radiation vulcanization of natural rubber latex loaded with carbon nanotubes
TRANSCRIPT
Radiation Vulcanization of Natural Rubber LatexLoaded with Carbon Nanotubes
Muataz Ali Atieh,1,5 Nazif Nazir,2 Faridah Yusof,3 Mohammed Fettouhi,3
Chantara Thevy Ratnam,4 Mamdouh Alharthi,1 Faraj A. Abu-Ilaiwi,5
Khalid Mohammed,2 and Adnan Al-Amer1
1Department of Chemical Engineering, King Fahd University of Petroleum and
Minerals, Dhahran, Saudi Arabia2Department of Biotechnology Engineering, International Islamic University
Malaysia, Kuala Lumpur, Malaysia3Chemistry Department, King Fahd University of Petroleum and Minerals,
Dhahran, Saudi Arabia4Malaysian Nuclear Agency Bangi, Kajang Selangor, Malaysia
5Centre of Research Excellent in Nanotechnology (CENT), King Fahd University
of Petroleum and Minerals, Dhahran, Saudi Arabia
Abstract: The radiation vulcanization of natural rubber latex (NRL) has been
carried out with 150 keV electrons beam with the presence of carbon nanotubes.
The NRL/CNTs were prepared by using solving casting method by dispersing
carbon nanotubes in a polymer solution and subsequently evaporating the
solvent. The load of the carbon nanotubes in the rubber was varied from 1–7wt%.
Upon electron beam irradiation, the tensile modulus of the nanocomposites
increases with the increase of carbon nanotubes content up to 7wt%. The
nanotubes were dispersed homogeneously in the SMR-L matrix in an attempt
to increase the mechanical properties of these nanocomposites. The properties of
the nanocomposites such as tensile strength, tensile modulus, tear strength,
elongation at break and hardness were studied.
Keywords: Carbon nanotubes, Nanocomposite, Natural rubber, Radiation
vulcanization
Address correspondence to Dr. Muataz Ali Atieh, Department of Chemical
Engineering, King Fahd University of Petroleum and Minerals, P. O. Box 5050,
Dhahran-31261, Saudi Arabia. E-mail: [email protected]
Fullerenes, Nanotubes and Carbon Nanostructures, 18: 56–71, 2010
Copyright # Taylor & Francis Group, LLC
ISSN 1536-383X print/1536-4046 online
DOI: 10.1080/15363830903291572
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1. INTRODUCTION
Research on new materials technology is attracting the attention of
researchers worldwide. Developments are being made to improve the
properties of the materials and to find alternative precursors that can
confer them desirable properties. Great interest is recently being shown in
the area of nanostructured carbon materials that are attaining consider-
able commercial importance since the discovery of buckminsterfullerene,
carbon nanotubes and carbon nanofibers. In this context, carbon
nanotubes (CNTs) exhibit unique mechanical, electronic and magnetic
properties, which have been the subject of a large number of studies (1–
3). CNTs are probably the strongest substances that will ever exist with a
tensile strength greater than steel but with only one-sixth of its weight (4).
Iijima discovered, for the first time, carbon nanotubes using the arc
discharge method (5, 6). Subsequently, other methods have been used
such as laser vaporization (7) and catalytic chemical vapor deposition of
hydrocarbons (8–10). Since carbon–carbon covalent bonds are among
the strongest bonds in nature, a structure based on a perfect arrangement
of these bonds oriented along the axis of nanotubes would produce an
exceedingly strong material. Nanotubes are strong and resilient structures
that can be bent and stretched into shapes without catastrophic structural
failure in the nanotube (11, 12). The Young’s modulus and tensile
strength rival that of diamond (1 Tera Pascal and ,200 Giga Pascal,
respectively) (13–16). This fantastic mechanical strength allows these
structures to be used as possible reinforcing materials. Just like current
carbon fiber technology, it is expected that CNTs could provide the
polymer matrix a better comprehensive performance due to their unique
and excellent mechanical and physical properties. Various nanocompo-
site materials with high ability for absorbing the applied load were
developed (17, 18). Initial experimental work on carbon nanotube-
reinforced rubber has demonstrated that a large increase in effective
modulus and strength can be obtained with the addition of small
amounts of carbon nanotubes. However, comparing the many studies on
the applications of CNTs in other polymers, there are fewer works
dealing with the applications of CNTs in rubbers for reinforcement (19–
24). Currently, the natural rubber (NR) vulcanized has a wide range of
properties of great interest from a technological point of view, such as
mechanical, damping, dynamic fatigue resistance, heat and age resis-
tance, low temperature flexibility, compression set, swelling resistance
and electrical properties (25–29). It has been recognized that the filler and
polymer play an equally important role in governing the rubber
properties. Carbon black fillers are functional materials or components
that have a significant effect on rubber performances and properties by
the changes of the morphology and the increase in polymer-filler
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interaction. They are used to achieve the level and range of properties to
provide an adequate amount of reinforcement such as tensile strength,abrasion resistance and tear resistance. They are most often looked upon
as quality enhancing and not as cost-cutting materials. In order to
achieve a high level of reinforcement, the amount of carbon black filler
loading has to be increased substantially, and it is not possible to enhance
all of these properties to the same optimal degree (30–35).
