elastomeric nanocomposites for tyre

Elastomeric Nanocomposites for TyreApplications

Kaushik Pal, Samir K. Pal, Chapal K. Das and Jin Kuk Kim

Abstract In this study the epoxidized natural rubber (ENR) and organoclay (Cloisite20A) composites were prepared by solution mixing process. The obtained nanocom-posites were incorporated in natural rubber (NR) and styrene butadiene rubber (SBR)blends in presence of varying types of carbon black as reinforcing fillers. Morphology,curing characteristics, mechanical and thermal properties were characterized andanalyzed. Also, the wear characteristics of the nanocomposites against Du-Pont andDIN abrader were determined and discussed. The morphology of the organoclayincorporated in ENR shows a highly intercalated structure. ISAF type of carbon blackshows a significant effect on curing and mechanical properties by reacting at theinterface between SBR and NR matrix. Blends containing ISAF N234 type of carbonblack shows high abrasion resistant properties against Du-Pont and DIN abrader.

1 Introduction

1.1 General

In automobile industry, the design of engine and other mechanical componentsreceives prime importance, while the tyres are often overlooked. Most means of

K. Pal (&) and J. K. KimPolymer Engineering and Science, School of Nano and Advanced Materials,Gyeongsang National University, Jinju, Gyeongnam, 660-701, South Koreae-mail: [email protected]@gmail.com

S. K. PalMining Engineering Department, IIT, Kharagpur 721302, India

C. K. DasMaterials Science Centre, IIT, Kharagpur 721302, India

V. Mittal et al. (eds.), Recent Advances in Elastomeric Nanocomposites,Advanced Structured Materials, 9, DOI: 10.1007/978-3-642-15787-5_8,� Springer-Verlag Berlin Heidelberg 2011

201

transport run by the use of tyres, thus, tyre plays an important and effective role intransportation. In automobiles, tyre is an inseparable assemblage of materials withvery wide range of properties whose manufacture demands great precision.

Tyre industries consume more than 60% of the rubber product; and the primefactors of consideration are safety and tread life. Thus, 30% of the mechanicalwork available from the fuel is dissipated in the tyres. A reduction of 15% in tyrerolling resistance may improve fuel consumption by 4.5% [1].

Throughout every industry, whatever type of machine is used, the concerns arethe same: lowering operating costs and maintaining or improving the level ofsafety on site. In both areas, tyres are an integral factor. Tyres are one of thegreatest consumable costs in surface mining and in underground trackless opera-tions, representing as much as 20% of the operating costs of some machines [2].

Increasing the abrasion resistance of rubbers and rubber products is one of theproblems of the rubber industry and is particularly important for the tyre industry. Ananalysis of the reasons, why tyres wear out?, from the data of tests on hundreds ofthousands of series of production tyres shows that from 60 to 90% of tyres go out ofuse because of wear in the tread. On modern scales of tyre production, every 10%increase in useful life means a saving of significant costs. It is therefore important tothe economy of the countries that the quality of tyres should be improved [3].

1.2 What is Tyre?

A tyre is a composite, in other words an inseparable assembly of materials withvery different properties, whose manufacture demands great precision.

Tyres, or tires (in American and British English, respectively), are eitherpneumatic enclosures, or solid items (including rubber, metals and plastic com-posites). They are used to protect and enhance the effect of road wheels.

Pneumatic tyres are used on many types of vehicles, from bicycles, motorcy-cles, cars, trucks, to earthmovers and aircrafts. Tyres enable vehicle performanceby providing for traction, braking, steering, and load support. Tyres provide aflexible cushion between the vehicle and the road, which smoothes out shock, andprovides comfort (Fig. 1).

Fig. 1 Structure of tyre

202 K. Pal et al.

It is made up of the following semi-finished products:

(1) The inner liner,(2) The casing ply,(3) The lower bead area,(4) Bead wires,(5) Sidewalls,(6) Bracing plies,(7) Tread

The fundamental tyre functions are:

(1) Proving load carrying capacity,(2) Provide cushioning and enveloping,(3) Transmit driving and breaking torque,(4) Producing tractive force,(5) Provide dimensional stability,(6) Resist abrasion,(7) Provide steering response,(8) Have low rolling resistance,(9) Provide minimum noise.(10) Permit minimum road vibration,(11) Be durable and safe.

1.2.1 Materials for Tyre

The basic materials for the production of tyres are:

(1) Base Rubber: NR, SBR, NBR, PBR, PUR, XNBR etc.(2) Fillers: carbon black, China clay(3) Additives: sulfur, peroxide, process oil (aromatic or aliphatic), wax, acceler-

ator (CBS, MBT, MBTS, TMTD, DCBS, TBBS, DPG etc.), acceleratoractivator (ZnO, Stearic acid etc.), antioxidant and antiozonant (IPPD, HQ, TQetc.), silica, nano fillers (clay, nanotuber, fibre etc.)

(4) Wire: steel, brass, nylon, polyvinyl, polyamide, polyester cord etc.

1.3 Tyre Life and the Causes of Tyre Wear

There are many factors that influence tyre life. Some of these are:

1. Cuts2. Contamination3. Dual tyre matching4. Tyre rotation5 Vehicle maintenance

Elastomeric Nanocomposites for Tyre Applications 203

a. Misaligned wheelsb. Wheel balancingc. Mechanical irregularities

6. Rolling resistance7. Inflation pressure

a. Over inflation/under loadingb. Under inflation/over loading

8. Heat9. Incorrect alignment

10. Grip11. Load12. Abnormal tyre wear

1.3.1 Damage and Wear of Tyres

Following are some reasons leading to damage and wear of tyres:

1. Tread detachment2. Air pockets3. Sidewall cuts and rupture of the sidewall4. Rupture resulting from a cut in the tread5. Impact ruptures6. Pinch rupture caused by road shock

Fig. 2 Different types of wounds in tyres

204 K. Pal et al.

7. Bead damage

a. Incorrect fitting or removalb. Due to over inflation

The images of several types of damages are depicted in the Fig. 2.

1.4 Phenomenon of Wear in Rubber

Wear is ‘the process in which the tyre becomes unfit for use during a certainminimum service life’, which to considerable extent results from, or is related to,continuous damage of the tread [4].

