Ekoloji 21, 82, 17-25 (2012) doi: 10.5053/ekoloji.2011.823

Eco-leather: Chromium-free Leather Production Using Titanium, Oligomeric MelamineFormaldehyde Resin, and Resorcinol Tanning Agents and the Properties of the Resulting Leathers Arife Candas ADIGUZEL ZENGIN1, Marian CRUDU2, Stelian Sergiu MAIER3, Viorica DESELNICU2, Luminita ALBU2, Gurbuz GULUMSER1, Behzat Oral BITLISLI1, Bahri BASARAN1, Mehmet Mete MUTLU1* 1Ege University, Engineering Faculty, Leather Engineering Department Bornova, Izmir- TURKEY 2National Research and Development Institute for Textile and Leather, Leather and Footwear Research

Institute 031215 Bucharest- ROMANIA 3Gheorghe Asachi Technical University of Iasi, Faculty of Textiles, Leather and Industrial Management

700050 Iasi- ROMANIA *Corresponding

author: [email protected]

Abstract The chromium salt tanning system, which is still the most popular leather tanning procedure, is under continuous pressure from environmental groups and international regulations. Therefore, for many years, numerous experiments have been carried out on chromium-free leather production. In this study, newly produced synthetic and inorganic chemicals were used as tanning and pre-tanning agents as an alternative to a chromium tanning agent. This new titanium based tanning agent, obtained from processing wastes from the industry of nonferrous metals, was used as a main tanning agent. In addition, an oligomeric melamine-formaldehyde resin and resorcinol type pre-polymer were used as pre-tanning agents for the production of chromium-free leathers. Physical tests and chemical analysis of the produced chromium-free leathers gave comparable results to leathers tanned with basic chromium sulphate. Keywords: Chromium-free leathers, Melamine-Formaldehyde resin, Resorcinol pre-polymer, Titanium based tanning agent. Eko-deri: Titanyum, Oligomerik Melamin Formaldehit Reçinesi ve Rezorsinol Tabaklama Maddeleri ile Kromsuz Deri Üretimi ve Elde Edilen Derilerin Özellikleri Özet Deri tabaklama işleminde hala en popüler olan krom tuzları ile tabaklama sistemi, çevre grupları ve uluslararası sınırlamalar tarafından sürekli baskı altındadır. Bu nedenle, krom içermeyen deri üretimi için yıllardır çok sayıda çalışma yapılmaktadır. Bu çalışmada, krom tabaklama maddesine alternatif olarak yeni üretilen sentetik ve inorganik kimyasal maddeler tabaklama ve öntabaklama maddeleri olarak kullanılmıştır. Demirsiz metal endüstrisinin atıkları işlenerek elde edilen bu yeni titanyum esaslı tabaklama maddesi ana tabaklayıcı olarak kullanılmıştır. İlaveten, kromsuz derilerin üretiminde oligomerik melamin-formaldehit reçinesi ve rezorsinol tipi bir pre-polimer ön tabaklama maddesi olarak kullanılmıştır. Üretilen kromsuz derilerin fiziksel ve kimyasal testleri, bazik krom sülfat ile tabaklanmış derilerle kıyaslanabilir sonuçlar vermiştir. Anahtar Kelimeler: Kromsuz Deri, Melamin Formaldehit Reçinesi, Rezorsinol, Titanyum esaslı tabaklama maddesi. Zengin ACA, Crudu M, Maier SS, Deselnicu V, Albu L, Gulumser G, Bitlisli BO, Basaran B, Mutlu MM (2012) Eco-leather: Chromium-free leather production using Titanium, Oligomeric MelamineFormaldehyde Resin, and Resorcinol Tanning Agents and the Properties of the Resulting Leathers. Ekoloji 21 (82): 17-25.

