Synthesis and application of aminophenyl-s-triazine derivatives as potential flame retardants in the modification of epoxy resins

Article abstract of DOI:10.1039/C8RA07450J

In order to develop new phosphorus-nitrogen-containing flame retardants, three novel compounds of DOPO-substituted aminophenyl-s-triazine (TAT-DOPO) with different P/N element ratios were synthesized by Kabachnik-Fields reaction. TAT-triDOPO possessing three DOPO units in one molecule exhibited the best thermal stability. Besides, TAT-triDOPO and TAT-hexaDOPO possessing active hydrogen displayed the characteristics of reactive-type retardant, while TAT-enneaDOPO without active hydrogen represented additive-type retardant. The influences of three TAT-DOPO compounds on the thermal stability, flame retardancy and mechanical properties of the cured EP composites were investigated. Compared with pure EP, the char yields of EP/TAT-DOPO were increased. The LOI value and the UL-94 ratings of EP/TAT-triDOPO with a 5% loading were improved to 34.0% and V-0 grade. The PHRR, THR and HRC of EP/TAT-triDOPO were decreased by 25.7%, 21.1% and 25.4%, respectively. Furthermore, the incorporation of TAT-triDOPO could also improve the flexural strength and elastic modulus of cured EP. It was explained that the existence of active hydrogen atoms within TAT-triDOPO molecule can act as effective chemical cross-linking points in EP network. Finally, based on the SEM and TGA-FTIR analysis of pyrolytic products of cured EP/TAT-DOPO, it was deduced that the prepared TAT-DOPO flame retardant could exhibit both condensed phase and gas phase flame-retardancy mechanism.

Full text of DOI:10.1039/C8RA07450J

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PAPER
Synthesis and application of aminophenyl-s-
triazine derivatives as potential flame retardants in
the modification of epoxy resins
Cite this: RSC Adv., 2018, 8, 37631
*
Haiyang Wu, Rong Hu, Birong Zeng,
Li Yang, Ting Chen, Wei Zheng, Xinyu Liu,
*
Weiang Luo and Lizong Dai
In order to develop new phosphorus–nitrogen-containing flame retardants, three novel compounds of
DOPO-substituted aminophenyl-s-triazine (TAT-DOPO) with different P/N element ratios were
synthesized by Kabachnik–Fields reaction. TAT-triDOPO possessing three DOPO units in one molecule
exhibited the best thermal stability. Besides, TAT-triDOPO and TAT-hexaDOPO possessing active
hydrogen displayed the characteristics of reactive-type retardant, while TAT-enneaDOPO without active
hydrogen represented additive-type retardant. The influences of three TAT-DOPO compounds on the
thermal stability, flame retardancy and mechanical properties of the cured EP composites were
investigated. Compared with pure EP, the char yields of EP/TAT-DOPO were increased. The LOI value
and the UL-94 ratings of EP/TAT-triDOPO with a 5% loading were improved to 34.0% and V-0 grade.
The PHRR, THR and HRC of EP/TAT-triDOPO were decreased by 25.7%, 21.1% and 25.4%, respectively.
Furthermore, the incorporation of TAT-triDOPO could also improve the flexural strength and elastic
modulus of cured EP. It was explained that the existence of active hydrogen atoms within TAT-triDOPO
molecule can act as effective chemical cross-linking points in EP network. Finally, based on the SEM and
TGA-FTIR analysis of pyrolytic products of cured EP/TAT-DOPO, it was deduced that the prepared
TAT-DOPO flame retardant could exhibit both condensed phase and gas phase flame-retardancy
mechanism.
Received 6th September 2018
Accepted 17th October 2018
DOI: 10.1039/c8ra07450j
rsc.li/rsc-advances
of weak bonds in ame retardants.15,16 Besides, a large loading
amount of DOPO may result in the declining in mechanical
properties of EP composites.17–19 Recently, some compounds
1. Introduction
As a member of thermosetting resins, epoxy resin (EP) has been
widely utilized in the elds of microelectronic packaging mate-
rials, surface coatings, adhesives and advanced composite
matrices due to its outstanding properties, such as good chem-
ical resistance, satisfactory electrical insulation, excellent
mechanical and adhesive properties.1–3 But ammability is the
main drawback of epoxy resin, which limits its further applica-
tion in some special elds.4–6 Because the use of traditional
halogenated ame retardants produce a series of environmental
concerns,7,8 researching possible halogen-free ame retardants
has attracted a great deal of attention. Some organophosphorus
compounds, especially 9,10-dihydro-9-oxa-10-phosphaphen-
anthrene-10-oxide (DOPO) and its derivatives, are known to be
a kind of additives with high reactivity and ame-retardant effi-
ciency, having the ability to capture free radicals, quench ames
and improve char formation during combustion.9–14 But it was
reported that the conventional DOPO-modied epoxy resins
showed a great decline in thermal properties due to the existence
containing triazine unit have received much attention due to the
excellent charring effect deriving from the abundant nitrogen
content. But the efficiency of ame-retardant additive with only
nitrogen was low.20 Therefore, some new strategies are required
to achieve comprehensive functionalities, such as combining
nitrogen-containing compounds like polyaniline21–23 or triazine
and organo-phosphorus together. These ame retardants con-
taining both phosphorus and nitrogen elements are popular
because of their high heat resistance24–26 and the phosphorus–
nitrogen synergistic effect.27,28 However, few works has been re-
ported to explore the inuences of different P/N ratios on the
performances of cured EP.
In this work, novel DOPO-substituted aminophenyl-s-
triazine (TAT-DOPO) compounds with controllable P/N element
ratio were synthesized by classical Kabachnik–Fields reaction
via the combination of 2,4,6-tris(p-aminoaniline)-1,3,5-triazine
(TAT), polyoxymethylene (POM), and 9,10-dihydro-9-oxa-10-
phospha-phenanthrene-10-oxide (DOPO), and were used as
ame retardants in the modication of epoxy resins. Following
are our considerations on the design of this novel ame retar-
dant system from three perspectives. Firstly, two functional
College of Materials, Fujian Provincial Key Laboratory of Fire Retardant Materials,
Xiamen University, Xiamen, Fujian, China 361005. E-mail: brzeng@xmu.edu.cn;
lzdai@xmu.edu.cn
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ame-retardant units like DOPO structure and triazine group
were integrated into one molecule to produce synergistic effect.
