Such links increase the effect of the repulsive strengths between the chains and so repress the swelling 2,3,4 in the same way as aldehydes act 5. Many have an aromatic structure, which produces bipolar forms, useful for a stabilisation of the structure by crosslinking. These discussions leave out the role developed by the so-called 'non-swelling' acids. Water controls the ionic concentration and, therefore, the employment of sufficient water quantities in the pickling is significant. In fact, the water flow, or its outflow, between the inside and the outside of the hide may or may not produce this phenomenon. Water has a substantial role in the swelling. It is known that a notable increase of the acid concentration in the pickling reduces the swelling until it corresponds to the isoelectric point. If there is a considerable increase in the ionic concentration of the external solution, the same result can be reached by the employment of products different from sodium chloride. The hide will be ready for mineral tanning. When the hides are adequately dehydrated, the carboxylic functions are completely uncharged and the aminic sites will be present in the form of salts. The process is completed when the concentration inside and outside the fibrils results in perfect equilibrium. The dilution produces a great dissociation of the acid and the salt, whose ions will be forced to separate through the interfibrillar spaces to the aminic and carboxylic groups of the side chains. A transfer of water, from the inside towards the outside, is caused with evident dilution. Such an increase makes the external solution hypertonic along with the water contained inside the fibre structure. The addition in the bath of sodium chloride increases the ionic concentration of the external solution by increasing the osmotic tension. The presence of sodium chloride with the acid avoids the development of this destructive phenomenon. Under these conditions, osmotic pressures, up to 400 atms, are developed, which can cause the complete breaking of the bonds among the protofibrils (elevated state of swelling) leading to a serious destabilisation of the structure 1. Therefore, spaces are generated where the water quickly penetrates, producing the swelling. Repulsive forces are then produced within the structure which keep the polypeptide chains at a distance from one another. As a result, it unbalances the equilibrium in favour of the positive charges of the side-aminic sites. Acid treatment promotes the deactivation of negative charges on the carboxylic groups of the collagen side chains. Unfortunately, actual technology and knowledge do not allow us to avoid its use because of the irreversible damages which would be produced on the hides during pickling. The extreme solubility of common salt and its great inactivity to precipitation could ultimately compromise the tanning industry. The elimination of sodium chloride from wastewaters represents one of the most difficult problems which the tanner has to resolve.
#Pressure tanning hides skin
Ongoing investigations at SSIPMC are looking into applying the salt free method to hide and skin preservation. The results were confirmed by semi-industrial trials under tannery conditions. The resultant leathers showed chemical, physical and organoleptic characteristics equal to those of the standard. In this case, swelling was reduced and the samples were perfectly comparable with the control treated with sodium chloride. The leathers, after retanning, dyeing, fatliquoring and staking, present chemical, physical and organoleptic properties which are as good as standard pickled pelts.Īn improved alternative of the studied process is also shown together with the acids with small amounts of chromium salts. In fact, the resulting swelling is almost completely suppressed in the following phase of chrome tanning. The results show that this is possible to achieve. The possibility of carrying out the pickling process in the absence of sodium chloride has been investigated.
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