BACKGROUND ART PTC means a characteristic that electrical resistance rapidly increases at a relatively narrow temperature range due to increase of temperature. PTC compositions have such PTC characteristics and they are generally used in a circuit protection element, which limits current of a circuit when the circuit with a heater, a positive-characterized thermistor, an ignition sensor, a battery or the like is blocked. The circuit protection element makes the circuit recovered when the cause of blocking is removed.

As another example employing the PTC compositions, there are PTC elements in which at least two electrodes are electrically connected to such compositions. Such PTC elements are used as an element for

protection of over current or overheat, which acts for self-control of temperature, as described above.

Over-current protection mechanism using the PTC elements is as follows.

At an ambient temperature, the PTC composition has sufficiently low resistance, so ensuring current flow through a circuit. However, if high current passes through the circuit for example by a short, Joule heat is generated in the PTC element due to such high current. It increases temperature and therefore resistance of the element by the PTC characteristics, which blocks current flow through the element, so protecting the circuit. This is generally referred as a current limiting property.

Such PTC element, or PTC composition, had better be provided with a current limiting property, which can repeatedly work under high voltage. Also, improvement of the current limiting property comes from sufficient decrease of an initial resistance of the PTC element as well as endowment of the effective PTC characteristics.

There are developed many kinds of PTC compositions. As an example, a PTC composition made by adding univalent or trivalent metal oxide to BaTiO3 is already well known. However, such composition has a problem that it allows current flow less than lmsec because it shows NTC (Negative Temperature Coefficient) characteristics, right after showing the PTC characteristics.

As an alternation, there has been developed a PTC composition,

which is made by dispersing electrical conductive particles such as carbon black, carbon fiber, carbon graphite or metal particles to an organic polymer such as polyethylene, polypropylene or ethylene-acrylic acid copolymer. Such PTC composition is generally made by blending necessary amount of electrical conductive particles into at least one resin, used as an organic polymer.

Reference may be made for example to U. S. Pat. No. 3,243,753 (Verent et al.), U. S. Pat. No. 3,823,217 (Kampe), U. S. Pat. No. 3,950,604 (Richard), U. S. Pat. No. 4,188,276 (Bernard), U. S. Pat. No. 4,272,471 (Walker), U. S. Pat. No. 4,414,301 (Ronald), U. S. Pat. No.

4,425,397 (Stephen), U. S. Pat. No. 4,426,339 (Hundi), U. S. Pat. No.

4,427,877 (Vijay), U. S. Pat. No. 4,429,216 (Raychem), U. S. Pat. No.

4,442,139 (Alan), and so on.

In addition, Korean Patent Publication No. 99-63872 discloses a technique of grafting conductive particulate fillers into denaturated polyethylene for making a PTC composition, which may show great adhesion to a metal electrode with a soft surface, recover its initial or lower resistance after repeated cycling (that is, changing from a low resistance state to a high resistance state and then returning), and extend a period of a tripped state.

However, any one among them does not show a technique to control a switching temperature and a trip time by adding polyethylene, which is denaturated into a maleic anhydride compound, into crystalline polymer compounds.

DISCLOSURE OF INVENTION Inventors of the present invention have discovered that it is possible to control a switching temperature and a trip time by adding low-density polyethylene (LDPE) or high-density polyethylene (HDPE), which is denaturated into maleic anhydride compound, into mixture of HDPE, LDPE, ethylene-ethyl acrylate copolymer (EEA), ethylene-acrylic-acid (EAA) or ethylene-vinyl-acetate (EVA).

An object of the present invention is to provide a PTC composition for easily controlling a switching temperature and a trip time thereof, and a method of controlling such PTC characteristics.

Another object of the present invention is to provide a PTC composition with good heat-stability and conductivity by a cross-linking reaction of conductive polymer compounds using a cross-linking agent.

In order to accomplish the above objects, the present invention provides a PTC conductive polymer composition, which includes organic polymer containing polyolefin components essentially consisting of 30 to 40% high density polyethylene (HDPE) by weight, 20 to 40% low density polyethylene (LDPE) by weight and 10 to 30% ethylene-acrylic-acid (EAA) or ethylene-vinyl-acetate (EVA) by weight, and additionally high or low density polyethylene which is denaturated into a maleic anhydride compound and is 20 to 30% by weight; electrical conductive particles dispersed into the organic polymer, the electrical conductive particles being 30 to 60% by weight of the organic polymer; and a peroxidic

cross-linking agent added for cross-linking reaction, the peroxidic cross-linking agent being 0.2 to 0.5% by weight of the organic polymer.

Therefore, the present invention makes it possible to control a switching temperature and a trip time of the PTC composition by suitably adjusting an added amount of the denaturated polyethylene.

The PTC conductive polymer composition may also include an antioxidant, which is 0.2 to 0.5% by weight of the organic polymer.

