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Research Article pubs.acs.org/journal/ascecg

A Chrome-Free and Chrome-Less Tanning System Based on the Hyperbranched Polymer Taotao Qiang,*,† Xin Gao,† Jing Ren,† Xiaoke Chen,‡ and Xuechuan Wang† †

Shaanxi Research Institute of Agricultural Products Processing Technology, Shaanxi University of Science & Technology, WeiYang District, Xi’an 710021, China ‡ China Leather and Footwear Industry Research Institute, Chao Yang District, Beijing 100015, China S Supporting Information *

ABSTRACT: This paper discusses the terminal carboxyl groups of the hyperbranched polymer (HPAE-C) complexing Al3+ in aluminum sulfate to synthesis a new chrome-free tanning agent (HPC-Al). The molecular structure of HPC-Al was characterized and analyzed by IR, UV, and XRD. In the tanning process, the results indicate the following: (1) The shrinkage temperature of HPC-Al tanning leather is 79.5 °C. (2) The shrinkage temperature is greater than 95 °C after chrome retanning and about 87 °C after zirconium retanning. (3) It has greatly improved the fullness, softness, and physical-mechanical properties of the leather. Meanwhile, the environmental impact assessment after tanning was evaluated. The results show that biochemical oxygen demand, total dissolved solids, and total suspended solids are reduced. This indicates that the use of HPC-Al as a tanning agent, with chrome-free or chrome-less tanning, can be achieved. KEYWORDS: HPAE-C, Complexing, Chrome-free tanning agent, Application

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have a large number of active groups.28 If we make the terminal groups of hyperbranched polymers coordinate with groups of hide collagen fiber macromolecules (such as hydroxyl, amino, carboxyl, etc.),29 it forms lots of firm chemical bonds and a multisites-cross-linked structure. Thus, we would find a new way to study chrome-free and chrome-less tanning based on hyperbranched polymers.30,31 In this paper, a chrome-free tanning agent from a hyperbranched polymer with terminal carboxyl groups and aluminum sulfate was synthesized. It has an excellent retanning effect and primarily overcomes the poor washablity and low shrinkage temperature10 of the aluminum tanning agent.

INTRODUCTION Chrome tanning methods are still very popular in the leather industry worldwide. With increasing concerns for the environment and establishment of strategies for sustainable development, many researches were studied on chromium pollution. Researchers found that Cr(III) will be converted to Cr(VI) with the existence of an oxidant.1,2 It is certainly harmful to the human body,3 water environment,4 and soil and has a potential risk to the natural environment. At the same time, the chromium resource is limited in the world. It is also very harmful to green and sustainable chemistry when we consume so much chromium in the tanning and electroplating industry5 because most of the chromium is nonrecyclable and it remains in the effluent. So the development of a chrome-free and chrome-less tanning agent becomes the current research’s hot topic.6−8 Aluminum tanning has a long history in the leather industry.9,10 The finished leather appears pure white and has great softness, elongation, and fine grains.10 However, it is unsatisfactory because of its low shrinkage temperature (73 °C) and poor tanning performance (weak tanning effect) .10 Aluminum tanning is mainly used in combination tannage and pretannage,11 such as when combined with an inorganic tanning agent (such as chrome, zirconium, and titanium)12−15 or organic tanning agent (such as vegetable, glutaraldehyde, and synthetic).16−23 This dramatically improves the shrinkage temperature. In recent years, hyperbranched polymers have been paid more attention by researchers concerned with novel structures, unique properties, and potential application prospects.24−27 Compared with ordinary polymers, hyperbranched polymers © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials and Equipment. Pickled pigskin was supplied by Liquan Shunji leather Co., Ltd. The hyperbranched polymer with terminal carboxyl groups (HPAE-C) was made by our laboratory.32 Sodium hydroxide(NaOH, ≥ 98.0%) was purchased from Shanghai Reagent Co., China. Maleic anhydride (C4H2O3, ≥ 99.0%) and p-toluenesulfonic acid (C7H8O3S·H2O, ≥ 98.5%) were purchased from Aladdin Industrial, Inc. Aluminum sulfate (Al2(SO4)3·18H2O, ≥ 99.0%) was purchased from Tianjin Fuchen Chemical Reagent Co., China. The chrome retanning agent Tankrom FS was purchased from SISECAM and was used without further modification. The zirconium retanning agent Tanfix SZS was obtained from Clariant Chemicals (China), Ltd., Guangzhou Branch. Received: April 27, 2015 Revised: December 5, 2015

