Assessing Epoxy-Acrylate/Chitosan Films for Banknote Coatings: Thermal, Mechanical, and Gloss Performance

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the present work, authors reported a investigation into the development of chitosan-enhanced epoxy-acrylate varnishes for banknote coatings, demonstrating a promising results regarding mechanical properties and gloss. Overall, this work was interesting, and a series of results have also been discussed. However, some issues should be addressed.

 

1, In abstract section, the novelty problem statement described by the authors should be emphasized to attract general readers by providing more insights on the experimental observations.

 

2, In addition, the key statistical findings should be included in the abstract, such as what is enhancement of the mechanical and antimicrobial performance.

 

3, The introduction provided a relevant background on banknote contamination but needs to better discuss the novelty of integrating chitosan into UV-curable varnishes compared to existing solutions.

 

4, In FTIR section, the characteristic peaks should be labeled in Figure 4. Moreover, the FTIR characterization effectively confirms chemical interactions but requires deeper interpretation of how chitosan incorporation influences the epoxy-acrylate curing mechanism.

 

5, The discussion in DSC about the thermal mechanism might refers to some previous work to support the explanation.

 

6, The DMA data demonstrate improved mechanical properties at low chitosan concentrations, yet the absence of statistical analysis or error margins weakens the reliability of the conclusions.

 

7, Some key research results in mechanical and thermal property of composites should be mentioned and cited so that we can provide a solid background and progress to the readers, such as Polymer, 2025, 334, 128772, and others.

 

8, The manuscript maintains a logical flow, but minor grammatical errors and inconsistent terminology should be addressed to improve clarity.

Author Response

We are deeply grateful for the reviewer's insightful observations, and we are confident that this will allow us to improve our work.

1, In abstract section, the novelty problem statement described by the authors should be emphasized to attract general readers by providing more insights on the experimental observations. Re: We thank the reviewer for this important observation. The abstract has been improved according to the reviewer's suggestions and are shaded in the manuscript on lines 15-31

2, In addition, the key statistical findings should be included in the abstract, such as what is enhancement of the mechanical and antimicrobial performance. Re: We have decided to change the title of the work to one that is consistent with what we reported in this first stage, where we evaluated the technical feasibility of incorporating chitosan into resins commonly used for banknote coatings, ensuring that this additive, which has demonstrated antimicrobial properties as shown by the state of the art, does not affect the mechanical properties or the gloss of the coatings in this application.

3, The introduction provided a relevant background on banknote contamination but needs to better discuss the novelty of integrating chitosan into UV-curable varnishes compared to existing solutions. Re: In the introduction, the background was improved as requested by the reviewer, attaching the references that support the inclusion of lines 103 to 115.

4, In FTIR section, the characteristic peaks should be labeled in Figure 4. Moreover, the FTIR characterization effectively confirms chemical interactions but requires deeper interpretation of how chitosan incorporation influences the epoxy-acrylate curing mechanism. Re: We have made a change to Figures 1-3, placing them in the supporting materials. Therefore, Figure 4 is now Figure 1, and we have assigned the bands described in the FTIR spectroscopy analysis. We have also improved the analysis as suggested by the reviewer.

5, The discussion in DSC about the thermal mechanism might refers to some previous work to support the explanation. Re: We appreciate the reviewer’s suggestion. The DSC discussion has been revised and now cites relevant published work supporting the observed thermal behavior. Studies on chitosan-modified polymer systems report that low chitosan concentrations can enhance reactivity or increase Tg due to hydrogen bonding and restricted chain mobility, whereas higher loadings often lead to reduced curing efficiency and diffusional limitations caused by micro-agglomeration. These mechanisms are consistent with our findings for the epoxy-acrylate/chitosan films. The manuscript has been updated accordingly. Ref [53-55]

6, The DMA data demonstrate improved mechanical properties at low chitosan concentrations, yet the absence of statistical analysis or error margins weakens the reliability of the conclusions. Re: We thank the reviewer for this observation. We agree that statistical analysis is important when working with measurements that exhibit significant sample-to-sample variability. However, in the case of Dynamic Mechanical Analysis (DMA), standard practice in polymer science differs from conventional mechanical testing for these reasons: DMA is an instrumental thermal–mechanical technique with very low experimental variability. DMA is designed to measure viscoelastic transitions with high precision using a controlled oscillatory deformation and does not rely on destructive failure of samples. When the sample preparation and curing conditions are identical, the variation between replicates is typically minimal, as documented in reference DMA methodology texts (e.g., Menard 2008). The DMA signals are intrinsic material properties. Unlike tensile or impact tests, DMA does not depend on specimen geometry imperfections, necking, strain localization or operator-dependent failure modes. For this reason, many publications routinely report single representative curves per formulation when sample variability is negligible. As described in the revised methodology section, DMA tests were carried out on multiple specimens from different regions of each film. Since the curves were essentially superimposable, the inclusion of error bars would have resulted in visually overlapping lines and no added interpretive value. The resolution of modern DMA instruments is sufficiently high that variations below 0.5–1% in storage modulus (E’) or <0.1 °C in Tg fall within the noise level of the instrument. Reporting statistical descriptors in such conditions does not improve scientific clarity and is not standard.

To address the reviewer’s concern, we have added a statement in the revised manuscript clearly noting that DMA measurements were performed on multiple specimens and that the curves were highly reproducible across replicates. This reproducibility supports the reliability of the observed trends.

7, Some key research results in mechanical and thermal property of composites should be mentioned and cited so that we can provide a solid background and progress to the readers, such as Polymer, 2025, 334, 128772, and others. Re: We thank the reviewer for this valuable comment. We have reviewed the reference that the reviewer recommended and fully agree that accelerated aging, humidity resistance, abrasion endurance, adhesion strength, and UV stability are essential parameters for evaluating document and banknote coatings. These properties are indeed critical for assessing long-term durability in real service conditions. At this stage, however, the present manuscript reports only the first phase of a broader research project. As clarified in our revised title and abstract, this phase focuses on establishing whether chitosan can be incorporated into an epoxy-acrylate UV-curable varnish without compromising the fundamental thermal, mechanical, and surface properties required for flexographic application on banknote substrates. This step is necessary before performing more resource and time-intensive durability tests. We fully acknowledge the importance of the reviewer’s suggestion. However, due to the limited time provided for the revision of the manuscript, and the extensive duration and equipment required for conducting standardized durability tests (e.g., accelerated aging chambers, UV exposure cycles, abrasion resistance, humidity cycling, and cross-cut adhesion tests), it was not feasible to perform these experiments within the current revision window. To address the reviewer’s concern, we have, revised the manuscript (Introduction and Conclusions) to state explicitly that durability assessment (including UV exposure, abrasion resistance, humidity cycling, and adhesion) is part of a planned second stage of the project, following the material feasibility confirmation presented here. Added a statement explaining that future work will include accelerated aging tests, abrasion resistance, adhesion strength, moisture and humidity resistance tests, and UV-weathering stability. Clarified that the present results should be interpreted as a preliminary evaluation of the physicochemical feasibility of incorporating chitosan into a UV-curable varnish for secure documents, which is a prerequisite for subsequent durability and biodefense analyses. We believe these additions clearly delimit the scope of the current manuscript and incorporate the reviewer’s recommendation into the ongoing research framework. We sincerely appreciate the reviewer’s suggestion, which strengthens the methodological roadmap of the project.

