Improving the performance of natural rubber using graphene and its derivatives

Improving the performance of natural rubber using graphene and its derivatives

In this research project, modified graphene was employed as filler to enhance the electrical conductivity and to reinforce mechanical properties of natural rubber (NR). The defect sites in the graphene sheets were investigated for further modification. The latex mixing and mechanical mixing methods to load functional graphene sheets into the NR matrix, improved the mechanical and electrical properties of the composite material. Graphene was prepared by a chemical oxidation-reduction approach to fill the NR matrix. The oxidation approaches were employed in progress, which will induce various defects in the final product. It is known that these defects decrease the properties of the graphene and graphene/natural rubber composites, which are prepared by traditional method as well. However, these defects could cause improvements in performance of the graphene composites with re-designed methods, the main focus of this thesis.

Before loading into NR matrix, the defect information of graphene oxide (GO) prepared using Hummers method was examined through positron testing, which is known to be highly effective in the study of the defects in graphite and its derivatives. The different types of defects were detectable, which revealed that the vacancy clusters and vacancy-oxygen group complexes were present on the GO sheets. No large open-volume hole was detected in GO.

The reduction of GO by potassium carbonate (K2CO3) as a green noble preparation approach was developed, and the oxygen groups dispersion status in the GO sheet was further investigated. K2CO3 was used as a reusable reduction agent to convert GO to reduced graphene oxide (RGO) in two steps, based on the conversion of the different types of oxygen groups detected. Carbon dioxide was the only by-product of this process, which was absorbed by K2CO3. In addition, the study further elucidates the structure of GO sheets. The oxygen groups on the GO sheets not only aligned but also randomly dispersed in different areas.

Antistatic NR nanocomposites with partly interconnected graphene architectures offer significant enhancement in various properties. RGO/NR composites were prepared using latex mixing and in-situ reduction process. The oxygen groups on the GO played a key role in attaching GO sheets to the surface of NR particles. Segregated current transfer routes were partly constructed in an NR matrix with an electrical conductivity of 0.1 S/m and reinforcing the tensile strength and elongation-at-break as well.

Silver nanoparticles (AgNPs) were used to decorate GO, which further increased the electrical conductivity of NR nanocomposites. Electrically conductive AgNPs/RGO filled NR with well-organized three-dimensional (3D) microstructures were prepared through electrostatic self-assembly integrated latex mixing. The oxygen groups in GO acted as an anchor for AgNPs growth, resulting in the electrical conductivity of 31000 S/m for the AgNPs/RGO. A honeycomb-like AgNPs/RGO 3D network was constructed in the NR matrix after freeze-drying and hot compression moulding. The AgNPs/RGO/NR nanocomposites show a percolation threshold of 0.63 vol.% and electrical conductivity of 196 S/m at AgNPs/RGO content of 4.03 vol.%.

The oxygen groups can not only be used to improve the electrical conductivity of NR but also used to reinforce mechanical properties. The effect of functionalized GO on the mechanical properties of NR was investigated through two strategies. In the first strategy, one layer of silica particles were attached to the GO surface through hydrogen bonds. The strength were reinforced because of well-dispersed SiO2/GO in the NR matrix. GO acted as a surfactant dispersed by silica into the NR matrix to reinforce the mechanical properties using latex mixing. Oxygen groups on the graphene sheets banded with silica to achieve the target.

In the second strategy, the strength reinforcement of NR nanocomposites was achieved by construction of an interpenetrating network between the NR molecules and porous graphene. In this project, porous graphene loaded NR nanocomposites were prepared through an ultrasonically assisted latex mixing and in-situ reduction process. The oxygen groups showed chemo-selectivity etched by potassium permanganate (KMnO4), forming pores possessing suitable dimensions in graphene sheets. Porous graphene/NR nanocomposites show strong interactions between the NR molecules and porous graphene than RGO/NR, which contributed to an increase in tensile strength compared to the RGO/NR nanocomposites. Furthermore, the scorch time compared to RGO/NR was decreased, and density of cross-linking was increased, which demonstrate the pores on the graphene sheets formed a mass transfer route, indicating an interpenetrating network was constructed.

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