Plasma Treatments to Improve the Bonding of Thermo-Treated Cherry Wood
Round 1
Reviewer 1 Report
The authors present a case of general interest to readers of the Coatings journal, applying plasma treatments to improve bonding strength of an undefined coating on cherry wood. Although the application is relevant and the experiments are comprehensive, there is a need to improve both, the description of the methods and the discussion of the results.
General remarks:
- Please consider, when to use "plasma pre-treatment", and when to use "plasma treatment". If no further preparation is carried out after plasma, the prefix "pre-" becomes meaningless. (E.g. P.1, l.11)
- Please check the language, occasionally occuring grammatical errors (mixing plural with singular forms; sentences with no fintie verbs; and such) should be corrected.
Abstract
P.1, l.9: Please specify: bonding strength with what?
Introduction
P.2, l.48: "wood-cherry wood", please rephrase sentence
Materials and Methods
P.2, l.60: "moisture" should read "moisture content"
P.2, l.62: Please refer to wood specimen or wood species, as appropriate.
P.2, l.80: Standard notation is to write "K Alpha" as Greek index.
P.2: You need to include a description of the plasma treatment process and device, as well as the gases used (manufacturer, purity).
P.2: Please state precisely, what coating you applied and what equipment was used to test the bonding strength.
Results and Discussion
P.3, l.97 & fig.1: Please change "0h" into "reference", "untreated" or similar to avoid confusion.
P.3, l.111-113: Are claims of nitrogen incorporation justified by XPS results? If so, please include stoichiometries. If not, please consider the well-known energy-transfer process in N2-O2 mixtures, which effectively lead to oxygen incorporation into surfaces even for technical grad cover gases. (At 40Pa, this is a Knudsen-type behavior, so that exchange processes within the gas still do play a major role.)
P.3, l.111-117: Contact angles on such complex materials such as wood show dependencies on various paramenters and include quite diverse effects. One major factor determining wetting behaviour and contact angles are morphology and surface structures. As you find later (3.3) that the morphology is changing esp. strong for N2, this might well explain the different findings of water contact angles after oxygen and nitrogen plasma treatments. Please include in the discussion.
P.4, fig.2: Please indicate all constraints used for fitting the C1s spectra. As single components do not resolve, constraints are duely needed to obtain valid results!
P.4, tab.1: Please include binding energies and peak width (FWHM) for all fits.
P.5, l.153-155: Please rephrase and shorten.
P.6, fig.5: What was the purpose of using XRD? First of all, you mention it being a surface property, which would only be true for GI-XRD, not conventional XRD. Secondly, only some crystalline parts of the cellulose contribute to the signal; for which you showed before in SEM that those were not influenced much. Without further discussion, this measurement does not add to the content of the paper and should thus be removed.
P.6, l.181-195: Chemically bound groups and radicals seem to be used synonymously in this part. For example, a hydroxyl radical would not seem to be bound to the surface and, thus, not add to the bonding strength; further, these would exist on surfaces in thermodynamic concentrations anyway, as all surfaces under atmospheric air are covered in a thin (typically several nm) water film... Please revise this part of the discussion and correct where necessary.
P.7, fig.6: There is no such thing as an "unheat" or an "unplasma". Please revise as plasma-treated vs. non-treated (or reference).
P.5, l.156: Flax or cherry wood? Which one is it?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Title: “bonding strength” is not a property of wood, like for instance the compression strength. Woods can be more or less difficult to bond, but the bonding strength is a property of the pieces bonded together and depends also on the adhesive, the pressing methods etc. (the term bonding quality, instead, is referred to wood-based products). For this reason, delete “strength” from the title. Check the entire text for the same modification, for instance at row 181 delete “strength”. Anyway, it is correct using sentences such as “bonding strength was determined”, “bonding strength resulted of…” when referring just to the bonding strength.
Title: the treatment performed is commonly named “thermal treatment”. The term "heat treatment" can create confusion with other and less intense treatments. Replace heat-treated with thermo-treated and check the entire text for this modification.
Row 31: it is well known that, when dealing with wood, durability and dimensional stability have to be considered. Replace “dimensional stability and durability is also an emergent problem to be solved” with “dimensional stability and durability of wood have to be addressed”.
Row 44: delete “properties”.
Row 48: why is this cherry wood “special”? Specify the reason or delete “special”. Replace “wood-cherry wood” with “cherry wood”
Row 50-51: thermal treatment usually ranges from 160 to 230°C and I would not say that 220°C is the conventional temperature. Replace “160 °C is lower than the conventional heat treatment temperature 220 °C” with “160°C, which falls in the lower range of commonly used treatment temperatures”.
Row 53: which film are the authors referring to? Please add a brief sentence to specify.
Row 60: The information about the moisture content of wood before thermal treatment is not needed, since the treatment brings wood to oven-dry state. Delete the sentence “These wood samples […] 12%-15%”.
Row 64: The standar GB/T 17517-1998 shall be added in References.
Row 87: which adhesive was used? This is a very important aspect since bonding quality can vary considerably on the basis of the adhesive. Please add a sentence to specify, for instance “Phenol-formaldehyde adhesive commonly available on the market was applied […]”. Even better if the authors can also detail the adhesive composition, for instance U/F ratio (if it is an UF adhesive) etc.
Row 88: which wooden surfaces of the two test pieces were bonded together? I believe not two transversal sections, so they should be two longitudinal radial or two longitudinal tangential sections. This information is relevant and shall be added in the text.
Row 89: to me, when referring to small test pieces it is better to use “specimens” rather than “samples”. Please check the entire text for this modification.
Row 89: how many specimens were tested? This is also relevant for the statistical analysis that has to be added (see comment for rows 181-206).
Row 89: how were the specimens tested? According to GB/T 17517-1998 cited above? Please specify the testing method also in this part of the text.
Row 102: statistical analysis shall be performed to compare average values. For instance, to compare contact angle after 1 h: NT vs oxygen plasma and vs nitrogen plasma, and oxygen plasma vs nitrogen plasma (and so on for all times). Almost all of the differences are very clear so the statistical analysis should show quite obvious results, but in particular it will enable to understand if the contact angle of nitrogen plasma after 4 days and 8 days is significantly different from contact angle of NT. To summarise, the authors shall perform the statistical analysis and write a sentence to state that the differences were significant.
Rows 181-206: the statistical analysis is missing here as well. A statistical analysis shall be performed to assess if significant differences exist among not thermo-treated, thermo-treated, and not plasma treated, nitrogen treated and oxygen treated. This is particularly relevant also because for bonding strength the results show a lesser clear trend compared to wettability. The text shall be modified according to the outcome of the statistical analysis.
Rows 181-206: when testing the bonding quality of wood, usually two parameters are taken into account: bonding strength (load at failure) and fibre release. The results of fibre release are missing in the text: is there a particular reason for that? The authors could add a sentence to explain why this parameter was not considered.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
General comment
The authors improved the manuscript considerably. All comments were successfully met, except for those on the statistical analysis. In my opinion, the manuscript can be accepted after detailing the statistical analysis (see comments below).
Detailed comments
1. First, a minor comment: please add the wood surface on which the wettability was tested (I imagine it is longitudinal tangential).
2. The authors added and specified the results, which is good, but the statistical analysis is still missing. For instance, they added (rows 221-223) "For the nonthermo-treated specimens, the bonding strength after oxygen plasma treatment was increased by 11.04% compared with that without treatment, and the bonding after thermal treatment was increased by 13.28%. It can be seen that nitrogen treatment can achieve better results." This is just a description of the values, but statistical significance is not indicated. Therefore, the authors should (please note that the following examples are just to give the idea):
add, in Materials and Methods, the description of the statistical analysis. For example: “Differences were investigated by ANOVA and LSD test as a post-hoc; significance was set at p<0.05”. Indicate which differences are significant. For example, referring to rows 221-223, the sentence could be “For the nonthermo-treated specimens, the bonding strength after oxygen plasma treatment was significantly increased by 11.04% compared with that without treatment, and the bonding after thermal treatment was significantly increased by 13.28”. The authors, if they wish, could also add the p-values. For example: “[…] after oxygen plasma treatment was significantly increased (p<0.01) by […]”
The above is valid for data presented in Figure 1 and Figure 6.
In Figure 1, where the differences are quite obvious, a general sentence at the beginning of section 3.1 could be enough. For example: “Differences in wettability resulted always significant (p<0.01), except for the difference in wettability between non-treated and oxygen-plasma treated wood after 7 days (p = 0.371)”. [just an example, maybe that also the difference between non-treated and oxygen treated wood after 7 days is significant].
In Figure 6 the differences are less clear than in Figure 1, hence I suggest reporting in the text the statistical significance for each comparison.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
