Digital inkjet printing: Can it solve the compatibility problem between water-based inks and paper?
For printing, water is a problem because inkjet printing, which replaces offset printing, has the opportunity to eject a lot of water on the paper. This type of inkjet ink is mainly water, which poses a problem for developers, on the one hand, how to reduce the dependence on water, and on the other hand, how to prevent it from causing damage to the paper. The obvious answer is "dry", which prompts the second question: how to dry?
The answer now is: it depends on the printer manufacturer, the ink itself, and the paper. The latter must be excluded, because for inkjet to replace offset printing in commercial printing, inkjet needs to be able to print on a wide range of substrates like offset printing.
Specialty inkjet paper will replace offset paper over time, but not until the market share of inkjet printing has grown significantly. This happened when offset replaced letterpress as the dominant printing process. At first, emerging technologies had to deal with substrates optimized for the printing method they were meant to replace. Optimized inkjet paper exists, but can be much more expensive than existing offset paper, so new technologies need to deal with it.
Using a primer is a commonly used method. It forms an intermediate surface between the ink and the paper to minimize the amount of ink that penetrates the fibers of the uncoated paper while preventing the ink from flowing across the coated paper. The primer binds to the paper and ink and forms a barrier layer to prevent or at least slow down the ink from entering the paper. It also helps provide ink chemists with a consistent, controlled surface to achieve their goals, rather than trying to develop a recipe that fits all the paper they may need.
The exact recipe for making an inkjet ink falls under the category of commercially sensitive information. The concept behind an ink is not: it transports pigment particles onto a substrate where they will bind to the material and hopefully remain in place. For this, the ink requires finely ground pigment particles (which can pass through the nozzle of the nozzle), a resin, a binder and a solvent to carry the pigment onto the substrate. In this case, water is used as the solvent, so the ink also needs a way to prevent the water from drying out and clogging the nozzle, namely a moisturizer.
Therefore, our inkjet recipe starts with 100g of carbon black and then adds 40g of styrene acrylic resin. They are mixed together at a very high speed. Thereafter, we add 50g of propylene glycol and 13g of potassium hydroxide and mix for another two hours. Finally, 464g of ion-exchanged water is added and we have the basic inkjet ink.
To make it commercially attractive, ink producers can add preservative compounds, viscosity regulators, pH regulators, chelating agents, plasticizers, antioxidants, and UV absorbers. The amount of ethylene glycol used as a moisturizer or wetting agent will also vary, up to 50% of the liquid in extreme cases. Moisturizers have a higher evaporation point than water, so more energy is required to remove them from paper.
In addition, the pigment particles will be provided in the form of a dispersant fluid. This is to prevent the nanoparticles from clumping together to form larger particles, which can affect print quality and jetting performance. To achieve this, small electrostatic charges can keep the particles apart as they swim through the dispersant fluid and may be coated with a polymer shell.
In an ideal world, the water content would be minimal and the viscosity would be higher, giving ink manufacturers more choice when creating formulations. However, ink manufacturers must find a balance between the ideal world and the real world to create inks that work reliably on the press and are inexpensive.
The ink manufacturer's goal is to produce a fluid with a viscosity of no more than 20 mPas, preferably around 7-8 mPas, so that it can flow through a customer-specified piezoelectric nozzle. This requires a lot of liquid, a lot of water, and the water needs to be removed from the paper being printed as quickly as possible.
In single-channel inkjet printing, water as a carrier is necessary because few piezoelectric nozzles can handle highly viscous fluids, so the viscosity can be increased to the point where the ink can work. In thermal inkjet, water is absolutely critical because the rapid heating of water causes the nozzles to expand instantaneously, forcing the ink out of the nozzle. Continuous inkjet also requires fluid that can easily flow through the nozzles, which means that the viscosity must be kept low. Since the flow of ink through each nozzle is constant, the wetting agent load can be lower because there is less opportunity for the ink to dry inside or on the nozzle. However, the lower the viscosity, the less pigment can be carried.
Alternatives to using UV-curable inks are impractical in commercial printing applications due to the additional cost of photoinitiators and the like. In packaging, UV will be avoided if possible, as the risks associated with migration are as high as the costs. In the past, solvent-based inks have been used in inks, but the issue of contamination and other issues caused by solvents need to be addressed. In comparison, water is harmless.https://www.risedongsheng.com/
Therefore, water is the future for almost all suppliers that produce inkjet presses, especially in high-speed applications involving packaging. These include companies such as HP, Koenig & Bauer, Canon, Screen, Ricoh, Kodak and Agfa. UV will continue to be used for printing on rigid and plastic materials as well as display materials, but its relevance is bound to decrease over time.
Tim Phillips, managing director of IMI, explains: "You can use solvent-based inks, which dry faster but can cause odour issues, especially when printing on paper. That's why developers are keen to use water-based inks that are also more environmentally friendly. Water can make up 60-70 per cent of the ink though, so whatever else is in the ink, the water has to dry, and you can't do much to help it dry."
"This greatly limits the choice of ink compounds," Agfa explains in a brochure about its own ink production facility. The company makes inks for its own presses and applications, as well as for third-party manufacturers. Its chemists select a batch of ingredients whose properties, as well as any interactions, are carefully documented. This is what Agfa calls the "pharmaceutical approach," helping the company achieve consistent quality, reducing the ink layers needed for desired colors, and extending shelf life. It also reduces the time to develop new inks.
Currently, although Agfa supplies the inks for Koenig & Bauer's Rotajet press, its experience in inkjet is mainly focused on large format multi-pass presses, rather than single-channel printing. However, through the acquisition of Inca, Agfa has acquired Speedset, a B1 single-channel press with UV inks for metal decoration. Next year it will launch water-based inks and target corrugated boxes and possible folding carton applications.
Mike Horston, ink marketing specialist at Agfa, says: "The biggest problem with offset printing is that lithography uses an oil-based ink. The oil doesn't absorb water, so it doesn't require much drying, the paper or corrugated paper doesn't expand or contract, the ink has good adhesion and the press can run at pretty good speeds."https://www.risedongsheng.com/
However, he believes that inkjet has its own advantages, at least to cope with the advantages of offset printing in terms of packaging. Groove corrugation minimizes the problem of material shrinkage, where the structure of the material provides stability, and then also needs to consider the operational and marketing advantages of digital printing. "We also know that shrinkage issues can be minimized through faster printing," says Hosten. "Speedset will run on 1,060mm wide paper at 150m/min, and print high-quality full B1 paper at about 12,000 sph, and we have seen some amazing quality."
That's thanks to the cradle-to-grave approach, which Agfa has shipped, developed inks and worked with printheads to optimise the waveform and performance to achieve its desired effect. "All ink manufacturers should be able to produce water-based inkjet inks, but in practice it's not easy," says Horsten. "It's like a thermal sublimation ink, there are only a few producers who use the same paste version of the ink for blending. For our thermal sublimation inks, we add more colour content, which means less water content, which reduces drying problems and increases drying speed." This method would be popular in commercial printing.https://www.risedongsheng.com/