An operable platform towards functionalization of chemically inert boron nitride nanosheets for flame retardancy and toxic gas suppression of thermoplastic polyurethane

Composites Part B 178 (2019) 107462

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Composites Part B journal homepage: www.elsevier.com/locate/compositesb

An operable platform towards functionalization of chemically inert boron nitride nanosheets for flame retardancy and toxic gas suppression of thermoplastic polyurethane Wei Cai a, 1, Bibo Wang a, 1, Longxiang Liu a, Xia Zhou a, Fukai Chu a, Jing Zhan a, b, Yuan Hu a, Yongchun Kan a, **, Xin Wang a, * a b

State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui, 230026, PR China School of Civil Engineering and Environmental Engineering, Anhui Xinhua University, Hefei, Anhui, 230088, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Thermoplastic polyurethane Boron nitride Lewis acid-base interactions Toxic gas suppression Flame retardant

The inherent chemical inertness presents a huge challenge for the functionalization of hexagonal boron nitride (h-BN), thus limiting its potential in flame retardant and toxic gas suppression of thermoplasticity polyurethane (TPU). Here, with the assistance of Lewis acid-base interactions, an operable platform formed by SiO2 coating is constructed onto the surface of h-BN nanosheets, offering an opportunity for introducing phytic acid (PA). The resultant h-BN nanohybrids present an enhancement effect for flame retardancy of thermoplastic polyurethane (TPU), confirmed by obvious reductions in peak value of heat release rate ( 23.5%) and total heat release ( 22.1%). Meanwhile, the smoke product rate and total smoke release of TPU composite containing 2.0 wt% hBN nanohybrids are decreased by 29.2% and 8.6%, respectively. Through CO2 detector and AtmosFIR, specially, it is found that the toxic gases (CO, CH4, C2H6, TOC) are turned into CO2. Through a series of analytic methods, it was found that the introduced PA suffered from a pre-gradation process to release P-containing compounds, reacting with TPU matrix to produce protective char. In addition, the presence of SiO2 was also contributed to improve the robustness of char residue. In view of high temperature condition, the catalysis effect of h-BN is responsible for the conversion of toxic gases. Therefore, the enhanced fire safety of TPU was attributed to the cooperation mechanism of h-BN, SiO2, and PA. Such a functionalization approach provides a novel route to overcome the chemical inertness of h-BN, thus promoting its application in fire safety fields of polymer materials.

1. Introduction In recent years, considerable research attempts have been devoted to directly exploit the superior performance of hexagonal boron nitride (hBN) nanosheets in various application fields, including hydrogen stor­ age, catalyst, pollution removal, gas separation, and anticancer drug delivery [1–5]. For instance, without the confine of Van der Waals’ force from adjacent nanosheets, excellent phonon transmission rate of h-BN nanosheets imparts higher thermal conductivity to polymer composites [6]. Recent research progresses have confirmed the fascinating proper­ ties of h-BN nanosheets and provide a strong impetus to continuously exploit its potential prospect. In comparison to the C atoms in a hex­ agonal graphitic lattice of graphene, alternating boron and nitrogen

atoms are existed in hexagonal lattice and construct a honeycomb-like layered structure [7]. However, the stack structure of h-BN is obvi­ ously different to the C atoms in graphite. The instinct electronegativity difference between B and N atoms makes the electron pair asymmetri­ cally dispersed in B–N domain, thus causing partially ionic character­ istic. Therefore, B atoms could interact with N atoms in the neighbor layers, leading to the peculiar B–N stacking characteristics of bulk h-BN. The chemical bonds formed as bridges between atoms of adjacent layers are commonly known as “lip–lip” interactions. Due to the partial ionic characteristic of B–N bonds and the “lip-lip” interactions between neighboring layers, therefore, the functionaliza­ tion route of h-BN nanosheets is severely confined by highly-degree chemical inertness, which causes a huge challenge for the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Kan), [email protected] (X. Wang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compositesb.2019.107462 Received 5 June 2019; Received in revised form 15 September 2019; Accepted 16 September 2019 Available online 21 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. A schematic illustration for the preparation of functionalized h-BN.

