Thermal degradation mechanism and flame retardancy of epoxy systems containing tris(3-nitrophenyl) phosphine

Article abstract of DOI:10.1007/s10973-018-7081-6

The aim of this work was to study the effect of tris(3-nitrophenyl) phosphine (NPPh3), which showed a good thermal stability and carbon-forming ability, on the flame retardancy and thermal degradation mechanism of epoxy resins. A series of diglycidyl ether of bisphenol A (DGEBA) loaded with tris(3-nitrophenyl) phosphine (NPPh3) were prepared. It was found that NPPh3 can effectively improve the flame retardancy and thermal stability of the composites. When the loading amount of NPPh3 was 14%, the LOI value of the DGEBA composites was 29.2% (about 1.53 times the corresponding value of the original DGEBA resin). Thermal stability was studied by thermogravimetric analysis, and the results showed that the addition of NPPh3 can improve char formation of this system both in nitrogen and in air atmosphere. Specifically, its combustion residue at 800?°C in nitrogen atmosphere was about 4.26 times of the original resin. Differential scanning calorimetry indicated that NPPh3 slightly decreased the glass transition temperature of epoxy resins. Additionally, the gaseous degradation products were analyzed by thermogravimetric analysis/infrared spectrometry, providing insight into the thermal degradation mechanism. Scanning electron microscopy and Fourier transform infrared were brought together to evaluate the morphology and structure of the residual char obtained after combustion.

Full text of DOI:10.1007/s10973-018-7081-6

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-018-7081-6(0123456789(). -, vo Vl
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Thermal degradation mechanism and flame retardancy of epoxy
systems containing tris(3-nitrophenyl) phosphine
Hang Luo1 Yunyun Yang1 Xilei Cao1 Xufu Cai1
•
•
•
Received: 10 April 2017 / Accepted: 14 February 2018
´
´
Ó Akademiai Kiado, Budapest, Hungary 2018
Abstract
The aim of this work was to study the effect of tris(3-nitrophenyl) phosphine (NPPh3), which showed a good thermal
stability and carbon-forming ability, on the flame retardancy and thermal degradation mechanism of epoxy resins. A series
of diglycidyl ether of bisphenol A (DGEBA) loaded with tris(3-nitrophenyl) phosphine (NPPh3) were prepared. It was
found that NPPh3 can effectively improve the flame retardancy and thermal stability of the composites. When the loading
amount of NPPh3 was 14%, the LOI value of the DGEBA composites was 29.2% (about 1.53 times the corresponding
value of the original DGEBA resin). Thermal stability was studied by thermogravimetric analysis, and the results showed
that the addition of NPPh3 can improve char formation of this system both in nitrogen and in air atmosphere. Specifically,
its combustion residue at 800 °C in nitrogen atmosphere was about 4.26 times of the original resin. Differential scanning
calorimetry indicated that NPPh3 slightly decreased the glass transition temperature of epoxy resins. Additionally, the
gaseous degradation products were analyzed by thermogravimetric analysis/infrared spectrometry, providing insight into
the thermal degradation mechanism. Scanning electron microscopy and Fourier transform infrared were brought together to
evaluate the morphology and structure of the residual char obtained after combustion.
Keywords Flame retardant Á Epoxy resin Á NPPh3
Introduction
flame-retardant epoxy resins are the main ways to improve
the flame retardancy of the epoxy resins [9].
Epoxy resins (EP) have been extensively used as semi-
conductor encapsulation, structural adhesives, laminates,
and coatings owing to its excellent mechanical properties,
remarkable adhesion, outstanding corrosion resistance,
high intensity, and good electrical performance [1–4].
However, the flammability is one of the major drawbacks
of conventional epoxy resins, which restricts their further
applications for safety consideration [5–7]. Therefore, for
the sake of meeting application requirements, it is signifi-
cant to improve the flame retardancy of epoxy resins [8].
Nowadays, adding flame retardant to the epoxy systems,
preparing of flame-retardant curing agent, and building
Over the past decades, halogenated flame retardants
were used in epoxy resins to obtain fire-retardant materials.
