Fabrication of green alginate-based and layered double hydroxides flame retardant for enhancing the fire retardancy properties of polypropylene

Journal Pre-proof Fabrication of Green Alginate-based and Layered Double Hydroxides Flame Retardant for Enhancing the Fire Retardancy Properties of Polypropylene Sheng Xu (Methodology) (Conceptualization) (Data curation) (Investigation) (Writing - original draft) (Funding acquisition), Si-Yu Li (Methodology) (Conceptualization) (Investigation), Min Zhang (Project administration) (Investigation), Hong-Yan Zeng (Writing review and editing) (Data curation) (Funding acquisition), Kun Wu (Investigation), Xian-Yao Tian (Investigation), Chao-Rong Chen (Investigation), Yong Pan (Funding acquisition)

PII:

S0144-8617(20)30065-5

DOI:

https://doi.org/10.1016/j.carbpol.2020.115891

Reference:

CARP 115891

To appear in:

Carbohydrate Polymers

Received Date:

2 September 2019

Revised Date:

16 January 2020

Accepted Date:

17 January 2020

Please cite this article as: Xu S, Li S-Yu, Zhang M, Zeng H-Yan, Wu K, Tian X-Yao, Chen C-Rong, Pan Y, Fabrication of Green Alginate-based and Layered Double Hydroxides Flame Retardant for Enhancing the Fire Retardancy Properties of Polypropylene, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115891

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Fabrication of Green Alginate-based and Layered Double Hydroxides Flame Retardant for Enhancing the Fire Retardancy Properties of Polypropylene Sheng Xu*, Si-Yu Li, Min Zhang, Hong-Yan Zeng*, Kun Wu, Xian-Yao Tian, Chao-Rong Chen, Yong Pan

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College of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan, China

Sheng Xu

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E-mail address: [email protected].

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*Corresponding Author

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Hong-Yan Zeng

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E-mail address: [email protected].

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Tel.: +86-731-58298175

Address: College of Chemical Engineering, University of Xiangtan, Xiangtan 411105, Hunan, China

Highlights  An efficient and bio-based alginate pillared layered double hydroxides (SA@LDHs) was successfully prepared.  LOI value of PP/30% SA@LDHs reached 30.9% and samples can pass UL-94 V-0 rating.  Synergistic flame-retarding effect of alginate and layered double

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hydroxides was found.

Abstract

An efficient and bio-based alginate pillared hydrotalcite (SA@LDHs) was fabricated via

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calcination-reconstruction manner with sodium alginate (SA) and hydrotalcite (LDHs-C), and used as novel flame retardant for polypropylene (PP). The morphologies and

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combustion properties of SA@LDHs and its hybrid with PP composites (PP/SA@LDHs)

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had been characterized by SEM, TGA, cone calorimetry, LOI and UL-94 measurements. With 30 wt% loading, the SA@LDHs achieved a LOI value of 30.9% and a UL-94 V-0 rating, whereas the LDHs-C exhibited only LOI value of 27.6% and a UL-94 V-1 rating.

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The peak heat release rate, total heat release and total smoke production of PP/SA@LDHs were 260.8 kW·m−2, 61.3 MJ·m−2 and 8.2 m2, respectively, which presented declines of

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69.2%, 42.8% and 32.2% compared with those of Neat PP. These improvements could be

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attributed to the presence of the radical-trapping effect of SA, which leading to promote PP chains to participate in the carbonization process.

Keywords Layered double hydroxides; Alginate; Flame retardant; Polypropylene

1. Introduction Polypropylene (PP), as one of the commercially available polyolefin, has been widely used in electronics industry, automotive, construction and packaging films as the matrix for its low cost, excellent mechanical properties, low toxicity, light weight, and ease of processing (Zheng, Liu, Dai, Meng, & Guo, 2019; Zhang et al., 2018; Wang et al., 2010). Unfortunately, virgin PP is a highly inflammable polymeric material with a low limited oxygen index (LOI) of only 18% and tends to drip upon being ignited with releasing smoke and poisonous gases,

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both of which threaten the safety of human life and restrict its use in high-risk applications (Wang, Niu, & Dong, 2017; Morgan, & Wilkie, 2007). Therefore, it is important to improve the flame retardancy and the smoke suppression of PP simultaneously. According to the

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studies, traditional halogen-based flame retardants are the most effective and commonly used flame retardants additives, but they have some significant drawbacks such as bioaccumulative,

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persistent in the environment, and a hazard for animals and humans (Xia, Su, & Li, 2019;

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Zhou et al., 2014; Venier, Salamova, & Hites, 2015). To overcome this issue, the development of eco-friendly halogen-free flame retardants additives and smoke suppression agents for the

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application of PP are in urgent need.

