Study on the pyrolysis characteristics of styrene-grafted polypropylene cable insulation material

Power cables are one of the crucial infrastructures in the power grid, and their safe and stable operation is essential for power transmission.¹ Compared with traditional cross-linked polyethylene (XLPE) insulation, polypropylene (PP) based insulation material has excellent electrical performance. PP has a melting temperature exceeding 160 °C, which is more than 40% higher than XLPE. This elevated maximum operating temperature, reaching 100–120 °C in long-term usage, is of significant importance for increasing the current-carrying capacity and voltage levels of cables.^2–4 In overload and short circuits, the temperature of the high voltage cable can rise rapidly in a short time, thus causing the breakdown accident of the cable body, which seriously threatens the stable operation of the power system.⁵ In order to avoid such cable failure caused by overheating, the thermal fault state assessment technology of the characteristic gas plays a key role in the cable state assessment by monitoring the changes in the characteristics of the cable gas product concentration.⁶ 

Yi et al. analyzed the characteristic substances released by the overheated polyvinyl chloride (PVC) cable and realized the early warning of electric fire.⁷ Zhou et al. conducted a gas detection study on the cable buffer layer and proposed a method to prevent high-voltage cable buffer layer erosion.⁸ Wang et al. used the SDT Q600 thermal analyzer and Fourier infrared spectroscopy (FTIR) to study the thermal behavior of flame retardant high voltage cable sheaths and insulation materials and obtained the content order of six main gas components produced by the decomposition of cable sheaths.⁹ Wan et al. used the tubular furnace reactor-mass spectrometry (MS) experimental platform to study the mixed samples at 300–700 °C and proposed a new decomposition path.¹⁰ All these studies introduce fault warning or condition assessment approaches based on the analysis of decomposition products from cable insulation materials, highlighting the valuable and meaningful application of thermal fault status assessment technology. However, there is currently relatively limited research on the thermal decomposition of polypropylene and modified polypropylene cable insulation materials. This study addresses this gap in the research.

The study of the pyrolysis process of the cable insulation material is helpful to deeply understand the gas production characteristics of the cable insulation layer under actual working conditions. So far, researchers have been working on studying the pyrolysis process of power cable materials. Thermogravimetric-Fourier infrared spectroscopy (TG-FTIR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) are common experimental methods to study the behavior and mechanism of pyrolysis.¹¹ Kong et al. conducted TG-MS analysis of the pyrolysis gas of each part of the cable materials and found that different cable faults showed different gas generation characteristics.¹² Beneš et al. conducted the thermal decomposition process at a temperature of 20–800 °C and found that the PVC main chain began to degrade in the range of 200–340 °C and released HCl, H₂O, CO₂, and benzene.¹³ Fedelich introduced a new method for combining TG with GC/MS and conducted a quantitative study of styrene butadiene rubber (SBR) pyrolysis gas, demonstrating the potential of this new combination.¹⁴ Du et al. studied the pyrolysis of crosslinked polyethylene using TG-DSC and FTIR techniques, and they found that olefins are the main pyrolysis product.¹⁵ The researchers mentioned above conducted thermal decomposition experiments on various insulation materials using Thermogravimetric Analysis (TG), obtaining the corresponding thermal decomposition products. Subsequently, they employed various analytical methods such as Fourier Transform Infrared Spectroscopy (FTIR), Gas Chromatography (GC), and Mass Spectrometry (MS) to analyze these decomposition products. This indicates that there is already a substantial research foundation for analytical methods based on thermal decomposition products. Their research work provides important information for understanding the pyrolysis characteristics of cable insulation materials, but no scholar has studied the pyrolysis characteristics of polypropylene cable insulation materials. This study addresses this gap by employing the TG-MS method to study the thermal decomposition characteristics of both polypropylene and modified polypropylene, filling this void in the research field.

