Properties of Oak Veneer Dyed with Supercritical CO₂ and Vacuum-Pressurized Assisted Natural Dyes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Reviewers,

It is attached.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The manuscript demonstrates an average level of English proficiency, which could be further improved.

Author Response

1. In the context of the present study, which focuses on SC-CO₂ pretreatment and dyeing methods, it is recommended that the existing literature on CO₂-based dyeing processes be reviewed more comprehensively. Such an expanded review would enhance the scientific depth of the work, allow for a comparative evaluation of the findings with similar studies in the literature, and contribute to more effectively emphasizing the sustainability dimension of the research.

-- Thank you for your suggestion to enhance the depth of the literature review. We have added more references related to SC-CO₂ staining, and have highlighted them in red font in the introduction section.

 

2. According to the above explanation, the magnification levels used in the SEM analyses should be clearly and explicitly stated, and it should be clearly specified whether the analyzed surfaces were coated or uncoated. This information is essential for the accurate interpretation of the obtained microstructure images and for ensuring the reproducibility of the study.

-- We agree with your point of view. Accurate interpretation of the microscopic structure images is of great importance. In the revised version 'Figure 12. SEM scanning electron microscopy.', we have provided supplementary explanations in bold font regarding the magnification and surface treatment used in the SEM analysis.

 

3. In order to enhance the scientific contribution of the study, it is recommended that the K/S values of the three different dyeing methods (conventional water bath dyeing, supercritical carbon dioxide (SC-CO₂) dyeing, and vacuum pressurized dyeing) be compared in pairs using the formulaThis comparison would numerically reveal theperformance differences among the methods and make it possible to identify the method that provides the highest increase in color depth. Furthermore, presenting the calculated percentage increase values in a tabular format would allow readers to more clearly observe the differences between the methods.

-- Thank you very much for your suggestion. We have added the calculation methods for the K/S percentage increments of the three methods in the revised draft, using bold font, and have presented the differences among the various methods in a visual table format.

 

4. The specific standards used for the washing fastness and light fastness tests should be clearly stated. If there is no internationally accepted standard for these tests, this should also be explicitly indicated in the study.

-- Thank you for bringing this matter to our attention. The washing fastness and light fastness test procedures respectively refer to the international standards ISO 105 C06; 2010 and ISO 105 B02: 2014. We have provided explanations in red font in the revised version.

 

5. When presenting plant names, the Latin names should be placed in parentheses and written in italics.

-- Thank you for your correction. We have made the necessary changes at the corresponding positions in the revised version.

 

6. For the identification of dyes bound to the oak samples, chromatographic analyses should be performed on the dyed specimens, and the three dyeing methods should be comparatively evaluated.

-- Thank you for your professional advice. We conducted FTIR tests on the dyed samples and analyzed the changes in functional groups of each sample before and after dyeing in the paper. We fully agree with your viewpoint. Chromatography technology can effectively identify each component in the mixture. However, due to time and practical reasons, we did not conduct supplementary chromatographic tests on the dyed samples. But we will pay more attention to this issue in future research. Thank you very much for your reminder!

 

7. It is recommended to clarify whether the components present in the oak samples, such as lignin, and the elements form a complex with the dyes used. Clarifying thisissue would contribute to a better understanding of the dyeing mechanism.

-- Thank you for bringing this crucial issue to our attention. To clarify the composition of the oak samples and the elements that form complexes with the dyes, we conducted a thorough investigation by thoroughly reviewing the literature and made a supplementary explanation in the introduction section.

 

8. In CIELab measurements, ΔE values are highly important for determining color differences. However, these values do not constitute a standard method for light and washing fastness tests. Furthermore, it is entirely natural for the L value to decrease as the color becomes darker; indeed, in the case of black, the L value is zero.

-- Thank you very much for pointing out this issue. We agree with your opinion. Since our color fastness tests follow the ISO standards, in the revised version, we have supplemented the table of gray scale classification for different dyeing methods under different pretreatment conditions using bold red font. Additionally, we agree with your viewpoint. Although the decrease in L value is completely natural, we still hope to test the CIELab of different dyes under different conditions to study the extent of influence of light exposure and water washing on the chromaticity values.

