Materials and Design 31 (2010) 1156–1164

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Materials and Design

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Influence of carbon blacks on butadiene rubber/high styrene rubber/naturalrubber with nanosilica: Morphology and wear

Kaushik Pal a,c,*, R. Rajasekar b, Dong Jin Kang a, Zhen Xiu Zhang a, Samir K. Pal c, Chapal K. Das b, Jin Kuk Kim a

a Polymer Engineering and Science, School of Nano and Advanced Materials, Gyeongsang National University, Jinju 660-701, Republic of Koreab Materials Science Centre, IIT Kharagpur 721 302, Indiac Department of Mining Engineering, IIT Kharagpur 721 302, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 July 2009Accepted 20 September 2009Available online 24 September 2009

Keywords:ElastomerCompositesMorphologyFrictionWearRock

0261-3069/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.09.037

* Corresponding author. Address: Polymer EngineNano and Advanced Materials, Gyeongsang NationRepublic of Korea. Tel.: +82 55 751 5299; fax: +82 55

E-mail address: pl_kshk@yahoo.co.in (K. Pal).

The effect of fillers on morphology and wear characteristics are studied in butadiene rubber (PBR)/highstyrene rubber (HSR)/natural rubber (NR) blends with different types of carbon black. SAF N110 withSRF N774 type of carbon black shows a significant effect on curing studies and mechanical propertiesby reacting at the interface between PBR, HSR and NR matrix. Blends containing the same carbon blacksshow high abrasion resistant properties against Du-Pont abrader, DIN abrader and different mining rocksurfaces and also is found to be the toughest rubber against all types of rock.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction cumulative growth of cracks and tearing. As regards physical ideas

Tyres used in mining vehicles are very costly and need regularmaintenance, as it is impossible to accept its replacement expensewithin very short term. The rugged working condition in minesreduces the life span of tyres on account of cuts, contamination,abrasion, wear, speed fluctuations, etc. There are several types ofdamage occurs in the dump-truck tyre such as tread detachment,sidewall cuts, impact ruptures and bead damage [1,2]. The simplestform of wear, which is particularly important in the friction of rigidmaterials, is abrasive wear and this had also been observed previ-ously [3–6] as the wear governed by the abrasion of the surfacelayer of materials by the sharp edges of hard projections fromthe rough surface of the abradant. An increase in the abrasionresistance of rubber products can be achieved by studying themechanism of wear of rubber under different operating conditions.

The wear of rubber is a complex phenomenon and dependent ona combination of processes such as mechanical, mechano-chemicaland thermo-chemical. Schallamach [7] and later, Grosch and Schal-lamach [8] reviewed abrasion of rubber and tyre wear. Schallamach[7] suggested that the saw teeth were bent back and abraded fromthin underside until torn off. Champ et al. [9] and Thomas [10] sug-gested that abrasion takes place through a cyclic process of

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ering and Science, School ofal University, Jinju 660-701,

753 6311.

on the nature of abrasion, Kragelskii and Nepomnyashchil [11] andSchallamach [12] were the first to examine the simple case of thefailure of rubber by the action of a hard projection moving overits surface. It has been observed [12] that during intense abrasionin sliding contact, a high temperature is developed, and conse-quently the abrasion resistance of the rubber depends, to a largeextent, on its resistance to high temperature and heat. The earlierliterature review has demonstrated about several types of damagesand its causes [13–33] in the tyre. The mechanism of wear providesa link between the abrasion resistance of rubber and its mechanicalproperties, which will predict the life of a product in service, andalso to develop the method of testing abrasion.

Polymer blends are being used extensively in numerous applica-tions; this statement is also true with rubber blends, especially intyre manufacture. Apart from blends of common rubbers, specialtyrubber is also being utilized, depending on service demands andcomponents of the tyre [34,35]. Natural rubber (NR) is known to ex-hibit numerous outstanding properties, such as good oil resistance,low gas permeability, improved wet grip and rolling resistance,coupled with high strength; having properties resembling thoseof synthetic rubbers. Natural rubber coming from latex is mostlypolymerized isoprene with a small percentage of impurities in it.This will limit the range of properties available to it, although addi-tion of sulfur and vulcanization are used to improve the properties.Synthetic rubber is any type of artificially made polymer materialwhich acts as an elastomer. Synthetic rubber serves as a substitutefor natural rubber in many cases, especially when improved mate-

