Conductive Polymer Compositions Having Improved Properties Under Electrical Stress

1. Field of the Invention

This invention relates to conductive polymer compositions and electrical devices containing them.

2. Summary of the Prior Art

Conductive polymer compositions comprising carbon black dispersed in a polymer are known. Depending on the polymer and the carbon black and the relative amounts thereof, the compositions may have room temperature resistivities ranging from less than 1 ohm. cm to 10.sup.8 ohm.cm or more, and may exhibit positive temperature coefficent (PIC) behavior, zero temperature coefficient (ZTC or constant wattage) behavior or negative temperature coefficient (NTC) behavior. The major uses for conductive polymer compositions are in the shielding of cables and the inhibition of electrostatic charges, but the compositions can also be used in electrical devices in which current passes through an element composed of the composition, e.g. in heaters and current-limiting devices. Compositions useful in electrical devices generally have different properties from compositions useful in shielding and electrostatic applications. Reference may be made, for example, to U.S. Pat. Nos. 3,823,217 (Kampe), 3,861,029 (Smith-Johannsen et al.), 3,950,604 (Penneck), and 4,177,376 (Horsma et al.) and to U.S. patent application Ser. Nos. 904,736 (Penneck et al.), now abandoned, 732,792 (Van Konynenburg et al.), now abandoned, 751,095 (Toy et al.), now abandoned, 798,154 (Horsma), 965,343 (Van Konynenburg et al.), 965,344 now U.S. Pat. No. 4,237,441 (Middleman et al.), 965,345 now U.S. Pat. No. 4,242,573 (Middleman et al.), 6,773 now U.S. Pat. No. 4,255,698 (Simon) and 75,413 now U.S. Pat. No. 4,304,987 (Van Konynenburg). The disclosures of these patents and applications are incorporated by reference herein.

In recent research into the use of circuit protection devices containing PTC conductive polymer elements, it was observed that previously proposed devices (e.g. those described in application Ser. No. 965,344) failed to give repeated and effective protection against fault conditions in which the device was subjected to a combination of high current and high voltage. Reference should be made in this connection to the application of Middleman et al filed contemporaneously with this application and entitled "Circuit Protection Devices", Ser. No. 141,987 now U.S. Pat. No. 4,413,301 the disclosure of which is incorporated by reference herein. I have found that the performance, under conditions of high electrical stress, of conductive polymer compositions containing carbon black or graphite as the sole conductive filler, can be markedly improved by adding to such compositions one or more of the additives which have previously been used to improve the tracking resistance of polymeric insulating compositions. Although it is not entirely clear precisely why such additives have this valuable effect, they are referred to herein as arc-controlling additives. It is thought that their efficacy is probably due, at least in part, to their ability to extinguish arcs after they have been formed, but the additives may also act to reduce the susceptibility of the composition to form arcs in the first place. In any event, it is to be noted that the prior use of these additives, which has been to extinguish arcs on the contaminated surface of an electrical insulator, involves a very different situation from the present one, where the additives are effective in controlling arcs within a mass of conductive polymer (as well as at the surface thereof).

In one aspect, the invention provides a conductive polymer composition which has a resistivity at 23.degree. C. of less than 10.sup.6 ohm. cm and which comprises

(a) a polymer component which is present in amount 20 to 91% by volume of the composition;

(b) a conductive filler component which consists essentially of carbon black or graphite or a mixture of carbon black and graphite, which is dispersed in said polymer component and which is present in amount 4 to 65% by volume of the composition; and

(c) an arc-controlling additive which is distributed in said polymer component and which is effective in reducing the susceptibility of the composition to damage when subjected to electrical stress sufficient to cause arcing in the absence of said additive.

In another aspect the invention provides an electrical device which comprises:

(a) an element composed of a conductive polymer composition as defined above, and

(b) at least two electrodes which can be connected to a source of electrical power and which, when so connected, cause current to flow through said element.

The compositions of the invention may exhibit PTC, ZTC, or NTC behavior; for example any of the compositions disclosed in the prior art and the earlier applications referred to above may be modified by the inclusion of at least one arc-controlling additive. The invention is especially valuable in relation to PTC compositions, particularly those having low resistivities at 23.degree. C., e.g. below 20 ohm. cm, preferably below 10 ohm. cm, especially below 2 ohm. cm, which are useful in circuit protection devices.

The preferred arc-controlling additives for use in the present invention are particulate materials, particularly inorganic materials, especially hydrated inorganic materials. Particularly good results have been obtained using alumina trihydrate, Al.sub.2 O.sub.3.3H.sub.2 O. Other inorganic materials include magnesia hydrate, magnesia and alumina.

