ERASABLE MARKING MEDIUM COMPOSITION

Abstract: An erasable writing medium composition suitable for use in porous tip pens and roller-ball pens is provided. The erasable writing medium comprises an emulsion having a continuous water phase and discontinuous phase that is comprised of a rubber. Pigment particles are exclusively associated with the discontinuous phase. Although the emulsion composition has a very low viscosity, very low surface tension, and very small rubber particle size, it is stable before use. Upon application of the composition to the highly ionic surface of standard stationary-type paper, the emulsion becomes unstable and the rubber agglomerates, and does not penetrate the paper. Upon drying, the pigment remains within the rubber phase thereby preventing the pigment from being absorbed by the writing surface. The rubber phase therefore forms a pigmented layer on the writing surface. The layer can be removed by erasure so that no pigment remains on the writing surface.

Description: ERASABLE MARKING MEDIUM COMPOSITION FIELD OF THE INVENTION This invention is an ink composition which makes erasable marks, especially transparent, colored marks. BACKGROUND OF THE INVENTION The present invention relates to an erasable marking or writing medium or ink that can be dispensed through a porous tip writing instrument, or by means of various types of roller-ball pens. The marks which result from application of the medium can be removed with comparative ease with a non-abrasive eraser, such as a common pencil eraser. When a person is producing written images, characters or highlight marks with writing instruments, it is not necessarily desirable to obtain a high degree of indelibility. For example, a person may want a writing medium which is easily removable from the writing surface by mechanical means, i.e., an erasable writing medium. To be truly erasable, the writing medium must be capable of being removed from the writing surface to which it has been applied without significant damage, such as abrasion, to the writing surface. Since the most commonly used writing surface is paper, a general discussion of the characteristics and composition of paper is helpful for an understanding of the present invention. Paper is essentially a mat of randomly distributed cellulose fibers. Because of the random orientation, the paper surface contains numerous voids which exist between the randomly oriented cellulose fibers. Therefore, for a writing medium to be truly erasable, at least that portion of the writing medium that contains the optically-effective portion (e.g., colorant) of the medium must be prevented from penetrating, to any substantial degree, into those voids. Otherwise, removal of the mark by erasing could not be accomplished without some damage in the form of abrasion to the writing surface. Therefore, a need exists for a writing medium that is not absorbed by the writing surface, namely paper, but which writing medium can be used in porous tip pens. The writing medium should also resist drying on the tip of the pen, but dry relatively rapidly when applied to a writing surface. Upon reflection, it can be argued that the development of an ink that will ready flow through a porous-tipped marker, but will not be indelibly absorbed into the paper, is an apparently impossible task. Generally, in order to allow the ink to effectively pass through the porous tip, the ink must have a very low surface tension, low viscosity, and any particles in the ink must be very small. These characteristics, however, will cause the ink to penetrate deeply into the paper. Furthermore, the ink composition must be very stable, under a wide range of conditions, in order to have a practical shelf life. Thus, an ink that can flow through a porous tip is probably not erasable, while an ink that can be made erasable will probably not pass through a porous tip and will have a poor shelf life. U.S. patent 4,297,260 to W. Ferree and G.V. Nguyen, addresses the need for an erasable ink composition for use in ball-point pens and felt-tipped markers. The patent was filed on November 19, 1979 and issued in the U.S. on October 27, 1981. The Ferree patent teaches that an erasable ink can be formed by chemically bonding a basic dye (a salt of a triarylmethyl cation) to a carboxylated styrene-butadiene rubber in a water emulsion. The resulting emulsion, when put on paper, allegedly forms a non-penetrating rubber layer on the paper with all of the dye chemically locked into the rubber layer. The rubber layer can then be removed by eraser, taking the dye with it. The Ferree patent teaches that it achieves its erasability by chemically bonding the specific family of chemically-active (highly-polar) dyes with a chemically-active (highly-polar) carboxylated rubber. Thus, the Ferree patent teaches away from the use of pigments or other colorants that do not react with the rubber. The Ferree patent also teaches away from using a non-reactive rubber such as a non-carboxylated styrene-butadiene rubber. The Ferree patent composition has been found to have a problem with sludge formation during formulation. This sludge must be filtered out of the ink, as noted in the patent, before the ink can be used. It appears that the sludge may be a by- product of the reaction between the dye and the rubber. The filter requirement creates cost and processing problems in the Ferree composition. The formation and isolation of the sludge creates disposal problems. Furthermore, it appears that the sludge formation reaction limits the amount of dye that can be bonded to the rubber and, therefore, unacceptably limits the intensity of color that can be realized using the Ferree concept. As a result, the preferred embodiments of the present invention avoid using a combination of reactive dyes and rubber. Either (or preferably both) the selected colorant ( e.g., pigment) and selected rubber (e.g., non-carboxylated SBR ) are relatively non-reactive. A pending patent application of R.M. LOFTIN, U.S. Serial No. 809,344, filed December 18, 1991, PCT application No WO 92US11127, filed on December 15, 1992, PCT publication No WO 9312175, published on June 24, 1993, addresses the need for an erasable ink composition for use in ball-point pens and felt-tipped markers. The LOFTIN patent application teaches the concept of incorporating a silicone-based release component into a latex having a Mooney value of above 90, to form an ink with a viscosity of 10-30 cps. The release component is adapted so that, as the ink is applied to the paper, the release component forms a barrier between the paper and the rubber phase of the latex. The barrier keeps the rubber from infiltrating into the paper and allows the rubber phase to be easily removed (erased) from the paper. The presence of the silicone-based release agent has a number of effects which may not be desirable. First, when the silicone-based release agent is present in concentration sufficient to have the effect taught by LOFTIN, the viscosity is forced above 10 cps. This viscosity can be too high for use in felt-tipped markers. Furthermore, the release agent can remain in the paper after erasure and interfere with subsequent writing on the same area of the paper. Finally, the silicone-based release agents are difficult and expensive to mix into and stabilize in the ink. For those reasons and because the "shock-out" effect of the present invention makes the release agent unnecessary, the preferred embodiments of the present invention do not include a release agent, or silicone-based release agent, or any other agent that separates the rubber from the paper. A Japanese patent application 4-56089, filed on February 7, 1992 and issued with a patent number of 5-214285 on August 24, 1993, to assignee Mitsubishi, concerns an erasable ink containing water, pigment, and a resin with a glass transition temperature less than 0 * C. The teaching is a broad recitation of the resins available in the selected glass transition temperature range. The teaching does not recognize the "shock out" effect of the present invention nor does it teach the resin systems which results in that effect. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention. It is, therefore, an outstanding object of the invention to provide a marker composition that can effectively pass through the porous tip of a marker. It is another outstanding object of the invention to provide a marker composition that can be effectively applied by the various types of roller-ball pens. Another object of this invention is the provision of a marker composition that forms a conventional (opaque) mark or highlight-type (colored, but transparent) mark, which mark is easily and completely erasable using a conventional eraser. A further object of the present invention is the provision of a marker composition which is highly stable under normal storage conditions. SUMMARY OF THE INVENTION In accordance with the present invention, a writing medium or ink is provided that comprises an emulsion having a discontinuous phase that comprises a rubber, a continuous phase of water, and a colorant that associates with the rubber. Careful selection of the rubber and colorant and a proper proportion of the rubber relative to the continuous phase results in an ink composition which forms a colored layer on a writing surface, which layer does not penetrate into the voids of a writing surface, such as stationary paper, and which layer is easily erasable from the writing surface for an indefinite period of time after application. The composition according to the present invention involves the use of colorant dispersions, specifically dispersions of hydrophobic, oliophilic pigments, dyes, or toners. In addition, the emulsion