Vanillin-Derived High-Performance Flame Retardant Epoxy Resins: Facile Synthesis and Properties

Article abstract of DOI:10.1021/acs.macromol.7b00097

Lignin derivative vanillin when coupled with diamines and diethyl phosphite followed by reaction with echichlorohydrin yields high-performance flame retardant epoxy resins. Biorenewable and environment-friendly flame retardant alternatives to bisphenol A epoxy resins (having plenty of applications such as coatings, adhesives, composites, etc.) have captured great attention due to their ecological and economic necessity. Vanillin, an industrial scale monoaromatic compound from lignin, is a promising sustainable candidate for high-performance polymers, while synthesis of diepoxies is challenging. Meanwhile, bio-based epoxy resins combining high performance and excellent fire resistance are more difficult to be achieved. In this paper, two novel bio-based epoxy monomers EP1 and EP2 were synthesized by one-pot reaction containing Schiff base formation and phosphorus-hydrogen addition between vanillin, diamines, and diethyl phosphite, followed by reacting with epichlorohydrin. Their reactivities are similar to bisphenol A epoxy resin DGEBA. After curing they showed excellent flame retardancy with UL-94 V0 rating and high LOI of ～32.8%, which was due to the outstanding intumescent and dense char formation ability. Meanwhile, it was found that the cured vanillin-based epoxies had exceedingly high T_gs of ～214 °C, tensile strength of ～80.3 MPa, and tensile modulus of ～2709 MPa, much higher than the cured DGEBA with T_g of 166 °C, tensile strength of 76.4 MPa, and tensile modulus of 1893 MPa; the properties of vanillin-based epoxies are easy to be regulated by using different coupling agents - diamines - during the synthesis process.

Full text of DOI:10.1021/acs.macromol.7b00097

Article
pubs.acs.org/Macromolecules
Vanillin-Derived High-Performance Flame Retardant Epoxy Resins:
Facile Synthesis and Properties
Sheng Wang,†,‡ Songqi Ma,*,† Chenxiang Xu,†,‡ Yuan Liu,†,‡ Jinyue Dai,†,‡ Zongbao Wang,§
Xiaoqing Liu,† Jing Chen,† Xiaobin Shen,†,‡ Jingjing Wei,† and Jin Zhu*,†
†Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Zhenhai
District, Ningbo 315201, P. R. China
‡University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing 100049, P. R. China
§Ningbo University, Ningbo 315201, P. R. China
ABSTRACT: Lignin derivative vanillin when coupled with
diamines and diethyl phosphite followed by reaction with
echichlorohydrin yields high-performance flame retardant epoxy
resins. Biorenewable and environment-friendly flame retardant
alternatives to bisphenol A epoxy resins (having plenty of
applications such as coatings, adhesives, composites, etc.) have
captured great attention due to their ecological and economic
necessity. Vanillin, an industrial scale monoaromatic compound
from lignin, is a promising sustainable candidate for high-
performance polymers, while synthesis of diepoxies is challenging.
Meanwhile, bio-based epoxy resins combining high performance and excellent fire resistance are more difficult to be achieved. In
this paper, two novel bio-based epoxy monomers EP1 and EP2 were synthesized by one-pot reaction containing Schiff base
formation and phosphorus−hydrogen addition between vanillin, diamines, and diethyl phosphite, followed by reacting with
epichlorohydrin. Their reactivities are similar to bisphenol A epoxy resin DGEBA. After curing they showed excellent flame
retardancy with UL-94 V0 rating and high LOI of ∼32.8%, which was due to the outstanding intumescent and dense char
formation ability. Meanwhile, it was found that the cured vanillin-based epoxies had exceedingly high Tgs of ∼214 °C, tensile
strength of ∼80.3 MPa, and tensile modulus of ∼2709 MPa, much higher than the cured DGEBA with Tg of 166 °C, tensile
strength of 76.4 MPa, and tensile modulus of 1893 MPa; the properties of vanillin-based epoxies are easy to be regulated by using
different “coupling” agentsdiaminesduring the synthesis process.
