Synthesis of a furfural-based DOPO-containing co-curing agent for fire-safe epoxy resins

Article abstract of DOI:10.1039/c9ra06425g

A furfural-based DOPO-containing flame retardant, 6,6′-(((methylenebis(4,1-phenylene))bis(azanediyl))bis(furan-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (MBF-DOPO), was synthesized and utilized as a co-curing agent of 4,4′-diaminodiphenyl methane (DDM) for fire-safe epoxy thermosets. For the cured epoxy resin containing 4.0% MBF-DOPO, the limiting oxygen index (LOI) reached 32.9% (with the V-0 rating in UL-94 test), and the peak heat release rate and total smoke production values were respectively decreased by 29.3% and 33.6%, compared to pure epoxy resin. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results confirmed that the furfural-based flame retardant MBF-DOPO promoted the charring formation of the epoxy matrix, which effectively isolated the gas and heat transfer during combustion and thus enhanced the fire-safety performance of the epoxy thermosets. This work provides an effective route for synthesizing a furfural-based flame retardant, which possesses great potential for application in fire-safe epoxy thermosets.

Full text of DOI:10.1039/c9ra06425g

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Synthesis of a furfural-based DOPO-containing co-
curing agent for fire-safe epoxy resins†
Cite this: RSC Adv., 2020, 10, 1956
a
ab
Weiqi Xie, ‡a Shiwen Huang,‡a Donglin Tang,
Shumei Liu
*
ab
*
and Jianqing Zhao
A furfural-based DOPO-containing flame retardant, 6,60-(((methylenebis(4,1-phenylene))bis(azanediyl))
bis(furan-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (MBF-DOPO), was synthesized
and utilized as
a
co-curing agent of 4,40-diaminodiphenyl methane (DDM) for fire-safe epoxy
thermosets. For the cured epoxy resin containing 4.0% MBF-DOPO, the limiting oxygen index (LOI)
reached 32.9% (with the V-0 rating in UL-94 test), and the peak heat release rate and total smoke
production values were respectively decreased by 29.3% and 33.6%, compared to pure epoxy resin.
Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results
confirmed that the furfural-based flame retardant MBF-DOPO promoted the charring formation of the
epoxy matrix, which effectively isolated the gas and heat transfer during combustion and thus enhanced
the fire-safety performance of the epoxy thermosets. This work provides an effective route for
synthesizing a furfural-based flame retardant, which possesses great potential for application in fire-safe
epoxy thermosets.
Received 16th August 2019
Accepted 7th December 2019
DOI: 10.1039/c9ra06425g
rsc.li/rsc-advances
improve the re-safety performance of epoxy resins.12–14 Among
them, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide
Introduction
(DOPO) and its derivatives are eco-friendly ame retardants
with many advantages (e.g., high efficiency and low toxicity).15,16
Recently, the preparation of a furfural-based ame retardant
has aroused extensive attention for sustainable develop-
ment.17–19 In order to meet the urgent needs for achieving
sustainable development, the preparation of DOPO derivatives
derived from furfural is a potential and sustainable strategy.20
However, the preparation of furfural-based DOPO derivatives
for epoxy resins is still in its infancy.
Recently, there has been more and more interest in the prepa-
ration of new materials and chemicals from furfural, which is
a multi-functional compound containing a furan ring and an
aldehyde group.1,2 It is mainly extracted from abundant agri-
cultural residues like plant hulls and corn cobs, thus enabling
furfural to be more feasible as a platform molecule for renew-
able chemicals and materials.3,4 Moreover, with a reactive
aldehyde group, furfural is deemed to be a promising renewable
resource for preparing various chemicals and materials, and
has been used in diverse research elds.5,6
Epoxy resins are a signicant class of polymers and have
been adopted in many different elds (i.e., coatings, adhesive,
and laminate materials) for their excellent chemical resistance
and mechanical properties.7–10 However, epoxy resins have the
drawback of poor re resistance performance, which greatly
limits their application in areas requiring high ame retard-
ancy.11 Various kinds of ame retardants (phosphorus-,
nitrogen- and silicon-containing compounds) were adopted to
Herein,
(((methylenebis(4,1-phenylene))bis(azanediyl))bis(furan-2-
a
novel furfural-based DOPO derivative, 6,60-
ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine
6-oxide)
(MBF-DOPO), is prepared and utilized as a co-curing agent of
4,40-diaminodiphenyl methane (DDM) for re-safe epoxy ther-
mosets. It is expected that the ame retardancy from furfural
and DOPO will have outstanding re resistance efficiency.
