Synthesis of a novel highly effective flame retardant containing multivalent phosphorus and its application in unsaturated polyester resins

Article abstract of DOI:10.1039/c6ra19798a

Unsaturated polyester resin (UP) as one of the most important thermoset materials often exhibits serious fire hazards. Conventional phosphorus-containing reactive flame ratardants could overcome the deficiency of additive flame retardants in UP, but they

Full text of DOI:10.1039/c6ra19798a

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Synthesis of a novel highly effective flame retardant
containing multivalent phosphorus and its
application in unsaturated polyester resins
Cite this: RSC Adv., 2016, 6, 86632
a
a
b
c
a
Yongqiang Lin,† Saihua Jiang,†* Zhou Gui, Guohui Li, Xingxing Shi,
a
a
Guohua Chen and Xiangfang Peng*
Unsaturated polyester resin (UP) as one of the most important thermoset materials often exhibits serious
fire hazards. Conventional phosphorus-containing reactive flame ratardants could overcome the
deficiency of additive flame retardants in UP, but they always present low flame-retardant efficiency.
Herein, a novel reactive flame retardant containing multivalent phosphorus (À1 and +5), 2-((((6-
oxidodibenzo[c,e][1,2]oxaphosphinin-6-yl)methoxy)(phenoxy)phosphoryl)oxy)ethyl acrylate (DHP), was
successfully synthesized and introduced into unsaturated polyester resin (UP). The structure of DHP
was confirmed by Fourier transform infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance
(NMR). The introduction of DHP (#20 wt%) significantly improved the flame retardancy of UP/DHP
composites, e.g., it increased the limiting oxygen index (LOI) from 23.0% to 29.0%, decreased the peak
heat release rate (PHRR) by 37.2% and passed the UL94 V-0 rating. Thermal degradation analysis by
thermogravimetric analysis (TGA) and thermogravimetric analysis-infrared spectrometry (TG-IR),
combined with the char analysis by Scanning Electron Microscopy (SEM) demonstrate that the presence
of multivalent phosphorus makes DHP in the composites not only reduce flammable gas amounts and
capture free radicals in the gas phase, but also catalyze char formation in the condensed phase. This
work, which combines gas phase strategy and condensed phase strategy in one flame retardant,
provides a promising approach for the flame retardance of polymers, and would also stimulate more
efforts to explore highly-effective flame retardants.
Received 5th August 2016
Accepted 6th September 2016
DOI: 10.1039/c6ra19798a
www.rsc.org/advances
Traditionally, UP was treated to improve ame retardancy by
introducing halogen-containing ame retardants (particularly
1
Introduction
Unsaturated polyester resin (UP) cured with styrene, one of the bromine and chorine). However, owning to the release of highly
most important thermoset materials, has been widely used in toxic and corrosive fumes during combustion, the halogen-
the automotive industry, the chemical industry, electrical containing ame retardants have been banned in many
2,6
engineering, building construction, etc., due to its prominent areas. In recent years, phosphorus-containing ame retar-
mechanical performances, electrical insulation properties, dants, as halogen-free ame retardants for polymeric materials,
1–3
process-ability and low cost. Styrene is used both as a cross- have aroused increasing attentions because of their nontoxicity
7,8
linking agent and a diluent in the process of curing UP, and high ame retardancy efficiency. Generally, phosphorus-
leading to a highly cross-linked structure and excellent containing ame retardants were added into UP by physical
4
mechanical properties. However, the intrinsic chemical incorporation. But the physical incorporation oen leads to
composition, molecular structure, and the high loading of poor mechanical property and transparency owning to the bad
styrene (ca. 35–40 wt%) give rise to high ammability of UP, distribution, and causes an environmental threat because of the
2,5
8,9
which restricts its further use in many desirable applications.
