Fabrication of two multifunctional phosphorus-nitrogen flame retardants toward improving the fire safety of epoxy resin
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e-Polymers 2022; 22: 430–444
Research Article
Yunxia Yang* and Dan Xiao*
Fabrication of two multifunctional
phosphorus–nitrogen flame retardants toward
improving the fire safety of epoxy resin
https://doi.org/10.1515/epoly-2022-0042 satisfactory corrosion resistance, dimensional stability,
received March 03, 2022; accepted March 28, 2022 electrical insulation, and mechanical properties (1–5).
Abstract: To improve the fire safety of epoxy resin (EP), Unfortunately, the high flammability and the release of
two novel phosphorus–nitrogen flame retardants, which a great number of toxic gases during combustion heavily
named as diphenyl allylphosphoramidate (DPCA) and limit the application of EP. Hence, it is critical to develop
N-allyl-P, P-diphenylphosphinic amide (DCA), were synthe- efficient flame retardants (FRs) to enhance the fire safety
sized by acyl chloride reaction and introduced into EP of EP.
for fabricating EP composites. The combustion tests The halogen compound, as a comonomer or additive,
showed that incorporation of 5 wt% DPCA or 5 wt% DCA has been widely used in fabricating FR EP composites.
into EP led to the exceptional limited oxygen index (LOI) However, halogen-containing FRs usually release toxic
value (27.1% or 31.6%). Besides, the peak of heat release fumes during combustion, which is extremely harmful
rate of EP-5 wt% DPCA and EP-5 wt% DCA was reduced to human health and the environment (6–8). As a result,
by 40.69% and 36.69%, respectively, compared to pure halogen-free FRs have attracted increasing interest from
EP. The enhanced fire resistance of EP was ascribed to researchers in enhancing the fire resistance of EP. It has
the trapping effect of fillers in the gas phase and the been reported that phosphorus-containing FRs, including
charring effect in the condensed phase. Furthermore, ultra- phosphide, organophosphorus compounds, polypho-
violet-visible spectra revealed that both EP-5 wt% DPCA sphate, etc., have been extensively used in varieties of
and EP-5 wt% DCA have considerable transparency. This polymers among all the halogen-free FRs due to the envir-
study is expected to broaden the application of EP in the onmental friendliness and outstanding flame retardancy
industrial area. (9–14). However, most of phosphorus-containing FRs are
liquid and it has some disadvantages (poor thermal sta-
Keywords: epoxy resin, flame retardancy, trapping effect, bility, high volatility, and poor compatibility) (15–17).
diluting action, transparency Besides, phosphorus-containing FRs have dissatisfac-
tory flame retardancy when incorporated into polymers
alone (18–20).
To overcome those intractable problems, phosphorus–
1 Introduction nitrogen (P–N) FRs have been developed for enhancing
the fire resistance of EP (21,22). Zhou et al. synthesized
Epoxy resin (EP), as one of the most important thermo- an innovative P–N FR (poly(piperazine phosphaphenan-
setting resins, has been extensively used in adhesive, threne) (DOPMPA)) and they found that the EP composites
electronic materials, aerospace industry, etc., due to its passed the vertical burning (UL-94) V-0 rating and had a
high LOI value (34%) with the incorporation of 13 wt%
DOPMPA (23). Yang et al. prepared an innovative benzimi-
* Corresponding author: Yunxia Yang, Key Laboratory of Polymer dazolyl-substituted cyclotriphosphazene filler (BICP) and
Materials and Products of Universities in Fujian, Department of incorporated 10.7 wt% BICP into EP to fabricate EP com-
Materials Science and Engineering, Fujian University of Technology, posite. The result showed that BICP not only endowed
Fuzhou, Fujian, 350108, China, e-mail: yangyx31628@yeah.net
EP with excellent flame retardance but also enhanced
* Corresponding author: Dan Xiao, Key Laboratory of Polymer
Materials and Products of Universities in Fujian, Department of
the thermal stability of EP (24). Zhu et al. synthesized
Materials Science and Engineering, Fujian University of Technology, a novel P–N FR phosphaphenanthrene/benzimidazo-
Fuzhou, Fujian, 350108, China, e-mail: 19872102@fjut.edu.cn lone-containing (POBDBI) including benzimidazolone
Open Access. © 2022 Yunxia Yang and Dan Xiao, published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
Fabrication of two multifunctional P–N flame retardants 431
and DOPO groups. They revealed that the EP–POBDBI (DDS) was provided by Shanghai Aladdin Chemical Reagent
composite with 13 wt% FR achieved the UL-94 V-0 rating Co., Ltd, China. Dichloromethane, DPC, DC, and triethyl-
and the LOI was increased to 36.5%. Besides, the peak of amine were acquired from Shanghai Sinopharm Reagent
heat release rate (PHRR) of EP/POBDBI composite was Co. Ltd, China.
