WO2012166149A1

WO2012166149A1 - Method of formulating an erasable ink

Info

Publication number: WO2012166149A1
Application number: PCT/US2011/039025
Authority: WO
Inventors: Raymond Adamic
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2011-06-03
Filing date: 2011-06-03
Publication date: 2012-12-06

Abstract

A method of formulating an erasable ink involves selecting a colorant from a group of human-friendly and environment-friendly colorants, and mixing the colorant in an inkjet ink vehicle to create the erasable ink. The ink vehicle includes a component to trigger a chemical reaction between the colorant and an erasure fluid. The ink vehicle enables the erasable ink to be printed from an inkjet pen to form an image on a medium, and the chemical reaction enables the image to be erasable by degradation of the colorant upon application of the erasure fluid.

Description

METHOD OF FORMULATING AN ERASABLE INK

BACKGROUND

The present disclosure relates generally to methods of formulating an erasable ink.

Inkjet printing is an effective way of producing images on a print medium, such as paper. The inkjet printing process generally involves ejecting ink droplets (formed, e.g., from one or more inks) from a nozzle at high speed by an inkjet printing system onto the paper to produce the image(s) thereon. In some instances, it may be difficult to effectively erase the inkjet ink(s) when the inks are established on the paper.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.

Fig. 1 is a flow diagram depicting an example of a method of formulating an erasable ink;

Fig. 2 is a flow diagram depicting an example of a method of erasing an image from a medium;

Fig. 3A schematically depicts an example of an inkjet printing system including a fluid ejector from which an erasure fluid is jetted onto a medium to erase an image from the medium;

Fig. 3B schematically depicts another example of an inkjet printing system including an example of a roll coater used to apply an erasure fluid onto a medium to erase an image from the medium; Fig. 3C schematically depicts an example of a sprayer from which an erasure fluid is sprayed onto a medium to erase an image from the medium;

Fig. 4 is a representation of a vial containing a solution of an FDC Red 40 dye before oxidation;

Fig. 5 is a representation of the vial of Fig. 4, but containing the solution of the FDC Red 40 dye after oxidation;

Fig. 6 is a representation of an image produced using the oxidized solution of the FDC Red 40 dye;

Fig. 7 is a representation of a number of blocks printed on a sheet of plain recycled paper using an example of an erasable inkjet ink;

Fig. 8 is a representation of the blocks from Fig. 7, but showing a portion thereof removed after applying an example of an erasure fluid;

Fig. 9 is a representation of another image printed over the removed portion of Fig. 8;

Figs. 10 and 1 1 are representations depicting the example of the erasable inkjet ink used for the process depicted in Figs. 7 through 9, where the ink was printed on brochure paper (Fig. 10), and the ink was removed from the brochure paper (Fig. 1 1 );

Figs. 12 and 13 are representations depicting an example of an image printed using another example of an erasable inkjet ink (Fig. 12) and erased using another example of an erasure fluid (Fig. 13);

Figs. 14 and 15 are representations depicting an example of an image printed using still another example of an erasable inkjet ink (Fig. 14) and erased using another example of an erasure fluid (Fig. 15); and

Figs. 16 and 17 are representations depicting an example of an image printed using a comparative inkjet ink (Fig. 16) which could not be erased using an example of an erasure fluid (Fig. 17). DETAILED DESCRIPTION

Several methods are currently available that may suitably erase images from the surface of a medium. For instance, inks that include dyes may be erased by subjecting the inks to a high intensity of radiation. This radiation generally causes the ink to degrade and, thus, the image that was formed by the ink upon printing the ink on the medium ultimately disappears from the medium. Oxidation and bleaching methods may also be used to erase an image. Further, the ink may be formulated to include thermal dyes that decolorize when exposed to secondary and tertiary components, typically in the presence of heat.

It has been found that the foregoing methods may have associated therewith certain issues pertaining, at least in part, to erasing an inkjet ink from the surface of a medium. For example, some oxidation methods may utilize relatively aggressive oxidants that may suitably erase the ink (and thus the image) from the medium, but may also deleteriously affect the integrity of the medium and/or the inkjet pen that was used to apply the oxidant to the medium. Additionally, methods utilizing radiation to fade the image may require a significant amount of energy (much of which may be wasted due at least to process inefficiencies) and/or time to break down the colorant(s) of the ink. As such, radiation methods may not be a sustainable solution, as these methods may expend more energy to erase than would be expended to make new paper. Further, inks containing thermal dyes (such as leuco dyes) are often complex and challenging to formulate, and in some cases, these inks may not be stable enough to be used for inkjet printing.

It has been found that at least some of the foregoing methods may affect the integrity of the medium upon which the ink(s) is/are established after a print, erase, and rewrite cycle. Further, it is uncertain how the components of the inkjet ink(s) removed from the medium after repeated oxidations or thermal degradations of the colorant may affect the surrounding environment.

Example(s) of the method as disclosed herein may be used to formulate an inkjet ink that, when established on a medium, is erasable from the medium. More specifically, the inkjet ink is erasable from the medium when the colorant of the ink (when in the solid or dry state, such as when the ink forms an image on the surface of a medium) interacts with an erasure component of an erasure fluid. Basically, the interaction between the colorant and the erasure component causes the molecular structure of the colorant to degrade, and the degradation of the colorant structure causes the colorant to disappear from the surface of the medium. As used herein, the colorant "disappears" from the surface of the medium when about 80% to about 100% of the image (i.e., the dried ink) is removed. Examples of the erasable inkjet ink formed by methods disclosed herein, as well as the erasure fluid used to remove the ink from a medium are provided below.

The inventor of the present disclosure has found that when examples of the erasable inkjet ink interact with examples of an erasure fluid appropriately formulated so that the interaction will effectively take place, images formed by the ink are erased (or removed) in a relatively "human-friendly" and "environment- friendly" manner. This may be due, at least in part, to the fact that the examples of the erasable inkjet ink and the examples of the erasure fluid are specifically formulated to include human-friendly and environment-friendly components. It is to be understood that as used herein, the terms "human-friendly" or the like and "environment-friendly" or the like are generally defined as components: listed as Generally Recognized As Safe (GRAS) by the United States Food and Drug

Administration (FDA); complying with the FDA's Federal Food, Drug and Cosmetic Act (FFDCA); appearing in the United States Environmental Protection Agency's (EPA) CleanGredients® list; and/or appearing in similar lists; and/or categorized in a similar manner.

Further, the formulation of the erasure fluid is tied, at least in part, to the nature of the colorant(s) incorporated into the erasable inkjet ink. For example, certain colorants have been found to be more erasable than others. Thus, the composition of the erasure fluid may be specifically designed to erase a particular colorant or a particular type of colorant so that the interaction between the two may effectively degrade the colorant(s) and remove the image from the medium. Accordingly, several different examples of the erasable inkjet ink may be

formulated, where each may be specifically designed to be used in combination with a particular erasure fluid to erase or remove the ink from the medium.

Likewise, several different examples of the erasure fluid may be formulated, where each may be specifically designed to be used in combination with a particular erasable inkjet ink to erase or remove the ink from the medium. Examples of the erasable inkjet ink will now be described, and examples of the erasure fluid will follow the description of the examples of the ink.

At the outset, examples of the erasable inkjet ink are designed to be erasable from a medium such as paper. The paper may be chosen from any cellulose-based paper, i.e., paper that includes cellulose fibers. For instance, the medium may be made from pulp fibers derived from hardwood trees (e.g., deciduous trees (angiosperms) such as birch, oak, beech, maple, and eucalyptus) and/or softwood trees (e.g., coniferous trees (gymnosperms) such as varieties of fir, spruce, and pine, (e.g., loblolly pine, slash pine, Colorado spruce, balsam fir and Douglas fir)), and these pulps may be prepared via any known pulping process. Further, the cellulose-based paper may include one or more fillers to control the physical properties of the medium. Examples of fillers include ground calcium carbonate, precipitated calcium carbonate, titanium dioxide, kaolin clay, silicates, and combinations thereof. It is to be understood that the cellulose-based paper may be referred to herein as plain paper.

Other examples of the paper medium include resin-coated papers (such as, e.g., photobase paper) and papers made from or including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polylactic acid (PLA), and/or the like, and/or combinations thereof.

In another example, the medium may be chosen from COLORLOK® papers (available from Hewlett-Packard Co., Houston, TX), which are plain papers having calcium chloride incorporated in the paper structure.