In recent years, layered double hydroxides (LDHs), as a promising class of layered material,

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also known as anionic clays, have attracted considerable interest in the field of eco-friendly halogen-free flame retardants additives and smoke suppression agents for polymers. The

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major advantage of LDHs with great flame retardancy and smoke suppression properties is that they are synthetic materials in which their unique layered structure and tunable chemical composition (Matusinovic, & Wilkie, 2012; Evans, & Duan, 2006; Zhang et al., 2018). Since Chen and Qu (Chen, & Qu, 2003) first investigated Mg/Al LDHs as flame retardant additives in polyethylene-graft-maleic anhydride nanocomposites, there has been a rapid growth in publications related to the synthesis and application of new flame retardant polymers-LDHs

nanocomposites. Recent research indicates that it is necessary to modify the pristine LDHs with intercalating flame retardant anions into its interlayer to enlarge the interlayer space, to alter the surface properties, to facilitate the dispersion of LDHs in the polymers and in order to further improve the flame retardancy of LDHs for the polymers simultaneously (Karami et al., 2019; Li, Liu, Dufosse, Yan, & Wang, 2018; Chhetri, Chandra Adak, Samanta, Chandra Murmu, & Kuila, 2018; Cai, Heng, Hu, Xu, & Miao, 2016). For instance, Wang et al. (Wang et al.,2013) have proved that the dispersion of Zn2Al-borate LDHs flame retardant in the

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PP/Zn2Al-borate LDHs composites was improved, and by adding 15 wt% Zn2Al-borate LDHs into PP, the peak heat release rate of PP/Zn2Al-borate LDHs composites could be decreased by 63.7% compared to pure PP. Besides, Kang et al. (Kang, & Wang, 2013) reported that

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anionic β-cyclodextrin derivate functionalized with carbon carbon double bond intercalated LDHs (interlayer distance of 2.30 nm) as flame retardant for PP showed significantly decrease

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in heat release rate and the total heat release. In our previous research (Xu et al., 2018),

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P3O105– intercalated LDHs (interlayer distance of 0.98 nm) as flame retardant for PP showed a high anti-flaming performances and an excellent disperse homogeneously in the PP matrix.

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Therefore, LDHs’s modifier with intercalating flame retardant anions into its interlayer would be very important to develop high performance LDHs-based for polymers matrix.

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Biomass, which has attracted considerable interest in the field of flame retardant for polymers, due to it is a series of natural materials, is more sustainable than fossil-based materials, use of

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it can decrease the emission of CO2 and environmental-friendly with less toxicity, no dioxin, no halogen acids by-product and low evolution of smoke during combustion (Ye et al., 2018; Zhao, Chen, & Chen, 2017; Liu et al., 2015). Alginate (SA), as a polyelectrolyte derived from seaweed, is considered to be good compatibility, rich reactive functional groups, high carbon content and biodegradable, which is an anionic copolymer comprised of β-1, 4-D-mannuronate (M) and α-1, 4-L-guluronate (G) units arranged in an irregular structure of

varying proportions of GG, MM and GM blocks (Wang et al., 2016; Liu et al., 2016). Recently, the development of alginate has been attracted increasing attention as a new generation of environmentally friendly and highly efficient flame retardants for polymers matrix (Wang et al., 2016; Li, X. L., Chen, & Chen, 2019; Chen, Shen, Chen, Zhao, & Schiraldi, 2016). For example, Zhang et al. (Zhang et al., 2011) have reported that alginate is known as an inherently flame retardant with a LOI value of 48.0%, an effective heat of combustion of 0.46 MJ·kg−1, a residues (at the time of 360 s) of 32.3% and a peak heat release

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rate of 4.99 kW·m−2 comparing favorably with the corresponding values of 20.0%, 12.06 MJ·kg−1, 10.3% and 168.75 kW·m−2 for viscose fiber. Generally, researchers have found that alginate as a synergistic flame retardant for polymers to improve the mechanical properties

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and flame retardancy of the matrix has been attracting increasing interest. Ionita et al. (Ionita, Pandele, & Iovu, 2013) have showed that a synergistic flame retardant containing sodium

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alginate and graphene oxide could significantly improve thermal stability and mechanical properties of the nanocomposite films, the results indicated that the tensile strength and

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Young’s modulus of sodium alginate films containing 6 wt% graphene oxide increased from 71 MPa and 0.85 GPa to 113 MPa and 4.18 GPa, respectively. Besides, TGA showed that the

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thermal stability of sodium alginate and graphene oxide composite films was better than that of neat sodium alginate film. Additionally, Wang et al. (Wang et al., 2016) have proved that

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brucite/3-aminopropyltriethoxysilane/nickel alginate (B/A/Nia) hybrid flame retardant (130

