Synergistic Flame Retardant Effect of an Intumescent Flame Retardant Containing Boron and Magnesium Hydroxide.

In this study, to develop an organic/inorganic synergistic flame retardant and to reduce the dosage and cost of flame retardants, organic/inorganic synergistic flame retardants, hexakis(4-boronic acid-phenoxy)-cyclophosphazene (CP-6B), and magnesium hydroxide (MH) were chosen. The flame retardant properties of CP-6B/MH in epoxy resin (EP) were discussed. EP/CP-6B/MH had better flame retardancy and heat resistance compared with EP/CP-6B and EP/MH. A limiting oxygen index of EP/3.0%CP-6B/0.5%MH of 31.9% was achieved, and vertical burning V-0 rating was achieved. Compared with EP, the cone calorimeter dates of EP/CP-6B/MH decreased. CP-6B/MH inhibited combustion and did little to damage mechanical properties. Besides, the flame retardant mechanism was studied by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and pyrolysis-gas chromatography-mass spectrometry. CP-6B/MH exerted good synergistic effects.

Introduction

Traditional halogen-containing fire retardants are widely used due to their good compatibility with materials and high flame retardancy.[1−3] However, halogen-containing flame retardants release a large number of toxic substances and corrosive smoke during combustion.[4−6] Halogen-free, environmentally friendly, low-toxic, and low-cost flame retardants are the focus of flame retardant research.[7−12]

Intumescent flame retardants (IFRs) are currently the research hotspots in the field of flame retardancy.[13−20] In general, IFR systems are mainly composed of three parts.[21−23] The acid source produces acidic substances, which can catalyze the dehydration and carbonization of carbon sources; the carbon source produces a highly stable char layer under acid catalysis to protect internal resin; the decomposition of the gas source releases non-flammable gas, dilutes oxygen and volatiles, decreases heat conductivity, and enhances insulation effects during burning.[24,25] However, to obtain good flame-retardant properties, a very large amount of traditional IFR is used in the substrate, which impair the mechanical properties of the material.[26,27]

An inorganic flame retardant is a halogen-free fire retardant with low toxicity, low smoke, good thermal stability, and abundant resources.[28−32] As a typical inorganic flame retardant, magnesium hydroxide (MH) is preferred in the industry.[33,34] The amount of MH added typically exceeds 50%; the polymer has flame retardant property, which makes it poorly compatible with the polymer and reduces the polymer’s mechanical properties.[35−37]

Most of IFRs are mainly composed of phosphorus and nitrogen.[38−40] In our previous study, organic boron IFRs [2,4,6-tris(4-boronic-2-thiophene)-1,3,5-triazine (3TT-3BA); 2,4,6-tris(4-boron-phenoxy)-(1,3,5)-triazine (TNB); and hexakis(4-boronic acid-phenoxy)-cyclophosphazene (CP-6B)] were synthesized.[41−44] 3TT-3BA, TNB, and CP-6B have good flame retardancy properties. Organic boron flame retardants can form a covering carbon layer containing boron to isolate volatiles and air.[41−46] It is possible for synergistic flame retardancy of IFR and MH to enhance the flame retardancy and decrease the amount of MH and IFR added; thereby, the mechanical properties of the material are not affected. Many literature studies have reported that the combination of MH and other IFRs containing phosphorus and nitrogen has successfully improved the flame retardancy.[27,36,37,47−50] However, the total amount of flame retardants still needs to be reduced, and the report on the combination of MH and organic boron IFRs was not many.

Epoxy resin (EP) is widely used engineering material. However, EP is easily combustible, which limits its wider application.[11,12,51−55] To develop organic/inorganic IFRs, EP was used as substrate material in this article. The synergistic flame retardant effect of CP-6B/MH on EP was studied, and the flame retardancy mechanism was discussed.

