GB2038549A - PTC devices having oxygen barrier encapsulation - Google Patents

Abstract

Access of air to a conductive polymer type of PTC element is restricted by a barrier 3 so that it absorbs oxygen at a rate of less than 10<-6> cc/sec/gram. The element may be used in a circuit control device or a self-limiting strip heater. The barrier 3 may be of metal, e.g. Al or of a polymeric material. In another arrangement of control device the barrier is a metal can spaced from the element which is supported by leads passing through a glass seal disc. <IMAGE>

Description

SPECIFICATION PTC devices comprising oxygen barriers This invention relates to electrical devices comprising conductive polymer PTC elements.

It is known, or has been described in Applicant's earlier patent applications, that PTC conductive polymer compositions [i.e. compositions which comprise a polymer and conductive particles dispersed therein and which exhibit positive temperature coefficient (PTC) behavior] can be used in a variety of electrical devices (see for example J. Pol. Eng. and Sci. 14, 706 (1974), U.S. Patent Nos. 3,351,882, 3,858,144 and 3,914,363 and German Offenlegungschriften Nos. 2,543,314.1, 2,755,077.2, 2,755,076.1, 2,821,799.4 and P2,903,442.2, and the applications filed contemporaneously with this application corresponding to U.S. Serial Nos.

965,334 and 965,345). Such devices often comprise a jacket of a polymeric material which insulates the device electrically and also provides physical protection.

We have now discovered that such devices have improved electrical stability if access of oxygen to the PTC element is restricted.

In one aspect the invention provides an electrical device which comprises (1) a PTC element which is composed of a composition which exhibits PTC behavior with a switching temperature Ts and which comprises a polymer and conductive particles dispersed therein; (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, preferably less than 4 X 10-7, especially less than 3 X 10-7, particuiarly less than 2 X 10-7, cc/sec/gram.

Especially when the device is one which is expected to operate in such a way that the barrier is maintained at elevated temperatures, it is preferred that the barrier should remain an effective oxygen barrier at elevated temperatures and thus prevent any excessive change in the resistance of the device.It is, therefore, preferred that if the resistance of the device at a temperature T"C is RT, and the resistance of the device at the same temperature after an active aging treatment as defined below is RTA, then RTA is from 0.5 RT to 3RT, preferably from O.SRT to 2RT, at at least one value of T which is between (Ts-1 1 0) C and Ts and preferably is between (Ts-60) C and Ts, especially at all values of T between (Ts-60) C and Ts, particularly at all values of T between (Ts-1 10) C and Ts. The active aging treatment just referred to consists of passing current through the device for a time t hours, the current being such that 12R heating of the device maintains the PTC element at a temperature between Ts and (Ts + 50)"C, and t being 100 hours, preferably 250 hours, particularly 500 hours, especially 1000 hours. For many devices, some or all of these criteria of resistance change will be met if RTA is from 0.5RT to 3RT, preferably 0.5RT to 2RT, at T = 25"C.

The PTC compositions used in the present invention may be any of the PTC conductive polymers disclosed in Applicant's earlier patent applications and in the prior art. The conductive particles preferably comprise carbon black, but other conductive particles, e.g. metal powders, metal oxides, inorganic salts and graphite, can be used. Preferred compositions comprise an organic polymer (the term polymer being used to include mixtures of polymers) having at least 10%, preferably at least 30%, crystallinity and having dispersed therein a conductive carbon black having a particle size of 20 to 250 millimicrons. The PTC composition may further comprise a non-conductive inorganic filler, e.g. zinc oxide, antimony trioxide or clay, and/or an antioxidant (e.g. a hindered phenol such as those disclosed in U.S.Patent No. 3,986,981 and those manufactured by Ciba Geigy under the trade name "Irganox") or any other additive which will stabilise the composition against thermo-oxidative degradation, the amount of such additive generally being 0.005 to 10%, for example 0.01 to 6%, preferably 0.5 to 4%, by weight based on the weight of the polymer. Some materials which are generally useful as antioxidants for polymers can have an adverse effect on electrical stability, but suitable antioxidants can readily be selected on a trial-and-error basis.

