Novel halogen-free flame retardants based on adamantane for polycarbonate

Article abstract of DOI:10.1039/c5ra10887j

A novel series of flame retardants (FRs) containing phosphate moieties attached to a bridgehead-substituted adamantane, 1-(diphenyl phosphate) adamantane (DPAd), 1,3-bis(diphenyl phosphate) adamantane (BDPAd), 1,3,5-tris(diphenyl phosphate) adamantane (TD

Full text of DOI:10.1039/c5ra10887j

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Novel halogen-free flame retardants based on
adamantane for polycarbonate
Cite this: RSC Adv., 2015, 5, 67054
*
Shu-Qin Fu, Jian-Wei Guo, Dong-Yu Zhu, Zhe Yang, Chu-Fen Yang, Jia-Xing Xian
and Xiong Li
A novel series of flame retardants (FRs) containing phosphate moieties attached to a bridgehead-
substituted adamantane, 1-(diphenyl phosphate) adamantane (DPAd), 1,3-bis(diphenyl phosphate)
adamantane (BDPAd), 1,3,5-tris(diphenyl phosphate) adamantane (TDPAd) and 1,3,5,7-tetrakis(diphenyl
phosphate) adamantane (TKDPAd), were systematically synthesized in an attempt to develop efficient FRs
for polycarbonate (PC). Their chemical structures were confirmed by Fourier Transform Infrared
Spectroscopy (FTIR), Nuclear Magnetic Resonance (1H and 31P NMR), elemental analysis and melting
point measurements. The flame-retarding efficiencies of the FRs were evaluated by Limited Oxygen
Index (LOI), UL-94 vertical burning experiments, Scanning Electron Microscopy (SEM), Cone Calorimeter
Test (CCT), Thermogravimetric Analysis (TGA), FTIR and TGA-FTIR. For TKDPAd, thermal decomposition
took place in a sharply two-step mechanism at high temperature above 381.5 ꢀC. A significant
improvement of flame-retardant performance in PC/TKDPAd among the PC/FR was observed with an
addition of 8 wt% TKDPAd in the presence of an anti-dripping agent (0.1 wt%). High thermal stability and
phosphorus content of flame retardant are believed to be of great importance for efficient flame-
retardant action of adamantane-based phosphates.
Received 8th June 2015
Accepted 28th July 2015
DOI: 10.1039/c5ra10887j
www.rsc.org/advances
amounts of TPP or RDP occurred an alcoholysis reaction with
the alcohol products from the decomposed PC, which are
related during the latter part of thermal degradation.18 None-
1. Introduction
Traditional materials such as metals and woods are now grad-
ually being replaced by polymers. However, polymers are easily
ammable and subjected to various mandatory controls for
safety reasons.1–3 The thermal stability of polycarbonate (PC)
meets the requirements for application in advanced electronics
and construction materials.4 Due to its relatively high tendency
of char formation, PC gives a limiting oxygen index (LOI) value
of typically 26% and a UL-94 V-2 rating. However, for safety
concerns, more stringent ame retardant performances of PC
composites are oen required.5,6 To decrease the combustibility
of the polymers, researchers have made great efforts to the
development of the ame retardants (FRs).7–11 As part of the
development of halogen-free FRs to replace bromo-aromatic
FRs, the development of phosphorus-containing compounds
generating less toxic gases and smoke gradually become the
main focus.12–15
theless, considerable amounts of TPP and RDP that evaporate in
the beginning mass loss region decrease their ability of stabi-
lizing the carbonate linkage of PC. The decomposition
temperature of the ame retardant is a major parameter
controlling the ame retardancy mode of action. It shows that
matching the decomposition temperature of PC promotes char
formation from PC in the pyrolysis zone.19,20 Marie-Claire Des-
pinasse et al. found that using an aryl phosphate mixture of BDP
and HDP to match the decomposition temperature of PC is
possible to enhance the charring of PC in the condensed
phase.21 The temperature matching is also likely by varying the
structure of the bridging unit or the end group of the aryl
phosphate oligomers. However, the bridging unit can vary in
chemical structure, such as can be rigid or exible, aromatic or
aliphatic, hydrophobic or hydrophilic. Congtranh Nguyen et al.
also showed that the decomposition temperature and char
residue of ethylene glycol bis(diphenyl phosphate) (ethylene
glycol as bridging unit) were lower than RDP (resorcinol as
bridging unit), which revealed the relationships of the effect of
the chemical structure of bridging unit on thermal stability and
ame-retarding ability.22 The substitution of the phenyl end
group also strongly inuences the reactivity of the ame retar-
dant.23 Further, phosphorus content is believed to be the most
vital factor on ame retardancy in recent years.24–26 However, the
Among the various phosphorus compounds, aryl phosphates
like triphenyl phosphate (TPP) and resorcinol bis(diphenyl
phosphate) (RDP) are the most widely used FRs and exhibit
fairly good re-retarding performances for polyesters and their
blends.6,16,17 Jang et al. reported that PC was not only stabilized
but delayed the degradation process by TPP or RDP; and part
School of Chemical Engineering
& Light Industry, Guangdong University of
Technology, Guangzhou 510006, P. R. China. E-mail: guojw@gdut.edu.cn
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relationships of re-retarding efficiencies and thermal degra- 1.58 ppm (s, 6H, Had-4,6,9); FT-IR (KBr) n: 2912, 2855, 1591,
dation behaviors among bridging unit, phosphorus content and 1489, 1284, 1196, 1010, 941, 764 cmꢂ1. Anal. calcd for
substitution amounts of the bridging unit were not clearly
revealed so far.
C
22H25O4P: C, 68.74, H, 6.56; found: C, 68.79, H, 6.62.