low price and enhanced environment benefits. There are
numerous bionewable resources including vegetable oils,10−14
cardanol,15 isosorbide,9,16 rosin,17,18 gallic acid,19−21 ferulic
acid,22 lignin, and its derivatives23−26 that have been
investigated as feedstocks for epoxy resins. Because of the
long flexible aliphatic chain and low reactivity of internal epoxy
groups, epoxidized vegetable oil and cardanol often exhibit poor
thermal and mechanical properties.6,27 Although isosorbide,
rosin, gallic acid, and ferulic acid-based epoxy resins exhibited
satisfied thermal and mechanical properties, isosorbide-based
one suffered from relatively high hydrophilicity,28 and rosin,
gallic acid, and ferulic acid have seldom spare capacity due to
their variety applications and limited resources. Lignin is the
second most abundant natural organic material accounting for
approximately 30% of organic carbon in the biosphere; it is also
the only scalable renewable feedstock consisted of aromatic
monomers,29 and it is highly underutilized.30 Epoxy resins
directly from lignin exhibit several drawbacks such as slow
curing rate, unstable properties, and issues with processability
INTRODUCTION
■
Epoxy resins, as one of the three most important thermosetting
polymers, have been widely employed in a multitude of fields
such as coatings, adhesives, laminated circuit board, electronic
component encapsulations, and advanced composites because
of their excellent adhesion, chemical resistance, mechanical
properties, and dielectric properties.1−5 Nowadays, almost all of
the epoxy resins are produced from fossil resources, and 90% of
the commercially available epoxy resins are diglycidyl ether of
bisphenol A (DGEBA) via the reaction of bisphenol A with
epichlorohydrin.6 Bisphenol A, fully dependent on fossil
resources, accounts for greater than 67% of the molar mass
of DGEBA.7 In addition, bisphenol A is a reprotoxic
compound;8 and even if chemically incorporated into polymers,
a small amount of bisphenol A can still release from the
polymers with time due to the not completely stable chemical
bonds linking bisphenol A.9 As a result, it is under close
monitoring, and its application has been restricted in many
countries.
Recently, the finite and rising price of fossil resources, climate
change from CO2 emission, and other environmental problems
have raised interests in polymers from biorenewable raw
materials which have a wide variety of biomass resources with
Received: January 16, 2017
Revised: February 20, 2017
© XXXX American Chemical Society
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due to their high molecular weights, insolubility, and
composition variability depending on the species or the time
of year,30−32 which greatly limited their utilization. Despite a
great deal of efforts, decomposition of lignin into small
molecules remains a challenge. Fortunately, the lignin-to-
vanillin process has been commercially utilized, and vanillin, as
the only monoaromatic compounds produced on an industrial
scale from lignin, has already exhibited tremendous potential to
be used as a renewable building block for polymers.33,34
However, vanillin has only one group suitable to be introduce
epoxy group; consequently, producing diepoxies is challenge.
Oxidation or reduction of vanillin’s formyl group was
conducted to achieve two suitable groups to introduce two
epoxy groups,25,26 and using pentaerythritol reacting with
formyl group and “coupling” two vanillin to achieve a
difunctional phenol followed by reacting with epichlorohydrin
is also a reported way to produce diepoxies.23 These methods
are relatively complex, and toxic compounds were utilized; the
fundamental mechanical properties of the vanillin-based epoxies
were not reported.
The second drawback of epoxy resins is their flammability,
which blocks their application in the fire resistance required
fields.35 Phosphorus-containing compounds have been re-
garded as effective and environment-friendly flame retardants
for polymers including epoxy resins, and phosphorus-
containing flame retardants have attracted intensive interests36
after some halogenated fire retardants were banned by the
European Union due to their toxicity.37 Recent years have
witnessed rapid development of phosphorus-containing epoxy
resins, while most of the phosphorus-containing epoxy systems
manifested a decrease in glass transition temperature (Tg) than
the phosphorus-free systems due to the limited chemical
structure design and relatively low cross-link density of the
cured epoxy resin.38 Sustainability, reduction of environmental
impacts, and green chemistry are increasingly guiding the
development of phosphorus-containing materials from renew-
able resources,39 while there is limited report on biobased
phosphorus-containing epoxy resins.
EXPERIMENTAL SECTION
■
Materials. Vanillin, 4,4-diaminodiphenylmethane (DDM),
tetrabutylammonium bromide, p-phenylenediamine (PDA), and
diethyl phosphite were purchased from Aladdin-reagent Co., China.
Zinc chloride, epichlorohydrin (ECH), ethanol, petroleum ether,
dichloromethane, and sodium hydroxide were obtained from
Sinopharm Chemical Reagent Co., Ltd., China. Epoxy resin
(DGEBA, trade name DER331, epoxy value 0.53) was supplied by
DOW Chemical Company.
Synthesis of Tetraethyl(4,4′-methylenebis(4,1-phenylene)-
bis(azanediyl))bis((4-hydroxy-3-methoxyphenyl)methylene)-
diphosphonate (DP1). DP1 was synthesized by a quintessential
condensation reaction between vanillin and 4,4-diamino-
diphenylmethane, followed by an addition reaction with diethyl
phosphite. 20 g (0.13 mol) of vanillin was dissolved in 150 mL of
ethanol in a 500 mL three-neck flask equipped with a magnetic stirrer
and a reflux condenser. 11.83 g (0.06 mol) of 4,4-diamino-
diphenylmethane dissolved in 50 mL of ethanol was added dropwise
into the flask over a period of 0.5 h at 25 °C, and they were reacted at
40 °C for 1 h. Then diethyl phosphite (24.86 g, 0.18 mol) and zinc
chloride (0.5 g) as the catalyst for the addition reaction were added
into the system, and the mixture was heated from 40 to 80 °C under a
nitrogen atmosphere and kept at 80 °C for 8 h. Finally, the mixture
was poured into 1000 mL of petroleum ether and stirred for 0.5 h after
being cooled to room temperature. A white solid was collected by
filtration; then the white crystalline product DP1 (29.7 g) with the
yield of around 93.3% was obtained after being washed with ethanol
twice followed by drying at 70 °C for 24 h in a vacuum oven. The
synthetic route is illustrated in Scheme 1.