INTRODUCTION During the last century, ecological awareness has increased in relation to public consciousness. Besides, consumers became suspicious and began considering the ecological and toxicological aspects of the goods they use in addition to their appearance, durability, colour, and other physical

characteristics. Legislation on the other hand, has led industrial producers to consider human health and the environment. The aspect of producing leather and textile products with no impact on the ecological balance, affecting both human and environmental health, is an important focal point to be pursued. Researches Received: 26.07.2011 / Accepted: 22.02.2012

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Ekoloji on the utilization of natural products or chemicals having less impact on the environment as substitutes of hazardous chemical products are gaining higher importance (Onem 2011). The chromium salt tanning system, which is still the most popular leather tanning procedure, is under continuous pressure from environmental groups and international regulations due to pollution and toxicology reasons unfairly associating the Chromium (III) commonly used with hazardous Chromium (VI). Tanners are finding it increasingly difficult to comply with ever emerging regulations with respect to the chrome content of effluent as well as the chrome containing solid wastes such as sludge, shavings, leather trimmings, and buffing dust. In some countries there are restrictions on the use of chrome-tanned leathers for certain purposes (Dasgupta 2002). The deep interest in clean technologies and stringent norms set by regulations, have led the tanners to increase their efforts to develop chromefree tanning agents. Chromium-free leathers have advantages, like the lack of chromium in the effluents, obtaining fully recyclable shavings and end-products for agricultural applications, no risk of Cr (VI) formation, metal free leathers, improved sorting in the pre-tanned stage, and white and light colored, brilliant leathers. Therefore, different markets require the manufacture of chrome-free leather to have comparable properties to chrome tanned leather such as feel, fullness, softness, and hydrothermal stability. It has been suggested that the environmental impact of chromium could be alleviated by substituting all or part of the offer by other metal tanning salts, when the following options are the likeliest candidates: Al(III), Ti(III)/(IV), Fe(II)/(III), Zr(IV), and lanthanide(III). Other alternative tanning options can be polyphenols, oxazolidine, formaldehyde, polymers, carbohydrates, etc. (Covington 2008). Modern tanning chemistry can be classified by mineral tanning, vegetable tanning, oil tanning, aldehyde tanning, syntans, and organic tanning (Covington 1997). Many alternative tanning agents have been explored for their solo tannages, however, no individual tanning agent has yet to match the properties of chromium (Saravanabhavan et al. 2004). The main objective of this study is to develop a new ecological tanning technology based on newly 18

Zengin et al. synthesized titanium and organic tanning agents based on pre-polymers and oligomers as an alternative to chrome tanning, in order to protect the environment and increase the quality of life. MATERIAL AND METHOD Ten (10) pickled sheep skins from the same origin were used for the chromium-free leather production for determining the effects of the newly synthesized leather tanning products. These newly synthesized titanium and organic tanning agents based on pre-polymers and oligomers shown in Figures 1, 2 and 3 were used as pre-tanning and tanning agents for this study (Crudu et al. 2010, Pruneanu et al. 2010) The skins were classified into 5 groups and tanned according to the experimental design given in Table 1. The pickled sheep skins were processed according to Table 2 and the recipes were differed during the pickling process by the addition of pretanning agents according to the experimental design of the tanning process. These recipes are given in Tables 3, 4, 5, 6, and 7. After the tanning process, half of the chromiumfree tanned leathers were dried and prepared for chemical analysis and physical tests for the determination of the characteristic properties of tanned leathers. Besides, the other half of the leathers were processed according to Table 8 in different drums for obtaining finished leathers to compare the change of their properties with the tanned leathers. After toggling, leather samples were prepared in accordance with “sampling location” and conditioned according to “sample preparation and conditioning” standards TS EN ISO 2418 and TS EN ISO 2419 (Anonymous 2006b and 2006c). For determination of the chemical and physical properties of the chromium-free leathers, TS 4119 EN ISO 3376: tensile strength (Anonymous 2006a), TS 4119 EN ISO 3376: elongation at break (Anonymous 2006a), TS 4118-2 EN ISO 3377-2: tear strength (Anonymous 2005a), color measurement of surface with spherical spectrophotometer, TS EN ISO 17226-1: formaldehyde determination with HPLC (Anonymous 2009d), TS 4120 EN ISO 3380: shrinkage temperature (Anonymous 2005b), TS EN ISO 4045 pH value (Anonymous 2009b), TS 4125 EN ISO 4047: ash content % (Anonymous 2009a), TS EN ISO 4048 determination of matter No: 82, 2012

Ekoloji

Eco-leather: Chromium-free leather production using...