K. Pal et al. / Materials and Design 31 (2010) 1156–1164 1157

rial properties are needed. A wide variety of particulate fillers areused in the rubber industry for various purposes, of which the mostimportant are reinforcement, reduction in material costs andimprovements in processing [36]. Reinforcement is primarily theenhancement of strength and strength-related properties, abrasionresistance, hardness and modulus [37,38]. The idea of blending syn-thetic rubbers with natural rubber is certainly not a new one, butonly now this can be shown to be possible with consistently posi-tive results, by the use of new techniques developed over the lastfive years. These compounds are capable of forming a chemical linkbetween these dissimilar rubbers to produce a technologically com-patible blend. The blends vulcanizates thus produce enhance phys-ical properties by judicious selection of the elastomeric ratio [39].The use of carbon black is synonymous with the history of tyres.Although it has lost some ground to other reinforcing fillers, suchas silica, nano clay, nano tube and nano fibre. by virtue of its unre-vealing performance, it is still the most popular and widely usedreinforcing filler. However, the primary properties of carbon blacksare normally controlled by particle size, surface area, structure, sur-face activity and they are in most cases interrelated [40].

The effort towards improving the efficiency of such transporta-tion system is essential for reducing the running costs and cuttingdown the end cost figures by using the PBR, HSR and NR rubberblends. In this study, the cure characteristics, morphological andmechanical properties of carbon black filled blends of NR, high sty-rene rubber (HSR) and poly butadiene rubber (PBR) with nanosilicaare investigated. Also, these rubber compounds are examined in aspecially fabricated experimental set-up for evaluating their wearresistance properties when abraded against various rock types.The parameters such as influence of composition of rubber vulca-nizates on wear characteristics and the mechanism of wear ofthese compounds against different rocks are reported. A literaturesearch shows a lack of studies about PBR, HSR with NR blendsalone. With this work, we pretend to partially fill this gap.

2. Experimental

2.1. Materials used in rubber preparation

In this study Buna CB 60 is a solution of high-cis polybutadienepolymer, lithium grade, coupled to a star shaped macro structure,not oil extended is used. It has Mooney Viscosity of 60ME, cis 1, 4

Table 1Compound formulations.

Compounds Sample codes

NPSU-1 NPSU-2

Weight in wt.%Natural rubber (NR) 80 80Butadiene rubber (BR) 10 10High styrene rubber (HSR) 10 10Carbon black

SAFa (N110) 15 –SRFb (N774) 20 –ISAFc (N234) – 35ISAF (N231) – –

Silica 5 5Si-69 1 1Stearic acid 2 2Sulfur 2 2IPPD 1 1CBS 0.7 0.7ZnO 5 5Paraffinic wax 1 1Elasto-710 2 2

a SAF – super abrasion furnace.b SRF – semi reinforcing filler.c ISAF – intermediate super abrasion furnace.

content is 38%, vinyl content is 11%, and density is approx0.91 g/cm3 and supplied by Lanxess India Pvt. Ltd., Pune, Mahara-stra. The high styrene rubber (HSR) used is Krylene HS 260, No.-5of 1948 grade of Bayer AG. Its styrene content is 63.5 ± 1.0, specificgravity is 0.94 and Mooney viscosity at 100 �C is 50 ± 5. Natural rub-ber (NR-RMA1X) is supplied by the Rubber Board, Kottayam, Kerala.

Ultrasil-VN3 is a premium grades of spray dried precipitated sil-ica powder of very high purity, is mainly used in tyres is purchasedfrom Degussa, India and has the specific surface area (BET) of180 m2/g. In this study, Silane (Si-69) is used as coupling agentsto adhere nanosilica to the polymer matrix and stabilize the com-posite material.

Zinc oxide, stearic acid, N-cyclohexyl-2-benzothiazyl sulfon-amide (CBS) and N-isopropyl-N-phenyl-p-phenylenediamine(IPPD) acts as anti oxidant and anti ozonant is supplied by Bayer(India) Ltd. Carbon black is supplied by Birla Carbon. Standard rub-ber grade process oil (Elasto 710) and paraffinic wax is purchasedfrom the Apar Industries, India and local market, respectively.

