Fabrication and Thermal Degradation Kinetics of PBT/BEO/Nano-Sb2O3 Composites

College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, China Service Center of Security Production Technology of Gansu Province, Lanzhou 730070, China Emergency Management Information Center of Gansu Province, Lanzhou 730070, China State Key Laboratory of Gansu Advanced Non-Ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China


Introduction
As a semicrystalline thermoplastic polymer, polybutylene terephthalate (PBT) possessed excellent mechanical properties and thermal property, which is widely used in engineering fields such as automobile, biomedical devices, and electronic and electrical industry [1,2]. But PBT is mainly composed of carbon and hydrogen, which leads to strong flammability and serious dripping as it burned. It is currently in the compound use of several flame retardants to improve thermal stability and flame retardancy for PBT composites [3]. However, these additive flame retardants are seriously affecting the mechanical properties and restricting the application of PBT composite. Therefore, the development of PBT composites with comprehensive physical and mechanical properties, thermal stability, and flame-retardant properties has become the keys to expand their application.
Nanometal oxide materials exhibit an unparalleled combination of chemical, electrical, magnetic, optical, and thermal properties [4,5], which have potential applications in flame retardants, electronics, and photonics industry. Sb 2 O 3 is one of nanometal oxides with excellent performance, which is often used as a flame-retarding additive for various plastics, rubber, and polymer materials [6]. Wang et al. reported the effect of MnCo 2 O 4 -GNS hybrid on PBT thermal degradation, and they concluded that the thermal stability of MnCo 2 O 4 -GNS/PBT composite improved greatly [7]. Feng et al. researched the thermal performance for polysulfonamide-based single polymer composites (PSA/SPCs) by thermogravimetric analysis (TG), and the results showed that thermal stability of PSA/SPCs was better than PSA resin composite [8]. Wang et al. investigated that the heating oxidation behaviors for short glass fiber reinforced nylon composite filled with brominated epoxy resin (BEO)/Sb 2 O 3 ; they concluded that the thermal stability of the composite was improved after aging at 160°C for 50 days, but after aging at 200°C for 50 days, the structure of the composite was seriously damaged and the thermal degradation behavior changed significantly [9]. Studies have shown that adding epoxy resin-coated ammonium polyphosphate microcapsules into polypropylene could increase the activation energy of polypropylene from 100.8 kJ/mol to 127.5 kJ/mol [10]. Qi et al. reported that aluminum hypophosphite/reduced graphene oxide (AHP/RGO) was added to the PBT matrix that could significantly reduce the heat release of the composites and improve the thermal stability and antidripping property of composites [11]. Many efforts have been made to develop the PBT composites with high thermal stability and flame-retardant properties. However, few studies focused on the effects of BEO and nano-Sb 2 O 3 on the thermal degradation of PBT. In addition, there is a correlation between the thermal stability and flame retardancy of nanocomposites. Therefore, it is also of great significance to study the thermal degradation kinetics of composites for improving its comprehensive properties.
In this paper, the PBT/BEO/nano-Sb 2 O 3 composites were prepared via methods of melt-mixing and injection molding. The effects of heating rate and nano-Sb 2 O 3 content on the thermal stability and thermal degradation kinetics of PBT composites were studied. With the increase of nano-Sb 2 O 3 particle content, the peak value of the thermal degradation rate of PBT composites gradually decreases, the thermal stability performance gradually improves, and the carbon residue rate also increases significantly.
Powders of PBT, BEO, and nano-Sb 2 O 3 particles were treated by ball milling at the conditions of revolving velocity which was 400 r/min for 6 hours to maintain better dispersion. BEO, nano-Sb 2 O 3 , and PBT were mixed with different mass ratios; mixtures were added into the extruder for melt blending and then into the injection machine. Mixture melts were injected into a model to prepare specimens. The component contents of specimens are listed in Table 1.

