PREVENTION OF SPOTTING IN THERMAL IMAGING COMPOSITIONS

Abstract: Thermally imageable composition comprising (a) at least one leuco dye, (b) a nitrate salt, (c) at least one base having a conjugate acid with pKa equal to or greater than zero. The base serves to prevent spotting or backgrounding of transparency films bearing said thermally imageable composition during the manufacturing process used in preparing the films.

Description: This invention relates to thermally imageable compositions and to stabilizers for these compositions. It is well known that dyes in their reduced leuco form can provide the basis of color image forming systems. The leuco dyes may initially be relatively colorless, but can return to a colored form when oxidized, e.g., by air under acidic conditions or any other suitable oxidizing agent. Examples of leuco dyes used in color image forming systems include triarylmethanes, xanthenes, styryl dyes, and azine dyes, such as, for example, phenoxazines, phenothiazines, and phenazines. In thermally sensitive materials of the type wherein at least one leuco dye is in reactive proximity with an inorganic nitrate salt, whereby imagewise application of heat causes said nitrate salt to oxidize said leuco dye to produce a change in color, a problem may arise from premature spotting or backgrounding of the thermally sensitive material during the drying step of the manufacturing process. As used herein, the term "spotting" means oxidation of the leuco dye to a colored dye form in random irregular spots; the term "backgrounding" means oxidation of the leuco dye to a colored dye form in uniform fashion, resulting in an evenly colored background. Either spotting or backgrounding can destroy the usefulness of a transparency film bearing a thermally imageable composition. One method for preventing spotting involves drying of the imageable coating at low temperatures. This method, however, requires long drying times, slow coating speeds, and high costs, and in most cases, does not offer a practical solution to the problem. In thermally imageable transparency films based upon combinations of leuco dyes and nitrate salts it is essential that the thermally imageable compositions show considerable stability to the thermal effects of the manufacturing process in order to have a useful shelf life. US 4,423,139 discloses thermally imageable compositions comprising a leuco dye, an inorganic nitrate salt, a polymeric binder and a stabilising combination comprising a) one or more aromatic compounds which form quinones, diimines or quinonimines upon oxidation and b) phenidone or phenidone derivatives. The stabilising combination stabilises the leuco dye of the thermally developable system against oxidation and premature reaction. Certain bases are disclosed in Example 11 all of which are outside the scope of the claims as filed. JP-A-5798392 discloses a thermosensitive recording composition comprising a leuco dye, a nitrate, a microencapsulated base e.g. sodium carbonate, a reducing agent e.g. thiourea and a thermoplastic resin. EP-A-124,296 discloses a thermographic imaging system comprising a leuco dye, a nitrate salt, and optionally an acid and a stabilising compound. The stablising compound may be the base phenidone or catechol or hydroquinone. US-A-3 460 964 discloses a heat-sensitive composition which can be used to prepare heat-sensitive recording elements comprising an aqueous solvent, a metal nitrate, a leuco-dye and optionally a thickener, a stabilizer, a sensitizer or similar agents. Certain bases are mentioned as suitable stabilizers such as 2, 2' - (phenylimino) - diethanol (equals N, N bis - (2-hydroxyethyl) - aniline) and amino diethanol. The function of the stabilizer is to maintain the heat-sensitive recording elements or sheets in a uniformally active condition for several weeks or more and thereafter for some time still operable at a gradually reduced sensitivity level. The present invention provides a thermally imageable composition comprising i) at least one leuco dye in reactive proximity to an inorganic nitrate salt, whereby imagewise application of heat causes said nitrate salt to oxidize said at least one leuco dye to produce a change in color, the cation of said nitrate salt being non-reactive with said leuco dye, and ii) at least one base whose conjugate acid has a pKa greater than or equal to zero, said at least one base being a compound in one of the categories: wherein R¹, R², and R³ can be the same or different and represent a member of the group selected from substituted or unsubstituted alkyl groups having 1 to 20 carbon atoms, substituted or unsubstituted alkenyl groups having from 1 to 16 carbon atoms, and substituted or unsubstituted aryl groups having up to 3 fused rings, subject to the proviso that if R¹, R² or R³ is a substituted group, its substitutent is neither hydroxy nor alkylthio; wherein R¹ is as defined above and where l is an inteqer from 0 to 6, inclusive, m is an integer from 0 to 6, inclusive, and n is an integer from 0 to 6, inclusive; where l and m are as defined above and n' is 0 or 4; where l and m are as defined above, and n'' is an integer from 0 to 4, inclusive; or where l' is 0 or 4, and m and n'' are as defined above; wherein R¹ is as defined above, and R⁴ represents CR⁵R⁶ where R⁵ and R⁶ represents a member of the class from which R¹, R², and R³ are selected, with the proviso that R⁵ and R⁶ need not be the same as R¹, R², or R³; wherein represents where x is 0, 1, or 2, y is an integer from 0 to 8, inclusive, and z is 0, 1, or 2, and R⁷ represents a member of the class from which R¹, R², and R³ are selected, with the proviso that R⁷ need not be the same as R¹, R², R³; or wherein R²⁰ and R²¹ can be the same or different and represent a member of the group selected from substituted or unsubstituted alkyl groups having 1 to 20 carbon atoms and substituted or unsubstituted alkenyl groups having 1 to 16 carbon atoms; subject to the proviso that if R²⁰ and R²¹ are a substituted group at least one of the substituent is not hydroxy wherein R²⁰ represents a member of the group selected from substituted or unsubstituted alkyl groups having 1 to 20 carbon atoms and substituted or unsubstituted alkenyl groups having 1 to 16 carbon atoms. wherein R¹ is as defined above; wherein R¹ and R² are as defined above; wherein R¹, R², and R³ are as defined above; wherein R¹, R², and R³ are as defined above; wherein R¹, R², and R³ are as defined above, and R⁸ represents a member of the class from which R¹, R², and R³ are selected, with the proviso that R⁸ need not be the same as R¹, R², or R³; wherein R¹ is as defined above, and A represents a metallic cation; wherein R¹ and A are as defined above; wherein R¹ and A are as defined above; wherein R⁹, R¹⁰, and R¹¹, can be the same or different and represent a member of the group selected from H, -NO₂, -CN, -COR¹², -COOR¹², and SO₂R¹² where R¹² is selected from the group consisting of phenyl group, naphthyl group, alkyl group having 1 to 4 carbon atoms, and A is as defined above; wherein R⁹ and A are as defined above and wherein a is 2, 3, or 4, or wherein b is 0 or 4; wherein A is as defined above, and R¹³ represents the phenyl or naphthyl group; wherein R¹, R², and A are as defined above; wherein R¹, R², and A + are as defined above, and wherein R¹, R², and R³ are as defined above. Addition of one or more of the above bases results in decrease or elimination of spotting thus allowing an increase in the drying temperature during the drying step of the manufacturing process, further resulting in faster drying higher coating rates decreased moisture sensitivity and lower manufacturing costs. When R¹, R², or R³ is a substituted alkyl or aryl group, the substituents can be any other than hydroxy or alkylthio which do not deleteriously affect the function of the thermographic system. Suitable substituents include halo groups, e.g. chloro, bromo, iodo, fluoro; cyano group; nitro group; alkoxy group having, for example, 1 to 20 carbon atoms; alkyl carbonyl group having, for example, 1 to 20 carbon atoms; alkylsulfonyl group, having, for example, 1 to 20 carbon atoms. R¹, R², R³ can be mono-, di-, tri-, or tetra- substituted. Dye classes which can be stabilized by the bases include styryl, phenoxazine, phenothiazine, and phenazine. Representative examples of styryl dyes are (a) 2,3-dihydro-1,3,3-trimethyl-2-[2-(2,4,6-trimethoxyphenyl)ethenyl]-1H-Indole; (b) 2-bromo-4-[2-(5-chloro-2,3-dihydro-1,3,3-trimethyl-1H-indol-2-yl)ethenyl]-N,N-dimethylbenzenamine; (c) 2,3-dihydro-1,3,3-trimethyl-2-[2-(4-dimethylamino)-phenyl-ethenyl]-1H-Indole; (d) 2,3-dihydro-1,3,3-trimethyl-2-[2-(4-N-morpholino-)phenyl-ethenyl]-1H-Indole; (e) 2,3-dihydro-1,3,3-trimethyl-2-[2-(4-N,N-bis-(2-cyanoethylamino)-phenyl-ethenyl]-1H-Indole. Representative examples of phenoxazine and phenothiazine dyes are: (f) 