SHAPED ARTICLES OF CROSSLINKED POLYMERS

Abstract: Electrical conductors which are insulated by an inner polymeric layer having little or no cross-­ linking and high elongation, and an outer polymeric layer having a relatively high level of cross-linking and low elongation. Preferably each of the components comprises a crystalline fluorocarbon polymer, espe­ cially an ethylene/tetrafluoroethylene copolymer. Such articles can be prepared by (1) melt-extruding a first polymeric composition which contains little or no cross-linking agent, and a second polymeric composition which contains a greater amount of cross-linking agent, (2) maintaining the two extrudates under conditions such that cross-linking agent migrates from the second to the first composition, and (3) cross-linking both compositions, preferably by radiation.

Description: This invention relates to insulated electrical conductors. It is known to use cross-linked polymeric com­ positions as electrical insulation on a wire or other conductor. Known insulated wires include wires coated with a layer of a radiation crosslinked fluorocarbon polymer, particularly an ethylene/tetrafluoroethylene copolymer (often referred to as an ETFE polymer), which are extensively used for the wiring in aircraft. Military Specification No. MIL-W-22759 sets various standards for such insulated wires. Reference may be made for example to U.S. Patents Nos. 3,763,222, 3,840,619, 3,894,118, 3,911,192, 3,970,770, 3,985,716, 3,995,091, 4,031,167, 4,155,823 and 4,353,961. Such wires have the significant disadvantage that if the outer surface of the insulation is damaged, subsequent flexing of the wire causes the damage to propagate through the insulation, at a rate which is highly unde­ sirable. This disadvantage is especially serious when the insulated wire is to be used in an aircraft or in other high performance situations where the consequen­ ces of insulation failure can be so serious. A quan­ titative measure of this disadvantage can be obtained from a notch propagation test such as that described below. We have discovered that this disadvantage can be substantially mitigated by insulation which comprises an inner layer of a polymer which has little or no cross-linking and an outer layer of polymer which has a relatively high level of cross-linking. Furthermore, this improvement is obtained with little or no substan­ tial deterioration of other important properties of the insulation, for example, resistance to scrape abrasion, resistance to crossed wire abrasion and resistance to cut-through. In one aspect, the present invention provides an insulated electrical conductor, especially a wire, which comprises (1) an electrical conductor; and (2) electrical insulation which comprises (a) an inner electrically insulating layer which (i) is composed of a first melt­ processed, cross-linked polymer com­ position wherein the polymer has a melting point of at least 200°C, and (ii) has a first M₁₀₀ value of 0 to 24.5 kg/cm² (0 to 350 psi); and (b) an outer electrically insulating layer which (i) is separated from the conductor by the inner layer, (ii) is composed of a second melt-processed cross-linked poly­ meric composition wherein the polymer has a melting point of at least 200°C, and (iii) has a second M₁₀₀ value which is at least 24.5 kg/cm² (psi) and at least 3.5 kg/cm² (50 psi) higher than the first M₁₀₀ value. The M₁₀₀ values given herein are modulus values measured at a temperature above the melting point of the polymer by the procedure described in detail below, and therefore reflect the level of cross-linking in the layer. A preferred method of making such an insulated conductor comprises (1) melt-shaping the first polymeric composition to form the first layer; (2) melt-shaping the second polymeric composition to form the second layer in contact with the first layer, the second composition containing a radiation cross-linking agent; (3) maintaining contact between the first and second layers under conditions such that part of the radiation cross-linking agent migrates from the second layer into the first layer; and (4) irradiating the first and second layers to effect cross-linking thereof. The polymeric component in the polymeric com­ positions used in the present invention preferably comprises, and more preferably consists essentially of, a melt-shapeable crystalline polymer having a melting point of at least 200°C, preferably at least 250°C, or a mixture of such polymers. The term "melting point" is used herein to denote the temperature above which no crystallinity exists in the polymer (or, when a mixture of crystalline polymers is used, in the major crystalline component of the mixture). Particularly preferred polymers are fluorocarbon polymers. The term "fluorocarbon polymer" is used herein to denote a polymer or mixture of polymers which contains more than 10%, preferably more than 25%, by weight of fluorine. Thus, the fluorocarbon polymer may be a single fluorine-­ containing polymer, a mixture of two or more