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Article

Fine-Tuning Flexographic Ink’s Surface Properties and Providing Anti-Counterfeit Potential via the Addition of TiO2 and ZnO Nanoparticles

by
Tamara Tomašegović
1,*,
Sanja Mahović Poljaček
1,*,
Ivona Jurišić
2 and
Davor Donevski
1
1
Faculty of Graphic Arts, University of Zagreb, Getaldićeva 2, 10000 Zagreb, Croatia
2
School of Graphics, Design and Media Production, Getaldićeva 2, 10000 Zagreb, Croatia
*
Authors to whom correspondence should be addressed.
Micro 2025, 5(2), 20; https://doi.org/10.3390/micro5020020
Submission received: 13 March 2025 / Revised: 15 April 2025 / Accepted: 23 April 2025 / Published: 28 April 2025
(This article belongs to the Section Microscale Materials Science)
Browse Figures
Review Reports Versions Notes

Abstract

:
The objective of this research was to fine-tune the surface properties of printed ink layers by incorporating TiO2 and ZnO nanoparticles into conventional flexographic ink. This modification aimed to improve print quality while simultaneously providing protection against counterfeiting. The presence of nanoparticles in the inks was indirectly detected through FTIR-ATR spectroscopy, which revealed changes in the fingerprint region of the ink spectrum when nanoparticles were added. This alteration enhanced the anti-counterfeiting potential of a produced print. The colorimetric measurements indicated that the addition of nanoparticles did not significantly affect the colorimetric properties of the print, since the maximal calculated ΔEab value was 2.83. However, the nanoparticles notably improved the ink coverage on printed line elements and allowed for the printing of elements without the characteristic outline associated with flexographic printing. The best results in terms of line definition and coverage were achieved with the addition of 2% rutile TiO2 and 1% ZnO to the ink: the measured line segment area covered in ink was 28.5% larger than the same area printed using unmodified ink. This improvement in print quality was attributed to the modified surface free energy (SFE) of the inks, which also influenced the adhesion parameters between the printed layer and the printing substrate. The lowest total SFE was calculated for the ink without added nanoparticles (40.31 mJ/m2), and the highest for the ink with the addition of 2% rutile TiO2 (48.33 mJ/m2). The work of adhesion increased after adding the nanoparticles to the ink, thereby improving the adhesion. The highest work of adhesion (79.36 mJ/m2) was calculated for the ink with 2% rutile TiO2. Interfacial tension was low and close to zero for all printed ink layers, and the lowest value was achieved for the ink without added nanoparticles (1.47 mJ/m2). The findings of this research demonstrated that fine-tuning the properties of flexographic inks using nanoparticles can yield several benefits in terms of optimizing the quality of and providing counterfeit protection for specific printed motifs.

