Buerkley, Donald D. (Inver Grove Heights Village, MN)
Lange, Heinz E. (St. Paul, MN)

05/169524

11/26/1974

08/05/1971

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Minnesota Mining and Manufacturing Company (St. Paul, MN)

283/95

106/31.180, 101/135, 283/114, 106/31.240, 434/328, 101/450.100, 106/31.290, 283/901

B41M3/00; C09D11/00; C09D11/00

106/14.5,21-22,27,32 117/36.2-36,36.6-36.9 252/62.1 35/9G

3363337	Method and devices for teaching writing skills	January 1968	Skinner et al.
3451143	SPIRIT DUPLICATION WITH VISIBLE AND CONCEALED IMAGES	June 1969	Thomas et al.
3666502		May 1972	Erickson

Schofer, Joseph L.

Gron T. S.

Alexander, Sell, Steldt & DeLaHunt

What we claim is

1. A quick set ink for providing printed, invisible entries on an absorbent offset grade paper-like sheet, said quick set ink comprising:

2. at least 10% by weight of a color-forming component consisting essentially of a solid particulate polyhydroxyaromatic iron-complexing color-forming reactant having the formula: ##SPC1##

3. at least 40% by weight of a color stable, two phase, quick set vehicle free of metallic driers having tack, viscosity, hydrophobicity, and pigment carrying capacity suitable for use in conventional wet lithographic offset printing, and

4. 0-3% by weight of wax; said quick set ink being hydrophobic and having a viscosity in the range of 300-3,500 poise at room temperature, the most volatile component in said quick set ink having a boiling point greater than 200°C.

5. A quick set ink according to claim 1 wherein said color-forming reactant is a gallic acid ester of an aliphatic alcohol containing 6 - 18 carbon atoms.

6. A quick set ink according to claim 1 wherein said composition further comprises a second iron-complexing compound capable of forming a stable, substantially colorless iron-containing complex.

7. A quick set ink according to claim 1 wherein said composition contains an aliphatic hydrocarbon oil having a boiling point greater than 200° C. and a Kauri-butanol value less than 28.

8. A quick set ink according to claim 1 wherein said quick set vehicle comprises a polyurethane alkyd having a viscosity greater than 500 poise.

9. A printed form comprising:

10. an offset grade, absorbent paper-like sheet containing less than 0.05% iron, based on the weight of said sheet,

11. a printed visible entry, and

12. a dry printed invisible entry derived from printing on said sheet a quick set ink comprising:

13. A printed form according to claim 6 wherein said paper-like sheet has a Sheffield smoothness number greater than 160 and an iron content of less than 0.03% based on the weight of said sheet.

14. A printed form according to claim 6 wherein said paper-like sheet comprises a fiber mixture containing a random distribution of fluorescent fibers.

15. A printed form according to claim 6 comprising a plurality of said printed invisible entries, one of said printed invisible entries being a solid block covering at least about 1% of the area of said paper-like sheet.

16. A printed form according to claim 6 comprising a plurality of said printed invisible entries, at least one of said entries being an image with informational content surrounded by a discontinuous pattern of dots covering up to about 15% of the area surrounding said image, said dots comprising said color-forming reactant and said quick set vehicle.

17. An invisible quick set ink for providing printed, invisible entries on an absorbent offset grade paper-like sheet, said quick set ink comprising,

18. 20-35% by weight of a color-forming component consisting essentially of a particulate gallic acid ester of an alkanol of 6-18 carbon atoms, said particulate ester having a numerical average particle size in the range of 0.5- 5 microns,

19. at least 55% by weight of a flowable color-stable two phase, quick set vehicle free of metallic driers having tack, viscosity, hydrophobicity, and pigment carrying capacity suitable for use in conventional wet lithographic offset printing,

20. at least 0.5% by weight of an iron-complexing agent comprising citric acid,

21. an aliphatic hydrocarbon oil having a boiling point greater than 225°C. and a Kauri-butanol value less than 28,

22. 0-3% by weight of a particulate wax, and

23. 0-5% finely divided silica; said quick set ink being hydrophobic and having a viscosity in the range of about 300 to about 3,500 poise at room temperature, the most volatile component in said ink having a boiling point greater than 200°C. and components (1)-(6) of said ink being sufficiently well blended to prevent settling or separation of any of said components.

