Review Recycling Waste into Ground 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: ; 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 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 ecofriendly recycling 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], 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 ,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 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 rubber (NR) (NR) an andd synthetic synthetic rubbers. rubbers. Rubbe Rubber,r, 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. resistance. The The carcass carcass and and the the beads beads are are composed composed of twistedof twisted metals metals or textile or textile cords cords and metaland metal alloy alloy coated coated steel steel wires, wires, respectively respectively [28,29 [28,29].].

FigureFigure 1.1. Typical tiretire structurestructure (reproduced(reproduced withwith permissionpermission fromfrom [[28]).28]).

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The tire composition depends on different parameters such as long distances, plane braking, roadThe quality tire and composition temperature depends since on the di tiresfferent need parameters to show extremely such as long high distances, resistance plane to severe braking, outdoor road qualityconditions and (chemical temperature reagents, since high the tires temperatures, need to show radiations extremely and shear high resistancestress) expected to severe during outdoor their conditionslifetime. As (chemical presented reagents, in Table high1, the temperatures, main components radiations of a tire and formulation shear stress) are expected natural and during synthetic their lifetime.rubbers, Ascarbon presented black, inmetal, Table textile1, the fabrics main components and other additives of a tire formulationwith specific areconcentration natural and depending synthetic rubbers,on the type carbon of tire black, [30]. metal, textile fabrics and other additives with specific concentration depending on the type of tire [30]. Table 1. Tire composition depending on the application (reproduced with permission from [28]). Table 1. Tire composition depending on the application (reproduced with permission from [28]). Material Car tire Truck tire Off-the-road tire Rubbers/ElastomersMaterial (wt.%) Car Tire47 Truck45 Tire Off-the-Road 47 Tire RubbersCarbon/ black and (wt.%)silica (wt.%) 47 22.5 45 21 22 47 Carbon blackMetals and silica (wt.%) (wt.%) 22.5 14 23.5 21 12 22 MetalsTextiles (wt.%) (wt.%) 14 5.5 23.5 1 10 12 VulcanizationTextiles (wt.%) agents (wt.%) 5.5 2.5 1 3 3 10 VulcanizationAdditives agents (wt.%) (wt.%) 2.5 8.5 6.5 3 6 3 Additives (wt.%) 8.5 6.5 6 The rubber fraction of a tire, mostly in the treads (32.5 wt.%) and the sidewalls (22 wt.%), is a mixtureThe rubberof NR (polyisoprene) fraction of a tire, an mostlyd synthetic in the treadsrubbers (32.5 such wt.%) as and the sidewalls (PB), (22 styrene- wt.%), is a mixture ofrubber NR (polyisoprene) (SBR) and butyl and rubber. synthetic The rubberschemical such structur as polybutadienee of each component (PB), styrene-butadiene is presented in Figure rubber 2 (SBR)[28,30]. and . The chemical structure of each component is presented in Figure2[28,30].

Figure 2. Chemical structure of rubbers used in tire composition (reproduced with permission fromFigure [28 2.]). Chemical structure of rubbers used in tire composition (reproduced with permission from [28]). Carbon black and silica play important roles in the mechanical reinforcement and abrasion resistanceCarbon of ablack tire depending and silica onplay their important size, structures roles in and the content. mechanical Recently, reinforcement silica compounds and abrasion gained moreresistance attention of a tire to replacedepending carbon on their black size, to producestructures more and ecofriendlycontent. Recently, tires. Metals, silica compounds such as steel gained and alloys,more attention are used toto reinforcereplace carbon the tires. black However, to produce textiles, more such ecofriendly as natural rayon,tires. Metals, polyamide such and as polyester,steel and arealloys, mostly are usedused in to car reinforce tires to replacethe tires. metals However, and produce textiles, lightweight such as natural tires [28 rayon,,31]. polyamide and polyester,Vulcanization are mostly agents used in (sulfur car tires and to replace sulfur metals compounds) and produce and di lightweightfferent additives tires [28,31]. (stabilizers, ,Vulcanization antiozonants, agents (sulfur extender and oils sulfur and waxes) compounds) are used and in adifferent tire formulation additives to (stabilizers, induce the crosslinkedantioxidants, structure antiozonants, and be extender resistant oils to and photochemical waxes) are decomposition,used in a tire formulation chemical reagents, to induce high the temperaturescrosslinked structure and biodegradability and be resistant [32 ].to Otherphotochemical additives decomposition, such as small amount chemical of reagents, elements, high like calcium,temperatures magnesium, and biodegradability sodium, potassium [32]. andOther chloride, additives can such be added as small to improve amount the of tireelements, properties like andcalcium, durability. magnesium, Because sodium, of their potassium very complex and formulation chloride, can (high be numberadded to of improve different the components tire properties with aand wide durability. range of concentration),Because of their recycling very complex of waste fo tiresrmulation through (high ecofriendly number and of different inexpensive components methods iswith a serious a wide challenge range of for concentrat the tire industriesion), recycling [33]. of waste tires through ecofriendly and inexpensive methods is a serious challenge for the tire industries [33].

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2. Tire Recycling Recycling of waste tires raises significant environmental concerns due to the highly crosslinked structure of vulcanized rubbers and their chemical composition containing toxic components like leachable heavy metals [25]. About 270 million discarded tires are annually generated in the USA alone. This leads to the necessity of finding easy, energy-efficient and cost-effective methods to recycle waste tires [34]. Different solutions have been introduced to reuse discarded tires in order to decrease the amount of disposable scrap tires as poor degradable waste materials in the environment. According to the US Tire Manufacturers Association [35], 16% of waste tires are still landfilled, which do not easily degrade and remain for long periods of time in the environment. However, 86% of scrap tires are being recycled in different ways such as retreading, incineration for energy recovery, pyrolysis to obtain gas and carbon black, as well as shredding to produce small particles used as fillers in a wide variety of matrices such as asphalt, concrete and polymers. These recycling methods not only help to keep the environment safe, but also contribute to the economic growth of several markets such as artificial reef, erosion control, breakwaters, floatation devices, athletic tracks, playground surface, rubberized composites and many more [27,36]. An ideal solution for waste tire management should not have any adverse effect on the environment and safeguard natural resources by using less raw materials, as well as creating industrial applications with commercial added-values. So far, several studies were devoted to improve the common recycling methods and introduce novel techniques for the management of waste tires (disposal issues). The main techniques to recycle waste tires can be classified into retreading, incineration, pyrolysis and blending (composites) through downsizing (particle size reduction) for their incorporation into matrices as explained later [6,37,38].

2.1. Retreading Retreading is a process to increase the lifetime of a used tire by stripping off its tread and applying a new one via cold or hot processes. However, this method is practical only to recycle tires having no damage to the carcass and passing a wear and tear inspection [39,40]. Usually, the tire carcass goes through a recapping system to introduce a new tread to the tire. The retreading process only needs 30% of the energy and 25% of the raw materials required to produce a new tire [41]. Not only is this method cost effective to recycle discarded tires, but is also an ecofriendly and waste-free method as rubber buffing is the only by-product which can be an appropriate filler for concrete and polymer composites. However, the disadvantages of retreading are low quality and safety concerns at high speeds, limiting the application of this recycling technique for passenger cars. Nevertheless, end-of-life truck tires can be easily retreaded [39,40,42].

2.2. Incineration

Incineration, as a self-supporting and exothermic process occurring above 400 ◦C, is used for energy recovery due to the high calorific value of waste tires compared to that of coal (18.6–27.9 MJ/kg). Waste tires, with a calorific value of 32.6 MJ/kg, are used as a fuel source for the production of steam, electrical energy, pulp, paper, lime and steel [43]. Also, Oriaku et al. [44] reported on the recovery of carbon black (CB) via incineration by burning the tire in a limited air supply. The recovered material can be used in small scale industries for the production of printing inks and paints. The main advantages of incineration are low energy production cost and maximum heat recovery. However, atmospheric contamination by flue gas and particle emission are sources of air pollution needing to be carefully addressed [45].

2.3. Pyrolysis Discarded tires are an excellent source of hydrocarbons that can be reused in the form of gas, oil and residues through pyrolysis. Pyrolysis can be classified into a catalytic or non-catalytic reaction, J. Compos. Sci. 2020, 4, 103 5 of 43

whichJ. Compos. is Sci. a thermal 2020, 4, x decomposition of the waste tires above 400 ◦C in an oxygen-free environment5 of [46 45]. Tire pyrolysis oil (40–60 wt.%), gas (5–20 wt.%) and char (30–40 wt.%) are the main products of waste tiretire pyrolysis.pyrolysis. The oil, gas and residues can bebe usedused forfor carboncarbon nanotubenanotube (CNT)(CNT) synthesis,synthesis, asas fuelfuel inin thethe pyrolysispyrolysis processprocess andand thethe productionproduction ofof porousporous activatedactivated carbon,carbon, respectivelyrespectively [[47].47]. The wastewaste tirestires pyrolysispyrolysis productsproducts andand mainmain applicationsapplications are presentedpresented in FigureFigure3 3.. However,However, thisthis solutionsolution forfor wastewaste tiretire managementmanagement requires large pyrolysispyrolysis plants,plants, which are costlycostly to buildbuild andand operateoperate (high(high temperaturetemperature withwith lowlow pressure)pressure) withwith limitedlimited industrialindustrial applicationsapplications atat largelarge scalesscales [[26].26].

Figure 3. WasteWaste tire pyrolysis products and their applicat applicationsions (reproduced with permission from [[26]).26]). 2.4. Material Composite 2.4. Material Composite Waste tires contain natural and synthetic rubbers, which are appropriate reinforcing materials Waste tires contain natural and synthetic rubbers, which are appropriate reinforcing materials for composite production. Blending waste tires with virgin matrices not only decreases the cost of for composite production. Blending waste tires with virgin matrices not only decreases the cost of the the final products, but also lowers the amount of virgin materials being used [27]. For example, the final products, but also lowers the amount of virgin materials being used [27]. For example, the addition of crumb tire as light fillers in asphalt is used in highways enhancing the road surface quality addition of crumb tire as light fillers in asphalt is used in highways enhancing the road surface quality (lower surface rutting), thermal stability and resistance to aging [48]. Scrap tires are also being used (lower surface rutting), thermal stability and resistance to aging [48]. Scrap tires are also being used for construction applications as fillers in cement mortar for the production of concrete compositions for construction applications as fillers in cement mortar for the production of concrete compositions more resistant to bending, dynamic loading and cracking [49]. They are also improving the thermal more resistant to bending, dynamic loading and cracking [49]. They are also improving the thermal insulation and acoustic properties, as well as decreasing moisture absorption and permeability to insulation and acoustic properties, as well as decreasing moisture absorption and permeability to chloride ions [50,51]. Blending waste tires with polymer matrices (thermosets, thermoplastics and chloride ions [50,51]. Blending waste tires with polymer matrices (thermosets, thermoplastics and rubbers) results in low cost and green composite blends with the possibility of commercialization as rubbers) results in low cost and green composite blends with the possibility of commercialization as substitutes for existing equivalent products. These polymer composites are being used in different substitutes for existing equivalent products. These polymer composites are being used in different applications such as mats, playground surfaces, athletic tracks, and automotive parts [38,52]. applications such as mats, playground surfaces, athletic tracks, and automotive parts [38,52]. Shredding of discarded tires into ground tire rubber (GTR) with small particle sizes and higher Shredding of discarded tires into ground tire rubber (GTR) with small particle sizes and higher specific surface areas improves the filler distribution into matrices and increases the chance of better specific surface areas improves the distribution into matrices and increases the chance of better bonding with rubber chains. Shredding waste tires, also known as downsizing or down-cycling, bonding with rubber chains. Shredding waste tires, also known as downsizing or down-cycling, requires the removal of the textile and steel reinforcements from the rubber particles by pneumatic requires the removal of the textile and steel reinforcements from the rubber particles by pneumatic separators and electromagnets during grinding, respectively [53]. Figure4 presents typical images of the separators and electromagnets during grinding, respectively [53]. Figure 4 presents typical images of rubber crumb particles and fibers separated during a grinding process [54]. Different grinding processes the rubber crumb particles and fibers separated during a grinding process [54]. Different grinding processes have been developed such as cryogenic, ambient, wet and water jet. Each method produces different particle sizes from shred (50–300 mm) to fine powder (<500 μm) with various surface

J. Compos. Sci. 2020, 4, 103 6 of 43

J. Compos. Sci. 2020, 4, x 6 of 45 have been developed such as cryogenic, ambient, wet and water jet. Each method produces different topography.particle sizes Both from parameters shred (50–300 are mm)known to fineto have powder a direct (<500 effectµm) withon the various final properties surface topography. of each compoundBoth parameters [25,55]. are known to have a direct effect on the final properties of each compound [25,55].

FigureFigure 4. 4. GeneralGeneral aspects aspects of of the the waste waste tire tire after after shredding: shredding: (a ()a crumb) crumb rubber rubber particles particles (1–10 (1–10 mm) mm) and and (b(b) )the the reinforcing reinforcing fibers fibers (reproduce (reproducedd with with permission permission from from [54]). [54]).

AmbientAmbient grinding grinding (air (air impact impact and and water water jet) jet) is is based based on on passing passing the the waste waste tires tires through through the the nip nip gapgap of of a atwo-roll two-roll mill mill to to decrease decrease the the particle particle size size.. During During this this milling milling step, step, the the temperature temperature may may rise rise upup to to 130 130 °C◦C[ [38,56].38,56 ].Increasing Increasing the the number number of of pass passeses leads leads to to smaller smaller particles particles size, size, but but increases increases the the processingprocessing costs. costs. Metal Metal and and polymer polymer fibers fibers can can be be removed removed from from the the tire tire chips chips (around (around 5 5cm) cm) from from a a previousprevious step andand thethe remaining remaining rubber rubber particles particles can becan used be forused further for sizefurther reduction size reduction via granulators, via granulators,cracker mills cracker and micromills mills and [ 5micromills]. On the other [5]. On hand, the wetother grinding hand, wet is based grinding on circular is based grinding on circular plates grindingmoving plates concurrently moving and concurrently lubricated and by water, lubricated which by is water, also used which to controlis also theused temperature, to control the and temperature,requiring a waterand requiring jet with pressure a water abovejet with 2000 pressure psi to stripabove the 2000 rubber. psi to However, strip the di rubber.fferent parameters,However, differentsuch as waterparameters, flow rate such and as thewater area flow over rate which and such the area pressurized over which water such is applied, pressurized can determinewater is applied,the effi ciencycan determine of this process the efficiency [57]. Also,of this solution process grinding [57]. Also, is solution widely usedgrinding by swelling is widely the used rubber by swellingchips in the a solvent rubber suchchips as in aromatic a solvent or such chlorinated as aromatic hydrocarbons or chlorinated before hydrocarbons being fed into before the being gap of fed the intogrinding the gap plates of the to grinding obtaintire plates powders to obtain (<1 tire mm) powders [58]. Finally, (<1 mm) cryogenic [58]. Finally, grinding cryogenic is using grinding liquified is usinggases liquified to change gases the elasticto change rubbery the elastic chips intorubbery brittle chips particles into brittle to eliminate particles the to rubber eliminate degradation the rubber due degradationto the heat buildupdue to the associated heat buildup with associated shearing at with ambient shearing conditions. at ambient In general, conditions. the rubberIn general, chips the are rubberimmersed chips in are liquid immersed nitrogen in liquid (LN2) nitrogen to convert (LN2) them to into convert brittle them materials into brittle below materials their glass below transition their glasstemperature transition (T temperatureg) followed by(Tg) grinding followed through by grinding a hammer through mill. a hammer Cryogenic mill. grinding Cryogenic benefits grinding from benefitshigher productionfrom higher rate production and lower rate milling and lowe energyr milling consumption energy consumption compared to compared ambient grinding to ambient and grindingsolution and processes solution [59 processes]. Figure [59].5 shows Figure that 5 show cryogenics that cryogenic grinding leadsgrinding to aleads smooth to a smooth rubber particlerubber particlesurface surface compared compared to ambient to ambient grinding. grinding. Therefore, Therefore, GTR particles GTR particles obtained obtained from cryogenic from cryogenic processes processeshave lower have surface lower roughness surface roughness and lower and specific lower surface specific area surface leading area to poor leading physical to poor bonding physical with bondingpolymer with matrices polymer when matrices blended. when Nevertheless, blended. Nevert cryogenicheless, grinding cryogenic operational grinding cost operational for the production cost for theof production finer particle of sizes finer (

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Figure 5. TypicalTypical particle surface state fo forr ground tire rubber (GTR) prod produceduced from different different grinding processes: ( a) ambient-mechanical, ( (b)) water jet, (c) cryogenic-pin mill, (d) ambient-rotaryambient-rotary mill andand ((e)) cryogenic-rotarycryogenic-rotary millmill (reproduced(reproduced withwith permissionpermission fromfrom [[38]).38]).

3. GTR in Blends 3. GTR in Blends Blending a polymer matrix and GTR (surface modified) with elasticity and impact resistance Blending a polymer matrix and GTR (surface modified) with elasticity and impact resistance might resultresult in in improved improved blend blend properties, properties, such assuch tensile as tensile strength strength and elongation and elongation at break, dependingat break, ondepending the level on of interfacialthe level of interaction interfacial between interaction the GTRbetween and the matrixGTR and [13 the,17, 22matrix,60]. The[13,17,22,60]. incorporation The ofincorporation even as little of as even 10 wt.% as little GTR as into 10 wt.% polymer GTR matrices into polymer (thermosets, matrices thermoplastics (thermosets, thermoplastics and rubbers) leads and torubbers) large consumption leads to large of consumption waste tires as of a waste partial tires replacement as a partial of virginreplacement polymers of virgin [38]. However,polymers GTR[38]. loadingHowever, in compositeGTR loading materials in composite is limited materials due to pooris limited interactions due to betweenpoor interactions GTR and between polymer GTR resulting and inpolymer a loss ofresulting physical in and a loss mechanical of physical properties and mechanical of the blends properties [13]. This of the is whyblends several [13]. studiesThis is havewhy beenseveral conducted studies have to find been proper conducted techniques to find toproper incorporate techniques higher to incorporate amounts of higher GTR (aboveamounts 50 of wt.%) GTR into(above polymer 50 wt.%) matrices into topolymer decrease matrices the costs to anddecrease uses of the fresh costs/virgin andmaterials. uses of fresh/virgin Different approaches, materials. suchDifferent as compatibilization, approaches, such surface as compatibilization, modification and surface regeneration, modification have and been regeneration, introduced tohave address been thisintroduced challenge to andaddress improve this challenge the interfacial and improve adhesion the between interfacial the componentsadhesion between [27]. Copolymers the components with similar[27]. Copolymers segments towith the similar blend componentssegments to canthe bl beend used components to compatibilize can be GTR used filled to compatibilize blends by acting GTR asfilled (physical blends/chemical) by acting bridges as in(physical/chemic immiscible polymersal) bridges improving in immiscible compatibility polymers [16]. For improving example, polyethylene-grafted-maleiccompatibility [16]. For example, anhydride polyethylene-grafted-maleic (PE-g-MA), as a well-known anhydride compatibilizer, (PE-g-MA), is as used a well-known to decrease thecompatibilizer, interfacial tension is used andto decrease stabilize the the interfacia blend morphologyl tension and of stabilize GTR filled the blend polyolefin morphology blends of [13 GTR,55]. Surfacefilled polyolefin modification blends leads [13,55]. to the Surface generation modification of peroxy, leads hydroperoxy, to the generation hydroxyl of and peroxy, carbonyl hydroperoxy, groups on thehydroxyl GTR surface and carbonyl to promote groups interaction on the with GTR polar surface polymers to promote or reactive interaction compatibilizers. with polar In polymers this method, or oxidizing agents, such as potassium permanganate (KMnO )[61], nitric acid (HNO ) and hydrogen reactive compatibilizers. In this method, oxidizing agents, such4 as potassium permanganate3 (KMnO4) peroxide (H O )[62], and sulphuric acid (H SO )[63], as well as energy radiation (microwave [21], [61], nitric acid2 2 (HNO3) and hydrogen peroxide2 (H4 2O2) [62], and sulphuric acid (H2SO4) [63], as well gammaas energy [64 radiation], and plasma (microwave [19]), and [21], ultrasonic gamma [64], waves and [65 plasma]) have [19]), been and used ultrasonic for GTR surface waves treatment.[65]) have Also,been rubberused for regeneration GTR surface is usedtreatment. to partially Also, destroy rubber the regeneration crosslinked is structure used to and partially induce destroy more chain the mobilitycrosslinked (molecular structure freedom) and induce for better more GTRchain bonding mobility with (molecular the matrix freedom) chains [ 66for]. better All these GTR techniques bonding willwith be the explained matrix chains in more [66]. detail All later,these includingtechniques a generalwill be discussionexplained in on more the e ffdetailect of later, GTR including on the final a rubbergeneral compounds discussion on properties. the effect of GTR on the final rubber compounds properties.

3.1. Modification of GTR Insufficient interfacial adhesion and bonding between the crosslinked or partially regenerated GTR and the continuous phase is the main problem associated with GTR introduction into different

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3.1. Modification of GTR Insufficient interfacial adhesion and bonding between the crosslinked or partially regenerated GTR and the continuous phase is the main problem associated with GTR introduction into different polymer matrices, especially at high concentrations (above 50 wt.%). This must be solved to achieve good adhesion and interfacial stress transfer to produce good mechanical strength and long term stability [16]. Therefore, GTR surface modification offers specific interaction sites between the phases resulting in improved interfacial adhesion (lower interfacial tension), avoiding particle agglomeration and morphology stabilization during processing [67]. Consequently, better compatibility in the blends leads to higher mechanical properties as a result of a strong interphase and smooth interfacial stress transfer. Compatibilization approaches can be classified as physical and chemical methods in which chemical compatibilization is divided into reactive and non-reactive methods [68]. Physical compatibilization is performed to improve the surface activity and roughness by applying external forces such as mechanical or thermo-mechanical shearing [69], microwave [70], γ radiation [64], ultrasonic waves [65], ultraviolet (UV) light [71], [67], plasma [19], and corona discharge [72] treatment to create a thick interphase and improve wetting. On the other hand, reactive compatibilization methods are associated to reactive molecules added during mixing to form a chemically linked interphase, while non-reactive methods are based on the introduction of copolymers (block or graft) and/or nanoparticles (NP) to improve the blend compatibility [14,20,23,73]. It should be mentioned that rubber regeneration also leads to some compatibilizing effect due to possible molecular entanglement with thermoplastic resins and curable rubbers [74].

