Friday Nwankwo Archibong

Olawale Monsur Sanusi

Pascal Médéric

Aït Hocine

An overview on the recycling of waste ground tyre rubbers in thermoplastic matrices: effect of added fillers

Keywords: Waste tyre rubbers, recycling, thermoplastics, blend, rigid fillers, nanocomposites

The increasing generation and accumulation of waste tyres threaten the ecosystem. Waste tyre landfills create potential fire hazards, environmental pollutions, and threats to human health, as waste tyre pileups harbor rodents, mosquitoes and reptiles, generating harmful diseases. Moreover, there is a great economic loss associated with the disposal of waste tyre rubbers. Therefore, this approach is not sustainable, making the recycling of waste tyre rubber necessary. Accordingly, several recycling strategies are being developed to obtain value-added waste tyre rubber. One of such strategies, which is of environmental conservative and consumes an appreciable quantity of the waste tyres, is the blending of thermoplastics with ground tyre rubber (GTR). However, achieving homogenous dispersion of GTR in matrix poses a great challenge owing to little or no affinity between GTR and thermoplastics. To overcome this problem, surface treatments of GTR, compatibilizers, additives, and energy treatments are used. Most recently, incorporation of rigid nanoparticles with or without combination of other techniques are adopted to facilitate GTR dispersion. In this review, we discuss the recycling of GTR in thermoplastic matrices. Specific attention

Introduction

Industrially, tyre rubbers are manufactured at a low cost through an efficient but irreversible process called vulcanization. Vulcanization can be defined as the creation of three-dimensional crosslinked network between rubber macromolecules via irreversible reactions, in the presence of curing agents (sulphur, peroxides), activators (stearic acid, zinc oxide), accelerators (thiazoles, sulphonamide), retarders (N-cyclohexylthio phthalimide) and other additives [START_REF] Luna | Incorporation of a recycled rubber compound from the shoe industry in polystyrene: Effect of SBS compatibilizer content[END_REF][START_REF] Junkong | Effect of fatty acids on the accelerated sulfur vulcanization of rubber by active zinc/carboxylate complexes[END_REF]. The sulphur vulcanization creates strong chemical bonds, and the sulphur content strongly dictates the resulting structural and physical properties of the rubber vulcanizates [START_REF] Sienkiewicz | Environmentally friendly polymer-rubber composites obtained from waste tyres: A review[END_REF][START_REF] Yangthong | Role of geopolymer as a cure activator in sulfur vulcanization of epoxidized natural rubber[END_REF]. Hence, rubber products like tyres are insoluble and infusible, which makes them difficult to be reprocessed unlike thermoplastics [START_REF] Sienkiewicz | Progress in used tyres management in the European Union: A review[END_REF]. Meanwhile, global industrialization is driving the rapid expansion of the automotive sector, leading to an increase in the production of tyres, about 1.5 billion units per year worldwide [START_REF] Czajczyńska | Use of pyrolytic gas from waste tire as a fuel: A review[END_REF][START_REF] Simon-Stőger | PE-contaminated industrial waste ground tire rubber: How to transform a handicapped resource to a valuable one[END_REF]. This consequently generates a large volume of end-of-life tyres, since they do not degrade naturally for centuries [START_REF] Gupta | Enhanced heavy metals removal and recovery by mesoporous adsorbent prepared from waste rubber tire[END_REF]Liu et al., 2020). The traditional dumping or unchecked burning of waste tyres pose great hazards to man and its environment, and thus raising attention from all nations [START_REF] Ghosh | Cradle-tocradle approach to waste tyres and development of silica based green tyre composites[END_REF].

Unlike thermoplastic waste used to obtain value-added new products, there is no known effective process to completely regain elastomeric waste monomers [START_REF] Adhikari | Reclamation and recycling of waste rubber[END_REF]. This makes the reclaimed rubber less viable commercially. Thus, the disposal of large quantity of such elastomeric materials, which alters the conservation of environment.

Nevertheless, a laudable alternative means of using worn-out tyres is simply the incorporation of the GTR in thermoplastic matrices to form blends [START_REF] Simon-Stőger | PE-contaminated industrial waste ground tire rubber: How to transform a handicapped resource to a valuable one[END_REF]. This approach is gaining scientific research momentum because it serves as the most straightforward rubber recycling means. It is cheap, easy and environmentally friendly.

Moreover, it consumes an appreciable quantity of waste tyres in the form of GTR [START_REF] Karger-Kocsis | Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers[END_REF]. More importantly, the renewability and recyclability of thermoplastic matrices support the ecosystem sustainability targets [START_REF] Ramarad | Waste tire rubber in polymer blends: A review on the evolution, properties and future[END_REF][START_REF] Nuzaimah | Recycling of waste rubber as fillers: A review[END_REF][START_REF] Sanusi | Clays and carbon nanotubes as hybrid nanofillers in thermoplastic-based nanocomposites -A review[END_REF]Sanusi et al., , 2021a)).

There are some reviews on the recycling of tyres in thermoplastics and other matrices focusing on different methods of treatments, preparations or applications. For instance, the review by [START_REF] Karger-Kocsis | Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers[END_REF] covers the recycling of waste tyre in thermoset, thermoplastic and elastomer matrices. [START_REF] Ramarad | Waste tire rubber in polymer blends: A review on the evolution, properties and future[END_REF] discuss incorporation of waste tyre rubber in both natural rubber and thermoplastics. [START_REF] Sienkiewicz | Environmentally friendly polymer-rubber composites obtained from waste tyres: A review[END_REF] focus on the methods of generation and treatment of rubber granules as well as implications on the properties of final composites. Beside waste tyre rubber, [START_REF] Nuzaimah | Recycling of waste rubber as fillers: A review[END_REF] extend their review to include waste condom and shoe rubbers. The paper by [START_REF] Fazli | Waste rubber recycling: A review on the evolution and properties of thermoplastic elastomers[END_REF] emphasizes the chemistry and kinetics of the compatibilization methods for rubber dispersion in thermoplastics. In this review, a distinctive effort is made to summarize, principally, the works dealing with the recycling of treated or untreated waste tyre particles in thermoplastic matrices, with or without additional phases (coupling agents, fillers), aiming at developing composites with competitive properties. Particular attention is paid to the effects of rigid micro and nanofillers on the structure and properties of thermoplastic/waste tyre blends.

Challenges and methods of waste tyre rubbers recycling

Challenges of waste tyre rubbers recycling

Over 70% of total natural and synthetic rubber materials are utilized for tyre manufacturing [START_REF] Myhre | Rubber recycling[END_REF][START_REF] Karger-Kocsis | Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers[END_REF]. When a tyre is no longer efficient or satisfactorily safe to be used, it becomes a waste tyre, that is equally referred to as end-of-life-tyre (ELT) [START_REF] Mangili | Mechanical and rheological properties of natural rubber compounds containing devulcanized ground tire rubber from several methods[END_REF]. Annually, a huge volume of tyres is scrapped worldwide [START_REF] Wayal | Use of waste plastic and waste rubber in aggregate and bitumen for road materials[END_REF][START_REF] Nuzaimah | Recycling of waste rubber as fillers: A review[END_REF][START_REF] Shulman | Tire Recycling[END_REF], that is over 17 million tons of worn-out tyres [START_REF] Formela | Assessment of microstructure, physical and thermal properties of bitumen modified with LDPE/GTR/elastomer ternary blends[END_REF]. In fact, more than 180 million and 150 million tyres were scrapped in European Union (EU) countries and North America, respectively [START_REF] Ayanoglu | Rotary kiln and batch pyrolysis of waste tire to produce gasoline and diesel like fuels[END_REF]. Unfortunately, far less than 1% of the entire waste tyres are naturally degraded over hundreds of years as a result of the complex cross-linked structure of rubber [START_REF] Adhikari | Reclamation and recycling of waste rubber[END_REF][START_REF] Gupta | Enhanced heavy metals removal and recovery by mesoporous adsorbent prepared from waste rubber tire[END_REF][START_REF] Liu | Recycling of waste rubber powder by mechanochemical modification[END_REF].

