Elsevier

Polymer Testing

Volume 63, October 2017, Pages 281-288
Polymer Testing

Material Behaviour
Influences of different dimensional carbon-based nanofillers on fracture and fatigue resistance of natural rubber composites

https://doi.org/10.1016/j.polymertesting.2017.08.035Get rights and content

Abstract

The dimensions of reinforcing filler is a key factor in influencing the fracture and fatigue of rubbers. Here, the fracture and fatigue resistance of natural rubber (NR) filled with different dimensional carbon-based fillers including zero-dimensional spherical carbon black (CB), one-dimensional fibrous carbon nanotubes (CNTs) and two-dimensional planar graphene oxide (GO) were explored. To obtain equal hardness, a control indicator in the rubber industry, the amounts of CB, CNTs, and GO were 10.7 vol%, 1.2 vol%, and 1.6 vol%, respectively. J-integral and dynamic fatigue tests revealed that NR filled with CB exhibited the best quasi-static fracture resistance and dynamic crack growth resistance. The much higher hysteresis loss of NR filled with CNTs weakened its fatigue resistance. The planar GO played a limited role in preventing crack growth. Furthermore, digital image correlation revealed that NR filled with CB had the highest strain amplification level and area at the crack tip, which dissipated the most local input energy and then improved the fracture and fatigue performance.

Introduction

Rubber products such as belts and tires are often working under long-term quasi-static and dynamic loading conditions. Failure can be mainly attributed to the crack nucleation and crack growth processes [1]. Thus, the quasi-static fracture and dynamic fatigue resistance dominated the reliability and service security of rubbers [2]. Based on the Griffith's fracture mechanical criterion [3], Rivlin et al. proposed the energy release rate theory, namely the tearing energy theory, showing that the fatigue crack growth rate was uniquely governed by the input tearing energy [4]. They also confirmed that quasi-static fracture occurred above a critical tearing energy, independent of specimen geometry [4]. J-integral theory, another energy balance criterion, describes the relationships between energy release rate and intensity of local crack tip fields [5]. Recently, J-integral has been proved to be an effective tool to evaluate the fracture resistance of rubber materials [6], [7], [8]. In our previous work, by combining J-integral theory with crack tip opening displacement (CTOD), the fracture resistance of rubber composites was evaluated using single-edge notched tension (SENT) specimens [9], [10], [11], [12]. Two J-integral parameters, critical J-value JIC and tearing modulus TR, were used to evaluate the quasi-static fracture initiation and propagation resistance, respectively.

The local strain field distribution at the crack tip has a close relationship with the fracture and fatigue resistance of rubber materials [13]. Due to the incorporation of fillers with higher modulus, the strain amplification effect occurs, leading to local stress concentration, which would have a great effect on the failure of rubbers [14]. Therefore, it is necessary to evaluate the strain field distribution. By utilizing the constitutive behavior of rubbers, the strain field distribution at the crack tip could be predicted by finite element simulation [15]. Digital image correlation (DIC) has been proved to be another effective and non-contacting tool to evaluate the strain field distribution of materials [16], [17]. Mzabi et al. have validated the existence of strain amplification area at the crack tip by DIC, and proposed a local tearing energy criterion to estimate the fatigue failure resistance of rubber materials [13]. Enlightened by this efficient method, the strain distribution and amplification at the crack tip of carbon black filled styrene butadiene rubber composites were evaluated by DIC in our previous work [18].

Many factors can influence the fracture and fatigue resistance of rubber materials. Mars summarized four main factors: mechanical loading history, environmental conditions, constitutive behavior, and rubber formulation [19]. For formulation, special attention should be paid to the types of rubber matrix and reinforcing filler. The strain-induced crystallizing (SIC) rubber had better fracture and fatigue resistance than non-SIC rubber due to the increased tendency of crack deviation [20]. For the most commonly used fillers in the rubber industry, low-structure CB was obviously superior to a high-structure one [19]. Also, CB with higher surface area was beneficial. Recently, one-dimensional fibrous carbon nanotubes (CNTs) and two-dimensional planar graphene oxide (GO) have attracted great attention for rubber due to their excellent reinforcing efficiency [21], [22], [23], [24]. It was proved that the addition of CNTs and GO could enhance the crack blunting tendency, deviation, and branching due to the orientation of fillers along the stretching direction at the crack tip [11], [12].

In this work, the effects of dimensional factor of carbon-based fillers on fracture and fatigue resistance of natural rubber (NR) composites were explored. Considering that hardness is one of the control indicators for the design of rubber products [11], the fracture and fatigue resistance of three filled composites were compared under equal hardness level. Due to the different reinforcing ability of CB, CNTs and GO, their contents were different in order to reach the same hardness. In our study, the contents were 10.7 vol% for CB, 1.2 vol% for CNTs, and 1.6 vol% for GO, respectively. To ensure fine filler dispersion, the NR compounds filled with spherical CB and fibrous CNTs were prepared by mechanical blending, and the NR filled with planar GO was prepared by latex blending and co-coagulation.

J-integral tests were carried out to evaluate the fracture resistance, and dynamic fatigue tests were applied to evaluate the fatigue resistance. The energy input and hysteresis loss during the fracture and fatigue process were also evaluated. Furthermore, the strain field distribution at the crack tip was measured by DIC to reveal the failure mechanism. The results are expected to understand the effects of carbon-based nanofillers with different shape factors on failure resistance of rubbers, and to provide new insights into the relationships between failure resistance and strain distribution at the crack tip.

Section snippets

Materials

The raw NR was obtained by coagulating natural rubber latex (NRL) and drying. The NRL with solid content of 60 wt% was supplied by Hainan Natural Rubber Industry Co., Ltd., China. CB N330 (average particle size of about 30 nm, specific surface area of about 83 m2/g) was purchased from Tianjin Cabot Chemical Products Co., Ltd., China. Highly one-dimensional aligned carbon nanotube bundles (Flotube ™ 7000) with a purity of 92%, average diameter of 6–8 nm and length of 50 μm were supplied by CNano

Dispersion morphology and nanofiller networking

The dispersion state of different nanofillers in the rubber matrix was observed by TEM. As shown in Fig. 3, the GO nanosheets, CNTs fibers and CB particles were all uniformly dispersed in the whole NR matrix. That was to say, mechanical blending ensured the uniform dispersion of CB and CNTs, and latex co-coagulation ensured the uniform dispersion of GO sheets. It should be pointed out that the dark spots in TEM graphs were the ingredients in the rubber matrix, such as zinc oxide. Therefore, the

Conclusions

The influences of dimensional factors of carbon-based fillers on fracture and fatigue properties of NR composites (under the equal hardness) were analyzed. The NR composites filled with 10.7 vol% CB, 1.2 vol% CNTs, and 1.6 vol% GO possessed equal hardness level. The mechanical blending and latex co-coagulation ensured the uniform dispersion of nanofillers in the rubber matrix. The CB-10.7 exhibited the most powerful fracture initiation and propagation resistance under quasi-static conditions

Acknowledgements

The authors would gratefully acknowledge the financial supports of the State Key Program of National Natural Science of China (Grant No. 51333004) and the National Key Technology Support Program of China (Grant No. 2013BAF08B03).

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