Synthesis and properties of phosphorus-containing bio-based epoxy resin from itaconic acid

Article abstract of DOI:10.1007/s11426-013-5025-3

A phosphorus-containing bio-based epoxy resin (EADI) was synthesized from itaconic acid (IA) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). As a matrix, its cured epoxy network with methyl hexahydrophthalic anhydride (MHHPA) as the curing agent showed comparable glass-transition temperature and mechanical properties to diglycidyl ether in a bisphenol A (DGEBA) system as well as good flame retardancy with UL94 V-0 grade during a vertical burning test. As a reactive flame retardant, its flame-resistant effect on DGEBA/MHHPA system as well as its influence on the curing behavior and the thermal and mechanical properties of the modified epoxy resin were investigated. Results showed that after the introduction of EADI, not only were the flame retardancy determined by vertical burning test, LOI measurement, and thermogravimetric analysis significantly improved, but also the curing reactivity, glass transition temperature (T _g), initial degradation temperature for 5% weight loss (T _d(5%)), and flexural modulus of the cured system improved as well. EADI has great potential to be used as a green flame retardant in epoxy resin systems.

Full text of DOI:10.1007/s11426-013-5025-3

SCIENCE CHINA
Chemistry
March 2014 Vol.57 No.3: 379–388
doi: 10.1007/s11426-013-5025-3
• ARTICLES •
Synthesis and properties of phosphorus-containing bio-based
epoxy resin from itaconic acid
MA SongQi, LIU XiaoQing*, JIANG YanHua, FAN LiBo, FENG JianXiang & ZHU Jin*
Ningbo Key Laboratory of Polymer Materials; Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences,
Ningbo 315201, China
Received August 15, 2013; accepted October 8, 2013; published online November 22, 2013
A phosphorus-containing bio-based epoxy resin (EADI) was synthesized from itaconic acid (IA) and 9,10-dihydro-9-oxa-10-
phosphaphenanthrene 10-oxide (DOPO). As a matrix, its cured epoxy network with methyl hexahydrophthalic anhydride
(MHHPA) as the curing agent showed comparable glass-transition temperature and mechanical properties to diglycidyl ether in
a bisphenol A (DGEBA) system as well as good flame retardancy with UL94 V-0 grade during a vertical burning test. As a re-
active flame retardant, its flame-resistant effect on DGEBA/MHHPA system as well as its influence on the curing behavior and
the thermal and mechanical properties of the modified epoxy resin were investigated. Results showed that after the introduc-
tion of EADI, not only were the flame retardancy determined by vertical burning test, LOI measurement, and thermogravimet-
ric analysis significantly improved, but also the curing reactivity, glass transition temperature (Tg), initial degradation temper-
ature for 5% weight loss (Td(5%)), and flexural modulus of the cured system improved as well. EADI has great potential to be
used as a green flame retardant in epoxy resin systems.
epoxy resin, bio-based, itaconic acid, DOPO, flame retardancy
is toxic to living organisms [6]. Nowadays, its application
1 Introduction
has been strictly limited in many countries. Therefore there
is an increasing interest in finding a bio-based or environ-
mentally friendly feedstock to synthesize epoxy without
bisphenol A.
Epoxy resins have been widely used as coatings, adhesives,
and structural composites as well as electronic materials due
to their superior mechanical properties, excellent thermal
resistance, and good processability [1–3]. As we know, di-
glycidyl ether of bisphenol A (DGEBA), which represents
more than 90% of the epoxy precursors in the world, is by
far the most widely used monomer to formulate epoxy net-
works [4]. Bisphenol A, which accounts for more than 67%
of the molar mass of DGEBA, is fully dependent on fossil
resources. In addition, bisphenol A appears to be both an
estrogen receptor agonist and an androgen receptor antago-
nist [5]. Because the chemical bonds linking bisphenol A in
polymer structures are not completely stable, over time the
polymer may release small amounts of bisphenol A, which
Plant oil is the most commonly used biomass to synthe-
size epoxy resins [7–11], but epoxidized plant-oil-based
materials often exhibit poor thermal and mechanical proper-
ties because of the low reactivity of epoxy groups and the
tendency of intramolecular bonding [4]. Besides plant oil,
some biomasses that are structurally similar to bisphenol A
such as lignin [12–16], rosin [17–19], cardanol [20], and
gallic acid [21–23] have also been used as feedstock to
synthesize epoxy resins. However, lignin-based epoxy res-
ins have a slow curing rate and unstable properties due to
the low mobility of the macromolecular species and com-
plex structures [12, 13]. Rosin-based epoxy resins usually
exhibit satisfactory thermal and mechanical properties [17],
*Corresponding authors (email: liuxq@nimte.ac.cn; jzhu@nimte.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com link.springer.com

