Properties of Water-Based Gravure Inks

A misconception that must be corrected from the start is that water-based inks will be cheaper than their conventional counter-parts. Although a high proportion of water is present, the resins that must be used to achieve the necessary properties are more expensive than those used in solvent-based inks. On the credit side, the water used for thinning is a lot cheaper than solvents. Printability can vary considerably from excellent to appaling and poor results are more common than those that are good. Mottle and dot skip are the major faults although scumming is more common than with solvent-based inks. These are caused primarily by the difficulty in getting water to wet the metal cylinder. Another major factor influencing printability is the grade of paper used. It may be that one has to investigate this problem in detail for the full answer to be found, although it is hoped that further developments in ink formulations will be able to make a significant contribution. The other property making a major contribution to the print quality - that of gloss has been improved considerably over the last couple of years but further improvement is still required to reach present solvent ink standards.

To wet and dry rub resistance of the dried ink films are generally both good when printed on paper and boards although again the substrate can have a considerable influence on the result. Concern has been expressed over problems with foaming, ink stability on storage and rusting of equipment. All can be troublesome but the user of flexo water-based inks does not find them impossible to overcome and foaming is less of a problem than with flexo.

The ink properties described so far are, of course, dependent on the actual formulations used and they can vary considerably. They may be based on either natural or synthetic resins, although it is likely that natural resin-based inks will give too many problems, particularly with variable performance. As with water flexo inks there are formulations on the pigments that may be used, primarily due to problems with stability on storage. To gain full benefit from water-based inks it will be desirable, if not entirely necessary, that the inks be completely free from organic solvents.

Paper Improvements

It is quite clear that there are problems to be overcome in the ink's performance but certain paper improvements are also required. The weakening effect of lithographic dampening solutions brought the introduction of special litho papers when those developed initially for letterpress proved unsatisfactory. Similarly, water-based inks weaken the surface of the coated papers normally supplied for gravure. It will be therefore necessary to introduce products that combine the surface properties necessary for gravure with wet pick strength. Having considered the properties, existing problems and potential benefits to be gained from using water-based gravure inks, what are the likely applications? The first and most obvious is for publication work. The most exciting prospect for the future is the use of water-based inks on plastic and cellulose films. Clearly, microwave drying will be a vital part of such work and therefore printing on foil will not be possible because it causes the microwave waveguide to short circuit. A great deal more work must be done to provide satisfactory adhesion to the different materials and it is probable that a variety of ink systems will be necessary for the different films. Laboratory trials have shown that satisfactory drying and adhesion can be achieved on polythene, although unfortunately the ink had other shortcomings.

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