Thermo-Oxidative Stability and Tribological Properties of Biolubricants Obtained from Castor Oil Fatty Acids and Isoamyl Alcohol
and Francisco Murilo T. Luna 1,*
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe article ‘Thermo-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol’ submitted by Luna and co-workers present an investigation of the thermo-oxidative stability and tribological behavior of bio-based lubricant samples synthesized from castor oil using isoamyl alcohol. My suggestion for this article is minor revision. Some other comments are as follows:
1. It is recommended to include the acid value of COFA to facilitate a comparison of the physicochemical properties before and after modification.
2. Has all the isoamyl alcohol been effectively eliminated from the final product? It is advisable for the author to furnish pertinent data, such as results from gas chromatography, to support this assertion.
3. The authors have already presented 1H NMR data. It is recommended to augment the analysis with 13C NMR to enhance the differentiation between COFA and the esters.
4. The worn surface morphologies revealed that the BL1-lubricated surface appeared smoother than the other samples. If feasible, it is advisable to provide an explanation for this observation.
Author Response
Manuscript ID: lubricants-2635459
RESPONSE TO REVIEWERS:
We would like to thank the reviewers for their thoughtful review of the manuscript. They raise important issues and their inputs are very helpful for improving the manuscript. We respond below in detail to each of the reviewer’s comments. Our point-by-point detailed responses are in blue and italics. Changes in the Ms text are shown in blue.
Reviewer #1
The article ‘Thermo-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol’ submitted by Luna and co-workers present an investigation of the thermo-oxidative stability and tribological behavior of bio-based lubricant samples synthesized from castor oil using isoamyl alcohol. My suggestion for this article is minor revision. Some other comments are as follows:
1. It is recommended to include the acid value of COFA to facilitate a comparison of the physicochemical properties before and after modification.
R.: We appreciate the comment. The acid value of COFA was added to Table 1.
2.Has all the isoamyl alcohol been effectively eliminated from the final product? It is advisable for the author to furnish pertinent data, such as results from gas chromatography, to support this assertion.
R.: We appreciate the reviewer’s comment. The isoamyl alcohol used in the esterification and oxirane ring opening reactions was removed by distillation in a Kugelrohr system under vacuum (3.10−2 mbar) at 90 °C. The GC technique was not used for monitoring the reaction products because the temperature for the injection of the samples (250-350ºC) could decompose the products and the chromatogram would not reflect the correct composition of the samples. FTIR and 1H NMR techniques have been commonly used for monitoring these types of products. In addition, an excess of non-reacted alcohol could be detected by our chemical characterization performed and by physicochemical properties, including the tribological behavior that would be drastically harmed.
3. The authors have already presented 1H NMR data. It is recommended to augment the analysis with 13C NMR to enhance the differentiation between COFA and the esters.
R.: We appreciate the suggestion of the reviewer, however, in this study we mainly focused on the 1H NMR technique. Using the 1H NMR spectra of the samples (Fig. 4) it was possible to confirm the esterification of COFA (appearance of peak V), the epoxidation of the ester (absence of peak VI, and appearance of peak VII), and the oxirane ring opening reactions (complete disappearance of peak VII). Thus, the 1H NMR data allowed adequate differentiation between our synthesized samples.
4. The worn surface morphologies revealed that the BL1-lubricated surface appeared smoother than the other samples. If feasible, it is advisable to provide an explanation for this observation.
R.: We appreciate the reviewer’s comment. We believe that the lower viscosity observed in BL1 and the presence of unsaturations in its molecule favor the sliding between the contacting surfaces of the balls, which may contribute to obtaining smoother surfaces. Probably the film that is formed with BL1 becomes thinner, indicating that the presence of a double bond favors the packing of the molecules on the surface of the balls [1-4].
Reviewer #2
This research contributes valuable insights into the development of bio-based lubricants with enhanced properties, some comments are given for improvements as:
1.Title: add the word : Castor oil since the findings are very specific
R.: We appreciate the suggestion and we have included the term "Castor oil" in the title to provide a more precise representation of the studied samples.
2. Introduction: try to cite only relevant works specially the ones work on : Castor oil, also, extends the section on : Castor oil to address the current trend and limitations showing the motivation of your work.
R.:We believe that the introduction section brings relevant studies about bio-based lubricants, castor oil and chemical modifications to improve the physicochemical and tribological properties of the products. A sentence was added in introduction section to better clarify the motivation of our study.
3. 2.3 138 1H NMR and FTIR measurements. Why you cite referencing no 33 and 34 ? there is no explanation ? !!
R.:References 33 and 34 were cited to mention previous reports that used the same experimental conditions for the FTIR measurement.
