ELECTRICAL CONDUCTOR COATED WITH POLYBUTYLENE TEREPHTHALATE

Abstract: An elongate conductor (4) has a layer of insulating or jacketing material (2) comprising an oriented polybutylene terephthalate composition having an initial elongation of at least about 25 %, most preferably at least about 75 %, and an elongation after heat aging at least about 25 %, most preferably at least about 75 % of its initial elongation. The conductor is provided with the surrounding layer by extruding the polybutylene terephthalate composition around the conductor as a loose tube and then drawing the loose tube, after cooling, down onto the conductor.

Description: E E CTR ICAL CONDUCTOR COATED WITH POLYBUTYLENE TEREPHTHALATE This invention relates to an elongate electrical conduc¬ tor a layer of insulating or jacketing material comprising oriented polybutylene terephthalate and to a method of pro¬ viding a layer of oriented polybutylene terephthalate insu¬ lating or jacketing material on an elongate electrical conductor. Polymers of polybutylene terephthalate, are well known and have many useful properties. However, their tendency to embrittle at elevated temperatures, i.e„ to lose elongation, renders them unsuitable for use in general as coatings for electrical conductors and in particular for use as wire insulation. A solution to this problem is disclosed in U.S. Patent No. 4,332,855, to Zingheim and Lunk, in which it is taught that the addition of a second polymer having a flex modulus of 500 to 100,000 psi and a specified solubility parameter to polybutylene terephthalate (also known as poly- tetramethylene terephthalate) improves the performance of polybutylene terephthalate as wire or cable insulation or jacketing. The second polymer is preferably a polyether/polyester block copolymer. The resulting com¬ position has improved retention of elongation at elevated temperatures. U.S. Patent No. 4,767,668 to Smith and Thornley relates to a composition comprising polybutylene terephthalate and a polyether/polyester block copolymer having a relatively high proportion of hard blocks. The composition is unexpectedly superior to comparable compositions containing a polyether/polyester block copolymers having a lower propor- tion of hard blocks in resistance to solvent stress- crazing when used as wire or cable insulating or jacketing material. The addition of such a second polymer to polybu¬ tylene terephthalate, as taught in these patents results in greatly improved wire and cable coated therewith but increases the cost of the composition and results in an insulation layer having a relatively low tensile strength. Japanese Patent Kokai No. 54-68990 to Nariki and Kawase discloses the use oriented polybutylene terephthalate as a wire coating. The method used in this Japanese patent to a P ly the wire coating results in a relatively low degree orientation of the polybutylene terephthalate. It is reported in the patent that the coated wire is capable of being bent about 180 Θ C without cracking. " Subsequently, it has been found that the coated wire becomes brittle (i.e. loses its elongation) when subjected to elevated tem¬ peratures for a short time. We have now unexpectedly discovered that an elongate electrical conductor such as a wire or cable can be insu¬ lated or jacketed with polybutylene terephthalate in such a manner as to provide an elongate electrical conductor having a polybutylene terephthalate layer which has surprisingly high tensile strength and improved retention of elongation when subjected to heat aging. One aspect of this invention comprises a method of pro¬ viding an elongate electrical conductor with a layer of insulating or jacketing material, which comprises: (a) extruding. a polybutylene terephthalate composition at a temperature, T]_, above the melting temperature of the composition so as to form a tube which surrounds the electrical conductor to be insulated or jacketed and has an inner diameter greater than the outer diameter of said conductor ? (b) cooling the extruded tube to a temperature, T2, below the melting temperature of the composition; (c) drawing the tube down onto the electrical conductor with a draw ratio greater than about 1.1. Another aspect of this invention comprises an elongate electrical conductor having a layer of insulating or jacketing material comprising an oriented polybutylene terephthalate composition having a tensile strength of at least about 9,000 psi, an initial elongation at 25°C of at least about 25% and an elongation after heat aging at 180°C for seven days of at least about 25% of its initial elonga¬ tion. FIG. 1 is a schematic view of an apparatus and process for applying a layer of oriented polybutylene terephthalate to an electrical conductor in accordance with this invention. FIG. 2 is a cross sectional view of the extruded