Thermal imaging material

Thermographic materials are colorless when unexposed, but provide an intense dark image when thermally addressed. The materials comprise white ferric organophosphate, ferric organophosphinate, or ferric organophosphonate in a clear binder with a colorless catechol or polycatechol held in said binder in solid solution. The choice of substituents on the catechol nucleus can give a change in the color of the thermal image together and provide good near infrared absorption. Use of mixed catechols can give achromatic black images. These combinations of materials show high stability at ambient temperatures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to thermographic materials. More particularly it 
relates to substantially colorless thermographic layers on a substrate. 
Many existing compositions exhibit a yellow or brown color cast which is 
caused by colored thermally sensitive metal compounds such as iron 
stearate. This invention uses compositions containing colorless iron salts 
which are thermally reactable to give a visible image. 
In commercial applications, thermally developed labels are sought which not 
only provide visible images but which are also capable of being read by 
optical scanners using near infrared radiation (NIR). The images resulting 
from catechols with certain substituents exhibit low discrimination in the 
NIR. Other catechols give good discrimination both visually and to NIR. 
2. Background of the Art 
For many years heat-sensitive imaging sheets have been used for copying, 
thermal printing, thermal recording, and thermal labeling. Many of these 
materials involve thermally increasing the reactivity of two or more 
components of a color forming reaction which do not react at normal 
ambient temperatures. Reactivity is often enhanced by melting of one or 
both reactants which are physically separated from one anoher. Separation 
is accomplished either by dispersion in a single coated layer or by being 
situated in two different coated layers. Several general classes of color 
forming reactants have been used, of which two common ones are (a) leuco 
lactone or spiropyran compounds reactable with phenolic compounds (e.g. 
U.S. Pat. Nos. 3,829,401 and 3,846,153) and (b) heavy metal salts of 
organic acids reactable with ligands to give colored complexes (e.g. U.S. 
Pat. Nos. 2,663,654, 3,094,620, 3,293,055 and 3,953,659). 
Commercial preference for the heavy metal salt class has often resulted 
from the high stability and near black color of the images produced (U.S. 
Pat. No. 4,531,141). Of the heavy metals used, iron, nickel, and cobalt 
are common and ferric iron appears to be preferred (U.S. Pat. Nos. 
2,663,654, 3,953,659 and 4,531,141). 
Two objections raised to the ferric salt-phenolic ligand systems are the 
colored nature of the unreacted ferric salt and the background stain often 
experienced because of the insidious, slow reaction of the two reactants 
on storage or during coating. Indeed, if the reactants are intimately 
mixed they often react rapidly at room temperature (U.S. Pat. Nos. 
3,442,682, 4,531,141). The first objection has led to the use of white 
fillers (U.S. Pat. No. 4,531,141) or other incident light scattering 
devices (e.g., "blushing" the surface of the layer as in U.S. Pat. No. 
3,953,659) to reduce the observed color tint of the coated layer. The 
second objection has led to the use of stabilizing compounds added to the 
reactive layer (U.S. Pat. Nos. 2,663,654, 3,442,682) and more particularly 
to the physical separation of the two reactants (U.S. Pat. No. 4,531,141) 
either by dispersion as separate micro-particles (U.S. Pat. Nos. 
2,663,654, 3,094,620, 3,111,423, and 3,293,055) or by separating the 
reactants in distinct but adjacent layers (U.S. Pat. Nos. 3,111,423 and 
3,442,682). 
As indicated earlier, a considerable list of heavy metals has been used in 
their organic acid salt form to give thermographic images (U.S. Pat. Nos. 
3,111,423 and 3,293,055). Some heavy metals giving colorless salts have 
been used and will be found amongst those listed in these two references. 
Such heavy metals (e.g., zinc), however, must be reacted with ligands 
which themselves contribute color to the image and may indeed have a color 
cast before reaction. These heavy metaIs have not successfuIIy provided 
satisfactory thermographic materials which are truly colorless and also 
give a deep near black color on thermal exposure. 
Thus the art discussed so far shows consistent interest in two problems of 
ferric chelate imaging (a) the colored nature of ferric organic acid salts 
and (b) the difficulty in controlling the room temperature reactivity of 
such salts with the range of ligands available. 
More recently there has been interest evinced in obtaining 
thermographically reactive iron salts which are colorless and which give 
sharp, high density images when reacted with a colorless ligand. 
Organo-phosphates of ferric iron are known in the art to be amongst the 
few colorless ferric salts (Smythe et al., J. Inorg. Nucl. Chem., 30 
1553-1561, (1968)). In U.S. Pat. No. 4,533,930 it is disclosed that such 
organophosphates and the equivalent thiophosphates can react with a 
variety of ligands under the influence of heat to give colored results. 
