Synergistic effect of ammonium polyphosphate and α-zirconium phosphate in flame-retardant poly(vinyl alcohol) aerogels

Journal Pre-proof Synergistic effect of ammonium polyphosphate and α-zirconium phosphate in flameretardant poly(vinyl alcohol) aerogels Yong Luo, Delong Xie, Yifan Chen, Tao Han, Renjie Chen, Xinxin Sheng, Yi Mei PII:

S0141-3910(19)30347-7

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.109019

Reference:

PDST 109019

To appear in:

Polymer Degradation and Stability

Received Date: 19 May 2019 Revised Date:

7 October 2019

Accepted Date: 26 October 2019

Please cite this article as: Luo Y, Xie D, Chen Y, Han T, Chen R, Sheng X, Mei Y, Synergistic effect of ammonium polyphosphate and α-zirconium phosphate in flame-retardant poly(vinyl alcohol) aerogels, Polymer Degradation and Stability (2019), doi: https://doi.org/10.1016/j.polymdegradstab.2019.109019. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Synergistic effect of ammonium polyphosphate and α-zirconium phosphate in flame-retardant poly(vinyl alcohol) aerogels Yong Luoa,b,c, Delong Xiea,b,c, Yifan Chena, Tao Hana,b,c, Renjie Chena,b,c, Xinxin Shengd,*, Yi Meia,b,c,** a Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China b Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, Kunming 650500, China c The Higher Educational Key Laboratory for Phosphorus Chemical Engineering of Yunnan Province, Kunming 650500, China d Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X.-X. Sheng),

1

[email protected] (Y. Mei).

Abstract To overcome the flammability and melt dripping issues of poly(vinyl alcohol) (PVA) aerogel, a combination of ammonium polyphosphate (APP) and α-zirconium phosphate (α-ZrP) was used to enhance the flame retardancy of PVA aerogels , which were fabricated using a simple mixing and freeze-drying approach. Results obtained from transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirm that α-ZrP was successfully exfoliated by ultrasonic stripping. The effects of the exfoliated α-ZrP nano-plates on the microstructure, composition, flame retardancy, fire behavior of the aerogels was investigated using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), limiting oxygen index (LOI), UL-94 vertical burning, and cone calorimetry test. It was found that the incorporation of α-ZrP improved the 3D network structure of the PVA aerogel. The results showed that a sample with 2 wt% α-ZrP and 13 wt% APP could reach the maximal LOI value of 43.1%, achieved the UL-94 V-0 rating, and the peak heat release rate (PHRR) and total heat release (THR) of the sample reduced by 82.6% and 76.3%, respectively. Thermogravimetric analysis (TGA) revealed that α-ZrP improved the thermal stability of PVA-APP system. The residues analysis revealed that α-ZrP accelerated the formation of a compact and graphitization structure of residues and catalyzed the charring of PVA during burning.

Keywords: α-zirconium phosphate (α-ZrP); Synergistic effect; Flame retardant; Poly(vinyl alcohol) (PVA)

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1. Introduction Aerogels are high-performance porous materials which are used in a wide variety of applications in fields such as architecture [1], aerospace [2], environmental protection [3, 4], and energy storage [5] due to their low density, low thermal conductivity, high porosity, and specific acoustic properties [6, 7]. Poly(vinyl alcohol) (PVA) is considered to be a promising candidate material for aerogel preparation because of its excellent properties such as biodegradability and biocompatibility [8, 9]. Nevertheless, serious flammability and melt dripping issues of PVA aerogels have severely restricted its applications. In order to mitigate this problem, the thermal stability and flammability of these aerogels may be enhanced through the incorporation of flame-retardant materials. Currently, materials commonly used for this purpose include phosphorus-/nitrogen-containing flame retardants[10-12], nanofillers[13-17], metal hydroxides [18, 19], and so on. Ammonium polyphosphate (APP), as a typical and promising phosphorus flame retardant due to its halogen free, low toxicity, high contents of phosphorus and nitrogen etc. APP is usually used as the acid source and blowing agent of intumescent flame retardant (IFR). The formation of intumescent char by IFR on the surface during burning, thus suppressing the transfer of fuel and heat between condensed phase and gas phase. To obtain better flame-retardant effects in polymer, metal compounds and nanomaterials have also been used as synergists for APP. Zhao et al. [20] proved that the incorporation of layered double hydroxide (LDH) resulted in the improvement on the flame retardancy of PVA-APP system. Feng et al. [8] synthesized 3

a metallic cyclodextrins (metal MC) and added into PVA/IFR composites as synergists. Lin et al. [21] revealed that montmorillonite (MMT) used to enhanced the flame retardancy of PVA-APP. Recently, the two dimensional (2D) layered nanomaterial α-zirconium phosphate (α-ZrP) has received attention due to its excellent chemical properties, thermal stability and potential high-performance catalytic properties [22, 23], and has also been employed as a promising flame-retardant additive for polymers [24]. Lu et al. [25] prepared PVA/ZrP nanocomposites with enhanced thermal properties and flammability performance; their results showed that compared to pure PVA, the peak heat release rate (PHRR) was decreased by 42% and the mass of residue was enhanced from 4.5% to 17%. Although the addition of nanofillers can effectively reduce the heat released, they are hardly improved the flammability test of polymer, such as LOI and UL-94. Therefore, the combination of α-ZrP and APP seems to be a meaningful work that take into consideration the drawback of α-ZrP. Yang et al. [26] reported that the synergistic effect between α-ZrP and IFR (APP and pentaerythritol) was obtained in the polypropylene (PP) matrix, and α-ZrP promoted the catalytic carbonization of PP. Xie et al. [27] reported that macromolecular charring agent (MCA)-modified α-ZrP could improve the flame retardancy of a PP-based IFR material. However, there is still no study on PVA-based aerogels. In the present study, to achieve stable dispersed α-ZrP nano-plates, we use the ultrasonic stripping method exfoliated layered α-ZrP into monolayer α-ZrP nano-plates. Due to PVA is a hydroxyl-rich polymer, it may be used as a 4

macromolecular carbonizing agent for IFRs. Consequently, designing a new IFR system with exfoliated α-ZrP nano-plates, thus enhancing the flame retardant of the PVA aerogels. A series of flame-retardant PVA aerogels loaded with different mass ratios of α-ZrP nano-plates and APP were fabricated using the freeze-drying method. The microstructures, thermal stabilities and flame retardancy of the composite aerogels were investigated. Additionally, the residues of the samples were analyzed in detail using Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques to elucidate the flame retardance mechanisms of the materials.

