Review Reports - Aqueous Polyhydroxyalkanoate Solution-Based Knife Coatings: An Alternative Approach to the Production of Coated Cotton Fabrics

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript, entitled ’Aqueous PHA Solution-Based Knife Coatings: A Sustainable
Approach to the Production of Coated Cotton Fabrics’, employs various formulations with and without additives to produce bio-based textile coatings. It demonstrates how the formulation composition and thermal treatment critically affect the coating's wettability, permeability, and durability, thus offering valuable insights for designing sustainable functional footwear materials. I believe this manuscript could be suitable for publication in Coating after implementing the following revisions.
1. Given the described progress in bio-based materials, what is the particular advantage or novelty of selecting PHBHHx over other biopolymers like PLA for this specific footwear coating application?
2. The authors note that the same knife gap (0.2 mm) was used for applying coatings with different PHBHHx concentrations (5%-20%). However, the final dry film thickness after solvent evaporation and curing is a critical parameter that directly influences coating properties like permeability and abrasion resistance.
3.Could the authors please provide the measured dry thickness for coatings made from different polymer concentrations?
4. It is important to clarify whether the observed performance differences are solely due to the polymer concentration and film morphology, or if variations in the final, physical thickness of the coating layer also play a significant role.
5. What is the binding mechanism between the cotton fabric and the PHBHHx coating?
6.Could the washing process preferentially remove or redistribute hydrophilic components from the extreme surface, thereby exposing a more hydrophobic pure PHBHHx layer?
7. It is strongly suggested that SEM images of the coating surfaces and cross-sectionsbe should be added in. The central hypotheses of this work, such as additive migration and the formation of a more uniform layer, require morphological validation.

Author Response

Given the described progress in bio-based materials, what is the particular advantage or novelty of selecting PHBHHx over other biopolymers like PLA for this specific footwear coating application?

R. The particular advantage of selecting PHBHHx over other biopolymers such as PLA for this footwear coating application lies in its unique combination of flexibility, toughness, and hydrophobicity. Unlike PLA, which is relatively brittle and prone to cracking under repeated bending, PHBHHx incorporates 3-hydroxyhexanoate units that act as soft segments, providing enhanced elasticity and impact resistance. This makes it particularly well-suited to endure the mechanical stresses and repeated flexing typical in footwear. In addition, PHBHHx is more hydrophobic, which improves moisture and wear resistance while maintaining biodegradability. Therefore, its novelty lies in offering a durable, flexible, and environmentally friendly coating that PLA and similar biopolymers struggle to achieve in footwear applications.

Furthermore, within the context of knife-coating techniques, and to the best of our knowledge, no prior studies have explored the application of PHAs. This represents a novel research avenue, creating an opportunity to systematically evaluate the potential of these materials for footwear coatings and to explore their properties step by step. The supporting discussion for these arguments has now been provided in the revised version on lines 83–87 and 102–105.

The authors note that the same knife gap (0.2 mm) was used for applying coatings with different PHBHHx concentrations (5%-20%). However, the final dry film thickness after solvent evaporation and curing is a critical parameter that directly influences coating properties like permeability and abrasion resistance.

R. We appreciate this valuable technical observation. All coatings were prepared with the same nominal thickness to ensure a fair and consistent comparison across the different formulations. Initially, when coatings were produced using xanthan gum as the thickener, the PHBHHx concentration was varied from 5% to 20% (w/v). These preliminary experiments were primarily exploratory and aimed at identifying the most suitable polymer concentration based on surface morphology and hydrophobicity. Therefore, not all properties were analysed at this stage. Based on these initial results, a PHBHHx concentration of 10% (w/v) was selected as the optimal formulation, as it provided the best balance between processability and coating performance. The thicknesses of this formulation, with and without additives, were measured before and after hot pressing, as well as following washing cycles. All corresponding explanations and data are now presented in the main manuscript. To further strengthen our results, we have explicitly included the measured film thickness values in the revised version. We are grateful for this comment, as it encouraged us to more thoroughly consider how coating thickness may influence the observed properties in each case.

3.Could the authors please provide the measured dry thickness for coatings made from different polymer concentrations?

R. As we mentioned in our previous response, we fully agree with this observation and acknowledge that it was an oversight on our part not to present the coating thicknesses initially. We have now included all thickness values in the main manuscript.

It is important to clarify whether the observed performance differences are solely due to the polymer concentration and film morphology, or if variations in the final, physical thickness of the coating layer also play a significant role.

