Exothermic conductive coating and heating device incorporating same

An exothermic conductive coating comprises carbon particles, a substantial portion being spherical particles having a diameter of not more than 500 .mu.m, and a synthetic resin. A conductive heating unit is obtained by coating or impregnating a desirably-shaped solid or solid surface with the coating and curing it. The heating unit provides a uniform temperature distribution at any selected temperature up to about 450.degree. C., and can be formed in various shapes.

BACKGROUND OF THE INVENTION 
(1) Technical Field 
The present invention relates to an exothermic conductive coating, 
particularly to an exothermic conductive coating for providing a 
conductive heating unit which can generate a uniform temperature 
distribution at any temperature up to about 450.degree. C. and the 
temperature of which is adjustable, and a conductive heating unit obtained 
therefrom. 
(2) Background Information 
Japanese Patent Publication No. 60-59131/1985 discloses a planar electric 
heating element comprising a synthetic resin film having a conductive fine 
powder of carbon, such as shell-like, flake-like, needle-like or 
fiber-like carbon, or graphite incorporated therein and electrode wires 
buried in the film at both ends in the longitudinal direction thereof. 
There is known a heating unit which comprises a solid lined with this 
element and the temperature of which can be increased to about 60.degree. 
C. 
However, for the heating unit containing the carbon black or graphite 
powder and the synthetic resin, the distance between electrodes on a 
coating film is narrow, for example, and a large heating surface having a 
uniform temperature distribution can not be obtained. In the heating unit 
wherein the conductive fine powder such as the conventional carbon or 
graphite powder is used, there is utilized the tape-shaped heating element 
which is formed by melt extrusion from the synthetic resin having this 
powder incorporated therein. Heretofore a paste or coating containing this 
conductive fine powder has not been used nor such coating applied to a 
surface to prepare a heating unit having a large heating surface. 
When heat radiation is is blocked from the heating surface, the 
conventional heating unit is in danger of local oxidation or damage by 
burning. Therefore, the temperature of this unit can only be increased to 
a temperature below about 60.degree. C. 
In a conventional heating unit, a substrate 1 is lined with a planar 
heating element (tape) 2 as shown in FIG. 10(a). The supply of electricity 
through metal terminals 3 causes the heating part (element 2) to be heated 
to produce a temperature distribution 6 on the substrate as shown in FIG. 
10(b). 
Thus, for the heating unit containing the conventional conductive powder 
such as a shell-like, flake-like, needle-like or fiber-like carbon or 
graphite powder, a large heating surface having a uniform temperature 
distribution can not be obtained. When the substrate is coated with the 
paste or coating containing such an conductive powder, the thickness of 
the coating film must be precisely controlled. This requires that the 
coating be applied by a suitable machine to achieve the precise thickness, 
for example, of 1/10 to 1/100 mm. The coating can not be manually applied. 
According to the conventional heating unit, the more electric current is 
supplied to the thicker portion when the thickness of the coating film is 
varied, and consequently the temperature of that portion is elevated. 
Further, the resistance slightly increases with an increase in temperature 
(FIG. 1(b)). Local overheating is therefore expected, when uneven action 
of heat radiation is exerted. In order to prevent this overheating, 
measures such as the use of thermostats and the incorporation of 
temperature controllers are taken. However, it is impossible to anticipate 
where heat radiation is locally prevented on the large surface. Moreover, 
it is impossible to estimate the number of such local portions and to 
mount a number of sensors thereto. Therefore, the planar electric 
resistance heating unit having these conventional conductive fine powders 
has not proven popular. 
According to the prior art, the paste or coating to be precisely applied by 
a machine. In an electric resistance heating unit having a curved surface, 
an inner surface of a hole or an uneven surface on which the paste or 
coating can not be applied by machine, the local overheating as described 
above takes place. It is therefore very difficult to produce the electric 
resistance heating unit satisfactorily by the prior art. 
Consequently, there has long been the need for an exothermic conductive 
coating or paste using improved conductive carbon material to provide an 
electric resistance heating unit with a large heating surface on which a 
uniform temperature distribution can be obtained, even if a substrate has 
a complex structure such as a curved surface, an inner surface of a hole 
or an uneven surface, and in which the substrate is coated with the paste 
or coating to a by hand or by impregnation, the local damage by melting or 
by burning does not take place, and the heating temperature can be freely 
controlled. 
