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Effect of Nanometer Antimony Trioxide on Properties of Flame Retardant Polypropylene Matrix Composites

Using antimony trioxide nanoparticles (nano-Sb2O3), montmorillonite (MMT) and brominated polystyrene (BPS) as additive and using polypropylene (PP) as matrix material, a novel nano-Sb2O3/MMT/BPS-PP flame retardant composite was synthesized via mechanical ball milling dispersion and melt blending method and characterized by X-ray diffraction, scanning electron microscopy, differential scanning calorimeter and tensile properties testing. The flame retardant performance of the composite was investigated by limiting oxygen index and vertical burning test (UL-94). The results show that the mechanical properties and flame retardant properties of the composites are improved compared to that of pure PP when PP matrix retardant composites are added with different mass fraction of nano-Sb2O3. When the mass fraction of nano-Sb2O3 is 3wt%, the PP matrix composite exhibits excellent tensile strength and crystallization properties. When the mass fraction of nano-Sb2O3 is more than 3wt%, the PP matrix composite exhibits excellent flame retardancy.


Introduction
With the rapid development of science and technology, polymer materials occupy an increasingly essential position in life and production. Polypropylene (PP) materials and their products are widely used in many fields due to their excellent properties of insulation, corrosion resistance and plasticity. However, PP materials and their products have some shortcomings such as poor flame retardancy, lower mechanical properties and low temperature, which limit the further development of PP matrix composites in engineering and life. Therefore, improving the flame retardant properties and related mechanical properties of PP materials and making PP more widely used in various fields have become an inevitable trend [1]. The antimony trioxide nanoparticles (nano-Sb 2 O 3 ) have an essential position in flame retardant materials because nano-Sb 2 O 3 has a good synergistic flame retardant effect with halogen flame retardants and organic flame retardants [2,3]. Since nano-Sb 2 O 3 particles have high surface activity, large specific surface area and high strength bonding with polymers, adding appropriate amount of nano-Sb 2 O 3 into the matrix material can not only maintain good flame retardancy, but also have little effect on the mechanical properties of the matrix material. Brominated polystyrene (BPS) is widely used in flame retardant engineering plastics as bromine organic flame retardant due to its high flame retardancy, thermal stability and light stability. BPS is made from  [4]. Montmorillonite (MMT) has also received extensive attention in the field of flame retardancy of polymers. Adding an appropriate amount of MMT to the composite material can improve the mechanical properties, flame retardancy and thermal stability of the material, which is becoming a hot spot of the flame retardant material with non-toxic and low-smoke [5]. On the one hand, the synergistic flame retardant effect of nano-Sb 2 O 3 and BPS will produce the effect of suffocating flame retardant and endothermic flame retardant [6]; On the other hand, MMT added into the matrix can improve to form a carbon layer on the surface of the substrate, which can owe forming a flame retardant effect to isolate the oxygen and heat [7]. Therefore, nano-Sb 2 O 3 /MMT/BPS-PP flame retardant composite material samples will be prepared in this work. The influences of different mass fraction of nano-Sb 2 O 3 on flame retardancy and mechanical properties of composites were researched.

Materials
MMT with 15 μm of particle size was provided by Guangzhou Yifeng Chemical Technology Co., Ltd. of China. Nano-Sb 2 O 3 were prepared and modified by ball milling, as reported in our study [8]. The particle size was 50-100nm. BPS with 1600 of average molecular weight was provided by Xingsheng Chemical Co., Ltd. of China. PP with 1 g/min of melt index was produced by China Petroleum & Chemical Corporation Beijing Yanshan Branch.