2.2.2 Synthesis of 1,3-bis(diphenyl phosphate) ada-
In this paper, incorporation of adamantane group into the mantane (BDPAd). Synthesis of BDPAd was similar to that in
molecular structures of FRs was performed by reacting aryl 2.2.1, except 1,3-dihydroxyadamantane (4.00 g, 23.80 mmol) was
phosphates with adamantane compounds to further improve used to instead of 1-hydroxyadamantane, giving white solid
1
the thermal stability of conventional FRs. The particular struc- BDPAd (12.50 g, 83.0% yield). H NMR (400 MHz, DMSO-d6) d:
ture of adamantane imparts its derivatives with many useful 7.38–7.20 (t, 20H, C6H5), 2.38–2.35 (m, 4H, Had-2,5,7), 2.00 (s,
chemical and physical properties, such as bulky and tetrahedral 8H, Had-4,8,9,10), 1.47 ppm (s, 2H, Had-6); FT-IR (KBr) n: 2926,
geometry, extreme lipophilicity, good thermal and oxidative 2867, 1589, 1492, 1300, 1281, 1195, 984, 957, 932, 782 cmꢂ1
.
stabilities, and innocuity, etc.27–31 Pyrolysis has been carried out Anal. calcd for C34H34O8P2: C, 64.56, H, 5.42; found: C, 64.57, H,
using TGA in air to simulate the real combustion. The rela- 5.47.
tionships among bridging unit (adamantane), phosphorus
2.2.3 Synthesis of 1,3,5-tris(diphenyl phosphate) ada-
content and substitution amounts of adamantane on thermal mantane (TDPAd). A solution of chromium trioxide (100.00 g,
stability, re performances and degradation behaviors of PC 1000.00 mmol) in distilled water (100 ml) was added dropwise
were investigated through TGA, TGA-FTIR, LOI, UL-94 vertical to the solution of adamantane (13.60 g, 100 mmol) in glacial
ꢀ
burning, CCT, SEM. The solid residues aer pyrolysis were acetic acid (100 ml) with continuous stirring at 95–98 C. Aer
analyzed by FTIR.
the completion of dropping, the mixture was stirred for an
additional 1 h at 100 C. The solution was evaporated and the
ꢀ
residue was neutralized by 40 wt% aq. KOH. The mixture was
extracted with ethyl acetate (500 ml) at 70 C for ve times to
2. Experimental
ꢀ
2.1 Materials
give white crystal 1,3,5-trihydroxyadamantane (8.50 g, 50.0%
yield). H NMR (400 MHz, DMSO-d6) d: 4.49 (s, 3H, OH), 2.11
1
Polycarbonate 301-10 was provided by Dow Chemical Company
(America) and was dried at 120 ꢀC for 8 h before use. Ada-
mantane (CP), 1-hydroxyadamantane and 1,3-dihydroxy-
adamantane were purchased from Hangzhou Yang Li
petrochemical co., LTD (Hangzhou, China). Diphenyl phos-
phoryl chloride (AR) was obtained from Aladdin Industrial
Corporation (Shanghai, China). Pyridine was distilled by anhy-
drous calcium chloride before use. All reagents commercially
available were used as received unless otherwise stated. 1,3,5-
Trihydroxyadamantane and 1,3,5,7-tetrahydroxyadamantane
were prepared according to the published procedures.32,33
(s, 1H, Had-7), 1.33–1.42 ppm (m, 12H, Had-2,4,6,8,9,10); anal.
calcd for C10H16O3: C, 65.19, H, 8.17; found: C, 65.19; H, 8.23.
Diphenyl phosphoryl chloride (10.30 g, 38.20 mmol) was
added in small portions over 30 min to a vigorously stirring
mixture of 1,3,5-trihydroxyadamantane (2.00 g, 10.90 mmol)
ꢀ
and pyridine (15.00 g, 189.00 mmol) at 70 C. The mixture was
then slowly heated to 90 ꢀC and held at that temperature for 7 h.
The solvent was evaporated at reduced pressure once the reac-
tion mixture was cooled to room temperature, and then the oil
resultant was dissolved in 100 ml of CH2Cl2. The solution was
washed successively with water 3 ꢁ 100 ml, HCl (1 M, 3 ꢁ
100 ml), aq. Na2CO3 (15 wt%, 3 ꢁ 100 ml), and distilled water
3 ꢁ 100 ml, dried over Mg2SO4, evaporated to dryness and
2.2 Synthesis
A series of adamantane-based phosphates are successfully heated for 12 h under vacuum at 50 ꢀC. The crude product was
synthesized and their structures are given in Table 1. The puried by silica gel chromatography eluted with ethyl
general synthetic routes are shown in Scheme 1.
acetate : n-hexane ¼ 1 : 10 to give product TDPAd (6.00 g, 62.5%
2.2.1 Synthesis of 1-(diphenyl phosphate) adamantane yield) as greenish paste. 1H NMR (400 MHz, CDCl3) d: 7.55–7.16
(DPAd). Diphenyl phosphoryl chloride (9.50 g, 30.00 mmol) was (m, 30H, C6H5), 2.52 (d, 6H, Had-2,4,9), 2.05–1.96 ppm (t, 7H,
added in small portions over 30 min to an intensively stirred Had-6,7,8,10). FT-IR (KBr) n: 2956, 2873, 1590, 1489, 1292, 1220,
mixture of 1-hydroxyadamantane (4.50 g, 29.00 mmol) and 1190, 1100, 1015, 952, 772 cmꢂ1. Anal. calcd for C46H43O12P3: C,
pyridine (11.70 g, 150.00 mmol) at 60 ꢀC. The mixture was then 62.73, H, 4.92; found: C, 62.71, H, 4.97.
slowly heated to 95 ꢀC and held at that temperature for 3 h. The
2.2.4 Synthesis of 1,3,5,7-tetrakis(diphenyl phosphate)
solvent was evaporated at reduced pressure once the reaction adamantane (TKDPAd). 1,3,5,7-Tetrabromoadamantane and
mixture was cooled to room temperature, and then the oil 1,3,5,7-tetrahydroxyadamantane were prepared by modifying
resultant was dissolved in 100 ml of ethyl acetate. The solution published procedures.33
was washed successively with water 2 ꢁ 100 ml, HCl (1 M, 2 ꢁ
Adamantane (4.86 g, 35.75 mmol) was added in small portions
100 ml), aq. Na2CO3 (15 wt%, 2 ꢁ 100 ml), and distilled water over 30 min to a stirring mixture of bromine (22.50 ml) and
ꢀ
2 ꢁ 100 ml, dried over Na2SO4 and evaporated to dryness and anhydrous aluminum chloride (5.00 g, 37.50 mmol) at 0–3 C.
