Synthesis and characterization of SPDSCD and its flame retardant application on epoxy resins

Article abstract of DOI:10.1039/d0ra05167e

In this study, a flame retardant agent, 2,4,8,10-tetraoxo-3,9-diphosphospiro[5.5]undecane spirophosphate-4,4-diaminopair benzene disulfone-1,3,5-himetriazine (SPDSCD), is synthesized through a direct polycondensation reaction. SPDSCD is a chemically expanded phosphorus-containing flame retardant in epoxy resins (EP). The molecular structure of SPDSCD and thermal stability are characterized by nuclear magnetic resonance, Fourier transform infrared spectroscopy, and thermogravimetric analysis, and EP/SPDSCD composites were investigated in detail. These properties are associated with the phosphorus-containing spiro structure on the main molecular chain which can promote condensation polymerization into carbon during pyrolysis, the new nitrogen-containing carbon source, and the triazine structure. SPDSCD shows good thermal stability and low flammability, the weight loss from 500 to 800 °C was only 6.1 wt%, and the residual mass at 800 °C was 48.9 wt%. With the addition of SPDSCD, the flame retardant quality of the composites was gradually enhanced, the carbon residue becomes denser, which isolates heat transfer and inhibits the volatilization of flue gas. The addition of 20 wt% SPDSCD in the EP sample was associated with a limited oxygen index (LOI) value of 26% and a vertical burning V-0 rating. Cone calorimeter test shows that the peak heat release rate is reduced by 75%; the heat release rate curve enters the heat release platform area with a value lower than the first peak, TSP is reduced by 24%, the ac-CO2Y value reduced by 25.6%, indicating that SPDSCD/EP produced less CO2, which obviously prevented the combustion of volatile gas. SPDSCD exerted a charring and barrier effect in the condensation phase. Using basic characterization and flame retardancy testing, this work determined that SPDSCD has good flame retardancy when added to EP.

Full text of DOI:10.1039/d0ra05167e

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Synthesis and characterization of SPDSCD and its
flame retardant application on epoxy resins
Cite this: RSC Adv., 2020, 10, 28654
a
Yi Zhang,ab Weiwei Yang, a Wei Zhao, c Fang Ruan,a Shulei Lia and Jiping Liu
*
In this study, a flame retardant agent, 2,4,8,10-tetraoxo-3,9-diphosphospiro[5.5]undecane spirophosphate-
4,4-diaminopair benzene disulfone-1,3,5-himetriazine (SPDSCD), is synthesized through direct
a
polycondensation reaction. SPDSCD is a chemically expanded phosphorus-containing flame retardant in
epoxy resins (EP). The molecular structure of SPDSCD and thermal stability are characterized by nuclear
magnetic resonance, Fourier transform infrared spectroscopy, and thermogravimetric analysis, and EP/
SPDSCD composites were investigated in detail. These properties are associated with the phosphorus-
containing spiro structure on the main molecular chain which can promote condensation
polymerization into carbon during pyrolysis, the new nitrogen-containing carbon source, and the triazine
structure. SPDSCD shows good thermal stability and low flammability, the weight loss from 500 to
800 ꢀC was only 6.1 wt%, and the residual mass at 800 ꢀC was 48.9 wt%. With the addition of SPDSCD,
the flame retardant quality of the composites was gradually enhanced, the carbon residue becomes
denser, which isolates heat transfer and inhibits the volatilization of flue gas. The addition of 20 wt%
SPDSCD in the EP sample was associated with a limited oxygen index (LOI) value of 26% and a vertical
burning V-0 rating. Cone calorimeter test shows that the peak heat release rate is reduced by 75%; the
heat release rate curve enters the heat release platform area with a value lower than the first peak, TSP is
reduced by 24%, the ac-CO2Y value reduced by 25.6%, indicating that SPDSCD/EP produced less CO2,
which obviously prevented the combustion of volatile gas. SPDSCD exerted a charring and barrier effect
in the condensation phase. Using basic characterization and flame retardancy testing, this work
determined that SPDSCD has good flame retardancy when added to EP.
Received 12th June 2020
Accepted 22nd July 2020
DOI: 10.1039/d0ra05167e
rsc.li/rsc-advances
components.6–8 Therefore, it is necessary to prepare high-ame
retardant EP materials with good physical properties and a high
1 Introduction
material compatibility. Remarkably, EP always generates large
amounts of CO2 and CO gases when burned. It is well known
that people can suffocate due to smoke when a re occurs.
Therefore, the preparation of a ame retardant that effectively
suppresses the release of smoke is essential.
