Tanning Alternatives

Abstract A titanium/chrome combination tanning system has been studied with a view to reducing the environmental concerns associated with using chromium salts alone for tanning. Globally, there is an increasing need for environmentally friendly upper material for footwear manufacture and titanium is a tanning agent found to be eco-friendly in nature. Hence, titanium tanning in combination with chrome for upper leather has been studied in this paper. The results indicate higher chrome uptake in combination with titanium salts and this leads to less discharge of chrome in the effluent. In addition, the strength and other functional properties of these leathers were found to be on par with conventional chrome tanned control leathers. This process also enjoys better fullness and dyeing characteristics as compared with control leathers. The present study clearly indicates the titanium/chromium combination tanning system for upper leather as a potential viable eco-friendly option for tanners. Introduction Advancement of science and technology brings about newer tanning methodologies and materials. Initially, vegetable tanning was the tannage of choice, followed by inorganic tannages, which now have prime position in leather production. Of these tanning materials, chrome takes first place but other inorganic tannages, such as aluminum, zirconium, titanium and silicates, are of interest for many researchers. Even though there are many advantages offered by chrome tanning and its widespread use in all types of leathers, there is worldwide interest in the development of a suitable alternative and sustainable tanning system because of growing environmental concerns. The drawbacks associated while using chromium salts alone for conventional tanning include possible poor exhaustion levels (40-70% in the worst cases), discharge of chromium in the effluent and availability factors associated with the chromium compounds. The motive behind the interest of this paper has centred principally on the possible development of titanium tannage in combination with chrome in small amounts, since the use of titanium compounds, which are abundant, cheap, and non-toxic, not only overcomes these problems but also has additional advantages. The chemistry and technology of titanium tanning salts have been reviewed previously¹.  Studies on the fixation of titanium in straight and in combination tanning systems have been made². A tanning material from titanium tetrachloride was prepared and used to produce white leathers³. The masking effect of anions with titanium sulfate has also been studied⁴. Titanium based tanning agents possess reasonable tanning properties but do not fulfil all other important properties when used alone. Therefore, a combination tanning salt, with a minimum level of chrome (0.5% Cr2O3), has been prepared by CLRI and patented⁵. Earlier preliminary studies were carried out on the Ti-Cr combination tanning for upper leather6. This paper analyses in detail the potential application and suitability of Ti-Cr combination tanning system for upper leather manufacture which have not been reported earlier. Overview of the titanium tanning salt Titanium is the ninth most abundant element in the earth's crust. Titanium reserves in India alone account for about 10% of the world's availability. It occurs as Ilemenite (FeTiO₃) and Rutile (TiO₂). For the leather industry, titanium dioxide finds extensive use as a white pigment either alone or after mixing with barium sulfate. As regards its inertness towards tanning materials in leather, it does not suffer from darkening on ageing. Titanium has the best corrosion resistance as a result of its passivity (< 0.007 mm/yr) and hence it imparts better corrosion resistance than any other materials usually employed in tanning systems. Leather with anti-corrosive properties produced by titanium tannage finds extensive utilisation in the production of protective gloves, upper leathers, garments and other outfit requirements to be used in corrosive atmosphere. With regards to its function as a tanning agent, the studies carried out so far have been based on predominantly anionic titanium and the leathers obtained possessed lower hydrothermal stability, abrasive resistance and strength. In these studies titanium exists as a double salt namely (NH₄)₂SO₄TiO(SO₄).xH₂O, which is predominantly anionic form. A literature survey indicates that Ti forms very stable complexes in its tetravalent state. Thus it seems interesting to study the complexes of Ti(IV) with reference to pre-tanning, retanning and simultaneous tanning system involving titanium, leading to fixation/stripping of the components of the systems taken up for such tannages. Chemistry of titanium tanning: Co-ordination chemistry The Ti(IV) complexes in water-sulfuric acid solution are represented by chain like structures as shown in Figure 1. The hydrolysis of such titanium complexes is accompanied by the coordination of hydroxo groups and formation of oxo bridges. The higher the concentration of Ti(IV) and sulfuric acid the more extensive the degree of polymerisation. Infra-red studies indicate that the titanium centred octahedral co-ordination compound with titanium will have a co-ordination number of six¹. Masking effect of anion with titanium sulfate solution The normal titanium sulfate solution precipitates at pH3.55 on the addition of 2M sodium bicarbonate. A solution meant for tanning hides/skins is effective only when it possesses high precipitation point and is capable of high thermal stability. The optimum pH value for titanium tannage where the availability of a reasonable number of carboxyl groups has been ensured and undesirable aggregation of mineral tanning agent does not take place is found to be around pH3.5-4.0. From the experimental studies⁴ it was found that the optimum precipitation pH of 3.9 can well be obtained with 0.167 eq/eq of titanium, when the basic titanium sulfate used is 50% basic. It is found that the complex formation of titanium with the hydroxy and carboxylic acid occurred through the formation of two links with the central metal cation, a phenomenon called ‘chelation'. From the experimental results1 with the hide powder, optimum conditions for titanium tannage are found to be 50% basicity, pH4 and 5% offer of TiO₂. Objectives of the present study This paper aims to investigate the possibility and applicability of titanium/chrome combination tanning system for upper leathers, thereby possibly reducing chrome in the spent tan liquor. In the present study, titanium -chromium combination tanning agent has been employed for upper leather material and compared with that of full chrome leather. Physical properties, chemical property analysis, spent liquor analysis and functional property analyses have been made. The specification of the combination tanning salt used in the present study is shown below

