The preparation of a novel intumescent flame retardant based on

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The preparation of a novel intumescent flame retardant based on supramolecular interactions and its application in polyamide 11 Xiaodong Jin, Jun Sun, Jessica Shiqing Zhang, Xiaoyu Gu, Serge Bourbigot, Hongfei Li, Wufei Tang, and Sheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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ACS Applied Materials & Interfaces

The preparation of a novel intumescent flame retardant based on supramolecular interactions and its application in polyamide 11 Xiaodong Jin a#, Jun Sun a#, Jessica Shiqing Zhang b, Xiaoyu Gu a, Serge Bourbigot c, Hongfei Li a, Wufei Tang a, Sheng Zhang a, d* a

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University

of Chemical Technology, Beijing, 100029, China b

The High School Affiliated to Renmin University of China, No. 37 Zhongguancun Street,

Haidian District, Beijing, 100080, China c

Univ. Lille, CNRS, ENSCL, UMR 8207, UMET, Unité Matériaux et Transformations, F-59

000 Lille, France d

State Key Laboratory of Inorganic-Organic Composites, Beijing University of Chemical

Technology, Beijing, 100029 #

Joint first authors

ABSTRACT The flammability and melt dripping of the widely used bio-based polyamide 11 (PA 11) have attracted much attention in the last decade, and they are still a big challenge for the fire science society. In this work, a novel single macromolecular intumescent flame retardant (AM-APP) which contains an acid source and a gas source was prepared by supramolecular reactions between melamine and p-aminobenzene sulfonic acid, followed by an ionic exchange with ammonium polyphosphate. The chemical structure of AM-APP was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Scanning electron microscopy (SEM). AM-APP and TiO2 were then introduced into PA 11 by melt compounding in order to improve the fire resistance of the composite. The fire performance of PA 11 composites was evaluated by limiting oxygen index (LOI), vertical burning (UL-94) and cone calorimetry tests respectively. The results

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showed that the presence of 22% AM-APP and 3% TiO2 increased the LOI value from 22.2 to 29.2%, upgraded the UL-94 rating from no rating to V-0, completely eliminated the melt dripping, and significantly decreased the peak heat release rate from 943.4 to 177.5 kW/m2. The thermal behaviors were investigated by thermogravimeric (TG) analysis and TG-FTIR. It is suggested that AM-APP produces intumescent char structure and release inert gases, while TiO2 may consolidate the char layers, leading to the improvement for the fire resistance of PA 11. KEYWORDS: polyamide 11, intumescent flame retardant, synthesis, supramolecule, ionic exchange 1. INTRODUCTION Polyamide 11 (PA 11) is a bio-based materials derived from renewable castor oil, 1-4 and has wide applications such as cable sheathing and flexible vehicle hosing. However, the highly flammable nature of PA 11 limits its application in many cases. Efforts have been undertaken to overcome these drawbacks.

5, 6

It has been reported that the presence of 30%

phosphorus-containing flame retardant could upgrade the UL rating to V-0, but only reduced the peak heat release rate (pHRR) of PA 11 by 44%.5 The introduction of montmorillonite (MMT) and carbon nanofiber (CNF) do reduce 30-70% pHRR value of PA 11, but could not improve the UL-94 rating.6 Therefore, it is necessary to develop a more efficient system/formulation to further improve the reaction to fire of PA 11 composites. Given environmental concerns, some halogen-containing flame retardants are gradually being replaced by halogen-free flame retardants. Among them, the most promising candidates are intumescent flame retardants (IFRs), which have been successfully used to improve the reaction to fire of polymers in many cases. 7-10 There are two major kinds of IFRs: physical IFRs and chemical IFRs. During combustion, the neat physical IFRs expand rapidly to form the protective layers without any reactions with other ingredients. 11 In our previous work, a typical physical IFR, expandable graphite, has been successfully employed to improve the reaction to fire of PA 11. The results showed that the limiting oxygen index (LOI) value was increased up to 28.5% and V-0 rating was achieved at UL-94 by the presence of 20% EG.12


