Production of soft leather for furniture upholstery

Introduction The production of high quality leather for furniture needs adequate preparatory treatment of hides during the entire wet end phase. The main features of this kind of material, ie softness, lightness and fullness, can be improved either in the beamhouse or at the fatliquoring stages. The combination of an appropriate mixed enzymatic treatment in the beamhouse with a new synthetic fatliquoring can produce a final leather with the required features. In this paper, we describe the results obtained with the use of a synergistic blend of enzymes (Lederzim PL/2) in the beamhouse. In the post tanning, there is a fatliquoring step with a new synthetic product (Morbidol BB). Enzymes in the leather industry Hides and skins are obvious natural substrates for proteases and lipases, because they are largely made of proteins and fats. Some of these substances have to be partially removed, or modified before the tanning operations can begin. In Table 1, Lamberti Lederzim products for the various steps of the beamhouse operation are shown (Martignone, 1997). Enzymes have been used in leather making for many years, mostly at the bating stage with crude trypsin obtained from pig pancreas. Bating is essentially a degradation process of non collagenous, globular proteins, carried out to soften hides and prepare them for the following tanning step. Currently, three factors are causing the use of pancreatic trypsin of animal origin to be discontinued: a) The production of human insulin by fermentation processes, thanks to genetic engineering, instead of by extraction from pig pancreas, has made the recovery of trypsin as a byproduct less competitive or economically unattractive b) The availability of novel bioengineered enzymes issued from biotechnology is opening new opportunities for innovative applications in the leather industry c) The rising concern for the possible presence of the BSE causative agent in material of bovine origin plays in favour of microbial fermentation products The most used families of enzymes for leather treatment are: Proteolytic enzymes in soaking, unhairing, bating of hides and, more recently, in acid bating of pickled skins and wet-blue (Schraeder et al, 1998; Paul et al, 2001); Lipolytic enzymes in the degreasing of hides under alkaline conditions at any stage of the beamhouse operation before tanning or of wet-blue at acidic pH (Christener, 1992). The results that we report in this paper show how the combination of the new protease and lipase (Lederzim PL/2), with selected specificity and n the appropriate ratio, can offer several advantages to the tanner. The correct use of Lederzim PL/2 allows a fast soaking/unhairing step, which combines cleaning, unhairing and degreasing, possibly avoiding the bating step. Fatliquoring The quality of modern furniture leather is generally characterised by great softness, fullness, lightness and a smooth, silky touch. The fatliquor must improve the evenness and brightness of dyeing, give excellent heat and light fastness, while leaving no bad smell on the leather. Among the traditional fatliquors used to produce furniture leathers, the most important are certainly the sulfonated animal or vegetable products, mainly based on fish oils and better known as 'sulfited oils'. Sulfited fish oils are frequently used for furniture because of the high degree of softness and penetration they can impart to the leather. There are drawbacks in the use of such products, such as: they generally have a bad smell, very poor heat and light fastness, and give soft but 'flat' leather (Kaussen, 1998). Very often they also need a separate special treatment in order to reduce their undesirable, peculiar, bad smell. A valid alternative to these fatliquors of natural origin is represented by synthetic products. Synthetic hydrocarbons, from petrochemical sources, can be suitably modified by chemical reactions in order to obtain synthetic fatliquors, with the same or better properties of sulfited fish oils, but without their negative features. Furthermore, because they are produced by synthetic routes, formulations can be obtained with constant physical/chemical properties and reproducible results on leather. Morbidol BB is a new synthetic fatliquor belonging to this class of products. The excellent degree of softness that Morbidol BB confers to leather is due to the presence of an 'alkyl-sulfosuccinate' among its components. Therefore, it is chemically quite similar to sulfited natural fish oil. This can explain several behavioural similarities of the two products. The chemical structure of a sulfosuccinate contains a strongly anionic group, the SO3- which confers the following properties: * the higher the polarity, the stronger the binding effect with leather fibres, which translates into a strong reduction of possible migration phenomena * deep penetration through the leather section provides high softness and fullness, thus preventing the leather from cracking even under harsh drying conditions * good stability of the emulsions guarantees a long shelf life of the product An adequate selection of the organic chain of the molecule, will help to: * obtain excellent heat and light fastness * contribute softness, fullness and a silky touch * good stability of the emulsion guarantees a long product shelf life The physical/chemical properties of Morbidol BB synthetic fatliquor are shown in Table 2, while in Table 3 results are shown which are obtained on a leather treated with Morbidol BB, using standard testing methods. With respect to performance, Morbidol BB offers a very deep penetration that makes it highly suitable for furniture and for all those leather articles where good softness and fullness are required. It also confers excellent light and heat fastness, and can be used alone or in combination with other fatliquors. Furthermore, from storage and handling standpoints, it maintains its fluidity down to approximately 10°C, making it suitable for automatic feeding systems. Application trials and physical/chemical characterisation The application of the two products, Lederzim PL/2 and Morbidol BB is described below, along with a few features of the leather obtained. Table 4 describes a shorter beamhouse process for soaking and liming, without bating, carried out with the use of Lederzim PL/2, a synergistic blend of proteolytic and lipolytic enzymes. It has previously been shown that a protease and a lipase, when used simultaneously under suitable conditions, can generate an excellent synergistic action (Palop et al, 2000; Ivanova, 2000). Therefore, Lederzim PL/2 was developed as a well-balanced blend of protease and lipase, accurately selected so that the comparable temperature and pH profiles of the two enzymes allows them both to be used under the same process conditions. The