There are two critical problems associated with waste tyre rubbers. The first one is the economic loss in terms of the wastage of rubber when discarded as waste, excluding an additional cost due to transportation to such disposal sites. Meanwhile, the unavailability of space has made tyre disposal by landfilling to be unrealistic with no potential in the nearest future [START_REF] Thomas | Recycling of waste tire rubber as aggregate in concrete: Durability-related performance[END_REF]. The second one is the environmental pollution caused by the abandoned waste tyres, which is disastrous to aquatic and terrestrial organisms [START_REF] Adhikari | Reclamation and recycling of waste rubber[END_REF][START_REF] Nuzaimah | Recycling of waste rubber as fillers: A review[END_REF]. For instance, mass dumping of tyres breeds potential dangers to the man and his environs. These landfills harbor dangerous reptiles, accumulates disease causative organisms, or creates fire hazard zones since the abandoned elastomers are combustible materials. Consequently, the EU imposed a ban on landfilling of scrapped tyres [START_REF] Sadaka | Controlled chemical degradation of natural rubber using periodic acid: Application for recycling waste tyre rubber[END_REF][START_REF] Karger-Kocsis | Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers[END_REF][START_REF] Grammelis | A Review on Management of End of Life Tires (ELTs) and Alternative Uses of Textile Fibers[END_REF]. Therefore, the attempts to recycle and re-utilize the waste tyre rubbers become necessary. Tyre recycling equally represents a source of raw material and offers a possibility of earnings for people that collect the waste tyres, thus support the economic development [START_REF] De Sousa | Vulcanization of natural rubber: Past, present and future perspectives[END_REF].

Techniques of waste tyres recycling

The complex composition and structure of tyres pose major restrictions to their recovery and recycling processes. Besides the 70-80% of sulphur-cured rubber component of tyres, fibre made up of polyester, nylon or cellulose, and steel cords are essential constituents used to reinforce tyres. Thus, separation of the fibres and steel components from the rubber is the first difficult step during recycling of tyres [START_REF] Ramarad | Waste tire rubber in polymer blends: A review on the evolution, properties and future[END_REF]. Then, the next stage of recycling of waste tyres is grinding to form GTR with different sizes and shapes [START_REF] Manuel | Standards for rubber granulates and powders[END_REF].

Since manufactured tyres are infusible, insoluble and technically complex to be reprocessed [START_REF] Zhang | Characterization of the properties of thermoplastic elastomers containing waste rubber tire powder[END_REF], large amount of GTR provides a source of energy. For instance, waste tyres are used as a heat source since they are made up of about 90% organic material with a heat of about 32.6 MJ/kg, far more than that of coal (18.6 -27.9 MJ/kg [START_REF] Fazli | Waste rubber recycling: A review on the evolution and properties of thermoplastic elastomers[END_REF]). Incineration of waste tyres is seen as an approach of eliminating a substantial amount of this post-consumer product to generate or recover energy, but this is with significant negative environmental impacts such as air pollution and global warming [START_REF] Mangili | Characterization and supercritical CO2 devulcanization of cryo-ground tire rubber: Influence of devulcanization process on reclaimed material[END_REF][START_REF] Cazan | The influence of temperature on the performance of rubber -PET-HDPE waste -based composites with different inorganic fillers[END_REF].

Furthermore, several efforts are made by the politicians and scientists to deal with the problem of the ever-growing waste tyres [START_REF] Rincón | Environmental performance of recycled rubber as drainage layer in extensive green roofs. A comparative Life Cycle Assessment[END_REF][START_REF] Sienkiewicz | Environmentally friendly polymer-rubber composites obtained from waste tyres: A review[END_REF][START_REF] Maris | Mechanical recycling: Compatibilization of mixed thermoplastic wastes[END_REF][START_REF] Liang | Utilization of wax residue as compatibilizer for asphalt with ground tire rubber/recycled polyethylene blends[END_REF]. The risks inherent in the scrap tyres necessitated the legislation of 1991 by United States Congress (Resource Conservation Recovery Act) and Intermodal Surface Transport Efficiency Act to mandate the use of a specified percentage of recycled tyres in any project funded by the federal government. In deviance to implementation, the funds would be withheld [START_REF] Shu | Recycling of waste tire rubber in asphalt and portland cement concrete: An overview[END_REF]. European Union (EU) issued the Landfill Directive "1999/31/EC" that prohibits the disposal of whole tyres and shredded tyres (50 -300 mm size) in [START_REF] Li | Mechanical properties of high-density polyethylene/scrap rubber powder composites modified with ethylene-propylene-diene terpolymer, dicumyl peroxide, and silicone oil[END_REF]and 2006, respectively (Karger-Kocsis et al., 2013). Thus, a significant use of tyres emerges in civil engineering for building and road construction, such as in insulation, sound deadening, soil stabilization, dam and road construction, artificial reefs, steel making and so on [START_REF] Wang | Asphalt modified by thermoplastic elastomer based on recycled rubber[END_REF][START_REF] Medina | Composites with recycled rubber aggregates: Properties and opportunities in construction[END_REF].

The use of waste tyres in this field is sometimes referred to as "downcycling" since the recycled materials have entirely different applications from the original purpose [START_REF] Karger-Kocsis | Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers[END_REF]. However, except for road paving, most of the downcycling applications are saturated and outdated. Hence, there is a necessity for identifying innovative and valueadded applications for waste tyres.

Reclaiming of tyres is another sophisticated technique of recycling tyres [START_REF] Dubkov | Reclamation of waste tyre rubber with nitrous oxide[END_REF][START_REF] Dobrotă | An innovative method in the regeneration of waste rubber and the sustainable development[END_REF]. Reclaiming of used tyres is the conversion and breaking down of the 3D interlinked, infusible and insoluble network of vulcanized rubber to soft, low modulus, easy processing and vulcanizable material covering properties of virgin rubber [START_REF] De Sousa | Vulcanization of natural rubber: Past, present and future perspectives[END_REF]. Recycling of tyres can be classified into physical reclaiming and chemical reclaiming processes.

The physical reclaiming method involves the application of external energy on used tyres to breakdown the 3D cross-linked network of rubber. The applied energy transforms the molecular structure of rubber into fragments of smaller molecular weight availing it the miscibility with virgin matrix or blend during compounding. Applying sufficient energy, which is enough to cleave the rubber crosslink bonds, results in high quality reclaimed rubber. The physical reclaiming approach encompasses mechanical, cryo-mechanical grinding, thermo-mechanical, ultrasonic and microwave techniques [START_REF] De Sousa | Devulcanization of waste tire rubber by microwaves[END_REF]. These techniques were well described in the review by [START_REF] Asaro | Recycling of rubber wastes by devulcanization[END_REF]. It should be noted that microwave technique, in particular, causes total breakdown of the polysulphide crosslinks. The remnant sulphur crosslinks exhibit largely mono-sulphide structures [START_REF] Molanorouzi | Reclaiming waste tire rubber by an irradiation technique[END_REF].

