Characteristics of Solvent-Based Gravure Inks and Their Relationship with Printing
Solvent-based gravure printing inks consist of pigments, resins, additives, and a significant amount of solvents, making the ink relatively thin. The drying process of solvent-based gravure inks is volatile, which makes them suitable for printing on paper, films, aluminum foil, and other materials. Given that gravure printing presses operate at high speeds and deposit a thick ink layer, it is essential to understand the impact of ink properties like thixotropy, flowability, viscosity, and drying speed on print quality. This knowledge is crucial for enhancing the quality of printed materials.
Thixotropy
Thixotropy refers to the change in the flow properties of ink under the influence of external forces. Specifically, it is the phenomenon in which a suspension or dispersion thins when stirred and thickens again when left undisturbed.
Generally, high-quality inks have low thixotropy, allowing them to be used directly on the press without special treatment. However, inks without thixotropy tend to experience sedimentation, which is not ideal. In general, white, yellow, and cyan inks have low thixotropy, while magenta and black inks exhibit higher thixotropy.
The thixotropy of inks is influenced by the following factors:
1. Properties of the Pigment: For example, carbon black pigments treated with ozone have significantly lower thixotropy compared to untreated ones.
2. Pigment Particle Size: Smaller pigment particles result in lower thixotropy, while larger particles cause higher thixotropy.
3. Pigment Quantity: Larger amounts of pigment can cause coagulation between pigment molecules, which increases thixotropy.
4. Molecular Weight of the Resin: Inks with high molecular weight resins but low n-heptane values tend to have higher thixotropy, and vice versa.
5. Pigment Wettability: Inks with better wettability of pigments generally have lower thixotropy. Some ink manufacturers pre-disperse pigments in the production of black inks to improve wettability and reduce thixotropy.
For solvent-based gravure printing inks, an appropriate level of thixotropy is required. If thixotropy is too high, the ink may exhibit poor leveling, viscosity control issues, and can result in printing defects like mottle or orange peel. If thixotropy is too low, dot gain may increase, causing text and lines to appear thicker.
Flowability
The flowability of solvent-based gravure printing inks is a comprehensive reflection of factors such as relative density, viscosity, thixotropy, and cohesion. To achieve optimal flowability, the ink must have suitable viscosity, good pigment wettability, and a balanced pigment concentration, while minimizing thixotropy.
When testing the flowability of ink, aside from measuring its flow time using a #4 or Zahn #3 cup, the amount of ink remaining on the cup wall compared to the amount that flowed out is also important. With a fixed viscosity, the smaller the ratio of retained ink to flowing ink, the better the flowability.
If the ink’s flowability is poor, printing faults such as uneven ink transfer or print defects are likely to occur.
Viscosity
Viscosity in solvent-based gravure inks is closely related to flowability, and maintaining the correct viscosity is key to ensuring effective ink transfer. Although many factors, such as resin type, ink adhesion, printing speed, pressure, substrate material, and environmental conditions, influence ink transfer, viscosity remains a critical factor. The relationship between viscosity and transfer rate is depicted in the diagram below.
Factors affecting viscosity in solvent-based gravure inks include:
1. Viscosity of the Resin Solution: Higher resin viscosity increases ink viscosity, and vice versa.
2. Pigment-Resin Compatibility: If pigments react with the resin and cause coagulation, the ink’s viscosity increases.
3. Solvent and Dispersant Compatibility: If solvents dilute the dispersants at any ratio, the ink’s viscosity will decrease.
4. Temperature: Lower temperatures increase ink viscosity, while higher temperatures reduce it.
Low viscosity in solvent-based gravure inks can result in issues such as pinholes, uneven color coverage, rough patches, or poor adhesion, leading to print defects. On smooth surfaces, ink adhesion may be weak, and on paper substrates, powdering or flaking may occur.
Conversely, high viscosity can cause issues like scratching, color bleeding, plate flooding, and “orange peel” defects.
Conclusion
In the design and production of solvent-based gravure printing inks, it is crucial to select compatible pigments, resins, and solvents to achieve the desired initial viscosity. Printers should use appropriate solvent blends, preferably provided by the original ink manufacturer, to adjust the ink to its optimal “working viscosity” for printing. This ensures high-quality printed results.
Furthermore, ink viscosity is also influenced by printing speed. Typically, faster printing speeds require lower ink viscosity to achieve optimal ink transfer.
Solvent-based gravure printing inks are volatile inks, meaning that when solvents evaporate from the wet ink film, it transitions from liquid to solid. The rate at which solvents evaporate directly affects the drying speed of the ink.
In designing and manufacturing solvent-based gravure printing inks, a mix of medium, low, and high boiling-point solvents is typically chosen to meet drying requirements. Incorrect resin selection, low softening points, poor solvent release from resins, or excessive high-boiling solvents can slow down solvent evaporation and result in residual solvent in the cured ink film.
When formulating solvent-based gravure printing inks, resins are typically selected first, followed by solvents. Solvents with similar solubility parameters to the materials being dissolved tend to dissolve more easily and evaporate faster. However, the presence of pigments reduces solvent evaporation rates, especially if the pigments are large.
Additionally, environmental factors such as higher ambient temperature, lower relative humidity, and faster air circulation increase the evaporation rate of solvents.
Common printing issues caused by improper drying include:
1. Ink drying too quickly at certain printing speeds can cause plate flooding, scratching, ink whitening, poor gloss, and leveling issues.
2. In multi-color printing, slow drying can result in roller adhesion or ink smearing during the next printing station, leading to print defects.
The evaporation rate of solvents in ink depends not only on their boiling point, vapor pressure, and latent heat of evaporation but also on factors such as printing environment temperature, relative humidity, air flow, and ink layer thickness. Therefore, ink solvent evaporation rates should be adjusted based on varying conditions.
Under normal production conditions, if ink dries too quickly, slow-drying diluents should be added. If ink dries too slowly, fast-drying diluents should be used. Where possible, printing environments should be controlled between 18-25°C, with relative humidity kept below 65% to stabilize ink performance.
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