DE2239177A1 - LATENTER IMAGE PRINT - Google Patents

Description

Minnesota Mining and Manufacturing Company, Saint Paul,

Minnesota 55101, V.St ₀ A ₀

Latent image printing

The invention relates to the printing of latent, i.e. hidden or practically invisible, images that have a Contain color-forming reactants, which with a metal salt or the like. Visible as co-reactants can be made. The invention relates to teaching aids or tools such as instruction sheets or worksheets, self-responsive ones Examination sheets and other "recoupling" systems, using sheets of paper. The invention is also aimed at making these teaching aids visible printed material, generally in the form of statements or questions, related to the latent material Provides "recouplings" or responses.

Most of the development in the latent image printing field has been for educational purposes. examples for recently developed paper-based systems for imparting expertise by developing or visualizing of latent or hidden images can be found in

309Ö07 / 1350

U.S. Patents 3 * 363 * 337 (Skinner et al., patent granted on January 16, 1968), 3,451,14J (Thomas et al., Patent issued June 24, 1969) and 3,516,177 (Skinner, Patent granted on June 23, 1970). From the known systems seem to be most suitable for those who use an "invisible color" (.e. a color, which contains a practically colorless dye precursor or a color-forming reactant), which is based on a paper-like Sheet printed and at any convenient time with a color-forming co-reactant can be developed. Typical "invisible colors" are described in U.S. Patent 3 * 363 * 337 (Skinner et al.) Mentioned above. and U.S. Patent 3,438,927 (Ehrlich, patent issued on April 15, 1969).

Despite the rapid development in the field of latent printing, there are still many problems to be solved. A problem, which one often encounters in the field of invisible colors or inks is the premature observation of the latent Image (i.e. finding a trail). Such observation is possible (l) when there is a difference between the background areas and the latent print areas are visible to the naked eye, i.e. when the image is different Has light reflectivity or turns yellow with aging, and (2) when accidental development of the latent image takes place through contamination in the ink, paper or printing device.

Even if the problem of this early observation is minor slow and insufficient development of the latent image can also be a problem. Longing Reaction times for converting the latent, practically colorless image into a colored, visible reaction product should be avoided as far as possible.

3D907 / 1350

_ 3 -

Another problem is the need for convenient printing processes to be able to apply, given. The most convenient printing method for large-scale production and for many types of teaching materials (especially those that are frequently / revised must be) is the lithographic (wet) offset printing process, i.e. the usual offset technique in which one makes use of the incompatibility of aqueous and oleoresin-like agents.

Special problems that are not present when working with a printing press or with "dry" offset printing processes, must be resolved to an "invisible" color (for latent printing) for common lithographic ("wet") offset printing processes To be made available: Existing offset presses (including the two-color offset presses) should advantageously be usable without costly changes; the vehicle of the color and the color-forming agent Reactants should be particularly suitable for conventional lithographic offset printing processes; undesirable chemical side reactions caused by conventional metal salt "drying agents" should be avoided as far as possible; some phenolic color-forming reactants are prone to oxidation and / or polymerization to inhibit common offset substrates; and a penetration or penetration of the invisible Color and wandering within the paper should be prevented as much as possible, without taking refuge resin-coated sheets of low porosity or high reflectivity, resulting in the formation of an image trail can (see U.S. Patent 3,438,927 mentioned above).

The invention aims to address these problems to solve by, among other things, the formulation for a latent printing ink is to be made available, which is used for

30 9 807/13 50

borrowed lithographic offset printing process is suitable, as well a suitable paper is to be created as a printing surface.