The vulcanization is a process that transforms the predominantly
thermoplastic or raw rubber into an elastic or hard ebonite-like state.
This process is also known as ‘‘crosslinking’’ or ‘‘curing’’ and involves theassociation of macromolecules through their reactive sites. The key
chemical modification is that sulfide bridges are created between adjacent
chains, as shown in Figure 1 (36).
The crosslinking imparts various properties to rubber. It improves its
tensile strength, becomes more resistant to chemical attack and is no
longer thermoplastic. It also makes the surface of the material smoother,
prevents it from sticking to metal or plastic chemical catalysts and
renders it impervious to moderate heat and cold. In addition, it is a goodinsulator against electricity and heat. These attractive physical and
chemical properties of vulcanized rubber have revolutionized its
applications. This heavily cross-linked polymer has strong covalent
bonds, with strong forces between the chains, and is therefore an
insoluble and infusible thermosetting polymer (36).
Natural rubber is sticky and nonelastic by nature. Crosslinking is a
reaction of polymers to form a three-dimensional network. Crosslinking
of rubber results in the vulcanization which makes it more elastic. Whenthe NR is irradiated by high energy radiation, hydrogen atoms of the
trunk chain, mainly of methylene group’s proportional to double bonds,
are ejected and radical sites are formed, and these radical sites are
combined into C-C crosslink (see Figure 2).
The addition of NR radicals to unsaturated C5C bond provides
formation of crosslinks. However, the radiation crosslinking efficiency of
NR is not high. This is believed to be due to the loose packing of NR
molecules with the cis structure and the presence of methyl group (36).The most important vulcanization agent for rubber is sulfur. Various
concentrations of sulfur are used to manufacture different kind of rubber
compounds. For every 100g of rubber, a dosage of 0.25–5.0g of sulfur is
Figure 1. Vulcanization — formation of sulfide bridges between adjacent chains
(36).
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used for the preparation of soft rubber goods, while for hard rubber
compounds the sulfur concentration is 25.0–40.0g. The rubber com-
pounds produced with a dosage of 5.0–25.0g have poor strength and
elastic properties and are of less importance for most applications. For
conventional vulcanization, one uses 1.5–2.5g sulfur with 0.5–1.0g
accelerator. If the content of the accelerator is increased, the sulfur
content has to be decreased to achieve the same crosslink density, as a
result of which the crosslinks with low sulfur content are formed. The
sulfur used for the vulcanization process should be at least 95% pure (ash
content , 0.5%) devoid of SO2 (37). The vulcanization specifications
usually require accelerator systems that have insufficient scorch safety,
toxic materials, short vulcanization times or high processing tempera-
tures; one often needs to retard the initiation of vulcanization (scorch
time) to ensure sufficient processing safety (39).
Radiation technology has emerged as one of the foremost techniques
for the processing of polymer materials. It has been an area of enormous
interest in the last few decades. Irradiation can induce reactions in
polymers, which may lead to polymerization, crosslinking, degradation
and grafting. Crosslinking is a reaction where polymer chains are joined
and a network is formed. Natural rubber can be crosslinked by
irradiation. When a polymer is subjected to high energy radiation such
as gamma rays and accelerated electron beams, it may undergo various
effects such as formation of free radicals, formation of hydrogen and low
molecular weight hydrocarbon, increase in saturation, formation of C –
C bonds between molecules, cleavage of C – C bond (chain scissoring),
breakdown of crystalline structure, coloration and oxidation, all of which
can lead to crosslinking or degradation of the polymer depending on its
structure and irradiation conditions. Most polymers fall into two distinct
classes: the ones which crosslink and the ones which degrade. Table 1
shows the classification of some polymers into the two groups when the
polymer is irradiated at ambient temperature in a vacuum. In the
presence of oxygen, oxidative degradation might occur (37).
