Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This publication examines the influence of ethyl cellulose on the tribological, physicochemical and rheological properties of P34HB-based lubricants. An increase in the mechanical stability of the lubricating system was noted after the addition of ethylcellulose, where the increase in ethylcellulose content results in rheological properties similar to those of oil gels. It was also reported that P34HB grease containing 5% ethyl cellulose had similar tribological properties to plastic grease.

 

The work is interesting, but I have the impression that the authors sent a bad version of the manuscript, which is not well edited and substantive, especially in terms of the literature introduction and discussion.

 

My comments are as follows:

1. The abstract needs to be corrected. I would include more specifics.

2. The literature is not revised according to MDPI guidelines. There are a lot of editing errors there.

3. The scale in the graphs in Figure 9 is unclear.

4. In my opinion, units should be used in the SI system.

5. Was the experiment planning method used?

6. Units in table 1. They should be corrected for editing, e.g. g/cm3, KJ/m2. Some units should be marked with a superscript.

7. Should literature citations be presented as items in square brackets with a superscript?

8. Table 2 should be corrected for editing.

9. In Chapter 3, I miss a deeper discussion in which the results of the research obtained are compared/compared with the results of other researchers.

Author Response

Dear Editor and Reviewers:

We deeply appreciate the time and effort you have spent in reviewing our manuscript (Manuscript Number: sustainability-2941202, title: Development of fully renewable lubricant: The effect of ethyl cellulose on the properties of polyhydroxyalkanoates (P34HB) based grease). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We revised the manuscript by the reviewers’ comments point to point and carefully proofread the manuscript to minimize typographical, grammatical, and bibliographical errors. Here below is our description of the revision according to the referees’ comments, and the revised portion is also marked in yellow in the manuscript.

Thanks again!

Response to Reviewer Comments

1. The abstract needs to be corrected. I would include more specifics.

Response: We appreciate the reviewers' comments on the manuscript. The abstract has been corrected, and more specifics have been included. The revised abstract is as follows:

In the context of the ongoing evolution of the global economy and increasing environmental awareness, green sustainable development has emerged as a pivotal pathway for future advancements in the lubrication industry. In this study, we prepared bio-based greases utilizing sustainable materials such as polyhydroxyalkanoate (P34HB) and ethyl cellulose as thickeners dispersed within castor oil. The findings demonstrate that the significant ethyl cellulose effectively enhances both the mechanical stability and colloidal stability of the grease system. Notably, the addition of 5 wt% ethyl cellulose leads to superior mechanical and colloidal stability, while increasing concentrations gradually result in rheological properties similar to oleogels. Moreover, the wear volume of the grease containing 5 wt% ethyl cellulose is reduced by 39.20% compared to that of reference P34HB grease, demonstrating its exceptional wear resistance. The present study prepares a novel grease comprising P34HB material and ethyl cellulose, thereby expanding the range of environmentally friendly greases formulated with biomass-based thickeners.

2. The literature is not revised according to MDPI guidelines. There are a lot of editing errors there.

Response: We express our gratitude to the reviewers for their valuable comments and apologize for our oversight. The reference errors have been revised according to MDPI guidelines in the revised manuscript.

3. The scale in the graphs in Figure 9 is unclear.

Response: Thank you for this valuable suggestion. The resolution of Figure 9 in our manuscript has been enhanced. The details are as follows:

 

Figure 9 Wear volume (a), depth of abrasion (b), and three-dimensional contours (c, P30E5; d, P32E3; e, P35) of complex P34HB-EC-based greases under test conditions of 25 N, 5 Hz, and 80 °C

4. In my opinion, units should be used in the SI system.

Response: We greatly appreciate your valuable comments. Based on your suggestion, we have made corrections to the units in our manuscript. The details are as follows:

Refined castor oil (kinematic viscosity at 40 °C: 248.3 mm²/s) was purchased from Inner Mongolia Weiyu Biotechnology Co., Ltd, as a biodegradable lubricating oil for greases. The following are different materials used as thickeners to prepare greases. P34HB powder (particle size about 15 μm, industrial grade) (Mw: ~300,000 g/mol) acquired from Tianjin Guoyun Biotechnology Materials Co., Ltd. The specific parameters of P34HB are provided by the manufacturer, as presented in Table 1. P34HB primarily consists of polymerized units of 3-hydroxybutyric acid (3HB) and 4-hydroxybutyric acid (4HB). Ethyl cellulose powder (analytical grade) was purchased from Tianjin Zhonglian Chemical Reagent Co., Ltd. Methyl cellulose powder (food grade) was sourced from Guangzhou Xinzhiwei Food Ingredient Mall. Bentonite powder (particle size about 10 μm, industrial grade) was procured from Hebei Hongyao Mineral Products Processing Co., Ltd.

During this period, the stirring speed was adjusted to 300-400 rpm and stirred for 10 min. Subsequently, the temperature was maintained at 175 °C with a stirring rate of 160 rpm for a reaction time of 1 h. A second thickening agent, such as ethyl cellulose, was then added and reacted under identical test conditions for an additional duration of 30 min.

5. Was the experiment planning method used?

Response: We sincerely appreciate your comments. After the exploration experiment, the fixed concentration of P34HB was 30% the fixed concentration of the second thickener was 5%, and the mechanical stability and colloidal stability of different thickener types of greases were compared and analyzed. It was observed that when ethyl cellulose was used as the thickener, the mechanical properties of the resulting grease were significantly enhanced compared to other thickeners. Therefore, through experimental design with a fixed total content of thickener and adjustable proportion between the two components, greases were prepared and their physicochemical, rheological, and frictional properties were systematically investigated. Subsequently, based on the experimental results, an evaluation of each component's influence on grease performance was conducted to determine the optimal ratio.

6. Units in table 1. They should be corrected for editing, e.g. g/cm3, KJ/m2. Some units should be marked with a superscript.

Response: We value your feedback and suggestions. The units such as g/cm3, KJ/m2 in Table 1 have been corrected.

7. Should literature citations be presented as items in square brackets with a superscript?

Response: We appreciate the reviewers' comments on the manuscript. In our manuscript, we have removed the superscript format from the citation.

8. Table 2 should be corrected for editing.

Response: We express our sincere gratitude to the reviewers for their invaluable feedback. In response, we have made amendments and additions to Table 2 in our manuscript. By comparing the mechanical stability of different thickeners, ethyl cellulose was selected as the best thickener. Therefore, we added the physicochemical properties data of ethyl cellulose in Table 2 to make the comparison more obvious. The specific modifications are outlined as follows:

Table 2 Supporting data from previous exploratory experiments

Type	unworked penetration (0.1 mm)	worked penetration (0.1 mm)	penetration variation (0.1 mm)	Oil separation (%)
P34HB (30 wt%)-Lignin (5 wt%)	317	424	107	11.58
P34HB (30 wt%)-Ethyl cellulose (5 wt%)	277	363	86	2.60
P34HB (30 wt%)-Lithium soap (5 wt%)	422	-	-	-
P34HB (30 wt%)-Calcium soap (5 wt%)	224	354	129	18.28
P34HB (30 wt%)-Methyl cellulose (5 wt%)	324	422	98	19.94

Subsequent attempts with other thickeners also demonstrated minimal improvement in the consistency or colloidal stability of the grease (as illustrated in Table 2). For example, the incorporation of 5 wt% bentonite into the grease formulation resulted in non-saponification and the formation of a compact yellow solid mass.

