Coaxial cables suitable for use at microwave frequencies

Abstract: Foamed polymers having low loss at microwave fre­ quencies are prepared through use of perfluorinated alkane foaming agents containing at least 3 carbon atoms. The invention is particularly valuable for polymers which are melt-processed at high temperatures, especially fluoropoly­ mers such as tetrafluoroethylene copolymers. The polymeric foams are particularly useful as electrical insulation, espe­ cially for coaxial cables suitable for use at microwave frequencies.

Description: This invention relates to foamed polymers having low loss at microwave frequencies. It is known to use foamed polymers, e.g. polyethylene, to provide lightweight electrical insulation, e.g. in coaxial cables. For a number of uses, it is also important that the polymer be foamed in order to reduce losses at microwave frequencies. The magnitude of these losses is dependent upon the loss tangent and permittivity of the insulation. By incorporation of voids (which are loss-free) in the dielectric material, as in foamed polymers, both the loss tangent and permittivity are lowered, yielding a substantial reduction in microwave losses. This result has been achieved in the past by the foaming of low loss polymers, e.g. polyethylene, with conventional blowing agents, e.g. through the decomposition of a chemical such as azobisformamide. Unfortunately, however, these conventional foams lack adequate thermal and/or chemical stability for a number of important uses. For example, a number of end uses require stability at 150°C, and polyethylene foam collapses at such temperatures. Similarly, stability of the foam when exposed to oxygen is often required, and the presence of the antioxidants needed to meet this requirement results in a foam having undesirably high loss. In attempting to make use of fluoropolymers of high melting point in order to provide foams having the desired thermal stability, we have found that perfluorinated alkanes containing at least 3 carbon atoms are excellent blowing agents for polymers which are to be converted into low loss foams, in particular for fluoropolymers and other high melting polymers. Perfluorinated alkanes containing 1 or 2 carbon atoms are not satisfactory because at room temperature they are gases which cannot be liquefied, and are therefore difficult to handle, and in particular are difficult to meter accurately. Hydrocarbon blowing agents such as n-heptane give rise to foams having undesirably high losses, apparently because they decompose and/or react, under the extrusion and foaming conditions, so that the foamed product contains materials having a high loss tangent. Known halogenated blowing agents containing at least one halogen other than fluorine, such as monochlorodifluoromethane (CHClF₂), give rise to corrosion (of the equipment used to extrude the polymeric composition and/or the metal conductors contacted by the polymeric foam in the final product) and/or suffer from other disadvantages. Accordingly, in its first aspect, the present invention provides a method of making a coaxial cable which is suitable for use at microwave frequencies, and which comprises a central conductor, a foamed polymer dielectric layer surrounding the central conductor and an outer conductive layer which is separated from the central conductor by the dielectric layer, which method comprises (1) mixing a melt-extrudable fluoropolymer having a melting point of at least 250°C and a dielectric loss tangent of less than 0.0015 at 10 GHz with a foaming agent having the formula C n F 2n+2 , wherein n is at least 3; and (2) melt-extruding around a central conductor the polymeric mixture obtained in step (1) under conditions which cause the foaming agent to expand and convert the polymer into a foamed dielectric having a dielectric loss of less than 17 dB/100 ft. (dB/30.5m); and (3) placing an outer conductive layer around the foamed dielectric layer. The melt-extrudable fluoropolymer preferably has a loss tangent of less than 0.0015 at all frequencies in the range 3 to 18 GHz. At least as initially produced, the foamed dielectrics produced by the above methods contain residues of the novel foaming agent, and can be recognized by this fact. Thus the invention further comprises a coaxial cable that