reinforcement effect of h-BN nanosheets in application fields [8]. Recently, a lot of research work adopted acid/alkali/oxidant/high temperature conditions to process h-BN nanosheets, for obtaining high active surface [9], which required complicate steps and large con­ sumption of solvent. Fortunately, Lewis acid-base interactions presented a simple and reliable strategy for modifying the surface characteristic of h-BN nanosheets [10,11]. The specific polarity of B–N bonds is gener­ ated from the electronegativity disparity, which leads to partial positive and negative charges for B and N atoms, respectively. Therefore, the electron lone-pair of the organic amine molecules (Lewis base) is capable of interacting with the vacant p-orbitals of the boron atoms (Lewis acid) distributed onto the h-BN surface, thus assisting the exfo­ liation and functionalization process [12]. Compared to the use of acid/alkali/oxidant/high temperature condition for making h-BN sur­ face active, the employment of Lewis acid-base interactions is easier to operate without high-demanding equipment [9,13–15]. Huang et al. reported the formation of chemical bonds between electron-deficient B atoms of h-BN nanosheets and electron-donating N atoms of poly­ ethyleneimine, rather than simple physical interactions [16]. Similar result has been also revealed in theoretical studies for the adsorption of NH3 and amine groups on BN nanotubes and nanosheets [17]. Taking into consideration superior thermal conductivity and low dielectric properties, h-BN nanosheets have been widely used in thermal management materials, in order to solve the rapidly increasing power density of modern electronic devices [18,19]. Sun et al. utilized hot-pressing induced orientation route to obtain orderly distributed h-BN nanosheets for providing a thermal transfer pathway [20]. During service, the largely accumulated heat between electronic devices and thermal interface materials would lead to a fire failure. Therefore, the research progresses for improving the flame retardancy of h-BN nano­ sheets toward guarantying the usability performance of the thermal interface materials is needed to be effectively pushed. The consumption amount of thermoplastic polyurethane (TPU) in thermal interface ma­ terial fields is gradually increased, especially at the soft pack of electric device. However, the flammable characteristic commonly found in polymeric material makes TPU-based thermal interface materials be ignited easily, leading to a potential fire hazards to people’s lives and

properties [21]. Due to the barrier effect of two dimensional nanosheet, the delivery of flammable pyrolysis products to fire is effectively sup­ pressed [22,23]. In previous works of our group, various layered nanomaterials have been used to enhance the flame retardancy of TPU, including graphene, MoS2, and carbon nitride [24,25]. For example, the addition of 4.0 wt% electrochemically prepared graphene reduced peak value of heat release rate and total heat release of TPU by 46.3% and 20.2%, respectively [26]. The combination of layered structure and superior thermal stability enabled h-BN to present significant potential to enhance the flame retardant of TPU [27–29]. Meanwhile, Si and P elements have been confirmed as highly-effect flame retardant elements, which could be incorporated into h-BN nanosheets to improve the flame retardancy [30]. Due to the existence of NH2- group, in this work, (3-aminopropyl) triethoxysilane (APTES) was used to interact with h-BN nanosheets to form a APTES-composed coating. Meanwhile, the cross-linked reaction between introduced oxyethyl group of APTES and tetraethyl orthosili­ cate was performed to produce a SiO2-based coating. In addition, phytic acid could be introduced into h-BN nanohybrids by chelating with the NH2 group of SiO2-based coating. Finally, Lewis acid-base interactions induced functionalization coating is prepared onto the chemically inert surface of h-BN nanosheets, aiming at imparting flame retardancy and toxic gas suppression for h-BN nanohybrids. The successful employment of Lewis acid-base interactions is directly observed by Transmission electron microscope (TEM) mapping images. Follow on, the resultant hBN nanohybrids are used to enhance the flame retardant and toxic gas suppression of TPU. 2. Experimental The detailed experimental information, including raw materials, preparation of h-BN@SiO2 nanosheets, h-BN@SiO2@PA, TPU compos­ ites, and characterization were provided in the Supplementary Material.

2

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Fig. 2. TEM photographs of bulk h-BN (a), h-BN@SiO2 (b and d), SiO2 spheres (c), and h-BN@SiO2@PA (e); elemental mapping (f) of h-BN@SiO2@PA, boron (f1), nitrogen (f2), silicon (f3), and phosphorus (f4). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

3. Result and discussion

presented higher Si element density than the surface of h-BN. If Lewis acid-base interactions were not constructed between APTES and h-BN, the distribution of N elements was same to the Si element. Comparing Fig. 2f2 and 2f3 difference, element loop similar to Si was not capable of being observing in N elements distribution. In addition, N element presented a similar core distribution to B element (Fig. 2f1). These re­ sults all indicated that APTES strongly combined h-BN nanosheets with Lewis acid-base interactions, rather than homogenous physical mixture. By the formed chelation bonds with proton amine, the successful detection of P element in TEM Mapping confirmed the surface of h-BN@SiO2 was covered by PA (Fig. 2f4). After being functionalized, the characteristic in-plane transverse optical modes (1384 cm 1) and out-of plane bending vibration (805 cm 1) of B–N were still able to be found in Fourier transform infrared (FTIR) spectroscopy spectra of h-BN@SiO2 and hBN@SiO2@PA (Fig. 3a) [32]. Based on the spectrum of SiO2, a new and broad peak from 1075 cm 1 to 1157 cm 1 in FTIR spectra of h-BN@SiO2 and h-BN@SiO2@PA were attributed to the Si–O vibration, confirming the successful synthesis of SiO2 coating [33]. It was frustrating that both P–O and C–P bonds of phytic acid weren’t detected in FTIR spectrum of h-BN@SiO2@PA, which may be due to the low addition of PA and low sign intensity. As exhibited by Fig. 3b, X-ray diffraction data (XRD) diffraction patterns of bulk h-BN displayed five peaks located at 26.7� , 41.7� , 44.0� , 50.2� and 55.1� , corresponding to the (002), (100), (101), (102), and (004) planes [34]. Usually, the diffraction peak intensity of XRD pattern is directly proportional to the re-stack degree of h-BN. Compared to the extremely strong reflection characteristic of bulk h-BN, the intensity of diffraction peak corresponding to (002) plane of h-BN@SiO2 nanohybrids was obviously decreased (Fig. 3b). Combining the result of other faint peaks including (100), (101), (102), and (004), it