loose tube of polybutylene terephthalate around the conductor prior to drawing. FIG. 3 is a cross sectional view of the conductor pro¬ vided with a layer of oriented polybutylene terephthalate after the tube has been drawn down onto the conductor. In accordance with this invention, the elongate electri¬ cal conductor is provided with a layer of insulating or jacketing material of oriented polybutylene terephthalate. The polybutylene terephthalate can be a homopolymer of buty- lene terephthalate or a copolymer thereof with an aromatic, aliphatic or alicyclic dicarboxylic acid and/or a diol or derivations of said acid or diol, for example isophthalic acid, 2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenylether dicarboxylic acid, diphenoxyethane dicarboxylic acid, adipic acid, sebacic acid, decane dicarboxylic acid, p-hydroxyethoxy benzoic acid, o-carboxycaproic acid, hexamethylene glycol, decamethy- lene glycol, dodecamethylene glycol, bis(-hydroxyethoxy- phenyl)sulfone , 2,2-bis(hydroxyethoxyphenyl)propane, diethylene glycol, cyclohexane dimethylol, cyclohexane dicar¬ boxylic acid and so on. A mixture of more than one kind of such compounds may be used also. A polyfunctional compound such as trimesic acid, tri- methylolpropane , or pentaerythritol and so on, and/or mono- fuinctional compound such as o-benzoyl benzoic acid or methoxypolyethylene glyol and so on may be copolymerized to the extent that the obtained polyester can maintain its linear character. Butylene terephthalate polymers can be prepared by any of the ordinary methods such as by ester exchange reaction or a direct polymerization method. In addition to polybutylene terephthalate, the com¬ position may contain various additives. Additives that can be used include, for example antioxidants such as alkylated phenols, e.g. those commercially available as Goodrite 3125, Irganox 1010, Irganox 1035, Irganox 1076, Irganox 1093, Vukanox BKF, organic phosphite or phosphates, e.g. dilauryl phosphite, bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite, Mark 1178; alkylidene polyphenols, e.g. Ethanox 330, thio-bis alkylated phenol, e.g. Santonox R, ortho coupled oligomer of 4,4'-thio-bis(3-methyl-6-t-butyl- phenol), dilauryl thio-dipropionate, e.g. Carstob DLTDP, dimyristyl thiodipropionate, e.g. Carstab DMTDP, distearyl thiodipropionate, e.g. Cyanox STOP, amines, e.g. Wingstay 29 etc; TJV stabilizers such as [2,2'-thio-bis(4-t-oσtyl- phenolato)] n-butylamine nickel, Cyasorb UV 1084, 3,5-ditertiarybutyl p-hydroxybenzoic acid, UV Chek Am-240; thermal stabilizers, such as polycarbodiimide; flame retar- dants such as decabromodiphenyl ether, perchloropen- tacyclodecane, l,2-bis(tetrabromophthalimido) ethylene; pigments such as titanium dioxide, antimony trioxide, zinc oxide, zinc sulfide, iron oxide, etc., and the like. Mixtures of such additives can be used. Typically, the composition comprises at least about 60% by weight, of polybutylene terephthalate, preferably at least about 80% and most preferably at least about 90% by weight, based on the weight of the composition. The composition can be prepared by mixing polybutylene terephthalate and the desired additives together in an internal mixer such as a banbury or Brabender mixer, a two- roll mill, an extruder or the like. Such mixing typically is conducted at a temperature of about 230°C to about 280°C, preferably about 240°C to about 260°C. The elongate electrical conductor coated with oriented polybutylene terephthalate in accordance with this invention can be for example a wire, cable, bus bar, or the like. Preferably the conductor is a wire and preferably is of a size of from about 4 to about 38 AWG, more preferably from about 10 to about 30 AWG. ' The conductor can be single- or multi-strand. The conductor can be coated with one or more inner layers, if desired before the layer of oriented poly¬ butylene terephthalate is applied in accordance with this invention. The use of the term conductor applies not only to the electrically conducting member but also to said member and said inner layer(s), if present. The inner layer may be, for example, polymeric compositions, including insu¬ lating compositions or compositions containing electrically conductive or magnetic particles, metallic materials such as metal braids, or the like. The term conductor also applies to a cable which comprises a plurality of conducting ele¬ ments, generally insulated from each other and optionally surrounded by shielding means. Further, the polybutylene terephthalate may itself be surrounded by additional layers, if desired. The polybutylene terephthalate composition is coated onto the conductor by extruding a "loose tube" of the com¬ position around the conductor. By "loose tube" is meant that a tube having an inner diameter greater than the outer diameter of the