Thermographic recording films

There is described, in thermographic recording films, the use of a compound containing at least two epoxide moieties in the protective layer and/or in a layer on top of the protective layer of thermographic recording films to reduce gouging and streaking of the printed image film and to reduce head build-up on the thermal printhead.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to thermographic recording films, and more 
specifically, it relates to the use of a crosslinking compound containing 
at least two epoxide moieties in a protective layer and/or in a layer on 
top of the protective layer of certain thermographic recording films which 
are to be imaged with a thermal printhead. The crosslinking compound helps 
to prevent gouging, to reduce head build-up on the thermal printhead, 
enhance print performance and to improve the image quality of the printed 
image. 
2. Description of the Related Art 
There are disclosed in the art a number of image-forming systems for use in 
thermographic recording films. One of these image-forming systems utilizes 
color-forming di- and triarylmethane compounds possessing certain 
S-containing ring closing moieties, namely a thiolactone, dithiolactone or 
thioether ring closing moiety as are disclosed in European Pat. No. 
250,558 and U.S. Pat. No. 5,196,297 of E. J. Dombrowski, Jr. et al. These 
dye precursors undergo coloration by contacting with Lewis acid material, 
preferably a metal ion of a heavy metal, particularly silver, capable of 
opening the S-containing ring moiety to form a colored metal-complex. 
As disclosed in the above-cited patents, the ability of these dye 
precursors to form a colored dye almost instantaneously when contacted 
with Ag+ renders them eminently suitable for use as color formers in 
thermal imaging systems employing organic silver salts, such as silver 
behenate. These thermographic recording films preferably include a 
heat-fusible organic acid material. U.S. Pat. No. 4,904,572 of E. J. 
Dombrowski, Jr. et al, issued Feb. 27, 1990, discloses 
3,5-dihydroxybenzoic acid as a preferred heat-fusible organic acid. 
The above described thermal color-forming system preferably employs a 
thermoplastic binder, e.g. polyvinylbutyral. When imagewise heating is 
accomplished by means of a thermal printhead, the thermoplastic binder is 
in direct contact with the thermal printhead during imaging. Since 
thermoplastic binders soften upon the application of heat, they tend to 
stick to the thermal printhead during imaging. This "sticking" interferes 
with the printing, adversely affects image quality, and can cause damage 
to the printhead. 
A number of ways to prevent sticking between a binder and a thermal 
printhead during printing have been suggested for various thermographic 
recording films. Many of these employ a protective or anti-stick topcoat 
comprising silica over the thermographic color-forming layer. These 
topcoats contact the thermal printhead during imaging to prevent 
"sticking". Another way to prevent sticking has been to employ a surface 
active agent to add anti-stick properties. However, these silica 
containing topcoats and surface-active agents have drawbacks and/or do not 
perform adequately when the binder employed in the coloring system is 
polyvinylbutyral and the support used for the thermosensitive recording 
film is a transparent support. 
For example, low surface energy materials such as silicone polymers exhibit 
good anti-stick properties. However, the useful silicone polymers are 
relatively low molecular weight silicone polymers which have a tendency to 
be migratory and thus cause problems, e.g., they transfer to the back of 
the film if it is rolled for storage or to the back of the adjacent film 
if stored in sheets. In addition, because these silicones are polymers, 
their properties change with changes in moisture and temperature and 
therefore, their performance is not consistent under all conditions. 
U.S. Pat. No. 4,583,103 issued Apr. 15, 1986 and U.S. Pat. No. 4,820,682 
issued Apr. 11, 1989 disclose protective topcoats for heat-sensitive 
recording papers containing a binder comprising silicon modified 
polyvinylalcohol and colloidal silica and/or amorphous silica. The above 
patents also disclose topcoats wherein said colloidal silica contains 
silica grains having an average particle size of from about 10 
millimicrons (m.mu.) to 100 m.mu.(1 m.mu.=1 nanometer (nm)) and the 
amorphous silica has primary grain size of about 10 micrometers (.mu.m) to 
30 .mu.m (1 .mu.m =10.sup.3 nm). These topcoats are disclosed as providing 
good printing densities, resistance to various chemicals, oils and water, 
and anti-sticking and anti-blocking properties. In addition, the latter 
patent discloses the topcoat as exhibiting excellent transparency and 
describes it for use on a transparent base. However, the lowest level of 
haze reported is 16%, a level which is higher than desirable for overhead 
transparency (OHT) applications. 
Published UK Patent Application No. 2,210,702 having a publication date of 
Jun. 14, 1989 and assigned to the same assignee as the latter two patents, 
discloses a heat-sensitive recording material which, when it employs a 
topcoat as described above, e.g., silicon modified polyvinylalcohol and 
colloidal silica, reports a level of haze as low as 8%. 
However, when polyvinylbutyral is used as the binder for the color-forming 
materials of this invention, and a topcoat as described above, i.e. 
silicon modified polyvinylalcohol and colloidal silica, is employed to 
prevent sticking, there is poor adhesion between the topcoat and 
underlying polyvinylbutyral layer, as well as poor scratch resistance of 
the resulting film. In addition, the silicon modified polyvinyl alcohol 
binder is water soluble and can be rubbed off with water. 
U.S. Pat. No. 4,985,394 issued Jan. 15, 1991 discloses a topcoat for a 
thermosensitive recording material which comprises at least one inorganic 
pigment selected from the group consisting of silica and calcium 
carbonate, each having an average particle diameter of 0.1 .mu.m or less, 
and a water-soluble binder, formed on the thermosensitive coloring layer. 
Many of these topcoats have problems of inadequate transparency and/or 
adhesion when coated over the polyvinylbutyral color-forming layer of the 
present invention. 