Moreover, the thermal and mechanical performance of epoxy
thermosets are also evaluated.
aSchool of Materials Science and Engineering, South China University of Technology,
Guangzhou, 510640, P. R. China. E-mail: liusm@scut.edu.cn; psjqzhao@scut.edu.
cn; Fax: +86-13611400566; +86-13609724000; Tel: +86-13611400566; +86-
13609724000
Materials and methods
Materials
4,40-Diaminodiphenyl methane (DDM), DOPO, and furfural
were obtained from Aladdin Reagent Co. Ltd., China. DGEBA
(epoxy value ¼ 0.51 mol/100 g) was obtained from SINOPEC
Baling company, China. 1,4-Dioxane and ethanol were obtained
from Guangzhou Chemical Reagent Factory, China.
bKey Laboratory of Polymer Processing Engineering, Ministry of Education, Guangzhou
510640, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06425g
‡ These authors contributed equally to this work.
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ꢀ
80 C for 20 min. Aerward, the reactiꢀon system was cured at
ꢀ
ꢀ
80 C for 2 h, 110 C for 2 h, and 180 C for 2 h to obtain the
epoxy networks. The preparation route for epoxy network is
depicted in Fig. 2.
Characterization
1H, 13C and 31P nuclear magnetic resonance (NMR) spectra were
collected with a Bruker NMR spectrometer (Billerica, MA, USA)
and deuterated dimethylsulfoxide (DMSO-d6) was used as the
solvent. The infrared spectra (FT-IR) were obtained with
a Vertex70 spectrometer (Bruker, Billerica, MA, USA) using KBr
pellets. Mass spectrometry (MS) spectra were collected on
a maXis impact mass spectrometer (Bruker).
Thermogravimetric analyses (TGA) were conducted using
a TG-209F1 TGA (Netzsch, Selb, Germany) at a heating rate of
10 ꢀC minꢁ1 (N2 atmosphere), and the temperature range is
from 50 to 700 ꢀC. Dynamic mechanical analysis (DMA) was
conducted using a TA instrument (DMA Q800, America) at
a heating rate of 3 ꢀC minꢁ1 (from 25 to 230 ꢀC). The dimensions
of the cured samples for measurement were 40 ꢂ 10 ꢂ 3.0 mm.
Tensile and exural properties were analyzed based on ASTM
D638-08 and ASTM D790-07, respectively, on an Instron-5967
universal electronic testing machine.
Fig. 1 Synthesis route of MBF-DOPO.
Synthesis of MBF-DOPO
DDM (9.91 g, 0.05 mol) and furfural (9.61 g, 0.10 mol) were
UL-94 vertical burning tests were conducted with a UL 94
ame chamber (Fire Testing Technology, UK) according to
ASTM D3801-10 (sample dimension of 125 ꢂ 13 ꢂ 3 mm).
Limiting oxygen index (LOI) tests were conducted using an
oxygen index instrument (Fire Testing Technology, UK)
according to ASTM D2863-97 (sample dimension of 150 ꢂ 6.5 ꢂ
3.2 mm). Cone calorimeter tests (CCT) were conducted using
a FTT cone calorimeter according to ISO5660 (sample dimen-
sion of 100 ꢂ 100 ꢂ 5 mm).
added to 200 mL of 1,4-dioxane under a N2 atmosphere. Aer
continuous stirring for 3 h at 80 C, the system was cooled to
ꢀ
room temperature, followed by the addition of 100 mL of 1,4-
dioxane solution containing DOPO (21.6 g, 0.10 mol) into the
system and then stirred at 55 ꢀC for 12 h. Aer that, the mixture
was transferred to ice water. The crude product was ltered,
washed (three times using ethanol) and vacuum dried (75 ꢀC for
24 h) to give a white powder (the yield was 92.4%). The synthetic
route of MBF-DOPO is depicted in Fig. 1.
