*2.4. Measurements*

To observe the condensation density, 29Si nuclear magnetic resonance (NMR) was used and performed by a Bruker DSX-400WB, Bremen, Germany. The samples were treated at 180 ◦C for 2 h and then ground into a fine powder. The vertical burning test was done inside a fume hood. The UL-94 standard was then used to evaluate the flame retardance of the hybrid material. Samples were held vertically with tongs at one end and burned from the free end. Three samples were prepared for every test. Samples were exposed to an ignition source for 10 s. Then, they were allowed to burn above cotton wool until both the sample and cotton wool were extinguished. Observable parameters were recorded to assess fire retardance. The UL-94 test classifies the materials as V-0, V-1, and V-2 according to the period needed before self-extinction and the occurrence of flaming dripping after removing the ignition source. V-0 is the most ambitious and desired classification. X-ray photoelectron spectroscopy (XPS) was adopted to determine the char composition to observe the mechanism of burning. The process for using the highresolution X-ray photoelectron spectrometer (HR-XPS, ULVAC-PHI, Inc., Kanagawa-ken, Japan) is as follows: the sample is crushed into powder and then adhered to the aluminum sheet with small round holes. This sheet is mainly used for detecting the sample's surface as well as its elemental composition and distribution in vertical directions, in addition to implementing analysis on the links of elemental substances. The morphology of the fractured surface of the composites was studied under a scanning electron microscope (SEM) (JEOL JSM 840A, Tokyo, Japan). Energy-dispersive X-ray spectroscopy (EDX) was used to observe the distributions of Si and P atoms in the hybrid, which were obtained from SEM EDX mapping (JEOL JSM 840A, Tokyo, Japan).

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

#### *3.1. 29Si NMR*

Solid-state 29Si NMR spectrometry was used to determine the structure of the PU/HBNPSi hybrid material, and the DOPO-BQ-IPTS reaction process and its degree of hydrolysis conden-

sation was obtained using the sol–gel method. Because one end of IPTS was terminated with three tri-alkoxy groups (T), the end of the prepolymer had a T-shaped structure. The other end was terminated with an organic chain—an –NCO functional group—that reacted with the –OH functional group in DOPO-BQ. The T-end further hydrolyzed and condensed to form a network structure of Si–O–Si, which was more stable. According to the degree of hydrolysis and condensation, the single-, double-, and triple-replacement absorption peaks located at −45 to −48 ppm, −56 to −62 ppm, and −66 to −69 ppm indicate the T<sup>1</sup> [12,13], T<sup>2</sup> [12,14], and T<sup>3</sup> [12,13] structures, respectively.

Figure 1 presents the solid-state 29Si NMR spectra of the PU/HBNPSi hybrid material. The hybrid material had the T-structure, primarily T3. Using peak separation treatments, the T1, T2, and T<sup>3</sup> areas could be identified. Then, the following equation was used to calculate the condensation density (Dc(%)) [13].

**Figure 1.** Solid-state 29Si nuclear magnetic resonance (NMR) spectra of PU/HBNPSi.

The Dc(%) of the hybrid material PU/HBNPSi was calculated to be 74.4%. Higher Dc(%) indicates that a material has a denser network structure. The Si–O–Si bonded into an excellent network structure, and the Si–O bond had high bond energy. The hybrid material thus had high thermal stability and flame retardance. The results are summarized in Table 1.



#### *3.2. P- and Si-Mapping of EDX*

This study determined the compatibility of pristine PU and HBNPSi. Two phases were obtained and demonstrated differences under different interaction forces. The P- and Si-mapping of energy-dispersive X-ray spectroscopy was utilized to determine whether the dispersibility and homogeneity were favorable or whether agglomeration occurred when the inorganic phase was mixed with the organic phase. Favorable dispersibility indicates that the two phases have excellent compatibility, which is crucial in a hybrid material.

Figure 2 displays P- and Si-mapping of 20% and 40% PU/HBNPSi. The images show the dispersion of the inorganic elements P and Si in their organic phases. Each bright dot represents P or Si. The Figure reveals that the inorganic substances in both the 20% and 40% PU/HBNPSi were dispersed favorably in the matrix and that the material was homogeneous. No agglomeration was observed, which indicated that the compatibility between the organic and inorganic phases was excellent. The amount of P and Si in the material increased as the HBNPSi content was increased.

