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stearate in layered double hydroxide: Toward flame retardant nanocomposites
Journal of Environmental Management 238 (2019) 235–242
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Research article
Intercalation of modified zanthoxylum bungeanum maxin seed oil/ stearate in layered double hydroxide: Toward flame retardant nanocomposites
T
Bin Lyua,b,∗, Yue-Feng Wanga, Dang-ge Gaoa, Jian-zhong Maa,b,∗∗, Yun Lic a
College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China Key Laboratory of Leather Cleaner Production, China National Light Industry, National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science & Technology, Xi'an, 710021, PR China c College of Chemistry and Chemical Engineering, Shandong, Yantai, 264005, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Zanthoxylum bungeanum maxin seed oil Layered double hydroxide Flame retardant Thermal stability
Zanthoxylum bungeanum Maxim Seed Oil (ZBMSO) is widely distributed in most parts of China, which cannot be edible and extensively consumed due to its high free fatty acids. This paper reports a rational route to utilization of ZBMSO in preparation of nanocomposites which can enhance leather flame retardancy and thermal stability. ZBMSO was synthesized through three-stage process, decoloration, acid reduction and sulfitation to prepare the modified ZBMSO fatliquoring agent (MZBMSO). Then nanocomposites based on MZBMSO and stearate-layered double hydroxide (s-LDH) were prepared via in-situ method. XRD and TEM results indicated that the MZBMSO intercalate into the galleries of s-LDH with uniform dispersion. Compared with MZBMSO, the leather treated by MZBMSO/s-LDH had a remarkable improvement on flame retardancy and superior softness which limiting oxygen index (LOI) increased from 23.6% to 28.0% and smoke density index decreased from 25 to 6.
1. Introduction Zanthoxylum bungeanum maxin (ZBM) is popular as a rutaceae and seasoning in China due to its unique special fragrance and taste characteristics. According to reports, ZBM cultivation area in China has reached 1.33 million hm2, the annual output of ZBM reaches one million tons. ZBM seeds (contains 27%–31% oil) as the main byproduct of ZBM, theoretically, which weight ratio of the peel is more than 20%, about 600,000 tons, have a great development value (Zhang et al., 2017). However, ZBM seeds oil (ZBMSO) contains about 25% free fatty acids leads to its unsuitable consumption for human (Zhang et al., 2015). At present, industrial application of ZBMSO concentrate mainly includes in three directions: production of edible oil (Xia et al., 2011), extraction of α-linolenic acid (Xue et al., 2013), and preparation of biodiesel (Zhang and Jiang, 2008; Zhang et al., 2015). Unfortunately, their high cost become the major obstacle to commercialization of ZBMSO. Therefore, exploring low-cost innovative application of ZBMSO is necessary. In recent decades, leather consumption in automobile cushion, aviation seat, furniture manufacturing and luxury goods is increasing rapidly. Fatliquoring agents, the largest amount of leather chemicals,
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based on oils, fat, or grease in emulsion form, are applied in tanned leather for improving leather softness and physical characteristics (Wang et al., 2016). Fatliquoring agents can be made of mineral oil, animal and plant oil (Lyu et al., 2016; Kim et al., 2006; Dang et al., 2016). Mineral oil is non-renewable resources, and annual production of animal fat is limited, so as a renewable material, vegetable oil is widely used. In this context, investigations are required to use ZBMSO as potential feedstock for fatliquoring agent. Generally, for existence of oil, the use of fatliquoring agent makes leather more flammable, reducing the safety of leather products. Traditional leather flame retardants contain halogen and phosphorus compounds, which are detrimental to the environment (Cao et al., 2010; Lyu et al., 2015; Huang et al., 2006; Bacardit et al., 2010). Therefore, it is important and necessary to develop effective fatliquoring agents that are flame-retardant and environmentally benign (Wang et al., 2006; Huang et al., 2011). Layered double hydroxide (LDH) is a category of anionic layered compounds. The main layer of LDH consists of two or more metal hydroxides, which can be thermally degraded to form metal oxides with large specific surface area to reflect good flame retardancy and smoke suppression (Chiu et al., 2014; Kalali et al., 2016; Li et al., 2014, 2016; Wang et al., 2015). MgAl-LDH as an example, Li et al. introduced 6% of
Corresponding author. College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China. Corresponding author. College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China. E-mail addresses: [email protected] (B. Lyu), [email protected] (J.-z. Ma).
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https://doi.org/10.1016/j.jenvman.2019.03.001 Received 22 November 2018; Received in revised form 26 February 2019; Accepted 1 March 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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metabisulfite solution was added and reacted for 2 h. Then the reaction system was cooled to 60 °C and added deionized water to adjust the solid content to 50 wt%. (Eq. (S3)).
MgAl-LDH into epoxy resin, which can reduce the peak heat release rate and total smoke production of EP by 62% and 45%, respectively (Li et al., 2016). LDH is hydrophilic and easy to disperse in water-soluble polymer. However, there is scarcely research about improving composite properties by LDH disperse in plant oil phase. In this paper, ZBMSO and stearate-LDH (s-LDH) are used as the raw materials to prepare a nanocomposite as leather fatliquoring agent. The purpose is to develop a low-cost use of ZBMSO and invest an environmentally friendly flame retardant leather fatliquoring agent. In order to achieve this goal, we performed the following experiment: firstly, we use stearate anions with a carbon chain length the same as ZBMSO molecule to modify LDH, which improves the compatibility between LDH and ZBMSO. Then the MZBMSO/s-LDH nanocomposites are prepared via an in-situ method. The effects of MZBMSO/S-LDH nanocomposites on the performance of fatliquored leather, including flame retardancy, thermal stability and softness, were investigated and compared with the performance of MZBMSO fatliquored leather. The flame retardant mechanism was proposed from two aspects of condensed-phases and gaseous-phases.
2.5. Leather fatliquoring process MZBMSO and MZBMSO/S-LDH were applied as fatliquoring agent in the fatliquoring process of wet-blue goat sheepskin. For comparison with the MZBSO and MZBMSO/s-LDH, the blue wet leather was divided into two parts according to the method shown in Fig. S1. Detailed experimental details have been added to the supplementary material S1. 2.6. Characterization The XRD patterns of the samples were recorded using a D8 Advance X-ray diffractometer (Bruker). Excitation by Cu Kα radiation generator; λ = 0.15406 nm; tube voltage 30 kV; tube current 30 mA; the scanning range of 2–70° and scanning rate of 10° min−1. Chemical structures of samples were identified using FTIR with KBr disk, recorded on a Perkin Elmer, FTI1650 Spectrum BX. TGA analysis was carried out using a thermogravimetric analyzer (TGA Q500 produced by TA company) in a nitrogen atmosphere and a heating rate of 10 °C/min. UV absorbance test of the ZBMSO was conducted on a TU1900 double beam ultraviolet visible spectrophotometer at wavelength range of 220–320 nm.
2. Experimental 2.1. Materials and instrument Stearic acid (analytical grade) was obtained from China Pharmaceutical Chemical Reagent Co., Ltd., MgAl-LDH (Mg/Al molar ratio = 2:1) was obtained from Beijing University of Chemical Technology. Sodium hydroxide, cis-butenedioic anhydride, sodium pyrosulfite, methanol, glycerol, sodium bisulfite, concentrated sulfuric acid (analytically pure) were purchased from Tianjin Hongyan Reagent Factory. ZBMSO was obtained from Hancheng, China. Activated carbon and activated clay (industrial grade) were purchased from Tianjin Dengfeng Chemical Reagent Factory.