 

9. It is recommended to review studies conducted on plants in relation to natural dyeing. In the conclusion section of the manuscript, a suggestion is made to use natural dyes in such products, with the term “sustainable” being included. I agree with this view; however, I believe that publications addressing the sustainability and international trade of natural dyes should also be reviewed. Including such references would enhance the scientific value of the study and enable the manuscript to reach a wider readership. I recommend reviewing some of the references listed below.

https://doi.org/10.1080/15440478.2023.2277836

https://doi.org/10.1080/15440478.2019.1588827

https://doi.org/10.1080/15440478.2024.2428385

-- Thank you for your suggestion regarding the multi-dimensional background of natural dyes. This is highly consistent with the application orientation of this study. In the introduction section, we have supplemented a review of research on the sustainability, international trade, and industrial chain of natural dyes.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. In relation to the Introduction section – why was oak veneer chosen in particular, given that it naturally has a distinctive and appealing coloration?
   Additionally, do the dyes used (Phellodendron Bark and Camphor Fruit) have a local or global character? Are they found only in China?
2. The introduction should be expanded to include a review of the methods used by other researchers and the results obtained in the supercritical environment, for example:

Solvent-Free Dyeing of Solid Wood in Water-Saturated Supercritical Carbon Dioxide

Julien Jaxel, Falk W. Liebner, and Christian Hansmann

ACS Sustainable Chemistry & Engineering 2020 8 (14), 5446-5451

DOI: 10.1021/acssuschemeng.0c01273

3. Material section - The selection of the wood species and dyes is still missing.
   Material sources – no information is provided regarding the origin of the dyes (e.g., whether they have been chemically characterized or standardized).
4. Typo – treatment temperature of 60 min. Additionally - parameters of vacuum-pressure treatment – no justification is provided for the choice of pressure and process duration.
5. Section 3.1. Lack of statistical analysis – no standard deviation or significance of differences is provided for the values of specific surface area, pore volume, and pore diameter.

Uncertainty regarding the causes of changes – the authors suggest selective migration of SC-CO₂ and removal of fillers as the main mechanism, but there is no confirmation, e.g., through chemical analysis of the extracts before and after treatment.

Lack of reference to the literature – it would be advisable to compare the magnitude of changes with results obtained by other researchers applying SC-CO₂ to wood.

 

6. Section 3.2. Reference error – data is missing.

Statistical analysis – information on the number of replicates, standard deviation, and significance tests between methods and samples is missing.

Explanation of differences between pigments and methods – it would be interesting to expand on hypotheses why SC-CO₂ works well with the pigment from camphor fruit but not with the pigment from Phellodendron bark. This may be due to differences in the chemical structure of the dyes or their solubility.

Comparison with literature – it would be beneficial to strengthen the discussion by relating the results to previous studies on wood dyeing using supercritical and vacuum-pressure methods.

Clarity and coherence of the narrative – some parts are quite confusing, for example, the phrase “supercritical carbon dioxide dyeing has shown side effects on the dyeing rate of oak instead” needs clarification and a more neutral wording (e.g., “negative impact on the dyeing degree”).

7.  

Lines 408-409 Typo ?

8. Section 3.3. The conclusions regarding the impact of SC-CO₂ or vacuum treatment on color are at times contradictory or counterintuitive (e.g., “did not significantly improve color difference but improved dyeing rate”) – it would be useful to elaborate on why this is the case. The text suggests that an improvement in dyeing efficiency does not necessarily translate to an increase in color difference (ΔE). This is an interesting conclusion, but it should be supported with arguments based on previous studies or literature. There is no information on whether the differences in ΔL, Δa, Δb, and ΔE values are statistically significant (e.g., ANOVA test, t-test). Without this, it is difficult to assess whether the conclusions are justified.
9. Section 3.4 -
   The analysis is based on reflectance values in different wavelength ranges; however, it lacks a more detailed explanation of why the pigments exhibit such optical behavior in the context of their chemical composition, light absorption, and interaction with wood. Adding a brief interpretation of these phenomena would increase the scientific value of the work.
   As in previous sections, there is no information on whether the differences in R (reflectance) values between samples are statistically significant. Introducing appropriate statistical tests would allow the reliability of the observed trends to be assessed.
   The text contains no references to previous studies or literature that could confirm or contrast the obtained results. This is important for placing the research in a scientific context and verifying its originality and credibility.
   The text repeatedly refers to figures (e.g., Fig. 8); however, the review text contains “Error! Reference source not found,” indicating a referencing error. Correct and clear insertion and description of the figures are essential so that the reader can independently verify the presented data.
10. Section 3.5. As in previous sections, it is not stated whether the differences in ∆E* values between dyeing methods and sample types are statistically significant. Including statistical analysis (e.g., ANOVA, post-hoc tests) would be essential to confirm the conclusions.