Scheme 1. Synthetic Routes of DP1 and DP2
Thus, in this paper, two novel high-performance flame
retardant bio-based epoxy resins containing phosphorus were
synthesized from vanillin. Diamines were used as the coupling
agents in this work, and the Schiff base intermediates
(produced from the coupling reaction between vanillin and
diamines), without being extracted from the systems, further
reacted with diethyl phosphite to obtain phosphorus-containing
difunctional phenols; and then the flame retardant epoxy resins
EP1 and EP2 were obtained by reacting these difunctianal
phenols with epichlorohydrin. The chemical structures of the
1
monomers were characterized by FTIR, H NMR, and 13C
NMR. DDM was used to cure EP1, EP2, and DGEBA (the
control) to obtain three thermosetting resins EP1-DDM, EP2-
DDM, and DGEBA-DDM. The curing behaviors of the systems
were examined by differential scanning calorimetry (DSC).
Flame retardancy and flame retarding mechanism of the epoxy
resins were investigated by vertical burning and limit oxygen
index test and char analysis using inductively coupled plasma
optical emission spectrometer (ICP-OES), X-ray photoelectron
spectroscopy (XPS), and scanning electron microscopy (SEM).
The thermal stability, glass transition temperature, and
mechanical properties were studied by thermogravimetric
analysis (TGA), DSC, and tensile test, respectively.
FTIR (KBr), cm−1: 3364 (N−H); 3225 (O−H); 1278 (PO);
1026 (P−O).
1H NMR (DMSO-d6, ppm): δ = 1.02 (t, 6H), 1.14 (t, 6H), 3.51 (d,
2H), 3.65 (m, 6H), 3.83 (m, 2H), 3.99 (m, 4H), 4.76 (dd, 2H), 5.96
(dd, 2H), 6.64 (dd, 6H), 6.75 (d, 4H), 6.84 (d, 2H), 7.07 (s, 4H), 8.85
(s, 2H).
13C NMR (DMSO-d6, ppm): δ = 16.68 (dd, 4C), 39.94 (s, 1C),
53.64 (s, 1C), 55.17 (s, 1C), 56.12 (s, 4C), 62.65 (dd, 2C), 113.09 (d,
2C), 114.07 (s, 4C), 115.40 (s, 2C), 121.38 (d, 2C), 128.00 (s, 2C),
129.18 (s, 4C), 130.78 (s, 2C), 145.76 (d, 2C), 146.34 (d, 2C), 147.69
(d, 2C).
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Synthesis of Tetraethyl(1,4-phenylenebis(azanediyl))bis((4-
hydroxy-3-methoxyphenyl)methylene)diphosphonate (DP2).
DP2 was also synthesized by a quintessential condensation reaction
between vanillin and p-phenylenediamine followed by an addition
reaction with diethyl phosphite. 20 g (0.13 mol) of vanillin was
dissolved in 150 mL of ethanol in a 500 mL three-neck flask equipped
with a magnetic stirrer and a reflux condenser. 6.4884 g (0.06 mol) of
p-phenylenediamine dissolved in 50 mL of ethanol was added
dropwise into the flask over a period of 0.5 h at 25 °C, and they
were reacted at 40 °C for 1 h. Then diethyl phosphite (24.86 g, 0.18
mol) and zinc chloride (0.5 g) as the catalyst for the addition reaction
were added into the system, and the mixture was heated from 40 to 80
°C under a nitrogen atmosphere and kept at 80 °C for 8 h. Finally, the
mixture was poured into 1000 mL of H2O and stirred for 0.5 h after
being cooled to room temperature. A light yellow solid was collected
by filtration; then the light yellow crystalline product DP2 (23.4 g)
with the yield of around 88.3% was obtained after being washed with
acetone twice followed by drying at 70 °C for 24 h in a vacuum oven.
The synthetic route is illustrated in Scheme 1.
Scheme 2. Synthetic Routes of EP-1 and EP-2
FTIR (KBr), cm−1: 3390 (O−H, N−H); 1285 (PO); 1040 (P−
O).
1H NMR (DMSO-d6, ppm): δ = 1.02 (t, 6H), 1.16 (t, 6H), 3.62 (m,
2H), 3.71 (s, 6H), 3.82 (m, 2H), 4.00 (p, 4H), 4.65 (dd, 2H), 5.39
(dd, 2H), 6.51 (d, 4H), 6.64 (d, 2H), 6.81 (d, 2H), 7.05 (s, 2H), 8.84
(s, 2H).
