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Fabrication of two multifunctional phosphorus–nitrogen flame retardants toward improving the fire safety of epoxy resin

  • Yunxia Yang EMAIL logo and Dan Xiao EMAIL logo
Published/Copyright: April 27, 2022
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e-Polymers
From the journal e-Polymers Volume 22 Issue 1

Abstract

To improve the fire safety of epoxy resin (EP), two novel phosphorus–nitrogen flame retardants, which named as diphenyl allylphosphoramidate (DPCA) and N-allyl-P, P-diphenylphosphinic amide (DCA), were synthesized by acyl chloride reaction and introduced into EP for fabricating EP composites. The combustion tests showed that incorporation of 5 wt% DPCA or 5 wt% DCA into EP led to the exceptional limited oxygen index (LOI) value (27.1% or 31.6%). Besides, the peak of heat release rate of EP-5 wt% DPCA and EP-5 wt% DCA was reduced by 40.69% and 36.69%, respectively, compared to pure EP. The enhanced fire resistance of EP was ascribed to the trapping effect of fillers in the gas phase and the charring effect in the condensed phase. Furthermore, ultraviolet-visible spectra revealed that both EP-5 wt% DPCA and EP-5 wt% DCA have considerable transparency. This study is expected to broaden the application of EP in the industrial area.

Keywords: epoxy resin; flame retardancy; trapping effect; diluting action; transparency

1 Introduction

Epoxy resin (EP), as one of the most important thermosetting resins, has been extensively used in adhesive, electronic materials, aerospace industry, etc., due to its satisfactory corrosion resistance, dimensional stability, electrical insulation, and mechanical properties (1,2,3,4,5). Unfortunately, the high flammability and the release of a great number of toxic gases during combustion heavily limit the application of EP. Hence, it is critical to develop efficient flame retardants (FRs) to enhance the fire safety of EP.

The halogen compound, as a comonomer or additive, has been widely used in fabricating FR EP composites. However, halogen-containing FRs usually release toxic fumes during combustion, which is extremely harmful to human health and the environment (6,7,8). As a result, halogen-free FRs have attracted increasing interest from researchers in enhancing the fire resistance of EP. It has been reported that phosphorus-containing FRs, including phosphide, organophosphorus compounds, polyphosphate, etc., have been extensively used in varieties of polymers among all the halogen-free FRs due to the environmental friendliness and outstanding flame retardancy (9,10,11,12,13,14,). However, most of phosphorus-containing FRs are liquid and it has some disadvantages (poor thermal stability, high volatility, and poor compatibility) (15,16,17). Besides, phosphorus-containing FRs have dissatisfactory flame retardancy when incorporated into polymers alone (18,19,20).

To overcome those intractable problems, phosphorus–nitrogen (P–N) FRs have been developed for enhancing the fire resistance of EP (21,22). Zhou et al. synthesized an innovative P–N FR (poly(piperazine phosphaphenanthrene) (DOPMPA)) and they found that the EP composites 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 benzimidazolyl-substituted cyclotriphosphazene filler (BICP) and incorporated 10.7 wt% BICP into EP to fabricate EP composite. The result showed that BICP not only endowed EP with excellent flame retardance but also enhanced the thermal stability of EP (24). Zhu et al. synthesized a novel P–N FR phosphaphenanthrene/benzimidazolone-containing (POBDBI) including benzimidazolone and DOPO groups. They revealed that the EP–POBDBI composite with 13 wt% FR achieved the UL-94 V-0 rating and the LOI was increased to 36.5%. Besides, the peak of heat release rate (PHRR) of EP/POBDBI composite was reduced by 48.9%, in comparison with pure EP (25). As mentioned above, P–N FRs show outstanding flame retardancy and smoke suppression properties. However, it has been reported the excessed P–N FRs showed an adverse effect on the transparency of materials (26,27). Thus, it is significant to develop multifunctional flame-retardant EP composites with outstanding flame retardancy, good thermal stability, and high transparency.

