New Transparent Flame-Retardant (FR) Coatings Based on Epoxy-Aluminum Hypophosphite Nanocomposites

Round 1

Reviewer 1 Report

This article reports a new transparent flame-retardant coating based on epoxy-aluminum hypophosphite nanocomposites. Epoxy coating containing AHP NPs shows satisfactory flame retardancy. Moreover, the thermal degradation of AHP NPs under nitrogen and air are deeply analyzed. The manuscript could be accepted for publication after minor revision. My detailed comments are as follows.

1. Section 1, “To date, to the best of the knowledge of the authors of this work, no study has focused on this subject and the FRs used were quite often of micron size.” However, it has been reported that the particle size of flame retardants is nanoscale, such as Nano-graphene.

2. Section 2, ethanol was used in the preparation of AHP nanoparticles, but the source and solvent purity of ethanol were not stated. Please supplement it.

3. Section 2, in the preparation of epoxy/AHP nanocomposites, why was 58 phr stoichiometric proportion of the hardener added?

4. Section 3.2, TG results showed that the residual char of EP/AHP increased at a stable growth rate during thermal decomposition. Why was the PHRR of 5wt.% of AHP higher than that of neat EP.

5. Section 3.3, Please supplement relevant parameters such as peak smoke production rate (PSPR), and total smoke production (TSP), average effective heat combustion (av-EHC), average carbon monoxide yield (av-CO) and fire growth index (FGI) in Table 4, and analyze them.

6. Please add a schematic diagram to show the flame-retardant mechanism of epoxy-aluminum hyphosophite nanocomposites on PLA.

7. There are a few spelling and grammar mistakes. Please check and revise accordingly.

Author Response

Many thanks to editors and reviewer for your recommendations and comments on our manuscript. We have revised the manuscript according to most of the reviewers’ comments (changes were highlighted in red), with detailed response given below.

Reviewer 1:

This article reports a new transparent flame-retardant coating based on epoxy-aluminum hypophosphite nanocomposites. Epoxy coating containing AHP NPs shows satisfactory flame retardancy. Moreover, the thermal degradation of AHP NPs under nitrogen and air are deeply analyzed. The manuscript could be accepted for publication after minor revision. My detailed comments are as follows.

1. Section 1, “To date, to the best of the knowledge of the authors of this work, no study has focused on this subject and the FRs used were quite often of micron size.” However, it has been reported that the particle size of flame retardants is nanoscale, such as Nano-graphene.

Answer: Thank you for this comment. Indeed, this sentence could be confusing since it only concerns nanoscale phosphorus-based flame retardants, but not nanoparticles in general. We have modified it for more clarity, as follow :

‘To date, to the best of the knowledge of the authors of this work, no study has focused on the use of phosphorus-based flame retardants at the nanoscale, and these agents were quite often of micron size.

2. Section 2, ethanol was used in the preparation of AHP nanoparticles, but the source and solvent purity of ethanol were not stated. Please supplement it.

Answer: We specify that we used technical ethanol from VWR

3. Section 2, in the preparation of epoxy/AHP nanocomposites, why was 58 phr stoichiometric proportion of the hardener added?

Answer: we wrote vaguely in the original manuscript, now we modified to “ … hardener was added to the so obtained blend at weight ratio of 100:58 (resin/hardener), based on stoichiometric molar proportion ….”

4. Section 3.2, TG results showed that the residual char of EP/AHP increased at a stable growth rate during thermal decomposition. Why was the PHRR of 5wt.% of AHP higher than that of neat EP.

Answer: The increase of PHRR observed with the composition EP/AHP-5 is mainly due to the nature of the decomposition products released during the composite decomposition. In fact, AHP, that is combustible product, could act as combustible or char forming agent, depending on its concentration, as it was demonstrated also by PCFC results, that evidenced that degradation products of EP/AHP-5 present higher PHRR that with neat EP while increasing AHP content induce PHRR reduction.

5. Section 3.3, Please supplement relevant parameters such as peak smoke production rate (PSPR), and total smoke production (TSP), average effective heat combustion (av-EHC), average carbon monoxide yield (av-CO) and fire growth index (FGI) in Table 4, and analyze them.