Investigation by Rymuza [5, 6] shows that the wear dynamics of polymer–polymer and polymer-metal systems is determined by properties of the polymersuch as surface energy, modulus of elasticity, specific heat, thermal conductivityand various operating conditions. Ratner [5, 7], Lewes [8], Rhee [9], Lancaster[10], Atkinson [11] and others have developed various forms of equations andrelationships for the wear of polymers using variables such as load/pressure, speed,sliding length, sliding duration, shear strength of polymer etc. In 1974, Kar andBahadur [12] developed a wear equation in terms of the sliding variables, pressure,speed, time and the material properties, modulus of elasticity, surface energy,thermal conductivity and specific heat.

A proper understanding is necessary for control and prediction of polymerperformance [13]. A vast amount of literature has appeared over the years in whichrelations between tribological performance and polymer properties are describedin terms of mechanical parameters, such as yield and shear stress, toughness (asdefined by the product of stress- and strain-to-break), plasticity index, Young’smodulus, and hardness [14–17]. These studies and reviews have helped in pre-dicting polymer behaviour in sliding wear and friction to a certain level.

1.4.1 Relation between Abrasion Resistance and Mechanical Propertiesof Rubber

Regarding physical ideas on the nature of abrasion, Schallamach [18, 19] was thefirst to examine the simple case of the failure of rubber by the action of a hardprojection. The coefficient of friction l can also be expressed as a function of theelastic and hysteresis properties of the rubber and of the configuration of theabrading surface [18, 19].

1.4.2 Effect of Temperature on the Resistance to Wear

In certain operating conditions of tyres e.g. sudden braking and acceleration, sharpbends and high speeds etc., high temperature develops in the contact area [20].


It has been observed [19] that during intense abrasion in sliding contact, hightemperature is developed, and consequently, the abrasion resistance of the rubberdepends to large extent on the resistance to high temperature and heat. One sug-gestion is that chemical degradation of the sliding interface of the rubber can beattributed to a thermal effect [21]. In the wear of tyres, the temperature of therubber at the interface needs to be considered in relation to abrability, which is afunction of temperature [22].

1.4.3 Mechanism of Wear of Tread Rubbers

Tread wear in pneumatic tyres may be measured as weight loss or decrease incrown thickness, or more commonly as a loss of tread depth over prolongedperiods. Pavement texture, as might be supposed, plays a major role in determiningthe extent and severity of the wear mechanism, in addition to driving habits,climatic conditions and operational factors. There are several mechanismsinvolved for tyre abrasion, such as,

(1) Fatigue, or hysteresis wear(2) Abrasive, or catastrophic wear(3) Cohesive tearing, or wear by roll formation(4) Tread reversion and blistering(5) Smearing of rubber(6) Threshold strength of rubber

1.4.4 Relation between Abrasion and Tensile Strength of Rubber

The investigation of abrasion by scratching rubber with needle led Schallamach tothe conclusion that wear of rubber on sharp abrasives was due to tensile failure[23, 24]. Buist and Davies [25] proposed an empirical relation between volume (V)of wear and physical properties of rubber as

V ¼ C0 þ C1 � Shore hardnessþC2 � Tensile strength ð1Þ

where, C0, C1, and C2 are constants. Thornly [26] has similarly been able tocorrelate tyre wear with hardness and tensile properties.

The effects of the particle size and structure of various carbon blacks on frictionand abrasion behavior of filled natural rubber (NR), styrene-butadiene rubber(SBR) and polybutadiene rubber (BR) compounds were investigated [27] using amodified blade abrader. Characteristic parameters like particle size and thestructure of carbon blacks were observed to have a linear relationship with theYoung’s modulus. The frictional coefficient depended not only on the particle size,but also on the structure of carbon black. The rates of abrasion were decreasedwith increasing surface area and developing structure of carbon blacks.

Parkins [28] showed that abrasion resistance of the carbon black filled rubberwould increase if the tensile strength is higher.

206 K. Pal et al.

Rattanasoma et al. [29] showed that the vulcanizates containing 20 and 30 phrof silica in hybrid filler exhibit better overall mechanical properties as well asabrasion resistance properties.

Persson [30] reported that friction force decreased as the velocity increased inthe high velocity range depending on the nature of the substrate, surface roughnessand the mechanical properties of rubber. Persson’s attention was focused onexamining the behavior of bulk rubber undergoing continuous motion.

Many attempts have been made to find a relationship between the rate ofabrasion loss and the physical and mechanical properties of rubber [22, 31–33].Uchiyama [33] found the following relationship for determining the wear volumeof rubber abrasion:

V ¼ k1lP

rBL ð2Þ

where V is the wear volume, l represents the friction coefficient, P denotes thenormal load, L the length of rubbing distance and k1 is a constant. The parameterrB is expressed by the following equation:

rB ¼ rN ð3Þ

where r is maximum amplitude of tensile stress and N is the number of cycles.It has been shown that the wear resistance could be correlated with the

mechanical properties of the vulcanizates [34].

1.5 Development of Wear Resistant Rubber Blends

Polymer blends are being used extensively in numerous applications, especially intyre manufacture.

In the time of the twelfth century, the source of the whole of the rubbersupplied to the industry was natural rubber. In the nineteenth century, CharlesGoodyear invented the process of ‘sulfur vulcanization’ and suggested the useof ground natural rubber to overcome its limitations [35]. As a consequence ofthe first world war, Germany introduced Buna rubbers which are purely syn-thetic rubber and it increased the curiosity of polymer chemists all over theworld. Various attempts were made in laboratory to enhance the properties ofnatural rubber thus transforming it into a material of desirable properties. As aresult, synthetic rubbers with tailor-made properties were born. Consequently,different chemicals and methods for vulcanization and processing weredeveloped.