Sample Preparation
Firstly, PP powder, BPS powder, MMT powder and nano-Sb 2 O 3 particles were dispersed by QM-3SP04 high-energy ball milling (Nanjing Laibu Science and Technology Industry Co. Ltd. of China) with 400 r/min of ball grinding speed for about 6 hours. Secondly, PP matrix composites were prepared by melted blending basing on a twin screw extruder (SJZS-10A, Wuhan ruiming Plastic machinery Co. of China). Finally, the test specimens were moulded by an injection mould machine (SZS-20, Wuhan Ruiming Machinery Manufacture Co. Ltd. of China). The compositions of experimental materials are listed in Table 1. The crystallization and melting behaviours of samples were investigated by QT-DSC-500C differential scanning calorimeter (DSC, Shanghai Qiantong Instruments Co. Ltd. of China). Samples of about 15 mg were encapsulated in aluminium pans and heated from room temperature to 220 ℃, and kept thermal for 5 minutes to erase the thermal history, then cooled rapidly to room temperature and subsequently heated again to 200 ℃. Heating rate is 10℃/min as well as cooling rate.  Figure 1 shows the XRD spectra of pure PP and experimental composites with different content of nano-Sb 2 O 3 . PP is a polymer with higher crystallinity and contains five different crystal forms of α, β, γ, δ and pseudo hexagon in its crystal structure, in which α crystal is the most common form [9]. As can be seen from figure 1 that 2θ angles were 13. With increasing of the mass fraction of nano-Sb 2 O 3 , the diffraction intensity of the PP matrix composites first increases and then weakens. The heterogeneous nucleation of the matrix material caused by the nanoparticles played a leading role resulting in increasing the crystallinity of the PP matrix composites when the mass fraction of Sb 2 O 3 was less (<3wt%); While the mass fraction of Sb 2 O 3 was higher (>3wt%), the nanoparticles hindered the movement of the PP molecular chain during the cooling and crystallization process of the PP matrix composites, which caused the matrix material insufficiently crystallized [10,11].

Non-isothermal crystallization behavior
The DSC crystallization curve and melting curve of PP and PP matrix composites are shown in figure 2. It can be comprehended from the figure that the crystallization curve of the PP matrix composites was similar to that of pure PP, demonstrating that the addition of MMT and nano-Sb 2 O 3 did not change the crystal form of the PP matrix. The crystallization peak position of the PP matrix composites was obviously shifted to the right, but the change of the melting peak position was not obvious. It revealed that the addition of MMT and nano-Sb 2 O 3 played a role of heterogeneous nucleation effect, which increased the initial crystallization temperature and crystallization temperature of PP material, but the effect on melting temperature was not obvious.   Table 2 shows the non-isothermal crystallization parameters of PP and PP matrix composites. T p is the peak temperature of crystallization; T m is the peak temperature of melting peak; ∆T is the degree of supercooling, and the value is the difference between T m and T p , reflecting the crystallization rate. The smaller the value of ∆T, the greater the crystallization rate of the composite [12]; ∆H is crystallization enthalpy; X(t) is the degree of crystallinity. The crystallinity of the sample can be calculated by the following formula: where ∆H m is a sample melting enthalpy, and ∆H is a sample crystallization enthalpy. ∆ is the melting enthalpy when the PP crystallinity is 100%, and its value is 207.1 J/g [13,14]. The results of Table 2 show that the crystallization rate and crystallinity of PP matrix composites are significantly higher than that of pure PP. While the mass fraction of nano-Sb 2 O 3 increased, the crystallization rate of PP matrix composites became getting faster and the crystallinity showed a tendency to rise first and then decrease. Pure PP had homogeneous nucleation and heterogeneous nucleation in the crystallization process, while PP matrix composites were mainly heterogeneous nucleation. As we all known homogeneous nucleus has longer nucleation time than heterogeneous nucleus. With increasing of the mass fraction of nano-Sb 2 O 3 , MMT and nano-Sb 2 O 3 particles obstructed the movement of macromolecular chains in the process of cooling and crystallization of matrix material, which caused the molecular chain movement insufficient leading to the crystallinity increases first and then decreases [15]. This is consistent with the results of the above XRD analysis.

