Heat sensitive recording material

A heat sensitive material which is comprised of a support having thereon a transparent heat sensitive layer and at least one layer selected from among a protective layer, a subbing layer and backing layer, wherein at least one of constituent layer contains fine grains of at least one crystalline metal oxide which has a volume resistivity ranging from 10.degree. to 10.sup.5 ohm.multidot.cm. An amount of said crystalline metal oxide is 0.0001 g-1 g per m.sup.2 and the best metal oxide is SnO.sub.2.

FIELD OF THE INVENTION 
The present invention relates to a heat sensitive recording material and, 
more particularly, to a heat sensitive recording material having a heat 
sensitive layer excellent in transparency. 
BACKGROUND OF THE INVENTION 
A heat sensitive recording method has many advantages, such as (1) no need 
of development, (2) embodying a quality akin to that of plain paper when a 
paper support is used, (3) easiness in handling, (4) high density of the 
developed color images, (5) simplicity and cheapness of a recording 
apparatus to be used thereof, (6) noiseless in the recording operation, 
and so on. Such being the case, this method has recently come into a rapid 
prevalence in the field of facsimile and printer, and further new uses of 
heat sensitive recording, e.g., the field of making labels to be used in 
POS, etc., are being enlarged. 
Under these circumstance, it has recently been desired to develop a 
transparent heat sensitive recording material on which images can be 
directly recorded with a thermal head for the purposes of adaptation to 
multicoloring, or application to an overhead projector (which is 
abbreviated as OHP, hereinafter). Therefore, we have previously proposed 
(in Japanese Patent Application No. 88197/87) a heat sensitive recording 
material having a substantially transparent heat sensitive layer formed by 
coating and then drying a composition comprising microcapsules containing 
a colorless or light colored electron donating dye precursor and an 
emulsified dispersion prepared by dispersing of emulsified dispersion, a 
color developer dissolved in an organic solvent slightly soluble or 
insoluble in water. 
On the other hand, as the conversion of materials constructing a heat 
sensitive paper carrying system from metallic materials to plastic ones 
has been driven with the advance of reduction in size and weight of 
apparatuses used for heat sensitive recording materials, e.g., facsimile 
and like, troubles due to electrification of heat sensitive paper through 
contact with a stock case, a tray, rollers for carrying and so on tend to 
frequently occur. Namely, the recording apparatuses of this type have 
defects, for instance, such that the electrified heat sensitive paper 
sheets cause electrostatic adhesion to one another or to the sheet 
carrying system to suffer from a so-called "Jamming" phenomenon, and it 
happens that thermal recording fails to be made on electrified heat 
sensitive paper sheets owing to interference from dust, such as paper 
powder, etc., sticked to the surfaces thereof. 
SUMMARY OF THE INVENTION 
Therefore, a first object of the present invention is to provide a heat 
sensitive recording material which ensures the smooth carrying inside a 
recording apparatus. 
A second object of the present invention is to provide a heat sensitive 
recording material which is hard to cause static electrification due to 
rubbing, or free from adhesion of dust thereto and so on, and thereby 
enable reliable recording. 
A third object of the present invention is to provide a highly transparent 
heat sensitive recording material which hardly causes static 
electrification due to rubbing to ensure the smooth carrying inside a 
recording apparatus, and has aptitudes for multicolor recording and OHP 
use. 
Above objects of the present invention were attained by a heat sensitive 
recording material which is comprised of a support having thereon 
constituent layers including a heat sensitive layer formed by coating a 
composition containing both microcapsules containing a colorless or light 
colored elecron donating dye precursor and an emulsified dispersion 
prepared by dispersing a color developer dissolved in an organic solvent 
slightly soluble or insoluble in water, and then drying the coated 
composition, and at least one layer selected from among a protective layer 
provided on the heat sensitive layer, a subbing layer provided beneath the 
heat sensitive layer and a backing layer provided on the back side of the 
support; wherein at least one of said constituent layers contains fine 
grains of at least one crystalline metal oxide selected from a group 
consisting of ZnO, TiO.sub.2, SnO.sub.2, Al.sub.2 O.sub.3, In.sub.2 
O.sub.3, SiO.sub.2, MgO, BaO, MoO.sub.3 and compound oxide constituted 
with two or more of said metal oxides, each of which has a volume 
resistivity ranging from 10.sup.0 to 10.sup.5 ohm.multidot.cm. 
The heat sensitive recording material of the present invention has an 
excellent antistatic property. Consequently, adhesion of dust like paper 
powder is hardly caused therein, and it dose not occur that there are some 
spots on which thermal printing failed due to dust sticked thereto. 
In addition, even when the heat sensitive recording material is rubbed with 
the parts of a recording apparatus in carrying it through the apparatus, 
static electrification is not caused therein, so paper stopping such as a 
jamming phenomenon dose not occur, that is to say, smooth travelling of 
the recording material through the recording apparatus becomes feasible, 
and omission of records and troubles in the apparatus can be prevented.

DETAILED DESCRIPTION OF THE INVENTION 
Electron donating dye precursors to be employed in the present invention 
are selected properly from known colorless or light colored compounds of 
the kind which can develop their colors by donating an electron or 
accepting a proton of an acid or the like. These compounds have such a 
skeleton as that of lactone, lactam, sultone, spiropyran, ester, amide, 
etc., as a part of their structures, and these skeletons undergo 
ring-opening or bond cleavage upon contact with a color developer. 
