Mechanism Of No-pickling Chrome Tannage And Its Related Problem

There exist some problems in traditional chrome tanning, such as low exhaustion of chrome, serious pollution of chlorine ion from pickling, and high BOD and COD of effluent produced by acid soluble and salt soluble proteins from pickling. In order to solve those problems, no-pickling chromitan C-2000, a new clean tanning agent, which can be used directly after bating with high exhaustion and leather with tight grain and fullness, was synthesized and produced in a leather chemical factory. But there are still some defects of this agent, which result in poor softness and deep color of leather. Since the conditions of pickling and no-pickling chrome tanning are different, the tanning effects of this two tanning agent are diversity. Generally, compared to the common chrome tanned leather, no-picking chrome tanned leather is thicker and has higher and evener chrome content, which indicates the difference of this two tanning mechanisms. The tanning mechanism of no-picking chrome tanning is an important part of chrome tanning mechanism, and up to now, the study on theory of no-pickling chrome tanning is incompletely and many problems have no enough evidences to be proved. So, it is necessary to study the mechanism of no-pickling chromitan through modern apparatus and new study methods. In this paper, the no-pickling chrome tanning mechanism and the coherent problems were studied from several aspects as following.1. In the preparation of no-pickling chromitan, glucose, catechin, gallic acid andSO2 were used as reducing material for bichromate. Their redox reaction kinetics was investigated by spectrum methods. The results indicate that the redox between glucose and Cr(Ⅵ) takes place mainly at the temperature of 100 □, while catechin, gallic acid and SO2 can be oxidized at room temperature. The reaction conditions such as temperature, acidity and concentration impact not only the reaction rate but also the composition of products. In the chrome tanning solution reduced by SO2, the introduced organic acid groups may have the capability to substitute some hydrones in the chrome complex and change its composition. The reduced product of gallic acid may be the mixture of some small organic acid, with fomate acid as main constitent, while the main product of glucose is acetic acid. The rate equation of kinetics about the redox between catechin and dichromate at room temperature can be formulated as: d[V(v)]/dt=k[Cr(Ⅵ)][cathechin]]/[H+]. But if the temperature is over 35 D, several primitive reactions may take place simultaneously.2. The solid and Cr2O3 content of no-pickling chromitan C-2000 used in this experiment are 92% and 20.7% respectively. Chrome tanning solution was investigated with ion exchange chromatogram. Results show that at the early stage of storing the chrome complexes are mainly neutral or negative and their charge may transfer into positive gradually. The Gel chromatography shows that during storage of tanning solution, the change of molecular weight is not obvious, but the distribution is more even.3. The molecular weight of collagen hydrolysate used as simulacrum was measured by electrophoresis method, and results show that the molecular weight of 78.79% collagen hydrolysate is less than 43KDa, the range from 43KDa to lOOKDa is 17.25%, and only 3.88% is more than 100kDa. Through experiments of circular dichroism and probability statistics of Fasman, the existence of α-helix structure in collagen hydrolysate is confirmed. At high pH, the amount of α-helix structure decreases, while β-fold structure increases.4. The regularities of no-pickling chromitan combined with gelatin, collagen hydrolysate, Type- D collagen and hide powder in different conditions weresystematically studied with IR-spectrum, UV-spectrum, and Fluorescent spectrum and so on. Moreover, the combination of groups at high pH was analyzed. Results show that the ideal pH for the interaction between no-pickling chromitan and gelatin is about 5.0. After the collagen hydrolysate is treated by no-pickling chromitan, the number of stable hydrogen bond in it reduces because the interaction between them is disadvantageous for the formation of hydrogen bonds. Moreover, the helical structure of collagen hydrolysate changes into random coil at high pH. With the addition of Cr2(SO4)3 and sodium formate, the fluorescent intensity of collagen hydrolysate is reduced at 315nm, and the reduction is more obvious at lower pH.5. The thermal alteration of modified hide powder whose hydrogen bonds were broken was studied before and after the no-pickling chrome tanning through differential scan calorimetry(DSC), and the effect of hydrogen bonds on the stabilization of collagen was analyzed. Results indicate that the transformation temperature of hide powder obviously reduces when it was tanned with no-pickling chromitan and then treated with urea.6. The interaction between gelatin and Cr(Ⅲ)compound in different conditions was investigated with Laser Sizer. some results are obtained as following: (1) there is a great difference of the particle size distribution between fresh C-2000 solution and C-2000 solution of pH 8.The former mainly focus at 58.7-78.8nm, and the latter moves to 78.8-531nm. (2) The particle size distribution of gelatin mainly focus at 8.7-825nm, however, after interaction with C-2000 for 4 hours, it moves to 1106-1484nm. (3) After gelatin has interacted with C-2000 for 0 hours, 2 hours and 4 hours, its zeta potential is -12mv, 8.9mv, and 9.28mv respectively. At 25℃, the pU of gelatin is 5.32, and its particle size and zeta potential both decrease with the reduction of pH; At 35℃ and 40℃, the pi is 6.61 and 7.00 respectively. (4) With the increase of concentration of Cr2(SO4)3 in gelatin solution, the size of gelatin particles increase gradually because of the bonding between Cr (□) and the external carboxyl groups of gelatin molecules.(5) In the course of acid base titration of Cr2(SO4)3 and gelatin solution, the range of fluctuation of size distribution by Z-average is from2200nm to 7800nm at 25℃, from 4700nm to 10000nm at 35℃ and from 4500nm to 15000nm at 40 ℃(6) the Zeta potential of gelatin solution decreases gradually with the increase of pH, and there is one pI of 7.27 at 25 ℃, and several pI at higher pH. (7) With the addition of C-2000, the Zeta potential of gelatin solution is equal to -5.5mv at the beginning, after 4 hours, it skips to 7mv. This phenomenon may be related to the plentiful external combination between the negative chromium complex and gelatin at the beginning.7. Goatskins were tanned with no-pickling chromitan in different conditions. Results show that the penetration and combination in tanning process almost reach balance during the first 2 hours. After 4 hours, the absorption rate of Cr2O3 can reach 87.41%; the bated skins have a high absorption of Cr2O3 at pH 7-8, however, the ratio of absorption will decrease at higher pH. Compared with the normal sample and deaminated leather, the shrinking temperature of esterified leather is very low. In the experiments of SEM, the intact fiber of deaminated wet blue is observed, and chrome stains can be seen clearly. Fibrous structure of esterified skin is not obvious, but it can absorb some no-pickling chromitan.8. The regularity of penetration and bonding were studied by UV spectrum, Chromatography, Electron Energy Spectrum and Differential Scan Calorimetry. Moreover, the tanning agents were eluted from leather by EDTA sodium salt to study the relative Cr2O3 content and the amount of combined acid. Results show that during the process of no-pickling tanning, there are an acid-alkali neutral course as well as the penetration and combination balance. At the beginning of tanning, pH of the middle layer of leather is higher than other layers. But the difference tends to disappear with the proceeding of tanning. The difference of pH between layers is slight after tanning for 2h to 3h, because the neutralization is balanced and most hydrolyzation and coordination finished. DSC of leather tanned for different times is varied. DSC peak values of different layers of Leather tanned for 0.5h are low and that of the middle layer is the lowest. With the increase of tanning time, peak values of the middle layer rise rapidly and the contrast of all layers become smaller andsmaller. That indicates that the bonding of no-pickling chromitain happened in hide from the inner to the surface. The molecular weight of chromitan increases gradually in tanning process especially during the prior 1.5h. During 3 to 6 hours the increasing will be continuous, but not so great. After alkalifying and heating, the increase of molecular weight is much obvious. The results of XPS show that the distribution of chrome in leather is even after tanning of 4h. Moreover, Ca2p was found in that leather sample, which may be the cause of flat leather.