**Figure 2.** Mapping photograph of the P- and Si-containing hybrid PU/HBNPSi.

#### *3.3. Flame Retardance Analysis*

In the UL-94 test, a standardized ignition specimen is combusted and the total burning duration (the sum of two burning durations) must meet the standard; additionally, during the burning process, dripping, which could ignite the cotton beneath the specimen, must not occur. The flame retardance of polymeric material is classified into the V-0, V-1, and V-2 levels. Figure 3 and Table 2 reveal that the pristine PU failed and did not achieve any level. Dripping was observed, and the drips ignited the cotton below. As the concentration of HBNPSi in the material was increased to 40%, the two flame burning durations of the PU/HBNPSi hybrid material were 0.7 and 16.1 s, respectively. The sum of these two burning durations was 16.8 s. However, because dripping occurred, the material was graded as V-2. HBNPSi had excellent flame retardance because during pyrolysis the N and P in the structure became gaseous, capturing free radicals and playing a role in the condensed phase, thereby catalyzing the formation of char. Additionally, Si migrated across the surface [14,15] and formed a dense SiO2 structure that protected the interior of the materials. The benzene ring of DOPO-BQ provided the source for char formation, and the char layer resisted burning under high temperature.

**Figure 3.** Effect of various HBNPSi concentrations on the UL-94 of the PU/HBNPSi hybrid.


**Table 2.** The flame retardance of the PU/HBNPSi hybrid by UL-94 values.

#### *3.4. Morphology Analysis*

SEM was used to observe the morphology of the hybrid material and determine its surface microtopography, particle size, and surface nucleation. Figure 4a shows that the surface of the pristine PU was smooth and homogeneous before burning, without phase separation. Figure 4b displays the surface of the burned pristine PU. Because pristine PU is highly flammable and molten after burning, some wave patterns can be observed in the SEM image. Figure 4c shows the surface of the 20% PU/HBNPSi before burning; it was generally smooth and without phase separation, despite the addition of HBNPSi. Figure 4d displays the surface of 20% PU/HBNPSi after burning. Some Si particles can be seen because Si migrated across the surface during burning. However, because insufficient char was formed, the heat blocking effect was weak, calling to mind UL-94. Therefore, 20% PU/HBNPSi was flammable. Figure 4e shows the surface of 40% PU/HBNPSi before burning and that the surface had discrete HBNPSi particles. Although the additional loading was 40%, no phase separation was observed. Figure 4f shows the surface of 40% PU/HBNPSi after burning. A dense char layer covered this surface because P was dehydrated and catalyzed into char and Si migrated across the surface while burning to form a dense char layer through the condensed phase. Both P and Si covered the material surface [13–15], which blocked the transmission of gas and flame and increased thermal stability.

#### *3.5. XPS Char Analysis*

XPS was used to examine the changes in chemical bonds in the PU/HBNPSi hybrid material at room temperature and after being burned at 800 ◦C in a high-temperature furnace. Additionally, after flame retardant was added to the PU the functional group changes, before and after the hybrid was burned, were observed. The peak separation method was used to calculate the material's anti-oxidation characteristics. The results are presented in Figures 5–10 and Tables 3 and 4.

**Figure 4.** SEM micrographs of composites (**a**) pristine PU (before burning) (×1 K), (**b**) pristine PU (after burning) (×1 K), (**c**) PU/HBNPSi 20% (before burning) (×1 K), (**d**) PU/HBNPSi 20% (after burning) (×1 K), (**e**) PU/HBNPSi 40% (before burning) (×1 K), (**f**) PU/HBNPSi 40% (after burning) (×1 K).

**Figure 5.** XPS survey spectra of (**a**) pristine PU, (**b**) PU/HBNPSi 10% at RT, (**c**) PU/HBNPSi 40% at RT, (**d**) PU/HBNPSi 40% at 800 ◦C.

