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Nano-emulsion based on acrylic acid ester co-polymer derivatives as an efficient pre-tanning agent for buffalo hide
⁎Corresponding author. farouk_chem_ekhmu@yahoo.com (Farouk Abd El-Monem)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Acrylic copolymer nanoemulsions were prepared based on methyl methacrylate (MMA) and butyl acrylate (BA). The prepared acrylic copolymer emulsions were characterized using solid content, rheological properties, molecular weight, MFFT and TEM. The prepared polymers were used as pre-tanning of the depickled hide to enhance the physico-mechanical properties of tanned leather. The key parameters which affect exhaustion and fixation of chrome tan as well as shrinkage temperature of the tanned leather were studied and evaluated using SEM, shrinkage temperature and the mechanical properties of the pre-tanned leather. The results showed that, the prepared polymers A & C are the best polymers in improving the physical properties of the treated leather. Furthermore, the shrinkage temperature and the mechanical properties of the tanned leather were improved. In addition, a significant enhancement in the texture of the leather treated by the polymers was noticed as proved by scanning electron microscopy (SEM).
Keywords
Acrylic acid ester copolymers
Nanoemulsion
Pre-tanning agent
Depickled hide
Chrome tan
Chromium uptake
1 Introduction
Chemically, polymers are long-chain molecules of very high molecular weight, often measured in hundreds of thousands. The trade literature sometime refers to polymers as resins (Sperling, 2006). The emulsion polymerization is carried out in liquid medium, which is almost always aqueous and produces milky fluid called latex (Yildirim Erbil, 2000).
Acrylic and methacrylic ester monomers have a combined worldwide production of near about 2 million tons/year (Lochmann and Lim, 1973; Lochmann et al., 1974) .The polymer products derived from these monomers, and n-butyl acrylate and methyl methacrylate are the most common monomers utilized. Poly (n-butyl acrylate) (PBA) (Tg = −55 °C) is soft, tacky, and rubbery while poly (methyl methacrylate) (PMMA) (amorphous, Tg = 105 °C) is hard, tough, and rigid.
Generally, methacrylates have higher tensile strength and lower elongation than acrylates. This difference in properties is attributed to the substitution of the α-hydrogen in acrylates by a methyl group in methacrylates, which results in restriction of motion and rotation of the polymer backbone (Lochmann and Lim, 1973; Lochmann et al., 1974; Mokhtari et al., 2013a,b).
The copolymerization of acrylates and methacrylates increasing the range of polymer properties available (Lochmann and Lim, 1973; Lochmann et al., 1974). The hardness or flexibility of the copolymer is a function of the composition of the monomer, and necessary adjustments are readily achieved by varying the (co)monomer composition. In general, acrylic ester copolymer nanoemulsion is of immense importance for industrial applications (Mokhtari and Pourabdollah, 2012a,b, 2013). Therefore, this work was devoted to explore their application for further use in the leather industry as pre-tanning agents (modifying agent) in leather industry processes.
Over several decades leather technology literature has been dominated by ways to reduce the environmental impact of leather production and techniques. Hides are obvious natural proteins which are easily attacked by organisms and prone to putrefaction.
Tanning operation is one of the most important steps in tanning industry, in which protein of raw hide is transformed into a stable fiber structure (leather). There are many types of tanning agents; the most applied is the chrome tan (Hauber and Germann, 1999). Chrome tannage has proven to be the effective method of tanning and is done in tanneries worldwide. It is used for the production of the great majority of various types of leathers such as upper garments and other light leathers (Wachsmann, 2001). Chrome tan gained the leather better characters other than tanning agents such as high thermal stability, light weight and high strength properties (Bieniewics, 1983; Srearam et al., 2003). Usually, it accounts more than 80% of the tanning industry consumption worldwide (Hauber and Germann, 1999).
Despite the many advantages offered by chrome tanning, there is a worldwide interest in containing the chrome waste. Therefore, there is a growing need for eco-benign tanning systems owing to stringent environmental regulations. Therefore, this work emphasizes the principle of prevention is better than treatment based on the preparation of acrylic copolymer nanoemulsions as an efficient pre-tanning agent for chrome tan.