limited linkage between additives and polymer matrix. An
alternative technique is the chemical incorporation via copoly-
merization. The relatively low content required to achieve
School of Mechanical and Automotive Engineering, South China University of sufficient ame retardancy, and the careful selection of reactive
Technology, Wushan Road 381, Guangzhou, 510641, P. R. China. E-mail: ame retardants, may keep detrimental changes to physical
a
meshjiang@scut.edu.cn; pmxfpeng@scut.edu.cn; Fax: +86-020-22236321; +86-020-
properties of UP at an acceptable level. The chemical bonds
8
7112503; Tel: +86-13678970588; +86-020-87111143
between matrix and ame retardants lead to a good distribution
of ame retardants, and then the ame retardancy of the
b
State Key Laboratory of Fire Science, University of Science and Technology of China,
Jinzhai Road 96, Hefei, Anhui, 230027, P. R. China
4,10–12
c
polymer can be kept longer.
Thus, exploiting novel
Tianjin Fire Research Institute of Ministry of Public Security, Tianjin 300381, China
phosphorus-containing reactive ame retardants to improve
†
These authors contributed equally to this work.
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the ame-retardancy of UP has become a promising avenue of
research.
2
Experimental
2.1 Materials
However, according to the available literature, the ame-
retardant efficiency of most conventional phosphorus- UP (commercial name 191), cobalt naphthenate and methyl
containing reactive ame retardants is low, due to their ethyl ketone peroxide (MEKP) were provided by Dongguan
single ame-retardant mechanism (gas phase mechanism or Nuoweitai Composite Materials Co., Ltd. (Dongguan, China).
10–12
condensed phase mechanism).
As described in our 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)
previous work, a combined gas and condensed phase action was supplied by Shenzhen Jinlong Chemical Co., Ltd. (Shenz-
occurred in UP could remarkably benet to the enhancements hen, China). Phenyl dichlorophosphate (PDCP) was supplied by
of ame-retardant efficiency, which can be achieved by multi- Changzhou Xuanming Chemical Industry Co., Ltd. (Changz-
13,14
valent phosphorus.
To address that, a novel reactive ame hou, China). Hydroxyethyl acrylate (HEA), triethylamine (TEA),
retardant containing multivalent phosphorus (À1 and +5), tetrahydrofuran (THF), toluene and paraformaldehyde were all
DHP, was successfully synthesized in this work, and then reagent grades and purchased from Sinpharm Chemical
introduced into UP matrix to prepare UP/DHP composites. The Reagent Co., Ltd. (Shanghai, China).
structure of DHP and UP/DHP was conrmed by NMR and
FTIR. The ammability of UP/DHP composites was evaluated
2
.2 Synthesis of 6-(hydroxymethyl)dibenzo[c,e][1,2]
by LOI test, UL-94 and microscale combustion calorimetry
MCC). Thermal degradation behaviors studied by TGA and
oxaphosphinine 6-oxide (DOPO-OH)
(
The synthesis of DOPO-OH was illustrated in Scheme 1a. Firstly,
DOPO (108 g, 0.5 mol) and toluene (200 mL) were added into
a 500 mL three-necked glass ask which was equipped with
a mechanical stirrer, a reux condenser and a nitrogen inlet.
TG-IR, combined with char analysis by SEM indicate that the
presence of DHP in UP/DHP composites could not only reduce

ammable gas amounts and capture free radicals (H$ and OH$)
in gas phase, but also catalyze the char formation of UP in
condensed phase, which are the main reasons for the ame
retardancy enhancements. The detailed mechanisms were
proposed.
ꢀ
The mixture was stirred and heated slowly to 80 C for 1.5 h
under nitrogen atmosphere until DOPO was dissolved
completely in toluene. Next, paraformaldehyde was added into
ꢀ
the mixture, and the reaction mixture was heated to 95 C and
kept at reuxing temperature for another 8 h. Finally, the
Scheme 1 Synthesis route and structure of DHP.
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precipitate was ltered and washed with toluene aer cooling to Table
1
Flame retardancy and thermal properties of UP/DHP
composites
room temperature, yielding a white powder. The powder was
ꢀ
dried under a vacuum at 70 C for 12 h.