reduced by 48.9%, in comparison with pure EP (25). As
mentioned above, P–N FRs show outstanding flame retar-
dancy and smoke suppression properties. However, it has
been reported the excessed P–N FRs showed an adverse 2.2 Synthesis of FRs
effect on the transparency of materials (26,27). Thus, it is
significant to develop multifunctional flame-retardant EP Allylamine (0.051 mol, 2.912 g), triethylamine (0.050 mol,
composites with outstanding flame retardancy, good 5.060 g), and dichloromethane (150 mL) were first intro-
thermal stability, and high transparency. duced into a three-necked flask with mechanical agita-
Herein, two innovative P–N-containing solid FRs were tion. Then, DPC (0.050 mol, 13.432 g) was dropwise added
synthesized from diphenyl chlorophosphate (DPC), diphe- into the above mixture at 0–5°C for 1 h. Then, the mixture
nylphosphinyl chloride (DC), and allylamine. After that, was stirred overnight at room temperature. After that,
FRs were introduced into EP to fabricate flame-retardant the solution was filtered to remove triethylamine hydro-
EP composites. The flammability, thermal property, and chloride, and the filtrate was washed no less than three
transparency of EP and its composites were studied. times with deionized water. The organic phase was dried
Besides, the FR mechanism of EP-diphenyl allylpho- by anhydrous magnesium sulfate and further evaporated
sphoramidate (DPCA) and EP-N-allyl-P, P-diphenylpho- under a vacuum at 40°C until the dichloromethane was
sphinic amide (DCA) was investigated. This study develops removed absolutely. Finally, the white solid (DPCA)
an innovative approach to fabricate EP composites with was obtained and the yield of DPCA was 90.60%. DCA
high flame retardancy, thermal stability, and transpar- was prepared by the similar method as that of DPCA and
ency, which is expected to broaden the industrial appli- the yield of DCA was 90.25%. The synthesis route of
cation of EP. DPCA and DCA is shown in Scheme 1.
2 Experimental section 2.3 Fabrication of FR EP composites
2.1 Raw materials Taking EP–DPCA composite as an example, specifically
EP and DDS were first mixed at 120°C by mechanical
Diglycidyl ether of bisphenol A (DGEBA, E-44) was provided stirrer until DDS was completely dissolved in EP. Then,
from Xingchen Synthetic Material Co. Ltd, China. Allylamine DPCA was incorporated into the above solution and
(A) was supplied from Chengdu Hongben Chemical stirred at 120°C for 30 min. Thereafter, the mixture was
Products Co. Ltd, China. The 4,4′-diaminodiphenylsulfone degassed using a vacuum at 120°C for 30 min and poured
Scheme 1: Graphical synthesis scheme for DPCA and DCA.
