**1. Introduction**

Due to the outstanding electric insulation, prominent mechanical properties, high chemical resistance, natural flame retardant effect, ease of processing, efficient recyclability, etc., Polyvinyl chloride (PVC), one of the most extensively used plastics, has been widely applied in the wire and cable industry as the main constituent of insulation and sheathing [1–3]. However, PVC plastic that is commercially applied in cable sheathing is considerably flammable, even when treated by stabilizer, lubricant, plasticizer, and flame retardant [1]. The fire statistics illustrate that cable faults are among the most common causes of electrical fires, which involves the new and aged cables [4]. The aging degradation will lead to changes in initial properties as a result of simultaneous chemical and physical processes, causing changes in chemical composition and structure of materials [5]. There must be some differences of fire protection properties between the new and aged cables [2,4]. The outer PVC sheath is recognized as the main combustible part of a cable, and the investigation of its pyrolysis and combustion behavior is key to the study of fire properties of cables [4,6]. It will be aged firstly and fiercely by used for a long time. Limited systematic work has focused on comparing the pyrolysis and combustion properties between new and aged cable sheaths. Besides, as cables of the type used in this study are mostly used indoors, the temperature is considered as the most important factor to make

them aged during the long-term service [2,5]. Thus, the thermal aging is supposed to simulate the natural indoors aging process.

Many studies have been conducted on the thermal degradation characteristics and combustion properties of typical PVC cable [7–15]. Benes et al. [16] used the thermogravimetric analysis (TG) coupled to mass spectrometry and Fourier transform infrared (FTIR) spectroscopy to study the thermal degradation of PVC cable under different atmospheres. It was proposed that the pyrolysis of PVC backbone is accompanied by the release of HCl, H2O, CO2 and benzene. Gao [17] explored the pyrolysis characteristics of insulative PVC materials applied for fire retardant cable. Wang et al. [10] performed several TG experiments coupled with FTIR analysis to determine the pyrolysis behavior of PVC sheath of flame-retarded cables. They proposed that the pyrolysis process for PVC sheath could be divided into two regions and the amount of six components were detected. Courty et al. [18] employed two tests method for the characterization of PVC/PVC cable pyrolysis and flammability. Fernandez-Pello et al. [19] studied the fire performance of seven types of complex cables, including PVC cable that is commonly used in electrical installations and focused on the ignition and flame spread. Andersson et al. [20] carried out both small and large scale fire experiments with PVC sheathed cable with PVC insulation around the individual wires under well-ventilated and vitiated conditions. McGrattan et al. [21,22] investigated the cable heat release, ignition, and spread in tray installations during a fire, corresponding to the PVC cable. Grayson et al. [23] studied the fire performance of several types of electric cables containing PVC cable to design improved standard testing methods to determine the fire property of cables. Matala et al. [1] investigated the effects of the modelling decisions and parameter estimation methods on the pyrolysis modelling of two PVC cables. It should be noted that all above works focus on the fresh PVC cables. Certainly, there are some studied on the aged PVC cable [2–5,24–27]. Quennehen et al. [24] analyzed the two sets of single core cables with PVC insulation to determine the aging mechanism that is responsible for this decrease of electrical properties. Jakubowicz et al. [5] studied the effects of accelerated and natural ageing on plasticized PVC cable and concluded that the accelerated ageing did not significantly affect the tensile properties of the insulation materials. Yu et al. [12] summarized thermal degradation of PVC waste. Emanuelsson et al. [2] studied the effect of accelerated aging on the fire performance of building wires involving one PVC-based cable and one flame-retarded polyolefin-based cable using cone calorimetry and FTIR. Wang et al. [28] performed experiments to estimate the fire characteristics of new and aged building wires using a cone calorimeter. Xie et al. [4] employed TG, FTIR, and microscale combustion calorimetry (MCC) to investigate the fire protection properties of PVC sheaths for new and old cables: the old one was taken from an old building's electric power system, which had been in use for more than ten years; the new one represented a typical PVC cable manufactured at present. The new and old cables have significant differences in the compositions and structures due to the different commercial companies made at different time. Whereas, the current work is an integral study to compare the pyrolysis and combustion behaviors of the same cable sheath at different thermal aging degrees.

In this study, one flame-retardant PVC cable with different thermal aging degrees was adopted. The PVC sheath part removed from the cable was prepared to the follow-up tests. TG experiments were carried out to study the pyrolysis properties of PVC sheaths of new and aged cables with different heating rates (5, 10, 20, 30 and 40 K min−1) in nitrogen atmosphere. The onset temperatures of pyrolysis, mass loss, mass loss rate, and residue mass were recorded. Meanwhile, the gaseous release during the thermal degradation process was analyzed by TG coupled with FTIR spectroscopy. The combustion characteristics including heat release rate and total heat release were experimentally analyzed by MCC. In addition, a cone calorimeter was applied to investigate the time to ignition of new and aged cable sheaths. Finally, a comparison of the pyrolysis and combustion properties between new and aged cable sheaths was made and discussed.
