Sympathetic Ink for Ink Jet Printer

1. Field of the Invention

The present invention is broadly concerned with an improved aqueous sympathetic ink composition particularly adapted for use with any non-impact printers using liquid inks, e.g., ink jet printers. More particularly, the invention pertains to such a composition, and a corresponding method of use, wherein the ink composition preferably includes therein a water soluble transition metal salt and an amount of n-butanol. The ink of the present invention is designed to dry as an essentially invisible image on a substrate, which can later be developed by application of a color precursor developer.

2. Description of the Prior Art

Modern day sympathetic ink systems of commercial importance are designed for a variety of uses, for example in self-testing educational booklets. In such uses, a student may be provided with a multiple choice answer sheet, with the correct answer being indicated by appropriate latent image indicia. In selecting an answer, the student marks one of the multiple choice possibilities with the developer substance; if his selection is correct, the latent indicia will reveal the student's correct choice and hence his progress. Other uses include certification testing, personnel selection and screening tests, novelty items such as games and toys, games of chance such as lotteries, children's books designed for educational and/or entertainment purposes, securing of documents, prize verification and promotional items.

Such sympathetic ink systems typically involve application of a latent image by means of a normally invisible printing ink. Thereafter, a developer substance is applied over the invisible image and a color-forming chemical reaction occurs to "develop" the image and render the same visible.

U.S. Pat. Nos. 4,631,203 and 4,784,876 describe particularly useful sympathetic ink/developer systems which give sharp, long-lasting, non-smearing images. However, these systems are designed for use with conventional lithographic, letter press, letterset, flexographic, silk screen or rotogravure printing equipment and hence the ink systems thereof are typically rather thick and viscous.

In many applications, however, it would be advantageous to employ non-impact printers such as ink jet printing equipment in the production of latent imaged items. Typical sympathetic ink systems of the type described in the aforementioned patents are not suitable for use in ink jet printers, because they have inappropriate viscosity and surface tension properties. As a consequence, it has heretofore been impossible to employ ink jet printers to give latent imaged products, even though the flexibility and printing characteristics inherent in such equipment would otherwise make it ideally suited for this application.

Broadly speaking, the ink compositions of the present invention are adapted for use in ink jet printers and comprise an aqeous dispersion including therein from about 4 to 20% by weight of a water soluble transition metal salt, together with of from about 2 to 4.5% by weight of n-butanol or 2-butanol. A particularly perferred salt is nickel sulfate.

In application procedures, an ink of the character described is employed in conjunction with otherwise conventional non-impact printing equipment, and the ink composition has viscosity and surface tension values appropriate for ink jet printing. If ink jet equipment is employed, the process includes forming droplets of the ink (typically having an average particle size on the order of from 6 to 10 mils.), electrostatically charging such droplets, and directing at least certain of the nondeflected charged droplets onto a substrate to form an image thereon which is invisible to the naked eye. Charged droplets not intended for substrate application are electrically deflected away from the substrate.

Preferred sympathetic ink compositions in accordance with the present invention include from about 4 to 20% by weight of a water soluble salt selected from the group consisting of the sulfates, nitrates, halides and acetates of cobalt, nickel and copper, and mixtures thereof, and more preferably of from 6 to 12% by weight of the salt. Additionally, the inks include of from about 2.0 to 4.5% by weight of n-butanol or 2-butanol, and more preferably of from about 2.2 to 4.4% by weight thereof. The balance of the inks hereof is normally water, although other substances may be added if desired.

The most preferred ink composition is formulated by first mixing 7,296 ml of water with 1,028 g. of nickel sulfate pentahydrate, followed by adding 304 ml of n-butanol. This corresponds to about 12% by weight of nickel sulfate, and about 2.9% by weight of n-butanol in the composition.

As indicated previously, the ink compositions of the invention should possess viscosity and surface tension properties enabling use of the compositions with ink jet printing equipment. To this end, the compositions of the invention should have viscosities in the range of from about 0.9 to 1.2 cps., and surface tensions of from about 30 to 50 dynes/cm.

After application of the inks of the invention using ink jet printing equipment, the resultant printed substrate includes images thereon which are and remain normally invisible to the naked eye for an indefinite period, and at a minimum period at least about three months. When it is desired to develop the latent images, it is necessary to apply a color-forming developer over the previously imprinted images. A variety of such developers may be used with the inks of the invention, and in particular the developers described in U.S. Pat. Nos. 4,631,203 and 4,784,876, both of which are incorporated by reference herein. The most preferred developer is the type described in the '876 patent, and includes of from about 50-90% by weight water, of from about 1-5% by weight dimethylglyoxime, and a minor amount of a base. Such a developer, when used in conjunction with a nickel-based sympathetic ink, gives a stable, long-lasting, sharp color image which is red in color. The '203 patent describes other developer systems useful in the context of the present invention, such developers including oxalic acid or a derivative thereof. Typically, the selected developer can be packaged as a liquid pen, as to facilitate application of the developer substance over latent images.

The inks of the invention may be used to imprint a variety of substrates, and indeed the only substrate limitation derives from the ink jet printing equipment itself, i.e., any substrate capable of being imprinted using this equipment may be imaged with the sympathetic inks of the invention.