Preferred examples of such compounds include triarylmethane compounds, 
diphenylmethane compounds, xanthene compounds, thiazine compounds, 
spiropyran compounds and so on. 
Particularly preferred compounds are those represented by the following 
general formula: 
##STR1## 
In the foregoing formula, R.sub.1 represents an alkyl group containing 1 to 
8 carbon atoms; R.sub.2 represents an alkyl or alkoxyalkyl group 
containing 4 to 18 carbon atoms, or a tetrahydrofuryl group; R.sub.3 
represents a hydrogen atom, an alkyl group containing 1 to 15 carbon 
atoms, or a halogen atom; and R.sub.4 represents a substituted or 
unsubstituted aryl group containing 6 to 20 carbon atoms. As substituent 
group for R.sub.4, alkyl, alkoxy and halogenated alkyl groups containing 1 
to 5 carbon atoms, and halogen atoms are preferred. 
Microencapsulation of the above-described color former in the present 
invention can prevent generation of fog during production of a heat 
sensitive material and, at the same time, can improve a freshness keeping 
quality of a heat sensitive material and a keeping quality of the record 
formed. Therein, the image density at the time of recording can be 
heightened by properly selecting a material and a method for forming a 
microcapsule wall. A preferred amount of the color former used is 0.05 to 
5.0 g per square meter. 
Suitable examples of wall materials for microcapsules include polyurethane, 
polyurea, polyester, polycarbonate, urea/formaldehyde resin, melamine 
resin, polystyrene, styrene/methacrylate copolymer, styrene/acrylate 
copolymer, gelatin, polyvinyl pyrrolidone, polyvinyl alcohol, and so on. 
These macromolecular substances can be used in combination of two or more 
thereof in the present invention. 
Of the above-cited macromolecular substances, polyurethane, polyurea, 
polyamide, polyester, and polycarbonate are preferred in the present 
invention. In particular, polyurethane and polyurea can bring about good 
results. 
Microcapsules to be employed in the present invention are preferably 
prepared by emulsifying a core material containing a reactive substance 
like a color former, and then forming a wall of a macromolecular substance 
around the droplets of the core material to microencapsulate the core 
material. Therein, reactants to produce a macromolecular substance are 
added to the inside and/or the outside of the oily droplets. For details 
of microcapsules which can be preferably employed in the present 
invention, e.g., for production methods of microcapsules which can be 
preferably used, descriptions in Japanese Patent Application (OPI) No. 
222716/84 (the term "OPI" as used herein means an "unexamined published 
application"), and so on can be referred to. 
An organic solvent to constitute the oily droplets can be arbitralily 
selected from known organic solvents, however, it is desired to use the 
aftermentioned organic solvents which are suitable to dissolve color 
developers are used, since these solvents are excellent as a solvent to 
dissolve the before mentioned electron donating dye precursor and can 
increase coloring density at heat recording, moreover, can decrease fog. 
Desirable microcapsules which are produced in the above-described manner 
are not those of the kind which are disrupted by heat or pressure, but 
those of the kind which have a microcapsule wall through which reactive 
substances present inside and outside the individual microcapsules 
respectively can permeate at high temperature to react with each other. 
Multicolored neutral tints can be effected by preparing some kinds of 
microcapsules having walls differing in glass transition point through 
proper selection of wall materials, and optional addition of glass 
transition point controlling agents (e.g., plasticizers described in 
Japanese Patent Application (OPI) No. 119862/85) to the wall materials, 
respectively, and further by combining selectively colorless electron 
donating dye precursors differing in hue with their respective color 
developers. Therefore, the present invention is not limited to a 
monochromatic heat sensitive recording material but can be applied to a 
two-color or multicolor heat sensitive recording material and a heat 
sensitive recording material suitable for recording of graded image. 
In addition, a photodiscoloration inhibitor as described, e.g., in Japanese 
Patent Application (OPI) Nos. 125470/85, 125471/85, and 125472/85 can be 
added, if desired. 
Color developers to be employed in the present invention, which undergo the 
color development reaction with electron donating colorless precursors 
when heated, can be those selected properly from known color developers. 
For instance, suitable examples of color developers to be combined with 
leuco dyes include phenol compounds, sulfur-contained phenolic compounds, 
carboxylic acid compounds, sulfon compounds, urea or thiourea compounds, 
and so on. Details of the color developers are described, e.g., in "Kami 
Pulp Gijutsu Times," pp. 49-54, and pp. 65-70 (1985). Of such color 
developers, those having melting points of 50.degree. to 250.degree. C., 
particularly phenols and organic acids which have melting points of 
60.degree. to 200.degree. C. and are hardly soluble in water are preferred 
over others. Combined use of two or more of color developers is desirable 
because of increase in solubility. 
Color developers preferred particularly in the present invention are 
represented by the following general formulae (I) to (IV): 
##STR2## 
m=0-2, n=2-11 
##STR3## 
R.sub.1 is an alkyl group, an aryl group, an aryloxy group, or an aralkyl 
group. In particular, methyl group, ethyl group and butyl group are 
preferred as R.sub.1. 
