Synergistic effect of montmorillonite and nano-Sb 2 O 3 /brominated polystyrene on the flame retardancy of polypropylene matrix composites

Abstract

Sb₂O₃ nanoparticles (nano-Sb₂O₃), montmorillonite (MMT) and brominated polystyrene (BPS) were used to enhance flame retardancy of polypropylene (PP) in the work, in which nano-Sb₂O₃ was modified by cetyl trimethyl ammonium bromide and polyethylene glycol and MMT was modified by silane coupling agent. PP matrix flame retardant composites were prepared by melt blending after pre-mixing by high energy ball milling technique. Then, the flame retardancy, thermal stability behaviour, crystallization performance and tensile strength of PP matrix composites (nano-Sb₂O₃/MMT/BPS-PP, nano-Sb₂O₃/BPS-PP) were investigated. The results show that MMT can significantly improve the thermal stability and flame retardancy of PP matrix due to improving the strength of char layer and forming barrier effect. Compared with 3.5%nano-Sb₂O₃/8%BPS-PP composites, the limiting oxygen index of 3.5%nano-Sb₂O₃/3%MMT/8%BPS-PP composites was increased from 26.9% to 29.0% and its UL94 grade was achieved to V-0 level. Moreover, MMT and nano-Sb₂O₃ can increase the crystallinity and tensile strength of PP matrix composites because of the heterogeneous nucleation effect of MMT and nano-Sb₂O₃ particles on the crystallization of PP matrix.

Keywords

Flame retardancy polypropylene montmorillonite brominated polystyrene

Introduction

Polypropylene (PP) is widely used in automobiles, textiles, electrical insulation, architectural materials, interior decorations and so on with high-cost performance, excellent mechanical and physicochemical properties.^1,2 However, PP is inflammable and may be dangerous closing to fair. It may also enhance fair propagation due to its dripping tendency during combustion. The application of PP in many fields is severely limited due to its flammability and dripping behaviours.³ Thus, the flame retardancy of PP has to be improved. A lot of work has been done in the past several decades to overcome the shortcoming of PP and some progresses have been achieved. The flame retardancy of PP is usually improved by adding flame retardants into PP or grafting flame retardant elements onto PP. The flame retardants include halogen, phosphorus, silicon, etc. As the synergistic system of brominated flame retardants and Sb₂O₃ (Br-Sb system) is considered as a more effective flame retardant system, its use in polymer materials can prevent oxygen, heat, flammable materials and thermal degradation reactions, thus achieving flame retardant effect.^4,5 For instance, The poly (pentabromobenzyl acrylate), Sb₂O₃ and modified magnesium hydroxide are compounded with PP to prepare flame retardant PP matrix composites. The results show that the PP matrix composites had excellent thermal stability and flame retardancy.⁶ In the Br-Sb system, the dispersion of Sb₂O₃ particles has a significant effect on the flame retardancy of polymer materials. Some research results show that the surface modification of Sb₂O₃ particles can effectively improve its dispersion in polymer materials and improve flame retardancy.^7,8

Brominated polystyrene (BPS) is a new high-temperature brominated flame retardant with higher bromine content, higher flame-retardant efficiency and higher molecular weight. In particular, BPS is non-toxic and is difficultly absorbed by the organism. Materials containing BPS does not produce dioxins during burning, which complies with EU RoHS directive and EPA regulations. The compounded flame retardant composed of BPS and Sb₂O₃ has good flame retardant efficiency due to the synergistic effect of Br-Sb system. The compounded flame retardant has been well applied in PBT and PET and has broad application prospects.⁹

The flame retardancy of PP is still realized by adding convention halogen flame retardant and Sb₂O₃. However, the kind of flame retardant will release a lot of black smoke, toxic gases and corrosive smoke during combustion, which will cause serious environmental pollution.¹⁰ In order to improve the environmental friendliness of flame retardant PP, nano-Sb₂O₃/MMT/BPS-PP and nano-Sb₂O₃/BPS-PP composites were prepared, respectively, by using Sb₂O₃ nanoparticles (nano-Sb₂O₃), BPS and montmorillonite (MMT) as flame retardant additives in the paper. And the effects of Br-Sb and MMT on flame retardancy, thermal stability behaviour, crystallization performance and tensile strength of PP matrix composites were studied.