R. We appreciate this insightful observation. This point has already been addressed in the main manuscript.

What is the binding mechanism between the cotton fabric and the PHBHHx coating?

R. The adhesion of PHBHHx coatings to cotton fabric is primarily governed by a combination of mechanical interlocking and interfacial interactions. During the coating process, the polymer infiltrates the surface irregularities and pores of the fibrous cotton substrate. Upon drying, this mechanical interlocking helps secure the polymer in place. In addition, van der Waals interactions between the PHBHHx chains and the hydroxyl groups of cellulose contribute further to adhesion. The relatively hydrophobic nature of PHBHHx also facilitates the formation of a continuous, uniform coating over the fibres. While chemical bonding is minimal in untreated cotton, surface modifications could enhance adhesion through hydrogen bonding or esterification. Taken together, these mechanisms ensure effective and stable attachment of PHBHHx coatings to cotton fabric, supporting the functional performance observed in our study.

Could the washing process preferentially remove or redistribute hydrophilic components from the extreme surface, thereby exposing a more hydrophobic pure PHBHHx layer?

R. Regarding the question of whether washing preferentially exposes a hydrophobic PHBHHx layer, our contact angle measurements indicate that this is not the primary mechanism. In several cases, contact angles decrease or water is absorbed instantly after washing, demonstrating that washing does not consistently enhance surface hydrophobicity. By examining the results, the observed increase in hydrophobicity, particularly in samples subjected to thermal treatment with additives, can be primarily attributed to reorganization and surface enrichment of the hydrophobic PHBHHx polymer. The hydrophilic additives present in the formulation are unlikely to directly contribute to higher contact angles; instead, thermal treatment facilitates the migration of PHBHHx chains toward the fibre surface, forming a more continuous and uniform hydrophobic layer. Moreover, the contact angle data suggest that thermal treatment can redistribute the hydrophilic additives within the coating, embedding them in the bulk and indirectly promoting surface hydrophobicity. Overall, the enhanced hydrophobicity observed is best explained by the combined effects of polymer reorganization, additive redistribution, and thermal treatment, rather than by selective removal of hydrophilic components during washing. This interpretation is further discussed in Section 3.5, which has now been highlighted in red for clarity. With these revisions, the influence of thermal treatment on hydrophobic properties is clearly described and easily accessible to the reader.

It is strongly suggested that SEM images of the coating surfaces and cross-sectionsbe should be added in. The central hypotheses of this work, such as additive migration and the formation of a more uniform layer, require morphological validation.

R. We appreciate the reviewer’s suggestion regarding SEM imaging. While SEM can provide high-resolution surface details, optical microscopy was sufficient to reveal the morphology and continuity of all coatings produced, allowing us to identify areas for improvement. In this initial phase of the work, SEM would primarily serve to support these observations; however, given that the coatings require further optimization to improve surface smoothness, SEM analysis is more appropriate for future studies. Regarding additive migration, SEM is not ideally suited for this purpose, as most organic additives have similar electron density and composition to the PHBHHx polymer, resulting in limited contrast. In contrast, the optical microscopy images clearly demonstrate the defects/uniformity of the coating layer, providing sufficient evidence to validate the proposed mechanisms of layer formation and polymer-additive organization. This study represents the first exploration of PHBHHx-based coatings for textiles, demonstrating the potential of this biopolymer as an alternative for sustainable textile finishing. Further optimization and high-resolution morphological analysis will be pursued in subsequent work to enhance coating smoothness and functional performance.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I have read the manuscript titled 'Aqueous PHA solution-based knife coatings: A sustainable approach to the production of coated cotton fabrics' and observed that it lacks some merits that limit it's possibility for publication in Coatings in the present form. Hence, the manuscript can be reconsidered for publication, provided that the authors are ready to resolve the following identified issues:

The authors should boost the grammatical structure, as some sentences in the manuscript are hanging.
Putting abbreviation in the title can limit the chances of discovering the manuscript and it's readerbility. Therefore, authors should write PHA in full in the title.
In as much as Keywords stand as indicators for looking and discovering the manuscript, authors need to come up with other words to replace those repeated from title words.
Authors should highlight the novelty of this study under Introduction section.
Section 3.1.2 reads more of methodology than results and discussion.
Authors only did presentation of results, rather than discussion of results as authors failed to compare their findings with those of similar studies reported in the literature. Authors are therefore encouraged to compare their findings with those of recent similar studies reported in the literature.
Citation of the following recent reference between lines 349 and 352, among others will enhance the quality of this manuscript: Chemistry 7(1):23 (2025).
For the research to be claimed to be sustainable: bio-degradable, eco-friendly, I suggest to the authors to carry out: -Biodegradability test using a referenced standard method -Toxicology test using a standard method: for validating eco-friendly claim.

