Hot water treatment for degreasing sheepskins

Abstract Current degreasing technologies are inconvenient in that they use highly polluting products such as organic solvents and/or surfactants and this is exacerbated by further use of large amounts of water. The use of enzymes does not provide a simple remedy as their action only develops in the presence of non-ionic surfactants. The purpose of the present study is for the elimination of the various drawbacks by utilising a very simple technology, that of using hot water. Fat melts at relatively low temperatures, 40-50°C on average. Pickled sheepskins from Iran with a mean average fat content of 25% were depickled, fleshed, repickled and then chrome tanned. The resultant leathers were drummed with hot water at a temperature of 60°C and then washed with water with the same temperature conditions. At the end of the process, the leathers gave a fat content no higher than 4-6%. They were shaved and then submitted to the processes of retanning, dyeing, fatliquoring and finishing. Semi-industrial trials, carried out in a tannery, showed that the final organoleptic aspect of the leathers complied perfectly with other standard leathers produced by the same tannery. All physical analysis of the leathers compared favourably with the tannery's normal production. It was shown that the removal of fats by melting did not affect any of the characteristics of the leather and, after cooling, the fats solidified and could be removed from the water's surface. Introduction The degreasing constitutes the final phase of the beamhouse or pickling process and is applied only on skins with a high fat content, ie lamb and sheepskins (30-40% fat). The use of surfactants and alkali in the liming of these skins or the utilisation of enzymes in the bating stage only achieves partial fat removal. This can cause a non-uniform distribution of tanning agents and dyestuffs and can lead to staining of the leather, which lowers the commercial value. Thus, surfactants, organic solvents and biochemical products are used in the degreasing stage to reduce the amount of natural grease. Various factors can influence the degreasing process, ie temperature, reagents concentration, pH of the bath and skin, and type of chemicals. The pH dependence results were particularly interesting. For instance, low pH values favour anionic surfactants linked to collagen, thus inactivating their emulsifying function. High pH values develop the same negative action on the emulsifying effectiveness with cationic surfactants. Surfactants are most used in the tannery, particularly during sheepskin processing. Their function is to reduce the surface tension of the water using their combined hydrophilic and hydrophobic characteristics. Their ability to reduce the contact angle between the liquid (water) and the solid (skin) phases favours the opening and the following inhibition of the fibres of the collagen structure. Their emulsifying action incorporates the natural fats and other residues of organic origin. The anionic surfactants mostly used are linear alkyl benzene sulfonates (LAS), alcohol sulfonates (AS), alcohol ethereal sulfonates (AES) and alkaline soaps of the fatty acids, which are sufficiently biodegradable. Non ionic surfactants are not always biodegradable, in particular ethoxylated fatty alcohols. Cationic surfactants, however, are mostly quaternary salts of ammonium and usually have a high toxicity. Finally, the amphoteric surfactants, commonly betaine and imidazolenic mixtures, show an elevated foaming activity and are effective, biodegradable but very expensive and require defoaming agents. Undoubtedly, non-ionic surfactants with nonylphenilic nucleus have particular importance in the tannery, due to their accessible cost and scarce ability to link to the collagen structure. Unfortunately, they have a notable polluting action on the environment, due to their high resistance to conventional wastewater treatments and low biodegradability. European legislation foresees a primary biodegradation value of at least of 80% according to OECD methods (Organisation for Economic Cooperation and Development which corresponds to the French OCSE). If the primary biodegradation does not proceed up to complete mineralisation, stable metabolites remain in the environment, thus polluting it. The EU has proposed to outlaw the nonylphenol ethoxylates for their sensitive resistance to the biodegradation. Such a proposal could become law, producing a crisis for the ovine leather manufacturing industry. Besides, the use of these chemicals or biochemical products requires the use of large quantities of water and does not always guarantee a satisfactory fat removal, producing only a superficial distribution. Even under the best conditions, the process must be repeated many times in order to reach a suitable degreasing, with a final fat content corresponding to 5-8%. Conditions for adequate degreasing can be reached by performing at least two treatments by 1.5% of nonylphenol ethoxylate in 450% of water, alternated by draining off and additional washings with 450% of water at 35°C¹. The present work has the aim of eliminating the environmental problems, applying a simple technology, which foresees only the employment of hot water. It is well known that fats melt at relatively low temperatures, 40-50°C, which makes their extraction possible through the use of hot water. Experimental Raw material: For the study we used pickled lambskins from the Middle East, particularly from Iran. Chemical: The chemicals were all commercially available. The chrome salts employed were chrome sulfate, 33% basic. Fat determination The skin fats have been determined as 'extractable matter by CH₂Cl₂' according to the