Chemical approach involves the utilization of chemical devulcanizing agents, with solvents serving as the reaction medium, and often carried out under elevated temperatures [START_REF] Gumede | Organic chemical devulcanization of rubber vulcanizates in supercritical carbon dioxide and associated less eco-unfriendly approaches: A review[END_REF]. In chemical devulcanization, the main goal is the penetration of devulcanizing reagent into the rubber matrix for cleavage reaction [START_REF] Liu | Devulcanization of waste tire rubber in supercritical carbon dioxide[END_REF]. Some of the recent significant solvent media used in devulcanizing GTR include ironic liquid [START_REF] Seghar | Devulcanization of styrene butadiene rubber by microwave energy: Effect of the presence of ionic liquid[END_REF], deep eutectic solvent [START_REF] Saputra | Devulcanisation of ground rubber tyre by novel ternary deep eutectic solvents[END_REF] and periodic acid [START_REF] Sadaka | Controlled chemical degradation of natural rubber using periodic acid: Application for recycling waste tyre rubber[END_REF]. Environmental concerns led to an innovative use of super-critical carbon dioxide as an alternative reaction medium for devulcanizing reagents like diphenyl disulphide [START_REF] Mangili | Characterization and supercritical CO2 devulcanization of cryo-ground tire rubber: Influence of devulcanization process on reclaimed material[END_REF]. Furthermore, thermochemical process has an extended application in pyrolysis, whereby used tyres are converted to oil, gas or char that are either deployed as a chemical feedstock or energy source [START_REF] Formela | Assessment of microstructure, physical and thermal properties of bitumen modified with LDPE/GTR/elastomer ternary blends[END_REF][START_REF] Guangjian | Study on application behavior of pyrolysis char from waste tires in silicone rubber composites[END_REF]. Oil from pyrolysis of tyre is competitive since it has fuel characteristics similar to diesel fuel [START_REF] Ayanoglu | Rotary kiln and batch pyrolysis of waste tire to produce gasoline and diesel like fuels[END_REF][START_REF] Kumaravel | Tyre pyrolysis oil as an alternative fuel for diesel engines -A review[END_REF]. Aside the complexity associated with control of the chemical reactions that limits its deployment in industry, this approach is expensive and does not completely eradicate toxic reagents [START_REF] Sadaka | Controlled chemical degradation of natural rubber using periodic acid: Application for recycling waste tyre rubber[END_REF].

Meanwhile, both chemical and physical approaches can lead to secondary pollution and involve urge energy consumption [START_REF] Li | Microbial desulfurization of ground tire rubber by Thiobacillus ferrooxidans[END_REF]. For instance, the chemical method involves high reaction temperature, complex instrumentation, and the use of reactor plants and chemicals. All these points constitute environmental and cost concerns [START_REF] Choi | Clean pyrolysis oil from a continuous two-stage pyrolysis of scrap tires using in-situ and ex-situ desulfurization[END_REF]. The use of microorganisms in desulphurization of vulcanized rubber (microbial desulphurization) is another alternative but it only breaks the sulphur crosslinks on the surface of powdered waste tyre rubbers. In addition, it is a slow process [START_REF] Li | Microbial desulfurization of ground tire rubber by Thiobacillus ferrooxidans[END_REF][START_REF] Yao | Microbial desulfurization of waste latex rubber with Alicyclobacillus sp[END_REF].

During the last decade, one of the simple routes of tyre recycling that reduces energy consumption as well as addressing environmental challenges is incorporating GTR in thermoplastic polymers [START_REF] Satapathy | Thermoplastic elastomers from waste polyethylene and reclaim rubber blends and their composites with fly ash[END_REF][START_REF] Fazli | Waste rubber recycling: A review on the evolution and properties of thermoplastic elastomers[END_REF]. This creates a multi-functional polymeric matrix that has the elasticity of rubber combined with the thermoplastic processability property [START_REF] Fazli | Waste rubber recycling: A review on the evolution and properties of thermoplastic elastomers[END_REF].

Recycling of waste tyre rubbers in thermoplastic matrices

Mixing GTR with thermoplastics matrices or thermoplastic blends to elaborate thermoplastic elastomers (TPE) is the most promising practical approach of GTR recycling, as the thermoplastic phase in the composite makes the reprocessing or recyclability of the resulting compound feasible (Kakroodi and Rodrigue, 2013;[START_REF] Sripornsawat | Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black[END_REF]). Yet, thermoplastic/GTR materials show poor mechanical properties due to low compatibility. This is explained by the weak entanglement of cross-linked rubber molecules and thermoplastic molecules, causing the phase separation which limits their applications [START_REF] Montagna | Evaluation of the effect of the incorporation of rubber tire waste particles on the properties of PP, HIPS and PP/HIPS matrices[END_REF]. Even though the utilization of GTR in thermoplastic consumes a reasonable quantity of GTR (sometimes >50% (Kakroodi and Rodrigue, 2013)) that addresses environmental concerns, the mechanical and thermal properties of the resulted TPE needs to be improved for competitiveness. Indeed, the composition of TPE (thermoplastic-based matrix, GTR), affinity between the different phases and the GTR particle size are critical in order to achieve highperformance thermoplastic elastomer materials.

It is thus clear that recycling of GTR as particles in different thermoplastic matrices or thermoplastic blends is a simple and inexpensive means of removing worn-out tyres from the environment with value addition. Although polymers like polyolefins and vulcanized rubbers can be recycled via chemical techniques, this type of recycling is expensive and comes with secondary pollution [START_REF] Datta | Morphology and properties of recycled polyethylene/ground tyre rubber/thermoplastic poly(ester-urethane) blends[END_REF]. Hence, the mechanical recycling, for which the waste materials are ground to granulated powder then mixed with the matrix, is adapted and, moreover, it supports ecological reasoning. More importantly, the resulting thermoplastic elastomer materials offer better material utilization as subsequent scraps and/or rejects can be easily recycled [START_REF] Pang | Influence of kenaf form and loading on the properties of kenaffilled polypropylene/waste tire dust composites: A comparison study[END_REF]. Nevertheless, several methods must be used to improve the interfacial adhesion and the degree of GTR dispersion: addition of compatibilizers/coupling agents, nanofillers, energy treatment, devulcanization and revulcanization [START_REF] Da Costa | Analysis and optimization of polypropylene (PP)/ethylene-propylene-diene monomer (EPDM)/scrap rubber tire (SRT) mixtures using RSM methodology[END_REF]Kakroodi and Rodrigue, 2013).

Effect of GTR dispersion

A homogenous dispersion of GTR in the matrix is an essential factor in obtaining competitive materials, because it improves, for example, the impact strength [START_REF] Da Costa | Analysis and optimization of polypropylene (PP)/ethylene-propylene-diene monomer (EPDM)/scrap rubber tire (SRT) mixtures using RSM methodology[END_REF]. The concentration of GTR in the matrix as well as its particle size influence the state of the GTR dispersion and the final blend properties [START_REF] Datta | Morphology and properties of recycled polyethylene/ground tyre rubber/thermoplastic poly(ester-urethane) blends[END_REF][START_REF] Halász | Development of Polypropylene-based Thermoplastic Elastomers with Crumb Rubber by Dynamic Vulcanization: A Potential Route for Rubber Recycling[END_REF]. GTR of about 1 µm disperses more easily in the matrix [START_REF] Fazli | Waste rubber recycling: A review on the evolution and properties of thermoplastic elastomers[END_REF]), but such a particle size is not easy to produce even with cryogenic technique [START_REF] Lievana | Use of ground tyre rubber (GTR) in thermoplastic polyolefin elastomer compositions[END_REF]. Equally, once the mass fraction of GTR in thermoplastic nears 10 -20 wt%, the composite mechanical performance deteriorates [START_REF] Candau | Poly (Lactic Acid)/Ground Tire Rubber Blends Using Peroxide Vulcanization[END_REF]. In fact, pristine GTR aggregates within matrix, leading to a deterioration of mechanical properties.

Hence, to obtain good dispersion of GTR in matrix and improve interfacial adhesion between the two phases, it is necessary to reduce the GTR particle size [START_REF] Candau | Poly (Lactic Acid)/Ground Tire Rubber Blends Using Peroxide Vulcanization[END_REF], achieve surface modification by compatibilizers or vulcanizing agents [START_REF] Li | Coupling effect of molecular weight and crosslinking kinetics on the formation of rubber nanoparticles and their agglomerates in EPDM/PP TPVs during dynamic vulcanization[END_REF][START_REF] Zedler | Reactive sintering of ground tire rubber (GTR) modified by a trans-polyoctenamer rubber and curing additives[END_REF], encapsulate GTR into neat rubber phase [START_REF] Lima | Partial replacement of EPDM by GTR in thermoplastic elastomers based on PP/EPDM: Effects on morphology and mechanical properties[END_REF] or use energy treatments of GTR [START_REF] Garcia | The effect of adding devulcanized rubber on the thermomechanical properties of recycled polypropylene[END_REF].

