ELECTRICAL WIRE AND CABLE

Abstract: An electrical wire comprises an electrical conductor (2), an insulating mineral layer (3) electrolytically formed on the conductor from chemically delaminated weathered mica, and a silicone polymer layer (3') and mineral layer. Preferably the layer of polymeric insulation (4) is extruded on the silicone layer. The electrical wire exhibits very good electrical properties for long periods at elevated temperatures, e.g. during a fire.

Description: ELECTRICAL WIRE AND CABLE This invention relates to electrical wire and cables . In certain fields where wire and cables are used, for example in military or mass transit applications, it is desired to use cables which are capable of func¬ tioning for a period of time during a fire without shorting or otherwise failing. These cables have been called circuit integrity cables or signal integrity cables depending on their use. The previously proposed cables have generally used the principle that the indi¬ vidual conductors should be separated from one another by mica tapes, by large volumes of packing materials, by relatively thick layers of silicone insulation or by combinations thereof in order to prevent the formation of short circuits during a fire. There is therefore a need for a cable that will retain its integrity for a period of time when subjected to a fire but which is relatively small and lightweight and which is relati¬ vely inexpensive to manufacture. According to the present invention, there is pro¬ vided an electrical wire which comprises a metallic electrical conductor, an insulating mineral layer electrolytically formed on the conductor from chemi¬ cally delaminated weathered mica, and a silicone polymer layer located on the mineral layer. It is known that several 2:1 layer phyllosilicate minerals form interlayer complexes with a wide range of charged and uncharged species of both organic and inorganic origins e.g. alkylammonium ions, amino acids and amino acid ^ cations. The inclusion of inter¬ calating species between the layers of the macrocrystal usually results in changes to the basal spacing which can be measured by X-ray diffraction techniques . Under certain circumstances an additional swelling can take place whereby further intercalation, by a wide range of polar and non-polar solvents, occurs. In special cases the degree of expansion can be so extensive as to produce 'gel-like' samples. The application of mild mechanical action to these exten¬ sively swollen systems can lead to the production of colloidal dispersions of the mineral on a dispersing solvent, this process being known as "chemical delamination" . This effect can be particularly apparent in a range of mica-type complexes containing n-alkylammonium ions, with water as a dispersing solvent. Whether additional interlayer expansion occurs depends on the layer change density separating successive layers on the mineral and the length of the alkyl chain of the associated intercallants . Minerals with a surface charge density in the range of 0.5 to 0.9, saturated with certain short chain n-alkylammonium ions e.g. n-propyl, n-butyl and isoamyl, behave exceptionally well in that they show extensive interlayer swelling in water. Crystals which show this type of behaviour can increase in volume by up to, and sometimes more than, 30 times their original volume and remain coherent and 'gel-like'. Interstrat- ified minerals containing mixed layers of exchangeable and non-exchangeable cations can be partially saturated with short chain alkylammonium ions and subsequently treated with water to swell 'macroscopically' only part of the layered structure. In either case mild mechanical shear will dela i- nate the swollen crystals along the macroscopically swollen cleavage plane where interlayer forces are minimised. This action can be used to produce a colloidal dispersion of thin high aspect ratio plate¬ lets . In the case whereby the starting mineral is of a homogenous nature the composition of the colloid will be consistent. However, if mixed layer minerals are used then there can be a wide variation of platelet composition and characteristics throughout the colloidal dispersion. Fractionation techniques, including sedimentation, can be used to isolate com¬ ponents of the dispersion which exhibit different che¬ mical and physical characteristics from each other and from the parent mineral. The term "weathered mica" is used herein to describe the weathering products of natural mica and includes minerals comprising vermiculite or minerals of a mixed layer type containing vermiculite layers as a major constituent. It includes any hydratable, layer latticed, expandable silicate structure, and primarily the three layer micas . The layers usually have a thickness of about 10 Angstrom units with the main ele¬ mental constituents being magnesium, aluminium, silicon and oxygen. It may be formed by replacement of non- exchangeable cations, e.g. potassium ions, by exchangeable cations, e.g. sodium or magnesium ions, in mica. Such replacement will normally occur through weathering of mica, but the term includes materials formed by other methods of cation exchange, e.g. by hydrothermal action or some synthetic micas . The term includes materials such as vermiculites and smectites in which there has been complete replacement of the non-exchangeable cations, and any intermediate materials such as formed by partial replacement of the non-exchangeable cations, provided, as explained below, that it is possible to form a colloidal dispersion from the material. The use of a weathered mica instead of unweathe-red mica has the advantage