Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl) fluorine and their reinforcements on the fire safety of polypropylene

Article abstract of DOI:10.1007/s10973-017-6729-y

Fabricating an effective synergism to improve the flame-retardant (FR) efficiency is recently considered to be a very promising way to prepare high-efficient FRs system. A novel FRs containing phosphorus and (4-hydroxyphenyl) fluorene have been synthesized and characterized by FTIR, ¹H NMR and MS. The FRs were then incorporated into polypropylene (PP) in different ratios, and then, ammonium polyphosphate (APP) was added to the FRs/PP system, with subsequent investigation into the synergistic effects between FRs and APP. Limited oxygen index (LOI), UL-94 test and cone calorimeter test were adopted to investigate the FR properties of the flame-retardant PP composites. The LOI value is as high as 31.0%, when FRs/APP is 2/3. Moreover, due to the synergistic FR effects between FRs and APP, the pHRR of PP7 decreases from 783?kW?m^?2 (PP0) to 110?kW?m^?2 (PP7), which is 85% lower than those of PP. Meanwhile, thermogravimetric analysis (TG) was used to study the thermal degradation characteristics of the PP composites. The char residues increased from 0.02 mass% to 15.85 mass% at 650?°C when FRs/APP is 3:2, indicating that there are synergistic flame-retarding effects between FRs and APP. Generally, the stable char layers of FR/APP/PP composites are responsible for the improved FRs properties, and the char layers could not only effectively prevent the release of combustion gases from releasing but also hinder the propagation of oxygen and heat into the interior substrate.

Full text of DOI:10.1007/s10973-017-6729-y

J Therm Anal Calorim
DOI 10.1007/s10973-017-6729-y
Synthesis of novel phosphorus-based flame retardants containing
9,9-bis (4-hydroxyphenyl) fluorine and their reinforcements
on the fire safety of polypropylene
Xiaodong Qian1 Nan Guo1 Yanhua Zhao2 Lingang Lu1 Huiya Wang1
•
•
•
•
•
Xuebao Wang1 Jing Jin1 Gaosong Shao1 Zhijia Hu3
•
•
•
Received: 7 May 2017 / Accepted: 21 September 2017
´
´
Ó Akademiai Kiado, Budapest, Hungary 2017
Abstract Fabricating an effective synergism to improve
the flame-retardant (FR) efficiency is recently considered
to be a very promising way to prepare high-efficient FRs
system. A novel FRs containing phosphorus and (4-hy-
droxyphenyl) fluorene have been synthesized and charac-
are responsible for the improved FRs properties, and the
char layers could not only effectively prevent the release of
combustion gases from releasing but also hinder the
propagation of oxygen and heat into the interior substrate.
1
terized by FTIR, H NMR and MS. The FRs were then
Keywords Polypropylene Á Flame retardants Á
Phosphorus Á Fire safety
incorporated into polypropylene (PP) in different ratios,
and then, ammonium polyphosphate (APP) was added to
the FRs/PP system, with subsequent investigation into the
synergistic effects between FRs and APP. Limited oxygen
index (LOI), UL-94 test and cone calorimeter test were
adopted to investigate the FR properties of the flame-re-
tardant PP composites. The LOI value is as high as 31.0%,
when FRs/APP is 2/3. Moreover, due to the synergistic FR
effects between FRs and APP, the pHRR of PP7 decreases
from 783 kW m-2 (PP0) to 110 kW m-2 (PP7), which is
85% lower than those of PP. Meanwhile, thermogravi-
metric analysis (TG) was used to study the thermal
degradation characteristics of the PP composites. The char
residues increased from 0.02 mass% to 15.85 mass% at
650 °C when FRs/APP is 3:2, indicating that there are
synergistic flame-retarding effects between FRs and APP.
Generally, the stable char layers of FR/APP/PP composites
Introduction
As one of the significant general commodities of polymers,
polypropylene (PP) has broad applications in nonwoven
fabrics, coverings, containers, automobiles, construction,
electronic and electric industry due to its outstanding
mechanical properties, low density and good chemical
resistance [1–4]. However, its limiting oxygen index (LOI)
value is only 17.8% and the characteristic of easy com-
bustibility of pure PP limits its further application in some
aspects at which required high flame retardancy [5, 6]. As a
result, the improvement in flame-retardant properties of PP
materials has become an urgent and attractive topic all over
the world. Among the various many ways of flame retar-
dation of PP, halogenated fire retardants is considered to be
the most efficient, but those halogen flame retardants also
bring a number of problems, taking raising the volume of
smoke and toxic gas for example.
