Abstract:
Novel, improved ink compositions are aqueous-based, shock-resistant, shear-thinning inks for writing instruments having a shear index greater than 0.6. The inks are a substantially uniform dispersion including colorant(s), a water dispersible, polymeric shear-thinning material selected from the group consisting of welan gum, rhamsan gum and combinations thereof, and a substantially water-soluble polymeric viscosity modifier in a polar solvent system. While the ink compositions have a high viscosity at low shear rates, they undergo shear-thinning and are readily flowable in response to shear rates produced in writing to provide a writing ink having an especially desirable combination of performance characteristics.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to novel, improved ink compositions and to writing instruments including such ink compositions. More precisely, described herein are aqueous ink compositions including water dispersible, polymeric shear-thinning providing materials and writing instruments including the ink compositions. 
     2. Description of the Related Art 
     Ink compositions including water dispersible, polymeric, shear-thinning providing materials are thickened liquids at rest. Being non-Newtonian liquids, such inks 3exhibit shear-thinning flow behavior or shear-thinning flow characteristics in use. In other words, they become thin, readily flowable liquids at shear rates produced in writing such as with a ball-point pen. The ink compositions include a water dispersible, polymeric shear-thinning providing material dispersed in a polar solvent system. Other materials such as coloring materials, surfactants, preservatives, corrosion inhibitors, humectants and the like can also be included in the inks. Such inks have been disclosed, for example, in U.S. Pat. Nos. 4,671,691 and 5,048,992, the disclosures of which are incorporated herein by this reference. 
     A common material used to provide shear-thinning characteristics to the inks is xantham gum. However, xanthum gum is not stable to high temperatures such as those that might be encountered during processing, storage or transport of the inks or pens containing the inks. 
     It would be desirable to provide aqueous ink compositions that exhibit shear-thinning flow characteristics and that are stable at the relatively high temperatures to which the ink might be exposed; e.g., during processing, storage, transport or by the user. 
     SUMMARY 
     The novel, improved ink compositions described herein are aqueous-based, shock-resistant, shear-thinning inks for writing instruments. The ink compositions exhibit non-Newtonian Flow characteristics and have a shear index greater than 0.6. The inks comprise a substantially uniform dispersion of colorant(s), a water dispersible, polymeric shear-thinning material selected from the group consisting of welan gum, rhamsan gum and combinations thereof, and a substantially water-soluble polymeric viscosity modifier in a polar solvent system. While the ink compositions have a high viscosity at low shear rates, they undergo shear-thinning and are readily flowable in response to shear rates produced in writing to provide a writing ink having an especially desirable combination of performance characteristics. 
     In the writing instruments of the invention, the ink compositions are contained in hollow, non-pressurized or pressurized tubes or reservoirs, and the ink is fed directly to the writing ball of a ball-point pen. The ink can be used in combination with a follower composition to help insure a smooth, continuous flow of ink to the ball as the ink supply is consumed. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present ink compositions include particular gums that are stable to high temperatures (e.g., greater than 100° C.), a water soluble polymeric viscosity modifier, a polar solvent system and a colorant. Other conventional ingredients (e.g., biocides, anti-corrosives, sequestrants, surfactants, etc.) can also be included in the present ink compositions. 
     The ink compositions have a shear index of 0.6 or greater. The shear-thinning index (n) is calculated by fitting shear stress (τ) and shear rate values (γ) obtained from Theological measurements on a viscometer to the empirical power law equation τ=Kγ n  (where K and n are calculated constants). Additionally, the shear-thinning index is obtained by measurements of an aqueous solution of the shear-thinning material at shear rates between about 0 to about 500 sect -1 . 
     The present ink compositions preferably have the following Theological profile: 
     
         ______________________________________Shear Rate (sec.sup.-1)              Viscosity (cps)______________________________________0.01               Less than about 10,0000.1                Less than about 6,0001.0                Less than about 2,00030                 Less than about 2002,000              Less than about 50______________________________________ 
    
     
         ______________________________________Shear Rate (sec.sup.-1)              Viscosity (cps)______________________________________0.01               Less than about 5,0000.1                Less than about 3,0001.0                Less than about 2,00030                 Less than about 2002,000              Less than about 100______________________________________ 
    