Apart from blends of common rubbers, specialty rubber is also utilized forsuch purposes, depending on service demands and components of the tyre [36,37]. Many reports covering a wide range of rubber blends have been published.The use of carbon black is synonymous with the history of tyres. Although it


has lost some ground to other reinforcing fillers such as silica, but, by virtue ofits unrivalled performance, it is still the most popular and widely used rein-forcing filler. However, the primary properties of carbon blacks are normallycontrolled by particle size, surface area, structure, surface activity and they arein most cases interrelated [38]. In tyre treads, silica can yield a lower rollingresistance at equal wear resistance and wet grip than carbon black [39]. Naturalrubber (NR) is a virgin rubber, having properties resembling those of syntheticrubbers. Natural rubber (NR) is known to exhibit numerous outstanding prop-erties; reinforcing fillers are necessarily added into NR in most cases in order togain the appropriate properties for specific applications. A wide variety ofparticulate fillers are used in the rubber industry for various purposes, of whichthe most important are reinforcement, reduction in material costs andimprovements in processing [40]. Reinforcement is primarily the enhancementof strength and strength-related properties, abrasion resistance, hardness andmodulus. It can offer unique properties such as good oil resistance, low gaspermeability, improved wet grip and rolling resistance, coupled with highstrength. A lot of research has been carried out on NR and SBR blends byvarying the quantity and composition of additives and fillers. The abrasionresistance of styrene-butadiene tread rubbers is observed to depend to largeextent on the molecular weight distribution, the average molecular weight andcertain other factors [41–43].

Solution styrene butadiene rubber (S-SBR) is used in a wide variety of appli-cations, including the production of tyres, footwear, conveyor belts, hoses, flooringand adhesives [44]. Solid solution polymerized styrene butadiene rubber, producedby anionic batch polymerization is available in a wide variety of styrene and vinylcontents. S-SBR rubbers provide excellent balance between wet grip, rollingresistance and dry handling in silica and carbon black compounds for high-per-formance tyres [45]. They are also used for the manufacture of high qualitytechnical rubber goods. As it is well known, the performance of motor car tyresmust be improved, whilst at the same time reducing the amount of natural rubberused. This problem applies even more to large truck tyres, and can be solvedprovided that new types of stereo-regular synthetic rubbers like isoprene [46–48]and butadiene [46, 49–51] are used.

The effects of mixing method, blend ratio, content, type of carbon black, andvulcanization system have also been compared. Tyres used in mining vehicles arevery costly and need regular maintenance, and it is expensive to replace themwithin a very short term. The rugged working conditions in mining industriesreduce the life span of tyres on account of cuts, contamination, abrasion, wear,speed fluctuations etc. There are several types of damage which occur in the dump-truck tyre such as tread detachment, sidewall cuts, impact ruptures, bead damageetc. [39, 52]. The idea of blending synthetic rubbers with natural rubber is certainlynot a new one, but it is only now that this can be shown to be possible withconsistently positive results, by the use of new techniques developed over the lastyears.

208 K. Pal et al.

1.5.1 Compatibility of Polymer Blends

Polymer blend is the intimate mixture of two or more polymers and/or copolymersresulting from common processing steps [53]. Throughout the last decades, sci-entific and technical literatures in this area have expanded remarkably as evi-denced by appearance of several books and proceedings of various conferences[54]. The word compatibility has been used by many investigators to describesingle phase behavior. However, the terms compatible and incompatible refer tothe degree of intimacy of the blends, which depends on the measurement proce-dure during examination. A blend could be considered as a compatible blend, if itdoes not exhibit the gross symptoms of component polymer segregation, whereas aheterogeneous blend at a macroscopic level is incompatible. Compatible system ofblends containing high molecular weight polymers have been identified usuallywhen a favorable specific interaction such as hydrogen bonding, dipole interactionor ionic interaction exists between two components. Although the majority ofthermoplastics/elastomer blends are heterogeneous, the components may bereferred to as compatible if some technically advantageous combination of prop-erties can be realized from the blends. Partial compatibility implies that above aparticular level either the minor or the major components remain as a dispersedphase.

There are some technical problems, which are frequently the result of sometype of mutual incompatibility, which provide an inferior set of properties whendissimilar polymers are blended together [55]. The blending process and thequality of the blends can be improved by adjusting the individual raw polymerviscosity. As a result, the effective viscosities of the phase will no longer mis-match. The thermodynamic incompatibility can be overcome if the surface energydifferences between polymers are small enough to permit the formation of verysmall micro domains of the individual polymer phases and there is sufficientadhesion between the phases by formation of crosslinks across the interface duringblending [56, 57]. The compatibility of various components and the generation ofsingle phase from multiphase system play a major role in influencing the physicalproperties of the polymer blends. The most significant need of the designer ofpolymer blends is to ensure good stress transfer between the components of themulti-component system which can only vouch for the efficient utilization ofcomponent physical properties of the blends. Numerous techniques have beenutilized to determine the compatibility of the blends but only a few predict goodresults [58, 59].

1.6 Nanofillers

Nanofillers have for many years high significance in the plastics industry.Nanofillers are basically understood to be additives in solid form, which differfrom the polymer matrix in terms of their composition and structure. They


generally comprise inorganic materials, more rarely organic materials. Inactivefillers or extenders raise the quantity and lower the prices, while active fillers bringabout targeted improvements in certain mechanical or physical properties. Theactivity of active fillers may have a variety of causes, such as the formation of achemical bond (e.g., cross linking by carbon black in elastomers) or filling of acertain volume and disruption of the conformational position of a polymer matrix,and also the immobilization of adjacent molecule groups and possible orientationof the polymer material [60].

There are many grades of nano fillers, e.g. carbon black, carbon nanotubes,carbon fiber, activated clay, natural clay (mined, refined, and treated), clay (syn-thetic), natural fiber, silica, zinc oxide etc. (Fig. 3).

1.6.1 Effects of Nanofillers

Nano filler are expected to improve the properties of materials significantly, moreeven at lower loading than conventional/micro-fillers. There are some reasons forthat [61]:

(1) The size effect: large number of particles with smaller inter-particle distanceand high specific surface area results in larger interfacial area with the matrix.

(2) The interactivity and potential reactivity of the nanofillers with the medium.

1.6.2 Nanofiller Reinforcement

The difference between the behaviors of micro and nano-reinforced polymers canbe analyzed by observing the specific changes in properties in nanoscale, in whichpolymer chain lengths approach the filler dimensions so that they might displayparticular interaction influencing the macroscopic behaviour of the materials.