2.7. Properties of MZBMSO/s-LDH fatliquored leather According to GB/T 2406.1–2008, LOI test was performed by HC-2C oxygen index meter (Shangyuan Company, China). The test samples were dried at room temperature and the size was 14 cm × 5.2 cm; testing vertical combustion on ZF-4 combustion test chamber (Shangyuan Company, China) with the sample dimension of 1.5 cm × 13.5 cm according to ALCA Method E 50; the smoke density tests were carried out on a smoke density tester with the sample dimension of 5 cm × 5.2 cm × 0.3 cm (YM-3, Nanjing Shangyuan Analytical Instruments Co., Ltd.) according to GB/T8627-2007. All of the above tests were performed on three samples and the results were averaged. Determination of the softness of fatliquored leather according to ISO 17235-2002 using GT-303 leather softness tester (Co., Ltd.), the leather samples were dried for 1 day at 20 °C and relative humidity 60 before testing. In the test, first used the metal disc to correct the dial to zero, and then randomly selected 9 parts in different parts of the leather to be tested for softness detection, and the results were averaged. The microscopic morphology of the leather samples was observed by TESCAN scanning electron microscopy (Czech). The acceleration voltage was 12 kV when observed. Elemental analysis was obtained by Energy dispersive X-ray (EDX) connected to the SEM. Thermogravimetric analysis connected with Fourier transform infrared spectroscopy (TG-FTIR) was carried out on STA449F3-1053-M thermal analyzer (NETZSCH). The analysis was carried out in an oxygen atmosphere, and the temperature of the link tube was maintained at 310–340 °C to prevent condensation of gaseous products.
2.2. Preparation of stearate modified layered double hydroxide (s-LDH) 1.00 g dried [Mg4 Al2 (OH)12]2+ (CO3)2- ⋅4H2O was added to a 750 mL sodium stearate solution in 0.003 M and stirred at room temperature for 24 h to obtain stearate-LDH. After filtration a white solid was obtained. The solid was washed three times (each time with 100 mL of deionized water) until the pH was about 7. Then dried in vacuo. 2.3. Decoloration and acid reduction of ZBMSO Decolorization: ZBMSO and decolorizing agent (2% of oil mass) were placed in a three-necked flask and heated at 110 °C for 30 min at 350 rpm. Then filtered with a 600-mesh filter after 8 h. Decolorizing agent consisted of 15.5 wt% of activated carbon and 84.5 wt% of activated clay. Acid reduction: Decolorized ZBMSO and glycerin (molar ratio of 1:1) were added to a three-necked flask, and concentrated sulfuric acid was added as a catalyst. The flask was heated at 130 °C for 3 h (Eq. (S1)). Then, the flask was cooled down to 70 °C before adding methanol (molar ratio of ZBMSO: methanol = 1:1), reaction lasted for 2 h to obtain reduced acid ZBMSO (Eq. (S2)).
3. Result and discussion
2.4. Preparation of MZBMSO/s-LDH
3.1. Analysis of stearate anions modified LDH
Firstly, ZBMSO (after acid reduction and decolorization) and s-LDH (0, 1%, 2%, 3%, 4%, 5%, according to oil mass) were added into a three-neck flask, which was heated to 75 °C for 30 min with a rotation rate of 350 rpm. Secondly, toluene-p-sulfonic acid sodium salt (as a catalyst) and maleic anhydride were added into the flask, and the reaction system was heated to 110 °C for 3 h. The pH was adjusted to 7, and the temperature was reduced to 80 °C, when 40% sodium
Fig. 1a showed the XRD patterns of the pristine and stearate -LDH in the range of 2θ degree from 2 to 70°. It was obvious that both LDH and stearate-LDH were crystalline and had a well-defined layered structure. The interlayer distance of the LDH or stearate-LDH were calculated by Bragg equation from the first diffraction peak, nλ = 2d sinθ, for the (003) peak n equals 1 and λ is the wavelength of Cu-Ka radiation, and θ is the degree of diffraction half angle. The intercalation of the stearate 236
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Fig. 1. (a) XRD spectra of pristine LDH and s-LDH. (b) FT-IR spectra of pristine LDH, sodium stearate and s-LDH.
Fig. 2. (a) FT-IR, (b) XRD spectra of MZBMSO/s-LDH; TEM image of MZBMSO/s-LDH (c) Magnified 100 K times, (d) Magnified 10.5 K times. Table 1 Flame retardancy tests of leather treated with MZBMSO/s-LDH composite. Dosage of s-LDH
After flame time (s)
The Length of charring (mm)
Mass loss rate (%)
LOI (%)
Smoke density (%)
0 1 2 3 4 5
70 54 47 40 36 37
81 55 55 51 38 35
60.32 44.27 44.35 37.74 30.54 30.29
23.6 ( ± 0.3) 26.7( ± 0.2) 27.3( ± 0.2) 27.7( ± 0.1) 28.0( ± 0.1) 28.0( ± 0.3)
25 18 15 13 6 8
( ± 6.1) ( ± 6.7) ( ± 4.6) ( ± 4.3) ( ± 2.4) ( ± 3.6)
( ± 8.3) ( ± 7.9) ( ± 4.3) ( ± 3.6) ( ± 2.3) ( ± 3.3)
( ± 11.66) ( ± 6.43) ( ± 3.68) ( ± 3.03) ( ± 1.67) ( ± 2.71)
at about 3400 cm−1. The symmetric vibration absorption band of CO32− could be observed at about 1357 cm−1 (Jiao et al., 2009; Chai et al., 2009). The lattice vibration band between metal element and oxygen element appeared below the 800 cm−1 regions (Eili et al., 2014). Seen from the s-LDH curve, the strong absorption bands at 28003000 cm−1 were corresponded to the characteristic adsorptions of the
anions was increased interlayer distance from 0.78 nm to 1.69 nm compared with pristine LDH. The intensity of (003) basal reflection of sLDH was much weaker than that of LDH, indicating that a slight amount of stearate anions had inserted into the interlayer of the LDH. Fig. 1b showed FT-IR spectra of the pristine LDH, sodium stearate and s-LDH. From pristine LDH curve, the -OH extensional vibration peak of metal hydroxide and interlayer water resulted to a broad peak 237
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Fig. 3. TGA curves of MZBSO/s-LDH fatliquored leather.
Fig. 5. FTIR spectra of the pyrolysis gaseous products of (a) the leather treated with MZBMSO, (b) leather treated with MZBMSO/s-LDH-4%.
ZBMSO was calculated by changing the UV absorbance of ZBMSO (the detection method was given in supplementary material S2). As shown in Fig. S2, the characteristic peak of ZBMSO at 203 nm. The decolorized rate of ZBMSO was 16.7%. Acid value reflects the amount of free fatty acids in the oil which can reflect the rancidity degree of oil. The higher acid value, the degree of rancidity is greater. There is a certain requirement for acid value of the oil used for leather fatliquoring, usually near 10 mgKOH/g. The method of measuring acid value of oil was given in supplementary material S3. As shown in Table S1, the acid value of ZBMSO could be reduced from 58.88 to 12.86 mg KOH/g, which could satisfy the preparation condition of fatliquoring agent. Saponification values indicate the average relative molecular mass of oil (a mixture of fatty acids and triglycerides). The test method of saponification value of ZBMSO was given in supplementary material S4. The saponification value of ZBMSO was 216.88 mgKOH/g, and its average relative molecular was about 778 g/mol−1. The length of the carbon chain of ZBMSO was C18, as the same as sodium stearate, which was selected as a modifier for LDH in order to improve the compatibility of LDH and ZMBSO as much as possible.
Fig. 4. Softness of leather fatliquored by MZBMSO/s-LDH.
3.3. Structural of MZBMSO/s-LDH nanocomposite fatliquoring agent
alkyl chain (C-H and –CH2), which could be attributed to superimposed in pristine LDH and sodium stearate (Costa et al., 2008). The strong absorption at about 1357 cm−1 and 1571 cm−1 could be attributed to the symmetric and asymmetric stretching vibrational peaks of carboxylates (Zhao et al., 2006; You et al., 2002; Olfs et al., 2009). This confirmed that sodium stearate has been modified pristine LDH preliminarily. In addition, the peak intensity of the comparative alkyl chain and -OH indicated that a large amount of stearate is adsorbed on the LDH layer. In summary, XRD and FT-IR analyses confirmed that the stearate anions were formed successfully modified MgAl-LDH. In addition, the pristine LDH was coated by stearate anions and stearate anions had intercalated into the interlayer of LDH.
