Controlled growth of in situ silica in a NR/CR blend by a solution sol-gel method and the studies of its composite properties

Article abstract of DOI:10.1039/c5ra08971a

Silica is grown in situ into a natural rubber (NR)/chloroprene rubber (CR) blend (at 40/60 ratio), by a solution sol-gel method, where the silica content in rubber blend is increased in a controlled manner exceeding the limit found for the same blend ratio in the soaking sol-gel method. Reaction conditions have been optimized to get adequate conversion of tetraethoxysilane (TEOS, a silica precursor) to silica. Rheological, thermal, mechanical and viscoelastic properties of all the composites are compared with those of the unfilled rubber blend at similar conditions. Thermal and mechanical properties of the composites are found to improve consistently as silica content in the composite increases owing to increased rubber-filler interaction as revealed in dynamical mechanical analysis (DMA). Further improvement in the properties is observed for a particular composite where a silane coupling agent ((γ-aminopropyl)trimethoxysilane, γ-APS) is used in the reactive sol-gel system during in situ generation of silica. This is attributed to the uniform distribution of silica in the rubber matrix and strong rubber-filler interaction, caused by bifunctionality of silane, as revealed by morphology and DMA studies respectively. The reinforcement effect of silica is evaluated by comparing the experimental results with theoretical values obtained from the Guth-Gold model and the modified Guth model. The present study supplements the in situ silica generation in NR/CR blend of 40/60 ratio, following the solution sol-gel method, to the earlier study involving the soaking sol-gel method where the maximum reinforcement was found for this composition.

Full text of DOI:10.1039/c5ra08971a

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Controlled growth of in situ silica in a NR/CR blend
by a solution sol–gel method and the studies of its
composite properties
Cite this: RSC Adv., 2015, 5, 53559
a
a
a
Naresh D. Bansod, Bharat P. Kapgate, Chayan Das, Debdipta Basu,b
*
Subhas Chandra Debnath,c Kumarjyoti Royc and Sven Wiessnerbd
Silica is grown in situ into a natural rubber (NR)/chloroprene rubber (CR) blend (at 40/60 ratio), by a solution
sol–gel method, where the silica content in rubber blend is increased in a controlled manner exceeding the
limit found for the same blend ratio in the soaking sol–gel method. Reaction conditions have been optimized
to get adequate conversion of tetraethoxysilane (TEOS, a silica precursor) to silica. Rheological, thermal,
mechanical and viscoelastic properties of all the composites are compared with those of the unfilled
rubber blend at similar conditions. Thermal and mechanical properties of the composites are found to
improve consistently as silica content in the composite increases owing to increased rubber–filler
interaction as revealed in dynamical mechanical analysis (DMA). Further improvement in the properties is
observed for a particular composite where a silane coupling agent ((g-aminopropyl)trimethoxysilane, g-
APS) is used in the reactive sol–gel system during in situ generation of silica. This is attributed to the
uniform distribution of silica in the rubber matrix and strong rubber–filler interaction, caused by
bifunctionality of silane, as revealed by morphology and DMA studies respectively. The reinforcement effect
of silica is evaluated by comparing the experimental results with theoretical values obtained from the
Guth–Gold model and the modified Guth model. The present study supplements the in situ silica
generation in NR/CR blend of 40/60 ratio, following the solution sol–gel method, to the earlier study
involving the soaking sol–gel method where the maximum reinforcement was found for this composition.
