Metallic effects from a thermal ink jet ink

US 2014/0170384 A1
Neutral gray reflective ink
Hewlett-Packard Development Company

Judging by the number of patents filed, it appears that the ink jet printing of expanded colours and appearances are sought after for home and office decorative printing. Reflective inks, in particular, have been disclosed with a particular emphasis on metal flakes – materials currently used in more traditional metal-effect printing processes. Thermal printheads have significant constraints regarding viscosity and particle size and so large metal flakes are a non-starter and a silver nano-particle approach would be expensive and questionable from a toxicological perspective.

The patent suggests that to look metallic, then only a certain threshold value for specular reflection needs to be achieved. It is suggested that 10% of the incident light must be specularly reflected, but greater than this is always desirable. With this specular reflectivity requirement relaxed, alternative non-metallic materials can be considered, such as magnetite. Magnetite (Fe3O4) can be made up into a stable nano-dispersion, suitable for thermal ink jet, and will give a metallic lustre when printed on glossy paper.

However, magnetite has an inherent yellow-orange hue and so to achieve a neutral silver effect a mixture of blue, cyan or magenta dyes must be added to counteract this. The exemplified ink jet formulations first prepared the magnetite dispersion in water by heavily bead milling a 5.6 wt% mixture of the oxide in water with a polyether alkoxysilane reactive dispersant at 0.5 wt%, relative to the pigment. This was then used (at 36.3 wt%) to make a simple thermal ink jet ink with 14 wt% humectant, 0.1 wt% neutralised styrene acrylic binder, 0.3 wt% surfactants, 5.4 wt% of a cyan and magenta dye mixture and water.

June-14aThis ink was loaded into a HP black cartridge from a Photosmart 8450 printer to print colour test patches. The chart left shows the colour coordinates of the neutral formulation above, compared against the non-colour corrected magnetite dispersion and a comparative silver nano-particle ink. It can be seen that the yellow tint has been fairly well counteracted in comparison to the metal ink samples.

The patent goes on to discuss the importance of the media in achieving a metallic effect and that the pore size of the surface of the media must be smaller than that of the magnetite pigment particles to ensure they remain on the surface as a contiguous layer. Fig 1 below shows this schematically with the dyes (16) absorbing into the media under the ceramic particles (14). There is an obvious issue to such a configuration in that the robustness of the printed image will be compromised if the pigment is left unprotected on the surface, but this is addressed in the patent application US 2014/0170395 A1.

June-14bAlthough it seems that the ultimate specular reflectivity of this approach is not quite that of a metallic silver ink, there do seem to be significant advantages in using such benign and low cost iron oxide materials.

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.

Optically clear nano-particle ink from Hewlett Packard

US 2013/0222496 A1
Optically clear fluid composition
Hewlett-Packard Development Company, L.P.

Uniform gloss on paper substrates requires a transparent ink to raise the gloss level of the chosen media to the same level of the coloured components of the ink.  If such an ink is to be used then there is a good argument for printing it over the full printed image.  This helps to improve mechanical properties, such as scratch and rub resistance, and potentially introduces other benefits such as UV absorption, therefore increasing the stability of the printed image.

This patent discusses a clear ink jet ink designed to be printed over the full image to equalise gloss, improve mechanical properties and absorb UV, without increasing the overall haze of the printed image.

The novelty of the patent is to use metal oxide nano-particles with a particle size below 140 nm.  This small particle size is necessary for transparency of these materials.  The patent suggests that it is also beneficial if the metal oxide average particle size is similar to that of the colorant pigment particles, in order to stack efficiently and therefore minimise coating roughness and haze.  Such nano-particulate materials also show benefits with respect to stability and printhead performance.

Additionally, these metal oxide dispersions should be compatible with the printed inks in order to prevent aggregation of these nano-particles which might increase haze.  Generally, such metal oxide dispersions are electrostatically stabilised and can be formulated to be stable at a pH of 8-10, the range for ink series such as HP 38 and HP 70.  Specifically the pH of the metal oxide dispersion should be within 0.5 pH units of the coloured inks.

The specific metal oxide particles used should be transparent to visible light and have a refractive index greater than 1.65.  Zinc and aluminium oxide work well for this technology, whereas silica (with a refractive index of 1.45) does not, as it apparently gives hazy coatings.

Exemplified coating formulations contain around 2 wt% of a polyurethane binder, 2 wt% (dry solids) of the metal oxide particles, 10 wt% of humectants (1,2-hexanediol, 2-pyrolidone or similar), ~0.5 wt% surfactants and biocides, with the remainder water.  The mechanical properties of such coatings were compared to a silver halide print, giving equal or better scratch resistance and very low haze levels.

