Indian Journal of Chemical Technology Vol. 7, May 2000, pp. 91-99

.. r •

Effects of compounding ingredients of a tyre tread of NR· based compound on physical properties, special reference to hardness

N 'Karak* & B Gupta ( n. ./.... . I

~u~ber-'fechoology Centre, ~ Institute of Technology, K pu:;'2

Receive~ovembeF 199.9;.QGGepled 7 March 2000

'PI effect of dose of ste~, ZnO and process oil; type of carbon black; dose of curatives like sulfur, and CBS , on hardnes's of a·tyre tread ofNR based compound has been investigated. Along with the hardness, other physical properties have also been measured with variation of the above compounding ingredients . Empirical relationships for the hardness of the vuIcanizates with the dose of stearic acid, ZnO and process oil , the particle size of carbon black, dose of sulfur and CBS

have been established.) . • . "I ., , ,-'? 1, « -- . .

..-/

The hardness of tyre treads plays an important role in deciding the performance of tyres . Indian road and weather conditions along with overloading put a severe· strain on truck tyres, as a result of which separation of tread and undertread layers occurs frequently. A tread with a high hardness results in (i) increased rolling resistance due to increase in friction coefficient, (ii) decreased riding comfort due to lowering of enveloping and shock absorbing capacity, (iii) decreased flexibility, (iv) rapid irregular wear and (v) increased heat build-up. It may also lead to stress cracking of the tread. On the other hand, low hardness may also give rise to some major problems like (i) high heat build-up, (ii) increased wear, cuts, blowout, etc. besides (iii) development of low strength of the tread. Matching the hardness of the joining substrates as closely as possible may gainfully alleviate severe stress concentration in the tread and undertread layers during loading and servicing; Howev~, the hardness values of carcass and the tread are not likely to be of the same magnitude; so, in such cases, the hardness of the compounds from layer-to-Iayer should be changed gradually across the thickness . Graded hardness, therefore, is an important criteria for achieving balanced tread properties and improved tyre performance.

ZnO is one of the most important compounding ingredients in tyre industries l

.5

• There are also a few reports3

.5 on the effect of stearic acid and ZnO on the

properties of the vu\canizate. The ability of carbon

*For correspondence:~emical Sciences Department, Tezpur University, Tezpur, Napaam 784 028

black to interact physically with elastomers is an

important aspect of reinforcement. There are a number of reports on the interaction of carbon black with elastomers6

.14 and its effect on the mechanical

properties of rubber compounds. The process oil has been reported to improve processing and influence physical properties of vu\canizates I 5

.19

. The reports on the effect of curatives on various aspects like crosslink density and cure rate of NR vulcanizates have been published in the relevant literature 20·22

In the present paper , the effect of dose of stearic acid, ZnO, process oil and the particle size of carbon black; and dose of sulfur, and CBS on hardness of a tyre tread of NR based compound. Other related physical properties of the vu\canizates have been assessed and reported. The hardness of the vu\canizate . has been correlated with the dose of stearic acid, ZnO, process oil and particle size of carbon black, dose of sulfur, and CBS using regression analysis .

Experimental procedure Materials-NR (50:50 blend of masticated RMA

and RMA 2); stearic acid (from Godrej) ; ZnO (from Kailash sales and service ); different types of carbon black (Philips Carbon Ltd .) viz. super abras ion furnace, SAF(N II 0), intermediate super abrasion furnace, ISAF (N220), high abrasion furnace, HAF (N330), fine effective furnace , FEF(N550 ), general purpose furnace, GPF(N660 ); process oil (mineral oi l, lOC); 1,2-Dihydro-2,2,4-trimethyl quinoline (TMQ- Bayer India Ltd .) ; sulfur, (Standard Chemicals

92 rNDIAN 1. CHEM. TECHNOL., MAY 2000

Table I-The variation of cure eflaracteristics and physical properties of the vulcanizates* with variation of dose of stearic acid

Properties Stock No.(dose in phr)

