When developing inkjet inks, the chemistry teams in companies around the world face a variety of challenges: often the only visibility of this is when a digital printing application falls short in the field and the search for someone to blame begins! Perhaps a more complete understanding of the difficulties being faced in today’s market would help the collaboration between ink developers and the other disciplines that are required for a successful development.
1. The fundamental trade-off
Any inkjet ink development is centred on the fundamental requirement that the ink be jetted reliably in the printing system of choice, while still delivering the expected application performance once printed. The problem is that in almost all cases the two parts of this requirement act against each other. For example, in a digital ceramic ink required to survive the high temperature firing production process, the pigment particles need to be inorganic (which makes them dense) and relatively large, which causes difficulties in creating a stable dispersion, and hence a usable ink. Secondly, inks for printing conductive tracks are usually made from fine particles of a conductive metal: these have to be dispersed in the ink for reliable printing, which means that each particle is coated in a dispersing medium. This means the printed ink does not conduct electricity unless subjected to further processing, thereby greatly complicating the process. Thirdly, when formulating inks to print vibrant colours for textile or other decorative applications, the more colorant deposited in a single pass, the better, but the concentration of colorant in the ink is generally limited by the printability parameters of the ink (either viscosity, or stability, or both). These are just some of the examples where the formulation chemist is found to be constantly on the horns of a dilemma.
2. Carrier choice
One of the first steps in development is to choose the carrier – the majority component of the ink – usually either water, solvent or UV curable monomer. Each of these ink types has its advantages and disadvantages, and in many cases some unattainable combination of advantages is what is really needed. Water based inks have the potential to be low-cost and have some environmental credentials, but can give reliability problems with single pass printing and are difficult to print onto non-absorbent substrates due to the drying requirement. Solvent-based inks are low cost and work well when printed onto polymer substrates, but can have reliability issues when formulated for fast drying and have relatively poor environmental impact because of the volatile organic constituents. UV inks are print reliably and work well on a wide variety of substrates, but are high cost and have a significant safety concern when used for food packaging applications. So again, formulation is inevitably a compromise.
3. Dispersion quality
Some inks use a dyestuff colorant that is dissolved in the ink, leading to a (relatively) simple formulation task. For other types of inks, including those using solid pigments as the colorant and many of the wide range of inks used to deposit functional materials, a dispersion of solid particles in the liquid ink is required. Without correct formulation such a two-phase system is unstable, leading to the particles of solid material settling out of the ink and blocking printhead nozzles, filters and even ink pipes. The utmost care must go into the dispersion process and the materials and equipment used to stand any chance of producing a product that will perform reliably in today’s demanding high speed single pass applications.
4. Characterisation and quality control
It is essential at all stages during an ink development and production process to ensure the necessary characterisation of key parameters using appropriate equipment. From checking raw materials on receipt from a supplier, through developing a prototype ink, all the way through to final checks before a product is shipped to a customer (and on-going checks on batches to validate shelf life) relevant performance parameters must be measured, and in the case of product manufacture, checked against agreed acceptable ranges for quality control purposes. Important parameters during ink formulation and manufacture include viscosity, surface tension, particle size and size distribution, drop formation, print quality and colour behaviour. All of these parameters require specialist equipment and techniques for measurement.
5. Application-specific issues
While the first four issues are generally applicable to a very wide range of applications, there are other issues that are more pertinent to a specific use. For many industrial printing applications, the printing process is integral to an overall production process, and so the digitally-printed result, as well as the process to achieve this, must be compatible with the other parts of the existing process in order to be adopted. This can put great strain on the different aspects of the digital printing system, not least the ink, as it adds further levels of requirement to an already difficult problem. Even for a single application area, such as flooring, disparate manufacturing processes in different factories make distinct demands, which potentially leads to an end to ‘one ink for all’ and to a much wider set of products to meet specific requirements.
In some flexible packaging, graphics and other applications, it is highly desirable to use water-based inks, but as mentioned above this is challenging when non-porous substrates are used, as without special attention print quality and adhesion may be poor and rapid drying difficult. Many companies are attacking this problem in different ways, but there is no clear solution at present.
Overall, increasingly demanding application requirements as well as the much broader range of digital applications now in production is leading to product fragmentation, where a larger number of inks are needed to meet niche requirements. This calls for increased ink development resource, and a lower return on investment for each product as sales volumes per product reduce. This is the ultimate challenge to the business model of the ink companies in today’s market.
Tim Phillips, IMI Europe
Many of the issues facing ink developers will be discussed at the IMI Europe Inkjet Ink Development Conference, to be held in Lausanne, Switzerland on 13-14 April 2016.