fluorine-­ containing polymers, or a mixture of one or more fluorine-containing polymers with one or more polymers which do not contain fluorine. Preferably the fluoro­ carbon polymer comprises at least 50%, particularly at least 75%, especially at least 85%, by weight of one or more thermoplastic crystalline polymers each containing at least 25% by weight of fluorine, a single such crystalline polymer being preferred. Such a fluorocar­ bon polymer may contain, for example, a fluorine-­ containing elastomer and/or a polyolefin, preferably a crystalline polyolefin, in addition to the crystalline fluorine-containing polymer or polymers. The fluorine-­ containing polymers are generally homo- or co-polymers of one or more fluorine-containing olefinically unsaturated monomers, or copolymers of one or more such monomers with one or more olefins. The fluorocarbon polymer has a melting point of at least 200°C, and will often have a melting point of at least 250°C, e.g. up to 300°C. Preferably the polymeric composition has a viscosity of less than 10⁵ poise at a temperature not more than 60°C above its melting point. A preferred fluorocarbon polymer is a copolymer of ethylene and tetrafluoroethylene and optionally one or more other comonomers, especially a copolymer comprising 35 to 60 mole percent of ethylene, 35 to 60 mole percent of tetrafluoroethylene and up to 10 mole percent of one or more other comonomers. Other specific polymers which can be used include copolymers of ethylene and chlorotrifluoroethylene; copolymers of vinylidene fluoride with one or both of hexafluoropropylene and tetrafluoroethylene, or with hexafluoroisobutylene; and copolymers of tetrafluoroethylene and hexafluoropropy­ lene. The polymeric composition can optionally contain suitable additives such as pigments, antioxidants, thermal stabilisers, acid acceptors and processing aids. The first and second compositions preferably contain the same polymeric components, and more pre­ ferably are substantially the same in all respects except for the level of cross-linking. The conductor is preferably a metal (e.g. cop­ per) wire, which may be stranded or solid. The wire may be for example from 10 to 26 AWG in size. The first layer preferably contacts the conductor. The second member is preferably also in the form of a layer which has the same general shape as the first layer or which serves to join together a number of wires each of which is surrounded by a first layer, thus forming a ribbon cable. The layers are preferably in direct con­ tact, but may be joined together by a layer of adhe­ sive. The first and second members are preferably formed by melt-extrusion, particularly by sequential extrusion, which may be tubular or pressure extrusion, so that the layers are hot when first contacted, in order to promote migration of the cross-linking agent. The polymeric compositions should preferably be selected so that at least the outer layer has a tensile strength of at least 3,000 psi (210 kg/cm²); and since a higher tensile strength is usually desired in the cross-linked product and there is frequently a loss of tensile strength during the irradiation step, a higher initial tensile strength is preferred, e.g. greater than 6,000 psi (420 kg/cm²), preferably at least 7,000 psi (490 kg/cm²), particularly at least 8,000 psi (560 kg/cm²). The thickness of the inner layer is generally 0.0075 to 0.038 cm (0.003 to 0.015 inch), preferably 0.0075 to 0.0225 cm (0.003 to 0.009 inch). The thickness of the outer layer is generally 0.01 to 0.063 cm (0.004 to 0.025 inch), preferably 0.01 to 0.023 cm (0.005 to 0.009 inch). Preferred radiation cross-linking agents con­ tain carbon-carbon unsaturated groups in a molar per­ centage greater than 15, especially greater than 20, particularly greater than 25. In many cases the cross-­ linking agent contains at least two ethylenic double bonds, which may be present, for example, in allyl, methallyl, propargyl or vinyl groups. We have obtained excellent results with cross-linking agents containing at least two allyl groups, especially three or four allyl groups. Particularly preferred cross-linking agents are triallyl cyanurate (TAC) and triallyl iso­ cyanurate (TAIC); other specific cross-linking agents include triallyl trimellitate, triallyl trimesate, tetrallyl pyromellitate, the diallyl ester of 1,1,3-­ trimethyl-5-carboxy-3-( p -carboxyphenyl) indan. Other cross-linking agents which are known for incorporation into fluorocarbon polymers prior to shaping, for example those disclosed in U.S. Patents referenced above, can also be used. Mixtures of cross-linking agents can be used. In the preferred method of preparing articles of the invention, in which cross-linking agent migrates from the second member to the first member, the first composition as extruded contains little or no cross-­ linking agent (e.g. 0 to 2% by weight, preferably 0%), and the second composition as extruded contains more than is desired during the cross-linking step, e.g. at least 5%, preferably 