Abstract:

Objective The green dyeing and finishing method has attracted a lot of attention. At present, the main technology used for in situ dyeing is biological enzyme method. However, the biological enzyme is generally expensive and the preservation condition is harsh, so it is important to find a new way to achieve dye-free dyeing for textile. In this work, the in situ catalytic dyeing of wool fabric was carried out by using K₇[MnV₁₃O₃₈]·18H₂O as catalyst and caffeic acid as substrate. Besides, the polymer was applied for wool in-situ dyeing to achieve coloring effect.

Method The in-situ dyeing process was performed by one-step one-bath method. Factors such as reaction time, temperature, polyoxometalate concentration and pH value as well as the polyethylene glycol were studied for their effects on dyeing. Consequently, the dyeing property and structure of the dyed wool was characterized and analyzed by color parameter, color fastness test and fourier-transform infrared spectrum, scanning electron microscope, UV-vis spectrum, respectively.

Results Based on the K/S value, the optimum conditions are obtained, namely, 0.9 g caffeic acid, 0.9 g polyethylene glycol 1000, pH = 4.0 acetic acid-potassium acetate buffer, 0.19 g polyoxometalate with temperature of 50 ℃ and reaction time 5 h (Fig. 4-7). The infrared spectra of polyoxometalate, caffeic acid, precipitations and dyed wool after dyeing reaction are contrasted (Fig. 1). The peak at 958 cm^-1 and the peaks in the range of 800-500 cm^-1 for crystals indicate that the catalyst is synthesized successfully. After the reaction, a large amount of black precipitate was produced in the bottle. In contrast, the peaks in the range of 3 600-2 500 cm^-1 indicate that the caffeic acid is polymerized after the reaction, and the polymer has strong hydrogen bond, while the peaks at 1 264 cm^-1, 1 086 cm^-1 and 1 023 cm^-1 indicate the formation of ether bonds in polymer. Besides, there are broad peaks in the spectrum of dyed wool similar to precipitate in the range of 3 600-3 000 cm^-1, and small peaks at 1 210 cm^-1 and 1 084 cm^-1 indicate the existence of polymer on the fabric surface.

The UV-vis spectrum of solution with different reaction time is obtained (Fig. 2), showing a strong peak at 293 nm, which is assigned to R band of benzene ring. Besides, the absorbance reaches minimum at 5 h, indicating that the polymer load most on the fabric, which is consistent with the results of K/S value of the fabric. The appearance of wool fabric before and after reaction is measured by SEM and cross-section of fiber (Fig. 3), indicating the better dyeing effect of the method. In addition, the wool fabric has a deep dyeing depth and appears black (Fig. 8). The color fastnesses on rubbing stress under dry and wet conditions are very well and reach 5 and 4-5, respectively. The color fastness on washing can reach 3 (Tab. 2).

Conclusion Polyoxometalate is chosen to be a catalyst for the oxidative polymerization of caffeic acid to produce polymeric colorants for the dyeing of wool, showing a dark black color. The colored wool is characterized well. The influence of five factors, including pH, reaction time, temperature, polyoxometalate concentration and polyethylene glycol, is evaluated on the K/S value of dyed wool. Additionally, the dyed wool exhibits good color fastnesses on rubbing stress and washing, which can be attributed to the polycaffeic acid coating and the strong covalent binding between polymer and wool. Importantly, the dyed wool undergoing chemical reaction remains the apparent feature. These good characteristics make the dyeing strategy a promising candidate for textile dyeing. Therefore, this study could lead to the successful development of catalyst for dyeing process.

Key words: polyoxovanadate, caffeic acid, catalyst, oxidation reaction, dyeing, wool fabric