Note: Descriptions are shown in the official language in which they were submitted.
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Description
CONDUCTIVE COMPOSITION FOR PRODUCING CARBON
FLEXIBLE HEATING STRUCTURE, CARBON FLEXIBLE
HEATING STRUCTURE USING THE SAME, AND MANU-
FACTORING METHOD THEREOF
Technical Field
[1] The present invention relates to a conductive composition in which the
weight ratio
between liquid silicon rubber and conductive carbon black is 100:115, a carbon
flexible heating structure which is obtained by molding the conductive
composition in
a particular shape or by coating the conductive composition on a mold having a
particular shape, and a method of manufacturing the carbon flexible heating
structure.
Background Art
[2] The importance of electrically conductive polymer as one of fields of
functional
polymer has been gradually increased. By providing electrical conductivity to
a
polymer material, the polymer material obtains useful physical and chemical
properties
and not only a functionally superior material, but also a cheap material in
view of
production costs can be obtained.
[3] In general, a number of polymer materials have been regarded as highly
insulating
materials. Although the polymer materials work well as electrically insulating
materials due to a low conductivity, they function as electrical conductors
when a filler
such as carbon black, carbon fiber, or metal powder is added.
[4] The added filler forms an electrical path in the polymer material which
works as a
passage of electrons so that the polymer material becomes an electrical
conductor.
[5] When the temperature increases, the interval between filler particles in
semicrystalline polymer including the conductive filler increases due to a
thermal
expansion in a melting area of the polymer so that the flow of electrons is
disturbed.
[6] Carbon black and carbon fiber are mainly used as the conductive filler
added to
provide a positive temperature coefficient (PTC) function to the polymer.
Crystalline
polymer such as polyethylene is mainly used as the polymer material.
[7] Accordingly, as the temperature increases, the resistance of the polymer
material is
suddenly increased greatly, which is referred to as a static characteristic
temperature
coefficient or a PTC phenomenon. That is, while resistance is relatively low
at a low
temperature, when the temperature reaches a predetermined degree, the
resistance
increases suddenly so that current is difficult to flow. The temperature at
which the
above sudden change occurs is referred to as a switching temperature or Curie
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WO 2006/004282 PCT/KR2005/000914
temperature.
[8] The switching temperature is defined as a temperature corresponding to
double the
minimum resistance value or a resistance value at a reference temperature
(25°C) and
is a major parameter in the property of the material.
[9] Also, changing the component of the material makes the switching
temperature
move toward a high temperature or a low temperature so that the material can
be used
for a variety of devices. For example, the material can be used for a
temperature sensor
or overheat protection using a resistance-temperature property, a heater using
a
current-voltage property, or a delay circuit or a demagnetic circuit using a
current at-
tenuation property.
[10] Of the above application fields, in the case of being used to prevent a
damage to a
product or an electronic circuit due to overheat or the flow of over-current,
the PTC
using polymer can greatly perform both protection functions with respect to
overheat
and overload.
[11] For a fuse used as an overload protection, although it has a superior
protection
function with respect to over-current, when current is discontinued as the
fuse is cut off
due to the over-current, the fuse needs to be replaced, so it is inconvenient.
For a
bimetal switch which provides a superior temperature protection function and a
restoring function, since it is not sensitive to over-charges, it is difficult
to use the
bimetal switch for a precise electronic circuit. Thus, it can be seen that the
PTC using
polymer has a superior property compared to the above members.
[12] The polymer PTC material can be used as a superior PTC material by
compensating
for drawbacks of a conventional ceramic PTC such as a low conductivity, high
process
costs, and a fixed shape. In particular, since the minimum resistance is quite
small and
a manufacturing shape is free, the polymer PTC material has already been
widely used
in designing small devices and the use thereof is fast increasing. The
temperature of
the polymer PTC decreases after heat or current is cut off. Also, the PTC
material has a
function of automatically restoring without being replaced when the over-
current is
removed.
