13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental Studies of Thermal Aging Effects on the Tensile and Tearing Fracture Behavior of Carbon Black Filled Rubbers Xiaoling Hu, Yan Li, Xiu Liu, Wenbo Luo* College of Civil Engineering and Mechanics, Xiangtan University, 411105, China * Corresponding author: luowenbo@xtu.edu.cn Abstract The effects of thermal aging on the tensile and tearing fracture behavior of carbon black filled rubbers with different filler loadings have been investigated in this paper. Thermal aging experiments were performed in a convection oven at 70oC and 120oC for various periods of aging time. Measurements of tensile strength, stretch to fracture and tearing strength were conducted with a 5943 Instron single column materials testing system as a function of aging time and aging temperature. It is found that the initial elastic modulus increases with increasing aging time, while the tensile strength decreases with the increase in aging time up to 100 hours and then increases when the material aged over 144 hours. The changes in tensile strength and stretch to fracture are found to be dependent on aging temperature, the higher the aging temperature, the more pronounced decrease in tensile strength and stretch to fracture. From the tearing fracture tests, the tearing strength generally is found to decrease with the increase in aging time. Keywords Rubber, Thermal aging, Tensile strength, Stretch to fracture, Tearing energy 1. Introduction Natural rubber is one of the most important elastomers, and its properties are usually reinforced by incorporation with carbon black (CB) of varying surface chemistry and aggregate size/aspect ratio to suit the application concerned [1]. CB filled rubbers are visco-hyperelastic materials, their mechanical properties are time- and temperature-dependent. Thermal aging causes both cross- linking and chain-scission. Cross-linking yields usually the increase in modulus and the decrease in extensibility of the material. Rubbers are susceptible to oxidative ageing due to their unsaturated carbon–carbon double bonds in the backbone. Elevated temperatures usually promote such an oxidative ageing [2]. The objective of this paper is to investigate the effect of thermal aging degradation upon the mechanical properties of filled rubbers with different CB loadings. The effects of aging are measured by the changes in tensile strength, stretch to fracture, initial elastic modulus and tear strength. 2. Experimental 2.1. Material and samples The materials used for mechanical testing in this work are CB filled natural rubber vulcanizates. The formulations of the rubber compounds are listed in Table 1.Thin rectangular strips as shown in Fig.1, having the length of 100 mm and width of 10 mm, used for uniaxial tension tests are cut from a 2 mm thick vulcanized rubber sheet, which is provided by the Zhuzhou Times New Material Technology Co., Ltd. in China. Table 1. Formulations of the rubber compounds (phr) Compound NR CB (N330) ZnO Stearic acid Antioxidant Sulfur Accelerator Aromatic oil A 100 11.2 * 5 3 1 2.5 0.7 3 B 100 37.6 * 5 3 1 2.5 0.7 3 * 11.2 phr CB = 5 vol% CB, 37.6 phr CB = 15 vol% CB
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