MATERIALS FOR PACKAGING PTC-BASED CIRCUIT PROTECTION DEVICES

Abstract: Circuit protection devices which comprise a PTC conductive polymer element and an enclosure which is spaced apart from the PTC element and at least a part of whose interior surface is composed of a material which has an oxygen index of at least 70 and comprises a thermoset polymer, preferably an alkyd resin, and a filler which, when heated in the absence of air, decom­ poses to give a gaseous by-product, e.g. alumina tri­ hydrate.

Description: This invention relates to circuit protection devices comprising PTC conductive polymer elements according to the pre-characterising portion of claim 1 as it is known from EP-A-0 087 884. Introduction to the Invention Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, are well known. Reference may be made, for example, to U.S. Patent Nos. 2,952,761; 2,978,665; 3,243,753; 3,351,882; 3,571,777; 3,757,086; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,950,604; 4,017,715; 4,072,848; 4,085,286; 4,117,312; 4,177,376; 4,177,446; 4,188,276; 4,237,441; 4,242,573; 4,246,468; 4,250,400; 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,473,450, 4,481,498, 4,502,929, 4,514,620, 4,517,449, 4,529,866, 4,534,889, and 4,560,498; J. Applied Polymer Science 19 , 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18 , 649-653 (1978), Narkis et al; and European Application Nos. 38,713, 38,714, 38,718; 74,281, 92,406, 119,807, 134,145, 84,304,502.2, 84,307,984.9, 85,300,415.8, 85,306,476.4 and 85,306,477.2. Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under the normal operating conditions of the circuit, but are "tripped", i.e. converted into a high resistance state, when a fault condition, e.g, excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in a high resistance state. The increase in resistance is accompanied by an expansion of the PTC element along an expansion axis. Such devices, and PTC conductive polymer compositions for use in them, are described for example in U.S. Patents Nos. 4,237,411, 4,238,812; 4,255,698; 4,315,237; 4,317,027; 4,329,726; 4,352,083; 4,413,301; 4,450,496; 4,475,138; and 4,481,498; in European Patent Publication Nos. 38,713, 134,145, and 158,410, and in the commonly assigned patent applications filed contemporaneously with this application corresponding to U.S. Serial Nos. 711,790 (MP0991), 711,907 (MP1021), 711-908 (MP1016), and 711,910 (MP1044). SUMMARY OF THE INVENTION We have been working on the use of circuit protection devices containing PTC conductive polymer elements in situations in which the device is mounted onto, or itself comprises, a wall which is spaced apart from the PTC element and through which the electrodes pass. The wall is usually part of an enclosure which encloses and is spaced apart from the PTC element. The wall can be associated with a plurality of protection devices whose electrodes pass through the wall. In our work we have found that the materials which have hitherto been used for such walls do not give satisfactory results under test condition which are designed to simulate actual fault conditions which may occur, for example when the device is used to provide secondary protection in subscriber loop interface circuits in telecommunications systems. We have found that improved results can be obtained, not only under the test conditions in question but also under other conditions, if the wall through which the electrodes pass is composed of a material which (a) comprises a thermoset polymer which is an alkyd resin and, dispersed in the resin, a filler which, when heated in the absence of air, decomposes to give a gaseous by-product, and (b) has an oxygen index of at least 70. Polymers which, when heated in the absence of air decompose to give a gaseous by-product are known. For example, DE1262391 describes arc-resistant polymer compositions comprising 60-90 parts by weight of a butadiene-styrene copolymer and 40-10 parts by weight of a finely divided non conductive oxide or hydroxide. In a first aspect, this invention provides apparatus which comprises (A) a circuit protection device which comprises (1) a PTC element composed of a conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component and, dispersed in the polymeric component, a particulate conductive filler comprising carbon black; and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element; and (B) an enclosure which (1) encloses and is spaced apart from the PTC element; and (2) comprises a wall through which the electrodes pass said wall comprising an interior surface which is composed at least in part of a material which (a) comprises a thermoset polymer which is an alkyd resin and, dispersed in the thermoset polymer, a filler which, when heated in the absence of air, decomposes to give a gaseous by-product, and (b) has an oxygen index of at least 70. BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawings, in which the Figure is an exploded perspective view of apparatus according to the invention. DETAILED DESCRIPTION OF THE INVENTION The thermoset polymers used in the present invention are alkyd resins. Such resins are well known to those skilled in the art and are commercially available. Reference may be made for example to Modern Plastics Encyclopedia, 1981-2, pages 54 and 55. A polyester which includes a high proportion of aromatic groups in its backbone is desirable. Preferably at least one, particularly both, of the acid precursor and the hydroxy precursor of the polyester contains an aromatic radical. The fillers which are dispersed in the thermoset polymers are particulate materials which when heated in the absence of air, will decompose to give a gaseous by-product, e.g. one or more of H₂O, CO₂ or N₂. Suitable fillers include hydrated inorganic materials, e.g. fully or partially hydrated metal oxides (this term being used to include materials which consist of or contain the corresponding metal hydroxide), for example alumina trihydrate and partial dehydration products thereof. As noted in U.S. Patent No. 4,481,498, failure of protection devices based on PTC elements comprising carbon black dispersed in a polymer can result from the formation of a conductive path between the elecrodes, as a result of the deposition of carbonaceous dust, evolved from the PTC element when it is tripped, onto a surface which joins the electrodes. We theorize that the fillers used in the present invention help to prevent the formation of such conductive paths because, when an arc is struck between the electrodes, via the carbon dust on the surface, the filler decomposes to give a gas which quenches the arc and/or blows the carbon dust away, and thus prevents the formation of a permanent low resistance conductive path between the electrodes. The filler is preferably a hydrated inorganic material, e.g. a hydrated metal oxide, particularly alumina trihydrate. The amount of the filler is generally at least 30%, e.g. 35 to 45%, by weight of the material. It is essential that the material comprising the alkyd resin and the filler has an oxygen index (as measured by ASTM D-2863) of at least 70, preferably at least 80, particularly at least 90. It is preferred that the material can be injection molded, since the precise configuration of the interior of the container can influence the performance of the device, and preferred configurations are most easily produced by injection molding. I obtained excellent results using the alkyd resin sold by Occidental Chemical Corp. under the trade name Durez 27962; another useful resin, though it does not mold as well as Durez 27962, is the polyester resin sold by Polyply Inc. under the trade name Polyply 453. On the other hand, not all polymers are suitable for use in the wall of the enclosure of the present invention, for example, the polyester resins sold by Plastics Engineering Co. under the trade names Plenco 1581 and 1535 and by Premix Inc. under the trade name Premidry 3130, are not satisfactory in the present invention. Contrary to the teaching of Patent No. 4,481,498, the materials useful in this invention do not, or at least do not necessarily pass the carbon burn-off test described in patent no. 4,481,498. In order for the benefits of this invention to be obtained, the filled thermoset polymer must form at least a part of the surface over which a low resistance carbonaceous path is most likely to form during use of the device. Generally, the material will provide at least part, and preferably all, of the surface which lies between the electrodes. Generally the wall through which the electrodes pass will consist essentially of the material. Preferably the whole of the container around the PTC element will be fabricated from the material, preferably by injection molding. Referring now to the drawing, this shows a container which comprises a wall portion 1 having pairs of exit ports 11, 12 passing therethrough and a cover portion 2 which can be fitted to the wall portion 1. The container is composed of an injection-molded filled thermoset polymer as defined above. The apparatus also includes five identical circuit protection devices 3, each comprising a pair of electrodes 31 and 32 which are embedded in a PTC conductive polymer element 33 and extend therefrom and fit through the exit ports 11 and 12 in the wall portion 1.