4. Referencing needed for ASTM D97
R.: We appreciate this observation. The reference was added to our Ms text.
5.This approach is similar to others reported elsewhere [23, 38]… what are their findings compared to the current !!!
R.: We used the same experimental apparatus to study the thermo-oxidative stability reported in references [23 and 38]. However, the use of isoamyl alcohol on COFA and the effect on stability has not been reported yet. Also, in references 23 and 39 (updated version) this approach was used for soybean fatty acids, with different chemical composition compared to the fatty acids of castor oil.
6. Again evaluated the rate of weight loss during heating versus temperature [39]. .. why you cite this ? !!!
R.: We agree with the reviewer’s comment. The reference was removed from our Ms text.
7. 3 rheometer (TA Instruments, USA) [40] .. add the ASTm standard as well
[11,72,73] , 11,74,75].. you are adding referencing without any explanation or arguments with them !!!
R.: We appreciate the reviewer’s comment. The ASTM standard used as reference for four-ball experiment was cited, but the experimental conditions were modified due to the rheometer limits. All used conditions were reported in our Ms text. We have revised the section 3.5 “Tribological results” to discuss the cited references.
8. Reduce unnecessary citations
R.: We believe that unnecessary citations were removed.
9. Conclusion is very general be specific and extracted from the findings
R.: The conclusion section was rewritten to be more specific about our findings.
Reviewer #3
In the paper “thermal-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol” the lubricating effect of new biolubricants obtained from castor oil has been studied. Authors have prepared the new lubricants by esterification and an oxirane ring-opening reaction using isoamyl alcohol. The characterization of the lubricants has been done by NMR, FTIR and different thermal analysis techniques. The tribological performance of the new samples obtained has been compared with respect castor oil fatty acids (COFA) and a commercial mineral oil (MOS). With respect to the results obtained, although friction values for BL2 lubricant are better than those obtained for the other lubricants, that is not achieved in the case of wear. However, the manuscript contains several aspects that must be improved to consider this work for publication.
1. It is necessary to explain if there are some differences in the composition of the Castor Oil Fatty Acids (COFA) used in this study and a commercial Castor Oil.
R.: We appreciate the reviewer’s comment. The sample of Castor Oil Fatty Acids (COFA) used in this study is a commercial product provided by Miracema-Nuodex (Brazil). The commercial or natural Castor Oil is a triglyceride mixture constituted mainly by ricinoleic acid in the tri-ester structure. The COFA samples were produced in industry through the hydrolysis of castor oil, with acid value of 116 mg KOH/g. This information was added to section 2.1 “Materials”.
2. Please, review spelling of Anton Paar in line 149.
R.: We thank the reviewer for pointing this out. The spelling has been corrected.
3. Please, include values of Flash Point for BL1 and BL2 lubricants.
R.: The focus of our study was on the evaluation of oxidative stability and tribological behavior of the synthesized samples. The flash points of BL1 and BL2 tend to be even higher than the value obtained for the COFA sample (216 ºC), because they are constituted by long-chain esters from COFA. In addition, the flash point value of COFA was higher than found in mineral oil sample (160 ºC). So, we did not measure this property for the samples obtained in this study.
4. It would be necessary to justify the conditions chosen for the tribological tests.
R.: We appreciate this comment. We have added in Section 2.7 that test conditions were based on ASTM D4172, but within the limits of the DHR-3 rheometer. We have inserted a sentence to clarify this fact in section 2.7 “Tribological evaluation”.
5. How do authors explain the higher stability of COFA with temperature in air atmospheric (Table 4 and Fig 6)
R.: The TGA technique in oxidant atmosphere provides an important result for characterization of the final bio-based samples to estimate the temperature range for an application. We observed that for more unsaturated carbon chain samples (COFA and BL1) the mass loss until 230ºC was lower than for the BL2 sample (more saturated sample). This occurs due the oxidation reactions, which lead to mass gains as oxygen is incorporated. This same behavior was already observed in the study carried out by Garcia et al. [5] for other unsaturated vegetable oils. However, the TGA curve for the COFA sample showed a more accentuated decay above 250ºC, indicating that the COFA sample was not so stable.
6. It would be necessary include the value of roughness of the balls after the tests, to confirm affirmation of line 384.
R.: We appreciate the reviewer’s comment. It is important to note that the method using an optical microscope has been used to evaluate the morphology and determine the wear scar diameter (WSD). This approach is adopted by other authors [6-9] to evaluate surface wear and corroborates our findings.
7. Please for better understanding, include an explanation of (a), (b) and (c).
R.: We believe that the reviewer suggestion is for the captions of Fig. 7. The captions were revised to include the description of each figure.
