CA1109609A

CA1109609A - Latent sensitizing ink

Info

Publication number: CA1109609A
Application number: CA325,904A
Authority: CA
Inventors: Heinz E. Lange; Robert J. Maloney; Dean A. Ostlie
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1978-05-17
Filing date: 1979-04-19
Publication date: 1981-09-29
Also published as: GB2021613B; AU527978B2; IT7922709A0; GB2021613A; FR2426074B1; IT1114225B; AU4709979A; JPS6213195B2; DE2919837C2; DE2919837A1; JPS54150211A; ZA792378B; FR2426074A1

Abstract

Abstract of the Disclosure Latent, sensitizing inks for application to selected areas of substrates by conventional printing methods. The inks comprise a vehicle component which is a nonpolymeric, oleophilic, organic acid anion having a cationic counterion and, in combination with said vehicle component, a thinner, a particulate filler and a polymeric, hydrocarbon, theremoplastic terpene resin which enhances imaging characteristics and allows the use of reduced filler loadings in the composition. The latent inks also include at least one color-generating component.

Description

' ~ 914,717 ~96~9 I~PROV~D LATENT SEN~ITIZING INK

The present invention relates to the art of sensitizin~ substrates with color generating components so that the sensitized areas, or portions of the sensitized areas, can be subsequently developed, i.e. rendered visible, by contact with a color-activatin~ material. More specifically, latent, sensitizing inks which can be applied to selected areas of substrates by a varlety of conventional printing methods are disclosed.

"Carbonless" papers and the art Gf applying undeveloped, color-generating materials to substrates are known. Previously, the application of these undeveloped or latent color-generating materials involved the coating of the complete surface of substrates usin~ aqueous coatin~
compositions and techniques~ These techniques do not lend themselves to the application of the color-generating materials in selected, patterned areas. Accordin~ly, in order to provide a substrate having imaLing capabilities on only a portion of the substrate, other techniques were developed such as desensitizing a portion of the substrate by the application of a desensitizing compound as disclosed in U.S.
Patent 3,809,668.
As the art of providing and developing latent images has progressed, particularly in the area of ; paper-based feedback systems for educational uses, the need for conveniently applying color-generating components in selected areas on substrates such as paper has become more acute. Vapor and pressure transfer techniques have been ", .,, . ~ ,

-2-employed wherein a latent coreactant is applied to selected areas of a paper sheet to provide a latent sensitized area which can be subsequently developed by the applicatlon of a second coreactant from a source such as a wax crayon applied to the areas of the sheet containing the latent coreactant.
These systems are exemplified by U.S. Patents 3,784,394,

3,725,104, 3,682,673, and 3,617,324. Spirit dupllcation techniques have also been employed to provide concealed lmages for subsequent development as exemplified in U.S.
~-~ 10 Patents 3,451,143 and 3,632,364.
U.S. Patent 3,632,364 also discloses the application of concealed images by lithographic, stencil, letterpress, gravure, hand stamp, and silk screen techniques.
The disclosed method involves the application of a special oxidlzing compound, capable of oxidizing an iodide to iodine, to selected areas of a sheet which contains starch or polyvlnyl alcohol. Subsequent application of an oxidizable iodlde compound ln the areas ln whlch the oxldlzlng compound has been applled to the substrate results ln a vlsible image ~; 20 belng formed.
While the prlor art vapor and pressure transfer and splrlt dupllcatlng technlques have proven useful in providing ~-small numbers of copies containing concealed images, a satisfactory means of providing large quantities of such copies by conventional printing methods using plain paper has been sought. The aforementioned U.S. Patent 3,632,364 ~' requlres a speclal iodide oxidation system in combination ~ .~
., ~6~9 with a prepared sheet containing starch or polyvinyl alcohol.
U.S. Patent 3,850,649 relates to printing a latent coreactant capable of forming colored complexes with metals by lithographic offset printing techniques. The coreactant is carried in a special two-phase, quick-set ink vehicle suitable for lithographic offset printing. U.S. Patents 3,540,909 through 3,540,914 also disclose "colorless"
printing inks containing a phenolic polymeric material dissolved in a petroleum solvent. After spot printing, the acidlc polymeric materlal can be contacted with a basic chromogenic material to form a visible image. U.S. Patent

4,063,754 issued December 20, 1977, discloses compositions and processes for providing solventless or "hot-melt" imaging compositions.
15The present invention has overcome the disabilitles of the prior art by providing latent, sensitizing inks comprising, based on the weight of the total ink compositlon, about 5 to 55% by welght vehlcle component which ls a non-polymerlc, oleophlllc, organic, proton donor acid anion having a cationlc counter ion, up to about 30% by weight thinner and up to about 70% by weight particulate flller and 20 to 30~ `by welght polymeric hydrocarbon, thermoplastic, terpene resin. The ink also includes as one ~; of the above components, or as an additional component, at least about 5% by weight of at least one color-generating ~; component. The ratio of filler to the vehicle component is from about 0.1:1 to 6:1. The polymeric hydrocarbon :

.

