Leather Fibre Structure and Strength
By A. Clare
Leather has for a long time been a popular material due to its unique properties and natural appearance. During industrialisation, the development of woven synthetic materials resulted in the replacement of leather in many applications. Synthetic materials began to rise due to their lower prices and ease of production, with many manufactured in a continuous production line. Despite these benefits, leather remains a superior material with properties unmatched by synthetic alternatives.
The basis of leather is animal skin – composed of an extracellular matrix fibrous collagen network. Collagen is a structure-forming protein; composed of three ɑ polypeptide chains in a triple-helix structure. The fibre network is composed of collagen fibrils which align next to each other to form fibril bundles. The fibril bundles are grouped together to form fibres (Figure 1), which sequentially forms fibre bundles. The fibre bundles then interweave to construct a highly cohesive 3-D network.
Figure 1 - Structure of a collagen fibre
The fibrous structure of leather has a density gradient from the grain (animal outer surface) to the reticular layer (reverse side). The coarse fibres in the central region (flesh and corium) provide high strength and are composed of type 1 collagen. The grain has a fine fibrous structure – formed by the deposition of a dense layer of mainly type 3 and 4 collagen. The fibres are randomly orientated perpendicular to the place of the skin; enabling the skin to near high strength resist strain in multiple directions.
Unlike leather, synthetic woven materials are composed of two fibres, (i.e, the warp and the weft) interlaced orthogonally in a consistent pattern to produce a sheet-like structure (Figure 2). The length of the structure consists of the warp, whereas the weft runs along the width of the material (intertwined around the warp). Woven materials do not have thick fibres and a hierarchy like that of the leather corium and, therefore, the structure is much weaker. the strength of leather materials is also greater due to the high level of interweaving between fibres. Knitted fabrics to go one step beyond weaving by using a knotting technique in the weave linking.
Figure 2 - Sheet-like structure of synthetic woven materials.
The strength of leather is improved further by linking during the tanning process. Linking involves the connection of collagen fibrils through chemical bonds, which enhance the structures strength and prevents the fibrils from moving apart. Furthermore, during fatliquoring, the leather fibres and fibrils are made slippery, which improves softness physical and mechanical properties.
Leather is admired for its unique properties, including high strength and elasticity, good water vapour permeability, abrasion resistance, durability and longevity. In woven materials, the fabric is not symmetrical and, therefore, the properties vary along the warp and the weft directions. To achieve the desired properties, the structure of woven materials often needs adapting. For example, water vapour-and air-permeability of woven fabrics depends on the porosity of the material (i.e., how tightly the structure is woven). Although woven materials can be adapted to achieve a desired property, this often results in other properties decreasing in performance. For example, increasing the thread density in woven materials increases the tear strength but reduces the tensile strength and wear properties.
Alternative synthetic materials may be of low cost and easily produced; however, their properties are incomparable to leather. The morphology and type of fibres used in woven synthetics significantly influence the materials mechanical properties. Although desired properties can be achieved with woven materials, this often results in a decrease in other characteristics. The complex fibrous structure of leather is unmatched by alternatives – resulting in a high strength material with no loss in flexibility and porosity.
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