UNI EN ISO 4048 2000. Considering that the fat quantity varies substantially not only from one animal to another but also within the same skin, we have analysed four samples from each skin, taking them always from the same region. The percentage values of fat reported constitute the mean values of the different measurements taken. Tanning process In order to obtain adequate preparation of the skin for tanning and fat extraction, two variables were evaluated in previous trials: 1. Using the material in the pickled state 2. Using the material after depickling, fleshing and repickling The best method was found to be the second so we used repickled skins. By depickling we obtained a good opening of the fibres and a soft swelling, as well as a more suitable fleshing, which allowed a better penetration inside the structure of the reagents, including the hot water. The result was less stains, especially on chrome tanned skins. Stains were also reduced in the basification phase. Basifying was performed by slowly adding the complexing salts including an oxalate and sodium acetate and, in the final step, by adding small amounts of sodium bicarbonate. Under these conditions, the subsequent extraction with hot water did not cause fat deposits on the grain surface. In Table 1, the tanning procedure is given. It is similar to that of the tannery that supplied the pickled skins with the exception of the basification step. Procedure for degreasing, retanning, dyeing and fatliquoring The following scheme shows the chronological order of the steps: * Washing of the pickled skins (without surfactants) * Depickling * Fleshing * Pickling * Chrome tanning * Shaving * Degreasing in two following phases: 200% H₂O at 60°C in two steps * Retanning, dyeing and fatliquoring The shaving operation also constituted a further variable in the process. The behaviour of the skins on the shaving machine both after chrome tanning, in order to favour the degreasing action, and after the degreasing the results were assessed. Degreasing method of the skins The first degreasing trials have been carried out in a single step using 200% water. The extraction yield was studied over time using three different temperatures: 55, 60 and 65°C. This range had been chosen in relation to the melting points of natural fat in skin, avoiding higher temperatures which could damage the skins, that are already subjected to strong mechanical stresses during degreasing. The effect obtained on the process by alkalisation to pH8 using additions of ammonia has also been studied. This alkalisation, theoretically, should improve the fat substance hydrolysis and ease the solubilisation and removal of the dermic structure. Successively, a series of consecutive extraction phases alternated with bath drain off and washing steps by 200% of water, have been carried out. In Table 2, all trials carried out on a laboratory scale are shown. Results and discussions Table 3 and Figure 2 show the results from the degreasing trials performed in one phase with water at 200% at different conditions. Besides the fat percentage values, the degreasing value efficiency is also shown. This last value has been calculated as a percentage of the original fat content. The values reported in the table refer to dry weight. The trends of percentage variations of the extracted fats versus the time are better expressed by Figure 1. According to the results, it emerges that the tests 2 and 1c allowed final extraction values that are sufficiently close to requirements. Therefore, the treatment at 60°C without the addition of ammonia was more interesting in terms of economic and an environmental impact. Nevertheless, in such case, a remarkable decrease in the percentage of fats was achieved only after five hours, an excessive amount of time. Better results may be obtained using a treatment of various degreasing processes in steps alternated by draining off the bath and washing with 200% water. In Table 4, the results of the degreasing process at 60°C, using both water only and water with added ammonia, are given. These results are compared with those obtained by a conventional degreasing process, with the use of a commercial mixture of anionic and non-ionic surfactants with values referring to skins in the pickled state. The results show clearly that a series of processes, followed by fast washing, is more advantageous than one extraction step only, obtaining high efficiencies over a short period of time. In Figure 3, the extraction effectiveness of all the performed trials is presented. For the trials carried out in one step, the obtained values for an extraction time of 180 minutes are highlighted, while for the trials performed in different steps the results refer to two, and not three, extraction steps (120 minutes of total extraction time). The efficiency value of conventional degreasing with surfactants is also shown. As previously mentioned, it emerges that: a) the extraction to pH8 with ammonia addition doesn't influence the process yield; b) no consistent variation can be seen between the standard and the samples 3 and 4. Therefore, the optimal conditions of the degreasing carried out after tanning, are as follows: i) Degreasing: 200% of water (% on pickled weight) at 60°C, 60' Drain off Washing: 200% Water at 40°C ii) Degreasing: 200% of water at 60°C, 60' Drain off Washing: 200% Water at 40°C On the subject of whether it is more convenient to shave before or after degreasing, our study has shown that prior shaving assures better results. In fact, this treatment improves the emulsifying action of the hot water and produces a partial removal of the fats present on the flesh side. In this case, particular care is necessary when using shaving machines in order