SHAPED ARTICLES COMPRISING CROSSLINKED FLUOROCARBON POLYMERS AND METHOD OF FORMING SUCH ARTICLES

Abstract: High strength, flexible compositions with improved mechanical properties at elevated temperatures for wire insulation coatings and other shaped articles used in hostile environments are disclosed, consisting of a high temperature fluorocarbon polymer, such as an ethylene-tetrafluoroethylene copolymer or the like, and from about 1% wt. to about 50% wt. of a polyvinylidene fluoride compound.

Description: The field of this invention is shaped articles made of crosslinkable fluorocarbon polymers and, in particular, high temperature wire coatings and the like. Various polymer compositions are known for electrical insulating purposes, such as wire insulation and mold-shaped insulating pieces. However, few compositions are capable of withstanding hostile environments such as those typically encountered in, for example, airplane wiring. In such environments, insulating compositions can encounter mechanical stress, wear, salt-laden moisture, corrosive cleaning fluids, oils and fuels, and low and high temperatures. One of the most important criteria for airplane wire is that it be able to withstand high temperatures without melting when a flash fire occurs, for example. Some of the existing polymer compositions for hostile environments are polyimide materials, such as Kapton®, an aromatic polyimide material manufactured by the Dupont Company of Wilmington, Delaware. The polyimide-based wire coatings have good thermal properties, but unfortunately suffer from cracking and embrittlement over time. Modifications which decreased the cracking problem in polyimide insulated wires apparently have lead to excessive stiffness and greater susceptibility to corrosion and chafing. The problem is so serious that a recent article in Defense Electronics , Jan. 1983, suggests that polyimide wiring harness insulation, especially in exposed areas, has caused short circuits in key aircraft systems. In another approach to developing durable insulators, efforts have been made to irradiation crosslink so-called "high temperature" fluorocarbon polymers, such as ethylene-tetrafluoroethylene copolymers (ETFE) and ethylenechlorotrifluoroethylene (E-CTFE) as the insulation. However, conventional radiation crosslinking promoters have not worked well with these fluorocarbon polymers. Because fluorocarbon polymers, such as ETFE and E-CTFE, have high melting points, volatile crosslinking promoters such as triallyl cyanurate and its isomer, triallyl isocyanurate, are ineffective. For a variety of fluorocarbon polymers, temperatures above 250°C. are required for extrusion or injection molding to fabricate shaped articles such as wire insulation, sheets, films, tubing, gaskets and boots. When promoters are added to high temperature fluorocarbon polymers prior to processing, the polymers tend to prematurely crosslink and to form gels or lumps, discolor and often to form voids in the final product. Various compounds have been proposed as substitutes for conventional crosslinking promoters to form durable, high temperature polymers. See, for example, US-A-3,840,619; US-A-3,894,118 and US-A-3,911,193 issued to Aronoff, which disclose the use of allylic esters of polycarboxylic acids in crosslining agents for fluorocarbon polymers. See also, US-A-3,970,770; US-A-3,985,716 and US-A-3,995,091 issued to Dhami, which disclose the use of esters of sulfonyl dibenzoic acid as crosslinking agents. Additionally, US-A-3,894,118 issued to Aronoff discloses crosslinking agents composed of esters of dimethacrylic acid. Despite these numerous disclosures the industry has not been totally satisfied by any of the available crosslinking promoters and many fluorocarbon polymers are still underutilized because they have not responded well to attempts at radiation-induced crosslinking using either the new classes of promoters or the more conventional promoters. JP-A-50-032245 discloses blends of ethylene-tetrafluoroethylene copolymer (ETFE) and polyvinylidene fluoride (PVDF) in the ratio 90-99.9 parts ETFE copoylmer : 0.1-10 parts PVDF. JP-A-60-260634 discloses a fluorocarbon resin composition for producing a crosslinked molding having a good appearance and excellent mechanical properties in a high-temperature atmosphere, prepared by adding neodymium oxide and a crosslinking agent to an ethylene/fluoro-olefin copolymer. Examples of the crosslinking agents used include triallyl cyanurate and tetraallyl pyromellitate. In US-A-4,353,961 issued to Gotcher, a method is disclosed for forming shaped articles from high temperature fluorocarbon polymers, wherein the polymer is first processed at or above its melting point and then permitted to cool and "imbibe" a promoter before being crosslinked by radiation. This