9. In Chapter 3, I miss a deeper discussion in which the results of the research obtained are compared/compared with the results of other researchers.

Response: We appreciate your suggestion on our manuscript. In our manuscript, we add some additional explanations and analyses to the conclusion.

As the ethyl cellulose content increased, the working penetration value of the grease decreased from 110 to 63, indicating a significant improvement in the mechanical stability of these greases. The mechanical stability of the P34HB-EC-based complex grease studied is marginally inferior to that of the oleogel or lithium-based grease prepared by Sanchez et al. [12].

In summary, the response of complex P34HB-EC-based greases to load and temperature exhibits distinct variations, thereby highlighting the superior tribological characteristics of P30E5 under light load and temperature conditions. We found through previous studies [35] that cellulose acetate butyrate-based oleogels with high viscosity and yield stress are only suitable for low-friction applications. In addition, under low load conditions, the prepared grease (P30E5) has the same frictional properties as the pure oleogel dispersion with high thickening agent content (7 wt% CAB) in the study.

References: 12. Sánchez, R.; Franco, J.M.; Delgado, M.A.; Valencia, C.; Gallegos, C. Development of new green lubricating grease formulations based on cellulosic derivatives and castor oil. Green. Chem. 2009, 11, 686-693.

Gorbacheva, S.N.; Yadykova, A.Y.; Ilyin, S.O. Rheological and tribological properties of low-temperature greases based on cellulose acetate butyrate gel. Carbohyd. Polym. 2021, 272, 118509.

We have tried our best to improve the manuscript and made some yellow-marked changes in the revised paper. We appreciate the editor and reviewers’ warm work earnestly and hope that the correction will be approved. The revised parts have been marked in yellow in the manuscript. Once again, thank you very much for your comments and suggestions.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is about developing eco-friendly greases based on polyhydroxyalkanoates (P34HB) in castor oil with the addition of ethyl cellulose. This three-component composition is considered innovative, however, P34HB is a fairly expensive material. It is interest to compare the properties of the obtained greases with those of commercially available and widely used compositions, such as NLGI 1-3 standard lithium greases. in the introduction, a lot of attention was paid to the environmental aspects of lubricants, so the properties that lubricants should have been left without attention. Also, the influence of different oils on the properties of greases is overlooked. Greases are utilized in a wide range of applications, each with its own material requirements. It is advisable for the authors to specify the intended application area for the developed composition. The authors should revise the introduction, addressing the emerging questions therein.

The work is executed at a commendable level, providing data that do not raise significant questions. However, there are redundant data that increase the volume of the article without contributing scientific value or influencing the final conclusions (Lines 163-192, including Tables 2 and 3). Perhaps the authors should consider removing these discussions from the article or relocating them to an appendix.

The formatting and presentation of the obtained results are at a low level, including technical edits that significantly impede readability. References and the bibliography are not formatted according to the journal's requirements, and there are also some remarks regarding the formatting of figures and text, as outlined below.

The work may be recommended for publication after major revisions.

Line 45: The phrase "Among them..." is not an appropriate linkage and can lead to misunderstanding. Authors should adjust the connection between the products containing cellulose and the conversion of cellulose.

Line 56: "..., and the same conditions with cel-" is unclear due to the absence of a predicate and subject, significantly complicating the comprehension of the text.

Line 69: "Among them..." is an incorrect linkage as there is nothing in the previous sentence to which "Among them" refers.

Line 75-77: The statement regarding the utilization of sustainable biological materials for the synthesis of oil and grease is unclear. Biological materials merely serve as additives (thickeners) and do not participate in the synthesis.

Line 79: Authors should specify whether synthesis or simple preparation of systems is conducted in the study.

Line 93: In what form was P34HB used? Mesh size is not a systemic quantity, and it should be indicated in SI units. The same comment applies to the description of bentonite (Line 100).

Table 1: The formatting is extremely careless. Are all the parameters listed in the table actually used in the study? Most of the methods are indicated as "A" or "1A," which is unclear to the reader.

Line 97: The molecular weight should be provided with units, e.g., "approximately 300,000 g/mol." Specifying the content in the range of 6-20% is inappropriate; either clarify or remove this information regarding the content range of 4HB.

Section "2.2. Preparation of biopolymer-based lubricating greases" contains repetitions. Authors should avoid this, as the general preparation methodology is the same, with minor variations in speed. Authors should provide a table with a comprehensive list of samples and their component concentrations.

Line 124: Absolute quantities of added powder do not provide information to the reader; it should be replaced with wt.%.

Line 150: Authors should specify the manufacturer of the "Optimol-SRV long-stroke high-frequency reciprocating test machine."

Line 176: How can hardness be evaluated based on photographs of lubricants?

Line 206: Castor oil viscosity cannot increase with the addition of ethyl cellulose; it may alter the viscosity of the system. Authors should clarify this formulation.

Figure 3: Authors should enlarge the scale bar. How did the authors distinguish ethyl cellulose and P34HB in the SEM images? The differences in microstructure are minor; authors should analyze a series of images for confirmation and provide them in the Appendix if available.

Figure 4: What causes the "plateau" formation on the spectra of triple systems in the 1500-900 cm^-1 range?

Line 273: The linear viscoelasticity range cannot be equal to an exact value, especially currency about 10^-4%.

Figure 5 (a,b): Representing viscosity, especially in linear coordinates does not make sense. Authors should remove these dependencies to make the graph clearer. These data graphs are necessary for determining the linear viscoelasticity range; therefore, authors should focus on low deformation regions, up to ~1%, as the high deformation region provides no information, and the sample structure is already destroyed.

Lines 277-279: Based on which data was determined the yield point? Is this value the yield strength for these systems?

Line 280: Which parameter's temperature dependence is being discussed?

Figure 5d: This graph is entirely incomprehensible to the reader, as the same notation G' and G'' is used, and the ordinate axis range extends up to 10⁷, although the experimental data ends at 10⁵, causing significant distortion of the data, and the difference is not noticeable. The Y-axis designation is missing from the inset, which hinders the graph's perception.

Figure 6a: Viscosity can change significantly, so a logarithmic scale should be used. What happens at shear rates exceeding 10 s^-1?

 

Line 330: Authors should use consistent terminology and specify "ring" or "loop."

Comments on the Quality of English Language

Line 45: The phrase "Among them..." is not an appropriate linkage and can lead to misunderstanding. Authors should adjust the connection between the products containing cellulose and the conversion of cellulose.

Line 56: "..., and the same conditions with cel-" is unclear due to the absence of a predicate and subject, significantly complicating the comprehension of the text.

 

Line 69: "Among them..." is an incorrect linkage as there is nothing in the previous sentence to which "Among them" refers.

