Synergistic Fire Retardancy of Bis ( 1-methoxy-2 , 2 , 6 , 6-tetramethylpiperidin-4-yl ) sebacate and Tris ( 2 , 4 , 6-tribromophenoxy )-1 , 3 , 5-triazine / Sb 2 O 3 in HIPS

Bis(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)sebacate (NORSM) was found to perform an exceptional synergistic effect with tris(2,4,6-tribromophenoxy)-1,3,5-triazine (TTBPC)/Sb2O3 in HIPS. 4e LOI of fire retardant HIPS (FR-HIPS) with 16wt% of TTBPC/Sb2O3 increased from 23.8% to 25.2%, the flame retardant rating of FR-HIPS was improved fromUL 94 V-2 to UL 94 V-0, and various heat release parameters such as peak heat release rate, total heat release, and mean effective heat of combustion were greatly lowered by combining NORSM of 0.50wt%. 4e Py-GC/MS results revealed that NORSM induced the role of synergistic effect; the main mechanisms were as follows: the active radicals such as methoxy radicals and aminyl radicals produced by the pyrolysis of NORSM promote the release of bromine radicals from TTBPC and the formation of HBr and CO2, which improves the flame retardancy of TTBPC/Sb2O3; the above active radicals, together with HBr, quench active free radicals, such as the hydroxyl radical (OH), and decompose the free radical source, which interrupts the chain reaction during combustion and results in a more efficient flame retardant effect in gaseous phase.

Although there are many reported flame retardants that can be used in HIPS, the primarily used flame retardants are brominated flame retardants, as they have better comprehensive performance. e most common brominated flame retardants are decabromodiphenyl ether (decaBDE) [14], tetrabromobisphenol A (TBBA) and its derivatives [15], decabromodiphenyl ethane (DBDPE) [14,16,17], and so forth. decaBDE was gradually restricted in use due to its environmental impact [18]. Besides, TBBA also involves environmental and safe impact [19]. Tris(2,4,6-tribromophenoxy)-1,3,5-triazine (TTBPC) is a novel brominated flame retardant containing nitrogen, which exhibits good melt fluidity, flame retardancy, thermal stability, antistatic ability, UV light stability, and so forth [20,21]. It is suitable for TTBPC in HIPS and other polymers. Nevertheless, it also needs to be used at a higher concentration (usually about 20 wt%) as compared to other brominated flame retardants. How to effectively improve the flame retardancy of brominated flame retardants, especially TTBPC in HIPS, is important to promote the development and application of HIPS.
In the last few years, it has been found that 1-alkoxy-2,2,6,6-tetramethylpiperidine-based compounds (NORs) not only exhibited UV stabilization but also contributed to the flame retardancy and synergistic flame retardant effect in polyolefin, especially polypropylene [22,23]. NOR116, the first commercial NORs product introduced by Ciba (now BASF) in 2000, primarily exhibits the following characteristics: (1) adequate performance as a flame retardant in polypropylene fiber and thin films [24]; (2) adequate performance as a synergist in traditional flame retardants such as brominated flame retardants [25,26], phosphorus flame retardants [25], melamine and its derivatives [27,28], intumescent flame retardants (IFR) [29,30], and inorganic flame retardants [31]; (3) lower smoke production and toxic gases release by replacing or decreasing the quantity of the commonly used synergist, Sb 2 O 3 , in halogen flame retardants; (4) being environment-friendly and safe in use because of its halogen-free nature and lower additive dosage; and (5) appreciable thermal stabilization as well as UV stabilization. Regardless of these properties, NOR116 exhibited some shortcomings: (1) adequate performance as a flame retardant only in thin polyolefin products; (2) discolored or yellowed the polyolefin when it was in use; (3) occurrence of dripping during burning; and (4) being exceedingly complex in structure and synthesis method [32], leading to increased cost of production and large three-waste emissions during synthesis.
To overcome the shortcomings of NOR116, some new NORs, with better performance and simpler molecular structure and synthesis process, have been developed domestically and abroad. One of these, a series of NORs containing AZO structure (AZONORs), was designed and synthesized by Aubert et al. inÅbo Akademi University in Finland [33,34]. e DIN4102 B2 flammability test showed that AZONORs had appreciable flame retardancy and selfextinguishing effect in polypropylene films and templates. Moreover, they proved capable of preventing dripping while burning. Besides, AZONOR compounds showed an exceptional performance and exhibited a significant synergistic effect with different brominated flame retardants and mineral fillers such as aluminum hydroxide (ATH).
Cao et al. [35], Zhejiang University, China, also designed and synthesized a silicon-containing NOR, which not only exhibits good synergistic effect with IFR but also can improve the UV stability and mechanical properties of the composite. Some other types of NORs had also better flame retardant effect in polypropylene than that of NOR116 [23,36,37].
NORSs is a kind of NORs with comparatively simpler chemical structure and synthesis process [38][39][40], the long chain in its structure leads to its good thermal stability and good dispersion in matrix [41], and its moderate molecular weight results in its somehow mobility resistance. Tinuvin NOR123, the trade name of bis(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl)sebacate, is a commercial product of Ciba, and it exhibits strong UV stabilization along with an appreciable synergistic flame retardancy. Charla, et al. found that Tinuvin NOR123 exhibited a good synergistic flame retardant effect with brominated phosphate in PP fiber, which could pass the FMVSS 302 test when the concentrations of Tinuvin NOR123 and brominated phosphate were 1 wt% and 2 wt%, respectively [42]. Except what is mentioned above, very few relative studies on the flame retardancy and the flame retardant synergistic effect of NORSs were reported, and no report on those performances in HIPS was found either. erefore, in this paper, more attention will be paid to the studies of the synthesis and synergistic flame retardancy of bis(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)sebacate (NORSM) with TTBPC/ Sb 2 O 3 in HIPS.

