Thermographic imaging composition and element comprising said composition

A thermographic imaging element comprises: PA1 (a) a support; PA1 (b) an imaging layer comprising: PA2 (i) a silver salt; PA2 (ii) a first reducing agent which has high activity with an activation energy of less than 10 Joules/sq.cm.; and PA2 (iii) a second reducing agent which has low activity with an activation energy of greater than or equal to 10 Joules/sq.cm.

FIELD OF THE INVENTION 
The present invention relates to thermographic compositions and elements 
for use in direct thermal imaging. 
BACKGROUND OF THE INVENTION 
Thermal imaging is a process in which images are recorded by the use of 
imagewise modulated thermal energy. In general there are two types of 
thermal recording processes, one in which the image is generated by 
thermally activated transfer of a light absorbing material, the other 
generates the light absorbing species by thermally activated chemical or 
physical modification of components of the imaging medium. A review of 
thermal imaging methods is found in "Imaging Systems" by K. I. Jacobson R. 
E. Jacobson--Focal Press 1976. 
Thermal energy can be delivered in a number of ways, for example by direct 
thermal contact or by absorption of electromagnetic radiation. Examples of 
radiant energy include infra-red lasers. Modulation of thermal energy can 
be by intensity or duration or both. For example a thermal print head 
comprising microscopic resistor elements is fed pulses of electrical 
energy which are converted into heat by the Joule effect. In a 
particularly useful embodiment the pulses are of fixed voltage and 
duration and the thermal energy delivered is then controlled by the number 
of such pulses sent. Radiant energy can be modulated directly by means of 
the energy source e.g. the voltage applied to a solid state laser. 
Direct imaging by chemical change in the imaging medium usually involves an 
irreversible chemical reaction which takes place very rapidly at elevated 
temperatures--say above 100.degree. C.--but at room temperature the rate 
is orders of magnitude slower such that effectively the material is 
stable. 
A particularly useful direct thermal imaging element uses an organic silver 
salt in combination with a reducing agent. Such systems are often referred 
to as `dry silver`. In this system the chemical change induced by the 
application of thermal energy is the reduction of the transparent silver 
salt to a metallic silver image. 
PROBLEM TO BE SOLVED BY THE INVENTION 
Prior art thermal imaging elements tend to have a relatively low dynamic 
range or relatively a narrow latitude which limits the number of tones or 
levels of gray that can be recorded. 
SUMMARY OF THE INVENTION 
One aspect of this invention comprises a thermographic imaging element 
comprising: 
(a) a support; 
(b) an imaging layer comprising: 
(i) a silver salt; 
(ii) a first reducing agent which has high activity with an activation 
energy of less than 10 Joules/sq.cm.; and 
(iii) a second reducing agent which has low activity with an activation 
energy of greater than or equal to 10 Joules/sq.cm. 
ADVANTAGEOUS EFFECT OF THE INVENTION 
This invention provides a heat-sensitive recording material suitable for 
direct thermal imaging having a high dynamic range (Dmax.gtoreq.2.5, 
Dmin.ltoreq.0.1, as described hereinafter) and a wide latitude (E1-E2, as 
described hereinafter) such that a large number of tones or levels of gray 
can be recorded.

DETAILED DESCRIPTION OF THE INVENTION 
The thermographic element and composition according to the invention 
comprise an oxidation-reduction image-forming composition which contains a 
silver salt, a high activity reducing agent, as defined herein) and a low 
activity reducing agent (as defined herein). 
The oxidizing agent is preferably a silver salt of an organic acid. 
Suitable silver salts include, for example, silver behenate, silver 
stearate, silver oleate, silver laureate, silver hydroxy stearate, silver 
caprate, silver myristate, silver palmitate silver benzoate, silver 
benzotriazole, silver terephthalate, silver phthalate saccharin silver, 
phthalazionone silver, benzotriazole silver, silver salt of 
3-(2-carboxyethyl-4-4-hydroxymethyl-4-thiazoline-2-thione, silver salt of 
3-mercapto-4-phenyl-1,2,4-triazole and the like. In most instances silver 
behenate is most useful. 