Table 1. Experimental design of the tanning processes.

Fig. 1. Complex of Ti (IV) with N-hydroxysuccinimide as ligand.

Fig. 2. Melamine-formaldehyde oligomer resin.

Table 2. Recipe for pre-tanning process.

Table 3. Tanning recipe of standard chromium tannage (D0).

Fig. 3. Pre-polymer (Resorcinol).

soluble in dichloromethane (%) (Anonymous 2009c), and TS 4126 chromium oxide (%) (Anonymous 1985) tests were performed and the properties of the chromium-free leathers tanned with newly synthesized tanning agents were compared with the properties of leathers tanned with chromium. The color measurements of the chromium-free and chromium leather samples were performed with a Minolta CM-2600d spherical spectrophotometer with the parameters of D65 day light and with an observation angle of 10 degrees. The colors of all leather samples and their color differences with the white standard were evaluated according to the CIE Lab color coordinate system. RESULTS Tensile strength, elongation at break, and tear strength results of the leather samples after tanning and retanning and fatliquoring processes are given in Table 9 and 10. After the tanning process, the tensile strength and tear strength values of samples D1, D2, D3, and D4, the chromium-free leathers, were found higher than sample D0 which is the standard chromium No: 82, 2012

Table 4. Tanning recipe of pretan TMW/1+ melamine formaldehyde resin (D1).

*Pretan TMW/1 contents 10% Titanium oxide

Table 5. Tanning recipe of pretan TMW/1 + pre-polymer (resorcinol) (D2).

*Pretan TMW/1 contents 10% Titanium oxide

tanned leather (Table 9). After retanning and fatliquoring process of the same leather samples, it was determined that the tensile strength values of 19

Ekoloji Table 6. Tanning recipe of pretan TMW/1 + melamine formaldehyde resin + pre-polymer (resorcinol) (D3)

*Pretan TMW/1 contents 10% Titanium oxide

Table 7. Tanning recipe of Pretan TMW/1(D4).

*Pretan TMW/1 contents 10% Titanium oxide

Table 8. Post-tanning (retanning/fatliquoring) recipe for all leather samples.

the chromium-free leather samples decreased comparing to the values obtained after the tanning process. However, these values were still similar to the tensile strength values of chromium leather. But the tear strength values of chromium-free leathers were determined to be relatively higher than the tear strength values of sample D0 (Table 10). Table 11 shows the CIE Lab color values, color differences (ΔE), and pseudo colors (generated by 20

