Flexographic inks are an integral part of ink technology.
Understanding and measuring their physical properties is essential for optimizing ink performance and addressing challenges.
In this article, we will explore the key properties of flexographic inks and how to measure them.
Physical prooperties of Flexographic Inks and How to Measure Them
Measuring and understanding the physical properties of flexographic inks is crucial for ink technology.
By studying ink performance on the press and its interaction with ingredients, ink formulators can improve inks and solve problems.
Care should be taken when setting print resistance specifications to avoid unnecessary safety margins that could restrict raw materials and result in lower print quality.
Viscosity and Dilution Measurement in Printing Inks
Printing inks are typically supplied with a viscosity higher than required for printing, serving two main purposes.First, this allows printers to make adjustments during the printing process. Second, it prevents pigment settlement during storage, which is more likely to occur with thinner inks.
The viscosity of ink plays a crucial role in the printing process, and the conventional method for measurement is using the efflux flow cup.
However, this method may yield inaccurate results for thixotropic inks due to their time-dependent viscosity behavior.
To address this, printers are advised to circulate the ink in the pump before taking the final viscosity reading to break down any ink structure.
Temperature also influences viscosity since ink generally becomes less viscous with an increase in temperature.
For ink formulators, analyzing a solvent-dilution curve provides more valuable information than a simple viscosity measurement after ink manufacturing.
By taking readings at various dilutions and plotting the results, formulators can understand the ink's behavior more comprehensively.
Steeper curves indicate a lower solvent requirement to achieve print viscosity, resulting in stronger print results.
Another critical factor affecting printing properties is the tackiness of wet ink, especially at high press speeds.
While it is relatively easy to measure tack for oil-based inks using a tackmeter, this becomes challenging for thin, volatile flexographic inks.
However, the impact of tack can be observed using a hand anilox proofer. Inks with high tack will exhibit more pull-through and strength during testing.
Drying tests
Drying Tests for Solvent-Based Inks
When it comes to solvent-based inks, their drying process is influenced by two main factors:
The evaporation rate of the solvents used and the solvent release characteristics of the binder.
Different resins may release solvents at different rates based on their chemical makeup, sometimes causing unexpected drying rates.
The hardness of the binder also affects how dry and tack-free the ink feels, even if some solvent remains in the ink film.
To improve drying, higher concentrations of pigment and extender can be used, which keeps the ink surface open without forming a skin and helps with evaporation.
Various additives and plasticizers can also impact solvent release.
To compare drying properties in the laboratory, researchers can make comparative drawdowns and observe the relative time it takes for the surface to become tack-free.
However, it's essential to remember that actual printing presses apply heat, which accelerates drying time.
Therefore, rapid drying in the lab may not directly correspond to real-world conditions where heat-assisted drying occurs.
Testing the final state of solvent retention after printing is typically done using gas-liquid chromatography (GLC).
The results are expressed as the amount of solvent present per square meter of print.
This equipment is valuable for ink-drying research.
Poorly dried prints can cause serious problems in printing.
Retained solvent can lead to unpleasant odors, and the presence of solvent can leave the resin in a semi-solid state, making it prone to blocking on the other side of the substrate.
Additionally, certain coated films may develop ghost images on the unprinted side due to solvent attack.
Drying of Water-Based Inks
Water-based inks for paper and board primarily dry through water penetration into the substrate, partially through evaporation.
The drying speed depends on the substrate's absorbency and the type of resin/water dispersion used in the formulation.
In the lab, it's challenging to measure drying speed accurately since water-based inks tend to dry too quickly.
However, rough comparisons against inks with known performance can be made by testing drawdowns on paper with one's finger as they dry.
For water-based inks on non-permeable substrates, drying occurs mainly through the evaporation of water and any solubilizing amino compounds in the formulation.
Drying comparisons can be made in the lab using similar methods as with solvent-based inks, but again, the presence of heat on the press will reduce drying time.
Special tests – Water-based inks
Water-based inks require special tests to assess their properties due to their unique nature.
While similar methods used for solvent inks can be applied, additional tests for foaming and pH levels are essential for water-based systems.
Foaming can lead to press-related issues, especially when the ink is subjected to high agitation during the printing process.
To check for foaming tendencies, a simple test involves vigorously shaking or stirring a diluted sample of ink and observing how long it takes for the foam to subside.
A comparison can be made with a satisfactory standard ink to evaluate the results.
Conventional water-based inks are typically alkaline, with pH readings above 7.
Conventional water-based inks are typically alkaline, with pH readings above 7.
By using a standard pH meter, inks can be tested, and the desired pH range is usually between 8 and 9.
Lower pH readings may indicate stability problems, while excessively high pH levels could result in poor ink drying.
The specific formulation used in the ink will influence the acceptable pH range, which can be determined through experimentation.
Dispersion State
Pigmented inks are made as a mixture called a dispersion, and how well the components are mixed together depends on the formulation and manufacturing process.To measure this, a common method uses a metal wedge called a grinding gauge, which is marked from 0 to 25 mm.
The ink is spread along the wedge with a blade until pigment particles start to create scratches.
This point is taken as the particle size. It's important to conduct this test before adding any waxes or additives, as they can affect the result.
Other indicators of dispersion quality are gloss, color development, and transparency, which are closely monitored.
Changes in the milling equipment settings can lead to variations in the dispersion quality, so it's essential to carefully control and avoid batch-to-batch differences.
Chip-based inks tend to be more consistent across batches than other highly dispersed inks, but they may reduce ink flow due to their high dispersion state.
While the manufacturing process plays a major role in achieving proper dispersion, the binder, pigment, and solvent also have significant influences and interdependence.
All three components are equally important for the final result, especially in terms of dispersion stability.
Finding the best conditions and formulation balance for optimal dispersion can be time-consuming and requires conducting many experiments.
It's generally preferred to produce bulk quantities of inks with a single pigment base, as this reduces the number of products that need to be studied.
Stability of the dispersion is crucial both when the ink is supplied in its original form and when it's diluted for use on a printing press.
To assess storage stability, an accelerated test at 50°C for one week can be used, while dilution stability can be tested by comparing a diluted sample left overnight with a freshly diluted sample.
Adhesion
Adhesion of ink to the substrate is crucial for the longevity of printed articles.
The most common test for adhesion is the adhesive tape test, but it should be complemented by other tests like scuff and scratch resistance for a comprehensive evaluation.
Ink formulations should consider heat sealing, rub resistance, and blocking resistance for reel-to-reel presses.
Flexographic printing for packaging demands diverse ink properties, taking into account the potential interaction with packaged products.
Ink manufacturers should be informed of the intended use and specific conditions to tailor formulations accordingly.
Testing simulates real-life conditions to avoid issues during manufacturing and ensure ink quality and performance.
Conclusion
Understanding and measuring flexographic inks' physical properties are vital for optimizing print quality.
Viscosity, dilution, drying, dispersion, and adhesion play key roles in ink performance.
Collaboration and innovation will drive continuous improvement in ink technology, revolutionizing the printing industry.
Let's push boundaries and unlock the full potential of flexographic inks for exceptional print results.
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