Electrical Devices Containing Conductive Polymers

The invention is illustrated in the accompanying drawing in which:

FIG. 1 is a side view of a device of the invention.

The microrough surface of the electrodes used in the present invention can be prepared in a number of different ways. The preferred method is electrodeposition, the microrough surface being the surface which is exposed to the electrolyte. For example, electrodeposited foils, particularly copper and nickel foils, are preferred for use in this invention. It is also possible to use other processes which result in a similar degree of roughness, e.g. irregularities which protrude from the surface by a distance of at least 0.03 microns, preferably at least 0.1 microns, particularly 0.1 to 100 microns, and which have at least one dimension parallel to the surface which is at most 500 microns, preferably at most 100 microns, particularly at most 10 microns, and which is preferably at least 0.03 micron, particularly at least 0.1 micron. The irregularities can be of the same shape as those produced by electrodeposition, e.g. generally spherical nodules protruding from the surface, or they can be of a different shape. Such processes can create the microrough surface by removal of material from a smooth surface, e.g. by etching, by chemical reaction with a smooth surface, e.g. by galvanic deposition, or by deposition of a microrough layer of the same or a different material on a smooth surface. A smooth foil can be treated by contact e.g. rolling or pressing with a patterned surface to generate a microroughness. The microrough surface can if desired be treated to change its chemical characteristics. For example, an electrodeposited metal foil can be passivated i.e. rendered inactive or less chemically reactive, by an appropriate treatment, e.g. one which provides a coating thereon of a water-stable oxide, especially a zinc-nickel or nickel treatment of an electrodeposited copper foil. Such treatment is for example desirable where the metal may catalyse degradation of the conductive polymer. Such treatment can also be carried out so as to provide appropriate acid-base interactions with the conductive polymer.

We have found that metal foils having a microrough surface which has a structure comprising "macronodules" which themselves comprise "micronodules" provide particularly good adhesion. Macronodules are irregularities which protrude from the surface and have one dimension parallel to the surface which is at most 50 microns, preferably at most 25 microns, particularly at most 15 microns, and is at least 3 microns. Micronodules are irregularities which may form part of a macronodule or protrude from the surface and which have one dimension parallel to the surface which is at most 4 microns, preferably at most 3 microns, particularly at most 2 microns, and is at least 0.1 micron, preferably at least 0.5 micron. During electrodeposition, the process which is preferably used to form this type of structure, micronodules grow together in clusters to form macronodules. The resulting surface thus comprises both macronodules and micronodules. The electrodes particularly useful for this invention have a surface which comprises at least 50 percent macronodules, preferably at least 60 percent macronodules, particularly at least 80 percent macronodules, most particularly consists essentially of macronodules. The surface comprising macronodules may be made of the same or a different material from the metal foil.

Through the use of microrough surfaces on the electrodes, the range of conductive polymers which can be used is increased. For example, when using a conventional metal foil, it is often necessary for the conductive polymer to include a polar copolymer or other polymeric ingredient which provides improved adhesion to the metal foil but whose presence detracts from the desired electrical characteristics. Thus this invention makes it possible to use a wider range of conductive polymers (both PTC and ZTC) in situations in which separation of the electrode and the conductive polymer is an anticipated problem, either as a result of flexing, different coefficients of expansion, exposure to solvents, e.g. diesel fuel, or thermal or electrical shock. Suitable conductive polymers are disclosed in the documents incorporated herein by reference. Preferrred conductive polymers include those based on polyolefins, particularly high density polyethylene, and those based on fluoropolymers, particularly polyvinylidene fluoride. Advantages of the improved adhesion include the ability to punch very small parts from foil laminates and substantially improved life, even when exposed to high voltages.

The suitability of a particular metal foil for use in this invention, in particular its ability to adhere adequately to the conductive polymer, can be assessed by the use of a standard test which is used in industry to measure the level of adhesion of an electrodeposited copper foil to an epoxy board, namely MIL 13149, "Epoxy Board FR-4 Peel". We have found that for electrodeposited nickel foils preferred results are achieved by using a nickel foil which, when used in place of an electrodeposited copper foil in MIL 13149, has a peel strength of at least 9 pounds/linear inch when measured on FR-4 epoxy board.

The invention can be used in any of the devices described in the documents incorporated herein by reference, including in particular circuit protection devices, particularly laminar devices having for example a resistance of less than 100 ohms, particularly less than 25 ohms, especially less than 1 ohm. Very small laminar devices, having at least one laminar dimension which is less than 0.2 inch, e.g. less than 0.15 inch, and even smaller, such as less than 0.1 inch, can be prepared by punching a foil laminate. Other useful devices include self-limiting heaters, especially flexible sheet heaters having a total surface area of at least 1.0 square inch.

