DEVICES COMPRISING PTC CONDUCTIVE POLYMERS

Abstract: Heat-recoverable electrical devices, particularly for covering telephone and other cable splices, comprise (a) a component composed of a PTC conductive polymer which has little or no cross-linking, (2) an adjacent heat-recoverable component composed of a PTC or ZTC conductive polymer which has a relatively high level of cross-linking, and (3) electrodes which can be connected to a power supply so that current passes through the components and causes recover of the device. Preferably a continuous laminar PTC component having little or no crosslinking is sandwiched between two crosslinked continuous laminar ZTC components. Such a device is illustrated in Figure 1.

Description: This invention relates to heat-recoverable electrical devices comprising PTC conductive polymer compositions. Conductive polymers are well known. They comprise a particulate conductive filler which is dispersed in, or otherwise held together by, an organic polymer. They can be used in circuits in which current passes through them, eg. in heaters and circuit protection devices, and in such use they may exhibit what is known as PTC (positive temperature coefficient) or ZTC (zero temperature coefficient) behavior. The term "PTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which, in the operating temperature range, has an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, preferably both, and which preferably has an R₃₀ value of at least 6, where R₁₄ is the ratio of the resistivities at the end and the beginning of the 14°C temperature range showing the greatest increase in resistivity, R₁₀₀ is the ratio of the resistivities at the end and the beginning of the 100°C temperature range showing the greatest increase in resistivity, and R₃₀ is the ratio of the resistivities at the end and the beginning of the 30°C temperature range showing the greatest increase in resistivity. The term "ZTC behavior" is usually used in the art, and is so used in the specification, to denote a composition which does not show PTC behavior in the operating temperature range; thus the term is used to include compositions which, in the operating temperature range of the device, eg. 0° to 100°C, (a) show no substantial change in resistivity, (b) show a substantial increase in resistivity but still do not have a R₁₄ or R₁₀₀ value as specified above, or (c) show a substantial decrease in resistivity [often denoted NTC (negative temperature coefficient) behavior]; these compositions can exhibit PTC behavior at temperatures above the operating temperature range. Documents describing conductive polymer compositions and devices comprising them include U.S. Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,986, 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, 4,481,498, 4,476,450, 4,502,929; 4,514,620, 4,517,449, 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, 133,748, 134,145, 144,187, 157,640, 158,410, 175,550 and 176,284. Devices of particular interest are heat-recoverable articles which comprise conductive polymer elements and which can be recovered, eg. around a telephone cable splice, by passing current through the conductive polymer, thus generating the heat needed to cause recovery; such devices can comprise a PTC conductive polymer element which controls the heating process so that overheating cannot take place. Reference may be made for example to U.S. Patent Nos. 4,223,209, 4,330,703, and 4,421,582, and British Patent No. 1,265,194. As is well known in the art, a polymeric article can be rendered heat-recoverable by cross-linking it, deforming the cross-linked article at a temperature above its melting point, and cooling the article in the deformed state (see for example U.S. Patent No. 3,086,242 2,027,962, 3,086,242, 3,253,618, 3,253,619, 3,379,218, 3,455,336 and 3,770,556). Unfo rtunately, however, when a cross-linked PTC conductive polymer is stretched, the increase in resistivity around the switching temperature (T s ) decreases. In many cases, if a cross-linked PTC conductive polymer is stretched more than 2X (ie. to twice its original dimension), the conductive polymer ceases to show an increase in resistivity which provides adequate assurance against runaway heating, for example ceases to exhibit PTC behavior as defined above. U.S. Patents Nos. 4,177,446 and 4,223,209 seek to overcome this drawback by means of an apertured PTC conductive polymer heater which can undergo dimentional change by changes in the shape of the apertures and which is attached to a non-conductive heat-recoverable article. This is a useful approach, but it suffers from the disadvantage that the heater itself does not provide a barrier against moisture etc. after recovery. We have discovered that these disadvantages can be overcome by combining together (1) a PTC conductive polymer element which has