PTC compositions

A PTC composition comprising at least one polymer, from 5% to 45% by volume of electrically conductive or semiconductive particles having a room temperature electric conductivity of at least 10.sup.2 [s/m] dispersed in said polymer, and from 0.2% to 20% by volume of thermally conductive particles having a room temperature electric conductivity of no more than 10.sup.-3 [s/m] and a thermal conductivity of at least 20 [w/m.multidot.k] dispersed in said polymer. A PTC device using such a PTC composition has a high ratio of peak resistance to room temperature resistance, and exhibits high safety.

BACKGROUND OF THE INVENTION 
This invention relates to an electrical material, a process for producing 
the same, and uses thereof and, more particularly, to a material 
composition having specific properties of sharply increasing its 
electrical resistance within a relatively narrow temperature range with 
increasing a temperature [PTC characteristics (positive temperature 
coefficient)], i.e., to a PTC composition. 
PTC compositions can be utilized in a heater wherein heat generation is 
ceased when it is raised to a specific temperature; in a PTC thermistor; 
in a heat-sensitive sensor; and a circuit protection device wherein when a 
circuit containing a cell or the like exhibits a short the current flowing 
through the circuit is restricted to a predetermined value or less due to 
the increase of a resistance value, whereas when its short is released the 
circuit is restored. Currently, various materials have been developed as 
the PTC compositions. Heretofore, there have been developed BaTiO.sub.3 
having a monovalent of trivalent metal oxide incorporated therein, and 
polymers such as polyethylene and ethylene-acrylic acid copolymers having 
electrically conductive particles such as carbon black uniformly dispersed 
therein. 
A process for preparing this PTC composition generally comprises 
incorporating a necessary amount of carbon black in one or more resins 
used as polymers and kneading them. 
Further, PTC composition is utilized in a PTC device wherein this 
composition is sandwiched or interposed between metallic electrode plates. 
Preferred characteristics of PTC compositions used as the PTC device or the 
like are a large resistance value at a high temperature (a peak 
resistance), and a low resistance value at room temperature (a room 
temperature resistance), i.e., a high ratio of peak resistance to room 
temperature resistance. Further, it is desirable to increase the spacing 
between electrodes in order to obtain devices having high safety, and to 
prevent a short between the electrodes. 
However, in the prior art PTC compositions and processes for producing the 
same, even if the thickness of the PTC composition sandwiched between the 
electrodes is increased in order to produce devices having high safety, a 
high peak resistance value in proportion to the thickness is not always 
obtained. In the case of PTC compositions having a certain thickness or 
above, the peak resistance value reaches a plateau. 
SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a PTC device having 
a high ratio of peak resistance to room temperature resistance and 
exhibiting high safety. 
Another object of the present invention is to provide a PTC composition 
capable of producing a PTC device having an increased thickness without 
reaching a plateau of a peak resistance even if the thickness is 
increased. 
A further object of the present invention is to provide a process for 
preparing a PTC composition capable of preventing discharge breakdown 
between device terminals. 
We have carried out various tests and studies in order to accomplish the 
objects described above. We have now found that, when an appropriate 
amount of thermally conductive particles is incorporated in a polymer, a 
composition having good characteristics is obtained. 
A PTC composition according to the present invention comprises at least one 
polymer, from 5% to 45% by volume of electrically conductive or 
semiconductive particles having a room temperature electric conductivity 
of at least 10.sup.2 [s/m] dispersed in said polymer, and from 0.2% to 20% 
by volume of thermally conductive particles having a room temperature 
electric conductivity of no more than 10.sup.-3 [s/m] and a thermal 
conductivity of at least 20 [w/m.multidot.k] dispersed in said polymer. 
In a preferred embodiment of a PTC composition of the present invention, 
thermally conductive particles can composed of at least one material 
selected from silicon, SiC, Si.sub.3 N.sub.4, beryllia, selenium, and 
alumina. 
In a preferred embodiment of a PTC of the present invention, thermally 
conductive particles can have an average particle size of from 1 to 200 
micrometers. 
In another embodiment of the present invention, a process for preparing a 
PTC composition comprises incorporating from 5% to 45% by volume of 
electrically conductive or semiconductive particles having a room 
temperature electric conductivity of at least 10.sup.2 [s/m] and from 0.2% 
to 20% by volume of thermally conductive particles having a room 
temperature electric conductivity of no more than 10.sup.-3 [s/m] and a 
thermal conductivity of at least 20 [w/m.multidot.k], in at least one 
polymer, and kneading the mixture in a temperature range of from the 
highest melting point Tm among the melting points of the polymers to be 
kneaded to Tm+80.degree. C. 
