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Article

Towards Expanding the Use of Paper Made from Recycled and Non-Woody Plants: Enhancing the Print Quality through the Application of Nano-Modified Offset Inks

by
Maja Strižić Jakovljević
*,
Sanja Mahović Poljaček
*,
Sonja Jamnicki Hanzer
,
Davor Donevski
and
Tamara Tomašegović
Faculty of Graphic Arts, University of Zagreb, Getaldićeva 2, 10 000 Zagreb, Croatia
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(11), 4785; https://doi.org/10.3390/su16114785
Submission received: 2 May 2024 / Revised: 28 May 2024 / Accepted: 31 May 2024 / Published: 4 June 2024
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Abstract

:
This study aims to investigate the feasibility of using paper made from eco-friendly recycled and non-woody plants in graphic technology, particularly in offset printing. Instead of changing the composition or modifying the surface properties of the paper, the focus was on enhancing the print quality by modifying the printing ink. By modifying the printing inks, the quality of the prints on recycled and non-woody paper can be optimized, which in turn reduces the need for paper made from primary fibers. This approach can expand the use of alternative materials in graphic technology and design. The objective was to optimize the print quality on these sustainable materials. Five types of uncoated paper were used, with high-quality uncoated offset paper based on virgin fibers serving as a reference. Laboratory tests of the basic and surface properties were carried out to measure the paper quality parameters that are important for offset printing. The influence of the paper composition on its optical and colorimetric properties was also investigated. The interaction between the selected papers and offset inks was examined through measurements of adhesion parameters and ink transfer, i.e., the paper’s ability to accept the ink. To enhance the applicability of the investigated papers as printing substrates in the graphic industry, SiO2 and TiO2 nanoparticles were added to the offset inks. The influence of the paper composition on the colorimetric properties of the prints was also investigated. The print uniformity, as an important quality characteristic, was determined by measuring the mottling index. The research findings indicate that incorporating SiO2 and TiO2 nanoparticles into offset inks can enhance the interaction between the paper and ink, leading to improved print quality. This study provides new perspectives on the possibilities of using recycled and non-woody plant paper in offset printing without significantly compromising the quality of the print.