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Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
although their rigid hydrogenated phenanthrene ring struc-
tures can induce severe brittleness [19]. Due to their long
flexible aliphatic chains, cardanol-based epoxy resins have
been preferred for use as toughening agents and diluents,
and the gallic-acid-based epoxy network has been shown to
be brittle due to its rigid benzene ring and exorbitantly high
crosslink density.
retardants act in the gas phase through flame inhibition and
in the condensed phase through charring. The UL94 of the
commercial epoxy resin RTM6 has been improved from HB
to V-1 and V-0 and the LOI has been increased from 25% to
34%–38% for the modified resin. Both the glass transition
) and initial decomposition temperature of
temperature (Tg
RTM6 were decreased after the introduction of DOPP and
DOPI. Wang et al. [46] synthesized a DOPO-substituted
organophosphorus oligomer (PDPDP) and found that the
incorporation of PDPDP could reduce the release of com-
bustible gas and induce the formation of a char layer, which
decreased the potential fire hazard and improved the flame
retardancy. However, the initial decomposition temperature
for 5% weight loss (Td(5%)) of the epoxy resin decreased
more than 60 °C. Apparently, the introduction of modified
DOPO by physical blending was still accompanied by a
decrease of glass transition temperature (Tg) and initial de-
composition temperature [28, 38, 43, 44, 46, 47] because of
the limited chemical structure design and a relatively low
cross-link density of the cured epoxy resin.
To overcome the above disadvantage, Wang et al. [40,
48] synthesized two DOPO-grafting epoxy monomers and
used them as a reactive flame-retardant additive to copoly-
merize with the epoxy/curing agent system. These results
demonstrated that the high Tg and initial decomposition
temperature of the modified epoxy resin remained and good
flame retardancy was obtained. Liu et al. [39, 49, 50] also
reported the synthesis of phosphorus-silicon containing
compounds from DOPO. Due to the reaction of their high
content of epoxy groups with the matrix resins, these phos-
phorus-silicon-containing compounds acted as effective fire
retardants for epoxy resin. In addition to better flame re-
tardancy, the Tg and initial decomposition temperature of
the cured epoxy were also improved. The proposed reason
was that the addition of phosphorus-silicon compounds with
high epoxy value could produce a high cross-link density
for the cured epoxy. Obviously, the above DOPO deriva-
tives could improve the flame retardancy of epoxy resins
and at the same time maintain or even increase their Tg and
initial decomposition temperatures. However, the main
feedstock used in these projects was from fossil sources and
some of them were toxic. According to the principles of
green chemistry [51, 52] and due to the increasing concern
about the depletion of fossil reserves and greenhouse gas
emissions, efficient fire retardants developed from biomass
should have great significance.
Itaconic acid (IA), also called methylenesuccinic acid, is
produced by fermentation of carbohydrates using aspergil-
lus terreus. Due to its great potential to replace the petro-
chemicals in the chemical industry, it has been selected as
one of the top 12 potential bio-based platform chemicals by
the U.S. Department of Energy [24]. In our previous work
[25], a bio-based epoxy resin with curable double bonds
(EIA) was synthesized by the esterification reaction be-
tween itaconic acid (IA) and epichlorohydrin (ECH). Com-
pared with DGEBA, this epoxy resin exhibited comparable
or better properties after being cured with methyl hexahy-
drophthalic anhydride which utilized the carbon-carbon
double bonds. After being copolymerized with comonomers,
its properties could be manipulated further. Along with its
ability to copolymerize with comonomers, the carbon-
carbon double bond of itaconic acid can also be utilized to
introduce functional structures such as phosphorus- or silicon-
containing structures to improve the properties of some
polyesters [26, 27].
As we know, the low flame-retardancy of epoxy resin
limits its end use in many fields. Traditionally, halogen-
containing compounds have been regarded as effective
agents to improve the flame retardancy of polymeric mate-
rials [28, 29]. However, halogen-containing materials have
been found to generate toxic and corrosive gases during
combustion, which result in serious health and environmen-
tal problems [28, 30]. With the ban of some halogenated
flame retardants such as polybrominated biphenyl (PBB)
and polybrominated diphenylether (PBDE) by the European
Union [31], the development of an applicable halogen-free
flame retardant has gained much more attention [32].
Compared with the halogen-containing compounds, or-
ganophosphorus reagents have demonstrated good flame
retardancy and generate less toxic gas or smoke during
combustion [33]. As a result, most of them have been be-
lieved to be environmental friendly flame retardants.
Among these organophosphorus compounds, 9,10-dihydro-
9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its
derivatives have attracted much attention due to their high
reactivity and applicability in epoxy resin [34–43]. Consid-
ering the fact that physical blending of DOPO into epoxy
resin will lead to the unnecessary consumption of reactive
oxirane groups [44, 45], chemical methods have usually
been employed for DOPO modification. For example, B.
Perret et al. [43] linked DOPO units to star-shaped aliphatic
ground body tetra-[(acryloyloxy)ethyl] pentaerythritol and
heterocyclic tris-[(acryloyloxy)ethyl] isocyanurate to get
two flame retardants, DOPP and DOPI. Both of these flame
In this paper, we recount our synthesis of a phosphorus-
containing bio-based epoxy resin (EADI) from itaconic acid
and DOPO. The epoxy network was obtained by an epoxy
ring-opening reaction with MHHPA as the curing agent; its
flame retardancy and thermal and mechanical properties
were thoroughly studied. For comparison, DGEBA cured
with the same curing agent was also investigated. EADI at
various ratios was also used to modify diglycidyl ether of
bisphenol A (DGEBA) with methyl hexahydrophthalic

Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
381
anhydride (MHHPA) as the curing agent. The objective of
this work was to synthesize a flame-retardant bio-based
epoxy resin and evaluate its potential to replace the com-
monly used DGEBA in the fire-resistant field.
Sinopharm Chemical Reagent Co., Ltd., China. Diglycidyl
ether of bisphenol A (DGEBA) with the epoxide equivalent
weight of 182–192 g eq1 was purchased from DOW
Chemical Company, America. All of the chemicals were
used as received.
2 Experimental
2.2 Synthesis of DOPO containing itaconic acid (DI)
2.1 Materials and methods
The DOPO containing itaconic acid (DI) was synthesized
according to the synthetic route reported by Lin et al. [28].
We introduced 216 g (1.0 mol) of 9,10-dihydro-9-oxa-
10-phosphaphenanthrene 10-oxide (DOPO), 690 g of xy-
lene, and 1.3 mL catalyst (1% H2PtCl6 in isopropyl alcohol)
into a three-necked round-bottomed flask equipped with a
reflux condenser, a thermometer, and a mechanical stirrer.
After the mixture was heated to 120 °C, 130 g (1.0 mol)
itaconic acid was added over 2 h. Then, the reaction was
maintained at this temperature for 20 h. The product was
filtered after cooling to room temperature and was washed
with acetone several times before it was dried in a vacuum
oven at 90 °C for 5 h. From this procedure we obtained DI
in the form of white powder that weighed 311 g (0.898 mol).
The synthetic route is shown in Scheme 1. FT-IR (KBr,
cm1): 3400–2400 (COOH), 1708 (C=O), 1176 (P=O),
The infrared spectrum (FT-IR) was recorded with a
NICOLET 6700 FT-IR (NICOLET, USA). 1H-NMR,
13C-NMR and 31P-NMR were performed on a 400 MHz
AVANCE III Bruker NMR spectrometer (Bruker, Switzer-
land) with acetone-d6 as a solvent. Non-isothermal curing
reaction was monitored on a Mettler-Toledo MET DSC
(METTLER TOLEDO, Switzerland) under a nitrogen at-
mosphere. A heating scan ranging from 25 to 225 °C was
performed at the heating rates of 2.5, 5, 8, and 10 °C min1,
respectively. The cured epoxy resins were cut into small
pieces and about 8 mg of sample was taken for the glass
transition temperature measurement. It was heated to
200 °C and held there for 3 min to eliminate any thermal
history. Then it was cooled to 25 °C at a cooling rate of
50 °C min1 and heated again to 200 °C at a rate of 20 °C
min1. The glass transition temperature was obtained from
the peak temperature of the differential curve of the second
heating curves of the cured epoxy resins. Vertical flame
tests of the cured epoxy resins were measured by an
AG5100B vertical burning tester instrument (Jiangning
Analysis Instrument Factory, China); the sample measured
130 mm × 13 mm × 3.0 mm according to the ASTM
D3801-00. The limiting oxygen index (LOI) was deter-
mined with an M606 oxygen index meter (Qingdao Shan-
fang, China) according to the ASTM D 2863-00, with a test
specimen bar measuring 70 mm × 6.5 mm × 3.0 mm.
Thermogravimetric analysis (TGA) was performed on a
Mettler-Toledo TGA/DSC1 Thermogravimetric Analyzer
(METTLER TOLEDO, Switzerland) with high-purity ni-
trogen or air as purge gas at a scanning rate of 20 °C min1
from 50 to 800 °C. Flexural properties were evaluated by a
three-point bending test conducted on an Instron 5567 Elec-
tric Universal Testing Machine (Instron, USA) with a span
length of 48 mm at a cross-head speed of 2 mm min1. The
specimens measuring 60 mm × 10 mm × 3.5 mm were used
for this evaluation. Itaconic acid (IA) was purchased from
Zhejiang Guoguang Biochemistry Co., Ltd., China. The 9,10-
dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)
we used was supplied by the flame retardant plant of
Shouguang Weidong Chemical Co., Ltd. China. Epibromo-
hydrin, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), ally bro-
mide, hexadecyl trimethyl ammonium bromide (HTAB or
CTAB), and m-chloroperoxybenzoic acid (m-CPBA) were
obtained from Aladdin Reagent, China. Methyl hexahy-
drophthalic anhydride (MHHPA) was obtained from
1
1430 (PC), 919 (POC). H-NMR (400 MHz, acetone-d6,
δ ppm): 10.99 (s, 2H), 8.17 (m, 2H), 7.99 (m, 1H), 7.80 (m,
1H), 7.63 (m, 1H), 7.47 (m, 1H), 7.31 (m, 2H), 3.23 (m, 1H),
2.68 (m, 2H), 2.39 (m, 2H). 31P-NMR (400 MHz, ace-
tone-d6, δ ppm): 34.85, 34.35.
2.3 Synthesis of the phosphorus-containing bio-based
epoxy resin (EADI)
As shown in Scheme 2, the synthesis of the phosphorus
containing bio-based epoxy resin (EADI) was performed
using two different routes.
Synthetic route 1: 60 g (0.173 mol) DI, 600 g (10.330
mol) acetone, 52 g (0.380 mol) epibromohydrin, and 58 g
(0.380 mol) 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
were placed into a three-necked round-bottomed flask with
a mechanical stirrer, a thermometer, and a reflux condenser.
The reactants were mixed and stirred at 25 °C for 3 h. After
removing most of the acetone, the raw product was diluted
with methylbenzene and washed with deionized water five
times. The final product EADI weighing 63.0 g (0.137 mol)
Scheme 1 The synthetic route of DOPO containing itaconic acid (DI).

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Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
organic layer was then dried over anhydrous sodium sulfate.
The final product EADI weighing 50.0 g (0.109 mol) was
obtained after removing the dichloromethane completely
(yield: 97.7%). The synthetic route is shown in Scheme 2.
The epoxy value of EADI determined by the hydrochloric
acid-acetone method was about 0.435. 1H-NMR (400 MHz,
acetone-d6, δ ppm): 8.17 (m, 2H), 7.98 (m, 1H), 7.81 (m,
1H), 7.64 (m, 1H), 7.48 (m, 1H), 7.31 (m, 2H), 4.34 (m, 2H),
3.83 (m, 2H), 2.5–3.2 (m, 11H). 13C-NMR (400 MHz,
acetone-d6, δ ppm): 172.44 (1C), 170.67 (1C), 149.03 (1C),
120–136 (11C), 65.49 (2C), 48.72 (2C), 43.65 (2C), 35.60
(3C).
2.4 Preparation of epoxy networks
Scheme 2 The synthetic routes of phosphorus-containing bio-based
epoxy resin (EADI).
The predetermined DGEBA and EADI, a stoichiometric
amount of MHHPA (the molar ratio of MHHPA to epoxy
group is 0.75/1) and 0.1 wt% CTAB (on the basis of the total
weight of the curing agent and epoxy) were stirred together
until a homogeneous mixture was obtained. Next, the mix-
ture was degassed in a vacuum oven at 80 °C for at least 10
min. Then the gas-free mixture was poured into a preheated
stainless steel mould and cured at 150 °C for 2 h and 190 °C
for 2 h. Then the cured DGEBA/MHHPA systems with dif-
ferent levels of EADI and EADI/MHHPA systems were
prepared. Their compositions and codes are listed in Table 1.
All of the samples were cured under the same conditions.
was obtained after removing the methylbenzene completely.
The yield was 79.3%. The synthetic route is shown in
Scheme 2. The epoxy value of EADI determined by hydro-
chloric acid-acetone method was about 0.434. FT-IR (KBr,
cm1): 1740 (ester C=O), 910, and 855 (oxirane ring).
1H-NMR (400 MHz, acetone-d6, δ ppm): 8.17 (m, 2H), 7.98
(m, 1H), 7.81 (m, 1H), 7.64 (m, 1H), 7.48 (m, 1H), 7.31 (m,
2H), 4.34 (m, 2H), 3.83 (m, 2H), 2.5–3.2 (m, 11H).
31P-NMR (400 MHz, acetone-d6, δ ppm): 33.74, 33.69,
33.65, 33.61, 33.40, 33.36.
Synthetic route 2: 50 g (0.144 mol) DI, 70 g (0.579 mol)
ally bromide, 44 g (0.318 mol) potassium carbonate, 500 g
(8.609 mol) acetone, and 100 g (1.368 mol) N, N-dimethyl-
formamide were placed into a three-necked flask equipped
with a magnetic stirrer, a thermometer, and a reflux con-
denser. Before the reactants were refluxed for 12 h, they
were mixed vigorously at room temperature for 10 min.
After removing the acetone and unreacted ally bromide, the
raw product was diluted with dichloromethane and washed
with deionized water five times. Allylated DOPO grafting
itaconic acid (ADI) weighing 50.7 g (0.119 mol) was ob-
tained after removing the water and dichloromethane (yield:
82.4%). The synthetic route is shown in Scheme 2.
1H-NMR (400 MHz, acetone-d6, δ ppm): 8.15 (m, 2H), 7.96
(m, 1H), 7.79 (m, 1H), 7.62 (m, 1H), 7.47 (m, 1H), 7.29 (m,
2H), 5.86 (m, 2H), 5.21 (m, 4H), 4.48 (m, 4H), 3.28 (m, 1H),
2.95 (m, 2H), 2.50 (m, 2H). 13C-NMR (400 MHz, acetone-
d6, δ ppm): 172.29 (1C), 170.55 (1C), 149.04 (1C), 117–136
(15C), 65.61 (2C), 36.65 (3C).
3 Results and discussion
3.1 Chemical characterization
In this study, we successfully synthesized phosphorus-
containing bio-based epoxy resin (EADI) using two differ-
ent routes. One synthetic route involved dehydrobromina-
tion between phosphorus-containing itaconic acid (DI) and
epibromohydrin. The other route used two steps: (i) obtaining
allylated itaconic acid (ADI) from dehydrobromination be-
tween DI and allyl bromide; and (ii) the epoxidation of ADI.
The chemical structure of EADI was determined by
1
FT-IR, H-NMR, 13C-NMR, and 31P-NMR. Figure 1 shows
the FT-IR spectrum of EADI. The peaks at 3064 cm1 cor-
respond to the stretching absorptions of Ph-H by the DOPO
ring; the peaks at 1600 cm1 belong to the stretching
Table 1 Composition and activation energy (Ea) of the different systems
47.6 g (0.112 mol) ADI, 481.4 g (5.668 mol) dichloro-
methane, and 56.7 g (0.329 mol) m-chloroperoxybenzoic
acid (m-CPBA) were placed into a three-necked round-
bottom flask equipped with a mechanical stirrer, a ther-
mometer, and a reflux condenser. The reactants were mixed
and stirred at 30 °C for 48 h. The solution was filtrated and
washed with a solution of 10% sodium sulfite followed by a
solution of 10% sodium carbonate and distilled water. The
Ea (kJ mol1)
Weight ratio of
GEBA/EADI
/MHHPA
100/0/67.61
87.7/12.3/66.06
75.6/24.4/64.54
52.2/47.8/61.60
0/100/55.04
Phosphorus
content
(%)
Samples
Kissinger Ozawa
DGEBA-P0
D/E-P0.5
D/E-P1.0
D/E-P2.0
EADI-P4.4
0
81.8