     All viscosities reported herein were measured using a Haake RS 100 with controlled stress mode using steady stress programs. 
     The aqueous ink compositions described herein contain welan gum or rhamsan gum or a combination thereof to provide shear-thinning flow characteristics to the ink. 
     Welan gum is the generic name for polysaccharide S-130, a microbial polysaccharide. Welan gum is commercially available, for example, under the designations Welan K1A96 and Welan Ex 7389 from Kelco Co., San Diego, Calif. 
     Rhamsan gum is a heteropolysaccharide resin composed of glucuronic acid, glucose and rhamose and is a fermentate of an alcaligenad microorganism. Rhamsan gum is commercially available, for example, under the designation K1A112 from Kelco Co., San Diego, Calif. 
     Welan and rhamsam gums will maintain the viscosity of the ink compositions to temperatures greater than 100° C. Such temperature stability is unachievable in conventional shear-thinning inks made using xanthum gum. The amount of welan and/or rhamsan gums present in the ink compositions will vary depending on a number of factors including the colorant employed and the nature of the polymeric viscosity modifier used. Normally, however, the welan and/or rhamsan gum will be present in an amount from about 0.1 to about 1.0 percent by weight of the entire composition, preferably about 0.1 to 0.3 weight percent. 
     The present ink compositions also include a viscosity modifier. The viscosity modifier is a substantially water soluble polymeric material that increases the high shear viscosity of the ink. Suitable viscosity modifiers include alginates (e.g., Kelgin MV from Kelco Co.), alkylene oxide polymers and copolymers (e.g., Polyox N-10 from Union Carbide) and polyvinylpyrrolidone (e.g., PVP K-90 from GAF Corp). Combinations of the foregoing can also be used. The viscosity modifier is present in the ink composition in an amount from about 0.1 to about 10 weight percent based on the weight of the entire composition. Preferably, from about 0.1 to about 1.0 weight percent of the viscosity modifier is employed. 
     The present ink compositions are based upon the use of a solvent system containing polar, non-aqueous liquids and water. Water can be used at levels of less than 50% of the solvent system. Suitable non-aqueous liquids include: (1) mono, di, tri, and higher alcohols (2) glycol ethers, (3) glycol ether acetates, and (4) glycol acetates. For a dye based ink, the solvent system constitutes from about 40 to about 90 weight percent of the ink, preferably from about 50 to about 75 weight percent of the ink. 
     Non-limiting examples of alcohols suitable for use in the solvent system are hexylene glycol, octylene glycol, 1,4 butanediol, thiodiglycol, 2,3-butyulene glycol, triethylene glycol, diethylene glycol, ethylene glycol, propylene glycol, glycerin, trimethylolethane, trimethylopropane, diglycerin, isopropyl alcohol, and sorbitol. 
     Non-limiting examples of glycol ethers suitable for use in the solvent system are ethylene glycol monoethylether, ethylene glycol monobutylether, diethylene glycolmonoisopropylether, propyleneglycolmonomethylether, diethyleneglycol, monomethylether, propylene glycol phenylether, ethylene glycol diethyl ether, and propylene glycol phenyl ether. 
     Non-limiting examples of glycol ether acetates suitable for use in the solvent system are propylene glycol mononethylether acetate; ethylene glycol monomethylether acetate, ethylene glycol monophenylether acetate, diethylene glycol monomethylether acetate, diethyulene glycol monoethylether acetate, diethylene glycol monobutylether acetate, diethylene, glycol monophenylether acetate, diethylene glycol monoisopropylether acetate, ethylene glycol diethylether acetate, diethylene glycol dimethylether acetate, and propylene glycol monomethyl ether acetate. 
     Non-limiting examples of glycol acetates suitable for use in the solvent system are ethylene glycol monoacetate ethylene glycol diacetate, and diethylene glycol diacetate. 
     The colorants to be used in the present ink compositions can be any of the water-soluble or dispersible dyes or pigments or mixtures thereof known to be useful in writing inks. From about 2 to 10%, and, preferably from about 4 to 6% by weight of a dye and/or from about 12 to 35%, preferably from about 18 to 22%, of a pigment dispersion is added to the ink formulation to obtain the desired color. Suitable colorants are available under the designation Bayer Special Black HF from Bayer, Pittsburgh, Pa., Violet 204-2 from Miliken Chemicals and Blue 204-3 from Miliken Chemicals. 