Secondly, the thermal and ame retardancy as well as
mechanical properties of the modied EP/TAT-DOPO compos-
ites could be adjusted by the number of DOPO units in the
corresponding molecular structure. Thirdly, the type of ame
retardants could be changed from reactive-type to additive-type
by different P/N element ratio and the number of active
hydrogen sites. Here, the chemical structures of the synthesized
three compounds of TAT-triDOPO, TAT-hexaDOPO and TAT-
enneaDOPO were characterized and veried by NMR, MS and
FTIR. Their thermal properties and reactivity were investigated
by TGA and DSC. The possible interaction between TAT-DOPO
and diglycidyl ether of bisphenol A (DGEBA) was discussed.
Aer incorporating TAT-DOPO into EP matrix, the modied
epoxy resins were prepared. The ame retardancy, thermal
degradation property and mechanical performance of the cured
EP/TAT-DOPO composites were tested by LOI, UL-94, SEM,
DMA, TGA-FTIR and three-point bending test etc. The ame
retardancy mechanism was also discussed. We hope that TAT-
DOPO compounds could not only improve ame retardancy of
the modied EP, but also maintain good thermal and
mechanical stabilities.
2. Experimental section
2.1 Materials
Cyanuric chloride, p-nitroaniline, acetic acid, Pd/C, dimethyl
formamide (DMF), and DOPO were purchased from Aladdin
Reagents Co. Ltd., China. Diglycidyl ether of bisphenol A
(DGEBA, trade name E51), a kind of liquid epoxy resin, was
supplied by Jinan Qingtian Chemical Co. Ltd., China. 4,40-Dia-
minodiphenylmethane (DDM) and polyoxymethylene (POM)
were purchased from Sinopharm Chemical Reagent Co. Ltd.,
China. The reagents used were of analytical grade unless
otherwise noted.
2.2 Synthesis
(1) Synthesis of 2,4,6-tris(p-aminoaniline)-1,3,5-triazine
(TAT). The synthetic route of TAT was shown in Scheme 1a.
The acetic acid solution of p-nitroaniline (0.045 mmol, 6.20 g)
and cyanuric chloride (0.01 mmol, 1.84 g) were mixed together
and vigorously stirred for 30 min. The green solid intermediate
product was obtained by hot-water washing and suction ltra-
tion, and then was introduced into DMF under N2 and heated to
100 ꢀC. When the mixture turned homogeneous, the Pd/C (0.1 g)
Scheme 1 The synthetic routes of (a) TAT and (b) three TAT-DOPO compounds of TAT-triDOPO, TAT-hexaDOPO and TAT-enneaDOPO with
different P/N ratio.
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and hydrazine hydrate (10 mL) were added. Aer stirring at 409EP by heating from room temperature to 800 ꢀC at a heating
100 ꢀC for 12 h, the mixture was washed with distilled water and rate of 10 ꢀC minꢁ1 under air and nitrogen atmospheres.
the white product of TAT was obtained nally.
Differential scanning calorimeter (DSC) was conducted with
TAT: 1H NMR (d ppm, DMSO-d6): 8.50 (3H, Ph–NH–Ph), 7.32 a Netzsch STA 449C by heating from room temperature to
ꢁ1
ꢀ
ꢀ
(6H, Ph-Hb), 6.45 (6H, Ph-Hc), 4.76 (6H, –NH2–Ph).
250 C at a heating rate of 10 C min under nitrogen atmo-
(2) Synthesis of DOPO-substituted aminophenyl-s-triazine sphere. The mass of each sample was approximately 4–10 mg.
(TAT-DOPO). The synthetic route of TAT-DOPO was shown in Dynamic mechanical analysis (DMA) was carried out by Netzsch
Scheme 1b. TAT (1 mmol, 0.399 g), DOPO (3 mmol, 0.65 g), POM DMA 242E instrument. The limiting oxygen index (LOI) was
(3 mmol, 0.09 g) and tetrahydrofuran (150 mL) were introduced determined with a JF-3 instrument (Jiangning, China), and the
into a 250 mL round-bottom glass ask equipped with specimen dimensions were 100 mm ꢂ 6 mm ꢂ 4 mm. The
a condenser magnetic stirrer and dry nitrogen inlet. The percentage of O2 in the O2/N2 mixture was taken as LOI value,
mixture was vigorously stirred at 60 ꢀC for 48 h under nitrogen which was just sufficient to sustain the ame. Vertical burning
atmosphere. It was concentrated to 10 mL by vacuum-rotary tests were performed according to UL-94 standard by an
evaporation and then dropped slowly into ethyl acetate (150 instrument of Fire Testing Technology UL-94, England. The
mL). A white precipitate was formed and isolated through average of ve measurements was reported as the nal results.
centrifuge followed by drying under vacuum at 70 ꢀC for 24 h to Microscale combustion calorimeter (MCC) tests were conducted
obtain the rst kind of ame retardant TAT-triDOPO. As the on Fire Testing Technology calorimeter of FAA-PCFC model.
ꢀ
feed ratio of [TAT]/[DOPO]/[POM] was changed to be 1 : 6 : 6 Samples of 3–5 mg were heated to 900 C at a heating rate of
and 1 : 9 : 9, the products of TAT-hexaDOPO and TAT-
1 .
ꢀC sꢁ1 in a stream of nitrogen owing at 80 mL minꢁ1
enneaDOPO were successfully synthesized, respectively.