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phr loading) for ethylene-vinyl acetate (EVA) could achieve a LOI value of 32.3% and a UL 94 V-0 rating, and the peak heat release rate and total heat released decreased by 41.5% and 8.9% compared with B/A and EVA composites. Meanwhile, the dispersion of B/A/Nia in EVA matrix significantly improved and the elongation at break increased by 97.5%. These improvements could be attributed to the synergistic effect of B/A and Nia hybrid flame retardant. Therefore, there is reason to believe that by adding alginate as a synergistic flame

retardant for polymers could improve the flame retardancy for matrix polymers during combustion. However, to the best of our knowledge, there have been no previous reports on the possibility of alginate with LDHs as synergistic flame retardant for PP matrix. In the present work, bio-based alginate pillared layered double hydroxide (SA@LDHs) was fabricated by calcination-reconstruction manner and used as novel flame retardant for PP matrix. The SA@LDHs samples were characterized by XRD, FT-IR, SEM and TG-DTG, in order to understand the structure, morphology and thermal stability. Then, the microstructure,

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dispersion, thermal properties and combustion properties of SA@LDHs and PP composites (PP/SA@LDHs) have been investigated in detail by XRD, SEM, TGA, cone calorimeter tests (CCT), LOI measurements and the vertical burning tests (UL-94).

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2. Experimental 2.1. Materials

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Polypropylene particles (K8303, melt flow rate: 2.6 g∙10 min–1 at 230 °C) with the particle

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size about 1 mm, were purchased from Yanshan Petrochemical I Co., Ltd. (Beijing, China). Sodium alginate (AR, 90%) was purchased from Shanghai Macklin Biochemical Co., Ltd.

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(Shanghai, China). All chemicals were of analytical grade and all solutions were prepared with boiled deionized water.

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2.2. Preparation of SA@LDHs

Intercalation of SA into LDHs was carried out as follows, and the schematic for the

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intercalation of SA@LDHs was illustrated in Scheme 1. Based on our previous works (Xu et al., 2018; Xu et al., 2019), the precursor hydrotalcite with Mg/Al molar ratio of 3.0 was prepared, which was designated as LDHs-C. Part of the LDH-C was calcined at 5°C·min−1 followed by keeping at 500 °C for 4 h in a muffle furnace, which was denoted as LDO. Then, SA@LDHs was prepared by calcination-reconstruction manner. 1 g of LDO was dispersed in 400 mL CO2-free deionized water in a 1000 mL three-necked flask before vigorously stirred

by a magnetic stirrer for around 20 min under a nitrogen atmosphere at 85 °C. Meanwhile, 1 g of SA was dissolved in 100 mL deionized water at initial pH of 7 and immediately added into the flask. After that, the mixture was stirred vigorously at 85 °C under a nitrogen atmosphere for 6 h. The product was centrifuged, washed to neutrality, and then dried at 80°C for 24 h, which was designated as SA@LDHs.

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Sodium Alginate

SA@LDHs

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LDO

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Scheme 1. Schematic diagram of SA@LDH composite materials synthesized by calcination-reconstruction method.

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2.3. Preparation of PP and SA@LDHs composites

PP and SA@LDHs composites (PP/SA@LDHs) were prepared by melting, where PP and SA@LDHs were mixed in a GH-10A high-speed mixer (Beijing Plastic Machinery Factory)

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at a rotor speed of 250 rpm at 230°C for 15 min. The admixture was molded into bar (120×10×4mm3) using a JK-WZM-I micro injection molding machine with a twin-screw

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extruder (SHJ-30A) (Beijing Heng Odd Instrument Co., Ltd.) for the testing. For convenience,

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the PP and LDHs-C composite was designated as PP/LDHs-C. 2.4. Characterization 2.4.1. Characterization of SA@LDHs XRD patterns were collected on a Rigaku D/max-2550PC with CuKα radiation (λ=1.5406Å). The scan step was 0.02° (2θ) with a filament intensity of 30 mA and a voltage of 40 kV. FT-IR was recorded on Perkin-Elmer Spectrum One B instrument using KBr pellet technique.

Mastersizer 2000 laser particle size analyzer was from the United Kingdom Malvern company. Scanning electron micrograph (SEM) was obtained with a JEOL JSM-6700F instrument was performed by a Noran SystemSix instrument. Thermogravimetric analysis was carried out in a nitrogen atmosphere with a Seiko TG-DTA 6300. The nitrogen gas flow rate for 50 cm3∙min−1, and the heating rate was 10 °C∙min−1. 2.4.2. Characterization of PP and SA@LDHs composites TGA was performed using a Perkin-Elmer Pyris-1. 6.0~10.0 mg of the sample was loaded in

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an open ceramic crucible, and heated within the temperature range from ambient to 800°C at the heating rate of 10 °C∙min−1 in air atmosphere. SEM with a JEOL JSM-6700F instrument was obtained to examine the morphology of the cryo-fractured surfaces. The specimens for

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TGA and SEM were prepared by cryogenic fracturing in liquid nitrogen.