The present invention has been found to be of particular value for circuit protection devices comprising columnar electrodes, e.g. solid or stranded wires, which are embedded in a PTC conductive polymer. We have found that in many such devices, when using conventional wires, it is desirable to coat the wires with a graphite-containing composition before contacting them with the molten conductive polymer, in order to obtain devices with satisfactory stability. By using electrodes having a microrough surface, devices of comparable or better stability can be obtained without the use of such a coating. Solid or stranded wires on which a layer of the same or a different metal has been formed by electrodeposition may be used. Particularly preferred are copper wires on which a nickel coating has been formed by electrodeposition.

The invention is illustrated by the following Examples.

The ingredients listed in the Table below were tumble-blended, mixed in a Banbury mixer, melt-extruded into a water bath and chopped into pellets. After drying, the pellets were extruded as a sheet 8.25 inch (21.0 cm) wide and 0.030 inch (0.076 cm) thick, and samples 6 inch (15.3 cm) square were cut from the sheet.

Each sample was laminated between two metal foils 6.times.6.times.0.0014 inch in a heated press at 260.degree. C. and 4,000 lb. pressure for 2 minutes, followed by 7,000 lb. pressure for 3 minutes. The metal foils were electrodeposited copper foils which had been passivated with nickel and zinc on the surface adjacent the sample. Such foils are available from Yates Industries under the trade name TWI.

Disc-shaped devices 0.125 inch (0.318 cm) in diameter were punched from the laminates. A 24 AWG nickel-plated steel lead was attached to each metal foil on each device. The devices were then encapsulated by an epoxy resin which was cured at 110.degree. C. for 3 hours.

The physical and electrical stability of the foil/conductive polymer interface remained excellent under a wide variety of conditions.

A device 10 of Example I is shown in FIG. 1. The device 10 comprises a laminar element 12 which is composed of a conductive polymer exhibiting PTC behavior and first and second electrodeposited metal foil electrodes (numerals 14 and 16), which electrodes 14 and 16 are in direct physical contact with a surface of the conductive polymer element 12.

Following substantially the same procedure as in Example 1, devices were made from a conductive polymer containing the ingredients listed in the Table below.

The following ingredients were mixed following the above procedure.

The pellets were extruded into a sheet 11 inch (27.9 cm) wide and 0.020 inch (0.051 cm) thick and were irradiated 20 Mrad using a 1.5 MeV electron beam. Samples 6 inch (15.3 cm) square were cut from the sheet. Two samples of polymer sheet were laminated between two (6.times.6 in) sheets of (1 ounce) electrodeposited copper foil by exposing to 1500 lbs pressure at 200.degree. C. for 4 minutes, 20,000 lbs pressure for 2 minutes, before cooling at 20,000 lbs pressure in a water-cooled press. Such foils are available from International Foils. The resulting sheet was 0.035 inch (0.089 cm). Flat 0.005 inch (0.013 cm) copper leads were soldered onto 1 inch (2.54 cm).times.2 inch (5.08 cm) samples cut from the slab. The heaters were then encapsulated in epoxy.

KF 1000 is a polyvinylidene fluoride available from Kureha.

Vulcan XC-72 is a carbon black available from Cabot with a particle size of 30 millimicrons and a surface area of 254 m.sup.2 /g.

Omya Bsh is CaCO.sub.3 available from Omya Inc.

The prorad used is triallylisocyanurate.

7945 g. of high density polyethylene (Marlex 6003 available from Phillips Petroleum), 7200 g. of carbon black (Statex G available from Columbian Chemicals) and 263 of antioxidant were mixed together, melt-extruded into a water bath and chopped into pellets. After drying, 12,185 g. of the pellets were mixed with 4726 g. of alumina trihydrate (Hydral 705 available from Alcoa), melt-extruded into a water bath and chopped into pellets. After drying, the pellets were melt-extruded through a crosshead die around two preheated 22 AWG solid copper wires that had been electroplated with nickel to a nominal thickness of 300 to 500 microinch (0.762-1.27 mm). The resulting strip (about 0.58 by 0.25 cm) was irradiated to 75 Mrad using a 1 MeV electron beam. The irradiated strip was cut into lengths of about 1 cm long, from which circuit protection devices were prepared in which the conductive polymer element was about 0.76 cm long and the electrodes extended from one side of the PTC element by about 0.25 cm.