432 Yunxia Yang and Dan Xiao
directly into the preheated mold. Finally, the samples Raman spectra were collected on a Dxr2xi micro
were step-cured at 160°C for 1 h, 180°C for 2 h, and Raman imaging spectrometer (American) with an argon
190°C for 1 h, and the samples were naturally cooled laser of 532 nm.
down to room temperature. In addition, pure EP and Pyrolysis-gas chromatography/mass spectrometry (Py-
EP–DCA sample was fabricated via the same process. GC/MS) testing was conducted on a GC/MS (Agilent 7890B-
The detailed formulas of all samples were summarized 5977AGC/MSD) equipped with a frontier pyrolyzer. Helium
in Table 1. was used as carrier gas. The injector initial temperature
was 40°C (3 min), and then at 10°C·min−1 from 40°C to
280°C. GC/MS had the interface temperature of 280°C
and the cracking temperature of 600°C.
2.4 Characterizations
Ultraviolet-visible (UV-Vis) transmission of samples
was recorded via an ultraviolet spectrophotometer (UV2600)
Fourier transform infrared (FTIR) analyses were collected
with a thickness of 1 mm.
on Nicolet 6700 infrared spectrometer (Thermo, USA).
1
H nuclear magnetic resonance (NMR) spectra were
performed on Bruker AV400 NMR spectrometer (Bruker,
USA) with chloroform (CDCl3) as the solvent.
Thermogravimetric analysis (TGA) was characterized
3 Results and discussion
on NETZSCH STA449F3 (NETZSCH-Gerätebau GmbH,
Germany) with a heating rate of 25–800°C under nitrogen 3.1 Characterization of DPCA and DCA
condition.
Differential scanning calorimetry (DSC) thermograms The FTIR spectra of FR are presented in Figure 1a and b.
were measured on DSC200F3 (Schneider) under N2 atmo- As shown in Figure 1a, three obvious absorption bands
sphere from 30°C to 250°C. situated at 1,589, 1,484, and 1,185 cm−1 can be found in
The LOI values were tested at room temperature on a the FTIR spectra of DPC, which are assigned to the
CH-2 oxygen index meter (Nanjing Jiangzhong Analysis stretching vibration of benzene ring, benzene ring, and
Instrument Co., Ltd, China) by using ASTM D2863, and P]O bond, respectively (28). After the reaction between
the sample size was 100 mm × 6.5 mm × 3 mm. DPC and allylamine, it is noted that DPCA shows the
Vertical burning (UL-94) tests were conducted on the same typical absorption band as that of DPC. Besides,
CZF3 instrument (Nanjing Jiangzhong Analysis Instrument two new peaks situated at 3,253 and 1,101 cm−1 are ascribed
Co., Ltd, China), and the sample size was 130 mm × 13 mm × to the stretching vibration of –NH and P–N–C groups,
3 mm. respectively (29). The result indicates that the preparation
Cone calorimeter test was analyzed on the Fire Testing of DPCA is successful. It is observed from Figure 1b that
Technology (FTT) cone calorimeter at the external heat DC and DCA display three characteristic peaks at 1,434,
flux of 50 kW·m−2. The sample size was 100 mm × 1,589, and 1,244 cm−1, which agree well with the benzene
100 mm × 3 mm. ring, benzene ring, and P]O bond, respectively (30). In
Scanning electronic microscopy (SEM) (NovaNano- addition, two obvious peaks at 850 cm−1 (P–N–C bond)
SEM450 OXFORD X-MaxN EDS, USA) was utilized to and 3,186 cm−1 (N–H group) appear in the FTIR spectra
analyze the microstructures of the residual chars after of DCA, demonstrating the successful preparation of
burning testing. DCA (31).