24. An invisible quick set lithographic offset ink for providing printed invisible entries on an absorbent offset grade paper-like sheet, said quick set ink comprising;

25. at least 40% by weight of a flowable color-stable two phase, quick set vehicle free of metallic driers having tack, viscosity, hydrophobicity, and pigment carrying capacity suitable for use in conventional wet lithographic offset printing, and, dispersed throughout said quick set vehicle,

26. at least 10% by weight of a color-forming component consisting essentially of a light colored, solid, particulate, water-insoluble color-forming reactant having a numerical average particle size within the range of about 0.5 to about 5 microns which reactant is capable of forming a strongly colored complex with a coreactant metal salt, said color-forming component being substantially free of any said correactant metal salt, said color-forming reactant being sufficiently dispersed throughtout said quick-set vehicle to prevent any substantial setting of said color-forming reactant, and said quick set ink being hydrophobic and having a viscosity in the range of about 400 to about 3000 poise at room temperature, the most volatile component in said quick set ink having a boiling point greater than 200°C.

FIELD OF THE INVENTION

This invention relates to the printing of latent, i.e. concealed or substantially invisible, images comprising a color-forming reactant which can be rendered visible with a coreactant metal salt or the like. An aspect of this invention relates to educational aids or tools such as work sheets, self-answering examination sheets, and other paper-based feedback systems. A further aspect of this invention is that these educational aids can contain visible printed material also, generally as statements or questions to which the latent material provides "feedback" or answers.

DESCRIPTION OF THE PRIOR ART

Most of the development in the art of latent image printing has centered around educational uses. Examples of recently developed paper-based systems for teaching skills through the development or revelation of latent or concealed images are: U.S. Pat. Nos. 3,363,337 (Skinner et al.), issued Jan. 16, 1968; 3,451,143 (Thomas et al.), issued June 24, 1969; and 3,516,177 (Skinner), issued June 23, 1970. The most promising prior art systems appear to be those which utilize an "invisible ink" (i.e. ink containing a substantially colorless dye precursor or color-forming reactant) which can be printed on a paper-like sheet and developed at any suitable time with a color-forming coreactant. Typical invisible inks are disclosed in the aforementioned Skinner et al. Pat. No. 3,363,337, and in U.S. Pat. No. 3,438,927 (Ehrlich), issued Apr. 15, 1969.

Despite the rapid development of the latent printing art, many problems remain to be solved. A problem which is often encountered with prior art invisible inks is a phenomenon called "clueing". Clueing occurs when a latent image is not, or ceases to be, totally beyond detection by the naked eye. Typical causes of clueing include: (1) any distinction between the background areas and the latent print areas, e.g. in light reflectance or color (some invisible ink compositions are subject to yellowing with age, resulting in a contrast with a white paper background), and (2) inadvertent development of the latent image by impurities in the ink, the paper, or the printing equipment.

Even if this clueing problem is minimized, slow and inefficient development of the latent image may still be a problem, particularly when the more convenient developing means are used, e.g. imaging crayons comprising an iron salt in a wax matrix. Many educational aids require more or less instant feedback of the answer. Long reaction times for converting the latent, substantially colorless image to a colored, visible reaction product hinder the attainment of this requirement.

A further problem is the need for convenient printing methods. Spirit duplication (see the Thomas et al. patent, previously cited) is suitable only for short runs -- up into the tens or hundreds of copies. Letterpress or dry offset (see the previously cited Skinner et al and Skinner patents) are suitable only for very long printing runs. The high production printing method of choice for many types of educational materials (particularly those in need of frequent revision) is lithographic (wet) offset, i.e. the conventional offset technique which takes advantage of the incompatibility of aqueous and oleoresinous media.