1. Introduction

In the field of graphic technology and design, there is a potential for the substitution of currently used papers produced from primary fibers with papers made of non-wood and recycled materials. The selection of such materials could have a positive impact on the environment, as they involve reused, recycled papers, and those produced from agricultural residues, aligning with the principles of a circular economy, as well as indirectly supporting all three pillars of sustainability. However, the choice of these new materials requires a feasibility study to compete with the materials currently used.
Using alternative materials to produce paper is important for several reasons. Firstly, it helps to reduce the environmental impact of papermaking by decreasing the reliance on trees as the primary source of wood fibers [1]. Also, alternative materials such as agro-waste and non-wood fibers have been found to have comparable properties to traditional wood pulp, making them suitable for paper production [2,3,4]. Additionally, using alternative materials can help address the raw material crisis faced by the paper industry, as well as promote local production and the circular economy by utilizing local resources [5]. Blending alternative materials with inferior-quality pulp can improve the physical properties of paper products. The use of alternative materials in papermaking offers a more sustainable and economically viable solution for reducing the consumption of wood pulp and minimizing the use of virgin fibers. Furthermore, using recycled and non-woody plants in paper production offers more advantages, like providing an environmentally friendly alternative to conventional wood-based paper manufacturing, reducing deforestation and CO2 production [5,6]. Moreover, it allows for the reuse and recycling of paper, contributing to waste reduction and resource conservation [7].
Alternative materials that can be used to produce paper include non-wood fibers such as pineapple crown, sandbox, pino macho, elephant grass, and teak leaves [8]. Other non-wood materials like Leucaena leucocephala, tagasaste, rice straw, Paulownia fortunei, Hesperaloe funifera, and empty fruit bunches have also been tested for paper production [9]. Invasive alien plant species like knotweed, goldenrod, and black locust have shown potential as feedstock for paper production [10]. Additionally, agricultural by-products like rice straw and millet husk can be used as raw materials for papermaking, reducing the need for virgin wood [11]. These alternative materials offer a sustainable and cost-effective solution for paper production, while also addressing environmental concerns related to deforestation [12].
Papers made from recycled wood fibers and non-woody plants, such as straw from wheat, barley, and triticale crops, show promise for applications in graphic technology and design. These papers can be used as printing substrates for different printing techniques, including digital UV inkjet, gravure printing, flexographic, and offset printing [5,13]. The quality of printed motifs on papers made from non-woody plants is comparable to those made on papers made exclusively from recycled wood pulp, with a similar reproduction quality and optimal reproduction of details [13]. Furthermore, the development of reusable and recyclable paper from natural materials, such as pollen, offers a sustainable alternative to conventional wood-based paper [7]. Handmade paper made from innovative raw materials, including agro-waste and seaweeds, also contributes to sustainability and has potential applications in graphic technology and design [14].
Besides the advantages mentioned above, there are a number of disadvantages to consider when using non-wood plants for paper production. For example, the ink penetration into the substrate may be slightly higher compared to commercial papers [15]. The use of recycled and non-woody plants in paper production offers sustainable and cost-effective alternatives, but careful consideration is needed to ensure that the desired printing and packaging properties are achieved.
The challenges in using paper made from recycled and non-woody plants for application in graphic technology and design include the need for an even density over the entire sheet, the availability of raw materials and environmental protection, and the influence of the fiber type on the vapor barrier properties and migration to foodstuffs [16,17,18]. Ensuring an even density in the paper is influenced by various parameters such as the pulp preparation technology, equipment, and selection of optimal raw materials [13]. The availability of raw materials, especially non-wood materials, is a challenge due to the pressure on forests and the decline in capacity utilization in the paper industry [6]. Additionally, the influence of the fiber type on the vapor barrier properties and migration to foods needs to be considered for food packaging applications. These challenges highlight the importance of addressing issues related to density, raw material availability, and food safety when using recycled and non-woody plant papers in graphic technology and design.
Due to the wide range of possible applications of papers made from recycled and non-woody plants, research into their properties in the field of graphic technology and design is of great importance. This is crucial for several reasons: firstly, to ensure the stability of these papers during the printing process and their impact on print quality, and secondly, to explore their potential for reproducing various motifs and incorporating them into the design of graphic products. Understanding the interactions of materials in the printing process therefore requires a highly detailed and analytical approach, as the surface properties of the materials involved, their control, and stability play a decisive role in all printing applications. The use of pigments and fillers on a nano scale offers various functional benefits to materials. These include an enhanced surface appearance, increased resistance to fading, UV and chemical resistance, improved thermal and electrical conductivity, and better corrosion resistance [19]. Important nanomaterials currently used in the printing ink and paint industry are titanium dioxide (TiO2), silicon dioxide (SiO2), zinc oxide (ZnO), aluminum oxide (Al2O3), silver oxide (Ag2O), and others of a nano size [20,21]. When the nanoparticles are incorporated into printing inks, they are adsorbed on the surface of the pigment particles, which significantly improves the lipophilicity and wettability of the ink, which in turn can improve the printability of the ink and the properties of the applied ink films [21,22]. However, they have little effect on the color of the ink, as they do not scatter white light but can absorb UV radiation [19]. Nanosilicate pigments are used in pigment-related applications as common extenders that support the functionalization of pigments [19]. They can be used for the rheology control of printing inks to improve the gloss, colorimetric properties, and scratch resistance of various ink systems [23,24,25]. It has been reported that the rheological properties of the offset ink (viscosity, yield value, flow, tack) are improved with the addition of nano-TiO2 [26].
Among the various forms of TiO2, pure anatase has proven to be the most promising material for photocatalytic applications due to its higher electron mobility, lower dielectric constant and lower density compared to rutile and brookite [26]. In addition, TiO2 increases the brightness of the final product and thus contributes to its optical attractiveness. Moreover, when nano-TiO2 is incorporated into printing ink, it leads to visual effects such as flashes, color transitions, and additional color variations. This not only enriches the surface color of the printed material, but also gives it a decorative touch, as nano-TiO2 continuously emits visible light, creating diverse visual effects [19]. Adding nano-SiO2 and nano-TiO2 to the ink can also improve the lightfastness of the modified ink, as these two substances have strong anti-ultraviolet and catalytic properties [19]. Heat resistance and adhesion are also improved to a certain extent [21]. In our previous research, we found that the modification of water-based UV-curable inks with nano-TiO2 and -ZnO had a positive effect on the adhesion properties of nano-modified inks and also on the mechanical and optical stability of the applied ink films on various paperboard substrates in flexography [22]. Furthermore, we studied the properties of a UV visible (daylight invisible) fluorescent coating for screen printing, modified by the addition of SiO2 and TiO2 nanoparticles [27]. It was found that the addition of nanoparticles to the printing inks had a positive influence on the surface, color properties, mechanical properties, and fading resistance of the resulting prints.
In this research, the quality of prints on recycled and non-woody papers was optimized by modifying the inks through the addition of SiO2 and TiO2 nanoparticles. The optimization process presented can reduce the need for papers made from primary fibers and thus expand the use of alternative materials in offset printing. Specifically, the aim was to improve the ink transfer and the associated properties of prints applied to different paper substrates by the modification of printing inks. To our knowledge, this type of research has not yet been published, and the results presented will provide new insights into the possibility of replacing papers made from primary fibers with papers made from recycled and non-woody plants in the offset printing process.
As part of this research, an experimental method to determine the paper’s receptivity to printing ink, i.e., quantifying the amount of ink transferred from the offset printing form to the paper substrate, the ink transfer test, was carried out. SiO2 and TiO2 nanoparticles were introduced to the printing ink to enhance ink transfer and impact the uniformity of the prints, thereby positively influencing the paper–ink interaction. The surface properties of the paper substrates, the adhesion of the modified printing inks on the substrates and the colorimetric properties of the prints were investigated. Additionally, print mottle, characterized by uneven ink distribution or non-uniform ink absorption across the paper surface, was also determined.

2. Materials and Methods

2.1. Paper

In this research, five types of uncoated papers are used, including high-quality uncoated offset paper based on virgin fibers serving as a reference, marked as MN 120-R. Papers RW 80 and NC 80 are specified as uncoated recycled papers suitable for all common printers and printing processes. While both are made from recycled fibers, NC 80 is produced entirely from post-consumer waste (PCW). PC 120 and MH 118 are specified as uncoated papers with rough surface, suitable for offset, UV offset, letterpress, and screen printing. Paper PC 120 is produced entirely from cotton fibers and has an uncoated surface. Paper MH 118 is pH neutral and acid-free, produced from 30% hemp and 70% virgin fibers. Here, hemp is used as a fast-growing plant for raw material in paper production, with a significant share of virgin fibers, to gain a high whiteness grade.