comparatively little or no cross-linking and (2) a heat-recoverable element which is preferably composed of a conductive polymer having a relatively high level of cross-linking. In its first aspect, this invention provides a heat-recoverable device which comprises (1) a first component which is heat-recoverable and which is composed of a cross-linked conductive polymer; (2) a second component which is composed of a conductive polymer and which exhibits PTC behavior; and (3) electrodes which can be connected to a source of electrical power; the electrodes and the first and second components being such that, when the electrodes are connected to a suitable source of electrical power, current passes through the first and second components so that the device recovers; and the second component having at least one of the following characteristics (a) it is substantially free from cross-linking; (b) it has been cross-linked by irradiation to a dosage of less than 6 Mrads, preferably less than 3 Mrads; (c) when the first component has been cross-linked by irradiation, the second component has been cross-linked by irradiation to a dosage which is at least 2 Mrads below, preferably at least 5 Mrads below, the dosage used to cross-link the first component; and (d) it has an M₁₀₀ value at a temperature 15°C above its crystalline melting temperature which is at most 0.65 times the M₁₀₀ value, at the same temperature, of a sample of the same material which (i) has a thickness of 40 mils, and (ii) has been irradiated on one surface with a 2.7 MeV electron beam to a dosage of 10 Mrads. Preferred devices comprise a third component which is heat-recoverable and is composed of a cross-linked conductive polymer, the second component being separated from the electrodes by the first and third components respectively, so that current passes sequentially through the first, second and third components. This ensures that the hot zone which forms in the PTC element during operation of the device is not adjacent an electrode and thus greatly reduces the possibility of arcing or an electrical short as the device recovers. Through use of the present invention it is possible to make heat-recoverable devices which has surprisingly high recovery ratios, eg. at least two or three and even higher. The new devices are particularly useful for covering a substrate, eg. a splice between telephone cables or other cables, the device being placed adjacent the substrate and then connected to a suitable power supply so that it self-heats and recovers towards the substrate. The heat-recoverable devices of the invention can be of any configuration, but they are preferably laminar heat-shrinkable devices comprising contiguous, and preferably continuous (ie. aperture-free), conductive polymer layers, and the invention will chiefly b e described by reference to such devices. The term "laminar" is used to include any device in which has two dimensions which are much larger than the third dimension; thus a laminar device can be planar, but can also, for example, have an annular cross-section. The laminar devices of the invention are preferably prepared in the form of flexible planar articles which can be used, for example, by wrapping them around a substrate to be covered, securing the wrapped ends, and powering the article so that it shrinks towards the substrate. The shrunken article can contact the substrate directly or indirectly, for example through an intermediate layer which can be a heat-responsive layer eg. of an adhesive or a mastic. In the novel devices, the components must of course be chosen so that the desired recovery can take place. Similarly, to the extent that the components are secured together before the heat-recoverable layers are tendered heat-recoverable, they must be selected so that the desired degree of heat-recoverability can be imparted. Preferred electrodes are those which can change shape to accommodate the recovery (and, if need be, expansion), particularly apertured electrodes which can change shape upon recovery through a change in the shape of the apertures, for example expanded metal mesh electrodes, pierced metal foil electrodes, and woven metal electrodes. However, rigid electrodes can be used provided that recovery takes place substan tially at right angles to any long dimension of the electrode. The conductive polymers must also be selected with care in order to ensure that the device will reach the recovery temperature of the heat-recoverable component(s), and will maintain such a temperature while sufficient recovery takes place. Recovery of heat-recoverable polymeric articles generally begins at about the crystalline melting point of the polymer, but is is desirable to attain somewhat higher temperatures, eg. at least 5°C higher, in order to ensure sufficient recovery in a reasonable time (say, less than five