13C NMR (DMSO-d6, ppm): δ = 16.68 (dd, 4C), 54.54 (s, 2C),
56.08 (s, 4C), 62.5 (m, 2C), 112.99 (s, 2C), 115.37 (s, 4C), 121.32 (s,
2C), 128.23 (s, 2C), 139.51 (d, 4C), 146.2 (s, 2C), 147.66 (s, 2C).
Synthesis of Tetraethyl(4,4′-methylenebis(4,1-phenylene)-
bis(azanediyl))bis((3-methoxy-4-(oxiran-2-ylmethoxy)phenyl)-
methylene)diphosphonate (EP1). 5 g of DP1 (0.0058 mol), 53.7 g
of epichlorohydrin (0.58 mol), and 0.5 g of tetrabutylammonium
bromide (10 wt % of DP1) as the catalyst were placed in a three-
necked flask equipped with a magnetic stirrer and a reflux condenser,
and the mixture was reacted at 80 °C for 3 h. After being cooled to 5
°C, 2.32 g of 40 wt % aqueous sodium hydroxide solution (0.058 mol
NaOH) was added dropwise into the flask and reacted for 5 h. The
solution was washed with distilled water three times after diluted with
dichloromethane; then a light yellow solid was collected after
removing dichloromethane and epichlorohydrin. Finally, the light
yellow solid product EP1 (5.3 g) with the yield of around 93.5% was
obtained after being washed with petroleum ether twice followed by
drying at 50 °C for 24 h in a vacuum oven. The synthetic route is
illustrated in Scheme 2.
FTIR (KBr), cm−1: 3383 (N−H); 1264 (PO); 1033 (P−O); 910
(epoxy).
1H NMR (CDCl3, ppm): δ = 1.14 (t, 6H), 1.27 (t, 6H), 2.74 (dd,
2H), 2.90 (t, 2H), 3.39 (s, 2H), 3.71 (m, 2H), 3.85 (s, 6H), 4.03 (m,
8H), 4.19 (dt, 2H), 4.55 (d, 2H), 6.43 (s, 4H), 6.92 (m, 6H).
13C NMR (CDCl3, ppm): δ = 16.37 (dd, 4C), 44.95 (s, 2C), 50.11
(s, 2C), 55.98 (d, 4C), 57.52 (s, 2C), 63.16 (m, 2C), 70.10 (s, 2C),
111.30 (s, 2C), 113.61 (s, 2C), 115.45 (s, 4C), 120.18 (d, 2C), 129.62
(s, 2C), 139.24 (d, 2C), 147.55 (s, 2C), 149.54 (s, 2C).
Preparation of the Cured Epoxy Resins. Both EP1 and EP2
were cured with a commonly used curing agent DDM. Epoxy
monomers were mixed with DDM in a 1:1 equiv ratio. The mixtures
were cured by a plate vulcanizer at 200 °C for 3 min (1 min for
removing bubbles, 2 min for heat pressing) to get films with thickness
of approximately 100 μm for thermal and mechanical properties
examination. The mixtures were also melted and poured into stainless
steel molds and cured at 160 °C for 2 h to obtain samples with
dimensions of 80 mm × 6.5 mm × 3 mm for flame retardancy
investigation. Both films and rectangular samples were postcured at
200 °C for 2 h and 230 °C for 2 h in a vacuum oven. For comparison,
commonly used bisphenol A epoxy resin (DGEBA) was also cured
with the same curing agent.
FTIR (KBr), cm−1: 3300(N−H); 1265 (PO); 1027 (P−O); 910
(epoxy).
1H NMR (CDCl3, ppm): δ = 1.07 (t, 6H), 1.27 (t, 6H), 2.72 (dd,
2H), 2.88 (t, 2H), 3.37 (s, 2H), 3.70 (m, 4H), 3.84 (s, 6H), 4.01 (m,
8H), 4.19 (dt, 2H), 4.62 (d, 2H), 6.47 (t, 4H), 6.91 (dd, 10H).
13C NMR (CDCl3, ppm): δ = 16.38 (dd, 4C), 40.06 (s, 1C), 44.92
(s, 2C), 50.13 (s, 2C), 55.17 (s, 1C), 55.96 (s, 4C), 56.68 (s, 1C),
63.23 (d, 2C), 70.12 (s, 2C), 111.37 (s, 2C), 113.71 (s, 2C), 113.93 (s,
4C), 120.19 (d, 4C), 129.41 (s, 4C), 131.72 (s, 2C), 144.48 (s, 2C),
147.60 (s, 2C), 149.59 (s, 2C).
