The term "plasma" is well known to us in its medical form, but, with the evolution of the new form of television screens, we now also know plasma to be "an electrically neutral ionised gas, composed of ions, electrons and neutral particles".
In plasma TV screens, for instance, an ionised gas produces the colour displays we see from the front of the screen by energising the same phosphors that, when bombarded by electrons fired from a cathode "gun", produce red, green and blue colours in the traditional CRT screens. But before we get too far into the subject, a word about what plasmas are and how they are generated.
What is plasma?
We are all familiar with the three common states of matter, solids, liquids and gases; plasmas are a special case of the latter. Under normal circumstances, the atoms in gases exist with an equal number of positive and negative charges. To put it another way, the nucleus of the atom is enclosed in a cloud of electrons such that charge is balanced. When gases have one or more electrons stripped from their atoms by the application of energy such as heat, electrical discharge, or powerful light (like a laser), they become a soup of atomic residues (ions) and electrons. This is a plasma. Such diverse areas as the corona of the Sun, lightning bolts and fluorescent lights all have plasmas associated with them.
Plasmas can be manipulated by both electric and magnetic fields, and this is where our research comes in. We are not seeking to generate plasmas (there are many companies making equipment to do just that), but we are putting plasmas into contact with materials like paper and other printable substrates to modify the properties of those materials. And whilst we are studying the use of plasmas to modify the surface chemistry of substrates, plasmas are also being used to modify the surface treatment of packaging films for printing and metallising.
In corona treatment of film a series of tiny electrical "sparks" gives rise to a localised plasma which can increase the surface energy of the film, both by physical rearrangement of the surface molecules and the addition of oxygen to the polymer chains. The supporters of plasma treatment argue that as the corona discharges are discrete little sparks, they work fine where they hit the film but there are also spots on the surface of the film that are "missed." By contrast, a glow discharge plasma is an all-encompassing gas, and so every part of the film is evenly treated by exposure to it.
For example, when polypropylene is metallised, aluminium vapour will condense onto the spots where the corona has hit but leave small voids where the film is effectively not treated. By contrast, the glow discharge plasma treated film will accept a uniform layer of aluminium, thus raising the degree of protection to light and vapour transmission that can occur through the film.
Plasma-generating equipment generally works in a near vacuum, and Program 1 at the ANU in Canberra has just such a unit. Under the watchful eye of Dr Armin Bauer, paper and paper-like surfaces are being exposed to a range of plasmas there. As the research of the Centre is strongly based on cooperative effort, we also have a new plasma unit at the CSIRO labs in Clayton where Program 2 is based. Dr S.K. Ooi is the principal operator of the new unit which has one significant difference: the Clayton unit runs in the atmosphere without the vacuum surround, a major boost to the use of plasma technology in industrial applications.
The paper that was presented at the Appita conference recorded collaborative experiments conducted on cellulose acetate with both the "low pressure" and "atmospheric" units. In the experiments described, the plasma unit in Canberra was coupled to a Mass Spectrometer, and the CSIRO atmospheric unit to an Optical Emission Spectrometer. The plasmas were generated from Argon gas, and the idea behind using these instruments was to measure the differences between the Argon fed into the plasma units and the mixture of chemical species that came out.
Without dwelling too much on the chemistry angle, a range of chemical entities including Hydrogen, CO and CO2, water, Nitrogen, and Oxygen appeared, and it was noted that both the N2 and O2 content increased after the plasma treatment for both the low pressure and atmospheric units. The most interesting chemical entities generated in the plasma reactions were hydroxyl, carbonyl, and carboxyl groups.
Effect on print
So, one may very well ask, what does all this clever stuff have to do with printing? Well, the plasma treatments at both laboratories gave a "significant decrease in the water contact angle" of the substrate". In simplest terms, the surface of the material exposed had become very much more hydrophilic, or in other words very much more easily wet by water. The opposite effect can also be achieved.
Future work will look at grafting new chemical groups onto the surface of paper and polymer films in order to precisely control the properties of the substrate and thereby to generate papers that are selectively easier to print on and will more accurately match the end application. One of the most interesting applications from a global perspective will be to generate new ways of modifying printing substrates to facilitate the removal of inks and coatings from printed surfaces in recycling.