Hewlett-Packard’s nano-colourant dispersion

US 2013/0284050 A1
Colorant dispersion for an ink
Hewlett-Packard Development Company L.P.

Ink jet inks usually contain organic colorants, as these materials when chosen correctly show strong absorption, low density and in the case of pigments are usually available at particle sizes suitable for incorporation into ink jet inks.  Inorganic pigments are used in speciality ink jet inks when the inherent stability to heat and UV is required – tile printing would be one example. However the range of available inorganic pigments is narrow and the colour of such materials tends to be dull.

This patent focuses on a nano-dispersion of Fe3O4 (iron(II,III)oxide) with a particle size of around 30 nm, which at this particle size has a burnt red colour unsuitable for black text printing.  When manufactured at a large particle size, iron(II,III) oxide is used as a black pigment (C.I pigment black 11).

The example takes an ~8 wt% Fe3O4 suspension in water synthesised by a basic precipitation process and adds Silquest A1230 (a reactive alkyleneoxide dispersant, 50 wt% on pigment) and citric acid (10 wt% on pigment).  This mixture was then bead milled to give a dark red dispersion with an average particle size of ~30 nm.  To this dispersion was then added 5 wt% gallic acid (3,4,5-trihydroxybenzoic acid) and it was heated to 60C for one hour, during which the colour changed from dark red to black.  This black dispersion was then made into a basic ink jet ink by dilution with water and 2-pyrolidone as humectant and successfully ink jet printed to give sharp text that was significantly more black than the pale yellow comparative ink produced without the gallic acid heating step.

From the examples it seems that although the heating step significantly improves the colour of the inorganic pigment, the intensity and neutrality is not quite the same as a comparative carbon black ink made to the same formulation.  However, scratch resistance and gloss were significantly better than the carbon black ink.

UV stability is unfortunately not mentioned, as it would be interesting to see if the pigment would revert to its former dark red colour.  If not this could be an interesting approach to the production of small particle size black pigments for extreme ink jet applications.