Zengin et al. the software) of the white reference and leather samples after tannage. Higher values of lightness (L) indicate closeness to white. Considering the L values, D4 shows the highest lightness value by showing a characteristic titanium tannage color. Another predominant color observed was yellow and it was detected by higher b values. Yellowness of D2 and D3 samples were caused by resorcinol usage. However for sample D1, melamine formaldeyde resin gave lower +b values compared to D2 and D3. It is well known that the color of leather is also affected by retanning and fatliquoring agents. Table 12 shows the CIE Lab color values, color differences (ΔE), and pseudo colors (generated by the software) of white reference and leather samples after retannage and fatliquoring processes. When comparing the ΔE values of tanned and finished leathers (Table 11 and Table 12), it was seen that the color of the D0 leather sample was not much different after the retanning and fatliqouring process (ΔE difference=1.04) although colors of all the other chromium-free leather samples changed from white to yellow towards red. Results of the chemical analysis performed to tanned and re-tanned and fatliquored samples are given in Table 13. When the matters of soluble in dichloromethane (%) results were examined, it can be seen that the results after the tanning and retanning and fatliquoring processes were found between 11.32%14.87%, and 19.75%-22.78% respectively. The pH value of chromium-free tanned leathers was determined as 3.9 and in the meantime the pH value of retanned and fatliquored chromium-free leather samples was found as 4.9. Besides, the pH value of the chromium leather chosen as the blank sample was 3.18 and 4.14 after the tanning and retanning and fatliquoring process respectively. The ash contents (%) of chromium-free leathers after the tanning process was found between 8.33 and 10.56%, but on the other hand, the ash content percentage of the leathers after the retanning and fatliquoring process decreased nearly half and was found between 4.11 and 4.59%. The formaldehyde content of the D0, D2, and D4 samples were found lower than the D1 and D3 samples due to the lack of melamine-formaldehyde resin in their processes. However, an increase was determined for the formaldehyde content of reNo: 82, 2012

Eco-leather: Chromium-free leather production using... Table 9. Physical test results of leathers after tanning process.

Table 10. Physical test results of leathers after re-tanning and fatliquoring process.

Table 11. Colors and color differences of leathers after tanning process comparing white reference.

Table 12. Colors and color differences of leathers after retannage and fatliquoring process comparing white reference.

Table 13. Results of chemical analysis.

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Ekoloji tanned and fatliquored samples which originated from the retanning materials. The highest formaldehyde content was found as 16.21 ppm for the D3 sample. The chromium oxide contents of the D0 sample were determined as 3.068% and 2.309% after the tanning and retanning and fatliquoring processes respectively. In the case of the shrinkage temperature, the chromium tanned leather has the highest value (96°C) as expected and on the other hand, the samples tanned with the new tanning agents provide values between 65 and 72°C. The retanning and fatliquoring process improved the shrinkage temperature of the chromium tanned leather and sample D2, but provided very little change with the other samples. Incomplete binding and/or washing off the tanning agent is thought to be the reason. DISCUSSION Although chromium is commonly regarded as a “perfect” tanning agent in terms of leather performance, there are some drawbacks as it is a limited natural resource, with uncontrolled oxidation which leads to toxic species and adverse environmental impacts (Skyes 1988). Many researches have been carried to overcome these drawbacks by various alternative cleaner tanning systems like ferrous, aluminum, titanium, zirconium, polyphenols, polymers, aldehydes, carbohydrates, tannic acid, etc (Saravanabhavan et al. 2004, Covington 2008, Hui et al. 2010). Among these, titanium is one of the most promising substitutes for chrome tanning in the leather industry because titanium is abundant in nature, easily obtainable, and posses little ecological impact. Titanium tanned leathers indicate high strength properties, have the same charge as chromium tanned leather, and are compatible with anionic retanning, fatliquoring agents, and dyestuffs which, enhance the titanium tanned leathers’ properties (Peng et al. 2007a). In this study, a new ecologic chromium-free tanning technology based on newly synthesized titanium, oligomeric melamine-formaldehyde resin, and resorcinol tanning agents has been developed and tested on pickled sheep skins, as an alternative to chromium tannage, for its potential usage for chromium-free leather production. The Tanning process has many definitions, but can be summarized as the stabilization of the 21