Examples of the erasable inkjet ink will now be described herein. In each of these examples, the ink generally includes an ink vehicle and a colorant added to the ink vehicle. As used herein, the term "ink vehicle" refers to the combination of at least one or more solvents and water to form a vehicle within which the colorant is added to form the erasable inkjet ink. The solvent(s) is/are basically used as a carrier for at least the colorant of the ink and may, in some examples, constitute the bulk of the erasable inkjet ink. In some examples of the inkjet ink, the solvent is chosen from 1 ,2-propanediol, glycerol, 1 ,2-hexanediol, 1 ,5-pentanediol, 1 ,6- hexanediol, 2-methyl-1 ,3-propanediol, trimethylolpropane, and combinations thereof. It is to be understood that the solvent (or combination of solvents) is desirably chosen from one or more solvents that are considered to be human- friendly and environment-friendly, as previously mentioned.

Some examples of human-friendly and environment-friendly solvents include 1 ,2-propanediol, glycerol, and combinations thereof. However, it is to be understood that the other solvents listed above may also be used in the examples of the erasable ink disclosed herein. Further, the solvent(s) may be present in the ink in an amount ranging from about 1 wt% to about 50 wt% of the erasable inkjet ink; and in another example, ranging from about 1 wt% to about 30 wt%. In yet another example, the amount of solvent(s) ranges from about 20 wt% to about 30 wt% of the erasable inkjet ink; and in still another example, ranges from about 1 wt% to about 15 wt%.

The vehicle may, in some examples, include an additive, which is a constituent of the ink that may operate to enhance performance, environmental effects, aesthetic effects, or other similar properties of the ink. Examples of the additive include surfactants, polymers, pH buffers, biocides, and/or the like, and/or combinations thereof. Some suitable examples of additives contemplated as being within the purview of the present disclosure may be found in the CleanGredients® list from the United States Environmental Protection Agency (EPA), and/or in other similar lists/categories described above. Some additives will be described hereinbelow in conjunction with some examples of the inkjet ink.

It is to be understood that, in some examples of the ink, the ink vehicle does not include an additive. As used herein, the term "colorant" refers to a constituent of the ink that imparts a color to the ink. In the examples of the inkjet ink disclosed herein, the colorant is chosen from those that are considered to be human-friendly and environment-friendly, as previously mentioned, and these colorants are readily degradable by chemical means such as via decolorization or mineralization techniques. Certain colorants that exhibit characteristics of high permanence, such as those that are often considered to be lightfast or waterfast, pigment-based colorants, and/or colorants typically used in inkjet inks were generally avoided. Rather, the colorants were chosen from those that tended to produce a stable color, but may be readily degraded in order to erase them. In other words, the human-friendly and environment-friendly colorants incorporated into the ink may be chosen from those that are susceptible to change, e.g., those that have intramolecular structures that may be broken down or degraded. In some instances, the intra-molecular bonds of the colorant may be broken in a controlled manner in order to minimize energy and chemical aggressiveness of the colorant.

The colorants were also chosen from those that tended to break down into products that minimally affect the potential reuse of the medium (upon which the ink was printed) after erasing. Thus, the medium upon which the ink is printed may be reused after erasing, at least in part because the erasing of the ink does not adversely affect the integrity of the medium. In fact, the medium may be used for a number of erasing and reprinting cycles (e.g., two, three, four, or even more cycles). In some instances, the medium may be reused after 5 to 10 erasing and reprinting cycles without adversely affecting the integrity of the medium.

Furthermore, it was found that colorants containing ionic complexes (which will be described in further detail below) may change between colored and non- colored states, at least in part because either the metal ion may change oxidation state, or it may be removed from the complex. It was found that this colorant may be used as part of an erasable black inkjet ink. For instance, FDC Red 40 is a red food dye additive often used in many food stuffs. About 0.7 wt% of the FDC Red 40 dye additive was used in combination with about 3 wt% ascorbic acid and about 0.3 wt% of iron obtained from iron (II) chloride salt. The dye exhibited a red color in solution R, as illustrated in Fig. 4. After heating at about 60°C for about 30 minutes, the red color of the solution R almost completely disappeared, leaving behind a practically clear fluid C, as illustrated in Fig. 5. A sample of the clear fluid C of Fig. 5 was pipetted onto a sheet of HP Recycled Office paper (available from Hewlett-Packard Co., Houston, TX), and the clear solution unexpectedly turned into a dark image on the paper, as illustrated in Fig. 6.

Without being bound to any theory, it is believed that, upon drying, materials in the almost clear fluid may have reacted or interacted strongly enough to form an iron ascorbate complex (which may be purple hued if the iron is present

predominantly in the 2⁺ form, or brown hued if the iron is present predominantly in the 3⁺ form), thus turning a dark color after having been placed on the paper. It is further believed, again without being bound to any theory, that the iron ascorbate complex may have been formed by one of the following reactions, or by a combination of the two. One potential reaction is the ascorbic acid, being a strong reducing agent, may have reduced the iron from the 3⁺ to 2⁺ state, and then its interaction with the ascorbic acid formed an iron ascorbate complex. Another potential reaction is oxygen from the air, as the solution was drying on the paper, oxidized the iron from the 2⁺ to the 3⁺ state to form an iron ascorbate complex.

Colored inks may be formulated using colorants commonly used in the food industry (e.g., natural food colorants and/or synthetic food colorants), the

pharmaceutical industry, and/or the cosmetic industry, at least in part because these colorants exhibit the color desired and the bonds of the colorant structure may be readily broken down.

It is to be understood that the composition of the components of the inkjet ink (which includes the concentration of each component) depends, at least in part, on the colorant selected. As previously mentioned, the colorant may be selected from one or more colorants that favorably interact with a particular erasure fluid to effectively erase an image formed when the inkjet ink is printed on a medium. In light of these considerations, several compositions of the erasable inkjet ink are contemplated herein, and some examples of these ink compositions are disclosed hereinbelow.

In one example, the colorant may be chosen from a mono-based colorant; i.e., a colorant that produces a neutral color when the ink is printed. The neutral color may be achieved, for instance, when the color space coordinate L^* is minimized, and the color space coordinates a^* and b^* are individually close to zero. L^* is minimized when L^* is as close to zero as possible, such as when L^* is less than 70. In another example, the L^* is minimized when L^* ranges from 20 to 40. Further, a^* and b^* are individually close to zero when each of the coordinates ranges from +1 to -1 . An example of a mono-based colorant is one containing an ionic complex such as, e.g., the iron-based ionic complex mentioned above. Other examples of mono-based colorants include those containing an ionic complex formed from other metal ions in combination with a ligand to form the ionic complex. Examples of the other metal ions include copper, manganese, cobalt, zinc, titanium, and tin.

Inks including an iron-based ionic complex colorant, for example, produce a color that may be classified as dark purple, and which may be referred to as a dark purple-hued mono ink. Again, the iron-based ionic complex may be used as a colorant to produce erasable black inkjet inks.

In an example, the purple color mentioned above may be produced by combining the ferrous (II) ion with ascorbic acid (which is the synthetic version of Vitamin C, and is often provided as a Vitamin C supplement) at a 2:1 ratio of ascorbic acid to Fe(ll) when in the solid state, and at about a 1 :1 or 2:1 ratio of ascorbic acid to Fe(ll) when in solution. This produces a dark purple-colored ink in solution as well as when printed on plain paper. This iron-based ionic complex is called iron ascorbate (which is available from Sigma Aldrich, St. Louis, MO), and the instant example of the inkjet ink containing this colorant has an ultraviolet and visible (UV-VIS) absorption ranging from about 350 nm to about 700 nm. In an example, when iron ascorbate is used as the colorant, the amount of the colorant present in the ink ranges from about 1 wt% to about 6 wt% of the ink; and in another example, ranges from about 2 wt% to about 4 wt% of the erasable inkjet ink. In yet another example, the amount of the iron ascorbate ranges from about 3 wt% to about 4 wt% of the ink.

It is to be understood that the pH of the inkjet ink is an important aspect of the ink composition. For instance, an inkjet ink formulated using iron ascorbate as the colorant having a pH greater than 7.5 may form ferric hydroxides that may form various stoichiometries of iron oxide. However, the colorant may be considered to be weak (i.e., may not produce the desired color when the ink is printed) when the pH is less than about 5 (which may occur, for example, when the pK_a of the ascorbic acid in combination with water in the ink drifts downwards). For example, at a pH of less than 5, the iron ascorbate may produce a color having a brownish hue, rather than a dark purple hue.

