CA1236299A - Prevention of spotting in thermal imaging compositions - Google Patents

Abstract

Abstract of the Disclosure 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

~236%9~

FUN 33,244 PREVENTION OF Spotting by THERMAL IMAGING COMPOSITIONS

Background of the Invention This invention relates to thermally image able compositions and to stabilizers for these compositions It is well known that dyes in their reduced Luke form can provide the basis of color image forming systems.
The Luke 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 Luke dyes used in color image forming systems include triarylmethanes, xanthenes, stroll dyes, and amine dyes, such as, for example, phenoxazines, phenothiaæines, and fanciness.
In thermally sensitive materials of the type wherein at least one Luke dye is in reactive proximity with an inorganic nitrate salt whereby images application of heat causes said nitrate salt to oxidize said Luke dye to produce a change in color, a problem may arise from premature spotting or backyrounding of the thermally sensitive material during the drying step of the manufacturing process.
As used herein, the term "spotting" means oxidation of the Luke dye to a colored dye Norm in random irregular spots; the term "back grounding" means oxidation of the Luke dye to a colored dye form in uniform fashion, resulting in an evenly colored background. Either spotting or back grounding can destroy the usefulness of a transparency film bearing a thermally image able composition One method for preventing spotting involves drying of the image able coating at low temperatures, This method, however, requires tony drying times, slow zoatiny speeds, and high costs, and in most cases, does not offer a practical solution to the problem, 9L~31~;2~

In thermally image able transparency films based upon combinations of Luke dyes and nitrate salts, it is essential that the thermally image able compositions show considerable stability to the thermal effects of the manufacturing process in order to have a useful shelf life.

Summary of the Invention ; This invention involves the prevention of spotting and back grounding of transparency films bearing thermally image able compositions that contain combinations of Luke dyes and nitrate salts. Spotting and back grounding which often occur during the drying step of the manufacturing process can be prevented by the addition of one or more Bronsted-Lowry bases! i.e. proton acceptors, which have conjugate acids with pea values of zero or higher and an oxidation potential such that i-t cannot be oxidized by silver ion (A+). These bases have conjugate acids with Kay values equal to or less than one. Effective additives include amine; amine oxides; asides; ureas; salts of phosphinic, phosphoric, and phosphoric acids; phosphines; salts of carboxylic acids; oxygen acids, e.g., alcohols and phenols; thioacids, e.g. mercaptans and thiophenols; salts or complexes of carbon acids; and inorganic bases the bases are not oxidizable to quinines, dominoes, or quinonimines. Addition of one or more of those bases to thermally image able compositions results in decreases 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.

~236~2~

Reduction and elimination of defects in the coated film resulting from premature dye color formation can also be brought about by adding the anti-spotting compounds of the present invention to the imaging compositions.
Detailed Description This invention involves compositions which are image able by thermal energy, e.g. infrared radiation, and pa --` ~236~

coated substrates prepared therefrom, which compositions comprise if) at least one Luke dye, (2) at least one inorganic nitrate salt, (3) a polymeric binder, (4) an optional acid, and (5) at least one anticipating compound selected from the group consisting of (A) tertiary amine, (B) secondary amine, (C) primary amine, (D) primary asides, (E) secondary asides, (F) tertiary asides, (G) tertiary amine oxides, (H) ureas, (It salts of carboxylic acids, (J) salts of alcohols, (K) salts of thiols~
(L) salts of complexes of carbon acids having pea values between 0 and 25, inclusive, (M) salts of organophosphoric acids, (N) salts of organophosphonic acids, (O) salts of organophosphinic acids r (P) phosphines, and (Q) inorganic salts.
These compounds can be represented by the following general formulas:
Al N-R2 (A) I

wherein Al, R2, and R3 can be the same or different and represent a member of the group selected from substituted or unsubstituted alkyd groups having 1 to 20 carton atoms, substituted or unsubstituted alkenyl groups having from 1 to 16 carbon atoms, substituted or unsubstituted aureole groups having up to 3 fused zings, ~23~
Jo The tertiary amine cay have the Lyon ucture:

Run \ R3 (Al) wherein Al is as defined above and R2) represents, for example, Shea\

C--( SHEA ) n /cJ
\ ( Shea m where 1 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;

(Shea) I
¦¦ (chenille /cJ

(shim I` ~æ36~

where 1 and m are as defined above and n' is 0 or 4, (SHEA) C C
(Sheehan"
/
C I
ICH2)m J

where 1 and m are I fake above on n" it an integer from 0 to 4, incl~iveT

( Shea ) ,,~, C I
( SHEA ) n "

fC I
(Shim J

where 1' is 0 or 4, m and n" are as defined above.