Results and Discussion

Thermogravimetry and Differential Thermogravimetry Analyses

Figure shows the thermogravimetry (TG) (a) and differential thermogravimetry analyses (DTG) (b) curves of the EP samples. Their characteristic thermal performance data are shown in Table S1 (Supporting Information). The residual char yield of pure EP was the lowest. Compared to pure EP, the degradation of EP/3%CP-6B was advanced because CP-6B decomposed to form a protective carbon layer. EP/3%CP-6B exhibited two decomposition peaks. The first stage reflected the main decomposition process in which CP-6B decomposed in advance to form the char layer containing the B–O–C structure. In the second stage, the char layer was further degraded to form a high stability char layer. The carbon residue of EP/3%CP-6B at 800 °C was 18.0%. When MH was added to EP/3% CP-6B, the initial decomposition temperature of EP/CP-6B/MH increased. Therefore, MH could improve the heat resistance of CP-6B. The thermal degradation of EP/CP-6B/MH had only one decomposition stage, and the residual char yield of EP/3%CP-6B/0.5%MH at 800 °C (W800°C) reached 21.8%. This result showed the formation of a stable carbon layer containing MgO when CP-6B and MH were added together. This layer protected the composite from further degradation and enhanced the heat stability of the resin. CP-6B and MH demonstrated good synergistic flame retardant effects.

Figure 1

(a) TG and (b) DTG curves for EP samples.

(a) TG and (b) pan class="Chemical">DTG curves for pan class="Chemical">EP samples.

Limiting Oxygen Index and Vertical Burning (UL 94)

Table shows the limiting oxygen index (LOI) and UL 94 rating of the EP samples. When adding 3 wt % MH and 3 wt % CP-6B to EP separately, the LOI of the EP sample increased from 22.8 to 25.2 and 30.8%, respectively. When MH and CP-6B were added together, EP/CP-6B/MH samples exhibited good flame retardant properties. The LOI of EP/3%CP-6B/0.5%MH attained 31.9%, and the UL 94 combustion grade reached V-0 without dripping. This result was ascribed to the synergistic effect of CP-6B and MH. CP-6B catalyzed EP to generate an intumescent char layer, and MH released water to form MgO. The char layer containing MgO could protect internal EP and prevent the formation of dripping. Simultaneously, the decomposition of MH produced water vapor, and the decomposition of CP-6B released nonflammable gases, which could dilute flammable volatiles and air.

Table 1

LOI and UL 94 Data of EP Samples

sample	LOI (%)	UL 94 (3.2 mm)a	dripping
EP	22.8	NR	yes
EP/3%MH	25.2	NR	no
EP/3%CP-6B	30.8	V-0	no
EP/3%CP-6B/0.5%MH	31.9	V-0	no
EP/3%CP-6B/1.0%MH	31.0	V-0	no
EP/3%CP-6B/1.5%MH	31.5	V-1	no

NR = no rating.

Cone Calorimeter Tests

Cone calorimeter can evaluate the combustion properties of pan class="Disease">flame retardant materials. The relevant combustion parameters of pan class="Chemical">EP samples are summarized in Table .

Table 2

Cone Calorimeter Data of EP Samples

sample	pk-HRR (kW/m2)	THR (MJ/m2)	av-EHC (MJ/kg)	FGI (kW/(m2·K))	av-MLR (g/s)	residual char yield (%)
EP	1091	83	24.0	12.8	0.76	6.8
EP/3%MH	751	80	23.3	8.8	0.75	7.8
EP/3%CP-6B	608	71	21.0	6.1	0.53	14.6
EP/3%CP-6B/0.5%MH	535	67	19.9	4.1	0.34	17.2

The effective heat combustion (EHC) represents the combustion intensity of volatile gases in the gas phase. Table shows the average EHC (av-EHC) of the EP samples. The av-EHC of EP/CP-6B and EP/MH were both lower than that of pure EP. With the addition of 3 wt % MH, the av-EHC of EP sample decreased from 24.0 to 23.3 MJ/kg. With the addition of 3 wt % CP-6B, the av-EHC of EP sample decreased from 24.0 to 21.0 MJ/kg. When 3 wt % CP-6B and 0.5 wt % MH were added simultaneously, the av-EHC of EP/3%CP-6B/0.5%MH was only 19.9 MJ/kg. The decomposition of CP-6B and MH generated a large amount of nonflammable gases and water vapor, which could reduce the contact of flammable volatiles with air. CP-6B promoted EP to produce an intumescent carbon layer. MH decomposed endothermically and released water to form MgO. The char layer-containing MgO inhibited internal resin decomposition. Table shows the fire growth index (FGI) of EP samples. FGI can assess the fire hazard of substrates. The FGI values of EP/3%MH and EP/3%CP-6B were 68.8 and 47.7% of that of pure EP, respectively. When 3 wt % CP-6B and 0.5 wt % MH were added together, the FGI of EP/3%CP-6B/0.5%MH was only about 32.0% of that of pure EP, thereby indicating that CP-6B/MH could control the spread of fire and reduce the fire intensity.

