Implication of CNT Fillers on Environmental Durability of GFRP Composites: An Evaluation of Microstructural Features and Mechanical Properties

Abstract

The excellent mechanical performance of the fiber reinforced polymer (FRP) composites in combination with its low density and corrosion resistance has made it a revolutionary material in the current structural world. However, the poor out of plane properties of these laminated FRP composites creates an interest for suitable modification of the matrix and/or interface by scientifically structured nanofillers. The paucity of structural defects in carbon nanotube (CNT) with unrivalled mechanical properties has always posed an interest to material scientists for its potential incorporation in soft polymer resins to achieve superior mechanical stability. But, the performance of these CNT modified FRP composites under various in-service environments must be well ensured before accepting these materials for different high end applications.

Present study starts with assessing the flexural behaviour of control glass fiber/epoxy (GE) and 0.3% multi-walled CNT (MWCNT) embedded GE (CNT-GE) composites at different in-service environmental temperatures. In-situ 3-point bend tests were performed on GE and CNT-GE composites at -80, -40, 20, 70 and 110 °C temperatures. The results revealed that at 110 °C temperature, the flexural strength of GE and CNT-GE composites was significantly decreased by 67% and 81% respectively in comparison to their strength at -80 °C temperature. Dynamic mechanical thermal analysis (DMTA) was carried out in the temperature range of -100 °C to 200 °C to correlate the mechanical and thermo-mechanical response of both the material systems. Addition of 0.3 wt. % MWCNT in GE composite resulted in lowering of glass transition temperature (Tg) by 12°C.

Further, emphasis was given on the elevated temperature durability of GE composites with various MWCNT contents (0, 0.1, 0.3 and 0.5 wt.% of epoxy). Flexural testing at room temperature revealed that addition of 0.1% MWCNT yielded maximum strength (+32.8% over control GE) and modulus (+11.5% over control GE) amongst all the composite systems. Further, all the CNT-GE composites resulted in accelerated degradation of mechanical performance with increasing temperature as compared to GE composite. 0.1% CNT-GE composite, which exhibited highest strength at room temperature, exhibited the poorest flexural performance at 110 °C. Fractographic analysis figured out various failure modes in all composites at various temperatures.

To evaluate the susceptibility of CNT-GE composites towards cryogenic shock, alteration in the flexural performance of GE composites with and without MWCNT was studied after liquid nitrogen conditioning for various time intervals. Decrease in strength and modulus was observed for short time span of liquid nitrogen conditioning and the rate of this decrement was higher for the CNT-GE composites. On the other hand, a longer conditioning time enhances the strength of CNT-GE composites more effectively than GE composite due to generation of clamping stress at CNT/epoxy interface.

The next objective is aimed to elucidate the temperature dependant reinforcement efficiency due to the chemical restructuring of the nano-filler/matrix interphase in CNT embedded GE composites. Carboxyl functionalized CNT embedded GE (FCNT-GE) composite exhibits 25% and 10% better strength than GE and unmodified (pristine) CNT embedded GE (UCNT-GE) composites respectively, at room temperature. Covalently bonded CNT/epoxy interface in oxidized nanotube modified epoxy matrix restricts the interfacial debonding to a better extent than pristine one upon excursion to elevated temperatures. On the contrary, mechanical gripping at the CNT/epoxy interface at lower temperatures maximizes its failure strength, resulting in an admirable structural material for low and cryogenic temperature applications. Thermo-mechanical properties and microscopic evidences of the interfaces at nanoscale (CNT/epoxy) and microscale (glass/epoxy) divulge the synergetic strengthening effect due to both chemical functionalization and low temperature environment.

The effect of diffusion temperature on the water uptake and subsequent degradation behaviour of GE and CNT-GE composite has also been studied. Presence of CNT in the GE composite significantly suppressed its water absorption propensity at lower temperature (25°C). It is attributed to the reduced free volume of polymer and hydrophobic nature of the CNTs, which forces the water molecules to follow a torturous extended path. However, CNT reinforcement in GE composite adversely affects its high temperature water resistance due to generation of unfavourable thermal and hygroscopic stresses at the CNT/polymer interfaces. Flexural testing of the water saturated samples revealed that diffused water exerts more detrimental effect on mechanical performance of CNT-GE composite than control GE composite. The extent of recovery in mechanical performance of the composites has also been evaluated after complete desorption of the water saturated samples.