heated for 12 h under vacuum at 45 ꢀC. The crude product was The mixture was then slowly heated to 70 ꢀC and held at that
puried by recrystallizing with ethyl acetate/n-hexane to give temperature for 24 h. Hydrogen bromide evolved vigorously
1
product DPAd (9.90 g, 88.2% yield) as white solid powder. H during the addition and heating. The reaction mixture was
NMR (400 MHz, DMSO-d6) d: 7.39–7.21 (t, 10H, C6H5), 2.15–2.05 treated subsequently with aqueous sodium sulte and hydro-
(d, 9H, Had-2,3,5,7,8,10, this “ad” means adamantane), chloric acid. The resulting solid was ltered, dried in vacuum,
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Table 1 Fundamental parameters of the FRs employed in this study
Abbreviation Full name
Molecular formula Structural formula
P contenta (%) MPa (ꢀC)
DPAd
1-(Diphenyl phosphate) adamantane
C22H25O4P
8.06
9.79
53.3
BDPAd
1,3-Bis(diphenyl phosphate) adamantane
C34H34O8P2
100.4
TDPAd
1,3,5-Tris(diphenyl phosphate) adamantane
C46H43O12P3
10.56
Liquid
TKDPAd
1,3,5,7-Tetrakis(diphenyl phosphate) adamantane C58H52O16P4
10.97
77.5
a
P content ¼ phosphorous content; MP ¼ melting pointing.
rinsed with water. The ltrate was neutralized with KOH and
evaporated. The resulting gray residue was dried and then
extracted 24 h with ethanol in a Soxhlet apparatus. Aer evap-
oration of the ethanol, the residue was dissolved in methanol
and ltered. Recrystallization from MeOH/acetone yielded a
white solid (2.88 g, 78.0% yield); MP: 316–318 ꢀC; 1H NMR
(400 MHz, DMSO-d6) d: 4.58 (s, 4H), 1.36 ppm (s, 12H); anal.
calcd for C10H16O4: C, 59.98; H, 8.05. Found: C, 59.92; H, 8.11.
Diphenyl phosphoryl chloride (12.80 g, 48.00 mmol) was
added in small portions over 30 min to an intensively stirring
Scheme 1 Synthetic routes for the FRs employed in this study.
mixture
of
1,3,5,7-tetrahydroxyadamantane
(2.00
g,
10.00 mmol) and pyridine (18.57 g, 235.00 mmol) at 70 ꢀC. The
mixture was then slowly heated to 90 ꢀC and held at that
temperature for 9 h. The solvent was evaporated at reduced
pressure once the reaction mixture was cooled to room
temperature, and then the resultant was dissolved in 100 ml of
CH2Cl2. The solution was washed successively with water 3 ꢁ
100 ml, HCl (1 M, 3 ꢁ 100 ml), aq. Na2CO3 (15 wt%, 3 ꢁ 100 ml),
and distilled water 3 ꢁ 100 ml, dried over Na2SO4 and evapo-
rated to dryness and heated for 8 h under vacuum at 50 ꢀC. The
and recrystallized from CH3CN to give tan crystals (10.10 g, 63.0%
yield); mp: 246–248 ꢀC; 1H NMR (400 MHz, CDCl3) d: 2.71 ppm (s,
12H); anal. calcd for C10H12Br4: C, 26.58; H, 2.68. Found: C, 26.66;
H, 2.68.
1,3,5,7-Tetrabromoadamantane (8.34 g, 18.44 mmol) and
Ag2SO4 (12.76 g, 40.90 mmol) were suspended in concentrated
sulfuric acid (20 ml) and the mixture was slowly heated to 80 ꢀC
and stirred at that temperature for 7 h. Aer cooling, the AgBr
precipitate was removed by ltration and the solution was
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crude product was puried by silica gel chromatography eluted magneto-dynamic oxygen analyzer, according to ASTM D2863-
with petroleum ether : diethyl ether ¼ 1 : 4 to give product 97 standard. The average values of LOI were obtained from
TKDPAd (6.52 g, 61.8% yield) as white crystal. 1H NMR the results of six tests.
(400 MHz, CDCl3) d: 7.29–7.16 (dt, 40H, C6H5), 2.54 ppm (s, 12H,
2.3.3 UL-94 vertical burning test. The UL-94 vertical
Had-2,4,6,8,9,10); FTIR (KBr) n: 3059, 1590, 1488, 1291, 1190, burning test was carried out based on AG5100A vertical-
1020, 958, 942, 772 cmꢂ1. Anal. calcd for C58H52O16P4: C, 61.71; horizontal burning apparatus according to ASTM D3801 stan-
H, 4.64. Found: C, 61.72; H, 4.60.
dard. The average values of UL-94 were obtained from the
2.2.5 Preparation of PC/DPAd, PC/BDPAd, PC/TDPAd and results of ve tests.
PC/TKDPAd composites. PC, FRs and anti-dripping agent were
2.3.4 Scanning electron microscopy (SEM). The SEM
dried in a vacuum oven at 110 ꢀC and 50 ꢀC for 6 h before used, observation was performed on a Hitachi S-3400N scanning
respectively. Then PC, FR and anti-dripping agent (0.1 wt%) electron microscope to investigate the morphologies of the
were melt-mixed in an internal mixer (KY-3220-2L, Dongguan residual chars. The char samples for SEM were obtained aer
Houjie Machinery Equipment factory, China) at 250 ꢀC for combustion in the LOI test and were made electrically
10 min. The prepared composites were molded under conductive by sputter coating with a thin layer of gold–palla-
compression (15 MPa) at 250 ꢀC for 10 min and cooled to room dium alloy. The images were taken in high vacuum mode with
temperature naturally to obtain PC/FR compositions sheets 15 kV acceleration voltage and a medium spot size.
with standard size for further testing. The formulations of the
PC/FR compositions are listed in Table 2.
2.3.5 Cone calorimeter test (CCT). The CCT was carried out
by using a cone calorimeter (Fire Testing Technology, UK)
according to ISO5660 standard procedures. Each specimen,
with a dimension of 100 mm ꢁ 100 mm ꢁ 3 mm, was wrapped
in an aluminum foil and exposed horizontally to an external
heat ux of 35 kW mꢂ2. All the samples were run in duplicate,
and the average value was recorded.