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DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

shrinks to one-third of its original length, this temperature was taken as the shrinkage temperature.34 Physical-Mechanical Property. The physical testing has been examined using the standard IULTCS methods.33 The mechanical properties of aluminum tanned leather, chrome retanned leather, zirconium retanned leather, and market leathers were analyzed by an electronic universal testing machine (UTM2102, Shenzhen Suns Technology Stock Co., Ltd.). The mechanical properties were compared after conditioning the leather for 48 h at 20 ± 2 °C with a relative humidity of 65 ± 2%. Thickening Rate. We conditioned the leather for 48 h at 20 ± 2 °C with a relative humidity of 65 ± 2%. We marked the thickness before tanning as d1 (mm) and the thickness in the same place after retanning as d2 (mm). Tp (thickening rate) can be calculated according to the following formula:

The PHS-3C pH meter was produced by Shanghai Precision & Scientific Instrument Co., Ltd., China. The MSW-YD4 digital leather Ts meter was produced by Shaanxi University of Science & Technology, China. The VECTOR 22 Fourier-transform infrared spectrometer was provided by Bruker Co., Germany. The D/max2200PC X-ray diffractometer was purchased from Japan. The Cary 50 Ultraviolet spectrophotometer was made by VARIAN Co., U.S.A. Preparation of HPAE-C. A weight of 0.1 mol hydroxyl-end hyperbranched polymer (HPAE-H) and 0.66 mol of maleic anhydride were added to the distillation flask, using an appropriate amount of p-toluenesulfonic acid as the catalyst, after the completion of feeding; the reaction was for 4 h at 80 °C. Then, under 130 °C to vacuum distillation of the filtrate, remove the unreacted maleic anhydride until no bubbles appear, and then obtain a yellow viscous liquid, which is HPAE-C. The reaction formula of HPAE-C is shown in Figure 1.

Tp =

d 2 − d1 × 100% d1

Environmental Impact Assessment. The pH, chromium content, chemical oxygen demand (COD), and biochemical oxygen demand (BOD) were analyzed by using the standard procedure (GB 30486-2013) for the tanned exhaust liquor from experimental processing.35 Total dissolved solids (TDS) and total suspended solids (TSS) were calculated after tanning and retanning. Leather Productive Process. Pickled pig skin, HPC-Al tanning agent, and aluminum sulfate were used in the tanning experiment. We tested the shrinkage temperature and thickness of pickled pig skin before the experiment. The leather productive process of tanning, retanning, and detections is shown in Figure 3, and tanning process parameters are shown in Table 1.

Figure 1. Reaction formula of HPAE-C.

