S22 - Session O3 - Study of plant-based colourants as raw materials for black ink

Authors: Nea Virta *, Päivi Laaksonen, Juha Jordan

The global printing ink industry exploits mainly synthetic pigments and dyes in the manufacture of inks. However, due to environmental challenges more sustainable alternatives are sought from natural resources. This research focuses on using plant-based raw materials as part of black printing inks. The study discusses the production process with different natural materials and the evaluation of their resistance to light, adhesion to printing substrate and ability to spread. The studied raw materials were Finnish spruce and willow barks obtained from local sawmills, commercial organic chokeberry powder and two different natural indigos (obtained from Isatis tinctoria and Persicaria tinctoria ). In addition, walnut shells, rubia tinctorum root and ferrous sulphate were used in the prepared inks. Walnut shells as well as spruce and willow bark were extracted with accelerated solvent extraction (ASE) to obtain tannins or tannin-like compounds. Tannins are largely found in the bark of conifers, which are widespread in the boreal forest, the world's largest land biome. Combined with iron salts, tannins can form a very dark, or even a black color. The lightfastness of the prepared inks was studied by subjecting the samples gradually to a xenon light with a spectrum similar to sunlight. After the xenon light stress, the solid ink films were evaluated both with visual inspection and with reflectance spectrophotometry according to the CIE L*a*b* -values. Additionally, the inks' spreadability, adhesion and resistance to water were examined on cardboard sheets. According to the results, the best light-fastness properties were found for inks containing indigo and ferreous sulphate. However, the adhesiveness of these formulations needed further improving. The addition of tannins improved the adhesiveness of the inks, and when combined with indigo and ferreous sulphate, remarkably dark inks, with L*-values of 10 and under, were created.