However, they are subjected to strict regulations in many
fields because of releasing toxic gases in the process of
combustion [10]. Therefore, from the viewpoint of envi-
ronmental protection, halogen-free epoxy resins have
attracted increasing attention [11]. Among them, organic
phosphorus compounds are important class of flame
retardants with high-retardant efficiency due to its radical
scavenger mechanism [12, 13]. It is an outstanding method
to modify epoxy resins by using a flame-retardant additive
with phosphorus [14].
Triphenyl phosphine (PPh3), a common phosphorus-
containing compound, is the triphenyl substitution product
of the phosphine, used on a large scale for pharmaceutical
industry, organic synthesis, and dye process. From the
structural point of view, its aromatic group and the phos-
phine element are beneficial to the flame retardant [15].
Previous studies also show that it has the advantages of low
& Xufu Cai
caixf2008@scu.edu.cn
1
Department of Polymer Science and Materials, The State Key
Laboratory of Polymer Materials Engineering, Sichuan
University, Chengdu 610065, China
123

H. Luo et al.
smoke, non-toxic, and halogen-free, suggesting that it is an
excellent polymer flame retardant and in line with the
current development direction of the flame retardant [16].
So, PPh3 and phosphorus flame retardant based on PPh3
have been developed rapidly.
O2N
NO2
P
Tris(3-nitrophenyl) phosphine (NPPh3), the derivative
of PPh3, shows a better thermal stability and carbon-
forming ability in TG analysis compared with PPh3.
However, in the area of flame retardant, the researches and
applications are rare. In this work, NPPh3 is used as flame
retardant for the epoxy resins, and the effect of NPPh3 on
the thermal and flame retardancy properties of epoxy resins
is investigated.
O2N
Scheme 1 The structure of tris(3-nitrophenyl) phosphine (NPPh3)
Then, the mixture was put into DGEBA and dried to
eliminate ethanol in rotary evaporators. Subsequently, the
curing agent DETDA was added relative to the amount of
DGTDA. After that, the blend was poured into the prepared
mold for curing and postcuring at oven as the following
Experimental
Materials
procedure: 120–140–160 °C 2 h-1
.
Diglycidyl ether of bisphenol
A (DGEBA, epoxy
value = 0.44) was purchased from Wuxi Bluestar Chemi-
cal Co., Ltd., China. Triphenyl phosphine (PPh3),
diethyltoluenediamine (DETDA), sodium bicarbonate,
sulfuric acid, and nitric acid were supplied by Chengdu
Kelong Chemical Reagent Factory. Ethanol was obtained
from Chengdu Changlian Chemical Reagent Co., Ltd.,
China.
Characterization
Fourier transform infrared spectra
Fourier transform infrared spectra (FTIR) were obtained on
a Nicolet Magna-FTIR 560 spectrometer (Nicolet Instru-
ment Co, USA) with KBr pellets.
Synthesis of tris(3-nitrophenyl) phosphine
Nuclear magnetic resonance
1HNMR (600 MHz) spectrum was recorded on a FT-80A
NMR using CDCl3 as the solvent and internal standard.
NPPh3 was synthesized through the nitration reaction of
PPh3, using already reported method [17]. PPh3 (2.62 g,
0.01 mol) and sulfuric acid (20 g, 0.2 mol) were added to a
100-mL three-necked round-bottom flask equipped with a
thermometer, a stirrer, and a constant pressure dropping
funnel. PPh3 was dissolved in sulfuric acid with stirring at
room temperature. After that, the temperature of the mixed
system was reduced to below 10 °C with an ice-salt bath
and the mixed solution of sulfuric acid (30 g, 0.3 mol) and
nitric acid (6 g, 0.06 mol) was slowly added dropwise over
30 min [18]. Then, the reaction mixture was warmed to
room temperature and then stirred for 6 h. The product was
precipitated by pouring the reaction mixture over an excess
of ice-cold water, neutralizing with a saturated solution of
sodium bicarbonate washing with water and filtering.
