A highly efficient gas-dominated and water-resistant flame retardant for non-charring polypropylene

Article abstract of DOI:10.1039/c7ra09868e

Traditional phosphorus-nitrogen (P-N) flame-retardant systems for polypropylene (PP) always act through joint action of the gaseous phase and condensed phase, and are accompanied with a decrease of the thermal stability and water resistance. In this work, a novel mono-component and gas-dominated flame retardant, named DPPIP, was prepared through an amidation reaction of diphenylphosphinyl chloride and piperazine, and used to flame retard PP. Experimental results confirmed that both the thermal stability and water resistance of PP/DPPIP were improved. The initial thermal decomposition temperature of PP/25 wt% DPPIP at 5 wt% weight loss was 287.5 °C under air atmosphere, which is higher than that of neat PP (266.1 °C). Besides, a water-resistance test verified that PP/25 wt% DPPIP had a weight loss of only about 0.52 wt%. More importantly, the flame retardant ability of PP/25 wt% DPPIP had been greatly improved, passing the V-0 rating (UL-94). Furthermore, after the water-resistance test, the LOI value of PP/25 wt% DPPIP exhibited nearly no difference and still passed the UL-94 V-0 rating. A cone calorimeter (CC) result indicated that DPPIP had a positive effect on inhibiting heat release of PP during combustion. All of these combustion tests displayed that there was no char left. The flame retardant mechanism of DPPIP was investigated with py-GC/MS and TG-FTIR. The results illustrated that the gaseous phase resulting from the thermal decomposition of DPPIP played the leading role in the self-extinguishing behavior of PP/DPPIP, which consisted of a large amount of inflammable gaseous products such as piperazine and its derivatives, and phosphorus-containing structures.

Full text of DOI:10.1039/c7ra09868e

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PAPER
A highly efficient gas-dominated and water-
resistant flame retardant for non-charring
Cite this: RSC Adv., 2017, 7, 51919
polypropylene
a
a
a
*
Zhu-Bao Shao,
and Rong-Kun Jian
Ming-Xin Zhang, Ye Han, Xu-Dong Yang,a Jing Jina
*
b
Traditional phosphorus–nitrogen (P–N) flame-retardant systems for polypropylene (PP) always act through
joint action of the gaseous phase and condensed phase, and are accompanied with a decrease of the
thermal stability and water resistance. In this work, a novel mono-component and gas-dominated flame
retardant, named DPPIP, was prepared through an amidation reaction of diphenylphosphinyl chloride
and piperazine, and used to flame retard PP. Experimental results confirmed that both the thermal
stability and water resistance of PP/DPPIP were improved. The initial thermal decomposition
temperature of PP/25 wt% DPPIP at 5 wt% weight loss was 287.5 ꢀC under air atmosphere, which is
higher than that of neat PP (266.1 ꢀC). Besides, a water-resistance test verified that PP/25 wt% DPPIP had
a weight loss of only about 0.52 wt%. More importantly, the flame retardant ability of PP/25 wt% DPPIP
had been greatly improved, passing the V-0 rating (UL-94). Furthermore, after the water-resistance test,
the LOI value of PP/25 wt% DPPIP exhibited nearly no difference and still passed the UL-94 V-0 rating. A
cone calorimeter (CC) result indicated that DPPIP had a positive effect on inhibiting heat release of PP
during combustion. All of these combustion tests displayed that there was no char left. The flame
retardant mechanism of DPPIP was investigated with py-GC/MS and TG-FTIR. The results illustrated that
the gaseous phase resulting from the thermal decomposition of DPPIP played the leading role in the
self-extinguishing behavior of PP/DPPIP, which consisted of a large amount of inflammable gaseous
products such as piperazine and its derivatives, and phosphorus-containing structures.
