Thermal printing head coating

A thermal printing head comprising an outermost protective layer and a coating on the outermost surface of the outermost protective layer, wherein the coating comprises a hydrolyzed silane, the silane having the general formula: SiR.sup.1 R.sup.2 R.sup.3 R.sup.4 wherein R.sup.1, R.sup.2 and R.sup.3 are independently a C1 to C4 alkoxy group, a substituted C1 to C4 alkoxy group, bromine or chlorine; and R.sup.4 is a C1 to C4 alkyl group; a substituted C1 to C4 alkyl group, a C1 to C4 alkoxy group or a substituted C1 to C4 alkoxy group; a coating process therefor; and a thermographic printing process utilizing said coated thermal printing head.

DESCRIPTION 
1. Field of the Invention 
The present invention relates to a coating for thermal printing heads used 
for the image-wise heating of thermographic materials. 
2. Background of the Invention 
Thermal imaging or thermography is a recording process wherein images are 
generated by the use of image-wise modulated thermal energy. In 
thermography three basic approaches are known: 
1. Thermal dye transfer printing wherein a visible image pattern is formed 
by transfer of a coloured species from an image-wise heated donor element 
onto a receptor element; 
2. Image-wise transfer of an ingredient necessary for the chemical or 
physical process bringing about changes in colour or optical density to a 
receptor element; and 
3. Direct thermal formation of a visible image pattern by image-wise 
heating of a recording material containing matter that by chemical or 
physical process changes colour or optical density. 
A survey of thermographic processes is given in "Unconventional Imaging 
Processes" by E. Brinckman, G. Delzenne, A. Poot and J. Willems, The Focal 
Press--London and New York (1978), Chapter 4 under the heading "4.10 
Thermography". 
Thermal dye transfer printing is a recording method wherein a dye-donor 
element is used that is provided with a dye layer wherefrom dyed portions 
or incorporated dyes are transferred onto a contacting receiver element by 
the application of heat in a pattern normally controlled by electronic 
information signals. Thermal dye transfer printing materials are 
described, for example, in EP-B 133 012 and EP-A 133 011. 
Processes in which image formation is obtained by the image-wise transfer 
of an ingredient necessary for the chemical or physical process bringing 
about changes in colour or optical density to a receptor element are, for 
example, described in EP-A 671 283. Materials for such processes are, for 
example, described in EP-A's 671 283, 671 284, 674 216, 677 775, 677 776, 
678 775, 682 438, 683 428 and 706 080. 
Direct thermal thermography is concerned with materials which are 
substantially not photosensitive, but are sensitive to heat or 
thermosensitive. Image-wise applied heat is sufficient to bring about a 
visible change in a thermosensitive imaging material. Most of the "direct" 
thermographic recording materials are of the chemical type. On heating to 
a certain conversion temperature, an irreversible chemical reaction takes 
place and a coloured image is produced. This irreversible reaction can be, 
for example, the reaction of a leucobase with an acid to produce the 
corresponding dye or the reduction of an organic or inorganic metal 
compound (e.g. silver, gold, copper or iron compounds) to its 
corresponding metal thereby producing a visible image. Such imaging 
materials are described, for example, in U.S. Pat. No. 3,080,254, EP-B 614 
770, EP-B 614 769, EP-A 685 760, U.S. Pat. No. 5,527,757, EP-A 680 833, 
U.S. Pat. No. 5,536,696, EP-B 669 876, EP-A 692 391, U.S. Pat. No. 
5,527,758, EP-A 692 733, U.S. Pat. No. 5,547,914, EP-A 730 196 and EP-A 
704 318. 
Image-wise heating is conventionally carried out with thermal printing 
heads. According to "Ullman's Encyclopedia of Industrial Chemistry", 5th 
completely revised edition, Vol. A13, edited by B. Elvers, S. Hawkins, M. 
Ravenscroft and G. Schulz, VCH Verlagsgesellschaft mbh, Weinheim, Germany 
(1989), pages 583-584 and S. Shibata et al., IEEE Trans. Components 
Hybrids Manuf. Technol. CHMT-7, 294-298 (1984), in high speed thin film 
thermal printing heads with a resolution of 16 dots/mm the resistive 
heating elements are sandwiched between a heat-insulating layer and a 
highly thermally conductive acid-resistant protective layer such as SiC, 
Ta.sub.2 O.sub.5, SiO.sub.2, Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4. 
According to JP-A 55-84683 and JP-A 57-89980 it is conventional practice 
to coat these high thermal conductivity protective layers with an 
abrasion-resistant layer by sputtering or evaporating a substance having a 
high hardness and high thermal conductivity and according to Shibata et 
al. SiC, Al.sub.2 O.sub.3, SiO.sub.2, Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5 
or TiO.sub.2 are suitable for this purpose. Head temperatures can vary 
from 250 to 550.degree. C. depending on the duration of the heating and 
the heating power applied thereto. 