Finally, 3.9 g light yellow product was obtained with the
yield of 89%. The structure of NPPh3 is shown in
Scheme 1.
Differential scanning calorimetry
The Q-200 (TA Instrument) was used for differential
scanning calorimetry (DSC) at 10 °C min-1; 8 mg of the
sample was examined under nitrogen atmosphere.
Scanning electron microscopy
Scanning electron microscopy (SEM) analyses were used
to study the morphologies of residues obtained from
combustion by UL-94 using a FEI Quanta-250 SEM.
Thermogravimetric analysis
The test of thermogravimetric analysis (TG) was conducted
on a Netzsch TG209 F1 with a heating rate of 10 °C min-1
from 30 to 800 °C under air and nitrogen atmosphere.
Preparation of the cured epoxy resins
In order to obtain the epoxy resins with uniform dispersion,
the NPPh3 was mixed with ethanol into a paste at first.
123

Thermal degradation mechanism and flame retardancy of epoxy systems containing…
Limiting oxygen index
the product was 1,3-substituted on the phenyl ring. The
information above confirmed that the target product was
synthesized successfully.
The limiting oxygen index (LOI) value was obtained on an
oxygen index instrument (XYC-75) produced by Chengde
Jinjian Analysis Instrument Factory according to
ASTMD2863-08 standard with a specimen dimension of
130 9 6.5 9 3.5 mm3.
1
To further determine the structure of NPPh3, HNMR
measurements were employed to characterize the product.
Figure 2 shows the 1HNMR spectrum of NPPh3(ppm):
8.52–8.55 ppm (a), 8.46–8.52 ppm (b), 8.15–8.24 ppm (c),
and 7.95–7.90 ppm (d). All these characteristics indeed
matched the NPPh3 structure.
Vertical burning tests
Vertical burning tests (UL-94) were measured by a CZF-2
instrument (Jiangning Analysis Instrument Company,
China) with sample dimension of 125 9 12.5 9 3 mm3
according to UL-94 ASTMD 635-77 standard.
Flammability tests of the cured epoxy resins
LOI, an important parameter for evaluating the ease of
extinguishment of polymeric materials in the same atmo-
sphere, was defined as the lowest volume concentration of
oxygen in an oxygen–nitrogen mixture [19]. The flame-
retardant properties of the cured DGEBA/DETDA and
DGEBA/NPPh3/DETDA systems were measured by LOI
and UL-94. Data are presented in Table 1. It can be seen
that the addition of NPPh3 was beneficial to improve the
flame retardancy of DGEBA. As can be observed, the pure
DGEBA was highly combustible. Its LOI value was only
19.0% and it was classed no rating (NR) in the vertical
burning (UL-94) test. However, the LOI value of the cured
DGEBA/NPPh3/DETDA was increased from 23 to 29.2%
(about 1.5 times of the LOI value of the DGEBA) with the
loading amount of NPPh3 increasing from 5 to 14%. When
the loadings of NPPh3 were 10, 12, and 14%, it passed the
UL94V-1 rating. The fact can be attributed to the prepared
NPPh3 which promoted the formation of char layer in the
condensed phase. The formed char layer can prevent
oxygen and heat from propagation to the underlying
polymer matrix during burning and protect the underlying
matrix from further combustion.
Thermogravimetric analysis/infrared spectrometry
Thermogravimetric analysis/infrared spectrometry (TG-
FTIR) was carried out on a Mettler Toledo TG/DSC 1
STARe System thermogravimeter couple with a Nicolet
IS10 FTIR spectrophotometer at a linear heating rate of
20 °C min-1 under air within 50–800 °C.