Experimental

Materials

Silane-modified MMT with 15 μm of average particle size was supplied by Sunda Chemical Co. Ltd. of China. Nano-Sb₂O₃ modified by Polyethylene glycol (PEG-4000) and cetyl trimethyl ammonium bromide (CTAB) with 50 nm of average particle size was made by our laboratory.¹¹ The main reagents used in the preparation process were PEG-4000, CTAB, hydrogen peroxide (H₂O₂, 30% of concentration) and distilled water that were all analytically pure. PP was purchased from Shihua Petrochemical Co. Ltd. of China. BPS containing more than 68.7% of bromine content was supplied by Qingdao Uchem Co. Ltd. of China.

Sample preparation

Firstly, BPS, MMT, modified nano-Sb₂O₃ and PP were mixed by QM-3SP04 ball miller (Nanjing Laibu Science and Technology Industry Co. Ltd., China) at 300 r/min of rotation speed for about 6 h. Then, melt blending of the mixed composites powder was completed by SJZS-10A twin-screw extruder (Wuhan Ruiming Machinery Manufacture Co., Ltd., China) with the temperatures of zone 1, zone 2, zone 3 and zone 4 of the extruder corresponding to 174°C, 191°C, 200°C and 205°C, respectively. Finally, the extruded materials were moulded to test specimens by SZS-20 injection mould machine (Wuhan Ruiming Machinery Manufacture Co. Ltd., China). The compositions of experimental materials are listed in Table 1.

Table 1.

The composition of experimental materials.

Samples No	Sample name	Component (wt%)
Samples No	Sample name	Nano-Sb₂O₃	MMT	BPS	PP
1	PP	0	0	0	100
2	1.5%nano-Sb₂O₃/6%BPS-PP	1.5	0	6	92.5
3	2%nano-Sb₂O₃/6%BPS-PP	2	0	6	92
4	3%nano-Sb₂O₃/6%BPS-PP	3	0	6	91
5	3.5%nano-Sb₂O₃/6%BPS-PP	3.5	0	6	90.5
6	1.5%nano-Sb₂O₃/8%BPS-PP	1.5	0	8	90.5
7	2%nano-Sb₂O₃/8%BPS-PP	2	0	8	90
8	3%nano-Sb₂O₃/8%BPS-PP	3	0	8	89
9	3.5%nano-Sb₂O₃/8%BPS-PP	3.5	0	8	88.5
10	1.5%nano-Sb₂O₃/3%MMT/6%BPS-PP	1.5	3	6	89.5
11	2%nano-Sb₂O₃/3%MMT/6%BPS-PP	2	3	6	89
12	3%nano-Sb₂O₃/3%MMT/6%BPS-PP	3	3	6	88
13	3.5%nano-Sb₂O₃/3%MMT/6%BPS-PP	3.5	3	6	87.5
14	1.5%nano-Sb₂O₃/3%MMT/8%BPS-PP	1.5	3	8	87.5
15	2%nano-Sb₂O₃/3%MMT/8%BPS-PP	2	3	8	87
16	3%nano-Sb₂O₃/3%MMT/8%BPS-PP	3	3	8	86
17	3.5%nano-Sb₂O₃/3%MMT/8%BPS-PP	3.5	3	8	85.5

Characterization

Flame retardancy

The limiting oxygen index (LOI) was analyzed by PX-01-005 limited oxygen index tester (Suzhou Phinix Quality Inspection Instrument Co. Ltd. of China) according to GB/T2406.2-2009. The UL94 grade was performed by STD-94 vertical burning tester (Shanghai Pan Standard Textile Testing Technology Co. Ltd. of China) according to GB/T2408-2008. The Catalytic Effectivity (CAT-EFF) of nano-Sb₂O₃ for PP matrix composites is defined as a variation of LOI with 1% mass fraction of metal ion at the optimal concentration of the catalyst.¹² The value is calculated by equation (1)

CAT EFF = \frac{Δ LOI}{{wt}_{M}} %

(1)

where ΔLOI is a variation of LOI and wt_M% is mass fraction of metal ion.

