Agrochemicals, water-treatment chemicals, detergents and the like are commonly supplied in unit-dose pouches where the active chemical is contained within a water-soluble (usually polyvinyl alcohol) film. Increasingly regulatory authorities are requiring that each of these unit-dose soluble packages are individually labelled. Thus, for example, it is proposed that in the EU that any detergent unit-dose package containing more than 25 ml of an irritant or corrosive chemical will need to be individually labelled as such.
Printing on the individual pouches may be done either on the web of film before the pouch has been made, when the image can be printed on either side of the film so that it ends up inside or outside the finished pouch, or after the pouch has been made when the image is on the outside of the pouch.
Detergent pouches can be produced either on rotary drum machines such as those produced by Cloud Packaging Equipment, Eme Engel Machinefabriek en Engineering B.V. or Green Sustainable Packaging Inc. Pouches can also be produced on a flat bed machine such as those produced by Harro Höfliger Verpackungsmaschinen GmbH. On rotary drum machines ink-jet printers can be installed on either one of the webs feeding the machine or placed to print on the formed pouches.
The colorants for use in printing on these water-soluble materials must meet many demanding criteria. Thus, they must display excellent adhesion to water-soluble materials (such as polyvinyl alcohol), they must give clear images with good scuff and scratch resistance and crucially they must be able to readily disperse/dissolve in water without leaving any residue or ink skin. It has been found that pigments give excellent performance.
Since the colorant must dissipate when the water-soluble pouch dissolves the pigments (which are in their native state insoluble) must be in a form able to readily disperse.
There are a number of different treatments which enable pigments to be self-dispersible. However, this technology is usually aimed at providing a pigment which can form a stable dispersion in a defined liquid medium. Dispersing a self-dispersible pigment in a defined liquid medium, such as an ink or paint, typically requires careful processing and the selection of solvents and other additives able to help disperse and stabilise the pigment.
This is a very different challenge to providing a pigment able to completely and rapidly self-disperse at room temperature in an aqueous environment and existing commercially available self-dispersible pigments are not able to satisfactorily disperse without leaving an undesirable residue.