Recent Trends in the Chemical Modification of Flax Fiber for Polymer Composite Materials: A Critical Review

The dynamics of the polymer composite materials market demonstrate a remarkable rate of adoption of organic fiber-reinforced plastics in major industrial sectors, medicine, sports and household equipment, and household goods. Natural fibers, particularly flax fibers, are of practical interest because of their low cost, environmental friendliness, biodegradability, acceptable mechanical properties, and availability. The expanding use of flax in composite materials imposes new requirements on the quality of flax fiber products. This progress highlights the need for broader studies and deeper knowledge of the possibilities for modifying the physical and mechanical properties of reinforced polymers through fiber pretreatment and selection of treatment conditions with consideration of the required characteristics and operating conditions of composite products. The objective is to analyze recent literature, published mainly over the past three years, addressing the principal issues that hinder the widespread use of flax fiber in the composite industry. Preservation and rational utilization of the natural potential of technical flax fiber in long-fiber forms of reinforcing fillers eliminates the adverse effect of anisotropy in the geometric and mechanical properties of elementary flax fibers. We analyze experience in the use of chemical treatments to overcome the natural porosity and hydrophilicity of flax materials, identify unresolved problems, and search for effective biomimetic approaches to overcoming these difficulties. Information on the structure and functions of the object of biomimicry, the xylem of the flax stem, provides the key to developing moisture-resistant composite materials based on advanced two-stage molding methods using different types of binders.