Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins

Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins

Accepted Manuscript Title: Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity s...

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Accepted Manuscript Title: Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins Author: Shuilai Qiu Xin Wang Bin Yu Xiaming Feng Xiaowei Mu Richard K.K. Yuen Yuan Hu PII: DOI: Reference:

S0304-3894(16)31087-1 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.11.057 HAZMAT 18211

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

30-7-2016 16-11-2016 19-11-2016

Please cite this article as: Shuilai Qiu, Xin Wang, Bin Yu, Xiaming Feng, Xiaowei Mu, Richard K.K.Yuen, Yuan Hu, Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.11.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins Shuilai Qiu ab, Xin Wang *a, Bin Yu ab, Xiaming Feng ab, Xiaowei Mu a, Richard K. K. Yuen bc and Yuan Hu *ab

a

State Key Laboratory of Fire Science, University of Science and Technology of

China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China b

USTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban

Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, P. R. China c

Department of Architecture and Civil Engineering, City University of Hong Kong,

Tat Chee Avenue, Kowloon, Hong Kong

Corresponding Authors *Yuan Hu. Fax/Tel: +86-551-63601664. E-mail: [email protected] *Xin Wang. Fax/Tel: +86-551-63601664. E-mail: [email protected];

Graphical Abstract

Highlights  

DOPO-based FR wrapped PZS nanotubes were synthesized via one-pot method. EP/[email protected] showed significant decrease in pHRR, THR and the amount of CO.



Incorporation of [email protected] provided a compact and stable char layer.



The barrier effect of distributed PZS network retards the heat and mass transfer.

Abstract: The structure of polyphosphazene nanotubes (PZS) is similar to that of carbon nanotubes (CNTs) before modification. For applications of CNTs in polymer composites, surface wrapping is an economically attractive route to achieve functionalized nanotubes. Based on this idea, functionalized polyphosphazene nanotubes ([email protected]) wrapped with a cross-linked DOPO-based flame retardant (FR) were synthesized via one-step strategy and well characterized. Then, the obtained [email protected] was introduced into epoxy resin (EP) to investigate flame retardancy and smoke toxicity suppression performance. Thermogravimetric analysis indicated that [email protected] significantly enhanced the thermal stability of EP composites. Cone calorimeter results revealed that incorporation of [email protected] obviously improved flame retardant performance of EP, for example, 46.0% decrease in peak heat release rate and 27.1% reduction in total heat release were observed in the case of epoxy composite with 3 wt% [email protected] The evolution of toxic CO and other volatile products from the EP decomposition was significantly suppressed after the introduction of [email protected], Therefore, the smoke toxicity associates with burning EP was reduced. The presence of both PZS and a DOPO-based flame retardant was probably responsible for this substantial diminishment of fire hazard.

Keywords: polyphosphazene nanotubes; flame retardant; nanocomposites; toxicity suppression; fire hazard

1. Introduction Epoxy resin (EP) is a widely used thermosetting material, which has an extensive range of application in laminating, potting, adhesive and coating areas, owing to its excellent chemical and mechanical properties [1]. However, the limitations of high flammability and the generation of a large amount of toxic gas and smoke during combustion

extremely

restrict

its

application

in

some

fields

such

as

electrical/electronic devices. To solve this issue, numerous strategies such as filler addition and chemical modification, have been utilized to enhance the fire retardancy of EP. Incorporation of nano-additives such as graphene, MoS2, carbon nanotubes and boron nitride, can improve the fire performance of EP [2-5]. The fire safety of OapPOSS-rGO/EP composites has been investigated that the incorporation of 2.0 wt% OapPOSS-rGO to EP brings about a 58% decrease in CO production rate, a 49% decrease in peak heat release rate (PHRR) and a 37% reduction in total heat release (THR) for combustion [6]. A modified layered double hydroxide has been used to fabricate a multi-modifier system, sCD-DBS-T-LDH, by addition of 6 wt% sCD-DBS-T-LDH in EP. The PHRR and THR values for the EP containing these additives were significantly lower by 66% and 34%, respectively, with respect to the same properties for EP alone [7]. Various kinds of nanofillers as flame retardant additives have been reviewed recently, as also summarized in Table S1. Polyphosphazenes are a family of versatile organic–inorganic hybrid materials that possess many superior properties [14,15]. It is well known that polyphosphazenes have been applied as biomaterials, optical materials, electrical materials, hybrid materials, etc., due to their outstanding thermal stability and structural diversity [16,17]. With the similar structure to carbon nanotubes, polyphosphazenes have attractive potential as flame-retardant additives. These materials exhibit high limiting oxygen index, which can be divided into three polymer types: linear polyphosphazenes or phosphazene polymers with cyclophosphazene units in main chain or side chain [18]. For linear polyphosphazenes, the main disadvantages such as