The conductive filler and the arc-controlling additive preferably have a total surface area of at least 1800, especially at least 3,000, particularly at least 4,000 m.sup.2 /100 cc of composition, with higher values, e.g. at least 8,000 m.sup.2 /100 cc at lease 10,000 m.sup.2 /100 cc and at least 12,000 m.sup.2 /100 cc being particularly preferred.

The composition should contain an effective amount of the arc-controlling additive, typically 5 to 65%, preferably 10 to 35%, by volume of the composition.

The composition can also contain further additives which are known to enhance the effectiveness of anti-tracking additives in insulating compositions. Examples of such additives include the phosphorus-containing compounds disclosed in U.S. Pat. No. 4,100,089 and U.S. patent application Ser. Nos. 869,244, now U.S. Pat. No. 4,219,607 and 869,268 now U.S. Pat. No. 4,223,071 and 869,269 U.S. Pat. No. 4,198,310, and the oxides of elements of the transition series, lanthanide series or non-transuranic actinide series disclosed in British Patents Nos 1337951 and 1337952 and U.S. application Ser. No. 434,126, now abandoned especially Fe.sub.2 O.sub.3 ; the disclosures of each of these patents and applications is incorporated herein by reference.

The conductive filler in the composition preferably consists essentially of at least one carbon black. The carbon black is selected with a view to the electrical characteristics desired in the composition, as taught by the various patents and applications referred to above. Thus for low resistivity PTC compositions, the carbon black preferably has a particle size, D, which is from 20 to 150 millimicrons and a surface area, S in m.sup.2 /g such that S/D is not more than 10 (See Ser. No. 965,343 now U.S. Pat. No. 4,237,444. When using such a carbon black, preferably the quantity ##EQU1## is less than 1.

The polymer component in the composition, which may comprise one or more polymers, preferably has a crystallinity of at least 1%, especially at least 5%, particularly at least 10%. Preferably the polymer component consists essentially of one or more crystalline polymers selected from polyolefins and copolymers of at least one olefin and at least one polar comonomer copolymerisable therewith, e.g. polyethylene or polypropylene. Other suitable polymers are referred to in the patents and applications referred to above.

The composition may be substantially free of cross-linking or may be cross-linked, e.g. to a gel fraction of at least 0.4 or 0.6. For some purposes, compositions free of cross-linking are preferred, because the presence of cross-linking tends to increase the likelihood of formation of carbonaceous conductive paths when arcing takes place.

The composition can be prepared by dispersing the carbon black or graphite, the arc-controlling additive and any other additives in the polymer component in any suitable way. The composition can be shaped by molding or extrusion or another melt-shaping technique into an element of the desired shape, any cross-linking thereof being carried out after such shaping.

The invention is illustrated by the following Examples.

The ingredients and amounts thereof given in the Table 1 below were used in this Example.

The ingredients for the master batch were dry blended and then mixed for 8 minutes in a Banbury mixer turning at high gear. The mixture was dumped, cooled and granulated. The final mix was prepared by dry blending 948 g of the Hydral 705 with 2439 g. of the master batch mixture, and then mixing the dry blend for 4-5 minutes in a Banbury mixer turning at high gear. The mixture was dumped, cooled, granulated and dried (at 70.degree. C., 1 Torr for 16 hours).

The granulated final mix was melt extruded in the form of a strip about 0.5 inch wide and about 0.105 inch thick, using a cross-head die, around a pair of pre-heated 20 AWG 19/32 stranded nickel-plated copper wires whose centers were 0.239 inch apart. The extruded product was cut into 1 inch lengths, and the polymeric composition removed from half of each length to produce a circuit control device as shown in FIG. 4 of the contemporaneously filed Middleman et al application referred to above.

The ingredients used in these Examples and the amounts thereof are shown in Table 2 below. The antioxidant is as specified in Table 1. Sterling NS and Sterling SO are available from Cabot, Hydral 705 from Alcan, Maglite D from Merck, and Kadox 15 from Gulf and Western, and they have the following properties

In Example 2, the Master Batch ingredients were blended in a pre-heated Banbury mixer, and the mixture dumped, cooled and granulated. 67 g of the granulated mixture was banded on a 3 inch electric roll mill, and the Hydral was added in portions to give a uniform mixture; mixing was continued for several more minutes and the mixture was then removed from the mill, cooled, granulated and compression-molded into slabs.

In Example 3, the Master Batch ingredients were blended in a pre-heated Banbury mixer, and the mixture dumped, cooled and granulated. 67 g of the granulated mixture was banded on a 3 inch electric roll mill, and the Hydral was added in portions to give a uniform mixture; mixing was continued for several more minutes and the mixture was then removed from the mill, cooled, granulated and compression-molded into slabs.

In Example 4 the procedure described for Example 2 was followed, using the different ingredients shown in Table 2, except that 50 g. of the granulated Master Batch was used and 50 g. of the filler (Maglite D) added to it.