In an example, the pH may be adjusted or otherwise maintained by incorporating a pH buffer as an additive into the ink vehicle. Examples of the pH buffer include 3-(N-morpholino)propanesulfonic acid (MOPS), 3-morpholino-2- hydroxy-propanesulfonic acid (MOPSO), 1 ,4-piperazinediethanesulfonic acid (PIPES), tris(hydroxymethyl)aminomethane (TRIS), and/or other similar biological buffers. Other examples of buffers include inorganic buffers such as sodium acetate, sodium phosphate, and/or sodium borate. In an example, the pH of an inkjet ink containing iron ascorbate ranges from about 5 to about 7.5; and in another example, ranges from about 6 to about 7. The foregoing ranges may be achieved, e.g., by incorporating the pH buffer into the ink vehicle of the respective inks in an amount ranging from about 0.2 wt% to about 3 wt% of the inkjet ink. In another example, the pH buffer may be present in an amount ranging from about 0.5 wt% to about 2 wt% of the inkjet ink.

In some cases, the ferrous ion having the +2 oxidation state (i.e., iron (II) or Fe⁺²) has a tendency to auto-oxidize to the +3 oxidation state (i.e., iron (III) or Fe⁺³). The ascorbic acid of the ionic complex may, for instance, act as a reducing agent to minimize the auto-oxidation of the ferrous ion in solution (e.g., when in the cartridge inside the printer) or in the solid state (e.g., when printed on the medium). In some cases, another reducing agent (such as, e.g., cysteine, sodium dithionite, and/or gallic acid) may be incorporated into the instant example of the erasable inkjet ink to prevent the auto-oxidation of the ferrous ion.

Since ionic complexes such as iron ascorbate are sensitive to oxygen, the ink vehicle may, in some examples, have to be deoxygenated before the iron ascorbate is added to the vehicle to form the inkjet ink. Generally, the

deoxygenation involves the removal of oxygen from the ink vehicle so that the oxygen does not degrade the colorant when the colorant is added to the vehicle. It has been found that sufficient deoxygenation may be accomplished by adding any of sodium bisulfite or sorbitol to the ink vehicle. It has further been found that the image quality of the ink when printed on the medium is improved by the presence (and, in some instances, the concentration) of sodium bisulfite alone, or in combination with sorbitol. In an example, the amount of sodium bisulfite present (if used in the ink) is less than about 1 wt%; and in another example, is less than about 0.5 wt% of the erasable inkjet ink. In another example, the amount of sorbitol present (if used in the ink) is less than about 6 wt%; and in another example, is less than about 2 wt% of the ink.

In some instances, it may be desirable to add a biocide to the ink vehicle, such as PROXEL® GXL (available from Arch Chemicals, Inc., Norwalk, CT) or KORDEK™ MLX (available from The Dow Chemical Co., Midland, Ml). The biocide may be added to the ink to protect the ink from bacterial growth. The amount of the biocide present in the inkjet ink, if one is incorporated, ranges from about 0.05 wt% to about 1 wt%; and in another example, ranges from about 0.05 wt% to about 0.25 wt%. In yet another example, the amount of the biocide, if used, ranges from about 0.05 wt% to about 0.15 wt%.

Another additive that may be added to the ink includes a surfactant. The surfactant may be included in the ink, for example, to assist in controlling the physical properties of the ink, such as jetting stability, waterproofness and bleeding. One or more surfactants may be used in the ink, and may be present in an amount ranging from about 2 wt% to about 5 wt%. The surfactant(s) may be chosen from nonionic surfactants or anionic surfactants, and are generally chosen from those that are human-friendly and environment-friendly, as previously defined. Several commercially available nonionic surfactants may suitably be used in the formulation of the ink, examples of which include ethoxylated alcohols such as those from the Tergitol® series (e.g., Tergitol® 15S5, Tergitol® 15S7, Tergitol® 15S9, Tergitol® TMN6) manufactured by Union Carbide, located in Houston, TX; surfactants from the Surfynol® series (e.g. Surfynol® 440 and Surfynol® 465) manufactured by Air Products and Chemicals, Inc., located in Allentown, PA; 2-diglycol surfactants, such as 1 ,2 hexanediol or 1 ,2- octanediol; or combinations thereof.

Some suitable anionic surfactant(s) that may be used in the ink

compositions include surfactants of the Dowfax® family (e.g., Dowfax® 8390) manufactured by Dow Chemical Company, located in Midland, Ml.

Further, polymers may be added to the ink for stabilizing the ink, and for achieving improved water and rub resistance, relatively good durability, relatively good gloss and low bronzing of the ink on the substrate/medium. Examples of polymers that may be used include polyethylene glycols having a weight average molecular weight of 1 ,000 to 20,000, and combinations thereof. Sugar components (such as, e.g., sorbitol, mannitol, and other related glycogens, which have a viscosity lower than about 5 cP), may also be added to the polymers, and are capable of interacting with the polymer(s) to increase the viscosity. In an example, the polymer(s) (if any are used) are present in an amount of about 2 wt% or less. Higher molecular weight polymers (e.g., having a weight average molecular weight greater than about 20,000) such as carboxy cellulose, methyl cellulose, and various starches may be used, e.g., at concentrations of about 1 wt% or less.

It is to be understood that water makes up the balance of the ink vehicle, and thus the balance of the example of the inkjet ink disclosed above.

Another example of the erasable inkjet ink includes a colorant chosen from an ionic complex in combination with a dye. It has been found that the

incorporation of the dye with the ionic complex (such as, e.g., the iron ascorbate identified above) may produce black or colored erasable inkjet inks having colors with a sharper hue than that produced with an ionic complex alone, and in some instances, exhibit improved color characteristics. It is to be understood that the color of the ink depends, at least in part, on the combination of the ionic complex and the dye. Typically, the ink will take the color of the dye, with perhaps some darkening due to the presence of the ionic complex (such as, e.g., the iron ascorbate). In this example of the ink, the ionic complex may be chosen from any of the examples of the ionic complexes disclosed above, and the dye may be chosen from natural dyes or synthetic dyes.

Some examples of natural dyes that may be used include anthocyanins

(which, in combination with the iron ascorbate produces a cyan color (with perhaps a tinge of purple due to the presence of the iron ascorbate)), saffron (such as ColorMaker® Natural Yellow available from ColorMaker, Inc., and which, in combination with the iron ascorbate produces a yellow color), turmeric (which, in combination with the iron ascorbate also produces a yellow color), cochineal (a red dye derived from the cochineal insect, which, in combination with the iron ascorbate produces a magenta color, and may also be referred to as carmine, carminic acid (e.g., ColorMaker® Natural Magenta available from ColorMaker, Inc., Anaheim, CA), and which is also known as natural red 40), indigo carmine (which, in combination with the iron ascorbate produces a cyan color), and combinations thereof.

Some examples of synthetic dyes (some of which may be derived from natural products) that may be used include acid blue 9 (which, in combination with the iron ascorbate also produces a cyan color), caramel coloring (E150) made from caramelized sugar, Annatto (E160b) (a reddish-orange dye made from the seed of the achiote), Chlorophyll (E140) (a green dye made from chlorella algae), Betanin (i.e., Beetroot Red, which is a red colorant extracted from beets), Curcuminoids (E100) (i.e., Turmeric), Carotenoids (E160a) (i.e., Saffron), Paprika (E160c), Elderberry juice (which is a red food colorant from elderberries), Pandan (a green food colorant), Butterfly pea (a blue food dye), FDC blue 1 , FDC blue 2, FDC yellow 5, FDC yellow 6, FDC red 3, FDC red 40, food black 2, Food Green 2, FDC Green 3, Food Yellow 3, Food Red 14, Natural Red 4 (such as Natural Red 2180 available from American Color Research Center, Inc. (San Dimas, CA)), red cabbage or other anthocyanin (such as Natural Blue 2179 also available from American Color Research Center, Inc.), and combinations thereof.

It is to be understood that one or more of the natural dyes identified above may also be synthetically made, and thus may, in some instances, also be considered to be a synthetic dye. In instances where the dye portion of the colorant is a combination of two or more dyes, the dye is referred to herein as a dye-blend.

