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Influence of Graphitization Degree of Carbon Microspheres on Properties of PET Flame Retardant.

INTRODUCTION

Halogen-free flame retardants have gained increasing interest in recent years due to their numerous advantages over existing flame retardants. In this respect, carbon nanomaterials have excellent thermal properties, and thus, great advantages in reducing the heat release rate (HRR) in polymers [1-5]. Moreover, carbon nanomaterials are halogen-free. Therefore, carbon nanomaterials have potential applications as flame retardants for polymers, especially those with higher processing temperatures such as polyamide 6 (PA6) [6] and poly (ethylene terephthalate) (PET).

Carbon nanomaterials mainly include fullerenes ([C.sub.60]), carbon black (CB), carbon nanotubes (CNTs), expandable graphite (EG), and graphene. A survey of the literature revealed that a number of investigations have been performed on carbon materials used in flame retardant polymers. For example, Kashiwagi et al. [7] studied the effect of CNTs content on flame retardancy of polypropylene (PP) and found that certain amounts of CNTs significantly reduce the HRR of PP. However, CNTs can form dense networking carbon layers, which hinder the transfer of heat and mass during the polymer combustion. Dittrich et al. [1, 5] prepared PP composites with different carbon materials including CB, EG, multiwalled carbon nanotubes (MWNTs), and graphene through the melt blending method and evaluated the effect of several parameters on the rheological properties, flame retardancy, and combustion behavior of PP. They proposed that both the structure and carbon content affect the melting rheological behavior and thermal conductivity of PP. They associated this tendency with factors like the limiting oxygen index (LOI) and vertical burn level (UL-94). Finally, Inuwa et al. [3] tested stripped graphene flame retardant PET/PP composites and concluded that the addition of 5% graphene improves the LOI from 21% to 31%, with a UL-94 rating of V-0.

Carbon microspheres (CMSs) are an important class of carbon materials which have fullerene cage spherical structures with multilayers of graphite, and have a low degree of graphitization. CMSs are used in various fields thanks to their unique structures, excellent physical/chemical properties, and high chemical/thermal stabilities [8-10]. Several preparation methods of CMSs have recently been reported [11-15], including chemical gas phase, hydrothermal, and template methods. These studies revealed that the morphology (particle size, surface pore size) and chemical structure (graphitization degree, surface oxide groups) of CMSs largely depend on the carbon source and the preparation method. In this view, the hydrothermal method based on glucose as a carbon source is advantageous in terms of low cost, large production scale, and shape-control. However, there is very little information known about CMSs used in flame retardant fields.

A number of studies have investigated the flame retardant properties of CMSs prepared by the hydrothermal method. They found that addition of CMSs as a flame retardant for PET using the melt blending method improves the LOI value of PET because CMSs act as heat shields for the underlying polymer [16]. Besides, the formed char layer after combustion was found to be relatively dense, which insulated the polymer from heat and oxygen [16, 17]. To provide effective heat shielding of polymers, CMSs have been graphitized, and the influence of their graphitization degree on the flame retardant properties was explored. In this regard, some studies demonstrated that the annealing treatment improves the graphitization degree of CMSs [18].

In this article, CMSs with different graphitization degrees were prepared using the annealing treatment, and the resulting annealed CMSs (TCMSs)/PET composites were fabricated by the melt blending method. The flame retardant performances of the PET composites were evaluated and the influence of the graphitization degree of CMSs on the flame retardancy of PET was analyzed and discussed.

EXPERIMENTAL

Materials

Glucose (C6H1206) and ethanol (C2H5OH) were purchased from Fengchuan Chemical Industries Co. Ltd. (Tianjin, China). Distilled water was obtained from our laboratory distillation system. PET chips (SD500) with a viscosity index of 0.68 dL/g were obtained from Sinopec Yizheng Co. Ltd. (China).

Preparation of CMSs

A glucose solution (100 mL, 0.42 mol/L) was added into a sealed stainless steel autoclave stirred at a constant speed. The autoclave was heated to 300[degrees]C for 8 h and then cooled down to room temperature. The reaction product was collected, filtered off, washed several times with ethanol and water, and then dried to yield CMSs.

Preparation of Annealed CMSs (TCMSs)

A certain amount of CMSs was first weighed in a quartz boat then placed in a tubular resistance furnace filled with nitrogen at a flow rate of 100 mL/min. The obtained CMSs powder was heated to 500[degrees]C, 700[degrees]C, and 900[degrees]C at the rate of 10[degrees]C/min, and then maintained for 2 h at 900[degrees]C. After cooling down to room temperature, the product was collected and named as TCMSs.

