EFFECT OF MULTI-WALL CARBON NANOTUBES ON THE MECHANICAL PROPERTIES OF STANDARD MALAYSIAN RUBBER LATEX (SMRL) WITH RADIATION VULCANIZATION

Muataz Ali Atieh*[1],

1Department of Chemical Engineering,

Abstract

Multi-Walled Carbon Nanotubes (MWNTs) were used to prepare standard Malaysian rubber latex (SMRL) nanocomposites with and without radiation vulcanization. Our first effort to achieve nanostructures in MWNTs/SMRL nanocomposites was formed by incorporating carbon nanotubes in a polymer solution and subsequently evaporating the solvent. Using this technique, nanotubes can be dispersed homogeneously in the SMRL 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. Mechanical test results show an increase in the initial modulus for up to 12 times in relation to pure NR.

Keywords: Carbon Nanotubes, Natural Rubber, Nanocomposite, Young’s modulus

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 bestow desirable properties on the materials. Great interest has recently developed in the area of nanostructured carbon materials. Carbon nanostructures are becoming of considerable commercial importance with interest growing increasingly rapidly over the decade or so since the discovery of buckminsterfullerene, carbon nanotubes, and carbon nanofibers. Carbon nanotubes (CNTs) exhibit unique mechanical, electronic and magnetic properties, which have caused them to be widely studied [1-3]. CNTs are probably the strongest substances that will ever exist with a tensile strength greater than steel, but only one sixth the weight of steel [4]. Iijima (1991) was first discovered carbon nanotubes (CNTs) using arc discharge method [5,6]. Following this discovery, a number of scientific researches have been initiated and variable methods have been used to synthesis CNTs and reported to be able to produce them, namely, arc discharge, laser vaporization [7] and catalytic chemical vapor deposition of hydrocarbons [8-10]. Since carbon–carbon covalent bonds are one of the strongest 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 property of mechanical strength allows these structures to be used as possible reinforcing materials. Just like current carbon fiber technology, these nanotubes reinforce would allow very strong and light materials to be produced. These properties of CNTs attracted the attention of scientists in all over the world because thier high ability for absorbing the load which is applied to nanocomposite materials [17-18]. Initial experimental work on carbon nanotube-reinforced CNT/Rubber has demonstrated that large increase in effective modulus and strength can be obtained with the addition of small amounts of carbon nanotubes [19-20]. The stress-strain curve of different percentages of pure carbon nanotube with SMR CV60 is shown in Fig 2.10. From these results, it shows that the CNTs play the role of reinforcement. As the CNT content in the rubber increases, the stress level gradually increases but at the same time the strain of the nanocomposites decreases. This increment occurs because of the interaction between the CNTs and the rubber. Therefore, the interface between the CNTs and the rubber is very important for a material to withstand the stress [19].

Fig. 2.1: Stress-strain of SMR CV60 with different percentage of CNTs [19].

Multi-walled carbon nanotubes (MWNTs) will be used to prepare standard Malaysian rubber latex (SMRL) nanocomposites. Our first effort to achieve nanostructures in MWNTs/SMRL nanocomposites will be formed by incorporating nanotubes in a polymer solution and subsequently evaporating the solvent. Using this technique, nanotubess will be dispersed homogeneously in the NR matrix in an attempt to increase the mechanical properties of these nanocomposites. The properties of the composites such as tensile strength, tensile modulus and elongation at break were studied.

2. Experimental

The carbon nanotubes were added to standard Malaysian rubber latex (SMRL) as Nanofiller. The natural rubber which was used in this study is standard Malaysian rubber latex (SMRL). The preparation of the nanocomposites was carried out by a solvent casting method using toluene as a solvent. The added amounts of the carbon nanotubes were 1, 3, 5 and 7 wt % of 10 grams of the total weight. This 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. A certain quantity of carbon nanotubes was added to a specific amount of toluene solution after carefully weighing (in order to maintain a specific weight ratio of nanotubes in the solution). This 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 or nanofibers. For this study, different CNT solutions were prepared (containing CNTs in various weight ratios):

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.