36~

thermoplastic terpene resins useful in the invention are known, are commercially avallable under the trade name "Piccolyte" Pennsylvania Industrial Chemical Corp.) and are composed essentially of polymers of pinene, especially beta-pinene. These polymeric terpene resins possess the unique property, in the environment of the present invention, to crack or "craze" on drying after application so that the surface of the ink remains open and receptive to color-forming coreactants. This effect is particularly surprising in view of the tendency of conventional polymeric ink vehicles, e.g., varnishes, to form films which seal the surface of the film and inhibit color-forming reactions.
Thus, the addition of this unlque polymeric component provides latent inks having improved imaging properties over latent inks not having this component. When this polymeric component is used it has been found that the level of filler can be reduced significantly. Thus, in certain formulations the ratio of filler to non-polymeric vehicle component can now be reduced to levels of about 0.1:1, preferably about ;1 20 0.2:1, as the lower limit. Other ingredlents such as additional tack and viscosity modifiers, antioxidants, wetting agents, optlcal brlghteners and the llke can be added as necessary.
The proton donor acld refers to the class of proton donor compounds which donate protons to water molecules in water solution.
As used herein, the term "color-generating . .
:``

component" refers to any of the materlals known in the carbonless paper art which will become colored or effect the visible coloring of a separate material when contacted with a color-activating component. Thus, the combination of materials is essential for color-formation. For purpose of convenience herein the components in the latent, sensitizing ink will be referred to as the color-generating component, while the component which is subsequently used to develop the color will be called the color-activating component, although the actual components can be interchanged as will be described hereinafter.
The vehicle components described above are preferably the aliphatic, aromatic and allcyclic carboxylic and sulfonic acids containing at least 6 carbon atoms and the cation contalning salts of these acids. These vehicle components have sufficiently oleophilic moieties to provide acceptable inking qualities and promote rapid and intense development of the sensitized areas when used in the formulations of thls lnventlon. They are partlcularly effective wlth the metal complexing color-generating/
- color-activating components such as the metal/dithiooxamide (DT0)/polyhydrlc phenol combinatlons which require a cosolvent reaction medium for rapid, lntense development of the æensitlzed area. Exemplary of these vehicle components are the rosin, stearic, oleic, 2-ethylhexoic, 2-phenyl-butyrlc, benzolc, hydrocinnamlc aclds and dinonylnaphthalg~e , ,, sulfonlc acids as well as the corresponding cation salts of , ,., ~

.

.`~6~9 these aclds.
The described vehicle components are generally olly liqulds or crystalline or~amorphous waxy solids and when dispersed or dissolved ln a thinner exhlblt the deslrable blndlng and vlscoslty modifying characteristics of conventional polymeric binders wlth respect to tack, adheslon, and the likeJ whlch are essentlal to the suitabillty of the inks for appllcation by conventional printing methods.
As noted above the color-generating component can be one of the recited lnk components or can be an addltlonal dlstlnct component. For example, the color-generating component can be the recited vehlcle component or the flller or both. Alternatlvely the lnk can include a separate transltlon metal salt as an addltlonal J dlstlnct color-generatlng component.
In one embodiment, the vehlcle component ls a `
color-generatlng vehlcle component whlch 18 an oleophlllc, ; organlc proton donor acld anlon contalnlng at least 6 carbon ;~. 20 atoms and havlng a transltlon metal counter-lon. The ; "
'~ transition metal counter-ion forms a colored complex when `~ contacted with a color-actlvatlng metal complexlng agent, ~ such as dlthiooxamide (DT0) and its derivatives and the c~
.~ polyhydric phenols.

The oleophillc anlon molety alds ln provldlng good " . ., ~ lnklng quallties and ln promoting the subsequent development i: ' ~X of the latent, sensltized lnk.