2.2. Printing Ink and Nanoparticles

Selected papers were printed with quick-drying and fast-setting sheet-fed offset ink (Inkredible eco-perfect-dry magenta, Huber Group Holding SE, Mühlhausen, Germany), using the IGT printability tester (IGT Testing Systems, Almere, The Netherlands). This is an ink that dries quickly through oxypolymerization, making it particularly appropriate for rapid post-finishing applications. The selected offset ink is suitable for printing on a wide variety of substrates and is cobalt- and mineral oil-free. The laboratory prints were prepared in accordance with ISO 2834-1:2020 [28]. The magenta process offset ink was printed in full tone on 25 mm wide paper strips. After printing, the prints were air-dried under standard laboratory conditions (23 ± 1 °C and 50 ± 2% relative humidity) for 48 h. During drying, the ink vehicle penetrates into the paper, and the resinous component with the pigments remains on the surface where it dries by oxidation. Besides producing prints with conventional ink, two additional series of prints were created using the same ink modified with nanosized TiO2 and SiO2. The purpose was to assess their influence on paper–ink interaction. The offset ink was thus modified with nanoparticles of colloidal silicon dioxide (SiO2, Aerosil 200) with an average primary particle size of 12 nm (CAS No. 112945-52-5) and titanium dioxide (TiO2) anatase nanoparticles (CAS No. 1317-70-0) with an average primary particle size of 15 nm, at a concentration of 1%. The concentration of nanoparticles in the ink was determined according to previously published results, which found that a higher concentration of nanoparticles can have a negative impact on the optical and visual properties of prints [22,27,29].

2.3. Ink Transfer Test

The ink transfer test was used to determine the amount of ink transferred from the printing form to the paper. There are several factors which affect the ink transfer, such as ink viscosity, the amount of ink, printing speed and printing force, surface absorption, and surface smoothness [28,30,31]. The influencing parameters and the challenges to be overcome in the optimization of offset printing on alternative materials were reported in [32,33,34]. In this research, the ink transfer method was implemented according to ISO 2834-1:2020, using IGT testing instruments [28,35]. An IGT ink pipette was used to apply an accurate quantity of 1 cm3 of ink to the inking device. The prints were made using 2 cm wide aluminum printing forms with smooth edges. An IGT A2 printability tester (IGT Testing Systems, Almere, The Netherlands) was used to make prints, with adjusted printing force of 800 N and constant speed of 0.2 m/s. The amount of ink transferred from printing form to the paper substrate was determined by weighing the printing form immediately before and after printing with an analytic balance (Enrico Toniolo, Milan, Italy), with 0.0001 reading accuracy. Ink transfer, e.g., transferred ink coverage, was calculated for each printed sample using following formula [35,36]:
C = (m1 − m2) × 10,000/P,
where
  • C is the ink coverage in g/m2;
  • m1 is the mass of the inked form before printing in g;
  • m2 is the mass of the form after printing in g;
  • P is the printed area in cm2.
Besides calculation of the ink transfer, printed samples were further analyzed for colorimetric properties, optical density, surface free energy, and mottling.

