Effect of sodium chloride on structure of collagen fiber network in pickling and tanning

Tannery wastewater usually contains a high salinity due to the use of sodium chloride (NaCl) in curing and pickling. Although some no-pickle tanning and salt-free pickling technologies were developed, few of them have been widely used due to relatively poor mechanical and bulk properties of the resultant leather. Therefore, the role of NaCl in pickling and tanning should be elucidated in the first place. In this paper, the hierarchical structures of collagen fiber network were observed by optical microscope and SEM, and the porosity of leather was measured. The bated pelts were pickled with salt-free pickling auxiliary and NaCl-acid, respectively. Then they were tanned by chrome tanning agent. The results suggested that the space among collagen fibers of pickled pelts became bigger, and fiber bundles were separated from others in the presence of NaCl. Although both tanned leathers achieved a regular arrangement of collagen fibers, the fibers of chrome tanned leather pickled with salt were dispersed better, and the porosity of the leather was higher. Moreover, the role of NaCl in organic tanning (using an amphoteric organic tanning agent) was investigated. The results also showed that NaCl used in tanning could improve the opening up of collagen fiber network and the porosity of the leather. These effects were probably due to the dehydration of collagen fiber. In general, NaCl used in leather processing presented a positive effect in leather quality.