To date, the art has apparently not developed an invisible (latent print) ink suitable for conventional lithographic offset printing methods, the available invisible inks being generally high volatility, low viscosity, low solids-content, at least partially hydrophilic compositions suitable for printing methods other than lithographic offset. Possible reasons why a lithographic latent print ink has not hitherto been available are: the inability to use existing offset presses without extensive modification; the inability to find a vehicle and/or a color-forming reactant adaptable to conventional lithographic offset techniques; the inability to avoid or design around undesirable chemical side reactions caused by conventional metal salt "driers", particularly in view of the tendency of phenolic color-forming reactants to inhibit oxidation and/or polymerization of conventional offset vehicles; the inability to use 2-color offset presses, thus necessitating a plurality of printing operations for visible and invisible textual material; and the inability to prevent invisible ink penetration and migration without resorting to resin-coated, low porosity, or other high reflectance sheets which can cause clueing (see the previously cited Ehrlich patent).

Accordingly, this invention contemplates a solution to these problems through, inter alia, the formulation of a latent print ink suitable for conventional lithographic offset techniques and the selection of a suitable paper for the printing surface.

Briefly, this invention involves (1) milling a suitable solid polyhydroxyaromatic compound having the ability to complex iron salts to form a strongly colored complex (e.g. an ester of gallic acid and a C ₆ - C ₁₈ alcohol), the milling being carried out until this compound is in the usual ink pigment size range (i.e. until the numerical average particle size lies in the range of 0.5 - 5 microns), preferably when the milling elements contain iron or steel, in the presence of an iron inhibitor such as citric acid, and (2) dispersing the milled iron-complexing, color-forming, polyhydroxyaromatic compound in a color stable (i.e. non-yellowing) lithographic quick set vehicle. The resulting viscous lithographic ink can be extended or modified in viscosity through the use of an aliphatic hydrocarbon oil, preferably a petroleum distillate or the like which boils at a temperature of at least 200°C. and preferably higher than 225°C. The ink can be further modified with flatting agents and rub resistance agents such as particulate (e.g. fumed or precipitated) silica. It is also preferable to add minor amounts of wax and polar organic additives to the ink to adjust tack and consistency and provide faster inking of a lithographic printing plate.

The ink of this invention thus comprises, in essence, a particulate color-forming reactant, preferrably a light colored, solid, particulate, water-insoluble color-forming reactant capable of forming a strongly colored complex with a coreactant metal salt, said reactant dispersed throughout a quick set vehicle, suitably modified, if desired, by any of the previously described additives. Due to the small particle size of the color-forming reactant material, the particles remain in suspension or dispersion and are prevented from settling, much as a pigment for a lithographic ink is prevented from settling. At least 40%, preferably at least 55% by weight of the ink composition is the quick set vehicle. To provide latent images which can be subsequently developed at sufficient speed, at least 10% and preferably 20 - 35% by weight of the ink composition should be the dispersed color-forming reactant particles. The lithographic ink of this invention permits the use of highly desirable conventional lithographic printing methods to print both visible and invisible entries on a suitable offset grade paper. Standard 2-color lithographic press equipment can be used to avoid the necessity of printing the visible and latent (i.e. invisible) images in separate operations. The result obtained by this invention is a printed form on offset grade paper which has printed visible entries and printed latent or invisible entries which can be developed with a suitable organic or inorganic iron salt-dispensing means, e.g. a chemical pen or wax crayon. These printed forms have the previously described utility in the field of education. A feature of these printed forms, most important from the standpoint of educational uses, is that they are not likely to be affected by the phenomenon of clueing, described previously, provided the paper is properly selected. It is a further feature of this invention that the iron content of the offset grade paper be less than 0.05%, preferably less than 0.03%, based on the weight of the sheet. As further aids to the elimination of clueing, the sheet preferably has a Sheffield smoothness number greater than 160 (preferably greater than 230) and can contain randomly distributed fibers capable of fluorescing under ultraviolet light. Other features of the invention will become readily apparent from the following detailed description. Surprisingly, the advantageous effects of this invention can be obtained without any of the operating difficulties which might be expended from the use of lithographic offset printing methods to print a latent image. For example, it has proved unnecessary to modify existing offset presses in any substantial manner to achieve the objects of this invention. Furthermore, the use of metal salt driers (e.g. the cobalt or manganese salts normally added to lithographic inks) has been eliminated in the present invention by using a quick set vehicle. This result is surprising in view of the fact that the polyhydroxyaromatic color-forming reactant, like other phenolic compounds, would be expected to inhibit oxidation and/or polymerization of the quick set vehicle, resulting in excessive penetration of the color-forming reactant into the paper sheet and loss of the ability to rapidly produce a legible image upon treatment of the paper surface with a suitable iron salt. The ability to eliminate the metal salt driers is highly advantageous, since these cobalt and manganese compounds have the ability to tie up the color-forming reactant and drastically reduce its effectiveness for the contemplated educational uses.

