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Peer-Review Record

Foaming with scCO₂ and Impregnation with Cinnamaldehyde of PLA Nanocomposites for Food Packaging

Processes 2022, 10(2), 376; https://doi.org/10.3390/pr10020376

by Adrián Rojas^1,2,*

, Alejandra Torres^1,2,3, Carol López de Dicastillo⁴, Eliezer Velásquez^1,2

, Carolina Villegas^1,2, Simón Faba^1,2

, Patricia Rivera⁵, Abel Guarda^1,2,3, Julio Romero⁵ and María José Galotto^1,2,3,*

Reviewer 1: Anonymous

Reviewer 2: Anonymous

Reviewer 3: Anonymous

Processes 2022, 10(2), 376; https://doi.org/10.3390/pr10020376

Submission received: 29 January 2022 / Revised: 11 February 2022 / Accepted: 11 February 2022 / Published: 16 February 2022

(This article belongs to the Section Chemical Processes and Systems)

Round 1

Reviewer 1 Report

Please see attached document.

Comments for author File: Comments.pdf

Author Response

Response to the comments of the Reviewer 1

The author describes how CO2 can be used to foam a material and impregnate a chemical into the material. The study presents an enough amount of experimental work to prove their approach. However, some necessary details regarding experiments and results are missing. Below are my questions, suggestions, and concerns:

1.- Terminology

Gas-assisted polymer processing is completely different from foaming with gas. What they did is foaming with CO2, not CO2-assisted foaming.

The title of the work was changed according the suggestion of the reviewer. The term CO₂-assisted foaming was changed by foaming with scCO2 in the revised version of the manuscript.

2.- Abbreviations should be defined for the first time of usage, for example P, Pc, T, Tc, etc.

The reviewer suggestion was taken into account and abbreviations were defined as was required.

3.- Typos

CO2 -> CO2, -> ºC

CO₂and °C were used along the entire body of the revised version of the manuscript.

4.- Line 43: Ci needs to be defined in the main text.

Ci was defined in the main text.

4.- Lines 108 and 110: what is C30B, what is this for, what are the dimensions, and why especially this product as nanoclay? Authors are not supposed to advertize a product, and thus it would be better to describe it. What is Ci, what is this for, and why especially this?

The reviewer's suggestion was taken into account and C30B was defined in the revised version of the manuscript. The geometry and typical dimensions of C40B were added in Section 2.1. The explanation about why C30B was used was added to the revised version of the manuscript. Ci was defined also. The support for why cinnamaldehyde is used is because it is one of the most used EOS derivatives to develop active packaging materials with antimicrobial properties because of its great activity against bacteria, yeast and filamentous yeast. This fact was commented in the revised version of the manuscript.

5.- Section 2.2: Extrusion itself will not make films. Then, how to make films? How to measure the thickness?

The films were made films using a Scientific Labtech LBCR-150 chill roll attachment (Samutprakarn, Thailand) at 1.8 rpm, this procedure was added to the section 2.2

6.- Was the same cell used for foaming and impregnation? If so, this should be stated.

Sample dimensions in the foaming cell?

The same 100 mL high-pressure cell was used for foaming and impregnation procedures. This fact was mentioned in the revised version of the manuscript.

7.- How to confirm saturation of PLA with CO2 before foaming?

In our work was guarantee using a saturation temperature (135 °C) near to the PLA melting temperature (154 °C) which dramatically increase the plasticization of PLA by CO₂, improving the CO2 transfer inside the polymer, allowing a fast saturation with CO₂.

8.- Any information regarding the diffusivity and solubility of CO2 in the sample at given condition?

There is not any report about the diffusivity of CO₂ at the conditions used in our study. The solubility of CO₂ in the samples at the conditions used in our study was expected near 17.51 wt.% as was reported by Aionicesei et al. (2008). This information was actualized in the revised version of the manuscript.

9.- Line 132: To claim that the film is nanocomposite, the nanoclay should be dispersed well. Any confirmation?

A previous study developed by our research group have confirmed the effective dispersion of C30B inside PLA film produced by extrusion (Villegas et al. 2019). The films made in our study were obtained using the same extrusion procedure and using the same materials (PLA and C30B). This information was actualized in the revised version of the manuscript.

10.- Lines 142 and 152: What is the normalized pressure drop rate? This is important since it affects foaming significantly. For example, https://doi.org/10.1063/5.0050649 shows how to determine the normalized pressure drop rate.

Unfortunately, we cannot calculate the normalized pressure drop rate because we only measure the time required to depressurize the system by opening the vent valve. Thus, we don’t have the data to graph ln P vs t in order to obtain the PDR. We appreciate the reviewer's suggestion and it will be considered in our next studies.