Epoxy resin (EP) is one of the most widely used thermosetting
polymers due to its multiple excellent properties, including
adhesion, mechanical strength, low shrinkage, electrical insu-
lation, and corrosion resistance.1–3 However, EP has a low
oxygen index and is extremely ammable, which makes it
a great re hazard.4,5 In addition, EP releases a lot of heat and
combustible volatile substances when burning, which brings
a great threat to people's lives and property. These factors limit
EP's scope of application in electronic packaging and other
elds. Recently, EPs have been used as engineering adhesives
and as the matrix of ber-reinforced polymers in applications
such as new energy automobiles and complex structural aircra
Double spiro SPDPC (2,4,8,10-tetraoxo-3,9-diphosphospiro5
undecanespirophosphate-1,3,5-himetriazine-4,4-diaminopair
benzene disulfone) is a highly symmetrical cage-like compound
which combines an acid source and a carbon source. It can
dehydrate into char during the combustion process. Due to its
excellent ame retardancy, derivatives of SPDPC have attracted
the attention of many scholars. Wang Xin et al. synthesized
a ame retardant PFR by mixing 10-(2,5-dihydroxyl-phenyl)-
9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-
BQ) and pentaerythritol diphosphonate dichloride (SPDPC).9
This ame retardant exhibits good ame retardant properties.
Remarkably, the limited oxygen index (LOI) value of EP/PFR is
30.2 and the UL-94 value is at the V-0 level when the content of
PFR is 10%. However, the effect of suppressing the release of
damaging gases is not high. Li et al.10 synthesized a novel ame
retardant (SPDV) containing phosphorus and silicon elements.
aSchool of Materials Science and Engineering, Beijing Institute of Technology, Beijing
100081, China. E-mail: emmazhangling@163.com; yangweiwei0811@163.com;
18255171997@163.com;
18515503841@163.com;
liujp@bit.edu.cn;
Tel:
+86-139-10788891
bDepartment of Biology and Chemical Engineering, Shandong Vocational College of
Science &Technology, Weifang 261053, China
cCAS Key Laboratory of Space Manufacturing Technology, Technology and Engineering
Center for Space Utilization, Chinese Academy of Sciences, Beijing, 100094, China.
E-mail: zhaowei@csu.ac.cn
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Spirocyclic pentaerythritol bisphosphonate disphosphoryl
chloride (SPDPC), synthesized through a simple dehydrochlo-
rination reaction of pentaerythritol (PER) and phosphorus
oxychloride (POCl3), was added into 9,10-dihydro-9-oxa-10-
phosphaphenanthrene-10-oxide (DOPO)/vinylmethyldimethoxy
silane (VMDMS) oligomer (DV) to form a novel ame retar-
dant. This was applied to ame retardant EVM materials,
showing good ame retardancy. Intermediate SPDPC has
a good ame retardant performance and has broad application
prospects.
2 Experimental
2.1 Materials
Pentaerythritol (PER) was purchased from Beijing Chemical Plant
(Beijing, China). Phosphorus oxychloride was purchased from
Tianjin Guangfu Fine Chemical Research Institute (Tianjin,
China).
4,4-Diaminodiphenylsulfone
(DDS),
4,4-dia-
minodiphenylmethane (DDM), and dimethylsulfoxide (DMSO)
were purchased from Sinopharm Chemical Reagent Beijing Co.,
Ltd (Beijing, China). Triethylamine and acetonitrile were
purchased from Beijing Chemical Plant (Beijing, China). Mela-
mine was purchased from Sann Chemical Technology (Shanghai)
Co., Ltd. (Shanghai, China). SPDPC was made in the laboratory.
Epoxy resin (DGEBA, commercial name: E-44) was supplied by
Sinopec Baling Petrochemical Branch (Yueyang, China). All
materials were of reagent grade and were used as received.
Phosphorus, nitrogen, silicon, and other elements can be
added to epoxy compounds to form epoxy compounds with
ame retardant elements, or they can react with amino-based
curing agents to introduce them into the curing agent.11,12
Phosphorus-containing curing agents can effectively promote
the formation of carbon on epoxy resins. The carbon layer plays
a protective role and isolates the internal matrix from external
heat and oxygen to exert a ame retardant effect.13–18 Phosphate
esters, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide
(DOPO), arylphosphine oxide, PEPA, and SPDPC units are
commonly used to add ame retardant structural units. Wang
et al. prepared a linear phosphite-based epoxy19 resin (LPN-EP)
through a six step reaction using linear phosphite, para-
benzaldehyde, and epichlorohydrin as the main raw materials.