Fabrication of CMSs/PET and TCMSs/PET Composites

The CMSs and TCMSs powders, as well as the PET chips, were firstly dried in a vacuum oven at 120[degrees]C for 14 h. The CMSs/PET and TCMSs/PET composites were then prepared in a two-screw extruder (CET35--40D). The temperature of the screw section varied from 255[degrees]C to 267[degrees]C at a screw rotation speed of 180 rpm. The resulting extrudates were cooled down with water, then cut into pellets, and finally processed into flame retardant materials by injection-molding equipment (52800NB-Aingbo Plastics Machinery Ltd., China) at temperatures of 280[degrees]C.

Materials Characterization

Fourier transform infrared (FTIR) spectra were recorded using a Tensor 27 spectrometer (Bruker) at wavelengths between 4000 and 700 [cm.sup.-1] and resolution of 4 [cm.sup.-1]. Fieldemission scanning electron microscopy (FESEM; JSM-6700F) was used to examine the morphologies of the different samples under an acceleration voltage of 7 kV. X-ray diffraction (XRD) was used to determine the structures of the composites, and was performed using a diffractometer (RigakuD/MAX-2500V/PV) at a Cu-K[alpha] radiation of 40 kV and 200 mA, scanning speed of 0.5[degrees]/min, 2[theta] range of 10[degrees]-80[degrees], and [lambda] = 0.154 nm. Raman spectra were obtained using a Renishaw Raman RM-1000 spectrometer with a 532 nm laser source and CCD. The thermal stability and mass percentage of the fillers and PET composites were determined by thermogravimetric analysis (TGA), which was performed on a Netzsch TG 209F3 instrument under nitrogen from 30[degrees]C to 800[degrees]C at a heating rate of 10[degrees]C/min. Measurements were performed on samples of 10 [+ or -] 1 mg with a purge gas flow rate of 30 mL/min.

The limiting oxygen index (LOI) values were measured by a TM606 oxygen index meter using specimens of 130 x 6.5 x 3 [mm.sup.3] according to the standard oxygen index testing methodology (ASTM [D.sup.2]863-97). Cone calorimeter measurements were carried out by means of an FTT Cone calorimeter according to the IS05660 procedure under an external heat flux of 35 kW/ [m.sup.2]. The dimensions of the samples were set to 100 x 100 x 3 [mm.sup.3]. The measurement of each specimen was repeated three times, and error values of typical cone calorimeter data were maintained within [+ or -]5% for reproducibility. The tensile tests were carried out using an Instron universal material testing system (model CMT6104) at room temperature with gauge length of 30 mm and crosshead speed of 50 mm/min. The values reported here represent an average of the results for five tests.

RESULTS AND DISCUSSION

Effect of Annealing Temperature on Morphology of CMSs

The microstructures of CMSs and TCMSs composites were characterized by SEM, and the results are shown in Fig. 1. It can be seen that pure CMSs were present as regular spherical shaped particles (Fig. 1al). As the CMSs were prepared by hydrothermal synthesis method with glucose as starting material, the CMSs surface contained a large number of oxygencontaining functional groups [16]. The average diameter determined from the statistical distribution of Fig. Ia2 was about 800 nm. After annealing at 500[degrees]C for 2 h, a few smaller sized TCMSs particles appeared in Fig. 1b 1. The average diameter of these particles estimated from the statistical distribution of Fig. Ib2 was reduced to about 720 nm. When the temperature was increased to 700[degrees]C, the size of the overall TCMSs particles significantly decreased to reach an average diameter of about 500 nm (Fig. Ici and c2). Finally, at a higher temperature of 900[degrees]C, no change in the size of the TCMSs was recorded (Fig. Id2), but an adhesion phenomenon appeared between the TCMSs, as shown in Fig. 1d1.

Effect of Annealing Temperature on Graphite Structure of CMSs

XRD is a relevant tool to detect structural changes within carbon materials. The graphitic degrees of CMSs and annealed TCMSs are depicted in Fig. 2. A unique and wide diffraction peak appeared at 25.6[degrees] for pure CMSs in Fig. 2a. This peak was assigned to the (002) peak of graphite [18], demonstrating that the CMSs sample was mainly composed of amorphous carbon with a low graphitic degree. After annealing, a new diffraction peak appeared in the XRD pattern at 46.1[degrees], which was similar to the characteristic (001) diffraction peak of graphite, indicating that some amorphous carbon was transformed into graphite carbon during the annealing process.