1. Introduction

Flexography is a printing technique utilized for various applications, today most commonly in packaging printing and printed electronics [1,2]. Initially, it was a printing technique primarily used for printing on corrugated cardboard and paper bags [3]. Nowadays, some specific applications of modern flexography include cold foil printing, perovskite solar cells, and wearable electronics, as well as providing an aspect of sustainability by using the flexographic printing plate waste from eco-adsorbent films for the removal of pollutants [4,5,6,7].
Flexography is characterized by the use of flexible polymer printing plates, which require low pressure between the printing plate and the substrate. It allows for the use of a wide range of printing inks with different viscosities and other properties [8,9,10]. The printing areas (surfaces that carry the image intended for printing) on the printing plate are raised and are aligned on the same plane, while the non-printing areas are dented (Figure 1a,b).
During the printing process, the flexible printing plate elastically deforms to an extent because of the printing pressure, which can result in the slight deformation of the image carried by the printing elements and, consequently, the “outline” being visible on a printed element (Figure 1c). This issue is particularly evident when printing on polymeric substrates, as they tend to have low surface free energy and do not absorb ink [11,12,13]. On the other hand, the ink tends to absorb and spread on absorbent substrates such as paper and board, creating a softer outline and making the issue less noticeable. However, this characteristic of flexographic printing can become problematic when printing fine details, such as the thin lines in barcodes that must be printed accurately for proper recognition by scanners [14]. To enhance the quality of a flexographic print and eliminate outlines, several methods can be used. These include adjusting the properties of the printing plate, careful adjustment of the printing pressure, and adjustment/modification of the substrate or printing ink properties [11,15,16,17]. Since flexography can utilize a wide range of printing inks with different properties, this versatility has led to significant research and advancements in the creation of specialized and tailored inks with novel materials in their composition. For example, different types of binders, such as cellulose-based and acrylic, have been investigated for application in water-based inks [18,19]; compositions of flexographic inks intended for food contact packaging have been researched, as have different crosslinkers in biodegradable coatings for use in flexography [8,20].
An important area of research on printing inks regards micro- and nano-sized additives for the optimization of properties and special applications when using various printing techniques, including flexography [5,21]. Specifically, titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles are of particular interest and are widely used for different applications in printing inks. The benefits of coatings and inks that incorporate these nanoparticles are numerous, including enhanced antibacterial properties, improved fire and scratch resistance, UV protection, increased hydrophobicity, and additional anti-corrosion features. Research in graphic technology has examined a range of applications for these nanoparticles, such as controlling the rheological properties of inks, mechanically protecting printed layers, and enhancing colorimetric stability over time (improved lightfastness).
TiO2 nanoparticles are particularly appealing for coating applications due to their photocatalytic activity and ability to provide UV protection. They can improve the mechanical strength, thermal stability, weathering resistance, and antibacterial properties of inks and coatings. Furthermore, rutile TiO2 nanoparticles can improve the impact strength and adhesion of the coatings [22]. The research examining titanium and composite metal/metal oxide titanium thin films on glass, focusing on the photocatalytic activity of these coatings, found that after a certain period of exposure to irradiation, all films exhibited superhydrophilicity [23]. TiO2 and ZnO nanoparticles were also synthesized to evaluate their impact on anticorrosive properties, antibacterial effectiveness, and self-cleaning efficiency [24]. The results of the research have shown that modifying inks with a low concentration of TiO2 nanoparticles enhanced their physicochemical properties. Moreover, TiO2 nanoparticles are being investigated and successfully used in the printing of solar cells, in 3D printing, in offset inks, and in specialized printing, such as ceramic layers [25,26,27,28]. Inks with ZnO nanoparticles are also being utilized in 3D printing and solar cells [29,30], with some other interesting applications such as printed pH sensors, electrohydrodynamic printing, and improving the performance of polyacrylate latex binders in water-based inks [31,32,33]. However, when using nanoparticles in printing inks, it is important to emphasize that any modification of the ink composition must be carefully assessed for its impact on colorimetric properties, surface characteristics, and other essential ink properties related to the printing process. Furthermore, the impact of the nanoparticles on the recyclability of the product and the impact on human health and the environment need to be considered. Regarding recycling, approximately 91% of TiO2 nanoparticles can be trapped in traditional water waste treatment plants, but 74–85% of total TiO2 nanoparticles are retained in the biological treatment system in the long run. Therefore, advanced methods such as alkali flux sintering with selective leaching and physico-chemical treatment technologies, such as aided coagulation, are sometimes the best options for TiO2 removal [34,35]. On the other hand, ZnO nanoparticles can easily be recycled [36,37]. The impact of the nanoparticles on human health certainly depends on their properties, such as the composition of the nanoparticle, size, surface functionality, crystallinity, and aggregation. Since it has been proven that nanoparticles’ toxic effects include cell death, the production of oxidative stress, DNA damage, apoptosis, and the induction of inflammatory responses [38], their effective removal from the environment is very important.
Nevertheless, nanoparticles have many uses and enormous potential in industrial and technological applications, including graphic reproduction. Besides the improvement of the functional properties of coatings and inks, nanoparticles have the potential to be used in security applications since they can be detected using forensic analysis [39,40,41]. With advancements in technology, counterfeiting has become a widespread issue affecting nearly all sectors of the economy. Today, virtually anything can be counterfeited—ranging from food and pharmaceuticals to parts for cars and machinery. Among the most common counterfeiting attempts in graphic technology is the forgery of valuable documents and specific packaging products, such as banknotes and packaging for medicines, cosmetics, and fragrances. To combat counterfeiting, or at least to make it less profitable, products are often safeguarded with protective elements. These may include specially designed printing substrates (usually paper), the use of various specialized security inks, and the incorporation of protective graphic elements such as holograms [42,43,44]. Security printing is one of the most challenging areas within graphic technology, necessitating constant adaptation to new technologies and proactive measures to stay ahead of counterfeiters. Conventional printing inks are usually not applicable in security printing. They are openly accessible and do not possess any security features present in specialty inks. Therefore, conventional prints, including flexographic ones, are a relatively easy target for counterfeiting.
In this research, anatase TiO2, rutile TiO2, and ZnO nanoparticles were added in specific concentrations to conventional inks for flexographic printing. The aim was to fine-tune the surface properties of the printed ink layers and simultaneously enhance protection against counterfeiting in conventional flexographic printing. The adjustment of the surface free energy components of the ink results in improved quality of the printed elements and a possibility of tailoring the interfacial interaction with different printing substrates, while the presence of nanoparticles can be indirectly detected using FTIR-ATR spectroscopy, thereby providing a security feature to conventional printing ink. This research marks an advancement in the functional modification of commercial flexographic printing inks. It utilizes various nanoparticles with the dual purpose of enhancing print security and improving the quality of printed fine elements. To our knowledge, this type of research has not been conducted before, and this work could serve as a foundation for future studies on methods for improving the quality of flexographic print: it aims to advance the flexographic printing system and explore new applications for flexographic inks and produced prints.

2. Materials and Methods

2.1. Materials Used for Printing

Polyester (PE) foil (Melinex) with a thickness of 105 µm was used as the printing substrate in this research, and the ink used was SunChemical UV-curable process black ink. Concentrations of 1% and 2% of the nanoparticles anatase TiO2 (99.9+%, metals basis, Alfa Aesar, Haverhill, MA, USA), rutile TiO2 (99.5% Sigma Aldrich, St. Louis, MO, USA), and ZnO (NanoArc ZN−0605, Alfa Aesar, Haverhill, MA, USA) were added to the printing inks with the aim of modifying their properties (Table 1).

