Synthesis and mechanical properties of nano-Sb2O3/BPS-PP composites

Abstract In view of the limitation of wide application of polypropylene(PP) with low strength, poor low-temperature brittleness and easy combustion, a kind of PP matrix nanocomposites was designed and prepared. Sb2O3 nanoparticles (nano-Sb2O3) modified by silane coupling agent of KH550 were dispersed into brominated polystyrene(BPS)-PP matrix by ball milling dispersion and melt blending method, respectively. And the nano-Sb2O3/BPS-PP composites samples were obtained by injection molding method. The effects of nano-Sb2O3 particles on mechanical properties of nano-Sb2O3/BPS-PP composites were investigated. The results showed that the surface of nano-Sb2O3 particles was successfully modified by the KH550 and the interfacial adhesion between nano-Sb2O3 and PP matrix was improved. With increasing of the mass fraction of nano-Sb2O3, the tensile strength and impact strength of nano-Sb2O3/BPS-PP composites were improved accompanying by increasing of crystallinity and refining grain of the composites. When the mass fraction of nano-Sb2O3 was 3 wt%, the tensile strength of nano-Sb2O3/BPS-PP composites was 43 MPa, which was 30.3% higher than that of PP. When the mass fraction of nano-Sb2O3 was 2 wt%, the impact strength of the composites was 44.19 kJ·m−2, which was 30.8% higher than that of PP.


Introduction
Polypropylene (PP) is an important engineering plastic. It has been widely used in many engineering fields because of its some advantages, such as good mechanical properties, corrosion resistance, easy formability, etc [1,2]. However, PP also has some disadvantages, such as low strength, poor low-temperature brittleness, easy combustion, etc [3]. In order to improve the comprehensive properties of PP, it is an effective method that some nanoparticles are filled into PP matrix. Nano-composites have excellent mechanical properties because nanoparticles have some characteristics of surface effect, volume effect and quantum size effect, especially nanoparticles have good interface combination ability with polymer matrix [4,5]. However, the particle size, shape and surface structure of nanoparticles have significant influences on the distribution of the nanoparticles in the matrix and the interfacial compatibility between the nanoparticles and the matrix. When the mass fraction of nanoparticles is too high, the nanoparticles easily form aggregates in the matrix, thus deteriorating the mechanical properties of nano-composites [6]. Therefore, the preparation of nanocomposites with excellent comprehensive properties has become an important target for material researches. Sb 2 O 3 nanoparticle (nano-Sb 2 O 3 ) is a new kind of synergistic flame retardant, which can reduce the amount of halogen flame-retardants in the synergistic systems of nano-Sb 2 O 3 and halogen flame-retardants. Nanocomposites can obtain excellent flame retardancy based on adding nano-Sb 2 O 3 [7]. We added nano-Sb 2 O 3 into PP matrix and found that nano-Sb 2 O 3 could improve the flame retardancy of PP matrix composites [8]. Sb 2 O 3 as nanoparticles in PP matrix has a positive effect on the mechanical properties of the matrix [9]. In order to thoroughly study the effects of nano-Sb 2 O 3 on the mechanical properties of polymer matrix composites, nano-Sb 2 O 3 /BPS-PP composites were prepared in the paper, and then the ef-fects of nano-Sb 2 O 3 on the mechanical properties of nano-Sb 2 O 3 /BPS-PP composites were studied.

Materials
The surface modifier was silane coupling agent KH-550 (Qufu Yishun Chemical Co., Ltd., China). The matrix material was homopolymer PP (Sinopec Beijing Yanshan Company, China) with molecular weight of 300000 and melting index of 1 g/10 min. Nano-Sb 2 O 3 with an average particle size of 50~100 nm were prepared by our research group using high-energy ball milling method. The flame retardants were brominated polystyrene (BPS) with molecular weight more than 200000 (Dow Chemical Company, U.S.A).

