer to the actions necessary to achieve the change towards a circular economy (CE) model. This model aims at extending the lifetime of the resources through their rational and efficient use to generate value repeatedly, reducing costs and waste. In this review, we examine the route followed by tires from the CE perspective, analyzing end-of-life strategies that aim to improve the circular flow of tire rubber materials. We discuss the most relevant studies on the "7Rs" concepts applied to tires, comparing different scientific approaches, as well as their industrial and commercial implementation. We also illustrate the drawbacks and feasibility of each of the R-hierarchy strategies. From the early stages of production to the post-consumption step, the path that tires trail within this CE model evidences the commitment and efforts towards the development of effective management schemes for achieving a real sustainable mobility.