2.3 Measurements and sample preparation
2.3.1 Spectroscopic analysis. 1H and 31P spectra of
synthesized intermediates and the nal product were obtained
by using a Bruker AVANCE III 400 MHz superconducting
Fourier in dimethyl sulfoxide-d6 (DMSO-d6) or deuteron-
chloroform (CDCl3) solution and were referenced to external
tetramethylsilane (TMS). FTIR spectra were obtained by using a
Thermo Electron Nicolet-6700 FTIR spectrometer with a scan-
ning number of 16. A nely ground, approximately 1% mixture
of a solid sample in KBr powders is fused into a transparent disk
for FTIR measurement using a hydraulic press. The elemental
analysis was performed with a Perkin Elmer Series II 2400
elemental analyzer.
2.3.6 Thermal analysis. The TGA experiments were per-
formed under an air atmosphere using a NETZSCH STA 409 PC
thermal gravimetric analyzer. The samples with a mass of about
8 mg were placed in an aluminum crucible and ramped from
room temperature up to about 800 ꢀC at a heating rate of 10 ꢀC
minꢂ1, while the ow of air was maintained at 50 ml minꢂ1
.
The TGA-FTIR experiments were carried out by using a 409
PC thermal analyzer (Netzsch, Germany) coupled with a Nicolet-
6700 FTIR (Thermosher, USA). About 8 mg of each sample was
heated from room temperature to 800 ꢀC at a heating rate of
10 ꢀC minꢂ1 under nitrogen (ow rate ¼ 50 ml minꢂ1). The
couple system between TGA and FTIR was a quartz capillary
kept at temperature of 220 ꢀC. The spectra were acquired in the
2.3.2 Limiting oxygen index (LOI) test. The LOI test was
performed by using a DM-4022 oxygen index apparatus with a
range of 4000 cmꢂ1 to 650 cmꢂ1
.
Table 2 LOI, UL-94 results for PC and PC/FR composites
UL-94
3. Results and discussion
3.1 Synthesis of the FRs
FR
P content (%) PC/FR (wt/wt) LOI (%) Ranking Dripping
DPAd
8.06
100/0
94/6
92/8
90/10
88/12
94/6
26.1
27.1
27.3
27.3
27.6
27.6
27.6
28.2
29.1
27.6
28.3
29.1
30.0
28.6
29.1
30.1
32.0
V-2
V-2
V-2
V-2
V-2
V-2
V-2
V-1
V-1
V-1
V-0
V-0
V-0
V-1
V-0
V-0
V-0
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
No
No
No
No
No
Synthesis of the FRs based on rigid adamantane ring structure,
were performed in good yields according to Scheme 1. For the
similar structures of the FRs, which were measured by FTIR, 1H
and 31P NMR displayed in Fig. 1, 2 and 3, respectively, TKDPAd
was taken as the only example to analyze its structure here.
FTIR spectrum of phosphorus-containing TKDPAd shown in
Fig. 1D exhibited the characteristic CH2 group absorption at
3059 cmꢂ1. The 1590 cmꢂ1 and 1488 cmꢂ1 to the vibration with
C]C. Stretches at 1291 and 1190 cmꢂ1 are attributed to the
absorptions of resonances of P]O and P–O–C (aromatic),
respectively, which are characteristic of phosphorus
compounds. The 772 cmꢂ1 stretch is caused by the attachment
of phosphonate group on the adamantane ring. The strong
absorptions at 1020 cmꢂ1, 958 cmꢂ1, 942 cmꢂ1 are ascribed to
P–O stretching.
BDPAd
TDPAd
9.79
92/8
90/10
88/12
94/6
10.56
92/8
90/10
88/12
94/6
92/8
90/10
88/12
TKDPAd 10.97
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Fig. 1 FTIR spectra of the FRs employed in this study: (A) DPAd, (B) BDPAd, (C) TDPAd, and (D) TKDPAd.
Fig. 2 1H NMR spectra of the FRs employed in this study: (A) DPAd, (B) BDPAd, (C) TDPAd, and (D) TKDPAd.
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Fig. 3 31P NMR spectra of the FRs employed in this study: (A) DPAd, (B) BDPAd, (C) TDPAd, and (D) TKDPAd.
1H and 31P NMR shown in Fig. 2D and 3D, respectively, were can be withdrawn from the results is that, the increase of P
employed to further conrm its chemical structure. The content and the substitution amounts of adamantane in the
chemical shis at 7.29–7.16 ppm are attributed to the aromatic FRs is effective for the enhancement of LOI of PC/FR
protons labeled (a–c). A strong resonance signal at 2.54 ppm is signicantly.
characteristic of the protons (labeled 2, 4, 6, 8, 9, 10) of CH2
The UL-94 vertical burning experiment is considered as
groups in adamantyl. The single peak observed at ꢂ16.99 ppm another important means for determining the upward burning
in 31P NMR spectrum suggesting the unique phosphorus characteristics of PC/FR. The results of UL-94 vertical burning
compound. These NMR data are self-consistent with the results test of these PC/FR compositions are summarized in Table 2. As
of FTIR described above, conrming the successful synthesis of was expected, PC/FR achieved the V-0 classication in UL-94 test
high-purity TKDPAd.
by loading different amounts of the FRs except DPAd, respec-
tively. Especially, addition of 8 wt% TKDPAd provided V-0 rating
and no dropping for PC/FR, which showed that the P content
3.2 Flammability characteristics
was actually a vital factor on ame retardancy of the
adamantane-based phosphates. At the end of burning experi-
ments, the surfaces of these PC/FR compositions were covered
with an expanded char structure, indicating that the PC/FR
compositions formed an effective char which was able to
prevent the heat transfer and ame spread during combustion.
3.2.2 Cone calorimeter test (CCT). Flame-retarding effi-
ciencies of the 8 wt% loading of FRs in PC were evaluated by a
cone calorimeter. CCT is becoming one of the most effective
methods for assessing the re behaviors of materials and brings
quantitative analysis to the ame retardancy of materials by
investigating some parameters, such as heat release rate (HRR),
peak of HRR (pHRR) and the total heat release (THR).34 The
HRR curves and THR curves of PC and PC/FR compositions are
showed in Fig. 4A and B, respectively.
3.2.1 Limiting oxygen index (LOI) and UL-94 vertical
burning test. The ammability characteristics of PC blended
with FRs added in amounts of 6–12 wt% in the presence of an
anti-dripping agent (0.1 wt%) were rst evaluated by the LOI
and UL-94 vertical measurements, respectively, and the corre-
sponding results were summarized in Table 2.
