Enhancement of an organic–metallic hybrid charring agent on flame retardancy of ethylene-vinyl acetate copolymer

An organic triazine charring agent hybrid with zinc oxide (OTCA@ZnO) was prepared and well characterized through Fourier transform infrared spectrometry (FTIR), solid-state nuclear magnetic resonance (SSNMR), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The flame retardancy and thermal behaviour of intumescent flame retardant ethylene-vinyl acetate (EVA) composites combining OTCA@ZnO and ammonium polyphosphate (APP) were investigated using limited oxygen index (LOI), UL-94 vertical burning, cone calorimetry and TGA. The structure and morphology of chars were investigated by scanning electron microscopy (SEM), FTIR, laser Raman spectroscopy analysis (LRS) and X-ray photoelectron spectroscopy (XPS). Results revealed that OTCA@ZnO exhibited excellent thermal stability and dispersity after hybridization. The flame retardancy and smoke suppression properties of EVA were significantly improved by introducing APP/OTCA@ZnO. TGA results indicated that APP/OTCA@ZnO presented an excellent synergistic effect and promoted the char formation of EVA composites. Residue analysis results showed more char with high quality connected by richer P–O–C, P–N and P–O–Si structures was formed in APP/OTCA@ZnO system than APP/HOTCA/ZnO system, which consequently suppressed more efficiently the combustion and smoke production due to the in situ catalytic carbonization effect of hybrid.

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-The authors claimed it is a metallic hybrid, but what they used is ZnO which is an oxide. This is very misleading.
-Many abbreviations such as SCTCA are not specified.
-The TEM images of Fig. 1 do not look good. The ZnO nanoparticles look more like aggregates. The SCTCA@ZnO looks entirely different than ZnO with different electron densities. However, SCTCA modification shouldn't change the TEM contrast of ZnO.
-The TGA curves in Fig. 8 do not really have any significant differences between samples. The minor difference could be just due to experimental variations.
-A lot of the measurements in the study are based on one-time measurement which is a potential issue.
-Each fitted peak in XPS spectra should be labeled.
-There are a lot of grammar mistakes and typos, even in the title of the manuscript. This would really need a lot of work on it.
-Besides the smoke production and release. Some other performance characterization as a flame retardant should be added, especially given that the mass loss seems very marginal without statistical analysis.

Reviewer: 2
Comments to the Author(s) 1. In abstract, page 2 line 40-42. The authors mentioned the comparison of the EVA/APP/SCTCA@ZnO and EVA/APP/SCTCA/ZnO systems, but in the subsequent text, they used hydrolyzed product HSCTCA/ZnO. Why did the authors use the hydrolyzed product HSCTCA/ZnO as a control rather than the compound of SCTCA with ZnO? 2. In introduction, extensive literature regarding current progress on the flame retardants EVA composites should be included. In addition, the research progress about ZnO as flame retardant of polymer should be reviewed. 3. Clarify the novelty of this study, compared with other similar research. 4. Clarify SCTCA. 5. How to determine the feeding ratio for the preparation of SCTCA? 6. Describe the reaction mechanism for the preparation of SCTCA with a schematic. 7. 2.2, what was used to adjust pH value? 8. P4, "Scheme 1 presented the routes of synthesis for SCTCA and SCTCA@ZnO", but the route for SCTCA synthesis was missed. 9. The full names for all abbreviations should be given when they appear for the first time, such as TGA, LRS and PHRR. 10. P7, what are HSCTCFA, HSCTFA/ZnO and SCTCFA-ZnO? They are not consistent with the expressions in the text and table! Please use the same expressions in full text, figures and tables! 11. Page 3 line 38-42 "Firstly, 20 mL DMAc solution containing 32 g γ-aminopropyltriethoxy silane was added into athree-necked flask equipped with a mechanical stirrer. Then, 27 g CYC and 14.6 g TEA in 20 mLDMAc was slowly added dropwise with vigorously stirring in an external ice bath". Was it ice bath or ice-water bath? What was the temperature? In addition, the reaction temperature was gone up to 50 0C after three hours. If ice bath was used, how to increase the temperature? The description is too confusing. 12. Page 3 line 51-52 "After dried to a constant weight under vacuum, the intermediate SCTCA (light yellow powder) was obtained". What was the drying temperature? 13. Page 4 Line 3-4, what is the purpose for activation of nano ZnO? Is there any reference? 14. Page 4 line 10, "The solid was washed three times with hot water", why was hot water used? 15. The authors should characterize the dispersion of the fillers in the composites by SEM or TEM.
Author's Response to Decision Letter for (RSOS-181413.R0) See Appendix A.