The second stage involved the dissolution of the rubber in a suitable organic solvent (toluene). A specific amount of rubber (50g) weighed using a balance was added to a certain quantity of organic solvent (600 ml of toluene) thereby maintaining a desired rubber weight ratio. This mixture was stirred and kept for certain duration of time until the rubber became uniformly dissolved in the solvent. The final step involved the thorough mixing of the solutions prepared in the first and second stages, resulting in a solution that consisted of a good blend of nanotubes in the rubber. The solution was poured on a plate. The toluene was evaporated from the solution and the nanocomposite was obtained.

Radiation Vulcanization of the Carbon Nanotubes / SMRL Nanocomposites

About 22-23g of CNT / SMRL 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. The samples were cut according to BS6746 standards with 1 mm thickness. The thickness of the samples was measured using a micrometer grew gauge. The samples 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

3.1 SEM Characterization

The carbon nanotubes were characterized extensively using SEM. Fig. 3.1 shows typical SEM images of carbon nanotubes. The SEM observation shows that these carbon nanotubes have a diameter in the range of 25-30 nanometers indicating that the CNTs are multiwalled. The images indicate high purity of the nanotubes since there is no sign of foreign nanoimpurities.

(a) (b)

Fig. 3.1: SEM Images of carbon nanotubes at (a) at low resolution, 5000X (b) at high resolution, 100000X.

3.2 TEM CHARACTERIZATION

TEM was carried out to characterize the structure of nanotubes. To prepare TEM samples, some alcohol was dropped on the nanotubes film followed by transferring the films with a pair of tweezers to a carbon-coated copper grid. From the Fig.3.2, it is observed that the nanotubes are hollow and tubular in shape. The TEM images indicate that the nanotubes are multiwalled.

(a) (b)

Figure 3.2: TEM Images of carbon nanotubes (a) at low resolution (b) at high resolution.

3.3 EFFECT OF CNTS ON THE STRESS-STRAIN VALUE OF SMRL WITHOUT 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 SMR L are shown in Fig. 3.3. The stress-strain curve indicated that the strength of the rubber nanocomposites was approx. 6 times of natural rubber at 7wt% of CNTs. This increment showed a great result of the strength and brittle properties of SMR L nanocomposites.

Fig. 3.3: Stress/Strain of SMR L with different percentage of CNTs without vulcanization.

The tensile strength increased radically as the amount of CNTs concentration increased. The general tendency was that the strain level decreased and the stress level was increased by the addition of CNTs, which play the role of reinforcement. Under load, the matrix distributes the force to the CNTs which carry most of the applied load. From these results, it was deduced that the reinforcing effect of CNTs was very marked thus making them good candidates as nanofillers.

3.4 EFFECT OF CNTS ON THE STRESS-STRAIN VALUE OF SMRL WITH RADIATION VULCANIZATION

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 Fig. 3.4. which indicate that the strength of irradiated nanocomposites of rubber with 7wt% of CNTs was approx. 1.8 times of irradiated natural rubber. The general tendency was that the stress level increased moderately by the addition of CNTs while the strain level decreased. It was deduced that radiation vulcanization has a marked effect on the tensile strength of the nanocomposites. The effect of addition of CNTs in increasing the tensile strength is not marked and they play the role of an additive in the rubber only. This is due to the cross-linking between the carbon atoms induced due to gamma irradiation, which reduces the interaction between the rubber chains and the CNTs.

Fig. 3.4: Stress/Strain of SMR L with different percentage of CNTs at 150kGy irradiation dosage.

3.5 EFFECT OF CNTS ON THE STRESS-STRAIN VALUE OF NATURAL RUBBER WITH CONVENTIONAL VULCANIZATION

The results obtained for the mechanical strength of the conventionally vulcanized nanocomposites of rubber and conventionally vulcanized SMR L are shown in Fig. 3.5. The strength of vulcanized nanocomposites of rubber with 7wt% of CNTs was approx. 0.9 times and that of vulcanized nanocomposites with 4wt% of CNTs was approx 1.17 times of the vulcanized natural rubber.