method, which requires immersion of the shaped product in a trough or the like filled with the promoter, poses handling problems and adds a time-consuming, additional step to the manufacturing process. There exists a need for fluorocarbon polymer compositions suitable for use in high temperature environments and which can be satisfactorily radiation crosslinked in an efficient manner. It has been discovered that ETFE fluorocarbon polymers can be blended with polyvinylidene fluoride and processed at high temperatures and, further that the resultant material can be highly crosslinked by radiation with or without promoters to produce wire coatings and the like. In particular fluorocarbon polymers may be mixed with polyvinylidene fluoride and then processed and crosslinked. According to the invention, a shaped article comprising an irradiation crosslinked composition on the basis of fluorocarbon polymers having a melting point prior to crosslinking of at least 240°C; the article having been subjected to at least one forming operation at temperatures in excess of the melting point of the composition prior to being crosslinked, and thereafter being crosslinked by radiation at a dose level of up to 400 kJ/kg, is characterized in that the composition contains 100 parts by weight of ethylenetetrafluoroethylene copolymer and 1 to 50 parts by weight of polyvinylidene fluoride, and contains no crosslinking agent at the commencement of said radiation other than such crosslinking agents as may have been contained therein during the forming operation. The shaped articles according to the invention, as wire coatings, can be processed and crosslinked without resort to difficult, time consuming, post-processing, immersion in promoters and have excellent electrical insulation properties, resistance to deformation at high temperatures, as well as flexibility, durability and thermal stability in hostile environments. Further, it has been found that small amounts (i.e. up to 4 percent) of promoters can be absorbed by powdered polyvinylidene fluoride and added to the composition prior to processing to yield a smooth non-porous extruded insulation coating which becomes highly crosslinked at lower radiation levels. Thus, preferably, the composition comprises a radiation crosslinking promoter absorbed in the polyvinylidene fluoride. The article may comprise an electrical conductor and an extruded insulation coating thereon, the coating comprising an irradiation crosslinked composition as defined above. The method of forming a shaped article of the above identified type, which is capable of withstanding high temperatures, by melt processing a mixture on the basis of fluorocarbon polymers and crosslinking by irradiation thereof, comprises the steps of a) preparing a mixture comprising 100 parts by weight of an ethylene tetrafluoroethylene copolymer and from 1 to 50 parts by weight of polyvinylidene fluoride, b) shaping an article from said mixture by said melt processing. Preferably step a) comprises (1) preparing the mixture of pellets of ethylenetetrafluoroethylene copolymer and polyvinylidene fluoride, (2) coating the pellets with a liquid radiation crosslinking promoter, (3) coating the resulting promoter-coated pellets with powdered polyvinylidene fluoride, and blending the pellets. A satisfactory crosslinking promoter is triallyl isocyanurate. The fluorocarbon polymers which may be blended with polyvinylidene fluoride to produce the high temperature compositions of this invention include ETFE fluorocarbon polymers, such as Tefzel® manufactured by the Dupont Company of Wilmington, Delaware. More generally, the fluorocarbon copolymers and terpolymers are defined as having carbon polymer backbones and about 10% or more fluorine, and having melting points of above about 240°C (as evidenced by a drop in viscosity and general lack of crystalline structure). These polymers also require high processing temperatures usually in excess of 250°C for forming into shaped articles by extrusion or molding. The polyvinylidene fluoride compounds useful in this invention may take a variety of forms and compositions. One preferred compound is the grade 460 polyvinylidene fluoride manufactured by Pennwalt, Inc. of Philadelphia, Pennsylvania and sold under the trademark Kynar®. The Kynar® 460 and 461 homopolymers have a specific gravity of about 1.75-1.78, a melting temperature of about 160°C and a melt viscosity of about 2800-250 Pa.s at 232°C and 100/s shear rate. The invention will next be described in connection with certain working examples and experimental results. Pigments, such as TiO₂ and ZnO, stabilizers, antioxidants, flame retardants, acid acceptors, processing aids and other additives can also be added to the compositions described herein. Conventional or new crosslinking promoters may be absorbed prior to processing in order to further improve crosslinking. While crosslinking by ionizing radiation is the preferred method of curing the compositions of this invention, other methods for irradiation crosslinking can also be employed. The dose of radiation necessary for curing typically will vary from about 50 kJ/kg to 250 kJ/kg, although in some instances a greater amount up to 400 kJ/kg may be necessary for certain properties. These doses can be found by those skilled in this art without undue experimentation. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following working and comparative examples are presented as illustrative of the compositions claimed herein: Example I Pellets of ethylene-tetrafluoroethylene (Tefzel® 280) were blended with pellets of polyvinylidene fluoride (Kynar® 460) in the ratio of five parts Kynar® to 100 parts Tefzel® and then fed into the hopper of a mixer. The mixed stock was extruded onto wire of a stock temperature of about 335°C. (Profile 305° to 365°). The coating was smooth and free of porosity, gels, lumps and sparkouts. The coating was then crosslinked at a radiation dose of about 250 kJ/kg to form a product with excellent resistance to deformation at temperatures as high as 300°C. Example II Pellets of ethylene-tetrafluoroethylene (Tefzel® 280) and pellets of polyvinylidene fluoride (Kynar® 460) were first coated with liquid triallylisocyanurate (TAIC) and then coated with powdered polyvinylidene fluoride (Kynar® 461) in the ratio of about 1 - 10 parts Kynar®, about 0.1 - 4.0 parts TAIC and 100 parts Tefzel®. Sufficient powdered Kynar® was added to absorb the excess TAIC. After blending with various compounding ingredients, the blend was fed into the hopper of an extruder and extruded onto wire at a melt temperature of about 335°C. (Profile 305° - 363°C.). A blend according to the formula in Table 1 was extruded to produce a smooth, porosity-free coating without sparkouts. When irradiated at about 200 kJ/kg, it exhibited excellent resistance to deformation at 300°C. Tefzel® 280 100.0 Kynar® 460 (pellets) 3.0 Kynar® 461 (powder) 2.0 TAIC 1.0 Compounding ingredients (ZnO/TiO₂ - a color concentrate 3.0 COMPARATIVE EXAMPLE I A blend of Tefzel and just TAIC, when extruded onto wire produced an extremely rough porous coating with little integrity and unsuitable for further consideration. This is also disclosed in prior art, e.g., US-A-4,353,961. COMPARATIVE EXAMPLE II Pellets of unmodified Tefzel® were mixed and extruded onto wire at a temperature of about 335°C. (Profile 305° to 365°C.). Attempts to crosslink the coating at low radiation doses failed as evidenced by melting. A measure of crosslinking was achieved at 500 kJ/kg but, as discussed below, the coating failed to meet the high temperature performance specifications because of a tendency to melt and flow. The wire coatings produced above were subjected to a variety of tests established by the wire and cable industry and Military specifications. For high temperature applications, the most important tests of the coatings were the solder iron test and the mandrel test. The solder iron test, which is described in MIL-W-16878 specification and used in the wire and cable industry to determine whether adequate crosslinking of the insulation has been achieved, consists of a soler iron fastened to an upright frame by a rigid hinge located on the solder iron handle. The solder iron tip has an angle of 45° and forms a flat surface with an asbestos sheet. The solder iron tip has a bearing surface of 12,7 mm. The iron is weighted to provide a 6.7 N force bearing down on the insulated wire (a 20 AWG conductor with a 254 µm wall). The apparatus includes equipment sufficient to measure and to control the temperature at the solder iron to within 345 - 10°C. The apparatus also has a 30 to 50 volt electric circuit arranged to indicate a burn-through or melt-through failure when the solder tip contacts the conductor. A satisfactory crosslinked insulation will withstand melt through for more than 6 minutes. The 7-hour at 300°C. mandrel test which is described in MIL-W-22759 specification as an accelerated aging test also measures the ability of the insulation to resist flow under pressure. It is carried out on a 61 cm sample of the finished wire which has 25 mm of insulation removed from each end. The central portion of the specimen then is bent at least halfway around a cylindrical, smooth, polished stainless steel mandrel having a 12.7 mm diameter. Each end of the conductor is loaded with 3,3 N such that the portion of the insulation between the conductor and the mandrel is under compression while the conductor is under tension. This specimen, so prepared on the mandrel, is placed in an air-circulating oven and maintained for a period of 7 hours