The text contains “Error! Reference source not found” placeholders, which prevent verification of the presented data. Figures should be correctly inserted and clearly captioned.

An interpretation should be added explaining why supercritical CO₂ pretreatment improves washing fastness (e.g., changes in pore structure, better dye penetration, more stable dye–lignin/cellulose bonding).

It would be useful to compare the obtained washing fastness values with previous research results to place the findings in the context of existing knowledge.

Although it is noted that Phellodendron Bark yields better washing fastness than camphor fruit, no explanation is provided as to why (e.g., differences in chemical composition, molecular weight, or hydrogen-bonding ability of the dyes).

11. Section 3.6 – Remarks as previous + In the final part, there is a contradiction: it is stated that there is a smaller color difference after solarization for camphor fruit compared to Phellodendron bark, which indicates better lightfastness, but the text refers to “washing fastness” — this is likely an error and should be corrected.

Author Response

Dear Reviewer,

Thank you very much for your meticulous and rigorous review of our research work. We have carefully read your professional opinions. Here are our responses to your suggestions. We have carefully checked the manuscript and made corresponding modifications and content additions in the revised version using bold font. Additionally, we acknowledge a major limitation of this study: due to the practical considerations of the laboratory, the sample size was insufficient during specific experiments and characterization tests, resulting in data that cannot be subjected to statistical testing. Therefore, we are temporarily unable to address certain issues of data credibility caused by a single test. Once again, thank you for your suggestions, which are of great significance for our future research work!

1.In relation to the Introduction section – why was oak veneer chosen in particular, given that it naturally has a distinctive and appealing coloration?

-- We chose oak as the research subject because oak wood itself has well-developed vessels and strong texture. After dyeing treatment, we can obtain a material with beautiful color and stable structure, which is favored by consumers and has a wide application market.

Additionally, do the dyes used (Phellodendron Bark and Camphor Fruit) have a local or global character? Are they found only in China?

-- The natural dyes we use are mainly from the Asian region; however, they are not limited to China. For instance, Phellodendron has a small distribution in Eastern Europe, and Camphor Fruit is also cultivated in regions such as South and North America, Australia, etc.

 

2.The introduction should be expanded to include a review of the methods used by other researchers and the results obtained in the supercritical environment, for example:

Solvent-Free Dyeing of Solid Wood in Water-Saturated Supercritical Carbon Dioxide

Julien Jaxel, Falk W. Liebner, and Christian Hansmann

ACS Sustainable Chemistry & Engineering 2020 8 (14), 5446-5451

DOI: 10.1021/acssuschemeng.0c01273

-- Thank you for your suggestion on supplementing the literature. This is crucial for clarifying the technical positioning of this research. We have retrieved more references and have highlighted them in red font in the revised version.

 

3.Material section - The selection of the wood species and dyes is still missing.

-- Thank you very much for bringing this issue to our attention. We have added the information about the types of wood and the sources of dyes in the revised draft, using bold font for clarity.

Material sources – no information is provided regarding the origin of the dyes (e.g., whether they have been chemically characterized or standardized).

-- Thank you for your professional review. In the revised version, we have provided additional information on the extraction of the dye sources. Moreover, in our previous research, we have focused on the standardization and chemical characterization of the use of two natural dyes, namely Phellodendron and Camphor Nut, for wood coloring. To avoid data duplication, we therefore do not elaborate on this in great detail in this article.

 

4.Typo – treatment temperature of 60 min. Additionally - parameters of vacuum-pressure treatment – no justification is provided for the choice of pressure and process duration.

-- Thank you for your meticulous review. We have made corresponding revisions and additions in the revised version using bold font.

 

5. Section 3.1. Lack of statistical analysis – no standard deviation or significance of differences is provided for the values of specific surface area, pore volume, and pore diameter.

