Bonding of Selected Hardwoods with PVAc Adhesive

Round 1

Reviewer 1 Report

The manuscript entitled „ Adhesion of selected hardwoods with PVAc glue” aims to demonstrate the adhesion properties of hardwoods with PVAc glue based on its surface properties, wettability and SFE.

Based on the presented information, I have to say that the experiment, mainly the SFE determination is set incorrectly. If the foundations for determination of SFE are set incorrectly, the results are questionable due to setting the output correlations.

Line 156: mL not ml.

Lines 155 – 164: how many measurements of water droplets for determination of contact angle were performed for each sample?

Lines 155-167: Which method was ude for SFE determination? OWRK, Wu, Acid base…? How many test liquids were used for the evaluation of the SFE? I can see only water, is that true? If is, than this part of the experiment is not relevant and cannot be taken into account. The SFE of solids should be calculated from at least three different test liquids, i.e. the liquids with different surface tension, especially for a chemically heterogeneous material like wood. In addition, when calculating the SFE of solids, the surface tensions of the test liquids should be stated since they may differ within used methods.

The mentioned references are rather “old” [43, 44] and are not applicable [44](for ice). Additionally, in the reference [43], the SFE was calculated from three liquids. The new research and methods describing the determination of SFE wood are available today. I suggests the authors to study new references regarding determination of SFE of wood and correct this part according to new methods.

Author Response

ANSWER TO REVIEWER 1:

First, we would like to express our thanks to the reviewer, who checked the manuscript thoroughly and completed its job with several valuable comments. We highly appreciate each comment or recommendation that could help to improve the manuscript.  

The manuscript entitled „ Adhesion of selected hardwoods with PVAc glue” aims to demonstrate the adhesion properties of hardwoods with PVAc glue based on its surface properties, wettability and SFE.

Based on the presented information, I have to say that the experiment, mainly the SFE determination is set incorrectly. If the foundations for determination of SFE are set incorrectly, the results are questionable due to setting the output correlations.

• On the base of your recommendation and with the aim to don’t give to readers a possible incorrect information, the part related to SFE we removed from the manuscript. A more detailed answer is given in the part of answers to the comments for Lines 155-167 and for References 43 and 44..

Line 156: mL not ml.

• It was corrected.

Lines 155 – 164: how many measurements of water droplets for determination of contact angle were performed for each sample?

• We performed 5 measurements for 6 samples / replicates of each hardwood species, i.e. the mean values of contact angles are from 30 measurements (it is mentioned in Note to Tab. 3).

Lines 155-167: Which method was ude for SFE determination? OWRK, Wu, Acid base…? How many test liquids were used for the evaluation of the SFE? I can see only water, is that true? If is, than this part of the experiment is not relevant and cannot be taken into account. The SFE of solids should be calculated from at least three different test liquids, i.e. the liquids with different surface tension, especially for a chemically heterogeneous material like wood. In addition, when calculating the SFE of solids, the surface tensions of the test liquids should be stated since they may differ within used methods.

The mentioned references are rather “old” [43, 44] and are not applicable [44](for ice). Additionally, in the reference [43], the SFE was calculated from three liquids. The new research and methods describing the determination of SFE wood are available today. I suggests the authors to study new references regarding determination of SFE of wood and correct this part according to new methods.

• I (the author No 4) have been committed by the authors No 1 and No 2 of the manuscript to address the answers on these comments of reviewer. I will tackle two problems: wood wetting with liquids and surface free energy evaluation. I apologize, but the explanation cannot be very brief.
• Considering the wetting as such, it is necessary to remember that wood has a porous-capillary structure and hydrophilic character. Consequently, when wood surface is wetted with liquids, it is not possible to determine experimentally the contact angle corresponding to the equilibrium conditions stated by the Young equation. We have already had experience with addressing the wood wetting from the past when we were focusing on quantitative and qualitative evaluation of the surface wetting process in various wood species. Our study finding at that time was that the contact angle values measured at the moment of liquid drop application onto the wood surface q₀ and the values at the moment of the advancing angle reversing to a receding one („equilibrium“  contact angle q_e) can be used for calculating the contact angle value for an absolute smooth surface θ_w. We suggest this angle as the most suitable for calculating the values of surface free energy and the components of this energy. The paper comprises the values of all three angles, as each angle has its own descriptive significance.
•   I agree with your statement that there are several methods (specified appropriately in your review) for surface free energy quantification. The given methods for thermodynamic characteristics calculation differ in the mathematic tools implemented and the number of liquids necessary to perform the calculations. In our case when water was used as a single wetting liquid, the corresponding surface free energy calculations could be performed with using the equation provided by Neuman et al. (1974). Accordingly, the disperse and polar components were calculated with the aid of equations proposed by Kloubek (1974). I have personally verified the correctness of deriving these equations and I can declare that they are O.K. - both in terms of mathematic and physics. Nevertheless, I must avow that two liquids would be a better approach also in this case. The types of the two liquids as well as cases when a single liquid is sufficient have been specified in the paper: Kúdela, 2014: Wetting of wood surface by liquids of a different polarity. Wood Research, 59(1): 11-24.
• I am convinced that the research results concerning the surface free energy values also represent valuable information in context of assessing the adhesive adhesion to wood. However, the data related have been excluded from the manuscript, after considering the objections of the reviewer.   

Author Response File: Author Response.pdf

Reviewer 2 Report

The article entitled “Adhesion of selected hardwoods with PVAc glue” has as the main argument the bonding of wood and aims to investigate the effect of selected characteristics of 13 hardwoods on the adhesion strength. The adhesion strength in dry state was not influenced by the density, cold-water extract and PH-value, but it was influenced by their lower roughness, lower water contact angles, and their higher surface free energies.

The comments are outlined below:

 

Line 15: it would be better to write “crucial” instead of “important” to reinforce the concept.

Line 19: It is preferred to not identify the density together with pH-value, wettability and roughness as only “structural characteristics”. Please, write “physical and structural characteristics” as it’s written in line 202 correctly.  

Line 44: Change “which play” with “that play..”

Line 35: It is better to not start the sentence with “..For glued wood products…”. It’s possible to start with “With regards to the glued wood products..”

Line 61: In the sentence “recommended to perform bonding process” please, add “the” before the word “bonding process”.

Line 62: At the end of line 62, it’s preferred to write “within the wooden structure”, otherwise the sentence turns out to be redundant. 

Lines 326-327-328: The sentence is too long. It is better to split it into two sentences to make it more understandable.

This article offers extensive and valid evidence. It is advisable to write more general information about PVAc adhesive, e.g. the demand for such adhesive, thus its importance and it’s recommended to add that information between lines 57 and 58. The chapter of a book indicated as follows might help to write more about PVAc.

• Ülker, O. (2016). Wood adhesives and bonding theory. Adhesives, IntechOpen, London, 271-288

The methodology is clear. The results are well discussed and supported by a quite comprehensive and thorough literature search. While there are broader implications when research work concerns wood bonding, the bibliography of the article may be sufficient to support this study. The style in which it is written should be improved, for example by finding synonyms for the word "important" otherwise many sentences are redundant.

The weakest part of the manuscript is related, in my opinion, to the lack of Anova test able to clarify the statistical difference among wood species. Dataset reported in Figure 3, can be considered the main part of the manuscripts but the few numbers of specimens submitted to test (6) could make the discussion and conclusion not appropriate (also checking evidence reported in figure 3).

I also noted in Table 4 (and correspondent figures) a low R2 value and I'm thinking that the statistical approach was not completely appropriate. By using 13 value (I imagine they come from the average dataset of each wood species) inevitably makes the results statistically confused. It stands to reason that the results of linear regressions (as reported by the authors on page 10, line 330-334) depend on this statistical approach.  

 

Author Response

 

ANSWER TO REVIEWER 2:

First, we would like to express our thanks to the reviewer, who checked the manuscript thoroughly and completed its job with several valuable comments. We highly appreciate each comment or recommendation that could help to improve the manuscript.

The article entitled “Adhesion of selected hardwoods with PVAc glue” has as the main argument the bonding of wood and aims to investigate the effect of selected characteristics of 13 hardwoods on the adhesion strength. The adhesion strength in dry state was not influenced by the density, cold-water extract and PH-value, but it was influenced by their lower roughness, lower water contact angles, and their higher surface free energies.

The comments are outlined below: 

Line 15: it would be better to write “crucial” instead of “important” to reinforce the concept.

• It was corrected.

Line 19: It is preferred to not identify the density together with pH-value, wettability and roughness as only “structural characteristics”. Please, write “physical and structural characteristics” as it’s written in line 202 correctly.

• It was corrected. 

Line 44: Change “which play” with “that play.”

• It was corrected.

Line 35: It is better to not start the sentence with “. For glued wood products…”. It’s possible to start with “With regards to the glued wood products.”

• It was corrected.

Line 61: In the sentence “recommended to perform bonding process” please, add “the” before the word “bonding process”.

• It was corrected.

Line 62: At the end of line 62, it’s preferred to write “within the wooden structure”, otherwise the sentence turns out to be redundant. 

• It was corrected.

Lines 326-327-328: The sentence is too long. It is better to split it into two sentences to make it more understandable.

• It was spilt and corrected.

This article offers extensive and valid evidence. It is advisable to write more general information about PVAc adhesive, e.g. the demand for such adhesive, thus its importance and it’s recommended to add that information between lines 57 and 58. The chapter of a book indicated as follows might help to write more about PVAc.

• Ülker, O. (2016). Wood adhesives and bonding theory. Adhesives, IntechOpen, London, 271-288

• Added was this reference together with a short description of PVAc adhesives.

The methodology is clear. The results are well discussed and supported by a quite comprehensive and thorough literature search. While there are broader implications when research work concerns wood bonding, the bibliography of the article may be sufficient to support this study. The style in which it is written should be improved, for example by finding synonyms for the word "important" otherwise many sentences are redundant.

• We tried to improve some verbal and technical expressions, e.g., now instead of “glue” is always used “adhesive”, in the Title of manuscript is used “Bonding” instead of “Adhesion”, etc. – see indications in the manuscript.

The weakest part of the manuscript is related, in my opinion, to the lack of Anova test able to clarify the statistical difference among wood species. Dataset reported in Figure 3, can be considered the main part of the manuscripts but the few numbers of specimens submitted to test (6) could make the discussion and conclusion not appropriate (also checking evidence reported in figure 3).

• Now the Duncan test was performed with the aim to define significantly higher values of the adhesion strength in the dry state of bonded specimens in relation to the bangkirai specimens having the lowest adhesion strength.
• Note: This type of statistical test, by our opinion, only for the adhesion strength determined in the dry state of specimens is an important for the basic aim of the manuscript. Effect of the moisture (Wet or Reconditioned states – Fig. 3) on the adhesion of individual wood species can be seen from mean values and SD in Tab.4 and from linear regressions / correlations – mainly from changes of the R² a p values – in Fig. 3. The total number of specimens for evaluation of each correlation was 78 (6x13) – by our opinion it could be enough for determination the changed or unchanged significance tendency related to adhesion strength of bonded hardwood species in the wet and re-conditioned states comparing to the dry state.  

I also noted in Table 4 (and correspondent figures) a low R2 value and I'm thinking that the statistical approach was not completely appropriate. By using 13 value (I imagine they come from the average dataset of each wood species) inevitably makes the results statistically confused. It stands to reason that the results of linear regressions (as reported by the authors on page 10, line 330-334) depend on this statistical approach.  

• In Table 5 (now it is redesigned = from all measurements) and corresponded Figures 4a-f (from mean values = with the aim of seeing what a total tendency was for 13 hardwoods?), there for the linear correlations in Table 5 were in more cases determined statistical significance (p <05) – see corrected Abstract, Discussion (lines 330-334; etc.), and Conclusions.
• So, the Figure 4 was not changed in a principle, but now in Table 5 are new regressions from all measured values.   

Author Response File: Author Response.pdf

Reviewer 3 Report

In general, article is quite interesting. Nevetheless, I have some questions and comments:

1. In Abstract you said: "The bonding of wood with assembly glues is important at manufacturing wooden composites such as solid wood panels, glulam, furniture parts, sport and musical instruments". I pretty sure that bangkirai and massaranduba have differetnt utilization. Please, supplemented or rephrase.
2. Materials ... : How did you do identification of Shorea wood? It was commercial products. Bankgirai and meranti are derived from a number of wood species of Shorea genus. According to my knowledge and experience, it is impossible to make identification to the species level. It should be detailed described. Are you really sure abot the species?
3. Line 142: Did you have wood "without bio-damages" ? For bangkirai it is  very difficult to obtained planks without larval holes. Thay are always in wood.
4. If you use European standards, I recommend to use EN 13556 regarding to name of woods.
5. I think that name Drożdżek for position 29 in literature is written down inappropriate

Author Response

ANSWER TO REVIEWER 3:

First, we would like to express our thanks to the reviewer who checked the manuscript thoroughly and completed its job with several valuable comments. We highly appreciate each comment or recommendation that could help to improve the manuscript.  

 

In general, article is quite interesting. Nevertheless, I have some questions and comments:

In Abstract you said: "The bonding of wood with assembly glues is important at manufacturing wooden composites such as solid wood panels, glulam, furniture parts, sport and musical instruments". I pretty sure that bangkirai and massaranduba have different utilization. Please, supplemented or rephrase.

• Yes we agree with you. But the aim of the first sentence in “Abstract” was to mention some typical types of wood composites made with help of assembly adhesives. Bangkirai and massaranduba wood species are not usually used in bonded wood composites – these species due to a high durability and hardness are usually applied as external decks, in terraces and road construction, etc. – however their bonding with assembly adhesives can sometimes be needed, e.g., for bridge constructions – so for the experiment we selected these two hardwood species, as well.      

Materials ... : How did you do identification of Shorea wood? It was commercial products. Bankgirai and meranti are derived from a number of wood species of Shorea genus. According to my knowledge and experience, it is impossible to make identification to the species level. It should be detailed described. Are you really sure about the species?

• All hardwoods used in the work were obtained from the trading company JAF Holz, Ltd. as the commercial products (boards / timbers). From the company we obtained the boards together with their commercial – English names and for more of them as well as their scientific names. We fully agree with you that in the Shorea genus is more species and in more cases is not possible to make identification the individual species level. However, the bangkirai species (Shorea obtusa ; Sh. Spp.) was determined by the electron-microscopy analysis (Mamoňová, M., Reinprecht, L. (2020): The impact of natural and artificial weathering on the anatomy of selected tropical hardwoods. IAWA Journal 4(3): 333–355. DOI: 10.1163/22941932-bja10028); and the species of meranti – dark red meranti (Shorea curtisii) we obtained from JAF Holz, Ltd. and this species was also confirmed by microscopy analysis (Vidholdová, Z., Reinprecht, L. (2019): The colour of tropical woods influenced by brown rot. Forests 10(4/322): 14 p.  ISSN 1999-4907, DOI: 10.3390/f10040322).           

Line 142: Did you have wood "without bio-damages" ? For bangkirai it is very difficult to obtained planks without larval holes. They are always in wood.

• It is true that the bangkirai boards from JAF Holz, Ltd. contained holes of insect with diameter of 1-1.5 mm (caused by insect impingement active only in standing tress). However, in the selected small samples 80mm x 20mm x 5 mm – used for determination of the wood characteristics and for preparation of the adhesion strength specimens – the insect holes were not (or were not in the parts of samples exposed to testing – e.g. insect holes at the adhesion test were not in the parts of samples bonded with PVAc adhesive).    

If you use European standards, I recommend to use EN 13556 regarding to name of woods.

• In the standard EN 13556: 2003 (Round and sawn timber. Nomenclature of timbers used in Europe) are names of hardwood species, but the ranges of their densities we obtained from the standard EN 350: 2016. However, in the Section 2.1.1 we now cited the EN 13556.

I think that name Drożdżek for position 29 in literature is written down inappropriate.

• Yes, the name was corrected.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors have responded to all problems.

Reviewer 2 Report

The authors performed a thorough revision of their manuscript and replied convincingly to the issues raised up during my evaluation. In my opinion, the manuscript is ready for publication.

Sincerely,