Vegetable derived-oil facilitating carbon black migration from waste tire rubbers and its reinforcement effect

Enhancing the sustainability of rubber materials: Dual benefits of wet mixing technology and recycled rubber's honeycomb reinforcement structure

The world's three leading tire manufacturers have proposed specific timelines for using recycled materials. For instance, Michelin targets an increase in the proportion of sustainable materials in tires to 40 % by 2030 and aims to produce 100 % of its tires from bio-based, renewable, or recyclable materials as of 2050. In such a context, this study introduced wet mixing technology to apply recycled rubber (RR) in highly wear-resistant tire tread compounds. This technique leverages the rubber's inherent crosslink density to enhance the mechanical performance of final products. The results indicated that wet mixing effectively addressed the high viscosity issue of RR. In the traditional dry mixing method, physical blending typically results in large particle sizes and suboptimal performance. In contrast, wet mixing reduced the rubber's hysteresis loss by 75 % and improved its rebound performance by 35.6 % at 23 °C, 60 °C, and 100 °C compared to traditional dry mixing. DIN volume abrasion was also reduced by 23.3 %. Remarkably, Akron abrasion nearly doubled its effect. Additionally, wet mixing regulated aggregate structure and formed a densely packed honeycomb-like structure within RR. Incorporating RR using wet mixing demonstrates noticeable advantages in carbon black/natural rubber/RR composite materials. This approach also presents a viable path to sustainable development in the rubber manufacturing industry.

Analyzing Thermal Degradation Effects on Devulcanized GTR-Based NR/SBR/NBR Rubber Compounds Reinforced with SiO2 Particles

The large number of tires produced annually demands new recycling methods. A key challenge associated with recycling elastomers is their crosslinking structure that prevents fusion. It is possible to reverse crosslinking through a process called devulcanization. Devulcanized elastomers can be blended with fresh rubber and revulcanized for reuse. This paper examines samples made from natural rubber (NR), styrene-butadiene rubber (SBR), and nitrile butadiene rubber (NBR), blended with varying proportions of devulcanized ground tire rubber (dGTR) and newly revulcanized rubber. SiO2, commonly present in tire formulations, is also added. Samples of these materials, with 0, 10, 20, and 40 phr of dGTR are subjected to accelerated degradation for 0, 30, 60, 120, and 240 h. The effects of this treatment, the influence of SiO2, and the presence of a silane-based devulcanization agent (TESPT) that promotes the interaction between the rubber and silica, are analyzed at the microstructural level (FTIR, TGA, SEM) and through mechanical properties testing. The microstructural results of the spectroscopy and thermal analysis show that interactions between dGTR, silica, and silane compounds form aggregates that impact the material properties and degradation of the tires. Mechanically, when the sample contained up to 20 phr of dGTR, the compound presented a more brittle behavior, due to the crosslinking induced by the degradation.

Characterization, Properties and Mixing Mechanism of Rubber Asphalt Colloid for Sustainable Infrastructure

Rubber asphalt has always been considered to have the most potential for the disposal of waste tires as sustainable infrastructure. However, the covalently cross-linked tire rubber presents an extreme challenge in reusing waste rubbers in roads. Rubberized asphalt with finely dispersed or colloidal structure has been regarded as a potential binder used as road material because of the improved properties in terms of storage stability, easy processing and high content of incorporation. However, the mixing mechanism between the finely dispersed rubber on micro-nano scale with asphalt is still not clear, which restricts its further development as value-added material. Devulcanized rubber (DR) was introduced to improve the compatibility between asphalt and rubber. The basic chemicals of DR and asphalt were introduced based on their structures. Furthermore, the interactions between DR and asphalt were discussed according to the functional elements at different levels, and the concept of DR as “the fifth component” of asphalt was put forward. Finally, high performance, environmental and economic effects and applications of devulcanized-rubber-modified asphalt (DRMA) were discussed. The review is expected to provide a guide for the wide application of DRMA, which is still restricted by poor compatibility and bad stability during processing, storage and recycling.