Received 5th September 2017
Accepted 20th October 2017
DOI: 10.1039/c7ra09868e
rsc.li/rsc-advances
thus protecting the substrate from burning.10,11 Therefore, these
ame-retardant systems for PP usually act through a gaseous
1. Introduction
phase and condensed-phase ame-retardant mechanism
together. Compared with the production of gas, the condensed-
phase formation of P–O–C structures during combustion is one
of the methods to improve the ame-retardant efficiency, and
the typical ame retardant system is a mixture of ammonium
polyphosphate and charring agent (CA).12–14 Wang et al.15
synthesized a novel charring agent (CA), namely MTEC, and
found that a PP/APP/MTEC system could achieve a LOI value of
28.5% and acquire a UL-94 V-0 rating at 20 wt% APP/CA through
the formation of P–O–C structures during combustion. The
condensed-phase formation of P–N–C structures during
combustion is also an efficient method to enhance the ame-
retardant efficiency of PP. In our previous papers,16–18 APPs
modied with ethylenediamine (EDA), ethanolamine (ETA) or
piperazine (PIP) were incorporated alone into PP. The results
also indicated that P–N–C structures were formed during
combustion, which could enhance the ame-retardant effi-
ciency for PP. In summary, APP is an essential component to
facilitate the formation of stable char during two P–N ame-
retardant methods. Unfortunately, the thermal stability and
water resistance of APP and modied APPs were not improved
Halogen-free ame retardant systems, such as metal hydrox-
ides, metal borates and intumescent ame retardants (IFR) etc.,
have been utilized to obtain ame-retardant PP composites.1–4
Among these halogen-free ame retardant systems, intumes-
cent ame retardants, especially phosphorus–nitrogen (P–N)
ame retardants, have attracted much attention in ame-
retardant PP due to their high ame-retardant efficiency.5–7
In order to obtain better ame-retardant efficiency of PP,
a lot of work has been done in past decades, especially research
on the ame-retardant mechanism. Generally, traditional P–N
ame retardants for PP mainly contain three components,
which are an acid source, blowing agent and charring agent,8,9
and when P–N ame retardants are heated beyond a critical
temperature they can generate multicellular swollen chars on
the surface of the polymer accompanied by decomposition of
the blowing agent, slowing down the heat and oxygen transfer,
aInstitute of Chemical Engineering, Changchun University of Technology, Changchun
130012, China. E-mail: zhu871227@163.com
bFujian Key Laboratory of Polymer Materials, College of Materials Science and Engineering,
Fujian Normal University, Fuzhou 350007, China. E-mail: jrkht1987@nu.edu.cn
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due to the production of large amounts of amine salts and the
The LOI value was measured using an HC-2C oxygen index
introduction of organic amine salts. Although lots of work has instrument (Jiangning, China) according to ASTM D2863-97.
been done, such as the modication of APP and micro- The sheet dimensions of all of the samples were 130 mm ꢂ
encapsulated APP,13,19 these problems still exist, which has 6.5 mm ꢂ 3.2 mm.
limited the application of APP in the practical eld. Thus, some
The UL-94 vertical burning level was tested on a CZF-2
researchers have considered whether PP could obtain an instrument (Jiangning, China) according to ASTM D 3801. The
excellent efficiency only acting through a gas-dominated ame dimensions of the samples were 130 mm ꢂ 13 mm ꢂ 3.2 mm.
retardant.
The ammability of the samples was measured on a CC
In the present work, a novel mono-component, only gas- device (Fire Testing Technology, UK). Samples with dimensions
dominated and P–N containing ame retardant, named of 100 mm ꢂ 100 mm ꢂ 3 mm were exposed to a radiant cone
DPPIP, was synthesized through the reaction of diphenylphos- at a heat ux of 50 kW mꢁ2
.
phinyl chloride and piperazine, and different measurements
The thermogravimetry-Fourier transform infrared spectros-
were used to conrm its chemical structure. The thermal copy (TG-FTIR) apparatus consisted of a TG 209 F1 (NETZSCH,
stability and water resistance of DPPIP were examined. Germany) thermogravimetric analyzer and a 170 SX FTIR
PP/DPPIP blends were prepared via physical blending, and their spectrometer (Nicolet, America). A sample (about 6 mg) was
thermal stability, water-solubility and ame retardancy were heated at a rate of 10 ꢀC minꢁ1 in the temperature range from 40
also studied. Finally, the ame retardant mechanism of DPPIP to 700 ꢀC under a nitrogen ow of 50 mL minꢁ1
.
was investigated with the aid of different measurements.
