Mechanical property evaluation of natural rubber/ vein graphite composites

Vein graphite filled natural rubber composites were prepared by keeping the total weight of composites constant in each using suitable dispersant, accelerators, and coagulants to use in high-end applications. All the composites were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The tensile properties, Young’s modulus and hardness of the composites were evaluated. XRD and FTIR analysis verified the presence of rubber and graphite in the composites without any noticeable structural changes. The SEM image of composites showed a uniform homogeneous surface of the composites. High tensile strength was observed at 10% of graphite incorporation in natural rubber. Young’s moduli and hardness of composites were observed to be improved with the addition of the dispersed graphite. The hardness of the composites increases with increasing graphite in the composite and optimized at 60% of graphite. The properties of natural rubber/graphite investigated in the present study is useful for many applications including electronic devices, aerospace, automobiles, toys, heavy equipment industry, battery, consumer products, etc.


INTRODUCTION
Recently, there is a huge demand for natural rubber (NR) worldwide in many applications.
NR products are mainly classified as latex base products and dry rubber products 1,2 . The common techniques for manufacturing useful products from NR latex are casting, coating, extrusion, molding, foaming, and dipping 3 . Latex dipping made a wide range of rubber products. The rubber produced in the world is largely used for producing tires and related items. Pneumatic tires and tubes, solid tires, tire flaps, retread material, and puncture repair kits are the tire products. Natural rubber is the ideal base material for aircraft, heavy-duty vehicles such as buses and trucks, racing cars and tractors. NR is used in many applications because it has remarkable and desirable properties. Also, NR has superior building track, better dynamic properties, fatigue resistance, low heat build-up, better resilience, high mechanical strength, and excellent flexibility. Natural rubber excels in both abilities, making it an ideal material for dynamic seals and gaskets 4 . Because of these properties, NR products are used in many fields such as engineering, sport, medial and household applications 5,6 .
But, for economic reasons, fillers are added into the NR to reduce the production cost by changing the volume and mass of the production. Normally, these fillers have a small effect on the mechanical and dynamic mechanical properties of the production. Also, fillers are used widely in thermoplastic and rubber industries to significantly improve modulus, tensile strength, tear resistance, abrasion resistance, and dynamic mechanical properties, etc [7][8][9] .
Graphene, graphite oxide (GO) reveals high modulus and strength 10,11 . Due to these behaviors GO is mixed with the rubber latex through the latex co-coagulation process. This composite indicated that the modulus of the NR increases with the addition of GO 12 . Fillers are added in to the NR to change the properties. Normally, fillers have small effect on mechanical and dynamic mechanical properties of the products 13 . Graphite is a natural raw material and has very good thermal properties. When graphite is applied as a filer to improve the properties such as thermal and electrical conductivity, the properties such as mechanical and tensile properties can be affected. All required properties for a particular application determine the quality of the final product such as solid tires [14][15][16][17] .
NR in latex form with vein graphite and then NR graphite composite series have prepared.
After that mechanical properties have evaluated. The homogeneous mixture of graphite was obtained through dispersion of graphite 18 .

Preparation of natural latex/ graphite composite dry sheets and vulcanization
Natural latex and disperse graphite were mixed for 10 minutes. After grinding well, the mixture was coagulated. Subsequently, the water is removed and dried in an oven at 105ºC to remove moisture. The composite dry sheets were prepared by changing the percentage of graphite.
Then the compounded sample was vulcanized using a hot press after adding sulfur into the dry rubber sheets.

Characterization
X-ray powder diffraction (XRD) analysis was conducted to identify the crystalline phase rubber graphite composite using Regaku ultima IV X-ray diffractometer with Cu Kl radiation (λ = 1.5406 Å) at 40 kV and 40 mA. The morphology of graphite, natural rubber, and natural rubber/ graphite composites was characterized by LEO 1420vp scanning electron microscopy (SEM). Functional groups attached to graphite, natural rubber, and natural rubber/ graphite composites were characterized using BRUKER Tenor 27, FTIR-ATR spectrometer.
The tensile properties and hardness were measured according to the ISO standards.

RESULTS AND DISCUSSIONS
XRD patterns of the natural graphite and dispersion graphite was depicted in figure 1. The characteristic peak for graphite was observed at 2=26.54º resulting from (002) reflection.
Normally, graphite is composed of stack layers of carbon planes in which each plane is orderly arranged with an interlayer of carbon planes of 3.355 nm. After dispersion, the interlayer spacing of natural graphite was 3.352nm with a sharp and high peak of (002) reflecting at 2=26.57º. There is no significant change in graphite before and after dispersion. Therefore, milling of graphite to prepare dispersion has not affected the crystalline nature of graphite. The FTIR spectrums of NR and dispersion graphite were shown in figure 3(A). The FTIR spectrum of NR contains some peaks known to be characteristic of the NR structure. It showed that the absorption peaks in the NR near 3398cm -1 and 1539cm -1 was stretching vibration absorption peaks of the N-H group and vibration compound peak of C-N group and N-H group in the amide group, respectively. These groups were caused by proteins in NR The characteristic bands of the saturated aliphatic 3 C-H bonds are seen at from 2960cm -1 to 2849cm -1 assigned to plane bending is observed at 1375cm -1 and it is characteristic of 2 deformation. The characteristic peak of the − 3 bending is seen at 1448cm -1 and 2 = bonds are seen at 834cm -1 . FTIR spectra of the composite revealed a spectrum similar to natural graphite when more graphite (50-60%) is added to rubber.

CONCLUSSION
The mechanical properties of natural graphite / natural rubber composite were investigated.
The graphite can be well dispersed in natural rubber by using a grinding process without any wastage. The natural rubber/ graphite composites were successfully prepared with the addition of graphite up to 60%. The highest tensile strength of the composites was found at 10% of graphite in natural rubber (NR). The Young's moduli of the composites increased with the increasing graphite percentage. The hardness of the natural rubber/ graphite composite was also increased with increasing percentage of graphite. The present investigation of mechanical properties of natural rubber/ graphite composite will be very useful in applications where the combination of properties expected from graphite and natural rubber is anticipated.

ACKNOWLEDGEMENT
The work was financially supported by Treasury Grants (TG)