Ekoloji collagen matrix to retain separated fiber structure and to increase the hydrothermal stability. For example, Hui et al. (2010) have described the tanning process as the stabilization of hide or skin against, wet heat, enzymatic attack, and thermomechanical stress. However, tanners and consumers expect additional properties from leather besides bacterial and hydrothermal resistance. Covington (2009) has stated that tanners do expect other changes and lists them as: appearance, handle, smell, rise in denaturation temperature, resistance to putrefaction by microorganisms, and a degree of permanence to changes. With Regards to these definitions and expectations, the pickled sheep skins were converted to leathers. The organoleptic controls showed that Ti-tanned leathers were not as soft as chromium tanned leathers; their grains were more flat and tight, and the thickness was found to be lower. However, these differences were compensated with retanning and fatliquoring processes. The characteristics of these chromium-free leathers were found acceptable after post tanning processes. Leathers should have enough strength properties depending on the usage area. UNIDO offers a minimum of 10N/mm2, 15 N/mm2, and 20 N/mm2 of tensile strength for chromium tanned garment leathers, linings, and shoe upper leathers respectively (Anonymous 1996). Peng et al. (2007b) found the tensile strength of Ti-tanned leathers as 30.72 N/mm2. Also, UNIDO offers a minimum of 15N/mm, 25 N/mm, and 30 N/mm of tensile strength for chromium tanned garment leathers, linings, and shoe upper leathers respectively (Anonymous 1996). Peng et al. (2007b) found the tensile strength of Ti-tanned leathers as 79.72 N/mm and our results are compatible with that of Peng et al. 2007b. This establishes that, Ti-tanned leathers have higher tensile and tear strength properties than chromium tanned leathers. Tanning materials give their own colors to leather, which can be easily observed on undyed leathers and even tanning materials can be recognized by their colorization on the leather. This study is a good representation of this. The leathers tanned with titanium (IV) salts are initially colorless although it tends to go pale yellow with aging (Covington 2009). Peng et al. (2007b) obtained white colored leathers with Ti-tanned leathers. The resultant leathers tanned with titanium were whiteivory colored in this study. 22

Zengin et al. Resorcinol and melamine-formaldehyde resins are mainly used in the retanning process to support the main tannage and enhance leather properties. However, they do also affect the color of the leather. The leather tanned with a tetrakis (hydroxymethyl) phosphonium chloride and resorcinol combination was characterized by a light color (Windus and Happich 1963), although the resorcinol-formaldehyde tanned skin gave the typical dark brown color (Jones and Windus 1968). Besides, some synthetic resins such as melamine urea formaldehyde resin produce light-colored leathers with high light fastness (Simon and Pizzi 2003). The resultant leathers tanned with these newly synthesized organic tanning agents were ivory and yellow colored, which were darker than titanium tanned leathers. This color change increased after the retanning and fatliquoring processes. The reason for the darkening and yellowing colors of chromiumfree leathers originated from the fatliquoring process and fatliquoring agents. In considering the results of the chemical tests, firstly we see an increase was observed in the percentage of the matters soluble in dichloromethane. This was due to the lubricating agents used in the fatliquoring process. Also, fatliquoring agents gave a softer handle and higher elasticity which can be also understood from the elongation at break (%) results shown in Table 9. For the elongation at break, fatliquoring contributes to its increase, since it gives a sliding effect to the fibers (Gutters and Santos 2009). The results of the pH analysis showed the characteristic pH values of the last processes. Peng et al., (2007b) ended the tanning process at pH 3.5 for Ti-tanned leathers. In our study, the pH of Titanned leathers was measured as 3.9. This pH increased to 4.9 after the retanning and fatliquoring processes. The ash content (%) of the chromium-free leathers, after the tanning process, decreased almost by half after the retanning and fatliquoring process. This decrease can be explained in two different ways, first, the ratio of inorganic matter in the leather samples decreased due to the increase of the organic contents in percentage such as the fatliquoring materials and second, the dissolution and rinsing of the tanning materials from the samples, due to the wet-end processes, could be another reason for this decrease. A certain amount No: 82, 2012

Eco-leather: Chromium-free leather production using...