LOI was measured using a JF-3 instrument (Nanjing, China) on bars 120×10×4 mm3

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according to the standard oxygen index test on a GB/T 2406.2-2009. The vertical burning

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UL94 test was carried out with 120×10×4 mm3 specimens based on the standard ANSI/UL-94-1985 and averaged over five measurements for each composites. The Cone calorimeter tests (CCT) were carried out using cone calorimeter (JCZ-2, Jiangning Analytic

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Instrument Company, China) according to ISO 5660. The specimen with dimension of 100 × 100 × 4 mm3 was irradiated horizontally at heat flux of 50 kW∙m−2.

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Elongation at break and tensile strength were based on the standard GB/T9341-2000 and

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GB/T1042-1992, respectively. Three specimens at least were repeated to determine the average values in order to obtain reproducible results.

3. Results and discussion 3.1. Characterization of the SA@LDHs samples 3.1.1. XRD analysis

ª

A

(110)

ª

d003=1.24nm

§SA ¨LDHs-C

(018)

ª

(200)

ª

(015)

ª

(009)

(006)

(003)

ªSA@LDHs

ª

ª

§

§

10

20

30

¨ 40

LDHs-C

¨ 50

¨

60

70

B

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2q ()

(110)

¨

(018)

(015)

¨

d003=0.76nm

(009)

(006)

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SA

(003)

Intensity(a.u.)

SA@LDHs

Fig. 1. X-ray diffraction patterns of LDHs-C, SA and SA@LDHs (A) and the size of SA block calculated by Materials Studio software (B). The powder XRD patterns for the LDHs-C, SA and SA@LDHs are showed in Fig. 1A. The

LDHs-C and SA@LDHs samples displayed the sharp and intense (003), (006), (009), (110) and (113) reflections and broadened (015) and (018) reflections corresponding the typical hydrotalcite-like materials which indicating that both the samples possessed a high degree of crystallinity and layered structure (Cavani, Trifiro, & Vaccari, 1991). Besides, the peaks of SA@LDHs at 2θ = 13.66° and 2θ = 20.98° plane were ascribed to the characteristic diffraction peak of typical alginate (as showed in Fig. 1A). Further analyses of the XRD patterns revealed some differences in the cell parameters between the two samples. For

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LDHs-C, the observed the interlayer space distance d003-value was 0.76 nm, which was the result of the LDHs host layer thickness of approximately 0.48 nm and the inter-layer spacing of 0.28 nm for CO32– intercalated hydrotalcite, indicating that the intercalated anions of

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LDHs-C were CO32– (Cavani, Trifiro, & Vaccari, 1991; Zeng, Deng, Wang, & Liao, 2009). For SA@LDHs, the interlayer space distance d003-value was 1.24 nm, meaning that the

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inter-layer spacing of LDH-NS was 0.76 Å, which was similarly to the size of monomer GM

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block (0.7554 nm) calculated by Materials Studio software (showed in Fig. 1B). Therefore, XRD results indicated that the monomer GM block of alginate had been successfully intercalated into the interlayer spaces of SA@LDHs.

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3.1.2. FT-IR analysis

The FT-IR spectrum for LDHs-C, SA and SA@LDHs are showed in Fig. 2, both the LDHs-C

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and SA@LDHs samples demonstrated typical hydrotalcite-like structures (Cavani, Trifiro, &

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Vaccari, 1991; Yang et al., 2015). The broad absorption peak between 3400 cm–1 and 3600 cm–1 was assigned to the O-H stretching vibration of the metal hydroxide layer andinterlayer water, and the bands recorded below 800 cm–1 can be attributed to the vibrations of metal oxides (M−O, O−M−O) (Cavani, Trifiro, & Vaccari, 1991; Rives, 2001). For LDHs-C, the 1630 cm–1 and 1360 cm–1 bands for the water bending vibrations and stretching vibration of CO32– (Cavani, Trifiro, & Vaccari, 1991; Zeng, Deng, Wang, & Liao, 2009). Compared with

the LDHs-C spectrum, the reflections assigning to CO32– disappear, and there were new reflections at around at around 1638, 1403 and 1103 cm–1 observed in SA@LDHs spectrum, which were similar to the characteristic peaks of the SA sample (as showed in Fig. 2). These new bands of SA@LDHs were assigned to what follows: 1638 and 1403cm–1 due to the symmetric stretching vibration of COO– and the stacking band of the typical antisymmetic vibration of COO– (Kang et al., 2014); 1103 cm–1 corresponding to the C−O−C stretching vibration (Pan et al., 2016). From the FT-IR spectra in combination with the XRD results, it

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can be concluded that the successful intercalating of MG block of alginate into the interlayer space of the SA@LDHs. Additionally, the characteristic absorption band of interlayer CO32– is almost completely absent in the case of the SA@LDHs, indicating that the CO32– was almost

3500

3000

452

621

621 475

1103

1403

1638 2000

1500

436

1360

1627 2500

1099

1400

1638

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3414

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LDHs-C

3449

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SA@LDHs

SA

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3409

Transmittance (a.u.)