2. Experimental 2.1 Materials Poly(vinyl alcohol) (PVA) (degree of polymerization 1700, 98-99 mol% hydrolysis) was purchased from Aladdin Industrial Corporation, China. Ammonium polyphosphate (APP) with average degree of polymerization n > 1000 was purchased from Klein, Germany. α-zirconium phosphate (α-ZrP) (99%) was supplied by Guangzhou Lihongji Chemical Industry Co., Ltd., China. Tetrabutylammonium hydroxide (TBAOH) (10%) was purchased from Sinopharm Chemical Reagent Co., Ltd, China. All the chemicals were used as received without further purification. 2.2 Preparation of exfoliated α-ZrP Exfoliated α-ZrP was prepared using an ultrasonic method, as follows: 1 g of primordial α-ZrP was dispersed in 10 mL of water under ultrasonication, followed by 5

the addition of 2.2 mL of TBAOH. The solution was mixed at 200 rpm in a centrifuge for 5 min before being diluted with 20 mL water and mixed ultrasonically for 30 min. After standing for 72 h, the solution was then centrifuged for 60 min at 5000 rpm, which the upper fluid was the exfoliated α-ZrP. 2.3 Preparation of flame retardant PVA composites PVA and APP were dried at 80 ℃ in vacuum for 12 h before use. A schematic of the experimental process for PVA-APP-ZrP aerogel fabrication is shown in Fig. 1. The total flame-retardant component of the PVA composites was maintained at 15 wt%, with the APP content varied between 11–15 wt% and α-ZrP between 0–4 wt%, as shown in Table 1. The samples were formulated as follows: 10 wt% PVA was dissolved in hot water at 85 ℃ while stirring vigorously for 1 h to form a transparent aqueous solution. The APP and exfoliated α-ZrP were then added to the PVA solution and mechanically stirred for 3 h. After stirring, the sample was poured into a plastic plate and frozen in the refrigerator for 12 h, following by freeze drying using a Christ ALPHA 1-2 LDplus lyophilizer at −50 ℃ under vacuum conditions (< 8 Pa). Following freeze drying, all samples were further dried at 70 °C for 24 h in a vacuum.

Fig. 1. Preparation process for PVA-APP-ZrP aerogel.

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Table 1 Formulation of flame-retardant PVA aerogels.

Sample

component PVA (wt%)

APP (wt%)

α-ZrP (wt%)

PVA

100

-

-

PVA-APP

85

15

-

PVA-APP/ 0.5ZrP

85

14.5

0.5

PVA-APP/ ZrP

85

14

1

PVA-APP/ 2ZrP

85

13

2

PVA-APP/ 4ZrP

85

11

4

2.4 Measurement and characterization The limiting oxygen index (LOI) text was performed on a COI oxygen index meter (Mordis Combustion Technology Co., Ltd., China), with sample dimensions of 100 × 10 × 3 mm3 according to the GB/T 2406.2-2009 test standard. The vertical burning test (UL-94) was performed on a CZF-6 horizontal and vertical combustion tester (Nanjing Jiangning Analytical Instrument Co., Ltd., China) with sample dimensions of 130 × 13 × 3 mm3 according to the GB/T 2408-2008 test standard. Fire behavior was measured using a PHINIX cone calorimeter (Suzhou Phoenix Quality Inspection Instrument Co., Ltd., China) under a heat flux of 35 kW/m2. According to the ISO 5660-1 method, sample dimensions of 100 × 100 × 10 mm3 were used and each sample was wrapped in aluminum foil. Thermogravimetric analysis (TGA) experiments were carried out using an STA449F3 thermogravimeter (NETZSCH Instruments Co., Ltd., Germany). The 40 – 800 ℃ and 40 – 1000 ℃ were measured in nitrogen and air atmosphere, respectively, with a heating rate of 10 ℃ min-1 under a flow of 100 mL min-1. 7

SEM-EDX experiments were carried out using an SU-8220 field emission scanning electron microscope (Hitachi Ltd., Japan) at 15 kV. Transmission electron microscopy (TEM) was performed on Tecnai G2 F20 S-TWIN (Thermo Fisher Scientific Inc., USA) with a 200 kV accelerating voltage. The atomic force microscopy (AFM) images were carried out using a Bruker MultiMode 8 (Bruker Optics Inc., Germany). The water suspension of the α-ZrP (0.1 mg/mL) were deposited on a mica plate. XPS experiments were performed on a PHI5000 VersaProbe-II spectrometer (Ulvac-Phi Inc., MN, USA) using Al Kα radiation and operated at 50 W and 15 kV. Raman spectroscopy was conducted on a Ramanscope 2000 Raman microscope (Renishaw Ltd., UK) with the wavenumber ranging from 500 to 2000 cm-1.

3. Results and discussion 3.1 Characterization of exfoliated α-ZrP SEM and TEM measurements are used to investigate the morphology of primordial α-ZrP and exfoliated α-ZrP nano-plates, respectively, and the results are shown in Figure 2. It can be clearly seen in Fig. 2a that the primordial α-ZrP sample consists of aggregations of α-ZrP sheets with thicknesses of about 120 nm. Apparently, the exfoliated α-ZrP is observed with an average size of nano-plates of 700−1000 nm (shown in Figure 2b). Fig. 2(c) and (d) shows the AFM image and the corresponding height profiles of α-ZrP nano-plates, respectively. This result shows that the obtained α-ZrP nano-plates have various lateral size in the ranges of 500–1000 nm, the 8

thickness of the nano-plates is 1.1–1.2 nm, indicating α-ZrP is fully exfoliated into single layer. Fig. 2e shows digital photographs of the effect of 36 h of storage on the appearance of both exfoliated and primordial α-ZrP suspensions in water with 4 wt% α-ZrP solids content. Following the storage period, it is clear that sedimentation has occurred in the primordial α-ZrP sample. However, the exfoliated α-ZrP solution maintains a stable appearance without noticeable sedimentation. Therefore, it may be inferred from these results that the α-ZrP is fully exfoliated and distributed in a regular arrangement following ultrasonic stripping.

Fig. 2. (a) SEM images of primordial α-ZrP; (b) TEM image of exfoliated α-ZrP; (c, d) AFM image of α-ZrP nano-plates and the corresponding height profiles; (e) Photographs of the exfoliated α-ZrP and primordial α-ZrP suspensions (α-ZrP content: 4%).

3.2 Characterization of aerogels SEM is used to investigate the microstructure of the PVA, PVA-APP, and PVA-APP/2ZrP aerogels (Fig. 3). As shown in Fig. 3a, the pure PVA surface is clean 9

and smooth, with a regular structure and uniform pore distribution. The viscosity of the PVA system increases after the addition of APP, resulting in pore disorder and destruction of the network structure caused by molecular chain restriction. However, from Fig. 3c, it may be observed that the aligned and continuous PVA-APP-ZrP main frames forms a three-dimensional (3D) pore network structure through the bridge of the ribbon-like short ligaments. The formation of these ligaments may be ascribed to partial α-ZrP nano-plates are captured by the rapidly advancing ice fronts, thus connecting the adjacent aligned α-ZrP nano-plates [28, 29]. The EDX mapping images (Fig. 3d and 3e) clearly show that Zr is only present in the PVA-APP/2ZrP aerogel and not in PVA-APP, which confirms the existence of α-ZrP moieties in the prepared aerogel samples. Further, each element is uniformly dispersed in the flame-retardant aerogels, which indicated the good compatibility between α-ZrP, APP, and the PVA matrix; hence, these samples provide a reliable basis for the study of aerogel properties.

