Hollow titanium oxide particles for white inks

WO 2015/047306 A1
White pigment dispersions
Hewlett-Packard Development Company

We have discussed the issues regarding white ink jet inks many times in these pages when we have described the many different approaches employed to get over the unavoidable issue of good white pigments being generally heavy, and the pigment size being necessarily large in order to scatter the light. This leads to settling, and potentially complex ink systems.

Some of the more promising white ink jet ink inventions have involved neutral density hollow resin particles – using low refractive index “bubbles” rather than high refractive index dense pigment particles. Others have used a combination of the two. This patent poses the question that if titanium dioxide is the best white pigment, but is too dense, then why not lower its density to give a pigment incorporating the best of both worlds.

The patent suggests two approaches to achieving this, both of which grow the titanium dioxide particles around a template, to give hollow pigment particles with an overall density less than the bulk. The first approach uses a carboxylic acid functional polymer dispersion, such as poly(methyl vinyl ether-alt-maleic acid), in the presence of titanium (IV) oxysulfate. The patent suggests that the carboxylic acid groups of the dispersion strongly coordinate with the Ti4+ groups, resulting in a titanium rich shell. Calcination of this mixture at 550C results in the inventive low density pigment.

The second approach uses an amazing sequence of chemical reactions to form the hollow titanium dioxide particles. A mixture of titanium (IV) oxysulfate is dissolved into water, hydrogen peroxide and urea. The hydrogen peroxide decomposes to water and oxygen, and onto these bubbles adsorbs the titanium oxo species, which are reduced in situ by ammonia (from the hydrolysed urea) to give titanium hydroxide. The resultant sludge is calcined at high temperature to yield the hollow titanium dioxide particles with a claimed 35% reduction in density over standard titanium dioxide.

Inks were made from these lower density pigments and successfully jetted through HP thermal printheads. Pigment settling was claimed to be undetectable during the 45 day experiment.

Improved printing of white backgrounds

US 2014/0354726 A1
Multi-layer printing on non-white backgrounds
Electronics for Imaging, Inc.

When printing on to a clear or coloured substrate it is normal to print a white layer, then print the image on top. However printing on to a bright white can require extra coloured ink to achieve full image density. Unfortunately the extra colour ink may lead to gloss banding effects.

Dec-14The proposed solution is to print with the white layer a proportion of the coloured image 31, then the remaining part of the colour image in the second pass 32. For instance 20% of the colour image may be printed with the white layer. Where colour ink will be printed less white is deposited, so that the total amount of ink laid down in the first pass is constant. The colour ink is printed into the white layer while it is still wet, and then the first layer is cured. The image is not particularly visible in the white layer at this stage. However, when the main part of the colour image is printed on top the colour density is higher than it would be if printed on a plain white layer, without loss of detail.

So instead of having to increase the amount of colour ink in areas of high image density to increase the saturation when printing on a uniform white layer, the amount of white ink is reduced in the areas of high image density.

There are other embodiments proposed. Instead of part of the colour image being printed with the white layer, only black or light black ink is used. This saves ink as only black is used, but the saturation is not quite so high. It works well in dense shadow or dark areas of the image.

The amount of the colour image printed with the white layer can also be varied according to the image or part of image. For instance where a photo appears, 60% of the colour image is printed with white, where a graphic appears 40%, and with text 0%.