Recycling of Waste Tyre into Silica-Rubber Compounds for Green Tyre Application

The prevention of detrimental effects to environment, owing to generation of a huge amount of rubber wastes, is a big challenge across the globe that warrants a thorough investigation of recycling and reuses waste of rubber products.In this spirit a sustainable development of a devulcanization process along with the production of value added devulcanized rubber is a task of hours.The present work describes a simultaneous devulcanization and chemical functionalisationprocessof waste solution styrene butadiene rubber (S-SBR). This kind of rubber is generally used as the main polymer component in silica filled tread rubber compounds for high-performance passenger car tyres. As-grown ethoxy groupson the functionalized devulcanized styrene butadiene rubber (D-SBR)are exploited for the coupling between silica and the devulcanized rubber chains. We compare the mechanical and dynamic mechanical performance of D-SBR with that of virgin SBR control composites. Covalently bonding interfaces developed from the pendent ethoxy groups of D-SBR and silanol groups on the silica surface offer a competitive and promising performance of the D-SBR based composites. We conclude that the present approach can be further utilized for the large-scale production of different rubber products with satisfied elastomeric performance.


Introduction
The development of an efficient devulcanization process of discharged crosslinked rubber products, for example, tyres and other various auto parts, is a challenging task in this 21 st century. The worldwide tyre production and waste generation scenario indicates that about 1.7 billion new tyres are produced in a year and over 1 billion waste tyres are spouted to the environment per year [1]. Due to three-dimensional covalently crosslinked network structure and the presence of many chemical additives, vulcanized rubber is not degradable by natural biological processes or by other chemical means like hydroxylation, oxidation etc. As a result, waste rubber creates serious environmental hazards [2]. So far,a number of different methods are introduced like mechanical [3][4][5][6], mechanochemical [7][8][9][10][11][12], microwave [13][14][15] and ultrasound [16,17] treatments. However, the desired level of devulcanization to regenerate the raw rubber has not been attained. It is worth mentioning that a straight complete recirculation of devulcanized rubber compounds intohigh-performance rubber products is not realized so far because of the poor mechanical performance of the devulcanized rubber products. However, a partial replacement of fresh rubber by devulcanized rubber is reported as possible and leads to adequate properties of the final rubber product [18,19].
A specific treatment route describes the mechano-chemical devulcanization of ground tyre rubber in presence of bitumen and three different types of additives such as peptizer (P300), organic peroxide [di(2-tert-butyl-peroxyisopropyl) benzene] and vulcanization accelerator (tetramethylthiuramdisulphide, TMTD) [20]. In this case the vulcanized rubber can be reclaimed to a reasonable degree. Detailed studies onthe extent of reclaiming clearly specify that the used additiveshave a significant influence on the rubber processing, as well on the physicomechanical and thermal properties of the reclaimed ground tyre rubber (GTR). Furthermore, it was shown that the GTR can be used as component in efficient rubber blends, e.g. in combination with fresh nitrile butadiene rubber (NBR).In another work, alow molecular weight disulfide based organic compound is used to devulcanize waste ethylene propylene diene monomer (EPDM) rubber which was then mixed with fresh EPDM in different proportions (20 -40 wt%) [21]. The results indicated that optimum cure-and scorch-time of the rubber blend were not affected uptoan added amount of 40 wt% of reclaimed EPDM.However, mechanical properties like tensile strength and elongation at break of the revulcanized blends were improved up to 14% and 26%, respectively. Different proportions (20 wt % to 60 wt %) of mechanochemically devulcanized GTR can be revulcanized in a composition with natural rubber (NR) and polybutadiene rubber (BR) in a 70:30 proportion [22]. Here, reclaiming of GTR can be carried out using TMTD in the presence of spindle oil, a paraffin-based rubber process oil. The evaluation of curing characteristics of NR-BR/devulcanized rubber (DR) vulcanizates indicates that the optimum cure time decreases with DR content, andthe observed improvement of mechanical properties of the vulcanizates is attributed to the enhanced interfacial interaction between DR and NR-BR matrix. A cost-effective rubber blend based on 92 phrfresh NBR and 8 phr devulcanizedNR is prepared and, subsequently, an evaluation of the mechanical properties of the vulcanizatewas performed by using highly reinforcing nanofiller. The nanofillerswere synthesized by condensation reactions between terephthaloyl chloride (TPC) with