Composite Films of Nanofibrillated Cellulose with Sepiolite: Effect of Preparation Strategy

Round 1

Reviewer 1 Report

Manuscript 1558236
Journal Coatings
Title Composite films of nanofibrillated cellulose with sepiolite: effect of preparation strategy
The manuscript entitled “Composite films of nanofibrillated cellulose with sepiolite: effect of preparation strategy” describes the effect of sepiolite incorporation into nanofibrillated cellulose (produced trough enzymatic or TEMPO oxidation pre-treatments, or mechanical tratment) films and the preparation strategy (solvent casting or filtration followed by hot-pressing) on the mechanical, optical, and barrier properties of the film. The paper is quite interesting but a major revision is requested. Now, the manuscript is not acceptable for publication. A better discussion of the results is necessary. Please follow the comments in the file.

Comments for author File: Comments.pdf

Author Response

-Reviewer #1:    

The manuscript entitled “Composite films of nanofibrillated cellulose with sepiolite: effect of preparation strategy” describes the effect of sepiolite incorporation into nanofibrillated cellulose (produced trough enzymatic or TEMPO oxidation pre-treatments, or mechanical tratment) films and the preparation strategy (solvent casting or filtration followed by hot-pressing) on the mechanical, optical, and barrier properties of the film. The paper is quite interesting but a major revision is requested. Now, the manuscript is not acceptable for publication. Please follow the comments below:

L2-3 Title Please rewrite the title that should reflect the main findings of the paper. A more interesting title is suggested. Please revise

Response: We are grateful to the reviewer for the comments made to our manuscript. The manuscript was improved according to the reviewer comments. However, we think that the title given reflects the content of the manuscript, that is, the effect of the preparation procedure and formulation on the film’s properties.

 

L20-35 Abstract Please revise the abstract. Some findings are not presented in the abstract (e.g., general decrease of mechanical performance after sepiolite addition). Please compare the actual abstract with the conclusions and add lacking results.

Response: The abstract was revised to include all the findings of the work.

 

L45-46 …can be improved by the addition of thin layers of other materials… Please provide some examples of these composite materials. Thanks

Response: Some examples were added to the manuscript text.

 

L58-59 Why cellulose-based materials are suggested? Other biopolymers are available (e.g., starch, chitosan, alginate and so on). Please add a scientific reason to use cellulose with a description of its advantages in comparison to other materials.

Response: Cellulose is the biopolymer more abundant on earth. Also, it is very stable and versatile, and thus it is a logical choice to prepare films due to the properties mentioned.

 

L61-64 Several cellulose derivatives are available: cellulose nanofibrils, cellulose nanocrystals, microcrystalline cellulose. Please describe all these materials with a comparison with NFC. The paper doi: 10.1016/j.carbpol.2020.117334 is suggested for your analysis. This review should be included in the paper.

Response: We are aware that different cellulose types and different cellulose derivatives are available and can be used to prepare films. However, the focus of the present manuscript is on the effect of the preparation method using cellulose nanofibrils and sepiolite on the film’s properties, and thus, other cellulose derivatives are out of the scope of the manuscript.

 

L74-75 Please better describe the paper “12” in which the thermal properties of PE/sepiolite films were investigated.

Response: The reviewer suggestion was followed and more information about the mentioned paper was added to the manuscript.

 

L78-79 The reviewer did not agree with this assumption. The literature describes the better performances of NCF/clay composites using planar clays such as MMT. Please delete this sentence

Response: The sentence was deleted.

 

L81-82 Please introduce the main methods to prepare cellulose/clay composites. The solvent casting method is the most used for the manufacturing at lab or pilot scale. The paper doi: 10.1016/j.fpsl.2021.100676 is suggested for your analysis and should be included in the paper for a general introduction.

Response: The main methods and the reference suggested by the reviewer were added to the manuscript.

 

L82 Why authors use a fibrous clay rather than planar clays? A deep discussion on this aspect should be included in the introduction. It is very important for the acceptance of the work.

Response: As mentioned in the introduction, most of the works dealing with CNF/clay composites have used planar/layered clays. We choose to work with fibre-like clays, not only to introduce novelty in our work, but also because we believe that, at least for the mechanical properties of the composites, this can be beneficial.