2.2. Printing Process

Before printing, the printing substrate was cut into samples measuring 5 × 70 cm and conditioned to a relative humidity of 55 ± 5% and a temperature of 23 ± 1 °C. The modified inks were prepared by incorporating the three mentioned types of nanoparticles into the process black (PB) ink. The nanoparticles were mixed into the inks using a Hielscher device for homogenization (Hielscher Ultrasonics GmbH, Teltow, Germany) at 100% amplitude for 2 min.
The printing process was carried out in the laboratory using the IGT Printability Tester F1 (IGT Testing systems, Almere, The Netherlands), under the same conditions of 55% relative humidity and 23 °C. An anilox roller with a screen ruling of 90 L/cm and a cell volume of 18 mL/m² was utilized. The printing plate used was Flint´s NEXT plate, and the printing pressure was set to 300 N. The motif transferred to the Melinex substrate via the printing plate consisted of fine positive and negative lines (needed for the microscopy and image analysis), as well as full-tone areas (needed for the measurement of the colorimetric values, surface properties, and FTIR-ATR spectroscopy). Each modification of the printing ink underwent four prints, with the first print of each series being discarded due to the differing initial interactions between the cleaned and dried flexographic printing plate and the ink.
After printing, the samples were cured in a Technigraf Aktiprint L 10–1 UV dryer (Techngraf GmbH, Hessen, Germany), with the emission of the UV source in the range from 210 to 380 nm (2870 ± 5 mW/cm2) and 320 to 420 nm (1150 ± 20 mJ/cm2), with a speed of 4 m/s and two passes under the UV source. Following a stabilization period of 24 h, all measurements and analyses were performed.

2.3. Methods of Measurement and Analysis

2.3.1. Colorimetric Measurements

Colorimetric measurements were performed using a Techkon SpectroDens spectrophotometer B703902 (TECHKON GmbH, Königstein, Germany). The CIE L*a*b* values were measured on the printed ink layers. Each sample was measured 10 times at different positions. The measurement conditions were set to radiation source D50, standard observer at 2°, and filter M1. Besides the CIE L*a*b* calculations, color differences between the prints obtained using the unmodified and modified inks were determined by calculating the ΔEab value. The CIE system of colorimetry is the internationally agreed metric for color measurement. All the official color-related international standards and specifications use the CIE system. In the CIELAB space, L* represents the lightness, and (a*, b*) the specific hues. Therefore, the coordinate (L*, a*, b*) represents a specific color in a three-dimensional space. The color difference in the CIELAB space (ΔEab) is calculated as the Euclidean distance between the points in this three-dimensional space [45]; the formula used for the calculation of ΔEab in this research is presented in Equation (1):
Δ E a b = ( L 1 − L 2 ) 2 +   ( a 1 − a 2 ) 2 +   ( b 1 − b 2 ) 2

2.3.2. FTIR-ATR Spectroscopy

FTIR-ATR spectroscopy was used to identify the presence of specific functional groups in the printed unmodified and modified ink layers. This method was particularly used to detect changes in the PB ink layers after incorporating nanoparticles to enhance protection against counterfeiting. FTIR-ATR analysis was performed using a Shimadzu IRAffinity−1 FTIR spectrophotometer (Shimadzu Corporation, Kyōto, Japan), with 15 scans performed for each sample.

2.3.3. Analysis of Surface and Interfacial Properties

The surface properties of the produced films were analyzed by measuring the contact angle of the water on the surfaces of the printed layers and calculating their surface free energy (SFE). The contact angles of probe liquids—demineralized water, diiodomethane, and glycerol—were measured. These liquids have known polar (γP) and dispersive (γD) surface tension components. The total, dispersive, and polar surface free energy components of the probe liquids, expressed in mJ/m², are presented in Table 2:
The measurements were PBerformed using a DataPhysics OCA 30 goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany). A volume of 1 µL was used for each liquid drop, and the calculations were performed using the sessile drop method. All contact angle measurements were taken with a delay of 5 s after the drop made contact with the sample surface, and the average value of ten measurements was calculated. The results of the contact angles of the water on the film surfaces were presented separately to determine if the nanoparticles in the inks influenced their hydrophilic/hydrophobic properties.
The contact angle measurements and the surface tension of all three probe liquids were used to calculate the surface free energy (SFE) of the produced films using the Owens, Wendt, Rabel, and Kaelble method (OWRK) [46]. This calculation included the total SFE, as well as its polar and dispersive components, which helped define the surface changes in the modified ink layers and investigate their interactions with the used substrate, as well as explore potential wettability and adhesion with other materials [46,47].
The adhesion parameters, which include the thermodynamic work of adhesion (W12), the wetting coefficient (S12), and the interfacial tension (γ12), were calculated [48]. To achieve optimal adhesion, the thermodynamic work of adhesion needs to be as high as possible, the wetting coefficient should be close to zero, and the interfacial tension should be either positive or equal to zero.

