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j.jclepro.2018.08.164

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Accepted Manuscript
Mechanism and Effect of Hydroxyl-terminated Dendrimer as Excellent Chrome
Exhausted Agent for tanning of Pickled Pelt
Qi Yao, Hualin Chen, Henghui Huang, Bailing Liu
PII:
S0959-6526(18)32507-1
DOI:
10.1016/j.jclepro.2018.08.164
Reference:
JCLP 13948
To appear in:
Journal of Cleaner Production
Received Date:
24 April 2018
Accepted Date:
16 August 2018
Please cite this article as: Qi Yao, Hualin Chen, Henghui Huang, Bailing Liu, Mechanism and Effect
of Hydroxyl-terminated Dendrimer as Excellent Chrome Exhausted Agent for tanning of Pickled
Pelt, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.08.164
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ACCEPTED MANUSCRIPT
Mechanism and Effect of Hydroxyl-terminated Dendrimer as Excellent Chrome Exhausted Agent
for tanning of Pickled Pelt
Qi Yaoa,c, Hualin Chenb*, Henghui Huanga,c, Bailing Liua
a Chengdu
b College
Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, 610041, China
of Chemistry & Environmental Engineering, Southwest Minzu University, Chengdu, 610041, China
c University
of Chinese Academy of Sciences, Beijing, 100049, China
* Corresponding author. E-mail address: aofly@163.com
Abstract
A combination chrome tanning technology which endows leather high chrome exhaustion and
thermal property based on hydroxyl-terminated dendrimer (HTD) is reported. The chrome
concentration in effluent, denaturation temperature (Td), enthalpy (?H), thermal degradation active
energy (Ea), crosslinking degree of wet-blue leathers are determined by inductively coupled plasma
atomic emission spectroscopy (ICP-AES), differential scanning calorimetry (DSC), thermal
gravimetry analysis (TG-DTG), and X-ray diffraction (XRD), respectively. The element (especially
chromium) content and distribution of wet-blue leather are also obtained by scanning electron
microscopy- energy dispersive spectrometer (SEM-EDS). The results show that the chrome uptake,
Td, ?H and Ea of wet-blue leather tanned with HTD and basic chrome sulphate (HTD-Cr) can
reach 94.1%, 117.4 ?C and 432.2 J/g, and 153.1 KJ/mol much higher than conventional chrome
tanning which is 66.1%, 112.5 ?C and 129.2 J/g and 147.2 KJ/mol, respectively. It is because HTD
could act as excellent masking agent with good alkaline stability, which slows down the basifying
process and promotes chrome penetration towards central of wet-blue leather, resulting in evenly
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dispersed along the grain layer, middle layer and flesh layer of wet-blue leather. Consequently,
crosslinking density of collagen increased, thereby inhibiting the outward diffusion of chrome and
improving the chrome uptake. In contrast, for blank trial, the chrome mainly aggregate at grain
layer and flesh layer. These results illustrate the chrome tanning promotion mechanism of HTD and
provide a valuable insight of developing high exhausted chrome agent based on dendrimer for more
sustainable future of leather industry.
Keywords: Hydroxyl-terminated dendrimer, chrome uptake, high exhausted chrome agent,
hydrothermal stability, thermal degradation active energy, chrome distribution.
1.Introduction
Collagen is composed of triple-helices structure with low hydrothermal stability, failed to meet
the actual practical usage, such as shoes, garment, furnish. Chrome tanning endows the leather with
highest thermal stability, comfortable feel and unparalleled properties, still irreplaceable by others
tanning agent(Covington, 2009; Zhang et al., 2016). However, tannery industry has been
categorized as one of the highly polluting industries due to its adverse influence on the
environment, especially chrome-containing pollutants(Cao, S. et al., 2018). In conventional chrome
tanning, only 60-70% of the total offered chrome is consumed in the tanning process while the other
30-40% remains in the effluent (Sundar et al., 2002; Zhang et al., 2018). Otherwise, the unstably
combined chrome will be released to wastewater during post tanning processes. The Cr3+ in chrome
containing pollutants can be oxidized and dissolved through various environmental conditions such
as heat, light(Dai et al., 2010), and humidity(Zhitkovich, 2011), leading to hazardous levels of
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aqueous Cr6+ in surface and ground water(Oze et al., 2007).
With increasing concerns for the environment, it is essential to increase chrome utilization so
as to reduce the chrome concentration in residual floats to a maximum degree. For the most part, a
number of researchers have developed various chrome exhaustion tanning additives, including
oxazolidines(Sundarapandiyan et al., 2011), hydroxymethyl phosphonium sulfate (THPS)(Li et al.,
2012), non-linear acrylic acid copolymer (Zhang and Lan, 2012), gallic acid(Ramamurthy et al.,
2014). Although these additives could enhance chrome uptake over 90%, high offer (almost 5-10%
of pelt mass) of them are required for meeting the expected chrome uptake(Yao et al., 2018).
