Synthesis of a novel phosphorus-containing flame retardant curing agent and its application in epoxy resins

Article abstract of DOI:10.1166/jnn.2016.11637

A novel phosphorus-containing compound diphenyl-(2,5-dihydroxyphenyl)-phosphine oxide defined as DPDHPPO was synthesized and used as flame retardant and curing agent for epoxy resins (EP). The chemical structure was well characterized by Fourier transform

Full text of DOI:10.1166/jnn.2016.11637

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 2811–2821, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
Printed in the United States of America
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Synthesis of a Novel Phosphorus-Containing
Flame Retardant Curing Agent and
Its Application in Epoxy Resins
Hongkun Zhang1ꢀ2ꢀ3, Miaojun Xu , and Bin Li
1
1ꢀ2ꢀ∗
1Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials,
College of Science, Northeast Forestry University, Harbin 150040, People’s Republic of China
2
Key Lab of Bio-Based Material Science and Technology, Northeast Forestry University,
Ministry of Education, Harbin 150040, People’s Republic of China
Department of Food and Environmental Engineering, East University of Heilongjiang,
3
Harbin 150086, People’s Republic of China
A novel phosphorus-containing compound diphenyl-(2,5-dihydroxyphenyl)-phosphine oxide defined
as DPDHPPO was synthesized and used as flame retardant and curing agent for epoxy resins (EP).
The chemical structure was well characterized by Fourier transform infrared (FTIR) spectroscopy,
1
13
31
H, C and P nuclear magnetic resonance. The flame retardant properties, combusting perfor-
mances and thermal degradation behaviors of the cured epoxy resins were investigated by limiting
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oxygen index (LOI), vertical burning tests (UL-94), cone calorimeter and thermogravimetric anal-
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ysis (TGA) tests. The morphologies and chemical compositions of char residues for cured epoxy
Copyright: American Scientific Publishers
resins were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spec-
troscopy (XPS), respectively. The water resistant properties were evaluated by putting the samples
ꢀ
into distilled water at 70 C for 168 h. The results revealed that the EP/40 wt% DPDHPPO/60 wt%
PDA thermosets successfully passed UL-94 V-0 flammability rating and the LOI value was as high
as 31.9%. The cone tests results revealed that the incorporation of DPDHPPO efficiently reduced
the combustion parameters of epoxy resins thermosets, such as heat release rate (HRR), total
heat release (THR) and so on. The TGA results indicated that the introduction of DPDHPPO pro-
moted epoxy resins matrix decomposed ahead of time compared with that of pure EP and led to
a higher char yield and thermal stability at high temperature. The morphological structures and
analysis of XPS of char residues revealed that DPDHPPO benefited to the formation of a sufficient,
compact and homogeneous char layer with rich flame retardant elements on the epoxy resins mate-
rials surface during combustion. After water resistance tests, EP/40 wt% DPDHPPO/60 wt% PDA
thermosets still remained excellent flame retardancy, the moisture absorption of epoxy resins ther-
mosets decreased with the increase of DPDHPPO contents in the thermosets due to the existing
of P–C bonds and the rigid aromatic hydrophobic structure.
Keywords: Curing Agent, Flame Retardant, Epoxy Resin, Synthesis, Water Resistance.
1
. INTRODUCTION
However, conventional epoxy resins are very flammable
and can not satisfy some applications which require high
flame retardant grade. Therefore, the enhancement of flame
retardancy for epoxy resins is becoming an urgent problem
and has attracted more and more attention. Tradition-
ally, halogenated compounds have been widely used as co-
monomers or additives with epoxy resins to obtain flame
retardant materials. Although the halogenated compounds
show excellent flame retardancy for epoxy resins and have
Epoxy resins have been widely used in coating, cast-
ing, adhesive, and composite, encapsulant for semiconduc-
tors, insulating materials for electric devices and laminates
due to their excellent characteristics of toughness, water
and solvent resistance, low shrinkage on cure, supe-
4
ꢀ5
1
–3
rior electrical and mechanical resistance properties.
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 3
1533-4880/2016/16/2811/011
doi:10.1166/jnn.2016.11637
2811

Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
Zhang et al.
been used widely for several decades, their use has been
curtailed in many countries because of the toxic and corro-
In this work, a novel phosphorus-containing com-
pound with P–C bonds diphenyl-(2,5 dihydroxyphenyl)-
phosphonate defined as DPDHPP was synthesized and
used as flame retardant curing agents to prepare flame
retardant epoxy resins. The flame retardant properties,
thermal degradation behavior, flame retardant mechanism,
water resistance and combustion behavior of the cured
epoxy resins were characterized and disclosed.