Figure 2. Synthesis of hyperbranched polymer tanning agent (HPC-Al). Preparation of HPC-Al. HPC-Al was synthesized by HPAE-C with aluminum sulfate as shown in Figure 2. Add HPAE-C (10 g) and aluminum sulfate (5 g) to a beaker (250 mL) along with 50 mL of water, and add sodium hydroxide (0.5 mol/L) to adjust the pH to 3.0. Then, the mixture was stirred for 3 h at 30 °C using a magnetic stirrer. After the reaction, transfer the mixture to a rotavapor to evaporate excess water until the system has no bubbles, and obtain a yellow jelly (HPC-Al). Gel Permeation Chromatography (GPC). Neutralize HPC-A1 to a pH of 6.5 ± 0.1 by adding 0.5 mol/L NaOH solution; the mobile phase was 0.1 mol/L NaNO3. Fourier Transform Infrared Spectroscopy (FT-IR). Before testing, the unreacted materials and solvent were removed by rotary evaporator and placed in vacuum drying oven for 5 h at 105 °C. Then, measured by the thin-film method, the wavenumber range was 500 to 4000 cm−1. UV−Visible Spectroscopy (UV−vis). After purification and drying, the sample was dissolved in water to form a solution of 40 mg/L and tested with water as a reference. X-ray Diffraction (XRD). After removing the unreacted materials and solvent, the sample was dried for 5 h at 105 °C. The data was collected in a 2θ-range of 5° to 60.0°, with a step size of 0.04°and an accompanying scan rate of 0.02°/s. Physical Testing. Shrinkage Temperature. The shrinkage temperature of leather was determined by using the standard IULTCS (International Union of Leather Technologists and Chemists Societies) method.33 The leather sample was suspended vertically in water, and the rate of heating was maintained at 2 ± 0.2 °C/min. When the leather

Figure 3. Leather productive process of tanning, retanning, and detections.

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RESULTS AND DISCUSSION Characterization and Performance. Determination of Relative Molecular Mass. Gel permeation chromatography (GPC) is one of the most widely used techniques for the determination of the molecular weight and polydispersity index (PDI) of the polymers. The GPC analysis was performed for the soluble samples prepared in water. Table 2 gives the GPC characteristics of HPC-Al. It was found that the molecular weight was divided into two levels (L1 and L2); peak molecular weight (Mp) is 2776 and 2200, respectively. Hyperbranched polymers were synthesized by a one-step method generally. The mix was not separated in accordance with relative molecular mass, and the products showing different GPC levels were normal. Molecular weights of HPC-Al B

DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Tanning Process Parameters of Pigskin Garment Leather process

material/g

dosage/%

temperature/°C

pretreatment

water salt formic acid pickled pig skin

100 8 0.1

28

water control pickling

water 100 salt 8 formic acid 0.5 HPC-Al/aluminum sulfate/chrome tanning agent AB 8 sodium bicarbonate 1.2 water 100 Stopped drum and stayed overnight water 200 degreasing agent DN 0.5 formic acid 0.3

tanning

degreasing

water retanning

pH

5 30

2.2

notes

25 Be. ≥ 6.5

40

3*10a 300 5*20 90

2.2 3.8

40 60 washed twice

water chrome retanning agent FS/zirconium retanning agent SZS sodium bicarbonate

100 2 0.4−0.6

40

water neutralizing agent NE105 sodium bicarbonate sodium formate

150 1 0.7 1

32

water neutralizing

120 3*20

3.5 4.0 washed twice

water filling

90

5.5−6.5 washed twice

water 150 40 acrylic retanning agent SN 2 dispersed tanning agent SR 2 lecithin EBS 2 50 synthetic oil PSW 2 electrode-stable fatliquor NUS 3 wool grease DSF 3 fish oil TSF 1 formic acid 1.5 Water, stopped drum, stayed overnight, dry, caging, togging

oiling

a

time/min

60

120 3*10 + 30b

3.6

3*10: Added in three portions, 10 min interval. b3*10 + 30: Added in three portions, 10 min interval, then changed to 30 min.

Table 2. GPC Characteristics of HPC-Al HPC-Al

Mn

Mw

Mp

Mz

Mz+1

PDI

L1 L2

2508 1971

2566 2031

2776 2200

2635 2092

2722 2152

1.02 1.03

are generally in agreement with theoretical values (PDI = 1.02−1.03). The pore size of leather fibers is certain; it decided the size of the leather tanning agent. If the size of tanning agent is small, effective cross-linking cannot be formed, and it is referred to as non-tannage.10 Conversely, some performance could be limited. For example, the relative molecular mass of vegetable tannins in leather is 500−3000.10 If it is below 500, the tanning property is not good, and if it is above 3000, it would affect the combination between tannins and leather fibers. Using the size of the molecules of the compound tanning agent and vegetable tanning agent as a reference, the size of HPC-Al molecules in this paper would not affect the penetration of HPC-Al in leather in theory. FT-IR Analysis. FT-IR spectra provide a rapid means for the characterization of functional groups of organic molecules. FT-IR spectra of HPAE-C and HPC-Al recorded at ambient temperature are shown in Figure 4.