Preparation of NORSM.
e synthesis methods of NORSs were reported in related literature [38][39][40], and one of the most effective methods was transesterification. In this paper, NORSM were synthesized by an improved catalyst transesterification with a novel methanol separation process.
With that temperature being unchanged, CuCl (6.36 mmol; 0.63 g) was added to the flask, and then the contents were heated up to 30°C and maintained for 24 h. ereafter, the unreacted acetone was removed by vacuum distillation, the residue was extracted with 60 ml dichloromethane, and the organic phase was washed thrice with deionized water (3 × 40 ml). e dichloromethane was removed from the organic phase by distillation, and the residue was dissolved with 60 ml methanol/water (volume ratio � 1 : 1) by heating; the contents were then cooled down to 0°C and maintained for 24 h. e crystallized product was filtered and washed thrice with deionized water (20 × 3 ml). e product was then dried to constant weight at 60°C, and about 8.75 g white powder product was obtained (theoretical 18.73 g). 46.7% yield of the product was obtained with purity of 99.1% and melting point of 86-89°C.
(2) Synthesis of NORSM: 0.11 mol 1-methoxy-2,2,6,6tetramethylpiperidin-4-ol (20.6 g), 0.09 mol finely ground LiOH (2.2 g), n-heptane (100 g), 0.05 mol dimethyl sebacate (11.5 g), and 0.003 mol tetrabutylammonium bromide (about 1 g) were added to a dried 250 ml flask equipped with a mechanical stirrer, a thermometer, a water separator, and a reflux condenser, and the contents were then heated up to reflux (about 98°C), with methanol being continuously separated; stop the reaction till there was no methanol remained (about 6-8 h). After the reaction, the contents were cooled to room temperature, the organic phase was washed thrice with deionized water (40 ml × 3) first and then washed twice with 35 g × 2 mixed solvent of methanol and water with the weight ratio of 1 : 1, the solvent and unreacted 1-alkoxy-2,2,6,6-tetramethylpiperidin-4ol were separated from the organic phase by vacuum distillation, and the distillation residue was washed trice with 35 g of the above mixed solvent. Finally, 21.6 g (theoretical 27.02 g) NORSM was obtained by removing residual mixed solvent through vacuum distillation. e yield of NORSM was 79.9% with purity of 96.3%.
For structural characterization, the Fourier transform infrared (FT-IR) spectra measurement of the product was performed by KBr tablet method in a wavelength range of 400 to 4000 cm − 1 using TENSOR-27 Infrared Spectrometer from Bruker Inc., Germany. e 1 H NMR and 13 C NMR spectra were recorded by an AVANCE-500M NMR Instrument from Bruker Inc., Germany, with deuterated chloroform as solvent. e elemental analysis of H, C, O, and N was performed with vario MACRO cube Element Analyzer made by Elementar Company (Germany). Melting points were determined with an XT4B Melting Point Meter made by Beijing Scientific Instruments Electro-optic Instrument Factory. Figures 1-3 correspond to the infrared spectrum (IR), 1 H NMR, and 13 C NMR of NORSM, respectively.
In Figure 1, the absorption peeks at 2975 cm − 1 correspond to υC-H of methyl; those at 2930 cm − 1 correspond to υC-H of methylene; those at 1735 cm − 1 can be attributed to υC � O; those at 1217 cm − 1 can be assigned to hydrogen-free υC-N; those at 1175 cm − 1 can be attributed to hydrogen-free υN-O; those at 1049 cm − 1 correspond to υC-O-C linking with carbonyl; and those at 719 cm − 1 can be attributed to υC-N of long-chain methylene [43]  It is shown from the above results that the synthesized product is NORSM.