Ferric salts of organophosphinic acid and organophosphonic acids are 
included. Some of these organophosphates and many of the thiophosphates 
have some color cast before reaction which appears to be obscured by the 
use of white filler in the thermosensitive compositions. In the 
thermographic materials disclosed, the reaction of the ferric salt with 
the ligand at ambient temperatures is precluded by either dispersing each 
reactant in microparticulate form in the binder or by providing separate 
but adjacent layers for the two reactants. These conditions are explicitly 
identified in the claims by the wording "said metal compound and said 
ligand compound being physically separated from one another . . . ". Also 
in this patent there are disclosed pressure-sensitive manifold papers in 
which at least one of the two reactants is encapsulated as a solvent 
solution. When the microcapsules are burst by pressure, the reactants come 
in contact and immediately react at room temperature to give a colored 
result. This patent further discloses the use of ferric organophosphates 
containing organic acid moieties as formed by the aqueous reaction of a 
ferric salt, an alkali metal organophosphate, and an alkali metal salt of 
an organic acid. These are disclosed as giving the initial material better 
"color forming properties" and giving better image colors (Column 5 lines 
38-39) than the simple organophosphates. Excess organic acid salt is 
disclosed as degrading the white color. It is of significance that the 
inventors do not consider the choice of the ferric salt used in the 
preparation to be important. In fact they specifically mention ferric 
chloride and ferric sulfate (Column 6 lines 10-17) and all of their 
examples use ferric chloride. 
SUMMARY OF THE INVENTION 
This invention provides thermographic layers which are colorless when 
unexposed and are stable at room temperatures but give intense dark colors 
when exposed to elevated temperatures. 
These layers comprise a transparent binder, and at least two thermal 
reactants which react with one another at elevated temperatures; one of 
these reactants is in solid solution in the binder, the other is dispersed 
in microparticulate form in the binder. Despite intimate contact between 
the two reactants, no reaction occurs until the temperature is elevated 
well above room temperature. 
The microparticulate reactant is chosen from a class of ferric iron 
complexes in which the ligand is chosen from organophosphates, 
organophosphinates, and organophosphonates which are colorless and which 
react with the second reactant only at elevated temperature. The second 
reactant is chosen from the class of catechols including polycatechols 
characterized by being colorless. Bis-catechols are particularly 
preferred. 
The thermographic layers are coated or extruded from coating mixes using 
non-aqueous solvents, which solvents enable efficient milling of the 
ferric organophosphates, and provide a solution mixture of the binder and 
the catechol. 
A principal aspect of the invention is to provide colorless thermographic 
sheets which give dark colored images when addressed with elevated 
temperatures. 
An aspect of the invention is to provide colorless thermographic sheets 
which are stable at room temperatures. 
A further aspect of the invention is to provide colorless thermographic 
materials which are stable during the process of coating and drying layers 
on a substrate. 
Yet another aspect of the invention is to provide colorless thermographic 
sheets which give thermal images exhibiting good discrimination when 
examined with near infrared radiation (NIR). 
Still another aspect of the invention is to provide colorless thermographic 
sheets which give black thermal images exhibiting good visual 
discrimination and also good NIR discrimination. 
Iron(III) is the preferred metal for the thermal reaction with catechol 
since it is capable of oxidizing the catechol and generating iron 
complexes that are both black in the visible and strongly absorbing in the 
near infrared. Definitions: 
"polycatechol" molecules containing more than one o-dihydroxybenzene 
moiety, the moieties being connected by an organic connecting link which 
does not provide electronic interaction between the moieties, such as a 
saturated organic group (e.g., alkyl, cycloalkyl). This group includes 
biscatechols. 
"ferric organophosphate" compounds of the form 
EQU Fe(O.sub.2 P(OR).sub.2).sub.3 
where R is an organic moiety such as alkyl, aryl, alphyl, alicyclic groups, 
etc. 
"ferric alkylphosphate" as above where R is an alkyl moiety. 
"chelate" in this case refers to the catechol and is normally bidentate but 
may be polydentate.

DETAILED DESCRIPTION OF THE INVENTION 
U.S. Pat. No. 4,533,930 discloses a wide range of ferric salts of organo 
phosphorus oxyacids and thioacids as useful in thermographic reactions 
with a range of ligands. They are presented as giving much whiter 
backgrounds than ferric salts previously used in this art. It is clear 
from the examples and confirmed from our own investigations that the 
organothiophosphates are highly colored and dark. Furthermore, a great 
many of their examples using organophosphates record appreciable 
coloration of the compounds and whiteness levels are achieved by the use 
of fillers such as zinc oxide, aluminum hydroxide, and calcium carbonate. 
This invention defines a preferred narrow range of ferric organophosphates 
which are entirely colorless, some of which are encompassed by the 
disclosure of U.S. Pat. No. 4,533,930 (I) whereas others are not (II). 
These compounds are di-alkylphosphates and have structures chosen from the 
general formulae 
EQU Fe[OOP(OR).sub.2 ].sub.3 1 (I) 
and 
EQU Fe[OOP(OR).sub.2 ].sub.3.X (II) 
in which each R is selected independently from alkyl groups and substituted 
alkyl bearing substituents such as those selected from alkyl, cycloalkyl, 
and aryl providing that such substituents do not act as ligands or 
chelates for ferric ions. 