     Sequestrants may be added for stabilizing components of the ink, e.g., the gum component. A sequestrant binds multivalent cations in the solution, enhancing solubility and stability of the gum. Suitable agents for this purpose are sodium salts of EDTA, such as tetrasodium EDTA or Versene 220 of Dow Chemical, trisodium phosphate, sodium hexametaphosphate, or sodium glucoheptanate. These sequestrants are normally added in the amount of 1-5 parts by weight of the gum. 
     Many ink components are subject to degradation by microorganisms such as bacteria, yeast, and mold. Such degradation is prevented by the addition of a biocide. Suitable biocides are known to those skilled in the art and include Methyl Parasept and Ethyl Parasept available from Bayer and Kathon PFM available from Rohm &amp; Haas. Combinations of these materials may also be used. Generally from 0.10 to 0.20%, preferably about 0.10% to about 0.5% of bacteriocide (which may contain some water) is added to the ink formulation to control the growth of microorganisms. 
     These inks containing water and polar non-aqueous liquids such as glycol can be corrosive to metals used in the roller ball and socket of pens. This corrosivity can be controlled or eliminated by the addition of a corrosion inhibitor, for example benzotriazole, tolytriazole, dicyclohexyl ammonium nitrate, and organic phosphate esters. Cobratec 99 available from PMC Specialties. Generally from about 0.10 to 1.0%, preferably from 0.13 to 0.20% by weight of a corrosion inhibitor is added to the ink formulation to prevent corrosion of the tip by the ink. 
     To ensure that the ink be capable of wetting the metal used in the tip of the pen to provide an acceptable delivery rate of ink to the writing surface, wetting agents or lubricants can be added to the inks described herein. Suitable wetting agents include, but are not limited to, surfactants such as sodium lauryl sulfate and fluorinated alkyl esters. Suitable surfactants are available, for example, under the designation Ethomeen C-15 from Akzo, Dobbs Ferry, N.Y.; Lubriphos LB 400 from Rhone Poulenc, Inc.; and Atlas G-711 from ICI Ame. Inc. One skilled in the art will appreciate that selection of the appropriate wetting agent should be based upon actual writing tests in the type of pen having the type of writing tip in which the ink is to be used. In general, however, in a dye or pigment based ink the amount of wetting agent is from about 0.10 to 1.0%, preferably 0.14 to 0.16% by weight. 
     The present inks can also contain a water soluble additive to improve the water-fastness of the ink. For example, an acrylic resin that is alkaline soluble will, upon drying, bind the colorant in the written script and therefore provide a water-fast script. The acrylic resin binder can also improve the rewettability of the residue on the pen tip and therefore greatly prolong capoff time and starting ease of the ink in the pen. A suitable amine-modified acrylic copolymer is available under the designation G-Cryl 5006 from Henkel Corp. 
     The pH of the finished ink is preferably higher than 5.0, and generally ammonia or an amine such as triethanolamine, morpholine, diethanolamine, ethylamine, or monoethylamine is added to the formulation to obtain the desired pH value. Generally, in a dye or pigment-based ink the amount of pH control agent added is from about 0.05 to 0.50%, preferably from about 0.09 to 0.11% by weight. 
     The ink compositions of this disclosure are easily prepared by combining the desired ingredients in a blender or mixing with a propeller stirrer until homogeneity is achieved. Preferably the polymeric shear-thinning providing material is first added to distilled water and this combination mixed for about two hours. The other ingredients, if present, are added in the following order: other polar solvents, sequestering agents, preservatives, surfactants, corrosion inhibitors, and dyes and/or pigments. In some cases, it is advisable to pre-wet the shear-thinning providing material with a material such as ethylene glycol, propylene glycol or glycerol. 
     Table 1 below shows the percent ranges of ink components based upon total weight of the ink. 
     