The samples for the py-GC/MS tests were rst pyrolyzed in
a pyrolyzer (CDS5200) under helium atmosphere. The relevant
samples (300 mg) were heated from ambient temperature to
600 ꢀC at a rate of 20 ꢀC msꢁ1 and kept for 15 s. Then, the
volatile products were carried through helium to a Perkin-Elmer
Clarus680GC-SQ8MS system coupled with the pyrolyzer. The
temperature of the capillary column (HP-5, 0.25 mm) of the GC
was held at 40 ꢀC for 3 min; aerward, the tempꢁe1rature was
2. Experimental section
2.1. Materials
Diphenylphosphinyl chloride (DPPC) was supplied by Shanghai
Changgen Chemical Reagent Co., Ltd. (Shanghai, China);
piperazine (AR), chloroform (AR) and triethylamine (AR) were
purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.
(Tianjin, China); polypropylene (T30S) was obtained from Petro
China Lanzhou Petrochemical Co., Ltd. (Lanzhou, China).
ꢀ
ꢀ
increased to 280 C at a heating rate of 10 C min and then
ꢀ
kept at 280 C for 5 min. The injector temperature was main-
tained at 280 ꢀC. The MS indicator was operated in the electron
impact mode at an electron energy of 70 eV, and the ion source
temperature was kept at 250 ꢀC. The identication of the
pyrolysis products was carried out by comparing to the NIST MS
2.2. Measurements
Fourier transform infrared spectroscopy (FTIR) tests were per- library.
formed on a Nicolet FTIR 170SX spectrometer (Nicolet, Amer-
The water-resistance of the ame retardant and modied PP
ica) using a KBr disk, and the wavenumber range was set from was tested according to the following procedure: 10 g of ame
4000–400 cmꢁ1
.
retardant was put into 100 mL of distilled water at 70 C and
ꢀ
1H nuclear magnetic resonance (1H-NMR) spectra and 31P stirred for 60 min. The suspension was then ltered. Next, the
nuclear magnetic resonance (31P-NMR) spectra were recorded obtained solid was dried to a constant weight at 100 ꢀC. Finally,
on a Bruker AV II-400 MHz spectrometer (Bruker, Switzerland) the solubility of the ame retardant in water was obtained.20
using CDCl3 as solvent. The sample (85 mg) was put into an The modied PP (the weight of M0) was put into distilled water
ꢀ
NMR tube, then a solution of CDCl3 (5 mL) was injected into the at 70 C, and kept at this temperature for 168 h (the water was
NMR tube, and next the NMR tube was shaken until the solids replaced at each interval of 24 h). Then the treated specimens
ꢀ
were dissolved in CDCl3. The concentrations of DPPIP, PIP or were taken out, and dried to a constant weight at 100 C. The
DPPC were about 1.7 g/100 mL. The NMR test was performed at weight of the dried sample is M1, so the weight loss can be
about 25 ꢀC, and TMS was applied. The peak which appeared at calculated based on the following equation: [(M0 ꢁ M1)/M0] ꢂ
0 ppm in the 1H-NMR spectra was ascribed to TMS.
X-ray photoelectron spectroscopy (XPS) spectra were recor-
ded on an XSAM80 spectrometer (Kratos Co, UK) equipped with
Al Ka excitation radiation (hn-1486.6 eV).