5 to 25%, particularly 7 to 12%, by weight. The time for which the layers should be maintained in contact prior to cross-linking depends upon the extent of migration which is needed and the temperature during such contact, which is preferably 5 to 150°C below the melting point of the polymer (or the lower melting polymer if there are two or more polymers in the layers). At the time of irradiation, the inner layer preferably contains 0 to 3% by weight of cross-­ linking agent and the outer layer preferably contains 3 to 10% by weight of crosslinking agent. The dosage employed in the irradiation step is generally below 50 Mrads to ensure that the polymer is not degraded by excessive irradiation, and the dosages preferably employed will of course depend upon the extent of cross-linking desired, balanced against the tendency of the polymer to be degraded by high doses of irradiation. Suitable dosages are generally in the range of 2 to 40 Mrads, for example 2 to 30 Mrads, pre­ ferably 3 to 30 Mrads, especially 5 to 25 or 5 to 20 Mrads, particularly 5 to 15 Mrads. The ionising radiation can for example be in the form of accelerated electrons or gamma rays. Irradiation is generally carried out at about room temperature, but higher tem­ peratures can also be used. The inner layer need not be cross-linked at all, but is preferably cross-linked so that it has an M₁₀₀ value of 2.8 to 17.5 kg/cm² (40 to 250 psi), par­ ticularly 3.5 to 10.5 kg/cm² (50 to 150 psi). The elongation of the inner layer is preferably at least 100%, particularly at least 125%, eg. at least 150%, especially 200 to 300%. The outer layer is preferably cross-linked so that it has an M₁₀₀ value of at least 28 kg/cm² (400 psi), particularly at least 31.5 kg/cm² (450 psi), with yet higher values of at least 42 kg/cm² (600 psi) being valuable in many cases. The elongation of the outer layer is preferably 40 to 150%, particularly 50 to 120%. The various physical properties referred to in this specification are measured as set out below. The Notch Propagation Values are measured on a piece of insulated wire about 30 cm (12 inch) long. A notch is made in the insulation, about 5 cm (2 inch) from one end, by means of a razor blade at right angles to the axis of the wire. The depth of the notch is controlled by mounting the razor blade between two metal blocks so that it protrudes by a distance which is 0.01 cm (0.004) inch or, if the insulation comprises two layers and the outer layer has a thickness t which is less than 0.018 cm (0.007 inch) thick, by a distance which is (t-0.0051) cm [(t-0.002) inch]. The end of the wire closer to the notch is secured to a horizontal mandrel whose diameter is three times the outer diameter of the insulation. A 0.675 kg (1.5 lb) weight is secured to the other end of the wire so that the wire hangs vertically. The mandrel is then rotated clockwise, at about 60 revolutions a minute, until most of the wires has wrapped around the mandrel. The mandrel is then rotated, counterclockwise, until the wire has unwrapped and most of the wire has again been wrapped around the mandrel. The mandrel is then rotated clockwise until the wire has unwrapped and most of the wire has again been wrapped around the mandrel. This sequence is continued until visual observation of the notched area shows the conductor to be exposed. If, at this time, the conductor is broken (or some or all of the strands of a stranded wire conductor are broken) then the failure is attributable to that breakage, not to propagation of the notch through the insulation. The number of cycles (half the number of times the rotation of the mandrel is reversed) is recorded. The M₁₀₀ values are determined by a static modulus test as described in U.S. Patent No. 4,353,961 carried out at about 40°C above the melting point of the polymer, (e.g. at about 320°C for ETFE polymers). The tensile strengths and elongations are determined in accordance with ASTM D 638-72 (i.e. at 23°C) at a testing speed of 50 mm (2 inch) per minute. The cross-wire abrasion values, the cut-through resistance values, and the scrape abrasion values, are measured by the tests described in U.S. Patent No. 4,353,961. The invention is illustrated by the following Examples, in which Example 1 is a comparative Example. Example 1 A 20 AWG (19/32) stranded tin-coated copper wire was insulated by melt-extruding over it, by sequential extrusion, an inner insulating layer 0.01 to 0.0125 cm (0.004 to 0.005 inch) thick and an outer insulating layer .018 to 0.020 cm (0.007 to 0.008 inch) thick. The layers were composed of the following compositions: The polymeric insulation was cross-linked by irra­ diating it to a dosage of 14 Mrads. Example 2 The procedure of Example 1 was followed except that the composition of the inner layer was ETFE polymer 99.2 (Tefzel from duPont) Additives 0.8 Triallyl isocyanurate - The products of the Examples were subjected to the various tests described above and the following results (averaged for a number of specimens) were obtained.