I 2 3 4 5

Cure' Characteristics (0.0) (2.0) (4.0) (6.0) (8 .0)

Del-torque (n-m) 2.03 2.25 2.31 2.30 2.25

Scorch time (min.) 2.39 2.73 2.96 3.05 3.07

Optimum cure time (min.) 15.0 14.0 16.0 17.0 19.0

Cure rate (n-mlmin.) 0.23 0.27 0.22 0.20 0. 17

Physical properties

Tensile strength (MPa) 19.31 22.01 23.53 23 .13 21.17

Modulus (MPa) 7.35 8.72 9.21 8.72 8.53

Elongation at break (%) 510.0 546.0 560.0 532.0 513.0

Resilience (%) 63.0 65.0 63.0 62.0 60.0

Hardness (Shore A) 56.0 60.0 63.0 64.0 66.0

Tear resistance (kglSTP*) 19.14 26.0 21.34 20.2 19.8

Fatigue to Failure (kc) 199.0 170.0 133.0 100.0 112.0

Abrasion Loss (m3x I 09) 134.0 114.0 109.0 106.0 112.0

Swelling (%) 275.0 260.0 254.0 245.0 246.0

Tand 0.058 0.056 0.056 0.055 0.057

Compression set (%) 18.0 11.0 13.0 14.0 19.0

*Base formulation: NR-I 00.0, TMQ-I .O, ZnO-5.0, HAF (N330)-40.0, Mineral oil-5 .0, Sulfur-2.5 and CBS-0.6 phr for all the cases. *STP- Standard test piece, as per IS3400, part XII , 1971

Table 2-The variation of cure characteristics and physical properties of the vu lcanizates* with variation of dose of ZnO

Properties Stock No. (dose in phr)

6 7 8 9

Cure Characteristics (2.5) (5 .0) (7.5) (10.0)

Del-torque (n-m) 2.15 2.35 2.47 2.38

Scorch time (min.) 2.57 2.41 2.54 2.26

Optimum cure time (min .) 14.0 13 .5 13.5 14.0

Cure rate (n-mlmin.) 0.27 0.31 0.32 0.30

Physical properties

Tensile strength (MPa) 22.74 23.43 23 .72 23.82

Modulus (MPa) 8.43 9.21 9.70 9.50

Elongation at break (%) 562.0 546.0 536.0 524.0

Resilience (%) 67.0 69.0 70.0 69.0

Hardness (Shore A) 59.0 62.0 63.0 64.0

Tear resistance (kglSTP) 29.8 24.8 29.7 27.7

Fatigue to Failure (kc) 161.0 170.0 130.0 17.0

Abrasion Loss (m'x I 09) 111.0 120.0 11 4.0 122.0

Swelling (%) 258.0 247.0 244.0 249.0

Tand 0.057 0.056 0.058 0.059

Compression set (%) 13.0 12.0 11.5 11.5

*Base formul ation: NR- I 00.0, TMQ-1.0, Stearic acid- 2.0, HAF(N330)-40.0, Mineral oil-5 .0, Su lfur-2.5 and CBS-0.6 phr for all the cases.

KARAK & GUPTA: EFFECTS OF COMPOUNDING INGRADIENTS ON PHYSICAL PROPERTIES OF TYRE TREAD 93

); cyclohexyl benzthiazyl sulfenamide (CBS, from Indian Explosives Ltd. ' ) were used as compounding ingredients as received.

Compounding-Mixing of the compounded rubber stocks was carried out on a two roll open mixing mill (0.228m x 0.457m) at a friction ratio of I: 1.09 for all the mixes, using conventional rubber nuXlOg procedure. The total time of mixing was 20 min and temperature of the rolls was initially maintained at 65 ± 5°C. When sulfur and accelerator were mixed, the roll temperature was brought down to 40-45°C.