Memjet inks and colour order to minimise nozzle plate mixing

US 2013/0070024 A1
Printer for minimizing adverse mixing of high and low luminance inks at nozzle face of inkjet printhead
Zamtec Limited

A key advantage of the Memjet printheads is that the nozzle rows are closely packed together. This means that 5 colours are printed within less than 1 mm. This considerably reduces the issues of maintaining speed and tracking of the media compared to printheads where the different colour heads have to be positioned 10’s of mm apart because of their size.

Memjet printhead
Memjet printhead

However, this patent application describes a downside to having the different colours so close – colour mixing on the nozzle plate. This requires a non- wetting coating to be applied to the nozzle face of the printhead, for instance a hydrophobic siloxane polymer. The spacing between colours d is 73 microns.

Cutaway view of printhead
Cutaway view of printhead

Here the Memjet printhead is shown as a cutaway, with the suspended heater below nozzle 102. Note that the surface of the nozzle plate is, for the main part, planarised.

Initially it was expected that adjusting the surface tension of the different colour inks would enable them to be controlled. However even when the black ink had a significantly higher surface tension than the colour inks, the black ink still flowed into colour inks on the nozzle plate surface.

The patent describes a surprising discovery, that control of the inks could be obtained by adjusting the amount of surfactant in each ink. If a colour ink contained at least 0.4% more surfactant than the black ink, mixing was far less likely to occur. This then leads to changing the colour order of the inks so that a low luminance ink (eg yellow) is flanked by a high luminance ink (eg black). Therefore the colour order should be CKMKY, or MKCKY etc.

Stretchable fluroelestomer ink from Xerox

EP 2 546 312 A1
Stretchable ink composition
Xerox Corporation

It is useful for an ink jet ink to be tolerant to a small amount of elongation in order to survive the flexing of the substrate media, but the ability to stretch a few hundred percent is another matter entirely.

This patent suggests that there is a demand for using ink jet to print onto flexible, stretchable materials such as latex gloves, tyres, tubes, cables etc.  The patent also suggests that these inks should be highly resistant to environmental factors in addition to a robustness comparable or better than the printed article itself.  All of which is a tough challenge.

To meet such stringent criteria, the patent suggests the use of fluroelastomer emulsions as the sole ink jet ink binders.  In more detail, 65 wt% of Technoflon TN latex was mixed with 25 wt% of a Pigment Green 7 dispersion (pre-dispersed in water using sodium dodecyl sulfate to give a 20% solids dispersion).  This mixture exhibited a particle size distribution from 40-300 nm with an average of 120 nm.

This mixture was then diluted down to 5 centipoise with water (no data given as to how much was needed), and then 10 wt% ethylene glycol was added as a humectant.  This ink was loaded into a Dimatix DMP-2800 ink jet printer equipped with 10 pl cartridges and printed onto a latex glove.  After printing, the ink was dried at 60C for 5 minutes to give an image that could stretch, repeatedly and robustly up to 500%, which was the limit of the substrate not the ink.

A second experiment substituted the latex glove for a silicone rubber sheet.  Silicones are exceptionally difficult materials to wet out and achieve any kind of  adhesion to, but surprisingly the ink adhered and demonstrated reliable stretching up to the limit of the substrate for hundreds of cycles.  Although this is a very simple ink formulation, the use of these fluoropolymer dispersions may be of use for some specialist applications, although the long term reliability in ink jet printheads would be a challenge.

Hewlett-Packard’s erasable ink technology

WO 2012/166149 A1
Method of forming an erasable ink
Hewlett-Packard Development Company

Several methods are currently available that may erase printed images from the surface of a medium. For example, inks that include dyes may be erased by subjecting the inks to high intensity UV radiation, causing the dye to break down to a colourless state. Oxidation and bleaching methods can also be used to erase an image with the correct colorants. Some inks can be formulated with thermal dyes that can decolorise when exposed to secondary and tertiary components in the presence of heat, the Toshiba e-blue technology for example.

These two patents present an erasable ink that is erased chemically with a second ink. A second patent (WO 2012/166160 A1) uses an additional electrolytic stage to accelerate the bleaching process. The main novelty over similar chemical whitening patents is the “relatively human friendly” nature of the materials used.

Thus the patent suggests that colorants containing ionic complexes that can change between coloured and non-coloured states on oxidation and reduction are ideal for this technology. A particularly favoured colorant mentioned in these patents is iron(II)ascorbate which has a dark purple colour in solution and when dry, with a  useful UV-vis absorption range from 350-700nm. This can give a workable “black” ink when formulated at around 3wt%.