8. In Figure 7, please review friction curve of BL1 at the end of the test, what is the reduction of the friction coefficient due to?
R.: We appreciate this comment. We have revised Figure 7 to correct this.
References cited in this response letter:
[1] Lundgren, S. M.; Ruths, M.; Danerlöv, K.; Persson, K.; Effects of unsaturation on film structure and friction of fatty acids in a model base oil. J. Colloid Interface Sci. 2008, 326(2), 530–536.
[2] Lundgren, S.; Persson, K.; Mueller, G.; Kronberg, B.; Clarke, J.; Chtaib, M.; Claesson, P. M.; Unsaturated Fatty Acids in Alkane Solution: Adsorption to Steel Surfaces. Langmuir 2007, 23(21), 10598–10602.
[3] Ruths, M.; Lundgren, S.; Danerlöv, K.; Persson, K.; Friction of Fatty Acids in Nanometer-Sized Contacts of Different Adhesive Strength. Langmuir 2008, 24(4), 1509–1516.
[4] Crespo, A., Morgado, N., Mazuyer, D., & Cayer-Barrioz, J. (2018). Effect of unsaturation on the adsorption and the mechanical behavior of fatty acid layers. Langmuir, 34(15), 4560-4567.
[5] C. C. Garcia; P. I. B. M. Franco; T. O. Zuppa; N. R. Antoniosi Filho; M. I. G. Leles THERMAL STABILITY STUDIES OF SOME CERRADO PLANT OILS. J Therm Anal Calorim 2007, 87, 645–648.
[6] Gupta, R.N.; Harsha, A.P. Friction and Wear of Nanoadditive-Based Biolubricants in Steel–Steel Sliding Contacts: A Comparative Study. J Mater Eng Perform 2018, 27, 648–658, doi:10.1007/s11665-018-3175-3.
[7] Reeves, C.J.; Menezes, P.L.; Jen, T.-C.; Lovell, M.R. The Influence of Fatty Acids on Tribological and Thermal Properties of Natural Oils as Sustainable Biolubricants. Tribol Int 2015, 90, 123–134, doi:10.1016/j.triboint.2015.04.021.
[8] Ossia, C.V.; Han, H.G.; Kong, H. Response Surface Methodology for Eicosanoic Acid Triboproperties in Castor Oil. Tribol Int 2009, 42, 50–58, doi:10.1016/j.triboint.2008.05.018.
[9] Liu, P.; Fang, J.; Wang, X.; Wu, J.; Chen, B. A Novel Boron-Nitrogen Modified Castor Oil as an Ecofriendly and Efficient Lubricant Additive. J Dispers Sci Technol 2021, 42, 180–189, doi:10.1080/01932691.2019.1667818.
Reviewer 2 Report
Comments and Suggestions for Authorsthis research contributes valuable insights into the development of bio-based lubricants with enhanced properties, some comments are given for improvements as:
Title: add the word : Castor oil since the findings are very specific
Introduction: try to cite only relevant works specially the ones work on : Castor oil, also, extends the section on : Castor oil to address the current trend and limitations showing the motivation of your work
2.3 138 1H NMR and FTIR measurements
Why you cite referencing no 33 and 34 ? there is no explanation ? !!
Referencing needed for ASTM D97
This approach is similar to others reported elsewhere [23, 38]… what are their findings compared to the current !!!
Again
evaluated the rate of weight loss during heating versus temperature [39]. .. why you cite this ? !!!
3 rheometer (TA Instruments, USA) [40] .. add the ASTm standard as well
[11,72,73] , 11,74,75].. you are adding referencing without any explanation or arguments with them !!!
Reduce unnecessary citations
Conclusion is very general be specific and extracted from the findings
Author Response
Manuscript ID: lubricants-2635459
RESPONSE TO REVIEWERS:
We would like to thank the reviewers for their thoughtful review of the manuscript. They raise important issues and their inputs are very helpful for improving the manuscript. We respond below in detail to each of the reviewer’s comments. Our point-by-point detailed responses are in blue and italics. Changes in the Ms text are shown in blue.
Reviewer #1
The article ‘Thermo-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol’ submitted by Luna and co-workers present an investigation of the thermo-oxidative stability and tribological behavior of bio-based lubricant samples synthesized from castor oil using isoamyl alcohol. My suggestion for this article is minor revision. Some other comments are as follows:
1. It is recommended to include the acid value of COFA to facilitate a comparison of the physicochemical properties before and after modification.
R.: We appreciate the comment. The acid value of COFA was added to Table 1.
