Background of the Invention
[0001] This invention relates to thermally imageable compositions and to stabilizers for
these compositions.
[0002] 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.
[0003] 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.
[0004] 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, ana nigh costs, and in most cases,
does not offer a practical solution to the problem.
[0005] In thermally imageable transparency films based upon combinations of leuco dyes ano
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.
Summary of the Invention
[0006] This invention involves the prevention of spotting and backgrounding of transparency
films bearing thermally imageable compositions that contain combinations of leuco
dyes and nitrate salts. Spotting and backgrounding 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 pKa values
of zero or higher. These bases have conjugate acids with Ka values equal to or less
than one. Effective additives include amines; amine oxides; amides; ureas; salts of
phosphinic, phosphonic, 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. Addition of one or more of
those bases which have conjugate acids with pKa values of zero or above 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,
[0007] 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
[0008] This invention involves compositions which are imageable by thermal energy, e.g.
infrared radiation, and coated substrates prepared therefrom, which compositions comprise
(1) at least one leuco dye, (2) at least one inorganic nitrate salt, (3) a polymeric
binder, (4) an optional acid, and (5) at least one antispotting compound 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.
[0009] These compounds can be represented by the following general formulas:

wherein
Rl,
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, substituted or unsubstituted aryl
groups having up to 3 fused rings.
[0010] The tertiary amine can have the following structure:

wherein R
1 is as defined above and

represents, for example,

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;

where 1 and m are as defined above and n' is 0 or 4;

where 1 and m are as defined above, and n" is an integer from 0 to 4, inclusive;

where 1' is 0 or 4, m and n" are as defined above.
[0011] The tertiary amine can also have the following structure:

wherein R
l is as defined above, and R
4 represents CR
5R
6 where R
5 and R
6 represents a member of the class from which R
1, R
2, and R
3 are selected, with the proviso that
R5 and R
6 need not be the same as
Rl,
R2, or
R3.
[0012] The tertiary amine can further have the following structure:

wherein

represents, for example,

where x is 0, 1, or 2, y is an integer from 0 to 8, inclusive, and z is 0, 1, or 2,
and R
7 represents a member of the class from which
R1,
R2, and R
3 are selected, with the proviso that R
7 need not be the same as
Rl,
R2,
R3;

wherein R
l and R
2 are as defined above.

wherein R
1 and R
2 are as defined above.

wherein R
1 is as defined above.

wherein R
1 and R
2 are as defined above.

wherein R
1, R
2, and R
3 are as defined above.

wherein R
1,
R2 and R
3 are as defined above.

wherein R
1, R
2, and R
3 are as defined above, and R
8 represents a member of the class from which R
1, R
2, and R
3 are selected, with the proviso that R
8 need not be the same as R
1, R
2, or R
3.

wherein R
1 is as defined above, and A+ represents a metallic cation, e.g. Li
+, Na
+, K
+, Mg
+2, Ca
+2, Mn
+2, Fe
+3, Ni
+2, Cu
+2, Zn
+2..

wherein R
1 and A+ are as defined above.

wherein R
1 and A+ are as defined above.

wherein R
9, R
10, and R
11 can be the same or different and represent a member of the group selected from H,
-NO
2, -CN, -COR
12, -COOR
12, and -SO
2R
12 where R
12 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.
[0013] The salts or complexes of carbon acids can also include the following structures

wherein R
9 and A+ are as defined above and

represent, for example,

wherein a is 2, 3, or 4;

where b is 0 or 4.
[0014] The salts or complexes of carbon acids can further include the following structure

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

wherein R
1, R
2, and A
+ are as defined above.

wherein R
l, R
2, and A+ are as defined above.

wherein R
1, R
2, and A+ are as defined above.

wherein R
1, R
2, and R
3 are as defined above.

wherein A+ is as defined above, and Y
- represents a member of the group selected from -
OH,
S,
HS,
C03, HC
03.
[0015] When R
l, R
2, or R
3 is a substituted alkyl or aryl group, the substituents can be any which do not deleteriously
affect the function of the thermographic system. Suitable substituents include halo
groups, e.g. chloro, bromo, iodo, fluoro; hydroxyl group; 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. In addition when
R1,
R2, or R
3 is a substituted aryl group, the substituent can be alkylthio, having, for example,
from 1 to 20 carbon atoms.
R1,
R2,
R3 can be mono-, di-, tri-, or tetra- substituted.
[0016] 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-lH-indol-2-yl)ethenyl]-N,N-dimethylbenzenamine;
(c) 2,3-dihydro-1,3,3-trimethyl-2-[2-(4-dimethylamino)-phenyl-ethenyl]-lH-Indole;
(d) 2,3-dihydro-1,3,3-trimethyl-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.

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.

[0017] Representative examples of phenazine dyes are; (h) 5,10-di- hydro-5-phenyl-10-benzoyl-3,7-bis-(N,N-diethylamino)phenazine
and (i) 5,10-dihydro-5-ethyl-10-benzoyl-3,7-bis-(N,N-dimethylamino)phenazine.

[0018] 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 (zr0
2), nickel, aluminum, chromium, iron, copper, tin, magnesium, lead, and cobalt, ammonium
nitrate, and cerous ammonium nitrate can be used.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] The thormally 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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 ( m), 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.
[0028] 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.
[0029] 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 1-15
[0030] 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(NO
3)
3·9H
2O 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 mil polyester film
at a 3 mil wet thickness and dried at 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.

[0031] As the pKa of the base's conjugate acid approaches zero, the additive is less effective
as an antispotting agent.
Examples 17-20
[0032] 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 1-16.
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 is listed in Table II:

Examples 21-24
[0033] 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 1-16 was used
and the results are listed in Table III:

Examples 25-27
[0034] 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 1-16 was used
and the results are shown in Table IV:

Example 28
[0035] This example demonstrates the effect of adding a salt of a carbon acid which has
a conjugate acid with pKa>4 to the thermally imageable 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:

Examples 29-33
[0036] 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 1-16 was used and the results are shown in
Table VI:

Examples 34-35
[0037] These examples demonstrate the effect of adding inorganic bases which have conjugate
acids with pKa>0 to the thermally imageable composition contemplated for this invention.
A procedure identical to that described in Examples 1-16 was used and the results
are shown in Table VII.

[0038] Various modifications and alterations of this invention will become apparent to those
skilled in the art without departing 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.
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,
the improvement wherein at least one base with a conjugate acid having a pKa greater
than or equal to zero is included in the composition.
2. 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
(0) inorganic salts.
3. 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 composition.
4. The composition of claim 1 wherein said at least one leuco dye is selected from
the group consisting of styryl, phenoxazine, phenothiazine, and phenazine.
5. The composition of claim 1 further including a binder.
6. The composition of claim 5 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.
7. The composition of claim 1 further including an organic acid.
8. The composition of claim 7 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.
9. The composition of claim 1 wherein there is at least 0.10 mole of nitrate ion per
mole of dye.
10. A thermally imageable transparency film comprising a substrate bearing on at least
one major surface thereof the composition of claim 1,