Because of the simple application process and good quality of leather, combination chrome
tanning with high chrome uptake, are considered as suitable solutions to overcome the problem
from single tanning process, has been proposed for years(Ding et al., 2015; Krishnamoorthy et al.,
2013). With the explosive development of hyper-branched polymer (HBP), HBP has been widely
studied in combination chrome tanning for leather industry, such as pre-tanning agents(Haroun et
al., 2006; Ibrahim et al., 2013), retanning agents(Chai et al., 2010), chrome tanning
auxiliaries(Qiang et al., 2016), and so on. Our research group have conducted serials of studies
including, combination behavior between carboxyl-terminated HBP (CHBP) and Cr3+, relationship
between the structure of CHBP and its promoting properties of chrome tanning, acting mechanism
of CHBP in improving chrome uptake, etc(Chen et al., 2007; Li Chenying et al., 2014; Yao et al.,
2017). Compared with common used high chrome exhausted agent, CHBP endow the leather up to
95% chrome uptake and higher thermal stability.
Similar to HBP, lower generation dendrimers have more well-defined structure which have
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many unique properties, such as higher solubility, higher chemical reactivity, large numbers of
functional end-groups (Inoue, 2000). Once incorporation in bath solution, low generation
dendrimers could dissolve and penetrate into intradermal easily for its lower viscosity and
hydrodynamic volume. Meanwhile large numbers of hydrogen bonds, coordination, and chemical
bonds will be formed between chrome, carboxyl group of collagen and low generation dendrimers.
These novel physical features render dendrimer ideal candidates in the application of leather
tanning.
Vegetable tannins are widely used especially in the production of natural leathers, for their
stabilization is based on multi-hydrogen links between the polyphenols and collagen (Onem et al.,
2017). Sparked from vegetable tannins, we has just found low generation hydroxyl-terminated
dendrimer (HTD) could act as high chrome exhausted agent which can par with CHBP(Yao et al.,
2018). As like as CHBP, low generation HTD can be synthesized by simple reactions and has a
more regular structure which endows the leather better properties. But little literature has reported
the employment of HTD in chrome tanning of pickled pelt. Moreover, the mechanism of HTD
acting in HTD-Cr tanning has also not been uncovered.
Herein, this paper, we reported the design and synthesis of HTD. The chrome uptake, chrome
distribution and thermal stability of HTD-Cr tanned wet-blue leather are evaluated compared with
that of conventional chrome tanning. The internalization mechanism of HTD in HTD-Cr tanning is
discussed in detail. The two processes, permeation and fixation of chrome, which determine the
chrome uptake and hydrothermal stability of the wet-blue leathers, are investigated based on the
tanning of hide powder and pickled pelt. Moreover the chrome concentration change during the
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chrome process is also recorded?tanning, basifying and heating process. Finally, the mechanism of
HTD acting on HTD-Cr tanning is proposed based on combinations of scanning electron
microscopy- energy dispersive spectrometry (SEM-EDS) and X-ray diffraction (XRD).
2. Experiment Section
2.1 Chemicals and Materials
The chemicals such as sodium chloride, formic acid, sulfuric acid, citric acid (CA) used in
chrome
tanning
were
technical
grade,
normally
used
in
leather
industry.
N,N'-
methylenebisacrylamide (MBA), ethylenediamine (EDA) and diethanolamine (DEOA) were
purchased from Aladdin Chemical Reagent Corporation (Shanghai, China) and used as received.
Basic chrome sulphate (TML chrome powder, Cr2O3%, 24� and basicity, 33�) was supplied
by Minfeng chemical CO., Ltd of Chongqing, China. Pickled pigskin was purchased from Shenghui
chemical CO., Ltd of Zhejiang, China. Hide powder was purchased from Institute of Forestry
Industry Chemical Industry, Chinese Academy of Forestry.
2.2 Method and Measurements
Synthesis of HTD
Hydroxyl-terminated dendrimer (HTD) was synthesized as literature(Yao et al., 2018). MBA
(6.16 g, 40 mmol) was dissolved in 30 mL methanol, and then core molecule EDA (0.6 g, 10 mmol)
methanol solution was added into at 20 ?C. After 24 h incubation, another DEOA (4.20 g, 40 mmol)
were added and reacted for 24h at 40?C. Thereafter, the viscous liquid HTD was obtained by
removing methanol through rotary evaporation. The chemical structure and synthesis of HTD is
5
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shown in Figure 1.
