Physical and Mechanical Properties of Poplar Wood Modified by Glucose-Urea-Melamine Resin/Sodium Silicate Compound

Round 1

Reviewer 1 Report

The paper investigates the impregnation modification of poplar wood with a mixture of glucose, urea, melamine, and sodium silicate. First, an organic resin was prepared by heating an aqueous, acidic solution of glucose to which the amino compounds and some minor components are added. For modification, wood specimens were impregnated with sodium silicate or mixtures of sodium silicate and the organic resin in a vacuum-pressure process and, subsequently, dried at temperatures up to 103 °C.

Modification with thermosetting resins such as melamine-formaldehyde and urea-formaldehyde can improve certain wood properties. Formaldehyde emissions of the modified wood and the use of fossil resources are major drawbacks of these treatments. Therefore, substituting glucose for formaldehyde would greatly improve the treatments. It remains unclear to me, what the authors expect from combining the organic resin with sodium silicate. Nevertheless, the topic is of relevance and general interest to the readers of the journal.

Unfortunately, I think that the paper has so many weak points that it needs a fundamental revision. These are the main points with a few examples:

• The paper does not provide sufficient information, for example:
  • Itaconic acid, boric acid, sodium hydroxide, hydrochloric acid are mentioned in the materials section (2.1), but it is unclear what they are used for.
  • In 2.2 it is mentioned that a ‘curing agent’ is added without stating the substance.
  • Physical and mechanical properties are determined according to Chinese standards. The standards should be cited in the literature and a basic description of the procedures should be added to the ‘Methods’-section.
  • Anatomical directions for mechanical tests are not stated.
  • Number of replicates for mechanical tests is missing.
• The paper involves some speculation, i.e. conclusions are drawn that are not backed by experimental results.
  • It is inferred from the IR spectrum and the colour change of the solution that glucose has been predominantly converted to HMF. However, no IR peak is assigned to the substance (e.g. the furan ring). Also, there are many more potential brown substances originating from caramelization and Maillard-reactions.
  • I agree that bulking indicates cell wall penetration of the resin. But I do not think that the cell walls of the treated wood are obviously thicker on the REM pictures.
  • Small differences of the mechanical properties probably do not allow conclusions on cell wall penetration and cross-linking of the resin, because they might be caused by simple lumen filling.
• The experimental design is incomplete:
  • It would have been very useful (and quite obvious) to include a control that is treated with the organic resin only.
  • The wood was treated in dimensions that were larger than the specimens used for testing. As the resin is usually unevenly distributed within the wood, specimens should be cut systematically, and the layout should be considered in the analysis. This is especially true, because it is assumed that the specimens are not completely treated but include untreated areas.
• The paper is not well written:
  • Words are used in a wrong (or at least ambiguous) way.
    • The solution that is prepared from melamine, urea and glucose is called ‘silicone’ resin, but the formation of silicones (= siloxanes) is impossible.
    • ‘Permeability’ is used to describe the ability of the solution to penetrate wood. However, it is a measure of the ability of a porous material (i.e. the wood) to allow fluids to pass through it.
  • There are verbose parts, e.g. lines 185 to 193.
  • linguistic errors
    • ‘modulus of rapture’, ‘fully swelled’
    • grammatical errors
  • The extensive use of imperative is untypical for research papers.

Author Response

Response to Reviewer 1 Comments

Point 1: Itaconic acid, boric acid, sodium hydroxide, hydrochloric acid are mentioned in the materials section (2.1), but it is unclear what they are used for. In 2.2 it is mentioned that a ‘curing agent’ is added without stating the substance.

Response 1: Itaconic acid and boric acid are curing agents. Sodium hydroxide and hydrochloric acid are used to adjust the pH at the end of the reaction, which has been supplemented in lines 86-98 in the text.

 

Point 3: Physical and mechanical properties are determined according to Chinese standards. The standards should be cited in the literature and a basic description of the procedures should be added to the ‘Methods’-section.