2.Has all the isoamyl alcohol been effectively eliminated from the final product? It is advisable for the author to furnish pertinent data, such as results from gas chromatography, to support this assertion.
R.: We appreciate the reviewer’s comment. The isoamyl alcohol used in the esterification and oxirane ring opening reactions was removed by distillation in a Kugelrohr system under vacuum (3.10−2 mbar) at 90 °C. The GC technique was not used for monitoring the reaction products because the temperature for the injection of the samples (250-350ºC) could decompose the products and the chromatogram would not reflect the correct composition of the samples. FTIR and 1H NMR techniques have been commonly used for monitoring these types of products. In addition, an excess of non-reacted alcohol could be detected by our chemical characterization performed and by physicochemical properties, including the tribological behavior that would be drastically harmed.
3. The authors have already presented 1H NMR data. It is recommended to augment the analysis with 13C NMR to enhance the differentiation between COFA and the esters.
R.: We appreciate the suggestion of the reviewer, however, in this study we mainly focused on the 1H NMR technique. Using the 1H NMR spectra of the samples (Fig. 4) it was possible to confirm the esterification of COFA (appearance of peak V), the epoxidation of the ester (absence of peak VI, and appearance of peak VII), and the oxirane ring opening reactions (complete disappearance of peak VII). Thus, the 1H NMR data allowed adequate differentiation between our synthesized samples.
4. The worn surface morphologies revealed that the BL1-lubricated surface appeared smoother than the other samples. If feasible, it is advisable to provide an explanation for this observation.
R.: We appreciate the reviewer’s comment. We believe that the lower viscosity observed in BL1 and the presence of unsaturations in its molecule favor the sliding between the contacting surfaces of the balls, which may contribute to obtaining smoother surfaces. Probably the film that is formed with BL1 becomes thinner, indicating that the presence of a double bond favors the packing of the molecules on the surface of the balls [1-4].
Reviewer #2
This research contributes valuable insights into the development of bio-based lubricants with enhanced properties, some comments are given for improvements as:
1.Title: add the word : Castor oil since the findings are very specific
R.: We appreciate the suggestion and we have included the term "Castor oil" in the title to provide a more precise representation of the studied samples.
2. Introduction: try to cite only relevant works specially the ones work on : Castor oil, also, extends the section on : Castor oil to address the current trend and limitations showing the motivation of your work.
R.:We believe that the introduction section brings relevant studies about bio-based lubricants, castor oil and chemical modifications to improve the physicochemical and tribological properties of the products. A sentence was added in introduction section to better clarify the motivation of our study.
3. 2.3 138 1H NMR and FTIR measurements. Why you cite referencing no 33 and 34 ? there is no explanation ? !!
R.:References 33 and 34 were cited to mention previous reports that used the same experimental conditions for the FTIR measurement.
4. Referencing needed for ASTM D97
R.: We appreciate this observation. The reference was added to our Ms text.
5.This approach is similar to others reported elsewhere [23, 38]… what are their findings compared to the current !!!
R.: We used the same experimental apparatus to study the thermo-oxidative stability reported in references [23 and 38]. However, the use of isoamyl alcohol on COFA and the effect on stability has not been reported yet. Also, in references 23 and 39 (updated version) this approach was used for soybean fatty acids, with different chemical composition compared to the fatty acids of castor oil.
6. Again evaluated the rate of weight loss during heating versus temperature [39]. .. why you cite this ? !!!
R.: We agree with the reviewer’s comment. The reference was removed from our Ms text.
7. 3 rheometer (TA Instruments, USA) [40] .. add the ASTm standard as well
[11,72,73] , 11,74,75].. you are adding referencing without any explanation or arguments with them !!!
R.: We appreciate the reviewer’s comment. The ASTM standard used as reference for four-ball experiment was cited, but the experimental conditions were modified due to the rheometer limits. All used conditions were reported in our Ms text. We have revised the section 3.5 “Tribological results” to discuss the cited references.
8. Reduce unnecessary citations
R.: We believe that unnecessary citations were removed.
9. Conclusion is very general be specific and extracted from the findings
R.: The conclusion section was rewritten to be more specific about our findings.
Reviewer #3
In the paper “thermal-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol” the lubricating effect of new biolubricants obtained from castor oil has been studied. Authors have prepared the new lubricants by esterification and an oxirane ring-opening reaction using isoamyl alcohol. The characterization of the lubricants has been done by NMR, FTIR and different thermal analysis techniques. The tribological performance of the new samples obtained has been compared with respect castor oil fatty acids (COFA) and a commercial mineral oil (MOS). With respect to the results obtained, although friction values for BL2 lubricant are better than those obtained for the other lubricants, that is not achieved in the case of wear. However, the manuscript contains several aspects that must be improved to consider this work for publication.
