Pigmented inks - Pre-treatment chemicals

Digital printing with pigment inks allows high colour fastness

Printing results in digital textile printing with qualities such as excellent edge sharpness, good colour fastness and a soft touch require a good prepress as well as corresponding pigment inks. However, it is not trivial to meet and reconcile these requirements. The German Institutes for Textile and Fiber Research (DITF) in Denkendorf have set themselves this task and are developing a chemical product preparation system for inkjet printing. Textilchemie Dr. Petry GmbH, together with the DITF, is researching the pre-treatment with "Pericoat" and "Perijet" for inkjet textile printing with these pigment inks and their properties.

The printing result in inkjet printing does not only depend on the resolution of the inkjet printer and the print heads used. Rather, the print quality depends primarily on the quality of the print pre-treatment. With a good pre-treatment and ideally matched textile auxiliaries, however, low-viscosity inks can be used to produce the desired properties such as sharp edges, clear contours and good colour fastness. At the same time, the thickener and binder systems used in pretreatment should not impair the fabric handle.

For good fastness results, a relatively high concentration of binder must be applied in pigment printing. It is not possible to formulate pigments plus binder completely in the ink at will. With higher binder quantities, the viscosity of the ink would increase so much that it can no longer be printed. The binders must therefore be applied in a separate process before printing with the pigment inks.

The market already offers a wide range of binders and pretreatment chemicals that are suitable for modifying the surface of the textile printing substrate and improving it for inkjet printing. However, due to the wide range of available chemical additives, each with its own mode of action, it may be difficult to achieve targeted improvements in digital printing results.

Setting process parameters

A screening of the chemicals in question makes it possible to identify particularly suitable active substances. In the subsequent development phase, these chemicals were therefore optimally adjusted with regard to their use as pretreatment chemicals in the injection pressure. Both the concentrations and the mixing ratios of the individual components were adjusted. Particularly important here was the adjustment of the flowability so that the chemical active ingredients were neither too thin nor too thick for application to the textiles.

The next step in development was to set the process parameters for applying the pretreatment chemicals. The number of chemicals applied, the type of application and the adaptation of intermediate drying phases – all these process steps ultimately have an effect on the printing result. According to the authors Reinhold Schneider and Ulrich Hageroth, the pigment inks used in the research project were developed in-house by DITF.

Finely divided pigment dispersions are produced from organic colour pigments. The addition of binding agents enables the pigments to adhere well to the textile substrate. The addition of additives can also influence other properties of the pigment inks, such as their hygroscopic or rheological characteristics.

Photo: amixstudio

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