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The chemical IFRs involve several chemical reactions with the different ingredients to form the protective char layers during combustion. They are usually constituted of three components: an acid source (e.g. ammonium polyphosphate (APP)), a carbon source (e.g. pentaerythritol (PER)) and a blowing or gas-forming agent (e.g. melamine (MEL)). The most typical chemical IFR is composed of APP, PER and MEL. The introduction of the chemical IFRs into polyolefin (polypropylene or polyethylene) has been successful during the past decades. 13 However, little work has been reported for PA 11/IFR composites due to the low flame retardant efficiency. During the combustion process, the formation of some heterocyclic compounds from PA is not beneficial to produce thermal stable char residues compared with that of the fully hydrocarbonized polyolefins. It was reported that the incorporation of 30% IFR into PA 1010 only decrease the pHRR by 24%. of 36% for PA 11 composites requires the addition of 30% or 40% APP.

14

15

The LOI value

However, in the

case of the synthesized single macromolecular IFRs, the flame retardant efficiency is usually better than that of typical IFR (APP, PER and MEL). With the 25% loading of the synthesized silicon and phosphorus containing FR (TPPSi), the PA 6 composites shows a 78% decrease on pHRR value.

16

Another work shows that the synthesized FR can upgrade the

UL rating of glass-reinforced PA 6 composite from no rating to a V-0 rating and have a LOI value of 30.1%.

17

Therefore, the synthesized single macromolecular IFRs have drawn more

and more attention in the flame retardant PA because of their better efficiency. However, the preparation process of these IFRs is difficult, and usually involves organic solvents. This work reports the synthesis and characterization of a novel intumescent flame retardant prepared by supramolecular reaction between MEL and p-aminobenzene sulfonic acid (ASC), followed by an ionic exchange with APP. The preparation process is regarded as environmental friendly because only water and ethanol have been used as solvent in the process. The synthesized IFR was then introduced into PA 11 to prepare intumescent flame retardant composites. The flame retardant property and thermal stability of PA 11 composites were investigated by LOI, UL-94 vertical burning, cone calorimeter (CONE) tests and thermogravimetric analysis (TGA). The morphology of residue char was observed and the flame retardant mechanisms were also discussed.



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2. EXPERIMENTAL SECTION 2.1. Materials. PA 11 (BESNO-TL) was provided by Arkema, France. APP (n=1000) was purchased from Shandong Jinyingtai Co., Ltd. Melamine (MEL) and p-aminobenzene sulfonic acid (ASC) was offered by Tianjin Fuchen Co., Ltd. Nanometric rutile-type TiO2, with an average particle diameter of 0.17 µm, was provided by Sichuan Longmang Company, China. All materials were dried for 24 h at 80oC before use. 2.2. The preparation of ASC-MEL (AM). AM was prepared by reacting MEL with ASC through π-π stacking and ionic bonding self-assembly. Firstly, MEL (30 g) was dissolved by 700 ml boiling deionized water in a three-necked flask equipped with a stirrer and a thermometer, and the pH value of the solution was then adjusted to 5-6 by 1 mol/L HCl solution. The solution was held at 90°C for half an hour, and then ASC (30 g) was added into the flask. The mixture was stirred at 90°C for about 5 hours, during which the color of solution was changed from colourless to red. Secondly, the solution was cooled to ambient temperature and held for 24 hours during which pale red crystals were precipitated. These crystals were then filtered out and washed with deionized water three times. Finally, the filter cake was dried at 80°C to constant weight (about 12h), yielding ASC-MEL assemblies (AM). The yield was about 90% at this stage. 2.3. The preparation of novel IFR (AM-APP). 60 g of APP was added and dispersed in a three-neck flask by 700 mL mixed solvent containing ethanol and deionized water with a volume ratio of 5:2, and the mixture was then stirred for about 30 min. After that, 50 g ASC-MEL was added into the flask. The mixture was heated to 70°C and held for about 6 hours. The product was then filtered immediately and washed with ethanol for three times before drying at 80oC for 24 hours. The yield was about 90% at this stage. The reaction process is shown in Figure 1. During the first stage, MEL can rapidly change into melaminium (M+) under the acid environment (PH value 4-5), and then subsequently interact with the negative charged ASC, followed by π-π stacking within triazine or benzene ring, leading to the formation of AM. 18 During the second stage, AM-APP was formed by the ionic exchange reactions between NH3+ from AM and NH4+ from APP.