A variety of reducing agents can be employed in the imaging composition of 
the invention. Typical reducing agents which can be used include, for 
example: 
(1) Sulfonamidophenol reducing agents in thermographic materials are 
described in U.S. Pat. No. 3,801,321 issued Apr. 2, 1974 to Evans et al., 
the entire disclosure of which is incorporated herein by reference, and 
sulfonamidoaniline reducing agents; 
(2) Other reducing agents are substituted phenol and substituted naphthol 
reducing agents. Substituted phenols which can be used include, for 
example, bisphenols, e.g., bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, 
bis(6-hydroxy-m-tolyl)mesitol, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 
4,4-ethylidene-bis(2-t-butyl-6-methylphenol) and 
2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane. Substituted naphthols which 
can be used include, for example, bis-b-naphthols such as those described 
in U.S. Pat. No. 3,672,904 of deMauriac, issued Jun. 27, 1972, the entire 
disclosure of which is incorporated herein by reference. Bis-b-naphthols 
which can be used include, for example, 2,2'-dihydroxy-1,1'-binaphthyl, 
6,-6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, 
6,6'-dinitro-2,2'-dihydroxy-1,1'-binaphthyl, and 
bis-(2-hydroxy-1-naphthol)methane. 
(3) Other reducing agents include polyhydroxybenzene reducing agents such 
as hydroquinone, alkyl-substituted hydroquinones such as tertiary butyl 
hydroquinone, methyl hydroquinone, 2,5-dimethyl hydroquinone and 
2,6-dimethyl hydroquinone, (2,5-dihydroxyphenyl)methylsulfone, catechols 
and pyrogallols, e.g., pyrocatechol, 4-phenylpyrocatechol, 
t-butylcatechol, pyrogallol or pyrogallol derivatives such as pyrogallol 
ethers or esters; 3,4-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, 
3,4-dihydroxybenzoic acid esters such as dihydroxybenzoic acid, methyl 
ester, ethyl ester, propyl ester or butyl ester; gallic acid, gallic acid 
esters such as methyl gallate, ethyl gallate, propyl gallate and the like, 
gallic acid amides; 
(4) aminophenol reducing agents, such as 2,4-diaminophenols and 
methylaminophenols can be used; 
(5) ascorbic acid reducing agents such as ascorbic acid and ascorbic acid 
derivatives such as ascorbic acid ketals can be used; 
(6) hydroxylamine reducing agents can be used; 
(7) 3-pyrazolidone reducing agents such as 1-phenyl-3-pyrazolidone can be 
used; 
(8) other reducing agents which can be used include, for example, 
hydroxycoumarones, hydroxycoumarans, hydrazones, hydroxaminic acids, 
indane-1,3-diones, aminonaphthols, pyrazolidine-5-ones, hydroxylamines, 
reductones, esters of amino reductones, hydrazines, phenylenediamines, 
hydroxyindanes, 1,4-dihydroxypyridines, hydroxy-substituted aliphatic 
carboxylic acid arylhydrazides, N-hydroxyureas, phosphonamidephenols, 
phosphonamidanilines, .alpha.-cyanophenylacetic esters 
sulfonamidoanilines, aminohydroxycycloalkenone compounds, N-hydroxyurea 
derivatives, hydrazones of aldehydes and ketones, sulfhydroxamic acids, 
2-tetrazolythiohydroquinones, e.g., 
2-methyl-5-(1-phenyl-5-tetrazolythio)hydroquinone, tetrahydroquinoxalines, 
e.g. 1,2,3,4-tetrahydroquinoxaline, amidoximes, azines, hydroxamic acids, 
2-phenylindan-1,3-dione, 1,4-dihydropyridines, such as 
2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridine. 
To determine the activity of a reducing agent the following procedure is 
conducted. A test formulation containing the following activity 
formulation #1 is prepared. 
______________________________________ 
ACTIVITY FORMULATION #1 
SILVER BEHENATE 0.88 millimole/sq. ft. 
(9.7 millimole/sq. m.) 
POLY(VINYL BUTYRAL) 400 mg/sq.ft 
(4400 mg/sq. m.) 