& Lingang Lu
wjxylulingang@sohu.com
Among the various halogen-free flame retardants,
phosphorus-based flame retardants have been a hot spot
and are considered as green environmental friendly flame
retardants [7–9]. Flame-retarded materials by phosphorus-
based flame retardants are essentially a special case of the
condensed phase flame-retardant mechanism. The phos-
phorus-based flame retardants exhibit flame retardation by
forming condensed char layer on the surface of polymer.
1
Chinese People’s Armed Police Force Academy, Xichang
Road 220, Langfang City 065000, Hebei Province, People’s
Republic of China
2
People’s Public Security University of China,
Beijing City 10000, People’s Republic of China
3
School of Instrument Science and Opto-Electronics
Engineering, Hefei University of Technology, Hefei 230009,
People’s Republic of China
123

X. Qian et al.
Synthesis of the flame retardants (FRs)
The stable char layers could protect the underlying polymer
matrix from further degradation. Therefore, the flame-re-
tardant efficiency of phosphorus-based flame retardants
mainly depends on the physical barrier effect of the char
layer [10, 11]. However, the flame-retardant efficiency and
the thermal stability of phosphorus-based flame retardants
are not high. To overcome this disadvantage, many
researchers have made great effect to develop novel
phosphorus-based flame retardants with high thermal sta-
bility and high flame-retardant efficiency.
In a 250-mL three-necked flask with a flux condenser,
thermometer, a mechanical stirrer and dropping funnel,
neopentyl glycol (20.8 g, 0.2 mol) and chloroform
(100 mL) were mixed. After the solids were dissolved
completely in ice-water bath, 39.8 g (0.26 mol) of phos-
phorus oxychloride was then slowly dropped into the above
reaction vessel within 2 h, and then, the temperature of the
mixtures was increased to refluxing temperature for 4 h.
After that, the solvent was removed by a rotary evaporator;
then, the white powders were obtained (intermediate II)
[17]. The schematic processes for the reactions are pre-
sented in Scheme 1(a).
Among the various phosphorus-based flame retardants,
phosphate ester-based flame retardants have attracted much
attention due to its special structural design [12, 13].
Compared with the aliphatic phosphate ester, the cyclic
phosphate ester has high thermal stability and excellent
charring properties, which is an important object of the
organic phosphorous flame-retardants research [11, 14].
Moreover, aromatic phosphate ester-based flame retardants
belong to polyaromatic ring compounds, and the own rigid
groups could impart the flame retardants with good
chemical stability and thermal stability. As far as we
concern, bisphenol fluorene has been widely used to
improve the performance of polymers such as thermal
stability and mechanical properties, but flame retardants
based on bisphenol fluorine are rarely reported [15, 16].
Thus, a new idea has arisen that whether the cyclic phos-
phate ester and the bisphenol fluorene can be combined in
one molecule as a novel thermal stable flame retardants.
Therefore, by considering all those aspects at present
work, we prepared a novel kind of phosphorus-based flame
retardants with cyclic phosphate ester and bisphenol fluo-
rene. Then, the novel synthetic flame retardant was used to
flame retard PP together with APP at the loading of
30 mass%, and the flame-retardant properties, thermal
degradation behavior and combustion behavior of the
composites were characterized and discussed.
The intermediate II 73.8 g (0.4 mol) and acetonitrile
(300 mL) were added into a three-necked flask with an ice
bath equipped with a mechanical stirrer, flux condenser,
dropping funnel and drying tube with calcium chloride.
Then, 35.0 g of 9,9-bis (4-hydroxyphenyl) fluorene
(0.1 mol) was added to the mixtures and the mixtures were
kept stirring in the ice-water bath. Triethylamine (1.5 mL)
was then slowly dropped into the above reactants, and the
temperatures of the mixtures were increased to 60 °C for
another 10 h. After that, the acetonitrile was removed by
rotary evaporator and the white products were obtained.
