Although chrome is considered as the major tanning agent in the production of all types of hides and leather worldwide, it represents a serious source of environmental pollution. Therefore, polyamidoamine hyperbranched polymer (HPAM) was involved in pretanning of the depickled hides to enhance the chromium uptake during the tanning process. The key parameters which affect the exhaustion and fixation of chrome tan including shrinkage temperature of the tanned leather were studied. The results showed a significant improvement in the chrome exhaustion, the shrinkage temperature, and the texture and softness of the leather treated by HPAM.

1. Introduction

At the last decades, hyperbranched polymers have attracted great attention due to their dendritic architecture as highly branched polymers and their unique properties including lower viscosity, higher solubility, and higher amount of reactive terminal groups, compared with their equivalent linear analogues. Furthermore hyperbranched polymers are synthesized via one pot reaction using various techniques such as step-growth polycondensation, self-condensing vinyl polymerization, ring-opening polymerization, self-condensing ring-opening polymerization, and proton transfer polymerization [15]. Several types of hyperbranched polymers were prepared such as polyphenylenes, polyethers, polyesters, polyamides, polycarbonates, poly(ether ketones), and polyurethanes [612]. Generally, hyperbranched polymers are readily feasible for different potential applications, such as toughening and cocuring agents, multifunctional initiators, rheology control compounds for surface modification, medical applications, nanofillers for polymer nanocomposites, nonlinear optics, nanoporous generators for low dielectric constant insulators, coatings, and drug delivery systems [1324]. Therefore, hyperbranched polymers are of immense importance for industrial applications. Accordingly, this work was devoted to explore their application for further use in the leather industry as pretanning agents (modifying agent) to enhance the uptake of chrome during tanning process.

In tanning process, protein of raw hides, which is easily attacked by organisms, is converted to a stable fiber structure, preventing their putrefaction. Although several types of metallic salts are employed for tanning of leather, chromium salts are still considered as the most important tanning agents used for all types of hides and leather. Generally, chromium salts are involved in more than 80% of the tanning industry worldwide [25]. However, under normal tanning conditions, chromium salts are not completely uptaken during tanning process from the tanning bath where about 40% of chrome tan does not react with the hide proteins and discharges in the spent tanning solution as wastewater [26, 27]. Moreover, about 11 wt.% of the total chrome in the wet tanned leather is released during sammying, washing, and retanning processes. Therefore, the tanning operation is disposed unsafely, and it poses a threat to the environment and wastes the valuable chromium salts [28]. As a result, only 47 wt.% of the used chrome tan is fixed with leather [29]. From the environmental point of view, chromium wastes cause a serious source of pollution in the tannery effluent where the trivalent chromium Cr(III) thereby, which is less toxic, transforms into hexavalent chromium Cr(VI) which is very toxic for both plants and animals by oxidation in combination with other factors such as extremes in pH of the tanning medium [28, 30]. Accordingly, several technologies for better management of chromium salts in tanneries have been developed through high exhaustion, recovery, and recycling processes, by using efficient tanning agents as alternatives to chromium salts and/or by safe utilization and disposal of chromium-bearing wastes [3134]. On the other hand, many attempts have been done to reduce the residual chrome in the tanning effluent by development of the primary tanning conditions and consequently to reduce the pollution of environment [3537].

In this paper, a multifunctional aminoterminated hyperbranched poly(amidoamine) (HPAM) was used as pretanning agent to develop the conditions of tanning process and hence to improve exhaustion and fixation of chrome tan. HPAM was prepared in one pot reaction through Michael addition of methyl acrylate (MA) monomer to ethylenediamine (EDA). The modification of depickled hide by the prepared HPAM polymer, as pretanning agent, was studied as a novel process for chrome tanning operation. The factors affecting chrome exhaustion and fixation, during tanning process, as well as shrinkage temperature of the tanned leather were extensively investigated.

2. Materials and Methods

2.1. Materials

Ethylenediamine (EDA) and methyl acrylate (MA) monomer were purchased from Sigma-Aldrich and were used as received. Local bovine hide (virgin leather) was selected for this study. Basic chromium sulphate (BCS) of 33% basicity (Bayer, Germany) was used as the chrome tan. All other reagents and chemicals were of pure grade and were used as received.

2.2. Methods
2.2.1. Preparation of Hyperbranched Poly(amidoamine) (HPAM)

0.9 mL (0.001 mol) of methyl acrylate (MA) was added drop-wise to ethylenediamine/methanol solution (0.001 mole of amine in 100 mL of methanol) with continuous stirring for 48 h at room temperature. The solvent was removed from the reaction mixture under reduced pressure as previously described in the literature [3840].

2.3. Tanning Process

The investigated raw leather samples were selected from bovine hides (depickled). The samples were worked up in the beam-house operations according to the conventional methods, as schematically illustrated in Figure 1 [41]. Briefly, depickled hides were firstly treated with 1% sodium formate solution for about 20 minutes. Afterward and before tanning process, the leather samples were treated with the prepared HPAM at different weight ratios (3, 5, and 8 wt.%) in aqueous media under continuous shaking for one hour. Then, HPAM-treated leather samples were treated with 12 wt.% of chrome tan (BCS) solution with continuous shaking for 8 hrs at room temperature. The percentages of the used reagents were calculated based on the dry weight of the depickled hide.

