US 3824328 A
Packaged PTC heater elements wherein the package includes an outer material such as a phenolic resin in the form of a casing and an inner potting material which is thermally conductive and electrically insulated and does not deteriorate the PTC material. The potting material is one that does not include nucleophiles such as amines and the like and therefore excludes the epoxy resins. The potting material used is a silicone resin, polyamides, polyimides and ceramics which are not amine initiated. The PTC device can be completely potted in one of the above noted potting materials or a barrier layer of one of the above potting materials can be placed around the PTC material to separate it from an external potting material and then the PTC material with the barrier layer therearound can be potted with the well known epoxy resin.
Claims available in
Description (OCR text may contain errors)
United States Patent 1 Ting et a1.
[ ENCAPSULATED PTC HEATER PACKAGES  Inventors: Youn H. Ting, Attleboro, Mass; 7 Hans Adolf Stoeckler, Pawtucket,
 Assignee: Texas Instruments Incorporated,
 Filed: Oct. 24, 1972  App]. No.: 300,022
 US. Cl 174/52 PE, 174/52 S, 219/210, 317/234 E, 317/235 Q, 338/22 R, 338/275  Int. Cl. H05k 5/06  Field of Search 219/210, 552; 338/275, 338/22, 23, 24; 174/52 S, 52 PE; 317/234, 235 Q; 336/96; 29/612; 323/20  References Cited UNITED STATES PATENTS 2,829,320 4/1958 Dimond 317/234 3,081,374 3/1963 Burch 174/52 PE 3,210,701 10/1965 Fastner et al. 174/52 PE X 3,214,719 10/1965 Turner 338/22 R 3,242,393 3/1966 Pauli 338/275X 3,251,015 10/1966 Denham 336/96 3,254,282 5/1966 West 174/52 PE X 3,375,774 4/1968 Fujimura. 338/22 R X 3,390,226 6/1968 Beyerlein 174/52 PE  3,824,328 [451 July 16,1974
Primary Examiner-Velodymyr Y. Mayewsky Attorney, Agent, or Firm-John A. Haug; James P. McAndrews; Edward J. Connors, Jr.
 ABSTRACT Packaged PTC heater elements wherein the package includes an outer material such as a phenolic resin in the form of a casing and an inner potting material which is thermally conductive and electrically insulated and does not deteriorate the PTC material. The potting material is one that does not include nucleophiles such as amines and the like and therefore excludes the epoxy resins. The potting material used is a silicone resin, polyamides, polyimides and ceramics which are not amine initiated. The PTC device can be completely potted in one of the above noted potting materials or a barrier layer of one of the above potting materials can be placed around the PTC material to separate it'from an external potting material and then the PTC material with the barrier layer therearound can be potted with the well known epoxy resin.
4 Claims, 2 Drawing Figures PATENIED JUL 1 88974 I 1 ENCAPSULATED PTC HEATER PACKAGES The disclosure relates to a packaging for PTC materials, and more specifically, to a potting material for use with doped PTC materials which does not include nuv cleophiles which degrade the PTC material.
In the prior art, epoxy resins have normally been used as potting materials for PTC devices. These potting materials have a tendency to degrade the PTC materials which are normally formed from doped 2arium titanate. The reasonfor this is that the epoxy resins are normally made from secondary amines or acid anhydrides which are nucleophiles and react with chemabsorbed oxygen atoms on the boundaries of the PTC device. These chemabsorbed oxygen atoms act as electron traps. In the event that these electron traps become either neutralized or desorbed from the grain boundaries, the PTC effect is destroyed in the sense that electron traps are no longer available to stop the current flow. In order to eliminate this detrimental effect, it is necessary to look for classes of packaging material's which do not contain nucleophiles.
The degradation takes place in the form of gradual flattening of the PTC resistance-temperature curve. The degradation of the resistance-temperature curve, when the organic polymer, such as the epoxy, acts as a reducing agent, is caused by depletion of the oxygen supply in the grain boundary. In accordance with the present invention, it has been noted that certain potting materials do not cause the above described degradation of the PTC materials and particularly the degradation of the resistance-temperature curve. Briefly, in accordance with the present invention, one embodiment thereof, the prior art epoxy potting materials are replaced by potting materials which do not degrade the PTC heating materials. These include the silicone resin, polyamides, polyimides and ceramics which are not amine initiated. In accordance with the second embodiment of the invention, the PTC material is coated with a barrier of a nondegrading material such as one of the above described potting materials and then the remainder of the package is potted with the well known epoxy material.
