GB2043671A - Latent sensitizing composition - Google Patents

Abstract

A humidity stable, latent sensitizing composition comprising (1) a color- forming, transition metal salt of an organic or inorganic acid, (2) a diterpene stabilizing agent, and (3) a substantially non-aqueous liquid vehicle. A solubilizing acid can be employed in combination with the sensitizing compositions. Stabilized, latent, sensitizing inks are also disclosed.

Description

SPECIFICATION Stabilized latent sensitizing composition The present invention relates to humidity-stable, latent sensitizing compositions and latent sensitizing inks prepared from these compositions. More particularly, stabilized, latent, flexographic sensitizing compositions are described.

Impact or pressure-sensitive self-marking carbonless transfer papers have been known for a number of years. The most common type of carbonless impact transfer paper comprises sheets having coated on the back surface (reverse surface) thereof tiny microscopic capsules containing liquid fill and having chemically reactive color-forming metal chelating compounds conventionally referred to as dye precursors herein.

These coatings are commonly referred to as CB coatings. The other surface of the sheet, generally the top surface or front surface (obverse surface), has a dry coating of a coreactant metal-containing chemical for the dye precursor. These sensitizing coatings are commonly referred to as CF coatings. When the CF and CB coatings are brought into contact, such as in a multi-sheet form, and pressure is applied so as to rupture the capsules on the CB surface and cause the dye precursor to migrate onto the CF coating, a chemical chelating reaction takes place and a colored complex is formed in the area of the ruptured capsules. The dye precursor in the capsules can be one of the known dye precursor metal chelating materials such as the N,N'-di-substituted dithiooxamide chelating agents which will form colored complexes with a transition metal cations.A particular substituted dithiooxamide derivative which has been found useful is the combination of N,N'-di-benzyl-dithiooxamide (DBDTO) and N,N'-bis(2-octanoyl oxyethyl) dithiooxamide (DOEDTO). These materials are usually present in an organic solvent, such as cyclohexane, within the microcapsule and are generally present in amounts of about 4% to 8% by weight of the capsule fill.

The CF sensitizing coating used with these dye precursor CB materials contains a coreactant chemical which is a transition metal salt. Nickel salts are preferred since they tend to provide fairly colorless coatings prior to reaction. Other metal cations, such as mercury, cadmium, lead, zinc, copper, cobalt and silver will also produce images. However, certain of these compounds such as cobalt, cadmium and zinc cation containing compounds react with the dithiooxamide derivatives to produce very nearly colorless products.

Consequently, where the use of white paper is desired, they form images having very little, if any, contrast with the background of the paper itself. The transition metal salt may be the salt of an organic or inorganic acid. Thus, nickel salts of, for example, nitric or sulfuric acid can be used. More preferably, the salts of organic acid are used. Nickel rosinate and nickel-2-ethylhexoate are two widely used color forming transition metal salts.

Because the CF and CB imaging compositions are generally relatively colorless when applied to sheets and form color only after reaction takes place, these materials are commonly referred to as latent sensitizing compositions.

Previously, the application of these latent sensitizing materials involved the coating of the complete surface of substrates using aqueous coating compositions and techniques. These techniques do not lend themselves to the application of the latent materials in selected, patterned areas. Accordingly, it is desired to provide latent, sensitizing compositions, particularly latent CF sensitizing compositions containing transition metal salts, which can be applied to selected, patterned areas of substrate by conventional printing and coating techniques.

One such sensitizing composition is described in U.S. Patent 4,111,462, issued September 5, 1978. It has been found, however, that such compositions, which by necessity must remain "open" and receptive to the dye precursor material released by the capsules on a CB sheet, also may exhibit reduced imaging speed and intensity on prolonged storage due to interaction with atmospheric moisture. Further, in certain circumstances, transfer of the sensitizing composition to the overlying CB sheet may occur. This "back transfer" can then cause objectionable coloration on the overlying CB sheet.

The present invention overcomes the difficulties encountered by prior art compositions by providing a transition metal salt color-forming component in combination with a stabilizing agent for said color-forming component such that the composition is stable in a substantially non-aqueous vehicle and is not degraded significantly by the effects of atmospheric moisture following application to a substrate. Inks formulated utilizing these compositions do not exhibit objectionable back transfer. The stabilized imaging composition comprises a blend of (1) a color-forming transition metal salt and, (2) a diterpene stabilizing agent, dissolved in a substantially non-aqueous (i.e. less than 50% by weight water), liquid vehicle.These latent compositions are referred to herein as "stable" or "stabilized" in that they retain a greater percent of their initial image speed and intensity over time, particularly when exposed to conditions of high relative humidity following application to a substrate, than do compositions not having the stabilizing agent.