minutes). The switching temperature (T s ) of a PTC conductive polymer is generally a little below the crystalline melting point, though a suitably designed device containing a PTC conductive polymer can have a T s which is above the inherent T s of the conductive polymer, as explained for example in U.S. Patents Nos. 4,017,715 and 4,177,376. Consequently, it is difficult to obtain high recovery ratios when all the heat-recoverable components are based on the same polymer as the PTC component. It is preferred, therefore, that at least one of the heat-recoverable components comprises a first crystalline polymer and that the PTC component comprises a second crystalline polymer whose crystalline melting point is above, preferably at least 5°C above, eg. 5 to 20°C above, the crystalline melting point of the first crystalline polymer. Orientation of a cross-linked PTC conductive polymer (such as takes place when such a conductive polymer is rendered heat-recoverable) reduces the size of the increase in resistivity which takes place on heating, and especially at high expansion ratios of 2X or more, can result in a heat-recoverable member which no longer exhibits PTC behavior but rather exhibits ZTC behavior. The invention includes devices comprising heat-recoverable components which exhibit PTC behavior, even though stretched, and heat-recoverable components which exhibit ZTC behavior and have been obtaining by stretching components which exhibit PTC behavior. However, there is a danger that such components, as they recover, will once again exhibit PTC behavior which will cause them to inhibit the flow of a current in a way which is not desired, particularly if they comprise a crystalline polymer whose melting point is lower than that of the crystalline polymer in the PTC component. It is preferred, therefore, that at least one of the heat-recoverable components should be composed of a conductive polymer which exhibits ZTC behavior i n the shrunken or unstretched state. Those skilled in the art will have no difficulty, having regard to their own knowledge and the disclosure herein, in determining suitable conductive polymers for carrying out the invention. Suitable compositions include those disclosed in the application filed corresponding to Application Serial No. 720,118 (Soni et al, MP1039), for example European Application No. 8630244.0 (Publication No. 197759), which comprise a melt-processable polymer, particularly polyethylene, having a weight average molecular weight, Mw, of at least 150,00, preferably 150,000 to 400,000; these compositions exhibit excellent physical properties, even at elevated temperatures and even when having little or no cross-linking, making them particularly suitable for use in the second component. Other suitable compositions are disclosed in European Patent Publication Nos. 153199 and 157640, which comprise polymers par ticularly sintered polymers, having a molecular weight, Mw, greater than 3 million. It is important the the PTC conductive polymer of the second component is free from cross-linking or has a relatively low level of cross-linking such that the component provides the necessary temperature control even after stretching. The higher the expansion ratio used to render the device heat-recoverable, the lower the permissible degree of cross-linking. The invention is particularly useful when the expansion ratio is above 3X, more preferably above 4X. The new devices can be prepared simply by laminating the various components together, the conductive polymer components already having a desired amount of cross-linking (or, in the second component, no cross-linking), and then subjecting the laminate to a treatment which renders it heat-recoverable. In this process each of the components can be present (prior to lamination) as an individual member. Alternatively, two or more of the conductive polymer components can be present as a single member which has been prepared by selective cross-linking of a laminate of two or more members or a single member. Such selective cross-linking can be obtained by the presence of radiation cross-linking agents or radiation cross-linking inhibitors in appropriate layers, and/or by making use of low energy radiation directed to one or both surfaces of a sheet of conductive polymer so that the cross-linking density is substantially higher at the surface of the sheet than it is at the center of the sheet. One measure of the extent to which a conductive polymer has been cross-linked is its M₁₀₀ value above its melting point. The second component preferably has an M₁₀₀ value at a temperature 15°C above its crystalline melting temperature which is at most 0.65 times, particularly at most 0.5 times, the M₁₀₀ value at the same temperature of a sample of the same material having a thickness of 40 mils, which has been irradiated on one surface with