DETAILED DESCRIPTION OF THE INVENTION

The term "quick-set offset ink" or "quick-set lithographic vehicle" is a commonly used term in ink technology. As pointed out in Ink Technology for Printers and Students by E. A. Apps (published 1963), page 187, quick-set lithographic inks "are based on a `two-phase` vehicle consisting of a stiff component and a thin component. The vehicle separates into these two phases on the paper surface; the thin component is absorbed into the stock, and the stiff component is left to set on the paper surface; the ink films set firm in a short period of time." Apps also points out that these inks "require less driers than do conventional inks, and drying is faster and more uniform". A more detailed description of lithographic vehicles including typical quick-set vehicles is contained in the article "Resins for Printing Inks" by J. W. Dyer, American Ink Maker, pages 57-68 and 116 (May, 1970). This article describes a wide variety of inks and ink vehicles, including the most common quick-set types; i.e., the rosin ester, maleic resin, phenolaldehyde, isomerized rubber, polyamide, and polyurethane types. Any of the common types of quick-set vehicles are useful in this invention, provided that they are reasonably color-stable and do not tend to yellow with age. For example, some of the phenol-aldehyde type quick-set vehicles are subject to yellowing over a period of time.

As is apparent from the description in the Apps reference, the quick-set offset inks, regardless of their chemical composition, work on the same principle. The "thin" component (e.g. a low volatility hydrocarbon oil and/or a flowable low molecular weight natural or synthetic polymer) is readily absorbed by the paper, while the "stiff" component (e.g. a higher molecular weight natural or synthetic resinous material) is relatively non-flowable and is not absorbed. The result is a phase separation, the thin component being filtered away in the capillaries of the paper and the stiff component being solidified on the surface of the paper and binding together the ink pigment particles.

In conventional lithographic inks of the quick-set type, a drier is required despite this phase-separation effect. It has been the experience of the printing industry that the quick-setting or phase separation effect is insufficient by itself to produce a fully dried printed entry on the paper in a sufficiently short period of time. Furthermore, a quick-set vehicle can have the tendency to carry some pigment into the paper stock. This pigment migration effect is highly detrimental in the art of latent or invisible ink printing, as is pointed out by the 3,438,927 (Ehrlich) patent, previously cited. Ehrlich's approach to the migration problem is to print on a resin-coated sheet. Although the resin coating can prevent migration, as taught by Ehrlich, it can also provide a relatively smooth or glossy background for the invisible entry and create conditions which tend to favor clueing. The present invention uses an approach diametrically opposed to Ehrlich's in that offset grade paper, a highly absorbent type of paper, receives the printed invisible entry. For reasons which are not entirely clear, the undesirable migration effects described by Ehrlich are not a problem in this invention.

Further properties of quick-set vehicles and inks, as is well known in the art, are tack, high viscosity (generally in the range of 300 - 3,500 poises at room temperature), hydrophobicity, and pigment-carrying capacity. By "hydrophobicity" is meant that the ink composition, and preferably each of the primary components included in it, have little or no solubility in water and are resistant to emulsification by water. As a general rule, lithographic inks are incapable of being emulsified in water to an extent greater than about 20% by weight, although a very small amount of emulsification (e.g. 10 - 20%) is permissible and even desirable. However, the quick-set vehicle itself is oleophilic or oleoresinous in nature and is therefore hydrophobic, as is any aliphatic hydrocarbon oil used as an extender or viscosity adjusting agent. In the present invention, the relatively small amount of iron inhibitor (e.g. citric acid) and aliphatic carboxylic acid (e.g. oleic acid, used to get faster inking up of the printing plate) and silica filler, do not significantly alter the hydrophobic characteristics of the ink. It has been found that the polyhydroxyaromatic color-forming reactant used in this invention should also be somewhat hydrophobic, and for this reason a hydrophobic substituent such as an aliphatic hydrocarbon radical having more than four carbon atoms should be substituted either directly on the aromatic nucleus or through an ester or carbonyl linkage. The ester and carbonyl linkages are strongly preferred for their relatively low level of dermitological activity as compared to the aforementioned directly substituted aliphatic radicals.