The FTIR spectra of ZBMSO, MZBMSO and MZBMSO/s-LDH were shown in Fig. 2a. Three functional groups, such as -COOH, -COO- and -SO3-, are different in ZMBSO and MZBMSO, so the discussion mainly focuses on the above three functional groups. Compared with the FTIR spectra of ZBMSO, the broad peaks at 3489 cm−1 in MZBMSO spectra was due to the stretching vibration peak of -OH of carboxylic acid dimer. Seen from MZBMSO spectra, the new absorption peaks around 1388 cm−1 and 1604 cm−1 were the asymmetric and symmetric stretching of carboxylate (C=O). The strong absorption at 1218 cm−1 was assigned to the C-O stretch of carboxylic acid. The strong absorption at 1049 cm−1 was associated with the S=O group stretching vibration of sulfonate. These results showed that hydrophilic group has successfully grafted onto ZBMSO. Besides, seen from MZBMSO/s-LDH spectra, the presence of new peak at low wavenumber (below 800 cm−1) region were assigned to the M-O (M = Mg and Al) lattice vibration of the LDH (Olfs et al., 2009). XRD results (Fig. 2b) provided the information to verify the composite structure of MZBMSO/s-LDH. The broad diffraction reflection could be attributed to diffraction of amorphous ZBMSO. In contrast, the XRD curve of MZBMSO/s-LDH composite showed some special
3.2. Physicochemical properties of ZBMSO The non-decolorized and decolorized of ZBMSO were shown in Fig. S2. After decolorized treatment, a clear light path could be observed from the side incident beam. Obviously, the degree of clarity had been enhanced and the solid impurities suspended in the non-decolorized ZBMSO was removed effectively. In addition, the decolorization rate of 238
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Fig. 6. Absorbance versus temperature curves of typical decomposition products for the leather treated with MZBMSO and MZBMSO/s-LDH-4%.
3.4. Flame retardancy of leather
diffraction peaks at 2θ values of 2.94, 15.84 and 27.96, which were similar to those of the LDH. The interlayer distance of MZBMSO/s-LDH was 3.08 nm (the interlayer distance of s-LDH was 1.69 nm) (Ding et al., 2015). The reason for this result may be that the molecular chain of MZBMSO contained -SO3- and -COO-, and the residual fatty acid of the oil contained -COO-. These functional groups may react with the layers of s-LDH and diffused into the interlayer of s-LDH under high temperature reaction, increasing the interlayer spacing of s-LDH (Carlino et al., 1994). The TEM micrograph could further verify the conclusion of XRD analysis and reflect the dispersion of s-LDH in MZBMSO. In Fig. 2c, it could be observed that LDH and MZBMSO form intercalated nanocomposites. In Fig. 2d, it could be seen that s-LDH had good dispersion in MZBMSO. Both morphological features obtained from XRD and TEM illustrated that MZBMSO/s-LDH had typical nano-composite structure, with good dispersion of s-LDH in MZBMSO.
Vertical combustion results of leather treated with MZBMSO/s-LDH were listed in Table 1. After-flame time is the duration after removal of the ignition source. Compared with leather treated with MZBMSO only, the after-flame time of MZBMSO/s-LDH treated leather reduced markedly, and after-flame time was decreased with increasing dosage of sLDH. The length of charring and mass loss rate intuitively reflect the flame retardant properties of leather. The length of charring and mass loss rate of MZBMSO/s-LDH treated leather were both lower than that of MZBMSO, decreasing with the higher dosage of s-LDH. The addition of s-LDH improved the flame retardancy and prevented the flame combustion of leather effectively. LOI means the minimum oxygen concentration required to maintain sample combustion. LOI can be used to judge how difficult it is to burn 239
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Fig. 7. SEM micrographs of char residues for (a) leather treated with MZBMSO, (b) leather treated with MZBMSO/s-LDH-4%.
Fig. 8. SEM and EDX of leather fatliquored by MZBMSO/s-LDH-4%.
a material when it comes into contact with a flame. The LOI value improved with increasing dosage of s-LDH. When 4% and 5% LDH were added, the highest LOI value was up to 28%. Smoke density measures the amount of smoke produced by the material under prescribed test conditions, which is quantified by the attenuation of light intensity through the smoke. The greater density of smoke is, the more disadvantageous to evacuees and extinguish of the fire. In comparison with MZBMSO treated leather, the smoke density of MZBMSO/s-LDH treated leather was reduced significantly. The lowest smoke density reached to 6 when 4% s-LDH was added. The smoke elimination performance can be attributed to form s-LDH a solid base with a high specific surface and large adsorption capability which can adsorb acid gases and particles produced in leather combustion (Li et al., 2016; Wang et al., 2015). In summary, the flame retardancy of MZBMSO/s-LDH fatliquoring leather was significantly improved compared to leather treated with MZBMSO.
which was pyrolysis of triglyceride (Xu et al., 2017). In the third stage, temperature ranging from 300 to 590 °C, major sub-reaction was taken place, which was the pyrolysis of collagen fibers (Xu et al., 2017). The thermal stability of the leather collagen fiber was examined using a temperature loss of 25% mass loss (T-25%) and the char residual percentage at 590 °C. The MZBSO/s-LDH treated leather had a higher decomposition temperature in the third stage than the MZBMSO treated leather. The maximum T-25% belonged to MZBSO/s-LDH- 4% fatliquored leather, increasing from 309.4 °C of MZBMSO fatliquored leather to 319.8 °C. Furthermore, the char yield was also improved gradually with s-LDH loading increases. The TGA results showed that the addition of s-LDH could improve the thermal properties of leather. This may be due to the introduction of LDH to protect the collagen fibers of the leather, so the thermal stability of the leather sample has improved.
3.6. Softness of leather The main purpose of fatliquoring is to make the neutral oil and active groups in the fatliquor penetrate into the leather and combine with collagen fibers to play the role of soft leather (Ma et al., 2014; Quadery et al., 2015). Fig. 4 showed softness results of leather fatliquored by MZBMSO/sLDH at different dosage of s-LDH. It can be seen from the figure that the softness of leather fatliquored by MZBMSO was about 7.6 mm. With the increased in the amount of s-LDH, softness of leather did not change
3.5. Thermal stability of leather The thermal decomposition behavior of leather treated with MZBSO/s-LDH was illustrated in Fig. 3. The mass loss of leather consisted three stages. The decomposition of the first stage was between 25 and 150 °C, which could be attributed to the evaporation of water and small molecules. Second mass loss phase between 150 and 300 °C, 240
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Fig. 9. Schematic illustration of the flame retardant mechanism of MZBMSO/s-LDH-4% on leather collagen fibers.
the leather treated with MZBMSO was much higher than that for the leather treated with MZBMSO/s-LDH-4%. The reduced amount of the hydrocarbon, aromatic, carbonyl and ether compounds (Fig. 6a, b, c and e) implied less “fuel” were fed back to the flame. The reduction in the amount of flammable gas released may be due to the presence of LDH or LDH-derived oxide which increase the strength of the leather char residue, thereby inhibiting the volatilization of the flammable gas. This effect could be expected to result in a reduction in the length of charring and mass loss rate in vertical burning ratings (Gao et al., 2014; Bahu et al., 2017). The microstructure of residual samples after vertical burning was observed by SEM. Leather treated with MZBMSO (Fig. 7a) loosed fiber structure and generated a broken porous residue after combustion, indicating its high flammability. Heat and oxygen easily pass through these holes and gaps, thus the leather sample burn violently. In contrast, leather treated with MZBMSO/s-LDH-4% (Fig. 7b), the residue, showing a perfect fibers structure, could effectively block the penetration of heat and oxygen into collagen fibers and improve the fire resistance of fibers. In order to prove the protective effect of s-LDH on the collagen fibers of leather. It must be demonstrated that s-LDH can be successfully penetrated and evenly distributed inside the leather. s-LDH contains Mg and Al elements. Therefore, the distribution of Mg and Al elements from the flesh to the grain side of MZBMSO/s-LDH-4% treated leather was characterized using line spectrum scanning. As shown in Fig. 8a, the distribution of Mg and Al inside the leather was relatively uniform. In addition, at higher magnifications (in Fig. 8b), it could be observed that a large amount of small white particles was coated on the surface of the leather collagen fibers. The elemental composition of these small particles was verified using EDX and the average contents of C, O, Mg and Al were 40.01%, 41.09%, 10.81% and 8.09%, respectively. This EDX result could prove the particles was s-LDH encapsulated by MZBMSO. The test results of Fig. 9 indicated that s-LDH has penetrated into the interior of the leather and was evenly distributed on the surface of the collagen fibers.