2.4. Characterization
2.4.1. Characterization of HPAM

Fourier transform Infrared spectroscopy (FT-IR) was used to verify the chemical composition of the prepared aminoterminated hyperbranched polymer (HPAM) using Jasco FT-IR-6100 spectrometer with resolution from 400–4000  . Proton nuclear magnetic resonance spectrum (1H NMR) of the prepared HPAM was recorded on Varian Mercury-Oxford-300 MZ using tetramethylsilane (TMS) as internal standard with CDCl3 as deuterated solvent. Gel permeation chromatography (GPC) coupled with refractive index (RI) detector was used to determine the number average molecular weight and the polydispersity index of the prepared hyperbranched polymer (HPAM). Samples were dissolved in N, N-dimethyl formamide (DMF) as the eluent. Measurements were carried out at 50°C with eluent flow rate 0.7 mL/min and injection sample volume 25 mL. GPC instrument used in the measurements was a modified HPLC, Waters 600 System Controller, 717 plus Autosampler; Columns: PhenomenexPhenogel 10 um 500 A,  mm PhenomenexPhenogel 5 um 50 A,  mm; Detection: Waters model 2410 Refractive index, .

2.4.2. Characterization of Tanning Process

Chrome exhaustion and fixation during tanning process are of paramount importance for evaluating performance, monitoring the quality of the tanned hide, and for controlling the remained chrome in the spent liquor. Chrome exhaustion was determined by measuring the chrome content in the tanning bath effluent, after tanning process, using atomic absorption (AA) measurement where the percentage of chrome fixation by leather was determined by immersing the tanned leather in deionized water for 3 days. Afterward, the released chromium in the solution was determined via Varian atomic absorption spectrometer (Specter AA 220). Acetylene/nitrous oxide flame was used with fuel flow rate of and oxidant flow rate of . Chrome was measured in the peak height mode at 357.9 nm, and deuterium background correction was applied for background fluctuation. The detection limit achieved with this line at the used operating conditions was 9.07 μg  with a linear dynamic range between 0.1 and 10 μg  . The recorded values are the average of three measurements.

Examination of the leather samples was carried out via scanning electron microscopy (SEM) where circular-shapeed specimens of 10 mm diameter were cut from the leather samples before and after treatment with HPAM and from chrome tanned leather after treatment with HPAM. These specimens were subjected to sputter-coating (Edwards’s model S 140A) of gold ions. Then, the sputter coated samples were scanned with JEOL Model JSM-T20 SEM.

Shrinkage temperature is considered as an important parameter, which gives an indication about the efficacy of the tanning agent, where is the temperature at which the hide has a significant shrinkage when it exposes to a heated medium. The reaction between chrome tan and raw hides results in stabilizing the protein fibers via their crosslinking with chrome. Thus, degree of stabilization is mostly investigated by measuring shrinkage temperature of the tanned hide. was measured according to the official reported methods and standards [4244]. The recorded value in each case is the average one of three measurements.

3. Results and Discussion

3.1. Preparation of HPAM

Multifunctional aminoterminated poly(amidoamine) hyperbranched polymer (HPAM) was prepared via Michael addition of methyl acrylate (MA) to ethylenediamine in one pot reaction, as shown in Scheme 1 [38]. The chemical structure of the prepared HPAM was confirmed by FT-IR, 1H NMR, and GPC analyses.


FT-IR (KBr) spectrum of HPAM is shown in Figure 2 where several characteristic bands appeared at 3286 and (ν NH2); (ν NH); 2941 and (ν CH2); (ν C=O) and (ν C–N). On the other hand, 1H NMR spectrum (CDCl3) of HPAM, as shown in Figure 3, revealed chemical shifts (δ, ppm) at 2.41 (NH2), 2.51–2.70 (COCH2, NHCH2, NH2CH2), and at 3.12–3.54 (NCH2). Additionally, number average molecular weight and polydispersity index PDI of the prepared HPAM were determined via GPC as 12249 g/mol and 1.9, respectively.

3.2. Chrome Tanning Process

As previously mentioned, the current research work was dedicated to enhance the chrome-uptake of the tanning process by changing the primary tanning conditions. This target was achieved by using the prepared aminoterminated hyperbranched polymer (HPAM) as a pre-tanning agent for leather (see Supplementary Material available online at http://dx.doi.org/10.1155/2013/120656). Before chrome-tanning process, the leather samples were immersed in HPAM aqueous solution under continuous shaking for one hour. Then, HPAM-treated leather samples were washed several times with distilled water to get rid of the unreacted HPAM with raw hides. Then, the tanning process was carried out according to the optimum conditions as reported in our previous studies [45, 46]. Briefly, HPAM-treated raw hides were immersed in acidic aqueous solution (at pH 2.5) containing 12 wt.% basic chromium sulphate with shaking (150 rpm) for 24 hrs. The shaking process served in flexing the raw hides and enhancing the penetration of chromium salts into the fiber structure [47, 48] allowing the chemical reactions between the chrome tan and hide proteins to take place efficiently. Also, it is worthy mentioning that the tanning process was performed at pH 2.5 because chromium(III) tanning salts are highly soluble in strong acids but usually precipitate as chromium hydroxide or hydrated chromium oxide at pH values above 4.0 [48, 49], in addition to the formation of large complexes which impedes their diffusion into the raw hide [50]. The key parameters which can influence the chrome exhaustion and fixation, using HPAM aminoterminated polymer, were studied.