[13] In addition to the above properties of the PTC, a negative temperature
coefficient
(NTC) phenomenon occurs in which resistance decreases greatly when a new con-
ductivity network is formed as the dispersion state of conductive particles in
a melting
state of polymer changes.
[14] Since the property provided to the conductive polymer by the PTC effect
can be lost
by the NTC phenomenon, the NTC phenomenon becomes a great hindrance to the PTC
phenomenon.
[15] The NTC phenomenon occurs when the conductive particles are moved by
cross-
linking in a melting state so that a new structure is formed. The cross-
linking forms a
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network to allow the conductive particles to strongly attract to each other
and restrict
motion of the conductive particles so that a structural stability can be
obtained.
[16] The polymer PTC material is used to prevent damage to electronic products
or
electronic circuits and has already been used in designing small devices
because the
manufacturing shape thereof is free. However, since a cross-linker is added to
restrict
the NTC phenomenon and then the polymer PTC material is cured so that it has a
hard
plastic structure, the polymer PTC material has a limit in the process and
purpose
thereof when being used for a general heating body.
[17] In the semicrystalline polymer including a conductive filler, as the
temperature
increases, since the interval between filler particles in the polymer
increases ac-
cordingly due to thermal expansion in the switching temperature area, an
amplitude
between thermal contraction and thermal expansion that repeat, continuously
occurs up
to a crystalline melting point so that the life span of products are
shortened.
Disclosure of Invention
Technical Problem
[18] To solve the above and/or other problems, the present invention provides
a carbon
flexible heating structure having superior physical and chemical properties
such as heat
resistance, winter-hardiness, ozone resistance, electricity insulation, and
flexibility, a
conductive composition used therefor, and a method of manufacturing the carbon
flexible heating structure.
[19] The present invention provides a method of manufacturing the carbon
flexible
heating structure which can reduce manufacturing costs by simplifying a manu-
facturing process.
[20] The present invention provides a carbon flexible heating structure in
which a
phenomenon of peeling off of the structure does not occur even when a periodic
change between thermal expansion and thermal contraction repeats, by mixing
and
agitating only a diluent and liquid silicon rubber that is the same material
as the
conductive composition and coating the mixture on a surface of the carbon
flexible
heating structure, as necessary, for insulation.
[21] The present invention provides a carbon flexible heating structure which
can be
used in a variety of fields by making a frame mold into a variety of shapes
such as a
mesh shape, a plate shape, a rod shape, a ring shape, or a bar shape during
the manu-
facturing of the carbon flexible heating structure.
Technical Solution
[22] According to an aspect of the present invention, a conductive composition
formed
of a mixture of liquid silicon rubber and conductive carbon black or liquid
silicon
rubber and graphite powder wherein weight ratios between the liquid silicon
rubber
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and the conductive carbon black and the liquid silicon rubber and the graphite
powder
are 100:115 and 100:10150, respectively.
[23] The thermal expansion coefficient of the liquid silicon rubber is 200x10
6~K-1
through 300x10 6~K 1
[24] The size of a particle of the conductive carbon black is 20 through 40 nm
and the
amount of absorption of dibutyl phthalate (DBP) is 300 through 50 ml/100g. The
size
of a particle of the graphite powder is 1 through 10 mm and electrical
resistance is
0.0005 through 0.08 S2~cm.
[25] A method of manufacturing a carbon flexible heating structure comprises
mixing a
conductive composition formed of liquid silicon rubber and a filler, agitating
a mixture
of the liquid silicon rubber and conductive carbon black by adding a diluent
at a rate of
1100% with respect to the weight of the liquid silicon rubber, and molding the
mixture into a particular shape and curing the molded mixture.
Advantageous Effects
[26] As described above, the carbon flexible heating structure according to
the present
invention and a conductive composition for manufacturing the same have
superior
phisical and chemical properties such as heat resistance, winter-hardiness,
ozone
resistance, and electricity insulation, and have a self-control resistance
heating function
and superior flexibility, so that the number of application fields of the
carbon flexible
heating structure according to the present invention are drastically
increased.
