RADIATION VULCANIZATION OF NATURAL RUBBER LATEX LOADED WITH
CARBON NANOTUBES
Muataz Ali Atieh1*, Nazif Nazir2, Faridah Yusof 3, Mohammed Fettouhi3, Chantara Thevy Ratnam4, Mamdouh
Alharthi1, Faraj A. Abu-Ilaiwi5, Khalid Mohammed2 , Adnan M. Jarallah Al-Amer6
1
Department of Chemical Engineering, Centre of Research Excellent in Nanotechnology
King Fahd University of Petroleum and Minerals P.O. Box 5050, Dhahran-31261, Saudi Arabia
2
Department of Biotechnology Engineering, International Islamic University Malaysia P.O.Box 10, 50728 Kuala
Lumpur, Malaysia
3
Chemistry Department, King Fahd University of Petroleum and Minerals P.O. Box 5050, Dhahran-31261, Saudi Arabia
4
Malaysian Nuclear Agency Bangi, 43000 Kajang Selangor, Malaysia
5
Centre of Research Excellent in Nanotechnology King Fahd University of Petroleum and Minerals
P.O. Box 5050, Dhahran-31261, Saudi Arabia
6
Department of Chemical Engineering,
King Fahd University of Petroleum and Minerals P.O. Box 5050, Dhahran-31261, 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 1-7 wt%.
Upon electron beam irradiation, the tensile
modulus of the nanocomposites keeps increasing with the increase of carbon nanotubes content up to 7 wt%.
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, Natural Rubber, Radiation Vulcanization, Nanocomposite
*
Corresponding Address : Tel: +966-3-(8607513 or 8606182)
Email : motazali@kfupm.edu.sa (Dr. Muataz Ali Atieh)
1
1.
Introduction
Research on new materials technology is attracting the attention of researchers all over the world.
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 considerable 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 (1991) has
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 one of 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, the nanotube reinforcement would allow very strong
and light materials to be produced. Various nanocomposite 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 [19-23]. Presently the natural rubber vulcanizates have a wide range
of properties, which are of great interest from a technological point of view such as mechanical, damping,
dynamic fatigue resistance, heat and age resistance, low temperature flexibility, compression set, swelling
resistance and electrical properties. 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 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 materials, not as cost cutting
ones. 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.
The vulcanization is a process, which 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 the association
of macromolecules through their reactive sites. The key chemical modification is that sulfide bridges are
created between adjacent chains (Kumar et al., 1997).
Figure 2.12: Vulcanization – Formation of sulfide bridges between adjacent chains (Kumar et al., 1997).
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The crosslinking imparts various properties to rubber. It improves its tensile strength, becomes more resistant to
chemical attack and it is not thermoplastic anymore. 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. It is also a good insulator 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
(Kumar et al., 1997).
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 (NR) can be crosslinked by irradiation
(Ratnam, 2007). 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 scissoning), breakdown of crystalline structure, coloration and oxidation. These
lead to crosslinking or degradation of the polymer depending on its structure and irradiation conditions. Thus
most polymers appear to fall into two distinct classes: the ones which crosslink and the ones which degrade.
Table 2.4 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 (Ratnam,
2007).
Natural rubber (NR) is sticky and non-elastic by nature. Crosslinking is a reaction of polymers to form a threedimensional network. Crosslinking of rubber results in the vulcanization which makes it more elastic. When the
natural rubber is irradiated by high energy radiation, hydrogen atoms of the trunk chain, mainly of methylene
groups proportional to double bonds, are ejected and radical sites are formed, and these radical sites are
combined into C-C crosslinks.
Figure 2.17: Natural Rubber (Cis 1,4 polyisoprene) (Ratnam, 2007).
The addition of NR radicals to unsaturated C=C 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. In 1954, Charlesby investigated the radiolysis of natural
rubber (smoked sheets) under reactor radiation. The distance between the crosslinks was found to decrease
linearly with the radiation dose, indicating that the extent of crosslinking increased linearly with dose.