Preferably R is selected from the group represented by the formula 
##STR1## 
where b&gt;a, b&gt;c, c is 1 to 10, and 3&lt;=a+b&lt;=18, and 
X is selected from F.sup.-,PF.sub.6.sup.-, Ph.sub.4 B.sup.-, 
BF.sub.4.sup.-, CH.sub.3 COO.sup.-, and C.sub.2 H.sub.5 COO.sup.-,C.sub.14 
H.sub.29 SO.sub.4.sup.- 
(where Ph=phenyl) 
Our preferred compound is in formula I with a=2, b=4, and c=2. 
Previously used iron carboxylates typically are too highly colored and 
cannot produce colorless backgrounds. Dialkylphosphates are the preferred 
ligand for iron(III) since the resulting complexes are completely 
colorless. Mixed dialkylphosphate/carboxylate iron complexes can be made 
to be less colored than iron carboxylates, but they still retain 
undesirable color because of the presence of the carboxylate. The iron 
complexes of the sulfur analogues of the carboxylates, phosphates, and 
their mixtures are particularly undesirable since they are highly colored, 
even black, materials. 
If trialkylphosphates are used as the main ligand, sufficiently stable iron 
complexes do not form, and if monoalkylphosphates (as well as inorganic 
phosphates) are used, generally undesirable, extensive crosslinking occurs 
between metal centers such that the resulting iron organophosphate is too 
stable to react with the catechol. Aromatic phosphates often provide an 
iron complex that is high melting, less reactive and more colored than the 
dialkylphosphates. 
The most preferred organophosphate ligands are branched chain 
dialkylphosphates, and especially di-2-ethylhexylphosphate (DEHP). Linear 
chain dialkylphosphates form colorless iron complexes that give images 
with catechols but are generally too unreactive (too highly crosslinked) 
to provide sufficient image density. The branch on the main chain should 
be sufficiently long and sufficiently close to the metal center that 
crosslinking between metal centers is inhibited. On the other hand, the 
branch should not be too long or too close to the phosphorus center since 
iron that is incompletely reacted with the phosphate may result in a 
colored iron source, and would probably be too reactive in the coating 
solution. From a practical aspect, the ideal structure is illustrated by 
DEHP. The range for the side chain length might be best put at about 1-10, 
the further from the connection point to the phosphorous the longer the 
chain. The length of the main chain is best illustrated by DEHP, that is, 
around 6-10. Chains as long as 18 are the practical maximum due to the 
required loading necessary to achieve suitable optical density (i.e., 
molecular weight becomes impractically high). Asymmetric dialkylphosphates 
provide lower melting iron complexes. 
Alkyl phosphinic acids (in which the alkyl groups are attached directly to 
the phosphorus) show good thermal reactivity with the catechols but are 
not preferred over the alkylphosphates. Apparently the higher pKa 
(insufficiently acidic) prevents them from forming a truly colorless, 
oligomeric complex. Ferric propyl(2-ethylhexyl)phosphinate, ferric 
cyclohexyl(2-ethylhexyl)phosphinate, and ferric dicyclohexylphosphinate 
have been made and found to be thermally reactive with catechols. 
Dialkyl-phosphonic acids (one R group attached directly to the phosphorus, 
the other attached via oxygen) have pKa's in between dialkylphosphates and 
dialkylphosphinic acids but have not been shown to be useful. DEHP not 
only works well, it is commercially available in large quantities of 
relatively good purity. 
Fe(DEHP).sub.3 is preferred in the iron organophosphate series. It is 
completely colorless, a major improvement over the iron carboxylates and 
mixed carboxylate/organophosphate iron complexes. In addition, unlike the 
general straight chain dialkylphosphate iron complexes, it is very 
thermally reactive with the bis-catechols. It is also insoluble in the 
organic solvents required to coat this type of thermal imaging 
construction, unlike mixed carboxylate/organophosphate iron complexes. 
The chelate compounds which we select as thermal reactants with these iron 
compounds, are chosen to be colorless, to be non-reactive with the iron 
compounds at room temperatures even on intimate contact, to be rapidly 
reactive at elevated temperatures above about 60.degree. C., and to be 
easily soluble in organic solvents which are also solvents for the binder 
used. In this invention these chelates are preferably chosen from 
polycatechols and heavily ballasted monocatechols. 
Common catechols are too reactive to be used with the preferred iron source 
in the preferred construction. Polyhydroxy catechols are similarly too 
reactive to be preferred. 
The preferred catechols are those in which two catechol (specifically 
o-dihydroxybenzene) groups are part of the same molecule but in which the 
connecting group insures minimum electronic interaction between the 
catechol rings. They should not therefore be parts of the same aromatic 
ring system. The preferred connection between catechols is alkylidene. 
Aromatic linkage would provide such electronic interaction between 
catechol groups that they would be too reactive. Aromatic linkages also 
give compounds which are colored and thus are precluded by the requirement 
that the chelate be colorless. Fused ring connections, as illustrated by 
the preferred catechol, are excellent. Preparation of this compound, 1,1'- 
spirobi[-1H-indene]-5,5'-6,6-tetrol-2,2'3,3',-tetrahydro-3.3. 