Figure 1 chemical structure and synthesis of HTD
Chrome tanning of pickled pelts
Chrome tanning process of pickled pelts which including pickling, tanning, basifying and
heating were performed in 250 mL flask on a shaker of 80cm (length)�cm(width)�cm(height).
Brie?y, the pickled pelts were soaked in 8%wt sodium chloride solution (the weight of sodium
chloride solution is twice that of pickled pelts), then 3% HTD (or 3% CA, called CA-Cr tanning)
based on the weight of pickled pelts was added (called HTD-Cr tanning). Adjust the bath solution
pH at 3.0 by 5%wt sulfuric acid and 10%wt formic acid mixed solution at 25 ?C. Secondly, tanned
with 8%wt TML chrome powder (based on weight of pickled pelt). After tanning for 6 hours, the
pH of the bath solution was raised to 4.0 by using 10 wt% of sodium bicarbonate solution.
Thereafter setting the temperature of the shaker to 45?C for another 4h, the shaker was stayed
overnight and shakes for another 1h in the next day. Then the leather was called ?wet-blue leather?
for its blue color.
Chrome tanning of hide powder
The tanning of hide powder also covers the pickling, tanning, basifying and heating process.
The detailed processes were shown in Table 1.
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Table 1 Tanning process for hide powder
Process
Chemical
hide powder
8% sodium
chloride solution
Pickling
HTD
Temperature
dosage
Time
(?C)
(g)
(h)
25
1.00
25
20 mL
25
0.03
Remark
2
Called HTD-Cr tanning
Formic acid and sulfuric acid was mixed
10% Formic acid
5% Sulfuric acid
for adjust the pH of bath solution
25
3�5
2
pH 2.9-3.0, stand over night
Chrome
TML chrome
tanning
powder
Basifying
Heating
Sodium
bicarbonate
water
25
0.08
25
8�5
45
20 mL
6
pH 4.0
2
Stay overnight and shake for one hour
next day.
Dry in vacuum oven at 40?C and for preparation for DSC.
Chrome concentration change during chrome tanning process
In order to simulate authentic chrome concentration change during traditional chrome tanning
(blank trial) and HTD-Cr tanning process, two pieces of 7 cm�cm pigskin were taken from the
same part and weight. In the HTD-Cr tanning, 3wt% of HTD was added during the pickling process
and pH was adjusted to 2.8-3.0. Next day, chrome powder was added and shacked for 1 h. Then,
250 ?L of chrome tanning solution was taken every hour to measure the chrome concentration. In
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total, the tanning, basifying, and heating process were carried out 4, 4, and 2 times, respectively.
The weight of samples is listed in Table 2.
Table 2 the weight of pickled pelts
Mass of 8% sodium chloride
sample
Leather weight/g
3%HTD /g
8% TML/g
solution/g
HTD-Cr
4.360
18
HTD
0.131
0.348
C
4.100
18
-
0.123
0.328
Note?Taking into account the smaller liquid ratio, the tanning effect of skin in the bottle is not good, and
eliminate analysis error of sampling, the liquid ratio of the pickling process is expanded to 4:1. C stands for
conventional chrome tanning or blank trial.
Shrinkage Temperature
The shrinkage temperature of leather was determined according to standard IUP 16(IUP,
2000), and literature (Morera et al., 2007). The leather sample was suspended vertically in a mixture
of water and glycerin, and the rate of heating was maintained at 2 � 0.2癈/min. When the leather
shrinks, this temperature was taken as the shrinkage temperature.
Chrome uptake
The tanned effluents were collected by centrifugation, digestion and analysis for the chrome
concentration using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES,
Optima 2100DV, Perkin Elmer, USA). Chrome uptake of the wet-blue leather or hide powder was
calculated by formula (1).
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chrome uptake=
added chromium-chromium in effluent
?100%
added chromium
(1)
Determination of denaturation and thermal degradation active energy
The tanned wet-blue leathers and hide powders were dried at ambient environment for 2 days.
Then their DSC thermograms were recorded on a DSC calorimeter (Q20 V23.12 Build 103) with a
temperature range from 50?C to 170 ?C with a scanning rate of 10?C /min under a nitrogen
atmosphere.
The TG-DTG curves of leather with blank trial and HTD-Cr tanning were plotted at the
heating rate 5 ?C /min, 10?C /min, 15 ?C /min and 20 ?C /min from 50?C to 700?C.