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completely replaced by the GM block of alginate.

1000

500

1

Wavenumber (cm )

Fig. 2. FT-IR spectras of LDHs-C, SA and SA@LDHs. 3.1.3. SEM analysis The LDHs-C and SA@LDHs are characterized by SEM analyzer and showed in Fig. 3 in

order to investigate the morphology. Both the LDHs-C and SA@LDHs samples exhibited characteristic hexagonal platelet structure of typical hydrotalcite morphology, and the LDHs-C presented well-developed hexagonal plates with a smooth surface as well as uniform sizes. Whereas, the SA@LDHs had a rich sheets structure on the surface, and the sheets and sheets cross with each other to form morphology similar to the "hydrangea shape". The results implied that the introduction of SA may change the surface morphology of SA@LDHs.

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SA@LDHs

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LDHs-C

Fig. 3. SEM images of LDHs-C and SA@LDHs, ×20,000.

3.1.4. TG-DTG analysis The thermal stabilities of LDHs-C, SA and SA@LDH are assessed by TG-DTG in nitrogen and TG-DTG curves are plotted in Fig. 4. As can be seen, LDHs-C showed three mass losses

in the thermal decomposition behaviors. The first decomposition step occurred between 80 and 220 °C, which was due to the loss of the surface and interlayer water molecules. The second step showed from 220 to 345 °C and was attributed to the loss of physically bound the OH– and CO32– .The third step occurred from 345 to 520 °C, which was corresponded to the dehydroxylation of the LDHs layers and decomposition of CO32– in the interlayer space (Cavani, Trifiro, & Vaccari, 1991; Yang, Kim, Liu, Sahimi, & Tsotsis, 2002). Compared with LDHs-C, the thermal decomposition process of SA was simpler, mainly display two steps

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process, the first step which was occurred between 30 to 100°C, can be attributed to the loss of free water and water linked through hydrogen bonds. The second step occurs between 215 to 280 °C and was mainly due to the thermal decomposition of SA skeleton (Chen, Shen,

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Chen, Zhao, & Schiraldi, 2016; Francavilla, Manara, Kamaterou, Monteleone, & Zabaniotou, 2015). As revealed in Fig. 4, the thermal decomposition of SA@LDHs mainly took place in

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four steps, which was different from LDHs-C and SA. The first stage, below 100 °C, can be

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due to the loss of free water and water linked through hydrogen bonds. The second step, from 100 to 215 °C, was corresponded to the loss of the physically absorbed and interlayer water molecules (Cavani, Trifiro, & Vaccari, 1991). The third step, from 215 to 280 °C, was

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attributed to the loss of the degradation alginate. The four stage occurred between 280 and 560 °C, the weight loss was attributed to the dehydroxylation of the SA@LDHs layers as well

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as to the degradation and combustion of alginate in the interlayer space (Chen, Shen, Chen,

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Zhao, & Schiraldi, 2016; Kang et al., 2014). It was worth noting that the temperature of the dehydroxylation and thermal decomposition of the SA@LDHs decreased, probably because of the introduction of thermally instable SA into LDHs (Zhang, Ji, Wang, Tan, & Xia, 2012; Zhang et al., 2011). Actually, there was reason to believe that earlier degradation enables SA@LDHs to produce char layers that can effectively protect the PP matrix from heat and flame (Liu et al., 2016; Du et al., 2019). The results further confirmed that SA have been

successfully intercalated into the interlayer space of the SA@LDHs and the intercalating of SA decreased the thermostability of SA@LDHs.

LDHs-C SA SA@LDHs

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60

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Weight (%)

80

40

200

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0.3

700

800

LDHs-C SA SA@LDHs

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0.9

0.6

600

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241

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Derivative weight (%·°C-1)

1.2

300 400 500 Temperature (°C)

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100

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20

64

194

64

194 241 287

422 387

0.0 100

200

300 400 500 Temperature (°C)

600

700

800

Fig. 4. TG-DTG curves of LDHs-C , SA and SA@LDHs. 3.2. Characterization of PP/SA@LDHs composites

3.2.1. SEM analysis In order to investigate the distribution results of LDHs-C and SA@LDH in PP matrix, the cryo-fractured surfaces of the PP/LDHs-C and PP/SA@LDHs composites samples are observed with SEM images in Fig. 5. The micrographs of the PP/LDHs-C showed that the virgin LDHs-C filler was not uniformly dispersed in the PP matrix, which the LDHs-C have been agglomerated and were distinctly separated from the PP matrix, suggesting that the interfacial interaction between the virgin LDHs-C and the PP matrix were deteriorated and

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lack of sufficient adhesion. In contrast, a clear difference was observed in the PP/SA@LDHs composites, the phase interface between SA@LDHs and PP matrix was relatively vague and SA@LDHs uniformly dispersed in the cross section of the PP matrix. Thus, it can be

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PP/LDHs-C

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compatibility of SA@LDHs in the PP matrix.