conductor is extruded around the conductor. The extrusion step is conducted at a temperature T-_ , above the melting temperature of the polybutylene terephthalate. This temperature is preferably in the range of between about 235 and about 300°C, more preferably between about 250 and about 280°C. The loose tube is then cooled to a temperature, T2, below the melting temperature of polybutylene terephthalate. Preferably T2 is between about 15 Θ C to about 210°C and more preferably between about 30°C and about 150°C. The loose tube may be cooled by the surrounding environment, passing it through a water bath or the like. The loose tube is then drawn down onto the conductor preferably by passing it through a drawing die, but other drawing methods such as a non-constrained draw initiated by local heating can be used. The drawing step is conducted at a temperature of about 15°C to about 210°C, more preferably of about 30°C to about 150°C. Typically the drawing step is conducted at ambient temperature, i.e. the temperature of the environment surrounding the equipment. To provide the desired degree of orientation, the draw ratio should be at least about 1.1, preferably at least about 1.2, most preferably at least about 1.4 and in par¬ ticular at least about 1.5. The draw ratio may be as high as 3.0, but draw ratios of below about 2.0 are preferred. The term "draw ratio" as used herein is determined by the following equation: draw ratio ~~ cross sectional area of the PBT tube prior to draw cross sectional area of the PBT layer after draw (where PBT stands for polybutylene terephthalate). An apparatus for applying a layer * of oriented polybuty¬ lene terephthalate in accordance with this invention is shown schematically in Fig. 1. In Fig. 1, a layer of poly¬ butylene terephthalate is extruded as a loose tube around conductor 4, through extruder die 6 (rest of extruder not shown). The loose tube/conductor assembly passes through take-off 8 and drawing die 10. The drawn insulated wire passes over take-off 12 and can then wound on a convention spool (not shown). The thickness of the polybutylene terephthalate coating on the electrical conductor after the drawing step varies depending on the particular conductor being coated. For example on 4 to 38 AWG wire, the polybutylene terephthalate coating is preferably from about 3 to about 40 mils thick, more preferably from about 5 to about 20 mils thick. The polybutylene terephthalate coating may be crosslinked, if desired. It may be crosslinked by means of chemical cross-linking agents or by irradiation, for example by exposure to high energy irradiation such as an electron beam of gamma rays. Radiation dosages in the range 2 to 80 Mrads, preferably 5 to 50 Mrads, e.g. 10 to 30 Mrads are in general appropriate. For the purpose of promoting cross-linking during irra¬ diation preferably from 0.2 to 5 weight percent of a prorad such as a polyfunctional vinyl or allyl compound, for example, triallyl σyanurate, triallyl isocyanurate or pen- taerythritol tetramethacrylate are incorporated into the composition prior to irradiation. The degree of cross-linking of the compositions may be expressed in terms of the gel content (ANSI/ASTM D2765-68) of the cross-linked polymeric composition, i.e. excluding non-polymeric additives that may be present. Preferably the gel content of the cross-linked composition is at least 10% more preferably at least 20%, e.g. at least 30%, more pre¬ ferably at least 40%. The oriented polybutylene terephthalate layer has an initial elongation at 25°C of at least about 25%, preferably at least about 50% and most preferably at least about 75%. The initial elongation- is the elongation of the polybutylene terephthalate after it has been heat set e.g. by heating it at a temperature of about 80°C to about 200°C for 1-30 minu¬ tes, removing the polybutylene terephthalate layer and measuring the elongation according to ASTM D3032. A unique feature of the article of this invention is that after heat aging at 180°C for 7 days, the polybutylene—terephthalate retains at least about 25% of its initial elongation, pre¬ ferably at least about 50% and most preferably at least about 75%. Another feature of the article of this invention is that the oriented polybutylene terephthalate has a ten¬ sile strength of at least about 9,000 psi as measured by ASTM D3032. Preferably the tensile strength is at least about 10,000 and most preferably at least about 11,000 psi. The excellent tensile strength of the oriented polybuty¬ lene terephthalate layer and its high elongation and its ability to retain its elongation at high temperatures makes it highly desirable as an insulating or jacketing material for wire or cable subjected to mechanical stress and ele¬ vated temperatures. For example it is contemplated that wire insulated with oriented