There are two classifications of thermoplastic/elastomer materials. Firstly, thermoplastic elastomer (TPE), which is elaborated from a mixing of rubber with a thermoplastic without any curative ingredients. Secondly, thermoplastic vulcanizate (TPV), which is prepared via dynamic vulcanization and requires additional curing agents [START_REF] Cañavate | Formulations for thermoplastic vulcanizates based on high density polyethylene, ethylene-propylene-diene monomer, and ground tyre rubber[END_REF][START_REF] Sripornsawat | Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black[END_REF]. Devulcanization and/or functionalization of GTR leads to an enhancement in compatibility with the host matrix [START_REF] Lievana | Use of ground tyre rubber (GTR) in thermoplastic polyolefin elastomer compositions[END_REF], thus, improving the degree of the GTR dispersion in matrices. The dynamic vulcanization treatment entails the production of thermoplastic/rubber mixture, wherein the rubber phase is selectively crosslinked [START_REF] Chiang | Improvement of the mechanical property and thermal stability of polypropylene/recycled rubber composite by chemical modification and physical blending[END_REF]. The process permits high rubber content of more than 50 -80% and produces high-performance TPV having a well-dispersed rubber phase in a low-content continuous thermoplastic phase [START_REF] Li | Coupling effect of molecular weight and crosslinking kinetics on the formation of rubber nanoparticles and their agglomerates in EPDM/PP TPVs during dynamic vulcanization[END_REF]. TPV materials are gaining momentum in advanced applications like sport, automobile, electronics, construction and medical fields [START_REF] Zheng | Structure evolution of biobased PLA/ENR thermoplastic vulcanizates during dynamic vulcanization processing[END_REF]. This is due to their easy processability, high elasticity, exceptional mechanical properties, and recyclability [START_REF] Sripornsawat | Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black[END_REF].

Effect of energy treatment

High-energy treatments such as gamma radiation (Hassan et al., 2014a), microwaves [START_REF] De Sousa | Blends of ground tire rubber devulcanized by microwaves/HDPE-Part B: Influence of clay addition[END_REF], plasma [START_REF] Zimmermann | Plasma modification of polyvinyltoluene and polystyrene latices[END_REF] and electron beam radiations [START_REF] Ramarad | Improved crystallinity and dynamic mechanical properties of reclaimed waste tire rubber/EVA blends under the influence of electron beam irradiation[END_REF][START_REF] Mohamad | Insight on the properties of thermoplastic elastomer-based natural rubber and recycled rubber post-treated with electron beam irradiation[END_REF] increase the concentration of oxygen on GTR surfaces and lead to better adhesion with the host matrix [START_REF] Li | Mechanical properties of high-density polyethylene/scrap rubber powder composites modified with ethylene-propylene-diene terpolymer, dicumyl peroxide, and silicone oil[END_REF][START_REF] Sonnier | Polyethylene/ground tyre rubber blends: Influence of particle morphology and oxidation on mechanical properties[END_REF]. [START_REF] Ramarad | Improved crystallinity and dynamic mechanical properties of reclaimed waste tire rubber/EVA blends under the influence of electron beam irradiation[END_REF] mixed GTR with EVA and thereafter treated the composite with different doses of electron beam irradiation. The EVA/GTR material has lower processing torque compared to neat EVA, suggesting easier processability of the product due to the GTR rubbery feature. Irradiation improved the dispersion of GTR entities within EVA matrix, by reducing their size. It was also reported that the irradiation induced formation of crosslink, leading to an increase of storage and loss moduli of EVA/GTR composite. In another work, [START_REF] Mohamad | Insight on the properties of thermoplastic elastomer-based natural rubber and recycled rubber post-treated with electron beam irradiation[END_REF] prepared TPE by mixing different ratio of natural rubber (NR), HDPE and GTR to form NR/HDPE/GTR composite. They studied the influence of irradiation on the properties of different TPE blends and reported that irradiation enhanced the bond between the rubbery phases and HDPE matrix, as highlighted in Fig. 1. Increasing the irradiation dosage seems to lead to higher adhesion force and uniform surface interaction of the constituents. This resulted in improved mechanical strength of the composites, as shown in Fig. 2. Similarly, Hassan et al. (2014a) reported that gamma irradiation improved the adhesion between PE and GTR, leading to an improvement in thermal and mechanical performances of the composite, in particular its hardness. Moreover, De Sousa et al. (2015) reported a less coarse morphology in the HDPE/GTR composite when it was exposed to microwaves for some period, which resulted in better mechanical properties of the composite.

Therefore, the importance of energy treatments in aiding the crosslinking of GTR, lowering the interfacial energy between GTR and matrices, and creating a finer structure of GTR is sometimes essential. This can create an improvement in the physical properties in thermoplastic/GTR composites.

Effect of additives

A third component incorporated into thermoplastic/GTR system may be used to improve the processability and end-use properties. For instance, it can help in the preparation of a thermoplastic blend as a matrix and/or increase the affinity between phases by playing the role of a coupling agent. [START_REF] Datta | Morphology and properties of recycled polyethylene/ground tyre rubber/thermoplastic poly(ester-urethane) blends[END_REF] utilized recycled polyethylene (rPE), GTR and thermoplastic poly(ester urethane) (TPU) to prepare rPE/GTR/TPU system by melt mixing. Increasing the concentration of neat TPU, while the rPE/GTR composition ratio was kept constant, improved the processability, due to the high melt flow index of TPU compared to rPE.

Unfortunately, while hardness property was unaffected, both tensile strength and elastic modulus decreased with an increase of the TPU concentration. In the same way, rebounding resilience, that is the ability of a material to absorb high impact energy, was also decreased.

They related the behavior to stronger interactions between GTR and rPE than between GTR and TPU. However, the decrease of TPU content, while increasing rPE content, resulted in more permanent plastic deformation (typical of polyolefins) but improved abrasion resistance due to the best rPE/GTR interfacial interaction. This study shows the difficulty in obtaining an efficient balance between processability and mechanical performance. [START_REF] Montagna | Evaluation of the effect of the incorporation of rubber tire waste particles on the properties of PP, HIPS and PP/HIPS matrices[END_REF] mixed 10 wt% of GTR in polypropylene (PP), high impact polystyrene (HIPS), and blend PP/HIPS via injection moulding, under an injection pressure of 60 bar and a temperature of 220°C. Firstly, the study established that the decrease of GTR particle size leads to improved performances of such materials. For PP/GTR blend, the morphology study revealed voids created as a result of the pullout of GTR particles, and the void size increased with GTR particle size, confirming poor interfacial adhesion between PP and GTR. Consequently, both tensile and flexural moduli of PP/GTR blend are reduced compared to neat PP. However, good interfacial interaction was observed between HIPS and GTR, which was associated to the presence of polybutadiene in HIPS, resulting in increased tensile and flexural moduli. On other hand, PP/GTR absorbed the highest impact energy of 23 kJ/m 2 compared to HIPS/GTR (6.3 kJ/m 2 ) or ternary composite PP/HIPS/GTR (13.1 kJ/m 2 ). Low impact energy observed for HIPS/GTR was related to high rigidity of the material.

The efficient role played by polybutadiene is also highlighted by [START_REF] Wang | Thermoplastic elastomer based on high impact polystyrene/ethylene-vinyl acetate copolymer/waste ground rubber tire powder composites compatibilized by styrene-butadiene-styrene block copolymer[END_REF]. High impact polystyrene (HIPS), ethylene-vinyl acetate copolymer (EVA) and GTR were mixed and the effect of styrene-butadiene-styrene copolymer (SBS) as a compatibilizer was studied.