SUCCINIMIDE 0.25 millimole/sq. ft. 
(2.75 millimole/sq. m.) 
TEST REDUCING AGENT 0.75 millimole/sq. ft. 
(8.25 millimole/sq. m.) 
______________________________________ 
The formulation is coated on a support and is thermally imaged using a thin 
film thermal head in contact with a combination of the imaging medium and 
a protective film of 6 micron thickness polyester sheet. Contact of the 
head to the element is maintained by-an applied pressure of 313 g/cm 
heater line. The line write time is 15 millisec. broken up into 255 
increments corresponding to the pulse width referred to above. Energy per 
pulse is 0.041 Joule/sq.cm. Individual picture elements are of a size 
corresponding to 300 dots per inch. 
The thermal sensitive coatings are treated with a linearly increasing 
pattern of pulses from 5 to 255 in 10 pulse increments. Densities of the 
resulting image steps are measured with an X-Rite 361 densitometer in the 
`ortho` mode. In the activity determination for low activity reducing 
agents, an additional test in which the average printing energy per pulse 
is increased to 0.085 Joules per sq.cm is required to generate sufficient 
density in the case of the low activity reducing agents. Measured activity 
values for high activity reducing agents, are the same in both tests. 
Plots of density versus pulse count can then be generated and the 
activity, E1, the `toe` of the curve, i.e., the onset of image density, 
can be read from the plot. The practical measure of E1 is the thermal 
energy which generates a density 0.1 greater than Dmin. Energies can be 
converted from pulse count to Joules/sq.cm. using the factors given above. 
Illustrative high activity reducing agents are given in Table 1. 
TABLE 1 
______________________________________ 
High Activity Reducing Agents 
Activation 
Energy, E1 
ID Reducing Agent (Joules/cm.sup.2) 
______________________________________ 
H1 
5.6 1## 
- H2 
4.9 2## 
- H3 
3.7 3## 
- H4 
5.3 4## 
- H5 
3.8R5## 
______________________________________ 
Preferred high activity reducing agents have an activation energy of less 
than about 6 Joules/sq.cm. In preferred embodiments of the invention, the 
high activity reducing agent has an activation energy between about 1 and 
10 Joules/sq.cm. and preferably between about 3 and about 6 Joules/sq.cm. 
Illustrative low activity reducing agents are given in Table 2. 
TABLE 2 
______________________________________ 
Low Activity Reducing Agents 
Activation 
Energy, 
E1 
(Joules/ 
ID Reducing Agent cm.sup.2) 
______________________________________ 
L1 
10.2 ## 
- L2 
13.9 ## 
- L3 
11.58## 
______________________________________ 
Low activity reducing agents have an activity, as defined herein, of equal 
to or greater than 10 Joules/sq.cm. The low activity reducing agents 
preferably have an activity between about 10 and about 20 Joules/sq.cm., 
more preferably between about 10 and about 15 Joules/sq.cm. 
Plots of the density versus pulse count for all the reducing agents of 
Tables 1 & 2 are given in FIG. 1. FIG. 1 shows the characteristic 
sensitometric curves obtained by plotting image density (D) versus the 
imaging thermal energy expressed as the number of thermal pulses applied. 
Labels identify the examples as high activity (H1 through H5) and low 
activity (L1 through L3) as shown in Tables 1 & 2. 
From the same plots of density versus pulse count, the D.sub.max, 
D.sub.min, E1, and E2 values, as described below and in FIG. 2, can also 
be obtained. The plots of density versus pulse count also provides 
contrast and tonal range. Contrast is an expression of the rate of change 
of image density versus imaging energy. Depending on the end use of the 
image different parts of the image range have greater or lesser 
importance. For the material herein described the whole of the density 
range is important so the applicable measure of contrast is over the range 
of densities from the `toe` (E1) or onset of image density, to the 
shoulder (E2) or onset of D.sub.max. The practical measure of E1 is the 
thermal energy which generates a density 0.1 greater than Dmin. Similarly 
the practical measure of E2 is the thermal energy that generates a density 
90% of D.sub.max. The tonal range is the value of E2-E1. 
