Use of glucose to improve the environmental aspects of traditional chrome tanning: development od a semi-industrial scaled process for high-quality bovine upper leather
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A tanning process based on a preliminary stabilization with glucose, followed by a chromium tannage, has been optimized on semi-industrial scale. A higher chromium content in the tanned leather and a better exhaustion of the floats indicate that the presence of glucose in the skin improves the penetration and fixation of chromium. The stabilization with glucose allows the production of chromium-free waste shavings that may be reutilized through more sustainable routes. The crust leather obtained is characterized by mechanical and technical properties comparable to those of the traditionally chromium tanned crust leather. As far as roundness, fullness and hand, the glucose stabilized leather display better performances. The results obtained assess the glucose stabilized leather as a good candidate for producing high quality bovine upper leather through a more environmental sound chromium tannage, in terms of lower chromium consumption and discharge in the wastewaters and of better reutilization of chromium-free wastes. Sommario E’ stato sviluppato, su scala semi-industriale, un processo di concia al cromo basato su una preliminare stabilizzazione con glucosio. La stabilizzazione con glucosio, oltre a favorire la penetrazione e fissazione del cromo nella pelle e quindi un aumento dell’esaurimento del bagno, conferisce alla pelle una sufficiente stabilità idrotermica che consente di poter effettuare la rasatura prima della concia al cromo. In questo modo si ottengono rasature esenti da cromo e pertanto più riutilizzabili con processo a più elevata sostenibilità. La pelle in crust ottenuta mostra proprietà meccaniche del tutto comparabili con la pelle tradizionalmente conciata al cromo; le proprietà organolettiche risultano addirittura migliori soprattutto in termini di rotondità, pienezza e mano. I risultati ottenuti indicano la stabilizzazione con glucosio come processo promettente per una produzione a più elevata sostenibilità ambientale di pelli bovine per tomaia di alta qualità in termini di minore consumo di cromo e minore quantità di cromo presente nelle acque reflue, nonché di migliore riutilizzo delle rasature esenti da cromo. Introduction The many advantages offered by chrome tanning justify its widespread use for production of almost all types of leather. However, the traditional chromium(III) tanning process is constantly under threat from the pressure of legislation and ever-tightening restrictions has resulted in tanners coming under pressure to minimize chromium-containing effluents discharge and chromium-containing wastes production. Research interest has thus centered on the development of environmentally sound chromium tannages. In a previous study, we developed a semi-industrial scaled tanning process to improve the environmental impact of chrome tanning in the production of high quality bovine shoe upper leathers, based on a twostep procedure in which a soluble sodium silicate is used as stabilizing agent and chromium salts are successively dosaged to complete the tanning process. Sodium silicate is capable of stabilizing delimed and bated pelts sufficiently to enable shaving, thus producing chrome-free wastes that can be used for the production of fertilizers, protein hydrolizates and/or pet food. Besides, the silicate pretreatment has been shown to favor chromium exhaustion. In order to avoid the use of a metal-based product, the use of carbohydrates to increase the hydrothermal stability of the collagen structure has been proposed in literature. In this study we report the results obtained by using glucose as stabilizing agent for the production of high quality bovine upper leather. The tanning procedure has been optimized and validated on a semi-industrial scale. The final leather obtained, after a standard fatliquoring/dyeing treatment, showed very good technical and physical-mechanical properties, quite similar to chrome tanned leather. The environmental improvements of the process consist in the preliminary stabilization of the leather so that it can be shaved by obtaining chromium free wastes; besides, the significant reduction of the chromium offer results in a drastic reduction of the chromium content of the exhaust effluent. The paper reports also the results of scanning electron microscopy (SEM) coupled with energy x-ray dispersion (EDX) used as investigations methods to follow the tanning process steps. Experimental procedures The glucose used as stabilizing agent was a commercial concentrate solution at 43° Bé. Basic chromium (III) sulfate (26% Cr2O3, 33% basicity) and chrome alum (15% Cr2O3, 0% basicity) were used as tanning agent. The semi-industrial scale tanning runs were conducted in a stainless steel drum (1.2 m diameter, 0.8 m length) by using heavy calf skins (8-12 kg) after the pickling stage (pH 2.6). In each run, the skins were divided into two sides: one