Rheometric study- The rheometric studies was carried out using a Monsanto Rheometer, MDR-2000 at 150°C for 30 min. To study the effect of temperature, experiments were performed at several assigned temperature for 60 min. Minimum torque, maximum torque, time to achieve 90% cure (t90), scorch time etc. were obtained directly from the rheometric chart. The optimum cure time and cure rate were calculated21 from the above data by taking optimum cure time = (t90+5) min and cure rate = {(90% maximum torque) - (minimum torque +2 )} / (optimum time - scorch time).

Vulcanization-Vulcanization of the stock . compounds was carried out in a double day light steam heated, hydraulic press (0.39 m x 0.38 m) at 150°C for the respective optimum cure time under a pressure of (35 ± 5) x 104 kg/m2

•

Physical test methods-For evaluation of each physical property, at least three test specimens were tested and the mean of the observed data was taken as the value of the property. Tensile strength, modulus at 300% strain and elongation at break were carried out according to the ASTM D 412-51 T using dumbbell specimens in a Mansanto Good Brand Tensile Testing Machine using a separation rate of grips 0.5 mlmin. Tear strength (crescent test piece) was tested as per IS3400, part XII, 1971, in the above machine. The hardness was measured by using a Shore-A durometer model SHR-MARK-II, as per the ASTM D 676-59 T standard procedure. Rebound resilience was measured by using a Dunlop tripsometer, as per BS 903, part 22, 1950. Abrasion loss of. the samples was measured using a DIN abrader according to DIN 53479 standard procedure . Compression set was determined according to the BS 903 part A6, 1969 procedure, at constant strain of 25%. Loss tangent factor (tanb) was measured at room temperature by Rheovibron Direct Reading Dynamic Viscoelastometer, model DDV -II­C at 11Hz frequency. The maximum length, thickness, width of the' sample used were 0.005 m,

0.0005m and 0.004 m respectively. Fatigue to failure (FIF) of the samples was measured by using a Monsanto Fatigue to failure tester at extent ratio 1.61 ± 0.04 according to the following formula: F / F= the sum of 50% of maXimum cycle, 30% of the second highest value and 10% each next subsequent two higher values. The percentage volume swell was measured by simple conventional procedure of volume swelling.

Results and Discussion The effect of stearic acid-The initial increase in

cure rate (Table 1) is due to better solubilization of Zn-ions which activate the accelerator in the curing reaction3

-4

. Tarosyuko and Maka 5 also reported that stearic acid controlled the amount of Zinc ion solubilized in the rubber which has significant effect on crosslink formation, numlr;r and type of the net­work bonds formation. But as stearic acid is an organic acid it retard the efficiency of CBS acc~lerator which is basic in nature and as a result of which there is steady falls in cure rate. As the cure ratl! initially increase and then gradually decreases, the scorch safety and optimum cure time initially decrease and then progressively increase with increase in the level of stearic acid. As the level of crosslinks increases, as supported by continuous decreasing swelling values, the difference in torque increases progressively up to loading of 4 phr of stearic acid.

As the doses of stearic acid increase in the mixes the tensile strength, modulus, tear resistance and resilience increase progressively due to increasing of the level of crosslink density of the vulcanizates . The reduction of these properties as well as that of del­torque (the difference of maximum to minimum in torque in rheometric curve), at higher doses of stearic acid is probably due to the change of nature and number of crosslinks. Since the resilience increases first and then decreases, the fatigue to failure values increases first, reaches a maximum and then decreases. The initial increase in elongation at break is probably due ' to presence of higher amount of polysulfidic linkages in the matrix at moderate doses of stearic acid as compared to that at higher doses. The final decrease ' in elongation at break and compression set values are due to change in nature of network structure and increase in crosslink density with increasing level of stearic acid in the compound.

