**2. Experimental**

#### *2.1. Sample Preparation*

The sheath sample used in the present study was obtained from a flame-retardant PVC cable (ZR-RVV) that was provided by Jiangsu Xinchangfeng Cable Co., Ltd. in Wuxi, China. The main components of the sheath were PVC, antimony trioxide (Sb2O3), plasticizers, etc. Flame retardant accounts for about 7 wt %. The flame-retardant PVC cable was subjected to different degrees of thermal aging treatment to obtain the aged sheath sample. A rough elemental analysis of the sample was conducted, and the measured results can be observed in Table 1. It can be found that the contents of carbon and chlorine are decreased while the content of oxygen is increased with thermal aging degree. That could be ascribed to the dehydrochlorination and oxidation of the material during the thermal aging [24,29]. Varied sample preparation methods were applied to the tests. For the microscale TG and MCC experiment, the samples were milled to less than 0.5 mm. For the bench-scale cone calorimetry experiment, the cable was cut into pieces 10 cm long with 10 mm thickness. Prior to testing, all of the samples underwent a drying procedure at 80 ◦C for approximately 1 h to remove moisture.

**Table 1.** Elemental analysis of new and aged cable sheaths.


#### *2.2. Thermal Aging*

Thermal aging of PVC sheaths was performed in a convection oven (GHX-100L, Hefei Anke Environmental Test Equipment Co., Ltd. in Hefei, China) in air at 100 ± 0.1 ◦C for 30 days and 60 days, respectively. After thermal aging, the samples presented a more pronounced brown color, as shown in Figure 1. Severe discoloration that occurred is due to the formation of chromophore groups [5]. The mass loss of PVC sheaths was monitored in the aging process using a precise balance, and the mass loss ratios were 10.2% for 30 days and 11.8% for 60 days when compared with the new sheath. Besides, the PVC sheath was also rigidized with prolonging the aging time. However, it is beyond the scope of the current study.

**Figure 1.** Digital photographs of polyvinyl chloride (PVC) sheaths before and after thermal aging.

#### *2.3. TG-FTIR Measurements*

The tests were conducted by a TA Instruments SDT Q600 thermal analyzer (TA Instruments in New Castle, PA, USA) coupled with FTIR spectroscopy (PerkinElmer in Waltham, MA, USA). The TG test ranged from 303 to 1073 K with a gas flowrate of 50 mL min−<sup>1</sup> in pure nitrogen atmosphere. Five typical heating rates 5, 10, 20, 30 and 40 K min−<sup>1</sup> were applied. The sample particles were placed into an alumina crucible with a mass of approximately 5 ± 0.2 mg in each test. A flow cell

with a recommended temperature of 280 ◦C was applied to the connection between the TG and FTIR spectrometer to prevent the condensation of gaseous products of the cable sheath. The spectral range was set as 4000–450 cm<sup>−</sup><sup>1</sup> with a scan frequency of eight times.

#### *2.4. Calorimeter Measurements*

The MCC tests of the samples were carried out using a microscale combustion calorimeter (MCC-2, Govmark in New York, NY, USA) based on ASTM D7309 [30] (Method A). A linear heating rate of 60 K min−<sup>1</sup> was applied in all tests. The temperature of the combustor was set at 1173 K. The heat release of the PVC sheaths of the new and aged cables was measured using the oxygen consumption principle. The heat release rate (HRR) as a function of time and sample temperature was experimentally analyzed and compared.

In general, the ignition of a cable essentially depends on the properties of the polymeric sheath. To obtain detailed information of the ignition risk of PVC sheathing, the time to ignition (TTI) of new and aged cables was measured based on a bench-scale cone calorimeter by Fire Testing Technology. Typical incident heat fluxes of 25, 35, 50, and 75 kW m<sup>−</sup><sup>2</sup> were selected for the ignition tests.

#### **3. Results and Discussion**

#### *3.1. Thermogravimetric Analysis*

Figures 2 and 3 describe the curves of TG and derivative thermogravimetric analysis (DTG) for PVC sheaths of new and aged cables under heating rates of 5, 10, 20, 30 and 40 K min−<sup>1</sup> in nitrogen atmosphere. It is noted that all TG curves for any sample show similar trends when ignoring the heating rates; the same result is obtained for the DTG curves. The thermal degradation process of the PVC sheath is generally defined as a two-step process, which has been reported and discussed by Zhu et al. [31] and Xie et al. [4]. In the first step, the dehydrochlorination of PVC polymer occurs accompanied by the formation of conjugated double bonds; the second step includes the continuous degradation of PVC polymer with cracking and pyrolysis, resulting in the formation of low hydrocarbons with linear or cyclic structures. Essentially, some weak points, including C–H, C–C, and C–Cl in PVC chains relate to the degradation of PVC polymer [32]. The pyrolysis residue has a conjugated polyene structure with cis- and trans-arrangements [10]. In the present TG and DTG profiles, three significant pyrolysis stages involving two strong peaks and one weak peak can be seen, as shown in Figures 2 and 3, whether in the new or aged cable sheath. The first DTG peak relates to the rapid volatilization and removal of HCl. The cleavage of the conjugated polyene chain and chain structural reconstruction in PVC and additives contribute to the second peak. The third peak may be attributed to the sluggish degradation of remaining residues. In addition, a displacement of TG curves and an increase of DTG peaks with heating rates are observed, as described by previous researchers [32–35].