low output and high cost, restrict their application [19,20]. Another kind of polyphosphazenes that has attracted considerable interest is cyclotriphosphazene, these materials possess a main chain of -P=N- units and exhibit self-extinction in flame tests [21]. Cyclotriphosphazenes may be utilized to synthesize micro- or nanoscale polymeric materials by condensation polymerization. These include polyphosphazene nanotubes, microspheres, nanofibers and nanochains [22,23]. Poly(cyclotriphosphazene-co-4,4’-sulfonyldiphenol) nanotubes (PZS), which were fabricated via a simple one-pot strategy with controllable morphology [24]. These synthesized nanomaterials may display superior thermal stability, flame retardancy and radiation resistance. Furthermore, amino or hydroxyl active groups may be achieved by selecting appropriate monomers and reaction conditions [25]. Combined with the tremendous controllability of the -P=N- units, the resulting PZS can be easily modified via covalent or noncovalent techniques. To illustrate, epoxy-group modified polyphosphazene nanotubes (EPPZTs) were introduced into EP to reinforce the matrix as a carbon nanotube (CNT) effect [26]. There are few previous reports about flame retardant impact of polyphosphazene nanomaterials in polymer matrices. Phosphorus-based flame retardants (FRs) are one of most efficient members of the versatile halogen-free FRs, which can act through a combination of vapor- and condensed-phase reactions [27,28]. They impart flame retardancy to polymers by promoting carbonization and char formation. Decomposition of the additive produces phosphoric acid which promotes cationic crosslinking. The surface char inhibits heat feedback to promote polymer pyrolysis and the formation of fuel fragments [29]. Thus 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

(DOPO),

as

a

efficient

phosphorus-based flame retardant (FR), has attracted increasing interests owing to its high thermal stability [30,31]. It is well known that DOPO and its derivatives mainly act in vapor-phase process, which might be augmented with a condensed-phase action through the introduction of designated functionalities in DOPO moiety [32,33]. Although the performance of DOPO-based FRs has been gratifying, some questions remain unanswered. For example, how may other FRs be combined with DOPO to further improve flame retardant efficiency. In recent years, the introduction of low levels of FRs on the surface of

nanomaterials to produce highly efficient flame retardant formulations has attracted interest. For example, the addition of both graphene nanosheets (GNS) and DOPO combined the advantages of thermal stability of GNS and flame retardant performance of DOPO, high flame retardant efficiency was achieved at low loading of additive [34]. In another example, specific phosphorus-containing multi-walled carbon nanotubes (PMWCNTs) were prepared via a ring-cleavage reaction between DOPO and MWCNTs. The PMWCNT/CE compositions display good flame retardant performance and smoke suppression [35]. The DOPO, POM and POSS have been combined to form a multi-element formulation that is an effective flame retardant in an epoxy matrix, An LOI value of 34.5% was observed for epoxy containing 20 wt% additives [36]. Since PZS and DOPO-based FR play roles in different combustion stages with different flame retardant action, the simultaneous integration of PZS and DOPO-based FR may combine the advantages of thermal stability of PZS and the flame retardant effect of DOPO-based FR. In this work, the DOPO-based FR, using the DOPO-HQ and phosphorus oxychloride as monomers, was synthesized via in situ condensation polymerization method, in the presence of the surface of PZS. The resultant flame-retardant-wrapped PZS ([email protected]) with the active hydroxyl groups on the surface was expected to greatly decrease the interfacial tension between [email protected] and EP matrix. The effect of PZS combined with the DOPO-based FR on the flame retardant and smoke toxicity suppression performances of EP were investigated.