SUMMARY OF THE INVENTION 
The present inventor has studied various exothermic conductive pastes or 
coatings for producing excellent heating units, particularly the type, the 
shape and the size of carbon powders which are most preferable in chemical 
resistance and sanitation as the conductive material, resins which are 
binders therefor, the compounding ratio thereof, and the combination of 
heat treating processes, coating processes and the like. As a result, it 
has been found that the problems described above are solved by a paste or 
coating comprising a synthetic resin and carbon particles, a substantial 
portion of such particles having a specific shape and crystalline 
structure, and that an excellent heating unit can be produced, thus 
arriving at the present invention. 
In accordance with the present invention, there are provided (1) an 
exothermic conductive coating comprising carbon particles, mainly 
spherical particles having a diameter of not more than 500 .mu.m, and a 
synthetic resin, (2) a conductive heating unit comprising an exothermic 
conductive coating film on a desirably shaped solid or solid surface 
having electrode terminals mounted therein, said film comprising carbon 
particles, mainly spherical particles having a diameter of not more than 
500 .mu.m, and a synthetic resin, (3) a conductive heating unit comprising 
an exothermic conductive coating film on a desirably shaped solid or solid 
surface having electrode terminals mounted thereon, said film comprising 
carbon particles, mainly spherical particles having a diameter of not more 
than 500 .mu.m, and a synthetic resin, and further comprising one or more 
exothermic layers laminated thereon, each of which has electrode terminals 
and an exothermic conductive coating film, (4) a process for producing a 
conductive heating unit, which comprises coating or impregnating a 
desirably shaped solid or solid surface having electrode terminals mounted 
thereon with an exothermic conductive coating or paste, said coating or 
paste comprising carbon particles, mainly spherical particles having a 
diameter of not more than 500 .mu.m, and a synthetic resin, and then 
curing the coating or paste to form an exothermic coating film, and (5) a 
process for producing a conductive heating unit, which comprises coating 
or impregnating a desirably shaped solid or solid surface having electrode 
terminals mounted thereon with an exothermic conductive coating or paste, 
said coating or paste comprising carbon particles, mainly spherical 
particles having a diameter of not more than 500 .mu.m, and a synthetic 
resin, then curing the coating or paste to form an exothermic coating 
film, subsequently further fixing electrode terminals thereon, followed by 
coating or impregnating treatment with said exothermic conductive coating 
or paste, and curing the coating or paste to form an exothermic layer, and 
repeating this procedure to laminate the plural exothermic layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Carbon particles used in the present invention are necessary to contain at 
least 60% by weight of spherical particles. The carbon particles used in 
the conventional heating unit are flake-like, needle-like, fiber-like or 
shell-like, or most of them have these shapes. There has been no instance 
in which spherical carbon particles are used as an exothermic coating. 
From the conventional coating in which the flake-like, needle-like, 
fiber-like or shell-like carbon particles are used, a heating unit having 
a large overheating surface without local heating and with a uniform 
temperature distribution can not be obtained. Further, a heating unit 
having the temperature self-controlling property has not been obtained, 
because the so-called temperature coefficient of electric resistance is 
low (FIG. 1, curve b). 
As the carbon particles used in the present invention, spherical graphite 
particles are preferable. 
The spherical carbon particles used in the present invention contain at 
least 60% by weight of particles having a diameter of not more than 500 
.mu.m, practically 1 to 200 .mu.m. If the carbon particles have a diameter 
of more than 500 .mu.m, the carbon particles are heterogeneously dispersed 
and accordingly temperature unevenness is unfavorably apt to take place. 
As the spherical carbon particles used in the present invention, there are 
used the particles of which interplanar spacing is decreased to not more 
than 3.425 to 3.358 .ANG., preferably 3.380 to 3.358 .ANG., by the heat 
treatment at a temperature of 1500.degree. to 3500.degree. C. (FIG. 7). 
The particles having an interplanar spacing of less than 3.358 .ANG. are 
more preferable, but the cost increases. If the spacing is higher than 
3.425 .ANG., the resistance increases and Watt/cm.sup.2 does not rise even 
if the voltage is raised (for example, up to 0.05 Watt/cm.sup.2). This is 
therefore unfavorable, because of difficulty of elevating temperature (for 
example, up to 20.degree. C.). The spherical graphite particles are 
preferable to be heat treated at a temperature of at least 1500.degree. C. 
and have a specific resistance of not more than about 1300 to 5000 
.mu..OMEGA. cm. The particles having a specific resistance of less than 
1300 .mu..OMEGA.cm are more preferable, but the cost increases. 