incorporated into the erasable writing medium compositions allows the writing medium to be easily erasable after application, but to adhere to the paper without chipping or cracking, even when the paper is bent or folded. Other components, including a plasticizer, surface tension reducers, and anti- drying agent (antioxidants and humectants), may also be incorporated into the compositions of the present invention. The marking composition is designed to exist as a stable emulsion except when deposited on the surface of conventional stationary-type paper. Such paper almost universally has a surface treatment which is acidic and contains water-soluble polyvalent cations, especially calcium, magnesium, and aluminum. The marking composition of the present invention is designed so that the emulsion, which is normally stable, becomes unstable and the rubber agglomerates, on contact with the paper surface. More specifically, the instability is triggered by the surface treatment components substantially universally present in stationary-type paper. Preferably, the stability is caused by a soap-type surfactant which stabilizes rubber-phase particles in a basic (pH > 7) aqueous medium. Contact of the emulsion with paper causes the medium to become more acid and to dissolve polyvalent cations from the paper. This deactivates the soap-type surfactant, because the caiboxylic acid end of the soap becomes an insoluble salt. The rubber particles break out of the emulsion and because the rubber has a glass transition temperature below room temperature, the particles agglomerate into a film. Because the film and the colorant associated with the film lie on the surface of the paper with minimal adhesion, the film and the colorant can be easily and completely removed. BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which: FIG. 1 is a diagrammatic view of a composition embodying the principles of the present invention as it is applied to a sheet of paper by a marker, FIG. 2 is a diagrammatic view of the composition on the paper prior to film formation and escape of the carrier, FIG. 3 is a diagrammatic view of the composition after escape of the carrier, and after film formation, FIG. 4 is a diagrammatic view of the film formed in FIG. 3 being removed by an eraser, and FIG. 5. is a graphical representation of the effect of ink system surfactant on ink surface tension. DETAILED DESCRIPTION OF THE INVENTION The erasable writing medium compositions of the present invention comprise an emulsion containing a pigment and preferably other components as hereinafter described. The typical operation of the present invention is shown diagrammatically in FIGS. 1-4. In FIG. 1, the marking composition denoted generally by the numeral 10 is shown being applied from the porous tip 11 of a porous tipped marker 12 and onto the writing surface 13 of a sheet of paper 14. As is normal in the porous tipped marking technology, the marking composition would be stored in a storage chamber 20 at the upper end of the tip 11. The composition would flow down through the porous tip 11 and would flow from the porous tip 11 when the porous tip 11 is placed in contact with a suitable writing surface 13. In FIG. 1, the marking composition is being applied in the form of a stripe 15, which is shown in FIG. 1 in its as - applied form, that is, as the non-dried emulsion. FIG. 2 shows the stripe 15 after it has been applied to the writing surface 13 of the sheet of paper 14, but prior to the escape of the continuous phase 16 from the non-continuous phase 17 which contains the pigment 18. At this point, the continuous phase 16 very quickly dissolves protons (H+) (acidity) and polyvalent cations (calcium, Ca+ + , magnesium, Mg+ + , and/or aluminum, A1+ + +) from the surface treatment components on the paper. This causes the emulsion to break, and the noncontinuous phase 17 to agglomerate into a film 21. Furthermore, the continuous phase 16 is absorbed into the paper 14. The mechanism for the almost instantaneous destabilization of the emulsion and agglomeration of the pigmented particles into a rubbery, coherent film is believed to involve the surfactant used to stabilize the latex from which the ink is made. Preferably, this surfactant is an amphipathic soap molecule, especially a sodium salt of a carboxylic acid. These surfactants function in an emulsion with a pH greater than 9.5. When the pH of the emulsion drops (becomes more acid) and when polyvalent cations are added to the emulsion, as occurs when the emulsion contacts the prepared surface of the paper, the surfactant becomes ineffective. The resulting salts of the carboxylic acids are water-insoluble and the hydrophilic character of the molecule is effectively lost. This effect on the ink deposited on the papers is almost instantaneous and quite remarkable. FIG. 3 shows the pigmented rubber film 21 which forms as the continuous phase 16 separates from the non-continuous phase 17 by absorption of the continuous phase 16 into the paper and/or by evaporation of the continuous phase 16. As is shown diagrammatically in FIG. 3, the pigmented film does not penetrate into the body of the paper but rather adheres to the outward projections of the surface of the paper. In this way, the pigmented film 21 is attached to the paper under normal circumstances but does not penetrate into the body of the paper to any significant degree. FIG. 4 shows the affect of applying a standard pencil eraser 25 on the end of a pencil 26 to the pigmented film 21. As can be seen, the shearing effect of the eraser 25 causes the pigmented film to disconnect from the high points of the writing surface and to be stripped off as a relatively unitary mass or a series of unitary masses. Because the pigmented film 21 is reasonably cohesive, it does not smear into the paper and tends to separate from the paper in clean chunks which can be easily removed from the writing surface without leaving any residual shadowing or without any pigment migration into the surface of the paper 13. The emulsion is the most important component of the writing medium compositions of the present invention, since it significantly affects characteristics such as flow, stability and erasability. According to the present invention, the emulsion is a rubber latex. It is diluted, in the final ink, to about 30 % to about 96 % water by weight. While there are numerous types of rubber latexes and, particularly, styrene-butadiene (SBR) latexes, care must be exercised in the selection of a rubber latex in order for the ink compromise to have the desired properties. Because the properties that an emulsion will have cannot always be exactly determined by mere reference to the components of the emulsion, and because the interactions and synergistic effects of the various components are not completely understood, it is anticipated that some experimentation may be necessary in selecting and compounding rubber emulsions that are suitable for use in accordance with the present invention. It is believed that the emulsion breaking effect which is critical to this invention is not dependant on the rubber. Thus the foπnulator is free to consider overall appropriateness in his or her selection of the rubber component. An important parameter in selecting an effective rubber for use in the emulsion is the glass transition temperature (Tg) of the rubber. This parameter provides a convenient indication of the critical room temperature properties of the rubber. A glass transition temperature which is too high might indicate a rubber which will be too stiff, brittle and non-tacky for an effective marking film on the paper. On the other hand, a glass transition temperature that is too low might indicate a rubber that will be smeared into the paper fiber by the eraser and will be so tacky that blocking will result. In should be understood that Tg does not necessarilly define these properties, and that Dalquist Modulus, molecular weight, and viscoelastic flow characteristics will be inportant in optimizing the product. For use in accordance with the present invention, the emulsion should possess certain characteristics and parameters which are hereinafter described. The preferred emulsion is a non-carboxylated styrene-butadiene latex. The styrene-butadiene inks of the present invention should have a room temperature viscosity in the range of from about 1 cps to about 35 cps and preferably from about 5 cps to about 10 cps and most preferably about 7 cps. Emulsions which are highly viscous, exhibit inadequate flow properties for use in writing instruments. Very low viscosity emulsions, while having better flow characteristics, tend to excessively penetrate paper fibers thereby adversely affecting erasability of the ink composition. Therefore, in order to be used in a standard felt-tipped marker, in general, emulsions having a viscosity of less than about 35 cps must be used. The rubber emulsions used in accordance with the invention, when dried on a paper surface, should have a "low adhesion" to the writing surface. As used herein, "low adhesion" means that the dried emulsion can be removed from the writing surface, such as paper, with a low abrasion eraser, such as a common pencil eraser, without causing more damage than erasure of a pencil mark would cause. Thus, the emulsion should, after drying (water evaporation and absorption into paper), have greater cohesion than adhesion to the writing surface. The emulsion should, when applied to a writing surface, form a cohesive film on the surface, generally within about 20 seconds. The characteristics of the emulsion (especially when dried) are determined in part by the ratio of the number of styrene units to the number of butadiene units in a copolymer chain. For use in accordance