Hardness-The hardness of the vulcanizates increases continuously with increasing loading of

94 INOlAN J. CHEM. TECHNOL., MAY 2000

Table 3-The variation of cure characteristics and physical properties of the vulcanizates* with variation of mineral oil dose

Properties Stock No. (dose in phr)

10 I I 12 13

Cure Characteristics (3 .0) (5.0) (8.0) (10.0)

Del-torque (n-m) 2.40 2.38 2.15 2.06

Scorch time (min.) 2.46 2.65 2.80 2.85

Optimum cure time (min. ) 14.5 14.0 14.5 15 .0

Cure rate (n-mlmin.) 0.27 0.29 0.25 0.24

Physical properties

Tensile strength (MPa) 22.74 23.43 20.58 20.19

Modulus (MPa) 8.43 8.23 7.74 7.45

Elongation at break (0/0) 540.0 560.0 570.0 578.0

Resilience (0/0) 64.0 63.0 62.0 61.0

Hardness (Shore A) 65.0 64.0 61.0 59.0

Tear resistance (kglSTP) 27.4 28.0 27.0 24.2

Fatigue to Failure (kc) 159.0 173.0 177.0 218.0

Abrasion Loss (m3x I 09) 130.0 125.0 118.0 120.0

Swelling (0/0 ) 255.0 250.0 250.0 249.0

Tano 0.055 0.056 0.057 0.058

Compression set (0/0 ) 13.0 14.0 13.0 11.0

*Base formulation: NR-I 00.0, TMQ-I.O, Stearic aci d-2.0, ZnO-5.0, HAF(N330)-40.0, Sul fur-2.5 and CBS-0.6 phr for all the cases.

Table 4-The variation of cure characteri stics and physical properties of the vulcanizates with vari ation of particle size of carbon bl acks

Pr.operties Stock No. (particle size, nm)

14 (20.0) 15 (26.0) 16 (28.0) 17 (39.0) 18 (50.0)

Cure Characteristics (SAF NI IO) (ISAF N220) (HAF N330) (FEF N550) (GPF N660)

Del-torque (n-m) 2.23 2.22 2. 15 2.10 2.05

Scorch time (min. ) 2.85 2.63 2.54 2.18 1.99

Opti mum cure time (min .) 15 .0 15.0 14.0 14.0 13.5

Cure rate (n -mlmin.) 0.22 0.24 0.243 0.25 0.251

Physical properties

Tensile strength (MPa) 21.96 23.72 23.52 21.66 21.08

Modulus (MPa) 8.82 9.21 9.60 9.12 8.53

Elongati on at break (0/0) 580.0 570.0 590.0 593.0 599.0

Resilience (0/0 ) 62.0 63 .0 67 .0 71.0 74.0

Hardness (Shore A) 67.0 66.0 64.0 61.0 60.0

Tear resistance (kglSTP) 29.7 30.5 28 .8 26.6 25.4

Fatigue to Faiiure (kc) 150.0 157.0 173.0 123.0 106.0

Abrasion Loss (m3x I 09) 143.0 136.0 139.0 162.0 173.0

Swelling (0/0) 240.0 245 .0 255.0 261.0 262.0

Tand 0.060 0.056 0.055 0.054 0.053

Compression set (0/0) 12.3 12.0 12.4 12.5 12.3

*Base fo rmulati on : NR-I 00.0, TMQ-I .O, Stearic acid-2.0, ZnO-5.0, Mincral oil-5.0, Su lfur-2.5 and CBS-0.6 phr for all the cases.

KARAK & GUPTA : EFFECTS OF COMPOUNDING INGRADI ENTS ON PHYSICAL PROPERTIES OF TYRE TREAD 95

- 6R r-- - - - -------, 166 :;M "" !!!. 62 :l 6() .. -ii 58 ; :'i 6 :c ~4 L-_~-~--~-~----l

u 2 8 10

Loading of Stearic acid (phr)

Fig.l-Vari ati on of hardness of the vulcanizates with dose of stearic acid

1>6 r----------,

J7 ~-~--~--~

2.' 7.' 10

Dose of ZnO (phr)

Fig.2-Vari ation of hardness of the vulcani zates with dose of ZnO

stearic ac id which is due to increase of crosslink density with the same.

