Nanopigment development and dispersion in aqueous systems

摘要

The main component of water based printing ink is its colour base, which can be pigment based or dye based. Pigment based inks consist of particles that attach to the substrate (e.g. paper) instead of molecules adsorbed into the paper fibers, similar to the dye based inks. This characteristic makes the printing with pigment based inks more durable and resistant to weather conditions. However, pigment based inks do not have the brightness and the colour range as dye based inks. Also the present of heavy metals and volatiles organic compounds in their formulations represent an environmental problem. Therefore, the challenge for new technologies is to produce pigment based inks with the same brilliancy of colours as dyes but free of heavy metals and volatile organic compounds. The main issue with pigments for printing applications is the low stability of the suspensions. Due to the hydrophobic character or high surface energy of the particles, they tend to aggregate into bigger particles and settle. This project investigates the production of pigments that meet the qualities of printing and coating technologies. The pigment particles termed as nanopigments, are produced from dye-clay interactions. For their use in ink formulations, they are required to form a stable suspension in an aqueous medium. The nanopigment particles were prepared by the intercalation of organic dye into inorganic clay. In addition to the production of the nanopigment particles, it was necessary to obtain a stable system where the particles are well dispersed. A stability study of the nanopigment suspension was carried out by encapsulating the particles with polyelectrolytes in order to put a barrier to stop the particles from agglomeration. This surface modification was done using polyelectrolytes. The stability of the nanopigment suspension was evaluated in terms of particle size, surface charge, UV-Vis absorbance and colour measurements under UV degradation. Once the stable suspension was obtained, a basic ink formulation with a rheological study was presented. The results can be summarized as follows: • Nanopigments were prepared by the adsorption of dye into clay particles. • Colloidal dispersions of nanopigments were produced for formulation of inks. • The nanopigment particles have an ionic characteristic and the suspension could be enhanced with the protonation of the particle surface followed by the coating with polyelectrolyte. • The protonation of the clay surface also increased the adsorption of the organic dye in the interlayer space of the clay particles .• The mechanism of the polyelectrolyte coating of nanopigment particles was examined to explain the reason for the dispersion enhancement. • The nanopigments suspension presented an improvement of the colour fastness compared with the dyes solutions. • The rheological studies indicated a shear thinning behaviour of the colour formulation, which is suitable for printing ink applications. As a final conclusion from this study, it can be stated that compared to pure dye solution, the nanopigment suspensions demonstrate an improvement in colour fastness for printing application on paper substrates.