that the cohesion of the resulting mineral layer is much larger than that of a deposited mica layer with the result that it is then possible to handle the wire more easily during manufac¬ ture and use, and in addition, much higher electrolytic deposition rates can be achieved with lower deposition voltages . Preferably the weathered mica is a mineral of a mixed layer type that contains mica layers interspersed with other layers that are formed by weathering. The weathered layers may comprise any hydratable, layer latticed, expandable silicate structure, e.g. hydro- biotite and hydrophlogopite layers , and preferably hydrophlogopite II layers although other layers may instead be present. The hydratable layers may comprise a major part of the original mineral although it is preferred for the major part (by weight) to be formed from* unweathered mica layers. Thus, the mineral used according to the invention may be regarded as formed from platelets that have a micaceous, or predominantly micaceous interior, and a surface that is formed from a hydrated silicate layer. The platelets preferably have an average thickness of not more than 500 Angstroms, more preferably not more than 300 Angstroms, especially not more than 200 Angstroms and most especially not more than 100 Angstroms, but preferably at least 20 Angstroms, more preferably at least 40 and especially at least 60 Angstroms . The wire will normally be provided with an outer protective layer or jacket which will protect the weathered mica layer from mechanical abuse during handling and which is preferably also electrically insulating so that it can provide further electrical insulation during normal operation. The protecting and insulating layer will normally be a polymeric layer which is formed on the coated conductor by an extrusion process although in some cases it may be preferred to apply the insulation by a tape wrapping process for example in the case of polytetrafluoroethylene or cer¬ tain polyimides . In other cases however, for example in the case of electric motor windings or transformer windings, where very thin, high temperature wire is required, it is possible to dispense with the polymeric insulation altogether . The wire according to the invention may be manu¬ factured in a particularly simple manner by passing an elongate electrical conductor through a dispersion of chemically delaminated weathered mica and applying an electrical potential to the conductor in order to depo- sit reconstituted weathered mica (hereinafter referred to simply as the "mineral" ) onto the conductor and drying the conductor and the mineral layer so formed. After the mineral layer has been dried, the silicone layer may be formed on the coated conductor by any appropriate method, e.g. by extrusion or dip-coating, and then curing the silicone layer so formed. The weathered mica dispersion may be formed by treating the weathered mica ore consecutively with an aqueous solution of an alkali metal e.g. a sodium salt, and especially sodium chloride, and an aqueous solution of a further salt, e.g. an organo substituted ammonium salt such as an n-butyl ammonium salt, in order to swell the ore for example as described in British Patent No. 1,065,385, the disclosure of which is incor¬ porated herein by reference. After the ore has been swelled to a number of times its original size in water, it is delaminated for example by means of a mill, a mixer, an ultrasonic agitator or other suitable device to form the majority of the expanded mineral into a colloidal dispersion. The colloidal dispersion so formed can be fractionated by sedimentation into several cuts . With a mineral such as vermiculite or other very highly weathered systems, as one moves from the 'fines' to the more coarse fractions the degree of hydration decreases through successive layers, the K2O content increases and the x-ray diffraction pattern moves closer to resembling the parent mineral. When partially weathered micas are used a distinctive increasing micaceous component can be easily identified and as one moves to the coarse unprocessable fraction of the mineral its x-ray diffraction pattern, TGA trace and elemental composition distinctly identifies it as pure mica. In the latter case it is possible to form a dispersion of predominantly micaceous lamellae by selecting the appropriate fractions of the colloid i.e. by discarding the coarse mica fraction and the highly hydrated vermiculitised fines . It is therefore possible to generate a dispersion of mica-like plate¬ lets as identified by XRD, TGA and elemental anaylsis by utilising the chemical exchangeability of ver¬ miculite interlayers on partially weathered interstra- tified layered minerals. In a typical process, the dispersion is permitted to stand for between 1 and 60 minutes, preferably 5 to 20 minutes, and the top fraction decanted to supply the working colloid. In many instances where partially weathered mica is employed, it will not be possible for all the mineral to be brought into suspension since the weathering process does not occur uniformly throughout the mineral, and the greater the degree of weathering or cationic replacement, the greater the proportion of mineral that can be dispersed. The particle size range of the decanted fraction typically is between 1 and 250 um, preferably between 1 and 100 μm. Preferably the suspension has a concentration of at least 0.5 and especially at least 1% by weight although lower con¬ centrations may be used provided that the concentration is not so low that flocculation occurs . The maximum concentration is preferably 8% and especially 4% by weight, beyond