The addition of SBS increased the tensile strength by 145% and elongation at break by 1280%. Sample HIPS/EVA/GTR revealed highly stiff behavior, while those of HIPS/EVA/GTR/SBS indicate soft and tough profound behavior due to the presence of SBS inclusion. They explained that the GTR toughened the HIPS plastic phase while the SBS improved the interfacial adhesion of GTR and HIPS. Thus, the presence of a high concentration of GTR (55 -70 wt%) increased elongation at break and decreased initial modulus of HIPS/EVA/GTR/SBS. [START_REF] Wang | Production of thermoplastic elastomers based on recycled pe and ground tire rubber: Morphology, mechanical properties and effect of compatibilizer addition[END_REF] reported a clear phase separation in the morphology of uncompatibilized PE/GTR blend, which resulted in poor mechanical properties. So, two different compatibilizers were selected and their individual effects were studied on the properties of the blend: Ethylene-octene copolymer (Engage 8180) and Trans-polyoctenamer (Vestenamer 8012). Addition of Engage 8180 created a homogenous morphology wherein the embedded GTR particles were difficult to be identified. This is the signature of a very good PE-GTR interaction that led to improved properties. The use of Vestenamer 8012 also created a homogenous morphology, but some GTR particles were observed in the blend, and weaker mechanical properties were obtained for the prepared composite. In fact, the "ethylene" component in Engage 8180 is compatible with PE, while the other component "octene" has a weak affinity with PE and mainly interacts with styrene butadiene rubber (SBR) contained in GTR. Hence, the good interaction between PE and GTR was obtained in the presence of Engage 8180, which facilitates better stress transfer. Thus, it is critical to select the right compatibilizer for each blend formulation.

Different coupling agents were also used, and their efficiency is attested. Kakroodi and Rodrigue (2013) prepared maleated polyethylene (MAPE)/GTR thermoplastic elastomer using high concentration of GTR (50 -90 wt%). The blend MAPE/GTR displayed more than 200% elongation at break and, thus, qualified it as a competitive material. Normally, a TPE is expected to display at least 100% elongation for it to be classified as a high-performance material. For comparison, GTR was incorporated in neat HDPE, but only 64% elongation at break was achieved in the HDPE/GTR composite. This was attributed to poor homogeneity of the morphology of the blend, which was improved with the incorporation of 5 wt% MAPE coupling agent to form the ternary HDPE/MAPE/GTR system. Similarly, the presence of MAPE improved the elastic properties. [START_REF] Hamim | Effect of pegmah on tensile properties and swelling behaviour of recycled high density polyethylene/ethylene vinyl acetate/waste tyre dust (r-HDPE/EVA/WTD) composites[END_REF] examined the effect of polyethylene grafted maleic anhydride (PE-g-MAH) on the mechanical and swelling behavior of HDPE/ethylene-vinyl acetate (EVA) copolymer/GTR blend. The tensile strength of HDPE/EVA/GTR material reduced with increasing GTR concentration because of incompatibility of HDPE/EVA and GTR, leading to fracture. A similar trend of property reduction with increasing GTR content was observed for HDPE/EVA/GTR/PE-g-MAH. But, at the same GTR fraction, the enhanced adhesion created by PE-g-MAH leads to strongly higher tensile strength of HDPE/EVA/GTR/PE-g-MAH material compared to HDPE/EVA/GTR one. On the other hand, the mass swell of both HDPE/EVA/GTR and HDPE/EVA/GTR/PE-g-MAH increased with GTR due to increase in elastomer content that absorbed the swelling liquid (toluene). However, for a given concentration of elastomeric phase, the mass of swollen HDPE/EVA/GTR was greater than that of HDPE/EVA/GTR/PE-g-MAH. The chemical bond created by the reaction of the PEg-MAH acrylic anhydride group and EVA functional group established a good interaction with HDPE, thus, limited the swelling property of the compatibilized sample.

Da Costa et al. (2010) used the response surface methodology (RSM) to formulate a ternary PP/EPDM/GTR system. A strong deterioration in mechanical strength, elongation and toughness with increasing GTR content was reported, owing to the poor interfacial adhesion between PP matrix and GTR. Indeed, the GTR without strong adhesion with the matrix could not play the role of transferring energy in material, hence, creating microvoids at blend interface, causing crack propagation under tensile loading. However, whenever the EPDM content is superior or equal to that of GTR in PP/EPDM/GTR, the elongation at break increased because EPDM plays a compatibilizing role between PP and GTR phases. Thus, EPDM served the role of compatibilizer, ensuring homogeneous morphology of the composite. However, since EPDM and GTR are softer materials compared to PP, an increase in their concentration resulted in a reduction of PP/EPDM/GTR stiffness.

Effect of cross-linking and vulcanization

Hejna et al. ( 2019) investigated the influence of three radical initiators (benzoyl peroxide (BP), dicumyl peroxide (DCP) and di-tert-butyl peroxide (DP)) on the crosslink density of HDPE/SBS/GTR blends. Irrespective of the peroxide kind, the measured sol fraction from the swelling test showed an increase in crosslink densities, compared to the untreated blend, in the following order: DCP > DB > BP. As shown in Fig. 3, this resulted in an increase in the mechanical strength of 80%, 50% and 0% for blends crosslinked with DCP, DB and BP, respectively. This was related to the fact that the use of DCP initiator in the HDPE/SBS/GTR blend caused the least change in melt flow index, which ensured good crosslinking at the GTR-SBS interface.

Cañavate et al. (2011) formulated a thermoplastic vulcanizate from HDPE, EPDM and

GTR by a combination of melt-mixing and dynamic vulcanization in the presence of peroxides. HDPE/GTR samples, without EPDM, displayed weak performance due to incompatibility of HDPE and GTR that resulted in poor adhesion. On adding EPDM to the HDPE/GTR composite, toughness and elongation at break increased, but tensile strength and modulus reduced. It was showed that peroxide only vulcanized EPDM, but not GTR. As peroxide content increased, HDPE/EPDM/GTR mechanical properties increased due to greater crosslinking effect of peroxide on EPDM, creating a network of co-continuous phase that encapsulated the GTR particles. Authors reported a reduction in swelling degree with increasing peroxide content, which confirmed the increased crosslinking reaction. When peroxide concentration is sufficiently high, the elongation at break drops, which could be explained by the possible peroxide reaction with the thermoplastic HDPE. These results show the importance of the peroxide vulcanization. [START_REF] Lu | Waste ground rubber tire powder/thermoplastic vulcanizate blends: Preparation, characterization, and compatibility[END_REF] prepared thermoplastic vulcanizate (TPV) from PP/EPDM, with different particle sizes and fractions of GTR, by using a two-roll mill. They observed that tear strength, tensile strength and elongation at break improved with GTR particle size reduction, while the same properties reduced with increase in GTR content. The incorporation of polypropylene grafted maleic anhydride (PP-g-MAH) compatibilizer increased significantly PP/EPDM-GTR interfacial adhesion, hence the enhancement of above-mentioned properties. Moreover, the rheological study indicated a lower apparent viscosity of compatibilized TPV compared to uncompatibilized TPV, which depicted that PP-g-MAH improved the processing performance. [START_REF] Zhang | Preparation of thermoplastic vulcanizates based on waste crosslinked polyethylene and ground tire rubber through dynamic vulcanization[END_REF] mixed crosslinked polyethylene (XLPE) and GTR in a high-shear mechanical mill to de-crosslink the constituents in solid-state before melt-mixing them. After 20 cycles of milling, the strength of XLPE/GTR increased from 5.5 to 6.0 MPa, with an elongation at break of 185%. Moreover, after dynamic vulcanization in the presence of sulphur, the strength and elongation at break increased to 9.1 MPa and 201%, respectively. [START_REF] Li | Mechanical properties of high-density polyethylene/scrap rubber powder composites modified with ethylene-propylene-diene terpolymer, dicumyl peroxide, and silicone oil[END_REF] cryogenically mixed GTR with HDPE and reported significant deterioration of impact strength and elongation at break of the obtained HDPE/GTR material, compared to HDPE matrix. Stress maxima occurring at the HDPE-GTR interface, due to incompatibility of the components, produced cracks and propagated, which resulted in the material break. By against, the impact strength and elongation at break of HDPE/GTR increased by 160% and 150%, respectively, when EPDM, peroxide and silicone oil additives were added. Meanwhile, HDPE/GTR and HDPE/EPDM binary materials present a reduced storage modulus, signifying the influence of the dynamic vulcanizates.