##STR4## 
R.sub.2 is an alkyl group. In particular, butyl group, pentyl group, heptyl 
group, and octyl group are preferred. 
##STR5## 
R.sub.3 is an alkyl group, an aryloxy group, or an aralkyl group. 
In the present invention, such a color developer is used in a form of 
emulsified dispersion. The dispersion can be prepared by dissolving color 
developers in an organic solvent slightly soluble or insoluble in water 
and mixing the resulting solution with an aqueous phase, which contains a 
surface active agent and a water-soluble high polymer as a protective 
colloid, to emulsify and to disperse the solution in the aqueous phase. 
An organic solvent to be used for dissolving the color developers can be 
properly selected from known oils. 
In the present invention, esters having high boiling point or before 
mentioned oils used for pressure sensitive materials are preferable. Some 
of desirable oils are compounds represented by the following general 
formulae (V) to (VII), triarylmethanes (such as tritoluylmethane, 
toluyldiphenyl-methane), terphenyl compounds (such as terphenyl), 
alkylated diphenyl ethers (such as propyldiphenyl ether), hydrogenated 
terphenyl compounds (such as hexahydroterphenyl), diphenyl ethers, 
chlorinated paraffins and so on. In particular, esters are more preferable 
from a view point of a stability of the color developer emulsion. 
##STR6## 
In the above formula, R.sup.1 represents a hydrogen atom, or an alkyl group 
containing 1 to 18 carbon atoms; R.sup.2 represents an alkyl group 
containing 1 to 18 carbon atoms; and p.sup.1 and q.sup.1 each represents 
an integer of 1 to 4, provided that the total number of alkyl groups 
therein is 4 or less. Preferred alkyl groups represented by R.sup.1 and 
R.sup.2 are those containing 1 to 8 carbon atoms. 
##STR7## 
In the above formula, R.sup.3 represents a hydrogen atom, or an alkyl group 
containing 1 to 12 carbon atoms; R.sup.4 represents an alkyl group 
containing 1 to 12 carbon atoms; and n is 1 or 2. p.sup.2 and q.sup.2 each 
represents an integer of 1 to 4. The total number of alkyl groups is 4 or 
less in case of n=1, while it is 6 or less in case of n=2. 
##STR8## 
In the above formula, R.sup.5 and R.sup.6, which may be the same or 
different, each represents a hydrogen atom, or an alkyl group containing 1 
to 18 carbon atoms. m represents an integer of 1 to 13. p.sup.3 and 
q.sup.3 each represents an integer of 1 to 3, provided that the total 
number of alkyl groups is 3 or less. 
Of alkyl groups represented by R.sup.5 and R.sup.6, those containing 2 to 4 
carbon atoms are particularly preferred. 
Specific examples of the compounds represented by the formula (V) include 
dimethylnaphthalene, diethylnaphthalene, diisopropylnaphthalene, and the 
like. 
Specific examples of the compounds represented by the formula (VI) include 
dimethylbiphenyl, diethylbiphenyl, diisopropylbiphenyl, 
diisobutylbiphenyl, and the like. 
Specific examples of the compounds represented by the formula (VII) include 
1-methyl-1-dimethylphenyl-1-phenylmethane, 
1-ethyl-1-dimethylphenyl-1-phenylmethane, 
1-propyl-1-dimethylphenyl-1-phenylmethane, and the like. 
Specific examples of esters include phosphates (e.g., triphenyl phosphate, 
tricresyl phosphate, butyl phosphate, octyl phosphate, cresyl-bi-phenyl 
phosphate), phthalates (e.g., dibutyl phthalate, 2-ethylhexyl phthalate, 
ethyl phthalate, octyl phthalate, buthlbenzyl phthalate, tetrahydro 
dioctyl phthalate, benzoates (e.g., ethyl benzoate, propyl benzoate, butyl 
benzoate, isopentyl benzoate, benzyl benzoate), abietates (e.g., ethyl 
abietate, benzyl abietate), dioctyl adipate, diethyl succinate, isodecyl 
succinate, dioctyl azelate, oxalates (e.g., dibutyl oxalate, dipentyl 
oxalate), diethyl malonate, maleates (e.g., dimethyl maleate, diethyl 
maleate, dibutyl maleate), tributyl citrate, sorbic esters (methyl 
sorbate, ethyl sorbate, butyl sorbate), sebacic esters (dibutyl sebacate, 
dioctyl sebacate), ethyleneglycol esters (e.g., formic acid monoesters and 
diesters, butyric acid monoesters and diesters, lauric acid monoesters and 
diesters, palmitic acid monoesters and diesters, stearic acid monoesters 
and diesters, oleic acid monoesters and diesters), triacetin, 
diethylcarbonate, diphenylcarbonate, ethylenecarbonate, 
propylenecarbonate, boric acid esters (e.g., tributyl borate, tripentyl 
borate). Of these esters, it is particularly preferred to use tricresyl 
phosphate from the standpoint of stabilization of emulsified dispersion of 
the color developers. 
Organic solvents having low boiling point can be added to the foregoing 
organic solvents. Some of these organic solvents are ethylacetate, 
isopropyl acetate, butyl acetate, methylene chloride, and the like. 