with the invention, the styrene-butadiene ratio should be in the range of from about 20:80 to about 55:45. The preferred styrene-butadiene ratio is from about 20:80 to about 45:55, with the preferred ration 24:76. If the styrene-butadiene ratio is much higher than 55:45 the ink composition has a greater tendency to become brittle when dried, this could result in cracking and chipping of the ink that has been applied to a writing surface. Therefore, use of styrene-butadiene ratio greater than 55:45 is not particularly advisable. In selecting or formulating an emulsion for use in the ink composition of the present invention, it is desirable that the emulsion have a relatively low surface tension as compared to the critical surface tension of the inner surface of the tip, thereby maximizing the ability the composition to penetrate the porous tip of a standard "felt-tipped" marker. Generally, the inks of the present invention have a surface tension of from about 10 to about 60 dynes/cm as acceptable, when used in conventional acrylic or polyester felt tips. 25 to 35 dynes/cm is preferred and 29.5 is most preferred. It is believed that the amount and type of emulsifier present to stabilize the latex may cause the surface tension of the emulsion to be to high. Thus, the type and amount of wetting agent should be adjusted to provide for the necessary low surface tension without so supplementing the emulsifier as to negate the emulsion breaking action which is critical to this invention. Higher surface tension inks can be used if the surface tension of the felt tip is increased as by plasma treating the inner surface of the tip or heating the tip. It is desirable that the emulsion be slightly tacky when dried so that it can easily adhere to, for example, a rubber eraser during the erasing process. If the emulsion is too tacky when dry, the paper sheets may stick together. It is also preferred that the emulsion have "freeze-thaw" stability and not deteriorate over long periods of shelf-life. The pH of the ink should generally be in the range of from about 9 to about 14. A pH outside of this range is not particularly desirable, since it may result in a composition that is unstable before use or composition in which the colorant cannot be dispersed. The most preferred styrene-butadiene emulsion is available from the Goodyear Tire and Rubber Company of Akron, Ohio under the trade designation "Pliolite LPF 2108" and from BASF Corporation under the trade designation "Butonal NS 103". This compound is a non-carboxylated, styrene-butadiene latex having a ratio of styrene- butadiene of 24:76, a solids content of about 40.0 percent by weight, a pH of about 11.4, and a surface tension of about 58 dynes/cm. The colorants or pigments present in the erasable ink compositions of the present invention are water dispersible pigments which are also oliophilic. These pigments are selected from industry standard products known to be appropriate when selecting pigments which are to be dispersed in water systems. They are selected from the group of hydrophobic pigments which are capable of dispersion in or in intimate association on the styrene-butadiene phase without significant penetration into a writing surface when the composition is applied to a writing surface, such as paper, for example. It is believed that this effect results because the pigment remains entirely occluded by or in intimate exclusive association with the rubber phase, which is selected so that it does not, itself, penetrate the paper. The pigment apparently does not enter the water phase, which does penetrate the paper. Specific pigments which can be used in accordance with the erasable writing medium compositions of the present invention include carbon black and ultra-fine fluorescent pigments and dyes and mixtures thereof. It is anticipated that other pigments, dyes and toners which behave in the same way, may be used in the compositions of the present invention. The colorant is generally formed into a stable suspension in water prior to being added into the ink composition. Generally, it has been found that colorants that are not stabilized prior to addition will not be stable in the ink. The pigment should be dispersed in the emulsion, but, when the pigment-containing emulsion is applied to a writing surface, such as paper, for example, the pigment should not penetrate into the paper sufficiently to form visible images in the paper. The pigment should not penetrate from the emulsion into the paper fibers or the voids between the paper fibers, since removal of the writing medium by erasing, without damage or removal of portion of the paper fiber, would then be precluded. Generally, the pigment will be present in an amount of from about 1% to about 50% by weight of the total writing medium composition. The size of the pigment particles is important to the effectiveness of the composition of the present invention. The particles preferably have a small diameter and/or chemistry which appears to cause them to so strongly favor the rubber phase and to form a stable ink. On the other hand, the particles must be large enough to create the desired optical effect. The size of the rubber particles in the emulsion is also important to the present invention. A size ranged about 700 Angstroms (0.07 microns) is preferred. Normally, particles that are this small would penetrate into the body of the paper and defeat erasability. The emulsion breaking effect of this invention is so rapid and dramatic that even these fine particles are forced to coalesce into a continuous coherent, rubbery film before paper penetration can occur. Although the precise mechanism of the invention's operation is not conclusively known, and a theory of operation is not crucial to practice the invention, set forth below is a description summarizing our beliefs as to the mechanism of operation. It is believed that when the pigments of the present invention are dispersed within the emulsion of the present invention, all of the pigment migrates to the rubber phase and remains attracted to that phase. This may be because the small surfaces of the fine pigment particles are oliophilic and hydrophobic. When the low viscosity emulsion of the present invention is placed on the surface of a sheet of paper, the aqueous continuous phase dissolves protons and polyvalent cation from the surface preparation components of the paper. This drops the pH of the emulsion and deactivates the soap-type surfactant-emulsif ier or other emulsifier which is deactivated by acid and/or polyvalent cations, which previously stabilized the emulsion. This is because the polyvalent cations form water-insoluble and/or non-charged salts with the carboxylic (or other) anions on the particle surfaces. This causes the rubber dispersion to collapse, and with the proper selection of Tg, to fuse into a continuous rubbery film. Simultaneously, the high absorbability of the paper quickly draws the continuous water phase, which contains no pigments, out of the emulsion, and into the paper surface. The non-continuous rubber phase (which contains, or absorbs, or adsorbs on its surfaces, the pigment) is collapsed and merges into a thin continuous coherent, rubbery layer on and attached to, but not absorbed by, the surface of the paper. This results in a pigmented mark which appears to mark the surface of the paper, while, in fact, only adheres to the surface of the paper. The layer is so loosely bound to the paper surface that physical abrasion will strip off the layer and the local pigment in the area of the abrading. This leaves the abraded portion of the paper apparently unmarked. In the preferred embodiment, an anti-drying agent (humectant) can be used to prevent the emulsion from drying on, for example, the marker tip when water from the emulsion evaporates, thereby facilitating the smooth flow of marking composition from the marker, especially after long periods of non-use. Suitable anti-drying agents include water-soluble organic ketones, esters and alcohols that do not have a significant deterious effect on the composition or its properties and whose boiling- point is relatively high, from about 140" C. to about 300° C. Specific compounds that can be use as anti-drying agents include, for example, 2-octanone, 5-methyl-2- hexanone, cellosolve acetate, glycerol, ethylene glycol, propylene glycol, diethylene glycol and butyl cellosolve (2-butoxyethanol). Of the foregoing anti-drying agents, sorbitol or butyl cellosolve is preferred. Since the anti-drying agent prevents drying of the emulsion, it also facilitates the spreading of the rubber and pigment on paper and can cause undesirable penetration of the rubber and pigment into, for example, paper fibers. Therefore, it is important that the concentration of the anti-drying agent be kept to a minimum, and in most cases, the concentration of anti-drying agent should not exceed 15 % by weight of the total erasable writing medium composition. The erasable writing medium compositions of the present invention may also optionally (though no preferably) include a plasticizer to increase the "tack" of composition and slow the film forming process to allow long lift-off times. As previously described, the ink composition is more easily erasable, and has adhesion to the eraaser,if it is very slightly tacky. The plasticizer increases the tackiness of the composition and, therefore, provides a composition which is more easily erasable with an ordinary non-abrasive eraser. Any plasticizer which is compatible with the compositions of the present invention may be utilized. The prefeπed plasticizer is dipropylene glycol dibenzoate which is marketed under the trade name Benzoflex 9-88 by the Velsicol Chemical Corporation of