U.S. Pat. No. 5,198,406 of J. M. Mack and K. Sun, assigned to the assignee 
of the present application, discloses a topcoat for transparent 
thermographic recording films using the above color-forming system. 
Specifically, the transparent thermographic recording films described 
therein comprise a transparent support carrying: 
(a) a dye image-forming system comprising a di- or triarylmethane 
thiolactone dye precursor, an organic silver salt, a heat-fusible organic 
acidic material, and polyvinylbutyral as the binder; and, 
(b) a protective topcoat layer positioned above said dye image-forming 
system and comprising a water-insoluble polymeric binder, a mixture of at 
least two colloidal silicas having different average particle diameters in 
the proportion, by weight, of 1 part of silica having an average diameter 
of 50 nm or smaller and 0.3 to 1 part of silica particles having an 
average diameter no more than 40% of the larger sized silica particles, 
the ratio of total silica to binder being at least 3 parts per weight 
silica to 1 part per weight binder. 
While the above described topcoat prevents sticking of the polyvinylbutyral 
color-forming layer(s) to the thermal printhead during printing, with 
certain high energy thermal printers, e.g. Model BX 500 high density 
printer, commercially available from Seikosha America, Inc., Mahwah, N.J. 
and Model TDU 850 commercially available from Raytheon Company, Submarine 
Signal Division, Portsmouth, R.I., there are the problems of gouging on 
the surface of the recording film and head build-up on the thermal 
printer. 
"Gouging" results in actual depressions or indentations in the recording 
film which can be either continuous or intermittent. Gouging is believed 
to be caused by high temperatures, pressure and/or sticking. 
"Head build-up" is the build-up of components of the thermographic 
recording film on the thermal printhead. Head build-up can cause streaking 
in the printed image, decreased image density with continued printing and 
damage to the thermal printhead. Head build-up can become so pronounced, 
particularly when a lubricant, e.g. polytetrafluoroethylene, is present in 
the topcoat, that it appears as "spiderwebs" on the thermal printer. 
"Streaking" is believed to be the result of the insulating effect of head 
build-up on the printing element(s) of the thermal printhead which 
interferes with printing causing linear discoloration ("streaking") in the 
printed image. 
The presence of a lubricant in the topcoat is generally desired to impart 
slip characteristics and to decrease gouging of the printed image, 
however, head build-up usually becomes more pronounced when a lubricant, 
e.g. polytetrafluoroethylene, is used in the topcoat. Generally, the 
greater the concentration of lubricant, the greater the degree of head 
build-up. 
The aforementioned U.S. Pat. No. 5,198,406 of J. M. Mack et al., discloses 
the use of organofunctional silanes in the topcoat or in a layer on top of 
the topcoat to react with both the silica and the binder(s) in the topcoat 
thereby functioning as a coupling agent to join the two and thereby 
reinforce and strengthen the silica/polymeric binder matrix. The addition 
of the organofunctional silane helps to reduce head build-up and improves 
the scratch resistance of the recorded image. 
SUMMARY OF THE INVENTION 
The thermographic recording film of the present invention includes an 
image-forming system and a protective layer comprising colloidal silica, 
preferably together with a binder material. The film also includes a 
multiepoxy compound, i.e., a compound containing at least two epoxide 
moieties, in the protective layer and/or in a layer on top of the 
protective layer. The multiepoxy compound strengthens and reinforces the 
thermographic recording film and thereby reduces gouging and head 
build-up, enhances print performance by decreasing density degradation and 
improves image quality by decreasing streaking. 
In a preferred embodiment, the protective layer comprises at least two 
different colloidal silicas having different average particle size 
diameters. 
It is, therefore, among the objects of the present invention to provide 
thermographic recording materials. 
DETAILED DESCRIPTION OF THE INVENTION 
The thermographic recording films according to this invention comprise a 
support carrying: 
(a) an image-forming system; and, 
(b) a protective layer comprising colloidal silica. The thermographic 
recording film additionally includes a multiepoxy compound in the 
protective layer and/or in a layer on top of said protective layer. The 
ratio (by weight) of colloidal silica to said multiepoxy compound is at 
least 2:1, and preferably in the range of from 2:1 to 15:1; a particularly 
preferred range is from 2.5:1 to 5:1. At ratios of less than 2:1 there is 
too little silica present so that sticking may occur. However, at ratios 
exceeding about 15:1 the integrity of the film tends to be compromised, 
e.g., crazing and/or cracking of the film may occur. 
The protective layer of the thermographic recording film may be arranged at 
different locations within the film dependent upon which surface of the 
film comes into contact with the thermal printhead during the imaging 
process. In the embodiment where a layer which is part of the 
image-forming system contacts the thermal printhead, the protective layer 
is positioned above the layer(s) comprising the image-forming system. In 
another embodiment where the support contacts the thermal printhead, such 
as in a dye diffusion thermal transfer system, the protective layer is 
arranged on the side of the support which is adjacent the thermal 
printhead during imaging. 
The protective layer preferably also includes a binder material, in which 
case the weight ratio of colloidal silica to the total amount of the 
multiepoxy compound and binder material combined is at least 2:1 and 
preferably in the range of from 2:1 to 15:1; a particularly preferred 
range is from 2.5:1 to 5:1. The absence of a binder in the protective 
layer generally results in higher levels of haze. Accordingly, the 
presence of a binder is particularly preferred in the embodiments of the 
invention where transparency of the imaged film is a concern such as in 
overhead transparency applications. 
The transparent supports that can be used in the present invention may be 
comprised of various materials and numerous suitable support substrates 
are known in the art and are commercially available. Examples of materials 
suitable for use as support substrates include polyesters, polycarbonates, 
polystyrenes, polyolefins, cellulose esters, polysulfones and polyimides. 