3.1. SiO2 coating formation and flame retardant functionalization of hBN nanosheets A schematic illustration for SiO2 coating formation and flame retardant functionalization of h-BN was presented in Fig. 1. This smooth edge structure was presented to pure h-BN (Fig. 2a). Through Lewis acid-base interactions between B atoms of h-BN and amine group, h-BN nanosheets were firstly coated by (3-aminopropyl)triethoxysilane (APTES). Then, appropriate pH value (9.0) promoted the hydrolytic condensation of introduced tetraethyl orthosilicate (TEOS) and APTES. Through the TEM photograph (Fig. 2b), it was found that the edge of hBN nanohybrids was embraced by a heterogeneous structure with a width of approximately 20 nm. The heterogeneous annular structure was speculated as SiO2. Without the presence of h-BN nanosheets, SiO2 spheres was produced by the cross-linked reaction between APTES and TEOS, indicated by Fig. 2c. In addition, the sole mixture of h-BN and TEOS was performed by Yao et al. [31]. It was found that large amount of SiO2 sphere was physically deposited onto the surface of h-BN nanosheets. Such a comparison implied the importance of APTES in the formation of SiO2 coating onto the surface of h-BN nanosheets. The formation of SiO2 coating could provide a highly-chemical active surface for the further functionalization of h-BN nanosheets. Due to the pro­ tonation of amine groups in aqueous solution, h-BN@SiO2 nanohybrids was able to combine with phytic acid by the coordinate linkages [30]. Compared to h-BN@SiO2, the boundary between h-BN nanosheets and SiO2 loop of h-BN@SiO2@PA was vaguer, which was due to the cover of PA (Fig. 2e). The elements distribution of produced h-BN@SiO2@PA was investigated by TEM EDS Mapping. As Fig. 2f3 indicated, SiO2 loop 3

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Fig. 3. FTIR spectra (a), XRD curves (b), XPS results (c) of bulk h-BN, h-BN@SiO2, SiO2 and h-BN@SiO2@PA; Deconvoluted XPS survey of N 1s of bulk h-BN (d), SiO2 (e) and h-BN@SiO2 (f); Deconvoluted XPS survey (g) of P 2p of h-BN@SiO2@PA; TGA curves (h) of bulk h-BN, SiO2, h-BN@SiO2, PA, h-BN@SiO2@PA at nitrogen atmosphere.

functionalization, these involving peaks intensity of h-BN@SiO2@PA were further decreased. These results all indicated that the re-stack phenomenon of h-BN@SiO2@PA was effectively suppressed.

was speculated that APTES molecule could strongly interact with h-BN nanosheets and react with TEOS to form SiO2 coating, thus hindering the re-stack phenomenon. In particular, after flame retardant

Fig. 4. SEM photographs of fracture surface of pure TPU (a), TPU/bulk h-BN-2.0 (b), TPU/SiO2-2.0 (c), TPU/h-BN@SiO2 (d), and TPU/h-BN@[email protected] (e); (f) FTIR spectra of pure TPU and TPU/h-BN@[email protected]. 4

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Fig. 5. TGA and DTG curves of TPU and its composites under air (a and b) and nitrogen (c and d) atmospheres.

In Fig. 3c, binding energy peaks of B 1s, N 1s, O 1s and C 1s could be detected in X-ray photoelectron spectrometer (XPS) results of bulk h-BN, h-BN@SiO2 and h-BN@SiO2@PA [35]. In addition, new characteristic peaks for Si (21.08 at%) and P (4.08 at%) elements were also presented in XPS results of h-BN@SiO2 and h-BN@SiO2@PA. The Lewis acid-base interactions between h-BN nanosheets and SiO2 coating was also confirmed by XPS. As presented by Fig. 3d, the N 1s curve of bulk h-BN was deconvoluted into two peaks near 398.9 eV and 398.2 eV, belong to free NH2- group and N–B bonds [36]. Meanwhile, free (401.3 eV) and bonded (399.3 eV) NH2- group were existed together for XPS result of SiO2, presented by Fig. 3e [37]. In addition, the ratio of bonded NH2group was less than free NH2- group. However, the ratio of free to bonded NH2- group was decreased in the XPS result of h-BN@SiO2 (Fig. 3f). According to other research reports, this phenomenon was attributed that electron-rich N atoms interacted with electron deficient B atoms of h-BN nanosheets by Lewis acid-base interactions, thus increasing the ratio of bonded N atoms [10,38]. Besides, high-resolution P 2p XPS spectrum of h-BN@SiO2@PA was presented in Fig. 3g [39]. The thermal stability and functionalized amount of functionalized hBN were determined by thermogravimetric analysis (TGA) test under nitrogen. As shown by Fig. 3h, bulk h-BN presents a roughly identical weight even temperature is up to 750 � C. This result indicated that h-BN was capable of holding layered structure under high temperature to present barrier effect. After being functionalized by SiO2 and PA, mass loss of 3.8 wt % and 14.5 wt% at 750 � C was observed and attributed to the thermal degradation of SiO2 and PA. Based on the mass loss of SiO2 and PA, the functionalized amounts of SiO2 and PA were 26.0 wt% and 13.7 wt%, respectively. Such a high thermal stability of h-BN and its nanohybrids indicated that the barrier function of layered structure was able to hinder the mass delivery during combustion.