When the amount of LPN-EP added was 30 wt% and the
phosphorus content of the system was 3.46 wt%, the LOI of
ame retardant EP increased from 22.3% to 31.8% of pure EP
and reached level UL-94 V-0, but previous research found that
the tensile strength and exural strength of ame retardant EP
decreased as the LPN-EP content increased. Huang20 et al.
added DOPO to 1,4-naphthoquinone and p-naphthoquinone to
prepare DOPO-based diols (DOPO-NQ and ODOPB) and mixed
them with DGEBA to prepare ame-retardant EP. When the
phosphorus content was 2.1 wt%, the ame retardant EP was
able to reach UL-94 V-0. The introduction of co-monomers
improved the ame retardant level of EP by 30 to 40 ꢀC
compared with pure EP, and the thermal stability of the system
improved. However, with an increase in the phosphorus
content, the thermal stability of O–P]O was weaker than that of
C–C, and the initial thermal decomposition temperature drops.
Accordingly, the development of a new reactive ame retardant
that can give EP a satisfactory ame-retardant performance
while simultaneously maintaining the inherent thermal prop-
erties and lowering gas emissions is of great signicance for
industrial application.
2.2 Synthesis of SPDPC
The synthesis route of SPDPC is shown in Scheme 1. The raw
materials were pentaerythritol and phosphorus oxychloride,
which were charged at a molar ratio of 1 : 5. White pentaery-
thritol powder (0.3 mol) was added to a 500 ml round-bottomed
ask, which was connected to a mechanical stirrer and
a condenser tube. Then, 1.5 mol of freshly distilled phosphorus
oxychloride was added to the round-bottomed ask quickly. A
lye absorption device was connected to absorb acidic gases
released during the reaction. Aer stirring at room temperature
ꢀ
for 10 minutes, the temperature was raised to 110 C, and the
reaction was continued for 10 hours at 110 ꢀC. Then heating
stopped and the reaction stopped. Aer the liquid in the round-
bottomed ask was cooled to room temperature, a white gran-
ular solid product was obtained following suction ltration,
which was washed with acetone several times. The obtained
product was dried in a vacuum oven for 6–8 hours at 80 ꢀC. The
yield reached 70%.
2.3 Synthesis of SPDSCD
Scheme 2 depicts the synthesis route of SPDSCD. A 500 ml
three-necked round-bottom glass ask was lled with nitrogen
In this work, we prepare a chemical intumescent ame
retardant containing phosphorus, carbon, and nitrogen for the
Scheme 1 Synthesis route of SPDPC.
1
rst time. SPDSCD is characterized by H NMR and 31P NMR
spectroscopy and applied to EP materials and is conrmed to
effectively reduce gas emissions by a ame retardant test. Spi-
rocyclic diacid chloride (SPDPC) prepared by our laboratory is
used as a starting material. The thermal stability and ame
retardancy of the synthesized SPDSCD and SPDSCD/EP are
investigated. And the effects of SPDSCD/EP with different
loading capacities on the thermal stability and ame retardant
performance of epoxy resin ammonium polyphosphate
composite materials are studied and compared.
Scheme 2 Synthesis route of SPDSCD.
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2.5.2 Nuclear magnetic resonance spectroscopy. 1H NMR
(400 MHz) and 31P NMR (163 MHz) were recorded on a Bruker
AVANCE NMR spectrometer. All 1H NMR and 31P NMR were
obtained in d6-DMSO and referenced to the residual protonated
solvent and phosphoric acid, respectively.
2.5.3 Gel permeation chromatography (GPC). GPC was
performed on a Waters Breeze™ 2 HPLC system with a 2489 UV/
Visible detector and a 1515 isocratic HPLC pump. Both the
Fig. 1 The preparation process of SPDSCD/EP spline.
ꢀ
column and the detector were maintained at 40 C during the
determination process.
for 5 minutes. Then, 300 ml of acetonitrile was added to SPDPC
and 4,4-diaminodiphenylsulfone (DDS) (1 : 1) in a three-necked
ask, and the start stirrer was turned on (stirring rate: 300
ml minꢁ1). Aer the reactants in the ask were mixed
uniformly, a small amount of triethylamine was added and the
2.5.4 Thermogravimetric analysis (TGA). TGA was per-
formed on
a Mettler-Toledo TGA/DSC thermogravimetric
analyzer under a nitrogen atmosphere. Approximately 5 mg of
sample in an alumiꢁn1a crucible and heated from 50 to 700 ꢀC at
ꢀ
a rate of 20 C min
.
ꢀ
heating rate was adjusted to 5 C per 10 min. The gas released
2.5.5 Fourier-transform infrared spectroscopy (FTIR). The
FTIR information of the pyrolysis gases of the samples was
obtained by a Bruker Tensor 27 Fourier transform spectropho-
tometer coupled with TGA. The FTIR analysis was performed at
a resolution of 4 cmꢁ1, ranging from 4000 to 550 cmꢁ1. The
thermogravimetric analyzer and FTIR spectrometer were con-
from the reaction system were absorbed with lye. The reaction
was maintained for 12 h at 110 ꢀC under the protection of
nitrogen. Cooling to room temperature, add 300 ml acetonitrile
into the solution, and then 18.4 g (0.1 mol) of cyanuric chloride
was added, keep the temperature steadily raised to 120 ꢀC. Aer
continuous reux for 24 h, stopped heating and cooled to room
temperature. Then, heating stopped and the system was cooled
to room temperature. A yellow-brown solid was obtained aer
rinsing three times with acetone and acetonitrile ꢀand suction
lter, and this was placed in a vacuum oven at 80 C for 8 h.
