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Silica Reinforcement of Epoxidised Natural Rubber of Varying Epoxy Content
Jacob K.Varkey*, Sadhan K.De** and K.T.Thomas**Rubber Research Institute of India, Rubber Board, Kottayam, Kerala, India
**Polymer Engineering Department, University of Akron, USA.
International Rubber Conference and IRRDB Meetings28-31 October 2012
Kovalam, Trivandrum
Natural Rubber
Renewable raw material
Excellent mechanical and dynamic properties
Broad range of applications – Major application-Tyres
Mostly used as filled vulcanizates
Carbon black (Petroleum product) and silica (mineral filler) Important reinforcing fillers used in rubber compounds.
Replacement of carbon black by silica in tyre will contribute to low rolling resistance and good wet grip.
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Carbon black and Silica as filler in natural rubber
Carbon black as filler in NR:
Good rubber-filler interaction Low filler-filler interaction Good reinforcement
Silica as filler in NR :
High filler-filler interaction Low rubber-filler interaction Inferior reinforcement
How to improve silica reinforcement of NR?
By the use of Silane coupling agent
By chemically modifying natural rubber so as to makeit polar so that rubber-silica interaction could beincreased.
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Silica –Rubber coupling through silane
Epoxidised natural rubber (ENR)
Chemically modified natural rubber
ENR more polar than NR due to the presence of epoxy group
Rubber-silica interaction is better in ENR than in NR
Silica filled ENR exhibits low rolling resistance and high wet grip
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Natural Rubber
Epoxidised natural rubber
Si OH + C C
O HH3C
H2C CH2
OH O
C
C
CH2
CH2
H3C
H
CH2
C
C
CH2
H
OHCH3
OSi
Si
Silanol ENR
Si OH + C C
O HH3C
H2C CH2
OH O
C
C
CH2
CH2
H3C
H
CH2
C
C
CH2
H
OHCH3
OSi
SiSi OH + C C
O HH3C
H2C CH2
OH O
C
C
CH2
CH2
H3C
H
CH2
C
C
CH2
H
OHCH3
OSi
Si
Silanol ENR
Proposed mechanism for Epoxy- Silanol interaction
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Objective of the study
To compare the silica reinforcement of NR and ENR of varying epoxy content
Materials Used:
Natural Rubber – ISNR 5
Epoxidised natural Rubber: ENR 10, ENR 17.5, ENR 25, ENR 50
Silica – Precipitated silica (Ultrasil VN3 of Degusa AG)
Silane coupling agent – ‘Si 69’ ((bis-triethoxysilylpropyl tetrasulphide)
Zinc oxide, Stearic acid, sulphur, Accelerators, anti oxidants and process oil – rubber grade
EXPERIMENTAL
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a) Rubber used for each experiment was initially masticatedin a laboratory mixing mill to a Mooney viscosity in therange of 65 ± 2.
b) Further mixing of rubber compound was done in threestages in a laboratory model internal mixer
c) Final homogenization of each mix was carried out in thelaboratory mixing mill.
Compound preparation
Testing
Compound testing
Cure characteristics
Using MDR 2000 Rheometer of M/s Alpha Technologies at 150 °C as per ASTM D-2084.
Vulcanizate testingMoulding of test samples –Using an electrically heated hydraulic press to their respective optimum cure time (t90) at 150 °C .Properties of the vulcanizates –Stress-strain properties – ASTM D 412, Tear strength – ASTM D 624 Hardness – ASTM D-2240, Heat build up – ASTM D 623Compression set – ASTM D 395 – Method B, Abrasion resistance – DIN 53516Resilience – ASTM D 1054 Silica dispersion – Using AFM
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Ingredients *
Compound number
1(N/S/S)
2(E10/S/S)
3(E17.5/S/S)
4 (E25/S/S)
5 (E50/S/S)
6(N/S)
7 (E50/S)
NR (ISNR 5) 100 - - - - 100 -
ENR 10 - 100 - - - - -
ENR 17.5 - - 100 - - - -
ENR 25 - - - 100 - - -
ENR 50 - - - - 100 - 100
Silica (Ultrasil VN3)
50 50 50 50 50 50 50
Silane (si 69)
4 4 4 4 4 - -
* Above compounds also contain Zinc oxide 4, Stearic acid 2, N-cyclohexyl-2-benzothiazolesulphenamide (CBS) 1.5, di phenylguanidine (DPG ) 1.5, Sulphur 2, Naphthenic oil 5, Mernox 6C (N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine ) 2
Formulation of mixes
Cure characteristics
Compound number1
(N/S/S)2
(E10/S/S)3
(E17.5/S/S)4
(E25/S/S)5
(E50/S/S)6
(N/S)7
(E50/S)
Min. Torque, ML,dN.m
1.5 1.3 2.0 2.6 2.1 2.3 2.3
Max. torque, MH, dN.m
19.5 23.6 18.9 21 20.5 32 22.8
Optimum cure time, (t90), min.
