Preparation, fire behavior and thermal stability of a novel flame retardant polypropylene system

Article abstract of DOI:10.1007/s10973-016-5352-7

In this study, a novel spirocyclic pentaerythritol bisphosphorate disphosphoryl-di-prop-2-en-1-amine (SPSA) compound containing a functional double bond (C=C) was synthesized and characterized by Fourier transform infrared spectroscopy and ¹H nuclear magnetic resonance spectroscopy (¹H NMR). In this paper, a new intumescent flame retardant (IFR) was designed by combining ammonium polyphosphate (APP) and SPSA (mass ratio of APP and SPSA was 2:1) and this IFR was mixed with polypropylene (PP) via twin-screw extruder to prepare a series of flame retardant PP composites. The thermal stability and fire behaviors of these flame retardant PP composites were investigated by thermogravimetric analysis, limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test. Furthermore, the morphology of the char residues of flame retardant PP composites after cone calorimeter test was studied by scanning electron microscopy with energy dispersive X-ray analysis (EDS). Aiming to further improve the thermal stability of the composites, electron beam irradiation has been used to treat these composites. The results showed that LOI value of flame retardant PP composite containing 30?% IFR was 24.0 and passed UL 94?V-0. In the cone calorimeter test, both peak heat release rate and total heat release results of PP/IFR system were lower than those of PP/APP system. In addition, this IFR provided relatively good quality of char residue in the combustion. Moreover, electron beam treatment of the composites improved the thermal stability of these designed flame retardant PP composites.

Full text of DOI:10.1007/s10973-016-5352-7

J Therm Anal Calorim
DOI 10.1007/s10973-016-5352-7
Preparation, fire behavior and thermal stability of a novel flame
retardant polypropylene system
Dan Xiao1,2 Zhi Li3 Sergio De Juan3 Uwe Gohs1 Udo Wagenknecht1
•
•
•
•
•
Brigitte Voit1,2 De-Yi Wang3
•
Received: 27 October 2015 / Accepted: 12 February 2016
´
´
Ó Akademiai Kiado, Budapest, Hungary 2016
Abstract In this study, a novel spirocyclic pentaerythritol
bisphosphorate disphosphoryl-di-prop-2-en-1-amine (SPSA)
compound containing a functional double bond (C=C) was
synthesized and characterized by Fourier transform infrared
spectroscopy and 1H nuclear magnetic resonance spec-
troscopy (1H NMR). In this paper, a new intumescent flame
retardant (IFR) was designed by combining ammonium
polyphosphate(APP) and SPSA (mass ratio of APP and SPSA
was 2:1) and this IFR was mixed with polypropylene (PP) via
twin-screw extruder to prepare a series of flame retardant PP
composites. The thermal stability and fire behaviors of these
flame retardant PP composites were investigated by thermo-
gravimetric analysis, limiting oxygen index (LOI), vertical
burning test (UL-94) and cone calorimeter test. Furthermore,
the morphology of the char residues of flame retardant PP
composites after cone calorimeter test was studied by scan-
ning electron microscopy with energy dispersive X-ray anal-
ysis (EDS). Aiming to further improve the thermal stability of
the composites, electron beam irradiation has been used to
treat these composites. The results showed that LOI value of
flame retardant PP composite containing 30 % IFR was 24.0
and passed UL 94 V-0. In the cone calorimeter test, both peak
heatreleaserateandtotalheatreleaseresultsofPP/IFRsystem
were lower than those of PP/APP system. In addition, this IFR
provided relatively good quality of char residue in the
combustion. Moreover, electron beam treatment of the com-
posites improved the thermal stability of these designed flame
retardant PP composites.
