Mechanism of High Chrome Uptake of Tanning Pickled Pelt by Carboxyl-Terminated Hyper-Branched Polymer Combination Chrome Tanning

Hyper-branched polymer (HBP) is extensively used in chrome tanning of leather as high chrome exhaustion auxiliary. In this paper, the promoting tanning effect of tanning pickled pelt is compared with different auxiliary, carboxyl-terminated HBP (CHBP-Cr), gallic acid (GA-Cr), and citric acid (CA-Cr), respectively. The results show that using of CHBP can endow the wet-blue leather up to 95.2% chrome uptake much higher than gallic acid (89.5%) and citric acid (79.3%) and control trial (69.3%). Furthermore, the thermal degradation stability and hydrothermal stability, which including denaturation temperature and enthalpy are also improved compared to blank trial. In order to explore the mechanism of CHBP acting in, the adding timing of CHBP, chrome concentration variation during the tanning process, the alkaline stability of CHBP as well as chrome distribution in wet-blue leather are also investigated. It is found that CHBP could expert its optimum effect when adding in the pickling process. In which, CHBP could act as excellent masking agent with good alkaline stability, which slow down the basification process and make the chrome dispersed evenly along the collagen. Moreover, the increased crosslinking density of collagen fibers could stabilize chrome and inhibit outward diffusion of chrome. These results lay a foundation of developing high exhausted chrome agent for clean chrome tanning.