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Application of rare earth salts for permanent stabilization of skin
K Dhayalan, R Aravindhan, K J Sreeram and J Raghava Rao*
Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, India Received 02 May 2008; revised 02 December 2008; accepted 02 January 2009
This study presents utility of rare earth metal salts with and without separation for bringing about a permanent stabilization of skin tanning. Hydrolytic stability, physico-chemical characteristics etc. have been evaluated. Combination tanning systems have also been worked out. Study indicates a potential use of rare earths salts for tanning, even without separation.
Keywords: Cerium, Leather, Organoleptic properties, Rare earth, Stabilization, Tanning
Introduction
Tanning increases shrinkage temperature (Ts) of leather and lateral cohesive forces of protein mainly by forming additional crosslinks1. Some tanning agents increase Ts but reduce mechanical strength. Hg, W etc.
offer high Ts2 to collagen. India and China have a large resource of rare earth (RE) metal ions3,4. REs, a group of 17 elements [lanthanum (La) and cerium (Ce) to lutetium (Lu)] in Periodic Table5, are metals in elemental state and form salts that are strong electrolytes in water. REs ionize in this medium to give triply charged ions5. Total world resources of REs, is about 35 million tonnes (MT) with process reserves being about 7 MT6,7.
This study evaluates properties of cerium (III) chloride vis-à-vis mixed rare earth chlorides (RECs) as tanning agent as replacements to chromium (Cr), either alone or in combination. Production of RE based combination tannages is also attempted.
Experimental Details
Materials
Goatskins with more compact structure and of larger area (4-6 sq ft) were chosen as raw material. All chemicals used for leather processing were of commercial grade and chemicals used for analysis of spent tan liquors were of analytical grade. Relugan RE, Basyntan DI, Basyntan FB6 and Lipoderm liquor SLW
were procured from M/s BASF Ltd, Germany.
Vernaminol liquor ASN and Vernol liquor SS were acquired from M/s ColorChem Ltd. Mixed RECs obtained from M/s Indian Rare Earths Ltd (IREL), as a proprietary product, is considerably cheaper.
Composition Analysis of Mixed RECs and Ce
Mixed REC was analyzed for Ce, La, praseodymium (Pr) and neodymium (Nd) by Inductively Coupled Plasma (ICP) analysis. Solubility and pH of 10% solution was also analyzed. To analyze Ce, sulfuric acid (10 ml) was added to REC salt solution (10-20 ml), followed by perchloric acid (15 ml). Solution was heated till conversion of Ce(III) to Ce(IV), indicated by a colour change of solution to orange. Solution was cooled and titrated against standardized ferrous ammonium sulfate using ferroin as indicator. End point was appearance of wine red colour.
Role of Masking in Hydrolytic Stability of Mixed Rare Earth Chloride
To study influence of masking agents on hydrolytic stability of REC, an aqueous solution (10%) of REC was treated with various masking agents (formate, acetate, tartarate, citrate at a molar ratio of 1:0.25). A potentiometric plot was made by titrating it against 0.1 N NaOH and measuring pH for every unit addition of NaOH at 25°C.
Tanning Studies
Three pickled goatskins were used for each experiment. Skins were treated with known quantities of
cerium (III) chloride or mixed REC such that the amount of RE oxide (10% w/w, on pelt weight) remained constant in both the cases. After addition of tanning salts, drumming was carried out for 2 h. A conventional basification (pH 3.8-4.0) was employed for both using sodium formate and sodium bicarbonate. Bath was drained and leathers washed and piled overnight. Ts was measured using Theis shrinkage tester8. Tanning trials were carried out by combining mixed REC with basic chromium (III) sulfate as well as zirconium oxychloride, such that the offer of total metal oxides remained constant. Ts and other environmental parameters were monitored.
Optimization of Tanning Process Using Mixed Rare Earth Salts
Tanning process employing mixed REC salts was optimized for high Ts coupled with high exhaustion of cerium and low emission loads. Pickled goatskins (4) were used for each trial. Four different percentages (5, 10, 15 and 20%) in relation to pelt weight of REC salts with water (50%) were used. The duration of treatment was 2 h. Final pH of leather was adjusted to be 3.8-4.2 employing basification process. Pelts were then piled for 24 h. Next day, hydrothermal stability of leathers was measured using Theis shrinkage tester8. In order to find ligands effect on Ts of tanned leather, similar experiments were carried out varying molar ratios (1:0-1:1) of ligands to that of mixed REC salts. Various ligands were sodium formate, sodium acetate, sodium citrate and sodium tartarate. Tanned leathers from goatskins were shaved to a uniform thickness (1.1 mm) and post tanned into upper crust leathers for control and experimental leathers9.
Physical Testing and Hand Evaluation of Leathers
Physical properties (tensile strength, % elongation at break, tear strength and grain crack resistance) of samples were examined10-13. Experimental and control crust leathers were assessed for softness, fullness, grain flatness, grain smoothness, grain tightness (break) and general appearance by hand and visual examination.