The graphitic degree of carbon materials can be evaluated by measuring the relative intensity and peak width of the carbon diffraction peak. In general, higher intensity and narrower peak shape indicate superior graphitic degree. For example, TCMSs showed higher graphitic degree at 700[degrees]C due to the higher relative intensity and narrower width of the peaks at 25.6[degrees] and 46.1[degrees]. These data combined with those from Effect of Annealing Temperature on Morphology of CMSs section indicated that the optimal annealing temperature was 700[degrees]C which provided uniform grain size and higher graphitic degree.

To quantitatively assess the graphitic degree of TCMSs at different annealing temperatures, the Raman spectra of CMSs and TCMSs were obtained, as shown in Fig. 3. Two important string peaks were observed at 1350 [cm.sup.-1] (D band) and 1580 [cm.sup.-1] (G band) [12, 19], which are attributed to disordered graphite structure and in-plane stretching [E.sub.2]g mode of graphite, reflecting the structural intensity of the [sp.sup.2]-hybridized carbon atoms of CMSs [20], In addition, the G band was mainly caused by the structural defects of CMSs. The D band corresponds to the typical C--C vibration mode of flake-like graphite layer. The peak intensity ratio of G band and D band ([I.sub.G]/[I.sub.D]) can describe the density of point defects in the graphite structure and reflect the graphitic degree [20], Smaller value of [I.sub.G]/[I.sub.D] indicates a higher density of point defects. The [I.sub.G]/[I.sub.D] of neat CMSs was 1.333. After annealing at different temperatures, the TCMSs showed different values of 1CHd. The IG/ID values of TCMSs annealed at 500[degrees]C and 700[degrees]C were 1.343 and 1.437, respectively, which are both higher than that of neat CMSs. However, when the annealing temperature increased to 900[degrees]C, the [I.sub.G]/[I.sub.D] of TCMSs decreased to 1.115. This illustrated that the structure of TCMSs was destroyed at 900[degrees]C. The graphitic degree of TCMSs at 700[degrees]C was the highest with the least point defects. Therefore, the optimal annealing temperature was confirmed to be 700[degrees]C.

Dispersion of CMSs and Graphitic TCMSs in PET Matrix

The dispersion of fillers in a polymer matrix also affects the properties of composites, so it is necessary to explore the distribution of fillers. The dispersion of CMSs and TCMSs in PET matrix was studied by FESEM observation as shown in Fig. 4, wherein the TCMSs sample was obtained at the annealing temperature of 700[degrees]C. Among the CMSs/PET composites with different contents of CMSs, it was observed that the CMSs were dispersed well in the PET matrix with the contents of 0.5% (al) and 1% (bl). Some aggregations between CMSs were present in the PET matrix, and the dispersion of CMSs with the mass fraction of 2% was poor, as seen in Fig. 4c 1. Compared to CMSs, the distribution of TCMSs in PET matrix (a2, b2, c2) was relatively homogeneous, even though the mass fraction was 2% (c2). With the same amount of fillers, the dispersion of TCMSs in PET matrix was better compared to CMSs. After annealing at high temperature, the groups on the CMSs surface were decomposed, which weakened the Van der Waals force and agglomeration between CMSs.

Effect of Graphitic Degree of CMSs on PET Flame Retardancy

Cone calorimeter (CONE) and LOI tests were also used to evaluate the burning behavior of the prepared polymer composites. The CONE results of CMSs/PET and TCMSs/PET composites are presented in Fig. 5 and Table 1. The heat release rate (HRR) and peak HRR (pk-HRR) are two important fire characteristic parameters of materials, where higher values mean higher fire risk [21]. Fig. 5(a) indicated that the addition of CMSs could reduce HRR of PET. The increase in amounts of CMSs (from 0.5, 1 to 2 wt%) reduced the PHRR values of PET composites by 20%. The reason could be that a residual protective char layer was formed when the CMSs/PET composites combusted, lowering the release of combustible volatiles and reducing the PHRR.