Answer: Unfortunately, with mass loss cone calorimeter, that determine HRR using thermopile rather that oxygen consumption used with cone calorimeter, we could not obtain data on the evolution of smoke and O2/CO, requested by the reviewer.

6. Please add a schematic diagram to show the flame-retardant mechanism of epoxy-aluminum hyphosophite nanocomposites on PLA.

Answer: We prepared a schematic diagram as requested as figure 11.

7. There are a few spelling and grammar mistakes. Please check and revise accordingly.

Answer: We revised the document and corrected the remaining mistakes. Thank you.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript entitled" New transparent flame-retardant (FR) coatings based on epoxy-aluminum hypophosphite nanocomposites" reports flame-retardant (FR) effect of transparent epoxy coating containing aluminum hypophosphite (AHP) nanoparticles (NPs) on polylactic acid (PLA) sheets for typical model of combustible polymeric material. It is prepared with systematical experiments and discussion. I recommend it for publication after addressing the following concerns.

1. The TGA residual product for AHP NPs is 10% larger than that of AHP MPs. The author should provide some possible reason. (Page 5, Line )

2. The authors claimed that "all curves show only one weight loss stage" (Page 9, line 287). However, a clear decrease of weigh starting from 180 oC can be observed which is needed to be explained.

Author Response

Many thanks to editors and reviewer for your recommendations and comments on our manuscript. We have revised the manuscript according to most of the reviewers’ comments (changes were highlighted in red), with detailed response given below.

Reviewer 2:

The manuscript entitled" New transparent flame-retardant (FR) coatings based on epoxy-aluminum hypophosphite nanocomposites" reports flame-retardant (FR) effect of transparent epoxy coating containing aluminum hypophosphite (AHP) nanoparticles (NPs) on polylactic acid (PLA) sheets for typical model of combustible polymeric material. It is prepared with systematical experiments and discussion. I recommend it for publication after addressing the following concerns.

1. The TGA residual product for AHP NPs is 10% larger than that of AHP MPs. The author should provide some possible reason. (Page 5, Line )

Answer: For instance, it is difficult for us to explain the difference of thermal stability between micro and nano AHP. We added the following section to highlight these changes and explain the need of further study.

The degradation pathway described in scheme 1 does not appear appropriate for AHP nanoparticles since the second decomposition step, corresponding to water loss, does not occur when AHP NANO are analyzed. Reduced particle size, from micro to nano-scale, has a significant impact on AHP's thermal behavior. To explain the difference in mass residue between both particles, a detailed study of the thermal behavior of AHP NPs is required. Additionally, this will explain why AHP NPs appear to gain mass during the second decomposition step.

2. The authors claimed that "all curves show only one weight loss stage" (Page 9, line 287). However, a clear decrease of weigh starting from 180 ^oC can be observed which is needed to be explained.

Answer: You are right. The following section has been added to correct and explain this degradation step.

Neat Epoxy thermal degradation occurred in two stages; (i) the first stage (150–275 °C) showing around 5% mass loss attributed to the loss of water via dehydration and eventual traces of solvent, followed by (ii) statistical rupture of the epoxy resin structure to form a char of about 11.8%. The first degradation step is mainly observed in the case of uncurred epoxy resin and corresponding to the condensation of curing agent during which some dehydration and elimination of low molecular weight molecules occurs [47].

Author Response File: Author Response.docx

Reviewer 3 Report

The reviewed paper deals with an interesting topic: aluminum hypophosphite nanocomposites as flame retardant in epoxy resin coatings.

The goal was to prepare a transparent, flame retardant coating based on epoxy resin. The authors clearly describe the basic intentions and the execution of these studies and prove their results by suitable measuring and characterization methods. The proportion of text and figures is well-balanced. The topic is of interest for readers of Coatings and the article should be considered for publication after a minor revision:

1) Please check if all abbreviations are written out at least once. Introduction 2) What is the advantage of nanoparticles compared to flame retardants that are soluble in epoxy resins? Methods

3) Part 2.5.3: How did you connect the thermocouple to front and back of the sample? You did not mention any temperatures measured on the front in your results. Results: Thermal analysis

4) Figure 2 and Figure 3: “micrometric”

5) Figure 3a: Please add a sentence to describe where this temperature was measured. How does the residue look like? Could a swelling of the sample lead to a contact of sample and thermocouple?

