Effect of Cyclic Stress while Being Dried on the Mechanical Properties and Thermostability of Leathers

Various mechanical processes are usually applied in leather making and using, which inevitably affect the structure and properties of leathers, such as mechanical performance and hydrothermal stability. Cyclic stress is very common in leather making, which may cause different changes to leather, compared with constant stress. However, very few results have been reported regarding the influence of cyclic stress on leathers. In the present work, cyclic stresses were applied to leather in drying. The influences of cyclic stress on the mechanical properties, hydrothermal stability and dry heat resistance of leathers were investigated. Also, the cross section of leather was observed with SEM, and the changes of hydrogen bonds in collagen fibers were characterized and discussed with the results of FT-IR. It was indicated that stress in drying leads to orientation of collagen fibers and increased mechanical strength. A balance is set up between tensile strength and elongation at break of leathers with the action cyclic stress. Longer stretching leads to higher tensile strength and lower elongation at break. Meanwhile, stress in drying may prevent the formation of hydrogen bonds inside collagen fibers and change the weaving structure of collagen fibers, resulting in decreased hydrothermal stability of leathers. Cyclic stress may provide leathers with better dry heat resistance than constant stress. Also, a simplified model of collagen fibers movement was introduced, to establish a relationship between processing and properties of leathers from viewpoint of collagen fiber structure.