at 300°C. After completion of the air oven test, the specimen is cooled to 23 - 3°C. within a period of 1 hour. The wire then is freed from tension, removed from the mandrel and straightened. When the specimen is submitted to a dielectric test, it must be capable of withstanding 2.5 kV for 5 minutes. It was found that after suitable irradiation each of the compositions described above containing the mixture of the high temperature fluorocarbon polymer and polyvinylidene fluoride with and without radiation crosslinking promoters passed both the solder iron test and the mandrel test while the composition which did not contain polyvinylidene fluoride did not pass the tests. Additional experiments were conducted with compounds containing Tefzel® and Kynar® in varying proportions. As Table II illustrates, the resistance to flow or deformation of the various extruded and irradiated compositions under the different temperature, pressure and time conditions of the two tests varied according to the Kynar® content and the irradiation dosage. The solder iron test was less severe than the mandrel test. For materials to pass the mandrel test, it was necessary that they posses a high degree of crosslinking but not an excessive amount. Too much irradiational crosslinking would cause premature aging and cracking under the temperature/time conditions of the mandrel test. The experiments also showed that there were limitations on the amounts of Kynar® that can be used in the blend on a practical basis. As the blend approached a Kynar® content of approximately 50%, it was observed that a rough coating with tendencies to shred to stripping was produced during extrusion. At 60% Kynar® and 40% Tefzel®, the extruded blend turned brown and cloudy and formed black decomposition deposits at the extruder tip. The resultant coating was brown and rough. These experiments were terminated at this point except to extrude a coating of Kynar® alone. This material required high levels of irradiation to obtain the limited degree of crosslinking needed to pass the less severe solder iron test. In US-A-4,353,961 issued to Gotcher there is disclosed treatment of mixtures of ethylene tetrafluoro ethylene (ETFE) and other fluorocarbons as listed, this list including polyvinylidene fluoride (PVDF). Although PVDF is included in the list given by Gotcher at Column 2, line 63-Column 3, line 6, we have found non-obvious advantages by using PVDF: namely, an acceptable product could be obtained without adding conventional crosslinking promoters, and, where needed, it permitted pre-extrusion addition of crosslinking promoters. Thus, we have selected that mixture from among many possibilities which has the unobvious result mentioned above. Far from suggesting that claimed mixture, the Gotcher disclosure teaches away from it by recommending polymers with melting points above about 200°C. Indeed, the passage implies that the disclosure is so limited. The PVDF in our mixture has a melting point well below 200°C. We have prepared data similar to that presented hereinabove, but using other fluorocarbons in Gotcher's list instead of Kynar® (PVDF). Commercially available equivalents of each of the fluorocarbon polymers described by Gotcher were blended with Ethylene Tetrafluoroethylene in various proportions, with the exception of two which were unavailable: namely, tetrafluoroethylene-vinylidene fluoride and vinylidene fluoride hexafluoroisobutylene. These blends and similar blends containing a crosslinking promoter were extruded on to wire and irradiated at various levels. The resultant insulated wires were tested for solder iron resistance and for performance in MIL-W-22759 Specification 7 hour/300°C Mandrel test. The fluorocarbon polymers evaluated in combination with Ethylene Tetrafluoroethylene (ETFE) were as follows: (in conformity with conventional nomenclature, the term "poly-" is usually omitted from the chemical names of these polymers). CHEMICAL NAME COMMERCIAL EQUIVALENT SUPPLIER Vinylidene Fluoride (PVDF Kynar® 460 Pennwalt Kynar® 2800 Ethylene Chlorotrifluoroethylene Halar® 500 Allied Chemical Tetrafluoroethylene Hexafluoro Propylene Teflon® FEP140 Dupont Vinylidenefluoride Hexafluoro Propylene Viton® A Dupont Vinylidenefluoride Hexafluoro Propylene Tetrafluoroethylene Fluorel® FT2481 3M ETFE from two sources, Tefzel® 280 from Dupont and Halon ET® from Allied Chemical were found to behave similarly in combination with PVDF. The only blend that extruded satisfactorily, in addition to being capable of being crosslinked by irradiation to form a useful product was that of ETFE and Vinylidene Fluoride. Table III compares the performance of the various blends in extrusion and as irradiated wire insulations. After performing the aforementioned tests, further tests were performed adding a small