Generally the barrier will be such that, when the device is placed in air, the only oxygen which can contact at least 95%, preferably substantially 100%, of the surface of the PTC element is oxygen which has passed through the barrier. The barrier is preferably composed of a material having an oxygen permeability rate at 25"C of less than 5 X 10-9, especially less than 10-9, cc(STP)/cm2/mm/sec/cm Hg, as measured by ASTM D 1434-75.Especially when the device is one which is expected to operate in such a way that the barrier is maintained at an elevated temperature, the physical properties of the barrier, including its oxygen permeability, at elevated temperatures are preferably such that the barrier retains its structural integrity and the device has the desired electrical properties after active aging as defined above. The thickness of the barrier should be sufficient to restrict the access of air o the PTC element to the desired extent and to prevent the formation of pinholes, e.g. at least 1 micron, and for polymeric materials is generally 0.0025 to 0.25 cm, preferably 0.013 to 0.13 cm, especially 0.025 to 0.075 cm.The barrier preferably protects the device against mechanical abuse, and for this reason is preferably composed of a material having a Young's Modulus greater than 7,000 kg/cm2. When using such a barrier, it is preferred, in order to avoid any danger of the barrier constructing the PTC element and thus changing the electrical performance of the device, that the barrier is separated from the PTC element by an air gap or a layer of another material of Young's Modulus less than 7,000 kg/cm2.

Suitable materials for the barrier include metals and polymeric compositions based on, for example, one or more polymers selected from polyvinylidene chloride, polyvinyl fluoride, polyethylene terephthalate, rubber hydrochloride, polychlorotrifluoroethylene, phenolformaldehyde resins, polyamides, epoxy resins, styrene/acrylontirile copolymers, polycarbonates, polystyrene, isobutylene/ isoprene copolymers, polyethylene, ethylene/tetrafluoroethylene copolymers, vinylidene fluoride/hexafluoropropylene polymers and fluorinated ethylene/propylene copolymers. The continuous surface temperature of the polymer should preferably exceed the Ts of the PTC element. These polymeric compositions can contain conventional additives, but should preferably not comprise material which will migrate into the PTC element and have an adverse effect on its properties.

In one preferred embodiment of the invention, the device is a circuit control device and the barrier is in the form of a self-supporting container, through whose walls the electrodes pass (via suitably sealed orifices) and within which the remainder of the device is supported or suspended out of contact with the walls of the container. The container preferably does not contain any oxygen; for example it may be evacuated or filled with an inert gas such as argon or nitrogen.

Typically the container will principally be made of metal, with the electrodes passing through a wall composed of a ceramic or rigid plastics material. In another preferred embodiment, the device is a heater or a circuit control device and the barrier is in the form of a layer of polymeric composition which surrounds the remainder of the device, with the volume enclosed by the layer being substantially free from voids. The barrier may be composed of a single material or two or more materials, either mixed together or as discrete components of the barrier, e.g. a laminate. One or both of the electrodes may be part of the barrier. The barrier should not of course provide an electrical connection between the electrodes.

The electrodes are generally composed of metal or some other material having a resistivity of less than 0.1 ohm. cm. Each of the electrodes may be in physical contact with the PTC element or wholly or partially separated therefrom by electrically conductive material, e.g. a conductive polymer composition which exhibits relatively constant wattage behavior, i.e. which does not exhibit PTC behavior at temperatures below the Ts of the PTC element. Alternatively the electrodes can be sandwiched between the PTC element and a relatively constant wattage conductive polymer composition. Preferably at least the outer surface of each of the electrodes is composed of a metal which does not catalyse degradation of the conductive polymer which it contacts. Thus the electrodes are preferably composed of nickel, tin, silver or gold, or one of these metals coated onto copper or another metal.When a planar electrode is required, electrodes in the form of an expanded metal or wire mesh are preferred. Other electrodes which can be used include solid wires, stranded wires and braids. When using stranded wire electrodes or other electrodes which contain voids, care should be taken to ensure that these voids do not provide a passageway for air to enter the device, e.g. by filling the voids or by sealing any exposed portions thereof. In preparing the device, care should be taken to minimise contact resistance.