The TG and DTG curves corresponding to new and aged PVC sheaths at different heating rates (5, 10, 20, 30 and 40 K min−1) in nitrogen atmosphere are depicted in Figures 4 and 5, respectively. Figure 4 illustrates that there is no difference in the TG curves of new and aged PVC sheaths in the initial pyrolysis stage. However, the TG curve of the new cable sheath decreases faster than that of the aged one with increasing sample temperature. The mass of lost residue from the new cable sheath is significantly less than that of the aged one. Namely, the new cable sheath experienced a more sufficient pyrolysis process than the aged one. In addition, the lower the heating rate, the less the residue mass. Overall, the pyrolysis difference between new and aged cable sheaths is clearly visible, regardless of the heating rates. Figure 5 shows that the onset temperature, which is related to the ignition time of polymer in fire, shifts slightly upward for the aged cable sheath at all heating rates. This indicates that the aged cable sheath pyrolyzes later than the new one. The values of DTG peaks for the aged cable sheath are higher than those of the new one. Table 2 lists the typical pyrolysis parameters of PVC sheaths of new and aged cables in nitrogen atmosphere with different heating rates. Note that the changes of chemical composition, structure, and additives in polymer after long-term thermal aging are the main reason for the pyrolysis difference of PVC sheaths between aged and new cables. However, it can be noted that the *Tonset* and *DTGpeak* for the aged sheath of 60 days is lower than that of 30 days, which may be attributed to the complex factors, including the dehydrochlorination, consumption of stabilizers and the aggravated chain scission and cross-linking.

**Figure 2.** Mass loss (thermogravimetric analysis (TG)) curves of PVC sheaths at different heating rates in nitrogen atmosphere: (**a**) new; (**b**) aged 30 days and (**c**) aged 60 days.

**Figure 3.** Mass loss rate (derivative thermogravimetric analysis (DTG)) curves of PVC sheath at different heating rates in nitrogen atmosphere: (**a**) new; (**b**) aged 30 days and (**c**) aged 60 days.

*Materials* **2018**, *11*, 1997

In addition, the effect of the flame-retardant on the thermal degradation of PVC sheath was also analyzed. As shown in Figures 4 and 5, the *Tonset* of new PVC sheath is lower than that of the pure PVC raw material when the flame retardant is added to the feedstock. However, the new PVC sheath exhibits a lower mass loss rate of while gives a more final residue, as compared with the pure PVC raw material. The whole pyrolysis process has been prolonged after the flame retardant added. Meanwhile, there is an obvious decrease of the main *DTGpeak* for pure PVC raw material. The results are primarily attributed to the formation of volatile SbCl3 and SbOCl between the antimony trioxide and PVC during the thermal degradation. Besides, it should be noted the losses of both chlorine and antimony content are clearly seen after thermal aging from the element analysis, which is agreemen<sup>t</sup> with the previous works [15,36]. They also proposed that SbCl3 production was possible by reaction of HCl with Sb2O3 without involving the high-temperature disproportionation reactions of intermediate SbOCl. Namely, the higher *Tonset*, larger *DTGpeak* and less HCl gas emission may be explained by this point, to some extent. The detailed study of this point is beyond the scope of the current work, taking the experimental design, thermal aging period, etc. into account. However, the relevant research will be conducted in the next plan.

**Figure 4.** Comparison of TG of PVC sheaths for new and aged cables at different heating rates in nitrogen atmosphere. (**a**) 5 K min−1; (**b**) 10 K min−1; (**c**) 20 K min−1; (**d**) 30 K min−1; (**e**) 40 K min−<sup>1</sup> and (**f**) 30 K min−<sup>1</sup> involving raw materials.

**Figure 5.** Comparison of DTG of PVC sheaths for new and aged cables at different heating rates in nitrogen atmosphere. (**a**) 5 K min−1; (**b**) 10 K min−1; (**c**) 20 K min−1; (**d**) 30 K min−1; (**e**) 40 K min−<sup>1</sup> and (**f**) 30 K min−<sup>1</sup> involving raw materials.


