Plywood Manufacturing Using Various Combinations of Hardwood Species

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is focused on investigation and evaluation of the potential of six hardwood species for manufacturing nine-layer plywood composites. In this respect, the manuscript is within the scope of the Special Issue “Novelties in Wood Engineering and Forestry—2nd Edition”, Section “Wood Science and Forest Products” of the Forests journal. In general, the manuscript is well-written, structured, and informative, but needs some improvements. Please, see below my comments on your work:

The title (lines 2-3), the abstract (lines 11-26) and the keywords (lines 27-28) correspond to the scope, aims and objectives of the manuscript. The abstract is informative and clearly presents the aim of the research. Here I’d recommend adding the botanical (Latin) names of the species used in order to avoid any misunderstanding.

Line 24: although the abbreviations used are well known, I’d suggest adding the full terms, i.e., modulus of elasticity and bending strength/modulus of rupture, then followed by the respective abbreviations.

Lines 31-33: The statement “The scientific investigation of utilising various hardwood species from mixed stands in a single product such as plywood and its mechanical and physical properties has been limited thus far.” is generally accurate, but there are some studies that have explored this area. For example, investigations have been conducted on the mechanical properties of plywood made from different hardwood species like birch, black alder, grey alder, and aspen. Please, check again and revise the statement.

Line 38: please support this statement with relevant reference(s): “Simultaneously, the timber industry is globally oriented and predominantly geared towards processing a limited range of wood species, primarily softwood.”.

Line 50: please, delete (Lee et al. 2021).

Overall, the Introduction part is well-written, informative, and provides relevant information and references on the research topic. The aim of the research is also clearly specified.

Line 177: please, provide some relevant characteristics of the adhesive used, e.g., viscosity, solids content, etc. Please add it in the main text of the manuscript, not only in the references.

Line 188: please, add the pressure applied during hot pressing. In addition, please explain/justify the selected pressing parameters (temperature, pressure, and duration).

Line 205: Figure 1 is very small and hard to read, please provide a larger figure, if possible.

Lines 246-255: the information in section 2.8. Layer thickness and compaction is repeated below, please remove this part.

Line 265: please, use () for equation 1.

In general, the Materials and Methods section is well-written and detailed, providing relevant information on the materials and methods used. However, it can be further elaborated based on the comments given above.

 

Line 317: Table 3 has a different font, please revise in accordance with the Instructions for Authors.

The same comment applies to Table 2. In addition, it is given after Table 3, please check table formatting throughout the manuscript.

Overall, the results of the study are properly presented and discussed with relevant research works in the field.

Lines 623-731: I’d recommend removing this part of the manuscript and providing the main findings of the research in the Conclusions section.

The Conclusion part (lines 732-771) reflects the main findings of the manuscript. Here I’d recommend adding short information about the potential for future research works in the field and the practical application of the results achieved.

The References cited are appropriate to the topic of the manuscript. However, most of them are not properly formatted, please revise in accordance with the Instructions for Authors.

Author Response

The title (lines 2-3), the abstract (lines 11-26) and the keywords (lines 27-28) correspond to the scope, aims and objectives of the manuscript. The abstract is informative and clearly presents the aim of the research. Here I’d recommend adding the botanical (Latin) names of the species used in order to avoid any misunderstanding.

The botanical names have been added.

 

Line 24: although the abbreviations used are well known, I’d suggest adding the full terms, i.e., modulus of elasticity and bending strength/modulus of rupture, then followed by the respective abbreviations.

Done

 

Lines 31-33: The statement “The scientific investigation of utilising various hardwood species from mixed stands in a single product such as plywood and its mechanical and physical properties has been limited thus far.” is generally accurate, but there are some studies that have explored this area. For example, investigations have been conducted on the mechanical properties of plywood made from different hardwood species like birch, black alder, grey alder, and aspen. Please, check again and revise the statement.

The statement has been adjusted and two references have been added.

 

 

Line 38: please support this statement with relevant reference(s): “Simultaneously, the timber industry is globally oriented and predominantly geared towards processing a limited range of wood species, primarily softwood.”.

