PTC Devices comprising oxygen barrier layers

• US 4,315,237 A
• Filed: 11/30/1979
• Issued: 02/09/1982
• Est. Priority Date: 12/01/1978
• Status: Expired due to Term

• Current Assignee: Littelfuse Incorporated
• Original Assignee: Raychem Limited (TE Connectivity Limited)
• Inventors: Gotcher, Alan J., Middleman, Lee M.
• Primary Examiner(s): Albritton, C. L.
• Application Number:
  US06/098,711
• Time in Patent Office: 802 Days
• Field of Search: 338/22 R, 338/225 D, 338/223, 338/224, 338/225, 338/275, 219/505, 361/106, 29/612, 252/502, 252/503, 252/512
• 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

Abstract

Electrical devices comprising PTC elements have improved electrical stability when they comprise an oxygen barrier which restricts access of air to the element so that the PTC element absorbs oxygen at a rate less than 10^-6 cc/sec/gram. The devices are for example circuit control devices or self-limiting heaters. Preferred PTC elements comprise a polymer having dispersed therein carbon black and an additive which stabilizes the polymer against degradation, especially an organic antioxidant. The oxygen barrier may for example be a layer of a polymeric composition or a self-supporting container principally made of metal and filled with an inert gas.

Claims

1. An electrical device which comprises

(1) a PTC element which is composed of a composition which exhibits PTC behavior with a switching temperature T_s and which comprises

(a) a macromolecular polymer;

and

(b) conductive particles dispersed in said polymer;

(2) 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 PTC element; and

(3) an oxygen barrier which, when the device is in air at standard temperature and pressure, restricts access of air to the PTC element so that the rate at which the PTC element absorbs oxygen is less than 10^-6 cc/sec/gram.

2. A device according to claim 1 which exhibits a change in resistance, at at least one temperature between (T_s -110)°

C. and T_s, of -50% to +200%, after having been subjected to an aging treatment which comprises passing current through the device for 250 hours, the current being such that I² R heating of the device maintains said PTC element at a temperature between T_s and (T_s +50)°

C.

3. A device according to claim 1 wherein said rate at which the PTC element absorbs oxygen is less than 4×

10^-7 cc/sec/gram.

4. A device according to claim 3 wherein said rate is less than 3×

10^-7 cc/sec/gram.

5. A device according to claim 4 wherein said rate is less than 2×

10^-7 cc/sec/gram.

6. A device according to claim 2 which exhibits a change in resistance, at at least one temperature between (T_s -110)°

C. and T_s, of -50% to +100%, after having been subjected to said aging treatment.

7. A device according to claim 2 which exhibits a change in resistance, at at least one temperature between (T_s -60)°

C. and T_s, of -50% to +100%, after having been subjected to said aging treatment.

8. A device according to claim 2 which exhibits a change in resistance, at at least one temperature between (T_s -110)°

C. and T_s, of -50% to +200%, after having been subjected to an aging treatment which comprises passing current through the device for 500 hours, the current being such that I² R heating of the device maintains said PTC element at a temperature between T_s and (T_s +50)°

C.

9. A device according to claim 1 wherein said barrier is composed of a material having an oxygen permeability rate of less than 5×

10^-9 cc(STP)/cm² /mm/sec/cmHg.

10. A device according to claim 9 wherein said oxygen permeability rate is less than 10^-9 cc(STP)/cm² /mm/sec/cmHg.

11. A device according to claim 9 wherein said barrier is 0.001 to 0.050 inch thick and comprises at least one layer of an electrically insulating composition which comprises at least one polymer and which has a Young'"'"'s Modulus greater than 100,000 psi.

12. A device according to claim 11 wherein said barrier layer is 0.01 to 0.03 inch thick.

13. A device according to claim 9 wherein said barrier comprises at least one polymer selected from the group consisting of polyvinylidene chloride, polyvinyl fluoride, polyethylene terephthalate, rubber hydrochloride, polychlorotrifluoroethylene, phenol-formaldehyde resins, polyamides, epoxy resins, styrene/acrylonitrile copolymers, cellulose acetate, butadiene/acrylonitrile copolymers, polycarbonates, polystyrene, isobutylene/isoprene copolymers, polyethylene, ethylene/tetrafluoroethylene copolymers, vinylidene fluoride/hexafluoropropylene polymers and fluorinated ethylene/propylene copolymers.

14. A device according to claim 9 wherein said barrier is such that, when the device is placed in air, the only oxygen which can contact at least 95% of the surface area of the PTC element is oxygen which has passed through the barrier layer.

16. A device according to claim 9 which is a circuit control device and wherein the barrier comprises a self-supporting container which is principally made of metal, with the electrodes passing through a wall composed of a ceramic or rigid plastics material.

17. A device according to claim 16 wherein said container is filled with a gas free from oxygen.

18. A device according to claim 9 which is free from voids between said PTC element and said barrier.

19. A device according to claim 1 wherein at least one of said electrodes provides a part of said barrier.

20. A device according to claim 1 wherein said PTC composition further comprises an additive which stabilises said macromolecular polymer against degradation and which reduces said change in resistance on aging.

21. A device according to claim 20 wherein each of said electrodes is in contact with a conductive polymer composition which comprises a macromolecular polymer and conductive particles dispersed in said polymer, and at least the outer surface of each of said electrodes is composed of a metal which does not catalyse degradation of said conductive polymer composition.

22. A device according to claim 20 wherein each of said electrodes is in contact with said PTC element and at least the outer surface of each of said electrodes is composed of a metal which does not catalyse degradation of said PTC element.

23. A device according to claim 22 wherein at least the outer surface of each of said electrodes is composed of a metal selected from the group consisting of nickel, tin, silver and gold.

24. A device according to claim 1 wherein said PTC composition comprises an organic polymer having at least 10% crystallinity and having dispersed therein a carbon black having a particle size of 20 to 250 millimicrons.

25. A device according to claim 24 wherein said organic polymer is a polyolefin.

26. A device according to claim 24 wherein said PTC composition also comprises a non-conductive inorganic filler.

27. A device according to claim 9 which is a circuit control device and wherein the barrier comprises a self-supporting container.

28. A device according to claim 27 wherein the barrier comprises a polymeric material.

29. A device according to claim 28 wherein the barrier also comprises a metal.

15. A device according to calim 14 wherein said barrier provides a hermetic seal around said PTC element.