6
ꢀ7
sive fumes released during combustion. Thus, the devel-
opment and application of halogen-free flame retardant has
been a subject of extensive investigation for epoxy resins.
A wide variety of halogen-free flame retardants containing
8
–10
11–15
16
17
silicon,
phosphorus,
nitrogen, carbon nanotube,
boron,1
8ꢀ19
20
21
layer double hydroxide, montmorillonite,
etc. have been explored to improve their flame retardancy
in the past decades.
2
. EXPERIMENTAL DETAILS
Among them, phosphorus-containing compounds is con-
sidered to be one promising method for flame retardant
epoxy resins because they can promote epoxy resins to
form a carbonaceous char, which acts as a physical bar-
rier to heat transfer from the flame to the polymer and
2
.1. Materials
Diphenylphosphine oxide was obtained from the
Fusilin Co., Ltd. (Qingdao, China). 1,4-Benzoquinone,
m-phenylenediamine, ethanol and methylbenzene were
purchased from the Tianjin Guangfu Fine Chemical
Research Institute (Tianjin, China). Epoxy resins (E-44,
epoxide equivalent weights = 213 g/epoxide) were sup-
plied by Fude Chemicals Industry (Guangzhou, China).
1
7ꢀ18
the diffusion of combustible gas and smoke.
As we
known, the mechanical properties of epoxy resin will
be deteriorated with the incorporation of flame retar-
dant additives due to the poor compatibility between
flame retardants and epoxy resins matrix, and the reac-
tive flame retardant will overcome these defects. For
example, Wang and Shieh reported that epoxy resins
cured by bis(3-hydroxyphenyl) phenyl phosphate exhib-
ited better flame retardancy and the thermosets passed
UL-94 V-0 rating with the phosphorus content as low
2
.2. Synthesis of Diphenyl-(2,5-dihydroxyphenyl)-
Phosphonate (DPDHPP)
The synthetic route of DPDHPP is shown in Scheme 1.
0.2 g (0.1 mol) DPO, 13.0 g (0.12 mol) Benzoquinone
2
and 120 mL dried toluene were added into 250 mL
four-necked flask equipped with a reflux condenser, a
thermometer and mechanical stirrer under a dry nitrogen
atmosphere. The mixture was heated to refluxing tempera-
ture and maintained for 20 h with stirring. Thereafter, the
reaction mixture was cooled down to room temperature
and a great deal of dark yellow precipitant appeared in
the solution. The precipitant was filtered and washed with
ethanol three times, and then was dissolved in potassium
hydroxide solution, hydrochloric acid was dripped into the
solution and white precipitant was produced. Finally, the
white solid particles were obtained by filtered, washed
2
2
as 1.5 wt% in the cured resin. 9,10-dihydro-9-aza-10-
phosphaphenanthrene-10-oxide (DAPO) was synthesized
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and reported, and the cured epoxy resins reached the
Copyright: American Scientific Publishers
2
3
UL-94 V-0 rating with phosphorus contents of 1.6 wt%.
2
4
Hergenrother et al. synthesized a novel phosphorus-
containing compound (bis(4-aminophenyl) methylphos-
phonate) and used as curing agent to prepare flame
retardant thermosets, and the LOI value of the thermosets
reached 31% with phosphorus contents of 3.9 wt%. How-
2
2–24
ever, these compounds
exhibited some inherent disad-
ꢀ
with ethanol and dried under vacuum at 60 C for 8 h
vantages, such as easy hydrolysis and poor resistance to
water due to the existence of P–O–C bonds in compounds,
which limit their widely application. An effective way to
improve flame retardant and comprehensive performance
for epoxy resins would be the incorporation of stable flame
retardants.
ꢀ
(
27.4 g, yield: 88.1%, m.p.: 212.5∼213.6 C).
2
.3. Preparation of the Cured Epoxy Resins
The epoxy resins thermosets were obtained via ther-
mally curing with various mass ratios curing agents of
DPDHPP and PDA (0/100, 10/90, 20/80, 30/70, 40/60,
Of the various phosphorus containing compounds, the
P–C bonds containing compounds are preferred as flame
5
0/50). The curing agents and epoxy resins in stoichiomet-
retardant.2
5–27
ric amounts were mixed homogeneously by mechanical
stirrer at 120 C. The liquid mixtures were poured into
a mold, cured at 130 C for 1 h and post cured at 160 C
They can promote char formation, have
ꢀ
high efficiency at low phosphorus concentration, and
ꢀ
ꢀ
have good thermal stability and resistance to acid and
base.2
6–28
Some P–C containing compounds have been syn-
thesized and used to enhance flame retardancy of poly-
mer materials, such as phosphine, phosphine oxide and
2
5ꢀ28–31
phosphonate.