amount of a crosslinking promoter to the aforementioned blends prior to extrusion. One (1) part (per 100 parts of ETFE) of a crosslinking promoter (TAIC) was added to the blends covered in Table III. They were again extruded on to wire and irradiated. The only blend that crosslinked appreciably with irradiation was that containing ETFE and Kynar®. For instance a blend containing 5 parts of Kynar® crosslinked at 150 kJ/kg sufficiently to pass the 7 hour/300°C. Mandrel Test, showing that the addition of a relatively small amount of TAIC enhances the effectiveness of irradiation on the properties of the blend. The addition of 1 part TAIC (per 100 parts of ETFE) to the remaining blends did not result in appreciable crosslinking with irradiation. All insulated wires still failed the 7 hour/300°C. Mandrel Test even with excessively high levels of irradiation (500 kJ/kg). A slight improvement was observed in the cases of the Halar® and FEP blends with ETFE containing 1 part TAIC in that the solder iron test was passed at 150 kJ/kg. Neither ETFE nor PVDF (unblended) with or without 1 part TAIC (per 100 parts of ETFE or PVDF, respectively) added performance satisfactorily on irradiation to pass either test. The foregoing data confirms the uniqueness of the ETFE/Kynar® blends as discloses and claimed herein. The coating of the pellets with powder after they were coated with TAIC makes them free flowing, as indicated by the following results which we have observed. i) The blend is made to flow freely from the hopper into the throat of the extruder. Without the powder the pellets tend to adhere to each other and to the sides of the hopper causing an uneven flow into the throat of the extruder and sometimes resulting in a complete stoppage of the flow of the blend into the extruder. ii) Once the pellets enter the screw of the extruder, it is desired that they respond to compressive forces in such a way that good conveyance and mixing occurs. When the powder coated pellets enter the screw of the extruder, they are conveyed evenly and efficiently through the feed, compression and metering sections of the screw. Without the powder, the liquid TAIC on the pellets gives lubricity which causes undesired slippage in response to compressive forces and prevents uniform conveyance of the blend particulary in the cooler feed section of the screw of the extruder. Thus the use of powder coated pellets not only allows a uniform flow from the hopper into the throat of the extruder but also assures a constant feed through the screw of the extruder to produce a wire coating that does not vary in properties and dimensions.

Claims: A shaped article comprising an irradiation crosslinked composition on the basis of fluorocarbon polymers having a melting point prior to crosslinking of at least 240°C; the article having been subjected to at least one forming operation at temperatures in excess of the melting point of the composition prior to being crosslinked, and thereafter being crosslinked by radiation at a dose level of up to 400 kJ/kg, the composition being characterized by containing 100 parts by weight of ethylenetetrafluoroethylene copolymer and 1 to 50 parts by weight of polyvinylidene fluoride, and containing no crosslinking agent at the commencement of said radiation other than such crosslinking agents as may have been contained therein during the forming operation. The article of claim 1 wherein the composition comprises a radiation crosslinking promoter absorbed in the polyvinylidene fluoride. The article of claim 1 wherein the article comprises an electrical conductor and an extruded insulation coating thereon, the coating comprising an irradiation crosslinked composition according to claim 1 or 2. A method of forming a shaped article as claimed in claim 1 and capable of withstanding high temperatures, by melt processing a mixture on the basis of fluorocarbon polymers and crosslinking by irradiation thereof, comprising the steps of a) preparing a mixture comprising 100 parts by weight of an ethylene tetrafluoroethylene copolymer and from 1 to 50 parts by weight of polyvinylidene fluoride, b) shaping an article from said mixture by said melt processing. A method according to claim 4, wherein step (a) comprises (1) preparing a mixture of pellets of ethylenetetrafluoroethylene copolymer and polyvinylidene fluoride, (2) coating said pellets with a liquid radiation crosslinking promoter, (3) coating the resulting promoter-coated pellets with powdered polyvinylidene fluoride, and blending the pellets. The method of claim 5 wherein said crosslinking promoter is triallyl isocyanurate.

Inventors: GLAISTER, FRANK J.
Applicants: HIGH VOLTAGE ENGINEERING CORPORATION
Application year: 1986
Main classification: H01B3
IPC codes: H01B3, C08L23, H01B7, C08J7, C08L101, C08L27, C08L1