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Figure 1. The preparation processes of AM and AM-APP. 2.4. The preparation of flame retardant PA 11 composites. PA 11 composites were prepared by melt compounding using a micro twin-screw extruder (Wuhan Rayzong Ming Plastics Machinery Co., Ltd, China) with a speed of 60 rpm. The temperature of 215oC, 225oC and 225oC from the hopper to die was used during the extrusion, and the processing temperatures were selected according to the TGA analysis of AM-APP (section 3.4.1), which ensured that AM-APP was thermally stable during the processing. The resulting extrudate was hot-pressed into standard samples under 235oC and 10 MPa for 20 min. The formulations of PA 11 composites are summarized in Table 1. Table 1. Formulations of PA 11 Composites Samples a

PA 11 (wt%)

APP (wt%)

AM (wt%)

AM-APP (wt%)

TiO2 (wt%)

S1 S2 S3 S4 S5 S6 S7 S8

100 80 75 80 75 75 75 75

0 12 15 0 0 0 0 0

0 8 10 0 0 0 0 0

0 0 0 20 25 24 22 20

0 0 0 0 0 1 3 5

a: The additive amount of APP and AM in S2 and S3 samples is in accordance with the corresponding amount of AM/APP in AM-APP used in S4 and S5 respectively.

2.5. Measurement. Fourier transform infrared (FTIR) spectra were recorded by a Nicolet Nexus 670 FTIR spectrometer under the resolution of 1 cm-1 in 128 scans by KBr disk with the wavenumber from 4000 to 500 cm-1. The X-ray photoelectron spectroscopy (XPS) spectra were recorded by an ESCALAB250 (ThermoVG, USA) spectrometer with Al Kα (1486.6 eV) radiation at pass energy of 12 kV and 15 mA. The elemental analysis (EA) was performed by a Vario EL cube elemental analyser from



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Elementar Analysensysteme GmbH (Hanau, Germany) The scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) results were obtained from a Hitachi S-4700 instrument. The beam voltage is 10 kV for fracture surface studies of PA 11 composites and 20 kV for other samples. Limiting oxygen index (LOI) values were measured using a JF-3 oxygen index apparatus (Nanjing Jiangning Analytical Instrument, China) according to ISO 4589. UL-94 vertical burning test was carried out on a Jiangning CZF-5 apparatus (Nanjing Jiangning Analytical Instrument, China) according to ISO 1210. The thickness of sample for UL-94 tests is 3.2 mm. The fire performance of the samples was measured by cone calorimetry (CONE) (Fire Testing Technology, UK) according to ISO 5660. The samples with the dimension of 100 mm × 100 mm × 3 mm were exposed to a radiant cone at a heat flux of 50 kW/m2. The thermogravimetric analysis (TGA) was carried out by a Q50 apparatus from TA Instruments at a heating rate of 10°C/min under N2 or air atmosphere with the temperature range from ambient temperature to 700°C. FTIR-TGA was used to characterize the decomposed gas products of the composites. A transfer steel line with an inner diameter of 1 mm was employed to connect TGA and the infrared cell. Both the transfer line and the gas cell were heated to avoid the condensation of the gas products. The spectra were recorded in the 4000-600 cm-1 range under the resolution of 4 cm-1 in 8 scans. 3. RESULTS AND DISCUSSION 3.1. Characterization of AM and AM-APP.


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Figure 2. FTIR spectra of MEL and AM (a), APP and AM-APP (b). 3.1.1. FTIR analysis. In the FTIR spectrum of MEL (Figure 2a), some distinct characteristic peaks at 3468, 3418 and 3131 cm-1 can be observed which are assigned to amino groups in MEL. However, these peaks shift to 3393 and 3129 cm-1 in the spectrum of AM, and the latter peak has a broad shoulder, indicating the formation of -NH3+-O- ionic bonds.19 Another characteristic peak of NH2 in MEL is located at 1651 cm-1, and it shifts to 1677 cm-1 in AM, which is due to the ionic interactions among the -NH2 groups.20 Moreover, the peak of triazine ring is shifted from 814 cm-1 of MEL to 788 cm-1 of AM, indicating the protonation of a ring nitrogen.19 For the other peaks in ASC spectrum, the peaks at 1912 and 1346 cm-1 are assigned to benzene, and the peaks at 1629, 1207 and 1159 cm-1 stands for NH2, S=O and S-O respectively. In the spectra of AM-APP (Figure 2b), some additional characteristic peaks of AM can be observed. Those additional peaks around 1675, 1251, and 788 cm-1 are attributed to NH2, S=O and triazine ring from AM respectively. Besides, the appearance of NH3+ bands at 3364, 3156 cm-1 and reduction of peaks intensities associated with NH4+ bands in the range of 3450 and 3030 cm-1 demonstrate the existence of -NH3+-O-P- bonds formed by reactions between AM and APP . 21



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Figure 3. XPS spectra of MEL and AM (a), APP and AM-APP (b).

Figure 4. N1s spectra of AM (a) and AM-APP (b). 3.1.2. XPS analysis. XPS spectra of different samples are shown in Figure 3. It can be seen that the characteristic peaks of MEL still exist in AM (Figure 3a). Furthermore, additional peaks at 168.1 and 531.6 ev can be observed, which are assigned to S2p and O1s of ASC. Similarly, it can be seen that all the characteristic peaks of APP can be found in AM-APP (Figure 3b). Moreover, the peak intensity of N1s in AM-APP sample is much stronger than that of APP, which is due to the introduction of AM. N1s spectra of AM (a) and AM-APP (b) are shown in Figure 4, and the N1s peak region can be fitted by several peaks according to the different binding energies of the relevant chemical bonds. For AM, the peak at 399.6 eV is assigned to the nitrogen in N=C double bonds of MEL, and the peak at 400.8 eV is assigned to the NH2 in ASC. Particularly, the peak at 400.5 eV corresponds to the NH3+ from ionic bonds formed by the reaction between ASC and MEL. For AM-APP, the peaks of N=C at 399.6 eV and the ionic bond -NH3+-O-S- at 400.5 eV can still be observed. After reacting with AM, the peak assigned to NH2 at 400.8 eV has


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disappeared, and an additional peak at 400.3 eV can be observed, which should be attributed to the formation of NH3+ in the -NH3+-O-P- by the ionic exchange between NH4+ in APP and NH3+ in AM. The peak at 401.2 eV is assigned to NH4+ from APP, which is consistent with literature results.22 In brief, the FTIR and XPS results confirm that AM-APP has been successfully prepared by the procedure shown in Figure 1. 3.1.3. Elemental analysis. The EA results of AM are given in Table 2. One can see that the experimental values of carbon, nitrogen, sulfur and hydrogen are nearly coincident with their theoretical values, indicating the successful preparation of AM as shown in Figure 1. Table 2. EA results of AM Content (wt%) Experimental values Theoretical values

C

N

S

O

H

38.08 39.07

21.53 19.54

13.92 14.88

21.72 22.32

4.75 4.19

Figure 5. SEM images of AM (a), APP (b) and AM-APP (c). 3.1.4. Surface morphology and dispersion. Figure 5 shows the surface morphology of AM (a), APP (b) and AM-APP (c). The morphology of the AM crystal shows cubic structure (Figure 5a). The pristine APP shows a smooth surface morphology (Figure 5b). However, the surface of AM-APP (Figure 5c) becomes rough and amorphous because of the randomness of ionic exchanges between NH3+ from AM and NH4+ along the surface of APP. Overall, from the discussions above (FTIR and XPS), it can be concluded that AM-APP has been successfully synthesized. Therefore, we can conclude the variation of particle morphology amounts to further evidence of the successful preparation of AM-APP.



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Figure 6. SEM images of the fracture surfaces of PA 11/APP/AM (a) and PA 11/AM-APP (b).

Figure 7. Proposed dispersion model of PA 11/APP/AM (a) and PA 11/AM-APP (b) samples. In order to investigate the dispersion and the interface compatibility between the flame retardants and the polymeric matrix, the morphologies of fractured sections of PA 11/APP/AM, PA 11/AM-APP composites were observed by SEM and the results are shown in Figure 6. Aggregates can be observed for the sample containing APP and AM (Figure 6a) and it is likely that AM tends to form self-aggregates because of the electrostatic attraction or π-π stacking of benzene and triazine rings in the molecular of AM during the extrusion process, leading to the immiscibility of AM/APP and decrease of interface compatibility with PA 11. For AM-APP (Figure 6b), the self-aggregation of AM-APP is significantly reduced as seen from the minor presence of aggregates in Figure 6b. The AM molecules are ionically bound to the APP molecular chains, which can restrain the aggregation of AM. The proposed dispersion models of flame retardants in the PA 11 matrix are illustrated in Figure 7.