Thermogravimetric analysis and infrared spectrometry (TGA-
1
TAT-triDOPO: H NMR (d ppm, DMSO-d6): 8.69 (3H, –NH– FTIR) was performed on TGA Q6500 that was interfaced to the
Ph), 8.20–7.11 (30H, Ph-H), 6.60 (6H, Ph-H), 5.51 (3H, –NH–C), Nicolet Magna IR560 FTIR spectrophotometer. About 10 mg
4.06–3.85 (6H, –CH2–); 31P NMR (d ppm, DMSO-d6): 32.42.
sample was put in an alumina crucible and heated from room
1
TAT-hexaDOPO: H NMR (d ppm, DMSO-d6): 8.92 (3H, Th– temperature to 800 ꢀC at a heating rate of 10 ꢀC minꢁ1 under N2
NH–Ph), 8.20–6.87 (54H, Ph-H), 6.62 (6H, Ph-H), 4.02–3.77 (12H, atmosphere. Three-point bending experiments were conducted
–CH2–); 31P NMR (d ppm, DMSO-d6): 32.01.
on an electronic universal testing machine (AGS-X, Shimadzu,
TAT-enneaDOPO: 1H NMR (d ppm, DMSO-d6): 8.08–7.02 Japan). The crosshead speed was 1 mm minꢁ1 and the typical
(78H, Ph-H), 6.61 (6H, Ph-H), 4.68–3.99 (18H, –CH2–); 31P NMR dimensions of the sample beams were 120 mm ꢂ 10 mm ꢂ 5
(d ppm, DMSO-d6): 31.10 and 29.23.
mm.
2.3 Preparation of EP/TAT-DOPO thermosets
3. Result and discussion
3.1 Structural characterization of TAT-DOPO
The thermosetting ame-retardant epoxy resin samples were
prepared via the thermal curing of DGEBA and stoichiometric
Three novel ame retardants of TAT-DOPO were synthesized via
the Kabachnik–Fields reaction. The chemical structures of TAT-
triDOPO, TAT-hexaDOPO and TAT-enneaDOPO were charac-
terized by 1H NMR, 31P NMR, FTIR and MS. It can be seen from
1H NMR spectra that the signal at 8.81 ppm was assigned to the
proton of P–H in DOPO (Fig. 1a), and the signal at 4.71 ppm was
assigned to –NH2 of TAT (Fig. 1b). Compared with DOPO and
TAT, the signal of P–H disappeared and the signal of –NH2
shied to 5.53 ppm for TAT-triDOPO (Fig. 1c), while both the
two signals disappeared for TAT-hexaDOPO (Fig. 1d) and TAT-
enneaDOPO (Fig. 1e). The new signals located in the range of
3.77–4.68 ppm for TAT-DOPO compounds were assigned to
methylene protons in the –N–CH2–DOPO group. In the 31P NMR
spectra (Fig. 1f–i), TAT-triDOPO and TAT-hexaDOPO displayed
single peak at 32.42 ppm and 32.01 ppm, respectively, while
weight of curing agent. DGEBA and
5 wt% TAT-DOPO
compounds were mixed at 125 ꢀC under continuous stirring
until the mixture became transparent. Aer the temperature
cooled to 85 ꢀC, DDM was added as curing agent. And then the
mixture was poured into an aluminium mould once the mixture
became homogeneous. A three-step curing procedure was
carried out to obtain the thermosetting resins. All samples were
thermally cured at 120 ꢀC for 4 h followed by 140 ꢀC for 2 h. They
were further post-cured at 180 ꢀC for 2 h to ensure the complete
curing reaction. Thereaer, all specimens were naturally cooled
to room temperature in order to avoid stress cracking. Mean-
while, pure cured epoxy resin was prepared as control samples.
2.4 Characterization
Nuclear magnetic resonance (1H NMR and 31P NMR) spectra DOPO itself only had one characteristic signal at 14.50 ppm. It
were recorded on a Bruker Advanced II AV400 MHz NMR can be seen from Fig. 1i that TAT-enneaDOPO displayed two
spectrometer with DMSO as the solvent. Mass spectra (MS) were signals at 31.10 ppm and 29.23 ppm, which was caused by
recorded using an Esquire 3000 plus mass spectrometer (Bruker DOPO unit in different spatial environments.18,29 Based on NMR
Daltonics). Attenuated total reectance Fourier transform data, it proved that the three compounds of TAT-triDOPO, TAT-
infrared (ATR-FTIR) spectra were obtained on a Nicolet Avatar hexaDOPO and TAT-enneaDOPO had been successfully
360 spectrophotometer. Scanning electron microscopy (SEM) synthesized.
images were obtained using an SU-70 microscope. Thermogra-
The FTIR spectra of DOPO, TAT and TAT-triDOPO were
vimetric analysis (TGA) was performed with a Netzsch STA shown in Fig. 2a. It can be seen that the absorption peak at
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Fig. 1 1H NMR spectra of (a) DOPO, (b) TAT, (c) TAT-triDOPO, (d) TAT-hexaDOPO, and (e) TAT-enneaDOPO; 31P NMR spectra of (f) DOPO, (g)
TAT-triDOPO, (h) TAT-hexaDOPO, and (i) TAT-enneaDOPO.
2386 cmꢁ1 of P–H bond in DOPO disappeared in TAT-triDOPO, TAT dissappeared in TAT-triDOPO, while the absorption peaks
indicating the complete reaction of DOPO. The absorption of –NH– (3334 cmꢁ1 and 3219 cmꢁ1) changed to a broaden peak
peaks of P]O at 1276 cmꢁ1 and Ph–O–P at 910 cmꢁ1 in TAT- with the absorption of 3314 cmꢁ1. Furthermore, the absorption
triDOPO conrmed the presence of DOPO units.30 Meanwhile, peaks at 1510 cmꢁ1 and 1081 cmꢁ1 corresponding to triazine
the absorption peaks of –NH2 (3427 cmꢁ1 and 3389 cmꢁ1) in ring skeleton were observed in TAT-triDOPO. This also
Fig. 2 (a) FTIR spectra of TAT, DOPO and TAT-triDOPO; (b) mass spectra of TAT-triDOPO and TAT-hexaDOPO.
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Fig. 3 (a) TGA curves and (b) DTG curves of TAT-triDOPO, TAT-hexaDOPO and TAT-enneaDOPO under N2 atmosphere.
approved the successful synthesis of TAT-triDOPO by the 380–560 ꢀC, respectively. The weight loss may be caused by the
combination of TAT and DOPO via Kabachnik–Fields reaction, bond breakdown of the weak bonds such as P–O–C and P–C, the
which was consistent with NMR results.
dissociation of triazine ring, and the escape of small molecular
ESI-MS was applied to further verify the results of chemical fragments or free radicals.18,27,31 During thermal degradation
structure deduced from NMR and FTIR. The identied molec- process, the maximum weight loss rate of TAT-triDOPO was the
ular ions corresponded to ionization with H+. It can be found smallest one among the three TAT-DOPO compounds. DTG
from Fig. 2b that the peak with m/z 1084.7 was assigned to the results showed that when the degradation temperature was
target product [TAT-triDOPO-H+], while the peak with m/z above 400 C, the maximum weight loss rates of TAT-triDOPO,
ꢀ
1768.2 was assigned to the target product [TAT-hexaDOPO-H+] TAT-hexaDOPO and TAT-enneaDOPO were 0.21, 0.46 and
under the testing mode. It was consistent with the calculated 0.67 wt% minꢁ1, respectively. In addition, the char residue of
molecular weight of TAT-triDOPO (C60H48N9O6P3) and TAT- TAT-triDOPO was as high as 46.8%, while the char residue of
hexaDOPO (C99H75N9O12P6). We had to mention that TAT- TAT-hexaDOPO and TAT-enneaDOPO was 35.0% and 32.2%,
enneaDOPO had a relative large molecular of 2451 beyond the respectively. These results indicated that the thermal stability of
detection limit of ESI-MS, so that the spectrum of TAT- TAT-triDOPO was better than TAT-hexaDOPO and TAT-
enneaDOPO was not presented. In a word, based on the above enneaDOPO, which would expect TAT-triDOPO to play a good
results of NMR, FTIR and ESI-MS, it was deduced that the target effect in the modication of epoxy resins.
compounds of TAT-DOPO had been successfully obtained.
From the view of molecular structure, the three TAT-DOPO
compounds had one similar design concept with the intra-
molecular integration of TAT center and DOPO units,
although TAT-triDOPO had one TAT center and three DOPO
units per molecule, while TAT-hexaDOPO and TAT-enneaDOPO
had six DOPO units and nine DOPO units. Based on the above
heat-resistance results, TAT-triDOPO exhibited the best thermal
stability. In order to investigate the advantage of this intra-
molecular design, we took TAT-triDOPO molecule to further
3.2 Heat resistance of TAT-DOPO
The heat resistances of TAT-triDOPO, TAT-hexaDOPO and TAT-
enneaDOPO under nitrogen atmosphere were measured by
TGA. The curves of TGA and DTG were displayed in Fig. 3. It can
be seen that the temperature range of weight loss was divided
ꢀ
into three parts, corresponding to 150–290 C, 290–380 ꢀC and
Fig. 4 (a) TGA and (b) DTG curves of TAT-triDOPO, TAT and DOPO under air atmosphere; (c) the calculated and experimental data of residue in
TAT-triDOPO.
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Table 1 Thermal decomposition data of TAT, DOPO and TAT-triDOPO. (R: wt% minꢁ1
)
Sample
R1/Tmax1
R2/Tmax2
R3/Tmax3
R4/Tmax4
Char%
TAT
DOPO
TAT-triDOPO
0.07/126 ꢀ
C
C
0.39/425 ꢀ
0.12/258 ꢀ
0.12/324 ꢀ
C
C
C
0.17/560 ꢀ
0.65/462 ꢀ
0.24/479 ꢀ
C
C
C
0.36/706 ꢀ
0.15/652 ꢀ
0.17/801 ꢀ
C
C
C
4.8
3.9
36
0.14/97 ꢀ
C
0.06/113 ꢀ
conduct the comparison of thermal stability between TAT, Charcal ¼ TATexp ꢂ wt% + DOPOexp ꢂ wt%.32 As shown in
DOPO and TAT-triDOPO. The TGA and DTG curves of TAT- Fig. 4c, the theoretical carbon residue of TAT-triDOPO was
triDOPO, TAT and DOPO under air atmosphere were shown in 6.9%, and the experimental carbon residue of TAT-triDOPO was
Fig. 4. The maximum weight loss rate R, the decomposition 36%. The experimental result was much higher than that of
temperature at the maximum weight loss rate (Tmax), and char theoretical calculation. It implied that the thermal stability of
ꢀ
yield at 800 C were summarized in Table 1. It could be seen TAT-triDOPO from the integration of DOPO with TAT by cova-
from Fig. 4(a and b) that the thermal degradation of TAT- lent bonds was much better than the simple mixture of TAT and
triDOPO at air atmosphere was complicated because oxygen DOPO. This proved the existence of phosphorus–nitrogen
was an important factor to accelerate the thermal degradation. synergistic effect in the designed molecules.
The whole degradation process of TAT-triDOPO, TAT and DOPO
included four stages. It was obvious that DOPO was the easiest
one to decompose because it had the lowest decomposition
temperature at each stage. In the temperature range of 300–
550 ꢀC, the maximum weight loss rate of DOPO with a value of
0.65 wt% minꢁ1 was bigger than those of TAT and TAT-DOPO. It
was explained by the existence of weak bonds of P–O–C and P–C
in DOPO. As for TAT, when the temperature was heated to above
500 ꢀC, its maximum weight loss rate reached the highest value
of 0.36 wt% minꢁ1 due to the decomposition of triazine ring.
Compared with DOPO and TAT, TAT-triDOPO exhibited a rela-
tively low decomposition rate. It was worth noting that at 800 ꢀC
the carbon residue of TAT, DOPO and TAT-triDOPO were 4.8%,
3.9% and 36%, respectively. This indicated that the integration
of DOPO with TAT by covalent bonds could not only improve the
thermal stability of DOPO units, but also produce synergistic
effect which was helpful to form more stable compounds during
the process of pyrolysis. Furthermore, in order to get more
understanding on the synergistic effect, we continued to
compare the experimental data and theoretical calculation of
the thermal-oxidative decomposition. We carried out the theo-
retical calculation by calculating the sum of TAT and DOPO. The
formula of the theoretical residual weight was expressed by
3.3 Curing reactivity of TAT-DOPO
In order to get more understanding of the chemical property of the
synthesized TAT-DOPO ame retardants, DSC was used to record
the curing reactivity of TAT-triDOPO, TAT-hexaDOPO and TAT-
enneaDOPO with diglycidyl ether of bisphenol A (DGEBA).