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.
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In this embodiment the vehicle component also acts as the color-generating component. Because of this dual characteristic of the vehicle component, these inks are particularly effective to provlde greater concentratlons of available color-generatlng component per unit area of the substrate to which the ink ls applied than is possible using color-generatlng components carrled ln conventlonal polymeric, film-forming vehicles. Thus, these inks can provide latent sensitized areas which can be more quickly and intensely developed by contact with color-activating components than is possible by formulating inks having color-generating components in conventional ink vehicles, such as conventional lithographic inks containing varnish.
The transition metal counter ion of these color-15 generating vehicle components is preferably selected from among nickel, copper, lron and cobalt~ Generally nlckel and ;~ iron are preferred because of the dark color these matals produce with conventional color-activating coreactants such as DT0 and its derivatives or the polyhydric phenols.
20 Representative color-generating vehicle components which can `~ be used in the present invention are the nickel, iron, and copper derlvatlves of aliphatic, aromatic and alicyclic carboxylic and sulfonic acids containing at least 6 carbon ; atoms and combinations thereof. Thus, nickel rosinate, 25 nickel calcium rosinate, nickel 2-ethylhexoate, nickel stearate, nickel 2-phenylbutyrate, nickel oleate, nickel benzoate, nickel hydroclnnamate, nickel dinonylnaphthalene . . ~ _ , , ~ 1-.. - ... . .
. ,. :~ . .

sulfonate, as well as the corresponding copper and iron salts of the above compounds, and mixtures of two or more of the above compounds are useful.
An essential ingredient of the latent, sensitizing

- 5 inks of the present in~ention is a particulate filler which can be dispersed in the liquid ink vehicle. These fillers are necessary to maintaln the sensitized area suitably receptive to the color-activating material used to develop the latent ink. These fillers can be any of the conventional pigments and extenders whlch are known in the printing art.
The flllers can be chosen so as to be nearly transparent when dispersed ln the ink vehicle or can be colored if desired.
Thus, when applled to a substrate, the latent, undeveloped ink can be transparent so as to be invisible or can have a :~
color which closely matches or which contrasts with the substrate to which the lnk is applled, depending upon the end use of the sensltized substrates.
The flller must be chosen with some care dependlng on the partlcular prlntlng method to be employed ln lts , 20 application. Thus, for certaln prlntlng technlques, such as wet offset prlntlng, a hydrophoblc filler should be used to provlde superior results. Inks formulated in thls manner can i also be used to advantage ln dry offset prlntlng. A slmple emulsification test can be used to determlne whether the fillers are suitably hydrophobic. This test can be performed by grinding the pigment into the desired oil and mixing the resulting dispersion with the desired aqueous offset fountain , 61~
g solution. After thoroughly mixing the pigment-oil combination with the ~ountain solution, the fountain solution is decanted and the remaining pigment-oil component weighed. If the weight gain is more than 20Z by weight the pigment is generally unsuitable for use in wet offset printing. For dry offset printing, a relatively non-hydrophobic ~iller, such as the colloidal silicas sold commercially under the trade ~ffle Cab-O-Sil (Cabot Corporation) may be used if desired.
The ultimate particle size of the flller in the final ink composition should be less than about 10 micro-meters and preferably less than 5 micrometers in order to be readily applied by conventional printing presses.
Representatlve flllers which can be used are fumed alumina, alumlna hydrate, and trihydrate, powdered and fumed anhydrous and particulate colloidal silica, such as the s'~
commercially available "Aerosils" and "Cab-O-Sils" (available from Degussa, Inc. and Cabot Corporation, respectively), calcium and magnesium carbonate, barium sulfate, kaolin clay, attapulgite clay, bentonite clay, zeolltes, zinc oxide, - 20 ureaformaldehyde pigment, and the like.
The filler can comprise up to about 70~ by welght ,: ,.; .
of the lnk compoæitlon. The larger amounts of filler may be ;~l necessary on non-absorptlve, smooth papers, whereas lower amounts of the fillers can be used where the paper readily absorbs the lnk or has a rough surface whlch aids ln keeplng the inked surface receptive to the coreactant. For most applications, the flller preferably comprises up to about 25 .. - .. :. .. - .. ~. . ..