The devices of the invention include circuit control devices, especially of the kind disclosed in application Serial No. 965,344 referred to above, and self-limiting heaters, including strip heaters.

In one class of devices according to the invention, generally circuit control devices, the PTC element is of relatively small size, having a volume of for example less than 20 cc., often less than 10 cc. or even smaller such as less than 5 cc or 1 cc., and the resistance of the device at 25"C is relatively small, for example less than 50 ohms, preferable less than 10 ohms., or even smaller such as less than 1 ohm. or 0.5 ohm.

In another class of devices according to the invention, generally heaters such as flexible strip heaters to be immersed in liquids, e.g. waterbed heaters, the barrier not only restricts access or oxygen to the PTC element but also is substantially impermeable to water or another liquid in which the device is to be immersed. When the device has an outer jacket of a plasticised polymer, eg. polyvinyl cholorde, the barrier can also serve to prevent migration of plasticiser from the jacket into the PTC element. For an immersible strip heater of this kind, the barrier is preferably provided by a laminate comprising a layer of metal, preferably aluminum, typically 0.0005 to 0.005 cm. thick, sandwiched between and bonded to two polymeric sheets, preferably of polyester, each sheet typically being 0.001 to 0.005 cm. thick. The laminate can be wrapped around the other components of the device so that it forms an overlapping envelope around them, and the overlapping portions sealed to each other. A preferred method is to make a progress overlapping wrap around an insulated strip heater with a tape laminate of the kind described and having a coating on the internal surface thereof of a heat-activatable adhesive, preferably a hot melt adhesive, and then heating the assembly to bond the adjacent wraps of tape to each other and to the strip heater. When the strip heater is provided with a grounding element, e.g. a braid, the PTC element is preferably separated from the braid both by a conventional insulating jacket, e.g. of a polyurethane, and by the oxygen and liquid barrier.The braid can be placed between the insulating jacket and the oxygen and liquid barrier, but when using a tape-wrapped barrier as described above, air can be retained in the interstices of the braid and have an adverse effect on the efficiency of the heater. The ends of the strip heater should preferably be sealed to ensure that no water or other liquid can contact the PTC element.

The invention is illustrated in the accompanying drawings, in which: Figures 1 to 4 show devices according to the invention; and Figure 5 shows the effect of active'aging on the resistance at 25"C of various devices of the invention.

Fig. 1 shows a circuit control device which comprises a PTC element 1 in the form of a round disc having round mesh electrodes 2 embedded in opposite faces thereof; leads 4 are attached to the electrodes 2; and barrier layer 3 encapsulates the PTC element 1 and the electrodes 2, with leads 4 passing through it. The interface between the barrier layer 3 and the PTC element 1 and the electrodes 2 is free from voids.

Fig. 2 shows a strip heater of constant cross-section comprising solid wire electrodes 2 embedded in PTC element 1 which is surrounded by barrier layer 3. The ends of such a strip heater are preferably covered by an oxygen barrier, but it is important to note that even if this precaution is not taken, the absence of voids between PTC element 1 and barrier layer 3 and in the electrodes 2 means that only a very limited proportion of the surface area of the PTC element is exposed to the air. By contrast, if voids are present between the jacket and the PTC element or if stranded wire electrodes are used, and the ends of the heater are not sealed, then even if the jacket is substantially impermeable to oxygen, air can percolate along the length of the PTC element and contact a substantial proportion of its surface.

