Fluorine Free Surface Modification of Microfibrillated Cellulose-Clay Composite Films: Effect of Hydrophobicity on Gas Barrier Performance

Round 1

Reviewer 1 Report

Finally, a small question is raised, hoping that the author can clearly point out the innovation of the research work in this paper at the end of the Introduction, that is, the difference between the research done in this paper and previous studies and the possible impact on the whole research field.

Minor revised。

Author Response

We agree and modified the Introduction accordingly in the revised manuscript.  

Author Response File: Author Response.docx

Reviewer 2 Report

The paper focuses on an interesting up-to-date topic. In general, it is well written, but I still have some comments that are listed below.

There are different fonts throughout the manuscript.

Introduction gives an overview of the topic, but the last paragraph should emphasize the novelty of the research. This should also be underlined in the conclusions.

What was the bonding mechanism between clay and MFC? Was it only hydrogen bonds? Were the components actually bonded or was it just a mix?

XRD: What was the spacing in the clay? The Authors mention exfoliation. Was it affected by heat treatment? I suggest adding XRD for MFC samples recorded at lower 2theta angle (5-10 deg.) as it may not be obvious for all the readers that there are no peaks. Also, the X-axis lacks unit. I am not sure if for the MFC sample the peak coming from the (004) plane should be marked as it is not present. In line 191 the Authors write that an increase of implies an increase of crystallite size. This should not be measured this way. Please use the Debye Scherrer equation, D = Kλ / βCosθ.

Table 1 should include only CI information, the other parameters were not measured by means of XRD. Also, there are no XRD patterns for modified samples.

Have the Authors performed the EDS analysis? What was the content of the modifying agents in the whole material?

DMA analysis: How many samples were tested? Was there only one test for each type of sample? Authors should also include some information about the origin of the plasticizing effect mentioned. Perhaps tensile testing would be handy. Obtaining good elasticity is an important aspect in the packaging industry.

The language of the manuscript is good. It is easy to follow, there are some small mistakes, but not that serious, making the manuscript difficult to understand.

Author Response

Reviewer #2

The paper focuses on an interesting up-to-date topic. In general, it is well written, but I still have some comments that are listed below.

• There are different fonts throughout the manuscript.

We made the correction in the revised manuscript. Notice that the equation editor uses a different font, and this will be fixed by the Journal.

• Introduction gives an overview of the topic, but the last paragraph should emphasize the novelty of the research. This should also be underlined in the

We revised the Introduction accordingly, and amended the Conclusions.

• What was the bonding mechanism between clay and MFC? Was it only hydrogen bonds? Were the components actually bonded or was it just a mix?

Interactions between clay and MFC are in the form of weak organic–inorganic hydrogen bonds (Jung et al., Polymers 2020, 12, 2399 and Li et al., Carbohydrate Polymers 2010, 80, 1, 270), as seen from the slight shift to higher wavenumbers of the cellulose O–H stretching in the FTIR spectrum (Fig. 1a). We added this information in the revised version.

• XRD: What was the spacing in the clay? The Authors mention Was it affected by heat treatment? I suggest adding XRD for MFC samples recorded at lower 2theta angle (5-10 deg.) as it may not be obvious for all the readers that there are no peaks. Also, the X-axis lacks unit. I am not sure if for the MFC sample the peak coming from the (004) plane should be marked as it is not present. In line 191 the Authors write that an increase of implies an increase of crystallite size. This should not be measured this way. Please use the Debye Scherrer equation, D = Kλ / βCosθ.

Following the suggestion of the reviewer, we calculated the d-spacing in the clay using Bragg’s equation and crystallite size of cellulose using Debye-Scherrer formula. The results reported in the revised manuscript confirm the increase of the d-spacing in the clay for the MFC/clay composites and of the crystallite size of MFC upon annealing. We recorded the XRD spectrum for MFC samples in the 10-40° range as we wouldn't have expected any peaks in the 5-10° range. Currently, we lack the facility to redo the XRD for MFC samples. We also removed the (004) mark in revised Figure 2.

• Table 1 should include only CI information, the other parameters were not measured by means of XRD. Also, there are no XRD patterns for modified samples.

Table 1 was modified with the addition of the d-spacing and crystallite size data. We moved the other parameters (RMS roughness and WCA data) in a new Table 2 in the revised manuscript.

• Have the Authors performed the EDS analysis? What was the content of the modifying agents in the whole material?

We did not perform EDS analyses. The quantitative analysis of silylation of MFC composites films was left apart for this study, which instead focused on the relationship between the films' surface hydrophobicity and permeability at high relative humidity conditions.

• DMA analysis: How many samples were tested? Was there only one test for each type of sample? Authors should also include some information about the origin of the plasticizing effect mentioned. Perhaps tensile testing would be handy. Obtaining good elasticity is an important aspect in the packaging industry.

For DMA, three samples were tested for each type of films, as mentioned in the revised version. We also added some information about the plasticizing effect resulting from silylation in the revised manuscript. This is due to the scission of intermolecular and/or intramolecular hydrogen bonds within the cellulose fibrils (Ishimura et al., Cellulose, 1998, 5, 135). We also added tensile testing data in the supporting information file (Figure S2), that are discussed in the revised version.

Reviewer 3 Report

The paper contains some interesting results  but the complexity of the interactions among different materials should be  more precisely discussed. Some points are reported below

surfaces-2902077

1).The indication Fluorine Free In the Title  is not convenient in my opinion. Normally in a Scientific Title ,one should indicate  what is present and not what it is lacking. Also it my create some misleading for literature checking. The value of the paper depends on results achieved with the material studied

2).line 83 . ….montmorillonite clay with Na+ surface…. Is not technically correct ina scientific paper

3).lines 86-88- This ense is trivial and  looks as a commercial promotion rather than a scientific statement.

4) lines 155-157: what happens to the disappeared hydroxyl groups ?

5) lines 165-166 The sense.. This demonstrates that the silylation process of the films  surface with HDTMS was achieved… and  the preceding discussion are affected by the preparative process consisting in mixing the various components without  any real process of purification. Clearly FTIR shows the presence of all the reactants used in the final material but does not provide  real evidence of stable binding  among them. Indeed the assertion  .. This demonstrates that the silylation process of the films  surface with HDTMS was achieved…. would need a better resolution of the spectra  below 1000cm-1 to better evidence for the  band at 791cm-1

6) The  variation of the  RX peak for the clay  claimed in the text cannot be evaluated from the  corresponding Figure 2. Moreover is certainly too modest to demonstrate exfoliation of the montmorillonite. Also the size of the cellulose fibrils  and the possible  interlayers separation in the clay should be evaluated and discussed.

7) XR patterns of Silylated MFC-clay  and Heat-treated and silylated MFC-clay are not shown in the figure 2. Why ??

8) The amount of loading of silane derivatives on the paper  is not reported

9) Also one should discuss the possibility that  silanol interacts with the clay which is per se hydrophilic

10) The  section on mechanical properties need to be discussed considering the effect of  the clay and its possible interaction of with cellulose ; also  the role of the sylilation  would need  a definition of the content. However considering  the features of the various components a feal mixing  is  not easy to predict and surface interactions should be mainly discussed.

11)   As shown in the table 1 the objective properties changes without a precise rule with  addition of clay, heat annealing and  sylilation. An attempt to better rationalize the whole process could be convenient.

Author Response

Reviewer #3

The paper contains some interesting results  but the complexity of the interactions among different materials should be  more precisely discussed. Some points are reported below

surfaces-2902077

1).The indication Fluorine Free In the Title  is not convenient in my opinion. Normally in a Scientific Title ,one should indicate  what is present and not what it is lacking. Also it my create some misleading for literature checking. The value of the paper depends on results achieved with the material studied

We would like to disagree with the opinion of the reviewer. Not only a very large number of published papers feature ‘fluorine-free’ in their title, including one from one of the co-authors of the present paper, but more importantly the search from alternatives to fluorinated compounds dictated by recent bans, in particular for food packaging, calls for more research on such chemistries. The creation of a fluorine-free superhydrophobic surface is moreover a major finding from our work, and we would like to stress that in the title of the paper. We thank the reviewer in advance for their understanding.

2).line 83 . ….montmorillonite clay with Na+ surface…. Is not technically correct ina scientific paper

We thank the reviewer for the comment and amended the corresponding text in the revised manuscript

3).lines 86-88- This ense is trivial and  looks as a commercial promotion rather than a scientific statement.