100%.20
2.3. Synthesis of DPPIP
Tensile testing was completed in accordance with the 47 g of diphenylphosphinyl chloride and 100 mL of chloroform
procedures in GB/T 1040.1-2006 at a crosshead speed of 20 were mixed in a 250 mL three-necked round-bottom ask
mm minꢁ1. The Izod impact properties were tested in accor- equipped with a reux condenser, dry N2 inlet, and a dropping
dance with the procedures in GB/T 1843-2008 and the depth of funnel. The mixture was agitated in an ice-bath under nitrogen
the nick was 2 mm.
atmosphere. Half an hour later, 8 g of piperazine dissolved in
Thermogravimetric (TG) analysis was carried out on a TG 209 chloroform (20 mL) was added over 1 h. Then 60 g of triethyl-
F1 (NETZSCH, Germany)ꢁt1hermogravimetric analyzer at a heat- amine was added to the solution. Aer completing the addition,
ing rate of 10 ꢀC min
under a nitrogen or air ow of the solution was heated to 75 ꢀC for 8 h until the reaction
50 mL minꢁ1 in the temperature range from 40 to 700 C.
completely nished. Finally, the solution was cooled to room
ꢀ
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are displayed in Fig. 2. All of the protons can be attributed to
1
the expected signals. Compared with the H-NMR spectrum
of PIP (Fig. 2b), the N–H signal at 1.75 ppm disappeared and
the peak at 2.80 ppm shied to 3.11 ppm in DPPIP, which is
assigned to –CH2– in the –CH2–N–P– group, indicating that
the P–N bond is formed. In the 31P-NMR spectra, there was
only one peak at 44.95 ppm for DPPC, which is ascribed to
the P in the –P–Cl group, and DPPIP displayed one charac-
teristic peak at 29.47 ppm, indicating that the P–N bond is
formed and a high purity of DPPIP. All of the 1H-NMR and
31P-NMR results demonstrate that DPPIP was obtained
successfully.
To further determine the structure of DPPIP, XPS measure-
ments of PIP and DPPIP were carried out. Their N1s spectra are
shown in Fig. 3. For PIP, the peak at 399.2 eV is ascribed to the N
in –C–NH–C–.17 While for DPPIP, a new peak at 402.0 eV is
attributed to the N in –P–N–C–, further indicating that DPPIP
was formed.
Scheme 1 The synthetic route of DPPIP.
temperature and then ltered. The ltrate was evaporated
under reduced pressure to obtain the crude product. The
product was washed with water repeatedly to remove the
impurities, then dried at 85 ꢀC for 12 h under vacuum to
a constant weight (84.3% yield). The reaction formula is
described in Scheme 1.
2.4. Sample preparation
ꢀ
DCPIP was dried in a vacuum oven at 85 C for 12 h. Then PP
blends lled with different ratios of DCPIP were prepared via
a twin-screw extruder (CTE 20, Kebeilong Keya Nanjing
Machinery Co., Ltd, Nanjing, China) with a rotation speed of
150 rpm with the following temperature protocol from the feed
zone to the die: 175, 180, 190, 185, 180 and 170 ꢀC. Finally, the
extruded pellets were hot-pressed into different samples by
a plate vulcanizer (Qingdao Yadong Rubber Viachinery Co. Ltd.
China).
3.2. Water resistance
Water resistance is an important property to assess the usability
of ame retardants in polymers. Thus, the water solubility of
DPPIP was tested and compared with our previously modied
APPs (PA-APP), and the corresponding data are listed in Table 1.
It could be observed that PA-APP is easily dissolved in 70 ꢀC
water due to the large amount of hydrophilic amine salt, and
the water solubility value is 10.0 g/100 mL of water. However,
because of the hydrophobic nature of P–N and the benzene
rings, the water solubility of DPPIP is less than 0.01 g/100 mL of
water. Moreover, the water resistance of PP/DPPIP and PP/PA-
APP was also evaluated through water-solubility experiments,
and the results are listed in Table 1. For PP/DPPIP, the weight
loss is only 0.52 wt%, which is far less than that for PP/PA-APP.
These results demonstrate that DPPIP and PP/DPPIP have an
excellent water resistance.
3. Results and discussion
3.1. Characterization of DPPIP
The FTIR spectrum of DPPIP is shown in Fig. 1. The peaks at
around 3025.6–3059.4 cmꢁ1 and 760.9 cmꢁ1 are ascribed to the
C–H vibrations of the benzene rings. The peaks located at
2948.6–2962.8 cmꢁ1 are assigned to the C–H stretching vibra-
tions of PIP. The peaks at 1116.4 cmꢁ1 and 722.5 cmꢁ1 corre-
spond to the P–N–C vibrations,16–18 and the peak at 1178.8 cmꢁ1
is assigned to the P]O vibration. The results show that DPPIP
had been prepared successfully.
