Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer

Article abstract of DOI:10.1016/j.polymer.2010.03.053

A series of flame-retardant epoxy resins (EP) with different content of poly(DOPO substituted dihydroxyl phenyl pentaerythritol diphosphonate) (PFR) were prepared. The PFR was synthesized via the polycondensation between 10-(2,5-dihydroxyl phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-BQ) and pentaerythritol diphosphonate dichloride (SPDPC). The structure of PFR was confirmed by Fourier transform infrared spectroscopy (FTIR) and ¹H nuclear magnetic resonance (¹H NMR). The flame retardancy and the thermal stability of the EP/PFR hybrids were investigated by limiting oxygen index (LOI) test and thermogravimetric analysis (TGA) in air. The results showed that the incorporation of PFR into EP can improve the thermal stability dramatically. The mechanical results demonstrated that PFR enhanced failure strain slightly accompanied by a decrease in tensile strength. The thermal oxidative degradation mechanisms of the EP/PFR hybrids were investigated by real time Fourier transform infrared spectra (RTFTIR) and direct pyrolysis/mass (DP-MS) analysis. X-ray photoelectron spectroscopy (XPS) was used to explore chemical components of the residual char of EP and EP/PFR hybrid. DP-MS analysis showed that the degradation process of EP/PFR hybrid was divided into two characteristic temperature regions, attributed to the decomposition of phosphate and aromatic structure.

Full text of DOI:10.1016/j.polymer.2010.03.053

Polymer 51 (2010) 2435e2445
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Flame retardancy and thermal degradation mechanism of epoxy resin
composites based on a DOPO substituted organophosphorus oligomer
,
,
c
,
a
a
a
d
d
Xin Wang a b, Yuan Hu a , Lei Song , Weiyi Xing , Hongdian Lu , Pin Lv , Ganxin Jie
*
a State Key Lab of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China
b Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
c Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, PR China
d State Key Laboratory of Environmental Adaptability for Industrial Products, China National Electric Apparatus Research Institute, Guangzhou 510300, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of flame-retardant epoxy resins (EP) with different content of poly(DOPO substituted dihydroxyl
phenyl pentaerythritol diphosphonate) (PFR) were prepared. The PFR was synthesized via the poly-
condensation between 10-(2,5-dihydroxyl phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-
oxide (DOPO-BQ) and pentaerythritol diphosphonate dichloride (SPDPC). The structure of PFR was
confirmed by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (1H
NMR). The flame retardancy and the thermal stability of the EP/PFR hybrids were investigated by limiting
oxygen index (LOI) test and thermogravimetric analysis (TGA) in air. The results showed that the
incorporation of PFR into EP can improve the thermal stability dramatically. The mechanical results
demonstrated that PFR enhanced failure strain slightly accompanied by a decrease in tensile strength.
The thermal oxidative degradation mechanisms of the EP/PFR hybrids were investigated by real time
Fourier transform infrared spectra (RTFTIR) and direct pyrolysis/mass (DP-MS) analysis. X-ray photo-
electron spectroscopy (XPS) was used to explore chemical components of the residual char of EP and EP/
PFR hybrid. DP-MS analysis showed that the degradation process of EP/PFR hybrid was divided into two
characteristic temperature regions, attributed to the decomposition of phosphate and aromatic structure.
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Received 19 January 2010
Received in revised form
13 March 2010
Accepted 27 March 2010
Available online 6 April 2010
Keywords:
Epoxy resin
Thermal degradation mechanism
Flame retardancy
1. Introduction
environment and human health, researchers are focused on
exploiting environmental friendly halogen-free fire retardant for
Epoxy resin (EP) is a very important thermosetting material
owing to its excellent mechanical and chemical properties. It is
used as a high performance material in many fields, such as
adhesive, coating, laminating capsulation, electronic/electrical
insulation, and composite applications [1e5]. These fields require
a remarkable flame-retardant grade, but the flammability is a major
drawback of epoxy resin and limits its applications, thus, the flame
retardance of EP is an urgent problem and has attracted much
attention [6]. Many methods have been developed to improve the
flame retardancy of EP. Traditionally, halogenated compounds are
widely used as co-monomers or additive with epoxy resins to
obtain fire-retardant materials. However, flame-retardant epoxy
resins containing bromine or chlorine can produce poisonous and
corrosive smoke and may give super-toxic halogenated dibenzo-
dioxins and dibenzofurans [7]. Consequently, to protect the
epoxy resins. It is reported that epoxy resins can be modified with
boron [8], phosphorus [9e13], silicone [14e16], layer double
hydroxide [17], melamine [18] and montmorillonite [19], etc. to
enhance their flame retardancy. Among them, epoxy resin modified
by phosphorus-containing compounds is considered as a promising
method due to its high flame-retardant efficiency.