The process for preparing the spherical carbon particles used in the 
present invention has been reported by Taylor et al. [Brooks and Taylor, 
Carbon 3, 185 (1965)]. There has recently been proposed the utilization of 
the spherical carbon particles for special carbon materials, intercalation 
compounds, adsorption materials, fillers and the like. However, there has 
been no instance in which-the spherical carbon particles are used as an 
exothermic coating, as described above. The present inventor has found out 
for the first time that an excellent effect is obtained by the spherical 
carbon particles. 
The spherical carbon particles used in the present invention may be 
prepared by any process, for example, by heating petroleum, coal or 
organic compounds to a high temperature, and carbonizing or coking them, 
followed by graphitization. 
For example, the spherical carbon particles are also prepared, according to 
the process of Taylor et al., by heat treating bitumen such as coal tar, 
coal tar pitch, petroleum heavy gravity oil or the like, at a temperature 
of 350.degree. to 500.degree. C. for a long period of time, repeating the 
polycondensation reaction of the low molecular compound to polymerize, and 
heat treating for reduction meso carbon micro beads obtained by separating 
optical an isotropic spherical particles from the resulting carbonaceous 
material or approximately spherical coke obtained by carbonizing a 
synthetic resin, at a temperature of one thousand and hundreds to three 
thousand and hundreds for graphitization. The specific resistance is in 
the range of 1300 to 6000 .mu..OMEGA.cm, and selected in accordance with 
the application for high resistance or low resistance. 
The carbon particles used in the present invention are preferable to be 
heat treated at a temperature of at least 1500.degree. C. The heat-treated 
carbon particles are necessary to obtain the practical electric 
conductivity of the coating film and the uniform dispersion of the carbon 
particles in a liquid coating comprising the carbon particles, a solvent 
and a synthetic resin or a powdery coating comprising the carbon particles 
and a synthetic resin. 
The synthetic resin used in the present invention, which is a binder, may 
be a thermoplastic, a thermosetting or an electron beam curable resin, and 
can be suitably selected according to the application fields of the 
heating unit. 
As the thermoplastic resin, there is used a resin having a softening point 
of at least 15.degree. C. and an average molecular weight of several 
thousands to several hundred thousands. As the thermosetting or the 
reaction type resin, there is used a resin having a molecular weight of 
not more than 200,000 in a state of a coating liquor. This resin is heated 
after coating and drying, and accordingly its molecular weight approaches 
infinity by the reaction such as condensation or addition. Further, there 
can be used the electron beam curable resin,in which the radical 
cross-linkable or polymerisable to dryness by the radiation exposure is 
contained or introduced in the molecule of the thermoplastic resin. Such a 
radical includes an acrylic double bond contained in acrylic acid, 
methacrylic acid or the esters thereof, which shows radical polymerisable 
properties, an allylic double bond contained in diallyl phthalate or the 
like and an unsaturated bond contained in maleic acid, the derivatives 
thereof or the like. 
As the synthetic resin, there can be mentioned, for example, a polyamide 
resin, a polyamide resin, a polyphenylene oxide resin, a silicone resin, a 
polytitanocarbosilane resin, a phenol resin, an epoxy resin, a 
polypararbanic acid resin, a polyurethane resin, a polyester resin, a 
polyether-etherketone resin, a polyphenylene sulfide resin, a 
fluorine-containing polymer, a polyolefin resin and a polyvinyl chloride 
resin. There can be selected a resin having a softening temperature or a 
decomposition temperature desired for the coating film. 
The ratio of the synthetic resin to the carbon particles is variously 
selected depending on the desired heating temperature, the area of the 
heating surface, the kind of carbon particles and synthetic resin, the 
combination thereof and the like. However, the synthetic resin is 
generally used in the ratio of 25 to 220 parts by weight, preferably 30 to 
200 parts by weight, to 100 parts by weight of the carbon particles. 
When the ratio of the synthetic resin is less than 25 parts by weight, the 
electric resistance value decreases and the temperature of the heating 
unit can be elevated (therefore, applicable to the heating unit having a 
large heating surface). However, the strength of the coating film is 
insufficient and the temperature coefficient of electric resistance is 
decreased to be liable to produce temperature unevenness. On the other 
hand, when the ratio of the synthetic resin is more than 220 parts by 
weight, the electric resistance value necessary for heating can not be 
obtained (because of the excessive electric resistance value), which 
causes the coating to be unsuitable for practical use. That is to say, 
when the electric resistance is less than 1 .OMEGA./.sub..quadrature. at 
ordinary temperature, wherein .OMEGA./.sub..quadrature. represents 
electric resistance value per square area, the electric current 
excessively flows, and accordingly the temperature becomes too high and 
uneven. In case of more than 6,000 .OMEGA./.sub..quadrature., the electric 
current flow becomes too little, and therefore the generation of heat is 
so depressed that a desired temperature is difficult to be obtained. 