8, The manuscript maintains a logical flow, but minor grammatical errors and inconsistent terminology should be addressed to improve clarity. Re: The manuscript was revised and improved

 

Reviewer 2 Report

Comments and Suggestions for Authors

1. The authors have focused mainly on mechanical (DSC, DMA) and optical (gloss) properties, with only a brief mention of antimicrobial functionality. They need to do antimicrobial assessment such as bacterial or fungal inhibition tests, and quantitative microbial reduction data. Without this, the claim of “providing antimicrobial properties” remains largely unverified.
2. Does the coating for documents resist aging, humidity, abrasion, and UV exposure? The authors didn’t mention accelerated aging, adhesion strength, or resistance to handling and environmental conditions, which are crucial for practical applications.
3. How did the authors make sure that the incorporation of chitosan into a varnish matrix, using a centrifuge, resulted to a uniform and homogeneous dispersion, especially at higher concentrations?
4. I would like to see some microscopic or surface morphology characterization such as SEM, and/or AFM to confirm uniform mixing or the presence of agglomerates.
5. Why did the authors only try the concentrations up to 5% and didn’t explore whether higher or optimized loadings could balance antimicrobial performance with acceptable optical and mechanical properties? The selection of 1% and 5% as “viable options” appears somewhat arbitrary to me without broader optimization data.
6. Is it only acetic acid that can be used to solubilize chitosan? If so, this may limit industrial scalability or compatibility with other varnish formulations. I am sure other solvents or pH-adjusted systems might yield different (perhaps better) dispersion or performance results!
7. Apparently, the authors assessed only film properties, not coated paper properties, which is a significant gap for practical implementation. For real-world document coatings, adhesion to paper or printed inks, print clarity, and writing/printing compatibility are key.
8. The authors did not report on the biodegradability, recyclability, or cost implications relative to standard varnishes.
9. While FTIR confirms chemical incorporation, I can only find little discussion on intermolecular interactions such as hydrogen bonding, and matrix compatibility. Th authors should elaborate more on how these factors influence observed mechanical and optical changes.

Author Response

We are deeply grateful for the reviewer's insightful observations, and we are confident that this will allow us to improve our work. We thank the reviewer for this pertinent questions.

 

