WO2000049625A1 - Fluoropolymer tubular structure - Google Patents

Abstract

Tubular structures such as wire insulation containing neutralized fluoropolymer ion exchange resin exhibit improved properties.

Description

TITLE

FLUOROPOLYMER TUBULAR STRUCTURE

FIELD OF THE INVENTION

This invention is in the field of electrical wire constructions having fluoropolymer insulation.

BACKGROUND OF THE INVENTION

Fluoropolymers are well known for outstanding combinations of properties. As a result, fluoropolymer resins are used in a wide variety of applications including wire insulation, cable jacket, hose, tubing, film, and the like. The service temperature in some ofthese appHcations can be high. As is common for thermoplastics, some properties of fluoropolymers change as temperature increases. Modulus and tensile strength, for example, typically decrease with increasing temperature, as does cut-through resistance when tested as tubing or wire insulation. Efforts have been made to improve the physical characteristics of fluoropolymers at elevated temperatures, largely by cross-linking. Approaches to cross-linking usually involve the incorporation of a cross-linking promoter, also called a coagent, such as triallyl cyanurate or triallyl isocyanurate (U.S. Patent 5,353,961) or metallic diacrylate (U.S. Patent 5,409,997) into a fluoropolymer such as an ethylene/tetrafluoroethylene copolymer, followed by treatment with ionizing radiation to effect the cross-linking.

Efforts have also been made to improve upon the mechanical properties of fluoropolymers by using other materials in conjunction with the fluoropolymers. Examples of such composite structures are the wire construction using a layer of high-modulus material, i.e.. a polyimide resin, disclosed by Harlow in U.S. Patent 4,801,501, and the wire construction using a braid of high-strength fibers disclosed by Randa in U.S. Patent 5,171,635.

Ways to achieve improved properties of fluoropolymers, in particular improved cut-through resistance of wire insulation and other tubular structures, and resistance to flow at elevated temperatures, while retaining predominantly fluoropolymer characteristics are still desired.

Ionic fluoropolymers have well-known commercial use in ion exchange processes. In addition to their principal uses as membranes in fuel cells and in electrolytic cells such as chloralkali cells, where their ion exchange properties are utilized and where it is critical that the membranes be used in the wet state, in which they contain substantial amounts of water or other polar solvents, they are also used for drying gases and liquids. This too is an application where their affinity for water and ability to transport water is advantageously employed. The use of fluoropolymers having ionic sites as coatings, particularly on nonionic fluoropolymer substrates, has been disclosed. These coatings provide surface activity, which fluoropolymers without ionic sites lack. This surface activity is used to improve printing characteristics and dyeability, as disclosed in U.S. Patent 3,692,569. Surface activity also acts to reduce accumulation of static electricity, as disclosed in Japanese Patent Application Kokai 10-195212, a use requiring electrical conductivity. For these surface activation applications, only superficial ionic sites are needed or desirable, both for reasons of economy and to maintain the properties of the substrate polymer. Therefore these applications disclose thin layers of fluoropolymer with ionic sites concentrated at the surface.

SUMMARY OF THE INVENTION

It has been discovered that some mechanical properties of tubular structures fabricated using fluoropolymer having ionic sites or precursors thereto are directly or indirectly improved when such structures are maintained in the dry state, which is contrary to the usual utility for such polymers as ion exchange membranes in a liquid environment. By direct improvement is meant that with certain counterions, fluoropolymer having ionic sites shows increased improved cut-through resistance. One object of the present invention is to provide improved cut-through resistance to a fluoropolymer, which is highly useful in such applications as tubing and electrical insulation for wire constructions. Another object of the present invention is to provide unchanging or increasing resistance to flow at elevated temperatures. By indirect improvement is meant that the ionic site precursors are precursors to the salts that provide the improved cut-through resistance and resistance to flow. Thus, for convenience of manufacture, the precursor of a fluoropolymer having ionic sites could be the alkyl ester or acid fluoride form, in which case, when converted to fluoropolymer having ionic sites, those sites would carboxylic, sulfonic or phosphonic acids, or salts thereof.

A first embodiment of the present invention is directed to a tubular structure of a fluoropolymer having ionic sites or precursors thereto.

In a preferred embodiment, this invention provides an improved wire construction comprising an electrical conductor and electrical insulation surrounding said conductor, wherein said insulation comprises fluoropolymer having ionic sites or precursors thereto. Preferably, at least some of the counterions associated with the ionic sites have valences of 2 or greater. The fluoropolymer having ionic sites or precursors thereto can be present as a component layer of an insulation having at least two component layers, at least one of which contains no ionic sites. The fluoropolymer having ionic sites can be present as a component of a blend with fluoropolymer having no ionic sites.

In a further embodiment, the invention provides a fluoropolymer blend, comprising fluoropolymer having no ionic sites and at least 20% by weight, based on total fluoropolymer, of fluoropolymer having ionic sites, wherein at least some of the counterions have valences of 2 or greater. Preferably, the fluoropolymer having no ionic sites is partially crystalline.

