Study on thermal degradation and combustion behavior of flame retardant unsaturated polyester resin modified with a reactive phosphorus containing monomer

Article abstract of DOI:10.1039/c6ra06544a

A halogen-free phosphorus-containing monomer (TAOPO) with a P-C bond was successfully synthesized and used as a co-curing agent to prepare intrinsic flame-retardant unsaturated polyester resin (FR-UPR) by radical bulk polymerization with different TAOPO content. The thermal degradation and flame retardancy of pure UPR and FR-UPR were investigated by thermogravimetric analysis (TGA), cone calorimetry tests and limiting oxygen index (LOI). As the phosphorus content increased to 3%, FR-UPR (URP-3) showed a lower peak heat release rate (PHRR) and total heat release (THR), reducing by 45.7% and 45.5% those of pure UPR, while the LOI value and char residue increased markedly. Besides, thermal-oxidative degradation behaviors of different UPR samples were characterized by real-time infrared spectrometry (RT-IR) and thermogravimetry-Fourier transform infrared (TG-FTIR) spectroscopy, revealing the degradation mechanism. Furthermore, the residual char of UPRs was investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results indicated that the incorporation of TAOPO into UPR could effectively prompt the UP matrix to form a more compact char layer which acted as a protective barrier to reduce heat release during combustion.

Full text of DOI:10.1039/c6ra06544a

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PAPER
Study on thermal degradation and combustion
behavior of flame retardant unsaturated polyester
resin modified with a reactive phosphorus
containing monomer
Cite this: RSC Adv., 2016, 6, 49633
a
a
ab
Ying Lin,a Bin Yu,ab Xin Jin, Lei Song and Yuan Hu
*
*
A halogen-free phosphorus-containing monomer (TAOPO) with a P–C bond was successfully synthesized
and used as a co-curing agent to prepare intrinsic flame-retardant unsaturated polyester resin (FR-UPR) by
radical bulk polymerization with different TAOPO content. The thermal degradation and flame retardancy of
pure UPR and FR-UPR were investigated by thermogravimetric analysis (TGA), cone calorimetry tests and
limiting oxygen index (LOI). As the phosphorus content increased to 3%, FR-UPR (URP-3) showed
a lower peak heat release rate (PHRR) and total heat release (THR), reducing by 45.7% and 45.5% those
of pure UPR, while the LOI value and char residue increased markedly. Besides, thermal-oxidative
degradation behaviors of different UPR samples were characterized by real-time infrared spectrometry
(RT-IR) and thermogravimetry-Fourier transform infrared (TG-FTIR) spectroscopy, revealing the
degradation mechanism. Furthermore, the residual char of UPRs was investigated by scanning electron
microscopy (SEM) and Raman spectroscopy. The results indicated that the incorporation of TAOPO into
UPR could effectively prompt the UP matrix to form a more compact char layer which acted as
a protective barrier to reduce heat release during combustion.
Received 11th March 2016
Accepted 12th May 2016
DOI: 10.1039/c6ra06544a
www.rsc.org/advances
ame retardant monomers into UP chain or prepolymer, and
they undergo cross-link copolymerization under appropriate
conditions.6 The incorporation of reactive monomers into
polymeric structure is recognized as a more efficient way to
obtain an excellent ame retardant polymer system than the
former. Reactive monomers can markedly enhance the re-
resistance efficiency of some polymer materials even at low
dosage, which also has little side effects to mechanical prop-
erties of polymer matrixes.2,7–9
In the recent decades, phosphorus-containing ame retar-
dants, such as phosphine oxides, phosphonium compounds,
phosphonates, phosphites and phosphate, have been exten-
sively studied as promising substitutes for the halogenated
compounds.10 The presence of which plays a signicant role in
the high performances of re-retarded polymers due to its
ability to inhibit ignition and promote char formation during
plastic matrix decomposition.11 Also, some phosphorus-
containing compounds or co-monomers can effectively
improve ame retardant properties and thermal stabilities of
unsaturated polyester resin. Ammonium polyphosphate (APP),
as an intumescent ame retardant, can lead to a signicant
inuence on thermal stability and ame retardant property of
UPR by promoting the formation of high quality char during
thermal degradation.12,13 Chen et al.6 reported a reactive ame
retardant UPR from a phosphorus-containing diacid (DDP),
which showed good thermal stability and higher LOI value
1. Introduction
Unsaturated polyester resin (UPR), as an important thermoset
material, has been widely used in the eld of coatings, trans-