The white products were washed by distilled water and
further recrystallized from distilled water and ethyl alcohol
for two times. Then, the white products were obtained
(49.46 g, 76.6%). The schematic processes of the reaction
are presented in Scheme 1(b).
1H NMR: 7.742–6.662 ppm, –C6H2– in the benzene
ring; 4.252–3.957 ppm, OCH2; 1.325, 0.918, 1.325 and
0.918 ppm –CH3.
MS: [M ? K]? = 687.4; thus, the molecular weight of
the FRs is 648.4.
Sample preparation
PP, APP and FRs were dried in vacuum at 80 °C overnight
before use. According to Table 1, PP and FRs were put into
the universal mill to mix evenly. The temperature of pre-
heating open-type double-roller rubber mixing machine
was increased to 170 °C, and then, PP and flame retardants
were added to the preheating open-type double-roller rub-
ber mixing machine for mixing. The PP composites were
then hot-pressed at 170 °C into sheets of suitable thickness,
and the flame-retardant PP composites were cut into pieces
of the required specifications.
Experimental
Materials
Polypropylene (PP) was supplied by Shanghai Secco
Petrochemical Co., Ltd. Phosphorus oxychloride (POCl3)
was distilled at 104–106 °C before use. (4-Hydroxyphenyl)
fluorine was supplied by Suqian City Wing Sing Pharma-
ceutical Co., Ltd. Neopentyl glycol was purchased from
Datianfengtuo Chemical Reagent Co., Ltd. (Beijing,
China). Anhydrous potassium carbonate was supplied by
Beifangtianyi Chemical Industry Co., Ltd. (Tianjin, China).
Other reagents were used as received without further
purification.
123

Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl)…
Scheme 1 Synthesis routes of
FRs
(a)
O
P
O
O
CH3
CH3
O
P
OH
OH
C
2HCl
Cl
Cl
C
Cl
Cl
(b)
O
O
CH3
C
P
Cl
O
O
O
CH3
O
O
O
P
P
OH
HO
O
O
CH3
H3C
O
C
C
H3C
CH3
on a CFZ-2-type instrument (Jiangning Analysis Instru-
ment Company, China).
Table 1 The composition of FR/PP and FR/APP/PP composites
Samples
PP/g
FRs/g
APP/g
The limiting oxygen index (LOI) values were measured
on an HC-2 oxygen index meter (Shangyuan Analysis
Instrument Co., China). The specimens used for the test
were of dimensions 100 9 6.5 9 3 mm3.
PP1
PP2
PP3
PP4
PP5
PP6
PP7
PP8
90
85
80
70
70
70
70
70
10
15
20
30
20
15
12
10
0
0
0
The flammability of the samples was also characterized
by cone calorimeter (Fire Testing Technology, UK)
according to ISO 5660. In order to investigate the ignition
times, all the samples were irradiated horizontally at a heat
flux of 35 kW m-1 rather than 50 kW m-1. Each specimen,
with the dimensions of 100 mm 9 100 mm 9 3 mm, was
mounted on aluminum foil and placed on a holder filled with
the mineral fiber blanket, so that only the upper face was
exposed to the radiant heater.
0
10
15
18
20
Measurements
Scanning electron microscopy (SEM) was used to
examine the surface morphology of the char residues after
the cone calorimeter tests. The surfaces of char were
sputter-coated with a conductive gold layer and the sam-
ples were tested.
The
FTIR
spectroscopy
(wavelength
range
4000–500 cm-1) was recorded with Nicolet 6700 FTIR
spectrophotometer using the thin KBr disk.
1H NMR measurements were performed on AVANCE
400 Bruker spectrometer at room temperature.
The measurements of tensile strength and elongation at
break were carried out according to the Chinese standard of
GB 13022-91 with a WD-20D electronic universal testing
instrument (Changchun Intelligent Instrument Co. Ltd.,
Changchun, China) at a crosshead speed of 20 mm/min,
and each sample is tested three times.
The limiting oxygen index (LOI) values were measured
on an HC-2 oxygen index meter (Jiangning Analysis
Instrument Company, China). According to ASTMD2863,
the specimens used for LOI were of dimensions
100 mm 9 6.5 mm 9 3 mm.