Chattanooga, Tenn. Another preferred plasticizer is marketed under the trade pame "Santicizer 8" by the Monsanto Industrial Chemicals Co. of St. Louis, Mo. "Santicizer" is a mixture of N-ethyl-ortho-toluene sulfonamides and N-ethyl-para-toluene sulfonamides. The compositions according to the present invention can be utilized in fiber tip marker, and, preferably, marking devices known as felt-tipped markers. In such markers, a body of porous felt-like material, usually of synthetic fabric, is encased in a marker case. A broad, flat tip extends from the marker case. The body is loaded with the ink of the present invention. When the tip is pressed against a paper surface and moved, an elongated band of the emulsion is applied to the surface. As the water in the emulsion is absorbed by the paper surface, the non-continuous rubber phase, which contains the pigment, merges into a layer of pigmented rubber imposed on and attached on the surface of the paper. This layer might be transparent, but colored, for highlighting, or might be opaque and black or, opaque and colored for conventional marking or writing. Whichever the case, the layer can be removed by erasing and this would remove all visible trace of the marking. EXAMPLE 1. ERASABLE MARKER -BLACK X 1188-69-1 FORMULATION: (T.N.V.) Dry Weight Wet Weight % (lbs) (lbs) Pigment Suspension: (Flexiverse Black LFD 4343) (46.0) 1.67 3.63 Distilled Water - - 10.00 Surfactant: (Fluorad FC-129) (50.0) 1.59 3.18 Latex: (Pliolite LPF 2108) (40.5) 24.44 60.35 Distilled Water - - 10.00 Humectant: (Sorbitol 70%) (70) 4.45 6.36 Distilled Water - _=_ 6.48 Total: 32.15 100.00 Specifications: VISC @ 60 RPM 7.0 ± 0.5 cps Surface Tension 29.5 + 0.5 dynes/cm T.N.V. stands for total non-volatiles. Instructions for Preparation: 1. Add Flexiverse to a clean stainless steel or glass lined container. 2. Set Lightning-type stirrer on at slow-to-medium speed and add the 10.0 lbs of distilled water. Continue mixing. 3. With stirrer at a medium to high speed, add the Fluorad FC 129 and continue mixing at medium to high speed until all of the Fluorad is thoroughly incorporated into the Flexiverse. Reduce speed to slow-to-medium and continue mixing. 4. In another clean stainless steel or glass lined container, weigh in the Pliolite, turn a stirrer on at slow-to-medium speed, and add 10 lbs. of distilled water. Continue stirring until homogeneous. 5. Add the diluted Pliolite to the Flexiverse container with the stirrer at medium speed. Continue stirring until homogeneous. 6. With the Flexiverse container stirrer at medium speed, add the sorbitol and continue stirring. 7. Achieve viscosity of 7.0 ± 0.5 cps @ 60 RPM with remaining distilled water. The resulting composition was placed in a standard felt-tipped marker. On application to standard stationery paper, a mark was formed identical in appearance to the mark from a standard marker. However, the mark was easily removable, with no visible residual marking, by conventional action of a normal pencil eraser. EXAMPLE 2 Correctable Black Marker Ink XI 188-92-1 Weight Flexiverse Black LFD 4343 Goodyear SBR Rubber Latex 2108 Fluorad FC-129 Sorbitol 70% Octolite 453 Demineralized Water Total 100.00 Viscosity - UL @ 60 RPM 7.5 +/- .5 cps Surface Tension 26 - 29 dynes/cm This formulation was prepared in the same way as Example 1 and resulted in an excellent erasable black mark. There are a number of other aspects to this invention. There will be a vinyl or rubber eraser affixed to the end of the marker to facilitate erasure of the markings. This eraser is not composed of any specific material. Most known eraser materials work fairly well. Colorant characterization: The colorant which is currently used in the development of the black ink is LPF 4343 Flexiverse Black from Sun Chemical. It contains a non-ionic surfactant, Joncryl 68 (an alkaline soluble acrylic), a bactericide, and carbon black. Oil soluble dyes were also tried initially with some success but were not as easy to disperse in the system as the pigmented colorants. The stability of the ink system using pigments is good due to the wide availability of water-based pigment dispersions in the market. This invention includes dispersions/emulsions of fluorescent colorants (dyes and toners) in water-based systems of this invention. In general, this invention includes use of these new small particle size emulsions and dispersions since they are particularly well adapted for use in the highlighter version of the erasable marker. A very effective version uses a Dian Co., Ltd. of Japan finished ink as the colorant. Resin emulsions: The following emulsions have been tried using the Flexiverse 4343 black dispersion from Sun Chemical. The general formula is 10% Flexiverse black pigment dispersion, 50% resin emulsion, 40% water (by volume). 0=not erasable, 5= completely Goodyear resin LPF 2108 is a non-carboxylated styrene-butadiene resin with a particle size of approximately 700 Angstroms (0.07 microns) (1 micron = 10,000 Angstroms). Goodyear LPF 5356 resin is an agglomerated version of the LPF 2108 resin and therefore has a wider range of particle sizes, most much larger than 700 Angstroms. This is reflected in the cream, higher solids nature of the 5356 compared to the almost translucent 2108 resin. Because of the larger particle size of the agglomerated 5356 (0.08 to 2 microns), that composition is not as easily let down with water. When the 5356 resin is used as a replacement for the 2108 resin, the resultant ink is not as stable and settles into layers upon standing for a few days. It nevertheless functions in this invention. The present invention includes an important discovery about the effect of the glass transition temperature (Tg) (film-forming temperature) of the rubber on erasability. It was previously believed that the rubber used as an erasable mark should have a glass transition temperature well above room temperature. It was believed that a relatively stiff, non-film-forming material would penetrate the paper less on application, would adhere less to the paper, and would flake off more cleanly during erasure. These ideal effects of high Tg do not, in fact, occur as well as expected. On the other hand, the present inventors have discovered that rubbers with sub-room temperature Tg's (e.g. -52 +/- 5 degrees C), surprisingly, work better. This superior behavior is particularly effective when combined with the shock agglomeration concept in which the destabilizable emulsion is instantly broken on contact with the paper. The low Tg allows even the tiniest rubber particles to instantly form a coherent, and non-penetrating film on the surface of the paper. Surprisingly, the highly rubbery film does not adhere unacceptably to the paper. Even more unexpectedly, the film performs a self-scavenging action during erasure, that is, it appears that any free particles of the film are absorbed from the paper texture by the body of film which forms during erasure. As a result, the entire erased area of the film falls cleanly from the paper as a small number of relatively large coherent masses. This self-scavenging effect results in a clean, shadow-free, erased paper surface. Mechanism of the erasable ink: If an erasable ink composed of 10% Flexiverse 4343 black pigment dispersion, 50% LPF 2108 SBr emulsion, 38% water, and 2% FC 129 surfactant (by volume) is applied to standard tablet paper, the marking is easily removed. The particle size of this ink is small (much less than 0.5 microns) and the surface tension is under 30 dynes/cm. It would be expected that this ink would not be erasable since the particles in the ink are dispersed and emulsified in water. It appears that the unexpected behavior of the LPF 2108 latex occurs because the latex emulsion is not very ion stable. Goodyear's LPF 2108 resin and the BASF competitive product Butonal NS 103 are latexes which share the property of ion instability. The emulsifier used in the latex is only effective at a pH of 9.5 or greater. The surface of most paper is acidic or cationic in nature and the latex is flocculated (the emulsion "breaks") immediately on contact with the surface of the paper. In addition to direct anionic-cationic effect, there are also many polyvalent coatings used in paper manufacture which would also flocculate the latex. As test of the surface flocculation theory, we tried the ink on filter paper. Ashless filter paper is substantially free of acid and polyvalent cations and when the erasable ink formulation was marked on this surface, the ink penetrated all the way through to the other side of the paper and erasability was lost. Carboxylated latexes are more ion stable and, consistent with the above theory, they tended to penetrate regular paper to a greater extent than the non-carboxylated latexes such as LPF 2108. In order for the flocculating ink to form a film on the surface of the paper, it is necessary or at least highly favorable for the water phase to be quickly removed from the resin. It is most probable that the wicking action of the paper combined with the evaporation of the water from the surface provides this speed. This drying action may also suddenly concentrate dissolved protons and polyvalent cations, thus adding to the shock-action of the film forming. From this proposed model, it is understandable why the high surface tension, highly cohesive, ion sensitive 2108 resin works so dramatically well in the low viscosity ink system which is the subject of this invention. This is critical because it is necessary to lower the overall surface tension of the ink with an ink system surfactant to facilitate the wetting out of the nib. It would, therefore, be best to utilize an ink system surfactant which will not increase the stability of the latex. Addition of