Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane

Journal Pre-proof Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane Wei Cai, Tongmin Cai, Lingxin He, Fukai Chu, Xiaowei Mu, Longfei Han, Yuan Hu, Bibo Wang, Weizhao Hu

PII:

S0304-3894(19)31925-9

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121971

Reference:

HAZMAT 121971

To appear in:

Journal of Hazardous Materials

Received Date:

19 October 2019

Revised Date:

7 December 2019

Accepted Date:

23 December 2019

Please cite this article as: Cai W, Cai T, He L, Chu F, Mu X, Han L, Hu Y, Wang B, Hu W, Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121971

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Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane

Wei Cai a,1, Tongmin Cai b,1, Lingxin He a, Fukai Chu a, Xiaowei Mu a, Longfei Han a,

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Yuan Hu a, Bibo Wanga*, Weizhao Hua*

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

Anhui 230026, PR China

KingFa Science and Technology Co. Ltd, Guangzhou 510663, China

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These authors contributed equally to this work.

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Corresponding authors.

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*E-mail: [email protected]; [email protected]; Fax/Tel: +86-551-63601664

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Graphical abstract

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Highlights  The ambient stability was imparted to BP nanosheets.  The radicals quenching effect of TA was developed to reduce the CO release.  The flame retardancy and toxic gases suppression of BP nanosheets was explored.

Abstract: Herein, as a natural antioxidant, tannin (TA) is firstly used to functionalize

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black phosphorous (BP) nanosheets to improve the ambient stability and toxic suppression, thus decreasing the fire hazards of polymer materials. Compared to pure

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BP nanosheets, higher temperature for thermal oxidation decomposition is achieved for TA-BP nanosheets, directly confirming the ambient stability of TA-BP nanosheets. Meanwhile, from high resolution TEM and XPS results, TA-BP nanosheets after being exposed at air for 10 days present well-organized crystal structure and low POx bonds content. Cone calorimeter results illustrate that the incorporation of 2.0 wt% 2

TA-BP nanosheets significantly decreases the peak value of heat release rate (-56.5 %), total heat release (-43.0 %), CO2 concentration (-57.3 %) of TPU composite. Meanwhile, with addition of low to 1.5 wt%, the release of highly-toxic CO gas is significantly suppressed, confirmed by lower peak value (0.52 mg/m3) and decreased total release amount (-55.1 %). The obviously enlarged tensile strength (36.7 MPa) and desirable elongation at break (622 %) are also observed. This strategy not only

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firstly adopts bio-based antioxidant to impart excellent environmental stability for BP nanosheets, but also promotes the promising potentials of BP nanosheets in the fire

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safety application of polymer composites.

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1. Introduction

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gas suppression; flame retardancy

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Key words: black phosphorous; natural antioxidant; environmental instability; toxic

As an emerging two-dimensional nanomaterial, black phosphorous (BP) nanosheets

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is gradually receiving considerable attention in academic and industrial fields[1]. Compared the zero-bandgap of graphene, the layer-dependent direct bandgap and high

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carrier mobility as well as large on/off ratio make few-layer BP a desirable candidate in information storage, chemical/biosensing, and optoelectronics[2, 3]. In addition, few-layer BP also presents potential application in flexible electronics, due to high aspect ratio and excellent mechanical strength, as well as electronic conductivity[4]. Lots of research works have been devoted to develop promising potential of BP 3

nanosheets, thus promoting the advances in technology. Compared to desirable stability of graphene and other 2D nanomaterials exposing to oxygen and water, the most crucial issue associated BP is the poor ambient stability[5-7]. Based on previous literatures, degradation mechanism of BP in ambient conditions is divided into three steps: firstly, the oxygen molecules adsorbed onto the surface of BP are turned into O2- by combining the free electron produced by

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illumination. Follow on, O2- dissociates onto the surface of BP and forms P=O bonds with phosphorous elements. Finally, through the hydrogen-bond interaction, water molecules draw the bonded O and remove P from the surface and break the top layer

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of black phosphorous[5, 6]. Therefore, the stability of raw few-layer BP nanosheets in

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ambient conditions is undesirable and seriously suppresses the application space of BP, where oxygen and moisture are impossible to avoid. As far as our information goes,

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the currently used protection mechanism to BP nanosheets are mainly divided into

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two approach, including establish isolated coating and passivate the lone-pair electron reactivity. For example, Avsar et al. employed h-BN nanosheets as encapsulating layer

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onto the surface of BP to isolate moisture and oxygen[8]. However, the cover of unmatched inorganic nanomaterials would deteriorate the special performance of BP

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nanosheets. It is certain that, regarded as white graphene, h-BN nanosheets will influence the photoelectron property of BP nanosheets. Taking into account reducing the reactivity of lone-pair electrons of P to O2-, Ag+ was adopted to spontaneously adsorbed on the BP surface via cation–π interactions, thus rendering BP more stable in air[9]. As initiator of the entire degradation process of BP, the elimination of O2- is 4

capable of directly stopping the degradation reaction at its source. However, recent research works didn’t pay attention to the capture of O2-. Therefore, a protection strategy combining the isolate and capture of O2- is urgent for BP nanosheets. For almost a thousand years, tea culture is gradually propagated in East Asian region and drinking tea has become an indispensable part of East Asians lifestyle[10]. Tea polyphenols, as the central constituent of tea, is capable of releasing hydrogen

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donors to scavenging superoxide radicals, thus presenting antioxidation effect[10, 11]. As analogue of tea polyphenols, tannic (TA) is also a naturally derived polyphenolic

compound and possesses much superior oxidation resistibility[11, 12]. Compared to

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tea polyphenols, TA is capable of modifying the materials surface through multiple

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interactions, including hydrogen-bond and π-π interactions. Aim at preventing the ambient degradation of BP nanosheets at the beginning, in this work, TA is used to

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coat onto the surface of BP nanosheets to eliminate superoxide radicals[13].

