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.
./....
~u~ber-'fechoology
.
N 'Karak* & B
Gupta
I
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
.,
..-/
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
reports 3. 5 on the effect of stearic acid and ZnO on the
properties of the vu\canizate. The ability of carbon
correspondence:~emical
*For
Sciences Department, Tezpur
Universit y, Tezpur, Napaam 784 028
-,
,-'?
1, «
. .
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 elastomers 6. 14 and its effect on the mechanical
properties of rubber compounds. The process oil has
been reported to improve processing and influence
19
physical properties of vu\canizates I 5. . 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-Dih ydro-2 ,2,4-trimethyl qui noline
(TMQ- Bayer India Ltd .) ; sulfur, (S tand ard 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
(0.0)
2
(2.0)
3
4
5
Cure' Characteristics
(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
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
Physical properties
Fatigue to Failure (kc)
199.0
170.0
133.0
100.0
112.0
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
18.0
11.0
13.0
14.0
19.0
3
9
Abrasion Loss (m x I 0
)
Compression set (%)
*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)
Cure Characteristics
6
7
(2.5)
(5 .0)
8
(7.5)
9
(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
22.74
23 .43
23 .72
23.82
Physical properties
Tensile strength (MPa)
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
9
111.0
120.0
11 4.0
122.0
Swelling (%)
25 8.0
247.0
244.0
249.0
Tand
0.057
0.056
0.058
0.059
13.0
12.0
11.5
11.5
Abras ion Loss (m'x I0
Compression set (%)
)
*Base formul ation: NR- I00.0, TMQ-1.0, Steari c 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
); 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 calculated 21 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,
93
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
4
reaction 3- . 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
II
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
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
Physical properties
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 (m 3 x I0 9 )
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
13.0
14.0
13.0
11.0
Compression set (0/0 )
*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 variati on of cure characteri stics and physi cal properties of th e vulcanizates with vari ati on of
particle size of carbon bl acks
Stock No. (particle size, nm)
Pr.operties
Cure Characteristics
14 (20.0)
15 (26.0)
16 (28.0)
17 (39.0)
18 (50.0)
(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
21.96
23.72
23.52
21.66
21.08
Physical properties
Tensile strength (MPa)
Modulus (MPa)
8.82
9.2 1
9.60
9.12
8.53
Elo ngati 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
30.5
28 .8
26.6
25 .4
Tear resistance (kglSTP)
Fati gue to Faiiure (kc)
3
Abrasion Loss (m x I 0
9
)
29.7
150.0
157.0
173.0
123.0
106.0
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
12.3
12.0
12.4
12.5
12.3
Compression set (0/0)
*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
-
6R
!!!.
""
62
166
:;M
..
-ii
r--
-
-
-
-------,
~
~EfSJ
5'
:l 6()
::l
58
; :'i 6
:c
95
~4 L-_~-~--~-~----l
u
10
8
2
Loading of Stearic acid (phr)
Fig.l-Vari ati on of hardness of the vulcanizates with dose of
stearic acid
1>6
r----------,
J 7 ~-~--~--~
2.'
7.'
10
Dose of ZnO (phr)
Fig.2-Vari atio n of hardness of th e vul cani zates with dose of
ZnO
stearic ac id which is du e 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 leve l 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 ex plained 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 increasin g ZnO level IS, optimum cure
time remains almost constant and difference in torque
value inc reases 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 co mpression
set, tear res istan ce, 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 le vel of cunng IIlcreases
continuously with increasing ZnO level and also at
-ii
~
GO
3
Dose
5
7
9
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 vin g
reduced energy input. The cure rate decreases,
therefore, with increasing mjneral oil level beyond 5
phr. As cure rate decreases, scorch safety increases
l s 17
and as level of cure and stock viscosity reduced - ,
the difference in torque decreases with increas in g
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, fatigu e to failure , abrasion loss,
tens ile strength, tear res istance and swelling are
improved due to better di spersion of filler and wettin g
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, so me voids are
fo rmed due to volatilizati on of oil reduction in ten sile
strength and tear resistance. The gradual decrease in
modulus and hardness are due to reducti on 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
inc reasin g chain flexibilit y. Thi s is reflected in
progressive increasing tan8 values. The compression
set values did not follow any definite trend which are
unex plainable to us at thi s stage.
The effect of carbon black type-The cure
characteri sti cs and physical properties of the
vul canizates 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
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 (m 3xI09)
144.0
140.0
133.0
130.0
Physical properties
Swelling (%)
285.0
244.0
229.0
212.0
TanCl
0.058
0.056
0.054
0.053
19.0
16.0
13.0
12.0
Compression set (%)
*Base formul ation : 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
(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
Cure Characteristics
Optimum cure time (min. )
17.0
15.0
12.5
11.0
Cure rate (n-mlmin.)
0.16
0.28
0.36
0.61
23.04
23.43
21.17
21.37
Physical properties
Tensile strength (MPa)
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
16.0
13.0
12.5
13.0
Compression set (%)
*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
I~t:=:: ~
~
~
~
~
~
E
170~
.,g
65
'"-60
B
n
c
"E~
==
~
U
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
ls
reduced cure rate . 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 black l9 filler.
Hardness-As the reinforcement of the carbon
black decreases due to increase of particle size, the
hardness of the vulcanizates decrease progressively.
2
2.S
)
DOle or Sulrur (phr)
Partide .ize of C-black (am)
Fig. 4--Variation of hardness of the vulcanizates with particle
size of the carbon blacks
97
Fig. 5-Variation of hardness of the vulcanizates with dose of
sulfur
1
68
~64
.
LJ
: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 graduall y
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 sulfur 21• 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 den sity. 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 W2S
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
H= 54 + 24 Wa - I 1.1 W2 a
where
0.3~Wa ~
[0.9866]
1.2
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 rel at 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.
[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)
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