Water soluble high polymers to be contained as a protective colloid in an 
aqueous phase, which is to be mixed with an oily phase wherein color 
developers are dissolved, can be selected properly from known anionic, 
nonionic or amphoteric high polymers. Of these high polymers, 
polyvinylalcohol, gelatin, cellulose derivatives and the like are 
preferred. 
Surface active agents to be contained additionally in the aqueous phase can 
be selected properly from anionic or nonionic surface active agents of the 
kind which do not cause any precipitation or condensation by interaction 
with the above-described protective colloids. As examples of surface 
active agents which can be preferably used, mention may be made of sodium 
alkylbenzenesulfonates (such as sodium laurylbenzenesulfonate), sodium 
dioctylsulfosuccinates, polyalkylene glycols (such as polyoxyethylene 
nonylphenyl ether) and so on. 
An emulsified dispersion of color developers to be used in the present 
invention can be prepared with ease by mixing an oil phase containing the 
color developers and an aqueous phase containing a protective colloid and 
a surface active agent with a general means for preparing a fine grain 
emulsion, such as a high-speed stirrer, an ultrasonic disperser or so on, 
to disperse the former phase into the latter phase. 
To the emulsified dispersion thus obtained, melting point depressants for 
the color developers can be added, if desired. Some of these melting point 
depressants have such a function as to control glass transition points of 
the capsule walls described hereinbefore, too. Specific examples of such 
melting point depressants include hydroxy compounds, carbamate compounds, 
sulfonamide compounds, aromatic methoxy compounds and so on. Details of 
these compounds are described in Japanese Patent Application No. 
244190/84. 
These melting point depressants can be used in an amount of 0.1 to 2 parts 
by weight, preferably 0.5 to 1 part by weight, per 1 part by weight of 
color developer whose melting point is to be depressed. It is to be 
desired that the melting point depressant and the color developer, whose 
melting point can be depressed thereby, should be used in the same place. 
When they are added to separate places, a preferred addition amount of the 
melting point depressant is 1 to 3 times of that of the above-described 
one. 
For the purpose of prevention of sticking to a thermal head, and 
improvement on writing quality, pigments such as silica, barium sulfate, 
titanium oxide, aluminium hydroxide, zinc oxide, calcium carbonate, etc., 
styrene beads, or fine particles of urea/melamine resin and so on can be 
added to the heat sensitive recording material of the present invention. 
In order to keep the transparency of the heat sensitive layer, a protective 
layer may be provided on the heat sensitive layer in a conventional manner 
for the purpose of acquisition of keeping quality and stability. 
Details of the protective layer are described, e.g., in "Kami Pulp Gijutsu 
Times", pp. 2 to 4 (September 1985). 
Particularly, it is desired to prepare the protective layer using mainly a 
combination of a polyvinyl alcohol modified with silicon and a colloidal 
silica in order to obtain the protective layer excellent in transparency 
and to improve transparency of a heat sensitive recording material 
remarkably. 
Also, waxes and metallic soaps can be used for the prevention of sticking 
phenomenon. A coverage of such additives is appropriately 0.2 to 7 
g/m.sup.2. 
In preparing the heat sensitive material of the present invention, a proper 
binder can be used for coating. 
Suitable example of binder which can be used include polyvinyl alcohol, 
methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, gum 
arabic, gelatin, polyvinyl pyrrolidone, casein, styrene-butadiene latex, 
acrylonitrile-butadiene latex, various kinds of emulsions such as one of 
polyvinylacetate, polyacrylic acid esters, ethylene-vinylacetate copolymer 
and so on. Such a binder is used at a coverage of 0.5 to 5 g/m.sup.2 on a 
solids basis. 
The heat sensitive recording material of the present invention is produced 
by providing a heat sensitive layer on a support, such as paper, a 
synthetic resin film, etc., coating and drying a coating composition, in 
which microcapsules enclosing a color former therein and a dispersion 
containing at least a color developer in an emulsified condition are 
contained as main components, and further a binder and other additives are 
incorporated, according to a conventional coating method, such as a bar 
coating method, a blade coating method, an air knife coating method, a 
gravure coating method, a roll coating method, a spray coating method, a 
dip coating method, or so on. A coverage of the heat sensitive layer is 
controlled to 2.5 to 25 g/m.sup.2 on a solid basis. 
As for the paper to be used as a support, neutralized paper which is sized 
with a neutral sizing agent like an alkylketene dimer and shows pH 6-9 
upon hot extraction (Japanese Patent Application (OPI) No. 14281/'80) is 
employed to advantage in the respect of long-range preservation. 
In order to prevent the penetration of a coating composition into paper, 
and in order to effect a close contact between a thermal recording head 
and a heat sensitive recording layer, paper described in Japanese Patent 
Application (OPI) No. 116687/82, which is characterized by Stokigt sizing 
degree/(meter basis weight).sup.2 .gtoreq.3.times.10.sup.-3 and Beck 
smoothness of 90 seconds or more, is used to advantage. 
