Radiation Vulcanization of Natural Rubber Latex Loaded With Carbon Nanotubes

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

56

Downloaded By: [University of Jordan] At: 18:14 15 January 2010

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

Radiation Vulcanization of Natural Rubber Latex 57


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).

58 M. Ali Atieh et al.


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).



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



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



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.



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.



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.



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.



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.



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.



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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Radiation Vulcanization of Natural Rubber Latex Loaded With Carbon Nanotubes

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