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Meanwhile, the radical quencher effect of TA is also expected to cut off the combustion reaction.

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Polymer materials are now part of most of the consumer goods but also find applications in very specific domains[14-16]. Among them, thermoplastic

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polyurethane (TPU) is particularly popular due to the soft elasticity and desirable break strength, special applies to wearable electron devices[17-21]. Similar to other polymer materials, high flammability gives a huge challenge for the engineering application of TPU, where demands high fire safety safeguards. Referring to previous literatures, various layered nanomaterials were used to improve the flame retardant of 5

TPU, including graphene, MoS2, carbon nitride, and h-BN[21-26]. BP nanosheets, consisting completely of P elements that is most efficient flame retardant element, is able to present a significant flame retardant effect that other layered nanomaterials couldn’t provide. Herein, we report a natural antioxidant functionalization strategy to scavenge the superoxide radicals which are onto the surface of BP nanosheets, thus hindering the

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ambient degradation of BP nanosheets at source. As a radical quencher, meanwhile,

the adopted TA is also capable of cutting off the consecutive combustion reaction to

decrease the heat release. Therefore, the enhancement effects of tannin-functionalized

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BP nanosheets for flame retardancy and toxic gases suppression of TPU are

2.1. Raw materials

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2. Experimental section

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investigated simultaneously.

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Tannin, N,N dimethylformamide (DMF, AP), red phosphorous (98.5 %) were supported by Aladdin Industrial Co., Ltd. (Shanghai, China) and used directly without

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further purification. Thermoplastic polyurethane (TPU) resin was obtained from Bangtai Material Co., Ltd., China.

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2.2. Preparation of TA-functionalized BP nanosheets (TA-BP) Bulk BP crystals were fabricated by vapor phase growth using red phosphorus as

precursor, based on previous literatures[27, 28]. After being obtained, 0.5 g bulk BP crystals were grinded to fine particle by manual operation. Follow on, fine BP crystal particles was added into 300 mL of ethyl alcohol solution containing 3 g tannin. For 6

the sufficient exfoliation, continuous stir and sonication was performed to the above dispersion for 6 h. From un-exfoliated and few-layered BP mixed dispersion, tannin-modified BP nanosheets were separated by centrifugation (1200r/min for 8 min). Finally, the BP suspension was processed by three times vacuum filtration to remove the excess tannin and produced TA-BP power was dried at vacuum oven. The pure BP nanosheets were prepared by same procedure without TA.

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2.3. Preparation of TPU/TA-BP composites

TPU/TA-BP composites were fabricated by solvent-blend method. Firstly, 49 g of

TPU resin was dissolved thoroughly in 200 mL of THF solution with a fierce stir at 50 o

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C. Firstly, 1.0 g of TA-BP was re-dispersed in 50 mL of THF solution by sonication

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and stir of 2 h. Following, above TA-BP nanosheets dispersion was poured into the TPU solution. The ultimately produced mixture was poured into watch glass and

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heated to 50 oC to remove THF. The fabricated TPU/TA-BP composite was named as

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TPU/TA-BP-2.0. The other samples were prepared with the similar procedure. All samples were hot-pressed into sheets of appropriate size under 10 MPa at 200 °C for

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10 min.

2.4. Measurements and characterization

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X-ray diffraction (XRD) measurements were performed with a Japan Rigaku

D/Max-Ra rotating anode X-ray diffractometer equipped with a CuKα tube and Ni filter (λ = 0.1542 nm).

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Raman spectroscopy was processed with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA) with excitation provided in backscattering geometry by a 514.5 nm argon laser line. Morphologies of fractured surface were observed with a XL-30 ESEM scanning electron microscope (SEM) at an acceleration voltage of 10.0 kV. Fourier transform infrared (FTIR) spectroscopy was carried out by a Nicolet 6700

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spectrometer (Nicolet Instrument Company) in 400–4000 cm-1 scope.

Transmission electron microscopy (TEM) was employed to study the morphology of BP nanosheets with a JEOL JEM-2100F transmission electron microscope at an

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accelerating voltage of 200 kV.

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The micro-sized combustion behavior of TPU and TPU composites were performed on a microscale combustion calorimeter.

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Thermogravimetric analysis (TGA) was executed with a TGA Q5000 IR

air atmosphere.

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thermogravimetric analyzer (TA Instruments, U.S.) at a heating rate of 20 °C min-1 in

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Thermogravimetric analysis-infrared spectrometry (TG-IR) was investigated with a TGA Q5000IR thermogravimetric analyzer linked to a Nicolet 6700 FTIR

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spectrophotometer from 20 to 700 °C at 10 °C min-1 (N2 atmosphere, flow rate of 30 mL min-1).

Combustion test was performed on a cone calorimeter (Fire Testing Technology, UK) according to ISO 5660 standard procedures, with 100 × 100 × 3 mm3 specimens.

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Each specimen was exposed horizontally to 35 kW/m2 external heat flux. Meanwhile, the produced heat, CO, and CO2 were detected during TPU combustion. X-ray photoelectron spectroscopy (XPS) test was performed to characterize the element of BP nanosheets with a VG ESCALB MK-II electron spectrometer. The excitation source was an Al Kα line at 1486.6 eV. Tensile testing of the composites was performed with an electronic universal testing

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instrument (MTS System Co., Ltd., China) at a crosshead speed of 50 mm/min.

The stiffness was recorded by dynamic mechanical analysis (DMA) with a DMA Q800 instrument (TA Instruments Inc.) at a constant temperature of 37 °C and a fixed

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frequency of 10 Hz.