Table 1: The detailed formulation of EP, EP–DPCA, and EP–DCA materials
Sample DGEBA (g) DDS DPCA (g) DCA (g) DPCA (wt%) DCA (wt%) P (wt%)
EP 100 30 0 0 0 0 0
EP-2 wt% DPCA 100 30 2.65 0 2 0 0.21
EP-5 wt% DPCA 100 30 6.84 0 5 0 0.54
EP-2 wt% DCA 100 30 0 2.65 0 2 0.24
EP-5 wt% DCA 100 30 0 6.84 0 5 0.60
mflame retardant 31
The formula for calculating the mass fraction of P in FR is provided as follows: P(wt%) = mEP + mDDS + mflame retardant
× Mflame retardant
× 100% (a).
Fabrication of two multifunctional P–N flame retardants 433
Figure 1: (a) and (b) FTIR spectra of DPCA and DCA. (c) and (d) 1H NMR spectra of DPCA and DCA in CDCl3. (e) and (f) MS spectra of DPCA
and DCA.
DPCA and DCA were investigated by 1H NMR spectra demonstrate that the thermal stability of DCA is better
to characterize the chemical structure, and the related than that of DPCA.
results are portrayed in Figure 1c and d. As shown in
Figure 1c, the chemical shift located at 7.39–7.12 ppm
and 7.19–7.10 ppm is assigned to the aromatic hydrogen
of DPCA. Besides, the obvious peaks at 5.77 and 3.75 ppm 3.2 Thermal stability of EP and its
are attributed to the C]C bond of DPCA. In the case of 1H composites
NMR spectra of DCA, the chemical shift at 8.04–7.86 and
7.57–7.38 ppm are ascribed to the aromatic hydrogen of The DSC data of EP, EP–DPCA, and EP–DCA are pre-
DCA. In addition, two sharp peaks can be found at 5.95 sented in Figure 2 and Table 2. It is noted that all the
and 3.64–3.53 ppm, corresponding to the hydrogen atom samples show only one glass transition stag, suggesting
of the C]C bond for DCA. To further study the chemical that the system had a cross-linking reaction, and the EP is
structure of FRs, DPCA and DCA were investigated by MS fully cured (32). Besides, as shown in Table 2, the Tg value
spectrum, and the results are portrayed in Figure 1e and f. gradually decreases with the increase of the additive,
As presented in Figure 1e and f, DPCA and DCA show the which is attributed to the decrease of cross-linking den-
equimolecular ion peak at 257.1 (C15H16NOP) and 288.9 sity of epoxy thermosetting materials (33). This result is
(C15H16NO3P), respectively, further demonstrating that ascribed to the interpretation that both DPCA and DCA
DCA and DPCA are synthesized successfully. participate in the curing process of EP through the reac-
The TGA curves of DCA and DPCA under nitrogen tion of amino group (–NH) with epoxy group (34).
atmosphere are presented in Figure A1 and Table A1 The thermal stability of EP, EP–DPCA, and EP–DCA
(Appendix). As can be seen from Table A1, T5% of DCA are assessed by TGA instrument, the thermogravimetric
and DPCA is 219.1°C and 213.9°C, respectively. Besides, it (TG) curves, derivative thermogravimetric (DTG) curves
is noted that the residual mass value of DCA (3.8%) is and related results are shown in Figure 3 and Table 2. As
higher than that of DPCA (2.0%). The above results shown in Table 2, EP, EP–DPCA, and EP–DCA show the
434 Yunxia Yang and Dan Xiao
one-step degradation, which is assigned to the degrada-
tion of macromolecular chains of EP (C–C bonds, aro-
matic ring, etc.). The T5% and Tmax of EP are 379°C and
416°C, respectively. With the addition of FR, the T5%
values of EP-5 wt% DPCA and EP-5 wt% DCA are lower
than pure EP. This result can be originated from the
early decomposition of the additive (35). However, it
is noted that the char residues of EP-5 wt% DCA and
EP-5 wt% DPCA are higher than that of pure EP. For
instance, the char residue value of EP-5 wt% DCA and
EP-5 wt% DPCA is increased by 20% and 40%, respec-
tively, relative to that of pure EP. This result reveals that
DCA and DPCA have the outstanding catalyze charring
effect, which is advantageous to enhance the fire safety of
polymers (36). The heat resistance index temperature
(THRI) is calculated by Eq. 1, and the results are shown
in Table 2 (37).
