Recent advances on waste tires: bibliometric analysis, processes, and waste management approaches

End of life tires (ELTs) are a pressing environmental concern due to their non-biodegradable nature and potential release of toxic chemicals, as confirmed by human health exposure studies. The expanding transport sector, driven by the automotive industry, has led to inadequate attention to safe tire disposal. This review extracted papers using keywords such as "waste tire rubber," "waste tire pollution," and "waste tire applications" from 2012 to 2023. Recycling publications have surged by 80% in the past decade, with China and the USA leading the research. Pyrolysis and devulcanization methods have emerged as key circular economy (CE) advancements, producing fuel and reusable rubber. Globally, 1.5 billion waste tires accumulate yearly, projected to increase by 70% in the next 30 years if unaddressed. Around 26 million tonnes of used tires are generated annually worldwide, while civil engineering and backfilling use 17 million tonnes of recycled rubber particles. These tires are complex polymer composites, primarily composed of natural and synthetic rubber. The amorphous nature of rubber results in a 50% loss of mechanical properties when exposed to chemicals and microbes, shortening its lifespan. This paper explores the applicability of waste tire rubber and polymer fabrication to offer eco-friendly and cost-effective solutions for proper disposal, mitigating environmental accumulation.

Phase morphology, mechanical, and thermal properties of fiber-reinforced thermoplastic elastomer: Effects of blend composition and compatibilization

In this work, recycled high density polyethylene (rHDPE) was compounded with regenerated tire rubber (RR) (35-80 wt.%) and reinforced with recycled tire textile fiber (RTF) (20 wt.%) as a first step. The materials were compounded by melt extrusion, injection molded, and characterized in terms of morphological, mechanical, physical, and thermal properties. Although, replacement of the rubber phase with RTF compensated for tensile/flexural moduli losses of rHDPE/RR/RTF blends because of the more rigid nature of fibers increasing the composites stiffness, the impact strength substantially decreased. So, a new approach is proposed for impact modification by adding a blend of maleic anhydride grafted polyethylene (MAPE)/RR (70/30) into a fiber-reinforced rubberized composite. As in this case, a more homogeneous distribution of the fillers was observed due to better compatibility between MAPE, rHDPE, and RR. The tensile properties were improved as the elongation at break increased up to 173% because of better interfacial adhesion. Impact modification of the resulting thermoplastic elastomer (TPE) composites based on rHDPE/(RR/MAPE)/RTF was successfully performed (improved toughness by 60%) via encapsulation of the rubber phase by MAPE forming a thick/soft interphase decreasing interfacial stress concentration slowing down fracture. Finally, the thermal stability of rubberized fiber-reinforced TPE also revealed the positive effect of MAPE addition on molecular entanglements and strong bonding yielding lower weight loss, while the microstructure and crystallinity degree did not significantly change up to 60 wt.% RR/MAPE (70/30).

Effect of Ground Tire Rubber (GTR) Particle Size and Content on the Morphological and Mechanical Properties of Recycled High-Density Polyethylene (rHDPE)/GTR Blends

This work investigates the effect of ground rubber tire (GRT) particle size and their concentration on the morphological, mechanical, physical, and thermal properties of thermoplastic elastomer (TPE) blends based on recycled high-density polyethylene (rHDPE). In our methodology, samples are prepared via melt blending (twin-screw extrusion followed by compression molding) to prepare different series of blends using GTR with three different particle sizes (0–250 μm, 250–500 μm, and 500–850 μm) for different GTR concentrations (0, 20, 35, 50, and 65 wt.%). The thermal properties are characterized by differential scanning calorimeter (DSC), and the morphology of the blends is studied by scanning electron microscopy (SEM). The mechanical and physical properties of the blends are investigated by quasi-static tensile and flexural tests, combined with impact strength and dynamic mechanical analysis (DMA). The SEM observations indicate some incompatibility and inhomogeneity in the blends, due to low interfacial adhesion between rHDPE and GTR (especially for GTR > 50 wt.%). Increasing the GTR content up to 65 wt.% leads to poor interphase (high interfacial tension) and agglomeration, resulting in the formation of voids around GTR particles and increasing defects/cracks in the matrix. However, introducing fine GTR particles (0–250 μm) with higher specific surface area leads to a more homogenous structure and uniform particle dispersion, due to improved physical/interfacial interactions. The results also show that for a fixed composition, smaller GTR particles (0–250 μm) gives lower melt flow index (MFI), but higher tensile strength/modulus/elongation at break and toughness compared to larger GTR particles (250–500 μm and 500–850 μm).