1
Characterization. H NMR and 13C NMR spectra were recorded
by an AVANCE III Bruker NMR spectrometer (Bruker, Switzerland)
with DMSO-d6 and CDCl3 as solvents, operating at 400 and 75.5
MHz, respectively. The infrared spectra (FTIR) were measured with a
NICOLET 6700 FTIR (NICOLET, America) using the KBr pellet
method. Differential scanning calorimetry (DSC) measurements were
performed under a nitrogen atmosphere on a Mettler Toledo Star 1
apparatus and a PerkinElmer DSC8000 apparatus for nonisothermal
curing kinetics and glass transition temperature (Tg), respectively. The
mixtures (around 3−5 mg) of epoxy monomers and DDM were taken
for the study of nonisothermal curing kinetics. A heat scan ranging
from 50 to 250 °C was performed at varying heating rates of 5, 10, 15,
and 20 °C min−1, respectively. Cured samples (around 8−10 mg) were
heated from 50 to 250 °C at a heating rate of 20 °C min−1 and held at
250 °C for 3 min to eliminate thermal history, and then they were
cooled to 50 °C at a cooling rate of 20 °C min−1 followed by being
heated again to 250 °C at a rate of 20 °C min−1. The Tg was obtained
from the second heating curve of the cured epoxy resins. Cured
samples with dimensions of 30 mm × 0.5 mm × 100 μm were used to
examine tensile properties by a Universal Mechanical Testing Machine
(Instron 5569A) with a cross-head speed of 0.5 mm min−1. The tensile
properties of each sample were reported as the average of five
Synthesis of Tetraethyl(1,4-phenylenebis(azanediyl))bis((3-
methoxy-4-(oxiran-2-ylmethoxy)phenyl)methylene)-
diphosphonate (EP2). 5 g of DP2 (0.0065 mol), 60.1 g of
epichlorohydrin (0.65 mol), and 0.5 g of tetrabutylammonium
bromide (10 wt % of DP2) as the catalyst were placed in a three-
necked flask equipped with a magnetic stirrer and a reflux condenser,
and the mixture was reacted at 80 °C for 3 h. After being cooled to 5
°C, 2.6 g of 40 wt % aqueous sodium hydroxide solution (0.065 mol of
NaOH) was added dropwise into the flask and reacted for 5 h. The
solution was washed with distilled water three times after diluted with
dichloromethane; then a light yellow solid was collected after
removing dichloromethane and epichlorohydrin. Finally, the white
solid product EP2 (4.3 g) with the yield of around 75.2% was obtained
after being washed with petroleum ether twice followed by drying at
50 °C for 24 h in a vacuum oven. The synthetic route is illustrated in
Scheme 2.
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3200−3400 cm−1 belonging to N−H and O−H, peaks at
around 1280 cm−1 ascribed to PO, and peaks at around 1030
cm−1 representing P−O. For the FTIR spectra of EP1 and EP2,
the peaks for O−H disappeared and the peaks for epoxy group
measurements. Thermogravimetric analysis (TGA) was carried using a
Mettler-Toledo TGA/DSC1 thermogravimetric analyzer (METTLER
TOLEDO, Switzerland). Approximately 3−5 mg samples were heated
from 50 to 700 °C at a heating rate of 10 °C min−1 under nitrogen and
air atmospheres. The limit oxygen index (LOI) was obtained on a JF-3
oxygen index instrument (Jiangning Analysis Instrument Company,
China) with sample dimensions of 80 mm × 6.5 mm × 3 mm
according to ASTM D2863-10. UL-94 vertical burning tests were
conducted according to ASTMD2863-97 on an AG5100B vertical
burning tester (Zhuhai Angui Testing Equipment Company, China)
with sample dimensions of 80 mm × 6.5 mm × 3 mm. The
morphologies of residues collected after LOI tests were observed by
scanning electron microscopy (SEM, EVO18) with an accelerating
potential of 20 kV. DGEBA-DDM could not self-extinguish, so it was
blown out during the test in order to investigate the morphology of its
surface during the burning. The residues of phosphorus content after
LOI tests were performed by inductively coupled plasma optical
emission spectrometer (ICP-OES) on a PerkinElmer Optima 2100DV
apparatus. 20 mL of aqua regia (10 mL of H2O, 7.5 mL of HCl, and
2.5 mL of HNO3) was added into a 120 mL volumetric flask with
approximately 0.02 g of carbon residue; then the systems were kept at
180 °C for 1 h; finally, water was added to make the systems reach 100
mL. 5 mL of the obtained solution was taken for test. Each sample was
examined twice under the same conditions, and the data were
averaged. The X-ray photoelectron spectroscopy (XPS) spectra of the
char residues (surface and inside (cut off the surface layer with
thickness of 2 mm)) were recorded with an AXIS ULTRA apparatus
(Kratos, England).
1
at around 910 cm−1 appeared. As seen from the H NMR and
13C NMR spectra of DP1, DP2, EP1, and EP2 (Figures 2 and
3), the chemical shift and integral area of all the peaks match
well with the protons and carbons of chemical structures for
DP1, DP2, EP1, and EP2. These results indicate that the target
compounds were synthesized successfully.
Curing Process. The nonisothermal curing kinetics of EP1-
DDM, EP2-DDM, and DGEBA-DDM were studied by DSC.
The Kissinger’s method (eq 1)40−42 and Ozawa’s method (eq
2)40−43 were used to obtain the apparent activation energy
during the curing process.