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The rheological behaviors of pure TPU and TPU/TA-BP-2.0 were studied using a rheometer (AR1500ex, TA instrument). An environmental test chamber (ETC) steel

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parallel-plate geometry with a diameter of 25 mm was used to carry out the

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measurements. Dynamic oscillation frequency was swept from 100 to 0.05 Hz at a strain of 1% within the linear viscoelastic range at 190 oC under an air atmosphere.

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3. Results and discussion

3.1. Characterization of TA-BP nanosheets

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Tannin (TA), one of the most common tea polyphenols, is a superoxide radical

scavenger that is capable of modifying the surface of the BP nanosheets to protect it from being attacked by superoxide radicals. The procedure for preparing TA-BP nanosheets is shown in Figure 1. The sonication exfoliation of BP nanosheets is carried out in ethyl alcohol which contained excess TA. The coupling effect between π 9

electrons of TA and lone pair electrons of BP nanosheets is the driving force of surface functionalization. During exfoliation process, TA molecules would interact with the surface of BP nanosheets fully. Therefore, the strong π-π interactions were built between BP nanosheets and TA, thus making TA cover onto the surface of BP nanosheets with a monolayer structure. The diffraction peaks near 17.0 o, 26.6 o, 34.3 o, 52.4 o, 55.9 o, and 56.9o corresponding to (020), (021), (040), (041), (060), (151), and

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(061) planes are observed in XRD curves of pure BP and TA-BP nanosheets, indicating the orthorhombic structure characteristic (Figure 2a). Moreover, the

intensity of several diffraction peak of TA-BP nanosheets is obviously decreased than

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that of pure BP nanosheets, including (040) and (060) planes, indicating a much less

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layer number. On the contrary, the peak intensity of (020) and (111) are increased. This phenomenon may be due to the successful exfoliation of TA-BP nanosheets, thus

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causing the exposure of special planes covered by bulk structure. The Raman

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spectrum of pure BP nanosheets shows the characteristic peaks of A1g, B2g, and A2g at 362.6 cm-1, 440.8 cm-1 and 467.3 cm-1, respectively, while, the peak locations of the

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TA-BP are near 361.0 cm-1, 437.2 cm-1 and 465.6 cm-1 (Figure 2b). According to previous literatures, the in-plane oscillation of P atoms is hindered by van der Waals

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forces provided by adjacent layer, thus causing a blue-shift phenomenon in Raman spectrum with decreasing layer[29, 30]. Therefore, the unique red-shift phenomenon could be attributed to the strong interfacial interaction between TA and BP nanosheets, instead of neighboring BP nanosheets, further suppressing P atoms oscillation. As presented by Figure 2c, FTIR spectra of TA and TA-BP nanosheets shows similar 10

characteristic peaks, which aren’t detected from FTIR spectrum of pure BP nanosheets. For instance, characteristic peaks at 1700 cm-1 and 1083 cm-1 attributing to the stretching vibrations of C=O and C–O–C bonds are presented in the FTIR spectra of TA and TA-BP, not the spectrum of pure BP nanosheets.[31] It is notable that the characteristic peaks of hydroxyl group are found in the spectrum of pure BP nanosheets, indicating the oxidation process of BP is unavoidable even expose a short

including Raman and FTIR spectra. 3.2. Analysis of ambient stability of TA-BP nanosheets

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time. These results confirm the successful functionalization of BP nanosheets with TA,

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The thermal oxidation stability of BP nanosheets was analyzed by TGA at air

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atmosphere. The obvious mass increase near ~450 oC is attributed to the oxidation of BP nanosheets (Figure 3a). It is obvious that the functionalization of TA can improve

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the thermal oxidation temperature of BP nanosheets, increased by 10.6 oC (Figure 3c).

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Compared to the complete loss of pure BP nanosheets, much more char residue was left at 800 oC for TA-BP nanosheets, up to 21.5 wt%. These results indicated that TA

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could react with BP nanosheets to produce thermally-stable char residue, which also demonstrated a potential application as flame retardant. In general, the modified

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amount was determined by TGA test under nitrogen (Figure 3b). It is found that the pure BP nanosheets didn’t suffer from thermal decomposition when temperature is lower than 350 oC. Besides, a significant mass loss is observed range to 490 oC from 350 oC. After being modified, the mass percentage of TA-BP is 96.5 wt % near 350 oC, which is due to the thermal decomposition of tannin. At 350 oC, the weight ratio of TA 11

is 46.5 wt%. Based on the TGA results, the modified amount of TA onto the surface of BP nanosheets is 6.5 wt%. As shown in DTG curves (Figure 3d), the temperature corresponding to maximum mass loss of TA-BP nanosheets is increased to 449.8 oC from 425.2 oC (pure BP), meanwhile the maximum rate of TA-BP nanosheets is decreased to 1.35 %/oC from 1.42 %/oC. Due to the thickness up to dozens of nanometers of bulk BP, TEM electron beam

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isn’t capable of piercing the layered structure. Therefore, grey and thick bulk

morphology is observed for the TEM photograph of bulk BP (inset in Figure 4a). Having said that, pure BP and TA-BP nanosheets analyzed in TEM and XPS have

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been exposed to ambient for 10 days. Compared to the thick layered structure of bulk

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BP, after being exfoliated, TA-BP nanosheets present thin layered structure (Figure 4a). Meanwhile, TEM electron contrast of TA-BP nanosheets is lower than that of

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bulk BP. These results indicat that bulk BP is successfully exfoliated by TA solution.