It is therefore an' object of this invention to provide a PTC heater package with potting material which does not degrade the surface of the PTC heater.
It is a further object of this invention to provide a PTC heater package using a potting material taken from the class consisting of silicone resins, polyamides, polyimides and ceramics which are not amine initiated.
It is a yet further object of this invention to provide a package for containing a PTC heater material wherein the PTC material is surrounded by a nondestructive barrier layer placed between the heater and an epoxy or other degrading potting material.
The above objects and still further objects of the invention will immediately become apparent to those skilled in the art after consideration of the following preferred embodiments, which are provided by way of example and not by way of limitation, wherein:
Referring first to FIG. 1, there is shown a case 1 which may be metallic or may be made out of a material s'uch as phenolic resin. Positioned within the case 1 is a PTC heater 3 of well known type to which has been secured a pair of lead wires 5 and 7. It is understood that the PTC material has been previously metallized in well known manner so that the leads 5 and 7 can be secured thereto by soldering or the like. The wire 5 includes insulation therearound which extends outwardly out of the case I and the wire 7 includes insulation ll therearound which extends out of the case in the same manner.
In the event that case I is metallic, it is a requirement that the PTC heater 3 be electrically insulated from the case and that the case and potting compound 13 be thermally conductive. The PTC heater 3 is therefore encapsulated inside the phenolic case 1 by using such FIG. 1 is a view in section of a first embodiment of a package in accordance with the present invention a potting or sealing compound 13 which also provides a tight seal around the PTC pill or heater, thus serving as a heat transfer medium. The potting compound 13 therefore also serves as a water-tight seal for the PTC device 3. In accordance with the present invention, the potting compound 13 is free of nucleophiles and is therefore formed from the class of compounds consisting of silicone resins, polyamides, polyimides and ceramics which are not amine initiated and which are capable of forming potted compounds of the type de-' scribed above. Some materials of this type which are well known are Novalac epoxy of Dow Chemical Co. which is a silicone rubber. These materials can be used as base potting materials and do not react with the PTC element, even at elevated temperatures of as high as C. A sealed potting material 15 can be added on the top of the base potting material 13 to provide a better water tight seal.
It can be seen that there is provided a package for a PTC pill or heater material which has the properties of the prior art epoxy and yet does not have any material in contact with the PTC heater which can in any way cause the heater to become degraded.
Referring now to FIG. 2, wherein like character references denote like elements, there is shown a second embodiment of the invention. Again, there is shown the case 1 and PTC heater 3 which is metallized as-in the prior embodiment and which includes leads 5 and 7 attached thereto with insulation 9 and 11 surrounding the leads 5 and 7 respectively. However, in accordance with the present embodiment, a layer of material which does not degrade the PTC material 3 and which is labelled 17 completely surround the PTC element. The entire PTC element 3 with the coating'l7 thereon is then placed into thecase l and the entire device with coating 17 thereon is now potted with a potting material 19. Since the PTC element now has an impervious barrier therearound, the potting material 19 can be the epoxy resins and the like which are specifically excluded in the first embodiment. The impervious or passivating coating 17 can be formed from silicon resins, RTV silicons, polyimides, polyimide-amide, etc. These materials do not degrade the PTC element in accordance with the second embodiment.
It has been found that the degradation of the PTC material by the potting agent is temperature related. It is therefore apparent, that many of the potting compounds which can be used in low temperature applications are not usable in higher temperature applications.
It is therefore intended that this concept also be included in accordance with the present invention.
Though the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications thereof will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be intcrpreted as broadly as possible in view of the prior art to I include all such variations and modifications.
What is claimed is:
l. A PTC heater package which comprises:
a. a PTC device,
b. a potting material encapsulating said PTC device, said potting material being taken from the class of potting materials which is free of nucleophiles, said potting material encapsulating said PTC device forms a layer contacting and surrounding said PTC device,
c. a further epoxy resin potting material encapsulates said PTC device and said potting material,
(1. a casing enclosing said PTC device and potting ma- I 4. A PTC heater package as set forth in claim 3,
wherein said PTC device is formed from doped barium titanate.