Scanning electron microscopy (SEM) experiments were con-
ducted with a NOVA NANOSEM 430 machine. The sample was
sputter-coated with gold before testing. X-ray photoelectron
Preparation of epoxy networks
The epoxy networks were prepared by the curing reaction of spectroscopy (XPS) was conducted using an Axis Ultra spec-
DGEBA with various weights of DDM and MBF-DOPO (see trometer (Kratos, England). Raman spectroscopy was conducted
Table 1, the molar ratio of the N–H in DDM and MBF-DOPO to using a DXR laser Raman spectrometer (532 nm Helium–Neon
epoxy groups is 1). DGEBA and MBF-DOPO were added to line) at room temperature. Thermogravimetry-Fourier trans-
ꢀ
a ask and stirred at 170 C for 60 min. Aer that, the system form infrared spectroscopy (TGA-FTIR) tests were conducted
was cooled to 80 ꢀC, followed by adding DDM. The mixture was with a STA449C/3MFC/G instrument (Bruker, USA) (N2 atmo-
stirred for 20 min, and poured into a mould and degassed at sphere, heating rate ¼ 20 C minꢁ1).
ꢀ
Table 1 Stoichiometric formulation of the epoxy system
Samplesa
DGEBA (wt%)
DDM (wt%)
MBF-DOPO (wt%)
DOPO (wt%)
P (wt%)
EP-0
EP-1.0
EP-2.0
EP-3.0
EP-4.0
EP-DOPO-4.0
EP-5.0
79.83
79.03
78.23
77.44
76.64
76.64
75.84
20.17
19.97
19.77
19.56
19.36
19.36
19.16
0
0
0
0
0
0
4.0
0
0
1.0
2.0
3.0
4.0
0
0.08
0.16
0.24
0.31
0.57
0.39
5.0
a
Sample name: EP-X, X represents the weight percentage of MBF-DOPO in the MBF-DOPO/DDM/DGEBA system. EP-DOPO-4.0 represents 4 wt%
DOPO in the DOPO/DDM/DGEBA system.
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Fig. 2 Route for preparation of the cured network MBF-DOPO/DDM/DGEBA.
in the DOPO spectrum, the peak of CH]O (d ¼ 9.62 ppm) in the
furfural spectrum and the peak of the primary amine (N–H, d ¼
4.79 ppm) in the DDM spectrum. Fig. S2† illustrates the
Results and discussion
Characterization of MBF-DOPO
1
assignment of peaks in the H-NMR spectrum of MBF-DOPO,
The structure of the obtained MBF-DOPO was characterized by
FT-IR, NMR, and MS techniques. The FTIR spectra of furfural,
DDM, DOPO and MBF-DOPO are shown in Fig. S1.† In the FTIR
spectrum of MBF-DOPO, a few peaks disappeared compared
with the spectra of furfural, DDM and DOPO, such as the peak
at 3430 cmꢁ1 (primary amine (N–H) stretching) in the DDM
spectrum, the peak at 2430 cmꢁ1 (P–H stretching) in the DOPO
spectrum and the peak at 1675 cmꢁ1 (C]O stretching) in the
furfural spectrum. Meanwhile, a peak corresponding to the
secondary amine (C–NH) stretching appears at 3320 cmꢁ1 in the
MBF-DOPO spectrum. The above results conrm the successful
synthesis of MBF-DOPO.
1H NMR (DMSO-d6, ppm): d ¼ 2.50 (DMSO), 3.33 (H2O), 3.47
(H16), 5.19 (H12), 6.11 (H11), 6.37 (H10), 6.43–6.74 (H14, H15, H17,
H18), 7.07–7.82 (H1–3, H6–9), 7.94 (H5), and 8.16–8.19 (H4, H13).