portation and building construction due to its comparatively
low price, easy processing and excellent mechanical properties
as well as good comprehensive performance aer curing.1–3
However, conventional UPR is susceptible to high temperature
and behaves poorly under re conditions (e.g., highly am-
mable and produces a large amount of smoke and toxic gases
when combusted) because of the intrinsic chemical composi-
tion and molecular structure, which seriously limits its further
application in some areas.4,5
To overcome the aforementioned drawbacks of UPR, many
approaches have been adopted to improve its thermal stability
and ame retardant properties. There are generally two effective
ways. One is additive-type ame-retarded modication that is to
blend the non-reactive additives and UPR matrixes; the other is
intrinsic ame-retarded modication—to introduce reactive
aState Key Laboratory of Fire Science, University of Science and Technology of China,
96 Jinzhai Road, Hefei, Anhui 230026, P. R. China. E-mail: leisong@ustc.edu.cn;
yuanhu@ustc.edu.cn; Fax: +86-551-63600081; +86-551-63601664; Tel: +86-551-
63600081; +86-551-63601664
bUSTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public
Safety, Suzhou Institute for Advanced Study, University of Science and Technology of
China, 166 Ren'ai Road, Suzhou, Jiangsu 215123, P. R. China
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(ꢀ29), and reached V-0 rating in UL-94 test when 18.1 wt%
DDP was incorporated. Kang et al.14 synthesized a cyclic
reactive monomer with high phosphorus content (EACGP)
that had reduced the mass loss rate and improved the char
yield of UPR when incorporated into matrix. Meanwhile,
EACGP had obvious effects on decreasing peak heat release
rate (PHRR) and total heat release (THR) of UPR. In addition,
based on the previously published literatures, phosphine
oxides containing P–C bond are more stable than the
compounds that only contain P–O–C bond among these
phosphorus-containing ame retardants.15–18 The former
possess higher thermal stability and resistance to moisture,
on account of better anti-hydrolysis ability of P–C bond.
Therefore, it can be an effective way to improve the ame
retardant property of polymers by incorporating chemical
units containing phosphine oxides with P–C bond into poly-
mer matrixes. Although, some of phosphine oxides have been
used as effective ame retardant for several polymer matrixes,
such as epoxy resin,17,18 little attempt was made to incorporate
reactive phosphine oxide monomers with P–C bond into
unsaturated polyester resins.
2.2 Synthetic procedures
Synthesis of Tris (hydroxymethyl) phosphine oxide (THPO).
THPO was synthesized based on the previously published
literatures.19 The synthetic route is shown in Scheme 1.
Synthesis of Tris (allyloxymethyl) phosphine oxide (TAOPO).
TAOPO was synthesized according to the method described in
the previous report.20,21 A three-necked round-bottom ask
equipped with magnetic stirrer, an additional funnel,
a circumference condenser and thermometer was employed. To
begin, benzyl-triethylammonium chloride (10% weight of
THPO) was well dissolved in THPO (0.1 mol) with stirring. Then
sodium hydroxide aqueous solution (30%) was added slowly,
ꢁ
maintaining the reaction temperature below 35 C. Aer that,
the mixture was cooled to 0 ꢁC in an ice bath. Next, allyl chloride
(0.36 mol) was added by dropwiseꢁinto the ask with stirring for
2 hours at low temperatures (<5 C). Subsequently, the system
ꢁ
temperature was increased to 45 C, maintaining for 10 hours,
then cooled to ambient temperature. Following this, crude
product was washed with a large amount of distilled water for
several times until chloride ion was completely undetectable by
Ba2+, and dried with anhydrous Na2SO4. Aer ltration, the
excꢁess reactants were removed by atmospheric distillation at
45 C. Finally, the remaining product was dried under vacuum
overnight. The detailed synthetic route is illustrated in Scheme
In this work, a ame retardant phosphine oxide monomer
with P–C bond was synthesized, and then copolymerized with
unsaturated bonds of UP in different ratios to obtain a ame-
retardant unsaturated polyester resin with excellent compre-
hensive performance. The re safety property, thermal degra-
dation and re-retardant mechanism were investigated.
1
1. Yield: 75%. H NMR (400 MHz, DMSO-d6), d: 4.07 ppm (6H,
P–CH2–O), 3.86 ppm (6H, O–CH2–C), 5.87 ppm (3H, –CH]),
5.18–5.31 ppm (6H, ]CH2). 31P NMR (400 MHz, DMSO-d6), d:
37.56 ppm (1P, O]P–C). FTIR (KBr): 3081.7 (]C–H), 2982.9
and 2857.1 (–CH2), 1645.3 (C]C), 1180.1 (P]O), 1090.4
(C–O–C).
2. Experimental
2.1 Materials
Orthophthalic unsaturated polyester (196# type, commercial
grade) was obtained from Hefei Chaoyu Chemical Co., Ltd.