3.1.1. Physical Methods Zhang et al. [69] reported an economical method of waste tires recycling by the preparation of compounds based on GTR and waste tire fibers using mechanical milling. As the pan-mill method exerts strong shear forces, a process similar to pulverization was obtained, leading to better dispersion and activation of the materials surface by scission of the vulcanized GTR structure. As shown in Table2, increasing the number of milling cycles generated a large number of oxygen containing groups on the GTR particle surface attributed to the reaction between the atmospheric oxygen and the free radicals produced during pan milling. Higher oxygen functional group content implies a higher number of polar groups improving the interfacial interactions (adhesion) between GTR and waste fibers.

Table 2. Variation of the elemental concentrations on the GTR surface during pan milling (reproduced with permission from [69]).

Relative Concentration (%) Sample COS Without milling 96.46 3.17 0.37 Milled for 15 cycles 94.82 4.74 0.44 Milled for 25 cycles 94.43 5.10 0.47

Low temperature plasma (LTP) is a promising approach for GTR surface modification at room temperature by applying energies less than 20 eV without significant damage of the bulk materials [75]. Different gases such as air, oxygen, nitrogen, hydrogen or ammonia can be used for hydrophilic modification, while tetramethyl silane, carbon tetrafluoride or glycidyl methacrylate are preferred for hydrophobic modification. The plasma conditions lead to easy cleavage of the surface chemical bonds and the formation of functional groups, surface roughness, crosslinks, graft polymerization and thin film coating. Figure6 presents the di fferent cases [76]. J. Compos. Sci. 2020, 4, 103 9 of 43 J. J.Compos. Compos. Sci. Sci. 2020 2020, 4,, x4 , x 9 of 459 of 45

Figure 6. Schematic representation of the surface modifications of polymers by plasma treatments: (a) Figure 6. Schematic representation of the surface modifications of polymers by plasma treatments: introductionFigure 6. Schematic of functional representation groups, (b) ofintroduction the surface of modifi surfacecations roughness, of polymers (c) crosslink by plasma formation, treatments: (d) (a) (a) introduction of functional groups, (b) introduction of surface roughness, (c) crosslink formation, surfaceintroduction graft polymerization of functional andgroups, (e) thin (b) filmintroduction coating (reproduced of surface roughness,with permission (c) crosslink from [76]). formation, (d) (surfaced) surface graft graft polymerization polymerization and and (e) (thine) thin film film coating coating (reproduced (reproduced with with permission permission from from [76]). [76 ]). Xiaowei et al. [77] studied the surface modification of GTR by oxygen plasma and the Xiaowei et al. [77] studied the surface modification of GTR by oxygen plasma and the polymerizationXiaowei ofet ethanolal. [77] tostudied introduce the surface surface func modificationtional groups. of AsGTR shown by oxygenin Figure plasma 7, electron and the polymerization of ethanol to introduce surface functional groups. As shown in Figure7, electron bombardmentpolymerization leads of toethanol the simultaneous to introduce dissociation surface func of Otional2 molecules groups. and As the shown rupture in of Figure C–X or 7, C=C electron bombardment leads to the simultaneous dissociation of O molecules and the rupture of C–X or C=C bondsbombardment on the GTR leads main to thechains simultaneous or branched dissociation chains. Next, of Othe22 molecules dissociated and ethanol the rupture molecules of C–X (free or C=C bonds on the GTR main chains or branched chains. Next, the dissociated ethanol molecules (free radicals)bonds onreact the with GTR the main functional chains groups or branched and are chains.grafted onNext, the theGTR dissociated hydrocarbon ethanol chains. molecules From this (free process,radicals)radicals) a react substantial with with the theincrease functional functional of the groups groupssurface and andro ughnessare are grafted grafted was on reported on the the GTR GTR as hydrocarbon a hydrocarbon result of ethanolchains. chains. FromLTP From this polymerizationthisprocess, process, a substantial a substantialand specific increase increase surface of the ofarea the surface surfaceof the ro GTRughness roughness increased was was reported reportedfrom 0.12 as as ato aresult result0.28 mof of2 /gethanol after LTP 2 modification.polymerizationpolymerization Also, andand ATR-FTIR specific specific surface and surface XPS area analysisarea of the of GTR resultsthe increased GTR confirmed increased from 0.12the fromformation to 0.28 0.12 m /goftoafter hydrophilic0.28 modification. m2/g after groupsAlso,modification. ATR-FTIRsuch as –COOH,Also, and ATR-FTIR XPS C–OH analysis and and –CHO results XPS on analysis confirmedthe GTR resultssurface the formation confirmedas the oxygenof the hydrophilic content formation increased groupsof hydrophilic from such as 8.1%–COOH,groups to 14.5% such C–OH asand –COOH, and enhanced –CHO C–OH wetting on the and GTR was–CHO surfaceconfirmed on the as GTR theby the oxygen surface decrease contentas the of oxygenthe increased liquid content droplet from increased 8.1% contact to 14.5% from angleand8.1% enhancedon to the 14.5% GTR wettingand surface enhancedwas from confirmed 122 wettingo to 34o by.was the confirmed decrease ofby the the liquid decrease droplet of the contact liquid angle droplet on the contact GTR o o surfaceangle on from the GTR 122 tosurface 34 . from 122o to 34o.

Figure 7. Treatment steps related to the low temperature plasma (LTP) process to modify the GTR Figure 7. Treatment steps related to the low temperature plasma (LTP) process to modify the GTR surface (reproduced with permission from [77]). surface (reproduced with permission from [77]). Figure 7. Treatment steps related to the low temperature plasma (LTP) process to modify the GTR Ozone (O3) is widely used for the surface oxidation of materials to increase the polarity resulting Ozonesurface (O (reproduced3) is widely usedwith permissionfor the surface from oxidation [77]). of materials to increase the polarity resulting in better interaction between the phases. Also, a combination of UV-ozone treatment is used for in better interaction between the phases. Also, a combination of UV-ozone treatment is used for the the surface modification of silicon rubber membranes [78], and micro-patterning application [79]. surfaceOzone modification (O3) is widely of silicon used rubberfor the surfacemembranes oxidation [78], ofand materials micro-patterning to increase application the polarity [79]. resulting Cataldo et al. [80] worked on the surface oxidation and functionalization of GTR using ozone as the Cataldoin better et interactional. [80] worked between on the the surface phases. oxidatio Also,n a and combination functionalization of UV-ozone of GTR treatment using ozone is used as the for the active agent. To prevent the risk of GTR spontaneous ignition in a fixed bed reactor, the reaction was activesurface agent. modification To prevent theof risksilicon of GTRrubber spontaneou membraness ignition [78], in anda fixed micro-patterning bed reactor, the reactionapplication was [79]. performed in a fluidized bed reactor. An evacuated large round bottomed flask was filled with an performedCataldo et in al. a [80]fluidized worked bed on reactor. the surface An evacua oxidatioted nlarge and round functionalization bottomed flask of GTR was usingfilled withozone an as the O /O mixture and was allowed to reach the desired degree of oxidation (ξoxy) according to: Oactive3/O3 2 mixture2 agent. andTo prevent was allowed the risk to reachof GTR the spontaneou desired degrees ignition of oxidation in a fixed (ξoxy bed) according reactor, to:the reaction was performed in a fluidized bed reactor. An evacuated large round bottomed flask was filled with an 𝜉ξoxy=mg= mgO(O⁄𝑔3)(rubber) /g (rubber ) (1) (1) O3/O2 mixture and was allowed to reach the desired degree of oxidation (ξoxy) according to:

Figure 8 presents the FTIR spectra where the changes in infrared absorption bands related to the Figure8 presents the FTIR spectra𝜉 where =mg theO changes⁄𝑔(rubber) in infrared absorption bands related to the(1) degree of oxidation and the formation of functional groups on the GTR surface can be seen. Increasing degree of oxidation and the formation of functional groups on the GTR surface can be seen. Increasing the degree of oxidation up to 26 mg/g led to a more intense absorbance peak of the ketone band at the degreeFigure of 8 presents oxidation the up FTIR to 26 spectra mg/g ledwhere to athe more chan intenseges in infrared absorbance absorption peak of bands the ketone related band to the at 1710 cm−1, which was also shifted to lower wavenumbers than the normal ketone band (1640 cm−1). 1710degree cm of1 oxidation, which was and also the shiftedformation to lower of functional wavenumbers groups thanon the the GTR normal surface ketone can be band seen. (1640 Increasing cm 1). Also, split− bands related to C-O stretching, together with peroxide and ozonide absorption, are − Also,the degree split bandsof oxidation related up to to C-O 26 mg/g stretching, led to a together more intense with peroxideabsorbance and peak ozonide of the absorption,ketone band are at 1710 cm−1, which was also shifted to lower wavenumbers than the normal ketone band (1640 cm−1). Also, split bands related to C-O stretching, together with peroxide and ozonide absorption, are

J. Compos. Sci. 2020, 4, 103 10 of 43 J. Compos. Sci. 2020, 4, x 10 of 45

−1 presentpresent aroundaround 10901090 cmcm− withwith increasing increasing levels of ozone trea treatment.tment. The The hydroxyl hydroxyl and hydroperoxide −1 stretchingstretching bandsbands alsoalso appearappear aroundaround 34103410 cmcm− [80].[80].

Figure 8. FTIR spectra of rubber crumb with different levels of oxidation. From top to bottom, the Figure 8. FTIR spectra of rubber crumb with different levels of oxidation. From top to bottom, the spectra were taken on pristine GTR for ξoxy = 11, 19 and 26 mg/g, respectively (reproduced with spectra were taken on pristine GTR for ξoxy = 11, 19 and 26 mg/g, respectively (reproduced with permission from [80]). permission from [80]). Martínez-Barreraa et al. [64] worked on GTR surface modification by gamma rays at 200, 250 and Martínez-Barreraa et al. [64] worked on GTR surface modification by gamma rays at 200, 250 300 kGy. SEM images in Figure9 show that the untreated GTR particles have a smooth and grated and 300 kGy. SEM images in Figure 9 show that the untreated GTR particles have a smooth and surface with some small particles. However, increasing the gamma radiation energy from 200 kGy to grated surface with some small particles. However, increasing the gamma radiation energy from 200 300 kGy increased the surface roughness with more pronounced cracks and some small cavities. It kGy to 300 kGy increased the surface roughness with more pronounced cracks and some small is expected that the ionizing energy of the gamma rays can form free radicals and chain scissions to cavities. It is expected that the ionizing energy of the gamma rays can form free radicals and chain produce oxygen functional groups. However, too high intensity leads to the degradation of the rubber scissions to produce oxygen functional groups. However, too high intensity leads to the degradation chains indicated by the presence of detached particles, cavities and cracks. of the rubber chains indicated by the presence of detached particles, cavities and cracks.

J. Compos. Sci. 2020, 4, x 11 of 45 J. Compos. Sci. 2020, 4, 103 11 of 43 J. Compos. Sci. 2020, 4, x 11 of 45

Figure 9. SEM images of untreated and irradiated (different energy levels) waste tire rubber particles FigureFigure 9. SEM9. SEM images images of of untreated untreated and and irradiatedirradiated (di(diffefferentrent energy levels) levels) wast wastee tire tire rubber rubber particles particles (amplification 1500×) (reproduced with permission from [64]). (amplification(amplification 1500 1500×)) (reproduced (reproduced with with permission permission fromfrom [[64]).64]). × 3.1.2.3.1.2. Chemical Chemical Methods Methods AsAs mentioned mentioned before, before, the the chemical chemical compatibilizationcompatibilizatcompatibilization ofof GTR GTR can can be be done done via via non-reactive non-reactive or or reactivereactive methods.methods. methods. Fazli Fazli and and Rodrigue Rodrigue reviewed reviewed theththe didifferentfferent chemical chemical compatibilization compatibilization methods methods to to improveimproveimprove thethe interfacialthe interfacial interfacial adhesion adhesionadhesion between betweenbetween GTR and GTRGTR thermoplastics and thermoplasticsthermoplastics for the production forfor thethe production ofproduction thermoplastic of of elastomerthermoplasticthermoplastic (TPE) compoundselastomer (TPE) (TPE) [27 ].compounds compounds For non-reactive [27].[27]. For methods,For non-reactive different methods,methods, block or different graftdifferent copolymers block block or or suchgraft graft as PE-g-MAcopolymerscopolymers [22 such], styrene-butadiene-styrenesuch as as PE-g-MA PE-g-MA [22], [22], styrene-butadi styrene-butadi block copolymerene-styrene (SBS) blockblock [81], ethylene-vinylcopolymercopolymer (SBS) (SBS) acetate [81], [81], ethylene- (EVA) ethylene- [82], andvinylvinyl polyolefin acetate acetate (EVA) (EVA) elastomer [82], [82], and and based polyol polyol onefin ethyleneefin elastomer elastomer octene basedbased copolymer on ethyleneethylene [14 ]octene octene have copolymer beencopolymer used [14] to [14] increasehave have been been the compatibilityusedused to toincrease increase of GTRthe the compatibility containingcompatibility blends. of of GTR GTR containing containing blends. ForFor example, example, Wang Wang etet et al.al. al. [[23]23 [23]] studiedstudied studied thethethe eeffeeffeffectscts ofof SBSSBS onon thethe mechanical mechanical and andand morphological morphologicalmorphological propertiespropertiesproperties ofof of highhigh high densitydensity density polyethylenepolyethylene polyethylene (HDPE)(HDPE)/GTR (HDPE)/GTR/GTR blends.blends. Figure Figure 10 1010 presents presentspresents typical typicaltypical stress-strain stress-strainstress-strain curvescurves of of the the blends blends for for didifferent differentfferent SBSSBS SBS loading.loading. loading. TheThe additionaddition of of 12 12 phr phr SBS SBS significantly significantlysignificantly improved improved the the tensile strength of HDPE/GTR blends from 11.8 MPa (0 phr SBS) to 15.0 MPa. Also, the elongation at tensiletensile strengthstrength ofof HDPEHDPE/GTR/GTR blendsblends fromfrom 11.811.8 MPaMPa (0(0 phrphr SBS)SBS) toto 15.015.0 MPa.MPa. Also, the elongation at break of the blends increased from 200% to 360% with 12 phr SBS. break ofof thethe blendsblends increasedincreased fromfrom 200%200% toto 360%360% withwith 1212 phrphr SBS.SBS.

Figure 10. Typical stress-strain curves of high density polyethylene (HDPE)/GTR compounds Figure 10. Typical stress-strain curves of high density polyethylene (HDPE)/GTR compounds Figurecompatibilized 10. Typical by styrene-butadiene-styrenestress-strain curves of high (SBS ) density(reproduced polyethylene with permission (HDPE)/GTR from [23]). compounds compatibilized by styrene-butadiene-styrene (SBS) (reproduced with permission from [23]). compatibilized by styrene-butadiene-styrene (SBS) (reproduced with permission from [23]).

J. Compos. Sci. 2020 4 J. Compos. Sci. 2020,, 4,, x 103 1212 of of 45 43

The fractured surface morphology of the uncompatibilized blends (Figure 11a,b) shows a The fractured surface morphology of the uncompatibilized blends (Figure 11a,b) shows a smooth smooth and clean surface due to easy GTR particle removal (pull-outs) from HDPE. However, SBS and clean surface due to easy GTR particle removal (pull-outs) from HDPE. However, SBS addition addition shows no tearing strips indicating better elastic recovery ability and better interaction shows no tearing strips indicating better elastic recovery ability and better interaction between the between the GTR and HDPE (Figure 11c,d) [23]. GTR and HDPE (Figure 11c,d) [23].

FigureFigure 11. FracturedFractured surfaces surfaces of of HDPE/SBS/GTR HDPE/SBS/GTR compounds compounds prepared prepared by by melt compounding (extrusion).(extrusion). The The compositions compositions (weight (weight ratio) ratio) are: are: (a (a) )30/0/70, 30/0/70, ( (bb)) 30/0/70, 30/0/70, ( (cc)) 30/12/70 30/12/70 and and ( (dd)) 30/12/70 30/12/70 (reproduced(reproduced with perm permissionission from [23]). [23]).

Also, the addition of nanoparticles (NP) such as nanoclays with large specific surface area and Also, the addition of nanoparticles (NP) such as nanoclays with large specific surface area and high aspect ratio (for natural montmorillonite clay, the specific surface area is 750 m2/g with an aspect high aspect ratio (for natural montmorillonite clay, the specific surface area is 750 m2/g with an aspect ratio of 200–1000, [46]) in rubber compounds have attracted numerous research interests due to the ratio of 200–1000, [46]) in rubber compounds have attracted numerous research interests due to the reinforcement potential giving superior mechanical, thermal and barrier properties [83,84]. Moreover, reinforcement potential giving superior mechanical, thermal and barrier properties [83,84]. the incorporation of NP (1–10 wt.%) into incompatible blends containing GTR results in a morphological Moreover, the incorporation of NP (1–10 wt.%) into incompatible blends containing GTR results in a stabilization by selective localization at the interface decreasing the interfacial tension and preventing morphological stabilization by selective localization at the interface decreasing the interfacial tension droplet coalescence during melt blending. The filler particle size and specific surface area, as well as its and preventing droplet coalescence during melt blending. The filler particle size and specific surface interactions with the matrix and good dispersion state, are controlling the NP efficiency in immiscible area, as well as its interactions with the matrix and good dispersion state, are controlling the NP polymer blends [85]. efficiency in immiscible polymer blends [85]. Shan et al. [86] reported on the reinforcing effect of exfoliated organoclay (3 phr) in NR/SB Shan et al. [86] reported on the reinforcing effect of exfoliated organoclay (3 phr) in NR/SB as as substantial improvement in tensile strength (92%) and tear strength (63%) was observed. The substantial improvement in tensile strength (92%) and tear strength (63%) was observed. The incorporation of 3 phr organoclay also decreased the scorch time and optimum curing time (T90, incorporation of 3 phr organoclay also decreased the scorch time and optimum curing time (T90, cure forcure 90%) for 90%)of of acrylonitrile butadiene butadiene rubber rubber (NBR)/SBR (NBR)/SBR from from 2.48 2.48 and and 14.08 14.08 min min to to 1.06 1.06 and and 8.03 8.03 respectively,respectively, indicating that the organoclay actedacted asas aa vulcanizationvulcanization accelerator.accelerator. SatyanarayanaSatyanarayana et et al. al. [73] [73] studied studied the the effect effect of modified modified montmorilloni montmorillonitete (MMT) (MMT) on on the the phase morphologymorphology of of incompatible incompatible polar polar carboxylated carboxylated nitrile rubber rubber (xNBR)/nonpolar (xNBR)/nonpolar NR NR evaluating the the X solubilitysolubility parameter parameter and and interaction interaction parameter parameter ( (X).). As As expected, expected, a a lower lower difference difference in interaction parameter between NR-toluene and Cloisite 15A-toluene (XNR/ toluene-X Cloisite 15A/toluene = 0.04) parameter between NR-toluene and Cloisite 15A-toluene (XNR/ toluene-X Cloisite 15A/toluene = 0.04) compared compared to that of xNBR-toluene and Cloisite 15A-toluene (XxNBR/toluene-XCloisite 15A/toluene = 0.17) to that of xNBR-toluene and Cloisite 15A-toluene (XxNBR/toluene-XCloisite 15A/toluene = 0.17) was observed as thewas majority observed of as Cloisite the majority 15A (at of Cloisite8 phr) migrated 15A (at 8 to phr) the migrated NR phase to theresulting NR phase in a resultingmorphology in a stabilization (suppressed NR droplet coalescence) leading to a better compatibilizing effect on the system.

J. Compos. Sci. 2020, 4, 103 13 of 43 morphology stabilization (suppressed NR droplet coalescence) leading to a better compatibilizing effect on the system. As mentioned above, reactive chemical compatibilization is performed through the formation of block or graft copolymers in situ acting as a bridge through covalent reactions of the functionalized components [27]. The main reactive compatibilizers are based on reactive groups randomly grafted onto the main chain inducing a linear structure. Graft compatibilizers have segments showing chemical affinity towards the blend components to decrease the interfacial tension, improving the droplet phase dispersion and stabilizing the morphology [87]. Maleic anhydride (MA) is widely used for the surface modification of rubbers to enhance compatibility and interfacial adhesion of immiscible blends [20,88,89]. Yassin et al. [20] worked on the graft polymerization of styrene and maleic anhydride (MA) onto the GTR surface using H2O2 as an initiator. The grafting level of styrene increased from 2% at 75 oC to 23% at 125 oC. Higher temperatures induced better chain mobility inside the GTR and increased the chance of styrene grafting on its active sites. However, temperatures above 125 oC led to a lower grafting level attributed to the degradation of the grafted monomers. Abou-helal et al. [89] reported on the positive effect of MA grafted on ethylene-propylene-diene monomer (EPDM) rubber to improve the compatibility of EPDM/NR compounds. As expected, the tensile strength and elongation at break of compatibilized EPDM/NR blends increased with higher MA concentration. For example, the tensile strength and elongation at break of EPDM/NR (25/75) increased from 2.4 MPa and 250% to 3 MPa and 290% respectively, by increasing the MA content from 1.5 to 6 phr. This behavior was attributed to a better penetration of the similar segments of the compatibilizers to that in rubbers at the interphase with increasing MA content. Thus better interfacial interaction provided better stress transfer and overall mechanical properties.

