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Recycling Waste Tires Into Ground Tire Rubber (GTR)/Rubber Compounds: a Review

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Recycling Waste Tires Into Ground Tire Rubber (GTR)/Rubber Compounds: a Review Review Recycling Waste Tires into Ground Tire Rubber (GTR)/Rubber Compounds: A Review Ali Fazli and Denis Rodrigue * Department of Department of Chemical Engineering, Université Laval, Quebec, QC G1V 0A6, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-418-656-2903 Received: 13 July 2020; Accepted: 29 July 2020; Published: 31 July 2020 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. Keywords: tire recycling; ground tire rubber; regenerated tire rubber; compounds 1. Introduction 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]. Despite the high life expectancy of new tires, which is at least 80,000 miles, a large number of scrap tires, about one billion end-of-life tires per year, are generated annually all around the world due to the increased number of cars on the roads [5]. 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. Landfilling, as the earliest waste management technique, causes J. Compos. Sci. 2020, 4, 103; doi:10.3390/jcs4030103 www.mdpi.com/journal/jcs J. Compos. Sci. 2020, 4, 103 2 of 43 J. Compos. Sci. 2020, 4, x 2 of 45 healthcauses andhealth environmental and environmental risks of risks accidental of accide firesntal and fires contamination and contamination of the undergroundof the underground water resourceswater resources [6]. Therefore, [6]. Therefore, regulations regulations for the recovery for the andrecovery disposal and of disposal waste tires of waste have beentires introducedhave been byintroduced governments by governments and environmental and environmental organizations organi to adoptzations more to adopt ecofriendly more recyclingecofriendly of recycling waste tires. of Also,waste from tires. an Also, economical from an point economical of view, point discarded of vi tiresew, discarded with low costtires can with be low used cost in several can be marketsused in suchseveral as tire-derivedmarkets such fuel as [7 ,tire-deriv8], asphalted pavements fuel [7,8], [ 9asphalt,10], concrete pavements [11,12 ],[9,10], plastic concrete composites [11,12], [13,14 plastic], and rubbercomposites compounds [13,14], [and15]. rubber Although compounds waste tires [15]. have Although high calorific waste valuestires have to be high used calorific as fuel, values scrap to tires be asused active as fuel, or inactive scrap tires fillers as (tougheningactive or inactive agents fillers for thermoplastics,(toughening agents thermosets for thermoplastics, and rubbers) thermosets are more profitableand rubbers) and are ecofriendly. more profitable In order and to ecofriendly. melt blend wasteIn order tires to withmelt polymers,blend waste grinding tires with methods polymers, are usedgrinding for downsizing methods are the used discarded for downsizing tires and the produce discarded ground tires tire and rubber produce (GTR) ground having tire smallrubber particle (GTR) sizeshaving (granulates small particle in the sizes range (granulates of 0.5–15 mmin the and range powders of 0.5-15 with mm sizes and less powders than 0.5 with mm) sizes [16 ].less However, than 0.5 poormm) bonding[16]. However, between poor GTR andbonding most polymerbetween matricesGTR and is leadingmost polymer to low mechanicalmatrices is strength leading and to lowlow durabilitymechanical of strength the resulting and low compounds, durability which of the isresu a significantlting compounds, challenge which limiting is a thesignificant performances challenge of thelimiting blends the [17 performances,18]. of the blends [17,18]. ToTo overcomeovercome thisthis challenge,challenge, variousvarious techniques,techniques, suchsuch asas GTRGTR surfacesurface modificationmodification [[19,20],19,20], devulcanizationdevulcanization [[21],21], dynamicdynamic vulcanizationvulcanization [[22],22], andand compatibilizingcompatibilizing agentagent additionaddition [[14,23],14,23], havehave beenbeen usedused toto increaseincrease thethe GTR-matrixGTR-matrix interfacialinterfacial adhesionadhesion generatinggenerating improvedimproved homogeneityhomogeneity andand processability,processability, as as well well as as mechanicalmechanical andand longlong termterm (durability)(durability) propertiesproperties ofof thethe compounds.compounds. RecentRecent advancesadvances andand opportunitiesopportunities onon thethe useuse ofof GTRGTR asas anan inexpensiveinexpensive fillerfiller inin constructionconstruction [[24,25],24,25], energyenergy storagestorage [[26],26], andand polymerpolymer blendsblends inin generalgeneral withwith aa focusfocus onon thermoplasticthermoplastic matricesmatrices [[16,27]16,27] havehave beenbeen reviewed.reviewed. However, However, there there is is very very limited limited work work focu focusingsing on onthe the recent recent advances advances and andevolution evolution of GTR of GTRapplication application in virgin in virgin rubbers. rubbers. Therefore, Therefore, to address to address this gap this in gap the in literature, the literature, this paper this paper not only not onlyreviews reviews the evolution the evolution of GTR of GTR recycling recycling methods, methods, downsizing downsizing techniques, techniques, GTR GTR devulcanization devulcanization and andsurface surface treatments, treatments, but butalso also reports reports onon the the recent recent development development and and opportunity opportunity of of using GTR inin rubberrubber formulations.formulations. Tire1.1. CompositionTire Composition WasteWaste automobileautomobile and and trucktruck tirestires are are thethe mainmain sourcessources ofof wastewaste rubbers.rubbers. FigureFigure1 1 shows shows a a typical typical structurestructure ofof aa tiretire withwith itsits components.components. TheThe exteriorexterior treadtread toto thethe interiorinterior lininglining ofof thethe tiretire consistsconsists ofof didifferentfferent materialsmaterials withwith specificspecific properties.properties. The tread, which is thethe partpart inin directdirect contactcontact withwith thethe road,road, isis mainly composed composed of of natural natural rubber rubber (NR) (NR) an andd synthetic synthetic rubbers. rubbers. Rubbe Rubber,r, steel steel and and textiles textiles can canbe used be used in the in the composition composition of ofthe the belts belts depending depending on on the the tire tire application application (car, (car, truck, truck, off-the-road, off-the-road, etc.).etc.). The materials in the sidewalls require goodgood resistanceresistance toto crackcrack propagationpropagation andand attackattack byby ozoneozone (O(O33)) inin air,air, whilewhile abrasiveabrasive wearwear resistanceresistance isis thethe mainmain requirementrequirement ofof rubberrubber treads.treads. The inner liner, responsibleresponsible forfor maintainingmaintaining thethe airair pressure,pressure, isis mademade ofof butylbutyl rubberrubber withwith aa lowlow airair permeabilitypermeability andand goodgood rollingrolling resistance.
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