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concluded that by intercalating SA into the layer of LDHs-C could improve the dispersion and

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PP/SA@LDHs

Fig. 5. SEM images of cryofractured sections for PP/LDHs-C and PP/SA@LDHs composites.

3.2.2 Mechanical properties The mechanical properties of the Neat PP, PP/LDHs-C and PP/SA@LDHs composites are listed in Table 1. From Table 1, it was found that the Neat PP exhibits a tensile strength of 23 MPa and an Elongation at Break of 70%. Compared with the Neat PP, the PP/LDHs-C and PP/SA@LDHs composites showed a reduction in tensile strength and elongation at break. In general, increased additive loading is likely to cause deterioration in the mechanical properties of the polymer matrix, due mainly to poor additive-matrix compatibility. Further analysis of

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the mechanical properties of PP/LDHs-C and PP/LDHs-C composites revealed that there was a difference in tensile strength and elongation at break. Compared with PP/LDHs-C, the tensile strength and elongation at break of PP/SA@LDHs were significantly improved, which

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was due to the improvement of compatibility between the PP matrix and the SA@LDHs particles after the modification of SA. The improvement in the elongation at break and tensile

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strength of PP/SA@LDHs composites further indicated that by intercalating SA into the layer

3.2.3. TGA analysis

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of LDHs-C could improve the dispersion and compatibility of SA@LDHs in the PP matrix.

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The thermal stability of the Neat PP, PP/LDHs-C and PP/SA@LDHs composites are evaluated by TGA under air atmosphere, and the obtained results are provided in Fig. 6 and

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Table 1. It was observed that the main degradation stage of Neat PP was in the range of 280~500 °C, and the thermal curves of PP/LDHs-C and PP/SA@LDHs composites were

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similar to Neat PP, for that the decomposition temperature at 10% mass loss (T10%), decomposition temperature at 50% mass loss(T50%) and char residue rate at 800°C were higher than that of the Neat PP, the results meant that the addition of the LDHs-C and/or SA@LDHs into PP improved the thermal stability of PP matrix. As reported in Table 1, it was observed that the T10% and T50% values of the PP/SA@LDHs were 314.4°C and 388.2 °C, while those of the corresponding PP/LDHs-C were 329.5 °C and 403.6 °C, as a result of the later initial

decomposition leading by the addition of SA@LDHs. Especially, the char residue rate at 800 °C for PP/LDHs-C and PP/SA@LDHs were 14.7% and 19.5%, respectively. The results demonstrated that SA@LDHs was a little better in enhancing the thermal stability of PP/SA@LDHs than LDHs-C for PP/LDHs-C due to the flame retardant fictionalization of SA, which promoted the formation of the char layer through dehydrogenation, hindered the transfer of oxygen and heat, and thus protected the underlying matrix from burning.

Neat PP PP/LDHs-C PP/SA@LDHs

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100 T10%

T50% 40

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60

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Weight (%)

80

0

200

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100

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20

300

400

500

600

Temperature (C)

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Fig. 6. TGA curves for Neat PP, PP/LDHs-C and PP/SA@LDHs composites under an air atmosphere.

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Table 1. Formulation, LOI, UL-94 result, thermal decomposition parameters, mechanical properties and cone calorimeter data of Neat PP, PP/LDHs-C and PP/SA@LDHs composites Mechanical properties

Content (wt%) LOI(%) LDHs-C

100

0

0

17.6±0.3

PP/LDHs-C(90/10)

90

10

0

21.1±0.2

PP/LDHs-C(85/15)

85

15

0

23.6±0.3

PP/LDHs-C(80/20)

80

20

0

PP/LDHs-C(75/25)

75

25

0

PP/LDHs-C(70/30)

70

30

0

PP/SA@LDHs(90/10)

90

PP/SA@LDHs(85/15)

85

PP/SA@LDHs(80/20)

80

PP/SA@LDHs(75/25)

75

PP/SA@LDHs(70/30)

70

Char residue rate at 800°C(%)

pHRR

THR

TSP

Break (%) 70±1.3

23±0.4

314.2

370.9

0

841±18

107±1.5

12±0.4

HB

61±2.5

21±0.6

--

--

--

--

--

--

V-2

55±1.9

20±0.4

--

--

--

--

--

--

Not pass

Pr

PP

T50%

Tensile strength (Mpa)

SA@LDHs

Elongation at

e-

PP

UL-94

pr

Samples

T10% (°C)

(°C)

