Method for heat treating PTC devices

• US 6,582,647 B1
• Filed: 09/30/1999
• Issued: 06/24/2003
• Est. Priority Date: 10/01/1998
• Status: Expired due to Fees

• Current Assignee: Littelfuse Incorporated
• Original Assignee: Littelfuse Incorporated
• Inventors: Weber, Michael J., Hall, Tom J.
• Primary Examiner(s): ORTIZ, ANGELA Y
• Application Number:
  US09/408,645
• Time in Patent Office: 1,363 Days
• Field of Search: 264/234, 264/340, 264/345, 264/347, 252/511, 252/512, 252/500, 252/510
• US Class Current: 264/345
• CPC Class Codes: H01C 17/232 Adjusting the temperature coefficient; Adjusting value of resistance by adjusting temperature coefficient of resistance
• CPC Class Codes: H01C 7/028 consisting of organic substances

Abstract

The present invention provides a method for heat treating a polymer PTC composition to increase the peak resistivity of the composition making it especially well suited for high voltage applications. A polymer PTC composition having a melting point temperature T_mp is provided. The temperature of the polymer PTC composition is increased at a rate, r₁, to a temperature greater than T_mp. The temperature of polymer PTC composition is held at the temperature greater than T_mp for a predetermined period of time. Then the temperature of the polymer PTC composition is decreased to a temperature less than T_mp at a rate, r₂, wherein r₂ is greater than r₁.

Claims

1. A method for heat treating a polymer PTC composition, the method comprising the steps of:

providing a polymer PTC composition having a melting point temperature T_mp;

increasing the temperature of the polymer PTC composition at a first rate, r₁, to a temperature greater than T_mp;

holding the polymer PTC composition at the temperature greater than T_mp for a predetermined period of time;

decreasing the temperature of the polymer PTC composition to a temperature less than T_mp at a second rate, r₂, wherein r₂ is greater than r₁.

2. The method of claim 1, wherein the temperature less than T_mp is room temperature.

3. The method of claim 1, wherein the temperature greater than T_mp is at least 5-10°

C. greater than T_mp.

4. The method of claim 1, wherein r₂ is at least two times greater than r₁.

5. The method of claim 1, wherein r₂ is at least four times greater than r₁.

6. The method of claim 1, wherein r₂ is at least eight times greater than r₁.

7. The method of claim 1, wherein the polymer PTC composition comprises a polyolefin having a crystallinity of at least 20%.

8. The method of claim 7, wherein the polyolefin has a crystallinity of at least 50%.

9. The method of claim 1, wherein the polymer PTC composition comprises polyethylene.

10. The method of claim 1, wherein the polymer PTC composition comprises polyvinylidene fluoride.

11. The method of claim 1, wherein the polymer PTC composition comprises a modified polyolefin.

12. A method for heat treating a polymer PTC composition having an initial peak resistivity, R_pi, and a melting point temperature, T_mp, the method comprising the steps of:

increasing the temperature of the polymer PTC composition at a first rate, r₁, to a temperature greater than T_mp;

holding the polymer PTC composition at the temperature greater than T_mp for a predetermined period of time;

decreasing the temperature of the polymer PTC composition to a temperature less than T_mp at a second rate, r₂, wherein r₂ is greater than r₁; and

after decreasing the temperature of the polymer PTC composition, the composition has a new peak resistivity, R_pn, which is at least 1.5×

R_pi.

13. The method of claim 12, wherein R_pn is at least 2×

R_pi.

14. The method of claim 12, wherein R_pn is at least 10×

R_pi.

15. A method for heat treating an electrical circuit protection device having a PTC element and two electrodes, the PTC element being composed of a polymer PTC composition having a melting point temperature, T_mp, and an initial peak resistivity, R_pi, the method comprising the steps of:

increasing the temperature of the polymer PTC composition at a first rate, r₁, to a temperature greater than T_mp;

holding the polymer PTC composition at the temperature greater than T_mp for a predetermined period of time;

decreasing the temperature of the polymer PTC composition to a temperature less than T_mp at a second rate, r₂, wherein r₂ is greater than r₁ such that the polymer PTC composition has a new peak resistivity, R_pn, which is at least 1.5×

R_pi.

16. The method of claim 15, wherein the predetermined period of time is in a range of approximately 10-15 minutes.