3,3'-tetramethyl, is known in the art. Heteroatomic fused ring connections 
are also acceptable. These are illustrated in Formula III. 
##STR2## 
where A is a saturated ring system optionally containing hetero atoms such 
as N, O, S, and 
R.sup.2, R.sup.3, R.sup.5 and R.sup.6 are independently chosen from forms 
which together alter the electronic character (donating or accepting) of 
the OH groups on the ring. Such groups include, but are not limited to H; 
halogen (F, Cl, Br, and I); groups with no more than 10 atoms in the 
backbone structure selected from C, N, S and O, (which may of course be 
further substituted by additional groups such as halogen); and aliphatic 
groups of up to 20 carbon atoms (e.g., alkyl, ethers, thioethers, etc.) 
The heterogroups with up to 10 carbon atoms includes heterocyclic and 
aromatic groups as well as linear and branched groups. Preferably these 
groups do not provide an acid hydrogen. Preferably the groups are not 
chosen from substituents comprising an acidic hydrogen linked to the 
aromatic ring through a single atom chosen from O, S, and N, and 
The two catechol units need not be symmetrical in their substituents or 
their positioning. 
The position of the o-hydroxy chelate site relative to the connection site 
between the two catechol groups is not critical. Substituents such as 
--OH, --SH, and --NH.sub.2 which contain acidic hydrogen can produce high 
reactivity of the o-dihydroxy substituents and are therefore not preferred 
in this invention. 
Molecules containing more than two catechol groups are also acceptable, as 
long as the connecting linkage between the catechols meets the above 
requirements. Thus in formulae IV, and V oligomers or polymers are 
illustrated which are useful chelates in this invention 
##STR3## 
where 
R.sup.2, R.sup.3 are as defined above 
R.sup.4 is a capping substituent preferably --H or alkyl 
R.sup.1 is a bivalent linking group preferably alkylene, or a chain which 
may contain N, S, O, but may be phenylene, naphthylene or combinations of 
these with the proviso that R.sup.1 does not facilitate electronic 
interaction between the catechol moieties, and 
n is an integer of 2 or more. 
##STR4## 
where 
R.sup.2, R.sup.3, and R.sup.5 are defined above, 
G is a unit in a polymer chain chosen from hydrocarbons, alkyd, acryloid, 
polyester, phenol formaldehyde resins etc. which are miscible with the 
binder used, and m is an integer of 3 or more, 
R.sup.1 is either as defined above, or if G does not facilitate electronic 
interaction between catechol moieties, then R.sup.1 need not be restricted 
in this way and in addition to the definition above may be chosen from a 
single bond and the groups defined for R.sup.2, R.sup.3 and R.sup.5 above 
Monocatechols are in general too reactive for use in this invention but if 
the ring is sufficiently ballasted with a non-reactive ballasting group 
such catechols can be used. Formula VI represents such ballasted 
monocatechols useful in this invention. 
##STR5## 
where R.sup.2, R.sup.3 and R.sup.5 are defined above and R.sup.7 is an 
alkyl chain of eight or more carbon atoms. 
The substituent groups R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 may 
serve three functions: (a) sterically constrain the molecule to enhance or 
inhibit interactions between the reacted metal centers, (b) modify the 
solubility and melting point of the catechol, and (c) modify the 
electronic character of the hydroxy groups by withdrawing or donating 
electron density to the chelating site. 
One of the most important functions of the R group is the control of the 
electronic properties of the catechol, in order to control the color of 
the final image. Commonly known electron donating R groups (such as alkyl, 
mono- or di-alkyl substituted amino, alkoxy, etc.) enable the catechol to 
be oxidized more readily by the iron, which is important for obtaining the 
infrared absorption properties (at 905 nm in particular) needed for bar 
code readers. A green complex results upon imaging this material with 
iron. Conversely, commonly known electron withdrawing R groups (such as 
nitro, ammonium, halogen, etc.) inhibit oxidation of the catechol by the 
iron. The resulting complex will tend to remain a violet-blue. The 
combination of catechols containing both electron donating and electron 
withdrawing groups provides for an imaging construction that is able to 
generate both a desirable black visible image and a high contrast image in 
the near infrared. The connecting linkage between the catechol groups may 
be used to control all three functions, (a)-(c), if the R groups are built 
into the connecting linkage. 
The proper choice of the substituents on each catechol in the bis-catechol 
or polycatechol molecule can give the desired mixture of visible and NIR 
absorption properties. Alternatively the physical mixture of catechols 
with the different substituents can give similar results. 
A number of bis-catechols are available commercially e.g., 
nordihydroguaiaretic acid. 
