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. 2020 Dec 24;6(1):666–674. doi: 10.1021/acsomega.0c05208

Flame-Retardant Performance of Epoxy Resin Composites with SiO2 Nanoparticles and Phenethyl-Bridged DOPO Derivative

Kui Wang 1,*, Hang Liu 1, Chong Wang 1, Weijiang Huang 1, Qin Tian 1, Qiuping Fu 1, Wei Yan 1,*
PMCID: PMC7807740  PMID: 33458519

Abstract

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Flame retardancy of epoxy resin (EP) plays a vital role in its applications. When inorganic nanomaterials form inorganic/organic nanocomposites, they exhibit special flame-retardant effects. In this study, EP nanocomposites were prepared by the incorporation of SiO2 nanoparticles and phenethyl-bridged 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative (DiDOPO), and the synergistic effects of SiO2 nanoparticles and DiDOPO on the flame-retardant performance of EP were discussed. Results indicated that the introduction of only 15 wt % SiO2 and 5 wt % DiDOPO in EP leads to the increase in the limiting oxygen index from 21.8 to 30.2%, and the nanocomposites achieve the UL-94 V-0 rating. Thermogravimetric analysis revealed that char yield increases with the increase in the SiO2 content of the nanocomposites and that an increased amount of thermally stable carbonaceous char is formed. SiO2 nanoparticles can improve the thermal stability and mechanical performance of EP; hence, the nanoparticles can serve as an efficient adjuvant for the DiDOPO/EP flame-retardant system.

Introduction

Epoxy resin (EP) is used in structural laminates, adhesives, and electrical devices. However, EP exhibits significant safety hazards due to its flammability when it is used in an application that requires good flame resistance.1−4 Therefore, it is crucial to improve the flame retardancy of EP composites in several fields. Currently, this is being done by the introduction of a functional group that contains a flame-retardant element (e.g., halogen element) and physical addition of a widely used flame retardant into the EP matrix. Although flame retardancy of EP composites is improved, halogen-containing flame retardants are deleterious to the environment.5−7 Besides, a high amount of additive flame retardants can effectively reduce the heat release rate and mechanical properties of flame-retardant EP composites.

Recently, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives have attracted widespread attention, and several studies on DOPO derivatives, including triazine, phenethyl, diphosphonate, silsesquioxane, and bismaleimide, have been reported.8−14 Their activities are mainly related to the PO· radical, produced by a gaseous-phase mechanism, and its interaction with the decomposing polymer, inducing charring in the condensed phase. Although DOPO derivatives exhibit good flame-retardant effects, they are typically added in high amounts. Hence, it is challenging to reduce their addition amount as well as costs associated with their use. Currently, the addition of synergists has become a facile and versatile strategy.

Inorganic synergists have been frequently used in the flame-retardant EP, exhibiting key characteristics of good dispersion and strong interfacial interaction. Several types of particles, such as titania, zinc oxide, carbon fillers, and phosphates, have been widely used in flame-retardant systems.15−19 As synergists, nanoparticles have attracted widespread attention due to their nanoscale effects. Owing to the high stability of silica at high temperatures, EP/SiO2 nanocomposites exhibit increased thermal stability due to the strong linkage between EP and SiO2.20−23 Besides, the effect of SiO2 on the mechanical performance of EP/SiO2 has been discussed, and the SiO2 content has been reported to exhibit different effects on the tensile modulus and fracture toughness of nanocomposites.24 Zhang and co-workers used polysiloxane flame retardants together with DOPO to prepare a flame-retardant system for EP and found the synergistic effect between DOPO and polysiloxane.25 Vu and co-workers extracted silica from rice husk and researched the effects of modified DOPO on the flammability and mechanical properties of an EP/silica system.26 Although SiO2-containing phosphorus EP nanocomposites have been researched, the content of DOPO and inorganic materials is higher, and the synergistic effect of colloidal nanosilica with DOPO derivatives (DiDOPO) on flame retardancy and char formation has not been investigated.

