Hydrothermal ageing of hydrophobic nano-calcium carbonate/epoxy nanocomposites

Highlights

• The combined effect of hydrophobic nano calcium carbonate content and temperature on water absorption of epoxy nanocomposites was investigated.

• Anomalous water absorption in the hydrophobic nano calcium carbonate/epoxy nanocomposites was observed.

• Hydrophobic nano calcium carbonate increased water absorption of epoxy nanocomposites.

• The tensile properties of hydrophobic nano calcium carbonate/epoxy nanocomposites were improved both in unaged and aged condition.

Abstract

In this study, we investigated the effects of the filler content and ageing temperature on the water absorption and tensile properties of hydrophobic nano-calcium carbonate (NCC)/epoxy nanocomposites. The NCC content of the nanocomposites was varied from 0 to 5 wt%, while the ageing temperature was set at 28, 45, and 55°C in a distilled water bath. The nanocomposites exhibited the non-Fickian behaviour at all the NCC contents and ageing temperatures. In general, the water absorption of the nanocomposites increased after the addition of NCC. Meanwhile, the diffusivity of the nanocomposites decreased with an increase in the NCC content; however, this decrease became less significant at higher ageing temperatures. Although the addition of 1–3 wt% NCC significantly increased the water uptake capability of the nanocomposites, it resulted in an improvement in their tensile properties at high temperatures and prolonged ageing times.

Introduction

Epoxy has been widely used as an adhesive, a coating, and a matrix of advanced composite materials owing to its good mechanical properties, low shrinkage, and high bonding to most materials [1]. A second phase material is often incorporated into epoxy to further improve its properties and achieve the desired properties, such as toughness and good thermal properties. Microparticles have been widely used to improve the stiffness, toughness, and thermal properties of epoxy. However, a large amount of microparticles is required to obtain the desired results, which increases the viscosity of epoxy and reduces the ease of processing. Furthermore, an increase in the microparticle content reduces the tensile strength of epoxy. To overcome these limitations of microparticle addition and to improve the tensile strength, modulus, toughness, and thermal properties of epoxy, the incorporation of nanoparticles such as nano-silica [2,3], nano-silicon carbide [1,4], carbon nano-tubes [5,6], nano-clay [1,7] and graphene [8,9] has been proposed.

Among nanoparticles, nano-calcium carbonate (NCC) is a low-cost filler with good mechanical properties. NCC exhibits low aspect ratio and large surface area, and hence shows strong interfacial interaction with epoxy. As reported by Jin and Park [10], the addition of 2–8 wt% NCC to epoxy improves its fracture toughness and impact strength without compromising its stiffness owing to the high NCC-epoxy interfacial strength. Jain et al. [11] reported that the incorporation of 4 wt% NCC into epoxy improves its rigidity, toughness, and thermal properties significantly. He et al. [12,13] incorporated 4 wt% silane-treated NCC in carbon fibre-reinforced plastic (CFRP) composites and found that the compressive strength, modulus, displacement at break, and fracture energy of the composites increased by 13.5, 6.1, 42.5, and 106.3%, respectively. Furthermore, Eskizeybek et al. [14] reported that the addition of 2 wt% NCC to CFRP composites improves their low-velocity impact properties. Yang et al. [15] evaluated the application of cubic- and rod-shaped NCCs as fillers for epoxy and found that both the types of NCCs improved the mechanical and thermal properties of epoxy. The rod-shaped NCCs showed better dispersion and mechanical and thermal properties than the cube-shaped ones.

The surface treatment of NCC is crucial for improving their dispersion and interfacial strength, leading to improved mechanical properties. He et al. [13], reported that the addition of silane-treated NCC improves the compressive strengths of unfilled cast epoxy and CFRP composites by approximately 13.5% and 14.1%, respectively, owing to its good dispersion. On the other hand, the addition of untreated NCC decreased the compressive strengths of the unfilled cast epoxy by about 8.4% and only improved the compressive strength of the unfilled CFRP composites by approximately 2.7%. Vrsaljko et al. [16] also reported that the addition of silane-treated NCC improves the tensile strength of polyurethane owing to the good NCC/polyurethane interfacial strength. Other researchers, including Shah et al. [17] reported that stearic acid-treated CC derived from eggshells shows good interfacial interaction with epoxy, and hence improves its thermal and tensile properties.

In most applications, epoxy composites are often subjected to moist environments and elevated temperatures. Epoxy can absorb humidity from the surrounding environment, and the absorbed water can have deleterious effects on its physical and mechanical properties. Therefore, the introduction of a secondary phase (either micro or nanoparticles) increases the complexity of the water uptake behaviour of epoxy, particularly the equilibrium water uptake and diffusivity. It is well-known that cement and fly ash particles increase and reduce the water uptake capability of epoxy at short ageing times, respectively. However, after prolonged ageing, both cement and fly ash increase the water uptake capability of epoxy because of their hydrolysis. Meanwhile, micron-sized CC decreases the water uptake capability of epoxy [18,19]. The addition of a small number of inert nanoparticles, such as carbon particles (expanded graphite, carbon black, multi-walled carbon nanotubes (MWCNTs), and nano-platelet graphite), increases the water uptake capability of epoxy. However, these nanoparticles decrease the diffusivity because of the tortuosity effect [20]. The addition of nano-clay, halloysite nanotubes, and nano-silicon carbide (up to 5 wt%) reduce the water uptake capability, diffusivity, and mechanical degradation of epoxy nanocomposites exposed to water [1]. The addition of up to 4 wt% silane-treated NCC also decreases the diffusivity and maximum water uptake capability of epoxy and improves its ageing flexural properties owing to the NCC barrier effect [21].

In addition to the nanoparticle content, the ageing temperature is an essential factor affecting the water uptake behaviour of epoxy. The effect of the ageing temperature on the water uptake behaviours of neat epoxy [22,23], CFRP/bismaleimide composites [24], and MWCNT/epoxy composites has been investigated [20,25]. The ageing temperature mainly increases the diffusivity of such composites, while the equilibrium water uptake capability can be increased or decreased depending on the epoxy system. Hydrophobic NCC can increase the water repellence of epoxy for application in coatings, adhesives, and composite matrices. However, to the best of our knowledge, the combined effect of the content of the hydrophobic NCC and the ageing temperature on the water absorption of epoxy has rarely been investigated. In this study, we investigated the water uptake behaviour of hydrophobic NCC/epoxy nanocomposites at various NCC contents and ageing temperatures. The tensile properties and fracture surfaces of the composites were also investigated.