Role of Small Fractions of Graphene Nanofillers on the Mechanical and Thermal Properties of Functionalized Cellulose Fiber-Reinforced Unsaturated Polyester Composites

Cellulose fiber-based polymer composites are lightweight, eco-friendly, and inexpensive, attributes that make them potential candidates to be used in the automotive, packaging and aerospace industry. Their major drawback is low strength, mainly due to hydrophilic nature of fibers and poor interfacial bonding between fibers and polymers. This work aims to improve strength of cellulose fiber-reinforced unsaturated polyester (UPE) composites by functionalization of fibers and incorporation of small quantities of graphene nanofillers. Functionalized cellulose fibers (FCF) were produced by treating fibers with maleated high oleic sunflower oil (MHOSO) to improve dispersion of fibers in the UPE. Graphene fillers (Graphene oxide [GO], thermally reduced graphene oxide [TRGO], and thermally reduced graphene flakes [TRGF]) were mixed at a concentration of 0.1, 0.2, and 0.5 wt.% in the pure UPE and also in 3 wt.% FCF/UPE composites by three roll mill followed by mechanical stirring to produce single filler and hybrid composites, respectively. The tensile testing results showed that in the case of 0.2 wt.% GO/UPE composite, the tensile strength increased by 48% compared to pure UPE (from ~23 to ~34 MPa). Hybrid composites with 0.1 wt.% GO and 3 wt.% fibers exhibited ~30% higher strength compared to unhybrid FCF/UPE composites. The incorporation of graphene fillers resulted in rough fractured surfaces due to good interfacial bonding of graphene fillers with the matrix as examined by scanning electron microscope (SEM). TGA analysis revealed that all graphene fillers increased degradation temperature of the UPE from 445°C to 470°C. The addition of GO to the FCF/UPE hybrid composites increased their tensile strength, whereas the incorporation of TRGO and TRGF did not enhance the composite strength but improved their thermal properties. Graphene fillers' content higher than 0.1 wt.% in hybrid composites increased viscosity of resin, which leads to more porosity and filler agglomeration, resulting in reduced tensile strength.