In this study, SiO2 with a diameter less than 50 nm combined with a phenethyl-bridged DOPO derivative (DiDOPO) was introduced in EP to improve its flame retardancy; the DiDOPO was designed and synthesized by our laboratory, and the synergistic effect of SiO2 and DiDOPO on the EP flame-retardant system was investigated. Scanning electron microscopy (SEM) was employed to observe the morphology and structure of SiO2 nanoparticles and the char layer of EP/DiDOPO/SiO2 nanocomposites. The cone calorimeter test (CCT), limiting oxygen index (LOI) method, and vertical UL-94 tests were employed to analyze the flame-retardant performance of EP/DiDOPO/SiO2 nanocomposites, and 3D TG-FTIR was employed for the chemical analysis of EP nanocomposites.. In addition, thermal stability and mechanical properties of EP and EP/DiDOPO/SiO2 nanocomposites were investigated.

2. Results and Discussion

2.1. Organic Modification of SiO2 Nanoparticles

Figure 1a shows the morphology of raw SiO2 nanoparticles: raw SiO2 tends to from aggregates due to nanoscale effects, and nanoparticles do not exhibit a regular spherical shape. The statistical distribution in Figure 1b revealed that the diameter of SiO2 nanoparticles is 33.58 ± 3.58 nm. To reduce the aggregation of SiO2 nanoparticles and further improve their compatibility with EP in acetone, SiO2 was treated with diluted HCl and CG-570 to prepare organic-modified nanoparticles. SEM results indicated that there is almost no change in the diameter and morphology of SiO2 nanoparticles before and after modification because the molecular weight and content of the modifier is relatively small. From the FTIR curves shown in Figure 1c, a broad absorption peak at 3410 cm–1 corresponded to the antisymmetric stretching vibration and symmetric stretching vibration of the −OH group, and the wide absorption band at 1061 cm–1 corresponded to the antisymmetric stretching vibration absorption of the Si–O–Si bond. After organic modification, stretching vibration peaks of the alkane CH bond were observed at 2980 cm–1 (−CH3), 2922 cm–1 (−CH2), and 2854 cm–1 (−CH). In addition, absorption peaks observed at 2359 and 2340 cm–1 corresponded to CO2, and the absorption peak observed at 1625 cm–1 corresponded to H2O in the environment. FTIR results indicated that SiO2 is successfully modified by the silane coupling agent. Furthermore, TGA results (Figure 1d) are consistent with the FTIR results: neither raw SiO2 nor modified SiO2 contained hydroxyl water and coordination water, and modified SiO2 was extremely stable at a temperature of less than ∼400 °C. Besides, after organic modification with CG-570, the mass loss of SiO2 increased, further indicating that SiO2 is successfully modified with CG-570.

Figure 1.

Figure 1

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(a) SEM image of raw SiO2 and the (b) statistical distribution of the diameter obtained from the corresponding SEM image, and (c) FTIR and (d) TG curves of raw SiO2 and modified SiO2.

2.2. Flame Retardancy of EP Composites

After organic modification, SiO2 was applied with DiDOPO to investigate the synergistic effect on the EP flame-retardant system. Table 5 lists the results for the LOI values and UL-94 grade of EP and EP/DiDOPO/SiO2 nanocomposites. The results revealed that the LOI value of EP is 21.8, with no UL-94 rating. However, after the incorporation of 5 wt % DiDOPO and a certain amount of SiO2, the LOI value increased, and the LOI values for EP/DiDOPO/SiO22, EP/DiDOPO/SiO25, EP/DiDOPO/SiO210, and EP/DiDOPO/SiO215 were 25.6, 27.7, 28.1, and 30.2, respectively. In addition, EP/DiDOPO/SiO25 exhibited a V-0 rating. The results indicated that EP and modified SiO2 exhibit good compatibility, and that SiO2 can help to improve the flame retardancy of the EP system. Figure 2 shows the heat release rate (HRR) and total heat release (THR) curves of EP and EP/DiDOPO/SiO2 nanocomposites evaluated by CCT. The results in Figure 2a revealed that a peak heat release rate (pHRR) of pure EP is 1101.7 kW/m2. After the incorporation of 5 wt % DiDOPO and a certain amount of SiO2, pHRR values for EP/DiDOPO/SiO22, EP/DiDOPO/SiO25, EP/DiDOPO/SiO210, and EP/DiDOPO/SiO215 were 1093.6, 1059.1, 801.5, and 644.1 kW/m2, respectively, and pHRR decreased by 0.7, 3.9, 27.2, and 41.5%, respectively. These results thankfully form protective residue layers. Table 1 summarizes CCT data, including the time to ignition (TTI), total smoke rate (TSR), average effective heat of combustion (av-EHC), pHRR divided by TPhrr (FGI), and TTI divided by pHRR (FPI). The THR value of EP/DiDOPO/SiO2 decreased by 4.4 and 5.4% after incorporating 2 and 10 wt % SiO2, respectively. For EP/DiDOPO/SiO215, the slightly increased THR value is attributed to the longer burning time. The TSR value is increased due to more incomplete combustion. Furthermore, a high value of TSR/av-EHC indicates the formation of an increased number of noncombustible components in the gas phase. Results revealed that EP nanocomposites exhibit a higher value of TSR/av-EHC than that of pure EP, indicating that an increased number of organic structures retained in the condensed phase participate in carbonization.27−30 Furthermore, the flame-retardant performance was quantitatively evaluated according to the following equation31−33

2.2. 1

where FRI is the “flame-retardancy index”. pHRR, THR, and TTI were estimated by CCT. Table 1 lists the calculation results. The results revealed FRI values of 1.05, 1.16, 1.42, and 1.86 for the four composites. Previous studies have reported that “poor,” “good,” and “excellent” fire-retardancy features correspond to FRI values of <1, 1–10, and >10, respectively. Hence, EP/DiDOPO/SiO2 nanocomposites retain a “good” flame-retardant performance in comparison with that of pure EP.

Table 5. Formulations and Flame-Retardant Results for EP and EP/DiDOPO/SiO2 Nanocomposites.

  formulation (g)
 flame retardancy
sample designation EP DiDOPO SiO2 LOI (%) UL-94
EP 100 0 0 21.8 N.R.
EP/DiDOPO/SiO22 100 5 2 25.6 V1
EP/DiDOPO/SiO25 100 5 5 27.7 V0
EP/DiDOPO/SiO210 100 5 10 28.1 V0
EP/DiDOPO/SiO215 100 5 15 30.2 V0
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Figure 2.

Figure 2

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(a) Heat release rate and (b) total heat release curves of EP and EP/DiDOPO/SiO2 nanocomposites.

Table 1. Cone Calorimeter Test (CCT) Results for EP and EP/DiDOPO/SiO2 EP Nanocompositesa.

sample designation TTI pHRR/ TPhrr/ THR/ av-EHC/ TSR/ FGI FPI FRI
(s) (kW/m2) (s) (MJ/m2) (MJ/kg) (m2/m2) pHRR/TPhrr TTI/pHRR (−)
EP 43 1101.7 180 147.7 23.4 5793.9 6.12 0.039  
EP/DiDOPO/SiO22 43 1093.6 165 141.2 21.7 6345.1 16.80 0.039 1.05
EP/DiDOPO/SiO25 47 1059.1 200 143.9 22.2 6544.3 5.29 0.044 1.16
EP/DiDOPO/SiO210 42 801.5 175 139.7 21.9 6796.9 4.58 0.052 1.42
EP/DiDOPO/SiO215 47 644.1 235 148.7 21.7 6568.9 2.74 0.073 1.86
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a

TTI: time to ignition; pHRR: peak heat release rate; Tphrr: time to peak heat release rate; THR: total heat release; av-EHC: average effective heat of combustion; TSR: total smoke rate; FGI: pHRR divided by TPhrr; FPI: TTI divided by pHRR; and FRI: flame-retardancy index.

2.3. Thermal Stability of EP Nanocomposites

Figure 3a,b shows the TG and DTG curves for EP and EP/DiDOPO/SiO2 nanocomposites. Table 2 lists the corresponding data. The polymeric matrix was completely broken down at a temperature of greater than 500 °C. The onset degradation temperature (T0) and the residues at 800 °C of EP composites are shown in Figure 3a, and the temperature of maximum weight loss rate (Tmax) of the composites are shown in Figure 3b. Along with the increase in the SiO2 content, T0 was basically the same, but the carbon residue rate increased; at a SiO2 content of 15 wt %, the residue char of the nanocomposite increased to 16.11 wt %. Besides, with the increase in the SiO2 content, Tmax decreased, indicating that low-temperature decomposition slightly promotes high-temperature char formation and that SiO2 nanoparticles promote the formation of residual carbon in the condensed phase.

Figure 3.

Figure 3

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(a) Thermogravimetry (TG) and (b) differential thermogravimetry (DTG) curves of EP and EP/DiDOPO/SiO2 nanocomposites under N2 at a heating rate of 10 °C/min.

Table 2. TG Data for EP and EP/DiDOPO/SiO2 Nanocomposites (Heating Rate of 20 °C/min).

sample designation T0 (°C) Tmax (°C) carbon residue (wt %)
EP 304 375 6.70
EP/DiDOPO/SiO22 305 372 8.72
EP/DiDOPO/SiO25 305 371 10.37
EP/DiDOPO/SiO210 303 368 13.48
EP/DiDOPO/SiO215 304 369 16.11
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2.4. Morphology and Chemical Analyses of the Residual Char after CCT

In addition to the gas-phase activity, the protective barrier produced from the expanded carbon layer also affects the flame-retardant performance of EP. Figure 4 shows the optical images of the residual char: pure EP comprised a low amount of residual char. However, after the incorporation of 5 wt % DiDOPO and a certain amount of SiO2, and the amount of residual carbon significantly increased with the increase in the SiO2 content, and the char layers became denser and more complete.

Figure 4.

Figure 4

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Optical images of the residual chars of (a) EP, (b) EP/DiDOPO/SiO22, (c) EP/DiDOPO/SiO25, (d) EP/DiDOPO/SiO210, and (e) EP/DiDOPO/SiO215.

Figure 5 shows SEM images and elemental analysis of the residual char. The residue char of pure EP (Figure 5a) exhibited a badly broken carbon layer structure. However, after the incorporation of 5 wt % DiDOPO and a certain amount of SiO2, the residual char became denser, and the holes were reduced along with the increase in the SiO2 content. This char layer surface may cover the inorganic SiO2 complex and the decomposition product of the phosphorus-containing group. In EP/DiDOPO/SiO2, the surface roughness of residual char gradually increased. According to elemental analysis, the C/O ratio of the residual char in pure EP was 6.07, and the C/O ratios of EP/DiDOPO/SiO22, EP/DiDOPO/SiO25, EP/DiDOPO/SiO210, and EP/DiDOPO/SiO215 were 0.40, 1.82, 0.91, and 1.59, respectively, indicating that the incorporation of SiO2 and DiDOPO can reduce the heat- and oxygen-exchange efficiencies. In addition, the measured phosphorus content was extremely low, indicating that a majority of phosphorus is released into the flame to form PO· radicals and that it exerts a quenching effect in the gas phase. In summary, the results suggested that gas- and condensed-phase working modes exist during the combustion of EP/DiDOPO/SiO2 nanocomposites.

Figure 5.

Figure 5

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SEM images and elemental analysis of the residual chars of (a) EP, (b) EP/DiDOPO/SiO22, (c) EP/DiDOPO/SiO25, (d) EP/DiDOPO/SiO210, and (e) EP/DiDOPO/SiO215. Insets show the higher-magnification SEM images.