So far, the phosphorus-containing with
P–C bonds flame retardant curing agent to improve flame
retardancy of epoxy resins has been seldom reported and
the flame retardant and water resistance mechanism should
be further investigated.
Scheme 1. Synthetic route of DPDHPP.
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Zhang et al.
Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
for 3 h. After curing, all the samples were cooled down to
room temperature and carried out LOI and vertical burning
2.8. Scanning Electron Microscopy (SEM)
Scanning electron microscopy (FEI QUANTA-200,
Netherlands) was used to investigate the residues surface.
The specimens were previously coated with a conductive
layer of gold.
(
UL-94) tests. The content of DPDHPP and phosphorus in
epoxy resins thermosets was listed in Table I.
2
.4. Characterization of DPDHPP
Fourier transform infrared (FTIR) spectra were obtained
using potassium bromide discs and Perkin Elmer 400
spectrometer (USA). Nuclear magnetic resonance (NMR)
spectra were obtained using 10–25% deuteron dimethyl
2.9. Water Resistance Tests
In order to perform the moisture resistance test, the cured
epoxy resins specimens were kept in water at 70 C for
168 h and the water was replaced every 24 h, then the
water on the surface of the specimens was wiped by filter
paper. The moisture absorption was evaluated in terms of
mass increase of samples. The specimens for flame retar-
ꢀ
sulfoxide (DMSO-d ꢁ and Bruker 300 NMR (300 MHz)
6
spectrometer (Germany). Proton and carbon chemical
shifts were reported with respect to tetramethylsilane
ꢀ
(
TMS) as an internal reference. Phosphorus chemical shifts
dancy tests were dried in a vacuum oven at 105 C for
were reported with respect to 85% aqueous phosphoric
acid solution as external reference.
72 h. The mass increase of the samples was tested with
3
the dimension of 50×50 ×1 mm . Flame retardancy was
tested with the same dimension as previously mentioned.
2
.5. Flame Retardancy Tests
The vertical burning ratings (UL-94) were measured on
a CZF-2 instrument (Jiangning, China) with sheet dimen-
sions of 125 mm × 12.5 mm × 3 mm according to the
American National Standard ANSI/UL 94-2010. The lim-
iting oxygen index (LOI) was measured on a JF-3 limit-
ing oxygen index chamber (Jiangning, China) with sheet
dimensions of 130 mm×6.5 mm×3 mm according to ISO
2.10. X-ray Photoelectron Spectroscopy (XPS)
XPS data were carried out in an ultrahigh vacuum sys-
tem equipped with a Kꢂ hemispherical electron analyzer
(Thermofisher Scienticfic Company), using a monochro-
−8
mated AlKꢂ source, at a base pressure of 1.0×10 mbar.
C, N, P and O elements were analyzed.
4
589-2:1996.
3
. RESULTS AND DISCUSSION
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3
.1. Characterization of DPDHPP
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2
.6. Cone Calorimeter Tests
Copyright: American S Tc ihe en tc i hf i ec mP i uc ab lli ss thr eu cr st ure of the synthesized DPDHPP was
The combustion behavior under ventilated conditions was
measured using a Fire Testing Technology cone calorime-
ter (West Sussex, UK), in conformance with ISO 5660-1
1
13
31
characterized by FTIR, H, C and P NMR, respec-
tively. The FTIR spectrum of the reactant DPO is shown
in Figure 1(a). The peak at 3051 cm corresponds to the
aromatic C–H stretching vibration, 2374 cm is assigned
to the P–H stretching. The peaks appear at 1589 and
−1
3
standard. Samples with size of 100 × 100 × 3 mm were
−1
−2
exposed to cone at a heat flux of 50 kW · m and one
specimen was tested for every sample.
−1
1
1
483 cm assigning to the stretching of benzene ring,
439 cm for the P–C stretching vibration and 1188 cm
−1
−1
2
.7. Thermogravimetric Analysis
The thermogravimetric analysis (TGA) was carried out
using a Perkin Elmer Pyris 1 Thermal Gravimetric
Analyzer (Massachusetts, USA) at a heating rate of
ꢀ −1 ꢀ
0 C min , heating from 50 to 800 C under nitrogen
with a flow rate of 20 mL min . All the thermal degra-
dation data were obtained from the TGA and derivative
thermal gravimetric analysis (DTG) curves.
1
−1
Table I. Formulations and flame retardancy of epoxy resin system.