Under the action of the applied thermal energy the density of the image 
increases from a minimum (D.sub.min) value to a maximum (D.sub.max) value. 
It is desirable for the D.sub.min to be as low as possible and the 
D.sub.max to be high enough that pleasing image density is achieved. For a 
transmission image D.sub.min of less than 0.1 and D.sub.max of greater 
than 2.5 are considered acceptable. The dynamic range of the thermal 
imaging material is D.sub.max -D.sub.min. 
Tonal and dynamic ranges are given for the high activity reducing agents in 
Table 3. 
TABLE 3 
______________________________________ 
Single Reducing Agent Dynamic & Tonal Range 
Dynamic Range 
Tonal Range 
Reducing Agent (.DELTA. density) (pulse count) 
______________________________________ 
H1 2.46 68 
H2 1.71 84 
H3 2.21 82 
H4 2.97 63 
H5 2.6 51 
______________________________________ 
The amount of high activity reducing agent used in the thermal imaging 
material of this invention is preferably about 0.005 to about 0.2 
millimoles/mole Ag, more preferably about 0.01 to about 0.1 and most 
preferable about 0.015 to about 0.05 mmoles/mole Ag. The amount of low 
activity reducing agent is preferably about 0.05 to about 2, more 
preferably about 0.1 to about 1 and most preferably 0.15 to about 0.5 
mmoles/mole Ag. Typically the ratio of the amount of high activity 
reducing agent to the amount of low activity reducing agent is about 1 to 
3 to about 1 to 30, particularly preferred is a ratio of about 1 to about 
10. 
The imaging composition and element of the invention can also contain a 
so-called activator-toning agent, also known as an accelerator-toning 
agent or toner. The activator-toning agent can be a cyclic imide and is 
typically useful in a range of concentration such as a concentration of 
about 0.10 mole to about 1.1 mole of activator-toning agent per mole of 
silver salt oxidizing agent in the thermographic material. Typical 
suitable activator-toning agents are described in Belgian Patent No. 
766,590 issued Jun. 15, 1971, the entire disclosure of which is 
incorporated herein by reference. Typical activator-toning agents include, 
for example, phthalimide, N-hydroxyphthalimide, 
N-hydroxy-1,8-naphthalimide, N-potassium phthalimide, N-mercury 
phthalimide, succinimide and/or N-hydroxysuccinimide. Combinations of 
activator-toning agents can be employed if desired. Other activator-toning 
agents which can be employed include phthalazinone, 2-acetyl-phthalazinone 
and the like. 
The thermographic imaging composition of the invention can contain other 
addenda that aid in formation of a useful image. 
A thermographic composition of the invention can contain various other 
compounds alone or in combination as vehicles, binding agents and the 
like, which can be in various layers of the thermographic element of the 
invention. Suitable materials can be hydrophobic or hydrophilic. They are 
transparent or translucent and include such synthetic polymeric substances 
as water soluble polyvinyl compounds like poly(vinyl pyrrolidone), 
acrylamide polymers and the like. Other synthetic polymeric compounds 
which can be employed include dispersed vinyl compounds such as in latex 
form and particularly those which increase dimensional stability of 
photographic materials. Effective polymers include water insoluble 
polymers of polyesters, polycarbonates, alkyl acrylates and methacrylates, 
acrylic acid, sulfoalkyl acrylates, methacrylates and those which have 
crosslinking sites which facilitate hardening or curing as well as those 
having recurring sulfobetaine units as described in Canadian Patent No. 
774,054, the entire disclosure of which is incorporated herein by 
reference. Especially useful high molecular weight materials and resins 
include poly(vinyl acetals), such as, poly(vinyl acetal) and poly(vinyl 
butyral), cellulose acetate butyrate, polymethyl methacrylate, poly(vinyl 
pyrrolidone), ethylcellulose, polystyrene, polyvinyl chloride, chlorinated 
rubber, polyisobutylene, butadiene-styrene copolymers, vinyl 
chloride-vinyl acetate copolymers, copolymers, of vinyl acetate, vinyl 
chloride and maleic acid and polyvinyl alcohol. 