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Pure BP nanosheets exfoliated by ethyl alcohol is also observed by TEM photograph (Figure 4c). As highlighted by red arrows, tortuous boundary and bubble hole that

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aren’t presented by TA-functionalized BP nanosheets, are shown in Figure 4c, which is induced by severe oxidation process[32, 33]. Meanwhile, the lattice fringes

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observed by high resolution TEM of pure BP and TA-BP present obvious distinction (Figure 4b and d). Unordered lattice structure takes up the large area of high resolution TEM of pure BP nanosheets, leaving minor ordered crystalline near boundary. Compared the low quality of pure BP nanosheets, well-order lattice fringes are observed and demarcated in TA-BP nanosheets, showing d-spacing of 2.2, 1.7, and 12

2.6 Å which are assigned to (020), (060) and (040) (Figure 4e1, e2, e3). In addition, high-resolution SAED patterns of low quality BP nanosheets are much obscurer than that of TA-BP nanosheets. In order to detect the chemical bond change of pure BP and TA-BP nanosheets after being exposed to air (10 day), X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical environment of P element. The initial P2p core level XPS

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spectrum of pure BP nanosheets is presented in Figure 5a. This result indicates that the pure BP nanosheets have the 2p3/2 and 2p1/2 doublet at 129.7 and 130.4 eV,

respectively, corresponding to crystalline BP. Besides, small oxidized phosphorus (i.e.,

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POx) sub-bands are also apparent at 133.6 eV, as observed in previous literature.

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These POx sub-bands are likely generated from oxygen defects or surface suboxides. After being exposed to air, the P2p core level XPS spectra of pure BP and TA-BP

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nanosheets are also studied (Figure 5b and c). The oxidized phosphorus (i.e., POx) is

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recognized by characteristic peak near 134.0 eV. It is found that the peak intensity of P-O band in TA-functionalized BP is much lower than that of pure BP nanosheets,

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which confirms the cover of TA is contributed to protect BP against ambient degradation. Together with a series measurement, including TGA, TEM, and XPS, a

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we can naturally arrive at the conclusion that TA is capable of protecting BP nanosheets by releasing hydrogen donors to eliminate superoxide radicals, thus hindering the ambient degradation of BP (Figure 5d). 3.3. Interfacial interactions and mechanical properties of TPU/TA-BP composites

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The freeze-fractured surfaces of pure TPU, TPU/TA-BP-2.0, and TPU/pure-BP-2.0 were observed by SEM photographs, for investigating the interfacial interactions between BP nanosheets and TPU matrix. The smooth surface of pure TPU is a typical characteristic of the brittle breakage, which indicates the weak interfacial interactions among TPU chains (Figure 6a). With the incorporation of TA-BP nanosheets, the fracture surface becomes very rough. Many raised TPU matrix is distributed onto the

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surface, observed in Figure 6b, meanwhile, obvious aggregation of TA-BP nanosheets

wasn’t found. Compared to the fracture surface of TPU/TA-BP-2.0, uncertain aggregation is easy to be found in fracture surface of TPU/pure-BP-2.0 (Figure 6c).

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Therefore, SEM EDS Mapping was used to determine the nature of aggregation

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(Figure 6d). The distribution of P element is same to the aggregation, thus confirming that the aggregation was re-stacked pure BP nanosheets. The rough surface around BP

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aggregation also demonstrates that the pure BP nanosheets also interact with TPU

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chains. However, the interfacial interactions supported by pure BP nanosheets are lower than that of TA-BP nanosheets, confirmed by partial smooth surface signed by

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red arrows. Together with the nonexistent aggregation of TA-BP nanosheets, it is reasonable to draw a conclusion that the functionalization of TA not only improves the

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interfacial interactions between TA-BP nanosheets and TPU matrix, but also promotes the dispersion of TA-BP nanosheets. The mechanical properties are of significant to the practical application of polymer composites. Therefore, the tensile test is carried out to study the tensile strength and elongation at break of pure TPU and it composites (Figure 7a and b). As a typical 14

elastomer material, high elongation at break (793 %) is observed in pure TPU with break strength of 29.0 MPa. With 0.5 wt% TA-BP nanosheets, the tensile strength of TPU composite is increased to 37.0 MPa (27.6 %), while the elongation at break is decreased to 624 %. This phenomenon has been reported by previous literatures, which is due to the strong interfacial interactions[34, 35]. When strong interfacial interactions between TA-BP and polymer chains are formed, the stress load is

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successfully transferred to robust TA-BP nanosheets from soft polymer chains, thus

obtaining higher tensile strength. How, the strong interfacial interactions likewise confine the movement of polymer chains, causing the decreasing elongation at break.

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When the addition amount is 2.0 wt %, the tensile strength and elongation at break for

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TPU/TA-BP-2.0 are up to 31.5 MPa and 738 %, respectively, correspondingly increased 17.3 % times and 13.3 % compared with the corresponding values of

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TPU/pure-BP-2.0. Compared to pure BP nanosheets, obviously, the functionalization

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of TA onto the surface of BP nanosheets is an efficient route for enhancing interfacial interactions with TPU, thus increasing the tensile strength.