In recent years, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-
10-oxide (DOPO) and its derivatives, as a novel kind of phosphorus-
containing flame retardants, have received considerable attention
due to their high reactivity. The technical literature extensively
reports the modification by DOPO as amine curing agents [20],
anhydride curing agents [21] and reactive co-monomers [22,23].
Nevertheless, flame retardants modified by DOPO reported above
are almost small molecular compounds. The small molecular
phosphorus-containing flame retardants display many drawbacks
such as leaching and poor compatibility with polymer matrix. The
polymeric flame retardant possessed higher phosphorus content
and richer aryl group structures than small molecular phosphorus-
containing flame retardant reported above and would overcome
some shortcomings of low molecular phosphorus-containing flame
* Corresponding author. State Key Lab of Fire Science, University of Science and
Technology of China, Hefei, Anhui 230026, PR China. Tel.: þ86 551 3601664.
E-mail address: yuanhu@ustc.edu.cn (Y. Hu).
0032-3861/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.03.053

2436
X. Wang et al. / Polymer 51 (2010) 2435e2445
Scheme 1. Synthesis route of SPDPC, DOPO-BQ and PFR.
retardants. The polymeric flame retardant modified by DOPO are
seldom reported up to now.
In this paper we presented a thermal and flame-retardant
analysis of the epoxy resins blended with different PFR concen-
trations. The synthesis, characterization, thermal stability and
flame-retardant properties of the EP/PFR hybrids were investigated
and dynamic FTIR and DP-MS were used to analyze thermal
oxidative degradation mechanism of EP/PFR hybrids.
2. Experimental
2.1. Materials
9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide
(DOPO) was supplied by Shandong Mingshan Fine Chemical
Industry Co. Ltd (Shandong, China). Epoxy resin (DGEBA,
commercial name: E-44) and polyamide (651) were supplied by
Hefei Jiangfeng Chemical Industry Co. Ltd (Anhui, China). DOPO,
DGEBA and polyamide (651) were all industrial grade. Phosphorus
oxychloride was reagent grade and provided by Shanghai HuaYi
Group HuaYuan Chemical Industry Co. Ltd (Shanghai, China). Pen-
taerythritol, p-benzoquinone, tetrahydrofuran (THF), acetonitrile,
trichloromethane and diethyl ether were all reagent grade and
purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai,
China). THF and acetonitrile were distilled at reduced pressure
before use.
Table 1
The composition and LOI values of EP/PFR hybrids with different PFR contents.
Sample
Epoxy resin
content (g)
Polyamide 651
content (g)
PFR content
(g)
PFR amount
(wt%)
LOI
(%)
EP
50.0
50.0
50.0
50.0
25.0
25.0
25.0
25.0
0
0
20.5
25.0
27.6
30.2
EP1
EP2
EP3
1.92
3.95
8.33
2.5
5.0
10.0
Fig. 1. (a) TG and (b) DTG curves of EP, EP1, EP2, and EP3 in air atmosphere.

X. Wang et al. / Polymer 51 (2010) 2435e2445
2437
Table 2
TG data of PFR, EP and EP/PFR hybrids in air.
