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.
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.