The statement has been revised and a reference has been added.

 

Line 50: please, delete (Lee et al. 2021).

The reference has been deleted.

Line 177: please, provide some relevant characteristics of the adhesive used, e.g., viscosity, solids content, etc. Please add it in the main text of the manuscript, not only in the references.

Relevant characteristics of the adhesive have been added.

 

Line 188: please, add the pressure applied during hot pressing. In addition, please explain/justify the selected pressing parameters (temperature, pressure, and duration).

During the production of the panels, the press was set to a target panel thickness of 16 mm. This resulted in a variable pressure that was adjusted by the press.

 

Line 205: Figure 1 is very small and hard to read, please provide a larger figure, if possible.

The figure has been changed.

 

Lines 246-255: the information in section 2.8. Layer thickness and compaction is repeated below, please remove this part.

The repetitions have been deleted.

 

Line 265: please, use () for equation 1.

The brackets have been inserted into the equation.

 

Line 317: Table 3 has a different font, please revise in accordance with the Instructions for Authors.

The font has been adapted in a standardised way.

 

The same comment applies to Table 2. In addition, it is given after Table 3, please check table formatting throughout the manuscript.

The tables and their order have been adjusted.

 

Lines 623-731: I’d recommend removing this part of the manuscript and providing the main findings of the research in the Conclusions section.

The sections were deleted and integrated into the Conclusions, albeit only to a limited extent.

 

The Conclusion part (lines 732-771) reflects the main findings of the manuscript. Here I’d recommend adding short information about the potential for future research works in the field and the practical application of the results achieved.

This has been added.

 

The References cited are appropriate to the topic of the manuscript. However, most of them are not properly formatted, please revise in accordance with the Instructions for Authors.

The references have been formatted.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript “Plywood manufacturing using various combinations of hardwood species” by Cordier et al. deals with an interesting and current topic - the expansion of the use of different hardwood species. In this article, the authors attempt to achieve this by combining different hardwoods for plywood products.

However, the manuscript still needs to be improved.

L17: Authors should use one spelling of the units and maintain it throughout the manuscript.

L206: Table 1 - in my opinion “sample” should be replaced by “variant” - this should be changed throughout the manuscript.

L216: The “-3” within the unit must be superscripted; check carefully throughout the manuscript.

L218: How was the strain measured for the MOE calculation? Extensometer,…?

L288: The authors need to reword the first sentence. The current sentence is misleading - “according to EN 314-1 and EN 314-2” seems as if the authors are performing two different tests. However, EN 314-2 contains “requirements”.

L235: Again, in my opinion the word sample should be replaced by variant (check carefully).

L246 – 262: There are two section 2.8 with partial different content.

L293: “Previous studies …” – the authors should cite these previous studies.

L309: “… by factors between 0.94 and 1.04 …” – according to Table 3, the factors are between 0.98 and 1.04.

L311: There is no sample 7 in Fig 2a. Why?

L317 – 323: Why is table 3 before table 2?

L321: Why was the density of the plywood higher compared to the veneers? If the authors state this, they should explain in one sentence that this is due to densification during gluing.

L351: The symbol for MOR is wrong.

L352: Modulus of rupture (MOR) – the authors should either use the full name or the abbreviation. It does not make sense to always use both. Check throughout the whole manuscript.

L354: The symbol is again wrong.

L376: A SD of 998 N mm^-2 seems pretty high. Are the authors sure that this is the correct value?

L386: It is correct, that the support span has an influence – however, for this study it should not have any influence as the span to height is the same for all specimens.

L393 -: Why is there no figure for MOE? The authors should discuss the MOE results in more detail – especially why is the MOE for sample 2 & 3 distinct higher compared to sample 1 - Sample 1 has a higher mean density and the top layer veneers lime in sample 2 & 3 has a low density (even after densification).

L397: Table 4: The authors should insert the exact value auf the MOEs with SD. Additionally, within the whole manuscript, authors are using N mm^-2 (MPa) for MOR and MOE. In Table 4, it seems as they changed to GPa for MOE without any note.

L493: Table 6 – there is no SD

L500: The authors mentioned, that the TS of plywood depends on the tangential swelling of the hardwood species. In section 2.1. the authors mentioned, that they were using rotary cut veneers. In my understanding, the thickness of rotary cut veneers are in radial direction. Moreover, in Fig 1 it looks as the year rings are lying.

L508: Is density really the decisive factor? According to Fig. 5, sample 1 has a fairly high density on the surface and in the outer layers. It is probably higher than in samples 2 to 4, so sample 1 would have the highest TS - especially as the uppermost layers of sample 1 have the highest compression. Therefore, birch plywood must swell more, right? What about the compression of the individual veneers, the resulting closure of their lumina and the known reset effect of compressed wood in the presence of water? -> In birch veneers with an average initial density of 569 kg m-3 and a densification of 17.7 - 30.5 %, the lumina are probably more closed than in lime veneers with an average initial density of 406 kg m-3 and a densification of 21.7 - 24.2 %. As the lumina are more open, water can penetrate the plywood layers more easily. Did the authors analyzed the micro-structure (lumina closure) of the plywoods?

Just a thought. However, the authors must discuss this section in more detail.

L522: Table 7 - Why no sample 6 & 7? Can the authors explain why in sample 2, 4 & 5, the inner core layer 3 has a higher compaction compared to the inner core layer 1 & 2?

L555: This table is duplicated

L577: Fig 5 – The font must be larger! Minimum and average density of ref samples – why no red line showing the maximum density? The grey line is almost not visible. Where are sample 6 & 7?

L608 - : Statistical analysis What about the other results? MOE, TSS, TS,…

Author Response

L17: Authors should use one spelling of the units and maintain it throughout the manuscript.

The spelling of the units has been standardised.

 

L206: Table 1 - in my opinion “sample” should be replaced by “variant” - this should be changed throughout the manuscript.

We have replaced ‘sample’ with ‘variant’ throughout the manuscript.

 

L216: The “-3” within the unit must be superscripted; check carefully throughout the manuscript.

Done.

 

L218: How was the strain measured for the MOE calculation? Extensometer,…?

The strain used for the MOE calculation was measured from the travel of the crosshead (relative position measurement); an extensometer was not used as its use can lead to problems with specimen fracture. The high stiffness of the testing frame relative to the expected sample stiffness ensures that the measured deformations are almost entirely attributable to the actual sample deflection. In addition, preliminary tests confirmed that the deviations between traverse measurements and direct deflection measurements are within acceptable tolerances for this investigation. This measuring method maintaining sufficient measurement accuracy for the comparative analysis of the specimens.

 

L288: The authors need to reword the first sentence. The current sentence is misleading - “according to EN 314-1 and EN 314-2” seems as if the authors are performing two different tests. However, EN 314-2 contains “requirements”.

It has been reworded.

 

L235: Again, in my opinion the word sample should be replaced by variant (check carefully).

Done.

 

L246 – 262: There are two section 2.8 with partial different content.

The repetitions have been deleted.

 

L293: “Previous studies …” – the authors should cite these previous studies.

The references have been added.

 

L309: “… by factors between 0.94 and 1.04 …” – according to Table 3, the factors are between 0.98 and 1.04.

The right factor is 0.85 (to 1.04).

 

L311: There is no sample 7 in Fig 2a. Why? 

The figure has been changed.

 

L317 – 323: Why is table 3 before table 2?

The order has been changed.

 

L321: Why was the density of the plywood higher compared to the veneers? If the authors state this, they should explain in one sentence that this is due to densification during gluing.

The explanation has been added.

 

L351: The symbol for MOR is wrong.

The symbol has been changed.

 

L352: Modulus of rupture (MOR) – the authors should either use the full name or the abbreviation. It does not make sense to always use both. Check throughout the whole manuscript.

It has been changed.

 

L354: The symbol is again wrong.

It has been corrected.

 

L376: A SD of 998 N mm-2 seems pretty high. Are the authors sure that this is the correct value?

The statement has been revised. It is related to MOE, not to flexural strength.

 

L386: It is correct, that the support span has an influence – however, for this study it should not have any influence as the span to height is the same for all specimens.

We mentioned that it was constant for all variants.

 

L393 -: Why is there no figure for MOE? The authors should discuss the MOE results in more detail – especially why is the MOE for sample 2 & 3 distinct higher compared to sample 1 - Sample 1 has a higher mean density and the top layer veneers lime in sample 2 & 3 has a low density (even after densification).

We have added a figure for the MOE.

 

L397: Table 4: The authors should insert the exact value auf the MOEs with SD. Additionally, within the whole manuscript, authors are using N mm-2 (MPa) for MOR and MOE. In Table 4, it seems as they changed to GPa for MOE without any note.

There is a misunderstanding. These numbers are not MOE values, but classifications according to the EN 636 standard. The table caption was corrected.

 

L493: Table 6 – there is no SD

The SD has been inserted.

 

L500: The authors mentioned, that the TS of plywood depends on the tangential swelling of the hardwood species. In section 2.1. the authors mentioned, that they were using rotary cut veneers. In my understanding, the thickness of rotary cut veneers are in radial direction. Moreover, in Fig 1 it looks as the year rings are lying.

The reviewer is right, it is radial swelling.

 

L508: Is density really the decisive factor? According to Fig. 5, sample 1 has a fairly high density on the surface and in the outer layers. It is probably higher than in samples 2 to 4, so sample 1 would have the highest TS - especially as the uppermost layers of sample 1 have the highest compression. Therefore, birch plywood must swell more, right? What about the compression of the individual veneers, the resulting closure of their lumina and the known reset effect of compressed wood in the presence of water? -> In birch veneers with an average initial density of 569 kg m-3 and a densification of 17.7 - 30.5 %, the lumina are probably more closed than in lime veneers with an average initial density of 406 kg m-3 and a densification of 21.7 - 24.2 %. As the lumina are more open, water can penetrate the plywood layers more easily. Did the authors analyzed the micro-structure (lumina closure) of the plywoods?

Just a thought. However, the authors must discuss this section in more detail.

Thanks to the reviewer for the suggestion. We have included it in the discussion.

 

L522: Table 7 - Why no sample 6 & 7? Can the authors explain why in sample 2, 4 & 5, the inner core layer 3 has a higher compaction compared to the inner core layer 1 & 2?

As described under 2.2, plywoods 1-5 and 6, 7 were produced in two batches, in which the number of panels produced varied. “A total of seven different variants were produced with three (variants 1 to 5) to five (variants 6 to 7) boards for each variant.” The investigations shown in Table 7 were only carried out on batch 1 (variants 1-5), as variants 6 and 7 were no longer available for these investigations. The same applies to the recording of the density profiles. The differential compaction pattern observed across inner core layers cannot be conclusively explained by our current investigations. This phenomenon likely results from an interaction between distinct densification mechanisms and minor variations in initial veneer thickness. More comprehensive studies would be necessary to verify the underlying causes.

 

L555: This table is duplicated

It has been deleted.

 

L577: Fig 5 – The font must be larger! Minimum and average density of ref samples – why no red line showing the maximum density? The grey line is almost not visible. Where are sample 6 & 7?

The graph has been revised. Additional lines for maxima were not inserted as the peaks in the graph are clearly identifiable, more so than the overlapping regions of the graphs at minima and mean values.

 

L608 - : Statistical analysis What about the other results? MOE, TSS, TS,…

The deliberate focus of our statistical analysis on the established relationship between density and flexural strength was methodologically justified as a preliminary assessment approach for the material. The overall experimental design of this study was not configured to generate data sets suitable for comprehensive statistical evaluation with adequate statistical power, as this represents an early exploratory research phase aimed at investigating fundamental feasibility parameters. As explicitly stated in the concluding section of our paper, future investigations with appropriate sampling designs would be required for more extensive statistical analyses of additional mechanical properties including MOE, TSS and TS.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors 

The article is very long and should be shortened especially in the discussion and conclusions where some sentences present considerations that are repeated. Incase also the chapter considering density profile of the panels could be eliminated.

Some Figures and/or Tables need to be revised. For example, the indications in Figure 1 are not very visible; in Figure 2 (a) the results of sample 7 are missing; Table 7 referred to in the text of the article is missing.

It would seem that the main objective is not to better valorize some wood species currently lesser used but rather to justify the validity of a plywood pressing system based on variable pressure for the final thickness control of the panel. Furthermore, the pressure values that determined the densification of the layers of veneer composed of lighter woods are not reported. In my  opinion less emphasis should be given to the aforementioned method by adding, at least in the discussion, that same results can be obtained by increasing the pressure even in the case of a traditional processing and pressing system.

Moreover, it would be appropriate to highlight that an homogeneous distribution of panel’s properties (mentioned at lines 652-655 and 666-668) can also be achieved using veneers of different thicknesses in its composition through an appropriate lay-up, like it is normally done by many plywood factories.

Some more detailed comments are in the attached pdf.

Comments for author File: Comments.pdf

Author Response

The article is very long and should be shortened especially in the discussion and conclusions where some sentences present considerations that are repeated. Incase also the chapter considering density profile of the panels could be eliminated.

The discussion has been removed and integrated into the conclusions to a much lesser extent.

 

Some Figures and/or Tables need to be revised. For example, the indications in Figure 1 are not very visible; in Figure 2 (a) the results of sample 7 are missing; Table 7 referred to in the text of the article is missing.

Revisions of all Figures has been performed.

 

It would seem that the main objective is not to better valorize some wood species currently lesser used but rather to justify the validity of a plywood pressing system based on variable pressure for the final thickness control of the panel. Furthermore, the pressure values that determined the densification of the layers of veneer composed of lighter woods are not reported. In my opinion less emphasis should be given to the aforementioned method by adding, at least in the discussion, that same results can be obtained by increasing the pressure even in the case of a traditional processing and pressing system.

The initial aim of the study was not to establish the validity of a plywood press system. In production, there are two options for pressing plywood: either constant pressure or constant travel. Since we wanted to produce uniformly thick plywood, we chose the latter. The press automatically adjusts the pressure, but this cannot be recorded. Therefore, the pressure may be lower or higher than in reference 1, depending on the density.

 

Moreover, it would be appropriate to highlight that an homogeneous distribution of panel’s properties (mentioned at lines 652-655 and 666-668) can also be achieved using veneers of different thicknesses in its composition through an appropriate lay-up, like it is normally done by many plywood factories.

We agree, but as we have removed the separate discussion due to the necessary cuts, this was not mentioned further. The study was initially only intended to show general feasibility. Detailed considerations should be the subject of further research, as mentioned in the Conclusions.

 

Some more detailed comments are in the attached pdf.

The detailed comments in the pdf file have been taken into account.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors adressed all my questions and comments! However, I noticed 2 more things:

L133-134: The authors deleted "thickness swelling", however, within the conclusion, "thickness swelling" is mentioned.

L308-310: "... raw densities for the variants 2 to 5 with similar raw densities differed from the reference by factors between 0.85 and 1.04 ..." - In table 2: the factors are between 0.98 and 1.04 for the variants 2 to 5. For variant 6 the factor is 0.85.

The auhtors should check these comments. 

 

Author Response

L133-134: The authors deleted "thickness swelling", however, within the conclusion, "thickness swelling" is mentioned.

We have deleted ‘thickness swelling’ in the Conlusions.

L308-310: "... raw densities for the variants 2 to 5 with similar raw densities differed from the reference by factors between 0.85 and 1.04 ..." - In table 2: the factors are between 0.98 and 1.04 for the variants 2 to 5. For variant 6 the factor is 0.85.

Thanks to the reviewer. I hadn't noticed that. Now the value has been changed to 0.98.