-- We sincerely thank you for the important comment on statistical analysis. We agree that including standard deviations and significance tests would enhance the rigor of the study. However, due to objective limitations in our laboratory, the number of samples available for characterization was insufficient, and with only eight days allowed for revision, it was not feasible to perform additional tests. Conducting statistical analysis with the current limited dataset might even be misleading.Instead, we have presented the results using mean values and graphical comparisons to illustrate the trends. This descriptive approach has also been adopted in related exploratory studies on wood treatment with supercritical CO₂ [1]. We acknowledge this limitation, but we still believe that the findings provide meaningful reference value and can serve as a foundation for future research.

[1] Nuralin, L.; Guru, M.; Cete, S. Extraction and Quantification of Some Valuable Flavonoids from Pinecone of Pinus Brutia via Soxhlet and Supercritical CO2 Extraction: A Comparison Study. Chem. Pap. 2021, 75, 5363–5373, doi:10.1007/s11696-021-01644-5.

Uncertainty regarding the causes of changes – the authors suggest selective migration of SC-CO₂ and removal of fillers as the main mechanism, but there is no confirmation, e.g., through chemical analysis of the extracts before and after treatment.

-- Thank you for raising this question. We conducted SEM, FTIR and XRD analyses on the oak samples before and after SC-CO2 pretreatment. The results showed that SC-CO2 mainly caused physical changes in the oak samples. Due to the length of the original text, we did not display it.

Lack of reference to the literature – it would be advisable to compare the magnitude of changes with results obtained by other researchers applying SC-CO₂ to wood.

-- Thank you for bringing this issue to our attention. Comparisons with other research results are crucial for highlighting the value of our own research findings. We have included supplementary comparisons in the revised draft by systematically reviewing the literature and marked them in red font.

 

6.Section 3.2. Reference error – data is missing.

-- Thank you very much for pointing out this mistake. We have rechecked the entire text and have made the data and figure titles consistent.

Statistical analysis – information on the number of replicates, standard deviation, and significance tests between methods and samples is missing.

— We agree with your opinion. However, due to the limited number of samples available for absorbance testing, it is difficult to conduct statistical analysis under the current conditions without risking misleading the readers.

Explanation of differences between pigments and methods – it would be interesting to expand on hypotheses why SC-CO₂ works well with the pigment from camphor fruit but not with the pigment from Phellodendron bark. This may be due to differences in the chemical structure of the dyes or their solubility.

— We agree with your viewpoint. The difference in molecular polarity of the two dyes might be the key factor leading to the different dissolution and diffusion behaviors in SC-CO₂, and thereby causing the differences in dyeing performance. We have expanded the corresponding assumption in the revised version using red font.

Comparison with literature – it would be beneficial to strengthen the discussion by relating the results to previous studies on wood dyeing using supercritical and vacuum-pressure methods.

-- Thank you for your valuable suggestion of strengthening the discussion through literature comparison. We have added the comparative content in the revised version.

Clarity and coherence of the narrative – some parts are quite confusing, for example, the phrase “supercritical carbon dioxide dyeing has shown side effects on the dyeing rate of oak instead” needs clarification and a more neutral wording (e.g., “negative impact on the dyeing degree”).

-- Thank you for bringing this issue to our attention. We have revised this sentence from a scientific perspective to make it clearer and more neutral. Additionally, we have carefully reviewed the entire text to avoid any unnecessary reading difficulties.

 

7.Lines 408-409 Typo ?

-- Thank you for your correction. We have already corrected this error in the revised version.

 

8. Section 3.3. The conclusions regarding the impact of SC-CO₂ or vacuum treatment on color are at times contradictory or counterintuitive (e.g., “did not significantly improve color difference but improved dyeing rate”) – it would be useful to elaborate on why this is the case. The text suggests that an improvement in dyeing efficiency does not necessarily translate to an increase in color difference (ΔE). This is an interesting conclusion, but it should be supported with arguments based on previous studies or literature. There is no information on whether the differences in ΔL, Δa, Δb, and ΔE values are statistically significant (e.g., ANOVA test, t-test). Without this, it is difficult to assess whether the conclusions are justified.

-- We sincerely thank you for the two valuable suggestions.

(1) Regarding the relationship between dyeing rate and color difference, we believe that there is no direct correlation between the two. The dyeing rate reflects the amount of dye fixed within the wood, whereas the color difference (∆E*) represents a comprehensive perceptual difference in color based on lightness (∆L) and chromaticity (∆a, ∆b). The asynchronous variations of these two parameters are a normal phenomenon, resulting from their different evaluation dimensions as well as the combined effects of dye distribution and parameter interactions, rather than a contradiction.