79.9

0.5
1.0
2.0
4.4


71.4
51.0
75.3
55.5

Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
383
absorptions of C=C by the benzene ring. Other characteris-
tic absorption peaks of EADI are at 1740 (C=O), 910, and
855 cm1 (epoxy group). In order to further identify their
structures, the 1H-NMR, 13C-NMR, and 31P-NMR spectra of
the peaks of carbon C12–23 on the DOPO ring at 117–149
ppm; and the peaks of carbons C4 and C7 on the ester bonds
–COO– at 170–173 ppm. The 31P-NMR spectrum of EADI
exhibits two multiple peaks, at 33.67 and 33.38 ppm; this is
because the 31P-NMR spectrum of DI shows two single
peaks at 34.85 ppm and 34.35 ppm that correspond to the
different stereo structures as reported by Zhang et al. [26],
and because the introduction of the epoxides increased the
number of stereo structures. These results demonstrate that
the target compound, EADI, was synthesized successfully.
1
EADI are also displayed in Figure 2. In the H-NMR spec-
trum of EADI, the characteristic peaks of protons H13–15
and H21–23 on epoxides CH2(O)–CH– at 2.52–3.16 ppm
and the peaks of protons H9–12 on the carbons next to
epoxides CH2(O)–CH–CH2– at 3.78–3.90 and 4.27–4.40
ppm can be observed. The 13C-NMR spectrum of EADI
shows the peaks of carbons C1, C2, C9, and C10 on epox-
ides CH2(O)–CH– at 44 and 49 ppm; the peaks of carbons
C3 and C8 next to epoxides CH2(O)–CH–CH2– at 64–66
ppm; the peaks of carbons C5, C6, and C11 at 35–36 ppm;
3.2 Curing behavior
Non-isothermal (dynamic) curing behaviors of DGEBA/
MHHPA systems with or without EADI and EADI/
MHHPA system were studied by DSC. Figure 3 shows the
DSC curing curves for these systems. Obviously, DGEBA-
P0, D/E-P2.0, and EADI-P4.4 displayed a single exothermic
peak that corresponds to the ring-opening reaction between
the epoxy and anhydride groups. As we know, under the
same curing conditions, the peak temperature of the DSC
exothermic curve is often taken as an indication that the
reactivity of the compound in curing reactions should be
evaluated. The lower the temperature of the peak is, the
higher the reactivity is [25, 53, 54]. Obviously, the epoxy
group of EADI (exothermic peak temperature is about
149.9 °C) was more active than that of DGEBA (exothermic
peak temperature is 168.5 °C) during the curing reaction
with MHHPA. With the introduction of EADI into the
DGEBA/MHHPA system (D/E-P2.0), the exothermic peak
for the ring-opening reaction of the epoxy group decreased
to a lower temperature (165.6 °C) as shown in Figure 3,
which indicated that the curing activity of DGEBA/
MHHPA system maintained or even increased slightly after
EADI was incorporated.
Figure 1 The FT-IR spectrum of EADI.
For accuracy, the activation energy of the curing reaction
was determined by both Kissinger’s [55] and Ozawa’s [56]
methods. According to Kissinger, activation energy can be
obtained from the peak temperatures at different heating
Figure 3 DSC thermograms of non-isothermal curing behaviors of
Figure 2 The 1H-NMR, 13C-NMR, and 31P-NMR spectra of EADI.
DGEBA-P0, D/E-P2.0, and EADI-P4.4 at the heating rate of 2.5 °C min1.