2.4. Characterization Methods

Images of the surfaces of the paper substrates were taken using an Olympus BX51 microscope (Olympus Corporation, Tokyo, Japan) at a magnification of 20× and 100×. The thickness (caliper) of the papers was measured with the DGTB001 thickness gauge (Enrico Toniolo, Milan, Italy), according to ISO 534:2011 [37]. Smoothness was measured using PTI line Bekk Tester (PTI Austria GmbH, Laakirchen, Austria) on 10 samples for each substrate (5 on the felt side and 5 on the wire side of the paper), according to ISO 5627:1995 [38]. The surface structure of paper substrates was measured by a MarSurf PS 10 gauge (Mahr GmbH, Germany) with the stylus method. Two different roughness parameters were measured: the parameter Ra, which expresses the arithmetic mean deviation of the surface profile, and Rz, which defines the vertical distance between the highest peak and deepest valley within the measuring length, according to ISO 21920-2:2021 [39]. Surface profilometry is widely used in the paper industry today as it enables the comparison of the results obtained with the various properties of the paper surface structures, which are important for the interactions within the papermaking and printing processes [40,41]. The size of the stylus was 2 µm and measuring force was 0.00075 N. Ten repetitions were made for each sample and the average values of the results were obtained.
The Cobb test is performed to determine the water absorptiveness of papers used in research, according to ISO 535:2023 [42]. This method is suitable for sized paper and board, under standard conditions (ISO 187:2022) [35]. The Cobb test is directly related to the sizing of the paper, as the sizing process plays an important role in achieving a certain degree of resistance to the absorption or penetration of liquids, especially water [43]. The ISO 535:2023 [42] test procedure includes placing a dry sample under the metal ring, pouring 100 mL of distilled water and leaving it to stand for 60 s. At 10 ± 2 s before the expiration of the test period, the water is poured out of the ring and the sample is then placed between standardized blotting papers and the excess water is removed with a roller. The sample is weighed before and after exposure to water, resulting in the paper absorptiveness value (g/m2). The test, in this research, is performed on the smoother side of the paper, selected for printing.
Optical properties of the paper substrates were measured with X-Rite (Grand Rapids, MI, USA), D50/2°, M1. Whiteness was measured over the entire visible range of the spectrum, while brightness was measured at 457 nm.
The surface free energy (SFE) calculation method according to Owens–Wendt–Rabel–Kaelble (OWRK) [44,45] was used to analyze the surface properties of papers and prints. The OWRK method is one of the more common approaches to determining the surface free energy of solids using contact angle measurements. The OWRK method considers polar and dispersive forces and uses the geometric mean approach for the calculation of the SFE of solids. Since the objective of the calculation is to find two unknowns, i.e., the dispersive and polar surface tension of the measured solid, contact angle measurements with at least two liquids with known dispersive and polar surface tension are needed [46]. One liquid should be dominantly polar (for example, water or glycerol) and one liquid should be dispersive (for example, diiodomethane). To calculate polar, dispersive, and total SFE of the papers, contact angles on samples were analyzed using the Data Physics OCA 30 goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany). Reference liquids of known surface tension (demineralized water, diiodomethane, and glycerol) were applied on the paper surfaces and measured using the sessile drop method. The volume of the drops was 1 µL. Ten drops of each reference liquid were applied on each sample and the average value was calculated. All measurements of the contact angles were performed 0.3 s after the drop had touched the measured substrate. Such a short period was required because of the absorptive properties of the papers. Therefore, to improve the accuracy of the SFE calculation, three reference liquids were used instead of the usual two. Polar, dispersive, and total surface tension components of reference liquids (expressed in mJ/m2) were, respectively, diiodomethane—0, 50.8, and 50.8; glycerol—30.0, 34.0, and 64.0; demineralized water—51.0, 21.8, and 72.8. Contact angles of the reference liquids were then used to calculate SFE components of the solids, i.e., papers and printed inks.
The obtained results of polar, dispersive, and total SFE of the papers and printed inks were used to calculate the adhesion parameters between the papers and inks. Specifically, the thermodynamic work of adhesion (W12), the spreading coefficient (S12), and the interfacial tension (γ12) were analyzed to predict the strength of interactions between the papers and different inks (unmodified and nano-modified). Generally, for optimal adhesion, the value of W12 should be as high as possible; γ12 should be positive and close to zero; and S12 should be positive in order to indicate spontaneous wetting [47,48]. When evaluating the adhesion between two layers, all three parameters should be considered.
CIE L*a*b* colorimetric measurements were performed on printed samples using the Techkon SpectroDens B703902 (Königstein, Germany) spectro-densitometer. It was set to CIE illuminant D50/2° with the M1 filter. Ten measurements were performed for each paper–ink combination. The purpose of these measurements was to determine the extents to which print colors differ between papers and between inks (with and without modifications). To calculate the color difference between the measured CIE L*a*b* values of the printed color samples, the CIEDE2000 formula was used [49], which is the most accurate and most complicated CIE color difference algorithm available to date and which corresponds to the way human observers perceive small color differences.
Print uniformity is an important quality characteristic. It can be assessed by various methods of determining mottling value or index. In this paper, we used the method presented in [50]. Printed samples were scanned using the EPSON PERFECTION V750 PRO scanner at 300 pixels per inch (PPI) resolution. For each sample, a 216 × 216 pixels patch was extracted from a scanned image. At 300 PPI, 216 pixels fit the length of 18,288 millimeters. This size was used since IGT A2 produces 20 mm wide strips with imperfect ink transfer on the edges. Since prints were made with magenta ink, RGB scans were converted to grayscale images using R and B channels, as described in [50]. Mottling values were calculated for “tiles” of sizes ranging from 2 × 2 pixels to 64 × 64 pixels.