Herein, two innovative P–N-containing solid FRs were synthesized from diphenyl chlorophosphate (DPC), diphenylphosphinyl chloride (DC), and allylamine. After that, FRs were introduced into EP to fabricate flame-retardant EP composites. The flammability, thermal property, and transparency of EP and its composites were studied. Besides, the FR mechanism of EP-diphenyl allylphosphoramidate (DPCA) and EP-N-allyl-P, P-diphenylphosphinic amide (DCA) was investigated. This study develops an innovative approach to fabricate EP composites with high flame retardancy, thermal stability, and transparency, which is expected to broaden the industrial application of EP.

2 Experimental section

2.1 Raw materials

Diglycidyl ether of bisphenol A (DGEBA, E-44) was provided from Xingchen Synthetic Material Co. Ltd, China. Allylamine (A) was supplied from Chengdu Hongben Chemical Products Co. Ltd, China. The 4,4′-diaminodiphenylsulfone (DDS) was provided by Shanghai Aladdin Chemical Reagent Co., Ltd, China. Dichloromethane, DPC, DC, and triethylamine were acquired from Shanghai Sinopharm Reagent Co. Ltd, China.

2.2 Synthesis of FRs

Allylamine (0.051 mol, 2.912 g), triethylamine (0.050 mol, 5.060 g), and dichloromethane (150 mL) were first introduced into a three-necked flask with mechanical agitation. Then, DPC (0.050 mol, 13.432 g) was dropwise added into the above mixture at 0–5°C for 1 h. Then, the mixture was stirred overnight at room temperature. After that, the solution was filtered to remove triethylamine hydrochloride, and the filtrate was washed no less than three times with deionized water. The organic phase was dried by anhydrous magnesium sulfate and further evaporated under a vacuum at 40°C until the dichloromethane was removed absolutely. Finally, the white solid (DPCA) was obtained and the yield of DPCA was 90.60%. DCA was prepared by the similar method as that of DPCA and the yield of DCA was 90.25%. The synthesis route of DPCA and DCA is shown in Scheme 1.

Scheme 1 Graphical synthesis scheme for DPCA and DCA.
Scheme 1

Graphical synthesis scheme for DPCA and DCA.

2.3 Fabrication of FR EP composites

Taking EP–DPCA composite as an example, specifically EP and DDS were first mixed at 120°C by mechanical stirrer until DDS was completely dissolved in EP. Then, DPCA was incorporated into the above solution and stirred at 120°C for 30 min. Thereafter, the mixture was degassed using a vacuum at 120°C for 30 min and poured directly into the preheated mold. Finally, the samples were step-cured at 160°C for 1 h, 180°C for 2 h, and 190°C for 1 h, and the samples were naturally cooled down to room temperature. In addition, pure EP and EP–DCA sample was fabricated via the same process. The detailed formulas of all samples were summarized in Table 1.

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

The formula for calculating the mass fraction of P in FR is provided as follows: P ( wt% ) = m flame retardant m EP + m DDS + m flame retardant × 31 M flame retardant × 100 % (a).

2.4 Characterizations

Fourier transform infrared (FTIR) analyses were collected on Nicolet 6700 infrared spectrometer (Thermo, USA).

1H 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 on NETZSCH STA449F3 (NETZSCH-Gerätebau GmbH, Germany) with a heating rate of 25–800°C under nitrogen condition.

Differential scanning calorimetry (DSC) thermograms were measured on DSC200F3 (Schneider) under N2 atmosphere from 30°C to 250°C.

The LOI values were tested at room temperature on a CH-2 oxygen index meter (Nanjing Jiangzhong Analysis Instrument Co., Ltd, China) by using ASTM D2863, and the sample size was 100 mm × 6.5 mm × 3 mm.

Vertical burning (UL-94) tests were conducted on the CZF3 instrument (Nanjing Jiangzhong Analysis Instrument Co., Ltd, China), and the sample size was 130 mm × 13 mm × 3 mm.

Cone calorimeter test was analyzed on the Fire Testing Technology (FTT) cone calorimeter at the external heat flux of 50 kW·m−2. The sample size was 100 mm × 100 mm × 3 mm.