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Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
rates. The equation is expressed as:
3.3 Flame retardancy of the cured epoxy resins
2

ln (q/Tp ) = Ea/RTp  ln(AR/Ea)
(1)
The flame retardancy of the cured epoxy resins were exam-
ined by a vertical flame test and by limiting the oxygen in-
dex (LOI) measurement. Table 2 lists the vertical flame test
results and the LOI values of the cured epoxy resins. Obvi-
ously, the DGEBA-P0 could not self-extinguish; it burned
out completely after ignition. However, after it was cured
with EADI and 0.5%–2.0% phosphorus was incorporated,
the resins became self-extinguishing. Although the UL-94
V0 grade (t1 + t2 < 10 s, no dripping) could not be reached,
the self-extinguishing time (t1 + t2) became shorter with the
increase in phosphorus content. At the same time, the LOI
values of the cured epoxy networks were increased from
19.6% for DGEBA-P0 to 31.4% for D/E-P2.0. As we know,
the higher the LOI value is, the better the flame retardancy
is. Undoubtedly, the flame retardancy of the DGEBA/
MHHPA system can be improved by the incorporation of
EADI. Images of the burning cured resins are shown in
Figure 5. The flame size of the resins shrank as the phos-
phorus content increased. As for the sample D/E-P2.0, only
a little part was lit up during the vertical burning test and
even this part self-extinguished in 27 s. This result shows
the potential of EADI to be used as a fire retardant for
epoxy resin systems. In Table 2, it can be also seen that the
(t1+t2) of EADI-P4.4 is less than 3 s with no dripping. In
addition, it reached the UL94 V-0 grade, which means that
EADI-P4.4 is a fire retardant epoxy network whereas the
LOI (22.8%) was much lower than the D/E-P2.0.
where q is the heating rate, Tp is the exothermic peak tem-
perature, Ea is the activation energy, R is the gas constant,
and A is the pre-exponential factor. After a plot showing
2
ln(q/Tp ) versus Ea/RTp is obtained, the apparent activation
energy can be calculated from the slope of the linear fitting
plot. Ozawa’s method can be expressed by the following
equation:
lnq = 1.052 × Ea/RTp + ln(AEa/R) – lnF(x) – 5.331 (2)
where F(x) is a conversion-dependent term. Therefore, Ea
can be calculated from the slope of the linear fitting plot of
lnq versus 1/Tp.
2
Figure 4 shows the plots of ln(q/Tp ) versus 1/Tp based
on Kissinger’s equation (Figure 4(a)), as well as lnq versus
1/Tp based on Ozawa’s theory (Figure 4(b)) for the DGEBA-
P0, EADI-P4.4, and D/E-P2.0 systems. The calculated Ea
are shown in Table 1. The Ea of the curing reaction of EADI
with MHHPA (51.0 kJ mol1/55.5 kJ mol1) was lower than
that of the DGEBA-MHHPA system (81.8 kJ mol1/79.9
kJ mol1), and the Ea of the DGEBA-MHHPA system was
decreased to 71.4 kJ mol1/75.3 kJ mol1 by introducing
EADI to the D/E-P2.0 system. To some extent, the curing
reactivity of EADI with MHHPA was higher than that of
DGEBA with MHHPA, and the curing reactivity of
DGEBA with MHHPA was increased by the incorporation
of EADI.
3.4 Thermal degradation behavior of the cured epoxy
resins
The thermal stability and degradation behavior of the cured
epoxy resins were investigated by thermogravimetric analy-
sis (TGA). The TGA curves under air and nitrogen are
shown in Figure 6. The values of the initial degradation
temperature for 5% weight loss (Td(5%)), the degradation
temperature for 50% weight loss (Td(50%)), and the residual
weight percentage (char yield) at 800 °C (R800) are listed in
Table 3. As shown in Figure 6 and Table 3, in air, Td(5%) of
the cured epoxy resins increased from 340.1 °C for
DGEBA-P0 to 360.1 °C for D/E-P2.0; in nitrogen, Td(5%) of
D/E-P0.5 was higher than that of DGEBA-P0, and D/E-P1.0
and D/E-P2.0 had similar Td(5%) compared with that of
Table 2 Vertical flame results and LOI of the cured epoxy resins
Burning time
after 1st ignition after 2nd ignition by flaming
(t1) (s)
not self-
extinguishing
99
Burning time Cotton ignited
LOI
(%)
Samples
(t2) (s)
drops
Yes
DGEBA-P0
19.6