detailed application recipe of Lederzim PL/2 is shown in Table 4, while a brief outline of its use is as follows: * after a thorough rinse of raw hides, the soaking bath is prepared 100% water 25°C 0.5% sodium carbonate 0.1% detergent rotation 15 min. pH9-9.5 0.1-0.15 % Lederzim PL/2 rotation 4 hrs * At the end of the soaking step, after checking for complete rehydration, add without draining the liquor bath and without adding further water: 0.1-0.15 % Lederzim PL/2 rotation 60 min. 2.5% sodium sulfide 1.5% lime rotation 60 min. pH12-12.5 1.8 % lime 30 % water 25° C rotation 20 min. automatic overnight * The next steps (ie deliming, pickling etc) can be carried out as usual. It is considered advisable to always evaluate the need for an eventual bating step, as the combined action of the two enzymes can produce sufficiently relaxed hides so that further bating might not be necessary. Indeed, the synergistic action of the enzymes produces well relaxed hides. Therefore, in all our trials we always judged it not necessary for the bating step, as reported in Table 4. The main properties of hide treated with Lederzim PL/2 are shown in Table 5. From the data of Table 5, it is evident that the physical/chemical features of a wet-blue treated with Lederzim PL/2 are comparable or even better than those of a wet-blue obtained by traditional soaking/liming steps. The higher chromium content, due to an improved uptake by the fibres, favours a more complete and thorough tanning, thereby giving improved tensile strength and elasticity of the leather fibres (Morera et al, 2000). In summary, the processing of raw hides with Lederzim PL/2 offers the following advantages: 1) Good soaking of all types of raw hides, removing scud and fats more thoroughly and efficaciously than only with detergents 2) Thorough soaking of very dried hides 3) Complete soaking, with reduced rotation time and liquor ratio, thus saving water and eliminating overnight soaking 4) Production of very clean and relaxed hides, well degreased and with excellent distension of wrinkles 5) Possible elimination of the bating step, because it may be needed according to the type of leather 6) Improvement of the uptake of chromium and of other chemicals added in the following steps, presumably because the more 'open' structure of the fibres offers a larger contact surface between the dermal substrate and the tanning substances 7) Production of a softer and lighter wet-blue, reducing the use of retanning and fatliquoring agents 8) Production of leathers with even surface in all parts, reducing flattening of flanks and hardness of neck areas 9) Evenness and brightness of dyeing, thanks to the optimal distribution of residual natural fats. The leather thus obtained from a recipe using Lederzim PL/2 was used for the following steps of fatliquoring and dyeing. Table 6 shows the application recipe for Morbidol BB. Along with physical and chemical properties, economical and environmental issues must also be taken into account when evaluating a fatliquor. The simplest way is to determine the 'float exhaustion' parameter. 'High float exhaustion' values mean that the following parameters are minimised: * effluent organic load * effluent disposal costs The trend of float exhaustion for Morbidol BB is described in Figure 1. Considering 100% of the initial Morbidol BB concentration in the float at the beginning of the fatliquoring stage, it can be seen that its concentration is reduced down to 3% of starting value, before acidification. This value signifies an excellent adsorption of the fatliquoring agent. In order to evaluate the actual light fastness behaviour of Morbidol BB, the test was performed on a leather sample before dyeing. Figure 2 shows the results obtained. According to the method UNI 7639, light fastness value ranges between 5 and 6 of the blue wool scale. In order to compare this result, Figure 3 shows the same test carried out on a leather sample treated with a traditional sulfited fish oil. In the latter case, the light fastness value is about 1-2. Figure 4 shows six leather samples, which represent, respectively: a) Morbidol BB referenc b) Morbidol BB submitted to 'tropical chamber' test (72 hrs, 80°C, 100% humidity) c) Morbidol BB submitted to heat fastness test (48 hrs, 100°C) d) Sulfited fish oil reference e) Sulfited fish oil submitted to 'tropical chamber' test (72 hrs, 80°C, 100% humidity) f) Sulfited fish oil submitted to heat fastness test (48 hrs, 100°C) The excellent performance of a synthetic product such as Morbidol BB can be easily compared with traditional sulfited fish oils; in the case of Morbidol BB, colour variations of samples compared with the reference are practically negligible. Even after high temperature treatments, leather sample surfaces are homogeneous, showing the high degree of stability of leather-fatliquor bonding. Conclusions The increased competition in the market of furniture leather has caused a growing demand for quality and fastness of the articles, aside from the traditional features of fullness, softness and touch. Heat and light fastness, such as low extractability and humidity resistance are the standard characteristics claimed for all tannery chemicals, together with their decreased environmental impact, reduction of water and energy wasting in the process. This paper has shown the outstanding properties of the new synthetic fatliquor Morbidol BB and how it allows compliance with those requirements. The experimental data have shown how it is possible to obtain high quality leather articles for furniture by using Lederzim PL/2 in the beamhouse and Morbidol BB in the fatliquoring. The use of this enzymatic product offers several advantages for the environmental impact and the cost effectiveness of the leather making process: * process flexibility and good reproducibility of results * lease-of-use and compatibility with other chemical products commonly employed in the various steps of leather making * good efficacy at a wide range of pH and temperature values * possibility to decrease the quantity of sulfide commonly used in unhairing, shortening the duration of the liming step and/or performing hair recovery * production of more relaxed hides by an enzymatic unhairing step, which translates into a better quality leather, as compared with a traditional process * remarkably positive effects on environmental pollution caused by the tannery: BOD, COD and suspended solids in the wastewater are decreased with substantial overall ecological benefits * replacement or significant decrease in the use of solvents or non-ionic detergents, such as ethoxylated compounds, with the proper use of lipase for degreasing. The authors wish to thank Dario Fornara for the organic synthesis, Fabrizio La Valle and Carlo Gonella for the application trials, and Lamberti management for support.