ABSTRACT
Rubber products are usually fabricated by adding fillers to meet demands in multi-faceted industrial applications. Reinforcing fillers like carbon black and silica provide excellent mechanical strength and dimensional stability to a rubber vulcanizate. Nevertheless, further enhancement of rubber properties by introduction of novel materials has been an exciting area of research, especially with reference to the development of lightweight radial tire and tire for electrical vehicles. Recently, graphene has been extensively explored for such applications. It is a two-dimensional sheet of sp2-hybridized carbon and has exceptionally high specific surface area and high charge carrier mobility. Due to these unique properties, graphene is considered to be an ideal, nano-sized, multifunctional filler for rubber nanocomposites used in tire industries. However, despite its high mechanical strength, graphene has not yet been known to improve rubber properties significantly, possibly due to its poor dispersibility in a rubber matrix. In the present work, four different graphenic materials – Graphene oxide (GO), Reduced Graphene Oxide (RGO), two types of Graphene nano-platelets (GP1, GP2) – have been taken for complete evaluation in natural rubber (NR) and styrene butadiene rubber (SBR) compounds. Pristine graphene fillers at high loading did not impart significant properties as reported from our earlier studies, and hence, hybrid fillers were used in this study. We have examined four different aspects, i.e. the effect of nature and loading (1, 3, 5 and 7 phr) of graphene in a hybrid filler system, keeping the total filler (carbon black (N234) and graphene) constant at 45 phr, the effect of loading of carbon black keeping the graphene level constant at 5 phr and the effect of nature of rubber for a fixed hybrid filler (40 phr N234:5 phr Graphene) system. A reference compound with 45 phr of N234, without graphene, was also prepared. Graphene nano-platelets were found to exhibit superior mechanical and dynamic properties as compared to GO and RGO. Incorporation of GP1 and GP2 in a NR compound had enhanced 300% modulus, tensile strength, tear strength and rebound resilience. The abrasion resistance of these compounds was significantly superior to the GO- and RGO-based compounds. The enhancement in properties was attributed to a greater degree of exfoliation of graphene nano-platelets and dispersion of carbon black in the rubber matrix as well as their rough surface characteristics. The properties of NR compound containing the graphene were found to reach an optimum at a loading of 5 phr of graphene and 40 phr of carbon black. A decline in tear strength was observed with the increasing loading of graphene. At higher loading of carbon black (60 phr), the effect of graphene was not always significant. The enhancement in rubber properties was also observed in SBR-based formulation containing 40 phr of N234 and 5 phr of graphene nano-platelet. The extent of improvement in SBR-based compound was compared with the NR-based formulation. Graphene also reinforces silica filled compounds. A mechanism of reinforcement by graphene in the hybrid has been discussed in the light of the existing theories.
