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Keywords:

  • flame retardant;
  • cyclic phosphate;
  • aromatic phosphate;
  • PC

SUMMARY

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

A series of organo phosphorus flame retardants (FRs) based on aromatic phosphate and cyclic phosphate were synthesized in an attempt to develop an efficient FR for polycarbonate. Their successful synthesis was confirmed by FT-IR and 1 H and 31P NMR. Their thermal stability and flame retarding efficiency as a single-component additive were investigated and compared with the commercial FR, resorcinol bis(diphenyl phosphate). The thermogravimetric analysis results revealed that the aromatic phosphate synthesized in this study, phloroglucinol diphenyl phosphate (PDP), shows a higher thermal degradation temperature and better flame retardancy even though it has a lower P content than cyclic phosphate-based FRs. The flame retarding efficiency was evaluated by the UL-94 test method. The V-0 rating was achieved at a PDP loading of 2 wt% in polycarbonate in the presence of an anti-dripping agent (1 wt%), which is better than that of resorcinol bis(diphenyl phosphate) and cyclic phosphate-based FRs. The high thermal stability and P–OH generation tendency is responsible for the better flame-retarding performance of PDP. The degradation path of PDP is also discussed. Copyright © 2012 John Wiley & Sons, Ltd.

INTRODUCTION

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

A great deal of effort has been made to develop environmentally friendly flame retardants (FRs), which can replace halogen compounds. Among the many FRs developed so far, phosphorus-based FRs are known to be the most promising [1-5]. Among the various phosphorus compounds, aromatic phosphates are the most widely used in industry, especially for polycarbonate (PC) and blends containing acrylonitrile-butadiene-styrene copolymer (ABS) [6, 7].

Among the various aromatic phosphates, triphenyl phosphate (TPP) has the simplest structure. The main weakness of TPP is its high volatility; thus, higher molecular weight versions, such as resorcinol bis(diphenyl phosphate) (RDP), bisphenol-A bis(diphenyl phosphate), and resorcinol bis(di-2,6-xylyl phosphate), have been developed as a replacement for TPP [8-12]. The modes of action in the flame retardancy of aromatic phosphates have been suggested to involve both condensed and gas phase mechanisms [13-15]. If phosphates are used in non-charrable polymers such as ABS, it can be considered that the gas phase action is the main fire retardant mechanism. If phosphates are used in a charrable polymer such as PC, it can be considered that the condensed phase action is the main fire retardant mechanism.

Recently, in a series of papers, it has been reported that the P content of the FR is the most important factor affecting its flame retardancy [11, 13-16]. In order to achieve a high P content, aliphatic phosphates are more desirable than aromatic phosphates. However, aliphatic structures become hydrophilic more easily, and aliphatic phosphates, in general, are less thermally stable than aromatic phosphates. Therefore, it is not plausible to achieve both a high P content and hydrophobic. Some cyclic phosphates with a high P content have good flame retardancy in the case of PC, because of their high charred residue formation ability, but have lower thermal stability compared with aromatic phosphates such as RDP [17].

Jang et al. reported that TPP and RDP stabilize PC and delay the degradation of PC and that some of the phosphates undergo an alcoholysis reaction with the alcohol products from the decomposed PC that are evolved during the latter's thermal degradation [18]. However, the considerable amounts of TPP and RDP that evaporate in the beginning mass loss region decrease their ability to stabilize the carbonate linkage of PC.

In this work, we synthesize a phloroglucinol-based aromatic phosphate and cyclic phosphate in order to obtain an FR with both higher P content and higher thermal stability compared with a commercial FR, RDP, and good hydro-stability. Its thermal degradation behaviors and flame-retarding performances are also studied to evaluate the effect of the P content and thermal stability of the FR on flame retardancy.

EXPERIMENTAL

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Materials

Phosphorus (V) oxychloride, diphenyl phosphoryl chloride, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, resorcinol, and phluroglucinol were purchased from Aldrich (St. Louis, MO63103, USA). Triethylamine, methylene chloride (MC), diethyl ether (ether), and tetrahydrofuran (THF) were purchased from Samchun pure Chemical Co. LTD. (Pyeongtaek City, Gyeonggi-do, Korea). PC and RDP of commercial grades were provided from the Cheil Industries, Euiwang, Gyeonggi-do, Korea.

Synthesis

Two cyclic phosphates were synthesized according to the literature procedures [17], and one novel phloroglucinol-based aromatic phosphate was synthesized. Their structures are given in Table 1, together with their generic names and abbreviations, which will be used afterwards. To synthesize these compounds, the synthesis routes shown in Scheme 1 were adopted.