Effect of zillc oxide (2nO)- The cure characteri stics and physical properties of the vulcanizates are shown in Table 2. In itially cure rate increases with increasing ZnO level in the compounds, as Zn-ion acts as activator of accelerator. The subsequent decrease in cure rate at hi gher dose of ZnO can be explained from the fact that at higher doses of ZnO, Zn-ion form chelate complex with sulfur accelerated rubber compound . Th is chelate complex strengthens the weak S-S bonds IS. The breaking of stronger S-S bonds during the curing process now requi res more energy resulting in hi gher activation energy as well as lowering of cure rate. As cure rate increases, the scorch safety decreases. But as level of curing increases with increasing ZnO level IS, optimum cure time remains almost constant and difference in torque value increases continuousl y.

The steady increase in tensile strength, resilience and modu lus due to increase in the level of cure with increasi ng the ZnO level in the compounds. As the crosslink density increases , the elongat ion at break and swelling decrease continuously. The compression set, tear res istance, abrasion loss and tan8 did not follow any trend may due to different nature of curing at lower and higher doses of ZnO.

Hardness-Since the level of cunng IIlcreases continuously with increasing ZnO level and also at

~

~EfSJ ::l GO

-ii 5' ~ 3 5 7 9

Dose or Process oil (phr)

Fig.3-Variation of hardness of the vulcanizates with dose of process oil

higher dose it may acts as inert filler so hardness progressively increases .

Effect of process oil-The cure characteristics and physical properties of the vulcanizates are shown in Table 3 with variation of process oil loading. The initial increase in cure rate due to better dispersion of the ingredients, including curatives in the matrix . But with increase of oil dose in the mixes, it form a film around the solid particles of different ingredients giving rise to higher amount of activation energy for curing reaction . The higher amount of process oil also results more slippage in rheometric disc gi ving reduced energy input. The cure rate decreases, therefore, with increasing mjneral oil level beyond 5 phr. As cure rate decreases, scorch safety increases and as level of cure and stock viscosity reduced l s

-17

,

the difference in torque decreases with increas ing level of oil. The change of optimum cure time is according to the cure rate variation.

As the level of oil in the compound increases, the e longation at break, fatigue to failure , abrasion loss, tens ile strength, tear res istance and swelling are improved due to better di spersion of filler and wetting of filler in presence of oil. But in presence of excess oi l there is deterioration of the properties like tensi le st rength and tear res istance with the increase in amount of oil in the compound, some voids are fo rmed due to volatilization of oil reduction in tensile strength and tear resistance. The gradual decrease in modulus and hardness are due to reduction of stock viscos ity with increase in the level of oil in the mixes. With increase in oil content in the matri x the capaci t of elastic store energy is reduced because of increasing chain flexibility. Thi s is reflected in progressive increasing tan8 values. The compression set values did not follow any definite trend which are unexplainable to us at this stage.

The effect of carbon black type-The cure characteri stics and physical properties of the vulcanizates have been shown in Table 4, with variation of carbon black type i.e . with change of particle size. The decrease in cure rate with

96 INDIAN.J. CHEM. TECHNOL., MAY 2000

Table 5-The variation of cure characteristics and physical properties of the vulcanizates with variation of dose of sulfur

Properties Stock No. (dose in phr)

19 20 21 22

Cure Characteristics (1 .5) (2.0) (2.5) (3.0)