polybutylene terephthalate in accordance with this invention is particularly useful as automotive wire, including wire for "under the hood" use. The following examples illustrate the coating of an electrical conductor with oriented polybutylene terephtha¬ late in accordance with this invention. Example 1 A commercially available grade of polybutylene terephthalate (inherent viscosity - 1.2) was compounded with flame retardants and stabilizers as designated below on a 20 mm counter-rotating twin screw ZDSK mixer at 250°C and pelletized. The formulation was as follows: Polybutylene terephthalate 84% (Celanex 1600A from Celanese Corp. ) - l,2-bis(tetrabromophthalimido)ethane 6% Antimony trioxide 6% Stabilizers and pigments 4% A flexible conductor made up of 19 strands of round .008 inches diameter, tin-plated copper wires with a bundle diameter of .039 inches was used. A li" Davis Standard extruder with a barrel length to diameter ratio of 24.1 fitted with general purpose screw (compression ratio - 3:1) was used for extruding a loose tube over the above described conductor. The extruder barrel, crosshead, die holder and the die was maintained at 265°C and the temperature of melt coming out of the die was 270°C as measured by a fine AWG thermocouple. Care was exercised to ensure that the conduc¬ tor did not come in contact with the molten polymer cone. The molten polymer cone was quenched with cold water (23°C) at li" away from the die exit. Length of the cold water bath was about 9 feet so that the tube was essentially cold to touch when it reached belt driven traction device set at 130 ft/min (labeled as Haul-off I in Figure I). The belt spacing in the traction device was adjusted such that it would firmly grip the polymer tube while the conductor could slide freely. This polymer tube (with the conductor inside) was passed through a two piece split die made of Tungsten Carbide, similar in design and set-up to the ones used in metal wire drawing operations, with adjustment available for opening and closing the die. The die opening size was .064 inches when fully closed. Initially the split die was kept open so that the polymer tube could easily pass through. Further down stream a two roll capstan (labeled as Haul-off II in Figure I) set at a line speed of 130 ft/min was used for continuously hauling away the polymer tube and was taken-up on a reel using a motorized shaft. The polymer tube dimensions were chosen to be at .051" inside diameter and .084" outside diameter. The polymer tube (with conductor inside) was next cold drawn by closing the split die to its size and accelerating " the capstan speed to 227 ft/min at the same time. The 'cold drawn' insulation was snug on the conductor and the insula¬ tion wall thickness was .0125 inches. This oriented wire was subsequently heat set at 185°C for 10 minutes in an air circulating oven, with both ends of the sample tied down to avoid any shrink back. This was labeled as Sample A. A control (unoriented) sample was prepared by simply extruding the same material but with the melt cone (at 270 Θ C) converging directly onto the same conductor. The molten polymer on the conductor was quenched 2" away from the die exit and the haul-off speed was set at 125 ft/min. The insulation was again .0125" and this sample was also heat set at 185 Θ C for 10 minutes (like the Sample A). This unoriented sample was labeled as B. The insulations were removed from both samples and suspended in an air circulating oven at 180°C for heat aging. Elongations were measured on an Instron with a ja - separation speed of 2 inches/min. Results are shown in Table I below. Another set of oriented samples was made with different draw ratios as shown in the following Table. After drawing, the samples were tested for tensile strength and elongation as specified above and were then heated at a temperature of 180 e C for 7 days. The elongation was then measured and the % retention of elongation was calculated. The results are shown in the following table. Table II %Elongation Retention of Cold Draw Tensile % Elongation After 1 week Initial Sample Ratio Strength (psi) Initial at 180°C Elongation C 1.0 8457 275 8 3% D 1.12 8880 160 40 25% E 1.20 9493 137 110 80% G 1.58 12530 112 112 100% H 1.78 14473 75 72 96% Example 3 (Comparative) This example illustrates the application of a layer of polybutylene terephthalate to a conductor in accordance with a prior art process. Commercially available virgin (no other ingredients added) polybutylene terephthalate (Celanex 1600A, commer¬ cially available from Celanese Polymer Corp.) dried at 80°C Tror 12 ~ fi urs was used in this run. The same reel of conduc¬ tor as described in example 1 was used in this trial. A 2" Davis Standard extruder fitted with a general purpose screw (compression ratio 4:1) was employed. The extrusion