Accordingly, the preferred iron-complexing color-forming reactants used in this invention contain at least these substituents substituted on an aromatic ring: a plurality of hydroxyl radicals substituted on an aromatic ring, at least two of these hydroxyls being located ortho to each other on the ring, and a higher alkanoyl radical or a radical of the formula --COOR, wherein R is a monovalent aliphatic radical of greater than four carbon atoms, preferably 6-18 carbon atoms. When the --COOR radical is present, as in the gallic acid esters, it has also been found that the use of a R with at least six carbon atoms appears to contribute to the color stability of these esters. The reason for the increased color stability cannot be readily explained, and although this invention is not bound by any theory, it is theorized that the esters of gallic acid with higher aliphatic alcohols are more resistant to oxidation or other chemical reactions which can cause undesirable color changes. A further benefit of introducing the higher molecular weight R radicals into the polyhydroxyaromatic molecule is that the room temperature vapor pressure of the compound is reduced, resulting in greater shelf life for the invisible entries printed on the examination sheets, workbooks, or the like. There appears to be no theoretical upper limit on the number of carbon atoms in R except that the imaging speed can be slowed down as the molecular weight increases. Furthermore, the availability of alkanols or other suitable aliphatic raw materials containing more than 18 carbon atoms is very limited.

Accordingly, preferred polyhydroxyaromatic compounds of this invention are those having two or three ortho hydroxyls such as suitable derivatives of gallic acid, pyrogallol, and catechol. The pyrogallol and catechol derivatives are preferably substituted with an alkanoyl radical having 6 - 18 (preferably 6-16) carbon atoms, e.g. decanoyl, lauroyl, myristoyl, or hexadecanoyl. The preferred gallic acid derivatives are the esters of gallic acid and an alkanol of 6 - 18 (preferably 6-16) carbon atoms. All of these polyhydroxyaromatic compounds form strongly colored complexes with coreactant organic and inorganic iron salts such as ferric chloride, ferric nitrate, and ferric salts of aliphatic or aromatic carboxylic acids such as ferric octoate, ferric benzoate, etc. Invisible entries printed according to this invention can therefore be developed by applying one of these coreactants to the printed surface with a suitable applicator. The coreactant can be dissolved in a solvent and applied from a fiber-tipped pen or rubbed on with frictional action using a wax crayon containing the coreactant.

It is of course preferred that the color-forming, iron-complexing reactant be light or pale in color, e.g. an off-white or gray color. Clueing can, in any event, be minimized by matching the color of the ink formulation as closely as possible to the color of the paper. Thus, pale yellow paper can be used with pale yellow ink, gray paper with gray ink, etc., provided the ink itself is color stable in the environment of the printed page. A particularly suitable reactant from a color standpoint is lauryl gallate (dodecyl gallate), which is off-white in color but gives a strongly colored gray-brown image when reacted with a suitable coreactant iron salt. As pointed out previously, the color-forming reactant should be ground or milled to form finely pg,12 divided particles in the size range characteristic of lithographic ink pigments. Particles larger than 10 microns, and even larger than 5 microns are not carried very well by a vehicle; consequently, the numerical average size of the color-forming reactant particles will be within the range of about 0.5 to 5 microns, preferably within the range of 1-3 microns.

The iron-inhibiting compound, hydrocarbon oil, wax, and flatting agent additive previously referred to will now be described in detail.

The hydrocarbon oil. Aliphatic hydrocarbon petroleum distillates having a K.B. (Kauri-butanol) value below 28 and a boiling point above 200°C. are preferably included in the ink formulation to adjust the viscosity and tack of the ink and/or, in the case of oil-soluble quick-set vehicles, as a solvent or extender. Up to about 5% by weight (based on the total ink composition) can ordinarily be used. Hydrocarbon oils having a boiling range located entirely above 225° C. are preferred. An example of a relatively low-boiling petroleum distillate is the so-called 440 oil, which boils in the range of 444 - 493° F. (229 - 256° C.). Higher boiling oils such as 535 oil are preferred; these oils generally have boiling ranges above 275° C.

