A Comparative Analysis of Tannin and Commercial Fire Retardants in Wood Fire Protection
and Elaine Cristina Lengowski 2,3,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn the construction of houses and furnishings, the most accessible material to humans is often used - wood. However, this material, while having a number of advantages, also has some disadvantages. For example, wood burns well and quickly loses its properties at high temperatures. The use of fire retardants and special fire-resistant impregnations and special agents is an urgent task. Typically, silicon- or phosphorus-containing compounds are used for such tasks. The use of tannins to impart special properties to wood is not an ordinary solution. The main task of special fire-retardants is to absorb heat, reduce the release of flammable gases, etc. The use of tannins has an undeniable advantage: they are non-toxic. The authors of the manuscript offer several options for processing wood samples. Treatment with a comparative industrial flame retardant is also carried out. After comparing the color characteristics of the treated samples, the thermal behavior of the original materials and wood processed in various ways is studied.
For the control sample, in my opinion, it is necessary to carry out the same treatment as for samples with tannin, etc. Those. treatment with water and drying under the same conditions. As is well known, the history of sample processing affects its structure and, as a consequence, its thermal behavior. Using cellulose as an example, this issue is discussed in this work - https://doi.org/10.1007/s10692-017-9786-x
In Figure 5, the course of the TGA curve is not clear; how do the authors explain such a strange behavior of the system?
L.414. "moisture and absorbed water are lost" is better replaced with "absorbed water are lost"
The conclusions reflect the results obtained in the work, but I still have a question: how will the wood behave after a certain period of time? Will the samples behave the same way after a week or a month? I would also like to see a comparison of the cost of tannin and the industrial design used?!
Author Response
Dear Reviewer,
We are very grateful for the comments and notes, they are certainly valuable contributions.
Attached are the suggestions that we accepted and also the ones that we preferred to contest.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsAfter careful evaluation, I found that this manuscript is suitable for publication in “Forests” after minor revision. The following observations have been made which are required to address in order to improve the quality of the manuscript.
1. Abstract can be rewrite with clear objectives.
2. Indicate the novelty in the end of introduction section.
3. How been the errors calculated for the data presented in Fig. 4?
4. Include statistical data in the conclusion section.
Author Response
Dear Reviewer,
We are very grateful for the comments and notes, they are certainly valuable contributions.
Attached are the suggestions that we accepted and also the ones that we preferred to contest.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsComments for author File: Comments.pdf
Please check your writing and ask a colleague proficient in English to check the language.
Author Response
Dear Reviewer,
We are very grateful for the comments and notes, they are certainly valuable contributions.
Attached are the suggestions that we accepted and also the ones that we preferred to contest.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsAuthors investigated the use of tannin as wood fire-retardant.
The subject is worth of investigation but the article need some major modifications.
Title
- The title of the study must be improved to specify more precisely the focus of the study and not sounds like a review paper.
Introduction :
- l. 78-79 : lack of references about tannins as FR, especially regarding hydrolysable tannins and considering the fact that tanninc acid is not classified as tannin (MW < 500 Da).
- l. 81 : tannins are also present in the wood (xylem)
- Interest of the color of wood products should be presented
Materials and methods:
- The chemical nature and composition of the commercial FR and varnish must be explained. Authors discussed about boron-based in the discussion section but informations about their chemical nature of these products are absent.
- As well as my previous remark, the chemical nature of the tannins used in this study is absent. Please describe the tannins composition considering the high variability of these molecules.
Results
- Control sample is sometime named "CT", and sometime "CR"
- Figure 5- (b) : how did the authors explain the TGA curve at 600°C, considering the fast weigth loss and the shift to lower tempertaure?
- regarding the TGA analysis, it is preferable to compare residual mass of the different samples at the same temperature for all.
Discussion
- l.381: what reaction between tannins and wood occurs here?
- l. 385-389: is boric acid present in the commercial FR?
Author Response
Dear Reviewer,
We are very grateful for the comments and notes, they are certainly valuable contributions.
Attached are the suggestions that we accepted and also the ones that we preferred to contest.