The tertiary amine can also have the following structure:

Run \\ (A) \R4 wherein R1 is as defined above, and R4 represents CROWER
where R5 and R6 represents a member of the class from which Al, R2, and R3 are selected, with the proviso that R5 and I need not be the same as Al, R2, or R3.

The tertiary amine can further have the following structure:

Run (A) ~3~2~

wherein Rlrepresents, for example, I

( 2)z Ho ) Y C C
Shucks / R

where x is 0, 1, or 2, y is an integer from O to 8, inclusive, and z is 0, 1, or 2, and R7 represents a member of the class from which Al, R2, and R3 are selected, with the proviso that R need not be the same as Al, R , R3;

(SHEA

I

I
ON H (B) wherein Al and R2 are as defined above.

N-H (C) wherein Al and R2 are as defined above.
None of the amine listed above are oxidizable to quinines, dim-ins, or quinonimines.

~362 H
RYAN / ( D ) H

wherein Al is as defined above.

I H
R -C-N\ (En wherein Al and R2 are as defined above.

If /
10~C-N\ (F) Al R3 wherein Al, R2, and R3 are as defined above.

Al R2 No (G) wherein Al, R2 and R3 are as defined above.

Al R3 NO (H) R2 ~R8 0 wherein Al, I and R3 ala a Dunn above, and R8 represent a myopia ox the kiwi prom which Al, R2, and R3 are equated with the proviso that R8 need not be the same as Al, R2, or R3.

25Rl-C-0-A+ (~) Lo wherein Al is as defined above, and A represents a metallic cation, e.g. Lit, No+, I Mg~2, Kiwi, Mn~2, Foe, Noah, Queue, Zn~2..

Rho A (J) wherein R1 and A+ are as defined above.

RlS A+ (K) wherein Al and A+ are as defined above.

Al C-A+ (L) Al wherein R9, R10, and R11 can be the same or different and represent a member of the group selected from H, -NO, -ON, -COREY, -COREY, and -SYRIA where R12 is selected from the group consisting of phenol lo group, naphthyl group, alkyd group having 1 to 4 carbon atoms, and A is as defined above.

The salts or complexes of carbon acids can also include the following structures Al C A+
(Al 1/

wherein R9 and A are as defined above and ~10 ( represent, for example, Roll Shea ,.
I -wherein a is 2, 3, or 4;

((SHEA
I; ' ` .
I' ..
where b is O or I

The salts or complexes of carbon acids can further include 10 the hollowing structure R13-C--C-A~

wherein A is as defined above, and R13 represents the phenol or naphthyl group.

R10 I) -OWE (M) R20~

wherein Al, R2, and A are as defined above.

Al o \ OWE (N) wherein I I and A are as defined above.

~2~6~Z~3 R _ +
P-O A JO) wherein Al, R2, and A are as defined above.

P R (P) wherein Al, R2, and R3 are as defined above.

ANY

wherein A is as defined above, and Ye represents a member of the group selected from -OH, S, HO, C03, HC03.

When Al, R2, or R3 is a substituted alkyd or aureole group, the substituents can be any which do not deleteriously affect the function of the thermographic system. Suitable substituents include halo groups, e.g. sheller, broom, idea, flyer; hydroxyl group; cyan group; vitro group; alkoxy group having, for example, 1 to 20 carbon atoms; alkyd carbonyl group having, for example, 1 to 20 carbon atoms; alkysulfonyl group, having, for example, 1 to 20 carbon atoms. In addition when Al, R2, or R3 is a substituted aureole group, the substituent can be alkyl-thio, having, for example, from 1 to 20 carbon atoms Al, R2, R3 can be moo-, do-, in-, or twitter- substituted.

