Fractural performance of epoxy nanocomposites reinforced with carbon aerogels in different structures

https://doi.org/10.1016/j.tafmec.2021.103079Get rights and content

Highlights

  • CNTs were used to increase the specific surface area and total pore volume of carbon aerogel.

  • Powdered carbon aerogels were used for reinforcement of epoxy nanocomposites.

  • The effect of changing the physical structure of the reinforcement on the fracture toughness of nanocomposites was evaluated.

  • Two direct and indirect methods were used to evaluate the fractural parameters.

Abstract

Home-synthesized carbon aerogels were used as reinforcements for epoxy nanocomposites in different weight contents (0.1, 0.3, and 0.5 wt%). The relationship between physical structure of aerogels (specific surface area and total pore volume) and mechanical properties of nanocomposites were evaluated. The fracture toughness of nanocomposites was investigated by both direct and indirect methods. The nanocomposites provided better mechanical properties than neat epoxy. Due to the penetration of polymer chains into the aerogels’ pores, the filler/polymer interface was increased, which lead to an effective increase in load transfer. The porous structure of the aerogels absorbs energy and external force applied, which ultimately plays a role in properties improvement. Due to the higher specific surface area of CNT-doped carbon aerogels (CNT-CA), Ep/CNT-CA nanocomposites showed a better performance. Improvement of fracture toughness in 3-point bending test was greater than impact test. Mechanical and topographical investigations of nanocomposites showed enhancements: 106–143% KIC, 241–372% GIC, 50–58% impact strength and 10.28–14.72 nm roughness, compared to the epoxy matrix.

Introduction

In recent decades, polymer nanocomposites have received much attention due to their unique properties. Increasing the surface area due to the transfer from microparticles to nanoparticles, low concentration of required filler for significant changes in the mechanical properties of nanocomposites has caused increasing attention to this category of materials [1], [2], [3]. Generally, the dispersion of nanoparticles and the optimal adhesion between the matrix and the particles play very important roles in determining the mechanical properties of polymer nanocomposites [4], [5], [6].

Epoxy resins are subset of thermoset resins and constitute polymer matrixes. There is a great interest in epoxy resins due to the different groups of epoxy for extensive reactions and outstanding properties of cross-linked polymers. These properties include high strength, very low creep, and excellent resistance to chemical agents, weather and heat. Disadvantages of thermoset polymers are their low final strain at rupture, which leads to low impact resistance and reduced fracture toughness [7], [8], [9], [10]. According to the studies reported so far, many methods have been proposed to improve the toughness of epoxy resins. Among these methods is the combination of epoxy resin with nanofillers and the production of polymer nanocomposites [11], [12], [13]. Chandrasekaran et al. [14] used Aerographite (AG) to reinforce the epoxy matrix. Their composites showed 133% improvement in energy absorption per unit volume, and 19% increase in fracture toughness (KIC). The mechanisms pull-out of arms of AG tetrapod, interface and inter-graphite failure have improved the toughness of composites. Salimian et al. [15] used silica aerogel to increase the toughness of epoxy nanocomposites by 126%. They attributed the toughness improvement to the mechanisms: pinning, crack deflection, debonding and plastic deformation. Park et al. [16] treated multi-walled carbon nanotubes (MWCNTs) surface with chemical functionalization and used them to make epoxy nanocomposites. Nanocomposites containing modified MWCNTs showed better fracture toughness than unmodified MWCNTs. Examination of the fracture surface of the samples showed increment in river lines and the fracture surface, and considerable increase in the fracture toughness of nanocomposites, which resulted due to the good dispersion of the modified particles throughout the matrix.

The aerogel word is used for sol-gel materials in which the liquid component of the gel is replaced with a gas, leaving a solid nanostructure without the collapse of pores, so that 90–99% of its volume is occupied by air. As a result, aerogels are highly porous solids with a high specific surface area and extremely low density [17]. Therefore, these materials are used for various applications. In general, due to the porous structure of aerogels, these materials are proposed as a new alternative for reinforcement of polymer nanocomposites in very low weight quantities. Thus, by placing these materials in the polymer bed and trapping the chains inside the pores, a high interaction between the polymer and the filler can be achieved [18]. Among the types of aerogels, carbon aerogels were widely welcomed due to their extremely high specific surface area and strong mechanical properties. Carbon nanotubes (CNTs) are one of the best candidates for making carbon aerogels due to their unique properties e.g. small size, low density, high aspect ratio, and considerable stiffness and strength. Due to the CNTs aspect ratio, carbon aerogels with high porosity and specific surface area can be produced from them or their combination with carbon particles, as a new nanomaterial for engineering applications [19], [20], [21], [22].

The KIC (fracture toughness) and GIC (fracture energy) parameters are used to investigate the fracture mechanics of nanocomposites. KIC is the degree of resistance of the material to crack propagation and GIC is defined as the amount of energy required to form 1 m2 of new surface [23]. In general, measurement of fracture parameters is evaluated both directly and indirectly. The direct method is based on calculations that eventually arrive at the true value of the parameters, while the indirect method provides an estimate of the actual value of the parameters [24].

In this work, epoxy nanocomposites were produced using two different types of carbon aerogels as reinforcements in various weight percentages (0.1, 0.3, and 0.5 wt%). Carbon aerogels were different in specific surface area and total pore volume. Three-point bending and impact test methods were used to evaluate the fracture parameters to compare direct and indirect methods, respectively. SEM and AFM observations were used to investigate the fractography and topography of the nanocomposites, respectively. The purpose of this study was to investigate the effect of physical structure of aerogels in different contents on fracture parameters using direct and indirect measurements. The results showed that only by changing the physical structure of the filler and very low weight percentages and creating physical intermingles between the filler and the matrix, the fracture toughness of nanocomposites has increased significantly.

Section snippets

Materials

CNT-doped carbon aerogel (CNT-CA) and Carbon aerogel (CA) were synthesized according to the methods described elsewhere [25], [26], [27]. Aerogel powders have porosity above 90% and particles diameter of 11.48 ± 4.00 μm. The specific surface area (measured by BET (SBET)), the average pores diameter (Dmean), the total volume of pores (Vt), skeleton densities (ρs) and apparent densities (ρa) of aerogels are presented in Table 1. Epoxy matrix had the following composition: Epoxy bisphenol A resin

Fracture toughness

Three-point bending test was used to determine energy absorption of the samples in static mode. The results are shown in Fig. 1 and Table 3. Fig. 1 (a, b) show force-displacement diagrams of nanocomposites (a) Ep/CA, (b) Ep/CNT-CA and Fig. 1 (c, d) show (KIC) and (GIC) of all the nanocomposites. Fig. 1 (a, b) show that bending properties of all the nanocomposites are improved compared to the pure epoxy matrix. In all the samples, by increasing the content of aerogels, the force and elongation

Conclusions

Epoxy nanocomposites were fabricated using carbon aerogels with different physical structures in different weight percentages (0.1, 0.3, and 0.5 wt%). Carbon aerogels improved fracture toughness (106–143%), fracture energy (241–372%), impact strength (50–58%) and roughness (10.28–14.72 nm) of nanocomposites, compared to the neat epoxy matrix. Examination of the fracture surface of nanocomposites indicated that the known mechanisms: pinning, crack deflection, river lines formation and debonding

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank the Ministry of Science, Research and Technology of Iran (MSRT) for financial support of this research. Also, Isfahan University of Technology, and Tarbiat Modares University are acknowledged for the provided laboratory facilities for accomplishment of this research work.

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