The requirement of constant print quality at high image resolution over the 
whole material width coupled with an acceptable lifetime for the thermal 
printing head in contact with different thermographic imaging materials 
places considerable demands upon the thermal printing head. Thin film 
thermal printing heads which are used with less thermally sensitive 
imaging materials are more susceptible to failure due to the higher 
printing head temperatures necessary for printing. Furthermore this 
susceptibility to failure increases with the applied heating power and 
hence with the temperature attained during the heating process. Thermal 
printing head failure can be due to physical abrasion and/or chemical 
erosion and chemical interaction between the protective layer of the 
thermal printing head and the ingredients of the imaging materials at the 
elevated temperatures at which image formation takes place. 
Physical abrasion of thermal printing heads can be reduced by incorporating 
lubricants into the imaging materials which come into contact with the 
thermal printing head as disclosed for example in EP-A 669 876. 
U.S. Pat. No. 4,396,684 discloses that reduction in the concentration of 
sodium and potassium ions in thermographic recording paper to less than 
601ppm results in reduced abrasion of thermal printing heads in contact 
therewith. O. P. Srivastava at the 3rd Annual Printing Workshop held at 
Cambridge, Mass., USA between 23 and 25 Mar. 1992 suggested that sodium 
and/or potassium ions in thermographic materials form their hydroxides 
with water present in the atmosphere during thermal printing and these 
hydroxides dissolve the protective glass coating of thermal printing heads 
and then migrate into the resistor material accelerating heating element 
failure. 
The presence of sodium, potassium and chloride ions in thermographic 
materials in direct contact with the thermal printing head during 
image-wise printing has an adverse effect on the operating lifetime of 
thermal printing heads which becomes more severe as the temperature at 
which printing is carried out is increased. For high printing temperatures 
a means of limiting or preventing the diffusion of these ions to the 
thermal printing head is therefore required. Furthermore, it is desirable 
for ecological reasons to coat thermographic materials from aqueous 
solutions or dispersions and it is extremely difficult to exclude these 
ions from water, hydrophilic dispersion agents and hydrophilic ingredients 
in general to attain a concentration of sodium and potassium ions below 
the 601ppm stipulated in U.S. Pat. No. 4,396,684. 
In the commercial production of thin film thermal printing heads it is 
impossible to avoid the occurrence of microscopically detectable thickness 
variation in the protective layer so-called pinholes or spots. These 
pinholes or spots are typically 3 to 10 .mu.m deep and 5 to 50 .mu.m in 
diameter. 
The failure of individual heating elements in thermal printing heads in the 
absence of substantial abrasion of the protective layer of the thermal 
printing head may be due to chemical interaction between the heating 
elements of the thermal printing head and ions or molecules from the 
imaging materials which have diffused through the thinner protective 
coating to or have reacted directly with the heating elements exposed at 
the bottom of pinholes. 
In order to extend the operating lifetime of thermal printing heads used 
for the image-wise heating of thermographic recording materials, it is 
therefore necessary to prevent diffusion of ions and molecules from the 
thermographic recording materials to the heating elements in general and 
through the pinholes in the protective layer in particular. 
OBJECT OF THE INVENTION 
It is therefore an object of the invention to provide a process for 
extending the lifetime of thermal printing heads used for the image-wise 
heating of substantially non-photosensitive thermographic recording 
materials without adversely affecting their printing characteristics. 
Further objects and advantages of the invention will become apparent from 
the description hereinafter. 
SUMMARY OF THE INVENTION 
The above mentioned object is realised by a thermal printing head 
comprising an outermost protective layer and a coating on the outermost 
surface of the outermost protective layer, wherein the coating comprises a 
hydrolyzed silane, the silane having the general formula: SiR.sup.1 
R.sup.2 R.sup.3 R.sup.4 wherein R.sup.1, R.sup.2 and R.sup.3 are 
independently a C1 to C4 alkoxy group, a substituted C1 to C4 alkoxy 
group, bromine or chlorine; and R.sup.4 is a C1 to C4 alkyl group; a 
substituted C1 to C4 alkyl group, a C1 to C4 alkoxy group or a substituted 
C1 to C4 alkoxy group. 
A process for coating a thermal printing head is also provided, the thermal 
printing head comprising a linear array of resistor elements with a 
heat-resistant support on one side of the resistor elements and at least 
one protective layer on the other side thereof, comprising the steps of: 
(i) applying a coating to the outermost surface of the outermost 
protective layer of the thermal printing head; (ii) heating the coating; 
and (iii) removing poorly adhering parts of the coating from the outermost 
surface of the thermal printing head by cleaning or printing, wherein the 
coating is applied as a solution or dispersion comprising water and a 
silane having the above-mentioned general formula. 