Samples
T
(ꢀC)
T
(ꢀC)
ꢁ50%
Tmax (ꢀC)
Char residue
(%) at 700 ꢀC
ꢁ5%
PFR
EP
EP1
EP2
EP3
286
305
303
299
291
652
414
398
402
432
384
42.6
1.9
3.6
7.3
20.8
365, 435, 548
360, 442, 556
356, 445, 556
348, 457, 539
2.2. Synthesis of 10-(2,5-dihydroxyl phenyl)-9,10-dihydro-9-oxa-
10-phosphaphenanthrene-10-oxide (DOPO-BQ)
64.8 g of DOPO and 32.4 g of p-benzoquinone were dissolved in
dried 200 ml THF and then introduced into a three-neck and round-
bottom 500 ml glass flask equipped with a nitrogen inlet, a reflux
condenser, and a mechanical stirrer. The mixture was stirred and
heated slowly to 60 ꢀC for about 1 h under a nitrogen gas atmo-
sphere until DOPO was dissolved completely in THF. The reaction
mixture was heated to 72 ꢀC, and then allowed to stir for 6 h under
a nitrogen gas atmosphere. After cooling to room temperature, the
precipitate was filtered and washed with THF. The white powder
was collected by filtration, and dried under reduced pressure. An
instrumental analysis of DOPO-BQ was carried out by Fourier
transform infrared (FTIR) spectroscopy and 1H nuclear magnetic
resonance (1H NMR). FTIR (KBr, cmꢁ1): 3418 (PheOH), 1596 (PePh),
1150 (P]O), 929 (PeOePh); 1H NMR (400 MHz, DMSO-d6, ppm):
9.39 (s, 1H, PheOH); 9.09 (s, 1H, PheOH); 8.25e8.13 (m, 2H, PheH);
7.68 (t, 1H, PheH); 7.59e7.34 (m, 3H, PheH); 7.30e7.17 (m, 2H,
PheH); 7.12 (dd, 1H, PheH); 6.83 (dd, 1H, PheH); 6.62e6.53 (m, 1H,
PheH).
Fig. 2. 3D TG-FTIR spectrum of gas phase in the thermal degradation of EP.
gradually heated and refluxed until no HCl gas was emitted. The
solvent was removed by rotary evaporator under reduced pressure.
The raw product obtained was washed by acetonitrile three times.
The product was then dried in vacuo at 60 ꢀC overnight. FTIR (KBr
cmꢁ1): 1596 (PePh), 1303, 1150 (P]O), 1028 (PeOeC), 929
(PeOePh); 1H NMR (400 MHz, DMSO-d6, ppm): 8.25e8.13 (m, 2H,
PheH); 7.68 (t, 1H, PheH); 7.59e7.34 (m, 3H, PheH); 7.30e7.17 (m,
2H, PheH); 7.12 (dd,1H, PheH); 6.83 (dd, 1H, PheH); 6.62e6.53 (m,
1H, PheH), 4.29e4.22 (dd, 8H, eOCH2e). The synthetic route of
above three steps is shown in Scheme 1.
2.5. Preparation of EP/PFR hybrids
Epoxy resin and PFR with the content of 2.5 wt% as total weight
were added into a 250 ml three-necked flask equipped with
a magnetic stirrer, a thermometer and a reflux condenser. The
temperature of the mixture was maintained at 120 ꢀC until PFR was
dissolved in epoxy resin completely. The curing agent polyamide
(651) was mixed into the EP/PFR with the content of 50 wt% as
epoxy resin subsequently. The reaction mixtures were cured in
a mould at 60 ꢀC for 1 h. After curing, samples were cooled to room
temperature. The composition of EP/PFR hybrids was listed in Table
1. Other samples were synthesized in the same procedures.
2.3. Synthesis of pentaerythritol diphosphonate dichloride (SPDPC)
A 500 ml three-neck and round-bottom glass flask equipped
with a temperature controller, magnetic stirrer and a reflux
condenser was charged with 306 g of phosphorus oxychloride and
49.5 g of pentaerythritol. The temperature of the reaction mixture
was raised to 75 ꢀC and maintained for 6 h under a nitrogen gas
atmosphere. Afterwards, the reaction mixture was heated slowly to
105 ꢀC and maintained for 12 h. After cooling to room temperature,
the precipitate was filtered and washed with trichloromethane and
diethyl ether. The white powder was collected by filtration, and
dried under reduced pressure. FTIR (KBr, cmꢁ1): 1303 (P]O), 1028
(PeOeC), 583, 541 (PeCl); 1H NMR (400 MHz, DMSO-d6, ppm):
4.29e4.22 (dd, 8H, eOCH2e).