2 Experimental
2.1 Materials
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Butyl acrylate (BA) and Methyl Methacrylate (MMA) were supplied by Sigma Aldrich and distilled with reduced pressure before use and stored at −20 °C.
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Potassium persulfate (KPS), nonyl phenyl ethoxylated (NP30), Cetyl alcohol (CA), sodium bicarbonate, ammonia water, acrylic acid (AA), acrylamide (AM) and Basic Chromium (III) Sulfate were supplied from Sigma chemicals. Distilled water was used. All chemicals were fine chemicals.
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Egyptian buffalo hides.
2.2 Experimental technique
2.2.1 Emulsion polymerization
2.2.1.1 Pre-emulsion
Distilled water, ionic surfactant and monomers were added in the flask equipped with a high speed homogenizer (ultra turax homogenizer) for 30 min (in three time portions). The acid monomer (AA) is added during continuous homogenization.
2.2.1.2 Emulsion polymerization
Emulsion polymerization of methyl methacrylate and/or butyl acrylate, was carried out in a 500 mL three-necked round bottomed flask equipped with a stirrer, a gas inlet system and a reflux condenser immersed in a water bath. The oxygen was removed by purging the flask by nitrogen.
30% of used distilled water was introduced into the glass reactor with non-ionic surfactant. The mixture was stirred under a blanket of nitrogen at 80 °C followed by the addition of sodium bicarbonate to adjust the pH of the reaction medium.
The prepared pre-emulsion was charged into the reactor. Then, the initiator system was gradually dropped. The reaction was allowed to proceed for 4 h under continuous stirring at 80 rpm with the addition of calculated amount of initiator. At the end of the pre-emulsion addition, the temperature was raised from 80 to 85 °C for 1 h to complete the polymerization reaction. The prepared emulsion was cooled to 40 °C and the pH adjusted using aqueous solution of ammonia to pH 8.
2.2.1.3 Molecular weight determination (Malihi et al., 1984)
In 2 mL of THF solvent 0.01 gm was dissolved, and filtrated by siring filter 0.45 micro then the sample but in GPC device (Agilent 1100 series, Germany, Detector: Refractive Index). Using three columns of pore type (100, 104, 105 A) on series, length 7.5 × 300 mm (Mw1000, 4,000,000) of THF solvent (polystyrene standard) Plgel particle size (5 μm).
2.2.2 Preparation of leather
The raw hide selected for the present investigation was commercially Egyptian buffalo hide. The samples were worked up in the beam house operation as usual. Slow additions of the reagents were done and all percentages of reagents were calculated on the dry weight of pickled hide.
2.2.2.1 Pre-tanning process
Pickled hides in each method were adjusted at pH = 2.5, then, 10% NaCl was added for 30 min as de pickled stage. The de-pickle pelt samples were modified with the prepared polymers as pre-tanning agents before being tanned with basic chromium sulfate (BCS, chromosal B 32.51% basicity). This means that the pre-tanning process was carried out using different doses (2%, 4% and 6%) of different types of polymers (A, B, C, D, E, and F). Then tanning process was carried out using 5% BCS for 2 h followed by 5% of BCS for another 2 h.
2.3 Testing and analysis
2.3.1 Transmission electron microscopy (TEM) (Groves, 1978)
The morphology of the polymer particles was examined using TEM. For the present work, the use of cryo-TEM was required because of the low glass transition temperature of the prepared polymer. To perform cryo-TEM analysis, the latex was diluted with distilled water. A drop of the diluted latex was placed on a carbon-coated grid and dried in a dissector. Then, 1–2 drops of a 0.8 wt% aqueous solution of phosphotungstic acid (PTA) were used to stain the particles.
2.3.2 Scanning electron microscope examination (Nashy et al., 2012)
Specimens for this study were cut from the samples. Specimen size was 10 mm diameter and it was circular in shape. These samples were subjected to sputter coating (Edwards’s model S 140A) of gold ions to have a conducting medium. Sputter coated samples were scanned with JEOL Model JSM-T20 SEM.
2.3.3 Mechanical properties
The tensile strength and elongation at break properties of the dumbbell samples were measured according to ASTM D 412, using Ziwick tensile testing machine, with crosshead speed of 200 mm/min, and the average value of mechanical properties was calculated using at least five samples.