While not wishing to be bound by any particular theory or mechanism, it is belived that the transition metal salts may undergo hydrolysis in the presence of atmospheric moisture forming undesirable side products which reduce the concentration of the color-forming component available. It is believed that the stabilizing agents used in the present invention tend to inhibit the hydrolysis or its undesirable effects and allow the imaging composition to substantially maintain its initial imaging characteristics even under conditions of high humidity. More specifically, it is believed that the diterpene stabilizing agents may react with free transition metal ions which may be formed by hydrolysis or dissociation of the color-forming transition metal salt to form a more stable, nickel diterpenate, which in itself is a color-forming compound.

Thus, by addition of an appropriate amount of the diterpene stabilizipg agent, stabilized sensitizing compositions can be provided and various liquid, latent, sensitizing compositions can be formulated, such as flexographic inks, gravure inks, off-set inks, lithographic inks and other coating and printing compositions.

The color-forming transition metal salts useful in the present invention can be the transition metal salts of organic or inorganic acids. The preferred transition metal salts are the nickel salts, although copper iron and other transition metals can be used to advantage. Inorganic acids which can be used to form the transition metal salts are acid which will provide salts with transition metals which dissociate to provide the transition metal for the color-forming reaction. Exemplary inorganic acids are nitric acid, sulfuric acid, hydrochloric acid and the like.

The organic acids which are useful in forming the transition metal salts of the present invention are the aliphatic and aromatic mono- and di- carboxylic acids, substituted aliphatic and aromatic monocarboxylic acids, and heterocyclic monocarboxylic acids. Monocarboxylic aliphatic acids containing about 6 to 10 carbon atoms are preferred. Nickel-2-ethylhexoate is a particularly preferred color-forming transition metal salt.Other nickel salts which have been investigated are nickel stearate, nickel rosinate, nickel laurate, nickel acetate, nickel-2-ethylhexoate, nickel decanoate, nickel cyclohexoate, nickel-2-ethylbutyrate, nickel caproate, nickel behenate, nickel-12-hydroxystearate, nickel benzoate, nickel o-chlorobenzoate, nickel chlorobenzoate, nickel 3, 4-dyhydroxybenzoate, nickel p-ethoxybenzoate, nickel 2-dimethyl aminobenzoate, nickel m-aminobenzoate, nickel salicylate, nickel succinate, nickel sebacate, nickel terephthalate, nickel camphorate, nickel 3-pyridine-carboxylate, nickel oleate, nickel itaconate, nickel fumarate, nickel n-octate, nickel 3-hydroxy-2-naphthoate, nickel linoleate, nickel undecylenate.As noted herein-above, nickel-2ethylhexoate is preferred for use in certain ink formulations since it provides superior properties with respect to solubility in the alcohol vehicles used in flexographic inks, initial image speed, and a low order of toxicity.

Nickel rosinate also has particularly desirable imaging properties, but is soluble at relatively low levels in many conventional ink vehicles.

The diterpene stabilizing agents which impart the improved humidity resistance to the sensitizing compositions of the present invention are components containing resin acids, alkali metal or alkaline earth metal salts of the resin acids, mixtures of such resin acids and their salts, and rosin compositions comprising a major fraction of resin acids, e.g., at least 25% by weight and preferably greater than 75% by weight, free resin acids.Representative rosin compositions meeting these requirements are the tall oil rosins, such as are commercially available from the Glidden Chemical Company under the trade name "Sylvaros", such as Sylvaros R Tall Oil Rosin which contains about 85% free resin acids, (22-25% of abietic acid) an acid number of about 161-165 and a softening point of about 81-85C. Tall oil rosin is often used because of its cost and ready commerical availablility.

The known resin acids are abietic acid, pimaric acid, the isomers of abietic and pimaric acids, elliotinoic acid and sandaracopimaric acid. See for example "Encyclopedia of Chemical Technology", Second Edition, (1968) published by Interscience Publishers, at Volume 17, pages 482 & 483. Exemplary salts are the sodium, potassium and calcium salts of these resin acids, particularly salts of abietic acid.

The diterpene stabilizing agent provides increasing stabilizing effect even at very low concentrations. It has been found that the stabilizing effect increases with increasing concentration of the stabilizing agent up to a ratio of about 1.75 parts stabilizing agent to 1 part transition metal ion, e.g., nickel ion. At weight ratios above this level, the effectiveness is only marginally increased. Amounts in excess of this level, for example, in a ratio of about 6 to 1 can be used, but may not be economically feasible. Depending on the composition to be formulated, amounts of the diterpene stabilizing agent in excess of the 1.75 ratio may not affect the composition properties significantly. In the preparation of flexographic inks, which are generally very low viscosity inks, the amount of diterpene agent can vary widely without affecting the properties of the ink.In other formulations, e.g., off-set inks, a weight ratio somewhat above the 1.75 ratio, may in fact be necessary to impart desirable viscosity and tack properties to the ink formulation. Thus, widely varying amounts of the stabilizing agent can be employed to advantage depending on the use for which the imaging composition is intended.