The radiation vulcanization of natural rubber latex (RVNRL) is an emerging technology in which radiation is
used in place of sulfur in the conventional prevulcanization process for the manufacture of dipper natural rubber
latex products (Makuuchi and Markovic, 1991). This technique has been under investigation since the late 50’s
(Kemp, 1959). The main components of NR latex are NR and water. Radiation vulcanization of NR latex
involves radiation-induced crosslinking of macroscopic particles of natural rubber dispersed in the aqueous
medium. Upon radiation, both NR molecules and water molecules absorb radiation energy independently. 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 (Ratnam,
2007). Numerous works have been reported on RVNRL including hydrogen coating of the RVNRL film by
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using irradiation Ratnam et al., 1999). The products obtained have shown to 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 (Ratnam, 2007).
To the best of our knowledge, no reports on standard Malaysian rubber latex/carbon nanotubes
nanocomposites are present in the literature. In this work, results on the preparation and mechanical
studies of standard Malaysian rubber latex (SMR-L)/Multi-walled carbon nanotubes nanocomposites are
presented. The properties such as tensile strength, tensile modulus and elongation at break are also
reported.
2.
Experimental
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 very 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 in order 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 was obtained by
dissolving 50 g 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
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the rubber. The solution was then poured onto a plate and the toluene evaporated to afford the
nanocomposite samples.
Radiation Vulcanization of the Carbon Nanotubes / SMRL nanocomposites
About 22-23 g of CNT / SMR-L nanocomposite samples were molded using Labtech hot and cold press
under high pressure of 14.7 MPa at 150˚C for 8 minutes. The samples were compression molded into
uniformly flat sheets of 140mm x 140mm x 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 beam current were 2 MeV and 2mA
respectively.
3.
Results and Discussion
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 7 wt %) of pure carbon nanotubes compared to natural rubber of SMRL are shown
in Figure 3.1. 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 nanocomposites of rubber compared to natural
rubber of SMR-L with an irradiation dosage of 150kGy are shown in Figure 3.2. The strength of irradiated
nanocomposites of rubber with 7wt% of CNTs is almost 1.8 times that of irradiated natural rubber. The
5
radiation vulcanization has a marked effect on the tensile strength of the nanocomposites. However the
effect of addition of CNTs in increasing the tensile strength is not significant and they appear to play the
role of an additive in the rubber only. While if we compare the stress-strain curve of an irradiated
CNT/SMRL with that unradiated at 7 wt % the strength increase almost 6 times and if we compare
irradiated CNT/SMRL with the blank rubber the strength increase 38 times due to the cross linking of the
between 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 Figures 3.1 and 3.2. However, the elongation of unradiate nanocomposites
seems to be less affected up compared to irradiate nanocomposites, which is shows considerably drops in
elongation up to 50 %.
3.3
EFFECT OF CNTS ON THE YOUNG’S MODULUS OF SMRL WITH AND WITHOUT RADIATION
VULCANIZATION
The Young’s Modulus of the nanocomposites with and without radiation vulcanization normalized with that
of the pure matrix (SMR-L) is presented in the Figure 3.3. As shown in the figure the Young’s Modulus
values increased 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 in increase in tensile strength.
3.4 EFFECT OF CNTS
VULCANIZATION
ON
THE
ENERGY ABSORPTION OF
SMRL
WITH
AND
WITHOUT
Figure 3.4 shows the toughness of the CNT/SMRL nanocomposites with and without radiation
vulcanization and considers the amount of energy required to fracture the material. The samples with 1,
6
4, and 7 wt% 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 to that of pure natural rubber (0.09 J) for unradiated samples
while for irradiate samples the energy absorption was 3.3376, 3.5225 and 4.18404 J respectively
compared to 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 7 wt % the energy required to fracture the sample is almost 11 times
while if we compare irradiated CNT/SMRL at 7 wt % with the blank rubber the energy required to fracture
the sample increase 47 times due to the cross linking of the between the carbon atoms in the rubber with
carbon nanotubes. 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 1-7 wt % of Multi-walled carbon nanotubes (MWCNTs). 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 1 wt % 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 the figure. At 7 wt % of
CNTs the stress value obtained reached 1.7 MPa which is 6 times that of pure natural rubber. 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 cross linking of natural rubber CNTs composites improved due to the
presence of nanosize 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. While if we
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compare the stress-strain curve of an irradiated CNT/SMRL with that unradiated at 7 wt % the strength
increase almost 6 times and if we compare irradiated CNT/SMRL with the blank rubber the strength
increase 38 times due to the cross linking of the between the carbon atoms in the rubber with carbon
nanotubes.
Acknowledgement
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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