XPS was employed to examine the chemical components in residual char of EP and EP/DiDOPO/SiO2 nanocomposites after CCT. In the XPS spectrum of EP/DiDOPO/SiO2 in Figure 6 and Table 3, the peaks at 134, 103, and 400 eV corresponded to P 2p, Si 2p, and N 1s, respectively. Peaks located at 284.8 eV corresponded to C–H and C–C bonds in aliphatic and aromatic species, respectively, and the peak at 532.8 eV corresponded to −O– in C–O–C, C–O–P, and/or C–OH groups.34−36 With the addition in DiDOPO and SiO2, the C and N contents were considerably less than that of EP, and the content of O was greater than that of EP. The nanoparticles tended to migrate to the polymer surface during combustion, which can form an effective char layer and delay the transformation of heat and oxygen.

Figure 6.

Figure 6

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XPS spectra of the residue char in EP and EP/DiDOPO/SiO2 nanocomposites.

Table 3. XPS Results for the Residue Char in EP and EP/DiDOPO/SiO2 Nanocomposites.

  EP
 EP/DiDOPO/SiO25
 EP/DiDOPO/SiO215
name B.E. (eV) atom (%) B.E. (eV) atom (%) B.E. (eV) atom (%)
C 1s 284.8 85.57 284.8 48.27 284.8 60.38
N 1s 400.39 3.71 400.19 2.15 400.66 1.81
O 1s 532.73 10.72 532.83 35.37 532.95 27
P 2p     134 0.5 134.18 0.65
Si 2p     103.42 13.71 103.52 10.16
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Figure 7a–c shows the 3D TG-FTIR spectra of the gas phase induced by the thermal degradation of EP composites. FTIR spectra of pyrolysis products at different reaction times were obtained, as shown in Figure 7d–f. The primary products of EP composites obtained by thermal decomposition were defined as: H2O (4000–3400, 2000–1500 cm–1), aliphatic C–H (2970, 2760 cm–1), CO2 (2366, 2334 cm–1), C–C (1506 cm–1), C–H of bisphenol-A (1241, 1125 cm–1), and RC=CH2 (833 cm–1).37−39

Figure 7.

Figure 7

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(a–c) 3D TG-FTIR profiles and (d–f) FTIR curves of the pyrolysis products obtained for EP, EP/DiDOPO/SiO25, and EP/DiDOPO/SiO215, respectively.

In Figure 7, most peaks corresponded to the bisphenol-A part with the EP component. Besides, different EP composites exhibited different peak ratios. As SiO2 and DiDOPO did not generate any novel peaks or shoulders with noticeable heights, Furthermore, any increase in the peak height may be related to the overlapping of the additional peak with the peaks originating from EP. The released amount of CO2 from EP composites was greater than that of pure EP, indicating that phosphorus is released into the gaseous phase with some minor additional changes in the number of products. The release of phosphorus led to the flame inhibition of the EP/DiDOPO/SiO2 nanocomposites. Figure 8 shows the schematic of flame-retardant mechanism of EP/DiDOPO/SiO2 nanocomposites; the addition of SiO2 and DiDOPO is conducive to form continuous char in the EP matrix, and the formed continuous char acts as a protective barrier and obstructs the diffusion of heat and the exchange of gas.

Figure 8.

Figure 8

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Schematic of the flame-retardant mechanism of EP/DiDOPO/SiO2 nanocomposites.

2.5. Mechanical Properties of EP Nanocomposites

Figure 9 shows the typical stress/strain curves of EP and EP/DiDOPO/SiO2 nanocomposites. The statistical results revealed that the strength and elongation at break of pure EP are 1.70 ± 0.21 GPa and 4.72 ± 0.61%, respectively. After incorporation with 5 wt % DiDOPO and a certain amount of SiO2, strength and elongation at break values for EP/DiDOPO/SiO22, EP/DiDOPO/SiO25, EP/DiDOPO/SiO210, and EP/DiDOPO/SiO215 were 1.94 ± 0.27 GPa and 4.42 ± 0.58%, 2.07 ± 0.25 GPa and 4.25 ± 0.51%, 2.53 ± 0.31 GPa and 3.44 ± 0.48%, and 2.60 ± 0.30 GPa and 3.16 ± 0.43%, respectively (Table 4). The results revealed that modified SiO2 can effectively increase the elastic modulus of EP but slightly reduce its elongation at break.