In case of the large heating surface, the coating showing a low electric 
resistance such as 1 .OMEGA./.sub..quadrature. at ordinary temperature is 
used. In case of the small heating surface, the coating showing a high 
electric resistance such as 6,000 .OMEGA./.sub..quadrature. at ordinary 
temperature is used. In general, the coating showing an intermediate value 
there between is used. According to the present invention, the surface 
temperature of the heating unit is stably heated at a desired temperature 
of at most 450.degree. C. (an environmental temperature+30.degree. C. to 
-40.degree. C.) for a long time according to the combination of the shape 
of graphite, the heating temperature, the compounding of the coating, the 
thickness of the coating film, the applied voltage and the like. 
This coating comprising the carbon powders and the synthetic resin is 
applied by the various coating methods such as brushing, roller coating, 
spray coating, electrostatic coating, electrode position coating and 
powder coating, or by the dipping method. To the coating, another additive 
or auxiliary agent may be added. 
The additive or auxiliary agent includes, for example, a diluting solvent, 
a suspending agent or a dispersant, an antioxidant, another pigment and 
another necessary additive. 
As the diluting solvent, there are employed the solvent usually used in the 
coating such as an aliphatic hydrocarbon, an aromatic petroleum naphtha, 
an aromatic hydrocarbon (toluene, xylene or the like), an alcohol 
(isopropyl alcohol, butanol, ethylhexyl alcohol or the like), an ether 
alcohol (ethyl cellosolve, butyl cellosolve, ethylene glycol monoether or 
the like), an ether (butyl ether), an acetate, an acid anhydride, an ether 
ester (ethyl cellosolve acetate), a ketone (methyl ethyl ketone, methyl 
isobutyl ketone), N-methyl-2-pyrrolidone, dimethylacetamide and 
tetrahydrofuran. The preferred solvent is suitably selected depending on 
the synthetic resin as the binder. The amount of the diluting solvent is 
selected in the range of 400 parts by weight or less per 100 parts by 
weight of the resin. 
As the suspending agent, there can be mentioned methyl cellulose, calcium 
carbonate, modified bentonite fine powder and so on. As the dispersant, 
there can be used various surface-active agents such as an anionic 
surface-active agent (a fatty acid salt, a liquid fatty oil sulfate salt), 
a cationic surface-active agent (an aliphatic amine salt, a quaternary 
ammonium salt), an amphoteric surface-active agent and a nonionic 
surface-active agent. In order to achieve solidification by drying or 
curing of the coating or paste with ease for a short time, a curing agent 
may be added. 
The curing agent is selected according to the resin used, and there is used 
the conventional curing agent such as an aliphatic or aromatic polyamine, 
a polyisocyanate, a polyamide, an amine or thiourea. 
In addition, a stabilizer, a plasticizer, an antioxidant or the like is 
suitably used. 
The solid made of a substrate such as a plastic material, a ceramic 
material, a woody material, a fibrous material, a paper material, a metal 
material coated with an electric insulator or the like is a desired shape 
or the surface thereof is coated with the present exothermic conductive 
coating or dipped in it to produce the heating unit. 
For example, the substrate made of a metal material coated with an electric 
insulator, a ceramic material, a plastic material, a woody material or the 
combination thereof, to which at least two metal terminals are securely 
attached, is coated with the coating of the present invention to a 
thickness of about 0.2 to 3.5 mm (the thickness of the coating film after 
curing is 0.1 to 0.3 mm). 
The shape of the plane surface or the curved surface of the substrate above 
described is not particularly limited. The heating unit may be produced 
from the linear, rod-like, cylindrical, plane or another 
three-dimensionally curved substrate. 
Although it is desirable to coat the substrate surface with a ceramic 
material, a woody material is sometimes usable if a desired temperature is 
below 150.degree. C. There is also usable a combined article such as a 
composite comprising a woody material, a plastic material or a metal 
material and a ceramic material applied thereon. 
When the solid surface to be coated is large and there is adopted brushing, 
roller coating or spray coating, the fluidity of the coating is increased 
to improve the workability. In this case, a solvent for dilution is 
preferably incorporated in an amount of less than 400 parts by weight per 
100 parts by weight of the conductive powder. If more solvent is 
incorporated, the coating is too much fluidized and it is difficult to 
obtain the prescribed thickness of the coating film. Therefore, the use of 
excessive solvent is unsuitable for obtaining a desired surface 
temperature of the coating film. 