The flame retardant efficiency (EFF) and the synergistic efficiency (SE) are calculated by the following equations¹³

EFF = \frac{Δ LOI}{FR} (%)

(2)

where ΔLOI is a difference value of composites LOI and matrix LOI and FR is the content of flame retardant. The higher value of EFF indicates good efficiency of the flame retardant

SE = \frac{{EFF}_{1}}{{EFF}_{2}}

(3)

where EFF₁ is the flame retardant efficiency of the synergistic system and EFF₂ is the flame retardant efficiency of the single flame retardant. Similar to EFF, SE can be used to compare two flame retardants or amongst more.

Thermal gravimetric analysis

Thermal decomposition of PP matrix composites was investigated by HS-TGA-101 thermal gravimetric analyzer (Shanghai Heson Instrument Co. Ltd. of China). The sample mass was approximately 10 mg. Thermal gravimetric analysis (TGA) testing was performed at 10 °C/min of heating rate in the temperature range of 20 ∼ 600°C in nitrogen gas.

Scanning electron microscopy

The surface morphology of the char layers of experimental materials after vertically burning was observed by JSM-6700F SEM (Japan Electron Optics Laboratory Co. Ltd.).

X-ray diffraction

X-ray diffraction (XRD) patterns of samples were recorded at room temperature by Ultima IV XRD (Rigaku Corporation of Japan) with 2θ diffraction angle range from 2° to 10° at 2°/min of scanning speed. The value of interplanar distance was calculated by Bragg’s law¹⁴

2 dsin θ = λ

(4)

The crystallite sizes of nano-Sb₂O₃ and MMT were calculated from full width at the half-maximum (FWHM) by Scherrer’s formula¹⁵

D = \frac{0.89 k}{bcos θ}

(5)

where k is the wave length (Cu Ka), b is the FWHM of XRD peaks and θ is the diffraction angle.

Differential scanning calorimetry

The crystallization and melting behaviours of samples were investigated by using a differential scanning calorimetry (QT-DSC-500C, Shanghai Qiantong Instruments Co. Ltd. of China). Samples of about 15 mg were encapsulated in aluminium pans and heated from room temperature to 200°C, and kept isothermal for 5 min to erase the thermal history, then cooled rapidly to room temperature and subsequently heated again to 200°C. Heating rate is 10 °C/min as well as cooling rate.

The melting temperature (T_m) of the samples was determined from the maxima of the fusion peaks. The crystallinity (X_c) of experiment materials was calculated by equation (6)¹⁶

X_{c} = \frac{Δ H_{c}}{W_{i} × Δ H_{0}}

(6)

where ΔH_c is the crystallization enthalpy, and W_i is the mass fraction of PP in experiment materials. ΔH₀ is the crystallization enthalpy of 100% crystalline polymer, and the value of pure PP is 187.7J/g.¹⁷

Mechanical properties

The tensile tests of samples were carried out by tensile tester (HS-100KN, Yangzhou Huahui Testing Instrument Co. Ltd. of China) with a crosshead speed of 20 mm/min. The results were the average values of at least five specimens.

Results and discussion

LOI and UL94 analysis

The LOI values and the UL94 grade of the experimental materials are listed in Table 2. Initially, the LOI values of nano-Sb₂O₃/BPS-PP composites increase with increasing of the content of nano-Sb₂O₃. When the content of Br-Sb reaches 11.5 wt%, the LOI value of the 3.5%nano-Sb₂O₃/8%BPS-PP composites increases from 17.4 to 26.9%. It might be that nano-Sb₂O₃ has two effects on the flame retardant PP matrix composites. On the one hand, nano-Sb₂O₃ can react with BPS to form the gas phase flame retardant system. On the other hand, nano-Sb₂O₃ can catalyse the reaction between Br and active radical (H and OH·). With increasing of the content of nano-Sb₂O₃, the concentration of gaseous SbBr₃ produced by the reaction between BPS and nano-Sb₂O₃ increases, resulting in improving the difficulty of reaction between flammable gas and air.¹⁸ Subsequently, the LOI value of the PP matrix composites increases from 26.9% (9#) to 29.0% (17#) when MMT with content of 3 wt% is added. MMT can effectively improve the flame retardancy of PP matrix composites, which the UL94 grade achieves V-0 level. It attributes to the self-extinguishing ability of MMT. In addition, MMT can promote the char residues density and play the role of flame retardant of condensed-phase.¹³

Table 2.

