Understanding the Effect of Sizing and Cellulose Nanocrystals on S2 Glass Fiber Sheet Molding Compounds

Abstract

Glass Fiber Reinforced Polymers (GFRPs) are increasingly more ubiquitous as research continuously pushes the limits of their mechanical performance. Excellent mechanical properties and low density coupled with increased ease of processibility are what distinguishes GFRPs over metallic materials. Emphasis has been given mainly to E-glass fibers as their low cost drove their demand. This work focuses on the more structurally graded S2-glass fibers which still are not performing as carbon fibers in terms of modulus, strength, or density, but they are seen as an alternative to E-glass fibers when carbon fibers are not an option. The question of interest is whether the performance of S2 glass fibers structural composites has reached its plateau or if there is still opportunity for improvement. This work seeks to investigative the potential of modifying glass fiber surface finish (sizing), and the addition of cellulose nanocrystals (CNCs) to improve the mechanical properties of S2-GFRPs. S2-GF/polyester SMC composites with three different types of glass fiber sizing were compared in terms of tensile, flexural and impact properties. The three types of sizing were characterized microscopically and using spectroscopic techniques, and differences in their chemistry (functional groups) were investigated. It was found that the tensile and flexural strength increased with increased conformability of the sizing film on the fibers, with the most conforming sizing resulting in composites with around 30% improvement in tensile and flexural strength, followed by the second best which exhibited around 20% improvement in tensile and flexural modulus, over the composite with the less conforming sizing film. No significant differences in the functional groups of the sizings were identified. This behavior is attributed to the enhanced interfacial interactions between the sizing and the fiber surface in the more conforming sizing films. The effect of CNCs on the mechanical properties of S2-GFRPs was marginal. However, the Tg of the composite increased ~ 6°C with the addition of 5 PHR CNCs due to the nanoparticles restricting the matrix polymer chains, and this was indicative of increased thermal stability in S2-GFRPs.