significantly, which indicated that the introduction of s-LDH has little effect on the leather softness. 3.7. Flame retardant mechanism In order analyze the specific effects of s-LDH on the flame retardant properties of leather, the mechanism was investigated for the condensed phase and the gaseous phase respectively. In this paper, TG-FTIR was utilized to study the thermal decomposition of volatiles during decomposition process. Through this part of the characterization, the gas phase components produced by the sample during thermal degradation could be obtained. Fig. 5 showed the normalized FTIR spectrum of the pyrolysis gas product of the leather treated with MZBMSO and the MZBMSO/s-LDH-4% at the maximum degradation rate. The spectra of MZBMSO and MZBMSO/s-LDH-4% treated leather had significant difference, which meant that the addition of s-LDH had a major effect on the decomposition of volatiles during thermal degradation. The gas phase products evolved from the leather treated with MZBMSO and MZBMSO/s-LDH-4% could be qualitatively identified by the peak of FTIR, such as phenol (around 36003700 cm−1), water (3212 cm−1), -C-H groups for small molecule organic combustible gas (2800-3100 cm−1), CO2 (2356 cm−1), carbonyl compounds (1749 cm−1), aromatic compounds (1508 cm−1), ethers (1170 cm−1) and unsaturated hydrocarbon (968 cm−1) (Liu et al., 2011; Dong et al., 2012; Huang et al., 2009; Gaan et al., 2009). It could be found in Fig. 6b that the flammable component released by leather treated with MZBMSO/s-LDH-4% was significantly reduced. This indicated that the addition of s-LDH can inhibit the release of flammable gas during thermal degradation of leather. To further prove the above conclusions, the absorbance of partial degradation products of MZBMSO and MZBMSO/s-LDH-4% treated leather with temperature were revealed in Fig. 6. The change of absorbance intensity of flammable gas (carbonyl, aromatic compounds and esters.) and non-flammable gas (CO2) at different temperatures were investigated. The absorbance intensity of the flammable gas for 241
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Upon the basis of the aforementioned combustion behavior, the flame retardant mechanism was proposed, as illustrated in Fig. 9. MZBMSO and MZBMSO/s-LDH penetrated into the leather and formed an oil film on the surface of collagen fibers. When encountering a flame, MZBMSO treated leather formed broken porous char residue on fibers surface, and thus, combustible volatile escaped from these cracks freely to feed the flame. Besides, the heat penetrated through these gaps to consume the underlying collagen fibers. In contrast, leather treated with MZBMSO/s-LDH appeared dense continuous char residue. Because the nanoparticles were coated on the fibers, which inhibit external heat, protecting the underlying fibers from being destroyed and inhibiting combustible gas escaping from the interior. Both barriers and thermal insulation effect were important to improve the fire resistance of collagen fibers.
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Synthesis and characterization of functional eugenol derivative based layered double hydroxide and its use as a nanoflame-retardant in epoxy resin. J. Mater. Chem. A. 3, 3471–3479. Li, C., Wan, J., Pan, Y.T., Zhao, P.C., Fan, H., Wang, D.Y., 2016. Sustainable, biobased silicone with layered double hydroxide hybrid and their application in natural-fiber reinforced phenolic composites with enhanced performance. Acs Sustain. Chem. Eng. 4, 3113–3121. Liu, Y., Zhao, J., Deng, C.L., Chen, L., Wang, D.Y., Wang, Y.Z., 2011. Flame-retardant effect of sepiolite on an intumescent flame-retardant polypropylene system. Ind. Eng. Chem. Res. 50, 2047–2054. Lyu, B., Duan, X.B., Gao, D.G., Ma, J.Z., Hou, X.Y., Zhang, J., 2015. Modified rapeseed oil/silane coupling agent-montmorillonite nanocomposites prepared by in-situ method: synthesis and properties. Ind. Crops Prod. 70, 292–300. Lyu, B., Gao, J., Ma, J., Gao, D., Wang, H., Han, X., 2016. Nanocomposite based on erucic acid modified montmorillonite/sulfited rapeseed oil: preparation and application in leather. Appl. Clay Sci. 121–122, 36–45. Ma, J.Z., Lv, X.J., Gao, D.G., Li, Y., Lv, B., Zhang, J., 2014. Nanocomposite-based green tanning process of suede leather to enhance chromium uptake. J. Clean. Prod. 72, 120–126. Olfs, H.W., Torres-Dorante, L.O., Eckelt, R., Kosslick, H., 2009. Comparison of different synthesis routes for mg–al layered double hydroxides (LDH): characterization of the structural phases and anion exchange properties. Appl. Clay Sci. 43, 459–464. Quadery, A.H., Uddin, M.T., Azad, K., Chowdhury, M.J., Deb, A.K., Hassan, M.N., 2015. Fatliquor preparation from Karanja seed oil (Pongamia pinnata L.) and its application for leather processing. IOSR J. Appl. Chem. 8, 54–58. Wang, Q.J., Duan, B.R., Sun, G.X., Yu, C.Z., Liu, Y.B., Zhu, F., 2006. Influence of fatliquoring agent on leather flame retardant. China Leather 35, 17–19 (In Chinses). Wang, X.C., Guo, X.X., Wang, H.J., Guo, P.Y., 2016. Effect of linear-hyperbranched amphiphilic phosphate esters on the collagen fibers. J. Agric. Food Chem. 65, 104–116. Wang, Xin, Kalali, E.N., Wang, D.Y., 2015. Renewable cardanol-based surfactant modified layered double hydroxide as a flame retardant for epoxy resin. Acs. Sustain. Chem. Eng. 3, 3281–3290. Xia, L., You, J., Li, G., Sun, Z., Suo, Y., 2011. Compositional and antioxidant activity analysis of zanthoxylum bungeanum seed oil obtained by supercritical CO2 fluid extraction. J. Am. Oil Chem. Soc. 88, 23–32. Xu, W., Li, J., Liu, F., Jiang, Y., Li, Z., Li, L., 2017. Study on the thermal decomposition kinetics and flammability performance of a flame-retardant leather. J. Therm. Anal. Calorim. 128, 1107–1116. Xue, H.L., 2013. Application of response surface methodology to optimize the separation technique for α-linolenic acid by β-cyclodetrin inclusion from pricklyash seed oil. Food Sci. (N. Y.) 34, 45–51 (In Chinses). You, Y., Zhao, H., Vance, G.F., 2002. Surfactant-enhanced adsorption of organic compounds by layered double hydroxides. Colloids Surf., A 20, 161–172. Zhang, J., Jiang, L., 2008. Acid-catalyzed esterification of zanthoxylum bungeanum seed oil with high free fatty acids for biodiesel production. Bioresour. Technol. 99, 8995–8998. Zhang, L., Wu, H.T., Yang, F.X., Zhang, J.H., 2015. Evaluation of soxhlet extractor for one-step biodiesel production from zanthoxylum bungeanum seeds. Fuel Process. Technol. 131, 452–457. Zhang, R.F., Chen, G.J., Li, W., Q. U, Kan, J.Q., 2017. Comparative analysis of fatty acids composition of seed oil in zanthoxylum bungeanum maxim.from different orgins. China Condiment 42, 1–4 (In Chinese). Zhao, Y., Li, F., Zhang, R., D. G. E, Duan, X., 2006. Preparation of layered double-hydroxide nanomaterials with a uniform crystallite size using a new method involving separate nucleation and aging steps. Chem. Mater. 14, 4286–4291.
