Pyrolysis Kinetics and Gas Evolution of Flame-Retardant PVC and PE: A TG-FTIR-GC/MS Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper "Pyrolysis Kinetics and Gas Evolution of Flame-Retardant PVC and PE: A TG-FTIR-GC/MS Study" explores the pyrolysis kinetics and gas evolution of flame-retardant PVC and PE. Key findings show flame-retardant PVC experiences two main pyrolysis stages: dehydrochlorination with quick HCl release and low activation energy, and main-chain scission forming aromatic compounds contributing to fire toxicity. In contrast, flame-retardant PE has a more stable pyrolysis process with random chain scission and a dense char layer, enhancing flame retardancy. FTIR and GC/MS analyses reveal PVC mainly generates HCl and aromatic hydrocarbons, while PE releases less toxic olefins and alkanes. The CART model accurately predicts mass loss behavior. This study highlights understanding these pyrolysis mechanisms is crucial for optimizing fire modeling and improving material design, offering insights for advancing fire hazard assessment and prevention strategies.The manuscript is well-structured and clearly written, but several issues should be addressed before publication:

1. The literature review offers a detailed summary of the current research on the pyrolysis of PVC and PE. However, some cited references are outdated. To provide readers with a more comprehensive understanding of recent advancements in the field, it is suggested to supplement the review with the latest research findings from the past five years.
2. In Section 2.1, it is recommended to supply more detailed information regarding the specific composition and properties of the flame - retardant additives present in the PVC and PE samples. This additional information will assist readers in comprehending the relationship between the additives and the observed pyrolysis behaviours.
3. The authors are encouraged to incorporate a discussion on the environmental and health impacts of the identified pyrolysis gases in Section 3.4.2, with a particular focus on the toxicity and flammability of these gases.
4. The usage of certain technical terms is either inaccurate or ambiguous. A thorough review and correction of such terms is necessary. For instance, concepts like "pyrolysis" and "cracking" may carry different meanings across various research contexts. It is crucial to ensure the consistent and precise usage of these terms throughout the manuscript.
5. When analysing the TG and DTG curves, the methodology employed to determine critical parameters such as the pyrolysis onset temperature and peak temperature should be clearly elaborated. The scientific validity and rationality of the adopted extraction methods must be ensured. Furthermore, a comparison of these methods with those used in prior studies is recommended to highlight potential discrepancies and the reasons behind the choice of methods in this research.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English could be carefully modified by native English speakers to more clearly express the manuscript titled "Pyrolysis Kinetics and Gas Evolution of Flame-Retardant PVC and PE: A TG-FTIR-GC/MS Study". 

Author Response

The paper "Pyrolysis Kinetics and Gas Evolution of Flame-Retardant PVC and PE: A TG-FTIR-GC/MS Study" explores the pyrolysis kinetics and gas evolution of flame-retardant PVC and PE. Key findings show flame-retardant PVC experiences two main pyrolysis stages: dehydrochlorination with quick HCl release and low activation energy, and main-chain scission forming aromatic compounds contributing to fire toxicity. In contrast, flame-retardant PE has a more stable pyrolysis process with random chain scission and a dense char layer, enhancing flame retardancy. FTIR and GC/MS analyses reveal PVC mainly generates HCl and aromatic hydrocarbons, while PE releases less toxic olefins and alkanes. The CART model accurately predicts mass loss behavior. This study highlights understanding these pyrolysis mechanisms is crucial for optimizing fire modeling and improving material design, offering insights for advancing fire hazard assessment and prevention strategies. The manuscript is well-structured and clearly written, but several issues should be addressed before publication:

Comments 1: The literature review offers a detailed summary of the current research on the pyrolysis of PVC and PE. However, some cited references are outdated. To provide readers with a more comprehensive understanding of recent advancements in the field, it is suggested to supplement the review with the latest research findings from the past five years.

Response 1: Thank you for your valuable suggestion. In accordance with your comment, we have added several recent studies published within the past five years to the literature review section, thereby improving the timeliness and comprehensiveness of the background discussion.