The radiation vulcanization of NR latex (RVNRL) is an emerging
technology in which radiation is used in place of sulfur in the
Figure 2. Natural rubber (Cis 1,4 polyisoprene) (36).
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conventional prevulcanization process for the manufacture of dipper NR
latex products. This technique has been under investigation since the late
1950s (37). The main components of NR latex are NR and water.
Radiation vulcanization of NR latex involves radiation-induced cross-
linking of macroscopic particles of NR dispersed in the aqueous medium.Upon radiation, both NR molecules and water molecules absorb
radiation energy independently (38). The radiolysis products of water
are diffused into NR particles and react with NR molecules. The OH
radicals, H radicals and hydrated electron formed are involved in the
radiation induced crosslinking of NR latex. The products obtained have
noticeable advantages over the conventionally vulcanized natural rubber
latex due to the absence of the carcinogenic nitrosamines derived from
the common accelerators of the sulfur vulcanized natural rubber latex.Due to radiation decomposition of NR latex proteins, the amount of
extractable proteins in RVNR is greater than in conventional sulfur
vulcanizate (39, 40).
To the best of our knowledge, no reports on standard Malaysian
rubber latex/carbon nanotubes nanocomposites exist in the literature. In
this work, results on the preparation and mechanical studies of standard
Malaysian rubber latex (SMR-L)/multi-walled carbon nanotubes nano-
composites are presented. The properties such as tensile strength, tensilemodulus and elongation at break are also reported.
2. EXPERIMENTAL
2.1. Production of Carbon Nanotubes
High purity multi-walled carbon nanotubes were produced by using achemical vapor deposition technique. The procedure of the production of
CNTs was reported by Muataz et al. (41). The produced carbon
nanotubes were characterized by using Field emission scanning electron
microscopy (FE-SEM) and transmission electron microscopy (TEM).
Table 1. Classification of some irradiated polymers (37).
Crosslinking Polymers Degrading Polymers
Polyethylene Polyisobutylene
Polystyrene Poly (a-methylstyrene)
Polyacrylate Polymethacrylates
Polyacrylamide Polymethacrylamide
Polyvinyl chloride Poly (vinylidene chloride)
Polyamide Cellulose and derivatives
Polyesters Polytetrafluoroethylene
Polyvinylpyrrolidone Polytetrafluoroethylene
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2.2. Preparation of Carbon Nanotubes/SMRL Nanocomposites
The carbon nanotubes were added to standard Malaysian rubber latex
(SMR-L) as nanofiller. The preparation of the nanocomposites was
carried out by a solvent casting method using toluene as a solvent.
The first phase involved the dissolution/dispersion of CNTs in
toluene in order to disentangle the nanotubes that typically tend to cling
together and form lumps, which become difficult to process. The required
quantity of carbon nanotubes was added to a specific amount of toluene.
The solution was further sonicated using a mechanical probe sonicator(Branson sonifier) capable of vibrating at ultrasonic frequencies to induce
an efficient dispersion of nanotubes. For this study, CNT solutions,
containing CNTs in various weight ratios, were prepared.
The added amounts of the carbon nanotubes were 1, 4 and 7 wt% of
50 grams of the total weight.
i) 1 wt% (0.5g) of CNTs in 50ml of toluene solution
ii) 4 wt% (2.0g) of CNTs in 50ml of toluene solution.
iii) 7 wt% (3.5g) of CNTs in 50ml of toluene solution.
In the second stage, the rubber was dissolved in toluene and the desired
weight ratio obtained by dissolving 50g of rubber in 600 ml of solvent.
The final step involved the thorough mixing of the solutions prepared in
the first and second stages, resulting in a solution consisting of a good
blend of nanotubes in the rubber. The solution was then poured onto a
plate and the toluene evaporated to afford the nanocomposite samples.
2.3. Radiation Vulcanization of the Carbon Nanotubes/SMRL
Nanocomposites
About 22–23g of CNT/SMR-L nanocomposite samples were molded
using Labtech hot and cold press under high pressure of 14.7 MPa at
150uC for 8 minutes. The samples were compression molded into
uniformly flat sheets of 140mm 6 140mm 6 1mm. They were cut
according to BS6746 standards with 1 mm thickness measured using a
micrometer grew gauge and were irradiated using a 3MeV electron beam
accelerator at a dose of 150kGy. The acceleration energy and beamcurrent were 2 MeV and 2mA, respectively.