~236~

The aromatic compounds which form quinines, dominoes, or quinonimines that are described in U.S. Patent 4,423,139 are well-known developers for silver halide, e.g., Axle, AgBr. In the process of developing silver halide, Ago ion oxidizes the developer to form a quinine, Damon, or quinonimine; at the same time, the Ago ion is reduced to silver metal, i.e., Ago. The aromatic compounds that are capable of developing silver halide are, of course, oxidizable by Ago ion. Because of their oxide-lion potential, they are quite useful as antioxidant, as they react with oxidizing agents more rapidly than do Luke dyes. The bases that are useful in the compositions of the present invention are incapable of being oxidized by Ago ion; consequently they typically operate by suppressing the effects of the Ho ion during the process of manufacturing thermally image able film or like material.
Dye classes which can be stabilized by -the bases include stroll, phenoxazine, phenothiazine, and phenazine.
Representative examples of stroll dyes are (a) dodder-trimethyl-2-[2-(2,4,6-trimethoxyphenyl)ethenyl]-lHHandel; (b)

2-bromo-4-[2-(5-chloro-2,3-dihydro-1,3,3-trimethyll-lH-indol-2-yl) ethenyl]-N,N-dimethylbenzenamine; (c) 2,3-dlhydro 1,3,3-trimethyl -2[2-(4-dimethylamino)-- lo -236Z~

phenyl-ethenyl]-lH-Indole; (d) 2,3~dihydro-1,3,3-trimeth~l-2-[2-(4-N-morpholino-)phenyl-ethenyl~-lH-Indole;
(e) 2,3-dihydro-1,3,3-trimethyl-2-[2-(4-N,N-bis-(2-cyanoethylamino)-phenyl-ethenyl]-lH-Indole.

SHEA SHEA OUCH SHEA SHEA By Of \ / C~3 N I U SHEA
SHEA OUCH SHEA
(a) tub) SHEA SHEA SHEA SHEA

N\ N O

SHEA SHEA
I . Ed) SHEA C~3 No CH2CH2cN ) 2 ZOO SHEA
(e) Representative examples of phenoxazine and phenothiazine dyes are: (f) 3,7-bis-(N,N-diethylamino~-10-benzoyl-phenoxa-zinc and (g) 3,7-bis-(N,N-dimethylamino)-10-benzoyl-pheno-thiamine, respectively.

I

No 5 (C2H5)2N N (C2Hs) 2 ( f ) `' ~C~

3 ) ON N ( OH 3 ) 2 (g) Representative example ox phenazine dye Roy (h) Dow-hydro-5-phenyl-10-benzoyl~3/7-bis-~N~N-di~thylaminno)phenazine and (i) 5,10-dihyd~o-5-ethy~10-~en~oy~-3!7-bi~-(N,N-dimethylamino)ph~nazine, 2~2~
. -13-I
N

(C2~5)2N . N(C2H5)2 (h) I

N
(SHEEHAN ¦ N(cH3)2 ( i ) 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 ox supplying the nitrate salt into the immune composition are satisfactory. For example, oarlock 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 (pro?), nickel, aluminum, chromium, iron copper, tin, ~%~
I
magnesium, lead end Boyle ammonium notate and Sirius ammonium nitrate can by 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 200P
(93C) for 60 seconds and preferably no greater than 160F
(71C) for 60 or most preferably 30 seconds. The salt must be chosen so that the cation thereof is non-reactive with the Luke 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, ferris 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. Non hydrated 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 I 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 Luke dye by the nitrate salt can be facilitated by the presence of an acid. The acids optionally useful in the thermos graphic system of this invention are acids as generally known to the skilled chemist. Organic acids, preferably ~3~2~D
those having carboxylic group, such as phthalic acid, are preferred, but inorganic acids can also be used. The acid can be present in a ratio of from O to lo times the amount of the nitrate ion.
The Luke dye, nitrate salt, base having a pea _ O, 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 p~ferred that the birder be selected from lo polymeric mutters Such ruin a polyvinyl acetals, e.g., polyvinyl Betty Pun eons polyvinylpyrrolidone, polyesters, polycarbonates, polyamides, polyacrylates, cellulose esters, copolymers and blends of these classes of resins, can be used. Saran, a vinyl chloride-vinylidene chloride eopolymer, is particularly preferred. Natural polymeric materials such as gelatin and gum Arabic can also be used.
The Luke dye should be present at a concentra-lion of at least 0.3% by weight, based on the weight of the binder, preferably at a concentration of at least lo by weight, based on the weight of the binder, and most preferably at a concentration of from 2 to lo 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 aissolvin~ the Luke dye, the metal nitrate, and the polymeric binder, together with an organic acid, and, optionally, a conventional stabilizing compound, e.g.
catcall, phenidone, along with the base whose conjugate acid has the required pea in an inert organic solvent, such as, for example, acetone/ methyl ethyl kitten, or tetrahydrofuran.
The formulation can by coaxed onto a supplier by methods well known in the art, such as, for example, warned rod, knife, or extrusion coating. Typical wet thickness of the layer can range from about lo to about loo micrometers ( m), and the layer can be dried in forced air ~æ36~