2.3.4 Thermal properties (Nashy et al., 2012)
The thermo gravimetric analysis (TGA) measurements for the obtained grafted samples were carried out at temperature range starting from 50 °C to 700 °C under nitrogen atmosphere with a heating rate of 10 °C/min using Shimadzu TGA-50, Japan.
2.3.5 Shrinkage temperature (Nashy et al., 2012)
A significant shrinkage is observed when it is exposed to a heating medium. The rise in shrinkage temperature of the tanned hide indicated the good effect of tanning agent. It was measured according to the conventional method and Egyptian Specification.
3 Results and discussion
This research aims to use nanoemulsion copolymer of poly (methyl methacrylate-co-butyl acrylate) as pre-tanning agent for buffalo hide to improve physical and mechanical properties of the tanned buffalo leather as well as to reduce the environmental impact of chrome tan.
The emulsion copolymerization of poly (methyl methacrylate-co-butyl acrylate) was carried out at different concentration ratios (MMA or BA) respectively in the presence of acrylic acid and/or acrylamide as thickening agents, potassium per sulfate as initiator and Cetyl alcohol and SLES as surfactants, Table 1. The above recipe contains pot. Persulfate as free radical initiator (0.6), Cetyl alcohol (2.4), and sodium lauryl ether sulfate (6.6). All chemicals weighted in gms.
Component
A
B
C
D
E
F
Methyl Methacrylate (gm)
90
75
60
90
75
60
Butyl acrylate (gm)
60
75
90
60
75
90
Acrylic acid (gm)
6
6
6
3
3
3
Acrylamide (gm)
0
0
0
3
3
3
H2O (gm)
150
150
150
150
150
150
3.1 Characteristics of the prepared emulsion copolymers of (MMA-co-BA) as pre-tanning agent
The prepared polymers were investigated by testing: solid content, M.wt, coagulum, viscosity, drying time, MFFT, TEM and SEM. The mechanical properties of the prepared leather samples were also examined to study the characteristics of the polymer. The physical, chemical as well as mechanical properties of the prepared polymer emulsions were examined according to the international standard as shown in Table 2.
Properties
Standard
Value
ASTM
A
B
C
D
E
F
pH
8.3
8.2
8.3
8.1
8.0
8.1
Solid Content (%)
D2369
50.1
49.6
50
50
49.5
50.2
Conversion (%), Piirma, 1982
100
99.4
100
99.8
99.7
100
Drying time (sec) at 23 °C, Nashy et al., 2012
70
95
115
75
100
120
Particle size (nm)
TEM
85
71
70
170
130
100
Brookfield Viscosity RVT #50 rpm, (cps)
D2196 – 99
55,000
73,000
90,000
22,000
27,000
30,000
3.1.1 Effect of butyl acrylate on viscosity of the prepared polymer
The data obtained from Table 2 were planned in Fig. 1 showing the relation between viscosity of the prepared samples and changing hydrophobic butyl acrylate concentrations with acrylic acid. It is obvious that the viscosity of copolymers increased as the concentration of butyl acrylate increased.
Butyl acrylate/methyl methacrylate (%).
3.1.2 Molecular weight and Molecular weight distribution
It is clear that the copolymer (D) (M.wt. = 3.1726e5 g/mol, Mn = 7.0307e4 g/mol) has a higher molecular weight and molecular weight distribution than copolymer (C) (M.wt. = 2.7435e5 g/mol, Mn = 6.9769e4 g/mol).
3.2 Transmission electron microscopy (TEM) of the prepared polymers
The morphology of the particles was examined using TEM. Figs. 2 and 3 show the TEM of the prepared emulsion lattices. It is clear that, the particle size of the prepared lattices ranged about 170 nm for polymer (D) which prepared with acrylic acid as thickening agent and high content of methyl methacrylate, while the particle size of polymer (F) ranged about 100 nm which prepared from acrylic acid and acrylamide as thickening agent and high content of butyl acrylate. It has also been reported that the hydrophobic/hydrophilic character of the monomer(s) used in emulsion polymerization has a decisive influence on particle morphology: according to the data obtained by Snuparek et al. (2005) macromolecules with an increase in hydrophilicity facilitate carboxyl ionization, resulting in higher particle swelling (increase in viscosity) and particles with high contents of AA and/or acrylamide being completely solubilized. Usually, carboxylic acids are added to improve the mechanical, freeze–thaw, and pigment mixing stability of the lattices.