ꢀ
nected by a quartz capillary at 300 C.
2.5.6 Scanning electron microscopy (SEM). SEM was per-
formed on the cross-section of EP nanocomposites using
a Hitachi SU8010 scanning electron microscope. The samples
were coated with a layer of gold prior to observation.
2.5.7 Cone calorimeter test. Flame retardant behavior was
carried out on a Fire Testing Technology apparatus with a heat
ux of 50 kW mꢁ2 according to ISO 5660, and the size of the
specimens was 100 ꢂ 100 ꢂ 1.2 mm3. All samples were moun-
ted on aluminum foil and tested three times, and the data were
averaged.
2.4 Preparation of SPDSCD/EP composites
Fig. 1 shows the process of the preparation of the SPDSCD/EP
composite. PA 250 ml beaker with 50 g of EP-44 was placed in
a constant temperature oil bath at 80 ꢀC and stirred for 30 min.
The formula dosage of ame retardant was added according to
Table 1 and stirred for 30 min. Then, we added 8.75 g of the
curing agent DDM to the beaker and continued stirring. Aer
the appearance of the system became uniform, the air in the
breaker was removed, and then injection mold curing was
carried out. The mold was placed in the oven at 120 ꢀC for 2
hours and then at 150 ꢀC for 4 hours and cured. The cured
sample of the SPDSCD/EP composite material was obtained.
The proportion of the ame retardant is shown in Table 1.
2.5.8 Limit oxygen index (LOI) measurement. The LOI
value was measured on a JF-3 oxygen index meter (Nanjing-
Jiangning Analysis Instrument Co., China) in accordance with
ASTM D2863. The specimens were prepared with dimensions of
130 ꢂ 6.5 ꢂ 3 mm3 by molding.
2.5.9 Vertical burning test. The vertical burning test was
conducted on a CZF-3 instrument (Nanjing Jiangning Analysis
Instrument Co., China) with dimensions of 125 ꢂ 12.7 ꢂ 3.2
mm3 in accordance with ASTM D3801.
2.5.10 X-ray photoelectron spectroscopy (XPS). The XPS
data were obtained from a PHI Quantera II SXM at 25 W under
a vacuum pressure lower than 10ꢁ6 Pa.
2.5 Characterization
2.5.1 Fourier transform infrared spectroscopy. FTIR
spectra were obtained over the range 4000–400 cmꢁ1 on
a Bruker Tensor 27 Fourier transform spectrophotometer from
samples in KBr pellets.
3 Results and discussion
3.1 Characterization of the structure and thermal stability of
SPDSCD
Table 1 The proportion of flame retardants
The synthesized SPDSCD was obtained via direct condensation
polymerization. For SPDSCD, trimethylamine was used as an
acid binder to remove the HCl during polycondensation.
Additional processes were used to split the by-product, trie-
thylamine hydrochloride. In order to study the structure–
performance relationships, SPDSCD with a narrow molecular-
weight distribution was needed. Therefore, a nitrogen atmo-
sphere was used to exhaust the HCl from the reaction. In U.S.
Samples
EP (wt%)
SPDSCD (wt%)
DDM (wt%)
EP-0
EP-1
EP-2
EP-3
EP-4
100
95
90
90
80
0
5
17.5
17.5
17.5
17.5
17.5
5 + 5 (APP)
10
20
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Fig. 2 1H NMR of the synthesized SPDSCD.
Fig. 4 FTTR spectra of SPDPC and SPDSCD flame retardant.
Waters Breeze™ 2 type high efficiency liquid chromatograph
ame retardants and intermediates, the molecular weight of the
compounds: Mw (sticky molecular weight), Mn (the number
average molecular weight, molecular weight distribution) (Đ).
The test conditions were as follows: at room temperature,
a small amount of DMSO was dissolved, and tetrahydrofuran
was used as mobile phase. The weight-average molecular weight
(Mw) of SPDSCD tested by gel permeation chromatography
(GPC) is 7897; the molecular weight dispersity (Đ) is 1.05. The
dispersity is usually a result of side effects, for instance, chlo-
rination and reaction with residual water or alkylation of amino
groups.21,22
hydrogen bonded to N in (CN)3–NH–(CH)6-position-NH–, the
peak appearing near 7.24 ppm corresponds to the hydrogen
bonded to N on P–NH–.24
From Fig. 3, the 31P-NMR spectrum shows that the chemical
shi of ꢁ7.73 ppm corresponds to the absorption peak of
phosphorus in the SPDSCD main chain, and the chemical shi
of ꢁ21.13 ppm corresponds to the absorption peak of phos-
phorus at the chain end.