3.2. Regeneration of GTR GTR regeneration is widely used to convert a three dimensional (3D) crosslinked, insoluble and infusible vulcanized rubber (100% gel content) into partially soluble materials with lower crosslink density having more chain mobility. The process leads to a material called regenerated tire rubber (RTR). The soluble fraction of RTR with high molecular weight (MW) results in sufficient bonding with the polymer matrix to improve interfacial adhesion between the phases [90]. Swelling degree estimation combined with the Flory–Rehner equation is used to determine the gel content to quantify the crosslink density [91]. Devulcanization and reclamation are also being used interchangeably related to the process of destroying the crosslinked structure of thermoset rubbers. However, devulcanization aims at S–S and C–S selective bonds scission for partial breakdown of the 3D network of the vulcanized tire to achieve plasticity, while reclamation is associated with the rubber backbone (C-C bonds) degradation to decrease the MW. However, the selective rupture of the crosslinked network is not possible without damage to the backbone chain since the required energies to break the S-S and C-S bonds (227 kJ/mol and 273 kJ/mol, respectively) are very close to the energy required to break the C-C bonds (348 kJ/mol) [66,92]. In this review, both reclaimed and devulcanized processes are associated with regeneration processes to produce RTR particles. GTR regeneration processes are classified as physical and chemical processes in which thermo-mechanical [93], microwave-assisted [21,94], ultrasonic-based [65,95,96], and thermochemical processes [97] are broadly studied. A detailed description of these methods is reviewed in numerous papers and patent applications [28,74,90,92]. In general, physical regeneration uses external energy sources to breakdown the vulcanized structure, while chemical regeneration is based on organic and inorganic reclaiming agents to react with the crosslinked rubber. Not only sulfide and mercaptan compounds are being used in chemical regeneration processes, but also non-sulfured agents have attracted attention due to ecological and economic benefits [92]. A brief description of the main GTR regeneration processes is presented next. J. Compos. Sci. 2020, 4, 103 14 of 43

3.2.1. Thermo-Mechanical Processes This process is based on the shear (mechanical energy) and heat around 200 oC (thermal energy) using mills and twin-screw extruders to decrease the rubber MW. There is also substantial heat generationJ. Compos. Sci. (friction) 2020, 4, x generated in these processes. The use of different solvents such as14 hexane, of 45 supercritical fluids or oils are effective for better heat transfer, as well as swelling the rubber to impose internal stresses making the crosslinks easier to break [98]. Although batch processes, like open mills, internal stresses making the crosslinks easier to break [98]. Although batch processes, like open mills, were used for GTR regeneration, continuous processes using extruders attracted more attention due were used for GTR regeneration, continuous processes using extruders attracted more attention due to better scalability for industrial application. Figure 12 presents a typical screw configuration for a to betterco-rotating scalability twin-screw for industrial extruder application.which is divided Figure into 12 three presents zones. a The typical first screw section configuration is composed forof a co-rotatingconveying twin-screw and kneading extruder elements which followed is divided by the intodevulcanization three zones. zone The in first the section second ispart. composed The last of conveyingpart consists and of kneading the discharge elements zone followed after a vacuum by the extraction devulcanization step to remove zone in the the volatiles second part.[66]. The last part consists of the discharge zone after a vacuum extraction step to remove the volatiles [66].

FigureFigure 12. 12.A typicalA typical screw screw configuration configuration for GTRfor GTR regeneration regeneration showing showing the di thefferent different processing processing sections (reproducedsections (reproduced with permission with permission from [66]). from [66]).

YazdaniYazdani et al.et al. [99 [99]] studied studied the the eff ecteffect of theof the barrel barrel temperature temperature and and the the screw screw speed speed of aof twin-screw a twin- extruderscrew extruder on the GTR on the regeneration GTR regeneration rate. It was rate. found It was that found increasing that increasing the temperature the temperature from 220 from to 280 220◦ C at constantto 280 °C at screw constant speed screw (120 speed rpm) (120 led rpm) to slightly led to lowerslightly regeneration lower regeneration rate from rate 88%from to88% 85% to 85% which mightwhich be might related be to related the degradation to the degradation of the backbone of the backbone chains insteadchains instead of the crosslinkedof the crosslinked network. network. Similar resultsSimilar were results reported were by reported other studies by other focusing studies on focusing the effect on of processingthe effect of parameters processing (compounding parameters sequence,(compounding feeding sequence, speed, screw feeding speed speed, and temperature screw speed profile) and ontemperature the GTR regeneration profile) on ratethe [GTR100]. regenerationFormela et rate al. [[100].93] studied the effect of the extruder operation by comparing a counter-rotating (screw profileFormela A) et and al. [93] a co-rotating studied the (screw effect profile of the B ex andtruder screw operation profile C)by screw.comparing They a reported counter-rotating a decrease of the(screw screw profile torque A) andand crosslinka co-rotating density (screw and profil hencee aB higher and screw regeneration profile C) rate screw. by increasing They reported the speed a ofdecrease the counter-rotating of the screw torque twin-screw and crosslink extruder. density They an usedd hence Horikx’s a higher theory regeneration to quantify rate by the increasing difference betweenthe speed crosslink of the scissioncounter-rotating and main twin-screw chain scission extrud duringer. They the used regeneration Horikx’s theory process to [101quantify]. Horikx’s the theorydifference describes between the relationship crosslink scission between and the main sol chai fractionn scission and changesduring the in therege crosslinkingneration process density [101]. after degradationHorikx’s theory via: describes the relationship between the sol fraction and changes in the crosslinking density after degradation via:  2 1 S1/2 v2 − 2 1 = 1 (2) − v −  1/⁄22 𝑣1 1 − 𝑆 1 S1 1− =1− − (2) 𝑣 ⁄ 1 − 𝑆 3 where v1 and v2 are the crosslinking density before and after regeneration (mol/cm ) respectively, while S1 and S2 represent the sol fraction before and after regeneration (%), respectively. According to the where v1 and v2 are the crosslinking density before and after regeneration (mol/cm3) respectively, sole fraction curves, the results lower than the theoretical values indicated a selective rupture of the while S1 and S2 represent the sol fraction before and after regeneration (%), respectively. According crosslinksto the sole (S-S fraction and C-S curves, bonds) the during results regenerationlower than the [102 theoretical]. Although values GTR indicated regeneration a selective was rupture achieved withof thethe counter-rotatingcrosslinks (S-S and twin-screw C-S bonds) extruder, during theregeneration co-rotating [102]. design Althou achievedgh GTR better regeneration RTR mechanical was propertiesachieved (closer with the to counter-rotating virgin rubber). twin-screw extruder, the co-rotating design achieved better RTR mechanicalMost GTR properties regeneration (closer based to virgin on thermo-mechanical rubber). processes use reclaiming agents. For example, Shi et al.Most [103 GTR] studied regeneration the effect based of di ffonerent thermo-mechanical regeneration factors, processes such use as temperature,reclaiming agents. shear For force, reactionexample, time, Shi regeneration et al. [103] studied atmosphere the effect and of regenerationdifferent regeneration agent, on factors, the GTR such structure as temperature, and properties. shear Theyforce, used reaction a regeneration time, regeneration activator (RAatmosphere 420; Henan and Jinfengregeneration Chemical agent, Industry, on the GTR Zhumadian, structure Henan,and China)properties. and diphenyl They used disulfide a regeneration (DD) as reagents activator and (RA observed 420; Henan that high Jinfeng temperatures Chemical (200–240Industry,◦ C) andZhumadian, strong shear Henan, forces inChina) a twin-screw and diphenyl extruder disulfide led to lower(DD) tensileas reagents strength and and observed elongation that athigh break attributedtemperatures to the (200–240 degradation °C) and of the strong main shear polymeric forces chainin a twin-screw and a high extruder sol fraction ledwith to lower low MW.tensile The strength and elongation at break attributed to the degradation of the main polymeric chain and a high sol fraction with low MW. The incorporation of RTR into NR increased the tensile strength and elongation at break of NR/GTR from 15 MPa and 350% to 21 MPa and 550%, respectively. These variations were related to the partial break-up of the crosslink network and good interfacial interactions between RTR and NR.

J. Compos. Sci. 2020, 4, 103 15 of 43 incorporation of RTR into NR increased the tensile strength and elongation at break of NR/GTR from 15 J.MPa Compos. and Sci. 350% 2020, 4 to, x 21 MPa and 550%, respectively. These variations were related to the partial15 break-upof 45 of the crosslink network and good interfacial interactions between RTR and NR. 3.2.2. Microwave Method 3.2.2. Microwave Method Microwave regeneration can be performed on GTR having polar groups by applying a 915 to 2450 MHzMicrowave frequency regeneration and 41 to 177 can Wh/lb be performed energy to on destroy GTR having the crosslink polar groupsbonds and byapplying produce RTR a 915 to with2450 properties MHz frequency very close and 41to tothe 177 original Wh/lb energyrubber to[90]. destroy Garcia the et crosslink al. [21] reported bonds and long produce microwave RTR with exposureproperties (7 verymin) closeof GTR to the(NR, original SBR and rubber carbon [90 blac]. Garciak) resulting et al. [in21 lower] reported gel content long microwave by breaking exposure the sulfur(7 min) crosslinks of GTR (NR,(C-S SBRand andS-S) carbonand carbon black) (C resulting-C) bonds in leading lower gelto contenthigher RTR by breaking chain mobility the sulfur (fluidity).crosslinks (C-S and S-S) and carbon (C-C) bonds leading to higher RTR chain mobility (fluidity). FigureFigure 13 13 shows shows SEM SEM micrographs micrographs of ofthe the GTR GTR shee sheetsts surface surface after after microwave microwave exposure exposure for 5, for 6 5, 6 andand 7 7 min. min. Homogenization Homogenization of ofthe the modified modified GTR GTR in inan anopen open two-roll two-roll mill mill enabled enabled the thesol solphase phase to to moremore efficiently efficiently wet wet the the gel gel phase. This This is is why GTR7 shows a smoothersmoother surfacesurface withwith smallersmaller voidsvoids due dueto its to higherits higher fluidity fluidity and and sol sol fraction fraction as aas result a result of aof higher a higher level level of regeneration.of regeneration. On On the the other other hand, hand,GTR5 GTR5 shows shows larger larger voids voids because because of weak of interactionweak interaction because because the rigid the particlesrigid particles (higher (higher crosslink crosslinkdensity) density) were not were able not to deformable to deform under mechanicalunder mechanical shearing shearing leading leading to low to interaction low interaction [21]. [21].

FigureFigure 13. 13. TypicalTypical SEM SEM images images of of the the surface surface sheets sheets of ofdifferent different GTR GTR microwave microwave treated treated for for 5 (GTR5), 5 (GTR5), 6 6(GTR6) (GTR6) andand 77 (GTR7)(GTR7) minutes (repro (reproducedduced with with permissi permissionon from from [21]). [21]).

ZanchetZanchet et et al. al. [104] [104 regenerated] regenerated SBR SBR via via microwav microwavee exposure exposure for for1, 2 1, and 2 and 3 min 3 min and and blended blended the the regeneratedregenerated SBR SBR (rSBR) (rSBR) with with virgin virgin SBR SBR (20 (20 phr) phr) for for subsequent subsequent revulcanization. revulcanization. As Aspresented presented in in TableTable 33,, thethe sol sol fraction, fraction, which which is ais direct a direct indication indication of the of regeneration the regeneration level, increasedlevel, increased with increasing with increasingexposure timesexposure and times temperature. and temperature.

TableTable 3. Temperature afterafter microwavemicrowave treatment treatment and and gel gel content content of the of regeneratedthe regenerated styrene-butadiene styrene- butadienerubber (rSBR) rubber for (rSBR) different for different exposure exposure time (1 time to 3(1 min) to 3 min) with with a power a power of 900of 900 W W (reproduced (reproduced with withpermission permission from from [104 [104]).]).

SampleSample Temperature Temperature (°C) (◦ C) Gel Gel content Content (%) (%) SBR-r - 93 ± 2 SBR-r - 93 2 ± rSBRrSBR 1 min 1 min 45 45± 2 2 90 90 ± 1 1 ± ± rSBRrSBR 2 min 2 min 60 60± 2 2 43 43 ± 2 2 ± ± rSBRrSBR 3 min 3 min 80 80± 4 4 33 33 ± 1 1 ± ± 3.2.3. Ultrasonic Method 3.2.3. Ultrasonic Method Continuous ultrasonic regeneration of GTR is a promising method to break-up the crosslinked Continuous ultrasonic regeneration of GTR is a promising method to break-up the crosslinked structure of vulcanized waste tires in a short time using cavitation bubbles induced by mechanical structure of vulcanized waste tires in a short time using cavitation bubbles induced by mechanical waves at high frequencies. The amplitude of the ultrasonic wave and the treatment time determine thewaves efficiency at high of frequencies. this GTR regeneration The amplitude process of the [95]. ultrasonic wave and the treatment time determine the efficiencyIsayevof et thisal. [96] GTR investigated regeneration the process effects [of95 GTR]. particle size (10 and 30 mesh) on the ultrasonic regenerationIsayev etefficiency. al. [96] investigated Ultrasonic regeneration the effects of at GTR 250 °C particle with an size amplitude (10 and 30of mesh)10 μm onrevealed the ultrasonic that theregeneration crosslink density efficiency. (0.1 Ultrasonickmole/m3) regenerationand gel content at 250(57%)◦C of with 30 mesh an amplitude (600 μm) of RTR 10 µ werem revealed lower than that the the crosslink density (0.2 kmole/m3) and gel content (67%) of 10 mesh (2000 μm) RTR particles indicating that higher regeneration rate can be achieved for smaller particles.

J. Compos. Sci. 2020, 4, 103 16 of 43 crosslinkJ. Compos. Sci. density 2020, 4,(0.1 x kmole/m3) and gel content (57%) of 30 mesh (600 µm) RTR were lower than16 of the 45 crosslink density (0.2 kmole/m3) and gel content (67%) of 10 mesh (2000 µm) RTR particles indicating The ultrasonic method can also be combined with an extruder for better efficiency as a that higher regeneration rate can be achieved for smaller particles. continuous process. Feng et al. [65] studied the ultrasonic regeneration of butyl rubber waste tires The ultrasonic method can also be combined with an extruder for better efficiency as a continuous using a grooved barrel ultrasonic extruder varying the ultrasonic amplitude (5, 7.5 and 10 μm) and process. Feng et al. [65] studied the ultrasonic regeneration of butyl rubber waste tires using a grooved the rubber flow rate (0.63, 1.26 and 2.52 g/s). As shown in Figure 14a, the complex viscosity of the barrel ultrasonic extruder varying the ultrasonic amplitude (5, 7.5 and 10 µm) and the rubber flow rate regenerated particles is lower than that of the untreated rubber particles attributed to the lower RTR (0.63, 1.26 and 2.52 g/s). As shown in Figure 14a, the complex viscosity of the regenerated particles is gel content indicating higher regeneration level. Moreover, increasing the ultrasonic amplitude led lower than that of the untreated rubber particles attributed to the lower RTR gel content indicating to lower RTR complex viscosity. However, the tan δ (ratio between the loss and storage moduli) of higher regeneration level. Moreover, increasing the ultrasonic amplitude led to lower RTR complex RTR increased after regeneration indicating a more viscous (less elastic) behavior attributed to a viscosity. However, the tan δ (ratio between the loss and storage moduli) of RTR increased after higher sol fraction. regeneration indicating a more viscous (less elastic) behavior attributed to a higher sol fraction.

Figure 14. (a) Complex dynamic viscosity and (b) loss tangent as a function of frequency for untreated Figure 14. (a) Complex dynamic viscosity and (b) loss tangent as a function of frequency for untreated and regenerated rubber obtained at various amplitudes and flow rates (reproduced with permission and regenerated rubber obtained at various amplitudes and flow rates (reproduced with permission from [65]). from [65]). 3.2.4. Thermo-Chemical Processes 3.2.4. Thermo-Chemical Processes Chemical reclaiming agents, such as such as benzoyl peroxide [105], tetrabenzylthiuram disulfideChemical [106], reclaiming diphenyl disulfideagents, such [107], as and such thiosalicylic as benzoyl acid peroxide [108], are [105], extensively tetrabenzylthiuram used in the rangedisulfide of 0.5–10[106], diphenyl wt.% for disulfide the chemical [107], and regeneration thiosalicylic of naturalacid [108], and are synthetic extensively rubbers. used in Figurethe range 15 presentsof 0.5–10 the wt.% chemical for the structure chemical of regeneration different organic of natural disulfides and synthetic and mercaptans rubbers. involved Figure 15 in thepresents chemical the regenerationchemical structure of waste of tires different [43]. organic disulfides and mercaptans involved in the chemical regenerationThermo-chemical of waste tires regeneration [43]. is based on applying thermal energy and mechanical mixing/ kneading,Thermo-chemical as well as adding regeneration chemical reclaiming is based agents on toapplying combine thermal the advantages energy of bothand regenerationmechanical processes.mixing/kneading, However, as elevatedwell as adding temperature chemical might reclaiming result in severeagents degradationto combine ofthe the advantages main chain of and both a dropregeneration of the mechanical processes. properties However, for elevated the regenerated temperature rubber might [93 ,109result]. in severe degradation of the mainAdhikari chain and et a al. drop [90] of reviewed the mechanical the list ofpr inorganicoperties for and the organic regenerated chemical rubber reclaiming [93,109]. agents used in GTR regeneration. Zhang et al. [97] synthesized thiobisphenols; 4,40-dithiobis(2,6-di-t-butylphenol) and used it as a novel reclaiming agent in an internal (batch) mixer at 180 and 200 ◦C. As shown in Figure 16, the RTR crosslink density decreased with increasing reclaiming agent content and temperature because of the break-up of both crosslink bonds and main backbone chains. They explained the inverse relation between crosslink density and MW between crosslinks by the reaction scheme presented in Figure 17.

J. Compos. Sci. 2020, 4, x 17 of 45

J. Compos. Sci. 2020, 4, 103 17 of 43 J. Compos. Sci. 2020, 4, x 17 of 45

Figure 15. Typical sulfides and mercaptans used in tire regeneration (reproduced with permission from [28]).

Adhikari et al. [90] reviewed the list of inorganic and organic chemical reclaiming agents used in GTR regeneration. Zhang et al. [97] synthesized thiobisphenols; 4,4′-dithiobis(2,6-di-t-butylphenol) and used it as a novel reclaiming agent in an internal (batch) mixer at 180 and 200 °C. As shown in Figure 16, the RTR crosslink density decreased with increasing reclaiming agent content and temperature because of the break-up of both crosslink bonds and main backbone chains. They FigureFigure 15.15. Typical sulfides sulfides and and mercaptans mercaptans used used in tire in tireregeneration regeneration (reproduced (reproduced with permission with permission explained the inverse relation between crosslink density and MW between crosslinks by the reaction fromfrom [[28]).28]). scheme presented in Figure 17. Adhikari et al. [90] reviewed the list of inorganic and organic chemical reclaiming agents used in GTR regeneration. Zhang et al. [97] synthesized thiobisphenols; 4,4′-dithiobis(2,6-di-t-butylphenol) and used it as a novel reclaiming agent in an internal (batch) mixer at 180 and 200 °C. As shown in Figure 16, the RTR crosslink density decreased with increasing reclaiming agent content and temperature because of the break-up of both crosslink bonds and main backbone chains. They explained the inverse relation between crosslink density and MW between crosslinks by the reaction scheme presented in Figure 17.

FigureFigure 16. 16.EffectEff ectof thiobisphenols of thiobisphenols content content and and temper temperatureature on on the the crosslink crosslink density density of reclaimedof reclaimed rubberrubber (reproduced (reproduced with with pe permissionrmission from from [97]). [97]).

Figure 17a,b show the reaction leading to the simultaneous break-up of both main chains and Figure 17a,b show the reaction leading to the simultaneous break-up of both main chains and crosslink bonds by thiobisphenols (Figure 17c) to form radicals, which can react with polymer crosslink Figurebonds 16. by Effect thiobisphenols of thiobisphenols (Figurecontent and 17c) temper toature form on radicals,the crosslink which density canof reclaimed react with polymer radicals.rubber As (reproduced shown in Figurewith permission 17d,e, thiobisphenolsfrom [97]). radicals may combine with polymer radicals and radicals.different As shown types of in chain Figure ends 17d,e, are generated thiobisphenols decreasing radicals the crosslink may combine bonds, resulting with polymer in lower crosslinkradicals and differentdensity. Figuretypes However, 17a,bof chain show low theends thiobisphenols reaction are generatedleading content to the decreasing si ledmultaneous to lower the break-up thiobisphenols crosslink of both bonds, main radical chains resulting concentration and in lower crosslinkandcrosslink lessdensity. chancebonds byHowever, for thiobisphenols reaction low with (Figurethiobisphenols polymer 17c) radicals. to form content radicals, Therefore, led which uncappedto canlower react polymer thiobisphenolswith polymer radicals canradical concentrationcombineradicals. together,As and shown less resulting in Figurechance in 17d,e, chainfor thiobisphenolsreaction extension with leading radicals polymer to may higher combineradicals. RTR MW with Ther and polymerefore, higher radicals crosslinkuncapped and density polymer radicals(Figuredifferent can 17 combinef)types [97]. of chaintogether, ends resultingare generated in chaindecreasing extension the crosslink leading bonds, to higher resulting RTR in MWlower and higher crosslink density. However, low thiobisphenols content led to lower thiobisphenols radical crosslinkconcentration density (Figureand less 17f)chance [97]. for reaction with polymer radicals. Therefore, uncapped polymer radicals can combine together, resulting in chain extension leading to higher RTR MW and higher crosslink density (Figure 17f) [97].

J. Compos. Sci. 2020, 4, 103 18 of 43

J. Compos. Sci. 2020, 4, x 18 of 45

FigureFigure 17. 17.Reaction Reaction scheme scheme for for the the regeneration regeneration ofof sulfursulfur vulcanized rubber rubber with with thiobisphenols thiobisphenols (reproduced(reproduced with with permission permission from from [97 [97]).]).

NotNot only only solvents, solvents, such such as alcoholas alcohol and and ketone, ketone, were were used used during during GTR GTR regeneration regeneration processes, processes, but supercriticalbut supercritical solvents solvents (water, (water, ethanol, ethanol, carbon carbon dioxide dioxide and toluene)and toluene) were were also also studied. studied. Li Li et al.et al. [110 ] investigated[110] investigated the effect the of temperature effect of temperature on the regeneration on the regeneration of waste sidewall of waste rubber sidewall from rubber passenger from car tirespassenger using supercritical car tires using ethanol supercritical and diphenyl ethanol and disulfide diphenyl (DPDS) disulfide as a(DPDS) regeneration as a regeneration agent. As agent. shown As shown in Figure 18, increasing the reaction temperature from 240 to 270 °C increased the RTR sol in Figure 18, increasing the reaction temperature from 240 to 270 C increased the RTR sol fraction fraction from 25 to 55% since ethanol reached its supercritical state maki◦ ng it easier to penetrate into from 25 to 55% since ethanol reached its supercritical state making it easier to penetrate into the GTR the GTR vulcanized structure to facilitate the crosslinked network breakdown. vulcanized structure to facilitate the crosslinked network breakdown.

J. Compos. Sci. 2020, 4, 103 19 of 43 J. Compos. Sci. 2020, 4, x 19 of 45

FigureFigure 18. E18.ff ectEffect of temperatureof temperature on on thethe solsol fraction of of regenerated regenerated sidewall sidewall rubber rubber obtained obtained from froma a passengerpassenger car tirecar (regenerationtire (regeneration reaction reaction conditions conditions are 8are MPa 8 forMPa 60 for min) 60 (reproducedmin) (reproduced with permissionwith frompermission [110]). from [110]).

