PIGMENTED AQUEOUS INK COMPOSITIONS AND METHOD

Abstract: Writing instrument ink compositions containing water as a principal ingredient and having excellent line intensity, water-fastness, light-fastness, and exceptional stability and which are prepared by employing a pigment which has been predispersed in an alkali-soluble resin or polymer.

Description: PIGMENTED AQUEOUS INK COMPOSITIONS AND METHOD BACKGROUND OF THE INVENTION Efforts have been made in the past to success- fully formulate ink compositions for writing instruments employing water as a major solvent component because of potential cost savings and a reduction in chemical toxicity concerns. One way to formulate such a product is to simply dissolve a water-soluble dye of the desired color in water and add a suitable thickener to provide the composition with the desired rheologiσal properties required by the type of writing instrument in which the composition is to be used. Such an approach, however, has a number of drawbacks. A major drawback is that the colored trace which is left upon drying of the ink is sensitive to moisture and can be easily smeared by perspiration on the fingers or under conditions of high ambient humidity. Another problem lies in the propensity of many water- soluble dyes to fade with time due to exposure to light. Attempts have been made to solve these problems by using finely divided, water-insoluble pigments along with or in place of the water-soluble dye component. While such compositions can provide an intensely colored trace of improved water fastness and light fastness, they are difficult to formulate into satisfactory inks because they must be subjected to high mechanical shear to deaggregate agglomerates so that the ink vehicle can wet the pigment particles. In addition, it is necessary to include a suspending agent to prevent settling of or reagglomeration of the dispersed pigment particles. Inks of this type are disclosed in Japanese early- disclosure Patent Publication No. 83/80368 in which a ' finely divided pigment component is maintained in a dispersed state in an aqueous ink composition by the use of a suspending agent comprising a water-soluble amine salt of a copolymer comprising 50 mol percent or more of a polymerizable hydrophobic monomer such as styrene along with less than 50 mol percent of acrylic acid or methacryliσ acid. SUMMARY OF THE INVENTION I have discovered that writing instrument ink compositions containing water as a principal ingredient and having excellent line intensity, water f stness, light fastness, and exceptional stability can be pre¬ pared by employing a pigment which has been predispersed in an alkali-soluble resin or polymer. The particulate predispersed pigment is formulated in an aqueous ink along with an alkaline material which solubilizes the resin or polymer encapsulating the pigment thus allowing the pigment to be dispersed into the ink vehicle with simple shear, The alkaline material is selected to be absorbed into the paper or to volatilize after appli¬ cation of the ink to thereby cause the resin component of the predispersed pigment to become water-insoluble, resulting in a water-resistant writing trace. Ink smudges on the skin or fabric stains can be removed, however, simply by aqueous washing with an alkaline detergent which redissolves the resin or polymer, per- mitting removal of the ink composition. DETAILED DESCRIPTION OF THE INVENTION The aqueous inks of this invention comprise dispersions of extremely fine particle-sized conventional pigments which have been formulated in water by simple ■ stirring and without the need for intensive milling, crushing, or grinding. These inks produce intensely colored, waterproof, washable and light-fast writing traces and can be dispensed from fine capillary nib or ball-point writing instruments. In contradistinction to typical aqueous inks based upon dispersed pigments which tend to settle and agglomerate, the ink composition of this invention have been found to be exceptionally stable. The inks of this invention are based upon the use of a particulate, predispersed, resin-pigment combination. Various methods for the production of predispersed pigments are known. They may be produced in the form of chips by heavy-duty milling on two-roll rubber mills to mill dry pigment and resin together to form sheets which are subsequently disintegrated to chips. Alternatively, dry pigment may be ground into a resin solution on, for example, a sigma-blade dough mixer, starting with a highly viscous resin solution and later adding more solvent to form a paste of low viscosity which can easily be removed from the mixer. A third method for the productiαa of predispersed pigments involves grinding in a suitable mixer an aqueous