Figure shows the heat release rate (HRR) and total heat release (THR) curves of EP samples. The peak HRR (pk-HRR) of pure EP, EP/3%MH, EP/3%CP-6B, and EP/3%CP-6B/0.5%MH were 1091, 751, 608, and 535 kW/m2, respectively. The THR of pure EP, EP/3%MH, EP/3%CP-6B, and EP/3%CP-6B/0.5%MH were 83, 80, 71, and 67 MJ/m2, respectively. Figure shows the mass loss curves of EP samples. When 3 wt % CP-6B and 0.5 wt % MH were simultaneously added, the av-MLR of EP/3%CP-6B/0.5%MH was the lowest, and the residual char yield of EP/3%CP-6B/0.5%MH was the highest. These observations were due to the fact that the char layer-containing MgO and the nonflammable gases exerted a protective effect on EP.

Figure 2

(a) HRR and (b) THR curves of EP samples.

Figure 3

Mass loss curves from cone calorimetry tests for EP samples.

(a) HRR and (b) pan class="Chemical">THR curves of pan class="Chemical">EP samples.

Mass loss curves from cone calorimetry tests for pan class="Chemical">EP sampn>les.

Figure shows the digital photos after the cone calorimetry tests. Pure EP was almost decompn>osed compn>letely. When 3 wt % MH was added, a small amount of char for EP/3%MH was observed, but most of the residue was MgO powder. With the addition of 3 wt % CP-6B, the char of EP/3%CP-6B was intumescent and hard, but there were a few small holes. After adding 3 wt % CP-6B and 0.5 wt % MH together, the char of EP/3%CP-6B/0.5%MH was continuous intumescent char layer, and the surface was surrounded by MgO coating. The protective layer formed a barrier to isolate oxygen and prevent heat exchange. Thus, EP/3%CP-6B/0.5%MH showed good flame retardancy.

Figure 4

Digital photos of residual chars from (a) pure EP, (b) EP/3%MH, (c) EP/3%CP-6B, and (d) EP/3%CP-6B/0.5%MH.

Scanning Electron Microscopy

Figure shows the scanning electron microscopy (SEM) images of the residual of EP/3%MH and EP/3%CP-6B/0.5%MH after cone calorimeter tests. The residual char of pure EP[44] had a lot of holes, and this carbon layer had no protective effect on EP. With the addition of 3 wt % MH, the residual char of EP/3%MH still contained large holes. With the addition of 3 wt % CP-6B, the residual char of EP/3%CP-6B[44] was compact with only a few holes. However, when 3 wt % CP-6B and 0.5 wt % MH were added together, the residual char of EP/3%CP-6B/0.5%MH was a continuous carbon layer without holes and cracks. The continuous char layer could prevent air from entering the internal resin and slow the exchange of combustion heat. Therefore, EP/3%CP-6B/0.5%MH showed better flame retardancy than the other samples. CP-6B and MH demonstrated good synergistic flame retardant effects.

Figure 5

SEM images of residual chars from (a) EP/3%MH (×200) and (b) EP/3%CP-6B/0.5%MH (×200).

Energy Dispersive X-ray Spectroscopy

The elemental analysis data of residual sample were shown in Table S2 (Supporting Information); EP/3%n>an class="Chemical">MH residual char showed higher oxygen and magnesium contents than pure EP residual char. These finding showed that the char layer contained MgO. EP/3%CP-6B showed higher phosphorus, nitrogen, and oxygen compared with pure EP, thereby indicating that the char layer contained phosphorus, nitrogen, and oxygen. When 3 wt % CP-6B and 0.5 wt % MH were added together, EP/3%CP-6B/0.5%MH showed higher oxygen, magnesium, phosphorus, and nitrogen contents compared with pure EP sample. EP/3%CP-6B/0.5%MH formed a more stable carbon layer during combustion, which had higher heat and oxidation resistance.

Morphologies of EP Samples at Different Temperatures

Figure shows the photographs of EP sampn>les that were stored at various tempn>eratures for 15 min in a muffle furnace. Pure EP had changed color at 250 °C, and little residual char was observed in the crucible at 650 °C. The change trend of EP/3%MH was basically the same as that of pure EP. The final carbon residue from EP/3%MH was only 0.61 wt %, which contained white MgO powder. This finding showed that the flame retardancy of MH was low, and low loads of MH did not result in good flame retardancy. The EP/3%CP-6B formed a continuous carbon layer at 400 °C, and the carbon residue yield of EP/3%CP-6B at 650 °C was 1.56 wt %. When 3 wt % CP-6B and 0.5 wt % MH were added together, EP/3%CP-6B/0.5%MH still retained its shape at 450 °C and generated a continuous carbon layer at 500 °C. The carbon residue yield of EP/3%CP-6B/0.5%MH at 650 °C was 3.99 wt %. The EP/3%CP-6B/0.5% MH had the highest thermal stability, and CP-6B and MH exerted a synergistic flame retardant effect.