According to the invention, it is shown in summary how Proceed as follows: First, a suitable solid polyhydroxyaromatic Compound that can form complex iron salts to form a strongly colored complex (such as ^ in

a

Quallic acid ester and / Cg- to C ₁ O-AlkOhOl), ground. The grinding is carried out until this compound has a partial size which corresponds to the usual size range for colored pigments (for example until the numerically average particle size is in the range from 0.5 to 5 μm). If the device parts used for grinding contain iron or steel, an iron inhibitor such as citric acid is preferably used during the grinding. Next, the ground iron complex-forming, color-forming polyhydroxyaromatic compound is dispersed in a color-fast (ie, non-yellowing) lithographic, fast-setting vehicle (vehicle for Sonellroken inks) described below. The viscous lithographic color obtained may be extended or the viscosity will be in terms modified by use of an aliphatic hydrocarbon oil, preferably a petroleum distillate, which preferably boils at a temperature of at least 200 ⁰ C and 225 ⁰ C. The color can then also be modified with matting agents and agents for making it abrasion resistant, such as particulate silica (eg fumed or precipitated silica). It is also advantageous to add smaller amounts of wax and polar organic additives to the ink in order to adjust the tack and the consistency and to achieve a more solid inking of a lithographic printing plate.

The paint according to the invention thus essentially contains a particulate, color-forming reactant, which is fully

309807/1350

is constantly dispersed in a fast-curing vehicle and, if desired, modified in a suitable manner with the additives indicated above. Because of the small particle size of the color-forming reactant, 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% by weight , preferably 55% by weight, of the color composition consist of the fast-curing vehicle. In order to provide latent images which can subsequently be developed with a sufficient speed, to at least 10 wt -.%, Preferably 20 to 35 wt .- ^ of the color mass of the dispersed particles composed of the color-forming reactants. The lithographic ink of the invention allows the use of very desirable lithographic printing processes to print both visible and invisible records on a grade of paper suitable for offset printing. A standard press for two-color lithographic printing can be used to obviate the need to print the visible and latent (ie "invisible") images in separate processes. The product obtained according to the invention consists of a printing form on a type of paper suitable for offset printing, which contains printed visible records and printed latent or invisible records, the latter with a device for distributing a suitable organic or inorganic iron salt, such as with a pen or with wax crayons. As indicated above, these printing forms are useful in the field of teaching materials. A feature of these printing forms which is of great importance for teaching purposes is that these printing forms do not easily give premature indications of the presence of the latent image, provided that the appropriate paper has been selected. According to another feature of the

30980 7 / t3BO

According to the invention, the iron content of the type of paper suitable for offset printing is less than 0.05%, preferably less than $ 0.03 based on the weight of the sheet. as The sheet preferably has a further aid for eliminating premature recognition of the latent image Sheffield smoothness number over 160 (preferably over 2 ^ 0) and may contain randomly distributed fibers that may fluoresce under ultraviolet light. Other characteristics of the Invention will be readily apparent from the more detailed description that follows. Surprisingly, you can the advantageous effects of the invention are achieved without any process difficulties arising, which might be expected using offset lithographic printing processes to print a latent image. E.g. it has proven unnecessary to use existing offset presses may be modified in any way to achieve the objects of the invention. Furthermore, the use of metal salt "drying agents" is no longer necessary. (such as the cobalt or manganese salts normally added to lithographic inks) in the invention through the use of a "fast setting" vehicle as discussed below will. This result is surprising in view of the fact that the polyhydroxyaromatic color-forming reactant, like other phenolic compounds, is expected would have to inhibit the oxidation and / or polymerization of the "fast curing" carrier, which excessive penetration of the color-forming reactant in the sheet of paper and a loss of the ability to quickly form a readable image after treating the paper surface to achieve with a suitable iron salt, would result. The possibility of the metal salt desiccant Leaving out is very beneficial because these cobalt and manganese compounds are the color forming reactants bind and drastically reduce their effectiveness for the intended teaching purposes.