Author Response

Dear Editor and Reviewers:

We deeply appreciate the time and effort you have spent in reviewing our manuscript (Manuscript Number: sustainability-2941202, title: Development of fully renewable lubricant: The effect of ethyl cellulose on the properties of polyhydroxyalkanoates (P34HB) based grease). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We revised the manuscript by the reviewers’ comments point to point and carefully proofread the manuscript to minimize typographical, grammatical, and bibliographical errors. Here below is our description of the revision according to the referees’ comments, and the revised portion is also marked in yellow in the manuscript.

Thanks again!

Response to Reviewer Comments

1. Line 45: The phrase "Among them..." is not an appropriate linkage and can lead to misunderstanding. Authors should adjust the connection between the products containing cellulose and the conversion of cellulose.

Response: We sincerely thank the reviewer for careful reading. The following modifications have been made:

Notably, the conversion of cellulose into high-value cellulose esters and ethers (such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, etc) has garnered significant attention in the realm of green and sustainable chemistry, thereby fostering advancements in environmental protection technologies [4].

2. Line 56: "..., and the same conditions with cel-" is unclear due to the absence of a predicate and subject, significantly complicating the comprehension of the text.

Response: We greatly appreciate your valuable comments. According to your comments, we have revised the manuscript as follows:

Based on the findings from our previous investigations, it has been observed that both P34HB grease and certain cellulose-based oleogel systems exhibit deficiencies in terms of their mechanical stability and colloidal stability.

3. Line 69: "Among them..." is an incorrect linkage as there is nothing in the previous sentence to which "Among them" refers.

Response: We appreciate your suggestion on our manuscript. The following modifications have been made:

It is noteworthy that when a grease containing a mixture of ethyl cellulose and methyl cellulose as a thickener is formulated, the resulting grease exhibits mechanical stability comparable to that of commercially available greases.

4. Line 75-77: The statement regarding the utilization of sustainable biological materials for the synthesis of oil and grease is unclear. Biological materials merely serve as additives (thickeners) and do not participate in the synthesis.

Response: We strongly agree with the reviewer's query. The following modifications have been made:

In summary, this innovative research holds great potential in mitigating environmental pollution and reducing reliance on non-renewable energy sources, aligning perfectly with both economic development strategies and the principles of sustainability.

This work investigates the pioneering effort in preparing a novel series of lubricating greases by employing sustainable biomaterials (P34HB) and ethyl cellulose as thickeners for the first time, with castor oil serving as the fundamental base oil.

5. Line 79: Authors should specify whether synthesis or simple preparation of systems is conducted in the study.

Response: We appreciate your suggestion on our manuscript. The lubrication preparation description has been supplemented in our manuscript, with the following modifications:

Following the methodologies proposed by Sanchez et al. [19] and Martín-Alfonso et al. [20], crucial experimental parameters such as reaction temperature, stirring rate, and duration were meticulously adjusted to prepare a series of lubricating greases. We have delineated the final procedure for fabricating the lubricating grease as follows: The mixture was prepared by heating the oil to approximately 160 °C, followed by the addition of P34HB powder, and further heating up to 175 °C.

6. Line 93: In what form was P34HB used? Mesh size is not a systemic quantity, and it should be indicated in SI units. The same comment applies to the description of bentonite (Line 100).

Response: We quite appreciate your favorite consideration and insightful comment. We modified and supplemented the shape and size of P34HB according to SI units. Specific modifications are as follows:

P34HB powder (particle size about 15 μm, industrial grade) (Mw: ~300,000 g/mol) acquired from Tianjin Guoyun Biotechnology Materials Co., Ltd. The specific parameters of P34HB are provided by the manufacturer, as presented in Table 1. P34HB primarily consists of polymerized units of 3-hydroxybutyric acid (3HB) and 4-hydroxybutyric acid (4HB). Ethyl cellulose powder (analytical grade) was purchased from Tianjin Zhonglian Chemical Reagent Co., Ltd. Methyl cellulose powder (food grade) was sourced from Guangzhou Xinzhiwei Food Ingredient Mall. Bentonite powder (particle size about 10 μm, industrial grade) was procured from Hebei Hongyao Mineral Products Processing Co., Ltd.

7. Table 1: The formatting is extremely careless. Are all the parameters listed in the table actually used in the study? Most of the methods are indicated as "A" or "1A," which is unclear to the reader.

Response: We extend our gratitude to the reviewer for their valuable suggestions. We have incorporated your comments and suggestions to revise Table 1. Given the diverse range and intricate composition of P34HB materials, we have included physical and chemical property parameters in Table 1 to provide readers with detailed information. Furthermore, after consulting relevant standards, we have identified that test methods A and 1A are based on standards. However, to enhance clarity for our readers, we have omitted this list of methods. The specific modifications made are as follows:

Table 1 Specific parameters of P34HB

Properties	unit	Property parameter	Testing standard
		P34HB
Density	g/cm3	1.2	ISO 1183-1:2004
Glass Transition Temperature	°C	2	ISO 11357-2: 1999
melting temperature	°C	155-165
Tensile strength	MPa	33	ISO 527-2: 1993
percentage of breaking elongation	%	10
Notched impact strength	KJ/m2	3.7	ISO 180: 2000
Flexural Strength	MPa	42	ISO 178: 1993
Flexural modulus	GPa	1.8	/
Thermal decomposition temperature	°C	286	Method TGA
Vicat softening temperature	°C	134	ISO 306: 1994

8. Line 97: The molecular weight should be provided with units, e.g., "approximately 300,000 g/mol." Specifying the content in the range of 6-20% is inappropriate; either clarify or remove this information regarding the content range of 4HB.

Response: We appreciate your suggestion on our manuscript. In our manuscript, we have included the unit of molecular weight to enhance the scientific rigor. Considering the lack of clarity regarding the range of 4HB content, we have omitted the relevant description of 4HB.  The specific modifications made are as follows:

Refined castor oil (kinematic viscosity at 40 °C: 248.3 mm²/s) was purchased from Inner Mongolia Weiyu Biotechnology Co., Ltd, as a biodegradable lubricating oil for greases. The following are different materials used as thickeners to prepare greases. P34HB powder (particle size about 15 μm, industrial grade) (Mw: ~300,000 g/mol) acquired from Tianjin Guoyun Biotechnology Materials Co., Ltd.

9. Section "2.2. Preparation of biopolymer-based lubricating greases" contains repetitions. Authors should avoid this, as the general preparation methodology is the same, with minor variations in speed. Authors should provide a table with a comprehensive list of samples and their component concentrations.

Response: We greatly appreciate your invaluable advice. We agree with your suggestion and in our manuscript, we have a concise description of the grease preparation process. The sample and its ingredients are listed in Tables 1, 2, and 3. The specific changes are as follows:

The reference grease possesses a thickener content of 35 wt% derived from P34HB grease. The preparation process of the reference grease is consistent with that of the composite P34HB-EC, except for the steps where a second thickener is not required. Subsequently, the prepared grease is designated as P35.

10. Line 124: Absolute quantities of added powder do not provide information to the reader; it should be replaced with wt.%.