more rubber emulsifier might stabilize the latex even in the presence of the polyvalent coating on the paper. The critical definition of the emulsion thus becomes one of defining destabilizable emulsions which can be part of a low surface tension ink. Humectants: We are using Sorbitol ( a polyhydric alcohol sold under the name "Sorbitol" by ICI Americas Inc. of Wilmington, Del.) as the humectant in the current system to improve the Cap Off performance of the marker. Cap Off is the measurement of the ability of the unit to withstand drying and still maintain functionality. The names of the humectants that are most effective are listed. Surfactants: As was mentioned above, an ink system surfactant will be used to lower the surface tension of the ink to the point where the nib can be wet out. Apparently, because of the very small (700 Angstrom) particle size of the LPF 2108 latex, a relatively large amount of surfactant is required to lower the surface tension of the system. Two percent of a fluorochemical surfactant is very high for such a highly active agent. The surfactant used is sold under the name FC129 by 3M, and is a potassium salt of a fluorinated alkyl-carboxylate, effective in alkaline systems. EXAMPLE 3, #1 - #7 XI 111-122 Erasable Marker Ink Evaluate various concentrations of FC-129 Prepare an Ink base as follows: Mix the following for five minutes: Flexiverse Black LFD 4343 Demineralized Water Mix the following for five minutes: Goodyear SBR Rubber Latex 2108 Demineralized Water Sorbitol 70% Combine above mixtures and mix for ten minutes Mix following for five minutes: #1 Ink base FC-129 #2 Ink base FC-129 #3 Ink base FC-129 #4 Ink base 19.6 2% 28.7 FC-129 0.4 #5 Ink base 19.4 3% 24.5 FC-129 0.6 #6 Ink base 19.2 4% 18.4 FC-129 0.8 #7 Ink base 61.5 The above data is graphed in FIG. 5 to show the effect of the FC-129 surfactant on surface tension of the ink. While low surface tension is the preferred way to enable the ink to wet the porous tip, those skilled in the art will recognize that other approaches can be employed. Viscosity: It is generally accepted that a marker ink should have a viscosity of under 5-10 cps. Antimicrobials or Preservatives, Bactericides, Fungicides: Even though we have not seen any evidence of microbial growth in the Flexiverse based ink formulation, we suggest that some sort of antimicrobial agent will be appropriate. Anti-foam Agents: Because of the extensive mixing which must be accomplished to formulate this product, a conventional anti-foaming agent may be employed to reduce foaming during manufacture. Antioxidant: The LPF 2108 which is used in the current formulations is sold by Goodyear without an antioxidant. This emulsion results in an ink which slowly oxidizes with exposure to air and makes the ink mark lose erasability. With the addition of antioxidants, the erasability can be extended indefinitely. Another important aspect of this invention is control of the indelibility or erasability time. Depending on the specific application, it may be desirable to compose the ink of the present invention to remain erasable after application to paper for periods varying from a short time (a few days) to a very long time (a few years). It has been found that, if no antioxidant is added to the ink, it becomes nonerasable in a few weeks, at normal room temperature. Addition of antioxidant causes a functional increase in the length of the time that the mark remains erasable. An antioxidant that works well in this system is sold under the commercial designation "Octolite 453" by Textile Rubber and Chemical Company of Dalton, GA. It is 55% solids and is employed up to about 3 percent (by weight) on dry basis, to maintain erasability for at least a few months. The product is said to be a 50:50 emulsified blend of two polymeric hindered phenol thioester antioxidants sold under the commercial names "Wing Stay L" and "Wing Stay SN1 " by Goodyear Tire and Rubber Company. Ghosting/residue: Using a microscope, the residue from a mark was analyzed. The residual color or "ghosting", which was visible on some substrates using the Flexiverse 4343 colorant with the LPF 2108 resin emulsion, is resin and colorant which is trapped in small caverns on the surface of the paper. The residue does not appear to be unbound colorant. The residue is resin/colorant matrix which is able to escape abrasion by the eraser. When a needle is used to remove the residue, there is no underlying stain on the paper. The rubber found most effective in this invention is designated LPF 2108 and sold by The Goodyear Tire & Rubber Company, Houston, Texas. The product is 40.0% solids. It has a residual styrene percentage of 0.019. It has a pH of 11.4 (highly basic). It has a surface tension, at 40% total solids content, of 58 dynes/cm. It contains 23.4% bound styrene. It has a 0.001 % coagulum. It has 0.008% stability MG. It contains 8.64% soap, which we believe is a potassium oleate. The effect of variation in the amount of rubber and of water in the ink is shown in Table 1. Rubber concentration below about 10 dry wt. % harms erasability. (INSERT TABLE 1) We begin with a pigment dispersion consisting of solid pigment particles suspended in alkaline aqueous medium containing a first surfactant. We add this to a latex which is an emulsion of rubber in an alkaline aqueous medium containing a second soap-type surfactant. The pigment dispersion and the latex are mixed with a substantial amount of a third surfactant which is sufficiently in excess of the amount needed to stabilize the mixture so that the surface tension of the total mixture is significantly reduced. This forms the ink. For the purposes of this specification, the term "ordinary writing paper" shall mean paper of the type normally used for commercial and domestic writing and typing and which has a surface which is treated or sized in a standard manner, and as a result, the surface releases ions when the surface is wetted. The term "ions" is used to denote water-soluble charged particles. The term "cations" is used to denote water-soluble charged particles having at least one positive charge. The term "polyvalent cations" is used to denote water-soluble charged particles having at least two positive charges. It is believed that the ink mixture is an aqueous emulsion of rubber particles in or on which particles of the colorant are exclusively associated. This emulsion is stable as long as the pH is high (basic) and ion content of the mixture is undisturbed, i.e., mono-valent cations (Na+ and K+). When the ink is applied to normal paper or ordinary writing paper, three things happen when ions present in the syrface treatment of the paper are dissolved into the ink. First, the acidity of the paper surface drops the pH of the ink, reducing the effectiveness of the soap-type second surfactant and causing the emulsion to break, i.e., causing the rubber and pigment to agglomerate. Second, this agglomeration is further accelerated by the presence of polyvalent cations (Ca+ +and Mg+ + and A1+ + +) in the paper surface. These cations form water-insoluble salts with the soap-type surfactant and void the effect of that surfactant. Just as the rubber and pigment in the ink are agglomerating, the water is absorbed into the paper. This causes the ink to form a pigmented film on the surface of the paper. Because the soap-type surfactant is essentially non-functional, the surface tension of the rubber is very high and the rubber has only minimal "wetting" on interaction with the paper surface. This is true even though the glass transition temperature of the preferred rubber is -52 +1- 5 degrees C. and therefore, the film would be expected to be quite flexible. Because the adhesion between the film and the paper is minimal, the pigmented film can be easily removed from the paper by friction. It is clear that not all of the available resin products will function to provide the advantages of the present invention. The following tables (Table 2 and Table 3) present the variation in performance of a range of available resin products. ( INSERT TABLE 2 ) ERASABLE INKS POLYMER EVALUATION TABLE 3 Erasability Rating 1) Movinyl 970 40% solids 50g 2.5 Day-Glo EP-17 colorant 25g 2) Formulation #1 with added 2.5 25g of water 3) Movinyl 970 40% solids 50g 2.5 Flexiverse 4343 carbon black dispersion lOg 4) Unocal resin Res 4171 98g 4.0 Basic Violet 3 dye 2g 5) Formulation #4 with added 3.5 water to bring vise, to 5 cps Another application of the erasable ink composition of the present invention would be in a class of writing instruments which will be referred to herein as " roller-ball " pens. This class of pens, which is very closely related to classic ball-point pens, developed in a manner which makes its useful to consider them a separate class (or perhaps a subclass) of writing instruments from the ball-point pen class of instruments. In order to effectively describe and classify the use of the erasable inks of the present invention in the roller-ball class of writing instruments, it is useful to provide a model of historical development of the roller-ball pen concept. It should be understood that this model, while consistent with facts, represents a simplification of the development process. Furthermore, it represents some speculation concerning the mental processes and motivation of the numerous parties involved in the development of the roller-ball technology. The classic ball-point pen writing instrument had developed into a fairly standard construction in which a tube of approximately two millimeter inner diameter was filled with highly viscous, organic-solvent-based ink, which was exposed to a rotating ball at one end of the tube. The non-aqueous inks which are used in this type of instrument were specifically