Figure 2: DSC curves of EP, EP-2 wt% DPCA, EP-2 wt% DCA, EP-5 wt%
DPCA, and EP-5 wt% DCA in nitrogen. THRI = 0.49⁎[T−5% + 0.6(T−30% − T−5%)] (1)
Table 2: TGA data of EP, EP-2 wt% DPCA, EP-2 wt% DCA, EP-5 wt% DPCA, and EP-5 wt% DCA in nitrogen
Samples T5% (°C) Tmax (°C) Char yields (%·°C−1) Rmax (%·°C−1) THRI
EP 379 416 20 1.7 194
EP-2 wt% DPCA 354 389 26 1.4 182
EP-2 wt% DCA 368 406 22 1.3 189
EP-5 wt% DPCA 339 372 28 1.3 176
EP-5 wt% DCA 354 398 24 1.2 183
Notes: T5%, Rmax, Tmax, and THRI, respectively, denote the temperature of mass loss for 5 wt%, the maximum mass loss rate, the maximum
weight loss temperature, and the heat resistance index temperature.
Figure 3: (a) TG and (b) DTG curves of EP, EP-2 wt% DPCA, EP-2 wt% DCA, EP-5 wt% DPCA, and EP-5 wt% DCA in nitrogen.
Fabrication of two multifunctional P–N flame retardants 435
It is clearly observed that EP–DPCA and EP–DCA to ignition (TTI) of EP-5 wt% DCA and EP-5 wt% DPCA is
show slightly lower THRI values, in comparison with lower than that of pure EP, which is attributed to the degra-
pure EP, demonstrating that the thermal stability of EP dation of polymers by the catalysis of fillers (38,39). This
is held. Besides, it is noted that the maxim degradation result is well consistent with the TGA results from Figure 3.
rate (Rmax) of EP–DPCA and EP–DCA is lower than that of As shown in Table A2, the PHRR and total heat release
pure EP, manifesting that the decomposition of EP is (THR) values of EP composites are decreased in compar-
inhibited by the additive. ison with pure EP. For instance, the PHRR of EP-5 wt%
DPCA and EP-5 wt% DCA are declined to 556.38 and
593.95 kW·m−2, respectively. Compared to pure EP, the
PHRR of EP-5 wt% DPCA and EP-5 wt% DCA is reduced
3.3 Flame retardancy of EP and its by 40.69% and 36.69%, respectively. Besides, the THR
composites values of EP-5 wt% DPCA and EP-5 wt% DCA are decr-
eased to 77.83 and 73.69 kW·m−2, which are declined by
The flame retardancy of EP, EP-5 wt% DPCA, and EP-5 wt% DCA 15.45% and 22.47%, respectively, in comparison with
was evaluated by vertical combustion test (UL-94) and pure EP. These results reveal that the heat release of
LOI test, the data are illustrated in Figure 4. As shown in EP composites is significantly decreased during com-
Figure 4, the pure EP shows the low LOI value (24.8%), and bustion by introducing DPCA or DCA.
it cannot pass the UL-94 rating. When the FR was added, To highlight the superior flame retardancy of DPCA
the LOI value of EP composites gradually increased. For and DCA, the fire-retardant performances of DCA and
instance, EP-5 wt% DCA composite shows the highest LOI DPCA are compared with prior literature, as shown in
value (31.6), which is 1.27 times as that of pure EP. Figure 6. As can be observed from Figure 6, these FRs
Besides, it is worth noting that EP-5 wt% DCA composite (lamellar-like phosphorus-based triazole–zinc complex
achieves the UL-94 V-1 rating. The above results demon- (Zn–PT), phosphorus/nitrogen-containing polycarboxylic
strate that the fire resistance is significantly improved acid (TMD), organic/inorganic phosphorus-nitrogen-
with the introduction of DPCA and DCA. Besides, it can silicon flame retardant (DPHK), etc.) play little effect in
be concluded from the UL-94 and LOI results that the suppressing the heat release of EP (40–45). For instance,
flame retardancy of EP-5 wt% DCA is better than that of Jian et al. prepared Zn–PT and revealed that the PHRR of
EP-5 wt% DPCA. This result is ascribed to the reason that EP composite was decreased by 22.26% via incorporating
the phosphorus content of DCA is higher than that 3 wt% Zn–PT (40). Ai et al. demonstrated that the PHRR of
of DPCA. EP composite was declined by 35.28% with the addition of
The FR performance of EP, EP-5 wt% DPCA, and EP-5 7.5 wt% organophosphorus-bridged amitrole (41). Luo et al.
wt% DCA was further evaluated by cone calorimeter reported 15.06% decrease in PHRR, by incorporating 2 wt%
testing (Figure 5 and Table A2). It is noted that the time DPHK into EP (42). Duan et al. incorporated 6.5 wt% TMD
into EP and demonstrated that the PHRR was declined
by 24.80%, compared to pure EP (43). In this study, the
PHRR of EP composite is dramatically reduced by 40.69%
and 36.69%, respectively, with the incorporation of DPCA
or DCA alone. Generally, these results demonstrate that
DPCA and DCA exhibit the outstanding effect in reducing
the release of heat.
The residual mass value of EP, EP-5 wt% DPCA, and
EP-5 wt% DCA is portrayed in Figure 5c. It is noted that
pure EP has little residual mass (2.91%) after the cone
testing. In contrast, the residual mass of EP-5 wt% DPCA
and EP-5 wt% DCA is obviously improved. For example,
the residual mass of EP-5 wt% DPCA is the highest
(13.29%) among all the EP composites, which is 4.6 times
as that of pure EP. The above result is due to the distin-
guished catalytic charring of DPCA, which is in line with
Figure 4: The LOI and UL-94 results of EP, EP-2 wt% DPCA, EP-5 wt% the TGA results. It has been reported that the fire growth
DPCA, EP-2 wt% DCA, and EP-5 wt% DCA. rate (FGR) and the fire performance index (FPI) were
436 Yunxia Yang and Dan Xiao
Figure 5: (a) HRR, (b) THR, (c) mass loss, and (d) COPR curves of EP, EP-5 wt% DPCA, and EP-5 wt% DCA.
used to evaluate the fire-resistance safety of polymers generation of toxic fume during the combustion of poly-
(46). The FGR and FPI are calculated via Eqs 2 and 3, the mers. The corresponding results of carbon monoxide for
data are shown in Table A2. EP, EP-5 wt% DPCA, and EP-5 wt% DCA are displayed in
FGR = PHRR/tPHRR (2) Figure 5d. It is clearly observed from Figure 5d that the
peak of carbon monoxide production rate (PCOPR) of the
FPI = TTI/ PHRR (3) pure EP is 0.0294 g·s−1. With the addition of 5 wt% DPCA,
Generally, the lower FGR and higher FPI indicate the the PCOPR of EP-5 wt% DPCA is reduced to 0.0204 g·s−1,
higher fire safety of polymer materials. It is noticeable which is declined by 30.6% compared to pure EP. The
that the FGR values of EP-5 wt% DPCA and EP-5 wt% DCA above data further demonstrate that the fire safety of EP
are less than that of pure EP. However, the FPI shows the composites is enhanced with the addition of FR.
opposite tendency as that of FGR. For example, the FPI of EP
composite is increased from 0.043 to 0.062 with the addition
of 5 wt% DCA. The above results are further manifest that 3.4 FR mechanism
the FR performance of EP is enhanced with the incorpora-
tion of DPCA or DCA. The digital pictures of char residues for EP, EP-5 wt%
Toxic fume is one of the most important parameters DPCA, and EP-5 wt% DCA are displayed in Figure A2.
