EP0038713B1

EP0038713B1 - Conductive polymer compositions containing fillers

Info

Publication number: EP0038713B1
Application number: EP81301764A
Authority: EP
Inventors: Joseph Hill Evans
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1980-04-21
Filing date: 1981-04-21
Publication date: 1987-09-02
Also published as: EP0038713A2; HK83089A; JPH0428744B2; DE3176415D1; CA1176453A; EP0038713A3; ATE29329T1; JPS56161463A; GB2074169B; GB2074169A

Abstract

The invention relates to conductive polymer compositions comprising carbon black or graphite dispersed in a polymer. The compositions of the invention further comprise an arc-controlling additive such as alumina trihydrate. Such compositions have improved performance when subjected to electrical stress and are, therefore, particularly useful in circuit protection devices.

Description

This invention relates to conductive polymer compositions and devices containing them, especially circuit protection systems.
Conductive and semi-conductive compositions comprising carbon black dispersed in a polymer are known. They may have room temperature resistivities ranging from less than 1 ohm - cm to 10⁸ ohm - cm or more, and may exhibit positive temperature coefficient (PTC) behaviour zero temperature coefficient (ZTC or constant wattage) behaviour or negative temperature coefficient (NTC) behaviour. Reference may be made, for example, to U.S. Patent Nos. 2,978,655, 3,243,753, 3,351,882, 3,823,217, 3,861,029, 3,950,604, 4,017,715, 4,177,376 and 4,246,468 (published 20.1.81), DE-A-2,413,475, 2,746,602, 2,755,076 and 2,821,570, to J. Applied Polymer Science 19 813-815 (1975), Klason and Kubat, and to Polymer Engineering and Science 18, 649-653 (1978), Narkis et al. Recent advances in this field are described in DE-A-2,948,350 (publ. 19.6.80), 2,948,281 (publ. 19.6.80), 2,949,173 (publ. 26.6.80) and 3,002,721 (publ. 7.8.80), in European Published Application Nos. 0,020,081 (publ. 10.12.80), 0,022,611 (publ. 21.1.81), 0,026,571 (publ. 8.4.81) and 0,028,142 (publ. 6.5.81), and in the European Patent Applications filed contemporaneously with this application and published as European Published Application Nos. 0,038,718, 0,038,714, 0,038,716 and 0,038,717.
US 3351882 discloses a device which comprises a PTC element composed of two electrodes and a conductive polymer composition, the composition comprising a polymer having carbon black dispersed therein.
In recent research into the use of circuit protection devices containing PTC conductive polymer elements, it was observed that previously proposed devices 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. We 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 mor.e 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, described, for example in GB Patent No. 831490, 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).
The invention provides an electrical device which comprises:

(1) a PTC element composed of a conductive polymer composition which exhibits PTC behaviour and which comprises an organic polymer component and, dispersed in the polymer component a conductive filler component which comprises carbon, and
(2) at least two electrodes for passing current through the PTC element, characterised in that the PTC element is composed of a composition which has a resistivity at 23°C of less than 10⁶ ohm. cm, and which comprises:
- (a) 20 to 91% by volume of an organic polymer component;
- (b) 4 to 65% by volume of a conductive filler component which consists essentially of carbon black and/or graphite; and
- (c) an arc controlling additive which is a hydrated inorganic particulate material which is dispersed in the polymer component.