3,7-bis-(N,N-diethylamino)-10-benzoyl-phenoxazine and (g) 3,7-bis-(N,N-dimethylamino)-10-benzoyl-phenothiazine, respectively. Representative examples of phenazine dyes are: (h) 5,10-dihydro-5-phenyl-10-benzoyl-3,7-bin-(N,N-diethylamino)phenazine and (i) 5,10-dihydro-5-ethyl-10-benzoyl-3,7-bis-(N,N-dimethylamino)phenazine. Nitrate salts suitable for this invention are themselves well known. They may be supplied as various chemical compounds, but are desirably provided as a metal salt, and most preferably provided as a hydrated metal salt. Most means of supplying the nitrate salt into the imaging composition are satisfactory. For example, organic salts, metal salts, acid salts, mixtures of acids and salts, and other means of supplying the ion are useful. Nitrates of zinc, cadmium, potassium, calcium, zirconyl (ZrO₂), nickel, aluminum, chromium, iron, copper, tin, magnesium, lead, and cobalt, ammonium nitrate, and cerous ammonium nitrate can be used. The nitrate salt component of the present invention must be present in a form within the imaging composition so that oxidant (i.e., decomposition products of the nitrate) will be provided within the composition when it is heated to a temperature no greater than 200°F (93°C) for 60 seconds and preferably no greater than 160°F (71°C) for 60 or most preferably 30 seconds. The salt must be chosen so that the cation thereof is non-reactive with the leuco dye. In the practice of the present invention, non-reactive salts are defined as those salts the cations of which do not spontaneously oxidize the dyes that they are associated with at room temperature. Preferred salts are the hydrated metal salts such as nickel nitrate hexahydrate, magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, ferric nitrate nonahydrate, cupric nitrate trihydrate, zinc nitrate hexahydrate, cadmium nitrate tetrahydrate, bismuth nitrate pentahydrate, thorium nitrate tetrahydrate, cobalt nitrate hexahydrate, gadolinium or lanthanum nitrate nonahydrate, and mixtures of these hydrated nitrates. Nonhydrated or organic nitrates may be admixed therewith. It is preferred to have at least 0.10 mole of nitrate ion per mole of dye. It is more preferred to have at least 0.30 or 0.50 mole of nitrate ion per mole of dye. The bases described in this invention can be used at as low a concentration as 0.05 equivalent of base per equivalent of nitrate ion, or as high as 1.0 equivalent of base per equivalent of nitrate ion. The preferred range is from about 0.3 to about 0.6 equivalent of base per equivalent of nitrate ion. The thermally stimulated oxidation of the leuco dye by the nitrate salt can be facilitated by the presence of an acid. The acids optionally useful in the thermographic system of this invention are acids as generally known to the skilled chemist. Organic acids, preferably those having carboxylic groups, such as phthalic acid, are preferred, but inorganic acids can also be used. The acid can be present in a ratio of from 0 to 10 times the amount of the nitrate ion. The leuco dye, nitrate salt, base having a pKa ≧ 0, and acid, when employed, are dissolved in a binder, which binder is neither strongly basic nor strongly acidic but which is sufficiently polar to hold the constituents in solution. It is preferred that the binder be selected from polymeric materials. Such resins as polyvinyl acetals, e.g., polyvinyl butyral, polyvinyl resins, polyvinylpyrrolidone, polyesters, polycarbonates, polyamides, polyacrylates, cellulose esters, copolymers and blends of these classes of resins, can be used. Saran, a vinyl chloride-vinylidene chloride copolymer, is particularly preferred. Natural polymeric materials such as gelatin and gum arabic can also be used. The leuco dye should be present at a concentration of at least 0.3% by weight, based on the weight of the binder, preferably at a concentration of at least 1% by weight, based on the weight of the binder, and most preferably at a concentration of from 2 to 10% or more by weight, based on the weight of the binder. A formulation which can be applied by conventional coating techniques can be produced by dissolving the leuco dye, the metal nitrate, and the polymeric binder, together with an organic acid, and, optionally, a conventional stabilizing compound, e.g. catechol, phenidone, along with the base whose conjugate acid has the required pKa in an inert organic solvent, such as, for example, acetone, methyl ethyl ketone, or tetrahydrofuran. The