The 31P NMR curves of SPDSCD show two prominent signals
(Fig. 3). The one at around ꢁ7.73 ppm corresponds to the
absorption peak of phosphorus in the main SPDSCD chain; the
other signal at around ꢁ21.13 corresponds to the absorption
peak of the blocked phosphorus element. All spectroscopic
curves conrm the synthesis of SPDSCD.
Fig. 4 depicts the FTTR spectra curve of the SPDPC and
SPDSCD retardants. The spectral curves of SPDPC show the
characteristic absorption peak of the primary DDS amino group
(–NH2) at 3453 cmꢁ1, the C]C stretching vibration absorption
peak on the benzene ring at 1599 and 1496 cmꢁ1, a stretching
vibration peak of S]O at 1155 cmꢁ1, and a strong absorption
peak at 1550 cmꢁ1. These characteristic peaks indicate the
existence of the triazine structure. For SPDPC curves, a P–O–C
The prepared SPDSCD was characterized via 1H NMR and 31
P
NMR (Fig. 2) spectrum curves. As shown in Fig. 2, the multiples
between 4.2 and 4.5 ppm correspond to the SPDSCD spiral ring,
–CH2, which corresponds to the 1H NMR curves of SPDPC. The
multiples between 7.60 and 7.95 ppm represent the benzene
ring, 8.15 and 8.22 ppm correspond to the P–NH– and DDS
bonds, and 6.9 and 7.24 ppm represent the P–NH– triazine ring
and DDS bond, respectively.23 The peaks appearing at 4.20–
4.5 ppm correspond to the hydrogen in SPDSCD spiro–CH2, the
peaks appearing at 6.9 ppm, 7.60–7.95 ppm correspond to the
hydrogen on the benzene ring in the medium DDS structure,
and appear at 8.15–8.22 ppm. The peak corresponds to the
Fig. 3 31P NMR of the synthesized SPDSCD.
Fig. 5 TGA curves of SPDSCD.
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Table 2 Thermogravimetric analysis (TGA) parameters of SPDSCD
Tx% (ꢀC) (x ¼ residual char, wt%)
Residual char
(500 ꢀC, wt%)
Residual char
(700 ꢀC, wt%)
Samples
99%
95%
90%
50%
SPDSCD
SPDPC
182 ꢀ
180 ꢀ
C
C
274 ꢀ
254 ꢀ
C
C
291 ꢀC
274 ꢀC
634 ꢀC
492 ꢀC
54
48
48.9
32
stretching vibration absorption peak appeared at 1026 cmꢁ1
,
the P]O stretching vibration absorption peak appeared at
1301 cmꢁ1, and both appear absorption peaks of P(OCH2)C
appeared at 835 cmꢁ1. Taken together, the NMR and FTTR curve
data conrm the successful production of SPDPC and
SPDSCD.25,26 At 1155 cmꢁ1 is the stretching vibration peak of
S]O; a strong absorption peak appears at 1550 cmꢁ1, indi-
cating the presence of the triazine structure. The POC stretching
vibration absorption peak appears at 1026 cmꢁ1, the P]O
stretching vibration absorption peak at 1301 cmꢁ1, and the
P(OCH2)C absorption peak appears at 835 cmꢁ1 at 921 cmꢁ1
.
The peaks appearing at 1851 cmꢁ1, 773 cmꢁ1 are characteristic
peaks of spirocyclic phosphate.27–31
Fig. 6 The LOI histogram of SPDSCD/EP composite.
The thermal stability of SPDPC and SPDSCD was also char-
acterized by the TGA test, and Fig. 5 and Table 2 illustrate the
results. As Fig. 5 depicts, the retardant sample decomposed at
a low rate at temperatures between 181 and 370 ꢀC, and a rapid
weight loss stage occurred at 380–410 ꢀC. Compared to the TGA
curve of SPDPC, the initial decomposition of SPDSCD was
advanced; however, attainment of the maximum weight loss
temperature was delayed. This phenomenꢀon is because the
triazine group started to decompose at 180 C and then gener-
ated non-combustion gas, which caused the initial weight loss
temperature of the SPDSCD to decrease. The weight loss rate
became slower than the temperature of SPDPC, reaching the
SPDSCD/EP. When we compared the LOI value of the 5%
ammonium polyphosphate and 5% SPDSCD to 10% SPDSCD,
10% SPDSCD/EP was found to have a higher LOI value. This
indicates that SPDSCD/EP can signicantly increase the ame
retardant performance of EP matrix.