The structure of MBF-DOPO is also conrmed by 13C and 31P
NMR and MS spectra (Fig. 3(b)–(d)). In Fig. 3(b), the expected
chemical shis of the C atoms are in good agreement with the
actual chemical shis, 13C NMR (DMSO-d6, ppm): d ¼ 51.2 (C9),
66.8 (C24), 109.8 (C22), 111.2 (C21), 114.1 (C4, C6), 120.4–120.6
(C2, C8), 121.9 (C11), 122.8 (C14), 123.8–126.0 (C12, C15, C19),
128.9–129.1 (C10, C17), 130.8–132.1 (C3, C7), 134.0–134.3 (C16,
C18), 135.9 (C13), 143.5 (C20), 145.2 (C5), and 149.1–149.5 (C1,
C23). The 31P NMR spectrum of MBF-DOPO shows two peaks at
27.4 and 29.3 ppm, which are due to the chiral structure of the P
atoms in MBF-DOPO.21 The mass spectrometry (MS) spectrum
shows an [M + Na+] ion peak at m/z 809.1944, suggesting
The 1H NMR spectra of furfural, DDM, DOPO, and MBF-
DOPO are shown in Fig. 3(a). The characteristic proton of N–
CH (d ¼ 5.19 ppm) is found in the MBF-DOPO spectrum,
following the disappearance of the peak of P–H (d ¼ 8.64 ppm)
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Fig. 3 (a) 1H NMR spectra of furfural, DDM, DOPO, and MBF-DOPO. (b) 13C-NMR, (c) 31P-NMR, and (d) HRESI-MS spectra of MBF-DOPO.
a molecular formula of C47H36N2O6P2. All these results fully MBF-DOPO exhibits a T5% of 312.0 ꢀC, suggesting that MBF-
conrm the successful synthesis of MBF-DOPO with a facile and DOPO possesses a higher thermal stability compared with
environmental-friendly route. The TGA curves of MBF-DOPO DOPO. The residue (at 700 ꢀC) of MBF-DOPO is 38.9% under
and DOPO (N2 and air atmosphere) are shown in Fig. 4. As a N2 atmosphere, which is far higher than DOPO (1.73%).
seen from Fig. 4(a), DOPO possesses an initial decomposition Similarly, in air atmosphere (Fig. 4(b)), MBF-DOPO shows
temperature (T5%) of 217.7 ꢀC under a N2 atmosphere, whereas a higher T5% (336.7 ꢀC) and residue (at 700 ꢀC, 41.8%) compared
Fig. 4 TGA curves of MBF-DOPO and DOPO under a (a) N2 and (b) air atmosphere.
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Fig. 5 DSC thermograms of (a) EP-0 and (b) EP-4.0 cured at varying heating rates.
with DOPO. The TGA results indicate that the furfural-based DOPO-4.0 shows a LOI value of 31.4% and passes the UL-94 V-1
compound MBF-DOPO possesses excellent charring ability rating. Compared with EP-DOPO-4.0, EP-4.0 (containing
associated with high thermal stability, which is mainly caused 4.0 wt% of MBF-DOPO) shows better re-safety properties with
by the thermally-stable benzene and furan ring structures in a V-0 rating of the UL-94 test and increases its LOI value to
MBF-DOPO.
32.9%. The results show that MBF-DOPO possesses a higher
ame-retardant efficiency for epoxy resins compared with
DOPO. It may be deduced that the unique combination of
phosphorus and nitrogen atoms in MBF-DOPO plays an
important role in enhancing the ame-retardant efficiency of
MBF-DOPO, and the synergistic effect between the phosphorus
and nitrogen elements endows MBF-DOPO with a high ame-
retardant performance in both the gas phase and condensed
phase.