−ln(q/T 2) = Ea/RT − ln(AR/Ea)
p
p
(1)
ln q = −1.052Ea/RT + ln(AEa/R) − ln F(x) − 5.331
p
(2)
where q is the heating rate, Tp is the exothermic peak
temperature, Ea is the activation energy, R is the gas constant, A
is the pre-exponential factor, and F(x) is a conversion-
2
dependent term. Figure 4 reveals the plots of −ln(q/Tp )
versus 1/Tp based on Kissinger’s equation and ln q versus 1/Tp
based on Ozawa’s theory for the EP1-DDM, EP2-DDM, and
DER331-DDM systems, and the Eas calculated from the slope
of the linear fitting plots are concluded in Table 1. It can be
seen that EP1-DDM and EP2-DDM presented similar Ea to
DGEBA-DDM, which is indicative of the comparable reactivity
of EP1 and EP2 to that of DGEBA toward the curing agent
DDM.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of DP1, DP2, EP1, and
EP2. In this paper, vanillin was coupled by diamines through
Schiff base condensation, and the generated Schiff base further
reacted with diethyl phosphite by phosphorus−hydrogen
addition reaction to yield phosphorus-containing vanillin-
based diphenols DP1 and DP2. Significantly, the Schiff base
formation and phosphorus−hydrogen addition reaction were
finished in one pot, which means that the Schiff bases produced
during the reaction did not need to be extracted from the
system. The two phosphorus-containing bio-based epoxy resins
EP1 and EP2 were synthesized by reacting DP1 and DP2 with
epichlorohydrin by a typical method.
In order to investigate the curing degree of the systems,
FITR spectra of EP1-DDM and EP2-DDM after curing are
exhibited in Figure 5. Compared with the FTIR spectra of EP1
and EP2, the peaks for epoxy groups at around 910 cm−1
disappeared and broad peaks ranged from 3200 to 3600 cm−1
representing O−H from the ring-opening reaction between
epoxy groups and amines appeared in the FTIR spectra of EP1-
DDM and EP2-DDM. This means that the epoxy systems were
cured well under the former mentioned conditions.
Flame Retardancy and Flame Retarding Mechanism
of the Cured Epoxy Resins. The flame retardancy of the
cured epoxy resins was investigated by vertical burning test and
limit oxygen index (LOI) examination. The UL-94 ratings and
LOI values of the cured epoxy resins are listed in Figure 6.
Obviously, the UL-94 V0 rating which was the highest flame
retarding rating by vertical burning test was achieved for the
EP1-DDM and EP2-DDM systems. Meanwhile, the LOI values
of EP1-DDM and EP2-DDM were 31.4% and 32.8%,
respectively. Nevertheless, the DGEBA-DDM showed no UL-
94 rating and low LOI value of 24.6%. These results illustrate
that EP1-DDM and EP2-DDM presented excellent flame
retardancy, while DGEBA-DDM was flammable.
The chemical structures of the synthesized monomers were
confirmed by FTIR, 1H NMR, and 13C NMR. Figure 1 presents
the FTIR spectra of DP1, DP2, EP1, and EP2. In the FTIR
spectra of DP1 and DP2, there are characteristic peaks at
Generally, the char residue structure after combustion can
reflect the flammability characteristics of polymer materials.
Digital photos and SEM photographs of the char residues after
LOI test are illustrated in Figure 7, and the phosphorus content
in char residues examined by inductively coupled plasma optical
emission spectrometer (ICP-OES) is concluded in Table 2. It
can be clearly seen in Figure 7a that EP1-DDM and EP2-DDM
formed great intumescent char residues, accounting for 35.8%
and 41.2% of the original weight of EP1-DDM and EP2-DDM,
Figure 1. FTIR spectra of DP1, DP2, EP1, and EP2.
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Figure 3. 13C NMR spectra of (a) DP1, (b) DP2, (c) EP1, and (d)
EP2.
1
Figure 2. H NMR spectra of (a) DP1, (b) DP2, (c) EP1, and (d)
EP2.
respectively. Moreover, the char layers of EP1-DDM and EP2-
DDM are extremely dense, as shown in Figure 7b. Besides, as
shown in Table 2, a large proportion of phosphorus as high as
∼53.5% was remained in the char, which suggests that the
degradation of phosphorus-containing groups produced
phosphate structure in the condensed phase and promoted
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Figure 6. LOI and UL-94 rating of the cured epoxy resins.
intumescent and dense char layers during combustion. While
DGEBA-DDM formed a litter bit of char layer with numerous
open holes, verifying the failure to prevent the flame and heat
transfer. Meanwhile, on account of the more phosphorus and
nitrogen content (Table 2), EP2-DDM exhibited better flame
retardancy than EP1-DDM reflected from its higher LOI value
than EP1-DDM.45
To further identify the chemical components of the char
residues, the surface part and the inside part of the char
residues were examined by XPS, and the XPS spectra are
illustrated in Figure 8. Obviously, for both EP1-DDM and EP2-
DDM, there are much more phosphorus and oxygen and less
carbon in the surface part of the char residues than in the inside
part of the char residues. This might be due to the reason that
during the combustion the phosphorus transferred to the
surface, captured the radicals containing oxygen, and left in the
surface layer of the residues, leading to the higher phosphorus
and oxygen content in the surface part of char residues for EP1-
DDM and EP2-DDM. There is more phosphorus in EP2-DDM
than that in EP1-DDM before burning, so there is more
phosphorus remained in char residues, and more radicals
containing oxygen were captured and remained in the char
residues, corresponding to the higher oxygen content of EP2-
DDM’s char residue than that of EP1-DDM’s char residue.