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Fig. 3. SEM images of (a) PVA, (b) PVA-APP, (c) PVA-APP/2ZrP aerogels and EDX elemental mapping of (d) PVA-APP, (e) PVA-APP/2ZrP aerogels.

3.3 Flame retardancy of aerogels The flame retardancy behavior of the aerogels is investigated using the LOI and UL-94 tests. The LOI value and UL-94 rating results are summarized in Table 2, whence it can be observed that the aerogels containing both the α-ZrP and APP exhibited noteworthy flame retardant properties. The LOI value of the PVA composite aerogels is as high as 43.1% for the sample with 2 wt% α-ZrP and 13 wt% APP. These results demonstrate not only that α-ZrP can enhance the flame-retardant properties of PVA-APP system, but that there exists a synergistic effect between α-ZrP and APP. However, when the α-ZrP content increases to 4%, the LOI value of the system decreases. One possible reason for this is that the excess α-ZrP catalyst crosslinks into the carbon and forms high-density bridge bonds, thus making the carbon layer brittle. Simultaneously, a large number of non-combustible gases degradation by APP break the equilibrium of the system and cause cracks in the expanded carbon layer. As a result, the barrier formed by the carbon layer is weakened, thereby reducing the flame retardancy of the material. As shown in Table 2, the UL-94 V-0 rating is achieved when the total flame retardant content of the material only 15 wt%. These results indicate that the presence of APP is effective for improving the fire-extinguishing properties of the aerogel. Table 2 LOI and UL-94 tests experimental results for pure PVA and the flame-retardant aerogels Sample

LOI (%)

UL-94

PVA

19.5 ± 0.5

NR

PVA-APP

37.5 ± 0.5

V0

11

PVA-APP/ 0.5ZrP

40.0 ± 0.5

V0

PVA-APP/ ZrP

42.3 ± 0.5

V0

PVA-APP/ 2ZrP

43.1 ± 0.5

V0

PVA-APP/ 4ZrP

41.3 ± 0.5

V0

The fire behavior of the aerogel samples is further investigated using the cone calorimetry method. Several characteristic parameters, such as time to ignition (TTI), PHRR, total heat release (THR), and mass loss (ML) are measured, which can be used to evaluate the behavior of aerogels in realistic fire scenarios. These data are presented in Table 3. Table 3 Cone calorimetric results for all aerogels. Mass

TTI

Sample

PHRR 2

av-EHC

TPHRR

FIGRA 2

THR

av-MLR 2

2

Residue

（g）

(s)

(kW/m )

(MJ/kg)

(s)

(kW/(m ·s))

(MJ/m )

(g/(m .s))

(wt%)

PVA

13.5 ± 0.1

40 ± 3

337.0 ± 19

23.3 ± 0.2

100 ± 2

3.37 ± 0.26

31.8 ± 1.7

3.90 ± 0.8

3.9 ± 0.3

PVA-APP

13.2 ± 0.2

136 ± 2

105.1 ± 12

14.3 ± 0.3

198 ± 4

0.53 ± 0.07

10.4 ± 1.8

1.48 ± 0.4

18.3 ± 1.5

PVA-APP/0.5ZrP

13.6 ± 0.1

146 ± 2

87.6 ± 13

12.5 ± 0.3

198 ± 2

0.44 ± 0.07

9.5 ± 1.4

1.44 ± 0.5

24.5 ± 1.3

PVA-APP/ZrP

14.4 ± 0.1

151 ± 1

73.4 ± 13

12.8 ± 0.1

215 ± 3

0.34 ± 0.07

8.1 ± 1.6

1.27 ± 0.3

27.9 ± 1.5

PVA-APP/2ZrP

13.1 ± 0.1

153 ± 1

58.8 ± 15

11.2 ± 0.3

223 ± 5

0.26 ± 0.08

7.5 ± 1.3

1.34 ± 0.3

31.5 ± 1.8

PVA-APP/4ZrP

14.0 ± 0.3

146 ± 2

80.2 ± 12

13.6 ± 0.1

212 ± 2

0.37 ± 0.07

8.7 ± 1.0

1.28 ± 0.4

26.0 ± 1.4

The heat release rate (HRR) has an effect on flame spread rate and fire intensity, with a low HRR value indicating a high-efficiency flame retardant system [30]. The HRR curves of the samples are shown in (Fig. 4a). The pure PVA sample is burns rapidly after ignition, and has a peak HRR (i.e. PHRR) value of 337.0 kW/m2 at 100 s. By comparison, the PHRR of the PVA-APP aerogel is significantly lower, at 105.1 kW/m2. In the flame-retardant PVA systems, partial substitution of APP by α-ZrP leads to further decreases in PHRR, the PVA-APP/2ZrP system has the lowest PHRR, which is decreases by 82.6% compared with the pure PVA. Additionally, the THR values of the flame-retardant PVA aerogels are much lower than those of the pure PVA aerogel as shown in Fig. 4b and Table 3. The introduction of APP into the pure 12

aerogel reduces its THR from 31.8 MJ/m2 to 10.4 MJ/m2, whereas the introduction of α-ZrP to the PVA-APP matrix further reduces this value, with the minimum THR value of 7.5 MJ/m2 shown by the PVA-APP/2ZrP sample, which represents a 27.9% decrease over PVA-APP. The reduction of PHRR and THR in PVA-APP-ZrP system demonstrates an excellent flame-retarding action of APP and α-ZrP.

Fig. 4. (a) HRR, (b) THR, and (c) Mass loss curves of the PVA aerogels obtained through cone calorimetry.