The PTC compositions defined above are particularly useful in circuit protection devices as described in the contemporaneously filed European Application Nos. 0,038,715, 0,038,716 and 0,038,717.
The compositions can be any of the PTC compositions which are disclosed in the prior art and the earlier applications referred to above, which have a resistivity at 23°C of less than 10⁶ ohm - cm, which comprise 20 to 91% by volume of a conductive filler which consists essentially of carbon black and/or graphite, and which have been modified by the inclusion of at least one arc-controlling additive as defined in Claim 1. The invention is especially valuable in relation to circuit protection devices, which usually have a resistance below 50 ohms, at 23°C as described in European Published Application No. 0,038,715, and in which the PTC composition has low resistivity at 23°C, e.g. below 20 ohm · cm, preferably below 10 ohm - cm, especially below 2 ohm - cm.
The preferred arc-controlling additive for use in the present invention is alumina trihydrate, Al₂O₃ · 3H₂0. Another additive is magnesia hydrate.
The conductive filler and the arc-controlling additive preferably have a total surface area of at least 1,800, especially at least 3,000, particularly at least 4,000, m2/100 cm³ of composition, with higher values, e.g., at least 8,000 m²/100 cm³, at least 10,000 m²/100 cm³ and at least 12,000 m²/100 cm³ 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 British Patent No. 1,575,465 and the oxide of elements of the transition series, lanthanide series or non-transuranic actinide series disclosed in British Patents Nos. 1,337,951 and 1,337,952.
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 nm (millimicrons) and a surface area, S in m²/g such that S/D is not more than 10.
When using such a carbon black, preferably the quantity
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.
Conductive polymer compositions comprising a crystalline polymer component, at least 4% by volume of carbon black and at least 4% by volume of a non-conductive particulate filler, the total surface area of the carbon black and filler being at least 1800 m²/100 cm³ of composition, are described and claimed in the contemporaneously filed application corresponding to European Published Application No. 0,038,718.
The invention is illustrated by the following Examples. The words Fumex, Marlex, Sterling and Maglite are UK Registered Trade Marks.

Example 1

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

Furnex N765 (available from City Services Co.) has a particle size (D) of 60 nm (millimicrons), a density of 1.8 g/cm³, and a surface area (s) of 32 m²/g.

Marlex 6003 is a high density polyethylene with a melt index of 0.3 which is available from Phillips Petroleum Co.
The antioxidant used was an oligomer of 4,4-thio bis (3-methyl-6-t-butyl phenol) with an average degree of polymerization of 3-4, as described in U.S. Patent No. 3,986,981.
Hydral 705 is alumina trihydrate.
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°C, 1 Torr for 16 hours; 1 Torr=233,3 Pa).
The granulated final mix was melt extruded in the form of a strip about 1.27 cm wide and about 0.27 cm thick, using a cross-head die, around a pair of pre-heated 20 AWG (diameter 0.095 cm) 19-32 stranded nickel-plated copper wires whose centers were 0.6 cm apart. The extruded product was cut into 2.5 cm lengths, and the polymeric composition removed from half of each length to produce a circuit control device as shown in Figure 4 of the contemporaneously filed application corresponding to European Published Application No. 0,038,716.

Examples 2-4

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 7.6 cm 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.

Claims

1. An electrical device which comprises: (1) a PTC element composed of a conductive polymer composition which exhibits PTC behaviour and which comprises an organic polymer component and, dispersed in the polymer component a conductive filler component which comprises carbon, and (2) at least two electrodes for passing current through the PTC element, charactersed in that the PTC element is composed of a composition which has a resistivity at 23°C of less than 106 ohm. cm, and which comprises:

(a) 20 to 91% by volume of an organic polymer component;

(b) 4 to 65% by volume of a conductive filler component which consists essentially of carbon black and/or graphite; and

(c) an arc-controlling additive which is a hydrated inorganic particulate material which is dispersed in the polymer component.

2. A device according to Claim 1, characterised in that the arc-controlling additive is present in amount 5 to 65%, preferably 10 to 35%, by volume of the composition.

3. A device according to Claim 1 or 2, characterised in that the arc-controlling additive is alumina trihydrate.

4. A device according to Claim 1, 2 or 3, characterised in that the conductive filler and the arc-controlling additive have a total surface area of at least 1,800 m² per 100 cm³ of composition.

5. A device according to any one of the preceding Claims 2 to 4, characterised in that the polymer component has at least 5% crystallinity and the conductive filler component (a) comprises carbon black having a particle-size, D, which is from 20 to 150 nm (millimicrons) and a surface area, S, in m²/g such that S/D is not more than 10, and (b) is present in amount such that the composition has a resistivity at 23°C of less than 20 ohm - cm.

6. A device according to Claim 5 characterised in that the quantity

is less than 1.

7. A device according to any of the preceding claims characterised in that it is a circuit protection device having a resistance at 23°C of less than 50 ohms, preferably 0.1 to 25 ohms.

8. A device according to Claim 7, characterised in that said conductive polymer composition has a resistivity of less than 10 ohm. cm and comprises

(a) 20 to 91% by volume of a polymer component having at least 10% crystallinity;

(b) 4 to 65% by volume of carbon black; and

(c) 5 to 65% by volume of alumina trihydrate the carbon black and the alumina trihydrate having a total surface area of at least 1,800 m²/100 cm³ of composition.