DHP contents PHRR
LOI
T0.1 Char at
À1
ꢀ
ꢀ
Sample
(wt%)
(W g ) UL-94 (%) ( C) 700 C (wt%)
2
.3 Synthesis of 2-((((6-oxidodibenzo[c,e][1,2]
UP-DHP0
0
452
349
324
284
NR
NR
NR
V-0
23.0 350 5.2
28.5 337 5.3
28.5 326 5.6
29.0 298 6.5
oxaphosphinin-6-yl)methoxy)(phenoxy)phosphoryl)oxy)ethyl
acrylate (DHP)
UP-DHP10 10
UP-DHP15 15
UP-DHP20 20
DHP was prepared according to Scheme 1b. PDCP (21.1 g,
0.1 mol) and THF (80 mL) were rstly fed into a 250 mL
three-neck ask equipped with a dropping funnel, a reux
condenser and a mechanical stirrer. The system was cooled to 2.5 Characterization
ꢀ
0
(
C. TEA (21.3 g, 0.21 mol), HEA (11.6 g, 0.1 mol) and THF
Fourier transform infrared (FTIR) spectrum was recorded in
transmittance mode using NEXUS 670 in a spectrometer in
20 mL) were added dropwise over a period of 2 h, and main-
tained stirring for 4 h. DOPO-OH (24.6 g, 0.1 mol) dissolved in
THF (20 mL) were then slowly added into the mixture, and
stirred for another 4 h. Next, the mixture was heated to room
temperature, and kept stirring for another 12 h. The precipi-
tated triethylamine hydrochloride was ltered, and the solution
was then rotary evaporated to remove the solvent and unreacted
reactants under reduced pressure, yielding a brown viscous
liquid.
À1
À1
a range of 4000–400 cm with a range resolution of 4 cm (KBr
disk). TGA was conducted on a TGA 209F3 IR thermal gravi-
metric analyzer (Netzsch, Germany) from room temperature to
ꢀ
ꢀ
À1
1
7
00 C at a heating rate of 20 C min under N atmosphere. H
2
31
NMR and P NMR spectra were registered with a Bruker AV400
NMR spectrometer (400 MHz) operating in the Fourier trans-
form mode using dimethylsulfoxide (DMSO) as solvent for
DOPO-OH and DHP. LOI values were conducted on a JF-4
oxygen index meter (Jiangning Analysis Instrument Company,
China) according to GB/T2406.2-2009, and the reproducibility
of LOI values was Æ0.5%. UL-94 vertical ame test was con-
ducted on a CFZ-2-type instrument (Jiangning Analysis Instru-
ment Company, China) according to GB/T2408-2008. MCC tests
were carried out on a Govmak MCC-2 microscale combustion
calorimeter. The test principle has been described in our
2
.4 Preparation of UP/DHP composites
As illustrated in Scheme 2, the UP/DHP composites were
prepared by blending unsaturated polyester with different
ratios of DHP, combined with cobalt naphthenate as an accel-
erator and MEKP as an initiator. Each sample was rstly stirred
at room temperature for 1 h until it formed a homogeneous
mixture. The mixture was then poured into a special Teon
mold and heated at 90 C for 48 h to complete the cured reac-
tion. The compositions of UP/DHP composites are given in
Table 1.
15
previous work. TG-IR was conducted on the TGA STA449C IR
thermo gravimetric analyser (Netzsch, Germany), which was
interfaced to the Tensor27 FTIR spectrophotometer (Bruker,
Germany). The detailed test condition has been reported in our
ꢀ
Scheme 2 The curing process of UP/DHP composites.
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13
previous work. SEM images were obtained with a scanning
electron microscope HITACHI S-3700N at an accelerating
voltage of 12.0 kV. Transmittance of UP and UP/DHP compos-
ites was measured by Ultraviolet-visible (UV-vis) spectropho-
tometer (UNICAM UV-500) in a range of 300–800 nm with
a range resolution of 2 nm.