Comments 2: In Section 2.1, it is recommended to supply more detailed information regarding the specific composition and properties of the flame - retardant additives present in the PVC and PE samples. This additional information will assist readers in comprehending the relationship between the additives and the observed pyrolysis behaviours.

Response 2: Thank you for the helpful suggestion. In response, we have revised Section 2.1 to include more detailed information on the specific flame-retardant additives used in both PVC and PE insulation. These additions clarify the chemical composition and functional mechanisms of the additives, which help explain the observed differences in pyrolysis behavior. The updated description enhances readers’ understanding of the material systems studied.

Comments 3: The authors are encouraged to incorporate a discussion on the environmental and health impacts of the identified pyrolysis gases in Section 3.4.2, with a particular focus on the toxicity and flammability of these gases.

Response 3: Thank you for your insightful suggestion. In response, we have added a discussion at the end of Section 3.4.2 addressing the environmental and health impacts of the identified pyrolysis gases. This includes specific commentary on the toxicity of HCl and aromatic compounds released from PVC, as well as the flammability of hydrocarbon gases released from PE. These additions help contextualize the practical safety implications of the experimental findings.

Comments 4: The usage of certain technical terms is either inaccurate or ambiguous. A thorough review and correction of such terms is necessary. For instance, concepts like "pyrolysis" and "cracking" may carry different meanings across various research contexts. It is crucial to ensure the consistent and precise usage of these terms throughout the manuscript.

Response 4: Thank you for pointing this out. In response, we have carefully reviewed the manuscript to ensure the accurate and consistent use of technical terms such as “pyrolysis” and “cracking.” Necessary corrections have been made where ambiguity or misuse was identified, and terminology has been standardized throughout the text to align with established definitions in thermal decomposition and polymer degradation research.

Comments 5: When analysing the TG and DTG curves, the methodology employed to determine critical parameters such as the pyrolysis onset temperature and peak temperature should be clearly elaborated. The scientific validity and rationality of the adopted extraction methods must be ensured. Furthermore, a comparison of these methods with those used in prior studies is recommended to highlight potential discrepancies and the reasons behind the choice of methods in this research.

Response 5: Thank you for your thoughtful suggestion. In response, we have clarified the methodology used to extract critical pyrolysis parameters such as the onset and peak decomposition temperatures from TG/DTG curves. The revised text (Section 3.2, first paragraph) specifies the extrapolation and peak identification approaches and briefly compares them with alternative methods used in the literature. This addition enhances the scientific validity and transparency of our thermal analysis.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript explores the flame retardant PVC and PE’s pyrolysis kinetics and the release behaviours of volatile products using TG-FTIR-GC/MS as the experimental baseline. Different simulation methods have been applied and combined to understand and predict the thermal decomposition behaviours of these two types of materials. FTIR and GC/MS analyses have been deployed to investigate the gaseous products during pyrolysis processes. Comparisons between the flame retardant PVC and PE have been conducted focusing on their thermal stability performance and thermal decomposition behaviours. Additionally, the released gas products have been identified, and their concentrations have been evaluated and compared. I found this manuscript has quite a lot of qualitative and quantitative work. However, I think the manuscript still should undergo further improvement with a major revision (especially the comments listed in question 6 should be critically revised and rebutted), before it could be considered for publication, which as noted below:

1. Please be consistent with the unit, i.e., K or °C?
2. Page 7 Line 225, please give a reference to back up this statement.
3. Page 7 Line 237, please revise the use of word "significant", as the mass loss is not extremely high compared to the first stage.
4. Please explain, why when the heating rate increases, the initial decomposition temperatures and DTG peaks increase as well.
5. Page 9 Line 299-301. Please correct if I am wrong. Eα is smaller means the reaction needs less energy to initiate, which means the reaction is easy to occur. If the reaction is easy to occur, why does this phenomenon suggest that further decomposition is inhibited?
6. Please revise some last sentences, which state the flame retardant performance of the samples, in some paragraphs. This manuscript used TG-FTIR-GC/MS under a nitrogen environment, which means it examines the thermal stability and pyrolysis behaviour of the sample. For the evaluation of flame retardancy, actual burning-related tests should be performed to determine the fire safety performance. So, these statements must be removed from the manuscript. Otherwise, fire safety tests should be performed to back-up these statements.

Comments for author File: Comments.pdf

Author Response

This manuscript explores the flame retardant PVC and PE’s pyrolysis kinetics and the release behaviours of volatile products using TG-FTIR-GC/MS as the experimental baseline. Different simulation methods have been applied and combined to understand and predict the thermal decomposition behaviours of these two types of materials. FTIR and GC/MS analyses have been deployed to investigate the gaseous products during pyrolysis processes. Comparisons between the flame retardant PVC and PE have been conducted focusing on their thermal stability performance and thermal decomposition behaviours. Additionally, the released gas products have been identified, and their concentrations have been evaluated and compared. I found this manuscript has quite a lot of qualitative and quantitative work. However, I think the manuscript still should undergo further improvement with a major revision (especially the comments listed in question 6 should be critically revised and rebutted), before it could be considered for publication, which as noted below:

Comments 1: Please be consistent with the unit, i.e., K or °C?

Response 1: Thank you for your observation. To maintain consistency throughout the manuscript, all temperature values have been uniformly expressed in Kelvin (K).

Comments 2: Page 7 Line 225, please give a reference to back up this statement.

Response 2: Thank you for your suggestion. A supporting reference has been added at Page 7, Line 225 to substantiate the statement.

Comments 3: Page 7 Line 237, please revise the use of word "significant", as the mass loss is not extremely high compared to the first stage.

Response 3: Thank you for your careful observation. The word “significant” has been revised to “noticeable” to more accurately reflect the degree of mass loss in comparison to the first stage.

Comments 4: Please explain, why when the heating rate increases, the initial decomposition temperatures and DTG peaks increase as well.

Response 4: Thank you for your insightful question. The increase in both the initial decomposition temperature and DTG peak temperatures with higher heating rates is attributed to the thermal lag effect. At faster heating rates, the sample requires more time to reach thermal equilibrium, resulting in a shift of thermal events to higher apparent temperatures. This behavior is commonly observed in non-isothermal thermogravimetric analysis and has been reported in many studies involving polymer pyrolysis.

Comments 5: Page 9 Line 299-301. Please correct if I am wrong. Eα is smaller means the reaction needs less energy to initiate, which means the reaction is easy to occur. If the reaction is easy to occur, why does this phenomenon suggest that further decomposition is inhibited?

Response 5: Thank you for your valuable comment. We agree with your point and have revised the sentence accordingly to clarify the relationship between the decreasing  values and the inhibition of further decomposition. The updated wording explains that the decline in  is due to the formation of char residue, which acts as a physical barrier that restricts mass and heat transfer, thereby slowing down further degradation.

Comments 6: Please revise some last sentences, which state the flame retardant performance of the samples, in some paragraphs. This manuscript used TG-FTIR-GC/MS under a nitrogen environment, which means it examines the thermal stability and pyrolysis behaviour of the sample. For the evaluation of flame retardancy, actual burning-related tests should be performed to determine the fire safety performance. So, these statements must be removed from the manuscript. Otherwise, fire safety tests should be performed to back-up these statements.

Response 6: Thank you for your important comment. We acknowledge that TG-FTIR-GC/MS analysis under a nitrogen atmosphere primarily reflects thermal stability and pyrolysis behavior rather than actual flame-retardant performance. Accordingly, we have removed the relevant statements referring to fire safety or flame-retardant performance from the corresponding paragraphs to avoid overinterpretation.

Reviewer 3 Report

Comments and Suggestions for Authors

This study systematically investigated the thermal decomposition kinetics and gas product release behavior of flame-retardant polyvinyl chloride (PVC) and polyethylene (PE) cable insulation materials. The authors employed the TG-FTIR-GC/MS combined technique, combined with the model free method (KAS) and the model fitting method (Malek method), to quantify the thermal decomposition stage, activation energy, and reaction mechanism of the two materials under different heating rates (10 - 40 K/min). The results showed that the flame-retardant PVC exhibited a three-stage thermal decomposition process, mainly involving dehydrogenation of hydrogen chloride (release of HCl) and main chain breakage (generation of aromatic compounds), with a relatively low residual carbon rate and higher toxicity of released gases; the flame-retardant PE showed a two-stage diffusion-controlled thermal decomposition process, mainly composed of alkanes/alkenes, with a high residual carbon rate and low toxicity gas release, attributed to its halogen-free flame retardant formula and the formation of a dense carbon layer due to radiation cross-linking structure. In addition, the authors innovatively introduced the classification regression tree (CART) model, successfully predicting the thermal weight loss behavior of the materials (R2 > 0.98), providing multi-scale data support for the optimization of fire model parameters and material design. However, before the publication of this paper, further revisions are still necessary.

1. Details of the experiment need to be supplemented: The number of repetitions for sample preparation and the range of data error (such as the standard deviation of the TG curve) have not been clearly defined. It is recommended to add statistical analysis to enhance the reliability of the results. The GC/MS analysis only selects the peak temperature point of DTG under a single heating rate (10 K/min), which may miss the dynamic changes of gas components in other temperature intervals. It is suggested to add comparisons at multiple temperature points or time-resolved GC/MS data.
2. The mechanism of flame retardants needs to be further explored: Although it is mentioned that Sb₂O₃ in PVC inhibits the release of HCl by generating SbOCl, there is a lack of quantitative evidence for this process (such as XPS characterization of the conversion rate of Sb/Cl elements). The halogen-free flame retardant formula for PE does not specify the exact components. It is suggested to supplement the material formula table or use FTIR/EDS to verify the formation mechanism of the carbon layer.
3. Relevance to actual fire scenarios: The experiments were conducted in a nitrogen atmosphere, but in real fires, oxygen is often involved. It is suggested to supplement TG-FTIR control experiments under air atmosphere to discuss the influence of oxidation reactions on the generation of toxic gases (such as CO). The physical structure of the residual carbon layer (such as porosity, degree of graphitization) was not characterized. SEM/BET analysis can be combined to explain the differences in flame retardant performance.

This paper innovatively integrates multi-scale analysis techniques with machine learning methods, providing important insights into the pyrolysis mechanism of flame-retardant cable materials and demonstrating clear engineering application value. The above suggestions aim to further enhance the depth and universality of the research. The paper is recommended for acceptance after revision.

Author Response

This study systematically investigated the thermal decomposition kinetics and gas product release behavior of flame-retardant polyvinyl chloride (PVC) and polyethylene (PE) cable insulation materials. The authors employed the TG-FTIR-GC/MS combined technique, combined with the model free method (KAS) and the model fitting method (Malek method), to quantify the thermal decomposition stage, activation energy, and reaction mechanism of the two materials under different heating rates (10 - 40 K/min). The results showed that the flame-retardant PVC exhibited a three-stage thermal decomposition process, mainly involving dehydrogenation of hydrogen chloride (release of HCl) and main chain breakage (generation of aromatic compounds), with a relatively low residual carbon rate and higher toxicity of released gases; the flame-retardant PE showed a two-stage diffusion-controlled thermal decomposition process, mainly composed of alkanes/alkenes, with a high residual carbon rate and low toxicity gas release, attributed to its halogen-free flame retardant formula and the formation of a dense carbon layer due to radiation cross-linking structure. In addition, the authors innovatively introduced the classification regression tree (CART) model, successfully predicting the thermal weight loss behavior of the materials (R2 > 0.98), providing multi-scale data support for the optimization of fire model parameters and material design. However, before the publication of this paper, further revisions are still necessary.

Comments 1: Details of the experiment need to be supplemented: The number of repetitions for sample preparation and the range of data error (such as the standard deviation of the TG curve) have not been clearly defined. It is recommended to add statistical analysis to enhance the reliability of the results. The GC/MS analysis only selects the peak temperature point of DTG under a single heating rate (10 K/min), which may miss the dynamic changes of gas components in other temperature intervals. It is suggested to add comparisons at multiple temperature points or time-resolved GC/MS data.

Response 1: Thank you for your valuable suggestions. In response, we have added information regarding the number of repetitions and statistical variation in the experimental section. Specifically, all TG experiments were conducted in triplicate, and the standard deviation of the TG mass loss curves was within 2.5%, indicating good repeatability.

Regarding the GC/MS analysis, we acknowledge the reviewer's point that selecting only the DTG peak temperature at a single heating rate (10 K/min) may limit the observation of dynamic changes. However, due to instrumental limitations and the need to avoid over-dilution or dispersion of gas components during low-concentration intervals, real-time GC/MS or multi-point sampling was not feasible within this study. Instead, we focused on the most representative decomposition point (i.e., the DTG peak) to ensure effective identification of dominant pyrolysis products. We appreciate your suggestion and will consider incorporating time-resolved GC/MS measurements in our future work to provide a more detailed gas evolution profile.

Comments 2: The mechanism of flame retardants needs to be further explored: Although it is mentioned that Sb₂O₃ in PVC inhibits the release of HCl by generating SbOCl, there is a lack of quantitative evidence for this process (such as XPS characterization of the conversion rate of Sb/Cl elements). The halogen-free flame retardant formula for PE does not specify the exact components. It is suggested to supplement the material formula table or use FTIR/EDS to verify the formation mechanism of the carbon layer.

Response 2: Thank you for your insightful suggestions. In response, we have added more detailed information regarding the flame-retardant formulations of PVC and PE insulation materials in Section 2.1. Specifically, we clarified that Sb₂O₃ and phosphate-based synergists are used in the flame-retardant PVC formulation, while halogen-free PE insulation primarily contains magnesium hydroxide (Mg(OH)₂) and aluminum hydroxide (Al(OH)₃) as inorganic flame retardants.

We fully agree that further quantitative evidence—such as XPS analysis of Sb/Cl conversion or EDS verification of char layer composition—would strengthen the understanding of the flame-retardant mechanisms. However, due to limitations in experimental scope and instrument access, such characterization was not performed in the current study. We have accordingly refrained from making mechanistic claims beyond the qualitative observations supported by TG-FTIR-GC/MS analysis. We appreciate this valuable suggestion and plan to explore these mechanisms more quantitatively in future work.

Comments 3: Relevance to actual fire scenarios: The experiments were conducted in a nitrogen atmosphere, but in real fires, oxygen is often involved. It is suggested to supplement TG-FTIR control experiments under air atmosphere to discuss the influence of oxidation reactions on the generation of toxic gases (such as CO). The physical structure of the residual carbon layer (such as porosity, degree of graphitization) was not characterized. SEM/BET analysis can be combined to explain the differences in flame retardant performance.

Response 3: Thank you for your thoughtful and constructive comments. We acknowledge the importance of simulating real fire conditions, including oxidative environments. However, the scope of this study was limited to pyrolysis behavior under a nitrogen atmosphere in order to isolate the intrinsic thermal degradation characteristics and gas evolution patterns of the flame-retardant materials without the interference of combustion reactions. Although we agree that TG-FTIR analysis under air could offer further insights into oxidation-induced gas generation (e.g., CO, CO₂), such control experiments were not performed due to equipment constraints and the study’s focus on anaerobic decomposition.

Similarly, we recognize that the microstructure of the residual carbon layer (e.g., porosity, graphitization degree) can significantly influence flame retardancy. Techniques such as SEM or BET analysis would indeed help to elucidate these structural features. However, such characterizations were beyond the analytical scope of this thermal degradation-focused investigation. We will consider integrating these advanced characterization methods in our future work to better correlate char structure with flame-retardant performance.

This paper innovatively integrates multi-scale analysis techniques with machine learning methods, providing important insights into the pyrolysis mechanism of flame-retardant cable materials and demonstrating clear engineering application value. The above suggestions aim to further enhance the depth and universality of the research. The paper is recommended for acceptance after revision.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have replied to all my comments. I believe this manuscript is ready for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

The author provided detailed answers to the questions I raised, and the paper has also reached a publishable level.