pigment filter cake with a liquid resin or a resin solution in the so-called flushing process to make the pigment leave the aqueous phase and enter into the organic phase, after which any surplus of water is decanted off and the remaining mass ground under vacuum to remove residual water and to get maximal grinding. Unlike conventional dry color milling processes, flushing catches the pigment before it has agglomerated into dry case- hardened particles , a process which once done cannot be undone. Techniques for forming predispersed pigments for use in this invention are well known in the art and are described, inter alia, in U.S. Patents 2,649,382 and 3,925,096. As mentioned above, the resin or polymer to be employed in forming the predispersed pigments for use in this invention must be an alkali-soluble material which is insoluble in neutral or acidic water but which has carboxylic or other acid groups on the polymer chains which can be neutralized with amines or other bases to render the polymer water-soluble when the pre¬ dispersed pigment is stirred into water containing the alkalizing agent. Many such alkali-soluble polymers are known to those skilled in the art and include, for example, polyacrylic acid; copolymers of acrylic acid with materials such as methacrylic acid; styrene; polyvinyl chloride-polyvinyl acetate copolymers; ethyl cellulose; block copolymers which have an ABA, (AB) X or (-AB)- arrangement of A (thermoplastic e.g. styrene) and B (rubbery e.g. butadiene) blocks; the glycerol ester of hydrogenated wood rosin, styrene-maleic acid copoly¬ mers, and modified shellacs. The predispersion of the pigment into the polymer not only simplifies subsequent ink processing but also promotes the wetting of the pigment particles by the polymer in preference to other ingredients contained in the finished ink formulation. This is in contrast to those systems in which the pigment is dis¬ persed into an ink vehicle consisting of polymer, fluids, and additives in which all species are competing for adsorption onto the pigment surface. It is thought that the location of polymer at the pigment surface is responsible for both the improved waterfast-was able performance of these inks and the exceptional dispersion stability resulting from both the ionic nature of the polymer and the entropic effects of the polymer chains emanating from the polymer surface. The predispersed pigments used in the practice of this invention may contain a broad range of proportion of pigment to polymer ranging from about 1:5 to about 5:1 by weight with optimum proportions depending upon the particular polymer used and the particle size and surface properties of the pigment employed. We prefer to use predispersed pigments in which the polymer component is CARBOSET XL-44, manufactured by B. F. Goodrich which is described as an all acrylic copolymer. The proportion of pigment to polymer in the preferred predispersed pigments ranges from about 2:3 to about 5:2. Any of the various pigments that are con¬ ventionally used in the formulation of inks for writing instruments may be likewise employed in the practice of this invention. Among such pigments may be mentioned organic pigments including azo, anthraquinone, condensed polyazo, thio indigo, metal complex, phthalocyanine, dioxazine, and quinacridone pigments, and inorgainc pigments such as carbon black, titanium white,and iron oxide. Specific preferred pigments include Phthalocyanine blue, Phthalocyanine green. Red Lake C, Red 2B, and carbon black. To be suitable for use in the practice of this invention, the final particle size of the predispersed pigment, i.e. the pigment combined with the polymer, should range from about 0.01 micron to about lβO micron with pigments ranging from about 0.02 micron to about 0.1 micron being preferred. In formulating the inks of this invention, from about 4.0% to about 12=0% by weight of the pre- dispersed pigment is stirred into water, preferably deionized, along with from about 0.5 to about 3.0% by weight of a solubilizing alkaline material capable of becoming separated from the pigment by either being absorbed into writing paper or by volatilizing after application of the ink. Such materials include, but are not limited to, triethylamine, ethylenediamine, 2-methylpiperazine, monoamylamine, dibutylamine, n-propylamine, isopropylamine, sodium hydroxide, ammonium hydroxide, triethanolamine, morpholine, and 2-amino-2-methyl-l-propanol. Conventional ink additives may be incorporated into the compositions of this invention to modify the rheological and writing properties of the inks as is well known in the art. These materials include the addition of surfactants such as TWEEN 20 and DOW #193 for the purpose of surface tension modification, sodium