Results and discussion
Characterization of NPPh3
In order to confirm the molecular structure of NPPh3, FTIR
was carried out. Figure 1 shows the FTIR spectrum of
NPPh3. For NPPh3, the absorption peaks at
3100–3000 cm-1 belonged to benzene ring. The absorp-
tions at 1525 and 1346 cm-1 were assigned to NO2. The
peaks at 1469 cm-1 were associated with the vibrations of
P-Ph. The peaks at 879, 733, and 674 cm-1 indicated that
O2N
NO2
b
d
b
a
a
P
c
c
d
d
c
b
O2N
3000–3100
a
1469
733
879
674
1525
1346
4000
3500
3000
2500
2000
1500
1000
500
10
8
ppm
6
Wavenumber/cm–1
1
Fig. 1 The FTIR spectrum of NPPh3
Fig. 2 The HNMR spectrum of NPPh3
123

H. Luo et al.
Table 1 LOI value and UL-94
test results of epoxy resins
Sample
DGEBA/g DETDA/g NPPh3/g P-content/% LOI/% UL-94
DGEBA/DETDA
100
100
19.6
19.6
19.6
19.6
19.6
–
–
19.0
23.0
26.1
27.5
29.2
NR
NR
V-1
V-1
V-1
DGEBA/5%NPPh3/DETDA
5
0.31
0.60
0.71
0.82
DGEBA/10%NPPh3/DETDA 100
DGEBA/12%NPPh3/DETDA 100
DGEBA/14%NPPh3/DETDA 100
10
12
14
Exp was the amount of residual carbon obtained from the
experiment, and Cal was the theoretical value calculated by
the formula:
Thermal properties of the cured epoxy resins
TG was widely utilized to study the thermal stability and
thermal degradation behavior of polymers [11]. Figure 3
shows the TG and DTG curves of NPPh3, cured DGEBA/
DETDA, and DGEBA/NPPh3/DETDA systems under
nitrogen atmosphere. The relative detailed data are listed in
Table 2. The T5% was defined as the temperature at 5%
mass loss (the initial decomposition temperature), and
Ycal ¼ wtDGEBA=DETDA% Â YDGEBA=DETDAexp þ wtNPPh3
 YNPPh3exp
%
wt% referred to the corresponding proportion of ingredi-
ents, Yexp was the actual amount of carbon residue.
From the DTG curves revealed in Fig. 3, it can be
observed that the whole degradation process of NPPh3 had
one stage and the maximum decomposition rate was about
9.75% °C-1 at 378.3 °C. The T5% of the synthesized
NPPh3 flame-retardant additive was 291.4 °C, and the char
residue at 800 °C was 28.16%, revealing greatly charring
ability at high temperature due to the presence of abundant
aromatic structure and nitro group in NPPh3 [20].
T1max and T2max were defined as the temperature of the first
maximum mass loss rate and the second maximum mass
loss rate, respectively. Yc was defined as the char residue,
100
80
The DGEBA/DETDA started to decompose at 363.7 °C,
and the char yield was only 4.13% at 800 °C. The thermal
degradation course of the DGEBA/DETDA presented only
one stage and the peak appeared at 391.7 °C with the loss
mass rate of 18.01% °C-1 due to the char forming and the
decomposition of the DGEBA/DETDA [21].
60
40
NPPh3
In Fig. 3, all cured resins had single-stage decomposi-
tion process, demonstrating that the original and modified
DGEBA resins had similar degradation process. With the
adding of NPPh3 content, the T5% of DGEBA/NPPh3/
DETDA systems decreased. The value was decreased from
363.7 °C of pure epoxy resins to 300.4 °C of DGEBA/
14%NPPh3/DETDA. This phenomenon originated from
the decomposition of NPPh3 at an inferior temperature
(291.4 °C) due to the low bond energy of P–C [22].
