**1. Introduction**

Polymeric materials have been widely used in electronic devices, construction, and transportation. However, most of the polymeric materials have been intrinsically inflammable [1,2]. Therefore, flame-retardant additives are important to mitigate the risk of fire [3,4]. Some halogen-based flame retardants have been banned because they form carcinogens during combustion [5–7]. Compared to traditional halogenated flame retardants, phosphorus-containing flame retardants have attracted much attention due to the advantages of having low smoke and low halogen content as well as being non-toxic [8–10]. Phosphorus flame retardants can be divided into inorganic phosphorus flame retardants and organophosphorus flame retardants. Organophosphorus flame retardants have low cost and good compatibility with polymers, but they have high volatility and poor thermal stability [11]. Inorganic phosphorus flame retardants have high phosphorus content, high flame retardant efficiency and low toxicity, but their particles are usually large, thus resulting in poor compatibility with polymer materials and uneven dispersion [12]. For example, red phosphorus needs to be modified or coated to increase its compatibility with polymer materials [13,14].

Black phosphorus (BP) is a new kind of 2D layered material that is only composed of the phosphorus element [15–17]. Recently, BP has been demonstrated to be a good flame retardant [18–20].

BP/BN/WPU.

The high specific surface area of the layered structure can result in an efficient barrier effect in the process of polymer combustion. Compared with volatile white phosphorus and amorphous red phosphorus, BP also exhibits higher thermal stability and phase-dispersion, which could improve the flame-retardant performance and reduce damage to the mechanical properties of the polymers. In previous research, we reported that BP can effectively enhance the thermal stability and fire resistance of polymers [21]. Yuan Hu, et al. synthesized the BP/carbon nanotube composite and demonstrated its synergistic flame retardant performance for epoxy resin [22]. However, some key indicators of flame-retardant property, such as the limit oxygen index (LOI), for the reported BP-based composite materials need to be further improved. *Polymers* **2020**, *12*, x FOR PEER REVIEW 4 of 14 **3. Results and Discussion** *3.1. Characterization of BP/BN/WPU* The TEM images reveal the micromorphology of BP and BN nanosheets prepared by liquid phase stripping. As shown in Figure 1a,b, it can be clearly seen that the bulk black phosphorus and

Hexagonal boron nitride (BN) nanosheet is a widely studied 2D material with high thermal stability, good mechanical strength, and large surface area. It has been found to be a good flame retardant for several polymers [23–25]. Considering the fact that the flame-retardant mechanism of BP materials has been mainly attributed to the formed radicals in the gas phase while the BN nanosheet mainly works in the condensed phase, the combination of BP and BN may have a good synergistic effect to further improve the flame-retardant performance and decrease the additive amount of the flame retardants. BN were peeled into a few layers of nanoflakes. We suggest that the BP nanosheets are arranged on the BN nanosheets surface, which were fully stripped with clear edges with a length of several micrometers. The Energy Dispersive Spectrometer (EDS) results (Figure 1c) confirmed the elements' distribution in the BP/BN nanosheets. Raman spectra and X-ray diffraction (XRD) were carried out for BP, BN powder, pure WPU and BP/BN/WPU composite, respectively. As shown in the Raman spectra (Figure 1d), the BP nanosheets show three characteristic peaks of A<sup>g</sup> 1 , B<sup>g</sup> 2 , and A<sup>g</sup> 2 , corresponding to the crystal orientation [26], thickness [27], and angle [28]. The BN shows the

Thus, in this paper, we designed a series of experiments to analyze the synergistic effect and flame-retardant mechanism of BP and BN by filling them into waterborne polyurethane (WPU). Through systematic characterization, we found that adding only 0.4 wt% BP/BN nanosheet could significantly raise the LOI of WPU from 21.7% to 33.8%. The improvement of flame retardancy involves the combination of the condensed phase and gas phase effects during combustion. In addition, such a small additive amount of BP/BN showed negligible color effect on the WPU (Figure 1), which broadens its real application range. in- plane ring vibration peak of BN (E2g vibration mode) at 1365 cm−1 [29]. The BP/BN/WPU composite shows four new peaks in comparison with pure WPU, indicating the successful introduction of BP and BN nanosheets into the WPU matrix. The XRD spectrum of BP reveals three obvious diffraction peaks (Figure 1e), corresponding to the (020), (040), and (060) plane, respectively [30,31]. The spectrum of BN also has three obvious diffraction peaks, indicating the good crystallinity of BN. The XRD spectrum of BP/BN/WPU retains the main peaks of BP and BN, which is consistent with the Raman results.

**Figure 1.** (**a**,**b**) TEM image of BP/BN nanosheets; (**c**) The EDS of BP/BN nanosheets; (**d**) Raman spectra of BP, BN, WPU and BP/BN/WPU; (**e**) XRD spectra of BP, BN, WPU and BP/BN/WPU; (**f**) and (**g**) SEM image of WPU and BP/BN/WPU; (**h**) phosphorus mapping in BP/BN/WPU and (**i**) boron mapping in **Figure 1.** (**a**,**b**) TEM image of BP/BN nanosheets; (**c**) The EDS of BP/BN nanosheets; (**d**) Raman spectra of BP, BN, WPU and BP/BN/WPU; (**e**) XRD spectra of BP, BN, WPU and BP/BN/WPU; (**f**) and (**g**) SEM image of WPU and BP/BN/WPU; (**h**) phosphorus mapping in BP/BN/WPU and (**i**) boron mapping in BP/BN/WPU.

