The Italian word for beamhouse, riviera, hints at the need for plenty of water during this leather processing phase. The goal is simple: to remove the hairs and free the dermis of all the superfluous material that could create or constitute defects in the finished leather. On the other hand, the chemical and physical phenomena that achieve this goal and quite literally set the quality of the leather are very complex. Given the volumes of water at play and the amount of organic and inorganic material released into the environment, this is the part of the tanning process with the greatest environmental impact.
In this article, we wish to point out the environmental impact of the various processing methods used and how they alter the organoleptic characteristics of leather.
Preservation under salt involves the abundant use of an electrolyte that is difficult to recover from the waste water resulting from the tanning process. In addition to performing a dehydrating action, salt roughly halves the globular protein content of a hide and also eliminates part of its natural fat, thus emptying it out and leaving it more relaxed than a fresh hide.
Using a fresh raw hide has the advantage of not using excess salt in the process, although it is also true that the use of up to 3% of sodium chloride for rehydration purposes favours the attack on soluble proteins. Fresh hides are basically more intact and therefore fuller than salted hides.
For logistical and commercial reasons, working with fresh hides alone does not provide a universal solution to the problem of using excess salt. Moreover, cooling to 4-5°C requires a considerable amount of power.
Soaking of fresh and salted hides
Rehydration must always be complete and must compensate for any problems due to poor preservation or unsuitable storage. The aim is to restore the natural hydration and smoothness of the fibres, thus benefiting from a mechanically durable and uniform dermis in its final appearance.
The working conditions in the soaking of fresh and salted hides are different and can be summarised as follows. In the case of salted hides, first they need to be desalted, the dirt must be washed off and the density of the water must be reduced to at least 3 Bè° in order not to hinder the enzymatic action. The aim is to rehydrate a material whose water content has been decreased by as much as 20% and whose various interfibrillary substances (albumin, globulin, proteoglycans, etc.) have been removed by at least 50%. Enzymes are widely used to accelerate the process, in addition to the conventional synthetic surfactants.
In the case of fresh hides, first they should be brought back to the working temperature. Their average physiological water content (about 65%) is still present, as are their interfibrillary substances. With the use of specific enzymes, these are rapidly removed, thus obtaining a condition similar to that of hides preserved under salted.
Bactericides are recommended to ensure effective protection in cases of poor preservation. In daily practice, they are absolutely essential. However, hypothetically, in a situation of ideal preservation, the fact of maintaining the working pH above 9 and the temperature of the water in the drum between 20 and 25°C – if necessary, cooling the bath every now and then – results in an adequate bacteriostatic effect. This operating condition also allows the liming pH to be approached gradually, while saponifying part of the fat of the connective tissue.
While fleshing after soaking rather than only after liming could be considered double the work, it actually offers significant benefits. Compared to the residual fleshing left after liming, this residual fleshing is a cleaner by-product and is suitable for other industrial uses as a source of fat and protein. The presence of traces of bactericide is tolerable, but use of dangerous chemicals should be avoided at all costs up to this point. It is particularly easy to remove the subcutaneous layer if the hide is treated with appropriate enzymes that make said layer gelatinous and improve its resistance to the blades of the fleshing machine. Reducing the pollution load results in a significant decrease (up to 20%) of the bath’s polluting potential, with a consequent reduction in the chemical products used. No less negligible from a quality perspective is the improved softness of the hide, which gains slightly in terms of square footage and in terms of easier and more uniform product penetration on the flesh side.
Unhairing and liming
It is a well-known fact that this is the beamhouse phase with the greatest environmental impact in terms of water consumption and pollution load, due to a high COD and the presence of sulphides, lime and possibly other chemicals. Traditional liming, using lime and sulphides, adopts a number of consolidated process variants, whose effects on the environment we will described briefly in this article, along with the resulting changes to the hide.
Let us assume that with hair-destroying liming, the COD settles at around 50-55,000 mg/litre. With immunisation and hair recovery, this figure can be lowered to 24-25,000, with the amount of sulphide and sulphydrate required being approximately halved. To obtain these values, removed hairs are immediately and continuously filtered. If filtration of a bath takes place the following day, the COD rises to around 37-40,000 mg/litre.
In any case, the organic load has a relative weight that is subordinate to the correct operation of the relevant purification plant. In general, considering all the polluting substances involved in the process, the parameters relating to tannery discharges are relative and depend on the regulations in force in the area where the company operates.
Hair accounts for approximately 6 to 8% of the weight of raw hide and is potentially an excellent raw material for other industrial uses. Exploiting its high keratin content or its fibrous structure, whose quality is inversely proportional to the damage caused by reducing agents, it may be treated and reused in agriculture, in cosmetics, in the construction industry and in textiles.
In industrial practice, unhairing through the use of enzymes alone does not guarantee total hair removal, and in the long run risks damaging the dermis. The use of sulphides ensures the desired unhairing effect, but makes the hair less attractive for further uses. A hybrid process involving enzymes followed by sulphides would allow the recovery of up to 90% of “clean” hairs and would preserve the quality of the dermis. Unhairing would then be completed by using a minimal amount of sulphides, with minimum by-product waste.
Worthy of note is what is known as oxidative unhairing which uses hydrogen peroxide and soda instead of lime and sulphides, making the process decidedly more ecological. While with this system the hides are absolutely clean and easy to process, it is more difficult to manage swelling, and this often has a negative impact on softness and accentuates the overall wrinkly effect.
At present, the issue of reusing hair is not particularly felt, and its commercial recovery is not encouraged. One of the reasons for this is the ambiguity in its classification as waste or by-product, therefore the legal position of those who produce it is not clear.