which the relatively high viscosity of the suspension may lead to unreproduceable coatings . The conditions that are employed to form the suspension will depend among other things on the particular type of mineral that is employed. The preferred method of forming the weathered mica dispersion is described in our copending patent application entitled "Mineral" and filed on even date herewith (Agent's reference RK375), claiming priority from British application No. 8813574. In order to coat the conductor, it is passed con¬ tinuously through a bath containing the mineral suspen¬ sion while being electrically connected as an anode with respect to a cathode that is immersed in the suspension, so that the weathered mica platelets are reconstituted electrolytically on the conductor in the form of a gelatinous coating. The fact that the coating is gelatinous and therefore electrically con¬ ductive means that it is not self-limiting in terms of the coating thickness and therefore enables relatively thick coatings to be formed. The plating voltage will depend on a number of factors including the residence time of the conductor in the bath, the desired coating thickness, the electrode geometry, the bath con¬ centration and the presence or otherwise of other spe¬ cies, especially ionic species, in the bath. The plating voltage will normally be at least 5V, more pre¬ ferably at least 10V and especially at least 20V since lower voltages usually require very long residence times in the bath in order to achieve an acceptable coating thickness . The voltage employed is usually not more than 200V and especially not more than 100V since higher voltages may lead to the production of irregular coatings and poor concentricity of the coating layer, to oxidation of the anode or electrolysis of the bath water and hence a poorly adhered coating. Such plating voltages will usually correspond to a current density of 0.1 to 6 mA mm"'--. After the coated wire has left the bath, and ore- ferably before being contacted by any rollers or other parts of the equipment, the coating is dried in order to remove residual water from the gel. This may be achieved by hauling the coated wire through a hot-air column or a column heated by infrared sources or hot filaments. Additional columns may be used if desired. The wire may then be hauled off for final use or to be provided with an outer protective insulation . The orientation of the platelets in a direction parallel to the underlying conductor means that relatively rapid drying methods can be used to collapse the gel to leave an integral, self-supporting inorganic layer. The silicone polymers used for forming the sili¬ cone polymer layer are preferably elastomeric and adapted for coating conductors by extrusion or dip- coating. It is preferred to use elastomers rather than solvent based resins because the resin will impregnate the mineral layer at least to some extent which will normally require a long drying period during manufac¬ ture of the wire. In addition it has been found that the use of a silicone elastomer layer will improve the fire performance of the wire as described below. Suitable forms of silicone polymer from which silicone elastomers may be derived include polymers of which at least some of the repeating units are derived from unsubstituted or substituted alkyl siloxanes, for example, dimethyl siloxane, methyl ethyl siloxane, methyl vinyl siloxane, 3,3 ,3-trifluoropropyl methyl siloxane, polydimethyl siloxane, dimethyl siloxane/- methyl vinyl siloxane co-polymers , fluoro silicones, e.g. those derived from 3 ,3, 3-trifluoropropyl siloxane. The silicone polymer may be, for example, a homopolymer or a copolymer of one or more of the above siloxanes, and is advantageously polydimethyl siloxane or a copo¬ lymer of dimethyl siloxane with up to 5% by weight of methyl vinyl siloxane. Silicone modified EPDM, such as Royalther (available from Uniroyal) and room tem¬ perature vulcanising silicones are also suitable materials . The silicone elastomer may, if desired, contain fillers, for example reinforcing fillers, flame retar- dants, extending fillers, pigments, and mixtures thereof. For example, suitable fillers include diato- maceous earth and iron oxide. It will be appreciated that such fillers may be used in addition to a rein¬ forcing filler such as silica that is added to silicone polymer to form the silicone elastomer. Other materials such as antioxidants , U V stabili¬ sers, thermal stabilisers, extending silicone oils, plasticisers and cross-linking agents, may be included. We have found that improvements in the mechanical performance of the wire may be achieved if a binder is incorporated in the mineral coating which can improve processability of the mineral-clad conductor. Thus, according to one preferred aspect of the invention, a binder is incorporated in the mineral dispersion and is deposited on the conductor along with the mineral in order to improve the processability of the clad conduc¬ tor. The material chosen for the binder should be inert, i.e. it should not corrode the conductor metal or react with the mineral coating and preferably it improves the bonding of the mineral layer to the con¬ ductor metal. It should also be electrophoretically mobile and non-flocculating. The binder may be disper- sible in the medium that is used to form the mineral suspension (water), for example it may comprise a water-dispersed latex, e.g. a styrene/butadiene/car- boxylic acid latex, a vinyl pyridine/styrene/butadiene latex, a polyvinyl acetate emulsion, an acrylic copo¬ lymer emulsion or an aqueous silicone emulsion. It is preferred to use