Unbleached Nanofibrillated Cellulose as Additive and Coating for Kraft Paper

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work is looking at the possibility of adding nanocellulose as an additive in paper fabrication. A similar study was also performed by the same group entitled Nanocellulose coating on kraft paper. I believe the current work is an extension of the previous findings. My comments are as follow:

1. To compare results with commercial kraft paper and biobased plastic materials

2. To perform comparison with other additives from other reported samples

2. Future work in the conclusion part

Author Response

Dear revisor, thanks for your considerations, follow the reply about your suggestions.

1. To compare results with commercial kraft paper and biobased plastic materials.

We did not consider making a comparison with commercial kraft paper because this material is very heterogeneous. The degree of delignification, species, fiber dimensions, degree of refining and conditions in paper manufacturing can influence the result. To remedy this gap, we included witness treatment, which is kraft paper made according to the standard conditions established by law.

We do not compare it with any biobased plastic, but we reproduce the film with the same weight that we use for the coatings and the mixture with the pulp, we consider this to be the main reference in our situation, once again the film formation conditions remain controlled.

Note that for thermal characteristics we made some comparisons with the polymeric materials most used in the food industry.

2. To perform comparison with other additives from other reported samples.

We made comparisons with the use of bleached nanocellulose, and in this round, we complemented these comparisons for most of the properties analyzed.

3. Future work in the conclusion part

A paragraph has been added to the Conclusions.

Reviewer 2 Report

Comments and Suggestions for Authors

Comments

In this paper, nanofibrillated cellulose (NFC) is used as an additive for paper and coatings for kraft paper for food packaging to improve its morphological, physical, mechanical and thermal properties. The topic of this paper involves the frontier of the discipline, which has important research significance and a wide range of application prospects. In general, the work is well done, while the conclusion is supported by the experimental and results. However, there are still some issues to be addressed before its acceptance.

1, It should be noted that the manuscript needs to be carefully edited by someone with expertise in technical editing in English, with special attention to English grammar, spelling, and sentence structure.

2, More background on the structures, properties and applications of NFC should be provided with supporting papers

3, Typographical error on line 287

4, The picture in Figure 4 needs to be modified to make it more concise and clear to better convey what the author wants to say to the reader.

5, In Table 3, for coated side and paper side contact angle variability is not highlighted and the data variability is small.

6, The authors are not rigorous enough in their presentation of the contact angles of CS and PS, and the principle of the increase in the contact angle is not reasonably explained, and the mechanism can be referred to and quoted in the above article： “Surface modifications towards superhydrophobic wood-based composites: Construction strategies, functionalization, and perspectives”.

7, Most of the references are old and cannot reflect the research status of this field well. Authors should cite more recent 3 years papers with high quality to show the novelty of this work.

Comments on the Quality of English Language

Need to be polished.

Author Response

Revisor 2

1, It should be noted that the manuscript needs to be carefully edited by someone with expertise in technical editing in English, with special attention to English grammar, spelling, and sentence structure.

We correct it and the modifications are highlight on the manuscriptp

2, More background on the structures, properties and applications of NFC should be provided with supporting papers

We correct it

3, Typographical error on line 287

We correct it

4, The picture in Figure 4 needs to be modified to make it more concise and clear to better convey what the author wants to say to the reader.

We decided to remove this figure

5, In Table 3, for coated side and paper side contact angle variability is not highlighted and the data variability is small.

We included this discussion:

The application of a coating caused a low variation in CA during the kinetics evaluated, showing that the wettability of the surface was altered with the addition of a coating, making the paper more hydrophobic. The addition to the mass did not significantly alter the wettability when compared to the control sample, as well as when it was evaluated on the side opposite the coating.

6, The authors are not rigorous enough in their presentation of the contact angles of CS and PS, and the principle of the increase in the contact angle is not reasonably explained, and the mechanism can be referred to and quoted in the above article： “Surface modifications towards superhydrophobic wood-based composites: Construction strategies, functionalization, and perspectives”.

We included this discussion:

The wettability of a surface is evaluated quantitatively by measuring the contact angle (CA) formed between a liquid droplet and the substrate. CA is defined as the angle θ formed by the tangent to the gas-liquid interface at the junction of the three phases, when the droplet encounters the solid surface, and the CA of the gas-solid-liquid phase reaches equilibrium. A smaller θ indicates stronger wettability of the substrate surface [ https://doi.org/10.1016/j.cis.2024.103142 ]..