Fig. 3 Nanosilica asobserved from TEM

210 K. Pal et al.

Indeed, many parameters can be taken into account for the reinforcement effi-ciency of filler into given medium (Fig. 4) [62, 63].

(1) chemical nature of fillers(2) shape and orientation of the fillers(3) average size, size distribution, specific area of the particle(4) volume fraction(5) dispersion state(6) interfacial area(7) respective mechanical properties of each phase

1.6.3 Several Uses of Nanofillers

• Solar energy—tougher, more efficient solar cells are already under develop-ment, with the promise of drastic cost reductions on the horizon. Some will evenproduce hydrogen.

• Fuel cells• Display technologies and e-paper—e-paper and carbon-nanotube-based field-

emission displays expected to be slugging it out with liquid–crystal displays(with carbon-nanotube-based backlights, of course) in the next 2 years.

• Nanotubes—both as raw materials and as products. Multi-walled nanotubes arealready used in composites, to increase conductivity at much lower filler loads.Single-walled nanotubes will have a much bigger effect in the longer term.

• Catalysis has a huge potential geopolitical impact, especially after recentdevelopments in the energy business.

• Nanocomposites—mainly clay-based for structural applications (increasedstrength) or with novel properties. These are already penetrating theautomotive and aerospace industries.

Fig. 4 Different phases ofnanocomposites


• Storage technologies—magnetic random access memory (RAM), nanotubeRAM and terabyte hard drives in the next few years.

• Nanocrystalline bulk materials or steels containing nanoparticulates—somecompanies are already using steel with nanoparticulate carbon added during therolling process.

• Coatings—extra hard or with special properties, such as being electrochromic orself-cleaning, are under investigation by everyone from car manufacturers toarchitects.

• Sensors—bio and chemical sensors made from nanowires and nanotubes arecurrently probed.

• Bio analysis—devices using atomic force microscopes and quantum dots arealready being readied for market.

• Textiles—nanofibres in stain-resistant trousers are already available, withelectrospun nanofibres and nanotube-enhanced fibers coming soon.

1.7 Tyre Retreading

Tyres that are fully worn can be re-manufactured to replace the worn tread.Retreading is the process of buffing away the worn tread and applying a new tread.Retreading is economical for truck tyres because the cost of the new tread is smallcompared to the cost of the tyre carcass. Retreading is less economical for pas-senger tyres because the cost is high compared to the cost of a new tyre. However,commercial truck drivers run the risk of ‘‘blow-outs’’, separation, and tread peelingfrom the casing, due to constant re-use of the casing.

Commercial trucking companies have taken their own initiative as well. Manyonly run retreads on their trailers, and keep ‘‘Virgin Casings’’ (new tyres) on theirSteer and Drive wheels. This ensures that in the event that a retread blows out, thedriver maintains control over the truck.

1.8 Objectives and Scope of Work

As mentioned earlier, apart from the blends of common elastomers, specialtyelastomers are also utilized for tyre applications, depending on service demandsand components of the tyre [36, 37] and carbon black is one the most commonlyused fillers [44, 64–66].

The primary aspect in preparing organoclay nanocomposites is to attain a veryhigh degree of dispersion of organoclay aggregates that afford to very large surfaceareas. Hence, it exhibits significant improvements in physical, mechanical andthermal properties in relation to the polymer host [67–69]. Though many organo-clay thermoplastics have been prepared and studied [70–73], less attention has been

212 K. Pal et al.

paid to use organically modified layered silicates in reinforcing elastomers [74–76].In this study, rubber-organoclay nanocomposites have been prepared and analyzed.The accomplishment of highly dispersed organoclay nanocomposites has tworequirements. The first requirement involves the compatibility between the polymerand nanoclay to obtain better dispersion of organoclay in the polymer matrix.Organoclay can be easily dispersed in polar polymers than in non-polar polymers.The formulation of organoclay/polar polymeric systems usually contains a poly-meric compatibilizer [77, 78] e.g. ENR. ENR is obtained by epoxidation of 1,4-polyisoprene, depicts higher glass transition temperature and increased polarity.Accordingly, as a fine dispersion of organoclay is needed, ENR was also chosen asa compatibilizer in this study. Arroyo et al. [52], Teh et al. [79] and Varghese et al.[80] have carried out few resplendent works using ENR as compatibilizer fororganoclay/natural rubber nanocomposites. In our previous literature, we havealready analyzed the effect of ENR as a compatibilizer in natural rubber-nanoclaygum compounds [81] and in presence of carbon black [82].

The second requirement is the preparation methods of nanocomposites. Variousmethods have been adopted for the preparation of rubber/organoclay nanocom-posites that include in situ polymerization intercalation [83], solution intercalation[84] and melt intercalation [85], and finally co-coagulation of rubber latex and clayaqueous suspension [86]. In this study, the solution intercalation process has beenused to make the organoclay nanocomposites.

Tyres used in mining vehicles are very costly and need higher abrasion resis-tance. An increase in the abrasion resistance of rubber products can be achieved bystudying the mechanism of wear of rubber under different operating conditions.The wear of rubber is a complex phenomenon and dependent on a combination ofprocesses such as mechanical, mechano-chemical and thermo-chemical etc.Schallamach [87] and later Grosch [88] reviewed abrasion of rubber and tyre wear.Champ et al. [89] and Thomas [90] suggested that abrasion takes place through acyclic process of cumulative growth of cracks and tearing. Kragelskii et al. [91]and Schallamach [92] examined the simple case of the failure of rubber by theaction of a hard projection moving over its surface. It has been observed thatduring intense abrasion in sliding contact, a high temperature is developed, andconsequently the abrasion resistance of the rubber depends, to a large extent, on itsresistance to high temperature and heat [92]. The earlier literature review hasdemonstrated several types of damages and its causes in the tyre [93–112].

