PTC compositions

• US 4,591,700 A
• Filed: 03/12/1984
• Issued: 05/27/1986
• Est. Priority Date: 05/19/1980
• Status: Expired due to Term

• Current Assignee: Tyco Electronics Corporation (TE Connectivity Ltd. (United Kingdom))
• Original Assignee: Raychem Limited (TE Connectivity Limited)
• Inventors: Sopory, Umesh K.
• Primary Examiner(s): Barr, Josephine L.
• Application Number:
  US06/589,032
• Time in Patent Office: 806 Days
• Field of Search: 252/511, 252/512, 252/518, 252/503, 252/506, 524/495, 524/496, 524/500, 524/544-546, 219/505, 219/541, 219/542, 219/543, 219/548, 219/552, 219/553
• US Class Current: 219/505
• 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

Abstract

Novel PTC conductive polymer compositions contain a mixture of two crystalline polymers of different melting points, the higher melting of the polymers having a melting point which is at least 160° C. and at least 25° C. higher than the melting point of the other polymer. The compositions do not increase in resistivity by a factor more than 2 when maintained at 150° C. for 1000 hours, and are therefore particularly suitable for self-limiting heaters which can be used on apparatus which is periodically subjected to high temperatures, e.g. during steam-cleaning thereof.

Claims

1. A self-limiting heater suitable for use at line voltages of about 120 volts or more which comprises

(a) a PTC element composed of a PTC conductive polymer composition which exhibits PTC behavior, which has a resistivity at 25°

C. of at least 10² ohm.cm and which comprises

(i) 50 to 85%, by weight of the composition, of a polymer component which comprises a mixture of a first crystalline fluorinated polymer having a first melting point, T₁, which is from 100°

C. to 175°

C. and a second crystalline fluorinated polymer having a second melting point, T₂, which is at least 200°

C., the ratio by weight of said first polymer to said second polymer being from 1;

3 to 3;

1, and

(ii) a particulate filler component which has been dispersed in said polymer component and which comprises carbon black in amount 8 to 40% by weight of the composition; and

(b) two electrodes which are are in electrical contact with said PTC element and which can be connected to a source of electrical power to cause current to flow through the PTC element;

said heater having a resistance at 25°

of R₂₅ ohms. and a resistance at 25°

C. after being maintained at a temperature of 160°

C. for 1000 hours, R₁₀₀₀, which is less than 2×

R₂₅.

2. A heater according to claim 1 wherein the first polymer is polyvinylidene fluoride.

3. A heater according to claim 2 wherein the second polymer is an ethylene/tetrafluoroethylene copolymer.

4. A heater according to claim 3 wherein the PTC element is substantially free from cross-linking.

5. A heater according to claim 1 wherein the ratio by weight of the first polymer to the second polymer is 1:

2 to 2;

1.

6. A heater according to claim 5 wherein said ratio is 0.5 to 1.

7. A heater according to claim 6 wherein said ratio is 0.6 to 0.8.

8. A heater according to claim 1 wherein said PTC composition contains 65 to 75% by weight of said polymer component, 10 to 15% by weight of carbon black and 10 to 25% by weight of non-conductive particulate filler.

9. A heater according to claim 1 wherein T₂ is at least (T₁ +70)°

C.

10. A heater according to claim 9 wherein T₂ is at least (T₁ +90)°

C.

11. A heater according to claim 1 wherein the carbon black is the sole conductive filler in the PTC composition.

12. A heater according to claim 1 wherein the amount of carbon black is 10 to 15% by weight.

13. A heater according to claim 1 wherein the PTC composition contain 10 to 25% by weight of a non-conductive filler.

14. A heater according to claim 1 wherein the first and second polymers are incompatible with each other.

15. A heater according to claim 1 which is a flexible elongate heating strip in which the PTC strip is elongate and flexible and has a resistivity of 10² to 10⁵ ohm.cm, and the electrodes are elongate, parallel, flexible metal wires which are embedded in the PTC element.

16. A heater according to claim 15 wherein the first and second polymers are incompatible with each other.

17. A heater according to claim 1 wherein the conductive polymer composition has a resistivity at 25°

C. of 10² to 10⁵ ohm.cm, the first polymer is polyvinylidene fluoride and the second polymer is an ethylene/tetrafluoroethylene copolymer.