4. Conclusion In this paper, a novel method of exploiting utilization of ZBMSO was developed. Nanocomposites based on ZBMSO and stearate-layered double hydroxide (s-LDH) were prepared via an in-situ method, which can use as leather fatliquoring agent to enhance the flame retardancy and thermal stability of leather. LDH could coat on the surface of leather collagen fibers, hindering the transfer of heat and flammable volatiles during combustion. MZBMSO/s-LDH nanocomposite has great application potential in leather fatliquoring, not only because of its lubrication property, but also its flame flame-retardation effects. Acknowledgments The research work was supported by Key project of the National Natural Science Foundation of China [21838007]; Shaanxi Provincial Education Department Serves Local Special Project [17JF002]; The Key Research Project of Shaanxi Province [2018GY-068]; The Key Research Project of Shaanxi Province [2017GY-187]. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2019.03.001. References Babu, H.V., Coluccini, C., Wang, D.Y., 2017. Functional layered double hydroxides and their use in fire-retardant polymeric materials. Novel Fire Retardant Polymers and Composite Materials 8, 201–235. Bacardit, A., Borras, M.D., Soler, J., Herrero, V., Jorge, J., Olle, L.L., 2010. Behavior of leather as a protective heat barrier and fire resistant material. J. Am. Leather Chem. Assoc. 105, 51–61. Cao, X.Y., Zhang, J.B., 2010. Analysis of flame retardant properties of phosphate fatliquoring agent. Leather Chem. 27, 13–16 (In Chinses). Carlino, S., Hudson, M.J., 1994. Reaction of molten sebacic acid with a layered (Mg/Al) double hydroxide. J. Mater. Chem. 4, 99–104. Chai, H., Xu, X., Lin, Y., Evans, D.G., Li, D., 2009. Synthesis and UV absorption properties of 2,3-dihydroxynaphthalene-6-sulfonate anion-intercalated Zn–Al layered double hydroxides. Polym. Degrad. Stabil. 94, 744–749. Chiu, C.W., Huang, T.K., Wang, Y.C., Alamani, B.G., Lin, J.J., 2014. Intercalation strategies in clay/polymer hybrids. Prog. Polym. Sci. 39, 443–485. Costa, F.R., Leuteritz, A., Wagenknecht, U., Jehnichen, D., Haubler, L., Heinrich, G., 2008. Intercalation of Mg–Al layered double hydroxide by anionic surfactants: preparation and characterization. Appl. Clay Sci. 38, 153–164. Dang, X.G., Chen, H., Wu, R.W., Shan, Z.H., 2016. Lipid nanoparticles and spontaneous precipitation of a fatliquoring agent. J. Soc. Leather Technol. Chem. 100, 190–197. Ding, P., Kang, B., Zhang, J., 2015. Phosphorus-containing flame retardant modified layered double hydroxides and their applications on polylactide film with good transparency. J. Colloid Interface Sci. 440, 46–52. Dong, Y., Gui, Z., Hu, Y., Wu, Y., Jiang, S., 2012. The influence of titanate nanotube on the improved thermal properties and the smoke suppression in poly(methyl methacrylate). J. Hazard Mater. 209–210, 34–38.
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Research article
Intercalation of modified zanthoxylum bungeanum maxin seed oil/ stearate in layered double hydroxide: Toward flame retardant nanocomposites
T
Bin Lyua,b,∗, Yue-Feng Wanga, Dang-ge Gaoa, Jian-zhong Maa,b,∗∗, Yun Lic a
College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China Key Laboratory of Leather Cleaner Production, China National Light Industry, National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science & Technology, Xi'an, 710021, PR China c College of Chemistry and Chemical Engineering, Shandong, Yantai, 264005, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Zanthoxylum bungeanum maxin seed oil Layered double hydroxide Flame retardant Thermal stability
Zanthoxylum bungeanum Maxim Seed Oil (ZBMSO) is widely distributed in most parts of China, which cannot be edible and extensively consumed due to its high free fatty acids. This paper reports a rational route to utilization of ZBMSO in preparation of nanocomposites which can enhance leather flame retardancy and thermal stability. ZBMSO was synthesized through three-stage process, decoloration, acid reduction and sulfitation to prepare the modified ZBMSO fatliquoring agent (MZBMSO). Then nanocomposites based on MZBMSO and stearate-layered double hydroxide (s-LDH) were prepared via in-situ method. XRD and TEM results indicated that the MZBMSO intercalate into the galleries of s-LDH with uniform dispersion. Compared with MZBMSO, the leather treated by MZBMSO/s-LDH had a remarkable improvement on flame retardancy and superior softness which limiting oxygen index (LOI) increased from 23.6% to 28.0% and smoke density index decreased from 25 to 6.
1. Introduction Zanthoxylum bungeanum maxin (ZBM) is popular as a rutaceae and seasoning in China due to its unique special fragrance and taste characteristics. According to reports, ZBM cultivation area in China has reached 1.33 million hm2, the annual output of ZBM reaches one million tons. ZBM seeds (contains 27%–31% oil) as the main byproduct of ZBM, theoretically, which weight ratio of the peel is more than 20%, about 600,000 tons, have a great development value (Zhang et al., 2017). However, ZBM seeds oil (ZBMSO) contains about 25% free fatty acids leads to its unsuitable consumption for human (Zhang et al., 2015). At present, industrial application of ZBMSO concentrate mainly includes in three directions: production of edible oil (Xia et al., 2011), extraction of α-linolenic acid (Xue et al., 2013), and preparation of biodiesel (Zhang and Jiang, 2008; Zhang et al., 2015). Unfortunately, their high cost become the major obstacle to commercialization of ZBMSO. Therefore, exploring low-cost innovative application of ZBMSO is necessary. In recent decades, leather consumption in automobile cushion, aviation seat, furniture manufacturing and luxury goods is increasing rapidly. Fatliquoring agents, the largest amount of leather chemicals,
∗
based on oils, fat, or grease in emulsion form, are applied in tanned leather for improving leather softness and physical characteristics (Wang et al., 2016). Fatliquoring agents can be made of mineral oil, animal and plant oil (Lyu et al., 2016; Kim et al., 2006; Dang et al., 2016). Mineral oil is non-renewable resources, and annual production of animal fat is limited, so as a renewable material, vegetable oil is widely used. In this context, investigations are required to use ZBMSO as potential feedstock for fatliquoring agent. Generally, for existence of oil, the use of fatliquoring agent makes leather more flammable, reducing the safety of leather products. Traditional leather flame retardants contain halogen and phosphorus compounds, which are detrimental to the environment (Cao et al., 2010; Lyu et al., 2015; Huang et al., 2006; Bacardit et al., 2010). Therefore, it is important and necessary to develop effective fatliquoring agents that are flame-retardant and environmentally benign (Wang et al., 2006; Huang et al., 2011). Layered double hydroxide (LDH) is a category of anionic layered compounds. The main layer of LDH consists of two or more metal hydroxides, which can be thermally degraded to form metal oxides with large specific surface area to reflect good flame retardancy and smoke suppression (Chiu et al., 2014; Kalali et al., 2016; Li et al., 2014, 2016; Wang et al., 2015). MgAl-LDH as an example, Li et al. introduced 6% of
Corresponding author. College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China. Corresponding author. College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China. E-mail addresses: [email protected] (B. Lyu), [email protected] (J.-z. Ma).
∗∗
https://doi.org/10.1016/j.jenvman.2019.03.001 Received 22 November 2018; Received in revised form 26 February 2019; Accepted 1 March 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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metabisulfite solution was added and reacted for 2 h. Then the reaction system was cooled to 60 °C and added deionized water to adjust the solid content to 50 wt%. (Eq. (S3)).
MgAl-LDH into epoxy resin, which can reduce the peak heat release rate and total smoke production of EP by 62% and 45%, respectively (Li et al., 2016). LDH is hydrophilic and easy to disperse in water-soluble polymer. However, there is scarcely research about improving composite properties by LDH disperse in plant oil phase. In this paper, ZBMSO and stearate-LDH (s-LDH) are used as the raw materials to prepare a nanocomposite as leather fatliquoring agent. The purpose is to develop a low-cost use of ZBMSO and invest an environmentally friendly flame retardant leather fatliquoring agent. In order to achieve this goal, we performed the following experiment: firstly, we use stearate anions with a carbon chain length the same as ZBMSO molecule to modify LDH, which improves the compatibility between LDH and ZBMSO. Then the MZBMSO/s-LDH nanocomposites are prepared via an in-situ method. The effects of MZBMSO/S-LDH nanocomposites on the performance of fatliquored leather, including flame retardancy, thermal stability and softness, were investigated and compared with the performance of MZBMSO fatliquored leather. The flame retardant mechanism was proposed from two aspects of condensed-phases and gaseous-phases.