Measurements and characterizations
The

Surface modification analysis of nano-Sb 2 O 3
The FT-IR spectra of nano-Sb 2 O 3 and the experimental materials are presented in Figure 1 The curve (c) and (d) in Figure 1 are FT-IR spectra of PP and the modified nano-Sb 2 O 3 /BPS-PP composites, respectively. It can be seen that the characteristic absorption peak of the nano-Sb 2 O 3 /BPS-PP composites containing modified nano-Sb 2 O 3 particles has some changes compared with those of PP. For example, the intensity of the characteristic absorption peak at 2929 cm −1 obviously increases, which indicates that the numbers of -CH 3 and -CH 2 -groups in the nano-Sb 2 O 3 /BPS-PP composites increases due to KH550 coating on the surface of the nano-Sb 2 O 3 particles. And a new characteristic absorption peak of C=O bond at 1720 cm −1 of the nano-Sb 2 O 3 /BPS-PP composites appeares because C-O-C group of KH550 coating on surface of nano-Sb 2 O 3 particles reacts with -CH 3 group in PP to form C=O bond. Figure 2 shows TEM micrographs of unmodified and modified nano-Sb 2 O 3 particles, respectively. It can be seen from Figure 2 (a) that there are obvious agglomeration phenomena of unmodified nano-Sb 2 O 3 particles and the size of aggregate is higher than 200 nm. These aggregates presents as a loose state and the particles clusters are not compact. Figure 2 (b) shows that nano-Sb 2 O 3 particles modified by KH550 disperse uniformly, which indicates that the dispersibility of nano-Sb 2 O 3 particles in the composites is significantly improved due to the surface modification of nano-Sb 2 O 3 particles. Surface modification of nanoparticles is also well-known as an effective way to improve the dispersion of nanoparticles in polymer matrix [10].     [11]. Therefore, the tensile strength of nano-Sb 2 O 3 /BPS-PP composites is not significantly affected negatively. When the mass fraction of nano-Sb 2 O 3 particles is more than 3 wt%, the tensile strength and static fracture toughness of nano-Sb 2 O 3 /BPS-PP composites decrease because nano-Sb 2 O 3 particles gradually aggregate in PP matrix and form some large aggregates. These aggregates can easily separate from PP matrix due to their large volume and then form large crack in the matrix when the nano-Sb 2 O 3 /BPS-PP composites are subjected to external force, which leads to sharply decrease on the tensile strength and static fracture toughness of the composites. Figure 4 shows the relationship between tensile strength of the experimental nano-Sb 2 O 3 /BPS-PP composites and the mass fraction of unmodified and modified nano-Sb 2 O 3 particles, respectively. With increasing of the mass fraction of nano-Sb 2 O 3 particles, the tensile strength of nano-Sb 2 O 3 /BPS-PP composites firstly increases and then decreases. The tensile strength of the modified nano-Sb 2 O 3 /BPS-PP composites is significantly higher than that of the unmodified nano-Sb 2 O 3 /BPS-PP composites containing the same mass fraction of the nano-Sb 2 O 3 particles. When the mass fraction of the unmodified nano-Sb 2 O 3 particles is 3 wt%, the tensile strength of the unmodified nano-Sb 2 O 3 /BPS-PP composites reaches the maximum value of 38 MPa, which is 15.2% higher than that of PP. If the modified nano-Sb 2 O 3 particles with 3 wt% of the mass fraction are used, the tensile strength of the nano-Sb 2 O 3 /BPS-PP composites reaches the maximum of 43 MPa, which is 30.3% higher than that of PP and 13.2% higher than that of unmodified nano-Sb 2 O 3 /BPS-PP composites with the same mass fraction of the unmodified nano-Sb 2 O 3 particles. The nano-Sb 2 O 3 particles modified by KH550 can obviously improve the interfacial bonding properties between nano-Sb 2 O 3 particles and PP matrix. Due to the better interface compatibility between the nano-Sb 2 O 3 particles and PP matrix, nano-Sb 2 O 3 particles can not easy to fall off from PP matrix and eliminate local stress under external force. Therefore, tensile strength of the composites is increased.     The fracture surface has some uneven distribution aggregates of Sb 2 O 3 particles, and the size of aggregates becomes larger. There are some larger gaps between the aggregates and PP matrix, which indicates that the interfacial compatibility between the Sb 2 O 3 particles and PP matrix becomes worse. The nano-Sb 2 O 3 particles with less than 3 wt% of mass fraction can be uniformly dispersed in PP matrix. When the nano-Sb 2 O 3 /BPS-PP composites is impacted, the Sb 2 O 3 particles dispersed uniformly in PP matrix can absorb a part of the energy as the stress concentration point accompanying with that the PP matrix also takes place yield to produce plastic deformation and absorbs large amount of impact energy [12]. Meanwhile, these rigid Sb 2 O 3 particles can also prevent the crack forming and promote the cracks tip passivation in cracks growth process, resulting in increasing the crack growth energy and preventing the destructive fracture of the composites by mean of the crack growth. Therefore, the impact strength of nano-Sb 2 O 3 /BPS-PP composites increases with increasing of the mass fraction of nano-Sb 2 O 3 particles when the mass fraction of nano-Sb 2 O 3 particles is lower. When the mass fraction of nano-Sb 2 O 3 particles reaches or even exceeds 3 wt%, the nano-Sb 2 O 3 particles begin to agglomerate and form some larger agglomerates with increasing of the mass fraction of nano-Sb 2 O 3 particles due to the higher amount and surface energy of nano-Sb 2 O 3 particles. The agglomerates formation reduces the specific surface area of Sb 2 O 3 particles and also decreases the interfacial bonding strength between the particles and PP matrix, which makes the interfaces between Sb 2 O 3 aggregates and PP matrix to become some weaker points of nano-Sb 2 O 3 /BPS-PP composites. The agglomeration of nano-Sb 2 O 3 particles becomes more significant with increasing of the mass fraction of nano-Sb 2 O 3 particles. These interfacial weaker points not only reduce the crack growth resistance, but also become some new micro-crack sources. The forming and rapid propagating of these new cracks easily make the nano-Sb 2 O 3 /BPS-PP composites to fracture on macroscopic, resulting in decreasing of impact strength of the composites [13]. The SEM images of the nano-Sb 2 O 3 /BPS-PP composites containing modified and unmodified nano-Sb 2 O 3 particles with the mass fractions of 3 wt% are presented in Figure 6 (c) and Figure 6 (f), respectively. For Figure 6 (c), the modified nano-Sb 2 O 3 particles are uniformly wrapped in PP matrix, and the interfacial compatibility between the particles and PP matrix is better. In Figure 6 Table 3, in which Tm is the melting peak temperature, Tc is the crystallization peak temperature and X(t) is the crystallinity. Com-pared with PP, the crystallization peak temperature and crystallinity of nano-Sb 2 O 3 /BPS-PP composites are higher than those of PP. When the mass fraction of nano-Sb 2 O 3 particles is 5 wt%, Tc of nano-Sb 2 O 3 /BPS-PP composites is 166.75 ∘ C and X(t) is 65.23%, which is 10.01 ∘ C and 46.16% higher than those of PP respectively. Nano-Sb 2 O 3 particles can improve the crystallinity and crystallization perfection of nano-Sb 2 O 3 /BPS-PP composites because it can promote the PP molecular chain to crystallize at a higher crystallization temperature increased by nano-Sb 2 O 3 particles. Nano-Sb 2 O 3 particles show the heterogeneous nucleation function in nano-Sb 2 O 3 /BPS-PP composites. The crystallinity of nano-Sb 2 O 3 /BPS-PP composites increases gradually with increasing of the mass fraction of nano-Sb 2 O 3 particles, but the increasing trend gradually slows down. This is due to the increasing of nucleation number in nano-Sb 2 O 3 /BPS-PP composites with increasing of the mass fraction of nano-Sb 2 O 3 particles, resulting in the in- The crystallinity also effects on the mechanical properties of nano-Sb 2 O 3 /BPS-PP composites in some extent. The high crystallinity can make the grain of the composites more refining, the number of grains more increasing and the grain size distribution more uniform. The increasing of crystallinity and grain refinement can eliminate the stress concentration caused by the uneven grain distribution of nano-Sb 2 O 3 /BPS-PP composites under the external force, and improve the deformation resistance capacity of the composites [14]. Thus the mechanical properties of the nano-Sb 2 O 3 /BPS-PP composites can be effectively improved, namely the tensile strength and impact strength of the composites are improved. However, the improving toughness effect of the composites attributing to the improvement of interfacial interaction between nano-Sb 2 O 3 particles and PP matrix is stronger than that of the composites attributing to grain refinement of PP matrix caused by nano-Sb 2 O 3 particles [15]. When the mass fraction of nano-Sb 2 O 3 particles exceeds 3 wt%, the reducing effect of the mechanical properties attributing to aggregation of nano-Sb 2 O 3 particles is stronger than improving effect attributing to grain refinement caused by nano-Sb 2 O 3 particles in nano-Sb 2 O 3 /BPS-PP composites. Therefore, the mechanical properties of nano-Sb 2 O 3 /BPS-PP composites containing nano-Sb 2 O 3 particles with more than 3 wt% of the mass fraction show a downward trend.