Blending of elastomers has been often used to obtain an optimum number ofdesirable combinations, physical properties, processability and cost. The elasto-mers selected in this study were natural rubber (NR) and styrene butadiene rubber(SBR). Since NR and SBR are non-polar, epoxidized natural rubber (ENR) is usedas a compatibilizer to improve the dispersion of organoclay for nanocompositepreparation. In this study, incorporation of organoclay in ENR was done bysolution mixing, in order to obtain uniform dispersion of the nanoclay in ENR. Theobtained ENR-organoclay nanocomposites were incorporated in the NR/SBRblends with varying types of carbon black. The cure characteristics, morphologi-cal, mechanical and thermal properties of nanocomposites were investigated.


These rubber compounds were examined in a specially fabricated experimentalset-up for evaluating their wear resistance properties, when abraded against vari-ous rock types.

1.9 Experimental

1.9.1 Materials Used in Rubber Preparation

Natural rubber (RMA-1X) was supplied by the Rubber Board, Kottayam, Kerala.The styrene butadiene rubber (SBR) used was Krylene HS 260, No.-5 of 1948grade of Bayer AG. Its styrene content is 23.5 ± 1.0, specific gravity is 0.94 andMooney viscosity at 100�C is 50 ± 5. Epoxidized natural rubber containing 47%epoxidation unit was supplied by Agricultural Product Processing ResearchInstitute, Zhangiang, PR China. Cloisite 20A, a natural montmorillonite (MMT)modified with a quaternary ammonium salt with cation exchange capacity of95 meq/100 g clay (Southern Clay, Inc, USA), was used as a nanofiller in thepreparation of the nanocomposites. Zinc oxide, stearic acid, N-cyclohexyl-2-benzothiazyl sulfonamide (CBS) and N-isopropyl-N-phenyl-p-phenylenediamine(IPPD) were supplied by Bayer (India) Ltd. Standard rubber grade process oil(Elasto 710) and paraffinic wax were purchased locally. Carbon black was sup-plied by Birla Carbon.

1.9.2 Solution Mixing Method

Epoxidized natural rubber was dissolved in methyl ethyl ketone (MEK). The ratioof the rubber to solvent was 1:3 (weight/volume). Continuous stirring was per-formed at room temperature, until the rubber was completely dissolved in thesolvent. Subsequently 100 wt% of nanoclay (Cloisite 20A) was added to the rubbersolution and stirring was continued. The resultant solution was then cast over in athoroughly cleaned plane glass plate. The sample was kept in the same conditionuntil the solvent was completely evaporated. Appearance of a transparent film wasobserved. The obtained nanocomposites contained 1:1 ratio of ENR and nanoclay.

1.9.3 Preparation of Nanocomposites

The compounds were prepared in two-roll mixing mill operated at room temper-ature. The speed ratio of the rotors was 1:1.4. Initially the natural rubber andstyrene butadiene rubber was masticated followed by incorporating ENR/nanoclaycomposites. The reinforcing filler (carbon black) was added along with the processoil followed by curatives and shown in Table 1.

Also, three types of carbon blacks and one semi reinforcing filler were used,such as, SAF N110, ISAF N231, ISAF N234 and SRF N774. For vulcanization, the

214 K. Pal et al.

amounts of additives such as sulfur, process oil, CBS were based on 100 wt% ofrubber and the samples had the codes ‘A’, ‘B’, ‘C’, ‘D’, ‘E’, and ‘F’, respectively.The physical properties of the different types of carbon black used in this studyhave already been discussed in our earlier literature [113].

1.9.4 Experimental Techniques

Experimental techniques followed in the present research are as follows.

Cure Characteristics of Rubber Compound

The cure characteristics of the rubber compound were studied with the help of aMonsanto Oscillating Disc Rheometer (ODR—100 s) at 150�C as per ASTMD-2084-07. From the graphs, the optimum cure time, scorch time and Cure RateIndex (CRI) could be determined.

Determination of Crosslink Density

The cross-link density was determined by immersing a small amount of sample in100 ml benzene for 72 h to attain equilibrium swelling. After swelling, the samplewas taken out from benzene and the solvent was blotted from the surface of thesample and weighed immediately. This sample was then dried out at 80�C toremove all the solvent, and reweighed. The volume fracture of rubber in swollen

Table 1 Compound formulation

Compounds Sample codes

A B C D E FWeight in wt%

Natural rubber (NR) 75 75 75 65 65 65Styrene butadiene rubber (SBR) 25 25 25 35 35 35Carbon blacks

1. SAF (N110) 20 – – 20 – –2. SRF (N774) 20 – – 20 – –3. ISAF (N234) – 40 – – 40 –4. ISAF (N231) – – 40 – – 40

ENR/nanoclay (Cloisite 20A) 10 10 10 10 10 10Stearic acid 2 2 2 2 2 2Antioxidant (HQ) 1 1 1 1 1 1Accelerator (CBS) 1 1 1 1 1 1Zinc oxide 5 5 5 5 5 5Process oil 2 2 2 2 2 2Sulfur 2 2 2 2 2 2


gel Vr, which represented the relative cross-link density of the vulcanizate, wasdetermined by the following equation.

Vr ¼m0Uð1� aÞ=qr

m0Uð1� aÞ=qr þ ðm1 � m2Þ=qs

ð4Þ

where m0 is the sample mass before swelling, m1 and m2 are sample masses beforeand after drying, U is the mass fraction of the rubber in the vulcanizate, a is theloss of gum EVM vulcanizate during swelling, qr and qs are the rubber and solventdensities respectively.

The physical cross-link density was calculated using modified [114] Flory-Rehner equation from swelling measurement in benzene reported earlier [115].

1Ml

c

¼ �½Vr þ vV2r þ lnð1� VrÞ�

qrVSðV1=3r � Vr=2Þ

ð5Þ

where, 1/M’c, Vr, Vs, v and qr are the physical cross-link density, volume fractionof rubber, molar volume of swelling medium, the Flory–Huggins solvent-rubberinteraction parameter and density of rubber respectively.

Lastly, the chemical cross-link density was determined from the followingequation.

12Mc

¼ 12Ml

c

þ 1M� 1:55� 10�5 ð6Þ

where 1/2Mc, 1/M’c and M are the chemical cross-link density, physical cross-linkdensity and molecular weight of rubber vulcanizate respectively.

