Phytic Acid-Silica System for Imparting Fire Retardancy in Wood Composites

Round 1

Reviewer 1 Report

The manuscript entitled „Phytic acid-Silica System for Imparting Fire Retardancy in Wood Composites” describes selected properties of wood composited modified with a newly developed fire retardant system.

The research was well-organised and described, and the results were presented clearly. I would only suggest adding more information about the economic and ecological aspects of the production and utilisation of new wood composites, as well as examples of their potential application to complete the picture.

Comments for author File: Comments.pdf

Author Response

Reviewer 1

 

Dear Reviewer,

We revised the section ‘’Conclusions’’, following the suggestions of the Reviewer. Thank you.

 

 

We thank all the reviewers for the useful comments.

 

 

Chia-feng LIN, PhD student (LTU) &

George I. MANTANIS, Professor (FIAWS, PhD)

Author Response File: Author Response.docx

Reviewer 2 Report

(1) In the introduction part, a literature review of the fire retardant treatment of wood materials with phytic acid–silica system should be added.

(2) The description of the methodology is not particularly clear, how the treatment process and preparation of the composites actually took place, the authors should detail this section as much as possible, including images of the process.

(3) CO is a asphyxiating gas, which can lead to human poisoning when the CO content in the air is higher than 0.1%. Therefore, it is very important to add the information on the production rate of CO in the text.

(4) It is suggested that the authors should summarize the flame retardant mechanism of the phytic acid-silica system on wood composites in the “Results and Discussion” part.

Author Response

Reviewer 2

 

Dear Reviewer,

 

(1) In the introduction part, a literature review of the fire retardant treatment of wood materials with phytic acid–silica system should be added.

We added an additional reference regarding phytic acid-silica system (10.3390/ma14154164) in the Introduction.

 

(2) The description of the methodology is not particularly clear, how the treatment process and preparation of the composites actually took place, the authors should detail this section as much as possible, including images of the process.

 

We added a new illustration (as Figure 1) to clarify this.

 

(3) CO is a asphyxiating gas, which can lead to human poisoning when the CO content in the air is higher than 0.1%. Therefore, it is very important to add the information on the production rate of CO in the text.

We agree that it is important to consider the potential risks associated with the production of CO; our study was not designed -however- to address this specific issue. Nonetheless, we appreciate your useful input and will take it into consideration for the future research that is being carried out now.

(4) It is suggested that the authors should summarize the flame-retardant mechanism of the phytic acid-silica system on wood composites in the “Results and Discussion” part.

 

We addressed this matter at the end of the fire performance section. Thank you.

 

 

 

 

We thank all the reviewers for the useful comments.

 

 

Chia-feng LIN, PhD student (LTU) &

George I. MANTANIS, Professor (FIAWS, PhD)

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript is of practical interest, but needs significant improvement.

Notes:

1. Abstract.

It is not clear what the authors mean by elemental Si? I think it's silicon oxide. Correction needed.

2. Introduction

The structural formula of phytic acid should be given.

Compare different classes of flame retardants for their flame retardant effect on different materials. For example, it is possible to indicate why the components chosen by the authors are better than flame retardants based on phosphazenes (10.3390/polym14245334, 10.3390/polym14173592, 10.3390/polym11071191), which, due to the cooperative effect of phosphorus and nitrogen, have an excellent fire retardant effect.

It is necessary to explain in more detail why it is necessary to use phytic acid and not other phosphorus acids as an acid catalyst for sodium silicate.

It should be clarified, due to which the synergistic effect of phytic acid and sodium silicate is expected. Give schemes of the corresponding reactions. I believe that phytic acid is not a catalyst, but a reactant.

3. Methods and materials

Ammonium nitrate is not a hardener, but a catalyst.

4. Results and discussion

It is necessary to present in the form of a table the composition of all samples to facilitate the perception of the text.

The conclusions regarding the release of acetic acid are doubtful, firstly because the authors did not determine the presence of acetate groups in wood samples. Second, acetic acid boils at 118 degrees Celsius and samples cure at 160 degrees. On the IR spectra, the bands, presumably corresponding to the acetate carbonyl groups, do not change (within the error). The change in pH, I believe, is due to the increase in the content of phytic acid in the samples.

