Deposition of carbon nanotubes onto glass fibers using ultrasound standing waves

Abstract

Carbon nanotubes (CNTs) are special types of carbon nanostructures with exceptional physical and mechanical properties such as high elastic modulus, high electrical and thermal conductivities, as well as high surface area and large aspect ratios [1]. Given their tubular structure and size, CNTs are conceptualized as one-dimensional nanostructures with a crystalline structure of carbon sp hybridization formed by carbon–carbon covalent bonds [1]. Extensive scientific investigations in nanotechnology have shown that the incorporation of CNTs in polymers can improve their thermal and mechanical properties and electrical conductivity [2]. The pronounced increase in these properties can occur at relatively low weight concentrations, which do not influence the final weight of the polymer composites, making them suitable for modern devices and lightweight structures [3]. Furthermore, as a result of their remarkable properties and synergy, CNTs can provide multifunctional characteristics (e.g., piezoresistive properties) to fiber-reinforced polymers (FRPs) [4–6], which are of great interest for a wide number of engineering applications and industries, such as marine, aircraft, automotive, energy, among others [7]. However, the final properties of polymer-based composites and FRPs containing CNTs are hindered by their difficulties in obtaining a homogeneous dispersion, uniform distribution, and effective alignment within the matrix. Although CNTs can be added into polymer matrices with relatively simple techniques [2], several research studies have indicated that the typical process used for adding these nanostructures into FRPs often causes severe difficulties during their manufacturing due to a large increase in the resin viscosity [8]. To overcome this limitation, scientists and researchers have proposed diverse alternative techniques based on the direct deposition and growth of CNTs on fibers to circumvent agglomeration and have more control during composite manufacturing. However, most of these techniques promote randomly oriented CNTs into the final laminate or, in such a case, they are not easily scalable to industrial applications. Thus, the manufacturing process of FRPs with aligned CNTs remains a challenge to date. The nanostructure orientation and uniform dispersion into the matrix are probably the main factors to achieve superb mechanical properties in nano-modified polymer composites and FRPs. In this regard, Gan et al. [9] reported that the shear-induced orientation of graphene oxide sheets in isotactic polypropylene plays the main role in the final properties of polymer composites. It was found that the incorporation of 2 wt.% of functional graphene oxide sheets in the polypropylene extruded by the shear-induced orientation extrusion process resulted in enhanced mechanical, gas barrier, and thermal properties of the nanomodified composites. In another work, Li et al. [10] reported that the uniform dispersion and orientation of short glass fibers in isotactic polypropylene through the shear-induced orientation extrusion system improved the mechanical properties of the composites. Although many studies have been focused on depositing/growing nanostructures on fibers [8] and align CNTs into the matrix [11] by techniques based on electric [12–14], magnetic [15–17], and ultrasound field [18–19], the microscale alignment of multi-walled carbon nanotubes (MWCNTs) on glass fibers using ultrasound standing waves has not been previously reported in the literature. Therefore, this work proposes a Centro de Ingeniería y Desarrollo Industrial, Departamento de Energía,