Keywords Polypropylene Á Flame retardancy Á Fire
behavior Á Thermal stability Á Electron beam irradiation
Introduction
Polypropylene (PP) has excellent mechanical and physic-
ochemical properties. Thus, it is used in a wide range of
applications, but its inherent combustibility limits its
broader application [1–3]. Thus, the flame retardancy of PP
has to be improved. The use of intumescent flame retar-
dants (IFR) is established in polymeric materials since it is
regarded as a safe and friendly flame retardant method [4].
IFR is generally made of three constituents: acid source
(e.g., ammonium polyphosphate, etc.), carbon source (e.g.,
pentaerythritol, sorbitol, etc.) and foaming agent (e.g.,
melamine, etc.).
Many researches have shown that synergistic agents can
effectively increase the strength and stability of char layer
as well as promote catalyzing the reactions among IFR
components in IFR-PP systems. Wang et al. [5] proposed to
use the catalytic action of phosphotungstic acid in the
synthesis of melamine salts of pentaerythritol phosphate in
order to solve the problems of conventional preparation
methods. Nie et al. [6] suggested the synthesis of a char-
forming agent and microencapsulated ammonium
polyphosphate was incorporated in PP via an intumescence
mechanism. Wang reported [7, 8] the synthesis of a novel
char former for intumescent system based on a charring
agent and ammonium polyphosphate. Bourbigot et al. [9]
investigated charring polyamide-6 clay nanocomposite
& De-Yi Wang
deyi.wang@imdea.org
1
Leibniz-Institut fu¨r Polymerforschung Dresden e.V., Hohe
Strasse 6, 01069 Dresden, Germany
2
¨
Organic Chemistry of Polymers, Technische Universitat
Dresden, 01062 Dresden, Germany
3
IMDEA Materials Institute, C/Eric Kandel, 2,
28906 Getafe, Madrid, Spain
123

D. Xiao et al.
Fig. 1 Synthesis of new
phosphorus-containing flame
retardant (SPSA)
O
O
O
O
O
O
HO
HO
OH
OH
O
O
O
O
O
O
H2N
P
P
POCI3
P
P
N
N
CI
H
CI
H
(SPSA)
studied by scanning electron microscopy (SEM) with
energy dispersive X-ray analysis (EDS). In the end, the
thermal stability of non-irradiated and electron beam irra-
diated flame retardant PP composites was investigated.
Table 1 Formulation of PP and flame retardant PP composites
Sample
PP/mass%
IFR
APP/mass%
SPSA/mass%
PP0
PP1
PP2
PP3
PP4
100
80
75
70
70
0
0
6.7
8.3
10
0
13.3
16.7
20
Experimental
Materials
30
Pentaerythritol, phosphoryl chloride, triethylamine, prop-2-
en-1-amine, trichloromethane, sodium hydroxide and
tetrahydrofuran were supplied by Sigma-Aldrich Chemical
Reagent Co., Ltd. and used without further purification.
Polypropylene (PP, homopolymer, HD 120MO) was supplied
by Borealis, Porvoo, Finland. Ammonium polyphosphate
(APP) was supplied by Budenheim Company. Spirocyclic
pentaerythritol bisphosphorate disphosphoryl-di-prop-2-en-
1-amine (SPSA) was prepared from pentaerythritol and
phosphoryl chloride, leading to spirocyclic pentaerythritol
bisphosphorate disphosphoryl chloride, which was then
reacted with prop-2-en-1-amine, as shown in Fig. 1.
hybrid (PA-6-nano) as a carbonization agent. Some phos-
phorus and nitrogen-containing intumescent agents were
synthesized, such as spiro and caged bicyclic phosphate
(SBCPO) [10], spirocyclic pentaerythritol bisphosphorate
disphosphoryl melamine (SPDPM) [11], poly(4,4-diamin-
odiphenyl methane spirocyclic pentaerythritol bisphos-
phonate) (PDSPB) [12] and poly(2-hydroxy propylene
spirocyclic pentaerythritol bisphosphonate) (PPPBP) [13].
