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| Gas Production Characteristics of PVC Electrical/Thermal Decomposition in Stages Based on Experiments and Molecular Simulations |
| Wang Zihao, Gong Jingwen, Ye Zhuocheng, Zhou Kai, Gong Wei |
| School of Electrical Engineering Sichuan University Chengdu 610041 China |
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Abstract The safe and reliable operation of polymer-insulated cables is of great concern to the power and industrial sectors, since cable-related fires and insulation breakdowns contribute disproportionately to catastrophic accidents and economic losses. Among insulating polymers, polyvinyl chloride (PVC) is widely used in low-voltage control and distribution cables due to its low cost, satisfactory electrical performance, and processing flexibility. However, PVC is particularly prone to degradation under combined thermal and electrical stresses, and its decomposition releases a complex mixture of volatile products. These gaseous by-products not only accelerate further degradation but also serve as potential diagnostic markers for the incipient failure of cables. Despite this, the systematic study of staged gas-generation behavior of PVC under both thermal and electrical conditions remains limited.
Experimentally, PVC insulation layers were stripped from 220 V-rated control cables to preserve industrially relevant composition. TG-IR experiments were carried out from 50 to 800 °C under air and argon to capture thermal decomposition pathways, while high-energy discharge tests were designed to mimic breakdown events by gradually increasing applied voltage at 1 kV/min until failure, with evolved gases sampled for FTIR and mass spectrometry (MS) analysis. Results show that under thermal stress, PVC begins to decompose at about 222℃, exhibiting three main mass-loss peaks at 302℃, 459℃, and 559℃ in air, while only two peaks appear in argon due to incomplete oxidation of char. The gases detected include HCl, aromatic esters, long-chain alkanes, CO, and CO2 in air; and in argon, chloroacetaldehyde is uniquely observed at 559℃. In contrast, electrical breakdown instantly generates methane, ethylene, acetylene, hydrogen, hydrogen chloride, and chloroacetaldehyde. Gas concentrations in air are an order of magnitude higher than in argon, and in repeated breakdown tests, the yields of hydrogen and acetylene decrease while hydrogen chloride remains relatively constant.
On the modelling side, a representative periodic simulation box was constructed with three PVC chains, two polyester plasticizer chains, and oxygen molecules. ReaxFF simulations at 1 250 K under both pure thermal and electro-thermal coupling conditions reveal distinct bond-breaking patterns. Thermal decomposition proceeds gradually, dominated by end-group scission and radical recombination, whereas electro-thermal coupling strongly accelerates chain scission and radical evolution, leading to a 3~5 fold increase in acetylene and hydrogen production. Importantly, the polyester plasticizer is found to react with chlorine radicals, forming chloroacetaldehyde and reducing the radical lifetime. This stabilizing role of plasticizer chains helps explain experimental detection of ester-derived volatiles and their interaction with chlorine. Thus, the combined simulation and experimental evidence clarifies the synergistic and competitive pathways in PVC decomposition.
To enable practical application, a library of potential marker gases was constructed using DFT vibrational frequency calculations at the B3LYP/def2-SVP level, with frequencies scaled by 0.961. This database provides characteristic infrared peaks with a detection limit of 10×10-4%. The use of a non-negative least squares fitting algorithm allows reliable deconvolution of overlapping composite spectra into concentrations of individual gases, ensuring physically meaningful results. Validation on actual cable samples subjected to 300℃ overheating and breakdown shows clear distinctions: overheating is dominated by HCl and CO2, while breakdown produces abundant acetylene, propyne, and water vapor. These differences in gas composition and ratios provide a practical diagnostic basis for distinguishing fault modes.
A novel concept introduced in this work is that of incremental gases, defined as new gaseous products appearing in each decomposition stage compared with the previous stage. Tracking incremental gases clarifies the staged nature of PVC decomposition: low-temperature dehydration and radical release, mid-temperature dehydrochlorination and plasticizer breakdown, and high-temperature oxidation or carbonization. Under electrical discharge, the incremental gases are skewed toward unsaturated hydrocarbons and hydrogen, underscoring the strong influence of electro-thermal coupling on decomposition pathways. The study elucidates the independent and synergistic roles of the plasticiser in electro-thermal decomposition, and establishes an infrared spectral database of characteristic gases covering both electrical and thermal regimes, providing theoretical and data support for early cable-fault diagnosis based on gas infrared detection and extending to sensor calibration, machine-learning model training and thermal-stability evaluation of PVC composites.
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Received: 12 May 2025
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2025, Vol. 40 
