Impact of Suberin Depolymerization Conditions on the Thermal Properties and Flammability of Rigid Polyurethane Foams
, Mikelis Kirpluks *, Daniela Godina, Arnis Abolins, Laima Vevere, Rudolfs Berzins, Maris Lauberts and Janis RizikovsRound 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this study, suberinic acids derived from birch bark were produced under different depolymerization conditions, and bio-based polyols were synthesized from these fractions and used in rigid polyurethane foams. The results showed that while depolymerization conditions affected mechanical and thermal properties, they did not lead to a significant difference in flammability.
The original contribution of the study is that it shows changes in polyphenolic content and molecular weight distribution do not significantly affect flammability, and that low-viscosity fractions obtained with FeCl₃ can improve processability while maintaining similar performance. This clearly demonstrates that flame-retardant behavior in bio-based PU systems does not inherently arise from natural content and that additional strategies are required.
Therefore, this study can be considered original and innovative, as it demonstrates that variations in polyphenolic content and molecular weight distribution do not significantly influence flammability, while also showing that FeCl₃-treated fractions can improve processability without compromising performance.
Here some revisions, after these revisions the paper can be published.
-For better consistency and readability, it is recommended that the font style used in the figures be aligned with the main text font.
-It should be clearly stated which standard is used for each test.
-Images of the compression test specimens before and after testing should be included in the manuscript.
-Repeated compression curves used to determine the compressive strengths should be included in the manuscript. In order to demonstrate the reliability of the manufacturing process, repeated experimental results should be presented and discussed.
-The results obtained from the compression tests should be validated by comparison with similar findings reported in the literature. In addition, the failure modes observed after testing should be presented, and the damage mechanisms of the manufactured specimens should be discussed by including microscope images.
Author Response
Response to Reviewer 1 Comments
Thank you very much for taking the time to review this manuscript. Please find the detailed responses below in green colour and the corresponding revisions/corrections in track changes in the re-submitted files.
In this study, suberinic acids derived from birch bark were produced under different depolymerization conditions, and bio-based polyols were synthesized from these fractions and used in rigid polyurethane foams. The results showed that while depolymerization conditions affected mechanical and thermal properties, they did not lead to a significant difference in flammability.
The original contribution of the study is that it shows changes in polyphenolic content and molecular weight distribution do not significantly affect flammability, and that low-viscosity fractions obtained with FeCl₃ can improve processability while maintaining similar performance. This clearly demonstrates that flame-retardant behavior in bio-based PU systems does not inherently arise from natural content and that additional strategies are required.
Therefore, this study can be considered original and innovative, as it demonstrates that variations in polyphenolic content and molecular weight distribution do not significantly influence flammability, while also showing that FeCl₃-treated fractions can improve processability without compromising performance.
Here some revisions, after these revisions the paper can be published.
-For better consistency and readability, it is recommended that the font style used in the figures be aligned with the main text font.
Font size used in the figures was aligned with the main text font (Palatino Linotype).
-It should be clearly stated which standard is used for each test.
We have reviewed the Materials and Methods section. Standards are stated for each test.
-Images of the compression test specimens before and after testing should be included in the manuscript.
Compression test specimens were not photographed before and after testing. We consider that inclusion of such images is not essential for the scope and objectives of the present study, as the compressive properties are fully supported by quantitative mechanical data in accordance with ISO 844:2021.
-Repeated compression curves used to determine the compressive strengths should be included in the manuscript. In order to demonstrate the reliability of the manufacturing process, repeated experimental results should be presented and discussed.
We thank the reviewer for this suggestion. By “repeated compression curves,” we understand that the reviewer refers to the individual stress–strain curves obtained from replicate specimens under compression testing.
In our study, compressive strength values presented in Manuscript Figure 9 are based on multiple parallel specimens, and the corresponding standard deviations already demonstrate the reproducibility and reliability of the manufacturing process.
The full individual stress–strain curves for all replicates were recorded during testing (presented in Figure 1 see attached file); however, they were not included in the manuscript to avoid excessive data volume and redundancy, as the averaged curves and statistical deviations sufficiently represent the mechanical behavior in accordance with ISO 844:2021.
-The results obtained from the compression tests should be validated by comparison with similar findings reported in the literature. In addition, the failure modes observed after testing should be presented, and the damage mechanisms of the manufactured specimens should be discussed by including microscope images.
We thank the reviewer for this suggestion. The compressive strength results obtained for the prepared rigid PU foams (apparent density 39–45 kg/m³) were compared with literature data reported for bio-based closed-cell PU foams used in thermal insulation applications. The obtained values are in good agreement with previously reported compressive strength ranges of approximately 0.15–0.40 MPa for rigid PU foams of comparable density. This confirms that the mechanical performance of the investigated materials is consistent with typical behaviour of rigid polyurethane foams.
However, microscopic analysis of failure modes and damage mechanisms was not performed in the present study, as this was outside the scope of the experimental work.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for Authors- Provide detailed conditions for FeCl₃ treatment (temperature, duration, and whether an inert atmosphere was used).
- Supplement the detailed process parameters for PU foam preparation (mixing speed, mold temperature, curing cycle).
- Clarify the support conditions of the sample in the cone calorimeter test.
- The 5% weight loss temperature of SA3-PU is 164°C, and the 10% weight loss temperature is 277°C, with a difference of 113°C between them. This is not very reasonable
- Figure 8 shows optical microscopy images but does not provide any quantitative data (e.g., average pore diameter, anisotropy ratio, pore size distribution)
- Introduction and Abstract It was anticipated that "SA1 with high polyphenol content should exhibit better flame retardancy," but the results (LOI 19-20, cone calorimeter data) showed no significant differences among the three groups. The authors merely stated the facts in the conclusion without delving into why the polyphenol content difference (3.84% vs. 1.12%) did not translate into variations in combustion performance.
- The abbreviation "ETOFA" must be defined upon its first occurrence in Section 2.3
- It is recommended to directly label the Tg value in the figure
- There is an issue with Reference 41
Author Response
Please see the attached file
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe corrections specified in the first round have been sufficiently implemented; the article is acceptable.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors of the paper did some analysis of the system and obtained relevant data. The paper has been revised to make it logically clear, with actual theoretical data to support its conclusions, the work is interesting, the manuscript is well organized, and the previous suggestions are answered point by point. Therefore, I recommend that the manuscript be published in the Polymers.