It has been found that the inkjet ink of the instant example (containing a dye or dye-blend) exhibits traits of an ink containing the ionic complex alone as the colorant. In an example, synthetic dyes (such as those identified above) may be present in an amount ranging from about 0.5 wt% to about 6 wt% of the inkjet ink; and in another example, ranges from about 2.5 wt% to about 4 wt%. In another example, natural dyes (such as those that are also indicated above) may be present in an amount ranging from about 2 wt% to about 12 wt%; and in another example, ranges from about 2 wt% to about 5 wt%. Further, the amount of the iron ascorbate when used in combination with a synthetic dye is present in an amount ranging from about 0.5 wt% to about 5 wt% of the ink, and when used in

combination with a natural dye is present in an amount ranging from about 2 wt% to about 12 wt% of the ink. In some instances, the dye-blend may include a natural dye in combination with a synthetic dye, and when the iron ascorbate is used in combination with this dye-blend, the iron ascorbate is present in an amount ranging from about 0.5 wt% to about 12 wt%, or in another example, in an amount ranging from about 2 wt% to about 4 wt%.

It is to be understood that the examples of the ink vehicle described above for the erasable inkjet ink formulated using the iron-based ionic complex alone may also be used in the erasable inkjet ink examples including a dye or dye-blend. Additionally, the pH of the inkjet ink including the colorant formed from an ionic complex in combination with a dye may be adjusted based, at least in part, on the dye selected for the combination. Typically, if the dye is such that the pH of the ink would not fall within the pH range of the ink including the iron ascorbate alone, the iron ascorbate will not be combined with that particular dye. In one example, the pH of the ink including a combination of iron ascorbate and a dye ranges from about 6 to about 7.5. In a more specific example, the pH of the ink for a

combination of iron ascorbate and anthocyanins ranges from 6.5 to 7.5, whereas the pH of an ink including a combination of iron ascorbate and saffron ranges from about 6 to about 7.5. Typically, inks containing a combination of iron ascorbate and a synthetic dye have a pH ranging from about 6 to about 7.

Yet another example of the erasable inkjet ink includes a colorant chosen from a non-ascorbic acid-based complex. Examples of the non-ascorbic acid- based complex include those formed from metal ions (e.g., Fe⁺², Cu⁺², Mn⁺², Zn⁺², etc.) in combination with a ligand chosen from a polyphenol. Examples of the polyphenol include caffeic acid, chlorogenic acid, gallic acid, hydroxytyrosol, protocatechuic acid, and the like, and combinations thereof. In another example, the ligand may be 1 ,3-trimethylxanthine (i.e., caffeine). In yet other examples, combinations of polyphenols and caffeine may be used. Further, the metal ions may be obtained from various salts containing those ions. For instance, the ferrous (II) ion may be obtained from ferrous chloride, ferrous nitrate, ferrous sulfate, or potentially other counter-ions of iron. In another example, zinc ions may be obtained from zinc chloride, zinc nitrate, etc.; whereas manganese ions may be obtained from manganese (II) chloride, manganese (II) nitrate, etc. It is to be understood that other metal ions may similarly be obtained from salts.

An example of a non-ascorbic acid-based complex that may be used as the colorant of the ink includes the ferrous ion in combination with caffeic acid or chlorogenic acid (which is the quinic acid ester of caffeic acid), and this colorant produces a dark colored ink. Another example of a non-ascorbic acid-based complex includes gallic acid in combination with iron to produce a bluish-black and/or brown color depending, at least in part, on the pH of the combination, the amount of ferrous ions present, the solubility of the gallic acid, and the amount of the gallic acid present in the ink.

It is to be understood that the examples of the ink vehicle described above for the erasable inkjet ink formulated using the iron-based ionic complex alone may also be used in the erasable inkjet ink examples including a non-ascorbic acid- based complex.

In an example, the pH of an inkjet ink containing a non-ascorbic acid-based complex formed from caffeic acid, gallic acid, chlorogenic acid, hydroxytyrosol, protocatechuic acid, or combinations thereof ranges from about 6 to about 8; and in another example, ranges from about 6.5 to about 7.5.

Still another example of the erasable inkjet ink includes one having a dye- blend mono-based colorant, which is a colorant formed from the mixture of two or more of the dyes previously disclosed. The dye-blend produces a color within a minimized color space coordinate L^*, and an a^* and b^* that are individually close to zero. It has also been found that the dye-blend mono-based colorants may additionally be used to form specifically colored inks, and an ink set (such as a cyan, magenta, and yellow (CMY) ink set) may be formed by the instant example of the inkjet ink.

The pH of the instant example of the inkjet ink may be adjusted as

previously described. In instances where the colorant includes a dye chosen from a synthetic dye (such as, e.g., indigo carmine, food black 2, FDC Yellow 5, and FDC red 40), the pH of the ink ranges from about 4 to about 9; and in another example, ranges from about 6 to about 8. In instances where the colorant includes cochineal or saffron, the pH of the ink ranges from about 5 to about 9; and in another example, ranges from about 6 to about 8. Further, in instances where the colorant includes anthocyanins, the pH of the ink ranges from about 6 to about 9; and in another example, ranges from about 6 to about 8. In another example, where the colorant includes anthocyanins, the pH of the ink ranges from about 7 to about 9. In an example, the erasable inkjet ink further includes an ink component that triggers an erasing property of the ink when an appropriate erasure fluid comes into contact with the ink dried on the surface of the medium. This ink component may be referred to herein as a catalyst, and the component triggers a chemical reaction between the colorant of the dried ink and the erasure fluid in order to erase the ink from the medium.

A few examples of such a reaction are as follows:

Fe(ll) + Ascorbic acid <> Fe(ll)Ascorbic acid (1 :2) or (1 :1 )

H₂O₂ (hydrogen peroxide) + Fe(ll) > 2OH-(radicals) + Fe(lll).

Thus, the Fe(ll) that reacts with ascorbic acid can catalyze the production of hydroxyl radicals, which can further react to degrade the colorant or create more Fe(lll).

The component of the erasable inkjet ink responsible for triggering the reaction may, for example, be a transition metal, and this metal may come from the colorant in the ink. In other examples, the transition metal may be separate from the colorant.

In one example, the transition metal may be iron in the ferrous form (e.g., Fe⁺²), which may come from an iron ascorbate colorant. The iron metal catalyzes the reaction between the colorant and the erasure fluid when the fluid contacts the dried ink (or image). Other examples of transition metals that may suitably be used to trigger the reaction include copper (e.g., in the Cu⁺² state), manganese (e.g., in the Mn⁺² state), zinc (e.g., in the Zn⁺² state), or the like.

It is to be understood that various complexes of metals with ascorbic acid, polyphenols, or caffeine are contemplated as being within the purview of the present disclosure. For example, one may use complexes of iron formed with any of ascorbic acid, caffeic acid, chlorogenic acid, gallic acid, hydroxytyrosol, protocatechuic acid, or caffeine, to form a colorant. Similarly, one may use complexes of copper formed with any of ascorbic acid, caffeic acid, chlorogenic acid, gallic acid, hydroxytyrosol, protocatechuic acid, or caffeine, to form a colorant. Further, one may use complexes of manganese formed with any of ascorbic acid, caffeic acid, chlorogenic acid, gallic acid, hydroxytyrosol, protocatechuic acid, or caffeine, to form a colorant. Yet further, one may use complexes of zinc formed with any of ascorbic acid, caffeic acid, chlorogenic acid, gallic acid, hydroxytyrosol, protocatechuic acid, or caffeine, to form a colorant.

Formulations of some examples of the erasable inkjet ink may be found in

PCT International Patent Application Ser. No. PCT/US1 1/39023 (Docket No.

201005855WO01 ), filed concurrently herewith, which is incorporated by reference herein in its entirety.

An example of a method of formulating the erasable inkjet ink is shown in Fig. 1 . The method generally involves selecting a colorant from the group of human-friendly and environment-friendly colorants described above (see reference numeral 100), and then mixing the colorant in the ink vehicle to create the erasable inkjet ink (see reference numeral 102).

Examples of the erasure fluid that may be applied to the dried ink are provided hereinbelow. It is to be understood that each of these examples includes a vehicle, and at least an erasure component incorporated into the vehicle. As used herein, the term "vehicle" for the erasure fluid refers to the combination of at least one or more solvents to form a vehicle within which the erasure component is incorporated to form the erasure fluid. In some examples, the vehicle may also include an additive, which is a constituent of the fluid that may operate to enhance performance, environmental effects, aesthetic effects, or other similar properties of the erasure fluid. Examples of the additive include surfactants, pH buffers, biocides, and/or the like, and/or combinations thereof. In other examples, the vehicle does not include an additive.

