While UV-curable inks boast excellent adhesion properties, a primer still comes in useful when printing on particular substrates. Some substrates are notoriously difficult to make ink adhere to. Take for example metal, glass, ceramics or polypropylene, which can be used for numerous applications, including decorative printing, direct-to-shape printing, and marking & coding. To ensure superior bonding of UV-curable inkjet inks to challenging media, dedicated digital primers can save the day.

How it Works

The chemistry of both the primer and the ink is acrylate-based. By applying and curing the primer and ink, some reactive groups in both fluids will bond. Eventually, the networks of the primer and the ink set will merge and they will stick together very closely. Partial curing of the primer is recommended, as it will leave more reactive groups available to bond with the ink.  

The primer must also stick to the substrate. To that end, it contains some very reactive acrylates. These reactive monomers are able to “attack” the substrate – such as plastic, metal or glass – and bind with the reactive groups on top of it.

Many attempts have been made to add these highly reactive monomers to an ink composition in order to develop inks that adhere directly to challenging substrates. Adhesion promoters with acidic groups effectively adhere to these substrates. Ultimately, however, all these trials have failed because they cause a destabilization of the ink’s pigment dispersion. The dispersion aid that helps stabilize the pigment particles in the ink is defeated by the adhesion promoter or reactive monomer, thus destabilizing the particles, and making the ink unstable. A primer, however, does not contain a pigment dispersion, which means the reactive monomers cannot damage it.

It is utopian to think that there is an ink available for every kind of substrate. A more efficient approach is to add a digital primer to the printing process and keep one high-quality ink set to print colours.

The Benefits of a Digital Primer

• Improves adhesion to difficult substrates – even in wet conditions

• Gets applied in-line and only where necessary, thus saving time and money

• Eliminates the need to switch to another ink, which is also often more aggressive

• High jetting performance & reliability

• Good curing speed

• Long shelf life

Traditional solutions to improve adhesion – such as pre-treatment (e.g. corona, plasma, flame) or primers applied using analog processes (e.g. gravure, flexography, screen printing) – are complicated as they require an additional process step and pose higher demands in terms of setting the right conditions.

The main function of primers is to improve the adhesion of the ink set to a difficult substrate. But they also improve the adhesion in wet conditions, for instance, when the printed object gets exposed to high humidity or cleaned with liquids. Especially for glass, which is often used in moist environments, the wet bonding behaviour of the ink is quite important. Digital primers get applied in-line with the printed images, and only where necessary. This makes them both time- and cost-efficient.

An additional print head must be implemented, and an additional UV-LED light source is needed to cure the primer, before the coloured inks are printed. Many systems have the space available to apply the digital primer in-line. Once implemented, it is a huge advantage to be able to enable or disable the application of the primer depending on the substrate to be printed on, rather than having to switch to another ink set that is specifically designed to print on the difficult substrate. In addition, ink sets for printing on glass or metals are hard to find and more aggressive to the system components, leading to more print head replacements and tube leaks.

More on Agfa’s Altamira and Arigi Primers

Agfa has developed dedicated digital primers that ensure superior bonding of its UV-curable Altamira and Arigi UV-curable inkjet inks to challenging media:

• Altamira Primer GP, a general-purpose primer for many challenging substrates (such as PMMA, polystyrene, polycarbonate, glass, ceramics and metals)

• Arigi Primer, a digital primer for polypropylene(-based) materials

These Agfa primers are distinguished by their extremely high jetting performance and reliability, and good curing speed, both with UV LED and with Hg bulb lamps. In addition, they have a long shelf life of 18 months.

 Authors

Rita Torfs, Product Manager Industrial Inks, Agfa, e-mail: rita.torfs@agfa.com

Marc Graindourze, Business Manager Industrial Inks, Agfa, e-mail: marc.graindourze@agfa.com