11.- Line 152: What is 3h for? Is depressurization for opening the cell or impregnation?

For impregnation. For a better understanding this fact was correctly declared in the revised version of the manuscript.

12.- Line 148: units for 1477.9? What is 25 for?

1477.9 µm and 25 cm² of foam sample. For a better understanding this information was added in the revised version of the manuscript.

13.- Line 152: Even if there is another place for detailed information, it would be better to show the impregnation principle here.

The reviewer's suggestion was taken into account and the principle of the impregnation process was added in the revised version of the manuscript.

14.- Line 277: An evidence or reference showing the relation between plasticization and nucleation needs to be shown.

The reviewer's suggestion was taken into account and for a better understanding “increase of plasticization” was changed by “decrease the viscosity”. A reference to support this idea was added in the revised version of the manuscript.

15.- Line 283: Isn’t there any other direct factor leading to the difference such as pressure drop rate, pressure, size and amount of particles, or temperature?

In line 283 we compared results for PLA without nanoparticles. Thus, the effect of different sizes and amounts of nanoparticles does not apply in this case. Moreover, CO₂ density depends on temperature and pressure. Thus, when we compare our results with the reports by Milanovic we are considering these parameters. Nevertheless, the reviewer is right, pressure drop could be a relevant factor. Particularly, pressure drop was lower in the study developed by Milovanovic (0.5 MPa min^-1), which certainly favored a higher residence time for CO₂ inside PLA. This fact was added in the revised version of the manuscript.

16.- Section 3.1.: Any figures showings the relation between foamed structure (diameter or number of cell) and amount of absorbed Ci?

The amount of Ci impregnated in the different foam samples was the same. The structure of the impregnated PLA samples is shown in Figure 2.

17.- Section 3.2: Any discussion regarding different spectral peaks for each different sample? What is the main difference between figures 3 and 4? Can only figure 4 be used?

Figures 3 and 4 were modified and a new figure 3 was included in order to show FTIR spectra of control and active foams including also characteristic peaks of PLA.

18.- Section 3.3: What happened to this section, accidentally deleted or Section 3.4. was supposed to be 3.3?

The reviewer is right. Section 3.4 was supposed to be 3.3. The change was made in the revised version of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article "CO₂.assisted foaming and impregnation with cinnamaldehyde of PLA nanocomposites for food packaging" by Rojas et. al describes a two step process for generating microcellular nanocomposite foams as potential active food packaging. The authors present details on the foam morphology, structural property analysis using FTIR and their thermal characterization, alongside with analysis of the cinnamaldehyde release kinetics. The results are presented clearly and excellently contextualized with other studies. However, the manuscript lacks some transfer to applicability of the presented packaging. It does not become clear why only C30B was chosen as additive and the authors decided against evaluating a broader range of montmorillonites, which should be discussed. The study would furthermore benefit from assessment of the antimicrobial properties comparing the foams only impregnated with Ci to the ones incorporating different percentages of C30B. However, the reviewer is aware that this might go beyond the scope of this study and is not required but rather recommended for publication. The result and conclusion parts are well structured and well explained, but the introduction could still be improved. This part will benefit from some rearrangements and shortening of long sentences. One possible improvement would be moving the discussion about antimicrobial coatings (P1 L39- P2 L47) to the end (P3 L105) and add some information about montmorillonites here. Some examples where active packaging is already in use would be interesting for the reader at this point too. Overall, the study is well-suited for publication in processes after the improving the points addressed above and some minor corrections summarized in the attached word document.

Comments for author File: Comments.pdf

Author Response

Response to the comments of the Reviewer 2

Reviewer 2

The article "CO₂.assisted foaming and impregnation with cinnamaldehyde of PLA nanocomposites for food packaging" by Rojas et. al describes a two-step process for generating microcellular nanocomposite foams as potential active food packaging. The authors present details on the foam morphology, structural property analysis using FTIR and their thermal characterization, alongside with analysis of the cinnamaldehyde release kinetics. The results are presented clearly and excellently contextualized with other studies.

1.- However, the manuscript lacks some transfer to applicability of the presented packaging. It does not become clear why only C30B was chosen as additive and the authors decided against evaluating a broader range of montmorillonites, which should be discussed.

The reviewer's suggestion has been taken into account. A paragraph was added to introduce the organo-modified montmorillonite Cloisite^® 30B nanoclay (C30B) and explain why was used in our study instead other commercial nanoclays.

2.- The study would furthermore benefit from assessment of the antimicrobial properties comparing the foams only impregnated with Ci to the ones incorporating different percentages of C30B. However, the reviewer is aware that this might go beyond the scope of this study and is not required but rather recommended for publication.