Curing behavior of the MBF-DOPO/DDM/DGEBA system
In the MBF-DOPO/DDM/DGEBA system, the main reaction for
the curing process is the ring opening of the epoxy by a reaction
with the amine groups (primary or secondary amine) of DDM
and MBF-DOPO.21 The nonisothermal curing kinetics of the
MBF-DOPO/DDM/DGEBA system are studied using DSC at
various heating rates (2.5, 5, 10, 15 and 20 C minꢁ1) and are
ꢀ
The re-safety performance of EP-0 and EP-4.0 is further
evaluated using the cone calorimeter test (CCT) (Fig. 7 and
Table S3†). As can be seen, the time to ignition (TTI) value of EP-
4.0 is lower than EP-0, which is due to the early decomposition
of unstable phosphorus-containing structures from MBF-
DOPO. The peak of the heat release rate (pHRR) of EP-4.0 is
decreased to 680 kW mꢁ2, which is 29.3% lower than that of EP-
0 (962 kW mꢁ2). Similarly, EP-4.0 possesses a 12.6% reduction
in the total heat release (THR) value compared to EP-0. The
depicted in Fig. 5. Kissinger's method is utilized for the calcu-
lation of the apparent activation energy,22
!
ꢀ
ꢁ
b
AR
Ea
Ea
ln
¼ ln
ꢁ
(1)
2
RTp
Tp
where b, R, and Tp respectively represent the heating rate, the
ideal gas constant, and the exothermic peak temperature. Ea
and A represent the apparent activation energy and the pre-
exponential factor, respectively. Table S1† lists the Tp values
of the EP-0 and EP-4.0 systems cured at various heating rates.
According to eqn (1), the curing activation energy (Ea) of EP-
2
0 and EP-4.0 is obtained from the linear plot of ln(b/Tp
)
versus 1/Tp plot (Fig. 6), as listed in Table S2.† It can be found
that the EP-4.0 curing system possesses lower Ea
a
(47.3 kJ molꢁ1) than the EP-0 curing system (50.4 kJ molꢁ1),
indicating that the introduction of MBF-DOPO in epoxy resins
can lower the activation energy of the system and accelerate the
crosslinking reaction.
Fire-safety performance
The re-safety performance of cured resins is assessed by LOI
and UL-94 vertical burning (UL-94) tests (see Table 2). The pure
epoxy resin EP-0 possesses the LOI value of 23.5% with melt-
dripping and no rating in the UL-94 test, and the LOI value
increases with the increase of the MBF-DOPO content in the
Fig. 6 Plot of ln(b/Tp2) versus 1/Tp of EP-0 and EP-4.0 according to
epoxy matrix. By adding 4.0 wt% of DOPO to the epoxy resin, EP- Kissinger's method.
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Table 2 UL-94 vertical burning and LOI test result of cured MBF-DOPO/DDM/DGEBA
UL-94
Samples
P (wt%)
LOI (%)
t1 + t2 (s)
Dripping
Rating
EP-0
EP-1.0
EP-2.0
EP-3.0
EP-4.0
EP-DOPO-4.0
EP-5.0
0
23.5 ꢃ 0.3
26.2 ꢃ 0.2
28.3 ꢃ 0.3
31.2 ꢃ 0.3
32.9 ꢃ 0.2
31.4 ꢃ 0.4
33.5 ꢃ 0.3
Lasting burning
78.5 ꢃ 3.1
35.6 ꢃ 1.7
19.5 ꢃ 3.4
7.5 ꢃ 2.1
Yes
No
No
No
No
No
No
NR
V-2
V-1
V-1
V-0
V-1
V-0
0.08
0.16
0.24
0.31
0.57
0.39
28.6 ꢃ 4.7
5.2 ꢃ 1.2
above results further verify the good re resistance performance which act as protective barriers to suppress the smoke
of MBF-DOPO for the epoxy matrix. As can be seen in Fig. 7(c), production of the epoxy matrix during combustion.
the residual mass of EP-4.0 aer the cone calorimeter test is
17.9 wt%, and is far higher than that of EP-0 (8.6 wt%), which
suggests that the introduction of MBF-DOPO may promote the
char forming during combustion. The smoke production from
combustion is a very signicant indicator for the re-safety
properties of epoxy resins. In Fig. 7(d), compared to EP-0, the
total smoke production (TSP) value of EP-4.0 is signicantly
decreased by 33.6%, suggesting that the introduction of MBF-
DOPO helps to form compact and protective char layers,
TG-FTIR analysis
TG-FTIR is utilized to understand the gas-phase ame-retardant
effect of MBF-DOPO for epoxy resins by detecting the gases
released from the thermal decomposition of EP-0 and EP-4.0.