From the Table 2, the LOI values increase with the addition
content of FRs. The LOI value of PC here is 26.1% and those of
composites containing 6–12 wt% FRs are 27.1–32.0%. The LOI
values of PC composites can be slightly enhanced with DPAd
from 26.1% (PC) to 27.3% (PC/12 wt% DPAd). However, it is
expected to notice that the PC/TKDPAd exhibits high values of
LOI over 28.6%, signicantly greater than that of the neat PC.
The conclusion about relative ame-retarding efficiency of FR
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Fig. 4 HRR (A) and THR (B) curves of PC and PC/FR composites.
The HRR, particularly the pHRR, has been believed to be one
Moreover, the THR curves of PC and PC/FR compositions are
of the most important parameters to evaluate the re safety of shown in Fig. 4B. It is observed that the THR decreases from
material.35 As can be seen in Fig. 4A, with the increasing emis- 61 ꢃ 15 MJ mꢂ2 (the neat PC) to 54 ꢃ 11 MJ mꢂ2 (PC/DPAd),
sion of ammable gases into the air, the neat PC burned ercely 50 ꢃ 10 MJ mꢂ2 (PC/BDPAd), 43 ꢃ 11 MJ mꢂ2 (PC/TDPAd)
at about 110 s of ignition to reach the pHRR of (306 ꢃ 30) kW and 41 ꢃ 13 MJ mꢂ2 (PC/TKDPAd), respectively, which indi-
mꢂ2 and exhibited a sharp peak with some small shoulders cates that part of the PC/FR compositions has not fully com-
aer. Under the radiation of a 35 kW mꢂ2 heat ux, the surface busted, possibly undergoing a char-forming process, which can
of the neat PC melted to form an integral but thin char layer be attribute to the addition of the FRs. In other words, the
rst, which corresponded to the appearance of the small adamantane-based phosphates could reduce the ammability
shoulders of the HRR curve of PC in Fig. 4A. Then the surface of and improve the ame retardant efficiency of PC compositions
the formed char layer was damaged by the emission of vigorous to be potential FRs. The ame-retarding efficiencies were
gases from the underlying compositions and the porous char improved signicantly with the increase of P content and
structure was formed gradually.36 Aer about 400 s as seen in substitution amounts on adamantane. And it is believed the
Fig. 4A, most of the PC/FR compositions have been burnt out lower HRR and THR are relate to the condensed phase and
and the HRR always keeps above zero till the test stopped, which mean the better ame retardation of FRs.38
is due to the slow degradation of the leaving char residue.
However, Fig. 4A shows that there is a mainly small shoulder
before reaching the pHRR of PC/FR, which means the FRs
3.3 Thermal degradation behaviors
3.3.1 TGA test. The thermal properties of the FRs and the
8 wt% loading of FRs in PC have been evaluated by TGA under
air atmosphere as shown in Fig. 5 and Table 3. Fig. 5A shows the
TGA curves (Fig. 5A1 for TG curves and Fig. 5A2 for DTG curves)
for the FRs employed in this study from room temperature to
employed in this study could facilitate PC decomposition in the
earlier stage. In other words, the FRs could promote char
formation of PC compositions during combustion. From the
HRR curves of PC/FR compositions in Fig. 4A, it presents a
steady decrease of HRR value aer an initial increase to form
the char layer. An obvious peak (pHRR) of 324 ꢃ 12 (PC/DPAd),
301 ꢃ 14 (PC/BDPAd), 255 ꢃ 12 (PC/TDPAd) and 240 ꢃ 20
(PC/TKBDPAd) kW mꢂ2 can be found in the HRR curves of the
FRs. Although the pHRR of PC/DPAd compositions is slightly
higher than that of the neat PC, the average of HRR (av-HRR) of
the former is lower (90 ꢃ 10 kW mꢂ2) than that of the latter
(94 ꢃ 10 kW mꢂ2), which means DPAd is still an efficient FR. It
is easy to nd that with the increase of P content and substi-
tution amounts on adamantane, the pHRR and av-HRR (89 ꢃ 8,
79 ꢃ 7, 79 ꢃ 7 kW mꢂ2 for PC/BDPAd, PC/TDPAd, PC/TKDPAd,
respectively) decreased signicantly. And a terse residue layer
covered on the aluminum foil could be observed. The decrease
of pHRR and av-HRR can be attributed to the formation of
compact and expansion of the residue layer stopped the spread
of the heat and oxygen, and the emissions of volatile products.37
The residue layer could keep the PC compositions thermal
stability and improve its ame retardancy.
800 C at a heating rate of 10 C minꢂ1 under air atmosphere.
There are two distinct thermal degradation behaviors to be
noticeable in Fig. 5A. DPAd and BDPAd show a gently two-step
degradation and little amounts of charred residues (6.4% for
ꢀ
ꢀ
ꢀ
DPAd and 9.0% for BDPAd) at 800 C while TKDPAd shows a
sharply two-step degradations leaving higher charred residue
(11.9%). However, TDPAd also shows a sharply two-step degra-
dations, but leaving very little amounts of charred residue
(4.5%) at 800 ꢀC. Next to a sufficient ame-retarding effect, FRs
for PC should meet two vital requirements: (a) good thermal
stability for high processing temperatures (225–310 ꢀC), and (b)
sufficient compatibility with PC. Moreover, the difference in
initial decomposition temperature (T5%) of the FRs became
more pronounced as the phosphorus content was increased.