309807/1350

The expression "fast-curing offset ink" or "tendon-curing lithographic carrier medium" corresponds to the expression commonly used in printing inks technology. As pointed out in "Ink Technology for Printers and Students" by EAApps (published 1963), page 187, fast setting lithographic inks (lithographic quick dry inks) are based on a two phase vehicle consisting of a rigid component and a thin component. The vehicle separates see in these two phases on the paper surface j the thin component is absorbed by the paper material, and the stiff component remains and hardens on the paper surface; the layers of paint harden to a solid state in a short period of time. Apps also describes that these paints require less desiccant than conventional paints and that drying is faster and more even. A more detailed description of lithographic carriers, including typical tendon-hardening carriers, is contained in the work "Resins for Printing Inks" by JWDyer, American Ink Maker, pages 57 to 68 and II6 (May 1970). In this work a wide variety of inks and color carriers, including the most common fast curing types, ie the rosin ester, maleic resin, phenol aldehyde, isomerized rubber, polyamide and polyurethane types, are described. Any fast setting vehicle of the conventional type is suitable for the invention provided that the vehicle is fairly colorfast and does not tend to turn yellow with age. For example, some of the fast-curing phenol-aldehyde-type vehicles are not preferred because of their tendency to yellow within a period of time.

As can be seen from the work of apps mentioned above, the fast-curing offset inks work without consideration on their chemical composition, on the same principle.

309807/1350

The "thin" component (e.g. a hydrocarbon oil with low volatility and / or a flowable natural or synthetic polymer of low molecular weight) is easily absorbed by the paper, while the "stiff" Component (e.g. a natural or synthetic resinous material with a higher molecular weight) is relatively non-flowable is and is not absorbed. This creates a phase separation, with the "thin" component in the The capillary of the paper is filtered off and the "stiff" component solidifies on the surface of the paper and binds the pigment particles of the paint together.

In the case of conventional lithographic inks of the fast-setting type, despite this phase separation effect, a "drying agent" is present. necessary. The experience in the printing industry has been that the rapid hardening or phase separation effect alone is not sufficient to produce a completely dry printed image in a sufficiently short period of time to achieve on paper. Furthermore, a fast-curing vehicle can carry some pigment into the paper material. This pigment migration effect is very detrimental in the field of latent or invisible color printing, such as the above-mentioned US patent specification 3,438,927 (Ehrlich) can be found. This patent attempts to eliminate the problem of migration by using a resin-coated Sheet is printed. Although, as shown in U.S. Patent 3,438,927, the resin layer does migrate can prevent the color, this resin layer also leads to a relatively smooth or shiny background for the latent print image and thereby creates conditions that facilitate the early discovery of this image material. The present invention makes use of it a route similar to that shown in U.S. Patent 3,438,927 Way is diametrically opposed by choosing a type of paper suitable for offset printing that is highly absorbent Paper type is the invisible print record

309807/1350

recording. For reasons that are not yet fully understood ' those described in the aforementioned United States patent Wandering effects are not a problem with the present invention.

Further properties of fast-setting vehicles and paints are, as is known, tack, high viscosity (generally in the range of 300 to 3500 P at room temperature), water repellency and the ability to carry pigments. "Water repellency" is to be understood as meaning that the paint, and preferably each of the main constituents contained therein, has little or no solubility in water and is resistant to emulsification with water. As a general guideline, lithographic inks in water does not apply to an extent greater than about 20 wt -.% Emulsified. although a very small degree of emulsification (e.g. $ 10-20) is allowable and even desirable. However, the fast-setting vehicle itself is oleophilic or oleoresin-like in nature and is therefore hydrophobic, as is any aliphatic hydrocarbon oil which is used as an extender or as an agent for adjusting viscosity. According to the invention, the relatively small amount of iron inhibitor (e.g. citric acid) and aliphatic ^ carboxylic acid (e.g. oleic acid, which is used to achieve faster printing on the printing plate) and silica filler do not significantly change the hydrophobic properties of the ink. In addition, it has been found that the polyhydroxyaromatic color forming reactant used in the invention should be somewhat hydrophobic and for this reason a hydrophobic substituent such as an aliphatic hydrocarbon radical of more than. 4 carbon atoms, either directly on the aromatic nucleus or via an ester or carbonyl bond. The ester * or carbonyl bonds are very preferred because of their relatively low degree of dermatological effectiveness compared to the aforementioned directly substituted aliphatic radicals.

3 0 9807/135: 0.