a 2.7 MeV electron beam to a dosage of 10 Mrads. The M₁₀₀ values referred to herein are determined by a dynamic modulus test carried out at a temperature 15°C above the crystalline melting temperature of the material. In this test the stress required to elongate a sample of the component by 100% (or to rupture if elongation to 100% cannot be achieved) is measured. Marks separated by 1 inch (2.54 cm) are placed on the center section of a dogbone-shaped sample (length 4 inches, thickness 40 mils, width of central region 0.125 inch, gauge length used in testing 1.5 inch), and the sample is stretched at a temperature 15°C above the crystalline melting temperature of the material at a rate of 2 inch per minute in an Instron hot box. The load at 100% elongation is measured, and the M₁₀₀ value is then calculated from the expression. The devices of the invention are normally surrounded by a layer of electrical insulation which does not prevent recovery of the device and which is preferably itself heat-recov erable. Referring now to the drawing, Figures 1 and 2 show a heat-shrinkable laminated device which comprises a first expanded metal electrode 1, a continuous, heat-recoverable layer 2 of a cross-linked conductive polymer exhibiting ZTC behavior, a continuous heat-recoverable layer 3 of a cross-linked conductive polymer, a continuous layer 4 of an uncrosslinked conductive polymer exhibiting PTC behavior, a continuous heat-recoverable layer 5 of a cross-linked conductive polymer, a continuous heat-recoverable layer 6 of a cross-linked conductive polymer exhibiting ZTC behavior, and a second expanded metal electrode 7. The invention is illustrated with reference to the following Example, which describes the method of manufacture of a device according to the invention. The method could be used, for example to make the device illustrated in Figures 1 and 2, wherein layers 3, 4 and 5 are provided by a single PTC layer. EXAMPLE First the compositions of the components of the device were prepared as follows. A) Insulation composition (not shown in Figs. 1 and 2) 97.0 wt% DEFD 1225 (linear very low density polyethylene) supplied by Union Carbide 2.0 wt% Statex MT (carbon black, supplied by Columbian Chemical Corp.) 1.0 wt% antioxidant B) Electrode mesh (member 1 in Figs. 1 and 2) 10-AL-10 575, (supplied by Delker Corporation) Mesh has a thickness of 10 mils. Also each strand of mesh has a width of 10 mils. There are 12 slits per inch. C) ZTC composition (for members 2 and 6 in Figs. 1 and 2) 20.0 wt% Ketjenblack 300J (carbon black supplied by Akzo Chemie) 79.0 wt% DPD 6169 (ethylene ethyl acrylate supplied by Union Carbide) 1.0 wt% antioxidant D) PTC composition (for members 3, 4 and 5 in Figs. 1 & 2) 41.0 wt% Statex GH (carbon black, supplied by Columbian Chemical Corp. 58.0 wt% Marlex HXM 50100 (high molecular weight, high density polyethylene supplied by Phillips) 1.0 wt% antioxidant The antioxidant used was an oligomer of 4,4-thio bis (3-methyl-6-t-butyl phenol) with an average degree of polymerization of 3-4, as described in U.S. Patent No. 3,986,981. All the polymeric compositions were prepared using a Banbury compounder and then a pelletizing extruder. Then the polymeric compositions were extruded into sheets (using a 2.5 inch screw diameter extruder having a sheet die width of 21 inches), the insulation layer being extruded at 300-350°C, the ZTC layer at 320-350°F, and the PTC layer at 380-450°F. The PTC compositions was extruded into sheets having a thickness of 40 mils and each of the ZTC and insulation sheets having a thickness of 35 mils. Two laminates were then prepared, each comprisng a sheet of the insulation composition, the electrode mesh, and a sheet of the ZTC composition, the electrode mesh being sandwiched between the polymer compositions. The lamination was carried out at 400°F using a nip roller operating at 10 feet per minute and exerting a force of 25 psi. The laminates were chilled at 32°C. The prepared laminates were irradiated from one side with a 2.7 MeV electron beam to a dose of 8 Mrads. The sheet of PTC composition was irradiated, from both sides, with a 175 KeV electron beam to a dose of 20 Mrads. A second lamination step was then carried out to laminate the irradiated PTC sheet between the two irradiated, previously prepared insulation/electrode/ZTC laminates. The second lamination step was carried out at 450°F using a nip roller operating at 3.5 feet per minute and exerting a force of 15 psi. The final laminate was chilled at 32°F. The PTC layer was preheated to 100°C for the lamination process. Finally the prepared laminate was expanded to render it heat-recoverable. Expansion was effected using an expander, the laminate being fed into the expander at 5 feet per minute and drawn from the expander at 15 feet p inute. The laminate was preheated to 230°F for the expansion process.

