Abstract
The application and mechanism study of microwave irradiation in traditional industries have attracted considerable attention owing to the unique thermal and athermal effects that could lead to unexpected benefits in high-efficiency and clean production. Herein, we report the investigation of the aluminum tanning under microwave irradiation upon using hide powder and skin pieces, respectively, as simulants of real hide or skin. The aluminum tanning process and the tanned products under microwave heating (MWH) were studied and compared with those of conventional water bath heating (WBH) as the controls. For the tanning system of hide powder, the tanning effluents were analyzed in terms of pH, conductivity, dielectric constant and aluminum content, and the tanned powder was investigated by differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, and FT-IR spectroscopy. For the skin piece system, the pH and aluminum content of tanning effluents were also determined, and at the same time, DSC, TG, SEM, FT-IR and shrinkage temperature were used to illustrate the actions of microwaves on the structure and properties of the tanned pieces. The results show that aluminum reactivity in the penetration and binding process of collagen fibers in hide powder and skin pieces improved using microwave treatment. The residual aluminum content was greatly reduced by microwave heating action, and the increased amount of aluminum with evener distribution was observed in the tanned products. Microwave irradiation also resulted in the tanned products with better thermal stability and thermal decomposition resistance. This work further promotes application of microwave treatments for aluminum-based tanning in leather industry.Graphical abstract
Highlights
Tanning is regarded as the most crucial process for converting raw hide into leather through crosslinking reactions between the reactive groups of collagen fibers and tanning agents, and this process improves the thermal and structural stability of hide collagen matrix [1]
3.1 Effect of microwave irradiation on the aluminum tanning of hide powder 3.1.1 Analysis of tanning effluents During the aluminum tanning process, the hydrolysis and olation of aluminum complexes take place, which leads to the increase of hydrogen ion (H+) and free charged ions in the tanning system, resulting of uneven charge distribution in the complex molecules [1]
With the increase of the reaction temperature, the decreased tendency of pH was observed for both microwave heating (MWH) and water bath heating (WBH), which is attributed to the improved hydrolysis and olation of the aluminum complexes
Summary
Tanning is regarded as the most crucial process for converting raw hide (or skin) into leather through crosslinking reactions between the reactive groups of collagen fibers and tanning agents, and this process improves the thermal and structural stability of hide collagen matrix [1]. Compared to heating using a water bath, the shrinking temperature was increased by about 1 °C, and the content of macromolecular tannin and phenol components in the tanning liquid was decreased The investigations of these authors confirm that microwave heating promotes the penetration of plant tannins into the skin and the binding to collagen fibers. For the skin piece system, the pH and aluminum content of tanning effluents were determined, and at the same time, DSC, TG, SEM, FT-IR and shrinkage temperature (Ts) were used to illustrate the action of microwave on the structure and properties of tanned pieces These analysis methods present a great benefit to provide information on aluminum tanning towards cleaning production and better leather performance with microwaves
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