Received 13th May 2015
Accepted 9th June 2015
DOI: 10.1039/c5ra08971a
www.rsc.org/advances
for practical application.2–6 Silica is an important non black type
of reinforcing ller which is generally added into rubber matrix
1 Introduction
by external mixing. However, good dispersion of silica in rubber
matrix to achieve proper reinforcement effect remains a major
issue for the external silica loading in rubber matrix.7–10 In this
context, incorporation of silica, in situ into rubber matrix, has
appeared to be a very efficient technique in recent times. There
are many reports available in literature on in situ silica incorpo-
ration in rubber including natural rubber, synthetic rubber or
rubber latex.11–15 But studies on rubber blends, lled with in situ
silica, are rather limited in literature.4 Recently, we reported the
dual role of in situ silica for NR/CR blend composite where in situ
silica enhanced the compatibility of otherwise immiscible NR/CR
blend in addition to imparting reinforcement effect.16 It may be
mentioned here that the blend compatibility is a major concern
for such heterogeneous blends comprised of polar and nonpolar
rubber as far as mechanical properties of the composite is con-
cerned.17–19 However, in that work, amount of silica generated, in
situ into rubber blend, is limited since we adopted soaking sol–
gel method. It may be mentioned here that, for a particular blend
ratio, amount of silica generated by soaking sol–gel method
remains xed at any particular reaction condition. Interestingly,
the best composite properties were shown by NR/CR blend at
ratio 40/60 although silica content was maximal for the NR/CR
Rubber blends comprising of natural rubber (NR) and chloro-
prene rubber (CR) in suitable proportion nd many useful
applications due to the combined features of the components in
the blend that cannot be exhibited by the individual compo-
nents.1–3 NR is well known for its good mechanical properties,
elasticity and dynamic properties while CR shows excellent
resistance to solvent, chemicals, re, heat and ozone attack.2–4
Blending of NR with a suitable proportion of synthetic rubber
such as CR, styrene butadiene rubber (SBR), nitrile rubber (NBR)
etc. has been proven to overcome the limitations of NR.2–4 For
example, the addition of CR into NR helps to overcome the main
shortcoming of NR like poor oil and ageing resistance. Never-
theless, incorporation of ller into rubber matrix is extremely
essential to achieve adequate mechanical properties to make it t
aDepartment of Chemistry, Visvesvaraya National Institute of Technology, Nagpur,
India. E-mail: chayandas@hotmail.com; chnds@rediffmail.com; Tel: +91-712-2801601
bDepartment of Elastomers, Leibniz-Institut fur Polymerforschung Dresden e.V.,
¨
Dresden, Germany
cDepartment of Chemistry, University of Kalyani, West Bengal, India
dTechnische Universitat Dresden, Institut fur Werkstoffwissenscha, Dresden,
¨
¨
Germany
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(Nagpur, India). Tetraethylorthosilane 98% (TEOS) and n-
butylamine were purchased from Across Organic (New Jersey,
USA). g-Aminopropyl trimethoxysilane (97%) (g-APS) was
purchased from Sigma-Aldrich. Toluene, zinc oxide (ZnO) and
stearic acid were purchased from sher scientic. Ethanol was
purchased from Merck. Ethylene thiourea (ETU) was purchased
from National chemicals (Vadodara, India). Magnesium oxide
(MgO), sulphur, N-cylohexylbenzothiazole-2-sulphenamide
(CBS) were received from Sara Polymer Pvt. (Nagpur, India).
2.2 In situ silica generation
Silica particles were generated in situ into NR/CR blends of 40/
60 ratio by solution sol–gel process, both in the presence and
absence of silane coupling agent (g-APS). 12 g of NR and 18 g of
CR were dissolved together in toluene and set to stirring for
homogenous mixing (for sample 2–6). Aer that varying amount
of TEOS, water, ethanol (TEOS : water : ethanol mole ratio ¼
1 : 2 : 4) and n-butylamine (0.096 mol with respect to TEOS) as
catalyst were added to the solution with continuous stirring for
15 min. For example, for composite 2, 18.1 ml of TEOS, 2.9 ml of
water, 1.8 ml of n-butylamine and 14.9 ml of ethanol were added
in rubber solution. For sample 6, 0.58 ml of g-APS was added in
rubber solution in addition to the other reagents. Then the
solution was reuxed for 1 h at 130 ꢀC. The solution was kept for
gelling at room temperature for 4 days followed by vacuum
Fig. 1 (a) Interaction between the silanol group of silica and chlorine
atom of CR (b) linkage between g-APS modified silica and CR.
ꢀ
blend at 80/20 ratio. More uniform dispersion of silica particle in
rubber matrix along with good degree of enhancement in blend
compatibility brought by in situ silica for that particular blend
ratio produced the best results for that particular composite. In
present work, we have further extended our studies on this
particular blend ratio (NR/CR at 40/60) adopting solution sol–gel
method, where amount of silica generation in the rubber matrix
is controlled to overcome the limitation of silica generation
associated with soaking sol–gel method. Detailed studies of NR/
CR composites, lled with varying amount of in situ silica (up to
30 phr) are done and the results are compared with unlled one.
Furthermore, silane treatment of silica in the reactive sol–gel
system was done for a selected composite using (g-aminopropyl)
trimethoxysilane (g-APS), as a silane coupling agent. It is known
that the use of appropriate silane in the reactive sol–gel system
during generation of in situ silica could effectively improve the
silica dispersion and enhance rubber–ller interaction
(Fig. 1).20–22 Studies on moisture curing of CR by g-APS had
established the existence of a strong interaction between CR and
g-APS.23,24 In fact, the silica dispersion and rubber–ller interac-
tion is found to improve drastically, in this study, by using this
silane with respective to the unsilanized silica lled composite at
same silica content. The results of silane treated silica lled
drying at 50 C.