**Table 2.** Pyrolysis parameters of PVC sheaths for new and aged cables in nitrogen atmosphere with different heating rates.

## *3.2. FTIR Analysis*

FTIR spectroscopy is an effective method to analyze the evolved gas products of materials in the pyrolysis process. Figure 6 gives the three-dimensional (3D) surface for the FTIR spectra of gases that are released by new and aged cable sheaths. It is clearly shown that the temperature range of gases released is well consistent with the evolution of DTG, regardless of the third relatively weak peak. In addition, gases are produced from the new cable sheath much earlier than the aged one. However, the kinds and concentrations of gases released in pyrolysis between new and aged cable sheaths have little difference. Table 3 presents typical gases released from PVC sheaths in nitrogen atmosphere at a heating rate of 30 K min−<sup>1</sup> at different times. It suggests that the main products including HCl, CO2, and H2O during the whole pyrolysis process are similar for new and aged cable sheaths. However, more dangerous products, such as alkene, benzene, styrene, etc., are not involved in current work. The spectrum band of 2400–2260 cm<sup>−</sup><sup>1</sup> is associated with CO2. HCl can be observed in the band range of 3100–2600 cm<sup>−</sup>1, and the absorption band of H2O corresponds to 1800–1300 cm<sup>−</sup><sup>1</sup> and 4000–3500 cm<sup>−</sup>1. The spectrum band in the range of 700–550 cm<sup>−</sup><sup>1</sup> is attributed to the stretching vibrations of C–Cl. The results show that the gases from the aged cable sheath are released later, but more quickly, than those by the new one, especially HCl gas.


**Table 3.** Gases released from PVC sheaths in nitrogen atmosphere at a heating rate of 30 K min−<sup>1</sup> with different times.

**Figure 6.** Typical three-dimensional thermogravimetric analysis-Fourier transform infrared (3D TG-FTIR) spectrogram of PVC sheath at a heating rate of 30 K min−1: (**a**) new; (**b**) aged 30 days; and, (**c**) aged 60 days.

## *3.3. MCC Analysis*

The HRR curves of PVC sheaths for both new and aged cables are shown in Figure 7. Two peaks of heat release rate (PHRRs) can be observed for new and aged sheaths. The value of the first HRR peak for the new cable sheath is greater than that of the aged one, and the value of the second HRR peak has a slight difference. The first HRR peak is produced by the combustion of pyrolysis gases, and the second one is formed by the char oxidation. The result indicates that the faster and more combustible gases were released for aged cable sheath during MCC tests, which can be supported by the measurements of FTIR. The temperature corresponding to two HRR peaks (TPHRRs) laterally shifts upward, and the duration from onset decomposition to PHRR of the aged cable sheath is longer than that of the new one. That means that the decomposition and combustion of aged cable sheath would be slightly weak due to the change of composition and structure. Figure 8 gives the integral HRR (total heat release (THR)) of PVC sheaths for new and aged cables. THR is related not only to the HRR peak, but also to the detailed pyrolysis process. Compared with the aged cable sheath, the THR value of the new cable sheath is relatively high. Table 4 presents the MCC data of PVC sheaths of new and aged cables. Where HRC is the capacity of heat release, PHRR is the maximum value of two peaks, TPHRR is the temperature corresponding to the first peak of HRR, and THR is defined as the value at the final temperature. It implies that the new cable sheath would burn more fiercely and amply, accompanied by more heat release, which is in good agreemen<sup>t</sup> with the above TG analysis.

**Figure 7.** Heat release rate of PVC sheaths for new and aged cables: (**a**) heat release rate (HRR) vs. time and (**b**) HRR vs. temperature.

**Figure 8.** Integral HRR (total heat release (THR)) of PVC sheaths for new and aged cables.

**Table 4.** MCC data of new and aged cable sheaths.


#### *3.4. Cone Calorimetry Analysis*

Figure 9 shows the time to ignition (TTI) of new and aged cable sheaths under the varied incident heat fluxes ranging from 25 kW m<sup>−</sup><sup>2</sup> to 75 kW m<sup>−</sup>2. The TTI value of the new cable sheath is greater than that of the aged one at different incident heat fluxes. The TTI difference between new and aged cable sheaths decreases with the increasing incident heat flux. The aged cable sheath has a maximal ignition time 93% greater than that of the new cable sheath at 25 kW m<sup>−</sup>2. This result is consistent with the onset temperature and thermal degradation process analysis of pyrolysis. It suggests again that the new cable sheath has a higher ignition risk.

**Figure 9.** Time to ignition (TTI) of PVC sheaths for new and aged cables with different incident heat fluxes.