 

L91-95 Why thermal properties were not evaluated? It is an important aspect for many applications of these films. Please explain

Response: We agree that thermal properties are important for many applications. However, for the intended uses, food packaging and printed electronics, the film’s thermal stability is good enough, because the films are stable until more than 200 °C. We are now doing the study of the thermal behaviour of the developed films, with the possibility of use these films in other applications.

 

L94-95 Results should not be presented in the introduction. Delete

Response: The sentence in the final of the introduction is not truly a result discussion. We are just justifying why we measured WVTR for the films prepared by filtration and not for the films prepared by solvent casting. Thus, in our opinion, the sentence should not be deleted.

 

L96-193 Please divide Materials and Methods in 4 sections: 4.1 Nanofibril preparation, 4.2 Nanofibril characterization, 4.3 Film preparation, 4.4 Film characterization

Response: The suggestion of the reviewer was followed, and the section “Materials and Methods” was now divided in four sub-sections.

 

L103-104…catalytic amounts of TEMPO and NaBr, according to a methodology developed by Saito et al.

Please provide more details related to the amounts of TEMPO and NaBr

Response: The details about the amounts of TEMPO and NaBr used in the oxidation procedure were added to the manuscript (0.016 g of TEMPO and 0.1 g of NaBr per g of fibres)

 

L109-110 Please provide the units of enzyme used per g of dry pulp. Please provide more details

Response: More information was added on the amount of the enzyme used per g of dry pulp.

 

L112-113 constant conductivity. Please add more details related to final values of conductivity. Thanks

Response: The final constant conductivity is the conductivity close to that of the distilled water. In the beginning of the washing process the conductivity values are very high due to the presence of high concentration of salts. After the removal of the salts by the several washing steps, the conductivity was reduced until a constant value close to the value of the distilled water was reached. This information was added to the manuscript.

 

L115-116 Please add the treatment duration 15 min, 30 min? Please add more details related to HPH treatment

Response: The time of treatment is very dependent on the high-pressure homogenizer (HPH) used. For example, a lab scale HPH can have a flow of less than 10L/h, and the HPH used in the present work, a pilot plant HPH has a flow of 120 L/h. Thus, we think that the time of treatment is not relevant because is very dependent on the equipment conditions available. The important information was previously added to the manuscript, the number of passes (two) ant the pressure used in each pass (the first at 500 bar and the second at 1000 bar).

 

L141-143 Delete

Response: The sentence was deleted.

 

L145-149 Please provide more details related to particle size

Response: Details related to particle size of sepiolite were added to the manuscript.

 

L160-165 The two methods provided films with different ratio weight/area (40 g/m2 or 25 g/m2). How is it possible to compare the two methods and the performances of the obtained films? Please explain in detail in the text. It is very important for the acceptance of the work

Response: We understand the concern of the reviewer. However, it is important to note that most of the measured properties (tensile strength, Young´s modulus, WVP) have into account the film thickness and as result, usually, the changes in the basis weight did not lead to a large variation on the properties of the prepared materials. For example, we prepared films of mechanical CNF with 20, 40 and 60 g/m² by filtration followed by hot pressing, and the tensile strength obtained for these three films was 122, 140 and 131 MPa, respectively. In our opinion, the major role is played by the preparation method and by the ratio clay/CNF, and not by the basis weight of the film. The WVP values were also added to manuscript in addition to the WVTR values.

 

L193-194 Please add a statistical section. The statistics should be included in Table 2 and Figure 3. Thanks

Response: We agree with the reviewer. Statistic was added to table 2 and figure 3.

 

L196-227 Please better describe the results related to CNF Enz samples.

Response: The results obtained for the fibrillation yield and degree of polymerization of CNF Enz were now described.

L228-333 Please divide this section in several sub-sections corresponding to each characterization (mechanical, barrier, optical properties). It is very important for the acceptance of the work. Thanks

Response: The section was divided in sub-sections.

 

L238-239 Again. The two methods provided films with different ratio weight/area (40 g/m2 or 25 g/m2). How is it possible to compare the two methods and the performances of the obtained films? Please explain in detail in the text.