Specific examples include polypropylene, cellulose acetate, and most 
preferably, polyethylene terephthalate. The thickness of the support 
substrate is not particularly restricted, but should generally be in the 
range of about 2 to 10 mils. The support substrate may be pretreated to 
enhance adhesion of the polymeric coating thereto. 
The thermographic recording films of the present invention may employ a 
reflective support in place of the transparent support. Typical suitable 
reflective supports include polyethylene clad paper such as that sold by 
Glory Mill Papers Limited (type 381), Glory Paper Mill, Wooburn Green, 
Wylombe, Buchingham Shire, England HP10 0DB; and Baryta coated paper such 
as that sold by Schoeller Technical Papers Inc. (type 527, Pulaski, N.Y. 
13142-0250. 
Any image-forming system which is suitable for use in thermographic 
recording films may be utilized in the recording element of the present 
invention including dye image-forming systems, dye transfer systems and 
systems where an image material, e.g., a metal complex, is formed as a 
result of a chemical reaction between two or more system components. A 
number of suitable image-forming systems are known in the art. Typical 
suitable image-forming systems which may be incorporated in the recording 
element of the invention include: 
A dye image-forming system wherein color-forming di- and triarylmethane dye 
precursors possessing certain S-containing ring closing moieties, namely a 
thiolactone, dithiolactone or thioether ring closing moiety, undergo 
coloration by contact with a Lewis acid material, preferably a metal ion 
of a heavy metal, particularly silver, capable of opening the S-containing 
ring moiety to form a colored dye metal complex. 
A dye image-forming system which utilizes a class of N-substituted 
triarylmethane sulfonamides which undergo reversible oxidation into the 
colored form and reversible reduction of the oxidized form into a 
colorless form as disclosed in U.S. Pat. 5,258,279. 
A dye image-forming system wherein a colorless or light-colored basic dye 
such as a phthalide derivative and a color developer, such as a phenol 
derivative, capable of causing color development upon contact with the dye 
are brought together in the presence of an aromatic secondary amine 
compound as described in U.S. Pat. 5,242,884. 
A dye image-forming system wherein a microencapsulated colorless or 
light-colored electron donating dye precursor is used in combination with 
a color developer dissolved in an organic solvent as described in UK 
patent application GB 2 210 702 A. 
A system which exploits redox reactions or metal complex formation 
reactions based on electron donor-acceptor combinations wherein at an 
increased temperature one of the components melts or diffuses and 
initiates a redox reaction to provide a colored species; typical of these 
systems are combinations of: (1) ferric stearate and pyrogallic acid and 
(2) silver behenate and a suitable reducing agent such as a phenolic 
compound. Various redox reactions are disclosed in Unconventional Imaging 
Processes, Focal Press Limited, 1978, page 128. 
A dye diffusion thermal transfer system wherein a donor layer including a 
preformed image dye is arranged in combination with an image-receiving 
layer and an imagewise pattern of the dye is transferred to the 
image-receiving layer with heat and pressure. As mentioned previously, in 
this embodiment the protective layer is positioned on the side of the 
support for the donor layer which is adjacent the thermal printhead during 
image processing. 
A system wherein a superacid is liberated from a superacid precursor and 
takes part in a reaction to provide a colored species as described in 
copending, commonly-assigned U.S. Pat. No. 5,286,612. 
It will be understood that various of these systems can be practiced by 
separating the reactive components from one another such as by placing 
them in different layers of the element and subsequently causing a desired 
amount of one reactive component from one layer to diffuse to another 
layer, as a function of the amount of heat applied, to react with a second 
component to provide the desired image. 
A particularly preferred image-forming system for use in the image 
recording element of the invention is that utilizing di- and 
triarylmethane thiolactone dye precursors as described in the 
aforementioned European Patent No. 250,558 and U.S. Pat. No. 5,196,297. 
The dye precursors may be represented by the formula 
##STR1## 
wherein ring B represents a substituted or unsubstituted carbocyclic aryl 
ring or rings, e.g., of the benzene or naphthalene series or a 
heterocyclic ring, e.g., pyridine or pyrimidine; G is hydrogen or a 
monovalent radical; and Z and Z' taken individually represent the moieties 
to complete the auxochromophoric system of a diarylmethane or a 
triarylmethane dye when said S-containing ring is open and Z and Z' taken 
together represent the bridged moieties to complete the auxochromophoric 
system of a bridged triarylmethane dye when said S-containing ring is 
open, i.e., when the ring sulfur atom is not bonded to the meso carbon 
atom. Usually, at least one of Z and Z' whether taken individually or 
together possesses as an auxochromic substituent, a nitrogen, oxygen or 
sulfur atom or a group of atoms containing nitrogen, oxygen or sulfur. 
In a preferred embodiment, B is a benzene ring and Z and Z' taken 
individually or together complete the auxochromophoric system of a 
triarylmethane dye. 
The dye precursor compounds used in this embodiment of the invention can be 
monomeric or polymeric compounds. Suitable polymeric compounds are those 
which, for example, comprise a polymeric backbone chain having dye 
precursor moieties attached directly thereto or through pendant linking 
groups. Polymeric compounds of the invention can be provided by attachment 
of the dye precursor moiety to the polymeric chain via the Z and/or Z' 
moieties or the ring B. For example, a monomeric dye precursor compound 
having a reactable substituent group, such as an hydroxyl or amino group, 
can be conveniently reacted with a monoethylenically unsaturated, 
polymerizable compound having a functional and derivatizable moiety, to 
provide a polymerizable monomer having a pendant dye precursor moiety. 