Author Response File: Author Response.pdf
Reviewer 5 Report
Comments and Suggestions for AuthorsThe work presents the use of tannin as fire retardant treatment, which can be classified as sustainable treatment for wood protection. The manuscript shows the result of a research which included the determination of mass gain due to the treatments, color changes, flame evaluation performance and thermal behavior. However, I have several comments regarding the implementation of some of the methods used in the paper. Also, I have doubt regarding the novelty of the work. Tannins have been reported as fire retardant treatments before, by several authors (few examples: https://link.springer.com/article/10.1007/s11998-021-00532-7 , https://www.mdpi.com/2079-6412/11/4/460 , https://link.springer.com/article/10.1007/s11998-020-00440-2 , https://www.scielo.br/j/mr/a/MzBrCVmdLTVQxnrZH3TxnSd/?lang=en ), but the authors do not make mention of that in their introduction and discussion. In consequence, I would like the authors include in the introduction a paragraph stating the novelty of their work and the contribution to the state of the art that their possible findings could imply. In addition, I have specific comments that need to be addressed.
Specific comments:
Line 46: Which one of the methods of evaluation may be more realistic when considering the protection of a building against a fire?
Paragraph in lines 50-63: This information can be found in any book of explaining wood combustion, it really does not add relevant information to support your work. You can either remove it or reduce it as much as possible.
Reorganize paragraph between lines 78 to 90, you may present first tannins and then explain their composition and why they are useful against fire.
Why are you using Simarouba amara? Is this a relevant wood specie? Is it important in wood construction? Add a paragraph clarifying that.
Line 100: why are you using only heartwood? State the reason for that.
Figure 1: it is unnecessary, a good description of the wood samples is enough.
Line 136: C*-color
Line 139, Mass retention: Any reference for this formula? Normally mass retention or mass gain is calculated as a percentage based on anhydrous mass of the sample. In this case there is no control of moisture in the samples, at least it is not mentioned in the manuscript. How do you know the increment is not merely given by the increment in moisture in the wood sample?
Line 159 Burning test: describe further this test. What are the conditions during the test. How do you make sure temperature is always the same in the ignition chamber? Same with air flux during the combustion? What about differences in moisture content in the samples, was that controlled as well? You need to clarify these points.
Lines 198 – 200: How changes in the saturation and hue angle affect the aesthetic value of wood surfaces? Why are they parameters of relevance to be reported along with color change?
Line 219 Mass retention: what was the solid content of the commercial antiflame treatment?
Fig 6a: CR is not a valid acronym for control sample, you said it was CT. Change also in the several point of the text.
Paragraph in lines 307-312 seems to have format problems. The acronym BVT is not correct.
Lines 344, 345: what VI stand for?
Discussion section miss a comparison with other studies dealing with the same subject, add a paragraph on that. Also discuss further how retention of tannin is affecting the results.
Lines 442-445: I don not agree with this, because sample with tannin did ignite.
Paragraph between 463-469 deliver same information than the previous one. Same with information in line 479 to 486.
Lines 471 to 478 deliver a result not a discussion. Move it to the results section.
Conclusion: I do not agree with information delivered in lines 496-497, because the test used was a low scale burning test. In my opinion you can not stand that in an event of fire tannin will be efficient a reducing the spread of fire. A fire is a major event, therefore, saying that the presence of tannin may help to reduce the spread of fire would be a more realistic affirmation based on the result of the study. Other option would be to conclude that presence of tannin reduced the spread of fire in condition of burning with short exposure to flame.
Author Response
Dear Reviewer,
We are very grateful for the comments and notes, they are certainly valuable contributions.
Attached are the suggestions that we accepted and also the ones that we preferred to contest.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe presented version of the manuscript requires minor modifications before publication. For example, it is better to replace the designation “º_” with the traditional “°”.
L. 54-57. Instead of "phase" it is better to use "stage".
In Figure 6, it is better to use the same color curves for the same sample, for example, BT.
L. 348. "vacuum (VT)" the abbreviation can be removed here.
Author Response
R:The presented version of the manuscript requires minor modifications before publication. For example, it is better to replace the designation “º_” with the traditional “°”.
A: We correct ir
R: L. 54-57. Instead of "phase" it is better to use "stage".
A: We correct it.
In Figure 6, it is better to use the same color curves for the same sample, for example, BT.
A: We don’t understand this recommendation, the colors are the same in image A and B
R: L. 348. "vacuum (VT)" the abbreviation can be removed here.
A:Other reviewer ask for us include this abbreviation, because that we decide for kept.
Reviewer 4 Report
Comments and Suggestions for AuthorsNo significant improve regarding the antoriority of previous literrature regarding the use of tannins as flame-retardant, it would have improve the quality of the article regarding chemical mechanism that occurs.
However, the article could be accepted in the present form
Author Response
A: We improve the discussion of the results. The follow texts were included in the discussion of results
“Due to its specific aromatic structure, tannin has high chemical and thermal stability, as well as low thermal conductivity (https://doi.org/10.1021/acssuschemeng.6b02596]. Natural fire resistance may be conferred to aid tree survival by tannins, and this can be attributed mainly to their reactivity similar to phenols, since phenoxy radicals can quench free oxygen radicals when the polymer is broken down during heating [https://doi.org/10.1021/acssuschemeng.8b04851].”
“Basically, the thermal breakdown of tannic acid can be categorized into three distinct phases. In the first phase, occurring between 20 and 182 °C, there is an increase in the rate of thermal decomposition of tannic acid from 100 °C onwards, due to the decomposition of the volatile components (https://doi.org/10.1016/j.tca.2015.02.016; https://doi.org/10.1016/j.jaap.2023.106111). In the temperature range between 182 and 328 °C, there is a significant loss of mass where the outer and inner benzene rings of tannic acid are converted into 3,4,5-trihydroxybenzoic acid (product 1) and 3,4-dihydroxybenzoic acid (product 2) (https://doi.org/10.1016/j.jaap.2023.106111). Product 1, because it has more electron-absorbing groups, decarboxylates more easily, resulting in the production of CO2 and catechol in the appropriate temperature zone. In the third stage, at temperatures between 328 and 700 °C, the central glucose molecules are broken down by the COC, producing CO and CH4 (https://doi.org/10.1016/j.tca.2015.02.016; https://doi.org/10.1016/j.jaap.2023.106111. Product 2 undergoes gradual decarboxylation, releasing CO2 and generating catechol. At high temperatures, catechol is deoxidized to H2O, while the rest of the benzene ring and its branched chains cross-link, promoting the formation of carbon (https://doi.org/10.1016/j.tca.2015.02.016). “
“The intumescent flame-retardant system is widely employed and typically comprises acid, carbon, and air sources [https://doi.org/10.1016/j.polymdegradstab.2022.109841]. The presence of carbon sources depends on both the carbon content and the number of active hydroxyl groups. Tannic) proves to be a suitable carbon source for incorporation into intumescent flame-retardant systems. Flame retardants based on tannic acid have demonstrated their effectiveness in reducing flammability in plastics like polyurethanes, commonly used in construction and packaging. For instance, Xinyi Chen et al. successfully developed self-expanding non-isocyanate polyurethanes based on glucose, utilizing condensed tannin as flame retardants [https://doi.org/10.1016/j.polymdegradstab.2020.109121 ]. Tannins exhibit a high efficiency in producing char during combustion, with condensed tannins yielding 55% char and tannic acid yielding 28% char, thereby forming a protective layer capable of blocking heat, oxygen, and combustible gases [https://doi.org/10.1016/j.jaap.2017.06.003]. Intumescent flame-retardant systems find application in refractory coatings, extensively employed for safeguarding metal structures and, notably, wood substrates [https://doi.org/10.3390/coatings11030280). “
“Tannin-boron wood preservatives intended for outdoor use exhibit notable mechanical strength and fire resistance. Their research highlighted that incorporating tannin as a flame retardant yielded consistently positive results across various fire tests. The inclusion of mimosa tannin in wood conferred a broad spectrum of beneficial flame-retardant effects. Overall, ignition and flame duration were significantly reduced, and weight loss was slower under continuous exposure to fire. (21 à 10.1007/s00107-012-0603-1)”
“The use of the hydrogen bond assembly system has been widely adopted in the construction of tannic assemblies due to its strong cohesion, simple synthesis process, flexibility for structural modifications and predictable recognizability. However, it is important to note that hydrogen bonding is an intermolecular force, and the materials resulting from this association often have poor thermal stability. To improve the thermostability of the matrix, it is possible to combine the hydrogen bonding system with high-efficiency flame retardants (10.3969/j.issn.0253-2417.2022.03.011; https://doi.org/10.1016/j.jaap.2023.106111)”
Reviewer 5 Report
Comments and Suggestions for AuthorsDespite the authors have made several changes in the manuscript in my opinion there are still problems that persist and must be addressed before the article can be accepted for publication.
Introduction must be improved including more information of previous work using tannins to improve fire resistance. To the moment the novelty of the work is basically sustained by the fact that tannins were used in wood specie that have been little studied. However, the authors refused to add a paragraph indicating the relevance of such wood specie. This makes no sense in my opinion.
I suggested changes in the order of the paragraph presenting tannins their composition and why they are useful against fire, but the authors decided to keep the paragraph as it was, which I still do not agree. Editor must decide.
I asked the authors to state why they are using heartwood. I’m ok with their answer, although I would like this to be included in the manuscript because not all people are familiar with the reality of wood industry at Brazil tropical forest.
I asked the authors to remove image 1 but the authors insist to keep it. I suggest they reduce the size; the image is even bigger than the one displayed at the results.
Discussion section must be improved including comparison with works made by other authors. If you do not put your results in context with similar or previous works, how will you be able to notice the contribution to the state of art that your work is doing?
Author Response
R: Despite the authors have made several changes in the manuscript in my opinion there are still problems that persist and must be addressed before the article can be accepted for publication.
R: Introduction must be improved including more information of previous work using tannins to improve fire resistance. To the moment the novelty of the work is basically sustained by the fact that tannins were used in wood specie that have been little studied. However, the authors refused to add a paragraph indicating the relevance of such wood specie. This makes no sense in my opinion.
A: We include some informations in introduction:
The Amazon Forest, situated in South America, stands as the largest equatorial forest globally, spanning an area of 5.5 million km² across nine countries. Brazil notably claims 60% of the Amazonian area (SNIF 2020). Within the Amazon thrive numerous species of towering trees with straight stems, many of which are endemic. It's estimated to harbor around 16,000 species of large trees (10.1126/science.1243092). In Brazil, regulated forest exploitation aims to drive economic, social, and environmental development in the region.
Brazil emerges as a key producer of tropical sawn timber, with the state of Mato Grosso playing a significant role in sector revenue (https://seer.sede.embrapa.br/index.php/RPA/article/view/245/205; https://doi.org/10.31413/nativa.v4i5.3426). Following tree extraction, logs undergo mechanical processing, being refined by the timber industry into rectangular or square pieces primarily used in construction (https://doi.org/10.1590/2179-8087.031116; https://doi.org/10.1007/s10668-020-00797-9). Notable among the Brazilian Amazon timber trade are: Hymenolobium petraeum Ducke, Apuleia leiocarpa J.F. Macbr., Hymenaea courbaril L., Astronium lecointei Ducke., Goupia glabra Aubl., Peltogyne angustifolia Ducke., and Dipteryx odorata (Aubl.) Forsyth f. (https://doi.org/10.1007/s10668-020-00797-9; http://dx.doi.org/10.5380/biofix.v7i1.82333).
Categorizing timber based on its intended use allows for optimal applications, whether for indoor or outdoor settings, structural or otherwise, taking into account market value (https://doi.org/10.1590/01047760201925042650). Simarouba amara (Aubl.) wood, among the species sourced from sustainable forest management, ranks among the most traded in the state. However, it's often utilized in less prestigious roles such as crates, plywood, charcoal, owing to limited knowledge of its properties.
R: I suggested changes in the order of the paragraph presenting tannins their composition and why they are useful against fire, but the authors decided to keep the paragraph as it was, which I still do not agree. Editor must decide.
R: I asked the authors to state why they are using heartwood. I’m ok with their answer, although I would like this to be included in the manuscript because not all people are familiar with the reality of wood industry at Brazil tropical forest.
A: We include the information: The accumulation of extracts that increase the flammability of wood occurs in heartwood, in addition, the greatest difficulty in impregnation occurs in the heartwood region, so if we can improve the wood in this region, we will also improve the sapwood. Furthermore, when it comes to construction timber from Brazilian tropical forests, there is no commercialization of sapwood.
R: I asked the authors to remove image 1 but the authors insist to keep it. I suggest they reduce the size; the image is even bigger than the one displayed at the results.
A: We reduce the size of image.
R: Discussion section must be improved including comparison with works made by other authors. If you do not put your results in context with similar or previous works, how will you be able to notice the contribution to the state of art that your work is doing?
A: We include following discussion of results.
Due to its specific aromatic structure, tannin has high chemical and thermal stability, as well as low thermal conductivity (https://doi.org/10.1021/acssuschemeng.6b02596]. Natural fire resistance may be conferred to aid tree survival by tannins, and this can be attributed mainly to their reactivity similar to phenols, since phenoxy radicals can quench free oxygen radicals when the polymer is broken down during heating [https://doi.org/10.1021/acssuschemeng.8b04851].
Basically, the thermal breakdown of tannic acid can be categorized into three distinct phases. In the first phase, occurring between 20 and 182 °C, there is an increase in the rate of thermal decomposition of tannic acid from 100 °C onwards, due to the decomposition of the volatile components (https://doi.org/10.1016/j.tca.2015.02.016; https://doi.org/10.1016/j.jaap.2023.106111). In the temperature range between 182 and 328 °C, there is a significant loss of mass where the outer and inner benzene rings of tannic acid are converted into 3,4,5-trihydroxybenzoic acid (product 1) and 3,4-dihydroxybenzoic acid (product 2) (https://doi.org/10.1016/j.jaap.2023.106111). Product 1, because it has more electron-absorbing groups, decarboxylates more easily, resulting in the production of CO2 and catechol in the appropriate temperature zone. In the third stage, at temperatures between 328 and 700 °C, the central glucose molecules are broken down by the COC, producing CO and CH4 (https://doi.org/10.1016/j.tca.2015.02.016; https://doi.org/10.1016/j.jaap.2023.106111. Product 2 undergoes gradual decarboxylation, releasing CO2 and generating catechol. At high temperatures, catechol is deoxidized to H2O, while the rest of the benzene ring and its branched chains cross-link, promoting the formation of carbon (https://doi.org/10.1016/j.tca.2015.02.016).
The intumescent flame-retardant system is widely employed and typically comprises acid, carbon, and air sources [https://doi.org/10.1016/j.polymdegradstab.2022.109841]. The presence of carbon sources depends on both the carbon content and the number of active hydroxyl groups. Tannic) proves to be a suitable carbon source for incorporation into intumescent flame-retardant systems. Flame retardants based on tannic acid have demonstrated their effectiveness in reducing flammability in plastics like polyurethanes, commonly used in construction and packaging. For instance, Xinyi Chen et al. successfully developed self-expanding non-isocyanate polyurethanes based on glucose, utilizing condensed tannin as flame retardants [https://doi.org/10.1016/j.polymdegradstab.2020.109121 ]. Tannins exhibit a high efficiency in producing char during combustion, with condensed tannins yielding 55% char and tannic acid yielding 28% char, thereby forming a protective layer capable of blocking heat, oxygen, and combustible gases [https://doi.org/10.1016/j.jaap.2017.06.003]. Intumescent flame-retardant systems find application in refractory coatings, extensively employed for safeguarding metal structures and, notably, wood substrates [https://doi.org/10.3390/coatings11030280).
Tannin-boron wood preservatives intended for outdoor use exhibit notable mechanical strength and fire resistance. Their research highlighted that incorporating tannin as a flame retardant yielded consistently positive results across various fire tests. The inclusion of mimosa tannin in wood conferred a broad spectrum of beneficial flame-retardant effects. Overall, ignition and flame duration were significantly reduced, and weight loss was slower under continuous exposure to fire. (21 à 10.1007/s00107-012-0603-1)
The use of the hydrogen bond assembly system has been widely adopted in the construction of tannic assemblies due to its strong cohesion, simple synthesis process, flexibility for structural modifications and predictable recognizability. However, it is important to note that hydrogen bonding is an intermolecular force, and the materials resulting from this association often have poor thermal stability. To improve the thermostability of the matrix, it is possible to combine the hydrogen bonding system with high-efficiency flame retardants (10.3969/j.issn.0253-2417.2022.03.011; https://doi.org/10.1016/j.jaap.2023.106111)