As previously mentioned, the erasure fluid vehicle includes at least one solvent, which is/are used as a carrier for the erasure component and may, in some examples, constitute the bulk of the erasure fluid. In an example, the solvent is chosen from 1 ,2-propanediol, glycerol, tetraethylene glycol, sorbitol, and combinations thereof. The solvent(s) may be present in an amount ranging from about 1 wt% to about 50 wt% of the erasure fluid. In another example, the solvent(s) is/are present in an amount ranging from about 1 wt% to about 25 wt%. In still another example, the solvent(s) is/are present in an amount ranging from about 10 wt% to about 25 wt% of the erasing fluid.

In one example, the solvent is chosen from a combination of 1 ,2-propanediol and glycerol, where the 1 ,2-propanediol is present in an amount ranging from about 1 wt% to about 25 wt% of the erasure fluid, and the glycerol is present in an amount ranging from about 1 wt% to about 25 wt%. In another example, the 1 ,2- propanediol and the glycerol are each present in an amount ranging from about 5 wt% to about 15 wt% of the erasure fluid; and in still another example, each are present in an amount ranging from about 5 wt% to about 10 wt% of the erasure fluid. Further, tetraethylene glycol, if used as a solvent in the vehicle, may be present in the erasure fluid in an amount ranging from about 1 wt% to about 25 wt%; and in another example, ranges from about 5 wt% to about 15 wt% of the erasure fluid. In still another example, the tetraethylene glycol may be present in an amount ranging from 5 wt% to about 10 wt%.

In an example, the erasure fluid vehicle may also include a surfactant that may be used, in part, as a wetting agent to wet the surface of the device (e.g., a roll coater) that may be used to apply the erasure fluid to the image formed on the medium. In this respect, the surfactant is chosen from a non-hydrophobic material. Further, the surfactant may also be incorporated into the erasure fluid to facilitate the removal of the colorant of the erasable inkjet ink from the medium (e.g., from the fibers of the plain papers or coated papers identified above). In this respect, the surfactant is also chosen from a group of surfactants that may contribute to the removal of the colorant from the fibers of the medium. Examples of the surfactant that may be incorporated into the vehicle include the surfactants of the

SURFYNOL® family (such as SURFYNOL® 465, available from Air Products, Inc., Lehigh Valley, PA), the surfactants of the TERGITOL® family (available from the Dow Chemical Co., Midland, Ml), SILWET® 7602 (available from Momentive Performance Materials, Albany, NY), and combinations thereof. The surfactant(s), if used in the erasure fluid, may be present in the erasure fluid an amount ranging from about 0.1 wt% to about 5 wt% of the erasure fluid. In another example, the surfactant(s) may be present in an amount ranging from about 0.1 wt% to about 1 wt%.

In another example, a biocide such as PROXEL® GXL (available from Arch Chemicals, Inc., Norwalk, CT), may be added to the erasure fluid to protect the fluid from bacterial growth. The amount of the biocide present in the erasure fluid, if one is incorporated, ranges from about 0.05 wt% to about 1 wt%.

Further, a pH buffer may also be incorporated into the erasure fluid vehicle. In an example, the pH buffer may be chosen from one or more of the pH buffers that may be incorporated into the examples of the erasable inkjet ink described above.

To reiterate from above, the erasure component of the erasure fluid is specifically chosen to interact with a particular colorant of the erasable inkjet ink used to form the image on the medium. In one example, the erasure component may be chosen from an oxidant/reductant that effectively interacts with the colorant of the erasable ink. Certain oxidants/reductants (such as, e.g., peroxides) may effectively interact with the colorant in the presence of oxygen molecules. It is believed that a degassed colorant (i.e., where no oxygen molecules are present) may be nonreactive, or have a very slow reaction rate when the colorant comes into contact with the erasure component. In an example, the oxygen molecules may come from air present in the surrounding environment within which the erasing process is being performed, or may be supplied to the medium (e.g., from an oxygen supply) during the erasing process.

Examples of oxidants/reductants that may be used for the erasure component include persulfate ions (e.g., from sodium persulfate, potassium persulfate, lithium persulfate, etc.), peroxymonosulfate ions (e.g., from sodium peroxymonosulfate, potassium peroxymonosulfate, lithium peroxymonosulfate, etc.), hydrogen peroxide, chlorate ions (e.g., from sodium chlorate, potassium chlorate, etc.), hypochlorite ions (e.g., from sodium hypochlorite, potassium hypochlorite, etc.), sodium ascorbate, and ascorbic acid. Further, the concentration of the erasure component depends, at least in part, on the erasability of the colorant and on desired environmental levels. For instance, it may be desirable to maintain the concentration level of the

oxidants/reductants to a value at or below 3 wt% to achieve the desired erasability of the ink and desired environmental levels, though lower concentration levels may also be used. It is to be understood, however, that the lower concentration level may affect the erasability of the ink. For instance, a concentration of the

oxidants/reductants of about 1 wt% may result in a 30% to 50% drop in the erasability of the ink. It may also be possible to increase the concentration of the oxidants/reductants to an amount above 3 wt% (such as, e.g., 5 wt%), but this may, in some instances, deleteriously affect the medium upon which the ink ws printed. One way of achieving a higher erasability without using an

oxidant/reductant concentration level higher than 3 wt% includes applying, during the erasing process, the erasure fluid having the lower concentration of

oxidants/reductants a multiple number of times (e.g., two or more times).

Despite the adjustability of the concentration of the erasure component depending on the colorant of the inkjet ink, the erasure component concentration still falls within a preset range. In an example, if the oxidant/reductant is chosen from persulfate ions, peroxymonosulfate ions, hydrogen peroxide, chlorate ions, and hypochlorite ions, the concentration of the oxidant/reductant ranges from about 0.25 wt% to about 6 wt% of the erasure fluid. In another example, the hydrogen peroxide is present in an amount ranging from about 2 wt% to about 4 wt% of the erasure fluid; and in yet another example, is present in an amount of about 3 wt%. The persulfate ions, peroxymonosulfate ions, chlorite ions, and hypochlorite ions may be present in an amount ranging from about 1 wt% to about 3 wt% of the erasure fluid; and in yet another example, are present in an amount of about 1 wt%. Furthermore, the ascorbic acid may be present in an amount ranging from about 1 wt% to about 10 wt%; in another example, is present in an amount ranging from about 2 wt% to about 5 wt%; and in yet another example, is present in an amount of about 4 wt%. It is believed that the oxidants/reductants identified above may, in some cases, require a catalyst to facilitate the chemical reaction between the erasure component and the colorant of the erasable inkjet ink. For example, the ferrous ion (Fe⁺²) (which may come from an iron ascorbate colorant (which is a dark, violet colorant) of the inkjet ink) may be used to catalyze a reaction between hydrogen peroxide and the iron ascorbate colorant to degrade the iron ascorbate colorant and erase the image formed by the ink from the surface of the medium. In this example, the iron ascorbate acts as both a colorant for the inkjet ink and as the catalyst for its own degradation during the erasing.

It is to be understood that other catalysts may be used to facilitate the reaction between the colorant and the erasure component, and these other catalysts may not necessarily be part of the colorant itself. Examples of other catalysts that may be used include manganese ions, cobalt ions, copper ions, and/or zinc ions. For instance, sodium peroxymonosulfate may be activated by a chloride ion (CI^") already present in certain coated papers, such as COLORLOK® papers (available from Hewlett-Packard Co.) to form the hypochlorite ion. It is believed that the hypochlorite ion then reacts with, and degrades many, if not all of the colorants of the erasable inkjet ink disclosed above.

