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Variation Effect of Carbon-Silica Dual Phase Fillers on the Rheological and Mechanical Properties of Natural Rubber Matrix Composites

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Variation Effect of Carbon-Silica Dual Phase Fillers on the Rheological and Mechanical Properties of Natural
Rubber Matrix Composites
Oladele Isiaka Oluwole*1, Ganiyu Soliu1 and Balogun Oluwayomi Peter2
1Metallurgical and Materials Engineering Department, Federal University of Technology, Akure. Nigeria.
2Prototype Engineering Development Institute (PEDI), National Agency for Science and Engineering Infrastructure.
Nigeria.
E-mail:wolesuccess2000@yahoo.com
Abstract: The effect of variation of Carbon-Silica Dual Phase Fillers (CSDPF) on the mechanical properties of
natural rubber compounding was investigated. The natural rubber for the compounding process was sourced locally.
Five different batches were prepared by varying the amount of carbon black and silica fillers present in them. These
samples were subjected to different rheological and mechanical tests from where the samples with optimum
rheological and mechanical properties were determined. It was found out that sample with higher proportion of
carbon black followed by sample with equal amount of the carbon black and silica has the best rheological and
mechanical properties. These observations were due to higher polymer-filler interactions and lower filler-filler
interactions taking place in the compounds.
[Oladele Isiaka Oluwole, Ganiyu Soliu and Balogun Oluwayomi Peter. Variation Effect of Carbon-Silica Dual
Phase Fillers on the Rheological and Mechanical Properties of Natural Rubber Matrix Composites]. Researcher,
2011; 3(8):37-42]. (ISSN: 1553-9865). http://www.sciencepub.net.
Keywords: carbon-silica dual phase; fillers; natural rubber; compounding; mechanical properties
1. Introduction
The pressure to meet the ever-more stringent
requirement for increase in wear resistance, reduction
in rolling resistance and improvement in wet skid
resistance for tyres in general have been transferred to
the filler supplies. These requirements can only be met
by new fillers that can impact high tensile strength at
high temperature and improved resilience of natural
rubber compounds. It has been established that the
wear resistance of filled rubber compounds is
essentially determined by the filler characteristics,
especially surface activity and morphology. High
surface area carbon black with large interfacial area
between fillers and the polymer generally shows high
interaction with the hydrocarbon polymer used in
tyres (Wang, et al,1991). It is also recognized that
highly structured product generally gives a higher
polymer- filler interaction when it is incorporated into
the polymer (Wolff, et al ,1993).
There are increasing reports in the literature that
significant improvements of multiple structural
functions can be achieved with new hybrid multiscale
composites which incorporate nanoscale
reinforcements as well as conventional micron scale
fibre or particle reinforcements. For example, while
fibre-dominated properties (i.e., longitudinal tensile
strength and elastic modulus) of conventional
unidirectional polymer composites with micron size
fibre reinforcements are excellent, the corresponding
matrix-dominated transverse tensile strength and
longitudinal compressive strength properties are often
poor. However, these traditionally poor properties can
be significantly improved by (a) replacing the neat
resin polymer matrix with a nanocomposite matrix,
and/or by (b) growing nanoreinforcements like carbon
nanotubes on the surface of the fibers (Vlasveld et al,
2005; Zhao et al, 2005; Uddin and Sun, 2008; Uddin
and Sun, 2010; Liu et al, 2008).
Carbon black posses high surface area with
concurrence of high surface activity, results in
improved wear resistance in the practical loading
range. This can be observed from the abrasion
resistance of filled natural rubber compounds. Tyre
rolling resistance has become a major concern during
the last decade as for environmental issues and energy
savings. For conventional carbon blacks there is
generally a trade-off between wear resistance and
rolling resistance. Carbon black impacts high abrasion
resistance to the compound but such compound has
low rolling resistance (Meng, et al, 2000)
Although, it has been reported that the
application of silica in tyre compounds has not been
successful due to its incompatibility with natural
rubber but in the last few years, silica has been used
with styrene butadiene rubber (SBR) in passenger
tread compounds to improve rolling resistance and
wet interaction. This source is attributed to its surface
modification with coupling agents. Compared to
carbon black, with similar surface area and structure,
silica fillers show a very low polymer-filler interaction
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38
and a strong tendency to agglomerate, forming a
developed filler network in the compounds. This
results in poor compound process ability, poor
mechanical properties such as low tensile strength and
abrasion resistance, and unacceptable dynamic
properties (Wang, et al ,1999). However, surface
modification with coupling agents such as bi-
functional saline, bis- (3-triethoxysilypropyl-)
tetrasulfane (TESPT) lowers the filler-filler interaction
by covering the polar surface with organo-grafts, and
enhances the polymer-filler through creation of
chemical linkages between the filler surface and
polymer chains (Wolff, et al ,1993).
The optimum fillers for natural rubber tread
compounds are the combination of fillers that posses
high polymer-filler and low filler-filler interactions.
The former ensures higher abrasion resistance while
the latter ensures low hysteresis, hence lower rolling
resistance. In this regard, the carbon-silica dual phase
fillers (CSDPF) provide the best balance of properties
for natural rubber –based tyres applications8. These
fillers, instead of containing typical 90-99% elemental
carbon as in traditional carbon black with oxygen and
hydrogen as minor constituents, there is silica phase
finely distributed in the carbon phase.
The choice of proper curing system materials to
accommodate the time require for mixing, forming,
moulding and processing of the selected materials to
form the product and at the same time minimized the
time necessary to cure the material to obtained the
optimal physical properties is the work of the
compounder (Alkadasi et al , 2004).
This work was carried out to study the effect of
the variation of carbon-silica proportions during
natural rubber compounding. This was done with the
aim of obtaining high polymer-filler and low filler-
filler interactions in the hydrocarbon elastomers,
compared to the conventional fillers used in tread
compound and, hence enhancement of the rheological
and mechanical properties of the final product.
2. Materials and Method
2.1. Materials
Materials employed in this research work include:
Natural rubber, Peptone 66, Zinc Oxide, Stearic acid,
Carbon black (N326), Silica (Si69), 2,2,4- Tri Methyl
Dihydroquuinoline (TMQ), Polyvinyl inhibitor (PVI)
and Sulphur (insoluble sulphur).
2.2. Equipment
The equipments used are: Monsanto Rheometer,
Monsanto Instron Machine, Two Roll Mill,
Compression Moulding Machine, Tripsometer,
Duraton, Mercer Gauge, Weighing Balance, Knife and
Duraton 2000I Tensometer.
2.3. Method
The total mass of the compound prepared per
batch was 600Kg. Five different batches were
produced by varying the mass of the filler in the
compound as shown in Table 1 below.
2.3.1. Mixing and Compounding
The mixing and compounding of the raw
materials was carried out on a two rolling mill. After
weighing, the natural rubber was charged into the mill
for several minutes to reduced mastication and particle
size of the rubber as well as increase cross linking
density so that it will allow high polymer-filler
dispersion. Peptone66, zinc oxide and stearic acid
(activator) were added to the rubber compound after
20 minutes of mixing. After proper blending, the filler
(either carbon black or silica or both depending on the
batch to be produce) was introduced into the mill
followed by the addition of TMQ, the accelerator
(insoluble sulphur) and the polyvinyl inhibitor to
complete the compounding process.
2.3.2. Material Testing
After compounding, samples were cut from
each batch of the composites produced and various
rheological property tests were carried out on the
samples at the different stages of the production
process. Mechanical tests were also carried out on the
cured samples.
3. Result Analysis
3.1. Rheological properties
Rheological tests are carried out on uncured
compounds to know whether the compound mix was
carried out correctly and to also provide some
information concerning the vulcanized product that
will be produced after curing. Rheological properties
are the properties that have to do with the flow of
materials in their liquid states. These properties have a
lot of influence on the properties that will ensue after
the materials have been cured.
Cure is often called vulcanization or cross-
linking. It is an intermolecular reaction caused by the
introduction of chemicals (usually sulphur and zinc
oxide or morfax) which link or tie independent chain
molecules together causing the polymer to form
molecular networks. The critical parameters relating
to the curing process are; the time elapsed before the
curing process starts, the rate at which the process
occurs and the extent to which the cross linking
occurs. There must be sufficient time before the
process begins to allow the mixing of all the
ingredients of the rubber compound, the forming of
the ultimate product and moulding before curing takes
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place. The process should be rapid but well controlled.