3
Results and discussion
3.1 Structural characterization
The chemical structure of DOPO-OH and DHP was character-
1
31
ized by H NMR, P NMR and FTIR. Fig. 1a and b present the
1
1
H NMR spectra of DOPO-OH and DHP, respectively. The H
NMR spectrum of DOPO-OH displays some characteristic
peaks: the peak at 3.30 ppm corresponds to the proton of –OH;
the peaks at 4.00–4.30 ppm correspond to the protons of –CH –;
2
the peaks at 7.26–8.27 ppm correspond to the protons of
Fig. 2 FTIR spectra of DOPO-OH and DHP.
À1
16,17
aromatic ring.
In the case of DHP, the peak at 4.12 ppm corresponds to the
protons of P–O–CH –C; the peak at 4.20 ppm is attributed to
C–CH –O; and the peak at 4.87 ppm is attributed to P–CH –O;
the peaks at 5.94–6.27 ppm correspond to the protons of vinyl
The peak at 2.50 ppm is attributed to DMSO.
À1
n
–OH and 1602 cm for d–OH), C–H vibration (2984–2910 cm
2
À1
for nC–H and 1445–1380 cm for dC–H), P]O stretching vibra-
2
2
À1
tion at 1224 cm , and P–O–C stretching vibration at 1068–965
À1
À1
cm . For DHP, new peaks at 1721 and 1637 cm correspond to
16,17
group; the peaks at 6.88–8.34 are attributed to aromatic ring.
C]O vibration and C]C vibration, respectively, and the
intensity of –OH vibration is signicantly reduced.
above results further conrm the successful synthesis of DHP.
3
1
Fig. 1c and d show the P NMR spectra of DOPO-OH and DHP,
respectively. DOPO-OH spectrum shows a single peak at 33.16
ppm. Aer reacted with HEA and PDCP, the phosphorus
chemical shi moves to 27.12 ppm. Additionally, a new peak at
À6.87 ppm is assigned to the phosphorus of PDCP part. These
results indicate that DHP has been successfully synthesized.
16–18
These
3.2 Transparency
Fig. 3 shows UV-vis transmission spectra of pure UP and the
Fig. 2 are FTIR spectra of DOPO-OH and DHP. The most composites (thickness of 0.2 mm). From Fig. 3, the copolymers
À1
important bands of DOPO-OH are –OH vibration (3230 cm for show a decline in transmission in the visible light region with
1
31
Fig. 1 H NMR and P NMR spectra of DOPO-OH and DHP.
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To further investigate the ame retardancy of UP/DHP
composites, LOI and UL-94 results of all samples are also
given in Table 1. Pure UP exhibits a relatively low LOI (23.0%)
and no UL-94 rating. By adding DHP, the LOI values increase
gradually from 23.0% to 29.0%. As DHP content reaches 20
wt%, the UP/DHP samples can pass UL-94 V-0 rating. These
results demonstrate the ame retardancy of the composites is
enhanced with the introduction of DHP. Moreover, when the
UP-DHP20 samples are conducted on UL-94 test, an interesting
phenomenon occurs: the ames is stable at rst and then
shakes up and down, and eventually quenches by the gas ejec-
ted from the char surface, le char residues with compact and
integrate structures. That means the combustion of DHP may
release some inammable free radicals to quench more H$ and
OH$ free radicals, thus reducing the combustion intensity as
19–21
well as the corresponding combustion heat.
The MCC, LOI
Fig. 3 UV-vis tranmittance for UP and UP/DHP composites.
and UL-94 results above conrm the ame retardancy
enhancement of UP/DHP composites.
the introduction of DHP, but they still retain relatively high
transparency. These results suggest the good compatiblity
between DHP and UP matrix.