bis(4aminophenyl) sulfone (BAPS) and bisphenol polyesteramidesulfone (PEAS) [23]. The results indicatethat the addition of PEAS improvesthe compatibility between NBR and reclaimed NR and enhances the effective crosslink density of the vulcanizates through physical interlocking leading to an increase of the tensile strength and decrease of the elongation at break of the vulcanizates. However, the amide and ester groups of PEAS affect the thermal stability of the vulcanizate up to 420 o Cas compared to neat NBR. Devulcanized rubber prepared from GTR using the microwave method can be mixed with styrene butadiene rubber (SBR) to develop new rubber compounds for further revulcanization [24]. It is observed that the devulcanized rubber/SBR composites offered improved frictional properties on nonabrasive surfaces and a significant rise in mechanical properties as compared to that of the SBR/GTR composites.Thermoplastic vulcanizates (TPVs) are prepared by mixing mechano-chemically devulcanized GTR (DR)/polypropylene (PP)/EPDMto study the effect of peroxide curing andradiation on mechanical, thermal and structural parameters of the developed composites [25].
Use of1 phrdicumyl peroxide (DCP) and 25 kGy -radiation dosecan offer promising mechanical properties of the composites.Thermo-mechanical devulcanizationof GTR is carried out at different temperatures (60, 120 and 180 o C) using a co-rotating twin screw extruder and the reclaimed rubbersare mixed with fresh SBR in 10 to 50 wt% [26]. It is found that the SBR vulcanizates containing devulcanized rubberlead to higher glass transition temperatures as comparedto the vulcanizates containing untreated GTR. Grinding of waste rubber in the mills with ultrasonic activation can lead to the formation of a crumb rubber with particle size distribution in the range of 100 to 150 which can be used in different rubber composition as a replacement of devulcanizedrubber [27]. The experimental results demonstrate thatreplacement of regenerated rubber by a large proportion crumb rubber can lead to significant enhancementoftensile strength and elongation at break of the vulcanizates. Foaming of devulcanizedground tyrerubber (DGTR) as an alternative to natural rubber was developed and various applications such as cushioning, heat insulation, sound absorption etc. were discussed [28]. The effects of ethylene vinyl acetate (EVA), sodium carbonate as blowing agent and dicumyl peroxide (co-curatives) on the DGTR propertiesare studied and it is found that the density and hardness of the foam decreased with increasing sodium bicarbonate content because of the formation of a higher number of voids by the released carbon dioxide gas. However, the density and hardness of the composites were found to be enhanced with increasing EVA content.
Crosslinked rubber powder was mechano-chemically modified through 2,2 /dibenzothiazoledisulfideat high shearing condition with the help of a twin-screw extruder at 100 o C [29]. In the study both unmodified rubber powder and chemically blended rubber powder were treated in the extruder to form mechanically modified and mechano-chemically modified rubber powder (MCRP), respectively. This chemical modification results a better processing characteristic and improved mechanical properties of MCRP/NR composites. A waste-to-product method was developed by hot-pressing technique to prepare membranes having gas permeance and separation characteristics using reclaimed tyre rubber as the polymer precursor [30].  [31]. They also reported that devulcanized GTR was very useful for the fabrication of a single electrode triboelectric nanogenerator (SETNG)which might produce electric energy during sliding friction. SETNG made of reclaimed rubber at 170 o C produced more output voltage than that of the reclaimed rubber at 190 o C, and exhibited distinct tactile sensation towards different objects such as human finger, latex rubber glove, cotton glove and polytetrafluroethtlene (PTFE) sheets.
In this work mechano-chemical devulcanization and simultaneous functionalization ofcommercially available waste SBR were done by using bis(3-triethoxysilyl propyl) tetrasulfide (TESPT) and subsequently the devulcanized rubber (D-SBR) is utilized to prepare new rubber composites using silica as additional reinforcing agent. It is known that silica-based rubber composites are found to be most suitable for energy efficient so-called green (PC) tyre production, and in this work D-SBR is expected to have chemical groups that can allow to establish a direct chemical coupling between externally added silica with rubber chains. With the developed recycled silica-rubber composites, systematic characterizations of chemical modification of the D-SBR, the effect of D-SBR on the structure property relationship are thoroughly investigated. Likewise, underlying strong reinforcement mechanism by the chemical interaction of D-SBR (with grafted TESPT fragments) and silica are demonstrated that may pave the way to produce silica based recycled rubber composites for green tyre applications.