at temperatures ranging from 20C to 50C. I is preferred that the coating thickness be selected to provide maximum image densities greater than 0.2, and more preferably in the range ox 0.5 to 1.5, as measured on a Macbeth Color Densitometer Model TO 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 example, 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 1-15 These examples demonstrate the effect of adding amine which have conjugate acids with a pea JO to the thermally image able company contemplated or this invention.

Ingredient Amount Bunyan dimethylamino)10-be$zoyl .12 phenothiazine ("CopyKem II") 2-(2H-benzotriazol-2-yl~-p-cresol ("Tinuvin P") .113 phthalic acid .06 5% by weight phenidone solution in .14 tetrahydrofuran ( THY ) I by weight catcall solution in TO .12 30 tetrahydrofuran 2.0 ethanol 1.0 Al(N03)3l9H20 0.12 15~ by weight vinylidene chloride/ ~.66 acrylonitrile copolymer ~Saran~F-310) in methyl ethyl kitten (ME) I k I

This formulation contains 0.32 millimole Allen 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 4 mix polyester film at a 3 mix wet thickness and dried at 162F
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 Luke dye, divided by the total area of coated film, multiplied by 100 .

Table I
., mount Percentage Ex~nple Sample Additlvea OK (Millimole) of Spotting_ 1 1 NUN bis-(2-hydrox~- 6 .05 25 2 ethyl aniline .10 0 3 .30 0 I 4 .50 0 2 5 3-quinuclidinol>11 .05 50 6 .10 2-3 7 .30 0 8 .50 0 3 9 trieth~laminellfO~ ,05 25 .10 15 .

4 13 N,N-diethyl-aniline 6.61 .05 95 14 .10 20 ,30 0 .50 0 ~3~2~

Table I (cont.) mount Percentage Example Spool Additive En (Millimole) of Sutton 17 aniline 4063 .05 20 18 .10 0 19 .30 0 .50 0 6 21 N-methyl-aniline4.85 ~05 10 22 .10 23 .30 0 24 .50 :

. .
7 25 diethylanine .10.49 .05 15 26 . .10 5 27 .30 0 28 .50 0 . _ .. .. _ _ 8 2g pyridazine 2.24 .05 100 .10 100 ~30 1-2 32 .50 0 ,~, . __ . . _ . _ .
9 33 2-methyl-pyrazine 1.45 .05 100 34 .10 95 : 35 .30 25 36 .50 5 ... .... _ _ . . . . . _ . _ 37 pardon . 5.25 .05 40 38 .10 25 39 .30 .0 .50 0 ..... _ .. _ _ .

`" 3~2~9 bye I (cont.) Hun Percentage Example Sample Additive pea lo mule) of Spotting if 41 1,3-diphenyl- 10.12 D05 70 42 guanidine lo 15 I .30 0 44 .50 0 .
12 45 bis~2r2,6,6-tetra~ 05 lo 46 meth~1-4-piperidin~l) lo lo lo 47 decanedioate .30 a O so o -13 49 quinazoline OWE ~05100 .10100 51 .30 52 owe 14 53 thiazole 2.44 Lowe 54 Lowe .30 95 56 .50 15 57 pardon Oxide 0.79.05 lo So Lyle 59 .30 80 .50 10 .. . .. ........ .. . . . . ...
16 61 quinaldine 5~83 .05 80 62 lo 20 63 ~30 0 So .50 0 -~23~

Table I keynote.) . .. .