TG is an effective method to study the kinetics of thermal decomposition of collagen
fibers(Yao et al., 2008). In the case of a program temperature increase, the reaction conversion rate
and the temperature (or time) were measured. Assume that the sample is homogeneous; there is no
overlap or parallel thermal decomposition reaction; the temperature is uniform throughout the
sample, there is no temperature gradient; and the gas generated by the decomposition has no
diffusion resistance (Budrugeac and Cucos, 2013; Zhang et al., 2014),
d?
? k (T )(1 ? ? ) n
dt
?2?
Where ? is the degradation rate?n is the reaction order and assumed to remain unchanged
during the reaction?k(T) is the reaction rate constant which is consistent with Arrhenius Equation:
k (T ) ? A exp(?
E
)
RT
?3?
Where, A and E are pre-exponential factor and heat degradation activation energy,
respectively.
Substituting Formula (3) into Formula (2), there is,
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d?
E
? A exp(?
)(1 ? ? ) n
dt
RT
?4?
Under conditions of program temperature increase, dT ? ? dt , ? is the heating rate, thus
d?
A
E
? exp(?
)dT
n
(1 ? ? )
?
RT
?5?
According to Kissinger?s method:
ln(
?
T
2
p
) ? ln(
AR
E
) ? ln[n(1 ? ? p ) n ?1 ] ?
E
RTP
(6)
Where, Tp and ?p are the thermodynamic temperatures and conversion rates when the thermal
decomposition rate is the highest, respectively. Plot with ln(
be obtained from the slope of the line, that is ? E
R
?
T
2
p
)?
1
. The activation energy (E) can
Tp
=Slope
Alkali stability of HTD
The alkali stability of the HTD and Cr3+ solutions were measuring by a PHS-3C pH meter
(INESA Scientific Instrument Co.,Lt, Shanghai) and titrated by 0.1 mol/L NaOH aqueous solution.
In order to simulate the true chrome tanning environment (8% of the TML was needed and the
liquid was two times of the weight of pickled pelt), 1 g/25 mL TML aqueous solution were
prepared.
20 mL of TML aqueous solutions were transferred into two 100 mL beaker. The pH of the
TML aqueous solutions was determined as the initial pH value. 10 mL distilled water and 0.3g
HTD was added into the beaker for blank and HTD-Cr trial, respectively. Then the solutions were
dropwise titrated by 1mL 0.1 mol/L NaOH aqueous solution every time. Five minutes later, the pH
of the mixed solution were recorded until the precipitation occurs.
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For a comparative study, the alkali stability of 0.5 mol/L CrCl3 solutions was also done.
Scanning electron microscopy- Energy Dispersive Spectrometer (SEM-EDS)
The cross section of wet-blue leather and main element distribution were observed by using
Scanning electron microscopy-Energy Dispersive Spectrometer (JSM-7500F/X-MAX50, JEOL,
Japan).
XRD of the wet-blue leather
The freeze-dried wet-blue leathers were cut into 2�cm pieces. XRD data were obtained by an
X-ray diffractometer (40 KV� mA) (XD-6, PERSEE General Analysis Instrument Co., Ltd,
Beijing) with Cu K? as the ray source. The sample was scanned in the range of diffraction angle 2?
from 5? to 90?C with a scanning rate of 4?C/min at ambient temperature and humidity.
3. Results and discussion
3.1 Tanning of hide powder and pickled pelt
There are distinct differences between the tanning of pickled pelt and hide powder. For the
tanning of pickled pelt, the crosslinking (or coordination) action happens under the condition that
chrome (or HTD) penetrates into derma. Here, the coordinated and uncombined chrome are
constrained in the collagen network in the forms of crosslinking or physical absorption(Zhang et al.,
2018). So the combination between chrome and HTD increases the chrome uptake of wet-blue
leather.
For hide powder, the collagen is much more loosely compared to pickled pelt. So the pendant
carboxyl of collagen can react with Cr3+ without hindrance of permeation. The combination of
11
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chrome with HTD still remains in the effluent. Hence, the formation of Cr3+-HTD complex is
accompanied with a significant decrease in the chrome uptake of hide powder. That is, for the
tanning of hide powder, the combination between Cr3+ and HTD, pendant carboxyl of the collagen
is a pair of competing reaction. The lower chrome uptake of the hide powder indicates the stronger
combination ability between HTD and Cr3+. The chrome uptake of hide powder tanned by HTD-Cr
and blank trial are shown in table 3.
Table 3 chrome uptake of hide powder tanned by HTD-Cr tanning and blank trial
Sample
Chrome uptake/%
HTD-Cr
76.0
C
93.4
It is obvious that the chrome uptake of hide powder of blank trial is much higher than that of
HTD-Cr tanning. It is contributed to the combination of Cr3+ to HTD in effluent, which reduce the
chrome content of hide powder. This consequence indicates the strong combination ability between
HTD and Cr3+.