The thermogravimetric analysis (TG) was carried out on
TGA Q5000 IR thermal gravimetric analyzer (TA Instru-
ments) at heating rates of 10 °C min-1. About 4–10 mg of
materials was heated from room temperature to 650 °C
under air or nitrogen purges. The specimens used for ver-
tical burning test are 100 mm 9 13 mm 9 3 mm, and test
was carried out on a CFZ-2-type instrument (Jiangning
Analysis Instrument Company, China).
Results and discussion
Characterization of FRs
The chemical structures of FRs were characterized by
1
FTIR, H NMR and MS. The FTIR spectrum of FRs is
shown in Fig. 1. The peak at 2968.32 cm-1 corresponds to
The specimens used for vertical burning test are
100 mm 9 13 mm 9 3 mm, and the test was carried out
the vibration absorption peak of methylene, and the peaks
123

X. Qian et al.
Fig. 1 FTIR spectra of FRs
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
3500
3000
2500
2000
1500
1000
500
Wavenumbers/cm–1
at 1613.22–1510.52 cm-1 belong to the stretching vibra-
tions of benzene rings. Moreover, the strong absorptions
band at 1174.75 cm-1 indicate the existence of P=O bonds
and the characteristic peak for P–O bond exists at around
carbon layers with the increase in temperature. The for-
mation of continuous carbon layers is very important dur-
ing flame-retardant process, because the carbon layers
could protect the underlying polymer matrix from further
degradation. Generally, the continuous carbon layers of
FRs at high temperature may contribute greatly to the
formation of condensed char layers during the thermal
degradation process of flame-retardant PP.
1
967.36 cm-1 [18]. Combining with the H NMR and MS
results (Figs. 2, 3), it is obvious that the FR has been
successfully synthesized.
The TG results usually reflect quantitative information
about the thermal stability and thermal degradation
behaviors of the flame retardants. The thermal decompo-
sition properties of FRs were investigated by TG in air
atmosphere at the heating rates of 10 °C min-1, as shown
in Fig. 4. It can be seen that there are two degradation
steps, and the first step is at the temperature range of
150–260 °C (mass loss 25%), while the second step is at
the temperature ranges of 260–370 °C (mass loss 55%).
The first step, which begins at around 150 °C, consists
essentially of degradation of phosphate ester and the for-
mation of phosphor materials due to the weak bond energy
of P–O–C bonds. The second step begins at around 250 °C
and has a maximum mass loss at around 300 °C. During
the second step, the 9,9-bis (4-hydroxyphenyl) fluorine
may degrade, which may be due to the catalyzing effect of
phosphorus. Moreover, it is found that the char residues at
500 °C are as high as 41 mass%, and the high char residues
indicate that the FRs may have the potential to be a high
efficiency flame retardants. In order to investigate the
thermal changes of FRs during the degradation process, the
surface morphologies of FRs at different temperatures are
investigated by SEM, as shown in Fig. 5. It can be found
that the FR changes from massive particles to continuous
Flame-retardant properties of FR/PP and FR/APP/
PP composites
LOI and UL-94 tests are used to evaluate the flame retar-
dancy of polypropylene. LOI is the minimum oxygen
concentration by volume for maintaining the burning of
polymeric materials; thus, it is an important parameter for
evaluating the flame-retardant properties of polymeric
materials. The UL94, which is vertical burning test method,
contains three ratings: V-0, V-1 and V-2. The LOI and
UL94 values of the samples are shown in Table 2. As for
PP, the LOI value is only 17.8%. However, when
30 mass% of FRs was incorporated into the PP matrix, the
LOI value increased to 24.8%, which is only 40% higher
than those values of PP. Those indicate that FRs can
improve the flame retardancy of PP. However, when APP
was added into the FRs/PP system, the LOI values
increased with the increased loading of APP. For example,
the LOI values of PP4 are only 24.8%, but when APP
replaces parts of FRs in the FRs/PP system, the LOI values
increased to 31.0% (PP7), which is a great improvement.