Fig. 3 shows a water-immersible strip heater of constant cross-section comprising electrodes 1 2 embedded in PTC element 14 which has a Ts such that the heater will maintain water in which it is immersed at a suitable elevated temperature, e.g. 65"C. Insulating jacket 16, e.g. of polyurethane, surrounds PTC element 14 and is itself surrounded by oxygen and water barrier 22 formed by progressively wrapping and bonding a tape laminate as described above. Woven braid grounding element 23 surrounds the barrier 22 and is itself surrounded by other insulating jacket 24, e.g. of plasticised polyvinyl cholride.

Fig. 4 shows a circuit control device in which the barrier is formed by a can of generally rectangular cross-section and having a metal top 1 and a base sealed thereto. The can is filled with nitrogen. The base comprises a metal ring 2, which has a peripheral sealing slot to which the top 1 is sealed, and a disc 4 which is sealed to the ring 2 and which is composed of glass or an epoxy resin. Pin leads 3 pass through disc 4 and support and are connected to rectangular electrodes between which is sandwiched a PTC element; the electrodes and PTC element are shown (in outline only) as 5.

The invention is illustrated in the following Examples, in which parts and percentages are by weight except where otherwise noted. In each of the Examples, devices were prepared and tested by the procedure described below. A PTC composition was prepared by mixing the ingredients shown in the Table below (the weight given being in grams); it should be noted that the polymers used were commercially available materials which contain a small quantity (about 0.5% by weight) of an antioxidant. The mixing was carried out at flux temperature for 5 minutes in a steam-heated Banbury mixer with a water-cooled rotor. The mixture was dumped from the mixer, allowed to cool to room temperature and chopped into small pieces. The chopped material was compression molded at a temperature of 1 80 C and a pressure of about 70 kg/cm2 for 5 minutes into a slab 0.2 cm. thick.Round discs, 1.9 cm. in diameter, were punched out of the slab. An electrode was formed on each face of each disk by molding into it a disc 1.9 cm. in diameter cut from an expanded metal mesh composed of nickel-coated copper.

The sample was irradiated to 20 megarads to cross-link the PTC composition. 20 AWG wire leads were attached to the electrodes. Where indicated in the Table, preparation of the device was completed by surrounding the sample with a barrier as specified in the Table. In Example 2, the sample was dipped into the epoxy resin composition, which was then dured at 80"C for 16 hours. In Examples 3 and 5 the sample was heated to 11 0'C and then dipped into a fluidised bed of the epoxy resin, which was then cured at 1 0 C for 1 6 hours. In Example 6, the sample was dipped into the silicone resin, which was then cured at 20 C for 1 6 hours. In Examples 2, 3, 5 and 6, the barrier layer was 0.025 cm. thick.

The electrical stability of the devices on active aging as defined above was tested as follows.

The leads of the device were attached to a variable voltage AC power supply. The voltage of the supply was maintained at 1 20 volts except when the device was first connected or reconnected to the power supply, when the voltage was 30-45 volts for the first 30 seconds and was then increased to 1 20 volts over a period of 2 minutes. At intervals during the aging, the device was disconnected from the power supply and allowed to cool to room temperature for 0.5 hour, and its resistance at room temperature was then measured.

The room temperature resistance of the devices after aging as specified above is shown in Fig.

5. It will be seen that the products of Examples 1, 4, 6 and 9, which do not comprise barriers according to the invention, have poor electrical stability, whereas the products of Examples 2, 3, 5, 7 and 8, which are in accordance with the invention, have excellent stability.

The presence of the barrier in the devices of the invention has the additional advantage that if the device is subjected to electrical stress which causes breakdown of the PTC composition, the likelihood of explosive failure or conflagration is substantially reduced.