Claims: Apparatus which comprises (A) a circuit protection device which comprises (1) a PTC element composed of a conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component and, dispersed in the polymeric component, a particulate conductive filler comprising carbon black; and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element; and (B) an enclosure which (1) encloses and is spaced apart from the PTC element; and comprises a wall through which the electrodes pass characterised in that (2) said wall comprises an interior surface which is composed at least in part of a material which (a) comprises a thermoset polymer which is an alkyd resin and, dispersed in the thermoset polymer, a filler which, when heated in the absence of air, decomposes to give a gaseous by-product, and (b) has an oxygen index of at least 70. Apparatus according to Claim 1 wherein said material has an oxygen index of at least 80. Apparatus according to Claim 1 wherein said material has an oxygen index of at least 90. Apparatus according to any ones of the preceding claims wherein said filler, when heated in the absence of air, decomposes to give H₂O, CO₂ or N₂. Apparatus according to Claim 4 wherein said filler is alumina trihydrate. Apparatus according to any one of the preceding claims wherein said material comprises at least 30% by weight of said filler. Apparatus according to any one of the preceding Claims wherein said thermoset polymer is a polyester resin in which at least one of the acid precursor and the hydroxy precursor comprises an aromatic group. Apparatus according to any one of the preceding claims wherein said enclosure consists essentially of said material and said material has been shaped by injection molding. Apparatus according to any one of the preceding claims wherein the enclosure encloses a plurality of substantially identical circuit protection devices.

Inventors: RATELL, JOSEPH M.
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1986
Main classification: H01C7
IPC codes: H01C1, H01C7