Mechanical and durability performance of hybrid flax fibres and graphene composites

Mechanical and durability performance of hybrid flax fibres and graphene composites

Interest into the research on natural fibre-reinforced polymer (NFRP) composites for different engineering applications has increased significantly because they are environmentally friendly and sustainable renewable materials. However, these NFRP composites degrade when used as outdoor applications due to several environmental conditioning factors including in-service elevated temperature and high moisture. Recently, nanoscale fillers are now being utilised to improve the mechanical and durability performance of these natural composites. Therefore, this study systematically investigated the physical, mechanical and durability properties of hybrid natural fibre composites produced by reinforcing epoxy resin with flax fibres and graphene nanoparticles in elevated temperature, high moisture and hygrothermal environments.

In the first study, hybrid flax composites with graphene at ratios of 0 %, 0.5 %, 1.0 % and 1.5 % by weight of the epoxy resin were prepared and their flexural and inter-laminar shear performance at different temperature conditions, i.e. 20 ℃ to 100 ℃ with increments of 20 °C was evaluated. The experimental results showed positive effect of graphene on the flexural and inter-laminar shear (ILSS) properties of hybrid flax composites with the maximum improvements were observed for 0.5 % graphene at room temperature by up to 62 % and 149 %, respectively. These improvements decreased with increasing graphene weight ratio due to filler agglomeration as observed in the scanning electron microscope (SEM). The mechanical properties of hybrid composites started to decrease with increasing temperature but are still significantly higher than those without graphene.

The second study investigated the degradation behaviour of hybrid composites in high moisture environment. Hybrid composites were immersed in water at room temperature for 1000, 2000, and 3000 hours and tested mechanically under flexural and ILSS loading. The moisture absorption and moisture diffusion of flax fibre-epoxy composites was found to be significantly reduce due to the graphene particles providing effective protection layers. While the mechanical properties were found to be affected by high moisture and exposure duration, the maximum retention of the flexural strength by 97 %, 92 % and 86 % and ILSS by 89 %, 84 % and 82 % was achieved in the hybrid composites with 0.5 % graphene for 1000, 2000 and 3000 hours, respectively. The failure mechanisms are also affected by the exposure to high moisture environment.

The combined effect of moisture and in-service elevated temperature on the long-term durability of hybrid composites was investigated as the last study. Hybrid composites were conditioned at a relative humidity of 98 % and a temperature of up to 60 °C for exposure durations up to 3000 hours. The results showed that the addition of graphene can minimise the adverse effects of hygrothermal environments on the flexural modulus, strength and ILSS of hybrid composites. However, the sensitivity of the flexural and ILSS properties of hybrid composites against in-service elevated temperature was much higher than the exposure duration. Arrhenius model predicted that hybrid composites can retain at least 57 % and 49 % of its flexural and interlaminar shear strength, respectively, after 100 years in service in hygrothermal environment at a temperature of 30 °C.

The results of this study provided a better understanding on the effect of graphene on the mechanical and durability properties of flax fibre-epoxy composites. The optimum ratio among the three tested ones is 0.5 % graphene by weight of the epoxy resin for maximum flexural and ILSS properties. These results also provided a useful guide for the natural fibre composite manufacturer on using additive manufacturing enhancing the long-term behaviour of such materials and be suitable for outdoor applications. It is recommended though to investigate other advance manufacturing process, in industrial scale, to scale up the outcome of this field of study.

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