In addition, paper having optical surface roughness of 8 microns or less 
and a thickness of 40 to 75 microns, as described in Japanese Patent 
Application (OPI) No. 136492/83; paper having a density of 0.9 g/cm.sup.3 
or less and optical contact rate of 15% or more, as described in Japanese 
Patent Application (OPI) No. 69097/83; paper which is prepared from pulp 
having received a beating treatment till its freeness has come to 400 cc 
or more on a basis of Canadian Standard Freeness (JIS P8121) to prevent 
permeation of a coating composition thereinto, as described in Japanese 
Patent Application (OPI) No. 69097/83; raw paper made with a Yankee paper 
machine, which is to be coated with a coating composition on the glossy 
side and thereby, improvements on developed color density and resolution 
are intended, as described in Japanese Patent Application (OPI) No. 
65695/83; raw paper which has received a corona discharge processing and 
thereby, its coating aptitude has been enchanced, as described in Japanese 
Patent Application (OPI) No. 35985/84; and so on can be employed in the 
present invention, and can bring about good results. In addition to the 
above-described papers, all supports which have so far been used for 
general heat sensitive recording papers can be employed as the support of 
the present invention. 
In the present invention, it is desired to use a transparent support. By 
using the transparent support, not only a recorded material can be 
utilized as OHP sheet etc., but also multi coloration can be realized 
easily by providing heat sensitive layers which color in different hue 
each other on both side of the transparent support. 
Now, a transparent support to be used in the present invention is 
illustrated below. 
The term transparent support as used herein is intended to include films of 
polyesters such as polyethylene terephthalate, polybutylene terephthalate 
and the like, cellulose derivative films like a cellulose triacetate film, 
polyolefin films such as a polystyrene film, a polypropylene film, a 
polyethylene film and the like and so on. These films may be used, 
independently or in a laminated form. 
A preferred thickness of such a transparent support is within the range of 
20 to 200 microns, particularly 50 to 100 microns. 
A subbing layer, which can be employed in the present invention, functions 
so as to heighten the adhesiveness between the transparent support and the 
heat sensitive layer. As a material for forming the subbing layer, mention 
may be made of gelatin, synthetic high polymer latexes, nitrocellulose, 
and so on. A preferred coverage of the subbing layer ranges from 0.1 to 
2.0 g/m.sup.2, particularly from 0.2 to 1.0 g/m.sup.2. When the coverage 
is below 0.1 g/m.sup.2, adhesion of the heat sensitive layer to the 
support is insufficient, whereas even when it is increased beyond 2.0 
g/m.sup.2, the adhesion power attains saturation to bring about only 
increase in cost. 
It is to be desired that the subbing layer should be hardened with a 
hardener because it sometimes swells in water contained in a coating 
solution to prepare the heat sensitive layer thereon to cause 
deterioration of image formed in the heat sensitive layer. 
As examples of hardeners which can be used in the present invention, 
mention may be made of: 
(1) active vinyl-containing compounds, such as divinylsulfone, 
N,N'-ethylenebis(vinylsulfonylacetamide), 
1,3-bis(vinylsulfonyl)-2-propanol, methylenebismaleimide, 
5-acetyl-1,3-diacryloyl-hexahydro-s-triazine, 
1,3,5-triacryloyl-hexahydro-s-triazine, 
1,3,5-trivinylsulfonylhexahydro-s-triazine, and the like, 
(2) active halogen-containing compounds, such as sodium salt of 
2,4-dichloro-6-hydroxy-s-triazine, 2,4-dichloro-6-methoxy-s-triazine, 
sodium salt of 2,4-dichloro-6-(4-sulfoanilino)-s-triazine, 
2,4-dichloro-6-(2-sulfoethylamino)-s-triazine, 
N,N'-bis(2-chloroethylcarbamyl)piperazine, and the like, 
(3) epoxy compounds, such as 
bis(2,3-epoxypropyl)methylpropylammonium-p-toluenesulfonate, 
1,4-bis(2',3'-epoxypropyloxy)-butane, 1,3,5-triglycidylisocyanurate, 
1,3-diglycidyl-5-(.gamma.-acetoxy-.beta.-oxypropyl)isocyanurate, and the 
like. 
(4) ethyleneimino compounds, such as 2,4,6-triethylene-s-triazine, 
1,6-hexamethylene-N,N'-bisethyleneurea, 
bis-.beta.-ethyleneiminoethylthioether, and the like, 
(5) methanesulfonate compounds, such as 1,2-di(methanesulfonoxy)ethane, 
1,4-di(methanesulfonoxy)butane, 1,5-di(methanesulfonoxy)pentane, and the 
like, 
(6) carbodiimide compounds, such as dicyclohexylcarbodiimide, 
1-cyclohexyl-3-(3-trimethylaminopropyl)carbodiimido-p-triethanesulfonate, 
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, and the like, 
(7) isooxazole compounds, such as 2,5-dimethylisooxazole perchlorate, 
2-ethyl-5-phenylisooxazole-3-sulfonate, 5,5'-(p-phenylene)bisisooxazole, 
and the like, 
(8) inorganic compounds, such as chrome alum, bolic acid, zirconium salt, 
chromium acetate, and the like, 
(9) dehydrating condensation type peptide reagents such as 
N-carboethoxy-2-isopropoxy-1,2-dihydroquinoline, 
N-(1-morpholinocarboxy)-4-methylpyridinium chloride and the like, and 
active ester compounds such as N,N'-adipolyldioxydisuccinimide, 
N,N'-terephthaloyl-dioxydisuccinimide and the like, 
(10) isocyanates, such as toluene-2,4-diisocyanate, 
1,6-hexamethylenediisocyanate and the like, 
(11) aldehydes such as glutaric aldehyde, glyoxal, dimethoxy urea, 
2,3-dihydroxy-1,4-dioxane and the like. 
Among these hardeners, especially aldehydes such as the glutaric aldehyde, 
the 2,3-dihydroxy-1,4-dioxane etc. and the boric acid are preferable. 
Such a hardner is added in a proportion ranging from 0.20 to 3.0 wt % to 
the weight of the materials to constitute the subbing layer. A proper 
amount to be added can be selected depending on the coating method, the 
intended degree of hardening. 
When the addition amount of a hardener is below 0.20 wt %, sufficient 
hardening can not be achieved however long the time elapsed is, and 
therefore the subbing layer swells upon coating of the heat sensitive 
layer, whereas when the hardener is added in a concentration higher than 
3.0 wt % the hardening proceeds too fast, and therefore the adhesiveness 
between the subbing layer and the support is lowered to result in peeling 
off the subbing layer from the support in the form of a film. 
According to the hardener used, the pH of a coating solution for the 
subbing layer can be rendered alkaline by the addition of sodium hydroxide 
or the like, or acidic by the addition of citric acid or the like, if 
needed. 
Further, a defoaming agent can be added in order to eliminate foams 
generated upon coating, and a surface active agent can also be added in 
order to level the surface of the coating solution in a good condition to 
result in prevention of coating streaks. 
Furthermore, an antistatic agent can be added, if needed. 
Before coating of the subbing layer, the surface of a support is preferably 
subjected to an activation processing according to known methods. As a 
method for the activation processing, mention may be made of an etching 
processing with an acid, a flame processing with a gas burner, a corona 
discharge processing, glow discharge processing, and so on. From the 
viewpoint of cost or simplicity, corona discharge processing described in 
U.S. Pat. Nos. 2,715,075, 2,846,727, 3,549,406 and 3,590,107, and so on 
are employed to the greatest advantage. 
In the present invention, the heat-sensitive recording material acquires 
antistatic properties by conductive grains constituting metal oxide(s) 
incorporated in at least one of its constituent layers or the support 
thereof to effect the smooth carrying inside the recording apparatus. 
Instead of or in addition to such layers, a backing layer having 
antistatic property may be provided on the back side of the support. 
Conductive fine grains of metal oxide(s) to be used in the present 
invention are at least one species selected from a group consisting of 
ZnO, TiO.sub.2, SnO.sub.2, Al.sub.2 O.sub.3, In.sub.2 O.sub.3, SiO.sub.2, 
MgO, BaO, MoO.sub.3 and compound oxides constituted by two or more of 
these oxides, which has a volume resistivity ranging form 10.sup.0 to 
10.sup.5 ohm.multidot.cm. Of these metal oxides, SnO.sub.2 is particularly 
preferred over others. 
The conductive fine grains of metal oxide(s) which can be used in the 
present invention are prepared with ease mainly using the following 
method. A first method involves preparing fine grains of metal oxide by 
burning, then subjecting the metal oxide to a heat treatment in the 
presence of foreign atoms capable of increasing conductivity. A second 
method involves carrying out the burning processing for preparing fine 
grains of metal oxide in the presence of foreign atoms capable of 
increasing conductivity. A third method involves introducing oxygen 
defects by lowering an oxygen concentration in the surrounding gas during 
the burning for preparation of fine grains of metal oxide. 
In the first method, the conductivity of the grains can be increased 
effectively at their individual surfaces, but there is a possibility of 
grain growth during the heat treatment. Therefore, a condition under which 
the heat treatment is performed should be selected carefully. In some 
cases, it is preferred to undergo the heat treatment under a reductive 
atmosphere. The second method is preferred, because the production seems 
to cost least. For instance, in a process of producing fine grains of 
SnO.sub.2 by spraying .beta.-stannic acid colloid (amorphous), or hydrate 
of SnO.sub.2, into a klin, the conductivity can be imparted to fine grains 
of SnO.sub.2 by allowing a hydrate of antimony chloride, antimony nitrate, 
antimony oxide or the like to be present in the .beta.-stannic acid 
colloid. In another process of producing SnO.sub.2 or TiO.sub.2 by 
oxidative decomposition of SnCl.sub.4 or TiCl.sub.4, that is, a so-called 
vapor phase process, conductive SnO.sub.2 or TiO.sub.2 can be obtained by 
making salts of foreign atoms be present at the time of oxidative 
decomposition. In still another process of producing metal oxides by 
pyrolysis of organic salts of metals, salts of foreign atoms are made to 
be present during the pyrolysis. As for the third method, there can be 
instanced a vacuum evaporation process carried out by evaporating metals 
in an atmosphere of oxygen to produce fine grains of metal oxides, wherein 
the metals or salts thereof are heated under an oxygen-deficient 
atmosphere, or without supplying sufficient oxygen to the evaporation 
system. 
It is desirable that a size of the conductive grains to be used in the 
present invention should be as small as possible. However, it often occurs 
that fine grains obtained in accordance with the above-described methods 
condense strongly to form coarse grains. In order to avoid this 
condensation phenomenon, it is effective in many cases that fine grains 
having no direct contribution for improvement on conductivity are made to 
be present as a finely graining aid in forming conductive grains. Specific 
examples of grains usable for this purpose include fine grains of metal 
oxides which have not been prepared with the intention of increasing the 
conductivity (e.g., ZnO, TiO.sub.2, SiO.sub.2, Al.sub.2 O.sub.3, MgO, BaO, 
WO.sub.3, MoO.sub.3, etc.), fine grains of sulfates such as BaSO.sub.4, 
SrSO.sub.4, CaSO.sub.4, MgSO.sub.4, etc., fine grains of carbonates such 
as MgCO.sub.3, CaCO.sub.3, etc., and so on. 
It is feasible to use the grains cited above in such a condition that they 
are dispersed in a binder together with conductive fine grains. Also, 
physical or chemical treatments can be performed for the purpose of 
removing most of fine grains for auxiliary use and most of coarse grains. 
Specifically, the grains obtained are thrown in liquid, and ground with a 
ball mill, a sand mill or the like, followed by selective collection of 
ultrafine conductive grains through filtration, centrifugation or the 
like. After the above-described grinding procedure, on the other hand, it 
is more effective to remove auxiliary grains alone by dissolution. It is a 
matter of course that ultrafine conductive grains can be obtained more 
efficiently by repeating or combining the above-described procedures. 
Moreover, previous addition of a surface active agent, a small amount of 
binder or a small amount of Lewis acid or base as a dispersing aid to a 
dispersion medium of the grains is much more effective in forming 
ultrafine conductive grains. 
It is obvious that grains usable as auxiliary ones cover a wider range of 
kinds when chemical procedures are employed together. 
Now, an instance of the manner of preparing fine grains is cited below. 
In 1,000 pts.wt. of ethanol were dissolved 65 pts.wt. of hydrated stannic 
chloride and 1.5 pts.wt. of antimony trichloride to make a homogeneous 
solution. To this solution, a 1N aqueous solution of sodium hydroxide was 
added dropwise till the pH of the resulting solution reached to 3. Thus, 
coprecipitate of colloidal stannic oxide and antimony oxide was obtained. 
This coprecipitate was allowed to stand for 24 hours at 50.degree. C. As 
the result, it was converted into reddish brown colloidal precipitate. 
This precipitate and the mother liquor were separated by centrifugation. 
Further, addition of water to the precipitate and centrifugation were 
repeated to achieve the removal of excess ions. 
100 pts.wt. of the thus obtained colloidal precipitate was mixed with 50 
pts.wt. of barium sulfate having an average grain size of 0.3 micron and 
1,000 pts.wt. of water, and sprayed into a klin heated to 900.degree. C. 
to yield powdery mixture of bluish stannic oxide having an average grain 
size of 0.1 micron and barium sulfate. A mixture of 10 pts.wt. of the thus 
obtained SnO.sub.2 powder, 50 pts.wt. of a 10% aqueous solution of 
polyvinyl alcohol (PVA 105, produced by Kurare Co., Ltd.) and 100 pts.wt. 
of water was subjected to a dispersion processing over a period of 1 hour 
using a paint shaker (produced by Toyo Seiki Seisakusho K. K.) to obtain a 
dispersion of fine grains. 
In another manner, a mixture of 100 pts.wt. of zinc oxide, 5 pts.wt. of a 
10% aqueous solution of Al(NO.sub.3).sub.3.9H.sub.2 O and 100 pts.wt. of 
water was irradiated with ultrasonic waves for 10 minutes to prepare a 
homogeneous dispersion. After drying at 110.degree. C. for 1 hour, the 
resulting dispersion was burnt for 5 minutes at 600.degree. C. under, 
1.times.10.sup.-4 Torr to yield zinc oxide grains having a specific 
resistance of 2.times.10.sup.2 ohm cm and a grain size of about 2 microns. 
These grains were ground with a ball mill to be made into fine grains 
having an average size of 0.7 microns. 
A mixture of 10 pts.wt. of thus obtained ZnO powder and 150 pts.wt. of 
water was subjected to a dispersion processing over a period of 1 hour 
using a paint shaker to prepare a homogeneous dispersion. From this 
dispersion, coarse grains were removed by centrifugation carried out at 
1,000 r.p.m. for 30 minutes. The remaining supernatant was further 
centrifuged at 2,000 r.p.m. for 1 hour to obtained ZnO paste consisting of 
fine grains. 10 pts.wt. of thus obtained ZnO paste was mixed with 25 
pts.wt. of a 10% aqueous solution of polyvinyl alcohol and 100 pts.wt. of 
water, and dispersed thereinto over a period of 1 hour using a paint 
shaker to prepare a dispersion of ZnO fine grains. 
In the present invention, at least one kind of grains among those obtained 
in the manners as described above is used. In general, these conductive 
compounds are used at a coverage of about 0.001 g to about 1 g, preferably 
about 0.05 g to 0.5 g, per square meter of the support. 
Although the action of these conductive compounds in preventing static 
electrification is not necessarily clarified, an antistatic property is 
imparted to a heat sensitive recording material by adding the compounds to 
at least one constituent layer of the recording material. 
However, the addition to a heat sensitive layer is accompanied by 
undesirable effects, such as lowering of the heat sensitivity and so on. 
Therefore, it is particularly desirable that at least one layer, such as a 
protective layer on the heat sensitive layer, a subbing layer beneath the 
heat sensitive layer or a backing layer on the back side of a support, 
should be provided together with the heat sensitive layer, and the 
foregoing conductive compound(s) should be incorporated into at least one 
among these layers. 
In a preferred embodiment of the present invention, therefore, the heat 
sensitive recording material is constructed by a support, a heat sensitive 
layer and at least one layer selected from among a backing layer, a 
subbing layer and a protective layer, and conductive fine grains are 
incorporated into at least one constituent layer other than the heat 
sensitive layer. 
EXAMPLES 
The present invention is illustrated in greater detail by reference to the 
following examples. However, the present invention should not be construed 
as being limited to these examples. 
EXAMPLE 1 
Preparation of Capsule Solution 
14 g of Crystal Violet lactone (leuco dye), 60 g of Takenate D 110N (Trade 
name of capsule wall material, produced by Takeda Yakuhin K. K.) and 2 g 
of Sumisoap 200 (an ultraviolet absorbent, produced by Sumitomo Kagaku K. 
K.) were added to a mixed solvent composed of 55 g of 
1-phenyl-1-xylylethane and 55 g of methylene chloride, and dissolved 
therein. The solution of this leuco dye was mixed with a water solution 
containing 100 g of a 8% aqueous solution of polyvinyl alcohol, 40 g of 
water and 1.4 g of a 2% aqueous solution of sodium salt of 
dioctylsulfosuccinate (dispersant), and emulsified using an Ace 
Homogenizer (made by Nippon Seiki K. K.) at 10,000 r.p.m. for 5 minutes. 
Thereto, 150 g of water was further added, and the reaction was run at 
40.degree. C. for 3 hours to prepare a capsule solution having a capsule 
size of 0.7 micron. 
Preparation of Emulsified Dispersion of Color Developer 
The color developers (a), (b) and (c) having the following structural 
formulae were dissolved in amounts of 8 g, 4 g and 30 g, respectively, 
into a mixed solvent composed of 8.0 g of 1-phenyl-1-xylylethane, and 30 g 
of ethyl acetate. The resulting color-developer solution was mixed with a 
water solution containing 100 g of a 8% aqueous solution of polyvinyl 
alcohol, 150 g of water and 0.5 g of sodium dodecylbenzensulfonate, and 
emulsified at ordinary temperature using an Ace Homogenizer (produced by 
Nippon Seiki K. K.) at 10,000 r.p.m. for 5 minutes to prepare an 
emulsified dispersion having a droplet size of 0.5 micron. 
##STR9## 
Preparation of Heat Sensitive Material 
A 5.0 g portion of the foregoing capsule solution, a 10.0 g portion of the 
foregoing emulsified dispersion of color developers and 5.0 g of water 
were mixed with stirring, and coated on one side of a 75 micron-thick 
transparent polyethylene terephthalate (PET) support at a coverage of 10 
g/m.sup.2 on a solids basis, and dried to form a heat sensitive layer. 
Then, a layer having the following composition was coated on the other 
side of the support as a backing layer at a coverage of 1 g/m.sup.2 on a 
solids basis. 
Composition of Backing Layer 
Styrene-maleic acid copolymer (Polymalon 385 produced by Arakawa Kagaku K. 
K.): 1 part (on solid basis), 
Fine grain dispersion consisting of 10 pts.wt. of SnO.sub.2 powder 
exemplified in this specification, 50 pts.wt. of a 10 wt % of aqueous 
solution of polyvinyl alcohol (PVA 105, produced by Kurare Co., Ltd.) and 
100 pts.wt. of water: 0.05 part. 
While thermal printing was performed on the thus obtained heat sensitive 
recording material using a commercially available printer under the dusty 
atmosphere of 10.degree. C., 40% RH, unprinted spots were not observed, 
and any problem was not found. 
EXAMPLE 2 
Another heat sensitive material was prepared in the same manner as in 
Example 1, except a subbing layer having the following composition was 
coated beneath the heat sensitive layer at a coverage of 1 g/m.sup.2 
instead of providing the backing layer. 
Composition of Subbing Layer 
Polyvinyl alcohol (PVA 117, produced by Kurare Co., Ltd.): 1 part 
Glutaraldehyde: 0.002 part 
Dispersion of ZnO fine grains prepared by mixing 10 pts.wt. of ZnO paste 
exemplified in this specification, 25 pts.wt. of a 10 wt % of aqueous 
solution of polyvinyl alcohol and 100 pts.wt. of water: 0.05 part. 
As the result of the same evaluation test as in Example 1, it was confirmed 
that no problem was found in thus obtained heat sensitive recording 
material also. 
COMATIVE EXAMPLE 
Still another heat sensitive recording material was prepared in the same 
manner as in Example 1, except the fine grain dispersion added to the 
backing layer in Example 1 was not used at all. 
While thus obtained heat sensitive recording material underwent the same 
evaluation test as in Example 1, adhesion of dust to the surface of the 
recording material occurred, and thereby some spots where a thermal head 
was not able to come to direct contact with the recording material surface 
were generated during thermal printing to result in generation of 
unprinted spots.