However, when the APP content is further increased, the
123

Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl)…
Fig. 2 1H NMR of FRS
10
9
8
7
6
5
4
3
2
1
0
ppm
Fig. 3 Mass spectrometry of
FRS
2000
1500
1000
500
0
400
500
600
700
800
900
1000
m/z
LOI value decreases to 28.0%. The LOI results indicate
that synergistic flame-retarding effects exist between FRs
and APP, and the FR/APP/PP systems exhibit improved
LOI values. As for the UL94 results, the experimental
results of the vertical burning rate indicate that the PP
systems gave no rating. When 30% FRs were introduced
into the PP matrix, the FRs/PP system can only reach the
V-1 rating. Those indicate that FR has flame-retardant
effect on the flame retardancy of PP. As for the APP/FRs/
PP system, the materials can reach the V-0 rating. Those
indicate that synergistic flame-retarding effects exist
between FRs and APP in the PP matrix, which is in
accordance with the LOI results. Generally, the LOI and
UL94 results indicate that FR has effects on the flame
123

X. Qian et al.
(a)
110
(b)
0.000
100
90
80
70
60
50
40
30
20
–0.001
–0.002
–0.003
–0.004
–0.005
–0.006
–0.007
100
200
300
400
500
600
100
200
300
400
500
600
Temperature/°C
Temperature/°C
Fig. 4 TG and DTG curves of FRs in air atmosphere
25 °C 300×
200 °C 300×
300 °C 300×
500 °C 300×
Fig. 5 The SEM of the char residues at different temperatures
PP has a small amount of volatiles when the temperature
increases to 265 °C. With the further increase in the tem-
perature, the mass loss increases remarkably and the
resulting char residue at 650 °C is only 0.02 mass%. As
shown in Fig. 7, the thermal decomposition of PP in air is
characterized by one decomposition steps, and FR/PP
composites exhibit improved thermal stability than that of
pure PP. Compared with pure PP, the incorporation of FRs
and APP can improve the thermal stability of PP. For
example, the T5% of PP is only 291.83 °C, but as those for
PP4 and PP7, the T5% increased to 314.50 and 311.83 °C
respectively. That is because the FRs could form a com-
plete connector (Fig. 5), the complete connector could
delay the inner PP matrix from degradation, so the T5% of
the flame-retardant PP increases significantly. Moreover, it
is interesting to observe that the char residues of flame-
retardant PP increase with the increased loading of FRs.
For example, PP0 has only 0.02 mass% char residues at
650 °C, while the char residue of PP4 at 650 °C is as high
as 4.72 mass%. As compared to FRs, FRs/APP system
performs much better on improving the char residues at
650 °C. PP7 has the highest char residues at 650 °C
(15.85 mass%), which indicates that there are synergistic
effect between FRs and APP on catalyzing the char for-
mation. Those results are in accordance with the LOI
Table 2 LOI and UL 94 values of PP, FR/PP and FR/APP/PP
composites
Samples
LOI/%
UL94
PP0
PP1
PP2
PP3
PP4
PP5
PP6
PP7
PP8
17.8
19.6
21.4
22.5
24.8
29.0
29.0
31.0
28.0
NR
V-2
V-2
V-2
V-1
V-0
V-0
V-0
V-0
retardancy of PP, but the effect is not significant. When the
FRs and APP were incorporated into PP system, the flame-
retardant effect is improved greatly due to the synergistic
flame-retarding effects existed between FRs and APP.
The thermal properties of FR/PP and FR/APP/PP
composites
TG profiles for PP and the flame-retardant PP composites
at the heating rate of 10 °C min-1 in air atmosphere are
shown in Fig. 6, and the data are shown in Table 3. Pure
123

Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl)…
PP0
PP5
PP6
PP7
PP8
100
80
60
40
20
0
PP0
PP1
PP2
PP3
PP4
100
80
60
40
20
0
100
200
300
400
500
600
100
200
300
400
500
600
Temperature/°C
Temperature/°C
Fig. 6 TG curves of PP, FR/PP and FR/APP/PP composites in air atmosphere
The combustion properties of FR/PP and FR/APP/
PP composites
Table 3 Thermogravimetric data of the PP composites in air
Samples
T
1%/°C
T5%/°C
Char 650 °C
FRs
PP0
PP4
PP7
133.33
265.50
270.13
286.00
175.33
291.83
314.50
311.83
39.04
0.02
Cone calorimetry is one of the most effective bench scale
methods to study the flammability of polymeric materials.
