Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
1999-07-30
2002-03-19
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S500000, C252S503000, C252S512000, C252S518100, C338S0220SD
Reexamination Certificate
active
06358438
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electrically conductive polymer composition exhibiting positive temperature coefficient (PTC) of electrical resistance behavior. Said composition can be used in PTC devices.
2. Introduction to the Invention
Conductive polymer compositions which exhibit PTC (positive temperature coefficient of resistance ) behavior are well-known for use in electrical devices such as circuit protection devices. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler such as carbon black or metal. The amount and type of filler in the composition are determined by the required resistivity for each application, as well as by the nature of the polymeric component. Compositions suitable for use in circuit protection devices have low resistivities at room temperature, e.g. less than 100 ohm-cm, and generally comprise relatively high levels of conductive filler.
Compositions with low resistivity are desirable for use in circuit protection devices which respond to changes in ambient temperature and/or current conditions. Under normal conditions, a circuit protection device remains in a low temperature, low resistance state in series with a load in an electrical circuit. When exposed to an overcurrent or overtemperature condition, however, the device increases in resistance, effectively shutting down the current flow to the load in the circuit. For many applications it is desirable that the device have as low a resistance as possible in order to minimize the effect on the resistance of the electrical circuit during normal operation. Although low resistance devices can be made by changing dimensions, e.g. making the distance between the electrodes very small or the device area very large, small devices are preferred because they occupy less space on a circuit board and generally have desirable thermal properties. The most common technique to achieve a small device is to use a composition that has a low resistivity.
The resistivity of a conductive polymer composition can be decreased by adding more conductive filler, but this process can affect the processability of the composition, e.g. by increasing the viscosity. Furthermore, the addition of conductive filler generally reduces the size of the PTC anomaly, i.e. the size of the increase in resistivity of the composition in response to an increase in temperature, generally over a relatively small temperature range. The required PTC anomaly is determined by the applied voltage and the application.
Japanese Patent Kokai Publication No. 172001/1996 (Heisei 08-172001) discloses that metal particles and metal-coated particles are used as the electrically conductive particles, because it is difficult to achieve electrically conductive material having a volume resistivity of at most 1 ohm-cm and good PTC anomaly when carbon black is used as the electrically conductive particles. However, the amount of the electrically conductive particles must be increased to decrease the resistivity. When the amount of the electrically conductive particles is increased, it is impossible to give sufficient PTC anomaly and molding of the composition is difficult due to poor flowability of the composition. Actually, the resultant value of the volume resistivity is limited.
Japanese Patent Kokai Publication No. 6309/1981 (Showa 56-6309) discloses a temperature sensor comprising electrically conductive particles dispersed in an insulative matrix. The insulative matrix comprises an aluminum soap added to a hydrocarbon wax. However, this temperature sensor does not exhibit sufficient PTC behavior.
Japanese Patent Kokai Publication No. 168005/1999 (Heisei 11-168005) discloses an organic PTC thermistor comprising an electrically conductive composition comprising a thermoplastic polymer matrix, a low molecular weight organic compound and electrically conductive particles. This publication describes that hydrocarbons, fatty acids, fatty acid esters, fatty acid amides, aliphatic amines and higher alcohols are used as the low molecular weight organic compound, but does not describe that a polymer is used as the low molecular weight organic compound. The electrically conductive composition has poor processability and does not have good PTC anomaly.
Hitherto, electrically conductive compositions having low volume resistivity have been obtained by adding a large amount of electrically conductive particles such as carbon black and metal powder to a matrix such as a polymer. However, electrically conductive compositions having satisfactory PTC anomaly cannot be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrically conductive composition having good flowability at high temperature and low resistivity at 20° C. and exhibiting good PTC anomaly.
In a first aspect, the present invention provides an electrically conductive polymer composition exhibiting positive temperature coefficient (PTC) of electrical resistance behavior and comprising:
(1) a polymer mixture comprising:
(i) at least 50% by volume of a first crystalline polymer having a weight-average molecular weight of at least 50,000, and
(ii) at most 50% by volume of a second crystalline polymer having a weight-average molecular weight of at most 10,000, and
(2) a particulate electrically conductive filler dispersed in the polymer mixture.
In a second aspect, the present invention provides a PTC device comprising:
(A) a PTC element (for example, a laminar PTC element) comprising the composition, and of the first aspect of the invention.
(B) two electrodes which can be connected to an electrical power source to pass an electrical current through the PTC element.
In a third aspect, the present invention provides an electrical circuit which comprises:
(I) the PTC device; of the second aspect of the invention.
(II) an electrical power source; and
(III) a load connected in series with the device and the power source.
DETAILED DESCRIPTION OF THE INVENTION
The electrically conductive polymer composition of the present invention comprises a polymer mixture comprising a first crystalline polymer and a second crystalline polymer, and a particulate electrically conductive filler, and exhibits positive temperature coefficient (PTC) of electrical resistance behavior.
The polymer mixture comprises a first crystalline polymer and a second crystalline polymer. Preferably, the amount of the polymer mixture is from 20 to 90% by volume, more preferably 20 to 70% by volume, especially 30 to 70% by volume, based on total volume of the electrically conductive polymer composition.
The first crystalline polymer has a weight-average molecular weight of at least 50,000. The lower limit of the weight-average molecular weight of the first crystalline polymer is 50,000, preferably 100,000. The upper limit of the weight-average molecular weight of the first crystalline polymer is generally 10,000,000, e.g. 3,000,000, preferably 1,000,000, more preferably 600,000.
The crystallinity of the first crystalline polymer may be at least 10%, preferably at least 20%, more preferably at least 30%, especially at least 40%, e.g. from 50 to 98%.
The first crystalline polymer is generally a thermoplastic resin. Preferably, the first crystalline polymer is a polymer comprising at least one monomer selected from olefins or olefin derivatives, e.g. a homopolymer or copolymer of ethylene. Suitable examples of the first crystalline polymer include polymers of one or more olefins such as high density polyethylene; copolymers of at least one olefin and at least one monomer copolymerisable therewith such as ethylene/acrylic acid, ethylene/ethyl acrylate, ethylene/vinyl acetate, and ethylene/butyl acrylate copolymers; melt-shapeable fluoropolymers such as polyvinylidene fluoride and ethylene/tetrafluoroethylene copolymers; and blends of two or more such polymers.
The amount of the first crystalline polymer is at least 50% by volume, e.g. at least 60% by volume, particularly at least 70% by volume, especially at least 80%
Isozaki Tsutomu
Jordan Susan Melsa
Stein Kevin Michael
Takahashi Masaaki
Gerstner Marguerite E.
Gupta Yogendra N.
Hamlin D G
Tyco Electronics Corporation
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