Meanwhile, the surface part of char residues had a little higher
nitrogen than the inside part of char residues for both EP1-
DDM and EP2-DDM, which implies that nitrogen also
transferred to the surface of the char residues during the
combustion.
2
Figure 4. Linear plots of (a) −ln(q/Tp ) versus 1/Tp based on
Kissinger’s equation and (b) ln q versus 1/Tp based on Ozawa’s
theory.
Table 1. Activation Energy (Ea) and Average Molecular
Weight between Cross-Link Points (Mc) of EP-DDM, EP2-
DDM, and DGEBA-DDM
Ea (kJ mol−1
)
sample
Kissinger
Ozawa
Mc
EP1-DDM
56.4
54.4
57.1
60.4
58.5
60.9
1907
1727
944
EP2-DDM
DGEBA-DDM
Thermal Stability. The TGA and DTG curves of the cured
epoxy resins under N2 and air atmospheres are illustrated in
Figure 9, and the data are summarized in Table 3. As shown in
Table 3, EP1-DDM and EP2-DDM exhibited lower degrada-
tion temperature for 5% weight loss (Td5%) and higher
degradation temperature for 30% weight loss (Td30%) under
both atmospheres. The lower Td5% was probably ascribed to the
fact that the OP−O bond in epoxy resin was more apt to
degrade than C−C bond.46 The degradation of the phosphorus
containing structure leads to the formation of phosphorus-rich
char with high thermal stability, which accumulated at the
surface of the epoxy matrix. The thick char was a superior
thermal insulating layer, which undergone slow thermal
degradation and prevented heat reaching the remaining
polymer,44,47 corresponding to the higher Td30% of EP1-DDM
and EP2-DDM than that of DGEBA-DDM. Although the char
layer could be further decomposed with a rise of temperature,48
the EP1-DDM and EP2-DDM still exhibited much higher char
Figure 5. FTIR spectra of the cured EP1-DDM and EP2-DDM.
the configuration of dense char layer to prevent the heat
transfer.35,44 Thus, the excellent flame retardancy of EP1-DDM
and EP2-DDM was ascribed to their good ability to form
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Figure 8. XPS spectra of the surface part and inside part of the char
residues of EP1-DDM and EP2-DDM.
resulted in the high char yields. The lower R700 under air
atmosphere than that under a nitrogen atmosphere was
attributed to the third stage degradation belonging to the
oxidative degradation of residual carbon under air atmosphere,
which can be seen in Figure 9. Meanwhile, the phosphorus
content of EP2-DDM is higher than that of EP1-DDM,
resulting in its higher Td30% and R700 than EP1-DDM. Higher
phosphorus content should lead to lower Td5% of the
phosphorus-containing systems, while EP2-DDM showed
higher Td5% than EP1-DDM. Our previous work proved that
increasing the cross-link density can improve the initial
degradation temperature.38 EP2-DDM has higher cross-link
density relative to EP1-DDM, which corresponds to its higher
Td5% than EP1-DDM.
Glass Transition Temperature of the Cured Epoxy
Resins. Glass transition temperature (Tg) is a major parameter
for thermosetting materials. The Tgs of EP1-DDM, EP2-DDM,
and DGEBA-DDM were determined by DSC, and the results
are summarized in Figure 10. EP1-DDM and EP2-DDM
showed higher Tgs of 183 and 214 °C, respectively, than
DGEBA-DDM with Tg of 166 °C. The Tgs of the cross-linked
polymers have a close tie with their cross-link density and the
rigidity of the chain segment structure.49,50 The higher the
cross-link density of the thermosets and rigidity of their chain
segment are the higher the Tg is. In order to estimate the cross-
link density of the cured epoxy resins, the average molecular
weight between cross-link points (Mc) (Table 1) was calculated
using eq 3.
nEPMEP + nDDMMDDM
Mc =
Figure 7. (a) Digital photos and (b) SEM photographs of the residues
after LOI test.