The erasure component may also or otherwise be chosen from a chelating agent, and this erasure component is useful for erasing inkjet inks containing ionically-complexed colorants that tend to have a stronger tendency to form an ion than to form a color-forming ligand. An example of such a colorant is an ionically- complexed colorant containing iron, for instance, iron ascorbate. Examples of chelating agents that may be used as the erasure component include citric acid, gluconic acid, sodium phosphate, sodium bicarbonate, ethylenediamine tetraacetic acid (EDTA), and combinations thereof. In an example, the chelating agent is present in an amount ranging from about 1 wt% to about 10 wt% of the erasure fluid. In another example, the citric acid, gluconic acid, sodium phosphate, and sodium bicarbonate (individually or in combinations thereof) may be present in an amount ranging from about 1 wt% to about 10 wt% of the erasure fluid; in another example, ranging from about 2 wt% to about 5 wt%; and in still another example, is about 4 wt%. EDTA may, for example, be present in an amount ranging from about 1 wt% to about 4 wt%; and in another example, from about 1 wt% to about 2 wt% of the erasure fluid.

In an example, the erasure fluid may further contain a polymer having a viscosity greater than 10 cP. It is believed that the use of a polymer having a large viscosity (i.e., a viscosity larger than 10 cP) in the erasure fluid allows the fluid to stay on the surface of the medium when the fluid is applied thereto during the erasing process. In another example, the erasure fluid may further contain a polymer having a viscosity less than 10 cP, which may allow the erasure fluid to be jetted from an inkjet printhead. It is further believed that the polymer also contributes to the efficiency of the erasing process compared to water in the fibers of the paper (which may render the medium as reactive for certain reactants).

Examples of polymers that may be incorporated into the erasure fluid (i.e., into the vehicle) include carboxymethylcelluloses having a weight average molecular weight ranging from 90,000 to 1 ,000,000 (which has a viscosity ranging from less than about 10 cP to about 2,000 cP, depending, at least in part, on the amount of polymer added), methyl celluloses (such as, e.g., methyl hydroxyethyl ether cellulose, which can achieve viscosities ranging from less than about 10 cP to greater than about 1000 cP, again depending on the amount of the polymer added), polyethylene glycols having a weight average molecular weight of 1 ,000 to 20,000 (which has a viscosity ranging from about 5 cP to about 100 cP, yet again depending on the amount of the polymer added), guar gum (which has a viscosity ranging from about 10 cP to about 1000 cP, again depending on the amount of the polymer added), starches (such as, e.g., rice starch, which have a viscosity ranging from less than about 10 cP to about 150 cP, yet again depending on the amount of the polymer added), and combinations thereof. Sugars (such as, e.g., sorbitol, mannitol, and other related glycogens, which have a viscosity lower than about 5 cP), may also be added to the polymers, and are capable of interacting with the polymer(s) to increase the viscosity. It is to be understood that the concentration of the polymer in the erasure fluid depends, at least in part, on the polymer chosen to be incorporated into the fluid. For instance, carboxymethylcelluloses and methyl hydroxyethyl ether cellulose may be present in an amount ranging from about 0.10 wt% to about 6 wt% of the erasure fluid; in another example, ranging from about 0.25 wt% to about 3 wt%; and in yet another example, ranging from about 1 wt% to about 2 wt%. The polyethylene glycols may be present in an amount ranging from about 1 wt% to about 20 wt% of the erasure fluid; in another example, ranging from about 5 wt% to about 15 wt%; and in still another example, ranging from about 10 wt% to about 15 wt%. Rice starch may be present in an amount ranging from about 1 wt% to about 10 wt% of the erasure fluid; in another example, ranging from about 2 wt% to about 6 wt%; and in yet another example, ranging from about 2 wt% to about 4 wt%. Sorbitol, for example, may be present in an amount ranging from about 1 wt% to about 20 wt% of the erasure fluid; in another example, ranging from about 2 wt% to about 10 wt%; and in yet another example, is about 5 wt%. Guar gum may be present in an amount ranging from about 1wt% to about 3 wt%. The sugar(s) may be present in an amount ranging from about 3 wt% to about 20 wt%; and in another example, ranging from about 5 wt% to about 10 wt% of the erasure fluid.

In an example, the balance of the erasure fluid is water.

Additionally, the inventor has found that the concentration of the solvent in the erasure fluid may contribute to the integrity of the medium, e.g., with respect to curl, cockle, reliability, and durability. Further, the polymers may impart a stiffening effect to the medium, which may balance the oily effect of the solvents used in the erasure fluid after repeated erasing cycles. Improvements in curl, for example, may be accomplished by balancing the amount of curl obtained with anti-curl solvents (e.g., 1 ,2-propanediol, glycerol, and tetraethylene glycol) and polymers of the erasure fluid with the amount of solvent absorbed by the medium during repeated printing and erasing cycles.

In some cases, the solvent absorbed by the medium after repeated cycles of erasing and reprinting increases, which may cause the medium to have an oily or greasy feel. In one example, a curl-to-oil balance may be achieved with a solvent concentration ranging from about 10 wt% to about 30 wt% of the erasure fluid. In another example, the curl-to-oil balance may be achieved with a solvent

concentration ranging from about 15 wt% to about 30 wt% of the erasure fluid; and in yet another example, a solvent concentration ranging from about 20 wt% to about 30 wt%. It is also believed that the application of the erasure fluid is substantially even (i.e., a substantially even amount of the fluid is coated across the surface of the medium), which may also contribute to an improvement in curl.

It is further believed that cockle may be managed by using appropriate solvents in the erasure fluid, and the durability/reliability of the medium may be managed by using the least amount of erasure fluid as possible to effectively erase the ink from the medium. In an example, the amount of the erasure fluid may be minimized by formulating the erasure fluid to perform more effectively when erasing the ink. For instance, effective performance of the erasure fluid may be achieved by increasing the viscosity of the erasing fluid (e.g., by adding higher viscosity polymer(s) to the fluid) so that the erasure fluid remains on the surface of the medium when applied thereto (noting, however, that if the erasure fluid is to be applied via inkjet printing, the viscosity of the erasure fluid should generally be less than about 10 cP).

It is to be understood that the effectiveness of the erasure fluid depends, at least in part, on certain variables of the fluid in addition to the erasure component selected, such as, e.g., the pH of the fluid. In an example, the pH of the erasure fluid should fall within a predefined range in order for the erasure component to effectively interact with a particular colorant of the inkjet ink. This is true, at least in part, because the chemical reaction that takes place between the colorant of the ink and the erasure component described above depends, at least in part, on the pH of the reacting medium. In some instances, it is desirable to maintain the pH of the erasure fluid above 4, whereas in other instances, a lower pH (such as 3 or lower) is also effective, for example, for applications other than for removing an inkjet ink from paper such as, e.g., in industrial applications that use non-paper substrates that can tolerate the lower pH values.

As one example, an erasure fluid containing hydrogen peroxide, persulfate ions, peroxymonosulfate ions, chlorite ions, or hypochlorite ions, should be formulated to have a pH ranging from about 2 to about 8; in another example, a pH ranging from about 4 to about 7.5; and in yet another example, a pH ranging from about 5 to about 7. An erasure fluid containing ascorbic acid should be formulated to have a pH ranging from about 3 to about 8; in another example, a pH ranging from about 4 to about 7.5; and in yet another example, a pH ranging from about 4 to about 6. Further, an erasure fluid containing citric acid should be formulated to have a pH ranging from about 3 to about 8; in another example, a pH ranging from about 4 to about 7; and in yet another example, a pH ranging from about 4 to about 5. Additionally, an erasure fluid containing gluconic acid should be formulated to have a pH ranging from about 4 to about 9; in another example, a pH ranging from about 6 to about 9; and in still another example, a pH ranging from about 7 to about 9.

Formulations of some examples of the erasure fluid may be found in PCT International Patent Application Ser. No. PCT/US1 1/39014 (Docket No.

201005863WO01 ), also filed concurrently herewith, which is incorporated by reference herein in its entirety.

Some combinations of erasable inkjet inks and erasure fluids specifically formulated to erase the ink are set forth in Table 1 below. Table 1 provides the erasure component present in the erasure fluid that may suitably degrade the colorant of the erasable ink. Table 1 : Example combinations of erasable inkjet inks and erasure fluids

An example of a method of erasing an image from a medium is shown in

Fig. 2. This method generally involves selecting a medium having an image formed thereon (see reference numeral 200). The image is formed by printing an example of the erasable inkjet ink onto the medium using an inkjet printing system. As shown in Figs. 3A and 3B, the inkjet printing system 10, 10' includes an inkjet printing device 12 (such as, e.g., a thermal inkjet (TIJ) device or a piezoelectric inkjet device) having one or more inkjet fluid ejectors 14. The fluid ejector 14 is fluidically coupled to an ink reservoir 16 that contains an example of the erasable inkjet ink (identified by reference numeral 18). The fluid ejector 14 is configured to eject the ink 18 onto a surface 22 of a medium, where the ink 18 is retrieved from the reservoir 16 during inkjet printing. The medium having the ink deposited thereon is referred to herein as a "printed" or "used" medium, and is identified in Figs. 3A, 3B and 3C (as well as in Figs. 6-17) by reference numeral 24.