2.3 Preparation of ame retardant UPR/TAOPO samples
(Anhui, China). Tetrakis (hydroxymethyl) phosphonium sulfate Different amount of reactive monomer (TAOPO) was added into
(THPS, 75% solution) was supplied by Aladdin Industrial Inc. UPR (according to the control of phosphorus content in UPR,
(Shanghai, China); Allyl chloride (CP) was provided by Xiya listed in Table 1) at room temperature with vigorous agitation
Chemical Industries Co., Ltd. (Sichuan, China); barium for about half an hour. Then radical initiator BPO (2 wt%) was
hydroxide (CP), hydrogen peroxide (30%, AR), sodium applied in homogeneous viscous liquid obtained in last step,
hydroxide (AR), chloroform (AR), anhydrous sodium sulfate stirring for another 20 minutes. Aer the removal of air bubbles
(AR), catalyst benzyltriethylammonium chloride (CP) and initi- in the mixture through the method of vacuum, casting
ator benzoyl peroxide (BPO, CP) were purchased from Sino- composition was rapidly poured into Teon mold, pre-cured at
pharm Chemical Reagent Co., Ltd. (Shanghai, China). BPO was 70 ꢁC for 3 h, post-cured at 120 ꢁC for 2 h. Finally, samples were
puried by recrystallization from chloroform and methanol cooled slowly to room temperature. The curing process was
before use. Other reagents were used as received.
shown in Fig. 1.
Scheme 1 The synthetic routes of THPO and TAOPO.
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Table 1 Formulations and appearance of UPR and UPR/TAOPO samples after curing
UPR content
(%)
TAOPO content
(%)
Phosphorus content
(%)
BPO contenta
(UPR %)
Appearance of
UPRs
Sample
UPR
100
0
0
2.0
2.0
2.0
2.0
Transparent
Transparent
Light yellow
Light yellow
UPR-1
UPR-2
UPR-3
91.6
83.2
74.5
8.4
16.8
25.5
1.0
2.0
3.0
a
BPO content is based on the weight percent of UPR.
Fig. 1 The curing process of flame retardant UPR samples.
The combustion behaviors of the samples (dimensions:
100 Â 100 Â 3 mm3) were investigated by a cone calorimeter
(Fire Testing Technology, UK) according to ISO 5660 standard.
Each specimen was wrapped in an aluminum foil and only the
upper surface was exposed horizontally to 35 kW mÀ2 external
heat ux.
Scanning electron microscopy (SEM) was used to investigate
the morphology of the char residue on a scanning electron
microscope AMRAY1000B. The specimens were previously
coated with a conductive gold layer.
2.4 Measurements
Thermogravimetric analysis (TGA) was carried out on a Q5000
IR thermogravimetric analyzer (TA instruments), heatiÀn1g from
ꢁ
ꢁ
ambient temperature to 700 C at a rate of 20 C min in air
and nitrogen atmosphere, respectively. The weight of each
sample was about 3–10 mg.
Thermogravimetry-Fourier transform infrared (TG-FTIR)
spectroscopy was for analyzing evolved gas of pyrolysis. It was
performed on the TGA Q5000 IR thermogravimetric analyzer that
was interfaced to the Nicolet 6700 FTIR spectrophotometer. The
samples were heated from room temperature to 700 ꢁC with the
heating rate of 20ꢁC minÀ1 under N2 condition during the test.
Real-time infrared spectroscopy (RTIR) was carried out using
a Nicolet MAGNA-IR 750 spectrophotometer equipped with
a heating device and temperature control apparatus. The
powdery samples were mixed with KBr, then pressed into
a tablet, following by placing them in a ventilated oven, heating
at a rate of 10ꢁC minÀ1 during the test.
Raman spectroscopy (RS) measurements were performed at
room temperature using a SPEX-1403 laser Raman spectrometer
(SPEX Co, USA) with excitation provided in back-scattering
geometry by a 514.5 nm argon laser line.
3. Results and discussion
3.1 Thermal degradation analysis
Limiting oxygen index (LOI) measurement was carried out at The thermal stability and degradation behaviors of ame
room temperature on a HC-2 LOI meter according to standard retardant unsaturated polyester (FR-UPR) and pure sample were
ASTM D2863. And the sample dimensions were 100 Â 6.5 Â 3 mm3. investigated by thermogravimetric analysis (TGA) under both
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air and nitrogen condition. Fig. 2 and 3 illustrate the TGA and nitrogen condition, and the char residue of all samples
differential thermogravimetry (DTG) curves of polymer material increased slightly than those in air, suggesting that oxygen
samples in air and N2 atmosphere, respectively. And the relative could facilitate the degradation of polymer materials. Also, it is
ꢁ
details are listed in Table 2. In addition, the initial degradation clear that the char yield at 600 C for pure sample and UPR-3
temperature (Td) is considered as the temperature at 5% weight was 7.84% and 10.74%, respectively. And the char residue of
loss, and Tmax, dened as the temperature at the maximum ame retardant samples improved obviously with the TAOPO
weight loss rate, was found in the DTG curves. And the yield of content increased, demonstrating the residual char of UPR
residual char at 600 ꢁC was obtained from TGA curves. It can be containing TAOPO was more stable in high temperature (>500
found from the TGA curves (Fig. 2(a)) that thermal oxidative ꢁC), which was similar to the results under air in Table 2.
