The Design, Preparation And Properties Of Carbon-based Nanofiller Filled Natural Rubber Composites

The failure of rubber products such as tires was mainly caused by the crack initiation and propagation. From the aspects of materials, rubber type and reinforced fillers were the main factors influencing the crack initiation and propagation. In this research, the influences of carbon-based fillers with different shape factors on dynamic fatigue and quasi-static fracture resistance of natural rubber (NR) composites were explored. The carbon-based fillers including spherical carbon black (CB), fibroid carbon nanotubes (CNTs) and platelike graphene oxide (GO) were chosen. The relationships between fracture and fatigue resistance and the failure mechanisms of NR composites were also analyzed. J-integral testing was adopted to evaluate the quasi-static fracture initiation and propagation resistance. Critical J-value JIC was as an indicator of crack initiation resistance, and tearing modulus TR was related to the crack propagation resistance of the composites. Meanwhile, fatigue tests were adopted to investigate the fatigue properties of NR composites. The main contents include the following four parts.(1) The influences of hybrid filler networks of CB and silica with different surface properties on fracture and fatigue resistance of NR composites were explored. With increasing silica/CB ratio, both JIC and TR were enhanced, indicating that the resistance to crack initiation and propagation resistance were improved. Fatigue testing under the constant tensile strain conditions demonstrated that the NR composite with higher silica/CB ratio exhibited lower cyclic crack growth rate and longer fatigue life.The enhanced fatigue resistance with increasing silica/CB ratio was induced by the decreased strain energy density at a fixed strain. The results of fatigue properties under constant strain conditions were in accordance with the results of TR, JIC and strain energy density of the composites. Therefore, quasi-static parameters including TR, JIC and strain energy density can be used to predict its fatigue properties under constant strain conditions.(2) The influences of hybrid filler networks of fibrous CNTs and spherical CB on quasi-static fracture and dynamic fatigue behaviors of NR composites were investigated. By using unequal replacement of 3 phr CB with 1 phr CNTs, CNTs/CB/NR composites with similar hardness were prepared.Results indicated that synergistic effects between CNTs and CB on mechanical properties, fracture and fatigue resistance were obtained. The NR composite filled with 3 phr CNTs exhibited the strongest fatigue resistance.The synergistic mechanisms and dominating factors on quasi-static and dynamic failure, such as dispersion state of CNTs, hybrid filler network structure, strain-induced crystallization, strain energy density, and viscoelastic hysteresis loss were discussed. The weakest fatigue resistance of composite filled with 5 phr CNTs was ascribed to its strikingly high hysteresis, which resulted in marked heat generation under dynamic fatigue conditions.(3) The influences of exfoliated GO nanosheets on fracture and fatigue resistance of NR composites were explored. Exfoliated GO reinforced NR composites were prepared by latex co-coagulation method, which realized the complete exfoliation and uniform dispersion of GO in NR matrix. Results revealed that with increase in GO sheet content, the mechanical properties,fracture initiation and propagation resistance were all highly improved. A much higher reinforcing efficiency of GO sheets than that of traditional fillers was realized. Fatigue crack growth resistance under constant strain conditions was remarkably enhanced with the incorporation of only 1 phr GO. The relatively weak fatigue resistance of NR composites filled with 3 phr and 5 phr GO was attributed to their high hysteresis loss and strain energy density.(4) The influences of different carbon-based fillers with different shape factors, including fibrous CNTs, platelike GO, and spherical CB, were investigated. NR composites with similar hardness level were prepared by traditional mechanical blending or latex co-coagulation, for obtaining the similar dispersion state of nanofiller. Results showed that NR composites filled with 25 phr CB, 3 phr CNTs and 4 phr GO performed the similar hardness level of 52. The composites filled with 25 phr CB exhibited the most excellent fracture initiation and propagation resistance under quasi-static condition and fatigue crack growth resistance under dynamic condition. For exploring the failure mechanisms of composites, digital image correlation was used to evaluate the strain field distribution and strain amplification at the crack tip. Results of strain field distribution showed that the composite filled with 25 phr CB exhibited the highest strain amplification level and amplification area, which would be helpful to dissipating the local input tearing energy and then improving the fracture and fatigue performance.The reinforced fillers can not only improve the crack growth resistance of rubber materials, but also improve the electrically conductive properties.The conductive rubber products were very important research areas for the functionalization of rubber composites. As mentioned above, GO nanosheets performed a limited ability to increase the fatigue resistance of rubber composites. However, after the reduction of GO, its reductant product RGO can endow the rubber materials with excellent conductive properties. In this research, the "pre-reduction" and "post-reduction" for GO nanosheets were respectively adopted to prepare conductive RGO/NR composites, and their applications in strain-sensing and solvent-sensing fields were explored. The main contents include the following two parts.(1) High-concentration RGO aqueous solution was produced using gelatin (Gel) as the stabilizer and sub-reductant, and hydrazine hydrate (HHA)as the main reductant. The irreversible restack of RGO was avoided due to the adsorbed gelatin. The Gel-HHA-RGO nanosheets exhibited excellent colloidal dispersibility and stability in alkaline condition. The Gel-HHA-RGO filled NR composites with well-organized interconnected RGO networks were prepared by water-based solution casting. The tensile modulus and dynamic storage modulus were improved by several orders of magnitude with increasing RGO content. Meanwhile, a dramatic increase in electrical conductivity with a low percolation threshold of 0.21 vol% was perceived.Strain-sensing tests revealed that the RGO/NR composites exhibited outstanding strain sensitivity and repeatability, which could be used to detect the cyclic movements of human joints.(2) The interfacial modifiers adsorbed on RGO nanosheets can decrease the conductive properties of RGO. The RGO/NR films with three-dimensional continuous RGO networks were prepared by blending GO suspension with natural rubber latex, followed by water-based solution casting and then in situ hydroiodic acid (HI) reduction. This simplified preparation technics avoided the use of surface modifiers. The electrical conductivity of HI-RGO/NR was as high as 49.3 S/m when filled with 5.00 vol % RGO, and the film exhibited an extremely low percolation threshold of 0.31 vol %. The solvent-sensitive properties were evaluated by detecting the variation in electrical resistance due to the swelling of NR matrix. It was noted that the RGO/NR hybrid films exhibited exceptional stimuli responses for organic solvents, meanwhile,different sensing responses for different solvents. Due to the reconstruction of continuous RGO networks after the solvent evaporation, the composites performed an excellent repeatability for solvent sensitivity.