2.6. Characterization
All 1H NMR spectra were performed at on a Bruker AV400 NMR
spectrometer (400 MHz) operating in the Fourier transform mode
2.4. Synthesis of poly(DOPO substituted dihydroxyl phenyl
pentaerythritol diphosphonate) (PFR)
In a 500 ml three-neck and round-bottom glass flask equipped
with a temperature controller, magnetic stirrer and a reflux
condenser, 29.7 g of SPDPC, 32.4 g of DOPO-BQ and 150 ml aceto-
nitrile were mixed at 70 ꢀC for 1 h. Thereafter, the mixture was
Table 3
Mechanical properties of unmodified and PFR-modified epoxy resins.
Samples
PFR amount
(wt%)
Tensile
strength (MPa)
Failure
strain (%)
EP
0
19.301
19.748
18.423
16.443
0.8
1.6
1.6
1.6
EP1
EP2
EP3
2.5
5.0
10.0
Fig. 3. 3D TG-FTIR spectrum of gas phase in the thermal degradation of EP3.

2438
X. Wang et al. / Polymer 51 (2010) 2435e2445
Fig. 4. FTIR spectrum of pyrolysis products for EP at the maximum decomposition rate
Fig. 5. FTIR spectrum of pyrolysis products for EP3 at the maximum decomposition
(27.6 min).
rate (19.5 min).
using DMSO-d6 as solvent. The thermogravimetric analysis (TGA) of
samples were carried out with Q5000 thermal analyzer (TA Co.,
USA) from 50 ꢀC to 700 ꢀC at a heating rate of 20 ꢀC/min. The
limiting oxygen index (LOI) was measured according to ASTM
D2863. The apparatus used was an HC-2 oxygen index meter
(Jiangning Analysis Instrument Company, China). The specimens
used for the test were of dimensions 100 ꢂ 6.5 ꢂ 3 mm3. The real
time Fourier transform infrared spectra (RTFTIR) were recorded
using a Nicolet MAGNA-IR 750 spectrophotometer equipped with
a heating device and a temperature controller. Powders of samples
were mixed with KBr powders, and the mixture was pressed into
a tablet, which was then placed in a ventilated oven. The temper-
ature of the oven was raised at a heating rate of 10 ꢀC/min. The
dynamic FTIR spectra were obtained in situ during the thermo-
oxidative degradation processes. Thermogravimetric analysis/
infrared spectrometry (TG-IR) of the samples was performed using
the TGA Q5000 IR thermogravimetric analyzer that was interfaced
to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of the
sample was put in an alumina crucible and heated from 30 to
600 ꢀC at a heating rate of 20 ꢀC/min (nitrogen atmosphere, flow
rate of 45 ml/min). DP-MS analysis was carried out with a Micro-
mass GCT-MS spectrometer using the standard direct insertion
probe for solid polymer materials, at a heating rate of 10 ꢀC minꢁ1
in the range of 30e600 ꢀC. The mass data were continuously
acquired at a scan rate of 0.1 s. Electron impact (EI) was used for the
mass spectra with 70 eV and the mass range of 10e1000m/z. All
Fig. 6. Absorbance of pyrolysis products for EP and EP3 vs time: (a) H2O; (b) hydrocarbons; (c) CO2; and (d) aromatic compounds.

X. Wang et al. / Polymer 51 (2010) 2435e2445
2439
Table 4
Assignment of FTIR spectra of EP and EP3.
Wavenumber (cmꢁ1
)
Assignment
EP
3392
2930
1607, 1513, 1458
1291
OeH stretching vibration of water or phenol
eCH3 and eCH2e stretching vibration
CeC stretching vibration of aromatic ring
eCH2e bending vibration
1240, 1035
1177
CeH vibration of eC6H4eOeCH2e
CeO stretching vibration
EP/PFR hybrids
3392
2930
1607, 1513, 1458
1291
OeH stretching vibration of water or phenol
eCH3 and eCH2e stretching vibration
CeC stretching vibration of aromatic ring
eCH2e bending vibration or P]O
stretching vibration
1240, 1035
1177
1458
CeH vibration of eC6H4eOeCH2e
CeO and/or PeOePh stretching vibration
PePh stretching vibration
degradation, including the free radical chain scission of the iso-
propylidene linkage and the branching and crosslinking reactions
of molecular chains induced by oxygen, besides the elimination of
the hydration water. The second and third stage are in the
temperature ranges of 410e490 ꢀC and 490e660 ꢀC corresponding
to Tmax2, Tmax3 of 435 ꢀC, 548 ꢀC, respectively. During the two
stages, the macromolecular chains of EP are further oxided and
many small molecular degradation products are released.