Figure 4. Infrared spectrum patterns of HPAE-C and HPC-Al.

Figure 4a shows the FT-IR spectra of HPAE-C, and there is a strong and broad band in the region of 3200−3500 cm−1, confirming a high concentration of the hydroxyl from carboxyl groups in HPAE-C.36 The absorptions in the range of 1726 cm−1 are double bonds between C and O in the carbonyl group. The absorptions of 2650−2950 cm−1 belong to the vibration of methyl and methylene groups.32 Compared with Figure 4b, there C

DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering is a sharp peak at 3700 cm−1, which represents the anti-stretching vibration or stretching vibration of Al with hydroxyl groups. This observation indicates that the carboxyl groups of HPAE-C were quickly reacted with Al3+ of aluminum sulfate. UV−Vis Analysis. The UV−vis absorption spectra of HPAE-C and HPC-Al are shown in Figure 5. As shown in Figure 5a, the

at 2θ = 29°, 34°, 41°, etc. The results show that the original crystalline structure has been changed26,38,39 due to the complexation of HPAE-C and Al3+. Application. Shrinkage Temperature. HPC-Al and aluminum sulfate were used in pigskin garment leather tanning. To reduce the error, we chose four pieces of pickled skins as samples to determine the indexes and took the average. The results are shown in Table 3. Table 3. Leather Shrinkage Temperature process

1

2

3

4

average

pickled skins 8% aluminum sulfate tanning agent 5%HPC-Al tanning agent 6%HPC-Al tanning agent 7%HPC-Al tanning agent 8%HPC-Al tanning agent 9%HPC-Al tanning agent 10%HPC-Al tanning agent

46.2 61.9

46.5 63.8

47.5 62.5

45.8 60.9

46.5 64.1

74.3 75.8 77.9 79.2 78.6 79.0

73.2 76.2 77.2 80.1 79.5 79.3

74.5 76.8 77.5 78.9 80.0 78.6

75.1 76.3 78.3 79.6 78.1 80.3

74.3 76.3 77.7 79.5 79.1 79.3

Figure 5. UV−vis absorption spectra patterns of HPAE-C and HPC-Al.

As shown in Table 3, the shrinkage temperature (Ts) of leather only can reach 64.1 °C after aluminum sulfate tanning. However, after using 5 wt % HPC-Al tanning agent, the Ts can be increased from 46.5 to 74.3 °C, and the Ts was on the rise along with the increase in the dosage of HPC-Al tanning agent. When the dosage was 8 wt %, the Ts reached 79.5 °C. However, when the content of the HPC-Al tanning agent was more than 8%, the shrinkage temperature does not continue to increase. Physical Properties. The mechanical properties of chrome retanned leather, zirconium retanned leather, aluminum tanned leather, and market leathers were analyzed in Table 4. As Table 4 shows, the Ts of HPC-Al tanned leather is remarkably improved after chromium retanning; it was greater than 95 °C. Compared with aluminum sulfate tanned leather, the Ts has increased 10 °C at least. The thickening of HPC-Al tanned leather also improved significantly; the thickening percentage is 25.32%. For zirconium retanned leather, the shrinkage temperature reached 87 °C, and the physical performance is higher than the market chrome-free leather. Chrome retanned leather has a similar mechanical strength with market chrome tanned leather, and zirconium retanned leather has a better mechanical strength than market chrome-free tanned leather. It can be observed from these results that tanned by HPC-Al and retanned with chromium are completely feasible ways to have a chrome-less tanning system and a chrome-free tanning system by retanning with zirconium. Organoleptic Properties. The organoleptic properties of tanned and retanned leathers are given in Table 5. From Table 5, it is observed that the chrome retanned leather exhibited the organoleptic properties with good tightness, softness, and smoothness compared chrome tanned leather. Because of less chromium tanning powder, performance of softness was slightly worse. For zirconium retanned leather, it had the same fullness and softness compared with chrome retanned leather, but it has the highest grain tightness and lowest smoothness. This is because zirconium(IV) has greater molecular weight than chromium(III) when the solution pH is 3.5−4.0; therefore, it can fill and firm (tighten) the grain. HPC-Al’s great tanning property is related with the special properties and structure of the hyperbranched polymer. The hyperbranched polymer has a special branched structure and many active functional groups. It can form lots of complexings