1. It is necessary to explain if there are some differences in the composition of the Castor Oil Fatty Acids (COFA) used in this study and a commercial Castor Oil.
R.: We appreciate the reviewer’s comment. The sample of Castor Oil Fatty Acids (COFA) used in this study is a commercial product provided by Miracema-Nuodex (Brazil). The commercial or natural Castor Oil is a triglyceride mixture constituted mainly by ricinoleic acid in the tri-ester structure. The COFA samples were produced in industry through the hydrolysis of castor oil, with acid value of 116 mg KOH/g. This information was added to section 2.1 “Materials”.
2. Please, review spelling of Anton Paar in line 149.
R.: We thank the reviewer for pointing this out. The spelling has been corrected.
3. Please, include values of Flash Point for BL1 and BL2 lubricants.
R.: The focus of our study was on the evaluation of oxidative stability and tribological behavior of the synthesized samples. The flash points of BL1 and BL2 tend to be even higher than the value obtained for the COFA sample (216 ºC), because they are constituted by long-chain esters from COFA. In addition, the flash point value of COFA was higher than found in mineral oil sample (160 ºC). So, we did not measure this property for the samples obtained in this study.
4. It would be necessary to justify the conditions chosen for the tribological tests.
R.: We appreciate this comment. We have added in Section 2.7 that test conditions were based on ASTM D4172, but within the limits of the DHR-3 rheometer. We have inserted a sentence to clarify this fact in section 2.7 “Tribological evaluation”.
5. How do authors explain the higher stability of COFA with temperature in air atmospheric (Table 4 and Fig 6)
R.: The TGA technique in oxidant atmosphere provides an important result for characterization of the final bio-based samples to estimate the temperature range for an application. We observed that for more unsaturated carbon chain samples (COFA and BL1) the mass loss until 230ºC was lower than for the BL2 sample (more saturated sample). This occurs due the oxidation reactions, which lead to mass gains as oxygen is incorporated. This same behavior was already observed in the study carried out by Garcia et al. [5] for other unsaturated vegetable oils. However, the TGA curve for the COFA sample showed a more accentuated decay above 250ºC, indicating that the COFA sample was not so stable.
6. It would be necessary include the value of roughness of the balls after the tests, to confirm affirmation of line 384.
R.: We appreciate the reviewer’s comment. It is important to note that the method using an optical microscope has been used to evaluate the morphology and determine the wear scar diameter (WSD). This approach is adopted by other authors [6-9] to evaluate surface wear and corroborates our findings.
7. Please for better understanding, include an explanation of (a), (b) and (c).
R.: We believe that the reviewer suggestion is for the captions of Fig. 7. The captions were revised to include the description of each figure.
8. In Figure 7, please review friction curve of BL1 at the end of the test, what is the reduction of the friction coefficient due to?
R.: We appreciate this comment. We have revised Figure 7 to correct this.
References cited in this response letter:
[1] Lundgren, S. M.; Ruths, M.; Danerlöv, K.; Persson, K.; Effects of unsaturation on film structure and friction of fatty acids in a model base oil. J. Colloid Interface Sci. 2008, 326(2), 530–536.
[2] Lundgren, S.; Persson, K.; Mueller, G.; Kronberg, B.; Clarke, J.; Chtaib, M.; Claesson, P. M.; Unsaturated Fatty Acids in Alkane Solution: Adsorption to Steel Surfaces. Langmuir 2007, 23(21), 10598–10602.
[3] Ruths, M.; Lundgren, S.; Danerlöv, K.; Persson, K.; Friction of Fatty Acids in Nanometer-Sized Contacts of Different Adhesive Strength. Langmuir 2008, 24(4), 1509–1516.
[4] Crespo, A., Morgado, N., Mazuyer, D., & Cayer-Barrioz, J. (2018). Effect of unsaturation on the adsorption and the mechanical behavior of fatty acid layers. Langmuir, 34(15), 4560-4567.
[5] C. C. Garcia; P. I. B. M. Franco; T. O. Zuppa; N. R. Antoniosi Filho; M. I. G. Leles THERMAL STABILITY STUDIES OF SOME CERRADO PLANT OILS. J Therm Anal Calorim 2007, 87, 645–648.
[6] Gupta, R.N.; Harsha, A.P. Friction and Wear of Nanoadditive-Based Biolubricants in Steel–Steel Sliding Contacts: A Comparative Study. J Mater Eng Perform 2018, 27, 648–658, doi:10.1007/s11665-018-3175-3.