Interestingly, it was found that CMSs contents of more than 1 wt% turned the PHRR value to "rebound," The changes in these data were consistent with those of MWNTs reported in the literature, which related the phenomena to defects in the nanotubes [22]. It has been reported that higher concentration of particles yielded more agglomerates instead of improving the residue structure [5], The decreased PHRR of CMSs/PET with 2 wt% CMSs was due to the poor dispersion of CMSs, combined with the results of Fig. 3. Compared to the CMSs/PET composites, TCMSs obviously decreased the HRR of PET, indicating that the residual protective char layer formed by TCMSs/PET was more effective. Besides, changing the amount of TCMSs from 0.5 to 2 wt% induced a drop in the PHRR values of the PET composites by 23.3%, 33.8%, and 37.5%, respectively. The "rebound" phenomenon observed with HRR was absent in the TCMSs/PET composites, which was attributed to the better dispersion of TCMSs in the PET matrix. Table 1 lists the relevant fire parameters of the PET composites. It was found that the total heat release (THR) values of CMSs/PET or TCMSs/PET composites with different contents were all lower than that of neat PET. Moreover, the TCMSs/PET with 2 wt% of TCMSs showed the lowest THR of 60.1 MJ/[m.sup.2]. At the same amount, the THR of TCMSs/PET was lower than that of CMSs/PET. Comparing the mean effective heat of combustion (MEHC) values of PET composites, it was found that the addition of CMSs in the PET matrix results in almost no change in MEHC, while the MEHC of TCMSs/PET composites decreased with the increase in TCMSs content. These results demonstrated that compared to CMSs, TCMSs are better in lowering the heat release, acting as a heat shield during the burning.

In addition, the addition of CMSs or TCMSs induced a delay in the TTI of PET from 90 to about 100 s. TTI is mainly affected by two factors: the thermal conductivity and heat absorption. An increase in the thermal conductivity should raise the thermal inertia, and thus, delay the TTI. Moreover, better heat absorption is believed to enhance early ignition [5], This indicated that CMSs or TCMSs improved the thermal inertia of PET, blocked the transfer of heat on the polymer surface layer, and decreased the accumulation of heat absorption on the top layer during early ignition.

Fire growth index (FGI) is another important parameter used to evaluate the thermal capacity of materials. It is defined as the ratio of PHRR to the time reaching PHRR (iPHrr) [2]. Higher FGI values mean faster fire spread and easier burn-out of a given material [23]. The results revealed that the FGI value of PET was the highest, and the FGI values for CMSs/PET composites were comparably lower. The addition of CMSs obviously improved the thermal stability of PET and lowered the fire spreading speed of PET. With similar contents, FGI of TCMSs/ PET composites were lower compared to CMSs/PET composites. This was related to the more efficient thermal conductivity of graphitized TCMSs compared to CMSs. Thus, the fire risk of TCMSs/PET composites was lower than that of CMSs/PET and pure PET. At flame out, the fire residue amount of CMSs/PET or TCMSs/PET composites also increased, especially for the TCMSs. With 2% filler content, the fire residue amount of TCMSs/PET composites was 19.29%, almost twice as much as that of neat PET (9.31%), and higher than that of CMSs/PET composites (13.73%). The increased residue amount implied that the addition of TCMSs improved the thermo-oxidative stability of PET and prevented the further decomposition of PET.

In conclusion, increased graphitization degree of CMSs can reduce the heat release rate, prolong the ignition time, slow the fire spread, and increase the fire residue amount during the PET burning.

To some extent, the difficulty in self-extinguishing can be measured by means of LOI testing. Materials with elevated values of LOI have better self-extinguishing property [21]. One crucial factor affecting the LOI values deals with the heat absorption. Increased heat absorption causes faster heating and pyrolysis of the polymer surface layer, and also shortens the time to reach the critical fuel release rate [22]. During burning of polymers, the carbon layer with greater number of aromatic carbon atoms can cause the fire to flame out. Table 1 indicates that the LOI values of CMSs/PET composites increased to more than 25% compared to the initial 21% of pure PET. This was related to the CMSs blocking the transfer of heat from the PET surface, decreasing the heat absorption, and increasing the self-extinguishing ability of the material. Furthermore, increased graphitization degree of CMSs slightly improved the LOI values of PET composites, confirming the advantage of TCMSs as heat shields.