tip (guide for the conductor) had an opening of .0395 inches while the extrusion die (for shaping and sizing of molten polymer around the conductor) had an opening of .064 inches. The die was fitted with a miniature thermocouple by drilling a .058 inches diameter hole, .25 inches deep, 0.19 inches away from die opening at the melt exit face. Melt pressure was monitored at a suitable location in the crosshead using a commercial pressure transducer. A 15 foot long cold water bath placed 3 inches away from the die exit was used for quenching the coated wire as it came out of the die. An extrusion run was conducted under the following con¬ ditions and the insulated wire thus obtained was labeled as sample I. Line speed = 1,000 ft/min Die Temp = 313°C Head Pressure = 5,000 psig Insulation Thickness = .011 inches Next the die was fitted with .125" diameter copper -tubing (coiled around the die) with provision for running cold water through the tube. Another extrusion run was made with this set-up using same conductor, polymer barrel/crosshead/die holder temperatures except that the extrusion die was cooled to a temperature of 206°C. The crosshead pressure was 7,500 psig under these conditions. The insulated wire made in this run was labeled as sample J. Both samples I and J were heat set at 185°C for 10 minu¬ tes and tested for percent elongation as set forth in Example 1 and flex durability by wrapping it around a mandrel to form a 180° bend. The results are summarized in table III below. Cracking of the samples indicates that the polybutylene terephthalate was embrittled and had low elongation.

Claims: What is claimed is: 1. A method of providing an elongate electrical conductor with a layer of insulating or jacketing material, which comprises: (a) extruding a polybutylene terephthalate composition at a temperature, T^, above the melting temperature of the composition so as to form a tube which surrounds the electrical conductor to be insulated or jacketed and has an inner diameter greater than the outer diameter of said conductor; (b) cooling the extruded tube to a temperature, 2, below the melting temperature of the composition; and (c) thereafter drawing the tube down onto the electri¬ cal conductor with a draw ratio greater than about 1.1. 2. A method in accordance with claim 1 wherein T^ is bet¬ ween about 235°C and about 300°C. 3. : A-method in accordance with claim 1 wherein T2 is bet¬ ween about 15°C and about 210°C. 4. A method in accordance with claim 1 wherein T2 is ambient temperature. 5. A method in accordance with claim 1 wherein the tube is drawn down onto the conductor by passing the tube and conductor through a drawing die. 6. A method in accordance with claim 1 wherein the draw ratio is greater than 1.2. 7. A method in accordance with claim 1 wherein the draw ratio is greater than 1.4. 8. A method in accordance with claim 1 wherein the draw ratio is greater than 1.5. 9. An article of manufacture comprising an elongate electrical conductor having a layer of insulating or jacketing material comprising an oriented polybutylene terephthalate composition having a tensile strength of at least about 9,000 psi, an initial elongation at 25°C of at least about 25% and an elongation after heat aging at 180°C for seven days of ate least about 25% of its initial elongation. 10. An article in accordance with claim 9 wherein said composition comprises at least about 60% polybutylene ter phthalate. 11. An article in accordance with claim 9 wherein said com¬ position- comprises at least about 80% polybutylene terephthalate. 12. An article in accordance with claim 9 having a tensile strength of at least about 10,000 psi. 13. An article in accordance with claim 9 having a tensile strength of at least about 11,000 psi. 14. An article in accordance with claim 9 having an initial elongation of at least about 50%. 15. An article in accordance with claim 9 having an initial elongation of at least about 75%. 16. An article in accordance with claim 9 having an elonga¬ tion after heat aging of at least 50% of its initial elongation. 17. An article in accordance with claim 9 having an elonga¬ tion after heat aging of at least about 75% of its ini¬ tial elongation. 18. An article in accordance with claim 9 having an initial elongation of at least about 75% and an elongation after heat aging of at least about 75% of its initial elongation. 19. An article in accordance with claim 9 wherein the poly¬ butylene terephthalate is from about 3 to about 40 mils thick. 20. An article in accordance with claim 9 wherein the poly¬ butylene terephthalate is from about 5 to about 20 mils thick.

Inventors: LUNK HANS, MEHAN ASHOK, LUNK, HANS, MEHAN, ASHOK
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1989
Main classification: B29C55
IPC codes: B29C47, B29C55, B29C63, B29K67, H01B13