TABLE EXAMPLE NO. 1 2 3 4 5 6 7 8 9 PTC COMPOSITION High density Polyethylene 699.1 699.1 699.1 741.3 741.3 699.1 699.1 699.1 699.1 Marlex 6003 (Phillips Petroleum) Ethylene/acrylic acid copolymer 873.9 873.9 873.9 925.7 925.7 873.9 873.9 873.9 873.9 EAA-455 (Dow Chemical) Carbon black 1391.5 1391.5 1391.5 1358 1358 1391.5 1391.5 1391.5 1391.5 Furnex N-765 (City Services) Added Antioxidant* 60.5 60.5 60.5 - - 60.5 60.5 60.5 60.5 BARRIER None None Epoxy Resin (Hysol EE 0067 HD 7054) - Yes - - - - - - (oxygen permeability less than 10-9) Epoxy Resin (REP 353 12-40) - - Yes - Yes - - - (oxygen permeability less than 10-9) Silicone Resin (Sylgard 170 A/B) - - - - Yes - - (oxygen permeability more than 50 x 10-9) Sealed Metal container under vauccm - - - - - Yes - Sealed Metal container filled with argon - - - - - - Yes Metal container having small hole - - - - - - - Yes * An oligomer of 4,4'-thiobis (3-methyl-6-t-butyl phenol) with an average degree of polymerisation of 3-4, as described in U.S. Patent No. 3,986,981.

Claims (12)

1. An electrical device which comprises (1) a PTC element which is composed of a composition which exhibits PTC behavior with a switch temperature Ts and which comprises a macromolecular polymer and conductive particles dispersed therein; and (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; characterised in that the device also comprises (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 characterised in that the rate at which the PTC element absorbs oxygen is less than 4 X 10-7 cc/sec/gram.

3. A device according to Claim 1 or 2 characterised in that it exhibits a change in resistance, at at least one temperature between (Ts1 10)"C and Ts, 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 12R heating of the device maintains said PTC element at a temperature between Ts and (Ts + 50)"C.

4. A device according to Claim 3 characterised in that it exhibits a change in resistance, at all temperatures between Ts and (T,-60)"C, of - 50% to + 100% after said aging treatment.

5. A device according to Claim 3 or 4 characterised in that it exhibits a change in resistance, at at ieast one temperature between (T,-1 1 0) C and Ts 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 12R heating of the device maintains said PTC element at a temperature between Ts and (Ts + 50)"C.

6. A device according to any one of the preceding claims characterised in that the barrier is composed of a material having an oxygen permeability rate of less than 5 X 10-9 cc (STP)/cm2/mm/sec/cmHg.

7. A device according to Claim 6 characterised in that the barrier is composed of a material having an oxygen permeability rate of less than 10-9 cc(STP)/cm2/mm/sec/cmHg.

8. A device according to Claim 6 or 7 characterised in that the barrier is 0.0025 to 0.13 cm. thick and comprises at least one layer of an electrically insulating composition which comprises at least one polymer.

9. A device according to Claim 8 characterised in that the barrier comprises a polymer having a continuous surface temperature in excess of the Ts of the PTC element and selected from 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/haxafluoropropylene polymers and fluorinated ethylene/propylene copolymers.

10. A device according to any one of the preceding claims characterised in that the barrier comprises a metal.

11. A device according to Claim 10 which is a circuit control device, charaterised in that said 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.

12. A device according to claim 10 characterised in that at least one of the electrodes provides a part of the barrier.

1 3. A device according to any one of the preceding claims characterised in that the barrier provides a hermetic seal around the PTC element.

1 4. A device according to any one of claims 1 to 10 which is a strip heater, characterised in that the barrier is impermeable to water.

1 5. A device according to any one of claims 1 to 1 0 and 14 characterised in that the barrier comprises a laminate comprising a layer of a metal sandwiched between and bonded to two polymeric sheets.

1 6. A device according to any one of the preceding claims characterised in that at least one of the electrodes is wholly or partially separated from the PTC element by a layer of a conductive polymer composition which exhibits relatively constant wattage behavior.