In terms of weight ratio of commonly used components, a weight ratio of diterpene stabilizing agents to nickel-2-ethylhexoate of about 0.33 or above provides the most desirable stabilizing effect.

The stabilized sensitizing compositions of the present invention can be advantageously used in formulating various liquid, latent, sensitizing compositions. Generally the stabilized imaging compositions are dissolved or dispersed in a vehicle having properties suitable for the particular application method contemplated. Representative ink and paint vehicles include the aromatic, aliphatic, chlorinated or nitrated hydrocarbons, such as toluene, VM & perchloroethylene, 2-nitropropane; ketones such as methyl ethyl ketone; esters such as ethyl acetate and butylcorbitol acetate, ethers such as diethyl ether; amides such as dimethylformamide; alcohols particularly the low molecular weight alcohols such as methanol, ethanol and diacetone alcohol; and glycols such as polypropyleneglycol.

The compositions of the present invention can be utilized to provide a variety of printing inks. In one embodiment of the present invention an ink comprising a vehicle component which is a non-polymeric, oleophilic, organic acid anion (that is, an anion of a proton donating acid) having a cationic counter-ion, a transition metal salt color-forming component, and a diterpene stabilizing component comprising a resin acid, alkali metal or alkaline metal earth resin acid salt or mixtures thereof. These resin acid components aid in providing acceptable tack and viscosity to the ink like a conventional ink vehicle, yet surprisingly, also aid the ink in maintaining imaging speed on exposure to atmospheric moisture which would otherwise reduce imaging speed.Thus, the diterpene stabilizing component provides latent inks having improved imaging properties over latent ink not having this component. These inks can be conveniently formulated so that they can be applied to selected areas of substrates by a variety of conventional printing methods, particularly offset and gravure methods.

These latent, sensitizing inks comprise, based on the weight of the total ink composition, about 5 to 55% by weight vehicle component which is a non-polymeric, oleophilic, organic, acid anion having a cationic counter ion, a stabilizing amount, e.g. 20 to 30% by weight, of diterpene stabilizing component, up to about 30% by weight thinner and up to about 70% by weight particulate filler. 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.

Other conventional ingredients such as additional tack and viscosity modifiers, antioxidants, wetting agents, optical brighteners and the like can be added as necessary.

The non-polymeric vehicle components described above are preferably the aliphatic, aromatic and alicyclic carboxylic and sulfonic acids containing at least 6 carbon atoms and the cation containing 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 this invention. They are particularly effective with the metal complexing color-generating/color-activating components such as the metal/dithiooxamide (DTO)/polyhydric phenol combinations which require a cosolvent reaction medium for rapid, intense development of the sensitized area.Exemplary of these vehicle components are the rosin, stearic, oleic, 2-ethylhexoic, 2-phenylbutyric, benzoic, hydrocinnamic acids and dinonylnaphthalene sulfonic acids as well as the corresponding cation salts of these acids.

The described vehicle components are generally oily liquids or crystalline or amorphous waxy solids and when dispersed or dissolved in a thinner exhibit the desirable binding and viscosity modifying characteristics of conventional polymeric binders with respect to tack, adhesion, and the like, which are essential to the suitability of the inks for application by conventional printing methods.

In one embodiment, the vehicle component is a color-generating vehicle component which is an oleophilic, organic acid anion containing at least 6 carbon atoms and having a transition metal counter-ion.

The transition metal counter-ion forms a colored complex when contacted with the color-activating metal complexing agent, such as dithiooxamide (DTO) and its derivatives and the polyhydric phenols.

The oleophilic anion moiety aids in providing good inking qualities and in promoting the subsequent development of the latent, sensitized ink.