2.2. Preparation of raw rubber

The compounding formulation for the PBR, HSR and NR blendswith its various ingredients are mixed in a two roll mill at a frictionratio of 1:2 following standard mixing sequence followed by ASTMD 3182-07. Compounding formulations based on changing of thepoly butadiene rubber (PBR), high styrene rubber (HSR), and natu-ral rubber (NR) contents are shown in Table 1. Also, three types ofcarbon blacks and one semi reinforcing filler are used, such as, SAF(N110), ISAF (N231 and N234) and SRF (N774). For vulcanizationthe amounts of additives, such as, sulfur, process oil, wax, CBS,IPPD are based on 100 wt.% of rubber and the samples have thecode name ‘NPSU-1’, ‘NPSU-2’, ‘NPSU-3’, ‘NPSU-4’, ‘NPSU-5’ and‘NPSU-6’, respectively. The physical properties of the differenttypes of carbon black used in this study have already been pub-lished in our earlier literature [41]. The reinforcing filler (carbonblack) is added along with the process oil and paraffinic wax fol-lowed by curatives.

2.3. Cure characteristics of rubber compound

The cure characteristics of the rubber compound are studiedwith the help of a Monsanto Oscillating Disc Rheometer

NPSU-3 NPSU- 4 NPSU-5 NPSU-6

80 70 70 7010 15 15 1510 15 15 15

– 15 – –– 20 – –– – 35 –35 – – 355 5 5 51 1 1 12 2 2 22 2 2 21 1 1 10.7 0.7 0.7 0.75 5 5 51 1 1 12 2 2 2

1158 K. Pal et al. / Materials and Design 31 (2010) 1156–1164

(ODR-100 s) at 150 �C as per ASTM D-2084-07. From the graphs,the optimum cure time, scorch time and rate of cure {tmax�tmin

(dN m)} could be determined.

2.4. Determination of cross-link density

The cross-link density is determined by immersing a smallamount (known mass) of sample in 100 ml of toluene to attainequilibrium swelling. After this, the sample is taken out from thetoluene and the solvent is blotted from the surface of the sampleand is weighed immediately. This sample is then dried out at80 �C to constant weight. Then the chemical cross-link density iscalculated by using the Flory–Rehner equation [42].

2.5. Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared Spectroscopy (FTIR) is performedusing a NEXUS 870 FTIR (Thermo Nicolet) instrument in dry airat room temperature. Spectra are taken in the range of 4000–500 cm�1 in transmission mode.

2.6. Mechanical characterization (tensile and tear)

Vulcanized slabs are prepared by compression molding, and thedumb-bell shaped specimens are punched out from a molded sheetby using ASTM Die C. The tests are done by means of a universaltensile testing machine (Hounsfield H10KS) under ambient condi-tion (25 ± 2 �C), following the ASTM D 412-06 and ASTM D 624-00(2007). The moduli at 100% and 300% elongation, tensilestrength, tear strength and elongation at break (%) are measuredat room temperature. The initial length of the specimens was25 mm and the speed of the jaw separation was 500 mm/min.

Nine samples are tested for each set of conditions, at the sameelongation rate. The values of the tensile strength, modulus at 100%elongation, 300% elongation and elongation at break are averaged.The relative error was below 5%. The hardness is measured byShore A hardness tester followed by ASTM D2240-05.

2.7. Differential scanning calorimetry (DSC)

DSC measurements are carried out using a Perkin–Elmer PYRISDiamond DSC instrument. The samples (610 mg), sealed underaluminum pans are scanned in the temperature range of �100 to50 �C. The heating rate is 10 �C min�1 under the nitrogen atmo-sphere with a flow rate of 40 ml min�1.

2.8. Thermal characterization (DTA–TGA)

TGA studies are carried out Shimadzu-DT-40 instrument inpresence of air at a rate of 10 �C min�1 from 50 �C to 650 �C tem-perature. Degradation temperature of the composites is calculatedfrom TGA plot.