In particular, such an ink can be relatively easily oxidised back to the colourless iron(III)ascorbate under mild conditions, lending itself to an easy decolorisation step. This ease of decolorisation does require the presence of a reducing agent  (<1% sodium bisulfite) in solution to prevent atmospheric oxygen decolorising the ink. The colour of the iron(II)ascorbate is also pH sensitive and so a buffer such as 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) should be included in the final ink formulation at about 0.2wt%. Other components included in the ink are the usual humectants, in this case 5wt% glycerol and 10wt% 1,2-propanediol and surfactants, 0.25% surfynol 465. This ink was printed onto HP recycled paper using the black ink cartridge in an HP Photosmart 8450 printer.

An erasure fluid was formulated with 3wt% hydrogen peroxide, 20wt% 1,2 propane diol and 5wt% glycerol and this was also printed using an HP Photosmart printer. When printed over the previous iron(II)ascorbate, the colour was bleached over a period of a few hours. The page was then reprinted with the “black” ink to give an image with similar density to the initial print.

Other fluid combinations gave slightly improved bleaching times which is where the second patent (WO 2012/166160 A1) comes in. This patent proposes to use a significantly less aggressive decolorising liquid coated onto the iron(II)ascorbate printed media, which is then contacted this liquid against a series of electrodes. When a voltage of up to 10V was applied across these electrodes the image was be decolorised in a matter of seconds.

These electrodes were positioned across the paper, but the main body of the patent discusses the use of a pair of spiral wrapped electrode wires contacting the wetted media surface, see fig below. In comparison to the the hydrogen peroxide bleaching agent, this approach works with simple surfactant solutions. The electrolysis of the water content of the decolorising solution generates hydrogen peroxide in situ, and judging by the rapid bleaching times.

Dec12

Although this approach to a re-usable media solution is interesting, there is the problem of a build up of the erasing solution in the paper after multiple cycles lending an oily feel. This is mentioned in the text, but not really fully addressed.

Doping ink to improve optical drop detection

WO 2012/044307 A1
Doped black ink with increased light scattering efficiency for nozzle health detection
Hewlett-Packard Development Company L.P. 

Single pass ink jet printers offer huge increases in print speed over their scanning counterparts, but the technical price of such speed is the lack of nozzle redundancy and so the loss of one nozzle is normally noticeable. To achieve adequate image or print quality, single pass ink jet printers require precise knowledge of the health condition of each nozzle before commencing print jobs or must rely on the maintenance routine being 100% effective. The latter is generally difficult to achieve without significant ink wastage and so the former is a favourable approach.

If the nozzle health can be determined before printing then a decision can be made as to whether maintenance is required, or whether some nozzles should be inactivated and other compensatory nozzles used instead. Light-scattering drop detection technology (LSDD) has emerged as one method for detecting the in-flight droplets and thus the nozzle health.

This technology uses a laser to illuminate the in-flight ink drops, using photodetectors to monitor the laser light scattered off the droplets. Efforts to increase the sensitivity of such a system have generally been focused on using higher intensity lasers, more sensitive photo detectors and more elegant beam optics. The schematic left shows a compact implementation of this LSDD technology, in this case using a light pipe detector arrangement.

Black ink jet inks are typically based upon carbon black dispersions, and are widely used for text and graphics printing. One reason for such widespread use of carbon black pigments is their excellent light absorbing properties across the whole visible range, rendering truly black ink. In practice, the carbon black pigment absorption range extends from the far UV, through visible into the infra red. Thus, the reliable detection of black ink jet ink droplets using the laser light scatter technology described here is challenging.

This patent proposes to add a dopant to the black ink jet ink in order to increase the signal from the lightscattering technique and thus improve signal to noise ratio. The chart shows both the optical density of the printed black ink and the backscattered laser signal at 650 nm for increasing levels of silver nano-particles. This shows a 100% increase in signal with 3.3% silver inclusion.

The silver nano-ink used in the examples is commercially available ink from Cabot Corp. This is at 20 wt% dispersion of 30-50 nm silver particles in a water/ethylene glycol mixture. Gold dispersions are also presented as a potential dopant, although as they must be included in the inks at similar levels to the silver dopants, the cost precludes this option in reality.

Although the backscatter signal is increased by this silver nano-particle additive, these metal inks are not cheap and would significantly add to the ink cost. In addition, no mention is made with respect to the ink performance or stability with the inclusion of these dopants.

Dr Phil Bentley