Comments on the Quality of English Language

Some sentences are hanging, authors should carefully read the manuscript and make the necessary corrections or involve the services of native English Language speaker.

Author Response

The authors should boost the grammatical structure, as some sentences in the manuscript are hanging.

R. We appreciate the reviewer’s comment regarding the manuscript’s grammatical structure. We have carefully revised the text to improve sentence flow, clarity, and overall readability, ensuring that all previously incomplete or awkwardly structured sentences have been corrected.

2. Putting abbreviation in the title can limit the chances of discovering the manuscript and it's readerbility. Therefore, authors should write PHA in full in the title.

R. We thank the reviewer for this suggestion. The title has been revised to spell out “polyhydroxyalkanoate (PHA)” in full, enhancing both discoverability and readability.

3. In as much as Keywords stand as indicators for looking and discovering the manuscript, authors need to come up with other words to replace those repeated from title words.

R. We thank the reviewer for this suggestion. In response, we have revised the keywords to enhance discoverability while avoiding repetition of the title words. Specifically, we removed “polyhydroxyalkanoates,” as it is already included in the title, and added “cotton fabric” along with other relevant terms. These updated keywords are expected to improve the visibility of the manuscript and facilitate its discovery by interested readers.

4. Authors should highlight the novelty of this study under Introduction section.

R. We thank the reviewer for the suggestion. The novelty of the study has now been explicitly highlighted in the Introduction section. We clearly state that, to the best of our knowledge, this is the first study to explore PHBHHx coatings for textile-based footwear components, including formulations with and without additives, and we emphasize its potential as a sustainable and high-performance alternative to conventional synthetic polymers and leather.

5. Section 3.1.2 reads more of methodology than results and discussion.

R. We understand this observation; however, the portion that appears methodological is limited to the beginning and is intended to clearly explain the logic of the article. This section clarifies that initially all experiments were conducted with all concentrations of PHBHHx and with xanthan gum, allowing the reader to follow the study’s steps without losing focus. Importantly, this methodological explanation spans only six lines (lines 244–250). Thereafter, we describe the differences observed on the surface of each coating, as well as the contact angles measured for these developing coatings.

6. Authors only did presentation of results, rather than discussion of results as authors failed to compare their findings with those of similar studies reported in the literature. Authors are therefore encouraged to compare their findings with those of recent similar studies reported in the literature.

R. To the best of our knowledge, there are currently no published studies reporting the use of PHBHHx as a coating material applied to textile substrates via knife coating. While PHBHHx has been widely investigated for films, packaging, and biomedical applications, its application as a textile coating using a knife coating technique represents a novel approach. Our work therefore provides a unique contribution by demonstrating the feasibility of this process, systematically evaluating the effects of additives, polymer concentration, and thermal treatment on the structural, morphological, and functional properties of the coated fabrics.

7. Citation of the following recent reference between lines 349 and 352, among others will enhance the quality of this manuscript: Chemistry 7(1):23 (2025).

R. We thank the reviewer for the suggestion. The reference led us to the article: Brazilian Plume (Justicia carnea L.) Leaf Wine-Colored Extract for Natural Dyeing of Mordant-Functionalized Cellulosic Fabric: Color Strength, Coordinates, and Fastness. However, from our perspective, this article does not support the information presented in lines 342–346 (formerly lines 349–352 – previous version). Therefore, we decided not to include it, as the contents are different and we did not find any relevant data in that study to substantiate the information in our manuscript.

8. For the research to be claimed to be sustainable: bio-degradable, eco-friendly, I suggest to the authors to carry out: -Biodegradability test using a referenced standard method -Toxicology test using a standard method: for validating eco-friendly claim.