[7] Reeves, C.J.; Menezes, P.L.; Jen, T.-C.; Lovell, M.R. The Influence of Fatty Acids on Tribological and Thermal Properties of Natural Oils as Sustainable Biolubricants. Tribol Int 2015, 90, 123–134, doi:10.1016/j.triboint.2015.04.021.
[8] Ossia, C.V.; Han, H.G.; Kong, H. Response Surface Methodology for Eicosanoic Acid Triboproperties in Castor Oil. Tribol Int 2009, 42, 50–58, doi:10.1016/j.triboint.2008.05.018.
[9] Liu, P.; Fang, J.; Wang, X.; Wu, J.; Chen, B. A Novel Boron-Nitrogen Modified Castor Oil as an Ecofriendly and Efficient Lubricant Additive. J Dispers Sci Technol 2021, 42, 180–189, doi:10.1080/01932691.2019.1667818.
Reviewer 3 Report
Comments and Suggestions for AuthorsIn the paper “thermal-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol” the lubricating effect of new biolubricants obtained from castor oil has been studied.
Authors have prepared the new lubricants by esterification and an oxirane ring-opening reaction using isoamyl alcohol. The characterization of the lubricants has been done by NMR, FTIR and different thermal analysis techniques. The tribological performance of the new samples obtained has been compared with respect castor oil fatty acids (COFA) and a commercial mineral oil (MOS). With respect to the results obtained, although friction values for BL2 lubricant are better than those obtained for the other lubricants, that is not achieved in the case of wear.
However, the manuscript contains several aspects that must be improved to consider this work for publication.
-It is necessary to explain if there are some differences in the composition of the Castor Oil Fatty Acids (COFA) used in this study and a commercial Castor Oil.
-Please, review spelling of Anton Paar in line 149.
-Please, include values of Flash Point for BL1 and BL2 lubricants.
-It would be necessary to justify the conditions chosen for the tribological tests.
-How do authors explain the higher stability of COFA with temperature in air atmospheric (Table 4 and Fig 6)
-It would be necessary include the value of roughness of the balls after the tests, to confirm affirmation of line 384.
-Please for better understanding, include an explanation of (a), (b) and (c).
-In Figure 7, please review friction curve of BL1 at the end of the test, what is the reduction of the friction coefficient due to?
Author Response
Manuscript ID: lubricants-2635459
RESPONSE TO REVIEWERS:
We would like to thank the reviewers for their thoughtful review of the manuscript. They raise important issues and their inputs are very helpful for improving the manuscript. We respond below in detail to each of the reviewer’s comments. Our point-by-point detailed responses are in blue and italics. Changes in the Ms text are shown in blue.
Reviewer #1
The article ‘Thermo-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol’ submitted by Luna and co-workers present an investigation of the thermo-oxidative stability and tribological behavior of bio-based lubricant samples synthesized from castor oil using isoamyl alcohol. My suggestion for this article is minor revision. Some other comments are as follows:
1. It is recommended to include the acid value of COFA to facilitate a comparison of the physicochemical properties before and after modification.
R.: We appreciate the comment. The acid value of COFA was added to Table 1.
2.Has all the isoamyl alcohol been effectively eliminated from the final product? It is advisable for the author to furnish pertinent data, such as results from gas chromatography, to support this assertion.
R.: We appreciate the reviewer’s comment. The isoamyl alcohol used in the esterification and oxirane ring opening reactions was removed by distillation in a Kugelrohr system under vacuum (3.10−2 mbar) at 90 °C. The GC technique was not used for monitoring the reaction products because the temperature for the injection of the samples (250-350ºC) could decompose the products and the chromatogram would not reflect the correct composition of the samples. FTIR and 1H NMR techniques have been commonly used for monitoring these types of products. In addition, an excess of non-reacted alcohol could be detected by our chemical characterization performed and by physicochemical properties, including the tribological behavior that would be drastically harmed.
3. The authors have already presented 1H NMR data. It is recommended to augment the analysis with 13C NMR to enhance the differentiation between COFA and the esters.
R.: We appreciate the suggestion of the reviewer, however, in this study we mainly focused on the 1H NMR technique. Using the 1H NMR spectra of the samples (Fig. 4) it was possible to confirm the esterification of COFA (appearance of peak V), the epoxidation of the ester (absence of peak VI, and appearance of peak VII), and the oxirane ring opening reactions (complete disappearance of peak VII). Thus, the 1H NMR data allowed adequate differentiation between our synthesized samples.
4. The worn surface morphologies revealed that the BL1-lubricated surface appeared smoother than the other samples. If feasible, it is advisable to provide an explanation for this observation.