Heat release rate (HRR), in particular peak HRR (pHRR),
is considered to be the most important parameter to eval-
uate the fire safety of polymer materials. The heat release
rate (HRR) and total heat release (THR) curves of PP, FR/
PP and FR/APP/PP composites are shown in Figs. 8 and 9,
respectively. As shown in Fig. 8, when FR was introduced
into PP matrix, the pHRR of FR/PP composites decreased
from 783 kW m-2 (PP0) to 545 kW m-2 (PP4), which is
30% decrease. As shown in Fig. 9, the THR of FR/PP
composites decreased from 198 MJ m-2 (PP0) to
139 MJ m-2 (PP4), which is 29% decrease compared with
PP. Therefore, it can be assumed that the FR exhibits good
flame retardancy on PP at the loading of 30 mass%. As
shown in Fig. 8, the ignition times (ITT) are slightly
decreased when the FRs and APP were incorporated into
the PP matrix. Such a decrease is attributed to the low
thermal stability and catalysis charring effect of FRs, which
speeded up the thermal degradation of PP. As compared to
FRs, FRs/APP system performs much better on improving
the flame-retardant properties of PP. When APP was
introduced into the FRs/PP system, both the pHRR and
THR were decreased. The pHRR of PP7 is only
110 kW m-2, which is 85% lower than those of PP. With
the further increase in APP content, the pHRR of FR/APP/
PP composites increases slightly. That is because the
suitable proportions of carbon sources in FRs and acid
sources exhibit synergistic flame-retarding effect, and the
synergistic flame-retarding effect is in accordance with the
LOI and TG results. Moreover, THR of PP decreased with
the increased loading of APP. For example, the THR of PP
4.72
15.85
0.000
–0.004
–0.008
–0.012
–0.016
–0.020
PP0
PP4
PP7
100
200
300
400
500
600
Temperature/°C
Fig. 7 DTG curves of PP0, PP4 and PP7 in air atmosphere
results. Generally, the char layers, which can protect the
underlying materials from heat and oxygen, are an
important factor in the flame retardancy of polymeric
materials [19, 20]. Due to the synergistic flame-retardant
effect, the FRs/APP system has a higher efficiency on
catalyzing the formation of char layers during the thermal
degradation process, and the higher char residues at high
temperature will result in the improved flame-retardant
effects such as LOI and UL-94 results.
is 197.2 MJ m-2, while THR of PP8 is only 37.9 MJ m-2
.
The reduced pHRR and THR are attributed to the
123

X. Qian et al.
(a)
(b)
PP0
PP5
PP6
PP7
PP8
PP0
PP1
PP2
PP3
PP4
800
600
400
200
0
800
600
400
200
0
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
Time/s
Time/s
Fig. 8 The HRR curves of PP, FR/PP and FR/APP/PP composites during the cone test
(a)
(b)
PP0
PP5
PP6
PP7
PP8
PP0
PP1
PP2
PP3
PP4
200
150
100
50
200
150
100
50
0
0
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
Time/s
Time/s
Fig. 9 The THR curves of PP, FR/PP and FR/APP/PP composites during the cone test
catalyzing charring effects of APP and FRs. Moreover, the
decreased pHRR and THR imply great improvement on the
fame retardancy of PP, which is in accordance with the
LOI and TG results.
but also hinder the propagation of oxygen and heat into the
interior substrate. As a result, the flame of FR/PP and FR/
APP/PP composites during the cone test is weaker than
those of PP. Generally, it can be found that the results of
cone and char morphology for the composites are in
accordance with the results of LOI and TG results. Thus, it
is believed that this condensed chars are responsible for the
improved thermal stability, enhanced flame retardancy and
higher char residues.
The char residues of PP, FR/PP and FR/APP/PP com-
posites during and after the cone tests are shown in Fig. 10,
which directly reflects the morphology of the char layers.
