PTC compositions and devices comprising them

• US 4,775,778 A
• Filed: 05/14/1985
• Issued: 10/04/1988
• Est. Priority Date: 10/15/1976
• Status: Expired due to Term

• Current Assignee: Littelfuse Incorporated
• Original Assignee: Raychem Limited (TE Connectivity Limited)
• Inventors: Lyons, Bernard J., van Konynenburg, Peter H.
• Primary Examiner(s): Barr, Josephine
• Application Number:
  US06/734,414
• Time in Patent Office: 1,239 Days
• Field of Search: 252/511, 252/512, 252/518, 204/159.11, 204/159.13, 524/495, 524/496, 524/492, 524/588, 219/549, 219/528, 219/548, 219/541, 219/542, 219/553, 219/550, 522/71, 338/22 R
• US Class Current: 219/549
• CPC Class Codes: C08K 3/04 Carbon
• CPC Class Codes: H01B 1/24 the conductive material comprising carbon-silicon compounds, carbon or silicon
• 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
• CPC Class Codes: H05B 3/56 Heating cables
• CPC Class Codes: H05B 3/565 flat cables

Abstract

Novel conductive polymer compositions which exhibit useful PTC behavior are produced by cross-linking processes. By correlating the polymer, the conductive filler dispersed therein, and the cross-linking conditions, PTC compositions having desired resistance/temperature characteristics can be prepared. In particular the cross-linking can be carried out so as to provide compositions having a switching temperature around the cross-linking temperature. The invention is applicable to both elastomeric and thermoplastic polymers, and provides for the first time a method of making PTC compositions exhibiting useful PTC behavior from elastomers.

Claims

1. An electrical device which comprises two or more electrodes and a PTC composition in contact therewith, the PTC composition comprising:

(1) a cross-linked elastomer prepared by cross-linking an elastomeric gum having a tensile stress of less than 10 psi at 20% elongation; and

(2) conductive particles enclosed with the cross-linked network of said elastomer, the particles having a size of at least 18 millimicrons;

which composition

(a) has a gel fraction of at least 0.6;

(b) exhibits PTC behavior with an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, and at least one T_s within the temperature range from room temperature to +250°

C.; and

(c) has a volume resistivity at temperatures below said T_s between 25 and 10⁵ ohm.cm.

2. A device according to claim 1 wherein the elastomer has been crosslinked with the aid of a chemical cross-linking agent.

3. A device according to claim 1 wherein the PTC composition is one whose T_s remains substantially unchanged when repeatedly subjected to thermal cycling which comprises heating the composition from a temperature below T_s to a temperature above T_s but below the peak temperature, followed by cooling to a temperature below T_s.

4. A device according to claim 1 wherein the conductive particles are carbon black particles having a size of 22 to 80 millimicrons.

5. A device ccording to claim 1 wherein the elastomer is a polysiloxane.

6. A device according to claim 5 wherein the PTC composition comprises a polysiloxane having silica particles dispersed therein.

7. A device according to claim 5 which is a strip heater comprising an elongate element of said PTC composition, two longitudinally extending, parallel electrodes embedded in said composition, and an outer layer of a protective and insulating composition.

11. A PTC composition according to claim 1 whose T_s remains substantially unchanged when repeatedly subjected to thermal cycling which comprises heating the composition from a temperature below T_s to a temperature above T_s but below the peak temperature, followed by cooling to a temperature below T_s.

12. A PTC composition according to claim 1 wherein the conductive particles are carbon black particles having a size of 22 to 80 millimicrons.

8. A PTC composition comprising:

(1) a cross-linked elastomer prepared by cross-linking an elastomeric gum having a tensile stress of less than 10 psi at 20% elongation; and

(2) conductive particles enclosed within the cross-linked network of said elastomer, the particles having a size of at least 18 millimicrons;

which composition

(a) has a gel fraction of at least 0.6;

(b) exhibits PTC behavior with an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, and at least one T_s within the temperature range from room temperature to +250°

C.; and

(c) has a volume resistivity at temperatures below said T_s between 25 and 10⁵ ohm.cm.

9. A PTC composition according to claim 8 wherein the elastomer has been cross-linked with the aid of a chemical cross-linking agent.

10. A PTC composition according to claim 9 wherein the elastomer has been cross-linked with the aid of a peroxide.

13. A PTC composition according to claim 8 which further comprises non-conductive particles enclosed within the cross-linked network of the elastomer.

14. A PTC composition according to claim 13 wherein the polymer is a polysiloxane and the non-conductive filler is silica.

15. A PTC composition according to claim 8 wherein the elastomer is a polysiloxane.

16. A PTC composition according to claim 15 wherein the conductive particles consist essentially of at least one carbon black.

17. A process for the preparation of a PTC composition which comprises:

(1) dispersing conductive particles having a size of at least 18 millimicrons in an elastomeric gum having a tensile stress of less than 10 psi at 20% elongation; and

(2) cross-linking the dispersion from step (1) to obtain a composition which has a gel fraction of at least 0.6 and which comprises conductive particles enclosed with the elastomer resulting from said cross-linking;

the particles, the elastomeric gum, and the cross-linking conditions being such that the cross-linked composition

(a) exhibits PTC behavior with an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, and at least one T_s within the temperature range from room temperature to +250°

C.; and

(b) has a volume resistivity at temperatures below T_s between 25 and 10⁵ ohm.cm.

18. A process according to claim 17 wherein the conductive particles comprise carbon black.

19. A process according to claim 17 wherein the elastomer is a polysiloxane.

20. A process according to claim 19 wherein the polysiloxane contain silica as a filler.

21. A process according to claim 19 wherein the conductive particles consist essentially of one or more carbon blacks.

22. A process according to claim 17 wherein the elastomeric gum is cross-linked by irradiation.

23. A process according to claim 17 wherein the elastomeric gum is cross-linked with the aid of a chemical cross-linking agent.

24. A process according to claim 17 wherein the cross-linking temperature is above 20°

C. and the conductive filler is a carbon black having a particle size of at least 20 millimicrons.

25. A process according to claim 17 wherein the cross-linking temperature is above 100°

C. and the conductive filler is a carbon black having a particle size of at least 30 millimicrons.

26. A process according to claim 17 wherein the cross-linking temperature is above 150°

C. and the conductive filler is a carbon black having a particle size of at least 40 millimicrons.