to threaten the health of human beings during a fire acci- As shown in Figure A2a and d, pure EP has few char
dent. Therefore, it is especially important to research the residues after combustion, which is due to the heavy
Fabrication of two multifunctional P–N flame retardants 437
by SEM, as shown in Figure 7. It is observed from Figure 7a–c
that the samples show the same external char residues, which
indicates that the internal char residues of EP-5 wt% DPCA
and EP-5 wt% DCA play the primary role in enhancing the fire
resistance in the condense phase. As portrayed in Figure 7d,
there are many holes and cracks in the internal char residues
of pure EP. After the incorporation of FR, the internal char
residues of EP composites become solid and continuous
(Figure 7e and f). Besides, almost no holes and cracks can
be discovered in the internal char residues of EP-5 wt% DPCA
and EP-5 wt% DCA. The above results further demonstrate the
prominent catalyze charring effect of DCA and DPCA, which is
beneficial to promote the forming of compact structure of char
residues in the condensed phase and thus enhancing the fire
Figure 6: Comparison of PHRR reduction with previous literature. resistance of polymeric material.
To better shed light onto the FR mechanism, the char
layer was analyzed by Raman spectrum, as disclosed in
flammability (35). However, it is noted that the carbonac- Figure 8. The signal G band (1,580 cm−1) is derived from
eous residues of EP-5 wt% DPCA and EP-5 wt% DCA are the vibration of the aromatic structure of C–C, and the
sharply increased with the introduction of FR. For instance, D band (1,350 cm−1) is attributed to the sp3 hybridized of
EP-5 wt% DPCA composite shows compact and coherent char C atoms (47,48). It is commonly used that the area ratio
residues, which is desirable to suppress the transfer of mass of ID/IG as an index negative to the graphitization degree
and heat. As can be seen from Figure A2c and f, EP-5 wt% DCA of the char layer (49). As shown in Figure 8a, the ID/IG
shows the same char residues as that of EP-5 wt% DPCA. value of EP is 2.58, which is higher than that of EP-5 wt%
Those results indicate that both DCA and DPCA have an excel- DPCA (2.22) and EP-5 wt% DCA (2.28). This result points
lent effect in catalyzing the char residues, which is in accord out that the graphitization degree of EP-5 wt% DPCA and
with the TGA results from Figure 3. EP-5 wt% DCA is better than that of pure EP. Besides, these
To further evidence the FR mechanism of EP com- data are in good consistency with the SEM results from
posites, the external and internal char residues of EP, Figure 7, which further indicates the superior catalyze
EP-5 wt% DPCA, and EP-5 wt% DCA were investigated charring effect of DCA and DPCA.
Figure 7: SEM images of char residues for (a, d) EP, (b, e) EP-5 wt% DPCA, and (c, f) EP-5 wt% DCA after UL-94 tests.
438 Yunxia Yang and Dan Xiao
Figure 8: Raman spectra of char residue of (a) EP, (b) EP-5 wt% DPCA, and (c) EP-5 wt% DCA composites.
To further study the FR mechanism of EP composites, which exert the diluting effect and quenching action
as shown in Figure 9, the pyrolysis products of DPCA and in the gas phase, respectively. For instance, m/z of frag-
DCA were analyzed by GC/MS and Py-GC/MS. It has been ments located at 47, 63, and 64 are ascribed to the PO˙,
reported that the phosphorus-containing fragments, such PO2˙, and HPO2˙ free radicals, respectively (51). It has
as PO, PO2, and HPO2 (m/z = 47, 63, 64), can interrupt been reported that these P-containing free radicals could
the chain reaction by capturing OH˙ and H˙ radicals and interact with H˙ and OH˙ free radicals, which led to the
thus result in the chain termination (50). Besides, the chain termination (52). Besides, two obvious peaks can
N-containing radicals, such as C3H3NO (m/z = 69) and be found at 69 and 46 m/z, which are attributed to the
NO2 (m/z = 46), take a major role in diluting the concen- C3H3NO and NO2, respectively. These nonflammable gases
tration of oxygen gas and flammable volatiles. It is noted can dilute the concentration of combustible gases and
that some N-containing nonflammable gases and P-con- thus further improving the fire resistance of polymers
taining free radicals are pyrolyzed from DPCA and DCA, (53). Overall, these above results reveal that DCA and
Figure 9: (a, d) Total ions chromatograph (TIC) and (b, c, e) Py-GC/MS spectra of main pyrolysis products of DPCA and DCA.