Upon selecting the printed medium 24, the method further involves applying an erasure fluid to the printed medium 24 to cover at least a portion of the image (see reference numeral 202 in Fig. 2). In the example shown in Fig. 3A, the erasure fluid (identified by reference numeral 20) is applied to the medium 24 by ejecting the fluid 20 onto the surface 22 of the medium 24 using other fluid ejector(s) 15 of the printing system 10. More specifically, the printing device 12 of the printing system 10 includes the other fluid ejector 15 (in addition to the fluid ejector 14 for the ink 18) that is fluidically coupled to another reservoir 17 that contains an example of the erasure fluid 20. The fluid ejector 15 is configured to eject the erasure fluid 20 onto the surface 22 of the printed medium 24 (upon feeding the printed medium 24 through the printing device 12), where the erasure fluid 20 is retrieved from the reservoir 17 during an erasing process involving the inkjet printing of the erasure fluid 20 onto the medium 24. It is to be understood that Fig. 3A is a schematic depiction, and in practice, the medium 24 generally would not be printed via ejector 14 and then erased directly thereafter via erasure fluid 20 from ejector 15. Rather, the printing and erasing steps (for any of the examples of the method described herein) generally take place at different times. Further, erasing may or may not be accomplished via the same or a similar device as with the printing.

In some instances, the erasure fluid 20 may be part of an ink set, which includes two or more erasable inkjet inks (e.g., two or more differently colored inks). In some cases, a single erasure fluid may be designed and used to erase any of the inks included in the ink set. It is also contemplated herein to incorporate more than one erasure fluid into the ink set, e.g., if a particular erasure fluid is required to erase a particular ink of the ink set. In another example, the

combination of a single erasable inkjet ink and the erasure fluid may form its own ink set, such as in the example depicted in Fig. 3A. In yet other instances, the erasure fluid may stand alone as a component of the printing system that is separate from the ink or from the inks of an ink set.

In another example, the erasure fluid 20 may be applied to the image formed on the medium during a post-processing coating process. For instance, the printed medium 24 may be fed into a post-processing coating apparatus, such as, e.g., a roll coater 28, and a thin (e.g., ranging from about 1 micron to about 15 microns) layer or film of the erasure fluid 20 may be applied to the medium 24 as the medium 24 passes through the roll coater 28. In the example schematically depicted in Fig. 3B, the roll coater 28 is incorporated into the printing system 10'. In this example, the medium 24 is fed back into the printing system 10', bypasses the fluid ejector 14, and the erasure fluid 20 is applied to the medium 24 via the roller coater 28. In another example, the roll coating apparatus 28 is separate from the printing system 10, 10', and in this example, the medium 24 is fed into a standalone roll coating apparatus 28.

The roll coating apparatus 28 generally roll coats the erasure fluid 20 onto the printed medium 24 to cover the image formed thereon. The roll coater 28 may, in one example, be configured to perform a gravure coating process, which utilizes an engraved roller running along a coating bath containing the erasure fluid 20. The engraved roller dips into the bath so that engraved markings on the roller are filled with the erasure fluid 20, and the excess fluid on the roller is wiped away using, e.g., a doctor blade. The fluid is applied to the printed medium 24 as the medium 24 passes between the engraved roller and a pressure roller.

Other roll coating processes that may be used include reverse roll coating (which utilizes at least three rollers to apply the erasure fluid 20 to the medium 24), gap coating (where fluid applied to the medium 24 passes through a gap formed between a knife and a support roller to wipe excess fluid 20 away from the medium 24), Meyer Rod coating (where an excess of fluid 20 is deposited onto the medium 24 as the medium 24 passes over a bath roller, the Meyer Rod wiping away excess fluid 20 so that a desired quantity of fluid 20 remains on the medium 24), dip coating (where the medium 24 is dipped into a bath containing the fluid 20), and curtain coating.

Yet another way of applying the erasure fluid 20 to the medium 24 involves spraying the fluid 20 (e.g., from a sprayer device 30) onto the medium 24 (e.g., as an aerosol), as schematically shown in Fig. 3C. Sprayer device 30 may generally include an aerosol generating mechanism and/or an air brush sprayer mechanism. A control mechanism associated with the sprayer device 30 may selectively control the delivery of the type of drops and the spray characteristics, such as, e.g., fine mist to fine bubbles to larger size droplets. In examples disclosed herein where the erasure fluid 20 is applied via a non-inkjet device, the erasure fluid 20 may include additional additives that are generally not ink jettable from an inkjet pen. For instance, the erasure fluid may contain additional additives that improve curl, cockle, reliability, and durability of the medium. Examples of these additives include high molecular weight polymers (e.g., polymers having weight average molecular weights greater than about 25,000) at concentrations greater than about 2 wt%, which may increase the viscosity of the erasure fluid to a value that is greater than about 10 cP (which viscosity is such that the fluid generally cannot effectively be printed from an inkjet pen). The erasure fluid may also or otherwise include a larger solids content (e.g., greater than about 5 wt%) in cases where the erasure fluid is applied to the medium by means other than by an inkjet pen.

In some instances, interaction between the erasure fluid 20 and the colorant(s) of the ink 18 may be accelerated by applying a prescribed amount of heat to the printed medium 24 after the erasure fluid 20 has been applied thereto (see reference numeral 204 in Fig. 2). In an example, the prescribed amount of heat ranges from about 60°C to about 80°C, and is applied for an amount of time ranging from about 5 seconds to about 30 seconds. In another example, the prescribed amount of heat ranges from about 100°C to about 120°C, and is applied for an amount of time ranging from about 1 second to about 10 seconds. In one example, the printing system 10, 10' may have a heating element 26 incorporated therein (as shown in Fig. 3A). In this example, the heating element 26 may be chosen from heated rollers, heated air, radiative sources (such as infra-red (IR) or visible light), standard heating elements using high resistance, and/or the like. In another example, the heating element may be part of a post-processing process that is separate from the printing system 10, 10'. In this case, the heat element may be, e.g., a heat gun, IR radiation, heat lamps, LED arrays, laser, and/or the like.

To further illustrate the present disclosure, examples are given herein. It is to be understood that these examples are provided for illustrative purposes and are not to be construed as limiting the scope of the disclosure. It is to be understood that the recitation of weight percents (wt%) herein is with respect to the total weight of the respective formulation (i.e., the erasable inkjet ink or the erasure fluid). EXAMPLES

Example 1

A sample of an erasable inkjet ink was formulated by incorporating about 3 wt% of an iron ascorbate complex colorant in a vehicle including about 10 wt% 1 ,2 propanediol, about 5 wt% glycerol, about 0.2 wt% MOPS, and about 0.25 wt% Surfynol 465. The pH of the ink was about 7. The ink sample was printed on HP Office recycled paper (available from Hewlett-Packard Co.) using the black print cartridge in an HP PhotoSmart 8450 printer (also available from Hewlett-Packard Co.). Example 2

The erasable inkjet ink sample formulated for Example 1 was used to print a number of blocks on a sheet of HP Office recycled paper 24, where the blocks were printed ranging from a 100% print density (the block on the far left) to a 55% print density (the block on the far right) using an HP PhotoSmart 8450 printer. An illustration of the blocks as printed is shown in Fig. 7.

An erasure fluid was printed (using the HP PhotoSmart 8450 printer) over the printed blocks to erase a portion of the blocks. The erasure fluid contained about 3 wt% hydrogen peroxide, about 20 wt% 1 ,2 propanediol, and about 5 wt% glycerol. As shown in Fig. 8, the erasure fluid effectively erased the portion of the blocks, revealing an image ("THE QUICK") where the erasure fluid removed the ink from the printed blocks. The erasing was accomplished over a period of time: within about 30-45 minutes, the blocks were about 75% erased; and after about 2-3 hours, greater than about 95% of the targeted printed ink was erased.