3.3. Thermogravimetric analysis
To conrm the formation of DPPIP, 1H-NMR and 31P-NMR
tests of DPPC, PIP and DPPIP were performed, and the results
TG and derivative thermogravimetric (DTG) curves of neat PP,
DPPIP and PP/DPPIP under N2 and air atmosphere are illus-
trated in Fig. 4, and the corresponding data are listed in Table 2.
There is only one degradation step for DPPIP ranging from 300–
420 ꢀC under N2 atmosphere, and the initial decomposition
temperature (T5%, dened as the temperature where the mass
loss is 5 wt%) of DPPIP is 328.9 ꢀC, which is lower than that of
ꢀ
neat PP (383.5 C). Consequently, with the addition of DPPIP,
the initial decomposition temperature of modied PP (N2)
decreased, and it corresponds well to that in the calculated
curve. However, the degradation of PP/DPPIP under air atmo-
sphere is very different, as the initial decomposition tempera-
ture of DPPIP (310.8 ꢀC) is higher than that of neat PP (266.1 ꢀC).
As shown in Fig. 4c and d, with the temperature rising, the
ꢀ
residues at 700 C are almost gone for the neat PP, DPPIP and
PP/DPPIP both under N2 and air atmosphere. The TG results
ꢀ
suggest that the char layer is not formed at 700 C, indicating
that DPPIP generated almost only gas-phase products during
the decomposition.
Fig. 1 FTIR spectrum of DPPIP.
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Fig. 2 1H-NMR and 31P-NMR spectra of DPPC (a), PIP (b) and DPPIP (c).
3.4. Mechanical properties of ame-retardant PP composites
The mechanical properties of PP/PA-APP and PP/DPPIP,
including the tensile strength, elongation at break, and
impact strength, are listed in Table 3. Compared with PP/PA-
APP, PP/DPPIP with an equal amount of ame retardant had
no apparent change in the impact strength, but both the tensile
strength and elongation at break for PP/DPPIP increased.
Therefore, it can be concluded that DPPIP is more conducive to
the improvement of the mechanical properties of ame-
retardant PP than PA-APP.
Fig. 3 N1s spectra of PIP (a) and DPPIP (b).
Table 1 Water solubility of DPPIP, PA-APP, PP/DPPIP and PP/PA-APP
3.5. Flame retardancy
Average water solubility
Sample (g/100 mL of water)
Average weight
loss (wt%)
The ame retardant performances of neat PP and PP/DPPIP
before and aer water-resistance tests were evaluated by LOI
and UL-94 testing, and the corresponding data are listed in
Table 4. The two tests showed that neat PP is a ammable
polymer. For PP/DPPIP, with an increasing amount of DPPIP,
Sample
DPPIP <0.01
PA-APP ¼10.0
PP/25 wt% DPPIP 0.52 ꢃ 0.02
PP/25 wt% PA-APP 6.53 ꢃ 0.04
Table 2 Thermogravimetric properties of PP, DPPIP and PP/DPPIP
N2
Air
Tmax (ꢀC)
Tmax (ꢀC)
Sample
T5% (ꢀC)
1
2
Residue (%)
T5% (ꢀC)
1
2
Residue (%)
Neat PP
DPPIP
PP/25 wt% DPPIP
Calculated data
383.5
328.9
340.1
368.0
451.9
416.3
404.6
422.5
0.0
0.0
0.0
0.0
266.1
310.8
287.5
268.5
338.4
387.3
332.1
340.1
0.0
0.1
0.1
0.0
451.0
452.5
398.2
387.5
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Table 4 LOI and UL-94 results of neat PP and PP/DPPIP before and
after water resistance testing
Not treated with water
Aer water treatment
Component
(wt%)
UL-94 (3.2 mm)
LOI
UL-94 (3.2 mm)
LOI
PP
DPPIP (%)
Rating Dripping (%)
Rating Dripping
100
80
75
0
18.0 NR
24.5 NR
27.0 V0
28.5 V0
Yes
Yes
No
No
—
—
—
—
—
—
No
—
20
25
30
26.9 V0
—
70
—
Fig. 4 TG and DTG curves of neat PP, DPPIP, and PP/25 wt% DPPIP
and the calculated curve of PP/25 wt% DPPIP at a heating rate of
10 ꢀC minꢁ1 under N2 (a, b) and air (c, d) atmosphere.