Fig. 1a and b also shows TG and DTG curves of the EP1, EP2 and
EP3 in air, respectively. The thermal oxidative degradation process
of all the hybrid samples has the similar three stages as the pristine
EP. It can be seen that PFR has remarkable influence on the thermal
degradation of EP. EP/PFR hybrids decompose faster compared with
EP at lower temperature. At higher temperature, T50% of the EP1,
EP2 is lower than that of EP, whereas T50% of the EP3 is higher than
that of EP. As the PFR content increases, the residue left at 700 ꢀC
increases significantly. Moreover, the char yield at 700 ꢀC for EP3 is
20.8% which is much higher than that of EP. These indicate that the
incorporation of PFR can improve the degradation stability of the
EP/PFR hybrids at higher temperature and premote the formation
of char residue.
Fig. 7. The dynamic FTIR spectra of EP at different pyrolysis temperatures.
pyrolysis products were identified using the software of the mass
spectrometer or by comparison with mass spectra in the literature.
X-ray photoelectron spectroscopy (XPS) spectra of the char residue
was recorded with a VG Escalab Mark II spectrometer (VG Scientific
Ltd, UK), using Al
Mechanical properties of flame-retardant EP were tested with
WSM-20KB universal testing machine (Changchun, China)
K
a
excitation radiation (h
y
¼ 1253.6 eV).
a
according to GB/T1040-92.
3. Results and discussion
3.1. Flame-retardant properties and thermal stability of EP
and EP/PFR hybrids
The LOI values of EP and EP/PFR hybrids are given in Table 1.
Table 1 presents that a higher LOI value is obtained with the
increase in phosphorus content. LOI increases drastically from 20.5
to 30.2 (meets the flame-retardant criterion) when PFR content is
increased from 0 to 10 wt %. The results indicate that PFR exhibits
good flame-retardant effect to epoxy resin.
The TG and DTG curves of PFR, EP and EP/PFR hybrids in air
atmosphere are shown in Fig. 1a and b. The onset degradation
temperature (Td) of samples which is evaluated by the temperature
of 5 wt% weight loss (Tꢁ5%), the mid-point temperature of the
degradation (Tꢁ50%), and the solid residue at 700 ꢀC are obtained
from the TG curves; the temperature of the maximum weight loss
rate (Tmax) of samples is obtained from the DTG curves. These data
are listed in Table 2.
As can be seen from Fig. 1, PFR has one stage of weight loss, and
accordingly there is only one differential thermogravimetric (DTG)
peak. PFR begins to degrade at 286 ꢀC, and the maximum mass loss
rate occurs at about 384 ꢀC. The stable solid residue at 700 ꢀC is
about 42.6 wt%. The Td of PFR is lower than that of pure epoxy resin,
which may be attributed to the fact that the O]PeO bond in PFR is
less stable than the common CeC bond. Therefore, the Td of EP/PFR
hybrids trends to decrease gradually with the increase of the PFR
addition.
As for pure EP, the thermal oxidative degradation process has
three stages. The first stage is in the temperature range of
180e410 ꢀC corresponding to a strong DTG peak at 365 ꢀC (Tmax1
)
and the weight loss is 47.9%. These indicate that some complex
Fig. 8. The dynamic FTIR spectra of EP3 at different pyrolysis temperatures.
chemical reactions take place at the first stage of thermal oxidative

2440
X. Wang et al. / Polymer 51 (2010) 2435e2445
In Fig. 2, peaks in the regions of around 3600e3700 cmꢁ1
around 2750e3150 cmꢁ1 around 2250e2400 cmꢁ1
around
,
,
,
1400e1650 cmꢁ1, and around 1100e1300 cmꢁ1 are noted. Some of
the gaseous decomposition products of the EP materials were
unambiguously identified by characteristic strong FTIR signals,
such as methane (3015 cmꢁ1), and water (3650 cmꢁ1) [25]. The
main share of the bands of the decomposition products is attrib-
uted to the functional groups with characteristic, unambiguous
band positions: compounds containing aromatic ring (1607,
1510 cmꢁ1
)
and phenol (746 cmꢁ1), and methyl-substituted
compounds (2930 cmꢁ1) [25e27].