Table 1. Structures and characteristics of FRs employed in this study.
StructureFull nameAbbreviated nameP content (%)Yield (%)Mp (°C)
  1. FRs, flame retardants; Mp,: melting point.

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Phloroglucinol tris(cyclic 2,2-dimethyl-1,3-propanediol phosphate)PCDMPP16.2975260.7
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Phloroglucinol tris(cyclic 1,3-propanediol phosphate)PCPP19.1170174.6
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Resorcinol bis(diphenyl phosphate)RDP10.7888Liquid
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Phloroglucinol tri(diphenyl phosphate)PDP11.3090100.4
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Scheme 1. Synthetic routes for the compounds employed in this study.

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Synthesis of phloroglucinol tris(cyclic 2,2-dimethyl-1,3-propanediol phosphate)

Synthesis of cyclic 2,2-dimethyl-1,3-propanediol phosphoryl chloride (CDMPP): CDMPP was synthesized according to the procedure found in the literature [19]. A solution of 15.3 g (0.100 mol) of POCl3 dissolved in dichloromethane (MC) was added dropwise to a two-neck flask containing a mixture of 10.4 g (0.100 mol) of 2,2-dimethylpropane-1,3-diol and 20.2 g (0.200 mol) of triethylamine in MC with continuous stirring at 0–5 °C. After the completion of dropping, the mixture was stirred for an additional 4 h at 50 °C. Triethylamine hydrochloride (Et3N.HCl) was removed by filtering, and the crude product was obtained by evaporating the filtrate. A 50 mL portion of THF was added to precipitate the residue triethylamine hydrochloride in the crude product. After filtration and the evaporation of THF, the crude CDMPP was obtained. The crude product was further washed with distilled water to give CDMPP of high purity (90% yield).

1 H NMR (CDCl3, δ): 0.93, 1.34 (s, CH3–C), 3.97–4.06, 4.23–4.27 (m, C–CH2–O).

A mixture of 12.6 g (0.100 mol) of phlroglucinol and 30.3 g (0.300 mol) of triethylamine dissolved in dry THF was placed in a two-neck flask equipped with a reflux condenser, a temperature controller, and a mechanical stirrer. To this solution, 59.0 g (0.320 mol) of CDMPP was added dropwise at room temperature. The mixture was stirred overnight at 60 °C. The precipitated solid containing the target material and triethylamine hydrochloride was filtered off and washed with distilled water many times to remove the triethylamine hydrochloride salt affording the pure product with high yield (75% yield).

1 H NMR (CDCl3, δ): 0.92, 1.34 (s, CH3–C), 3.95–4.03, 4.23–4.27 (m, d, C–CH2–O), 7.27 (s, H–Ar); 31P NMR (CDCl3, δ): −13.5 (s).

Synthesis of phloroglucinol tris(cyclic 1,3-propanediol phosphate)

Synthesis of cyclic 1,3-propanediol phosphoryl chloride (CPPC): CPPC was synthesized following the procedure found in the literature [20]. A solution of 15.3 g (0.100 mol) of POCl3 dissolved in diethyl ether (ether) was added dropwise into a two-neck flask containing a mixture of 7.60 g (0.100 mol) of 1,3-propandiol and 20.2 g (0.200 mol) of triethylamine dissolved in ether at 0–5 °C. After the completion of dropping, the mixture was stirred overnight at room temperature. Triethylamine hydrochloride was removed by filtering, and CPPC was crystallized by evaporating the solvent and obtained with high purity.

1 H NMR (CDCl3, δ): 1.81–1.86, 2.39–2.49 (m, C–CH2–C), 4.50–4.59 (m, C–CH2–O).

Synthesis of phloroglucinol tris(cyclic 1,3-propanediol phosphate) (PCPP): A solution containing 12.6 g (0.100 mol) of phloroglucinol and 30.3 g (0.300 mol) of triethylamine in THF was added dropwise to 50.0 g (0.320 mol) of CPPC in THF at 0–5 °C. The mixture was stirred overnight at 60 °C. The precipitated solid was filterred off and washed with distilled water to give pure PCPP (70% yield).

1 H NMR (DMSO, δ): 1.86–1.91, 2.16–2.28 (m, C–CH2–C), 4.47–4.58 (m, C–CH2–O), 5.65, 7.15, 8.91 (s, H–Ar); 31P NMR (DMSO, δ): –12.9 (s).