2. Experimental section 2.1. Materials. Anhydrous

ethanol,

triethylamine

(TEA),

tetrahydrofuran

(THF),

4,4′-diaminodiphenylmethane (DDM), acetone and p-benzoquinone were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Phosphorus oxychloride (POCl3) was purchased from Dong-fang Chemical Co. Ltd (Beijing, China). Hexachlorocyclotriphosphazene (HCCP) and 4,4’-sulfonyldiphenol (BPS) were purchased from Aldrich Chemical Co. Ltd. (U.S.). DOPO was supplied by Shandong Mingshan Fine Chemical Industry Co. Ltd (China). EP (DGEBA, E-44) was provided

by Anhui Jiangfeng Chemical Industry Co. Ltd. (China). 2.2. Fabrication of PZS nanotubes with hydroxyl groups [37] Firstly, TEA (4.16 g, 41.2 mmol) and given amount of BPS were dissolved in 250 mL THF, and then added into a 500 mL three-necked flask, which equipped with dropping funnel and mechanical stirrer. Then HCCP (2.4 g, 6.9 mmol) dissolved in 100 mL THF was added dropwise to the flask during 1 h. The above system was stirred under ultrasonication for an additional 6 h, and the temperature was controlled at 40 oC accurately. After completion of the reaction, the solvent was removed and the precipitate product was washed with anhydrous ethanol and deionized water, respectively. Finally, the solid products were dried under vacuum at 60 oC. 2.3. Fabrication of flame retardant wrapped PZS nanotubes ([email protected]) DOPO-HQ was synthesized from DOPO according to the method in previous work [38]. [email protected] was fabricated by a simple one-pot strategy, shown in Scheme 1. Firstly, 1.0 g of PZS, 0.26 g of POCl3 and extra TEA were added to a 500 mL flask filled with 300 mL THF, and then conducted with ultrasonication (53 kHz) under room temperature for 0.5 h. Subsequently, 0.59 g of DOPO-HQ dissolved in 10 mL of THF was added dropwise into the flask during 2 h. Afterwards, the mixture was controlled at 70 oC with ultrasonication for additional 12 h. Eventually, the obtained product was filtered and washed with THF and anhydrous ethanol for three times, respectively, and then dried under vacuum at 40 °C overnight. 2.4. Preparation of EP/[email protected] nanocomposites Typically, preparation process of EP composite with 3 wt% [email protected] loading was illustrated below: 1.35 g of [email protected] was dispersed in 30 mL of acetone solution under ultrasonication for 1 h. Subsequently, 35.85 g of EP was poured into the above mixture with mechanical stirring for 2 h. After that, the solvent was removed in a drying oven at 80 oC for 6 h. Subsequently, 7.80 g of DDM was melt and mixed with the above mixture by a vigorous stirring for 1 min. Finally, the sample named as EP/[email protected] was cured at 100 °C for 2 h and then 150 °C for 2 h. After the completion of curing process, the sample was allowed to cool to room temperature. For the preparation of pure EP, EP/[email protected] (0.5 wt%), EP/[email protected] (1.0 wt%) and EP/PZS3.0 (3.0 wt%) composites, a similar process was utilized except the

variation of the nanoadditives. 2.5. Characterization Fourier transform infrared (FTIR) spectra were conducted on a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). X-ray diffraction (XRD) were performed on an X-ray diffractometer (Rigaku Co., Japan), using Cu Kα radiation (λ= 0.15418 nm). X-Ray photoelectron spectroscopy (XPS) spectra were obtained from a VG ESCALAB MK-II electron spectrometer (V.G. Scientific Ltd., UK). Transmission electron microscopy (TEM) was carried out using a JEM-2100F transmission electron microscopy (Japan Electron Optics Laboratory Co., Ltd., Japan). Thermogravimetric analysis (TGA) was performed on a Q5000 thermo-analyzer instrument (TA Instruments Inc., USA), at a linear heating rate of 10 oC min-1 from 20 to 800 oC under nitrogen atmospheres. The fire performance of EP and its nanocomposites were conducted on a cone calorimeter based on ASTM E1354/ISO 5660. Every specimen was exposed horizontally under a heat flux of 35 kW/m2. Microstructures of the residual char and fracture surface were investigated by high-resolution JEOL JSM-6700 field-emission scanning electron microscopy (FE-SEM). The structure and components of the residual char of EP composites were evaluated by a LABRAM-HR laser confocal microRaman spectrometer (Jobin Yvon Co., Ltd., France) with an argon laser of 514.5 nm. Thermogravimetric analysis/infrared spectrometry (TG-IR) was carried out on a TGA Q5000 thermogravimetric analyzer, which used a stainless steel transfer pipe to combined with a Nicolet 6700 FT-IR spectrophotometer. The real time Fourier transform infrared spectra (RTFTIR) were conducted using a Nicolet MAGNA-IR 750 spectrophotometer.

3. Results and discussion 3.1. Characterization of [email protected] nanotubes TEM and FE-SEM images of PZS and [email protected] are shown in Fig. 1, which provides information on the morphology and size of PZS and its derivatives. As shown in TEM images of PZS (Fig. 1A) and [email protected] nanotubes (Fig. 1B and 1C), the neat PZS and [email protected] nanotubes present hollow tube structures and closed tube terminals. The length of these tubes is estimated to be several micrometers. From the

SEM images of PZS (Fig. 1D) and [email protected] nanotubes (Fig. 1E), it can be observed that the neat PZS and [email protected] with fiber shape and entangle with each other. Moreover, the surfaces of neat PZS nanotubes are clean and smooth. For the [email protected], the surfaces are relatively rough, wrapped by a layer of FR as visible extra phase (Fig. 1B and E). From the SEM images, it is distinct that the diameters of nanotubes increase from PZS to [email protected] The diameter of PZS is 40-50 nm, while the diameter of [email protected] is 50-60 nm. Clearly, TEM and FE-SEM results demonstrate that DOPO-based flame retardants are successfully wrapped on the surface of PZS. FTIR analysis provides crucial structural information of PZS and [email protected] nanotubes, as presented in Fig. 2A. From the IR spectrum of pure PZS, it can be observed that two distinct peaks at 1488 and 1589 cm−1 are attributed to the absorption of C=C groups in the benzene ring units. The characteristic peaks at 1153 and 1293 cm -1 correspond to O=S=O groups. Besides, The absorption peaks at 883 and 1186 cm-1 are assigned to the stretching vibration of P–N and P=N groups, respectively. The absorption peak for P–O–Ar groups appears at 941 cm-1. After the wrapping of PZS with DOPO-based FR, the new absorption peak at 1240 cm-1 associated to P=O, along with the peak at 1041 cm-1 assigned to P-O-C, is presented in the spectrum of [email protected], indirectly revealing that PZS was wrapped by FR. XPS offers diversified information about surface composition and chemical state of PZS and [email protected], which can further figure out their structures. Fig. 2B-2D presents XPS survey scans of PZS and [email protected]; C1s XPS spectra of PZS and [email protected] Obviously, the surface of neat PZS were composed of C, O, N, P, and S elements, meanwhile, the atomic percentage of N, P, and S is 5.3%, 6.0% and 4.6%, respectively. From the XPS survey scans of [email protected] (Fig. 2B), [email protected] shows increased intensity in the P2p and P2s peak compared to PZS. These results are attributed to the wrapping of DOPO-based FR on the surfaces of PZS during the condensation polymerization. As can be observed from Fig. 2C, the C1s XPS spectrum of PZS is deconvoluted into several carbon series, containing C-C (284.6 eV), C-O-P (285.0 eV), C-O (286.6 eV) and C-S (287.0 eV). After the surface wrapping reaction, the C1s spectrum of [email protected] (Fig. 2D) exhibits a newly appeared peak at 286.1 eV assigned to the C-P bond in the DOPO-HQ units. Moreover, the