Leathers were rated on 0-10 points scale for each functional property by four experienced tanners, where higher points indicate better property.
Results and Discussion
Mixed Rare Earth Chlorides
This work derived its scope from the possible use of Ce or mixed RE metal ions for tanning, as a suitable
replacement for Cr (III). Total RE elements in salt (35.36%, dry wt) had following elements: Ce, 15.09;
La, 5.78; Pr, 4.36; and Nd, 10.13%. Moisture content in sample was 3%. Mixed REC salt had a chloride content of 450 mg/kg of salt.
Role of Masking on Hydrolytic Stability of Rare Earth Ions
A mole ratio of RE: masking agent of formate and acetate (1:0.25) offered an increase in hydrolytic stability by 0.5 units (Table 1). However, citrate and tartarate reduced hydrolytic stability of REC salts, attributed to poor solubility of cerium citrate and cerium tartarate.
Comparative Assessment of Cerium (III) against Mixed Rare Earth Chloride for Tanning
Ts of tanned leathers, which was evaluated using Ce (III) as against those using mixed RECs (RE oxide 2%) without any masking, was found to be 63°C and 67°C, respectively. Uptake of Ce (III) from both trials was found to be 85%, indicating suitability of mixed RECs as a potential replacement for Ce (III) in tanning.
Influence of Masking Agents on Shrinkage Temperature of Mixed Rare Earth Tanned Leathers
Masking agent was varied such that mole ratio of mixed RE to ligand was 1:0.25 to 1:1, with mixed RECs (10%). Ts (82±2°C) by a formate masking (Table 2) has been found comparable to earlier reports14,15 of mixed RECs for cerium (III) sulfate (80±2°C).
Influence of Offer of Mixed Rare Earth Chloride on Leathering Characteristics
Mixed REC (5-20% on pelt weight) was offered such that RE element content varied from 1.76 to 7.02%.
Increasing offer of mixed RECs resulted in higher Ts (Table 3). Use of masking agents improved Ts of final leathers. Among masking salts used, sodium formate provided better results. Tanning system employing 10%
Table 1—Hydrolytic stability of rare earth salts on treating with various masking agents
Masking agents pH of precipitation Mixed rare earth Cerium (III)
Without masking 5.35 5.0
Formate 6.0 5.4
Acetate 6.5 6.0
Citrate 2.7-3.0 2.3-2.5
Tartarate 2.8-3.0 2.5-2.7
Table 2—Effect of masking agents on shrinkage temperature Masking agent Shrinkage temperature
Mol ratio, 1:0.25 Mol ratio, 1:0.5 Mol ratio, 1:0.75 Mol ratio, 1:1.0
Formate 82±2ºC 78±2ºC 68±2ºC 64 ±2ºC
Acetate 68±2ºC 68±2ºC 64±2ºC 64±2ºC
Table 3—Influence of RE offer on shrinkage temperature of leathers Offer of mixed REC Shrinkage temperature, ºC
% on pelt weight No masking Formate, 1:0.25 Acetate, 1:0.25
5 62±1 74±1 68±1
10 66±2 82±1 68±1
15 68±1 78±2 70±2
20 70±1 77±2 72±2
Table 4—Spent tan liquor analysis
Total solids (TS) Chemical oxygen Exhaustion
mg/l demand (COD), mg/l of cerium, %
Control* 60000±120 2700±24 67#
5% Rare earth 74388±82 1566±32 89
10% Rare earth 95623±143 5696±21 88
15% Rare earth 140783±127 8784±14 84
20% Rare earth 152225±151 11534±18 54
*8% BCS offer; # Chromium
mixed RECs and sodium formate (1:0.25) resulted in higher Ts.
Effluent Characteristics
Uptake of RE ions was found to be around 88% at an offer of 10% mixed REC on pelt weight (Table 4). An increase was observed in total solids (TS) content and COD as against that for conventional Cr tanning, could be attributed to presence of chloride ions in mixed REC salt. Modulations in process line so as to avoid pickling and utilize inherent chlorides available with tanning agent could bring down COD and TS loads.
Combination Tanning Employing Mixed Rare Earth Chloride and Chromium
Chrome tanned leathers were better than RE tanned leather. Thus, combination tanning would be required to attain comparable properties to that of chrome-tanned leathers. Metal oxide content in combination tanning system was kept constant and equal to metal oxides employed for RE tanning. Major drawback of RE tanning is Ts and poor feel16. In present work, combination of RECs with Cr and zirconium (Zr), respectively have been attempted. At an offer of BCS (2%) and mixed REC (8%), Ts was 92±2°C (10oC rise over mixed RE tanning); this
was further enhanced (110±2°C) when BCS offer was 4% and mixed RECs offer was 6% on pelt weight.
Characteristics of RE-Cr Combination Tanning System
Tensile, tear and grain crack strength measurements were carried out for conventional chrome tanned leathers (8% BCS offer) and RE-Cr tanned crust leathers, both along and across backbone line (Table 5). Tensile strength of combination tanned leathers is comparable to that of chrome tanned leathers, in spite of a reduction in offer of Cr by 50% (as Cr2O3).