The chrome tanning effects of the pickled pelt includes chrome concentration of effluent,
chrome uptake and shrinkage temperature of wet-blue leather, which is shown in Table 4.
Table 4 chrome tanning effect of wet-blue leather tanned with HTD-Cr and conventional method
Sample
Chrome concentration of wastewater/ppm
Chrome uptake/%
Shrinkage temperature/?C
C
1097.3
66.1
103.5
CA-Cr
874.5
79.3
105.7
GA*
400-550
92-94
105 � 3
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HTD-Cr
106.7
94.1
108.3
Note: The GA * dates were taken from literature(Ramamurthy et al., 2014). GA is the abbreviation of Gallic acid.
It can be seen from table 4 that the chrome uptake of wet-blue leather with HTD-Cr is 94.1%,
higher than blank trial CA-Cr and GA. It has demonstrated that HTD could improve the chrome
uptake and hydrothermal stability of the wet-blue leather.
3.2 Denaturation temperature and enthalpy of wet-blue leather
The peak in the DSC profiles indicates the denaturation temperature(Tang et al., 2003). For the
denaturation processes, enthalpy (?H) is the heat absorbed by the wet-blue leather through external
heat transfer. ?H is equal to area surrounding by baseline and DSC curve.
The DSC profiles of wet-blue leather treated with HTD-Cr, CA-Cr and blank trial are shown in
Figure 2.
0.0
Heat Flow (W/g)
-0.5
-1.0
C
CA-Cr
HTD-Cr
-1.5
-2.0
-2.5
-3.0
20
Exo up
40
60
80
100
120
140
Temperature (癈)
Figure 2 the DSC profile of the wet-blue leather
From Figure 2, we can see that Td of the wet-blue leather tanned with HTD-Cr shows the
higher value than CA and blank trial. It is consistent with shrinkage temperature (Ts) results. The
actual Td and ?H of the leathers are calculated and the corresponding results are shown in Table 5.
13
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For the blank trial, the Td and ?H of the blank leather is 112.5 ?C, 129.2 J/g, respectively, lower
than that of the CA-Cr and HTD-Cr tanned leather. Moreover, HTD-Cr tanning endows the leather
highest Td and ?H, compared to CA and blank trial. Because HTD can not only form hydrogen
bonds with collagen(Yao et al., 2018), also increase the chrome uptake which can improve the
thermal stability of leather both. These results infer that the HTD is incorporated into the collagen
and significantly improve the hydrothermal stability of wet-blue leather.
Table 5 the denaturation and ?H of the wet-blue leather
Sample
Denaturation temperature/?C
?H/J/g
C
112.5
129.2
CA-Cr
107.0, 115.4(double peak)
264.5
HTD-Cr
117.4
432.2
3.3 Thermal degradation stability and Ea of wet-blue leather
The TG-DTG curves of the wet-blue leather tanned with HTD-Cr and blank trial are shown in
Figure 3.
(a)
100
90
-2
DTG/(%/min)
Mass/%
80
70
60
50
(b)
0
5 K/min
10K/min
15K/min
20K/min
40
5 K/min
10K/min
15K/min
20K/min
-4
-6
-8
-10
30
-12
0
100
200
300
400
500
600
700
0
T/?
100
200
300
400
T/?
14
500
600
700
ACCEPTED MANUSCRIPT
(c)
100
90
80
-2
5 K/min
10K/min
15K/min
20K/min
-4
DTG(%/min)
70
Mass/%
(d)
0
5 K/min
10K/min
15K/min
20K/min
60
50
-6
-8
-10
40
-12
30
20
-14
0
100
200
300
400
500
600
700
0
100
200
300
T/?
400
500
600
700
T/?
Figure 3 the TG-DTG curves of the wet-blue leather at different heating rate
(a) TG, (b) DTG of blank trial; (c)TG, (d) DTG of HTD-Cr tanning
The temperature at maximum thermal degradation rate and residual mass of wet-blue leathers
at each heating rate are shown in Table 6.
Table 6 the temperature at maximum thermal degradation rate, residue mass at different heating rate
The temperature at maximum heating degradation rate/?C
Residue mass/%
Heating rate/ K/min
HTD
C
HTD
C
5
318.5
308.3
41.4
35.3
10
327.6
321.7
33.9
29.5
15
334.4
330.0
44.4
41.9
20
343.7
333.4
44.5
41.6
From Table 6, it can be seen that the thermal degradation stability of the wet-blue leather
increases tanned with HTD-Cr, and the residual mass also increases, which is mainly due to the
increment of chrome content in the wet-blue leather.