6) Scheme 2: arrows are lopsided. Results: Characterization of epoxy/AHP nanocomposites

7) You showed a mechanism for neat AHP – is the residue of Epoxy/AHP more than the sum of the residues of the single components (the theoretical residue)? Could you see a synergism in terms of char formation?

8) Would it be useful to compare the total heat release additionally? Results: FR properties of PLA sheets coated with epoxy/AHP nanocomposites

9) Figure 6: “15AHP” looks yellowish? Is there an explanation?

10) MLC tests: How did the mass of the samples differ? Did you check if the PLA sample without coating had the same mass as the coated samples? In our experience, mass differences have a huge impact on the outcome of cone calorimetric tests.

11) The introduction mentions UL94 as an important method since it shows flame retardant properties in a vertical test. There is a comparable test for coatings. The paper could improve a lot by those results (especially since the authors described the test in the introduction as important).

Further: 12) For a possible application as a coating, different properties like water uptake or mechanical properties are important.

You also mentioned in the introduction that obtaining appropriate mechanical properties while keeping transparency could be an advantage of nanoscale flame retardants. Do you have any results that you could add to this paper?

Author Response

 Many thanks to editors and reviewer for your recommendations and comments on our manuscript. We have revised the manuscript according to most of the reviewers’ comments (changes were highlighted in red), with detailed response given below.

Reviewer 3

The reviewed paper deals with an interesting topic: aluminum hypophosphite nanocomposites as flame retardant in epoxy resin coatings.

The goal was to prepare a transparent, flame retardant coating based on epoxy resin. The authors clearly describe the basic intentions and the execution of these studies and prove their results by suitable measuring and characterization methods. The proportion of text and figures is well-balanced. The topic is of interest for readers of Coatings and the article should be considered for publication after a minor revision:

1) Please check if all abbreviations are written out at least once. Introduction

Answer: done. Thank you.

2) What is the advantage of nanoparticles compared to flame retardants that are soluble in epoxy resins?

Answer: Not all flame retardants are soluble in epoxy. As a result, nano-FR is advantageous because it allows testing and evaluation of other non-soluble products at nanoscale.

3) Part 2.5.3: How did you connect the thermocouple to front and back of the sample? You did not mention any temperatures measured on the front in your results. Results: Thermal analysis

Answer: We held both thermocouples under the weight of the sample holder. Monitoring the exposed face temperature did not distinguish between samples and oscillated around the cone temperature of 650-700°C.

4) Figure 2 and Figure 3: “micrometric”

Answer: Many thanks. We corrected these two errors.

5) Figure 3a: Please add a sentence to describe where this temperature was measured. How does the residue look like? Could a swelling of the sample lead to a contact of sample and thermocouple?

Answer: This temperature was measured by a thermocouple located inside the TGA furnace. No contact between the sample and the thermocouple was observed. The mass curve should be altered when there is contact between the sample and the thermocouple, but this was not observed in our experiment.

6) Scheme 2: arrows are lopsided. Results: Characterization of epoxy/AHP nanocomposites

Answer: Many thanks. The scheme has been modified.

7) You showed a mechanism for neat AHP – is the residue of Epoxy/AHP more than the sum of the residues of the single components (the theoretical residue)? Could you see a synergism in terms of char formation?

Answer: The difference between theoretical and experimental char formed during TGA analysis was not so high. Unfortunately, No clear synergistic effect, in term of char formation, was observed.

8) Would it be useful to compare the total heat release additionally? Results: FR properties of PLA sheets coated with epoxy/AHP nanocomposites

Answer: Values of THR have been added to table 4. The following section has been also added to comment these results.