Fabrication of two multifunctional P–N flame retardants 439
DPCA play significant roles in improving the FR perfor- which effectively enhances the fire resistance of EP com-
mance of EP composites in the condense and gas phase. posites. In the gas phase, P-containing radicals could
As mentioned in above results, the FR mechanism of capture H-containing free radicals (H˙, HO˙, etc.), which
EP composites is illustrated in Figure 10. In the con- prevent the further combustion of polymers. Besides,
densed phase, the compact and coherent carbon layers combustible gas could be diluted by these free radicals
are catalyzed by the FRs. Then, the release of heat and (NOx˙, POx˙), which further leads to the enhanced fire
toxic fumes are suppressed by the compact carbon layer, resistance of EP composites. Overall, the excellent FR
Figure 10: The possible FR mechanism of EP–DPCA and EP–DCA.
Figure 11: Digital photos of (a) EP, (b) EP-5 wt% DPCA, and (c) EP-5 wt% DCA samples. (d) UV-Vis spectra curves of EP samples in visible
region. (e) Normalized transmittance of EP and its composites.440 Yunxia Yang and Dan Xiao
performance of EP composites is attributed to the mul- composites. This study develops a novel multifunctional
tiple functions of FRs in the gas phase and condensed EP composite with high flame retardancy and transparency
phase. and thus it is expected to expand the application of EP.
Funding information: Authors state no funding involved.
3.5 Transparency of EP and its composites Author contributions: Yunxia Yang: writing – original draft,
formal analysis, investigation; Dan Xiao: writing – review,
It has been reported that EP has been extensively used supervision.
as optical material due to its high optical transmittance
(54–56). The related optical transmittance data of digital Conflict of interest: Authors state no conflict of interest.
photographs for EP, EP-5 wt% DPCA, and EP-5 wt% DCA
are indicated in Figure 11a–c. As shown in Figure 11a–c,
the background words and logos under all the samples can
be clearly observed, which indicates the excellent optical References
transmittance of EP, EP-5 wt% DPCA, and EP-5 wt% DCA.
To further investigate the optical transmittance, the optical (1) Wang P, Chen L, Xiao H, Zhan TH. Nitrogen/sulfur-containing
transmittance of EP, EP-5 wt% DPCA, and EP-5 wt% DCA DOPO based oligomer for highly efficient flame-retardant
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444 Yunxia Yang and Dan Xiao
Table A1: The TGA data of DPCA and DCA under nitrogen
atmosphere
Sample T5% (°C) Tmax (°C) Residue at 700°C (%·°C−1)
700
DPCA 213.9 293.9 1.9879
DCA 219.1 291.6 3.80615
Table A2: The cone calorimeter data of EP and its composites
Sample TTI PHRR FPI FGR THR PCOPR char
(s) (kW·m−2) (MJ·m−2) (g·s−1) (%)
Error ±4.66 ±40.37 — — ±1.49 ±0.003 ±0.42
EP 40 938.09 0.043 7.50 92.05 0.03 2.91
EP-5% DPCA 32 556.38 0.058 5.30 77.83 0.02 13.29
EP-5% DCA 37 593.95 0.062 4.75 73.69 0.03 11.04You can also read