peak at 291.7 eV is associated to the π–π* shake-up satellite of aromatic structure or conjugated systems. Moreover, in the C1s spectrum of [email protected], the band related to C-S is not perceptible, because of the surface wrapping effect of FR. XRD pattern of PZS and [email protected] further support these results. From Fig. 3A, it can be observed that a broad peak at 2θ values of 15.0° is assigned to the reflection peak of neat PZS [37]. However, after PZS nanotubes were functionalized with FR, XRD pattern of [email protected] is similar to neat PZS except that the intensity of the peak becomes weaker, indicating the successful modification. The results above indicate that PZS was successfully wrapped by DOPO-based FR. 3.2. Thermal properties of EP and its Composites The thermal properties of PZS and [email protected] were investigated by TGA in nitrogen. The temperatures at maximum mass loss at which 5% mass loss occurs are defined as the maximum degradation temperature (Tmax) and the initial degradation temperature (T−5%), respectively. As can be observed in Fig. 3B, neat PZS shows a one-step weight loss under nitrogen. The T−5% of neat PZS is over 480 °C, with a char yield of 58 wt% at 800 °C, indicating that the PZS possesses outstanding thermal stability. The [email protected] has a two-stage decomposition process. The first stage from 240 to 360 °C is assigned to the degradation of unstable DOPO-based FR, while the higher temperature stage is ascribed to the decomposition of PZS. TGA and DTG curves of EP and its nanocomposites under nitrogen are presented in Fig. 4. The detailed data from TGA curves are shown in Table S2. EP/[email protected] nanocomposites presents a one-stage degradation process (Fig. 4A), exhibiting the similar decomposition behaviors to bare EP. The T−5% is decreased with the incorporation of [email protected], and the earlier degradation of FR in EP matrix shifts the T−5% to relatively lower temperature, which accelerates the decomposition of the EP matrix. As for the char residue at 800 °C, the char yield value of bare EP is approximately 13.9%, which is significantly increased with the introduction of [email protected] Furthermore, EP/[email protected] exhibits higher char residue than that of EP/PZS3.0, owing to the catalytic charring effect of FR and polyphosphazene substrate. The high char residues are beneficial to the inhibition of the oxygen exchange and heat and mass transfer between vapor- and condensed phase. The DTG

curves (Fig. 4B) indicate that the addition of [email protected] distinctly reduces the maximum mass loss rate of EP nanocomposites during the decomposition process. The improvement is attributed to two causes: the random distributed PZS tangle with each other and form crosslinking network structure, which can acts as physical barrier to efficiently retard the heat and mass transfer; the formation of compact char residues suppress the release of pyrolysis volatiles. Compared to EP/PZS3.0, EP/[email protected] presents a slightly lower T−5%, with higher char residue at 800 °C and lower maximum mass loss rate. The catalyzing effect of FR leads to the lower thermal stability of EP/[email protected] at relative low temperature, however, the higher residual char of EP/[email protected] results in lower mass loss rate. To understand the compatibility between [email protected] nanotubes and EP matrix, the freeze-fractured surface microstructures of EP composites were evaluated by FE-SEM. The fractured surface roughness of the polymer nanocomposites reflects the dispersion level and interfacial interaction. From Fig. 5A and 5B, it can be clearly observed that the pure EP shows a smooth fractured surface. The fractured surfaces of EP/PZS (Fig. 5C and 5D) and EP/[email protected] (Fig. 5E and 5F) are much rougher than that of pure EP. No obvious pulled-out PZS and [email protected] nanotubes is under observation and obvious “scratch” can be observed, which is attributed to the strong interfacial interaction between nanoadditive and matrix. In addition, it is observed that several [email protected] nanotubes are embedded on the fracture surfaces of EP matrix uniformly. Nevertheless, a few nanotubes agglomerates in the SEM images of EP/[email protected] sample are also observed (Fig. 5F). 3.3. Fire performance of EP and its composites. As is well known, cone calorimeter is a commonly used tool for evaluating combustion performance of various materials under real-world fire condition. The HRR and THR vs. time curves of EP composites are presented in Fig. 6, and several important parameters including PHRR, THR, the time to peak heat release rate (TPHRR), maximum average heat rate emission (MAHRE), the peak smoke production rate (PSPR) and the total smoke release (TSR) values from cone calorimeter are summarized in Table S3. The pure EP is highly flammable, with a PHRR value of 1820.7 kW/m2. An apparent reduction in the PHRR can be observed for the