3. Results and Discussion

3.1. Properties of Paper Substrates

The results of the measured basic and structural properties of the paper substrates are presented in Table 1. Before the measurements, the preparation of the samples was conducted in accordance with the ISO 187:2022 standard for conditioning paper samples at 23 ± 1 °C and 50 ± 2% relative humidity [35]. The results of the caliper (thickness), density, and specific volume (bulk) of the papers are presented in Table 1. The grammage and thickness values of each individual sample were used to calculate the density of the paper using the following formula [37]:
ϒ = x/d × 1000 (g/cm3),
where
  • x is the basis weight (g/m2);
  • d is the caliper (mm).
The density of the paper sheet represents the mass of one cubic centimeter of the tested sample. It is influenced by various additives used in paper production, such as fillers, sizing agents, and dyes, and the fiber type, separation, and mechanical treatments like refining, drying, and calendering. This parameter significantly affects the paper’s optical and mechanical properties, including its structure, porosity, and compactness.
In addition, the paper density is an indicator of the relative air content in the paper [51]. The results presented in Table 1 show the highest density of 0.84 g/cm3 was that of the reference sample MN 120-R, followed by RW 80 (0.83 g/cm3), NC 80 (0.79 g/cm3), MH 118 (0.70 g/cm3), and PC 120 (0.60 g/cm3). This property of paper varies inversely with the specific volume (bulk), which is calculated using the following formula [37]:
1/ϒ = d/x × 1000 (cm3/g)
The specific volume of paper (bulk) is the volume per mass unit of paper (cm3/g). A lower specific volume typically indicates denser paper. This property is important in the printing industry as it affects ink absorption, air permeability, and the final appearance of the printed material. The paper’s specific volume is closely related to ink absorbency in printing processes. The ink absorption capacity of paper can be influenced by parameters such as the type of pulp used, basis weight, sizing agent, and beating degree [52]. For example, research performed by Dong et al. showed hardwood pulp exhibits a higher performance in terms of paper absorptivity compared to softwood pulp [53].
The results indicate that PC 120 has the highest bulk of 1.66 cm3/g, followed by MH 118 of 1.43 cm3/g; NC 80 of 1.26 cm3/g; RW 80 of 1.21 cm3/g; and reference paper MN 120-R, having the smallest bulk of 1.19 cm3/g (Table 1). The results show that papers of the same grammage can result in a different bulk because of the difference in the thickness. For example, PC 120 is thicker than MN 120-R and less dense, resulting in a higher bulk. This means that the reference paper MN 120-R has a higher density, lower relative air content, and is more compact than PC 120, which is more porose. Similar results were found for NC 80 and RW 80. These results are consistent with previously published studies that have found that the density of paper is closely related to its porosity. According to the results presented, the density value is inversely correlated with the porosity of the paper [54]. This means that a high density value implies a low porosity, and vice versa. In addition, low density values were found to be more common in the recycled paper and paper made from non-woody plants than in paper made from virgin fibers, which is due to the larger voids and gaps between the fibers in the recycled paper and non-woody papers [55,56].
The results of the paper smoothness using the Bekk method are shown in Table 2 and represent the mean values of five (5) measurements for each individual type of paper tested on the wire and felt side (marked as A and B). The surface properties of the paper have a significant effect on the print reproduction and quality. Uncoated papers exhibit a different surface smoothness on the wire and felt side, which is a result of the paper manufacturing process. Since high-quality printing and reproduction requires a high degree of smoothness, it is recommended to print on the smoother side of uncoated paper whenever possible. The results in Table 2 demonstrate the highest degree of smoothness was found for the RW 80 B sample, with a value of 34 s. The smoothness of NC 80 A is 27.9 s, followed by MN 120-R A (20.4 s), MH 118 A (8.5 s), and PC 120 A (2.3 s). The smoother side of each paper was selected for printing (marked in green in Table 2) and analyzed in further tests.
Figure 1 displays the relation between the Bekk smoothness (Figure 1a) and basic roughness parameters (Ra and Rz) of the papers used in this research (Figure 1b). Although the direct negative correlation between smoothness and roughness was not expected since the principles of measurements are different, there is a visible inverse proportionality in the relation of the values. The highest average roughness Ra and Rz parameters, as well as the lowest smoothness (2.3 s), were found for the PC 120 paper (4.85 µm and 32.05 µm, respectively). The lowest average Ra and Rz parameters were found for the RW 80 paper (1.90 µm and 14.09 µm, respectively). The highest smoothness, of 34 s, was found for the same paper. It can be seen that the roughness and smoothness vary significantly among the papers used in this research. These results are of great importance for understanding the interaction of the paper with the printing ink, as many studies have shown that these parameters are responsible, among others, for the functional properties of the inks and the colorimetric properties of the prints. In some cases, the surface of the paper can be coated to achieve an optimal print quality, as it is known that the additional coating of the paper can improve the mechanical and colorimetric properties of the prints [57]. Alternatively, additional fillers can be used in the papermaking process to improve the mechanical and surface properties of the paper [56]. It has also been shown that a low value of smoothness and a high value of roughness parameters affect the optical density of prints, the gloss, and the general paper properties [40].