A thermographic printing process is further provided comprising the steps 
of: (i) bringing a substantially non-photosensitive thermographic 
recording material into thermal contact with a thermal printing head 
having the coating described above; (ii) image-wise heating the 
thermographic recording material with the thermal printing head; and (iii) 
separating the thermographic recording material from the thermal printing 
head. 
Further preferred embodiments of the present invention are disclosed in the 
dependent claims. 
DETAILED DESCRIPTION OF THE INVENTION 
Process for Coating a Thermal Printing Head 
According to the present invention the outermost surface of the outermost 
protective layer, preferably silicon nitride, of a thermal printing head 
is coated with a coating comprising water and a silane having the general 
formula: SiR.sup.1 R.sup.2 R.sup.3 R.sup.4 wherein R.sup.1, R.sup.2 and 
R.sup.3 are independently a C1 to C4 alkoxy group, bromine or chlorine; 
and R.sup.4 is a C1 to C4 alkyl group, a substituted C1 to C4 alkyl group, 
a C1 to C4 alkoxy group, bromine or chlorine. The substituted C1 to C4 
alkyl groups may be substituted with any substituents including, for 
example, the following groups: acryloyloxy, methacryloyloxy, amino, 
substituted amino, carboxyl, carboxyester, acyl, epoxy, glycidyloxy, 
epoxyoxy, oxoamine, nitrile, vinyl, substituted vinyl, hydroxy, thiol, 
thiooxoalkyl, thiooxoaryl, fluoroalkyl etc. Particularly preferred silanes 
according to the present invention are selected from the group consisting 
of .gamma.-glycidyloxypropyltrimethoxysilane, 
methacryloyloxypropyltrimethoxy-silane, tetramethylorthosilicate and 
tetraethylorthosilicate. The silanes of the present invention may be on 
their own or in admixture with one of more other silanes. The use of a 
mixture of .gamma.-glycidyl-oxypropyltrimethoxysilane and 
tetraethylorthosilicate or a mixture of 
methacryloyloxypropyltrimethoxysilane and tetraethylorthosilicate in the 
coating composition is particularly preferred. 
The coating may further comprise curable silicone compounds, which when 
used on their own to coat the protective layers of thermal printing heads 
produce coatings which are too soft to withstand the abrasive effect of 
printing thermographic recording materials; a catalyst to promote the 
hydrolysis of the silanes of the present invention such as an acid, e.g. 
hydrochloric acid, methanesulfonic acid or formic acid, a basic catalyst, 
e.g. an alcoholate of titanium, zirconium or aluminium, imidazole etc.; 
colloidal inorganic particles, such as oxides, hydrated oxides, nitrides 
and carbides of silicon, boron, aluminium and transition metals such as 
zinc, titanium and zirconium with a specific surface area of preferably at 
least 100 m.sup.2 /g, for example, silica, aluminium oxide, zinc oxide, 
bentonite, boehmite and the like; an aromatic polyol with a molecular 
weight less than or equal to 1000, such as bisphenol A, bisphenol F, 
bisphenol S and 1,5-dihydroxynaphthalene, vinyl or substituted vinyl 
monomers such as methyl methacrylate, binders, such as polyvinyl alcohol 
which react with the hydrolysed silane, inorganic compounds, such as metal 
nitrates, metal chlorides etc., organometallic compounds, such as metal 
acetates, metal formates, metal alkoxides etc., metal chelates, such as 
metal acetylacetonates, organic compounds, such as amines, tensides which 
may be cationic, anionic or non-ionic and additives which improve the 
frictional properties of the resulting coating, such as particles of clay, 
talc etc, particularly china clay, and other well known thermally stable 
lubricants. 
The dispersion medium or solvent can be deionized water, one or more 
organic solvents or a mixture of water and one or more organic solvents. 
Examples of suitable organic solvents are methanol, ethanol, n-propanol, 
isopropanol, butanol, sec-butanol, tert-butanol, methanone, 2-butanone 
etc. 
Curing of the coating is usually performed thermally, but radiation curing 
may also be carried out with appropriate ingredients, for example silanes 
comprising substituted vinyl groups. 
Upon contact with water the silanes hydrolyze and the viscosity of the 
solution or dispersion begins to increase. Coating may be carried out 
before significant viscosity increase has taken place or when the 
viscosity of the solution or dispersion has increased by prepolymerization 
to the required coating viscosity. The coating may at the end of the 
coating process, i.e. after any cleaning step, cover the outermost surface 
of the outermost protective layers of the thermal printing head or only a 
part thereof, but must at least fill any pinholes present in the outermost 
surface. The process for coating a thermal printing head of the present 
invention may be carried out as many times as is necessary to achieve the 
objects of the present invention, the coating thickness resulting from 
each process cycle preferably not exceeding 1 .mu.m. Substantially 
non-photosensitive thermographic recording material 
According to the present invention, a substantially non-photosensitive 
thermographic recording material comprises a material or set of materials 
necessary for image formation. 