3.3. GTR3.3. GTR in Curable in Curable Rubbers Rubbers RamaradRamarad et al.et al. [16 [16]] and and Karger-Kocsis Karger-Kocsis et al. [38] [38] reviewed reviewed the the evolution, evolution, properties properties and future and future of wasteof waste tire tire rubber rubber as as a fillera filler/r/reinforcementeinforcement inin polymer blends. blends. Also, Also, Fazli Fazli and and Rodrigue Rodrigue presented presented a reviewa review on on waste waste rubber rubber recycling focusing focusing on onsize sizereduction reduction and melt and blending melt blending of GTR/RTR of GTRwith /RTR withthermoplastic thermoplastic resins resins to toproduce produce thermoplastic thermoplastic elastomer elastomer (TPE) (TPE) compounds compounds [27]. [Even27]. Eventhough though thermoplastic matrices containing GTR/RTR are being studied by several researchers, very few thermoplastic matrices containing GTR/RTR are being studied by several researchers, very few publications reviewed the progress of blending GTR/RTR with virgin rubbers. publicationsGTR, reviewed with and without the progress regeneration, of blending have been GTR rarely/RTR mixed with virgin with virgin rubbers. rubbers such as natural rubberGTR, with(NR), and styrene without butadiene regeneration, rubber (SBR), have polybu beentadiene rarely mixedrubber with(BR) and virgin acrylonitrile rubbers suchbutadiene as natural rubberrubber (NR), (NBR) styrene to produce butadiene lower rubber costs (SBR), compounds polybutadiene [38]. The introduction rubber (BR) of and GTR/RTR acrylonitrile into virgin butadiene rubberrubbers (NBR) is mainly to produce leading lower to the costssacrifice compounds of some properties [38]. The of the introduction final compounds. of GTR /RTR into virgin rubbers isAs mainly expected, leading particle to the size sacrifice and regeneration of some properties level of ofthe the GTR final directly compounds. affect the rubber compoundsAs expected, properties. particle sizeAs described and regeneration later, smaller level pa ofrticle the GTRsize show directly less aeffectffect theon the rubber mechanical compounds properties.properties As loss described due to better later, dispersion smaller particle and higher size specific show lesssurface effect area on for the sufficient mechanical interaction properties with loss duethe to betterrubber dispersionmatrix chains and [111]. higher On the specific other hand, surface RTR is area offering for su betterfficient mechanical interaction strength, with thermal the rubber stability and curing behavior compared to GTR. This is attributed to the soluble fraction of RTR matrix chains [111]. On the other hand, RTR is offering better mechanical strength, thermal stability responsible for a good interfacial adhesion with rubber molecules and possible co-crosslinking at the and curinginterphase behavior between compared RTR and rubber to GTR. matrix This [17,22]. is attributed Recently, to more the soluble research fraction attention of was RTR directed responsible for ato good find interfacialthe optimum adhesion GTR/RTR with loading rubber in moleculesrubber matrices and possiblewith less co-crosslinkingproperty loss compared at the interphase with betweenvirgin RTR rubber and properties. rubber matrix The next [17 section,22]. Recently, is reporting more on the research effect of attention GTR/RTR wasaddition directed into rubber to find the optimummatrices GTR and/RTR reviewing loading the in literature rubber matricesrelated to with the curing less property characteristics, loss compared rheological, with mechanical, virgin rubber properties.aging, thermal, The next dynamic section mechanical is reporting and on sw theelling eff propertiesect of GTR of/RTR such additioncompounds. into rubber matrices and reviewing the literature related to the curing characteristics, rheological, mechanical, aging, thermal, dynamic3.3.1. mechanicalCure Characteristics and swelling properties of such compounds. Curing properties of rubber compounds can be determined by evaluating the maximum and 3.3.1.minimum Cure Characteristics torque beside scorch time and cure time using a rheometer. GTR incorporation into virgin rubber is expected to modify the cure characteristics since the materials already have some Curing properties of rubber compounds can be determined by evaluating the maximum formulation/history. In general, GTR introduction leads to lower maximum torque and scorch time, and minimum torque beside scorch time and cure time using a rheometer. GTR incorporation while increasing the minimum torque [112]. On the other hand, RTR particles leads to a lower intoincrease virgin rubber in the minimum is expected torque to compared modify the with cure GTR characteristics [113]. This is attributed since the to materialsthe partial break-up already have someof formulation the crosslinked/history. RTR structure In general, having GTR better introduction flow and leadsfewer toagglomerates lower maximum in the rubber torque matrix, and scorch time,decreasing while increasing the compound the minimum viscosity [114]. torque Not [112 only]. better On the chain other mobility hand, of RTR the RTR particles particles leads changes to a lower increase in the minimum torque compared with GTR [113]. This is attributed to the partial break-up of the crosslinked RTR structure having better flow and fewer agglomerates in the rubber matrix, decreasing the compound viscosity [114]. Not only better chain mobility of the RTR particles changes J. Compos. Sci. 2020, 4, 103 20 of 43 the minimumJ. Compos. Sci. torque, 2020, 4, x but also the presence of 10–15% of processing oil in their formulation20 induces of 45 a lower increase of the minimum torque, which implies better processability of RTR compared to the minimum torque, but also the presence of 10–15% of processing oil in their formulation induces GTR [113,115]. Also, the presence of carbon black in both GTR and RTR leads to higher minimum a lower increase of the minimum torque, which implies better processability of RTR compared to torque [116]. On the other hand, the maximum torque, as a measure of the elastic modulus, did not GTR [113,115]. Also, the presence of carbon black in both GTR and RTR leads to higher minimum decreasetorque much [116]. with On the RTR other loading hand, because the maximum its shorter torque, chains as a measure acted as of the elastic combinedmodulus, did with not the presencedecrease of some much processing with RTR loading oil [113 because,117]. GTR its shorter incorporation chains acted leads as toplasticizers a reduction combined of the maximumwith the torque,presence which of stabilizes some processing upon further oil [113,117]. addition GTR of incorporation crosslinked GTRleads particlesto a reduction [113]. of In the general, maximum lower torquetorque, for RTR which containing stabilizes rubberupon further compounds addition is of related crosslinked to the GTR break-up particles of the[113]. crosslinked In general, structure lower and lowertorque crosslinkfor RTR containing density compared rubber compounds to GTR [16 is]. related to the break-up of the crosslinked structure andRubber lower compounds crosslink density crosslinking compared starts to GTR at the [16]. scorch time, which decreases upon the addition of GTR/RTRRubber into the compounds rubber matrix. crosslinki Thisng behavior starts at isthe more scorch prevailing time, which with decreases RTR, which upon is the attributed addition to of the presenceGTR/RTR of active into the crosslinking rubber matrix. sites This or crosslink behavior precursorsis more prevailing and unreacted with RTR, curatives, which is facilitatingattributed to the crosslinkingthe presence reaction of active upon crosslinking heating [118 sites]. For or crosslink example, precursors accelerators and as unreacted crosslink curatives, precursors facilitating migrate to the rubberthe crosslinking matrix from reaction GTR/ RTRupon particles heating and[118]. decrease For example, the scorch accelerators time, which as crosslink implies anprecursors early start of themigrate crosslinking to the rubber reaction matrix as anfrom undesirable GTR/RTR particle propertys and in decrease the industry the scorch [113, 119time,]. Severalwhich implies researches an early start of the crosslinking reaction as an undesirable property in the industry [113,119]. Several have been done to overcome this challenge and control the cure time of rubber compounds. For researches have been done to overcome this challenge and control the cure time of rubber example, De et al. [120] reported on the positive effect of 0.25 phr N-cyclohexyl thiophthalimide as a compounds. For example, De et al. [120] reported on the positive effect of 0.25 phr N-cyclohexyl prevulcanizationthiophthalimide inhibitor as a prevulcanization (PVI) in NR/RTR inhibitor (50/50) (PVI) blends in NR/RTR to increase (50/50) the scorchblends timeto increase by 150% the (fromscorch1.5 to 2.25time min) by and150% uniform (from 1.5 curing to 2.25 in min) thick and products uniform such curi asng tires. in thick Also, products Nelson such et al. as [ 115tires.] reportedAlso, Nelson longer scorchet al. time [115] and reported cure time longer for scorch NBR/RTR time blends and cure by time grafting for NBR/RTR MA onto blends the RTR by grafting surface. MA Incorporation onto the of 20RTR wt.% surface. modified Incorporation RTR increased of 20 wt.% the modified cure time RTR of NBRincreased/RTR the compounds cure time of from NBR/RTR 4 to 7 compounds min after the additionfrom of4 to modified 7 min after RTR the particles.addition of This modified was attributedRTR particles. to theThis presence was attributed of anhydride to the presence in the of MA structureanhydride delaying in the the MA curing structure process dela (longerying the onset curing of process the curing (longer process) onset andof the decreasing curing process) the cure and rate resultingdecreasing in higher the cure cure rate times. resulting in higher cure times. TheThe optimum optimum cure cure time time is anotheris another parameter parameter forfor thethe analysisanalysis of of the the rubber rubber cure cure characteristic characteristic revealingrevealing the the required required time time to achieveto achieve optimum optimum physical physical propertiesproperties of of the the rubber rubber compounds. compounds. The The optimumoptimum cure cure time time varies varies upon upon the the incorporation incorporation ofof GTRGTR/RTR/RTR similar to to the the scorch scorch time, time, which which decreasesdecreases with with filler filler content content [113 [113].]. However, However, further further increase increase ofof GTR content content leads leads to to the the stabilization stabilization of the optimum cure time due to the crosslinked GTR structure limiting the crosslink ability [116,121]. of the optimum cure time due to the crosslinked GTR structure limiting the crosslink ability [116,121]. A summary of all the changes in the cure characteristic of GTR/RTR containing rubber blends is A summary of all the changes in the cure characteristic of GTR/RTR containing rubber blends is presented in Figure 19. presented in Figure 19.

Figure 19. Effect of tire rubber content on the cure characteristic of rubber blends (reproduced with Figure 19. Effect of tire rubber content on the cure characteristic of rubber blends (reproduced with permission from [16]). permission from [16]). 3.3.2. Rheological Properties

The incorporation of waste tire rubber into a rubber matrix increases the Mooney viscosity of the rubber compounds due to the crosslinked gel fraction and the presence of carbon black limiting

J. Compos. Sci. 2020, 4, x 21 of 45

3.3.2. Rheological Properties The incorporation of waste tire rubber into a rubber matrix increases the Mooney viscosity of J. Compos. Sci. 2020, 4, 103 21 of 43 the rubber compounds due to the crosslinked gel fraction and the presence of carbon black limiting chain mobility. Increasing the GTR loading implies the presence of a more crosslinked gel content andchain carbon mobility. black Increasing content resulting the GTR in loading further implies increase the of presence the rubber of a morecompound crosslinked viscosity gel content [121,122]. Debapriyaand carbon et blackal. [121] content reported resulting on the in increasing further increase Mooney of viscosity the rubber of compoundNR-polybutadiene viscosity rubber [121,122 (PBR)]. compoundsDebapriya etupon al. [121 RTR] reported addition on due the increasingto the increase Mooney of carbon viscosity black of NR-polybutadiene loading inducing rubber more (PBR)stiffness andcompounds processing upon difficulty RTR addition of the compounds due to the increase (Figure of 20). carbon black loading inducing more stiffness and processing difficulty of the compounds (Figure 20).

Figure 20. Mooney viscosity of NR-PBR/RR blends as a function of carbon black/regenerated tire rubber Figure(RTR) loading20. Mooney (reproduced viscosity with of permissionNR-PBR/RR from blends [121]). as a function of carbon black/regenerated tire rubber (RTR) loading (reproduced with permission from [121]). Similar to the Mooney viscosity, the incorporation of waste tire rubber influences the shear viscosity trendSimilar in a similar to the way Mooney and shows viscosity, a pseudoplastic the incorporat behavior.ion Extrudateof waste swelltire rubber or Barus influences effect is described the shear viscosityby the ratio trend of thein a extrudate similar way diameter and shows to the capillarya pseudoplastic die diameter. behavior. This Extrudate phenomenon swell is attributed or Barus effect to isnormal described stresses by the released ratio whenof the the extrudate material emergesdiameter from to the a capillary capillary die die [123 diameter.]. Sombatsompop This phenomenon et al. [122] is attributedstudied the to e ffnormalect of RTR stresses incorporation released into when two NRthe matrices material (STRVS60 emerge ands from STR20CV) a capillary using adie sulfur [123]. Sombatsompopvulcanization system et al. [122] to analyze studied the the extrudate effect of swell. RTR incorporation As shown in Figureinto two 21 ,NR increasing matrices the (STRVS60 RTR andloading STR20CV) led to lowerusing extrudatea swell due to the system higher to carbon analyze black the content extrudate decreasing swell. As the shown rubber in chain Figure 21,mobility increasing and elasticitythe RTR ofloading the compound. led to lower Also, extrud molecularate swell interactions due to the between higher the carbon rubber black molecules content J. Compos. Sci. 2020, 4, x 22 of 45 decreasingand the carbon the rubber black might chain be mobility responsible and for elasticity lower elasticity of the compound. (normal stresses). Also, molecular interactions between the rubber molecules and the carbon black might be responsible for lower elasticity (normal stresses).

FigureFigure 21.21. ExtrudateExtrudate (die) swell swell as as a afunction function of of RTR RTR content content for for NR NR (STRVS60 (STRVS60 and and STR20CV) STR20CV) at a at a -1 -1 temperaturetemperature ofof 8080 ◦°CC for 10 min min of of mastication mastication and and a ashear shear rate rate of of3.6 3.6 s s(reproduced(reproduced with with permission permission fromfrom [ 122[122]).]).

3.3.3. Mechanical Properties Different investigations were devoted to the effect of GTR incorporation into rubber matrices to evaluate the final compound mechanical properties such as tensile strength, hardness, abrasion resistance, resilience and compression set. As presented in Table 4, GTR incorporation into NR leads to a drop of tensile strength (TS) and elongation at break (EB), while blending GTR with NBR resulted in an improvement of both properties [115,116]. However, the incorporation of GTR into SBR did not show clear trends in mechanical property changes since sometimes TS and EB were improved [117], while other times they were deteriorated [114]. Lower TS and EB of the compounds based on GTR/RTR can be attributed to the low level of blend homogeneity and weak interfacial adhesion between the phases. Increasing the GTR loading implies an increase of the gel fraction with a crosslinked structure that cannot be dispersed as a continuous matrix in the virgin rubber, thus acting as a stress concentrating point resulting in lower TS and EB.

Table 4. Common mechanical properties of GTR/RTR rubber blends.

Tensile Tear Abrasion GTR/RTR Elongation Compression Resilience Blends strength strength resistance loading at break (%) set (%) (%) (MPa) (N/mm) (cc/h) 20 15 610 27 10.6 18 58 NR/RTR 40 12.5 580 29 10.7 25 52 [112] 60 12 60 30 10.7 25.2 50 20 15.9 500 NR/GTR 40 13.4 417 Not reported [114] 60 11.6 260 20 2.5 320 27.9 2.5 18 30 NBR/RTR 40 3.0 380 27.5 5 21 29 [116] 60 5.5 500 27.3 8 26.5 28 NBR/GTR 20 4 325 22 4 15.5 30.5 [124] 40 6 310 27 7 17.5 27 20 2.5 350 13 14.5 4.5 46 SBR/RTR 40 3.8 525 17 19 5.0 45 [117] 60 5.5 600 21 20 6.2 38 20 17.5 350 SBR/RTR 40 14.8 317 Not reported [114] 60 8.7 260

J. Compos. Sci. 2020, 4, 103 22 of 43

3.3.3. Mechanical Properties Different investigations were devoted to the effect of GTR incorporation into rubber matrices to evaluate the final compound mechanical properties such as tensile strength, hardness, abrasion resistance, resilience and compression set. As presented in Table4, GTR incorporation into NR leads to a drop of tensile strength (TS) and elongation at break (EB), while blending GTR with NBR resulted in an improvement of both properties [115,116]. However, the incorporation of GTR into SBR did not show clear trends in mechanical property changes since sometimes TS and EB were improved [117], while other times they were deteriorated [114]. Lower TS and EB of the compounds based on GTR/RTR can be attributed to the low level of blend homogeneity and weak interfacial adhesion between the phases. Increasing the GTR loading implies an increase of the gel fraction with a crosslinked structure that cannot be dispersed as a continuous matrix in the virgin rubber, thus acting as a stress concentrating point resulting in lower TS and EB.

Table 4. Common mechanical properties of GTR/RTR rubber blends.

Tensile Elongation Tear Abrasion GTR/RTR Compression Resilience Blends Strength at Break Strength Resistance Loading Set (%) (%) (MPa) (%) (N/mm) (cc/h) 20 15 610 27 10.6 18 58 NR/RTR 40 12.5 580 29 10.7 25 52 [112] 60 12 60 30 10.7 25.2 50 20 15.9 500 NR/GTR 40 13.4 417 Not reported [114] 60 11.6 260 20 2.5 320 27.9 2.5 18 30 NBR/RTR 40 3.0 380 27.5 5 21 29 [116] 60 5.5 500 27.3 8 26.5 28 NBR/GTR 20 4 325 22 4 15.5 30.5 [124] 40 6 310 27 7 17.5 27 20 2.5 350 13 14.5 4.5 46 SBR/RTR 40 3.8 525 17 19 5.0 45 [117] 60 5.5 600 21 20 6.2 38 20 17.5 350 SBR/RTR 40 14.8 317 Not reported [114] 60 8.7 260

Sombatsompop et al. [122] investigated the effect of RTR addition on the properties of two natural rubber compounds (STRVS60 and STR20CV) with respect to the RTR concentration and mastication time. As shown in Figure 22, the tensile stress at break and elongation at break of both rubber compounds decreased with increasing RTR content due to the difficult distribution of the high filler concentration (80 wt.% RTR) and insufficient interaction with the rubber matrix due to restricted chain mobility of partially regenerated RTR. As shown in Figure 23, SEM micrographs of the fractured blends surface (STRVS60/RTR (20/80)) show less homogeneity generating defects at the interphase leading to lower mechanical properties. Also, the presence of carbon black in the RTR formulation acts as a stress concentration point inhibiting the molecular orientations and chain mobility of the rubber chains leading to compound failure at lower TS and EB. Debapriya et al. [121] investigated the cure characteristics and tensile properties of NR-PBR compounds filled with different concentrations of tire rubber powders. They reported a tensile modulus increase with increasing rubber powders content due to the higher gel content and crosslink density of the highly filled (up to 60 wt.%) compounds (Figure 24). As reported earlier, increasing the filler content with higher crosslink density restricts the chain mobility so higher load is required for elongation increasing the modulus of the blends. Increasing the proportion of rubber powders also increases the presence of carbon black as an effective reinforcing filler restricting the mobility of the rubber chains under tension; i.e., higher modulus [74]. The incorporation of rubber powders into J. Compos. Sci. 2020, 4, x 23 of 45

J. Compos.Sombatsompop Sci. 2020, 4, x et al. [122] investigated the effect of RTR addition on the properties23 ofof 45two natural rubber compounds (STRVS60 and STR20CV) with respect to the RTR concentration and masticationSombatsompop time. As shown et al. [122]in Figure investigated 22, the tensilethe effect stress of RTRat break addition and elongationon the properties at break of oftwo both J. Compos.rubbernatural Sci. compounds 2020rubber, 4, 103compounds decreased (STRVS60 with increasing and STR20CV) RTR conten witht due respect to the to difficult the RTR distribution concentration of the and high23 of 43 fillermastication concentration time. As(80 shownwt.% RTR) in Figure and insufficient22, the tensile interaction stress at breakwith the and rubber elongation matrix at duebreak to of restricted both chainrubber mobility compounds of partially decreased regenerated with increasing RTR. RTR As conten shownt due in toFigure the difficult 23, SEM distribution micrographs of the highof the virgin rubber affects the homogeneity of the rubber compounds, which can be corroborated from SEM fracturedfiller concentration blends surface (80 wt.% (STRVS60/RTR RTR) and insufficient (20/80)) interactionshow less withhomogeneity the rubber generatingmatrix due todefects restricted at the micrographs. Increasing the RTR loading increased the number of crack paths and holes with less interphasechain mobility leading of topartially lower mechanicalregenerated propertiesRTR. As shown. Also, inthe Figure presence 23, ofSEM carbon micrographs black in ofthe the RTR homogeneityfractured makingblends surface the compounds (STRVS60/RTR more (20/80)) vulnerable show under less homogeneity mechanical stress.generating Asshown defectsin atFigure the 25, formulation acts as a stress concentration point inhibiting the molecular orientations and chain interphase leading to lower mechanical properties. Also, the presence of carbon black in the RTR increasingmobility of the the RTR rubber content chains from leading 20 to to 60 compound wt.% decreased failure at the lower blend TS and homogeneity EB. of NR-PBR/RTR blendsformulation attributed acts to as the a stress poor concentration filler distribution point withinhibiting higher the crosslinkedmolecular orientations fraction(gel and content)chain at 60 wt.%mobility [121 ].of the rubber chains leading to compound failure at lower TS and EB.

FigureFigure 22. 22.Tensile Tensile stress stress at at break break andand elongationelongation at brea breakk as as a a function function of of RTR RTR content content in inNR NR (STRVS60 (STRVS60 Figure 22. Tensile stress at break and elongation at break as a function of RTR content in NR (STRVS60 andand STR20CV) STR20CV) compounds compounds (reproduced (reproduced withwith permissionpermission from [122]). [122]). and STR20CV) compounds (reproduced with permission from [122]).

J. Compos. Sci. 2020, 4, x 24 of 45

less homogeneity making the compounds more vulnerable under mechanical stress. As shown in FigureFigureFigureFigure 23. 25, 23. SEM23. increasing SEM SEM micrographs micrographs micrographs the RTR of ofofcont some somesomeent rubberrubber from 20 co compounds mpoundstompounds 60 wt.% related related relateddecreased to to toFigure Figure Figurethe 22:blend 22: 22(a )(:( aSTRVS60, homogeneity)a STRVS60,) STRVS60, (b) ( RTRb ) (of bRTR ) NR- RTR PBR/RTRand (c) STRVS50/RTRblends attributed (20/80) to (reproduced the poor withfiller permission distribution from with [122]). higher crosslinked fraction (gel andand (c )(c STRVS50) STRVS50/RTR/RTR (20 (20/80)/80) (reproduced (reproduced with with permissionpermission from [122]). [122]). content) at 60 wt.% [121]. DebapriyaDebapriya et et al. al. [121] [121] investigatedinvestigated the cure characteristicscharacteristics and and tensile tensile properties properties of ofNR-PBR NR-PBR compounds filled with different concentrations of tire rubber powders. They reported a tensile compounds filled with different concentrations of tire rubber powders. They reported a tensile modulus increase with increasing rubber powders content due to the higher gel content and crosslink modulus increase with increasing rubber powders content due to the higher gel content and crosslink density of the highly filled (up to 60 wt.%) compounds (Figure 24). As reported earlier, increasing the density of the highly filled (up to 60 wt.%) compounds (Figure 24). As reported earlier, increasing the filler content with higher crosslink density restricts the chain mobility so higher load is required for filler content with higher crosslink density restricts the chain mobility so higher load is required for elongation increasing the modulus of the blends. Increasing the proportion of rubber powders also elongationincreases increasingthe presence the of moduluscarbon black of the as anblends. effectiv Increasinge reinforcing the filler proportion restricting of rubberthe mobility powders of the also increasesrubber chainsthe presence under tension;of carbon i.e black., higher as anmodulus effectiv [74].e reinforcing The incorporation filler restricting of rubber the powders mobility into of the rubbervirgin chains rubber under affects tension; the homogeneity i.e., higher of modulusthe rubber [74]. compounds, The incorporation which can ofbe rubbercorroborated powders from into virginSEM rubbermicrographs. affects Increasing the homogeneity the RTR loadingof the ru incrbbereased compounds, the number which of crack can paths be corroborated and holes with from SEM micrographs. Increasing the RTR loading increased the number of crack paths and holes with

FigureFigure 24. Stress-strain 24. Stress-strain behavior behavior of of NR-PBR NR-PBR/RR/RR blends (reproduced (reproduced with with perm permissionission from from[121]). [ 121]).