Scanning electronic microscopy (SEM) (NovaNanoSEM450 OXFORD X-MaxN EDS, USA) was utilized to analyze the microstructures of the residual chars after burning testing.

Raman spectra were collected on a Dxr2xi micro Raman imaging spectrometer (American) with an argon laser of 532 nm.

Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) testing was conducted on a GC/MS (Agilent 7890B-5977AGC/MSD) equipped with a frontier pyrolyzer. Helium 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.

Ultraviolet-visible (UV-Vis) transmission of samples was recorded via an ultraviolet spectrophotometer (UV2600) with a thickness of 1 mm.

3 Results and discussion

3.1 Characterization of DPCA and DCA

The FTIR spectra of FR are presented in Figure 1a and b. As shown in Figure 1a, three obvious absorption bands situated at 1,589, 1,484, and 1,185 cm−1 can be found in the FTIR spectra of DPC, which are assigned to the stretching vibration of benzene ring, benzene ring, and P═O bond, respectively (28). After the reaction between DPC and allylamine, it is noted that DPCA shows the same typical absorption band as that of DPC. Besides, two new peaks situated at 3,253 and 1,101 cm−1 are ascribed to the stretching vibration of –NH and P–N–C groups, respectively (29). The result indicates that the preparation of DPCA is successful. It is observed from Figure 1b that DC and DCA display three characteristic peaks at 1,434, 1,589, and 1,244 cm−1, which agree well with the benzene ring, benzene ring, and P═O bond, respectively (30). In addition, two obvious peaks at 850 cm−1 (P–N–C bond) and 3,186 cm−1 (N–H group) appear in the FTIR spectra of DCA, demonstrating the successful preparation of DCA (31).

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.
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 to characterize the chemical structure, and the related 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 are attributed to the C═C bond of DPCA. In the case of 1H 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 DCA. In addition, two sharp peaks can be found at 5.95 and 3.64–3.53 ppm, corresponding to the hydrogen atom of the C═C bond for DCA. To further study the chemical structure of FRs, DPCA and DCA were investigated by MS spectrum, and the results are portrayed in Figure 1e and f. As presented in Figure 1e and f, DPCA and DCA show the equimolecular ion peak at 257.1 (C15H16NOP) and 288.9 (C15H16NO3P), respectively, further demonstrating that DCA and DPCA are synthesized successfully.

The TGA curves of DCA and DPCA under nitrogen atmosphere are presented in Figure A1 and Table A1 (Appendix). As can be seen from Table A1, T 5% of DCA and DPCA is 219.1°C and 213.9°C, respectively. Besides, it is noted that the residual mass value of DCA (3.8%) is higher than that of DPCA (2.0%). The above results demonstrate that the thermal stability of DCA is better than that of DPCA.

3.2 Thermal stability of EP and its composites

The DSC data of EP, EP–DPCA, and EP–DCA are presented in Figure 2 and Table 2. It is noted that all the samples show only one glass transition stag, suggesting that the system had a cross-linking reaction, and the EP is fully cured (32). Besides, as shown in Table 2, the T g value gradually decreases with the increase of the additive, which is attributed to the decrease of cross-linking density of epoxy thermosetting materials (33). This result is ascribed to the interpretation that both DPCA and DCA participate in the curing process of EP through the reaction of amino group (–NH) with epoxy group (34).

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.
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.

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 T 5% (°C) T max (°C) Char yields (%·°C−1) R max (%·°C−1) T HRI
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: T 5%, R max, T max, and T HRI, 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.

The thermal stability of EP, EP–DPCA, and EP–DCA are assessed by TGA instrument, the thermogravimetric (TG) curves, derivative thermogravimetric (DTG) curves and related results are shown in Figure 3 and Table 2. As shown in Table 2, EP, EP–DPCA, and EP–DCA show the one-step degradation, which is assigned to the degradation of macromolecular chains of EP (C–C bonds, aromatic ring, etc.). The T 5% and T max of EP are 379°C and 416°C, respectively. With the addition of FR, the T 5% 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%, respectively, 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 (T HRI) is calculated by Eq. 1, and the results are shown in Table 2 (37).