D/E-P0.5
D/E-P1.0
D/E-P2.0
EADI-P4.4
4
4
No
No
No
No
25.2
28.5
31.4
22.8
81
27
2
36
2
Figure 4 (a) Linear plot of ln(q/Tp ) versus 1/Tp based on Kissinger’s
< 1
equation; (b) ln(q) versus 1/Tp based on Ozawa’s theory.

Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
385
Figure 6 TGA curves of the cured epoxy resins under (a) air and (b)
nitrogen.
Table 3 Thermal properties of the cured epoxy resins
Td(5%) (°C)
Td(50%) (°C)
In air In N2
424.7 428.2
420.0 423.2
417.1 419.0
411.2 413.4
342.4 339.5
R800 (%)
Figure 5 Images of DGEBA-P0- and EADI-modified DGEBA systems
during a vertical burning test.
Samples
Tg (°C)
In air In N2
In air In N2
DGEBA-P0 340.1 368.3
D/E-P0.5 347.4 373.8
D/E-P1.0 358.0 368.8
D/E-P2.0 360.1 366.5
EADI-P4.4 288.5 290.0
0.16
1.00
2.21
5.55
5.29
4.58
7.35
8.05
8.41
6.49
109.5
110.5
114.9
119.8
101.9
DGEBA-P0. This result indicates that the thermal stability
of DGEBA/MHHPA system reflected by the initial degra-
dation temperature (Td(5%)) is maintained or even increased
after EADI introduction. Due to the easily thermally de-
gradable PO, PC of phosphorus compounds, DOPO con-
taining flame retardants often decreases the initial degrada-
tion temperature of epoxy networks [18, 20]. As we know,
the initial degradation temperature not only has a relation-
ship with the chemical structure of the network, it also has
close ties with crosslink density of the network. The main-
tained or even increased initial degradation temperature of
EADI-modified epoxy resins might be due to the increased
crosslink density of the epoxy network. In order to prove
this conjecture, the degradation behaviors of DGEBA and
EADI monomers were also examined by TGA (Figure 7).
The Td(5%) of the EADI monomer was lower than that of the
DGEBA monomer due to the easily thermally degradable
bond between PO, PC and ester. Thus the shorter mole-
cule chain of EADI (compared with DGEBA) can increase
the crosslink density of the DGEBA/MHHPA systems,
which corresponds to the similar or even higher Td(5%) than
Figure 7 TGA curves of DGEBA and EADI monomers under air and
nitrogen.