R. We thank the reviewer for this important suggestion. We agree that performing biodegradability and toxicology tests using referenced standard methods would provide strong validation of the sustainability and eco-friendliness of the coatings. However, the current study represents an initial phase of the project, focusing on the development and characterization of PHBHHx-based coatings and their functional performance on cotton fabrics. At this stage, the coatings still require further optimization to improve uniformity, durability, and overall performance. Consequently, the biodegradability and toxicology assessments will be carried out in subsequent phases of the project, once the coatings reach a fully optimized formulation. We have clarified this point in the revised manuscript to indicate that these sustainability evaluations are planned as the next steps in the research.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Textiles made from natural fibers are often treated with special compounds. This study proposes applying PHA to the surface of cotton products. The abbreviation "PHA" should not be used in the manuscript title; it is better to replace "PHA" with the full name "polyhydroxyalkanoate." The same applies to "PHBHHx" in the abstract. The solution described in this paper is intended for use in the footwear industry. Thus, some shoe components can be replaced with systems made from natural materials with a special coating. This primarily involves replacing components made from synthetic polymers.
The abstract reflects the main content of the paper.
In the keyword list, remove the abbreviations "Knife-coating" and add "cotton."
In the section on materials and methods, detailed characteristics are required for the former. Also, in line 135, "the first layer was applied with a nominal thickness of 0.0 mm" means there is no layer. Therefore, it should be written as "no coating."
Lines 234-247. Why weren't the optimal formulations selected at the rheological behavior assessment stage? It would also be advisable to include the obtained rheological data, including the results of dynamic tests, in the paper. Such results can also be added to the supplementary materials of the manuscript. Why wasn't degassing of the systems performed?
I recommend beginning the review of the research results with the IR spectroscopy spectra. These data will further clarify many of the properties and results of other studies.
Table 1 is more of a figure formatted as a table than a table.
The spectra in Figure 1 do not indicate the regions characterizing OH groups, which are actively discussed in the manuscript. I also recommend noting that the ATR method is used, and the axis is labeled "Transmittance."

The conclusions reflect the obtained results, but their novelty and scientific significance are difficult to assess due to the lack of comparative results. Am I correct in assuming that similar studies have not been conducted previously and are not presented in the literature (possibly for other systems)? How toxic is the coating, which, as shown, is partially removed by aqueous treatment? The manuscript contains a small number of typos and errors that need to be corrected. It would be better to use a consistent style in the figures provided; for example, in the spectra for the cotton sample, it would be better to use a single curve color.

Author Response

Textiles made from natural fibres are often treated with special compounds. This study proposes applying PHA to the surface of cotton products. The abbreviation "PHA" should not be used in the manuscript title; it is better to replace "PHA" with the full name "polyhydroxyalkanoate." The same applies to "PHBHHx" in the abstract.

R. We are grateful for this observation, and these details have already been revised. They are highlighted in red.

The solution described in this paper is intended for use in the footwear industry. Thus, some shoe components can be replaced with systems made from natural materials with a special coating. This primarily involves replacing components made from synthetic polymers.
The abstract reflects the main content of the paper.

R. We thank the reviewer for the positive feedback. We are pleased that the relevance of our work to the footwear industry and the main focus of the abstract are clearly recognized.

In the keyword list, remove the abbreviations "Knife-coating" and add "cotton."

R. We appreciate this comment. We have removed the term “knife coating” and replaced it with “cotton fabric.

In the section on materials and methods, detailed characteristics are required for the former.

R. We thank the reviewer for this observation. We have added more details to the Materials and Methods section and included information about xanthan gum, which was previously missing.

Also, in line 135, "the first layer was applied with a nominal thickness of 0.0 mm" means there is no layer. Therefore, it should be written as "no coating."

R. rate surface, improve adhesion, and act as a reference point for measuring the thickness of subsequent layers. Essentially, it “primes” the surface so that the next layers can be applied accurately and uniformly. From this base layer, the additional layers were deposited to achieve the final thicknesses reported (0.2 + 0.2 mm). Therefore, even though the nominal thickness is indicated as 0.0 mm, this first layer is essential for ensuring uniformity, adhesion, and reproducibility of the coating process.

Lines 234-247. Why weren't the optimal formulations selected at the rheological behavior assessment stage? It would also be advisable to include the obtained rheological data, including the results of dynamic tests, in the paper. Such results can also be added to the supplementary materials of the manuscript.

R. We thank the reviewer for this valuable suggestion. The initial selection of formulations was based on surface morphology and hydrophobicity, as our first goal was to understand how different PHBHHx concentrations affect the cotton surface and its properties. Rheological characterization was subsequently performed to complement this information. Nevertheless, we agree that including the rheological data, including dynamic test results, would strengthen the manuscript. Therefore, we have added these results to the supplementary materials to provide a comprehensive overview of the formulations’ rheological properties.