##STR6## 
Preparation of polycatechols is disclosed in Rodgers et al. J. A. C. S., 
107, 4094 (1985) and in Anderson & Hiller, "Development of Iron Chelators 
for Clinical Use" DHEW Publ. No. (NIH) 76-994 p. 137. These polycatechols 
may be represented by the formula: 
##STR7## 
wherein B comprises the atoms in an organic bridging group necessary to 
complete a cyclic structure with the included catechol moiety or moieties 
and m is 1 to 10, preferably 1 to 4. B is preferably comprised of C and N 
ring atoms and is more preferably selected from 
##STR8## 
wherein n is 1 to 20, preferably 1 to 4. Rodgers et al., supra, shows the 
formation of monomers, dimers, trimers, tetramers, pentamers, and hexamers 
having these diaminoalkine linkages. 
Of particular importance to this invention is that the chelate and the 
binder are soluble in a common solvent and that after coating and drying 
off the solvent the chelate remains in solid solution in the binder. The 
ferric alkylphosphates are not soluble either in the solvent or in the 
binder and are thus dispersed in the latter as microparticles which are in 
intimate contact with the chelate in solid solution. These two reactants 
exhibit very poor reactivity even at elevated temperatures if they are 
physically separated in the binder by using dispersed microparticles of 
the chelate as well as the ferric alkylphosphate. In the practice of this 
invention these classes of chelate exhibit very low reactivity at room 
temperature but good reactivity at elevated temperatures. 
Binders suitable in this invention are polyacrylate and methacrylate and 
their copolymers vinyl resins, styrene resins, cellulose resins, polyester 
resins, urethanes, alkyl resins, silicones, and epoxy resins. Generally 
the resins must be miscible with non-aqueous solvents and have a melting 
point above the reaction temperature of the ferric compound and chelate. 
The binder should also be transparent. 
A coating composition suitable to make a thermal recording sheet can be 
made in the following manner. The ferric alkylphosphate (I or II) is 
dispersed in a solvent such as acetone, methyl ethyl ketone, ethanol, 
etc., by ball milling. To this dispersion a polymer binder and a chelate 
(III, IV V or VI) both soluble in the chosen solvent are added and 
agitated until dissolved. The coating composition may then be coated on a 
suitable substrate and dried at temperatures below thermal reaction 
temperatures. 
Substrates which may be used are films of transparent, opalescent, or 
opaque polymers, paper, optionally with white or colored surface coatings, 
glass, ceramic, etc. The substrate must be stable and undistorted at the 
thermal reaction temperatures which are preferably between 60.degree. and 
200.degree. C. and more preferably between 80.degree. and 150.degree. C. 
We have found that the preparation of the colorless ferric organophosphate 
compounds I is not as simple as U.S. Pat. No. 4,533,930 suggests. Their 
method involves mixing aqueous solutions of an alkali metal salt of the 
organophosphoric acid and a ferric salt of a strong mineral acid such as 
hydrochloric and sulfuric, which results in a precipitate of the ferric 
organophosphate. It has been found that ferric chloride which is preferred 
by the patent gives slightly colored precipitate even with alkyl 
phosphates whereas those from ferric nitrate are completely colorless. The 
preferred preparation therefore uses ferric nitrate to give compounds I 
and II. 
Ferric alkylphosphate compounds II where 
X=fluoride, hexafluorophosphate, tetraphenylborate, tetrafluoroborate, 
tetradecylsulfate, acetate, and propionate. 
This may be prepared by mixing required equivalent quantities in aqueous 
solution of ferric nitrate, alkali metal salt of the alkylphosphoric acid, 
and the alkali metal salt of the acid HX. Compound II then precipitates. 
When X=acetate, however, the acetate ion is too soluble in water to remain 
attached to the ferric alkylphosphate and the result is the compound I 
again. However, if the ferric nitrate and alkali metal alkylphosphate are 
dissolved in glacial acetic acid, then compound II for X= acetate is 
precipitated. This compound and the fluoride may also be prepared using 
ethyl alcohol as solvent and adding potassium acetate or sodium fluoride 
to the ferric nitrate and alkali metal phosphate in required equivalent 
amounts. 
It is of interest to note that the disclosure of U.S. Pat. No. 4,533,930 
says that any carboxylic or thiocarboxylic acid may be used to form 
"composite iron salt" by reacting ferric chloride, an organic phosphoric 
acid and the carboxylic acid in an aqueous medium. The patent says that 
non-white salts are precipitated if the carboxylic acid is in excess. 
Their preferred carboxylic acids contain 5 or more carbon atoms. Our 
experimental evidence is that the composite iron salts obtained by their 
methods are of the form (leaving thio equivalents aside) 
EQU Fe(OOPR'.sub.2).sub.y (OOCR).sub.x 
where y+x=3 and R, R' can be a wide range of aliphatic and aromatic 
substituents. R=CH.sub.3 cannot be obtained by their preparation. 
The following are preparative examples for the ferric alkylphosphates I and 
II. 
EXAMPLE A 
Preparation of Fe(DEHP).sub.3 
1. The method is similar to (but using ferric nitrate instead of ferric 
sulfate) the literature preparation of L. E. Smythe, T. L. Whateley and R. 