Another embodiment of the present invention is a process for improving the cut-through resistance of a tubular structure comprising installing a tubular structure comprising fluoropolymer having ionic sites and their accompanying counterions in an enclosure, and maintaining a low humidity environment in said enclosure.

DETAILED DESCRIPTION It has been discovered that fluoropolymer having ionic sites, normally used in a wet state for ion exchange purposes in a wet environment such as in electrolytic cells, or in fuel cells, or for electrodialysis, can be used effectively in a dry state in a dry environment to obtain improved mechanical properties. In these fluoropolymers having ionic sites, the sites may be either anionic or cationic. Examples of anionic sites are sulfonates, carboxylates, and phosphonates. U.S. Patent 3,282,875 describes the preparation of fluoropolymers having anionic sites of the sulfonate type. An example of a cationic site is quaternary ammonium. U.S. Patent 4,900,420 describes the preparation of fluoropolymers of this type. These ionic sites are accompanied by ions of the opposite charge so that electroneutrality is maintained. These accompanying ions are known as

"counterions". Fluoropolymers having anionic sites have cationic counterions, such as hydrogen ion, sodium or other alkali metal ion. magnesium or other alkaline earth cation, or ions of higher valency such as ferric ion or chromic ion. If the fluoropolymer has cationic sites, the counterions will be anions, such as chloride, nitrate, sulfate, or phosphate. Because the counterions can easily be exchanged, these fluoropolymers with ionic sites are commonly known as fluorinated ion exchange polymers. Such polymers have well-known commercial use in ion exchange membranes for chloralkali and other processes, and are available, e.g., as Nafion^® perfluorinated membrane (DuPont Company, Wilmington Delaware USA).

Fluoropolymers with anionic sites or the precursors thereto are preferred as usually being simpler and more economical to produce, and usually having greater thermal and chemical stability. More preferred are fluoropolymers with sulfonate or carboxylate groups or the precursors thereto. Most preferred are fluoropolymers with sulfonate groups or the precursors thereto.

In particular, it has been discovered that wire insulation incorporating fluoropolymer having ionic sites exhibits improved cut-through resistance and resistance to flow, especially when at least some of the counterions have a valence of 2 or greater. Furthermore, these properties are notably improved as the temperature rises. This is unexpected, since it is usual for polymer properties to deteriorate with increasing temperature. Since wire insulation is tubular in form, closely fitted about a metal conductor, the term "tubular structure" as used herein includes wire constructions. Tubular structures of the present invention also include tubing, including convoluted tubing, and similar structures such as hose comprising fluoropolymer having ionic sites with counterions of valency of 2 or greater. Collectively, tubular structures that are not wire constructions are identified herein as "tubing". The wire construction of the present invention comprises an electrical conductor, such as copper, plated copper, aluminum, or carbon, and electrical insulation surrounding the conductor. The metallic conductor can be solid or stranded, as is well known in the art. While the solid conductor and the stranded conductors normally have circular cross sections, conductors having other cross sections can be used.

The wire construction has electrical insulation surrounding the conductor, said insulation comprising at least one layer of fluoropolymer having ionic sites. The insulation can consist of only a single layer of fluoropolymer having ionic sites, or it can have in addition one or more layers of polymer having no ionic sites. This polymer having no ionic sites may be a fluoropolymer or a nonfluoropolymer, including but not limited to, cross-linked polyolefin. In one embodiment of the present invention, the wire construction has at least one layer of fluoropolymer having ionic sites and at least one layer of fluoropolymer having no ionic sites, with either the outer layer (that layer farthest from the conductor) or the inner layer (that layer closest to the conductor) of fluoropolymer being fluoropolymer having no ionic sites. Thus, in the simplest structure of this embodiment, the conductor is surrounded by a first layer of fluoropolymer having no ionic sites adjacent to the conductor, and the first layer of fluoropolymer is surrounded by a layer of fluoropolymer having ionic sites. The insulation of the wire construction of the present invention can be applied to the metallic conductor by any of several techniques known to those skilled in the art of wire construction operations, or combinations of such techniques. The techniques include extrusion and tape wrapping, the latter technique usually being followed by a high temperature exposure to fuse adjacent layers of tape and/or to fuse tape to layers of insulation applied previously or subsequently. Fluoropolymers in tape form can be melt-fabricable or non-melt- fabricable. As one skilled in the art will recognize, composite constructions can be formed by applying one insulation component by one technique, e.g., melt extrusion, and another insulation component by another technique, e.g., tape wrapping. Composite constructions can, of course, have more than two insulation component layers. It may be convenient to apply the fluoropolymer that in the finished product has ionic sites in the form of a precursor polymer, and after application convert the precursor polymer to the ionic form, that is. to the form in which the fluoropolymer has ionic sites. For convenience, the precursor polymer is designated "precursor ionic polymer". After the precursor ionic polymer is in place, and either before or after other layers of fluoropolymer having no ionic sites are added, the precursor ionic polymer is treated to convert the precursor polymer to polymer with ionic sites. Preferably, for better process efficiency, the precursor ionic polymer is treated and converted to polymer having ionic sites before any layer of non-ionic polymer is applied over the precursor ionic polymer.