The coating film is cured at a temperature ranging from about 70.degree. to 
350.degree. C. or dried to solidification, or cured by electron beams 
(radiation). 
When the drying to solidification or the curing is conducted at a 
temperature ranging from 70.degree. to 350.degree. C. for an ample time, 
the smooth film having a prescribed thickness can be obtained. The 
solidification or the curing at a temperature higher than that is 
undesirable, because foaming, flowing and deterioration are liable to take 
place, and the solidification or the curing at a temperature lower than 
70.degree. C. is also undesirable, because it requires a lot of time. 
When the coating is applied to a thickness of about 0.2 to 3.5 mm and then 
allowed to react for curing at a temperature of not more than 350.degree. 
C., the coating film dried to solidification and having a thickness of 
about 0.1 to 3.0 mm is obtained. This electric resistance heating coating 
film generates high temperature as well as low temperature. It is 
preferred that the coating is applied to a thickness of about 0.1 to 3.0 
mm. If the thickness is less than 0.1 mm, the electric resistance 
increases too high, the wattage per unit area decreases too low, and 
further the film strength is insufficient. When the thickness is more than 
3.0 mm, segregation is liable to occur by the precipitation of particles 
and therefore the uniform coating film is difficult to be obtained. The 
electric resistance between the metal terminals on this coating film is 1 
to 6000.OMEGA./.sub..quadrature. at ordinary temperature as described 
above. When the electric resistance is low, this film also becomes a 
conductive film. 
If there is a fear of leak, the exothermic coating film is covered with an 
electric insulating film thinly so far as the strength is maintained. Too 
thick a film results in disturbance of heat transfer. 
The heating unit is similarly prepared by treating a fibrous material or a 
paper material with the coating or paste of the present invention 
comprising the spherical graphite and the synthetic resin. 
Also, the heating unit having excellent surface properties can be obtained 
by the use of the electron beam (radiation) curable resin. 
According to the exothermic conductive coating of the present invention, 
the temperature of the heating unit is adjustable to a desired 
temperature, by the selection of the kind of carbon particles and 
synthetic resin, the compounding ratio, the thickness of the coating film 
and the combination thereof, and further the selection of the heating area 
or the applied voltage. 
This is due to the selection of the spherical carbon particles in the 
present invention. The conventional heating unit in which the flake-like, 
needle-like, shell-like or fiber-like carbon or graphite is used can not 
possibly obtain this effect. 
When the exothermic conductive coating of the present invention is used, 
the heating unit can be obtained by laminating the exothermic films, 
whereby the electric resistance is adjustable and the exothermic area can 
be doubled at the same temperature as described below. Further, the 
heating unit having the same exothermic area at the same temperature can 
be obtained by laminating the exothermic films to adjust the voltage. 
______________________________________ 
Exo- 
thermic 
Exo- 
Vol- Resist- Exothermic 
tem- thermic 
tage ance value perature 
area 
______________________________________ 
First 120 V 20.OMEGA./.sub..quadrature. .fwdarw. 
720 Watt 
220.degree. C. 
1200 cm.sup.2 
layer 
Second 
120 V 10.OMEGA./.sub..quadrature. .fwdarw. 
1440 Watt 
220.degree. C. 
2400 cm.sup.2 
layer 
Third 120 V 6.66.OMEGA./.sub..quadrature. .fwdarw. 
2160 Watt 
220.degree. C. 
3600 cm.sup.2 
layer 
First 120 V 20.OMEGA./.sub..quadrature. .fwdarw. 
720 Watt 
220.degree. C. 
1200 cm.sup.2 
layer 
Second 
85 V 10.OMEGA./.sub..quadrature. .fwdarw. 
720 Watt 
220.degree. C. 
1200 cm.sup.2 
layer 
Third 69 V 6.66.OMEGA./.sub..quadrature. .fwdarw. 
720 Watt 
220.degree. C. 
1200 cm.sup.2 
layer 
______________________________________ 
As the electrode terminal used in the heating unit of the present 
invention, any type of terminal can be used. For example, the electrode 
terminal of metal wire or metal net can be used (FIGS. 11(a) and 11(b)). 
Particularly, the terminal of metal net as shown in FIG. 11(b) is 
preferred, which includes, for example, a copper net having an opening 
size of 0.3 mm.times.0.3 mm and composed of Ni-plated copper wires having 
a diameter of about 0.2 mm. This terminal of the metal net permits the 
heating unit having more stable exothermic temperature to be obtained. 
