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Synthesis and application of a triazine derivative containing boron as flame retardant in epoxy resins
⁎Corresponding authors. mcpliu@scut.edu.cn (Ping Liu)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Abstract
A halogen-free, organic boron/nitrogen compound, 2,4,6-tris-(4-boronphenoxy)-(1,3,5)-triazine (TNB), was synthesized. The thermal and flame-retardant properties of epoxy resins (EP) containing TNB were investigated based on TGA, limiting oxygen index (LOI), vertical burning (UL 94) and cone calorimeter tests. The results showed that the residual char of EP increased after TNB was present. The LOI value of EP/20%TNB reached 31.2% and UL 94 V-0 rating was achieved. The peak heat release rate and total heat release of EP/20%TNB reduced to 305.3 kW/m2 and 58.0 MJ/m2, respectively. In addition, the flame-retarding mechanism was investigated using X-ray diffraction, scanning electron microscopy, laser Raman spectroscopy, elemental analysis, and thermogravimetric analysis-infrared spectroscopy. The results show that TNB is an efficient flame retardant, which is effective in the gas and condensed phases simultaneously.
Keywords
Triazine derivative
Boron/nitrogen flame retardant
Epoxy resin
Flame-retarding mechanism
1 Introduction
Epoxy resins (EP), which are typical thermosetting resins, have been used in a range of applications, e.g., fiber-reinforced materials, general adhesives, laminates, and anticorrosive encapsulating materials, because of their excellent mechanical properties, high adhesion, and chemical and electrical resistances (Hashimoto et al., 2012; Jin et al., 2015; Yang et al., 2016; You et al., 2015). However, like other ordinary polymeric materials, regular EP burn easily and the flames are difficult to extinguish. Their poor fire resistance limits their wider application (Huo et al., 2016; Perret et al., 2011; Ratna, 2005). It is therefore necessary to improve the flame-retardant properties of EP.
Over the past few decades, halogen flame retardants have been extensively used to reduce the flammability of polymer materials, especially brominated flame retardants (BFRs), which are widely used industrial chemicals (Cao et al., 2016). However, based on the inherent shortcomings of BFRs, especially the release of corrosive and toxic gases during combustion, they are used with caution or avoided in many industries. The European Union banned the use of polybrominated biphenyls and polybrominated diphenyl ethers on July 1, 2006 (Dasari et al., 2013; Rakotomalala et al., 2010; Squires, 2008). The development of non-halogen flame retardants is therefore important. Among various halogen-free flame retardants, intumescent flame retardants caused a serious concern due to their high flame retardant efficiency, low smoke and ant-dripping (Huang and Shi, 2007; Krishnadevi and Selvaraj, 2015; Liu et al., 2016; Peng et al., 2008; Tao et al., 2011; Wang et al., 2009; Yang et al., 2008; Zhao et al., 2012).
Inorganic boric acid and borate salts have been used as flame retardants. They not only have less smoke and less toxicity, but also have lower accumulation (Martín et al., 2006; Korkmaz 2011; Schoderboeck et al., 2011). However, the flame-retardant performances of organic boron compounds have been less investigated than those of organic halogen and phosphorus compounds (Armitage et al., 1996; Blasi et al., 2007). The boron based flame-retardants dehydrate at high temperature, which can reduce the temperature of combustible materials. Furthermore, if they contain more than one boronic acid functional group, they may form boroxine or boronic acid anhydride during the combustion process, which may result in the formation of high residual char and the protection of the remaining substance from further oxidation degradation (Peng et al., 2014; Zhang et al., 2016a, 2016b; Zhong et al., 2015). In addition, it is reported that triazine derivatives have good thermal stability and abundant nitrogen, which often act as gas source in the intumescent flame retardant (Dai and Li, 2010; Ke et al., 2010; Wu et al. 2009).
In order to develop novel organic boron/nitrogen flame retardants, this paper reports the synthesis of compound 2,4,6-tris-(4-boronphenoxy)-(1,3,5)-triazine (TNB). The flame-retardant properties of EPs containing TNB were investigated, and the possible flame-retarding mechanism was also investigated.
2 Experimental section
2.1 Materials
Cyanuric chloride (CNC) was purchased from Energy Chemicals (Shanghai, China). 4-Hydroxyphenylboronic acid was bought from the Zhengzhou Alpha Chemical Co., Ltd. Diaminodiphenylmethane (DDM) was purchased from the Shanghai Macklin Biochemical Technology Co., Ltd. An EP (E-44) with an epoxide equivalent weight of 210–240 g/equiv was supplied by the Xiya Reagent Co., Ltd., China. Other reagents were commercial products of analytical grade, and used without further purification.