Curing agent compositions
(DPDHPP/PDA)
LOI
P% (%)
UL-94
rating
Sample
Dripping
EP-0
EP-1
EP-2
EP-3
EP-4
EP-5
0/100
10/90
20/80
30/70
40/60
50/50
0.00 17.8 No rating
0.60 26.5 No rating
1.15 30.2 No rating
Yes
No
No
No
No
No
1.66 31.4
2.11 31.9
2.53 32.1
V-1
V-0
V-0
Figure 1. FTIR spectrum of DPO (a) and DPDHPPO (b).
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016
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Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
Zhang et al.
attributing to the P O stretching vibration. The FTIR
spectrum of DPDHPP present in Figure 1(b), the char-
the epoxy resins. The LOI value of the EP/10 wt%
DPDHPP/90 wt% PDA thermosets was increased from
17.8% for pure EP thermosets to 26.5% when the phos-
phorus content was only 0.6%. The LOI values of the
EP thermosets were gradually increased with increasing
the DPDHPP content. The EP/40 wt% DPDHPP/60 wt%
PDA thermosets with the phosphorus content of 2.11%
successfully passed UL-94 V-0 flammability rating and
the LOI value was as high as 31.9%. In the previous
reports, numerous phosphorus compounds have been used
to improve the flame retardancy for epoxy resins. For
example, the LOI value of the EP thermosets cured with
hyperbranched (3-hydroxyphenyl) phosphate (HHPP) was
30.0% and there were no report on the vertical burning
−1
acteristic absorption peak at 2374 cm assigning to the
P–H bonds disappears, and the strong absorption peaks at
−1
3
143 cm assigning to O–H bonds is observed compar-
ing with the FTIR spectrum of DPO. The results indicate
that the P–H bonds in DPO has been reacted with 1,4-
Benzoquinone.
The 1H NMR spectrum of DPDHPP was shown in
Figure 2, the peaks appear at 7.507 to 7.586 ppm were
attributed to protons (1, 2, 3) in the benzene ring. The
signals appear at 7.619 to 7.683 ppm corresponded to the
proton (4) in the benzene ring. The peaks at 6.736, 6.973
and 6.894 ppm were attributed to protons (5), (7) and (8),
respectively. The peaks appear at 9.134 and 9.785 ppm
3
2
tests. The LOI value of the epoxy resins cured with bis(4-
aminophenoxy)phenylphosphine oxide was 33% when the
phosphorus content reached 4.2%, but the thermosets had
1
3
were attributed to the protons (6, 9) of –OH. The
C
NMR spectrum of DPDHPP is shown in Figure 3, the
peaks at 131.71, 128.44, 131.38 and 132.65 ppm are
attributed to the carbon atoms (1∼3) in the benzene ring,
respectively. The peaks at 121.38, 152.39, 118.50, 117.56,
3
3
no rating in the UL-94 test. Compared with the reported
results, the synthesized DPDHPP curing agent provided
an efficient flame retardany for epoxy resins. In fact, the
phosphorus-containing curing agent upon degradation pro-
duced phosphoric, polyphosphoric acid after the addition
of DPDHPP, and then the produced acids accelerated the
decomposition of the epoxy resins and formed an insu-
lating protective char layer, which prevented the heat and
oxygen from transferring to the surface of the materials
and enhanced the flame retardancy of the epoxy resins.
1
49.84 and 115.55 ppm correspond to the carbon atoms
3
1
(
5∼10) in the benzene ring. The P NMR spectrum of
DPDHPP is shown in Figure 4, the single peak appeared
at 38.78 ppm, suggesting that there exists only unique
phosphorus-containing compound.
The structure of DPDHPP was well confirmed by
1
13
31
FTIR, H NMR, C NMR and P NMR spectroscopy.
These results indicate that the product was synthesized
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As well known, the vertical burning test and limit-
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successfully.
Copyright: American Scientific Publishers
ing oxygen index test are effective methods to rank the
flammability of the material type in a small-scale fire,
while cone calorimeter is regarded as one of the most
effective bench-scale methods to measure many impor-
tant parameter of a real fire. The data and curves of
EP/PDA and EP/40 wt% DPDHPP/60 wt% PDA ther-
mosets obtained from the cone calorimeter test at a heat
flux of 50 kW· m are shown in Table II and Figure 5.
The TTI (time to ignition) is used to determine the influ-
ence of a flame retardant on the ignitability, which can be
measured from the onset of an HRR curve. As revealed in
3
.2. Flame Retardancy of the Cured Epoxy Resins
To evaluate the effects of DPDHPP on the flame retar-
dancy of epoxy resins, LOI and UL-94 vertical burning
tests were conducted. The relative data are presented in
Table I. The LOI value of the EP/PDA system was only
−2
1
7.8% and there was no rating in the vertical burning
tests, indicating that the pure epoxy resins were highly
combustible. The LOI values were dramatically increased
when the curing agent DPDHPP was incorporated into
1
Figure 2. H NMR spectrum of DPDHPPO (600 MHz, DMSO-d ).
6
2814
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Zhang et al.
Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
1
3
Figure 3.
C NMR spectrum of DPDHPPO (600 MHz, DMSO-d ).
6
Table II, the TTI of EP/40 wt% DPDHPP/60 wt% PDA
thermosets decreased from 63 s for pure EP to 51 s. The
DPDHPP flame retardant additive was not only decom-
posed ahead of time itself but also promoted the epoxy
resins matrix to degrade at a lower temperature. In certain
conditions, the degradation behaviors ahead of time was
attributed to the charring earlier during combustion and
those for the EP/PDA thermosets. The first HRR peak of
EP/40 wt% DPDHPP/60 wt% PDA thermosets at 80 s
−2
with a PHRR of 256.3 kW m is corresponding to the
decomposition of the DPDHPP additive at an early stage
on heating, which stimulated the epoxy resins matrix to
degrade and char at an early stage. The insulation of char
layer was formed on the surface of matrix, that is, the
favored a better flame retardan Dc ye l oi vf et rh ee dE bP yt Ihn e gr me no tsae t tso. : Univ eh r es ai tt yi no sf u Nl aet iwn gS oc hu at hr Wc a ua sl ee ds a rapid increase in the sur-
The HRR is recognized to be the mI P o: s 5t .i 8m . 4p 4o r. 1t a 5n 4t pOa rn a: mT -hu, 2f 8a c Ae pt er m2 p0 e1 r 6a t u0 r9e : 5a 9n d: 4 a4 ccelerated the decomposition of EP
Copyright: American Scientific Publishers
eter to quantify the size of fire, and an effective flame
retardant system normally shows a lower HRR value.
Figure 5(a) shows the HRR (heat release rate) curves of
EP/PDA and EP/40 wt% DPDHPP/60 wt% PDA ther-
mosets. The cured EP/PDA thermosets are observed to
be burned rapidly after ignition and two HRR peaks
appeared during the combustion process. The first HRR
peak appeared at 185 s with a PHRR (peak heat release
matrix on the surface of the EP composites. The second
HRR peak of EP/40 wt% DPDHPP/60 wt% PDA ther-
mosets corresponding to the combustion of EP thermosets
is observed at 170 s, the PHRR is decreased obviously
−2
−2
from 383.7 kW m for pure EP to 732.8 kW m . It is
confirmed that DPDHPP can promote the EP thermosets
to form char due to the charring capacity of epoxy resins
themselves and the char with high quality formed by the
degradation of DPDHPP, which prevents the heat trans-
fer from the samples during the ignition process and thus
caused the decreased intensity of the combustion pyroly-
sis reactions and decreased heat release. With the incor-
poration of DPDHPP into epoxy resins, the total heat
−2
rate) of 732.8 kW m due to the combustion of epoxy
resins thermosets. The second HRR peak observed at 250 s
−2
with a PHRR of 316.9 kW m was attributed to the crack
of the formed char layer with poor strength and low ther-
mal stability. As shown in Figure 5(a), the HRR curve
of the EP/40 wt% DPDHPP/60 wt% PDA thermosets is
observed to have two peaks. The peaks appear earlier than
−
2
release is decreased from 103.1 MJ m for pure EP to
−2
68.1 MJ m , as revealed in Figure 5(b) and Table II.
3
1
Figure 4.
P NMR spectrum of DPDHPPO (600 MHz, DMSO-d ).
6
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016
2815

Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
Zhang et al.
Table II. Cone calorimeter data of EP/PDA system, EP/40 wt%
DPDHPP/60 wt% PDA system.
The EHC (effective heat of combustion) is the heat
released from combustion of the volatile portion of the
polymer. The lower EHC value revealed noncombustible
gases existing in the gas phase. As shown in Table II, the
averaged value of the EHC (AEHC) for the EP/40 wt%
DPDHPP/60 wt% PDA system was decreased from 22.7
Samples
EP/40 wt%
Properties
TTI (s)
EP/PDA
DPDHPP/60 wt% PDA
63
732ꢃ8
185
316ꢃ9
250
103ꢃ1
22ꢃ7
1186ꢃ2
0ꢃ11
51
256ꢃ3
80
383ꢃ7
170
68ꢃ1
13ꢃ2
786ꢃ5
0ꢃ09
−1
to 13.2 MJ kg in the EP/PDA system. The reason is that
Peak -HRR (kW·m 2ꢁ
−
1
the EP/40 wt% DPDHPP/60 wt% PDA system released
noncombustible gases, such as volatile phosphide, N , NH
t
(s)
peak1-HRR
Peak -HRR (kW·m 2ꢁ
−
2
3
2
3
4
t
(s)
and CO during the combustion process, which diluted
peak2-HRR
2
THR (MJ·m 2ꢁ
−
the flammable gases and thus reduced the oxygen con-
centration around the surface of thermoset. Although the
chemical mechanism can not be depicted in this paper,
the nonflammable gases could play an important role in
the gas phase.
−
−1
1
AEHC (MJ·kg
ꢁ
2
ASEA (m ·kg
)
−1
AMLR (g·s
ꢁ
The lower THR value indicates that part of the EP/40 wt%
DPDHPP/60 wt% PDA thermosets was not completely
combusted and possibly undergone a char-forming pro-
cess. It is believed that lower HRR and THR are related
to the condensed phase and contributed to a better flame
retardancy of the epoxy resins thermosets.
3
.3. Thermal Degradation Behavior
TGA (Thermogravimetric analysis) can provide direct
information about the thermal stability, char formation and
degradation behavior of the samples. It can also provide
indirect information about the potential flame retardancy
of the cured epoxy resins largely related to the formation
of a char residue during heating. The initial decomposition
temperature is defined as the temperature at which 1 wt%
weight loss occurs and the Tmax is defined as the tempera-
ture at a maximum weight loss rate. The TGA and deriva-
tive thermogravimetric (DTG) curves of the cured pure
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EP (sample EP-0) and EP/40 wt% DPDHPP/60 wt% PDA
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thermosets (sample EP-4) under nitrogen atmosphere are
Copyright: American Scientific Publishers
presented in Figure 6, and the concerned thermal degrada-
tion data are summarized in Table III. The EP/PDA ther-
mosets starts to decompose at 333.4 C and the char yield
ꢀ
ꢀ
is only 1.5 wt% at 800 C. The thermal degradation course
can be divided into two stages, as reveals in Figure 6(b),
the peak of maximum thermal decomposition rate for
ꢀ
step one appears at 386.3 C with a weight loss rate of
1
−1
1.72% min due to the decomposition and char-forming
of epoxy resins. The second stage of decomposition occurs
ꢀ
ꢀ
from 500 C to 800 C and the peak appears at about
6
ꢀ
34.0 C, which can be assigned to the decomposition of
the formed char residues with a lower thermal stability.
The initial decomposition temperature of the EP/30 wt%
ꢀ
DPDHPP/70 wt% PDA system decreased from 333.4 C
ꢀ
for EP/PDA system to 310.7 C, the residual char was
increased from 1.5 to 13.6% at 800 C. When the phospho-
ꢀ
rus content was increased in the cured epoxy resin system,
the Tinitial and Tmax of the EP/40 wt% DPDHPP/60 wt%
ꢀ
PDA composites were 304.4 and 380.6 C, respectively,
and the Tinitial was lower than that of the EP/30 wt%
DPDHPP/70 wt% PDA system, but the char residue was
ꢀ
enhanced to 18.6% at 800 C. It was obvious that the
char yield was increased and Tinitial was decreased with
increasing the DPDHPP content in the epoxy resins, which
may be mainly attributed to the phosphorus groups in EP
thermosets, which were firstly decomposed to form phos-
phorus rich residue such as polyphosphoric acid and the
Figure 5. HRR (a) and THR (b) curves of EP/PDA system and
EP/40 wt% DPDHPPO/60 wt% PDA system.
2816
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016

Zhang et al.
Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
at an early degradation to form a compact char, which
delayed the further thermal decomposition of EP ther-
mosets at high temperatures.
The FTIR spectrum of the residual char of the
EP/40 wt% DPDHPP/60 wt% PDA theromsets after heat-
ꢀ
ing at 600 C in Muffle furnace was presented in Figure 7.
−1
The absorption band at 1430 cm corresponding to the
P–C bonds is disappeared, confirming that the P–C bonds
−1
were broken. Meanwhile, the peak appears at 3438 cm
was attributed to the stretching vibration of the –O–H
groups in phosphoric or polyphosphoric acid and the peak
−1
at 1402 cm was due to the P O group, indicating that
the DPDHPP occurred thermal oxidative decomposition
and produced phosphoric or polyphosphoric acid in a con-
−1
densed phase. The peaks appear at 1630 and 720 cm
were assigned to the stretching and bending vibration of
C–H bonds in the polyaromatic compounds in the char
residues. The FTIR spectra provided positive evidence
that the EP/40 wt% DPDHPP/60 wt% PDA system pro-
duced phosphoric or polyphosphoric acids during thermal
degradation in air, which acted as the dehydration agents
and stimulated the formation of the char layer by car-
bonization. The result was in accordance with the above-
mentioned burning tests.
