Effect of Carbon Nanotube Functionalization and Alignment on the Out-of-plane Mechanical Performance and Environmental Durability of GFRP Composites

Abstract

Globally Glass fiber reinforced polymer (GFRP) composites are one of the most widely used material for critical applications like structural, renewable energy, marine, automotive, and aerospace applications as a substitute for traditional metallic materials. They have an added advantage over traditional metals in terms of high corrosion resistance, greater in-plane strength and modulus. However, this laminated composites are comparatively weak when subjected to out-of-plane (perpendicular to the plane of glass fiber fabric) and in-plane shear (parallel to the plane of glass fiber fabric) loading. This weakness makes them particularly susceptible to delamination and interfacial debonding, due to the absence of reinforcement in the thickness direction (also known as the "z" direction). The improvement in the out-of-plane properties can be carried out by introducing a third phase into the composites i.e., nanofillers. Research on the use of carbon nanotubes (CNTs) to improve the out-of-plane properties and environmental durability study of GFRP composites has been ongoing for roughly one to two decades, with more intensive research and development in recent years. In our past studies, it was observed that adding CNTs to glass fiber reinforced epoxy (GE) composites improves its water resistance, with the extent of improvement dependent on the water bath temperature[1]. Moreover, functionalizing the CNTs resulted in even higher enhancements in the mechanical and creep performance of GE composites, compared to using pristine CNTs[2]. Building on this work, the present investigation begins with assessing the hydrothermal conditioning behavior of GE composites with varying wt. % (0.1, 0.3, and 0.5) of pristine and functionalized multi walled carbon nanotubes (CNTs and FCNTs) at two different water bath temperatures: (i) 15℃ (Low-Temperature Hydrothermal Conditioning (LTHC)) and (ii) 50℃ (Elevated-Temperature Hydrothermal Conditioning (ETHC)). The gravimetric analysis revealed that, FCNTs greatly hinders the water absorption through the interfaces at LTHC. At LTHC, the water saturated 0.1FCNT-GE composites exhibited superior flexural strength than GE and 0.1CNT-GE composites. At ETHC, generation of unfavorable hygroscopic and thermal stresses at the weak CNT/polymer interface adversely affected the water resistance of 0.1CNT-GE composites compared to 0.1FCNT-GE composites with stronger FCNT/polymer interface. The elevated in-service environmental temperature had posed a significant decrement in the flexural strength of all the composites compare to room temperature (RT) testing. The extent of recovery in the flexural strength was evaluated by complete desorption of water-saturated specimens. FTIR and DSC were conducted to study the changes in chemical bonding characteristics and glass transition temperature of GE composite due to above mentioned factors. Since actual environmental temperatures may not be static, it is important to investigate the effect of fluctuating temperatures on the durability of GE composites embedded with CNTs and FCNTs. To address this, the impact of repeated hydrothermal cycling on the out-of-plane durability of GE composites modified with CNTs and FCNTs, compared to control GE composites was assessed. Each HC consists of 24 hours of conditioning in a water bath maintained at 15 °C, followed by 24 hours of conditioning in a water bath maintained at 50 °C. Control GE, 0.1CNT-GE, and 0.1FCNT-GE were exposed to 20, 40, and 60 HC cycles to assess their durability. Initially, for a lower number of HC cycles, the 0.1FCNT-GE composite showed the best water resistance, followed by 0.1CNT-GE and GE composites. However, after 40 and 60 HC cycles, the trend changed. Flexural testing was conducted to assess the mechanical strength of these composites upon 20, 40, and 60 HC cycles. Additionally, the extent to which the flexural properties of the composites recovered after 60 HC cycles was evaluated by conducting a desorption process. The cyclic changes in water bath temperature caused expedited interfacial debonding at the weak CNT/matrix interface, resulting in accelerated water absorption and reduced flexural performance in 0.1CNT-GE composite compared to 0.1FCNT-GE composite. The water absorption by the composites had a detrimental effect on their Tg and the matrices' chemical bonding. Changes in the failure modes of these composites before cycling and after 60 HC cycles were compared using scanning electron microscopy (SEM) analysis. Conventional mechanical mixing techniques typically result in a random orientation of CNTs within GE composites. However, in order to effectively enhance the out-of-plane mechanical performance of these materials, it is beneficial to achieve through thickness CNTs alignment in composites to create a 3D reinforcement structure. To achieve this objective, an electric field alignment technique was utilized with CNTs and FCNTs in GE composites. The optimization process involved evaluating flexural properties at varying electric voltage and CNTs/FCNTs content. The use of aligned FCNTs in the GE composite showed the best performance, asconfirmed by a series of mechanical and thermo-mechanical tests, including flexural properties, interlaminar shear strength (ILSS), mode-I and II interlaminar fracture toughness (ILFT), and dynamic mechanical analyser (DMA). The key finding of this work includes an enhancement in the critical energy release rate under crack opening mode (i.e., GIC) as high as ~82% in the case of A-0.1FCNT-GE (0.1 wt.% FCNT aligned at 600V in GE composite) over the control GE composite. Fractographic studies reveal the evidence of CNT alignment along with various strengthening and toughening mechanisms responsible for the improved mechanical properties of the composites due to alignment and functionalization of CNTs. In continuation, the impact of in-situ environmental temperature on the out-of-plane durability of random and aligned CNT/FCNT modified GE composites was studied. For this reason, flexural and short beam shear (SBS) tests on neat and modified GE composites with random and aligned (at 600V) 0.1 wt.% of CNT/FCNT at both in-situ cryogenic and elevated (70℃) environmental temperatures (CT and ET). It was revealed that the well-being of the matrix and its interfaces with the reinforcement, i.e., fibers and nanotubes, was significantly influenced by the in-situ environmental temperature. The A-0.1FCNT-GE composite exhibited remarkable improvement, with an increase of approximately 41% and 47% in flexural strength, 29% and 35% in flexural modulus, and 30% and 24% in ILSS over the neat GE composite at CT and ET, respectively. The synergistic effect of functionalization and alignment of CNTs provided a much enhanced load-carrying capacity along the through-thickness direction which effectively prevented delamination at both CT and ET. To better understand the mode of failure, SEM was employed to observe the tested samples.