Response: We greatly appreciate your professional comments on our articles. According to your suggestions, we have revised the wording of the added amount, which is as follows:

The reference grease possesses a thickener content of 35 wt% derived from P34HB grease. The preparation process of the reference grease is consistent with that of the composite P34HB-EC, except for the steps where a second thickener is not required. Subsequently, the prepared grease is designated as P35.

Revised for other analogous cases have been made in our manuscript.

11. Line 150: Authors should specify the manufacturer of the "Optimol-SRV long-stroke high-frequency reciprocating test machine."

Response: Thanks for your suggestion. We have added the manufacturer of the instrument, and the specific modifications are as follows:

Friction tests were conducted using the Optimol-SRV long-stroke high-frequency reciprocating test machine (Germany Optimol Instrument Equipment Co., LTD) under varying temperatures, loads, and frequencies.

12. Line 176: How can hardness be evaluated based on photographs of lubricants?

Response: We would like to express our sincere gratitude for your professional review of our article. We have made specific modifications to address this issue as follows:

The prepared greases are illustrated in Figure 1. The appearance of the complex P34HB-EC-based grease undergoes a transition from dark yellow to a lighter shade as the ethyl cellulose content increases.

Complex P34HB-EC-based greases with ethyl cellulose contents of 0 wt%, 3 wt%, 5 wt%, 7 wt%, and 10 wt% were prepared, as illustrated in Figure 2. With an increase in ethyl cellulose content, these greases exhibited a distinctive semi-fluid state with specific viscosity. Fundamental physicochemical properties of complex P34HB-EC-based greases were tested, as presented in Table 4.

13. Line 206: Castor oil viscosity cannot increase with the addition of ethyl cellulose; it may alter the viscosity of the system. Authors should clarify this formulation.

Response: We sincerely appreciate your valuable comments. According to your comments, we have revised the statement that the increase of ethyl cellulose may improve the viscosity of the system, and the modification is as follows:

This suggests that within a specific range, ethyl cellulose effectively enhances the colloidal stability of those greases by potentially reducing oil separation through increased grease system viscosity [12].

14. Figure 3: Authors should enlarge the scale bar. How did the authors distinguish ethyl cellulose and P34HB in the SEM images? The differences in microstructure are minor; authors should analyze a series of images for confirmation and provide them in the Appendix if available. 

Response: We quite appreciate your favorite consideration and insightful comment. In our manuscript, we have included some enlarged SEM images (Figure A in Appendix) depicting the microstructure of grease. It was difficult to differentiate the morphology of ethyl cellulose and P34HB using SEM; however, this similarity in morphology is one of the reasons why we selected these two materials for preparing the grease. Nonetheless, minor variations in grease morphology were analyzed through SEM images and are presented as supplementary material. The following modifications have been made:

As the ethyl cellulose content reaches 10 wt%, the microstructure of composite P34HB-EC-based grease gradually transforms from a relatively irregularly stacked lamellar aggregate to a larger lamellar structure. The observed phenomenon indicates that an increase in ethyl cellulose content leads to the structural transformation of the grease.

Appendix A: The microstructure of greases with varying ethyl cellulose content exhibits distinct winding degrees, as evidenced in Annex A1. Through electron microscopy analysis of adhesives A1f and A2f, when the ethyl cellulose content reaches 10 wt%, the microstructure of the composite P34HB-EC-based grease consists of distinctly irregularly aggregated lamellar assemblies. The microstructure of this grease gradually forms larger chunks and cohesive layers, with a significant increase in the volume of interlamellar voids. This indicates that an increase in ethyl cellulose content induces structural transformation in the grease. It is noteworthy that the microstructure of P25E10 exhibits adherence to both small and large sheet structures, with the small size sheet structure on its surface resembling that of ethyl cellulose (see Figure A1). Furthermore, a cellulose strip-like structure can be observed in the lower right corner of Figure A1. This phenomenon suggests that ethyl cellulose can modulate the microstructure of grease.

 

Figure A1 Microscopic morphology of greases with different thickener ratios magnified 10,000 times (a, P35; b, Ethyl cellulose; c, P32E3; d, P30E5; e, P28E7; f, P25E10)

 

Figure A2 Microscopic morphology of greases with different thickener ratios magnified 50,000 times (a, P35; b, Ethyl cellulose; c, P32E3; d, P30E5; e, P28E7; f, P25E10)

15. Figure 4: What causes the "plateau" formation on the spectra of triple systems in the 1500-900 cm^-1 range?

Response: Thank you very much for your guidance on our content. Through quantitative analysis of infrared spectroscopy, we observed the continuous formation of ternary system "platforms" in the range of 1500-900 cm^-1. Therefore, it is assumed that there is a strong infrared absorption in this particular spectral region.

 

16. Line 273: The linear viscoelasticity range cannot be equal to an exact value, especially currency about 10^-4%.

Response: We appreciate your suggestion on our manuscript. The interpretation of the linear viscoelastic region in our manuscript has been revised and supplemented as follows:

Oscillation mode tests were conducted at controlled temperatures of 25 °C and 80 °C with a fixed angular frequency (ω) of 10 rad·s^-1 through amplitude sweep tests. Subsequently, the grease undergoes small amplitude oscillatory shear (determined by an amplitude scanning experiment) at a fixed strain (γ) of 0.0403% and an angular frequency (ω) ranging from 0.1 to 100 rad·s^-1 within the linear viscoelastic region.

The experimental results presented in Figure 5 illustrate the amplitude scanning and frequency scanning experiments conducted on complex P34HB-EC-based grease.

17. Figure 5 (a,b): Representing viscosity, especially in linear coordinates does not make sense. Authors should remove these dependencies to make the graph clearer. These data graphs are necessary for determining the linear viscoelasticity range; therefore, authors should focus on low deformation regions, up to ~1%, as the high deformation region provides no information, and the sample structure is already destroyed.

Response: I would like to thank the reviewers for their comments sincerely. We shorten the shear strain range and retain the data in the low deformation area to make the chart clearer. The specific changes are as follows:

 

Figure 5 Modulus and shear stress variation curves of the complex P34HB-EC-based grease at different temperatures (a, 25 °C; b, 80 °C) and flow Shear stress value at flow point (c); (d) Frequency scanning curve of the complex P34H B-EC-based grease with different thickening agent additives

18. Lines 277-279: Based on which data was determined the yield point? Is this value the yield strength for these systems?

Response: Thank you very much for your advice. According to your suggestions, we have made supplementary explanations to the interpretation of structural strength and flow point in the paper, and the specific modifications are as follows:

The experimental results presented in Figure 5 illustrate the amplitude scanning and frequency scanning experiments conducted on complex P34HB-EC base grease. It is widely known that the energy storage modulus (G') and the loss modulus (G'') respectively represent the elastic potential energy and viscous potential energy of grease. When G' = G'', it indicates the structural transformation point (flow point) of the grease has been reached, leading to its transition into a fluid state (G' < G''). Therefore, the shear stress at this transition point reflects the structural strength of the grease [27]. At a temperature of 25 °C, an increase in ethyl cellulose content leads to a reduction in the gap between G′′ and G′ for this grease system, indicating that ethyl cellulose content influences its structural characteristics.