mount Percentage Example Sample Additive kiwi (Millimole) of Spot 65 no additive 100 a In Examples 1, 3-16 the additive was dissolved in tetrahydrofuran prior to its being Dick to thy ebb oqmpo~it~on~ In Example 2, the additive was dissolved it methanol prior to its being added to the image able composition, As the pea of the base's conjugate acid approaches zero, the additive is less elective a an anticipating agent, Examples These examples demonstrate the effect of adding asides and ureas which have conjugate acids with pea values between about 0 end 2 to the thermally image able composition contemplated for this invention Samples were prepared as described in Examples 1-16. 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 1-16. The percentage of spotting was determined for each sample, and I is listen in Table II:

23~2~

Table II

.. . . _ _ .
mount Percentage Example Sample dative pea tMillimole) of Sutton_ 17 69 acet~mide 0.63 .05 95 .10 75 71 .30 20 72 .50 2 . . . _ _ . _ 18 73 dimethylacetamide 0.~6 .05 100 74 .10 100 .30 I
76 .50 20 .. . . _ . . _ . .
19 77 urea 0.10 .05 100 78 .10 80 79 .30 10 I ~50 _ _ _ . . _ _ . .
81 1,1,3,3-tetramethyl- 1 .05 100 82 urea .10 100 83 ,30 95 84 ~50 50 a In Examples 17, 18 and 20, the additive was dissolved in tetrahydrofuran prior to its being added to the image able composition, In example 19 thy additive was dissolved in methanol prior its being added to the image able composition.

Examples 21-24 These examples demonstrate the effect ox adding salts of carboxylic acids which have conjugate acids with pKa>0 to the thermally image able composition contemplated for this invention. A procedure identical to that described in Examples 1-16 was used and the results are listed in Table III:

I
;, Table III

. . . . . .
mount Percentage Example Sample Additive pea (Millimole?__f Spotting 21 89 lithium phthalate 5.51 .05 100 ~10 60 91 .30 92 ~50 0 . . _ . .
22 93 lithium salicylate 2.97 .05 100 I .10 100 .30 96 .50 0 ... _ _ _ , . __ . . _ .. . .
23 97 lithium nrheptanoate 4.89 ~05 90 98 .10 75 99 .30 0 100 .50 0 -- . .. _ _ _ . .. .. . _ . . _ 24 101 manganese acetate 4~75 ~05 100 102 .10 I
103 ,30 0 104 ,50 0 a In Examples 21-24, each additive was dissolved in methanol prior Jo its briny added to the image able composition.

I
These example demonstrate the effect of adding salts of alcohols or thiols which have conjugate acids Will pKa>5 to the thermally image able composition contemplated for this invention, procedure iden~lcal to that described in Examples 1-16 was used and the results are shown in Table IV:

I

viable IV

.. . . I,.. Jo mount Percentage Example Sample Additive pea (Millimole? of Spotting_ 109 sodium methylate16 .05 100 110 . 10 100 111 .30 15 112 .50 0 _ 26 113 lithium finality 9.95 Owe 100 114 .10 100 115 .30 0 116 .50 0 27 117 lithium 8 .05 100 118 thiocresolate .10 70 - 119 ~30 I
120 .50 0 a In Example 25, the additive was partially dissolved in methanol and was added to the image able composition as a homogeneous suspension at a concentration of 0.25 millimole/g total solution. In Example 26, the additive was dissolved in methanol prior to its briny added I to the legibly c~n~osition. In Example 27, the additive was dissolved in tetrahydrofuran prior to it hying Ed to the image able composition.

Example 28 This example demonstrates the effect of adding a salt of a carbon acid which has a conjugate acid with pKa>4 to the thermally image able composition contemplated for this invention. A procedure identical to that described in Examples 1-16 was used and the results are shown in Table V:

I
Table V

hmsun~ Percentage 28 125 nickel pinion 9 ,05 100 126 donate ~10 95 127 .30 2 12~ .50 0 a ye additive was dissolved in t~trahydrofuran and added to the image able composition at a concentration of 0.125 millimole/g total lo solution.