2.2 Synthesis
Scheme 1 shows the synthesis of 2,4,6-tris-(4-boronphenoxy)-(1,3,5)-triazine (TNB).
2.2.1 Synthesis of TNB
4-Hydroxyphenylboronic acid (15.5 mmol) and sodium hydroxide (31 mmol) were dissolved in tetrahydrofuran (120 mL) in a three-necked flask, and the mixture was stirred at room temperature for 2 h under a nitrogen atmosphere. CNC (5 mmol) dissolved in tetrahydrofuran (30 mL) was added dropwise to the flask, continuously and slowly, under a nitrogen atmosphere at room temperature. The reaction mixture was heated slowly to the reflux temperature, and then stirred vigorously for 16 h. The mixture was cooled to room temperature and the precipitate was separated by filtration. The precipitate was dissolved in water (200 mL) and hydrolyzed with acid for 1 h (pH < 2). The pale-red precipitate was collected by filtration, purified on a silica-gel column using dichloromethane/ethanol (20:1, v/v), and dried at 70 °C for 8 h. The yield was 86.5%.
2.3 Preparation of samples
Various weights of TNB were added to EP, the mixture was heated to 80 °C with frequent agitation. Then the curing agent DDM at a 1.2:1 equivalent of EP was added. The mixture was stirred until the DDM had dissolved completely, degassed in a vacuum oven at 60 °C for 3 min, rapidly poured into preheated molds, and cured at 80 °C for 2 h, 120 °C for 2 h, and 150 °C for 2 h. The samples were allowed to cool slowly to room temperature.
2.4 Characterization
FTIR spectra were recorded using a Nicolet 6700 FTIR spectrometer (Madison, WI, USA). 1H and 13C NMR spectra were recorded using a Bruker AVANCE-600 MHz NMR spectrometer (Billerica, MA, USA) with DMSO-d6 as the solvent. 11B NMR spectra were recorded at 193 MHz using the same instrument.
Differential scanning calorimetry (DSC) was performed using a Netzsch 204 F1 analyzer at a heating rate of 10 °C/min under nitrogen atmosphere. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) were performed using a Netzsch 209 F3 thermal analyzer (Selb, Germany) at a heating rate of 10 °C/min.
The limiting oxygen index (LOI) was determined using a Fire Testing Technology (FTT, East Grinstead, UK) instrument, according to ASTM D2863. The dimensions of each sample were 80 mm × 10 mm × 4 mm. Vertical burning (UL 94) tests were performed with a FTT UL 94 instrument, using samples of dimensions 125 mm × 12.7 mm × 3.2 mm, according to ASTM 3801. The burning grades were classified as either V-0, V-1, V-2, or no rating (NR), depending on the self-extinguishing time and dripping. Cone calorimeter (cone) tests were performed using a FTT instrument according to ISO-5660, with an incident flux of 50 kW/m2. The dimensions of the samples were 100 mm × 100 mm × 3 mm. Each specimen was placed in an aluminum tray, and only the upper surface was exposed to the radiant heater.
X-ray diffraction (XRD) patterns were recorded using a MERCURY CCD X-ray diffractometer (D/max-III, Japan). The morphologies of the residual chars from the cone tests were examined using scanning electron microscopy (SEM; Carl Zeiss, Germany) at an accelerating voltage of 10 kV. Laser Raman spectroscopy (LRS) was performed at room temperature using a laser Raman spectrometer (RenishawinVia, Renishaw). The elements in the residual chars from the cone tests were analyzed using a Vario EL Cube elemental analyzer (Elemental, Germany). Thermogravimetric analysis-infrared (TG-IR) spectroscopy was performed using a combination of an STA-449c FTIR spectrophotometer (Netzsch, Germany) and a TENSOR-27 FTIR spectrophotometer (Bruker, Germany).
The impact strengths were measured with a ZCJ 1320 impact testing machine (Guangdong, China) for which the sample dimensions were 80 mm × 10 mm × 4 mm.
3 Results and discussion
3.1 Characteristics of TNB
Fig. 1 shows the FTIR spectra of TNB. The peak at 3059 cm−1 is attributed to the C⚌C—H stretching vibration. The peak at 1573 cm−1 and 1378 cm−1 is assigned to the vibration of C⚌N and C—N, respectively. The C—O stretching vibration is observed at 1217 cm−1. The characteristic absorption peaks of B-C at 1276, 1016, and 658 cm−1.