maximum absorption wavelength is 310 nm in UV−vis. Figure 5b show that the maximum absorption wavelength of the complex between HPAE-C with aluminum sulfate was at 354 nm. Comparing Figure 5a with b, the maximum absorption wavelength shifts 44 nm bathochromically. That means when the carboxyl groups of HPAE-C reacted with Al3+ of aluminum sulfate to form a complex,37 it weakened the conjugated system of HPAE-C. The energy level transition was reduced in UV−vis absorption spectra, and the maximum absorption wavelength moved toward the longer wavelength, i.e., red-shifted. It also proved that the coordination reaction occurred. XRD Analysis. XRD patterns of HPAE-C and HPC-Al are shown in Figure 6. In Figure 6a, there is a certain ordered

Figure 6. XRD patterns of HPAE-C and HPC-Al.

structure in HPAE-C, and the characteristic diffraction peaks are presented at 2θ = 22° nearby.26 In Figure 6b, the diffraction peak at 2θ = 22° is slightly moved to the right near 2θ = 25°, the strength decreased significantly, and the peak shape became broadened. The introduction of Al3+ reduced the regularity of the macromolecular chain of HPAE-C, resulting in the decline of the characteristic peak’s intensity and displacement. There are sharp diffraction peaks of aluminum appearing obviously D

DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 4. Properties of Market Leathers and Tanned Leathers by Different Tanning Methods

a

properties

HPC-Ala (retanned by CrIIIb)

HPC-Ala (retanned by ZrIVb)

aluminum tanned leathera

market chrome-free leathers

market chrome leathers

Ts after retanning/°C tensile strength/(N·mm−2) breaking elongation/% tearing strength/(N·mm−1) thickening percentage/%

> 95 21.21 ± 0.12 90.21 ± 0.11 24.68 ± 0.14 25.32 ± 0.09

87 ± 1.5 12.23 ± 0.03 72.18 ± 0.20 23.93 ± 0.16 22.56 ± 0.21

85.1 ± 2 14.53 ± 0.17 82.32 ± 0.12 20.01 ± 0.11 12.35 ± 0.09

89 ± 1 9.97 ± 0.04 57.00 ± 0.15 17.63 ± 0.10 −

> 100 16.29 ± 0.20 95.92 ± 0.32 26.36 ± 0.15 −

Dosages of tanning agents were both 8 wt %. bDosage of retanning agents was 2 wt %.

green, and sustainable chemistry. Chemical oxygen demand (COD), biochemical oxygen demand (BOD), total dissolved solids (TDS), and total suspended solid (TSS) are the main parameters for assessing the quality of tanning exhaust liquors. The environmental testing parameters after tanning and retanning process are shown in Table 6. It is shown in Table 6 that the COD value after HPC-Al tanning exceeds chrome tanning. Meanwhile, it is evident that the tanning method reduces the BOD, TDS, and TSS loads by 22%, 22%, and 8%, respectively. The molecular weight of the HPC-Al tanning agent is more than 2000, and it has a larger molecular weight than other tanning agent, like D-lysine aldehyde34 or oxazolidines.10 The HPC-Al tanning agent is a large molecular weight tanning agent, so some of it remained in the waste liquid, leading to the high value of COD. Environmental parameters are maintained at a low level after retanning by chromium and zirconium. After a comprehensive consideration of tanning and retanning, we believe it is a reasonable system for clean production, and execution of this tanning system could bring significant changes to the tanning industry to achieve a chrome-free or chrome-less tanning process.