Curing is a process in which polymer molecules are
cross-linked by the addition of sulphur or other
equivalent curative agents. This makes the bulk
materials harder, durable and more resistance to
chemical attack. From Table 1 and Figure 1, it was
observed that sample A with carbon black of 180.82
Kg of the total weight of the composite has the least
curing time of 0.9 minute and hence, the best curing
time. This early curing time was due to high polymer-
filler interaction that exists between the natural rubber
and the carbon black. This was followed by sample C
with equal mass of carbon black and silica (90.41 Kg
each) which cured at 1.35 minutes. Sample B with
180.82 Kg silica takes the longest time to cure and
does not cure as at 1.6 minutes but burnt to form ashes
due to the lower polymer-filler interaction which
makes it difficult for silica to disperse effectively into
the matrix of the polymer.
3.1.1. Curing Time
Figure 3 show the minimum and maximum
torque of the natural rubber compounds containing
different filler contents. Minimum torque is a
measure of the reinforcing efficiency of the
vulcanizates and the number of sulphur cross link that
will be form in the compound. It is the lowest points
on the graph that also measure the viscosity of the
uncured compounds. Viscosity is the resistance of a
fluid to flow under stress. It is a temperature
dependant property. Materials are less viscous at high
temperature. From the Figure, it was observed that
sample C with equal mass carbon black and silica and
sample D with 108.49 Kg of carbon black and 72.33
Kg of silica has the highest minimum torque of 2.58 lb
while sample B has the least value of 1.05 lb.
The maximum torque measures the stiffness
of the cured compound and the number of sulphur
cross link that are formed in the compound. A
compound with higher stiffness will have good rolling
resistance properties which are indicated by higher
maximum torque. From Figure 3, it was observed that
sample D followed by sample A has the best
maximum torque of 12.04 lb and 10.79 lb
respectively.
3.1.2. Scorch Time
Scorch time is the parameter that is used to
monitor premature cross linking between the filler and
polymer when sulphur and accelerator have not been
added (Alkadasi et al, 2004). It is the time required at
a specific temperature or heat history for a rubber to
form incipient cross linking. It was observed that
sample E with 108.49 Kg silica and 72.33 Kg of
carbon black have the best scorch time of 0.87 minute
followed by sample C with equal amount of both
carbon black and silica (90.41 Kg) having a scorch
time of 0.71 minute while sample B with 180.82 Kg
silica has the least scorch time of 0.18 minute. The
scorch time indicate some interference effect to the
cross linking formation caused by the presence of the
silica in sample B as shown in the Figure 2. Monsanto
Rheometer was used to determine the scorch time and
from the results, it was observed that all the sulphur
cured formulations adopted have sufficient time to
permit mixing, forming and flowing of the compound
into the mould before curing except for sample B.
Table 1. Composition of the samples.
Samples Natural Rubber(Kg) Carbon Blac k(Kg)
Silica(Kg)
Others(Kg)
A 384.71 180.82 -
B 384.71 - 180.82
C 384.71 90.41 90.41
D 384.71 108.49 72.33
E 384.71 72.33 108.49
34.47
34.47
34.47
34.47
34.47
Figure 1. Curing Time for the Different Batches of the
Samples Produced.
Figure 2. Scorch Time for the Different Batches of the
Samples Produced.
3.1.3. Torque at a given Time
Figure 3. Torque Properties of the Different Batches
of the Samples Produced.
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3.1.4. Specific Gravity
Figure 4. Variation of Specific Gravity with Samples
Figure 4 revealed the variation of specific
gravity with the samples. Specific gravity is the ratio
of the weight of a compound to the weight of an equal
volume of water at a standard condition. It is a
measure of thorough mixing and incorporation of
compounds. From the Figure, sample C followed by
sample D have the best specific gravity values of
1.124 and 1.112 respectively. These samples have the
best mixing and incorporation.
