The Impact of Fungicides, Plasma, UV-Additives and Weathering on the Adhesion Strength of Acrylic and Alkyd Coatings to the Norway Spruce Wood

Round 1

Reviewer 1 Report

The paper is generally well-written and I must admit I really enjoyed reading it. There are some minor issues I would like you to consider:

1. In the introduction, the literature review is well handled but I find some critical point missing. Namely, the motivation of this study should be well provided. Although the purpose of this study has been given explicitly, it is hard to judge what kind of novelty, in specific, this research paper brings. Why Norway spruce wood was studied? Why is that spiecies important? Why the additivies.coating were chosen?
2. Could you please elaborate on why surface finish seems not have a significant impact of the adhesion? It appears that different kind of surface texture (also geometrical anisotropy) should influence the wetting and penetrating behaviour during coating if other material properties were identical (which might be hard because the wood is quite non-homogenized material).
3. In materials and methods, could you please specify if the grinding was done manually? What were the allowances removed by each finishing operations? Can you provide any surface roughness measurements to support that the actual surface texture was similar for all samples?
4. If you use correlation analysis for linear or any other model it would be necessary to check if the results could be statistically discriminated. I would suggest doing ANOVA with post-hoc test first and then proceed, if possible, with regression analysis.
5.  

Author Response

ANSWER TO REVIEWER No 1:

Thank you for your professional revision of our manuscript.

_______________________________________________________________

The paper is generally well-written and I must admit I really enjoyed reading it. There are some minor issues I would like you to consider:

1 A/In the introduction, the literature review is well handled but I find some critical point missing. A/ Namely, the motivation of this study should be well provided. Although the purpose of this study has been given explicitly, it is hard to judge what kind of novelty, in specific, this research paper brings. B/ Why Norway spruce wood was studied? Why is that spiecies important? Why the additivies.coating were chosen?

• 1A) The motivation of this study was added before the aim of the work. Permanent quality of painted wood is connected with its colour stability, resistance to creation of cracks, to thickness reduction and to biological attacks, as well as with a good and long-term adhesion of the coating system with the wood surface. In exterior exposures the durability of wood surfaces painted with the transparent coatings is often times limited to just their adhesion strength to wood substrate. Adhesion of coatings to wood is commonly suggestible by weathering conditions, but less information is about the effect of wood pre-treatment with fungicides and plasma and also about the presence of UV-additives in coatings.   
• 1B) Why spruce wood and additives in coatings? The Norway spruce is common and very important wood species, mainly in the Central Europe, used for industrial and building structures, bridges, furniture, musical instruments, sport equipment and other uses. From this point of view, research aimed at improving its surface properties was performed in this work. UV-additives are needed in composition of transparent coatings used for painting of wood products exposed outdoor, with the aim to secure their colour stability. Impact of these additives on the adhesion strength was in an interest of this work.

2A/ Could you please elaborate on why surface finish seems not have a significant impact of the adhesion? B/ It appears that different kind of surface texture (also geometrical anisotropy) should influence the wetting and penetrating behaviour during coating if other material properties were identical (which might be hard because the wood is quite non-homogenized material).

• 2A) Yes, we tried to elaborate it. As it is well known the adhesion of coatings to wood is influenced by several factors, first of all by the kind of wood surface, the type of coating and the technology of coating application. The adhesion strength was not influenced by the UV-additives in coatings, probably because their presence did not affect the contact angle with coating (these measurements for individual coating types have not been made) and water resistance of final coating films. The adhesion of alkyd coatings, in comparison to acrylic ones, was a partly better – without weathering only by 4.7% but after 42 weeks of outdoor weathering already by 19.8%. It means that in the initial state before weathering the effect of coating type was not dominant, however, the acrylic coatings occurred less durable to sunlight, water and other weathering agents.
• 2B) Yes we agree and edited the text as follows. Differences in the wood surface texture of various wood species or of specimens of the same wood species are given by (a) the geometrical level, such as the radial or tangential surface, knots, roughness influenced by machining, etc., (b) the morphological and anatomical levels, such as the diameter of cell elements “fibers, vessels, rays, …” and their lumens, etc., and (c) the molecular level, such as the type and amount of hydroxyl and other polar functional groups, crystallinity of cellulose, polarity of lignin-polysaccharide components and various extractives, migration of extractives, etc. Surface texture of wood significantly influences the wettability and penetration processes of liquid coatings in wood as well as the adhesion of created coating films to wood.

3 A/ In materials and methods, could you please specify if the grinding was done manually? B/ What were the allowances removed by each finishing operations? A/ Can you provide any surface roughness measurements to support that the actual surface texture was similar for all samples?

• 3A) Sentences in the Materials and Methods we modified as follows. Top surfaces of the naturally dried and planed Norway spruce (Picea abies Karst L.) boards have been ground along the grain on a belt sander gradually with 80-grit and 120-grit sandpapers, and then freed from wooden dust with compressed air. Following, on a circular saw were from boards prepared samples: (a) with a dimension of 80 mm × 45 mm × 8 mm (axial × radial × tangential) for the artificial weathering test in Xenotest, and (b) with a dimension of 375 mm × 78 mm × 20 mm (axial × radial × tangential) for the outdoor weathering test. Selected samples, i.e., without biological damages, juvenile wood and growth inhomogeneities, were conditioned at a temperature of 20 ± 2 °C and a relative air humidity of 65 ± 5% achieving their equilibrium moisture content of 12 ± 2%. Density of spruce samples used in the experiment ranged from 450 to 475 kg^.m^-3. The roughness parameters Ra (arithmetic mean deviation) and Rz (arithmetic mean of the heights and depressions of the profile at the basic length), determined for selected samples in accordance with the Standard EN ISO 4287 [50] perpendicular to the grain using the profilometer Surfcom 130A (Carl Zeiss, Jena, Germany), ranged in the intervals of 6.04 – 7.42 mm and 50.2 – 57.6 m
• 3B) No in this experiment. The allowances removed by each finishing operations (of the wood from spruce boards with 80- and 120-grit sandpaper, as well as of the 1-st and 2-nd coating layers from painted surfaces with 240-grit sandpaper) were not measured and determined.
• 3C) Hydrophilicity and wettability. Polar wood surfaces consisting of lignin-polysaccharide macromolecules are characterized by small contact angle with polar liquids and by higher polar component of the surface free energy. Average value of the initial contact angle g₀ of redistilled water with the surface of un-pre-treated natural spruce samples was 55.5°. It due to exposure to plasma for 60 s decreased actually to 13°, however, after 24 h it again partly increased to 35°.

4. A/ If you use correlation analysis for linear or any other model it would be necessary to check if the results could be statistically discriminated. I would suggest doing ANOVA with post-hoc test first and then proceed, if possible, B/ with regression analysis.

• 4A) Yes we agree. The multiple variance analysis is commonly used for determination how the group factor influence the measured property, i.e., in our case how the adhesion strength between coating and wood was influenced by the UV-additive, fungicide, plasma and weathering, and also their combinations, at which ANOVA with Duncan test is used for identification if there is a meaningful difference (at a 95% confidence level) among the groups. Experimental results we analysed as well as with these statistical methods, however, for the sake of “simpler and clearer” presentation of the results, we presented them in form the regression analyzes.
• 4B) Adding parameter “p” which identify the statistical significance of regression analyses. Into the Figures 2 a 3 we added, except the coefficient of determination R², as well as the parameter “p” for identification the statistical significance of the individual regression equations.    

Author Response File: Author Response.pdf

Reviewer 2 Report

1. Please explain the utility of this work and how this work can be applied to industrial products?
2. Please thoroughly check your manuscript for the use of the English language.
3. Fig 1 is unclear to me, please provide labeling and add scale.
4. Line 185: What is the pull-off test tensile strength method? Please explain.
5. Line 198: The equation used is very simplistic in nature. Can you please explain how you derived this equation?
6. Fig 2: Please increase the resolution and font size
7. Line 257: What is this nonsignificant effect?
8. Line 288: Please elaborate on your reasoning.
9. Line 354: Options of some researchers?? What does this even mean?
10. Line 356: ‘By several researchers?’ Kindly rectify the phrases used.

Author Response

ANSWER TO REVIEWER No. 2:

Thank you for your time and the profesional review of our manuscript. 

________________________________________________________

1) Please explain the utility of this work and how this work can be applied to industrial products?

• Permanent quality of painted wood is connected with its colour stability, resistance to creation of cracks, to thickness reduction and to biological attacks, as well as with a good and long-term adhesion of the coating system with the wood surface. In exterior exposures the durability of wood surfaces painted with the transparent coatings is often times limited to just their adhesion strength to wood substrate. Adhesion of coatings to wood is commonly suggestible by weathering conditions, but less information is about the effect of wood pre-treatment with fungicides and plasma and also about the presence of UV-additives in coatings. The Norway spruce is common and very important wood species, mainly in the Central Europe, used for industrial and building structures, bridges, furniture, musical instruments, sport equipment and other uses. From this point of view, research aimed at improving its surface properties was performed in this work.

2) Please thoroughly check your manuscript for the use of the English language.

• Yes, we tried to improve the English language in the manuscript – manuscript was thoroughly checked, errors were corrected and changes to the document were recorded by the Revisions tool.

3) Fig 1 is unclear to me, please provide labeling and add scale.

• Yes, labelling and scale was added to Fig 1.

4) Line 185: What is the pull-off test tensile strength method? Please explain.

• Yes, the formulation was not correct, and we improved it. “ …determined by the pull-off test for adhesion …”

5) Line 198: The equation used is very simplistic in nature. Can you please explain how you derived this equation?

• This type of exponential equation can be used for description of non-linear dependencies, for such y (adhesion) values which non-linearly decrease with x (time of weathering time), and it was for us suggested by mathematician of the Technical University in Zvolen.  

6) Fig 2: Please increase the resolution and font size

• Yes, it was done in Fig. 2 and also in Fig. 3, together with insertion of the significance p values for the correlations.

7) Line 257: What is this nonsignificant effect?

• It was corrected (also in Table 1): No effect of the UV-additives HALS and BTZ on the adhesion strength between the coatings and the plasma modified Norway spruce wood – confirmed by the small coefficient of determination R² equal to 0.01 and p >1 for the linear correlations.

8) Line 288: Please elaborate on your reasoning.      

• The adhesion of alkyd coatings, in comparison to acrylic ones, was a partly better – without weathering only by 4.7% but after 42 weeks of outdoor weathering already by 19.8%. It means that in the initial state before weathering the effect of coating type was not dominant, however, the acrylic coatings occurred less durable to sunlight, water and other weathering agents.

9) Line 354: Options of some researchers?? What does this even mean?

• Thank you, yes, it should be “Opinions of some researchers”. Word Options was incorrectly used (due to a typo) instead of the word Opinions. This error has already been fixed in the text.

10) Line 356: ‘By several researchers?’ Kindly rectify the phrases used.

• Phrase was changed to „Several studies report that“.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you for incorporating my comments.