Effect of added fillers

Generally, the primary objectives of the use of GTR to fabricate TPE are to recycle rubbers, and to obtain materials with competitive properties and cost. To extend applications of TPE/TPV and considering the clean environment, without compromising the targeted properties, the incorporation of different fillers is currently practiced in the development of composites [START_REF] Noriman | Properties of styrene butadiene rubber (SBR)/recycled acrylonitrile butadiene rubber (NBRr) blends: The effects of carbon black/silica (CB/Sil) hybrid filler and silane coupling agent, Si69[END_REF][START_REF] Naderi | Effect of modified single-wall carbon nanotubes on mechanical and morphological properties of thermoplastic elastomer nanocomposites based on (polyamide 6)/(acrylonitrile butadiene rubber)[END_REF][START_REF] Irez | Design and Mechanical-Physical Properties of Epoxy-Rubber Based Composites Reinforced with Nanoparticles[END_REF][START_REF] Sripornsawat | Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black[END_REF][START_REF] Deriszadeh | Epoxy/GO nanocomposite adhesives reinforced with waste blends of PET/GTR: Evaluation of mechanical properties[END_REF][START_REF] Favakeh | Dynamically vulcanized thermoplastic elastomer nanocomposites based on linear low-density polyethylene/styrene-butadiene rubber/nanoclay/bitumen: morphology and rheological behavior[END_REF]. Incorporated fillers sometimes perform a dual role of compatibilizer and reinforcement. Fillers can reduce the GTR sizes and facilitate their homogeneous distribution in the host matrix. This may therefore allow microstructural synergy between the composite constituents, which consequently improve the physical properties of the final product [START_REF] Vahidifar | Thermoplastic vulcanizate nanocomposites based on polyethylene/reclaimed rubber: A correlation between carbon nanotube dispersion state and electrical percolation threshold[END_REF][START_REF] Papadopoulos | Comparative study of crystallization, semicrystalline morphology, and molecular mobility in nanocomposites based on polylactide and various inclusions at low filler loadings[END_REF]Sanusi et al., 2021b).

Micrometric fillers

To obtain durable and more eco-friendly materials, natural cellulose fibres, such as kenaf [START_REF] Pang | Tensile properties, water uptake, and thermal properties of polypropylene/waste pulverized tire/kenaf (PP/WPT/KNF) composites[END_REF], hemp, flax and wood flours [START_REF] Cosnita | The influence of inorganic additive on the water stability and mechanical properties of recycled rubber, polyethylene terephthalate, high density polyethylene and wood composites[END_REF], are now being incorporated in thermoplastics/GTR to create high-property recyclable composites [START_REF] Mohanty | Composites from renewable and sustainable resources: Challenges and innovations[END_REF]. [START_REF] Pang | Influence of kenaf form and loading on the properties of kenaffilled polypropylene/waste tire dust composites: A comparison study[END_REF] studied the dependence of kenaf particle fibres (KNFs) and kenaf particle powders (KNFp) on the processing and properties of PP/GTR blends. The stabilization torque is higher during the mixing of PP/GTR/KNFs system. This is attributed to the higher viscosity of PP/GTR/KNFs system, which allows a better dispersion of KNFs within the blend, compared to that of KNFp. The morphology of both systems, observed by scanning electron microscopy (Fig. 4), confirms this claim, as the KNF powder was completely pulled out of the PP/GTR/KNFp composite, leaving voids. The increase of dispersion degree leads to a higher mechanical performance of PP/GTR/KNFs composite compared to PP/GTR/KNFp. ternary composite PU/GTR/BSG composite. The authors suggest that BSG fillers hinder the polymer chains movement, thus increasing the stiffness of the composite. [START_REF] Zainal | The effects of short glass fibre (SGF) loading and a silane coupling agent on properties of polypropylene/waste tyre dust/short glass fibre (PP/WTD/SGF) composites[END_REF] put short glass fibre (SGF) in PP/GTR blends. With the use of silane-coupling agent γaminopropyltrimethoxysilane (γ-APS), the thermal, tensile and swelling resistance properties of PP/GTR/SGF composite were significantly increased due to improved PP/GTR-SGF interaction.

At last, [START_REF] Moghaddamzadeh | The effect of polyester recycled tire fibers mixed with ground tire rubber on polyethylene composites. Part I : Morphological analysis[END_REF] utilized two waste components derived from worn-out tyres, i.e. GTR and recycled tyre fibre (RTF), and incorporated them in linear low-density polyethylene (LLDPE). In the presence of the RTF particles and compatibilizer MA-g-SEBS, GTR were well dispersed in the LLDPE matrix, resulting in an increase in the impact resistance of the LLDPE/GTR/RTF composite. Thus, it is clear that the incorporation of natural fibres and other micro-sized fillers can enhance the properties of thermoplastic/GTR blends. Ecologically, renewable composites are created by using natural fibres as reinforcements in blends. Nevertheless, optimal attention should be paid to the size, shape and mass fraction of the fillers as an inappropriate filler choice can deteriorate material performances. Further process treatments or agents incorporation may also be required to enhance the targeted properties. Table 1 details the summary of thermoplastic/GTR blends incorporated with different types of micrometric fillers as earlier discussed.

Nanometric fillers

The addition of nanofillers in polymeric blends leads to high-performance material, as there is the advantage of combining blends and nanocomposites properties [START_REF] Yeetsorn | Experimental study on heat dissipative ability in recycled thermoplastic vulcanizate and reclaimed rubber composites[END_REF]Sanusi et al., 2021aSanusi et al., , 2021c)). It seems that the nanoparticles act as compatibilizer, which reduces interfacial tension and promote reduction of GTR particle size during processing [START_REF] De Sousa | Blends of ground tire rubber devulcanized by microwaves/HDPE-Part B: Influence of clay addition[END_REF]. Thus, the particles coalescence is reduced, and increase in morphology refinement is achieved. Besides, dynamic vulcanization is adopted to significantly improve the interaction of matrix, GTR and nanoparticles [START_REF] Razmjooei | Preparation of dynamically vulcanized thermoplastic elastomer nanocomposites based on LLDPE/reclaimed rubber[END_REF][START_REF] Paran | Enhancement of mechanical, thermal and morphological properties of compatibilized graphene reinforced dynamically vulcanized thermoplastic elastomer vulcanizates based on polyethylene and reclaimed rubber[END_REF]. Meanwhile, when nanoparticles such as carbon black (CB) are added in the matrix, carbon crystallites form, leading to particles aggregation, which causes reduction in the specific surface area of the nanofillers [START_REF] Huang | Carbon black filled conducting polymers and polymer blends[END_REF]. Therefore, appropriate preparation techniques, like surface treatment of the particles, should be achieved to facilitate agglomerate-free dispersion of nanoparticles in matrix. 2. Remarkably, SiO2

gives the superior properties at negative temperature, while CaO supports high-temperature condition. The behaviors of the composites were correlated to specific ionic character of each oxide.

Table 2

Mechanical properties of polymer blends composites subjected to different temperature conditions [START_REF] Cazan | The influence of temperature on the performance of rubber -PET-HDPE waste -based composites with different inorganic fillers[END_REF]. -tensile strength, -Young's modulus, -compresive strength, and impact energy. [START_REF] Sripornsawat | Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black[END_REF] mixed thermo-chemical devulcanized GTR and copolyester (COPE) and examined the effect of carbon black (CB) on the microstructure and mechanical behavior of COPE/GTR composite. They found that an increase in CB content facilitated the transformation towards the co-continuous structure of COPE/GTR and favored a uniform dispersion of smaller vulcanized GTR domains in the COPE matrix (Fig. 5). The incorporated CB particles, which facilitated the rubber-filler interaction, significantly increased the viscosity of the COPE/GTR blend. The resulting shear effects during mixing were enhanced, leading to better CB dispersion and hence strengthening of the composite.

