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Journal of Cleaner Production 17 (2009) 1603–1606

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

A salt-free pickling regime for hides and skins using oxazolidine

Kai Li a,b, Hui Chen a, Yajuan Wang a, Zhihua Shan a,*, Jeff Yang c, Patrick Brutto c

a The Key Laboratory of Leather Chemistry and Engineering of the Ministry of Education, Sichuan University, Chengdu 610065, P.R. Chinab Hunan Entry-Exit Inspection and Quarantine Bureau, P.R. Chinac The Dow Chemical Company – Dow Biocides, Midland, MI 48674, USA

a r t i c l e i n f o

Article history:Received 12 July 2008Received in revised form10 June 2009Accepted 12 June 2009Available online 21 June 2009

Keywords:Salt-free picklingSwellingChrome uptakeOxazolidine

* Corresponding author.E-mail address: likai152142@163.com (Z. Shan).

0959-6526/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.jclepro.2009.06.004

a b s t r a c t

During the last few decades new materials and technologies have being studied and applied in order toreduce the ecological impact of leather production. Severe restrictions imposed by the pollution controlauthorities on the disposal of chromium, total dissolved solids and chlorides in tannery effluents haveforced the tanners and researchers to look for eco-benign pickling and tanning processes. In this study,we would focus on the novel process development of pickling with the presence of oxazolidine basedproduct. Investigations indicated that oxazolidine is a potential material to replace neutral salts inpickling and, realize high chrome exhaustion in the subsequential chrome tanning.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In the conversion of hides and skins to chromium tannedleather, an intermediate pickling process is normally performedbefore tanning, which brings the entire collagen matrix to uniformchemical and physical conditions to prevent the rapid combinationof the skin substrate with chromium compound [1], thereforeenables the even penetration (without precipitation) of chromiuminto the collagen matrix. Salt, usually sodium chloride or some-times sodium sulphate and the salts of the organic acids such assodium formate and sodium acetate, are previously added torestrict the swelling of the collagen in the acidic medium [2]. Theacidic treatment minimizes the negative charge of the carboxylgroups and maximizes the positive charge of the amino groups ofthe collagen peptides thus making the pelt positively charged. Therepellent effect of like charge widens the spacing of the collagenpeptides resulting in collagen swelling. What’s more, the conglu-tination and crosslinking of the collagen fibres is partially destroyedand the stability of the collagen weakened. Such swelling anddisruption of crosslinking can probably be avoided with the pres-ence of neutral salt. The prior addition of neutral salt into thepickling bath increases the ion concentration resulting in highosmotic pressure. This enhances the migration of water from theinside of the collagen to the outside, resulting in a decrease of the

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ion concentration in the pickling bath. The ionization of the acidand the salt is thereby increased. The ions then penetrate thecollagen under osmotic pressure, passivating the carboxyl groupsand making the amino groups exist as amine salts. The swelling ofthe collagen is avoided [3].

Although it is effective and inexpensive, the traditional processdescribed above suffers in that high salt concentrations in thetannery effluent can be a serious environmental problem:contributes to about 35% total dissolved solids (TDS) in the effluents[1]. The TDS load in the waste water is high due to the greatapplication of salt in preservation and tanning stages of life cycle ofleather [4]. Salinity and TDS in tannery effluents is a major envi-ronmental concern where the effluents are used for irrigation. Highsalinity in irrigation water causes high osmotic pressure, resultingin dehydration and retarded growth of salt-intolerant crops. Inaddition, high concentrations of sodium ions in irrigation water canadversely affect soil structure by causing dispersion of clay [5].What’s more, in the conventional pickling process the temperatureis usually maintained at or below 28 �C because raw skins havea low shrinkage temperature, usually below 40 �C at this stage.Higher temperatures would lead to loose grain and shrinkage of theskin. As a result, the pickling process usually takes a long time andoften requires workers to add ice to control the temperature. So theprocess is labor intensive and higher cost.