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 provide greater concentrations of available color-generating component per unit area of the substrate to which the ink is applied than is possible using color-generating components carried in conventional 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-generating vehicle components is preferably selected from among nickel, copper, iron and cobalt. Generally nickel and iron are preferred because of the dark color these metals produce with conventional color-activating coreactants such as DTO and its derivatives or the polyhydric phenols. Representative color-generating vehicle components which can be used in the present invention are the nickel, iron and copper derivatives of aliphatic, aromatic and alicyclic carboxylic and sulfonic acids containing at least 6 carbon atons and combinatins thereof.Thus, nickel rosinate, nickel calcium rosinate, nickel-2-ethylhexoate, nickel stearate, nickel 2-phenylbutyrate, nickel oleate, nickel benzoate, nickel hydrocinnamate, nickel dinonylnaphthalene 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 offset or gravure ink according to the present invention is a particulate filler which can be dispersed in the liquid ink vehicle. These fillers are necessary to maintain 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 which are known in the printing art. The fillers can be chosen so as to be nearly transparent when dispersed in the ink vehicle or can be colored if desired. Thus, when applied 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 ink is applied, depending upon the end use of the sensitized substrates.

The filler must be chosen with some care depending on the particular printing method to be employed in its application. Thus, for certain printing techniques, such as wet offset printing, an oil-receptive hydrophobic filler should be used to provide superior results. Inks formulated in this manner can also be used to advantage in dry offset printing. A simple oil absorption test can be used to determine whether the fillers are suitably oil-receptive. An oil absorption test such as ASTM D-281 -31 can be used. The results of these tests are generally expressed as parts by weight of oil absorbed per 100 parts by weight of filler. It has been found that a filler tested using linseed oil according to ASTM D-281-31 should have an oil absorption value of at least about 5 in order to be suitable for wet offset printing.For dry offset printing, a relatively non-hydrophobic filler, such as the colloidal silicas sold commerically under the trade name Cab-O-Sil (Cabot Corporation) may be used if desired.

The ultimate particle size of the filler in the final ink composition should be less than about 10 micrometers and preferably less than about 5 micrometers in order to be readily applied by conventional printing presses.

Representative fillers which can be used are fumed alumina, alumina hydrate, and trihydrate, powdered and fumed anhydrous and particulate colloidal silica, such as the commerically available "Aerosils" and "Cab-O-Sils" (available from Deguss, Inc. and Cabot Corporation, respectively), calcium and magnesium carbonate, barium sulfate, kaolin clay, attapulgite clay, bentonite clay, zeolites, zinc oxide, ureaformaldehyde pigment, and the like.

The filler can comprise up to about 70% by weight of the ink composition. The larger amounts of filler may be necessary on non-absorptive, smooth papers, whereas lower amounts of the filler can be used where the paper readily absorbs the ink or has a rough surface which aids in keeping the inked surface receptive to the coreactant. For most applications, the filler preferably comprises about 5 to about 25 percent by weight of the ink 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 0.1 :1 to about 4:1. When filler amounts below the 0.1 :1 ratio are used, the sensitized areas may develop with less speed and intensity. Ratios above about 6:1 are generally not satisfactory for use on conventional printing presses.

Certain of the filler components can function as color-generating components. The acidified clays, e.g., kaolin, attapulgite and bentonite clays, and the natural and synthetic zeolites can bused to provide both the color-generating and filling function in the ink composition since they possess the ability to provide an intense color when contacted with the color-activating 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 weight percent of the filler should be about 15% by weight or greater to obtain satisfactory color development in the sensitized areas.

The thinners which are used in combination with the aforementioned vehicle components in the inks of the present invention are materials which 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 printing 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 thinner is about 10to about 25% by weight ofthe total inkcomposition.

Although the use in this invention of the aforementioned vehicle components in combination with thinners eliminates the need for conventional varnishes in order to obtain satisfactory inking properties, varnishes can be used in the formulations if desired to modify further the tack, viscosity and other rheological properties where these properties are difficu It to obtain with normal combinations of the vehicle component and thinner. Useful varnishes are 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 composition since these varnishes tend to inhibit sunsequent development of the latent inks due to theirfilm-forming properties.

Agents which inhibit the effect of oxygen on the components of the ink, i.e. antioxidants, can also be added to stabilize the ink components and the sensitized areas after development. Useful antioxidants are well known in the printing 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 undesirable coloring of the sensitized area.

Exemplary of the antioxidants which can be used are thiourea, hydroquinone, hindered phenols such as alkylated hydroxytoluene, and the like. Preferably the antioxidant comprises less than 10% by weight and most preferably less than about 2.5% by weight of the total ink composition.

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 aforementioned acidifed clays or zeolites, the filler itself can be a color-generating component in lieu of or in addition to the 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-generating components, an additional color-generating component or mixture of color-generating components can be incorporated into the ink formulation. Thus, the ink formulation can comprise a transition metal salt such as the nitrates, sulfates or halides of the transition metals. A preferred transition metal salt is nickel nitrate.

If more than one color-generating component is used in the ink formulation, care must be taken to seen that the components do not prematurely react with each other. Thus generally a transition metal salt and a DTO derivative would not be incorporated in the same formulation. Similarly an acidified 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 DTO or other metal complexing compound or by a leuco dye or both.