Del-torque (n-m) 1.61 2.06 2.39 2.58

Scorch time (min.) 3.31 2.86 2.75 2.59

Optimum cure time (min.) 13.5 14.0 14.5 15.0

Cure rate (n-mlmin.) 0.24 0.26 0.28 0.27

Physical properties

Tensile strength (MPa) 22.94 23.33 23.62 22.35

Modulus (MPa) 5.98 8.23 8.82 9.41

Elongation at break (%) 590.0 580.0 560.0 549.0

Resilience (%) 66.0 69.0 71.0 74.0

Hardness (Shore A) 52.0 60.0 65.0 66.0

Tear resistance (kglSTP) 30.3 31.25 32.5 30.9

Fatigue to Failure (kc) 155.0 166.0 176.0 123.0

Abrasion Loss (m3xI09) 144.0 140.0 133.0 130.0

Swelling (%) 285.0 244.0 229.0 212.0

TanCl 0.058 0.056 0.054 0.053

Compression set (%) 19.0 16.0 13.0 12.0

*Base formulation : NR-lOO.O, TMQ-1.0, Stearic acid-2.0, ZnO-5.0, HAF (N330)-40.0, Mineral oil-5.0, and CBS-0.6 phr for all the cases.

Table 6--The variation of cure characteristics and physical properties of the vulcani..zates with variation of CBS dose

Properties Stock No. (dose in phr)

23 24 25 26

Cure Characteristics (0.3) (0.6) (0.9) (1.2)

Del-torque (n-m) 1.98 2.39 2.60 2.87

Scorch time (min.) 2.84 2.75 2.68 2.67

Optimum cure time (min. ) 17.0 15.0 12.5 11.0

Cure rate (n-mlmin.) 0.16 0.28 0.36 0.61

Physical properties

Tensile strength (MPa) 23.04 23.43 21.17 21.37

Modulus (MPa) 7.55 8.82 9.70 9.80

Elongation at break (%) 582.0 560.0 543.0 532.0

Resilience (%) 62.0 70.0 71.0 72.0

Hardness (Shore A) 60.0 65.0 66.0 67.0

Tear resistance (kglSTP) 29.9 31.5 25 .5 22.3

Fatigue to Failure (kc) 145.0 176.0 130.0 121.0

Abrasion Loss (m3x I 09) 139.0 132.0 126.0 97.0

Swelling (%) 232.0 230.0 228.0 211.0

TanCl 0.056 0.054 0.053 0.052

Compression set (%) 16.0 13.0 12.5 13.0

*Base formulation : NR-lOO.O, TMQ-1.0, Stearic acid-2.0, ZnO-5.0, HAF(N330)-40.0, Mineral oil-5 .0, and Sulfur-2.5 phr for all the cases.

KARAK & GUPTA : EFFECTS OF COMPOUNDING INGRADIENTS ON PHYSICAL PROPERTIES OF TYRE TREAD 97

I~t:=:: ~ ~ ~ ~ ~ ~ E ~

Partide .ize of C-black (am)

Fig. 4--Variation of hardness of the vulcanizates with particle size of the carbon blacks

decreasing of particle size of the carbon blacks is due to the fact that as the particle size decreases both surface area and surface activity of the blacks increased. This results in adsorption and chemisorption of a part of the curatives and giving reduced cure ratels. So cure rate increases with increasing particle size of the blacks. As cure rate increases, scorch safety and optimum cure time decrease with increasing particle size of the blacks. The decrease in del-torque values with increasing the particle size is due to decrease in filler-polymer interaction because of lower surface activity of the fillers .

The decrease in tensile strength, tan8, tear resistance and increase in abrasion loss, swelling and resilience with increasing particle size of the carbon black are explained on the basis of decreasing reinforcement of rubber matrix by decreasing filler­polymer interaction. The reinforcement increases the hysteresis behaviour which results storing of a part of input energy in the matrix; thereby requiring large amount of energy to failure. The compression set which has been found to be independent of the particle size of the blacks, is possible due to more or less same level of crosslink density in all the cases. However, the increase in swelling may be arttibuted to the decreasing bound rubber as the particle size increases. As the hysteresis (tan8 ) increases due to jncreasing reinforcement the fatigue life decreases from HAF (N330) to SAF (N II 0) black. The decrease of fatigue life from HAF (N330) to GPF (N660) may be due to non-interaction of some high particle size filler with rubber matrix .