The exothermic conductive coating has temperature self-controlling 
function. Thus, the thickness of the coating film need not be precisely 
uniform and the coating film can be manually formed on the solid surface 
of a desired shape. Further, the heating unit can be produced by dipping 
of the impregnatable solid material having a desired shape such as a 
fibrous material or a paper material. Therefore, the heating unit of the 
present invention can be widely utilized in various fields such as an 
interior wall application, flooring, roofing, a furnace inner surface use, 
pipe inner and outer surface applications, carpets, blankets, simplified 
heaters, warmers and antifreezers. Particularly, this heating unit is 
excellent as the parts for room heating, hot insulation and heating. 
The exothermic conductive coating mainly comprises the spherical carbon 
particles and the synthetic resin. Therefore, there can be produced 
therefrom the heating unit which has the temperature self-controlling 
function, the temperature of which can be adjusted up to about 450.degree. 
C., and further has a uniform temperature distribution over a large 
heating surface as well as a small heating surface, in various shapes and 
surfaces containing an uneven surface and the like. Further, the heating 
unit can be constituted by plural laminated layers of the coating film. 
Therefore, the heating unit thus obtained is suitable for wide 
applications, such as for an interior wall application, flooring, roofing, 
pipe inner and outer surface applications, a furnace inner surface use, 
heaters and carpets. 
The present invention will now be described in detail with reference to the 
following examples that by no means limit the scope of the invention. 
EXAMPLE 1 
Using PTFE (polytetrafluoroethylene) as a synthetic resin for a binder, a 
coating (a) was prepared by mixing therewith 1 part by weight of spherical 
graphite particles of the present invention with diameters of 20 to 50 
.mu.m per 0.9 part by weight of the resin solid. On the other hand, a 
coating (b) was prepared by mixing 1 part by weight of the conventional 
needle-like graphite powder having sizes of 10 to 60 .mu.m with the same 
resin solid. Each coating was used as an exothermic conductive coating. 
These coatings (a) and (b) were applied on solid surfaces, respectively, to 
a thickness of about 0.6 mm to produce heating units. 
The relationships between the electric resistance .OMEGA./.sub..quadrature. 
of these heating units and the surface temperature thereof are shown in 
FIG. 1. 
As apparent from FIG. 1, in case of the exothermic conductive coating (a) 
of the present invention, an about 30-fold increase of the electric 
resistance was observed at 120.degree. C. This sudden increase of a 
temperature coefficient of electric resistance at 100.degree. C. shows the 
action of the temperature self-control. 
In contrast, with respect to the coating (b) in which the conventional 
needle-like graphite powder is used, the electric resistance was little 
increased with an increase of temperature. This shows that the 
conventional needle-like graphite provides a very low temperature 
coefficient of electric resistance. Therefore, when a heat insulating 
member is placed on the heating unit, the electric current does not 
decrease and the continuous temperature increasing produces overheated 
spots. The flake-like, fiber-like and shell-like graphite powders also 
showed the same tendency as that of the needle-like graphite powder. 
As shown in FIG. 2, a heat insulating member 4 (ceramic wool) was placed on 
the surface of the coating film 2 through which the electric current was 
passed to heat at 120.degree. C., and the temperatures at the point A and 
the point B under the heat insulating member were measured. FIG. 3 shows 
the temperature differences between the temperatures at the point B and 
the point A of the heating units obtained from the coatings (a) and (b) 
according to the heating time when 0.55 Watt/cm.sup.2 of electric power is 
fed. The heating unit obtained from the exothermic conductive coating (a) 
of the present invention showed only an increase of about 3.degree. C. 
(123.degree. C.-120.degree. C.=3.degree. C.). In contrast, the heating 
unit of the conventional exothermic conductive coating (b) showed an 
increase of about 104.degree. C. (222.degree. C.-118.degree. 
C.=104.degree. C.). As apparent from this, it was shown that the 
exothermic film of the exothermic conductive coating of the present 
invention had the temperature self-controlling function without the 
generation of overheating, even if the heat radiation was locally 
disturbed. 
EXAMPLE 2 
A heating unit having a 1.5 mm-thick coating film was obtained from a 
coating in which 2.2 parts by weight of PTFE solid was mixed with 1 part 
by weight of spherical graphite particles of which maximum diameter was 
600 .mu.m and mean diameter was 500 .mu.m. When a voltage of 100 V was 
applied to this heating unit, a sudden increase of electric resistance 
caused no rise of temperature. When room temperature was 30.degree. C., a 
temperature unevenness of 70.+-.30.degree. C. took place on an exothermic 
surface of 100 cm.sup.2, and only a local rise of temperature was 
observed. In a similar experiment in which 2 parts by weight of PTFE solid 
was mixed with 1 part by weight of spherical graphite particles of which 
maximum diameter was 500 .mu.m and mean diameter was 400 .mu.m, a 
temperature unevenness was reduced to 75.+-.12.degree. C. This example 
showed the limits of size of the graphite particles and compounding amount 
of the synthetic resin for homogenizing temperature. 