Effect of Graphitic Degree of CMSs on PET Thermal Decomposition Behavior

The thermal decomposition behaviors of the samples were investigated under nitrogen atmosphere, and the results with neat CMSs, TCMSs, and PET composites under a nitrogen atmosphere are presented in Fig. 6 and Table 2. It was observed that the oxygen-containing functional groups present on the CMSs surface started to decompose at a loss rate of 3%. The initial decomposition temperature of CMSs (Ts%) was determined as 376.5[degrees]C. As the temperature increased, the carbon structure of CMSs started to decompose until the residual char of CMSs reached 64.38%. After the annealing treatment, the thermal stability of the TCMSs obviously improved and the onset decomposition temperature reached 714.8[degrees]C. The final residual char of TCMSs rose to 95.5%, demonstrating that the graphitic layer present on TCMSs could effectively prevent the carbon decomposition.

To investigate the effect of CMSs and TMCSs on the thermal stability of PET, thermal degradation behaviors of PET composites with 1% CMSs or TCMSs were evaluated under an inert atmosphere, and the results are shown in Fig. 6 and Table 2. Compared to pure PET, the initial decomposition temperature ([T.sub.5%]) of the CMSs/PET composite increased by only 2[degrees]C but that of TCMSs/PET composite increased by 13[degrees]C. These data combined with those of DTG in Fig. 6(b) revealed that the temperature of maximum weight loss rate ([T.sub.max]) of TCMSs/PET composites shifted to higher temperatures (477.1[degrees]C). These shifts were nearly 12[degrees]C and 9[degrees]C higher than those observed with CMSs/PET and pure PET, respectively. The latter suggested that TCMSs played the main role in the thermal decomposition behavior of PET.

By contrast, the residual amounts present at 700[degrees]C reduced the char of pure PET to 15.18% and increased the char of TCMSs/PET composites by 3%. In summary, TCMSs improved the initial decomposition temperature, delayed the decomposition speed, and played a catalytic charring effect on the PET, which is in agreement with the results of TTI testing shown in Dispersion of CMSs and Graphitic TCMSs in PET Matrix section.

Effect of Graphitic Degree of CMSs on Residual Carbon Structure of PET

To explore the flame mechanism of CMSs and TCMSs in the PET matrix, the morphology and structure of residual carbon of PET composites were investigated. The contents of CMSs or TCMSs in the PET matrix were all 1 wt%. Figure 7 displays the digital photos and SEM images of PET composites residues after the CONE testing. The obtained morphology and density of the residues suggested that the residual layer formed by combustion of pure PET in Fig. 7al was far from intact since it contained a large number of sheet pieces and lacked larger free areas. Figure 7a2 showed the appearance of large pores with a loose carbon layer, implying the formation of a weak carbon layer unable to withstand heat and volatile combustible products. This, in turn, induced further decomposition of the inner polymer and weakened the flame retardancy of PET. By contrast, the carbon layer of CMSs/PET composites appeared completely formed, except for a few small apparent holes on the surface (Fig. 7b 1 and b2). The carbon layer was thicker and expanded randomly through the large amount of gas produced during the burning process of CMSs/PET composites, where the carbon layer played a role in cutting off the gas.

Compared to PET and CMSs/PET, Fig. 7c 1 and c2 revealed that the density of the formed carbon layer was the best as it contained the least amount of apparent holes. Moreover, the barrier property and strength of the carbon layer of TCMSs/PET significantly increased. The graphitized TCMSs effectively decelerated the decomposition rate of PET. Also, the char layer appeared denser, indicating that TCMSs played a condensed phase flame retardancy role. Also, an effective heat shield was formed during the burning of TCMSs/PET composites.

The chemical structure of residual char of PET composites after cone testing was further examined by FTIR. Figure 8a shows absorption bands at 2322 and 1700 [cm.sup.-1] for PET, which are assigned respectively to the carbon dioxide stretching vibration and C=0 stretching of PET main chain [16]. The peak at 1587 [cm.sup.-1] was associated with the C=C stretching of the aromatic nucleus absorption peak. The two peaks at 1159 and 755 [cm.sup.-1] were attributed to the stretching vibration of C(O)--O of the ester group and C--H bond of the benzene ring, respectively.