Pure EP was found to have no rating in UL-94 tests, the LOI
was 19%, and it produced little char residue. When the amount
of SPDSCD added was increased to 20%, the char residue of the
EP/SPDSCD composites increased remarkably. It is worth
mentioning that burning had a blowout effect when the addi-
tion was 20%. Char residue is able to protect the EP matrix,
preventing heat transfer to the pyrolysis zone and ammable
gases transfer to the ame zone. The UL-94 test for 20%
SPDSCD/EP reached level V-0, and there was no dripping
phenomenon, which showed the anti-dropping effect of
SPDSCD on EP during combustion. For the LOI test, we
compared the UL-94 level of 10% SPDSCD/EP and 5% APP/5%
SPDSCD/EP and showed that prepared SPDSCD has a higher
level and this matched well with the LOI test.
ꢀ
weightless platform stage at a temperature of 530 C. Further-
more, the residual carbon percentages of SPDSCD and SPDSCD
were ꢃ49% and 29% respectively. This is because of the pres-
ence of triazine groups, the simultaneous addition of SPDPC,
and because triazine has the advantage of increasing the
amount of residual carbon. Additionally, the carbon layer
generated during the pyrolysis and expansion of the sample was
dense and showed favorable thermal stability. Accordingly, the
prepared ame retardant SPDSCD showed higher thermal
stability and had a better charring performance, which depicts
that it could perform better as a barrier for materials.
3.3 Cone calorimeter test
3.2 LOI and vertical burning test
The cone calorimeter test was applied to investigate the ame-
The LOI test was used to investigate the ame retardancy of retardant effects of SPDSCD/EP and APP/SPDSCD/EP. This test
SPDSCD/EP, as depicted in Fig. 6. It is well known that the LOI can provide a signicant deal of information on materials'
value can support a convenient numerical assessment of ame- combustion characteristics through simulating a real re
retardant stability for polymers.32,33 However, the LOI value is atmosphere. Detailed information from the cone test is listed in
not a strict performance indicator of the material, as it is Table 4. This includes the time to ignition (TTI), the peak of
affected by environmental factors and experimental behavior. total mass loss (TML), the average effective heat of combustion
Fig. 6 shows that the LOI value of the SPDSCD/EP materials (av-EHC), the average heat release rate (av-HRR), the average
increased observably as the amount of SPDSCD increased. The CO2 yield (av-CO2Y), and the total heat release per total mass
LOI value increased from 19% for pure EP to 26% for 20% loss (THR/TML). The heat release rate (HRR) images and total
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10% SPDSCD and showed that the latter has a lower PHRR
value. Therefore, SPDSCD ame retardant performs better.
Because of the reduction in PHRR, there is a small gap between
5% APP/5% SPDSCD/EP and 10% SPDSCD. Interestingly, the
HRR curves show two peaks in the sample; this phenomenon
occurs due to the intumescent ame retardant (IFR)
characteristic.
Fig. 8 shows that SPDSCD/EP signicantly amended the THR
reduction of EP. Furthermore, the THR images of 5% SPDSCD/
EP show much lower values than those of pure EP, and the
images of 5% SPDSCD/EP and 10% SPDSCD/EP show much
higher values than those of 20% SPDSCD/EP, during the re
regeneration period. Less heat release indicates a lower speed of
re spread and a longer effective time period for the response,
thereby enhancing the escape time. The test also compared 5%
APP/5% SPDSCD/EP with 10% SPDSCD, showing that the latter
has relatively low PHHR and THR values. Therefore, SPDSCD
has more advantages in terms of ame retardant performance.
SPDSCD does not melt in EP or DDM. The SPDSCD/EP cured
system always exists; therefore, with the addition of SPDSCD,
the crosslink density of the system decreases and the initial
degradation temperature changes decreases. The pyrolysis gas
in the SPDSCD/EP is ignited earlier, and accordingly, the TTI
becomes smaller. The THR/TML represents the product of the
available heat from burning the volatiles and the burning effi-
ciency.34 The THR/TML ratio for the SPDSCD/EP composites
decreased by 22.4% because of ame inhibition. Furthermore,
Fig. 7 Heat release rate (HRR) curves of EP/SPDSCD.