to avoid damage to skins. The skins tanned and dyed did not show any difference in comparison with the standard both for the aesthetic proprieties or for chemical and physical properties. In order to confirm the results, a semi-industrial experiment has been performed on 15 lambskins after tanning and degreasing. The skins were taken to the same tannery, which had sourced the skins in the pickled state. They were then retanned, dyed, fatliquored and finished with a normal stock of production. Finally, the technical staff assessed and compared the look and handle of the final leathers, which were degreased with hot water and the normal production control. No difference was noticed, either in the state of crust or finished. Results of the comparative analysis of chemical and physical properties are given in Tables 5a and 5b. Discussion The chemical characteristics and the physical and mechanical properties of the skins, treated by new degreasing technology are perfectly comparable with the standard. There are good reasons for the practical industrial application of the procedure, independent of the possible decisions by the European Community², which could banish the use of different surfactants, particularly of the nonylphenol ethoxylate. The advantage of such a technology would offer the possibility of using surfactants, solvents and biochemicals. In fact, the action of the surfactants in the degreasing process is less favourable due to their affinity to the protein of the skin. If we consider that anionic surfactants have the tendency to link to the basic groups of the collagen under the isoelectric point, their use in the degreasing of the pickled skins cannot guarantee good results^3,4,5. This situation cannot be improved even by a depickling at pH5-6. In this case, a suitable elimination of the positive charges can be obtained. However, the maximum activity conditions cannot be met. Better results can be achieved by treating the pickled skins directly with cationic or non-ionic surfactants6, which show less affinity to the acidified skin. Yet, if in the case of the cationic surfactants, we have not achieved optimal results, we can obtain elevated performances using non-ionic surfactants, particularly if they are combined with solvents. But, the use of these, as already described, could be banned because of their high environmental toxicity. A good solution for the degreasing process could be the use of enzymatic systems based on protease or lipase. The protease action, in particular, requires the presence of suitable ionic surfactants⁷ for its evolution. In fact, they don't directly act on the fats, but produce only a lysis of the protein membrane, which contains them. Then the lipid matter is easily emulsified and removed by the surfactant. Nevertheless, this process must be well balanced and checked in order to avoid unsatisfactory degreasing or high skin degradation due to high enzyme concentrations^8-15. Yet, technically advantageous results could be reached by using a combination of lipase, protease and surfactants to improve the natural fat release, hydrolysis and removal^16,17. But these treatments have not yet been adopted industrially into the tannery. In any case the action of the enzymes combined with the surfactants would not solve the environmental problems associated with them. Finally, other solvents do not offer satisfactory results, in spite of the promising laboratory investigations¹⁸. In fact, the presence of water in the skin causes low effectiveness, as the water creates a barrier that hinders the access of the solvent to the fats and, therefore, their extraction. Good results could be reached by utilising a synergetic solvent - anionic¹⁹ or non-ionic^1,20,21 surfactant systems. The anionic surfactant bonds with the collagen by chemisorption, producing hydrophobic sites, which improve the solvent penetration in the structure. The non-ionic surfactant, instead, facilitates the water-solvent-fats in to an emulsion. In this last case, important results have already been reached by a treatment and two following washings with surfactants. But, the solvent used in this case is expensive and above all recovery by distillation is needed. Conclusion Current degreasing methods show many inconveniences caused by the use of highly polluting products, such as organic solvents and surfactants; and the situation is made worse by the use of large amounts of water. Since the EU proposal to outlaw the nonylphenol ethoxylates and other chemicals due to their notable resistance to the biodegradation new products or techniques have to be studied in order to reach satisfactory conditions for fat removal from the skins. The problems faced by the tanner could be resolved using the solubility of the fats by melting. Such a method would be ecologically compatible, without modifying the final quality of the finished leather. SSIP investigations have shown that degreasing with hot water is a valid alternative to the conventional degreasing techniques. The proposed method obtains satisfactory laboratory results and is ecologically compatible. Trials of the new method, carried out in a tannery, have pointed out that the final organoleptic aspect of the leathers using the traditional process are perfectly comparable with the leathers produced by the same tannery. As well as the treatment of the end degreasing baths, the new process is favourable as cooling to room temperature allows an easy separation of the extracted fats, which solidify and appear on the surface, from which they can be easily removed and recycled. The recovered fats could be used in the same industrial tannery as material for fatliquoring agent preparation²² after solfonation and for other chemical treatments.