Preparation of devulcanized SBR (D-SBR)
100 g of SBR crumb is mixed with 6 mL of bis(3-triethoxysilyl propyl) tetrasulfide (devulcanizing agent) and 10 g of aromatic oil at room temperature and subsequently transferred to a two-roll mixing mill followed by mixing at a friction ratio of 1:1.25 for 40 min at ~70 to 80 o C.It is seen that with progress of milling a band formation occurs in the roll and the scrap powder material is transformed to homogeneous elastomeric material. The physical characteristics of D-SBR like sol content, crosslink density, and degree of devulcanization are found to be 25%, 0.306 × 10 −3 mol/cm 3 , 72.3% respectively. The Mooney viscosity ML(1+4) 100 o C was found to be 22.

Preparation of SBR/D-SBR/Silicacomposites
Virgin SBR, different proportions of D-SBR and compounding additives such as ZnO, stearic acid, sulphurand CBS were carried out for 10 min at room temperature on a two-roll mixing mill. Compound formulations are presented in Table 1. In order to study the polymer-filler interaction as well as the crosslinking density of the composites, the tensile stress-strain data wereused to the Mooney-Rivlin equation [33,34]: where the term in the left-hand side is known as the reduced stress ( ) and the term 1 where is the volume fraction of silica.To avoid complexity the volume fraction of the filler from the original crumb rubber is not considered here. The extension ratio is represented by λ and is evaluated from λ = 1 + ε , where is the strain.
A Mooney-Rivlin crosslink density ( ν MR ) can be evaluated from the constant 1 with the following equation [36] ν MR = 1 where and are the universal gas constant and the measurement temperature in Kelvin scale, respectively. In

Curing characteristics
The cure behavior of the compounds is shown in Fig. 1.It is evident from this figure (

Mechanical properties
The stress-strain behavior of SBR/D-SBR/Silica composites containing varying proportion of D-SBR is presented in Fig. 2a. Corresponding values such as modulus at 100%, 200%, 300% elongation, tensile strength, elongation at break, tear strength and hardness are summarized in Table 2. It is seen that moduli at 100, 200 and 300% elongation steadily increase with D-SBR    Hardness (Table 2) also steadily increases with increasing D-SBR content. The tear strength presented in Table 2

Thermogravimetry analysis
The thermogravimetric curves (TG and DTG) of SBR (control) and different SBR/D-SBR/silica vulcanizates are presented in Fig. 3a. The degradation temperature at various weight loss, onset and end temperature, the temperature at maximum weight loss, the rate of change of weight loss at and char residue are shown in Table 4. The initial minor weight loss at around 145-240 o Cwas due to the presence of volatile matter like stearic acid and absorbed water at around 300 o C [41]. Although the SBR (control) vulcanizates shows one stage degradation but SBR/D-SBR/silica vulcanizates shows two stage degradation. The first stage of degradation takes place in the region of 360-425 o Cand the second stage of degradation occurs around 425 -515 o C.The first one related to the decomposition of D-SBR and the second one due to the SBR polymer decomposition [42]. There is no weight loss above 550 o C which indicatesoxidation of carbon black present in D-SBR, originated from scrap SBR does not take place [43]. The degradation temperature at 10, 50 and 70% weight loss of SBR/DeVulcSBR/silica vulcanizates are much higher than that of the SBR (control) vulcanizates due to strong rubber-filler interaction. In SBR/D-SBR/Silica vulcanizates, the temperature at 10% weight loss and the maximum degradation temperature decrease with D-SBR content because devulcanized rubber contain large amount of processing additives. Therefore, with increasing devulcanized rubber content 10% weight loss occurs at low temperature. The temperature corresponding to 50% and 70% weight loss increases with D-SBR content due to interfacial interaction between