The preferred iron-complexing, color-forming reactants, which are used in the invention contain at least the following substituents on one aromatic ring: Several hydroxyl radicals attached to one aromatic ring as a substituent, at least two of these hydroxyl groups being in the ortho position are to each other on the ring, and a higher alkanoyl radical or a radical with the formula -COOR, where R is a monovalent is an aliphatic radical with more than 4 carbon atoms, preferably with 6 to 18 carbon atoms. If the -COOR residue is present, as with the GallussMureestern, the use of an R group of at least 6 carbon atoms for color fastness appears to have been found this ester to be responsible. The reason for the increased color fastness is not easy to explain, and although the invention is not bound by theory, it can be assumed that the esters of gallic acid with higher aliphatic alcohols against oxidation or other chemical reactions that cause undesired coloring change can cause, are more resilient. Another advantage of introducing the higher molecular weight R radicals in the polyhydroxyaromatic molecule is given by the fact that the vapor pressure of the compounds at room temperature is decreased, resulting in a longer life of the invisible printed images on e.g. examination sheets or in instruction books. For the number of carbon atoms there appears to be no upper limit in the R group, except that the rate of image formation to the extent that the molecular weight increases, can be slowed down. In addition, there are alkenols or other suitable aliphatic starting materials with more than 18 carbon atoms are only available to a very limited extent.

The preferred polyhydroxyaromatic compounds are shown below of the invention are therefore those which have 2 or 3 ortho-Hydroxyl

309807/1350

- li -

have groups, such as suitable derivatives of gallic acid, the pyrogallol and the catechol. The pyrogallol and pyrocatechol derivatives preferably contain as substituents an alkanoyl radical having 6 to 18 (preferably 6 to Id) carbon atoms, such as decanoyl, lauroyl, myristoyl or hexadecanoyl. The preferred gallic acid derivatives are the esters of gallic acid and a 6-18 alkanol (preferably 6 to 16) carbon atoms. All of these polyhydroxyaromatic Forms compounds with organic and inorganic iron salts acting as co-reactants, such as iron (III) chloride, iron (III) nitrate and iron (III) salts strongly colored by aliphatic or aromatic carboxylic acids such as ferric octoate, ferric benzoate, etc. Complexes. Invisible printed images according to the invention can therefore be created by applying one of these co-reaction participants can be developed onto the printed surface with a suitable applicator. The participant in the reaction can dissolved in a solvent and applied with a writing tube with a fiber tip, or with rubbing movement using a wax crayon that contains the co-reactant, rubbed on »

It is of course preferred that the color-forming, iron complex-forming Reagent is light or pale in color, such as off-white or gray. It is also preferred to match the color of the paint formulation as closely as possible to the color of the paper. E.g. a pale yellow paper of a pale yellow color, a gray paper of a gray color, etc. can be used be provided that the color itself is in the area

is

of the printed sheet stable in color ^ From the color point of view Lauryl gallate (dodecyl gallate) is a particularly suitable reactant because it has an off-white _, Has color but gives a strongly colored grayish-brown image when, with a suitable co-reactant

309807/1350.

acting Elsenalz is implemented. As stated above, the color-forming reactant is said to be comminuted or ground so that it is finely divided into shape Particles are present, with the size range characteristic of lithographic parabolic pigments. Particle, greater than 10 μm and even greater than 5 μm are made by one Carrier means not worn very well; therefore it is numerical average size range of the particles from the color-forming reactant within the range of about 0.5 to 5 μm and preferably within the range of 1 to j5 μηΐ

The iron-inhibiting compound, the hydrocarbon oil, the wax and matting agent given above are explained in detail.

The hydrocarbon oil - Aliphatic hydrocarbon petroleum distillates with a KB value (kauri-butanol value) below 28 and a boiling point above 200 C are preferably included in the paint formulation in order to adjust the viscosity and the stickiness of the paint and / or in the case of oil-soluble to serve as solvents or extenders for fast-curing vehicles. Up to about 5% by weight (based on the total composition of the paint) can generally be used. Hydrocarbon oils with a boiling range which is completely above 225 ^° C. are preferred. An example of a relatively low boiling petroleum distillate is the so-called 44O-Ö1 which boils in the range 229-256 ⁰ C. Higher boiling oils such as the 535-01 are preferred. These oils generally have boiling ranges above 275 ⁰ C. (The terms 44O-Ö1 and 535-01 represent substantially descriptive terms that relate to the lower end of the boiling range in ⁰ F).