TS and COD (Table 6) are comparatively equal for conventional leathers and those tanned using BCS (4%) and REC (6%). The % exhaustion of Cr in chrome (2%) and chrome (4%) were more or less 90%, whereas in case of control leather, it was 67-70%. No decrease in Cr uptake was found on combination tanning with RE, which is an indicative of nature of binding of Cr and RE metal ions. RE metal ions do not compete for the sites of preference to Cr or that penetration of mixed RE salts is poorer than that of Cr. Therefore, an increase in Ts and leathering characteristics was observed when a
combination tanning with Cr was carried out. At BCS (4%), organoleptic properties were at par with that of conventional chrome tanned leathers.
Chromium Free Combination Tanning Using Zirconium and Mixed Rare Earth Chlorides
A complete replacement of Cr in tanning was evaluated through combination tanning employing mixed REC and zirconium oxychloride. In this case also metal offered for tanning was kept constant, and equal to the metal content of RE salt. At an offer of Zr (2%) and RE (8%), Ts observed was 84±2oC, while with Zr (4%) and RE (6%), Ts observed was 92±2oC, indicating that penetration of RECs into collagen matrix may not be sufficient enough to bring about adequate crosslinking. However, combination with RE ions, Ts of Zr tanned leathers was above 90oC, which is an indicative of the nature of binding of RE ions. RE ions bind to carboxyl or amino groups of collagen, whereby an enhancement in Zr is observed. Zr being bound to the matrix predominantly through H-bonds, would provide for leather the much desired fullness.
Table 5—Physical strength characteristics of rare earth-chromium tanned leathersa Tanning system Tensile Extension at Tear strength Grain crack
strength break, % Kg/cm Load Distension
Kg/cm2 Kg mm
Control (8% BCS) 230±5 53±6 52±3 30±1 8±1
8% RE 240±3 51±5 55±2 21±3 6.0±1
Expt 1 (2% BCS & 8% RE) 211±9 51±4 48±4 20±2 6.2±1
Expt 2 (4% BCS & 6% RE) 261±7 49±4 49±4 25±2 7.5±1
amean values of two leathers, in each leather two determinations were performed [at 95% confidence interval (P=0.95)]; RE, rare earth
Table 6—Effluent characteristics of RE-Cr tanned leather
Offer of salts COD TS Exhaustion of
% ppm ppm cerium, %
8% BCS (control) 2500±23 50000±48 67#
2% BCS + 8% RE salt 4628±21 82769±55 92
4% BCS + 6% RE salt 3132±14 54346±45 85
# Chromium
The strength characteristics of tanned leathers using a combination of Zr and REC was comparable to that of Cr (Table 7). Further, a higher offer of RE ions indicates a lower loading and hence higher strength properties to leather than when Zr offer is higher. Spent tan liquor analysis of effluent arising from RE- ZrOCl2 combination tanning has been carried out to evaluate impact on environment. TS and COD are comparatively equal to conventional chrome tanned leathers (Table 8). A higher uptake of metal ions (Zr and Ce) in the process indicates that binding sites for two metal ions could be different.
Leather tanned using mixed RE and Zr salts provided better organoleptic properties on par with that of control chrome tanned leathers.
Conclusions
An evaluation of RE ions as possible replacement for Cr in tanning has been carried out. Ce (III) ions have been found to behave in a similar manner to that of mixed RE salts, wherein no separation of Ce from other lanthanides has been carried out. Mixed RECs behave in a similar or better manner than cerium (III) chloride, with respect to hydrolytic stability and leathering characteristics and offer
Table 7—Physical strength characteristics of rare earth-zirconium tanned leather Tanning system Tensile Extension at Tear strength Grain crack
strength break, % kg/cm Load Distension
kg/cm2 kg mm
Control (Cr) 230±5 53±6 52±3 30±1 8±1
Expt 1 (2% ZrOCl2 & 8% RE) 242±6 61±2 43±4 33±1 10.9±1
Expt 2 (4% ZrOCl2 & 6% RE) 220±8 54±4 38±6 26±4 9.3±1
amean values of two leathers, in each leather two determinations were performed (at 95% confidence interval (P=0.95))
Table 8—Effluent characteristics of RE-Zr tanned leather Offer of salts (%) COD, ppm TS, ppm exhaustion, %
8% BCS (control) 2500±23 50000±48 67# 2% Zr + 8% RE salt 3702±25 127200±112 75 4% Zr + 6% RE salt 3132±21 81879±87 84
# Chromium
a cheaper alternative to Cr. Mixed RECs masked with formate provides for a shrinkage temperature of 82°C and meets strength requirements of upper leather. A combination of Cr with REC indicates that mixed REC salts can replace Cr partially in tanning, without any impairment to shrinkage temperature, organoleptic and physical properties. This also reduces Cr load on environment upon disposal of used leather.
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