**Figure 6.** C1s spectra of PU/HBNPSi 10%: (**a**) RT; (**b**) under an air atmosphere at 800 ◦C for 30 min.

**Figure 7.** C1s spectra of PU/HBNPSi 40%: (**a**) RT; (**b**) under an air atmosphere at 800 ◦C for 30 min.

**Figure 8.** O1s spectra of PU/HBNPSi 40%: (**a**) RT; (**b**) under an air atmosphere at 800 ◦C for 30 min.

**Figure 9.** P2p spectra of PU/HBNPSi 40%: (**a**) RT; (**b**) under an air atmosphere at 800 ◦C for 30 min.

**Figure 10.** Si2p spectra of PU/HBNPSi 40%: (**a**) RT; (**b**) under an air atmosphere at 800 ◦C for 30 min.

**Table 3.** Binding energy (eV) and relative peak intensities (%) of the various components of C1s peak-fitted signals.


**Table 4.** The values of Cox/Ca of composites at room temperature (RT) and 800 ◦C.


Figure 5 displays XPS rough scans illustrating the elemental composition of the pristine PU, 10% PH/HBNPSi at room temperature, 40% PH/HBNPSi at room temperature, and 40% PU/HBNPSi after being burned at 800 ◦C. The scan in Figure 5a indicates three elements: C, O, and N. For the hybrid material containing 10% flame retardant (Figure 5b), P and Si were observed in addition to C, O, and N. The XPS rough scan, obtained after DOPO-BQ and IPTS were reacted, verifies that the hybrid material contained P and Si. Figure 5c presents the rough scan for the hybrid material containing 40% of HBNPSi. Because the additive content was higher than for the 10% hybrid material, the P and Si content were higher. Figure 5d shows that the hybrid material was 40% HBNPSi after being burned. Burning at a high temperature led to a slight increase in the amount of P and Si in the hybrid material because during the burning process P was dehydrated and formed char and Si migrated across the surface [14,15]. Together, these elements formed a layer of char containing P and Si that protected the material. Nitrogen mainly exerts a flame-retardant effect in the gas phase, which can dilute the oxygen concentration and capture free radicals in the gas phase, so in the XPS spectrum, the nitrogen element disappears in Figure 5d.

Figures 6–10 present scans showing Cls, Ols, P2p, and Si2p bonds in the materials at room temperature and after being burned at 800 ◦C. Six functional groups can be observed in the Cls spectra: those at 284 eV (C–C and C–H) [16], 285.4 eV (C–N) [17], 288.5 eV (C=O) [18], 284.5 eV (C=C) [19], and 286 eV (C–O) [20]. Because the material additive contained silicide, the spectrum contains a peak at 282.9 eV (C–Si) [21]. After heat oxidation at 800 ◦C, the material was mostly graphitized. The C=C content was higher, as illustrated in Figures 6 and 7. The formation of graphitized carbon–carbon double bonds is very important for the improvement of thermal stability. It takes more than 3000 degrees to destroy its structure and form a high thermal stability protective layer on the surface of the material.

Figure 8 shows the Ols spectra, in which four functional groups can be observed, namely 531.2 eV (C=O), 532.2 eV (C–O), 531.5 eV (P=O), and 530.6 eV (Si–O) [22–27]. This indicates the change in bonding type after high-temperature heat oxidation. In Figure 8a, the Si–O–Si bonds are formed by the sol–gel reaction. In Figure 8b, it can be clearly seen that a large number of Si–O bonds are formed, mainly to form silicon dioxide, which is resistant to high temperatures. The heat-resistant layer will protect the polymer matrix on the surface of the material.

Figure 9 shows that the P2p spectrum has peaks at 131.5 eV (P–C) [28] and 133 eV (P–O–C) [27]. The corresponding bonds formed because of the addition of DOPO-BQ flame retardant. After high-temperature deep oxidation, the peaks were at 135 eV (P2O5) and 133.1 ± 0.3 eV (P2O7 <sup>4</sup>−) [29], corresponding to inorganic phosphides, forming char protective material. The two aforementioned inorganic phosphides were converted from the bonds before burning. P2O5 is a kind of glass, which is a non-combustible material. It will melt and cover the surface of the material to achieve insulation resistance and material transfer, which can improve the flame-retardant properties of the material.

The Si2p spectra presented in Figure 10a,b, obtained before and after burning, respectively, have considerable differences in bonding strength. Figure 10a reveals that the siloxane coupling agen<sup>t</sup> caused the hydrolysis condensation reaction. Three bonds were identified: those corresponding to peaks at 100.7 eV (Si–C), 101.6 eV (Si(–O1)), and 102.2 eV (Si(–O2)). After burning and oxidation, the Si–C peak has disappeared whereas peaks have appeared at 101.6 eV (Si(–O1)), 102.2 eV (Si(–O2)), and 103.5 eV (Si(–O4)) [30] (Figure 10b). Because silicide was converted into SiO2, the peak intensity of Si(–O4) increased. This result indicates that after burning, the hybrid material PU/HBNPSi had char layers that covered and protected the matrix. Silica is an inorganic glass that will not burn and will melt at high temperatures, covering the surface of the material to resist high-temperature attacks.

We further analyzed the differences between PU/HBNPSi 10% and PU/HBNPSi 40% at room temperature and 800 ◦C for 30 min under an air atmosphere. After calculating the area ratio of each bond species individually using the Cls spectrum from Figure 6 to Figure 7, the Cox (oxidized carbons)/Ca (aliphatic, aromatic carbons) ratio was obtained to show the antioxidant effect of the material [31]. The results are shown in Tables 3 and 4. The ratio of Cox/Ca at room temperature of the mixed material with 10% and 40% concentration are 0.33 and 0.27, respectively. After high-temperature combustion, the ratio decreases to 0.24 and 0.11, respectively. In summary, the higher the concentration, the better the oxidation resistance, thus improving the thermal stability of hybrid materials.