2.9. Scanning electron microscopy (SEM)

The tensile fracture surface of the samples are studied in a scan-ning electron microscope (JSM-5800 of JEOL Co.; Acceleration volt-age: 20 kV; type of coating: gold) at 1000 times zooming, Scanningelectron microscopy has been used to study the morphology of thetensile fracture surface of the samples prepared.

2.10. DIN and Du-Pont abrasion test

DIN abrasion test is done by the DIN abrasion tester for deter-mining the abrasion resistance of compounds of vulcanized rubberfollowed by ASTM D5963-96.

Du-Pont abrasion test is done by the Du-Pont Craydon typeabrasion tester for determining the abrasion resistance of vulca-nized rubbers followed by ASTM D394.

2.11. Heat build up study

Heat build up study is carried out using Goodrich Flexometerfor the compounds followed by ASTM D623. The test pieces areprepared in cylindrical shape having diameter of 17.8 ± 0.15 mmand height of 25 ± 0.25 mm by compression molding machine at150 �C. The test pieces are kept at initial temperature of 50 �Cand stoke of 4.45 ± 0.03 mm. The temperature and the load arekept constant throughout the study. The temperature attained bythe samples after the time periods of 10 and 20 min has beenrecorded.

2.12. The experimental set-up

The indigenous experimental set-up is specially fabricated inour lab and the full arrangements of the experimental set-up, prep-aration of rubber specimen and rock samples are already discussedin our earlier literature [41] during wear testing of the rubber sam-ple of XNBR-NR blend and the machine schematic diagram isshown in Fig. 1. At first, rubber disc is fixed tightly to the shaft ofthe step cone pulley by the set-screw. The smoothly cut rock sam-ple is then clamped in the rock holder. The length of projected por-tion of the rock sample outside the rock holder is adjusted so as toobtain a reading of 2.5 mm on the dial gauge attached to top sur-face of the cantilever beam which ensure its horizontality.

Total weight of 450 g, 900 g and 1350 g is placed on the hangerof the cantilever beam, so as to produce normal load of 4.41 N,8.82 N and 13.23 N at the rock–rubber contact and the test run isconducted for 500 revolutions with sandstone sample placed inthe rock holder. The temperature, normal load and frictional forceare recorded on the computer during the wheel rotation. The aboveprocedure is repeated on rubber discs of different PBR–HSR–NRcombination with carbon black variation using other types of rocksas abrader. The mass loss of the rubber samples after 500 revolu-tions (M500) is measured at room temperature and dynamic coef-ficient of friction (l) and abrasion loss (V) are computed.

3. Results and discussion

3.1. Cure characteristics of the rubber compounds

The optimum cure time for NPSU-1 and NPSU-4 rubber sampleis higher than other rubber vulcanizates due to the mixing of SAFN110 and SRF N774 carbon black with PBR, HSR and NR of thethree types of blend system is shown in Table 2. The rate of cure(tmax�tmin) always increases with increasing concentration ofHSR and PBR. This increase in cure rate may be due to the fact thatan increasing concentration of HSR and PBR content causes the vul-canization reaction to increase and create more active cross-linksites in the rubber compound.

Maximum torque can be considered as a measure of stock mod-ulus [43]. The torque difference (MH �ML), which shows the extentof cross linking [44], is found to have lesser variation from onecompound to other. The lesser torque difference is found for thecompound containing 80 wt.% of NR, which contain less gel frac-tion [45] (Table 2) compare to other rubber vulcanizates, which re-duces the maximum rheometric torque. The obtain cross-linkdensity values get correspond with the variation in torque differ-ences. The lower cross-link density is found in 70 wt.% of NR, whichcontains 15 wt.% of PBR and 15 wt.% of HSR for NPSU-6, hinders theformation of chemical cross-links and physical cross-links are

Fig. 1. Schematic diagram of the experimental setup.

Table 2Cure characteristics of the blends.

Properties NPSU-1 NPSU-2 NPSU-3 NPSU-4 NPSU-5 NPSU-6

Min. Torque (dN m) 13.55 13.81 12.30 10.80 11.30 9.79Max. Torque (dN m) 43.96 41.93 43.19 42.43 49.72 40.68TS2 induct time (min) 3.78 3.63 3.18 3.55 2.93 3.07TS5 scorch time (min) 4.98 4.01 4.32 4.70 3.65 3.98TC90 opt. cure Time (min) 19.12 17.05 16.32 17.22 13.55 13.30Opt. cure (min) 45.22 38.22 40.1 45.27 38.87 37.49Cure rate index (min�1) 6.52 7.36 7.61 7.41 9.42 9.77Cross-link density (moles/g) � 10�5 5.05 4.64 4.72 5.04 3.76 2.84

Fig. 2. FTIR study of different types of blend.