side was conventionally tanned with chrome, the other side was tanned according to the innovative recipes. The exhausted floats after tanning were sampled; their chromium content was determined by a PERKIN-ELMER PLASMA 400 ICP-AES analyser. The chrome content of the tanned skin was determined according to the I.U.C./8 standard method. Chrome distribution in the section of the tanned skin was analyzed by scanning electron microscopy (SEM) coupled with energy x-ray dispersion (EDX) by using a JEOL 5600 LV electron scanning microscope, that was used also to observe the collagen cross-linking. After tannage, the two halves followed a chrome-retannage/dyeing-fatliquoring cycle to obtain crust leathers currently used by the tannery to produce upper leathers. The crust leathers were evaluated by their main physical and technical properties. Physical testing were conducted according to Italian standards (UNI 10594) for upper leather, while technical properties were assesses by the technical personnel of Po.Te.Co. The shrinkage temperature was determined according to the UNI-ISO 3380 method by using a Giuliani IG/TG/THEISS equipment. The data reported are the mean of two determinations. The tearing load was determined according to the UNI ISO 3377 method by using a dinamometer Pegasil Model Marte. The data reported are the mean of six determinations. The grain distension was determined according to the UNI ISO 3379 method by using a lastometer Pegasil Model EL-51E. The data reported are the mean of three determinations. Results and discussion Due to the presence of hydroxyl groups in the molecule, glucose has been selected as a possible stabilizing agent of the collagen structure. By selecting a proper pH, the hydroxyl groups may, in fact, cross-link the carboxyl groups (aspartic and glutamic acids) of the side chains of collagen through hydrogen bonds and increase the hydrothermal stability of the skin structure so that the skin may be submitted to the mechanical stress connected to the shaving operation. Preliminary test runs have evidenced that the optimal pH to allow the interaction between glucose and collagen is close to 5.5-6, so that the carboxylic groups are partially dissociated and may give hydrogen bonds. An increase of pH above 6 may cause an excessive swelling of the hide. The addiction of glucose effectively allows a partial stabilization of the hide. The shrinkage temperature of the pickled hide is raised from 52°C up to 60°C. By this increase of the hydrothermal stability, the shaving of the hide is thus allowed. Besides, the preliminary tests have evidenced that a better stabilization may be obtained if adopting a dry process, i.e. if using the glucose solution without adding additional water. Variation of the chrome dosage In these runs, chromium dosage was varied from 2 to 8 % (as chromium sulfate 26/33) on the skin weigh, according to the recipes reported in Table 1. After stabilization with glucose at pH 5.5-6, chromium was dosed either as chromium sulfate either as chrome alum to control the pH. The possibility of shaving the hide before chromium tannage gives, as first result, a significant reduction of the consumption of chromium based tanning agents on the same basis of pickled weigh processed. As reported in Table 2, the final shrinkage temperature of the leather previously stabilized by glucose is similar to that of the leather traditionally tanned by chromium. The stabilizing effect of glucose is confirmed. As expected, the hydrothermal stability increases as the chromium dosage is increased. Fig. 1 reports the SEM analyses of the cross section of the hides after stabilization with glucose and after chromium tannage. A preliminary compacting effect of the glucose may be observed. A complete reticulation of the collagen fibrils is achieved after chromium cross-linking. The presence of glucose in the collagen structure does not prevent a uniform penetration of chromium at all the dosages, as reported in Fig. 2. Following these promising results, a chromium retannage/dyeing-fatliquoring was applied in order to obtain crust leather to submit to mechanical and technical assessment. Table 3 reports the results of the mechanical tests of the crust leathers obtained. The results of the assessment of the technical properties of the glucose-chromium crust leathers, in comparison with the conventionally chrome tanned leather, are reported in Table 4. A conventional scale of grades ranging from 1 (worst performance) to 5 (best performance) has been used. The crust leathers comply very well with the mechanical standards required for high quality bovine upper leather. The technical properties improve as the chromium dosage is increased but, however, the crust leathers obtained by stabilization with glucose show worse technical properties in comparison with the corresponding chromium-only tanned leather, that do not allow their utilization as high quality upper leather. Optimization of the tannage procedure Following the results obtained in the previous runs, the tannage procedure was optimized by selecting the d