Recently, Wang et al. [14, 15] found that P–O–C and P–N–
C structures could greatly improve the residues layer dur-
ing combustion. Since traditional IFR is non-reactive type
compounds to polymers, it may result in the low thermal
stability of flame retardant polymer due to the initial
decomposition of the components in IFR.
Synthesis of spirocyclic pentaerythritol
bisphosphorate disphosphoryl chloride [21]
In a 500-mL glass flask equipped with a magnetic stirrer, a
thermometer, a heating bath and circumference condenser,
pentaerythritol (68 g, 0.5 mol) and phosphoryl chloride
(618 g, 4.0 mol) were mixed. The mixture was stirred
It is well known that electron beam (EB) technology is
an environment-friendly method [16–18]. It can be used to
chemically modify polymer [19]. Its main advantage is the
spatial and temporal precise generation of free radicals in
accordance with the requirement of the chemical reaction
without any use of chemicals as well as without any
dependence on the temperature and the state of aggregation
[20]. Formed the cross-linked structure may be used to
improve the thermal stability of polymers.
0.0
0.2
0.4
0.6
0.8
1.0
In this work, a novel halogen-free intumescent flame
retardant containing a functional double bond (C=C) that
may form cross-linking structure with PP in the electron
beam treatment, spirocyclic pentaerythritol bisphosphorate
disphosphoryl-di-prop-2-en-1-amine (SPSA), was devel-
oped and mixed in the PP system via melt compounding.
The thermal stability and fire behavior of these flame
retardant PP composites were investigated by thermo-
gravimetric analysis (TG), limiting oxygen index (LOI),
vertical burning test (UL-94) and cone calorimeter test.
Furthermore, the morphology of the char residues of flame
retardant PP composites after cone calorimeter test was
4000
3500
3000
2500
2000
1500
1000
cm–1
Fig. 2 FTIR spectra of SPSA
123

Preparation, fire behavior and thermal stability of a novel flame retardant polypropylene…
about 100 °C until HCl evolution subsided. Then, the
Table 2 LOI values and UL94 results of PP and flame retardant PP
composites
mixture was gradually heated and refluxed until no HCl gas
was emitted. The raw product obtained was filtered and
purified using a filter with chloroform sequentially. The
product was dried to constant mass at 55 °C in vacuum.
Thus, spirocyclic pentaerythritol bisphosphorate disphos-
phoryl chloride, a white solid powder, was obtained (108 g,
yield: 73 %).
Characterization: 1H NMR (500 MHz, d6-DMSO)
(ppm): 4.23–4.20 (d, -CCH2O-PO-, 8H). FTIR (cm-1):
1305 (vs, P=O), 1025 (vs, P–O–C).
Sample
LOI
UL94
Dripping in
UL94 test
PP0
PP1
PP2
PP3
PP4
17.6
21.0
22.5
24.0
20.4
N.R.
N.R.
N.R.
V-0
Yes
Yes
Yes
No
N.R.
Yes
N.R. no rating
Synthesis of new phosphorus-containing flame
retardant (SPSA)
for the feed zone to the die: 175, 180, 190, 185, 180 and
170 °C. Finally, the specimens were applied for LOI,
UL-94, and cone calorimeter test was carried out by
injection molding (Arburg 320 C) at a processing temper-
ature around 200 °C.
A mixture of prop-2-en-1-amine (28.5 g, 0.5 mol) and
triethylamine (70 mL, 0.5 mol) was added dropwise to a
stirred solution of spirocyclic pentaerythritol bisphosphor-
ate disphosphoryl chloride (60 g, 0.2 mol) in anhydrous
tetrahydrofuran (100 mL) under nitrogen gas at 0–5 °C.
The resulting mixture was allowed to warm to room tem-
perature and left stirring for 12 h. Then, the mixture was
filtered off. Finally, the filtrate was concentrated at reduced
pressure. Thus, the product SPSA (57 g, yield: 85 %) was
distilled under reduced pressure to provide the desired
compounds. The reactions are shown in Fig. 1.