Tensile properties
The tensile strength and Young's modulus of PP and PP matrix composites are shown in figure 3. It can be known from the figure that the tensile strength of the PP matrix composites was significantly higher than that of the pure PP because the addition of the MMT and the nano-Sb 2 O 3 particles increased the tensile strength of the matrix material. First and foremost, the specific diameter of MMT was larger, which played a role in fiber reinforcement during the stretching process of the matrix material. Last but not least, the nano-Sb 2 O 3 particles were separated from the matrix material during the stretching process to produce voids, which were effective in preventing the diffusion of cracks and could absorb the tensile stress of the matrix material. This played a leading role in increasing the tensile strength of the matrix material. It can't ignored in figure 3 that the tensile strength of the PP matrix composites was different with different of the mass fraction of nano-Sb 2 O 3 . When the mass fraction of nano-Sb 2 O 3 was 3wt%, the tensile strength of 3.0%nano-Sb 2 O 3 /3%MMT/8%BPS-PP composites had a maximum value, which was 22.6% higher than that of pure PP. When the mass fraction of nano-Sb 2 O 3 was 3.5wt%, the tensile strength of 3.5%nano-Sb 2 O 3 /3%MMT/8%BPS-PP composites was slightly lower than that of the former. Similar to tensile strength, the Young's modulus value of the PP matrix composite material was also significantly higher than that of the pure PP, and the Young's modulus value of PP matrix composites reached a maximum value when the mass fraction of nano-Sb 2 O 3 was 3wt%. Figure 4 is tensile fracture SEM images of PP matrix composites containing nano-Sb 2 O 3 with 2.0wt% and 3.5wt% of mass fraction. As can be seen from the figure, a peeling phenomenon occurred between the nano-Sb 2 O 3 particles and the substrate under the action of the tensile force when the mass fraction of nano-Sb 2 O 3 was 2.0wt%. The appearance of voids could effectively alleviate the tensile stress on the matrix material, and could effectively prevent the crack from spreading. At this time, the tensile strength of the PP matrix composites was obviously increased. When the mass fraction of nano-Sb 2 O 3 was 3.5wt%, the agglomeration took place due the higher content of nano-Sb 2 O 3 with the higher surface activity of the nanoparticles. During the tensile process, the agglomeration of the nano-Sb 2 O 3 caused stress concentration and produces cracks in matrix material, resulting in decreasing of the tensile strength of the PP matrix composites [8,16].

Flame retardant performance
In the vertical burning test, the UL-94 grade of PP matrix composites containing MMT and nano-Sb 2 O 3 was significantly better than that of pure PP. For pure PP, it was flammable accompany with serious low drop phenomenon and had a large fire risk, which its UL-94 grade reached V-2 degree. When the mass fraction of the nano-Sb 2 O 3 was 1.5wt%, although the 1.5% nano-Sb 2 O 3 /3%MMT/8%BPS-PP composite had droplets in the vertical combustion, the burning time of the composite was reduced and its UL-94 grade reached V-2 degree. When the mass fraction of nano-Sb 2 O 3 was 2.0wt%, MMT and nano-Sb 2 O 3 could effectively suppress the droplets generation of the 2.0%nano-Sb 2 O 3 /3%MMT/ 8%BPS-PP, which the UL-94 grade reached V-1 degree. When the mass fraction of nano-Sb 2 O 3 was higher than 3.0wt%, the PP matrix composites was extinguished within 10s of the fire source, which their UL-94 grade reached V-0 degree without droplets.
The LOI values and UL-94 grades of PP and PP matrix composites are shown in Table 3. It can be seen from the table that the LOI value of the PP matrix composite material was significantly higher than that of pure PP. The LOI value of the PP matrix composites increased with the increase of the mass fraction of nano-Sb 2 O 3 . The reason is that the noncombustible gas generated by the BPS and nano-Sb 2 O 3 in the PP matrix composites had a suffocating flame retardant effect on the combustion. At the same time, nano-Sb 2 O 3 could catalyze BPS during combustion, which generated Br ions to react with active free radicals H + and OHions in the combustion process, and absorbed heat to play a flame retardant role [17,18]. Moreover, MMT has also improved the flame retardancy of PP matrix composites. MMT formed a high temperature resistant layer during combustion, and gradually moved to the surface layer of the combustion body to form a dense carbon layer as the gas and the solution flow generated during the combustion process. The formation of the carbon layer conduces to hinder the contact of the combustion body with the air and heat transfer, thus providing a good flame retardant effect [19,20]. Therefore, MMT and nano-Sb 2 O 3 /BPS show a good synergistic flame retardant effect.   (1) The incorporation of nano-Sb 2 O 3 could improve the crystallization temperature and crystallization of the PP matrix. When the mass fraction of nano-Sb 2 O 3 was 3.0wt%, the crystallinity of the PP matrix composite was highest with a value of 69.88%. (2) Due to the addition of MMT and nano-Sb 2 O 3 , the tensile properties of the PP matrix composites were significantly increased. When the mass fraction of nano-Sb 2 O 3 was 3wt%, the tensile strength of the PP matrix composites reached the maximum, which was 22.6% higher than that of pure PP. (3) The flame retardancy of the PP matrix composites was remarkably improved due to the incorporation of nano-Sb 2 O 3 and MMT. As the mass fraction of nano-Sb 2 O 3 reached 3.5wt%, the LOI value of the PP matrix composites was 28.8 %, and its UL-94 grade reached the V-0 level.