X-Ray Diffraction Measurements (XRD)

X-ray diffraction was performed with a PW 1840 X-ray diffractometer with acopper target (Cu-Ka) at a scanning rate of 0.050 2h/s, chart speed 10 mm/2h,range 5,000 c/s, and a slit of 0.2 mm, applying 40 kV, 20 mA to assess the changeof crystallinity of the blends as a function of blend ratio [116]. The range of 2hscanning of X-ray intensity employed was 1.5�–10�.

The degree of crystallinity (vc) was measured using the following relationship:

vc ¼ Ic=ðIa þ IcÞ ð7Þ

where, Ia and Ic are the integrated intensity of the crystalline and amorphousregion, respectively.

The crystallite sizes (P) and the interplaner distance (d) are calculated asfollows:

P ¼ Kk=b cos h ð8Þ

216 K. Pal et al.

D ¼ k=2 sin h ð9Þ

where b is the half height width (in radian) of the crystalline peak, k is thewavelength of the X-ray radiation (1.548 for Cu), and k is the Scehrrer constanttaken as 0.9 [117].

Mechanical Characterization (Tensile and Tear)

Vulcanized slabs were prepared by compression molding, and the dumb-bellshaped specimens for tensile tests and crescent shaped specimens for tear testswere punched out from a molded sheet by using ASTM Die C. The tests wereperformed on a universal tensile testing machine (Hounsfield H10KS) underambient condition (25 ± 2�C), following the ASTM D 412-06 and ASTM D 624-00 (2007). The modulus at 100, 300 and 500% elongation, tensile strength, tearstrength and elongation at break (%) were measured at room temperature. Theinitial length of the specimens was 25 mm and the speed of the jaw separation was500 mm/min. Samples were tested five times for each set of conditions, at thesame elongation rate. The values of the tensile strength, modulus at 100% elon-gation, 300% elongation, 500% elongation and elongation at break were averaged.The relative error was below 5%.

The hardness was measured by Shore A hardness tester following ASTMD2240-05 standards.

Thermal Characterization

Thermal characterization (TGA) studies were carried out Shimadzu-DT-40instrument in presence of air at a rate of 10�C/min, using temperature range of 25 to650�C. Degradation temperature of the composites was studied through this anal-ysis. It is well known that because of the high flexing of tyre off the road (dump-truck), the temperature rises. In tropical countries, the temperature can go as high as150�C. Hence, the study of thermal resistance of the compounds is desirable.

Transmission Electron Microscopy (TEM)

The dispersion morphology was observed in the high resolution transmissionelectron microscope (HRTEM, JEOL 2100). The samples were ultramicrotomed at-20�C for ENR/nanoclay films and -70�C for ENR/nanoclay composites in NR.

Scanning Electron Microscopy

The tensile fracture surface of the samples was studied in a scanning electronmicroscope (JSM-5800 of JEOL Co.; acceleration voltage of 20 kV; gold coating


and 500 and 1000 times magnification. Scanning electron microscopy (SEM) wasused to study the morphology like filler dispersion and indentation of the abraderon the abraded rubber surface of the samples prepared.

Du-Pont Abrasion Test

Du-Pont abrasion test was done by the Du-Pont Craydon type of abrasion tester fordetermining the abrasion resistance of compounds of vulcanized rubber recom-mended by the Indian Standards Institution vide IS:3400 (Part-III)—1965.

DIN Abrasion Test

DIN abrasion test was done by the DIN abrasion tester for determining theabrasion resistance of compounds of vulcanized rubber recommended by theIndian Standards Institution vide IS:3400 (Part 3)—1987.

Heat Buildup Study

Heat build up study was carried out using Goodrich Flexometer for the selectivecompounds having higher tensile strength.

The test pieces were prepared in cylindrical shape having diameter17.8 ± 0.15 mm and height of 25 ± 0.25 mm by compression molding at 150�C.The test pieces were kept at initial temperature of 50�C and stoke of4.45 ± 0.03 mm. The temperature and the load were kept constant throughout thestudy. The temperature attained by the samples after the time periods of 10 and20 min was recorded.

1.10 Results and Discussions

1.10.1 Cure Characteristics of the Rubber Compounds

The optimum cure time (t90) for ‘C’ and ‘E’ rubber sample is higher than otherrubber vulcanizates as shown Table 2. It is possibly due to the mixing of 25 wt%of SBR irrespective of carbon black grade. The t90 of the compounds sharplydecreased may be due to the amine functionalities in the filler after the modifi-cation process or ion exchange process. The rate of cure {tmax - tmin} alwaysincreased with increasing concentration of NR. This increase in cure rate can bedue to the fact that an increasing concentration of NR caused the increase invulcanization reaction and created more active cross link sites in the rubbercompound.

218 K. Pal et al.

Maximum torque can be considered as a measure of stock modulus [118]. Thetorque difference (MH–ML) which shows the extent of cross linking, [119] is foundto have higher variation from one compound to other. The lesser torque differenceis found for the compound ‘B’ and ‘D’ which contain less gel fraction [120](Table 2) compared to other rubber vulcanizates, which reduces the maximumrheometric torque. The obtained cross-link density values from Flory-Rehnerequation [121] correspond with the variation in torque differences.

The lower cross-link density in 10 wt% of ENR/nanoclay, which contains 3.09wt% of nanoclay for ‘B’ hinders the formation of chemical cross-links andphysical cross-links are formed by the clay bundles [122]. As a result of this, thedecrease in MH–ML value is observed. By the use of semi reinforcing type ofcarbon black the scorch time reduces. This decrease in scorch time was due topresence of active cross-linking sites in the vulcanized rubber [123]. Faster curerate index is observed in Table 2 for the compounds containing 25 wt% of SBRand 75 wt% of NR. The decrease in cure rate may be due to the greater thermalhistory formed during mixing, as a result of their higher compound viscosities.Also, the possible formation of a Zn complex in which sulfur and ammoniummodifier participate may facilitate for the increase in rate of cure [124].

1.10.2 XRD Analysis

X-ray diffraction patterns of epoxidized natural rubber with 100 wt% of nanoclayloading are shown in Fig. 5. The d-spacing (spacing between the planes in theatomic lattice) values were calculated using the Bragg’s Law.

