Coir/glass hybrid fiber reinforced thermoset polymer composite laminates with room-temperature self-healing and shape memory functions

Abstract

Natural fibers have a lot of potentials as renewable reinforcements for polymers. However, their lower load-carrying capacity compared with synthetic fibers made it a challenge to use them as polymer reinforcements. When a laminated composite beam is bent, the normal stress is the highest on the top and bottom layers and zero in the midplane due to the (non)linear distribution of normal strain. Even though the shear stress is the highest in the midplane, most polymers can tolerate the high shear stress. Therefore, hybrid composite laminates with strong synthetic glass fibers stacked over the core of weak natural coir fiber reinforcement have been designed. Even using a significant portion of coir fibers in the composites, the samples with hybrid fiber reinforcement showed almost similar mechanical properties compared with the samples with glass fiber reinforcement alone. Delamination and matrix crack induced by low velocity impact inside the composite panels were repeatedly healed at room temperature due to the self-healing capability of the diglycidyl 1,2-cyclohexanedicarboxylate (DCN) and polyethylenimine (PEI) polymer matrix. The healing efficiency was up to 99% for the first healing cycle and 72% for the fourth healing cycle. The composites also had good shape memory properties, with a shape fixity ratio up to 51% and a shape recovery ratio up to 93%, which were well maintained even after the several damage/healing cycles of the composites. This study opens new opportunities for applications of natural fibers in multifunctional load-bearing composites.