To overcome the environmental problems of TDS related toneutral salts in the effluent, recycle/reuse of pickle liquor for thesubsequent batches as well as resorting to pickle free alum-chromecombination tanning systems have been studied [6]. Salt free

K. Li et al. / Journal of Cleaner Production 17 (2009) 1603–16061604

pickling using naphthalene sulfonic acid is well known andextensively studied [7]. Pickle-less tanning system using chromesyntan and modified basic chromium sulfate (BCS) has beenestablished [1,8]. Pickle and basification free tanning system usingcommercial BCS has also been developed for cowhides [9]. Anenzyme-driven three-step tanning process applicable to hides andskins [10,11–14] has been developed that involves enzymatic de-hairing, enzyme-based fiber opening, and pickle-free chrometanning at pH 4.0–8.0. An approach of pickle-less enzymaticvegetable tanning was developed by Kurian Joseph and N. Nithya todecrease TDS and chlorides using pickle-less vegetable tanning byavoiding the use of salt in the pickling process [15]. An integratedchemo-enzymatic methodology has been explored by R. Aravind-han and his colleagues resulted in decreasing of COD and TS (totalsolids) loads by 67 and 78% [16].

The purpose of our research and paper is to develop a novelpickling process to reduce or eliminate the use of neutral saltsthereby avoiding the shortcomings of the traditional picklingprocess.

Research completed during the past few decades has shown thatsalt-free pickling processes should allow blocking of collagenamino groups at relatively low pH (around 3.0 or lower). Thisprevents protonation of the amino groups thereby avoiding like-charge repulsion and swelling of the collagen.

2. Experiments and results

2.1. Aims of experiments

Experiments in this study were focus on developing a novelpickling method with the presence of oxazolidine based product toreduce the environmental impact from neutral salts in picklingstep. The novel regime should be practicable and economical.

Table 1Percent increase in substrate weight as a measure of swelling.

Pickling regime Increase inweighta (%)

Ts1

(�C)Increase inweightb (%)

Ts2

(�C)

Acid only (no salt) 70–908% Saltþ acid

(standard pickling)�35 to �40 61.8 �30 102.0

1% ZEþ acid 8.3 76.0 6.4 103.82% ZEþ acid �1.6 78.8 5.9 106.73% ZEþ acid �2.7 81.1 2.3 106.94% ZEþ acid �6.7 83.7 0.4 107.5

Ts1 ( �C): Ts of skin after pickling (with & without oxazolidine); Ts2( �C): Ts of skinafter chrome tanning.

a Percent weight gain (loss) before/after pickling.b Percent weight gain (loss) before/after tanning.

2.2. Methods and procedures

Oxazolidine chemistry was first introduced to the leatherindustry by Samir DasGupta and patented in the 1970s [17]. Themain utilization of oxazolidines in the leather industry has been intanning, retanning and finishing processes. Leathers obtained fromprocesses using oxazolidines are soft and light in color.

Oxazolidines can rapidly raise the shrinkage temperature of skinover 60 �C, often in less than 30 min, and tend to give highershrinkage temperatures at higher pH. Addition of oxazolidine intothe delimed pelt allows higher temperatures during pickling thanthe conventional processes. That is to say, there is not as muchconcern about skin shrinkage and loose grain when oxazolidine isused prior to pickling.

In this novel pickling process oxazolidine was introducedstarting from delimed pelt using the following steps (offers are ondelimed weight):

Acetic acid (1:10 v/v aqueous solution) until pH 5.0–6.0;ZOLDINE ZE 1–4% (based on the weight of pelt) to desired time;Formic acidþ sulphuric acid (1:10 v/v aqueous solution) untilpH 3.0� 0.1;Stand overnight, chrome tanning, dyeing/fatliquoring.

In order to have a comparative study, normal pickling to pH3.0� 0.1 was conducted using 8% (wt) sodium chloride. Subse-quent tanning with 6% (wt) chrome salt allowed us observe theexhaustion of chromium. The chrome-tanned crust was thensubjected to standard dyeing/fatliquoring processes. Theshrinkage temperature of the ZE-treated skin and chrome-tanned

crust were measured, and the data were reported as the mean oftwo determinations.

The delimed pelt, pickled pelt and chrome tanned crust wereweighed to determine the degree of swelling. The Cr2O3 content ofthe tanned crust was also determined and the uptake of chromesalt calculated. The data were reported as the mean of twodeterminations.

