Abstract
Inkjet is a digital printing process where the ink is ejected directly onto a
substrate from a jet device driven by an electronic signal. Most inkjet inks
have a low viscosity and a low surface tension, which put high demands on
the coating layer's porosity and absorbency characteristics.
The aim of this study has been to gain an increased knowledge of the
mechanisms that control the sorption and fixation of inkjet inks on coated
papers. The focus has been on printability aspects of high print quality
(although not photographic quality) laboratory-coated inkjet papers for
printers using aqueous-based inks.
Papers coated solely with polyvinyl alcohol (PVOH) and starch presented
excellent gamut values and good print sharpness over the uncoated substrate,
due to good film-forming characteristics observed by light microscopy and
ESCA. ESEM analyses showed the complexity and variation of PVOH surface
structures, which has probably explained the wide scatter in the colour-to-
colour bleed results. Pure PVOH coatings also gave a surface with high gloss
variations (2-8 times greater than that of commercial inkjet papers), prolonged
ink drying time, and cracked prints when using pigmented inks. When an
amorphous silica gel pigment (with broad pore size distribution) was used in
combination with binder, a new structure was formed with large pores in and
between the pigments and a macro-roughness generated by the large particles.
The inkjet ink droplets could quickly penetrate into the large pores and the
time for surface wicking was reduced, which was beneficial for the blurriness.
However, the macro-roughness promoted bulk spreading in the coarse surface
structure, and this tended to increase the line width. Finally, when the ink
ends up within the coating, the colourant is partly shielded by the particles,
and this reduced the gamut area to some extent. The binder demand of the
silica pigments was strongly related to their pore size distributions. Silica gel
required two to three times the amount of binder compared to novel
surfactant-templated mesoporous silica pigments (with small pores and
narrow pore size distribution). This finding was attributed to the significant
penetration of PVOH binder into the pores in the silica gel, thereby,
increasing its binder demand. Furthermore, this binder penetration reduced
the effective internal pore volume available for rapid drainage of the ink
vehicle. Consequently, the surfactant-templated pigments required
significantly lower amounts of binder, and gave improvements in print
quality relative to the commercial pigment.
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