2.3.4. Microscopy and Image Analysis

Microscopic images of the printed lines were captured using an Olympus BX 51 microscope (Olympus, Tokyo, Japan) at a magnification of 50×. The widths of the selected lines, both positive and negative, were measured ten times for each line with the support of Stream Motion software. Image analysis of the printed lines was performed to calculate the changes in the ink coverage of the printed lines after the addition of the nanoparticles. Image analysis of the printed lines was performed to calculate the changes in the ink coverage of the printed lines after the addition of the nanoparticles. Calculations were performed in Python 3.12.10 programming language using Python Imaging Library and Numpy packages. The microscopic images contain a scale in micrometers. The scale was used to determine the length of pixels’ sides by dividing the scale’s length expressed in micrometers by its length expressed in the number of pixels. The image must be converted to binary using the appropriate threshold to calculate the area of a line from its image. The images of the lines were first converted from RGB to grayscale mode. Grayscale images were then processed with the “autocontrast” function, which remaps an image’s pixels such that the darkest pixel becomes black and the lightest pixel becomes white. Grayscale images were then converted to binary using the 80% threshold. That is, pixels with values ≥80% black were mapped to black, and those with values <80% black were mapped to white. The area of a line was then calculated by multiplying the number of black pixels in the image by the area of a single pixel.

3. Results and Discussion

3.1. Colorimetric Properties of the Printed Layers

Figure 2 presents the CIE L*a*b* values measured on the unmodified and modified prints. It can be seen that the lightness (L*) and b* values were most affected when the nanoparticles were added to the ink.
Specifically, the lowest average L* value was measured on the unmodified print (10.13), while the maximal L* value of 12.87 was measured on the print with 2% ZnO nanoparticles. Furthermore, a* values ranged from 0.34, measured on the print with 2% rutile TiO2, to 0.76 for the unmodified print. The lowest b* value of −0.78 was measured on the print with 2% rutile TiO2, and the highest b* value was on the unmodified print (0.1). It can be concluded that the addition of 2% rutile TiO2 to the printing ink resulted in the most expressed changes in colorimetric values of the printed layer among the measured samples. The measurement of the color differences between the modified and unmodified prints by determining the ΔEab value according to [45] showed that the ΔEab values were lower than 3 for all prints produced using the unmodified ink (Table 3). Specifically, the highest ΔEab was calculated on the print with 2% ZnO (2.83). The lowest ΔEab value of 2.23 was calculated on the print with 2% rutile TiO2. A well-known fact is that values of ΔE lower than 3 are determined as being under the limit of eye perception for most people.
Considering the range of obtained CIE L*a*b* measurements and calculated ΔEab values, it can be concluded that the addition of the anatase/rutile TiO2 or ZnO nanoparticles to the black UV-curable flexographic ink in the concentration up to 2% did not significantly affect the colorimetric properties of the printed PB ink layer. However, if modifying other ink colors (for example, cyan, magenta, yellow, or some other color), it is advisable to carry out colorimetric measurements and determine whether nanoparticles have a significant influence on the changes in CIE L*a*b* values for those colors.

3.2. FTIR-ATR Spectroscopy

Changes in the FTIR-ATR spectra after adding a 2% concentration of TiO2 and ZnO nanoparticles to the PB ink are presented in Figure 3. The presented spectra are characteristic of the composition of flexographic UV-curable inks [49,50]. Asymmetric stretching of the C-H bond is observable between 2800 and 3000 cm−1. C=O bond stretching is present in all observed samples at ~1720 cm−1. The bands around 1100–1200 cm−1 can be attributed to the vibrations of the C-O bond [51].
It can be seen that subtle changes in the modified ink spectra are present mainly in the fingerprint region, specifically between 954 cm−1 and 1028 cm−1.
Specifically, with the addition of TiO2 nanoparticles, the band at 954 cm−1 shifted towards 975 cm−1. For the ink with ZnO nanoparticles, this shift did not occur; however, the peak at 954 cm−1 decreased in intensity compared to the spectrum of the ink without added ZnO nanoparticles. The band at 1020 cm−1, present in the spectrum of the ink without nanoparticles, shifted to 1028 cm−1 after nanoparticles were added to the ink. It is possible to conclude that with accurate knowledge of the FTIR-ATR spectrum of the unmodified PB ink, it is possible to identify the differences in the fingerprint area and thereby recognize that the ink has been modified using the nanoparticles. FTIR-ATR spectroscopy (as well as other types of spectroscopic analyses, for example, Raman, UV-Vis, etc.) can therefore be used as a method for the detection of intentionally modified ink for anti-counterfeiting purposes.