Meanwhile, the Rpeak was decreased from 18.01% °C-1 for
the DGEBA/DETDA to 6.39% °C-1 when the contents of
NPPh3 were 14%. This may be attributed to the previously
formed char layer, protecting the underlying epoxy resins
matrix from further degradation [23]. It was worth noting
that the char yields at 800 °C remarkably increased from
4.13 to 17.60% with NPPh3 content increasing from 0 to
14%. And this phenomenon can be due to the formed char
which had high thermal stability, resulting the increasing of
the char residue at high temperature. Furthermore, as listed
in Table 2, the theoretical char residue was lower than the
experimental value at 800 °C. And the more the amount of
NPPh3 added, the larger the difference was. This also
DGEBA/DETDA
20
DGEBA/5% NPPh3/DETDA
DGEBA/10% NPPh3/DETDA
DGEBA/12% NPPh3/DETDA
DGEBA/14% NPPh3/DETDA
0
100
200
300
400
500
600
700
800
Temperature/°C
0
–10
–20
NPPh3
DGEBA/DETDA
DGEBA/5% NPPh3/DETDA
DGEBA/10% NPPh3/DETDA
DGEBA/12% NPPh3/DETDA
DGEBA/14% NPPh3/DETDA
100
200
300
400
500
600
700
800
Temperature/°C
Fig. 3 The TG and DTG curves of the cured systems under N2
atmosphere
123

Thermal degradation mechanism and flame retardancy of epoxy systems containing…
Table 2 The TG and DTG data
of different systems under N2
Sample
T5%/°C
Rpeak/%/°C
Tmax/°C
Yc at 800 °C/%
atmosphere
Cal
Exp
NPPh3
291.4
363.7
336.5
329.0
304.6
300.4
9.75
18.01
12.90
12.05
7.87
378.3
391.7
391.5
387.2
382.5
381.2
–
28.16
4.13
DGEBA/DETDA
–
DGEBA/5%NPPh3/DETDA
DGEBA/10%NPPh3/DETDA
DGEBA/12%NPPh3/DETDA
DGEBA/14%NPPh3/DETDA
5.27
6.31
6.70
7.08
9.44
10.01
12.41
17.60
6.39
showed that the addition of NPPh3 increased the mass of
residual carbon content significantly.
decomposition rate appears at 384.9, 538.6, and 708.5 °C
with the loss rate of 7.39, 0.68, and 1.14% °C-1. Initial
decomposition occurred when the temperature reached
319.5 °C, and the char yield was 36.48 and 13.62% when
the temperature was 500 and 800 °C.
TG and DTG curves of NPPh3, cured DGEBA/DETDA,
and DGEBA/NPPh3/DETDA systems in the temperature
range of 30–800 °C under an air atmosphere are shown in
Fig. 4, and the detailed data are summarized in Table 3.
As can be seen, the degradation of NPPh3 in atmosphere
was observed in three stages which occurred at the tem-
perature ranges of 240–460 °C, 460–600 °C, and
600–800 °C, and the peak of maximum thermal
Compared with only one decomposition stage taken
place in nitrogen atmosphere, the DGEBA/DETDA and
DGEBA/NPPh3/DETDA systems had two decomposition
stages existed in air atmosphere. The first stage was the
thermal degradation of the polymer network, and the sec-
ond stage was due to the char formed in the early degra-
dation that behaved unstable thermal stability and
reoxidized [24]. Figure 4 shows that in the presence of
increasing amount of NPPh3, the values of T5%, T1max, and
R1peak were decreased. This trend was the same as that
obtained in the nitrogen atmosphere, suggesting that the
incorporation of flame retardants affected the decomposi-
tion process of epoxy resins matrix. In addition, the value
of T2max increased with the addition of NPPh3 content and
R2peak except for DGEBA/14%NPPh3/DETDA was
decreased. This may be due to the physical barrier of the
oxygen diffusion caused by the accumulation of NPPh3
moieties on the surface of the residue, which reduced the
amount of oxygen available for the oxidation of underlying
char and thus delayed the oxidation process as a whole. As
shown in Fig. 4 and Table 3, char yields at 500 °C of the
flame-retardant thermosets increased with the increasing
content of NPPh3. When the amount of NPPh3 increased
from 0 to 14%, the char residue increased from 18.14 to
32.54%, and the experimental residual carbon content at
500 °C was higher than the theoretical value. However, the
residual carbon content at 800 °C was decreased with the
addition of NPPh3. Meanwhile, the experimental value
obtained at this time was lower than the theoretical value.
This phenomenon suggested that the addition of NPPh3 did
promote the formation of the carbon layer before 500 °C.
With the continuous increasing of temperature, the addition
of NPPh3 had a certain influence on the decomposition of
carbon layer [9, 25].