nDDM
(3)
where n and M represent the molarity and molar mass of the
corresponding component in the epoxy formulations.51 As
shown in Table 1, EP1-DDM and EP2-DDM exhibited higher
Mcs than DGEBA-DDM, which means that they possessed
lower cross-link density than DGEBA-DDM. Thus, the higher
Tgs of EP1-DDM and EP2-DDM than that of DGEBA-DDM
mainly resulted from the higher rigidity of EP1-DDM and EP2-
yield at 700 °C (R700) as high as ∼44.7% under air atmosphere
and ∼58% under a nitrogen atmosphere than DEGBA-DDM
with R700 of 0% under air atmosphere and 14.0% under a
nitrogen atmosphere. As mentioned previously, the phospho-
nate group as a promoter for “char formation” formed the
phosphorus-rich layer during the degradation process, which
Table 2. Phosphorus and Nitrogen Content of EP-DDM, EP2-DDM, and DGEBA-DDM before and after Burning
before burning
after burning
sample
phosphorus content (wt %) nitrogen content (wt %) weight residue (wt %) phosphorus content (wt %) phosphorus retention (%)
EP1-DDM
6.50
7.18
0
4.40
4.86
2.86
35.8
41.2
6.99
9.32
38.5
53.5
EP2-DDM
DGEBA-DDM
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Figure 9. (a) TG and (b) DTG curves under a nitrogen atmosphere and (c) TG and (d) DTG curves under an air atmosphere of the cured epoxy
resins.
content than EP1-DDM, resulting in the higher Tg of EP2-
DDM relative to that of EP1-DDM.
Mechanical Properties. Figure 11 presents the tensile
stress−strain curves of the cured epoxies, and their tensile
Table 3. TGA Data of the Cured Epoxy Resins
Td5% (°C)
Td30% (°C)
R700 (%)
sample
air
N2
air
N2
air
N2
EP1-DDM
286
287
356
340
353
382
427
482
390
435
450
403
26.6
44.7
0
53.0
58.0
14.0
EP2-DDM
DGEBA-DDM
Figure 11. Stress−strain curves for the cured epoxy resins.
properties are summarized in Table 4. All of the epoxy systems
exhibited rigid tensile behaviors without a yield point.
Figure 10. Nonisothermal DSC curves and Tgs of the cured epoxy
resins.
Table 4. Mechanical Properties of the Cured Epoxy Resins
DDM than that of DGEBA-DDM. Although the EP1-DDM
and EP2-DDM have flexible phosphate groups −(P(O)−(O−
C2H5)2), they possessed more rigid aromatic rings and strongly
polar N−H originated intramolecular hydrogen bonds,52
leading to the higher rigidity of EP1-DDM and EP2-DDM
than that of DGEBA-DDM. EP2-DDM displayed lower Mc,
corresponding to the higher cross-link density and higher N−H
tensile strength
(MPa)
tensile modulus
(MPa)
elongation at break
(%)
sample
EP1-DDM
EP2-DDM
80.3
60.6
76.4
5
3
6
2114 132
2709 110
1893 140
5.2 0.4
2.6 0.2
7.4 0.5
DGEBA-
DDM
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ORCID
Obviously, the moduli of EP1-DDM and EP2-DDM were
much higher than that of the cured DGEBA. As Tg, modulus of
thermoset also has a close tie with its cross-link density and
rigidity of chain segment structure,49,50 and the higher moduli
of EP1-DDM and EP2-DDM were ascribed to their higher
rigidity from the more rigid aromatic rings and strongly polar
N−H originated intramolecular hydrogen bonds52 than the
cured DGEBA. Owing to the higher cross-link density and
more N−H of EP2-DDM than those of EP1-DDM and EP2-
DDM presented higher modulus than EP1-DDM. The higher
rigidity of the EP1-DDM and EP2-DDM led to the lower
mobility of the chain segments of the networks, corresponding
to the lower elongation at break of the two bio-based epoxy
networks than that of DGEBA-DDM. EP2-DDM has higher
cross-link density and more N−H originated intramolecular
hydrogen bonds than EP1-DDM, resulting in its lower
elongation at break than EP1-DDM. Owing to its more strong
aromatic rings and N−H originated intramolecular hydrogen
bonds, EP1-DDM showed higher tensile strength than
DGEBA-DDM. While for the EP2-DDM, it exhibited lower
tensile strength than DGEBA-DDM. This is probably due to
the more internal stress produced from the curing process for
EP2-DDM. The internal stress is related to Tg. When the curing
temperature is higher than Tg, the internal stress will be low.
When the curing temperature is lower than Tg, the internal
stress will be high.53 For EP1-DDM and DGEBA-DDM, their
Tgs are much lower than the curing temperature (230 °C),
while the temperature 230 °C locates in the glass transition
range of EP2-DDM. As a result, EP2-DDM has high internal
stress, leading to the comparatively low tensile strength.
Songqi Ma: 0000-0002-9652-1016
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for the financial support from Project
51473180 supported by the National Natural Science
Foundation of China, Natural Science Foundation of Zhejiang
Province (LY15E030004), and the Ministry for Industry and
Information of the People’s Republic of China under grant
agreement no. [2016] 92.
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Two novel bio-based epoxy resins with excellent flame
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Corresponding Authors
*(S.M.) E-mail masongqi@nimte.ac.cn; Tel 86-0574-
87619806; Fax 86-0574-86685186.
*(J.Z.) E-mail jzhu@nimte.ac.cn; Tel 86-0574-87619806; Fax
86-0574-86685186.
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