Fig. 8 Total heat release (THR) of SPDSCD/EP and APP/SPDSCD/EP.
heat release (THR) data are depicted in Fig. 7, 8, and the
detailed representative parameters of the composite samples
are catalogued in Table 3. We can see from Fig. 7 that the peak
HRR (PHRR) of 5% SPDSCD/EP is close to that of 10% SPDSCD/
EP, and both are below that of EP without ame retardant,
which burns instantly aer ignition, causing the HRR value to
rapidly reach a sharp peak of 1177 kW mꢁ2. When the addition
of SPDSCD was 20%, the PHRR peak decreased to 28% of the
pure EP cure. The av-HRR for SPCSCD/EP samples in order of
decreasing re risk is as follows: 20% SPDSCD/EP < 5% APP/5%
SPDSCD/EP < 10% SPDSCD < 5% SPDSCD < EP (Table 3). We
also compared the HRR of 5% APP/5% SPDSCD/EP with that of
Fig. 9 The TGA curve of SPDSCD/EP in the N2 atmosphere.
Table 3 Cone calorimeter results for SPDSCD/EP composites
P-HRR (kW
THR/TML (MJ
av-HRR (kW
mꢁ2
av-CO2Y
(kg/kg)
av-EHC (MJ
kgꢁ1
Sample
TTI (s)
mꢁ2
)
THR (MJ mꢁ1
)
mꢁ2 gꢁ1
)
)
)
EP-0
EP-1
EP-2
EP-3
EP-4
51
41
31
26
24
1177
491
610
458
327
66
58
60
58
54
2.81
2.44
2.30
2.23
2.18
178.06
148.57
154.71
145.53
131.65
2.03
1.64
1.63
1.51
1.46
24.66
21.09
20.39
19.09
18.25
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Table 4 TGA parameters of SPDSCD/EP in N2 and air atmosphere
3 shows that the ac-CO2Y value reduced by 25.6%, indicating
that SPDSCD/EP produced less CO2, which obviously prevented
the combustion of volatile gas, acting as a gas phase ame
retardant. According to Table 3, we can conclude that the
prepared SPDSCD has a better ame retardant effect than the
normal ame retardant APP applied on EP.
Residuals at 500
Samples
Tonset (ꢀC)
Tmax (ꢀC)
ꢀC (wt%)
EP-0
EP-1
EP-2
EP-3
EP-4
388
340
340
338
328
408
346
364
366
351
17
23
23
26
29
3.4 TGA of EP and SPDSCD/EP composites
The TGA curves of SPDSCD/EP (Fig. 9) and APP/SPDSCD/EP give
information about the thermal stability and thermal degrada-
tion performance. Table 4 lists some detailed information. In
the N2 atmosphere, the initial decomposition temperature of
ꢀ
pure EP is 388 C; however, as SPDSCD and APP/SPDSCD were
added, the initial decomposition temperature gradually
decreased. Because of the earlier decomposition and catalysis
function of SPDSCD and APP/SPDSCD, the Tonset and Tmax were
lower than those of pure EP. The amount of residual char at
500 ꢀC was about 17 wt%, whereas that of SPDSCD/EP at 500 ꢀC
was from 23% to 29% with increased added SPDSCD mass.
Therefore, the addition of SPDSCD ame retardant has
a promotion effect on the charring yield of the EP matrix.
3.5 Morphology and ingredient analyses of the residues
from the cone calorimeter test
Fig. 10 shows digital images of the residues produced in the
cone calorimeter test. According to the images, pure EP
(Fig. 10a) was almost completely combusted, and we only
observed a small amount of EP residue aer the cone test. Aer
SPDSCD was incorporated into EP, as shown in Fig. 10(b and c),
some tiny holes could be clearly observed in the picture of 5%
SPDSCD. As the amount of ame retardant SPDSCD added
increased, the carbon layer gradually became denser, as seen by
the naked eye. We compared 5% APP/5% SPDSCD/EP to 10%
SPDSCD (insert in Fig. 10c) and found that the latter had
a denser structure. The carbon layer of the former was similar to
a lamellar structure. The residue char yield and quality of 20%
SPDSCD obviously enhanced and was the densest (Fig. 10d).
SPDSCD facilitated the charring process of EP during burning
and the carbon residue layer carried out a barrier function in
the condensed phase. There were no open pores visible to the
Fig. 10 Digital photos of EP residues from cone calorimeter. (a); EP (b);
EP + 5% SPDSCD. (c); EP + 10% SPDSCD. (d); EP + 20% SPDSCD.
the av-EHC values of all samples decreased when the amount of
added SPCSCD increased. EHC was used to test the burning
ratio of ammable volatile gas. The decline in the av-EHC values
means that the addition of SPDSCD can effectively suppress the
burning of volatile gases and prevent the free radical chain
reaction of the gaseous phase. The amount of CO2 released has
a signicant effect on the escape time when a re occurs. Table
Fig. 11 SEM photos of 20% SPDSCD residue from cone calorimeter. (a) Scan bar 100 mm; (b) scan bar 20 mm.