Fig. 4.5: Stress/Strain of SMR L with different percentage of CNTs vulcanized conventionally.

The general tendency was that the stress level increased moderately by the addition of CNTs while the strain level decreased. It was deduced that conventional vulcanization has a marked effect on the tensile strength of the nanocomposites. The effect of addition of CNTs in increasing the tensile strength is not marked and they only act as an additive in the rubber. This is due to the sulphur induced cross-linking between the carbon atoms which reduces the interaction between the rubber chains and the CNTs. The nanocomposite samples containing 7wt% CNTs become more brittle due to which slippage occurs as a result of which they fracture at a smaller stress level.

3.6 EFFECT OF CNTS ON THE YOUNG’S MODULUS OF SMR L WITHOUT VULCANIZATION

The Young’s Modulus of the nanocomposites normalized with that of the pure matrix (SMR L) is presented in the Fig. 4.6. The result indicated that the Young’s Modulus increased with an increase in the amount of CNTs in the formulation leading to an increase in the degree of stiffness of the rubber nanocomposites. The increase in Young’s Modulus led to a decrease in the extension of the rubber nanocomposite which indicated high stiffness.

Fig. 4.6: Young’s Modulus of SMR L at different percentage of CNTs without vulcanization.

3.7 EFFECT OF CNTS ON THE YOUNG’S MODULUS OF SMR L WITH RADIATION VULCANIZATION

A similar phenomenon was observed for Young’s Modulus in the nanocomposites vulcanized by irradiation. The results presented in Fig. 3.7 indicate that the Young’s Modulus increased with an increase in the concentration of CNTs, which increased the level of stiffness in the rubber nanocomposites. This led to a decrease in the extension of the rubber nanocomposite due to increment in Young’s Modulus.

Fig. 3.7: Young’s Modulus of SMR L for different percentage of CNTs at 150 kGy irradiation dosage.

3.8 EFFECT OF CNTS ON THE YOUNG’S MODULUS OF SMR L WITH CONVENTIONAL VULCANIZATION

The results in Fig. 3.8 show that there was a sharp increase in the value of Young’s Modulus when the CNT concentration in the pure matrix (SMR L) was increased from 1 to 4 wt% indicating increase in degree of stiffness. However, there was a sharp decline in the modulus when the CNT concentration was increased to 7wt%. The inability of the nanocomposite sample to withstand the load was due to a high degree of brittleness because of high concentration of CNTs which led to slippage of the nanocomposite samples and consequently the failure to resist the load.

Fig. 3.8: Young’s Modulus of SMR L at different percentage of CNTs vulcanized conventionally.

3.9 EFFECT OF CNTS ON THE ENERGY ABSORPTION OF SMR L WITHOUT VULCANIZATION

Fig. 3.9 shows the toughness of the CNT/SMR L nanocomposite and considers the amount of energy required to fracture a material. From the analysis of Fig. 3.9, it was evident that by increasing the amount of CNTs in the SMR L, the energy of absorption needed to fracture the material increased. Since the 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 energy given by:

(Strength × Strain) α (Force × Distance) = Energy

Fig. 4.9: The toughness as a function of wt% of CNTs without vulcanization.

In general, stress increased with the amount of CNT concentration from 1-7%. As a result, the energy required to fracture the material increased with an increase in the CNT concentration. However, there was an insignificant decrease in strain from 1 to 7 wt% implying that the ductility was virtually preserved at these percentages. The observed decrease in strain in Fig.3.9 from 1 to 7 wt% had no effect on the overall strength of the carbon nanotubes due to the much higher increase in stiffness. As shown in the Fig., the energy of absorption at 1, 4, and 7 wt% showed a general trend of increase in stiffness with increase in energy, which was 0.17, 0.328, and 0.395 respectively compared to that of pure natural rubber which is 0.09 J. This increase can be attributed to the reinforcing property of carbon nanotubes, which in turn increases the strength of the rubber.