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When suffering from low and repeated mechanical stress, the stiffness coefficient of TPU composites would be increased, which seriously limits its application in

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wearable devices. For detecting this situation, the stiffness of neat TPU, TPU/TA-BP-2.0 and TPU/pure-BP-2.0 during 100 k dynamic stretch cycles is measured by DMA test (Figure 7c). Obviously, the stiffness of neat TPU is gradually increased with the increasing cycle index. After 100 000 dynamic loading, the stiffness of neat TPU is increased to 0.36 %, indicating a self-stiffen phenomenon, 15

which is due to the crystallization behavior of TPU chains. It is found that the stiffness of TPU/pure-BP-2.0 is higher than those of pure TPU and TPU/TA-BP-2.0, when cycle index is range from 20 000 to 80 000. However, the stiffness of TPU/TA-BP-2.0 achieves a same increase to TPU/pure-BP-2.0 (0.41 %) at 10 0000 cycles. Based on previous literatures, these results demonstrate that the sole incorporation of pure BP nanosheets is contributed to the crystallization of TPU chains during dynamic stretch

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loading, while the strong interfacial interactions between TA-BP nanosheets and

polymer chains confine the polymer chains movement to crystallization. Therefore, it is reasonable that the stiffness of TPU/TA-BP-2.0 is lower than that of

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TPU/pure-BP-2.0. After 100 000 dynamic compressive cycles, the interfacial

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interactions between TA-BP nanosheets and polymer chains are destroyed by dynamic stress, liberating the polymer chains again. Therefore, the ultimate stiffness of

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TPU/TA-BP-2.0 is increased to the same level of TPU/pure-BP-2.0. Obviously, the

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functionalization of TA is contributed to restrict the self-stiffness phenomenon of TPU composites, thus ensuring the practical application.

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Rheological measurements at 190 oC were carried out to investigate the interfacial interactions between TPU matrix and TA-BP nanosheets. The complex viscosity (η*)

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is recorded as a function of oscillation frequency (left axis of Figure 7d). As presented in low frequency, the η* decrease of pure TPU and TPU/TA-BP-2.0 with increasing oscillation frequency is slight, which is attributed to the characteristic of Newtonian fluid. This phenomenon indicates that the entanglement and disengage process in low frequency of polymer chains is in balance. With the increasing of oscillation 16

frequency, η* is obviously decreased, presenting a typical shear thinning phenomenon. In addition, the incorporation of TA-BP nanosheets increases the η* in all frequency ranges. This result confirms that TA-BP nanosheets strongly interact with TPU chains, thus hindering the chains mobility and increasing the complex viscosity. In addition to the η*, tan δ is also presented in right axis of Figure 7d. Generally, the lower tan δ is corresponding to the higher elasticity[36]. It is found that the incorporation of TA-BP

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nanosheets significantly decreases the value of tan δ, which indicates that the

increased elasticity of TPU melts. Due to the micro-phase separation of TPU matrix, a peak value of tan δ is capable of being observed. The peak position is moved to 0.13

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Hz from 0.20 Hz, which indicates more time is needed for relaxation time of molecule

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chains of TPU/TA-BP-2.0. Such a result likewise confirms the formation of strong interfacial interaction between TA-BP and TPU chains.

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3.4. Thermal pyrolysis performance of TPU/TA-BP composites

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The thermal degradation process of pure TPU and its composites was analyzed by TGA. At air atmosphere, all TPU samples presented three thermal decomposition

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stages, corresponding to the degradation of the principal chains, the further degradation of polyols and isocyanates, and oxidation of the char residue[37, 38]. As

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presented in Figure 8a, the addition of pure-BP and TA-BP nanosheets don’t influence the first two decomposition process of TPU composites. The temperature corresponding to 5wt% mass loss is defined as initial degradation temperature (T5wt%). As presented by Table 1, the influence of BP-based flame retardant is slight for the T5wt%. Meanwhile, the maximum of mass loss rate (MLRmax) of all samples are similar. 17

In addition, the char residue at 700 oC is gradually increased with the increasing addition of TA-BP nanosheets. The residual weights at 700 oC of TA-BP nanosheets and pure TPU are 26.0 wt% and 0.53 wt%. Taking into consideration the 2.0wt% addition of TA-BP nanosheets, the calculated residual weight of TPU/TA-BP-2.0 is 0.52 wt%. However, the practical residual weight at 700 oC of TPU/TA-BP-2.0 is 2.68 wt%, much higher than the calculated results. This result confirms that the reaction

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between TA-BP nanosheets and TPU matrix could promote the formation of char

residue. It is found that the char residue at 700 oC of TPU/pure-BP-2.0 (3.28 wt %) is

higher than that of TPU/TA-BP-2.0 (2.68 wt %). The higher phosphorous content in

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pure BP nanosheets is contributed to the formation of thermal stable char residue[39].

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Therefore, the final char residue at 700 oC of TPU/pure-BP-2.0 is highest. As presented in Figure 8c, the T5wt% of TPU/TA-BP-2.0 and TPU/pure-BP-2.0 are

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increased to 306.1 oC and 302.9 oC from 285.2 oC at nitrogen atmosphere, (pure TPU).

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Similar to the TGA results under air, the change of MLRmax is also slight with the addition of BP-based flame retardant. Besides, the char residue at 600 oC of

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TPU/pure-BP-2.0 (6.40 wt%) is still higher than that of TPU/TA-BP-2.0 (6.32 wt%). These results confirm that the addition of BP-based flame retardant is capable of

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increasing the initial degradation temperature and char residue. Meanwhile, the char residue weight is mainly determined by the additional amount of BP nanosheets, demonstrating the reaction between BP nanosheets and TPU matrix is the dominating contributor for the improvement of flame retardancy. 3.5. Fire safety analysis of TPU/TA-BP composites 18

Microscale combustion calorimeter (MCC) was used to measure the heat release of combusted gases evolved during controlled heating of the samples in a small-scale.[40] It is obvious that the pure TPU is highly flammable, with a high peak value of heat release rate (HRR) of 420 W/g (Figure 9a and b). Peak values of HRR of all samples with pure BP and TA-BP nanosheets are decreased to an approximate value (near 370 W/g), without the influence of addition amount. Based on the principle of oxygen

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consumption, cone calorimeter is developed to investigate the heat release behavior of materials under real-world fire scene.[41] As presented in Figure 9c and Table 2, time to ignition (TTI), time to peak value (TTP),

heat release rate (HRR), total heat

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release (THR), and fire growth index (FGI) data are given. It is found that pure TPU