is suitable for use at microwave frequencies, which comprises a central conductor, a foamed polymer dielectric layer surrounding the central conductor and an outer conductive layer which is separated from the central conductor by the dielectric layer, the dielectric layer having a dielectric loss of less than 17 dB/100ft (dB/30.5m) at a frequency of 10GHz, and having been formed from a melt extrudable fluoropolymer having a melting point of at least 250°C and a dielectric loss tangent of less than 0.0015 at 10GHz, the dielectric layer containing residues of a foaming agent used to foam the polymer, the foaming agent having the formula CuF 2n+2 , wherein n is at least 3. The term "residues" is used in this specification to mean the forming agent itself and/or, if the foaming agent decomposes or reacts, decomposition products of the foaming agent or reaction products of the polymer and the foaming agent (or decomposition products thereof). The invention is illustrated in the accompanying drawings, in which Figure 1 is a cross-section through a coaxial cable prepared according to the method defined above, and Figure 2 shows the loss characteristics of the foamed dielectrics prepared in the Examples. To achieve low microwave losses in a polymer dielectric, the loss tangent and permittivity of the unfoamed dielectric material should be as small as possible. The polymers used in this invention, before foaming, have loss tangents of less than 0.0015, preferably less than 0.0010, and preferably have dielectric permittivities of less than 2.5, especially less than 2.1, at a frequency of 10 GBz, and preferably at all frequencies in the range of 3 to 18 GHz. The microwave losses are also affected by the dimensions of the foamed dielectric and the extent of foaming. These and any other factors which affect microwave losses can be effectively summarized by saying that in this invention, the loss contribution of the foamed dielectric is generally less than 17, more preferably less than 15, particularly less than 14, especially less than 11, dB/100 ft. (dB/30.5m) at a frequency of 10 GHz, and is preferably less than 18, particularly less than 17, especially less than 16, at all frequencies in the range 3 to 18 GHz. The invention is particularly useful when the polymers are processed at temperatures which result in decomposition and/or reaction of conventional foaming agents, particularly at least 200°C, especially at least 250°C, e.g. at least 280°C. Any suitable method of processing the polymer can be used, but melt-shaping, and particularly melt-extrusion, are preferred. The polymers used are fluoropolymers having melting points of at least 250°C, particularly fluoropolymers which are substantially free of hydrogen, especially melt-processable copolymers of tetrafluoroethylene with suitable comonomers such as hexafluoropropylene and perfluoroalkoxyalkenes. Suitable commercial available copolymers include those sold by E.I. duPont de Nemours under the trade names Teflon FEP and Teflon PFA. The foaming agents used in this invention are perfluroalkanes of the formula C n F 2n+2 , where n is at least 3, preferably 4 to 6. Straight chain compounds are preferred, in particular perfluorobutane, perfluropentane and perfluorohexane. The foaming agent in liquid form is injected, e.g. using a conventional metering pump, into the molten polymer before it is shaped. Referring now to Figure 1, this shows a coaxial cable having a center metal conductor 1, a foamed dielectric 2, and an outer metal conductor 3. The invention is illustrated by the following Examples, in which parts and percentages are by weight except where otherwise noted. Examples 2 to 7 are comparative Examples and are not in accordance with the invention. In each of the Examples, the dielectric layer of a coaxial cable was formed by conventional single screw melt extrusion of a mixture of a tetrafluoroethylene/hexafluoropropylene copolymer (FEP 100 available from du Pont) and 1% of a fine powder boron nitride nucleating agent. The blowing agent was injected under pressure through a port in the extruder barrel at a rate to produce a concentration of approximately 1% blowing agent in the melt. The polymer melt was extruded through a conventional crosshead die onto a 16 AWG stranded silver-plated copper wire (diameter about 0.147 cm). The resulting shaped dielectric had an outer diameter of 0.143 inch (0.36 cm) and a void level of 65%. The copper wire with the surrounding foamed dielectric was placed coaxially into the interior of a high conductivity aluminum tube with an outer diameter of 0.312 inch (0.79 cm) and an inner diameter of 0.284 inch (0.72 cm) and then drawn through a 0.173 inch (0.44 cm) cold drawing die. The aluminum tube was thereby snugly formed around the shaped dielectric, leaving no void space between the dielectric and the aluminum, and causing no distortion of the shaped dielectric. The coaxial cable was terminated with properly matched SMA connectors and transmission losses were measured in the frequency range from 3 to 18 GHz in Examples 1-4 and at 10 GHz in Examples 5-7. The dielectric loss in dB/100 ft (dB/30.5m) was calculated from the following equation: where α D = dielectric loss (dB/100 ft.) (dB/30.5m) α T = total transmission loss (dB/100 ft.) (dB/30.5m) f = frequency (GHz) Z o = characteristic impedance (ohm) R cc = resistivity of center conductor (microohm/cm) R oc = resistivity of outer conductor (microohm/cm) d cc = diameter of center conductor (inches) D d = diameter of shaped dielectric (inches) K s = center conductor stranding loss factor, taken as 1.3 from literature data. The second term on the right hand side of this equation represents the loss contribution of the conductors to the total transmission loss. The blowing agents used in the various Examples, and the properties of the foamed products, are shown in the Table below and in Figure 2. Ex. No. Blowing Agent Loss Tangent at 10 GHz Curve in Fig 2 Dielectric Loss at 10 GHz Comment 1 n-C₅F₁₂ 0.00090 A 10.7 2 n-C₇H₁₆ 0.00135 B 16.0 Discolored foam 3 Benzene 0.00120 C 14.3 4 CH₂Cl₂ 0.00125 D 14.8 Discolored foam, Tarnished conductor 5 CCl₃F 0.00110 - 13.1 Tarnished conductor 6 C₂Cl₃F₃ 0.00115 - 13.7 Tarnished conductor 7 n-C₅H₁₂ 0.00120 - 14.3 Discolored Foam

Claims: A method of making a coaxial cable which is suitable for use at microwave frequencies, and which comprises a central conductor, a foamed polymer dielectric layer surrounding the central conductor and an outer conductive layer which is separated from the central conductor by the dielectric layer, which method comprises (1) mixing a melt-extrudable fluoropolymer having a melting point of at least 250°C and a dielectric loss tangent of less than 0.0015 at 10 GHz with a foaming agent having the formula C n F 2n+2 , wherein n is at least 3; and (2) melt-extruding around a central conductor the polymeric mixture obtained in step (1) under conditions which cause the foaming agent to expand and convert the polymer into a foamed dielectric having a dielectric loss of less than 17 dB/100 ft. (dB/30.5m); and (3) placing an outer conductive layer around the foamed dielectric layer. A method according to claim 1, wherein the polymer is a copolymer of tetrafluoroethylene and hexafluoropropylene or a perfluoroalkoxyalkene. A method according to claim 1 or claim 2, wherein the foaming agent is perfluorobutane, perfluoropentane or perfluorohexane. A coaxial cable that is suitable for use at microwave frequencies, which comprises a central conductor, a foamed polymer dielectric layer surrounding the central conductor and an outer conductive layer which is separated from the central conductor by the dielectric layer, the dielectric layer having a dielectric loss of less than 17 dB/100ft (dB/30.5m) at a frequency of 10GHz, and having been formed from a melt extrudable fluoropolymer having a melting point of at least 250°C and a dielectric loss tangent of less than 0.0015 at 10GHz, the dielectric layer containing residues of a foaming agent used to foam the polymer, the foaming agent having the formula CuF 2n+2 , wherein n is at least 3. A cable according to claim 4, wherein the polymer is a copolymer of tetrafluoroethylene and a hexafluoropropylene or a perfluoroalkoxyalkene. A cable according to claim 4 or 5, wherein the foaming agent is a compound of the formula C n F 2n+2 wherein n is an integer from 4 to 6 inclusive.

Inventors: LUNK HANS EDMUND, REED DONALD ALAN, LUNK, HANS EDMUND, REED, DONALD ALAN
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1984
Main classification: H01B3
IPC codes: C08J9, H01B13, H01B3