From Table 6, we can plot the Kissinger curves of wet-blue leather of HTD-Cr tanning and
blank trial
15
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-9.8
(a)
-9.8
y=20.23-18.44x
R2=0.971
y=19.32-17.71x
R2=0.989
-10.0
-10.2
-10.2
-10.4
-10.4
ln(?/Tp2)
ln(?/Tp2)
-10.0
(b)
-10.6
-10.6
-10.8
-10.8
-11.0
-11.0
-11.2
-11.2
1.63
1.64
1.65
1.66
1.67
1.68
1.69
1.70
1.71
1.64
1.65
1.66
1.67
1/Tp/ K-1?103
1.68
1.69
1.70
1.71
1.72
1.73
1/Tp/ K-1?103
Figure 4 the fitting Kissinger curve of wet-blue leather
(a) is HTD-Cr tanning; (b) is the blank trial
From Figure 4, we can calculate the thermal degradation active energy by slope of the fitting
line. The thermal degradation activation energy of wet-blue leather tanned with HTD-Cr is 153.3
KJ/mol higher than that of blank trial which is 147.2 KJ/mol.
3.4 Alkali stability
Since Cr3+ is sensitive to pH value, the pH of effluent strictly controlled at about 4.0 in the
basifying process. The alkali stability of the HTD-Cr mixed solution is pivotal important for chrome
tanning. The curves of pH of the solutions vs NaOH addition volume are plot in Figure 5.
5.5
CrCl3
TML
CrCl3+HTD
5.0
4.5
pH
4.0
3.5
3.0
2.5
2.0
-2
0
2
4
6
8
10
12
14
16
VNaOH/mL
Figure 5 the pH titration curves for different Cr3+ systems
16
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The hydrolysis and polymerization of [Cr(H2O)6]3+ is shown in Figure 6 (Covington, 1997).
[Cr(H2O)6]3+
[Cr(H2O)5OH]2+ + H+
[Cr(H2O)5OH]2+
[Cr(H2O)4(OH)2]+
[Cr(H2O)4(OH)2]+ + H+
Cr(OH)3
+ 3 H2O + H+
Figure 6 the hydrolysis diagram of [Cr(H2O)6]3+
With the addition of NaOH solution, the equilibrium shifts to the right, which promotes the
hydrolysis and polymerization of [Cr(H2O)6]3+ and eventually forms Cr(OH)3 precipitates. The pH
of precipitation point of different solutions is measured to examine their alkali stability(Janardhanan
et al., 2013).
From the titration curve of CrCl3 solution in Figure 5, it can be seen that precipitation occurred
when 9 mL of NaOH was added. For TML chrome powder solution, the increasing rate of pH is
lower than CrCl3 solution. TML chrome powder, which is synthesized by cane sugar and sodium
dichromate, may contain trace amounts formic acid or acetic acid. While for HTD-CrCl3 mix
solution, the increasing rate of pH is significantly reduced. Moreover the precipitation point reaches
when 16 mL of NaOH is added. It is attributed to formation of poly-nuclear chrome complexes with
HTD in the solution. That is, the addition of HTD can effectively slow down the basifying process
so that more Cr3+ will permeate the leather and get homogeneous dispersion along the leather.
3.5 The permeation and combination of chrome
The change of chrome concentration in the effluent with chrome tanning process is plotted in
Figure 7.
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6
HTD-Cr
C
Cr concentration (mg/mL)
5
4
3
Heating
Basification
Tanning
2
1
0
0
2
4
6
8
10
time (h)
Figure 7 the relationship between chrome content of effluent and chrome process
From Figure 7, it can be clearly seen that the chrome concentration change in the effluent
shows three stages, which is tanning, basifying and heating process.
In tanning stage, the pH of the bath is low (pH 2.9-3.0) and chrome firstly adsorbs on the
pickled pelt surface and then gradually penetrates into the pickled pelt(Renner et al., 2009).
Throughout the tanning process, the chrome concentration in the bath does not decrease
significantly.
According to the Fick-Einstein diffusion law equation?
dq RT
1
dc
?
?
?S ?
dt N A 6?? r
dx
Where,
(7)
dq
dc
is the diffusion rate of tannin, is the concentration gradient, f= 6?? r ,f refer to
dt
dx
resistance diffusion coefficient of tannin. S is the diffusion area(Chen and Li, 2011).
The diffusion rate is decided by the concentration gradient of tannin. So the chrome
concentration change of bath solution keeps the same during tanning process for HTD-Cr and blank
trial.