3.2.1. Effect of HPAM Polymer Concentration

To determine the optimum conditions for enhancing exhaustion and fixation of chrome tan, HAPM polymer was used at different concentrations (3, 5, and 8 wt.%), as pre-tanning agent for treatment of the depickled hides, and then these samples were followed by chrome-tanning process. The samples were denoted as HP-CTL 1, HP-CTL 2, and HP-CTL 3, respectively. Chrome-tanned leather sample (CTL) untreated with HPAM was used as a blank for comparison. As clearly evidenced from Table 1 and Figures 4 and 5, the treatment of the raw hides by HAPM led to a remarkable improvement in chrome exhaustion and consequently to a drastic reduction in the residual chrome in the tanning bath. Furthermore, the optimum concentration of HPAM was found to be 5.0 wt.% which was equal to 99.8% of chrome exhaustion and 2.15 mg/L of the residual chrome in the tanning bath.

These remarkable results were attributed to the treatment of leather fibers by HPAM which imparts extra reactivity, through its reactive peripheral aminogroups with collagen fibers and consequently with more chrome salts in the tanning bath. More interestingly, it is highly favorable from the environmental point of view to reduce the amount of the residual chrome in the tanning bath, which means less pollution.

3.3. Shrinkage Temperature

Shrinkage temperature , as previously mentioned, is an indicative parameter for evaluating the efficiency of tanning process. In other words, it reflects the reaction between chrome tanning agent and hide proteins which results in stabilizing the fibers by crosslinking. Accordingly, the increase in of the tanned hides refers to the improvement of exhaustion and fixation of chrome during tanning process. As indicated in Table 2, of the chrome-tanned hides was remarkably increased by using HPAM polymer as pre-tanning agent. In addition, increased in parallel with HPAM concentration.

3.4. Investigation of the Effect of HPAM as Pretanning Agent via SEM

In order to investigate the effect of the prepared HPAM as pre-tanning agent on the grain surface and protein fiber bundles of leather, a comparative morphological study was carried out on the chrome-tanned leather before (CTL) and after pre-tanning with HPAM (HP-CTL2). SEM micrographs of the cross-section (100x) and grain surface (50x) of CTL and HP-CTL2 samples are shown in Figure 6.

As clearly seen from Figure 6(a), the porous surface of chrome-tanned leather sample (CTL) was filled up upon treatment with HPAM (Figure 6(b)). In addition, the CTL pretanned with HPAM (HP-CTL2) had a significant improvement in softness of the grain surface, as detected by handle inspection. Furthermore, the cross-section SEM photos represented that the protein leather fibers of CTL samples (Figure 6(c)) were completely separated from each others. In the case of HP-CTL2 (Figure 6(d)), leather protein fibers aggregates were noticed and referred to the interaction of HPAM with leather protein fibers. Similar results were obtained in case of using acrylate copolymer emulsion as pre-tanning agent, as reported in our previous published work [51]. Generally, during tanning process, chrome tan (Cr(III)) reacts with the polypeptide chains of leather via coordination bonds with the aminogroups of leather protein (collagen) fibers forming crosslinked structures [44]. The compatibility between chemical structure of (poly(amidoamine)) HAPM and that of leather protein fibers (polypeptides) made it suitable to be used as pre-tanning agent and improved the interaction of HPAM with the protein fibers. Accordingly, the treatment of raw hides by aminoterminated HPAM led to increasing the number of metal binding sites (amino groups) on fiber’s surface, and consequently enhanced chrome tan uptake, as shown in Scheme 2.


4. Conclusions

Aminoterminated hyperbranched poly(amidoamine) (HPAM) had remarkable effect as pre-tanning agent for leather, which led to distinct improvement in chrome fixation and exhaustion during chrome-tanning process. This in turn was reflected on the reduction of chrome loss in the tanning bath, which should have great impact on the environment. Furthermore, leather samples showed improvement in leather smoothness and soft texture upon treatment with HPAM in addition to increasing their shrinkage temperature. In fact, HPAM can be used as a multipurpose pretanning agent for leather and more interestingly, it may serve in protection of the environment from inorganic heavy metals like chrome which is considered as a dangerous source of pollution.

Supplementary Materials

Employing of poly aminoamine hyperbranced polymer (HPAM) as pre-tanning agent for leather.

  1. Supplementary Material