“Indeed, nothing more beautifully simplifying has ever happened in the history of science than the whole series of discoveries culminating about 1914 which finally brought practically universal acceptance to the theory that the material world contains but two fundamental entities, namely, positive and negative electrons, exactly alike in charge, but differing widely in mass, the positive electron—now usually called a proton—being 1850 times heavier than the negative, now usually called simply the electron.” These are the words of Robert Andrews Millikan, who received the 1923 Nobel Prize in Physics for the measurement of the elementary electronic charge.
These electrons are the very protagonists of the talk on electron beam (EB) curing on the agenda for the IMI Europe Inkjet Ink Summer School 2017 in Ghent, Belgium. The school is an excellent opportunity to connect with professionals in the inkjet industry, and learn about different, existing inkjet curing techniques. As one of the speakers, I will discuss EB curing by first introducing the physics concepts related to the electrons, then showing how these extremely small but powerful particles can be exploited for the benefit of the printing industry. Specifically, as inkjet is the drive of the recent developments in the digital printing world, I will explain how the EB industry is innovating towards compact, fast and dynamic digital applications.
Back to basics
An electron is a subatomic elementary particle. This means that everything around us made of molecules and atoms contains electrons. An electron is electrically charged and its charge is considered to be the elementary charge. In other words, it is a fundamental physic constant. An electron has a mass 34 orders of magnitude less than a Swiss cow, and so far scientists have discovered no smaller components within the electrons.
Electrons behave like waves as well as particles: difficult to imagine, but (relatively) easy to control. Thanks to the charge, electrons can be accelerated, guided and focused. Thanks to the mass they carry energy, they penetrate into materials and can be pictured as, well… balls! No disrespect to all the physicists, engineers and chemists, who have worked on more accurate simulations, but the simplest way to describe electrons and their interaction, is to think about ball games like tennis, for instance. The player hits balls which carry (kinetic) energy, and the balls reach the net to score. This is exactly what happens in an EB system: electrons are generated then accelerated to finally deliver their energy to the substrate to treat it. The stronger the balls are thrown, the farther they travel and the more energy they carry. Similarly, the stronger the electrons are accelerated, the farther they travel and the more energy they deliver. Generally, in EB systems electrons travel at half the speed of the light, therefore all the treatments are almost immediate.
Electron beam curing
What does the electron do for printing? In the printing industry, we exploit electrons to cure inks, coatings and adhesives. EB curing is an alternative to thermal drying and UV curing. The printed substrates enter the EB system carrying a wet image, pass through an electron cloud, and instantly exit the EB system carrying a completely dry image.
To continue with the ball metaphor, if the ball impacts an unstable brick, for instance a domino piece, the effect on the first brick will generate a chain reaction. Ultimately, looking from distance, the audience sees a wonderful image created thanks to that little ball. This is a way to illustrate what happens when electrons react with the components of an EB ink: electrons interact with monomers and oligomers which immediately form radicals. The latter are very unstable species, which generate a chain reaction, to create polymers. The conversion from monomers and oligomers to polymers corresponds to the phase transition from a liquid ink to a dry ink. At the end of the treatment, the consumer will see a wonderful image with a uniformly dry ink layer thanks to that little electron.
The energy that electrons carry is a fundamental advantage when using EB technology. This energy is capable of triggering the radical formation, described above, and consequently the polymerisation of the inks, without the need of photoinitiators. In addition, since the electrons can travel through the entire layer of the inks, the adhesion of the inks to substrates like plastic films, paper and aluminium is enhanced. These properties make electrons perfect for curing inks on food packaging.
EB curing is already being used in the printing industry for flexo and offset applications. The demands of the inkjet technology presented several challenges to the EB systems manufacturers: small installation footprint for the curing unit, high throughput and cost efficiency. Only since last year, have these challenges been overcome, thanks to the introduction of small and compact EB curing stations. It is now time to explore the potential of this technology.
And this is just the beginning
The range of applications is incredibly wide. Combining inkjet’s versatility with the curing performance of EB technology is revolutionising the way brand owners think about food packaging. That’s why I am so thrilled to be part of the summer school to engage with other experts about the market’s expectations of electron beam technology. My belief is this negatively-charged particle truly possesses the potential to bring enormous positive change!
Elsa Callini, ebeam Technologies
Elsa will be presenting electron beam curing as part of the Inkjet Drying & Curing Course, being held at the IMI Europe Inkjet Summer School, 12-16 June in Ghent, Belgium.