2.3 Formulation and compounding of rubber
In situ silica lled NR/CR blends were masticated on a two roll
mill for 5 minute before compounding with other crosslinking
ingredient. Then all crosslinking ingredients (ZnO, MgO, stearic
acid, CBS, ETU and sulphur) were added stepwise and mixed for
10 minutes to obtain homogenous rubber mixture. This process
was followed for all the in situ silica lled composites (2–6).
Similarly unlled sample (1) was prepared by solution blending,
NR and CR in 40/60 ratio dissolved together in toluene and set
to stirring for homogenous mixing. Then the dried rubber sheet
was compounded with other crosslinking ingredient on a two
roll mill without any ller. Formulation of all the composites is
given in Table 1. The resultant compounds were then vulca-
ꢀ
nized, using compression moulding machine, at 160 C as per
their respective cure time (t90) given in Table 3.
Table 1 Formulation of rubber compounds (NR/CR)a in phr (parts by
weight per hundred parts of rubber)b
Samples
1
2
3
4
5
6
composites are compared with those with unsilanized silica lled In situ silica
—
—
—
—
10
—
20
—
25
—
30
—
30
2
g-APS
composite.
TEOS (g)c
16.9 37.8 47.3 63.3 63.3
70 66 66 56 56
% conversion of TEOS to SiO2
a
NR and CR are at 40 and 60 phr respectively for all the above
2 Experimental
2.1 Materials
b
compounds. Crosslinking ingredients for all the above compounds
(in phr): zinc oxide – 5; magnesium oxide – 4; stearic acid – 1; CBS –
1; sulfur – 2 and ethylene thiourea – 0.5. Amount of TEOS added in
g for 30 g of blend (12 g of NR + 18 g of CR).
c
Chloroprene rubber (LANXESS Bayprene 116, XD grade) and
natural rubber (RSS 4) were obtained from Heritage Rubber
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3.6 Dynamic mechanical properties (DMA)
3 Characterizations
Dynamic mechanical properties were measured over tempera-
ture range ꢁ100 ꢀC to 140 ꢀC using an Eplexor 2000 N dynamic
mechanical analyzer (Gabo Qualimeter, Ahlden, Germany) by
using a constant frequency of 10 Hz. All the measurements were
done in tension mode. For the measurement of complex
modulus, E*, a static load of 1% per strain was applied and then
the samples were oscillated to a dynamic load of 0.5% strain. All
the measurements were done with a heating rate of 2 ꢀC minꢁ1
under liquid nitrogen ow.
3.1 Curing studies
The minimum torque (ML), maximum torque (MH), delta tor-
que, cure rate index (CRI), cure time (t90) and scorch time (ts2)
for all the compounds were determined by using a Monsanto
Rheometer R-100 at an amplitude of 3ꢀ. The measurements
ꢀ
were carried out at 160 C for 60 min.
3.2 Scanning electron microscopy (SEM)
The surface morphology of NR/CR blends were investigated by
using scanning electron microscope (Zeiss Ultra Plus; Carl Zeiss
Microscopy GmbH, Jena, Germany). The samples were cut with
an ultramicrotome and sputter coated with 3 nm platinum to
prevent charging on the surface.
3.7 Differential scanning calorimeter measurements (DSC)
Measurements were performed using a differential scanning
calorimeter, DSC 204 F 1 (NETZSCH, Germany). The samples of
7–10 mg were placed in a standard aluminum pan with pierced
lid. The pan was placed into the apparatus at room temperatuꢁre1
ꢀ
ꢀ
and heated immediately upto 200 C at a rate of 30 C min
and kept ve minute at this temperature to remove the volatiles
3.3. Thermogravimetric analysis (TGA)
ꢀ
impurities. They were quenched to ꢁ70 C and DSC scan was
ꢁ1
ꢀ
ꢀ
ꢀ
Thermal stability and silica content of the rubber vulcanisates
were determined by thermogravimetric analysis and derivative
thermogravimetry (DTG) by using a thermal analyzer TG-DTA
7200 (Hitachi, Japan). About 5–10 mg of sample was kept on a
platinum pan and heated in the temperature range 40 to 500 ꢀC
under nitrogen atmosphere and heating was continued upto
performed from ꢁ70 C to +70 C at a heating rate 5 C min
under nitrogen atmosphere.