Response 3: Thank you for your valuable comments. We have added specific experimental method information in lines 121-122, and added test standard references in lines 336-353.

 

Point 4: Anatomical directions for mechanical tests are not stated.

Response 4: During the experiment, the size and anatomical direction of the test piece are set according to the test standard, and supplementary explanations are made in lines 125-127 in the text.

 

Point 5: Number of replicates for mechanical tests is missing.

Response 5: The number of repetitions of the mechanics and density test of the specimen are all 15, which has been explained in line 127 in the text.

 

Point 6: The paper involves some speculation, i.e. conclusions are drawn that are not backed by experimental results. It is inferred from the IR spectrum and the colour change of the solution that glucose has been predominantly converted to HMF. However, no IR peak is assigned to the substance (e.g. the furan ring). Also, there are many more potential brown substances originating from caramelization and Maillard-reactions.

Response 6: Thank you for your valuable suggestions. I agree with you. This is a complicated reaction process including caramelization and Maillard. The conversion rate of glucose to 5-HMF is low. Although the infrared absorption peak is not significant, it is still visible. In order to describe it more accurately, we added ‘The two peaks appeared at 1561 cm^-1 and 775 cm^-1 can be attributed to conjugated C=C bond and skeletal vibration of 2,5-disubstituted furan rings, respectively’ in lines 147-149 in the text.

 

Point 7: I agree that bulking indicates cell wall penetration of the resin. But I do not think that the cell walls of the treated wood are obviously thicker on the SEM pictures.

Response 7: Thank you for your valuable suggestions. ‘Cell wall thickening’ here is to emphasize: the adhesion and filling effect of the modifier on the wood cell wall and cell cavity wall. Since this description might be inappropriate, it has been deleted.

 

Point8: Small differences of the mechanical properties probably do not allow conclusions on cell wall penetration and cross-linking of the resin, because they might be caused by simple lumen filling.

Response 8: Thank you for your suggestion. We are working on the modification mechanism. In this paper, we mainly want to discuss the performance changes of wood before and after modification. According to your suggestion, the conclusion has been changed to ‘The filling effect of the compound modifier on wood cell walls and lumens improved the mechanical properties of the modified wood’.

 

The experimental design is incomplete:

Point 9: It would have been very useful (and quite obvious) to include a control that is treated with the organic resin only.

Response 9: Thank you for your suggestion. In this paper, we mainly tried to improve the size stability of sodium silicate inorganically modified wood through compounding with organic resin, we will consider your valuable suggestions in the following experiments.

 

Point 10: The wood was treated in dimensions that were larger than the specimens used for testing. As the resin is usually unevenly distributed within the wood, specimens should be cut systematically, and the layout should be considered in the analysis. This is especially true, because it is assumed that the specimens are not completely treated but include untreated areas.

Response 10: Thank you for your valuable suggestions. The physical and mechanical specimens used in this research are all made in accordance with the corresponding national standards. Poplar has good permeability. Under vacuum-pressure impregnation, the weight gain rates of specimens of different sizes are almost the same, indicating that they have been fully penetrated. In line 101 of the article, it has been added that ‘Before impregnation, all samples were processed into standard sizes’.

 

The paper is not well written:

Point 11: Words are used in a wrong (or at least ambiguous) way.

The solution that is prepared from melamine, urea and glucose is called ‘silicone’ resin, but the formation of silicones (= siloxanes) is impossible.

Response11: This is caused by a writing error in the text. The solution prepared by glucose urea and melamine is called glucose-urea-melamine resin (MUG). First, MUG is prepared, and then MUG, sodium silicate and distilled water are mixed at 40°C to prepare waterborne silicone resin (G₁₀S₂₀ and G₂₀S₁₀).

 

Point 12: ‘Permeability’ is used to describe the ability of the solution to penetrate wood. However, it is a measure of the ability of a porous material (i.e. the wood) to allow fluids to pass through it.