However, when putting these results in the context of the print properties and quality (which is shown in the results presented in the continuation; results of the ink transfer, variation in ΔE, and print mottle), it can be concluded that the roughness/smoothness alone do not relate to the increased or decreased quality of the print. This means that there is no direct limitation regarding the paper surface profile when choosing paper made from recycled and non-woody plants as a potential substitute for common virgin fiber-based uncoated paper in offset printing.
Figure 2 displays the water absorptiveness of the paper used in this research, according to the Cobb method. The results show the highest water absorptiveness, of 166.2 g/m2, was found for MH 118. For hard-sized papers intended for offset printing, the Cobb value usually does not exceed 100 g/m2 [58], indicating that this paper may not be ideal for offset printing due to its excessive water absorbency. Paper MH 118 probably belongs to the class of soft-sized papers (Cobb value > 100 g/m2). Soft-sized (or weak-sized) papers tend to absorb more water and have a more open structure (such as newsprint paper). They are suitable for printing techniques that require greater ink absorption. The water absorptiveness of papers made from recycled fibers, RW 80 and NC 80, showed different absorptiveness values compared to the reference sample (MN 120-R), which are most likely due to different additives in paper production, such as fillers and sizing agents, which reduce the water absorption capacity. This is very important for offset papers, because of the fountain solution, which can potentially affect the dimensional stability of the substrate. The results are also influenced by the paper composition, fiber type, and surface roughness. The importance of the water absorptiveness and print stability in offset printing has already been investigated in [59,60,61]. In those studies, the influence of different types of papers with standardized and non-standardized water absorptiveness values on the requirements of the printing process, on the stability of the paper sheets during storage, and on the print quality were analyzed in detail. The result of a water absorptiveness of 48 g/m2, determined for the reference paper MN 120-R in this research, can be regarded as the ideal Cobb value for offset papers for multicolor printing.
The total, dispersive, and polar SFE components of the papers are presented in Figure 3. Paper MN 120-R has the highest total, dispersive, and polar SFE among the papers used in this research (36.01, 31.63, and 4.38 mJ/m2, respectively). The lowest total and dispersive SFEs were found with PC 120 (21.3 and 19.18 mJ/m2, respectively). A low polar SFE (<5 mJ/m2) is characteristic of cellulose-based paperboard surfaces [62]. The significantly lower SFE of PC 120 (in comparison to the other papers) could be associated with the presence of fillers that decrease the SFE (CaCO3, clay, and/or talc) [63,64]. Specifically, the components in paper such as hydrophobic sizing agents, wood extractives, and fillers (for example, CaCO3, clay, and talc), decrease the surface free energy [65]. There is no significant difference in the total or dispersive SFE of papers MH 118, RW 80, and NC 80. The lowest polar SFE (0.62 mJ/m2) was found for paper MH 118. It can be expected that the adhesion between the papers and printing inks, as well as other surface interactions resulting in the changed quality of the prints, will be affected by differences in the SFE components of the papers.
Indeed, the values recorded for paper PC 120 are orders of magnitude greater than the values recorded for other papers, which is reflected in the results for the mottling values shown in the continuation, which could be related to the lowest SFE of that paper. Low SFE values of the surface can result in the poor wetting of the printing ink and consequently increased mottling since the wetting of a liquid onto a substrate is essential for the spreading of the liquid on the substrate [64].
Figure 4 shows the microscopic images at 20× and 100× magnification as well as the line profiles of the paper substrates. As can be seen in Figure 4a, the surface structure of the reference sample MN 120-R, a paper based on virgin fibers, is uniform and interspersed with well-interwoven fibers of different widths. After evaluating the image at 100× magnification, it can be seen that the fibers extend to a maximum width of 50 μm. Sample PC 120 (Figure 4d) has a surface structure that is the most irregular compared to all the observed samples, and is clearly visible on the line profile. The fibers are of a uniform width, long and without visible breaks; the fiber width is about 20 μm or more. Sample MH 118 has an interwoven fiber structure consisting of 30% hemp and 70% virgin fibers. The hemp fibers are narrow and interwoven between virgin fibers, which have a uniform shape without breaks. The papers made from 100% recycled fibers (RW 80 and NC 80) have a similar profile, with a surface structure of thin fibers interwoven in all directions in the surface structure. The particles of recycled material are visible in the RW 80 sample. The width of the fibers is different for both papers; the fiber width is between 20 μm and a maximum width of 50 μm.
The optical properties of the paper substrates were measured on the smoother side of each paper type, and included the whiteness, brightness, and opacity (Table 3). The highest degree of whiteness among the tested samples was measured for the reference MN 120-R paper (129.4%), and PC 120 (125.8%). These papers also have high values of brightness, exceeding 100%, but the opacity value of reference MN 120-R is 3.03% higher than that for PC 120. Paper RW 80 shows the lowest whiteness degree, of 56.1%, and the highest opacity, of 98.2%, which is the result of the recycled fibers used in paper production. These results confirm the visual impression of the paper. In an effort to achieve a higher degree of whiteness and brightness, OBAs (Optical Brightening Agents) are added to the raw material, resulting in values exceeding 100% when measuring the mentioned properties with the CIE standard illuminant D65 (outdoor light) that contains considerable UV energy. The OBAs can absorb radiation between 300 and 400 nm and re-emit most of the absorbed energy into the visible (blue) part of the spectrum, between 400 and 500 nm.