4 Results and discussion
4.1 Controlled loading of in situ silica into NR/CR rubber
ꢁ1
ꢀ
ꢀ
blend
700 C under air at a heating rate of 10 C min
.
In present study, we have adopted solution sol–gel method to
incorporate silica in situ in NR/CR blend at 40/60 ratio. Here, NR
and CR were rst dissolved in toluene in proper proportion and
3.4 Mechanical characteristics
Stress–strain studies were carried out using a Zwick tensile mixed thoroughly by mechanical stirring followed by addition
tester (model 1456, Z010, Ulm Germany) using dumb-bell sha- of measured quantity of TEOS, the silica precursor. In sol–gel
ped test specimen at uniform speed of 200 mm minꢁ1. The process, tetraethoxysilane undergoes hydrolysis and poly-
hardness of the vulcanized sample was measured by a Shore A condensation reactions, in catalytic condition, to form silica
durometer (BSE testing machine India). All tests were per- particles (Fig. 2). There are several factors such as nature of
formed at ambient temperature.
solvent, type and amount of catalyst, type of silica precursor,
reaction time and other physical condition that may have
signicant effect on the sol–gel reaction and consequently can
generate silica with different content, shape/size and surface
characteristics.26 In this work, TEOS was used as a silica
precursor and n-butylamine was used as a catalyst as they are
widely used in similar studies.12 Other than these, we have
optimized several reaction parameters like choice of solvent,
concentration of catalyst and reaction time to get appreciable
amount of silica generation in rubber matrix (described in
details in Experimental section). The results are summarized in
Table 2. It is evident here that silica generation was more when
THF was used as a solvent compared to other two solvents. This
is due to the polar nature of THF that accounts for higher water
solubility and hence provides better miscibility of water and
TEOS in the rubber matrix. This in turn increases the rate of
hydrolysis and condensation reaction which leads to more silica
generation.5 However, addition of ethanol in calculated
amount, which could act as a homogenizing agent,27 was found
3.5 Swelling study
The crosslinking density of all the vulcanized samples was
determined by the equilibrium swelling method using toluene
as solvent. The cured samples of known weight were allowed to
swell for 7 days in fresh toluene at room temperature. Aer each
24 hour solvent were changed with fresh toluene. Aer 7 days,
samples were taken out and adhered liquid was rapidly
removed by blotting with tissue paper and weighed immedi-
ꢀ
ately. It was vacuum dried at 80 C till a constant weight was
obtained. Crosslinking density n, dened by the number of
elastically active chains per unit volume was determined by
Flory–Rehner eqn (1)25
Â
Ã
2
ꢁ lnð1 ꢁ VrÞ þ Vr þ xVr
n ¼
(1)
ꢀ
ꢁ
1
3
Vs Vr ꢁ Vr=2
where, Vs is the molar volume of the toluene (106.2), Vr is the to improve the miscibility between toluene and water in the
volume fraction of rubber in swollen gel and c is the Flory– rubber matrix enhancing the hydrolysis of TEOS. Thus,
Huggins rubber–solvent interaction parameter.
combination of toluene and ethanol yielded more silica than
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Fig. 2 Hydrolysis and condensation reaction of tetraethoxysilane (TEOS).
THF alone, when used as solvent, under similar conditions it was expected to facilitate the silica dispersion and rubber–
which appeared to be a better alternative approach from ller interaction (Fig. 1(b)).
economical point of view. It may be noted that the excessive use
of ethanol could lead to rubber phase separation from the
mixture. Concentration of catalyst, n-butylamine, was varied
from 0.032 to 0.18 mole ratio with respect to TEOS, keeping all
other parameter constant and it was found that silica genera-
tion was increased with increase in catalyst concentration upto
0.096 mole ratio. Moreover, the reaction time was varied to see
it's effect on the silica generation and it was found that there
was no signicant improvement in silica generation beyond 60
min. Thus in the optimised condition, amount of silica in the
NR/CR blend was varied up to 30 phr by varying the amount of
TEOS added. From Table 2, it is found that there is 70%
conversion of TEOS into silica for composite (2) (10 phr silica).
However, percentage conversion dropped to certain degree for
higher silica content and 56% conversion was achieved for
composite 5 and 6 (30 phr silica) (Table 1).
4.2 Curing characteristics
Curing characteristics of all the compounds are illustrated in
Fig. 3 and Table 3. It is observed that the minimum torque
values of the silica lled compounds are higher than that of
unlled compound and the value gradually increases with
increase in silica content. This indicates the increased viscosity
i.e. stiffness resulted with the incorporation of silica into rubber
blend. Higher maximum torque values with increased silica
content are well supported by the crosslinking density values of
the compound calculated by Flory–Rehner equation (eqn (1)).