Fig. 5a showed the curing curves without the addition of usual
curing-agent DDM. It can be seen that the mixture of DGEBA/TAT-
triDOPO displayed an exothermic peak at about 120 ꢀC, which was
caused by the reaction of –NH– in TAT-triDOPO with the epoxy
groups in DGEBA.20,33 Compared with the mixture of DGEBA/TAT-
triDOPO, the exothermic peak of DGEBA/TAT-hexaDOPO shied
ꢀ
to about 135 C because TAT-hexaDOPO possessed more DOPO
units per molecule than TAT-triDOPO did. More DOPO units,
stronger steric hindrance. The steric effect led to a higher reaction
temperature for the reaction between TAT-hexaDOPO and DGEBA.
However, for the mixture of DGEBA/TAT-enneaDOPO, the
exothermic peak almost disappeared due to no chemical reaction.
The variation in the exothermic behavior was ascribed to the
numbers of active hydrogen atoms in the three compounds. In
detail, TAT-triDOPO had six –NH– groups and TAT-hexaDOPO only
had three –NH– groups, resulting in less curing reactivity for TAT-
hexaDOPO than TAT-triDOPO. Because TAT-enneaDOPO did not
Fig. 5 (a) The DSC curing curves of DGEBA, DGEBA/TAT-triDOPO, DGEBA/TAT-hexaDOPO, and DGEBA/TAT-enneaDOPO without adding
DDM; (b) the DSC curves of the cured epoxy resins with adding DDM.
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have any –NH– group, no chemical reaction happened between serve as cross-linking agent to increase the cross-linking density
DGEBA and TAT-enneaDOPO. Both TAT-triDOPO and TAT- of EP, and then increase the Tg values of the modied EP.
hexaDOPO could react with the epoxy groups to participate in
the curing reaction of DGEBA, therefore, they belonged to the
reactive-type ame retardant, but TAT-enneaDOPO belonged to
3.4 Thermal properties of the cured epoxy resins
a kind of additive-type ame retardant.
TGA tests were also carried out to investigate the thermal
stability and decomposition behavior of the cured epoxy resins.
The TGA and DTG curves under both air and N2 atmospheres
were shown in Fig. 6 and the relative thermal decomposition
data were summarized in Table 2. As shown in Fig. 6a, under air
atmosphere the whole degradation process of all the cured EP
samples could be divided into two stages. However, under N2
atmosphere the degradation of all the cured EP samples
nished completely in only one stage, as displayed in Fig. 6b.
Because of the thermal-oxidative interaction, the initial
decomposition temperatures Td of all the samples under air
atmosphere were lower than those in N2 atmosphere. It can be
seen from Fig. 6a that the rst decomposition process of all
samples appeared at around 300 ꢀC, implying that the same
thermal decomposition pathway occurred to these samples.
This was probably induced by the dehydrogenation and
aromatization of alkyl groups in the range of 220–375 ꢀC.29 The
second degradation stage appeared at 375–725 ꢀC, which was
considered to be the thermo-oxidative degradation of the
unstable carbon layer resulting from the rst stage of degra-
dation. During the thermal degradation process, the nitrogen-
containing segments of TAT-DOPO might generate gas mole-
cules and triazine skeleton structures, and the phosphorus-
containing segments might produce phosphorus-based
Since the curing activities of the three TAT-DOPO compounds
were different, their inuences on the modied EP thermosets
were further investigated by DSC. The glass transition tempera-
ture is an important parameter for the application of EP ther-
mosets. Here, the cured EP/TAT-DOPO samples with
stoichiometric weight of curing-agent DDM were prepared by the
general curing program under high temperature, and the DSC
curves of the different cured EP samples were shown in Fig. 5b. It
is known that the glass transition is ascribed to the segmental
motion of polymeric networks, and Tg is determined by the
degree of freedom for segmental motion, crosslinking and
entanglement constraints of polymer chains. Fig. 5b showed that
the Tg value of neat cured EP was 165 ꢀC, while that of EP/TAT-
enneaDOPO decreased to 154 ꢀC. It was explained that the
incorporation of the additive-type TAT-enneaDOPO possessing
several rigid benzene rings led to the increasing of the free
volume of epoxy resins, which enlarged the motion space and
improved the motion ability of segmers, thus resulting in
a decline of Tg. However, it was noteworthy that the cured EP/
TAT-triDOPO and EP/TAT-hexaDOPO thermosets displayed
ꢀ
ꢀ
a little higher Tg values of 167 C and 168 C than that of neat
cured EP. It was explained that the reactive-type TAT-triDOPO
and TAT-hexaDOPO could easily participate in curing reaction,
Fig. 6 TGA curves of the cured EP composites under (a) air and (b) N2; DTG curves of the cured EP composites under (c) air and (d) N2
atmospheres.
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Table 2 Thermal decomposition data of the cured epoxy resins
Air atmosphere
N2 atmosphere
Sample
Td (ꢀC)
Tmax1 (ꢀC)
Tmax2 (ꢀC)
Char %
Td (ꢀC)
Rmax (wt% minꢁ1
)
Tmax (ꢀC)
Char %
EP
279
280
275
273
303
304
304
303
565
582
585
584
0.19
0.83
0.48
0.36
351
327
329
336
1.46
1.09
1.23
1.26
382
373
374
372
15.79
23.51
19.86
18.75
EP/TAT-triDOPO
EP/TAT-hexaDOPO
EP/TAT-enneaDOPO
Table 3 LOI values and UL-94 data of the cured epoxy resin
ratio of phosphorus to nitrogen being 0.74 played the best
optimal effect on the thermal stability of cured EP/TAT-DOPO
materials at a low phosphorus content of 0.43%.
