Abstract
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.
Highlights
Vulcanized rubbers, as thermoset materials, show low elasticity and yield strain as well as high Young’s modulus
Maleic anhydride (MA) modification of regenerated tire rubber (RTR) led to longer cure time, scorch time and lower cure rate in all blends since anhydrides grafted on the RTR surface as cure retarders delay the onset of cure reaction resulting in higher cure times
Increase in minimum torque and Mooney viscosity of the compounds by RTR addition due to higher carbon black content resulting in higher stiffness and chain mobility restriction, as well as lower optimum cure time related to the presence of active crosslinking sites in RTR
Summary
Vulcanized rubbers, as thermoset materials, show low elasticity and yield strain as well as high Young’s modulus. The vulcanization process results in the formation of a crosslinked structure inside the rubber that can resist against intensive shear and temperature applications, as well as environmental agents [1]. Rubbers have been extensively used in various applications ranging from household, healthcare, military, automotive and construction [2,3]. Automotive and truck tires are the main application of vulcanized rubber because of their high resistance to severe outdoor conditions (chemical reagents, high temperatures, radiations and shear stress) during their lifetime [4]. From an environmental point of view, recycling waste tires with a non-biodegradable structure and a high volume of production raises concerns about waste tires management approaches.
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