8.4 7.5 7.7 6.2 7.6 11.9 8.6
Scorch time,(ts2),min.
4.1 3.5 3.7 2.6 2.5 0.4 3.1
Cure rate index (CRI), 100/t90-t2
23.4 25.3 25.1 27.7 19.5 8.65 18.1
Cure characteristics
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Property Compound number
1(N/S/S)
2(E10/S/S)
3(E17.5/S/S)
4 (E25/S/S)
5 (E50/S/S)
6(N/S)
7 (E50/S)
100%mod., N/mm2
2.2 3.4 3.2 3.4 3.5 1.4 3.2
200%mod., N/mm2
5.7 8.4 8.8 9.7 10.4 2.8 8.8
300%mod., N/mm2
10.4 14.6 15.8 - - 5.3 15.8
Tensile strength, N/mm2
29.2 23 24.5 14.2 14.5 22.2 17.0
Elongation at break,%
595 430 425 260 260 630 320
Stress – strain properties
Stress – strain properties - comparison
Stress - strain properties
0
5
10
15
20
25
30
35
0 100 200 300 400 500 600 700
Elongation, %
Mod
ulus
, N/m
m2
NR/Silica/silane
ENR10/Silica/silane
ENR17.5/silica/silane
ENR25/Silica/silane
ENR50/Silica/silane
NR/Silica
ENR50/Silica
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Variation of tear strength
0
20
40
60
80
100
120
NR/Silic
a/Sila
ne
ENR10/S
ilica
/Sila
ne
ENR17.5
/Sili
ca/S
ilane
ENR25/S
ilica
/Sila
ne
ENR50/S
ilica/S
ilane
NR/Silic
a
ENR50/S
ilica
Tear
str
engt
h, N
/mm
Property Compound number1
(N/S/S)2
(E10/S/S)3
(E17.5/S/S)4
(E25/S/S)5
(E50/S/S)6
(N/S)7
(E50/S)
Hardness, Shore A
64 67 67 71 74 66 71
Resilience,% 67.3 66.6 61.9 57.4 32.8 61.2 54.7
Compression set,%, (70 °C, 22 hrs.)
38.8 41 41 41.2 43.7 50.4 49
Demattia flex -Initiation, (Cycles-lakhs)
2.98 1.96 1.98 0.23 0.10 0.52 0.37
Demattia flex -Failure, (cycles-lakhs)
5.42 3.81 3.39 1.68 0.26 1.76 0.54
Technological properties
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Variation of heat build-up
13
1112
14
16
19 19
0
2
4
6
8
10
12
14
16
18
20
NR/Sili
ca/Sila
ne
ENR10/S
ilica
/Sila
ne
ENR 17.5
/Sili
ca/S
ilane
ENR25/S
ilica
/Sila
ne
ENR50/S
ilica
/Sila
ne
NR/Sili
ca
ENR50/S
ilica
He
at
bu
ild
-up
, d
eg
ree
C
Variation of Abrasion loss, mm3
128 132141
167
192174
196
0
50
100
150
200
250
NR/Sili
ca/Sila
ne
ENR10/S
ilica
/Sila
ne
ENR17.5
/Sili
ca/S
ilane
ENR25/S
ilica
/Sila
ne
ENR50/S
ilica
/Sila
ne
NR/Sili
ca
ENR50/S
ilica
Ab
rasi
on
lo
ss,m
m3
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ATOMIC FORCE MICROSCOPY STUDIES (Phase image)
N/S/S E10/S/S
E17.5/S/S E25/S/S E50/S/S
SAMPLE -1-PHASE -3D SAMPLE – 2 -3D PHASE
SAMPLE -3 – 3D-PHASE SAMPLE 4 -3D – PHASE SAMPLE 5 – 3D PHASE
N/S/S E10/S/S
E17.5/S/S E25/S/S E50/S/S
ATOMIC FORCE MICROSCOPY STUDIES (Phase 3Dimage)
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Conclusions
Cure characteristics of silane modified silica filled NR andENR of varying epoxy content were comparable.
NR/Silica exhibited delayed cure and poor processability,whereas ENR 50/Silica showed reasonale cure rate.
Silane modified Silica filled ENR exhibited a gradualchange in properties with the extent of epoxidation.
Modulus , hardness and compression set of vulcanizatesshowed an increasing trend with extent of epoxidation
Tear strength, elongation at break, abrasion resistance,resilience and flex resistance exhibited a decreasingtrend with extent of epoxidation
NR, ENR10 and ENR 17.5/silica/Silane compositesexhibited low and comparable heat buid-up. Compositeswith higher levels of epoxidation exhibited higher heatbuild-up.
AFM studies revealed that Silica dispersion is better inENR than in NR.
Silane modified silica filled ENR 10 and ENR 17.5exhibited comparatively better overall properties
Conclusions contd……….
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