K. Pal et al. / Materials and Design 31 (2010) 1156–1164 1159

formed by the silica bundles [46]. As a result of this, the decrease inMH�ML value is observed. Using of ISAF type of carbon black thescorch time reduces. This decrease in scorch time is observeddue to presence of active cross-linking sites in the vulcanized rub-ber [47].

Faster cure rate index is depicted in Table 2 for the compoundscontaining ISAF N231 type carbon black, whereas, moderate curerate is found for the compounds that contain ISAF N234 type car-bon black. The decrease in cure rate may be due to the greater ther-mal history formed during mixing, as a result of their highercompound viscosities. It is also observed that compounds contain-ing 70 wt.% of NR, 15 wt.% of PBR, and 15 wt.% HSR, has got highercure rate index may be due to the rise in temperature at the time ofmixing. The possible formation of a Zn complex in which sulfur andammonium modifier participate may facilitate for the increase inrate of cure [48].

3.2. FTIR spectral analysis

The IR Spectra of the different samples are shown in Fig. 2. Thepeak at 3430–2900 cm�1 is found may be due to the O–H bond cre-ates from the presence of stearic acid. A distinct peak at the rangeof 3730–3500 cm�1 is seen, which is may be due to the asymmetricstretching peak of C–H bond of NR is seen. The peak at 3430 cm�1

is N–H bond for PBR. The peak at 2913 cm�1 is due to the symmet-ric stretching of the C–H bond. The peak at 2360 cm�1 is due to theC–H bond of HSR and 1800 cm�1 may be due to the C–H bond ofNR. The peak at 1650 cm�1 is due to the C@C bond of isoprene.The peak is also seen at frequency range of 1021 cm�1 is may bedue to the styrene.

Another important observation is that unlike Padella et al. [49]there is no peak in the range of 1730 cm�1 (>C@O stretching peak),which indicated that the oxidation of main polymeric chain did notoccur at the time of rubber milling with the help of cracker cummixing mill at high temperature.

3.3. Mechanical properties of the rubber samples

Tensile strength, modulus, elongation at break, tear strength forall the compounds are shown in Table 3. The tensile strength of

Table 3Mechanical properties of the vulcanizates.

Sample code Tensile strength (MPa) Elongation at break (%) 100% modulus (MPa) 300% modulus (MPa) Tear strength (N/mm) Hardness (Shore A)

NPSU-1 11.80 523 1.89 5.70 36.00 55–60NPSU-2 9.42 528 1.32 4.26 34.13 54–56NPSU-3 9.95 545 1.43 4.37 23.68 54–57NPSU-4 11.21 583 1.59 4.73 40.54 60–64NPSU-5 9.43 533 1.59 4.74 52.60 62–65NPSU-6 8.19 538 1.44 3.96 34.29 55–63

1160 K. Pal et al. / Materials and Design 31 (2010) 1156–1164

NPSU-1 is higher addition of SAF N110 and SRF N774 type of car-bon black within the 10 wt.% of PBR, 10 wt.% of HSR and 80 wt.%of NR blend system, and NPSU-4 is higher with the addition ofsame carbon black within the 15 wt.% of PBR, 15 wt.% of HSR and70 wt.% of NR blend system. Tensile strength increases with carbonblack SAF N110 and SRF N774, may be due to the outstanding reac-tivity of the nano size carbon black acting as filler, increases theproperties of the samples.

The tear strength for all the samples is moderate, but it varieswith varying the matrix ratio. But, tear strength is increased forthe sample containing 15 wt.% of PBR, 15 wt.% of HSR and70 wt.% of NR blend system. NPSU-5 containing 15 wt.% of PBR,15 wt.% of HSR and 70 wt.% of NR blend system with carbon blackISAF N234 gives higher tear strength in the blend system.