3.2. Properties of Printed Samples

The results of the measured ink transfer for all the selected papers printed with the unmodified and modified offset ink are shown in Figure 5.
The results are expressed as the mean values of the four measurements of ink transfer for each sample, showing the amount of ink transferred from the printing form to the paper substrate. The results of the ink transfer test for all the selected papers printed with offset ink modified with SiO2 and TiO2 nanoparticles indicate an increase in ink transfer compared to the reference unmodified ink, with the exception of NC 80 printed with TiO2 modification. Modification with SiO2 nanoparticles results in a slightly greater increase in ink transfer than with TiO2, compared to the reference. The highest increase in ink transfer was observed on the RW 80 paper printed with ink modified by SiO2 nanoparticles when compared to unmodified ink at an amount of 0.4 g/m2. The reference MN 120-R shows the smallest increase in ink transfer with SiO2 modification of the ink compared to the other samples. The lowest ink transfer value, of 4.4 g/m2, occurred for PC 120 and increased with ink modified with TiO2 for 0.3 g/m2. The results of the ink transfer for MH 118, RW 80, and NC 80 are higher than those achieved the reference MN 120-R. This result indicates that the same amount of ink used in offset printing will cause greater ink transfer on recycled and paper made of hemp fibers, than on the one produced from virgin fibers.
The adhesion parameters between the papers and printed unmodified or modified inks are presented in Table 4. Note that adding the nanoparticles to the ink did not cause a significant change in either adhesion parameter, regardless of the paper or nanoparticle type. However, the differences in ink adhesion are observed when comparing different papers. It is clear that the lowest, and therefore, the most optimal interfacial tension, was generally calculated for MN 120-R and the inks, as well as between MH 118 and the inks. The highest work of adhesion was calculated for MN 120-R and the inks.
The wetting coefficient was negative for all the paper–ink combinations, which is a result of the relatively low surface free energy of the papers. However, this occurrence does not pose a problem for the ink’s adhesion on the substrate, since the papers used in this research are porous and absorptive. It can be observed that the best adhesion with ink (overall) was achieved when printing on MN 120-R or MH 118. The poorest adhesion was achieved between the NC 80 paper and the inks. This result was to be expected, as the NC sample consists entirely of PCW, the composition of which cannot be classified in detail and was responsible for the relatively low adhesion properties in this case.
The colorimetric properties of the printed samples are compared in Figure 6. The mean of ten CIE L*a*b* values measured on the MN 120-R paper printed with unmodified ink was taken as a reference. Each boxplot represents the CIEDE2000 difference between ten measurements for a given substrate and ink, and the reference. Note that Figure 6a, unlike the others, does not display the results for the unmodified ink, since this is the reference condition. The boxplots reveal that different inks printed on different substrates do not result in considerable colorimetric differences compared to the reference conditions, since most measurements fall well below ΔE00 = 1, which is considered to be a just-noticeable difference (JND). For all the papers except the reference, it can be noted that the interquartile ranges of the boxplots for the modified inks do not overlap with those for the unmodified inks, which could indicate the presence of statistically significant differences between the modified and unmodified inks. However, the absolute values of the colorimetric differences are small with respect to the JND. To conclude, neither the choice of paper nor the ink resulted in considerable colorimetric differences with respect to the reference conditions.
Figure 7 displays the print mottle values. Each plot represents a different paper and shows the mottling values for the three inks. For each paper and ink, the mottling values were calculated for a range of different “tile” sizes, as described in Section 2.4. The plots reveal that the mottling values generally decrease with increasing tile size. That is, the variability is greater between the smaller printed areas. The larger areas are more uniform. Comparing the different papers, the mottling values recorded on the PC 120 paper are orders of magnitude larger than those recorded on the other papers.
It is also worth noting that the smallest mottling values were recorded on RW 80, a recycled paper. Comparing the mottling values between the different inks reveals that the modified inks result in smaller mottling values than the unmodified ink. However, the extent of this improvement differs between the papers. On the MN 120-R paper, the effects were negligible. On PC 120, such improvements would be most beneficial, but they were moderate and did not produce visually acceptable results. The largest improvement was achieved on MH 118 paper. Comparing MH 118 and NC 80, the mottling values produced with the unmodified ink were larger on MH 118. Using the modified inks on the MH 118 paper produced smaller mottling values than those obtained with the unmodified ink on the NC 80 paper. However, this is the only such case. While the modified inks may result in several times smaller mottling values compared to the unmodified ink printed on the same paper, the differences of mottling values between the papers are, in most cases, larger than the differences in the mottling values between the inks.
The microscopic images of the samples are shown in Figure 8. When comparing the individual paper samples printed with the unmodified ink, it can be seen that the observed ink completely covers the surface of the MN 120-R and RW 80 papers. The surface structure of the paper is most clearly visible in sample PC 120, which is comparable with the results of the roughness and smoothness of this sample. Paper PC 120 has the lowest smoothness (2.3 s) and shows the greatest unevenness in its surface structure compared to the other paper samples (Ra = 4.85 µm and Rz = 32.05 µm). Looking at the changes on the surface of the paper printed with the modified inks, it can be said that the addition of nanoparticles has a positive effect on the coverage of the printing inks. The positive effect of the nanoparticles is more pronounced in all the samples with the addition of nano-TiO2 than with the addition of SiO2. These results are consistent with the ink transfer results, where the positive effect of adding SiO2 nanoparticles is visible. The mottling results showed a similar result, with the addition of nanoparticles having the least effect on the PC 120 paper samples, as can be seen in the microscopic images. It can be said that these results play an important role in the development of good printability on recycled and non-woody papers in offset printing.

4. Conclusions

This research explored the potential for the wider use of non-woody and recycled fibers in papers for offset printing, with a focus on modifying the ink by adding nanoparticles. This study utilized five types of uncoated papers, with high-quality uncoated offset paper made from virgin fibers serving as a reference.
This study showed that the addition of SiO2 and TiO2 nanoparticles to the ink significantly improved the ink transfer for all the paper substrates, except for NC 80 when printed with SiO2 modification. The SiO2 nanoparticles excelled slightly over TiO2 in enhancing the ink transfer, indicating potential ink savings when printing on recycled and non-woody papers. Additionally, the printing inks modified with nanoparticles did not show significant colorimetric differences (ΔE00 < 1) compared to the reference, ensuring a consistent print quality across different papers.
This study offers original contributions to the field by improving print quality through the modification of printing inks with nanoparticles, rather than altering the composition or surface properties of the paper or using a protective layer. The integration of SiO2 and TiO2 nanoparticles into offset inks to enhance the print quality on eco-friendly papers is a novel approach in graphic technology.
The main challenges include the economic feasibility of incorporating nanoparticles into commercial printing practices, which requires further investigation. Additionally, there is a need to balance the improvement to the print quality with the maintenance of environmentally friendly practices in both paper and ink production.
In summary, the use of offset inks modified with SiO2 and TiO2 nanoparticles can optimize the quality of prints on recycled and non-woody papers. These findings could lead to a decrease in the use of papers made from primary fibers and broaden the utilization of sustainable materials in graphic technology and design.

Author Contributions

Conceptualization, M.S.J., S.M.P., S.J.H., D.D. and T.T.; methodology, M.S.J., S.M.P., S.J.H., D.D. and T.T.; software, D.D.; validation, M.S.J., S.M.P., S.J.H., D.D. and T.T.; formal analysis, M.S.J., S.M.P., S.J.H., D.D. and T.T.; investigation M.S.J., S.M.P., S.J.H., D.D. and T.T.; resources, M.S.J., S.M.P., S.J.H., D.D. and T.T.; data curation, M.S.J., S.M.P., S.J.H., D.D. and T.T.; writing—original draft preparation, M.S.J.; writing—review and editing, M.S.J., S.M.P., S.J.H., D.D. and T.T.; visualization, M.S.J., S.M.P., S.J.H., D.D. and T.T.; supervision, M.S.J., S.M.P. and S.J.H.; project administration, M.S.J., S.M.P. and S.J.H.; funding acquisition, M.S.J. and 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