L. Werner (J Inorg. Nucl. Chem., 30, 1553 (1968)). To 2.0 g KOH in 175.0 
ml H.sub.2 O is added 10.0 g DEHP. This solution is added over 5 minutes 
to 35.0 ml of water containing 4.0 g Fe(NO.sub.3).sub.3.9H.sub.2 O with 
vigorous stirring. The mixture is stirred 10 minutes, filtered, washed in 
fresh water with stirring, filtered and dried under vacuum at 70.degree. 
C. to a constant weight. An-off white solid is obtained . The infrared 
spectrum shows the expected phosphate stretches, as well as small amounts 
of OH, and the characteristic ethyl group presence at 1466.1 cm.sup.-1. 
The Differential Scanning Calorimeter (DSC) revealed a melting point 
endotherm of this complex at 120.degree. C. 
2. To 1.0 liters of water is added 16.8 g KOH. To a separate 1.5 liters of 
water is dissolved 11.0 g NaBF.sub.4, then 38.2 g 
Fe(NO.sub.3).sub.3.9H.sub.2 O. While the iron salt is dissolving, 97.7 g 
DEHP is added to the rapidly stirred KOH solution The thick phosphate 
solution is added rapidly to the iron solution with mechanical stirring. 
The pure white, rubbery solid is filtered, washed with stirring, filtered 
and dried. It is important that the source of iron(III) is not ferric 
chloride as it gives a yellow product. DSC shows a melting point endotherm 
of the material at 145.degree. C. The infrared spectrum of a vacuum dried 
(room temperature) sample reveals the expected phosphate stretches, but 
unlike 1 (above) there is no presence of OH. 
3. Example Al may also be carried out at elevated temperatures (60.degree. 
C.) with no disadvantageous effects. The same endotherm behaviour in the 
DSC is obtained as in Al. 
EXAMPLE B 
Preparation of Fe(DEHP).sub.3 (acetate) 
Powdered Fe(NO.sub.3).sub.3.9H.sub.2 O, 80.8 g, is dissolved in 800 ml 
glacial acetic acid. As soon as a clear solution is obtained, 193.0 g 
bis-(2-ethylhexyl)phosphate (DEHP) is added in a rapid dropwise manner 
with vigorous stirring. Less than a stoichiometric amount of DEHP gives a 
more colored product; an excess of DEHP is not disadvantageous. The white 
product is filtered, washed with acetic acid and dried under vacuum. The 
approximate yield is 84%. The product is found to be rubbery and may be 
recrystallized by precipitation from cyclohexane by acetone. It is 
important that FeCl.sub.3 not be used since a clear yellow acetic acid 
solution results. 
Alternative preparation from ethanol: To 40 ml of absolute ethanol is added 
2.0 g Fe(NO.sub.3).sub.3.9H.sub.2 O. Upon dissolution, 5.0 g DEHP are 
added, and the clear solution stirred 5 minutes. An aqueous solution of 
potassium acetate (0.5 g in 4.5 g H.sub.2 O) is added dropwise. The 
mixture is stirred 2 minutes, filtered, redispersed in water, stirred an 
additional 20 minutes, filtered and vacuum dried. The infrared spectrum is 
identical to that prepared from acetic acid. 
Characterization: The infrared spectrum clearly shows the coordinated 
organophosphate (1000-1200 cm-1) and carboxylate (1551.0 cm-1 asymmetric 
stretch, the symmetric stretch is under other peaks), and the absence of 
Fe-O-Fe stretches. DSC shows a small exotherm centered around 215.degree. 
C. followed by the main endotherm centered at 282.degree. C. The complex 
is readily soluble in cyclohexane, and is an excellent film forming 
material when coated on a substrate (clear, colorless film). Elemental 
analysis is consistent with the presence of one carboxylate, and confirms 
the 3:1 P:Fe ratio. Magnetic susceptibility determined by the Evan's NMR 
method (J. Chem. Soc., 2003 (1959), demonstrates a high spin iron complex. 
The complex was also found to be conductive in cyclohexane solution. 
EXAMPLE C 
Preparation of Fe(DEHP).sub.3 F 
1. To 500.0 g H.sub.2 O is added 6.0 g KOH. To a separate 500.0 g H.sub.2 O 
is added 12.0 g Fe(NO.sub.3).sub.3.9H.sub.2 O followed by 0.62 g NaF. To 
the aqueous base solution is added 32.0 g DEHP, which is then added 
rapidly to the mechanically stirred iron solution. The pure white iron 
complex is filtered, washed and vacuum dried. 
2. To 300 ml ethanol is added 16.13 g Fe(NO.sub.3).sub.3.9H.sub.2 O. Upon 
dissolution, 40.0 g DEHP is added rapidly dropwise (3 minutes). The clear 
solution is stirred 5 minutes then 3.2 g NaF in 32 g H.sub.2 O are added 
dropwise (5 minutes). The white solid is stirred, then diluted with 400 
ml H.sub.2 O, stirred 30 minutes and filtered. A colorless solid results. 