–2

–2

(kW·m )

(MJ·m )

(m2)

V-2

43±1.6

19±0.1

--

--

--

--

--

--

26.2±0.3

V-2

31±2.1

17±0.6

--

--

--

--

--

--

27.6±0.1

V-1

17±1.5

16±0.2

314.4

388.2

14.7

579±20

100±0.7

11±0.5

0

10

23.8±0.3

V-2

68±1.5

23±0.2

--

--

--

--

--

--

0

15

26.7±0.2

V-2

60±1.8

22±0.7

--

--

--

--

--

--

0

20

28.4±0.2

V-1

51±2.1

22±0.1

--

--

--

--

--

--

0

25

29.5±0.3

V-1

39±1.6

20±0.9

--

--

--

--

--

--

0

30

30.9±0.1

V-0

26±1.3

19±0.6

19.5

260±11

97±0.6

8.0±0.3

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25.4±0.1

329.5 403.6

3.3. Combustion properties of PP/SA@LDHs composites 3.3.1. Burning Behaviors (LOI and UL-94 Tests) The flammability of the Neat PP, PP/LDHs-C and PP/SA@LDHs composites are evaluated by the LOI and UL-94 tests, and the final results are summarized in Table 1. As can be seen from Table 1, Neat PP presented a low LOI value of 17.6% and no classification in the UL-94 vertical, which was a highly flammable material. Whereas, adding LDHs-C and/or SA@LDHs significantly improved the LOI value and UL-94 rating for PP/LDHs-C or

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PP/SA@LDHs composites, and PP/SA@LDHs showed better flame retardancy than PP/LDHs-C at the same loading level. For PP/LDHs-C composites, the LOI value reached up to 27.6% with a UL-94 V-1 rating by adding 30 wt% LDHs-C filler. However, it was noted

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that adding 30 wt% SA@LDHs as filler, the PP/SA@LDHs composites exhibited a higher LOI of 30.9% and achieved the UL-94 V-0 rating. This reduction in flammability was

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believed to be related to better dispersion and compatibility of SA@LDHs than the LDHs-C

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in PP matrix, and the flame retardant functionalization of SA. Thus, the results confirmed that adding SA@LDHs than LDHs-C as the flame retardant filler evidently improved the inflammability of PP matrix. To understand the flame retardant properties of PP/LDHs-C and

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PP/SA@LDHs composites, the more detail analyzed by other techniques as the cone calorimeter should be further tested.

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3.4.2. Cone Calorimeter Tests

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In order to further evaluate the flame retardancy properties of PP/LDHs-C and PP/SA@LDHs composites at 30 wt% LDHs-C and/or SA@LDHs loading level, CCT are carried out for measuring the burning behaviors of composites in the bench-scale. The heat release rate (HRR), total heat release (THR) and total smoke production (TSP) curves of the samples were plotted in Fig. 7 and the related data were listed in Table 1. From Fig. 7A, it found that Neat PP burnt very rapidly after ignition and the peak heat release

rate (pHRR) value was 846.5 kW·m–2, meaning that Neat PP showed a very poor fire resistance. Compared with Neat PP, the pHRR value for PP/LDHs-C composites decreased to 581.4 kW·m–2 and the reduction of pHRR was 31.3%. Furthermore, the PP/SA@LDHs composites burnt relatively slowly and the pHRR decrease to 260.8 kW·m–2, which exhibited reduction of 69.2% compared with that of Neat PP. The remarkable reduction in pHRR of PP/SA@LDHs composites suggested that SA@LDHs had a significant improvement on flame retardancy of PP matrix. As expected, THR demonstrated a similar trend to HRR for

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Neat PP, PP/LDHs-C and PP/SA@LDHs composites (showed in Fig. 7B). At 400 seconds after the ignition, the THR value of Neat PP was 107.2 MJ·m–2. After adding LDHs-C and/or SA@LDHs as flame retardancy fillers for PP, the THR values of PP/LDHs-C and

-p

PP/SA@LDHs composites droped down to 100.7 and 61.3 MJ·m–2, respectively. The THR value of PP/SA@LDHs composites evidently decreased, which was due to the effect of SA

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leading to promote PP chains to participate in the carbonization process, resulting in less

lP

volatile products form “fuel” to go into the combustion zone (Wang, Kalali, & Wang, 2015). As showed in Fig. 7C and Table 1, it could be seen that the TSP for Neat PP was the highest with the value of 12.1 m2, and the TSP values for PP/LDHs-C and PP/SA@LDHs composites

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reduced to 11.2 and 8.2 m2, respectively. The results demonstrated that addition of LDHs-C (or SA@LDHs) had a significant influence on reducing the smoke production of PP matrix.

