Synthesis and characterization of a novel P/N containing flame retardant and its effect on flame-retardancy,thermal and mechanical properties of epoxy/clay nanocomposites

Abstract

A P/N containing flame retardant DOPO-QN was synthesized by a reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 8-hydroxyquinoline (QN). Its chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) like ¹H, ¹³C, ³¹P NMR, and high-resolution mass spectrometry (HRMS) techniques. The synthesized DOPO-QN was employed as an additive type flame retardant in combination with clay nanomer 1.34TCN (NC) in diglycidyl ether of bisphenol A/4,4ʹ-diaminodiphenlsulfone (DGEBA/DDS) to synthesize epoxy thermosets. The fire behavior of synthesized epoxy thermosets was studied by UL-94, limiting oxygen index (LOI), and cone calorimeter tests. The EP/DOPO-QN (1.5%P)/NC sample passed the UL-94 test with a V-0 rating and LOI value of 26.5%. A decrease in peak heat released, carbon monoxide, and carbon dioxide emissions was observed in the cone calorimeter test. Thermogravimetric analysis (TGA) of epoxy thermosets showed an increase in thermal stability at a higher temperature range on the formation of nanocomposites. Also, a remarkable improvement in storage modulus was observed for the EP/DOPO-QN (1.5%P)/NC sample containing 2.0% NC in dynamic mechanical analysis (DMA).

Keywords

Epoxy clay flame retardance

Introduction

Epoxy resins are an important class of thermosetting polymers and have been used indisputably for a long time in electronic devices, laminates, encapsulations, transportation, coatings, and adhesives, etc. owing to their good mechanical properties, electrical insulation, and excellent dimension stability, and also resistance to chemical corrosion, moisture, and heat.^1–4 However, epoxy resins are highly flammable and release a large amount of smoke when heated during their use in many aspects.^3–8 Hence, it is extremely crucial to reduce the flammability of epoxy resins by incorporating flame-retardants to retard combustion and avoid the spread of fire after ignition for safety.⁹

In beginning, halogen-containing compounds have been widely used as flame retardants due to their low-cost, ease of processing, and effective gas-phase quenching, but halogen-containing flame retardants have hazardous effects on living beings and the environment due to their toxicity.¹⁰ So, the need for the development of environment-friendly flame retardants is increasing day by day. Recently, many halogen-free flame-retardants containing phosphorus,^11–13 boron,^14,15 nitrogen,^16–18 and functionalized nanoclay^19–21 have been developed for use in epoxy thermosets. Among these, the phosphorus-containing DOPO is considered a good flame-retardant by acting in the gaseous and condensed phases. Synergistic effects were observed when DOPO is combined with compounds containing nitrogen, sulfur, and silicon elements, which decreases the loading of flame-retardants and maintains high flame efficiency as well. In recent times, nanofillers like clay (montmorillonite-MMT), carbon nanotube, and graphene had been used to boost the flame retardancy of epoxy resin.^22–24 Between these MMT is extensively used in combination with phosphorus-containing (DOPO) flame retardants to enhance flame retardancy and simultaneously reduce the smoke released on burning due to its low cost, ease of availability, and greater surface area and cation exchange ability.^25–29 For example, Xiangdong et al.³⁰ synthesized a DOPO-MMT nano compound and utilized it as a flame retardant in epoxy resin, with 6 wt% DOPO-MMT in EP a V-0 rating on UL-94 test was obtained with an LOI value of 33.4%, due to the synergistic effect of DOPO and MMT. Yan et al.³¹ synthesized a phenyl-bridged DOPO derivative, DiDOPO, and a combination of DiDOPO and OMMT was incorporated into EP. The sample containing 3.5 wt% DiDOPO/3.5 wt% OMMT resulted in an increase in LOI value from 21.8% to 32.2% and a V-0 rating in the UL-94 test.

Wang et al.³² synthesized a phosphorus/nitrogen-containing flame retardant DDM-DOPO which achieved a V-0 rating in the UL-94 vertical burning test and an LOI value of 30% in the LOI test at 0.25 wt% phosphorus. Peng et al.³³ synthesized a tetra-DOPO derivative as a flame retardant for epoxy resin, which resulted in a V-0 rating in the UL-94 vertical burning test and LOI value of 30% with 5 wt% loading.