Fig. 4c shows the curves of mass loss versus burning time for the samples. At flame-out, the residual mass of PVA, PVA-APP, PVA-APP/0.5ZrP, PVA-APP/ZrP, PVA-APP/2ZrP, PVA-APP/4ZrP were 3.9 wt%, 18.3 wt%, 24.5 wt%, 27.9 wt%, 31.5 wt%, 26.0 wt%, respectively. It can be seen that the residual mass of PVA-APP with α-ZrP samples are higher than that of PVA-APP, indicating that more the PVA sample is not completely burnt and undergoing a charring process. In addition, we can be observed that a massive mass loss before ignition for the flame-retardant aerogels, which is a result of the massive gas and smoking release during the thermal decomposition and charring of the aerogel [31]. Under the radiation of the radiation 13

cone heat, the surface of the aerogel is thermally decomposed into a large amount of small molecular gases and residues, resulting in a massive mass loss. And we can clearly see that the ignition time can be effectively delayed by the flame-retardant aerogel. The reason for this phenomenon may be that the existence of the flame retardant enhances the decomposition of non-combustible gases and dilutes the concentration of flammable gases, which is further suppressing the ignition of the substrate. Furthermore, as can be seen from Table 3 that the loading of APP and α-ZrP can decrease the average mass loss rate (av-MLR), suggesting that APP and α-ZrP can effectively inhibit the decomposition of the aerogels and form stable residues. And the residues can prevent the mass and the heat transfer [32], thus decrease the PHRR and related parameters. The reduction in average effective heat of combustion (av-EHC) is used to monitor fuel dilution in the gas phase and disclose the flame inhibition of volatiles in the gas phase during the burning. As shown in Table 3, the pure PVA show the highest av-EHC of 23.3 MJ/kg. With the incorporation of APP and α-ZrP, the av-EHC is decreased, suggesting that the burning intensity is suppressed in the gaseous phase. Due to APP decomposed to release NH3 and H2O during the burning, thus diluting the flammable gas in the gas phase and decreasing the heat of combustion in the gas phase. The fire growth rate index (FIGRA), which can estimate the hazard of developing fires, is the maximum ratio of the HRR(t) to the corresponding time of burning of the sample [33, 34]. A higher value of FIGRA means a higher fire hazard coefficient. As shown in Table 3, the FIGRA value of the samples notably decreased with the incorporation of APP and α-ZrP. For instance, the FIGRA value of the PVA-APP/2ZrP aerogel decreased from 3.37 to 0.26 compared to the pure PVA aerogel, which would inhibit the fire hazard quality of the materials. Table 4 PHRR, THR, LOI and UL-94 results of ZrP based polymer composites reported in prior work and this work. Polymer

Content

PHRR

THR

LOI

(wt%)

reduction (%)

reduction (%)

(%)

Flame retardant matrix

UL-94

14

Ref.

PP

APP/ZrP-d-MCA

15/5

81

56

32.5

V-0

[27]

PP

IFR/O-ZrP

22.5/2.5

-

-

37

V-0

[26]

PLA

F-ZrP

10

42.0

41.8

26.5

V-0

[35]

PS

IFR/ZrP-CTBA

18/2

61.9

20

-

-

[36]

PS

S-R-ZrP

1

25

7

-

-

[37]

HIPS

O-ZrP

3

17

-

-

-

[38]

EVA

IFR/OZrP

23/2

57.5

20.6

-

-

[39]

PA6

α-ZrP

2

6.8

3.4

33.5

V-0

[40]

PA6

AS/ZrP

2/2

-19.5

5.1

-

-

[40]

PA-46/PPO

O-ZrP/CF

2/20

65.6

48

42

-

[41]

PVA

ZrP-EA

8

42.1

16.7

-

-

[42]

PVA

HAC

20

-

-

30.7

V-0

[43]

PVA

APP/α-ZrP

13/2

82.6

76.3

43.1

V-0

This work

Poly(lactic acid) (PLA); Polystyrene (PS); High-impact polystyrene (HIPS); ethylene vinyl acetate copolymer (EVA); Polyamide 6 (PA6); Polyamide 46/polyphenylene oxide (PA46/PPO); ammonium sulfamate (AS); carbon fiber (CF); ZrP-d-MCA , O-ZrP, ZrP-CTBA , S-R-ZrP , ZrP-EA, HAC are Organically modified α-ZrP

In addition, to highlight the effective reinforcement by using exfoliated α-ZrP nano-plates and APP, the flame retardant properties of PVA-APP/2ZrP aerogels in present work are compared with the results of previously reported α-ZrP based polymer composites (Table 4). As we know, the incorporation of α-ZrP can enhance the flame retardant properties of the polymer. Obviously, although addition alone of α-ZrP or modified α-ZrP is conductive to improving the LOI and UL-94 lever of polymers, it could not effectively reduce PHRR and THR [35, 37, 38, 40, 42]. Moreover, the available reported methods on the fabrication of flame-retardant polymers usually required chemical modification [27, 35, 37, 38, 42, 43], or loading of carbonizing agent [26, 36, 39, 41], etc. In this work, our strategy without any chemical modification, the introduction of exfoliated α-ZrP and APP to PVA leads to the remarkable reduction in PHRR (82.6%), THR (76.3%), which are better than most reported results in the literature, indicating the superior flame retardant properties.

15

3.4 Thermal degradation behavior of aerogels The thermal decomposition of the pure PVA aerogel and the flame-retardant PVA aerogels in nitrogen and air are investigated using TGA. Fig 5 shows the TGA and differential thermogravimetric (DTG) curves of all samples, and the corresponding results are shown in Table 5. The thermal degradation of all samples under nitrogen atmosphere contains two stages. It can be observed that both of the initial degradation temperatures (T5%) and the first maximum degradation temperature (Tmax-1) of the flame-retardant PVA aerogels is lower than that of pure PVA aerogel, which may be ascribed to the degradation of APP [41], while the second maximum degradation temperatures (Tmax-2) without obvious change. It is noteworthy that the residue at 800 °C sharply increases with the incorporation of α-ZrP. The residue of PVA-APP/2ZrP aerogel is 35.2%. It should be mentioned that the results of residues of samples in TGA (under nitrogen) is consistent with cone calorimeter (under fire), indicating that the pyrolysis under fire is dominated by an anaerobe decomposition. Under air atmosphere, all the samples show four decomposition stages in the degradation process, which is similar to the result in the literature [44]. The first stage of weight loss is caused by the loss of water, which is most likely present due to the large number of hydroxyl groups on the surface of the aerogel facilitating the absorption of moisture [8, 9]. The second weight loss stage occurs between 200 ℃ and 400 ℃, and is caused by the scission and cyclization of the PVA chains [45]. Furthermore, the APP begins to decompose, forming polyphosphoric acid (PPA) alongside the release of ammonia and water [46]. The third weight loss stage at 16

400−600 ℃, which is caused by the decomposition of polyene. The last weight loss stage occurs between 600 ℃ and 900 ℃, which may be the oxidation of residues. The residue of PVA-APP at 1000 °C is only 6.9 wt%, whereas the residue of PVA-APP/2ZrP is much higher, at 14.4 wt%. Obviously, there is an optimum loading of α-ZrP where a high synergistic effect exists between APP and α-ZrP. The incorporation of excessive α-ZrP leads to the decrease in the thermal stability of material. There are two possible reasons for this phenomenon. On the one hand, the concentration of the added APP is diluted by the incorporation of α-ZrP. Hence, the amount of APP that as the main carbonization ingredient is decreased by the excessive increase of the α-ZrP content. On the other hand, further increasing the loading of α-ZrP may inhibit the swelling behavior and carbonization action of the PVA-APP systems and decrease the residue of aerogel [47, 48].