As noted previously, the inks can contain ingredients, such as varnish, to modify further the tack, viscosity and other rheological properties of the ink. Additional conventional ingredients 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% by weight of the total composition. For example, the ink may require up to about 7% by weight antioxidant. 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 about 0.5% or less by weight.

A preferred ink formulation would comprise about 5 to 55%, and preferably about 30 to 45%, by weight vehicle component which is a non-polymeric, oleophilic, organic acid anion having a cationic counter ion, such as nickel-2-ethylhexoate, a stabilizing amount of diterpene stabilizing component, about 5 to 15% by weight particulate filler, up to about 30% by weight thinner, and about 0.5% by weight antioxidant. The ink must include at least about 5% by weight transition metal salt color generating component.

The inks of the type described above can be prepared using conventional ink milling equipment. Generally the ink is prepared by admixing all ofthe 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 filler can be added before the ink is milled. The admixture is then dispersed in a conventional agitating means 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 adjust the tack, viscosity and other rheological properties of the ink as required.

In addition to the inks described above, flexographic inks can also be formulated. Flexographic printing techniques are particularly well adapted to the manufacture of forms and other printed products and latent flexographic inks can be provided in accordance with the present invention. Flexographic printing generally involves the use of a rotary press comprising usually one or more hard and/or rubber blanketed rolls to apply a flexographic ink over the entire surface of a paper substrate, such as in tinting a substrate or to provide a pattern or other message on the substrate. Flexographic inks generally comprise a low viscosity, polar vehicle, typically a denatured alcohol vehicle such as methanol or ethanol which may or may not contain some water, having dissolved or dispersed therein various dyes, pigments, and resins, etc. to provide an acceptable flexographic ink.Due to the large amount of low viscosity liquid vehicle, these inks tend to be very low viscosity and water-like. In attempting to provide latent, sensitizing compositions for flexographic printing a color-forming component must be incorporated in the ink vehicle together with other colored dyes and pigments if desired. Merely adding a conventional color-forming material such as a transition metal salt to alcohol or other conventional flexographic ink vehicle does not provide satisfactory inks since a loss in the speed of image formation after several days exposure to 80% relative humidity at 80"F is realized. Under more extreme conditions, a loss in imaging intensity may also be exhibited. However, the stabilized imaging composition of the present invention provide means for formulating stabilized, latent, sensitizing flexographic inks.

It has been found by the present inventor that in formulating flexographic inks and certain other liquid sensitizing compositions some additional problems may be encountered which are peculiar to these compositions. This is believed due to the tendency of the transition metal salt to dissociate and form a transition metal diterpenate, such as nickel diterpenate, to an extent which may exceed the solubility product constant of the transition metal diterpenate in the vehicle and cause precipitation of the nickel diterpenate.

This tendency has been overcome in the present invention by the addition of a solubilizing acid comprising the family of acids which have the following properties; 1. The solubilizing acid must itself be soluble in the vehicle.

2. The solubilizing acid should be of sufficient strength or have a sufficiently high dissociation constant in the vehicle to drive the resin acid equilibrium to a less dissociated level so as not to exceed the solubility product constant in the vehicle of the transition metal diterpenate, e.g. nickel diterpenate, and cause precipitation of the transition metal diterpenate.

3. The transition metal salt of the solubilizing acid must be soluble in the vehicle at least to the point where the transition metal is suppressed from precipitating as the diterpenate. In other words, the solubility product constant of the transition metal salt of the solubilizing acid in the vehicle must be high enough so that a sufficient amount of the solubilizing acid can be added to suppress formation of transition metal diterpenate in excess of its solubility product constant in the vehicle.

From the foregoing discussion, it can be seen that the solubilizing acid must be chosen so that the solubility product constant of the transition metal salt of the solubilizing acid in the vehicle is not below the solubility product constant of the transition metal diterpenate in the vehicle.

The proper selection of an acid according to the above criteria will allow the use of the stabilized imaging compositions in formulating flexographic inks and other sensitizing compositions. In effect, the use of these acids will allow sufficient diterpene stabilizing agent to be added to stabilize the transition metal color-forming salt without causing precipitation of the transition metal diterpenate which may be formed through dissociation of the transition metal salt. A number of acids which meet the criteria described above in varying degrees have been found. Exemplary of such acids are o-chlorobenzoic acid, benzoic acid, salicylic acid, 2-naphthalene sulfonic acid, benzene sulfonic acid, nitric acid, lactic acid, acetic acid, and citric acid.For reasons discussed hereinafter, the preferred solubilizing acids for formulating flexographic inks are the o-chlorobenzoic acid, benzoic acid and salicylic acid, with o-chlorobenzoic acid being the most preferred.