of tanning material passes into the float in the wetend processes after tannage and this can be reduced by forming a complex with a second tanning material (Colak et al. 2005). Variable limits exist for free formaldehyde content, depending on the legislation and the usage of leather products in a country. It is not recommended that the products for general applications, products in contact with skin, and baby leather products contain formaldehyde higher than 200, 75, and 20 ppm respectively (Anonymous 2008). The formaldehyde content of chromiumfree leather samples were found to be below these risky limit values. The minimum chromium oxide limit for finished leather is suggested as 2.5% (UNIDO 1996) and the samples tanned with chromium in this study, contains chromium within these limit values. Shrinkage temperature of raw mammalian skin varies between 58 and 64°C. In most cases, tannage enhances the shrinkage temperature. However, some leathers like oil-tanned chamois skins and brain tanned elk skins, stabilizes the process and does not increase the shrinkage temperature, but these products exhibit all the characteristics of true leathers (Thomson and Kite 2006). High shrinkage temperatures, like 90°C, can be achieved with Titanium solo tanning, however, this requires large quantities and this causes the leather to be overfilled, and yet remain soft (Covington 2006). Peng et al. (2007b) has tanned goat skins with a 7.5%

Ekoloji TiO2 offer and the ending shrinkage temperature was 84°C after tannage. In our study, we used a 10% titanium offer (10% active matter) over leather weight which is equal to a 1% TiO2 offer and ending shrinkage temperature was 72°C after tannage. Finally the following conclusions have been drawn: The whitest colored leathers were obtained by using Titanium based tanning material solo. Also, the physical test results of Ti-tanned leathers were better among all tanning trials. However, the handle of these leathers needs to be improved. The use of a melamine-formaldehyde based tanning agent gave light colored leathers with an ivory color. But, the use of resorcinol caused a yellowness to the samples. The physical test and chemical analysis of all chromium-free leathers produced comparable results to standard chromium leather except for shrinkage temperatures. Since color change is an important factor, a Ti-tanning agent and melamineformaldehyde based tanning agent combination is advised for obtaining light colored and chromiumfree leather with the closest properties to chromium leathers. ACKNOWLEDGEMENT The authors would like to thank Tübitak and ANCS for their financial support to this project (Project Number: 109M393) and the State Planning Organization (DPT) along with the Ege University Scientific Research Projects Office (BAP) for their instrumental support (Project number 2007-DPT001 and 2010-MUH-009).

REFERENCES Anonymous (1985) TS 4126 Determination of Chromic Oxide (Cr2O3). Turkish Standard (TSE), Ankara, Turkey. Anonymous (1996) UNIDO Acceptable quality standards in the leather and footwear industry. ISBN: 92-1-106301-9, Vienna. Anonymous (2005a) TS 4118-2 EN ISO 3377-2 Physical and mechanical tests, Determination of tear load - Part 2: Double edge tear. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2005b) TS 4120 EN ISO 3380 Physical and mechanical tests. Determination of shrinkage temperature up to 100 C. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2006a) TS 4119 EN ISO 3376 Physical and mechanical tests, Determination of tensile strength and percentage extention. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2006b) TS EN ISO 2418 Chemical, physical and mechanical and fastness tests, Sampling location. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2006c) TS EN ISO 2419 Physical and mechanical tests, Sample preparation and conditioning. Turkish Standard (TSE), Ankara, Turkey.