However, Table 3 shows that DPAd already starts to decompose
ꢀ
at 215.7 C under an air atmosphere. In contrast to DPAd, the
T5% of decompositions for BDPAd, TDPAd and TKDPAd are
shied towards
a
higher temperature, indicating an
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Fig. 5 TGA curves of FRs (A1 and A2) and PC and PC/FR (B1 and B2) under air atmosphere.
enhancement of the thermal stability upon more intercalation PC, the initial thermal degradation occurs at a lower tempera-
of phosphate. DPAd showed T5% at 215.7 ꢀC in air whereas ture. In contrast to literature report,40 TGA experiments only
BDPAd and TDPAd degraded between 284.8 ꢀC and 334.9 ꢀC. showed scarcely any charring for the neat PC (0%) at 800 ꢀC. The
Notably, a two-stage dramatic weight loss of TKDPAd occurred decomposition mechanism of PC under air was investigated by
at 377.5 ꢀC. The results indicated that TKDPAd has the highest Jang et al.15 and was found to mainly take place via chain scis-
char yield of that of the other three FRs, even greater than the sion of the isopropylidene linkage. However, the TG results
commercial RDP,39 which means that the introducing of the presented in Fig. 5B1 clearly show that the amount of charred
bridging unit, adamantane, can improve the thermal stability of residue of composites containing FR is directly proportion to
the FR. Moreover, the char yields increased as more phosphorus the char yielding behaviors of FRs discussed in previous
and substitution amounts are introduced except TDPAd.
section. For compositions of PC with DPAd and TDPAd,
The onset of degradation occurring at lower temperature respectively, slightly increased amounts of residual char were
with 8 wt% add-on FRs in contrast with the neat PC at below found (0.9 and 0.5 wt%, respectively). In the case of the
ꢀ
422.5 C were presented in Fig. 5B (Fig. 5B1 for TG curves and PC/BDPAd and PC/TKDPAd compositions, the amount of
Fig. 5B2 for DTG curves). When 8 wt% of FRs was added to the residual char aer heating to 800 ꢀC was relatively high (1.2 and
Table 3 TGA data of the FRs, PC and PC/FR in this study
Sample
T5% (ꢀC)
Tmax1 (ꢀC)
Tmax2 (ꢀC)
Tmax3 (ꢀC)
Tmax4 (ꢀC)
Residue at 800 ꢀC (wt%)
DPAd
BDPAd
TDPAd
TKDPAd
PC
PC/DPAd
PC/BDPAd
PC/TDPAd
PC/TKDPAd
215.7
284.8
334.9
377.5
461.7
376.4
331.5
352.0
422.5
235.7
287.8
336.9
381.5
340.7
358.8
343.4
391.5
6.4
9.0
4.5
11.8
0.0
0.9
1.2
0.5
1.6
519.2
514.4
506.0
504.5
499.0
612.2
599.9
558.0
566.5
565.5
449.4
433.5
439.0
448.0
346.0
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Fig. 6 Three-dimensional TGA-FTIR spectra of (A) PC, (B) PC/DPAd, (C) PC/BDPAd, (D) PC/TDPAd and (E) PC/TKDPAd.
discernible increase in the amount of charred residue is
observed, the superior ame-retarding performance of TKDPAd
is believed to be closely related to its char forming ability and
high thermal stability. That is, the condensed phase mecha-
nism is known to be the dominating action for the ame
retardancy of PC.18 Therefore, the ame retardancy of
phosphorus-based FRs depends not only on the phosphorous
content but also on the thermal stability of the phosphorus-
based FRs.
3.3.2 TGA-FTIR analysis. The volatilized products from
TGA furnace were inspected by a FTIR spectrometer simulta-
neously. Fig. 6 shows the three-dimensional (3D) TGA-FTIR
spectra of gaseous phase in the thermal degradation of PC
and PC/FR compositions, respectively. Fig. 7 shows the FTIR
spectra of pyrolysis products of PC, PC/DPAd, PC/BDPAd,
PC/TDPAd and PC/TKDPAd at their rst maximum weight loss
rates, respectively. The TGA-FTIR technique can give more
information of volatilized products, which can help to better
understand the thermal degradation mechanisms of PC/FR
compositions.41 Fig. 6 exhibits that the most obvious absor-
bance band in TGA-FTIR spectra of PC and PC/FR compositions
is similar around 2379 cmꢂ1, which is the main product, carbon
dioxide (CO2). Furthermore, PC and PC/FR compositions
exhibit nearly the same infrared absorbing time. In addition to
the above information, no more can be directly detected from
the 3D FTIR spectra. Fig. 7 shows that the other main volatilized
products of the decomposition of PC and PC/FR compositions
Fig. 7 FTIR spectra of pyrolysis products of PC, PC/DPAd, PC/BDPAd,
PC/TDPAd and PC/TKDPAd at their maximum weight loss rates.
1.6 wt%, respectively). An interesting phenomenon can be seen
that PC/TDPAd has four maximum decomposition tempera-
tures, whereas PC/DPAd, PC/BDPAd and PC/TKDPAd have three
of that. Furthermore, the amount of residual char of PC/TDPAd
was found to be the least among the FRs in this study, which
may be attributed to the resin state of TDPAd. Even though no
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Fig. 8 SEM micrographs of the residual chars obtained from the LOI teat for (A) PC, (B) PC/DPAd, (C) PC/BDPAd, (D) PC/TDPAd and (E)
PC/TKDPAd.
are released and also similar, such as water (H2O, around with PC mainly in the condensed phase as well as slightly in the
2379 cmꢂ1), carbon monoxide (CO, around 2180 cmꢂ1) and gaseous phase during thermal degradation. And most
methane (CH4, around 3016 cmꢂ1). Otherwise, some other phosphorous-containing components such as P–O–C, P–OH
important but not obvious peaks are listed: –OH (mainly and P–H components were retained in the char residual rather
include H2O, phenol; around 3600 cmꢂ1), compounds con- than that in the gaseous phase during the thermal degradation.
taining aromatic ring (around 1600 cmꢂ1), and hydrocarbons TGA results had been showed that the incorporation of
(C–H stretching at around 1190 cmꢂ1).42 Finally, the peak adamantane-based phosphates into PC compositions promoted
around 1787 cmꢂ1 indicates the volatilized products containing the formation of char residual, which trapped some of the
carbonyl are released. These compounds are identied as volatile products and made them participate in the charring
carboxylic acid RCOOH (2977–2880 cmꢂ1, around 1787 cmꢂ1).42 reaction. The char layer become a barrier and covered on the PC
Moreover, the intensity of the carbonyl absorbance band matrix which is effective to prohibit the heat ux and air
(1787 cmꢂ1) to its nearby band (around 1600 cmꢂ1, aromatic incursion. This surface structure can improve the ame
rings vibration absorbance) in the spectra of PC/FR is higher retardancy and thermal stability of PC compositions.
with the increase of P content and substitution amounts of
adamantane. It indicates that more carbon char structures are
presented in PC/FR. Jang et al. also found that phosphate may
stabilize carbon char from oxidative degradation in air atmo-
sphere.18 Besides, the weak peaks absorbance at 950 cmꢂ1 and
1641 cmꢂ1 which can be correspond to the phosphates (P–O)
and P–OH stretching vibration, and the peak absorbance at
2320 cmꢂ1 was assigned to P–H.43,44 It is because the FRs reacted
3.4 Analysis of residual char
3.4.1 Scanning electron microscopy (SEM). To further
investigate how the FRs works in the PC/FR compositions, the
morphology of the chars for the residues that le aer LOI test
are tested by SEM. As shown in Fig. 8A, there exists macro-
porous structure on the exterior surface of the neat PC.