FTIR spectra of pyrolysis products of EP at maximum decom-
position rates are shown in Fig. 4. The main products of the thermal
decomposition of EP are compounds containing eOH (such as H2O,
phenol; 3500e3600 cmꢁ1), CO2 (2360 cmꢁ1), aromatic ethers
(1260 cmꢁ1), hydrocarbons (CeH stretching at 1172 cmꢁ1),
compounds containing aromatic rings (1607, 1510 cmꢁ1), etc
[25e27]. It is well-known that depolymerization is the main
process associated with the thermal degradation of polymers. In
the process of depolymerization, the main decomposition products
are CO2, phenol, and hydrocarbons, etc.
Fig. 9. Total ion current (TIC) curve of the decomposition process of EP3.
Fig. 3 shows the 3D TG-FTIR spectrum of gas phase in the
thermal degradation of EP3. As shown in Fig. 5, the evolved gas
analysis for EP3 at maximum decomposition rates exhibited char-
acteristic bands of H2O and/or phenol (3650 cmꢁ1), CO2
(2362 cmꢁ1), hydrocarbons (eCH3 and eCH2e groups: 2950e2850
and 1100e1300 cmꢁ1) and compounds containing aromatic rings
(1607, 1510 cmꢁ1) [25e27] which is similar to that of EP. In fact, the
new absorption band at 924 cmꢁ1 appears which is due to the
structures containing ePeOePh. Moreover, it is probably that other
new absorption bands, such as 1260 cmꢁ1 (P]O), 1118 cmꢁ1
(ePeOePeO), almost coincides with the characteristic peaks of the
EP matrix. It can be concluded that the polyphosphate structures
are formed by decomposition of PFR. These polyphosphate struc-
tures can react with other pyrolysis products (such as phenol or
bisphenol) to form the PeOePh bond.
The absorbance of pyrolysis products for EP and EP3 vs time is
revealed in Fig. 6. It can be seen that the pyrolysis products for EP3
begin to release at about 15 min, whereas those for EP begins to
release at about 23 min. It can be interpreted that PFR can catalyze
the thermal decomposition of EP. However, the absorbance inten-
sity of pyrolysis products for EP3 is lower than that for EP, especially
hydrocarbons. Consequently, the addition of PFR can reduce the
release of combustible gas and the weight loss. The results of the
pyrolysis products release correspond well to thermal analysis and
are explained by the proposed decomposition models discussed
below.
3.2. Mechanical properties
The data for the tensile strength, and failure strain of the
unmodified and PFR-modified epoxy resins were obtained from the
computer controlled universal tensile testing machine. The results
for the mechanical properties are summarized in Table 3. It can be
seen that the tensile strength strain can be slightly enhanced by the
addition of PFR with appropriate contents (2.5 wt%). The incorpo-
ration of PFR gives reinforcement impact on the EP firstly. Never-
theless, the tensile strength of EP/PFR hybrids begins to decrease at
higher PFR load. As is known, the phosphorus-containing flame
retardant at high content usually cause a negative impact on the
mechanical properties of the polymer matrix. This phenomenon is
also reported by earlier literatures [1,24]. Moreover, the PFR-
modified epoxy resins exhibit relatively ductile behaviors
compared to the unmodified ones.
3.3. TG-FTIR analysis of EP and EP/PFR hybrids
TG-FTIR was used to analyze the gas products during the
thermal degradation. The 3D TG-FTIR spectra of gas phase in
the thermal degradation of EP and EP3 are shown in Figs. 2 and 3.
3.4. Thermal degradation of EP and EP/PFR hybrids
Dynamic FTIR was employed to evaluate the solid pyrolysis
products of EP and EP3. The FTIR spectra of EP at different degra-
dation temperatures are shown in Fig. 7. The assignment of
dynamic FTIR spectra of EP and EP3 is presented in Table 4. It can be
seen thaꢁt1peaks at 3392, 2930, 1607, 1513, 1458, 1240, 1177 and
1035 cm are the characteristic absorptions of EP. The band at
3392 cmꢁ1 nearly disappears at the temperature of 250 ꢀC, and this
can be explained by the release of water. The relative intensities of
other characteristic peaks do not change below 350 ꢀC. While the
temperature rises up to 380 ꢀC, it’s found that the absorption peaks
at 2930, 1607, 1458, 1240 and 1035 cmꢁ1 disappear, indicating that
the main decomposition happens at this stage. This is consistent
with the TGA results.