Figure 9.

Figure 9

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Typical stress/strain curves of EP and EP/DiDOPO/SiO2 nanocomposites.

Table 4. Tensile Performance of EP and EP/DiDOPO/SiO2 Nanocomposites.

sample designation strength (GPa) elongation at break (%)
EP 1.70 ± 0.21 4.72 ± 0.61
EP/DiDOPO/SiO22 1.94 ± 0.27 4.42 ± 0.58
EP/DiDOPO/SiO25 2.07 ± 0.25 4.25 ± 0.51
EP/DiDOPO/SiO210 2.53 ± 0.31 3.44 ± 0.48
EP/DiDOPO/SiO215 2.60 ± 0.30 3.16 ± 0.43
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3. Conclusions

In this study, SiO2 nanoparticles and phenethyl-bridged DOPO derivative (DiDOPO) were used to prepare EP nanocomposites, and the synergistic effects of SiO2 and DiDOPO on the flame-retardant performance, thermal stability, and the flame-retardant mechanism of the EP system were discussed. With the increase in the SiO2 content from 2 to 15 wt %, the UL-94 result changed from N.R. to V0 rating, the LOI value increased from 21.8 to 30.2%, and pHRR decreased from 1101.7 to 644.1 kW/m2 in comparison to pure EP. FRI results indicated that compared to pure EP, nanocomposites retain a “good” flame-retardancy performance. In addition, SEM results revealed that the residual char becomes more continuous and compact with the increase in the SiO2 content. Besides, elemental analysis indicated that flame fillers were transferred to the matrix surface during combustion to form an effective char layer and delay the permeation of heat and toxic gases. This work revealed that the EP/DiDOPO/SiO2 nanocomposites exhibit gas- and condensed-phase flame-retardant effects, which may provide a new route to improve the flame retardancy and thermal stability of the EP system.

4. Experimental Section

4.1. Materials

SiO2 nanoparticles were purchased from Beijing Anbiqi Biological Technology Co., Ltd., China, and its morphology and diameter were measured by SEM. A silane coupler CG-570 (≥98%) with a density of ∼1.070 g/cm3 and a refractive index of ∼1.425 at 25 °C was purchased from Nanjing Chengong Silicone Material Co., Ltd., China. The flame-retardant phenethyl-bridged DOPO derivative (DiDOPO) was synthesized in our laboratory, according to a previously reported method.40Figure 10 shows the chemical structure of EP, D230, CG-570, and DiDOPO. Diglycidyl ether of bisphenol-A (DGEBA, EP: epoxide value of 0.48–0.52 mol/100 g) was purchased from Laizhou Baichen Insulation Materials Co. Ltd., China. O,O′-Bis(2-aminopropyl)polypropylene glycol (D230: 98%) was purchased from Beijing Huawei Ruike Chemical Co., Ltd. China. Ultrapure water (Millipore Milli-Q grade) with a resistivity of 18.0 MΩ was used in all experiments. All chemicals were used as received without further purification.

Figure 10.

Figure 10

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Chemical structures of (a) EP, (b) D230, (c) CG-570, and (d) DiDOPO.

4.2. Organic Modification of SiO2 Nanoparticles

SiO2 nanoparticles were modified by a conventional strategy with modifications according to our previous study.41 The detailed experimental procedure was as follows: first, SiO2 was washed three times with ultrapure water and dried at 60 °C for 12 h, followed by the addition of dried SiO2 to HCl/H2O (v/v 1:7) and stirring at room temperature for 1 h. After the reaction was continued for 24 h, the mixture was washed three to five times with ultrapure water, followed by drying at 60 °C for 12 h to prepare acidified SiO2. Second, the silane coupler CG-570 was dissolved in ethanol/H2O (v/v 18:1), and the solution pH was adjusted to 4–6 with formic acid. Finally, the CG-570 solution was added to an acidified SiO2 aqueous solution with 8 wt % of CG-570 relative to SiO2 and subsequently stirred at 60 °C for 5 h. Purification of the final products by centrifugation and drying afforded organic-modified SiO2 nanoparticles.