In addition, the PTC conductive polymer composition may have a specific resistance, which is between 0.8 and 2.0 Q-cm at an ambient temperature.

In order to fulfill the above object, the present invention also provides a method of controlling positive temperature coefficient (PTC) characteristics of the above PTC conductive polymer composition, in which the method controls a switching temperature and a trip time of the PTC conductive polymer composition by adjusting an added amount of the HDPE or the LDPE, which is denaturated into the maleic anhydride compound.

At this time, as an amount of the denaturated polyethylene added to the organic polymer increases, the switching temperature decreases and the trip time increases.

In order to achieve the above object, the present invention also provides an electrical device, which includes a PTC element having the above PTC conductive polymer composition; and a pair of electrodes connectable to a power source, respectively, the electrodes allowing

current to flow through the PTC element when being connected to the power source.

In the electrical device, when testing a current-time characteristic of the electrical device with 1,000 successive cyclic tests under the condition that the trip time is set to a time when a resistance of the device becomes 10Q and a overload current added is set to 5A, a ratio R 1/RO is preferably maintained between 1.0 and 1.5 at every test, where R1 is a resistance after the test and RO is a resistance before the test.

Also, in the current-time characteristic test, the ratio Rl/RO is preferably maintained between 1.0 and 2.5 after 10 hours since the electrical device is in a tripped state.

Moreover, when testing a temperature-resistance characteristic of the electrical device with 10 successive cyclic tests, a ratio R2/RO is preferably maintained between 1.0 and 2.0 at every test, where R2 is a resistance after the test and RO is a resistance before the test.

Furthermore, the ratio R2/RO is preferably maintained between 1.0 and 2.0 at every test even when a ratio of a maximum resistance to a resistance at an ambient temperature is more than 106.

Still furthermore, in a temperature-resistance test, a ratio R3/RO is preferably maintained more than 105 at 140°C or more, where R3 is a resistance peak and RO is an initial resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, in which like components are referred to by like reference numerals. In the drawings: FIG. 1 shows a section view of an electrical device according to the present invention; FIG. 2 shows temperature-resistance graphs for embodiments 1 to 4 of the composition according to the present invention; FIG. 3 shows temperature-resistance graphs for embodiments 1, 5,6 and 7 of the composition according to the present invention; and FIG. 4 shows temperature-resistance graphs for embodiments 2 and 5 of the present invention and a comparative example using a cross-linking agent.

BEST MODES FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Suggested in this invention are a PTC (Positive Temperature Coefficient) conductive polymer composition, which has a specific resistance between 0.8 and 2.0 Q-cm and good temperature-resistance characteristics at an ambient temperature and maintains its initial specific resistance when the temperature repeatedly increases and decreases, and an electrical device using the PTC conductive polymer

composition.

More concretely, the PTC conductive polymer composition is made by adding electrical conductive particulate fillers containing LDPE (Low-Density Polyethylene), HDPE (High-Density Polyethylene), carbon black, and so on into an organic polymer containing HDPE, LDPE, EEA (Ethylene-ethyl Acrylate Copolymer), EVA (Ethylene-Vinyl-Acetate), EAA (Ethylene-Acrylic-Acid), and so on, which is denaturated into a maleic anhydride compound, and then cross-linking the mixture with a cross-linking agent. The PTC composition may also include an antioxidant, an inert filler, a stabilizer, a dispersing agent, and so on, additionally.

The organic polymer contains 30 to 40% HDPE by weight, 20 to 40% LDPE by weight and 10 to 30% of EAA, EVA or EEA by weight.

A suitable amount of the HDPE or the LDPE, which is denaturated into a maleic anhydride compound, is about 20 to 30% by weight.

In addition, the conductive particulate filler may be, preferably, powder nickel, gold dust, powder copper, silvered power copper, metal-alloy powder, carbon black, carbon powder or carbon graphite.

Particularly, the carbon black is selected as the conductive particulate filler in the present invention.

An added amount of the carbon black is preferably about 30 to 60% by weight.

An amount of the peroxidic cross-linking agent added for

cross-linking reaction is suitably about 0.2 to 0.5% by weight.

And, a preferred amount of the antioxidant added as an additional agent is 0.2 to 0.5% by weight.

The PTC conductive polymer composition described above can be disposed between two metal film electrodes to make an electrical device with PTC characteristics. Such electrical device with PTC characteristics is described in FIG. 1. As shown in FIG. 1, the electrical device includes two metal film electrodes 1 and a PTC element 2 united between them. Such PTC element 2 has the PTC conductive polymer composition described above.

Hereinafter, the PTC conductive polymer composition and a process of making the electrical device with the PTC composition are described in detail.

At first, organic polymer containing polyolefin components essentially consisting of 30 to 40% HDPE by weight, 20 to 40% LDPE by weight and 10 to 30% EAA or EVA by weight, and additionally HDPE or LDPE which is denaturated into a maleic anhydride compound and is 20 to 30% by weight; 30 to 60% electrical conductive particles by weight of the organic polymer, which are dispersed into the organic polymer; and 0.2 to 0.5% peroxidic cross-linking agent by weight of the organic polymer, which is added for cross-linking reaction are blended in a Banbury mixer for 20 to 30 minutes at above a melting temperature.

The blended mixture is molded at a temperature of 140°C for 2 minutes under a pressure of 300kg/cm2 to make a PTC element of 5mm

thickness.

This PTC element is bonded to the metal electrodes at a suitable temperature, and then cross-linked and cooled to eventually make the electrical device as shown in FIG. 1.

The electrical device has the PTC element (or, conductive complex) surrounded by two metal film electrodes, in which the metal electrodes are 15 to 50pm in thickness and the PTC element is 150 to 400pu in thickness. The finally made electrical device has a disk shape, and more preferably, has a doughnut shape with a suitable-sized hole at its center.

Now, embodiments of the present invention are described.

Embodiment 1 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent by weight of an organic polymer, to the organic polymer which contains 35% HDPE (High-Density Polyethylene) by weight with 0.95 to 0.965 g/cm2 density and 3 to 6 melt index, 35% LDPE (Low-Density Polyethylene) by weight with 0.90 to 0.93 g/cm2 density and 3 to 6 melt index, and 30% EVA (Ethylene-Vinyl Acetate) by weight.

Embodiment 2 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent

by weight of an organic polymer, to the organic polymer which contains 30% HDPE by weight with 0.95 to 0.965 g/cm2 density and 3 to 6 melt index, 30% LDPE by weight with 0.90 to 0.93 g/cm2 density and 3 to 6 melt index, 10% EVA by weight, and 30% LDPE by weight which is denaturated into maleic anhydride compound with 3 to 6 melt index.

Embodiment 3 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent by weight of an organic polymer, to the organic polymer which contains 30% HDPE by weight with 0.95 to 0.965 g/cm2 density and 3 to 6 melt index, 30% LDPE by weight with 0.90 to 0.93 g/cm2 density and 3 to 6 melt index, 10% EVA by weight, and 20% LDPE by weight which is denaturated into maleic anhydride compound with 3 to 6 melt index.

Embodiment 4 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent by weight of an organic polymer, to the organic polymer which contains 40% HDPE by weight with 0.95 to 0.965 g/cm2 density and 3 to 6 melt index, 40% LDPE by weight with 0.90 to 0.93 g/cm2 density and 3 to 6 melt index, 10% EVA by weight, and 10% LDPE by weight which is denaturated into maleic anhydride compound with 3 to 6 melt index.

Embodiment 5 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent by weight of an organic polymer, to the organic polymer which contains 30% HDPE by weight with 0.95 to 0.965 g/cm2 density and 3 to 6 melt index, 30% LDPE by weight with 0.90 to 0.93 g/cm2 density and 3 to 6 melt index, 10% EVA by weight, and 30% HDPE by weight which is denaturated into maleic anhydride compound with 3 to 6 melt index.

Embodiment 6 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent by weight of LDPE, to the LDPE which is denaturated into maleic anhydride compound with 0.90 to 0.93 g/cm2 density and 3 to 6 melt index.

Embodiment 7 Make a PTC conductive polymer composition by adding 35% carbon black, 0.3% antioxidant and 0.2% peroxidic cross-linking agent by weight of HDPE, to the HDPE which is denaturated into maleic anhydride compound with 0.95 to 0.965 g/cm2 density and 3 to 6 melt index.

Comparative Example 1

Do not add the peroxidic cross-linking agent to the organic polymer of the embodiment 2, so making a PTC composition without cross-linking reaction.

Comparative Example 2 Do not add the peroxidic cross-linking agent to the organic polymer of the embodiment 5, so making a PTC composition without cross-linking reaction.

Hereinafter, temperature-resistance characteristics and current-time characteristics of the PTC composition in each embodiment and each comparative example are presented.

Test 1 A test method and experimental instruments for testing the temperature-resistance characteristics are as follows.

1) sample The sample for the test 1 is obtained by uniting the PTC compositions of the embodiments 1 to 4 with the metal electrodes, cross-linking the united device with pressure for 20 to 30 minutes and then cooling it for 10 minutes.

2) test method -temperature range for measurement :-40°C ~ 180°C -temperature interval for measurement: 10°C

-wait time at each measurement temperature : 15 minutes 3) experimental instruments -temperature rising/falling rate in a chamber : at least 1°C/min -resistance measuring device: HP 34401A (test current: less than 1 mA, measuring range: 0. lmQ-100MQ) Results of the test 1 for the temperature-resistance characteristics of the sample according to the embodiments of the present invention are well shown in FIG. 2.

As shown in FIG. 2, it can be easily understood that a switching temperature of the PTC composition increases as an added amount of the polyethylene, which is denaturated into maleic anhydride compound, decreases. In addition, it can be easily found that a switching temperature of the embodiment 4 is greater than that of the embodiment 2.

Here, the switching temperature means a temperature at the point that a resistance suddenly increases depending on changing temperature. Therefore, it should be acknowledged that the switching temperature could be determined as desired by adjusting an added amount of the polyethylene, which is denaturated into maleic anhydride compounds.

And, FIG. 3 shows temperature-resistance characteristics of the electrical devices in each case that the PTC composition contains only polyolefin (embodiment 1), polyolefin and the denaturated polyethylene (embodiment 5), and only the denaturated polyethylene (embodiments 6

and 7). As shown in FIG. 3, it can be found that the electrical device shows different temperature-resistance characteristics at each case.

Also, a resistance after repeated measurement of the temperature-resistance characteristics (R2) and a resistance before the measurement (RO) are compared. The electrical device of the present invention maintains a ratio R2/RO less than 2.0 at every test until 1,000 times of the test, and preferably between 1.0 and 2.0.

Moreover, the electrical device also maintains the ratio R2/RO between 1.0 and 2.0 even when a ratio of a maximum resistance to a resistance at an ambient temperature is more than 106.

Test 2 A test method and experimental instruments for testing the current-time characteristics are as follows.

1) sample The sample for the test 2 is obtained by uniting the PTC compositions of the embodiments 1 to 7 with the metal electrodes, cross-linking the united device with pressure for 20 to 30 minutes and then cooling it for 10 minutes.

2) test method -set voltage: 15V DC (depending on conditions) -set current: 10A DC (depending on conditions) -time interval for measurement: 1 Oms 3) experimental instruments

-power supply: 20V/40A DC -voltage and current measuring device: shunt (0. 01V/0. 01A resolution) 4) trip time The trip time is defined as the time required for reducing a fault current as much as 1/2. For example, if voltage and current are set as 15V/10A, the trip time is a time required to decrease the current to 5A.

At this time, the resistance of the PTC element becomes 3Q.

Results of the test 2 for the current-time characteristics of the sample according to the embodiments of the present invention are described in Table 1 below.

Table 1 Embodiment 1 2 3 4 5 6 7 Triptime 4-5 7-8 6-7 5-6 7-8 8-9 9-10 (sec) As shown in Table 1, it can be easily understood that a trip time of the PTC composition decreases as an added amount of the polyethylene, which is denaturated into maleic anhydride compound, decreases. In particular, the trip time decreases as an added amount of LDPE, which is denaturated into maleic anhydride compound, decreases. However, if the PTC composition consists of only polyethylene, which is denaturated into maleic anhydride, like the

embodiments 6 and 7, the trip time rather tends to increase.

Also, a resistance after repeated measurement of the temperature-resistance characteristics (R1) and a resistance before the measurement (RO) are compared. The electrical device of the present invention maintains a ratio Rl/RO less than 1.5 at every test until 1,000 times of the test, and preferably between 1.0 and 1.5.

Moreover, the electrical device also maintains the ratio R 1/R0 between 1.0 and 2.5 after 10 hours in a tripped state.

Test 3 In this test, temperature-resistance characteristics for electrical devices containing the PTC compositions of the embodiments 2 and 5 and electrical devices containing PTC compositions of the comparative examples 1 and 2, which are different from the PTC compositions of the embodiments 2 and 5 in that they are made without cross-linking reaction, with the same method as the test 1.

Results of the test 3 are well shown in FIG. 4. As shown in FIG.

4, the electrical devices according to the embodiments 2 and 5 maintain resistance of 1,000Q at more than 140°C, while the electrical devices of the comparative examples have resistance less than 1, OOOQ at more than 140°C.

In other words, supposing that a resistance of an electrical device at more than 140°C is R3 and an initial resistance at an ambient temperature is R0, the electrical devices of the embodiments 2 and 5

maintains a ratio R3/RO more than 105, while the electrical devices of the comparative examples shows the ratio R3/RO less than 105.

Therefore, the electrical device using the PTC conductive polymer composition of the present invention has an advantage that its PTC characteristics can be controlled as desired by adjusting an added amount of polyethylene, which is denaturated into maleic anhydride.

In particular, as an added amount of the denaturated polyethylene decreases, the switching temperature increases and the trip time decreases.

In addition, the electrical device of the present invention, which is made using chemical cross-linking reaction with peroxidic cross-linking agent, shows excellent heat stability rather than other electrical devices, which have not experienced the cross-linking reaction.

The PTC conductive polymer composition, the method of controlling the PTC composition and the electrical device containing the PTC composition according to the present invention have been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.