Abstract:

Objective In order to solve the problems of keratin hair fibers damage and allergy caused by the oxidant and keratin hair colorants in the current routine of keratin hair fibers dyeing process, the keratin hair fibers were dyed in situ by laccase catalyzed polymerization of dopamine hydrochloride to achieve blackening effect, accompanied by ecological restoration, health and fastness features.
Method The in-situ dyeing process was performed in one step with 2.5 g/L dopamine hydrochloride, 0.9 U/mL laccase at 50 ℃ for 4 h. Factors, such as reaction time, temperature and laccase concentration, were investigated on the influence on determining the binding of pigment molecule to keratin hair fibers. Besides, the structure and properties of the dyed keratin hair fibers were characterized and analyzed by color parameter, single fiber strength, Fourier-transform infrared spectroscopy, scanning electron microscope, anti-UV and color fastness test.
Results To understand the effect of pre-bleaching on keratin hair fibers coloring, the untreated natural white keratin hair fibers and white keratin hair fibers pretreated with hydrogen peroxide were dyed (Fig. 5, Fig. 6). After the pre-processing, more binding points were exposed to facilitate the penetration of the pigment formed in the in-situ dyeing process into the keratin hair fibers, reaching a darker color and good fastness. The results of SEM and K/S value as well as color fastness showed that the dyeing process was more suitable to the pretreated white keratin hair fibers, owing to the weaken cuticle layer of pretreated white keratin hair fibers. After dyeing, the cuticle layer of the keratin hair fibers was not clear, attributing to the uniform and dense polydopamine coating formed by a reaction catalyzed by laccase covering on the surface of the keratin hair fibers (Fig. 7). Comparing to the IR spectra of keratin hair fibers before dyeing (Fig. 8), fibers after dyeing showed a broad peak at 3 273 cm^-1, indicating that there were polydopamine pigments on the surface of keratin hair fibers. Peaks at 1 634 and 2 921 cm^-1 could be ascribed to the stretching vibration of C=O and C—H, revealing that the coupling reaction occurred between benzene ring carbon radicals. On the other hand, the K/S value of keratin hair fibers after dyeing reached to 22.1, while the K/S value is 21.7 after treated with DMF. The slight variation indicated pigment combines with keratin hair fibers by covalent bonds. Therefore, the color fastness on rubbing stress under dry and wet conditions were very well and reached 4 and 3-4, respectively. The color fastness on washing can reaches 4-5. Besides, owing to the formation of polydopamine on the surface of the keratin hair fibers, the dyed keratin hair fibers exhibited excellent anti-ultraviolet property (UPF: 100⁺, Fig. 9). In addition, in order to test the mechanical robustness, the tensile strengths of non-dyed and dyed keratin hair fibers were reported with a 7.8 % increase, which results from the generation of polydopamine pigment. It is clear that the pigment plays a certain role in repairing the damaged spots of the bleached keratin hair fibers, further improving the strength of keratin hair fibers.
Conclusion Laccase was chosen to be a catalyst for the oxidative polymerization of dopamine hydrochloride to produce polymeric colorants for the dyeing of keratin fibers and shows a dark black color. The colored keratin hair fibers were characterized fully. Besides, the dyed keratin hair fibers exhibits excellent anti-ultraviolet property, color fastness on rubbing stress and washing, which can be attributed to the polydopamine coating and the strong covalent binding between polydopamine and keratin hair fibers. It is important to note that the dyed keratin hair fibers undergoing chemical modification remains the natural physical feature. The proposed eco-friendly enzymatic polymerization provides an efficient and green strategy for biological dyeing of keratin hair fibers. These good characteristics make the dyeing strategy a promising candidate for textile dyeing and even other field. Therefore, this study could lead to the successful development of biologically dyed materials with multiple functions.

Key words: keratin hair fiber, dyeing, laccase, catalysis, hydroxytyramine, repairing