binders in the form of emulsions because they may be dried quickly with only a few seconds residence time in the drying tower, whereas with aqueous solutions much longer drying times are necessary, and, if drying is forced, bubbles may be formed in the mineral layer that will cause imperfec¬ tions in the resulting dried layer. In addition at least some binders that are hydrophobic have the advan¬ tage that they can prevent or reduce the uptake of moisture by the mineral layer after it has been dried. This is particularly useful where the weathered mica has a relatively high degree of cationic replacement, i.e. where it contains a relatively high degree of ver¬ miculite, so that undesired exfoliation of the mineral layer when subjected to a fire can be eliminated. The binder is preferably non-curable since curable binders do not significantly improve the performance of the wire and will normally reduce the speed at which the wire can be manufactured. We have observed that the presence of a polymeric binder usually has a detrimental effect on the electri¬ cal resistance of the mineral layer, usually during the first one or two minutes that the wire is subjected to a fire, after which the effect becomes insignificant, with the result that any wires that have been tested for circuit integrity performance at reasonably high voltages e.g. 200V, will either fail within the first minute or two or will survive for a number of hours at the test temperature. It is believed that the reduc¬ tion in resistance of the wire is due to carbonisation of the binder as the temperature rises and/or to the generation of gaseous conductive species from the binder or any other organic components in the cable, and that this effect rapidly dies away as the carbon so formed is oxidized. However, the detrimental effect on the resistance caused by most of the binders may usually be ameliorated by the presence of the thin silicone layer. It is believed that the silicone layer acts as some form of electrical and/or mechanical barrier which prevents the char from the binder forming an electrical short circuit. Thus, for the first minute or so of the test, the electrical performance of the wire is usually dominated by that of the silicone layer. By the time the silicone layer has ashed, the char from the binder will normally have completely oxi¬ dized away and will no longer have any effect on the wire performance. Thus, according to another aspect, the invention provides a flame resistant electrical wire which comprises a metallic electrical conductor and electrical insulation which comprises an insulating mineral layer that is formed from weathered mica and contains an organic binder, and, located on the mineral layer, a layer of a material that will provide a tem¬ porary barrier when the wire is subjected to a fire which will reduce or eliminate the detrimental effect of char formed from the binder on the electrical resistance of the wire insulation. The binder is preferably used in quantities in the range of from 5 to 30%, and especially from 10 to 25% by weight based on the weight of the weathered mica. The use of smaller quantities may not sufficiently improve the processability of the conductor and/or may not improve the adhesion of the mineral layer to the metal conductor adequately while the use of larger quantities of binder may lead to the generation of too much char for the silicone layer to mask. Also, it is preferable not to use binders such as neoprene that generate large quantities of char. Preferably the binder has a carbonaceous char residue of not more than 15%, more preferably not more than 10% and especially not more than 5%. The char residue can be measured by the method known as thermogravimetric analysis, or. TGA, in which a sample of the binder is heated in nitrogen or other inert atmosphere at a defined rate, e.g. 10°C per minute to a defined temperature and the residual weight, which is composed of char, is recorded. The char residue is simply the quantity of this residual char expressed as a percentage of the initial polymer after having taken into account any non polymeric vola¬ tile or non-volatile components. The char residue values quoted above are defined as having been measured at 850°C. As stated above, an outer protective layer, pre¬ ferably a polymeric insulating layer, may be provided in order to protect the underlying mineral layer from mechanical abuse and in order to provide the required insulating and dielectric properties during normal use. Examples of polymers that may be used to form the outer layer include olefin homopolymers and copolymers of olefins with other olefins and with other monomers e.g. vinyl esters , alkyl acrylates and alkyl alkacrylates , e.g. low, medium and high density polyethylene, linear low density polyethylene and ethylene alpha-olefin copolymers, ethylene/propylene rubber, ethylene vinyl acetate, ethylene ethyl acrylate and ethylene acrylic acid copolymers, and styrene/butadiene/styrene, styrene/ ethylene/butadiene/styrene block copolymers and hydrogenated versions of these block copolymers. A particularly preferred class of low charring polymers is the polyamides . Preferred polyamides include the nylons e.g. nylon 46, nylon 6, nylon 7, nylon 66, nylon 610, nylon 611, nylon 612, nylon 11 and nylon 12 and aliphatic/aromatic polyamides, polyamides based on the condensation of terephthalic acid with trimethylhexa- methylene diamine (preferably containing a mixture of 2,2,4- and 2,4,4-trimethylhexamethylene diamine isomers ) , polyamides formed from the condensation of one or more bisamiήomethylnorbornane isomers with one or more aliphatic, cycloaliphatic or aromatic dicarboxylic acids e.g. terephthalic acid and optionally including one or more amino acid or lactam e.g. £~caprolactam comonomers , polyamides based on units derived from laurinlactam, isophthalic acid and bis-(4-amino-3-methylcyclohexyl) methane, polyamides based on the condensation of 2,2-bis-(p-aminocyclo- hexyl) propane with adipic and azeleic acids, and polyamides based on the condensation of trans cyclo- hexane-1,4-dicarboxylic acid with the trimethylhexa- methylene diamine isomers mentioned above. Other aliphatic polymers that may be used include polyesters e.g. polyalkylene terephthalate and especially poly¬ tetramethylene terephthalate, and cycloaliphatic diol/terephthalic acid copolymers e.g. copolymers of terephthalate and isophthalate units with 1,4-cyclo- hexanedimethyloxy units, polyethers e.g. polybutylene ether copolymers, and especially polyether esters such as those having polytetramethylene ether and poly(tetramethylene terephthalate) blocks; aliphatic ionomers e.g. those based on metal salts of ethylene (meth)acrylic acid copolymers or sulphonated olefins such as sulphonated EPDM, and the like. Preferred aliphatic polymers include polyethylene, polybutylene terephthalate, ionomers based on metal salts of methacrylated polyethylene, acrylic elastomers e.g. those based on ethyl acrylate, n-butyl acrylate or alkoxy-substituted ethyl or n-butyl acrylate polymers containing a cure site monomer and optionally ethylene comonomer, and block copolymers having long chain ester units of the general formula: 0 0 II -OGO-C-R-C- and short-chain ester units of the formula 0 0 II II -ODO-C-R-C- in which G is a divalent radical remaining after the removal of terminal hydroxyl groups from a polyalkylene oxide) glycol, preferably a poly (C2 to C4 alkylene oxide) having a molecular weight of about 600 to 6000; R is a divalent radical remaining after removal of carboxyl groups from at least one dicarboxylic acid having a molecular weight of less than about 300; and D is a divalent radical remaining after removal of hydroxyl groups from at least one diol having a molecular weight less than 250. Preferred copolyesters are the polyether ester polymers derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butane diol. These are random block copolymers having crystalline hard blocks with the repeating unit: CH2) and amorphous , elastomeric polytetramethylene ether terephthalate soft blocks of repeating unit having a molecular weight of about 600 to 3000, i.e. n = 6 to 40. Other preferred aliphatic polymers include those based on polyether and polyamide blocks, especially the so called a "polyether-ester amide block copolymers" of repeating unit: -C-A-C-O-B-O- ιι II 0 0 wherein A represents a polyamide sequence of average molecular weight in the range of from 300 to 15,000, preferably from 800 to 5000; and B represents a linear or branched polyoxyalkylene sequence of average molecu¬ lar weight in the range of from 200 to 6000, preferably from 400 to 3000. Preferably the polyamide sequence is formed from alpha,omega-aminocarboxylic acids, laσtams or diamine/dicarboxylic acid combinations having C4 to C]_4 carbon chains, and the polyoxyalkylene sequence is based on ethylene glycol, propylene glycol and/or tetramethylene glycol, and the polyoxyalkylene sequence constitutes from 5 to 85%, especially from 10 to 50% of the total block copolymer by weight. These polymers and their preparation are described in UK Patent Specifications Nos. 1,473,972, 1,532,930, 1,555,644, 2,005,283A and 2,011,450A. The polymers may be used alone or in blends with one another or with other polymers and may contain fillers e.g. silica and metal oxides e.g. treated and untreated metal oxide flame retardants such as hydrated alumina and titania. The polymers may be used in single wall constructions or in multiple wall construc¬ tions e.g. as described in British Patent Application No. 2,128,394A the disclosure of which is incorporated herein by reference. The polymers may be uncrosslinked or may be crosslinked for example by chemical cross- linking agents or by electron or gamma irradiation, in order to improve their mechanical properties and to reduce flowing when heated. They may also contain other materials e.g. antioxidants , stabilizers, cross- linking promotors , processing aids and the like. In some cases polymer insulation or at least the inner wall of the insulation may be substantially halogen- free. In addition, it has been found that certain halogen-containing polymers may generate electrically conductive species during a fire and so cause the wire to fail prematurely. In those cases the insulation preferably contains not more than 5% by weight halo¬ gens, especially not more than 1% by weight halogens and most especially not more than 0.1% by weight halo¬ gens. However, in other cases, for example in the case of airframe wire where high temperature ratings are desirable, it may be appropriate for the outer wall or primary jacket of the insulation to include a halogen- ated polymer. One class of halogenated polymer that is particularly useful is the fluorinated polymers, pre¬ ferably those containing at least 10%, more preferably at least 25% fluorine by weight. The fluorinated polymer may be a single fluorine containing polymer or a mixture of polymers one or more of which contains fluorine. The fluorinated polymers are usually homo-or copolymers of one or more fluorinated, often per- fluorinated, olefinically unsaturated monomers or copo¬ lymers of such a comonomer with a non-fluorinated olefin. The fluorinated polymer preferably has a melting