2.5. Leather fatliquoring process MZBMSO and MZBMSO/S-LDH were applied as fatliquoring agent in the fatliquoring process of wet-blue goat sheepskin. For comparison with the MZBSO and MZBMSO/s-LDH, the blue wet leather was divided into two parts according to the method shown in Fig. S1. Detailed experimental details have been added to the supplementary material S1. 2.6. Characterization The XRD patterns of the samples were recorded using a D8 Advance X-ray diffractometer (Bruker). Excitation by Cu Kα radiation generator; λ = 0.15406 nm; tube voltage 30 kV; tube current 30 mA; the scanning range of 2–70° and scanning rate of 10° min−1. Chemical structures of samples were identified using FTIR with KBr disk, recorded on a Perkin Elmer, FTI1650 Spectrum BX. TGA analysis was carried out using a thermogravimetric analyzer (TGA Q500 produced by TA company) in a nitrogen atmosphere and a heating rate of 10 °C/min. UV absorbance test of the ZBMSO was conducted on a TU1900 double beam ultraviolet visible spectrophotometer at wavelength range of 220–320 nm.
2. Experimental 2.1. Materials and instrument Stearic acid (analytical grade) was obtained from China Pharmaceutical Chemical Reagent Co., Ltd., MgAl-LDH (Mg/Al molar ratio = 2:1) was obtained from Beijing University of Chemical Technology. Sodium hydroxide, cis-butenedioic anhydride, sodium pyrosulfite, methanol, glycerol, sodium bisulfite, concentrated sulfuric acid (analytically pure) were purchased from Tianjin Hongyan Reagent Factory. ZBMSO was obtained from Hancheng, China. Activated carbon and activated clay (industrial grade) were purchased from Tianjin Dengfeng Chemical Reagent Factory.
2.7. Properties of MZBMSO/s-LDH fatliquored leather According to GB/T 2406.1–2008, LOI test was performed by HC-2C oxygen index meter (Shangyuan Company, China). The test samples were dried at room temperature and the size was 14 cm × 5.2 cm; testing vertical combustion on ZF-4 combustion test chamber (Shangyuan Company, China) with the sample dimension of 1.5 cm × 13.5 cm according to ALCA Method E 50; the smoke density tests were carried out on a smoke density tester with the sample dimension of 5 cm × 5.2 cm × 0.3 cm (YM-3, Nanjing Shangyuan Analytical Instruments Co., Ltd.) according to GB/T8627-2007. All of the above tests were performed on three samples and the results were averaged. Determination of the softness of fatliquored leather according to ISO 17235-2002 using GT-303 leather softness tester (Co., Ltd.), the leather samples were dried for 1 day at 20 °C and relative humidity 60 before testing. In the test, first used the metal disc to correct the dial to zero, and then randomly selected 9 parts in different parts of the leather to be tested for softness detection, and the results were averaged. The microscopic morphology of the leather samples was observed by TESCAN scanning electron microscopy (Czech). The acceleration voltage was 12 kV when observed. Elemental analysis was obtained by Energy dispersive X-ray (EDX) connected to the SEM. Thermogravimetric analysis connected with Fourier transform infrared spectroscopy (TG-FTIR) was carried out on STA449F3-1053-M thermal analyzer (NETZSCH). The analysis was carried out in an oxygen atmosphere, and the temperature of the link tube was maintained at 310–340 °C to prevent condensation of gaseous products.
2.2. Preparation of stearate modified layered double hydroxide (s-LDH) 1.00 g dried [Mg4 Al2 (OH)12]2+ (CO3)2- ⋅4H2O was added to a 750 mL sodium stearate solution in 0.003 M and stirred at room temperature for 24 h to obtain stearate-LDH. After filtration a white solid was obtained. The solid was washed three times (each time with 100 mL of deionized water) until the pH was about 7. Then dried in vacuo. 2.3. Decoloration and acid reduction of ZBMSO Decolorization: ZBMSO and decolorizing agent (2% of oil mass) were placed in a three-necked flask and heated at 110 °C for 30 min at 350 rpm. Then filtered with a 600-mesh filter after 8 h. Decolorizing agent consisted of 15.5 wt% of activated carbon and 84.5 wt% of activated clay. Acid reduction: Decolorized ZBMSO and glycerin (molar ratio of 1:1) were added to a three-necked flask, and concentrated sulfuric acid was added as a catalyst. The flask was heated at 130 °C for 3 h (Eq. (S1)). Then, the flask was cooled down to 70 °C before adding methanol (molar ratio of ZBMSO: methanol = 1:1), reaction lasted for 2 h to obtain reduced acid ZBMSO (Eq. (S2)).
3. Result and discussion
2.4. Preparation of MZBMSO/s-LDH
3.1. Analysis of stearate anions modified LDH
Firstly, ZBMSO (after acid reduction and decolorization) and s-LDH (0, 1%, 2%, 3%, 4%, 5%, according to oil mass) were added into a three-neck flask, which was heated to 75 °C for 30 min with a rotation rate of 350 rpm. Secondly, toluene-p-sulfonic acid sodium salt (as a catalyst) and maleic anhydride were added into the flask, and the reaction system was heated to 110 °C for 3 h. The pH was adjusted to 7, and the temperature was reduced to 80 °C, when 40% sodium
Fig. 1a showed the XRD patterns of the pristine and stearate -LDH in the range of 2θ degree from 2 to 70°. It was obvious that both LDH and stearate-LDH were crystalline and had a well-defined layered structure. The interlayer distance of the LDH or stearate-LDH were calculated by Bragg equation from the first diffraction peak, nλ = 2d sinθ, for the (003) peak n equals 1 and λ is the wavelength of Cu-Ka radiation, and θ is the degree of diffraction half angle. The intercalation of the stearate 236
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Fig. 1. (a) XRD spectra of pristine LDH and s-LDH. (b) FT-IR spectra of pristine LDH, sodium stearate and s-LDH.
Fig. 2. (a) FT-IR, (b) XRD spectra of MZBMSO/s-LDH; TEM image of MZBMSO/s-LDH (c) Magnified 100 K times, (d) Magnified 10.5 K times. Table 1 Flame retardancy tests of leather treated with MZBMSO/s-LDH composite. Dosage of s-LDH
After flame time (s)
The Length of charring (mm)
Mass loss rate (%)
LOI (%)
Smoke density (%)
0 1 2 3 4 5
70 54 47 40 36 37
81 55 55 51 38 35
60.32 44.27 44.35 37.74 30.54 30.29
23.6 ( ± 0.3) 26.7( ± 0.2) 27.3( ± 0.2) 27.7( ± 0.1) 28.0( ± 0.1) 28.0( ± 0.3)
25 18 15 13 6 8
( ± 6.1) ( ± 6.7) ( ± 4.6) ( ± 4.3) ( ± 2.4) ( ± 3.6)
( ± 8.3) ( ± 7.9) ( ± 4.3) ( ± 3.6) ( ± 2.3) ( ± 3.3)
( ± 11.66) ( ± 6.43) ( ± 3.68) ( ± 3.03) ( ± 1.67) ( ± 2.71)
at about 3400 cm−1. The symmetric vibration absorption band of CO32− could be observed at about 1357 cm−1 (Jiao et al., 2009; Chai et al., 2009). The lattice vibration band between metal element and oxygen element appeared below the 800 cm−1 regions (Eili et al., 2014). Seen from the s-LDH curve, the strong absorption bands at 28003000 cm−1 were corresponded to the characteristic adsorptions of the
anions was increased interlayer distance from 0.78 nm to 1.69 nm compared with pristine LDH. The intensity of (003) basal reflection of sLDH was much weaker than that of LDH, indicating that a slight amount of stearate anions had inserted into the interlayer of the LDH. Fig. 1b showed FT-IR spectra of the pristine LDH, sodium stearate and s-LDH. From pristine LDH curve, the -OH extensional vibration peak of metal hydroxide and interlayer water resulted to a broad peak 237
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Fig. 3. TGA curves of MZBSO/s-LDH fatliquored leather.
Fig. 5. FTIR spectra of the pyrolysis gaseous products of (a) the leather treated with MZBMSO, (b) leather treated with MZBMSO/s-LDH-4%.
ZBMSO was calculated by changing the UV absorbance of ZBMSO (the detection method was given in supplementary material S2). As shown in Fig. S2, the characteristic peak of ZBMSO at 203 nm. The decolorized rate of ZBMSO was 16.7%. Acid value reflects the amount of free fatty acids in the oil which can reflect the rancidity degree of oil. The higher acid value, the degree of rancidity is greater. There is a certain requirement for acid value of the oil used for leather fatliquoring, usually near 10 mgKOH/g. The method of measuring acid value of oil was given in supplementary material S3. As shown in Table S1, the acid value of ZBMSO could be reduced from 58.88 to 12.86 mg KOH/g, which could satisfy the preparation condition of fatliquoring agent. Saponification values indicate the average relative molecular mass of oil (a mixture of fatty acids and triglycerides). The test method of saponification value of ZBMSO was given in supplementary material S4. The saponification value of ZBMSO was 216.88 mgKOH/g, and its average relative molecular was about 778 g/mol−1. The length of the carbon chain of ZBMSO was C18, as the same as sodium stearate, which was selected as a modifier for LDH in order to improve the compatibility of LDH and ZMBSO as much as possible.
Fig. 4. Softness of leather fatliquored by MZBMSO/s-LDH.
3.3. Structural of MZBMSO/s-LDH nanocomposite fatliquoring agent
alkyl chain (C-H and –CH2), which could be attributed to superimposed in pristine LDH and sodium stearate (Costa et al., 2008). The strong absorption at about 1357 cm−1 and 1571 cm−1 could be attributed to the symmetric and asymmetric stretching vibrational peaks of carboxylates (Zhao et al., 2006; You et al., 2002; Olfs et al., 2009). This confirmed that sodium stearate has been modified pristine LDH preliminarily. In addition, the peak intensity of the comparative alkyl chain and -OH indicated that a large amount of stearate is adsorbed on the LDH layer. In summary, XRD and FT-IR analyses confirmed that the stearate anions were formed successfully modified MgAl-LDH. In addition, the pristine LDH was coated by stearate anions and stearate anions had intercalated into the interlayer of LDH.
The FTIR spectra of ZBMSO, MZBMSO and MZBMSO/s-LDH were shown in Fig. 2a. Three functional groups, such as -COOH, -COO- and -SO3-, are different in ZMBSO and MZBMSO, so the discussion mainly focuses on the above three functional groups. Compared with the FTIR spectra of ZBMSO, the broad peaks at 3489 cm−1 in MZBMSO spectra was due to the stretching vibration peak of -OH of carboxylic acid dimer. Seen from MZBMSO spectra, the new absorption peaks around 1388 cm−1 and 1604 cm−1 were the asymmetric and symmetric stretching of carboxylate (C=O). The strong absorption at 1218 cm−1 was assigned to the C-O stretch of carboxylic acid. The strong absorption at 1049 cm−1 was associated with the S=O group stretching vibration of sulfonate. These results showed that hydrophilic group has successfully grafted onto ZBMSO. Besides, seen from MZBMSO/s-LDH spectra, the presence of new peak at low wavenumber (below 800 cm−1) region were assigned to the M-O (M = Mg and Al) lattice vibration of the LDH (Olfs et al., 2009). XRD results (Fig. 2b) provided the information to verify the composite structure of MZBMSO/s-LDH. The broad diffraction reflection could be attributed to diffraction of amorphous ZBMSO. In contrast, the XRD curve of MZBMSO/s-LDH composite showed some special
3.2. Physicochemical properties of ZBMSO The non-decolorized and decolorized of ZBMSO were shown in Fig. S2. After decolorized treatment, a clear light path could be observed from the side incident beam. Obviously, the degree of clarity had been enhanced and the solid impurities suspended in the non-decolorized ZBMSO was removed effectively. In addition, the decolorization rate of 238
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Fig. 6. Absorbance versus temperature curves of typical decomposition products for the leather treated with MZBMSO and MZBMSO/s-LDH-4%.
3.4. Flame retardancy of leather
diffraction peaks at 2θ values of 2.94, 15.84 and 27.96, which were similar to those of the LDH. The interlayer distance of MZBMSO/s-LDH was 3.08 nm (the interlayer distance of s-LDH was 1.69 nm) (Ding et al., 2015). The reason for this result may be that the molecular chain of MZBMSO contained -SO3- and -COO-, and the residual fatty acid of the oil contained -COO-. These functional groups may react with the layers of s-LDH and diffused into the interlayer of s-LDH under high temperature reaction, increasing the interlayer spacing of s-LDH (Carlino et al., 1994). The TEM micrograph could further verify the conclusion of XRD analysis and reflect the dispersion of s-LDH in MZBMSO. In Fig. 2c, it could be observed that LDH and MZBMSO form intercalated nanocomposites. In Fig. 2d, it could be seen that s-LDH had good dispersion in MZBMSO. Both morphological features obtained from XRD and TEM illustrated that MZBMSO/s-LDH had typical nano-composite structure, with good dispersion of s-LDH in MZBMSO.
Vertical combustion results of leather treated with MZBMSO/s-LDH were listed in Table 1. After-flame time is the duration after removal of the ignition source. Compared with leather treated with MZBMSO only, the after-flame time of MZBMSO/s-LDH treated leather reduced markedly, and after-flame time was decreased with increasing dosage of sLDH. The length of charring and mass loss rate intuitively reflect the flame retardant properties of leather. The length of charring and mass loss rate of MZBMSO/s-LDH treated leather were both lower than that of MZBMSO, decreasing with the higher dosage of s-LDH. The addition of s-LDH improved the flame retardancy and prevented the flame combustion of leather effectively. LOI means the minimum oxygen concentration required to maintain sample combustion. LOI can be used to judge how difficult it is to burn 239
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Fig. 7. SEM micrographs of char residues for (a) leather treated with MZBMSO, (b) leather treated with MZBMSO/s-LDH-4%.
Fig. 8. SEM and EDX of leather fatliquored by MZBMSO/s-LDH-4%.
a material when it comes into contact with a flame. The LOI value improved with increasing dosage of s-LDH. When 4% and 5% LDH were added, the highest LOI value was up to 28%. Smoke density measures the amount of smoke produced by the material under prescribed test conditions, which is quantified by the attenuation of light intensity through the smoke. The greater density of smoke is, the more disadvantageous to evacuees and extinguish of the fire. In comparison with MZBMSO treated leather, the smoke density of MZBMSO/s-LDH treated leather was reduced significantly. The lowest smoke density reached to 6 when 4% s-LDH was added. The smoke elimination performance can be attributed to form s-LDH a solid base with a high specific surface and large adsorption capability which can adsorb acid gases and particles produced in leather combustion (Li et al., 2016; Wang et al., 2015). In summary, the flame retardancy of MZBMSO/s-LDH fatliquoring leather was significantly improved compared to leather treated with MZBMSO.
which was pyrolysis of triglyceride (Xu et al., 2017). In the third stage, temperature ranging from 300 to 590 °C, major sub-reaction was taken place, which was the pyrolysis of collagen fibers (Xu et al., 2017). The thermal stability of the leather collagen fiber was examined using a temperature loss of 25% mass loss (T-25%) and the char residual percentage at 590 °C. The MZBSO/s-LDH treated leather had a higher decomposition temperature in the third stage than the MZBMSO treated leather. The maximum T-25% belonged to MZBSO/s-LDH- 4% fatliquored leather, increasing from 309.4 °C of MZBMSO fatliquored leather to 319.8 °C. Furthermore, the char yield was also improved gradually with s-LDH loading increases. The TGA results showed that the addition of s-LDH could improve the thermal properties of leather. This may be due to the introduction of LDH to protect the collagen fibers of the leather, so the thermal stability of the leather sample has improved.
3.6. Softness of leather The main purpose of fatliquoring is to make the neutral oil and active groups in the fatliquor penetrate into the leather and combine with collagen fibers to play the role of soft leather (Ma et al., 2014; Quadery et al., 2015). Fig. 4 showed softness results of leather fatliquored by MZBMSO/sLDH at different dosage of s-LDH. It can be seen from the figure that the softness of leather fatliquored by MZBMSO was about 7.6 mm. With the increased in the amount of s-LDH, softness of leather did not change
3.5. Thermal stability of leather The thermal decomposition behavior of leather treated with MZBSO/s-LDH was illustrated in Fig. 3. The mass loss of leather consisted three stages. The decomposition of the first stage was between 25 and 150 °C, which could be attributed to the evaporation of water and small molecules. Second mass loss phase between 150 and 300 °C, 240
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Fig. 9. Schematic illustration of the flame retardant mechanism of MZBMSO/s-LDH-4% on leather collagen fibers.
the leather treated with MZBMSO was much higher than that for the leather treated with MZBMSO/s-LDH-4%. The reduced amount of the hydrocarbon, aromatic, carbonyl and ether compounds (Fig. 6a, b, c and e) implied less “fuel” were fed back to the flame. The reduction in the amount of flammable gas released may be due to the presence of LDH or LDH-derived oxide which increase the strength of the leather char residue, thereby inhibiting the volatilization of the flammable gas. This effect could be expected to result in a reduction in the length of charring and mass loss rate in vertical burning ratings (Gao et al., 2014; Bahu et al., 2017). The microstructure of residual samples after vertical burning was observed by SEM. Leather treated with MZBMSO (Fig. 7a) loosed fiber structure and generated a broken porous residue after combustion, indicating its high flammability. Heat and oxygen easily pass through these holes and gaps, thus the leather sample burn violently. In contrast, leather treated with MZBMSO/s-LDH-4% (Fig. 7b), the residue, showing a perfect fibers structure, could effectively block the penetration of heat and oxygen into collagen fibers and improve the fire resistance of fibers. In order to prove the protective effect of s-LDH on the collagen fibers of leather. It must be demonstrated that s-LDH can be successfully penetrated and evenly distributed inside the leather. s-LDH contains Mg and Al elements. Therefore, the distribution of Mg and Al elements from the flesh to the grain side of MZBMSO/s-LDH-4% treated leather was characterized using line spectrum scanning. As shown in Fig. 8a, the distribution of Mg and Al inside the leather was relatively uniform. In addition, at higher magnifications (in Fig. 8b), it could be observed that a large amount of small white particles was coated on the surface of the leather collagen fibers. The elemental composition of these small particles was verified using EDX and the average contents of C, O, Mg and Al were 40.01%, 41.09%, 10.81% and 8.09%, respectively. This EDX result could prove the particles was s-LDH encapsulated by MZBMSO. The test results of Fig. 9 indicated that s-LDH has penetrated into the interior of the leather and was evenly distributed on the surface of the collagen fibers.
significantly, which indicated that the introduction of s-LDH has little effect on the leather softness. 3.7. Flame retardant mechanism In order analyze the specific effects of s-LDH on the flame retardant properties of leather, the mechanism was investigated for the condensed phase and the gaseous phase respectively. In this paper, TG-FTIR was utilized to study the thermal decomposition of volatiles during decomposition process. Through this part of the characterization, the gas phase components produced by the sample during thermal degradation could be obtained. Fig. 5 showed the normalized FTIR spectrum of the pyrolysis gas product of the leather treated with MZBMSO and the MZBMSO/s-LDH-4% at the maximum degradation rate. The spectra of MZBMSO and MZBMSO/s-LDH-4% treated leather had significant difference, which meant that the addition of s-LDH had a major effect on the decomposition of volatiles during thermal degradation. The gas phase products evolved from the leather treated with MZBMSO and MZBMSO/s-LDH-4% could be qualitatively identified by the peak of FTIR, such as phenol (around 36003700 cm−1), water (3212 cm−1), -C-H groups for small molecule organic combustible gas (2800-3100 cm−1), CO2 (2356 cm−1), carbonyl compounds (1749 cm−1), aromatic compounds (1508 cm−1), ethers (1170 cm−1) and unsaturated hydrocarbon (968 cm−1) (Liu et al., 2011; Dong et al., 2012; Huang et al., 2009; Gaan et al., 2009). It could be found in Fig. 6b that the flammable component released by leather treated with MZBMSO/s-LDH-4% was significantly reduced. This indicated that the addition of s-LDH can inhibit the release of flammable gas during thermal degradation of leather. To further prove the above conclusions, the absorbance of partial degradation products of MZBMSO and MZBMSO/s-LDH-4% treated leather with temperature were revealed in Fig. 6. The change of absorbance intensity of flammable gas (carbonyl, aromatic compounds and esters.) and non-flammable gas (CO2) at different temperatures were investigated. The absorbance intensity of the flammable gas for 241
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Upon the basis of the aforementioned combustion behavior, the flame retardant mechanism was proposed, as illustrated in Fig. 9. MZBMSO and MZBMSO/s-LDH penetrated into the leather and formed an oil film on the surface of collagen fibers. When encountering a flame, MZBMSO treated leather formed broken porous char residue on fibers surface, and thus, combustible volatile escaped from these cracks freely to feed the flame. Besides, the heat penetrated through these gaps to consume the underlying collagen fibers. In contrast, leather treated with MZBMSO/s-LDH appeared dense continuous char residue. Because the nanoparticles were coated on the fibers, which inhibit external heat, protecting the underlying fibers from being destroyed and inhibiting combustible gas escaping from the interior. Both barriers and thermal insulation effect were important to improve the fire resistance of collagen fibers.
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4. Conclusion In this paper, a novel method of exploiting utilization of ZBMSO was developed. Nanocomposites based on ZBMSO and stearate-layered double hydroxide (s-LDH) were prepared via an in-situ method, which can use as leather fatliquoring agent to enhance the flame retardancy and thermal stability of leather. LDH could coat on the surface of leather collagen fibers, hindering the transfer of heat and flammable volatiles during combustion. MZBMSO/s-LDH nanocomposite has great application potential in leather fatliquoring, not only because of its lubrication property, but also its flame flame-retardation effects. Acknowledgments The research work was supported by Key project of the National Natural Science Foundation of China [21838007]; Shaanxi Provincial Education Department Serves Local Special Project [17JF002]; The Key Research Project of Shaanxi Province [2018GY-068]; The Key Research Project of Shaanxi Province [2017GY-187]. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2019.03.001. References Babu, H.V., Coluccini, C., Wang, D.Y., 2017. Functional layered double hydroxides and their use in fire-retardant polymeric materials. Novel Fire Retardant Polymers and Composite Materials 8, 201–235. Bacardit, A., Borras, M.D., Soler, J., Herrero, V., Jorge, J., Olle, L.L., 2010. Behavior of leather as a protective heat barrier and fire resistant material. J. Am. Leather Chem. Assoc. 105, 51–61. Cao, X.Y., Zhang, J.B., 2010. Analysis of flame retardant properties of phosphate fatliquoring agent. Leather Chem. 27, 13–16 (In Chinses). Carlino, S., Hudson, M.J., 1994. Reaction of molten sebacic acid with a layered (Mg/Al) double hydroxide. J. Mater. Chem. 4, 99–104. Chai, H., Xu, X., Lin, Y., Evans, D.G., Li, D., 2009. Synthesis and UV absorption properties of 2,3-dihydroxynaphthalene-6-sulfonate anion-intercalated Zn–Al layered double hydroxides. Polym. Degrad. Stabil. 94, 744–749. Chiu, C.W., Huang, T.K., Wang, Y.C., Alamani, B.G., Lin, J.J., 2014. Intercalation strategies in clay/polymer hybrids. Prog. Polym. Sci. 39, 443–485. Costa, F.R., Leuteritz, A., Wagenknecht, U., Jehnichen, D., Haubler, L., Heinrich, G., 2008. Intercalation of Mg–Al layered double hydroxide by anionic surfactants: preparation and characterization. Appl. Clay Sci. 38, 153–164. Dang, X.G., Chen, H., Wu, R.W., Shan, Z.H., 2016. Lipid nanoparticles and spontaneous precipitation of a fatliquoring agent. J. Soc. Leather Technol. Chem. 100, 190–197. Ding, P., Kang, B., Zhang, J., 2015. Phosphorus-containing flame retardant modified layered double hydroxides and their applications on polylactide film with good transparency. J. Colloid Interface Sci. 440, 46–52. Dong, Y., Gui, Z., Hu, Y., Wu, Y., Jiang, S., 2012. The influence of titanate nanotube on the improved thermal properties and the smoke suppression in poly(methyl methacrylate). J. Hazard Mater. 209–210, 34–38.
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