Ground tire rubber functionalization as a promising approach for the production of sustainable adsorbents of environmental pollutants

Waste tires management and further utilization are currently one of the biggest concerns regarding the environment and human health protection. At present, shredding, grinding, or pulverization of waste tires are the most popular options for industrial recycling. Although many solutions for ground tire rubber (GTR) applications were checked and verified so far, their further implementation at an industrial scale is still very limited. In this brief review work, ground tire rubber functionalization strategies as a promising approach for the production of sustainable adsorbents of environmental pollutants were presented and discussed. Our findings indicate that suitable functionalization of GTR significantly improves adsorption capacity or selectivity of prepared GTR-based adsorbents. However, it should be mentioned that most of the performed research based on multi-step and time-consuming protocols of GTR functionalization is performed usually in the presence of solvents, which results in very low efficiency and as a consequence high-cost and limited applications. Current research trends showed that reactive extrusion can be considered as efficient, solvent-free, and pro-ecological alternative for commonly investigated periodic methods of GTR functionalization. This work shows that reactive extrusion is a promising method for further development of GTR-based adsorbents dedicated to environmental pollutants.

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

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.

Setting Relationships between Structure and Devulcanization of Ground Tire Rubber and Their Effect on Self-Healing Elastomers

The use of devulcanized tire powder as an effective reinforcement in self-healing styrene-butadiene rubber (SBR) compounds has been investigated for the first time in this work. For this purpose, the evolution of the microstructure of the rubber from end-of-life tires (ELTs) was studied during granulation, grinding and devulcanization through an exhaustive characterization work in order to relate the final microstructure with the mechanical response of the repaired systems. Different morphologies (particle size distribution and specific surface area) obtained by cryogenic and water jet grinding processes, as well as different devulcanization techniques (thermo-mechanical, microwave, and thermo-chemical), were analyzed. The results demonstrated the key influence of the morphology of the ground tire rubber (GTR) on the obtained devulcanized products (dGTR). The predictions of the Horikx curves regarding the selectivity of the applied devulcanization processes were validated, thereby; a model of the microstructure of these materials was defined. This model made it possible to relate the morphology of GTR and dGTR with their activity as reinforcement in self-healing formulations. In this sense, higher specific surface area and percentage of free surface polymeric chains resulted in better mechanical performance and more effective healing. Such a strategy enabled an overall healing efficiency of more than 80% in terms of a real mechanical recovery (tensile strength and elongation at break), when adding 30 phr of dGTR. These results open a great opportunity to find the desired balance between the mechanical properties before and after self-repair, thus providing a high technological valorization to waste tires.

Recycling of Rubber Wastes as Fuel and Its Additives

Economic, social, and urban developments generally require improvements in the transportation sector, which includes automobiles such as trucks, buses, trailers, airplanes, and even bicycles. All these vehicles use rubber tires. After consumption, these tires become waste, leading to enlarged landfill areas for used tires and implying additional harm to the environment. This review summarizes the growth of rubber recycling application and the sustainability of using waste rubber in the construction field. Furthermore, we provide methods to convert rubber waste to fuel or fuel additives by using tire-derived fuel and concentrate to pyrolysis, which are environmentally friendly and efficient ways. The related parameters such as temperature, pressure, and feedstock composition were studied. Most research papers observed that 500 °C is the optimal temperature at atmospheric pressure in the presence of a specific type of catalyst to improve pyrolysis rate, oil yield, and quality.

GTR/NBR/Silica Composites Performance Properties as a Function of Curing System: Sulfur versus Peroxides

In this work, conventional sulfur and two types of organic peroxides (dicumyl peroxide (DCP) and di-(2-tert-butyl-peroxyisopropyl)-benzene (BIB)) curing systems were used to investigate the possibility for tailoring of the performance properties of GTR/NBR blends reinforced with a variable content of highly dispersive silica (0-30 phr). The curing characteristics, static mechanical and acoustical properties, swelling behavior, thermal stability, and microstructure of the prepared composites were investigated. The results show that regardless of the curing system used, increasing the content of highly dispersive silica resulted in the improvement of the mechanical properties of the studied materials. It was observed that sulfur-based systems are the best choice in terms of cross-linking efficiency determined based on torque increment and cross-link density parameters. However, further analysis of the physico-mechanical properties indicated that the cross-linking efficiency does not match the performance of specimens, and the materials obtained using organic peroxides show higher tensile properties. This is due to the improved physical interactions between the GTR/NBR matrix and highly dispersive silica when using peroxide systems. It was confirmed using the analysis of the Wolff activity coefficient, indicating the enhanced synergy.