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is ignited rapidly at 51s and arrived at largest heat release rate with 152s. Meanwhile, the peak value of HRR (PHRR) is significantly up to 1293 kW/m2 (Figure 9a). The

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ratio of PHRR to TTP is defined as FGI, which is usually used to estimate the fire

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safety of materials. Along with FGI value (8.50), this high and broad curve shape indicate that pure TPU is highly-degree inflammable.[42] At the end of burning, pure

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TPU releases a THR of 79.9 MJ/m2, accompanied by char residue of 4.3 wt%. The incorporation of 0.5 wt % TA-BP nanosheets slightly decrease the TTI (-3s) and TTP

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value (-17s). The earlier ignition and heat release phenomenon are also presented in other samples containing BP nanosheets. However, the degradation temperature of BP nanosheets is significantly higher than that of TPU resin. Therefore, this result is caused by the reaction between BP nanosheets and TPU matrix, rather than the self-thermal degradation of BP nanosheets. Meanwhile, incorporated 0.5wt% TA-BP 19

nanosheets decreases PHRR and THR by 15.5 % and 11.1 % (Figure 9d). The weight of residual char is also increased to 6.8 wt% from 4.3 wt%. However, the sharp type of heat release curve is still presented by TPU/TA-BP-0.5, demonstrating the improvement in fire safety is attributed to the high P loading and radical capture effect of TA-BP nanosheets, rather than barrier effect. With the gradually increasing addition, the flame retardancy of TPU composites is further enhanced. With the addition of

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2.0wt% TA-BP nanosheets, the TTI and TTP are decreased to 39s and 125s from 51s

and 152s. Meanwhile, values of PHRR and THR are decreased to 562 kW/m2 and 45.5 MJ/m2, respectively, corresponding to 56.5 % and 43.0 % reduction compared to

-p

pure TPU. More importantly, the FGI is significantly decreased to 4.50

re

(TPU/TA-BP-2.0) from 8.50 (pure TPU), confirming the suppression of fire growth. It is found that the char residue weight of TPU/TA-BP-2.0 and TPU/pure-BP-2.0 are 9.7

lP

wt% and 9.8 wt%. In comparison to pure TPU, much more char residue is left for

na

TPU/TA-BP-2.0 and TPU/pure-BP-2.0 after cone calorimeter test, increased by 125.6 and 127.9 %. Such a result confirms that the modification of TA isn’t contributed to

ur

the formation of char residues. It is the BP nanosheets which are main contributor for the production of protective char layer. Therefore, it is reasonable that the weight of

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char residue at TPU/TA-BP-2.0 and TPU/pure-BP-2.0 is similar. From the observation of HRR curves of TPU/TA-BP-1.5, TPU/TA-BP-2.0, and TPU/pure-BP-2.0, the typical sharp curve indicating highly-degree inflammable of polymer isn’t observed, confirming that the appearance of barrier effect of BP nanosheets. As expected, with the addition amount of 2.0wt%, the sole addition of pure BP nanosheets don’t receive 20

desirable flame retardancy, compared to TA-BP nanosheets, with PHRR of 640 kW/m2 and THR of 67.2 MJ/m2. These results indicated that the better dispersion state caused by the modification of TA is capable of making BP nanosheets play a barrier effect in hindering the pyrolysis products delivery. Besides, during the degradation process of TA, phenoxy radicals are produced to quench oxygen free radicals the polymer structure, thus hindering the combustion reaction.[43]

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For further investigating the fire hazards of TPU combustion, CO and CO2 concentration versus time curves were also obtained by an external sensor (Figure 10 and Table 3). The time integral of CO2 concentrations is defined as the total release,

-p

which is similar to the definition of heat release. It was found that the peak value and

re

total release of CO2 release of pure TPU are 15.7 % and 1024 %*s, respectively. Meanwhile, the maximum concentration of CO2 is gradually decreased with the

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addition of TA-BP nanosheets. For example, the peak values of CO2 concentration of

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TPU/TA-BP-0.5, TPU/TA-BP-1.0 and TPU/TA-BP-1.5 are 14.5 %, 12.3 %, and 8.3 %. The most decrease is observed in TPU/TA-BP-2.0, with CO2 concentration of 6.7 %.

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However, the total release of CO2 concentration is close in TPU/TA-BP-1.5, TPU/TA-BP-2.0 and TPU/pure-BP-2.0. This result confirms that the flame retardancy

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improvement is attributed to the barrier effect of BP nanosheets, which couldn’t significantly decrease the total amount of inflammable products. During combustion, obvious reduction in the peak value of CO concentration is also observed. With addition of as low as 0.5wt % TA-BP, the total release of CO is reduced by 18.6%, from 64.4 ((mg/m3)*s) to 52.4 ((mg/m3)*s). It’s worth noting that total CO release 21

curve of TPU/TA-BP-1.0 and TPU/pure-BP-2.0 is highly similar and overlapped. By the addition of 2.0 wt% TA-BP nanosheets, maximum concentration and total release of CO are decreased to 0.55 mg/m3 and 40.1 ((mg/m3)*s) from 0.80 mg/m3 and 64.4 ((mg/m3)*s) (pure TPU). There is the cooperation effect of BP nanosheets and TA that significantly decreased the release of CO2 and toxic CO. The barrier effect of BP nanosheets firstly suppresses the delivery of heat and inflammable pyrolysis products

ro of

between TPU matrix and fire, thus hindering the combustion reaction. BP nanosheets

and TA are then reacted with pyrolysis products to form solid products, including smoke particles and char residue. Less combust consumption of pyrolysis products

-p

indicated less release of heat and CO as well as CO2. The simultaneous reductions in

re

parameters, which stand for the fire hazards, confirm that the fire safety of TPU composite could be effectively improved, thereby further reducing casualties and

