Cracking grain15 October 2003
In use, leather can be subjected to considerable stresses, eg during lasting it is heated to temperatures which can be in excess of 100°C and stretched with considerable force.
If the grain layer is weakened in any way, this can result in the grain cracking open. Sometimes it is not the leather itself that cracks, but the finish on the surface which breaks up in use. Causes and prevention Cracky grain There are very many potential causes of weak cracky grain in leather. Some possibilities are: * High angle of weave of the fibre structure, either natural or process induced * Excessive removal of the corium resulting in a high ratio of grain to corium * Heat or chemical degradation of the collagen * Putrefaction * Over liming * Insufficient tannage * Inappropriate use of enzymes * Incorrectly adjusted machinery, eg samming, staking machine or during lasting * Insufficient penetration of fatliquor into the grain * Vacuum drying at too high a temperature Some of these possibilities are self explanatory and need no further discussion, whereas others are rather more complex issues and are explained in more detail. High angle of fibre weave Normally in bovine leathers, the collagen fibre bundles are orientated at an angle of approximately 45°C from the horizontal. If the angle of the fibre is very high, eg more than 75° from the horizontal, the fibres are unable to interweave with one another properly and the leather becomes weak (Figure 2). Occasionally, this can be a result of insufficient depletion of the hide with deliming. However, it is more commonly caused by a naturally occurring defect known as vertical fibre defect. This is a genetic defect affecting predominantly Hereford cattle and it causes the collagen fibres of the upper corium to have an almost vertical orientation with very little interweaving. When the hide is split to the required substance, the more normally orientated fibres of the lower corium are split away, just leaving the vertical ones. Unfortunately, it is not possible to determine which hides have the defect until the weakness becomes apparent. It has been estimated that 2% of UK hides, 13% of US hides and 23% of Australian hides are affected. High grain to corium ratio It is the large collagen fibres in the corium layer of a hide or skin that give leather its strength; the grain layer on its own is relatively weak. Therefore, if an excessive amount of the corium is removed by splitting or shaving then the leather becomes weak and the grain is more liable to crack when stressed. Ideally, the grain to corium ratio should not exceed 50% grain. Therefore, if a thin leather is required, it is best to select thin hides or skins since they will naturally have a thinner grain layer. Consequently, when they are split to substance, they will retain more corium to provide strength. Heat or chemical damage Severe thermal damage causes gelatinisation of the collagen resulting in a hard and brittle grain surface. Collagen is at its most vulnerable to heat damage when at extremes of pH. Therefore, at very high and low pH, eg during liming or pickling, the temperature should not exceed 30°C, otherwise the risk of damage to the grain surface is increased significantly. Strong acids generate a considerable amount of heat when they are diluted and are a common cause of heat/chemical damage resulting in cracking of the grain (Figure 3). Acids should be well diluted and allowed to cool before adding. Fatliquor penetration Fatliquors lubricate the leather fibres and allow them to move over one another when the leather is stretched in any way. Therefore, insufficient penetration of fatliquor into the grain results in brittleness and a tendency to crack. Possible causes of insufficient fatliquor penetration are: * Poor fatliquor emulsification due to: a) Unsuitable particle size of the fatliquor b) Poor emulsification technique c) Incorrect pH of float or leather * Incorrect temperature * Too much retannage * Inadequate mechanical action * Insufficient application time * Poor removal of the hair and/or epidermis Cracking finish Cracking of the finish on the surface of the leather can be caused by: * Application of too brittle a finish * Faulty application of foam finishes * Deterioration of the finish by inappropriate cleaning or accidental contamination of the leather in use Brittle finish Incorrect formulation of the finish can result in a brittle finish which is prone to cracking when flexed, especially at lower temperatures. This can be caused by incorrect ratio of binders to pigment and wax and fillers etc. Selecting too hard a resin or application of too thick a finish film will also result in crackiness. Foam finishes If large bubbles are present in the finish, they can open out when the finish film is stretched resulting in the finish surface breaking open and cracking. Drying foam finishes at too high a temperature can result in cracking and trap moisture inside the finish film. Medium drying temperatures and a longer drying tunnel are advisable for this type of finish. Contamination or cleaning Anything that will degrade the polymers used in the finish could lead to cracking of the finish. Work at BLC has shown that even some proprietary care products on the market for use on leather work very well on some leathers, but are not necessarily safe to use on all leather types. If a cleaning product is to be recommended, it is essential that it is tested for compatibility with leathers that it is being recommended for. Potentially, leather can become contaminated with a wide range of damaging substances in use, many of which have deleterious effects on the finish, eg nail varnish remover.