Ink jet printing, for example, drop-on-demand ink jet printing, involves forming characters on a substrate by ejecting ink droplets from a printhead having one or more nozzles. The inks may be hot melt inks that are solids at room temperature but in a liquid state at jetting temperatures. Hot melt jet inks can be partially or fully worn off the surface of a substrate after they solidify in response to, for example, rubbing, scratching, or flexing.

Ink jet printers may be used for printing codes (e.g., manufacturing and/or expiration dates, lot number, manufacturing location) on packaging materials for consumer products like candy bars, milk cartons, and other food products, as well as directly on some types of products such as PVC pipe, electrical wiring, plastic bottles, etc.

Summary of the Invention The invention relates to inks for ink jet printing that provide a substantially permanent color on a substrate even when the ink vehicle wears off the substrate.

There are a number of aspects to the invention. In one aspect, the invention features a hot melt jet ink printing process that involves using a hot melt ink in which the colorant is capable of locking to a substrate. The colorant is substantially soluble in the ink when the ink is in a liquid state but relatively insoluble in the ink when the ink solidifies. When the ink solidifies after printing on the surface of the

substrate a sufficient quantity of colorant migrates from the ink to the substrate surface and locks to the substrate surface to provide it with a substantially permanent readable color. "Locking" to the substrate surface, as used herein, means that the colorant attaches to the substrate material (e.g., by hydrogen bonding, ionic bonding, acid base interaction, van der Waals forces, London dispersion etc.) so that a substantially permanent color is obtained on the substrate surface under the ink.

"Substantially permanent" color, as used herein, means that the color does not significantly lose resolution (e.g., from feathering), after the ink vehicle (and components included within the ink vehicle that have not locked to the substrate surface) is rubbed off the substrate within three months of printing, for at least six months (and preferably at least one year) when stored under ambient conditions in the dark. Ambient conditions are, for example, room temperature (22°C - 25°C) and atmospheric pressure.

"Readable color", as used herein, means a color that is unambiguously readable by someone with normal eyesight under normal light conditions.

In another aspect, the invention features a printed product including a substantially permanent readable color indicium. The product includes a substrate, and a solid ink defining an ink mark indicium on the surface of the substrate. The ink includes a colorant, and the product further includes a sufficient quantity of the colorant locked to the substrate surface to provide a substantially permanent readable color indicium generally corresponding to the ink mark indicium.

In another aspect the invention features a hot melt ink composition that is solid at ambient

temperatures but melts at an elevated temperature. The hot melt ink includes a vehicle and a colorant that is substantially soluble in the ink when the ink is in a liquid state but relatively insoluble in the ink at ambient conditions. When the ink is ejected onto a polyamide surface, a sufficient quantity of the colorant migrates and locks to the substrate surface to provide a substantially permanent readable color.

In another aspect, the invention features a printing process that includes printing a liquid ink onto a surface. The ink includes a vehicle and a colorant, and the colorant is substantially dissolved in the liquid ink. The process further includes solidifying the ink on the surface. Solidification may involve cooling the liquid ink if the ink, for example, is a hot melt ink, or curing the liquid ink if the ink includes monomers or prepolymers that are curable, for example, by UV light, heat, and/or other agents necessary to cause curing. The colorant migrates from the ink to the surface and locks to the surface to provide a relatively permanent readable mark.

In another aspect, the invention features a hot melt ink composition including an oil soluble anthraquinone dye and a non-polar wax vehicle in which the dye is relatively insoluble at ambient conditions.

In another aspect, the invention features a method for marking coded characters (e.g., dates, location, lot number, etc.) on a surface of a packaging material (e.g., a wrapper) used to package a product, or on the surface of the product itself. The method includes printing a liquid ink onto the surface in the form of coded characters. The ink includes a vehicle and a colorant substantially dissolved in the liquid ink. The ink adsorbs and solidifies on the surface. The colorant is relatively insoluble in the solidified ink, and the

colorant provides substantially permanent readable color generally corresponding to the coded characters on the surface.

Inks that can provide a substantially permanent readable mark on a substrate can be designed by selecting a colorant that locks to a particular type of substrate surface and by selecting a vehicle in which the colorant is relatively insoluble when the ink solidifies. The surface may be, for example, polyamide, nitrocellulose, polyester, ionomeric, ethylene vinyl acetate, polyvinyl acetate, polyurethane, polyvinylidene chloride, polyvinylidene chloride/aerylonitrile, polystyrene, aerylate, polypropylene, polyethylene, or mixtures thereof. The inks may exhibit low shrinkage on solidification and, if not worn off, provide a sharp image with high resolution.