References: 27. Xu, N.; Wang, X.; Ma, R.; Li, W.; Zhang, M. Insights into the rheological behaviors and tribological performances of lubricating grease: entangled structure of a fiber thickener and functional groups of a base oil. New. J. Chem. 2018, 42, 1484-1491.

19. Line 280: Which parameter's temperature dependence is being discussed?

Response: We sincerely thank the reviewer for careful reading. We have clarified and supplemented the expression of temperature dependence in the manuscript, specifically as follows:

Moreover, with increasing temperature, significant variations in flow points are observed between two temperatures for P35, P32E3, P30E5, P28E7, and P25E10, which correspond to 206.8 Pa, 210.1 Pa, 330.9 Pa, 356.0 Pa, and 2,427.2 Pa respectively. The findings demonstrate that the intermolecular distance of the thickener in the microstructure can be significantly increased by temperature and shearing, leading to a weakening of intermolecular forces and resulting in a pronounced alteration in structural strength. A gradual increase in the temperature sensitivity of the structure strength of complex grease with an elevated ethyl cellulose content. This phenomenon provides empirical evidence for predicting both the drop point of grease and the oil separation trend on steel mesh when the ethyl cellulose content exceeds 10 wt%.

20. Figure 5d: This graph is entirely incomprehensible to the reader, as the same notation G' and G'' is used, and the ordinate axis range extends up to 10⁷, although the experimental data ends at 10⁵, causing significant distortion of the data, and the difference is not noticeable. The Y-axis designation is missing from the inset, which hinders the graph's perception.

Response: We deeply appreciate the invaluable insights provided in your comments. In our manuscript, we use different symbols to distinguish between the energy storage modulus G' and the loss modulus G''. We refined the range of ordinate values in the chart to ensure a clear representation of data within the icon. We modify Figure 5d as follows:

It is widely known that the energy storage modulus (G') and the loss modulus (G'') respectively represent the elastic potential energy and viscous potential energy of grease.

 

21. Figure 6a: Viscosity can change significantly, so a logarithmic scale should be used. What happens at shear rates exceeding 10 s^-1?

Response: We greatly appreciate your valuable comments. After conducting an extensive shear thixotropic ring test, we have observed that the trend in the high shear rate range remains consistent with our previous findings. Consequently, we have replaced and elucidated the data accordingly. The specific modifications are outlined as follows:

It is worth noting that at high shear rates, the microstructure of P35 undergoes irreversible damage. The results indicate that P35 exhibits a limited affinity for base oil binding, inadequate mechanical stability, and diminished recovery capability. However, upon external force recovery, the microstructure of the prepared composite P34HB-EC grease reorganizes into a new stable spatial network structure characterized by disordered thixotropic loop changes and macroscopic thixotropic areas. This suggests that ethyl cellulose may have the ability to regulate the thixotropic behaviour of the lubricating grease. The calculated thixotropic values for the P35, P32E3, P30E5, P28E7, and P25E10 are 75,142.8 Pa·s^-1, 67,557.8 Pa·s^-1, 292,870.1 Pa·s^-1, 237,375.5 Pa·s^-1, and 453,645.7 Pa·s^-1, respectively. This suggests that the thixotropic property of the lubricating grease gradually improves as the ethyl cellulose content decreases, which is consistent with the structural strength test of the grease. The reason behind this phenomenon lies in the fact that greases with lower ethyl cellulose content exhibit reduced structural strength and possess larger cavity volumes within their microstructure. Consequently, these greases demonstrate enhanced recovery capabilities after experiencing damage, thereby leading to an improvement in thixotropic performance [34].

 

Figure 6 Thixotropic loops (a) and their area histograms (b) for different nanometres of complex P34HB-EC-based greases with different thickener ratios

22. Line 330: Authors should use consistent terminology and specify "ring" or "loop."

Response: We sincerely thank the reviewer for careful reading. As suggested by the reviewer, we have corrected the “ring” to "loop". Specific modifications are as follows:

Figure 6 Thixotropic loops (a) and their area histograms (b) for different nanometres of complex P34HB-EC-based greases with different thickener ratios

Comments on the Quality of English Language

23. Line 45: The phrase "Among them..." is not an appropriate linkage and can lead to misunderstanding. Authors should adjust the connection between the products containing cellulose and the conversion of cellulose.

Response: We sincerely thank the reviewer for careful reading. As suggested by the reviewer, we have corrected the "Among them" to "Notably". The following modifications have been made:

Notably, the conversion of cellulose into high-value cellulose esters and ethers (such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, etc) has garnered significant attention in the realm of green and sustainable chemistry, thereby fostering advancements in environmental protection technologies [4].

24. Line 56: "..., and the same conditions with cel-" is unclear due to the absence of a predicate and subject, significantly complicating the comprehension of the text.

Response: We greatly appreciate your valuable comments. According to your comments, we have revised the manuscript as follows:

Based on the findings from our previous investigations, it has been observed that both P34HB grease and certain cellulose-based oleogel systems exhibit deficiencies in terms of their mechanical stability and colloidal stability.

25. Line 69: "Among them..." is an incorrect linkage as there is nothing in the previous sentence to which "Among them" refers.

Response: We appreciate your suggestion on our manuscript. As suggested by the reviewer, we have corrected the "Among them" to "Notably". The following modifications have been made:

It is noteworthy that when a grease containing a mixture of ethyl cellulose and methyl cellulose as a thickener is formulated, the resulting grease exhibits mechanical stability comparable to that of commercially available greases.

We have tried our best to improve the manuscript and made some yellow-marked changes in the revised paper. We appreciate the editor and reviewers’ warm work earnestly and hope that the correction will be approved. The revised parts have been marked in yellow in the manuscript. Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This article investigates the synthesis of a series of lubricating greases with varying amounts of 80 P34HB and ethyl cellulose as thickeners, along with castor oil as the base oil through experimental investigations. Comparative analyses were conducted to assess the impact of thickener content on the colloidal stability, mechanical stability, rheological behavior, and tribological performance of the lubricating greases. However, there are still many key issues that need to be addressed in the article.

1. The experimental section is inadequate, lacking essential details crucial for reproducibility and scientific integrity. Structural characterization methods, which are vital for understanding the composition and properties of the synthesized greases, have not been provided. This omission greatly undermines the utility of the research.

2. The paper lacks innovation. The use of commercially available materials and conventional experimental methods does not contribute substantially to the advancement of knowledge in the field of lubricating greases. Without introducing novel concepts, approaches, or insights, the manuscript fails to justify its contribution to the existing literature.

3. The depth of research conducted in the article is insufficient. Despite claiming to anticipate an interplay among properties "Furthermore, analysis and discussion were carried out in anticipation of the interplay among their properties.", the analysis and discussion remain superficial, with only a cursory presentation of experimental results. Speculative assertions, such as the role of hydrogen bonds in the friction process, are not substantiated with empirical evidence or simulation data, detracting from the scientific rigor of the study.