• Solubility in water - freely soluble
• Titanium content (%TiO2) - 3.0%
• Total chromium content (Cr2O3) - 0.5%
• pH of 1% solution - 2.8
• Nature of titanium tanning species

% Cationic - 52 % Anionic - 40 % Non-ionic - 8 Titanium tanning - An eco-friendly system The titanium tannage is absolutely free from pollution, unlike other mineral-tanning agents. The short-term bioassay study on the effect of the exhaust liquor from titanium tannage on aquatic life in different media and varied concentrations has shown no toxic effects¹. Experimental Materials and methods Five wet salted cow-hides of good quality, weighing about 10-12 kg/piece, were taken for the process. They were trimmed, cut into sides and marked on each corresponding left and right hand sides. Left hand sides were taken for the Ti -Cr combination tanning system and right hand sides were tanned with conventional chrome tanning system. The sides were weighed and soaked in three changes of water for three hours. Then liming was carried out in a paddle for two days using 300% water, 3% sodium sulfide and 8% lime. The paddle was run for 10 minutes every hour. After liming, the hides were unhaired, fleshed and scudded. The fleshed weight of the pelts was noted. Then the pelts were delimed using 150% water and 1.5% ammonium chloride for 1 hour and bated using 1% alkaline bate for 1 hour. The completion of deliming was ascertained using a phenolphthalein indicator for complete disappearance of pink colour. The pelts were then pickled with 80% water, 8% salt, 1% sulfuric acid and 0.5% formic acid. The pH was checked to be 2.8-3.0 at the cross section. The pelts were left in the pickle bath over night. Next day, the drum was run for 30 minutes. Titanium-chrome combination tanning The pickled pelts were tanned with 8% of Ti-Cr combination tanning salt for 1 hour and then flooded with 100% water and the drum run for a further 30 minutes. The conventional chrome tanning was performed using 8% basic chromium sulfate in a separate drum. Then the bath was basified with 1% sodium formate (10% w/v solution) for 30 minutes and 1% sodium bicarbonate (10% w/v solution) in three feeds of 10 minutes interval for total time 1 hour. The spent tan liquors were collected and analysed. The pH of the cross section was noted to be 3.8-4.0. The leathers were washed in plain water and piled overnight. Next day, the leathers were neutralised using 1% sodium formate and 0.5% sodium bicarbonate to pH 5.2 at the cross section. The leathers were then thoroughly washed in water. The leathers were retanned with 8% syntan (3% vegetable; 3% phenolic; 2% acrylic), fatliquored with 10% fatliquor (4% vegetable; 3% semi synthetic; 3% synthetic) and dyed with acid brown dye 2%. The leathers were then assessed with physical as well as chemical testing being carried out. Analytical methods Strength property analysis Strength characteristics of the dyed crust leathers such as tensile strength⁷ and tongue tear strength⁸ were tested using an Instron tensile tester and grain crack and grain burst⁸ were tested using a lastometer. Leather samples for the physical testing were taken parallel to the backbone from the circular dyed leather samples following the IUP/1 procedure for sampling and testing⁹. Shrinkage temperature (Ts) measurement Shrinkage temperature, which is the measure of degree of tanning, has been analysed for Ti- Cr tanned as well as for control leathers. The measurements were carried out using a shrinkage tester as per IULTCS official testing method IUP16. Chemical testing analysis of leather Chemical quantitative analysis of leather was made for Cr₂O₃, TiO₂, oils & fats, moisture content, ash content, water solubles as per SLTC - IULTCS (IUC) official methods of chemical analysis⁷. Spent tan liquor analysis Collected spent tan liquor was analysed for titanium content (as TiO₂) and chrome content (Cr₂O₃) as per American Wastewater Association (AWWA) method¹⁰. The percentage exhaustion of the tanning agent in the pelt was calculated using the formula: Functional property analysis Dyed crust leathers were subjected to visual/feel assessment for functional properties such as softness, fullness, grain smoothness and tightness (break), dyeing characteristics and general appearance by hand and visual examination. The leathers were rated based on a scale of 0-10 grade points for each property by experienced professionals in the leather field. The higher grade points indicate better property for the subsequent end usage of leather. Results and discussions Physical testing characteristics Physical testing characteristics such as tensile strength and tear strength are shown in Table 1. The results indicate that strength properties are comparable for experimental and control leathers. Shrinkage temperature Shrinkage temperature of the leathers tanned with Ti-Cr combination tanning and conventional chrome tanning method are shown in Table 2. The results indicate that a shrinkage temperature of 95°C was achieved for the Ti-Cr combination tanning method. Chemical testing of leather Chemical analysis of leather indicates 8.53 and 1.85% of TiO₂ and Cr₂O₃ content (% based on dry weight of leather) for Ti-Cr combination tanned leathers as shown in Table 3. The other chemical analysis results are comparable for experimental and control leathers. Environmental benefits - Spent liquor analysis The spent liquor analysis indicates better chrome exhaustion in the pelt, 87% as compared with 55% for control process, as shown in Table 4. The exhaustion of titanium is also found to be 95%. Therefore, there is a significant reduction in the chrome load in the effluent due to adopting the Ti-Cr combination tanning system. Quality improvements - functional property analysis The general functional properties of the experimental leathers were found to be comparable with control as shown in Table 5.  Dyeing characteristics of the Ti-Cr tanned leathers are found to be better as compared to control leathers. This may be due to the fact that titanium tanning gives whiteness as a base colour improving the brilliancy and dyeing properties. Mechanism of titanium - chrome combination tanning From the earlier studies², with modified hide powders such as deaminated, decarboxylated hide powders, it was shown that carboxyl groups of collagen play a major part in the fixation of cationic titanium. Since the mechanism for the fixation of Cr is the same there may be competitive-ligand environment for both these central metal ions in the formation of co-ordinate covalent bonding leading to better fixation and exhaustion of Cr and Ti. Conclusions The present study clearly indicates that use of the titanium - chrome combination tanning system in the tanning of upper leather provides leather with comparable physical, chemical and functional properties. Dyeing characteristics of the Ti-Cr tanned leathers are found to be better as compared with control leathers. Spent liquor analysis indicated there is a substantial reduction of chrome in the effluent. There may be competitive-ligand environment for both these metal ions in the formation of co-ordinate covalent bonding leading to better fixation of chrome and titanium. Therefore, titanium - chrome combination tanning can be employed successfully for upper leathers with following advantages,

• an eco-friendly system: better exhaustion of Ti and Cr and better fixation
• a filler tannage which is suitable for upper leathers
• improved dyeing property
• comparable strength properties
• tighter and smoother grain

References 1. M P Swamy, S Bangaruswamy, S N Chatterjea, J B Rao. The chemistry and technology of titanium in leather science - A review and further prospects - Part I. Leather science. 1983; 30(10): 291.  2. M P Swamy, S Bangaruswamy, S N Chatterjea, J B Rao. Studies on the fixation of titanium: Straight and combination tanning systems - Part V. J Soc Leather Technol Chem 1985; 69(2): 44.  3. J J VanBenschoten, D G Tasset, R Eversole, W E Kallenberger. The production of white leather and boil-stable brown leather using titanium. J Am Leather Chem Assoc. 1985; 80(9): 237.  4. M P Swamy, S Bangaruswamy, S N Chatterjea, J B Rao. Studies on the masking effect of anion with titanium sulfate solution - Part II. Leather science. 1983; 30(11): 325.  5. CSIR (Council of Scientific & Industrial Research), India. Process for the preparation of a tanning agent containing titanium and chromium for using in leather processing. 1996: Indian Patent. No. 193285.  6. V Sivakumar, K Jeyaraj. Titanium chrome combination tannage for upper leathers, BTech Project Report, Anna University, Chennai, 1994.  7. SLTC (Society of Leather Technologists and Chemists). Official methods. UK: IULTCS; 1996.  8. SATRA. Test procedures.  Northamptonshire: UK; 1992.  9. IUP (International union of leather chemists' societies physical testing commission). IUP Recommended physical test methods -IUP/1. J Soc Leather Technol Chem Soc. 1958; 42: 382. 10. AWWA (American water works Association). Standard methods for the examination of water and wastewater. 20th Ed: USA; 1998. Acknowledgements The authors thank Dr T Ramasami, Secretary, DST & DG, CSIR, Government of India for motivation. Prof Dr A B Mandal, acting director, CLRI, India for encouragement. They also thank Dr D H Kamath and Dr M P Swamy (retired), CLRI scientists, Chennai, for their valuable guidance. Dr S Sadulla, head, education and training, CLRI, also gave his valuable support. For additional tables and equations, please see the print version of Leather International June 2008