The total heat release (THR) is not significantly reduced in the presence of AHP. First, it is critical to note that the deposition of the virgin epoxy layer significantly increases the THR from 100 for PLA alone to 130 MJ/m². Adding 15 and 20% AHP NPs gradually re-duces this value to about 107. The THR values of the latter two compositions remain at the same level as that of pristine PLA. However, we can also notice that pHRR decreases and heat is released over a longer time range. The total heat release (THR) is not significantly reduced in the presence of AHP. First, it is critical to note that the deposition of the virgin epoxy layer significantly increases the THR from 100 for PLA alone to 130 MJ/m². Adding 15 and 20% AHP NPs gradually re-duces this value to about 107. The THR values of the latter two compositions remain at the same level as that of pristine PLA. However, we can also notice that pHRR decreases and heat is released over a longer time range. It should also be noted that the use of an epoxy layer, which is itself combustible, on PLA plates leads to an increase in the amount of combustible material tested. This can greatly affect the THR. In fact, PLA plates coated with epoxy weigh on average about 60 g, while PLA plates alone weigh about 45 g.

9) Figure 6: “15AHP” looks yellowish? Is there an explanation?

Answer: Of course, this plates present some yellow color but we are not able to clearly explain the reason behind this coloration.

10) MLC tests: How did the mass of the samples differ? Did you check if the PLA sample without coating had the same mass as the coated samples? In our experience, mass differences have a huge impact on the outcome of cone calorimetric tests.

Answer: You are right. Thanks for this comment. In fact, mass change. We added the following section to consider mass change.

The total heat release (THR) is not significantly reduced in the presence of AHP. First, it is critical to note that the deposition of the virgin epoxy layer significantly increases the THR from 100 for PLA alone to 130 MJ/m². Adding 15 and 20% AHP NPs gradually re-duces this value to about 107. The THR values of the latter two compositions remain at the same level as that of pristine PLA. However, we can also notice that pHRR decreases and heat is released over a longer time range. The total heat release (THR) is not significantly reduced in the presence of AHP. First, it is critical to note that the deposition of the virgin epoxy layer significantly increases the THR from 100 for PLA alone to 130 MJ/m². Adding 15 and 20% AHP NPs gradually re-duces this value to about 107. The THR values of the latter two compositions remain at the same level as that of pristine PLA. However, we can also notice that pHRR decreases and heat is released over a longer time range. It should also be noted that the use of an epoxy layer, which is itself combustible, on PLA plates leads to an increase in the amount of combustible material tested. This can greatly affect the THR. In fact, PLA plates coated with epoxy weigh on average about 60 g, while PLA plates alone weigh about 45 g.

11) The introduction mentions UL94 as an important method since it shows flame retardant properties in a vertical test. There is a comparable test for coatings. The paper could improve a lot by those results (especially since the authors described the test in the introduction as important).

Answer: We did not perform such tests because we had only limited quantities of AHP nanoparticles available. We are not able to present the results of the UL-94 tests.

Further: 12) For a possible application as a coating, different properties like water uptake or mechanical properties are important.

Answer: Of course. This is a first results we obtained on this system. We hope to be able to find another opportunities to continue this research.

Author Response File: Author Response.docx

Reviewer 4 Report

Review Report Journal Name: coatings (ISSN 2079-6412) Manuscript Title: New transparent flame-retardant (FR) coatings based on epoxy-aluminum hypophosphite nanocomposites

Manuscript ID: coatings-2101862-peer-review-v1

Manuscript type: Research paper

The author(s) of the paper proposed a New transparent flame-retardant (FR) coatings based on epoxy-aluminum hypophosphite nanocomposites. The paper is of little interest and innovation, so need some improvements which are as follows:

1. What’s the new perspectives about the work on how to design and AHP for PLA plates?

2. Can you depicts the procedures of aluminum hypophosphite nanoparticles?

3. Grammatical errors (tense, sentence formulation), abbreviation, space problem and typos should be corrected.

4. Can you compare the other results about “the epoxy coating was able to ensure significant reduction in the pHRR and appropriate insulating thermal properties of coated PLA plates”?

5. Can you using particle size distribution and energy dispersive X-ray spectrometer analysis of the containing aluminum hypophosphite (AHP) nanoparticles?

6. what's different between using air and N2?

7. The format is improper; please refer to the standard format of journal. kindly correct it.

8. The content is abundant, but a portion of the reference literatures is somewhat out of date (in 5 years). Important works should be cited as thoroughly as possible.

In this section, please also clarify the novelty and application implications of your work. I recommend that authors consult the most recent publications from "MDPI" and other flame-resistant journals. However, please do not exceed 30% of total citations from MDPI.