The amount of the acid necessary to suppress the precipitation of the transition metal diterpenate varies depending on the acid used. In effect, the correct amount of acid must be chosen to provide a "solubility window". That is, as stated above, a sufficient amount of the acid must be added to prevent precipitation of the transition metal diterpenate, but not so much as to cause precipitation of the transition metal salt of the solubilizing acid. This "solubility window" varies with each acid but can be readily determined by routine tests. For example, a formulation comprising the stabilized imaging composition described hereinabove in an alcohol vehicle can be formulated and various amounts of the solubilizing acid added to determine when precipitation occurs.For example, a solution comprising 10% nickel-2-ethylhexoate, 3.3% by weight tall oil rosin as a solution in anhydrous ethyl alcohol can be prepared and various weight percents of the various acids added to determine the "solubility window". Results of these tests indicate that the "solubility window" for this particular formulation when benzoic acid is used as the acid, is between about 0.8 to 1.5% acid based on the total weight of the composition. Thus, with formulations containing less than about 0.8% by weight benzoic acid, the nickel diterpenate will precipitate out of solution. With formulations containing above about 1.5% by weight benzoic acid, nickel benzoate will precipitate out of solution.

Salicylic acid in a similar formulation provides a "solubility window" between about 0.7% and 1.2% by weight.

o-chlorobenzoic acid on the other hand, is rather unique in that amounts in excess of about 2.8% by weight in the above formulation are required to prevent the precipitation of nickel diterpenate. This then represents the lower edge of the "solubility window" for o-chlorobenzoic acid in this formulation. On the other hand, amounts greatly in excess of this amount of o-chlorobenzoic acid can be added with no resulting precipitation of the nickel salt of o-chlorobenzoic acid. Thus, the "solubility window" for o-chlorobenzoic acid in this formulation is extremely wide once the lower level has been reached. o-chlorobenzoic acid is unique in another respect in that the nickel salts of this acid are extremely soluble in water. Thus, if alcohol vehicles containing some water, as opposed anhydrous alcohols, are used, no undesirable precipitates will be formed.For these reasons, o-chlorobenzoic acid is the preferred solubilizing acid for many sensitizing compositions including flexographic ink formulations.

As noted above, in formulating various sensitizing compositions, an organic vehicle is normally used as the major vehicle component. In practice, concentrates or syrups are often prepared by dissolving or dispersing the stabilized imaging composition and the solubilizing acid described above in a vehicle, such as an alcohol vehicle. This concentrate or syrup is then diluted at the point of use by the printer or other user to provide the flexographic ink or other sensitizing composition. Dilution is performed by adding an amount of the vehicle, such as ethanol, to provide a composition having a solids content of about 5-15% by weight, depending on the particular printing or coating equipment employed and the substrate to be printed or coated.In addition to the latent, stabilized, imaging concentrates described herein, conventional color concentrates can be added to provide a colored latent composition for tinting or printing.

Preparation of the coating compositions is accomplished by formulating a concentrate or syrup comprising (1) a color-forming transition metal salt as described above, (2) a diterpene stabilizing agent, (3) a solubilizing acid, and 14) a solvent for the above components. Generally the solvent will be an organic solvent, for example an alcohol such as methanol or ethanol, naphtha or one of a number of other common solvents or vehicles described hereinabove.

The concentrate is generally formulated to provide a solution of about 80% or more solids and containing about 40 to 75% by weight color-forming transition metal salt. This concentrate is diluted by the printer by adding a sufficient amount of liquid vehicle particularly adapted to the coating technique to be employed to allow the desired amount of transition metal salt to be applied to the substrate. It is generally desired to provide about 0.0085 Ibs. of nickel ion per 1300 square feet of paper (about 3.2 x 10.2 grams nickel ion per square meter of paper).

Flexographic inks formulated according to the present invention can be applied to paper substrates with conventional flexographic printing ortinting equipment. These substrates can be imaged using conventional CB sheets containing encapsulated dithiooxamide-based dye precursors as known in the art. Upon exposure to atmospheric moisture, e.g. seven days at 80 relative humidity and 80" Fthe image speed is retained at a satisfactory level. Other coating compositions can be readily formulated and applied by other conventional techniques.

The sensitizing compositions 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. Applying these ink compositions with conventional printing equipment makes it practical to pattern or spot coat substrates, and thus, selectively sensitize paper or other substrates for business forms applications where there is a desire to have an image appear only on certain portions of a form or on certain 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.