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serious economic losses during fire.[44]

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3.6. Flame retardant mechanism

The production amount of gas volatiles of pure TPU, TPU/pure-BP-2.0, and

ur

TPU/TA-BP-2.0 were investigated by TG-FTIR test under nitrogen atmosphere (Figure 11). As observed in Figure 11a, the characteristic adsorption peaks in the

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FTIR spectra of three samples are same, indicating the similar thermal decomposition products[45, 46]. After being normalized with the total sample mass, the absorbance intensity of total and individual characteristic peaks is compared to determine the release amount of pyrolysis products (Figure 11b). Distinguishing to other nanomterials, it is found that the intensity of characteristic peaks is significantly 22

enhanced with the incorporation of pure BP nanosheets and TA-BP nanosheets, including hydrocarbons (2977 cm-1), CO2 (2310 cm-1), carbonyl (1760 cm-1), and ethers (1150 cm-1)[47, 48]. Even, the pyrolysis products are detected earlier. It is speculated that pure BP nanosheets and TA-BP nanosheets will suffer from thermal pyrolysis process to produce radical scavenger, thus reacting with carbon radicals and producing more solid products. In addition, compared to the TG-IR result of

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TPU/pure-BP-2.0, the addition of TA-BP nanosheets increases the intensity of

characteristic peaks at lower level. These results forecasts that more inflammable products may be turned into solid products, without being burned. In comparison to

-p

the sole addition of pure BP nanosheets, the functionalized TA is attributed to the

re

formation of solid product in condensed phase, thus decreasing the peak intensity corresponding to gaseous pyrolysis products[49].

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In general, the chemical composition and morphology of char residue of the

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polymer composites, after combustion, could provide involving important messages to analyze the flame retardant mechanism[50]. As expected, pure TPU leaves little char

ur

residue with a cracked aluminized paper, indicating the thorough combustion (Figure 12a). Adding pure BP nanosheets and TA-BP nanosheets into TPU matrix obviously

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increase the char residue (Figure 12b and c). These results demonstrate that the enhanced fire safety of TPU composites is due to the formation of protective char layer. In addition, the chemical bond characteristic of char residue is analyzed by FTIR spectra (Figure 12d). The characteristic peaks at 1578 cm-1 and 1200 cm-1 of char residue of pure TPU are signed to the stretching vibration of C=O and C-O bonds, 23

respectively.[51, 52] With the incorporation of pure BP and TA-BP nanosheets, the characteristic peak at 1200 cm-1 is disappeared and a new peak near 1100 cm-1 is presented, which indicate that the formation of P-O bands in char layer of TPU composites.[53] The graphitization degree of char residue is directly related to the mechanical strength of protective char layer, which is analyzed by Raman spectra (Figure 12e).[54]

ro of

Char residue of pure TPU, TPU/pure-BP-2.0 and TPU/TA-BP-2.0 all display overlapped peaks with intensity maxima at 1350 cm-1 (called D band) and 1585 cm-1 (called G band), which stand for disordered graphite and organized graphitic

-p

structures, respectively. The intensity ratio of G band to D band is usually used to

re

compare the graphitization degree.[55] It is found that the IG/ID ratio of TPU/pure-BP-2.0 (0.334) is similar to that of pure TPU (0.363), indicating the

lP

incorporation of pure BP didn’t effectively increase the graphitization degree.

na

However, char of TPU/TA-BP-2.0 shows the significantly increased IG/ID value (0.792). This result confirms that the incorporation of TA-BP nanosheets can improve

ur

the graphitization degree of char residue. Likewise, high resolution C 1s XPS spectra of char residue of pure TPU and TPU/TA-BP-2.0 are used to investigate the

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graphitization degree (Figure 12f and g). It is found that the percentage of C-C bonds is increased to 67.8 % (TPU/TA-BP-2.0) from 60.4 % (pure TPU), indicating higher graphitization degree. Besides, characteristic peaks corresponding to P-C, P-O, and P=O, are found at high resolution P2p XPS spectrum at 133.9, 134.7, and 135.6 eV, respectively (Figure 12h). Compared to TA-BP nanosheets, the movement of 24

characteristic peaks of P 2p indicates that the oxidation reaction of TA-BP nanosheets during TPU combustion. As presented in flame retardant mechanism (Figure 13), due to the functionalization of TA, strong interfacial interaction is responsible for the well-dispersed state of BP nanosheets, thus presenting barrier effect in each place of polymer matrix to hinder the delivery of inflammable pyrolysis products. Follow on, TA-BP nanosheets also

ro of

suffer from thermal degradation process to produce radical quenching agent, thus reacting with the carbon radicals to cut off the combustion reaction, confirmed by the

presence of P-O, P-C, and P=O bonds. Meanwhile, the adsorbed TA is also

-p

contributed to the formation of char residue by reacting with chain radicals. In

re

addition, higher graphitization degree makes char residue mechanically strong, thus

4. Conclusion

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protecting the bottom polymer from fire.

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The functionalization and surface protection of BP nanosheets were successfully achieved with the use of natural antioxidant, which combines isolation and capture of

ur

superoxide radicals. Through TGA tests, higher temperature for thermal oxidation decomposition was found for TA-BP nanosheets, than those of pure BP nanosheets.