This is attributed to the strong interaction between the silanol
group of silica and chlorine atom of CR which provides addi-
tional crosslink's to the rubber matrix (Fig. 1(a)).16 For the silane
treated silica lled compound (6), minimum torque is found to
be higher compared to the compound lled with similar silica
It would be worthwhile to mention here that in soaking sol–
gel method, silica content for NR/CR (40/60) rubber blend was
found to be 20 phr that was xed for a particular blend ratio, at a
given reaction condition, as the mechanism of silica loading
was different in that case. In soaking sol–gel method, sheets of
rubber blend are immersed in liquid TEOS (tetraethoxysilane)
for sufficient time for swelling of TEOS inside the rubber matrix
and the swelled sheets are immersed in catalyst solution at
standard condition for sol–gel reaction to occur. Silica genera-
tion is governed by the imbibed TEOS inside the rubber blend
matrix which is inuenced by blend composition.16 NR, being
non polar in nature, has greater TEOS uptake capability than CR
leading to more silica generation in the former phase. Thus
silica generation in the rubber blend increases with increase in
NR proportion in the NR/CR blend in soaking sol–gel method
but amount of silica remains xed for a particular blend
proportion. Whereas, in solution sol–gel method, silica content
can be varied by controlling the amount of silica precursor in
the reaction mixture as done in the present study. Furthermore,
a composite (6 at silica content 30 phr) was prepared using g-
APS (2 phr), in the reactive sol–gel system, to see the effect of
silane integration on the composite properties at higher silica
loading. The specic silane was chosen for its ability to interact
with chlorine atom of CR via the amino functional end and thus
Table 2 Optimization of sol–gel reaction conditions for in situ silica
loading
n-Butylamine
as catalyst
Reaction time
%
Sr. no. Solvent
(mole ratio to TEOS) (minute)
conversion
Effect of solvent
1
2
3
4
THF
Toluene 0.096
Xylene 0.096
Toluenea 0.096
0.096
60
60
60
60
60
46
35
70
Effect of reaction timea
1
2
3
4
Toluene 0.096
Toluene 0.096
Toluene 0.096
Toluene 0.096
30
60
90
120
60
70
71
72
Effect of catalyst concentrationa
1
2
3
4
5
Toluene 0.032
Toluene 0.064
Toluene 0.096
Toluene 0.128
Toluene 0.180
60
60
60
60
60
60
67
70
70
71
a
Ethanol was added in the mixture (TEOS : ethanol ¼ 1 : 4 mole ratio).
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microscopy (SEM) and the micrographs are shown in Fig. 4. It is
evident that, the silica particles are homogenously dispersed
throughout the rubber matrix with few aggregates at 10 phr
silica content in composite 2 (Fig. 4(a)). With increase in silica
loading (20 and 30 phr), the aggregation tendency of silica
particles increases and it is much dominant for the sample (5)
lled with 30 phr (Fig. 4(c)). Notably, much better silica
dispersion along with signicant reduction in particle size at
same silica loading (30 phr) is found in silane treated silica
lled composite 6 (Fig. 4(d)). For composite (5), the particle size
distribution is in the range of 100–150 nm for smaller particles
while size of larger silica aggregates lies in the range of 300–450
nm (Fig. 4(c)). On the other hand, in composite (6), size of the
silica particles is signicantly reduced in the range 50–70 nm.
Uniform and homogeneous distribution of silane treated silica
particles into rubber matrix for composite 6 is attributed to the
bifunctionality of silane that might enhance the rate of sol–gel
reaction.22,28 The hydrolysable trimethoxy groups of silane,
present at one end, react with surface hydroxyl (silanol) groups
of silica and prevent the aggregation (ller–ller interaction) of
silica.20,21 The organofunctional amino groups, on the other end
of silane, interact with the chlorine atom of CR and provide
additional crosslink's resulting in strong rubber–ller interac-
tion as shown in Fig. 1(b).23,24
Fig. 3 Curing characteristics of the unfilled and in situ silica filled
compounds (1–6).
content but without silane (5). This could be due to the partial
crosslinking, which might take place before actual vulcaniza-
tion starts, through strong rubber–ller interaction facilitated
by bifunctionality of silane.20–22 This is also well supported by
the crosslinking density values.