3.3. Surface and Interfacial Properties of the Prints

3.3.1. Contact Angle of Water

The measured contact angles of the water on the unmodified and modified prints are presented in Figure 4. The changes in the contact angles upon the addition of nanoparticles to the PB ink are indicative of the changed hydrophilicity of the printed ink layers.
It is visible that the highest average value of the water contact angle among the measured surfaces was obtained on the unmodified print (74.93°), leading to the conclusion of this print having the lowest hydrophilicity among the prints. The lowest contact angle of water was measured on the print with 1% rutile TiO2 (54.62°). In general, the decreased contact angle of water, i.e., increased hydrophilicity after adding the nanoparticles to the ink, was expected due to their polar nature. Among the printed ink layers modified with nanoparticles, the highest contact angle of water was measured on the print with 1% ZnO (68.12°) and 2% rutile TiO2 (61.81°). These values are higher than the values of the water contact angles on the other modified prints, which could be explained by the nanoparticle type and size (Table 1), as well as the possibility of nanoparticle agglomeration, which could have decreased their specific surface area and polarity to an extent [52,53]. Observed in the framework of material interactions in the graphic reproduction process, the polarity of the printed ink layer can influence the quality of the print and the adhesion of an overprinted layer, and certainly influence the adhesion of the ink to the substrate.

3.3.2. Surface Free Energy of Printed Layers

Figure 5 presents the surface free energy (SFE) components of the printed unmodified and modified inks. The lowest total SFE was calculated for the ink without added nanoparticles (40.31 mJ/m2), and the highest for the ink with the addition of 2% rutile TiO2 (48.33 mJ/m2).
The lowest value of dispersive SFE was calculated for the ink without the nanoparticles (36.39 mJ/m2), and the highest value was observed for the ink with the addition of 2% rutile TiO2 (40.42 mJ/m2). Furthermore, the lowest value of polar SFE was calculated for the ink without the addition of nanoparticles (3.91 mJ/m2), and the highest value was calculated for the ink with the addition of 1% anatase TiO2 (10.42 mJ/m2).
Although the changes in the total SFE of the printed ink layers upon adding nanoparticles are not very clear, it is noticeable that the increase in total SFE of the modified prints occurred mainly due to the increased polar SFE component. The range of SFE components obtained only by adding different nanoparticles in different concentrations to the basic PB ink can be used to fine-tune the surface properties of the ink and optimize the quality of the print. In this way, with the possibility of the addition of higher concentrations of nanoparticles, SFE components of the printing inks can be adjusted for printing on different substrates and for using printing plates with different surface properties. Even the modest adjustment of the polar and dispersive ink SFE can result in better ink transfer from the printing plate to the printing substrate and improve the quality of the print, which is observable in the reproduction quality of the printed lines in this research.

3.3.3. Adhesion Parameters

The adhesion parameters between the printed ink layers and Melinex printing substrate (work of adhesion (W12), wetting coefficient (S12), and interfacial tension (γ12)) were calculated using the SFE components of all surfaces in contact. The calculated total, dispersive, and polar SFE components of Melinex were 35.5 mJ/m2, 34.9 mJ/m2, and 0.6 mJ/m2, respectively. The adhesion parameters are presented in Figure 6.
Observing Figure 6a, it can be concluded that the work of adhesion increased after adding the nanoparticles to the ink, thereby improving the adhesion. The highest work of adhesion (79.36 mJ/m2) was calculated for the ink with 2% rutile TiO2. Interfacial tension was low and close to zero for all printed ink layers (Figure 6b), and the lowest value was achieved for the ink without added nanoparticles (1.47 mJ/m2). The wetting coefficient, on the other hand, was negative for all printed ink layers. This was expected since the polar component of SFE for the Melinex substrate was very close to zero (0.6 mJ/m2). The wetting coefficient with the smallest negative value (−6.28 mJ/m2) was calculated for the ink without the added nanoparticles. The obtained negative values of the wetting coefficients are not unusual for the flexographic ink–polymer substrate interaction because of the low polarity of polymeric substrates in flexography. For this reason, surface treatments of such printing substrates are usually recommended [54].
Overall, the best adhesion among the inks with added nanoparticles was achieved between the substrate and the ink with 1% ZnO, where the work of adhesion was among the highest, the value of interfacial tension was the lowest, and the smallest negative value of the spreading coefficient was present. It is possible to conclude that the modification of UV-curable flexographic PB ink using nanoparticles did primarily improve the work of adhesion between the ink and the used Melinex substrate.
To illustrate some possibilities of tailoring and optimizing the ink–substrate interaction, the adhesion parameters between the inks used in this research and a polymer material that can be used as a printing substrate but, contrary to Melinex, has a polar SFE component (polymethylmethacrylate–PMMA), were calculated and are presented in Table 4.
Since there are many variations in PMMA polymers and their surface free energy components can differ, for these calculations, a total SFE of 41.1 mJ/m2, a dispersive SFE of 29.6 mJ/m2, and a polar SFE of 11.5 mJ/m2 were used. Observing Table 4, it can be concluded that the addition of the nanoparticles to the PB ink would decrease the interfacial tension between the ink and PMMA, and noticeably increase the work of adhesion, thereby improving the interaction between the ink and the substrate. The wetting coefficient, on the other hand, would still achieve negative values, but they would not be as expressed as for Melinex, since PMMA’s surface is more polar. The lowest interfacial tension between the ink and PMMA would be achieved when anatase TiO2 is added to the ink, and the highest work of adhesion would be achieved for the ink with added rutile TiO2. There are many polymer substrates with surface free energy components that fall between those of Melinex and PMMA, so the choice and concentration of the used nanoparticles should be tailored for them. Furthermore, surface treatments of the substrates, such as corona and plasma, would alter the surface free energies of the substrates and enable further optimization of the interaction between the modified ink and the substrates.
It can be concluded that adding nanoparticles to flexographic ink makes it possible to modify the surface properties of the ink, allowing for easier adjustments when printing on various substrates that have a polar SFE component. This could further expand the range of applications for flexographic printing inks for different types of graphic products.