2.2.
Characterization. PBT composites were treated by spraygold. The JSM 6700 Scanning Electron Microscope (SEM) was used to study the microstructure of specimens. Thermodynamic behavior was investigated with Mettler Toledo Thermogravimetric Analysis (TG-DSC). In the experiments, the range of quality was 5~10 mg. Specimens were heated from room temperature to 600°C; heating rates were 5°C/min, 10°C/min, 20°C/min, and 40°C/min.  Figure 1, the temperature of initial decomposition, maximum decomposition rate, and thermal weight loss rate of pure PBT all move to high temperature with the increase of heating rate. However, the heating rate has little effect on the final thermal decomposition of pure PBT, and the TG trends of the four curves have not changed. The main reason for this phenomenon is that the temperature variation changes between furnace and specimens with the increasing heating rate and the temperature gradient are generated in the internal of specimens. Figure 2 shows the TG-DSC curves of pure PBT at the heating rate of 5°C/min (Figure 2(a)), 10°C/min (Figure 2(b)), 20°C/min (Figure 2(c)), and 40°C/min (Figure 2(d)). There are two endothermic peaks in the TG-DSC curves of pure PBT. The first endothermic peak appears in the range of 223~226°C. However, there is no corresponding weight loss on the TG. The endothermic peak in this range is the phase transition endothermic peak of neat PBT from solid to liquid, and PBT composites do not begin to break and decompose. Different heating rates have no effect on the phase transition temperature of PBT. The second endothermic peak appears at the temperature of around 400°C; there is a significant thermal degradation process. The main chain of PBT begins to break down into polymer fragments, then further produces small molecule combustible gases, and escapes small molecule combustible gases. As the heating rate increases, the endothermic peak gradually shifts to a higher temperature, which is consistent with the DTG curve of neat PBT (Figure 1(b)).

Results and Discussion
The maximum degradation rate of pure PBT is about 400°C; the obvious rapid degradation process occurred. With the increase of heating rate, the carbon residue rate increases gradually in the range of 350°C to 400°C. This shows that with the increase of heating rate, the thermal degradation of PBT also has a hysteresis effect due to the thermal hysteresis effect of the temperature gradient. When the temperature exceeds 400°C, the thermal hysteresis effect is gradually eliminated. With the increase of heating rate, the thermal stability of PBT decreases, the decomposition rate of PBT increases correspondingly, and the residual carbon rate decreases.

Thermal Stability Effects of Heating Rates on PBT/BEO
Composites. The TG and DTG curves of PBT/BEO composites with 16 wt% BEO flame retardant at different heating      Figure 4. Each of the four TG-DSC curves at different heating rates has endothermic peak values, and the first phase transition endothermic peak of which appears at 221°C, indicating that the phase-change temperature is not affected by heating rate. The second endothermic peak appears at about 400°C, and the TG process is evident at this stage. The endothermic peak temperature gradually shifts to high temperature with the increase of heating rate, which is basically consistent with the TG-DSC curve of neat PBT.    When the content of nano-Sb 2 O 3 reaches more than 3 wt%, a phenomenon can be found by comparing Figures 1 and 3.

Journal of Nanomaterials
A slowing band of the thermal degradation rate appears in DTG curves near 320°C, which prevents further thermal degradation of PBT composites. It indicates that there is a synergistic effect between BEO and nano-Sb 2 O 3 after the addition of 3 wt% nano-Sb 2 O 3 . In the early stage of PBT/BEO/nano-Sb 2 O 3 composite pyrolysis, coflame retardants play a role, improving the thermal stability of PBT/BEO/nano-Sb 2 O 3 composites and the flame-retardant properties at the initial stage of combustion.
The representative PBT/BEO/nano-Sb 2 O 3 3 wt% composite was selected to study the effect of heating rate on the thermal stability of the PBT composite. As the heating rate increases, the thermal degradation trend of PBT/BEO/nano-Sb 2 O 3 3 wt% composite is similar to that of PBT/BEO. The results show that the addition of nano-Sb 2 O 3 does not change the thermal degradation process of PBT/BEO composites but improves the thermal stability of PBT/BEO composites, and the carbon residue rate increases significantly. The TG-DSC curves of PBT/BEO/nano-Sb 2 O 3 3 wt% composites at different heating rates are shown in Figure 6. At high heating rates, peak fluctuations are not obvious. The DSC curve peak of the PBT/BEO/nano-Sb 2 O 3 3 wt% composite is similar to that of the PBT/BEO composite. The endothermic peak temperature gradually shifts to high temperatures with an increase in heating rate.