The scorch time and cure time of the in situ silica lled
compounds are found lower for silica lled compounds
compared to that of unlled compound that gradually decrease
with increase in silica content. This is little unexpected, as it is
known that surface silanols of silica could adsorb the cure
accelerators and inhibit the crosslinking reaction.2,7 However,
the strong interaction between the silanol groups of silica and
chlorine atom of CR in the blend might suppress the curatives
adsorption by silanols.16 This effect becomes more signicant
for silane treated silica lled compound (6). Cure rate index
(CRI) values for the in situ silica lled compounds are found
higher compared to that of unlled sample which indicates the
faster curing process in former case. Further increase in CRI,
brought by silane treatment, indicates enhancement in cure
rate. This may be attributed to the strong rubber–silica and
rubber–silane–silica interaction (Fig. 1).16,20
4.4 Thermogravimetric study
Thermogravimetric analysis was performed to evaluate the
thermal stability and silica content of the composites (Fig. 5). In
situ silica content of the rubber composites (2–6) was deter-
mined from the residual weight percentage of a particular
composite, relative to unlled one as obtained from thermog-
ravimetric results (Table 1). In all the composites (1–6), initial
weight loss beginning around 100 ꢀC, is due to the combustion
of organic additives added for vulcanization and elimination of
polycondensation residues that starts with increase in temper-
ature. The onset temperature (300–335 ꢀC) that arises from the
degradation of rubber phase, increases slightly as silica content
increases. The differential thermogravimetric (DTG) curves of
unlled and lled samples show three peaks. The rst peak in
the temperature range 335 ꢀC to 350 ꢀC corresponds to the
maximum weight loss of the NR phase in the blend and the
4.3 Morphology
ꢀ
ꢀ
second peak in the temperature range 430 C to 440 C corre-
Morphology of the composites at different silica loading viz. at sponds to the maximum weight loss of CR phase in the blend. It
10 phr, 20 phr and 30 phr was investigated by scanning electron is known that the double bonds in CR are less reactive towards
Table 3 Curing characteristics of unfilled and in situ silica filled compounds
Sample code
1
2
3
4
5
6
Minimum torque (dNm)
Maximum torque (dNm)
Dtorque (dNm)
4
38
34
9.5
1.75
12.9
1.4
4.5
60
55.5
8.0
1.5
5.2
65
6
75
69
6.7
0.75
16.80
4.11
6.5
79
72.5
6.5
0.5
8.0
81.75
73.75
5.5
0.5
20
59.8
6.9
0.72
16.18
3.19
Cure time t90 (min)
Scorch time (min)
Cure rate index (minꢁ1 a
)
15.38
2.02
16.66
5.4
Crosslinking density (n ꢂ 104)
6.6
a
Cure rate index (CRI) ¼ 100/(cure time ꢁ scorch time).
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to the drop in tensile strength of the corresponding rubber
composites. We wish to mention here that the level of
improvement in modulus and tensile strength relative to
unlled sample obtained from solution sol–gel method, in the
present study, does not differ much with the results obtained
for similar composites with NR/CR blend at 40/60 ratio at 20 phr
silica loading, prepared by soaking sol–gel method in our earlier
studies.16 But in present work, silica content in NR/CR blend is
increased in a controlled manner and further improvement in
mechanical properties is gained which was not feasible in case
of soaking sol–gel method for this blend ratio. In present case,
moduli at 100% and 300% strain increase approximately four
times at 30 phr silica loading compared to those of unlled
sample. Moreover, the values become still higher by silane
treatment of silica for composite 6, at same silica content (30
phr). This is in accordance with the results found for this
particular composite in morphology study where uniform
dispersion of silica throughout the rubber matrix was observed.
This is due to the presence of the organosilane, (g-APS) that
prevents the silica aggregation (ller–ller interaction) and
provides additional crosslinks through its organofunctional
amino groups.
Fig. 4 Scanning electron microscopy (SEM) images of in situ silica
filled NR/CR composites: (a) composite 2 (10 phr silica), (b) composite
3 (20 phr silica), (c) composite 5 (30 phr silica), (d) composite 6 (30 phr
silica treated with g-APS).
Hardness values of the elastomeric composite are closely
related to the modulus at low strain.20 As expected, hardness
values (Shore A) of the lled composites are found to increase
with increase in situ silica loading (Fig. 6(b), Table 4). Never-
theless, the maximum hardness value is shown by composite
(6). This may be attributed to the highest silica content and
enhanced crosslinking density brought up by the silane treat-
ment for this composite in the series.
To understand the reinforcement effect of in situ silica on the
rubber blend, dependence of modulus enhancement (EC/E0) on
the volume fraction of ller has been evaluated experimentally
and the results are compared with the theoretical values
determined from Guth–Gold model and modied form of Guth
model considering shape factor (f ¼ 2.53) (given below).15,30,31
Fig. 5 Thermogravimetric curves of the unfilled and in situ silica filled
composites (1–6).
