Fracture toughness of the nano-particle reinforced epoxy composite

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Abstract

Although thermoset polymers have been widely used for engineering components, adhesives and matrix for fiber-reinforced composites due to their good mechanical properties compared to those of thermoplastic polymers, they are usually brittle and vulnerable to crack. Therefore, ductile materials such as micro-sized rubber or nylon particles are added to thermoset polymers are used to increase their fracture toughness, which might decrease their strength if micro-sized particles act like defects.

In this work, in order to improve the fracture toughness of epoxy adhesive, nano-particle additives such as carbon black and nanoclay were mixed with epoxy resin. The fracture toughness was measured using the single edge notched bend specimen at the room (25 °C) and cryogenic temperature (−150 °C). From the experimental results, it was found that reinforcement with nano-particles improved the fracture toughness at the room temperature, but decreased the fracture toughness at the cryogenic temperature in spite of their toughening effect.

Introduction

Thermoset polymers have been widely used for engineering components, adhesives and matrix for fiber-reinforced composites due to their good mechanical properties compared to those of thermoplastic polymers. However, since they are usually brittle and vulnerable to crack, ductile thermoplastic materials such as micro-sized rubber or nylon particles are added to the polymers to increase their fracture toughness, which compromises the strength of thermoset polymers.

The addition of rigid micro-scale fillers to polymers often increases its strength, but decreases the toughness since the fillers or agglomerates may induce stress concentration, which initiates cracks and make them become larger than the critical crack size that causes failure. Therefore, it is a good way to reinforce the polymers with nano-particles in order to increase the fracture toughness without sacrificing the mechanical strength of the polymers because well-dispersed nano-particles are much smaller than the critical crack size to initiate failure. Thus, they provide an avenue for simultaneously toughening and strengthening polymers [1].

Nano-particle reinforced polymer composites have been widely studied and some researchers already studied the improvement of the fracture toughness of polymers. Carbon nanotubes (CNT) have shown a high potential to improve the mechanical properties of polymers as well as electrical properties [2], [3]. Gojny et al. [4] reported that DWCNT (double walled CNT) could increase both tensile strength and fracture toughness. Florian et al. [5] studied the influence of different carbon nanotubes on the tensile properties as well as fracture toughness and explained the contribution of nanomechanical mechanisms to enhancement of the fracture toughness. However, CNT has not been widely used to improve the mechanical properties because of its high material cost. Since the first discovery by Toyota researchers on the reinforcing effect of relatively cheap nanoclay on the polymeric material [6], many researchers have focused on it: Weiping et al. [7] reported that nanoclay could increase the fracture toughness of epoxy by 2.2 and 5.8 times. Lei et al. [8] studied the dependences of Young’s modulus and fracture toughness on clay concentration using the tensile and 3-point bending methods. Qi et al. [9] investigated the effect of several nanoclay additives, which were mixed with DGEBA epoxy resin using a mechanical stirrer, on tensile modulus, tensile strength and fracture toughness of the nanocomposite. Ho et al. [10] increased the tensile strength and Vickers’ hardness value of the epoxy using nanoclay mixed by mechanical stirring method. Carbon black also has been used for reinforcement of polymers, which has been mainly used for reinforcing elastomer and for UV protection, electromagnetic interference shielding and anti-static shielding of polymers [11]. Novak [12] studied the electro-conductive HDPE/CB composite with improved toughness. However, mechanical reinforcing effects of the carbon black have not been widely studied yet.

As mentioned above, most of researchers have been interested in the mechanical properties of nanocomposites at the room temperature. However, only few researchers studied the cryogenic properties of epoxy and its composite [13], [14], [15].

A study on the nanocomposite is important since it can affect the structural characteristics of a composite structure when it is used as a matrix of the laminates or the reinforcement of a foam core. It was reported that the characteristics of a composite structure could be improved when the nanoclay reinforced epoxy was used as a matrix of laminates. Antonio et al. [16] improved the damping coefficient and the energy dissipation characteristic of the glass/epoxy composite using nanoclay particles. Hosur et al. [17] improved the impact characteristic of the composite sandwich structure using the nanoclay infused foam.

In this study, carbon black and nanoclay were mixed with epoxy to investigate their toughening effect. The fracture toughness was measured using the single edge notched bend (SENB) specimens with respect to the particle content at the room (25 °C) and cryogenic temperature (−150 °C). In order to investigate their toughening mechanism, the fracture surface was also observed with SEM.

Section snippets

Materials

The epoxy matrix used in this study was the modified bisphenol-A type epoxy resin (YD-114F, Kukdo Chemical, Korea) and polyetheramine hardener (JEFFAMINE D-230, Huntsman, US) were used as a curing agent. The proper mixing ratio was 10:3. The conductive carbon black (Ketjenblack EC-300J, Ketjen Black International Co., Japan) and the nanoclay (Cloisite 93A, Southern Clay Products, US), which was natural montmorillonite modified with quaternary ammonium salt, were used as reinforcements. Table 1

Experiments

The mode I fracture toughness, KIc, was determined by the 3-point bending method with SENB specimens and the fixture as shown in Fig. 2. The tests were performed using INSTRON 5583 (INSTRON corp., MA, USA) equipped with an insulation chamber. The crosshead speed was 10 mm/min as recommended by ASTM D5045-99, which was fast enough to prevent the viscoelastic behavior of the epoxy [20].

The KIc values were determined using the following relationship [20]:KQ=PQBW1/2f(x),W=2Bf(x)=6x1/2[1.99-x(1-x)(

Effect of the residual stress around the crack tip and the crack length

ASTM recommends either inserting a fresh razor blade by tapping or sliding a new razor blade across the notch root to initiate a pre-crack [20]. However, it can generate large amount of residual stress around the crack tip. Though many researchers have suggested the fracture toughness of polymeric materials, they have not inspected the crack tip visually or investigating the effect of the residual stress. In addition, it is not easy to make the specimen with the same crack length. In order to

Conclusion

In this work, the toughening effect of carbon black (Ketjenblack EC-300J, Ketjen Black International Co., Japan) and nanoclay (Cloisite 93A, Southern Clay Products, US) on the modified bisphenol-A type epoxy resin (YD-114F, Kukdo Chemical, Korea) was investigated at the room (25 °C) and cryogenic (−150 °C) temperatures.

At the room temperature, the carbon black (CB) of 3.0 wt% could increase the KIc value by 23% on the average due to the toughening mechanisms of nano-scale crack branching and

Acknowledgements

This work has been supported by Ministry of Science and Technology in part by NRL (TBP), and BK21. Their supports are gratefully acknowledged.

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