3.2. Characteristic and properties of TPU composites 3.2.1. Fractured surface characteristic of TPU composites The interfacial interactions of h-BN@SiO2@PA nanohybrids with polymer matrix is of significant to enhance the performance of polymer composites [40–42]. Therefore, fracture surfaces of TPU composites obtained under liquid nitrogen were investigated by scanning electron microscope (SEM). The cross-section of pure TPU was extremely smooth, and no crack stripe could be found in Fig. 4a. However, the incorpora­ tion of bulk h-BN makes the fracture surface become rougher, causing apparent irregular stripes appear, confirming the formation of strong interfacial interactions [43]. This phenomenon may be attributed to the hydrogen-bond interactions between N atoms of h-BN and urethane groups of TPU. Meanwhile, the aggregation of bulk h-BN was also observed in inset of Fig. 4b. It was notable that the introduced SiO2 sphere didn’t lead to an obvious distinction for fracture surface (Fig. 4c). After being covered by SiO2, h-BN@SiO2 produced more protuberances in fracture surface of TPU composites (Fig. 4d). This result may be due to the SiO2 coating causes a better dispersion state to h-BN@SiO2 nano­ hybrids. In contrast to fracture surface of pure TPU and TPU with bulk h-BN, both well-dispersed h-BN@SiO2@PA nanohybrids and irregular stripes were observed in Fig. 4e. It wasn’t hard to find that h-BN@SiO2@PA nanohybrids were homogenously distributed in the fracture surface (signed by red arrows). Meanwhile, h-BN@SiO2@PA nanohybrids aggregation in TPU matrix was not exist. In addition, as shown in Fig. 4f, a significant red-shift phenomenon (moved to 3376 cm 1 from 3436 cm 1) was found for the characteristic peak of –NH– in TPU chains, from FTIR spectra of pure TPU and TPU/h-BN@­ [email protected] [44]. Due to the presence of PA and aminated-SiO2, abundant hydroxyl and amino groups were existed onto the surface of h-BN@SiO2@PA. In view of the presence of urethane groups, it isn’t hard to understand the formation of hydrogen-bond interactions 5

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Table 1 TGA data of TPU and its composites at air and nitrogen atmospheres. Air

Pure TPU TPU/bulk h-BN-2.0 TPU/SiO2-2.0 TPU/[email protected] TPU/h-BN@[email protected]

N2

T5wt% � 1 (oC)

T50 wt% � 1 (oC)

Residual mass at 750 C � 0.2 (wt %)

T5wt% � 1 (oC)

T50 wt% � 1 (oC)

Residual mass at 750 � C � 0.2 (wt %)

305.8 305.3 309.7 308.4 300.8

397.4 396.2 401.7 400.7 395.4

0.33 1.78 2.33 0.92 1.42

296.6 299.3 291.4 306.1 300.9

396.1 395.0 383.1 398.6 399.5

3.93 5.31 4.75 4.78 6.11

�

Fig. 6. HRR (a), THR (b), SPR (c), and TSR (d) versus time curves of TPU and its composites.

between hydroxyl and amino groups and urethane groups. These results all demonstrated that h-BN@SiO2@PA nanohybrids were capable of forming strong interfacial interactions with molecule chains of TPU.

extremely high thermal resistance of h-BN and SiO2, this slight increase in residual mass was attributed to nanofiller itself (2.0 wt%). As a kind of low molecular compounds, the thermal stability of phytic acid was lower than those of TPU and h-BN as well as SiO2. Therefore, it wasn’t strange that the PA suffers from earlier decomposition and reacts with TPU chains, thus causing lower T5wt% ( 5 � C) for TPU/h-BN@[email protected] [26,46]. Meanwhile, higher residual char confirmed the reaction of PA with TPU chains facilitated the formation of char layer (inset in Fig. 5a). Compared to pure TPU, with the addition of h-BN@SiO2@PA, the maximum mass loss rate at second stage of TPU composites was decreased to 0.73%/oC from 0.79%/oC. The thermal stability of TPU composites was also investigated under nitrogen atmosphere, which related to the char residue of TPU com­ posites after combustion (Fig. 5c and d). As presented in Table 1, in addition to the SiO2 nanoparticles, the inclusion of bulk h-BN, hBN@SiO2, and h-BN@SiO2@PA all increased T5.0 wt% of TPU compos­ ites. In view of the higher thermal stability and layered structure of hBN, the increased T5.0 wt% was attributed to the barrier effect of h-BN. After being functionalized by SiO2, well-dispersed h-BN@SiO2 further

3.2.2. Thermal stability of TPU composites The thermal stability of neat TPU and TPU composites was moni­ tored by TGA test at air atmosphere. The residual and derived weight versus temperature curves were shown in Fig. 5a and b, and according data were shown in Table 1, including the characteristic degradation temperature at weight loss of 5 wt% (T5wt%) and 50 wt% (T50 wt%), as well as residual mass at 750 � C. Pure TPU presented three decomposition stages under air atmosphere, which were corresponding to the main chains degradation, pyrolysis of polyols and isocyanates, residual char oxidation [45]. Comparing to the TGA curve of pure TPU, similar TG and DTG curves of TPU composites indicated that the incorporation of nanofillers didn’t cause significant distinction for the thermal stability of TPU. When bulk h-BN and SiO2 as well as h-BN@SiO2 were added solely, the thermal degradation process of TPU composite was same to that of pure TPU, even with a higher residual mass at 750 � C. In view of 6

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composites at high temperature.