The wax. Any suitable wax used in lithographic inks can be added to adjust tack consistency and to decrease film-forming tendencies of the ink. For example, up to 3% by weight of the ink formulation can be a particulate polyethylene-type wax.

The iron inhibiting agent. When the color-forming reactant is milled in contact with iron or steel-containing rolls, or in contact with other metals that can have undesirable chemical effects upon the color-forming reactant, it is preferred to add at least 0.5 %, preferably at least 0.7%, by weight of a compound having the ability to form a colorless or substantially colorless complex with iron. A particularly suitable example of such a compound is anhydrous citric acid. The citric acid reacts with any foreign iron forming a colorless reaction product in the process of mixing, milling, and also after printing on the writing surface. If desired, the citric acid may be combined with phosphoric acid for this purpose.

The flatting agent is particularly useful to reduce the glossiness of the printed ink when a high percentage of quick-set vehicle is used. The inks of this invention typically comprise more than 55% by weight quick-set vehicle to ensure good pigment-carrying capacity. At proportions higher than about 65%, the printed ink may have excessive gloss, even when some flatting agent is used. (A further disadvantage of these high proportions of quick-set vehicle is the relatively slow rate of invisible image development.)

The flatting agent. The preferred flatting agent is finely divided micron-size or sub-micron size silica particles. Preferably the silica makes up no more than about 5% by weight of the total ink composition. In addition to providing flatting effects which reduce the glossiness of the invisible ink deposits, the silica improves rub resistance and adjusts the physical properties of the ink to obtain ink stability on the press. Synthetically produced silica is available in a wide variety of particle sizes ranging from a fraction of a micron to several microns, 3 - 5 micron-size silicas being preferred.

Other recommended additives. Whether or not the quick-set vehicle is a polyurethane alkyd, some additional liquid polyurethane alkyd, up to about 4% of the total ink composition, is preferably added for viscosity adjustment, added stability on the press, and better rub resistance for the ink once printed on the paper. Amounts of highly fluid polyurethane alkyds in excess of 4% or 5% result in too much fluidity over a period of time so that the printed ink migrates into the paper fibers and is no longer available for reaction with the coreactant metal salt applied to the paper surface. Surprisingly, however, when the quick-set vehicle itself is a polyurethane alkyd and contains stiff material as well as thin material, the migration of printed invisible ink into the paper fibers is sufficiently small to avoid any difficulty in developing the invisible image after printing.

Another desirable additive for the inks of this invention is a relatively long chain organic compound having a polar group at one end of the chain. Suitable compounds of this type are the long chain aliphatic carboxylic acids such as oleic acid. These carboxylic acids provide faster inking of the lithographic printing plate.

The ink formulations of this invention contain no active drying agents because of the inherent property of the color-forming reactant to tie up oxidizing agents, e.g. the cobalt and manganese salts commonly used as driers in lithographic inks. The formulations of this invention dry by absorption primarily, although some very slow evaporation of the aliphatic hydrocarbon oil can occur over a period of 3 to 10 days, or longer. Because of this absorption or filtration drying method, the ratios of color-forming reactant, vehicle, and the other additives are important to the objective of obtaining a printable ink that will dry sufficiently and not change with time because of penetration into the paper fibers. The drying rate of the invisible ink formulation has, however, been found to be somewhat less critical than that of black and colored inks.