Flame retardancy of the experimental materials.

Samples No	1	2	3	4	5	6	7	8	9	10
UL94	V-2	V-2	V-2	V-2	V-1	V-2	V-2	V-1	V-0	V-2
LOI(%)	17.4	21.4	22.3	24.2	25.2	22.6	23.2	26.3	26.9	23.1
ΔLOI	0	3.7	4.9	6.8	7.8	4.3	5.8	8.9	9.5	5.7
Samples No	11	12	13	14	15	16	17	—	—	—
UL94	V-2	V-1	V-0	V-2	V-1	V-0	V-0	—	—	—
LOI(%)	23.9	25.8	28.2	24.3	25.2	27.5	29.0	—	—	—
ΔLOI	6.5	8.4	10.8	6.9	7.8	10.1	11.6	—	—	—

The values of EFF, CAT-EFF and SE are listed in Table 3. It can be seen that nano-Sb₂O₃ is a high-efficiency flame retardant for PP. As the content of nano-Sb₂O₃ increases, the CAT-EFF values of nano-Sb₂O₃/BPS-PP composites increase firstly and then decrease. Though, nano-Sb₂O₃/BPS-PP composites containing 3.5 wt% of nano-Sb₂O₃ has the highest LOI value, and nano-Sb₂O₃/BPS-PP composites containing 3 wt% of nano-Sb₂O₃ has the highest CAT-EFF value. The result indicates that nano-Sb₂O₃ (3 wt%) has an optimal synergistic effect for the nano-Sb₂O₃/BPS-PP composites with 8 wt% of BPS. Moreover, there is a good synergy between MMT and Br-Sb. This may be attribute to two reactions, namely, the vapour phase reaction of Br-Sb and the condensed phase reaction of MMT. MMT and Br-Sb play a good synergistic flame retardant effect in PP, which can enhance the flame retardancy of PP compared with the single Br-Sb flame retardant system in PP. Therefore, MMT-Br-Sb is a superior flame retardant system.

Table 3.

Synergistic flame retardant efficiency between MMT and Br-Sb.

Samples No	W(Br)/%	ΔLOI	EFF	CAT-EFF(Sb₂O₃)	SE (Sb₂O₃)	SE (MMT)
1	0	—	—	—	—	—
6	5.496	4.3	0.78	2.87	—	—
7	5.496	5.8	1.06	2.90	1.35	—
8	5.496	8.9	1.58	2.97	2.02	—
9	5.496	9.5	1.73	2.71	2.21	—
17	5.496	11.6	2.11	—	—	1.22

The synergistic flame retardancy effect of Br-Sb system is attributed to the chemical reaction of Sb₂O₃ and gas phase HBr, which can product SbBr₃ and water. The reaction is as follows

{Sb}_{2} O_{3} (s) + 6HBr (g) \to {2 SbBr}_{3} {(g) + 3 H}_{2} O (g)

(7)

When the mole ratio of bromine atoms to antimony atoms is 3:1, the synergies effect is the best. It can be calculated as follows¹⁹

m_{1} \times \frac{W_{1}}{M_{1}} {= 3 m}_{2} \times \frac{W_{2}}{M_{2}}

(8)

where M₁ is the relative atomic mass of bromine atoms, and its value is 79.904. W₁ is the mass fraction of Br in the BPS. M₂ is the relative atomic mass of Sb, and its value is 112.760. And W₂ is the mass fraction of antimony atoms in the Sb₂O₃, and its value is82.451%. Then, these above values are substituted in equation (8), the mass ratio of BPS to Sb₂O₃ is 2.55:1 accompanying by optimum synergistic efficiency of Br-Sb system composites. The mass ratio of BPS to Sb₂O₃ of sample 8# is 2.66:1, which is close to the optimal mass ratio. The value of SE of sample 8# is 2.02, which is 49.62% higher than that of sample 7#.

Thermal gravimetric analysis

The effects of nano-Sb₂O₃ and MMT on the thermal stability of the experimental materials were studied by TGA under nitrogen atmosphere. Figure 1 shows the TGA curves of the experimental PP and PP matrix composites and Table 4 lists the characteristic parameters of theses TGA curves. In Table 4, T_1%, T_5% and T_50% correspond to the temperatures when thermal mass loss is 1 wt%, 2 wt% and 3 wt%, respectively. The thermal degradation of PP takes place through one-step process corresponding to random C-C bond scission mechanism.²⁰ The decomposition temperature of the experimental nano-Sb₂O₃/BPS-PP composites is lower than that of pure PP due to advance decomposition of BPS. Adding MMT particles, the decomposition temperature of the experimental nano-Sb₂O₃/MMT/BPS-PP composites is increased, which is assigned to tight combining between PP and MMT and thereby limiting the mobility of polymer chains.²¹ In addition, MMT can increase T_50% of decomposition temperature of PP because MMT can promote the formation of a complete and solid char residue layer on the surface of PP matrix composites, which the amount of char residue of the experimental composites increases with increasing of content of MMT. In Table 4, the char residue of 3.5%nano-Sb₂O₃/3%MMT/8%BPS-PP composites (sample 17#) is 9.24 wt%, 2%nano-Sb₂O₃/8%BPS-PP composites (sample 7#) is 1.52 wt% and 1.5%nano-Sb₂O₃/3%MMT/8%BPS-PP composites (sample 14#) is 2.17 wt%. It shows that not only MMT can increase char residue, but also nano-Sb₂O₃ can improve thermal stability of PP.

Figure 1.

Thermal gravimetric analysis curves of PP and PP matrix composites.

Table 4.

Thermal degradation data of PP and PP matrix composites.

Samples No	T1%/°C	T5%/°C	T50/°C	Residue (500°C)/%
1#	314.5	368.6	427.8	0
7#	286.7	308.2	411.7	1.52
9#	287.8	314.6	413.4	2.53
14#	289.8	366.3	441.8	2.17
15#	336.6	364.5	442.2	3.17
16#	354.9	364.7	436.3	5.98
17#	354.4	364.7	436.9	9.24

Morphology of char residues

The Scanning electron microscopy (SEM) images of the char residues of the experimental PP matrix composites under vertical burning are presented in Figure 2. Figures 2(a) and (b) show SEM images of the char residues of sample 7# and sample 9#, respectively. It can be seen from Figure 2(a) that there are lots of holes and voids on the surface of char residues, which means that the volatilization rate of the surface of PP matrix in combustion is very fast. What’s more, the surface of the char residues of sample 7# easily damaged due to their weak loose structure, which could not effectively protect the PP matrix against fire. Figure 2(b) shows that the surface of the char residues of sample 9# is continuous and compact. When the content of nano-Sb₂O₃ is higher, nano-Sb₂O₃ can react with BPS to form a more complete and firm char layer. Nano-Sb₂O₃ can promote the char layer formation and the layer can insulate the heat transfer, thus improving the flame retardancy of the experimental composites and reducing the heat release rate of the matrix.

Figure 2.

Scanning electron microscopy images of char layers after vertical burning (a)7#, (b)9#, (c)15#, (d)17#, (e)15#.