Technical aspects of liming
In order to obtain leather having the characteristics required by the market, the fibrous tissue needs to be loosened using different methods depending on the original breed. Alkaline swelling removes unnecessary substances such as non-collagen proteins (which glue the fibres together and hinder their loosening) and part of the natural fats, such as waxes, phospholipids and triglycerides (which can give rise to surface efflorescence or rancidity). The dermis’ reactive sites should be cleared in order to be able to insert chemical tanning and retanning products. Moreover, the epidermal layer also needs to be removed in order to obtain a dyeable grain that is permeable to chemicals.
Simultaneously, disulphide bonds are attacked in order to remove the hair or obtain its complete solubilisation. The maximum reducing potential is active at a very alkaline pH, with hide swelling reaching its optimal peak between 12.5 and 12.7.
At this pH, without the simultaneous action of sulphide, the cysteine bonds are transformed into lanthionine, making the hair immune to any following attack by reducing agents.
Dermal swelling, hair immunisation and hair removal all occur upon raising the pH. Their different speed of reaction is exploited in order to obtain the best quality result. It must be said that the paddle does not have the physical effect of compression that the drum has, and therefore the process is much easier to manage.
Dermal swelling is connected with the amount of water in the bath in relation to the mechanical action exerted. The intensity of the mechanical action exerted, and hence the speed of swelling, is variable and depends on the loading conditions and on the drum’s speed of rotation.
Under the action of reducing agents, hair removal takes about an hour. The first part of the hair to give way is the root. In the long run, the hair can solubilise completely.
Hair immunisation, instead, takes up to 6 hours. Once this time has elapsed, the hair can no longer be attacked by chemical agents. Hair removal can be successfully achieved, either with the recovery or the destruction of the hair, in a time that is substantially shorter than that required for immunization.
In an alkaline environment, the hide is negatively charged, and the collagen molecules repel each other. By moving away from one another, they favour the absorption of water which, at a pH of 12.5, can increase by as much as 30%. Swelling causes the hide fibres to become taut. If this does not occur uniformly and gradually, the grain can be marked by looseness or accentuated unevenness. Moreover, any neck and abdomen wrinkles that are not smoothed out at this stage will remain visible until the end of the entire working process, thus penalising the quality of the material. In this state, the hide is very delicate, and the grain can easily be damaged due to mechanical rubbing and high temperatures. The collagen shrinkage temperature is at its lowest: all it takes is 35°C to make the fibres irreversibly gelatinous. The amount of water present must prevent friction, and the temperature must remain below a precautionary 28°C. If swelling exceeds the limit required for successful completion of the process, the structure will break open; in addition to detaching the grain, this also causes a drastic deterioration in mechanical resistance.
Sulphide and sulphydrate
Apart from lime, which is always used in excess, the combination of sulphide and sulphydrate sets the type of liming and the finished article. Just 3%, deriving from the combination of the two, is enough to solubilise the hair completely, with the sulphydrate quota ranging from 0 to 1.5%. Hair recovery requires lower quantities. Sulphide and sulphydrate cause the fibres to swell, thus changing the physical state of the pelt already subject to alkali swelling. Their respective quota changes based on the raw hide to be processed and the organoleptic characteristics of the finished article required. The parameter that ultimately defines the physical characteristics of swelling is turgescence, which is assessed empirically.
Excess sulphide leads to the formation of caustic soda, which causes more swelling than is necessary. Sulphide causes greater hide turgescence than sulphydrate, which is less alkaline. This translates into a crust that is fuller, rounder and less relaxed. In principle, these are the features required for the footwear and automotive industries. Conversely, by decreasing the level of sulphide to the partial advantage of sulphydrate, the liming process results in softer, spongier hides that are more suitable for furniture and soft leather goods.
Effects of auxiliaries
Successful liming requires just a few but effective auxiliaries.
Anti-wrinkle agents: Such auxiliaries tend to quickly remove the soluble proteins. The tension that hinders fibres from spacing out is decreased and the dermal structure, therefore, tends to swell more evenly. Based on their specific formulation, they can even slow the swelling down.
When working in drums, it is not advisable to aim for zeroing the TKN brought by these auxiliaries in order not to penalise the quality of the hides. One solution would be to recover part of the old liming bath. To prevent excessive differences in the composition between the original bath and the following ones, the recovery can be done over a few cycles.
Organic reducing agents: These aid the unhairing process and improve the superficial cleanliness of the grain. They achieve optimal results, levelling out the typical differences in the raw hide mantle and only slightly increasing the pollution load.
Emulsifiers: These are needed to keep the natural fats in emulsion given that, if they were to precipitate due to a processing mistake, they would be very difficult to remove. In any case, they should be used in small quantities.
Polyphosphates: Used in the final stages, they avoid the formation of calcium carbonate – and therefore permanent stains – on the surface.
The use of auxiliaries makes it possible to achieve the highest leather quality. While, on paper, their use increases the pollution load, it is also true that, when used opportunely, they can reduce the level of COD (based on the above assumptions, in hair-destroying liming, the correct use of a well-formulated anti-wrinkle agent results in up to 10% less COD). Moreover, it is possible to exploit their effectiveness in relation to the different speeds at which swelling and the breaking of disulphide bonds occur. Basically, they should be left to act for as short a time as possible so as to guarantee the same effect on the hide. At the same time, a lower COD is recorded due to the fact that their rapid action focuses on the hair follicles, without damaging the rest of the hair structure.
Following the liming process, the structural fibres will be liberated and well separated, preparing them for subsequent chemical treatments – first of all, the tanning agent, which will take the place of the globular proteins and take over their stabilising functions.
Work in progress
We are currently experimenting with a new oxidative unhairing process using an oxidant other than hydrogen peroxide combined with enzymes. This system leaves the removed hair intact and therefore suitable for further industrial uses (felt, insulation, etc.) without having to undergo chemical modification.