Author Response

Reviewer 4:

The author(s) of the paper proposed a New transparent flame-retardant (FR) coatings based on epoxy-aluminum hypophosphite nanocomposites. The paper is of little interest and innovation, so need some improvements which are as follows:

1. What’s the new perspectives about the work on how to design and AHP for PLA plates?

Answer: As mentioned in the paper, PLA sheets were selected as a combustible reference material to test the fire retardancy of our transparent coatings containing the AHP NPs. Thus, we see no interest in applying epoxy to PLA. On the other hand, we see interest in developing transparent coatings on metals or wood, for example, where the esthetic aspect is important.

2. Can you depicts the procedures of aluminum hypophosphite nanoparticles?

Answer:

2.2. Preparation of AHP nanoparticles

AHP microparticles were used for the preparation of AHP NPs by a two–step milling process by SDTech Nano France (Alès, France), in ethanol using microbeads Netzsch Labstar mill. First, AHP microparticles size was reduced up to 1 µm using grinding balls with a diameter of 1 mm and a grid mesh with the size of 400 µm. During the second step, smaller grinding balls (diameter of 200 µm) and grid mesh (diam-eter of 100 µm) were used for obtaining AHP NPs with a diameter lower than 60 nm.

3. Grammatical errors (tense, sentence formulation), abbreviation, space problem and typos should be corrected.

Answer: The document has been proofread and we have corrected the mistakes.

4. Can you compare the other results about “the epoxy coating was able to ensure significant reduction in the pHRR and appropriate insulating thermal properties of coated PLA plates”?

Answer: We added the following section to complete the discussion of the flame retardant section:

The total heat release (THR) is not significantly reduced in the presence of AHP. First, it is critical to note that the deposition of the virgin epoxy layer significantly increases the THR from 100 for PLA alone to 130 MJ/m². Adding 15 and 20% AHP NPs gradually re-duces this value to about 107. The THR values of the latter two compositions remain at the same level as that of pristine PLA. However, we can also notice that pHRR decreases and heat is released over a longer time range. The total heat release (THR) is not significantly reduced in the presence of AHP. First, it is critical to note that the deposition of the virgin epoxy layer significantly increases the THR from 100 for PLA alone to 130 MJ/m². Adding 15 and 20% AHP NPs gradually re-duces this value to about 107. The THR values of the latter two compositions remain at the same level as that of pristine PLA. However, we can also notice that pHRR decreases and heat is released over a longer time range. It should also be noted that the use of an epoxy layer, which is itself combustible, on PLA plates leads to an increase in the amount of combustible material tested. This can greatly affect the THR. In fact, PLA plates coated with epoxy weigh on average about 60 g, while PLA plates alone weigh about 45 g.

5. Can you using particle size distribution and energy dispersive X-ray spectrometer analysis of the containing aluminum hypophosphite (AHP) nanoparticles?

Answer: We know how many AHP NPs are present in each composition, since we prepare them.

6. what's different between using air and N2?

Answer: We performed TGA analysis of AHP under N2 and O2 for better understanding the effect of particle size on its thermal stability and also because AHP presents some fire and explosion risk. TGA performed on EP-AHP composite were done under N2 to evaluate the anaerobic composite thermal decomposition since it is the main decomposition mode occurring during the combustion.

7. The format is improper; please refer to the standard format of journal. kindly correct it.

Answer: Thank you. We checked the standard format of the journal.

8. The content is abundant, but a portion of the reference literatures is somewhat out of date (in 5 years). Important works should be cited as thoroughly as possible.

Answer: we added ref 47. 31 papers on 49 cited in the paper have less than 5 years. We tried our best to equilibrate and cite adapted references.

In this section, please also clarify the novelty and application implications of your work. I recommend that authors consult the most recent publications from "MDPI" and other flame-resistant journals. However, please do not exceed 30% of total citations from MDPI.

Answer:

As mentioned in the paper, PLA sheets were selected as a combustible reference material to test the fire retardancy of our transparent coatings containing the AHP NPs. Thus, we see no interest in applying epoxy to PLA. On the other hand, we see interest in developing transparent coatings on metals or wood, for example, where the esthetic aspect is important.

Author Response File: Author Response.docx