The Wax - Any suitable wax used in lithographic inks can be used to adjust the tackiness

30980 7/13 50

.The consistency and to reduce the film forming ability color be added to _n example to up to 3 wt -.% Of Farbforraulierung of particulate! Consist of polyethylene type wax.

The iron inhibitor - If the color-forming reactant is ground in contact with ferrous or steel-containing balls or comes into contact with other metals which may have undesirable chemical effects on the color-forming reactant, it is advantageous to use at least 0.5 ßew, - $, ' % to add a compound which can form a colorless or substantially colorless complex with iron - preferably at least 0.7 wt.. A particularly suitable example of such a compound is anhydrous citric acid. Citric acid reacts with any foreign iron to form a colorless reaction product during mixing, grinding and also after printing on the printing surface. If desired, citric acid can be combined with phosphoric acid for this purpose. The matting agent is particularly suitable for reducing the gloss of the printing ink when a high percentage of fast-curing vehicle is used. The colors of the invention typically contain about 55 wt -.% Fast curing support means for a good carrying capacity for the pigment to ensure. At proportions above about $ 65, the printing ink can have an excessive gloss, even if some matting agent is used (another disadvantage of such high proportions of fast-curing vehicle is the relatively slow speed at which the latent image develops.)

The Matting Agent - The preferred matting agent consists of finely divided silica particles (silica particles) that are micron or sub-micron in size. Apart from the matting effect, which enhances the sheen of the invisible color.

7/1350

If precipitation decreases, the silica improves the abrasion resistance and restores the physical properties the color in such a way that the color is stable in the press. Synthetic silica is in many particle sizes available by the fraction of one micron to several microns, with a particle size of the silica of 3 to 5 HW being preferred.

Other recommended additives - Regardless of whether the fast-curing vehicle is a polyurethane alkyd or not, further liquid polyurethane alkyd is preferably added, up to about 4 $ of the total paint mass can be added to adjust the viscosity, the resistance in the press and to give better scrub resistance of the ink when this has been printed on the paper, proportions of highly liquid polyurethane alkyds above $ 4 or $ 5 lead to excessive flowability within a certain period of time, so that the printing ink migrates into the paper fibers and is no longer available for a reaction with the metal salt serving as co-reactant which is applied to the paper surface. Surprisingly, however, when the fast-curing vehicle itself is a polyurethane alkyd and contains both stiff material and thin material, the migration of the printed latent color into the paper fibers is so largely small that any difficulty in developing the invisible image after Printing is avoided.

Another beneficial additive for the paints of the invention is a / relatively long-chain organic compound polar group at one end of the chain, suitable compounds of this type are the long chain aliphatic carbon? - acids, such as oleic acid. These carboxylic acids lead to faster coloring of the lithographic printing plate »

309807/1350

The paint formulations of the invention do not contain active ones Desiccant, because of the inherent ability of the color-forming reactant to bind oxidizing agents, such as the cobalt and manganese salts commonly used as drying agents in lithographic inks "The formulations according to the invention dry mainly by absorption, although a very slow evaporation of the aliphatic hydrocarbon oil within a period of 5 to 10 days or longer can take place. Little of this absorption or the filtration dry method are the proportions of the color forming reactant, vehicle and the others Additives important to achieve a printable color that dries adequately and does not stand out because of one Penetration into the paper fibers changes over time. The drying speed however, the formulation for the latent color is, as has been found, somewhat less critical, than that of black or colored paints.