The FTIR spectra of the EP3 at different degradation tempera-
tures are shown in Fig. 8. The FTIR spectrum of EP3 shows very
Fig. 10. EI-MS spectra of compounds evolved from EP3 at different temperatures.

X. Wang et al. / Polymer 51 (2010) 2435e2445
2441
Table 5
Structural assignments in the DP-MS of EP3.
m/z
Structure
m/z
Structure
CH3
C
OH
O
P O
324
135
CH3
HO
OH
M135
M324
CH3
C
CH2OH
CH2
CH2OH
M119-2
CH3
HOCH2
C
HO
C
O CH2 CH2
254
244
119
107
CH3
CH3
M254
M119-1
O
O
O O
H2C
OH
H
P
P OH
O
O
M244
M107
O
OH
O
C
HO
P OH
HO
OH
O
P
O
231
99
OH2
M99
CH3
OH
M231-1
M231-2
CH3
HO
C
OH
OH
H2C
228
213
91
82
CH3
M91
M228
O
HO
C
HO
P
OH
CH3
M213
H
M82
CH3
C
H
CH3
197
H
H
77
M77
M197
168
153
65
51
O
M65
M168
M51
M153
(continued on next page)

2442
Table 5 (continued)
X. Wang et al. / Polymer 51 (2010) 2435e2445
m/z
Structure
m/z
Structure
CH2OH
HOCH2
C
CH2OH2
137
39
CH2OH
M137
M39
Fig. 11. Oversimplified mass fragmentations of EP3.

X. Wang et al. / Polymer 51 (2010) 2435e2445
2443
Fig. 12. Schematic outline of the degradation process of EP/PFR hybrids.
similar features to that of EP. In this figure, the characteristic peaks
3.5. DP-MS analysis of EP and EP/PFR hybrids
of PFR are not obvious because its characteristic bands probably
coincide with the characteristic bands of the EP matrix. It is worth
noting that at 350 ꢀC the intensities of most peaks decrease
sharply, which means that the presence of PFR catalyzes the
thermal degradation of EP and formation of char. The residual char
can prevent the materials from further degradation during
combustion. This conclusion is also suggested by the TG-FTIR
analysis above.
Fig. 9 shows the total ion current (TIC) chromatogram of EP3 by
DP-MS. It can be observed that the EP3 has twoTIC peaks at 358 and
538 ꢀC. The mass spectra corresponding to the TIC peaks at
different temperatures are presented in Fig. 10 and the products
identified are listed in Table 5 [28].
The first peak at around 358 ꢀC in the mass spectrum is mainly
attributed to the decomposition products of PFR, such as 324m/z for
Fig. 13. (a) C1s, (b) O1s, (c) N1s and (d) P2p XPS spectra of the char residue of EP.

2444
X. Wang et al. / Polymer 51 (2010) 2435e2445
Fig. 14. (a) C1s, (b) O1s, (c) N1s and (d) P2p XPS spectra of the char residue of EP3.
C18H13O4P and 224m/z for C5H10O7P2. And the peaks at 99m/z and
82m/z are due to the degradation of phosphate. In the mass spec-
trum at 538 ꢀC, two new strong peaks at 119m/z corresponding to
the mixture ions of M119-1 and M119-2, and 213m/z corresponding
to M213 appear. And the peaks at 254m/z for M254, 228m/z for
M228, and135m/z for M135 can be assigned to the fragment ions
that mainly resulted from the degradation of epoxy resin matrix.
The oversimplified fragment of EP3 is shown in Fig. 11 with the
data from the mass fragmentations of the key TIC ions shown
collectively in Fig. 10. Associating with the former paragraph as has
been discussed, it can be found that the degradation of EP3 can be
divided into two steps. From 300 to 400 ꢀC, the degradation is
mainly attributed to phosphate groups and mostly carbonyl group
in the PFR. When raising temperature over 450 ꢀC, alkyl chain and
some aromatic molecules of epoxy resin matrix are decomposed.