 

4. Additionally, the language expression throughout the manuscript is subpar, riddled with punctuation errors, wording issues, and unclear phrasing, such as: “mechanical stability, rheological behaviour, and tribological performance” “thereby demonstrating a favourable binding effect.” “while its colour transitions from”. These linguistic deficiencies not only impede comprehension but also reflect poorly on the professionalism and attention to detail of the author.

Comments on the Quality of English Language

The presence of multiple revision marks indicates a lack of diligence and care in preparing the manuscript for submission, raising concerns about the author's commitment to the study. Such oversights compromise the overall quality and credibility of the research presented.

Author Response

Dear Editor and Reviewers:

We deeply appreciate the time and effort you have spent in reviewing our manuscript (Manuscript Number: sustainability-2941202, title: Development of fully renewable lubricant: The effect of ethyl cellulose on the properties of polyhydroxyalkanoates (P34HB) based grease). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We revised the manuscript by the reviewers’ comments point to point and carefully proofread the manuscript to minimize typographical, grammatical, and bibliographical errors. Here below is our description of the revision according to the referees’ comments, and the revised portion is also marked in yellow in the manuscript.

Thanks again!

Response to Reviewer Three Comments

1. The experimental section is inadequate, lacking essential details crucial for reproducibility and scientific integrity. Structural characterization methods, which are vital for understanding the composition and properties of the synthesized greases, have not been provided. This omission greatly undermines the utility of the research.

Response: Thank you very much for your guidance on our content. We have made a detailed supplementary explanation of the structural characterization method in our manuscript, and the specific modifications are as follows:

Furthermore, the microstructure of the lubricating greases was examined using a scanning electron microscope (JSM-7610F). SEM pretreatment methods are categorized into the soaking method (compound P34HB-EC-based grease system) and the powder method (ethyl cellulose). For the soaking method, an appropriate amount of grease is applied to the copper grid, followed by a 48 h immersion in n-heptane. The solvent is gradually separated, and the sample is dried at room temperature before being coated with gold for SEM characterization. As for the powder method, a small portion of vacuum-dried raw material is mixed with 7 ml of ethanol and subjected to ultrasound treatment for 5 min. This process is repeated three times after centrifugation. Finally, ethanol is added and shaken, and a drop of the mixture is placed on a copper grid for gold coating drying at room temperature before characterization testing. Fourier transform infrared spectroscopy (FTIR, Tensor 27, Bruker, Germany) was used to perform qualitative and quantitative tests on the samples. The precise and quantitative Fourier infrared test device developed by Zhongke Runmei Co., Ltd. is adopted. The window was a potassium bromide slice with a resolution of 4.0 cm^-1, the scanning times were 32, the fixed optical path was 100 μm, and the infrared spectrum acquisition range was 4,000-400 cm^-1 with air as the background.

2. The paper lacks innovation. The use of commercially available materials and conventional experimental methods does not contribute substantially to the advancement of knowledge in the field of lubricating greases. Without introducing novel concepts, approaches, or insights, the manuscript fails to justify its contribution to the existing literature.

Response: We quite appreciate your favorite consideration and insightful comment. Currently, the P34HB material finds extensive applications in the fields of food, plastics, medicine, and others. Our innovation is the first to harness the potential of P34HB material combined with ethyl cellulose as a grease thickener. Furthermore, we have conducted preliminary investigations and improvements in the preparation process based on previous experiments. Our primary focus has been exploring the properties of other biomass-based thickeners to expand the range of environmentally friendly greases. This is reflected in the abstract and introduction of our manuscript.

3. The depth of research conducted in the article is insufficient. Despite claiming to anticipate an interplay among properties "Furthermore, analysis and discussion were carried out in anticipation of the interplay among their properties.", the analysis and discussion remain superficial, with only a cursory presentation of experimental results. Speculative assertions, such as the role of hydrogen bonds in the friction process, are not substantiated with empirical evidence or simulation data, detracting from the scientific rigor of the study.

Response: We extend our gratitude to the reviewer for their valuable suggestions. Currently, we have conducted preliminary investigations into the potential of utilizing the novel thickener P34HB and ethyl cellulose for grease production. The results demonstrate that the incorporation of ethyl cellulose effectively enhances the mechanical stability and colloidal stability of P34HB-EC grease. Additionally, infrared analysis revealed alterations in hydrogen bond states. However, quantifying the strength of hydrogen bonding under current conditions and methodologies remains challenging. Therefore, we anticipate that our future investigations will provide further insights into the hydrogen bonding mechanism and frictional behavior exhibited by these greases. For instance, a comprehensive investigation and analysis of molecular dynamics models and stoichiometric models can effectively simulate the variations in hydrogen bonding among greases with varying concentrations of thickening agents.

4. Additionally, the language expression throughout the manuscript is subpar, riddled with punctuation errors, wording issues, and unclear phrasing, such as: “mechanical stability, rheological behaviour, and tribological performance” “thereby demonstrating a favourable binding effect.” “while its colour transitions from”. These linguistic deficiencies not only impede comprehension but also reflect poorly on the professionalism and attention to detail of the author.

Response: We strongly agree with the reviewer's query. We have made great efforts to revise the normalization and professionalism of some language descriptions in our manuscript, and the changes are highlighted in bright yellow in the manuscript.

The impact of thickener content on the colloidal stability, mechanical stability, rheological behavior, and tribological characteristics of the lubricating greases was assessed through comparative analyses.

The appearance of the complex P34HB-EC-based grease undergoes a transition from dark yellow to a lighter shade as the ethyl cellulose content increases.

Revised for other analogous cases have been made in our manuscript.

5. Comments on the Quality of English Language

The presence of multiple revision marks indicates a lack of diligence and care in preparing the manuscript for submission, raising concerns about the author's commitment to the study. Such oversights compromise the overall quality and credibility of the research presented.

Response: We appreciate the valuable suggestions from the reviewer. We have tried our best to polish the language in the revised manuscript. Previously, we submitted the manuscript with annotations to enhance its alignment with the sustainability theme. We will take note of the details in this regard in the subsequent revision.

We have tried our best to improve the manuscript and made some yellow-marked changes in the revised paper. We appreciate the editor and reviewers’ warm work earnestly and hope that the correction will be approved. The revised parts have been marked in yellow in the manuscript. Once again, thank you very much for your comments and suggestions.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript could be accepted after minor revision

- Remove (800 mesh) present in the lines 112 and 124.

- Fixed the used units for example: rad·s⁻¹ or rad/s

- Check The sentence in line 162.

- Combine Table 3 with 4, and illustrate the Dropping Point and Oil Separation for samples P20E15, P15E20, P10E25, and P5E30.

Comments on the Quality of English Language

English language required minor editing

Author Response

Response to Reviewer Four Comments

1.-Remove (800 mesh) present in the lines 112 and 124.

Response: We deeply appreciate the invaluable insights provided in your comments. According to the reviewer's suggestions, we have made the following modifications:

The mixture was prepared by heating the oil to approximately 160 °C, followed by the addition of P34HB powder, and further heating up to 175 °C.