In one embodiment thereof the substantially non-photosensitive 
thermographic recording material comprises a donor ribbon comprising a 
sublimable dye layer and a receiving layer for the sublimable dye. 
In another embodiment thereof it comprises a donor ribbon and a receiving 
layer, the donor ribbon comprising a component which under the influence 
of heat is transferred to the receiving layer and undergoes a 
colour-forming reaction with a component thereof. For example the donor 
ribbon might contain a toning agent and/or one or more reducing agents and 
the receiving layer might contain a reducible silver source. 
In a still further embodiment thereof it is a substantially 
non-photosensitive thermographic recording material comprising a support 
and at least one thermosensitive element. 
Thermosensitive Element 
The thermosensitive element, according to the present invention which under 
the influence of imagewise heating forms an image either in the element 
itself or after peeling off a material delaminated during the image-wise 
heating. The element may comprise a layer system and any active 
ingredients may be present in different layers although in thermal working 
relationship with one another during the thermal development process. 
Such thermosensitive elements may, for example, comprise leuco dyes with 
acid-releasing ingredients, substantially light-insensitive reducible 
silver sources and reducing agents therefor in thermal working 
relationship therewith or delaminatable pigment layers. In a particular 
embodiment of the present invention the element comprises a substantially 
light-insensitive reducible silver source, a reducing agent therefor in 
thermal working relationship therewith and a binder. 
Reducible Silver Sources 
Preferred substantially light-insensitive reducible silver sources 
according to the present invention are organic silver salts. Preferred 
organic silver salts, according to the present invention, are silver salts 
of aliphatic carboxylic acids known as fatty acids, wherein the aliphatic 
carbon chain has preferably at least 12 C-atoms, e.g. silver laurate, 
silver palmitate, silver stearate, silver hydroxystearate, silver oleate 
and silver behenate, with silver behenate being particularly preferred. 
Such silver salts are also called "silver soaps". In addition silver 
dodecyl sulphonate described in U.S. Pat. No. 4,504,575; and silver 
di-(2-ethylhexyl)-sulfo-succinate described in EP-A 227 141, modified 
aliphatic carboxylic acids with thioether group as described e.g. in GB-P 
1,111,492 and other organic silver salts as described in GB-P 1,439,478, 
e.g. silver benzoate and silver phthalazinone, may be used likewise to 
produce a thermally developable silver image. Further are mentioned silver 
imidazolates and the substantially light-insensitive inorganic or organic 
silver salt complexes described in U.S. Pat. No. 4,260,677. 
Reducing Agents 
Suitable organic reducing agents for the reduction of the substantially 
light-insensitive organic heavy metal salts are organic compounds 
containing at least one active hydrogen atom linked to O, N or C, such as 
is the case with, aromatic di- and tri-hydroxy compounds; aminophenols; 
METOL (tradename); p-phenylenediamines; alkoxynaphthols, e.g. 
4-methoxy-1-naphthol described in U.S. Pat. No. 3,094,41; 
pyrazolidin-3-one type reducing agents, e.g. PHENIDONE (tradename); 
pyrazolin-5-ones; indan-1,3-dione derivatives; hydroxytetrone acids; 
hydroxytetronimides; hydroxylamine derivatives such as for example 
described in U.S. Pat. No. 4,082,901; hydrazine derivatives; and 
reductones e.g. ascorbic acid; see also U.S. Pat. Nos. 3,074,809, 
3,080,254, 3,094,417 and 3,887,378. 
Among useful aromatic di- and tri-hydroxy compounds having at least two 
hydroxy groups in ortho- or para-position on the same aromatic nucleus, 
e.g. benzene nucleus, hydroquinone and substituted hydroquinones, 
catechol, pyrogallol, gallic acid and gallic acid esters are preferred. 
Particularly preferred catechol-type reducing agents are disclosed in EP-A 
692 733, e.g. 3,4-dihydroxybenzoic acid esters such as ethyl and butyl 
3,4-dihydroxybenzoate. 
Auxiliary Reducing Agents 
The above mentioned reducing agents being considered as primary or main 
reducing agents may be used in conjunction with so-called auxiliary 
reducing agents. Such auxiliary reducing agents are e.g. sterically 
hindered phenols, that on heating become reactive partners in the 
reduction of the substantially light-insensitive organic heavy metal salt 
such as silver behenate, such as described in U.S. Pat. No. 4,001,026; are 
bisphenols, e.g. of the type described in U.S. Pat. No. 3,547,648; or are 
sulfonamidophenols. The auxiliary reducing agents may be present in the 
imaging layer or in a polymeric binder layer in thermal working 
relationship thereto. 
Film-forming Binders of the Thermosensitive Element 
The film-forming binder of the thermosensitive element containing the 
substantially light-insensitive reducible silver source may be all kinds 
of natural, modified natural or synthetic resins or mixtures of such 
resins, wherein the organic heavy metal salt can be dispersed 
homogeneously: e.g. cellulose derivatives such as ethylcellulose, 
cellulose esters, e.g. cellulose nitrate, carboxymethylcellulose, starch 
ethers, galactomannan, polymers derived from .alpha., .beta.-ethylenically 
unsaturated compounds such as polyvinyl chloride, after-chlorinated 
polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, 
copolymers of vinyl chloride and vinyl acetate, polyvinyl acetate and 
partially hydrolyzed polyvinyl acetate, polyvinyl alcohol, polyvinyl 
acetals that are made from polyvinyl alcohol as starting material in which 
only a part of the repeating vinyl alcohol units may have reacted with an 
aldehyde, preferably polyvinyl butyral, copolymers of acrylonitrile and 
acrylamide, polyacrylic acid esters, polymethacrylic acid esters, 
polystyrene and polyethylene or mixtures thereof. 
The above mentioned binders or mixtures thereof may be used in conjunction 
with waxes or "heat solvents" also called "thermal solvents" or 
"thermosolvents" improving the reaction speed of the redox-reaction at 
elevated temperature. 
By the term "heat solvent" in this invention is meant a non-hydrolyzable 
organic material which is in solid state in the recording layer at 
temperatures below 50.degree. C. but becomes a plasticizer for the 
recording layer in the heated region and/or liquid solvent for at least 
one of the redox-reactants, e.g. the reducing agent for the organic heavy 
metal salt, at a temperature above 60.degree. C. 
Toning Agents 
In order to obtain a neutral black image tone in the higher densities and 
neutral grey in the lower densities the recording layer contains 
preferably in admixture with the organic silver salts and reducing agents 
a so-called toning agent known from thermography or photothermography. 
Suitable toning agents are the phthalimides and phthalazinones within the 
scope of the general formulae described in U.S. Pat. No. 4,082,901. 
Further reference is made to the toning agents described in U.S. Pat. Nos. 
3,074,809, 3,446,648 and 3,844,797. Other particularly useful toning 
agents are the heterocyclic toner compounds of the benzoxazine dione or 
naphthoxazine dione type described in GB-P 1,439,478 and U.S. Pat. No. 
3,951,660. 
A toner compound particularly suited for use in combination with 
polyhydroxy benzene reducing agents is 
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine described in U.S. Pat. No. 
3,951,660. 
Other Ingredients 
The recording layer may contain in addition to the ingredients mentioned 
above other additives such as free fatty acids, surface-active agents, 
antistatic agents, e.g. non-ionic antistatic agents including a 
fluorocarbon group as e.g. in F.sub.3 C(CF.sub.2).sub.6 CONH(CH.sub.2 
CH.sub.2 O)--H, silicone oil, e.g. BAYSILONE Ol A (tradename of BAYER 
AG--GERMANY), ultraviolet light absorbing compounds, white light 
reflecting and/or ultraviolet radiation reflecting pigments, silica, 
and/or optical brightening agents. 
Support 
The support for the thermal imaging material according to the present 
invention may be transparent, translucent or opaque, e.g. having a white 
light reflecting aspect and is preferably a thin transparent resin film, 
e.g. polyethylene terephthalate. 
The support may be in sheet, ribbon or web form and subbed if need be to 
improve the adherence to the thereon coated thermosensitive recording 
layer. The support may be made of an opacified resin composition. Should a 
transparent base be used, the base may be colourless or coloured, e.g. 
have a blue colour. Outermost layer in contact with the thermal printing 
head assembly 
The outermost layer of the substantially non-photosensitive thermographic 
recording material on the same side of the support as the thermosensitive 
element, according to the present invention, may be a protective layer 
applied to the thermosensitive element to avoid local deformation of the 
thermosensitive element and to improve resistance against abrasion or the 
outermost layer of the thermosensitive element. Such protective layers may 
also comprise particulate material, e.g. talc particles, optionally 
protruding from the protective outermost layer as described in WO 
94/11198. Other additives can also be incorporated in the protective layer 
e.g. colloidal particles such as colloidal silica. 
A maximum dynamic frictional coefficient between the thermal printing head 
assembly and the outermost layer in contact with the thermal printing head 
assembly of less than 0.3 can be attained by one skilled in the art by a 
combination of one or more matting agents, as described in WO 94/11198 
with one or more thermomeltable particles optionally with one or more 
lubricants, as described in WO 94/11199, or with at least one solid 
lubricant having a melting point below 150.degree. C. and at least one 
liquid lubricant in a binder, wherein at least one of the lubricants is a 
phosphoric acid derivative. 
Binder for Outermost Layer of the Thermographic Material 
According to an embodiment of the present invention the outermost layer of 
the recording material on the same side of the support as the 
thermosensitive element comprises a water-dispersible, a water-soluble or 
a water-soluble and a water-dispersible binder. Suitable water-soluble 
binders for the outermost layer in contact with the thermal printing head 
assembly are, for example, gelatin, polyvinylalcohol, cellulose 
derivatives or other polysaccharides, hydroxyethylcellulose, 
hydroxypropylcellulose etc., with hardenable binders being preferred and 
polyvinylalcohol being particularly preferred. Suitable water-dispersible 
binders are, for example, polymer lattexes. 
Crosslinking Agents for Outermost Layer 
According to an embodiment of the present invention the outermost layer of 
the recording material in contact with the thermal printing head assembly 
may be crosslinked. Crosslinking can be achieved by using crosslinking 
agents such as described in WO 95/12495 for protective layers, e.g. 
tetra-alkoxysilanes, polyisocyanates, zirconates, titanates, melamine 
resins etc., with tetraalkoxysilanes such as tetramethylorthosilicate and 
tetraethylorthosilicate being preferred. 
Matting Agents for Outermost Layer 
The outermost layer of the recording material in contact with the thermal 
printing head assembly according to the present invention may comprise a 
matting agent. Suitable matting agents are described in WO 94/11198 and 
include e.g. talc particles and optionally protrude from the outermost 
layer. 
Lubricants for Outermost Layer 
The outermost layer of the recording material according to the present 
invention may comprise at least one lubricant. Examples of suitable 
lubricating materials are surface active agents, liquid lubricants, solid 
lubricants which do not melt during thermal development of the recording 
material, solid lubricants which melt (thermomeltable) during thermal 
development of the recording material or mixtures thereof. 
The lubricant is preferably selected from a group consisting of silicon 
derivatives, polyolefins, fatty acid derivatives, fatty alcohol 
derivatives and phosphoric acid derivatives. 
Antistatic Layer 
The thermographic recording material comprising a support and a 
thermosensitive element, of the present invention, may further comprise an 
outermost antistatic layer on the opposite side of the support to the 
thermosensitive element. Suitable antistatic layers therefor are 
described, for example, in U.S. Pat. No. 5,354,613. 
Coating 
The coating of any layer of the recording material of the present invention 
may proceed by any coating technique e.g. such as described in Modern 
Coating and Drying Technology, edited by Edward D. Cohen and Edgar B. 
Gutoff, (1992) VCH Publishers Inc. 220 East 23rd Street, Suite 909 New 
York, N.Y. 10010, U.S.A. 
Processing Assemblies 
As described in "Handbook of Imaging Materials", edited by Arthur S. 
Diamond--Diamond Research Corporation--Ventura, Calif., printed by Marcel 
Dekker, Inc. 270 Madison Avenue, New York, N.Y. 10016 (1991), p. 498-502 
in thermal printing image signals are converted into electric pulses and 
then through a driver circuit selectively transferred to a thermal 
printhead. The thermal printhead consists of microscopic heat resistor 
elements, which convert the electrical energy into heat via Joule effect. 
The electric pulses thus converted into thermal signals manifest 
themselves as heat transferred to the surface of the thermal paper wherein 
the chemical reaction resulting in colour development takes place. The 
operating temperature of common thermal printheads is in the range of 300 
to 400.degree. C. and the heating time per picture element (pixel) may be 
50 ms or less, the pressure contact of the thermal printhead with the 
recording material being e.g. 100-500 g/cm of linear array of resistor 
elements to ensure a good transfer of heat. 
In a preferred embodiment of the thermographic printing process, according 
to the present invention, a pressure of at least 100 g per cm of linear 
array of resistor elements is applied between the substantially 
non-photosensitive thermographic recording material and the thermal 
printing head assembly. 
In a particular embodiment of the method according to the present invention 
the thermographic image-wise heating of the recording material proceeds by 
Joule effect heating in that selectively energized electrical resistors of 
a thermal printing head array are used in contact or close proximity with 
the recording layer. 
The image signals for modulating the current in the micro-resistors of a 
thermal printhead are obtained directly or from an intermediary storage 
means, optionally linked to a digital image work station wherein the image 
information can be processed to satisfy particular needs. Activation of 
the heating elements can be power-modulated or pulse-length modulated at 
constant power. 
According to EP-A 622 217 relating to a method for making an image using a 
direct thermal imaging element, wherein the activation of the heating 
elements is executed line by line with a duty cycle .DELTA. representing 
the ratio of activation time to total line time in such a way that the 
following equation is satisfied: 
EQU P.ltoreq.P.sub.max =3.3 W/mm.sup.2 +(9.5 W/mm.sup.2 .times..DELTA.) 
wherein P.sub.max is the maximal value over all the heating elements of the 
time averaged power density P (expressed in W/mm.sup.2) dissipated by a 
heating element during a line time. 
Thermographic imaging can be used for both the production of transparencies 
and reflection type prints. In the hard copy field recording materials on 
a white opaque base are used. Black-imaged transparencies with transparent 
bases are used in both the graphics and medical diagnostic fields. In the 
graphics field dots and lines are printed using thermographic recording 
materials with a hard gradation and the transparencies are used as masks 
in the exposure of photosensitive compositions on printing plate bases in 
the process of printing plate preparation. In the medical diagnostic field 
black-imaged transparencies are widely used in inspection techniques 
operating with a light box. 
While the present invention will hereinafter be described in connection 
with a preferred embodiment thereof, it will be understood that it is not 
intended to limit the invention to that embodiment. On the contrary, it is 
intended to cover all alternatives, modifications, and equivalents as may 
be included within the spirit and scope of the invention as defined by the 
appending claims. 
PREATION OF A SUBSTANTIALLY NON-PHOTOSENSITIVE THERMOGRAPHIC RECORDING 
MATERIAL FOR TESTING THERMAL PRINTING HEADS TREATED ACCORDING TO THE 
PRESENT INVENTION 
Thermosensitive Element 
A subbed polyethylene terephthalate support having a thickness of 175 .mu.m 
was doctor blade-coated with a coating composition containing 2-butanone 
as a solvent and the following ingredients so as to obtain thereon, after 
drying for 1 hour at 50.degree. C., a layer containing: 
silver behenate: 4. 91g/m.sup.2 
polyvinylbutyral (Butvar.TM. B79 from Monsanto): 19.62 g/m.sup.2 
silicone oil (Baysilonem.TM. MA from Bayer AG): 0.045 g/m.sup.2 
benzo [e] [1,3] oxazine-2,4-dione, a toning agent: 0.268 g/m.sup.2 
7-(ethylcarbonato)-benzo [e] [1,3] oxazine-2,4-dione dione, a toning agent 
(see formula II below): 0.138 g/m.sup.2 
butyl-3,4-dihydroxybenzoate, a reducing agent: 1.003 g/m.sup.2 
tetrachlorophthalic anhydride: 0.157 g/m.sup.2 
adipic acid: 0.352 g/m.sup.2 
benzotriazole 0.130 g/m.sup.2 
##STR1## 
Coating of the Thermosensitive Element with a Protective Layer 
The thermosensitive element was then coated with an aqueous composition 
with the following composition expressed as weight percentages of 
ingredients present: 
polyvinylalcohol (Mowiviol.TM. WX 48 20 from Wacker Chemie): 2.5% 
Ultravon.TM. W (dispersion agent from Ciba Geigy) converted into acid form 
by passing through an ion exchange column: 0.09% 
talc (type P3 from Nippon Talc): 0.05% 
colloidal silica (Levasil.TM. VP AC 4055 from Bayer AG, a 15% aqueous 
dispersion of colloidal silica): 1.2% 
silica (Syloid.TM. 72 from Grace): 0.10% 
mono [isotridecyl polyglycolether (3 EO)] phosphate (Servoxyl.TM. VPDZ 
3/100 from Servo Delden B.V.): 0.09% 
mixture of monolauryl and dilauryl phosphate (Servoxyl.TM. VPAZ 100 from 
Servo Delden B.V.): 0.09% 
glycerine monotallow acid ester (Rilanit.TM. GMS from Henkel AG): 0.18% 
tetramethylorthosilicate hydrolyzed in the presence of methanesulfonic 
acid: 2.1% 
The pH of the coating composition was adjusted to a pH of 3.8 by adding 1N 
nitric acid. Those lubricants in these compositions which were insoluble 
in water, were dispersed in a ball mill with, if necessary, the aid of a 
dispersion agent. The compositions were coated to a wet layer thickness of 
85 .mu.m and were then dried at 40.degree. C. for 15 minutes and hardened 
at 45.degree. C. for 7 days.

COMATIVE EXAMPLE 1 
A printing run was carried out with sheets of the above-described 
substantially non-photosensitive thermographic recording material in an 
experimental printer equipped with a thin film thermal printing head with 
a pinhole in its outermost protective layer. 
Printing was carried out with a printer in which the above-mentioned thin 
film thermal printing head had been installed in which the sheets of 
substantially non-photosensitive thermographic material were fed at a 
speed of 4 mm/s onto a drum past the thermal printing head mounted in such 
a way as to contact the substantially non-photosensitive thermographic 
material. The thermal printing head was operated at a line time of 19 ms 
(the line time being the time needed for printing one line), during which 
it received constant power, and at an average printing power, being the 
total amount of electrical energy used for printing one line divided by 
the line time and the surface area of the heat-generating resistors, of 
1.25 mJ/dot, being sufficient to obtain maximum density in said recording 
material. 
A defective heating element, corresponding in position to the pinhole in 
the outermost protective layer of the thermal printing head was detected 
in the prints as a white line after 50 prints. 
INVENTION EXAMPLE 1 
A mixture of 14.5 g of a copolymer consisting of 80 mol % of methyl 
methacrylate and 20 mol % of methacryloyloxypropyltrimethoxysilane, 2.5 g 
of tetraethylorthosilicate, 5 g of glycidyloxypropyltrimethoxysilane 
(GPTS), 15 ml of 2-butanone and 0.5 g of deionized water was stirred for 1 
hour at 25.degree. C. to produce dispersion A. 
Dispersion A was then applied from a pipette via an injection needle to the 
outermost protective layer of the same type of thin film thermal printing 
head as used in COMATIVE EXAMPLE 1 to fill the pinholes therein. The 
excess of dispersion A was removed from the thermal printing head either 
by applying a current to the heating elements thereof for about 7 minutes, 
removing the excess dispersion with a cloth moistened with 2-butanone and 
then heating the thermal printing head for 40 minutes at 130.degree. C. 
Performance of Coating During Printing 
A printing run of 1000 prints was carried out with a thermal printing head 
with a pinhole using sheets of the above-described substantially 
non-photosensitive thermographic material as described for COMATIVE 
EXAMPLE 1. Evaluation of the thermal printing head with a microscope 
afterwards showed that the pinholes in the protective layer had remained 
filled with the coating and that no discolouration of the heating elements 
in the thermal printing head had taken place. Furthermore none of the 
heating elements of the thermal printing head was defective after this 
printing run of 1000 prints, which shows that application of a coating, 
according to the present invention, to the protective layer of a thermal 
printing head with a pinhole has prevented the premature failure of the 
heating elements observed with a thermal printing head with a pinhole 
which had not been subjected to the above-described treatment, see 
COMATIVE EXAMPLE 1. 
INVENTION EXAMPLE 2 
A mixture of 75 g of a 21.7% by weight solution a copolymer consisting of 
80 mol % of methyl methacrylate and 20 mol % of 
methacryloyloxypropyltrimethoxysilane in 2-butanone, 6.7 g of 
tetraethylorthosilicate, 1 g of formic acid and lg of deionized water was 
stirred for 1 hour at 25.degree. C. to produce dispersion B. 
Dispersion B was then applied from a pipette via an injection needle to the 
outermost protective layer of the same type of thin film thermal printing 
head as used in COMATIVE EXAMPLE 1 to fill the pinholes therein. The 
excess of dispersion B was removed from the thermal printing head either 
by applying a current to the heating elements thereof for about 7 minutes, 
removing the excess dispersion with a cloth moistened with 2-butanone and 
then heating the thermal printing head for 40 minutes at 130.degree. C. 
In a preliminary evaluation 30 prints were made as described in COMATIVE 
EXAMPLE 1. Evaluation of the thermal printing heads with a microscope 
afterwards showed that the pinholes in the protective layer had remained 
filled with the coating. 
INVENTION EXAMPLE 3 
A mixture of 25 g of tetraethylorthosilicate and 250 g of 2-butanone was 
prehydrolysed with the following quantities of deionized water and 
catalyst by stirring for 4 hours at 25.degree. C. to produce dispersions 
C, D and E: 
dispersion C: with 2.16 g of H.sub.2 O and 0.576 g of methanesulfonic acid 
dispersion D: with 2.16 g of H.sub.2 O and 0.408 g of imidazole 
dispersion E: with 3.88 g of H.sub.2 O and 0.408 g of imidazole 
Dispersions C, D and E were then each applied from a pipette via an 
injection needle to the outermost protective layer of different thin film 
thermal printing heads each with a pinhole in its outermost protective 
layer to fill the pinholes therein. The thermal printing heads were then 
heated for 15 minutes at 65.degree. C. and the excess of the dispersions 
removed by applying a cloth moistened with a mixture of 2-butanone and 
isooctane. 
In a preliminary evaluation 5 prints were made as described in COMATIVE 
EXAMPLE 1. Evaluation of the thermal printing heads with a microscope 
afterwards showed that the pinholes in the protective layer had remained 
filled with the coating. 
Having described in detail preferred embodiments of the current invention, 
it will now be apparent to those skilled in the art that numerous 
modifications can be made therein without departing from the scope of the 
invention as defined in the following claims.