Claims: 1. An insulated electrical conductor, especially a wire, which comprises (1) an electrical conductor; and (2) electrical insulation which comprises (a) an inner electrically insulating layer which (i) is composed of a first melt-processed, cross-linked polymer composition wherein the polymer has a melting point of at least 200°C, and (ii) has a first M₁₀₀ value of 0 to 24.5 kg/cm² (0 to 350 psi); and (b) an outer electrically insulating layer which (i) is separated from the conductor by the inner layer, (ii) is composed of a second melt-processed cross-linked polymeric com­ position wherein the polymer has a melting point of at least 200°C, and (iii) has a second M₁₀₀ value which is at least 24.5 kg/cm² (350 psi) and at least 3.5 kg/cm² (50 psi) higher than the first M₁₀₀ value. 2. An insulated conductor according to Claim 1 wherein the polymer in at least one of the first and second poly­ meric compositions consists essentially of a crystalline fluorocarbon polymer, preferably a copolymer of 35 to 60 mole percent of units derived from ethylene, 35 to 60 mole percent of units derived from tetrafluoroethylene and 0 to 10 mole percent of units derived from at least one addi­ tional copolymerizable comonomer. 3. An insulated conductor according to Claim 1 or 2 wherein the inner layer has an M₁₀₀ value of 0 to 17.5 kg/cm² (0 to 250 psi), preferably 3.5 to 10.5 kg/cm² (50 to 150 psi), and the outer layer has an M₁₀₀ value of at least 28 kg/cm² (400 psi), preferably at least 42 kg/cm² (600 psi). 4. An insulated conductor according to any one of the preceding claims wherein the inner layer has an elongation of at least 100%, preferably 200 to 300%, and the outer layer has an elongation of 40 to 150%, preferably 50 to 120%. 5. An insulated conductor according to any one of the preceding claims wherein the conductor is a metal wire; the inner layer contacts the wire, is 0.0075 to 0.038 cm (0.003 to 0.015 inch) thick and has an elongation of at least 125%; and the outer layer contacts the inner layer, is 0.01 to 0.063 cm (0.004 to 0.025 inch) thick and has an elongation of 50 to 120%. 6. An insulated conductor according to any one of the preceding claims wherein each layer has been cross-linked by irradiation with the aid of at least one of triallyl cyanurate and triallyl isocyanurate. 7. A process for the preparation of an insulated con­ ductor as claimed in any one of the preceding claims which comprises (1) melt-shaping the first polymeric composition to form the first layer; (2) melt-shaping the second polymeric composition to form the second layer in contact with the first layer, the second composition containing a radiation cross-linking agent; (3) maintaining contact between the first and second layers under conditions such that part of the radiation cross-linking agent migrates from the second layer into the first layer; and (4) irradiating the first and second layers to effect cross-linking thereof. 8. A process according to Claim 7 wherein the first polymeric composition is substantially free of radiation cross-linking agent when it is melt-shaped and the second polymer composition contains at least 5% by weight of radiation cross-linking agent immediately after it has been melt-shaped.

Inventors: ENAULT, NEAL, LUNK, HANS E., MEHAN, ASHOK
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1986
Main classification: H01B7
IPC codes: H01B3, H01B13, H01B7