Table 2 Cone calorimeter data of TPU and its composites.

Pure TPU TPU/bulk h-BN-2.0 TPU/SiO2-2.0 TPU/[email protected] TPU/hBN@[email protected]

Tig (s)

pHRR (kW/ m2)

THR (MJ/ m2)

SPR (m2/s)

TSR m2

57 50 57 54 44

830 729 715 661 635

94.6 82.4 85.5 87.0 73.7

0.24 0.17 0.20 0.12 0.17

15.1 14.2 16.2 13.8 13.8

3.2.3. Fire safety investigation of TPU composites The time to ignition (Tig), heat release rate (HRR), and total heat release (THR) curves obtained by cone calorimeter test (CC) are pre­ sented in Fig. 6, coming as a support for Table 2. It was found that, with the addition of SiO2 nanoparticles, Tig was still kept at same value to pure TPU (57s). However, the incorporation of bulk h-BN and hBN@SiO2 decreased Tig of TPU composites by 7s and 3s, respectively, which was due to the presence of h-BN. This result demonstrated that high thermal conductive nanofiller would promote the thermal transfer to decrease time to ignition. In addition, the largest decrease of Tig was in TPU/h-BN@SiO2@PA, where Tig was decreased by 13s. Based on the TGA results, the decreased Tig of TPU/h-BN@SiO2@PA was due to the low thermal stability of PA, which was decomposed earlier to release large amount of phosphorous-containing compounds, which promotes the degradation of TPU matrix to form char residue. Compared to PHRR of pure TPU (830 kW/m2), the peak HRR values of TPU composites

increased the T5.0 wt%, increased to 306.1 � C from 296.6 � C. Due to the earlier degradation of PA, however, T5.0 wt% of TPU/h-BN@[email protected] was decreased to 300.9 � C. The residual mass at 750 � C of TPU/hBN@[email protected] (6.11 wt%) was higher than those of other sample, especially for TPU/bulk h-BN-2.0 (5.31 wt%), whose nanofiller was thermally stable bulk h-BN. This result demonstrated that the intro­ duction of PA was attributed to the formation of residual char of TPU

Fig. 7. Gas release curves versus time detected by CO2 sensor and AtmosFIR of pure TPU and its composites, including CO2 (a), CO (b), TOC (c), CH4 (d), and C2H6 (e). 7

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Table 3 Peak value of combustion gas concentration of pure TPU and its composites. Table 4 The time integral of combustion gas concentration of pure TPU and its composites.

Pure TPU TPU/bulk-BN-2.0 TPU/SiO2 -2.0 TPU/h-BN@SiO2 -2.0 TPU/h-BN@[email protected]

CO2 (%)

CO (%)

CH4 (%)

C2H6 (%)

TOC (%)

4.62 4.41 4.05 3.82 4.11

0.41 0.41 0.40 0.46 0.30

0.064 0.068 0.068 0.090 0.037

0.0044 0.0048 0.0055 0.0065 0.0019

0.149 0.151 0.160 0.202 0.072

Table 5 Results of LOI and UL-94 test for pure TPU and its composites. Pure TPU TPU/bulk-BN-2.0 TPU/SiO2 -2.0 TPU/h-BN@SiO2 -2.0 TPU/h-BN@[email protected]

LOI

UL-94

22.0 22.5 22.5 22.5 22.5

No rating No rating No rating V-2 V-2

Table 4 The time integral of combustion gas concentration of pure TPU and its composites.

Pure TPU TPU/bulk-BN-2.0 TPU/SiO2 -2.0 TPU/h-BN@SiO2 -2.0 TPU/hBN@[email protected]

CO2 (% *s)

CO (% *s)

CH4 (% *s)

C2H6 (% *s)

TOC (% *s)

2235 2199 2271 1858 2005

195 166 168 179 98

18.4 18.4 22.9 24.0 9.05

1.26 1.21 1.76 1.66 0.49

53.7 53.1 65.8 67.2 23.9

containing bulk h-BN (729 kW/m2) and SiO2 (715 kW/m2) are decreased by 12.2% and 13.9%, respectively. Meanwhile, the calculated THR values per unit surface area of TPU/SiO2-2.0 (85.5 MJ/m2), TPU/[email protected] (87.0 MJ/m2) are close to THR of pure TPU (94.6 MJ/m2). Due to the existence of NH2 group, PA successfully combined with the hBN@SiO2 nanosheets by the coordinate linkages [47]. The flame retar­ dant functionalization makes h-BN nanohybrids possess a highly effi­ cient flame retardancy, confirmed by lower pHRR (635 kW/m2, decreased by 23.5%) and THR values (73.7 MJ/m2, decreased by 22.1%) in TPU/h-BN@SiO2@PA-2. The smoke product rate (SPR) and total smoke release (TSR) are another two critical parameters to evaluate the fire safety of materials since toxic smoke not only makes it difficult for trapped people to escape from the fire, but also can easily be inhaled by the people trapped in the fire and lead to poisoning. As presented in Fig. 6c and d, pure TPU resin presented SPR of 0.24 m2/sand TSR of 15.1 m2. It is worth mentioning that the TSR is the time integral of SPR. The TSR value at 200 s integral of SPR was chosen. With addition of 2.0 wt% bulk h-BN, SPR and TSR were decreased by 29.2% and 6.1%, comparing to those of pure TPU. Meanwhile, both SiO2 and h-BN@SiO2@PA also caused a suppression effect for the smoke release during TPU combustion, confirmed by lower SPR. Though the SPR of TPU/h-BN@[email protected] was higher than that of TPU/bulk h-BN-2.0, the TSR was obviously lower. When 2.0 wt% hBN@SiO2@PA was added, a desirable smoke suppression effect was obtained, confirmed by decreases of 15% and 13.5% in SPR and TSR, respectively.

Fig. 8. Photographs of pure TPU and TPU composites after UL-94 test: (a) pure TPU; (b) TPU/bulk h-BN-2.0; (c) TPU/SiO2-2.0; (d) TPU/[email protected]; (e) TPU/h-BN@[email protected]

concentration, which are 4.41% and 4.05%, respectively. It was found that incorporated h-BN@SiO2 not only decreased the maximum CO2 concentration, but also suppress the release process of CO2 (Fig. 7a). The time integral of CO2 concentration in TPU/h-BN@SiO2 is 1858%*s, much lower than other samples (Table 4). It was notable that the introduction of 2.0 wt% h-BN@SiO2@PA results in increase for the CO2 release, which was confirmed by higher concentration (4.11%) and time integral (2005%*s). Compared to other samples, however, maximum concentration and time integral of toxic gases were significantly decreased in TPU/h-BN@SiO2@PA. For example, incorporated h-BN@SiO2@PA reduced the time integral values of CO, CH4, C2H6, TOC concentration to 98%*s, 9.05%*s, 0.49%*s, 23.9%*s, from 195%*s, 18.4%*s, 12.6%*s, 53.7%*s (pure TPU). Such significant decreases indicated that the gaseous toxicity of TPU combustion was effectively suppressed with the addition of h-BN@SiO2@PA. 3.2.5. LOI and UL-94 test results LOI and UL-94 tests were also performed to evaluate the flame retardancy of pure TPU and its composites. Table 5 presented the results of LOI and UL-94 tests, meanwhile the samples after UL-94 test were presented in Fig. 8. LOI value of pure TPU was 22.0, which was similar to the previous literature [49]. However, even with the addition of nano­ fillers, the LOI value of TPU composites weren’t effectively increased. The slight increase was attributed to the low addition of nanofillers [50]. As presented by residual samples (Fig. 8), TPU composites were seri­ ously burned during UL-94 test. In addition, due to low melt viscosity, the dripping flame was existed for pure TPU and its composites and ignited the bottom cotton. Though the total two combustion time of pure TPU, TPU/bulk h-BN-2.0, and TPU/SiO2-2.0 was higher than 30s, the addition of h-BN@SiO2 and h-BN@SiO2@PA was able to decrease the combustion time, thus obtaining V-2 rating.

3.2.4. Toxic gases analysis Even a miniscule intake amount, CO produced by materials com­ bustion makes the person poisoning and occupies main cause of death in fire [48]. Once concentration is too high, some other gases also cause severe harm to the human body, including CH4, C2H6, and TOC. As presented in Fig. S1, the produced gas was extracted and detected by CO2 detector and AtmosFIR, meanwhile according concentration versus time curves were exhibited in Fig. 7, including CO2, CO, C2H6, CH4, and total organic carbon (TOC). In addition, maximum gas concentration and the time integral of gas concentration (as a reference for total release) were presented in Table 3 and Table 4, respectively. The peak value of CO2 concentration was obtained for pure TPU, which was 4.62%. The addition of bulk-BN and SiO2 slightly decreased CO2

3.2.6. Flame retardant mechanism Microscale combustion calorimeter (MCC) was carried out to directly monitor the flammability of pyrolysis gases evolved during controlled 8

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Fig. 9. HRR curves (a) of TPU and its composites through MCC; 3D TG-IR spectra of pure TPU and TPU/h-BN@[email protected] (b and c); (d) FTIR spectra of the pyrolysis gaseous products emitted from neat TPU and its composites at the maximum degradation rate; Absorbance of pyrolysis products for neat TPU and its composites vs time (f and e).