Response: The explanation was given in the previous point raised by the reviewer. Regarding the optical properties, there was a sentence in the text already discussing this issue, “It was also observed that, for each formulation type, the films prepared by filtration showed lower transparency than the films obtained by solvent casting, which is believed to be mainly a consequence of their larger basis weight (ca. 40 vs. 25 g/m²).”

 

Table 2 Add statistics

Figure 3 Add statistics

Response: Statistic was added to table 2 and figure 3.

 

L333-334 Overall which is the best combination of manufacturing method and concentration of sepiolite for the mechanical, barrier and optical properties? Please give a single option to be used in a specific sector.

Response: The goal of the present work was not to find the best combination of manufacturing method and concentration of clay; instead, we think that is more important to give insight on the effect of the preparation method and of the presence or not of clay (sepiolite), as well of its concentration. This can allow each reader to develop new composites without following a pre-defined formulation. Notwithstanding, it can be said that the films prepared by filtration + hot pressing of CNF Enz and CNF TEMPO containing a low amount of sepiolite (10-20%) have overall a better balance between mechanical, optical and barrier properties.

 

Discussion Please add other papers to discuss your results. The literature cited iv very limited

Response: More references were considered in this manuscript to discuss the results, as suggested by the reviewer.

 

Conclusion The filtration followed by hot-pressing could be not cheap compared to solvent casting, especially at industrial level. Please add this consideration in the conclusion.

Response: The consideration pointed out by the reviewer was added to conclusions.

 

L370-426 References. Please add the suggested references and revise the list and the numbering in the manuscript. Thanks

Response: References were added, and the references list and the numbering in the manuscript were updated.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The article entitled "Composite films of nanofibrillated cellulose with sepiolite: effect of preparation strategy" deals with a very interesting topic of using nanocellulose as a basis for preparing sustainable substituents petrochemical-derived polymers.

The article is well designed, and the research is thorough and appropriate conclusions that support the results derived are presented; the methodology is well described.

I have two very minor questions: Table 1 – what do you mean by "solid content," and how is it relevant to your research? No comment is given for this value in the article, and it seems it is unnecessary to actually include it here; Point 3 should be, in fact, named "Results and Discussion".

Author Response

-Reviewer #2:    

The article entitled "Composite films of nanofibrillated cellulose with sepiolite: effect of preparation strategy" deals with a very interesting topic of using nanocellulose as a basis for preparing sustainable substituents petrochemical-derived polymers.

The article is well designed, and the research is thorough and appropriate conclusions that support the results derived are presented; the methodology is well described.

I have two very minor questions: Table 1 – what do you mean by "solid content," and how is it relevant to your research? No comment is given for this value in the article, and it seems it is unnecessary to actually include it here; Point 3 should be, in fact, named "Results and Discussion".

Response: We thank the reviewer for the positive comments. About the first question raised by the reviewer, “solid content” means the dry residue of the CNF aqueous suspensions obtained after drying at 105 °C. We agree that these values were not used during the discussion and thus were removed from table 1 data. The section title was changed according to the reviewer suggestion.

 

Author Response File: Author Response.docx

Reviewer 3 Report

This work reports the preparation of nanocomposite films of cellulose nano fibers (CNCs) with sepiolite, a clay mineral having fibril morphology. Preparation methods varied from solvent casting, to filtration, or filtration+oxidation and pressing. Composite were investigate by different methods, including optical and mechanical properties. These composites are unique in that previous studies focused on using CNCs with 2D clay materials. Hence, this study provides a new direction for composites with enhanced mechanical stability. Results are interesting as these composites are potential biocompatible and biodegradable replacements for oil-based plastics. These membranes find applications as gas barriers and gas separations, as well as potential for water purification. Thus, this work is of interest to a broad audience and the subject is within the scope of this journal. I only have minor comments, and once these are addressed, the manuscript is recommended for publication. Comments:

1. See Young's moduli and tensile strengths for CNCs and layered clay in Biomacromolecules 2012, 13, 6, 1927–19320, and compare present results with other CNC layered materials having carboxylic groups: ACS Appl. Mater. Interfaces 2021, 13, 3, 4463–4472
2. Compare materials properties, including water permeability with CNC-sepiolite membranes prepared using ultrasound Applied Clay Science Volume 189, May 2020, 105538
3. The following work investigated rheological properties of sepiolite and CNC suspensions, which may aid authors in explaining the formation mechanism of each series of composite membranes prepared: Cellulose volume 24, pages 1815–1824 (2017). Moreover, this review discusses the different treatments on CNCs and clays for composites: Advances in Colloid and Interface Science Volume 272, October 2019, 101994
4. How does water permeability for the present composite compare to a common used plastic for packaging? See references: Food Packaging 2017, Pages 147-184, and Chem. Mater. 2014, 26, 19, 5459–5466, for example
5. Finally, add permeability results to Table 2

Author Response

-Reviewer #3:      

This work reports the preparation of nanocomposite films of cellulose nano fibers (CNCs) with sepiolite, a clay mineral having fibril morphology. Preparation methods varied from solvent casting, to filtration, or filtration+oxidation and pressing. Composite were investigate by different methods, including optical and mechanical properties. These composites are unique in that previous studies focused on using CNCs with 2D clay materials. Hence, this study provides a new direction for composites with enhanced mechanical stability. Results are interesting as these composites are potential biocompatible and biodegradable replacements for oil-based plastics. These membranes find applications as gas barriers and gas separations, as well as potential for water purification. Thus, this work is of interest to a broad audience and the subject is within the scope of this journal. I only have minor comments, and once these are addressed, the manuscript is recommended for publication. Comments:

See Young's moduli and tensile strengths for CNCs and layered clay in Biomacromolecules 2012, 13, 6, 1927–19320, and compare present results with other CNC layered materials having carboxylic groups: ACS Appl. Mater. Interfaces 2021, 13, 3, 4463–4472

Response: We thank the reviewer for the positive comments on our manuscript and the suggestion given. The references given report the preparation of films of TEMPO-oxidised CNF and montmorillonite (MTM), a planar clay mineral, and the values obtained for the tensile strength (TS) and Young’s modulus are higher than the ones obtained in the present work. It is important to note that this kind of layered clay mineral can be easily exfoliated, and form exfoliated nanocomposites with good properties. However, a similar trend was observed with the increment of clay content in the nanocomposite, in the present work, being observed an increase in the TS with the addition of low amounts of clay to TO-CNF, until 10%. This discussion and a reference were added to the manuscript.

 

Compare materials properties, including water permeability with CNC-sepiolite membranes prepared using ultrasound Applied Clay Science Volume 189, May 2020, 105538

Response: In fact, this reference has been used in our manuscript to discuss the WVTR results. Now, it was also used to discuss the mechanical properties. Our composite films presented higher values in the mechanical properties compared to the reference. The discussion related with the comparison of our results and the results reported in the suggested reference was added to the manuscript.

 

The following work investigated rheological properties of sepiolite and CNC suspensions, which may aid authors in explaining the formation mechanism of each series of composite membranes prepared: Cellulose volume 24, pages 1815–1824 (2017). Moreover, this review discusses the different treatments on CNCs and clays for composites: Advances in Colloid and Interface Science Volume 272, October 2019, 101994

Response: We thank the suggested works. The review article is from our authorship and it has been already mentioned. The mixture of charged CNF (TO-CNF) with clay minerals can result in a better dispersion state of the minerals and allow a good entanglement of clay fibres and CNF. This can probably explain the enhancement of the mechanical properties of the TO-CNF/sepiolite composite with the addition of low amounts of sepiolite (10%). This discussion was included in the manuscript.

 

How does water permeability for the present composite compare to a common used plastic for packaging? See references: Food Packaging 2017, Pages 147-184, and Chem. Mater. 2014, 26, 19, 5459–5466, for example

Response: Our composite films present WVP values in line or even lower than plastics like PLA but higher than others such as LDPE or PCL. This information was added to the manuscript, as well as the references recommended by the reviewer.