As particle size increases, elongation at break also increases from ISAF (N220) to GPF (N660) due to decreasing physicochemical interaction of filler with the polymer. The small increase in elongation at break from ISAF (N220) to SAF(N 110) may be due to softening effect of higher reinforcing blackl9 filler.

Hardness-As the reinforcement of the carbon black decreases due to increase of particle size, the hardness of the vulcanizates decrease progressively.

170~ .,g 65

'" -60

B n c "E~ == U 2 2.S )

DOle or Sulrur (phr)

Fig. 5-Variation of hardness of the vulcanizates with dose of sulfur

168

LJ ~64 .. :llll c "ES6 == 0.3 0.6 0.9 1.2 1.5

DOle or CBS (phr)

Fig. 6-Variation of hardness of the vulcanizates with dose of CBS

Effect of sulfur -At higher dose of sulfur the decrease in cure rate (Table 5) as compared to lower doses is due to the fact that at lower doses of sulfur, the crosslinks are mainly polysulfidic· but at higher doses, a substantial portion of sulfur is combined in cyclic monosulfides. Again the composition of the polysulfidic structure is transformed into structure containing intramolecular cyclic monosulfide groups. During these transformations, the free cyclic monosulfides are continuously being formed, which do not result in crosslink formation. Therefore, although the total sulfur present in the matrix is higher than that of lower doses, it does not give higher cure rate. There is also more free sulfur present in the matrix. As cure rate increases, scorch safety decreases. But as the level of curing increases, optimum cure time and difference in torque increase continuously . Similar type results are also reported earlier 20.

The increase in crosslink density considerably influences the different mechanical properties of the vulcanizate. The modulus and resilience increase continuously whereas abrasion· loss, compression set, elongation at break, tan8 and swelling gradually decrease. The properties related to energy to break for example tensile strength and tear resistance increase with increasing the number of network structure formation and hysteresis (tan8) of the vulcanizates. But, since hysteresis decrease as more network are developed, hence these properties reach a maxi mum value at some intermediate crosslink density. At higher dosage (3 phr), the crosslink lengths become shorter and there is also more free sulfur in the matrix.

98 INDIAN J. CHEM. TECHNOL., MAY 2000

The shorter crosslink length reduces the chain flexibility and presence of free sulfur increases the heat build-up. These two characteristics are responsible for lower the fatigue life.

Hardness-Since the crosslink density continuously increases with increasing sulfur doses, so the hardness of the vulcanizates also increased progressively, tllat is also supported by continuous increased of difference in torque values.

Effect of CBS--The continuous increase of cure rate with increasing accelerator leve l (Table 6) is due to the fact that it lowers the activation energy by complex formation in the curing reaction and thus accelerate the action of sulfur21

• As cure rate increased, scorch safety and optimum cure time decreased. Since the level of cure increases with increasing level of accelerator the difference in torque also increases .

The variations in the different mechanical properties of the vulcanizates with incre.asing the level of accelerator are due to increase in level of the crosslink density. At higher dosages of accelerator, the crosslink lengths become shorter which reduce the chain flexibility and are responsible for lowering the fatigue life. The reason behind the changes in the mechanical properties, including hardness is same as described earlier under the effect of sulfur.

Empirical analysis for hardness-In this section an attempt has been made to generate the empirical mathematical relationships of hardness with the dose of stearic acid, ZnO, process oil and the carbon black type (particle size) and the dose of sulfur, and CBS. The relationships are established by help of a computer using regression analysis. The empirical relationships developed are as follows :

(I) Hardness as a function of stearic acid dose (Ws in phr) H= 56.1 + 2.1 Ws - O. II W2

S where rO.9948] 0~Ws~8

(11 ) Hardness as a function of ZnO dose (Wz in phr) H= 55.5+ 1.64 Wz - 0.11 W2 z where [0.9948]

2.5~ Wz~ IO

(Ill) Hardness as a function of process oil dose (WI' in phr) H= 66.2- 0.23 WI' + 0.05 Wp 2 where [0.9983]