Flame Retardant Performance Test.
e limiting oxygen index (LOI) value was measured on the specimens of 100 × 6.5 × 1.6 mm 3 according to the standard oxygen index test ISO4589-1984 using a JF-3 oxygen index meter (made by Jiangning Analytical Instrument Company, Nanjing, China); each sample was retested twice; vertical burning test was conducted on sheets of 100 × 13 × 1.6 mm 3 according to the standard UL-94 test ASTM D635-77 using a CZF-3 horizontal vertical burning tester (made by Jiangning Analytical Instrument Company, Nanjing, China), and each sample was retested twice; the cone calorimeter test (CCT) was carried out at an incident heat flux of 50 kW/m 2 with specimens of 100 × 100 × 1.6 mm 3 according to ISO5660 standard procedures by using an FTT Standard Cone Calorimeter (made by FTT Company, UK). All measured data were processed by relative software such as Origin.

ermogravimetric Analysis.
ermogravimetric analysis (TGA) was carried out using TA-SDTQ600 ermogravimetric Analyzer (made by TA Company, USA) under nitrogen flow rate of 50 ml/min and the heating rate of 10°C/ min from ambient temperature to 700°C. e used sample weight was 5-10 mg consistently. e samples were dried for 2 h at 150°C before testing.

Py-GC/MS Analysis.
e pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was carried out using Pyrolyzer EGA/PY-3030D (made by Frontier Laboratories Ltd., Japan) and Gas Chromatograph Mass Spectrometer GCMS-QP2010 Ultra (made by Shimadzu, Japan). e samples were cracked at 400°C under helium atmosphere.

ermal Stability of NORSM.
e thermal stability of NORSM was characterized by TGA. TGA and DTG curves of NORSM under nitrogen atmosphere are represented in Figure 4. As shown in Figure 4, the temperature at 2% weight loss (T 2wt% ), 5% weight loss (T 5wt% ), and 10% weight loss (T 10wt% ) and the maximum thermal degradation rate (T max ) of NORSM are 273.2°C, 281.9°C, and 293.2°C and 301.5°C, respectively, and the residual of NORSM at 700°C is 5.97%. e above results indicate that NORPM exhibits a good thermal stability.

Synergistic FR-Effect of NORSM and TTBPC/Sb 2 O 3 .
To evaluate the synergistic effect of NORSM with TTBPC/ Sb 2 O 3 in HIPS, LOI was determined and UL-94 tests were conducted. e proportions of used samples and the results were presented in Table 1. As shown in Table 1

Fire Performance of HIPS and FR-HIPS.
e CCT based on the oxygen consumption principle has been widely conducted to evaluate the flammability of polymeric materials because CCT results have good correlation with real fire disaster and can be used to predict the combustion behavior. Various heat release and mass loss parameters of the neat HIPS and the FR-HIPS samples, such as heat release rate (HRR), total heat release (THR), mass loss rate (MLR), mass loss (ML), smoke production rate (SPR), total smoke production (TSP), and the CO production (COP) rate, were measured by CCT and used to investigate the effect of the novel flame retardant system on the combustion behavior of the FR-HIPS composites in real fires. e above-mentioned parameters as a function of time are shown in Figure 5, and some typical parameters are listed in Table 2.
As shown in Figure 5 and Table 2, the THR and HRR of pure HIPS increase rapidly, its HRR curve displays a sharp peak, and it burns out within 280 s after ignition, but those of FR-HIPS increase slowly, and there is no sharp peak in their HRR curves. Compared with pure HIPS, the peak heat release rate (PHRR), the THR, and the mean heat release rate (MHRR) of sample-2 are reduced by 70. 45 e decrease of various heat release parameters is beneficial to suppress the pyrolysis of materials and then to effectively inhibit combustion of materials. In addition, compared with    neat HIPS, the peak mass loss rate (PMLR) of FR-HIPS is obviously reduced, but the mean mass loss rate (MMLR) even increases somehow on the contrary, which may be due to the reduction of the stability of HIPS by the addition of the flame retardant.
e above results reveal that the flame retardant property of sample-4 is the best, that of sample-6 is slightly poorer than sample-4 but better than sample-2, which means that TTBPC/Sb 2 O 3 has good flame retardancy, and combining NORSM can improve its flame retardancy. e smoke production along with the HRR is also considered as another critical parameter in fire. As shown in Figure 2 and Table 2, total smoke production (TSP), smoke production rate (SPR), and CO production (COP) rate of all samples increase rapidly, but those of FR-HIPS do more rapidly. Compared with pure HIPS, TSP, MSPR, and MCOP of sample-2 increase by 62.94, 100.00, and 67.57%, respectively, those of sample-4 increase by 91.07, 100.00, and 67.57%, respectively, and those of sample-6 increase by 83.48, 85.71, and 67.57%, respectively. e results show that the addition of TTBPC/Sb 2 O 3 brings about an increase of smoke and CO during burning and goes up with the concentration of TTBPC/Sb 2 O 3 . Brominated flame retardant/Sb 2 O 3 induces the flame retardancy mainly through gaseous phase mechanism, making the pyrolysis produce incomplete combustion, so TSP, SPR, and COP of the FR-HIPS increase markedly. NORSM make the pyrolysis produce more incomplete combustion; then TSP further increases in the presence of NORSM.