degradation process of polymer samples consisted of three
The TGA and DTG results revealed that chemically incorpo-
stages. The initial degradation step was in the temperature rating ame retardant monomer with UPR could affect the
range of 200–330 ꢁC, resulting from mass loss of water dehy- degradation behavior of resin matrix. Although the initial
dration. While the second degradation stage, occurring at 300– decomposition temperature of FR-UPR declined slightly, the
420 ꢁC, could be ascribed to chain scission of polymer frag- thermal stability and char yield improved at relatively higher
ments (involving polystyrene and polyester), corresponding to temperature region, indicating that the phosphine oxide might
a strong peak in DTG curves at around 395 ꢁC. This thermal play a positive role in promoting charring because of the
degradation process led to the formation of a metastable formation of phosphoric acid during pyrolysis. Furthermore,
carbonaceous char which can be degraded further at higher this phosphorated residual char exhibited high thermal
temperature regions above 550 ꢁC in the third stage, and in this stability, and acted as a barrier to prevent heat and combustible
last step, as the TAOPO content raised, the char residue le gases transferring, thus protecting underlying materials.
increased markedly (Table 2). Specically, the char yield of
sample without TAOPO only remained 0.68 wt% at 600 ꢁC, while
that of UPR-3 reached to 8.01 wt%, indicating the phosphorus
3.2 TG-FTIR analysis on evolved gas of pyrolysis
containing monomer can be effective to promote char- In order to explore the primary thermal degradation mecha-
formation of UPR matrices.
nism in gas phase of UPRs, a further study on gaseous pyrolysis
However, under nitrogen condition, the degradation behav- products of UPR and FR-UPR during combustion was carried
iors of all samples were signicantly different from those in air. out by TG-IR technique which could signicantly identied the
As shown in Fig. 3(a), the thermal degradation process of each various gaseous products from decomposition. Fig. 4(a) and 5(a)
sample consisted of two stages. The rst step, occurring at 200– show the FTIR spectra of evolved gas products from pyrolysis of
320 ꢁC, was ascribed to water elimination reaction and the UPR and UPR-3 at the maximum mass loss rate (MMLR),
decomposition of TAOPO. Meanwhile, compared Td for UPRs at respectively. And the relative DTG curves were presented in (b)
ꢀ250 ꢁC, FR-UPRs degraded earlier than neat resin in lower (Fig. 4 and 5). Fig. 6 depicted the differences in TG-IR spectra of
temperature. It could be due to the decomposition of relatively gaseous products during thermal degradation at MMLR of pure
weak chemical bond P–C–O in TAOPO molecular at lower UPR and UPR-3. Comparing those gures (Fig. 4(a), 5(a) (curve
temperature, which was consistent well with the pyrolysis 1) and 6), it is clear that no obvious differences in pyrolysis
behavior of phosphorus-based ame retardants reported product species between two samples except some weak
previously.5,22 Moreover, Tmax of FR-UPRs was slightly higher absorbances at 1170 cmÀ1, 1488 cmÀ1 and 775 cmÀ1 that may
than that of pristine resin with relatively lower maximum mass be attributed to compounds containing phosphorus (corre-
loss rate, suggesting the UPR with TAOPO degraded more slowly sponding to functional groups of P]O and P–C), indicating the
and possessed higher thermal stability. Besides, the third introduction of TAOPO had little effect on the main species of
degradation process in air atmosphere disappeared under evolved gas during pyrolysis. Meanwhile, the gaseous small
Fig. 2 (a) TGA and (b) DTG curves of different UPR samples in air atmosphere.
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Fig. 3 (a) TGA and (b) DTG curves of different UPR samples in N2 atmosphere.
Table 2 TGA data of UPR and flame retardant UPR in air and nitrogen
atmosphere
Char (600
Td (ꢁC)
Tmax (ꢁC)
ꢁC, wt%)
Sample
P content (wt%)
Air
N2
Air
N2
Air
N2
UPR
0
1
2
3
238
259
236
206
283
250
268
225
383
392
398
397
394
403
404
396
0.68
3.38
6.33
8.01
7.84
8.31
9.28
UPR-1
UPR-2
UPR-3
10.74
molecules can be easily identied by the characteristic peaks in
the infrared region. Specically, the signals at around 3550–
3800 cmÀ1, 3150–2800 cmÀ1, 2400–2200 cmÀ1, 1900–1650 cmÀ1
and 1500–600 cmÀ1 tted well to the reported FTIR features of
volatile products such as H2O (3500–3800 cmÀ1), hydrocarbons
(2950–2850 cmÀ1), CO2 (2400–2300 cmÀ1), CO (2300–2200
cmÀ1), anhydride (1900–1840 cmÀ1) and carbonyl compounds
(1800–1700 cmÀ1).23,24
On the other hand, compared with the DTG data in Fig. 4(b)
and 5(b), two peaks appeared in the DTG curves at around 220
ꢁ
ꢁC and 380 C of UPR-3, and only one absorption signal at 390
ꢁC can be observed in DTG curve of pure UPR, indicating that
thermal degradation process of UPR-3 consisted of two stages,
while that of neat UPR presented only one. In addition, the FTIR
spectra of gaseous products at the rst degradation stage of
UPR-3 (see Fig. 5(a) curve 1) exhibited several characteristic
absorption at 1480 cmÀ1 and 800 cmÀ1, suggesting the gas
phase was mixed with phosphorus containing compound.