UL-94
Samples
LOI (%) Burning grade t1 (s) t2 (s) Dripping
3.5 Flame retardancy of the cured epoxy resins
EP
26.2
34.0
32.8
NR
V-0
V-0
V-1
>30
1
1
—
1
1
Yes
No
No
No
EP/TAT-triDOPO
EP/TAT-hexaDOPO
EP/TAT-enneaDOPO 32.1
The ame retardancy of the cured epoxy resins was measured by
LOI and UL-94 vertical burning tests and the concerned data
were illustrated in Table 3. It can be seen that the LOI of pure EP
was 26.2%, while the LOI values of the cured EP/TAT-triDOPO,
EP/TAT-hexaDOPO and EP/TAT-enneaDOPO composites were
increased to 34.0%, 32.8% and 32.1%, respectively. Both the
cured EP/TAT-triDOPO and EP/TAT-hexaDOPO passed through
the UL-94 V-0 rating with no dripping and exhibited self-
extinguishing. But the cured EP/TAT-enneaDOPO only ach-
ieved a UL-94 V-1 rating. These results indicated that the
incorporation of TAT-DOPO could improve the ame retardancy
of the EP thermosets, and the coexistence of phosphorous and
nitrogen in TAT-DOPO played synergistic ame retardant effect.
The heat release rate (HRR) curves of the cured epoxy resins
were demonstrated in Fig. 7, the peak heat release rate (PHHR),
the total heat release (THR) and the heat release capability
(HRC) were listed in Table 4. It was worth noting that the values
of PHHR, THR and HRC of the cured epoxy resins showed the
similar trend as the results of LOI test. The PHHR and the THR
of pure EP were 452.9 W gꢁ1 and 25.1 kJ gꢁ1, respectively. When
TAT-triDOPO was added, the PHHR and the THR of EP/TAT-
triDOPO decreased to 336.4 W gꢁ1 and 19.8 kJ gꢁ1, respec-
tively. Furthermore, the HRC of EP/TAT-triDOPO also
24
2
radicals and polyphosphoric acid. The possible inert gases and
phosphorus-based radicals could act in the vapor phase to
interrupt the combustion process of EP composites. Simulta-
neously some solid layers containing polyphosphoric acid and
triazine compounds formed at the surface of EP matrix. The
char layers acted as physical barrier, which hindered combus-
tible volatiles and heat transfer between the gas and condensed
phases, and then prevented the EP matrix from further
combustion. In addition, Fig. 6c and Table 2 showed that for EP/
TAT-triDOPO, EP/TAT-hexaDOPO and EP/TAT-enneaDOPO, the
temperature of maximum weight loss rate at the second stage
(Tmax2) were 582, 585 and 584 ꢀC, respectively, which was higher
than pure EP thermoset with a Tmax2 value of 565 ꢀC. This
implied that the rst pyrolysis products of TAT-DOPO could
effectively improve the thermal stability of the EP/TAT-DOPO
thermosets.
As shown in Fig. 6(b and d), under N2 atmosphere, the Td,
Tmax and maximum weight loss rate of pure EP were around
351 ꢀC, 382 ꢀC, and 1.46 wt% minꢁ1, respectively. However, the
Td and Tmax values of all the modied samples decreased
slightly, which mainly caused by the decomposition of weak
bonds (P–O–C and P–C) in DOPO unit.20 The maximum weight
loss rates of cured EP/TAT-triDOPO, EP/TAT-hexaDOPO and EP/
TAT-enneaDOPO were 1.09, 1.23 and 1.26 wt% minꢁ1, respec-
tively. Obviously the maximum weight loss rates of the EP/TAT-
DOPO composites were lower than that of pure EP. Further-
more, the char residue of cured EP/TAT-triDOPO, EP/TAT-
hexaDOPO and EP/TAT-enneaDOPO composites at 800 ꢀC
were increased to 23.51%, 19.86% and 18.75%, respectively, in
comparison with 15.79% of pure EP. These results showed that
the incorporation of TAT-DOPO could reduce the maximum
weight loss rate of EP/TAT-DOPO composites and promote the
char formation in the process of thermal degradation. Overall,
the three kinds of ame retardants could improve the thermal
stability of epoxy resin to some extent. Among the three TAT-
DOPO compounds, the addition of TAT-triDOPO with the
Fig. 7 HRR curves of the cured epoxy resins.
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Table 4 Microscale combustion calorimeter data of the cured epoxy
resins
phosphorus-containing free radicals and polyphosphoric acid,
and nitrogen-containing compounds released nitrogen-
containing radicals, inert gases, and triazine skeletons. The
non-combustible gases were likely to escape to the outside
during the combustion process, whereas the viscous carbon
layer formed by polyphosphoric acid and triazine skeletons
reduced the release rate of the gases, thus formed this kind of
honeycomb structure. These special structures could reduce the
efficiency of heat and oxygen exchange. The barrier effect of the
char layer and strong quenching effect of high concentration of
free radicals led to the promoted ame retardant property.
However, as for cured EP/TAT-hexaDOPO and EP/TAT-
enneaDOPO samples, the honeycomb structures of the inte-
rior carbon layer became smaller and even disappeared. This
may be attributed to the decreasing of non-combustible gases
during combustion and the reduction of the triazine structure
in the condensed phase with the decreasing of nitrogen content
in cured EP/TAT-hexaDOPO and EP/TAT-enneaDOPO samples.
Sample
PHRR(W gꢁ1
)
THR(kJ gꢁ1
)
HRC(J gꢁ1
)
EP
452.9
336.4
368.6
448.1
25.1
19.8
20.1
24.8
453
338
370
450
EP/TAT-triDOPO
EP/TAT-hexaDOPO
EP/TAT-enneaDOPO
signicantly decreased. It could be deduced that the ame
retardancy of the cured EP/TAT-triDOPO, EP/TAT-hexaDOPO
and EP/TAT-enneaDOPO were better than that of pure EP due
to the addition of TAT-DOPO. Among three cured EP/TAT-
DOPO, EP/TAT-triDOPO exhibited the best performance due
to the effective synergetic effect of phosphorus and nitrogen.