Effect of Nano-Sb 2 O 3 Content on the Thermal Stability of PBT Composites.
To study the effect of nano-Sb 2 O 3 con-tent on the thermal stability of PBT composites, the thermal stability properties of neat PBT and its composites were tested at 600°C with a heating rate of 5°C/min; the corresponding thermal performance data are summarized in Table 2. TG and DTG curves of neat PBT and its composites at a heating rate of 5°C/min are presented in Figure 7. The results show that the PBT composites still have a single weight-loss stage, and the DTG curves have only one large weight loss peak after adding BEO and nano-Sb 2 O 3 particles from Table 2 and Figure 7. This indicates that the thermal degradation process of PBT composite is mainly the catalytic fracture of the primary key. However, the thermal degradation of neat PBT and its composites can be divided into two stages: a rapid decomposition stage and a slow degradation stage of a stable carbon layer.   6 Journal of Nanomaterials The first stage mainly occurs between 320°C and 430°C. The initial decomposition temperature of nano-Sb 2 O 3 particles occurs during the slow degradation stage of the PBT stable carbon layer. After adding the BEO flame retardant, the thermal decomposition temperature of the PBT composite is increased, and the carbon residue rate is improved. With the addition of nano-Sb 2 O 3 in different contents, the initial decomposition temperature (T 10% ) of PBT/BEO/nano-Sb 2 O 3 composites is decreased by 25.8°C (1 wt%), 30.9°C (3 wt%), and 44.1°C (5 wt%) compared to the pure PBT, respectively ( Table 2). With the increase of nano-Sb 2 O 3 content, the maximum thermal decomposition peak temperature and thermal decomposition rate of PBT/BEO/nano-Sb 2 O 3 composites decreased gradually. This may be due to the catalytic effect of metal oxides on macromolecular chain cleavage, which promotes the rapid oxidation of PBT matrix materials [15]. The interaction between nano-Sb 2 O 3 and BEO flame retardants and PBT matrix materials promotes the thermal decomposition of PBT composites at lower temperatures, resulting in the formation of some high-quality carbon residue layers. At this stage, the high-temperature    Journal of Nanomaterials heat energy promotes the cleavage of various bonds in the molecular chain of PBT materials. As a result of the degradation of bond cleavage, the mass loss of polymers increases rapidly, which is reflected in the TG and DTG curves.
In the second stage, with the increase of pyrolysis temperature, the initial carbon layer of PBT composites is gradually decomposed, but the residual amount of PBT composites is still higher than that of neat PBT material. Studies show that the effective carbonization process of flame-retardant polymer materials occurs at a temperature higher than the processing temperature, but much lower than the decomposition temperature of the polymer matrix material [16]. Therefore, in the first stage of degradation, the effective carbonization layer is formed by the early pyrolysis of PBT composites. It is beneficial to delay the thermal degradation of PBT matrix materials in a higher temperature range and improve the thermal stability and flame-retardant properties of PBT composites. Table 2 indicates that the carbon   To further analyze the relationship between nano-Sb 2 O 3 content and thermal stability of PBT/BEO/nano-Sb 2 O 3 composites, the surface morphology SEM images of solid residues of neat PBT (Figure 8(a)) and PBT/BEO/nano-Sb 2 O 3 3 wt% (Figure 8(b)) after combustion are shown in Figure 8. Compared with neat PBT, a large number of lamellar and agglomerate structures appear on the surface of solid residue after the combustion of PBT/BEO/nano-Sb 2 O 3 3 wt%, forming a protective film with a network structure. This kind of network barrier structure can hinder the conduction of combustible volatiles and heat produced in the thermal degradation process of PBT composites and protects the thermal degradation of PBT.

