Optically Transparent Flax Fiber Reinforced Composite Materials

Abstract

Transparent Natural Fiber Composite Materials may pose a more environmentally friendly alternative regarding energy efficiency and low CO2 emissions in contrast to traditional composites, exhibiting transparency for future smart building or photovoltaic applications. The present study involved a research effort for the development of a lightweight flax fiber-reinforced composite material, which will exemplify high optical transparency. A detailed focus is conducted, on minimizing light absorption phenomena with delignification methods of flax fibers, eliminating light scattering via a Refractive Index Matching procedure between the reinforcement and the polymer matrix and via fiber modifications which tackle several issues such as enhancing the imperfect interphase between the hydrophilic reinforcement and the hydrophobic matrix, or providing finer polymer impregnation of the fibers. Acetylation, nitric acid, or TEMPO-oxidation treatments that modify the fiber’s nature and morphology are implemented to tackle these obstacles. Transparent Flax Fiber Reinforced Composites of 35-45 % fiber volume fraction were fabricated with wet-lamination and hot press forming techniques, indicating a maximum total light transmittance of approximately 70% (at low thickness) and 56-58% (at high thickness), depending on the fiber modification implemented. Regarding mechanical properties, a maximum specific flexural strength of 153 MPa/(g/(cm3)) and a specific flexural stiffness of 11.95 GPa/(g/(cm3)) was achieved for these materials. The transparent FFCMs showed comparable values of total light transmission correlated to reference materials like GFRPs or other natural fiber-reinforced composite materials, with similar volume fractions, signifying the success of achieving high optical transparency.