Elemental analysis is consistent with a 3:1:1 P:Fe:F ratio. 
EXAMPLE D 
Preparation of Fe(DEHP).sub.3 (tetradecylsulfate) 
A mixture of 1.06 g tetradecylsulfate in 100 g H.sub.2 O with 1.2 g 
Fe(NO.sub.3).sub.3.9H.sub.2 O yields an orange precipitate which is 
immediately treated with 3.2 g DEHP and 0.6 g KOH in 50 ml H.sub.2 O. 
After stirring 3 days a white solid is filtered and air dried. The 
infrared spectrum is consistent with the proposed material. 
EXAMPLE E 
Preparation of Fe(DEHP).sub.3 (tetraphenylborate) 
To 1.1 g sodium tetraphenylborate and 1.0 g Fe(NO.sub.3).sub.3.9H.sub.2 O 
in 40 ml H0 is added rapidly 3.2 g DEHP and 0.73 g KOH in 80 ml H.sub.2 O. 
The mixture is filtered, dispersed in water, stirred, filtered and air 
dried. The infrared spectrum is consistent with the proposed material. 
EXAMPLE F 
Preparation of Fe(DEHP).sub.3 (Fe(CN).sub.6) 
This example illustrates that the choice of the counter ion is important in 
determining the color of the complex. Because it is colored, this complex 
is not preferred in this invention. 
To 25.0 g H.sub.2 O is added 0.61 g KOH, 3.2 g DEHP and then 1.2 g 
K4(Fe(CN).sub.6).3H.sub.2 O. A total of 1.2 g Fe(NO.sub.3).sub.3.9H.sub.2 
O are added, and the mixture shaken over 6 days. A brown solid results 
which is filtered and dried. It exhibits an infrared spectrum that shows 
the presence of the phosphate and the Fe(CN).sub.6 groups. 
The following are examples of the preparation of ferric organophosphinates 
useful in this invention. 
EXAMPLE G 
Preparation of ferric n-propyl(2-ethylhexyl)phosphinate 
To a solution of 25 g of n-propyldichlorophosphineoxide in 300 ml of 
petroleum ether, 28 g of diethylamine in 150 ml of petroleum ether was 
added over 4 hours. The petroleum ether was removed by distillation and 
the remaining n-propyl (diethylamine)chlorophosphine oxide was distilled 
off under vacuum. 
The Grignard of 1-bromo-2-ethylhexane (31 g) was prepared in ether, and 
26.4 g of the n-propyl(diethylamine)chlorophosphine oxide was added to it 
at room temperature and refluxed for 72 hours. The resulting solution was 
treated with 5M hydrochloric acid and refluxed overnight. On cooling the 
n-propyl (2-ethylhexyl)phosphinic acid was extracted with petroleum ether 
and distilled to give a colorless liquid (B.P.=172.degree.-180.degree. C. 
at 0.12 mm Hg). 
To 1.3 g of Fe(NO.sub.3).sub.3.9H.sub.2 O dissolved in 5 g of glacial 
acetic acid, 2.7 g of the prepared organophosphinic acid was added. This 
solution was diluted with 9 parts of water rapidly. The ferric 
n-propyl(2-ethylhexyl)phosphinate appeared as a white solid precipitate 
which was filtered off, washed with water, and dried in air. 
EXAMPLE H 
Preparation of ferric dicyclohexylphosphinate 
The dicyclohexylphosphinic acid was made by the method disclosed in Smythe 
et al., supra. A solution of 1.3 g of Fe(NO.sub.3).sub.3.9H.sub.2 O 
dissolved in 50 ml water was prepared. In a solution of 0.66 g of KOH in 
10 g of water, 2.35 g of the phosphinic acid was dissolved. This was 
diluted with 50 ml water and added rapidly to the solution of ferric 
nitrate. A fine yellow precipitate occured which was filtered off, washed 
with water, and air dried to give the ferric dicyclohexylphosphinate. 
EXAMPLE I 
Preparation of ferric cyclohexyl(2-ethylhexyl)phosphinate 
Using the method described in Example G, 30 g of 
cyclohexyldichlorophosphine oxide was used in place of the n-propyl 
dichlorophosphine oxide to give a thick colorless oil The white ferric 
cyclohexyl(2-ethylhexyl)phosphinate was obtained by the treatment 
described in Example H. 
The following are examples of thermographic materials according to this 
invention. 
Definition of Terms used in Examples 
Bkgd - total optical reflectance density of the unimaged sheet using a 
MacBeth RD504 or MacBeth TR924 densitometers. 
I.D. - Maximum optical reflection density of the image areas. 
This is color of the background. Measured with Hunter Labscan II using 2 
degree Observer for Illuminant C and specified in "L-a-b" units. 
L,a,b - the luminance and the two color coordinates for the measured 
surface color using the "L-a-b" color solid. 
PCR - Print contrast ratio at wavelength of 900 nm where 
##EQU1## 
and R are the reflectance values equivalent to the indicated measured 
densities measured with MacBeth PCM-II Print/contrast meter or RJS 
Enterprises Codascan 3600. 
Initiation Temperature is temperature at which an optical density of 0.05 
above the background is reached. 
EXAMPLE 1--Green Image 
A dispersion of the iron tris(di-2-ethylhexyl)phosphate (I) was formed by 
ball milling for 24 hours 25 grams of (I) in 75 g of acetone using flint 
glass marbles. To 4.0 g of this dispersion was added 3.27 g of 15% 
ethylacrylate methyl methacrylate copolymer resin in acetone and 0.5 g of 
the 
1,1'-spirobi[-1H-indene]-5,5'-6,6-tetrol-2,2'3,3,'-tetrahydro-3.3.3'3,'-te 
tramethyl(II). This was agitated till (II) dissolved. This was coated on 2 
mil opaque titanium dioxide filled polyester at 2.5 mil oriface using a 
knife coater and allowed to air dry. The resulting thermal recording sheet 
exhibited excellent whiteness giving a blue image which changed to green 
within four hours. 
Bkgd=0.09 I.D.=0.84 initiation temperature=129.degree. C. Initial PCR=0.45; 
PCR after 12 hours=0.76; Color with Hunter 2C, L=92.37, a=-2.07, b=3.56. 
EXAMPLE 2--Purple Image 
Same as Example 1 using tannic acid (MCB reagent) in place of (II). This 
gave a thermal recording sheet with a white background and a purple image 
which was stable and did not change color. 
Bkgd=0.12; I.D.=1.09; Initiation temp.=120.degree. C.; PCR=0.19 measured 12 
hours after imaging. Color wih Hunter 2 C, L=89.84, a=-1.19, b=5.06. 
EXAMPLE 3--Black Image 
A thermal recording sheet was prepared as in Example 1 substituting 0.4 g 
of (II) and 0.1 g of tannic acid for (II). The thermal recording sheet 
exhibited a white background with a bluish-purple image which turned black 
within 4 hours. 
Bkdg=0.11; I.D.=1.13; Initiation temp=122.degree. C.; PCR=0.45 increasing 
to 0.58 within 12 hours. Color with Hunter 2 C, L=91.09, a=-1.60, b=3.56. 
EXAMPLE 4--Peroxide Green Image 
A thermographic recording sheet was prepared following Example 1 but with 
the addition of 0.08 g of t-buty(peroxy-benzoate (Aldrich Chem) and 
immediately coated. The resulting sheet had a light green background with 
an immediate vibrant green image upon imaging. 
Bkgd=0.12; I.D.=1.02; PCR=0.87; Color with Hunter 2 C, L=83.97, a=-7.08, 
b=8.50. 
EXAMPLE 5--Peroxide Black Image 
Same as Example 3 but with 0.08 g of t-butylperoxybenzoate and coating 
immediately. The resulting thermal recording sheet had a very light green 
background which gave an immediate black image color. 
Bkgd=0.12; I.D.=1.24; PCR=0.73; Color with Hunter 2C, L=84.43, a=-5.05, 
b=5.74. 
EXAMPLE 6 --Phenidone 
A thermal recording sheet was prepared as in Example 1 with the addition of 
0.5 g of a 5% solution in acetone of phenidone A 
(1-phenyl-3-pyrazolidinone, 95% Aldrich) onto the formulation. This gave 
excellent pot life eliminating any premature reaction and also gave 
improved sheet stability. This noticeably whiter thermal recording sheet 
had the following properties. 
Bkgd=0.11; I.D.=0.71; Initiation temp.=131.degree. C.; Initial PCR=0.50 
increasing to 0.70 within 12 hours. Color Hunter 2 C; L=92.79, a=-1.48, 
b=3.83; Side by side control with this example showed Bkgd=0.12; ID=0.84. 
EXAMPLE 7--Paper 
Same as Example 5 but coated on a 46 lb paper (24.times.36.times.500 basis) 
giving a sheet with good whiteness. 
Bkdg=0.09; ID=0.72; Initiation temperature=125.2.degree. C., Color with 
Hunter 2C, L=91.03, a=-1.28, b=-5.31. 
EXAMPLE 8--Acetate 
A dispersion of 25 g iron tris(di-2-ethylhexyl) phosphate Acetate was made 
with 48.75 g Acetone and 1.25 g cellulose acetate by ball milling with 
flint glass balls for 24 hours. A coating dispersion was prepared from 6.0 
g of this dispersion, 5 g of a 12% solution of cellulose acetate in 
acetone, and 9.0 g of a 10% solution of 
1,1',spirobi[-H-indene]-5,5',6,6,-tetol-2,2',3,3'-tetrahydro-3,3,3',3,tetr 
am ethyl(II) in acetone coated at 2 mils wet thickness on 46# 
(24".times.36".times.500) paper, and air dried. This thermal recording 
sheet gave a blue image changing to green within 4 hours. 
Bkg=0.16, ID=0.52, initiation temp. of 152.degree. C.