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3.2. Flame retardancy. Table 3. LOI and UL-94 Tests Results of PA 11 and PA 11 Composites Samples S1 (PA 11) S2 (PA 11/12%APP/8%AM) S3 (PA 11/15%APP/10%AM) S4 (PA 11/20%AM-APP) S5 (PA 11/25%AM-APP) S6 (PA 11/24%AM-APP/1%TiO2) S7 (PA 11/22%AM-APP/3%TiO2) S8 (PA 11/20%AM-APP/5%TiO2)

LOI (%)

∆LOI (%)

22.2±0.3 24.5±0.3 24.9±0.3 26.2 ±0.3 28.3±0.5 28.9±0.4 29.2±0.4 28.1±0.3

0 2.5 2.7 4.0 6.1 6.7 7.0 5.9

UL-94 t1/t2

a

15.4/7.8/24.2 3.7/17.2 3.4/12.7 2.1/6.3 5.2/10.4

Dripping

Rating

Y Y Y Y Y N N Y

F F F V-2 V-2 V-1 V-0 V-2

b

a: t1, t2 represent the after flame time after the first and second 10 s flame application, and “-” means the sample burns completely. b: F means the sample cannot pass any rating during the UL-94 test.

3.2.1. LOI and UL-94 test results. Table 3 lists the LOI values and UL-94 ratings of PA 11 and PA 11 composites. The LOI value of neat PA 11 is 22.2%, and it is slightly increased to 24.9% for the sample containing 15% APP and 10% AM. In consideration of the 25% additive amount, it can be concluded that the use of both APP and AM are not very effective in enhancing the fire resistance of PA 11. However, the LOI values of PA 11 containing AM-APP (26.2% for S4, 28.3% for S5) are higher than those of samples containing APP and AM together ( 450 oC), more thermal stable char residues can be formed by the interactions between the degradation products of AM and the polyphosphoric acids.

Table 5. FTIR Spectra of AM-APP at Different Temperature Temperature (oC)

Absorption range (cm-1)

Groups

Analysis

3400

NH

The degradation of -NH3+-O-P- with the release of H2O: normally, APP or synthesized FRs based on APP releases NH3 during the first degradation process; however, AM-APP also releases H2O at the initial degradation process.

3359 3160

NH3+

The incomplete degradation of -NH3+-O-P-.

3071

P-OH

The degradation of APP chains into P-OH with the release of NH3.

1675

NH2

1250

S=O

The existence of these peaks combined with the disappearance of NH3+ at 1513 cm-1 show that the separation of AM into benzenesulfonic acid and melamine.

3169

P-NH2

The complete desorption of AM from AM-APP.

3359 3160 1513

NH3+

The degradation of AM-APP along with the release of NH3.

1002 770

P-N-C ring

The char residues are cross-linked by P-N-C. The formation of melem and melon.

1675

NH2

The formation of the melem and melon, and

1002

P-N-C

the char residues are cross-linked by P-N-C.

250

400

500 and 600



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Figure 14. The possible degradation process of AM-APP. 3.4.3. Char structure analysis. Figure 15 shows the SEM-EDX results of outer and inner char residues of S3, S5 and S7 samples. Some open voids can be found in the outer char residues of S3 (Figure 15a) and S5 (Figure 15b) samples, which cannot effectively isolate the mass and heat transfer. The poor protection of char layers also leads to the low C content in the inner char residue of PA 11 composites (Figure 15a’ and b’). For S7 sample, the char layers of both outer and inner display totally different morphologies. Dense, compact and continuous char layers can be observed, which can provide an excellent barrier effect. As a result, the C content in the inner char of S7 sample reaches 42.4%. As discussed earlier, AM-APP will release H2O during the decomposition, and the peptide linkages of PA 11 molecular chain can be easily hydrolyzed by the presence of H2O, leading to a drastic decrease in PA 11 molecular mass and an increase in polymeric end-group contents. 33 The reaction is illustrated scheme 1.


