Feeling weak?24 April 2002
The problem One of the key advantages leather has over other materials is that it possesses great strength without losing its flexibility and porosity - few man made products can rival it! However, occasionally things go wrong and leather items fail in use; the leather trousers may burst at the knee in use, or the seat cushion may split open when sat upon (Figure 1). Tear strength is one of the key elements of most internationally recognised leather specifications. The cause Leather gets its strength from its unique and complex fibre structure - innumerable collagen fibrils twisted around each other to form fibril bundles, which in turn twist around each other to form fibres. These then twist together to form fibre bundles. The fibre bundles then interweave in a three dimensional manner (Figure 2), forming a fibre structure so cohesive it takes great force to pull it apart - unless something goes wrong. Common factors which can affect the strength of leather are: * High angle of weave of the fibre structure, either natural (vertical fibre) or process induced * Excessive removal of the corium, giving a high grain to corium ratio * Heat or chemical degradation of the collagen * Putrefaction * Over liming * Insufficient tannage * Inappropriate use of enzymes * Insufficient penetration of fatliquor * Over drying (especially vacuum drying) One of the most frequently occurring factors is the high angle of fibre weave, or vertical fibre. This can be process induced if the hides became particularly swollen in the lime and were not subsequently depleted when delimed. If this is the case, very large numbers of hides are likely to be affected from the production. However, it is much more common for the fault to be due to a genetic problem in the animal. Some breeds of beef cattle, particularly Herefords, can carry the gene for vertical fibre which causes a vertical orientation of the upper corium fibres. When the hide is split, the normal fibre structure of the lower corium is split away leaving only the vertical fibres. Because they do not interweave like normal fibres, the leather has relatively little strength. The problem is not so prevalent in UK hides because Hereford cattle are often crossed with dairy breeds, thus diluting the gene. However, in countries such as North America and Australia, where there are large herds of mainly Hereford beef cattle, the problem is much more prevalent. The proteolytic enzymes produced by bacteria can break the bonds between the collagen protein molecules, thus weakening them, resulting in the fibre structure losing some of its integrity and strength. Invariably, it is necessary to determine if a leather's weakness is due to putrefaction by using a microscope to see the bacteria. However, if putrefaction is advanced, there may be other tell-tale signs such as patches of grain damage or loose fibre structure. Also be aware that excessive use of enzymes in the process can produce similar features. In life, the grain layer's principal role is to grow hair. Therefore, not only does the grain layer contain many hair follicles, it also contains all the structures which are associated with them - sebaceous glands, sweat glands, elastin fibres and many small blood vessels which not only supply the hair follicles with blood but are also important for temperature regulation. As the grain layer is filled with so many structures, there is relatively little space left for the collagen fibres which give leather its strength. Consequently, the grain layer on its own has little strength compared with that of the corium layer. Particularly in the case of bovine leathers, if too much corium is removed to provide a grain leather of suitable substance, weakness will ensue. As a 'rule of thumb', if the total thickness comprises of much more than 50% grain, there is a risk of reducing the tear strength significantly (Figure 5). If a leather of thin substance is needed, it is best to select a thin hide as less corium will need to be removed to achieve the required thickness. An extra note of caution here; although it is the corium which provides leather with its strength, when using too thin a split, there may be insufficient space for complete interweaving of the fibre bundles, ie longer fibre bundles are cut through before they weave amongst other bundles. This results in failure due to the fibre bundles being pulled apart when force is applied. During the liming process, the collagen becomes very swollen, principally due to the osmotic uptake of water. In this turgid state, the bonds in the collagen molecule are under great strain (Figure 6), therefore, it takes relatively little effort to break them. Degradation of the collagen can occur at temperatures as low as 35°C at liming pH. An excess of alkali, or too much mechanical action during liming, can also disrupt the collagen fibre structure and cause weakness. Damage due to over-liming, be it too warm, too long or too strong, is characterised by a very open and 'woolly' fibre structure, particularly in the bellies. If the corium fibres are not lubricated sufficiently, they are more likely break under strain rather than flex and move over one another as they should. Such a lack of lubrication can arise if insufficient fatliquor is offered or lack of fatliquor penetration because of: a) Poor emulsification due to: unsuitable particle size of the fatliquor; poor emulsification technique; incorrect pH of float or leather b) Incorrect temperature c) Too much retannage d) Inadequate mechanical action e) Insufficient application time The cure Ensure that raw hides are well preserved; if there are any suspicions that they may have suffered any putrefaction, they may have hair slip, red heat or an offensive odour for example, they should be rejected or processed immediately to prevent further degradation. Choose your raw material carefully; use thin hides for thin leathers. Carefully monitor your liming process; keep running times to a minimum and vessel speeds slow. Check the temperature of incoming water with a thermometer at the inlet occasionally; don't always rely on the water delivery system's thermocouple, they are not always accurate. Do not allow limed hides to be exposed to ttmperatures much above 30°C, preferably keep it less than this. Use enzymes in accordance with the manufacturer's guidelines; remember that a small increase in concentration or temperature can result in a large increase in activity. Check that sufficient fatliquor is being used and that the penetration is adequate. The penetration of fatliquor can be monitored easily on undyed or pale coloured leather with Sudan Black1.