EP/PZS3.0 sample, a 36.7% reduction compared to pure EP. Introducing [email protected] into EP further apparently decreases the PHRR values. As a result, the PHRR of EP/[email protected] is decreased by 46.0% than that of EP, revealing the highest fire safety performance among these samples. The improvement is attributed to catalytic carbonization effect of DOPO-based FR and barrier effect of PZS. Fig. 6B indicates that the THR of EP composites exhibit a similar decreasing trend to PHRR. EP/[email protected] shows a lower THR value compared to EP/PZS3.0. With increasing the [email protected] loading, the THR values of the EP/[email protected] composites are gradually decreased. After adding 3.0 wt% loading of [email protected] in EP, the THR of EP/[email protected] is reduced to 72.4 MJ/m2, a 27.1% decrease than that of pure EP. The notable decrease in the fire hazards of EP/[email protected] is attributed to the gas and condensed phase activity: the catalytic carbonization effect of the wrapped FR decreases the release of degradation products; the physical barrier effect of random distributed PZS network structure retards the heat and mass transfer and the escape of pyrolysis volatile. The smoke production rate (SPR) and total smoke release (TSR) curves of EP nanocomposites are presented in Fig. 7. Owing to its specific multi-aromatic structures, pure EP exhibits high yield of toxic smoke with high PSPR and TSR values. EP/PZS shows slight reduction in the PSPR and TSR. Moreover, the introduction of [email protected] to EP significantly reduces the PSPR value. It is decreased from 0.47 m2/s for pure EP to 0.21 m2/s for EP/[email protected] with a reduction of 55.3%. EP/[email protected] sample also shows the lowest TSR with a reduction of 44.1%, compared to that of pure EP. The above results reveal that the EP/[email protected] composite exhibits the best flame retardancy among all the samples, which indicates the denser protective char layers as physic barriers constituted by the cooperation of polyphosphazene and DOPO-based FR in EP/[email protected] composites were more effective than the EP/PZS. 3.4. Gas phase analysis To further evaluate the toxic effluents elimination behavior of [email protected], the toxic gases released from EP, EP/[email protected], EP/PZS3.0 decomposition were detected by using a TG-FTIR technique. As shown in Fig. 8, 3D TG-FTIR and FTIR

spectra were obtained from thermal decomposition process of EP nanocomposites at the maximum evolution rate. Several toxic pyrolysis products are remarkably identified by typical FTIR signals. The characteristic peaks of the gaseous decomposition products appear in the regions of 3500–4000 cm-1, 2750–3200 cm-1, 2200–2400 cm-1, 1750–1900 cm-1, 1250–1600 cm-1 and 600–1000 cm-1. Some toxic pyrolysis products signals are weaker identified by characteristic FTIR absorption, after incorporated [email protected] FTIR spectra of the pyrolysis products for EP (A) and EP/[email protected] (B) at different temperatures are presented in Fig. 9. As can be observed, the FTIR spectra of EP/[email protected] are similar to that of neat EP. Several characteristic peaks are assigned to the pyrolysis products of EP and EP/[email protected], such as 1510 cm-1 (aromatic compounds), 1740 (carbonyl compounds), 2190 cm-1 (CO), 2360 cm-1 (CO2), 2930 cm-1 (hydrocarbons) and 3650 cm-1 (absorbed water) [39]. Moreover, the EP/[email protected] composites release the pyrolysis products relatively earlier than that of pure EP, revealing that introduction of [email protected] catalyzes the thermal decomposition process of EP. The intensities of total pyrolysis volatiles and representative pyrolysis products for EP and its composites are shown in Fig. 10 and Fig. 11. Total pyrolysis volatiles of EP/[email protected] nanocomposite are lower than pure EP, EP/PZS3.0 composites. With the incorporation of 3.0 wt% [email protected], the maximum absorbance intensity of pyrolysis products are shifted to lower values, including hydrocarbons (Fig. 10B), carbonyl compounds (Fig. 10C), aromatic compounds (Fig. 10D), CO (Fig. 11A) and CO2 (Fig. 11B), compared to those of the pure EP and EP/PZS3.0 samples. The decreased intensity of pyrolysis products is attributed to more compact and cohesive char layer as reinforced barrier, retards the escape of pyrolysis products. Moreover, the major source of smoke particles, originates from the organic volatile products such as hydrocarbons, carbonyl and aromatic compounds. The decrease in these volatiles contributes to the suppression of smoke, and the CO decrease leads to the reduction in smoke toxicity, which is beneficial to the improvement of fire safety. 3.5. Condensed phase Analysis The real time Fourier transform infrared is performed to further evaluate the thermal degradation process of pure EP and EP/[email protected] composite. As shown in