Action Mechanisms.
e synergistic action mechanism of the NORSM with TTBPC/Sb 2 O 3 in HIPS was discussed by the thermogravimetric analysis, the analysis of the residues obtained in CCT, and Py-GC/MS analysis of the samples. e thermal stability of the HIPS samples was studied by TGA.
e results of TGA are presented in Figure 6 and Table 3. As shown in Figure 6 and Table 3, T 2wt% , T 5wt% , T 10wt % , and T max of the FR-HIPS decrease obviously compared with pure HIPS, which means that the addition of flame retardants significantly reduces the thermal stability of HIPS but slightly increases the residual of the composites at 700°C from 1.3% to about 3.5%. ere appear two T max in FR-HIPS; T max1 might result from the decomposition of the flame retardants. In addition, T 2wt% of samples decreases markedly in the presence of NORSM, which is perhaps owing to the relative lower decomposition temperature of NORSM.
For the analysis of the residues obtained in CCT, images of the residues obtained in CCT are presented in Figure 7.
e results show that pure HIPS is almost completely burned out, while FR-HIPS has a small amount of unexpanded black residues, which is almost consistent with the TGA results. e residuals of sample-2, sample-4, and sample-6 are 2.58%, 5.76%, and 3.56%, respectively, and increase with the concentration of TTBPC/Sb 2 O 3 , but all no-intumescent layers are formed, which results in weak flame retardancy of TTBPC/Sb 2 O 3 in condensed phase. Adding of NORSM slightly increases the residual, which revealed that NORSM has relatively a small effect in condensed phase.
Next, we discuss Py-GC/MS analysis of samples. In order to further investigate the action mechanism of the new flame retardant system, the pyrolytic products of HIPS samples were analyzed by GC/MS, and the pyrolysis reaction was deduced.
e MS analysis showed that the main cracking products of HIPS (sample-1) were styrene (3), styrene dimers (9, 10, 12, 14, 16), trimers (13), and a small amount of diphenylpropane (8). Styrene might result from the scission of a head-to-head bond, from random scission, or by both random scission and chain-end depolymerization, while styrene dimmers and trimers might be formed due to β-scission reactions [45]. e main pyrolytic products of HIPS/TTBPC/Sb 2 O 3 (sample-2) were also styrene (3), styrene dimers (9, 10, 12, 14, 16, 17, 19), trimers (13,20), and diphenylpropane (8). Besides, the pyrolysis products of TTBPC, such as tribromophenol (7; about 3.28 wt%), dibromophenol (6; about 1.41 wt%), and CO 2 (1; about 1.68 wt%), were observed. e production of dibromophenol indicated that TTBPC released bromine free radicals, which captured hydrogen from polymers and produced hydrogen bromide during thermal degradation, which is shown in Scheme 2. Besides, CO 2 might be due to the decomposition of the triazine ring in TTBPC (Scheme 2). e analysis of the pyrolysis products shows that TTBPC/Sb 2 O 3 mainly performs flame retardancy through the following mechanisms: the oxygen in the gas phase is diluted by HBr and CO 2 ; meanwhile, the active radicals such as the hydroxyl radical (·OH) and hydrogen free radicals in the gas phase during combustion are captured by HBr, which interrupts the chain reaction during combustion [46], thus imparting a flame retardant effect in the gas phase. SbBr 3 produced through the reaction of HBr with Sb 2 O 3          covers the surface of the material and isolates oxygen and heat, which results in flame retardant effect [46]. e pyrolysis products of HIPS/TTBPC/Sb 2 O 3 /NORSM (sample-6) mainly consisted of styrene (3), styrene dimers (9, 10, 11, 12, 14, 16, 17, 19), trimers (13,15,18,20), and diphenylpropane (8). In addition, dibromophenol (6; about 1.55 wt%), bromophenol (5; about 1.28 wt%), and CO 2 (1; about 7.27 wt%) were produced through TTBPC pyrolysis. As the concentration of NORSM in sample-6 was only 0.50 wt%, no pyrolysis product of NORSM in sample-6 was detected. Compared with sample-2, no tribromophenol but bromophenol was found in the pyrolysis products of sample-6, which indicated that NORSM could promote the decomposition of TTBPC and the release of bromine free radicals, which was similar to the phenomenon described in the literature [23,[47][48][49]. Although NORSM itself can pyrolyze to CO 2 , its concentration is very small; therefore, the marked content increase of CO 2 from 1.68 wt% to 7.27 wt% indicated that NORSM could obviously promote the decomposition of the triazine ring in TTBPC. As a result, it could be inferred that TTBPC was decomposed as shown in Scheme 2 in the presence of NORSM. e main pyrolysis products of NORSM were methanol (21), N-alkoxyamine (22, 28-31), amines (23-27), and sebacic acid and its esters (32)(33)(34). Methanol might be produced by methoxy free radicals capturing of hydrogen. e aminyl radical produced from NORSM took hydrogen to form 34, which broke into 27, and sebacic acid, 23-26 were the further crackates of 27, and sebacic acid was further decomposed into CO 2 , short chain hydrocarbons, and water. In addition, 29, 10-(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)oxy)-10-oxodecanoic acid, and sebacic acid were also the pyrolysis products of NORSM, 10-(1-methoxy-2,2,6,6-tetramethylpiperidin-4-yl)oxy)-10-oxodecanoic acid was further pyrolyzed into 31, and 31 was then further decomposed into 22, 27, 29, methanol (methoxy free radicals formed first), and a series of hydrocarbons. Moreover, 29 was further decomposed into 27 and methanol (methoxy free radicals formed first). It can be seen that the pyrolysis reaction of NORSM is very complex, and the possible pyrolysis pathway is shown in Scheme 3. e above analysis also shows that NORSM produced methoxy radical and aminyl radical (Scheme 4) in thermal decomposition. As the free radicals were very active, they were rapidly transformed to stable compounds; thus, they could not be detected by MS [23,[47][48][49]. e above-mentioned analysis of the pyrolysis products shows that NORSM imparts the synergistic effect mainly through the following mechanisms: active radicals such as methoxy radicals and aminyl radicals produced by the pyrolysis of NORSM promote the release of bromine radicals from TTBPC and the formation of HBr and CO 2 , which improves flame retardant effect of TTBPC/Sb 2 O 3 . e methoxy radicals and aminyl radicals together with HBr quench active free radical such as hydroxyl radical (·OH) and decompose the free radical source-peroxide. us, the chain reaction is interrupted during combustion and results in a more highly efficient flame retardant effect in the gaseous phase. [47][48][49].