A nephelometer was used to measure progress of the fatli-quoring step. The initial turbidity of the fatliquoring bath wasmeasured on a dilution of 1 mL fatliquor bath in 50 mL; this wasrepeated every 15 min. The fatliquoring agent was a blend ofseveral kinds of common commercially available fatliquoringagents, added at 16% on the weight of tanned crust.

The dyeing speed was investigated using a spectrometer tomeasure initial dye absorption and absorption at 15 min intervalsthereafter. Exactly 1 mL of dyeing liquor was diluted to 100 mL forthese measurements. The dye was a common commerciallyavailable black dye, added at 2% based on the weight of tannedcrust.

2.3. Materials and reagents

Oxazolidine. ZOLDINE ZE, a bisoxazolidine from ANGUS Chem-ical Company, is supplied at approximately 95% actives. The pH ofthis material (0.1 M aqueous at 20 �C) is 10.2.

Basic chrome sulfate. Common commercially available materialcontaining 28.33% (wt)Cr2O3;

Hide and skin substrates. Delimed and pickled sheepskin peltswere purchased from local tanneries. The delimed pelts werereceived at pH 7.8 and the pickled pelts were received at pH 3.0;

Ts of delimed pelt: 58.7 �C;Ts of pickled pelt: 61.8 �C;

Drums. Laboratory scale trials were conducted on piecesmeasuring 20� 25 cm, with average weight 100 g. The pieces wereprocessed in cylindrical stainless steel drums (40 cm diameter,20 cm length) which were rotating in a temperature controlledwater bath.

2.4. Results

The results obtained are presented in Tables 1 and 2, Figs.1 and 2.As expected, treatment of the delimed pelt with ZE controlled

swelling during the pickling process with the absence of neutralsalts. Table 1 shows that 1% ZE is not enough to prevent swellingduring pickling, there was still nearly 10% increase in weight.However there was a decrease of swelling after chrome tanning

Fig. 3. Assumed mechanism of oxazolidine–collagen combination. (P-NH2 presents theamino group of the collagen side-chain.)

Table 2Effects of different pickling regimes on chrome uptake.

Pickling regime Cr2O3 (%) Uptake (%)

8% Salt (standard pickle)þ acid 4.21 71.91% ZEþ acid 4.78 80.42% ZEþ acid 5.27 88.53% ZEþ acid 5.37 90.34% ZEþ acid 5.42 91.1

Cr2O3%: Cr2O3 content of completely dried chrome-tanned crust.

0

20

40

60

80

100

0 15 30 45 60 75 90 105

Time, min

Upt

ake,

8 salt + acid2 ZE + acid

Fig. 1. The uptake of fatliquor under different pickling processes.

K. Li et al. / Journal of Cleaner Production 17 (2009) 1603–1606 1605

indicating that even 1% ZE has a positive effect. With a ZE dosage of2% or more the pelt does not swell at all during pickling. The Cr2O3

content and shrinkage temperature of crust increase withincreasing offer of ZE.

Considering the cost and application effect, 2% ZE is enough tocarry out the novel pickling process to replace the neutral salts. Theresults of subsequent dyeing and fatliquoring process are presentedin Figs. 1 and 2.

Figs. 1 and 2 indicate that the pickling process with ZOLDINE ZEdid not adversely affect the subsequent dyeing and fatliquoringprocesses. Chrome tanned crust treated with ZOLDINE ZE beforepickling exhibited the same affinity to fatliquor and dye as tradi-tional pickled crust did.

As reported in the literature [18,19], we can explain the resultsas follows. The covalent bonding of oxazolidine and the amino

0

20

40

60

80

100

0 15 30 45 60 75 90

Time, min

Upt

ake,

8 salt + acid2 ZE + acid

Fig. 2. The uptake of dye under different pickling processes.

groups of collagen is favored by relatively high pH relative to theisoelectric point of the collagen (close to pH 5.0), according tofollowing mechanism (Fig. 3): hydrolysis – ring-open – binding.