The relatively small amount of invisible ink laydown, combined with the inherent circumstance that the latent print content of a printed sheet of this invention is usually a small percentage of the area of the sheet, has the effect of requiring close control of the amount of ink metered to the ink train from the ink supply fountain of the offset press. It is difficult to maintain a water/ink balance with this low level of ink being supplied and printed. Two methods of solving this problem are (1) to include a solid bar or block of invisible ink content, preferably near an edge of the paper, and covering at least one percent of the surface area of the sheet, for example, a 1/2 × 8 inch bar can be printed at the bottom of a standard 81/2 × 11 inch sheet; (2) to include a screened-in background, i.e. a pattern of scattered dots of invisible ink content. The screened background area should surround any latent images on the sheet having informational content, thereby also helping to reduce clueing by camouflaging the latent informational content. A screened background area covering generally up to about 15% and in no event greater than 30% of the latent print informational content camouflages this content without interfering with the vivid contrast of the activated or developed invisible image. Furthermore, the screening allows the pressman to carry more ink on ink rollers on the press, because more ink is being applied per sheet of paper. An excessive amount of invisible ink in the informational content of the invisible entries can make it possible for one to read the latent image content before development with a coreactive metal salt.

For quick set vehicles having a viscosity at room temperature greater than 500 poises, the optimum color-forming reactant content in the inks of this invention is 25 - 31% by weight. Higher amounts have poor stability in the ink train, and lower amounts provide slower image development and, after development, relatively low color density.

The following non-limiting examples illustrate the principle and practice of this invention. All parts are by weight unless otherwise indicated.

Example 1

The following formulation illustrates the use of a phenolic-based quick-set vehicle containing a drying oil. The vehicle, "Kemset" 300-101 quick-set varnish, has a solids content of 62% by weight, a specific gravity of 0.939, a viscosity of 350 poises, and acid number in the range of 5 - 10 mg KOH per gram. This quick-set vehicle is extendible with 535 oil, but can be used as the sole vehicle where speed of drying is more important. In this Example, the Kemset was modified with "SKP-104" (Sun Chemical Corp.), a polyurethane alkyd of 100% solids and a specific gravity of 1.00 ± 0.05. Its acid number is 2 mg KOH per gram.

______________________________________ Parts by Weight Ingredient ______________________________________ 61.0 "Kemset" 300-101 Varnish - Sun Chemical Corp. 2.0 "SKP 104" (Polyurethane Alkyd) Sun Chemical Corp. 1.0 "Micropoly" 800-111 (Polyethylene type wax) Sun Chemical Corp.* 31.5 Lauryl Gallate 0.8 "Syloid 72", Grade 73 micron-sized silica flatting agent (W. R. Grace & Co.) 1.7 Citric acid, anhydrous 1.2 Magie Bros. 535 oil 0.8 Oleic acid 100.0 ______________________________________ *"Micropoly" 800-111 wax is a finely powdered modified polyethylene-type wax having a Gardner (Hellige) color of 1-2, and a particle size range of 5-15 microns.

Example 2

The following is an example of a non-yellowing quick-set lithographic ink of this invention. In this formulation, "Uroset" varnish is a urethane-modified alkyd completely free of metal salt driers. This alkyd-type material has the advantages of fast drying, rub resistance, and good pigment wetting qualities of the isocyanate modified materials. The viscosity of this vehicle at 77° F. (25° C.) is 530 - 650 poises. Its acid number is less than 9 mg KOH per gram and its weight per gallon is 7.85 lbs. (specific gravity 0.95).

______________________________________ Parts by Weight Ingredient ______________________________________ 63.0 "Uroset" quick-set vehicle - Lawter Chemicals, Inc. 3.0 SKP 104 (Polyurethane Alkyd) - Sun Chemical Corp. 1.0 "Micropoly" 800-111 (Polyethylene type wax) Sun Chemical Corp. 28.5 Lauryl gallate 0.5 "Syloid 72", Grade 73 (4 micron sized) silica flatting agent (W. R. Grace & Co.) 1.7 Citric acid, anhydrous 1.5 Magie Bros. 535 oil 0.8 Oleic acid 100.0 ______________________________________

Examination sheets printed with this lithographic invisible ink and a second, visible ink composition, were printed on offset paper of about 0.02% iron content, based on the weight of the paper. The surface of this paper was toothy -- a rough vellum finish having a Sheffield smoothness number in the range of 240 - 270. These examination sheets showed no evidence of clueing over an extended period of time. Coated or calendared glossy, smooth sheets had reflectance differences, however, such that the invisible image was visible without developing. Thus, rough and matte-like finishes on the paper were shown to be preferable.