On aniline leather grain damage can result in shade differences and dull patches. On finished leather it can be the underlying cause of differences in gloss and penetration of impregnants. Some tanners say that the grain surface of hides is becoming more sensitive and more easily damaged. However, the suspicion is that it is more a case of production techniques are now more intensive and customers are becoming more discerning.
The cause
The grain surface can be damaged during the life of the animal e.g. scratches, scars and parasitic attack. There are also very many possible causes of grain damage occurring during the processing of the leather, the most common being:
* Putrefaction
* Too high temperature, particularly when the hide is at extremes of pH
* Excessive liming (too long, too warm or too strong)
* Excessive mechanical action, particularly when at high pH
* Inappropriate use of enzymes
These are the areas which will be discussed in this article.
As soon as the hide or skin is removed from the carcass the exposed flesh surface is susceptible to attack by bacteria. Unless this attack is controlled, the putrefactive enzymes produced by the bacteria can soon reach the grain surface. Putrefaction in hides and skins can occur due to:
* A delay between flaying and curing/processing
* Insufficient salting
* Poor penetration of the salt due to heavy fat/flesh deposits
* Insufficient draining of the salted hide or skin allowing liquors to pool and reduce salt concentrations
* Poor storage conditions of the raw salted hide or skin e.g. exposed to the elements allowing salt to be washed off or excessively warm conditions
* Prolonged storage, particularly under warm conditions
* Soaking the raw hides or skins without sufficient biocide protection
When the hide is in a swollen state, as it is during liming, the delicate grain surface is at its most vulnerable to high temperatures, chemical attack and abrasion. This is because the osmotic uptake of water by the collagen molecule forces the polypeptide molecules apart thus destabilising the links between them (Figure 3). In this destabilised state, it takes very little to cause the links to break resulting in grain damage. Work at BLC has found that the shrinkage temperature of collagen at liming pH can be less than 35°C.
Enzymes added during leather manufacture are used to partially digest certain components within the skin to aid processing eg some bating enzymes digest elastin to help produce a softer leather. However, these enzymes are not wholly specific towards their target, most will have some collagenase activity and quite easily begin to digest the grain surface if used in excess.
Prevention of grain damage
Minimise the risk of grain damage by ensuring that raw hides and skins are as good a quality as possible:
* Ensure that there is no delay between flaying and curing/processing
* Salt adequately – use around 30% of the hide weight for storage
* Allow proper drainage of the salted stock; it takes several days for the salt to draw out liquors after which the hides should be re-salted if necessary before folding onto pallets for storage
* Keep salted hides in cool, dry conditions away from direct sunlight if possible
* Process rawstock as quickly as possible and use up in order of arrival
If there are any suspect hides they should be moved to the front of the queue unless they can be returned to the supplier
* Use a biocide if your soaking procedure is more than 5 hours long, especially if elevated temperatures are used.
In the beamhouse tight process controls are imperative:
* Avoid excessive undis solved lime which is a gritty substance and could abrade the surface
* Avoid low floats; don’t leave the drum draining for longer than is necessary
* Run limed hides at slow speeds (less than 5 rpm) intermittently eg 5 minutes every half hour
* Minimise swelling by ensuring that there is not an excess of alkali present during liming. Add salt if necessary to suppress swelling, particularly during washing off
* Do not allow the temperature to exceed 30°C during liming or during the early part of deliming. Check the temperature inside the drum and incoming water periodically with a thermometer; do not always rely on the water system’s ther mocouple to be accurate
* Ensure that the vessel is turning when adding warm or concentrated liquors to ensure that they are quickly dispersed
* Pre-dilute pickling acid well and allow to cool thoroughly before adding