Figure 25. Tensile fractured surface of the compounds of Figure 24: (a) NR-PBR (100), (b) NR- PBR/RTR (80/20), (c) NR-PBR/RTR (60/40) and (d) NR-PBR/RTR (40/60) (reproduced with permission from [121]).

However, Sreeja et al. [116,117] reported an increasing trend of the tensile strength with RTR incorporation into NBR and SBR matrices in contrast to the decreasing trend of the tensile strength of NR/RTR compounds [112]. The tensile strength of NBR and SBR samples increased from 1.8 and 1.9 to 6.3 and 5.1 MPa, respectively, with the addition of 80 parts RTR [116,117]. However, increasing the RTR loading led to a tensile strength drop for NR/RTR compounds indicating a higher

J. Compos. Sci. 2020, 4, x 24 of 45

less homogeneity making the compounds more vulnerable under mechanical stress. As shown in Figure 25, increasing the RTR content from 20 to 60 wt.% decreased the blend homogeneity of NR- PBR/RTR blends attributed to the poor filler distribution with higher crosslinked fraction (gel content) at 60 wt.% [121].

J. Compos. Sci. 2020, 4, 103 24 of 43 Figure 24. Stress-strain behavior of NR-PBR/RR blends (reproduced with permission from [121]).

FigureFigure 25. Tensile 25. Tensile fractured fractured surface surface of the of compounds the compounds of Figure of Figure 24:( 24:a) NR-PBR(a) NR-PBR (100), (100), (b) NR-PBR(b) NR- /RTR (80/20),PBR/RTR (c) NR-PBR (80/20),/RTR (c) (60NR-PBR/RTR/40) and (d (60/40)) NR-PBR and (/dRTR) NR-PBR/RTR (40/60) (reproduced (40/60) (reproduced with permission with permission from [ 121]). from [121]). However, Sreeja et al. [116,117] reported an increasing trend of the tensile strength with RTR However, Sreeja et al. [116,117] reported an increasing trend of the tensile strength with RTR incorporation into NBR and SBR matrices in contrast to the decreasing trend of the tensile strength of incorporation into NBR and SBR matrices in contrast to the decreasing trend of the tensile strength NR/RTR compounds [112]. The tensile strength of NBR and SBR samples increased from 1.8 and 1.9 to J. Compos.of NR/RTRSci. 2020, 4compounds, x [112]. The tensile strength of NBR and SBR samples increased from 1.8 and25 of 45 6.3 and1.9 5.1 to 6.3 MPa, and respectively, 5.1 MPa, respectively, with the with addition the addition of 80 of parts 80 parts RTR RTR [116 [116,117].,117]. However, However, increasing increasing the RTR loading led to a tensile strength drop for NR/RTR compounds indicating a higher degradation degradationthe RTR level loading (scission led to ofa tensilerubber strengthchain) ofdrop RTR for inNR/RTR the regeneration compounds processindicating [112]. a higher Also, the level (scission of rubber chain) of RTR in the regeneration process [112]. Also, the difference between difference between the tensile strength of NBR and SBR compounds compared to NR compounds the tensile strength of NBR and SBR compounds compared to NR compounds containing RTR can containing RTR can be attributed to the non-crystallizing nature of the NBR and SBR leading to poor be attributed to the non-crystallizing nature of the NBR and SBR leading to poor strength. Also, the strength. Also, the tensile strength of NBR and SBR might be related to the presence of reinforcing tensile strength of NBR and SBR might be related to the presence of reinforcing fillers in the RTR fillers in the RTR producing a higher dilution effect of the NR matrix than a reinforcement one. Also, producing a higher dilution effect of the NR matrix than a reinforcement one. Also, the EB increased the EB increased with increasing RTR content, which is attributed to a plasticization effect caused by with increasing RTR content, which is attributed to a plasticization e ect caused by the presence of the presence of the processing oil in RTR [116,117]. As shown in Figureff 26, the elongation at break of theNBR/RTR processing and oilSBR/RTR in RTR compounds [116,117]. As increased shown in from Figure 278% 26 ,and the elongation304% at 0 parts, at break to 506% of NBR and/RTR 629% and at SBR80 parts/RTR of compounds RTR, respectively increased [116,117]. from 278% and 304% at 0 parts, to 506% and 629% at 80 parts of RTR, respectively [116,117].

Figure 26. Elongation at break as a function of reclaim rubber content for acrylonitrile butadiene rubber Figure 26. Elongation at break as a function of reclaim rubber content for acrylonitrile butadiene (NBR) (left) and styrene butadiene rubber (SBR) (right) compounds (reproduced with permission rubber (NBR) (left) and styrene butadiene rubber (SBR) (right) compounds (reproduced with from [116,117]). permission from [116,117]).

The presence of reinforcing filler in GTR enhances the energy dissipation and deviation of the tear path resulting in higher intrinsic strength. The reinforcing fillers can alter the crack path by resisting or delaying the crack growth to stabilize or even improve the tear strength of blends containing GTR. As shown in Figure 27, the incorporation of GTR into an SBR matrix led to the deviation of uniform pattern spacing and deflection of the unidirectional crack path [119].

Figure 27. Abrasion pattern of SBR samples (normal load = 25 N): (A) without GTR, (B) with GTR (particle size = 420–600 μm; content = 30 phr) (reproduced with permission from [119]).

Han et al. [119] reported on the abrasion resistance improvement of NR/GTR compounds due to the presence of vulcanized GTR particles with higher modulus. However, the incorporation of RTR with a weaker surface and lower MW (shorter fragments and smaller chains) led to the deterioration of the abrasion resistance [113,125]. Rattanasom et al. [125] studied the effect of conventional vulcanization (CV) and efficient vulcanization (EV) on NR/RTR compound properties using sulfur (S):N-tert-butyl-2-benzothiazolesulfenamide (TBBS) ratios of 1.75:0.75 and 0.75:1.75 for CV and EV systems. The addition of 10 phr RTR resulted in 100 and 90 mm3 volume loss of CV and EV vulcanizates, respectively, which are inversely proportional to the abrasion resistance. So higher abrasion resistance of NR/RTR compounds was obtained by the CV system through vulcanization of the RTR surface with the virgin NR matrix. Since the crosslink density, hardness and modulus all influence the abrasion resistance, a CV process increasing the crosslink density should also increase

J. Compos. Sci. 2020, 4, x 25 of 45

degradation level (scission of rubber chain) of RTR in the regeneration process [112]. Also, the difference between the tensile strength of NBR and SBR compounds compared to NR compounds containing RTR can be attributed to the non-crystallizing nature of the NBR and SBR leading to poor strength. Also, the tensile strength of NBR and SBR might be related to the presence of reinforcing fillers in the RTR producing a higher dilution effect of the NR matrix than a reinforcement one. Also, the EB increased with increasing RTR content, which is attributed to a plasticization effect caused by the presence of the processing oil in RTR [116,117]. As shown in Figure 26, the elongation at break of NBR/RTR and SBR/RTR compounds increased from 278% and 304% at 0 parts, to 506% and 629% at 80 parts of RTR, respectively [116,117].

Figure 26. Elongation at break as a function of reclaim rubber content for acrylonitrile butadiene rubber (NBR) (left) and styrene butadiene rubber (SBR) (right) compounds (reproduced with J. Compos. Sci. 2020, 4, 103 25 of 43 permission from [116,117]).

TheThe presence presence of of reinforcing reinforcing filler filler in in GTR GTR enhances enhanc thees the energy energy dissipation dissipation and and deviation deviation of the of tear the pathtear resultingpath resulting in higher in intrinsichigher intrinsic strength. strength. The reinforcing The reinforcing fillers can fillers alter the can crack alter path the bycrack resisting path orby delayingresisting theor crackdelaying growth the tocrack stabilize growth or evento stabiliz improvee or theeven tear improve strength the of blendstear strength containing of blends GTR. Ascontaining shown in GTR. Figure As 27 shown, the incorporation in Figure 27, of the GTR inco intorporation an SBR matrixof GTR led into to an the SBR deviation matrix of led uniform to the patterndeviation spacing of uniform and deflection pattern spacing of the unidirectional and deflection crack of the path unidirectional [119]. crack path [119].

FigureFigure 27. 27.Abrasion Abrasion patternpattern ofof SBRSBR samplessamples (normal(normal loadload= = 2525 N):N): (A(A)) withoutwithout GTR,GTR, (B(B)) with with GTR GTR (particle(particle size size= =420–600 420–600µ μm;m; contentcontent= = 3030 phr)phr) (reproduced(reproduced withwith permissionpermission fromfrom [[119]).119]).

HanHan et et al. al. [[119]119] reportedreported onon thethe abrasionabrasion resistanceresistance improvement improvement of of NR NR/GTR/GTR compounds compounds due due to to thethe presence presence of of vulcanized vulcanized GTR GTR particles particles with with higher higher modulus. modulus. However,However, thethe incorporation incorporation ofof RTR RTR withwith a a weaker weaker surface surface and and lower lower MW MW (shorter (shorter fragments fragments and and smaller smaller chains) chains) led led to to the the deterioration deterioration ofof thethe abrasionabrasion resistanceresistance [113[113,125].,125]. Rattanasom et al.al. [[125]125] studiedstudied thethe eeffectffect ofof conventionalconventional vulcanizationvulcanization (CV)(CV) andand eefficientfficient vulcanizationvulcanization (EV)(EV) onon NRNR/RTR/RTR compound compound propertiesproperties usingusing sulfursulfur (S):N-tert-butyl-2-benzothiazolesulfenamide(S):N-tert-butyl-2-benzothiazolesulfenamide (TBBS)(TBBS) ratiosratios ofof 1.75:0.751.75:0.75 andand 0.75:1.750.75:1.75 forfor CVCV andand EVEV 3 systems.systems. The The addition addition of 10of phr10 RTRphr resultedRTR resulted in 100 andin 100 90 mm and volume90 mm loss3 volume of CV andloss EV of vulcanizates,CV and EV respectively,vulcanizates, which respectively, are inversely which proportional are inversely to the prop abrasionortional resistance. to the abrasion So higher resistance. abrasion resistanceSo higher ofabrasion NR/RTR resistance compounds of NR/RTR was obtained compounds by the CVwas system obtained through by the vulcanization CV system through of the RTR vulcanization surface with of thethe virgin RTR surface NR matrix. with Sincethe virgin the crosslink NR matrix. density, Since hardness the crosslink and modulusdensity, hardness all influence and the modulus abrasion all resistance,influence athe CV abrasion process increasingresistance, thea CV crosslink process density increasing should the also crosslink increase density the hardness should and also modulus increase leading to better abrasion resistance. As CV results in the formation of crosslink bridges between shorter fragments from the RTR surface and the NR matrix, longer chain length for effective stress transfer to the continuous matrix is obtained. The effect of GTR particle size on the mechanical properties of the final compounds has been reviewed in different publications [16,27]. Han et al. [119] studied the effect of four different GTR particle sizes (30–40, 60–80, 100–120 and 170–200 mesh) on the mechanical properties of SBR/GTR compounds. As shown in Figure 28, the tensile strength of SBR compounds containing 30 phr GTR decreased from 240 to 160 kg/cm2 with an increase in GTR particle size from 100 to 500 µm (140 to 35 mesh). The incorporation of smaller GTR particles with higher specific surface area and uniform dispersion resulted in strong bonding and good stress transfer leading to higher tensile properties. However, using smaller particles to improve the tensile properties becomes negligible at high GTR/RTR content (above 50 wt.%) since the interfacial adhesion, rubber particles agglomeration and void formation at the interface between GTR/RTR and rubber matrix are controlling the mechanical strength of highly filled compounds [16]. In general, RTR shows higher TS and EB than GTR. Zhang et al. [124] reported a 69% (23.2 from 13.7 MPa) and 47% (612% from 416%) improvement of TS and EB, respectively, in NR/RTR compounds compared to NR/GTR compounds (10 wt.%). Partial regeneration of the RTR led to better tensile properties as a result of improved interfacial bonding between the RTR and the NR matrix. As schematically shown in Figure 29, the sol fraction of RTR is responsible for co-crosslinking with the NR matrix to form a strong interphase leading to better stress transfer and mechanical properties. J. Compos. Sci. 2020, 4, x 26 of 45 the hardness and modulus leading to better abrasion resistance. As CV results in the formation of crosslink bridges between shorter fragments from the RTR surface and the NR matrix, longer chain length for effective stress transfer to the continuous matrix is obtained. The effect of GTR particle size on the mechanical properties of the final compounds has been reviewed in different publications [16,27]. Han et al. [119] studied the effect of four different GTR particle sizes (30–40, 60–80, 100–120 and 170–200 mesh) on the mechanical properties of SBR/GTR compounds. As shown in Figure 28, the tensile strength of SBR compounds containing 30 phr GTR decreased from 240 to 160 kg/cm2 with an increase in GTR particle size from 100 to 500 μm (140 to 35 mesh). The incorporation of smaller GTR particles with higher specific surface area and uniform J. Compos. Sci. 2020, 4, x 26 of 45 dispersion resulted in strong bonding and good stress transfer leading to higher tensile properties. the hardness and modulus leading to better abrasion resistance. As CV results in the formation of crosslink bridges between shorter fragments from the RTR surface and the NR matrix, longer chain length for effective stress transfer to the continuous matrix is obtained. The effect of GTR particle size on the mechanical properties of the final compounds has been reviewed in different publications [16,27]. Han et al. [119] studied the effect of four different GTR particle sizes (30–40, 60–80, 100–120 and 170–200 mesh) on the mechanical properties of SBR/GTR compounds. As shown in Figure 28, the tensile strength of SBR compounds containing 30 phr GTR decreased from 240 to 160 kg/cm2 with an increase in GTR particle size from 100 to 500 μm (140 to 35 J.mesh). Compos. The Sci. 2020incorporation, 4, 103 of smaller GTR particles with higher specific surface area and uniform26 of 43 dispersion resulted in strong bonding and good stress transfer leading to higher tensile properties.

Figure 28. Tensile properties of SBR/GTR compounds as a function of GTR particle sizes at 30 phr (reproduced with permission from [119]).

However, using smaller particles to improve the tensile properties becomes negligible at high GTR/RTR content (above 50 wt.%) since the interfacial adhesion, rubber particles agglomeration and void formation at the interface between GTR/RTR and rubber matrix are controlling the mechanical strength of highly filled compounds [16]. In general, RTR shows higher TS and EB than GTR. Zhang et al. [124] reported a 69% (23.2 from 13.7 MPa) and 47% (612% from 416%) improvement of TS and EB, respectively, in NR/RTR compounds compared to NR/GTR compounds (10 wt.%). Partial regeneration of the RTR led to better tensile properties as a result of improved interfacial bonding between the RTR and the NR matrix. Figure 28.28. Tensile properties of SBRSBR/GTR/GTR compounds as a function of GTR particle sizes at 30 phr As schematically(reproduced(reproduced shown withwith permissionpermin Figureission 29,fromfrom the [[119]).119 sol]). fracti on of RTR is responsible for co-crosslinking with the NR matrix to form a strong interphase leading to better stress transfer and mechanical properties. However, using smaller particles to improve the tensile properties becomes negligible at high GTR/RTR content (above 50 wt.%) since the interfacial adhesion, rubber particles agglomeration and void formation at the interface between GTR/RTR and rubber matrix are controlling the mechanical strength of highly filled compounds [16]. In general, RTR shows higher TS and EB than GTR. Zhang et al. [124] reported a 69% (23.2 from 13.7 MPa) and 47% (612% from 416%) improvement of TS and EB, respectively, in NR/RTR compounds compared to NR/GTR compounds (10 wt.%). Partial regeneration of the RTR led to better tensile properties as a result of improved interfacial bonding between the RTR and the NR matrix. As schematically shown in Figure 29, the sol fraction of RTR is responsible for co-crosslinking with the NR matrix to form a strong interphase leading to better stress transfer and mechanical properties.

FigureFigure 29. 29.SchematicSchematic representation representation of thethe partial partial regeneration regeneration process process and itsand eff ectits oneffect the morphologyon the morphologyof rubber of blends rubber (reproduced blends (reprodu with permissionced with permission from [124 from]). [124]).

In another work, the incorporation of 20 wt.% RTR in SBR led to 10% and 12% improvement of TS and EB, respectively, compared to the addition of the same concentration of GTR. SEM micrographs of the tensile fracture surfaces of the blends show poor interfacial adhesion of GTR to the SBR indicated by a clear gap between both phases (Figure 30a,b) [126]. On the other hand, no significant void between RTR and SBR can be seen due to better wettability of RTR associated with better compatibility and stronger interface resulting in improved deformation and strength (Figure 30c,d) [126]. Li et al. [127] reported similar observations about the positive effect of partially regenerated RTR particles on better

compatibility and co-crosslinking at the interphase between RTR and virgin NR matrix leading to improvedFigure TS 29. and Schematic EB. representation of the partial regeneration process and its effect on the Inmorphology general, GTR of rubber is a vulcanized blends (reprodu materialced with with permission restricted from chain [124]). mobility and weak interaction with polymer matrices compared to RTR, which is a partially regenerated material that can form a good interphase as well as allowing for co-crosslinking between RTR and matrix chains. Contrary to a 100% gel fraction for GTR, RTR benefits from the presence of a soluble fraction responsible for the interaction or bonding with the polymer matrix to improve interfacial adhesion and hence the tensile strength and elongation at break of the final compounds. Also, regeneration results in lower crosslink density of RTR and lower MW (smaller fragment and shorter chains) compared to GTR, which corresponds to a lower modulus of RTR blends. The effect of GTR/RTR incorporation on the tensile properties of rubber compounds (mainly NR) is summarized in Figure 31. J. Compos. Sci. 2020, 4, x 27 of 45

In another work, the incorporation of 20 wt.% RTR in SBR led to 10% and 12% improvement of TS and EB, respectively, compared to the addition of the same concentration of GTR. SEM micrographs of the tensile fracture surfaces of the blends show poor interfacial adhesion of GTR to the SBR indicated by a clear gap between both phases (Figure 30a,b) [126]. On the other hand, no significant void between RTR and SBR can be seen due to better wettability of RTR associated with better compatibility and stronger interface resulting in improved deformation and strength (Figure 30c,d) [126]. Li et al. [127] reported similar observations about the positive effect of partially regenerated RTR particles on better compatibility and co-crosslinking at the interphase between RTR J. Compos. Sci. 2020, 4, 103 27 of 43 and virgin NR matrix leading to improved TS and EB.

Figure 30. SEM micrographs of the tensile fracture surfaces of SBR compounds filled with GTR (a,b) or J. Compos.FigureRTR Sci. (30.c ,2020d )SEM (30, 4, phr) xmicrographs (reproduced of withthe tensile permission fracture from surf [126aces]). of SBR compounds filled with GTR (a,28b) of 45 or RTR (c,d) (30 phr) (reproduced with permission from [126]).

In general, GTR is a vulcanized material with restricted chain mobility and weak interaction with polymer matrices compared to RTR, which is a partially regenerated material that can form a good interphase as well as allowing for co-crosslinking between RTR and matrix chains. Contrary to a 100% gel fraction for GTR, RTR benefits from the presence of a soluble fraction responsible for the interaction or bonding with the polymer matrix to improve interfacial adhesion and hence the tensile strength and elongation at break of the final compounds. Also, regeneration results in lower crosslink density of RTR and lower MW (smaller fragment and shorter chains) compared to GTR, which corresponds to a lower modulus of RTR blends. The effect of GTR/RTR incorporation on the tensile properties of rubber compounds (mainly NR) is summarized in Figure 31.

Figure 31. The effect of tire rubber content on the mechanical properties of rubber blends (reproduced Figurewith permission 31. The effect from of [ 16tire]). rubber content on the mechanical properties of rubber blends (reproduced with permission from [16]). In general, the hardness of a rubber compound is influenced by the elastic modulus and crosslink density.In general, This is the why hardness a difference of a rubber is again compound expected betweenis influenced GTR by and the RTR. elastic As modulus shown in and Figure crosslink 32, density.the incorporation This is why of RTRa difference increased is theagain crosslink expect densityed between (higher GTR gel and fraction) RTR. ofAs rubber shown blends in Figure with 32, theincreasing incorporation content of resulting RTR increased in higher the chain crosslink mobility density restriction (higher and gel blend fraction) rigidity. of rubber This isblends why the with increasinghardness values content (Shore resulting A) of in NR higher/RTR compounds chain mobility increased restriction with RTRand contentblend rigidity. [122]. This is why the hardness values (Shore A) of NR/RTR compounds increased with RTR content [122].

Figure 32. Crosslink density (left) and hardness (right) of NR/RTR compounds as a function of RTR content (reproduced with permission from [122]).

3.3.4. Aging Properties Materials of waste tires age and degrade at different levels. This is why the investigation of the aging properties of GTR/RTR rubber compounds is very important. However, very few studies investigated the effect of GTR/RTR on the aging behavior of rubber compounds. Debapriya et al. [121] reported an increase of the 200% moduli retention of NR-PBR/RTR with increasing RTR content from 20 to 60 phr after aging at 70 °C for 24, 48 and 72 h. The variation was attributed to increasing crosslink density and the formation of new crosslinks due to the presence of active sites in RTR (Figure 33). They also reported better aging resistance of RTR containing compounds compared to that of the virgin rubber. Better thermal aging behavior of RTR is related to the hydrocarbon chains stabilization induced by the regeneration process through heating and mechanical shearing, as well

J. Compos. Sci. 2020, 4, x 28 of 45

Figure 31. The effect of tire rubber content on the mechanical properties of rubber blends (reproduced with permission from [16]).