The organically modified nanoclay patterns showed an intense peak around2h = 3.144�, corresponding to the basal spacing of 2.58 nm (d001). The EC patternshowed that the d001 main diffraction peak shifted towards the lower angle2h = 2.12�, corresponding to the basal spacing of 3.79 nm (d001). The peak shiftto a lower angle corresponds to the increased distance between interlayers. Thehigher d-spacing value is observed with ENR which signified an intercalatedstructure, suggesting that rubbery polymer was incorporated into the interlayerspacing. XRD data showed that the extent of intercalation was a function of thepolarity of the rubber.

Table 2 Cure characteristics

Properties A B C D E F

Min. Torque (dN m) 11.39 12.19 11.32 10.40 11.35 10.83Max. Torque (dN m) 37.58 32.75 59.39 31.29 46.87 43.39TS2 induct time (min) 1.71 1.82 1.60 2.15 1.81 1.91TS5 scorch time (min) 2.32 2.40 2.37 2.75 2.51 2.33TC90 opt. cure time (min) 5.64 5.97 10.07 6.85 9.15 7.85Opt. cure (min) 35.23 29.62 53.75 38.58 40.13 37.81Cure rate index (min-1) 23.86 22.54 13.04 19.32 14.88 16.94Crosslink density (moles/g) 9 10-5 2.27 1.89 4.02 2.66 3.12 3.68


1.10.3 Mechanical Properties of the Rubber Samples

Tensile strength, modulus, elongation at break, tear strength for all the compoundsare shown in Table 3. The tensile strength of ‘A’, ‘E’ and ‘F’ is higher in thesystem, because in the case of rubber vulcanizates, the rubber chains orientthemselves in the direction of stretching creating crystallites. These crystallites tietogether a large number of network chains and contribute to high tensile strengthand elongation. But for ‘E’ sample, the tensile strength increases with carbon blackISAF N234, possibly due to the outstanding reactivity of the carbon black acting asfiller, thus enhancing the properties of the samples. While for other blend types,the tensile and tear strength starts to decrease, the filler is uniformly dispersed inthe natural rubber matrix which can be attributed to the aggregation of claynanolayers [125], also confirmed by the SEM images shown in Fig. 7. Theaggregation leads to the formation of weak points in the NR matrix, accordinglyreducing the elastomeric strength [126, 127]. The filler has high aspect ratio whichleads to improved interfacial bonding and form filler-rubber interactions becauseof the high specific surface area of the filler. The mechanical properties of rubbervulcanizates markedly depended on the number of conjugate double bonds

Fig. 5 XRD study of ENRand nano clay

Table 3 Mechanical properties of the blends

Samplecode

Tensilestrength(MPa)

Elongationat break (%)

100%modulus(MPa)

300%modulus(MPa)

Tearstrength(N/mm)

Hardness(Shore A)

A 12.19 577 2.25 6.61 50.6 61B 9.44 457 2.01 5.01 25.2 57C 8.55 505 1.85 3.45 19.7 64D 10.19 526 2.44 5.66 32.4 69E 13.17 529 3.01 7.77 33.8 78F 12.65 524 2.89 6.32 36.6 70

220 K. Pal et al.

(sample ‘E’ and ‘F’). These observations suggest that more SBR reacts with thecarbon–carbon double bonds, slower is the reversion reaction rate and henceincreases the mechanical properties of the vulcanizates. The tear resistance ofelastomers is mainly dependent on the processes by which stress dissipation nearthe tip of the growing crack takes place. Several processes such as slippage orbreakage of crosslinks or chain entanglements or arresting of the growing crack byfiller particles take place during the tear failure of elastomers [128]. The tearstrength for all the samples is moderate except sample ‘A’, but it varies withvarying the matrix ratio. So the system with carbon black SAF N110 and SRFN774 gives better reinforcing effect as well as tear strength. It is believed that atlower filler content, the filler can be dispersed well in the rubber matrix and thefiller can extend further propagation. However, at higher filler content, the fillertends to form agglomerates, thus decreases the tear properties of composite. Themodulus of all the NR vulcanizates increased with increasing concentration ofSBR. This was because of the following possible reasons: the restriction ofmolecular chain mobility, and an increase in the cross-link density. The maximumtorque (MH) is generally correlated with the durometer hardness and modulus. Thisindicates that the incorporation of organoclay filler increases the stiffness of therubber. The hardness of ‘D,’ ‘E’ and ‘F’ rubber sample was higher than otherblends as shown in Table 3. The increase in hardness of those rubber samplesprobably increases the cross-link density.

1.10.4 Thermal Analysis

High temperature Thermal Analysis (TGA) (50–650�C) curves for the sample areshown in Table 4. The temperature for the onset of degradation (T1), the tem-perature at which 10% degradation occurred (T10), the temperature at which 50%degradation occurred (T50) and the temperature at which 90% degradationoccurred (T90) were calculated from the TGA plots.

It was observed that the onset degradation temperature was more or less samefor all the samples except sample ‘B’. The onset degradation temperature therebyprobably decreased in the case of rubber sample due to a decrease in cross linkdensity (Table 4). Cross linking increased the rigidity of the system, which in turnincreased the thermal stability [129, 130]. This proves that increasing percentageof SBR content is responsible for the increase in thermal stability.

Table 4 Thermal propertiesof the rubber compounds (�C)

Sample code T10 T50 T90

A 315 339 558B 307 339 558C 315 339 552D 317 410 549E 315 414 540F 318 413 558


1.10.5 TEM Study

TEM is the most lineal method to observe the dispersion of nanoclay. TEM imageof ENR with nanoclay composites is shown in Fig. 6. TEM photomicrographsshowed uniform distribution of the organo-clay in the NR-SBR matrix. Nanoclayclusters were observed from the image. The dark lines are the silicate layers, inwhich bulks of the nanoclay dispersion are in the intercalated state. The claystructures did not break down during mixing in the NR-SBR matrix and interca-lation was observed from the TEM photomicrographs. It reveals that nanoclaydispersion is in the intercalated state, which affirms the better dispersion ofnanoclay in ENR [124].