Traditional experimental methods make it difficult to capture structural changes at the atomic level and cannot fully explain the formation mechanism of pyrolysis gas. With the development of computer technology and simulation technology, the study of chemical reactions from a microscopic perspective is also increasing. Theoretical calculation methods such as reaction molecular dynamics (MD) and density functional theory (DFT) show great potential in elucidating the chemical reaction mechanism occurring in a very short time.^16–18 The most effective method is the MD simulation based on the ReaxFF force field, which describes larger systems and dynamic chemical reaction processes. The precision of ReaxFF near density functional theory, especially in assessing reaction barriers and reaction energies, can be used to study macromolecular systems involved in chemical reactions, which plays a key role in interpreting experimental phenomena and predicting theoretical results. Furthermore, ReaxFF combines the bond order concept and a polarizable charge model able to simulate bond breaking and formation and associated charge rearrangements during energy-conserved MD simulations. He and Chen employed reactive force field molecular dynamics (ReaxFF MD) in conjunction with the auto-reaction mechanism analysis (AutoRMA) tool to investigate the pyrolysis reaction mechanisms of typical waste plastics, including polyethylene (PE), polypropylene (PP), and polystyrene (PS), from three aspects: dynamics, pyrolysis products, and the pyrolysis reaction process. The study revealed that with an increase in pyrolysis temperature, there was a notable increase in the yields of hydrogen (H₂) and methane (CH₄). However, the production rates of ethylene (C₂H₄) and propylene (C₃H₆) exhibited a pattern of initially rising and then decreasing for the respective products.⁵ Shao et al. used GC-MS, ReaxFF simulation, and DFT calculation to study the hydroxy radical depolymerization cellulose mechanism, indicating that the GC-MS based method to determine the components of the depolymerization product and ReaxFF reaction dynamics simulation are effective methods to study the reaction pathway.¹⁹ Liu et al. studied the high-density polyethylene system using ReaxFF MD, analyzed the detailed reaction mechanism in the simulation trajectory, and proved that ReaxFF MD was applied to better deepen the systematic understanding of the polymer pyrolysis process.²⁰ Guo et al. studied the pyrolysis process of polypropylene cable insulation and the generation pathway of pyrolysis gases using TG-GC experiments and reactive molecular dynamics simulations, indicating that the pyrolysis gases primarily consisted of H₂, CO, C₂H₄, and CH₄.²¹ The aforementioned researchers conducted in-depth studies on polymeric compounds through Molecular Dynamics (MD) simulations and Density Functional Theory (DFT) calculations. By simulating the decomposition process of polymeric compounds, it is demonstrated that MD simulations are feasible for investigating the decomposition of polymeric compounds. Higher temperatures were applied in the ReaxFF MD simulations to facilitate sufficient atomic motion and molecular collisions to accelerate the reaction process. This acceleration is widely used in the study of oxidation, pyrolysis, and gasification of hydrocarbons, coal, and biomass, with no effect on the reaction mechanism, and many studies show that the results obtained by increasing the temperature in ReaxFF MD are consistent with the experimental results. In conclusion, the ReaxFF MD method can effectively reflect the pyrolysis process of macromolecular organic matter.

In this study, the thermogravimetric-gas chromatography (TG-GC) experiment was used to analyze the pyrolysis of styrene-grafted polypropylene (PP-g) and pure polypropylene (PP) cable insulation material. The pyrolysis processes of polypropylene cable insulation materials and pure PP were simulated using reactive force field molecular dynamics (ReaxFF MD). The resulting pyrolysis products were analyzed, and a comparison was made between the types and proportions of products before and after the modification of PP. This comparison revealed the relationship between decomposition products, decomposition temperature, and the thermal properties of PP. This study addresses the gap in the research field of thermal decomposition studies on polypropylene cable insulation materials using the GC-MS method. The findings of this study will provide theoretical support for the insulation status monitoring of polypropylene-based cables.

The styrene-grafted polypropylene medium voltage power cable was purchased from Ningbo Dongfang Cable Co., Ltd. Pure polypropylene cable was customized by the same company and utilized the same additives. After dismantling the cable, samples are taken from the insulation layer, and the insulation material is sliced to produce granular samples. The sampling process is illustrated in Fig. 1.

Thermal decomposition characteristics were studied using a Mettler Toledo thermal gravimetric analyzer (METTLER TGA/DSC 3+) produced by the Swiss company Mettler Toledo, with a heating rate of 20 °C/min in an oxygen atmosphere.

In the TGA experiment, the sample weighed 10 mg (±0.05 mg), thermal decomposition experiments were performed in a 50 ml/min oxygen atmosphere, and the sample temperature was heated from 30 to 800 °C to obtain the thermal weight loss curve of pure PP and styrene-grafted polypropylene cable insulation material in oxygen.

For the thermal decomposition experiment, the sample weighed 10 mg (±0.05 mg) and was used in a dry air atmosphere of 50 ml/min. The sample was heated from 30 to 270, 280, 290, and 300 °C in a nitrogen atmosphere, respectively. Then, the gas was switched to dry air, and the temperature was maintained for 1 h. The pyrolysis gas was collected at the outlet of the furnace body at 5 min intervals. The GC 5890N gas chromatography was produced by Nanjing Kejie Analytical Instrument Co., Ltd. (Fig. 2).

Polypropylene (PP) is a polymer with extremely long molecular chains. Considering the limitations of computing ability, the model should be built with an appropriate chain length and number of atoms. In order to ensure the unity of the relative molecular mass, the model of styrene-grafted polypropylene (PP-g) was built first. Then, based on its relative molecular mass, a model for polypropylene (PP) is constructed.

Establishing the graft-modified polypropylene (PP-g) model: a long chain of ten propylene monomers was used as a polypropylene single chain, as shown in Fig. 3(d), to replace the hydrogen element on the tertiary carbon atoms of the grafted styrene monomer to obtain the grafted polypropylene subchain, as shown in Fig. 3(e).

Building the PP-g/O₂ System: Five optimized PP-g long chains and 50 oxygen molecules were randomly placed into a 3D periodic crystal cell with an initial density set to 0.92 g/cm³, which is close to the actual density. The model is shown in Fig. 3(f). Then, the geometry of the established model was optimized for annealing cycles of 298–450 K to obtain the final PP-g/O₂ system.

The relative molecular mass of PP-g cells was calculated, and the same approach was used to construct a PP/O₂ system with a similar relative molecular mass. The model is depicted in Fig. 4.

In ReaxFF MD simulations, the NVT ensemble was employed, and the N–H thermostat method was used. Reaction molecular dynamics simulations were conducted at temperatures of 3050, 3300, 3550, 3800, and 4050 K, with a time step of 0.25 fs and a simulation duration of 1000 ps.

The TG curves of PP and PP-g cable insulation materials are shown in Fig. 5. Define the temperature corresponding to a 5% mass loss of the material as the decomposition temperature.

There is an obvious difference between the thermogravimetric curves of PP and PP-g cable insulation materials. The initial decomposition temperature of PP is 268 °C, while the initial thermal decomposition temperature of PP-g is 275.4 °C, indicating that the grafting modification improves the heat resistance of PP. In addition, in the later stages of pyrolysis, PP-g undergoes an earlier completion of thermal decomposition, with its residual components slowly decomposing at higher temperatures, continuing until complete decomposition occurs only at 500 °C. Therefore, we can infer that graft modification generally enhances the heat resistance of polypropylene. Simultaneously, during the modification process, it also generates products that are more difficult to decompose. The determination of the thermal decomposition temperature will guide the temperature selection for the subsequent thermal decomposition experiments.

In the thermal decomposition experiments of pure PP and PP-g cables at different temperatures, the trend of mass percentage over time is shown in Figs. 6 and 7. As can be seen from Figs. 6 and 7, after one hour of decomposition experiments, the residual mass of both materials showed a trend of decreasing with the increase in experimental temperature, and the percentage of PP-g cable insulation material at all four temperatures was higher than the percentage of the residual mass of pure PP after thermal decomposition at the same temperature.

The decomposition process of PP cable insulation material is shown in Fig. 6. It can be seen that the decomposition process at 270 and 280 °C shows a certain difference from the trend at 290 and 300 °C. The decomposition process of the former is relatively stable in terms of mass decline, while at higher temperatures (290 and 300 °C), the trend of mass decline is more significant.

The decomposition process of PP-g cable insulation material is shown in Fig. 7. It can be seen that the change in temperature has a more uniform influence on the decomposition process compared with PP.

The trend of percent residual mass after 1 h of pyrolysis with temperature is shown in Fig. 8. It can be seen that with the increase in temperature, the remaining mass of PP-g cable insulation material is higher than that of pure PP.

The order of the main pyrolysis gas content is H₂ > CO > CH₄ > C₂H₄ > C₃H₆ > C₂H₆.

Two materials’ trends in generating H₂ and CO through thermal decomposition vary with temperature, as shown in Figs. 9 and 10.

The concentration of CH₄, C₂H₄, C₃H₆, and C₂H₆ generated from the thermal decomposition of two materials at different temperatures is illustrated in Figs. 11 and 12.

In the process of thermal decomposition, the long chain of the polymer breaks first, forming substances with a shorter chain length and then small molecule products. The comparison of Figs. 9–12 shows that the content of H₂ and CO in the product is significantly higher than that of hydrocarbons, and the proportion of H₂ and CO in the decomposed gas increases with the increase of temperature, and the concentration ratio of hydrocarbons is significantly decreased with the increase of temperature.

As can be seen from Figs. 11 and 12, the concentration of CH₄ and C₂H₄ decreases with increasing temperature, while the concentration of C₂H₆ increases with temperature. In addition, these trends are more pronounced in the decomposition products of PP-g. Simultaneously, the decreasing trend in the proportion of hydrocarbons in the total decomposition products with rising temperature highlights the change in C₂H₆ concentration. The concentration of C₃H₆ does not change significantly with temperature. At lower temperatures, it is easier to produce CH₄ and C₂H₄, while at higher temperatures, it is easier to produce C₃H₆. This aligns with the rule that molecules with higher relative molecular masses have higher thermal decomposition temperatures, whereas molecules with lower relative molecular masses have lower thermal decomposition temperatures.

Figure 13 illustrates the trend of the ratio between C₂H₆ and (CH₄ + C₂H₄) in the thermal decomposition experiment with C₂H₆, varying with temperature. In the decomposition products of both the PP-g insulating material and pure PP, the ratio of the molecular weights of C₂H₆ to (CH₄ + C₂H₄) demonstrates an increasing trend with rising temperature, with the trend being more pronounced in the case of the PP-g insulating material.

Simulations were conducted at five evenly distributed temperatures between 3050 and 4050 K. The trends in the simulated products remained consistent across all five temperatures. We chose to showcase the results at the midpoint of these five temperatures, 3550 K, as we believe it represents the simulation outcomes across varying temperatures. Table I shows the main gaseous products after the completion of thermal decomposition at 3550 K. The primary gaseous products from the thermal decomposition of PP and PP-g cable insulation materials are H₂, CH₄, C₂H₄, and CO. Among the decomposition products, the higher content of hydrocarbons includes C₂H₂, C₂H₄, CH₄, and C₃H₆, where the relative content of C₂H₄ is close to that of CH₄, while C₃H₆ is relatively closer to the content of CH₃ free radical.

The trends in the quantities of CH₄, C₂H₄, and C₃H₆ molecules at 3550 K are depicted in Figs. 14–16. The yellow area represents the transitional phase, while the white area signifies the stable stage.

The experiment showed a relatively high C₂H₆ content; however, the simulation results did not display C₂H₆. Upon the breaking of long chains of polypropylene to form C₂H₆ in the high-temperature environment, the subsequent step involves the bond breaking to create CH₃ radicals and then the formation of CH₄. In the experimental setup conducted within a gas flow, when the decomposition reached the stage of C₂H₆, the molecules were already carried away from the high-temperature region by the gas flow, thus retaining their original state. Consequently, in the simulation results, we can use the quantity of CH₃ to reflect the quantity of C₂H₆. The trend in the quantity of CH₃ radicals is depicted in Fig. 17, and C₂H₆ exhibits a similar trend.

From the trends observed in the aforementioned products, we can infer that in the initial stages of the reaction, the production rates of CH₄, C₂H₄, and C₂H₆ in the PP-g/O₂ system are lower compared to the PP/O₂ system. Moreover, during the stable phase of both systems, the quantities of these three products are higher in the PP-g/O₂ system than in the PP/O₂ system. Among them, the difference in the quantity of C₂H₆ molecules during the stable phase is particularly noticeable. This suggests that the introduction of styrene grafts alters the decomposition pathway of PP, as illustrated in Fig. 18. The proportion of reactions leading to the generation of C₂H₆ increases, causing a shift in the proportions of the resultant products.

The trend of the number of the four molecules with the simulation temperature is shown in Figs. 9–12.

It can be observed from Figs. 19 and 20 that the number of molecules of CH₄ and C₂H₄ decreases as the temperature rises, while the content of C₃H₆ fluctuates up and down with increasing temperature without showing a clear trend (Fig. 21). The quantity of CH₃ radical molecules, as shown in Fig. 22, also demonstrates an increasing trend with rising temperature. This indicates that the quantity of C₂H₆ molecules also increases with temperature.

The trend of the ratio of C₂H₆ to (CH₄ + C₂H₄) molecular weights with temperature variation in the simulation is illustrated in Fig. 23. It can be observed that in both the PP-g/O₂ and PP/O₂ systems, the ratio of C₂H₆ to (CH₄ + C₂H₄) molecular weights shows an upward trend with increasing temperature. The upward trend is more pronounced in the PP-g/O₂ system.

Comparing Figs. 13 and 23, it is evident that the simulation and experimental results are consistent: the ratio of C₂H₆ to (CH₄ + C₂H₄) in the thermal decomposition products of both PP and PP-g exhibits an increasing trend with rising temperature. Moreover, the ratio of C₂H₆ to (CH₄ + C₂H₄) in the thermal decomposition products of PP-g cable insulation material shows a more pronounced increase with temperature. This demonstrates that graft modification has enhanced the proportion of C₂H₆ in the thermal decomposition products, thereby achieving an increase in the thermal decomposition temperature of PP.

Comparing Figs. 13 and 14 shows the consistency between the simulation and the experiment: in the thermal decomposition product of PP and PP-g, the ratio of C₂H₆ to (CH₄ + C₂H₄) molecular weights both increases with the rise in temperature, and the ratio rises more significantly with the rise in temperature in the thermal decomposition product of PP-g cable insulation material. Proved that the grafting modification has improved the C₂H₆ in the proportion of thermal decomposition products so as to achieve the purpose of raising the PP thermal decomposition temperature.

1. PP-g cable insulation materials demonstrate a higher thermal decomposition temperature in oxygen compared to pure PP.

2. Both PP-g and PP cable insulation materials produce similar compounds during thermal decomposition. As temperature rises, there is a consistent decrease in the proportion of hydrocarbons, a reduction in CH₄ and C₂H₄ yield, and an increase in C₂H₆ yield.

3. Simulation results indicate that introducing styrene grafting onto polypropylene increases the temperature required for decomposition. With rising simulation temperature, proportions of CH₄ and C₂H₄ in decomposition products decrease, while the quantity of CH₃ radicals notably increases.

4. In PP-g cable insulation materials, there is a more pronounced increase in C₂H₆ content relative to CH₄ and C₂H₄ in thermal decomposition products compared to pure PP. Graft modification enhances the proportion of C₂H₆ among hydrocarbons, thereby elevating the thermal decomposition temperature of PP.

This job is supported by the Innovation Fund Project of the China Electric Power Research Institute (Grant No. SZ83-22-004).

The authors have no conflicts to disclose.

Chao Peng: Conceptualization (equal); Data curation (equal); Methodology (equal). Fanwu Chu: Writing – original draft (equal); Writing – review & editing (equal). Mingzhong Xu: Funding acquisition (equal); Project administration (equal); Supervision (equal). Junping Hou: Formal analysis (equal); Software (equal). Wei Zhang: Methodology (equal); Supervision (equal). Kai Deng: Funding acquisition (equal); Methodology (equal); Software (equal); Visualization (equal). Jian Gong: Formal analysis (equal); Investigation (equal); Software (equal); Validation (equal). Zihang Qin: Investigation (equal); Software (equal); Visualization (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.