Fig.
4
(a) FTIR spectrum of pyrolysis products for UPR at the
maximum degradation rate and (b) corresponding DTG curve.
Therefore, considering the similar structure of two samples, the be caused by the reaction of TAOPO oligomer (& matrix) in UPR-
results above revealed that the preceding degradation step of 3 during degradation, which led to decrease in gaseous prod-
ame retardant UPR is mainly about the decomposition of ucts intensity. Also, a large amount of char layer formed in the
TAOPO oligomer, which demonstrated the ame retardant period of decomposition acted as a barrier to minimize the
mechanism based on gas phase may occur as expected.
evolved gas and heat release. Moreover, the maximum degra-
Fig. 7 illustrates the selected FTIR spectra of some volatilized dation rate signals of volatile products for UPR-3 came earlier
products (hydrocarbons, CO2, anhydride and carbonyl than UPR, indicating that the ame retardant UPR degraded
compound) from pyrolysis of UPR and UPR-3 at MMLR, earlier, which accorded well with the TG results. Consequently,
respectively. It is obvious that the UPR with TAOPO shows much the incorporation of TAOPO into UPR could improve the ame
lower absorbance intensity than that of pristine sample. It may retardant property based on gas phase.
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Fig. 6 TG-IR spectra of pyrolysis products at maximum degradation
rate of UPR and UPR-3 (comparison diagram).
Compared to the pure UPR, UPR-3 showed somewhat
different in the degradation behavior at relatively higher
temperature. And the RTIR spectra of UPR-3 are given in
Fig. 5(b). Although most characteristic absorption peaks can be
found in the spectrum at ambient temperature, the absorption
of P]O (1280 cmÀ1) and P–C (1480 cmÀ1) were difficult to
discriminate clearly, because they overlapped with the C–O–C
stretching vibration peak and aromatic absorption band,
respectively.28 Whereas the absorption peaks at 1280 cmÀ1, 1083
cmÀ1, 880 cmÀ1 still exist in a higher temperature above 420 ꢁC,
which differed from those of pure UPR. These peaks could be
assigned to asymmetric stretching vibration of P–O–P.29 This
demonstrated that phosphine oxide had poor heat resistance
and transformed into more stable polyphosphates at higher
temperature.30 Meanwhile, the absorption bands at 748 cmÀ1
and 699 cmÀ1 indicated the formation of aromatic residue char
Fig.
5 (a) FTIR spectra of pyrolysis products for UPR-3 at the
maximum degradation rate and (b) corresponding DTG curve.
3.3 RT-IR analysis on condensed product of pyrolysis
The real time infrared spectra (RT-IR) are employed to investi-
gate the different thermal-oxidative degradation behaviors of
UPRs before and aer incorporation of TAOPO via analyzing the
solid-state pyrolysis products of them. Fig. 8 reveals the RT-IR
spectra of neat UPR and UPR-3. As shown in Fig. 8(a), the
characteristic absorptions of pure UPR are listed as follows:
3490 cmÀ1, stretching vibration of –OH; 2957 cmÀ1, asymmet-
rical stretching vibration of –CH2; 1732 cmÀ1 and 1272 cmÀ1
&
ꢁ
during the pyrolysis in high temperature zone (>420 C). And
the compact char layer could prevent the further degradation of
polymer matrix in the combustion.
1148 cmÀ1, ester group (stretching of C]O and symmetrical
stretching of C–O–C, respectively);25 1448 cmÀ1, 747 cmÀ1 and
696 cmÀ1, aromatic ring. And all these absorption peaks were
obvious at ambient temperature. But as the temperature rose,
some alterations appeared. Particularly, the relative intensity of
peak at 3490 cmÀ1 declined signicantly at 250–300 ꢁC, and
completely vanished at 350 ꢁC, which ascribed to the evolution
of H2O and the loss of hydroxyl groups. Besides, only little
3.4 Flame retardant and combustion behavior analysis
Limiting oxygen index (LOI) is dened as the minimum volume
percentage of oxygen in a mixture which consists of oxygen and
nitrogen to just support the combustion of sample strip.31 This
parameter is used to evaluate the ammability of material. The
results of LOI test for ame retardant unsaturated polyester and
pure samples are presented in Table 3. The UPR without TAOPO
showed a low LOI value at 20.5%, whereas the ame-retardant
UPRs exhibited a higher LOI value which arose with the
increasing mass fraction of TAOPO. When the phosphorus
content of the ame retarded polymer sample (UPR-3)
enhanced to 3 wt%, its LOI value reached up to 27%. The
ꢁ
variation can be observed when temperature below 350 C for
other characteristic peaks. However, with the temperature
increasing further, the intensity of all absorption bands reduced
drastically, indicating that the main thermal-oxidative degra-
dation of unsaturated polyester and polystyrene chains
happened in the temperature range of 350–420 ꢁC. What's
more, all peaks almost disappeared when the temperature
reached 420 C, suggesting that the UPR degraded totally.26 All
results above are consistent well with TGA results.27
ꢁ
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Fig. 7 TG-FTIR spectra of (a) hydrocarbons, (b) CO2, (c) anhydride and (d) carbonyl compound at the maximum degradation rate for UPR and
UPR-3.