Other features and advantages of the invention will be apparent from the description of the preferred embodiment thereof, and from the claims. Description of the Preferred Embodiment

The preferred ink is a hot melt ink that includes a wax vehicle, an antioxidant, and a colorant. The ink preferable has a melt viscosity of at least 5 cps, preferably at least 10 cps, and more preferably from 15 cps to 25 cps, at a temperature of at least 105°C, preferably from 115°C to 165°C, and more preferably from 130°C to 140°C.

The wax vehicle is the carrier for the colorant. Typically the wax vehicle contains a blend of waxes that provide the ink with the appropriate melting point, toughness, flexibility, substrate adhesion, etc.. Preferably the wax vehicle has a melting point generally lower than the temperature at which the ink jet printer operates. The ink preferably includes between about 50% and about 98% of the wax vehicle by weight.

Examples of waxes include stearic acid, lauric acid, linear polyethylene, behenic acid, stearone, carnauba wax, microcrystalline waxes, paraffin waxes, polyethylene wax, candelilla wax, montan wax, Fischer- Tropsch waxes, bisamide waxes, amide waxes, hydrogenated castor oil, synthetic ester waxes, oxidized polyethylene waxes, oleamides, stearamides, lauramides, erucamides, glycerol esters, chlorinated waxes, urethane modified waxes, and other synthetic and natural waxes. The wax vehicle is selected so that, taking into account the properties of the other components of the ink, the colorant is soluble in the molten ink but relatively insoluble in the solidified ink. As a general rule, polar colorants may be insoluble in solidified non- polar vehicles, and non-polar colorants may be insoluble in solidified polar vehicles. Thus, if the colorant is polar the wax vehicle preferably is substantially non- polar.

The antioxidant inhibits oxidation of ink components. Sufficient antioxidant should be included to inhibit oxidation, but not so much should be included that the other properties of the ink are adversely affected. The ink preferably includes between about 0.1% and about 5%, more preferably between about 0.5% and about 3%, of the antioxidant by weight. Suitable antioxidants include those conventionally used in the art, for example, dibutyl hydroxy toluene compounds.

The colorant provides the substantially permanent opaque color to the substrate when the ink is printed on the substrate. The colorant should lock to the substrate material on which the ink is intended for use. Preferred colorants include polar dyes like oil-soluble anthraquinones (e.g., Nitrofast Blue 2B) that have groups available for locking to the substrate surface. The ink should include a sufficient quantity of colorant so that

the ink has adequate color, and so that adequate colorant is available to migrate from the solidified ink to the substrate surface. Preferred inks include between about 0.1% and about 10%, more preferably between about 1% and about 3%, of the colorant by weight.

The ink optionally may include other conventional hot melt ink ingredients such as resins and flexibilizers/plasticizers. Examples of resins include aliphatic hydrocarbons, aromatic hydrocarbons, ethylene vinyl acetate, rosin derivatives (includes gum, Tall oil, and wood rosins) , polyamides, and other synthetic and natural resins. Examples of flexibilizers/plasticizers include aromatic sulfonamides, phthalates, acetates, adipates, amides, azelates, epoxides, glutarates, laurates, oleates, sebacates, stearates, sulfonates, tallates, and tri ellitates. A sufficient quantity of an optional ingredient may be included in the ink to provide the desired property.

The hot melt inks generally are prepared by combining the wax vehicle components and the antioxidant, heating the combination to its melting point, and slowly stirring until the liquified combination is homogeneous. The colorant then is added to the mixture with stirring. The molten ink then is filtered to remove particles larger than 1 micron in size.

A hot melt ink was prepared that included 20 parts Paraffin SP Wax 130/135 (available from Strahl and Pitsch) , 38 parts Polywax 500 (available from Petrolite Polymers) , 42 parts AC 617 (available from Allied Signal) , 1 part Igranox 1010 (available from Ciba-Geigy) , and 1 part Nitrofast Blue 2B (available from Sandoz) . The AC-617 (viscosity 180 cps at 135°C) is a homopolymer of polyethylene and provides toughness to the wax blend. The Polywax 500 (viscosity 3.8 cps at 135°C) also is a homopolymer of polyethylene and reduces the brittleness

of the Paraffin SP Wax (viscosity 3 cps - 5 cps at 135°C) .

Other embodiments are within the claims.

What is claimed is: