Abstract:

Objective Down is a renewable natural keratin fiber material with abundant resources. However, down is difficult to be biodegraded in natural conditions, and a large amount of waste down causes serious environmental problems. At present, the technology based on chemical and physical extraction faces challenges such as low one-time dissolution rate and unstable process. Therefore, it is important to improve the one-time dissolution rate of down. Based on application of lactic acid/cysteine deep eutectic solvent in dissolution of down, this study is to evaluate the dissolution process and working mechanism of down waste.

Method Down samples were washed to remove floating dust from the surface and dried until the mass was constant. Then, the samples were added into the lactic acid/cysteine eutectic solvent and dissolved assisted by magnetic stirring at 105 ℃. The dissolution solutions were extracted at different times and the morphological changes and spectral characteristics of the dissolved solutions were evaluated by ultra-depth of field three-dimensional microscope and ultraviolet visible spectrophotometer. The dissolved solution was poured into the dialysis bags for treatment at room temperature. The deionized water was replaced at an interval of 5 h, and the dialysis lasted for 72 h. When the conductivity of the dialysis water was reduced to less than 1.2 μS/cm, the down keratin powder was prepared by freeze-drying method from keratin solution. Down keratin obtained by dialysis and freeze-drying processes were analyzed and examined carefully by using high performance liquid chromatography (HPLC), Fourier transform infrared spectrometer and X-ray diffractometer to detect the molecular weight distribution, chemical composition and aggregation structure of the prepared samples.

Results The prepared lactic acid/cysteine deep eutectic solvent was a transparent liquid with a certain apparent viscosity. After placing it in dark environment for 10 h, the color and apparent viscosity of the solution did not show significant change and obtained material was exhibiting excellent stability. The grinding down fibers were immersed in a lactic acid/cysteine deep eutectic solvent and dissolved at 105 ℃. The down samples were gradually dissolved with the extension of dissolution time, and the color of the deep eutectic solvent changed to yellow while the transparency decreased (Fig. 2). The down sample was completely dissolved after 7 h, during which the down fibers were firstly swollen and then fractured until completely dissolved (Fig. 3). Furthermore, the deep eutectic solvent for dissolving down waste exhibited characteristic absorption at 371 nm, and the absorption intensity gradually increased with the extension of dissolution time of the down samples (Fig. 4). According to the high performance liquid chromatography test, the molecular weight of the down keratin was small, showing highest molecular weight as only 11 309 with a mass ratio of only 0.38%, the amount of keratin with molecular weight distribution from 1 470 to 6 522 u accounted for 57.93%, and the fraction with molecular weight less than 750 u reached 41.69% (Fig. 5). Characterizations using X-ray diffractometer and Fourier transform infrared spectrometer showed that compared with down fibers, the β-sheet diffraction peak intensity of down keratin was enhanced, the α-helix structure diffraction peak intensity was decreased, and no disulfide bond characteristic absorption peak was observed (Fig. 6 and Fig. 7).

Conclusion The research confirmed that the prepared lactic acid/cysteine deep eutectic solvent is a transparent viscous liquid with stable viscosity. The dissolved solution can be completely dissolved in a short time. Down keratin was prepared by dialysis and freeze-drying techniques, and HPLC test showed that the molecular weight of down keratin was small. Compared with down fibers, the β-sheet diffraction peak intensity of down keratin was enhanced, the α-helix structure diffraction peak intensity was decreased, exhibiting no characteristic absorption peak of disulfide bond. The lactic acid component of lactic acid/cysteine deep eutectic solvent demonstrated a good softening, expansion and penetration effects on the cuticle, which promotes the expansion of down fibers. At the same time, the strong polarity of deep eutectic solvent breaks the hydrogen bond between the molecular chains of keratin, and disulfide bond breaks under cysteine reduction. The dissolution of down mainly includes fiber expansion, fiber stripping, fiber fracture and dissolution.

Key words: down, deep eutectic solvent, lactic acid, cysteine, extraction process, down keratin, dissolution mechanism