Fig. 5 HRR (a) and THR (b) plots of PP/25 wt% DPPIP.
the LOI value increased from 18.0% (neat PP) to as high as
28.5% with the addition of 30 wt%. Furthermore, with only
25 wt% addition of DPPIP, the ame-retardant PP can pass the
UL-94 V0 rating and dripping is not observed. Aer the water
treatment, the LOI value of ame-retardant PP decreased
slightly, and when the addition of DPPIP was 25 wt%, the LOI
value decreased from 27.0% to 26.9%, and this can still pass the
UL-94 V0 rating. These results indicate that DPPIP is an effective
ame retardant for PP, which could further meet the needs of
practical application.
CC testing is an effective method to assess the combustion
behavior of materials. The corresponding heat release rate
(HRR), total heat release (THR), smoke production rate (SPR)
and total smoke production (TSP) curves of neat PP and PP/
25 wt% DPPIP are shown in Fig. 5 and 6, and the corresponding
data are presented in Table 5. As shown in Table 5, the time to
ignition (TTI) of the neat PP (20 s) is far lower than that of PP/
DPPIP (58 s) due to the early decomposition of neat PP at the
beginning of combustion, which is in accordance with the TGA
results in air atmosphere. Therefore, the ignition-resistance of
the composite is improved. With the addition of DPPIP, both
the peak of the HRR (PHRR) and the THR of PP/25 wt% DPPIP
decreased compared with those of neat PP, especially for the
PHRR, which decreased from 904.4 kW mꢁ2 to 487.7 kW mꢁ2
Fig. 6 SPR (a) and TSP (b) plots of PP/25 wt% DPPIP.
with a reduction of 46.1% compared with that of PP, indicating
that the presence of DPPIP is favorable to suppress the heat
release of neat PP.
Based on the HRR curves, the re growth rate (FGR) has been
calculated to assess the re hazard of the composite according
to the following equation:21,22
FGR ¼ PHRR/tPHRR
Table 3 Mechanical properties of PP/DPPIP and PP/PA-APP
Flame retardant
(wt%)
Tensile strength
(MPa)
Elongation at
break (%)
Impact strength
(kJ mꢁ2
Sample
)
PP/DPPIP
20
25
30
20
25
30
38.8 ꢃ 0.3
33.2 ꢃ 0.2
29.5 ꢃ 0.3
33.0 ꢃ 0.4
30.6 ꢃ 0.3
25.2 ꢃ 0.2
113.3 ꢃ 5.4
87.5 ꢃ 6.3
60.4 ꢃ 4.5
92.4 ꢃ 8.2
80.7 ꢃ 5.4
43.2 ꢃ 5.5
1.83 ꢃ 0.04
1.95 ꢃ 0.03
1.98 ꢃ 0.04
1.85 ꢃ 0.02
1.92 ꢃ 0.03
2.02 ꢃ 0.03
PP/PA-APP17
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Table 5 CC data of neat PP and PP/25 wt% DPPIP composites
products of DPPIP during the thermal decomposition process
under N2 atmosphere are shown in Fig. 8. The absorption peaks
of the benzene rings at 3063.3 cmꢁ1 and 695.1 cmꢁ1 appeared at
328 ꢀC, and some nitrogen-containing and carbon–hydrogen-
containing products were released at 328 ꢀC, as they pre-
sented the relevant characteristic absorption peaks at
2951.2 cmꢁ1 (–CH–, –CH2–, –CH3) and 1437.6 cmꢁ1 (–NH–,
–NH2). Meanwhile, the absorption peaks at 1246.2 cmꢁ1 (P]O),
1125.2 cmꢁ1 and 960.6 cmꢁ1 (P–C) and 1125.2 cmꢁ1 and
727.5 cmꢁ1 (P–N–C) appeared.17,18,23 With increasing time, these
absorbing peaks increased in intensity in the range from 328 to
418 ꢀC. The results indicate that some non-ammable volatiles,
such as benzene ring containing, nitrogen-containing, carbon–
hydrogen-containing and phosphorus-containing products are
generated during the thermal degradation of DPPIP.