~1~96~9 percent by weight of the ink composition and most preferably about 5 to 15 percent by weight of the composition.
Within the range of compositions disclosed above, it has been found that the ratio of filler to the aforementioned vehicle component is important. In order to obtain the desired printing characteristics along with superior imaging speed and image intensity when the sensitized areas are contacted with a color-activating component, the ratio of filler to vehicle component should be from about 0.1:1 to about 6:1 and preferably about 0.2:1 to ; 1:1.
As stated previously, certaln of the filler components can function as color-generating components. The acldified clays, e.g., kaolin, attapulgite and bentonite clays, and the natural and synthetic zeolites can be used to provlde both the color-generating and filling functlon in the ~- ink compositlon slnce they possess the ablllty to provide an ~` intense color when contacted with the color-activatlng basic chromogenic materials, i.e. the leuco dyes, which are well known in the carbonless paper art. Generally when these fillers are used as color-generating components the welght percent of the flller should be about 15% by weight or greater to obtain satisfactory color development in the sensltized areas.
The thlnners which are used in combination with the aforementioned vehicle components in the inks of the present lnventlon are materials whlch are known in the : . , .

, ~ :' ' : ' ,' .,. :
.

printing art. These materials are solvents, diluents, and low viscosity oils which are added to ink to reduce their consistency and tack, thereby modifying the rheological properties of the ink as required for use in a particular printing method. Typical thinner materials are liquid hydrocarbons, castor oils, dialkyl phthalates, trialkyl phosphates such as tributyl phosphate, alkyl carboxylates, low molecular weight alcohols, fatty alcohols, and the like.
The amount and type of thinner to be used in any particular composition varies, depending primarily upon vehicle component and the oil-receptive filler employed. A
sufficient amount of the thinner is incorporated into the formulation to provide the proper viscosity and tack for the particular prlnting method employed. Determination of ink tack values as is conventional in the art can be used to determine the proper amount of thinner to be added.
Generally up to about 30% by weight of thinner can be used.
Preferably the amount of thlnner is about 10 to about 25% by :
; welght of the total lnk composltlon.
Although the use ln thls lnventlon of the aforementloned vehicle components ln combination wlth thlnners ellmlnates the need for conventlonal varnishes in ` order to obtain satlsfactory lnklng propertles, varnlshes can be used ln the formulatlons if deslred to modlfy further the tack, vlscosity and other rheologlcal properties where these properties are difficult to obtaln with normal comblnatlons of the vehicle component and thinner. Useful varnishes are .

~1~96~9 the drying oils and other naturally occurring and synthetic polymers known in the art such as the phenolics, linseed oils, alkyds, and modified alkyds, nitrocellulose, tung oil, cellulose acetate, ethyl cellulose, and the like. Up to about 40% by weight of varnish, based on the weight of the total ink composition, may be used. Preferably the varnish comprises less than about 10% by weight of the ink compositlon since these varnishes tend to inhibit subsequent ; development of the latent inks due to thelr fllm-formlng propertles.
Agents whlch lnhlblt the effect of oxygen on the components of the ink, i.e. antioxidants, can also be added to stabllize the ink components and the sensitlzed areas a~ter development. Useful antioxidants are well known in the prlnting ink art, and any of the known antioxidants which do not react with the color-generating component can be used.
If an antioxidant which reacts with the color-generating component is used, the ink may be undesirably "desensitized"
or a colored reaction product may be formed which causes undeslrable coloring of the sensitlzed area.
Exemplary of the antioxldants which can be used are thlourea, hydroquinone, hindered phenols such as alkylated hydroxytoluene, and the like. Preferably the antioxidant comprlses less than 10% by welght and most preferably less than about 2.5% by welght of the total lnk compositlon.

- . . . . ~ .

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.

6~9 Other conventional ink additives can be added to the ink formulation if desired, such as lubricants, optical brighteners, dyes, waxes, buffers, wetting agents and odorants and the like, to improve the application, performance and aesthetic qualities of the ink. Generally these additives comprise less than about 2% by weight of the ink composition and most preferably comprise less than about 0.5% by weight of the total ink composition.
When the filler is, or contains, one of the aforementloned acldified clays or zeolites, the filler itself can be a color-generating component in lieu of or in addition to a color-generating vehicle component. If both the vehicle and filler contain a color-generating component, the latent, sensitizing ink may be developed by a plurality of distinct color-activating components.
In addition to or in lieu of the above-noted ~, color-generatlng components, an additlonal color-generatlng component or mixture of color-generating components can be ~; incorporated lnto the ink formulatlon. Thus, the lnk ; 20 formulatlon can comprlse a transltlon metal salt such as the nltrates, sulfates or hallde of the transltlon metals, DTO
or DTO derivatives, the polyhydrlc phenols or the leuco dyes all o~ which are known in the art as color-generating/
color-activatlng components.
If more than one color-generatlng component is used in the lnk formulatlon, care must be taken to see that the components do not prematurely react with each other.