While for other blend types, mainly use of ISAF type of carbonblack, the tensile and tear strength is decreased, may be due tothe effect of carbon black which suppresses the PBR and HSR effectin the system and the filler is uniformly dispersed in the naturalrubber matrix that can be attributed to the aggregation of PBRand HSR [50]. The aggregation leads to the formation of weak pointin the NR matrix, accordingly reduces the elastomeric strength[51,52].

The modulus of all the NR vulcanizates are increased withincreasing concentration of HSR. This is may be for one or two pos-sible reasons: the restriction of molecular chain mobility, and anincrease in the cross-link density. Elongations at break values arehigher for the blends enriched with HSR and PBR, because carbonblack plays a vital role with HSR and PBR. The hardness value ofthe vulcanizates indicates the same trend as the modulus values.

The mechanical property of rubber vulcanizates markedly de-pends on the number of conjugate double bonds and it is foundfor the sample ‘NPSU-1’, and ‘NPSU-4’. These observations suggestthat more NR react with the carbon–carbon double bonds of SAFN110 and SRF N774 and slower the reversion reaction rate andhence increases the mechanical properties of the vulcanizates.

Fig. 3. DSC studies of different types of blends.

The hardness of ‘NPSU-1’ and ‘NPPV-4’ rubber sample is ob-tained higher than other blends by the influence of SAF type of car-bon black, whose results are shown in Table 2. The increase inhardness of those rubber samples probably by the increase incross-link density.

3.4. DSC study

To study the thermal response of the blends, differential scan-ning calorimetry has been performed. The DSC curves of all rubbervulcanizates are shown in Fig. 3. The criterion for compatibility orincompatibility in polymer blends is the presence of a single glasstransition temperature Tg for the material, which is intermediatebetween the Tg of the pure components and the existence of twoTgs in the DSC thermograms for incompatible polymer blends. Alltraces show a reasonable sharp Tg transition dependent on compo-sition; however, the graph for blend DSC traces are not shown forbrevity.

In Fig. 3 entire sample show the Tg value at around�60 �C but incase of other samples especially like NPSU-2, and NPSU-5 the Tg va-lue get shifts to the lower side. All the samples show only singlemelting peak on the DSC curve; this is attributed to the same back-bone structure of the matrix and the carbon black acts as reinforc-ing filler. Since miscible polymer blends should exhibit a single Tg

between the Tgs of pure components [53]. This is consistent withthe effect of blend composition on miscibility when the chemicalstructure and the molecular mass of components are fixed [54].But in case ISAF N234 type of carbon black, the melting peakappreciably shifts to the lower side. This may be due to the dilutionof matrix as a result of the incorporation of fillers.

3.5. Thermal analysis

High temperature TGA (50–650 �C) curves for the sample areshown in Fig. 4. The temperature for the onset of degradation(T1), the temperature at which 10% degradation (T10), 50% degrada-

Fig. 4. TGA plot of different types of blends.

Fig. 6. (a) DIN abrasion and (b) Du-Pont abrasion results.

K. Pal et al. / Materials and Design 31 (2010) 1156–1164 1161

tion (T50) and 90% degradation (T90) is occurred, is calculated fromthe TGA plots.

It has been observed that the onset degradation temperature isalmost same for all the samples. The onset degradation tempera-ture thereby probably decreases in the case of NPSU-5, due to a de-crease in cross-link density. Cross linking increases the rigidity ofthe system, which in turn increases the thermal stability [55,56].The rate of degradation is almost same up to 90% degradation forall the samples. Also, it is found that for samples, NPSU-4 toNPSU-6 the degradation temperature is decreased may be due tothe increase in concentration of PBR and HSR. Thus, PBR and HSRare used mainly for the high performance applications.

3.6. SEM study

The tensile fracture samples are scanned after gold coating, andare represented in Fig. 5. The smooth fracture surfaces and smoothfiller dispersion and unidirectional tear path oriented along thedirection of flow, where the additives are clearly seen; the appear-ance is associated with a low tensile strength, which is smoothrubbery in nature is observed for ‘NPSU-4’ and ‘NPSU-5’ rubbersamples [57]. But for ‘NPSU-1’, ‘NPSU-2’, ‘NPSU-3’ and ‘NPSU-6’, fa-tigue type of failure is clearly observed from the figure. It may alsoconclude that rough surface fracture has been seen for the samplecontaining ISAF N234 type of carbon black.