heating of the samples under N2 atmosphere [51]. This obtained heat release versus temperature curves were displayed in Fig. 9a. In consis­ tent with TGA results in nitrogen, two heat release peaks observed indicated that TPU suffered from two-step thermal degradation process [52]. Compared to PHRR of pure TPU (370 W/g), the introduction of bulk h-BN couldn’t effectively decreased the PHRR of TPU/bulk h-BN-2.0 (350 W/g). Due to the presence of Si element, SiO2 nano­ particles and h-BN@SiO2 could react with TPU matrix to form char residue, thus decreasing the release of inflammable products. Therefore, PHRR values of TPU/SiO2-2.0 and TPU/h-BN@SiO2 were decreased to 328 W/g and 322 W/g, respectively. As expected, the most significant decrease for PHRR was obtained by the incorporation of h-BN@PA@­ SiO2 nanosheets, reduced by 19.5%. These results demonstrated that the formation of char residue and decreased pyrolysis products were mainly due to the introduction of SiO2 and PA, rather than h-BN nanosheets. In the interior of polymer matrix, thermal degradation process continuously release gaseous volatile under anaerobic atmosphere. Therefore, TG-IR was also used to simulate and monitor the release process [53]. This visualized evidence for the decreased gas release caused by the suppression effect of h-BN@SiO2@PA was directly depicted by 3D TG-IR spectra (Fig. 9b and c) [30,51]. It was found that the sign intensity of pyrolysis products from TPU/h-BN@[email protected] was much lower than that of pure TPU. In addition, the 3D TG-IR spectra of other samples were presented in Fig. S2. Furthermore, FTIR absorp­ tion spectra obtained in strongest sign moment displayed similar char­ acteristic peaks for the pyrolysis gas from the thermal decomposition of pure TPU and its composites under nitrogen atmosphere, indicating the gas types still kept consistent before and after the incorporation of h-BN@[email protected] (Fig. 9d). After being divided by sample mass, the

intensity of characteristic absorption peaks was used to compare the release amount of pyrolysis products. The incorporation of h-BN@SiO2@PA is contributed to reduce the release of pyrolysis gases, confirmed by decreased intensity of total and characteristic absorption peak, including aromatic compounds (1608 cm 1), carbonyl (1774 cm 1), isocyanates (2275 cm 1), and hydrocarbon (2977 cm 1) [54]. Based on above analysis, it could draw a conclusion that incor­ porated h-BN@[email protected] was capable of suppressing the interior thermal decomposition of bulk TPU material, thus decreasing release of pyrolysis gas. Such a suppression effect may be attributed to synergistic effect between SiO2 and PA. The morphologies and composition of residual residue after CC test were investigated to analyze condensed phase mechanism [55,56]. It was found that the char yields of pure TPU, TPU/bulk h-BN-2.0, and TPU/h-BN@[email protected] were gradually increased (Fig. 10a, b, and c). Extremely cracked and scattered char residue bulk was observed in Fig. 10d, further indicating the produced char layer of pure TPU wasn’t capable of inhibiting delivery of flammable gas. Even though holes and cracks were still presented, char residue wasn’t scattered bulk any more after adding bulk h-BN. In addition, more dense and integrated char layer was produced by incorporating h-BN@SiO2@PA (Fig. 10f). These morphologies observation revealed a truth that the incorporation of SiO2 and PA could promote the formation of dense and robust char layer, indicating the presence of crosslinking reaction between pyrolysis products and flame retardant. The chemical bonds and element composition change of TPU/hBN@[email protected] were determined by FTIR and XPS tests, before and after combustion. It was found that characteristic peaks of h-BN and SiO2 wasn’t presented in TPU/h-BN@[email protected] before combustion, 9

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Fig. 10. Photos of the chars of (a) neat TPU and (b) TPU/bulk h-BN-2.0 and (c) TPU/h-BN@[email protected] composites; SEM micrographs of the char residue of (d) neat TPU and (e) TPU/bulk h-BN-2.0 and (f) TPU/h-BN@[email protected] composites; FTIR spectra (g) and XPS spectra (h)TPU/h-BN@[email protected] composites before and after combustion; high-resolution spectra of Si 2p (i) as well as P 2p (j) regions of the char residues of TPU/h-BN@[email protected].

Fig. 11. Scheme of proposed flame-retardant mechanism.

which was due to the package of TPU matrix (Fig. 10g). After combus­ tion, stretching vibration of Si–O bond (1210 cm 1) and P–O bond (1060 cm 1) were also detected in FTIR spectrum of char residue of TPU/h-BN@[email protected] [32,57]. A phenomenon was presented by XPS spectra of TPU/h-BN@[email protected] before and after combustion

(Fig. 10h). Before combustion, the content of B and Si elements of TPU/h-BN@[email protected] were 0.26 at% and 12.85 at%. In view of the probe depth of XPS test, such a low content of B element indicated that the h-BN nanosheets were covered by SiO2. However, after combustion, the percentages of B and Si elements were 25.93 at% and 11.20 at%. 10

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effect for the breaking strength, only increased by 8.2%. Meanwhile, the elongation at break of TPU/SiO2-2.0 was decreased by 16.1%. This undesirable enhancement for the break strength may be attributed that the mechanical robustness of SiO2 are lower than those of h-BN, including Young’s modulus and tensile strength [62]. Therefore, it wasn’t hard to understand that h-BN@SiO2 presented a worse enhancement result to mechanical property than that of bulk h-BN. The distinction in the decreased elongation at break of TPU/bulk h-BN-2.0 and TPU/SiO2-2.0 was due to the geometry characteristic of h-BN and SiO2. The slide between h-BN layers is more favorable for the hold of excellent ductility. With the introduction of PA, hydrogen interactions were effectively formed between h-BN@SiO2@PA and TPU matrix, thus building strong interfacial interactions with TPU matrix. Therefore, TPU/h-BN@[email protected] presented an increase of up to 55.9% for breaking strength and a decrease of only 8.58% in elongation at break. It was found that the introduction of nanofillers also increased the tensile modulus of TPU composites (Table 6). The tensile modulus of pure TPU was 8.0 MPa. With the addition of bulk h-BN, tensile modulus was increased by 116%. Similar to the result of break strength, SiO2 nano­ particles also presented a lower enhancement effect for tensile modulus of TPU (91%), due to lower mechanical robustness. The cover of PA onto the surface of h-BN@SiO2 improved the interfacial interaction, thus increasing the tensile modulus again (104%). These results indicated that, due to strong interfacial interaction, incorporated h-BN@SiO2@PA obviously increased the break strength and tensile modulus of TPU composites.

Fig. 12. Tensile curves of pure TPU and its composites. Table 6 Results of LOI and UL-94 test for pure TPU and its composites.

Pure TPU TPU/bulk h-BN2.0 TPU/SiO2-2.0 TPU/[email protected] TPU/hBN@[email protected]

Breaking strength � 0.5 (MPa)

Elongation at break � 10.0 (%)

Tensile modulus �2% (MPa)

19.5 27.5

781.9 781.0

8.0 17.3

21.1 26.0

655.4 727.8

15.3 12.7

30.4

714.5

16.3

4. Conclusion With the aid of Lewis acid-base interactions, chemically inert h-BN was successfully functionalized for obtaining flame retardancy and toxic gas suppression. Due to the pre-degradation process of PA, P-containing compounds were produced to react with TPU matrix to obtained pro­ tective char, thus isolating the fire. Together with barrier effect of h-BN nanosheets and char reinforcement effect of SiO2, the flame retardancy and toxic gas suppression of TPU was obviously improved. The PHRR and THR of TPU/h-BN@[email protected] composites were effectively reduced by 23.5% and 22.1%, respectively. Meanwhile, the smoke product rate and total smoke release of TPU composite containing 2.0 wt % h-BN@[email protected] were decreased to 0.17 m2/s and 13.8 m2 from 0.24 m2/s and 15.1 m2 (pure TPU). Through CO2 detector and Atmos­ FIR, specially, it was found that the toxic gases (CO, CH4, C2H6, TOC) was turned into CO2 by catalysis effect of h-BN@SiO2@PA. Through results of TGA, MCC, and TG-IR, one conclusion could be obtained that it was the introduction of Si and P elements to promote the formation of char residue. There is no doubt, such an employment of Lewis acid-base interactions could overcome chemical inertness nature of h-BN, thus offering a operable platform for the functionalization of h-BN and exploiting the potential prospect in flame retardant and toxic gas suppression.

This result confirmed that, during combustion, SiO2 was thermally decomposed and h-BN nanosheets were exposed. The decomposed SiO2 was capable of reacting with TPU matrix to form stable char layer. Through high-resolution spectrum of Si 1s, Si–C and Si–O bonds were also detected (Fig. 10i) [58]. Meanwhile, the chemical environment of P 2p was divided into P–O (134.5 eV) and P–C (133.6 eV), respectively (Fig. 10j) [59]. These results indicate that Si 2p and P 2p participate in the formation process of char residue, facilitating the formation of robust residual char. Based on the above analysis, a reliable flame retardant mechanism was put forward. As presented in Fig. 11, due to superior resistance to thermal, layered structure of h-BN nanosheets acts as first barrier line to prevent the delivery of flammable pyrolysis gas to fire seat. Then, py­ rolysis gases blocked by h-BN nanosheets reacted with pre-graded PA to produce char residue, thus reducing the release of toxic gases. Due to the presence of Si element, the mechanical robustness of char residue was much improved. The final produced char layer was further capable of restraining the permeation of flammable products and acting barrier and shield effect to heat. Meanwhile, due to the high temperature of flame, the electron hole onto the surface of h-BN was produced and capable of oxidizing these toxic and reducing gases, including CO, CH4, C2H6, and TOC [60,61].

Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Acknowledgement

3.2.7. Mechanical property In tensile test, mechanical characteristics of typical elastomer were presented in Fig. 12 and Table 6. It was also observed that the incor­ poration of bulk h-BN obviously increased the breaking strength (41.0%), which much higher than pure TPU (19.5 MPa). In addition, the elongation at break of TPU/bulk-BN-2.0 wasn’t deteriorated, which is still up to 781.0%. In comparison to the result of TPU/bulk h-BN-2.0, the sole incorporation of SiO2 sphere couldn’t achieve same enhancement

The authors are grateful for financial support from National Natural Science Foundation of China (21604081), Natural Science Research Project of Anhui Province (KJ2018A0593), China Postdoctoral Science Foundation (2018M642541).

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Appendix A. Supplementary data

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