Differential scanning calorimetry analysis
The DSC thermograms of different SBR/D-SBR/silica composites are shown in Fig. 3b and the respective values of glass transition, heat capacity increment, transition width and fraction of immobilized polymer chains are presented in Table 5. The devulcanized rubber content does not significantly affect the calorimetric glass transition temperature ( ) but the heat capacity increment ∆ in the glass transition region decreases with increasing D-SBR content whichindicates that the addition of the D-SBR limited the movement of the rubber molecular chains as the surface of the silica filler produced a restricted molecular layer due to interfacial interaction between D-SBR and silica [44]. The heat capacity increment in the glass transition region of neat rubber was maximum as SBR molecular chains were not affected by silica fillers and could move freely with high degree of freedom and as a result showing highest enthalpy. By considering ∆ as a measure of the quantity of devulcanized rubber that takes part in the glass transition, this decrease of ∆ is usually considered in terms of an immobilized layer of devulcanized rubber around the silica particles. The fraction of immobilized rubber   im  can then be evaluated by the following equations [45]: where,

Dynamic mechanical analysis
The interfacial interaction between rubber-filler and filler-filler greatly affects the  Table 6. However, the room temperature storage modulus of SBR (control) vulcanizates is much inferior than that of the D-SBR containing vulcanizates due to strong filler-filler interaction as a result silica agglomerates are formed in the rubber matrix which is responsible for poor compatibility of silica filler in the fresh SBR, whereas, in D-SBR containing vulcanizates the respective values are much higher due to interfacial interaction between the ethoxy group of D-SBR and hydroxyl group of silica.  Table 6 it is seen that is consistently decreased with increased D-SBR content indicating more non-elastic component in the rubber matrix. More bound rubber and higher reinforced state of the rubber matrix could be the reason behind such behavior.
The impact of filler reinforcement can also be explained in terms of Cole-Cole type plots.
Here, the moduli plots E'' vs. E' from the temperature sweepsof SBR/D-SBR/silica vulcanizates are presented in Fig. 4d.  The strain sweep analysis at room temperature is shown in Fig. 4e. It can be found that, as expected, the D-0 shows a little dependency of the storage modulus on the dynamic strain amplitude, however, the modulus further increases with increasing D-SBR loadings. This substantiates the formation of both reinforcing networks: filler-filler and filler-polymer networks which the last one is defined as the Payne effect [47]. The overall dynamic performance here is mainly governed by different factors like the chemical activity of the active groups of the devulcanized rubber, the presence of sulphur curatives in the recycled rubber, pre-crosslinked rubber molecules, fragments of the devulcanized rubber chains and the final results are the complex interplays between all those factors. More studies are further needed to understand the overall mechanical properties of the devulcanized rubber composites. It is also seen from the D-SBR containing micrographs that in higher D-SBR loaded composites (Fig. 5c & 5d) the maximum matrix tearing line and surface roughness is observed compared to that of the Fig. 5b having lower D-SBR content. A superior rubber-filler interaction in Fig. 5c & 5d altered the fracture path, which led to increased resistance to fracture propagation that resulted an increase in tensile modulus, tensile strength and hardness of the composites as evident from the performance of the mechanical properties of the composites. The micrographs of the tensile fractured surfaces were in good concurrence with the results studied by other researchers [50,51] who stated that surface roughness and matrix tearing lines in the fractured surface occurred due to increase in rupture energy.

Conclusion
The research describes a unique technology to devulcanize waste rubber and its subsequent   Table of Content