TEM of prepared emulsion copolymer (D).
TEM of prepared emulsion copolymer (F).
Carboxylic acid monomers are often completely soluble in water; however, they will still distribute to vary extents into the organic phase depending on their relative hydrophilicity.
3.3 Emulsion polymers of MMA-co-BA as pre-tanning agent
Pre-tanning process was carried out with two groups of polymers using different doses of 2%, 4%, and 6%, respectively. Pickled hide was treated with sodium format and sodium chloride to adjust the pH through the all its thickness as well as to eliminate out the acid and salts. This process (depickling) is a very important process to prevent the acid or salt hydrolysis of protein fibers. The depickled pelt samples were modified with the prepared polymers as pre-tanning agents before being tanned with basic chromium sulfate (BCS).
The pre-tanning process of buffalo hides was done as follows:
Treatment of pickled hide with 10% NaCl for 30 min as depickled agent.
The depickled pelt was pre-tanned by 2%, 4% and 6% doses of different types of polymers.
The full tanning process by (BCS) was carried out through adding 5% BCS for 2 h followed by 5% BCS for another 2 h. Tanning allows stabilization of the collagen fiber through a cross-linking action with active sites of proteins. The tanned hides and skins are tradable intermediate products (wet-blue).
3.4 Characteristics of the pre-tanned leather
3.4.1 Mechanical properties
Mechanical properties have generally been given the greatest consideration in the evaluation of leather. The mechanical characters include the measurement of the tensile strength and elongation at break. These characters were carried out according to the Egyptian standard method (ES-123) and official method. An average value of at least five tests was taken for each item. In general Figs. 6–9 showed an improvement in the mechanical properties of the treated leather by the prepared polymers than untreated leather. This may be due to the good adhesion effect of the polymer on hide fiber of the leather and also due to the filling action on grain layer.
Tensile strength of pre-tanned buffalo hide (transverse).
Tensile strength of pre-tanned buffalo hide (longitudinal).
Elongation at break of pre-tanned Buffalo hide (transverse).
Elongation at break of pre-tanned buffalo hide (longitudinal).
SEM of grain surface of chrome tanned leather (X45).
SEM of the cross-section of chrome tanned leather (X80).
SEM of grain surface of polymer (C) tanned leather (X45).
SEM of the cross-section of polymer (C) tanned leather (X80).
3.4.2 Tensile strength
Figs. 4 and 5 show tensile strength of the tanned leather samples at both transverse (Fig. 4) and longitudinal directions (Fig. 5) by different types of polymers.
Fig. 4 shows tensile strength of the tanned leather samples at transverse direction by different types of polymers with doses 2%, 4% and 6% respectively, it is obvious from Fig. 4 that; the polymer (A) achieved good tensile strength at 4% concentration compared with the other polymers.
Fig. 5 shows that; the polymer (A) achieved good tensile strength compared with the other polymers. It can be concluded from Figs. 4 and 5 that the polymer (A) is the best one in its effect on tensile strength at both longitudinal and transverse directions of 4% concentration.
3.4.3 Elongation at break
Figs. 6 and 7 show elongation at break of the tanned leather samples at both transverse (Fig. 6) and longitudinal directions (Fig. 7) by different types of polymers with doses 2%, 4% and 6% respectively.
Fig. 6 shows that; the polymer (C) achieved good elongation at break, compared with the other polymers.
Fig. 7 is obvious that; the polymer (F) achieved good elongation at break, compared with the other polymers.
In general it was noticed that the polymer (A) showed improved tensile strength compared with the other polymers. However, the elongation at break was improved for polymer (C) in the transverse section while elongation at break was improved in polymer (F) in the longitudinal section. The improvement of elongation by polymers (C and F) can be attributed to the lubricating effect of polymer. The lubrication effect reversed to these two polymers (C and F) has a higher ratio of butyl acrylate which has elastic properties than methyl methacrylate and also has smaller particle size compared with the other polymers.