PE-contaminated industrial waste ground tire rubber: How to transform a handicapped resource to a valuable one

Sustainability and enhancement of recycling differing polymer waste has become a leading driver for industries associated with this type of waste. However, polymer waste streams that have not seen as 'typical' have existed in smaller but not insignificant amounts. This study has focused on the recycling of such a waste resource that is not classified as a typical one in developed countries but appears in other locations globally where opportunities for careful waste pretreatment are hindered, therefore creating a challenge for waste handling and the application of modern techniques. Compatibilizing is a strategy employed to recycle ground tire rubber (GTR) by blending with waste high density polyethylene (w-HDPE). Such processing methods and measurement techniques have been chosen to allow easy access without extra costs. For enhanced incorporation of the filler into the matrix olefin-maleic-anhydride copolymer based additives have been synthesized and have succeeded in creating a more homogenous blend with samples having a good surface appearance and mechanical properties. Outstanding Charpy impact strength at room temperature (10.1 kJ/m²) has been achieved in compatibilized 70/30 w-HDPE/GTR (containing 20% PE-contaminant), while elongation at break and tensile strength have been 10.3% and 14.9 MPa. Morphological structure of rubber resources and blends have been assessed by SEM while analytical properties and other features of experimental compatibilizing additives have been studied by e.g. FT-IR.

Recycling Waste Tires into Ground Tire Rubber (GTR)/Rubber Compounds: A Review

Recycling and recovery of waste tires is a serious environmental problem since vulcanized rubbers require several years to degrade naturally and remain for long periods of time in the environment. This is associated to a complex three dimensional (3D) crosslinked structure and the presence of a high number of different additives inside a tire formulation. Most end-of-life tires are discarded as waste in landfills taking space or incinerated for energy recovery, especially for highly degraded rubber wastes. All these options are no longer acceptable for the environment and circular economy. However, a great deal of progress has been made on the sustainability of waste tires via recycling as this material has high potential being a source of valuable raw materials. Extensive researches were performed on using these end-of-life tires as fillers in civil engineering applications (concrete and asphalt), as well as blending with polymeric matrices (thermoplastics, thermosets or virgin rubber). Several grinding technologies, such as ambient, wet or cryogenic processes, are widely used for downsizing waste tires and converting them into ground tire rubber (GTR) with a larger specific surface area. Here, a focus is made on the use of GTR as a partial replacement in virgin rubber compounds. The paper also presents a review of the possible physical and chemical surface treatments to improve the GTR adhesion and interaction with different matrices, including rubber regeneration processes such as thermomechanical, microwave, ultrasonic and thermochemical producing regenerated tire rubber (RTR). This review also includes a detailed discussion on the effect of GTR/RTR particle size, concentration and crosslinking level on the curing, rheological, mechanical, aging, thermal, dynamic mechanical and swelling properties of rubber compounds. Finally, a conclusion on the current situation is provided with openings for future works.

Waste Rubber Recycling: A Review on the Evolution and Properties of Thermoplastic Elastomers

Currently, plastics and rubbers are broadly being used to produce a wide range of products for several applications like automotive, building and construction, material handling, packaging, toys, etc. However, their waste (materials after their end of life) do not degrade and remain for a long period of time in the environment. The increase of polymeric waste materials’ generation (plastics and rubbers) in the world led to the need to develop suitable methods to reuse these waste materials and decrease their negative effects by simple disposal into the environment. Combustion and landfilling as traditional methods of polymer waste elimination have several disadvantages such as the formation of dust, fumes, and toxic gases in the air, as well as pollution of underground water resources. From the point of energy consumption and environmental issues, polymer recycling is the most efficient way to manage these waste materials. In the case of rubber recycling, the waste rubber can go through size reduction, and the resulting powders can be melt blended with thermoplastic resins to produce thermoplastic elastomer (TPE) compounds. TPE are multi-functional polymeric materials combining the processability of thermoplastics and the elasticity of rubbers. However, these materials show poor mechanical performance as a result of the incompatibility and immiscibility of most polymer blends. Therefore, the main problem associated with TPE production from recycled materials via melt blending is the low affinity and interaction between the thermoplastic matrix and the crosslinked rubber. This leads to phase separation and weak adhesion between both phases. In this review, the latest developments related to recycled rubbers in TPE are presented, as well as the different compatibilisation methods used to improve the adhesion between waste rubbers and thermoplastic resins. Finally, a conclusion on the current situation is provided with openings for future works.