Nano-enhanced interface in carbon fibre polymer composite using halloysite nanotubes

Abstract

The carbon fibres (CFs) sizing has a crucial impact on final properties of CFs reinforced polymer composites. Herein, an industrially-adaptable process for sizing of CFs was employed through a dip-coating process which includes using epoxy sizing solutions mixed with halloysite nanotubes (HNTs). Potential changes in the mechanical properties of the CFs and the characteristics of the resulting composites were evaluated. Epoxy sizing containing various concentrations of HNTs changed the surface roughness, and friction of CFs. The obtained results demonstrated that HNTs does not significantly affect the tensile strength of CFs, however, surface energies, obtained by an inverse gas chromatography technique, are increased. To demonstrate the effect of HNTs on the fibre/epoxy matrix interactions, single fibre fragmentation tests (SFFT) were conducted and consequently apparent interfacial shear strength (IFSS) were calculated. The IFSS increased up to 23% compared to a pure sized CFs and up to 61% compared to an unsized-CFs.

Introduction

Since 1960s, carbon fibre has been a promising reinforcement in a wide range of applications including aerospace, sporting goods, military field, and automotive industries [1], [2]. For instance, polymer composite reinforced with carbon fibre can be employed in automotive body parts including hoods, chassis, deck lids, front end, doors, bumpers and leaf spring [3], [4]. The increasing attention to carbon fibre composite is due to the fact that carbon fibre possesses exceptional properties such as high tensile strength ranging from 2 to 7 GPa, high modulus (∼200–900 GPa), and low density. Nonetheless, there have been some limitations which has hindered the true potential of carbon fibre in high performance applications. The limitations are mainly associated to the smooth graphitic surface, chemical inertness, low surface energy, and stable nonpolar structures inert which in turn lead to a weak interfacial adhesion between fibre and matrix [5]. Due to the challenges with interface, over decades, various approaches have been investigated to solve such performance mismatch between carbon fibre and polar matrices such as epoxy resins [6], [7]. From chemical aspect, these methods include both oxidative based methods and non-oxidative approaches. On the other hand and given the technical aspects, these classifications can be narrowed down to wet and dry modifications. In this context, wet modification can be done through acidic treatment, electro-polymer coating, sizing and electrochemical modification. Approaches involve in dry method mostly deal with plasma surface modification, high energy irradiation modification, nickel surface coating, thermal and miscellaneous dry treatments. Although the aforementioned wet methods can to a great extent improve compatibility between carbon fibre and matrix, they are accompanied with some shortcomings such as poor diffusion of sizing, reduction of fibre strength, and lack of precise control and optimization. On the other hand, when it comes to dry method, cost is a major limiting factor.

In the era of development of nanotechnology solutions for various applications, the utilization of nanomaterial to improve fibre-matrix adhesion in carbon fibre composites could potentially lead to enhanced mechanical interlocking as well as chemical interaction [8], [9], [10], [11]. The introduction of nanomaterials onto the fibre surface can be done through either dip coating of fibre into the water based solutions containing nanoparticles or direct grafting onto the fibre surface (e.g., chemical vapor deposition, chemical vapor infiltration, and injection chemical vapor deposition). Nonetheless, there have been still some limitations such as optimization and reduction of fibre strength. Additionally, when it comes to the large production scales, these methods, can be easily scaled up but the cost of carbonous nanomaterials (e.g., graphene, carbon nanotube and carbon nanofiber) is fairly high [12], [13], [14], [15]. On the other hand, inorganic nanomaterials such as halloysite nanotubes (HNTs), are often inexpensive and possess high aspect ratio (due to tube-shaped thin particles), considerable mechanical strength and modulus, good wetting with epoxy resins (due to hydrophilic nature), electrical insulation, low thermal conductivity, and low thermal expansion coefficient [16], [17]. These desirable features make HNTs a good candidate for nano-reinforced interface with the potential of scaling up.

In this study, a facile sizing process using HNTs was used to improve the interface in carbon fibre- epoxy resin. Halloysite nanotubes, was selected to produce a nano-enhanced sizing agent. The aim herein is to find out whether the cost-effective halloysite nanotubes has the potential to elevate the interfacial performance in carbon fibre-epoxy composites. To this end, carbon fibre surface properties and morphologies as well as interfacial characterization and reinforcing mechanism were investigated.