Fig. 5. TGA and DTG curves of all samples under (a, c) nitrogen and (b, d) air atmosphere.

17

Table 5 TGA results for all samples in nitrogen and air atmosphere. Sample

Nitrogen

Air

T5%

Tmax-1

Tmax-2

Residue at

T5%

Tmax-1

Tmax-2

Tmax-3

Tmax-4

Residue at

(℃)

(℃)

(℃)

800 ℃ (wt%)

(℃)

(℃)

(℃)

(℃)

(℃)

1000 ℃ (wt%)

PVA

264

298

445

5.9

268

311

442

508

-

0

PVA-APP

217

251

447

26.1

214

241

431

517

699

6.9

PVA-APP/0.5ZrP

224

253

445

28.0

212

234

443

509

684

10.0

PVA-APP/ZrP

222

254

444

33.3

216

236

439

524

681

12.3

PVA-APP/2ZrP

220

254

443

35.2

217

237

427

520

688

14.4

PVA-APP/4ZrP

220

254

443

30.8

214

241

433

529

688

10.8

3.5 Analysis of the residue Fig. 6 shows the digital photographs of residues of pure PVA and the flame-retardant PVA aerogels at flame-out after burning under 35 kW/m2. The pure PVA exhibits a small amount of residue after burning and the flame-retardant aerogels are forming a thick and intumescent residue. Usually, the compact residues can contribute to the suppression on burning of polymer, due to its barrier effect on the diffusion of the gaseous volatiles and oxygen. [20] There are some obvious small holes on the outer surface of PVA-APP sample. With the incorporation of α-ZrP, the integrated and compact residues can be observed on the outer surface.

Fig. 6. Photographs of residues of (a) PVA-APP, (b) PVA-APP/0.5ZrP, (c) PVA-APP/ZrP, (d) PVA-APP/2ZrP, and (e) PVA-APP/4ZrP at flame-out after burning under 35 kW/m2.

The microstructures of the outer and inner residues of the PVA-APP and 18

PVA-APP/2ZrP aerogels after cone calorimetry are investigated using SEM (Fig. 7). For the PVA-APP sample, it may be observed from the figure that there is a continuous residue with lots of holes and cracks in the outer and inner residues. On the hand, for the PVA-APP/2ZrP aerogel, both the residues exhibit an integrated and compact structure, with a lamellar microstructure formed on the surface of the residues by the stacking of α-ZrP nano-plates. The microstructure contains Zr4+ as frames, which is non-flammable and has excellent catalytic carbonization properties, with the condensed residues as filler to provide a physical barrier protecting the underlying aerogels. Furthermore, in order to clarify the chemical compositions and distribution of the burning residues in the inner and outer surfaces and further analyze the flame-retardant mode of action, EDX was applied (Fig. 8 and Table 6). As the table shows, the amount of carbon (atom%) in the residues increases with the amount of α-ZrP incorporated into the aerogel matrix, indicating that α-ZrP effectively catalyzes the degradation products of the aerogel into carbon. In addition, the EDX elemental mapping images of the residue shown in Fig. 8 provide evidence of an enriched and well-distributed P, N, and Zr-containing burning residues, which further enhances the thermostability of the material during a fire.

19

Fig. 7. SEM images of the residues of the (a) outer and (b) inner of the PVA-APP and PVA-APP/2ZrP samples after cone calorimetry.

Fig. 8. EDX elemental mapping images of the residues of the (a) outer and (b) inner of the PVA-APP and PVA-APP/2ZrP samples after cone calorimetry. Table 6 Elemental content of the residues of the (a) outer and (b) inner of the PVA-APP and PVA-APP/2ZrP samples after cone calorimetry. Sample

C (at%)

O (at%)

N (at%)

P (at%)

Zr (at%)

PVA-APP-a

46.55

40.24

4.46

8.75

-

PVA-APP-b

23.87

56.79

1.90

17.44

-

PVA-APP/2ZrP-a

76.67

16.19

3.84

2.40

1.62

PVA-APP/2ZrP-b

53.66

4.00

36.24

4.48

0.90

20

The degree of graphitization and the thermally stable residues structures of the carbonaceous materials are investigated using Raman spectroscopy. Fig. 9 shows the Raman spectra of the residues of all the aerogel samples following cone calorimetry. Two main characteristic bands are observed in all the spectra, namely the G band at approximately 1580 cm-1 and the D band at approximately 1350 cm-1. Generally, the ratio of the integrated intensities of the two bands (ID/IG) is used to evaluate the graphitization degree of the residues [49]. A lower value of ID/IG implies a higher degree of graphitization and greater thermal stability of the carbon residue. Here, the ID/IG ratio follows the sequence of PVA-APP/2ZrP < PVA-APP/4ZrP < PVA-APP/ZrP < PVA-APP/0.5ZrP < PVA-APP. The IG/ID ratio of the composite aerogels with α-ZrP loading was significantly lower than that of the PVA-APP aerogel, indicating that α-ZrP enhances the formation of graphitized residues. The high thermal stability of the residues effectively prevents the thermal degradation and flame spread of the aerogel, and promotes the flame retardancy of the material [50]. The Raman spectra shown in Fig. 9 demonstrate that the highest graphitization degree of the PVA-APP/2ZrP aerogel leads to improvements in flame retardancy and thermal stability of aerogel. Therefore, a moderate loading of α-ZrP can improve the carbonization performance of the PVA-APP system.

21

Fig 9. Raman spectra of the residues of (a) PVA-APP, (b) PVA-APP/0.5ZrP, (c) PVA-APP/ZrP, (d) PVA-APP/2ZrP, and (e) PVA-APP/4ZrP following cone calorimetry.

To further investigate the elemental composition in the residues, XPS is conducted. The C 1s, O 1s, N 1s, P 2p, and Zr 3d spectra of the residues for samples PVA-APP and PVA-APP/2ZrP are shown in Figs. 10 and 11, respectively. Comparing between the two samples, it can be seen that for C 1s an additional peak at 282.4 eV is present for the PVA-APP/2ZrP sample, which is attributed to C-Zr; this indicates that the decomposition products of PVA are captured by the Lewis acid sites (Zr4+) of α-ZrP [27]. The binding energies at 284 eV and 284.6 eV are assigned to the C=C and C-C bonds of the graphite-type structure, respectively [51], whereas the band at 285.2 eV is attributed to C-N. Owing to the existence of α-ZrP in the residue of PVA-APP/2ZrP, the area of the peak at 284 eV is significantly larger than the corresponding peak for PVA-APP. This phenomenon demonstrates that the Brønsted acid sites (H+) of α-ZrP are able to catalyze the dehydration, dehydrogenation, 22

cross-linking and cyclization reactions occurring during burning [27]. The O 1s spectrum reveals two peaks at around 531.7 eV (corresponding to C=O and P=O) and 532.9 eV (corresponding to C-O, P-O-P, and C-O-P) [32], suggesting the generation of phosphorus crosslinks. The N 1s spectrum is split into two peaks: 400.6 eV (corresponding to C-N) and 402 eV (corresponding to N-H) [45]. For the P 2p spectrum, the binding energy of 134.2 eV may be attributed to the P-O-C or PO3 groups, which are formed during the generation of phosphorous-rich crosslinks [20]. The residues of PVA-PP/2ZrP reveals the presence of Zr containing molecules, with the regular Zr 3d doublet (at 181.2 eV and 183.7 eV) being coincident with the characteristic absorption band for Zr (IV) [52]. This indicates that incorporating α-ZrP into the PVA system results in excellent catalytic effects, thereby enhancing the carbonation and flame retardancy of the resulting aerogels.

Fig. 10. (a) C 1s, (b) O 1s, (c) N 1s, and (d) P 2p XPS spectra of the residues for sample PVA-APP. 23

Fig. 11. (a) C 1s, (b) O 1s, (c) N 1s, (d) P 2p, and (e) Zr 3d XPS spectra of the residues for sample PVA-APP/2ZrP.

3.6 Flame retardance mechanism On the basis of the results presented so far, a possible flame retardance mechanism (as illustrated in Fig. 12) is proposed to explain the synergy shown between α-ZrP and APP in the PVA aerogels. As the figure shows, the first step in the burning process is the thermal degradation of APP, forming PPA along with the release of ammonia and water, and self-crosslinking reaction of partial PPA (Fig.12 Reaction 1) [8, 46]. After that, the PVA chains are degraded by a dehydration reaction with the phosphate ester (Fig. 12 Reaction 2) [20]. In the two stages, the intumescent residue began to form. Moreover, the incorporation of α-ZrP as a synergistic agent may affect the formation of residues through chemical catalyst effects. α-ZrP is considered to be a layered acidic catalyst, which possesses both Brønsted (H+) and 24

Lewis (Zr4+) acidic sites on its surface [25, 53]. The PVA chain tends to be catalytically carbonized by the former during pyrolysis, while the degradation products of PVA are captured by the latter. The presence of Zr4+ will further accelerate the dehydrogenation, crosslinking, cyclization, condensation and aromatization reactions to form a substantial graphitization residues (Fig. 12 Reaction 3) [26, 27, 54]. Therefore, the presence of α-ZrP can enhance the formation of an integrated and compact intumescent residue in condensed phase during burning, thus effectively inhibit the transfer of heat and mass.

Fig. 12. Possible flame-retardant mechanism for APP-ZrP in PVA aerogels.

4. Conclusion In this work, the PVA/IFR system was designed to use APP as the acid source and blowing agent, PVA as a carbonization agent, and α-ZrP as a catalyst for carbonization. we found that the existence of APP and α-ZrP can synergistically improve the flame retardancy of PVA aerogel. Characterization of the morphology and chemical composition of the aerogels using SEM-EDX techniques revealed that 25

uniform dispersion of α-ZrP and APP in the material was achieved and that the incorporation of α-ZrP resulted in a complete 3D network structure. The fire test of the material with different ratios of α-ZrP and APP were investigated, and it was found that the optimum α-ZrP loading appeared to be 2 wt%, which improved the LOI value of the composite material to 43.1%. Further, all the aerogels containing α-ZrP achieved V-0 in UL-94 classification tests and the PHRR and THR values were reduced significantly, compared to the pure PVA aerogel. And the thermal decomposition behavior suggested that the α-ZrP also can significantly enhance the thermal stability of PVA-APP system. In addition, investigation of the residues showed that the integrated and compact structures formed provided a physical barrier to protect the underlying aerogels during burning. A mechanism was proposed to explain the synergistic combination of APP and in improving the flame-retardant properties of PVA aerogels. Hence, these materials show great potential as building materials.

Acknowledgement This work was supported by a grant from the National Natural Science Foundation of China (No. 21663015, 51603096). References [1] J. He, H.Y. Zhao, X.L. Li, D. Su, H.M. Ji, H.J. Yu, Z.P. Hu, Large-scale and ultra-low thermal conductivity of ZrO2 fibrofelt/ZrO2-SiO2 aerogels composites for thermal insulation, Ceramics International 44(8) (2018) 8742-8748. [2] H.M. Kim, Y.J. Noh, J. Yu, S.Y. Kim, J.R. Youn, Silica aerogel/polyvinyl alcohol (PVA) insulation composites with preserved aerogel pores using interfaces between the superhydrophobic aerogel and hydrophilic PVA solution, Composites Part A: Applied Science and Manufacturing 75 (2015) 39-45. 26

[3] J. Dai, T. Huang, S.Q. Tian, Y.J. Xiao, J.H. Yang, N. Zhang, Y. Wang, Z.W. Zhou, High structure stability and outstanding adsorption performance of graphene oxide aerogel supported by polyvinyl alcohol for waste water treatment, Materials & Design 107 (2016) 187-197. [4] H. Maleki, Recent advances in aerogels for environmental remediation applications: A review, Chemical Engineering Journal 300 (2016) 98-118. [5] J.J. Mao, J. Iocozzia, J.Y. Huang, K. Meng, Y.K. Lai, Z.Q. Lin, Graphene aerogels for efficient energy storage and conversion, Energy Environmental Science 11(4) (2018) 772-799. [6] F.C. Cao, L.L. Ren, X.A. Li, Synthesis of high strength monolithic alumina aerogels at ambient pressure, RSC Advances 5(23) (2015) 18025-18028. [7] V. Gibiat, O. Lefeuvre, T. Woignier, j. Pelous, j. Phalippou, Acoustic properties and potential applications of silica aerogels, Journal of Non-Crystalline Solids 186 (1995) 244-255. [8] J.X. Feng, X.M. Zhang, S.Q. Ma, Z. Xiong, C.Z. Zhang, Y.H. Jiang, J. Zhu, Syntheses of metallic cyclodextrins and their use as synergists in a poly (vinyl alcohol)/intumescent flame retardant system, Industrial & Engineering Chemistry Research 52(8) (2013) 2784-2792. [9] M. Wang, X.W. Han, L. Liu, X.F. Zeng, H.K. Zou, J.X. Wang, J.F. Chen, Transparent aqueous Mg (OH)