Examples 29-33 These examples demonstrate the effect ox adding salts of organophosphoric acids, ox organophosphonic acids, or of organophosphinic acids, or phosphines which have pK~0 to the thermally image able composition contemplated for this invention. A procedure identical to that described in Examples 1-16 was used and the results are shown in Table VI:

~23~
I
able VI

., .
kmountPercentage Example Simple Additive __ pea (Millimole) of Spottln~L
29 133 lithium depth 2 .05 100 134 hexyl)phosphate .10 70 135 .30 0 136 .50 0 . . _ _ 137 nickel deathly ,05 100 138 hexyl~phosphate ~10 100 139 .30 10 1~0 .50 0 . . _ _ .
31 141 phosphoric acid, Lowe .05 100 142 ~[3~5-bis(l,l-di- .10 100 143 methyl- ~30 0 144 hydroxyphenyl] .50 0 methyl]monoethyl ester, nickel ( I ) salt .. .. .. ., __ 32 145 lithium din- 2.5-3.5 .05 100 146 decyl-phosphinate .10 90 1~7 .30 14~ .50 0 . . . _ _ .
33 14~ diphenyl-phosphine 0.03 .05 90 150 a 10 lo 151 .30 0 152 .50 0 _ _ _ _ . _ . _ _ _ T . _ . _ . _ . _ _ _ . _ _ . _ . _ . . . _ _ . . _ a The additive in Examples 29 and 32 were dissolved in a mixture containing 85~ ethanol and 15% water and added to the irnageable composition at a concentration of 0.25 millimole/g total solution.
In Examples 30, 31 and 33, the additive was dissolved in tetrahydroEuran prior to its being added to the image able c~nposition.

I
Examples These examples demonstrate the effect of adding inorganic bases which have conjugate acids with pea to the thermally image able compositic3n contemplated for this invention. A procedure identical to that described in Examples 1-16 was used and the else aye shown in Table VII.

Table VII

mount Percentage 10 Example Sample Additive _ (Mill oily of Spotting ,. _ . _ __ 34 153 potassium hydroxide .05 100 154 .10 100 155 .30 5 156 ~50 0 157 sodium sulfide .05 100 158 .10 100 159 .30 0 160 .50 a In Example 34, the additive way dissolved in methanol prior to its Baja added to the jubilee composition In Example 35, the additive was partially dissolved in methanol aye added to tile image able composition as a homogeneous suspension at a concentration owe mil]imole/g total solution.

Various modifications and alterations ox this invention will become apparent to those slcilled in the art without copyrighting from the scope and spirit 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 (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A thermally imageable composition comprising 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, and an effective amount of at least one base characterized as having a conjugate acid having a pKa greater than or equal to zero and an oxidation potential such that it cannot be oxidized by silver.

2. The composition of claim 1 wherein said at least one base is not oxidizable to a quinone, diimine, or quinonimine.

3. The composition of claim 1 wherein said at least one base is selected from the group consisting of (A) tertiary amines, (B) secondary amines, (C) primary amines, (D) primary amides, (E) secondary amides, (F) tertiary amides, (G) tertiary amine oxides, (H) ureas, (I) salts of carboxylic acids, (J) salts of alcohols, (K) salts of thiols, (L) salts of complexes of carbon acids having pKa values between 0 and 25, inclusive, (M) salts of organophosphoric acids, (N) salts of organophosphonic acids, (O) salts of organophosphinic acids, (P) phosphines, and (Q) inorganic salts.

4. The composition of claim 1 wherein said at least one base is present at a concentration of from about .05 to about 1.0 equivalent per equivalent of nitrate ion present in said composit-ion.

5. The composition of claim 1 wherein said at least one leuco dye is selected from the group consisting of styryl, phenox-azine, phenothiazine, and phenazine.

6. The composition of claim 1 further including a binder.

7. The composition of 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.

8. The composition of claim 1 further including an organ-ic acid.

9. The composition of claim 8 wherein the concentration of organic acid is present in a ratio of from 0 to 10 times the amount of nitrate ion present in said composition.

10. The composition of claim 1 wherein there is at least 0.10 mole of nitrate ion per mole of dye.

11. A thermally imageable article comprising a substrate bearing on at least one major surface thereof the composition of claim 1.

12. The article of claim 11 wherein said substrate is selected from the group consisting of paper, glass and polymeric film.

13. The article of claim 12 wherein said substrate is transparent polymeric film.