Surface energy pertains to the rise in free energy within a system upon the creation of a new surface per unit area. Ehe Owens-Wendt-Kaelble method, explain the essential principles for engineering superhydrophobic surfaces can be summarized into two primary strategies: firstly, altering rough substrates using materials with low surface free energy; and secondly, constructing hierarchical structures at both micro- and nanoscales on surfaces that are inherently hydrophobic. [ https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/1521-3773%2820020402%2941%3A7%3C1221%3A%3AAID-ANIE1221%3E3.0.CO%3B2-G; https://doi.org/10.1016/j.apsusc.2011.11.045  ]. When we add NCF to the mass and as a coating, we reduce the roughness and porosity, increasing the surface energy and therefore making the paper more hydrophobic.

The Fowkes method [https://pubs.acs.org/doi/abs/10.1021/ie50660a008]posits that the surface tension of the liquid and the free energy of the solid surface consist of two main components: the intermolecular dispersion force (primarily involving interactions between non-polar molecules) and the polarity component (mainly encompassing dipole interactions and hydrogen bonds, whose strength is influenced by the surface's polarity factor). In this test, both water and glycerol are polar substances, but water is more polar due to its simple structure and the greater difference in electronegativity between the hydrogen and oxygen atoms. Polar liquids tend to interact more strongly with polar surfaces. In this case, water, being more polar, forms hydrogen bonds with polar groups on the solid surface and is absorbed quickly. Glycerol, on the other hand, was less absorbent than water because it was more polar. Another aspect is the surface tension of the liquid: glycerol has a higher surface tension of 64 mN/m at 20°C, while water has a surface tension of 72 mN/m, making it more difficult to be absorbed by the paper surface (Adamson, A. W., & Gast, A. P. (1997). Physical Chemistry of Surfaces (6th ed.). Wiley-Interscience.)

7, Most of the references are old and cannot reflect the research status of this field well. Authors should cite more recent 3 years papers with high quality to show the novelty of this work.

We included some new references but most of lower than 4 years.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript addresses an interesting topic related to the application of unbleached nanofibrillated cellulose (NFC), both as an additive and a coating, aiming to confer a notable enhancement in the performance of kraft paper for food packaging.

All sections are well-written, in a concise and clear style. The experimental plan is well-designed and discussed. The results are presented in correlation with the appropriate literature data. The References section is related to the addressed topic.

I have some minor comments on the manuscript and these are inserted in the attached PDF form.

Comments for author File: Comments.pdf

Author Response

Dear Review

Thank you for your corrections. We include all modifications suggested by you on the manuscript. They are highlighted by yellow color.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Major revision recommended.

-novelty of this work is not articulated. Please include a paragraph describing what is different/new in this study in comparison to other researchers’ research work.

-Authors claim in Introduction that barrier properties are determined. In fact, no barrier properties were measured. Water adsorption alone is not sufficient for that purpose. Diffusion must be investigated as well. Adsorption AND diffusion bot determine barrier properties.  Figure 4 images interpretation is too generic to get insights and explain permeability.

-Figure 4 needs to be revised. More detailed, “closer look”, images would be desirable. Hard to see in the way they are presented.

-please provide standard deviations in a conventional way, such as 11.1± 1.1

-please provide standard deviations for data in Table 6

-please eliminate excessive digits after decimal for most data in all Tables.

-DSC analysis prior and after NFC incorporation would be helpful to explain changes of morphology

Comments on the Quality of English Language

can be proofread

Author Response

1-novelty of this work is not articulated. Please include a paragraph describing what is different/new in this study in comparison to other researchers’ research work.

We included one paragraph with the novelty.

2-Authors claim in Introduction that barrier properties are determined. In fact, no barrier properties were measured. Water adsorption alone is not sufficient for that purpose. Diffusion must be investigated as well. Adsorption AND diffusion bot determine barrier properties. Figure 4 images interpretation is too generic to get insights and explain permeability.

The barrier properties that were determined were permeance to air and water, as well as using the contact angle to assess hydrophobicity to water and glycerol, two polar liquids but with different surface tensions. Regarding image 4, we decided to remove it from the work.

3 -Figure 4 needs to be revised. More detailed, “closer look”, images would be desirable. Hard to see in the way they are presented.

We decided to remove Figure 4.

4 -please provide standard deviations in a conventional way, such as 11.1± 1.1

This information is coefficients of variation, in percentage (see table legend), not standard deviations.

5 -please provide standard deviations for data in Table 6

These values represent peaks of a curve obtained by derivative, there are no dispersion values for DSC results.

6 -please eliminate excessive digits after decimal for most data in all Tables.

We will maintain the use of two places after the comma to maintain precision in observation.

7-DSC analysis prior and after NFC incorporation would be helpful to explain changes of morphology

The morphology was well explained using SEM and TEM, and we did not consider it necessary to include DSC to describe the morphology.

Author Response File: Author Response.pdf