In general, the hardness of a rubber compound is influenced by the elastic modulus and crosslink density. This is why a difference is again expected between GTR and RTR. As shown in Figure 32, the incorporation of RTR increased the crosslink density (higher gel fraction) of rubber blends with increasing content resulting in higher chain mobility restriction and blend rigidity. This is why the J. Compos.hardness Sci. 2020 values, 4, 103 (Shore A) of NR/RTR compounds increased with RTR content [122]. 28 of 43

Figure 32. Crosslink density (left) and hardness (right) of NR/RTR compounds as a function of RTR Figure 32. Crosslink density (left) and hardness (right) of NR/RTR compounds as a function of RTR content (reproduced with permission from [122]). content (reproduced with permission from [122]). 3.3.4. Aging Properties 3.3.4. Aging Properties Materials of waste tires age and degrade at different levels. This is why the investigation of Materials of waste tires age and degrade at different levels. This is why the investigation of the the aging properties of GTR/RTR rubber compounds is very important. However, very few studies aging properties of GTR/RTR rubber compounds is very important. However, very few studies investigatedinvestigated the etheffect effect of GTR of GTR/RTR/RTR onthe on agingthe aging behavior behavior of rubber of rubber compounds. compounds. Debapriya Debapriya et al.et [al.121 ] reported[121] anreported increase an increase of the 200% of the moduli 200% moduli retention retention of NR-PBR of NR-PBR/RTR/RTR with with increasing increasing RTR RTR content content from 20 tofrom 60 phr 20 afterto 60 agingphr after at70 aging◦C forat 70 24, °C 48 for and 24, 72 48 h. and The 72 variation h. The variation was attributed was attributed to increasing to increasing crosslink densitycrosslink and the density formation and the ofnew formation crosslinks of new due crosslinks to the presence due to of the active presence sites inof RTRactive (Figure sites in33 ).RTR They also(Figure reported 33). better They agingalso reported resistance better of RTRaging containing resistance of compounds RTR containing compared compounds to that compared of the virgin to rubber.that Better of the thermalvirgin rubber. aging behaviorBetter thermal of RTR aging is related behavior to theof RTR hydrocarbon is related chainsto the hydrocarbon stabilization chains induced J. Compos. Sci. 2020, 4, x 29 of 45 by thestabilization regeneration induced process by the through regeneration heating process and mechanical through heating shearing, and mechanical as well as shearing, residual additivesas well formas residual the original additives processing. form the original Therefore, processing. Therefore, addition antioxidant is not required addition for is the not production required for of rubberthe production compounds of rubber due to compounds the intrinsic due aging to the properties intrinsic of aging the RTR properties [90]. of the RTR [90].

Figure 33.33.E Effectffect of of RTR RTR content content on the on retention the retention of the 200% of the modulus 200% ofmodulus NR-PBR /ofRR NR-PBR/RR blends (reproduced blends with(reproduced permission with from perm [121ission]). from [121]).

As shown in Table5 5,, Sreeja Sreeja et et al. al. [ [116]116] reportedreported aa 120%120% retentionretention inin tensiletensile strengthstrength ofof NBRNBR compared toto thatthat ofof curedcured NBRNBR/RTR/RTR compounds compounds indicating indicating that that the the state state of of cure cure (crosslink (crosslink density) density) is increasingis increasing while while aging. aging. The The presence presence of RTRof RTR in the in rubberthe rubber blends blends led toled some to some degradation degradation since since RTR wasRTR obtainedwas obtained from an from NR source,an NR whichsource, was which more wa prones more to degradation prone to degradation under elevated under temperature elevated andtemperature lower tensile and lower strength tensile of the strength filled compounds. of the filled compounds.

Table 5. Tensile strength of NBR/RTR compounds before and after aging (reproduced with permission from [116]).

Tensile strength (MPa) Sample Retention (%) Before aging After aging NBR 1.8 2.2 120 NBR/RTR20 2.6 2.3 89 NBR/RTR40 2.6 2.5 92 NBR/RTR60 5.0 4.7 94 NBR/RTR80 6.3 5.7 91

3.3.5. Thermal Properties The thermal stability of rubber compounds is highly important with respect to waste tire addition into rubber formulations since the materials are already degraded, affecting the overall compound thermal stability. The presence of volatile materials in GTR leads to lower thermal degradation onset temperature. However, increasing the GTR loading results in lower weight loss of the rubber compounds [16]. Debapriya et al. [128] reported improved thermal stability of SBR/RTR compounds with RTR incorporation as the char residue of SBR increased from 5.3% to 22.6% with the addition of 60 wt.% RTR in SBR/RTR (40/60) compounds. As shown in Figure 34, the increase of initial weight loss of SBR under an N2 atmosphere from 3.6% to 10% after the addition of 60 wt.% RTR was attributed to the volatilization of the processing oil associated with the regeneration process. The presence of RTR in NR produced two distinct peaks in the derivative (DTG) curve similar to NR/SBR blends [129].

J. Compos. Sci. 2020, 4, 103 29 of 43

Table 5. Tensile strength of NBR/RTR compounds before and after aging (reproduced with permission from [116]).

Tensile Strength (MPa) Sample Retention (%) Before Aging After Aging NBR 1.8 2.2 120 NBR/RTR20 2.6 2.3 89 NBR/RTR40 2.6 2.5 92 NBR/RTR60 5.0 4.7 94 NBR/RTR80 6.3 5.7 91

3.3.5. Thermal Properties The thermal stability of rubber compounds is highly important with respect to waste tire addition into rubber formulations since the materials are already degraded, affecting the overall compound thermal stability. The presence of volatile materials in GTR leads to lower thermal degradation onset temperature. However, increasing the GTR loading results in lower weight loss of the rubber compounds [16]. Debapriya et al. [128] reported improved thermal stability of SBR/RTR compounds with RTR incorporation as the char residue of SBR increased from 5.3% to 22.6% with the addition of 60 wt.% RTR in SBR/RTR (40/60) compounds. As shown in Figure 34, the increase of initial weight loss of SBR under an N2 atmosphere from 3.6% to 10% after the addition of 60 wt.% RTR was attributed to the J. Compos.volatilization Sci. 2020, 4, x of the processing oil associated with the regeneration process. The presence of RTR in30 of 45 NR produced two distinct peaks in the derivative (DTG) curve similar to NR/SBR blends [129].

Figure Figure34. Thermogravimetric 34. Thermogravimetric analysisanalysis (TGA (TGA/DTG)/DTG) of SBR /ofRR compoundsSBR/RR compounds (reproduced with(reproduced permission with from [128]). permission from [128]). Cañavate et al. [70] evaluated the effect of GTR surface treatment on the thermal stability of CañavateNBR/NR /GTRet al. compounds. [70] evaluated The incorporationthe effect of of GT GTRR surface increased treatment the residues on at the 600 thermal◦C of NBR stability/NR of NBR/NR/GTRcompounds compounds. from 15% to 30%The withincorporation the addition of 50GTR phr GTR.increased GTR addition the residues increased at the600 crosslinking °C of NBR/NR compoundsdegree from resulting 15% in to better 30% thermal with stability,the addition and also, of microwave50 phr GTR. treatment GTR ledaddition to a shift increased of the the crosslinkinginitial decompositiondegree resulting temperature in better (T 5%thermal) to higher stab temperaturesility, and also, indicating microwave increased treatment thermal stability led to a shift attributed to the regeneration that improved the crosslinking in agreement with similar observations of the initial decomposition temperature (T5%) to higher temperatures indicating increased thermal stability attributed to the regeneration that improved the crosslinking in agreement with similar observations in SBR/GTR blends [130]. However, GTR exposure to microwave radiation above 5 min resulted in a thermal stability drop due to excessive treatment degrading the GTR main chains [70]. Xavier et al. [131] determined the stability of NR/GTR compounds using microwave treated GTRcar and GTRtruck (Figure 35). As shown in Table 6, truck tires contain more NR and less carbon black than passenger car tires due to their specific requirements. The temperature for 5% weight loss (T5%) of NR/GTRcar compounds was higher than that of NR/GTRtruck compounds due to higher GTRcar thermal stability. In both cases, the incorporation of 50 phr GTR increased the char residues at 550 °C of NR from 26.6% to 30.8% and 29.4% for NR/GTRcar and NR/GTRtruck respectively, which was related to the presence of carbon black and SiO2 in GTR. Despite reports on the positive effect of GTR regeneration on improving the thermal stability, a negligible effect of the regeneration was reported on the thermal stability of NR/GTR compounds.

J. Compos. Sci. 2020, 4, 103 30 of 43 in SBR/GTR blends [130]. However, GTR exposure to microwave radiation above 5 min resulted in a thermal stability drop due to excessive treatment degrading the GTR main chains [70]. Xavier et al. [131] determined the stability of NR/GTR compounds using microwave treated GTRcar and GTRtruck (Figure 35). As shown in Table6, truck tires contain more NR and less carbon black than passenger car tires due to their specific requirements. The temperature for 5% weight loss (T5%) of NR/GTRcar compounds was higher than that of NR/GTRtruck compounds due to higher GTRcar thermal stability. In both cases, the incorporation of 50 phr GTR increased the char residues at 550 ◦C of NR from 26.6% to 30.8% and 29.4% for NR/GTRcar and NR/GTRtruck respectively, which was related to the presence of carbon black and SiO2 in GTR. Despite reports on the positive effect of GTR regeneration on improving the thermal stability, a negligible effect of the regeneration was reported on J. Compos. Sci. 2020, 4, x 31 of 45 the thermal stability of NR/GTR compounds.

Figure 35. TGA and DTG curves of: (a) NR/GTRcar and (b) NR/GTRtruck samples (N2 atmosphere and

a heatingFigure rate 35. TGA of 20 and◦C/min) DTG (reproducedcurves of: (a) withNR/GTR permissioncar and (b) from NR/GTR [131truck]). samples (N2 atmosphere and Tablea heating 6. Typical rate compositionof 20 °C/min) of(reprodu passengerced with and permission truck tires from (reproduced [131]). with permission from [131]).

Table 6. TypicalComposition composition of passenger and truck tires Passenger (reproduced Car with Tire permission Truckfrom [131]). Tire Natural rubberComposition Passenger 25% car tire Truck tire 35% Synthetic rubberNatural rubber 32% 25% 35% 25% Carbon blackSynthetic rubber 33% 32% 25% 30% SiO2 Carbon black 5% 33% 30% 6% Other additives (curing system, processingSiO2 aids, etc.) 5%5% 6% 4% Other additives (curing system, processing aids, etc.) 5% 4% Garcia et al. [21] reported that the improved thermal stability for RTR compounds can be related Garcia et al. [21] reported that the improved thermal stability for RTR compounds can be related to the barrier effect of the carbon black adsorbing low molecular weight volatile products formed to the barrier effect of the carbon black adsorbing low molecular weight volatile products formed duringduring the the thermal thermal degradation, degradation, hence hence improving improving thethe thermal stability. stability. However, However, the the preparation preparation of of NR/NR/RTRRTR compounds compounds using using an an internal internal mixer mixer and and compressioncompression molding molding at at an an elevated elevated temperature temperature (160(160◦C for °C 12for min) 12 min) ledto led some to some RTR thermalRTR thermal degradation degradation and theand evaporation the evaporation of low of molecularlow molecular weight volatileweight products. volatile products. Also, using Also, a compatibilizerusing a compatibiliz sucher assuch MA as increasedMA increased the the blend blend compatibility compatibility and and interfacial bonding between GTR/RTR and virgin rubber matrices resulting in higher thermal stability of the compounds. Medhat et al. [55] reported that the addition of 5% MA can substantially increase the initial decomposition temperature of NR from 181 to 237 °C for NR/RTR/MA (30/70/5).

3.3.6. Dynamic Mechanical Properties Dynamic mechanical analysis (DMA) can provide information about the loss modulus (E’’), storage modulus (E’) and damping factor (tan δ = E’’/E’), which are essential to understand the viscoelastic behavior of rubber compounds. The loss and storage moduli are related to the maximum heat dissipation per unit deformation and the maximum energy that can be stored in a period of time

J. Compos. Sci. 2020, 4, 103 31 of 43 interfacial bonding between GTR/RTR and virgin rubber matrices resulting in higher thermal stability of the compounds. Medhat et al. [55] reported that the addition of 5% MA can substantially increase the initial decomposition temperature of NR from 181 to 237 ◦C for NR/RTR/MA (30/70/5).

3.3.6. Dynamic Mechanical Properties Dynamic mechanical analysis (DMA) can provide information about the loss modulus (E”), storage modulus (E’) and damping factor (tan δ = E”/E’), which are essential to understand the viscoelastic behavior of rubber compounds. The loss and storage moduli are related to the maximum heat dissipation per unit deformation and the maximum energy that can be stored in a period of time reflecting the degree of elasticity/rigidity (crosslinking density), respectively. The value of tan δ is an indication of the ability of rubber compounds to absorb and diffuse energy [3]. As reported before, increasing the GTR/RTR content in a rubber matrix leads to higher crosslink density and carbon black content, as well as further chain mobility restriction leading to increased rigidity and storage modulus of the compounds. This is why Li et al. [126] reported lower storage modulus for RTR/SBR than GTR/SBR, which was attributed to the lower crosslink density of RTR (named DGTR) (Figure 36).

Figure 36. Storage modulus as a function of temperature for SBR/GTR and SBR/DGTR at 30 phr (reproduced with permission from [126]).

Also, the loss modulus of rubber compounds increase with increasing GTR/RTR content, indicating enhanced viscoelasticity of the compounds with further filler content. The tan δ peak corresponds to the glass transition temperature (Tg), which is related to the mobility of pendant groups and molecular chains in the rubber. Similar to the effect of GTR loading on E’ and E”, the tan δ peak increases with increasing GTR content because of higher crosslink density and the presence of carbon black limiting chain mobility. Lower intensity of the tan δ peak with increasing GTR content implies improved compound elasticity. So the molecular chain motion needs lower energy as it approaches the transition from a glassy to a rubbery state. Li et al. [126] reported higher elasticity of RTR compounds compared to GTR ones as determined by the lower peak height of the former.

3.3.7. Swelling Properties The swelling degree of rubber compounds represents the sorption and desorption behavior of a solvent, such as toluene or cyclohexane, to determine the crosslink density of the sample. The swelling degree (Q) of rubber compounds is measured at equilibrium in a solvent as:

mt m Q = − 0 100% (3) m0 ×

1

J. Compos. Sci. 2020, 4, 103 32 of 43

where mt and m0 are the mass of the swollen sample (g) and its initial mass (g), respectively. Also, the Flory–Rehner equation is used to evaluate the crosslink density as [91]:

1 ! 2 3 2Vr ln(1 Vr) Vr XV = 2Vsη V (4) − − − − r swell r − f where Vr and Vs are the rubber volume fraction in the swollen gel and the molar volume of the toluene 3 (106.2 cm /mol), respectively. X is the rubber-solvent interaction parameter, while ηswell is the crosslink densityJ. ofCompos. the Sci. rubber 2020, 4,(mol x /cm3) and f is the functionality of the crosslinks (f = 4 in sulphur33 of 45 curing system). Equation (5) is used to determine the volume fraction of a rubber network in the swollen phase: system). Equation (5) is used to determine the volume fraction of a rubber network in the swollen   phase: W2 d V = 𝑊2 r  W    (5) 1 𝑑+ W2 𝑉 = d d 𝑊1 𝑊2 (5) + 𝑑 𝑑 where W1 and W2 are the weight fraction of the solvent and the polymer in the swollen sample respectively,where whileW1 andd 1Wand2 ared 2theare weight the density fraction ofof thethe solventsolvent and the the polymer polymer, in respectively.the swollen sample GTRrespectively, is composed while of d the1 and sol d2 andare the gel density fractions of the formed solvent by and free the chains polymer, (uncrosslinked) respectively. and crosslinked chains, respectively.GTR is composed The swelling of the degree sol and of gel a rubber fractions compound formed by indicates free chains its state(uncrosslinked) of cure [132 and]. So the crosslinked chains, respectively. The swelling degree of a rubber compound indicates its state of cure incorporation of GTR results in increased crosslink density of the compounds due to the presence of [132]. So the incorporation of GTR results in increased crosslink density of the compounds due to the active crosslinkingpresence of sitesactive in crosslinking the GTR to formsites in further the GT crosslinkingR to form duringfurther mixing.crosslinking Kumnuantipa during mixing. et al. [133] reportedKumnuantipa a fast increase et al. of [133] the reported toluene a uptake fast increase in NR of/RTR the toluene before uptake reaching in NR/RTR a plateau before (equilibrium reaching a state). Figure 37plateau shows (equilibrium that compounds state). Figure filled 37 with shows 80 wt.%that compounds RTR reached filled an with equilibrium 80 wt.% RTR faster reached than lessan filled samplesequilibrium because they faster have than theless lowestfilled samples toluene becaus uptake.e they Other have the works lowest reported toluene uptake. a similar Other result works of higher GTR contentreported needing a similar less result time of high for theer GTR rubber content compounds needing less to time reach for anthe equilibriumrubber compounds state to while reachhaving an equilibrium state while having a lower swelling degree [134]. So an inverse relation between the a lower swelling degree [134]. So an inverse relation between the swelling degree and the crosslink swelling degree and the crosslink density exists as lower swelling degree implies higher crosslink densitydensity. exists as lower swelling degree implies higher crosslink density.

FigureFigure 37. Eff 37.ect Effect of RTR of RTR content content on on the the toluene toluene sorptionsorption (25 (25 °C)◦C) and and desorption desorption (70 °C) (70 curves◦C) curves of of RTR/NR(STRVS60)RTR/NR(STRVS60) compounds compounds (reproduced (reproduced with with permission permission from from [133]). [133 ]).

SolventSolvent penetration penetration through throug vulcanizedh vulcanized rubbers rubbers with high high cros crosslinkslink density density and restricted and restricted chain chain mobilitymobility is very is diveryfficult. difficult. However, However, a swellinga swelling experimentexperiment must must be be performed performed until until an equilibrium an equilibrium state is reached. Therefore the swelling ratio does not depend on the kinetics of solvent molecule state ispenetration, reached. Therefore but on the the length swelling of chain ratio segments does notbetween depend crosslink on the points; kinetics i.e., ofthe solvent amount moleculeof penetration,crosslink but points on the per length unit volume of chain (crosslink segments density) between and the crosslink polymer-solvent points; i.e., interaction the amount parameter. of crosslink points perFigure unit 38 volumepresents a (crosslink comparison density) between andGTR theand polymer-solventRTR on the swelling interaction degree of NR parameter. compounds. Figure It 38 presentscan a comparisonbe seen that RTR between induces GTR a higher and RTRswelling on thedegr swellingee compared degree to GTR, of NR which compounds. is attributed It to can the be seen lower crosslink density of the partially regenerated RTR [124].

J. Compos. Sci. 2020, 4, 103 33 of 43 that RTR induces a higher swelling degree compared to GTR, which is attributed to the lower crosslink J.density Compos. Sci. of the2020 partially, 4, x regenerated RTR [124]. 34 of 45

Figure 38. Effect of GTR and RTR content on the swelling ratio of NR compounds soaked in toluene at Figure 38. Effect of GTR and RTR content on the swelling ratio of NR compounds soaked in toluene room temperature for 72 h (reproduced with permission from [124]). at room temperature for 72 h (reproduced with permission from [124]). Data analysis of the swelling curves can be used to determine the transport mechanism inside Data analysis of the swelling curves can be used to determine the transport mechanism inside the rubber compounds. For example, the solvent mass absorbed (Mt) as a function of time (t) and the thevalue rubber at equilibrium compounds. (M For) canexample, be related the solvent via [135 mass]: absorbed (Mt) as a function of time (t) and the ∞ value at equilibrium (𝑀) can be related via [135]: Mt n 𝑀 = Kt (6) M ∞=𝐾𝑡 (6) 𝑀 or  M  or log t = log K + n log t (7) M ∞ where the slope (n) and intercept (K) are obtained𝑀 by linear regression. The value of K is a constant log = log𝐾 + 𝑛log𝑡 (7) associated to rubber/solvent interaction, while𝑀 the value of n is an indication of the solvent diffusion mode through the rubber blends, which is normally between 0.5 and 1 [135]. A value of n = 0.5 where the slope (n) and intercept (K) are obtained by linear regression. The value of K is a constant indicates a Fickian mode of transport where the diffusion coefficients are functions of concentration associated to rubber/solvent interaction, while the value of n is an indication of the solvent diffusion alone. When n = 1, the mechanism is non-Fickian, Case II (relaxation controlled) transport, while a mode through the rubber blends, which is normally between 0.5 and 1 [135]. A value of n = 0.5 value between 0.5 and 1 represents an anomalous transport behavior. The values of n and K for rubber indicates a Fickian mode of transport where the diffusion coefficients are functions of concentration vulcanizates can be obtained from a plot of log(Qt/Q ) against log(t)[136]. alone. When n = 1, the mechanism is non-Fickian, Case∞ II (relaxation controlled) transport, while a Kumnuantip et al. [133] reported that K increases with increasing RTR content in NR/RTR value between 0.5 and 1 represents an anomalous transport behavior. The values of n and K for rubber compounds because of higher rubber/solvent interaction associated to the increased crosslink density vulcanizates can be obtained from a plot of log(Qt/Q∞) against log(t) [136]. with RTR addition (Table7). However, increased interaction can lead to more contacts between the Kumnuantip et al. [133] reported that K increases with increasing RTR content in NR/RTR rubber and solvent and faster time to equilibrium. So fewer possibilities are provided for the solvent compounds because of higher rubber/solvent interaction associated to the increased crosslink density molecules to penetrate the rubber structure, decreasing its swelling. The n values slightly decreased with RTR addition (Table 7). However, increased interaction can lead to more contacts between the with increasing RTR content attributed to a more restricted diffusion of the solvent into the rubber rubber and solvent and faster time to equilibrium. So fewer possibilities are provided for the solvent matrix resulting in a lower degree of swelling. molecules to penetrate the rubber structure, decreasing its swelling. The n values slightly decreased Finally, to provide the reader with a general overview of the current literature, Table8 presents with increasing RTR content attributed to a more restricted diffusion of the solvent into the rubber a list of the works performed on GTR/RTR containing rubber compounds. Based on the results matrix resulting in a lower degree of swelling. summarized in Table8, the development of GTR /RTR containing rubber compounds is a promising approach for waste tire recycling for low cost and negligible environmental impact products, as well as Table 7. Values of n and K (Equation (5)) for RTR/NR blends (compounding by a two-roll mill at 25 achieving good (comparable) properties with virgin rubber compounds. °C) (reproduced with permission from [133]).