Claims: 1. A heat-recoverable device which comprises(1) a first component which is composed of a cross-linked conductive polymer and which is heat-recoverable;(2) a second component which is composed of a conductive polymer and which exhibits PTC behavior; and(3) electrodes which can be connected to a source of electrical power;the electrodes and the first and second components being such that, when the electrodes are connected to a suitable source of electrical power, current passes through the first and second components so that the device recovers; and the second component having at least one of the following characteristics (a) it is substantially free from cross-linking; (b) it has been cross-linked by irradiation to a dosage of less than 6 Mrads; (c) when the first component has been cross-linked by irradiation, the second component has been cross-linked by irradiation to a dosage which is at least 2 Mrads below the dosage used to cross-link the first component; and (d) it has an M₁₀₀ value at a temperature 15°C above its crystalline melting temperature which is at most 0.65 times the M₁₀₀ value, at the same temperature, of a sample of the same material which (i) has a thickness of 40 mils, and (ii) has been irradiated on one surface with a 2.7 MeV electron beam to a dosage of 10 Mrads. 2. A device according to Claim 1 which further comprises(4) a third component which is composed of a cross-linked conductive polymer and which is heat-recoverable,and wherein the second component lies between the first and third components so that when current passed between the electrodes it passes sequentially through the first, second and third components. 3. A device according to Claim 2 which is heat-shrinkable and wherein the first, second and third components are if the form of continuous laminar sheets. 4. A device according to claim 2 or 3 which has a recovery ratio of at least 2, preferably at least 3. 5. A device according to any one of the preceding claims which comprises a heat-shrinkable component which is composed of a conductive polymer having a first crystalline melting point and which exhibits ZTC behavior, and a second component which exhibits PTC behavior and which is composed of a conductive polymer having a second crystalline melting point which is at least 5°C higher then the first melting point. 6. A device according to claim 5 wherein the first melting point is 5 to 20°C higher than the first melting point. 7. A device according to any one of the preceding claims which comprises, in order, (a) a heat-shrinkable sheet which is composed of a cross-linked conductive polymer and which exhibits ZTC behavior in the heat-shrinkable state and in the shrunken state, (b) a heat-shrinkable sheet which is composed of a cross-linked conductive polymer and which exhibits PTC behavior in the shrunken state, (c) a heat-shrinkable sheet which is composed of a substantially uncrosslinked conductive polymer and which exhibits PTC behavior in the heat-shrinkable state and in the shrunken state, (d) a heat-shrinkable sheet which is composed of a cross-linked conductive polymer and which exhibits PTC behavior in the shrunken state, and (e) a heat-shrinkable sheet which is composed of a cross-linked conductive polymer and which exhibits ZTC behavior in the heat-shrinkable state and in the shrunken state. 8. A device according to any one of the preceding claims which has been prepared by a process which comprises (A) irradiating a sheet composed of conductive polymer under conditions such that, after irradiation, the sheet comprises an inner layer which exhibits PTC behavior and which has a relatively low degree of cross- linking, and two outer layers which have a relatively high degree of cross-linking; and (B) subjecting the irradiated sheet from step (A) to a treatment which renders it heat-shrinkable. 9. A device according to any one of the preceding claims wherein the second component is composed of a conductive polymer which comprises polyethylene having a molecular weight of 150,000 to 400,000. 10. A method of covering a substrate, preferably a splice in a telephone cable, which comprises (A) placing adjacent the substrate a heatrecoverable electrical device as claimed in any one of the preceding, and (b) connecting the electrodes to a power supply so that the device recovers towards the substrate.

Inventors: CHAN, RANDOLPH W., JACOBS, STEPHEN M., SONI, PRAVIN L., VAN KONYNENBURG, PETER H., WARTENBERG, MARK
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1986
Main classification: B29C61
IPC codes: H05B3, B29C61, C08L101, H01C7