Thus, the composite COPE/GTR/CB exhibits increased stress relaxation, tensile strength and hardness compared to COPE/GTR, however with little a reduction in elongation at break. The authors also found that CB inclusion improved the damping property of composite. Moreover, percolation threshold was identified at a low concentration of 1 wt% MWCNT. In fact, at this concentration, the nanofillers formed a network structure along blend to achieve high electrical conductivity property. [START_REF] Kazemi | Highly stretchable conductive thermoplastic vulcanizate/carbon nanotube nanocomposites with segregated structure, low percolation threshold and improved cyclic electromechanical performance[END_REF] developed MA-g-PE/GTR/MWCNT nanocomposite for an electrical application. They observed that the Fourier transform infrared spectroscopy (FTIR) characteristic peaks of maleic anhydride (1714 and 1791 cm -1 ) in neat MA-g-PE are significantly reduced in MA-g-PE/GTR. This suggests that the maleic anhydride groups grafted on PE formed a chemical link with the C=C bonds of GTR particles, which favors the interfacial adhesion. Moreover, the incorporation of MWCNT facilitates the dispersion of GTR in MA-g-PE. So, the MA-g-PE/GTR/MWCNT nanocomposite exhibited good mechanical performance. In fact, the strong compatibility achieved with the help of MWCNT prevents crack propagation through the interface. If MWCNT facilitates the dispersion of GTR in MA-g-PE, GTR in return lowers the amount of MWCNT required to obtain significant electrical conductivity. This is partially explained by the fact that MWCNT cannot penetrate into the complex 3D-structure of GTR and is localized in the MA-g-PE phase of the ternary system, easily creating 3D networks of MWCNT. Thus, the electrical conductivity of the material is enhanced, while the required concentration of the MWCNT is reduced.

As far as nanoplatelets are concerned, [START_REF] Deriszadeh | Epoxy/GO nanocomposite adhesives reinforced with waste blends of PET/GTR: Evaluation of mechanical properties[END_REF] 2018) analyzed the effect of montmorillonite (MMT) nanoparticles and electron beam irradiation on the properties of EVA/GTR blend. They found that the nanoclay, at low mass fractions, in EVA/GTR exhibited an intercalated structure that favored its dispersion and resulted in high mechanical strength, whereas an increase in the MMT concentration led to clay agglomeration that resulted in strength reduction. On the further treatment of EVA/GTR/MMT nanocomposite with irradiation, the interaction between the blend and the MMT nanoparticles was improved. [START_REF] Hassan | Synergistic effect of montmorillonite-clay and gamma irradiation on the characterizations of waste polyamide copolymer and reclaimed rubber powder nanocomposites[END_REF] studied the combined effect of MMT nanoparticles and gamma irradiation on the thermal and mechanical properties of the blend constituted with polyamide (PA) and GTR. It was reported that the gamma radiation remarkably increased in the tensile strength and thermal stability of the PA/GTR/MMT nanocomposite compared to untreated nanocomposite. The gamma radiation treatment facilitated the release of free radicals on GTR surfaces that attacked the thermoplastic PA to form covalent bonds within the two phases. In addition, the radiation induced the crosslinking that created interatomic interaction by hydrogen bond, while the MMT nanoparticles were homogeneously dispersed, leading to improved properties.

On a notably ecological importance, [START_REF] Cosnita | The influence of inorganic additive on the water stability and mechanical properties of recycled rubber, polyethylene terephthalate, high density polyethylene and wood composites[END_REF] mainly used GTR, recycled PET and little amount of HDPE (5 wt%) to prepare a PET/HDPE/GTR blend for outdoor applications. The ternary blend was filled with wood dust and rigid calcium oxide (CaO)

nanofillers, aiming at achieving low density, high mechanical properties and dimensional stability of the final composite. Two significant roles of the CaO nanoparticles were identified: First, they ensure dimensional stability of PET/HDPE/GTR/wood composite by absorbing water molecules. Second, the absorbed water molecules make the CaO ordering degree increased, which promotes its interactions with the other composite constituents (GTR, PET and wood). The wood dust plays the role of a coupling agent at PET-HDPE-GTR interface, whereas the dispersed CaO absorbs the wood dust moisture (and acidity) to achieve outstanding water resistance material.

Therefore, the role of nanofillers in facilitating GTR particle size reduction and dispersion in matrices is an asset for creating high performance materials. Nanofillers can act as a barrier, coupling agent as well as a conductive material depending on the intended application. Table 3 summarizes various thermoplastic/GTR/nanofillers materials as previously discussed above.

Conclusion

The mixing of waste tyre particles (GTR) with thermoplastic matrices to fabricate thermoplastic elastomers (TPE) is an efficient, promising, and practical means of recycling large amount of the worn-out tyres. The development of recyclable composites is further possible with the incorporation of renewable reinforcements. Although the incorporation of GTR in thermoplastic consumes reasonable quantity that addresses environmental concerns, the properties of the final TPE composites need to be improved for competitive advantage, especially if higher percent of GTR (>50%) is to be used. Several methods have been highlighted to improve the interfacial adhesion: addition of compatibilizers/coupling agents, application of dynamic vulcanization techniques, high-energy irradiation treatments and so on. These approaches, individual or coupled, improve the compatibility of crosslinked rubber (GTR) molecules making them entangle with thermoplastic molecules and favoring their dispersion to form high-performance composites. More particularly and more recently, adding nanoparticles in thermoplastic/GTR blends improves the GTR dispersion in thermoplastic phases and facilitates the reduction of GTR size in the TPE, leading to better nanocomposite performance. The recyclability and renewability of thermoplastic/GTR can also be achieved by using natural cellulose fibres as reinforcing phase without altering the mechanical properties of the composites.

The developed thermoplastic/GTR composites have been applied in various engineering fields. For instance, these materials have been used in high-thermal foam [START_REF] Formela | Structural, thermal and physico-mechanical properties of polyurethane/brewers' spent grain composite foams modified with ground tire rubber[END_REF], ferromagnetic [START_REF] Sridhar | Fly ash reinforced thermoplastic vulcanizates obtained from waste tire powder[END_REF], impact resistance [START_REF] Başboğa | Determination of some technological properties of injection molded pulverized-HDPE based composites reinforced with micronized waste tire powder and red pine wood wastes[END_REF], electric conductive [START_REF] Kazemi | Highly stretchable conductive thermoplastic vulcanizate/carbon nanotube nanocomposites with segregated structure, low percolation threshold and improved cyclic electromechanical performance[END_REF] and adhesive [START_REF] Deriszadeh | Epoxy/GO nanocomposite adhesives reinforced with waste blends of PET/GTR: Evaluation of mechanical properties[END_REF] applications. Aside other conventional applications of thermoplastic/GTR composites in the construction and sports sectors, their relevance in defence and security technology may be another interesting application area. Specifically, in the field of armour and ballistic engineering, thermoplastic/GTR nanocomposites could be a potential alternative due to their reported impact resistance, thermal and mechanical properties. Thermoplastic/GTR nanocomposites could also be relevant in acoustic and sound applications. In the meantime, more strict regulations against indiscriminate disposal of waste tyres need to be considered worldwide.

Aggressive awareness creation and research support for more innovative research development around waste tyre recycling and management should be pushed across all nations.

Fig. 1

 1 Fig. 1 SEM of NR/HDPE/GTR composite irradiated at 0, 50, 100, and 150 kilograys (kGy) absorption doses (Mohamad et al., 2021).

Fig. 2

 2 Fig.2Tensile strength of NR/HDPE/GTR composites irradiated at 0, 50, 100, and 150 kGy absorption doses: RR stands for GTR[START_REF] Mohamad | Insight on the properties of thermoplastic elastomer-based natural rubber and recycled rubber post-treated with electron beam irradiation[END_REF].

Fig. 3

 3 Fig.3Stress-strain curves of HDPE/SBS/GTR (S) blends mixed with different peroxides[START_REF] Hejna | Towards understanding the role of peroxide initiators on compatibilization efficiency of thermoplastic elastomers highly filled with reclaimed GTR[END_REF].

Fig. 4

 4 Fig. 4 SEM images of fractured surface of PP/WTP/KNF composites filled with (a) 10 phr KNFp; (b) 20 phr KNFp; (c) 10 phr KNFs; (d) 20 phr KNFs composites (Pang and Ismail, 2014).

  [START_REF] Cazan | The influence of temperature on the performance of rubber -PET-HDPE waste -based composites with different inorganic fillers[END_REF] mixed polyethylene terephthalate (PET), HDPE and GTR. It was shown that the incorporation of different oxides (TiO2, CaO, ZnO and SiO2) in the ternary PET/HDPE/GTR material influences the interface performance towards improving mechanical properties at different temperatures, as summarized in Table

Fig. 5 .

 5 Fig. 5. SEM images showing the average size of GTR domain in COPE/GTR/CB at different concentration of CB: {0, 5, 10, 15, 20} (wt%)[START_REF] Sripornsawat | Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black[END_REF]

Table 1 476

 1 Thermoplastic/GTR blends incorporated with different types of micrometric fillers

	477
	Blend	GTR	GTR	Filler type	Filler	Additional	Processing	Properties analyzed	Reference
		particle	concentration		concentration	treatment/	method
		size	range		range	agent
	PP/GTR	-	65 phr	Fly ash	10-50 phr	SEBS-g-MAH Extrusion	Morphology,	(Sridhar et al., 2009)
								Tensile,
								DMA,
								Thermal,
								Rheology,
								Magnetic property
	PP/GTR	0.2-150	30 phr	Kenaf	5-20 phr	-	Melt mixing Morphology,	(Pang and Ismail, 2014)
		µm						Tensile,
								Thermal,
								Water uptake test
	HDPE/GTR 250 µm 7.5-30 wt%	Flax	7.5-30 wt%	SEBS-g-MAH Melt mixing Morphology,	(Kakroodi et al., 2012)
								Tensile,
								Flexural,
								Density
	MAPE/GTR 50-850	5-26 wt%	Hemp	10-60 wt%	-	Extrusion	Morphology,	(Kakroodi et al., 2013)
		µm						Tensile,
								Impact test
								Water uptake and ageing,
								Hardness,
								Thermal,
								Density
	PU/GTR	1 mm	5-20 wt%	Brewers'	5-20 wt%	Flame	Mechanical	Foaming parameter,	(Formela et al., 2017)
				spent grain		retardant,	stirring	Morphology,
						Foaming		Tensile,
						agents		Structure,
								DMA,
								Thermal stability

Samples kept at high temperature: +10 to +50 °C)

		(oxides)	(MPa)	(MPa)	(MPa)	(kJ/m 2 )
	PET/HDPE/GTR (35:5:60)	0	1.57	2.56	69.7	7.12
	PET/HDPE/GTR/Oxide	TiO2	1.10	17.9	72.1	8.70
	(No-aged samples)	CaO	1.12	11.7	70.4	11.4
		ZnO	0.92	20.9	82.4	10.6
		SiO2	1.29	25.3	82.7	12.1
	PET/HDPE/GTR/Oxide (TiO2 CaO	1.25 1.35	14.1 21.6	43.7 71.9	10.5 11.0
		ZnO	0.82	3.15	35.2	10.1
		SiO2	0.96	6.23	68.8	10.9
	PET/HDPE/GTR/Oxide
	(
	Blend composition	Nanofillers

Samples kept at low temperature: -40 to -10 °C)

	TiO2	1.38	8.56	74.6	9.78
	CaO	1.04	4.03	70.2	9.51
	ZnO	1.08	7.59	86.5	12.3
	SiO2	1.70	9.77	80.2	9.92
	NB:

Table 3 583

 3 Thermoplastic/GTR nanocomposites filled with different types of rigid nanofillers.

	584
	Blend	GTR	GTR	Filler	Filler	Additional	Processing	Properties analyzed	Reference
		particle	concentration	type	concentration	treatment/	method
		size	range		range	agent
	COPE/GTR	40	50 wt%	CB	0-40 phr	Dynamic	Melt mixing	Mechanical,	(Sripornsawat et al.,
		mesh				vulcanization		DMA,	2018)
								Rheology,
								Morphology
	EVA/GTR	-	20 phr	MMT	2-10 phr	Electron beam	Extrusion and	Structure,	(Bee et al., 2018)
						irradiation	compression	Gel content test,
							molding	Tensile,
								Thermal,
								Morphology
	HDPE/GTR	-	80 wt%	MMT	5 wt%	Dynamic	Melt mixing	Tensile,	(De Sousa et al., 2015)
						vulcanization		Rheology,
								Morphology,
								DMA
	LLDPE/GTR	11-14	30-50 wt%	MWCNT 1-3 wt%	PE-g-MA,	3-step melt	Morphology,	(Vahidifar et al., 2019)
		µm				Dynamic	mixing	Tensile,
						vulcanization		Hardness,
								Thermal,
								Rheology
	MAPE/GTR	<50 µm 50 wt%	MWCNT 0.5-1.0 wt%	PE-g-MA	Solution,	Structure,	(Kazemi et al., 2018)
							melt casting	Morphology,
								Calometric,
								Tensile,
								Electrical
	PET/HDPE/GTR	1 mm	60 wt%	TiO2,	0.25-2 wt%	-	Melt mixing	Tensile,	(Cazan et al., 2019)
				CaO,				Thermal,
				ZnO,				Contact angle,
				SiO2				Structure,
								Morphology
	PET/HDPE/GTR	1 mm	78-80 wt%	CaO	0.5-2 wt%	-Wood sawdust	Melt mixing	Physicochemical	(Cosnita et al., 2017)

Acknowledgement

The Tertiary Education Trust Fund (TETFund) is acknowledged for the PhD sponsorship.

 [START_REF] Kakroodi | Mechanical and morphological properties of flax fiber reinforced high density polyethylene/recycled rubber composites[END_REF]incorporated lignocellulosic flax fibre in HDPE/GTR in the presence of coupling agent (SEBS) to improve matrix-rubber-fibre surface adhesion. Substantial physical properties improvement was reported. Then, Kakroodi et al. (2013) prepared a ternary composite of maleic anhydride grafted polyethylene/GTR/hemp.

Significantly, the addition of hemp fibres increased the mechanical and thermal properties of the system, which were sustained even after ageing in water. More recently, wood-plastic composites based on HDPE and GTR were prepared using either red pine wood flours (RPF) [START_REF] Başboğa | Determination of some technological properties of injection molded pulverized-HDPE based composites reinforced with micronized waste tire powder and red pine wood wastes[END_REF] or sugarcane bagasse (SB) [START_REF] Rodríguez-Anda | Composite based on recycled highdensity polyethylene-sugarcane bagasse-tire rubber waste: formulation, mechanical and workability evaluation[END_REF]. In the later study, it was found that the best mechanical properties were achieved with the sample having 50/20/30 wt% (HDPE/GTR/SB) concentrations.

Aside the natural cellulose fibres discussed above, other fillers like natural mineral (e.g., feldspar), industrial wastes (fly ash from coal thermal plants, brewer spent grain) and glass fibre are among the fillers incorporated in thermoplastic/GTR blends to achieve highperformance composites [START_REF] Sridhar | Fly ash reinforced thermoplastic vulcanizates obtained from waste tire powder[END_REF]Hassan et al., 2014b;[START_REF] Bukit | Mechanical properties of composite thermoplastic hdpe / natural rubber and palm oil boiler ash as a filler[END_REF][START_REF] Allouch | Experimental investigation on the mechanical behavior of recycled rubber reinforced polymer composites filled with aluminum powder[END_REF]. However, careful material design, selection and treatment process must be ensured to avoid properties degradation. For instance, [START_REF] Sridhar | Fly ash reinforced thermoplastic vulcanizates obtained from waste tire powder[END_REF] incorporated GTR in PP thermoplastic and studied the effect of fly ash on performances of the resulting composite. The fly ash showed more affinity towards the rubbery GTR phase than PP, which created widespread debonding of fly ash particles. The weak interaction between the PP and fly ash particles leads to a reduction of strength by ~3% for 10 wt% added fly ash. The authors assumed that the trend could be reversed by treatment of fly ash with coupling agents. [START_REF] Formela | Structural, thermal and physico-mechanical properties of polyurethane/brewers' spent grain composite foams modified with ground tire rubber[END_REF] prepared a composite foam composed of a polyurethane (PU) filled with GTR and brewer spent grain (BSG). Incorporation of GTR into PU/BSG composite raised its strength higher by 50% and accelerated its foaming reaction. In the same way, dynamic mechanical properties and thermal stability were increased with increasing GTR in