Electron beam irradiation
Flame retardant PP composites were irradiated at room
temperature in nitrogen atmosphere with an average dose
32 kGy using an ELV-2 electron accelerator (BINP,
Novosibirsk, Russia). The electron energy was 1.5 MeV,
and the electron current amounted to 4 mA.
Experimental devices description
Preparation of flame retardant PP composites
The Fourier transform infrared (FTIR) spectrum of SPSA
sample was recorded using the Bruker Vertex 80 V spec-
APP and SPSA were dried in a vacuum oven at 80 °C for
12 h. Then, PP and IFR (mass ratio of APP and SPSA at
2:1) were compounded by a twin-screw extruder (KETSE
20/40 EC, Brabender) (compositions see Table 1) with a
rotational speed of 150 rpm at a fixed temperature protocol
trometer in the wave number range of 600–4000 cm-1
.
1H nuclear magnetic resonance (1H NMR) was obtained
on a Bruker spectrometer using CDCl3 as a solvent.
The thermogravimetric analysis (TG) was carried out
using a TA Instruments TGA Q 5000 in the range between
O
O
O
O
O
O
P
P
c
N
N
H
d
H
e
1000
a
b
Pure PP
PP + 20 % IFR
PP + 25 % IFR
H2O
PP + 30 % IFR
PP + 30 % APP
800
e
600
400
CDCI3
200
0
d
c
a
b
0
100
200
300
400
500
600
700
800
8
7
6
5
4
3
2
1
0
ppm
Time/s
Fig. 3 1H NMR spectra of SPSA
Fig. 4 Heat release rate plots of pure PP and its composites
123

D. Xiao et al.
room temperature and 700 °C. The experiments were done
Fig. 2. The characteristic absorption peak at 3311 cm-1
was attributed to the N–H bond segments in structure of
SPSA. Also, the characteristic absorption peaks for car-
bon–carbon double bonds (1635 cm-1) were clearly
observed. The peak at 987 cm-1 was assigned to P–O–C
bond on the structure of SPSA segments.
in nitrogen atmosphere at heating rate of 10 K min-1, and
the flow rate of the nitrogen was 60 mL min-1
.
The UL-94 test was performed using a vertical burning
instrument (Fire Testing Technology, UK), and the speci-
mens in this work for testing were of dimensions
130 mm 9 13 mm 9 3.2 mm.
Limiting oxygen index (LOI) was measured on sheets
(120 mm 9 6.5 mm 9 3.2 mm) according to the standard
DIN EN ISO 4589-1 and 4589-2. It describes the minimum
concentration of oxygen in nitrogen that supports the com-
bustion of a polymer. The unit of concentration is percentage.
The cone calorimeter tests (CC) were carried out on a
cone calorimeter (FTT, UK). The squared specimens
(100 mm 9 100 mm 9 4 mm) were wrapped with alu-
minum foil and placed in a frame without grid. The spec-
imens were irradiated at a heat flux of 50 kW m-2. Each
sample was tested three times.
1H NMR spectroscopy
Further structural conformation for SPSA was made by
1
means of H NMR spectroscopy. As shown in Fig. 3, the
peak groups at 5.80–5.11 ppm could be assigned to the
protons on the carbon–carbon double bond on the structure
of SPSA segments (corresponding to a, b in Fig. 3). The
peak groups at 4.51–3.00 ppm could be assigned to the
protons on methylene groups of the structure of SPSA
segments (corresponding to d, e in Fig. 3).
Scanning electron microscopy (SEM) (microscope
model: Ultra Plus, Carl Zeiss SMT) with energy dispersive
X-ray analysis (EDS) was used to study morphological
features of the powdered samples. The samples were placed
on a sample holder using conducting copper tape and then
coated with a thin layer of platinum (layer thickness 3 nm)
using a sputter coater (BAL-TEC SCD 500 sputter coater).
