Sustainable Biopolymer Based on Tung Oil and Tung Free Fatty Acids for Hair Tensile Enhancement

Tung oil (TO) and its derivatives, characterized by a high content of α-eleostearic acid, exhibit significant potential as renewable feedstocks for the development of sustainable biopolymers, particularly for structural keratin repair applications. In this study, Tung free fatty acids (TF) were generated in a one-step process from TO via a vortex fluidic device (VFD)-assisted saponification process, enabling rapid, solvent-minimal conversion with high efficiency while preserving the reactive conjugated triene system. TO, TF, and a hybrid TO-TF (1:1) copolymer were then subsequently formed under natural solar irradiation. Morphological analysis revealed that the TO-TF copolymer yields a homogeneous, conformal film, attributed to the steric modulating effect of flexible TF chains, which regulates excessive crosslinking and mitigates the brittle network formation characteristic of neat TO polymers. Structural analysis confirmed a chemically saturated surface in the copolymer, suggesting enhanced resistance to oxidative degradation. Differential scanning calorimetry revealed that the copolymer exhibited a higher post-curing transition temperature (165 °C) compared to the TO polymer (142 °C) and TF polymer (134 °C), indicating enhanced thermal network stability. Rheological measurements demonstrated improved viscoelastic performance, with a strain yield approximately seven times greater than that of the TF polymer, confirming a superior balance between flexibility and mechanical resilience. When formulated as an emulsion and applied to bleached hair under thermal styling conditions, the TO-TF (1:1) emulsion treatment produced uniform surface coverage and effective cuticle sealing on the hair surface, leading to significant mechanical recovery. Notably, yield stress increased from 89 to 124 MPa, and Young's modulus improved by over 40% without compromising fiber elongation at break. These results demonstrated that copolymerizing TO with VFD-based TF yields a structurally optimized, sustainable biopolymer capable of reinforcing damaged hair. The established structure-performance relationship highlights its potential for next-generation eco-friendly hair-repair applications.