📄 chemistry

Study on Multiphase Pyrolysis Kinetics and Fire Early Warning Characteristics of Cable Sheaths and Insulating Layers

This study integrates thermal analysis and kinetic modeling to elucidate how graded mineral fillers in multilayer cable sheaths govern pyrolysis mechanisms and volatile emissions, ultimately establishing a two-tier fire early-warning paradigm based on specific temperature thresholds and characteristic gas markers for enhanced urban utility tunnel safety.

Original authors: Yujie YIN, Lei Lei, Zeyu Wang

Published 2026-09-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Yujie YIN, Lei Lei, Zeyu Wang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Beneath the streets of modern cities lies a complex network of utility tunnels, carrying the electrical power that keeps our lights on and our hospitals running. Within these dark, confined spaces, thick bundles of cables snake through the infrastructure, protected by layers of plastic and rubber. These outer layers, known as sheaths, and the inner insulation are the first line of defense against fire. However, when these materials overheat, they do not simply melt; they undergo a chemical breakdown called pyrolysis, where heat forces the solid plastic to decompose into gases. Some of these gases are toxic, while others are flammable, potentially turning a small electrical fault into a catastrophic blaze. Understanding exactly how and when these materials break down is critical for designing early warning systems that can detect a fire before it spreads, buying precious time for evacuation and intervention.

A team of researchers from the China People's Police University and Hangzhou Public Transport Co., Ltd. set out to map this hidden chemical process with greater precision than ever before. Instead of looking at cable materials as simple, uniform blocks, they treated them as complex, multi-layered systems where the amount of mineral filler inside the plastic changes how it behaves under heat. They focused on four specific types of materials found in real-world cables: a standard plastic sheath, a flame-retardant sheath with a moderate amount of mineral additives, a fire-resistant sheath packed with a high concentration of minerals, and the inner insulation core made of cross-linked polyethylene. By heating these samples in a controlled environment while simultaneously measuring how much mass they lost and analyzing the specific gases they released, the researchers could watch the breakdown happen in real time.

The study revealed that the inner insulation and the outer sheaths fail in fundamentally different ways. The inner insulation, which protects the electrical current, behaves like a single, solid unit. When it reaches a critical temperature between 400 and 500 degrees Celsius, it undergoes a rapid, single-stage collapse, breaking apart all at once and releasing large amounts of methane and other hydrocarbon gases. In contrast, the outer sheaths, which are designed to protect the cable from the outside world, degrade in a more complex, multi-stage process. The researchers found that the amount of mineral filler inside these sheaths acts as a regulator for this breakdown. In a standard plastic sheath with no added minerals, the material breaks down quickly at a lower temperature, releasing a massive amount of weight as toxic hydrogen chloride gas and flammable fragments.

As the researchers increased the amount of mineral filler, the behavior of the sheath changed dramatically. In the flame-retardant sheath with a moderate amount of minerals, the breakdown slowed down, and the release of dangerous gases was reduced. The minerals acted like a diluent, spreading out the combustible plastic and making it harder for the heat to travel through the material. In the fire-resistant sheath, which contained the highest concentration of minerals, the effect was even more profound. The minerals did not just slow the process; they fundamentally altered the chemistry of the breakdown. At high temperatures, these minerals fused together to form a hard, ceramic-like barrier that trapped the gases inside. This barrier was so effective that it reduced the release of methane, a key fuel for fire, by over two orders of magnitude compared to the standard plastic.

The researchers also discovered that the energy required to break the chemical bonds in these materials shifted depending on the filler content. For the standard plastic, breaking the bonds required a high amount of energy, but as the mineral content increased, the apparent energy needed to start the breakdown actually decreased. This counterintuitive result happened because the minerals disrupted the continuous structure of the plastic, forcing it to break apart in smaller, localized pockets rather than as a whole sheet. However, in the final stage of the fire-resistant sheath's breakdown, the energy requirement jumped back up. This was because the minerals had to undergo a difficult physical transformation, sintering together to form that protective ceramic shell, a process that consumes significant heat.

By combining these observations of heat, mass loss, and gas release, the team established a new, two-tiered system for detecting cable fires. The first warning stage, or Tier I, occurs between 270 and 310 degrees Celsius. At this point, the outer sheaths begin to degrade, releasing hydrogen chloride and carbon monoxide. This is the signal that the protective jacket is failing. The second, more critical stage, or Tier II, happens around 450 degrees Celsius, when the inner insulation begins to collapse. This stage is marked by a sudden release of methane, ethylene, and acetylene. This two-stage approach allows monitoring systems to distinguish between a minor overheating event that can be managed and a catastrophic failure that requires immediate emergency response. The study confirms that by understanding the specific chemical signatures of different cable materials, engineers can build smarter, more sensitive safety systems for the underground tunnels that power our cities.

Technical Summary: Multiphase Pyrolysis Kinetics and Fire Early Warning Characteristics of Cable Sheaths and Insulating Layers

Problem Statement
Current fire safety protocols for power cables in confined spaces, such as urban utility tunnels, often rely on reductionist experimental paradigms that analyze single-component polymer systems. This approach fails to resolve the interdependent, multi-phase degradation mechanisms inherent in multilayered cables, where inorganic filler concentration gradients significantly modulate thermodynamics and kinetics. Consequently, existing monitoring systems are limited to binary, single-threshold alarm logic, lacking the predictive capability to distinguish between sheath degradation and catastrophic insulation failure. This study addresses three specific knowledge gaps: (i) the absence of quantitative comparisons between filler-tuned outer sheaths and thermally robust XLPE insulation under fire-relevant heating; (ii) insufficient insight into the evolution of apparent activation energy ( $E_a$ ) and diagnostic gas species as a function of filler content; and (iii) a lack of integrated structure–property–response relationships linking microstructural evolution to gas-phase signatures.

Methodology
The research systematically evaluated four representative cable materials: conventional PVC outer sheath, flame-retardant ZR-YJLV outer sheath (moderate inorganic filler loading), fire-resistant N-BV outer sheath (high inorganic filler loading), and the ZR-YJLV cross-linked polyethylene (XLPE) insulation core. Samples were prepared via cryogenic ball milling and dried to ensure compositional integrity.

Thermal decomposition was analyzed using a synchronized Thermogravimetric Analysis–Fourier Transform Infrared (STA–FTIR) hyphenated system under non-isothermal conditions (heating rates of 10, 15, 20, and 25 °C/min in an air atmosphere). The experimental workflow captured macroscopic mass loss (TG/DTG), kinetic profiles, and real-time 3D FTIR gas evolution.

To quantify the kinetics, the study employed model-free (isoconversional) methods, specifically the Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) techniques. These methods were used to calculate the apparent activation energy ( $E_a$ ) across conversion degrees ( $\alpha$ ) from 0.10 to 0.90, ensuring statistical robustness without pre-assuming a reaction mechanism.

Key Results

Thermogravimetric Behavior:
- Insulation (XLPE): Exhibited a single-step decomposition process centered between 400–500 °C, characterized by a sharp mass loss peak at 458.9 °C (at 10 °C/min), indicative of random scission in the crosslinked aliphatic backbone.
- Sheaths (PVC, ZR-YJLV, N-BV): All displayed multi-step degradation dominated by an initial dehydrochlorination phase (270–310 °C). The inclusion of inorganic fillers significantly attenuated the initial mass loss rate: conventional PVC lost 65.1% of its mass in the first stage, compared to 45.1% for ZR-YJLV and 41.6% for N-BV.
Kinetic Analysis ( $E_a$ ):
- Filler Effect: In the second decomposition stage, $E_a$ decreased progressively with increasing filler content: 208.53 kJ/mol (PVC) > 114.07 kJ/mol (ZR-YJLV) > 91.53 kJ/mol (N-BV). This trend confirms a "progressive inert dilution effect," where dense mineral fillers disrupt polymer continuity and lower the macroscopic energy barrier for mass loss.
- High-Temperature Transition: The N-BV sheath exhibited a unique kinetic shift in the final stage, where $E_a$ increased to 160.96 kJ/mol. This is attributed to the high energy consumption required for a large amount of inorganic powder to overcome solid-phase migration resistance and undergo microscopic sintering into a dense ceramized skeleton.
- Insulation Kinetics: The XLPE insulation showed a unimodal $E_a$ profile, peaking at 115.33 kJ/mol during primary degradation before declining to 94.90 kJ/mol as the structure collapsed without char formation.
Gas Evolution (FTIR):
- Sheaths: Early-stage emissions (270–310 °C) were dominated by HCl. As filler content increased, the release of hydrocarbons (CH₄) was suppressed by over two orders of magnitude in N-BV due to the formation of a low-permeability ceramic char barrier. Medium-to-high fillers (ZR-YJLV) showed enhanced H₂O vapor release, attributed to the endothermic dehydration of hydroxide fillers.
- Insulation: The XLPE core released significant quantities of CH₄ and aromatic compounds at high temperatures (~455 °C), signaling backbone cleavage.

Significance and Contributions
The primary contribution of this work is the establishment of a two-tier early-warning paradigm for tunnel cable fire monitoring, derived from the distinct kinetic and chemical signatures of sheath versus insulation degradation:

Tier I (Precursor Alert, 270–310 °C): Utilizes HCl and nascent CO as indicators of sheath degradation (dehydrochlorination). This stage signals the onset of thermal failure in the outer protective layers.
Tier II (Critical Escalation Alert, > 450 °C): Utilizes the abrupt release of CH₄, C₂H₄, and C₂H₂ as markers of irreversible XLPE insulation breakdown.

By integrating these hierarchical inhibition mechanisms with the intrinsic vulnerability of the insulation core, the study provides a defensible basis for moving beyond binary alarms to multi-parameter, predictive fire monitoring systems. The findings offer fundamental thermo-kinetic parameters necessary for designing adaptive mitigation strategies and risk assessment models for underground utility tunnels, specifically addressing the limitations of current single-threshold detection logic.

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