However, the interior char residue of the neat PC presents a
Fig. 9 FTIR curves of PC and PC/8 wt% FR before (A) and after (B) combustion under a synthetic air atmosphere.
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smooth and continual char layer which means the rapid vola- adamantane ring. It therefore becomes clear that the O–H
tilization over the surface. This structure cannot provide a good structure is contained in the charred residue of the FRs.
barrier to stop the spread of the heat, oxygen and the emissions
Combine with the TGA and SEM results, the decomposition
of volatile products. Compared to the sample of the neat PC, the of the FRs proceeds with the cleavage of the P–O bond and
surface of the sample of PC/FR compositions (Fig. 8B–E) mainly consequent generation of phosphorus compounds, which
presents a continuous and protective carbon layer on which are promote the formation of a continuous and protective carbon
dispersed bubbles and folds. And the compact order of the layer. As shown previously, it can be inferred that the formation
formed carbon layer is: PC/DPAd < PC/BDPAd < PC/TDPAd < of the sharp degradation products from TKDPAd are the main
PC/TKDPAd, which means that the quality of the formed carbon reason for the observation of larger amount of charred residue
layer raises with the increase of the P content and the substi- from TKDPAd than the other FRs. It is speculated that the
tution amounts of adamantane. That also comes with the mainly sharp one degradation path of TKDPAd is responsible
results of CCT. This means that the incorporation of the for the generation of phosphorus compounds, which leads to
adamantane-based phosphates restricts the rapid volatilization the formation of a continuous and protective carbon layer.46
at the surface and enhances the ame resistance of the char
residue. From the analysis of CCT and SEM, the thermal
degradation of the PC matrix proceeds ercely under a high
4. Conclusions
temperature in the process of combustion. So the combustion
behaviors of FRs could be summarized: a fast degradation is
In an attempt to investigate the factors affecting the ame-
retarding behaviors of phosphorus-based FRs, a series of
preceded under high temperature, and then a char layer was
adamantane-based phosphorus derivatives were successfully
formed on the surface of the PC matrix. The FRs quickly
synthesized and used to impart ame resistance to PC, their
thermal degradation and ame-retarding performances were
decomposed to form phosphorus compounds in the PC matrix
and released more combustible small molecules such as CO2
compared with each other. In TGA results, the FRs containing
dissociated from the FRs, so that the time to ignite the PC
more diphenyl phosphoryl substituents were degraded at
compositions was decreased, which was shown in Fig. 4A.
higher temperatures and produced a moderate amount of char
However, part of the FRs is pushed on the surface of the PC
yields at elevated temperature, which was related to the gener-
matrix by the volatile products such as H2O, CO2 and phenol
ation of phosphorus compounds that lead to the formation of
derivates.45 The addition of the FRs incorporated the ada-
the continuous and protective carbon layer in the charred
mantane as the bridging unit into PC enhanced thermal
residues. This tendency to generate a large amount of phos-
oxidative stability of the char layer formed on the surface of the
phorus compounds is benecial to the condensed phase ame
retardancy mechanism, and the TKDPAd shows the best ame
PC matrix. Then the layer char prevented heat and oxygen into
underlying polymeric substrate to block the series of thermo-
retardancy (UL-94 V-0 ratings at 8 wt% loading) on PC compare
oxidative reactions and the release of volatile products of
to the other three FRs. And the analysis results of TGA and TGA-
underlying polymeric substrate. So the value of HRR of the
FTIR show that the ame retardancy mechanism of these
adamantane-based phosphates are mainly in the condensed
phase as well as slightly in the gaseous phase during the
PC/FR compositions decreases. The efficiency of the FRs on the
combustion and enhancement of thermal stability of the char
layer result in the signicantly ame retardant efficiency of the
thermal degradation. It is concluded that the ame retardancy
PC/FR compositions.
of phosphorus-based FRs depends not only on their phospho-
rous content but also on their thermal stability. And even
though the phosphorous content of TKDPAd and RDP is nearly,
but the introduction of adamantly increased TKDPAd's thermal
3.4.2 FTIR analysis. To understand better the char forma-
tion of these phosphorus-containing compounds, the degra-
dation of these FRs was carried out under air. Aer that, the
charred residues of PC/FR were analyzed spectroscopically by
stability compared with RDP in this study. The results revealed
employing FTIR. The FTIR spectra of before and aer
that TKDPAd was an efficient ame retardant for PC.
combustion of PC and PC/FR compositions were shown in
Fig. 9. Fig. 9A exhibited the characteristic P–O–C (aromatic)
group absorption at 1103 cmꢂ1. The 947 cmꢂ1 is attributed to Acknowledgements
the absorptions of resonances of P]O and the 766 cmꢂ1 stretch
This work was supported by National Natural Science Founda-
is caused by the attachment of phosphonate group on the
tion of China (Grant no. 21476051) and Science and Technology
Project of Guangdong Province (Grant no. 2013B010403028).
combustion of PC and PC/FR compositions. The intensities of
adamantane ring. Fig. 5B show the FT-IR spectrum of aer
the absorption bands at 3425 and 1003 cmꢂ1 are attributed to
the stretching of P–OH group, and the absorption peak at
Notes and references
1162 cmꢂ1 is attributed to P]O and P–H.45 A weak character-
istic band at 2360 cmꢂ1 is attributed to the P–H stretching
vibration. Stretch at 3167 cmꢂ1 is attributed to the absorptions
of resonances of aromatic. The absorption band at 1003 cmꢂ1
corresponds to P–O–C stretching. Moreover, the 749 cmꢂ1
stretch of the PC/FR is caused by phosphonate group on the
1 S. Y. Lu and I. Hamerton, Prog. Polym. Sci., 2002, 27, 1661–
1712.