(1) T HRI = 0.49 ⁎ [ T − 5 % + 0.6 ( T − 30 % − T − 5 % ) ]

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.
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.

It is clearly observed that EP–DPCA and EP–DCA show slightly lower T HRI values, in comparison with pure EP, demonstrating that the thermal stability of EP is held. Besides, it is noted that the maxim degradation rate (R max) of EP–DPCA and EP–DCA is lower than that of pure EP, manifesting that the decomposition of EP is inhibited by the additive.

3.3 Flame retardancy of EP and its composites

The flame retardancy of EP, EP-5 wt% DPCA, and EP-5 wt% DCA was evaluated by vertical combustion test (UL-94) and LOI test, the data are illustrated in Figure 4. As shown in Figure 4, the pure EP shows the low LOI value (24.8%), and it cannot pass the UL-94 rating. When the FR was added, the LOI value of EP composites gradually increased. For instance, EP-5 wt% DCA composite shows the highest LOI value (31.6), which is 1.27 times as that of pure EP. Besides, it is worth noting that EP-5 wt% DCA composite achieves the UL-94 V-1 rating. The above results demonstrate that the fire resistance is significantly improved with the introduction of DPCA and DCA. Besides, it can be concluded from the UL-94 and LOI results that the flame retardancy of EP-5 wt% DCA is better than that of EP-5 wt% DPCA. This result is ascribed to the reason that the phosphorus content of DCA is higher than that of DPCA.

Figure 4 The LOI and UL-94 results of EP, EP-2 wt% DPCA, EP-5 wt% DPCA, EP-2 wt% DCA, and EP-5 wt% DCA.
Figure 4

The LOI and UL-94 results of EP, EP-2 wt% DPCA, EP-5 wt% DPCA, EP-2 wt% DCA, and EP-5 wt% DCA.

The FR performance of EP, EP-5 wt% DPCA, and EP-5 wt% DCA was further evaluated by cone calorimeter testing (Figure 5 and Table A2). It is noted that the time to ignition (TTI) of EP-5 wt% DCA and EP-5 wt% DPCA is lower than that of pure EP, which is attributed to the degradation of polymers by the catalysis of fillers (38,39). This result is well consistent with the TGA results from Figure 3. As shown in Table A2, the PHRR and total heat release (THR) values of EP composites are decreased in comparison 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 by 40.69% and 36.69%, respectively. Besides, the THR values of EP-5 wt% DPCA and EP-5 wt% DCA are decreased to 77.83 and 73.69 kW·m−2, which are declined by 15.45% and 22.47%, respectively, in comparison with pure EP. These results reveal that the heat release of EP composites is significantly decreased during combustion by introducing DPCA or DCA.

Figure 5 (a) HRR, (b) THR, (c) mass loss, and (d) COPR curves of EP, EP-5 wt% DPCA, and EP-5 wt% DCA.
Figure 5

(a) HRR, (b) THR, (c) mass loss, and (d) COPR curves of EP, EP-5 wt% DPCA, and EP-5 wt% DCA.

To highlight the superior flame retardancy of DPCA and DCA, the fire-retardant performances of DCA and DPCA are compared with prior literature, as shown in Figure 6. As can be observed from Figure 6, these FRs (lamellar-like phosphorus-based triazole–zinc complex (Zn–PT), phosphorus/nitrogen-containing polycarboxylic acid (TMD), organic/inorganic phosphorus-nitrogen-silicon flame retardant (DPHK), etc.) play little effect in suppressing the heat release of EP (40–45). For instance, Jian et al. prepared Zn–PT and revealed that the PHRR of EP composite was decreased by 22.26% via incorporating 3 wt% Zn–PT (40). Ai et al. demonstrated that the PHRR of EP composite was declined by 35.28% with the addition of 7.5 wt% organophosphorus-bridged amitrole (41). Luo et al. reported 15.06% decrease in PHRR, by incorporating 2 wt% 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.