In the present work, a novel P/N containing flame retardant abbreviated as DOPO-QN was synthesized via Atherton Todd³⁴ reaction and characterized by various spectral techniques. The DOPO-QN was employed as an additive type flame retardant in combination with 2.0% clay nanomer 1.34TCN (NC) in diglycidyl ether of bisphenol A/4,4ʹ-diaminodiphenlsulfone (DGEBA/DDS) to synthesize epoxy thermosets having phosphorus in the range of 1.0 to 1.5% via thermal treatment. The effect of DOPO-QN and NC on fire behavior, thermal stability, and mechanical properties of epoxy thermosets was studied using UL-94/LOI/cone calorimeter, TGA, and DMA techniques, respectively.

Experimental

Materials

DGEBA, DDS, Nanomer 1.34TCN (NC) montmorillonite clay modified with 25to 30 wt.% methyl dihydroxyethyl hydrogenated tallow ammonium and triethyl amine were purchased from Sigma Aldrich Co Ltd 8-hydroxyquinoline (QN) was purchased from Alfa Aeser Co Ltd (India). 9,10-dihydro-9-oxa-10-phosphenathrene-10-oxide (DOPO) was purchased from TCI chemical (India). Carbon tetrachloride and dichloromethane were obtained from Himedia Co Ltd (India) and Mould releasing agent was purchased from TAG chemicals GmbH (Giant Sales Corporation, Gurugram, India). All these commercial materials were used as received without further purification.

Synthesis of 6-(quinolin-8-yloxy)-6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO-QN)

DOPO (43.2 g, 0.20 mol), 8-hydroxyquinoline (29.0 g, 0.20 mol), and triethylamine (30.7 mL, 0.22 mol) dissolved in 200 mL of dichloromethane were charged in a 500 mL three-neck round-bottom glass flask fitted with a stirrer, condenser, and a dropping funnel. Then, the mixture was cooled to 0°C stirring. Later on, CCl₄ (21.3 mL, 0.22 mol) was added dropwise to the mixture while keeping the reaction temperature below 15°C. After the complete addition, the solution was allowed to come at room temperature and continued stirring overnight. Finally, the desired product (Yield 85%; mp: 104 to 108°C) was extracted with dichloromethane (25 × 2 mL), evaporated under reduced pressure, and dried with sodium sulfate (Na₂SO₄). The synthesis route of DOPO-QN is depicted in Scheme 1.

Scheme 1.

Synthesis route of DOPO-QN.

Preparation of EP thermosets

EP thermosets were prepared by mixing DGEBA, DDS, DOPO, DOPO-QN, and NC as per the proportions given in Table 1. The mixture was heated under constant stirring at 120°C. Then the mixture was cooled to 100°C and a stoichiometric amount of DDS (curing agent) was added. The whole mixture was stirred continuously until it became homogeneous. The melted homogeneous mixture was poured into a pre-heated mold and degassed for 30 min in a vacuum oven at 130°C and then post-cured in a hot air oven at 150°C and 170°C for 2 h each. After the completion of the curing process, the samples were cooled to room temperature.

Table 1.

Proportions of EP thermosets.

Sample	DGEBA (g)	DDS (g)	DOPO-QN (g)	DOPO (g)	NC (g)	P Content (wt%)
EP	40.62	14.46	-	-	-	0.00
EP/DOPO(1.5%P)	36.32	12.93	-	5.75	-	1.50
EP/DOPO-QN (1%P)	35.85	12.76	6.37	-	-	1.00
EP/DOPO-QN (1.25%P)	34.69	12.35	7.96	-	-	1.25
EP/DOPO-QN (1.5%P)	33.50	11.93	9.56	-	-	1.50
EP/DOPO-QN (1%P)/NC	35.05	12.47	6.37	-	1.10	1.00
EP/DOPO-QN (1.25%P)/NC	33.90	12.06	7.96	-	1.10	1.25
EP/DOPO-QN (1.5%P)/NC	32.67	11.64	9.56	-	1.10	1.50

Characterization methods

Fourier transform infrared spectrum (FTIR) were recorded in the range 4000-500 cm⁻¹ by mixing the sample with KBr powder on a Perkin Elmer Spectrum, BX II, FTIR spectrophotometer.

The ¹H, ¹³C and ³¹P NMR spectra were recorded on Bruker AVANCE III instrument at 400 MHz, 101 MHz and 161 MHz, respectively using deuterated chloroform as solvent (CDCl₃). Coupling constants values were recorded in Hertz (Hz) and the multiplicity of signals was assigned as singlet (s), doublet of doublet (dd), doublet of doublets of doublets (ddd), triplet (t), and multiplet (m).

The high-resolution mass spectrum of the sample (DOPO-QN) was measured on SCIEX TripleTOF 5600 mass spectrometer.

The completion of the reaction and purity of the synthesized compound DOPO-QN were monitored by thin-layer chromatography using silica gel plates.

Thermogravimetric analysis (TGA) was recorded on a simultaneous thermal analyser (STA) 6000 (Perkin Elmer) at a heating rate of 10°C per minute from ambient temperature to 700°C under nitrogen and air atmosphere, respectively.

UL-94 vertical burning test and limiting oxygen index (LOI) tests were used to evaluate the flame retardancy of samples. LOI values of samples with size 130.0 × 6.5 × 3 mm³ were measured according to ASTM D-2863. UL-94 test was performed with samples of size 130.0 × 13.0 × 3.0 mm³ according to ASTM D3801.

A cone calorimeter device is used to determine the combustion behavior of samples with a three-dimensional size of 100.0 × 100.0 × 3.0 mm³ exposed to a radiant cone at a heat flux of 35 kW/m² according to ISO 5660-1.

X-Ray diffraction (XRD) studies of the epoxy/clay nanocomposites were carried out using Anton Paar SAXS point 2.0 Small angle X-ray Scattering diffractometer with a sample size of 10 × 10 × 1 mm³, in the range of 1–30° on the 2θ scale.

A JEOL JEM 2100 plus High-resolution transmission electron microscope working at 200 kV is used to investigate the degree of dispersion (intercalation or exfoliation) of the EP thermosets.

Dynamic mechanical analysis (DMA) was carried out using DMA 800 Perkin Elmer instrument. The dimensions of the samples taken were 40 × 10 × 2 mm³. The samples were tested in a three-point bending mode with a frequency of one Hz at a heating rate of 10°C per minute from room temperature up to 250°C.

Results and discussion

Characterization of DOPO-QN

The chemical structure of DOPO-QN was characterized by FTIR, ¹H NMR, ¹³C NMR, ³¹P NMR, and HRMS. The FTIR spectra of DOPO and DOPO-QN are depicted in Figure 2. The characteristic peak of P-H of DOPO was observed at 2386 cm⁻¹ in the FTIR spectrum, which was not observed in the case of DOPO-QN. The ¹H, ¹³C, ³¹P NMR and HRMS spectra of DOPO-QN are depicted in Figure 3. In the ¹H NMR spectrum, the chemical shift of all the aromatic protons appeared in the region δ 8.79–7.22 ppm. In the ¹³C NMR spectrum, the peaks appeared in the region δ 150.19–120.41 ppm. In the ³¹P NMR spectrum, a singlet due to phosphorus was observed at δ 7.94 ppm. Further, in the HRMS spectrum [m/z + H⁺], the peak of the compound DOPO-QN was found at 360.1267, which is in good agreement with the theoretically predicted value of 360.0791. From the above results, it is evident that DOPO-QN was successfully synthesized.

Figure 2.

FTIR spectra of DOPO and DOPO-QN.

Figure 3.

¹H-NMR (a), ¹³C-NMR (b), ³¹P-NMR (c) and HRMS (d) spectra of DOPO-QN.

Flame retardancy of EP thermosets

The flame retardancy of EP thermosets was assessed by UL-94 vertical burning and LO tests. The results are displayed in Table 2. UL-94 vertical burning is studied via self-extinguishing and classified as V-0, V-1, and V-2 ratings depending on the burning, dripping, and burning times. The LOI is the minimum concentration of oxygen (vol%) that is sufficient to sustain a stable combustion of a polymer sample. Table 2 indicated that pure epoxy (EP) is highly flammable as per the LOI value of 18% and has no rating on the UL-94 test. LOI value of EP/DOPO-QN (1%P) elevated from 18 to 23%. Further increase in phosphorus content to 1.5% in EP/DOPO-QN (1.5%P) sample increased LOI value to 25.1% whereas, EP/DOPO(1.5%P) gave a V-1 rating on UL-94 and an LOI value of 23%. Hence, DOPO-QN has better flame retardancy for EP than DOPO. The addition of NC resulted in a little increase in the LOI value of EP/DOPO-QN (1.5%P)/NC to 26.5%.

Table 2.

LOI and UL-94 rating data of EP thermosets.

Sample	LOI (%)	UL-94 rating	Dripping
EP	18.0	NR*, BC	Yes
EP/DOPO(1.5%P)	23.0	V-1	No
EP/DOPO-QN (1%P)	23.0	V-1	No
EP/DOPO-QN (1.25%P)	23.5	V-1	No
EP/DOPO-QN (1.5%P)	25.1	V-0	No
EP/DOPO-QN (1%P)/NC	24.1	V-1	No
EP/DOPO-QN (1.25%P)/NC	25.0	V-1	No
EP/DOPO-QN (1.5%P)/NC	26.5	V-0	No

*NR = no rating, BC = burn up to clamp.

UL-94 vertical burning test, gave no rating for pure EP, while the EP/DOPO-QN (1.5%P) sample having 1.5% phosphorus passed the UL-94 test with the highest V-0 rating. Whereas, no effect of the addition of nanoclay in thermosets is observed in the UL-94 test.

Cone calorimeter is the most effective method to identify the flammability behavior of epoxy thermosets. The cone calorimeter test was carried out for only three samples based on their performance in UL-94 and LOI tests. The parameters like peak heat release rate (PHRR), total heat release (THR), average effective heat of combustion (av EHC), the maximum average rate of heat emission (MARHE), total smoke production (TSP), CO₂ yield and CO yield are listed in Table 3. The pure EP burns readily and releases a large amount of heat as evidenced by a PHRR value of 408.1 kW/m². For EP/DOPO-QN (1.5%P) sample, the PHRR decreased to 377.8 kW/m². The yields of carbon dioxide and carbon monoxide decreased to 2668.33 mg/m³ and 25.81 mg/m³, respectively, on the incorporation of flame retardant having 1.5% phosphorus. The addition of nanoclay (NC) was observed very effective in reducing the toxic gases (CO₂ and CO) due to the formation of nanocomposites, which acted as a barrier for gas escape.

Table 3.

Cone calorimeter results of synthesized EP thermosets.

Sample	PHRR kW/m2	THR MJ/m2	Av EHC MJ/kg	MARHE kW/m2	TSP M−2	CO2 Yield mg/m3	CO yield mg/m3
EP	408.1	45.9	9.1	89.2	40.2	2680.3	26.7
EP/DOPO-QN (1.5%P)	377.8	43.5	8.7	80.4	36.7	2668.3	25.8
EP/DOPO-QN (1.5%P)/NC	387.2	40.3	8.0	76.7	39.4	2531.1	23.3

Thermal analysis

Thermogravimetric analysis (TGA) was conducted in nitrogen and air atmospheres to investigate the thermal stability and decomposition behavior of EP thermosets. The TGA and DTG thermograms of DOPO-QN and three EP thermosets as representatives are shown in Figure 4. The temperature of 5% weight loss (T_5%) defined as the initial degradation temperature, the temperature of maximum decomposition rate (DTG peak, T_max), and char yield at 600°C was noted from these thermograms and summarised in Table 4. In the nitrogen atmosphere, a one-stage decomposition curve was obtained for the EP thermosets. The initial degradation temperature of T_5% for pure EP was observed at 306°C, while T_5% in the case of EP/DOPO-QN (1.5%P) and EP/DOPO-QN (1.5%P)/NC decreased by about 20°C due to less thermal stable DOPO-QN as observed in TGA thermogram in Figure 4. Whereas the presence of nano clay showed no effect on initial degradation temperature. Similarly, the T_max for EP/DOPO-QN (1.5%P) and EP/DOPO-QN (1.5%P)/NC decreased by 45–50°C, respectively due to interaction between EP matrix and phosphaphenanthrene group of DOPO-QN. The char yield at 600°C of EP/DOPO-QN (1.5%P) raised to double in percent in comparison to pure EP due to the condensed phase action of the DOPO-QN and thermal stable quinoline group.

Figure 4.

TGA and DTG curves of DOPO-QN and various EP thermosets under nitrogen (a, b) and air (c, d) atmospheres.

Table 4.

TGA data of DOPO-QN and synthesized EP thermosets.

Sample	T_5% (°C) N₂ air		T_max (°C) N₂ air		Char (wt%) N₂ air
EP	306	281	412	348, 557	10.6	0.00
DOPO-QN	174	151	316	296, 573	36.2	23.1
EP/DOPO-QN (1.5%P)	287	252	368	316, 568	19.9	0.00
EP/DOPO-QN (1.5%P)/NC	286	285	360	307, 562	22.9	1.10

In the air atmosphere, a two-stage decomposition was found. The first stage from 220-385°C corresponds to the decomposition of the epoxy chain, while the second stage from 400-650°C corresponds to the degradation and complete oxidation of the left residue.³⁵ In pure EP, T_5% was observed at 281°C with T_max at 348°C and 557°C, respectively. In the air atmosphere, the residue left after the first stage degradation of EP matrix shows some stability up to about 500°C due to the formation of stable bonds of phosphorylated moiety before it is completely oxidized and evaporated leading to zero char residue except the sample containing thermal stable nano clay residue.

XRD analysis

X-Ray diffraction analysis was conducted to identify the dispersion of NC in the EP thermoset. The XRD spectra of the samples were shown in Figure 5. The interlayer spacing of nano clay was obtained from Bragg’s law ( $n λ = 2 d \sin θ$ ) in 2θ range of 1–30°. The characteristic diffraction peak of NC was found at 2θ = 4.67° (d₀₀₁ = 1.89 nm). This characteristic diffraction was found to disappear in EP/DOPO-QN (1.5%P)/NC sample. This might be due to shifting of the peak below 2θ = 1°, suggesting intercalation or exfoliation which was beyond the limit of the used XRD instrument or due to less presence of clay content in the analyzed sample. Therefore, the dispersion (intercalation or exfoliation) of nanoclay in the EP matrix was studied by TEM.

Figure 5.

XRD spectra of NC and EP/DOPO-QN (1.5%P)/NC in 2θ range (a) 1–5° and (b) 5–30°.

TEM analysis

TEM analysis of nanoclay (NC) and EP-DOPO-QN (1.5%P)/NC was conducted using an accelerated voltage of 200 kV to investigate the dispersion of nanoclay. The dark lines represented nano clay layers, which were seen in Figure 6. The TEM micrographs of NC as depicted in Figure 6 (a, b, c) showed the presence of stacks of silicate layers. While TEM micrograph of EP-DOPO-QN (1.5%P)/NC depicted in Figure 6 (d, e, f) showed dispersion of nanoclay in the epoxy matrix in the form of intercalation shown in Figure 6 (f) at I and single layer exfoliation at II to some extent.

Figure 6.

TEM micrographs of NC (a, b, c) and EP/DOPO-QN (1.5%P)/NC (d, e, f).

Thermomechanical properties

Thermomechanical properties of various EP thermosets were studied by DMA. The thermomechanical spectra of cured EP thermosets for the storage modulus (Eʹ), loss modulus (Eʹʹ), and tan δ as a function of temperature were displayed in Figure 7. The glass transition temperature, storage modulus, loss modulus, and crosslinking density were summarised in Table 5. The glass transition temperature of the EP and various samples was determined by the peak of tan δ value. Generally, the storage modulus value was used to measure the rigidity of materials.³⁶ The data suggests that EP/DOPO-QN (1.5%P) and EP/DOPO-QN (1.5%P)/NC had higher Eʹ values than EP in the temperature range up to 100°C (Figure 7(a)). At room temperature (30°C) EP/DOPO-QN (1.5%P) had an Eʹ value of 3417.7 MPa, a 49% higher than EP (2295.9 MPa). The EP/DOPO-QN (1.5%P)/NC had shown more higher Eʹ value of 13,469.2 MPa. The increased Eʹ value confirmed that EP/DOPO-QN (1.5%P) was more rigid than EP because of the rigid ring structure of DOPO-QN. The rigidity of the EP/DOPO-QN (1.5%P)/NC sample increased further due to the large surface area provided by rigid nano clay for interfacial interaction. In addition, the loss modulus spectra of the EP thermosets as shown in Figure 7 (b), there was no significant variation in the Eʹʹ value of EP and EP/DOPO-QN (1.5%P) (45 MPa) in the glassy state. However, EP/DOPO-QN (1.5%P)/NC showed a five times higher loss modulus than that of EP in the glassy state. A higher Eʹʹ value near room temperature means higher damping energy dissipation, which will result in the transformation of likely damageable vibration energy to heat. Therefore, EP/DOPO-QN (1.5%P)/NC was very promising for use as a hard thermoset with a high modulus and damping. As the temperature increased, the Eʹ value of samples started decreasing due to increased chain mobility. EP/DOPO-QN (1.5%P) sample gave a higher Eʹ value than EP till 133°C thereafter the value decreased due to lower crosslink density. The crosslink density of the samples was estimated by the following equation.³⁷

E = 3 R T_{r} v_{e}

(2)

Where E represents storage modulus at T_r taken as 30°C above the glass transition temperature. R is the ideal gas constant and ν_e is the crosslink density. The calculated results were shown in Table 5. The crosslink density of EP/DOPO-QN (1.5%P) is 1125.0 mol m⁻³ which was found to be lower than EP (2229.2 mol m⁻³). Usually, the glass transition temperature decreased with a decrease in crosslink density, which was consistent with the T_g measured by DMA (186.8°C vs 167.4°C). However, a remarkable increase in crosslink density was observed for EP/DOPO-QN (1.5%P)/NC, while the two networks showed very close glass transition temperatures (186.8°C vs 175.8°C).

Figure 7.

DMA spectra for (a) storage modulus, (b) loss modulus and (c) tan δ against temperature of EP thermosets.

Table 5.

DMA data of EP thermosets.

Sample	Tg (°C)	Eʹ at 30°C (MPa)	Eʹʹ at 30°C (MPa)	ν_e (mol m⁻³)
EP	186.8	2295.9	45.0	2229.2
EP/DOPO-QN (1.5%P)	167.4	3417.7	44.5	1125.0
EP/DOPO-QN (1.5%P)/NC	175.8	13,469.2	286.0	9144.0

Conclusions

In the present work, a P/N-containing flame retardant (DOPO-QN) was synthesized by a reaction between phosphaphenanthrene and 8-hydroxyquinoline which is used as an additive type flame retardant in EP. The flammability tests (LOI, UL-94 and cone calorimeter) results showed that the addition of DOPO-QN with 1.5% phosphorus had imparted flame retardancy to the EP matrix with a decrease in toxic gas emissions. Thermogravimetric analysis (TGA) of epoxy thermosets showed an increase in thermal stability at a higher temperature range on the formation of nanocomposites. The glass transition temperature of EP/DOPO-QN (1.5%P) (167.4°C) decreased with a decrease in crosslink density in comparison to EP (186.8°C). A remarkable improvement in storage modulus and crosslink density was observed with the addition of 2.0% nano clay for the EP/DOPO-QN (1.5%P)/NC sample.

Supplemental Material

Supplemental Material - Synthesis and characterization of a novel P/N containing flame retardant and its effect on flame-retardancy, thermal and mechanical properties of epoxy/clay nanocomposites

Supplemental Material for Synthesis and characterization of a novel P/N containing flame retardant and its effect on flame-retardancy, thermal and mechanical properties of epoxy/clay nanocomposites by Vishal Soni and Jai Bhagwan Dahiya in High Performance Polymers

Footnotes

Acknowledgements

This work was supported by CSIR, New Delhi, India as a junior/senior research fellowship to one of the authors (Mr Vishal Soni).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Jai Bhagwan Dahiya

Supplemental Material

Supplemental material for this article is available online.

References

Shieh

Wang

. Synthesis of novel flame retardant epoxy hardeners and properties of cured products. Polymer 2001; 42: 7617–7625. DOI: 10.1016/s0032-3861(01)00257-9

Wang

Shi

. Kinetics study of thermal decomposition of epoxy resins containing flame retardant components. Polym Degrad Stab 2006; 91: 1747–1754. DOI: 10.1016/j.polymdegradstab.2005.11.018

Song

, et al. Synthesis and characterization of a functional polyhedral oligomeric silsesquioxane and its flame retardancy in epoxy resin. Prog Org Coat 2009; 65: 490–497. DOI: 10.1016/j.porgcoat.2009.04.008

Nakamura

Yamaguchi

Okubo

, et al. Effects of particle size on mechanical and impact properties of epoxy resin filled with spherical silica. J Appl Polym Sci 1992; 45: 1281–1289. DOI: 10.1002/app.1992.070450716

Deng

Shi

Liu

, et al. Solidifying process and flame retardancy of epoxy resin cured with boron-containing phenolic resin. Appl Surf Sci 2018; 427: 894–904. DOI: 10.1016/j.apsusc.2017.07.278

Jangra

Dahiya

. Effects of nanoclay and flame retardant additives on glass transition temperature, thermal stability and flammability of epoxy composites. Mater Res Innov 2018; 22: 387–395. DOI: 10.1080/14328917.2017.1325059

Lin

Chang

, et al. Benzoxazines with tolyl, p-hydroxyphenyl or p-carboxyphenyl linkage and the structure–property relationship of resulting thermosets. Polymer 2009; 50: 2264–2272. DOI: 10.1016/j.polymer.2009.02.042

Sponton

Mercado

Ronda

, et al. Preparation, thermal properties and flame retardancy of phosphorus-and silicon-containing epoxy resins. Polym Degrad Stab 2008; 93: 2025–2031. DOI: 10.1016/j.polymdegradstab.2008.02.014

Ekpe

Choo

Barcelo

, et al. Introduction of emerging halogenated flame retardants in the environment. Compr Anal Chem 2020; 88: 1–39. DOI: 10.1016/bs.coac.2019.11.002

10.

Shaw

Blum

Weber

, et al.

Halogenated flame retardants: do the fire safety benefits justify the risks?

Rev Environ Health 2010; 25: 261–305.

11.

Zhang

Wang

, et al. Highly effective P-P synergy of a novel DOPO-based flame retardant for epoxy resin. Ind Eng Chem Res 2017; 56: 1245–1255. DOI: 10.1021/acs.iecr.6b04292

12.

Green

. A review of phosphorus-containing flame retardants. J Fire Sci 1992; 10: 470–487. DOI: 10.1177/073490419201000602

13.

Ren

Sun

, et al. Synthesis and properties of a phosphorus-containing flame retardant epoxy resin based on bis-phenoxy (3-hydroxy) phenyl phosphine oxide. Polym Degrad Stab 2007; 92: 956–961. DOI: 10.1016/j.polymdegradstab.2007.03.006

14.

Unlu

Dogan

. Comparative study of boron compounds and aluminum trihydroxide as flame retardant additives in epoxy resin. Polym Adv Technol 2014; 25: 769–776. DOI: 10.1002/pat.3274

15.

Martin

Lligadas

Ronda

, et al. Synthesis of novel boron-containing epoxy–novolac resins and properties of cured products. J Polym Sci A Polym Chem 2006; 44: 6332–6344. DOI: 10.1002/pola.21726

16.

Zang

Wagner

Ciesielski

, et al. Novel star‐shaped and hyperbranched phosphorus‐containing flame retardants in epoxy resins. Polym Adv Technol 2011; 22: 1182–1191. DOI: 10.1002/pat.1990

17.

Qiu

Qian

. Flame-retardant effect of a novel phosphaphenanthrene/triazine-trione bi-group compound on an epoxy thermoset and its pyrolysis behaviour. RSC Adv 2016; 6: 56018–56027. DOI: 10.1039/c6ra10752d

18.

Leu

Wang

. Synergistic effect of a phosphorus–nitrogen flame retardant on engineering plastics. J Appl Polym Sci 2004; 92: 410–417. DOI: 10.1002/app.13689

19.

Sudhakara

Kannan

Obireddy

, et al. Organophosphorus and DGEBA resins containing clay nanocomposites: flame retardant, thermal, and mechanical properties. J Mater Sci 2011; 46: 2778–2788. DOI: 10.1007/s10853-010-5152-6

20.

Nazare

Kandola

Horrocks

. Flame‐retardant unsaturated polyester resin incorporating nanoclays. Polym Adv Technol 2006; 17: 294–303. DOI: 10.1002/pat.687

21.

Liu

Qiu

Qian

, et al. Strengthen flame retardancy of epoxy thermoset by montmorillonite particles adhering phosphorus‐containing fragments. J Appl Polym Sci 2020; 137: 47500. DOI: 10.1002/app.47500

22.

Rajaei

Kim

Bickerton

, et al. A comparative study on effects of natural and synthesised nano-clays on the fire and mechanical properties of epoxy composites. Composites Part B: Engineering 2019; 165: 65–74. DOI: 10.1016/j.compositesb.2018.11.089

23.

Zhang

Jiang

Sun

, et al. Preparation of flame retardant and conductive epoxy resin composites by incorporating functionalized multi‐walled carbon nanotubes and graphite sheets. Polym Adv Technol 2021; 32: 2093–2101. DOI: 10.1002/pat.5239

24.

Liu

Zhao

Gao

, et al. Recent advances in the flame retardancy role of graphene and its derivatives in epoxy resin materials. Composites Part A: Applied Science and Manufacturing 2021; 149: 106539. DOI: 10.1016/j.compositesa.2021.106539

25.

Kiliaris

Papaspyrides

. Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy. Prog Polym Sci 2010; 35: 902–958. DOI: 10.1016/j.progpolymsci.2010.03.001

26.

Gao

Yao

, et al. Improving flame retardancy of linear low-density polyethylene/nylon 6 blends via controlling localization of clay and intumescent flame-retardant. Polym Degrad Stab 2018; 153: 75–87. DOI: 10.1016/j.polymdegradstab.2018.04.022

27.

Kong

Zhang

, et al. Improving flame retardancy of IFR/PP composites through the synergistic effect of organic montmorillonite intercalation cobalt hydroxides modified by acidified chitosan. Appl Clay Sci 2017; 146: 230–237. DOI: 10.1016/j.clay.2017.05.048

28.

Zhu

Zhou

Kabwe

, et al. Exfoliation of montmorillonite and related properties of clay/polymer nanocomposites. Appl Clay Sci 2019; 169: 48–66. DOI: 10.1016/j.clay.2018.12.006

29.

Zhang

Liao

, et al. Structure and properties of poly (oxypropylene) diamine intercalated montmorillonite/epoxy composites. Journal of Macromolecular Science Part B 2019; 58: 877–889. DOI: 10.1080/00222348.2019.1662989

30.

Zhang

Yang

. The characterization of DOPO/MMT nanocompound and its effect on flame retardancy of epoxy resin. Composites Part A: Applied Science and Manufacturing 2017; 98: 124–135. DOI: 10.1016/j.compositesa.2017.03.020

31.

Yan

Zhang

, et al. Enhanced flame retardancy of epoxy resin containing a phenethyl-bridged DOPO derivative/montmorillonite compound. J Fire Sci 2018; 36: 47–62. DOI: 10.1177/0734904117740820

32.

Wang

Kan

, et al. Two high-efficient DOPO-based phosphonamidate flame retardants for transparent epoxy resin. High Perform Polym 2019; 31: 249–260. DOI: 10.1177/0954008318762037

33.

Peng

Nie

, et al. A tetra-DOPO derivative as highly efficient flame retardant for epoxy resins. Polym Degrad Stab 2021; 193: 109715. DOI: 10.1016/j.polymdegradstab.2021.109715

34.

Le Corre

Couthon-Gourvès

Berchel

, et al. Atherton–Todd reaction: mechanism, scope and applications. Beilstein J Org Chem 2014; 10: 1166–1196. DOI: 10.3762/bjoc.10.117

35.

Wang

Song

, et al. Thermal degradation mechanism of flame retarded epoxy resins with a DOPO-substitued organophosphorus oligomer by TG-FTIR and DP-MS. J Anal Appl Pyrolysis 2011; 92: 164–170. DOI: 10.1016/j.jaap.2011.05.006

36.

Yamunadevi

Palaniradja

Thiagarajan

, et al. Characterization and dynamic mechanical analysis of woven roven glass fiber/cerium-zirconium oxide epoxy nanocomposite materials. Mater Res Express 2019; 6: 095057. DOI: 10.1088/2053-1591/ab2f64

37.

Wan

Gan

, et al. A novel biobased epoxy resin with high mechanical stiffness and low flammability: synthesis, characterization and properties. J Mater Chem A 2015; 3: 21907–21921. DOI: 10.1039/c5ta02939b

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

0.00 MB

0.81 MB