Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1998-12-16
2001-04-03
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S203000, C338S328000, C338S013000, C029S612000, C029S610100
Reexamination Certificate
active
06211771
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to devices comprising conductive polymer elements, in particular electrical devices such as circuit protection devices in which current flows between two electrodes through a conductive polymer element.
2. Introduction to the Invention
It is well known to make compositions which comprise a polymeric component and, dispersed therein, electrically conductive particles. The type and concentration of the particles may be such that the composition is conductive under normal conditions, e.g. has a resistivity of less than 10
6
ohm-cm at 23° C., or is essentially insulating under normal conditions, e.g. has a resistivity of at least 10
9
ohm-cm at 23° C., but has a non linear, voltage-dependent resistivity such that the composition becomes conductive if subjected to a sufficiently high voltage stress. The term “conductive polymer” is used herein to describe all such compositions. When the polymeric component comprises a crystalline polymer, the composition will usually exhibit a sharp increase in resistivity over a relatively narrow temperature range just below the crystalline melting point of the polymer, and such compositions are described as PTC compositions, the abbreviation “PTC” meaning positive temperature coefficient. The size of the increase in resistivity is important in many uses of PTC compositions, and is often referred to as the “autotherm height” of the composition. PTC conductive polymers are particularly useful in circuit protection devices and self-regulating heaters. Conductive polymers can contain one or more polymers, one or more conductive fillers, and optionally one or more other ingredients such as inert fillers, stabilizers, and anti-tracking agents. Particularly useful results have been obtained through the use of carbon black as a conductive filler.
For details of known or proposed conductive polymers and devices containing them, reference may be made, for example, to the documents incorporated herein by reference in the Detailed Description of the Invention below.
When a melt-processed, sintered, or otherwise shaped conductive polymer element is to be divided into smaller pieces, this has in the past been achieved by shearing (also referred to as “dicing”) the conductive polymer element. For example, many circuit protection devices are made by shearing a laminate comprising two metal foils and a laminar PTC conductive polymer element sandwiched between the foils.
SUMMARY OF THE INVENTION
We have discovered that in many cases, important advantages can be obtained by dividing a conductive polymer mass into a plurality of parts by a process in which at least part of the division is effected by causing the conductive polymer element to break, along a desired path, without the introduction of any solid body into the conductive polymer element along that path. The resulting cohesive failure of the conductive polymer produces a surface (referred to herein as a “fractured” surface) which is distinctly different from that produced by a shearing process, which necessarily results in deformation of the conductive polymer by the cutting body. In order to control the path along which the conductive polymer element breaks, we prefer to provide one or more discontinuities which are present in one or more members secured to the conductive polymer, and/or in the conductive polymer itself, and whose presence causes the conductive polymer to fracture along desired paths which are related to the discontinuities.
The invention preferably makes use of assemblies in which a conductive polymer element is sandwiched between metal members having corresponding physical discontinuities in the form of channels. When such an assembly is bent in the regions of the channels, the conductive polymer element will fracture along paths which run between the corresponding channels in the metal members. However, the invention includes the use of other types of physical discontinuity and other kinds of discontinuity which will interact with a physical or other force to cause fracture of the conductive polymer along a desired path.
We have found the present invention to be particularly useful for the production of devices from a laminar assembly comprising a laminar PTC conductive polymer element sandwiched between metal foils. We have found that, such devices, especially when they are small (e.g. have an area of less than 0.05 inch
2
), have a slightly higher resistance and a substantially higher autotherm height than similar devices produced by the conventional shearing process. The invention is particularly useful for the production of devices of the kind described in Ser. Nos. 08/121,717 and 08/242,916.
In one preferred aspect, the present invention provides a device comprising an element which
(a) is composed of a composition which comprises (i) a polymeric component and (ii), dispersed in the polymer, electrically conductive particles, and
(b) has at least one fractured surface.
A preferred embodiment of this aspect of this invention is a device which comprises
(1) a laminar conductive polymer element which
(a) is composed of a composition which comprises (i) the polymeric component and (ii) the electrically conductive particles in an amount such that the composition has a resistivity at 23° C. of less than 10
6
ohm-cm, and
(b) has a first principal face, a second principal face parallel to the first face, and at least one transverse face which runs between the first and second faces and at least a part of which has a fractured surface;
(2) a first laminar electrode which has (i) an inner face which contacts the first principal face of the conductive polymer element, and (ii) an outer face; and
(3) a second laminar electrode which has (i) an inner face which contacts the second principal face of the conductive polymer element, and (ii) an outer face.
In another preferred aspect, the present invention provides a method of making a device, which method comprises
(1) making an assembly which (a) comprises an element composed of a composition comprising (i) a polymeric component, and (ii), dispersed in the polymeric component, electrically conductive particles, and (b) has one or more discontinuities in or adjacent to the conductive polymer element; and
(2) separating the assembly into two or more parts by a treatment which causes cohesive failure of the conductive polymer element along a path which is related to the discontinuity.
A preferred embodiment of this aspect of the invention is a method wherein the assembly comprises
(A) a laminar conductive polymer element which
(a) is composed of a composition which comprises a polymeric component and, dispersed in the polymeric component, electrically conductive particles in an amount such that the composition has a resistivity at 23° C. of less than 10
6
ohm-cm, and
(b) has a first principal face and a second principal face parallel to the first face,
(B) a plurality of upper laminar conductive members, each of which has (a) an inner face which contacts the first principal face of the conductive polymer element and (b) an outer face, the upper conductive members defining, with intermediate portions of the conductive polymer element, a plurality of upper fracture channels, and
(C) a plurality of lower laminar conductive members, each of which has (a) an inner face which contacts the second principal face of the conductive polymer element, and (b) an outer face, the lower conductive members defining, with intermediate portions of the conductive polymer element, a plurality of lower fracture channels; and
wherein step (2) of the process comprises applying physical forces to the assembly which cause the conductive polymer element to fracture along a plurality of paths each of which runs between one of the upper fracture channels and one of the lower fracture channels.
In another preferred aspect, this invention provides an assembly which can be divided into a plurality of devices by method of the invention, and which comprises
(A) a laminar conductive polymer element wh
Beadling William Cardwell
Thompson Mark S.
Toth James
Zhang Michael
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