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Table 5 X-ray energy spectrum analysis of EP and SPDSCD/EP
Sample
C (%)
N (%)
O (%)
P (%)
EP-0
EP-1
EP-2
EP-3
EP-4
83.69
81.04
75.90
83
1.2
15.11
12
15.58
9.79
15.84
—
5.84
6.41
5.69
6.19
1.11
1.31
1.51
2.62
75.35
Fig. 13 The FTIR curve of residual carbon.
naked eye. The volume of the object was the smallest. Accord-
ingly, with an increase in the amount of ame retardant
SPDSCD, the structure of the ame retardant EP cured carbon
layer was denser, and the resistance to gas evolution also later has a higher phosphorus content, in accordance with
increased, which is in accordance with the cone test data.
results of the cone test. Due to the addition of 5% APP and 5%
The phosphorus–nitrogen ratio (P/N) of 20% of the samples SPDSCD in EP-2 and 20% SPDSCD in EP-4, the phosphorus
was 1 : 10. The gas release rate of this type of chemical expan- content of ame retardant samples EP-2 and EP-4 are 1.82 wt%
sion agent matches the speed of the ame retardant polymer, and 0.91 wt%, respectively, the triazine group content is
forming a carbon layer. The carbon formation rate and viscosity 1.46 wt%, 5.85 wt%, as shown in Table 5, the C, N, O, P in the
of the melt correspond to the system gas generation rate during samples of the two groups of ame retardant samples aer cone
the process of crosslinking, forming a carbon layer. Therefore, calorimetry test The content is not much different. The residual
the carbon layer foams evenly, forming a homogeneous closed- carbon content of EP-4 even reaches 2.62%, which is higher
cell foam carbon layer.
than EP-2, which conrms that the ame retardant SPDSCD has
SEM analysis of 20% SPDSCD was performed in order to a synergistic effect between the phosphorus-containing ame
observe the microscopic morphology of the char residue retardant structure and the new carbon source, which can be
further, and the results are depicted in Fig. 11. Fig. 11a shows improved. The ame retardant effect of phosphorus element
that the carbon residue layer performed as a foam-like solid promotes the retention of phosphorus element in the residual
formed by non-penetrating holes with a level of 40–100 mm, and carbon. A small amount of sulfur was found in the samples,
the residual hole distribution in the eld of view was relatively which is due to the DDS curing agent. It is reported that the
uniform and continuous. This structure is relatively regular sulfur-based products could accelerate the oxidation reaction,
(Fig. 11b), and it can provide thermal insulation protection for leading to the formation of char precursors. In addition, the
unburned substrate. It also hinders the escape of CO2 gas. This results also depict that there is a synergistic function between
result is in accordance with the results of the cone test analysis. the hetero-atoms and the phosphonamide structure. At the
The element contents of the carbon layer from SPDSCD/EP same time, the decomposition of the P–C bond structure can
aer the cone test were characterized by XPS. The result generate a phosphorus-containing gas in the gas phase, exerting
details are listed in Table 5. The XPS curve depicts that in the a gas phase ame retardant effect,35 which matches the above
char layer generated from the cone test, C, N, O, S, and P characteristics well.
elements were observed (Fig. 12). As the SPDSCD content
The FTIR test was performed on the residual carbon formed
increased, the phosphorus in char residue showed an obvious by the EP cured product and the ame-retardant EP cured
increase, indicating that more phosphorus penetrated into the product aer the cone calorimetry test, and the obtained curve is
char process, facilitating the formation of phosphorus-rich shown in Fig. 13. The residual carbon infrared spectra of EP-0,
residue and, therefore, generating better heat and combustion EP-2, and EP-3 all show peaks at 1555 cmꢁ1, which correspond
functions. In terms of comparison of the 5% APP/5% SPDSCD/ to fused ring aromatics. In the residual carbons of ame-
EP composite with 10% SPDSCD/EP, Table 5 shows that the retardant samples, the peaks at 1179 cmꢁ1 and 975 cmꢁ1 repre-
sent the existence of P]O, P–O–C and P–O–P bonds. The peak at
3409 cmꢁ1 represents the water peak, conrming that there is
moisture absorption during the placement of the residual carbon
of the sample; the peak at 2356 cmꢁ1 represents the C]O bond;36
the peak at 1555 cmꢁ1 represents –C]N–, –C]C– bond,37 cor-
responding to fused ring aromatic hydrocarbons; the peak at
975 cmꢁ1 in the residual carbon of samples EP-2, EP-3, EP-4
represents P]O; in the sample. The peaks at 1237 cmꢁ1 and
1067 cmꢁ1 in EP-4 residual carbon represent the presence of PN,
POC and POP bonds.38 In the sample EP-4 residual carbon, there
is an absorption peak near 2800–3200 cmꢁ1, conrming the
presence of –CH3, –CH2, –CH in the residual carbon.
Fig. 12 XPS spectrum of residual carbon full range energy spectrum
and P2P.
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