In basifying stage, carboxyl of the collagen dissociate into COO- with raising the pH of the
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effluent(Cao, Shan et al., 2018). The dissociated carboxyl combines with chrome rapidly, resulting
in the linear decrease of chrome concentration of effluent. By linear fitting of the curve, the slope of
the curve are -0.521 mg/mL穐 and -0.558 mg/mL穐 for HTD-Cr and blank trial, respectively.
Actually, in the operation of basifying process, it can also be found that the raising rate of pH value
of the effluent in the HTD-Cr tanning is slow. For HTD-Cr tanning, the chrome presents as monodentate or dimeric chrome ligands, resulting in increased penetration (towards central of wet-blue
leather) rate(Yao et al., 2017). In contrast, for blank trial, Cr3+ immediately combine with
dissociated carboxyl of surface of the leather when pH of bate solution rise(Xia et al., 2016). In the
fourth hour of basifying, the chrome concentration of bath solution nearly keeps the same at pH
closed to 4.0.
In heating stage, the binding capacity between collagen and Cr3+ strengthen by increasing
volume and temperature of effluent. In turn, the combination of chrome and collagen fibers will
further drive the penetration of chrome from effluent into the intradermal, which will further reduce
the chrome concentration of the effluent.
Most of the chrome in the blank trial bound to the surface of the collagen, hindering the further
penetration of chrome into the intradermal, resulting in lower decrease rate of chrome concentration
in the heating stage. In contrast, for HTD-Cr trial, the chrome is evenly dispersed in the intradermal
and does not block the ?channel? in which chrome of the effluent further entered. In additional,
masking action of HTD can increase the pH at which the chrome salt precipitates; in these
circumstances the final pH of the tannins may be elevated beyond that of blank trial, thereby
enhancing the reactivity of the collagen(Covington, 1997). So the crosslink density of HTD-Cr
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tanned collagen increase. Moreover, the formation of hydrogen bond between HTD and collagen,
further results in reducing out-diffusion of the uncombined chrome. Enhance, the chrome uptake of
leather of HTD-Cr tanning is much higher than blank trial.
In conclusion, the basifying process has a pronounced effect on the distribution of chrome. The
slower the basifying process, the more dispersed the chrome. HTD can effectively slow down
basifying process, and does not block the permeation ?channel? of chrome during the heating
process. In addition, the crosslink density of collagen increased by increasing the chrome and HTD,
thereby inhibiting the outward diffusion of chrome.
3.6 Morphology of wet-blue leather
In order to observe the morphology of wet-blue leather, the freeze-dried wet-blue leathers were
cut into 1�cm, stuck to cooper plate and sprayed with golden. And the results at magnification
scanning of 500 are shown in Figure 8.
Figure 8 cross section of wet-blue leather. (a) Blank trial. The entire graph was divided to three regions by red
arrow and dotted line. Each region which indicates the degree of dispersion of collagen bundle was marked with
black text below. (b) HTD-Cr tanning. The collagen bundles in the entire graph are in good dispersion.
20
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From Figure 8, the weave of collagen bundles of wet-blue leather can be clearly seen. For
blank trial (Figure 8(a)), central collagen bundles bind tightly which names aggregation regions. In
the marginal regions, the collagen bundles are in good dispersion, names dispersion regions. For
HTD-Cr tanning, the collagen bundles are in good dispersion all the regions of Figure 8(b).
Literatures (Lyu et al., 2018; Zhang et al., 2016) have demonstrated that good dispersion of
collagen bundle attributes to effective crosslinking of collagen.
Thus, for blank trial, we can infer that the chrome does not penetrate into the central of the
wet-blue leather. As above text has also illustrated that in the basifying process, chrome combined
with collagen of the surface of leather. In contrast, HTD-Cr tanned leather is well crosslinked along
all the leather.
3.7 Chrome distribution in the wet-blue leather
The SEM-EDS technique was used to determine the element content and distribution
(especially chrome) of wet-blue leather based on cross-sections. Figure 9 shows the full picture of
the freeze-dried wet-blue leathers of HTD-Cr tanning and blank trial. In Figure 9, red arrows mark
length of the successive region which collagen bundle bind together. It can be seen that the length
of arrows in sample No.3 (blank trial) are relative longer than sample No.4 (HTD-Cr tanning). That
is, the degree of dispersion of collagen bundle of HTD-Cr tanning leather is bigger than blank trial,
which is consistence with SEM result.
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Figure 9 complete cross section of wet-blue leather. No.3 is the Blank trial. No.4 is the HTD-Cr tanning.
Full element scanning of complete fracture was done and the distribution of each element
(except Cr) in the cross sections is shown in Figure 10.