Figure 6

Digital photographs of EP samples after storage at various temperatures for 15 min in a muffle furnace: (a) pure EP, (b) EP/3%MH, (c) EP/3%CP-6B, and (d) EP/3%CP-6B/0.5%MH.

Digital photographs of EP sampn>les after storage at various tempn>eratures for 15 min in a muffle furnace: (a) pure EP, (b) EP/3%MH, (c) EP/3%CP-6B, and (d) EP/3%CP-6B/0.5%MH.

Fourier Transform Infrared Spectroscopy

Figure shows the Fourier transform infrared spectroscopy (FTIR) spectra of residual chars. The decomposition of pure EP was extremely thorough.[44] When 3 wt % MH was added, EP was also decomposed thoroughly. However, many infrared absorption peaks were found from residual EP/3%CP-6B and residual EP/3%CP-6B/0.5%MH. These peaks revealed that some structures were still retained after EP/3%CP-6B/0.5%MH decomposed.

Figure 7

FTIR spectra of residual chars: (a) EP/3%MH, (b) EP/3%CP-6B, and (c) EP/3%CP-6B/0.5%MH.

X-ray Diffraction

In a previous study,[44] CP-6B was found to form intumescent char layers containing BPO4 when added to EP. Figure shows the X-ray diffraction (XRD) pattern of the residual EP/3%CP-6B/0.5%MH. The residual EP/3%CP-6B/0.5%MH had a diffraction peaks at 2θ = 24.5°, which was BPO4. The residual EP/3%CP-6B/0.5%MH also contained MgO, and its corresponding diffraction peaks was at 2θ = 42.9° and 62.3°. These results indicated that residual EP/3%CP-6B/0.5%MH contained BPO4 and MgO.

Figure 8

XRD patterns of residual char of EP/3%CP-6B/0.5%MH.

Pyrolysis–Gas Chromatography–Mass Spectrometry

Figure is the PY–GC–MS curves of pure EP[44] and EP/3%CP-6B/0.5%MH at 450 (a) and 700 °C (b). The structures corresponding to the peaks in the mass spectrum of EP/3%CP-6B/0.5%MH are shown in Table S3 (Supporting Information). After the polymer material was ignited, the chain reaction occurred in the gas phase. During the combustion process, the EP polymer chain was broken, and a large amount of active radicals H• and •OH were generated. These active radicals continued to promote the chain scission of the EP molecular chain and accelerate decomposition. Therefore, the amount and intensity of pure EP vapor volatiles were very high. The small molecule radicals generated by the decomposition of CP-6B could capture H• and •OH to form stable amines, alcohols, and phenols; and MgO could also adsorb free radicals. Therefore, the amount and intensity of EP/3%CP-6B/0.5%MH vapor volatiles were much lower than pure EP.

Figure 9

PY–GC–MS of EP and EP/3%CP-6B/0.5%MH at (a) 450 and (b) 700 °C.

Flame Retardant Mechanism

Figure is the flame retardant model of EP sample. When only MH was added, it decomposed endothermically, released water and generated MgO. MgO could form a physical protective layer of MgO residue through physical accumulation. However, the continuous decomposition of the internal substrate, which released heat and volatile gases, caused the external MgO carbon layer to crack. These cracks led to the reduction in heat insulation and oxygen isolation efficiency of the carbon layer. Thus, MH required a high amount of load to achieve intense flame retardancy. When only CP-6B was added, it could promote the formation of shrinkable intumescent carbon layers; moreover, no cracks were formed, but several small holes were observed. By contrast, when CP-6B and MH were added together, CP-6B first catalyzed the formation of a shrinkable carbon layer, and MgO produced by MH decomposition was dispersed, forming a compact and continuous intumescent char layer without cracks and holes. Therefore, better flame retardant properties were obtained with the synergistic effect of CP-6B and MH.

Figure 10

Flame retardant model of CP-6B and MH, (a) EP/MH, (b) EP/CP-6B, and (c) EP/CP-6B/MH.

Mechanical Property

Table shows the mechanical properties of EP sampn>les. With the addition of 3 wt % CP-6B, the impact energy and impact strength of EP sample increased. EP was a thermosetting resin with high strength and easy to crack under stress. CP-6B could absorb impact energy, adding a small amount of CP-6B could improve the toughness of EP sample. When 3 wt % MH was added, the impact energy of EP sample decreased from 0.31 to 0.24 J, and the impact strength of EP sample decreased from 7.7 to 6.1 kJ·m–2. This phenomenon indicated that MH would damage the mechanical properties of EP materials. The impact energy of EP/3%CP-6B/0.5%MH was 0.39 J, and the impact strength of EP/3%CP-6B/0.5%MH was 9.7 kJ·m–2. Therefore, CP-6B and a small amount of MH could effectively suppress combustion without decreasing the mechanical strength.

Table 3

Mechanical Properties of EP Samples

samples	impact energy (J)	impact strength (kJ·m–2)
EP	0.31	7.7
EP/3%MH	0.24	6.1
EP/3%CP-6B	0.55	13.8
EP/3%CP-6B/0.5%MH	0.39	9.7
EP/3%CP-6B/1.0%MH	0.32	7.9
EP/3%CP-6B/1.5%MH	0.30	7.6

Conclusions

To develop organic/inorganic IFRs and reduce the dosage and cost of flame retardants, MH and an organic compound containing phosphorus, nitrogen, and boron, namely, CP-6B, were selected. The decomposition of CP-6B generated shrinkable intumescent char layers and nonflammable gases. MH could decompose endothermically and release water, and it formed a physical protective layer of MgO residue. Results showed that when CP-6B and MH were added together, the flame retardancy of EP was better than when only one of them was added. The results demonstrated that the flame retardant properties of CP-6B and MH induced good synergistic effects.

Experimental Section

Materials

EP (E-44) was purchased from Xiya Reagent Co., Ltd., China. Diaminodiphenylmethane (DDM) was obtained from Shanghai Macklin Biochemical Technology Co., Ltd. MH was supplied by Sinopharm Chemical Reagent Co., Ltd. CP-6B was synthesized by our laboratory.[44]

Characterization

TG analysis was tested using a Netzsch 209 F3 thermal analyzer (Selb, Germany) under a nitrogen atmosphere, and the heating rate was 10 °C/min. The LOI was tested using an oxygen index tester (FTT, the United Kingdom), in accordance with ASTM D2863 (80 × 10 × 4 mm3). Vertical burning tests were tested using a vertical burning tester (FTT, the United Kingdom), in accordance with ASTM 3801 (125 × 12.7 × 3.2 mm3). The UL 94 grades were divided into NR, V-2, V-1, and V-0. Cone calorimeter tests were tested using a cone calorimeter (FTT, the United Kingdom) following the ISO-5660 guidelines (100 × 100 × 3 mm3), and the incident flux was 50 kW/m2.

The morphology of the residual chars was observed by SEM (Carl Zeiss, Germany). Elemental analysis of the residual chars was tested by energy dispersive X-ray spectroscopy (Carl Zeiss, Germany). XRD was analyzed using a pan class="Chemical">MERCURY CCD X-ray diffractometer (D/max-III, Japn>an). FTIR was tested using a Nicolet 6700 FTIR spectrometer (WI, USA). PY–GC–MS was tested using a GCMS-QP 2010 plus pyrolysis-gas chromatograpn>hy mass spectrometer (Japn>an).

The impact test was tested by a ZCJ 1320 cantilever beam impact testing machine, and the impact test sample dimension was 80 × 10 × 4 mm3.

Preparation of EP Samples

In this work, the CP-6B prepared in our laboratories was also used.[44]Table lists the composition of the EP samples. The mixture was stirred uniformly at 80 °C according to the ratio, then poured into the mold to cure, and cured at 80, 120, and 150 °C for 2 h, respectively.

Table 4

Composition of the EP Samples

samples	E-44 (g)	DDM (g)	CP-6B (g)	MH (g)
EP	100	25	0	0
EP/3%MH	100	25	0	3.750
EP/3%CP-6B	100	25	3.75	0
EP/3%CP-6B/0.5%MH	100	25	3.75	0.625
EP/3%CP-6B/1.0%MH	100	25	3.75	1.250
EP/3%CP-6B/1.5%MH	100	25	3.75	1.875