Fig. 12A, it can be observed that the peaks at 3409, 2961, 2870, 1609, 1508, 1459, 1363, 1240, 1180, 1040 and 829 cm-1 are the characteristic absorption peaks of pure EP [40]. The absorption peak at 1363 cm-1, corresponds to the C(CH3)2 group. With the temperature increasing from 250 to 300 °C, the intensity of the peak gradually decreases and disappears completely over 350 °C, which can be attributed to the release of methyl groups in EP. The peak at 3409 cm-1 nearly disappears at 250 °C, revealing the absorbed water of EP released. When the temperature increases to 380 °C, it can be observed that the characteristic peaks at 2961, 2870, 1609, 1508, 1240, 1180 and 1040 cm-1 disappear, indicating that the main degradation of EP occurs in this stage, in accordance with the TGA results. By contrast, the RTFTIR spectra of EP/[email protected] are shown in Fig. 12B. The characteristic peaks of EP/ [email protected] include 3409, 2961, 2870, 1609, 1508, 1459, 1363, 1239, 1183, 1040 and 829 cm-1. As can be observed, the intensity of the peaks at 2961 and 2870 cm-1, corresponding to CH3 stretching vibration, and the peak at 1363 cm-1, corresponding to CH3 deformation vibration, gradually decreases from 250 to 300 °C and disappear nearly over 350 °C. Furthermore, the absorption peak at 941 cm-1 corresponding to P– O–C bond decreases rapidly over 250 °C and then disappears nearly over 380 °C, revealing that P–O–C bonds in the [email protected] are stable under heating. The absorption peak at 1293 cm-1 is attributed to the O=S=O groups and the peak at 900 cm-1 is assigned to the absorption of P–N, both of which are decreased rapidly over 300 °C and then disappear entirely over 380 °C. It is worthy noted that the characteristic peaks at 1609, 1508, 829 cm-1 still exist at high temperature range (over 380 °C), indicating the formation of aromatic structure. To understand the flame-retardant mechanism, the char residues of EP and its nanocomposites were investigated. During the cone calorimeter test, neat EP was completely melted to warping and burned dramatically, with breaking char residue left. The incorporation 3.0 wt% of PZS and [email protected] leads to the remarkable improvement of the char yield. EP/[email protected] sample could swell into a foam-like structure with a continuous and compact surface when exposed to fire (Fig. 13A, 13B and 13C). Furthermore, Fig. 13D-13F shows the SEM images of microstructures of external residual char for EP, EP/PZS3.0 and EP/[email protected] nanocomposites. It is

obviously observed that a continuous and compact char surface is generated after the EP/[email protected] combustion. Thus, the char surface with a more compact and cohesive layer is effective to inhibit the heat and mass transfer between condensed and vapor-phase, as a result of enhancing the flame retardant performance. Raman spectroscopy was performed to investigate the component and structure of the char residues. The spectrum for EP (Fig. 14A) depicts two bands at 1596 cm-1and 1365 cm-1, which are defined as G and D peak, respectively. The intensity ratio of D to G band (ID/IG) is used to evaluate the graphitization degree of the char residue. Relatively lower ID/IG value indicates higher graphitization degree [41]. The EP composites exhibit the similar spectra to neat sample (Fig. 14B and 14C). The value of ID/IG for neat EP is 2.83, whereas the EP/PZS3.0 and EP/[email protected] samples exhibit lower value (2.71 and 2.45), respectively, revealing the higher graphitization degree. The present results indicate the formation of the graphitized carbon during EP composites combustion, owing to the catalytic charring effect of DOPO-based [email protected]