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Figure 15. SEM-EDX results of outer and inner char residues of S3 (a, a’), S5 (b, b’) and S7 OOOO

OOOO

OOOO

(c, c’) samples.

HHHH NNNN

CCCC 9999

2222

HHHH CCCC

2222

HHHH CCCC

HHHH NNNN

CCCC0000 1111 2222 HHHH CCCC

HHHH NNNN

CCCC

OOOO 2222 HHHH ffff oooo eeee cccc nnnn eeee tttt ssss iiii xxxx eeee eeee hhhh tttt yyyy BBBB OOOO

OOOO

OOOO

HHHH NNNN

CCCC 9999

2222

HHHH CCCC

2222

HHHH CCCC

NNNN 2222 HHHH

HHHH OOOO

CCCC 0000 1111 2222 HHHH CCCC

HHHH NNNN

CCCC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(1) These end-groups contained segments can further participate in the char forming process with the degradation products of AM-APP such as polyphosphoric acids, leading to the formation of cross-linked network. The outer char residues of S7 sample (Figure 15c) is further consolidated by the migration of TiO2 during combustion. According to the previous investigation, 34 TiO2 can react with the nitrogenous and phosphorus compound in the char



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layers at high temperature with the formation of the stable titanic oxide and the increased amount of phosphides in the condensed phase, which leads to the highly cross-linked thermal stable P contained network and hence effectively isolate the matrix from heat and oxygen. The reaction can also release NH3 and H2O, as shown in scheme 2. It can also be found that the contents of Ti (wt%) in outer and inner char layers of S7 sample are 9.3 and 2.7 wt% respectively (Figure 15c’), which is due to the migration of TiO2 into the surface, demonstrating the TiO2 can also reinforce the residues by the physical migration processes. 2TiO2 + (NH4)4P4O12

2TiP2O7 + 4NH3 + 2H2O

(2)

The possible flame retardant mechanism is proposed in Figure 16. During stage I (< 250oC), AM-APP first releases H2O, which further promotes the degradation of PA 11 by hydrolyzing the amide groups of PA11, leading to the increased contents of the end-groups containing segments. These segments further cross-link with the degradation products of AM-APP, leading to the formation of cross-linked network. During stage II (250-450oC), more non-flammable gases can be released to participate in the blowing because of the introduction of AM. These incombustible gases can offer effective protection to the substrate through dilution of the fuel and cooling effect caused by the endothermic process. Meanwhile, the TiO2 will migrate to the surface to strengthen the protective barriers by both physical and chemical interactions. Therefore, these barriers are strong enough to withstand the pressure induced by the gases release, leading to high expansion height of char layer. During stage III (> 450 oC), these strengthened char residues act as the physical barriers, separating the polymeric matrix from heat and oxygen. Overall, the AM-APP improves the flame retardant properties of PA 11 through both the gas phase and the condensed phase mechanism.


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Figure 16. The possible flame retardant mechanism of AM-APP and TiO2 in PA 11. 4. CONCLUSIONS A novel single macromolecular intumescent flame retardant, AM-APP, has been successfully prepared by supramolecular reaction and ionic exchange. The flame retardancy of PA 11 was significantly improved by the presence of AM-APP and TiO2. For the PA 11 composite sample containing 22% AM-APP and 3% TiO2, the LOI increased to 29.2%, UL-94 test rating was upgraded to V-0, and the peak heat release rate reduced to only 177.5 kW/m2. The introduction of AM-APP altered the thermal degradation pathway of PA 11, and increased the residue amount to 21.5% at 700oC. It is suggested that AM-APP mainly takes effects by releasing non-flammable gases and promoting char formation, while the TiO2 can consolidate the char layers through both the physical migration to the surface and the chemical interactions with (NH4)4P4O12.

AUTHOR INFORMATION Corresponding Author *Tel: +86 (10) 64436820.

E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The current work was financially supported by National Natural Science Foundation of China (Grant No. 21674008).



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Intumescent Flame Retardant


for Polypropylene:


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Table of Contents:


The preparation of a novel intumescent flame retardant based on

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