1. The authors have focused mainly on mechanical (DSC, DMA) and optical (gloss) properties, with only a brief mention of antimicrobial functionality. They need to do antimicrobial assessment such as bacterial or fungal inhibition tests, and quantitative microbial reduction data. Without this, the claim of “providing antimicrobial properties” remains largely unverified. Re: We thank the reviewer for this important observation. We agree that antimicrobial functionality must eventually be demonstrated through quantitative microbial reduction assays. However, the present manuscript corresponds to the first stage of a broader research project, and its primary objective is to verify the technical feasibility of incorporating chitosan into an epoxy-acrylate UV-curable varnish intended for banknote paper coatings. Accordingly, and in line with the reviewer’s comment, we have made the following clarifications and adjustments: The title has been modified to reflect the actual scope of this study and to avoid suggesting that antimicrobial efficacy is evaluated here. The revised title emphasizes the thermal, mechanical and gloss properties of the epoxy-acrylate/chitosan films. The abstract has been rewritten to clearly state that: antimicrobial performance will be assessed in a subsequent phase of the Project. The current work focuses exclusively on thermal (DSC), mechanical (DMA) and optical (gloss) characterization to ensure that the incorporation of chitosan does not compromise the functional requirements of varnishes used on banknote substrates. In the Introduction and Conclusions, we now explicitly state that the purpose of this first stage is to determine whether chitosan can be successfully integrated into the UV-curable varnish matrix without affecting its core mechanical and surface properties, which is a prerequisite before conducting standardized antimicrobial testing. The manuscript no longer claims that the films “provide antimicrobial properties.” Instead, it states that chitosan is a biopolymer with well-known intrinsic antimicrobial potential and that the evaluation of antimicrobial activity of the optimized formulations (1% and 5% chitosan) will be the focus of a dedicated follow-up study. This staged approach is common in the development of multifunctional coatings: the initial step ensures that the mechanical integrity, thermal transitions, film formation, and optical quality remain compatible with industrial requirements, before moving on to biological performance test, We trust that these clarifications, title adjustment, and text revisions adequately address the reviewer’s concern and ensure that the scope of the manuscript is now fully aligned with the results presented.
2. Does the coating for documents resist aging, humidity, abrasion, and UV exposure? The authors didn’t mention accelerated aging, adhesion strength, or resistance to handling and environmental conditions, which are crucial for practical applications. Re: We thank the reviewer for this valuable comment and fully agree that accelerated aging, humidity resistance, abrasion endurance, adhesion strength, and UV stability are essential parameters for evaluating document and banknote coatings. These properties are indeed critical for assessing long-term durability in real service conditions. At this stage, however, the present manuscript reports only the first phase of a broader research project. As clarified in our revised title and abstract, this phase focuses on establishing whether chitosan can be incorporated into an epoxy-acrylate UV-curable varnish without compromising the fundamental thermal, mechanical, and surface properties required for flexographic application on banknote substrates. This step is necessary before performing more resource and time-intensive durability tests. We fully acknowledge the importance of the reviewer’s suggestion. However, due to the limited time provided for the revision of the manuscript, and the extensive duration and equipment required for conducting standardized durability tests (e.g., accelerated aging chambers, UV exposure cycles, abrasion resistance, humidity cycling, and cross-cut adhesion tests), it was not feasible to perform these experiments within the current revision window. To address the reviewer’s concern, we have, revised the manuscript (Introduction and Conclusions) to state explicitly that durability assessment (including UV exposure, abrasion resistance, humidity cycling, and adhesión) is part of a planned second stage of the project, following the material feasibility confirmation presented here. Added a statement explaining that future work will include, accelerated aging tests, abrasion resistance, adhesion strength, moisture and humidity resistance tests, and UV-weathering stability. Clarified that the present results should be interpreted as a preliminary evaluation of the physicochemical feasibility of incorporating chitosan into a UV-curable varnish for secure documents, which is a prerequisite for subsequent durability and biodefense analyses. We believe these additions clearly delimit the scope of the current manuscript and incorporate the reviewer’s recommendation into the ongoing research framework. We sincerely appreciate the reviewer’s suggestion, which strengthens the methodological roadmap of the project.
3. How did the authors make sure that the incorporation of chitosan into a varnish matrix, using a centrifuge, resulted to a uniform and homogeneous dispersion, especially at higher concentrations? Re: Several methodological and analytical procedures were implemented to ensure that chitosan was uniformly dispersed within the epoxy–acrylate varnish matrix, Chitosan was fully solubilized in dilute acetic acid under controlled pH and continuous stirring before mixing with the varnish. This step prevents the presence of undissolved particles and promotes molecular-level dispersion of the biopolymer. The solubilization behavior of chitosan in acidic media is well established in the literatura. The pre-solubilized chitosan solution was incorporated into the resin using controlled centrifugation parameters (3000rpm, 180 seconds). Centrifugation enhances wetting of biopolymers, eliminates micro-agglomerates, and removes entrapped air, improving dispersion quality particularly in viscous oligomeric matrices. Taken together, the combination of controlled solubilization, centrifugation, microstructural inspection and surface gloss consistency provides strong evidence that chitosan was homogeneously dispersed in the varnish matrix.
4. I would like to see some microscopic or surface morphology characterization such as SEM, and/or AFM to confirm uniform mixing or the presence of agglomerates. Re: We thank the reviewer for this valuable suggestion. Following this recommendation, we have incorporated Scanning Electron Microscopy (SEM) analysis into the revised manuscript to examine the surface morphology and evaluate the dispersion of chitosan within the cured varnish films. Because AFM was not available within the limited revision period, SEM was selected as the most appropriate and accessible technique to meet the reviewer’s request. SEM provides sufficient resolution and compositional contrast to detect micro-agglomerates or phase separation in polymer–biopolymer systems. The new SEM micrographs and corresponding discussion have been added to the Results section (Figure 4), and a detailed description of the methodology has been included in the Materials and Methods section. Analysis was performed using a JEOL JSM-IT100 microscope at 10 kV with composition backscattered electrons (BSE) to enhance contrast between the epoxy–acrylate matrix and chitosan domains. The images demonstrate that films containing 1% and 5% w/w chitosan exhibit a largely uniform morphology with no visible micro-agglomerates, while higher concentrations (10–20% w/w) show localized domains consistent with reduced dispersion, supporting the thermal–mechanical interpretation discussed earlier. We believe that the inclusion of SEM analysis strengthens the manuscript and addresses the reviewer’s request for microstructural evidence of chitosan dispersion.
5. Why did the authors only try the concentrations up to 5% and didn’t explore whether higher or optimized loadings could balance antimicrobial performance with acceptable optical and mechanical properties? The selection of 1% and 5% as “viable options” appears somewhat arbitrary to me without broader optimization data. Re: We appreciate the reviewer’s thoughtful comment. The selection of 1% and 5% chitosan as viable concentrations is grounded in the experimental evidence obtained in this first stage of the work, rather than being arbitrary. Higher concentrations were indeed evaluated in the study (10% and 20% w/w), and the results demonstrated clear limitations. DSC showed reduced curing rate and decreased conversion at low temperatures, consistent with diffusion-limited kinetics due to micro-agglomeration of chitosan. DMA revealed a marked increase in rigidity, lower ductility, and reduced toughness at 10% and 20% w/w, indicating that these films do not meet the mechanical requirements for banknote paper coatings. These results were also supported by SEM micrographs, which revealed morphological heterogeneity and localized chitosan domains at 10–20% w/w, confirming reduced dispersion and explaining the deterioration in properties. In contrast, the 1% and 5% w/w films exhibited improved or preserved thermal and mechanical behavior, adequate ductility and toughness, minimal changes in gloss and translucency, and homogeneous microstructure, as observed by SEM. Therefore, 1–5% w/w concentrations represent the upper limit at which the varnish maintains the performance standards required for banknote paper coatings, and this conclusion arises directly from comparative data. The results conceptually narrow the viable range to 1–5%, which is the only region where all functional requirements are preserved. We agree that the balance between antimicrobial activity and material performance must be optimized. For this reason, the next phase of the project will include, quantitative antimicrobial testing, evaluation of intermediate concentrations within the 1–5% range, and durability assessments (accelerated aging, abrasion, humidity, UV exposure).

 

6. Is it only acetic acid that can be used to solubilize chitosan? If so, this may limit industrial scalability or compatibility with other varnish formulations. I am sure other solvents or pH-adjusted systems might yield different (perhaps better) dispersion or performance results!

Re: We thank the reviewer for this insightful comment. We fully agree that the choice of solvent for chitosan solubilization is relevant for both material performance and industrial scalability. Our decision to use acetic acid in this first stage was based on that acetic acid is the standard and most widely documented solvent for chitosan dissolution. Chitosan requires protonation of its amino groups (–NH₂ → –NH₃⁺) to become soluble, and dilute acetic acid is the most commonly used protonating medium due to its efficiency, mild acidity, low toxicity, low cost, and compatibility with polymeric systems. This is extensively supported in the literature (Ref [3]). Acetic acid was selected to ensure maximum reproducibility and controlled dispersion during this feasibility phase. Since the primary objective of this study was to verify whether chitosan can be incorporated into a UV-curable epoxy–acrylate varnish without compromising its thermal, mechanical, or optical properties, using the conventional solvent minimized variables and ensured that differences observed across formulations were attributable to chitosan concentration (not to solubilization chemistry). We fully acknowledge that other acidic systems or pH-adjusted solvents may improve dispersion or performance. Indeed, alternatives such as lactic acid, citric acid, formic acid have been reported to modify chitosan–polymer interactions or influence viscosity and film morphology. However, exploring solvent effects was outside the scope of the present first-stage study. We appreciate the reviewer’s point, as it highlights an important direction for future optimization.

7. Apparently, the authors assessed only film properties, not coated paper properties, which is a significant gap for practical implementation. For real-world document coatings, adhesion to paper or printed inks, print clarity, and writing/printing compatibility are key.