                       TABLE 1______________________________________INGREDIENT       RANGE OF CONCENTRATION______________________________________Welan or Ramsan Gum            0.1-0.3%Viscosity Modifier            0.1-0.3%(e.g. Sodium Alginate)Acrylic Resin    1-10%Organic Amine    0.5-6%Surfactant       0.2-1.0%Sequestering Agent            0.1-0.5%Anticorrosive Agent            0.1-0.5%Biocide          0.1-0.5%Smoothness Additive            0.5-1.0%Colorant (pure color)            2-10%Non-Aqueous Polar Solvent,            20-60%(e.g., Glycol)Water            20-90% (inc. H.sub.2 O in other            components)Total H.sub.2 O Content %            20-90%H.sub.2 O % in Total Solvents            30-80%Other (Solids) % 15-20%______________________________________ 
    
    
    
     EXAMPLES 
     The following examples are presented to illustrate specific embodiments of the present compositions. These examples should not be interpreted as limitations upon the scope of the invention. 
     Example 1 
     As specific ink composition is prepared having the formulation given below. Ranges for each ingredient are also listed. 
     
         ______________________________________COMPOUND       % WT       % RANGE______________________________________Water          34.00      10-40%Benzyl Alcohol 2.05       0-5%G-CRYL 5006    4.00       0-8%Triethanol Amine          4.73       Adjust PH 7.0-8.5Whelan Gum - K1A96          0.20       0.1-0.3%Na Alginate-Kelgin MV          0.10       0.1-0.5%Ethylene Glycol          19.27      15-40%Glycerine      4.00       3-10%Lubriphos LB400          0.70       0.3-0.0%Versene 220    0.25       0.2-0.5%Corbratec 99   0.20       0.1-0.5%Methyl Parasept          0.05       0.05-0.10%Ethyl Parasept 0.07       0.05-0.10%KH.sub.2 PO.sub.4          0.50       0.1-1.0%Violet 204-2   22.28      20-30%Blue 204-3     7.60       5-15%______________________________________ 
    
     Examples 2-5 
     The following Table II provides ink formulations in accordance with this disclosure: 
     
                       TABLE II______________________________________      Ex. 2  Ex. 3    Ex. 4    Ex. 5______________________________________Water        37.81    37.78    27.00  27.05Ethylene Glycol        32.44    32.15    35.00  35.00Glycerine    5.65     5.80     6.63   5.95Triethanol Amine        0.11     --       --     0.05Ethomeen C-15        0.39     0.39     0.15   0.10Welan K1A96  0.12     0.22     0.14   0.25PVP K-90     --       --       0.88   1.40Kelgin MV    0.11     0.21     --     --Versene 220  0.23     0.23     0.34   0.34Cobratec 99  0.22     0.21     0.22   0.22Methyl Parasept        0.06     0.05     0.07   0.07Ethyl Parasept        0.07     0.07     0.08   0.08Kathon PFM   0.10     0.11     0.15   0.15Bayer Special Black HF        22.69    22.76    --     --Violet 204-2 --       --       21.86  21.86Blue 204-3   --       --       7.48   7.48Shear Thinning Index        0.67     0.62     0.71   0.61Viscosity @ 0.3 S.sup.-1        920      4,200    1500   6,20Viscosity @ 10 S.sup.-1        170      450      200    500Viscosity @ 100 S.sup.-1        40       100      50     120Viscosity @ 500 S.sup.-1        20       50       30     60______________________________________ 
    
     Examples 6-10 
     The ink formulations in Table III are prepared in accordance with the present disclosure. 
     
                       TABLE III______________________________________      Ex. 6 Ex. 7   Ex. 8   Ex. 9 Ex. 10______________________________________Water        28.47   28.72   28.19 27.73 20.21Ethyleneglyaol/        32.74   34.46   33.17 32.29 36.26Propylene GlycolGlycerine    5.92    3.49    5.38  6.29  8.74Lubriphos LB400        0.74    0.74    0.73  0.73  0.73Welan gum-EX 7389        0.18    0.175   0.21  --    --Kelgin M.V.  0.20    0.205   --    0.79  --PVP K-90     --      0.5     1.08  0.69  1.73Polyox N-10  --      --      --    --    0.99Triethanol   0.77    0.77    0.76  0.76  0.76AmineCobratec 99  0.20    0.22    0.22  0.22  0.22Versene 220  0.34    0.34    0.34  0.34  0.33Methyl Parasept        0.07    0.07    0.07  0.07  0.07Ethyl Parasept        0.08    0.08    0.08  0.08  0.08Kathon       0.12    0.12    0.12  0.17  0.15KH.sub.2 PO.sub.4        0.69    0.69    0.66  0.68  0.68Violet 204-2 21.98   21.92   21.60 21.73 21.64Blue 204-3   7.50    7.50    7.39  7.43  7.41Shear Thinning        0.63    0.62    0.65  0.85  0.95Index______________________________________ 
    
     
                       TABLE 4______________________________________  Ex. 11        Ex. 12  Ex. 13  Ex. 14                              Ex. 15                                    Ex. 16______________________________________Ethylene 37.39   38.00   38.00 38.00 50.00 45.00GlycolGlycerine    13.71   12.35   12.35 12.35 15.00 5.00PVP K-90 2.88    --      2.00  2.00  1.00  1.50Alginate-    --      0.40    --    --    --    --Kelgin MVWelan gum    0.07    0.09    0.09  0.09  0.10  0.10K1A96Water    17.15   20.33   17.59 17.59 3.72  13.22Kathon PFM    0.14    0.15    0.10  0.10  0.10  0.10Atlas G-711    --      --      --    0.15  --    --Eastman EB    --      --      --    --    --    5.0Ethomeen C-15    0.12    0.15    0.15  --    0.13  0.13Cobratec 99    0.21    0.20    0.22  0.22  0.20  0.20Versene 220    0.30    0.50    0.30  0.30  0.30  0.30Methyl   0.07    0.06    0.07  0.07  0.07  0.07ParaseptEthyl    0.08    0.07    0.08  0.08  0.08  0.08ParaseptViolet 204-2    20.78   21.00   21.65 21.65 21.85 21.85Blue 204-3    7.11    7.00    7.40  7.40  7.45  7.45Total Water    39.46   42.73   40.83 40.83 27.16 36.66Total Solvents    51.10   50.35   50.35 50.36 65.00 55.00% Water in    43.57   45.91   44.78 44.78 29.47 39.99Total Solvents______________________________________ 
    
     It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.