2

nanodispersion for transparent and flexible polymer film with enhanced flame-retardant

property, Industrial & Engineering Chemistry Research 54(51) (2015) 12805-12812. [10] L. Wang, M. Sánchez-Soto, M.L. Maspoch, Polymer/clay aerogel composites with flame retardant agents: Mechanical, thermal and fire behavior, Materials & Design 52 (2013) 609-614. [11] Y.T. Wang, S.F. Liao, K. Shang, M.J. Chen, J.Q. Huang, Y.Z. Wang, D.A. Schiraldi, Efficient approach to improving the flame retardancy of poly(vinyl alcohol)/clay aerogels: incorporating piperazine-modified ammonium polyphosphate, ACS Applied Materials & Interfaces 7(3) (2015) 1780-6. [12] P. Shen, H.B. Zhao, W. Huang, H.B. Chen, Poly(vinyl alcohol)/clay aerogel composites with enhanced flame retardancy, RSC Advances 6(111) (2016) 109809-109814. [13] M. Yousefi, E. Noori, D. Ghanbari, M. Salavati-Niasari, T. Gholami, A facile room temperature synthesis of zinc oxide nanostructure and its influence on the flame retardancy of poly vinyl alcohol, Journal of Cluster Science 25(2) (2014) 397-408. [14] V. Swapna, K. Suresh, V. Saranya, M. Rahana, R. Stephen, Thermal properties of poly (vinyl alcohol)(PVA)/halloysite nanotubes reinforced nanocomposites, International Journal of Plastics Technology 19(1) (2015) 124-136. [15] R. Surudžić, A. Janković, M. Mitrić, I. Matić, Z.D. Juranić, L. Živković, V. Mišković-Stanković, K.Y. Rhee, S.J. Park, D. Hui, The effect of graphene loading on mechanical, thermal and biological properties of poly(vinyl alcohol)/graphene nanocomposites, Journal of Industrial and Engineering Chemistry 34 (2016) 250-257. [16] W.W. Guo, J.J. Liu, P. Zhang, L. Song, X. Wang, Y. Hu, Multi-functional hydroxyapatite/polyvinyl alcohol composite aerogels with self-cleaning, superior fire resistance and low thermal conductivity, Composites Science and Technology 158 (2018) 128-136. [17] H.B. Chen, Y.Z. Wang, D.A. Schiraldi, Preparation and flammability of poly(vinyl alcohol) composite aerogels, ACS Applied Materials & Interfaces 6(9) (2014) 6790-6796. [18] M. Goudarzi, D. Ghanbari, M. Salavati Niasari, Room temperature preparation of aluminum hydroxide nanoparticles and flame retardant poly vinyl alcohol nanocomposite, Journal of Nanostructures 5 (2015) 110-115. [19] P.J. Liu, S.B. Bai, Q. Wang, Preparation of aluminum hydroxide/aluminum phosphinate 27

flame-retardant poly(vinyl alcohol) foam through thermal processing, Journal of Applied Polymer Science 132(23) (2015) 42020. [20] C.X. Zhao, Y. Liu, D.Y. Wang, D.L. Wang, Y.Z. Wang, Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly(vinyl alcohol), Polymer Degradation and Stability 93(7) (2008) 1323-1331. [21] J.S. Lin, Y. Liu, D.Y. Wang, Q. Qin, Y.Z. Wang, Poly (vinyl alcohol)/ammonium polyphosphate systems improved simultaneously both fire retardancy and mechanical properties by montmorillonite, Industrial & Engineering Chemistry Research 50(17) (2011) 9998-10005. [22] J. Alongi, A. Frache, Flame retardancy properties of α-zirconium phosphate based composites, Polymer Degradation and Stability 95(9) (2010) 1928-1933. [23] Q.L. Tai, Y.C. Kan, L.J. Chen, W.Y. Xing, Y. Hu, L. Song, Morphologies and thermal properties of flame-retardant polystyrene/α-zirconium phosphate nanocomposites, Reactive and Functional Polymers 70(6) (2010) 340-345. [24] H.L. Xie, X.J. Lai, H.Q. Li, X.R. Zeng, Remarkably improving the fire-safety of polypropylene by synergism of functionalized ZrP nanosheet and N-alkoxy hindered amine, Applied Clay Science 166 (2018) 61-73. [25] H.D. Lu, C.A. Wilkie, M. Ding, L. Song, Thermal properties and flammability performance of poly (vinyl alcohol)/α-zirconium phosphate nanocomposites, Polymer Degradation and Stability 96(5) (2011) 885-891. [26] D.D. Yang, Y. Hu, L. Song, S.B. Nie, S.Q. He, Y.B. Cai, Catalyzing carbonization function of α-ZrP based intumescent fire retardant polypropylene nanocomposites, Polymer Degradation and Stability 93(11) (2008) 2014-2018. [27] H.L. Xie, X.J. Lai, H.Q. Li, X.R. Zeng, Fabrication of ZrP nanosheet decorated macromolecular charring agent and its efficient synergism with ammonium polyphosphate in flame-retarding polypropylene, Composites Part A: Applied Science and Manufacturing 105 (2018) 223-234. [28] Z.Y. Wang, N.M. Han, Y. Wu, X. Liu, X. Shen, Q.B. Zheng, J.K. Kim, Ultrahigh dielectric constant and low loss of highly-aligned graphene aerogel/poly (vinyl alcohol) composites with insulating barriers, Carbon 123 (2017) 385-394. [29] Z.Y. Wang, X. Shen, N.M. Han, X. Liu, Y. Wu, W.J. Ye, J.K. Kim, Ultralow electrical percolation in graphene aerogel/epoxy composites, Chemistry of Materials 28(18) (2016) 6731-6741. [30] X.S. Li, Z.L. Zhao, Y.H. Wang, H. Yan, X.Y. Zhang, B.S. Xu, Highly efficient flame retardant, flexible, and strong adhesive intumescent coating on polypropylene using hyperbranched polyamide, Chemical Engineering Journal 324 (2017) 237-250. [31] T. Gong, Q.Y. Xie, X.Y. Huang, Fire behaviors of flame-retardant cables part I: Decomposition, swelling and spontaneous ignition, Fire Safety Journal 95 (2018) 113-121. [32] B.H. Yuan, A. Fan, M. Yang, X.F. Chen, Y. Hu, C.L. Bao, S.H. Jiang, Y. Niu, Y. Zhang, S. He, H.M. Dai, The effects of graphene on the flammability and fire behavior of intumescent flame retardant polypropylene composites at different flame scenarios, Polymer Degradation and Stability 143 (2017) 42-56. [33] B. Sundström, The Development of a European Fire Classification System for Building Products, Department of Fire Safety Engineering, Lund University, Sweden

(2007).

[34] B. Schartel, T.R. Hull, Development of fire℃retarded materials—interpretation of cone calorimeter data, Fire Materials: An International Journal 31(5) (2007) 327-354. [35] X.Q. Liu, D.Y. Wang, X.L. Wang, L. Chen, Y.Z. Wang, Synthesis of functionalized α-zirconium 28

phosphate modified with intumescent flame retardant and its application in poly (lactic acid), Polymer Degradation and Stability 98(9) (2013) 1731-1737. [36] H.D. Lu, C.A. Wilkie, Study on intumescent flame retarded polystyrene composites with improved flame retardancy, Polymer Degradation and Stability 95(12) (2010) 2388-2395. [37] G.P. Cai, H.D. Lu, S.R. Xu, Z.Z. Wang, C.A. Wilkie, Fire properties of silylated α℃zirconium phosphate composites based on polystyrene, Polymers for Advanced Technologies 24(7) (2013) 646-652. [38] D.D. Yang, Y. Hu, H.T. Li, J.R. Wang, L. Song, H.P. Xu, J.R. Chen, Z.W. Lu, Flammability and carbonization of high-impact polystyrene/α-zirconium phosphate nanocomposites, Iranian Polymer Journal 24(12) (2015) 1069-1075. [39] H.D. Lu, C.A. Wilkie, The influence of α℃zirconium phosphate on fire performance of EVA and PS composites, Polymers for Advanced Technologies 22(7) (2011) 1123-1130. [40] H.X. Xiang, L.L. Li, W. Chen, S.L. Yu, B. Sun, M.F. Zhu, Flame retardancy of polyamide 6 hybrid fibers: Combined effects of α-zirconium phosphate and ammonium sulfamate, Progress in Natural Science: Materials International 27(3) (2017) 369-373. [41] J.C. Ran, J.D. Qiu, H.L. Xie, X.J. Lai, H.Q. Li, X.R. Zeng, Combination effect of zirconium phosphate nanosheet and PU-coated carbon fiber on flame retardancy and thermal behavior of PA46/PPO alloy, Composites Part B: Engineering 166 (2019) 621-632. [42] H. Lu, C.A. Wilkie, M. Ding, L. Song, Thermal properties and flammability performance of poly (vinyl alcohol)/α-zirconium phosphate nanocomposites, Polymer Degradation and Stability 96(5) (2011) 885-891. [43] L.F. Xu, C.H. Lei, R.J. Xu, X.Q. Zhang, F. Zhang, Hybridization of α-zirconium phosphate with hexachlorocyclotriphosphazene and its application in the flame retardant poly (vinyl alcohol) composites, Polymer Degradation and Stability 133 (2016) 378-388. [44] H.F. Pan, X.D. Qian, L.Y. Ma, L. Song, Y. Hu, K.M. Liew, Preparation of a novel biobased flame retardant containing phosphorus and nitrogen and its performance on the flame retardancy and thermal stability of poly (vinyl alcohol), Polymer Degradation and Stability 106 (2014) 47-53. [45] D. Guo, Q. Wang, S.B. Bai, Poly(vinyl alcohol)/melamine phosphate composites prepared through thermal processing: thermal stability and flame retardancy, Polymers for Advanced Technologies 24(3) (2013) 339-347. [46] Z.B. Shao, C. Deng, Y. Tan, M.J. Chen, L. Chen, Y.Z. Wang, An efficient mono-component polymeric intumescent flame retardant for polypropylene: preparation and application, ACS Applied Materials & Interfaces 6(10) (2014) 7363-70. [47] M. Lewin, Synergism and catalysis in flame retardancy of polymers, Polymers for Advanced Technologies 12(3℃4) (2001) 215-222. [48] C.M. Feng, Y. Zhang, S.W. Liu, Z.G. Chi, J.R. Xu, Synergistic effect of La2O3 on the flame retardant properties and the degradation mechanism of a novel PP/IFR system, Polymer Degradation and Stability 97(5) (2012) 707-714. [49] Y.Q. Shi, B. Yu, L.J. Duan, Z. Gui, B.B. Wang, Y. Hu, R.K.K. Yuen, Graphitic carbon nitride/phosphorus-rich aluminum phosphinates hybrids as smoke suppressants and flame retardants for polystyrene, Journal of Hazardous Materials 332 (2017) 87-96. [50] B.H. Yuan, Y. Hu, X.F. Chen, Y.Q. Shi, Y. Niu, Y. Zhang, S. He, H.M. Dai, Dual modification of graphene by polymeric flame retardant and Ni(OH)2 nanosheets for improving flame retardancy of polypropylene, Composites Part A: Applied Science and Manufacturing 100 (2017) 106-117. 29

[51] W.J. Li, X.Z. Tang, H.B. Zhang, Z.G. Jiang, Z.Z. Yu, X.S. Du, Y.W. Mai, Simultaneous surface functionalization and reduction of graphene oxide with octadecylamine for electrically conductive polystyrene composites, Carbon 49(14) (2011) 4724-4730. [52] S. Ardizzone, C.L. Bianchi, XPS characterization of sulphated zirconia catalysts: the role of iron, Surface and Interface Analysis 30(1) (2000) 77-80. [53] Y. Liu, L.G. Chen, T.J. Wang, Y. Xu, Q. Zhang, L.L. Ma, Y.H. Liao, N. Shi, Direct conversion of cellulose into C6 alditols over Ru/C combined with H+-released boron phosphate in an aqueous phase, RSC Advances 4(94) (2014) 52402-52409. [54] R.J. Song, Z.W. Jiang, H.O. Yu, J. Liu, Z.J. Zhang, Q.W. Wang, T. Tang, Strengthening carbon deposition of polyolefin using combined catalyst as a general method for improving fire retardancy, Macromolecular Rapid Communications 29(10) (2008) 789-793.

30

Highlights: • Flame-retardant of composite aerogels was fabricated by freeze drying method. • The flame retardancy and thermal stability were significantly enhanced. • The high synergistic flame-retardant was achieved by the loading of 13 wt% APP and 2 wt% α-ZrP.

Journal: Polymer Degradation and Stability Ms. Ref. No.: PDST-D-19-00487 Title: Synergistic effect of ammonium polyphosphate and -zirconium phosphate in flame-retardant poly (vinyl alcohol) aerogels Author(s): Yong Luo, Delong Xie, Yifan Chen, Tao Han, Renjie Chen, Xinxin Sheng*, Yi Mei** Declaration of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Synergistic effect of ammonium polyphosphate and -zirconium phosphate in flame-retardant poly (vinyl alcohol) aerogels”. Prof. Dr. Yi Mei

Faculty of Chemical Engineering, Kunming University of Science and Technology, Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, The Higher Educational Key Laboratory for Phosphorus Chemical Engineering of Yunnan Province, Kunming 650500, China. E-mail: [email protected]