In general, the formation of acetic acid is not of interest. The authors should study the resin curing process in more detail depending on the amount of additives, since the mechanical properties are highly dependent on the structure of the polymer matrix. Make an assumption about it, but provide evidence. I believe that it is necessary to conduct IR studies of the original resin, cured resin and resin with additives. Compare the spectra and evaluate the conversion of methylol groups depending on the concentration of additives. For even more accurate confirmation, mechanical testing of the cured resin with and without additives should be carried out. Compare results with wood-filled composites.

It is necessary to conduct TGA of separately cured resin and wood in order to understand which component makes the main contribution to thermal degradation and which, to a greater extent, stabilizes the fire retardant. Draw appropriate conclusions.

Provide digital photographs of samples when measuring LOI and residual chars, IR-Fourier spectra of pyrolysis products at maximum decomposition rate for unmodified wood composite and fire-resistant wood composites.

5. Conclusions

Indicate which sample has the best properties and where the authors plan to apply the fire-resistant composite.

English needs some improvement

Author Response

Reviewer 3

 

Dear Reviewer,

 

1. Abstract.

 

It is not clear what the authors mean by elemental Si? I think it's silicon oxide. Correction needed.

We revised it; elemental silicon. The EDX elemental mapping suggested the aggregation of elemental silicon but not oxygen, even though the elemental silicon mostly originates from silicon oxide (silica gel). The authors suppose that elemental silicon is a more accurate word than silicon oxide in this sentence.

 

2. Introduction

 

The structural formula of phytic acid should be given.

We highlighted this, in yellow colour, in the Introduction.

 

Compare different classes of flame retardants for their flame retardant effect on different materials. For example, it is possible to indicate why the components chosen by the authors are better than flame retardants based on phosphazenes (10.3390/polym14245334, 10.3390/polym14173592, 10.3390/polym11071191), which, due to the cooperative effect of phosphorus and nitrogen, have an excellent fire retardant effect.

Phosphate-based compounds are generally known to be water-soluble and acidic; which makes them suitable for the purpose of neutralizing the alkali sodium silicate. However, we would like to point out that phosphazene-based compounds, on the other hand, are strong and uncharged bases. While they are commonly dissolved in organic solvents, their water solubility depends upon their structure. Since the wood panel production primarily uses water-borne solutions, the use of organic solvents would require additional ‘recycling’, which obviously is not a practical approach. Therefore, we think that it may not be easy to adapt this type of compound for the current application.

 

It is necessary to explain in more detail why it is necessary to use phytic acid and not other phosphorus acids as an acid catalyst for sodium silicate.

As the trend of utilizing sustainable compounds continues, we think that the bio-based phytic acid is a promising option as a phosphate source. The majority of phosphoric acid is typically derived from minerals, such as phosphorite.

We have emphasized this in the text of the paper.

It should be clarified, due to which the synergistic effect of phytic acid and sodium silicate is expected. Give schemes of the corresponding reactions. I believe that phytic acid is not a catalyst, but a reactant.

We added a new additional reference in the paragraph concerning the Phytic acid.

 

The acid source, e.g. phytic acid, can catalyze the condensation of sodium silicate with the formation of silica gel. We considered this to be an aspect of a ‘’catalyst’’, rather than a ‘’reagent’’ itself.

 

Ammonium nitrate is not a hardener, but a catalyst.

This correction was made in the text. Thank you.

 

4. Results and discussion

 

It is necessary to present in the form of a table the composition of all samples to facilitate the perception of the text.

We thank the reviewer for this suggestion. As for the structure of the analysis, we have provided a brief description of it in the final paragraph of the Introduction. The majority of the analysis in the article is focused on unmodified, 6-FR, 9-FR, and 12-FR wood composites, which are clearly presented in most of the figures. We strongly think that these figures make the understanding of the text more easier.

The conclusions regarding the release of acetic acid are doubtful, firstly because the authors did not determine the presence of acetate groups in wood samples. Second, acetic acid boils at 118 degrees Celsius and samples cure at 160 degrees. On the IR spectra, the bands, presumably corresponding to the acetate carbonyl groups, do not change (within the error). The change in pH, I believe, is due to the increase in the content of phytic acid in the samples.

 

The wood composite was realised from -softwood- Norway spruce particles, which actually contain galactoglucomannan as the principal hemicellulose. When exposed to high temperature, the acetyl group of galactoglucomannans undergoes hydrolysis, leading thus to autohydrolysis of the hemicellulose and the subsequent formation of acetic acid. We agree with the point that the pH of phytic acid could have an influence on the shifting of IR spectra. This text, therefore, was highlighted and revised.

 

In general, the formation of acetic acid is not of interest. The authors should study the resin curing process in more detail depending on the amount of additives, since the mechanical properties are highly dependent on the structure of the polymer matrix. Make an assumption about it, but provide evidence. I believe that it is necessary to conduct IR studies of the original resin, cured resin and resin with additives. Compare the spectra and evaluate the conversion of methylol groups depending on the concentration of additives. For even more accurate confirmation, mechanical testing of the cured resin with and without additives should be carried out. Compare results with wood-filled composites.

Thank you for your suggestion. We appreciate your feedback and will consider it in future research.

However, in this study, our focus was mainly on the effect of fire retardants upon the properties of wood composites, including the possible influence on the panel mechanical properties.

As for the resin curing process, we have considered some speculations, based on the existing literature and our own understanding of the system, but we fully agree that further studies upon the cured resin with and without additives, can provide in the near future more accurate confirmation of the critical effect of additives on the structure and properties of the polymer matrix.

It is necessary to conduct TGA of separately cured resin and wood in order to understand which component makes the main contribution to thermal degradation and which, to a greater extent, stabilizes the fire retardant. Draw appropriate conclusions.

 

Thank you for this valuable suggestion.

The TGA showed that FR additives significantly increased the char residue in a proportion to its concentration. In our previous study, the amino resin itself did not significantly improve (increase) the char residue of the wood (please see; 10.1021/acsomega.1c01044).

 

Provide digital photographs of samples when measuring LOI and residual chars, IR-Fourier spectra of pyrolysis products at maximum decomposition rate for unmodified wood composite and fire-resistant wood composites.

 

Regarding the request for photographs of residual char and LOI, we would like to clarify that while we understand this point, we believe that the numerical value of LOI is the most important result, and therefore we decided not to include photographs in the paper; it is not common also.

As for the TGA-FTIR analysis, we agree that it would be interesting to study the release of gas. However, we would like to inform you that we do not currently have access to the necessary equipment to perform such a study.

 

5. Conclusions

 

Indicate which sample has the best properties and where the authors plan to apply the fire-resistant composite.

The text was revised in that respect.

 

 

We thank all the reviewers for the useful comments.

 

 

Chia-feng LIN, PhD student (LTU) &

George I. MANTANIS, Professor (FIAWS, PhD)

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors claimed "Additionally, the combination of phytic acid and sodium ions from sodium silicate, with the formation of sodium phytate, creates a synergistic effect that suppresses smoke production more effectively than phytic acid alone " in line 333 to 335, however, the study cannot lead to this conclusion.

Author Response

Reviewer 2.

The authors claimed "Additionally, the combination of phytic acid and sodium ions from sodium silicate, with the formation of sodium phytate, creates a synergistic effect that suppresses smoke production more effectively than phytic acid alone " in line 333 to 335, however, the study cannot lead to this conclusion.

 

We agree with the reviewer’s point. We corrected it in the text.

 

 

 

We thank all the reviewers for the useful comments.

 

 

Chia-feng LIN, PhD student (LTU) &

George I. MANTANIS, Professor (FIAWS, PhD)

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

The authors made some changes to the manuscript, but the authors did not provide answers to a number of significant questions.

In its present form, the manuscript cannot be accepted for publication.

    Abstract.

1. Silica gel is a gel based on silicic acids. It is not clear how elemental silicon is formed from them. It is necessary to add a reaction scheme to the appropriate section, which describes the formation of the element and its aggregation.

Introduction

2. The authors added the gross formula of phytic acid. It was necessary to add a structural formula. Fix please.

  3. Comparison of different classes of flame retardants has not been given. The authors indicated that water-soluble flame retardants are used as fire retardants for wood. However, water-soluble flame retardants are practically not used anywhere, since they adsorb moisture from the air, are washed out of the material by water, etc., thereby deteriorating the properties of the material. The literature describes a lot of phosphorus-containing synergistic flame retardants, including those that are insoluble in anything at all. Therefore, you should consider various synergistic flame retardants, conduct a comparative analysis of them and argue why your system is better.

4. The authors claim that phytic acid catalyzes the condensation of sodium silicate, and does not participate in chemical reactions. An appropriate scheme of the condensation reaction should be given and the mechanism of catalysis should be given.

5. The authors also did not provide a diagram of the synergistic action of phytic acid and sodium silicate, which increases the fire resistance of wood.

  Results and discussion

6. The authors describe in detail the hydrolysis of acetate groups in wood, but this does not matter, since the purpose of this study is to study the effect of phytic acid and sodium silicate on the properties of the material. However, the results of the chosen goal are reflected extremely poorly. Since the synergistic system is introduced into the polymer matrix, the study of additives, first of all, should be carried out on a pure polymer. Nevertheless, it is necessary to study the processes taking place in the resin during the introduction of additives. It is necessary to investigate the resulting unfilled materials, evaluate their characteristics and their combustion products. Compare results with wood-filled polymer.

English needs minor editing

Author Response

Reviewer 3.

1. Silica gel is a gel based on silicic acids. It is not clear how elemental silicon is formed from them. It is necessary to add a reaction scheme to the appropriate section, which describes the formation of the element and its aggregation.

This sol-gel reaction was briefly described in the Introduction; for further details, the readers can refer to the cited reference no. 14 (10.1016/j.jcis.2004.04.051).

We added the reaction scheme, in as Figure 1(b). 

 

2. The authors added the gross formula of phytic acid. It was necessary to add a structural formula. Fix please.

We added the structural formula in Figure 1(b). 

 

 

3. Comparison of different classes of flame retardants has not been given. The authors indicated that water-soluble flame retardants are used as fire retardants for wood. However, water-soluble flame retardants are practically not used anywhere, since they adsorb moisture from the air, are washed out of the material by water, etc., thereby deteriorating the properties of the material. The literature describes a lot of phosphorus-containing synergistic flame retardants, including those that are insoluble in anything at all. Therefore, you should consider various synergistic flame retardants, conduct a comparative analysis of them and argue why your system is better.

We would like to clarify that water-soluble fire-retardants, such as phosphate-based, are commonly used in the wood industry, especially for indoor products where water-leaching resistance is much less of a concern. The use of organic-based fire-retardants is not preferred due to the requirement and impracticality of solvent recycling. We agree that a comparative analysis of different flame retardant systems would be informative, but this would be absolutely beyond the scope of our study.

 

4. The authors claim that phytic acid catalyzes the condensation of sodium silicate, and does not participate in chemical reactions. An appropriate scheme of the condensation reaction should be given and the mechanism of catalysis should be given.

We appreciate the reviewer’s point. We added it in Figure 1(b).

 

 

5. The authors also did not provide a diagram of the synergistic action of phytic acid and sodium silicate, which increases the fire resistance of wood.

Although we have provided some information on the mechanism in the fire performance section, we consider that further analysis is needed in order to draw a diagram. We think that the cited reference no. 15 is a good resource respecting the synergistic action of phytic acid and sodium silicate, please see DOI; 10.3390/ma14154164.

 

6. The authors describe in detail the hydrolysis of acetate groups in wood, but this does not matter, since the purpose of this study is to study the effect of phytic acid and sodium silicate on the properties of the material. However, the results of the chosen goal are reflected extremely poorly. Since the synergistic system is introduced into the polymer matrix, the study of additives, first of all, should be carried out on a pure polymer. Nevertheless, it is necessary to study the processes taking place in the resin during the introduction of additives. It is necessary to investigate the resulting unfilled materials, evaluate their characteristics and their combustion products. Compare results with wood-filled polymer.

We appreciate your suggestion to study the additives on a pure polymer and to investigate the processes during the incorporation of additives.

However, as the aim of this study was to investigate the effect of phytic acid and sodium silicate on the properties of wood composites, which are typically produced using amino-based resins and wood particles. The improvement of the fire-retardancy of such material was the starting point of the study. Nonetheless, we take your suggestion into consideration for future studies.

Regarding the hydrolysis of acetate group, this reflects the thermal degradation of wood in high- temperature environments and can lead to changes in the mechanical properties. We believe that investigating  this aspect is important for a comprehensive understanding of the behavior of the composite material.

 

 

We thank all the reviewers for the useful comments.

 

 

Chia-feng LIN, PhD student (LTU) &

George I. MANTANIS, Professor (FIAWS, PhD)

 

Author Response File: Author Response.docx