Omadine or propyl parasept as fungicides, benzotriazole as a corrosion inhibitor, and humectant materials such as glycols, urea, or thiourea to reduce dry-out problems if the pen is not promptly recapped after use. The following Examples are illustrative of the preparation of the inks of this invention. Example 1 Seventy-one and one-half parts of deionized water, nineteen and nine-tenths parts of ethylene glycol, four parts of triethanolamine, and six-tenths part of TWEEN 20 were added to a Du Pont Sorvall-Omni mixing vessel. Four parts of CVB-330M, a predispersed acrylic resin-pigment preparation produced by Custom Chemicals of Elmwood Park, New Jersey were then added. The CVB-330M pigment preparation consisted of six parts of a styrenated, alkali soluble, acrylic resin and four parts of a red shade, phthalocyanine blue pigment. The composition was mixed for one-half hour in the Sorvall mixer and was then filtered through a 0.8 micron filter to remove coarse residual particles. The filter retained less than 0.25 weight percent of the total pigment preparation present. The resultant ink exhibited a pH of 8.5, a surface tension of 34.5 dyne/cm. and a viscosity of 2.3 cps. at 25°C. The ink was dispensed from pens containing polyester reservoirs through a variety of molded or extruded plastic or fiber pen nibs, as well as rolling ball type points, and provided intense, water-resistant, solvent-resistant, and light-fast ink traces which could be removed from skin with soap and water. The pens containing t e ink wrote out a full ink supply initially as well as after six weeks storage at ambient conditions and 60°C. Inks of similar physical and performance properties were made from predispersed pigment prepara¬ tions containing the same polymer but differing in pigment type including Carbon Black, Red Lake C, and a phthalocyanine green. Example 2 Seventy-one and four-tenths parts of deionized water, fifteen parts of ethylene glycol, and one and six-tenths parts of 2-amino-2-methyl-l-propanol were added to a Sorvall mixing vessel. Twelve parts of Predisol Blue 1460 W, a predispersed acrylic resin- pigment preparation, manufactured by KVK U.S.A. Inc., New Brunswick, New Jersey, were added. The Predisol Blue 1460 W consisted of six parts of phthalocyanine blue pigment and four parts of Carboset XL-44 alkali- soluble acrylic resin manufactured by B. F. Goodrich. The composition was mixed for one-half hour in a 3c:r"-ll mixer with the resultant ink exhibiting a pH of 9.29, a surface tension of 31.2 dynes/cm. and a viscosity of 2.85 centipoise at 25°C. The ink was dispensed from pens containing polyester reservoirs through a variety of pen nibs, and provided intense, water-fast, solvent-fast, and light- fast ink traces which could be removed from skin by washing with soap and water and from cloth by washing with detergent. The ink was freely dispensed from the pen initially and after storage for ten months at ambient conditions. Ink of similar properties were compounded from pigment preparations consisting of the same polymer but containing Red 2B in place of the phthalocyanine blue.

Claims: C L A I M S 1. An ink composition for writing instruments, characterized by comprising a pigment dispersion in an aqueous solution of an alkali-soluble resin and an alkaline material, said pigment and said resin having been introduced together into said solution in the form of a predispersed combination. 2. An ink composition as described in claim 1, characterized in that said predispersed combination has a range of proportion of pigment to polymer ranging from about 1:5 to about 5:1 by weight. 3. An ink composition as described in claim 1 or 2, characterized in that said predispersed combination has a particle size before introduction into said solution of from about 0.01 to about 1.0 micron. 4. An ink composition as described in any one of claims 1 to 3, characterized in that the amount of said pre¬ dispersed combination is about 4.0 to about 12.0% by weight of said ink composition and in which the amount of said alkaline material is about 0.5 to about 3.0% by weight of said ink composition. 5. An ink composition as described in any one of the preceding claims, characterized in that said alkaline material is capable of becoming separated from said pigment by either being absorbed into writing paper or by volatilizing after application of said ink composition. 6. A method of preparing an i k composition for writing instruments comprising combining water, an alkaline material, and a predispersed combination of an alkali-soluble resin and a pigment.

Inventors: FERGUSON EDWARD JOHN, FERGUSON, EDWARD, JOHN
Applicants: GILLETTE CO, THE GILLETTE COMPANY
Application year: 1985
Main classification: C09D11
IPC codes: C09D11