4. Conclusions In this study, the DOPO-based flame retardant wrapped polyphosphazene nanotubes were designed and synthesized successfully by in situ polycondensation. The morphology, structure and thermal property of [email protected] were well characterized by TEM, SEM, FTIR, XRD, XPS and TGA. Owing to the FR wrapped PZS with hydroxyl groups, [email protected] was dispersed well in EP and formed strong interfacial interaction with matrix. Incorporation of [email protected] into EP matrix decreased the maximum mass loss rate and enhanced the char yield at 800 oC, revealing the enhanced thermal stability. Furthermore, the presence of 3.0 wt% [email protected] significantly decreased the PHRR and THR values by 46.0% and 27.1%, respectively, compared to those of pure EP. TG-FTIR results presented that the yield of toxic CO and other volatile products from the EP decomposition was significantly suppressed after introducing the [email protected] hybrids, implying a reduced smoke toxicity. The distinct improvement in the fire hazards was primarily attributed to the reason that the incorporation of [email protected] provided a compact and stable char barrier, which slowing

down the heat release, retarded the release of volatile products, and formed graphitized char layer to protect the internal polymer from outward flames.

Acknowledgements This study was supported by the National Natural Science Foundation of China (No. 21374111 and 51323010), the Fundamental Research Funds for the Central Universities (WK2320000032), and the grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (GRF Project number CityU 11215314 and Theme-based Research Scheme Project Number T32-101/15-R, respectively).

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Fig. 1. TEM images of PZS (A) and [email protected] (B, C); SEM images of PZS (D) and [email protected] (E).

Fig. 2. FTIR spectra (A) and XPS survey spectra (B) of PZS and [email protected]; high-resolution C1s XPS spectra of PZS (C) and [email protected] (D).

Fig. 3. XRD patterns (A) and TGA curves (B) of PZS and [email protected] under nitrogen.

Fig. 4. TGA (A) and DTG (B) curves of pure EP and its nanocomposites in nitrogen.

Fig. 5. SEM images of the fracture surfaces of pure EP (A, B); EP/PZS (C, D) and EP/[email protected] (E, F).

Fig. 6. HRR (A) and THR (B) vs. time curves of EP and its nanocomposites obtained from cone calorimeter.

Fig. 7. SPR (A) and TSR (B) vs. time curves of EP and its nanocomposites obtained from cone calorimeter.

Fig. 8. 3D TG-FTIR spectra (A) and FTIR spectra (B) of pyrolysis products for pure EP, EP/PZS3.0 and EP/[email protected] at the maximum evolution rate.

Fig. 9. FTIR spectra of the pyrolysis products for EP (A) and EP/[email protected] (B) at different temperatures.

Fig. 10. Absorbance of pyrolysis products for EP and its nanocomposites vs. time: (A) total pyrolysis products; (B) hydrocarbons; (C) carbonyl and (D) aromatic compounds.

Fig. 11. Absorbance of pyrolysis products for EP and its nanocomposites vs. time: (A) CO; (B) CO2.

Fig. 12. Real time FTIR spectra of EP (A) and EP/[email protected] (B) at different pyrolysis temperatures.

Fig. 13. Digital photos and SEM images of the char residues from EP (A, D), EP/PZS3.0 (B, E) and EP/[email protected] (C, F).

Fig. 14. Raman spectra of the char residues from EP (A), EP/PZS3.0 (B) and EP/[email protected] (C).

Scheme 1. Synthetic route of PZS and [email protected] nanotubes.