A thermographic element according to the invention comprises a thermal 
imaging composition, as described above, on a support. A wide variety of 
supports can be used. Typical supports include cellulose nitrate film, 
cellulose ester film, poly(vinyl acetal) film, polystyrene film, 
poly(ethylene terephthalate) film, polycarbonate film and related films or 
resinous materials, as well as glass, paper, metal and the like supports 
which can withstand the processing temperatures employed according to the 
invention. Typically, a flexible support is employed. 
The thermographic imaging elements of the invention can be prepared by 
coating the layers on a support by coating procedures known in the 
photographic art, including dip coating, air knife coating, curtain 
coating or extrusion coating using hoppers. If desired, two or more layers 
are coated simultaneously. 
Thermographic imaging elements are described in general in, for example, 
U.S. Pat. Nos. 3,457,075; 4,459,350; 4,264,725 and 4,741,992 and Research 
Disclosure, June 1978, Item No. 17029. 
The components of the thermographic element can be in any location in the 
element that provides the desired image. If desired, one or more of the 
components can be in more than one layer of the element. For example, in 
some cases, it is desirable to include certain percentages of the reducing 
agent, toner, stabilizer and/or other addenda in an overcoat layer. This, 
in some cases, can reduce migration of certain addenda in the layers of 
the element. 
The thermographic imaging element of the invention can contain a 
transparent, image insensitive protective layer. The protective layer can 
be an overcoat layer, that is a layer that overlies the image sensitive 
layer(s), or a backing layer, that is a layer that is on the opposite side 
of the support from the image sensitive layer(s). The imaging element can 
contain both a protective overcoat layer and a protective backing layer, 
if desired. An adhesive interlayer can be imposed between the imaging 
layer and the protective layer and/or between the support and the backing 
layer. The protective layer is not necessarily the outermost layer of the 
imaging element. 
The protective overcoat layer preferably acts as a barrier layer that not 
only protects the imaging layer from physical damage, but also prevents 
loss of components from the imaging layer. The overcoat layer preferably 
comprises a film forming binder, preferable a hydrophilic film forming 
binder. Such binders include, for example, crosslinked polyvinyl alcohol, 
gelatin, poly(silicic acid), and the like. Particularly preferred are 
binders comprising poly(silicic acid) alone or in combination with a 
water-soluble hydroxyl-containing monomer or polymer as described in the 
above-mentioned U.S. Pat. No. 4,828,971, the entire disclosures of which 
are incorporated herein by reference. 
The thermographic imaging element of this invention can include a backing 
layer. The backing layer is an outermost layer located on the side of the 
support opposite to the imaging layer. It is typically comprised of a 
binder and a matting agent which is dispersed in the binder in an amount 
sufficient to provide the desired surface roughness and the desired 
antistatic properties. 
The backing layer should not adversely affect sensitometric characteristics 
of the thermographic element such as minimum density, maximum density and 
photographic speed. 
The thermographic element of this invention preferably contains a slipping 
layer to prevent the imaging element from sticking as it passes under the 
thermal print head. The slipping layer comprises a lubricant dispersed or 
dissolved in a polymeric binder. Lubricants the can be used include, for 
example: 
(1) a poly(vinyl stearate), poly(caprolactone)or a straight chain alkyl or 
polyethylene oxide perfluoroalkylated ester or perfluoroalkylated ether as 
described in U.S. Pat. No. 4,717,711, the disclosure of which is 
incorporated by reference. 
