Effects of chemical treatments on mechanical properties of unidirectional flax fiber reinforced high density polyethylene composites

Flax fiber based thermoplastic composites offer environmentally sustainable option for structural components. However, inadequate bonding between natural fibers and synthetic polymers, as well as their high tendency to absorb moisture, restricts the extensive use of these materials in large-scale manufacturing. High-density polyethylene (HDPE), widely used for its recyclability, chemical resistance, and toughness, offers a promising matrix for such composites. Although chemical surface treatments have been widely studied for flax fiber composites, limited attention has been given to their application in unidirectional flax fiber–reinforced HDPE systems. This study investigates the mechanical properties of unidirectional flax fiber reinforced HDPE composites under quasistatic loading by subjecting them to various chemical treatments. The chemical treatments applied to flax fibers encompass several commonly employed techniques used on natural fibers, including alkaline treatment, silane treatment, dicumyl peroxide treatment, and acetic anhydride treatment. Results show that alkaline treatment significantly enhanced both tensile strength (from 41 MPa to 59 MPa) and strain to failure (from 1.6% to 14.4%), though it reduced stiffness (from 5501 MPa to 1372 MPa). Silane treatment on alkaline pretreated mats partially restored stiffness (4563 MPa) but introduced interfacial brittleness, resulting in reduced tensile strength (48 MPa) and strain to failure (7%). Dicumyl peroxide (DCP) treatment yielded the poorest performance, with tensile strength reduced to 19 MPa, modulus to 1084 MPa, and strain to 8.8%. Acetic anhydride (ACAN) treatment led to moderate modulus recovery (2024 MPa) and improved ductility (10%), but tensile strength decreased to 24 MPa. The study provides comparative insights into the effect of chemical surface treatment on the mechanical response of unidirectional flax fiber–reinforced HDPE composites.