results demonstrated that the reactive monomer showed
certain ame retardant effect on UPRs.
Table 3 LOI values and cone data (PHRR and THR) of UPR and FR-
UPR
Cone calorimeter test is widely applied to investigate the
combustion behavior of polymeric materials.32 From which, the
statistics of heat release rate (HRR) and total heat release (THR)
can be obtained. The curves of those are shown in Fig. 9. And
the relative data is given in Table 3. It is clear that the incor-
poration of TAOPO into UPR had a signicant effect on
combustion behavior of polymer samples. With the introduc-
tion of ame retardant, the peak heat release rate (PHRR) and
THR of UPRs decreased markedly. Specically, the neat sample
Sample
LOI (%)
PHRR (kW mÀ2
)
THR (MJ mÀ2
)
UPR
20.5
25
26
849
585
547
461
49.9
37.7
34.9
27.2
UPR-1
UPR-2
UPR-3
27
Fig. 8 FTIR spectra of (a) UPR and (b) UPR-3 at different temperatures.
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Fig. 9 (a) HRR and (b) THR curves of UPR and FR-UPR samples under a 35 kW mÀ2 heat flux.
presented a rapid combustion with high PHRR (849 kW mÀ2
)
adding amount,6 attributed to aromatic structures that produce
and THR (49.9 MJ mÀ2) values (see Table 3), and remained only more smoke when burning. Moreover, layered double hydrox-
a small amount of char residues aer the test (Fig. 10). Unlikely, ides and some metal compounds can be as smoke suppressing
when TAOPO was incorporated to UPR with 3 wt% phosphorus agents for UPR.27,34,35
content, a much lower PHRR (461 kW mÀ2) and THR (27.2 MJ
mÀ2) can be observed, having fallen by about 45.7% and 45.5%
3.5 The characterization of residual char
than those of pure UPR, respectively. Meanwhile, UPR-3
possessed the most char yield of all aer burning, which
accorded with TGA results. Besides, when the phosphorus
The compact carbonaceous char, constructing during burning
process, is a protective barrier which acts as a thermal insulation
layer to prevent matrix pyrolysis and inhibit the transfer of
ammable gases and heat between both gas and condensed
content of re retardant polymer samples increased from 0 to 2
wt%, PHRR and THR values of UPR-1 drop to 585 kW mÀ2 and
phases during combustion.36 Therefore, the morphology of
37.5 MJ mÀ2, respectively. While those of UPR-2 show relatively
residual char is of great value in evaluating the ame retardancy
lower (547 kW mÀ2, 34.9 MJ mÀ2), indicating that TAOPO can
of materials.37 And the residues were obtained from pyrolysis of
promote UPR matrix to form a protective char layer and decline
ꢁ
polymer samples in a muffle furnace at 600 C for 10 minutes.
heat release during combustion effectively.
Fig. 11 shows SEM images of the outer char residue of pure UPR
Besides, the smoke release property is also considered as
and TAOPO/UPR samples. As shown in Fig. 11(a), many holes
and pits can be observed on the char surface, which may serve
limited insulation effect. On the contrary, the image of UPR-3
a valuation on re safety of polymer materials.33 Conventional
UPR is highly ammable and produces a large amount of smoke
during combustion, mainly due to styrene component and
char exhibited a much more compact and smooth surface with
some other aromatic structures.30 Although phosphorus-
containing ame retardants can signicantly improve the
ame-retarded property of UPR, they are not all effective in
reducing smoke emission. Furthermore, some phosphorus-
containing monomers and compounds with aromatic struc-
tures may raise the smoke emission rapidly as the increase of
Fig. 10 Photos of (a) UPR and (b) UPR-3 after the cone calorimetry Fig. 11 SEM images of the outer char residue of (a) UPR, (b) UPR-1, (c)
test.
UPR-2 and (d) UPR-3.
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Fig. 12 Raman spectra of char residues for FR-UPR samples.
only few pores (see Fig. 11(d)), which could play an effective part
on heat barrier. Besides, fewer holes on the char surface can be
observed with the increase of TAOPO content on ame retardant
UPRs (Fig. 11). As a result, it indicated that the ame retardant
component can promote the polymer matrix to construct
a protective char layer on the surface during pyrolysis and
combustion, reducing the production of combustible volatiles.