Table 5. Organoleptic Properties of Tanned and Retanned Leathers

a

leather

fullnessa

grain tightnessa

grain smoothnessa

softnessa

chrome tanned chrome retanned zirconium retanned market leathers

9 7.5 7.5

8 8 9

9 9 7.5

8 8 8

7

8

9

9

Larger number = better properties; the best property is 10.

with Al3+ and hydrogen bonds with collagen after it is in the leather interior, so it is significantly better than the general aluminum tanning agent in thickness. The schematic diagram is shown in Figure 7. Also because of the cross-linking, it

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CONCLUSIONS In summary, a way to achieve a chrome-free and chrome-less tanning system has been designed and applied in leather cleaner production based on a hyperbranched polymer (HPAE-C) complexing Al3+. Besides exhibiting good performance after chrome retanning and with zirconium and a comparatively ideal environmental impact assessment, the leathers achieved different organoleptic properties compared with common chrome tanned leather. Furthermore, the shavings after tanning do not contain chromium, and the disposal process is easier and more diverse. In addition, HPAE-C can be used as high-exhaustion chrome tanning auxiliaries with its large number of terminal carboxyl groups. After all, chrome tanning is widely used in the leather industry. The latter still needs more exploration.

Figure 7. Bound forms of HPAE-C with collagen and Al3+.

effectively restrains the deformation of the leather fiber from axial tension and improves the tear strength and tensile strength of the leather. Meanwhile, it can disperse the fiber bundles, making leather grain soft and full. Environmental Impact Assessment. Chromium Content. EDTA complexometric titration was used for measuring the content of chromic oxide in liquid waste. Results show that when HPC-Al was used as the tanning agent, the chromium content of the waste liquid is 0.883g/L; the absorption rate of chrome is ∼87% after chrome retanning. This means that HPC-Al also has a great effect on the high-absorbency of chrome-tanning, and it would be used in chrome tanning as a high exhaustion chrome tanning auxiliary to reduce chromium pollution and realize clean production. Environmental Parameters. The environmental impact analysis of the tanning process is necessary to determine the possible positive or negative impact on environmental, social,

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00917.

Table 6. Environmental Testing Parameters after Tanning and Retanning Process exhaust liquors chrome tanning HPC-Al tanning aluminum tanning chrome retanning zirconium retanning

COD/mg·L−1

pH 3.83 3.76 3.75 4.10 4.03

± ± ± ± ±

0.05 0.05 0.06 0.08 0.03

1427 ± 55 1613 ± 34 1862 ± 33 373 ± 20 422 ± 19

BOD/mg·L−1

TDS/mg·L−1

TSS/mg·L−1

± ± ± ± ±

48281 ± 34 37829 ± 26 42675 ± 42 7231 ± 33 7626 ± 27

580 ± 15 532 ± 12 593 ± 10 161 ± 13 187 ± 8

858 665 934 182 186 E

19 15 23 16 18

DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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2D and 3D chemical formula of the hyperbranched polymers with terminal hydroxyl, subsection model of the different size of leather tanning agents between the fibers, TG patterns of HPAE-C and HPC-Al, measures of the content of chromic oxide in liquid waste, MSW-YD4 digital leather Ts meter, more information on materials in the tanning process, and physical properties of market leather. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-029-86132530. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51403120), project for postdoctoral sustentation (2012M521733), Key Scientific Research Group of Shaanxi province (2013KCT-08), scientific research group of Shaanxi University of Science and Technology (TD12-04), and the innovation foundation of graduate student in Shaanxi University of Science and Technology. The authors are thankful for the teachers and students from Green Chemicals Institute for Leather and Synthetic Leather (http://www. leather420.cn/) for their self-less help with GPC, FT-IR, UV− vis, and XRD.

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DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.5b00917 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX