Oxidative Degradation Mechanism of Zinc White Acrylic Paint: Uneven Distribution of Damage Under Artificial Aging

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Many thanks for your highly appreciated study i only suggest some comments, please consider them

Comments for author File: Comments.pdf

Author Response

Responses

We sincerely thank the reviewer for their time and insightful feedback on our manuscript. This feedback has been invaluable in helping us improve the quality and clarity of our work. We have carefully considered all suggestions and have revised the manuscript accordingly. Below we detail our responses to each of the reviewers' points.

Reviewer #1.

The paper title is too long and can be improved with an informative short one, such as “The oxidative degradation of zinc white based acrylic paint; uneven  distribution damage under artificial aging’

Agree. Corrected.

We have changed the title to: " Oxidative Degradation Mechanism of Zinc White Acrylic Paint: Uneven Distribution of Damage Under Artificial Aging".

The abstract needs some updates. Please do not include abbreviations without a previous definition. What is P.W4?

Agree. Corrected.

We have revised the abstract to spell out all abbreviations upon first use.

"P.W.4" has been changed to: "Zinc oxide (ZnO)”.  We have added a brief clarification in (2.1. Samples Preparation) section that the ZnO pigment belong to PW4 (Pigment White 4) which is the standard Color Index international designation for zinc oxide-based pigments.

 

-Also, please unify ΔE as a total color difference, and also notice that the naked eyes notice the total color difference that is more than 3, not 2.

-Please check more references about the colorimetric measurements, as the human eye can discriminate the total color change that is more than 3

 

Agree.

Please check the citations below. Based on many citations from reliable references, I found that:

ΔE ≤ 1.0: Not perceptible to the human eye under normal viewing conditions.

ΔE 1.0 - 2.0: Perceptible only through close observation or under controlled scrutiny.
ΔE 2.0 - 10.0: Perceptible at a glance.
ΔE > 10.0: Colors are clearly different.

https://zschuessler.github.io/DeltaE/learn/,

https://www.sensientindustrial.com/emea/color-college/how-to-choose-color/how-do-we-calculate-a-perceptible-difference

 

 

-Please refer to the reason for the yellowness value. Why did they show different values?

Agree.

Yellowness is primarily due to the formation of conjugated carbon-carbon double bonds in the binder or intermediates during oxidation that exhibit an n→π* transition, such as in C=O bonds [1]. In this study, the observed slight shift (decrease and increase) in the b* values (yellowness index) over time are characteristic of the complex, multi-step process of photodegradation catalyzed by ZnO, where the obtained mechanism confirming the formation of conjugated bonds as intermediates. Therefore, the formation vs. the destruction of chromophores was the cause of this shift.

 

 

-Mentioning CaCO3 was misleading to me, as I do not know if it was compared with ZnO as a white paint or from the ground layer, so if the study is comparative, please mention that in the abstract.

-Please clarify from where CaCO3 appeared?! Was it added as an extender to the product, and is it similar to the historical ones? Please clarify? You prepared the samples on aluminum sheets, so from where does CaCO3 come? Please clarify the used product so as not to mislead the reader. “SEM image _ results part”

 

Agree.

 

We thank the reviewer for pointing this out.
 The source of the CaCO3 was the paint formulation, where it was used as an additive, not from the ground layer or another paint for comparison.  In ZnO paint formulations, ZnO is often partially replaced with calcium carbonate, which acts as an extender. This substitution reduces raw material costs and increases the total volume of paint produced. Chalk or gypsum are traditionally added to paints to reduce costs, improve opacity, and enhance hydrophobic properties. In particular case, opacity is more likely the primary parameter.

We have revised the abstract for clarity as the following: "Accelerated artificial aging of zinc oxide (ZnO)-based acrylic artist's paint, filled with calcium carbonate (CaCO₃) as an extender, was carried out…."

 

Please specify why you chose these artificial aging conditions with references.

 

Agree.

 

While some studies utilize UV-specific sources (e.g., UV fluorescent lamps) or Xenon Arc Lamp with a UV filter for shorter durations (400–800 hours)[2][3] to isolate UV damage, this work utilized a xenon-arc lamp to replicate the full spectrum of solar radiation. This setup provides a superior simulation of the sunlight spectrum, including both ultraviolet and visible light, which is essential for accurately replicating real-world photodegradation mechanisms, especially for materials like ZnO that are sensitive to a broad range of wavelengths (below ~385 nm). Based on climate data compiled by IILSS[4], this accelerated aging period (1963h) is estimated to be equivalent to approximately one year of natural outdoor sunlight and approximately 91 years in a museum with light at 100 lux. The moderate temperature (38°C) and humidity (65% RH) were chosen to accelerate photooxidative reactions—the primary degradation mechanism for acrylic paints—without causing purely thermal or hydrolysis degradation, which would be less representative of real-world conditions.

Bibliography.

[1]      P. M. Whitmore and V. G. Colaluca, “The natural and accelerated aging of an acrylic artists’ medium,” Stud. Conserv., vol. 40, no. 1, pp. 51–64, 1995, doi: 10.1179/sic.1995.40.1.51.

[2]      M. T. Doménech-Carbó et al., “Study of behaviour on simulated daylight ageing of artists’ acrylic and poly(vinyl acetate) paint films,” Anal. Bioanal. Chem., vol. 399, no. 9, pp. 2921–2937, 2011, doi: 10.1007/s00216-010-4294-3.

[3]      P. Aguilar-Rodríguez, A. Mejía-González, S. Zetina, A. Colin-Molina, B. Rodríguez-Molina, and N. Esturau-Escofet, “Unexpected behavior of commercial artists’ acrylic paints under UVA artificial aging,” Microchem. J., vol. 160, no. November 2020, 2021, doi: 10.1016/j.microc.2020.105743.

[4]      “Annual sunshine hours of the world map – IILSS-International institute for Law of the Sea Studies.” Accessed: Jul. 10, 2025. [Online]. Available: https://iilss.net/annual-sunshine-hours-of-the-world-map/

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is a very interesting work, as the authors didn’t just stay at simple spectral characterization after applying artificial aging, but they also present spectral processing changes and chemometrics, resulting to very interesting findings regarding degradation phenomena. The manuscript is quite well written, and it is easy to follow. The whole manuscript needs to be refined according to the following suggestions.

 

 

 

1. Lines 8-12. Please check the journal’s template for indicating the authors emails.
2. Line 14. Artist’s paint.
3. Lines 14 and 16, and 144, and table 1. I am confused here. Since the accelerated aging lasted 1963 hours, why the color change for just 1725 hours is mentioned, and not for the whole aging period? In line 144 we see “970 hours”, while in table 1 it lasts up to 1725 hours. The same for lines 154, , while in lines 164, 206, 243 1963 hours are mentioned.
4. Line 18. ATR-FTIR. Please, explain the abbreviations upon their first mentioning, not only in the main text, but in the abstract also.
5. Line 18. Please, change to micro-Raman spectroscopy.
6. Lines 30-31. Keywords act so as a potential reader to discover the study. There is reason to use same things that already appear in the title, such as acrylic artists’ paint. Please, revise them to promote your work.
7. Lines 44-45. “uniform in color” and “texture” repetition.
8. Line 68. Attenuated total reflectance
9. Line 68. Micro-Raman spectroscopy
10. Line 71. (PCA)
11. Line 72. ATR-FTIR
12. Line 83. Was the paint from a particular manufacturer?
13. Line 108. Reflectance. And please change the position of the parenthesis “Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy”
14. Line 111. 4000-600 cm−1.
15. Section 2.6. Please use micro-Raman spectroscopy. Check the whole manuscript.
16. Section 2.7. As PCA analysis is mentioned in this section, why is this title used and not something like PCA analysis, or Chemometrics? Which softwares were used for spectral processing and PCA analysis?
17. Line 125. Outliers.
18. Line 142. Colorimetry, maybe?
19. Line 152. “∆E* must be >2 to be., while …”. Please rephrase, this is awkward.
20. Line 234. “The use of averaged spectra (Figure 3) is due”
21. Entire manuscript and tables. Please check the journal’s template for referencing in the manuscript, for example [1,2,3] instead of [1][2][3], using spaces and dots at the right places.
22. Section 2.6. Please, add the resolution with which the spectra were collected.
23. Table 2, first line. Please, correct the 31130 cm-1.
24. Figures 3, 4, and supplementary file. Please plot FTIR spectra to descending wavenumbers (4000-600)
25. Line 267. Better replace measured with collected.
26. Line 268. The authors used a 50x lens for their measurements, giving a spot size of ~1 μ ZnO and CaCO3 grain size may vary between 300 nm – 2 μm, while formed agglomerates should be taken into account, as they can reach up to ~10 μm. For this reason, and after checking the corresponded micro-Raman spectra in the supplementary file, I would not be so assertive regarding attributing such small fluctuations to degradation phenomena. On the other hand, PCA results seem to indicate that organic degradation is recorded. Please, discuss this.
27. Lines 306-307. 1850-600 and 3700-2700
28. Line 312. “(figure 6a, where…”
29. Line 519. Values and value repetition.
30. Supplementary file. Please use images of better resolution.

Author Response

 

Responses

We sincerely thank the reviewer for their time and insightful feedback on our manuscript. This feedback has been invaluable in helping us improve the quality and clarity of our work. We have carefully considered all suggestions and have revised the manuscript accordingly. Below we detail our responses to each of the reviewers' points

 

 

Reviewer  #2

This is a very interesting work, as the authors didn’t just stay at simple spectral characterization after applying artificial aging, but they also present spectral processing changes and chemometrics, resulting to very interesting findings regarding degradation phenomena. The manuscript is quite well written, and it is easy to follow. The whole manuscript needs to be refined according to the following suggestions.

 

Lines 8-12. Please check the journal’s template for indicating the authors emails.

 Agree.

Corrected.

 

Line 14. Artist’s paint.

Agree.

Corrected.

 

Lines 14 and 16, and 144, and table 1. I am confused here. Since the accelerated aging lasted 1963 hours, why the color change for just 1725 hours is mentioned, and not for the whole aging period? In line 144 we see “970 hours”, while in table 1 it lasts up to 1725 hours. The same for lines 154, , while in lines 164, 206, 243 1963 hours are mentioned.

 

Agree.

Corrected.

 

(see “970 hours”) I have corrected this.

The colorimetric samples were large (45 x 45 mm), but the analysis (Raman spectroscopy, ATF IR spectroscopy) was performed on a separate, smaller sample cut from the entire paint film during aging. Synchronization of the measurements was impossible due to the use of different instruments located in different locations. In any case, this desynchronization is not significant for the fundamental conclusions of the work.
Line 18. ATR-FTIR. Please, explain the abbreviations upon their first mentioning, not only in the main text, but in the abstract also.

Agree.

Corrected.

 

Line 18. Please, change to micro-Raman spectroscopy.

Agree.

Corrected.

 

Lines 30-31. Keywords act so as a potential reader to discover the study. There is reason to use same things that already appear in the title, such as acrylic artists’ paint. Please, revise them to promote your work.

Agree.

Corrected.

 

Lines 44-45. “uniform in color” and “texture” repetition.

Agree.

Corrected.

 

Line 68. Attenuated total reflectance

Agree.

Corrected.

 

Line 68. Micro-Raman spectroscopy

Agree.

Corrected.

 

Line 71. (PCA)

Agree.

Corrected.

 

Line 72. ATR-FTIR

Agree.

Corrected.

 

Line 83. Was the paint from a particular manufacturer?

 

      PW4:  Zinc Oxide (ZnO) 100, Acrylic Nevskaya Palitra 

      https://masterclass-acrylics.com/

 

Line 108. Reflectance. And please change the position of the parenthesis “Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy”

 

Agree.

Corrected.

 

Line 111. 4000-600 cm−1.

Agree.

Corrected.

 

 

Section 2.6. Please use micro-Raman spectroscopy. Check the whole manuscript.

 

Agree.

Corrected.

 

Section 2.7. As PCA analysis is mentioned in this section, why is this title used and not something like PCA analysis, or Chemometrics? Which softwares were used for spectral processing and PCA analysis?

Agree.

Corrected.

 

Spectral data acquisition was performed using software built into the Raman spectroscope-microscope and the ATR-FTIR spectroscope

Pre-processing of spectral data and their subsequent mathematical analysis were carried out using home-made scripts written in Python or MatLab using the SVD, PSA functions and the Savitsky-Golay algorithm.

 

Line 125. Outliers.

Agree.

Corrected.

 

Line 142. Colorimetry, maybe?

Agree.

Corrected.

 

Line 152. “∆E* must be >2 to be., while …”. Please rephrase, this is awkward.

Agree.

Corrected.

 

Line 234. “The use of averaged spectra (Figure 3) is due”

Agree.

Corrected.

 

Entire manuscript and tables. Please check the journal’s template for referencing in the manuscript, for example [1,2,3] instead of [1][2][3], using spaces and dots at the right places.

Agree.

Corrected.

 

Section 2.6. Please, add the resolution with which the spectra were collected.

Agree.

Corrected.

 

Table 2, first line. Please, correct the 31130 cm-1.

Agree.

Corrected.

 

Figures 3, 4, and supplementary file. Please plot FTIR spectra to descending wavenumbers (4000-600)

Agree.

Corrected.

 

Line 267. Better replace measured with collected.

Agree.

Corrected.

 

Line 268. The authors used a 50x lens for their measurements, giving a spot size of ~1 μ ZnO and CaCO3 grain size may vary between 300 nm – 2 μm, while formed agglomerates should be taken into account, as they can reach up to ~10 μm. For this reason, and after checking the corresponded micro-Raman spectra in the supplementary file, I would not be so assertive regarding attributing such small fluctuations to degradation phenomena. On the other hand, PCA results seem to indicate that organic degradation is recorded. Please, discuss this.

Agree.

 

This is a valuable point. We agree that agglomerate size is a consideration.

Our finding here is based on the comparison of Raman spectra before, during, and after the aging. We based our interpretation on many factors:

Probing Depth: The effective probing depth indeed exceeds 10 µm, making each spectrum a bulk-average measurement.

The Protective Role of Inorganic Particles: CaCO3 particles can reflect and scatter the light, protecting the surrounding binder. 

Formulation Chemistry: The paint formulation includes surfactants and dispersants specifically designed to minimize agglomerates and promote a homogeneous distribution. Although heterogeneity was detected in our paint, the absence of Raman spectra corresponding to pure CaCO₃ confirms that the inorganic particles are coated by and embedded within the organic binder matrix, and are not present as large, uncoated agglomerates."

 

Lines 306-307. 1850-600 and 3700-2700

Agree.

Corrected.

 

Line 312. “(figure 6a, where…”

Agree.

Corrected.

 

Line 519. Values and value repetition.

Agree.

Corrected.

 

Supplementary file. Please use images of better resolution.

Agree.

Corrected.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Comments:

It is recommended that the authors should get a minor revision for the paper in accordance with the above suggestions; After that, the paper can be accepted for publication.

The experimental design is rigorous and employs diverse methods. It utilizes accelerated artificial aging tests to simulate the natural aging process of coatings, with regular sampling and monitoring to ensure data timeliness. Meanwhile, a variety of advanced analytical techniques—such as ATR-FTIR, Raman spectroscopy microscopy, and SEM-EDS—are integrated to analyze the degradation process from multiple dimensions, including macroscopic color changes, microscopic morphology, and chemical composition. Additionally, principal component analysis is introduced to process complex spectral data, effectively eliminating interfering information and enhancing the reliability and scientific validity of the results.

It is proposed that the deep oxidation of organic intermediates and the volatilization of organic substances are the core reasons for the significant changes in chemical structure despite the ΔE* value (< 2) not reaching the threshold perceptible to the human eye. This revises the traditional cognitive bias of judging the aging degree of coatings solely based on color changes.

 

Suggestions:

1. For the Introduction section, it is suggested to include several more references, which will help understand how the aging mechanism of such materials is analyzed in the relevant field. There are numerous experimental papers on the photocatalytic performance of inorganic semiconductor materials; thus, comparative analysis should be conducted before proposing the aging mechanism.
2. In the sample preparation process, coating thickness is a key parameter affecting the aging rate of coatings. Different thicknesses may lead to differences in oxygen permeability and light absorption, thereby influencing the repeatability of degradation results. Relevant quantitative information needs to be supplemented.
3. The impact of differences between experimental conditions and natural aging environments on the applicability of conclusions has not been discussed. In accelerated aging tests, there are significant differences between high-intensity light, constant temperature and humidity, and the fluctuating light intensity as well as seasonal changes in temperature and humidity in natural environments. It is necessary to analyze the potential degradation pathway deviations caused by such differences and clarify the applicable scope of the research conclusions in natural aging scenarios.
4. Check the images in the article to ensure that labels such as scale bars are clear.

Author Response

Responses

We sincerely thank the reviewer for their time and insightful feedback on our manuscript. This feedback has been invaluable in helping us improve the quality and clarity of our work. We have carefully considered all suggestions and have revised the manuscript accordingly. Below we detail our responses to each of the reviewers' points

 

 

Reviewer  #3

 

It is recommended that the authors should get a minor revision for the paper in accordance with the above suggestions; After that, the paper can be accepted for publication.

The experimental design is rigorous and employs diverse methods. It utilizes accelerated artificial aging tests to simulate the natural aging process of coatings, with regular sampling and monitoring to ensure data timeliness. Meanwhile, a variety of advanced analytical techniques—such as ATR-FTIR, Raman spectroscopy microscopy, and SEM-EDS—are integrated to analyze the degradation process from multiple dimensions, including macroscopic color changes, microscopic morphology, and chemical composition. Additionally, principal component analysis is introduced to process complex spectral data, effectively eliminating interfering information and enhancing the reliability and scientific validity of the results.

It is proposed that the deep oxidation of organic intermediates and the volatilization of organic substances are the core reasons for the significant changes in chemical structure despite the ΔE* value (< 2) not reaching the threshold perceptible to the human eye. This revises the traditional cognitive bias of judging the aging degree of coatings solely based on color changes.

 

1. For the Introduction section, it is suggested to include several more references, which will help understand how the aging mechanism of such materials is analyzed in the relevant field. There are numerous experimental papers on the photocatalytic performance of inorganic semiconductor materials; thus, comparative analysis should be conducted before proposing the aging mechanism.

Agree.

Corrected

1. In the sample preparation process, coating thickness is a key parameter affecting the aging rate of coatings. Different thicknesses may lead to differences in oxygen permeability and light absorption, thereby influencing the repeatability of degradation results. Relevant quantitative information needs to be supplemented.

Agree.

This is a valuable point. We agree that coating thickness is a consideration.

Our finding here is based on the comparison of Raman spectra before, during, and after the aging. We based our interpretation on many factors:

Probing Depth: The effective probing depth indeed exceeds 10 µm, making each spectrum a bulk-average measurement.

The Protective Role of Inorganic Particles: CaCO3 particles can reflect and scatter the light, protecting the surrounding binder. 

Formulation Chemistry: The paint formulation includes surfactants and dispersants specifically designed to minimize agglomerates and promote a homogeneous distribution. Although heterogeneity was detected in our paint, the absence of Raman spectra corresponding to pure CaCO₃ confirms that the inorganic particles are coated by and embedded within the organic binder matrix, and are not present as large, uncoated agglomerates."

 

 

1. The impact of differences between experimental conditions and natural aging environments on the applicability of conclusions has not been discussed. In accelerated aging tests, there are significant differences between high-intensity light, constant temperature and humidity, and the fluctuating light intensity as well as seasonal changes in temperature and humidity in natural environments. It is necessary to analyze the potential degradation pathway deviations caused by such differences and clarify the applicable scope of the research conclusions in natural aging scenarios.

Agree.

 

1. Check the images in the article to ensure that labels such as scale bars are clear.

Agree.

 

While some studies utilize UV-specific sources (e.g., UV fluorescent lamps) or Xenon Arc Lamp with a UV filter for shorter durations (400–800 hours) [1,2] to isolate UV damage, this work utilized a xenon-arc lamp to replicate the full spectrum of solar radiation. This setup provides a superior simulation of the sunlight spectrum, including both ultraviolet and visible light, which is essential for accurately replicating real-world photodegradation mechanisms, especially for materials like ZnO that are sensitive to a broad range of wavelengths (below ~385 nm). Based on climate data compiled by IILSS[3], this accelerated aging period (1963h) is estimated to be equivalent to approximately one year of natural outdoor sunlight and approximately 91 years in a museum with light at 100 lux. The moderate temperature (38°C) and humidity (65% RH) were chosen to accelerate photooxidative reactions—the primary degradation mechanism for acrylic paints—without causing purely thermal or hydrolysis degradation, which would be less representative of real-world conditions.

[1] M. T. Doménech-Carbó et al., “Study of behaviour on simulated daylight ageing of artists’ acrylic and poly(vinyl acetate) paint films,” Anal. Bioanal. Chem., vol. 399, no. 9, pp. 2921–2937, 2011, doi: 10.1007/s00216-010-4294-3.

[2]      P. Aguilar-Rodríguez, A. Mejía-González, S. Zetina, A. Colin-Molina, B. Rodríguez-Molina, and N. Esturau-Escofet, “Unexpected behavior of commercial artists’ acrylic paints under UVA artificial aging,” Microchem. J., vol. 160, no. November 2020, 2021, doi: 10.1016/j.microc.2020.105743.

[3]      “Annual sunshine hours of the world map – IILSS-International institute for Law of the Sea Studies.” Accessed: Jul. 10, 2025. [Online]. Available: https://iilss.net/annual-sunshine-hours-of-the-world-map/

 

Author Response File: Author Response.pdf