(2) a polyethylene glycol having a number average molecular weight of about 
6000 or above or fatty acid esters of polyvinyl alcohol, as described in 
U.S. Pat. No. 4,717,712 the entire disclosure of which is incorporated 
herein by reference; 
(3) a partially esterified phosphate ester and a silicone polymer 
comprising units of a linear or branched alkyl or aryl siloxane as 
described in U.S. Pat. No. 4,737,485 the entire disclosure of which is 
incorporated herein by reference; 
(4) a linear or branched aminoalkyl-terminated poly(dialkyl, diaryl or 
alkylaryl siloxane) such as an aminopropyldimethylsiloxane or a 
T-structure polydimethylsiloxane with an aminoalkyl functionality at the 
branch-point, as described in U.S. Pat. No. 4,738,950, the entire 
disclosure of which is incorporated herein by reference; 
(5) solid lubricant particles, such as poly(tetrafluoroethylene), 
poly(hexafluoropropylene) or poly(methylsilylsesquioxane, as described in 
U.S. Pat. No. 4,829,050, the entire disclosure of which is incorporated 
herein by reference; 
(6) micronized polyethylene particles or micronized polytetrafluoroethylene 
powder as described in U.S. Pat. No. 4,829,860, the entire disclosure of 
which is incorporated herein by reference; 
(7) a homogeneous layer of a particulate ester wax comprising an ester of a 
fatty acid having at least 10 carbon atoms and a monohydric alcohol having 
at least 6 carbon atoms, the ester wax having a particle size of from 
about 0.5 .mu.m to about 20 .mu.m, as described in U.S. Pat. No. 
4,916,112, the entire disclosure of which is incorporated herein by 
reference; 
(8) a phosphonic acid or salt as described in U.S. Pat. No. 5,162,292, the 
entire disclosure of which is incorporated herein by reference; 
(9) a polyimide-siloxane copolymer, the polysiloxane component comprising 
more than 3 weight % of the copolymer and the polysiloxane component 
having a molecular weight of greater than 3900, the entire disclosure of 
which is incorporated herein by reference; 
(10) a poly(aryl ester, aryl amide)-siloxane copolymer, the polysiloxane 
component comprising more than 3 weight % of the copolymer and the 
polysiloxane component having a molecular weight of at least about 1500, 
the entire disclosure of which is incorporated herein by reference. 
In the thermographic imaging elements of this invention can contain either 
organic or inorganic matting agents. Examples of organic matting agents 
are particles, often in the form of beads, of polymers such as polymeric 
esters of acrylic and methacrylic acid, e.g., poly(methylmethacrylate), 
styrene polymers and copolymers, and the like. Examples of inorganic 
matting agents are particles of glass, silicon dioxide, titanium dioxide, 
magnesium oxide, aluminum oxide, barium sulfate, calcium carbonate, and 
the like. Matting agents and the way they are used are further described 
in U.S. Pat. Nos. 3,411,907 and 3,754,924. 
The concentration of matting agent required to give the desired roughness 
depends on the mean diameter of the particles and the amount of binder. 
Preferred particles are those with a mean diameter of from about 1 to 
about 15 micrometers, preferably from 2 to 8 micrometers. The matte 
particles can be usefully employed at a concentration of about 1 to about 
100 milligrams per square meter. 
The imaging element can also contain an electroconductive layer which, in 
accordance with U.S. Pat. No. 5,310,640, is an inner layer that can be 
located on either side of said support. The electroconductive layer 
preferably has an internal resistivity of less than 5.times.10.sup.11 
ohms/square. 
The protective overcoat layer and the slipping layer may either or both be 
electrically conductive having a surface resistivity of less than 
5.times.10.sup.11 ohms/square. Such electrically conductive overcoat 
layers are-described in U.S. Pat. No. 5,547,821, incorporated herein by 
reference. As taught in the '821 patent, electrically conductive overcoat 
layers comprise metal-containing particles dispersed in a polymeric binder 
in an amount sufficient to provide the desired surface resistivity. 
Examples of suitable electrically-conductive metal-containing particles 
for the purposes of this invention include: 
(1) donor-doped metal oxide, metal oxides containing oxygen deficiencies, 
and conductive nitrides, carbides, and borides. Specific examples of 
particularly useful particles include conductive TiO.sub.2, SnO.sub.2, 
V.sub.2 O.sub.5, Al.sub.2 O.sub.3, ZrO.sub.2, In.sub.2 O.sub.3, ZnO, 
TiB.sub.2, ZrB.sub.2, NbB.sub.2, TaB.sub.2, CrB.sub.2, MoB, WB, LaB.sub.6, 
ZrN, TiN, TiC, WC, HfC, HfN, ZrC. Examples of the many patents describing 
these electrically-conductive particles include U.S. Pat. Nos. 4,275,103, 
4,394,441, 4,416,963, 4,418,141, 4,431,764, 4,495,276, 4,571,361, 
4,999,276, and 5,122,445; 
(2) semiconductive metal salts such as cuprous iodide as described in U.S. 