designed to be highly- viscous in order that they did not flow out of the open end of the tube and in order that they did not leak out of the relatively large clearance between the ball and toroidal seat in which the ball was held. This relatively large space between the ball and seat was probably required because of the practical tolerance problems associated with the manufacturing methods. The organic inks were also specifically selected to have minimal evaporation from the tube in order that the ink did not dry out prior to use either through the open-back end of the tube or at the ball end. The latter would, of course, clog the ball's action. It was apparently recognized that the high-viscosity inks which seemed necessary in this ball-point pen construction did not provide the most pleasantly feeling writing effect. It was apparently recognized that a low-viscosity ink would allow the ball¬ point pen to function with far less force required to move the ball and ink across the paper surface. However, for reasons mentioned above, low-viscosity inks were simply not practical in the classic ball-point pen construction. One of the first of the roller-ball class of pens resolved the problems of low viscosity ink by absorbing the ink reservoir in a fibrous, porous filler. Porous ink feed rods were provided to lead the ink from the filler to the rotating ball at the end of the pen. These feed rods controlled the rate of flow of the ink and thereby, reduced the potential seriousness of leaking of the ink around the periphery of the ball, and over-flowing and "globbing" of ink at the start and stop of writing. While these constructions provided a way to use low viscosity inks in a practical rotating ball application system, it was found that the traditional organic-based inks, when prepared in low viscosity, had a tendency to bleed through normal paper to an unacceptable degree. On the other hand, it was found that water-based inks did not unacceptably bleed through the paper. As a result, there was significant activity in the development of water-based ink roller-ball pens having porous fillers, feed rods and a rotating ball. Examples of prior art patents employing such systems to control the flow of aqueous inks in a roller-ball writing instrument includes British patent specification 1 , 139,038, and U.S. Patent No.'s: 3,446,564; 3,533,708; 3,572,954; 3,873,218; and 4,145,148. The disadvantages of using an ink-storage and -feeding system of the typed described above ("filler, nib, ball system"), are several. The first, it is frequently the case that the feed rods or nib fails to provide a sufficiently continuous flow of ink to the ball for rapid use. Furthermore, the use of the filler to physically stabilize the ink in the reservoir significantly reduced (to about 1/2) the amount of ink available in the writing instrument. Furthermore, the use of inks which employ pigments as their colorant, as opposed to soluble dyes, can clog the capillary passages in the fibrous reservoir and feed rods further inhibiting and interrupting the rate of flow and the amount of ink that is delivered to the ball. The disadvantages of the reservoir-filler led to the development of various filler- less roller-ball pens which are herein called "filler-less, nib, ball systems" . While this type of system does provide more efficient storage of the ink, it also possesses all of the difficulties which one would expect as long as the feed-rod concept is maintained. It remains the objective of writing instrument developers to formulate a concept which would eliminate the need for the filler and feed rods or nib while avoiding the leakage and evaporation problems which exists when a smooth-writing low-viscosity ink is used in such a structure. One approach to the solution of this design problem for the "fillerless, nibless, ball systems" is set out in a series of "gelled ink" U.S. patents, namely U.S. Patent No.'s: 4,671,691; 4,686,246; 4,786,198; and 5,013,361. This series of patents sets out a system for employing aqueous inks in a fillerless, nibless, ball system and which strives to achieve the smooth feel of low-viscosity ink. In essence, this system employs an ink which is characterized as pseudoplastic or shear-thinning. The inks of this approach, which are also referred to herein as "jelled inks" are said to have the Theological property that they are highly viscous in the normal standing state, although typically less viscous than the ink used in classic ball point pens. However, when they are exposed to the high shear rates produced in writing with the ball-point pen the viscosity drops to less than 100 cps. The normal high-viscosity reduces the incidents of leakage, while the low-viscosity during and in the vicinity of the writing operation provides the smooth feel of low-viscosity ink. In practice, the system requires the presence of a high-viscosity filler plug between the back end of the ink reservoir and atmosphere in order to control evaporation and drying of the reservoir ink. The erasable ink system of the present invention can be applied effectively to all three classes of the roller-ball technology. Another development in ball-roller-point pen technology is particularly beneficial in the context of the present water-based erasable ink invention. Traditionally, the ball which is used in writing instruments is formed of metal. Because water-based inks do not "wet" the metal surface of the ball as effectively as certain existing organic-based ink formulations, water-based inks frequently had somewhat inferior writing characteristics when used in metal-ball-point pens. The use of ceramic balls in the ball-roller-point pen systems provides a superior method for applying water- based inks. The ceramic surface frequently is much more effectively "wetted" by the water-based inks and, therefore, provides a smoother, more uniformed application of the ink. When this ceramic ball technology is applied to the water-based erasable inks of the present invention, superior results can occur. Significantly, a higher surface tension ink can be used and that will improve erasability. The ball tends to roll more smoothly and the ink tends to be applied with less force by the writer, and the mark applied by the ink has a more uniformed smoother form. Furthermore, a textured ball tends to create less shear in a given situation, which will be seen to be an advantage as set out below. In the first class of roller-ball systems, namely the filler, nib, ball system, the ink of the present invention is essentially the same formulation and behaves in about the same manner as that used in the felt tip marker applications. The composition of the present invention, substantially in the same form as described for felt-tipped markers, can be used in the second class of roller-ball products, namely the fillerless, nib, ball system, and in the third class of roller-ball pens, namely the fillerless, nibless, ball pen, designed for the gelled ink. EXAMPLE 4. A Class 1 commercially-available roller-ball pen, sold under the trademark "BIC METAL POINT ROLLER FINE POINT" was obtained and disassembled. It included a tube, a filler, a nib, and a ball. The parts were all cleaned of ink, the pen was loaded with test ink, and the pen was reassembled. The test ink had the following composition. Carbon Black KS5725 3.23 gm. FC 129 surfactant 1.1 gm. Pliolite 2108 latex 42.0 gm. Sorbitol/glycerine 3.5 gm. Octolite 453 0.4 gm. Demineralized water 9.82 gm. The working Sorbitol/glycerine mixture was two parts by weight Sorbitol and one part by weight glycerine. The mixture was prepared using the standard procedure described in earlier examples above. The viscosity was about 5.3 cps. The surface tension was about 23.7 dynes/cm. The resulting product wrote in an acceptable manner and the resulting mark was erasable. After five days, the product would still write and the resulting mark was erasable. EXAMPLE 5. A Class 2 commercially-available roller-ball pen, sold under the trademark "PILOT PRECISE ROLLER BALL V5" was obtained and disassembled. It included a tube, no filler, a nib, and a ball. The parts were all cleaned of ink, the pen was loaded with test ink, and the pen was reassembled. The test ink had the following composition. Carbon Black KS5725 3.23 gm. FC 129 surfactant 1.1 gm. Pliolite 2108 latex 42.0 gm. Sorbitol/glycerine 3.5 gm. Octolite 453 0.4 gm. Demineralized water 9.82 gm. The working Sorbitol/glycerine mixture was two parts by weight Sorbitol and one part by weight glycerine. The mixture was prepared using the standard procedure described in earlier examples above. The viscosity was about 5.3 cps. The surface tension was about 23.7 dynes/cm. The resulting product wrote in an acceptable manner and the resulting mark was erasable. After five days, the product would still write and the resulting mark was erasable. EXAMPLE 6. A Class 3 commercially-available roller-ball pen, sold under the trademark "SAKURA BALL SIGN" was obtained and disassembled. It included a tube, no filler, no nib, and a ball. The parts were all cleaned of ink, the pen was loaded with test ink, and the pen was reassembled. The test ink had the following composition. Carbon Black KS5725 3.23 gm. FC 129 surfactant 1.1 gm. Pliolite 2108 latex 42.0 gm. Sorbitol/glycerine 3.5 gm. Octolite 453 0.4 gm. Demineralized water 9.82 gm. The working Sorbitol glycerine mixture was two parts by weight Sorbitol and one part by weight glycerine. The mixture was prepared using the standard procedure described in earlier examples above. The viscosity was about 5.3 cps. The surface tension was about 23.7 dynes/cm. The resulting product did not write in an acceptable manner. It would "skip" , that is, make a noncontinuous mark, and