Synthesis of phloroglucinol diphenyl phosphate

Synthesis of phloroglucinol diphenyl phosphate (PDP): A solution containing 12.6 g (0.100 mol) of phloroglucinol and 30.3 g (0.300 mol) of triethylamine in THF was added dropwise to 80.7 g (0.300 mol) of diphenyl phosphoryl chloride in THF at 0–5 °C. The mixture was stirred overnight at 60 °C. Triethylamine hydrochloride was removed by filtering, and PDP was crystallized by evaporating the filtrate.

Measurements and sample preparation

Spectroscopic analysis

1 H and 31P NMR spectra were performed on a Varian Unity Inova 500 NB spectrometer (Varian, Inc., Palo Alto, CA, USA) using CDCl3 or DMSO as a solvent and tetramethylsilane as a reference. The chemical shifts of the 31P NMR spectra are relative to the external standard of 85% H3PO4. The FT-IR spectra were obtained by means of a Nicolet 380 (Thermo Electron Corporation 5225 Verona Road Madison WI 53711-4495 USA) using KBr pellets.

Thermal analysis

Differential scanning calorimetry (Auto Q-20 instrument; TA Instruments, Newcastle, DE, USA) was carried out on 7- to 10-mg samples under a nitrogen atmosphere at a heating rate of 10 °C/min. Thermogravimetric analysis (TGA) was performed on 7- to 10-mg samples under a nitrogen atmosphere at a heating rate of 10 °C/min using a thermogravimetric analyzer (Q-50 instrument; TA Instruments, Newcastle, DE, USA). Each sample was examined twice, and the difference was less than 2 wt% of charred residue.

Sample preparation for UL-94 test

The mixture of the synthesized FR with the polymer at the designated composition was processed in a Haake PolyDrive mixer at 60 rpm for 7 min at 230 °C for ABS and at 240 °C for PC.

UL-94 measurement

The fire retardancy performance was evaluated according to the FMVSS 302/ZSO 3975 testing procedure with test specimen bars 127 mm in length, 13.0 mm in width, and having a maximum thickness of 3.2 mm.

RESULTS AND DISCUSSION

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Synthesis of phloroglucinol diphenyl phosphate

Phloroglucinol diphenyl phosphate was synthesized via the reaction presented in Scheme 1. Figure 2(A,B) show the 1 H NMR and 31P NMR spectra of the PDP compound, respectively. The 1 H-NMR spectrum of PDP showed multi-peaks at 7.12, 7.20, and 7.30 p.p.m., which were assigned to the protons of the aromatic phenyl group. Its 31P NMR spectrum also shows the presence of a single phosphorus compound with a single peak at −17 p.p.m.

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Figure 2. The (A) 1 H NMR and (B) 31P NMR spectra of phloroglucinol diphenyl phosphate.

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To confirm its successful synthesis, the FT-IR spectrum of PDP was analyzed spectroscopically by FT-IR, and the results are presented in Figure 3. In Figure 3, the adsorption band of the C–H stretching from aromatic bonding is observed at 3000 cm−1, and the aromatic C–C stretching vibration band is observed at 1590–1606 cm−1. Moreover, weak bands are observed at 1635–1690 cm−1 corresponding to the aromatic C═C bonding due to the highly symmetric structure of PDP. The peaks at 1300 and 1200 cm−1 can be attributed to the stretching bands of P═O and P–O–C (aryl), respectively. The strong adsorption at 969 cm−1 is attributed to the P–O stretching vibration. These results confirm the successful synthesis of high-purity PDP.

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Figure 3. The FT-IR spectra of phloroglucinol diphenyl phosphate.

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Thermal degradation behaviors

Figure 4 shows the TGA results obtained under a nitrogen atmosphere for the FRs employed in this study.

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Figure 4. Thermogravimetric analysis curves for four flame retardants employed in this study under nitrogen at a heating rate of 10 °C/min. RDP, resorcinol bis(diphenyl phosphate); PDP, phloroglucinol diphenyl phosphate; PCDMPP, phloroglucinol tris(cyclic 2,2-dimethyl-1,3-propanediol phosphate); PCPP, phloroglucinol tris(cyclic 1,3-propanediol phosphate).

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It is noticed from Figure 4 that the thermal degradation behavior of FR is very strongly dependant on the type of pendent phosphate attached to the center aromatic, that is, resorcinol or phloroglucinol. Considering the initial degradation temperature (Tonset), the PDP containing the aromatic phosphate shows the highest Tonset, and those of the FR containing cyclic phosphates are far lower than those of RDP and PDP. From these results, it is clearly confirmed that, with regard to the initial decomposition of the FRs, those containing the aromatic phosphate exhibit higher thermal stability than those containing the cyclic phosphate.