point of at least 150°C, often at least 250°C and often up to 350°C, and a viscosity (before any crosslinking) of less than 10^ Pa.s at a temperature of not more than 60°C above its melting point. Preferred fluorinated polymers are homo- or copolymers of tetra¬ fluoroethylene, vinylidine fluoride or hexafluoro- ethylene, and especially ethylene/tetrafluoroethylene copolymers e.g. containing 35 to 60% ethylene, 35 to 60% tetrafluoroethylene by mole and up to 10% by mole of other comonomers , polyvinylidine fluoride, copoly¬ mers of vinylidine fluoride with hexafluoropropylene, tetrafluoroethylene and/or hexafluoroisobutylene, poly- hexafluoropropylene, and copolymers of hexafluoropropy- lene and tetrafluoroethylene. Alternatively 1-C4 perfluoroalkoxy substituted perfluoroethylene homopoly- mers and copolymers with the above fluorinated polymers may be used. In addition, the polymeric insulation, or the inner layer of any polymeric insulation, preferably has a carbonaceous char residue of not more than 15% by weight as determined by thermogravimetric analysis. Such wires are the subject of our copending British patent application entitled "Electrical Wire" filed on even date herewith (Agent's ref: RK341" the disclosure of which is incorporated herein by reference. The wire according to the invention may be formed using most commonly available electrical conductor materials such as unplated copper and copper that has been plated with tin, silver or chromium. In addition, if desired the conductor may be coated with an electri¬ cally conductive refractory layer, for example as described in European Patent Application No. 190,888, the disclosure of which is incorporated herein by reference. One embodiment of a wire in accordance with the present invention and a method of manufacturing it will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is an isometric view of part of a wire in accordance with the invention with the thicknesses of the layers of insulation exaggerated for the sake of clarity; and Figure 2 is a schematic view of apparatus for forming the wire of figure 1; and Figures 3a to c are graphical respresentations showing the effect of a binder and a silicone layer on the circuit integrity performance of the wires . Referring to the accompanying drawings, an electrical wire 1 comprises a 22 AWG seven strand copper conductor 2 which has been coated with a 50 micrometre thick layer 3 of a partially weathered mica, a 50 micrometre thick silicone polymer layer 3* and followed by a 0.15mm thick extruded layer of polymeric insulation 4 based on a blend of polytetramethylene terephthalate and a polytetramethylene ether terephthalate/polytetramethylene terephthalate block copolymer. The wire may be formed by means of the apparatus shown schematically in figure 2. In this apparatus the conductor 2 is fed into a bath 5 that contains a colloidal suspension of the weathered mica and binder, the suspension being fed from a supply bath 5', and agitated in order to maintain uniform mixing of the dispersion. The conductor passes down into the bath, around a roller 6 and then vertically upwards as it leaves the bath. A hollow tube 7 is positioned around the part of the conductor that leaves the bath and a hollow electrode 8 is located inside the hollow tube 7 so that the weathered mica is deposited on the rising part of the conductor . This prevents the mineral coating so formed being damaged as the conductor is Dassed around roller 6. After the coated conductor leaves the bath it passes through a drying tower 8 about 1.5 metres in length that is heated by a counter current of warm air so that the top of the drying tower is at a temperature of about 200°C while the bottom is at about 160°C. After the mineral coating has dried the coated conduc¬ tor is passed through a coating pot 10 that contains a silicone polymer. After a layer of silicone polymer is applied to the wire, it is passed through a further w-rm air drying tower 11 arranged to have a temperature of about 130°C at the top and 90°C at the bottom. When the silicone layer has been applied and dried the wire may then be spooled to await the provision of an insulating top-coat or a top-coat may be provided in-line for example by means of an extruder 12. The eed rate of the conductor 2 to the coating apparatus will depend on the thickness of the intended coating, the electrophoresis potential and the con¬ centration of the weathered mica in the bath. Feed rates in the range of from 2 to 20, and especially 5 to 10 metres per minute are preferred although increases in the feed rate should be possible, for example by increasing the dimensions of the bath in order to main¬ tain the same residence time with higher conductor speeds . Figures 3a to 3c show the effect of both the binder and the silicone layer on the electrical perfor¬ mance of the wire insulation. In each case a 1 metre long twisted pair of wires was heated to 900°C in a gas flame, and the electrical resistance between the wires was recorded, and is shown along the ordinate, as a function of time since the heating commenced, shown along the abscissa. Figure 3a shows the performance of wires insulated only by means of a 25 micrometre thick layer of weathered mica that contained no binder The resistance fell when the wire was heated to a value slightly below 10 7 ohms in about 60 seconds, and remained at that level until the end of the test. Although this insulating layer had satisfactory electrical performance, it had inadequate mechanical performance and could not be manufactured at economic wire and cable processing rates. Figure 3b shows the performance of wires in which the mineral layer contains 15% by weight of a styrene butadiene styrene block copolymer binder. The mechani¬ cal properties were excellent and the wire could easily be mechanically handled through wire and cable pro¬ cessing operations at rates of up to 50m minute--!