State-of-the-Art of Cellulose Nanocrystals and Optimal Method for their Dispersion for Construction-Related Applications

Round  1

Reviewer 1 Report

This manuscript shows a brief state-of-the-Art on cellulose nanocrystal (CNC) dispersion and its prospective use for diverse applications. The experimental part deals with the influence of processing parameters on the resulting aspect ratio and surface charge of CNCs. I think the combination of a review part together with an experimental part is a good idea and may be of interest for the readers of the Journal. In any case, I think the review part should have a more in deep analysis (withy further referencing) and the experimental part should also analyze the morphology of CNCs. Once this is done, I suggest accepting the manuscript for publication.

· The title should be improved as it is currently focused on the dispersion of CNCs, but no reference to the dispersing media (liquid, polymer matrix…) is done.

·  Some sentences need to be referenced; for example in line 48 “CNCs are known to be stronger than steel” (refer to: Materials Today 16 (2013) 220-227).

·  I think authors should make clear the difference between the term nanocellulose (referring to nanoscale cellulosic particles such as CNFs and CNCs) and CNCs.  Moreover, it is my opinion that at some point authors do not properly use the terms microcrystalline cellulose (MCC) or CNCs. See for further details: European Polymer Journal 59 (2014) 302-325.

·  In lines 94-96, when making reference to the work of Heux et al. about the electrostatic repulsion between cellulose micro-crystals, I think authors should report further works which aimed to reduce the strong hydrogen bonding between cellulosic particles. For example, it has been shown that CNC grafting significantly contributes to the good dispersion in the polymeric matrix, increasing Young´s modulus (refer to: Carbohydrate Polymers 142 (2016) 105–113) or storage moduli (refer to: Cellulose 25 (2018) 331–345).

·  Section “2. Review of existing research” provides interesting features about the efforts made so far in the field. Unfortunately, its current form may result quite tedious for the reader. Therefore, I suggest arranging this data into a Table and shortening the associated text.

·  In Table 1, I suggest including the use of cellulosic membranes for catalytic purposes (for example: Cellulose 25 (2018) 2547–2558), fluorescence sensing (Materials Chemistry Frontiers 1 (2017) 979-987) or photonic applications (ACS Appl. Nano Mater. 1 (2018) 2175–2183).

·  Section 3.2 needs further referencing.

·  Add scale bar in Figure 3. Moreover, authors should report a TEM image of CNCs in Figure 3.

· Together with the particle size determination showed, it will be great if authors could provide further insights on the morphology of CNCs as this would add value to the work. This can be accomplished by AFM or TEM analyses.

· Although the changes in surface charge are not dramatic, authors should provide a plausible explanation of its change with particle size (or magnetic/high-pressure dispersion time).

· I also suggest including a scheme as the last figure within the paper which highlights obtained main findings in an eye-catching fashion. This would serve to summarize the whole work.

Author Response

Thank you very much for your suggestions, which have helped us improve out manuscript. 
Our answers to your individual questions follow below:

1. The title should be improved as it is currently focused on the dispersion of CNCs, but no reference to the dispersing media (liquid, polymer matrix…) is done.
A: A review of the existing literature on the dispersion of CNCs has been added in the manuscript. 
Cao et al. dispersed CNCs by ultrasonication, which is a mechanical method. With an increase in the treatment time, the transparency of the dispersion increased, and the shear stress decreased, as determined by rheological measurements. In most previous studies, ultrasonication has been used for the mechanical dispersion of CNCs. Therefore, a comparative analysis of the different approaches for dispersing CNCs was not possible. Hence, in this study, we aimed to evaluate different nanomaterial dispersion methods by quantitatively comparing magnetic stirring and high-pressure dispersion.
 
2. Some sentences need to be referenced; for example in line 48 “CNCs are known to be stronger than steel” (refer to: Materials Today 16 (2013) 220-227)
A: We have added the relevant references in the manuscript. 
 
3. I think authors should make clear the difference between the term nanocellulose (referring to nanoscale cellulosic particles such as CNFs and CNCs) and CNCs. Moreover, it is my opinion that at some point authors do not properly use the terms microcrystalline cellulose (MCC) or CNCs. See for further details: European Polymer Journal 59 (2014) 302-325.
A: The contents of the reference documents related to the intellectual matters are summarized again.  In the mentioned study, unmodified CNCs and silylated CNCs (SCNCs) were prepared and used with PLLA to solution-cast nanocomposite films. The effect of the surface silylation on the morphology, nonisothermal and isothermal crystallization behaviors, and mechanical properties of the synthesized nanostructured composites were investigated. The unmodified CNCs formed agglomerates, whereas the SCNCs were well dispersed in PLLA. Further, the tensile modulus and tensile strength of the PLLA/SCNC nanocomposite films with only 1 wt% SCNCs were more than 20% higher than those of pure PLLA, owing to crystallinity effects and the fact that the CNCs were well dispersed in the former.