3.6 Char residue analysis of the cured epoxy resins
The microstructure of char residues of the cured epoxy resins
were characterized by SEM and shown in Fig. 8. It can be seen
from Fig. 8(a and b) that the residual char of pure EP exhibited
3.7 Investigation of ame retardant mechanism
a granular loose outer layer and a smooth folded sheet-like In order to investigate the possible ame retardant mechanism
inner layer. The drawback of this kind of structure was that for the EP/TAT-triDOPO composites, the thermal degradation
the heat and oxygen could easily penetrate through this char and combustion products were analyzed by FTIR. The FTIR
layer and enter into the inner, consequently increased the spectra of the volatile products produced by EP/TAT-triDOPO
combustion intensity. However, from Fig. 8(c, e and g), we around the temperature range at the maximum decomposi-
found that the char residue of the cured EP/TAT-DOPO dis- tion rate were presented in Fig. 9a. The main absorption peaks
played a continual and compact exterior char layer, which was were ascribed to the characteristic bands of water (3646 cmꢁ1),
helpful to prevent the volatile gas from thermal degradation carbon dioxide (2256 cmꢁ1
) and compounds containing
from spreading. Especially, the exterior char layer of EP/TAT- aromatic ring (1700–1500 cmꢁ1). In addition, the absorption
triDOPO contained some holes. It was presumed that the free peaks at 1259, 1174 and 1076 cmꢁ1 were attributed to the
radicals were hindered by the continuous and compact char characteristic absorption peaks of P]O and P–O–C in gas. It
residue at rst, and nally concentratedly released to exert was obvious that phosphorus-containing products were gener-
a strong quenching effect, thus resulting in some holes on the ated from the cured EP/TAT-triDOPO. The characteristic
surfaces. Besides, as shown in Fig. 8d, the interior carbon layer absorption peaks of C–N and NH3 were observed at 1370, 984
of EP/TAT-triDOPO exhibited a honeycomb with some broken and 925 cmꢁ1, respectively, conrming the formation of
bubbles on the surface. The reason for this phenomenon may ammonia and other nitrogen-containing compounds.34 On the
be that phosphorus-containing compounds produced basis of the chemical structure of TAT-triDOPO, it could be
Fig. 8 SEM images of char residue: (a) EP, exterior; (b) EP, interior; (c) EP/TAT-triDOPO, exterior; (d) EP/TAT-triDOPO, interior; (e) EP/TAT-
hexaDOPO, exterior; (f) EP/TAT-hexaDOPO, interior; (g) EP/TAT-enneaDOPO, exterior; (h) EP/TAT-enneaDOPO, interior.
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Fig. 9 FTIR spectra of (a) volatiles generated at the maximum decomposition rate, and (b) residual char obtained from UL-94 test for the cured
EP/TAT-triDOPO.
deduced that the phosphorus-containing products were gener- effect. From the view of condensed phase, the TAT-triDOPO
ated by the DOPO units and the nitrogen-containing products decomposed to produce phosphate, polyphosphate and triazine
were produced from the triazine groups. The generated gases skeleton, which promoted the charring of EP matrix to form the
could dilute the oxygen concentration, and phosphorus- phosphorus-rich viscous char layer with triazine groups. Mean-
containing free radicals could quench other essential active while, the residual char played a crucial role in preventing the
free radicals, and then weaken or terminate the chain reaction, transfer of heat and oxygen and hindering the combustion greatly.
which prevented the matrix from burning. Hence, during The effect of non-combustion gases, quenching effect of high
combustion of cured EP/TAT-triDOPO, these volatiles products concentration of free radicals and the barrier effect of the ther-
were produced and played an important role in re inhibition mally stable char layer were responsible for improving ame-
through the gaseous phase.
retardant performance of the cured EP/TAT-triDOPO.
The FTIR spectra of the char residue of EP/TAT-triDOPO ob-
tained from UL-94 test were demonstrated in Fig. 9b. The peak
observed at 3351 cmꢁ1 corresponded to N–H and O–H stretching
vibrations. The peaks at 1447, 1510 and 1609 cmꢁ1 corresponded
to the absorbance peaks of polyaromatic carbon resulting from the
degradation of the cured EP/TAT-triDOPO. The absorbance peaks
of C–N at 1360 cmꢁ1 and the characteristic peak of triazine ring at
1507 cmꢁ1 (overlapping with that of aromatic carbon) indicated
the existence of triazine structure reserved in the residual char.18
Meanwhile the characteristic peak at 1104 cmꢁ1 corresponded to
the stretching vibrations of PO2/PO3 from the phosphoric acid
carbon composites, and the absorbance peaks at 935 and
756 cmꢁ1 identied the existence of P–O–Ph in the residual char.
These results indicated that during the thermal degradation of
cured EP/TAT-triDOPO, DOPO groups decomposed to generate
phosphate and polyphosphate and the triazine groups remained
in the char residue. The different functional groups of TAT-
triDOPO synergistically acted in the condensed phase to form
the phosphorus-rich viscous char layer with triazine groups, which
possessed high cross-linking density and thermal stability.
Overall, TAT-triDOPO could improve the ame retardancy of EP
thermosets effectively due to the combination of gas phase and
condensed phase ame retardant mechanisms. From the view of
gas phase mechanism, TAT-triDOPO decomposed and released
phosphorus-containing and nitrogen-containing free radicals and
inert gases, which played an important role in weakening or
terminating the combustion. And when the amount of pyrolysis
products exceeded the holding capacity of the char layer, the free
radicals were released concentratedly to exert a strong quenching
3.8 Mechanical properties of cured epoxy resins
The inuence of three TAT-DOPO ame retardants on the
mechanical property of the modied EP thermosets was inves-
tigated. The mechanical properties of cured epoxy resins were
evaluated by dynamic mechanical analysis under double
cantilever model. The curves of storage modulus and loss factor
tan d were shown in Fig. 10, and the data were also given in the
Table 5. The average storage modulus of EP, EP/TAT-triDOPO,
EP/TAT-hexaDOPO and EP/TAT-enneaDOPO at 40 ꢀC was
Fig. 10 DMA curves of the cured epoxy resins.
