Electrical devices comprising cross-linked conductive polymers

• US 4,857,880 A
• Filed: 02/08/1988
• Issued: 08/15/1989
• Est. Priority Date: 03/14/1985
• Status: Expired due to Term

• Current Assignee: Littelfuse Incorporated
• Original Assignee: Raychem Corporation (TE Connectivity Limited)
• Inventors: Fahey, Timothy E., Au, Andrew N., Deep, Marguerite E., Jacobs, Stephen M.
• Primary Examiner(s): Goldberg, E. A.
• Assistant Examiner(s): LATEEF, MARVIN M
• Application Number:
  US07/153,178
• Time in Patent Office: 554 Days
• Field of Search: 338/22 R, 338/225 D, 338/23, 338/212, 264/104, 264/105, 219/549, 219/506, 219/510, 219/511, 219/553
• US Class Current: 338/22.00R
• CPC Class Codes: H01C 7/027 consisting of conducting or semi-conducting material dispersed in a non-conductive organic material
• CPC Class Codes: H05B 3/146 Conductive polymers, e.g. polyethylene, thermoplastics

Abstract

Electrical devices containing PTC conductive polymers which have been cross-linked in two steps, preferably by radiation. The conductive polymer is heat-treated above the temperature at which it begins to melt between the two cross-linking steps, and/or the cross-linking steps are such that a center section of the conductive polymer, intermediate the electrodes, is substantially more cross-linked than the conductive polymer adjacent the electrodes. The process is particularly useful for the preparation of circuit protection devices which are subject to high voltage faults.

Claims

1. A process for the preparation of an electrical device which comprises

(1) a laminar PTC element composed of a cross-linked conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component comprising a crystalline polymer and, dispersed in the polymeric component, a particulate conductive filler;

and

(2) two laminar electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element, which process comprises the steps of;

(a) melt-extruding the conductive polymer composition to form a laminar PTC element which does not contain an electrode;

(b) subjecting at least part of the PTC element to a first cross-linking step;

(c) heating at least part of the crosslinked PTC element to a temperature above T_I, where T_I is the temperature at which the conductive polymer starts to melt;

(d) cooling the cross-linked and heated PTC element to recrystallize the polymer;

(e) subjecting at least part of the cross-linked, heated and cooled PTC element to a second cross-linking step to effect further cross-linking thereof; and

(f) securing laminar electrodes to the PTC element.

2. A process according to claim 1 wherein the PTC element is cross-linked by irradiation in step (b) and in step (e).

3. A process according to claim 2 wherein the whole of the PTC element is irradiated in step (b) and in step (e).

4. A process according to claim 2 wherein the radiation dose in step (b) is 5 to 60 Mrad, and the radiation dose in step (e) is at least 10 Mrad.

5. A process according to claim 2 wherein the radiation dose in step (b) is 10 to 50 Mrad, and the radiation dose in step (e) is 50 to 180 Mrad.

6. A process according to claim 2 wherein the radiation dose in step (b) is 15 to 40 Mrad, and the radiation dose in step (e) is 50 to 100 Mrad.

7. A process according to claim 1 wherein in step (c) the cross-linked PTC element is heated to a temperature above T_M, where T_M is the temperature at which melting of the conductive polymer is complete.

8. A process according to claim 1 wherein in step (d) the cross-linked and heated PTC element is cooled at a rate of less then 4°

C. per minute over the temperature range in which recrystallization takes place.

9. A process according to claim 2 wherein the electrical device is a circuit protection device having a resistance at room temperature of less than 100 ohms and the conductive polymer composition has a resistivity at 23°

C. of less than 50 ohm-cm.

10. A process according to claim 9 wherein the conductive polymer comprises carbon black dispersed in polyethylene.

11. A process according to claim 9 wherein the radiation doses in steps (b) and (e) are such that when the device is converted into a high temperature, high resistance state by passing through the device a current of 1 amp from a power source of 600 volts AC, the PTC element reaches a maximum surface temperature which is at most 1.2 times T_M, where T_M is the temperature in degrees C at which melting of the conductive polymer is complete.

12. A process according to claim 2 wherein step (f) is carried out after step (a) and before steps (b) to (e).

13. A process according to claim 2 wherein step (f) is carried out after steps (a) to (e).

14. A process according to claim 2 wherein step (f) is carried out after steps (a) and (b) and before steps (c) to (e).

15. A process according to claim 1 wherein the PTC element is cross-linked by chemical cross-linking in step (b) and by irradiation in step (e).

16. A process according to claim 1 wherein the PTC element has a thickness of at least 0.040 inch.

17. A process according to claim 16 wherein the PTC element has a thickness of at least 0.060 inch.

18. A process according to claim 17 wherein the PTC element has at thickness of at least 0.100 inch.

19. A circuit protection device which has a resistance of less than 100 ohms and which comprises

(1) a laminar PTC element composed of a cross-linked conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component comprising a crystalline polymer and, dispersed in the polymeric component, a particulate conductive filler;

and

(2) two laminar electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element;

the cross-linking of said conductive polymer composition being such that, when said circuit protection device is converted into an equilibrium high temperature, high resistance state by passing through the device a current of 1 amp from a power source of 600 volts AC, said PTC element has a maximum surface temperature in the equilibrium state which is at most 1.2 times T_M, where T_M is the temperature in degrees C at which melting of the conductive polymer is complete.

20. A device according to claim 19 wherein said maximum surface temperature is at most 1.1 times T_M.