Therefore, on the basis of the results for volatile pyrolysis
products and residue pyrolysis products, a possible route of the
degradation process of EP/PFR hybrids, as shown in Fig. 12 [29], has
been postulated and summarized for the main decomposition
pathway for EP3.
spectrum has two peaks at around 531.8 and 532.7 eV. It is reported
that the peak at 531.8 eV can be attributed to the ]O in phosphate
or carbonyl groups and the peak centered at 532.7 eV is assigned to
eOe in CeOeC, CeOeP, and/or CeOH groups [26]. For the N1s
spectra, the peak at 399.1 eV can be assigned the nitrogen in imide
groups formed by the condensation of low molecular polyamide
curing agent at higher temperature. The peak at 400.5 eV corre-
sponds to formation of some oxidized nitrogen compounds. The
single peak between 134 and 135 eV in P2p spectrum can be
assigned to the pyrophosphate and/or polyphosphate [26,31].
From Table 6, it can be found that the addition of PFR influences
the flame retardancy and thermal degradation of the composites
greatly. The atom percent of carbon in the char residues of EP and
EP3 is 50.91% and 76.94%, respectively. And the atom percent of
oxygen in the char residues of EP3 is lower than that in the char
residue of EP. These mean that the incorporation of PFR into EP can
Table 6
XPS results of the residual char of EP and EP3.
System
Binding energy (eV)
Atom (%)
3.6. Chemical components of the residual char of EP and EP/PFR
hybrids
EP
C1s
O1s
N1s
P2p
284.7, 286.0, 287.2
531.8, 532.7
399.1, 400.5
134.7
50.91
38.79
2.81
The chemical components of the residual char for EP and EP3
(heated in muffle furnace for 10 min at 600 ꢀC) were investigated
by XPS. As shown in Figs. 13 and 14, three bands are observed from
C1s spectra: the peak at around 284.7 eV is attributed to CeH and
CeC in aliphatic and aromatic species, the peak at around 286.0 eV
is assigned to CeO (ether and/or hydroxyl group), and another peak
at around 287.2 eV corresponded to carbonyl groups [30]. O1s
0.05
EP3
C1s
O1s
N1s
P2p
284.7, 286.0, 287.2
531.8, 532.8
399.1, 400.5
133.8
76.94
19.23
2.76
1.07

X. Wang et al. / Polymer 51 (2010) 2435e2445
2445
premote the formation of char residues and enhance the thermal
oxidative resistance of the EP matrix. The effect of PFR on
combustion behaviors could be summarized as follows: under the
high temperature, PFR can increase the char yield remarkably and
improve the thermal oxidative stability of the char layer. The char
layer builds up on the surface of the burning polymer, insulates the
heat transformation and the dispersion of oxygen into underlying
polymeric substrate, and prevents the release of volatile products of
underlying polymeric substrate from the matrix. Thus, the
combustion is retarded.
Laboratory of Environmental Adaptability for Industrial Product,
and the Program for the graduate innovation fund in University of
Science and Technology of China.
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4. Conclusions
A series of epoxy resin/PFR hybrids containing 0 wt%, 2.5 wt%,
7.5 wt% and 10 wt% content of PFR were prepared. The high phos-
phorus content and rich aryl group structures of PFR contribute an
excellent flame retardancy to epoxy resins without a considerable
decrease of the mechanical properties. EP/PFR hybrids possess
lower initial decomposition temperatures and higher char residues
than pure EP. The degradation of EP/PFR hybrids can be divided into
two main steps. The first stage can be assigned to the decomposi-
tion of phosphate and carbonyl group in the PFR, whereas the
second is due to the thermal pyrolysis of alkyl chains and some
aromatic structures. The degradation products of PFR, including
pentaerythritol (carbon agent), phosphorus acid and phosphoric
acid or poly(phosphoric acid) (acid source), associate with amine
curing agent (blowing agent) to constitute a intumescent flame-
retardant system. Thus, the intumescent char layer is formed to
protect the inner polymer matrix.
Acknowledgements
The work was financially supported by the Program for
Specialized Research Fund for the Doctoral Program of Higher
Education (200803580008), the National Natural Science Founda-
tion of China (no. 50903080) and the Program for Science and
Technology of SuZhou (SG-0841), the Opening Project of State Key