Milling Effects on Mechanical Behaviors of Leather

Authors: Liu, Cheng Kung; Latona, Nicholas - Nick; Dimaio, Gary; Cooke, Peter

Submitted to: Journal of American Leather Chemists Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2007
Publication Date: 6/1/2007
Citation: Liu, C., Latona, N.P., Dimaio, G.L., Cooke, P.H. 2007. Milling Effects on Mechanical Behaviors of Leather. Journal of American Leather Chemists Association. 102(6):191-197.

Interpretive Summary: Milling is being practiced in the tannery to mechanically tumble and therefore soften leather for adequate stiffness and feel. However, there is no report regarding its effects on the structure change and physical properties of leather. This investigation is aimed to address these concerns by the leather industry. Our studies showed milling caused a significant decrease in the stiffness, but brought very little change in mechanical strength and toughness. Electron scanning microscopic observation showed that the milled leather has more of an opened fiber structure; the fibers are well separated from each other, whereas the non-milled samples show the fibers are still stuck together. The opened fiber structure is the key for gaining softness. The results of this research will benefit leather producers to gain a better understanding of the effects of milling and tools to monitor these effects. To consistently produce high quality leather, it is essential that the tanner understands the impact and effects of every mechanical operation in the leather-making process.

Technical Abstract: Milling is a key process to soften leather for adequate compliance. It is, however, still not well understood for its effects on the leather structure and mechanical properties. To consistently produce high quality leather, it is essential that the tanner understands the impact and effects of every mechanical operation in the leather-making process. This investigation is devoted to gain a better understanding of its effects on pliability and some other properties concerned by the leather industry, such as mechanical strength and toughness. Observation showed milling leads to a decrease in Young's modulus, consequently an improved compliance and softness. Data, however, also showed little change in tensile strength and toughness. Moreover, leather products in service are constantly being stretched. For understanding the mechanical behavior of leather products under cyclic stretching, we measured the energy loss (hysteresis) during the cyclic tensile tests using an advanced computing program. Data showed that hysteresis is the greatest for the first cycle; thereafter, the rest of hysteresis values are relatively unchanged. We also discovered that drum milling significantly decreases the hysteresis. This implies a structural change occurs during milling, resulting in a removal of the residual stress that was introduced during the leather making process.