As for the char residues of pure PP, it can be found that the
PP was ignited easily and the flame highness of PP is
higher than those of PP4 and PP7. Moreover, there are
almost no char residues for PP after the cone test. When FR
was incorporated into PP, the char layers of FR/PP com-
posites expanded distinctly, indicating that condensed char
layers were formed during combustion. The catalytic
charring effect of FRs plays an important role in the char
formation, but the char layers of FRs/PP are lower than
those of FR/APP/PP composites. This phenomenon is due
to the introduction of APP in the PP matrix, and APP has
better catalyzing charring effect during the thermal
degradation process of PP. The stable char layer could not
only effectively prevent the release of combustion gases
The changes of the chemical structures in the condensed
phase during the thermal oxidative degradation of PP
composites were measured by the FTIR after heated at
different temperatures, as shown in Fig. 11. As for PP4 at
300 °C, the peaks at 2968.98 and 2917.78 cm-1 are due to
the stretching vibration of methyl and methylene, respec-
tively. The characteristic peak at 1634.45 cm-1 is due to
the stretching vibration of benzene ring, and the peaks at
1463.34 and 1376.74 cm-1 are due to the bending vibra-
tion of C=C bonds and C–H bonds. Moreover, the peak at
1112.64 cm-1 is due to the O–P–O bonds. When the
temperature was further increased to 400 °C, the intensity
123

Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl)…
PP0
PP4
PP7
100 S
400 S
600 S
Char residues
Fig. 10 The char residues of PP, FR/PP and FR/APP/PP composites during and after the cone tests
10.0
PP4
PP7
600 °C
3.5
3.0
2.5
2.0
1.5
1.0
1.5
1.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
600 °C
500 °C
500 °C
400 °C
400 °C
300 °C
1.5
–1.0
–1.5
–2.0
300 °C
2.5
3500
3000
2500
2000
1500
1000
500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers/cm–1
Wavenumbers/cm–1
Fig. 11 The FTIR of PP4 and PP7 at different temperatures
of the absorption peaks of O–P–O bond decreased, result-
ing in weaker broad peaks. As for the composites at
500 °C, the characteristic absorption peaks of methyl and
methylene disappeared, indicating that molecular chains of
PP had broken completely. When the temperature
increased to 600 °C, the band for P=O at 1229.25 cm-1
still existed, indicating that phosphorus-based elements
existed at high temperature and the phosphorus-based
elements could reinforce the char layers greatly. As for PP7
at 400 °C, the intensities of peaks at 1463.03 cm-1 for
C=C bonds and 1376.84 cm-1 for C–H bonds increased,
and the peaks at around 1200–1000 cm-1 for the O–P–O
and P=O bonds were enhanced, indicating that FRs and
APP decomposed and formed phosphoric acid or
metaphosphoric acid. When the temperature of PP7
increased to 500 °C, the characteristic peaks of methylene
bond disappeared, but the peaks for the O–P–O and P=O at
around 1200–1000 cm-1 still existed, indicating phospho-
rus-based char layers formed at this temperature.
In order to further investigate the relationships between
the morphology of the formed char layers and flame-re-
tardant properties of APP/FR/PP composites, scanning
electron microscopy (SEM) was used to investigate the
microstructure of char layers after the cone test, as shown
in Fig. 12. The char layer of PP4 is uneven and could not
form an integral carbon layers. Thus, the barrier effect of
the char layers is very limited during the combustion of
PP4. That is because the catalytic charring effect of FRs is
limited, when the amount of gas under the carbon layer
reaches a certain value, the gas breaks through the carbon
layer overflow, making the integrity of carbon layer dam-
aged [21, 22]. As those for PP7, the surface of the char
layers is more continuous and has inflated holes, which is
mainly attributed to the APP in the FRs/PP system. The
results indicate that the addition of APP can enhance the
123

X. Qian et al.
Fig. 12 The SEM of the char
residues after the cone test
×
×
PP4 400
PP4 100
×
×
PP7 400
PP7 100
quality of the char layers. The condensed char layers could
effectively reduce the escape of flammable volatile com-
ponents, protect the under lying materials form oxygen,
suppress the further degradation and combustion of PP
matrix and then lead to outstanding flame retardancy for
APP/FR/PP composites. Generally, APP has a good syn-
ergistic effect with the flame-retardant FRs, and the flame-
retardant PP exhibited reduced pHRR and THR.
smoke toxicity of PP. That may be due to the fact that the
FRs could inhibit the complete combustion of PP. How-
ever, compared with PP4, the introduction of APP into
FRs/PP system can reduce the av-SEA and the av-COY
slightly. The av-SEA of PP7 decreased to 457.00 m2 kg-1
,
and av-COY of PP7 decreased to 0.04 kg kg-1. From the
char residues of PP7 after the cone test, it can be found that
the addition of APP can benefit to the formation of con-
densed carbon layers, and the condense carbon layers could
reduce the release of smoke toxicity to a certain extent.
