Surface Analyses of Thin Multiple Layer Barrier Coatings of Poly(vinyl alcohol) for Paperboard
Round 1
Reviewer 1 Report
This article addresses a very important but often overlooked issue - the effects of defects on the performance of barrier coatings for food packaging. There are many studies that have focused on various issues related to what could be achieved with various chemical and mechanical strategies under ideal conditions on ideally smooth base paper. But real paper is rough, and the authors have been patient in their present work to sort out various effects of this on coating quality issues. They have shown that despite various advantage of preparing multiple coating layers, the barrier still seems incomplete, and this can be traced back to persistent defects that are not overcome even by multiple layers. Though the research is very practical, it also scores high for novelty and for theoretical content. Also, the authors utilized analytical methods in an admirable fashion and explained themselves clearly. The following specific points represent ways to further improve the article:
Lines 37-39: The authors seem to be missing something here. When papermakers use the term "coating", they are very often meaning conventional aqueous coatings comprising pigments (such as clay), binders (such as latex or starch), and other additives. That's a very mature technology, and such coatings ought to be very beneficial as a foundation for barrier coatings that can benefit from a smooth surface in which there are no protruding fibers.
Lines 63-67: It would be worth pointing out to readers that the shape of CNC is inherently inefficient for the purpose of blocking a diffusing molecular of liquid or gas. A pencil-like object would allow the diffusing entity to just go around it, not having a big expected effect on tortuosity, especially if the main part of the nanocomposite is the polyvinyl alcohol. I found a wonderful article that considers this issue in a practical, quantitative manner: Wolf, J. R., and Strieder, W. (1990). “Surface and void tortuosities for a random fiber bed - Overlapping, parallel cylinders of several radii,” J. Membrane Sci. 49(1), 103-115. DOI: 10.1016/S0376-7388(00)80781-4
Line113: What was behind the decision to adjust the pH to that value? Was that because the base sheet contained calcium carbonate, for instance? Readers are likely to be curious.
Line 135: Is a soft applicator system intended to achieve a contour-type coating rather than to scape the applied coating down to the level of protruding fibers?
Line 149: Does the sticky nature of damp PVOH make it unfeasible to use conventional dryer cans?
Line 197: At this point in the experimental description, or maybe earlier, it would be helpful to have a description of the equilibration of the paper at standard conditions. Typically this is done for at least 24 hours before testing. This would be very important for the oxygen permeability tests.
Line 301: The term microfibrillated cellulose (MFC) can cover a huge range of materials. For example, some MFC may be like highly refined conventional kraft pulp fibers. At the other extreme, the inventor Tubak used the term NFC to include nanofibrillated cellulose. Readers will want more information so that they can understand what the authors added.
As a general comment, it would make logical sense to try to optimize the type of highly fibrillated cellulose such that it has the effect of creating a tougher film that is less vulnerable to cracking or other such effects leading to passageways for air. This could be discussed in the article. Do the authors have any idea of what range of MFC size or morphology might be expected to be helpful?
Line 541: Too thin to reflect light? It would make sense that such a distance might be one or two wavelengths of light, which can be easily computed for the range of light wavelength employed. This seems like a rich area for potential future work.
Line 617: The final sentence in the article would be better supported if the topic were discussed much earlier in the article. Readers will wonder, if the authors are prepared to do multiple PVOH coatings, why don't they just make the first coating a clay-based coating, etc. Is there a practical or economic reason for wanting to avoid this?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Surface Analyses of Thin Multiple Layer Barrier Coatings of 2 Poly(vinyl alcohol) for Paperboard
This study addresses the application of PVOH coatings from aqueous solution onto uncoated paperboard at high machine speeds using an integrated short-dwell type application/metering apparatus equipped with a resilient metering tip.
This study applied PVOH coating from aqueous solution to uncoated cardboard at high machine speeds. This was mainly achieved by an integrated short residence time coating/metering device equipped with an elastic metering tip. The barrier properties and surface structures of PVOH coatings produced by multi-layer, thick single and double-layer coatings were analyzed. The results showed that the thin multilayered coating approach yielded a lower number of pinholes and superior grease resistance compared to thick single or double coating.
The innovation of this study is worthy of recognition. However there are still some deficiencies as follows. My suggestion is to make some minorrevisions to get it published in the “Coatings”.
[1] In introduction part, it is necessary to introduce the advantages and disadvantages of hot melt lamination or extrusion when discussing the two most common coating methods for paper and paperboard.
[2] Please point out common water-soluble polymers before drawing out PVOH. It is also necessary to explain why other polymers are not suitable for reducing the barrier properties of paper and paperboard.
[3] “Most extant research papers focus on laboratory-scaled studies where free-standing films, casting, or drawn-coatings at low speeds have been used to produce PVOH films or PVOH coatings.” Please cite references for free-standing films, casting or low-speed drawing-coatings.
[4] It is suggested to summarize the research progress of single layer, double layer and multi-layer in the introduction.
[5] “This may be a consequence of the reduced uptake of water into the substrate after the first pass, since water sorption may promote fiber swelling and induce subsequent and fiber movement (fiber rising) during the drying process.” It is recommended to use the water contact angle to detect the hydrophilicity of the coating surface for further proof.
[6] Please use ultra-depth of field 3D microscope (UDFM) and atomic force microscope (AFM) to further characterize the surface roughness of the coating.
[7] Figure 3 showed the ratio between Sc and Sv for ML series and uncoated paperboard. The bar chart showed a trend of fluctuation and rise, which has not reached the critical point. It is suggested to increase the measurement sites to testify the real validity of the selected points.
Comments for author File: 
Comments.docx
Moderate editing of English language required
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
This manuscript reported the fabrication of thin multiple-layer barrier coatings of poly(vinyl alcohol) for paperboard. The experiments and analyses are supportive. There are still some issues that should be addressed.
The units of the Y-axis in Figures 2, 4, 5, and 6 should be corrected; μm.
Other advanced technologies to build high-precision functional coatings and engineered materials can be added to the Introduction section of the manuscript. For instance, biomimetic coating with mild and versatile merits (Science 2007, 318 (5849), 426-430. Chem. Rev. 2022, 122, 13, 11432–11473), Atomic layer deposition with precise and controllable features (Matter 2021, 4(11), 3515-3548. Adv. Funct. Mater. 2020, 30(15), 1910062), and surface functionalization with advanced nanomaterials (Coord. Chem. Rev., 2021, 436: 213846).
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