EXAMPLE 3 
There was formed a 1 mm-thick exothermic conductive coating film in which 
0.3 part by weight of PEEK (polyetheretherketone resin) solid was mixed 
with 1 part by weight of carbon particles containing 0.6 part by weight of 
spherical graphite particles with an average particle diameter of 30 .mu.m 
(an interplanar spacing of 3.36.+-.0.02 .ANG.) and 0.4 part by weight of 
needle-like graphite particles with an average particle diameter of 30 
.mu.m. When 0.7 Watt/cm.sup.2 of electric power was applied to this film, 
the electric resistance was about 210 .OMEGA./.sub..quadrature. even at 
260.degree. C., which was 7 times that at ordinary temperature. When heat 
insulating wool was locally placed on the film, the temperature thereunder 
rose to 290.degree. C. In a coating film containing 0.25 part by weight of 
PEEK, the electric resistance became 105.OMEGA./.sub..quadrature. at 
260.degree. C., which was 4 times that at ordinary temperature. When heat 
insulating wool was locally placed on the film, the temperature thereunder 
rose beyond 300.degree. C., which caused the deterioration of the film. 
60% by weight of the spherical graphite particles (in the carbon 
particles) and 30 parts by weight of the synthetic resin (per 100 parts by 
weight of the carbon particles) are lower limit values at which the 
temperature self-controlling function acts. 
EXAMPLE 4 
Exothermic conductive films having a thickness of 0.5 mm were formed from 
the coatings containing 100 parts by weight of spherical graphite 
particles and up to 200 parts by weight of each synthetic resin of 
polyester, epoxy, polyamide, polyimide, polyethylene, fluorine-containing 
polymers, polyetheretherketone, polyphenylene sulfide, silicone and 
polytitanocarbosilane resins. When the electric resistance at 30.degree. 
C. was measured, the resistance increased with an increase of the 
synthetic resin, as shown in FIG. 4. The coarse particles (100 .mu.m) 
showed lower values (a), and the fine particles (1 to 8 .mu.m) showed 
higher values (b). The compounding of 30 to 200% by weight of the 
synthetic resin provides arbitrary resistance in the range of 1 to 6000 
.OMEGA./.sub..quadrature.. 
When the resistance is 6000.OMEGA./.sub..quadrature., the temperature of a 
square of the surface with each side 5 cm long can be raised to 20.degree. 
C. at room temperature of 0.degree. C., 100 V and 1.7 W (1.7 W/5.times.5 
cm.sup.2 =0.07 Watt/cm.sup.2). When the resistance is 3000 
.OMEGA./.sub..quadrature., the temperature of a square of the surface with 
each side 7 cm long can be raised to 20.degree. C. at 100 V and 3.3 W. 
Further, when the resistance is 10 .OMEGA./.sub..quadrature., the 
temperature of a square of the surface with each side 42 cm long is raised 
to 120.degree. C. on applying a voltage of 100 V. 
EXAMPLE 5 
Using exothermic conductive coatings containing 200 parts, 100 parts and 70 
parts by weight of PTFE per 100 parts by weight of spherical graphite 
particles with a diameter of about 50 .mu.m, 0.5 mm-thick coating films 
were formed, and the resistance and the exothermic temperature thereof 
were measured (FIG. 5). As apparent from FIG. 5, the higher content of the 
synthetic resin provides the lower exothermic temperature. When the 
content of PTFE is 200 parts by weight, the maximum exothermic temperature 
is about 30.degree. C. at room temperature of 0.degree. C. ((a) in FIG. 
5). The exothermic temperature rises with a decrease of the synthetic 
resin content. When the content is 100 parts by weight, the exothermic 
temperature is about 120.degree. C. ((b) in FIG. 5). Further, when the 
content is 70 parts by weight, the temperature can be raised to about 
220.degree. C. ((c) in FIG. 5). 
When a heat-resistant polytitanocarbosilane resin is used as this synthetic 
resin, the high temperature up to about 450.degree. C. can be achieved. 
As described above, according to the present invention, the exothermic 
temperature is freely and easily adjustable up to 450.degree. C., 
depending upon the diameter of the spherical carbon particles, the 
compounded amount of the synthetic resin and the kind of synthetic resin. 