Raman spectroscopy is used to evaluate the structural order
degree of carbonaceous char layer. And two characteristic bands
appear on the spectra: D and G bands, at ꢀ1366 cmÀ1 and 1600
cmÀ1, which are typical of disorganized carbon and graphitic
phases, respectively.38,39 According to the reports previously, the
relative intensity ratio of two bands (ID/IG) can reect the
graphitization degree of carbon materials.40,41 Fig. 12 shows the
Raman spectrum which was dealt with peak tting to obtain 2
Gauss bands of each ame retardant UPR sample. Generally,
the increasing of the ID/IG ratio can indicate a decrease of
graphitization degree in char layer.42 It can be seen that (in
Fig. 12) the ID/IG ratios of three ame retardant UPR, in decrease
order, were UPR-1(2.987) > UPR-2(2.589) > UPR-3(2.347), sug-
gesting char layer of UPR-3 possessed the highest graphitization
degree and the best performance in char forming, which can be
employed to explain its highest char yield in TGA test under air
and N2 atmosphere. Moreover, the ID/IG ratio of UPR-2 was
lower than that of UPR-1, corresponding to the TGA results that
4. Conclusions
In this work, a reactive ame retardant phosphine oxide
monomer with P–C bond (TAOPO) was successfully synthesized.
TAOPO copolymerized with unsaturated polyester in different
ratios, resulting in ame-retardant UPRs with excellent
comprehensive performance. All the test results of ame-
retarded UPR indicated high temperature thermal stability,
combustion property and ame retardancy of TAOPO/UPR have
improved greatly. TGA data revealed that TAOPO can strengthen
the thermal stability of UPR matrix and produced additional
carbonaceous residue in higher temperature region. PHRR and
THR value of UPR-3 from cone calorimetry experiments reduced
signicantly, corresponding to the increase of LOI value. The
ame retardant mechanism was investigated by TG-FTIR and
RT-IR spectroscopy, indicating that the phosphine oxide olig-
omer in ame retardant UPR can decline the release of volatile
gas during pyrolysis and transformed into more thermally
stable polyphosphates species at higher temperature, giving rise
to highly structured char residue which prevented the further
degradation of the underlying polymer matrix, fully according
with SEM results.
Acknowledgements
more char residue le for UPR-2 aer pyrolysis. Raman results This work was nancially supported by National Basic Research
were consistent well with TGA data in both air and nitrogen Program of China (973 Program) (2014CB931804), the National
environment.
Natural Science Foundation of China (51473154) and
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Paper
Fundamental Research Funds for the Central Universities 21 L. J. Higham, M. K. Whittlesey and P. T. Wood, Dalton Trans.,
(WK2320000032).
2004, 4202–4208, DOI: 10.1039/b411701h.
22 X. Wang, Y. Hu, L. Song, W. Xing, H. Lu, P. Lv and G. Jie,
Polymer, 2010, 51, 2435–2445.
23 T. Rajkumar, C. T. Vijayakumar, P. Sivasamy and
C. A. Wilkie, Eur. Polym. J., 2008, 44, 1865–1873.
24 X. Chen, J. Zhuo and C. Jiao, Polym. Degrad. Stab., 2012, 97,
2143–2147.
References
1 B. K. Kandola, A. R. Horrocks, P. Myler and D. Blair,
Composites, Part A, 2002, 33, 805–817.
2 Z. Bai, L. Song, Y. Hu and R. K. K. Yuen, Ind. Eng. Chem. Res.,
2013, 52, 12855–12864.
25 E. Wawrzyn, B. Schartel, H. Seefeldt, A. Karrasch and
3 D. H. Builes and A. Tercjak, RSC Adv., 2015, 5, 96170–96180.
¨
C. Jager, Ind. Eng. Chem. Res., 2012, 51, 1244–1255.
´
4 M. C. S. Ribeiro, S. P. B. Sousa, P. R. O. Novoa, C. M. Pereira
26 L. Tibiletti, C. Longuet, L. Ferry, P. Coutelen, A. Mas,
J.-J. Robin and J.-M. Lopez-Cuesta, Polym. Degrad. Stab.,
2011, 96, 67–75.
and A. J. M. Ferreira, IOP Conf. Ser.: Mater. Sci. Eng., 2014, 58,
012020.
5 Z. Bai, L. Song, Y. Hu, X. Gong and R. K. K. Yuen, J. Anal. Appl.
Pyrolysis, 2014, 105, 317–326.
6 C. Zhang, J. Y. Huang, S. M. Liu and J. Q. Zhao, Polym. Adv.
Technol., 2011, 22, 1768–1777.