The second point worth noticing in Figure 4 is that the FRs containing the cyclic phosphates show at least two-step degradation and leave a remarkable amount of charred residue (36% for phloroglucinol tris(cyclic 2,2-dimethyl-1,3-propanediol phosphate) and 56.8% for PCPP after treatment at 600 °C), whereas the FRs containing the aromatic phosphates degraded in sharp one step and left a lower amount of charred residues (0.2% for RDP and 8.6% for PDP after 600 °C). Most of the phosphate eventually evolves during thermal degradation, and there is no significant char enhancement after 600 °C [18]. However, the degradation of PDP generates greater amount of charred residue than that of RDP but less than that of the FRs containing cyclic phosphates. It has been reported that the first degradation of FRs containing cyclic phosphates is due to the cleavage of their cyclic structure and consequent generation of arylphosphoric (or diarylphosphoric) acids and that the resulting formation of a network structure is the main reason for the observation of a large amount of charred residues [17], whereas the decomposition of RDP proceeds with the cleavage of the P–O bond and consequent generation of phosphinic acid, which leads to the formation of a network structure through the esterification of the phosphinic acids [18]. From this result, it can be inferred that the formation of two major degradation products from PDP, followed by two different degradation paths, are the main reason for the observation of a large amount of charred residue from PDP. The first degradation path of PDP is the generation of diphenyl phosphonic acid, and the second is the generation of P–OH-rich products, which lead to the formation of a network structure consisting of charred residues. This degradation mechanism is presented in Scheme 2.

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Scheme 2. Degradation path for phloroglucinol diphenyl phosphate.

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To confirm the aforementioned assertions, the charred residue of PDP was analyzed spectroscopically by employing FT-IR. The degradation of a specific PDP was carried out under nitrogen in a TGA apparatus. After the temperature reached 600 °C, the residue was taken out and examined by the investigator using an FT-IR instrument. The FT-IR spectrum of the residue of PDP is shown in Figure 6. In Figure 6, the intensities of the adsorption bands at 1590–1606 cm−1, which are attributed to the C–C stretching of the aromatic species, remain unchanged in the spectrum of charred residue, and the adsorption band of the C═C stretching of the aromatic species at 1650 cm−1 appears when the symmetric structure of PDP is degraded. Moreover, the absorption band at 1200 cm−1, which corresponds to P–O–C (aryl) stretching, diminishes in intensity at 600 °C, and the peak at 969 cm−1 attributable to the P–O stretching shifts to a higher frequency of 990 cm−1. It therefore becomes clear that the P–O–P structure is contained in the charred residue of PDP. These findings strongly support our previous interpretation of the TGA results and the degradation mechanism presented in Scheme 2.

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Figure 6. FT-IR spectra of charred residue of phloroglucinol diphenyl phosphate obtained after degradation at 600 °C.

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Flame-retarding performances

The flame-retarding performances of the FRs investigated in this study were evaluated on charrable PC. The FRs are added in amounts of 2–3 wt% in the presence of an anti-dripping agent (1 wt%), and the UL-94 results for the various formulations are listed in Tables 2. To facilitate the discussion, TGA experiments were carried out for the mixtures of PC with the various FRs, and the results are given in Figure 7.

Table 2. UL-94 results for the mixtures of PC with various FRs.
FRP (%) in FRPC/FR (wt/wt)P (%) in formulationUL-94Tig1/Tig2 (s/s)Dripping
  1. PC, polycarbonate; FR, flame retardants; RDP, resorcinol bis(diphenyl phosphate); PDP, phloroglucinol diphenyl phosphate; PCDMPP, phloroglucinol tris(cyclic 2,2-dimethyl-1,3-propanediol phosphate); PCPP, phloroglucinol tris(cyclic 1,3-propanediol phosphate).

100/0V-15/15No
RDP10.7898/20.22V-14/12No
RDP10.7897/30.32V-00/9No
PDP11.3098/20.23V-03/4No
PDP11.3097/30.34V-01/4No
PCDMPP16.2998/20.33V-12/14No
PCDMPP16.2997/30.49V-01/8No
PCPP19.1198/20.38V-11/12No
PCPP19.1197/30.57V-02/6No
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Figure 7. Thermogravimetric analysis curves of mixtures of polycarbonate with various flame retardants under nitrogen at a heating rate of 10 °C/min (The flame retardant loading is 3 wt%). RDP, resorcinol bis(diphenyl phosphate); PDP, phloroglucinol diphenyl phosphate; PCDMPP, phloroglucinol tris(cyclic 2,2-dimethyl-1,3-propanediol phosphate); PCPP, phloroglucinol tris(cyclic 1,3-propanediol phosphate).