(2) Concerning the lack of statistical validation for ∆E* value differences, we fully agree with the reviewer’s comment. Statistical analysis is indeed essential for ensuring the scientific validity and reliability of the conclusions on dyeing performance. We have therefore conducted supplementary analysis in Section 3.3 of the revised manuscript and improved the corresponding discussion accordingly.

9. Section 3.4 -

The analysis is based on reflectance values in different wavelength ranges; however, it lacks a more detailed explanation of why the pigments exhibit such optical behavior in the context of their chemical composition, light absorption, and interaction with wood. Adding a brief interpretation of these phenomena would increase the scientific value of the work.

-- We agree with your professional opinion, and in the revised version, we have added supplementary content in bold font to explain the reasons why the two dyes exhibit different optical behaviors within different wavelength ranges from the perspectives of chemical composition and light absorption.

As in previous sections, there is no information on whether the differences in R (reflectance) values between samples are statistically significant. Introducing appropriate statistical tests would allow the reliability of the observed trends to be assessed.

— Thank you once again for the valuable suggestion regarding statistical significance. The measurement of R values is subject to the same limitation of insufficient sample size. In our future work, we plan to increase the sample size to ensure that statistical analysis can be properly conducted.

The text contains no references to previous studies or literature that could confirm or contrast the obtained results. This is important for placing the research in a scientific context and verifying its originality and credibility.

— We agree with your suggestion of adding literature comparisons, and have already included the comparative content in red font in Section 3.4.

The text repeatedly refers to figures (e.g., Fig. 8); however, the review text contains “Error! Reference source not found,” indicating a referencing error. Correct and clear insertion and description of the figures are essential so that the reader can independently verify the presented data.

-- Thank you for your correction. We have conducted a more comprehensive and meticulous review of the revised version and have corrected the incorrect citations.

 

10.Section 3.5. As in previous sections, it is not stated whether the differences in ∆E* values between dyeing methods and sample types are statistically significant. Including statistical analysis (e.g., ANOVA, post-hoc tests) would be essential to confirm the conclusions.

-- Thank you for pointing out the issue of the lack of statistical validation for the ∆E* value differences. We agree with your suggestion. Statistical analysis is the core that ensures the scientificity and reliability of the staining effect conclusion. We have conducted supplementary analysis and revised the manuscript to address this problem in the revised version.

The text contains “Error! Reference source not found” placeholders, which prevent verification of the presented data. Figures should be correctly inserted and clearly captioned.

-- Thank you for bringing this issue to our attention. We have revised the text by using bold font for the relevant parts.

An interpretation should be added explaining why supercritical CO₂ pretreatment improves washing fastness (e.g., changes in pore structure, better dye penetration, more stable dye–lignin/cellulose bonding).

-- We agree with your suggestion. Necessary explanations can make the text more readable and clarify the research value. We have supplemented the reasons for the positive impact of SC-CO2 pretreatment on the water washing fastness of dyed wood in the revised version using bold font.

It would be useful to compare the obtained washing fastness values with previous research results to place the findings in the context of existing knowledge.

-- We agree with your opinion. Wash fastness is the core indicator for evaluating the practicality of wood dyeing processes. By comparing the results of this study with previous research, the academic value of this process can be more clearly highlighted. We have added the comparative content in the revised version.

Although it is noted that Phellodendron Bark yields better washing fastness than camphor fruit, no explanation is provided as to why (e.g., differences in chemical composition, molecular weight, or hydrogen-bonding ability of the dyes).

-- Thank you for your professional and comprehensive evaluation. We have added the reasons for the different washing fastness of the two dyes in the revised version.

 

11.Section 3.6 – Remarks as previous + In the final part, there is a contradiction: it is stated that there is a smaller color difference after solarization for camphor fruit compared to Phellodendron bark, which indicates better lightfastness, but the text refers to “washing fastness” — this is likely an error and should be corrected.

-- Thank you very much for your correction. We have revised the text by using bold font for the relevant parts.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I would like to thank the Authors for implementing all the suggested revisions to the manuscript and for thoroughly addressing the reviewer’s comments. The revised version adequately responds to the concerns raised and significantly improves the clarity and overall quality of the paper. I have no further remarks, and in my opinion the article is suitable for publication in the journal.