Guth–Gold equation (1938): EC/E0 ¼ 1 + 2.5f + 14.1f2 (2)
Modified Guth equation (1945): EC/E0 ¼ 1 + 0.67ff + 1.62f2f2
thermal oxidative degradation as compared to the double bond
in NR.16,29 The third peak in the temperature range 550 ꢀC to 580
ꢀC, obtained at heating under air ow, is due to the decompo-
sition of carbonaceous residue.20 It is evident that the addition
of silane has little effect on the Tmax of both NR and CR phases
as well as on the thermal stability of the composite 6.
(3)
where EC and E0 are the young modulus of the lled and unlled
rubber samples respectively, f is the volume fraction of the ller
in rubber composite
The experimental as well as predicated values (obtained by
both the above models) of EC/E0 are plotted against volume
fraction of ller and best tting curves are shown in Fig. 7. It is
interesting to note that the experimental values are found to be
much higher than those of the predicted values calculated by
using eqn (2) and (3) for the composites (2–5).15,30,31 The curve
that best ts with experimental data corresponds to volume
fraction f ꢃ 2.25. The reinforcement of rubber matrix caused by
ller is a very complex process and depends on several param-
eters like volume fraction, aspect ratio (shape factor) of ller
and their interaction with the rubber matrix. In present case, it
is observed that the calculated values obtained by using both
4.5 Stress–strain studies
The inuence of in situ silica loading on the mechanical prop-
erties of the composites was investigated by stress–strain
measurement. The test results are given in Table 4 and stress–
strain curves are depicted in Fig. 6. It is evident that the moduli
at 100% and 300% strains become higher by incorporation of
silica and gradually increases with increase in silica content in
the blend. However, tensile strength becomes less at higher
silica content. This is attributed to the large aggregates of silica
in the rubber matrix at higher silica content, as found in SEM
images, which may impart stiffness to the composite and leads
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Table 4 Mechanical properties of the unfilled and in situ silica filled composites (1–6)
Sample code
1
2
3
4
5
6
s100% (MPa)
s300% (MPa)
Tensile strength (MPa)
Elongation at break (%)
Hardness (Shore A)
0.78
1.62
10.10
636
1.52
4.35
11.55
469
1.86
5.04
15.37
529
2.15
5.45
6.43
342
68
3.01
5.83
5.86
306
70
3.30
8.08
10.47
383
52
58
64
75
Fig. 6 Mechanical properties of the unfilled and in situ silica filled composites (1–6): (a) stress–strain curves and (b) Shore A hardness values and
s100% moduli.
transition temperature (Tg), compared to the unlled sample.
Moreover, in the rubbery region, the storage moduli increase as
the silica loading increases. Therefore, it is realized that the
silica particles substantially reinforces the compounds. Inter-
action between the chlorine atom of CR and surface hydroxyl
group of silica leads to formation of strong rubber–ller
network and restricts the movement of rubber chains.32–34 The
maximum increase in storage modulus is observed for
composite 6 which was prepared by the integration of silane
during silica generation for this composite.
Fig. 7 Plots of modulus enhancement (EC/E0) versus volume fraction
(f).
volume fraction and shape factor of silica particles are low
compared to the experimental results. This modulus enhance-
ment (EC/E0) is attributed to the strong rubber–ller interaction
which is supported by DMA analysis.
4.6 Dynamic mechanical analysis
The temperature dependence of storage modulus (E0) and loss
tangent (tan d) of all the composites are shown in Fig. 8 and 9
respectively. It is observed that the storage modulus values are
Fig. 8 Storage modulus versus temperature plot of the unfilled and in
higher for all the silica lled composites (2–6), beyond the glass situ silica filled composites (1–6).
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NR phase of the blend in lled composites, show marginal shi
towards higher temperature side with slight reduction in peak
height. This indicate the development of rubber–ller interac-
tion for the in situ silica lled composites that in turn reinforces
the rubber matrix and lead to improved mechanical properties
as observed in stress–strain studies. On the other hand, tan d
peaks corresponding to CR phase show signicant reduction in
peak height with increasing in situ silica content in the
blend.35–37 Maximum reduction in tan d peak is observed for
composite 6, lled with silane treated in situ silica, for both the
NR and CR phases relative to the unlled sample (26% reduc-
tion in NR phase and 55% reduction in CR phase). This clearly
shows that very strong rubber–ller interaction is developed for
this particular composite. This observation is consistent with
the other results found for this composite.
