Submitted to: Journal of American Leather Chemists Association
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/2/2009
Publication Date: 7/1/2009
Citation: Liu, C., Latona, N.P., Lee, J., Cooke, P.H. 2009. Microscopic observations of the leather looseness and its effects on mechanical properties. Journal of American Leather Chemists Association. 104(7):230-236. Interpretive Summary: Looseness is a very troublesome issue to tanners and results in the downgrading of the leather to be used in lower end products. Tanners go to great lengths to prevent loose leather from occurring in the tanning process. This investigation has gained a better understanding of the looseness effects on the fibrous structure and mechanical properties of concern to the leather industry, such as tensile strength, elongation, and toughness. Our observations showed that a separation gap exists between the interstitial layer (middle layer) and the corium layer (flesh side) in the loose leather. Also, the grain (hair side) appeared to be separated into sheet like structures. These sheet-like structures when bent will stack up on one another, thus providing the wrinkles on the grain surface and one characteristic of loose leather. In contrast, in the tight leather, the grain layer is tightly packed together and the fiber bundles seem to be much smaller than the loose leather. Mechanical property studies showed looseness leads to a decrease in toughness while there is little change in tensile strength and elongation. A stress relaxation experiment, on the other hand, showed loose leather has a greater initial stress than the tight leather, indicating a higher stiffness than tight leather. The results from this study have illustrated the loose structure and its resulting properties and provide the tanneries the insight on why they need to develop measures to correct this defect such as fine tuning the leather making processes, lowering the degree of fiber opening and adding fillers to the leather.
Technical Abstract: Looseness is a very troublesome issue regarding the structure of leather as well the final leather product and it is a great concern to the tannery to which tanners will go to great lengths to address by adding different chemicals. However, its fine structure and effects on the mechanical properties are not fully understood. This investigation is devoted to gaining a better understanding of looseness based on microscopic observations of its fibrous structure and its effects on mechanical properties such as tensile strength and toughness. Microscopic studies showed there was an evident gap visible between the interstitial layer and the corium layer in the loose leather. Moreover, the grain showed a sheet-like structure, whereas in normal leather, the grain layer is tightly packed together and the fiber bundles seem to be much smaller than the loose leather. Mechanical property studies showed looseness leads to a decrease in toughness. However, measurements also showed there is little change in tensile strength and elongation. Moreover, viscoelasticity studies showed loose leather has very similar stress relaxation curves as tight leather. Both leathers demonstrate a rapid decrease in stress for the first few seconds followed by a much slower decay thereafter. The loose leather, nevertheless, has a greater initial stress than the tight leather, indicating a higher stiffness than tight leather. This study has provided a better understanding about looseness, which could lead to developing measures to correct this defect such as adding fillers to the leather.