The reaction of oxazolidine with amino groups of the collagenside-chains prevents them from combining with hydrogen ions.Swelling of the pickling pelt is therefore prevented. And thecombining of collagen with chromium during the subsequentchrome tanning step increases because of the previously exposedcarboxyl groups, resulting in relatively higher chrome uptake.

Further investigations were launched into the physical-mechanical properties of the leather obtained from this new pick-ling process. The ZE pickled leather was treated with traditionaltanning/dyeing/fatliquoring processes, and the tensile strength,elongation and tear strength of the finished leather weremeasured; the results are reported below in Table 3.

As indicated in Table 3, the novel pickling method with ZEprovides leather with identical tensile strength to that producedwith traditional pickling. The leather produced with ZE has supe-rior elongation, and although there is a significant difference in thetear strength the value is far above the National Standard. All thingsconsidered, leather obtained from our novel pickling regime withZE complies with the main Chinese standard requirements and ISOstandard for garment leather.

3. Industrial application

Oxazolidine based product has being widely used in thetanneries both in the north and south of China. About 1 ton of suchproduct is consumed every day in the processes of salt-free picklingand chrome-free combination tannages.

The above mentioned salt-free pickling regime was conducted inNingbo Feifan Leather Production Company and Wenzhou WeishuoLeather Production Company since May of 2006, 3000 sheep skinsevery day for each company. The main differences between thetraditional and the novel regime are displayed in Table 4 below.

Table 3Effects of different pickling regimes on the physical-mechanical properties.

Pickling regimesa Tensile strength (MPa) Elongation (%) Tear strength (N/mm)

Pickling with ZE 22.934 82.710 71.242Pickling with salt 22.727 72.565 94.317National standardb �6.5 20–60 �18ISOc – – �20

a Goat skin, tanned with 6% basic chrome sulfate.b China Light Industry Standard Collection – Garment Leather (QB 1872–1993).c ISO/FDIS 14931: 2003 Leather – Guide to the selection of leather for apparel

(excluding furs).

Table 4The main differences between the two pickling regimes.

Items Traditional regime Novel regime

Neutral salt 8% 0Oxazolidine ZE 0 2%Ice (control the temperature) 20% 0Chromium (tanning and retanning)Dosage 10% 7%Uptake 70–75% >90%Glutaraldehyde (retanning) 2% 0Water (washing off the neutral salt) 400% 100%COD (before treatment) 1500–2000 1200–1800BOD (before treatment) 400–1000 800–1200Total dissolved chlorides 10,000–15,000 5000–10,000Total dissolved solids 35,000–55,000 20,000–35,000Undegradable pollutants Neutral salt and chromium ChromiumVolume of effluent (Comparatively) 1 0.8Erodibility to equipment Serious SlightTs >90 �C 4–8 �C higher

K. Li et al. / Journal of Cleaner Production 17 (2009) 1603–16061606

In the industrial production, no neutral salt is needed to controlthe swelling when Oxazolidine ZE is introduced and it doesn’taffect the following processes. It is obvious that the cost of Oxa-zolidine is higher than neutral salt, but when neutral salt isreplaced by Oxazolidine, it means lower cost on ice, water, tanningand retanning agent, equipment, labor and effluent management.So the total cost of the novel pickling is not necessarily higher thantraditional regime. Since Oxazolidine and its derivatives have beingwidely used in medicine, agriculture and biochemistry, no litera-ture about its toxicity is published till today, so there is no evidenceof the harm on the human health if Oxazolidine treated leather isused.

4. Conclusions

1. Pretreatment with 2% ZOLDINE ZE before pickling controlsswelling of pelt in the picking process with the absence ofneutral salts, it doesn’t adversely affect the subsequent dyeingor fatliquoring process and furthermore improves chromeabsorption and the hydrothermal stability of resulting crust.

2. The physical properties of the resulting crust are well withinthe requirements of the Chinese and ISO standards for garmentleather.

3. According to the information from the tanneries, the cost of thesalt-free pickling regime would be slightly lower than thetraditional one.

Acknowledgements

Cooperative research program between the Dow ChemicalCompany through its Biocide Business and Sichuan University.

Financial support from the Dow Chemical Company, Jeff Yangand Patrick Brutto from the Dow Chemical Company are gracefullyacknowledged.

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