Fig. 2 shows the 1H, 13C and 11B NMR spectra of TNB. In Fig. 2a, the aromatic protons were appeared at 7.19–7.82 ppm, and the presence of OH protons was confirmed by the peak at 8.09 ppm. In Fig. 2b, the presence of aromatic carbon was confirmed by the peaks observed at 120.8–153.4 ppm, and the carbon signal at 173.5 ppm assigning to triazine ring. In Fig. 2c, a single peak appearing at −2.3 ppm in the 11B NMR spectrum confirmed the presence of boric acid in TNB.
3.2 Thermal properties of TNB
Fig. 3 shows the TGA and DTG curves of TNB under nitrogen atmosphere. The data are summarized in Table 1. The residual char yield of TNB at 800 °C was 30.6%. TNB began to decompose at 127.4 °C and thermal degradation occurred in two stages. The peak in stage one, at 175.3 °C, corresponds mainly to conversion of the boronic acid group to boroxine. The second degradation stage occurred at 327.3 °C, and is attributed to the degradation of boroxine to form a B—O—C char (Bhat et al., 2011; Kua and Gyselbrecht, 2008).
| Samples | Tonseta (°C) | Tmax1b (°C) | Tmax2 (°C) | Tmax3 (°C) | W800°Cc (%) | W800°C_theod (%) |
|---|---|---|---|---|---|---|
| TNB | 127.4 | 175.3 | 327.3 | – | 30.6 | – |
| EP | 315.2 | 390.7 | – | – | 9.0 | – |
| EP/1%TNB | 272.6 | 339.6 | 392.6 | 513.8 | 11.7 | 9.2 |
| EP/5%TNB | 304.4 | 340.6 | 394.2 | 522.9 | 16.9 | 10.1 |
| EP/10%TNB | 280.5 | 340.5 | 395.0 | 529.5 | 19.0 | 11.2 |
| EP/15%TNB | 255.6 | 335.1 | 394.6 | 526.6 | 22.1 | 12.2 |
| EP/20%TNB | 220.3 | 340.6 | 404.3 | 524.2 | 24.3 | 13.3 |
The DSC curves of TNB are shown in Fig. 4. TNB had a wide endothermic peak during the first heating process, in the temperature range 123–209 °C. No exothermic and endothermic peaks were observed during the first cooling and second heating processes. The formation of boroxine is an irreversible process, therefore, based on the TGA curves, it can be concluded that TNB decomposed to boroxine at about 175.3 °C.
3.3 Characteristics of EP thermosets
3.3.1 TGA and DTG
Fig. 5 shows the TGA and DTG curves of the pure EP and EPs with TNB under nitrogen atmosphere. Their thermal decomposition data are summarized in Table 1. The results show that pure EP had only one decomposition stage. The decomposition temperatures of the EPs with TNB were lower than that of pure EP. The reason was that TNB dehydrated and promoted EP decomposition in advance, forming a protective carbon.
The thermal degradation behavior of the EPs with TNB occurred in three stages, i.e., it differed from that of pure EP. The first degradation stage corresponds to TNB decomposition, leading to the formation of boroxine. The second degradation stage is assigned to degradation of cured EP and boroxine, forming B—O—C chars and generating the initial char residue. The third thermal degradation stage is attributed to destruction of this char residue. However, the thermal degradation was weaker, and the residual char yields of the EPs with TNB were significantly higher, by 30–170% compared with that of pure EP. In addition, the residual char yield at 800 °C (W800°C) was significantly higher than the theoretical residual char yield (W800°C_theo). These results indicate that TNB promoted the formation of a protective char and acted as a flame retardant.
Fig. 6 shows the TGA and DTG curves of the pure EP and EPs with TNB under air atmosphere. Their thermal decomposition data are summarized in Table 2. The initial decomposition temperatures of the EPs with TNB were lower than that of pure EP. The reason was that TNB promoted the decomposition of EP in advance to form a char layer. However, the Tmax2 and Tmax3 of EP/TNB system were higher than that of pure EP. This was mainly due to the initial char layer formed had a protective effect on EP. Note that, the residual char yield of EP at 800 °C was 0. However, the residual char yields of the EP/TNB system were significantly increased after adding different amount of TNB. Obviously, TNB has good flame retardance of EP, whether under nitrogen or air atmosphere.