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Table 5 DMA data of the cured epoxy resins
Sample
Tg/ꢀC
E0-40 ꢀC/MPa
E0-190 ꢀC/MPa
tan d
EP
164.6 ꢃ 0.3
166.4 ꢃ 0.5
169.7 ꢃ 0.5
154.6 ꢃ 0.4
1256.7 ꢃ 18.7
1393.4 ꢃ 15.9
1347.4 ꢃ 20.3
1377.3 ꢃ 22.5
16.8 ꢃ 0.3
17.5 ꢃ 0.2
19.6 ꢃ 0.3
18.1 ꢃ 0.3
0.67 ꢃ 0.01
0.45 ꢃ 0.01
0.44 ꢃ 0.01
0.53 ꢃ 0.01
EP/TAT-triDOPO
EP/TAT-hexaDOPO
EP/TAT-enneaDOPO
Table 6 Flexural strength of cured epoxy resins
triDOPO had one TAT center and three DOPO units in one
molecule, while TAT-hexaDOPO and TAT-enneaDOPO had six
DOPO units and nine DOPO units. All the three compounds
were used as ame retardant to modify epoxy resins. It was
interesting that TAT-triDOPO and TAT-hexaDOPO displayed the
characteristics of reactive-type retardant, while TAT-
enneaDOPO represented additive-type retardant. Besides, TAT-
triDOPO with the P/N ratio of 0.74 exhibited the best thermal
stability and curing activity with DGEBA. Compared with the
pure EP, the LOI value and char yields of the cured EP/TAT-
DOPO composites were both increased and EP/TAT-triDOPO
was the most remarkably promoted one. The LOI value and
the UL-94 ratings of the cured EP/TAT-triDOPO with a 5%
loading were raised up to 34.0% and V-0 grade. Meanwhile, the
PHRR, THR and HRC of the cured EP/TAT-triDOPO were
decreased by 25.7%, 21.1% and 25.4%, respectively. The
mechanical performance tests showed that the incorporation of
TAT-triDOPO could also improve both the exural strength and
the elastic modulus of the modied EP/TAT-triDOPO. This may
be attributed to the existence of active hydrogen atoms within
TAT-triDOPO molecule, which could act as effective chemical
cross-linking points in EP network. Finally, based on the SEM
and TGA-FTIR analysis of pyrolytic products of cured EP/TAT-
DOPO composites, it was deduced that TAT-DOPO ame
retardants could exhibit both condensed phase and gas phase
ame-retardancy mechanism. During the combustion,
phosphorus-containing and nitrogen-containing free radicals
and gases were released, which not only diluted the concen-
tration of combustible gases, but also promoted the expansion
of residual char with honeycomb-like morphology. Simulta-
neously the polyphosphoric acid and compounds with triazine
skeleton were generated and covered on the surface of
substrate, resulting in forming a protective layer with excellent
thermal stability, which effectively hindered the transmission of
heat and oxygen and prevented further ablation of substrate. In
a word, this work could make contributions to the development
of novel ame retardants and could be helpful to get more
understandings on the synergistic ame retardancy mechanism
of this kind of modied epoxy resins.
Flexural
Percentage of increased
Sample
strength (MPa)
exural strength (%)
EP
70.02 ꢃ 4.03
79.67 ꢃ 3.07
66.39 ꢃ 3.98
69.08 ꢃ 3.37
0
EP/TAT-triDOPO
EP/TAT-hexaDOPO
EP/TAT-enneaDOPO
13.8
ꢁ5.1
ꢁ1.3
1256.7, 1393.4, 1347.4 and 1377.3 MPa, respectively. This
implied that at glass state, the incorporation of TAT-DOPO
could enhance the dynamic mechanical properties of the
cured EP/TAT-DOPO. When the temperature increased to
190 ꢀC, the EP thermosets were transformed into a rubber state.
The average storage modulus of pure EP was 16.8 MPa, while
those of EP/TAT-DOPO thermosets increased to 17.5, 19.6 and
18.1 MPa, respectively, indicating all the storage modulus of the
cured EP/TAT-DOPO were enhanced to a higher level compared
with pure EP. It was explained that the rigid macromolecular
structures of TAT-enneaDOPO could act as physical cross-
linking points. Meanwhile TAT-triDOPO and TAT-hexaDOPO
with active hydrogen could act as chemical crosslinking
points, and then enhanced the storage modulus of the EP
composites under the rubber state.
In order to further investigate the mechanical property of the
cured epoxy resins, three-point bending tests were adopted to
measure the exural strengths and the data were demonstrated
in Table 6. The results showed that the exural strength of pure
EP was about 70.02 MPa while that of EP/TAT-triDOPO was
79.67 MPa. The percentage of increased exural strength was
about 13.8%. This improvement may be attributed to the active
hydrogen atoms in TAT-triDOPO molecules acting as effective
chemical cross-linking points. However, the addition of TAT-
hexaDOPO and TAT-enneaDOPO led to a decrease in the ex-
ural strength. It was explained that TAT-hexaDOPO and TAT-
enneaDOPO had less active hydrogen atoms and had more
rigid benzene rings with strong space steric hindrance, resulting
in the reduction of cross-linking density of the modied EP
composites. Based on the DMA and three-point test results, the
EP/TAT-triDOPO thermoset was the best one to have the potential
application in some elds with the requirement of high ame
retardancy and good mechanical property.
Conflicts of interest
There are no conicts to declare.
4. Conclusions
Acknowledgements
Three novel TAT-DOPO compounds possessing the functional
groups of phosphaphenanthrene and triazine were successfully This work was supported by the National Natural Science
synthesized via classical Kabachnik–Fields reaction. TAT- Foundation of China (51573150), the Natural Science
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Foundation of Fujian Province of China (2018J01079, 17 S. Y. Sun, Y. D. He, X. D. Wang and D. Z. Wu, J. Appl. Polym.
2016J01257), and the Xiamen Science and Technology Project
(3502Z20183001, 3502Z20183014).
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