In order to further investigate the relationship between
the morphology of the formed char layer and the flame
retardant properties of the EP/40 wt% DPDHPP/60 wt%
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Copyright: American Scientific Publishers
residues from the EP/PDA and EP/40 wt% DPDHPP/
0 wt% PDA systems were observed by SEM after cone
6
tests. After the EP/PDA system burned, numerous large
pores were observed and played a key role as the chan-
nels, where the oxygen and the volatile material can flow
and interact, thus promoting the combustion. There are no
flame retardation char layers existing in the surface of char
Figure 6. TGA (a) and DTG (b) curves of EP/PDA, EP/40 wt% DPDH-
PPO/60 wt% PDA and EP/40 wt% DPDHPPO/60 wt% PDA system.
formed acid react with epoxy resins. The reaction stim-
ulated the epoxy resins degradation at an early stage on
heat, leading to the decrease of the initial thermal decom-
position temperature and charring formation of thermosets
ahead of time. The higher char yield will enhance the
flame retardancy of the EP thermosets and the thermal
degradation results are consistent with the aforementioned
flame retardancy tests. A conclusion can be drawn that
DPDHPP may stimulate the thermal degradation of EP
Table III. Thermogravimetric properties of EP/PDA, EP/30 wt%
DPDHPP/70 wt% PDA and EP/40 wt% DPDHPP/60 wt% PDA system.
Char residues
Tinitial
ꢀ
R1peak/T1peak
−1
R2peak/T2peak
−1
at (wt%)
ꢀ
ꢀ
ꢀ
Samples
( C) (% min / C) (% min / C)
800
C
EP/PDA
333.4 11.72/386.3
1.76/634.0
–
1ꢃ5
EP/30 wt%
DPDHPP/70 wt%
PDA
EP/40 wt%
DPDHPP/60 wt%
PDA
310.7
10.9/387.6
13ꢃ6
18ꢃ6
304.4
16.5/380.6
–
Figure 7. FTIR of residues of EP/40 wt% DPDHPPO/60 wt% PDA
ꢀ
system at 600 C.
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016
2817

Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
Zhang et al.
residue, Figure 8(a). On the contrary, the morphology of
the char layer for the EP/40 wt% DPDHPP/60 wt% PDA
thermosets appears more uniform and compact than that
of cured pure EP, Figure 8(b). The results suggest that the
introduction of DPDHPP into the EP thermosets stimu-
lated EP matrix to form char layer during the combustion
and resulted in a more sufficient and compact char layer
with a better mechanical performance. Consequently, the
char layer exhibited a higher thermal stability and strength,
which effectively prevented the heat and oxygen being
transferred into the interior matrix and flammable volatiles
into flame zone, suppressing the further degradation and
combustion of the EP matrix and leading to excellent
flame retardancy of EP/40 wt% DPDHPP/60 wt% PDA
thermosets.
moisture absorption percentage and the DPDHPP con-
tent in the epoxy resins is presented in Figure 9. The
incorporation of DPDHPP into epoxy resins is observed
to reduce the moisture adsorption behavior. For example,
the percentage of moisture uptake for the pure EP ther-
mosets is 1.95% after the water resistance tests, whereas
for the cured EP/10 wt% DPDHPP composites, the value
of the moisture absorption percentage was increased to
1.6%. The percentage of moisture adsorption for the cured
EP/DPDHPP composites gradually was decreased with
the increase of the DPDHPP content and the percent-
age of moisture uptake of the EP/50 wt% DPDHPP ther-
mosets was decreased to 1.02%. It could be concluded
that the DPDHPP incorporation played an important role
in improving the resistance to moisture absorption. The
reduction of this parameter was due to the existing abun-
dant inherent hydrophobic nature of the P–C bonds and
3
.4. Moisture Resistance
3
5
aromatic hydrophobic in the DPDHPP.
Epoxy resins are hygroscopic because of the inher-
ent hydrophilic nature, and the moisture adsorption
would have a negative impact on the flame retardancy
and mechanical property. The relationship between the
Table IV gives the results of the LOI and UL-94 verti-
cal burning tests of the cured EP/DPDHPP/PDA compos-
ites with different DPDHPP contents after water resistance
tests. Compared with the results before water resistance
tests, as revealed in Table I, the corresponding LOI val-
ues marginally was decreased and the UL-94 rating was
not changed, indicating that the cured EP/DPDHPP/PDA
composites exhibited excellent water resistance. This fact
may be contributed to the P–C bond with a good ther-
Delivered by Ingenta to: University of New South Wales
mal stability and strong capability of moisture resistance
in DPDHPP additive.