The relatively small amount of invisible ink given off during printing, combined with the fact that in the printing sheet of the invention, the proportion of the latent print image generally makes up a small percentage of the sheet area, necessitates a precise adjustment of the amount of ink that is required for the printing process the ink supply of the offset press is required. It is difficult to maintain a water / ink balance with this small amount of ink to be fed and printed. Two methods of solving this problem are to (l) provide a solid strip or block containing invisible paint, preferably near the edge of the paper, with that strip or block covering at least 1% of the surface area of the sheet ( For example, a 1 cm χ 20 cm on the lower part of a standard sheet with a size of 20 χ 30 cm can be printed) or (2) provides a screened-in background, such as

30 9 8 07 / 135.0

a pattern of scattered dots with an invisible content Colour. The screen-like background area is intended to have any latent images with informative content on the sheet surrounded, which also supports the camouflage of the latent information. A screen-like covered background area, which generally goes up to about $ 15 and in none If more than j50 # of the information in the form of the latent print image disguises this information without obscuring the stark contrast of the activated or developed latent image disturb. Furthermore, the screen-like covering of the background allows the printer to put more ink on the inking rollers of the press to be applied because more color is released per sheet of paper. An excessive amount of invisible color in the information area of the invisible image can be as described above Create problem of early detection of the image (i.e. finding a trail).

In the case of fast-curing carriers with a viscosity at room temperature above 500 P, the optimum content of color-forming Respondents in the colors of the invention 25 to 31% by weight. Higher proportions lead to a lower one Resistance of the paint application and lower proportions result in slower image development and a relatively low color density after development.

The following examples, which are not intended to limit the invention, illustrate the essential and practical Implementation of the Invention. All parts are based on weight, unless otherwise stated.

example 1

The following formulation explains the use of a fast-curing phenol-based carrier with a compound

309807 / 13BO

stop at drying oil. The vehicle, a fast-curing lacquer "Kemset" 300-101 (trade mark), has a pesticide content of 62% by weight , a specific gravity of 0.959, a viscosity of 350 P and an acid number in the range from 5 to 10 mg KOH per g. This fast-curing carrier is extended with 535-Ö1, but can also be used as such as a carrier if the drying speed is of greater importance. In this example, the carrier "Kemset" was modified with a polyurethane alkyd having a solids content of $ 100 and a specific gravity of 1.00 0.05 ("SKP-IOV, trademark of Sun Chemical Corp.). Its acid number is 2 mg KOH per g.

Parts by weight of ingredients

61.0 "Kemset" paintwork 300-101 (trademark for ·

one from Sun Chemical Corp. produced fast-curing lacquer)

2.0 "SKP 104" (trademark for polyurethane

alkyd, manufactured by Sun Chemical Corp.)

1.0 "Micropoly" 800-111 (Trademark of

Sun Chemical Corp. for a finely powdered wax from the modified one Polyethylene type with a Gardner (light) parbe of 1 to 2 and with a particle size range of 5 to 15 μm)

31 * 5 lauryl gallate

0.8 "Syloid 72" (Trademark), a Mattie

silica with a particle size in the micron size range, Grade 73 (W.R. Grace & Co.)

1.7 Citric Acid Anhydrous

1,2 535-01 (one used by Magic Bros.

descriptive term for a hydrocarbon oil with a boiling range generally above 275 C)

0.8 . oleic acid

309807/1350

Example 2

The following is an example of a non-yellowing, fast setting lithographic ink of the invention. In this formulation, the paint "Uroset" (trademark) is a urethane-modified alkyd that is completely free of metal salt desiccants is. This alkyd type material has advantages of quick drying, abrasion resistance and the good pigment wetting properties of isocyanate modified materials. The viscosity of this vehicle at 25 ° C. is 530 to 650 P. The acid number of this carrier is below 9 mg KOH per g, and its specific weight is 0.95.

Parts by weight components

65.0 "Uroset" (Trade Mark), quick cure

of the vehicle, Lawter Chemicals, Inc.

3.0 "SKP 104" (trademark for polyurethane

thanalkyd, manufactured by Sun Chemical Corp.)

1.0 "Micropoly" 800-III (trademark for

a polyethylene type wax from Sun Chemical Corp.)