3.3.2 LOI and UL 94 tests
LOI and UL 94 tests are used to investigate the flame-retardant performances of materials. The results for the pure EP and EPs with TNB are presented in Table 3. The pure EP burnt easily, accompanied by dripping, and did not pass the UL 94 test. The LOI values of EPs with various TNB loadings were higher than that of pure EP. When 1% TNB was added, the LOI value of the EP increased from 22.9% to 26.1%, and achieved a UL 94 V-1 rating, i.e., it improved the flame retardance of EP. When 20% TNB was added, the LOI value of EP reached 31.2%, and the sample achieved the UL 94 V-0 rating. In addition, the incorporation of TNB into EP composites suppressed resin dripping during burning. These results indicate that TNB had a flame-retardant effect for EP. A possible reason is the synergistic effect of the boroxine and triazine moieties.
3.3.3 Cone tests
Cone tests are widely used to evaluate the fire hazards posed for materials. The parameters include the time to ignition (TTI), heat release rate (HRR), peak heat release rate (PHRR), total heat release (THR) and average mass loss rate (av-MLR). Table 4 lists typical cone data for the pure EP and EPs with TNB. The data show that the EPs with TNB had shorter TTIs, consistent with the Tonset values for TNB and EPs with TNB as described above. TNB therefore decomposes first, and promotes the degradation of EP at lower temperatures.
| Samples | TTI (s) | PHRR (kW/m2) | THR (MJ/m2) | av-MLR (g/s) | W (wt%) |
|---|---|---|---|---|---|
| EP | 35 | 1064.9 | 80.3 | 0.662 | 5.5 |
| EP/1%TNB | 23 | 685.9 | 68.1 | 0.396 | 10.0 |
| EP/5%TNB | 22 | 426.6 | 64.1 | 0.323 | 14.7 |
| EP/10%TNB | 20 | 323.6 | 59.3 | 0.135 | 15.2 |
| EP/15%TNB | 22 | 308.6 | 58.3 | 0.083 | 17.0 |
| EP/20%TNB | 22 | 305.3 | 58.0 | 0.053 | 21.2 |
Fig. 7 shows HRR curves for the pure EP and EPs with TNB. Before about 60 s, the HRR values of the EPs with TNB were higher than that of the pure EP, because TNB promoted the decomposition of EP ahead of time. When TNB was incorporated into the EP, the total heat release (THR) decreased, and two PHRRs were observed in the HRR curves. The first PHRR is assigned to thermal degradation of the flame retardant, generating an initial carbon residue. The second PHRR is mainly attributed to destruction of the initial carbon layer on exposure to high temperatures, producing a large amount of heat and forming a protective char residue. The addition of 20% TNB to the EP decreased the PHRR value from 1064.9 to 305.3 kW/m2, and decreased the THR value from 80.3 to 58.0 MJ/m2.
Mass loss rate (MLR) represents the rate of mass loss during the combustion of a material, and can be used to clarify the flame-retarding mechanism in the condensed phase. The data in Table 4 show that the av-MLR values of the EPs with TNB were 40.2–92.0% lower than that of the pure EP. Fig. 8 shows that the residual char yields from all the EPs with TNB were lower than that from the pure EP before 60 s, but were greater than that from the pure EP at higher temperatures. The charring ability gradually improved with increasing weight of TNB, which is consistent with the TGA and DTG results. These results are confirmed by digital photographs of the residual chars after the cone tests, shown in Fig. 9. The pure EP had almost no residual carbon layer after the cone test, indicating that the pure EP burned completely. However, the residual char from EP/20%TNB was intumescent and hard, and its shape was well maintained, which helped to prevent dripping. These phenomena indicate that boron and nitrogen have a good synergistic flame-retardant effect. The boronic acids dehydrated and formed to a protective char residue, which can prevent oxygen and heat reaching the internal material and delay the degradation of substrate. The decomposition of triazine moieties produce non-combustible gases, which can dilute the oxygen and combustible gases produced through the decomposition of EP. The synergy between boroxine and triazine moieties can inhibit the decomposition of the internal resin.
3.4 Flame-retarding mechanism
3.4.1 Morphologies of residual chars from pure EP and EPs with TNB
Fig. 10 shows digital photographs of pure EP and EP/20%TNB kept at various temperatures for 15 min in a muffle furnace. The photographs show that pure EP began to expand at 350 °C and could not maintain its shape above this temperature. In addition, pure EP had many fluffy pores, associated with melting. The results indicate the absence of protective carbon, therefore thermal degradation occurred easily. When the pure EP was kept at 650 °C for 15 min, there were little residue in the crucible, and the pure EP was completely combusted. In contrast, EP/20%TNB retained their shape at 350 °C. When the temperature reached 450 °C, the thermoset began to swell, without melting, and formed a compact residual char. After being kept at 650 °C for 15 min, the amount of residual char from EP/20%TNB were still as high as 6.13 wt%.
Fig. 11 shows the FTIR spectra of the residual chars of pure EP and EP/20%TNB after being kept at 650 °C for 15 min. No absorption peaks are observed in the FTIR spectrum of the pure EP residual char, indicating that pure EP combustion was complete. However, many absorption peaks are present in the FTIR spectrum of the EP/20%TNB residual char, including those from C⚌N (1639 cm−1), B—O—C (1380 cm−1), and C—O (1060 cm−1). This shows that the organic structure of the EPs with TNB was not completely degraded and was well protected. In addition, new absorption peaks appeared at 1223 and 704 cm−1 in the FTIR spectrum of the EP/20%TNB residual char, corresponding to the stretching vibration of B—O in B2O3. This was confirmed by the XRD pattern.
Fig. 12 shows the XRD pattern of the origin and residual chars of pure EP and EP/20%TNB. The residual char from EP/20%TNB shows two diffraction peaks, at 2θ = 14.6° and 27.8°, which correspond to B2O3. B2O3 is produced during the degradation process, and is considered to contribute to the flame-retardant properties of EP/TNB systems.
3.4.2 SEM
Fig. 13 shows SEM micrographs of the surface morphologies of pure EP and EP/20%TNB residual chars after cone tests. The pure EP residual char was loose and porous, and could not form a protective char to retard heat transfer. However, the residual char from EP/20%TNB was relatively compact and continuous, and was able to prevent oxygen and heat reaching the internal material. However, it also had some small holes, which were produced by the emission of non-combustible gases during EP/20%TNB combustion. This contributed to the formation of a swollen carbon layer, and prevented the dripping of material during burning. These results indicate that the TNB flame retardant was active in the gas and condensed phases.
3.4.3 LRS
LRS can sensitively reflect changes in the carbon crystal structure, and can be used to determine the sequence of carbon residue formation. Fig. 14 shows the LR spectra of the residual chars from pure EP and EP/20%TNB after cone tests. There are D and G band in the LRS. The ratio of the accumulated intensities of the D and G bands (ID/IG) can be used to determine the degree of graphitization and the degree of graphite structural integrity. Normally, the lower ID/IG is, the better the structure of the carbon material. Fig. 14 shows that ID/IG decreased from 1.69 for pure EP to 1.49 after addition of 20% TNB; this indicates an increase in the degree of graphitization of the remaining carbon. These results show that the intumescent carbon layer had a higher degree of graphitization, which improved the strength of the carbon layer, thereby improving its barrier properties and suppressing the transfer of combustion heat to the internal material. TNB therefore has a good carbonization capacity, and has a flame-retardant effect in the condensed phase.
3.4.4 Elemental analysis
The contents of carbon (WC) and nitrogen (WN) in the residual char after the cone tests were determined using elemental analysis. The data in Table 5 showed that WN did not change significantly with increasing TNB content, but WC decreased and the amount of carbonaceous char increased correspondingly. This is because the nitrogen element was released into the air in forms such as NH3 and N2, and these inert gases acted as flame retardants in the gas phase, as discussed in Section 3.4.2. Thermally unstable structures decompose under heating and sufficient oxygen, leaving substances that are highly stable to thermal oxidation. As discussed in Section 3.4.1, the XRD pattern showed that the EP/20%TNB thermoset generated boron oxide compounds during combustion, leading to the formation of B—O—C, B—O—N, and B—O—B structures. These structures, which have good stability against thermal oxidation, protect the underlying substance from further degradation, promote the formation of a compact carbon layer, and act as flame retardants in the condensed phase. These results show that TNB played an important role in both the gas and condensed phases.