The authors appreciate the reviewer's comments. Unfortunately, we cannot add to this contribution the study of the antimicrobial properties of the PLA nanocomposite foams loaded with carvacrol. This study will be part of a future contribution.

3.- The result and conclusion parts are well structured and well explained, but the introduction could still be improved. This part will benefit from some rearrangements and shortening of long sentences. One possible improvement would be moving the discussion about antimicrobial coatings (P1 L39- P2 L47) to the end (P3 L105) and add some information about montmorillonites here. Some examples where active packaging is already in use would be interesting for the reader at this point too. Overall, the study is well-suited for publication in processes after the improving the points addressed above and some minor corrections summarized in the attached word document.

The reviewer's comments have been considered and the introduction has been highly improved showing shortening long sentences, and adding new information related to C30B and some examples of the use of active polymer foams in food packaging.

4.- P2 L47:

Probably word lost at the end of sentence. Should this be filamentous fungi?

The reviewer is right and the word “fungi” was incorporated in the revised version of our manuscript.

5.- P2 L 56:

“has open in the last year” but reference is from 2015. Maybe change to last decade?

The reviewer's suggestion has been taken into account and the word year has been changed by decade in the revised version of our manuscript.

6.- P2 L 55-58:

Long sentence, consider splitting

The reviewer's suggestion has been taken into account and the alluded phrase has been shortened in the revised version of the manuscript.

7.- P2 L 61:

Requirements should be specified, or authors should at least point to a reference where this can easily be accessed.

A reference has been considered in the original paper (Hoc et al. 2021) as reference point.

8.- P2 L70:

Transition between the two paragraphs is very sharp, consider adding another sentence for better readability.

The reviewer's suggestion has been taken into account and another phrase has been added in the revised version of the manuscript to a better transition between the alluded two paragraphs.

9.- P2 L73:

Change “like” to “comparable”

The reviewer's suggestion has been taken into account and the word “like” has been changed to “comparable”.

10.- P2 L75:

“Improved mass transport” is not really connected to sentence before

The reviewer's suggestion has been taken into account and the phrase “improved mass transport” has been deleted in this revised version of the manuscript.

11.- P2 L76:

Change “absorption” to “adsorption”

The reviewer's suggestion has been taken into account and the word “absorption” has been changed to “adsorption”

12.- P2 L80:

Reference missing

The reference was added.

13.- P2 L87:

The techniques mentioned here need some explanation

The reviewer's suggestion has been taken into account and the alluded phrase was improved for a better understanding of the methods in the revised version of the manuscript.

14.- P2 L99- P3 L104:

Long sentence, consider splitting

The reviewer's suggestion has been taken into account and the phrase alluded has been shortened in the revised version of the manuscript.

15.- P3 L 105:

Add some examples for active foam applications here

The reviewer's suggestion has been taken into account and the two published papers on foams for active food packaging applications were cited in the revised version of the manuscript.

16.- P3 L 106:

Remove “In this framework”

The reviewer's suggestion has been taken into account and “In this framework” was delete it.

17.- P3 L110:

C30B was not properly introduced beforehand. Add paragraph about montmorillonites in introduction.

The reviewer's suggestion has been taken into account. A paragraph was added to introduce the organo-modified montmorillonite Cloisite^® 30B nanoclay (C30B) and explain why was used in our study.

18.- P2 L116:

Replace “of Ci” by “with Ci”

“of Ci” was replaced by “with Ci”

19.- P4 L164:

Replace “were” by “was”

“were” was replaced by “was”

20.- P5 L223:

Detector missing, please use HPLC-UV instead of only HPLC. You cannot detect anything only with HPLC.

HPCL was changed to HPCL-UV

21.- P6 L253:

SEM lacks explanation of abbreviation

The author's suggestion was considered and SEM was defined.

22.- P6 Table 1:

Split sample name in two columns whereof one contains the % 30B

The author's suggestion was considered and the C30B content of the samples was shown in another column in Table 1.

20.- P7 L280:

Shortly highlight differences in system used by Milovanovic and the one used in this study that might explain why the results are so different.

The author's suggestion was taken into account and the differences between our study and Milovanovic were highlighted. Particularly, CO₂ density depends on temperature and pressure. Thus, when we compare our results with the reports by Milanovic in terms of density we are considering these parameters (T and P). Nevertheless, the reviewer is right, other factors could have influenced the different results. Pressure drop could be a relevant factor. Particularly, pressure drop was lower in the study developed by Milovanovic (0.5 MPa min^-1) than the used in our study, which certainly favored a higher residence time for CO₂ inside PLA. This fact was added in the revised version of the manuscript.