The FTIR spectra of gases obtained from the thermal decom-
position of EP-0 and EP-4.0 at the initial degradation tempera-
ture (370 ꢀC for EP-4.0, 390 ꢀC for EP-0) and the maximum
degradation temperature (390 ꢀC for EP-4.0, 405 ꢀC for EP-0) are
Fig. 7 (a) HRR, (b) THR, (c) residual mass, and (d) TSP curves of EP-0 and EP-4.0.
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Fig. 8 FTIR spectra of the pyrolysis products of EP-0 and EP-4.0 at (a) the initial and (b) maximum degradation temperatures.
presented in Fig. 8. As can be seen, the gaseous products of EP- EP-0, there is a new peak in the spectrum of EP-4.0, belonging to
4.0 and EP-0 are identied by the absorptions at 1170 and P–O–P (1057 cmꢁ1),25 which is generated from the decomposi-
1251 cmꢁ1 (ethers), 1598, 1503 and 1332 cmꢁ1 (aromatic tion of MBF-DOPO in EP-4.0 during combustion, indicating that
substances), 2343 and 2302 cmꢁ1 (CO2), 3037 and 2960 cmꢁ1 the phosphorus-containing gaseous products play an important
(hydrocarbons), and 3658 cmꢁ1 (phenols).23,24 However, unlike role in the gas-phase ame-retardant effect of MBF-DOPO.
Fig. 9 SEM photographs of external ((a) EP-0, (b) EP-4.0) and internal char layers ((c) EP-0, (d) EP-4.0) after CCT.
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residue aer CCT is analyzed by XPS (see Table S4†). Both EP-
0 and EP-4.0 contain C, O and N elements. Compared to EP-0,
the P element is found in EP-4.0 with a content of 0.32 wt%,
which agrees with the FT-IR results and further conrms that
MBF-DOPO plays an important role in the condensed phase,
which forms a phosphorus-rich char layer to inhibit the heat
and gas transfer during combustion.
Thermal and mechanical properties
TGA is used for assessing the thermal stability of EP-0 and EP-
4.0 under N2 (Fig. 11) and air conditions (Fig. 12), the related
data are shown in Table S5.† As shown in Fig. 11(a), in N2
atmosphere, EP-0 and EP-4.0 have only one weight loss stage.
Compared with EP-0, EP-4.0 has a slightly lower T5% and
maximum decomposition temperature (Td,max) due to the less
stable structures (P–O–C and P]O) from MBF-DOPO in EP-4.0.
In Fig. 11(b), the maximum decomposition rate (Rmax) of EP-4.0
is 27.4% minꢁ1, which is far lower than EP-0 (36.5% minꢁ1),
suggesting that MBF-DOPO has a good inhibition effect on the
thermal degradation of the epoxy matrix. The residual mass (at
700 ꢀC) of EP-0 is 14.7%, whereas EP-4.0 is increased to 21.8%.
The above results show that although the phosphorus-
containing groups in the MBF-DOPO molecule decompose at
a relatively low temperature at the early and middle stages of
thermal degradation, the phosphorus-containing compounds
formed by MBF-DOPO can inhibit the thermal degradation of
epoxy matrix at high temperatures and promote the formation
of a stable char residue, which leads to a high residual mass and
ame retardancy. Under air atmosphere, as shown in Fig. 12,
both EP-0 and EP-4.0 possess two degradation stages. The rst
degradation stage corresponds to the degradation of the epoxy
matrix, the second degradation process corresponds to the
further oxidation of char residues. Similarly, the introduction of
MBF-DOPO helps to catalyze the degradation of the epoxy
thermoset under air conditions, which leads to the decrease of
EP-4.0 in T5% and Td. The early degradation of phosphorus-
containing groups in MBF-DOPO is able to generate
phosphorus-based acid, which is helpful to promote the char
Fig. 10 FT-IR spectra of char residues after CCT.