| Samples | WN (%) | WC (%) | Wother (%) | Carbonaceous char (%)a |
|---|---|---|---|---|
| EP | 4.5 | 83.4 | 12.1 | 4.6 |
| EP/1%TNB | 5.0 | 84.2 | 10.8 | 8.4 |
| EP /5%TNB | 4.4 | 80.1 | 15.5 | 11.7 |
| EP/10%TNB | 4.0 | 77.7 | 18.3 | 11.8 |
| EP/15%TNB | 4.0 | 74.7 | 21.3 | 12.7 |
| EP/20%TNB | 4.1 | 67.4 | 28.5 | 14.3 |
3.4.5 TG-IR spectroscopy
TG-IR spectroscopy is widely used to clarify thermal degradation mechanisms because it can detect the volatilization products during thermal degradation. The FTIR spectra of the volatilization products at different temperatures are shown in Fig. 15. For pure EP, there was no obvious absorption peaks at 132 and 350 °C. When the temperature reached 378 °C, weak absorption peaks assigned to water (3650, 1602, and 744 cm−1), aromatic compounds (3048 and 1251 cm−1), aliphatic compounds (2973 and 1184 cm−1), and nitrogen oxides (1500 cm−1) were observed. At 456 °C, all the peaks became stronger, and no new peaks appeared, indicating that pure EP degradation involved only one degradation process; this is consistent with the TGA results. Note that some peaks were still observed at 603 °C, indicating continuous degradation of EP.
The degradation process for EP/20%TNB is different from that for pure EP. At 132 °C, weak absorption peaks assigned to water (3650, 1602, and 744 cm−1) were observed. This is because the TNB flame retardant was dehydrated to form boroxine. The water formed would help to reduce the temperature of the substrate surface. At 350 °C, the water peaks strengthened and some new peaks emerged, including NH3 (3546 and 3507 cm−1), aromatic compounds (3048 and 1252 cm−1), aliphatic compounds (2973 and 1184 cm−1), CO2 (2354, 2287, and 678 cm−1), and nitrogen oxides (1500 cm−1). The TNB flame retardant contributed to the formation of CO2 and NH3, which can decrease the oxygen and combustible gases concentrations around materials, inhibit substrate combustion, and promote the formation of an intumescent carbon layer. When the temperature rose to 603 °C, no clear absorption peaks were observed in the spectrum, in contrast to the EP spectrum at 603 °C. It indicated that the addition of TNB was beneficial to protect the degradation of the EP.
Overall, it can be deduced that TNB acts as a flame retardant in the gas and condensed phases, via stimulation of thermoset decomposition at lower degradation temperatures and the release of non-combustible gases; these factors are conducive to the formation of a sufficient and compact residual char layer. In addition, B2O3 is produced during combustion, which improves the thermal stability of the residual carbon. All of these are conducive to achieving good flame retardancy.
3.5 Mechanical properties
Table 6 showed the mechanical properties of pure EP and EP/TNB. When 1% TNB was added, the impact energy and impact strength of EP increased from 0.39 J to 0.57 J, and 9.8 kJ·m−2 to 14.3 kJ·m−2, respectively. In general, EP is easy to crack. TNB could consume deformation work and prevent crack propagation when TNB was added. This effect enhanced the fracture toughness of EP. As the content of TNB continues to increase, the mechanical properties of EP began to decline. This indicated that excess TNB could degrade the mechanical properties of EP.
| Samples | Impact energy (J) | Impact strength (kJ·m−2) |
|---|---|---|
| EP | 0.39 | 9.8 |
| EP/1%TNB | 0.57 | 14.3 |
| EP/5%TNB | 0.42 | 10.5 |
| EP/10%TNB | 0.38 | 9.5 |
| EP/15%TNB | 0.34 | 8.5 |
| EP/20%TNB | 0.29 | 7.3 |
4 Conclusions
A halogen-free flame retardant, namely TNB, containing boron and nitrogen was synthesized and characterized. The addition of TNB promoted the decomposition of EP at a lower temperature and improved the residual carbon. EP/20%TNB achieved a UL 94 V-0 rating, with a LOI value of 31.2%, showing that it has excellent flame-retardant properties. TNB also reduces the PHRR, THR, and av-MLR values of EP thermosets. TNB exerted flame-retardant effects in the gas and condensed phases simultaneously, by releasing non-combustible gases, improving the carbonation capacity, forming an intumescent and compact residual char, and producing B2O3. Organic boron/nitrogen flame retardants are promising alternative halogen-free flame retardants with efficient flame-retardant properties.
Acknowledgment
This research was financially supported by the NSFC (Grant Nos. 20674022, 20774031, and 21074039), the Natural Science Foundation of Guangdong (Grant Nos. 2010A090100001, 2014A030313241, 2014B090901068, and 2016A010103003), the Natural Science Foundation of Guangzhou (Grant Nos. 201604010034), and the Ministry of Education of the People’s Republic of China (Grant No. 20090172110011).
Notes
The authors declare no competing financial interest.
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