Scanning electron microscopy images of the char residues of sample 15# and sample 17# are presented in Figures 2(c) and (d), respectively. Figure 2(e) is a magnified image of C1 region of sample 15#. MMT can easily form a peculiar structure of char layer during burning, namely, cortex-honeycomb structure of the layer.²² The upper layer of the char layer is a relative dense cortex. And the lower layer is a honeycomb layer, which is char residue structure with more gaps accumulated by some aggregates of large size of layered char residues. It can be seen from Figures 2 (c) and (d) that the lamellar aggregates of MMT in the cortex presents a three-dimensional network structure with higher mechanical strength, which can protect the inner materials and avoid the damage of char residue under thermal action. At the same time, the cortex has better heat insulation and gas-resisting, and then effectively relieves the decomposition of inner material. In Figure 2(e), the wall of the honeycomb layer is formed by the aggregation of MMT, resulting in forming a honeycomb char layer as a whole. It can improve the thickness of char layer and then effectively play the role of flame retardant layer. Therefore, MMT can improve flame retardancy of nano-Sb₂O₃/MMT/BPS-PP composites by means of char residue layer.

Melting and crystallization behaviour

X-ray diffraction analysis

Figure 3 shows the XRD patterns of pure PP, MMT and PP matrix composites. The grain size of nano-Sb₂O₃ was calculated by Scherrer’s formula, expressed as equation (5). The grain size of nano-Sb₂O₃ on (222) crystal face is about 46.7 nm and that of MMT on (001) crystal face is about 3.7 nm. The interplanar distance of MMT is about 2.1 nm at 4.22° of 2θ calculated by Bragg’s equation. For 3%nano-Sb₂O₃/3%MMT/8%BPS-PP composites, there is not a diffraction peak of MMT in range of 2θ from 2° to 35°, which indicates that the intercalation structure of MMT exfoliates and inserts into PP matrix. It also shows that the MMT presents exfoliated structure in nano-Sb₂O₃/MMT/BPS-PP system composites prepared by mechanical mixing. In the XRD patterns of nano-Sb₂O₃/BPS-PP composites, it is clearly seen that the pattern still has both of diffraction peaks of pure PP and Sb₂O₃ nanoparticles. It indicates that the crystal structure of PP and Sb₂O₃ nanoparticles do not change after being prepared into the experimental composites.

Figure 3.

X-ray diffraction patterns of PP, MMT and PP matrix composites.

Differential scanning calorimetry analysis

Figure 4 shows the Differential scanning calorimetry (DSC) crystallization and melting curves of pure PP and the experimental PP matrix composites and Table 5 lists the corresponding DSC characteristic data. It can be seen that the T_m,T_c and X_c of PP matrix composites are higher than those of pure PP. It shows that the nano-Sb₂O₃ particles with high dispersibility act as nucleating agents. However, the dispersed inorganic particles with high concentration will prevent to form large crystalline region due to a large number of nucleation points and result in limiting the movement of polymer chains.²³ For the sample 8# and sample 9#, the crystallinity of PP matrix of the composites decreases sharply with increasing of the content of nano-Sb₂O₃. When the content of nano-Sb₂O₃ is higher, nano-Sb₂O₃ particles can easily agglomerate, which can reduce the number of nucleation points. So, the crystallinity increases firstly and then decreases with increasing of the content of nano-Sb₂O₃. For nano-Sb₂O₃/MMT/BPS-PP system composites, MMT with 3 wt% of content can increase sharply the crystallinity of PP matrix of the composites because MMT causes an increase of its nucleation substrate.

Figure 4.

Differential scanning calorimetry of pure PP and PP matrix composites (a) Crystallization curve, (b) Melting curves.

Table 5.

DSC data of PP and PP matrix composites.

Samples No	Tm (°C)	Tc (°C)	ΔHm (J/g)	ΔHc(J/g)	Xc (%)
1#	160.85	118.35	47.43	58.23	31.02
7#	166.13	122.19	57.64	74.35	45.27
8#	166.17	123.75	59.15	76.40	47.33
9#	166.36	124.84	51.71	71.29	43.66
17#	166.41	125.62	54.34	80.02	49.86

DSC: Differential scanning calorimetry.

