Characterization of Reinforced Structural Composites with Carbon Nanotubes Grown Directly on the Fibers/Fabrics Using the PopTube Approach

Fiber reinforced polymers (FRPs) have been extensively implemented across an array of engineering fields, from the aerospace and automotive industries to the areas of renewable energy and civil infrastructure. This material consists of three main components: the reinforcing fibers (typically glass or carbon), the matrix (commonly epoxy), and the interphase zone developed between the fibers and the matrix[1]. FRPs possess a number of advantageous functional features associated with their fiber-dominated properties (like longitudinal strength and modulus), such as: superior stiffness and specific strength, lighter weight, and enhanced corrosion resistance. However, the matrix and interphase zone-dominated properties present drawbacks to the widespread use and implementation of FRPs in a variety of areas. In this way, a need exists to reinforce the matrix and interphase zone within FRPs to effectively overcome the aforementioned disadvantageous features. An ideal candidate to reinforce the matrix and interphase zone within composites is carbon nanotubes (CNTs). First discovered by Ijima in 1991 [2], CNTs possess outstanding mechanical properties (such as superior stiffness, high specific strength, low weight density, and improved corrosion resistance) as well as excellent thermal and electrical properties [3]. With regard to the ultimate incorporation of CNTs in composite materials, the most commonly used method – direct dispersion of bulk CNTs in the matrix material – has been shown to be very difficult [4,5]. This difficulty is due primarily to the matrix material’s generally high viscosity and the CNTs’ natural tendency to agglomerate with the matrix during mixing. These two characteristics inhibit 1.) The effectiveness and use of commonly used economical manufacturing techniques such as vacuum assisted resin transfer molding (which requires relatively low matrix viscosities), and 2.) The general effectiveness of the reinforcing scheme itself (due to the non-uniform distribution of CNTs within the matrix). A more effective way to reinforce composite materials using CNTs is to grow CNTs directly on the reinforcing fibers/fabrics. In this way, the most critical area for CNT reinforcement – the interphase zone – is intrinsically targeted. To this end, a novel technique used to grow CNTs directly on fibers/fabrics has been developed and patented at the University of Alabama and Auburn University [6,7]. This method, referred to as the PopTube Approach, uses microwave irradiation to grow CNTs at room temperature in air, without the need for inert gas protection or additional feedstock gases. Compared to existing production methods of CNTs, the PopTube Approach enjoys many advantages, including: minimal damage/chemical alteration in the reinforcing fibers; the viability of high yield, large scale manufacturing that utilizes simple equipment and processes; and enhanced energy efficiency and cost effectiveness. The objective of this study is to evaluate the potential of a novel technique, the PopTube Approach, to produce reinforced structural composites with carbon nanotubes grown directly on the fibers/fabrics to achieve superior mechanical performance and long-term durability. To accomplish these goals, a series of mechanical tests were carried out to assess the effects of the PopTube Approach CNT synthesis method on carbon fiber/epoxy laminate composites. Where applicable, characterization methods were also used in order to further understand how this technique affects said composites. Based on the results of this series of studies, several significant conclusions can be drawn with respect to the PopTube Approach CNT synthesis method. These conclusions are summarized below. • Compared to existing CNT synthesis methods, the PopTube Approach induces much less damage in the fibers. The PopTube Approach led to a 11.4% reduction in characteristic fiber tensile strength, which is much lower compared to some existing methods – which have reported reductions of up to 55%. • For three principal reasons, the PopTube Approach leads to a reduction in composite tensile strength. The PopTube Approach led to a 28.5% decline in composite tensile strength, which was due damage to the fibers themselves (from the CNT synthesis process), a reduction in fiber volume fraction (presumably due to the presence of CNTs on the fibers), and a shift in failure mode due to the reinforcing effect of the CNTs. • The PopTube Approach leads to sizable improvements in Mode I and Mode II fracture toughness, as well as short-beam shear strength. Mode I fracture toughness upon crack initiation was improved by 48%, while Mode II fracture toughness was improved by 10% due to the PopTube Approach. Short-beam shear strength, which gives a representation of interlaminar shear strength, was improved by 31.6%. • In a drop-weight impact loading event, the PopTube Approach reduces the delamination area in composite facesheets and leads to an improvement in compression after impact strength. The damaged (delaminated) area was reduced by 13.6% due to the PopTube Approach, and the compression after impact strength was improved by 9.2%. • At the laboratory scale, the PopTube Approach appears to be a viable CNT synthesis technique, but needs further development to become useful for potential end-users. Mechanical testing has shown promising results for materials produced on a small scale, but ultimately, scale-up will be critical for this technology to move forward. If this technique can be scaled up in manner that improves the quality of CNTs produced (via manufacturing equipment with more precise controls) while maintaining sufficient quality control, this technology could become viable for use by commercial end-users.