Suitable monoethylenically unsaturated compounds for this purpose include 
acrylyl chloride, methacrylyl chloride, methacrylic anhydride, 
2-isocyanatoethyl methacrylate and 2-hydroxyethyl acrylate, which can be 
reacted with an appropriately substituted dye precursor compound for 
production of a polymerizable monomer which in turn can be polymerized in 
known manner to provide a polymer having the dye precursor compound 
pendant from the backbone chain thereof. 
The thiolactone dye precursors can be synthesized, for example, from the 
corresponding lactones by heating substantially equimolar amounts of the 
lactone and phosphorus pentasulfide or its equivalent in a suitable 
solvent. The silver behenate may be prepared in a conventional manner 
using any of various procedures well known in the art. 
The polymeric binder for use in this dye-imaging forming system may be any 
of those binders described in the aforementioned European Patent No. 
250,558 and the aforementioned U.S. Pat. No. 5,196,297. The preferred 
polymeric binder is polyvinylbutyral. 
The organic silver salts which can be employed in this color-forming system 
of the present invention include any of those described in the 
aforementioned European Patent No. 250,558 and U.S. Pat. No. 5,196,297. 
Preferred silver salts are the silver salts of long chain aliphatic 
carboxylic acids, particularly silver behenate which may be used in 
admixture with other organic silver salts if desired. Also, behenic acid 
may be used in combination with the silver behenate. 
The preparation of such organic silver salts is generally carried out by 
processes which comprise mixing a silver salt forming organic compound 
dispersed or dissolved in a suitable liquid with an aqueous solution of a 
silver salt such as silver nitrate or a silver complex salt. Various 
procedures for preparing the organic silver salts are described in U.S. 
Pat. Nos. 3,458,544, 4,028,129 and 4,273,723. 
The heat-fusible organic acidic material which can be employed in this 
embodiment of the invention is usually a phenol or an organic carboxylic 
acid, particularly a hydroxy-substituted aromatic carboxylic acid, and is 
preferably 3,5-dihydroxybenzoic acid. A single heat-fusible organic acid 
can be employed or a combination of two or more may be used. 
As previously described, the protective layer may include one or more 
colloidal silicas. The average diameter of the colloidal silicas which may 
be incorporated in the thermographic recording films of the invention can 
be up to about 100 nm. It is preferred to utilize colloidal silicas having 
an average diameter between about 5 nm and about 50 nm. Particularly 
preferred colloidal silicas are those which have an average diameter of 
from about 5 nm to about 20 nm. 
The use of colloidal silicas having an average diameter above 50 nm can 
result in thermographic recording films which have relatively higher 
levels of haze and thus which are not as transparent as would be the case 
when colloidal silicas with smaller average diameters are used. For 
overhead transparency (OHT) applications, it is desired that the 
thermographic recording films have a measured level of haze less than 10% 
and preferably less than 5% Thus, for films intended for such 
applications, it is preferred to utilize colloidal silicas having an 
average diameter of 50 nm or less. For other applications where haze is of 
less concern, for example, in reflective thermographic recording films or 
where the thermal recording film is imaged and subsequently used as a 
photomask to expose another material, e.g. in the production of circuit 
boards or diazo prints, etc., a higher level of haze may be tolerated. It 
should also be noted here that the haze level may be reduced to some 
extent where a binder is present by choosing a binder which has an index 
of refraction substantially the same as that of the colloidal silica 
particles, thus reducing light scatter and resulting haze. 
One of the colloidal silicas employed in the protective layer of the 
present invention may be a fumed colloidal silica. Fumed colloidal silica 
is branched, three-dimensional, chain-like agglomerates of silicon 
dioxide. The agglomerates are composed of many primary particles which 
have fused together. Fumed silica is produced by the hydrolysis of silicon 
tetrachloride vapor in a flame of hydrogen and oxygen. The fumed colloidal 
silica is referred to as "fumed" silica because of its smoke-like 
appearance as it is formed. If fumed colloidal silica is employed, an 
average particle diameter in the range of 14-30 nm is generally used, 
preferably 14-15 nm. 
When one colloidal silica is used in the protective layer, cracking of the 
film may be encountered. Accordingly, in high clarity (transparency) 
applications, it is preferred to include a binder material in the layer 
and to select the amount of binder so as to overcome any tendency of the 
film to suffer cracking. A particularly preferred protective layer 
composition comprises polyvinylalcohol, a diepoxide compound and 5 nm 
colloidal silica. Such layers exhibit very low haze levels and no, or 
substantially no, cracking. 
In a preferred embodiment of the invention, the protective layer comprises 
a mixture of at least two colloidal silicas having different average 
particle diameters in the proportion, by weight, of 1 part of silica 
having an average diameter of 50 nm or less, and about 0.3 to 2 parts of 
silica particles having an average diameter no more than about 40% of the 
larger sized colloidal silica particles. The use of two different 
colloidal silicas helps to prevent cracking in the film. In this 
embodiment, it is preferred that the largest colloidal silica particles be 
at least 20 nm in diameter unless fumed colloidal silica is used as the 
largest sized silica, in which case it is preferred that the fumed 
colloidal silica be at least 14 nm in diameter. 
When fumed colloidal silica is employed as the largest sized colloidal 
silica, it is preferred that the colloidal silicas be present in the 
proportion, by weight, of 1 part of fumed colloidal silica and 1 to 2.0 
parts of silica particles having an average diameter no more than 40% of 
the larger sized fumed colloidal silica particles. If fumed colloidal 
silica is not used, it is preferred that the mixture of silicas have 
different average particle diameters in the proportion, by weight, of 1 
part of silica having an average diameter of 50 nm or smaller and 0.3 to 1 
part of silica particles having an average diameter no more than 40% of 
the larger sized silica particles. 
The mixture of silicas can be utilized to give the hardness and durability 
necessary to prevent sticking of thermoplastic binder material such as 
polyvinylbutyral to the thermal printhead, to inhibit scratching on the 
surface of the thermographic recording film and to limit crazing, i.e., 
cracking on the surface of the film. 
The colloidal silicas used in the present invention are produced 
commercially and typically are provided as an aqueous colloidal dispersion 
of silica particles in the form of tiny spheres of a specified average 
diameter. Preferably, the colloidal silicas are aqueous alkaline 
dispersions, e.g., ammonia stabilized colloidal silica. The fumed 
colloidal silicas used in the present invention are aqueous dispersions of 
fumed colloidal silica commercially available under the name 
Cab-O-Sperse.RTM. from Cabot Corporation, Cab-O-Sil Division, Tuscola, IL. 
Colloidal silicas and fumed colloidal silicas low in sodium content are 
preferred since sodium can cause corrosion of the thermal printhead. 
The binders which can be used in the protective layer of the present 
invention include both water-soluble and water-insoluble binders. Poor 
adhesion between the protective layer and color-forming layers with 
water-soluble binder material has been a problem when a water-soluble 
binder is used in the absence of the compound containing at least two 
epoxide moieties. 
A single binder or a combination of one or more binders can be employed in 
the protective layer. 
Examples of water-insoluble binders for use in the protective layer of the 
present invention include aliphatic polyurethanes, styrene-maleic 
anhydride copolymers, polyacrylic acid, polyacrylic latex emulsions, 
polyvinylidene chloride copolymer emulsions and styrene-butadiene 
copolymer emulsions. Examples of water-soluble binders suitable for use in 
the protective layer include polyvinylalcohol, polyacrylamide, 
hydroxyethyl- cellulose, gelatin and starch. 
To prevent interaction of the components in the protective layer with those 
in the solvent soluble color-forming layer beneath it, and to ameliorate 
the environmental concerns associated with coating from solvents, the 
protective layer of this invention is preferably coated out of aqueous 
systems. If the binders employed are water-insoluble, they are either 
coated as latex emulsions or they are made water soluble by mixing with 
alkali, preferably aqueous ammonia which is lost upon drying. 
The coating amount of the protective layer is in the range of about 100 to 
400 mg/ft.sup.2. 
The protective layer preferably contains at least one lubricant, e.g. a 
wax, a polymeric fluorocarbon such as polytetrafluoroethylene or a metal 
soap. The preferred lubricant is a polymeric fluorocarbon, e.g. 
polytetrafluoroethylene. The presence of a lubricant imparts slip 
characteristics to the thermographic recording film and helps to reduce 
gouging of the recording film. 
The protective layer may contain other additives provided the additives do 
not hinder the anti-stick function of the protective layer, do not damage 
the thermal printhead or other wise impair image quality. Such additives 
include surfactants, preferably nonionic surfactants and more preferably 
nonionic fluorosurfactants; plasticizers; anti-static agents; and 
ultraviolet absorbers. 
The multiepoxy compound may be any compound containing at least two epoxide 
groups provided that the multiepoxy compound is water soluble or water 
dispersible. Multiepoxy compounds found to be particularly useful in the 
present invention are diepoxy crosslinking compounds. Examples of suitable 
diepoxy crosslinking compounds include cycloaliphatic epoxides, e.g., 
3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, vinyl 
cyclohexene dioxide, 
2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexanemetadioxane and 
bis(3,4-epoxycyclohexyl)adipate; 1,4-butanediol diglycidyl ether; 
1,2,5,6-diepoxycyclooctane; and 1,2,7,8-diepoxyoctane. 
When present in the protective layer or in a separate layer on top of the 
protective layer of the recording films of the present invention, the 
multiepoxy compounds may be crosslinking with the binder and/or the silica 
and/or they may be reacting with themselves. 
The multiepoxy compound may be present in the protective layer itself or in 
a separate layer on top of the protective layer or it may be present in 
both the protective layer and in a separate layer on top of the protective 
layer. Where a multiepoxy compound is present in both the protective layer 
and a separate layer on top of the protective layer, two different 
multiepoxy compounds may be used, however, it is preferred that the same 
multiepoxy compound be used in both layers. 
The presence of the multiepoxy compound in either layer results in a 
stronger, more robust protective layer without any substantial impact on 
the level of haze. The strengthened protective layer results in decreased 
gouging and enhanced reduction of head build-up. The reduction in head 
build-up is particularly advantageous when a lubricant is employed in the 
protective layer. The presence of a lubricant, while often desirable to 
impart slip characteristics and to decrease gouging, generally increases 
head build-up. As mentioned earlier, head build-up can cause streaking in 
the printed image, density degradation over time with continued printing 
and damage to the thermal printhead. In addition to the above, the 
presence of the multiepoxy compound provides for both a water and 
fingerprint resistant film surface. 
When the multiepoxy compound is present in both the protective layer and in 
a layer on top of the protective layer, there is generally a more 
pronounced reduction in head build-up than when the multiepoxy compound is 
present in only one layer. 
When the multiepoxy compound is added in the protective layer, the amount 
employed is calculated to yield, after drying, a coated coverage in the 
range of 2-40 mg/ft.sup.2, and preferably 5-15 mg/ft.sup.2. 
Where the multiepoxy compound is added in a separate layer on top of the 
protective layer, it is added as an aqueous solution or an aqueous 
dispersion and the amount of multiepoxy compound employed is calculated to 
yield, after drying, a coated coverage in the range of 5-20 mg/ft.sup.2, 
preferably 10 mg/ft.sup.2. Generally, a surfactant is added to the aqueous 
solution or dispersion of the multiepoxy compound to be coated over the 
protective layer. The amount of surfactant used is added in an amount 
calculated to yield, after drying, a coated coverage of 2-5 mg/ft.sup.2. 
It has been found that in some instances increased haze levels may be 
encountered when the coating fluid, containing the multiepoxy compound, 
for the protective layer is allowed to stand for some period of time, 
e.g., a few hours, prior to coating the layer. Accordingly, it is 
preferred to add the multiepoxy compound to the coating dispersion just 
prior to coating the layer. 
A preferred protective layer of the present invention comprises a mixture 
of two different sized colloidal silica particles wherein the largest 
sized colloidal silica is a fumed colloidal silica having an average 
particle diameter in the range of 14-30 nm, preferably 14-15 nm and the 
smaller sized colloidal silica has an average particle diameter of 4 or 5 
nm, a diepoxy crosslinking compound added in an amount calculated to 
yield, after drying, a coated coverage of 15-35 mg/ft.sup.2, a lubricant, 
preferably polytetrafluorethylene, and a water-insoluble binder. 
Fumed colloidal silica has been found to be particularly preferred in 
thermographic recording films which are imaged with high energy thermal 
printers such as Model TDU 850 commercially available from Raytheon 
Company, Submarine Signal Division, Portsmouth, Rhode Island and Model BX 
500 commercially available from Seikosha America, Inc., Mahwah, N.J. 
The present invention is illustrated by the following specific examples. 
Examples 1-16 represent recording elements prepared by coating various 
protective layer formulations according to the present invention over the 
identical imaging system. Examples 17 and 18 represent comparative 
protective layer formulations, which do not contain a multiepoxy compound 
in or on the protective layer, coated over the same imaging system 
employed in Examples 1-16. 
The imaging system employed in each of the examples was prepared by coating 
Layer One onto a transparent 2.65 mil polyethylene terephthalate substrate 
pretreated with a solvent adherable subcoat (ICI 505, commercially 
available from ICI Americas, Inc., Wilmington, Del.) by the slot method, 
followed by air drying. Layer Two was then coated on top of Layer One in 
the same manner and air dried. It will be appreciated that while slot 
coating was employed, any appropriate coating method could be used, e.g. 
spray, air knife, gravure, silkscreen or reverse roll. Both Layer One and 
Layer Two were coated from a solvent mixture comprised of 80% of methyl 
ethyl ketone and 20% of methyl propyl ketone. The amounts of components 
used in each of the layers were calculated to give, after drying, the 
indicated coated coverages. 
______________________________________ 
Coverage (mg/ft.sup.2) 
______________________________________ 
Layer One: 
Polyvinylbutyral 386 
(Butvar B-72, available from 
Monsanto, St. Louis, Mo.) 
3,5-Dihydroxybenzoic acid 
80 
Layer Two: 
Polyvinylbutyral 475 
(Butvar B-76, available from 
Monsanto, St. Louis, Mo.) 
*Silver behenate dispersion 
156 (as silver 
behenate 
Blue Dye Precursor 1 
Red Dye Precursor 2 
Black Dye Precursor 50 
______________________________________ 
Blue Dye Precursor 
##STR2## 
Red Dye Precursor 
##STR3## 
Black Dye Precursor 
##STR4## 
______________________________________ 
*The silver behenate dispersion was prepared according to the procedure 
described on page 29 of the aforementioned European Patent No. 250,558 of 
E. J. Dombrowski, Jr. et al.

Each of the following Examples describes a protective layer formulation 
which was prepared and coated, either as an aqueous dispersion or as an 
aqueous solution, over the above described imaging system. The amounts of 
components used in each protective layer formulation were calculated to 
give the indicated coated coverages. 
EXAMPLE 1 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
(33% total solids (TS), available 
from ICI Resins, Wilmington, MA) 
Cab-O-Sperse A205 80.0 
(a fumed colloidal silica 
having an average particle 
diameter of 14 nm, available 
from Cabot Corporation, 
Cab-O-Sil Division, Tuscola, IL) 
Nalco 2326, 5 nm Silica dispersion 
80.0 
(17% TS, available from Nalco 
Chemical Co.) 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion, (60% TS, 
available from Hoechst-Celanese, 
Chatham, NJ) 
Zonyl FSN, perfluoroalkyl polyethylene 
5.0 
oxide non-ionic surfactant available from 
DuPont, Wilmington, DE) 
1,4-Butanediol diglycidyl ether 
20.0 
(commercially available as Araldite 
DY 026 SP from Ciba-Geigy Limited 
(Plastics Division). 
______________________________________ 
EXAMPLE 2 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
35.0 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
25.0 
______________________________________ 
EXAMPLE 3 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
38.4 
Cab-O-Sperse A205, fumed colloidal 
71.3 
silica 
Nalco 2326, 5 nm Silica dispersion 
98.7 
Hostaflon 5032, polytetra- 
5.5 
fluoroethylene dispersion 
Zonyl FSN 5.5 
1,4-Butanediol diglycidyl ether 
27.4 
______________________________________ 
EXAMPLE 4 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
80.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
80.0 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
20.0 
______________________________________ 
EXAMPLE 5 
A recording element was prepared according to example 4, above, and was 
subsequently coated with an aqueous mixture of 1,4-butanediol diglycidyl 
ether and Zonyl FSN. The amounts of each component used were calculated to 
give the indicated coated coverages after drying at 145.degree. F. 
(.about.63.degree. C.) for 3 minutes: 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
1,4-Butanediol diglycidyl ether 
10 
Zonyl FSN 3 
______________________________________ 
EXAMPLE 6 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
80.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
80.0 
Zonyl FSN 5.0 
______________________________________ 
The above prepared recording element was subsequently coated with an 
aqueous mixture of 1,4-butanediol diglycidyl ether and Zonyl FSN as 
described in Example 5. 
EXAMPLE 7 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
10.0 
______________________________________ 
EXAMPLE 8 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
30.0 
Cab-O-Sperse A205, fumed colloidal 
96.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
96.0 
Zonyl FSN 6.0 
1,4-Butanediol diglycidyl ether 
24.0 
______________________________________ 
The above prepared recording element was subsequently coated with an 
aqueous mixture of 1,4-butanediol diglycidyl ether and Zonyl FSN as 
described in Example 5. 
EXAMPLE 9 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
Polyvinyl alcohol, Vinol 350 
25.0 
(available from Monsanto, St. Louis, Mo.) 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
20.0 
______________________________________ 
EXAMPLE 10 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
35.0 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion 
Zonyl FSN 5.0 
Bis(3,4-epoxycyclohexyl)adipate 
25.0 
(commercially available from 
Union Carbide Corp., Danbury, CT) 
______________________________________ 
EXAMPLE 11 
A recording element was prepared according to example 9, above, and was 
subsequently coated with an aqueous mixture of 1,4-butanediol diglycidyl 
ether and Zonyl FSN as described in Example 5. 
EXAMPLE 12 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Hostaflon 5032, polytetra- 
1.0 
fluoroethylene dispersion 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
10.0 
______________________________________ 
EXAMPLE 13 
A recording element was prepared according to example 11, above, and was 
subsequently coated with an aqueous mixture of 1,4-butanediol diglycidyl 
ether and Zonyl FSN as described in Example 5. 
EXAMPLE 14 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
Cab-O-Sperse A205, fumed colloidal 
80.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
80.0 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
20.0 
______________________________________ 
EXAMPLE 15 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion 
Zonyl FSN 5.0 
______________________________________ 
The above prepared recording element was subsequently coated with an 
aqueous mixture of 1,4-butanediol diglycidyl ether and Zonyl FSN as 
described in Example 5. 
EXAMPLE 16 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
Nalco 2326, 5 nm Silica dispersion 
180.0 
Vinol 540 (polyvinylalcohol 
25.0 
(available from Monsanto, St. Louis, MO) 
Zonyl FSN 5.0 
1,4-Butanediol diglycidyl ether 
20.0 
______________________________________ 
COMATIVE EXAMPLE 17 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
65.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
90.0 
Hostaflon 5032, polytetra- 
0.5 
fluoroethylene dispersion 
Zonyl FSN 5.0 
______________________________________ 
COMATIVE EXAMPLE 18 
______________________________________ 
Coverage 
(mg/ft.sup.2) 
______________________________________ 
NeoRez R966 Polyurethane Latex 
25.0 
Cab-O-Sperse A205, fumed colloidal 
80.0 
silica 
Nalco 2326, 5 nm Silica dispersion 
80.0 
Zonyl FSN 5.0 
______________________________________ 
Each of the recording elements prepared above, except for the one prepared 
in Example 3, were imaged by means of a Model TDU 850 direct thermal 
printer, commercially available from Raytheon Company, Submarine Signal 
Division, Portsmouth, R.I. Example 3 was imaged with a Model BX 500 direct 
thermal printer, commercially available from Seikosha America, Inc., 
Mahwah, N.J. When using a Model BX 500 printer to image, the thermographic 
recording media of the present invention preferably include a lubricant in 
the topcoat in amount to give a coated coverage after drying of 4.0 to 6.0 
mg/ft.sup.2. When using other high energy printers, e.g., the Model TDU 
850, a lesser amount of lubricant, i.e. 0.25 to 1.0 mg/ft.sup.2 , is 
generally employed. 
The streaking, % haze, the amount of gouging and the head build-up were 
determined for each imaged film. The results are recorded in Table 1. 
The haze measurements were determined using a Spectrogard II 
Spectrophotometer made by Gardner-Neotec Instruments, Silver Spring, Md. 
Streaking, gouging and head build-up were each ascertained visually. 
For streaking, "excellent" describes those recording films for which there 
was no observable streaking after 50 feet of printing; "very good" 
describes those recording films for which there was only slight, but 
noticeable streaking after 50 feet of printing; "good" describes recording 
films for which there was moderate streaking visible after 50 feet of 
printing; "fair" is used to describe those recording films for which there 
was heavy streaking before 50 feet of printing accompanied by significant 
density loss; and, "poor" describes those recording films for which 
streaking was so severe that 50 feet of recording film could not be 
successfully printed--the heating elements were insulated to an extent 
which seriously interfered with printing. 
For gouging, "excellent" describes those recording films for which there 
was no observable gouging after 50 feet of printing; "fair" describes 
those recording films for which infrequent gouging was observed in the 
high density areas of the images; and, "poor" describes those recording 
films for which severe gouging was observable at the onset of printing. 
For head build-up, "excellent" describes those situations in which there 
was only very slight if any head build-up on the thermal printhead after 
50 feet of printing; "good" describes those situations where there was a 
slight to moderate accumulation of material on and/or after the print 
elements after 50 feet of printing; "fair" describes those situations for 
which there was substantial accumulation of material on and/or after the 
print elements after 50 feet of printing; and, "poor" describes those 
situations in which there was an exorbitant amount of material directly on 
and after the print elements. 
TABLE I 
__________________________________________________________________________ 
HEAD 
% HAZE STREAKING 
GOUGING BUILD-UP 
__________________________________________________________________________ 
EXAMPLE 
1 8.2 very good 
excellent 
excellent 
2 8.7 very good 
excellent 
excellent 
3 4.8 excellent 
excellent 
good 
4 7.0 very good 
fair good 
5 6.9 excellent 
fair excellent 
6 8.2 good fair good 
7 5.9 very good 
excellent 
good 
8 8.0 very good 
fair excellent 
9 24.7 excellent 
excellent 
excellent 
10 8.2 very good 
excellent 
good 
11 4.5 excellent 
excellent 
excellent 
12 4.8 fair excellent 
fair 
13 4.5 good excellent 
fair 
14 17.0 good excellent 
good 
15 5.6 fair excellent 
fair 
16 2.7 excellent 
excellent 
excellent 
Comparative 
Examples 
17 5.8 fair excellent 
poor 
18 8.3 poor poor poor 
__________________________________________________________________________ 
The level of haze in examples 9 and 14 is noted as being relatively higher 
than that reported for the other examples. The high level of haze in 
example 9 is believed to be due to crosslinked polyvinylalcohol coming out 
of solution during the drying process when the film was formed. The high 
level of haze in example 14 is attributed to the absence of binder in the 
topcoat. 
As can be seen from the results shown in Table 1, the thermographic 
recording films of Examples 1-16 according to the present invention were 
superior in terms of gouging (for those recording films which did not 
contain any lubricant), head build-up, and streaking to comparative 
Examples 17-18 which did not contain a diepoxy crosslinking compound in 
the protective layer and/or in a layer on top of the protective layer. 
To further illustrate the present invention, recording films prepared as in 
Examples 2, 4, 5, 6 and 16 were continuously imaged with a test pattern 
having an eight-step gray tone scale. Measurements of the optical 
transmission density (O.D.) of each of the gray steps were made. Tables 
2-6 show the initial density of each of the gray steps, the density of the 
gray steps after imaging 50 feet of recording film and the difference 
between the two measurements (O.D. .DELTA.) for each of examples 2, 4, 5, 
6 and 16 respectively. The densities reported after 50 feet of printing 
were obtained after continuously printing for 50 feet, stopping, allowing 
the printer to cool for 10 minutes, restarting the printing and measuring 
the resulting transmission density. This was done to compensate for any 
density loss attributable to the thermal printer. The built-in electronics 
of the thermal printhead do not sufficiently compensate for heat build-up 
in the head itself and consequently some density loss tends to occur upon 
continued printing, independent of the particular thermographic recording 
film. 
As a control, the experiment was repeated using a recording film prepared 
according to comparative example 17; the results are reported in Table 7. 
TABLE 2 
______________________________________ 
Example 2 
Step Initial O.D. O.D. 50 ft 
O.D. .increment. 
______________________________________ 
1 0.28 0.29 -0.01 
2 0.35 0.35 0.00 
3 0.42 0.44 -0.02 
4 0.48 0.46 0.02 
5 0.54 0.55 -0.01 
6 0.71 0.69 0.02 
7 0.92 0.95 -0.03 
8 1.76 1.79 -0.03 
______________________________________ 
TABLE 3 
______________________________________ 
Example 4 
Step Initial O.D. O.D. 50 ft 
O.D. .increment. 
______________________________________ 
1 0.33 0.32 0.01 
2 0.40 0.42 -0.02 
3 0.50 0.50 0.00 
4 0.57 0.56 0.01 
5 0.65 0.66 -0.01 
6 0.78 0.78 0.00 
7 1.01 1.01 0.00 
8 1.84 1.85 -0.01 
______________________________________ 
TABLE 4 
______________________________________ 
Example 5 
Step Initial O.D. O.D. 50 ft 
O.D. .increment. 
______________________________________ 
1 0.32 0.32 0.00 
2 0.40 0.41 -0.01 
3 0.49 0.48 0.01 
4 0.56 0.54 0.02 
5 0.66 0.65 0.01 
6 0.80 0.79 0.01 
7 1.03 1.00 0.03 
8 1.83 1.81 0.02 
______________________________________ 
TABLE 5 
______________________________________ 
Example 6 
Step Initial O.D. O.D. 50 ft 
O.D. .increment. 
______________________________________ 
1 0.29 0.19 0.10 
2 0.35 0.26 0.09 
3 0.46 0.35 0.11 
4 0.50 0.39 0.11 
5 0.64 0.55 0.09 
6 0.74 0.68 0.06 
7 0.99 0.92 0.07 
8 1.84 1.79 0.05 
______________________________________ 
TABLE 6 
______________________________________ 
Example 16 
Step Initial O.D. O.D. 50 ft 
O.D. .increment. 
______________________________________ 
1 0.24 0.27 0.03 
2 0.32 0.27 -0.05 
3 0.48 0.46 -0.02 
4 0.54 0.57 0.03 
5 0.66 0.72 0.06 
6 0.79 0.84 0.05 
7 1 1.19 0.19 
8 1.61 1.76 0.15 
______________________________________ 
TABLE 7 
______________________________________ 
Comparative Example 17 
Step Initial O.D. O.D. 50 ft 
O.D. .increment. 
______________________________________ 
1 0.14 0.05 0.09 
2 0.20 0.10 0.10 
3 0.27 0.12 0.15 
4 0.31 0.14 0.17 
5 0.44 0.20 0.24 
6 0.57 0.39 0.18 
7 0.78 0.55 0.23 
8 1.44 1.28 0.16 
______________________________________ 
As can be seen from the foregoing data, the recording films of the present 
invention which contain a multiepoxy compound in the protective layer 
and/or in a layer on top of the protective layer, decrease the density 
degradation which may occur over time with continued printing. It is noted 
that Example 6, which had only 10 mg/ft.sup.2 of 1,4-butanediol diglycidyl 
ether in the protective layer, showed some density degradation with 
continued printing. However, the density loss was less than that observed 
in comparative example 17, which contained no multiepoxy compound in the 
protective layer. 
Since certain changes may be made in the above subject matter without 
departing from the spirit and scope of the invention herein involved, it 
is intended that all matter contained in the above description and the 
accompanying examples be interpreted as illustrative and not in any 
limiting sense.