Figure 10 and Table V present the cone test results
IP: 5.8.44.154 On: Thu, 28 Apr 2016 09:59:44
Copyright: American Scientific Publishers
of the EP/40 wt% DPDHPP/60 wt% PDA system before
and after the water resistance test. The values of THR
and AEHC increased slightly after the water resistance
test and the tpeak-HRR of the samples did not change,
while TTI and PHRR were reduced slightly after the
water resistance test. Overall, the test did not destroy
its flame retardancy even though when the sample was
Figure 8. SEM images of the char residue obtained from CONE test:
a) EP/PDA system, (b) EP/40 wt% DPDHPPO/60 wt% PDA system.
Figure 9. The relationship between moisture absorption and DPDH-
PPO content in epoxy resins thermosets.
(
2818
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016

Zhang et al.
Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
Table IV. Flame retardancy of cured epoxy resins after water resistance
Table V. Cone calorimeter data of EP/40 wt% DPDHPP/60 wt% PDA
test.
system before and after water resistance test.
Curing agent compositions
(DPDHPP/PDA)
UL-94
rating
Properties
TTI (s)
Before the test
After the test
Sample
LOI (%)
Dripping
51
256ꢃ3
80
383ꢃ7
170
68ꢃ1
13ꢃ2
786ꢃ5
0ꢃ09
49
250ꢃ6
80
385ꢃ2
170
69ꢃ2
13ꢃ4
785ꢃ7
0ꢃ08
Peak
-HRR (kW·m−2ꢁ
EP-0
EP-1
EP-2
EP-3
EP-4
EP-5
0/100
10/90
20/80
30/70
40/60
50/50
17.7
26.5
30.0
31.3
31.8
32.0
No rating
No rating
No rating
V-1
V-0
V-0
Yes
No
No
No
No
No
1
t
peak1-HRR (s)
-HRR (kW·m 2ꢁ
−
Peak
t
2
(s)
peak2-HRR
THR (MJ·m 2ꢁ
−
−
−1
1
AEHC (MJ·kg
ꢁ
2
ASEA (m ·kg
)
AMLR (g·s−
1
ꢁ
placed into a hostile condition. The morphologies of the
char layer for the EP/40 wt% DPDHPP/60 wt% PDA
system were observed by SEM before and after the
water resistance test. As shown in Figure 11, the sur-
face morphology of the char layer is observed contin-
uous and compact, indicating that the water resistance
test did not destroy the char layer forming after combus-
tion. In this study, the results of the water resistance test
indicated that the EP/40 wt% DPDHPP/60 wt% PDA sys-
tem possessed excellent water resistance due to aromatic
and P–C hydrophobic structures.
3
6ꢀ37
3
.5. Chemical Compositions of the Char Residue
XPS is an effective measurement method to study the sur-
face chemical structure of the samples without destruction.
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Copyright: American Scientific Publishers
Figure 11. SEM image of char layer of EP/40 wt% DPDHPPO/60 wt%
Figure 10. HRR (a) and THR (b) curves of EP/40 wt% DPDH-
PPO/60 wt% PDA system before and after water resistance test.
PDA system layer from combustion before and after water resistance test:
(a) before the test, (b) after the test.
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016
2819

Synthesis of a Novel Phosphorus-Containing Flame Retardant Curing Agent and Its Application in EP
Zhang et al.
Table VI. Element relative content of the char layer for cured epoxy
resins by XPS analysis.
combustion, resulting in an efficient flame retardancy for
cured epoxy resins thermosets.
Char layer of
EP/40 wt% DPDHPP/60 wt%
PDA thermoset
Char layer of
EP/PDA thermoset
Acknowledgments: The authors appreciate the funds
from National Nature Science Foundation of China (Grant
No. 21174025), China Postdoctoral Science Founda-
tion (2012M510907, 2013T60339), Heilongjiang Postdoc-
toral Grant (LBH-Z11252) and Youth Science Fund of
Heilongjiang Province (QC2011C102).
Peak
Atomic
Peak
Atomic
Element
BE (ev)
percentage (%)
BE (ev)
percentage (%)
C1s
N1s
O1s
P2p
298.28
410.28
545.28
144.28
84ꢃ66
1ꢃ07
13ꢃ99
–
298.38
410.38
545.38
144.38
89ꢃ82
1ꢃ22
7ꢃ31
1ꢃ64
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2
2
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Received: 23 April 2015. Accepted: 26 May 2015.
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 16, 2811–2821, 2016
2821