Ink jet technology is increasingly accepted for wide format printing on materials such as paper, card, glass and plastics. The development of suitable inks is a challenge since a large number of factors have to be taken into account.
Characterized by many advantages, ink jet technology is one of the fastest growing print processes and is also being used increasingly for wide format printing. Besides providing an excellent printed image, it enables even small runs to be produced at acceptable cost and offers high flexibility. Ink jet printing is suitable for a wide range of substrates and formats, providing an optimal solution for every situation.
Worldwide consumption of ink jet inks in 2003 for wide format printing is currently estimated at almost 14 million liters, approximately 25 percent of the total market for ink jet inks. Experts forecast a further significant increase since the technology’s advantages will open up new areas of application in addition to poster and banner printing.
Two basic forms of ink jet technology have become established. In the “Drop-On-Demand” process (DOD), the ink is directed electronically in droplet form via a nozzle onto the substrate. This is achieved by means of, for example, piezoelectric crystals or, as in the case of a bubble jet printer, with a heating element.
For industrial printing, printers operating on the continuous principle are usually used. In such systems, the ink jet emerging at high pressure (up to 30 bar) is subjected to high frequency pressure waves by a piezoelectric device. The emerging jet of liquid breaks up into very fine micro droplets. Depending on the geometry of the nozzle, these can be as small as a few micrometers and have a velocity of up to 40 m/s. Since only part of the ink jet is required for the printed image, the excess droplets are deflected by electrostatic charge and removed before they reach the substrate. The excess ink is then fed back into the reservoir.
Precisely Tailored Ink Formulations
Each ink is specially developed for the intended application. It must be precisely matched to the printing technique. Three fundamental types are available: water-based, UV curing and, as described in the following, solvent-based ink formulations. These inks are preferred for wide format printing. Their main advantage over water-based inks is their short drying time. In addition, it is possible to print without difficulty on non-absorbent substrates with good adhesion and high rub resistance.
In order to guarantee excellent print quality and optimum life, further factors must be taken into account when developing inks. The environmental conditions, pigment particle size, the carrier used in the pigment preparations and, of course, the solvent system itself, are of importance. Their influence is described in the following.
1. Dye or pigment?
When choosing the colorant for the ink, consideration must be given as to where the printed article will be used. Indoor use, for example, does not require any special fastness properties. Solvent-based dyes are entirely adequate. Product ranges are available in every imaginable shade and offer brilliant print quality.
The situation is different if outdoor end-use is intended. In such cases, both the ink and the substrate must exhibit excellent lightfastness and weather resistance. Substrates such as polyvinyl chloride (PVC) are therefore used. Dyes are not suitable for use in inks under these conditions. On the one hand they are neither lightfast nor weather resistant and, on the other hand, they migrate into the plasticizer in the plastic. Suitable pigments are the answer to this problem. Besides many other advantages, they offer migration resistance and excellent fastness characteristics.
Nevertheless, weathering conditions must be precisely investigated for each individual case. Depending on the geographic region, time of year and local weather conditions, the effects of wind, sun and rain will always differ. Fig. 1 shows the distribution of global solar radiation measured in Kly. (1 Kly corresponds to 1 kcal/cm2; 1 Luxh corresponds to 0.581 x 10-3 Ly).
Units such as the Xenotest X 1200W (manufacturer: Atlas) are used to simulate climactic conditions. They enable the ambient temperature, atmospheric humidity and water spraying time to be precisely controlled. Evaluation is made in accordance with international standards. Lightfastness of pigments is thus quoted in accordance with the international wool scale (Blue Wool Scale) in eight grades ranging from “excellent lightfastness“ (grade 8) to “inadequate” (grade 1). Determination of weather resistance is based on ISO 54001. The Grey Scale ranks the weather resistance in 5 grades. The exposure time required to achieve the middle grey value (3) is taken for comparisons. How the results from simulated weathering correlate with real exposure is shown in Table 1 using Germany as an example.
2. Particle size and carrier in pigment preparations
The use of pigment powders is entirely feasible with good results, provided dispersion is good, nevertheless pigment preparations are becoming more established. In many cases they offer significant advantages over untreated pigment powders, which after synthesis, drying and milling, are no longer present as primary particles but usually as aggregates or agglomerates (Figure 2). This results in serious disadvantages since the inadequately-dispersed pigments can disrupt the printing process. Pigment agglomerates thus block printhead nozzles which usually have a diameter of 80 µm and less. With increasing particle size, the color intensity, brilliance and transparency of the printing ink decrease (Figure 3).
With pigment preparations from Clariant, a special dispersion process permanently suppresses the tendency to form agglomerates: After wetting the pigment surface, the agglomerates are broken down using high shear forces into aggregates and primary particles. Subsequently, the small particles are stabilized in a final step. Depending on the pigment type, this enables the d50 value of the pigment particle to be lowered to around 100 nm. Particles of this size are suitable for commercial printhead nozzles and also offer the required excellent print quality in terms of brilliance, transparency and color intensity (Fig. 3).
The carrier contained in pigment preparations also contributes decisively to the quality of the ink and print. Matching the carrier to the substrate makes it easier to adjust ink viscosity and improves adhesion to the substrate. For example, pigment preparations based on a PVC/PVAC copolymer are particularly suitable for PVC. Clariant offers a wide choice of pigment preparations with this carrier under the trade name Permajet. The products are suitable for inks based on esters and ketones.
3. Combining solvents
The third and final important parameter affecting the print quality of solvent-based formulations is the choice and combination of solvents.
Normally, a mixture of two solvents is used. Extensive tests at Clariant prove that the combination should be chosen so that one solvent dissolves the carrier in the pigment preparation very well, the second should not exhibit such good solubility characteristics in this respect.
Other variants have not been successful. For example, the use ofonly strong solvents results in the carrier being stripped off. The undesirable consequences are pigment reagglomeration and sedimentation. If, on the other hand, the solvent is too weak for the carrier, the latter swells up on the surface resulting in sedimentation or excessive ink viscosity.
Finally, the design and material used for the printhead may limit the choice of solvents. In case of doubt, the manufacturer of the printhead should be consulted.
These points explain why there is particular demand for pigment preparations which are compatible with a large number of solvents and thus do not place additional restrictions on this already difficult choice. The solvent resistance of each pigment preparation must be tested since every product behaves differently. Typical solvents in combination with very solvent-resistant pigment preparations from Clariant are listed in Table 2.
Many factors influence the quality of ink jet printing. Besides the properties of the colorants and the peculiarities of the printing technique, the effect of the solvent and influence of the carrier must also be taken into account when formulating inks. The complex interaction among individual parameters requires detailed knowledge of these relationships and the use of first class raw materials. Only then can an ink be precisely tailored to the individual application. That is why Clariant also offers specially developed custom products in addition to its wide standard range of dyes and pigment preparations for wide format printing. These tailor-made products are developed in close cooperation with customers and guarantee optimum print results.
1. The Source of Ink Jet Printing Excellence, Clariant GmbH, March 2003 (DP 8518E)