21.- P9 L341:

Replace “Soft lines” by “bright lines”

“Soft lines” was replaced by “bright lines”

22.- P10 Table 2:

Why are there missing values in the table? Explanations of variables should be added to the table Header

The cells showing a hyphen in table 2 for some samples means that the sample did not present a thermal transition or peak associated with the thermal parameter indicated in the corresponding column. For more clarity, this explanation was included in the legend of the table.

The meaning of all variables (abbreviations for thermal parameters) was explained in the experimental section. Thus, some corrections associated with that issue were done in the discussion section (3.3), legend and captions of table 2, and in the corresponding experimental methodology (Section 2.3.3).

Author Response File: Author Response.pdf

Reviewer 3 Report

This submission by María José Galotto and co-workers describe the CO₂-assisted foaming and impregnation with cinnamaldehyde of polylactic acid nanocomposites with different organo-modified montmorillonite C30B concentrations for food packaging. Morphological, structural, thermal, and release properties of the developed foams were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Furthermore, kinetic models of Higuchi and Korsmeyer-Peppas were drawn.

I have read in details and carefully consider the present manuscript.

I am not an expert in the field and to my point of view, the scientific soundness and overall merit of this investigation is of high quality, however I am not sure about its originality and novelty as regard, for example, to previous studies by the same group. E.g., ref. [20‒21].

Composites Part B: Engineering 2019, 176, 107336.

Food Research International 2017, 99, 650–659.

I thus recommend to the editorial office, also sending the manuscript to experts in the fields of polymeric foaming, impregnation, and studies of the release kinetics. For the latest, I recommend scientists in the field of molecular imprinted polymers, for instance.

Please subscript the 2 of CO2 (L20, 33, and ref. 11, 13, 23, 28, 31, 33, 34, 42, 61.

L79: is the sentence = ……non-toxic, non-flammable, and it is inexpensive because it is a by-product in ethanol and…..

L80: P > Pc = 7.38 MPa (spaces before and after P, =)

L81: T > Tc = 7.38 MPa (spaces before and after T, =)

L121: Merck

L132: equal than or equal to?

L182 must be on next page

L185: 6-8; - must be inserted via the insert symbol button and is bigger (‒).

L270: spacing needed between table 1 and L270.

L272: ….10.5x10¹²…. Spaces before and after x, furthermore x is inserted via the insert symbol button. It is not a x like in xylophone but a multiplication sign (×).

L272: …cm^-3….- is inserted via insert symbol and is bigger (‒).

L296: ….173.4x10^-12….See comments for L272.

L325: …C‒O

L355 to L358: Please reconsider phrasing, meaning.

L368: Spacing between L367 and 368 mandatory.

L446: …0.18‒0.49….- is inserted via insert symbol. It is not the - below 6 in AZERTY keyboards. The sign is bigger, e.g., (–).

L447: …0.14-0.34…. see comments for L446.

L430: Spacing needed between table 3 and L430.

L470: Decrease size of symbols.

L491 and L494: See comments of L446.

L565, ref. 20: Add page 107336.

L570, ref. 22: Add page 1945.

L655, ref. 55: Add page 1502 and doi.

Author Response

Response to the comments of the Reviewer 3

Reviewer 3

I have read in details and carefully consider the present manuscript.

1.- I am not an expert in the field and to my point of view, the scientific soundness and overall merit of this investigation is of high quality, however I am not sure about its originality and novelty as regard, for example, to previous studies by the same group. E.g., ref. [20‒21].

Composites Part B: Engineering 2019, 176, 107336.

Food Research International 2017, 99, 650–659.

The authors appreciate the reviewer's comments. The novelty of this article lies in the different physical structuring of PLA. In our previous works (Composites Part B: Engineering 2019, 176, 107336; Food Research International 2017, 99, 650–659.) PLA nanocomposite films were impregnated with cinnamaldehyde using scCO2. Meanwhile, in this new study the PLA nanocomposite films were previously foamed using scCO₂ and then impregnated with cinnamaldehyde which resulted in porous PLA structures with very different physical and cinnamaldehyde release properties than the reported for commonly used PLA films, that encourages their potential use for food packaging. These facts, evidence the originality and novelty

of our work. 2.- Please subscript the 2 of CO2 (L20, 33, and ref. 11, 13, 23, 28, 31, 33, 34, 42, 61.

The corrections were made in the revised version of the manuscript.

3.- L79: is the sentence = ……non-toxic, non-flammable, and it is inexpensive because it is a by-product in ethanol and…..

The suggested corrections were made in the revised version of the manuscript.

4.- L80: P > Pc = 7.38 MPa (spaces before and after P, =)

The suggested corrections were made in the revised version of the manuscript.

5.- L81: T > Tc = 7.38 MPa (spaces before and after T, =)

The suggested corrections were made in the revised version of the manuscript.

6.- L121: Merck