Tuning inkjet printability of hydroxypropylated-starch-based coatings by mineral selection

Abstract

Coatings based on hydroxypropylated starch (HPS) provide extraordinary print density with dye-based inkjet inks, but their suitability for pigment-based inks can be limited due to inadequate carrier medium (water) absorption and colorant fixation. In the present work, HPS-based coatings were tailored for both a pigmentbased and a dye-based ink by adding silicate minerals or acid clay (both Lewis and Brönstedt acid sites present in the acid clay). Substrate-ink interaction and colorant distribution were investigated via uncoated gloss and print quality indicators such as print density, ink bleeding tendency and delta gloss after printing with desktop printers. Three-dimensional z-stack CLSM images taken from printed samples revealed substantial differences in ink holdout and penetration characteristics between the studied materials. Pigment-filled HPS coatings showed up to 60% increase in print density compared to uncoated reference substrate and an increase of up to 35% compared to plain HPS coating with dyebased inks. In case of pigment-based ink, the increase was 17% between plain HPS coating and acid-clay-filled HPS coating. In addition, a combination of HPS and silicate mineral decreased ink bleeding substantially, but lower print gloss compared to substrate initial gloss was observed with the majority of experimental coatings. It was found that the pigment type affects substantially the unprinted gloss, which is naturally high in the case of pure HPS coating, but also the observed drastic difference in substrate permeability may explain the differences in print quality. The findings suggest that coating morphology, together with chemical interactions between starch and mineral and coated substrate and ink, has a key role in achieving high gloss and good print quality.