Finally, add permeability results to Table 2

Response: The WVTR and WVP results were added to table 3, following the reviewer suggestion.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Authors addressed large part of reviewer's comments. However, minor revisions are suggested prior the acceptance. Follow minor comments below:

Revise letters in Table 2 and Figure 3. They are lacking on some data. Please check carefully. Thanks

L160-165 The two methods provided films with different ratio weight/area (40 g/m2 or 25 g/m2). How is it possible to compare the two methods and the performances of the obtained films? Please explain in detail in the text. It is very important for the acceptance of the work Response: We understand the concern of the reviewer. However, it is important to note that most of the measured properties (tensile strength, Young´s modulus, WVP) have into account the film thickness and as result, usually, the changes in the basis weight did not lead to a large variation on the properties of the prepared materials. For example, we prepared films of mechanical CNF with 20, 40 and 60 g/m² by filtration followed by hot pressing, and the tensile strength obtained for these three films was 122, 140 and 131 MPa, respectively. In our opinion, the major role is played by the preparation method and by the ratio clay/CNF, and not by the basis weight of the film. The WVP values were also added to manuscript in addition to the WVTR values.

OK. Please add this aspect (In our opinion, the major role is played by the preparation method and by the ratio clay/CNF, and not by the basis weight of the film) in the text.

L333-334 Overall which is the best combination of manufacturing method and concentration of sepiolite for the mechanical, barrier and optical properties? Please give a single option to be used in a specific sector. Response: The goal of the present work was not to find the best combination of manufacturing method and concentration of clay; instead, we think that is more important to give insight on the effect of the preparation method and of the presence or not of clay (sepiolite), as well of its concentration. This can allow each reader to develop new composites without following a pre-defined formulation. Notwithstanding, it can be said that the films prepared by filtration + hot pressing of CNF Enz and CNF TEMPO containing a low amount of sepiolite (10-20%) have overall a better balance between mechanical, optical and barrier properties.

OK. Please add this conclusion in the abstract and in the conclusion section. Thanks

Author Response

Reviewer’s Comments:

 

-Reviewer #1:    

Authors addressed large part of reviewer's comments. However, minor revisions are suggested prior the acceptance. Follow minor comments below:

 

Revise letters in Table 2 and Figure 3. They are lacking on some data. Please check carefully. Thanks

Response: We thank again the reviewer for the comments made to our manuscript. The letters in Table 2 and Figure 3 were carefully revised. Previously we only are comparing the properties “inside” the same type of CNF. Now we expand the analysis, and the properties were compared considering all the films, independently of the CNF type.

 

L160-165 The two methods provided films with different ratio weight/area (40 g/m2 or 25 g/m2). How is it possible to compare the two methods and the performances of the obtained films? Please explain in detail in the text. It is very important for the acceptance of the work Response: We understand the concern of the reviewer. However, it is important to note that most of the measured properties (tensile strength, Young´s modulus, WVP) have into account the film thickness and as result, usually, the changes in the basis weight did not lead to a large variation on the properties of the prepared materials. For example, we prepared films of mechanical CNF with 20, 40 and 60 g/m2 by filtration followed by hot pressing, and the tensile strength obtained for these three films was 122, 140 and 131 MPa, respectively. In our opinion, the major role is played by the preparation method and by the ratio clay/CNF, and not by the basis weight of the film. The WVP values were also added to manuscript in addition to the WVTR values.

 

1. Please add this aspect (In our opinion, the major role is played by the preparation method and by the ratio clay/CNF, and not by the basis weight of the film) in the text.

Response: The sentence was added to the manuscript as suggested by the reviewer.

 

L333-334 Overall which is the best combination of manufacturing method and concentration of sepiolite for the mechanical, barrier and optical properties? Please give a single option to be used in a specific sector. Response: The goal of the present work was not to find the best combination of manufacturing method and concentration of clay; instead, we think that is more important to give insight on the effect of the preparation method and of the presence or not of clay (sepiolite), as well of its concentration. This can allow each reader to develop new composites without following a pre-defined formulation. Notwithstanding, it can be said that the films prepared by filtration + hot pressing of CNF Enz and CNF TEMPO containing a low amount of sepiolite (10-20%) have overall a better balance between mechanical, optical and barrier properties.

 

1. Please add this conclusion in the abstract and in the conclusion section. Thanks

Response: The conclusion was added to the manuscript as suggested by the reviewer.

 

Author Response File: Author Response.pdf