Response 12: Thank you for your valuable suggestions for making us aware of this problem. In line 272 and 280, we have changed the ‘permeability of the modifier’ to ‘It can fully penetrate into the wood tissue’and ‘MUG resin has better ability to penetrate wood’ according to your suggestions.

 

Point 13: There are verbose parts, e.g. lines 185 to 193.

Response 13: These sentences have been simplified according to your opinion.

 

Point 14:

linguistic errors:‘modulus of rapture’,‘fully swelled’.

Response 14 Thank you for your careful review. It is a slip of pen. ‘Modulus of rapture’ has been corrected as ‘modulus of rupture’, ‘the cell wall is fully swelled’ has been changed to ‘the cell wall is swelled’.

 

Point 15:

Grammatical errors: The extensive use of imperative is untypical for research papers.

Response15: Thank you for your careful guidance. Your suggestions have made us progress. According to your comments, the sentence and grammar of the article have been revised in detail.

Author Response File: Author Response.docx

Reviewer 2 Report

The objective of this manuscript was to develop an environmentally friendly inorganic-organic hybrid wood modifier to improve the mechanical performance of poplar wood

The paper is innovative and the topic is interesting, although the price of this technology might not compensate in comparison with other wood modification technologies (doi: 10.3832/ifor2380-010).

The introduction section needs to be reorganized. Some sentences must be organized from line 47.

The synthesis procedure of the waterborne silicone resin is somehow confusing. The procedure for determination of water solubility of resin is not indicated. The size of test pieces that were impregnated is not indicated. The combination of MUG resin with sodium silicate solution was tested for G10S20 or G20S10. It is not explained why these formulations were tested. Why G20S20 was not prepared ?

The results are not always well explained.

It is indicated that “From Table 1, the densities of S20, G10S20 and G20S10 treated 170 specimens are 0.529, 0.522, 0.515 g·cm-3, respectively, all higher than the untreated wood. The highest density of S20 treated wood is caused by its severe shrinkage and smaller volume”. These differences are statistically significant ? In table 1 it is not indicated what is the control ? Non-treated solid poplar wood ?

In Fig 3, the lines between points do not make sense, because this figure does not present an evolution with the quantity of the quantity of sodium silicate.

In line 188 please explain “Among the modified wood, G20S10 has the smallest swelling rate, indicating that its size is the most stable”. Why for G10S20 formulation the swelling coefficients are similar to the control, with no stability improvement ?

In line 199 it is written “Some hydroxyl groups are cross-linked and the hygroscopic groups are reduced”. Which proofs are presented to justify this conclusion?

In line 205 it is written “cellulose microfibrils in the internal structure play a role in supporting the wood”. Please rewrite the sentence because it is not correct. In fact cellulose microfibrils plays a support function in the tree.

In line 208, what is the meaning of “high content of colloidal wood fibers”?

In line 226 it is written “It shows that the organic-inorganic synergistic effect produced by the compound of sodium silicate solution and glucose resin makes it easier for the modifier to penetrate the wood”. Why ?

In the conclusions the sentence in line 261 “the MUG resin has good permeability, and the composite package and consolidation of Na2SiO3 by the MUG resin achieves the effect of the wood cell wall expansion and restriction” is very confusing. Please explain better.

Some minor revisions are also indicated in the manuscript.

 

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 2 Comments

Point 1: The introduction section needs to be reorganized. Some sentences must be organized from line 47. Introduction

Response 1: Thank you for your suggestion to make the article more logical, and the language has been reorganized as required.

 

Point 2: The synthesis procedure of the waterborne silicone resin is somehow confusing. The procedure for determination of water solubility of resin is not indicated. The size of test pieces that were impregnated is not indicated. The combination of MUG resin with sodium silicate solution was tested for G₁₀S₂₀ or G₂₀S₁₀. It is not explained why these formulations were tested. Why G20S20 was not prepared? 

Response 2: 1. First, MUG resin was prepared. Then its water solubility was measured according to national standard GB/T 14074-200697. The method has been described as ‘its water solubility was tested with a graduated glass tube of 10ml. When 1 ml resin was fully blended with 9 ml distilled water at 23 °C, the blend solution is still clear without precipitation, and the water solubility multiple of the resin is considered to be more than 9 in lines 89-92.

2. In line 101 of the article, it has been added that ‘The size of the impregnated specimen was the size of the according standard specimen’. The size and quantity of impregnated specimens are described in paragraph 2.4 of the text.
3. According to the practical application experience, the treatment effect is better when the mass fraction of the treatment solution is 30%. Therefore, different proportions of resin and sodium silicate solution are prepared mainly based on the mass fraction of the common solution of 30%. As wood treated with 30% sodium silicate solution has great shrinkage, a smaller mass fraction is used.

 

Point 3: It is indicated that “From Table 1, the densities of S₂₀, G₁₀S₂₀ and G₂₀S₁₀ treated 170 specimens are 0.529, 0.522, 0.515 g·cm^-3, respectively, all higher than the untreated wood. The highest density of S₂₀ treated wood is caused by its severe shrinkage and smaller volume”. These differences are statistically significant? In table 1 it is not indicated what is the control? Non-treated solid poplar wood?

Response 3: All densities were tested according to the standard GB/T 1933-2009, and the statistical average of 15 samples was taken. It is a common phenomenon that S₂₀ treated wood has severe shrinkage and smaller volume. These differences are statistically significant. In table 1 the control is the untreated poplar wood which is listed in the first row of the table.

 

Point 4: In Fig 3, the lines between points do not make sense, because this figure does not present an evolution with the quantity of the quantity of sodium silicate.

Response 4: In Figure 3, it has been changed to a histogram showing changes in the data according to your suggestion.

 

Point 5: In line 188 please explain “Among the modified wood, G₂₀S₁₀ has the smallest swelling rate, indicating that its size is the most stable”. Why for G₁₀S₂₀ formulation the swelling coefficients are similar to the control, with no stability improvement? Response 5: When the swelling rate is smaller, i.e. the size change of wood is smaller, so the wood dimensional stability is better. Figure 4 shows that the swelling coefficient of G₁₀S₂₀ is slightly higher than that of control, indicating that its dimensional stability is worse than that of the control, which is due to the high hygroscopicity of its more sodium silicate.

 

Point 6: In line 199 it is written “Some hydroxyl groups are cross-linked and the hygroscopic groups are reduced”. Which proofs are presented to justify this conclusion? Response 6: Thank you for your suggestion. Considering the size change of wood is mainly caused by its hygroscopic group, we speculate this conclusion from its great decreased hygroscopicity. In order to ensure the preciseness of the paper, this sentence has been deleted.

 

Point 7: In line 205 it is written “cellulose microfibrils in the internal structure play a role in supporting the wood”. Please rewrite the sentence because it is not correct. In fact cellulose microfibrils plays a support function in the tree.

Response 7: Thanks for your suggestion, it has been modified in line 225.

 

Point 8: In line 208, what is the meaning of “high content of colloidal wood fibers”? Response 8: The ‘colloidal wood fibers’ in the article is ‘gelatinous fiber’. Line 227 of the article has been modified. In fact, ‘gelatinous fiber’ refers to wood fibers that have not been lignified and the inner wall of the cell cavity is gelatinous. The high content of gelatinous fiber in fast-growing poplar is a growth defect.

 

Point 9: In line 226 it is written “It shows that the organic-inorganic synergistic effect produced by the compound of sodium silicate solution and glucose resin makes it easier for the modifier to penetrate the wood”. Why?

Response 9: It can be seen from table 1 that the WPG of G₂₀S₁₀ treated wood is higher than that of G₁₀S₂₀ with the same mass fraction of the solution, so we speculate that the more resin might improve this. In order to ensure the preciseness of the paper, this sentence has been deleted.

 

Point 10: In the conclusions the sentence in line 261 “the MUG resin has good permeability, and the composite package and consolidation of Na₂SiO₃ by the MUG resin achieves the effect of the wood cell wall expansion and restriction” is very confusing. Please explain better.

Response 10: Thank you for your valuable suggestions, it has been changed as ‘MUG resin has better ability to penetrate wood, and the waterborne glucose silicone resin will bulk wood cell wall and fill cell lumen, significantly reduces the shrinkage and moisture absorption of the Na₂SiO₃ modified wood, and greatly improves its dimensional stability.’ in the text at lines 272-275.

Reviewer 3 Report

This paper presents an interesting approach. This paper covers well the state of the art, the descriptinon of the objectives and the discussion of the results.

I recommend following changes:

Introdution: between line 47 and 59: it is not clear what you want to introduce. Please rewrite and try to be more clear.

Please also reformulate the chapter methods in a more scientific way, avoiding terms like put, .... 

 

Author Response

Response to Reviewer 3 Comments

Point 1: Introduction: between line 47 and 59: it is not clear what you want to introduce. Please rewrite and try to be more clear.

Response 1: Thank you for your suggestion. The introduction has been revised, mainly to introduce the development and problems of resin or sodium silicate impregnation for wood modification. Then the compound modification of sodium silicate and glucose resin is put forward to solve these problems. Then the properties of the compound modified wood is discussed in this study.

 

Point 2: Please also reformulate the chapter methods in a more scientific way, avoiding terms like put....

Response 2: Thank you for your suggestions, we have made changes in section 2.2 of the text.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear authors,

I think that the revised paper still needs some changes/clarification:

• ‘The term silicones is used for compounds in which silicon atoms are linked via oxygen atoms, each silicon atom bearing one or several organic groups’ (ULLMANN'S Encyclopedia of Industrial Chemistry). Please make clear if you expect that these compounds form upon mixing sodium silicate and MUG. If so, please explain why; e.g. by citing literature in which silicone has been synthetized in the same way or by detecting the respective structures in the mixture. If the term does not refer to the structure described above, but is rather used to describe a mixture of water glass and organic compounds, please add this definition.
• line 60: Do you really mean ‘silicide’. Maybe the term ‘silicon compounds’ would be better.
• line 74: There is a typo: ‘radical’
• line 74: I find it confusing that it is referred to the wood blocks as specimens. Actually, the specimens are cut from the wood block. Therefore, the dimensions given 2.1 seem irrelevant and may be removed.
• lines 168-173: Without leaching, the higher WPG of G20S10 does not indicate a better fixation. Also, the WPG does not allow to draw any inferences on crosslinking.
• line 187: I think, it should be ‘swollen’ instead of swelled.
• lines 204-212: I suggest to condense this paragraph into one or two sentences, because it mainly consists of trivial statements like: smaller swelling rate indicates more stable size; large swelling rate means low ASE; swelling increases in order radial, tangential, volume. Also, it is unnecessary to repeat the figures from the table in the text.
• lines 214-222: The paragraph is printed twice.

Author Response

Response to Reviewer  Comments

Point 1: ‘The term silicones is used for compounds in which silicon atoms are linked via oxygen atoms, each silicon atom bearing one or several organic groups’ (ULLMANN'S Encyclopedia of Industrial Chemistry). Please make clear if you expect that these compounds form upon mixing sodium silicate and MUG. If so, please explain why; e.g. by citing literature in which silicone has been synthetized in the same way or by detecting the respective structures in the mixture. If the term does not refer to the structure described above, but is rather used to describe a mixture of water glass and organic compounds, please add this definition.

Response 1: Thank you for your suggestion. The “waterborne glucose silicone resin” in this article is to express the compound synergistic effect of organic MUG resin and inorganic sodium silicate. We agree with your opinion. We have changed the “waterborne glucose silicone resin” in the article to “MUG resin / sodium silicate compound modifier”.

 

Point 2: line 60: Do you really mean ‘silicide’. Maybe the term ‘silicon compounds’ would be better.

Response 2: Special thanks to you for your comments. We agree with your suggestion and have changed ‘silicide’ to ‘silicon compounds’ in line 60 of the text.

 

Point 3: line 74: There is a typo: ‘radical’.

Response 3: Thank you for your careful review. It is a slip of pen. In the 115 line of the text, ‘radical’ has been corrected as ‘radial’.

 

Point 4: I find it confusing that it is referred to the wood blocks as specimens. Actually, the specimens are cut from the wood block. Therefore, the dimensions given 2.1 seem irrelevant and may be removed.

Response 4: Thank you for your careful guidance. We have deleted ‘specimen size is 300 × 25 × 25 mm³’ in line 73-74 of the text.

 

Point 5: lines 168-173: Without leaching, the higher WPG of G₂₀S₁₀ does not indicate a better fixation. Also, the WPG does not allow to draw any inferences on crosslinking.

Response 5: Thank you for your valuable suggestions. We want to express that the higher the WPG of the modifier, the better its penetration ability into wood. The conclusion on crosslinking and fixation has been deleted in line 170-174 of the text.

 

Point 6: line 187: I think, it should be ‘swollen’ instead of swelled.

Response 6: Thank you for your careful guidance. ‘Swelled’ has been corrected as ‘swollen’ in the 188 line of the text.

 

Point 7: lines 204-212: I suggest to condense this paragraph into one or two sentences, because it mainly consists of trivial statements like: smaller swelling rate indicates more stable size; large swelling rate means low ASE; swelling increases in order radial, tangential, volume. Also, it is unnecessary to repeat the figures from the table in the text.

Response 7: Thank you for your guidance. These sentences have been simplified according to your opinion.

 

Point 8: lines 214-222: The paragraph is printed twice.

Response 8: Thank you for your advice. The repeated paragraph has been deleted.

Author Response File: Author Response.docx

Reviewer 2 Report

Most of the revisions were done according to the reviewer’s comments.

However, the term “Waterborne Glucose Silicone resin” must substituted by other, e.g “an organic-inorganic modifier” or a "melamine-urea-glicose resin modified with sodium silicate". Silicone resins are a type of silicone material which is formed by branched, cage-like oligosiloxanes with the general formula of RnSiXmOy. In the manuscript, it is is written that “Waterborne glucose silicone resin (G20S10, G10S20) were prepared by blending MUG and sodium silicate at 40 °C”. No evidence is presented to prove that a structure like that is formed. The structure of MUG resin was investigated by FTIR, but the modification of MUG resin with the silicate was not studied. 

Some minor revisions are also indicated in the manuscript.

 

Comments for author File: Comments.docx

Author Response

Response to Reviewer  Comments

Point 1: However, the term “Waterborne Glucose Silicone resin” must substituted by other, e.g “an organic-inorganic modifier” or a "melamine-urea-glucose resin modified with sodium silicate". Silicone resins are a type of silicone material which is formed by branched, cage-like oligosiloxanes with the general formula of RnSiXmOy. In the manuscript, it is is written that “Waterborne glucose silicone resin (G₂₀S₁₀, G₁₀S₂₀) were prepared by blending MUG and sodium silicate at 40 °C”. No evidence is presented to prove that a structure like that is formed. The structure of MUG resin was investigated by FTIR, but the modification of MUG resin with the silicate was not studied.

Response 1: Special thanks to you for your comments. The “waterborne glucose silicone resin” in this article is to express the compound synergistic effect of organic MUG resin and inorganic sodium silicate. We agree with your suggestion because there is no data to prove the existence of silicone resin structure in the article. We have changed the “waterborne glucose silicone resin” in the article to “MUG resin / sodium silicate compound modifier”.