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Besides, it is obvious that PP/SA@LDHs exhibited a lower TSP value compared with

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PP/LDHs-C, indicating that adding the same amount of SA@LDHs can remarkable decreased the TSP. The reason may be that the presence of the radical-trapping effect of SA for PP matrix, thereby reducing the amount of total smoke release in the gas phase (Wang et al., 2018). Therefore, it can be concluded that addition of the SA@LDHs had a significant effect on the flame retardant properties compared to the LDHs-C, which was in agreement with the results from LOI and UL-94 rating analyses.

1000

Heat Release Rete (kW·m2)

(A)

Neat PP PP/LDHs-C PP/SA@LDHs

800

600

400

200

0 0

200

400

600

80

-p

60

re

40

20

0 0

lP

Total Heat Release (MJ·m2)

Neat PP PP/LDHs-C PP/SA@LDHs

(B)

100

ro of

Time (s)

800

200

400

600

800

Time (s)

(C)

12

Neat PP PP/LDH-C PP/SA@LDH

10

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Total Smoke Production (m2)

na

14

8 6 4 2 0

-2 0

200

400

600

800

Time (s)

Fig. 7. Heat release rate (A), total heat release (B) and total smoke production (C) curves of Neat PP, PP/LDHs-C and PP/SA@LDHs during the cone calorimeter tests.

3.2.3. Morphology of the Char Residues The morphology of the char residues for Neat PP, PP/LDHs-C and PP/SA@LDHs composites are analyzed, and the photographs and SEM images of char residues after CCT were showed in Fig. 8. In Fig. 8A, there was almost no residue left for Neat PP, indicating that the Neat PP was completely burned. As showed in Fig. 8B and Fig. 8C, there were residues in both PP/LDHs-C and PP/SA@LDHs, the density and yields on the char residue of PP/SA@LDHs significant increased than those of PP/LDHs-C. Compared with the

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PP/LDHs-C, the PP/SA@LDHs exhibited a more consolidated and thick char layer, which demonstrated that SA@LDHs acted as an effective barrier carbon layer for both heat flow and gas transport, resulting in the remarkable reduction of HRR, THR and TSP (as showed in

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Fig. 6 and Table 1). In addition, it was obvious that there were differences between the microstructure of char residue of PP/LDHs-C (Fig. 8D) and PP/SA@LDHs (Fig. 8E), the

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former was more fragile and cracked, and the surface of the carbon is rough with particles,

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which provides less protection for the underlying PP matrix. Whereas, the latter char layer was a more compact and continuous surface without cracks, indicating that SA@LDHs can prevent the collapse of the char layer and play an important role in the process of dense char

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formation because of its improved dispersion in the PP matrix. This dense and continuous char layer can prevent fuel volatilization and hinder oxygen and heat feedback during burning

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of PP, resulting in more char residue and better fire retardant properties for PP/SA@LDHs.

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Fig. 8. Photographs of Neat PP (A), PP/LDHs-C (B) and PP/SA@LDHs (C) after the cone calorimeter tests, SEM images of PP/LDHs-C (D) and PP/SA@LDHs (E) after the cone calorimeter tests.

4. Conclusions

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In summary, a novel and highly efficient alginate pillared layered double hydroxides (SA@LDHs) flame retardant was fabricated via calcination-reconstruction manner by

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intercalating sodium alginate (SA) into the interlayer space of hydrotalcite (LDHs-C), and then the polypropylene (PP) and SA@LDHs composites (PP/SA@LDHs) were prepared by

na

melt blending. XRD, FT-IR, SEM and TG-DTG characterizations of SA@LDHs indicated that the monomer GM block of SA had been successfully intercalated into the interlayer

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spaces of SA@LDHs, and the SA@LDHs presented a weaker thermostability than LDHs-C.

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Furthermore, the characterization results of PP/SA@LDHs composites revealed that SA@LDHs was more effective than LDHs-C on improving the flame retardancy and thermal stability for PP matrix. Meanwhile, the peak heat release rate, total heat release and total smoke release of PP/SA@LDHs composites were reduced slightly compared to the Neat PP. In short, an efficient and bio-based green flame retardant had been produced using a simple method in this work.

Credit Author Statement Sheng Xu: Methodology; Conceptualization; Data curation; Investigation; Writing-original draft; Conceptualization;

Funding acquisition. Si-Yu Li: Methodology;

Investigation.

Min

Zhang:

Project

administration;

Investigation. Hong-Yan Zeng: Writing-review & editing; Data curation ; Funding acquisition. Kun Wu: Investigation. Xian-Yao Tian: Investigation.

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Chao-Rong Chen: Investigation. Yong Pan: Funding acquisition.

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Acknowledgements

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This work is supported by Natural Science Foundation of Hunan Province (2019JJ50613), Key Research and Development Program of Hunan Province (2018SK20110) and Hunan

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2011 Collaborative Innovation Center of Chemical Engineering & Technology with

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Environmental Benignity and Effective Resource Utilization.

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