Conclusions
NORSM exhibits a good synergistic flame retardant effect in HIPS. When the concentration of NORPM is 0.5 wt%, the LOI of FR-HIPS with 16 wt% of TTBPC/Sb 2 O 3 increased from 23.8% to 25.2%, t 1 was reduced from 16.2 s to 2.4 s and t 2 was reduced from 6.5 s to 1.5 s, respectively, dripping was prevented during burning, and the flame retardant rating of FR-HIPS could be improved from the UL 94 V-2 to the UL 94 V-0 rating. e PHRR, the THR, and the MHRR were reduced by 21.64, 14.44, and 28.07%, respectively. e TGA, analysis of the residues obtained in CCT, and Py-GC/MS analysis of FR-HIPS revealed that NORSM imparted the synergistic effect with TTBPC/Sb 2 O 3 mainly through the following mechanism: active radicals such as methoxy radicals and aminyl radicals produced by NORSM pyrolysis promote the release of bromine radicals from TTBPC and the formation of HBr and CO 2 , which improves flame retardant effect of TTBPC/Sb 2 O 3 . e methoxy radicals and aminyl radicals together with HBr quench active free radical such as hydroxyl radical (·OH) and decompose the free radical source-peroxide.
us, the chain reaction is interrupted during combustion and results in a more highly efficient flame retardant effect in the gaseous phase. [47][48][49].
Data Availability e data (IR of NORSM, 1 H NMR of NORSM, 13

Conflicts of Interest
e authors declare that they have no conflicts of interest.