 
4. In lines 94-96, when making reference to the work of Heux et al. about the electrostatic repulsion between cellulose micro-crystals, I think authors should report further works which aimed to reduce the strong hydrogen bonding between cellulosic particles. For example, it has been shown that CNC grafting significantly contributes to the good dispersion in the polymeric matrix, increasing Young´s modulus (refer to: Carbohydrate Polymers 142 (2016) 105–113) or storage moduli (refer to: Cellulose 25 (2018) 331–345).
A: Previous studies on CNCs are now summarized in tables, including the study by Heux et al.
 
5. Section “2. Review of existing research” provides interesting features about the efforts made so far in the field. Unfortunately, its current form may result quite tedious for the reader. Therefore, I suggest arranging this data into a Table and shortening the associated text.
A: For the benefit of the readers, summaries of previous studies on CNCs are now placed in tables. Further, the summaries have been revised accordingly.
 
6. In Table 1, I suggest including the use of cellulosic membranes for catalytic purposes (for example: Cellulose 25 (2018) 2547–2558), fluorescence sensing (Materials Chemistry Frontiers 1 (2017) 979-987) or photonic applications (ACS Appl. Nano Mater. 1 (2018) 2175–2183).
A: Thank you for your suggestion. We have included these studies in the manuscript. 
 
7. · Section 3.2 needs further referencing.
A: We have added the relevant references.
 
8. · Add scale bar in Figure 3. Moreover, authors should report a TEM image of CNCs in Figure
A: A TEM image that shows the shape of the dispersed CNCs has been included in the manuscript.
 
9. Together with the particle size determination showed, it will be great if authors could provide further insights on the morphology of CNCs as this would add value to the work. This can be accomplished by AFM or TEM analyses.
A: A TEM image of the CNCs in the dispersion formed by magnetic stirring for 180 min (the longest duration) has been added in the manuscript.
 
10. Although the changes in surface charge are not dramatic, authors should provide a plausible explanation of its change with particle size (or magnetic/high-pressure dispersion time).
A: The results of the analysis of magnetic stirring as a dispersion method have been modified. In the case of high-pressure dispersion, it was concluded, that as the pressure is increased, the diameter of the CNCs decreases. That is to say, their dispersibility increases. Further, the difference in the diameters corresponding to the different numbers of dispersion passes is insignificant.
 
11. I also suggest including a scheme as the last figure within the paper which highlights obtained main findings in an eye-catching fashion. This would serve to summarize the whole work.
A: We believe the conclusion section has been modified to summarize the whole work. Thank you for your comment.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper entitled "State-of-the-Art Research on Cellulose Nano-Crystals and their Optimal Dispersion for Construction Applications " by Hyung-Joo Lee et al describes the possibility to obtain a composite concrete using colloidal nanocellulose (CNC) through two different dispersion methods. The topic and the results are interested but in this form the manuscript is a technical report not a scientific article. For this reason the paper should be reconsidered after major revisions to address the following issues.

The points listed below need to be addressed.

1)  The abstract is not clear, should be re-elaborate.

2)  The introduction should be reduced and re-written considering the guideline of Applied Sciences journal.

3)  In the “Results and discussion”  section there isn’t a consistent discussion of the results. This section should be re-elaborated by comparing and better supporting the results with scientific literature.

4)  The conclusion section is confused, it has to be re-elaborated.

Author Response

Thank you very much for your suggestions, which have helped us improve out manuscript. 
Our answers to your individual questions follow below:
 
1. The abstract is not clear, should be re-elaborate.
A: We have modified the abstract to make it more readable. 
 
2. The introduction should be reduced and re-written considering the guideline of Applied Sciences journal.
A: All duplicate content has been deleted, and the Introduction section has been rewritten.
 
3. In the “Results and discussion”  section there isn’t a consistent discussion of the results. This section should be re-elaborated by comparing and better supporting the results with scientific literature.
A: We have partially rewritten the Results and Discussion section.
 
4. The conclusion section is confused, it has to be re-elaborated.
A: In the Conclusions section, we have added a supplementary comment on mechanical dispersion methods and state that high-pressure dispersion was determined to be the optimal method for dispersing CNCs. 

Author Response File: Author Response.docx

Round  2

Reviewer 2 Report

The revised paper can be accepted in the present form.