2.-Fixed the used units for example: rad·s⁻¹ or rad/s

Response: Thank you for your meticulous examination. Following your suggestion, we have adjusted the unit to rad·s^-1. The following modifications have been made:

Oscillation mode tests were conducted at controlled temperatures of 25 °C and 80 °C with a fixed angular frequency (ω) of 10 rad·s^-1 through amplitude sweep tests.

3.-Check The sentence in line 162.

Response: Thanks for your suggestion. We modified the 162-line sentence as follows:

According to previous experiments, greases containing 35 wt% P34HB reached a stable state of cured base oil. Consequently, for this experiment, the total concentration of the thickening agent was chosen as 35 wt%. Previous studies have shown that a combination of ethyl cellulose and methyl cellulose can enhance mechanical stability. However, initial experiments with methyl cellulose did not significantly improve mechanical or colloidal stability. Subsequent attempts with other thickeners also demonstrated minimal improvement in the consistency or colloidal stability of the grease (as illustrated in Table 2). For example, the incorporation of 5 wt% bentonite into the grease formulation resulted in non-saponification and the formation of a compact yellow solid mass. Therefore, keeping the total thickening agent content fixed at 35 wt%, an exploration was conducted by adding different amounts of ethyl cellulose, specifically 0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt%. The prepared greases are illustrated in Figure 1. The appearance of the complex P34HB-EC-based grease undergoes a transition from dark yellow to a lighter shade as the ethyl cellulose content increases.

4-Combine Table 3 with 4, and illustrate the Dropping Point and Oil Separation for samples P20E15, P15E20, P10E25, and P5E30.

Response: We appreciate your suggestion on our manuscript. We have included a supplementary note on the dropping point and oil separation for samples P20E15, P15E20, P10E25, and P5E30 in our manuscript. The specific contents are outlined as follows：

Notably, the complex P34HB-EC-based grease (with 5 wt% ethyl cellulose and 30 wt% P34HB) provided better mechanical stability and colloidal stability. Through the analysis of Table 3 and Table 4, it can be inferred that the dropping point of P20E15, P15E20, P10E25, and P5E30 exhibits a gradual decrease, while the oil separation rate of the steel mesh demonstrates a progressive increase. This phenomenon may be due to the increased sensitivity of the ability of the oil to bind the base oil to temperature changes after the oil reaches a certain threshold of ethyl cellulose concentration.

Moreover, with increasing temperature, significant variations in flow points are observed between two temperatures for P35, P32E3, P30E5, P28E7, and P25E10, which correspond to 206.8 Pa, 210.1 Pa, 330.9 Pa, 356.0 Pa, and 2,427.2 Pa respectively. The findings demonstrate that the intermolecular distance of the thickener in the microstructure can be significantly increased by temperature and shearing, leading to a weakening of intermolecular forces and resulting in a pronounced alteration in structural strength. A gradual increase in the temperature sensitivity of the structure strength of complex grease with an elevated ethyl cellulose content. This phenomenon provides empirical evidence for predicting both the drop point of grease and the oil separation trend on steel mesh when the ethyl cellulose content exceeds 10 wt%.

5-Comments on the Quality of English Language

English language required minor editing

Response: We thank the reviewer for the good comment. we have double-checked the English to polish the language throughout the manuscript.

We have tried our best to improve the manuscript and made some yellow-marked changes in the revised paper. We appreciate the editor and reviewers’ warm work earnestly and hope that the correction will be approved. The revised parts have been marked in yellow in the manuscript. Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors of the publication followed all my comments and answered my questions satisfactorily.

Author Response

We greatly appreciate your valuable comments, which have significantly enhanced the quality of our manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

I express gratitude to the authors for their thorough examination of the critiques and meticulous revision of the manuscript. The undertaken efforts have notably enhanced the article's quality, and no substantial issues were identified. This study exhibits a commendable scientific standard, with the findings lucidly articulated. Consequently, I endorse this article for publication.

Author Response

We greatly appreciate your valuable comments, which have significantly enhanced the quality of our manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

Although the author has responded to the review comments carefully, it is regrettable that the key issues I am concerned about have not been resolved.

The experiments in the article mostly use unique raw materials and self built methods, which makes it difficult to replicate this research. In addition, there are significant differences from the research on conventional lubricants, making it difficult to see the universality and reference value of the research content.

More importantly, the authors also need to carefully consider the rigor of the mechanism research, rather than relying mainly on speculation to infer conclusions.

Comments on the Quality of English Language

The author has made some revisions to the language, but it is still not standardized and fluent enough, requiring overall polishing.

Author Response

Dear Editor and Reviewers:

       We deeply appreciate the time and effort you have spent in reviewing our manuscript (Manuscript Number: sustainability-2941202, title: Development of fully renewable lubricant: The effect of ethyl cellulose on the properties of polyhydroxyalkanoates (P34HB) based grease). Those comments are valuable and helpful for revising and improving our paper and the important guiding significance to our research. We revised the manuscript based on the reviewers' comments point to point and carefully proofread it to minimize typographical, grammatical, and bibliographical errors. Here below is our description of the revision according to the referees' comments, and the revised portion is also marked in yellow in the manuscript.

Thanks again!

Response to Reviewer Comments

1、Comments and Suggestions for Authors

Although the author has responded to the review comments carefully, it is regrettable that the key issues I am concerned about have not been resolved.

（1）The experiments in the article mostly use unique raw materials and self built methods, which makes it difficult to replicate this research.

Response: We sincerely appreciate your comments. We found that P34HB material has the potential of thickening through preliminary research experiments, and we also conducted some related experiments on P34HB thickening base oil. However, the relevant experiments found that the grease prepared by thickening base oil with P34HB material has some problems, such as poor mechanical stability. Therefore, we selected P34HB material and traditional bio-based thickener (ethyl cellulose) to prepare grease. The P34HB and ethyl cellulose utilized in this study are not unique raw materials; they are both commercially available and easily accessible. In addition, the detailed parameters of P34HB material are shown in Table 1. Ethyl cellulose is a general reagent for analytical purity. All these indicate that the material we used is not unique. The method of experimental preparation of grease is based on Sanchez et al. [19] and Martin-Alfonso et al. 's method, which is a common process for preparing bio-based greases, indicating that the experiments in our paper are not based on self-built methods. In summary, our research can be easily replicated.

References: 19. Sánchez, R.; Franco, J.M.; Delgado, M.A.; Valencia, C.; Gallegos, C. Thermal and mechanical characterization of cellulosic derivatives-based oleogels potentially applicable as bio-lubricating greases: Influence of ethyl cellulose molecular weight. Carbohyd. Polym. 2011, 83, 151-158.

20. Martín-Alfonso, J.E.; Valencia, C.; Franco, J.M. Composition-property relationship of gel-like dispersions based on organo-bentonite, recycled polypropylene and mineral oil for lubricant purposes. Appl. Clay. Sci. 2014, 87, 265-271.

（2）In addition, there are significant differences from the research on conventional lubricants, making it difficult to see the universality and reference value of the research content.