. - ., - ~ ., . .: .~ .
:. .. , . . . .: ~ , : . .. ., . ., , .. : . : :.. , ~ ,.... . -6~ 9 Thus generally a transltion metal salt and a DT0 derlvative would not be incorporated in the same formulation. Simllarly an acidi~ied clay and a leuco dye would not be combined.
Typically, the ink may contain a transition metal ion and an acidified clay so that the ink can be developed by either a DT0 or other metal complexing compound or by a leuco dye or both.
As noted previously, the inks can contain ingredients, such as varnish, to modlfy further the tack, viscosity and other rheological properties of the ink.
Additional cnventional lngredlents such as antioxidants, lubricants, optical brighteners, dyes, waxes, buffers, wetting agents, odorants and the like can be added as necessary and generally comprise in total less than about 10%
~-~ 15 by weight of the total composition. For example, the ink may :., requlre up to about 7% by weight antloxldant. However, the amount is generally about 2.5% by weight or less, with the remaining additives ranging up to about 2% by weight and preferably less than about 0.5% by welght.
A preferred improved lnk formulatlon would comprise about 5 to 55%, and preferably about 30 to 45% by weight vehicle component which is a non-polymeric, oleophillc, organic acld anion havlng a catlonic counter ion, such as nlckel-2-ethylhexoate, 20 to 30% by weight polymeric, hydrocarbon thermoplastic terpene resin, e.g. Piccolyte resin, about 5 to 15% by weight particulate filler, up to about 30Z by weight thinner, and about 1% by weight - .
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-, . ~
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9~ 9 antioxidant.
The inks of the present invention can be prepared using conventional ink milling equipment. Generally the ink is prepared by admixing all of the ingredients and blending to form a homogeneous mixture of working consistency suitable for an ink mill. In some cases the amount of particulate filler may be such that only a portion of the flller can be added before the ink is milled. The admixture is then run through an ink mill such as a three-roll ink mill or other ~ 10 conventional ink mill until a homogeneous tacky-viscous fluid `: i8 formed. Any remaining filler is then added and the mixture further milled to form a homogeneous dispersion. The particle size in the final mixture should be about 10 micrometers, or less, for ease of application by conventional printing methods. Other liquid ingredients as previously noted can be added to the milled ink to ad~ust the tack, viscosity and other rheological properties of the ink as required.
- The inks of this invention can be applied to selected areas of substrates such as paper by conventional printing techniques, including lithography, flexography, letterpress, dry offset, rubber plate, intaglio, silk screen, rotogravure, and the like. Applylng these ink compositions wlth conventlonal printing equipment makes it practical to pattern or spot coat substrates, and thus, selectively sensitlze paper or other substrates for business forms appllcatlons where there is a desire to have an lmage appear ... . . ,, ", ., .. - .. , :. , ,, ... ,~ : - .

-16- ~ `9 only on certain portions of a form or on certaln sheets of a multi-sheet form, such as invoice and purchase order forms where price or cost information is needed only on certain sheets of the multi-sheet form.
; 5 The sensitized areas of the substrates can then be developed by contacting the sensitlzed area, or a portion of the area, with a color-activating component. Typically, the color-activating component is contained as an encapsulated component on a sheet of paper, e.g., a CB (coated back) sheet, so that when the CB sheet is placed in contact with ; the sensitized sheet and the capsules ruptured, as by writing or typing on the obverse surface of the CB sheet, the color-activatlng component contacts the sensitized area, or a portlon thereof, containing the color-generating component and a visible image is formed.
Alternatively, the sensitized areas can be contacted with a crayon, or developing solution contalned in a marklng pen, which contalns a color-actlvating component and the sensitized area vlslbly developed.
Buslness forms whlch are multi-sheet forms can comprlse one or more CB sheets overlylng and in register with one or more sensitized CF (coated front) sheets. Thus a multi-sheet form may have a CB sheet as the top sheet overlying one or more sheets in register which are CB sheets havlng at least a part of the obverse surface sensltlzed.
Writlng or typing on the obverse surface of the top sheet causes llke lnformatlon to be recorded in the obverse l~g6~9 sensitized portions of the underlying sheets which are in register with the written or typed information entered on the ; top sheet of the multi-sheet form.
.
In addition to the latent, sensitizing ink applied to the surface of the sheets, conventional printing can be ; applied to the sheets in the sensitized or unsensitized areas to provide forms for business entries, paper based feedback systems for educational use and the like.
The following examples further illustrate the .~ 10 present invention. In these Examples, all parts and percents are by welght, unless otherwise lndicated.