RSOS-181413.R1 (Revision)
Review form: Reviewer 1 Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Recommendation?
Accept as is

Comments to the Author(s)
The authors have responded to the previous concerns and the reviewer does not have additional comments except that the authors used the phrase "organic-metallic hybrid". Is this on purpose to be distinct from metal organic framework (MOF)? The term as used by the authors is not frequently used. It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email. Comments to the Author(s) The authors have responded to the previous concerns and the reviewer does not have additional comments except that the authors used the phrase "organic-metallic hybrid". Is this on purpose to be distinct from metal organic framework (MOF)? The term as used by the authors is not frequently used.

Dear Editors,
Thank you for your letter and the reviewers' comments concerning our manuscript entitled "Enhancement of an organic-metallic hybrid charring agent on flame retardacy of EVA".
Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have uploaded a revised manuscript plus responses to the reviewers' comments that we feel addresses adequately all the issues proposed. Words in green are the changes for the comments of Referees in the modified manuscript. The corrections and responses to the reviewer's comments are as following:

Responses to the reviewers' comments:
Reviewer: 1 Comment 1: Abbreviations should not be used in the title.
Response: Thanks for the reviewer's kind advice. According to the reviewer's suggestion, the title has been changed to "Enhancement of an organic-metallic hybrid charring agent on flame retardancy of ethylene-vinyl acetate copolymer" in the modified manuscript.

Comment 2: The authors claimed it is a metallic hybrid, but what they used is ZnO which is an oxide. This is very misleading.
Response: Thank you very much for your suggestions. Based on the definition, hybrid materials are composites consisting of two constituents at the nanometer or molecular level. Commonly one of these compounds is inorganic (including inorganic oxide, metal oxide, etc) and the other one organic in nature and they are connected by the covalent bonds. The synthetic idea of hybrid polymer materials is that if there are groups on the polymer chain that can participate in the hydrolysis and condensation process, through the hydrolysis and condensation of these functional groups with inorganic precursors, the organic polymer and inorganic hybrid polymer materials can be formed by covalent bonding. Thus, they differ from traditional composites where the constituents are at the macroscopic (micrometer to millimeter) level. Mixing at the microscopic scale leads to a more homogeneous material that either shows characteristics in the two original phases or even new properties. Specific to this work, nano-ZnO as a nanometer scale inorganic metal oxide was modified by the triazine oligomer containing functional groups of silicon ethoxy and the two could be connected by the covalent bonds generated through the hydrolysis and condensation of these functional groups with hydroxyl on nano-ZnO, forming a nanometer scale composite.
Based on this, the composite in this work was regarded as metallic hybrid material.
At the same time, we have also consulted some previous works and found some metallic hybrids with metal oxides as raw materials. Response: Thanks for your comment. As seen from Fig. 1(a), nano-ZnO possessed shapes like long rods and strongly aggregated into clusters of several micrometers in size due to the high surface energy and lots of hydroxyl groups on the surface. The average particle size of 80 nm × 15 nm of nano-ZnO and yet the aggregation was measured by a 500 nm scale. In fact, the aggregation for nanometer metal oxide particles commonly appears due to the high surface energy and they will agglomerate into clusters much larger than 500 nm rather than being dispersed at a particle size of nanometer each, which will strongly affect the dispersion and distribution of particles in the matrix. Thus the organic surface modification of nanometer particles may be an optimized way to obstruct the aggregation of nanometer particles. Just for this reason, this paper hybridizes nano-zinc oxide with organic charring agent in order to improve the dispersion of nano-zinc oxide, which is the primary goal of this work.
In fact, the goals of TEM tests were to identify the influence of hybridization on the dispersion of nano-ZnO, though the contrast difference of the test results may be because the operators are different twice, responded by the testing organization for TEM images in Figure   1. From Figure 1(a, b), the contrast of particles´ morphology and size before and after hybridization of nano-ZnO was enough obvious.
OTCA@ZnO showed a distribution like "seeds" in "melons", and we selected a seed to enlarge, as shown in Figure 1(c). It was not difficult to see from the enlarged Figure 1(c) that although the nano-zinc oxide still agglomerated, its particle size had been reduced from much more  also normal for the char residues of both to be similar, even if the char residues of OTCA@ZnO was higher or lower than that of IFR, because the amount of char was not the exclusive crucial factor on the properties of flame retardants.
Secondly, for the two curves of APP and IFR-Cal, it could be clearly seen that the calculated IFR-Cal was closer to the thermal weight loss process of APP. As you said, the two curves are almost the same and it was really easy to be regarded as certain experimental errors. In fact, this was because APP accounted for a large part (2/3) of IFR, so that the calculated theoretical curve was very close to APP curve. On the contrary, it was the fact that the two almost identical curves proved the addition of OTCA@ZnO had greatly demonstrated the synergy effect between OTCA@ZnO and APP, rather than simply mixing.      Lots of previous works also referred these characterizations and maybe the performance characterizations in previous references were less than those in this work. If you insist on still lacking performance characterizations, please kindly tell us the specific methods, which would be carried out in the future.
Thank you very much for your comments and suggestions again. Response: Thank you very much for your question. It was also our fault that the drying temperature of the products was not given in the manuscript. The drying temperature was 100 o C. This has been modified in the revised manuscript and marked in green. Response: Thank you very much for your comment. When the crude products after reaction were post-processed, we considered the feedstock may have the greater solubility in hot water than cold water according to the common sense, and thus hot water was chose to wash the crude product. Maybe cold water was also usable, which has not been tried in this experiment.
Comment 15: The authors should characterize the dispersion of the fillers in the composites by SEM or TEM.
Response: Thank you very much for your kindly suggestion.
According to your suggestion, the dispersion of the fillers in the composites has been characterized using SEM and the results and discussion have been added into the modified manuscript marked in green. Here is an excerpt.

Dispersion state and compatibility of fillers
The dispersion state and compatibility of APP/OTCA@ZnO ( Figure   10) and APP/HOTCA/ZnO ( Figure 10) in EVA was investigated by SEM. Comparing Figure 10 (a) and (c), it could be seen that EVA/IFR-6 had plenty of small holes and bulges, apparently as a result of poor compatibility of pristine nano-ZnO particles with the matrix, while EVA containing OTCA@ZnO didn't have this appearence. The dispersion around APP was presented in Figure 10 (b) and d and it could be observed that the compatibility between the char forming agent and APP was not bad. From Figure 10 (d), it was worth noting that a large amount of nano-zinc oxide was accumulated around APP and the charring agent, and most of them existed in the form of agglomerate. Such a distribution not only deteriorated the compatibility of the fillers with the matrix, but also many nano-ZnO were not sufficiently in contact with APP and the char-forming agent, which resulted in a significant reduction of flame-retardant efficiency.
In contrast, the hybrid nano-zinc oxide was encapsulated by OTCA, showing a better compatibility in the matrix and APP. In this way, nano-zinc oxide together with the char forming agent could be more uniformly dispersed in matrix, thereby improving the flame retardant efficiency. This distribution also explained why the initial decomposition temperature of EVA/IFR-3 was higher than that of EVA/IFR-6. Thanks very much for your kind work and consideration of our work again.
We greatly appreciate the efficient, professional and rapid processing of our manuscript by your team. If there is anything else we should do, please do not hesitate to let us know.
Thanks again and best regards.