then stopped writing . The ball appeared clogged. EXAMPLE 7. A Class 3 commercially-available roller-ball pen, sold under the trademark "SAKURA BALL SIGN" was obtained and disassembled. It included a tube, no filler, no nib, and a ball. The parts were all cleaned of ink, the pen was loaded with test ink, and the pen was reassembled. The ball end was replaced by the ball end from a product sold under the trademark "PARKER VECTOR ROLLER BALL", because the Sakura ball have become clogged in earlier experiments. The test ink had the following composition. DAYGLO MAGENTA EP21 (11 gm. solids in water) 25 gm. FC 129 surfactant 0.5 gm. Pliolite 2108 latex 50 gm. Sorbitol glycerine 25 gm. The working Sorbitol glycerine mixture was two parts by weight Sorbitol and one part by weight glycerine. The mixture was prepared using the standard procedure described in earlier examples above. The viscosity was about 12 cps. The surface tension was about 42.5 dynes/cm. The resulting product wrote in an excellent manner and the resulting mark was erasable. After five days, the product would still write and the resulting mark was erasable. Of particular significance, however, is the excellent performance characteristics which the erasable inks of the present invention, when formulated in relatively high- viscosity form ( from the approximately 5 cps of the standard formula to 15 to 20 cps, by adding Sorbitol in place of water), function in the third class of roller-ball writing instruments, namely the filler-less, nibless, ball systems. It is not presently clear that the erasable inks of the present invention possess the Theological properties normally associated with the pseudo plastic or shear-thinning ("jelled") inks described above. In fact, tests thus far indicate that the present inks are Newtonian and perhaps even dilatant. However, it has been found that the high-viscosity versions of the inks of the present invention work very well in this class of writing instruments. Furthermore, it has been found that the high-viscosity versions of this product have excellent erasability characteristics and as a result, essentially provide a whole new product category of erasable roller-ball writing instruments. It would be expected that the Newtonian rheology of the present inks would be so incompatible with the shear-thinning model used to explain the effectiveness of "jelled ink" in the third class of roller-ball pens, that the present inks would not be expected to work as well as they do work. It has also been found that third class of roller-ball pens can effectively employ a "jelled" ink form of the ink formula recited above with the addition of the shear- thinning, shock-resisting, and water-fast enhancing additives recited in the "jelled ink" patents recited above. When the water-based inks of the present invention are employed in this third class of roller-ball products with a ceramic ball having a hydrophilic surface, the performance can be enhanced significantly over traditional metal surfaced balls. The effectiveness of the inks of the present invention in the gelled-ink-type class three roller-ball pens appears to be effected by shear instability ( rather than shear- thinning) of the present inks. It has been observed that the shear that occurs at the working end of a felt tipped marker is about 6000 sec 1 . The large ball of a classic ball-point pen creates a shear of about 6000 sec 1 . At this level of shear, the standard formulation of the present ink appears to be non-shear-stable. There is evidence that the emulsion starts to become unstable and break when the marker or pen is used to write. In a felt-tipped pen, this effect does not interfere with the operation, because the break occurs outside of the tip and the "glob" of ink is transfered to the paper. In a roller-ball pen, however, the "glob" is carried back to the ball socket and tends to clog the ball and socket. When the roller-ball designs having a relatively small diameter ball are used, especially in the class three roller- ball pens, the ball creates high shear as the ink is transported around the ball and onto the paper. This shear is in the order of 9000 sec '1 . When inks of the standard formulation of the present invention are exposed to to increasing shear, and especially shears of about 6000 sec 1 , the emulsion breaks, and the ink is fed to the paper in a two-phased, water-rubber form. This two-phased form tends to clog the ball in its socket, even a large-diameter ball, and to interfere with the initial part of the writing process. It also causes "skipping", that is, unintended breaks in the written mark. This emulsion breaking is destructive of the emulsion and not reversible. Fortunately, we have found that this problem can be resolved by adding shear stabilizing additives to the ink formulation. Ideally , the shear-stabilizing additives must stabilize the emulsion against the shear created by the pen in which the ink is to be used, without unduly increasing the ion- instability which is so beneficial to the, operation of the invention. It has been found that the polyhyric alcohols, for example Sorbitol and glycerol, when added in concentrations equal to or greater than would typically be required by their role as humectants in the formula of this invention, stabilize the emulsion to the point that the resulting ink can be used without clogging, in the higher shear ball pens. This high amount of shear stabilizer also increases the cap-off time, that is, the time that the pen can be left without a cap before the ink on the tip becomes too dry for the pen to be used. The concentration of shear stabilizer must be optimized because too much humectant can extend the drying time of the written mark to an unacceptable length and cause "ghosting" that reduces erasability. Experiments with resin-water-sorbitol-glycerine systems has resulted in some very surprising increases in cap-off time on roller-ball instruments. Standard commercial roller-ball-ink systems become unsatisfactory if the cap is left off the writing tip for a day. This appears to result from evaporation of the volatile ink components through the clearance around the ball and irreversible clogging of the ball. The roller-ball formulas of the present invention form a delicate, but quite gas-tight, seal around the ball when writing is stopped. This seal is broken and flows with the ink on to the written mark when writing is resumed. In the meantime, however, the seal allows the pen to remain useful for cap-off times of approximately a month. EXAMPLE 8 In a test, the unit contained ink number X 1265-90-1 from our lab notebook. This ink was composed of the following (wt %): 60% BASF NS 103 SBr latex emulsion 20% Sorbitol/Glycerine (2:1 ratio) 10% Flexiverse 4343 carbon black dispersion 10% Deionized water (with Triethanolamine added to increase the pH to 8.5) This roller-ball ink has more rubber emulsion than our standard marker ink. This ink also has more sorbitol/glycerine than our standard marker ink to prevent drying of the roller tip and to lubricate the roller tip. Two gel-type roller-ball units were left uncapped for 30 days and then we tried using the units. In light of the fact that standard roller-ball/units are expected to last only a few days with the cap-off, it was a surprise when the two erasable units wrote perfectly well after thirty days of cap-off testing. After studying the tip of a 30 day cap-off unit prior to reuse, it was discovered that the latex had formed a sealing ring in the ball-to-socket interface. On top of (or outside) this seal, the sorbitol/glycerine had formed a clear protective layer that coated the exposed surface of the ball and edge of the socket. When the unit is used, the rubber seal and the sorbitol/glycerine layer are easily rolled innocuously onto the surface of the paper. The air-tight seal formed by the rubber explained why the unit had such a long cap-off time and such excellent immediate writability after the cap-off test. The sealing action of the rubber combined with the anti-drying and lubricating action of the sorbitol/glycerine proved to be a very effective roller-ball combination. It is important to note that the erasable ink in the gel-type roller-ball system is currently out-performing even the standard non-erasable inks in the cap-off testing. It appears that standard non-erasable roller-ball inks could be improved by adding a small amount of latex and sorbitol/glycerine. The resulting roller-ball ink would not penetrate into the fibers of the paper and would have an extended cap-off time. Though not erasable, the new ink would allow roller-ball units to compete with high molecular weight solvent inks which do not require a cap. The viscosity of the ink has been found to be one of the critical parameters in optimizing the ink for a particular writing instrument. Because the variation of viscosity can effect other critical parameters, the achievement of a functional ink with optimum viscosity for a given writing instrument is sometimes difficult. It has been found useful to divide writing inks into three ranges; low viscosity (less than 10 cps) for capillary type writing instrument such as felt-tipped markers, high viscosity (greater than 30 cps) for bulk flow type instruments such as ball-point pens and gel- type roller-ball pens, and mid-range viscosity (10-30 cps). By experimentation (set out below) it was established that the "shock out" effect and the resulting superior erasability of the ink, can be achieved in all three viscosity ranges. In examples 9, 10, and 11, erasable inks of various viscosities were tested in "Bic round stic medium" ball pen parts. The parts were rinsed in MEK to free them of solvent-based ink residue. The only difference between these parts and the parts used in the gel-type systems on the market is the quantity of ink delivered to the ball. In the Bic parts, the ink tube is smaller than a typical gel ink tube. This size difference is required to compensate for the larger volume of ink which is delivered to the paper in present gel ink units. EXAMPLE 9 Ink X1265-105-2 has a viscosity of 64 cps and is composed of the following: 60g BASF NS 103 SBr latex lOg Flexiverse 4343 carbon black dispersion 15g Sorbital/Glycerine (2:1) 14g .5% Tragacanth gum solution in demineralized water (i.e., approx. 