Pat. Nos. 3,245,833, 3,428,451 and 5,075,171; 
(3) a colloidal gel of vanadium pentoxide as described in U.S. Pat. Nos. 
4,203,769, 5,006,451, 5,221,598, and 5,284,714; and 
(4) fibrous conductive powders comprising, for example, antimony-doped tin 
oxide coated onto non-conductive potassium titanate whiskers as described 
in U.S. Pat. Nos. 4,845,369 and 5,116,666. 
The following examples illustrate the thermographic elements and 
compositions of this invention. 
EXAMPLE 1 
A support of polyethylene terephthalate having a thickness of 178 micron 
was doctor blade coated from a coating composition containing methyl ethyl 
ketone as solvent and the listed components so as to give the final dry 
weights as shown. 
______________________________________ 
SILVER BEHENATE 400 mg/sq. ft (4.4 g/m.sup.2) 
POLYVINYL ACETAL 400 mg/sq. ft (4.4 g/m.sup.2) 
PHTHALAZINONE 40 mg/sq. ft (.44 g/m.sup.2) 
REDUCING AGENT 1 AS LISTED mg/sq. ft (g/m.sup.2) 
REDUCING AGENT 2 AS LISTED mg/sq. ft (g/m.sup.2) 
______________________________________ 
Coatings were imaged using the procedure defined above. Dynamic range is 
simply D.sub.max -D.sub.min. Tonal Range is E2-E1 expressed in units of 
pulse count. Table 4 sets forth the reducing agents used, the amounts of 
reducing agents and the dynamic and tonal ranges obtained. 
TABLE 4 
______________________________________ 
Reducing agent Mixtures - Dynamic & Tonal Range 
EX- REDUCING REDUCING 
AMPLE AGENT 1 AGENT 2 DYNAMIC TONAL 
ID ID AMT ID AMT RANGE RANGE 
______________________________________ 
C1 H1 10 (0.11) -- -- 0.93 41 
D1 H1 10 (0.11) L1 100 (1.1) 2.95 92 
D2 H1 10 (0.11) L2 320 (3.5) 2.63 73 
D3 H1 10 (0.11) L3 180 (2.0) 1.99 82 
C2 H2 8 (0.08) -- -- 0.76 87 
D4 H2 8 (0.08) L1 100 (1.1) 2.47 113 
D5 H2 8 (0.08) L2 280 (3.1) 2.66 107 
D6 H2 8 (0.08) L3 140 (1.5) 2.51 124 
C3 H3 20 (0.22) -- -- 0.96 56 
D7 H3 20 (0.22) L1 100 (1.1) 2.74 121 
D8 H3 20 (0.22) L2 320 (3.5) 2.68 106 
D9 H3 20 (0.22) L3 180 (2.0) 2.09 126 
C4 H4 10 (0.11) -- -- 0.85 39 
D10 H4 10 (0.11) L1 100 (1.1) 2.6 78 
D11 H4 10 (0.11) L2 320 (3.5) 2.01 91 
D12 H4 10 (0.11) L3 180 (2.0) 1.77 80 
C5 H5 10 (0.11) -- -- .82 35 
D13 H5 10 (0.11) L1 100 (1.1) 2.12 106 
D14 H5 10 (0.11) L2 320 (3.5) 2.64 82 
D15 H5 10 (0.11) L3 180 (2.0) 1.93 104 
______________________________________ 
In FIGS. 3-7 each of the strong reducing agents is combined with each of 
the weak reducing agents as defined in Table 4. In every case the dynamic 
and tonal range of the mixture is greater than the sum of the strong 
reducing agent by itself and the weak reducing agent by itself. 
The invention has been described in detail with particular reference to 
preferred embodiments, but it will be understood that variations and 
modifications can be effected within the spirit and scope of the 
invention.