Re: We appreciate the reviewer’s observation and fully agree that evaluating the coating directly on paper substrates (especially those used for security documents) is essential for practical implementation. These assessments include adhesion to cellulose fibers, interaction with printed inks, print clarity, and compatibility with writing and handling. However, as clarified in the revised title, abstract, and conclusions, the present manuscript corresponds to the first stage of a broader research project, focused exclusively on verifying the physicochemical feasibility of incorporating chitosan into an epoxy–acrylate UV-curable varnish. The aim of this initial phase was to determine whether chitosan affects the curing behavior, thermal transitions, mechanical properties, and optical appearance of the varnish itself before applying it to banknote paper. Pre-evaluation of varnish films is common practice in coatings science because it allows isolating the intrinsic effects of formulation changes before introducing the variability associated with complex substrates such as banknote paper, which contains cotton fibers, security threads, and multiple layers of ink. Security-grade banknote paper is not always readily available, and standardized adhesion tests (e.g., ASTM D3359 cross-cut, tape adhesion, print clarity analysis) require uniform substrates and controlled printing conditions. These resources were not available within the limited revision period provided. Although the current stage assesses only free-standing films, this was an intentional and necessary step to ensure that introducing chitosan does not compromise fundamental material performance. A complete evaluation on paper substrates (Form 3NT-31) is planned and has been explicitly incorporated into the manuscript.

 

 

8. The authors did not report on the biodegradability, recyclability, or cost implications relative to standard varnishes. Re: We thank the reviewer for this valuable comment and fully agree that biodegradability, recyclability, and cost implications are crucial considerations for the long-term adoption of bio-based or hybrid varnish systems. However, the present manuscript corresponds to the first stage of a broader project and is focused exclusively on evaluating the thermal, mechanical, and optical feasibility of incorporating chitosan into a UV-curable epoxy–acrylate varnish. Biodegradability and recyclability assessments require a validated formulation and this manuscript does not claim environmental superiority, only technical compatibility. The incorporation of chitosan was evaluated here strictly from a materials-performance perspective, not from a sustainability or cost perspective. The revised Title, Abstract, and Conclusions now emphasize this focus to avoid overextension of claims. These additions have now been incorporated into the manuscript to clearly delineate the scope of the present work and its planned continuation.
9. While FTIR confirms chemical incorporation, I can only find little discussion on intermolecular interactions such as hydrogen bonding, and matrix compatibility. Th authors should elaborate more on how these factors influence observed mechanical and optical changes. Re: We thank the reviewer for this important observation. Following this suggestion, we have expanded the FTIR and Discussion sections to address intermolecular interactions and their influence on the mechanical, thermal, and optical behavior of the chitosan–varnish films. Chitosan contains protonated amino groups (–NH₃⁺) and hydroxyl groups (–OH) capable of forming hydrogen bonds with carbonyl, ether, and hydroxyl functionalities present in the epoxy–acrylate resin. Literature has shown that such interactions can restrict segmental mobility, influence Tg, and modify viscoelastic behavior. In the revised manuscript, we now explain that the observed increase in Tg at 1–5% w/w chitosan is consistent with this type of intermolecular interaction and higher effective cross-link density. Although no new covalent bonds are formed, we now describe how shifts in the O–H/N–H stretching region broadening of the hydrogen-bonding bands (~3200–3500 cm⁻¹) and slight changes in C=O stretching intensity suggest hydrogen-bond formation and improved matrix compatibility at low concentrations. This explanation has been added to the FTIR section of the Results. A dedicated paragraph has been inserted into the FTIR subsection, and a more complete explanation has been added to the Discussion section connecting hydrogen bonding and compatibility with thermal transitions (DSC), viscoelastic behavior (DMA), and optical appearance (gloss). These additions provide a clearer mechanistic understanding of how intermolecular interactions between chitosan and the epoxy–acrylate matrix govern the property changes observed in this first-stage formulation study.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Can be published now.