Char analysis
The SEM morphology of the char residues aer CCT is depicted
in Fig. 9. For EP-0, both the external and internal char layers
exhibit a cracked char residue with open holes. The external
char layer of EP-4.0 presents a compact and homogeneous
structure without broken bubbles, and the internal char layer
shows a continuous and skeletal morphology with some small
pores. The formation of the strong char layer in EP-4.0 inhibits
the transmission of heat and gas during combustion, which
endows EP-4.0 with excellent re-safety performance.
The chemical structures of the char residue are measured
with the FTIR technique. As shown in Fig. 10, both EP-0 and EP-
4.0 have a peak at 3452 cmꢁ1 (O–H or N–H stretching) and
a peak at 1631 cmꢁ1 (carbonized compound stretching).
Compared with EP-0, two new characteristic peaks occur at
1103 cmꢁ1 (P–O–C stretching) and 1280 cmꢁ1 (P]O stretching)
in the spectrum of EP-4.0, showing that the char residue of EP-
4.0 contains phosphorus- and nitrogen-containing compounds,
which contribute to the charring of the epoxy matrix and lead to
a compact and intumescent char with highly carbonized
aromatic networks. The element composition of the char
Fig. 11 (a) TGA and (b) DTG curves of EP-0 and EP-4.0 under N2 conditions.
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Fig. 12 (a) TGA and (b) DTG curves of EP-0 and EP-4.0 under air conditions.
Fig. 13 (a) Storage modulus and (b) tan d curves of EP-0 and EP-4.0.
forming of the epoxy matrix. As shown in Fig. 12(b), the Rmax lower than that of EP-0. Thus, EP-4.0 maintains the outstanding
(21.8% minꢁ1
) of EP-4.0 is also far lower than that thermal stability of EP-0. The cross-link densities of EP-0 and
(31.3% minꢁ1) of EP-0 under air atmosphere, suggesting that EP-4.0 are calculated based on the literature,27,28 as listed in
the phosphorus-based acid formed by MBF-DOPO can also Table S6.† The EP-4.0 shows a cross-link density of 4.99 ꢂ
decrease the degradation rate of the epoxy networks.
103 mol mꢁ3, which is slightly lower than that of EP-0, sug-
DMA is utilized for evaluating the thermomechanical gesting that the introduction of MBF-DOPO leads to a slight
performance of EP-0 and EP-4.0. The storage modulus (E0) and decrease of the cross-link density of the epoxy resins.
loss factor (tan d) curves are depicted in Fig. 13, the related data
The mechanical properties of EP-0 and EP-4.0 are evaluated
are shown in Table S6.† EP-4.0 possesses higher E0 values (3.04 (Table 3). Compared to EP-0, the tensile and exural strengths
GPa) than EP-0 (2.89 GPa) at room temperature (30 ꢀC), indi- of EP-4.0 are respectively decreased by 2.2% and 1.6%. These
cating that the rigid structures (phosphaphenanthrene and differences are mainly due to the lower cross-link density of EP-
furan structure) in MBF-DOPO enhance the stiffness of the 4.0. Meanwhile, the tensile and exural moduli of EP-4.0 are
epoxy thermosets. In Fig. 13(b), EP-4.0 possesses a Tg (peak respectively increased by 6.2% and 8.3%, which is mainly
temperature in tan d curve26) of 159.5 C, which is only 2.7 C a result of the more rigid structures (e.g., phosphaphenanthrene
ꢀ
ꢀ
Table 3 Tensile and flexural properties of EP-0 and EP-4.0
Samples
Tensile strength (MPa)
Tensile modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
EP-0
EP-4.0
71.8 ꢃ 2.1
70.2 ꢃ 1.9
2.89 ꢃ 0.11
3.07 ꢃ 0.09
105.5 ꢃ 2.2
103.8 ꢃ 3.1
2.78 ꢃ 0.12
3.01 ꢃ 0.10
1964 | RSC Adv., 2020, 10, 1956–1965
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RSC Advances
and furan structure) in MBF-DOPO.29–31 In summary, the small
content of MBF-DOPO has little impact on the mechanical
performance of the EP-0 network.