In another embodiment of the present invention, a PTC device using a PTC 
composition comprises a material having PTC characteristics disposed 
between electrodes, wherein said material is a PTC composition comprising 
at least one polymer, from 5% to 45% by volume of electrically conductive 
or semiconductive particles having a room temperature electric 
conductivity of at least 10.sup.2 [s/m] dispersed in said polymer, and 
from 0.2% to 20% by volume of thermally conductive particles having a room 
temperature electric conductivity of no more than 10.sup.-3 [s/m] and a 
thermal conductivity of at least 20 [w/m.multidot.k] dispersed in said 
polymer.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be more fully described. 
Polymer 
Examples of the polymers which can be used in the present invention include 
polyethylene, polyethylene oxide, polybutadiene, polyethylene acrylates, 
ethylene-ethyl acrylate copulymers, ethylene-acrylic acid copolymers, 
polyerters, polyamides, polyethers, polycaprolactam, fluorinated 
ethylene-propylene copolymers, chlorinated polyethylene, chlorosulfonated 
polyethylene, ethyl-vinyl acetate copolymers polypropylene, polystyrene, 
styrene-acrylonitrile copolymers, polyvinyl chloride, polycarbonates, 
polyacetals, polyalkylene oxides, polyphenyl oxide, pulysulfrenes, 
fluoroplastics, and blend polymers of at least two polymers selected from 
the polymers described above. In the present invention, the type of the 
polymers and compositional ratios can be varied depending on desired 
performance, uses or the like. 
ELECTRICALLY CONDUCTIVE TICLES 
Electrically conductive or semiconductive particles (hereinafter referred 
to as electrically conductive particles) dispersed in the polymer are 
composed of electrically conductive materials having a room temperature 
electric conductivity of at least 10.sup.2 [s/m]. Examples of such 
particles which can be used herein include particles of electrically 
conductive materials such as carbon black, silver powder, gold powder, 
carbon powder, graphite, copper powder, carbon fibers, nickel powder, 
silver plated fine particles. It is desirable to vary the particle size 
and specific area of the electrically conductive particles depending upon 
the uses and desired characteristics of the PTC composition. 
THERMALLY CONDUCTIVE TICLES 
In the present invention, thermally conductive particles dispersed in the 
polymer are composed of thermally conductive materials having a room 
temperature electric conductivity of no more than 10.sup.-1 [s/m], 
preferably no more than 10.sup.-3 [s/m] and a thermal conductivity of at 
least 20 [w/m.multidot.k]. Examples of such thermally conductive particles 
include semiconductors and electrically insulating materials such as at 
least one material selected from silicon, selenium, SiC, Si.sub.3 N.sub.4, 
BeO and Al.sub.2 O.sub.3, and mixtures thereof. The particle size, and 
specific area of the thermally conductive particles can be varied 
depending on the uses and desired characteristics of the PTC composition. 
For example, some thermally conductive particles have an average particle 
size of from 1 to 200 microns. 
PTC Composition 
In preparing the PTC composition, optional various additives can be admixed 
in addition to the polymer, the electrically conductive particles and 
thermally conductive particles. Examples of such additives include flame 
retardants such as antimony-containing compounds, phosphorus-containing 
compounds, chlorinated compounds and brominated compounds, antirexidants 
and stabilizers. 
In the present invention, a PTC composition is prepared by blending and 
kneading its raw materials, a polymer, electrically conductive particles, 
thermally conductive particles and other additives in predetermined 
ratios. A PTC composition can be prepared by incorporating electrically 
conductive particles in a polymer and then incorporating thermally 
conductive particles therein. A PTC composition can also be prepared by 
incorporating thermally conductive particles in a polymer and then 
incorporating electrically conductive particles therein. Further, a PTC 
composition can be prepared by incorporating thermally conductive 
particles and electrically conductive particles in a polymer at the same 
time. When at least two polymers are used, kneading the polymers with 
electrically conductive particles and thermally conductive particles can 
be carried out by preblending each polymer with electrically conductive 
particles and thermally conductive particles and then kneading each 
preblend in a predetermined ratio. This kneading is carried out by 
kneading the polymer with the electrically conductive particles and the 
thermally conductive particles. While the blend ratios of polymer to 
particles can be varied depending on the content of particles in a desired 
composition, the type of a polymer, the type of a mixer or kneader, or the 
like, in the present invention, the amount of the electrically conductive 
particles is from 5% to 45% by volume, preferably from 23% to 38% by 
volume and the amount of the thermally conductive particles is from 0.2% 
to 20% by volume, preferably from 0.2 to 5% by volume. In the present 
invention, pretreatments such as grinding, heating and mixing can be 
carried out prior to kneading. The kneading temperature is from the 
melting point of the polymer to be kneaded to a temperature higher by 
80.degree. C., preferably 50.degree. C. than the melting point of the 
polymer. This is because the polymer to be kneaded can gel to uniformly 
disperse the electrically conductive particles therein. 