The sensitized areas of the substrates can then be developed by contacting the sensitized 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 activating component contacts the sensitized area, or a portion 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 contained in a marking pen, which contains a color-activating component and the sensitized area visibly developed.

Business forms which are multi-sheet forms can comprise one or more CB sheets overlying 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 having at least a part of the obverse surface sensitized. Writing or typing on the obverse surface of the top sheet causes like information to be recorded in the obverse 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 composition 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 present invention. In these Examples, all parts and percents are by weight, unless otherwise indicated.

Example 1 An ink for sensitizing selected areas of paper with a latent coreactant was prepared having the following formulation: Wt% Nickel-2-ethylhexoate 36.0 Hydrophilic Silica Filler 14.2 Tall Oil Rosin 25.2 Butylated Hydroxy Toluene 0.5 Hydrocarbon Oil 24.1 The nickel-2-ethylhexoate, tall oil rosin and antioxidant were dissolved in the hydrocarbon oil. The filler was then dispersed in the solution and the final dispersion carried out in a conventional heated, agitated kettle. The composition had a tack of 12.3 and a viscosity of 100,000 centipoise and could be satisfactorily printed on a wet offset or dry offset press.

The ink was applied to 16 Ib (7.2 kg) bond paper in selected areas of the sheet. After drying at room temperature, the sensitized sheets were developed by placing a sheet coated with an encapsulated dithiooxamide derivative ("3M" Brand Carbonless Paper, Type 200 CB [coated back] sheet) so that the capsule coating and the inked surfaces were in direct contact. The sheets were run through a steel pressure roll to develop uniformly a 1.2 cm wide strip of the sensitized sheet. The developed area had an acceptable blue-purple color as determined by optical density measurements.

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

Sensitized sheets were exposed to 80% relative humidity at 80"F. for a period of 12 days with a decrease in image intensity from a value of about 32 to a value of about 41 while under the same conditions a similar ink without the stabilizing component showed an image intensity decreasing from about 34 to about 63 (30 second image time).

Example 2 An ink for sensitizing selected areas of paper with a latent coreactant was prepared having the following formulation: Wt% Nickel-2-ethylhexoate 36.0 Hydrophilic Silica Filler 13.7 Tall Oil Rosin 13.9 Sodium Abietate 9.7 Butylated Hydroxy Toluene 0.5 Hydrocarbon Oil 26.2 The nickel-2-ethylhexoate, tall oil rosin, sodium abietate and antioxidant were dissolved in the hydrocarbon oil. The filler was then dispersed in the solution and the final dispersion carried out in a conventional heated, agitated kettle. The composition had a tack of 13.0, a viscosity of 192,000 centipoise and could be satisfactorily printed on a wet offset or dry offset press and was satisfactorily developed by a CB sheet as described in Example 1.

Sensitized sheets were exposed to 80% relative humidity at 32"C for a period of 12 days with a decrease in image intensity from a value of about 34to about 46 while under the same conditions a similar ink without the stabilizing component showed a decrease in image intensity from about 34 to about 63 (30 second image time).

Example 3 A stabilized flexographic ink was prepared in a heated kettle by dissolving 51.4 parts nickel-2-ethylhexoate in 16.2 parts of anhydrous ethyl alcohol. Subsequently 15.4 parts of o-chlorobenzoic acid was dissolved followed by 17.0 parts tall oil rosin. The composition was an 83.3% solids syrup.

Aflexographic ink was prepared by diluting the concentrate prepared above with anhydrous ethyl alcohol.

The 83.8% solids syrup was combined with 1.8 parts of a colored flexographic ink and 86.6 parts of ethyl alcohol to provide a tinted flexographic ink. The ink was applied to forms paper on a laboratory flexographic rotary press to provide a coating weight of about 0.7 Ibs, ink per 1300 ft2 of substrate (31 8g/1 20m2).

The CF coated sheet, when dried, provided rapid intense imaging with a conventional CB sheet containing substituted dithiooxamide in cyclohexane as the microcapsule fill. Initial 30 second speed image tests (reflectance 30 seconds after imaging) gave values of about 28 to 30. A clear control ink without the stabilizing agent of the present invention provided a 30 second speed image of about 32. After 7 days aging at 80% R.H. and 27eC the 30 second speed image value for the control ink had only increased to a reflectance value of about 72, whereas the sheet coated with the stabilized flexographic ink had increased to a value of less than about 40. This demonstrates that the stabilized inks of the present invention are resistant to the effects of atmospheric moisture.

Example 4 A stabilized flexographic ink was prepared by dissolving 42.2 parts of nickel-2-ethylhexoate and 29.5 parts of tall oil rosin in 28.3 parts of hydrocarbon oil (Magiesol 52) to give a 71.7% solids syrup.