We understand the comment of the reviewer but again, we would disagree: the relation between superhydrophobicity and the presence of fluorinated moieties is not trivial, in particular due to the prominent role of the surface topology, besides that of its surface energy,

and so is the toxicity of such compounds. At no point we aimed at a commercial promotion of the investigated chemicals. Nevertheless we slightly amended the text to emphasize its scientific relevance.

4) lines 155-157: what happens to the disappeared hydroxyl groups ?

The decrease in the number of free hydroxyl groups is a consequence of the increase of crystalline cellulose (Korkut et al., Wood Research, 2010, 55, 67). This information was added to the revised version of the paper.

5) lines 165-166 The sense.. This demonstrates that the silylation process of the films  surface with HDTMS was achieved… and  the preceding discussion are affected by the preparative process consisting in mixing the various components without  any real process of purification. Clearly FTIR shows the presence of all the reactants used in the final material but does not provide  real evidence of stable binding  among them. Indeed the assertion  .. This demonstrates that the silylation process of the films  surface with HDTMS was achieved…. would need a better resolution of the spectra  below 1000cm-1 to better evidence for the  band at 791cm-1

This is a good point and we amended the manuscript to better explain this achievement. In fact, we did follow a systematic procedure, in which the modified films were rinsed with ethanol 3 times to fully remove unreacted moieties. We do confirm the chemical grafting of MFC with HDTMS, as revealed from the FTIR bands at 2840–2918 cm^-1 and at 791 cm^-1.

6) The  variation of the  RX peak for the clay  claimed in the text cannot be evaluated from the  corresponding Figure 2. Moreover is certainly too modest to demonstrate exfoliation of the montmorillonite. Also the size of the cellulose fibrils  and the possible  interlayers separation in the clay should be evaluated and discussed.

We understand the comment of the reviewer, and have calculated the d-spacing in the clay and the crystallite size of cellulose for the various MFC/clay samples. The equations and data, and their discussion are reported in the revised version. We only claim a partial exfoliation and intercalation of cellulose microfibrils within the clay galleries (Wu et al., Biomacromolecules 2012, 13, 1927).

7) XR patterns of Silylated MFC-clay  and Heat-treated and silylated MFC-clay are not shown in the figure 2. Why ??

In our previous work we have found that the crystal structure of cellulose in the MFC was not changed upon silylation (Poothanari et al., Polymer International, 2021, 70, 1316). We added this information in the revised version.

8) The amount of loading of silane derivatives on the paper is not reported

The quantitative analysis of silylation of MFC composites films was left apart for this study, which instead focused on the relationship between the films' surface hydrophobicity and permeability at high relative humidity conditions.

9) Also one should discuss the possibility that  silanol interacts with the clay which is per se hydrophilic

This is correct. The surface silylation reaction of films with HDTMS also involves the hydroxyl groups of clays. The hydroxyl groups located at the edges of clays are the most reactive sites for silane grafting (He et al., Applied Clay Science, 2013, 71, 15; Penaloza et al., Journal of Silicate Based & Composite Materials, 2019, 71). We amended the corresponding text in the revised manuscript.

10) The section on mechanical properties need to be discussed considering the effect of  the clay and its possible interaction of with cellulose ; also  the role of the sylilation  would need  a definition of the content. However considering  the features of the various components a feal mixing  is  not easy to predict and surface interactions should be mainly discussed.

We amended Section 3.3 on mechanical analyses with additional explanations and tensile testing data. Indeed the results do not allow to discriminate the respective influences of the multiple interactions between clay and MFC and within individual phases, and their evolution upon annealing.

11)As shown in the table 1 the objective properties changes without a precise rule with  addition of clay, heat annealing and  sylilation. An attempt to better rationalize the whole process could be convenient.

We believe that we did attempt to rationalize the combination of influences, essentially in the discussion of the permeation data in Section 3.4, and in the Conclusions. Now it is clear that the process-structure-property relations of such multiphase materials are extremely intricate and much more work is needed to improve our understanding.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

In the revised manuscript Authors have included the required information. Of course, the introduction and presentation of the result could still have been modified, but at this point I find it acceptable.

no

Reviewer 3 Report

The authors amended propeprly the ms according to most of my remarks. I accept they on two points we have different opinion, but is not a very significant difference