3.4. Microscopy and Image Analysis of the Printed Lines

The microscopy analysis aimed to determine the influence of nanoparticles on the appearance of line edges (i.e., the existence of the outline) and the effect of the nanoparticles on the change in their width due to the different spreading behaviors of the ink on the printing substrate caused by the changes in the inks’ SFE components. Utilizing microscope software support, the thin lines with a nominal width of 200 µm were measured on the prints obtained using unmodified and modified inks, printed in positive and negative (Table 5).
By observing the results of the measured line widths in the positive mode, it could be seen that the widths of the lines printed using the unmodified ink were higher than the widths of the lines printed using the modified inks. All lines printed using modified inks showed smaller line widths than the nominal width. This phenomenon occurred because of the changes in the polar and dispersive SFE components of the modified inks, as well as the changes in their ratio. Due to the low polarity of the Melinex substrate, inks with a higher total SFE and its more prominent polar component do not spread on the substrate, resulting in a thinner printed line. This phenomenon is by no means unfavorable, but it should be taken into account when forming and adjusting the image on the printing plate for reproduction with a modified PB ink. The widths of the lines closest to the values of the lines printed by the unmodified ink were measured for the inks with 2% rutile TiO2 and 1% ZnO. Precisely, these modified inks had the lowest polar SFE components and, at the same time, the highest dispersive SPE components among all modified inks (Figure 5).
The changes in the width of the lines printed in negative mode are analogous to the changes in the values of the lines printed in positive mode. A higher spreading of the ink on the substrate would result in a thinner line printed in negative mode (negative deformation). The measured widths of these lines are fully consistent with the results and the interpretation of the line widths in the positive mode: the line widths closest to the values obtained using the unmodified ink were achieved on prints produced using the inks with 2% rutile TiO2 and 1% ZnO. It was shown that by modest manipulation of the SFE components of the inks, it is possible to influence the dimensions of the printed line elements in the positive and negative, highlighting the critical role of the surface properties of the printing ink for print quality optimization, especially for printing elements where the precise width is of great importance (for example, bar codes).
The changes in the SFE components did not only affect the widths of the printed lines, but also visibly contributed to the quality of the printed line in terms of its visual appearance due to improved ink transfer (Figure 7).
Figure 7 shows images of lines with a nominal width of 200 µm, taken with a microscope under a magnification of 50× and obtained by printing with all modifications of the PB ink. As mentioned earlier, on flexographic prints, an outline often appears on the line edges due to the deformation of the printing element on the printing plate during printing and the plate–ink interaction. This phenomenon can present a problem for reading barcodes that are printed on polymer substrates. By adding the nanoparticles to the PB ink, a reduction in or complete elimination of the outline was achieved. When using the ink with the addition of 1% ZnO (Figure 7f), a complete elimination of outline was achieved, and a similar effect was achieved after the addition of 2% rutile TiO2 (Figure 7e). A degree of improvement in the appearance of the printed lines and increased ink coverage are visible on the prints obtained with all modified inks compared to the print obtained using unmodified ink (Figure 7a). Specifically, by increasing the polar and total SFE of the ink, it was possible to reduce the ink run down the edge of the printing element on the printing plate and allow the ink to be retained at the top of the printing element. Adequate pressure during the printing process thus resulted in a successful transfer of the ink to the substrate almost completely or completely without outlines, which significantly contributed to the quality of the print.
Figure 8 presents the printed lines displayed in Figure 7 after thresholding to calculate the ink coverage on the captured line segments.
The changed interactions of the modified inks with both the printing plate and printing substrate resulted in the changes in the ink coverage on the printed lines. Specifically, the elimination of the outline on the print and changes at the plate–ink interface upon adding the nanoparticles to the ink resulted in increased ink coverage on the lines (i.e., improved ink transfer). Using the scale from the microscope’s program support, the area covered in ink was calculated for the line segments presented in Figure 8, and the results are displayed in Table 6.
It can be seen that the highest ink-covered line area was achieved on the lines printed using the inks with 2% rutile TiO2 and 1% ZnO. The ink coverage on the microscoped line segments for these inks increased by approximately 28.5% compared to the coverage on the line printed using unmodified ink. It can be concluded that the particular modification of the dispersive and polar SFE present in these two inks (i.e., modest increase in polar SFE and the simultaneous increase in the dispersive SFE (Figure 5)) was best suited for improving the interaction of the ink with the materials used in the printing system in this research. It can be concluded that by being familiar with the surface properties of the materials in a specific printing system and by knowing how to achieve the specific targeted modification of the printing ink’s surface properties using nanoparticles, the qualitative properties of prints can be significantly improved. Future research will focus on various aspects of graphic reproduction systems employing nano-modified printing inks. This will include an analysis of the modified inks, specifically examining different nanoparticles and their dispersion within the ink. Additionally, future research will explore printability when various substrates, types of printing inks, and nanoparticles are used, as well as the interactions between the modified inks and different polymeric printing substrates.