Figure 10 the element distribution in wet-bule leather. 3, blank trial; 4, HTD-Cr tanning
From Figure 10, the distribution of Na, Cl, and C element of wet-blue leather is uniform and
there is no significant difference between HTD-Cr tanning and blank trial. The spectrum of each
element and distribution of Cr in cross-sections of the leather are shown in Figure 11. The content
of each element in cross section of the leathers are shown in Table 7.
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Figure 11 the element spectrum and Cr distribution of wet-blue leather. (a) Element spectrum of blank trial. (b)
Element spectrum of HTD-Cr tanning. (c) Chrome distribution of blank trial. (d) Chrome distribution of HTD-Cr
tanning. In the (c) and (d), the grain, middle, and flesh layer are marked in red text.
Table 7 element content and proportion based on the fracture of wet-blue leather. 3 blank trial. 4 HTD-Cr tanning
wt%
atom percentage
Element
Standard sample label
3
4
3
4
C
58.89
56.27
70.93
69.07
C Vit
O
23.36
24.74
21.12
22.80
SiO2
Na
5.19
4.33
3.27
2.78
Albite
Cl
9.18
9.02
3.74
3.75
NaCl
Cr
3.38
5.64
0.94
1.60
Cr
total
100.00
100.00
100.00
100.00
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From Figure 11 (a), (b) and Table 7, the content of Cr in sample No. 4 (HTD-Cr tanning) is
5.64%, which is obviously higher than 3.38% of No. 3 sample (blank trial). Moreover, The Na+
content in the HTD-Cr tanned leather is lower than blank trial. It can be postulated that Na+ in wetblue leather of HTD-Cr tanning is excluded, and more chrome enters and crosslinks the collagen,
resulting in good dispersion of collagen fibers.
In addition, the chrome distribution in the fracture of leather is shown in Figure 11 (c) and (d).
Chrome in HTD-Cr tanned leather disperses evenly along the fracture surface. In contrast, for blank
trial, the chrome is mainly concentrated in grain layer and flesh layer, and the middle layer is less.
3.8 Structure of wet-blue leather
Collagen is a polymer material with a certain degree of crystallinity, and X-ray diffraction is an
effective means to understand the different structural layers of collagen(Zhang et al., 2018).
HTD
5000
4500
4000
3500
Intensity
3000
C
2500
2000
1500
1000
500
0
0
10
20
30
40
50
60
70
80
90
2??
Figure 12 the XRD spectrum of wet-blue leather
There are 4 main diffraction peaks on the X-ray diffraction pattern of chrome tanning collagen.
The first occurs at 7-8?(2?), which refers to the horizontal distance between the collagen fibers
(Maxwell et al., 2006). The diffuse peaks in 22? caused by the reflection of the irregular weave of
24
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collagen fibers. If the regularity of the dimensional structure of collagen decreases, the diffraction
intensity also decreases and the crosslinking degree of the collagen fibers increased(Maxwell et al.,
2006). From Figure 12, it was also found that the diffuse peaks appeared in 22? for blank trail. But
the peaks near 22 ?disappeared for HTD-Cr tanning. The degree of crosslinking between the
collagen fibers increased for HTD-Cr tanning.
The diffraction peaks at 32? (~0.287) is mainly related to the axial distance between the amino
acid residues in the collagen triple-helices structure. The peaks of the HTD-Cr tanned leather moves
to the low angle, indicating the axial distance of adjacent collagen is slightly increased(Sionkowska
et al., 2004). That is, the collagen bundle was more dispersed. Moreover, enlargement of the
intensity of the peaks, indicates the hydrogen bond was formed between HTD and collagen fibers.
The peak locating at 45.3? is the peaks of poly-chrome complex. The peak intensity of the leather
tanned with HTD-Cr is larger than that of the blank trial, which indicates that more chrome enters
and crosslinks the collagen.
4. Conclusion
The cleaner approach to less chrome discharge based on HTD has been explored. The HTD-Cr
tanning could improve the chrome uptake of wet-blue leather to 94.1%, much higher than previous
work such as gallic acid, THPS, and oxazolidines, which closed to 90%. In additional, incorporation
of HTD in pickling process will benefit the chrome uptake compared to addition in basifying
process. More importantly, HTD with good alkali stability could slow down the basifying process,
which make the chrome dispersed evenly along the collagen and increase the crosslinking density of
wet-blue leather. Therefore, HTD holds huge promise in application of clean chrome tanning to
25
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improve the chrome exhaustion and can further inspire researchers in the field of exploring the
using of HTD in leather industry.
Acknowledgement
The authors acknowledge financial support from the National Natural Science Foundation of
China (Project no. 21474114 and 21776231), and Youth Scientific Innovation Research Group of
Sichuan (2017TD0024).
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