Data will be provided upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a,b) Bekk smoothness and Ra and Rz parameters of the papers.
Figure 1. (a,b) Bekk smoothness and Ra and Rz parameters of the papers.
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Figure 2. Cobb absorptiveness (t = 60 s) of the papers.
Figure 2. Cobb absorptiveness (t = 60 s) of the papers.
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Figure 3. Total, dispersive, and polar SFE of the papers.
Figure 3. Total, dispersive, and polar SFE of the papers.
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Figure 4. Micrographs (mag. 20×, 100×) and line profiles of paper substrates, according to the [66]. (a) MN 120-R, (b) RW 80, (c) NC 80, (d) PC 120, (e) MH 118.
Figure 4. Micrographs (mag. 20×, 100×) and line profiles of paper substrates, according to the [66]. (a) MN 120-R, (b) RW 80, (c) NC 80, (d) PC 120, (e) MH 118.
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Figure 5. The results of ink transfer test for all selected papers printed with unmodified offset ink and ink modified with SiO2 and TiO2 nanoparticles.
Figure 5. The results of ink transfer test for all selected papers printed with unmodified offset ink and ink modified with SiO2 and TiO2 nanoparticles.
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Figure 6. Colorimetric properties of all printed samples. (a) MN 120-R, (b) RW 80, (c) NC 80, (d) PC 120, (e) MH 118.
Figure 6. Colorimetric properties of all printed samples. (a) MN 120-R, (b) RW 80, (c) NC 80, (d) PC 120, (e) MH 118.
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Figure 7. Print mottle values. (a) MN 120-R, (b) RW 80, (c) NC 80, (d) PC 120, (e) MH 118.
Figure 7. Print mottle values. (a) MN 120-R, (b) RW 80, (c) NC 80, (d) PC 120, (e) MH 118.
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Figure 8. Micrographs of prints made with unmodified and modified inks (mag. 5×). (a) Unmodified ink, (b) modified ink with SiO2, (c) modified ink with TiO2.
Figure 8. Micrographs of prints made with unmodified and modified inks (mag. 5×). (a) Unmodified ink, (b) modified ink with SiO2, (c) modified ink with TiO2.
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Table 1. The results of measured basic and structural properties of selected papers.
Table 1. The results of measured basic and structural properties of selected papers.
Paper Grammage (g/m2)Caliper (µm)Bulk (cm3/g)Density (g/cm3)
MN 120-R1201441.190.84
RW 8080971.210.83
NC 80801001.260.79
PC 1201201991.660.60
MH 1181181691.430.70
Table 2. The results of measured Bekk smoothness of the paper surface.
Table 2. The results of measured Bekk smoothness of the paper surface.
Paper Bekk A (s)Bekk B (s)
MN 120-R20.417.4
RW 8018.134.0
NC 8027.924.1
PC 1202.31.8
MH 1188.55.9
Table 3. The results of measured optical properties of the selected papers.
Table 3. The results of measured optical properties of the selected papers.
Paper Whiteness (%)Brightness (%)Opacity (%)
MN 120-R129.4101.696.5
RW 8056.166.998.2
NC 8010085.995.6
PC 120125.8105.793.5
MH 118116.7102.593.7
Table 4. Adhesion parameters between papers and inks.
Table 4. Adhesion parameters between papers and inks.
Adhesion Parameters (mJ/m2)γ12W12S12
MN 120—R
Unmodified ink0.38 79.39 −8.13
SiO2—modified ink0.51 80.63 −9.62
TiO2—modified ink0.47 78.58 −7.49
RW 80
Unmodified ink1.81 71.81 −16.89
SiO2—modified ink1.89 69.4 −14.66
TiO2—modified ink2.97 72.36 −19.76
NC 80
Unmodified ink2.97 58.28 −21.62
SiO2—modified ink3.04 58.68 −22.16
TiO2—modified ink3.88 57.32 −22.48
PC 120
Unmodified ink0.75 68.45 −10.89
SiO2—modified ink1.01 69.87 −12.81
TiO2—modified ink0.76 67.99 −10.43
MH 118
Unmodified ink0.66 70.72 −7.34
SiO2—modified ink0.49 71.29 −7.56
TiO2—modified ink0.22 69.59 −5.32
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Strižić Jakovljević, M.; Mahović Poljaček, S.; Jamnicki Hanzer, S.; Donevski, D.; Tomašegović, T. Towards Expanding the Use of Paper Made from Recycled and Non-Woody Plants: Enhancing the Print Quality through the Application of Nano-Modified Offset Inks. Sustainability 2024, 16, 4785. https://doi.org/10.3390/su16114785

AMA Style

Strižić Jakovljević M, Mahović Poljaček S, Jamnicki Hanzer S, Donevski D, Tomašegović T. Towards Expanding the Use of Paper Made from Recycled and Non-Woody Plants: Enhancing the Print Quality through the Application of Nano-Modified Offset Inks. Sustainability. 2024; 16(11):4785. https://doi.org/10.3390/su16114785

Chicago/Turabian Style

Strižić Jakovljević, Maja, Sanja Mahović Poljaček, Sonja Jamnicki Hanzer, Davor Donevski, and Tamara Tomašegović. 2024. "Towards Expanding the Use of Paper Made from Recycled and Non-Woody Plants: Enhancing the Print Quality through the Application of Nano-Modified Offset Inks" Sustainability 16, no. 11: 4785. https://doi.org/10.3390/su16114785

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