Graphene/polyurethane nanocomposite coatings – Enhancing the mechanical properties and environmental resistance of natural fibers for masonry retrofitting

Natural fibers form economic and environmentally friendly substitutes for synthetic fibers in civil engineering composites. However, natural fibers present flaws, irregular morphology and reduced durability in alkaline media. The present work reports the application of a new nanocomposite-based coating for flax and hemp fibers that enhances their mechanical properties and environmental resistance. The nanocomposite coating is based on graphene nanoplatelets (GNPs) and a waterborne polyurethane (WPU). Nanocomposites were prepared with pristine and surface modified GNPs, the latter obtained by polydopamine self-polymerization, tailoring the surface chemistry for strong interfaces with polyurethane. The coated fibers demonstrate enhanced tensile properties and low water absorption. Hemp yarns present 120% and 163% increases in tensile strength and elastic modulus, respectively, compared to the as-received yarns. The yarn properties achieved are adequate for their application as reinforcement of brittle and hydrophilic matrices such as lime, thus setting a new milestone in natural fiber composites manufacturing.