3. RESULTS AND DISCUSSION
3.1. Characterization of Carbon Nanotubes
A high purity of multi-walled carbon nanotubes were achieved with a
chemical vapor deposition (CVD) technique. The produced carbon
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nanotubes were observed by suing FE-SEM and TEM. The diameter of
the produced carbon nanotubes were varied from 20–40 nm with the
average diameter at 24 nm, while the length of the CNTs was up to few
microns. Figure 3(a) shows the SEM image of carbon nanotubes at
low magnification, and figure 3(b) shows the SEM image of carbon
nanotubes at high magnification. From the SEM observation, the
product is pure and only carbon nanotubes were observed.
TEM was carried out to characterize the structure of nanotubes
(Figure 4). To prepare TEM samples, some alcohol was dropped on the
nanotubes films, then these films were transferred with a pair of tweezers
to a carbon-coated copper grid. It is obvious from the images that all the
nanotubes are hollow and tubular in shape. In some of the images,
catalyst particles can be seen inside the nanotubes. TEM images indicate
that the nanotubes are of high purity, with uniform diameter distribution
and contain no deformity in the structure. Figure 4(b) shows the High
Resolution Transmission Electron Microscope (HRTEM) of the carbon
nanotubes and that a highly ordered crystalline structure of CNT is
present.
3.2. Effect of CNTS on the Stress-Strain Value of SMRL with and without
Radiation Vulcanization
The results obtained for the mechanical strength of the nanocomposites
of rubber with different percentages (1, 4 and 7wt%) of pure carbon
Figure 3. SEM Images of carbon nanotubes at (a) at low resolution (b) at high
resolution.
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nanotubes compared to natural rubber of SMRL are shown in Figure 5.
The stress-strain curve indicates that the strength of the rubber
nanocomposites at 7wt% of CNTs is approximately 6 times that of
natural rubber.
The tensile strength increased significantly as the amount of CNTs
concentration increased. The general tendency was that the strain level
decreased and the stress level increased by the addition of CNTs, which
appears to play the role of reinforcement. Under load, the matrix
distributes the force to the CNTs, which carry most of the applied load.
The observed reinforcing effect of CNTs makes them good candidates as
nanofillers.
The results obtained for the mechanical strength of the nanocompo-
sites of rubber compared to natural rubber of SMR-L with an irradiation
dosage of 150kGy are shown in Figure 6. The strength of irradiated
nanocomposites of rubber with 7wt% of CNTs is almost 1.8 times that of
irradiated natural rubber. The radiation vulcanization has a marked
effect on the tensile strength of the nanocomposites. However, the effect
of the addition of CNTs in increasing the tensile strength is not
significant, and the CNTs appear to play the role of an additive in the
rubber only. If we compare the stress-strain curve of an irradiated CNT/
SMRL with that unradiated at 7wt%, the strength increases almost 6
times; if we compare irradiated CNT/SMRL with the blank rubber the
strength increases 38 times due to the crosslinking of the carbon atoms in
the rubber with carbon nanotubes.
As the tensile strength increases for both irradiate and unradiate
nanocomposites, the elongations are expected to drop as shown in
Figure 4. TEM Images of carbon nanotubes (a) at low resolution (b) at high
resolution.
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Figures 5 and 6. However, the elongation of unradiate nanocomposites
seems to be less affected compared to irradiate nanocomposites, which
show considerable drops in elongation up to 50%.
3.3. Effect of CNTS on the Young’s Modulus of SMRL with and without
Radiation Vulcanization
The stiffness measured of an isotropic elastic material is called Young’s
modulus (E). It is also known as the Young modulus, modulus of
elasticity, elastic modulus (though Young’s modulus is actually one of
several elastic moduli such as the bulk modulus and the shear modulus)
or tensile modulus. It is defined as the ratio of the stress over the strain in
which Hooke’s Law holds. This can be experimentally determined from
the slope of a stress-strain curve created during tensile tests conducted on
a sample of the material.
Young’s modulus, E, can be calculated by dividing the tensile stress
by the tensile strain:
Figure 5. Stress/Strain curve of SMR-L with different percentage of CNTs
without vulcanization.