formulation can be coated onto a support by methods well known in the art, such as, for example, wire-wound rod, knife, or extrusion coating. Typical wet thickness of the layer can range from about 10 to about 100 micrometers (µm), and the layer can be dried in forced air at temperatures ranging from 20°C to 50°C. It is preferred that the coating thickness be selected to provide maximum image densities greater than 0.2, and more preferably in the range of 0.5 to 1.5, as measured on a MacBeth Color Densitometer Model TD 504 using the color filter complementary to the dye color. The support material can be selected from a wide range of materials, including paper, glass, polymeric film, and the like, depending upon the particular imaging requirement. Preferred materials include polymers having good heat stability, such as polyesters. A particularly preferred polyester is polyethylene terephthalate. The following examples, which are illustrative rather than limiting or delineative of the scope of the invention, serve to describe the compositions and properties of the present invention. Examples 1-3 These examples demonstrate the effect of adding amines which have conjugate acids with a pKa >0 to the thermally imageable composition contemplated for this invention. This formulation contains 0.32 millimole Al(NO₃)₃·9H₂O or 0.96 milliequivalents of nitrate ion. Stock solutions of additives were made containing 0.5 millimole/g of total solution and the amounts indicated in Table I were added to the samples. The resulting samples were coated on 101.6 µm (4 mil) polyester film at a 76.2 µm (3 mil) wet thickness and dried at 72°C (162°F) for 3 minutes. The percentage of spotting resulting from the foregoing step was determined for each sample and is shown in Table I. The term "percentage of spotting" is defined here as the ratio of the area of coated film which is colored due to premature oxidation of the leuco dye, divided by the total area of coated film, multiplied by 100. As the pKa of the base's conjugate acid approaches zero, the additive is less effective as an antispotting agent. Examples 3-6 These examples demonstrate the effect of adding amides and ureas which have conjugate acids with pKa values between about 0 and 2 to the thermally imageable composition contemplated for this invention. Samples were prepared as described in Examples 1-3. Again stock solutions of additives were made containing 0.5 millimole/g of total solution and the amounts indicated in Table II were added to the samples. All samples were coated and dried identically to those described in Examples 1-3. The percentage of spotting was determined for each sample, and is listed in Table II: Examples 7-8 These examples demonstrate the effect of adding salts of carboxylic acids which have conjugate acids with pKa>0 to the thermally imageable composition contemplated for this invention. A procedure identical to that described in Examples 1-3 was used and the results are listed in Table III: Examples 9-11 These examples demonstrate the effect of adding salts of alcohols or thiols which have conjugate acids with pKa>5 to the thermally imageable composition contemplated for this invention. A procedure identical to that described in Examples 1-3 was used and the results are shown in Table IV: Examples 12-15 These examples demonstrate the effect of adding salts of organophosphoric acids, of organophosphonic acids, or of organophosphinic acids, or phosphines which have pK>0 to the thermally imageable composition contemplated for this invention. A procedure identical to that described in Examples 1-3 was used and the results are shown in Table V: Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims: A thermally imageable composition comprising i) at least one leuco dye in reactive proximity to an inorganic nitrate salt, whereby imagewise application of heat causes said nitrate salt to oxidize said at least one leuco dye to produce a change in color, the cation of said nitrate salt being non-reactive with said leuco dye, and ii) at least one base whose conjugate acid has a pKa greater than or equal to zero, said at least one base being a compound in one of the categories: wherein R¹, R², and R³ can be the same or different and represent a member of the group selected from substituted or unsubstituted alkyl groups having 1 to 20 carbon atoms, substituted or unsubstituted alkenyl groups having from 1 to 16 carbon