The char structure of CMSs/PET is shown in Fig. 8b. The strength of the stretching vibration peak of C=0 became weaker, implying an incomplete decomposition of the CMSs/PET. A new stretching vibration peak appeared at 1327 [cm.sup.-1] corresponding to crosslinked carbon [24], which further demonstrated that CMSs could catalyze PET to form a crosslinked carbon structure. This crosslinked structure played an important role in protecting the polymer from heat and oxygen, thus inhibiting the decomposition of PET. As a result, the absorption peak of PET in the decomposition product increased in intensity and the crosslinked carbon structure evolved. Compared to CMSs/PET, the peak at 1327 [cm.sup.-1] also appeared in the char structure of TCMSs/PET and strength of the crosslinked carbon structure increased, indicating the formation of more crosslinked carbon during the burning process of TCMSs/PET which acted as a strong "barrier" of heat.

The thermal oxidative stabilities of the carbon layer of PET composites were investigated by XRD analysis. The graphitization degree of the carbon layer determines the thermal oxidative stability [25]. The residue morphology data in Fig. 9a revealed that the carbon layer was subjected to fragmentation. This resulted in no apparent characteristic diffraction peak of carbon in the XRD profile shown in Fig. 9a. The latter confirmed the poor thermal oxidative stability of the carbon layer formed by PET. A new broad diffraction peak appeared at 0.3543 nm, which is similar to previous reports [25]. This peak was close to the characteristic peak of turbostratic carbon (0.3445 nm), corresponding to the (002) peak of graphitic carbon. The turbostratic carbon contained a number of stacked aromatic layers, each with a completely random orientation to the normal layer. In addition, a new weak diffraction peak associated with the (100,101) of graphitic carbon appeared at 0.20 nm. These two new peaks confirmed the FTIR data, indicating that CMSs changed the residue carbon structure of PET during the burning process to form crosslinking carbon.

The diffraction peak at 0.3543 nm of the TCMSs/PET composites became narrower, suggesting that the graphitization degree of the carbon layer was higher than that of the CMSs/PET composites. The thermal oxidative stability of the carbon layer formed by TCMSs/PET was stronger than those of CMSs/PET and pure PET. These data were consistent with the residue morphology results of Fig. 7c and the FTIR analyses. Therefore, the flame retardancy of TCMSs/PET was the best among all the composites, as confirmed by several features, including an increase in the initial decomposition temperature and ignition time during early combustion, decrease in heat release rate, catalysis of PET to form crosslinked carbon, improvement in thermal oxidation stability of the carbon layer during late burning, and increase in both the residual carbon amount and LOI value of PET.

Tensile Strength of CMSs/PET and TCMSs/PET Composites

In addition to flame retardant property, the mechanical property of PET composites was also investigated. Figure 10 shows the tensile strength of CMSs/PET and TCMSs/PET composites with different amounts of fillers. It was found that, at the same filler amounts, the tensile strength values of TCMSs/PET composites were all higher than those of CMSs/PET composites. As seen from the results of dispersion analysis in Fig. 3, the dispersion of TCMSs in the PET matrix was better than that of CMSs, which was the main reason for improving the tensile strength. The tensile strength values of TCMSs/PET with different filler amounts all exceeded 33 MPa. The tensile strength of general engineering plastic is 20 to 80 MPa, so the TCMSs can be used in engineering plastic applications. In addition, based on the thermal stability of TCMSs, they can withstand high processing temperatures. Moreover, TCMSs/PET can also be made into PET composite fibers by melt spinning method.

CONCLUSIONS

This study demonstrated that the graphitization degree of CMSs could be changed by annealing treatment. The graphitized TCMSs acted as an effective heat shield to improve the initial decomposition temperature and ignition time of PET, and also formed a dense turbostratic carbon layer. Higher graphitization degrees of CMSs could improve the flame retardancy of PET materials. This was reflected in several features, including the effective decrease in heat releasing rate, reduction in fire risk, catalysis of PET to form crosslinked carbon, and improvement in thermal oxidation stability of the carbon layer. With the addition of 2% TCMSs, the LOI of TCMSs/PET composites increased from 21% (neat PET) to 26.3%, the PHRR value decreased from 531.90 to 332.46 kW/[m.sup.2], and the residue amount increased from 9.31% to 19.31%. From a practical point of view, compared with other carbon materials (such as carbon nanotube, graphene, etc.), the manufacturing technology of the TCMSs is simple, low cost, and convenient. Overall, these findings suggest that graphitized TCMSs are efficient flame retardant materials which are promising for future use in firefighting applications.