27 E. Kandare, B. K. Kandola, D. Price, S. Nazare and
R. A. Horrocks, Polym. Degrad. Stab., 2008, 93, 1996–2006.
28 K. Dai, L. Song, S. Jiang, B. Yu, W. Yang, R. K. K. Yuen and
Y. Hu, Polym. Degrad. Stab., 2013, 98, 2033–2040.
29 M. Bugajny, S. Bourbigot, M. B. Le and R. Delobel, Polym.
Int., 1999, 48, 264–270.
´
`
7 M. Sacristan, T. R. Hull, A. A. Stec, J. C. Ronda, M. Galia and
´
V. Cadiz, Polym. Degrad. Stab., 2010, 95, 1269–1274.
8 K. Dai, L. Song, R. K. K. Yuen, S. Jiang, H. Pan and Y. Hu, Ind.
Eng. Chem. Res., 2012, 51, 15918–15926.
30 Z. Bai, S. Jiang, G. Tang, Y. Hu, L. Song and R. K. K. Yuen,
Polym. Adv. Technol., 2014, 25, 223–232.
¨
9 R. M. Perez, J. K. W. Sandler, V. Altstadt, T. Hoffmann,
31 Y. Zhang, X. Li, Z. Cao, Z. Fang, T. R. Hull and A. A. Stec, Ind.
Eng. Chem. Res., 2015, 54, 3247–3256.
¨
D. Pospiech, M. Ciesielski and M. Doring, J. Mater. Sci.,
2006, 41, 341–353.
10 S. Lu and I. Harmerton, Prog. Polym. Sci., 2002, 27, 1661–
32 G. You, Z. Cheng, Y. Tang and H. He, Ind. Eng. Chem. Res.,
2015, 54, 7309–7319.
1712.
33 S. Nazare, B. K. Kandola and A. R. Horrocks, Polym. Adv.
Technol., 2006, 17, 294–303.
´
`
´
11 M. Sponton, J. C. Ronda, M. Galia and V. Cadiz, Polym.
Degrad. Stab., 2008, 93, 2158–2165.
34 S. Nazare, B. K. Kandola and A. R. Horrocks, J. Fire Sci., 2008,
26, 215–242.
35 C. M. C. Pereira, M. Herrero, F. M. Labajos, A. T. Marques
and V. Rives, Polym. Degrad. Stab., 2009, 94, 939–946.
36 Y. Yu, S. Jiang and F. Sun, Ind. Eng. Chem. Res., 2014, 53,
16135–16142.
12 J. W. Gu, G. C. Zhang, S. L. Dong, Q. Y. Zhang and J. Kong,
Surf. Coat. Technol., 2007, 201, 7835–7841.
13 L. L. Pan, G. Y. Li, Y. C. Su and J. S. Lian, Polym. Degrad. Stab.,
2012, 97, 1801–1806.
14 K. Dai, L. Song and Y. Hu, High Perform. Polym., 2013, 25,
938–946.
37 Q. Tai, Y. Hu, R. K. K. Yuen, L. Song and H. Lu, J. Mater.
Chem., 2011, 21, 6621.
¨
15 Q. Wu, J. Lu and B. Qu, Polym. Int., 2003, 52, 1326–1331.
16 U. Braun, A. I. Balabanovich, B. Schartel, U. Knoll, J. Artner,
38 A. Fina, H. C. L. Abbenhuis, D. Tabuani and G. Camino,
Polym. Degrad. Stab., 2006, 91, 2275–2281.
¨
M. Ciesielski, M. Doring, R. Perez, J. K. W. Sandler,
¨
V. Altstadt, T. Hoffmann and D. Pospiech, Polymer, 2006,
39 X. Wang, S. Zhou, W. Xing, B. Yu, X. Feng, L. Song and Y. Hu,
J. Mater. Chem. A, 2013, 1, 4383.
47, 8495–8508.
17 M. Xu, W. Zhao and B. Li, J. Appl. Polym. Sci., 2014, 131,
41159.
40 Z. Bai, X. Wang, G. Tang, L. Song, Y. Hu and R. K. K. Yuen,
Thermochim. Acta, 2013, 565, 17–26.
18 Y. Li, H. Zheng, M. Xu, B. Li and T. Lai, J. Appl. Polym. Sci.,
2015, 132, 42765.
19 M.-J. Chen, C.-R. Chen, Y. Tan, J.-Q. Huang, X.-L. Wang,
L. Chen and Y.-Z. Wang, Ind. Eng. Chem. Res., 2014, 53,
1160–1171.
41 Q. Wu and B. Qu, Polym. Degrad. Stab., 2001, 74, 255–261.
42 S.-H. Liao, P.-L. Liu, M.-C. Hsiao, C.-C. Teng, C.-A. Wang,
M.-D. Ger and C.-L. Chiang, Ind. Eng. Chem. Res., 2012, 51,
4573–4581.
20 Z. Tan, C. Wu, M. Zhang, W. Lv, J. Qiu and C. Liu, RSC Adv.,
2014, 4, 41705–41713.
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