It may be recalled here that signicant enhancement in
compatibility of NR and CR phases in the NR/CR blend, induced
by in situ silica, was observed in the composites obtained from
soaking sol–gel method which was ascertained by the shi of
tan d peaks of NR and CR phases towards each other.16 In
present case, however, tan d peaks of NR phase slightly move
towards higher temperature while those of CR phase almost
remain constant. It is obvious that silica reinforcement mech-
anism in these two approaches is different. In soaking sol–gel
method, silica is mostly grown in NR phase and migrates
towards CR phase during mixing with other ingredients
resulting its accumulation at the interface which enhances the
compatibility of the blends.16 On the other hand, in solution
sol–gel method, growth of silica is more likely to take place in
CR phase owing to strong CR–silica interaction. This is sup-
ported by the strong reinforcement effect of in situ silica in CR
phase as reected from the signicant reduction in tan d peak
height for this phase (Fig. 9 and Table 5).
Fig. 9 Loss tangent (tan d) versus temperature plot of the unfilled and
in situ silica filled composites (1–6).
Table 5 Glass transition temperatures, Tg (ꢀC) and tan d peak of the
unfilled and in situ silica filled composites (1–6)a
tan dmax
Sample code
Tg(NR)
Tg(CR)
DTg
NR phase
CR phase
1
2
3
4
5
6
ꢁ49.9
ꢁ44.9
ꢁ46.7
ꢁ46.9
ꢁ47.0
ꢁ46.7
ꢁ28.1
ꢁ27.9
ꢁ26.5
ꢁ26.7
ꢁ26.8
ꢁ26.9
21.8
17.0
20.2
20.2
20.2
19.7
0.35
0.36
0.36
0.35
0.32
0.26
1.83
1.59
1.40
1.37
1.07
0.82
a
Tg(NR)
: glass transition temperature of NR phase; Tg(CR): glass
transition temperature of CR phase; DTg: separation of Tg.
Furthermore, the reduction in peak height for both NR and
CR phases, in the tan d versus temperature plot, indicates the
reinforcement effect of ller in rubber matrix (Fig. 9 and Table
5).15,35 It is observed here that tan d peaks, corresponding to the
4.7 Differential scanning calorimeter (DSC)
The DSC curves indicate two separate glass transition temper-
atures (Tg) corresponding to NR phase and CR phase for all the
composites (1–6) (Fig. 10.) The Tg of the NR and CR phases for
Fig. 10 DSC thermograms of the unfilled and in situ silica filled composites (1–6).
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unlled sample are found at ꢁ61.4 ꢀC and ꢁ45.5 ꢀC respec-
tively. It may be noted that, the Tg values obtained from DSC
study are different from those found in DMA analysis. This
variation may happen due to the different principles involved in
these two techniques. DSC measurement is associated with the
change in heat capacity as rubber chains evolve from the frozen
to unfrozen state, whereas DMA analysis measures the change
in mechanical response by the rubber chains.38 From the DSC
thermograms, it is observed that the Tg of the NR phase is
slightly inuenced by the silica loading whereas Tg of the CR
phase remains almost unaffected. This small variation for NR
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5 Conclusions
Controlled growth of sol–gel derived in situ silica in NR/CR
blend (40/60 ratio) is achieved by solution sol–gel process. In
soaking sol–gel method, silica content for this blend ratio is
limited to 20 phr. Here, silica content in the NR/CR blend is
increased by increasing the concentration of silica precursor in
optimised reaction condition to gain further improvement in
the mechanical properties of the composites. Therefore,
composite with silica content up to 30 phr is prepared where
mechanical properties improve consistently with increase in
silica content. Moreover, silane treatment of silica in the reac-
tive sol–gel system causes much more improvement in the
properties of the highest silica lled composite compared to the
composite containing same amount of untreated silica. This is
attributed to the uniform distribution of silica in rubber matrix
as evidenced by SEM image and strong rubber–ller interaction
as revealed by DMA study for this composite. Improvement in
compatibility of the rubber blend relative to unlled blend is
found to be, however, less than that found in soaking sol–gel
method. Different mechanism, regarding incorporation and
distribution of silica in the rubber blend, occurring in solution
and soaking sol–gel method could be the primary reason
behind this fact. Thus, maximum reinforcement effect of in situ
silica is found for NR/CR blend of composition 40/60 with 20
phr silica in soaking sol–gel method while more silica is
generated for this blend composition by solution sol–gel
method to further enhance the reinforcement effect.
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