Jo

Compared to pure BP nanosheets, exposing to ambient for 10 days, the lattice fringes and chemical bonds of TA-BP nanosheets weren’t destroyed, due to the hydrogen donors release of TA which could eliminate superoxide radicals. Moreover, introduced TA could enhance the interfacial interactions and dispersion state of BP nanosheets in TPU matrix, thus promoting function of BP nanosheets in suppressing the fire hazards 25

of TPU. As a result, the peak values of HRR, THR, CO2, and CO during TPU combustion were effectively decreased by 56.5 %, 43.0 %, 57.3%, 31.3 %, respectively. Specially, the total release of deadly CO was significantly decreased by 1.5 wt% TA-BP, up to 55.1 %, indicating the suppressed fire hazards. In addition to the increase of tensile strength (22.7 %), during service process, the self-stiffness phenomenon was also suppressed to ensure the use of TPU composites in dynamic

ro of

condition. It is believed that this natural antioxidant functionalization will not only provide a general route to overcome the instability of 2D materials under oxygen, but also push the practical use of BP nanosheets in fire safety application of polymer

Jo

ur

na

lP

re

-p

composites.

26

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Table and Figure captions Table 1 TGA data of TPU and its composites. Table 2 PHRR, THR, and residual char data of TPU and its composites. Table 3 The peak values and time integral values of CO2 and CO of TPU and its composites.

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Figure 1 A schematic illustration for the preparation of TA-functionalized BP

Figure 2 XRD curves of pure BP nanosheets and TA-BP nanosheets (a); Raman

spectra of pure BP nanosheets and TA-BP nanosheets (b); FTIR spectra of pure BP

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nanosheets and TA as well as TA-BP nanosheets (c).

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Figure 3 TGA and DTG curves of pure BP nanosheets, TA, and TA-BP nanosheets at air (a and c); TGA and DTG curves of pure BP nanosheets, TA, and TA-BP

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nanosheets at nitrogen atmosphere (b and d).

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Figure 4 TEM photographs and high resolution TEM images of TA-BP nanosheets (a and b) and pure BP nanosheets (c and d) exposed to ambient for 10 days; Sign spectra

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of selected lattice fringe of TA-BP nanosheets (e1, e2, and e3). Figure 5 Deconvoluted XPS survey of P 2p of initial BP nanosheets (a), pure BP

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nanosheets (b), and TA-BP nanosheets (c) exposed to ambient for 10 days; Protective mechanism of TA for BP nanosheets (d). Figure 6 SEM photographs of fracture surface of pure TPU (a), TPU/TA-BP-2.0 (b), TPU/pure-BP-2.0 (c), SEM Mapping image of TPU/pure-BP-2.0 (d). Figure 7 Tensile curves and data of pure TPU and its composites (a and b); The 36

stiffness increase curves versus cycles of pure TPU, TPU/TA-BP-2.0, and TPU/pure-BP-2.0 (c); The η* and Tan θ curves of pure TPU and TPU/TA-BP-2.0. Figure 8 TGA curves of TPU and its composites under air (a and b) and nitrogen (c and d). Figure 9 HRR versus time curves of TPU and its composites through MCC (c and d); HRR (c) and THR (d) curves versus time of TPU and its composites through CC test.

CO (b and d) of TPU and its composites through CC test.

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Figure 10 The concentration and total release curves versus time of CO2 (a and c),

Figure 11 FTIR spectra of the pyrolysis gaseous products emitted from neat TPU and

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TPU/TA-BP-2.0 at the maximum degradation rate (a); Absorbance spectra of

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pyrolysis products for neat TPU and TPU/TA-BP-2.0 versus time (b). Figure 12 Photos of the char of (a) pure TPU, (b) TPU/pure-BP-2.0, and (c) TPU/

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TA-BP-2.0 composites; FTIR spectra (d) and Raman spectra (e) of char residue of

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pure TPU, TPU/pure-BP-2.0, and TPU/TA-BP-2.0 composites; high-resolution XPS spectra of C 1s of char of pure TPU (f); high-resolution XPS spectra of C 1s (g) and P

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2p (h) of char residue of TPU/TA-BP-2.0.

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Figure 13 Scheme of proposed flame-retardant mechanism.

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Table 1 Nitrogen T5wt% MLRmax Char residue (oC) (%/oC) at 600 oC (wt%) 285.2 1.37 3.77 297.7 1.36 4.14 297.8 1.42 5.20 302.3 1.43 6.06 306.1 1.42 6.32 302.9 1.25 6.40

Table 2 TTP (s)

PHRR (kW/m2)

THR (MJ/m2)

51 48 37 44 39 43

152 135 120 124 125 132

1293 1092 884 678 562 640

79.9 71.0 61.7 59.7 45.5 67.2

Table 3

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CO2 (%) Pure TPU 15.7 TPU/TA-BP-0.5 14.5 TPU/TA-BP-1.0 12.3 TPU/TA-BP-1.5 8.3 TPU/TA-BP-2.0 6.7 TPU/pure-BP-2.0 8.4

re CO (mg/m3) 0.80 0.84 0.81 0.52 0.55 0.92

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FGI Residual 2 (kW/m s) char (wt%) 8.50 4.3 8.09 6.8 7.37 6.8 5.47 7.7 4.50 9.7 4.85 9.8

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TTI (s)

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Pure TPU TPU/TA-BP-0.5 TPU/TA-BP-1.0 TPU/TA-BP-1.5 TPU/TA-BP-2.0 TPU/pure-BP-2.0

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Air T5wt% MLRmax Char residue (oC) (%/oC) at 700 oC (wt%) 291.3 0.88 0.53 Pure TPU 292.0 0.89 1.08 TPU/TA-BP-0.5 294.3 0.85 1.51 TPU/TA-BP-1.0 291.1 0.93 2.26 TPU/TA-BP-1.5 293.3 0.84 2.68 TPU/TA-BP-2.0 3.28 TPU/pure-BP-2.0 293.8 0.91

CO2 (%*s) 1024 859 801 671 671 679

CO ((mg/m3)*s) 64.4 52.4 52.4 28.9 40.1 52.4

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