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Baoxia Xue (ID), (1,2,3) Mei Niu, (1,3) Yongzhen Yang, (1,2) Weiya Wang, (3) Yun Peng, (3) Yinghao Song, (3) Xuguang Liu (1,4)

(1) Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, Taiyuan 030024, China

(2) Research Center on Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China

(3) College of Textile Engineering, Taiyuan University of Technology, Yuci 030600, China

(4) College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Correspondence to: M. Niu; e-mail: niumei@tyut.edu.cn or Y.Z. Yang; e-mail: yyztyut@l26.com

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51302183; U1607120; U1610255. DOI 10.1002/pen.24728

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. SEM images of annealed TCMSs at different temperatures: (al, a2) CMSs, (bl, b2) TCMSs at 500[degrees]C, (c1, c2) TCMSs at 700[degrees]C, and (dl. d2) TCMSs at 900[degrees]C. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. XRD spectra of CMSs and annealed TCMSs at different annealing temperatures: (a) CMSs, (b) TCMSs at 500[degrees]C, (c) TCMSs at 700[degrees]C, and (d) TCMSs at 900[degrees]C. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Raman spectra of CMSs and annealed TCMSs at different annealing temperatures: (a) CMSs, (b) TCMSs at 500[degrees]C, (c) TCMSs at 700[degrees]C, and (d) TCMSs at 900[degrees]C. [Color figure can be viewed at wileyonlinelibrary. com]

Caption: FIG. 4. FESEM images of CMSs/PET and TCMSs/PET with different contents: (a1) 0.5% CMSs, (a2) 0.5% TCMSs, (b1) 1% CMSs, (b2) 1% TCMSs, (c1) 2% CMSs, and (c2) 2% TCMSs.

Caption: FIG. 5. HRR profiles of (a) CMSs/PET and (b) TCMSs/PET composites. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. TG-DTG curves of CMSs, TCMSs, and PET composites under nitrogen atmosphere. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Digital photos and SEM images of residual char of PET composites after the CONE testing: (a1, a2) Pure PET, (b1, b2) 1 wt% CMSs/PET, and (c1, c2) 1 wt% TCMSs/PET. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. FTIR spectra of residual char of PET composites after the CONE testing. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. XRD spectra of PET composites after the CONE testing. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. The CONE parameters of CMSs/PET and TCMSs/PET composites.

                                         Time to PHRR
Sample             LOI (%)   TTI (s)   [t.sub.PHRR] (s)

PET                  21        90            195
CMSs/PET    0.5%    24.5       104           250
            1%      26.1       104           235
            2%      25.8       103           225
TCMSs/PET   0.5%    25.1       102           225
            1%      26.4       109           245
            2%      26.3       98            215

                        PHRR             THR          MEHC
Sample             (kW/[m.sup.2])   (MJ/[m.sup.2])   (MJ/kg)

PET                    531.90            74.2         20.84
CMSs/PET    0.5%       423.23            69.1         20.01
            1%         401.51            64.8         20.16
            2%         488.47            66.2         20.19
TCMSs/PET   0.5%       407.92            67.7         19.53
            1%         352.09            64.3         18.52
            2%         332.46            60.1         17.36

                         FGI           Residue
Sample             (kW/[m.sup.2] s)   amount (%)

PET                     2.7277           9.31
CMSs/PET    0.5%        1.6969          11.49
            1%          1.7086          15.41
            2%          2.1710          13.73
TCMSs/PET   0.5%        1.8130          16.47
            1%          1.4731          18.08
            2%          1.5463          19.29

FGI = PHRR/[t.sub.PHRR].

TABLE 2. Thermal properties of CMSs, TCMSs, and PET composites.

Sample         [T.sub.5%]    [T.sub.max]    700[degrees]C
              ([degrees]C)   ([degrees]C)    Char (wt%)

CMSs             376.5          573.1           64.38
TCMSs            714.8            --            95.50
PET              422.4          465.8           15.18
l%CMSs/PET       424.1          468.1           16.85
l%TCMSs/PET      435.1          477.1           18.60

FIG. 10. Tensile strength of CMSs/PET and TCMSs/PET composites.
[Color figure can be viewed at wileyonlinelibrary.com]

       Tensile Strength (MPa)

       CMSs/PET   TCMSs/PET

0.5%    35.22      39.23
1%      28.41      35.56
2%      24.50      33.21

Note: Table made from bar graph.
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Author:Xue, Baoxia; Niu, Mei; Yang, Yongzhen; Wang, Weiya; Peng, Yun; Song, Yinghao; Liu, Xuguang
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2018
Words:5931
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