-. In this case the electrical resistance of the wire fell to a value of about 10-*- ohms after 30 seconds, whereupon the resistance rose slowly until it reached about 10 7 ohms after 150 to 200 seconds and remained at this level until the test was terminated. The resistance drop to 10 ■ -'ohms would greatly restrict the voltage range to which such a wire could be specified. Figure 3c shows the performance of the wires of figure 3b with an additional 50 micrometre layer of a silicone elastomer to give a total thickness of 75 micrometres . The resistance falls to slightly over 10 ' ohms at 100 seconds after commencement of the test and remains at that level until the test is terminated. Thus the deleterious effect of the organic binder is completely removed. The mechanical performance of the insulation was good, the limits being determined by the strength of the silicone layer. The wire could easily be provided with a further layer of polymeric insula¬ tion. The following Examples illustrate the invention: In all the Examples the working colloid that was used for coating the conductor was formed as follows: 800 gramms of a weathered mica in accordance with our co-pending British patent application entitled "Wire" (Agents ref: RK342) filed on even date herewith, was washed with boiling water for about 30 minutes and the resulting liquid was decanted to remove the clay frac¬ tion. The mineral was then refluxed for 4 to 24 hours in saturated sodium chloride solution to replace the exchangeable cations with sodium ions . This was then washed with distilled or deionised water to remove excess sodium chloride until no further chloride ions could be observed by testing with silver nitrate. The material was then refluxed for 4 to 24 hours with molar n-butyl ammonium chloride solution followed by further washing with distilled or deionised water until no chloride ions could be detected with silver chloride. The swollen material was then worked in a Greaves mixer for 20 minutes to shear the mineral and was allowed to stand for 20 minutes to sediment the unpro¬ cessed mineral. The top fraction was used as the working colloid. Example 1 A colloid having 4% by weight weathered mica and 15% by weight carboxylated styrene-butadiene-styrene rubber based on the weight of the weathered mica, was used as the plating bath. A 20 AWG wire was passed through a 40 cm long bath of the colloid at a speed of 5 metres minute"-!- while the weathered mica was electrophoretically deposited on the conductor at a 4.2V plating voltage and a 165 rtiA current. The coated wire was then passed through a drying tower as shown in the drawing to form a mineral layer of 30 micrometre dry thickness . The wire was then passed through a bath of a two part silicone (KE1204 ex Shinetsu) and cured again as * hown in the drawing to form a 50 micrometre thick silicone layer. Thereafter a 100 micrometre thick single wall insulation formed from low density polyethylene containing 8% by weight decabromodiphenyl ether and 4% antimony trioxide flame retardant was extruded onto the wire. The wire was tested for circuit integrity by twisting three wires together and connecting each wire to one phase of a three phase power supply, and then heating the wire to 900°C for a test period of three hours in accordance IΞC 331. The wire was able to sup¬ port 300V phase-to-phase for the entire test at 900°C without failing (i.e. without blowing a 3A fuse). Examp-les 2 to 5 Example 1 was repeated with the exception that the following binders were used: Example 2 polyvinyl acetate Example 3 acrylic copolymer emulsion Example 4 polyvinylidine chloride Example 5 vinylpyridine terminated styrene-butadiene-styrene rubber The wire was tested as described in Example 1 and in each case the wires were able to support 300V phase- to-phase at 900°C for 3 hours. Example 6 Example 1 was repeated with the exception that the silicone layer was formed from an extended polydimethyl siloxane based formulation. The silicone composition was room-temperature extruded onto the coated conductor to give a 75 to 100 micrometre thick layer and was vulcanised in a tube furnace at 300°C (20.5 second residence time). The wire was tested as described in Example 1 and supported 440V phase-to-phase for 3 hours at 900°C. Example 7 Example 6 was repeated with the exception that the plating voltage of the deposition bath was 15.5V ( 300mA) which gave a mineral layer thickness of 40 micrometres . The wire supported 440V phase-to-phase for 3 hours at 900°C. Example 8 Example 1 was repeated with the exception that the silicone used was a dip-coated solventless silicone (sylgard 184) applied to a thickness of 70 micrometres. The wire supported 300V phase-to-phase for 3 hours at 900°C. Example 9 Example 1 was repeated with the exception that the low density polyethylene insulation was replaced with a 100 micrometre thick layer comprising: parts by weight polybutylene terephthalate ( PB ) 80 Surlyn ionomer 20 decabromodiphenyl ether 8 antimony trioxide 4 Irganox 1010 2 triallyl isocyanurate cross- 5 linking promotor The wire supported 300V phase-to-phase for 3 hours at 900°C. Example 10 Example 9 was repeated with the exception that the PBT/Surlyn layer contained no flame retardant (decabromodiphenyl ether/Sb2θ3. and that an additional polymeric layer of thickness 100 micrometres was pro¬ vided on top of the PBT/Surlyn layer. The additi * - al layer had the composition: parts by -weight polybutylene terephthalate (PBT) 70 polybutylene terephthalate - 30 polybutylene ether tereph¬ thalate block copolymer ethylene bis-tetrabromo- 10 phthalimide antimony trioxide 4 magnesium hydroxide 20 The wire supported 300V phase-to-phase for 3 hours at 900°C. Example 11 Example 7 was repeated with the exception that the low density polyethylene insulation was replaced by the additional layer of Example 10. The wire supported 440V phase-to-phase for 3 hours at 900°C. Example 12 Example 6 was repeated with the exception that the low density polyethylene insulation was replaced with a 100 micrometre thick layer of un-flame retarded high density polyethylene. The wire supported 300V phase- to-Dhase for 3 hours at 900°C. Example 13 Example 1 was repeated with the exception that the binder used was a vinyl acetate/ethylene copolymer, the plating voltage was 12.5V and current 422 mA, the line speed was 10 metres minute " -*- and the silicone layer and polymer insulation had the compositions shown below: Silicone composition polybutylene terephthalate 43.5 butylene terephthalate/ polybutylene oxide terephthalate copolymer 15.8 polycarbodimide 2.8 decabromo diphenyl ether 9.5 antimony trioxide 3.8 244-26 3.9 antioxidant (Irganox 1010) 1.9 magnesium hydroxide 18.8 The silicone layer had a thickness of 100 um and the polymer layer had a thickness of 125 um. The wire was tested as described in Example 1 and was able to support 440V (3A) bhase-to-phase for the entire test at 900°C. Example 14 Example 13 was repeated with the exception that the polymer insulation had the composition: Composition Parts by weight polybutylene terephthalate 43.5 butylene terephthalate/ polybutylene 15.6 oxide terephthalate copolymer 15.8 polycarbodimide 2.8 decabromodiphenyl ether 9.5 antimony trioxide . 3.8 244-26 3.9 antioxidant (Irganox 1010) 1.9 magnesium hydroxide 18.8 The plating voltage was 11.5V and current was 365 mA. The mineral layer has a thickness of 25 um and the silicone layer had a thickness of 125 um. The wire was able to support 440V (3A) phase-to- phase for the entire test (3 hours) at 900°C.

Claims: CLAIMS : 1. An electrical wire which comprises a metallic electrical conductor, an insulating mineral layer electrolytically formed on the conductor from chemi¬ cally delaminated weathered mica, and a silicone polymer layer located on the mineral layer. 2. A wire as claimed in claim 1, which includes an outer protective jacket that is -located over the sili¬ cone layer . 3. A wire as claimed in claim 2, wherein the pro¬ tective jacket is an electrically insulating jacket that is formed from an organic polymer. 4. A wire as claimed in claim 2 or claim 3, wherein the protective jacket comprises an inner and an outer layer, the inner layer being substantially halogen- free. 5. A wire as claimed in any one of claims 1 to 4 , wherein the mineral layer includes a binder. 6. A wire as claimed in claim 5, wherein the binder has a carbonaceous char residue of not more than 15% by weight . 7. A wire as claimed in claim 5 or claim 6, wherein the binder is present in the mineral layer in the range of from 5% to 30% by weight based on the total mineral weight . 8. A wire as claimed in any one of claims 5 co 7 , wherein the binder has been incorporated in the mineral layer in the f r of an organic latex. 9. A wire as claimed in any one of claims 5 to 8 , wherein the binder comprises a styrene/butadiene/- carboxylic acid polymer or a vinylpyridine/styrene/- butadiene polymer . 10. A wire as claimed in any one of claims 1 to 9 , wherein the silicone polymer is an elastomer. 11. A wire as claimed in any one of claims 1 to 10, wherein the mineral layer has a thickness in the range of from 5 to 125 micrometres . 12. An electrical cable which comprises a bundle of wires as claimed in any one of claims 1 to 11 enclosed in a cable jacket. 13. A method of forming an electrical wire, which comprises: (i) passing an elongate electrical conductor through a suspension of chemically delaminated weathered mica and applying an electrical potential to the conductor in order to deposit the weathered mica onto the conductor; (ii) drying the layer of weathered mica so formed; (iii) applying a layer of a silicone polymer to the surface of the weathered mica; and (iv) drying the silicone polymer layer. 14. A method as claimed in claim 13, which includes the step of forming an insulating protective jacket on the coating conductor. 15. A method as claimed in claim 13 or claim 14 , which includes incorporating a binder in the suspen¬ sion . 16. A method as claimed in claim 15, wherein the binder is incorporated into the suspension in the form of a latex. 17. A method as claimed in any one of claims 13 to 16, which includes the step of forming an organic polymer layer on the silicone layer. 18. A flame resistant electrical wire which compri¬ ses a metallic electrical conductor and electrical insulation which comprises an insulating mineral layer that is formed from chemically delaminated weathered mica and contains an organic binder, and, located on the mineral layer, a layer of a material that will pro¬ vide a temporary barrier when the wire is subjected to a fire which will reduce or eliminate the detrimental effect of char formed from the binder on the electrical resistance of the wire insulation.

Inventors: BARRETT SHAUN MICHAEL, LUDDEN MICHAEL JOSEPH, BARRETT, SHAUN, MICHAEL, LUDDEN, MICHAEL, JOSEPH
Applicants: RAYCHEM LTD, RAYCHEM LIMITED
Application year: 1988
Main classification: H01B7
IPC codes: H01B3, H01B7