2 C. Hoffendahl, G. Fontaine, S. Duquesne, F. Taschner,
M. Mezgerb and S. Bourbigot, RSC Adv., 2014, 4, 20185–
20199.
67064 | RSC Adv., 2015, 5, 67054–67065
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
3 S. J. Chen, J. Li, Y. K. Zhu and S. P. Su, RSC Adv., 2014, 4, 25 K. A. Salmeia, J. Fage, S. Liang and S. Gaan, Polymers, 2015, 7,
32902–32913. 504–526.
4 H. Li, J. Q. Zhao, S. M. Liu and Y. C. Yuan, RSC Adv., 2014, 4, 26 H. Vothi, C. Nguyen and J. Kim, Polym. Degrad. Stab., 2010,
10395–10401.
95, 1092–1098.
5 S. V. Levchik and E. D. Weil, J. Fire Sci., 2006, 24, 137–151.
27 R. C. Fort and P. R. Schleyer, Chem. Rev., 1964, 64, 277–300.
6 B. Swoboda, S. Buonomo, E. Leroy and J. M. Cuesta Lopez, 28 Y. Hu and S. B. Sinnot, Surf. Sci., 2003, 526, 230–242.
Polym. Degrad. Stab., 2008, 93, 910–917.
29 L. Bauer and K. K. Khullar, J. Org. Chem., 1971, 36, 3038–
7 A. Toldy, B. Szolnoki, I. Csontos and G. Marosi, J. Appl. Polym.
Sci., 2014, 131, 40105–40112.
8 S. V. Levchik and E. D. Weil, J. Fire Sci., 2006, 24, 345–364.
3040.
30 W. Azzam, A. Bashir and O. Shekhah, Appl. Surf. Sci., 2011,
257, 3739–3747.
9 M. Yin, L. Yang, X. Y. Li and H. B. Ma, J. Appl. Polym. Sci., 31 A. Z. Halimehjani, K. Marjani, A. Ashouri and V. Amani,
2013, 130, 2801–2808. Inorg. Chim. Acta, 2011, 373, 282–285.
10 W. C. Zhang, X. M. Li, H. B. Fan and R. J. Yang, Polym. 32 H. Zhu, J. W. Guo, C. F. Yang, S. Liu, Y. D. Cui and X. Zhong,
Degrad. Stab., 2012, 97, 2241–2248. Synth. Commun., 2013, 43, 1161–1167.
11 J. P. Ding, Z. Q. Tao, X. B. Zuo, L. Fan and S. Y. Yang, Polym. 33 S. Liu, J. W. Guo and D. M. Jia, CN 1,974,515 A, issued June
Bull., 2009, 62, 829–841. 06, 2007.
12 Z. Wang, W. Wu, Y. H. Zhong, M. Z. Ruan and L. L. Hui, J. 34 J. S. Wang, G. H. Wang, Y. Liu, Y. H. Jiao and D. Liu, Ind. Eng.
Appl. Polym. Sci., 2015, 132, 41545–41555. Chem. Res., 2014, 53, 6978–6984.
13 K. A. Salmeia and S. Gaan, Polym. Degrad. Stab., 2015, 113, 35 J. W. Gilman, C. L. Jackson and A. B. Morgan, Chem. Mater.,
119–134. 2000, 12, 1866–1871.
14 V. Benin, X. M. Cui, A. B. Morgan and K. Seiwert, J. Appl. 36 J. Feng, J. W. Hao, J. X. Du and R. J. Yang, Polym. Degrad.
Polym. Sci., 2015, 132, 42296–42305. Stab., 2012, 97, 605–614.
15 N. N. Tian, J. Gong, X. Wen, K. Yao and T. Tang, RSC Adv., 37 P. Y. Jia, M. Zhang, C. G. Liu, L. H. Hu, G. D. Feng, C. Y. Bo
2014, 4, 17607–17614. and Y. H. Zhou, RSC Adv., 2015, 5, 41169–41178.
16 Z. J. Jiang, S. M. Liu, J. Q. Zhao and X. B. Chen, Polym. 38 Z. Hu, L. Chen, B. Zhao, Y. Luo, D. Y. Wang and Y. Z. Wang,
Degrad. Stab., 2013, 98, 2765–2773. Polym. Degrad. Stab., 2011, 96, 320–327.
17 K. H. Pawlowski and B. Schartel, Polym. Int., 2007, 56, 1404– 39 B. N. Jang and C. A. Wilkie, Thermochim. Acta, 2005, 433, 1–
1414. 12.
18 B. N. Jang and C. A. Wilkie, Thermochim. Acta, 2005, 433, 1– 40 B. N. Jang and C. A. Wilkie, Thermochim. Acta, 2005, 426, 73–
12. 84.
19 B. Perret, K. H. Pawlowski and B. Schartel, J. Therm. Anal. 41 T. Xu and X. M. Huang, Fuel, 2010, 89, 2185–2190.
Calorim., 2009, 97, 949–958.
20 E. Wawrzyn, B. Schartel, M. Ciesielski, B. Kretzschmar,
42 X. D. Qian, L. Song, Y. Hu, R. K. Yuen, L. J. Chen, Y. Q. Guo,
N. N. Hong and S. H. Jiang, Ind. Eng. Chem. Res., 2011, 50,
1881–1892.
¨
U. Braun and M. Doring, Eur. Polym. J., 2012, 48, 1561–1574.
21 M. C. Despinasse and B. Schartel, Thermochim. Acta, 2013, 43 A. I. Balabanovich, Thermochim. Acta, 2005, 435, 188–196.
563, 51–61.
44 L. Song, Q. L. He, Y. Hu, H. Chen and L. Liu, Polym. Degrad.
Stab., 2008, 93, 627–639.
22 C. Nguyen and J. Kim, Polym. Degrad. Stab., 2008, 93, 1037–
1043.
45 J. Wang, J. S. Yi and X. F. Cai, J. Appl. Polym. Sci., 2011, 120,
968–973.
46 V. Hai, H. Soekmin, N. Congtranh, B. Imhyuck and
K. Jinhwan, Fire Mater., 2014, 38, 36–45.
23 M. C. Despinasse and B. Schartel, Polym. Degrad. Stab., 2012,
97, 2571–2580.
24 D. Q. Hoang, J. Kim and B. N. Jang, Polym. Degrad. Stab.,
2008, 93, 2042–2047.
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