Tensile strength

Table 6 lists the tensile strength of the experimental materials. There is a better interfacial adhesion between the copolymer matrix and nano-Sb₂O₃ particles due to strong electrostatic interaction.²⁴ Excellent interface and lots of nano-Sb₂O₃ particles with better dispersion consequently can enhance mechanical strength, resulting in that the tensile strength of the experimental materials increases with increasing of the content of nano-Sb₂O₃. However, when the content of nano-Sb₂O₃ is more than 3 wt%, the tensile strength of PP matrix composites decreases due to agglomerate of nano-Sb₂O₃ particles. The incorporation of 3 wt% MMT within PP matrix increases the mechanical properties because MMT can exfoliate and form intercalation structure in PP matrix. The tensile strength of nano-Sb₂O₃/MMT/BPS-PP composites show an increase trend from 38.38 Mpa to 41.27 Mpa, which is 16.08%–25.17% higher than that of pure PP. In brief, the experimental composites have higher tensile strengths due to adding MMT and nano-Sb₂O₃ particle.

Table 6.

Tensile strength of PP and PP matrix composites.

Samples No	1	2	3	4	5	6	7	8	9	10
Tensile strength (Mpa)	32.97	37.74	38.78	40.38	38.91	37.04	37.62	38.53	37.95	38.38
Samples No	11	12	13	14	15	16	17	—	—	—
Tensile strength (Mpa)	40.36	40.87	39.42	38.27	39.69	41.27	39.15	—	—	—

Flame retardant mechanism

Figure 5 shows the flame retardant mechanism of the experimental flame retardant PP matrix composites. The Br-Sb system and MMT can play roles of flame retardant effects. For Br-Sb system, BPS plays two roles in flame retardancy of the experimental composites. Firstly, BPS can quench the active free-radicals. In the early stage of combustion, PP can decompose via β-scission and generate alkyl free-radicals under heat and oxygen.²⁵ Simultaneously, BPS can generate bromine radicals through thermal degradation and oxidizing reaction. The bromine radicals can react with H• and OH• free radicals, which can block the chain scission reaction and then prevent the heat feeding back to PP matrix.²⁶ Meanwhile, the incombustible gases (H₂O) can dilute the combustible gas and reduce the flame heat. Secondly, BPS and nano-Sb₂O₃ show good synergistic flame retardant effect in PP matrix composites during combustion. Nano-Sb₂O₃ reacts with the HBr generated by the degradation of BPS to form SbBr₃ and H₂O. Combining with the dilution effect of SbBr₃, the bromine radicals can effectively suppress the flame and even extinguish it. For MMT, it plays a role of condensed phase flame retardant in the experimental composites. MMT can improve the density of the char residues on the combustion surface, which can increase the isolation effect of the char layer and prevent the transmission of combustible gas, oxygen and heat. The formation of char layer makes the protective effect more effective and enhances the flame retardant effect of the condensed phase.

Figure 5.

Flame-retardant mechanism of PP matrix composites.

Conclusion

(1) Sb₂O₃ nanoparticles (nano-Sb₂O₃) and BPS have good synergistic flame retardant effect, which the maximum of SE is 1.82. Not only nano-Sb₂O₃ and BPS have gas phase flame retardant effect via reaction, but also the reaction product SbBr₃ has condensed phase flame retardant effect.

(2) Incorporation of MMT, nano-Sb₂O₃ and BPS within PP matrix has good flame retardant effect, which the maximum of flame retardant efficiency EFF of experimental materials achieves 2.11. Montmorillonite can not only improve the thermal stability of PP matrix composites, but also increase the density of char layer on the burning surface of the composites, so as to improve the flame retardant effect of condensed phase.

(3) Nano-Sb₂O₃ and MMT can improve the crystallinity of PP matrix composites due to their heterogeneous nucleation. And they can improve tensile strength of PP matrix composites to some extent because of dispersion strengthening effect of nano-Sb₂O₃ and intercalation structure formed by MMT and PP matrix.

(4) The 3.5%nano-Sb₂O₃/3%MMT/8%BPS-PP composites has excellent flame retardant properties and tensile strength, which its LOI is 29% and its UL94 grade reaches V-0 level and its tensile strength is 39.15 Mpa.

Footnotes

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (No. 51761025).

ORCID iD

Jianlin Xu

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