Generally, FRs could increase the smoke release of PP, but
the incorporation of APP into the FRs/PP system could
reduce the smoke release to a certain extent, which may
due to the condensed flame-retardant mechanism.
The smoke release and mechanical properties of FR/
PP and FR/APP/PP composites
In order to further estimate the fire risks of the PP, FR/PP
and FR/APP/PP composites, the smoke data obtained from
the cone test were analyzed. Figure 13 shows the specific
extinction area (SEA) and carbon monoxide release rate
(COY) of PP, PP4 and PP7. It can be found that the
introduction of FRs and APP can influence the smoke
release of PP significantly. Compared with PP0, the SEA
and CO release increased with the addition of flame
retardants. As for PP, the av-SEA was 226.20 m2 kg-1 and
the av-COY was 0.02 kg kg-1. However, when FRs are
introduced into PP matrix, the av-SEA of PP4 increased to
589.17 m2 kg-1 and the av-COY increased to
0.05 kg kg-1. Those indicate that the addition of FRs could
increase the smoke release, resulting in the increased
The mechanical properties of PP and the flame-retardant
PP composites are evaluated by tensile tests, as shown in
Fig. 14. According to the tensile strength of PP and the
flame-retardant PP composites, it is very interesting to find
that FR has the effect of improving the tensile strength of
PP: 30 mass% FRs in the PP matrix which leads to a 63%
increase in the tensile strength of PP, but the elongation at
break of the flame-retardant PP decreases when FRs and
APP are incorporated into PP matrix. Generally, FRs can
act as plasticizer, and plasticizer can improve the pro-
cessing performance of PP, resulting in improved tensile
strength.
123

Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl)…
(a)
(b)
PP0
PP4
PP7
PP0
PP4
PP7
1800
1500
1200
900
600
300
0
0.16
0.12
0.08
0.04
0.00
0
100 200 300 400 500 600 700 800 900
Time/s
0
100 200 300 400 500 600 700 800 900
Time/s
Fig. 13 The SEA and av-COY curves of PP0, PP4 and PP7 during the cone test
35 33.72
71.04
32.55
67.98
70
60
50
40
30
20
10
0
66.46
61.19
30
25
60.45
56.03
55.8
20.88
43.67
19.72
19.12
20
15
10
5
33.38
13.96
13.56
10.32
6.4
0
PP0 PP1 PP2 PP3 PP4 PP5 PP6 PP7 PP8
Samples
PP0 PP1 PP2 PP3 PP4 PP5 PP6 PP7 PP8
Samples
Fig. 14 The mechanical properties of PP, FR/PP and FR/APP/PP composites
suppression properties due to the stable char layer of FR/
APP/PP composites. Generally, the stable char layer of
FRs/APP/PP system could not only effectively prevent the
release of combustion gases but also hinder the propagation
of oxygen and heat into the interior substrate.
Conclusions
A novel flame retardants (FRs) containing phosphorus and
(4-hydroxyphenyl) fluorene have been synthesized and
characterized. The FR combined with APP was then
incorporated into PP at different ratios. The flame-retardant
PP exhibited both good flame retardancy and thermal
properties when the two components of flame retardants
existed at an appropriate proportion. The flame-retardant
PP can reach UL-94 V0 rating, and the LOI is as high as
31% when the mass fraction ratio of FRs/APP is 3:2. The
improved flame-retardant effect may be attributed to the
synergistic flame-retardant effect between FRs and APP.
The char residues at 650 °C in an air atmosphere increased
from 0.02 to 15.85 mass% notably when FRs/APP is 3:2,
indicating an improvement of thermo-oxidation resistance.
From the cone test, it is found that both the THR and pHRR
of the flame-retardant PP are reduced, and there are syn-
ergistic effects between FRs and PP in the flame retardancy
of PP. Compared to the FR/PP composites, av-SEA and av-
COY results indicate that APP also exhibits good smoke
Acknowledgements This work was supported by the Natural Science
Foundation of China (Nos. 21472241, 21704111), Natural Science
Foundation of Hebei Province (Nos. E2016507032, E2016507027)
and Hebei Science and Technology Project (16211248).
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