100
80
60
40
NPPh3
DGEBA/DETDA
20
DGEBA/5% NPPh3/DETDA
DGEBA/10% NPPh3/DETDA
DGEBA/12% NPPh3/DETDA
DGEBA/14% NPPh3/DETDA
0
100
200
300
400
500
600
700
800
Temperature/°C
0
–10
–20
NPPh3
DGEBA/DETDA
DGEBA/5% NPPh3/DETDA
DGEBA/10% NPPh3/DETDA
DGEBA/12% NPPh3/DETDA
DGEBA/14% NPPh3/DETDA
100
200
300
400
500
600
700
800
Temperature/°C
Figure 5 gives DSC curves of DGEBA/DETDA and
DGEBA/14%NPPh3/DETDA. They exhibited single glass
transition temperature (Tg). It was well known that an
Fig. 4 The TG and DTG curves of the cured systems under air
atmosphere
123

H. Luo et al.
Table 3 The TG and DTG data of different systems under air atmosphere
Sample
T5%/°C
R1peak/% °C-1
T1max/°C
R2peak/% °C-1
T2max/°C
Yc/%
550 °C
Cal
800 °C
Exp
Cal
Exp
NPPh3
319.5
333.0
307.6
298.3
297.7
296.6
7.39
9.68
6.40
5.25
5.04
4.72
384.9
396.7
386.0
381.1
383.1
376.9
1.14
2.87
2.50
2.31
2.68
3.05
708.5
550.3
569.1
586.9
582.7
582.3
–
36.48
18.14
22.77
26.34
27.8
–
13.62
1.94
1.53
1.21
1.3
DGEBA/DETDA
–
–
DGEBA/5%NPPh3/DETDA
DGEBA/10%NPPh3/DETDA
DGEBA/12%NPPh3/DETDA
DGEBA/14%NPPh3/DETDA
19.10
19.80
20.11
20.39
2.50
3.00
3.19
3.37
32.54
1.07
14%NPPh3/DETDA are shown in Figs. 6 and 7. As shown
in Fig. 6, the addition of NPPh3 did not affect the type of
volatile products. The DGEBA/14%NPPh3/DETDA
released the similar decomposition products to that of
DGEBA/DETDA. The major pyrolytic gases were water
(3650 cm-1), aromatic components (2969, 1604, 1509, and
699 cm-1) [26], CO2 (2356 and 2310 cm-1), CO (2181
and 2107 cm-1) [27], and ester/ether components (1747,
1258, and 1177 cm-1).
140.73 °C
From Fig. 7, it can be seen that the pyrolysis products
from DGEBA/14%NPPh3/DETDA released earlier than
DGEBA/DETDA, which had no pyrolysis product below
300 °C. This can be interpreted that NPPh3 catalyzed the
thermal decomposition of epoxy resins [2]. In the tem-
perature range of 300–500 °C, for DGEBA/DETDA, its
peaks strength of CO2 and aromatic components firstly
increased with increasing temperature. Then, the strength
124.54 °C
DGEBA/DETDA
DGEBA/14% NPPh3/DETDA
30
60
90
120
150
180
Temperature/°C
Fig. 5 DSC curves of DGEBA/DETDA and DGEBA/14%NPPh3/
DETDA
DGEBA/DETDA
DGEBA/14% NPPh3/DETDA
absolutely miscible blend presents only one Tg peak. This
single Tg indicated a homogeneous morphology of these
systems. Moreover, DGEBA/14%NPPh3/DETDA com-
posites exhibited lower Tg value than DGEBA/DETDA.
This phenomenon can be partially explained by two pos-
sible factors, which were the crosslink density of epoxy
resins decreased due to the addition of NPPh3 and the
presence of NPPh3 particles in epoxy resins loosened the
cohesive forces between the epoxy molecules.