Some samples like ‘NPSU-1’, ‘NPSU-4’, ‘NPSU-5’ and ‘NPSU-6’show rupture type of failure in SEM study. More serious effects

Fig. 5. SEM pictures of different types of blend: (a) NPSU-1, (b) NPSU-2, (c) NPSU-3, (d) NPSU-4, (e) NPSU-5 and (f) NPSU-6.

1162 K. Pal et al. / Materials and Design 31 (2010) 1156–1164

of hysteresis arise from chemical changes to the rubber structure athigher sustained temperatures; these effects include the rubbercross-linking system, and thermal degradation leading to explosiverupture (blowout). This phenomenon is studied by Gent and Hindi[58]. They heated rubber specimens in a microwave oven andshowed that blowout is due to the generation of gases in the inte-rior of rubber components.

Fig. 8. Comparison of mass losses of rubber compounds in DIN abrader againstsandstone.

3.7. DIN and Du-Pont abrader test results

Fig. 6a refers to the DIN abrasion test result in terms of massloss of rubber compounds. Compounds ‘NPSU-1’ and ‘NPSU-4’shows higher abrasion resistance mainly due to the presence ofSAF N110 and SRF N774 type of carbon black with the blend sys-tem. The compound containing 70 wt.% of NR, 15 wt.% of PBR,and 15 wt.% of HSR exhibits good abrasion resistance property. Itis also seen that ‘NPSU-2’ and ‘NPSU-3’ shows the high abrasiononly because of the use of different ISAF types of carbon black inthe system.

The mass loss of rubber against Du-Pont abrader is given inFig. 6b. The mass loss of rubber for ‘NPSU-1’ and ‘NPSU-5’ is lowercompared to other four types of blends under the same condition,since at particular normal load. It is clearly understood that higherabrasion resistance mainly due to the presence of SAF N110 andSRF N774 type of carbon black in the blend system. It is also seenthat samples containing 80 wt.% of NR, 10 wt.% of HSR and 10 wt.%of PBR blend shows the high abrasion resistant property with ISAFN231 type of carbon black.

From both DIN and Du-Pont abrasion studies, the compounds‘NPSU-1’, ‘NPSU-4’ and ‘NPSU-5’ have produced lesser volume loss.From these studies, a balancing correlation has obtained for thesame compounds under standard abraders.

3.8. Rock–rubber abrasion results

Based on DIN abrader and Du-Pont abrader test results, Fig. 7shows the abrasion loss of different rubber compounds whenabrades against various rocks at different loads. Out of all typesof rocks the coal has been identified as the major abrader for al-most all types of rubber compounds. The rubber compound con-taining 80 wt.% of NR, 10 wt.% of PBR and 10 wt.% of HSR withSAF N110 and ISAF N774 type of carbon black is found to be thetoughest rubber against all rock types under the present study.Coal being the softest rock causes major abrasion to all types ofrubber compounds under the present study. Sandstone and graniteis another rock which has abraded almost all the rubber com-pounds extensively and may be considered as an important rockfor rock–rubber abrasion studies.

Fig. 8 refers to the DIN abrasion test result in terms of mass lossof rubber compounds against sandstone, because the type of abra-

Fig. 7. Mass loss of rubber compounds against different rocks at different loads.

siveness of the DIN abrader and sandstone is more or less similar innature. Compound ‘NPSU-1’ shows higher abrasion resistancemainly due to the presence of 10 wt.% of HSR and 10 wt.% of PBRin NR with SAF N110 and SRF N774 type of carbon black. The com-pound ‘NPSU-4’ also exhibits good abrasion resistance, which alsocontains 15 wt.% of HSR and 15 wt.% of PBR in NR with same typeof carbon black.