Fire behaviors
In the following, polypropylene (PP)/intumescent flame
retardant (IFR) composites have been prepared in different
compositions as given in Table 2 and the composites have
been compared with pure PP in the fire behavior. Ammo-
nium polyphosphate (APP) alone and together with SPSA
in a 2:1 ratio were used as IFR.
Results and discussion
Structure characterization of SPSA
FTIR spectroscopy
LOI and UL94
Aiming to evaluate flammability of PP and flame retardant
PP composites, usually, LOI and UL-94 tests are used
[22–24]. In this study, both LOI and UL94 were used to
investigate the flammability of the composites and the
results are given in Table 2. Obviously, LOI value of virgin
PP was only 17.6 %, showing high flammability. At a load
of 30 mass% of APP, the LOI value of PP/APP composite
increased to 20.4 %, but there was no rating in the UL 94
test. It meant that APP alone was not an efficient flame
retardant to PP. In comparison, the LOI values of PP/IFR
composites containing 20, 25 and 30 mass% IFR were
21.0, 22.5 and 24.0, respectively. More importantly,
PP ? 30 % IFR passed UL 94 V-0, indicating that this IFR
was more efficient than APP alone.
In order to identify the structure of SPSA, firstly, FTIR
spectroscopy was used to characterize SPSA, as shown in
180
Pure PP
PP + 20 % IFR
PP + 25 % IFR
160
PP + 30 % IFR
PP + 30 % APP
140
120
100
80
60
40
20
0
Cone calorimeter test
While the LOI and UL 94 tests are widely used to evaluate
the flame retardation of materials, especially for screening
flame retarded formulations of polymers, the cone
calorimeter provides a wealth of information on the com-
bustion behavior [25]. Some cone calorimeter results cor-
relate well with those obtained from large-scale fire tests
and can be used to predict the behavior of materials in real
0
100
200
300
400
500
600
Time/s
Fig. 5 Total heat release (THR) of PP and flame retardant PP
composites
123

Preparation, fire behavior and thermal stability of a novel flame retardant polypropylene…
Fig. 6 Digital photographs of residue char for PP (a), PP composite with 30 % APP (b), PP composite with 30 % IFR (c)
(a)
(b)
(c)
0
1
2
3
4
5
6
7
Full scale 1596 cts cursor: 0.000
keV
Fig. 7 a SEM images of the residues for outer residues surface of PP/30 % IFR; b SEM images of the inner residues surface of PP/30 % IFR;
c EDS spectra of the char residues of PP/30 % IFR
fires [26]. The heat release rate (HRR) has come to be
recognized as one of the most important fire properties of a
material [27], and it is a measure of the heat release per
unit of surface area of burning materials. Figure 4 shows
the HRR curves of the pure PP and flame retardant PP
composites. It was noted that the pure PP resin burned very
123

D. Xiao et al.
(a)
(a)
Pure PP
PP + 20 % IFR
PP + 25 % IFR
PP + 30 % IFR
EB PP + 20 % IFR
EB PP + 25 % IFR
EB PP + 30 % IFR
PP + 20 % IFR
PP + 25 % IFR
PP + 30 % IFR
PP + 30 % APP
100
80
60
40
20
0
100
80
60
40
20
0
100
200
300
400
500
600
700
Temperature/°C
100
200
300
400
500
600
700
(b)
Temperature/°C
Pure PP
3.0
(b)
PP + 20 % IFR
PP + 25 % IFR
PP + 30 % IFR
PP + 30 % APP
PP/20 % IFR
PP/25 % IFR
PP/30 % IFR
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
EB PP/20 % IFR
EB PP/25 % IFR
EB PP/30 % IFR
1.0
0.5
0.0
100
200
300
400
500
600
Temperature/°C
100
200
300
400
500
600
700
Temperature/°C
Fig. 8 TG (a) and DTG (b) curves of PP and its composites
Fig. 9 TG (a) and DTG (b) curves of non-irradiated and irradiated
flame retardant PP composites at 32 kGy
fast after ignition and a sharp HRR peak appears with a
peak PHRR of 842 kW m-2. In the case of PP with
30 mass% of APP, the PHRR decreased to 489 kW m-2
.