2356
2310
699
1177
1509
1747
1604
1258
2969
3650
2181
2107
Pyrolysis products analysis of DGEBA/DETDA
and DGEBA/14%NPPh3/DETDA
In order to further investigate the thermal degradation
mechanism, the pyrolysis products during thermal
decomposition were analyzed by TG-FTIR in an air
atmosphere. FTIR spectra of the volatilized products at
maximum decomposition rate and 3D TG-FTIR spectra of
the pyrolysis products of DGEBA/DETDA and DGEBA/
4000
3500
3000
2500
2000
1500
1000
Wavenumber/cm–1
Fig. 6 The TG-FTIR spectra of gas phase in the thermal degradation
of DGEBA/DETDA and DGEBA/14%NPPh3/DETDA at maximum
decomposition rates
123

Thermal degradation mechanism and flame retardancy of epoxy systems containing…
decomposition, and a large amount of phosphorus com-
(a)
pound stayed in condensed phase [19].
Investigation of the char formation
Morphologies of the residual chars
Figure 8 presents SEM micrographs of char residues of the
cured DGEBA/DETDA and DGEBA/14%NPPh3/DETDA
collected from vertical burning tests. It was obvious that
for the cured DGEBA/DETDA, there were many small
crevasses and holes on the surface due to insufficient char
formation during the burning process. This char cannot
effectively protect DGEBA from heat and flammable
volatiles. However, with the incorporation of NPPh3, the
char residues of DGEBA/14%NPPh3/DETDA exhibited a
continuous, compact, and strong structure. The above
phenomenon suggested that NPPh3 can catalyze the for-
mation of a dense char layer, which contributed to the
improvement in the flame-retardant properties of the epoxy
resins.
(b)
FTIR study of the residual chars
In order to further explore the formation of condensed
phase, the solid products of cured DGEBA/DETDA and
DGEBA/14%NPPh3/DETDA collected after UL-94 test
were measured by FTIR, and the spectra are shown in
Fig. 9. The absorbance peaks at 3444, 2800–3000, 1627,
and 878 cm-1 appeared simultaneously in both spectro-
grams. The broad peaks around 3444 cm-1 were N–H and
O–H bonds, which is attributed to the ammonium com-
pounds and the pyrolysis products of the hydroxyl com-
pounds [29]. The absorption peaks appearing at
2800–3000, 1627, and 878 cm-1 can be assigned to aro-
matic compounds [30]. Besides, new absorbance peaks at
1508, 1300, 1238, 1176, and 1030 cm-1 appeared in the
FTIR spectrum of DGEBA/14%NPPh3/DETDA. The
absorbance peaks at 1508 and 1300 cm-1 indicated the
existence of N–O bond in the char residue, which meant
the nitro group did not completely decompose after burn-
ing. It was worth noting that the evident absorbance peaks
of P=O and P–O can be discerned at 1238 cm-1 and 1176
and 1030 cm-1, suggesting that the char layer structure
contained polyphosphoric acid and also proved the flame-
retarded effect of NPPh3 in condensed phase. The FTIR
results coincided with the TG-FTIR tests discussed above.
Fig. 7 The 3D TG-FTIR spectra of pyrolysis products for a DGEBA/
DETDA and b DGEBA/14%NPPh3/DETDA
decreased gradually after the maximum degradation rate.
The intensity of this area was stronger than that of
DGEBA/14%NPPh3/DETDA, which meant that the rate of
chain scission of DGEBA/DETDA in 300–500 °C was
higher.
Compared with DGEBA/DETDA, the peaks strength of
CO2 and aromatic components of DGEBA/14%NPPh3/
DETDA were higher in the temperature ranged from 500 to
800 °C, especially in the temperature after 650 °C. This
stage was assumed as the oxidation process of the char
moiety [28]. It was clear that the addition of NPPh3 made
the oxidation of the carbon char intensified. It was worth
noting that there was no characteristic peak of phosphorus-
containing fragment in the FTIR spectra of volatilized
products of DGEBA/14%NPPh3/DETDA volatiles, which
meant that the interaction between NPPh3 and epoxy resins
occurred in the condensed phase in the time of thermal
123

H. Luo et al.
Fig. 8 SEM of the char layer of a DGEBA/DETDA and b DGEBA/14%NPPh3/DETDA
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