386
Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
that of DGEBA-P0. This result was similar to the results
reported by Wang et al. [48].
density and the chemical structure of the chain segment.
A higher cross-link density or more-rigid chain segment
will produce a higher Tg. Although there were a lot of rigid
DOPO rings in the network of the EADI/MHHPA system,
the steric hindrances of EADI and MHHPA might cause an
incomplete curing reaction of EADI with MHHPA, which
would correspond to relatively lower crosslink density in
the network. These two competing effects ultimately led to
the slightly lower Tg of EADI-P4.4 than of DGEBA-P0.
High char yield has been related to high flame retardancy
to a certain degree [57]. Based on Table 3, R800 (char yield)
of the cured epoxy resins increased with the increase of
phosphorus content in both air and nitrogen. The improved
char yields (R800) of the cured epoxy networks were due to
the high char yield of the EADI monomer (Figure 7). Simi-
larly to the vertical burning and LOI results, this result
demonstrates that EADI can improve the flame retardancy
of the DGEBA/MHHPA system. We can also see, in Figure
6 and Table 3, that either in air or in nitrogen, the Td(50%)
decreases after the introduction of EADI.
With EADI as a matrix, the cured epoxy network EADI-
P4.4 showed much lower Td(5%) and Td(50%) than other sam-
ples in both air and nitrogen; in addition, the R800 was not as
high as the D/E-P2.0. This might be because for EADI-P4.4,
the backbone of the epoxy network has a lot of easily
cleavable ester bonds and no thermal stable rigid aromatic
ring other than the DOPO structure, which would mean that
the rapid thermal degradation of EADI-P4.4 occurred at a
much lower temperature than other samples and corre-
sponded to the much lower Td(5%) and Td(50%) of other sam-
ples; and that the R800 of EADI-P4.4 could result only from
the DOPO structure in EADI, which resulted in the rela-
tively lower R800 of EADI-P4.4 than that of D/E-P2.0.
3.6 Mechanical properties of the cured epoxy resins
The flexural stress-strain curves for the cured epoxy resins
are shown in Figure 9; the data for flexural strength and
flexural modulus as well as strain at break are listed in Table
4. The stress of EADI-P4.4 and EADI modified DGEBA
systems showed linear increases that strain the maximum
value without any yielding, which implies brittle fractures
[58] similar to those of DGEBA-P0. Due to the rigid DOPO
structure of EADI, EADI-P4.4 exhibited higher flexural
modulus (4004 MPa) than DGEBA-P0. The strain at break
(3.28%) and flexural strength (125.5 MPa) of EADI-P4.4
were slightly lower than those of DGEBA-P0. For the
3.5 Glass transition temperature of the cured epoxy
resins
We measured DSC in order to investigate the glass transi-
tion temperature (Tg) of the different cured systems in our
study. The DSC heating curves for the cured resins are
shown in Figure 8 and the Tgs obtained from the peak tem-
peratures of these curves are shown in Table 3. It has been
reported that physically blending DOPO into epoxy resin
would lead to the unnecessary consumption of reactive
oxirane groups, resulting in a lower network density and
reduced Tg of the cured resin [44, 45]. However, based on
Figure 8 and Table 3, it is easy to notice that with the intro-
duction of EADI, the Tg of the DGEBA/MHHPA system
increased from 109.5 °C for DGEBA-P0 to 119.8 °C for
D/E-P2.0. As we know, the Tg of an epoxy network is
mainly dependent on its cross-link density and the chemical
structure of the chain segment. A higher cross-link density
or more-rigid chain segment will produce a higher Tg. In our
study, we thought that the shorter molecular chain of EADI
relative to DGEBA might increase the cross-link density of
the cured epoxy resins and that the rigid DOPO structure
could hinder the movement of the epoxy chain segment,
which might be the reason for the increased Tg of EADI-
modified epoxy resins. However, the Tg of EADI-P4.4
(101.9 °C) was a little lower than that of DGEBA-P0
(109.5 °C). We all know that the glass transition tempera-
ture of epoxy networks is mainly dependent on its cross-link
Figure 8 DSC curves of the cured epoxy resins.
Figure 9 Flexural stress-strain curves of the cured epoxy resins.

Ma SQ, et al. Sci China Chem March (2014) Vol.57 No.3
387
Table 4 Flexural properties of the cured epoxy resins
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Flexural strength
(MPa)
Flexural modulus Strain at break
Samples
9
(MPa)
(%)
DGEBA-P0
D/E-P0.5
D/E-P1.0
D/E-P2.0
EADI-P4.4
131.6 ± 23.21
132.2 ± 8.38
119.7 ± 11.64
117.6 ± 15.81
125.5 ± 12.8
3390 ± 33.7
3479 ± 24.3
3596 ± 4.2
3672 ± 32.0
4004 ± 49.3
4.48 ± 1.27
4.14 ± 0.36
3.55 ± 0.36
3.43 ± 0.50
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4 Conclusions
A phosphorus-containing itaconic acid-based epoxy resin
(EADI) was synthesized successfully. Its cured resin
showed high flame retardancy (UL-94 V0-grade), and
comparable glass transition temperature (Tg) and mechani-
cal properties relative to a cured DGEBA system. As a reac-
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increased flame retardancy, Tg, and curing reactivity as well
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EADI has great potential to be used as a green candidate for
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This work was financially supported by the National Natural Science
Foundation of China (51203176), the National Basic Research Program of
China (2010CB631100), the Natural Science Foundation of Ningbo
(2012A610095), the China Postdoctoral Science Foundation (2013M-
540504), the Postdoctoral Science Foundation of Zhejiang Province
(Bsh1201011), and the Director Funds of the Ningbo Institute of Materials
Technology and Engineering, Chinese Academy of Sciences (Y20224-
QF06).
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