The total heat release (THR) is another important param-
eterforfire hazardevaluation. THR value of pure PP, PP/APP
and PP/IFR composites are shown in Fig. 5. In comparison
with pure PP, all the flame retardant PP composites (PP1,
However, the PHRR of PP/IFR containing 20, 25, and
30 mass% of IFR was 473, 349 and 430 kW m-2
,
respectively. It confirmed that this IFR was a more efficient
flame retardant to PP compared to APP.
Table 3 Data obtained from TG and DTG curves
Samples
Tonset/°C
Tmax/°C
Rmax/%°C-1
Residue/%
500 °C
750 °C
PP
374
265
315
314
309
340
337
325
427
439
425
414
430
440
438
437
2.5
2.0
1.4
1.1
2.8
1.9
1.8
1.8
1.1
21.2
6.6
1.0
8.6
PP ? 30 % APP
PP ? 20 % IFR
PP ? 25 % IFR
PP ? 30 % IFR
EB PP ? 20 % IFR*
EB PP ? 25 % IFR*
EB PP ? 30 % IFR*
6.5
7.3
7.2
7.6
7.5
10.9
7.3
10.8
7.2
7.5
7.4
* The composites were irradiated with a dose of 32 kGy
123

Preparation, fire behavior and thermal stability of a novel flame retardant polypropylene…
PP2, PP3 and PP4, see Table 1) showed lower THR within
the first 350 s. Consequently, these flame retardant PP com-
posites showed a lower heat release during the combustion
compared to that of pure PP. Further, the difference in the
THR between PP3 and PP4 indicates that SPSA acts as an
excellent flame retardant synergist to traditional APP since
both composites contain 30 mass% flame retardant additives.
In order to further understand the relationship between
the microstructure of intumescent chars and flame retar-
dancy of PP composites, SEM micrographs of the char
residues of the outer surface (Fig. 7a) and inner surface
(Fig. 7b) of PP/30 % IFR were imaged. It is noted that the
char morphology on the outer surface of PP/30 % IFR
showed the formation of a continuous and compact char
layer, which would act as an insulating barrier to heat and
would prevent the access of oxygen during combustion.
Aiming to further understand elements distribution on the
char residues, energy dispersive spectrometry (EDS) was
employed to characterize the residue. The results are shown
in Fig. 7c. It is noted that in the elemental composition of
the char residue of the PP/30 % IFR, the content of C, O,
N, P were 34.37, 7.97, 36.78, 20.88 mass%, respectively.
This means that rich P and N-based compounds are formed
on the surface via varied decomposition and reconstruction
approaches.
Morphology and structure of the char residues
From the digital photographs (as shown in Fig. 6) of the
specimens after cone calorimeter test, there was almost no
residue for pure PP. In comparison, after adding the flame
retardant to PP, more char residues remained in the sample
holder. In particular, by introducing IFR into PP, the
quantity was increased and quality of formed char was
improved. These improved char layers will act as good
barrier to prevent the transmission of fuel and oxygen.
