Review Reports

Round 1

Reviewer 1 Report

As for this paper, it is involved in development and evaluation of crosslinked casein films for sustainable footwear, to increase the sustainability of footwear and its alignment with the circular bioeconomy. In can be seen that there is a pressing need to develop novel bio-based materials to replace the synthetic polymers currently used in this fields. However, with regard to casein-based films, the preparation by using the crosslinker and plasticizer is not that so new to us. Especially, casein as the film-former in leather industry, i.e. casein-based leather finishing agent have been widely explored so far, as referenced in following articles:

 

l Bio-template synthesis of MgAl layered double hydroxide with enhanced flame retardant property for leather finishes. Applied Surface Science, 2021(551): 149-409.

l Bio-based core-shell casein-based silica nano-composite latex by double-in situ polymerization: Synthesis, characterization and mechanism.Chemical Engineering Journal, 2013, 228: 281-289.

l Antibacterial Water Resistant Casein-based ZnO Nanomposite Coatings Via Double In-Situ Route[J], Progress in Organic Coatings, 2019, 134: 40-47

l Fabrication of Antibacterial Casein-based ZnO Nanocomposite for Flexible Coatings [J]. Materials and Design.2017 (113 ): 240–245

l Hollow casein-based polymeric nanospheres for opaque coatings[J]. ACS Applied Materials & Interfaces, 2016 8 (18): 11739-11748

l Facile Synthesis of Casein-based TiO₂ Nanocomposite for Self-cleaning and High Covering Coatings: Insights from TiO2 dosage[J]. Progress in Organic Coatings.2016,(99): 223-229

l Facile Synthesis of Casein-based Silica Hybrid Nano-composite for Coatings: Effects of Silane Coupling Agent. Progress in Organic Coatings, 2015 (88): 1-7

 The authors should further clarify the significance of develop casein films in footwear, for example, to show the sustainability and biodegradability; meanwhile, it will be better if the authors can tell how to and whether has overcome the defects of pristine casein films, such as its sensitivity to bacteria or mould, and to the moisture, thus to assure the service life of the footwear.

  In addition, the mechanism and principle during the film-forming process is not so clear. For example, the influence of synthesis parameters on the appearance , mechanical property should be deep investigated.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, Casein-based films were prepared by solution casting with tannic acid crosslinker and glycerol plasticizer. The casein films were used in sneaker and ballerina style shoes as components of the insock and decorative design elements on the vamp,which is innovative. However, the experimental design is not scientific enough. The reviewer recommends reconsideration after major revision. Here are some comments/questions:

1. As a control, a film of casein-only was prepared, while there is no corresponding performance to compare the advantages of casein films crosslinked by tannic acid. Please supplement corresponding experiment.

2. The applying method of films in shoes should be described in detail in the Methods.

3. Flexing endurance and mechanical properties is crucial to shoes during wearing process. The manuscript did not describe the properties of casein-based films applied to shoe material. Whether the strength of film is consistent with the shoes upper material?

4.Casein-based films were directly applied in shoes/insock, lack of practical operability in leather processing. The local film of the upper has little effect on the water vapor permeability of shoes. Usually, the crosslinked casein solution is highly recommended to be sprayed/coated on leather and investigate the flexing endurance, mechanical properties, water vapour permeability and other properties as mentioned in the paper.

5. Why the casein film was used as decorative design elements on ballerina style shoes?

6. The performance should be further explained according to the experimental design principles, especially in conjunction with tannic acid cross-linking effect.

7. As shown in Table 5, why the water vapour permeability of film D (G60/T10) was higher than that of film C (G60/T5)？

8. In line 289, Section 3.6 is at the end of Figure 5 caption.

 

Comments for author File: Comments.doc

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript entitled “ The initial development and evaluation of crosslinked casein films for sustainable footwear” has been peer-reviewed. In this study, the possibility of casein-tannic acid gels for application in footwear has been investigated, but unfortunately, it cannot meet the desired properties for this application. Despite writing the manuscript in fluent English, it has nothing new to share with the reader: The crosslinking with tannic acid and preparation of the film has been reported in a previous study with far better characterization and a well-organized article. A number of tests without any appropriate description and goal have been conducted and the samples have not been optimized to fulfil the requirement for footwear application. Therefore, the manuscript in this format cannot be accepted by this journal. 

Some specific remarks have been addressed in the following:

 The novelty of the study: There is no novelty in the chemical structure and film production. The authors just want to perform the same idea ( main material ( casein), crosslinker( tannic acid) and plasticizer( glycerol)) in another application and disregard the fact that for any application the priorities are different and the chemical structure should be re-engineered and optimized again( see Ref 63 : Casein films crosslinked by tannic acid for food packaging applications https://doi.org/10.1016/j.foodhyd.2018.06.028) ). A bunch of tests with a minimum level of planning make the whole manuscript vague. Keep in mind that the tensile strength of film produced in the above-mentioned ref(63) is in order of Mega (MPa, not Pa )while the authors have reported tensile strength of about 5 Pa. there is a huge difference which can be certainly stemmed from the synthetic approach. Furthermore Ref 63 : Casein films crosslinked by tannic acid for food packaging  applications (https://doi.org/10.1016/j.foodhyd.2018.06.028) should be investigated in the introduction as the main reference article.

Introduction:

It is informative but it needs to be summarized

If you name a polymer as a toxic material, the exemplified toxic compound ( like residual monomer) should correspond to that polymer: correct this “Perhaps the greatest concern is around their possible effects on hu- 53man health, as some synthetic polymers, such as polyvinyl chloride, polyester and poly- 54 carbonates contain toxic compounds including residual monomers such as bisphenol A, 55 heavy metals (particularly antimony,…”

Figures 1 and 2 should be reformed and adopted for the current study. Right now, these figures seem irrelevant and are just derivative. Instead of those, it is strongly recommended to schematically display 1-the gelation reaction (between tannic acid and casein), 2-film formation and application procedure.