3
.4 Flame-retardant mechanism analysis
To study the ame-retardant mechanism of UP/DHP compos-
ites, TGA test of all samples was employed. Fig. 5 shows the TGA
curves of all samples under N atmosphere, and the corre-
2
3.3 Flame retardancy
sponding data are listed in Table 1. T0.1 is dened as the
temperature at 10 wt% weight loss, and the solid residual char
yields at 700 C are obtained from TGA curves. Compared to
To evaluate ame retardancy of UP/DHP composites, all
samples were characterized by MCC. Heat release rate (HRR)
obtained from MCC tests, especially for PHRR, is regarded as
the most important parameter for re hazard evaluation of
materials. Fig. 4 presents the HRR curves of UP and UP/DHP
composites vs. temperatures, and the corresponding PHRR
values are listed in Table 1. Pure UP resin is highly combustible,
ꢀ
pure UP, all composites with the introduction of DHP decom-
pose earlier, and T0.1 decreases as DHP content rises, which may
be attributed to the lower stability of P–O–C bond than common
12
C–C bond. The char yields of UP/DHP samples are increased
slightly with the increasing content of DHP. For example, the
ꢀ
À1
char yield of UP-DHP20 at 700 C is 6.5 wt%, which is higher
and its PHRR value is as high as 452 W g . With the intro-
than that of pure UP (5.2 wt%). That is because DHP can
catalyse the thermal degradation of UP matrix at the initial stage
and enhance char formation at higher temperature. The char
formed on the condensed phase can inhibit the transfer of
oxygen, heat and ammable gaseous products, which is
duction of DHP, the PHRR values of all composites are shied
to lower values, and decrease as DHP loading rises. For
example, when 20 wt% DHP is incorporated, the PHRR value is
shied to 284 W g , decreased by 37.2% when compared to
pure UP resin. These results suggest the good ame-retardant
effect of DHP on UP resins.
À1
Fig.
5 TG curves of UP and UP/DHP composites under N
2
Fig. 4 Heat release rate of UP and UP/DHP composites.
atmosphere.
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Fig. 6 SEM images (Â100): (a) exterior char residues and (b) interior
char residues of UP, (c) exterior char residues and (d) interior char
residues of UP-DHP20.
2–5
benecial to the ame retardancy enhancement. To further
study the char formation of UP/DHP composites during
combustion, Fig. 6 shows the SEM images of the outer and
inner char residues for UP and UP-DHP20. Compared to pure
UP, a much more compact and integrate char is formed in UP-
DHP20. In addition, the char residue of UP-DHP20 exhibits
a good cell-like structure, which would contribute to the inhi-
22,23
bition of heat and mass transfer in the burning area.
However, compared to the signicant enhancement of ame
retardancy, the increase of char content in condensed phase is
limited. DHP may exert more ame-retardant effect in gaseous
phase. To demostrate that, TG-IR of UP/DHP composites was
also conducted. Fig. 7 presents the 3D TG-IR spectra of
UP-DHP0 and UP-DHP20. The thermal degradation of UP-
Fig. 7 3D surface graphs for TG-IR spectra of the evolved gases
produced by (a) UP-DHP0 and (b) UP-DHP20.
DHP20 is different from that of UP-DHP0. In addition to the hydrocarbons is conducive to the ame retardancy enhance-
À1
characteritic peaks of UP matrix, such as CO
2
À1
(2357 cm ), ments. On the other hand, owing to incomplete combustion,
À1
À1
hydrocarbons (3078 cm , 2969 cm , 2860 cm ), anhydride small molecular gases such as CO2 and hydrocarbons are
2738 cm , 1808 cm , 910 cm ), ester (1760 cm , 1264
cm ) and styrene (1616 cm ), some new peaks appear in the
TG-IR spectra of UP-20. FTIR spectra of pyrolysis products of UP-
DHP0 and UP-DHP20 at the maximum decomposition rates are
shown in Fig. 8. As can be observed in Fig. 8, new adsorption
À1
À1
À1
À1
(
À1
À1
4
À1
À1
bands of P]O (1257 cm ), P–O (1120 cm ) appear in UP-
DHP20 which were referred to PO in H PO , indicating that
there are phosphate and its derivatives in the gaseous phase. It
was reported that the phosphorus-containing substance PH
H PO could generate free radicals such as P$, PO$, and so on,
2
3
2
3
,
3
2
which captured the OH$ and H$ to interrupt the radical action
19–21
as halogen did.