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~ , . . , , - .. . : . , .:-, . ,. .: - . , . , --18~ S6~g ..-3 Example 1 An lnk for sensitizing selected areas of paper with r a latent coreactant was prepared having the following formulation: -Weight %
- 5 Nickel-2-ethylhexoate 38.5 Hydrophobic silic~a filler 11.5 (Aerosll R-972, Degussa, Inc.) Polymeric terpene resin 25.4 (Piccolyte S-135~

Butylated hydroxy toluene 1.0 (antloxidant) 10 Hydrocarbon Oil (Magie Oil #520) 23.6 The nickel-2-ethylhexoate, terpene resin and antioxidant were dlssolved in the hydrocarbon oil. The filler was then dispersed in the solution and the final dispersion milled on a three-roll ink mill to a particle size of about 3 micrometers or less.
The ink was applied to 16 lb. bond paper by a hand roller in sele¢ted areas of the sheet. After drylng at room temperature, the sensltized sheets were developed by placlng a sheet coated wlth an encapsulated dlthlooxamlde derivatlve ("3M" Brand Carbonless Paper, Type 200 CB
[coated back] sheet) so that the capsule coating and the inked surfaces were ln direct contact. The sheets were run through a steel pressure roll to develop uniformly a 1.2 cm wlde strlp of the sensitlzed sheet. The developed area had an acceptable blue-purple color as determined by optical denslty measurements.

:
Sensitied sheets were also developed by placing ~ a sheet coated with an encapsulated dlthiooxamide ;~ derivative (CB sheet described above) so that the capsule coating and the inked surfaces were in direct contact.
Images were produced on the sensltized sheet by writing on the obverse surface of the CB sheet with a ball-point pen :,.
~ using normal hand pressure. Similarly, typing on the ,~ obverse surface o~ the CB sheet produced an image on the sensitized sheet. In both cases image formation was very rapid.

Example 2 An lnk for sensitizing selected area~ of paper wlth a latent coreactant and particularly adapted for - application by a dry o~fset press was prepared having the 1~ formulation shown ln Example 1 except that a non-hydro-phoblc slllca filler tCab-O-Sll M-5) was substltuted for the Aerosll R-972 ~iller. This composltion was satls-factorlly prlnted on a dry offset press and was satisfactorily developed by a CB sheet as in Example 1.

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Claims

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A latent printing ink for providing latent, sensitized areas on substrates, which ink can be applied to a paper substrate by a printing press and subsequently developed with a pressure-releasable, encapsulated, color-activating compound carried on a substrate, characterized in that said ink comprises, based on the weight of the total ink composition, about 5 to 55% by weight of a vehicle component which is a non-polymeric, oleophillic, organic acid anion having a cationic counter-ion, 20 to 30% by weight of a polymeric, hydrocarbon, thermoplastic terpene resin, up to about 70%
by weight particulate filler and up to about 30% by weight thinner and wherein said ink includes at least 5% by weight of at least one color-generating component, the ratio of said filler to said non-polymeric vehicle component being from about 0.1:1 to 6:1.

2. A latent ink according to Claim 1 wherein said anion is selected from the group consisting of aliphatic, alicyclic and aromatic carboxylic and sulfonic cold anions and mixtures thereof.

3. An ink according to Claim 1 wherein said cationic counter-ion is a transition metal ion.

4. An ink according to Claim 3 wherein said metal ion is selected from the group consisting of nickel, copper, iron and cobalt.

5. An ink according to Claim 1 wherein said filler comprises an acidified clay.

6. An ink according to Claim 1 wherein said thinner is a hydrocarbon oil.

7. A sheet having selected areas on at least one major surface sensitized with the latent, sensitizing ink of Claim 1.

8. A sheet according to Claim 7 wherein the sensitized surface is the obverse surface of the sheet and the reverse surface carries a color-activating component.

9. A multi-sheet form comprising a first sheet having on the reverse surface a color-activating component and, underlying said first sheet, a second sheet having at least a portion of the obverse surface sensitized with the latent, sensitizing ink of Claim 1.