Why wasn't degassing of the systems performed?

R. We thank the reviewer for this observation. In our study, degassing of the coating formulations was not performed. Despite the presence of potential air bubbles, the formulations showed sufficient flowability and uniformity when applied with the knife coating system. The thinness of the applied layers, combined with the spreading action of the blade, allowed any small bubbles to escape during application, resulting in defect-free and reproducible coatings, as confirmed by surface morphology analysis. For future work, we acknowledge that degassing could further minimize the risk of bubbles, especially for thicker coatings or formulations with higher viscosity.

I recommend beginning the review of the research results with the IR spectroscopy spectra. These data will further clarify many of the properties and results of other studies.

R. We appreciate the reviewer’s suggestion. However, we believe that beginning the results section with IR spectroscopy may not provide the clearest narrative. Our study first presents coating morphology, thickness, and functional properties, establishing the physical and performance context. IR spectroscopy is then presented to confirm the presence of the polymeric coating on the textile substrates, validating both the chemical composition and successful deposition of PHBHHx. This sequence ensures that the structural and functional observations are directly supported by the chemical evidence.

Table 1 is more of a figure formatted as a table than a table.

R. We appreciate this observation. However, we considered it more appropriate to present it as a table rather than a figure, given the structured format and the way the information is systematically organized for comparison.

The spectra in Figure 1 do not indicate the regions characterizing OH groups, which are actively discussed in the manuscript. I also recommend noting that the ATR method is used, and the axis is labelled "Transmittance."

R. We thank the reviewer for this comment. In Figure 1, the OH regions have now been highlighted to clearly indicate their positions. The figure caption has also been updated to note that the spectra were obtained using the ATR (Attenuated Total Reflectance) method. Although ATR measures absorption via internal reflection at the crystal interface, the resulting spectra are commonly presented as %Transmittance. This is standard practice in FTIR spectroscopy and facilitates interpretation of the data.

R. We thank the reviewer for this observation. To the best of our knowledge, no previous studies have specifically investigated PHBHHx coatings on cotton fabrics under the formulations and conditions presented here. While related polymer coatings have been explored, our work is novel in its combination of natural and synthetic thickeners, the range of PHBHHx concentrations, and the comprehensive evaluation of surface morphology and wettability. This has been clarified in the revised manuscript to highlight the originality and scientific significance of our study.

How toxic is the coating, which, as shown, is partially removed by aqueous treatment?

R. The toxicity of the coatings was not evaluated in this study, as this work represents an initial stage focused on developing and characterizing the formulations. Nevertheless, we plan to assess toxicity in future studies to provide a more comprehensive evaluation and complete the overall investigation of the material’s suitability for practical applications.

The manuscript contains a small number of typos and errors that need to be corrected.

R. We thank the reviewer for this observation. We have carefully proofread the manuscript and corrected all typos and errors to ensure clarity and accuracy throughout the text.

It would be better to use a consistent style in the figures provided; for example, in the spectra for the cotton sample, it would be better to use a single curve color.

R. We thank the reviewer for this helpful suggestion. We have already modified the IR spectra of the cotton sample now use a single curve colour to ensure a consistent and clear presentation across all figur

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript can be considered for acceptance in its current form. I have no major criticisms.

Author Response

"The manuscript can be considered for acceptance in its current form. I have no major criticisms."

R: We sincerely thank the reviewer for his/her positive evaluation and encouraging feedback. We are very pleased that the revised version of the manuscript meets the reviewer’s expectations. We highly appreciate the time and effort dedicated to the review process.

Reviewer 2 Report

Comments and Suggestions for Authors

Authors are strong advised to carryout Biodegradability and Toxicological studies on their product for the word 'sustainability' in the title to be validated or otherwise remove 'sustainable' from the title.

Author Response

"Authors are strong advised to carryout Biodegradability and Toxicological studies on their product for the word 'sustainability' in the title to be validated or otherwise remove 'sustainable' from the title."

R. We thank the reviewer for this valuable suggestion. Following your recommendation, we have removed the word “sustainable” from the title and replaced it with “alternative”, to more accurately reflect the scope of the study, which focuses on the development of an aqueous coating approach. We fully acknowledge that conducting biodegradability and toxicological studies is important to validate claims of sustainability. However, at this stage, such investigations are not feasible, as the formulations are still being optimized. These studies will be addressed step by step in future work as the development progresses.