EXAMPLE 6 
Exothermic conductive films having a thickness of 0.5 mm were formed from 
the coatings containing 100 parts by weight of spherical graphite 
particles with a diameter of 30 .mu.m and an interplanar spacing of 3.358 
to 3.425 .ANG., and 50 parts, 100 parts and 150 parts by weight of each 
synthetic resin of polyester, epoxy, polyamide, polyimide, polyethylene, 
fluorine-containing polymer, polyetheretherketone, polyphenylene sulfide, 
silicone and polyitanosilane resins. The resistance at 30.degree. C. was 
measured. The results are shown in FIG. 6. As apparent from FIG. 6, 
.OMEGA./.sub..quadrature. rapidly increased at an interplanar spacing of 
3.40 to 3.425 .ANG., and the temperature did not rise, even if a high 
voltage was applied. This is therefore unsuitable for a surface heating 
unit. 
EXAMPLE 7 
As shown in FIG. 8, a solid 1 having a corrugated uneven surface was coated 
with a heat-resistant ceramic material 5, to which Ni-plated copper net 
bands with a width of 7 mm and a net size of 0.2 mm were fixed in parallel 
with each other as electrode terminals 3. An exothermic conductive coating 
was applied thereon in which 100 parts by weight of a one-liquid type 
epoxy resin per 100 parts by weight of spherical graphite particles having 
an average diameter of 30 .mu.m was compounded to fix a cured coating film 
2 having a thickness of about 0.4 mm thereto. 
When a voltage of 100 V was applied between terminals spaced 30 cm apart, 
an approximately uniform temperature distribution 6 of 80.degree. C. (room 
temperature 30.degree. C.+50.degree. C.).+-.4.degree. C. over the whole 
surface was obtained. 
EXAMPLE 8 
As shown in FIG. 9, metal terminals 3 were securely fixed to a 
frusto-conical ceramic body 1 with a wide taper, wherein the diameter of 
the top was 200 mm, the diameter of the base was 300 mm and the height was 
500 mm. Using an exothermic conductive coating in which 0.6 parts by 
weight of PTFE per 1 part by weight of spherical graphite particles with 
an average diameter of 30 .mu.m, there was fixed a cured coating film 2 
having a thickness of 0.5 mm at the smaller diameter portion, a thickness 
of 0.8 mm at the larger diameter portion and an average thickness of about 
0.65 mm. By applying a voltage of 120 V between the terminals, an 
approximately uniform temperature of 220.degree. to 240.degree. C. was 
obtained at room temperature. The use of ten Ni-plated copper wires with a 
diameter of 0.3 mm as the terminals caused an increase of the resistance, 
while continuously heated for a long time. However, the use of nets (with 
a net size of 0.3 mm and a net width of 7.5 mm) composed of Ni-plated 
copper wires with a diameter of 0.2 mm stabilized the resistance, which 
did not change for several thousand hours. When the same net leads of 
copper wires and the same exothermic film were further fixed on this 
exothermic film, the electric resistance was halved. Consequently, the 
approximately similar temperature was obtained, even if the voltage was 
reduced from 120 V to 85 V. 
EXAMPLE 9 
Band leads of nets having a net size of 0.8 mm and composed of Ni-plated 
copper wires with a diameter of 0.3 mm were fixed as the terminals on the 
both end of a square of a 30 mm-thick ceramic plate with each side 1 m 
long. Using an exothermic conductive coating containing 0.6 part by weight 
of PTFE per 1 part by weight of spherical graphite particles with an 
average diameter of 20 .mu.m, a coating film having a thickness of about 
0.8 mm was fixed thereon. When a voltage of 130 V was applied between the 
copper net terminals, a temperature of about 145.degree. C. was obtained. 
Band leads of nets having a net size of 0.8 mm and composed of Ni-plated 
copper wires with a diameter of 0.3 mm were fixed as the terminals on the 
both ends of a square of a 30 mm- thick ceramic plate with each side 1.4 m 
long. Using an exothermic conductive coating containing 0.6 part by weight 
of PTFE per 1 part by weight of spherical graphite particles with an 
average diameter of 20 .mu.m, a coating film having a thickness of about 
0.8 mm was fixed thereon. When a voltage of 130 V was applied between the 
copper net terminals, a temperature of about 70.degree. C. was obtained. 
The same leads were overlapped on that leads and fixed thereon, and the 
same 0.8 mm-thick film was fixed thereon by use of the same coating. The 
upper and lower leads were tied to one. When a voltage of 130 V was 
applied thereto, a temperature of about 103.degree. C. was obtained. The 
lamination of three layers could provide an exothermic surface having a 
three-fold area at the same voltage (FIG. 12).