Current progress in waste tire rubber devulcanization

Vulcanized rubber, due to its superior mechanical properties, has long been used in various industries, especially automotive. The rubber industry has evolved and expanded over the years to meet the increasing global demands for tires. Today tires consist of about 19% natural rubber and 24% synthetic rubber, while plastic polymer and metal, filler and additives make up the rest. Over 1.6 billion new tires are produced annually and around 1 billion waste tires are generated. Tires are extensively designed with several complex processes to make them virtually indestructible. Since tire rubber does not decompose easily, their disposal at the end of service life creates a monumental environmental impact. However, waste tire rubber (WTR) consist of valuable rubber hydrocarbon, making its recovery or regeneration highly desirable. The conventional recovery method of WTR tends to produce undesirable products due to the destruction of the polymeric chain and exponentially degenerates the vulcanizates' physical properties. Since then, multiple devulcanization processes were introduced to effectively and selectively cleave vulcanizate's crosslinks while retaining the polymeric networks. Different devulcanization methods such as chemical, mechanical, irradiation, biological and their combinations that have been explored until now are reviewed here. Besides, an overview of the latest development of devulcanization by ionic liquids and deep eutectic solvents are also described. While such devulcanization technique provides new sustainability pathway(s) for WTR, the generated devulcanizate also possesses comparable physical properties to that of virgin products. This further opens the possibility of novel circular economic opportunities worldwide.

Reactive Sintering of Ground Tire Rubber (GTR) Modified by a Trans-Polyoctenamer Rubber and Curing Additives

The proposed method of ground tire rubber (GTR) utilization involves the application of trans-polyoctenamer rubber (TOR), a commercially available waste rubber modifier. The idea was to investigate the influence of various curing additives (sulfur, N-cyclohexyl-2-benzothiazole sulfenamide (CBS), dibenzothiazole disulfide (MBTS) and di-(2-ethyl)hexylphosphorylpolysulfide (SDT)) on curing characteristics, physico-mechanical, thermal, acoustic properties as well as the morphology of modified GTR, in order to evaluate the possibility of reclaiming GTR and the co-cross-linking between applied components. The results showed that the presence of the modifier without the addition of curing additives hinders the physico-mechanical properties of revulcanized GTR. The addition of SDT, CBS, MBTS and sulfur change the melting kinetics of TOR, indicating partial degradation and/or co-cross-linking between components. In the studied conditions, the best mechanical properties were obtained by the samples cured with sulfur. The morphology analysis, combined with the physico-mechanical results, indicated that when the surface of the GTR is more developed, obtained by the addition of TOR, the properties of the GTR improve.

Ground Tire Rubber Modified by Ethylene-Vinyl Acetate Copolymer: Processing, Physico-Mechanical Properties, Volatile Organic Compounds Emission and Recycling Possibility

Ground tire rubber (GTR) was reclaimed and modified with 10 phr of ethylene-vinyl acetate copolymer via low-temperature extrusion process. Processing, physico-mechanical properties, volatile organic compounds emission, and recycling possibility were investigated. In order to better understand the impact of used modifiers, their efficiency was compared with trans-polyoctenamer, which is an additive that is commercially dedicated to waste rubber recycling. The results showed that a relatively small amount of ethylene-vinyl acetate copolymer improves the mechanical properties of modified reclaimed GTR and also allows further recycling by multiple processing without the deterioration of performance after three cycles.