28.5 lauryl gallate

0.5 "Syloid 72" (Trademark), a Mattie

agent made of silica with a particle size of 4 μm, grade 73 (W.R. Grace & Co.)

1.7 Citric Acid Anhydrous

1.5 535-01 according to Magie Bros, (corresponding to

Example 1)

0.8 oleic acid

Exam sheets invisible with this lithographic or latent color and a second, visible color mass were printed using offset paper with an iron content of about $ 0.02 based on that

309807/1350

Wound of paper, made. The surface of this paper was rough - a rough parchment paper surface with a Sheffield Smoothness Number in the range of 240 to 270. These exam sheets did not indicate any premature marks Discovery of the invisible image - recognize within a longer period of time «, coated or calendered, glossy, smooth sheets, however, showed differences in reflection, so that the invisible image without Development was noticeable. Therefore rough and stone-like surface textures of the paper are preferred.

Claims

309807/1350

Claims (1)

- 20 claims 1.) Invisible or latent, for the lithographic offset printing process suitable printing ink that contains a carrier and a solid compound that forms with iron a stable color complex, characterized in that the solid compound is in the form of particles with a numerically average particle size is present in the range from 0.5 to 5 μm and the formula R _ where η is 0 or 1 and R (l) is a monovalent aliphatic radical with more than 4 carbon atoms or (2) is a radical of the formula ~ (- CO -) - OR ¹ or - (- CO -) - R ¹ , wherein R * is a monovalent aliphatic radical having more than 4 carbon atoms, and at least 40 wt -. consist% of the latent print color from a fast-curing vehicle that sufficient tackiness, viscosity, a sufficient water repellency and sufficient capability for carrying the pigment for one application in the lithographic offset printing process has that the latent printing ink is hydrophobic and has a viscosity in the range from 300 to 3500 P and that the most volatile component in the latent printing ink has a boiling point above 20O ⁰ C. 2.) Printing ink naoh Anspruoh 1, characterized in that the color-forming reactant is an ester of the Gallic acid and an aliphatic alcohol with 6 to 18 carbon atoms is. 5.) Printing ink according to claim 1, characterized in that it also contains anhydrous citric acid. 30 9 807/1350 4.) Printing ink according to claim 1, 2 or ~ 5, characterized in that it contains an aliphatic hydrocarbon oil with a boiling point above 200 ⁰ C and a Kauri-butanol value below 28. 5.) Printing ink according to one of claims 1 to K _1, characterized in that the fast-curing carrier contains a polyurethane alkyd with a viscosity of over 500 P. 6.) A method for producing the printing ink according to claim 1, characterized in that said fixed connection ground until the numerically average particle size of the particles obtained is in the range from 0.5 to 5 μm, and the milled particles in one quick dispersed hardening vehicle. f.) Printing form, containing a visible print image and an invisible or latent print image, wherein the latent print image can be developed into a visible image by reaction with an iron salt, characterized in that the paper used for the printing form is a type of paper for offset printing , and this paper contains less than $ 0.05 iron, based on the weight of the paper, and that the latent print image consists essentially of (a) a polyhydroxyaromatic iron complex-forming, color-forming reactant with several hydroxyl radicals as substituents on an aromatic ring, with at least two of the Hydroxyl radicals are located ortho to one another on said ring, and (b) there is at least k0 QevSo% of a lithographic fast-curing carrier. 8.) Printing form according to claim ¹ J _1, characterized in that the paper of a type for offset printing has a Sheffield smoothness number over 160 and an iron content of less than $ 0.0j5, based on the weight of the paper. 309807/1350 9.) Printing forme according to claim 7 or 8, characterized in that the paper-like sheet contains a fiber mixture that contains fluorescent fibers in random distribution. 10.) Printing forme according to claim 7 or 8, characterized in that that part of the latent print image consists of a solid print block which is at least about 1 $ of the area of a Sheet covered from the paper. 11.) Printing form according to claim 7 or 8, characterized in that the latent print image consists of an image with information Salary surrounded by an incoherent period of dots that is up to $ 15 of the range that the called picture surrounds. 309807 / 135P