nanosheets (Figure 1g), which could be attributed to the additives BP and BN. As shown in

The SEM images of pure WPU (Figure 1f) are observed on the fractured surface. It is shown to

#### **2. Materials and Methods**

#### *2.1. Materials*

In this study, BP was prepared by a mineralization transformation method from red phosphorus with the help of iodine and tin in a quartz tube under argon atmosphere. The prepared BP crystals were washed with toluene to remove residual mineralizer, followed by washing with water and acetone. The utilized red phosphorus, iodine, tin, toluene and acetone were analytically pure. The BN was purchased from Shanghai Huayi Group Huayuan Fine Chemicals Co., Ltd., Shanghai, China with a particle size lower than 30 µm. The WPU latex with a solid content of 25 wt% was purchased from Anhui Huatai New Material co. Ltd., Hefei, China The water used was deionized water.

#### *2.2. Fabrication of BP and BN Nanosheets*

BP was ground for 2 h into powder, and then 0.5 g of the powder was added into a conical flask with 500 mL of deionized water. The conical flask was sealed with argon gas to stop oxidation of the BP. Then the dispersion was added to a working ultrasonic device (50 Hz, 200 W) for 24 h with the temperature controlled below 30 ◦C. Afterwards, the dispersed liquid was centrifuged at 3500 rpm for 15 min by a centrifugal machine (TGL-16C, Shanghai Anting Scientific Instrument Factory, Shanghai, China). Finally, the supernatant liquid was collected and condensed by suction filtration. An argon atmosphere should be used to prevent the oxidation of phosphorene throughout the whole experimental process. The dispersion of boron nitride was also obtained by liquid phase stripping. In order to calculate the concentration of the dispersion, freeze drying was used to remove the moisture. Then the remaining solid was weighed.

#### *2.3. Preparation of BP*/*BN*/*WPU Composite Materials*

The obtained BP and h-BN dispersion were added into a beaker with WPU. After stirring for a few minutes, the beaker was sealed by filling with an argon atmosphere. The mixture was ultrasonicated for 2 h at low temperature with an ice bath. Then the obtained suspension was poured onto a plate of the size of 120 × 120 mm and dried under vacuum at 22 ◦C. After it was dried completely, the BP/BN/WPU material was formed. The additive amount of BP/BN is 0.2%/0.2%. For comparison, the BP/WPU with 0.4% BP and BN/WPU with 0.4% BN were synthesized using the same method (Table 1).


**Table 1.** The additive amount of the BP and BN nanosheets in the samples.

#### *2.4. Analytical Test*

#### 2.4.1. Structure Characterizations

Transmission electron microscopy (TEM, Philips CM100, Amsterdam, The Netherlands) was conducted to observe the BP and BN nanosheets at an acceleration voltage of 100 kV.

An X-ray diffraction device (XRD, PANalytical Empyrean, Almelo, The Netherlands) was used to analyze the BP, BN powders, as well as the WPU and its composite materials, respectively.

Raman spectra were obtained on a LabRAM-HR Confocal Raman Microprobe (HORIBA Scientific Co., Palaiseau, France) with excitation provided in backscattering geometry by a 633 nm argon laser line.

Scanning electron microscopy (SEM, Bruker Nano, Bruker, Karlsruhe, Germany) was used to analyze the microstructure of the materials and the charred residue. The WPU and its composite materials with BP and BN nanosheets were fractured by liquid nitrogen, then the fracture surfaces of the samples were uniformly coated with a layer of gold and then observed by SEM. In order to present the distribution of phosphorene in the polymers, elemental mapping tests were also conducted on another two specimens without a coating of a gold layer. In addition, An EDS test of the charred residue was applied to determine the content of BP and BN in the condensed phase.

X-ray photoelectron spectroscopy (XPS) analysis was performed using Al radiation as a probe (K-alpha, Thermo Fisher Scientific, Waltham, MA, USA) to measure the valence states and chemical composition of the residue of BP/BN/WPU.

#### 2.4.2. Thermal Properties Measurement

Thermogravimetric analysis (TG) of the materials was performed on a thermal analyzer (NETZSCH STA449F3, NETZSCH, Selb, Germany) with a gas flow rate of 80 mL/min under nitrogen atmosphere. The heating rate was 10 ◦C/min and the temperature ranged from 40 ◦C to 800 ◦C.

Thermogravimetric analysis–Fourier transform infrared spectroscopy (TG–FTIR) was performed via a TGA/DSC3 thermogravimetric analyzer (METTLER TOLEDO, Greifensee, Switzerland) linked with a Nicolet FTIR IS50 spectrometer (Thermo Fisher) at a heating rate of 10 ◦C/min within a temperature range of 40 ◦C to 800 ◦C under a N<sup>2</sup> flow of 50 mL/min. The temperature of the transfer line in the TG–FTIR system was 200 ◦C.

#### 2.4.3. Flammability Property Measurement

The limiting oxygen index (LOI) values were measured according to the standard oxygen index test ASTM D2863-77 by using the device of COI from Motis combustion technology co. LTD (Kunshan, China).

In order to study the combustion behavior, cone calorimetry (CC, PX-07-007, Suzhou phoenix quality inspection instrument co. LTD, Suzhou, China) was performed at a heat flux of 35 kW/m<sup>2</sup> . The material specimens were made into a square with the size of 100 × 100 × 3 mm. After wrapping in a piece of aluminum film, the specimens were set on fire on the CC.

#### **3. Results and Discussion**
