Enhancing properties of damaged hair through moisture control via construction of highly hydrophobic surface

Hair dyeing and perming processes damage could significantly reduce the stability and mechanical properties of keratin, making hair rough and fragile. Different hair repair ingredients have been developed by strengthening the chemical bonds inside hair. In this work, a new repair strategy of altering hair water content by construing highly hydrophobic surface was proposed. This was achieved by treating damaged hair with maleic-modified amino silicone oil (MASO), which was synthesized through the amidation reaction of maleic acid with poly[dimethylsiloxane-co-(3-aminopropyl)methylsiloxane] (ASO). Due to the thiol-Michael click reaction with free thiol groups on reduced hair surface, MASO presented high deposition efficiency at a low dosage of 0.1 wt%, which was confirmed by SEM and contact angle measurements. After MASO treatment, hair forms a highly hydrophobic surface with a contact angle of 137 °, significantly higher than that of virgin hair. The highly hydrophobic surface hinders the penetration of moisture into the hair, especially reducing the bound water content from 4.46 % to 2.81 %. This was jointly proven by the disappearance process of water droplets on hair strands, TGA testing, and DVS testing. The reduced water content in MASO-treated hair significantly lowered the combing work, with the wet combing work decreasing from 336.12 J to 43.52 J, giving a 87.05 % decrease, and enhanced the thermal stability of keratin, ultimately increasing Young's modulus and tensile strength by 13.99 % and 9.13 %, respectively. This finding demonstrates that the impact of water on tensile properties can even exceed the adverse effects caused by the breakage of disulfide bonds for reduce damaged hair.