R.: We appreciate the reviewer’s comment. We believe that the lower viscosity observed in BL1 and the presence of unsaturations in its molecule favor the sliding between the contacting surfaces of the balls, which may contribute to obtaining smoother surfaces. Probably the film that is formed with BL1 becomes thinner, indicating that the presence of a double bond favors the packing of the molecules on the surface of the balls [1-4].
Reviewer #2
This research contributes valuable insights into the development of bio-based lubricants with enhanced properties, some comments are given for improvements as:
1.Title: add the word : Castor oil since the findings are very specific
R.: We appreciate the suggestion and we have included the term "Castor oil" in the title to provide a more precise representation of the studied samples.
2. Introduction: try to cite only relevant works specially the ones work on : Castor oil, also, extends the section on : Castor oil to address the current trend and limitations showing the motivation of your work.
R.:We believe that the introduction section brings relevant studies about bio-based lubricants, castor oil and chemical modifications to improve the physicochemical and tribological properties of the products. A sentence was added in introduction section to better clarify the motivation of our study.
3. 2.3 138 1H NMR and FTIR measurements. Why you cite referencing no 33 and 34 ? there is no explanation ? !!
R.:References 33 and 34 were cited to mention previous reports that used the same experimental conditions for the FTIR measurement.
4. Referencing needed for ASTM D97
R.: We appreciate this observation. The reference was added to our Ms text.
5.This approach is similar to others reported elsewhere [23, 38]… what are their findings compared to the current !!!
R.: We used the same experimental apparatus to study the thermo-oxidative stability reported in references [23 and 38]. However, the use of isoamyl alcohol on COFA and the effect on stability has not been reported yet. Also, in references 23 and 39 (updated version) this approach was used for soybean fatty acids, with different chemical composition compared to the fatty acids of castor oil.
6. Again evaluated the rate of weight loss during heating versus temperature [39]. .. why you cite this ? !!!
R.: We agree with the reviewer’s comment. The reference was removed from our Ms text.
7. 3 rheometer (TA Instruments, USA) [40] .. add the ASTm standard as well
[11,72,73] , 11,74,75].. you are adding referencing without any explanation or arguments with them !!!
R.: We appreciate the reviewer’s comment. The ASTM standard used as reference for four-ball experiment was cited, but the experimental conditions were modified due to the rheometer limits. All used conditions were reported in our Ms text. We have revised the section 3.5 “Tribological results” to discuss the cited references.
8. Reduce unnecessary citations
R.: We believe that unnecessary citations were removed.
9. Conclusion is very general be specific and extracted from the findings
R.: The conclusion section was rewritten to be more specific about our findings.
Reviewer #3
In the paper “thermal-oxidative stability and tribological characteristics of bio-based lubricants synthesized using isoamyl alcohol” the lubricating effect of new biolubricants obtained from castor oil has been studied. Authors have prepared the new lubricants by esterification and an oxirane ring-opening reaction using isoamyl alcohol. The characterization of the lubricants has been done by NMR, FTIR and different thermal analysis techniques. The tribological performance of the new samples obtained has been compared with respect castor oil fatty acids (COFA) and a commercial mineral oil (MOS). With respect to the results obtained, although friction values for BL2 lubricant are better than those obtained for the other lubricants, that is not achieved in the case of wear. However, the manuscript contains several aspects that must be improved to consider this work for publication.
1. It is necessary to explain if there are some differences in the composition of the Castor Oil Fatty Acids (COFA) used in this study and a commercial Castor Oil.
R.: We appreciate the reviewer’s comment. The sample of Castor Oil Fatty Acids (COFA) used in this study is a commercial product provided by Miracema-Nuodex (Brazil). The commercial or natural Castor Oil is a triglyceride mixture constituted mainly by ricinoleic acid in the tri-ester structure. The COFA samples were produced in industry through the hydrolysis of castor oil, with acid value of 116 mg KOH/g. This information was added to section 2.1 “Materials”.
2. Please, review spelling of Anton Paar in line 149.
R.: We thank the reviewer for pointing this out. The spelling has been corrected.
3. Please, include values of Flash Point for BL1 and BL2 lubricants.
R.: The focus of our study was on the evaluation of oxidative stability and tribological behavior of the synthesized samples. The flash points of BL1 and BL2 tend to be even higher than the value obtained for the COFA sample (216 ºC), because they are constituted by long-chain esters from COFA. In addition, the flash point value of COFA was higher than found in mineral oil sample (160 ºC). So, we did not measure this property for the samples obtained in this study.
4. It would be necessary to justify the conditions chosen for the tribological tests.
R.: We appreciate this comment. We have added in Section 2.7 that test conditions were based on ASTM D4172, but within the limits of the DHR-3 rheometer. We have inserted a sentence to clarify this fact in section 2.7 “Tribological evaluation”.
5. How do authors explain the higher stability of COFA with temperature in air atmospheric (Table 4 and Fig 6)
R.: The TGA technique in oxidant atmosphere provides an important result for characterization of the final bio-based samples to estimate the temperature range for an application. We observed that for more unsaturated carbon chain samples (COFA and BL1) the mass loss until 230ºC was lower than for the BL2 sample (more saturated sample). This occurs due the oxidation reactions, which lead to mass gains as oxygen is incorporated. This same behavior was already observed in the study carried out by Garcia et al. [5] for other unsaturated vegetable oils. However, the TGA curve for the COFA sample showed a more accentuated decay above 250ºC, indicating that the COFA sample was not so stable.
6. It would be necessary include the value of roughness of the balls after the tests, to confirm affirmation of line 384.
R.: We appreciate the reviewer’s comment. It is important to note that the method using an optical microscope has been used to evaluate the morphology and determine the wear scar diameter (WSD). This approach is adopted by other authors [6-9] to evaluate surface wear and corroborates our findings.
7. Please for better understanding, include an explanation of (a), (b) and (c).
R.: We believe that the reviewer suggestion is for the captions of Fig. 7. The captions were revised to include the description of each figure.
8. In Figure 7, please review friction curve of BL1 at the end of the test, what is the reduction of the friction coefficient due to?
R.: We appreciate this comment. We have revised Figure 7 to correct this.
References cited in this response letter:
[1] Lundgren, S. M.; Ruths, M.; Danerlöv, K.; Persson, K.; Effects of unsaturation on film structure and friction of fatty acids in a model base oil. J. Colloid Interface Sci. 2008, 326(2), 530–536.
[2] Lundgren, S.; Persson, K.; Mueller, G.; Kronberg, B.; Clarke, J.; Chtaib, M.; Claesson, P. M.; Unsaturated Fatty Acids in Alkane Solution: Adsorption to Steel Surfaces. Langmuir 2007, 23(21), 10598–10602.
[3] Ruths, M.; Lundgren, S.; Danerlöv, K.; Persson, K.; Friction of Fatty Acids in Nanometer-Sized Contacts of Different Adhesive Strength. Langmuir 2008, 24(4), 1509–1516.
[4] Crespo, A., Morgado, N., Mazuyer, D., & Cayer-Barrioz, J. (2018). Effect of unsaturation on the adsorption and the mechanical behavior of fatty acid layers. Langmuir, 34(15), 4560-4567.
[5] C. C. Garcia; P. I. B. M. Franco; T. O. Zuppa; N. R. Antoniosi Filho; M. I. G. Leles THERMAL STABILITY STUDIES OF SOME CERRADO PLANT OILS. J Therm Anal Calorim 2007, 87, 645–648.
[6] Gupta, R.N.; Harsha, A.P. Friction and Wear of Nanoadditive-Based Biolubricants in Steel–Steel Sliding Contacts: A Comparative Study. J Mater Eng Perform 2018, 27, 648–658, doi:10.1007/s11665-018-3175-3.
[7] Reeves, C.J.; Menezes, P.L.; Jen, T.-C.; Lovell, M.R. The Influence of Fatty Acids on Tribological and Thermal Properties of Natural Oils as Sustainable Biolubricants. Tribol Int 2015, 90, 123–134, doi:10.1016/j.triboint.2015.04.021.
[8] Ossia, C.V.; Han, H.G.; Kong, H. Response Surface Methodology for Eicosanoic Acid Triboproperties in Castor Oil. Tribol Int 2009, 42, 50–58, doi:10.1016/j.triboint.2008.05.018.
[9] Liu, P.; Fang, J.; Wang, X.; Wu, J.; Chen, B. A Novel Boron-Nitrogen Modified Castor Oil as an Ecofriendly and Efficient Lubricant Additive. J Dispers Sci Technol 2021, 42, 180–189, doi:10.1080/01932691.2019.1667818.
Round 2
Reviewer 2 Report
Comments and Suggestions for Authorsthe authors revised the paper according to the given comments
Reviewer 3 Report
Comments and Suggestions for AuthorsAuthors of “Thermo-oxidative stability and tribological characteristics of bio-based lubricants obtained from castor oil fatty acids and isoamyl alcohol” have considered the suggestions provided and they have also kindly answered to the comments and doubts.
The manuscript can be published, but before, please, review "Rancimat" reference in Conclusions, where it is possible to read "Racimat".