3.1.5. Ultimate Tensile Strength
Figure 5. Variation of Ultimate Tensile Strength with
the Samples
The ultimate tensile strength of the samples
was shown in Figure 5. Tensile stress is the force per
unit cross sectional area of the tensile test specimen. It
is a measure of the stress required to fracture or break
the material. Ultimate tensile strength is the
maximum stress the material can withstand before
fracture or failure occurs. Monsanto Instron Machine
was used for the determination of the tensile
properties. It was observed from the test that sample A
followed by sample D have the best ultimate tensile
strength values of 19 MPa and 16 MPa respectively.
As shown in Figure 5, the high tensile strength
observed in the two samples (A and D) is due to the
high carbon black content. Generally carbon black
possesses high abrasion resistance which is impacted
into the composite.
3.1.6. Young’s Modulus of Elasticity
Modulus of elasticity measures the load
require to stretch the polymeric substance to a set
extension per cross sectional area. It expresses the
resistance to stiffness of the composites. The results of
the test as shown in Figure 6 revealed that sample D
has the best Young’s modulus of elasticity of 14 MPa
followed by sample A with a value of 12 MPa. The
high modulus of elasticity of the two samples was due
to the high polymer-filler interaction and low filler–
filler interaction that exist in the compounds while
sample E shows the least modulus of elasticity
because of poor polymer-filler interaction and high
filler-filler interaction.
Figure 6. Variation of Young’s Modulus of Elasticity
with the Samples
3.1.7. Elongation at Break
Figure 7. Variation of Elongation at Break with the
Samples
This is the maximum stretch a material can
attain before fracture. The result of the test in Figure 7
shows that sample D with a value of 432 mm has the
best elongation at the break followed by sample E
with a value of 392 mm. These samples possess the
best capability to stretch to the maximum length
before breaking.
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41
3.1.8. Resilience
Figure 8. Variation of Resilience with the Samples
Resilience is the ability of a material to return
to its normal energy level when the applied stress that
causes its deformation is removed. From Figure 8, it
was observed that sample A and sample D with values
of 69 J and 65 J respectively have the best resilience
properties.
3.1.9. Hardness
Figure 9. Variation of Hardness with the Samples
The hardness of a material is a measure of the
resistance of the material to indentation. Duration
2000I was used for the determination of the hardness
values. It was observed from Figure 9 that, sample D
and sample A with values 63 HDN and 61 HDN
respectively gave the best hardness value.
4. Conclusion
This research work was carried out to study
the reinforcement efficiency of carbon-silica dual
phase filler for the improvement of the mechanical
properties of natural rubber compounding for the
production of automobile tyres.
From the results of the various rheological tests
carried out, it was observed that sample C which is the
mixture of equal amount of carbon and silica has the
best rheological properties followed by sample D that
contains 108.49 Kg of carbon black and 72.33 Kg of
silica. These show that the mixture of these two fillers
enhanced the rheological properties of the blends.
The results of the mechanical tests show that
sample D that contains 108.49 Kg of carbon black and
72.33 Kg of silica has the best mechanical properties
followed by sample C that contains equal amount of
carbon black and silica and, sample A that contains
only carbon black of 180.82 Kg.
The results show that the mixture of carbon
and silica to form dual phase compounds in the
production of carbon-silica dual phase filler reinforced
natural rubber composites enhanced the rheological
and the mechanical properties of the composites more
than the individual fillers. Carbon black was observed
to be more effective in the reinforcement of the
natural rubber than silica.
Thus, for a filled compound, carbon-silica
dual phase fillers gave the best performance and
improvements in production of tread compounds than
the conventional fillers (carbon or silica) due to the
high polymer-filler interaction and lower filler-filler
interaction that exist in the formulation. The mixture
of carbon black with silica brings about improvement
in the bonding strength between the matrix and the
filler and, hence, the ensuing properties.
Correspondence to:
Oladele I.O.
Metallurgical and Materials Engineering, Federal
University of Technology,PMB 704, Akure. Nigeria
Telephone: +2348034677039
Email: wolesuccess2000@yahoo.com
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Wang, M.J., Wolff, S. and Donnet, J.B. Rubber Chemical Technology, 64. 714, 1991.
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Wolff, S., Wang, M.J. and Tan, E.H. Rubber Chemical Technology, 66.163, 1993.
Paper Presented at the Meeting of Rubber Division
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