Abstract:

Objective The smoothness of digital inkjet printing depends on the pigment ink's particle size, but at the moment, research on the relationship between particle size and flow primarily focuses on the suspension system of large particles, and the choice of pigment ink particle size is largely determined by engineering expertise. Consequently, it is crucial to develop a realistic particle size range appropriate for inkjet. However, most convenitional pigments are prepared by grinding or other physical methods, and their size and morphology distribution is relatively random, making quantitative research impossible.

Method As a new polymer, latex particle size is easy to control and the performance is stable, making it a suitable choice for the study of particle size and inkjet fluidity. Monodisperse spherical polystyrene (PSt) and poly(styrene-co-styrene sulfonate) (P(St-co-SS)) latex particles of different sizes were prepared by mini-emulsion polymerization and soap-free emulsion polymerization, respectively, which were configured into PSt latex particle dispersions and P(St-co-SS) latex particle dispersions. The relationships among latex particle size, storage stability and inkjet fluidity of the dispersions were investigated.

Results In this research, monodisperse spherical PSt and P(St-co-SS) latex particles with controllable particle size of 50-250 nm were successfully prepared by changing the addition of potassium persulfate (KPS), sodium dodecyl sulfate (SDS) and sodium p-styrene sulfonate (SS) in the reaction components. The prepared latex particles were uniform in size and had a spherical structure (Fig.2). The results of storage stability tests suggested the temperature range of 4-50 ℃ for PSt latex particle dispersion and P(St-co-SS) latex particle dispersion, respectively (Fig.4). It was seen that the storage stability of both PSt and P(St-co-SS) latex particle dispersions was higher than 96% in this temperature range, and that the smaller the particle size of latex particles, the better the storage stability of the dispersions. Subsequently, the dispersion was filtered using a filter membrane with an absolute pore size of 1 μm, and the results of the filtration rate reals that the filtration flow rate of the dispersion became higher as the particle porosity ratio decreased (Fig.5). When the particle porosity of the PSt and P(St-co-SS) systems was smaller than 8.5% and 9.5%, respectively, the filtration flow rate of the dispersion appeared to be higher than 2 mL/s. DFT simulation was carried out using the finite element analysis method. The results from the simulations showed that the increase of the particle porosity ratio led to the decrease of the actual dispersion flow radius (Fig.6), which eventually resulted in blockage of the filter membrane pores. Direct grafting of sulfonic acid groups on the particle surface provided more effective mutual repulsion between particles than adding dispersant, thus better hindering the agglomeration and sinking of particles in high-speed flow and increasing the flow of the dispersion (Fig.7).

Conclusion In the case of particles dispersed by dispersant alone at a solid content of 8.5%, the dispersion is essentially unable to flow when the particle porosity ratio exceeds 10%, while P(St-co-SS) latex particle has a fixed sulfonic acid group on its surface, that the particle porosity ratio threshold can rise to about 9.5%. Meanwhile, grafting or modifying the surface of the particles with hydrophilic functional groups enables better cold or hot storage stability of the particles as opposed to adding surfactants to the dispersion. Therefore, an appropriate reduction in solid particle size together with an increase in the number of particles with hydrophilic functional groups can improve the dispersion storage and the inkjet printing smoothness.

Key words: polystyrene, poly(styrene-co-styrene sulfonate), latex particle, inkjet printing smoothness, monodisperse system, pigment ink, inkjet printing