Experimental :

Materials should be introduced in a separate subsection. The type of casein (For example α_s2-casein or β-casein and …) needs to be elaborated.

For the film preparation part, Ref 63: Casein films crosslinked by tannic acid for food packaging applications (https://doi.org/10.1016/j.foodhyd.2018.06.028)  should be referenced.

The section on Mechanical properties (subsection 2.2.2) should be comprehensive: which test by which apparatus (equipment) in which rate, the employed load cell, sample size and shape ( and possibly the corresponding standard ) needs to be concisely mentioned and then referenced. Bulk referencing without any description for tests is not accepted at all.

Subsection 2.2.4  Water vapour permeability:“…was based on the movement of water vapour through the films (30 mm diameter)” Do the authors mean that the Gravimetric Method - Wet Cup or not?

Result and discussion:

In the result and discussion section, diverse tests have been employed but the reasons behind the analyses are blurred. Certainly, the main goals for those measurements along with a brief description of the corresponding test are required. Therefore, the results can be discussed more precisely. Additionally, the tensile strength, tear strength, as well as hardness, grain crack and burst tests, should be pictorially (selected digital images of samples during the conducting test) depicted and inserted in the supporting information file. Unfortunately, Figure 3 does not give a precise understanding of the formed gel film.

For investigation of film transparency (Figure 3): a written text under the films can visually delineate the degree of transparency for the observer. It can be added to Figure 3.

In Table 3, did you aim to report the Stitch tear strength or maximum force?  The term and its unit are contradictory (N unit for strength). In the Stitch tear strength term, both the tensile stress and the thickness of the film are important.

Stitch tear strength = tearing load / Thickness

Why have the authors used Shore D for analyses when it is obvious that gels are much softer than plastics ( shore D is recommended for harder materials( Plastics) such as PTFE, and PEEK)?

Besides, the results do not satisfy each other: based on the tensile strength values for the samples, the gel network is weak and very soft. how come get you the number of around 80 for shore A? this value is equivalent to eraser rubber hardness (Shore A)!!! additionally, by comparing the shore A and D values, the value of 80 for shore A is usually somewhere between 30-40 for shore D. How do the authors describe these discrepancies?”

 In subsection 3.4 Water vapour permeability, the sample and analytical approach have not been explained. As far as I know, the films with different ingredients were the only test samples, otherwise, a clear description is needed for the other sample for the test i.e. application of film on other surfaces. 

One row table is not an appropriate choice for the data in Table 5. It can be depicted in columnar format.

Generally, there are too many tables in the manuscript!!

Subsection 3.5. Fourier transform infrared spectroscopy (FTIR) should be the first section in the result and discussion section to discuss the gelation reaction and gel chemical structure. It has a huge impact on better elaboration and reasoning for further analyses.

“FTIR absorption band assignments of the films are shown in Table 6 and the 280 spectra are presented in Figure 6.” Wrong address! Figure 5 contains FTIR spectra, not figure 6.

The conclusion needs to be summarized.

 

The number of references is adequate.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have revised most of the comments and offered plausible explanation as the reviewer’s suggestion. It can be accepted after supplementing the following questions.

1.The author said they have replied the comment 5 in line 271-275. However, we didn't find the corresponding modification in the revised draft in line 271-275.

2.The author emphasized that this work is a proof-of-concept study and ignore scientific explanations for the experimental data (such as comment 6). We recommend adding appropriate explanations for the resulting data, which can make the paper worth publishing.

 

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The revised version of the manuscript entitled “The initial development and evaluation of crosslinked casein films for sustainable footwear” has been peer-reviewed. Respecting and appreciating the revision, a few essential changes (based on previous comments) have not been done. Unfortunately, some minor revisions, which were based on the author’s preference, cannot change the novelty issue of the manuscript, optimization and data-gathering approach. The main goal of us, either authors or reviewers, is to share reliable data through a smooth and scientific approach for further development in the world of science and technology. Overall, the manuscript cannot be accepted after major revision based on the following comments.

·        Figure 1 must be adjusted for this certain study and hold details of the synthetic and analytical process: what needs to be elaborated is that it is not a review article to use referencing in the caption. Commonly, this sort of referencing (in the caption of figure 1 ) is seen in review articles while any borrowed picture/scheme/ diagram should have copyright permission. Therefore, it is more rational to scheme a new reaction based on your specific situation. Herein coexistence of any relevant items sheds light on the incorporated approach and strategy and assists the reader to follow up the manuscript efficiently and smoothly.

·        Pictures of the samples during analyses and data gathering are vital and without those, all the processes are absolutely ambiguous and imprecise. For instance, in the introduction after referencing 49, the authors claim that: “but not into stand-alone components of footwear to replace leather/synthetics.” Do you mean that in this manuscript the prepared film has been used stand-alone as leather or other synthetic substances?!!! in fact, the FTIR diagram does not have much to say and prove what has been claimed. The only way is to display the sample entity and a snapshot of its performance during the test pictorially; The pictures can be simply taken by replicating the test.

·        Optimization of the film in terms of mechanical properties is required. The release of plasticizer (glycerol) from the film and the brittleness of the film during service needs to be investigated.

·        Talking about Tables; changing them to columnar plots (diagram) would be helpful.

·        The authors claimed that the introduction section has been summarized while its size has not been altered. at least two or three paragraphs should be omitted.

·        Another claim for the addition of a brief description of the analyses, the mechanical properties section is intact and any lucid explanation has not been added to attract the reader specifically junior researchers.

 

 

Author Response

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Author Response File: Author Response.pdf