atoms, and substituted or unsubstituted aryl groups having up to 3 fused rings, subject to the proviso that if R¹, R² or R³ is a substituted group, its substituent is neither hydroxy nor alkylthio; wherein R¹ is as defined above and where l is an integer from 0 to 6, inclusive, m is an integer from 0 to 6, inclusive, and n is an integer from 0 to 6, inclusive; where l and m are as defined above and n' is 0 or 4; where l and m are as defined above, and n'' is an integer from 0 to 4, inclusive; or where l' is 0 or 4, and m and n'' are as defined above; wherein R¹ is as defined above, and R⁴ represents CR⁵R⁶ where R⁵ and R⁶ represents a member of the class from which R¹, R², and R³ are selected, with the proviso that R⁵ and R⁶ need not be the same as R¹, R², or R³; wherein represents where x is 0, 1, or 2, y is an integer from 0 to 8, inclusive, and z is 0, 1, or 2, and R⁷ represents a member of the class from which R¹, R², and R³ are selected, with the proviso that R⁷ need not be the same as R¹, R², R³; or wherein R²⁰ and R²¹ can be the same or different and represent a member of the group selected from substituted or unsubstituted alkyl groups having 1 to 20 carbon atoms and substituted or unsubstituted alkenyl groups having 1 to 16 carbon atoms; subject to the proviso that if R²⁰ and R²¹ are substituted group at least one of the substituent is not hydroxy wherein R²⁰ represents a member of the group selected from substituted or unsubstituted alkyl groups having 1 to 20 carbon atoms and substituted or unsubstituted alkenyl groups having 1 to 16 carbon atoms. wherein R¹ is as defined above; wherein R¹ and R² are as defined above; wherein R¹, R², and R³ are as defined above; wherein R¹, R², and R³ are as defined above; wherein R¹, R², and R³ are as defined above, and R⁸ represents a member of the class from which R¹, R², and R³ are selected, with the proviso that R⁸ need not be the same as R¹, R², or R³; wherein R¹ is as defined above, and A represents a metallic cation; (J) R¹O⁻A n+ wherein R¹ and A are as defined above; (K) R¹S⁻A n+ wherein R¹ and A are as defined above; wherein R⁹, R¹⁰, and R¹¹, can be the same or different and represent a member of the group selected from H, -NO₂, -CN, -COR¹², -COOR¹², and -SO₂R¹² where R¹² is selected from the group consisting of phenyl group, naphthyl group, alkyl group having 1 to 4 carbon atoms, and A is as defined above; wherein R⁹ and A are as defined above and wherein a is 2, 3, or 4, or wherein b is 0 or 4; (L2) R¹³-C≡C⁻A n+ wherein A is as defined above, and R¹³ represents the phenyl or naphthyl group; wherein R¹, R², and A are as defined above; wherein R¹, R², and A + are as defined above, and wherein R¹, R², and R³ are as defined above. A composition as claimed in claim 1 wherein said at least one base is present at a concentration of from 0.3 to 1.0 equivalent per equivalent of nitrate ion present in said composition. A composition as claimed in claim 1 or claim 2 wherein said at least one leuco dye is selected from the group consisting of phenoxazine, phenothiazine, and phenazine. A composition as claimed in claim 3 wherein said at least one leuco dye is 3,7-bis-(N,N-diethylamino)-10-benzoyl-phenoxazine; 3,7-bis-(N,N-dimethylamino)-10-benzoyl-phenothiazine; 5,10-dihydro-5-phenyl-10-benzoyl-3,7-bis-(N,N-diethylamino)phenazine or 5,10-dihydro-5-ethyl-10-benzoyl-3,7-bis-(N,N-dimethylamino)phenazine. A composition as claimed in claim 1 or claim 2 wherein said dye is a styryl dye. A composition as claimed in any preceding claim further including a binder. A composition as claimed in claim 6 wherein said at least one leuco dye is present at a concentration of at least 0.3% by weight, based on the weight of the binder. A composition as claimed in any preceding claim further including an organic acid. A composition as claimed in claim 8 wherein the organic acid is present in an amount up to 10 times the amount of nitrate ion present in said composition. A composition as claimed in any preceding claim incorporating at least 0.10 mole of nitrate ion per mole of dye. A composition as claimed in any preceding claim wherein said at least one base is not a primary amine. A thermally imageable transparency film comprising a substrate bearing on at least one major surface thereof a composition as claimed in any preceding claim.

Inventors: MILLER, ALAN G. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1985
Main classification: B41M5
IPC codes: B41M5