STRVS60 STR20CV RTR content (%) n K × 102 (g/g minn) n K × 102 (g/g minn) 0 0.40 1.44 0.39 1.39 20 0.34 1.58 0.38 1.47 40 0.35 1.65 0.34 1.52 60 0.32 1.81 0.32 1.63 80 0.27 2.07 0.28 1.93

J. Compos. Sci. 2020, 4, 103 34 of 43

Table 7. Values of n and K (Equation (5)) for RTR/NR blends (compounding by a two-roll mill at 25 ◦C) (reproduced with permission from [133]).

STRVS60 STR20CV RTR Content (%) n K 102 (g/g minn) n K 102 (g/g minn) × × 0 0.40 1.44 0.39 1.39 20 0.34 1.58 0.38 1.47 40 0.35 1.65 0.34 1.52 60 0.32 1.81 0.32 1.63 80 0.27 2.07 0.28 1.93 J. Compos. Sci. 2020, 4, 103 35 of 43

Table 8. Effects of GTR/RTR treatment, composition, regeneration, and compounding on the properties of rubber compounds.

GTR/RTR Compound Mixing Results Ref. Content NR regeneration by diallyl disulfide (DADS) at RTR (25, 40, 50 60 C for 35 min on an open two-roll mixing mill. Increasing RTR content resulted in decreased scorch time and optimum cure time. Addition NR/RTR ◦ [120] and 60 phr) Compounding in a laboratory size two-roll mill of N-cyclohexylthiophthalimide as PVI increased the scorch time of NR/RTR (50/50) blend. based on ASTM D 15-54T (1954). MA grafting on RTR at 150 C in a Brabender ◦ MA modification of RTR led to longer cure time, scorch time and lower cure rate in all blends RTR (20, 40, 60 Plasticorder at 30 rpm for 3 min. Blends NBR/RTR since anhydrides grafted on the RTR surface as cure retarders delay the onset of cure reaction [115] and 80 phr) preparation on a laboratory size two-roll mill resulting in higher cure times. based on ASTM D 3182. 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 Regeneration of GRT by tetramethyl thiuram as lower optimum cure time related to the presence of active crosslinking sites in RTR. RTR (20, 30, 40, disulfide (TMTD) in the presence of spindle oil. NR/PBR/RTR Increase in tensile modulus with increasing RTR content attributed to the higher gel fraction [121] 50 and 60 phr) Mixing of NR/PBR/RTR compounds for 15 min via and crosslink density of the compounds. Also, increase in 200% moduli retention of laboratory size two-roll mill at room temperature. NR/PBR/RTR with increasing RTR loading after aging attributed to higher crosslink density and the formation of new crosslinks due to the presence of active sites in RTR. NR mastication on a laboratory two-roll mill for 10, Shear viscosity increased with RTR content, but decreased with mastication time. Increase in RTR (20, 40, 60 NR/RTR 20 or 30 min followed by blending with RTR for tensile modulus with RTR loading due to the variation of crosslink density and chain mobility [122] and 80 phr) 10 min at 25 ◦C. of RTR, as well as better blends homogeneity. Increasing trend of elongation at break with RTR incorporation attributed to a plasticization RTR (20, 40, 60 Compound mixing according to ASTM D 3182 on effect of the processing oil in RTR. NBR/RTR [116] and 80 phr) a two-roll laboratory size mixing mill. Decrease in aging resistance of the blends after RTR addition as it contains NR which is more prone to degradation during mixing. Microbial desulfurization of the GTR led to increased GTR sol fraction from 4.69-8.68%. Higher storage modulus and rigidity of SBR/GTR compounds than SBR/RTR compounds SBR/GTR and GTR and RTR (10, SBR mastication on a two-roll mill and blending attributed to the higher crosslink density, carbon black content and chain mobility restriction [126] SBR/RTR 20, and 30) phr) with various contents of GTR or RTR for 10 min. of GTR. Higher elasticity of RTR compounds determined by the lower tan δ peak height of RTR. Also, better wettability of RTR due to more compatibility and good interfacial interactions between RTR and SBR. Lower scorch time and optimum cure time of NR/RTR blends compared to NR/GTR blends due to more unsaturated rubber in the RTR. GTR regeneration via a pan-mill type NR/RTR GTR and RTR (10, Higher tensile strength and elongation at break of NR/RTR blends because of better mechano-chemical reactor for 25 cycles. [124] NR/GTR 30 and 50 phr) compatibility and homogeneity of the blends. Blending with NR using a two-roll mill. Also, RTR induced a higher degree of swelling compared to GTR attributed to the lower crosslink density of the partially regenerated RTR. J. Compos. Sci. 2020, 4, 103 36 of 43

Table 8. Cont.

GTR/RTR Compound Mixing Results Ref. Content

GTRcar and GTRtruck were exposed to microwave Better thermal stability of NR/GTRcar than NR/GTRtruck attributed to the higher content of irradiation for 0-10 min. Preparation of NR/GTR synthetic rubbers in GTRcar and also the evaporation of low MW volatile products formed NR/GTR GTR (50 phr) blends at 70 ◦C using an internal batch mixer with during the thermal degradation of GTR. [131] rotational speed of 100 rpm. Improvement of tensile properties of modified GTR containing blends due to improved interfacial interaction confirmed by SEM images. GTR regeneration in a microwave reactor for 3, 5 Microwave modification of GTR enhanced the tensile properties due to good interaction and 10 min with a power of 700 W. Blends NBR/NR/GTR between the RTR and the matrix. preparation at 70 C using an internal batch mixer and GTR (50 phr) ◦ GTR microwaves treatment resulted in an improvement of the crosslinking and hence better [70] with a rotational speed of 100 rpm. The curing NBR/NR/RTR thermal stability of the blends after revulcanization, but microwave radiation for more than system was based on oxide, stearic acid, TBBS, 5 min led to the degradation of the GTR main chains. TMTD and sulphur. Increase in minimum torque and Mooney viscosity of the SBR/RTR compounds with GRT regeneration by tetramethyl thiuram disulfide increasing RTR content, while the scorch time remained unchanged. RTR (20, 30, 40, (TMTD) in the presence of spindle oil in an open RTR addition led to increase in tensile strength by about 19% for 20 parts filler and 115% for SBR/RTR [128] 50 and 60 phr) two-roll mill. Mixing of fresh SBR and RTR on an 60 parts RTR in compounds. Enhanced thermal stability of SBR/RTR compounds with RTR open two-roll mill at room temperature for 15 min. incorporation as char residue of SBR increased from 5.3% to 22.6% with the addition of 60 parts RTR. The plasma treatment modification of GTR was The water contact angle of the modified GTR decreased from 116 to 0 after 10 s of irritation carried out before mixing with NBR for 2 min with ◦ ◦ GTR (5, 10, 15 inducing a hydrophilic nature to the modified GTR. NBR/GTR a plasma discharge power in range of 30 to 80 W. [19] and 20 phr) The TS and tear strength of NBR/GTR improved by 42% and 21% respectively, due to NBR/GTR compounding was performed on a increased interfacial bonding between the plasma modified GTR (20 wt.%) and NBR matrix. two-roll mill for 10 min. The SBR/RTR blends were prepared in a laboratory The tensile strength of SBR/RTR compounds increased from 1.9 MPa to 5.1 MPa with the RTR (20, 40, 60 size two-roll mill and vulcanization was carried addition of 80 parts RTR. SBR/RTR 2 [117] and 80 phr) out at 150 ◦C and 180 kg/cm using an electrically Also, the elongation at break of SBR increased from 506% to 629% at 80 parts of RTR due to a heated hydraulic press. plasticization effect caused by the presence of the processing oil in RTR. NR masticating was performed on a laboratory two-roll mill for 10 min followed by the addition The swelling degree of the compounds at the equilibrium state decreased with RTR content RTR (20, 40, 60 of RTR and further mastication for 2 min. due to the increased crosslink density and the polymer–solvent interaction. NR/RTR [133] and 80 phr) NR compounding with RTR was performed on a Also, increasing the RTR content led to a higher elastic behavior (reduced tan δmax) and Tg two-roll mill for 8 min at 25 ◦C. attributed to the increase in crosslink density and the presence of carbon black in RTR.

GTR surface regeneration was performed using a GTR regeneration resulted in 30% increase of oxygen content on the GTR surface and a biological treatment by Thiobacillus sp. NR was GTR and RTR (0, reduction of the GTR contact angle from 120.5 to 93.5 (RTR) after regeneration. NR/GTR and masticated on a two-roll mill and then blended ◦ ◦ 10, 20, 30 and NR/RTR compounds showed better compatibility and co-crosslinking at the interphase [127] NR/RTR with GTR or RTR for 10 min followed by 40 phr) between RTR and virgin NR leading to better tensile properties. Addition of 10 phr RTR in vulcanization in a press at 15 MPa and 150 C ◦ the compounds retained 91% and 92% of their original TS and EB, respectively. according to ASTM D 2084. J. Compos. Sci. 2020, 4, 103 37 of 43

4. Conclusions Based on the current environmental situation and social acceptance, discarded tires should no longer be seen as a pollutant and useless waste material, but rather as a durable and inexpensive raw material for the production of different and innovative parts. The incorporation of even a small fraction of waste tires into polymer matrices (thermosets, thermoplastics and rubbers) can lead to a substantial consumption of discarded tires as a partial replacement of virgin raw materials with advantages such cost reduction and sustainable compound production. Waste tires can be finely shredded to obtain ground tire rubber (GTR) particles with higher specific surface area to improve the interaction with the corresponding matrices. GTR particle size and surface topography depend on the residence time/number cycles and temperature of the grinding process such as ambient, wet or cryogenic methods. In general, GTR particles obtained from an ambient grinding process have higher surface roughness and higher specific surface area promoting better bonding with polymer matrices. However, the GTR particles are incompatible with most polymer matrices due to a lack of sufficient chain mobility and interaction with the corresponding matrix limiting the use of high GTR concentrations (above 50 wt.%). To address this issue and promote smooth stress transfer between the blend components, GTR surface modification (compatibilization) can be used to decrease the surface tension, suppress droplet coalescence and obtain uniform GTR dispersion in the polymer matrix. Physical surface treatments were developed to increase the surface roughness and wetting properties by introducing polar (oxygen) functional groups on the GTR surface to better interact with polar polymers. Chemical surface modification can also be applied to improve the blend compatibility through in situ generation of a compatibilizer during mixing or by the addition of block/graft copolymers and nanoparticles to modify the interface. Moreover, GTR regeneration is extensively used to partially breakdown the crosslinked structure of vulcanized waste tires to increase the chain mobility (molecular freedom) of GTR for better interaction with the polymer matrix molecules. It can be concluded that a regeneration process can produce a partially vulcanized rubber (RTR) with lower crosslink density and more chain mobility, as well as physico-chemical properties similar to that of a virgin rubber depending on the regeneration process. The addition of GTR/RTR in rubber matrices results in changes of the curing, rheological, mechanical, aging, thermal, dynamic mechanical and swelling properties of rubber compounds. The introduction of RTR into a rubber formulation results in improved flowability and fewer agglomerates in the rubber matrix compared to GTR addition, hence decreasing the compound viscosity and the minimum torque, leading to better processability. Considering the mechanical properties, increasing the GTR concentration in compounds decreases the tensile strength and elongation at break because of higher gel fractions acting as stress concentrating points. Since waste tires are aged recycled materials exposed to severe conditions (chemical, mechanical, physical and thermal degradation) during their lifetime and recycling processes, the tensile strength of the compounds changes after aging. The thermal stability of the compounds filled with GTR/RTR is still an active research area requiring more attention. The addition of GTR/RTR results in enhanced thermal stability of the rubber compounds due to the barrier effect of carbon black in waste tire formulation. Based on dynamic mechanical analysis, the rigidity and storage modulus of the compounds increase with GTR/RTR loading. It should be mentioned that regeneration leads to lower storage modulus of RTR/rubber compounds compared to that of GTR/rubber ones attributed to the lower crosslink density of RTR. The swelling behavior of the rubber compounds is directly related to the degree of crosslinking. It is concluded that RTR particles show higher swelling degree compared to that of GTR particles attributed to the lower crosslink density of partially regenerated RTR particles and less active sites for crosslinking during mixing. So higher free volume is available for the solvent molecules to enter and diffuse. It is expected that in the near future, material recycling in general, and waste tires in particular, will attract more attention for research projects and government investments as a promising approach to improve the circular economy and sustainability of rubber tires. J. Compos. Sci. 2020, 4, 103 38 of 43

Depletion of natural resources like virgin/natural rubber consumed by the tire industries is expected to accelerate the research efforts and industrial dedication towards waste tire recycling for potential applications in artificial reef, erosion control, breakwaters, floatation devices, mats, playground surfaces and athletic tracks. As stated earlier, waste tires (GTR and RTR) can be used as a source of valuable raw materials in different polymeric matrices for the manufacture of low cost products and a cleaner environment. However, a complete understanding of the phenomena involved in bond break-up mechanisms and the interaction at the interphase of GTR/RTR and virgin rubber are critical steps to control the processing conditions and optimize the final compounds properties. In the near future, more research should be directed to improve the interfacial interaction between all the components. Also, GTR surface modification and compounding/mixing processes should be more environmentally friendly and cost effective following the concepts of green chemistry. Based on the review performed, there is a clear lack of literature about mixing GTR/RTR with virgin rubber, especially for other matrices than NR. This should be the focus for more research on the preparation and characterization of such rubber compounds.

Author Contributions: Conceptualization, D.R.; formal analysis, A.F.; investigation, A.F.; resources, D.R.; data curation, D.R.; writing—original draft preparation, A.F.; writing—review and editing, D.R.; supervision, D.R.; project administration, D.R.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. McKeen, L.W. The Effect of Long Term Thermal Exposure on and Elastomers; William Andrew: Norwich, NY, USA, 2014. 2. Nuzaimah, M.; Sapuan, S.M.; Nadlene, R.; Jawaid, M. Recycling of waste rubber as fillers: A review. In Proceedings of the IOP Conference Series: Materials Science and Engineering, Selangor, Malaysia, 21–23 November 2017; Volume 368, p. 012016. 3. Rubber, S.N. Natural rubber and reclaimed Rubber composites–A Systematic Review. Polymer 2016, 2, 7. 4. Fiksel, J.; Bakshi, B.R.; Baral, A.; Guerra, E.; DeQuervain, B. Comparative life cycle assessment of beneficial applications for scrap tires. Clean Technol. Environ. Policy 2010, 13, 19–35. [CrossRef] 5. De Sadhan, K.; Isayev, A.; Khait, K. (Eds.) Rubber Recycling; CRC Press: Boca Raton, FL, USA, 2005. 6. Sienkiewicz, M.; Kucinska-Lipka, J.; Janik, H.; Balas, A. Progress in used tyres management in the European Union: A review. Waste Manag. 2012, 32, 1742–1751. [CrossRef][PubMed] 7. Khan, S.R.; Zeeshan, M.; Masood, A. Enhancement of hydrocarbons production through co-pyrolysis of acid-treated biomass and waste tire in a fixed bed reactor. Waste Manag. 2020, 106, 21–31. [CrossRef] [PubMed] 8. Banar, M.; Akyıldız, V.; Ozkan, A.; Cokaygil, Z.; Onay, O. Characterization of pyrolytic oil obtained from pyrolysis of TDF (Tire Derived Fuel). Energy Convers. Manag. 2012, 62, 22–30. [CrossRef] 9. Shu, X.; Huang, B. Recycling of waste tire rubber in asphalt and portland cement concrete: An overview. Constr. Build. Mater. 2014, 67, 217–224. [CrossRef] 10. Ge, D.; Yan, K.; You, Z.; Xu, H. Modification mechanism of asphalt binder with waste tire rubber and recycled polyethylene. Constr. Build. Mater. 2016, 126, 66–76. [CrossRef] 11. Yung, W.H.; Yung, L.C.; Lee, H.H. A study of the durability properties of waste tire rubber applied to self-compacting concrete. Constr. Build. Mater. 2013, 41, 665–672. [CrossRef] 12. Aiello, M.; Leuzzi, F. Waste tyre rubberized concrete: Properties at fresh and hardened state. Waste Manag. 2010, 30, 1696–1704. [CrossRef] 13. Kakroodi, A.R.; Rodrigue, D. Highly filled thermoplastic elastomers from ground tire rubber, maleated polyethylene and high density polyethylene. Plast. Rubber Compos. 2013, 42, 115–122. [CrossRef] 14. Wang, Y.-H.; Chen, Y.-K.; Rodrigue, D. Production of Thermoplastic Elastomers Based on Recycled PE and Ground Tire Rubber: Morphology, Mechanical Properties and Effect of Compatibilizer Addition. Int. Polym. Process. 2018, 33, 525–534. [CrossRef] J. Compos. Sci. 2020, 4, 103 39 of 43

15. Araujo-Morera, J.; Hernández, M.; Verdejo, R.; López-Manchado, M.A. Giving a Second Opportunity to Tire Waste: An Alternative Path for the Development of Sustainable Self-Healing Styrene–Butadiene Rubber Compounds Overcoming the Magic Triangle of Tires. Polymers 2019, 11, 2122. [CrossRef][PubMed] 16. Ramarad, S.; Khalid, M.; Ratnam, C.T.; Abdullah, L.; Rashmi, W. Waste tire rubber in polymer blends: A review on the evolution, properties and future. Prog. Mater. Sci. 2015, 72, 100–140. [CrossRef] 17. Shaker, R.; Rodrigue, D. Rotomolding of Thermoplastic Elastomers Based on Low-Density Polyethylene and Recycled Natural Rubber. Appl. Sci. 2019, 9, 5430. [CrossRef] 18. Moghaddamzadeh, S.; Rodrigue, D. The effect of polyester recycled tire fibers mixed with ground tire rubber on polyethylene composites. Part II. Prog. Rubber Plast. Recycl. Technol. 2018, 34, 128–142. [CrossRef] 19. Zhang, X.; Zhu, X.; Liang, M.; Lu, C. Improvement of the properties of ground tire rubber (GTR)-filled nitrile rubber vulcanizates through plasma surface modification of GTR powder. J. Appl. Polym. Sci. 2009, 114, 1118–1125. [CrossRef] 20. Aggour, Y.A.; Al-Shihri, A.S.; Bazzt, M.R. Surface Modification of Waste Tire by Grafting with Styrene and Maleic Anhydride. Open J. Polym. Chem. 2012, 2, 70–76. [CrossRef] 21. Garcia, P. Devulcanization of ground tire rubber: Physical and chemical changes after different microwave exposure times. Express Polym. Lett. 2015, 9, 1015–1026. [CrossRef] 22. Esmizadeh, E.; Bakhshandeh, G.R.; Fasaie, M.R.; Ahmadi, S.; Naderi, G. Reactively compatibilized and dynamically vulcanized thermoplastic elastomers based on high-density polyethylene and reclaimed rubber. Polym. Sci. Ser. B 2017, 59, 362–371. [CrossRef] 23. Wang, L.; Lang, F.; Li, S.; Du, F.; Wang, Z. Thermoplastic elastomers based on high-density polyethylene and waste ground rubber tire composites compatibilized by styrene–butadiene block copolymer. J. Thermoplast. Compos. Mater. 2013, 27, 1479–1492. [CrossRef] 24. Thomas, B.S.; Gupta, R.C. A comprehensive review on the applications of waste tire rubber in cement concrete. Renew. Sustain. Energy Rev. 2016, 54, 1323–1333. [CrossRef] 25. Mohajerani, A.; Burnett, L.; Smith, J.V.; Markovski, S.; Rodwell, G.; Rahman, T.; Kurmus, H.; Mirzababaei, M.; Arulrajah, A.; Horpibulsuk, S.; et al. Recycling waste rubber tyres in construction materials and associated environmental considerations: A review. Resour. Conserv. Recycl. 2020, 155, 104679. [CrossRef] 26. Sathiskumar, C.; Karthikeyan, S. Recycling of waste tires and its energy storage application of by-products —A review. Sustain. Mater. Technol. 2019, 22, e00125. [CrossRef] 27. Fazli, A.; Rodrigue, D. Waste Rubber Recycling: A Review on the Evolution and Properties of Thermoplastic Elastomers. Materials 2020, 13, 782. [CrossRef] 28. Bockstal, L.; Berchem, T.; Schmetz, Q.; Richel, A. Devulcanisation and reclaiming of tires and rubber by physical and chemical processes: A review. J. Clean. Prod. 2019, 236, 117574. [CrossRef] 29. Shulman, V. Chapter 21-Tyre Recycling; Waste, Academic Press: Boston, MA, USA, 2011; pp. 297–320. 30. Evans, A.; Evans, R. The Composition of a Tyre: Typical Components; The Waste & Resources Action Programme: Banbury, UK, 2006; p. 5. 31. Shulman, V.L. Tire Recycling. Waste 2019, 489–515. [CrossRef] 32. Akiba, M. Vulcanization and crosslinking in elastomers. Prog. Polym. Sci. 1997, 22, 475–521. [CrossRef] 33. Vergnaud, J.-M.; Rosca, I.-D. Rubber Curing and Properties; CRC Press: Boca Raton, FL, USA, 2016. 34. Morin, J.E.; Farris, R.J. Recycling of 100% Cross Linked Rubber Powder by High Temperature High Pressure Sintering. Encycl. Polym. Eng. 2000, 37, 95–101. 35. Association, United States Tire Manufacturers, U.S. Scrap Tire Disposition 2017. Available online: https: //www.ustires.org/ (accessed on 30 July 2020). 36. Imbernon, L.; Norvez, S. From landfilling to vitrimer chemistry in rubber life cycle. Eur. Polym. J. 2016, 82, 347–376. [CrossRef] 37. Czajczynska, D.; Krzy˙zy´nska,R.; Jouhara, H.; Spencer, N. Use of pyrolytic gas from waste tire as a fuel: A review. Energy 2017, 134, 1121–1131. [CrossRef] 38. Karger-Kocsis, J.; Mészáros, L.; Bárány, T. Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers. J. Mater. Sci. 2012, 48, 1–38. [CrossRef] 39. Zebala, J.; Ciepka, P.; Reza, A.; Janczur, R. Influence of rubber compound and tread pattern of retreaded tyres on vehicle active safety. Forensic Sci. Int. 2007, 167, 173–180. [CrossRef][PubMed] 40. Lebreton, B.; Tuma, A. A quantitative approach to assessing the profitability of car and truck tire remanufacturing. Int. J. Prod. Econ. 2006, 104, 639–652. [CrossRef] J. Compos. Sci. 2020, 4, 103 40 of 43

41. Gieré, R.; Smith, K.; Blackford, M. Chemical composition of fuels and emissions from a coal+tire combustion experiment in a power station. Fuel 2006, 85, 2278–2285. [CrossRef] 42. Tsang, H.-H. Uses of scrap rubber tires. In Rubber: Types, Properties and Uses; Nova Science Publisher: New York, NY, USA, 2013; pp. 477–491. 43. Saha, P. Rubber Recycling: Challenges and Developments; Royal Society of Chemistry: London, UK, 2018. 44. Oriaku, E. Waste to wealth through the incineration of waste tyres and recovery of carbon black. Int. J. Multidiscip. Sci. Eng. 2013, 4, 30–36. 45. Singh, S.; Nimmo, W.; Gibbs, B.; Williams, P. Waste tyre rubber as a secondary fuel for power plants. Fuel 2009, 88, 2473–2480. [CrossRef] 46. Yu,Q. Application of nanomaterials in alkali-activated materials. In Nanotechnology in Eco-Efficient Construction; Elsevier: Amsterdam, The Netherlands, 2019; pp. 97–121. 47. Gnanaraj, J.S.; Lee, R.J.; Levine, A.M.; Wistrom, J.L.; Wistrom, S.L.; Li, Y.; Li, J.; Akato, K.; Naskar, A.K.; Paranthaman, M. Sustainable Waste Tire Derived Carbon Material as a Potential Anode for Lithium-Ion Batteries. Sustainability 2018, 10, 2840. [CrossRef] 48. Navarro, F. Thermo-rheological behaviour and storage stability of ground tire rubber-modified bitumens. Fuel 2004, 83, 2041–2049. [CrossRef] 49. Medina, N.F.; Garcia, R.; Hajirasouliha, I.; Pilakoutas, K.; Guadagnini, M.; Raffoul, S. Composites with recycled rubber aggregates: Properties and opportunities in construction. Constr. Build. Mater. 2018, 188, 884–897. [CrossRef] 50. Benazzouk, A.; Douzane, O.; Langlet, T.; Mezreb, K.; Roucoult, J.; Quéneudéc, M. Physico-mechanical properties and water absorption of cement composite containing shredded rubber wastes. Cem. Concr. Compos. 2007, 29, 732–740. [CrossRef] 51. Oikonomou, N.; Mavridou, S. Improvement of chloride ion penetration resistance in cement mortars modified with rubber from worn automobile tires. Cem. Concr. Compos. 2009, 31, 403–407. [CrossRef] 52. Hejna, A.; Korol, J.; Przybysz-Romatowska, M.; Zedler, Ł.; Chmielnicki, B.; Formela, K. Waste tire rubber as low-cost and environmentally-friendly modifier in thermoset polymers—A review. Waste Manag. 2020, 108, 106–118. [CrossRef] 53. Sunthonpagasit, N.; Duffey, M.R. Scrap tires to crumb rubber: Feasibility analysis for processing facilities. Resour. Conserv. Recycl. 2004, 40, 281–299. [CrossRef] 54. Fernández, A.; Barriocanal, C.; Alvarez, R. Pyrolysis of a waste from the grinding of scrap tyres. J. Hazard. Mater. 2012, 203, 236–243. [CrossRef] 55. Hassan, M.M.; Mahmoud, G.A.; Hegazy, E.-S.A.; El-Nahas, H.H. Reinforced material from reclaimed rubber/natural rubber, using electron beam and thermal treatment. J. Appl. Polym. Sci. 2007, 104, 2569–2578. [CrossRef] 56. Dierkes, W. Untreated and treated rubber powder. In Rubber Recycling; CRC Press: Boca Raton, FL, USA, 2005; pp. 151–175. 57. Darrel, R., Sr. Process for Recycling Vehicle Tires. U.S. Patent 5,115,983, 26 May 1992. 58. Dufton, P. End-Of-Life Tyres: Exploiting Their Value; iSmithers Rapra Publishing: Akron, OH, USA, 2001. 59. Adhikari, J.; Das, A.; Sinha, T.K.; Saha, P.; Kim, J.K. Grinding of Waste Rubber. Green Chem. Ser. 2018, 59, 1–23. 60. Kumar, C.; Fuhrmann, I.; Karger-Kocsis, J. LDPE-based thermoplastic elastomers containing ground tire rubber with and without dynamic curing. Polym. Degrad. Stab. 2002, 76, 137–144. [CrossRef] 61. Sonnier, R.; Leroy, E.; Clerc, L.; Bergeret, A.; Lopez-Cuesta, J. Polyethylene/ground tyre rubber blends: Influence of particle morphology and oxidation on mechanical properties. Polym. Test. 2007, 26, 274–281. [CrossRef] 62. Yehia, A.; Mull, M.A.; Ismail, M.N.; Hefny, Y.A.; Abdel-Bary, E.M. Effect of chemically modified waste rubber powder as a filler in natural rubber vulcanizates. J. Appl. Polym. Sci. 2004, 93, 30–36. [CrossRef] 63. Colom, X.; Cañavate, J.; Carrillo, F.; Velasco, J.I.; Pagès, P.; Mujal, R.; Nogués, F. Structural and mechanical studies on modified reused tyres composites. Eur. Polym. J. 2006, 42, 2369–2378. [CrossRef] 64. Martínez-Barrera, G.; Del Coz-Díaz, J.J.; Álvarez-Rabanal, F.P.; Gayarre, F.L.; Martínez-López, M.; Cruz-Olivares, J. Waste tire rubber particles modified by gamma radiation and their use as modifiers of concrete. Case Stud. Constr. Mater. 2020, 12, e00321. [CrossRef] J. Compos. Sci. 2020, 4, 103 41 of 43

65. Feng, W.; Isayev, A.I. Recycling of tire-curing bladder by ultrasonic devulcanization. Polym. Eng. Sci. 2005, 46, 8–18. [CrossRef] 66. Seghar, S.; Asaro, L.; Rolland-Monnet, M.; Hocine, N. Thermo-mechanical devulcanization and recycling of rubber industry waste. Resour. Conserv. Recycl. 2019, 144, 180–186. [CrossRef] 67. Fan, P.; Lu, C. A Study on Functionalization of Waste Tire Rubber Powder Through Ozonization. J. Polym. Environ. 2011, 19, 943–949. [CrossRef] 68. Maris, J.; Bourdon, S.; Brossard, J.-M.; Cauret, L.; Fontaine, L.; Montembault, V. Mechanical recycling: Compatibilization of mixed thermoplastic wastes. Polym. Degrad. Stab. 2018, 147, 245–266. [CrossRef] 69. Zhang, X.; Lu, C.; Liang, M. Preparation of rubber composites from ground tire rubber reinforced with waste-tire fiber through mechanical milling. J. Appl. Polym. Sci. 2006, 103, 4087–4094. [CrossRef] 70. Cañavate, J. Influence of microwave treatment conditions of GTR on physico-mechanical and structural properties of NBR/NR/GTR composites. Afinidad 2019, 76, 171–179. 71. Shanmugharaj, A.; Kim, J.K.; Ryu, S.H. UV surface modification of waste tire powder: Characterization and its influence on the properties of polypropylene/waste powder composites. Polym. Test. 2005, 24, 739–745. [CrossRef] 72. Romero-Sánchez, M.D.; Pastor-Blas, M.M.; Martın-Martınez, J.M. Treatment of a styrene-butadiene-styrene rubber with corona discharge to improve the adhesion to polyurethane adhesive. Int. J. Adhes. Adhes. 2003, 23, 49–57. [CrossRef] 73. Satyanarayana, M.; Bhowmick, A.K.; Kumar, K.D. Preferentially fixing nanoclays in the phases of incompatible carboxylated nitrile rubber (XNBR)-natural rubber (NR) blend using thermodynamic approach and its effect on physico mechanical properties. Polymer 2016, 99, 21–43. [CrossRef] 74. Akca, E.; Gursel, A.; Sen, N. A Review on Devulcanization of Waste Tire Rubber. Period. Eng. Nat. Sci. 2018, 6, 154–160. [CrossRef] 75. Cheng, X.; Long, D.; Huang, S.; Li, Z.; Guo, X. Time effectiveness of the low-temperature plasma surface modification of ground tire rubber powder. J. Adhes. Sci. Technol. 2015, 29, 1330–1340. [CrossRef] 76. Yoshida, S.; Hagiwara, K.; Hasebe, T.; Hotta, A. Surface modification of polymers by plasma treatments for the enhancement of biocompatibility and controlled drug release. Surf. Coatings Technol. 2013, 233, 99–107. [CrossRef] 77. Xiaowei, C.; Sheng, H.; Xiaoyang, G.; Wenhui, D. Crumb waste tire rubber surface modification by plasma polymerization of ethanol and its application on oil-well cement. Appl. Surf. Sci. 2017, 409, 325–342. [CrossRef] 78. Park, H.B.; Han, D.W.; Lee, Y.M. Effect of a UV/Ozone Treatment on Siloxane-Containing Copolyimides: Surface Modification and Gas Transport Characteristics. Chem. Mater. 2003, 15, 2346–2353. [CrossRef] 79. Diaz-Quijada, G.A.; Wayner, D.D.M. A Simple Approach to Micropatterning and Surface Modification of Poly(dimethylsiloxane). Langmuir 2004, 20, 9607–9611. [CrossRef] 80. Cataldo, F.; Ursini, O.; Angelini, G. Surface oxidation of rubber crumb with ozone. Polym. Degrad. Stab. 2010, 95, 803–810. [CrossRef] 81. Formela, K.; Korol, J.; Saeb, M.R. Interfacially modified LDPE/GTR composites with non-polar elastomers: From microstructure to macro-behavior. Polym. Test. 2015, 42, 89–98. [CrossRef] 82. Li, Y.; Zhang, Y.; Zhang, Y. Morphology and mechanical properties of HDPE/SRP/elastomer composites: Effect of elastomer polarity. Polym. Test. 2004, 23, 83–90. [CrossRef] 83. Naderi, G.; LaFleur, P.G.; Dubois, C. The influence of matrix viscosity and composition on the morphology, rheology, and mechanical properties of thermoplastic elastomer nanocomposites based on EPDM/PP. Polym. Compos. 2008, 29, 1301–1309. [CrossRef] 84. Amani, M.; Sharif, M.; Kashkooli, A.; Rahnama, N.; Fazli, A. Effect of mixing conditions on the selective localization of graphite oxide and the properties of polyethylene/high-impact polystyrene/graphite oxide nanocomposite blends. RSC Adv. 2015, 5, 77723–77733. [CrossRef] 85. Sahakaro, K. Mechanism of reinforcement using nanofillers in rubber nanocomposites. In Progress in Rubber Nanocomposites; Elsevier: Amsterdam, The Netherlands, 2017; pp. 81–113. 86. Shan, C.; Gu, Z.; Wang, L.; Li, P.; Song, G.; Gao, Z.; Yang, X. Preparation, characterization, and application of NR/SBR/Organoclay nanocomposites in the tire industry. J. Appl. Polym. Sci. 2010, 119, 1185–1194. [CrossRef] 87. Zhang, C.-L.; Feng, L.-F.; Gu, X.-P.; Hoppe, S.; Hu, G.-H. Efficiency of graft copolymers as compatibilizers for immiscible polymer blends. Polymer 2007, 48, 5940–5949. [CrossRef] J. Compos. Sci. 2020, 4, 103 42 of 43

88. Botros, S. Preparation and characteristics of NR/EPDM rubber blends. Polym. Technol. Eng. 2002, 41, 341–359. [CrossRef] 89. Abou-Helal, M.O.; El-Sabbagh, S. A Study on the Compatibility of NR-EPDM Blends Using Electrical and Mechanical Techniques. J. Elastomers Plast. 2005, 37, 319–346. [CrossRef] 90. Adhikari, B. Reclamation and recycling of waste rubber. Prog. Polym. Sci. 2000, 25, 909–948. [CrossRef] 91. Flory, P.J.; Rehner, J. Statistical Mechanics of Cross-Linked Polymer Networks I. Rubberlike Elasticity. J. Chem. Phys. 1943, 11, 512. [CrossRef] 92. Markl, E.; Lackner, M. Devulcanization Technologies for Recycling of Tire-Derived Rubber: A Review. Materials 2020, 13, 1246. [CrossRef] 93. Formela, K.; Cysewska, M. Efficiency of thermomechanical reclaiming of ground tire rubber conducted in counter-rotating and co-rotating twin screw extruder. Polimery 2014, 59, 231–238. [CrossRef] 94. Zanchet, A.; Carli, L.N.; Giovanela, M.; Crespo, J.D.S.; Scuracchio, C.H.; Nunes, R.C. Characterization of Microwave-Devulcanized Composites of Ground SBR Scraps. J. Elastomers Plast. 2009, 41, 497–507. [CrossRef] 95. Yun, J.; Yashin, V.V.; Isayev, A.I. Ultrasonic devulcanization of carbon black-filled ethylene propylene diene monomer rubber. J. Appl. Polym. Sci. 2003, 91, 1646–1656. [CrossRef] 96. Isayev, A.I.; Liang, T.; Lewis, T.M. Effect of particle size on ultrasonic devulcanization of tire rubber in twin-screw extruder. Rubber Chem. Technol. 2014, 87, 86–102. [CrossRef] 97. Zhang, X.; Saha, P.; Cao, L.; Li, H.; Kim, J.K. Devulcanization of waste rubber powder using thiobisphenols as novel reclaiming agent. Waste Manag. 2018, 78, 980–991. [CrossRef] 98. Wang, X.; Shi, C.; Zhang, L.; Zhang, Y. Effects of shear stress and subcritical water on devulcanization of styrene-butadiene rubber based ground tire rubber in a twin-screw extruder. J. Appl. Polym. Sci. 2013, 130, 1845–1854. [CrossRef] 99. Yazdani, H.; Karrabi, M.; Ghasmi, I.; Azizi, H.; Bakhshandeh, G.R. Devulcanization of waste tires using a twin-screw extruder: The effects of processing conditions. J. Vinyl Addit. Technol. 2011, 17, 64–69. [CrossRef] 100. Tao, G.; He, Q.; Xia, Y.; Jia, G.; Yang, H.; Ma, W. The effect of devulcanization level on mechanical properties of reclaimed rubber by thermal-mechanical shearing devulcanization. J. Appl. Polym. Sci. 2013, 129, 2598–2605. [CrossRef] 101. Horikx, M.M. Chain scissions in a polymer network. J. Polym. Sci. 1956, 19, 445–454. [CrossRef] 102. Seghar, S.; Asaro, L.; Hocine, N.A. Experimental Validation of the Horikx Theory to be Used in the Rubber Devulcanization Analysis. J. Polym. Environ. 2019, 27, 2318–2323. [CrossRef] 103. Shi, J.; Jiang, K.; Ren, D.; Zou, H.; Wang, Y.; Lv, X.; Zhang, L. Structure and performance of reclaimed rubber obtained by different methods. J. Appl. Polym. Sci. 2012, 129, 999–1007. [CrossRef] 104. Zanchet, A.; Carli, L.; Giovanela, M.; Brandalise, R.; Crespo, J.D.S. Use of styrene butadiene rubber industrial waste devulcanized by microwave in rubber composites for automotive application. Mater. Des. 2012, 39, 437–443. [CrossRef] 105. Rooj, S.; Basak, G.C.; Maji, P.K.; Bhowmick, A.K. New Route for Devulcanization of Natural Rubber and the Properties of Devulcanized Rubber. J. Polym. Environ. 2011, 19, 382–390. [CrossRef] 106. Mandal, S.K.; Alam, N.; Debnath, S.C. Reclaiming of ground rubber tire by safe multifunctional rubber additives: I. tetra benzyl thiuram disulfide. Rubber Chem. Technol. 2012, 85, 629–644. [CrossRef] 107. Kojima, M.; Tosaka, M.; Ikeda, Y. Chemical recycling of sulfur-cured natural rubber using supercritical carbon dioxide. Green Chem. 2004, 6, 84. [CrossRef] 108. Thaicharoen, P.; Thamyongkit, P.; Poompradub, S. Thiosalicylic acid as a devulcanizing agent for mechano-chemical devulcanization. Korean J. Chem. Eng. 2010, 27, 1177–1183. [CrossRef] 109. Diaz, R.; Colomines, G.; Peuvrel-Disdier, E.; Deterre, R. Thermo-mechanical recycling of rubber: Relationship between material properties and specific mechanical energy. J. Mater. Process. Technol. 2018, 252, 454–468. [CrossRef] 110. Li, X.; Deng, X.-Q.; Dong, C. Effect of Temperature on Devulcanization of Waste Sidewall Rubber by Supercritical Ethanol. J. Braz. Chem. Soc. 2018, 29, 2169–2179. [CrossRef] 111. Ismail, H.; Awang, M.; Hazizan, M.A. Effect of Waste Tire Dust (WTD) Size on the Mechanical and Morphological Properties of Polypropylene/Waste Tire Dust (PP/WTD) Blends. Polym. Technol. Eng. 2006, 45, 463–468. [CrossRef] 112. Sreeja, T.D.; Kutty, S.K.N. Cure characteristics and mechanical properties of natural rubber/reclaimed rubber blends. Polym. Technol. Eng. 2000, 39, 501–512. [CrossRef] J. Compos. Sci. 2020, 4, 103 43 of 43

113. Li, S.; Lamminmäki, J.; Hanhi, K. Effect of ground rubber powder and devulcanizates on the properties of natural rubber compounds. J. Appl. Polym. Sci. 2005, 97, 208–217. [CrossRef] 114. Yehia, A.; Ismail, M.N.; Hefny, Y.A.; Abdel-Bary, E.M.; Mull, M.A. Mechano-Chemical Reclamation of Waste Rubber Powder and its Effect on the performance of NR and SBR Vulcanizates. J. Elastomers Plast. 2004, 36, 109–123. [CrossRef] 115. Nelson, P.; Kutty, S. Studies on Maleic Anhydride Grafted Reclaimed Rubber/Acrylonitrile Butadiene Rubber Blends. Prog. Rubber Plast. Recycl. Technol. 2003, 19, 171–188. [CrossRef] 116. Sreeja, T.; Kutty, S. Studies on acrylonitrile butadiene rubber. J. Elastomers Plast. 2002, 34, 145–155. [CrossRef] 117. Sreeja, T.D.; Kutty, S.K.N. Styrene butadiene rubber/reclaimed rubber blends. Int. J. Polym. Mater. 2003, 52, 599–609. [CrossRef] 118. Lamminmäki, J.; Li, S.; Hanhi, K. Feasible incorporation of devulcanized rubber waste in virgin natural rubber. J. Mater. Sci. 2006, 41, 8301–8307. [CrossRef] 119. Han, S.-C.; Han, M.-H. Fracture behavior of NR and SBR vulcanizates filled with ground rubber having uniform particle size. J. Appl. Polym. Sci. 2002, 85, 2491–2500. [CrossRef] 120. De, D.; Maiti, S.; Adhikari, B. Reclaiming of rubber by a renewable resource material (RRM). III. Evaluation of properties of NR reclaim. J. Appl. Polym. Sci. 2000, 75, 1493–1502. [CrossRef] 121. De, D.; Panda, P.K.; Roy, M.; Bhunia, S. Reinforcing effect of reclaim rubber on natural rubber/polybutadiene rubber blends. Mater. Des. 2013, 46, 142–150. [CrossRef] 122. Sombatsompop, N.; Kumnuantip, C. Rheology, cure characteristics, physical and mechanical properties of tire tread reclaimed rubber/natural rubber compounds. J. Appl. Polym. Sci. 2003, 87, 1723–1731. [CrossRef] 123. Vlachopoulos, J. Die Swell and Normal Stresses: An Explanation. Rubber Chem. Technol. 1978, 51, 133–138. [CrossRef] 124. Zhang, X.; Lu, C.; Liang, M. Properties of natural rubber vulcanizates containing mechanochemically devulcanized ground tire rubber. J. Polym. Res. 2008, 16, 411–419. [CrossRef] 125. Rattanasom, N.; Poonsuk, A.; Makmoon, T. Effect of curing system on the mechanical properties and heat aging resistance of natural rubber/tire tread reclaimed rubber blends. Polym. Test. 2005, 24, 728–732. [CrossRef] 126. Li, Y.; Zhao, S.; Wang, Y. Microbial Desulfurization of Ground Tire Rubber by Sphingomonas sp.: A Novel Technology for Crumb Rubber Composites. J. Polym. Environ. 2011, 20, 372–380. [CrossRef] 127. Li, Y.; Zhao, S.; Wang, Y. Improvement of the properties of natural rubber/ground tire rubber composites through biological desulfurization of GTR. J. Polym. Res. 2012, 19, 9864. [CrossRef] 128. De, D.; De, D. Processing and material characteristics of a reclaimed ground rubber tire reinforced styrene butadiene rubber. Mater. Sci. Appl. 2011, 2, 486. [CrossRef] 129. Fernandez-Berridi, M.J.; González, N.; Mugica, A.; Bernicot, C. Pyrolysis-FTIR and TGA techniques as tools in the characterization of blends of natural rubber and SBR. Thermochim. Acta 2006, 444, 65–70. [CrossRef] 130. Formela, K.; Formela, M.; Thomas, S.; Haponiuk, J. Styrene-Butadiene Rubber/Modified Ground Tire Rubber Blends Co-Vulcanization: Effect of Accelerator Type. Macromol. Symp. 2016, 361, 64–72. [CrossRef] 131. Colom, X.; Marin-Genesca, M.; Mujal, R.; Formela, K.; Cañavate, J. Structural and physico-mechanical properties of natural rubber/GTR composites devulcanized by microwaves: Influence of GTR source and irradiation time. J. Compos. Mater. 2018, 52, 3099–3108. [CrossRef] 132. De, D.; De, D.; Singharoy, G. Reclaiming of ground rubber tire by a novel reclaiming agent. I. virgin natural rubber/reclaimed GRT vulcanizates. Polym. Eng. Sci. 2007, 47, 1091–1100. [CrossRef] 133. Kumnuantip, C.; Sombatsompop, N. Dynamic mechanical properties and swelling behaviour of NR/reclaimed rubber blends. Mater. Lett. 2003, 57, 3167–3174. [CrossRef] 134. Balasubramanian, M. Cure modeling and mechanical properties of counter rotating twin screw extruder devulcanized ground rubber tire—Natural rubber blends. J. Polym. Res. 2008, 16, 133–141. [CrossRef] 135. Kraus, G. Modification of Asphalt by Block Polymers of Butadiene and Styrene. Rubber Chem. Technol. 1982, 55, 1389–1402. [CrossRef] 136. Igwe, I.O.; Ezeani, O.E. Studies on the Transport of Aromatic Solvents through Filled Natural Rubber. Int. J. Polym. Sci. 2012, 2012, 1–11. [CrossRef]

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