Fig. 10 Possible cross-linking mechanism
123

D. Xiao et al.
Thermal stability of flame retardant PP composites
double bond (C=C) that may form cross-linked structure
with PP in the electron beam treatment was synthesized
and characterized successfully. Then, an intumescent
flame retardant (IFR) based on aluminum polyphosphate
(APP) and SPSA (mass ratio of APP and SPSA at 2:1)
was developed and used in PP system to prepare fire
retardant PP composites. LOI value of flame retardant PP
composite containing 30 % IFR was 24.0, while the LOI
value of PP ? 30 % APP was only 20.4. More impor-
tantly, PP ? 30 % IFR passed UL 94 V-0, indicating that
this IFR was more efficient than APP alone. In the cone
calorimeter test, both PHRR and THR results of PP/IFR
system were lower than those of PP/APP system. It
proved that this IFR was a more efficient flame retardant
to PP compared to APP. In addition, the analysis of char
residues proved that this IFR provided relatively good
quality of char residue in the combustion that acted as
good barrier to prevent the transmission of fuel and
oxygen. Moreover, it could be shown that electron beam
treatment of the composites is an efficient approach to
improve the thermal stability of these designed flame
retardant PP composites. Further investigations on the
optimized dose of electron beam irradiation will be
reported in the future. This irradiated flame retardant PP
system might be a promising formulation for the indus-
trial application such as insulated material of wire, cable,
etc.
The effect of IFR on the thermal stability of PP composites
was revealed through TG test. TG and DTG curves of PP
and its composites are shown in Fig. 8. The corresponding
data are given in Table 3. From Fig. 8, it was observed that
all samples studied showed only one degradation step
within the experimental temperature range. It was clear
from Fig. 8 and Table 3 that the addition of IFR had an
obvious effect on the thermal stability of PP. The onset
degradation temperature of pure PP, Tonset, defined as the
temperature at which 5 % mass loss occurred, was 374 °C.
In the case of PP with 30 mass% of APP, the onset
degradation temperature decreased to 265 °C. In contrast,
the Tonset of flame retardant PP composites containing 20,
25 and 30 % IFR were 315, 314 and 309 °C, respectively.
This indicates that the addition of IFR decreased the ther-
mal stability of PP-based composites. However, as given in
Table 3, Tmax of PP amounts to 427 °C, while the maximal
decomposition temperature Tmax of PP with 20, 25 and
30 % IFR was 425, 414 and 430 °C. Consequently, there is
only a small impact on the Tmax of PP after adding IFR.
Effect of electron beam irradiation on the thermal
stability of flame retardant PP composites
Figure 9 presents the TG and DTG curves of non-irradiated
and irradiated flame retardant PP composites, and the
corresponding values are listed in Table 3.
Acknowledgements One of the authors ((Mr. Dan Xiao) would like
to thank the financial support from the China Scholarship Council
(Ph.D. Program No. 201308530045) and STSM support from COST
MP1105 (MP1105-090315-053686). This research is also partly
supported by Spanish Ministry of Economy and Competitiveness
Obviously, the thermal stability of flame retardant PP
composites was remarkably improved via electron beam
(EB) treatment. For example, Tonset of PP ? 20 % IFR was
increased from 315 to 340 °C after EB treatment. For the
other flame retardant PP composites, a similar trend was
shown. Moreover, the maximal decomposition temperatures
(Tmax) of flame retardant PP composites were also increased
via EB treatment. Herein, a possible grafting and/or cross-
linking mechanism between the novel flame retardant PP
was proposed, as shown in Fig. 10. Firstly, electron beam
treatment resulted in the generation of PP macroradicals.
Then, the macroradical reacts with the C=C group presented
in the SPSA to generate the first grafting reaction. The same
reaction can be induced by the macroradicals at the second
C=C group of SPSA. Finally, a cross-linked structure was
generated in the flame retardant PP composites.
´
(MINECO) under Ramon y Cajal fellowship (RYC-2012-10737) and
COMETAD project (MAT2014-60435-C2-2-R).
Funding Mr. Dan Xiao would like to thank the financial support
from the China Scholarship Council (Ph.D. Program No.
201308530045) and the STSM support from COST MP1105
(MP1105-090315-053686). Dr. De-Yi Wang acknowledges this
´
research is partly supported by Ramon y Cajal fellowship (RYC-
2012-10737) and COMETAD project (MAT2014-60435-C2-2-R).
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict
of interest.
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