[27] The carbon flexible heating structure according to the present invention
can provide
an economic manufacturing method by simplifying the manufacturing steps to
lower
the manufacturing costs.
[28] In the carbon flexible heating structure according to the present
invention, a
phenomenon of peeling off of the structure does not occur even when a periodic
change between thermal expansion and thermal contraction repeats, by mixing
and
agitating only a diluent and liquid silicon rubber that is the same materal as
the
conductive composition and coating the mixture on a surface of the carbon
flexible
heating structure, as necessary, for insulation.
[29] When manufactured, the carbon flexbile heating structure may be used in a
variety
of fields by molding the structure into a variety of shapes in the step of
molding or by
making a frame mold into a variety of shapes such as a mesh shape, a plate
shape, a
rod shape, a ring shape, or a bar shape.
Brief Description of the Drawings
[30] FIG. 1 is a flow chart for explaining a manufacturing process of a carbon
flexible
heating structure according to an embodiment of the present invention;
[31] FIG. 2 is a plan view illustrating a structure of a carbon flexible
heating mesh
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according to an embodiment of the present invention;
[32] FIG. 3 is a cross-sectional view illustrating a fine structure of carbon
flexible
heating mesh of FIG. 2;
[33] FIG. 4 is a view illustrating a fine structure of a conductive
composition according
to an embodiment of the present invention;
[34] FIG. 5 is a view illustrating a fine structure of the conductive
composition shown in
FIG. 4 in a state in which the temperature is higher than room temperature;
[35] FIG. 6 is a graph showing a temperature-resistance property of a
conventional PTC
device; and
[36] FIG. 7 is a graph showing a temperature-resistance property of the carbon
flexible
heating structure of FIG. 1.
Best Mode for Carrying Out the Invention
[37] With reference to the accompanying drawings, an embodiment of the present
invention will be described in detail with respect to a case in which a carbon
flexible
heating structure using a conductive composition obtained by mixing liquid
silicon
rubber and conductive carbon black is molded in the form of a mesh.
[38] FIG. 1 is a flow chart for explaining a manufacturing process of a carbon
flexible
heating structure according to an embodiment of the present invention.
Referring to
FIG. 1, the manufacturing process includes mixing liquid silicon rubber and
conductive carbon black (Operation 110), agitating by adding a diluent to a
mixture of
liquid silicon rubber and conductive carbon black (Operation 120), and molding
and
curing by pasting or coating the mixture on a structure having a particular
shape
(Operation 130).
[39] In the mixing operation 110, liquid silicon rubber and conductive carbon
black are
mixed at a mixture ratio of about 100:115 based on a weight ratio thereof.
Next, in
the agitating operation 120, a diluent is added to the mixture of liquid
silicon rubber
and conductive carbon black and the mixture is agitated. Toluene or xylene is
mainly
used as the diluent. The diluent added to the mixture in the agitating
operation 120 is
preferably within a range of about 0~ 100% with respect to the weight ratio of
the
liquid silicon rubber. In the agitating operation 120, when the content of
carbon black
is small, flexibility of the conductive composition is obtained without adding
the
diluent. However, since the flexibility is deteriorated as the content of
carbon black
increases, the flexibility of the conductive composition is improved by adding
the
diluent and agitating the mixture. The conductive composition underwent the
mixing
operation 110 and the agitating operation 120 undergoes the molding and curing
operation 130 so that a carbon flexible heating structure befitting a desired
use is
obtained.
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[40] The conductive composition that is the agitated mixture is molded into a
particular
shape and then cured, or pasted or coated on a mold having a particular shape
and then
cured. A structure having a variety of shapes such as a mesh shape, a plate
shape, a rod
shape, a ring shape, or a bar shape may be used as the particular shape or the
mold
having a particular shape.
[41] Table 1 below shows curing time after the conductive composition is
coated on the
mold having a particular shape.
[42] Table 1
Curing Temperature Curing Time
Room Temperature 4 days ~ 1 week
150C 510 minutes
250C 1~5 minutes
[43] Referring to Table 1, when the conductive composition is cured, a curing
time of
4-7 days is needed at room temperature, which can be reduced to 1-5 minutes at
a
temperature of 200°C.