Response: We greatly appreciate your invaluable advice. Currently, there is a growing societal concern regarding the implementation of environmental conservation principles and strategies for sustainable development. In response, our research focuses on utilizing biological materials to expand the application scope of bio-based thickeners, thereby contributing valuable insights in this field. Most of the studies on bio-based thickener greases focus on cellulose derivative oil gel dispersions [1], which have the potential to be utilized as bio-based greases. However, there are still some studies that employ plastic raw materials (such as polypropylene [2], polyester resin [3], triethyl citrate [4], etc.) for grease preparation. Therefore, based on our preliminary investigation, we have identified P34HB material with characteristics such as thickening ability [5], mechanical properties similar to polypropylene, low cost, renewability, biodegradability, and structural diversity. Consequently, it can be widely applied in relevant fields as an alternative to petroleum-based materials [6]. Overall, we believe that P34HB material exhibits promising potential for base oil thickening and can serve as a suitable thickening agent for grease formulation. Nevertheless, previous exploration experiments revealed issues concerning the poor mechanical stability of the prepared greases. Subsequently, following a comprehensive assessment of the available bio-based thickeners documented in the literature, we opted to enhance the grease's performance by synergistically incorporating ethyl cellulose with P34HB. We conducted tests on the physical and chemical properties, rheological properties, and friction properties of the prepared lubricating grease, which showed no significant differences from the research on conventional lubricants. Through the analysis of the rheological properties of the lubricating grease, it is evident that the prepared lubricating grease exhibits rheological characteristics akin to those observed in oil gel dispersions, thereby demonstrating its potential as a bio-based lubricant. Based on the microstructure diagram analysis of the grease, it is evident that the composite P34HB-EC-based grease exhibits an irregularly stacked sheet structure akin to that of bentonite grease and partially polyurea grease [7]. Furthermore, upon analyzing the appearance of the grease, it can be observed that the compound P34HB-EC-based grease possesses a distinctive semi-flow state characteristic of greases. The above results indicate that the thickening characteristic of the prepared lubricating grease thickener is similar to that of traditional lithium soap. The research focuses on exploring the potential of P34HB material as a biodegradable lubricant thickener, providing a theoretical basis and data accumulation for preparing essential lubricants as a substitute for non-degradable materials in the future. Therefore, this study has a certain degree of universality, guiding significance, and reference value for developing new biodegradable lubricating greases.

References: 1. Gallego, R.; Arteaga, J.F.; Vaencia, C.; Diaz, M.J.; Franco, J.M. Gel-Like dispersions of HMDI-cross-linked lignocellulosic materials in castor oil: Toward completely renewable lubricating grease formulations. ACS. Sustain. Chem. Eng. 2015, 3, 2130-2141.

2. Muller, D.; Matta, C.; Thijssen, R.; bin Yusof, M.N.; van Eijk, M.C.P.; Chatra, S. Novel polymer grease microstructure and its proposed lubrication mechanism in rolling/sliding contacts. Int.2017, 110, 278-290.
3. Vafaei, S.; Jopen, M.; Jacobs, G.; König, F.; Weberskirch, R. Synthesis and tribological behavior of bio-based lubrication greases with bio-based polyester thickener systems. Clean. Prod.2022, 364, 132659.
4. Ilyin, S.O.; Gorbacheva, S.N.; Yadykova, A.Y. Rheology and tribology of nanocellulose-based biodegradable greases: Wear and friction protection mechanisms of cellulose microfibrils. Int.2023, 178, 108080.
5. Yin, F.; Zhang, X.; Li, D.; Ma, X. Preparation and characterization of poly (3-hydroxybutyrate-co-4-hydroxybutyrate)/porous cellulose aerogel b Bioresources2019, 14, 4748-4759.
6. Dong, C.-L.; Li, S.-Y.; Wang, Y.; Dong, Y.; Tang, J.Z.; Chen, J.-C.; Chen, G.-Q. The cytocompatability of polyhydroxyalkanoates coated with a fusion protein of PHA repressor protein (PhaR) and Lys-Gln-Ala-Gly-Asp-Val (KQAGDV) polypeptide. Biomaterials2012, 33, 2593-2599.
7. Wu, C.; Xiong, R.; Ni, J.; Yao, L.; Chen, L.; Li, X. Effects of CuO nanoparticles on friction and vibration behaviors of grease on rolling bearing. Int.2020, 152, 106552.

（3）More importantly, the authors also need to carefully consider the rigor of the mechanism research, rather than relying mainly on speculation to infer conclusions.

Response: We immensely appreciate your thoughtful consideration and insightful comment. Because the research on P34HB material as a novel bio-based lubricant thickener and its compatibility with other materials such as ethyl cellulose is still preliminary, its mechanism still needs further exploration and research. In future research, we will further investigate the lubrication mechanism of the system through tests such as the optical interference oil film measuring instrument and pay more attention to the rigor of the mechanism research. Our manuscript has made rigorous language revisions, and the specific modifications have been highlighted in yellow in the article.

2、Comments on the Quality of English Language

The author has made some revisions to the language, but it is still not standardized and fluent enough, requiring overall polishing.

Response: We greatly appreciate your valuable comments. The revised manuscript has undergone meticulous proofreading to enhance its linguistic and grammatical aspects. We have tried our best to polish the language in the revised manuscript. The specific content of the revision has been indicated in yellow and bright colors in the article, and the language of the revision does not affect the content and conclusion of the manuscript.

 

        We appreciate the editor and reviewers' warm work earnestly and hope the corrections will be approved. The revised parts have been marked in yellow in the manuscript. Once again, thank you very much for your comments and suggestions.

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Although the author's response to the review comments may not fully satisfy me, the overall quality of the article has indeed greatly improved.

It is recommended to carefully check the language of the entire article.

Comments on the Quality of English Language

There is still repetition in minority languages, and the author needs to verify the entire text.

Author Response

Dear Editor and Reviewers:

       We deeply appreciate the time and effort you have spent in reviewing our manuscript (Manuscript Number: sustainability-2941202, title: Development of fully renewable lubricant: The effect of ethyl cellulose on the properties of polyhydroxyalkanoates (P34HB) based grease). Those comments are valuable and helpful for revising and improving our paper and the important guiding significance to our research. We carefully checked and verified the manuscript based on the reviewer's comments. Here below is our description of the revision according to the referees' comments, and the revised portion is also marked in yellow in the manuscript.

Thanks again!

Response to Reviewer Comments

1、Although the author's response to the review comments may not fully satisfy me, the overall quality of the article has indeed greatly improved.

It is recommended to carefully check the language of the entire article.

Response: We immensely appreciate your thoughtful consideration and insightful comment. We carefully checked the language throughout the entire article to ensure it was clear and accurate.

2、Comments on the Quality of English Language

There is still repetition in minority languages, and the author needs to verify the entire text.

Response: We greatly appreciate your valuable comments. We have carefully verified the entire article and made changes to similar repetitive languages. The specific content of the revision has been indicated in yellow and bright colors in the article, and the language of the revision does not affect the content and conclusion of the manuscript.

      We appreciate the editor and reviewers' warm work earnestly and hope the corrections will be approved. The revised parts have been marked in yellow in the manuscript. Once again, thank you very much for your comments and suggestions.