Kinetic Parameters of Nano-Sb 2 O 3 /BEO/PBT Composites.
The Kissinger method and Friedman method were used to calculate the thermal degradation kinetic parameters of the composites. The straight lines of composite materials calculated by the Kissinger method are demonstrated in Figure 9. The activation energy E a and frequent factors lnA were calculated according to the slope and intercept of the straight line which are summarized in Table 3. At the heating rate of 10°C/min and 40°C/min, the critical transition temperature of the neat PBT is 402.8°C and 431.8°C, respectively. After adding 3 wt% nano-Sb 2 O 3 particles into PBT/BEO, the critical transition temperature of PBT/BEO/nano-Sb 2 O 3 3 wt% is 398.2°C at the heating rate of 10°C/min, and the transition temperature is reduced by about 5°C. Here, nano-Sb 2 O 3 particles have a catalytic effect on the molecular chain of PBT. It can be seen from Figure 9 and Table 3 that the addition of nano-Sb 2 O 3 particles has a certain effect on the thermal stability of PBT.
With the increase of nano-Sb 2 O 3 content, the E a of PBT composites increases at first and then decreases. When the content of nano-Sb 2 O 3 is 3 wt%, the E a is the highest, which is 66.18 kJ/mol (31.43%) higher than that of pure PBT. At this time, the thermal degradation of PBT composites is the most difficult, which indicates that the addition of nano-Sb 2 O 3 particles hinders the movement of molecular chains and enhances the thermal stability of PBT.
The degradation kinetic parameters E a and reaction order n obtained by the Friedman method are shown in Figure 10 and Table 4. It can be seen from Table 4 that the thermal degradation of the PBT composites belongs to the first-order reaction. With the increase of nano-Sb 2 O 3 content, the activation energy of thermal degradation first increases and then decreases, which is consistent with the change rule of activation energy obtained by the Kissinger method. When the content of nano-Sb 2 O 3 is 3 wt%, the activation energy E a increased by 71.96 kJ/mol (36.72%) compare with neat PBT. From the increase of activation energy E a value, it can be seen that there is an interaction between nanofiller and PBT matrix material, which reduces the movement of distal chain in PBT nanocomposites and improves its thermal stability [17].
The thermal degradation reaction of PBT can be regarded as zero order in the early stage, and the degradation reaction under a high weightlessness rate can be regarded as the first order [18][19][20][21][22][23][24]. The thermal degradation of pure PBT and its composites can be divided into two stages. In the early stage of the reaction, the reaction order is zero. With the increase of thermal degradation temperature, the reaction order changes from zero to the first order in the second stage.

Conclusions
With the increase of heating rate, the thermal hysteresis effect of the temperature gradient is produced, and the thermal degradation of PBT composite also has a hysteresis effect. When the temperature exceeds 400°C, the thermal hysteresis effect is gradually eliminated. With the increase of nano-Sb 2 O 3 particle content, the thermal degradation activation energy E a of PBT/BEO/nano-Sb 2 O 3 increases at first then decreases, and its maximum value at 3 wt% nano-Sb 2 O 3 . The results indicate that the addition of nano-Sb 2 O 3 particles hinders the movement and regular stacking of molecular chains. Therefore, the BEO and nano-Sb 2 O 3 particles can enhance the thermal stability of PBT and increase the maximum activation energy of thermal degradation.

Data Availability
The data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest
The authors report no conflicts of interest in this work.