Waste tire rubber as low-cost and environmentally-friendly modifier in thermoset polymers - A review

Nowadays, waste tire rubber (WTR) management is a growing and serious problem. Therefore, research works focused on the development of cost-effective and environmentally-friendly methods of WTR recycling are fully justified. Incorporation of WTR into polymer matrices and composite materials attracts much attention, because this approach allows sustainable development of industrially applicable waste tires recycling technologies. Generally, utilization of WTR as a filler for polymer composites noticeably reduces materials costs, while suitable modification/functionalization of WTR may significantly enhance the performance of plastics and rubbers. This work aims to summarize the literature reports related to the thermoset/WTR composites based on various matrices such as: polyurethanes, epoxy and other resins. It particularly focuses on compatibilization strategies in thermosets/WTR systems and their impact on the chemistry and physical interfacial interactions between thermoset matrix and WTR filler phase, what significantly affecting performance properties of prepared materials. Moreover, future trends and limitation related to thermoset/WTR composites development are discussed.

Devulcanization Technologies for Recycling of Tire-Derived Rubber: A Review

In general, composite materials are difficult to recycle. Tires belong to this class of materials. On top, one of their main constitutents, vulcanized rubber, is as elastomer, which cannot be remolten and hence is particularly challenging to put to a new use. Today, the main end-of-life routes of tires and other rubber products are landfilling, incineration in e.g., cement plants, and grinding to a fine powder, generating huge quantities and indicating a lack of sustainable recycling of this valuable material. True feedstock recycling is not feasible for complex mixtures such as tires, but devulcanization can be done to reactivate the cross-linked polymer for material recycling in novel rubber products. Devulcanization, i.e., the breaking up of sulfur bonds by chemical, thermophysical, or biological means, is a promising route that has been investigated for more than 50 years. This review article presents an update on the state-of-the art in rubber devulcanization. The article addresses established devulcanization technologies and novel processes described in the scientific and patent literatures. On the one hand, tires have become high-tech products, where the simultaneous improvement of wet traction, rolling resistance, and abrasion resistance (the so-called "magic triangle") is hard to achieve. On the other hand, recycling and sustainable end-of-life uses are becoming more and more important. It is expected that the public discussion of environmental impacts of thermoplastics will soon spill over to thermosets and elastomers. Therefore, the industry needs to develop and market solutions proactively. Every year, approximately 40 million tons of tires are discarded. Through the devulcanization of end-of-life tires (ELT), it is possible to produce new raw materials with good mechanical properties and a superior environmental footprint over virgin products. The devulcanization process has become an interesting technology that is able to support the circular economy concept.

Cell metabolomics to study the cytotoxicity of carbon black nanoparticles on A549 cells using UHPLC-Q/TOF-MS and multivariate data analysis

Carbon black nanoparticles (CBNPs) are core component of fine particulate matter (PM2.5) in the atmosphere. It was reported that the particle in the atmosphere with smaller size and the larger the specific surface area are easier to reach the deep respiratory tract or even the alveoli through the respiratory barrier and cause lung injury. Therefore, it has been believed that ultrafine or nanometer particles with more toxic than those with larger particle sizes. Moreover, it was confirmed that CBNPs could induce inflammation, oxidative stress and changes in cell signaling and gene expression in mammalian cells and organs. However, the cytotoxicity mechanism of them has been uncertain so far. The aim of the present study was to explore the underlying mechanism of cytotoxicity induced by CBNPs on A549 cells. In the current research, the viabilities of A549 cells were detected by Cell Counting Kit-8 (CCK-8) assay. The further metabolomics studies were conducted to detect the cytotoxic effect of CBNPs on A549 cells with an IC50 value of 70 μg/mL for 48 h. Potential differential compounds were identified and quantified using a novel on-line acquisition method based on ultra-liquid chromatography quadrupole time-of-flight mass spectrometry(UHPLC-Q-TOF/MS). The cytotoxicity mechanism of CBNPs on A549 cells was evaluated by multivariate data analysis and statistics. As a result, a total of 32 differential compounds were identified between CBNPs exposure and control groups. In addition, pathway analysis showed the metabolic changes were involved in the tricarboxylic acid (TCA) cycle, alanine, aspartate and glutamate metabolism, histidine metabolism and so on. It is also suggested that CBNPs may induce cytotoxicity by affecting the normal process of energy metabolism and disturbing several vital signaling pathways and finally induce cell apoptosis.