4. Conclusions

The development of graphic technology and the utilization of a wide range of novel materials in printing, including different nanoparticles, enable improvements in quality and provide potential for printing products with added value. The objective of this research was to enhance the surface properties of printed ink layers by incorporating TiO2 and ZnO nanoparticles into the conventional ink formulation. This modification aimed to improve print quality while also offering protection against counterfeiting. Various measurements on printed ink layers were performed, including colorimetric measurements, water contact angle measurements, calculations of surface free energy and adhesion parameters, microscopy of the printed elements, and image analysis.
The presence of nanoparticles in the inks was indirectly confirmed through FTIR-ATR spectroscopy, which showed changes in the fingerprint region of the ink spectrum after the addition of nanoparticles, opening a possibility for improving the security of conventional flexographic ink.
The influence of the nanoparticles on the visual appearance of the printed color could not be detected visually, since the ΔEab value was under 3 for all printed modified ink layers. The results of the SFE calculations showed that it is possible to use nanoparticles as modifiers of the polar and/or dispersive components of SFE to improve the qualitative properties of the flexographic print. By the targeted modification of SFE components, it was possible to influence the dimensions and appearance of the printed elements, remove the outline present at the element edge, and improve the ink coverage, i.e., the ink transfer to the print. For the specific printing substrate used in this research, the additions of 2% rutile TiO2 and 1% ZnO had the greatest effect on improving the quality of the printed lines.
It can be concluded that the application of flexographic printing inks modified with nanoparticles is extremely applicable for printing on a wide range of polymer substrates with different surface properties, including biodegradable ones. Also, the application of conventional inks protected by nanoparticles is highly applicable in the field of specific packaging products, for example, in pharmaceuticals, as well as for all other protections against product counterfeiting and preserving the integrity of brands. The simultaneous protection and improvement of the quality of printed thin lines is important for motifs such as barcodes and other similar elements. This research opened the possibility of expanding the use of various types and concentrations of nanoparticles in conventional and specialized inks within the field of graphic technology.

