Waterborne Coating Binders Based on Self-Crosslinking Acrylic Latex with Embedded Inorganic Nanoparticles: A Comparison of Nanostructured ZnO and MgO as Crosslink Density Enhancing Agents
Round 1
Reviewer 1 Report
The manuscript submitted for the publication in the journal Coatings represents a comprehensive study related to the formulation and preparation of acrylic coating binders with embedded nanoparticles. The paper also compares the effect of MgO and ZnO nano-sized particles on the main properties/characteristics of the waterborne coatings. The research was carried with the aid of appropriate characterisation techniques.
In the text of the manuscript, some sentences are difficult to grasp due to the wrong order of the words. Therefore, the authors should seek an assistance from a native English speaker to address multiple issues in the entire manuscript.
The reviewer had provided a number of hand-written comments (in red ink) in the text of the submission and asking the authors to correct or clarify them.
In addition, the following comments should be addressed by the authors during the revision:
- The ICP-OES is a quantitative method that provides an elemental contents (as an alternative to the Atomic Adsorption spectroscopy). The authors decided to use the results of this analytical technique to explain the chemical transformation and interpret them. The chemical nature of the compounds, i.e. oxides, hydroxides and sulfates cannot be established by this method. The authors should be very careful and correct the sentences in the entire text, which state that the chemical nature was established using ICP-OES.
- The thickness of dry films was 1 mm. How this was measured? Later in the text, the authors provide different film thicknesses, e.g. 60 micron.
- The thickness of the wet coatings was 120 microns. Again, it is not clear why the wet thickness was lower than the one for the dry films. How the thickness (in microns) was determined and what is the error in measuring the thickness? I would imagine this would vary from one coating to another. Is it an average? Error of measurement should be given then.
- How the relative humidity of 50% was achieved? Please specify the equipment.
- Please specify the periods of time.
- Some chemical transformations do occur but ICP-OES method does not confirm it. Other analytical techniques should be used to confirm the presence of hydroxide and sulfate ions.
- Please provide chemical schemes, corresponding to chemical conversions of oxides to hydroxides to sulfates. Any indication from the FT-IR about sulfates presence?
Also attached the marked manuscript with the comments to be addressed.
Comments for author File: 
Comments.pdf
Author Response
Dear reviewer,
Thank you very much for your remarks and hand-written comments in the text of our MS. We appreciate your effort to help us very much and we tried to revise the manuscript according to all your comments. The specific response to your comments is as follows:
Point 1: The ICP-OES is a quantitative method that provides an elemental contents (as an alternative to the Atomic Adsorption spectroscopy). The authors decided to use the results of this analytical technique to explain the chemical transformation and interpret them. The chemical nature of the compounds, i.e. oxides, hydroxides and sulfates cannot be established by this method. The authors should be very careful and correct the sentences in the entire text, which state that the chemical nature was established using ICP-OES.
Response 1: On the basis of your comment, we have rewritten the entire text dealing with this topic and we also added results of XRD analysis to establish the chemical nature of nanoparticles. Nevertheless, we still left the results of ICP-OES analysis (including the content of Sulphur, which was determined for to estimate the content of presumed sulfates) in the revised version to support the information from XRD.
Point 2: The thickness of dry films was 1 mm. How this was measured? Later in the text, the authors provide different film thicknesses, e.g. 60 micron.
Response 2: We apologize for confusing (and in some cases false) information regarding the coating thickness and the confusing terms describing coating films/samples. We prepared “thin” coatings applied on glass substrates and “thick” free-standing films which were prepared by casting into silicone molds. The thickness of the latter samples was given by the dimensions of the mold (depth of 1 mm). The thickness of these dry samples was usually around 0.6 - 0.75 mm. In our opinion, we do not consider the accurate thickness determination to be essential, as these “thick” film samples were made especially for the testing of inherent coating properties and material structure (e.g. Tg, gel content, crosslink density, FT-IR analysis, ICP-OES analysis, SEM on fracture) which are not supposed to be significantly affected by coating thickness. The thickness of the “thin” coatings (cast on glass substrates) was given by the blade applicator with the slot width of 150 mm. The value of 120 mm was a mistake that unfortunately got into the old version MS. We feel ashamed for our carelessness.
Therefore, we have rewritten the text carefully and we used terms “free-standing film“ for the thick coating films prepared using silicone molds and “coating” for the coating films cast on glass substrates and we hope this would not mystify and confuse the reader any more.
Point 3: The thickness of the wet coatings was 120 microns. Again, it is not clear why the wet thickness was lower than the one for the dry films. How the thickness (in microns) was determined and what is the error in measuring the thickness? I would imagine this would vary from one coating to another. Is it an average? Error of measurement should be given then.
Response 3: The wet thickness of the “thin” coatings applied on glass substrates was 150 microns. After drying, their final thickness was measured using a three-point instrument (BYK-Gardner, Germany) and the thickness around 60 microns was achieved.
We tried to rewrite the text to include and clarify this question and we added the information regarding thickness measurement and accurate thickness values for all the coatings into Table 7.
Point 4: How the relative humidity of 50% was achieved? Please specify the equipment.
Response 4: We let the coatings (cast on glass substrates) dry in a climatic test chamber (CTC Memmert, Germany). We clarified this point in the revised version of our MS.
Point 5: Please specify the periods of time.
Response 5: The periods of time were specified.
Point 6: Some chemical transformations do occur but ICP-OES method does not confirm it. Other analytical techniques should be used to confirm the presence of hydroxide and sulfate ions.
Response 6: We performed XRD spectroscopy to confirm the chemical nature of the compounds and we presented these results in the revised text.
Point 7: Please provide chemical schemes, corresponding to chemical conversions of oxides to hydroxides to sulfates. Any indication from the FT-IR about sulfates presence?
Response 7: We provided chemical schemes in the text. According to your advice, we have tested the nanoparticles by means of FT-IR as well, where the sulfates were confirmed clearly in the case of ZnO nanoparticles after the polymerization-like treatment (a characteristic absorption band at around 1100 cm-1 appeared in the spectrum). Nevertheless, the peaks of metal-oxygen vibrations in the range around 400-500 cm-1 were close/beyond the measuring range of the instrument. Therefore, we decided to present the XRD results which seem more informative to us.
Reviewer 2 Report
The authors prepared acrylic latex with ZnO and MgO nanoparticle for coating applications. Some information and explanation of this study still lacked.
- Page2, Line 67-68. “there has been no evidence on MgO nanoparticle toxicity so far.” However, previous studies mentioned MgO nanoparticles induced cellular apoptosis and intracellular reactive oxygen species and revealed prominent oxidative stress at the high dose level for the particles. The toxicokinetic analysis showed significant levels of Mg accumulation in the liver and kidney tissues. Mechanistic investigational reports are warranted to document safe exposure levels and health implications post-exposure to high levels of nanoparticles.
Ref: Ecotoxicology and Environmental Safety 122 (2015) 260-267; Regulatory Toxicology and Pharmacology 90 (2017) 170-184.
- Page3, Line104-106. “The detailed composition of monomer feeds forming all latexes was as follows: 42 g MMA, 53 g BA and 5 g MAA dosed in the first step and 41 g MMA, 52 g BA, 2 g MAA and 5 g DAAM dosed in the second step.” Please explain why to use these ratios of the reactants and nanoparticles.
- Table 6. Viscosities of samples are between 19.2 and 46.1 mPa.s; the low viscosity coating led to sagging.
- Figures 6 and 7. How many repeated measures do the authors test? Please add the error bars.
- Please show the appearance of the polymeric films.
- Please show the statistical results of all data.
- Please check the typographical and grammatical errors.
Author Response
Dear reviewer,
Thank you very much for your comments, remarks and suggestions that we tried to incorporate into our manuscript as best as we could.
Point 1: Page2, Line 67-68. “there has been no evidence on MgO nanoparticle toxicity so far.” However, previous studies mentioned MgO nanoparticles induced cellular apoptosis and intracellular reactive oxygen species and revealed prominent oxidative stress at the high dose level for the particles. The toxicokinetic analysis showed significant levels of Mg accumulation in the liver and kidney tissues. Mechanistic investigational reports are warranted to document safe exposure levels and health implications post-exposure to high levels of nanoparticles.
Response 1: Thank you for this comment and information that we (should have known but) we had not known before. We feel ashamed for that. We have rewritten the text dealing with this issue in the Introduction part to involve this fact.
Point 2: Page3, Line104-106. “The detailed composition of monomer feeds forming all latexes was as follows: 42 g MMA, 53 g BA and 5 g MAA dosed in the first step and 41 g MMA, 52 g BA, 2 g MAA and 5 g DAAM dosed in the second step.” Please explain why to use these ratios of the reactants and nanoparticles.
Response 2: We have added an explanation of the ratios of particular reactants into the Experimental section.
Point 3: Table 6. Viscosities of samples are between 19.2 and 46.1 mPa.s; the low viscosity coating led to sagging.
Response 3: According to our experience, (even after a long-term storing the latexes at elevated temperatures) involved this information in discussion dealing with latex stability.
Point 4:Figures 6 and 7. How many repeated measures do the authors test? Please add the error bars.
Response 4: We specified the water absorption testing and water-whitening measurements in the Experimental part to be clear for a reader that for each film sample, 4 specimens were tested in the case of water absorption and 3 coatings were evaluated in the case of water-whitening experiments.
In the case of water absorption (Figs. 7 and 8), we apologize, but for the sake of clarity, we do not add error bars in curves of water absorption (because the initial parts of curves would become unclear). Nevertheless, we have added the information about standard deviations of water absorption measurements into the captions of these figures. In the case of water-whitening (Fig. 9), we added the error bars.
Point 5: Please show the appearance of the polymeric films.
Response 5: We provided a demonstrative picture of coating films.
Point 6: Please show the statistical results of all data.
Response 6: Standard deviations of all measurements were presented except the crosslink density values which were calculated from Mc. (In our opinion, the statistics can be estimated from standard deviations of the corresponding Mc values.)
Point 7: Please check the typographical and grammatical errors.
Response 7: We used the assistance of a native speaker and English was corrected.