To further demonstrate the ame retardant mechanism of
DPPIP, the prolyzed products of DPPIP were characterized with
py-GC/MS measurement and the results are shown in Fig. 9,
Table 6 and Scheme 2. It was found that the pyrolysis behavior
of DPPIP is very complicated. When DPPIP is heated up to
600 ꢀC, a small molecule like CO2 is produced at rst. With
increasing time, more than twenty-three kinds of pyrolysis
product were detected, which could be mainly summarized as
three kinds of component. The rst one was composed of
piperazine and its derivatives, including the peaks at 2 (piper-
azine), 4 (pyrazine), 6 (3-ethylpyrrole), 7 (2-methylpyrazine), 8
((E)-but-2-ene), 11 (2-methylimidazole) and 16 (4,40-bipyridine),
which were obtained from direct breaks of the piperazine ring
and its rearrangement into decomposition segments.23,24 The
Sample
Neat PP
PP/DPPIP
TTI (s)
20
904.4
140
6.46
126.2
0.5
58
Peak HRR (kW mꢁ2
Time to PHRR (s)
FGR (kW mꢁ2 s)
)
487.7
135
3.61
87.5
1.1
THR (MJ mꢁ2
Char residue (wt%)
)
Generally, a lower FGR value indicates that the time to
ashover is delayed.21 For PP/DPPIP, the FGR decreased from
6.46 kW mꢁ2 s to 3.61 kW mꢁ2 s, which was decreased by 44.1%
compared with that of neat PP. This result suggests that the
introduction of DPPIP could signicantly reduce the FGR of the
PP composite, and consequently could signicantly extend the
time to escape in a real incident.
The SPR and TSP curves of neat PP and PP/DPPIP are pre-
sented in Fig. 6, and the corresponding data are summarized in
Table 5. It is clear that the SPR and TSP of PP/DPPIP increased
more signicantly compared with those of neat PP. The results
illustrate that PP/DPPIP could produce lots of smoke, and
suggest that DPPIP generates gas-phase products during the
decomposition.
The digital photos of the residues for neat PP and PP/DPPIP
aer the CC test are shown in Fig. 7, and the char residues are
listed in Table 5. There is almost nothing le aer burning,
which is in accordance with the TGA results in air and N2
atmosphere, indicating that the gaseous phase production from
the decomposition of DPPIP plays a positive role in the ame
retardancy of PP. This can be described as follows: during
combustion the decomposition of DPPIP occurs, then a lot of
inammable gas is produced which could dilute the oxygen
concentration. As a consequence, the formation of inammable
gas should be the most important factor to achieve a better
ame retardancy of PP.
3.6. The ame retardant mechanism of DPPIP
As mentioned above, to reveal the ame-retardant mechanism
of DPPIP, the gaseous-phase activity of DPPIP is suggested.
Thus, the gaseous phase products of DPPIP were investigated
through TG-FTIR, and the FTIR spectra of the gaseous phase
Fig. 8 FTIR spectra of the gaseous products of DPPIP during the
thermal degradation under N2 atmosphere.
Fig. 7 Digital photographs of residues of neat PP (a) and PP/25 wt%
DPPIP (b) composites after the CC test.
Fig. 9 Pyrograms of DPPIP.