Conclusions
Using Sb 2 O 3 nanoparticles (nano-Sb 2 O 3 ) modified by silane coupling agent of KH550 as the additives, nano-Sb 2 O 3 /BPS-PP composites were prepared in the paper. The effects of nano-Sb 2 O 3 on the mechanical properties of nano-Sb 2 O 3 /BPS-PP composites were studied. The research obtained the following conclusions: 1. Silane coupling agent KH550 could effectively modify the surface of nano-Sb 2 O 3 particles, and then improve the dispersibility of the particles in nano-Sb 2 O 3 /BPS-PP composites and the interfacial compatibility between the particles and PP matrix. 2. The mechanical properties of nano-Sb 2 O 3 /BPS-PP composites were improved with increasing of the mass fraction of nano-Sb 2 O 3 particles. When the mass fraction of nano-Sb 2 O 3 particles was 3 wt%, the tensile strength of nano-Sb 2 O 3 /BPS-PP composites was 43 MPa, which was 30.3% higher than that of PP. When the mass fraction of nano-Sb 2 O 3 particles was 2 wt%, the impact strength of nano-Sb 2 O 3 /BPS-PP composites was 44.19 kJ·m −2 , which was 30.77% higher than that of PP. 3. The modified nano-Sb 2 O 3 particles could improve the crystallinity and grain refinement of nano-Sb 2 O 3 /BPS-PP composites, thus improve their mechanical properties. 4. There are two reasons for the improvement of the mechanical properties of nano-Sb 2 O 3 /BPS-PP composites with increasing of the mass fraction of nano-Sb 2 O 3 particles. On the one hand, the heterogeneous nucleation of nano-Sb 2 O 3 particles in PP matrix can refine grain and increase crystallinity of the composites. On the other hand, uniformly dispersed nano-Sb 2 O 3 particles can absorb part of the energy when the composites is subjected to external forces, and prevent micro-cracks propagating.