The mechanical properties of leather in relation to softness

The deformation behaviour of chrome tanned crust leather (with or without fatliquor) and finished leather has been measured in three-point bend, tensile and compression tests. Relations between the experimental results and the perceived softness of the leather were investigated. The instrumentally assessed softness of a set of leathers was represented by parameters obtained from these tests and compared with a quantified subjective assessment. Differential Scanning Calorimetry (DSC) was used to analyse the water status in crust leathers. The influence of water content prior to staking on its softening effect was investigated. An orthogonal experiment was applied to investigate the influence of various factors (e.g. amplitude and frequency) on the softening effect of mechanical action to obtain an optimum combination of these factors. The bending force, F, increased in a linear way with increasing sample width, w, in accordance with the standard theory of beam bending. However the change in F with increasing sample thickness, t, departed (markedly in some cases) from the standard theory. The theory of laminated materials offered a quantitative description of the dependency provided that the grain and corium layers were regarded as the laminae. F was approximately proportional to the actual bending length raised to a power between -2 and -3. An empirical equation based on the results of the bend testing was developed. A parameter, Se/tn, can be used to rank the softness of leather, where Se is equivalent to the bending stiffness in standard bending theory, and n is the approximate index in the power function describing the relationship of the bending force F to the sample thickness t for a particular type of leather. A model describing the compression behaviour of crust leather is proposed which involves four stages of compression: (i) hair-like fibres being bent flat onto the surface and buckling of vertical fibre bundles, (ii) closing up of voids between fibre bundles, (iii) closing up of voids between fibres, and (iv) compressing of solid fibres. DSC results indicate that water has a stronger interaction with collagen molecules in an unfatliquored leather than in a fatliquored one. Fatliquor appears to function as a shield between water and collagen. The influence of water content prior to staking on the resulting softening effect was clearly reflected in bending and tensile tests, but was not nearly so evident in a compression test. It is shown that a maximum softening effect can be obtained in the staking process when moisture content is about 30-40%.