Figure 6 Comparison of PHRR reduction with previous literature.
Figure 6

Comparison of PHRR reduction with previous literature.

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 distinguished catalytic charring of DPCA, which is in line with the TGA results. It has been reported that the fire growth rate (FGR) and the fire performance index (FPI) were used to evaluate the fire-resistance safety of polymers (46). The FGR and FPI are calculated via Eqs 2 and 3, the data are shown in Table A2.

(2) FGR = PHRR / t PHRR

(3) FPI = TTI / PHRR

Generally, the lower FGR and higher FPI indicate the higher fire safety of polymer materials. It is noticeable that the FGR values of EP-5 wt% DPCA and EP-5 wt% DCA are less than that of pure EP. However, the FPI shows the 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 the FR performance of EP is enhanced with the incorporation of DPCA or DCA.

Toxic fume is one of the most important parameters to threaten the health of human beings during a fire accident. Therefore, it is especially important to research the generation of toxic fume during the combustion of polymers. The corresponding results of carbon monoxide for EP, EP-5 wt% DPCA, and EP-5 wt% DCA are displayed in Figure 5d. It is clearly observed from Figure 5d that the peak of carbon monoxide production rate (PCOPR) of the pure EP is 0.0294 g·s−1. With the addition of 5 wt% DPCA, the PCOPR of EP-5 wt% DPCA is reduced to 0.0204 g·s−1, which is declined by 30.6% compared to pure EP. The above data further demonstrate that the fire safety of EP composites is enhanced with the addition of FR.

3.4 FR mechanism

The digital pictures of char residues for EP, EP-5 wt% DPCA, and EP-5 wt% DCA are displayed in Figure A2. As shown in Figure A2a and d, pure EP has few char residues after combustion, which is due to the heavy flammability (35). However, it is noted that the carbonaceous residues of EP-5 wt% DPCA and EP-5 wt% DCA are sharply increased with the introduction of FR. For instance, EP-5 wt% DPCA composite shows compact and coherent char residues, which is desirable to suppress the transfer of mass and heat. As can be seen from Figure A2c and f, EP-5 wt% DCA shows the same char residues as that of EP-5 wt% DPCA. Those results indicate that both DCA and DPCA have an excellent effect in catalyzing the char residues, which is in accord with the TGA results from Figure 3.

To further evidence the FR mechanism of EP composites, the external and internal char residues of EP, EP-5 wt% DPCA, and EP-5 wt% DCA were investigated 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 resistance of polymeric material.

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.
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.

To better shed light onto the FR mechanism, the char layer was analyzed by Raman spectrum, as disclosed in Figure 8. The signal G band (1,580 cm−1) is derived from the vibration of the aromatic structure of C–C, and the D band (1,350 cm−1) is attributed to the sp3 hybridized of C atoms (47,48). It is commonly used that the area ratio of I D/I G as an index negative to the graphitization degree of the char layer (49). As shown in Figure 8a, the I D/I G value of EP is 2.58, which is higher than that of EP-5 wt% DPCA (2.22) and EP-5 wt% DCA (2.28). This result points out that the graphitization degree of EP-5 wt% DPCA and EP-5 wt% DCA is better than that of pure EP. Besides, these data are in good consistency with the SEM results from Figure 7, which further indicates the superior catalyze charring effect of DCA and DPCA.

Figure 8 Raman spectra of char residue of (a) EP, (b) EP-5 wt% DPCA, and (c) EP-5 wt% DCA composites.
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, as shown in Figure 9, the pyrolysis products of DPCA and DCA were analyzed by GC/MS and Py-GC/MS. It has been reported that the phosphorus-containing fragments, such as PO, PO2, and HPO2 (m/z = 47, 63, 64), can interrupt the chain reaction by capturing OH˙ and H˙ radicals and thus result in the chain termination (50). Besides, the N-containing radicals, such as C3H3NO (m/z = 69) and NO2 (m/z = 46), take a major role in diluting the concentration of oxygen gas and flammable volatiles. It is noted that some N-containing nonflammable gases and P-containing free radicals are pyrolyzed from DPCA and DCA, which exert the diluting effect and quenching action in the gas phase, respectively. For instance, m/z of fragments located at 47, 63, and 64 are ascribed to the PO˙, PO2˙, and HPO2˙ free radicals, respectively (51). It has been reported that these P-containing free radicals could interact with H˙ and OH˙ free radicals, which led to the chain termination (52). Besides, two obvious peaks can be found at 69 and 46 m/z, which are attributed to the C3H3NO and NO2, respectively. These nonflammable gases can dilute the concentration of combustible gases and thus further improving the fire resistance of polymers (53). Overall, these above results reveal that DCA and DPCA play significant roles in improving the FR performance of EP composites in the condense and gas phase.

Figure 9 (a, d) Total ions chromatograph (TIC) and (b, c, e) Py-GC/MS spectra of main pyrolysis products of DPCA and DCA.
Figure 9

(a, d) Total ions chromatograph (TIC) and (b, c, e) Py-GC/MS spectra of main pyrolysis products of DPCA and DCA.

As mentioned in above results, the FR mechanism of EP composites is illustrated in Figure 10. In the condensed phase, the compact and coherent carbon layers are catalyzed by the FRs. Then, the release of heat and toxic fumes are suppressed by the compact carbon layer, which effectively enhances the fire resistance of EP composites. In the gas phase, P-containing radicals could capture H-containing free radicals (H˙, HO˙, etc.), which prevent the further combustion of polymers. Besides, combustible gas could be diluted by these free radicals (NO x ˙, PO x ˙), which further leads to the enhanced fire resistance of EP composites. Overall, the excellent FR performance of EP composites is attributed to the multiple functions of FRs in the gas phase and condensed phase.

Figure 10 The possible FR mechanism of EP–DPCA and EP–DCA.
Figure 10

The possible FR mechanism of EP–DPCA and EP–DCA.

3.5 Transparency of EP and its composites

It has been reported that EP has been extensively used as optical material due to its high optical transmittance (54,55,56). The related optical transmittance data of digital 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 transmittance of EP, EP-5 wt% DPCA, and EP-5 wt% DCA. To further investigate the optical transmittance, the optical transmittance of EP, EP-5 wt% DPCA, and EP-5 wt% DCA was detected by UV-Vis spectrometry, as shown in Figure 11d. It is noted that the optical transmittance of pure EP is 54% at 650 nm. With the addition of DCA or DPCA, the EP-5 wt% DCA and EP-5 wt% DPCA are slightly decreased to 44% and 47%, respectively. Besides, the calculated data of normalized optical transmission related to pure EP at 650 nm are portrayed in Figure 11e. As observed from Figure 11e that the EP-5 wt% DCA and EP-5 wt% DPCA show the high normalized optical transmission value (81% and 87%), which keeps the optical transmittance more than 90% of pure EP. These results demonstrate that EP-5 wt% DCA and EP-5 wt% DPCA composites still hold distinguished transparency and thus have promising application in the field of optical materials.

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.
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.

4 Conclusion

In this study, DPCA and DCA were introduced into the EP/DDS system, and the effects on the thermal stability, flame retardancy, and transparency of the composites were investigated. The results showed that the UL-94 rating of EP composite with 5 wt% DCA was V-1, the LOI was increased to 31.6%. Besides, compared to pure EP, the PHRR was decreased by 40.69% and 36.69% for EP-5 wt% DPCA and EP-5 wt% DCA, respectively. The quenching effect, dilution effect, and catalytic charring effect of FR in gas phase and condensed phase led to the enhanced fire safety of EP. Moreover, the addition of two FRs has positive effect on the thermal stability of the composites. This study develops a novel multifunctional EP composite with high flame retardancy and transparency and thus it is expected to expand the application of EP.

  1. Funding information: Authors state no funding involved.

  2. Author contributions: Yunxia Yang: writing – original draft, formal analysis, investigation; Dan Xiao: writing – review, supervision.

  3. Conflict of interest: Authors state no conflict of interest.

Appendix

Figure A1 TG curves of DCA and DPCA under nitrogen conditions.
Figure A1

TG curves of DCA and DPCA under nitrogen conditions.

Figure A2 (a–c) Top view and (d–f) side view of EP, EP-5 wt% DPCA, and EP-5 wt% DCA.
Figure A2

(a–c) Top view and (d–f) side view of EP, EP-5 wt% DPCA, and EP-5 wt% DCA.

Table A1

The TGA data of DPCA and DCA under nitrogen atmosphere

Sample T 5% (°C) T max (°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 11.04

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Received: 2022-03-03
Revised: 2022-03-25
Accepted: 2022-03-28
Published Online: 2022-04-27

© 2022 Yunxia Yang and Dan Xiao, published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2022-0042
Keywords for this article
epoxy resin; flame retardancy; trapping effect; diluting action; transparency
Creative Commons
BY 4.0

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  25. A machine learning investigation of low-density polylactide batch foams
  26. A comparison study of hyaluronic acid hydrogel exquisite micropatterns with photolithography and light-cured inkjet printing methods
  27. Multifunctional nanoparticles for targeted delivery of apoptin plasmid in cancer treatment
  28. Thermal stability, mechanical, and optical properties of novel RTV silicone rubbers using octa(dimethylethoxysiloxy)-POSS as a cross-linker
  29. Preparation and applications of hydrophilic quaternary ammonium salt type polymeric antistatic agents
  30. Coefficient of thermal expansion and mechanical properties of modified fiber-reinforced boron phenolic composites
  31. Synergistic effects of PEG middle-blocks and talcum on crystallizability and thermomechanical properties of flexible PLLA-b-PEG-b-PLLA bioplastic
  32. A poly(amidoxime)-modified MOF macroporous membrane for high-efficient uranium extraction from seawater
  33. Simultaneously enhance the fire safety and mechanical properties of PLA by incorporating a cyclophosphazene-based flame retardant
  34. Fabrication of two multifunctional phosphorus–nitrogen flame retardants toward improving the fire safety of epoxy resin
  35. The role of natural rubber endogenous proteins in promoting the formation of vulcanization networks
  36. The impact of viscoelastic nanofluids on the oil droplet remobilization in porous media: An experimental approach
  37. A wood-mimetic porous MXene/gelatin hydrogel for electric field/sunlight bi-enhanced uranium adsorption
  38. Fabrication of functional polyester fibers by sputter deposition with stainless steel
  39. Facile synthesis of core–shell structured magnetic Fe3O4@SiO2@Au molecularly imprinted polymers for high effective extraction and determination of 4-methylmethcathinone in human urine samples
  40. Interfacial structure and properties of isotactic polybutene-1/polyethylene blends
  41. Toward long-live ceramic on ceramic hip joints: In vitro investigation of squeaking of coated hip joint with layer-by-layer reinforced PVA coatings
  42. Effect of post-compaction heating on characteristics of microcrystalline cellulose compacts
  43. Polyurethane-based retanning agents with antimicrobial properties
  44. Preparation of polyamide 12 powder for additive manufacturing applications via thermally induced phase separation
  45. Polyvinyl alcohol/gum Arabic hydrogel preparation and cytotoxicity for wound healing improvement
  46. Synthesis and properties of PI composite films using carbon quantum dots as fillers
  47. Effect of phenyltrimethoxysilane coupling agent (A153) on simultaneously improving mechanical, electrical, and processing properties of ultra-high-filled polypropylene composites
  48. High-temperature behavior of silicone rubber composite with boron oxide/calcium silicate
  49. Lipid nanodiscs of poly(styrene-alt-maleic acid) to enhance plant antioxidant extraction
  50. Study on composting and seawater degradation properties of diethylene glycol-modified poly(butylene succinate) copolyesters
  51. A ternary hybrid nucleating agent for isotropic polypropylene: Preparation, characterization, and application
  52. Facile synthesis of a triazine-based porous organic polymer containing thiophene units for effective loading and releasing of temozolomide
  53. Preparation and performance of retention and drainage aid made of cationic spherical polyelectrolyte brushes
  54. Preparation and properties of nano-TiO2-modified photosensitive materials for 3D printing
  55. Mechanical properties and thermal analysis of graphene nanoplatelets reinforced polyimine composites
  56. Preparation and in vitro biocompatibility of PBAT and chitosan composites for novel biodegradable cardiac occluders
  57. Fabrication of biodegradable nanofibers via melt extrusion of immiscible blends
  58. Epoxy/melamine polyphosphate modified silicon carbide composites: Thermal conductivity and flame retardancy analyses
  59. Effect of dispersibility of graphene nanoplatelets on the properties of natural rubber latex composites using sodium dodecyl sulfate
  60. Preparation of PEEK-NH2/graphene network structured nanocomposites with high electrical conductivity
  61. Preparation and evaluation of high-performance modified alkyd resins based on 1,3,5-tris-(2-hydroxyethyl)cyanuric acid and study of their anticorrosive properties for surface coating applications
  62. A novel defect generation model based on two-stage GAN
  63. Thermally conductive h-BN/EHTPB/epoxy composites with enhanced toughness for on-board traction transformers
  64. Conformations and dynamic behaviors of confined wormlike chains in a pressure-driven flow
  65. Mechanical properties of epoxy resin toughened with cornstarch
  66. Optoelectronic investigation and spectroscopic characteristics of polyamide-66 polymer
  67. Novel bridged polysilsesquioxane aerogels with great mechanical properties and hydrophobicity
  68. Zeolitic imidazolate frameworks dispersed in waterborne epoxy resin to improve the anticorrosion performance of the coatings
  69. Fabrication of silver ions aramid fibers and polyethylene composites with excellent antibacterial and mechanical properties
  70. Thermal stability and optical properties of radiation-induced grafting of methyl methacrylate onto low-density polyethylene in a solvent system containing pyridine
  71. Preparation and permeation recognition mechanism of Cr(vi) ion-imprinted composite membranes
  72. Oxidized hyaluronic acid/adipic acid dihydrazide hydrogel as cell microcarriers for tissue regeneration applications
  73. Study of the phase-transition behavior of (AB)3 type star polystyrene-block-poly(n-butylacrylate) copolymers by the combination of rheology and SAXS
  74. A new insight into the reaction mechanism in preparation of poly(phenylene sulfide)
  75. Modified kaolin hydrogel for Cu2+ adsorption
  76. Thyme/garlic essential oils loaded chitosan–alginate nanocomposite: Characterization and antibacterial activities
  77. Thermal and mechanical properties of poly(lactic acid)/poly(butylene adipate-co-terephthalate)/calcium carbonate composite with single continuous morphology
  78. Review Articles
  79. The use of chitosan as a skin-regeneration agent in burns injuries: A review
  80. State of the art of geopolymers: A review
  81. Mechanical, thermal, and tribological characterization of bio-polymeric composites: A comprehensive review
  82. The influence of ionic liquid pretreatment on the physicomechanical properties of polymer biocomposites: A mini-review
  83. Influence of filler material on properties of fiber-reinforced polymer composites: A review
  84. Rapid Communications
  85. Pressure-induced flow processing behind the superior mechanical properties and heat-resistance performance of poly(butylene succinate)
  86. RAFT polymerization-induced self-assembly of semifluorinated liquid-crystalline block copolymers
  87. RAFT polymerization-induced self-assembly of poly(ionic liquids) in ethanol
  88. Topical Issue: Recent advances in smart polymers and their composites: Fundamentals and applications (Guest Editors: Shaohua Jiang and Chunxin Ma)
  89. Fabrication of PANI-modified PVDF nanofibrous yarn for pH sensor
  90. Shape memory polymer/graphene nanocomposites: State-of-the-art
  91. Recent advances in dynamic covalent bond-based shape memory polymers
  92. Construction of esterase-responsive hyperbranched polyprodrug micelles and their antitumor activity in vitro
  93. Regenerable bacterial killing–releasing ultrathin smart hydrogel surfaces modified with zwitterionic polymer brushes
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