3.9. Heat buildup study

The values of heat buildup for the compounds, those are havinggood abrasion resistant properties against DIN and Du-Pont abra-der, are shown in Table 4. For both the compounds, the tempera-ture development is higher, due to the presence of 35 wt.% ofcarbon black and 5 wt.% of nanosilica. This may be due to the dis-proportionate breaking of the carbon black structure and reforma-tion of the inter-aggregate bonds of carbon black. The compoundNPSU-1 shows lesser heat buildup compare to NPSU-4 andNPSU-5. The compound ‘NPSU-1’ contains SAF N110 and SRFN774, whereas the compound ‘NPSU-4’ contains the same carbonblack but with the increasing content of HSR and PBR contentand NPSU-5 contains ISAF N234, high structured carbon black with15 wt.% of PBR and 15 wt.% of HSR. The use of semi reinforcing fillerand 80 wt.% of NR may be responsible for low heat buildup. Thesehigh temperature containing samples accelerate the fatigue of rub-ber components [59]. Higher tyre temperature usually means high-er energy dissipation and thus higher fuel consumption [60]. Henceit is proved that lower percentage of PBR and HSR leads to mini-mum heat buildup. It may be concluded, that there is an importantconnection between heat buildup and the crosslink system. Thus,the higher degree of network stability gives by low sulfur systemgenerally causes less heat generation. Heat generation tests beforeand after aging indicate where a low degree of heat buildup can beexpected, even when the degree of crosslink densities is keptsimilar.

3.10. Temperature generation at rock–rubber interface

Since the low thermal conductivity of rubbers can result in avery high temperature at the interface, it is important to investi-

Table 4Heat build up of the rubber samples.

Sample Code Temperature (�C)

Initial 10 min 20 min

NPSU-1 50 57 62NPSU-4 50 58 64NPSU-5 50 59 66

Table 5Temperature ranges of different rubber samples during abrasion against rocks.

Sample code Temperature range (�C) up to 500 revolutions at 4.41 N load

Sandstone Concrete Granite Shale Coal

NPSU-1 24–63 27–63 24–53 25–57 25–90NPSU-4 23–72 28–69 24–57 25–52 25–86NPSU-5 24–71 27–64 24–51 25–56 25–86

K. Pal et al. / Materials and Design 31 (2010) 1156–1164 1163

gate the effects of frictional heating on the sliding friction of rub-bers [61]. Ettles and Shen [62] have presented a paper concerningthe effects of heat generation on the level of friction at theinterface.

Table 5 shows the range of temperature generation during fric-tion of rocks with different rubber compounds. In general the tem-perature generation has been found to be higher in case of therubber compounds containing SAF N110 and SRF N774 type of car-bon black.

The temperature generation during friction in rubber com-pound ‘NPSU-1’ is slightly lower, which may be attributed to thepresence of SRF N774 type of carbon black. The highest tempera-ture is observed when rubbers are abraded against coal surfaces,whereas the lowest temperature could be noticed for shale. Forother rocks the temperature generation is found to be in the mod-erate range.

4. Conclusions

Preparation of abrasion resistant tyre tread rubber with the helpof an open two-roll-mixing mill represents a novel method formaking valuated rubber products. The NR with addition of PBRand HSR obtained from this process has very good abrasion resis-tant properties which can be withstand the rugged working condi-tion of dump-truck tyre. The optimum cure time is higher for thesample containing SAF N110 and SRF N774 carbon black. Fastercure rate index is observed for the compounds containing ISAFN231 type carbon black. Tensile strength increases with carbonblack SAF N110, may be due to the outstanding reactivity of thenano size carbon black acting as filler, increases the properties ofthe samples. DSC study reveals that Tg shifts to the lower side forthe samples containing ISAF N234 type of carbon black. It may con-clude that rough surface fracture has been seen for the sample con-taining ISAF N234 type of carbon black in SEM study. It is clearlyunderstood that higher abrasion resistance against Du-Pont andDIN abrader is mainly due to the presence of SAF N110 and SRFN774 type of carbon black in the blend system. Also silica plays aleading role with the blends. The rubber compound containing80 wt.% of NR, 10 wt.% of PBR and 10 wt.% of HSR with SAF N110and ISAF N774 type of carbon black is found to be the toughest rub-ber against all rock types under the present study.

Acknowledgements

The authors are deeply grateful to Coal India Limited, Kolkataand I-Cube Centre, Gyeongsang National University for their kindassistance. Also, special thanks to the members of this researchand key laboratory of Materials Science Centre and Central Re-search Facility at IIT, Kharagpur, India.

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