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It can be seen in Table 2 that the flame-retarding behavior of PC is improved by the addition of the FR. PC itself is categorized as a moderate-to-slightly flammable polymer. It is classified as UL-94 V-1 in the presence of Teflon (1 wt%) without the addition of an FR, and a V-0 rating can be achieved by the addition of a small amount of one. In Table 2, it can be seen that PDP shows the best performance in terms of the UL-94 test result. In the previous section, it was concluded that PDP is also the best in terms of its thermal stability and leaves 8.6 wt% of charred residue. Therefore, it is postulated that the observed best flame-retarding performance of PDP is related not only to its ability to stabilize the carbonate but also to its charring ability for a charrable polymer such as PC. It is well known that aromatic phosphates such as TPP and RDP enhance the carbonate linkages in PC, but the phosphate degradation begins at a lower temperature than the degradation temperature of PC, which interrupts the flame retardancy of the phosphate groups. As mentioned in relation to Figure 4, PDP provides the highest stability of the thermal degradation against the evaporation and a moderate amount of charred residue, which enhances the stability of the phosphate groups significantly, leading to the stabilization of the PC and delaying its degradation. This result shows that the best flame-retarding performance was obtained for PDP in the UL-94 test, as shown in Table 2 (V-0 rating at a PDP loading of 2 wt%).

In Figure 7, PC leaves more than 25 wt% of charred residue at temperature above 600 °C, indicating that it really is a charrable polymer. When 3 wt% of FR is added to the PC, the initial thermal degradation occurs at a lower temperature, independent of the kind of FR that is added. PC generates phenolic derivatives after its decomposition, and the formation of a rigid solid-like charred residue on the surface of the flaming specimen through its further reaction with a phosphate-type FR such as RDP is responsible for the flame inhibition mechanism. Even though no discernible increase in the amount of charred residue is observed, the superior flame-retarding performance of PDP is believed to be closely related to its char forming ability and high thermal stability. That is, the condensed phase mechanism is known to be the dominating action for the flame retardancy of PC [11]. Therefore, the flame retardancy of phosphorus-based FRs depends not only on the P content but also on the thermal stability of the phosphorus-based FRs.

To confirm the aforementioned assertions, the charred residues of the mixtures of PC/PCPP and PC/PDP were analyzed spectroscopically by FT-IR. The degradation was carried out under nitrogen in a TGA apparatus, and the FT-IR spectra are shown in Figure 8. In Figure 8, it is clearly observed that the strong adsorption at 3457 cm−1 corresponding to the phenolic OH stretching and the vibration of P–O at 969 cm−1 shift to higher frequencies and overlap with the strong bands of PC, because of the low amount of PDP in the mixture. From these results, it can be concluded that PDP also has the ability to generate a P–OH-rich charred residue, which is responsible for its flame retardancy in the condensed phase.

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Figure 8. FT-IR spectra of charred residue of mixtures of (A) polycarbonate/phloroglucinol tris(cyclic 1,3-propanediol phosphate) and (B) polycarbonate/phloroglucinol diphenyl phosphate obtained after degradation at 600 °C.

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CONCLUSIONS

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

In an attempt to investigate the factors affecting the flame-retarding behaviors of phosphorus-based FRs, a series of organo-phosphorus FRs based on the aromatic phosphate and cyclic phosphate were synthesized, and their thermal degradation and flame-retarding performances were compared with the commercial FR, RDP. From TGA experiments, it is clearly observed that the FRs containing the aromatic phosphates start to be degraded at higher temperatures than the cyclic phosphate-based FRs and that PDP leaves a moderate amount of charred residue at elevated temperatures, which is related to the generation of P–OH-rich products that lead to the formation of a network structure in the charred residues. This tendency to generate a large amount of P–OH is beneficial to the condensed phase flame retardancy mechanism, and the PDP shows good flame retardancy on PC (UL-94 V-0 ratings at 2 wt% loading). It is concluded that the flame retardancy of phosphorus-based FRs depends not only on their P content but also on their thermal stability. Furthermore, these FRs are solid and water insoluble, which are additional advantages of these synthesized compounds.

ACKNOWLEDGEMENT

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

The authors appreciate the financial support from Cheil Industries, Korea.

REFERENCES

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES