Basalt fiber–epoxy laminates with functionalized multi-walled carbon nanotubes

Abstract

Cross-ply laminates reinforced with basalt fibers and functionalized multi-walled carbon nanotubes (MWCNTs) were fabricated from unidirectional epoxy prepregs. MWCNTs with varied surface conditions were prepared by oxidization or esterification, and then dispersed into a DGEBA epoxy system. The dispersion of the MWCNTs in the epoxy was improved by surface modification, resulting in improved composite mechanical properties as well. Significant increases in elastic modulus and strength were observed for epoxies with functionalized MWCNTs, especially for esterified species. When MWCNT – filled epoxies were used as matrices for basalt fiber/epoxy laminates, however, the reinforcement effects of MWCNTs on the composite elastic modulus exceeded micromechanics based semi-empirical predictions and were independent of surface functionalization. SEM morphological observations and the results of the micromechanical model revealed that nanotube re-distribution and orientation during processing was responsible for the enhancement of fiber-dominated mechanical properties. This work demonstrated the feasibility of in situ alignment and dispersion of functionalized nanotubes in multi-scale composite laminates.

Introduction

Several approaches can be employed to improve the properties of fiber-reinforced epoxies. One approach is to select fiber reinforcements which optimize specific thermal, mechanical, chemical, electrical, or optical properties [1], [2], [3]. A second approach is to toughen the matrix and overcome the inherent brittleness of epoxy systems through techniques such as rubber/thermoplastic toughening or epoxy composition modulation [4], [5]. A third alternative is to optimize the fiber–matrix interface to enhance the stress transfer properties [6], [7]. More recently, multi-scale approaches have been explored to design optimized microstructures at multiple reinforcement length scales [8], [9], [10], [11]. These approaches typically attempt to incorporate both nano-scale and micro-scale reinforcements.

Fiber-reinforced epoxy composites filled with nano-particles afford the opportunity to improve the bulk composite properties with minimal sacrifice of other properties of the composites. Among nano reinforcements, multi-walled carbon nanotubes (MWCNTs) stand out because of the ultra-high strength/stiffness [12], large aspect ratio, and relative affordability. Examples of multi-scale composites utilizing MWCNT-filled epoxies in conventional fiber-reinforced composites (FRCs) have been reported. For example, Thostenson et al. [13], [14] demonstrated that the anchorage of MWCNTs onto carbon fibers through either direct CVD growth or electrophoresis selectively tailored the “interface properties”. In other work, Qiu et al. [15] and Gojny et al. [16] reported that MWCNTs enhanced the electrical properties and the interlaminar shear strength respectively, while preserving or enhancing tensile properties, thus improving “matrix properties”.

Despite the temptation to assemble or organize ordered nanotube arrays into composites [17], [18], direct dispersion of MWCNTs with tailored surface properties, offers the most practical route because of the simplicity and compatibility with existing composite processing methods. Three key processing challenges are associated with direct CNT incorporation – nanotube dispersion, interfacial bonding, and alignment [19], [20]. The dispersion and interfacial bonding issues can be addressed by identifying a suitable nanotube surface modification, which not only disperses the MWCNTs in the resin system but also facilitates stress transfer at the interface through the formation of covalent bonds [21], [22]. In addition, high shear force, electric field, and magnetic field have been used successfully to align CNTs in polymer matrices [23], [24], [25]. However, these studies have not addressed the influence of processing conditions and CNT surface modification on CNT dispersion and alignment, although this is understandable, given the complexity of composite systems with multiple reinforcement length scales and processing stages.

In the present study, multi-walled carbon nanotubes were grafted with epoxy-compatible surface modifiers [26] before being dispersed directly into the epoxide. The resultant nanotube-filled epoxies were used as matrices in composite laminates reinforced with continuous basalt fibers, selected for the unique thermal, mechanical, and chemical characteristics. The morphology and mechanical behaviors of the nanotube reinforced epoxies (NEs) were analyzed and related to those of NE/basalt laminates in order to elucidate the composition, processing, structure, and property relationship in these hierarchically reinforced composites.