[44] Table 2 below shows a thermal property of polyethylene and liquid silicon
rubber
according to the present invention. Table 3 below shows the life span of use
of the
silicon rubber according to a temperature.
[45] The liquid silicon rubber is used for the conductive composition because
it exhibits
superior heat resistance, winter-hardiness, ozone resistance, electricity
insulation, and
flexibility. As shown in Table 2, since the thermal expansion coefficient of
the liquid
silicon rubber that is 270x10 6~K 1 is higher, by about two times, than that
of
polyethylene that is 150 x 10 6~K 1, the carbon flexible heating structure has
a self-
control resistance heating function.
[46] Table 2
Item Liquid Silicon RubberPolyethylene (HDPE)
Specific Crravity 1.04 0.940.97
Glass Transition -118-132C -30C
Temperature (Tg)
Crystal Melting 137C
Temperature (Tm)
Thermal Expansion 270 150
Co-
efficient ( 10-6/k-1)
Continuous Use 190C 8090C
Temperature
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[47] Table 3
Temperature Range Expected Life-Span of Use
-50~-30C 10 years or more
-30~ 150 C semi-permanent (20 years or
more)
150200C 510 years
200250C 1~2 years
250300C 1~2 months
300400C several weeks to several months
[48] Since the carbon flexible heating structure according to the present
invention uses
the liquid silicon rubber, it exhibits a superior flexibility so that the
application fields
of the carbon flexible heating mesh according to the present invention
drastically
increase. Also, silicon rubber can be used over 20 years or semi-permanently
according
to a range of temperature in which the silicon rubber is used.
[49] Table 4 below shows typical properties of the conductive carbon black
according to
the present invention.
[50] Table 4
Item Air Space Rate Primary ParticleNumber of Primary
(%)
Diameter (mm) Particle(x 1015
piece/
g)
Conductive Carbon60 40 38
Black
[51] It is the typical properties of the conductive carbon black that the size
of a particle
is 40 manometers, a porosity is 60%, and the number of particles is 38x 1015
per gram.
This means that the conductive carbon black has a high conductive structure in
which
the absorption amount of dibutyl phthalate (DBP) is between 300500 ml/100g.
[52] FIG. 2 illustrates a structure of a mesh type of a carbon flexible
heating structure
according to an embodiment of the present invention (hereinafter, referred to
as the
"carbon flexible heating mesh"). FIG. 3 is a cross-sectional view the carbon
flexible
heating mesh of FIG. 2.
[53] A carbon flexible heating mesh 200 is a fabric made of a woof 230 and a
warp 220.
Port portions 210a and 210b are formed longer than the woof 230 and the warp
220 of
the fabric as ports to supply electric power to both end portions of the woof
230 or the
warp 220. The port portions 210a and 210b are formed of a conductive metal
wire
exhibiting superior conductivity and a tin-plated copper wire or a silver wire
exhibiting
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superior conductivity are used as the conductive metal wire. A conductive
composition
250 is preferably coated or pasted on a frame structure 240 to a thickness of
0.05
through 0.15 mm.
[54] Meanwhile, a mixture obtained by mixing liquid silicon rubber and a
diluent only
and agitating the same can be coated on a surface of the carbon flexible
heating mesh
200, as necessary, for insulation. Since an insulation coating 260 is formed
of the
liquid silicon rubber that is the same material as the conductive composition
250, even
when there is a periodic change between thermal expansion and thermal
contraction
that repeatedly occur, a peeling-off phenomenon of the mesh 200 does not
occur.
[55] Next, the self-control resistance heating mechanism will be described in
detail with
reference to FIGS. 4 and 5.
[56] FIG. 4 is a view illustrating a fine structure of a conductive
composition according
to an embodiment of the present invention at room temperature. FIG. 5 is a
view il-
lustrating a fine structure of the conductive composition shown in FIG. 4 in a
state in
which the temperature is higher than the room temperature. FIGS. 4 and 5 show
a
degree of orientation of a conductive carbon black 310 in a liquid silicon
rubber 320.