The 71.7% solids syrup was combined with 89.9 parts of VM & naphtha (flash point 28"F). The ink was applied to forms paper on a laboratory flexog raphic rotary press to provide a coating weight of approximately 0.7 Ibs ink per 1300 ft2 of substrate (3189/120 m2).

The CF coated sheet, when dried, provided rapid intense imaging with a conventional CB sheet containing substituted dithiooxamide in cyclohexane as the microcapsule fill. Initial 4 second speed image tests (reflectance 4 seconds after imaging) gave values of 53.5 compared to the value for a control ink, without the stabilizing agent of the present invention, of 52.5. After 4 days aging at 80% RH and 27"C, the 4 second speed image value for the control ink had increased to a reflectance value of about 80.5, whereas the sheet coated with the stabilized flexographic ink had only increased to 67.

The concentrated syrup described in Example 4, above, may be diluted with a variety of solvents, such as perchloroethylene, diacetone alcohol, butyl carbitol acetate, methyl ethyl ketone, ethyl acetate, toluene, polypropyleneglycol 1025, dimethylformamide, diethyl ether, 2-nitro propane, or various mixtures thereof, with comparable results.

Claims (18)

1. A latent, stabilized, sensitizing composition characterized in that said composition comprises a solution of a color-forming, transition metal salt and a stabilizing amount of diterpene stabilizing agent in a substantially non-aqueous liquid vehicle.

2. A composition according to claim 1 wherein said diterpene stabilizing agent is present in an amount sufficient to provide at least about 1.75 parts diterpene stabilizing agent per part of transition metal ion.

3. A composition according to claim 2 wherein said diterpene stabilizing agent is selected from the group consisting of resin acids, alkali metal and alkaline earth metal salts of resin acids, tall oil rosin and mixtures thereof.

4. A composition according to claim 1 wherein said transition metal salt is nickel-2-ethylhexoate and said diterpene stabilizing agent is tall oil rosin present in an amount to provide a weight ratio of tall oil rosin to nickel-2-ethylhexoate of at least about 0.33.

5. A composition according to claim 4 wherein said vehicle is alcohol.

6. A stabilized sensitizing composition comprising the composition of claim 1 further characterized by having in combination therewith a solubilizing acid soluble in said vehicle and having a dissociation constant in said vehicle which is sufficient to maintain the presence of the transition metal diterpenate below the precipitation point and wherein the solubility product constant of the transition metal salt of said solubilizing acid in said vehicle exceeds the solubility product constant of the transition metal diterpenate in said vehicle.

7. A composition according to claim 6 wherein said vehicle is alcohol and said solubilizing acid is selected from the group consisting of o-chlorobenzoic acid, benzoic acid and salicylic acid.

8. A stabilized, latent, flexographic sensitizing ink comprising the composition of claim 1 diluted with a liquid flexographic vehicle.

9. A paper substrate having at least a portion of one major surface sensitized with the latent, stabilized, sensitizing composition of claim 1.

10. A paper substrate having at least a portion of one major surface sensitized with the stabilized latent flexographic ink of claim 8.

11. A latent printing ink for providing latent, sensitized areas on substrates, characterized in that said ink can be applied to a paper substrate by a printing press and subsequently developed with a pressurereleasable, encapsulated, color-activating compound carried on a substrate, said ink comprising, based on the weight of the total ink composition, about 5 to 55% by weight of a vehicle component which is a non-polymeric, oleophilic, organic acid anion having a cationic counter-ion, a stabilizing amount of a diterpene stabilizing component, 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 transition metal salt color-forming component, the ratio of said filler to said non-polymeric vehicle component being from about 0.1:1 to 6:1.

12. An ink according to claim 11 wherein said stabilizing component is selected from the group consisting of resin acids, alkali metal and alkaline earth metal resin acid salts, tall oil rosin, and mixtures thereof.

13. An ink according to claim 11 wherein said cationic counter-ion is a transition metal ion.

14. A sheet having selected areas on at least one major surface sensitized with the latent, sensitizing composition of claim 1.

15. A multi-sheet form comprising a first sheet having on at least portions of the reverse surface a dye precursor and, underlying said first sheet, one or more additional sheets characterized by having at least a portion of the obverse surface in register with the dye precursor on said reverse surface of said first sheet and sensitized with the stabilized, latent, sensitizing composition of claim 1.

16. A latent, stabilized. sensitizinq composition according to claim 1 and substantially as herein described.

17. Sensitizing ink substantially as described in the examples herein.

18. A substrate having at least one major surface thereof sensitized with a composition or ink according to claim l6orclaim 17.