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Anonymous (2008) Restricted substances-formaldehyde. Leather International Magazine 7: 44-45. Anonymous (2009a) TS 4125 EN ISO 4047 Determination of sulphated total ash and sulphated waterInsoluble ash. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2009b) TS EN ISO 4045 Chemical tests, Determination of pH. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2009c) TS EN ISO 4048 Chemical tests, Determination of matter soluble in dichloromethane and free fatty acid content. Turkish Standard (TSE), Ankara, Turkey. Anonymous (2009d) TS EN ISO 17226-1 Chemical determination of formaldehyde content, Part 1: Method using high performance liquid chromatography. Turkish Standard (TSE), Ankara, Turkey. Colak S, Ozgunay H, Mutlu M M, Akyuz F (2005) Reducing the Amount of Tanning Materials Passing into Wastewater in Post-tanning Processes. Journal of the American Leather Chemists Association 100: 111-118. Covington (1997) Modern tanning chemistry. Chemical Society Reviews 26: 111-126. Covington (2006) The Chemistry of Tanning Materials. In: Kite M, Thomson R (eds), Conservation of Leather and Related Materials, Elsevier, Oxford, 29-30. Covington (2008) Alternative Tanning Options. Leather Magazine, July, 16-22. Covington (2009) Tanning Chemistry-The Science of Leather. The Royal Society of Chemistry, Cambridge. Crudu M, Deselnicu V, Mutlu, MM, Gulumser G, Bitlisli BO, Basaran B, Zengin ACA (2010) New Tanning Agents Based on Titanium and Zirconium. In: Albu L, Deselnicu V (eds), Proceedings of the 3rd International Conference on Advanced Material and Systems, 16-18 September 2010, Bucharest, 27-32. Dasgupta S (2002) Chrome free tannages: Part I Preliminary studies. Journal of the Society of Leather Technologists and Chemists 86 (5): 188-194. Gutters M, Santos LMD (2009) Study of Fatliquoring Parameters Using Experimental Design. Journal of the Society of Leather Technologists and Chemists 93: 171-175. Hui C, Jun G, Zhi-Hua S (2010) A Cleaner Chrome-Free Tanning Regime: Sulfonated Urea-PhenolFormaldehyde Condensed Polymer and Ferrous Sulfate Tanning. Journal of the American Leather Chemists Association 105: 18-24. Jones H W, Windus W (1968) A Comparison of the Tanning Action of Dithioresorcinol and Resorcinol. Journal of the American Leather Chemists Association 63: 452-456. Onem E, Gulumser G, Ocak B (2011) Evaluation of Natural Dyeing of Leather with Rubia Tinctorum Extract. Ekoloji 20 (80): 81-87. doi: 10.5053/ekoloji.2011.8011 Peng B Y, Shi B, Ding K, Fan H, Shelly CD (2007a) Novel Titanium(IV) Tanning for Leathers with Superior Hydrothermal Stability Part II The Influence of Organic Ligands on Stability and Tannin Power of Titanium Sulfate Solutions. Journal of the American Leather Chemists Association 102: 261-270. Peng B Y, Shi B, Ding K, Fan H, Shelly CD (2007b) Novel Titanium(IV) Tanning for Leathers with Superior Hydrothermal Stability Part III Study on Factors Affecting Titanium Tanning and an EcoFriendly Titanium Tanning Method. Journal of the American Leather Chemists Association 102: 297-305. Pruneanu M, Maier SS, Maier V, Deselnicu V, Mutlu MM, Gulumser G, Bitlisli BO, Basaran B, Zengin ACA (2010) Oligomeric Melamine-Formaldehyde Resin as Pre-Tanning Agent, In: Albu L, Deselnicu V (eds), Proceedings of th 3rd International Conference on Advanced Material and Systems: 1618 September 2010, Bucharest, 113-124. Saravanabhavan S, Fathima NN, Rao JR, Nair BU (2004) Combination of white minerals with natural tannins chrome-free tannage for garment leathers. Journal of the Society of Leather Technologists and Chemists 88 (2): 76-81. Simon C, Pizzi A (2003) Lightfast and High Shrinkage Temperature Leather Produced Using Vegetable Tannins and Novel Melamine-Urea-Formaldehyde Tanning Formulations. Journal of the American Leather Chemists Association 98: 83-96. Skyes R L (1988) Improvement of Chromium potentials in leather industry. Journal of the Society of Leather Technologists and Chemists 72: 1-6. 24

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Thomson R, Kite M (2006) Conservation of Leather and Related Materials. Elsevier, Oxford. Windus W, Happich W F (1963) A New Tannage Tetrakis (Hydroxymethyl) Phosphonium ChlorideResorcinol. Journal of the American Leather Chemists Association 58: 638-645.

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