[57] Particles of the conductive carbon black 310 are distributed with a
narrow gap
which is filled with the liquid silicon rubber 320. The narrow gap works as a
potential
barrier and electrons are tunneled though the narrow gap by thermal
fluctuation so that
electrical conductivity is exerted.
[58] The self-control resistance heating function according to the present
invention uses
tunneling current as described above. The tunneling current flows through the
narrow
gap when the narrow gap made of the silicon rubber 320 is maintained to be 1
nm or
less and is very sensitive to a distance so that it changes in inverse
proportion and ex-
ponentially with respect to a change in the distance.
[59] When the temperature increases, as shown in FIG. 5, the narrow gap filled
with the
silicon rubber 320 increases so that electrical conductivity is lowered. Thus,
a
resistance value rises so that the narrow gap works as an electrical
insulator.
[60] An embodiment of the carbon flexible heating structure operating as above
is
described in detail with reference to FIGS. 6 and 7. FIG. 6 is a graph showing
a
temperature-resistance property of a conventional PTC device. FIG. 7 is a
graph
showing a temperature-resistance property of the carbon flexible heating
structure
according to an embodiment of the present invention.
[61] Referring to FIGS. 6 and 7, a carbon flexible heating mesh test sample
including a
content of carbon black 10% and a carbon flexible heating mesh test sample
including
a content of carbon black 8% are used in Embodiment 1 and Embodiment 2, re-
spectively. A temperature-resistance property is measured for each embodiment
and
the results of measurements are shown below in Table 5.
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[62] Table 5
Temperature (C) Resistance rate
(p~cm)
Embodiment 1 Embodiment 2
20 91
30 129 150
40 144 220
50 156 267
60 170 312
70 187 416
80 208 468
90 250 625
100 267 939
110 312 1300
120 407
[63] FIG. 7 shows a temperature-resistance characteristic curve of a general
polymer
PTC device as a comparative example. As shown in FIG. 6, the temperature-
resistance
characteristic curve of the conventional PTC device shows that the heat
temperature of
the PTC device is determined by a crystalline melting temperature Tm of each
polymer
material and that the resistance rate no longer increases at a particular
temperature after
passing the switching temperature.
[64] However, as shown in FIG. 7, the carbon flexible heating mesh according
to the
present invention, unlike the conventional PTC device, exhibits a self-control
resistance heating property, that is, the resistance rate gradually increases
as the
temperature increases.
[65] In another embodiment, graphite powder can be used instead of the
conductive
carbon black. When the graphite powder is used as the filler, since graphite
have a
superior lubricity to the conductive carbon black, the graphite powder can be
easily
mixed with the liquid silicon rubber.
[66] It is preferred that the weight ratio between the liquid silicon rubber
and the
graphite powder is 100:10150 in a conductive composition made of a mixture of
the
liquid silicon rubber and the graphite powder. The average particle size of
graphite
powder is 110 mm and electrical resistance is 0.00050.08 S2~cm.
[67] A short staple can be used as a reinforcing material for the conductive
composition
obtained by mixing the liquid silicon rubber and the conductive carbon black
or
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graphite powder as the filler. The short staple may be glass fiber, carbon
fiber, or
graphite fiber having a diameter of 1 through 50 mm. By adding the short
staple, not
only the liquid conductive composition can be reinforced, but also molding the
conductive composition into a desired shape without the frame structure is
made easy.
[68] The conductive composition and the carbon flexible heating structure
according to
the present invention can be applied to the fields of a temperature sensor, a
temperature
compensation device, protection against overheat, a heater, and an electric
circuit for
protection of over-current and are not limited to the above-described
embodiments.
[69] The foregoing embodiments are merely exemplary and are not to be
construed as
limiting the present invention. The description of the present invention is
intended to
be illustrative, and not to limit the scope of the claims. Many alternatives,
modi-
fications, and variations will be apparent to those skilled in the art.
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