Author Contributions

Conceptualization: S.M.P. and T.T.; methodology: S.M.P. and T.T.; software: D.D.; validation: S.M.P., T.T., and D.D.; formal analysis: S.M.P., I.J., and T.T.; investigation: S.M.P., T.T., and I.J.; resources: T.T. and I.J.; writing—original draft: T.T., I.J., S.M.P., and D.D.; writing—review and editing: S.M.P., D.D., and T.T.; visualization: T.T. and D.D.; supervision: S.M.P. and T.T.; project administration: S.M.P.; funding acquisition: S.M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Flexographic printing plate; (b) cross-section of a flexographic printing plate; (c) an element printed using flexographic printing plate.
Figure 1. (a) Flexographic printing plate; (b) cross-section of a flexographic printing plate; (c) an element printed using flexographic printing plate.
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Figure 2. CIE L*a*b* values of unmodified and modified flexographic prints.
Figure 2. CIE L*a*b* values of unmodified and modified flexographic prints.
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Figure 3. FTIR−ATR spectra of unmodified print and modified flexographic prints with 2% concentration of nanoparticles.
Figure 3. FTIR−ATR spectra of unmodified print and modified flexographic prints with 2% concentration of nanoparticles.
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Figure 4. Contact angles of water on unmodified and modified flexographic prints.
Figure 4. Contact angles of water on unmodified and modified flexographic prints.
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Figure 5. Total, dispersive, and polar surface free energy of unmodified and modified flexographic prints.
Figure 5. Total, dispersive, and polar surface free energy of unmodified and modified flexographic prints.
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Figure 6. Adhesion parameters between the printed layers and printing substrate: (a) work of adhesion; (b) interfacial tension and spreading coefficient.
Figure 6. Adhesion parameters between the printed layers and printing substrate: (a) work of adhesion; (b) interfacial tension and spreading coefficient.
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Figure 7. Printed lines (positive) with nominal width of 200 µm, 50× magnification: (a) unmodified PB ink, (b) PB ink + 1% TiO2 (A), (c) PB ink + 2% TiO2 (A), (d) PB ink + 1% TiO2 (R), (e) PB ink + 2% TiO2 (R), (f) PB ink + 1% ZnO, (g) PB ink + 2% ZnO.
Figure 7. Printed lines (positive) with nominal width of 200 µm, 50× magnification: (a) unmodified PB ink, (b) PB ink + 1% TiO2 (A), (c) PB ink + 2% TiO2 (A), (d) PB ink + 1% TiO2 (R), (e) PB ink + 2% TiO2 (R), (f) PB ink + 1% ZnO, (g) PB ink + 2% ZnO.
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Figure 8. Printed lines (positive) with nominal width of 200 µm after thresholding for ink coverage analysis: (a) unmodified PB ink, (b) PB ink + 1% TiO2 (A), (c) PB ink + 2% TiO2 (A), (d) PB ink + 1% TiO2 (R), (e) PB ink + 2% TiO2 (R), (f) PB ink + 1% ZnO, (g) PB ink + 2% ZnO.
Figure 8. Printed lines (positive) with nominal width of 200 µm after thresholding for ink coverage analysis: (a) unmodified PB ink, (b) PB ink + 1% TiO2 (A), (c) PB ink + 2% TiO2 (A), (d) PB ink + 1% TiO2 (R), (e) PB ink + 2% TiO2 (R), (f) PB ink + 1% ZnO, (g) PB ink + 2% ZnO.
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Table 1. Specification of the used nanoparticles.
Table 1. Specification of the used nanoparticles.
NanoparticleNameCAS NumberAverage Nanoparticle Size (nm)
TiO2 (A)Titanium (IV) oxide, anatase1317–70−015
TiO2 (R)Titanium (IV) oxide, rutile13463–67−7<100
ZnOZinc oxide1314–13−240–100
Table 2. Surface free energy components of probe liquids.
Table 2. Surface free energy components of probe liquids.
Probe LiquidTotal SFE
(mJ/m2)		Dispersive SFE (mJ/m2)Polar SFE (mJ/m2)
Water72.821.851.0
Diiodomethane50.850.80.0
Glycerol64.034.030.0
Table 3. ΔEab values for prints obtained using nano-modified printing inks.
Table 3. ΔEab values for prints obtained using nano-modified printing inks.
Mass Concentration of Nanoparticles in the Ink (%)Anatase TiO2Rutile TiO2ZnO
1%2.292.482.53
2%2.662.232.83
Table 4. Adhesion parameters between printing inks and PMMA.
Table 4. Adhesion parameters between printing inks and PMMA.
Printing Ink γ 12 (mJ/m2) W 12 (mJ/m2) S 12   ( mJ / m 2 )
PB2.3679.05−1.57
1% TiO2 (A)0.4087.69−6.30
2% TiO2 (A)0.4187.47−6.09
1% TiO2 (R)0.4488.07−6.75
2% TiO2 (R)1.1188.32−8.34
1% ZnO1.6484.61−5.69
2% ZnO0.4886.34−5.10
Table 5. Width of the printed lines in positive and negative modes (nominal width of 200 µm).
Table 5. Width of the printed lines in positive and negative modes (nominal width of 200 µm).
InkMeasured Line Width—Positive (µm)Measured Line Width—Negative (µm)
PB200.36 ± 4.79191.33 ± 1.02
PB + 1% TiO2 (A)177.31 ± 5.49196.06 ± 2.53
PB + 2% TiO2 (A)180.99 ± 2.49195.95 ± 2.00
PB + 1% TiO2 (R)183.51 ± 3.24196.05 ± 1.21
PB + 2% TiO2 (R)195.34 ± 2.64193.56 ± 0.90
PB + 1% ZnO199.66 ± 2.38192.35 ± 1.03
PB + 2% ZnO185.23 ± 3.95195.57 ± 1.45
Table 6. Calculated areas of the printed line segments.
Table 6. Calculated areas of the printed line segments.
Ink Used for PrintingMeasured Line Segment Area (µm2)
PB429,401.02
PB + 1% TiO2 (A)484,629.29
PB + 2% TiO2 (A)566,771.37
PB + 1% TiO2 (R)495,325.05
PB + 2% TiO2 (R)600,903.94
PB + 1% ZnO601,092.04
PB + 2% ZnO552,346.60
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MDPI and ACS Style

Tomašegović, T.; Mahović Poljaček, S.; Jurišić, I.; Donevski, D. Fine-Tuning Flexographic Ink’s Surface Properties and Providing Anti-Counterfeit Potential via the Addition of TiO2 and ZnO Nanoparticles. Micro 2025, 5, 20. https://doi.org/10.3390/micro5020020

AMA Style

Tomašegović T, Mahović Poljaček S, Jurišić I, Donevski D. Fine-Tuning Flexographic Ink’s Surface Properties and Providing Anti-Counterfeit Potential via the Addition of TiO2 and ZnO Nanoparticles. Micro. 2025; 5(2):20. https://doi.org/10.3390/micro5020020

Chicago/Turabian Style

Tomašegović, Tamara, Sanja Mahović Poljaček, Ivona Jurišić, and Davor Donevski. 2025. "Fine-Tuning Flexographic Ink’s Surface Properties and Providing Anti-Counterfeit Potential via the Addition of TiO2 and ZnO Nanoparticles" Micro 5, no. 2: 20. https://doi.org/10.3390/micro5020020

APA Style

Tomašegović, T., Mahović Poljaček, S., Jurišić, I., & Donevski, D. (2025). Fine-Tuning Flexographic Ink’s Surface Properties and Providing Anti-Counterfeit Potential via the Addition of TiO2 and ZnO Nanoparticles. Micro, 5(2), 20. https://doi.org/10.3390/micro5020020

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