3~ Wi'~IO

(IV) Hardness as a function of size of the carbon black (di n nm) H= 74.8- 0.52d + 0.OO54~ where [0.9999]

20~d~50

(V) Hardness as a function of the sulfur dose(Ws in phr) H=51.95 + 19.9 (Ws-f.5) - where [0.9998] 7 (Ws _1 .5)2 1.5 ~Ws~ 3.0

(VI) Hardness as a function of the CBS-accelerator dose (Wa in phr)

H= 54 + 24 Wa - I 1.1 W2 a where

0.3~Wa ~ 1.2 [0.9866]

In all the cases, H is the hardness and the number given in the third bracket is correlation coefficient. From the above equation and also from Figs I - 6, it can be seen that hardness of the tread type compound is critically depends on the dose of stearic acid, sulfur and CBS, but marginally depends on dose of ZnO, process oil, particle size of carbon blacks.

Conclusion

From the present study, it has also been found that the hardness for NR vulcanizates increases with increasing dose of stearic acid, ZnO, sulfur, CBS and decreases with increasing particle size of carbon blacks, dose 9f process oil. The above compounding variables also have prominent effect on the physical properties and cure characteristics of the vulcanizates. The results are also supported by the reported literature. To get optimum performance of a tyre tread, therefore, one has to carefully select the amount and type of compounding ingredients.

The empirical relat ionships developed can be used to predict the hardness of NR vulcanizates. The correlation coefficients for those equations are very close to unity showing very small deviation from experimental values. Thus these relationships may be used to obtain the theoretical values of hardness of the vulcanizates with the given dose of the discu ssed compounding ingredients without performing the actual practical experiment.

Acknowledgement The support of thi s work by lIT, Kharagpur and

Dunlop India Ltd., Sahagang IS gratefully acknowledged. The authors wish to thank J Bhattacharjee of Dunlop India Ltd . for his kind coguidance of this work.

References I Grove G E, Rubber Age, 103 (1971) 55 2 Sage A K, Rubber Age. 103 (1971) 61 3 Mallik A K & Shah R D. Rubber Chem Review, 14 (1984) 17 4 Radhakri shnan K S & Das C K. Rubber Chem Review. 9

(1980) 17 5 Tarosyko D & Maka T , lnt Polm Sci Tech, 9 ( 1982) T/9 6 Trexler H E & Lee C H. Rubber Chem Techno!. 58 ( 1985)

863 7 Pal P K, Bhowmick A K & De S K. Rubber Chem Techno!,

5S (1982) 23 8 Rigbi Z, Rubber Chem Techno!. 55 (1982) I 180

KARAK & GUPTA: EFFECTS OF COMPOUNDING INGRADIENTS ON PHYSICAL PROPERTIES OF TYRE TREAD 99

9 Bhowmick A K & De S K, Rubber Chern Techno!, 53 (1980) 960

10 Bhowmick A K & De S K, Rubber Chern Technol, 52 (1979) 985

II Hess W M & Lamp W K, Rubber Chern Technol , 56 (1983) 390

12 Medalia A I, Rubber Chern Technol, 60 (1987) 45 13 Ayala J A et al , Rubber Chern Techno!, 64 (1991) 19 14 Gessler A M, Rubber Age, 101 (1969) 54 15 Geshwind G H, Rubber Age~ 10 I (1969) 57

16 Corman B G & Depierri W G, Rubber Chern Techno!, 43 (1967) 1522

17 Stout W J & Eaton R L, Rubber Age, 99 ( 1967) 82 18 Coran A. Y, Rubber Chern Techno!, 99 (1967) 82 19 Blow C M & Hepburn C (Ed) , Rubber Technology,

Manufacture (Butterworth, London),1982 20 Coran A Y, Rubber Chern Techno!, 37 (1964) 689 21 Hofman W, Vulcanization and Vulcanizing Agents

(Palmenrton, New York), 1961 22 Morita E, Rubber Chern Technol, 53 ( 1980) 393