When additives are incorporated in the PTC composition, the additives can 
be added before or after premixing, before or after kneading, or during 
premixing or kneading. 
The PTC composition obtained by the present invention can be used in 
various uses. The PTC composition can be used to produce a PTC device 
having the PTC composition disposed between electrodes. When the PTC 
composition is used in a PTC device, the PTC device can be produced by 
forming the PTC composition into a film, hot pressing metallic foil 
electrodes to the upper and lower surfaces of the film to form a laminate, 
cutting this laminate into a predetermined size and electrically 
connecting a lead wire to the surface of each of the electrodes. 
Because the present invention is constituted as described above, it acts as 
follows: 
In the PTC composition wherein the electrically conductive particles such 
as carbon black are dispersed in the polymer such as polyethylene, 
polyethylene has a low thermal conductivity of 3,4 (w/m.multidot.k) and 
carbon black also has a low thermal conductivity (15.5 w/m.multidot.k). 
Accordingly, the thermal conductivity of the PTC composition is inferior 
and the heat distribution occurs in a direction perpendicular to the 
equipotential surface. Only a portion of the PTC composition exhibits PTC 
characteristics to become a high restance due to the heat distribution. 
Accordingly, it is believed that the peak resistance is not increased in 
proportion to the thickness even if the thickness of the PTC composition 
is increased, and that the peak resistance reaches a plateau in the case 
of a certain thickness or above. It is also believed that the heat 
distribution is present in a surface direction, thereby only a portion of 
the PTC composition is raised to a higher temperature to occur the 
breakdown of the device and that higher portions and lower portions in 
resistance value occur and the peak resistance is lower than the peak 
resistance inherent to the device. In the present invention, the thermally 
conductive particles are further dispersed into the polymer, and therefore 
the heat conduction of the PTC composition is improved, and the heat 
distribution in the PTC composition is relaxed. Partially high resistance 
is eliminated and no peak resistance reaches a plateau. Furthermore, the 
thermally conductive particles have a low electric conductivity and 
therefore the peak resistance is not reduced. 
EXAMPLE 
In order to indicate more fully the nature and utility of this invention, 
the following examples are set forth, it being understood that these 
examples are presented as illustrative only and are not intended to limit 
the scope of the invention. All parts used herein are by weight unless 
otherwise specified. 
EXAMPLE 1 
Six parts of Si powder (available from Wako Junyaku Co. under the tradename 
No. 198-05455) were added to 17.6 parts of high density polyethylene 
(hereinafter referred to as HDPE; available from Toyo Soda Co. under the 
tradename Niporon Hard 5100), 17.6 parts of an ethylene-acrylic acid 
copolymer (hereinafter referred to EAA; available from Mitsubishi Yuka Co. 
under the tradename A201K) and 28 parts of carbon black (available from 
Cabot Co. under the tradename STERLING SO). The mixture was kneaded at a 
temperature of 180.degree. C. by means of a twin-screw roll mill, and 
formed into a film. Nickel foils each having a thickness of 60 micrometers 
were hot pressed to both the surfaces of the film of the PTC composition 
to prepare a PTC device. The size of the device was 10.5.times.10.5 
millimeter, and the thickness of the PTC composition was 0.25 millimeter. 
Current was passed through the resulting PTC device to occur self-heat 
generation and the peak resistance was measured. As a result, the peak 
resistance was 6 kilohms. The room temperature resistance was 120 
milliohms. 
PTC devices were prepared and their peak resistance (kilohm) and room 
temperature resistance (milliohm) were measured as described above except 
that the amount of Si powder was changed. The results are shown in FIG. 1. 
As can be seen from this FIG, the peak resistance increases with 
increasing the amount of Si powder added. 
PTC devices were prepared and their peak resistance (kilohm) and room 
temperature resistance (milliohm) were measured as described above except 
that the amount of Si powder was changed. The results are shown in FIG. 1. 
As can be seen from this FIG., the peak resistance increases with 
increasing the amount of Si powder added. 
PTC devices were prepared and their peak resistance (kilohm) and room 
temperature resistance (milliohm) were measured as described above except 
that the thickness of the PTC compositions was changed. The results are 
shown in FIG. 2. As can be seen from this FIG., the peak resistance 
increases with increasing the thickness of the PTC compositions, and the 
peak resistance does not reach a plateau. 
EXAMPLE 2 (COMATIVE EXAMPLE) 
PTC compositions were prepared by prior art. Forty eight parts of carbon 
black were added to 26 parts of EAA and 26 parts of HDPE, and the mixture 
was kneaded to prepare PTC compositions. The PTC compositions were tested 
as in Example 1 for their characteristics. The results are shown in FIG. 
3. 
As can be seen from comparison of Examples 1 and 2, the peak resistance 
does not reach a plateau in Example 1, and thus the PTC composition 
according to the present invention has excellent characteristics.