3.4.4 Thermal study (TGA)
The TGA and Dr-TGA curves are shown in Table 3 and the TGA analysis shows that, the decomposition temperature of the polymer tanned leather is of higher degrees than those of the chrome tanned leather. Thus, the incorporation of the polymer into leather increases the thermal stability of the polymer leather over that of the chrome tanned leather. This improvement in thermal stability can be attributed to the formation of polymer – collagen composite, which can be explained by brought about multiple weak hydrogen bonding between the numerous carbonyl groups (C⚌O) of the polymers and the countless hydrogen atoms of (NH) peptide groups, which support of the junction between the grain and corium. These results indicate that polymers fill up the empty parts of leather and lubricate the leather fibers.
Temperature at (°C)
Weight loss (%)
Chrome pre-tanned leather
Tanned leather by polymer C
Tanned leather by polymer F
Tanned leather by polymer A
Tanned leather by polymer D
75
15.188
10.978
11.581
14.583
11.851
300
33.471
26.256
26.013
28.754
24.820
500
62.156
74.55
69.536
67.126
63.899
720
90.056
93.602
84.004
82.836
78.995
3.4.5 Scanning electron microscope (SEM)
The morphological study of the chrome tanned leather was carried out in comparison with tanned leather by copolymers (A, B, C, D, E and F). SEM can be used to assess the penetration of the polymer through the leather and into the hierarchy of the structure and is thus a useful technique for evaluating the effects of various treatments on the skin. SEM of the grain surface (X45) and the cross-section of the (X80) of the skin with and without polymers was carried out to show the effect of the prepared polymers on the grain and fiber bundles as pre-tanning agents (Figs. 8–10a and b). The SEM of the cross section of the leather fibers tanned by the polymers showed a significant lubrication of fiber bundles (Figs. 8–10b) and surface grain is fine (Figs. 8–10a). It was observed that the SEM of the samples pre-tanned by polymers (C&A) has a smooth fiber, firmness grain and modified handle, which are good evidence for the penetration and lubrication of copolymers onto the leather fibers and grain surface. The pre-treated chrome tanned leather by polymers gives better grain smoothness, soft fibers, filling and modified handle. The filling of the grain layer improves buff ability for uses as corrected grain leather.
SEM of grain surface of polymer (A) tanned leather (X45).
SEM of the cross-section of polymer (A) tanned leather (X80).
3.4.6 Shrinkage temperature
Fig. 11 and table 4 represent shrinkage temperature of the tanned leather samples by chrome and by different types of prepared polymers at the same dose (4%). It is obvious that; the polymer (A) achieved good shrinkage, compared with other samples treated with polymers (F, C, D and Cr).
Shrinkage Temp. for copolymers as pretanning agent.
Pre-tanning agent
Cr 10%
F 4% & 10% Cr
C 4% & 10% Cr
A 4% & 10% Cr
D 4% & 10% Cr
Shrinkage
87
70
90
93
60
Shrinkage temperature was measured according to the conventional method and Egyptian Specification. The treated leather with polymers (A, D, C and F) as well as tanned leather were subjected to shrinkage testing. A significant shrinkage is noticed when it is exposed to a heating medium. The rise in shrinkage temperature of the pre-tanned hide with polymers (A and C) indicated the good effect of tanning agent.
4 Conclusions
The obtained results of this work indicated that, the characters of the pre-tanned leather are improved in the following respects:
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Emulsion polymerization is a flexible process by which a wide range of practical materials can be made, and in each case, the process is tailored to optimize the performance properties of the final product.
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The visual properties of the treated leather through fullness and tight grain were modified due to the filling action of nano-particle copolymer.
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The best tensile strength was obtained with polymer (A) when applied as pre-tanning agent due to a high content of MMA.
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The polymers (C and F) improve the elongation at break when applied as pre-tanning agent depending upon its lubrication.
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Replacing chrome by 4% polymer (A or C) for pre-tanning process increased leather shrinkage temperature.
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Enhancement the thermal stability of the treated leather.
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