Thereafter, the erasable inkjet ink was reprinted over the previously erased portion of the blocks, as shown in Fig. 9. It is to be understood that Fig. 9 is an illustration of the printed page, and as such, due to illustrating constraints, the printing of "THE QUICK" with dark ink could not fully be shown. However, it is to be noted that the ink density of the printing illustrated in Fig. 9 was substantially the same as the iron ascorbate mono ink originally printed on plain paper.

Example 3

The same erasable inkjet ink formulated for Example 1 was printed on a sheet of HP brochure coated media 24 using the HP PhotoSmart 8450 printer, and an illustration of the image produced is shown in Fig. 10. Then, the same erasure fluid as described above in Example 2 was printed over the image of Fig. 10 to remove the image from the media 24. As illustrated in Fig. 1 1 , the image was effectively erased within about 5 minutes to about 10 minutes. It is believed that the decrease of erasing time compared to that for Example 2 above may be attributed to the more acidic nature of the media.

Example 4

Another erasable inkjet ink sample was formulated by incorporating about 4 wt% Natural Red 1029 obtained from American Color Research Center, Inc.

(Walnut, CA) colorant in a vehicle including about 15 wt% 1 ,2 propanediol, and about 0.1 wt% citric acid. The pH of the ink was about 5. The ink was printed on HP Office recycled paper using an HP PhotoSmart 8450. An illustration of the printed image is shown in Fig. 12. Then, an erasure fluid was printed (using the HP PhotoSmart 8450 printer) two times over the image. The erasure fluid contained about 1 .5 wt% NaHCO3, about 20 wt% 1 ,2 propanediol, about 5 wt% glycerol; and about 0.5 wt% Surfynol 465. The erasure fluid had a pH ranging from about 7 to about 8. The medium 24 having the erasure fluid applied thereon was placed in an oven for about 1 minute at about 60°C. As illustrated in Fig. 13, the image was about 80% erased from the medium. However, a residual, brownish color remained on the erased medium, which is believed to be due to other components of the stock dye solution, such as sugar or other natural components. Example 5

Another erasable inkjet sample was formulated including a combination of the colorant used for the erasable ink of Example 1 (about 2 wt% iron ascorbate) blended with about 1 wt% Natural Color 2179 Blue. The ink vehicle and the pH of the ink were the same as in Example 1 . A series of blocks were printed on HP Office Recycled paper using an HP PhotoSmart 8450 printer as illustrated in the background of Fig. 14. Then, two erasure fluids were applied. The first erasure fluid was the same as in Example 2. About 5 minutes after the first erasure fluid was applied, a second erasure fluid was applied. The second erasure fluid was a mixture of about 5 wt% sodium phosphate, about 5 wt% citric acid, about 20 wt% 1 ,2 propanediol, about 5 wt% glycerol, and about 2 wt% Crodafos N3 surfactant (an anionic surfactant commercially available from Croda International PLC). The pH of the second erasure fluid was about 6. The erasure fluids were applied in an offset manner to the blocks to effectively erase the ink, as illustrated in the foreground of Fig. 14.

Example 6

The erasable inkjet ink of Example 5 was again used to print a series of blocks on HP Office Recycled paper using an HP PhotoSmart 8450 printer. Then, two erasure fluids were applied. The erasure fluids were the same as those used in Example 5, however the order of application was reversed, i.e., the second erasure fluid was applied first, and about 5 minutes later, the first erasure fluid was applied. The applied erasure fluids effectively erased the ink, as illustrated in Fig. 15.

Comparative Example

A comparative inkjet ink was formulated including an AB9 cyan dye, which is a permanent dye typically used in inkjet ink compositions. The ink was printed as a series of blocks on HP Office recycled paper using an HP PhotoSmart 8450 printer, as illustrated in Fig. 16. An erasure fluid containing about 3 wt% hydrogen peroxide, about 20 wt% 1 ,2 propanediol, and about 5 wt% glycerol was applied three times to the ink printed on the paper 24. As shown in Fig. 17, most of the originally printed ink remained on the paper. This shows the resistance of this cyan dye to the erasure fluid as compared to examples of the other colorants described above.

It is to be understood that concentrations, amounts, and other numerical data have been presented herein in range format. It is to be understood that this range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a weight range of about 2 wt% to about 50 wt% should be interpreted to include not only the explicitly recited concentration limits of about 2 wt% to about 50 wt%, but also to include individual concentrations such as 10 wt%, 22.5 wt%, 35 wt%, etc., and sub-ranges such as 10 wt% to 40 wt%, 15 wt% to 25 wt%, etc. As a further example, a viscosity range of less than about 10 cP should be interpreted to include 9.9 cP, 8 cP, 5 cP, 1 cP, etc., and sub-ranges such as 1 cP to 8 cP, 2 cP to 6 cP, etc. Furthermore, when "about" is utilized to describe a value, this is meant to encompass minor variations (up to +/- 5%) from the stated value.

It is further to be understood that, as used herein, the singular forms of the articles "a," "an," and "the" include plural references unless the content clearly indicates otherwise.

Additionally, the term "any of, when used in conjunction with lists of components (e.g., solvents, additives, etc.) refers to one of the components included in the list alone or combinations of two or more components. For instance, the term "any of, when used with reference to a colorant, includes i) a natural food colorant, ii) a synthetic food colorant, iii) a cosmetic colorant, iv) a pharmaceutical colorant, or v) a combination of two or more of the colorants listed above.

While several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is not to be considered limiting.

Claims

What is claimed is:

1 . A method of formulating an erasable ink, the method comprising:

selecting a colorant from a group of human-friendly and environment-friendly colorants; and

mixing the colorant in an inkjet ink vehicle to create the erasable ink, the ink including a component to trigger a chemical reaction between the colorant and an erasure fluid;

wherein the ink vehicle enables the erasable ink to be printed from a thermal inkjet pen to form an image on a medium, and wherein the chemical reaction enables the image to be erasable by degradation of the colorant upon application of the erasure fluid.

2. The method as defined in claim 1 wherein the colorant includes any of a natural food colorant, a synthetic food colorant, a cosmetic colorant, or a

pharmaceutical colorant.

3. The method as defined in claim 1 wherein the component of the inkjet ink vehicle is chosen from a transition metal.

4. The method as defined in claim 1 wherein the medium is chosen from plain paper or coated paper.

5. A method of erasing an image from a medium, comprising:

selecting a medium having an image formed by an ink, the ink having a colorant from a group of human-friendly and environment-friendly colorants and a component to trigger a chemical reaction between the colorant and an erasure fluid; and

applying the erasure fluid to the medium to cover at least a portion of the image to create the chemical reaction triggered by the component, the chemical reaction degrading the colorant to erase where the image is covered by the erasure fluid on the medium.

6. The method as defined in claim 5 wherein the applying of the erasure fluid to the medium is accomplished by any of i) depositing the erasure fluid onto the image using a thermal inkjet pen or a piezoelectric inkjet pen, ii) roll coating a layer of the erasure fluid onto the medium, or iii) spraying the erasure fluid as an aerosol on the medium.

7. The method as defined in claim 5, further comprising accelerating an interacting of the erasing fluid with the colorant by applying a prescribed amount of heat to the medium after the erasure fluid has been applied thereto.

8. The method as defined in claim 5 wherein the medium is chosen from plain paper or coated paper, and wherein the medium is reusable upon erasing the image.

9. An inkjet printing system, comprising:

an inkjet fluid ejector fluidically coupled to a reservoir, the reservoir containing an inkjet ink, including:

an inkjet ink vehicle; and

a colorant dispersed in the ink vehicle, the colorant being chosen from a group of human-friendly and environment-friendly colorants that degrade in response to a chemical reaction with an erasure fluid;

wherein the inkjet fluid ejector deposits the inkjet ink onto a medium to form an image thereon; and

an other reservoir, from which the erasure fluid is retrieved by an erasure fluid application device, wherein the erasure fluid application device applies the erasure fluid to the medium to cover at least a portion of the image.

10. The inkjet printing system as defined in claim 9 wherein the erasure fluid application device is an other inkjet fluid ejector fluidically connected to the other reservoir containing the erasure fluid, and wherein the other inkjet fluid ejector is configured to deposit the erasure fluid onto the medium.

1 1 . The inkjet printing system as defined in claim 9 wherein the erasure fluid application device contains architecture to roll coat the erasure fluid onto the medium.

12. The inkjet printing system as defined in claim 9, further comprising a heating element to apply a prescribed amount of heat to the medium having the erasure fluid applied thereon.