With inkjet printing rapidly proving itself as a true production process within the area of industrial printing, we as inkjet ink developers need to keep up with the pace and develop new fluids for these applications. Aqueous inks for industrial inkjet printing are potentially advantageous for many different applications. However, delivering the right functionality in a formulation with the constraints imposed by inkjet printing is no easy task.
Inkjet inks by nature are low viscosity, which means that the amount of functional chemistry that can be included in the formulation is limited. Generally speaking, an increase in functionality (adhesion, colour, abrasion resistance, wash fastness) of an ink requires the inclusion of higher viscosity products which can put the ink formulation outside the parameter window required for reliable jetting.
For many industrial applications, pigments are often the colorants of choice due to their increased light fastness behaviour compared to their soluble dye-based counterparts. Aqueous pigment dispersions, however, require significant processing to become suitable for inkjet applications. For many years, high quality pigment dispersions in water have been available that are suitable for inkjet printing. However, these formulations were developed for desktop printing on paper. This means that they are not always ideally matched for industrial printing technologies, where the colorant type is crucial for a number of different reasons.
Two market areas in which aqueous pigmented inkjet usage is set to increase rapidly are décor printing and textile printing. These are two very different markets with different drivers for converting from analogue to digital. They also have very different technical barriers to this conversion.
With over 50% of the world’s printed textiles currently using conventional analogue printing with pigment inks, it is no wonder that pigment inkjet ink for textile has been a hot topic of conversation for many years. So why has this technology not gained greater traction?
Pigment textile inks require the deposition of solid pigment particles and a binder pre-polymer on the textile. When cured with heat, a polymeric matrix is created that is strong enough to trap the colorant and bond it to the textile fibre. The technical barrier is often the complexity of ink. Combining a pre-polymer and pigment particles in an aqueous low viscosity inkjet ink requires a complex formulation which needs care to avoid printability issues and inconsistent jetting. Inkjet ink design is always a fine balance; in this case, too much binder can cause printability issues, while too little impacts the wash fastness of the finished ink.
Some companies have removed the binder from the ink to simplify the formulation and improve jettability, using pre-treated fabrics with a coating of binder. This step reduces the flexibility for the end user, in that all production fabrics need pre-treating.
Recent developments are showing much promise in the use of binder systems that are capable of printing at higher speeds through a number of printhead platforms. There are also early signs of pigmented inks being used in single pass inkjet engines – this would be a significant breakthrough in inkjet ink development.
Compared to textile printing with pigmented inks, designing an inkjet ink for décor printing should easier. The bulk of décor printing uses a paper substrate that is then laminated. In this case, the ink can be rather simple in design, with little or no binder included. Therefore compared to textile printing, one problem has been eliminated and the ink formulation is somewhat simplified. However, the problem with décor inks is more in the pigments themselves.
Décor printing (for the purposes of this article we will use laminate flooring as the example end use) is currently dominated by the rotary gravure process. The pigments used in this process have been used for many years, and there is a reluctance to change pigment types in order to move to digital printing. The reluctance is technical and to a certain extent cultural.
Technically, the selected pigments need to fulfil the following criteria:
- Correct colour gamut
- High resistance to the lamination process (both chemical and physical effects)
- Low metamerism properties (the effect where a mixture of pigments gives different colour properties under different lighting conditions)
Gravure printing rarely uses a CMYK process colour set as is habitually used in digital printing – more often it is a combination of specific spot colours for each design. For inkjet ink developers, this poses a problem. Reproduction of a gravure process using up to eight different pigments which are often not commonly used in inkjet inks is no easy task. This requires large investments in pigment dispersing technology and expertise, and leads to a wider ink set than is commonly used in inkjet printing. If this means ink changes between design print runs with inkjet, it reduces one of the advantages of digital printing as set-up will take longer.
SINGLE PASS PRINTING
If inkjet is to contend seriously with analogue printing processes, then single pass printing will be required to match or even approach the speeds producers are currently used to. We have already seen the advent of single pass printers in a number of different applications, including ceramics and textile. However, aqueous pigmented systems with functional binders still require further development for reliable and widespread use in single pass applications.
My talk at the IMI Europe Inkjet Ink Development Conference in Lausanne, 15-16 March 2017 will investigate the technical barriers preventing market penetration of aqueous pigmented inks and look at some solutions that innovative companies are investigating to overcome these problems.
Andy Hancock, Mexar Limited