Thus, it is concluded that the gas-phase
mechanism play a key role in the ame retardancy enhance-
ments of UP/DHP composites.
Furthermore, Fig. 9 displays the absorbance intensity of
hydrocarbons, anhydride and CO
It is evident for UP-DHP20 that the release of CO
2
for UP-DHP0 and UP-DHP20.
and hydro-
2
carbons are signicantly decreased compared with pure UP,
which reveals that more carbon is le in the condensed phase
consistent with the TGA test. The reduction of ammable Fig. 8 TG-IR spectra of gas phase during the thermal decomposition
of UP-DHP0 and UP-DHP20 at the maximum decomposition rates.
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2
Fig. 9 Absorbance of pyrolysis products for UP-DHP0 and UP-DHP20 vs. temperature: (a) hydrocarbons, (b) CO , and (c) anhydride.
suppressed and macromolecules anhydrides are formed which TAOPO increases LOI value to 27%, and exhibits no rating in
1
0–12
have a dehydration capacity under high temperatures, and UP94 test.
further promote the dehydration and carbonization reaction in
matrix. Therefore, the presence of DHP could not only reduced
4
ammable gas amounts and capture free radicals in gas phase,
Conclusions

but also catalyze the char formation of UP in condensed phase,
corresponding well to thermal analysis and ammability tests.
Based on the analysis above, we propose a suggested ame-
retardant mechanism. A schematic representation of the
mechanism is shown in Scheme 3. During the UP/DHP
composites combustion, some gaseous pyrolytic products
such as hydrocarbons are released. Meanwhile, a protective
charred ceramic surface layer is formed as the result of the DHP
catalyze the charring of UP matrix. The char layers can inhibit
the diffusion of ammable gas products into the ame zone and
In this work, a novel reactive ame retardant containing
multivalent phosphorus was successfully synthesized and
incorporated into UP matrix through radical bulk polymeriza-
tion. The UP/DHP composites obtained exhibit relatively high
transparency and signicantly improved ame retardancy, such
as increased LOI values to 29.0%, decreased PHRR by 37% and
passing UL94 V-0 level. The presence of DHP in the composites
could not only reduce ammable gas amounts and capture free
radicals in gas phase, but also catalyze the char formation of UP
in condensed phase, which are the main reasons for the prop-
erty enhancements. This work which combines condensed-
phase mechanisms with gas-phase mechanisms in a reactive
2
that of the O and heat into the inner of polymers. On the other
hand, the degradation of DHP can produce free radicals such as
P$, PO$, and so on, which captured the OH$ and H$ to interrupt
the radical action of combustion in gas phase. The combination
of the ame-retardant strategies above leads to the ame
retardancy enhancement of UP/DHP composites. When
compared with other reactive ame retardants such as EACGP,
TAOPO and TRIPOD-DOPO, DHP in our work has been in the
top class, if not the best, to improve the ame retardancy of UP
resins. For instance, 20 wt% EACGP added into UP could
increase LOI to 26.5%, and present no rating in UP94 test. The
addition of 20 wt% TRIPOD-DOPO increases LOI value to 26%,
and only achieves UL94 V-2 level. The introduction of 25.5 wt%

ame retardant not only provided a promising approach for the
ame retardancy enhancement of UP resin, but also would

stimulate more efforts for highly effective ame retardants of
polymers.
Acknowledgements
This work was supported by the Natural Science Foundation of
Guangdong Province (2014A030310122), the Natural Science
Foundation of China (51503067), the Natural Science Founda-
tion of Guangdong Province (2014A030310190), the Opening
Fund of Key Laboratory of Building Fire Protection Engineering
and Technology of MPS and the Fundamental Research Funds
for the Central Universities.
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