7 F. Zhou, Z. Guo, W. Wang, X. Lei, B. Zhang, H. Zhang and
Q. Zhang, Compos. Sci. Technol., 2018, 167, 79–85.
8 F. Jeyranpour, G. Alahyarizadeh and A. Minuchehr, Polymer,
2016, 88, 9–18.
9 F.-L. Jin, X. Li and S.-J. Park, J. Ind. Eng. Chem., 2015, 29, 1–11.
10 W.-Q. Xie and X.-S. Chai, Polym. Test., 2017, 59, 113–117.
11 S. Wang, S. Ma, C. Xu, Y. Liu, J. Dai, Z. Wang, X. Liu, J. Chen,
X. Shen and J. Wei, Macromolecules, 2017, 50, 1892–1901.
12 S. Yang, J. Wang, S. Huo, J. Wang and Y. Tang, Polym. Degrad.
Stab., 2016, 126, 9–16.
13 Y. Chen, H. B. Zhang, Y. Yang, M. Wang, A. Cao and Z. Z. Yu,
Adv. Funct. Mater., 2016, 26, 447–455.
14 L. Zhang, Y. Wang, Q. Liu and X. Cai, J. Therm. Anal.
Calorim., 2016, 123, 1343–1350.
15 M.-J. Xu, G.-R. Xu, Y. Leng and B. Li, Polym. Degrad. Stab.,
2016, 123, 105–114.
16 P. Wang and Z. Cai, Polym. Degrad. Stab., 2017, 137, 138–150.
17 L. Costes, F. Laoutid, S. Brohez and P. Dubois, Mater. Sci.
Eng., R, 2017, 117, 1–25.
Conclusions
Herein, a new furfural-based DOPO-containing ame retardant,
namely 6,60-(((methylenebis(4,1-phenylene))bis(azanediyl))bis(-
furan-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine
6-
oxide) (MBF-DOPO), was successfully prepared and utilized as
a co-curing agent of 4,40-diaminodiphenyl methane (DDM) for
re-safe epoxy thermosets. For the epoxy resin sample con-
taining 4.0% MBF-DOPO, the limiting oxygen index (LOI)
reached 32.9% (with a V-0 rating in the UL-94 test). Besides this,
the mechanical and thermal performance of EP-4.0 were well
maintained. Hence, MBF-DOPO has great potential for appli-
cation in re-safe epoxy thermosets. This work provides an
effective and facile strategy to prepare a furfural-based DOPO-
containing ame retardant.
18 X. Zhao, D. Xiao, J. P. Alonso and D.-Y. Wang, Mater. Des.,
2017, 114, 623–632.
Conflicts of interest
´
19 R. Menard, C. Negrell, M. Fache, L. Ferry, R. Sonnier and
The authors declare that there are no conicts of interest.
G. David, RSC Adv., 2015, 5, 70856–70867.
20 C.-M. Lin, C.-H. Chen, C.-H. Lin and T.-Y. Juang, Eur. Polym.
J., 2018, 108, 48–56.
21 W. Xu, A. Wirasaputra, S. Liu, Y. Yuan and J. Zhao, Polym.
Degrad. Stab., 2015, 122, 44–51.
22 F. Ferdosian, Y. Zhang, Z. Yuan, M. Anderson and C. C. Xu,
Eur. Polym. J., 2016, 82, 153–165.
23 Y. Wen, Z. Cheng, W. Li, Z. Li, D. Liao, X. Hu, N. Pan, D. Wang
and T. R. Hull, Polym. Degrad. Stab., 2018, 156, 111–124.
24 C. Dong, A. Wirasaputra, Q. Luo, S. Liu, Y. Yuan, J. Zhao and
Y. Fu, Materials, 2016, 9, 1008.
Acknowledgements
We gratefully acknowledge support from the National Natural
Science Foundation of China (51573054), the Key Project in
Science and Technology of Guangzhou (No. 201902010065), and
the Opening Project of Key Laboratory of Polymer Processing
Engineering (South China University of Technology), Ministry
of Education, China (No. KFKT1702).
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RSC Adv., 2020, 10, 1956–1965 | 1965