It should also be noted that TEM showed that the clay layers were dispersed inthe NR matrix at the nano level but XRD indicated that there were some non-exfoliated MMT layers in the NR matrix.

1.10.6 SEM Study

The tensile fracture samples were scanned after gold coating, and are representedin Fig. 7. The smooth fracture surfaces and smooth filler dispersion and unidi-rectional tear path oriented along the direction of flow, which is smooth rubbery innature [131], were observed for all rubber samples. Extended nanoclay plateletswhich are partially wrapped by the matrix due to the adsorption of the polymer onnanoclay with some tear line in branching were observed. The micrographs of theentire rubber sample are characterized by a smooth, rubbery failure (which is asmooth failure in the case of rubber samples without the formation of necking)

Fig. 6 TEM pictograph ofENR and nanoclay

222 K. Pal et al.

where the additives are clearly seen and the appearance is associated with a lowtensile strength. But for ‘A’, ‘E’ and ‘F’ fatigue, intermolecular and ductile type offailure was clearly observed. In ‘F’, many holes compared with the fracture surfaceof other samples were noticed. This hole formation may be assigned to low rubber-filler interaction as a result of detachment of the filler from the natural rubber[132]. Such holes could act as initial flaws leading to localised stress concentrationduring deformation. Finally, premature failure of the rubber compound occurred.This perhaps explains the reduction of both tensile and tear strength with higherfiller content. Some samples like ‘E’ and ‘F’ have also shown rupture type offailure. More serious effects of hysteresis arise from chemical changes to therubber structure at higher sustained temperatures; these effects include the rubbercross-linking, and thermal degradation leading to explosive rupture (blowout).

Fig. 7 SEM images of different types of blends


This phenomenon was studied by Gent and Hindi [133]. They heated rubberspecimens in a microwave oven and showed that blowout was due to the gener-ation of gases in the interior of rubber components.

1.10.7 Du-Pont Abrader Study

The mass loss of rubber against Du-Pont abrader is given in Fig. 8. The mass lossof rubber for E and F samples are lower compared to other four type blends underthe same conditions. It is clear that higher abrasion resistance is mainly due to thepresence of ISAF N234 type of carbon black in 65 wt% of NR, 35 wt% of SBRand 10 wt% of ENR/nanoclay in the blend system. It is also seen that samplescontaining 65 wt% of NR, 35 wt% of SBR and 10 wt% of ENR/nanoclay showedgood abrasion property, where as SAF N110 and SRF N774 type of carbon blackwith 75 wt% of NR, 25 wt% of SBR and 10 wt% of ENR/nanoclay showed thehigh abrasion against Du-Pont abrader.

1.10.8 DIN Abrader Study

Figure 9 refers to the DIN abrasion test result in terms of mass loss of rubbercompounds. Compounds ‘B’ and ‘F’ showed higher abrasion resistance mainlydue to the presence of ISAF N234 type of carbon black for first one and ISAFN231 type of carbon black for later. The compound C, D, and E exhibited mod-erate abrasion resistance property, where as SAF N110 and SRF N774 type ofcarbon black with 75 wt% of NR, 25 wt% of SBR and 10 wt% of ENR/nanoclayshowed the high abrasion against DIN abrader.

1.10.9 Heat Buildup Study

The values of heat build-up for the compounds, which showed good abrasionresistant properties against DIN and Du-Pont abrader, are shown in Table 5. For

Fig. 8 Du-Pont abrasionresults

224 K. Pal et al.

both the compounds, the temperature development is higher, due to the presence of40 wt% of carbon black and 10 wt% of nano clay. This may be due to thedisproportionate breaking of the carbon black structure and reformation of theinter-aggregate bonds of carbon black. The compound A shows lesser heat buildupcompared to B and F. The compound ‘A’ contains SAF N110 and SRF N774,whereas the compound ‘B’ and ‘F’ contain ISAF N234 high structured carbonblack and ISAF N231 low structured carbon black, respectively. The use of semireinforcing filler and 80 wt% of NR may be responsible for low heat buildup.These high temperature containing samples accelerate the fatigue of rubbercomponents [134]. Higher tyre temperature usually means higher energy dissi-pation and thus higher fuel consumption [135]. Hence, it is proved that lowerpercentage of HSR leads to less heat build-up. It may be concluded that there is animportant connection between heat build-up and the crosslinking system. Thus, thehigher degree of network stability given by sulfur system generally causes highheat generation. Heat generation tests before and after aging indicate a low degreeof heat build-up can be expected, even when the degree of crosslink densities iskept similar.

1.11 Summary

Preparation of abrasion resistant tyre tread rubber with the help of an open two-roll-mixing mill represents a novel method for making high value rubber tyre

Fig. 9 DIN abrasion results

Table 5 Heat build-up of therubber samples

Sample code Temperature (�C)

Initial 10 min 20 min

B 50 57 62D 50 59 66F 50 58 64


tread. The NR and SBR with addition of organoclay nanocomposites obtainedfrom this process has very good mechanical properties which can withstand therugged working condition of automobile tyre.

From this study, faster scorch time and cure time has been observed for thecompounds having ISAF N231 type of carbon black. In addition, they showincrease in maximum torque, which correlates with the cross-link density results.The morphology of the organoclay dispersion in ENR by solution mixing dem-onstrates the higher intercalation of organoclay based on the XRD results andTEM images. The FTIR study proves the interaction between ENR and organo-clay. The overall mechanical properties increases for the compounds containingISAF type carbon blacks. In DSC study, the all the samples show the same Tg

except ‘E’, may be due to the ISAF type of carbon black reinforcement with 35wt% SBR. Higher thermal stability is found for the nanocomposites containing 35wt% SBR content. It was found that onset degradation temperature was higher forsamples containing 35 wt% of SBR. From the SEM micrographs, fatigue, ductileand intermolecular fracture type of failure is clearly observed for ‘A’, ‘E’ and ‘F’,respectively. Samples containing ISAF N234 type of carbon black show higherabrasion resistance property against Du-Pont and DIN abrader. Also, samplecontaining 25 wt% SBR with ISAF N234 type of carbon black shows the lowestheat generation among all the samples.

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