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Table 6 Compounds identified from the pyrograms of DPPIP
Peak
1
Retention time (min)
1.70
m/z
Compounds
Assigned structure
44
Carbon dioxide
O]C]O
2
3
4
5
6
2.39
2.85
3.99
4.47
5.38
86
78
80
92
95
Piperazine
Benzene
Pyrazine
Toluene
3-Ethylpyrrole
7
8
5.60
6.37
94
56
2-Methylpyrazine
(E)-But-2-ene
9
6.94
7.46
104
110
Ethenylbenzene
10
Phenylphosphineinhexane
11
12
13
7.64
8.03
9.35
82
2-Methylimidazole
White phosphorus
Methylphenylamine
124
107
P4
14
15
14.84
15.58
154
168
Biphenyl
2-Methylbiphenyl/4-methylbiphenyl
16
17
18
15.78
17.60
18.07
156
186
200
4,40-Bipyridine
Diphenylphosphin
Methyldiphenylphosphine
19
20
18.84
20.66
214
201
Ethyldiphenylphosphine
Diphenylphosphinoamine
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Table 6 (Contd.)
Peak
Retention time (min)
23.25
m/z
Compounds
Assigned structure
21
22
262
Triphenylphosphine
24.25
25.54
242
260
Diphenylpropenyl phosphine oxide
Ethylenediamine phosphine oxide
23
can capture active free radicals during the burning.26 Further-
more, the three kinds of inammable volatile gases can reduce
the oxygen content and capture active free radicals during
combustion through the gaseous ame retardance mode,
resulting in a superior ame-retardant efficiency of PP.
4. Conclusion
A
mono-component ame retardant named DPPIP was
prepared in this work. PP/DPPIP showed excellent thermal
stability and water resistance: the thermal decomposition
temperature of the PP/25 wt% DPPIP composite at 5 wt% weight
ꢀ
loss is 287.5 C under air atmosphere, and the weight loss was
only about 0.52 wt% aer the water treatment. Meanwhile,
PP/DPPIP exhibited excellent ame-retardant efficiency. The
LOI value of PP/DPPIP reached 27.0%, and passed the V-0 rating
at 25 wt% DPPIP. The CC data showed that the PHRR and low
THR etc., especially the FGR, largely decreased compared with
the corresponding values of neat PP. Furthermore, there was
almost no char residue le. The study on the ame retardant
mechanism of DPPIP indicated that lots of inammable
gaseous phase products are produced, such as piperazine and
its derivatives, and phosphorus-containing structures, which
nally lead to a better ame retardant performance for PP.
Scheme 2 Speculative pyrolysis products of DPPIP at 600 ꢀC.
second one consisted of the compounds with aromatic struc-
tures (peaks at 3, 5, 9, 13, 14 and 15), including the benzene ring
and its derivatives. These aromatic derivatives occurred
following the rupture of P–Ar. And last, lots of phosphorus-
containing structures were detected, corresponding to the
peaks at 10, 12, 17, 18, 19, 20, 21, 22 and 23.24–26 The
phosphorus-containing structures could produce PO free radi-
cals to capture active free radicals such as H and hydrocarbon
free radicals during combustion.26 Generally, the result of Py-
GC/MS is consistent with those of the TG-FTIR and CC tests.
Based on the Py-GC/MS and FTIR-TG results, the effect of the
gaseous phase on the ame retardation of PP is concluded as
follows. With increasing temperature, lots of piperazine and its
Conflicts of interest
There are no conicts to declare.
Acknowledgements
derivatives deriving from direct breaks of the piperazine ring This work was nancially supported by the National Natural
and its rearrangement into decomposition segments are Science Foundation of China (grant number 51703011;
formed and released rstly; meanwhile, the P–Ar bonds break 21504015; 51603018); Project of Jilin Provincial Department
and aromatic structures are produced, which can further of Education (grant no. 2016329); Program of Jilin
increase the amount of inammable volatile gas. Besides, the Provincial Department of Science & Technology (grant no.
rest of the P–Ar bonds continue to be degraded and rearranged, 20170520125JH); and Natural Science Foundation of Fujian
and some structures containing P are nally generated, which Province of China (grant number 2015J05094).
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15 Z. J. Wang, Y. F. Liu and J. Li, ACS Sustainable Chem. Eng.,
2017, 5, 2375–2383.
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