0.07 gm gum) lg fc-129 surfactant This ink had excellent writability and excellent erasability when applied from a pen formed of the Bic parts. EXAMPLE 10 Ink X1265-105-3 (a repeat mixing of ink X1265-90-1), has a viscosity of 26 cps and is composed of the following: 60g BASF NS 103 SBr latex lOg Flexiverse 4343 carbon black dispersion 20g Sorbital Glycerine (2:1) lOg Demineralized water This ink, as before, had excellent writability and excellent erasability in the Bic parts. EXAMPLE 11 Ink X1625-105-4 has a viscosity of 559 cps, because of the use of higher amounts of tragacanth gum and is composed of the following: 60g BASF NS 103 SBr latex lOg Flexiverse 4343 carbon black dispersion 15g Sorbital/Glycerine (2:1) .3g Tragacanth gum 14.7g Demineralized water This ink also had excellent writability and erasability in the Bic parts. The upper viscosity limit of the present invention will be determined by the maximum amount of pigment and rubber that can be added to the formulation. Very thick inks will require higher pigment and rubber loadings. In Brennenman et al., U.S. patent 4,721,739, viscosity ranges are given for erasable, as well as non¬ erasable, inks. For standard non-erasable ball pens, the viscosity range is 50-150 poise (5,000-150,000 cps). For erasable inks the range is 48-500 poise (4,800-50,000 cps). These erasable inks sometimes use a pressurized cartridge. Effective versions of our erasable inks could be formulated in these higher ranges with the use of various thickeners available in the art. While the invention has been described with respect to preferred embodiments, it is evident that upon reading the specification, numerous changes, modifications and substitutions will be apparent and are intended to be within the scope of the appended claims. TABLE I ' . COMPOSITION ERASABILITY CAP-OFF PROPERTIES by paper (minutes) Sample percent percent percent bond copy yellow 30 60 90 120 over vise surface drainback # dry SBr dry col. water lined night cps tension stability *** dyn/cm 1625-102 Transparent Orange * Ingredients (wt. % solid) ** Flexiverse 45.9 Erasability rating guide: 4343 pigment 0 5 dispersion no erasability complete erasability Dian FP 47 Cap-off guide: orange 0 pigment dry,no mark no loss in marking dispersion *** Drainback guide: Goodyear 39.9 0 2108 SBr no color no loss in color intensity latex ERASABLE INKS - POLYMER EVALUATION TABLE RATING: 0 - none; 5 - excellent FORMULATION: % of Diy Weight Abreviations: Sty » St>τene; ACN » Acrylonitnle; SB - Styrene Butadiene; VDc - Vmylidene chloride; FAS - Fatty Acid Soap Colorant (44%T.S.) 21.95 Surfactant (50% active) 0.50 Polymer 7.55 Viscosity Range: 3 - 10 cps@60RPM Surface Tension Range: 20 - 40 dynes/cm Marker - pen style; filler - polyester, nib - acrylic chisel point; ink fill . 1.3 - 1.7 gr

Claims: CLAIMS 1. A writing medium composition that is erasable from ordinary writing paper by an ordinary pencil eraser, comprising, (a) (i) an emulsion comprised of a discontinuous rubber phase and a continuous aqueous carrier phase, (ii) the emulsion including an emulsifier which holds the rubber phase in suspension when the composition is not in contact with ordinary writing paper, said emulsion omiting a silicone-based barrier material, (iϋ) characterized in that when said emulsion contacts ordinary writing paper, it rapidly transforms from a first state in which the emulsion is stable to a second state in which the emulsion is unstable and the rubber phase rapidly agglomerates, forming a cohesive rubber phase film having a low adhesion to the surface of the paper, said film being capable of complete removal from the paper by rubbing the film with an ordinary pencil eraser, and (b) a plurality of colorant elements in the emulsion, the elements being characterized by constant association with the rubber phase and with the said film. 2. A composition as recited in Claim 1, wherein the composition has a viscosity of less than 10 cps. 3. A composition as recited in Claim 1, wherein the composition has a viscosity of 10-30 cps. 4. A composition as recited in Claim 1, wherein the composition has a viscosity of greater than 300 cps. 5. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion includes an emulsifier which holds the rubber phase in suspension, but which is rendered ineffective in that role when a factor dissolved by the emulsion from ordinary writing paper reacts with the emulifier. 6. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion includes an emulsifier which holds the rubber phase in suspension, but which is rendered ineffective in that role when ions dissolved by the emulsion from ordinary writing paper react with the emulifier. 7. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion includes an emulsifier which holds the rubber phase in suspension, but which is rendered ineffective in that role when cations dissolved by the emulsion from ordinary writing paper react with the emulifier. 8. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion includes an emulsifier which holds the rubber phase in suspension, but which is rendered ineffective in that role when polyvalent cations dissolved by the emulsion from ordinary writing paper react with the emulifier. 9. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion includes a soap which holds the rubber phase in suspension, but which is rendered ineffective in that role when a factor dissolved by the emulsion from ordinary writing paper reacts with the soap. 10. A composition as recited in Claim 1 , 2, 3, or 4, wherein the emulsion includes a soap which holds the rubber phase in suspension, but which is rendered ineffective in that role when ions dissolved by the emulsion from ordinary writing paper react with the soap. 11. A composition as recited in Claim 1 , 2, 3, or 4, wherein the emulsion includes a soap which holds the rubber phase in suspension, but which is rendered ineffective in that role when cations dissolved by the emulsion from ordinary writing paper react with the soap. 12. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion includes a soap which holds the rubber phase in suspension, but which is rendered ineffective in that role when polyvalent cations dissolved by the emulsion from ordinary writing paper react with the soap. 13. A composition as recited in Claim 1 , 2, 3, or 4, wherein the emulsion includes a surfactant, said surfactant having a transformable portion which is hydrophilic in an alkaline aqueous medium, but which can be rendered hydrophobic by acidity and polyvalent cations dissolved by the emulsion from ordinary writing paper. 14. A composition as recited in Claim 1, 2, 3, or 4, wherein a surface active agent is included to reduce the surface tension of the composition, but does not eliminate the instability of the emulsion. 15. A composition as recited in Claim 1, 2, 3, or 4, wherein the rubber is selected from the following: styrene-butadienne copolymer latex, PLIOLITE LPF 2108 latex or equivalent, and BUTANOL NS 103 latex or equivalent. 16. A composition as recited in Claim 1, 2, 3, or 4, wherein the rubber has a glass transition temperature less than 40 * C. 17. A composition as recited in Claim 1, 2, 3, or 4, wherein the emulsion is free of a release agent capable of forming a barrier layer on the paper. 18. A composition as recited in Claim 1, 2, 3, or 4, wherein colorant is a pigment. 19. A writing medium composition that is erasable from ordinary writing paper by an ordinary pencil eraser, comprising, (a) (i) an emulsion comprised of a discontinuous non-carboxylated-styrene-butadienne- rubber phase having a glass transition temperature of less than 40 * C and a continuous aqueous carrier phase, (ii) and emulsion being free of a silicone-based release agent capable forming as barrier layer on the paper, (iii) the emulsion including an emulsifier which holds the rubber phase in suspension, but which is rendered ineffective in that role when a factor dissolved by the emulsion from ordinary writing paper reacts with the emulsifier to neutralize the emulsifying power of the emulsifier, (iv) so that said emulsion is characterized by its ability to be rapidly transformed form a first state in which the emulsion is stable to a second state in which the emulsion is unstable and in which the rubber phase rapidly agglomerates, (v) so that the agglomerated rubber phase does not penetrate the paper surface, even in the absence of the said silicone-based barrier layer, (vi) said transformation thus being caused by contact of the emulsion with writing paper, (vii) and, thereby, to form a cohesive rubber phase film having a low adhesion to the surface of the paper, and being capable of complete removal from the paper by rubbing the film with an ordinary pencil eraser, (viii) said emulsion having a viscosity of less than 10 cps or 30 to 300 cps, and (b) a plurality of pigment elements in the emulsion, the elements being characterized by constant association with the rubber phase and with the said film.

Inventors: ABBER HERMAN, SMITH MARK T, ABBER, HERMAN, SMITH, MARK, T.
Applicants: AVERY DENNISON CORP, AVERY DENNISON CORPORATION
Application year: 1993
Main classification: C09D11
IPC codes: C09D11