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E:tensile stress
tensile strain~
s
e~
F=A0
DL=L0~
FL0
A0DL
where E is the Young’s modulus (modulus of elasticity), F is the force
applied to the object, A0 is the original cross-sectional area through whichthe force is applied, DL is the amount by which the length of the object
changes and L0 is the original length of the object.
The Young’s Modulus of the nanocomposites with and without
radiation vulcanization normalized with that of the pure matrix (SMR-L)
is presented in Figure 7. As shown in Figure 7, the Young’s Modulus
values increase with the increase in CNTs content in both cases, leading
to an increase in the degree of stiffness of the nanocomposites. Both
nanocomposites exhibit the same trend with increasing CNTs content.
This clearly indicates that the intercalation of CNTs into the SRML
layers has improved the compatibility of the two materials and resulted inincrease in tensile strength.
3.4. Effect of CNTS on the Energy Absorption of SMRL with and without
Vulcanization
Figure 8 shows the toughness of the CNT/SMRL nanocomposites with
and without radiation vulcanization and considers the amount of energy
Figure 6. Stress/Strain curve of SMR-L with different percentage of CNTs at
150kGy irradiation dosage.
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required to fracture the material. Since strength is proportional to the force
needed to break the sample, and strain is measured in units of distance
(i.e., the distance the sample is stretched), then strength times strain is
proportional to force times distance which in turn equals to energy.
Strength|strain*force|distance~energy absorption
The samples with 1, 4, and 7wt% showed a general trend of increase in
stiffness with an increase in energy, which was 0.17, 0.328, and 0.395 J,
respectively, compared with that of pure natural rubber (0.09 J) for
unradiated samples. For irradiate samples the energy absorption was
3.3376, 3.5225 and 4.18404 J, respectively, compared with that of
irradiated natural rubber (3.1612 J). The same phenomenon was observed
for the energy absorption as function of the CNTs. If we compare the
energy absorption curve of an irradiated CNT/SMRL with that unradiated
at 7wt% the energy required to fracture the sample is almost 11 times, while
if we compare irradiated CNT/SMRL at 7wt% with the blank rubber the
energy required to fracture the sample increase 47 times due to the
crosslinking of the carbon atoms in the rubber with carbon nanotubes.
Figure 7. Young’s Modulus of SMR-L for different percentage of CNTs at
150 kGy irradiation dosage.
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This can be attributed to the reinforcing property of carbon nanotubes,
which in turn increases the strength of the rubber.
4. CONCLUSION
In summary, we have demonstrated the successful fabrication of
nanocomposites consisting of standard Malaysian rubber latex (SMR-
L) with 1–7wt% of multi-walled carbon nanotubes. Carbon nanotubes
were applied as an interface nano-reinforcement in advanced commercial
carbon/rubber composite. This is the first attempt such a work is
reported. The preparation of the nanocomposites was carried out by a
solvent casting method with toluene as a solvent. It is clear shows that the
maximum stress of pure SMR CV60 is 0.2839 MPa. With the addition of
1wt% of CNTs to the unradiated rubber, the stress level for the
nanocomposite material increased from 0.2839 MPa to 0.56413 MPa.
Addition of the CNTs to the natural rubber increased the stress level
gradually as shown in Figure 5. At 7wt% of CNTs, the stress value
obtained reached 1.7 MPa which is 6 times that of pure natural rubber.
Figure 8. The toughness as a function of wt% of CNTs with and without
radiation vulcanization.
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The result indicates that by increasing the amount of CNTs into the
rubber, the ductility decreased and the material become stronger and
tougher but at the same time more brittle. The clear trend observed here
is that as the nanotube amount increases, the fiber breaking strain
decreases. The physical and mechanical properties of radiation-induce
crosslinking of natural rubber CNTs composites improved due to the
presence of nanosized CNTs in the natural rubber matrix. It is clear
shown that by using radiation process at 150 kGy there is tremendous
increase in the mechanical properties of the SMRL with CNTs. If we
compare the stress-strain curve of an irradiated CNT/SMRL with that
unradiated at 7wt%, the strength increases almost 6 times. If we compare
irradiated CNT/SMRL with the blank rubber the strength increases 38
times due to the crosslinking of the carbon atoms in the rubber with
carbon nanotubes.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the King Fahd University for
Petroleum and Minerals (KFUPM) and International Islamic University
Malaysia (IIUM) for their support of this study.
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