4.3. Preparation of Flame-Retardant EP Composites

The flame-retardant EP/DiDOPO/SiO2 nanocomposites were prepared by a modified method according to that reported by our group, and the corresponding preparation routes were almost similar, just replacing the inorganic materials OLDH with SiO2 nanoparticles.40 Typically, modified SiO2 was first dispersed in acetone (1 g/mL) by ultrasonication at room temperature for 1 h. Then, EP was added to the solution, and the mixture was stirred at 80 °C for 3 h. Next, the mixture was transferred to an oil bath and continuously stirred at 130 °C for 0.5 h to evaporate the excess acetone, followed by the addition of DiDOPO to the mixture and stirring for 0.5 h. Then, the solution was cooled to 50 °C, and the curing agent D230 was added to the solution. The final homogeneous solution was degassed in a vacuum oven at 50 °C for 15 min. Next, the solution was poured into preheated molds and cured at 80 °C for 1 h and then at 140 °C for 2 h. In addition, all samples were prepared using the same strategy. Table 5 lists the designated samples and corresponding content of each component.

4.4. Characterization

Field-emission SEM (FEI Quanta 250 FEG, FEI Inc.) under high vacuum at a voltage of 20 kV was employed to observe morphologies of all testing samples. FTIR spectroscopy (Nicolet IS50, Thermo Fisher Scientific Inc.) was employed to characterize raw and modified SiO2. The resolution was 4 cm–1, the number of scans was 32, and the test range was 400–4000 cm–1. A TG 219 F3 thermal analyzer (Netzsch Instruments Co., Ltd., Germany) was utilized at a constant scanning rate of 10 °C/min under nitrogen at temperatures ranging from 50 to 800 °C to evaluate the thermal stability of all testing materials. UL-94 tests (CZF-2, Jiangning, China; dimensions of 130 mm × 13 mm × 3.2 mm) were performed as per ASTM D3801. LOI (according to ASTM D2863-77) was measured using a JF-3 oxygen index meter (Jiangning, China), with a sample size of 100 mm × 6.5 mm × 3.2 mm. CCT (FTT, U.K.) was conducted as per ASTM E1354/ISO 5660. Specimens with dimensions of 100 mm × 100 mm × 6 mm and an external heat flux of 50 kW/m2 were selected. A TG-FTIR instrument comprised a TG 219 F3 system (Netzsch Instruments Co., Ltd., Germany), an FTIR spectrometer (Nicolet IS50, Thermo Fisher Scientific Inc.), and a transfer tube with an inner diameter of 1 mm connected to the TG and IR cells. Measurements were conducted from 30 to 600 °C at a linear heating rate of 20 °C/min under a nitrogen flow of 30 mL/min. X-ray photoelectron spectroscopy (XPS) profiles of the char residue were recorded on a Thermo Escalab 250Xi system (Thermo Fisher Scientific Inc.) using Al Kα excitation radiation (hν = 1253.6 eV). CMT6104 (MTS, Tianjin, China) was utilized to conduct tensile performance tests (sample size: 75 mm × 5 mm × 1.88 mm).

Acknowledgments

This study was financially supported by the Natural Science Foundation of China (52063005), the Guizhou Youth Science and Technology Talents Project (KY2019085), the Talent Introduction Project of Guiyang University by Guiyang City Financial Support Guiyang University (2019039510822), the Discipline and Master’s Site Construction Project of Guiyang University by Guiyang City Financial Support Guiyang University (HC-2020), and the Innovation Group Project of Guizhou Provincial Department of Education (2020024).

The authors declare no competing financial interest.

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