The insulation of the wire construction of the present invention can also be applied to the conductor, or to conductor covered by one or more layers of polymer having no ionic sites, by applying the ionic polymer or precursor ionic polymer from solution. The preparation of these solutions is disclosed in U.S. Patents 5,290,846; 4,433,082; and 3,692,569. The fluoropolymer having ionic sites can be applied to the wire construction from these solutions by dipping, spraying, or other coating techniques known in the art. The coating can be applied in one pass through the coating device, or built up by multiple passes. The solvents should be removed by drying before subsequent steps are undertaken. Heat may be applied during drying to accelerate the process and to promote complete removal of solvent. Heat may also be necessary to fuse or coalesce the coating if this does not occur without heating. If a solution of precursor ionic polymer is used, then after the precursor ionic polymer is in place surrounding the conductor, and either before or after any other layers of fluoropolymer having no ionic sites are added, the precursor ionic polymer is treated to convert it to polymer with ionic sites. Such treatment is not necessary for coating applied from aqueous or alcoholic solutions of fluoropolymer in the ionic form. The thickness of the insulation, or of each insulation layer, can vary widely, depending on the size of the conductor and the desired degree of insulation. For example, for AWG 22 conductor, a total insulation thickness in the range of about 0.004-0.012 inch (0.1-0.3 mm) is typically used. If such insulation has two layers, e.g., a first layer of fluoropolymer having no ionic sites and a second layer of fluoropolymer having ionic sites, the thickness of each layer is typically in the range of about 0.0005-0.008 inch (0.013-0.2 mm). Since cut- through resistance increases with counterion valence, as discussed below, the layer of fluoropolymer having ionic sites can be thinner, e.g., as thin as

0.0003 inch (0.008 mm), when at least some of the counterions have valences of 2 or greater.

Wire constructions of the present invention can include other features known in the art, such as fiber braid, either for protective or reinforcing purposes, jacketing, and the like. If a fiber braid is used between fluoropolymer insulation layers, and is thus a component of the insulation system as disclosed in U.S. Patent 5,171,635, the fiber will be of an electrically insulating material such as a polymeric material or glass and desirably will be impregnated with an insulating material such as fluoropolymer resin. If a braid is external to the insulation, the fiber can be metallic or of other high strength material.

The treatment of the precursor ionic polymer generally includes treatment to convert the polymer to fluoropolymer having ionic sites. Thus, for example, when the functional group is sulfonyl fluoride, it is hydrolyzed to convert it to the sulfonate group. Hydrolysis is carried out by immersing the wire in an alkaline solution, preferably a 25% by weight aqueous solution of alkali metal hydroxide, preferably potassium hydroxide, for about 16 hours at a temperature of about 90°C followed by rinsing the wire twice in deionized 90°C water using about 30 minutes to about 60 minutes per rinse. Another possible method employs an aqueous solution of 6-20% of an alkali metal hydroxide, preferably potassium hydroxide, and 5-40% polar organic solvent such as dimethyl sulfoxide (DMSO) with a contact time of at least 5 minutes at 50°-100°C followed by rinsing for 10 minutes. When the functional group is more easily hydrolyzed, as in the case of the esters of carboxylic acids, milder hydrolysis conditions, including simple water or water with a solvent such as DMSO, or steam treatment, will be adequate. The resulting fluoropolymer with carboxylate groups may be used in this form, which is a fluoropolymer having anionic sites, or it may be further reacted according to the methods of U.S. Patent 4,900,420, to convert the carboxylate to a quaternary ammonium function, which is a fluoropolymer having cationic sites. The extent of hydrolysis can be measured by cutting a thin cross-section of the insulated wire, optionally removing the metal component, dipping the cross- section in a 1-5% aqueous hydrochloric acid solution, rinsing in deionized water. and then dipping in a 1% aqueous alcoholic solution of malachite green or crystal violet dyes. The hydrolyzed area of the layer having fluoropolymer having ionic sites will be stained by the dye. Unhydrolyzed areas, those that remain in the form of the precursor ionic polymer, will be unstained by the dye. The degree, as opposed to the spatial extent, of hydrolysis can be determined by techniques known in the art, e.g., by titration.

Hydrolysis should be at least 25% complete, that is, at least 25% of the precursor ion exchange sites should be hydrolyzed. It is preferable that hydrolysis be at least 50% complete. It is more preferable that hydrolysis be at least 75% complete. It is most preferable that hydrolysis be at least 95% complete. Following the treatment of the precursor ionic polymer to convert it to fluoropolymer having ionic sites, the ionic sites are associated with the counterion that is characteristic of the method of preparation. For example, precursor ionic polymer hydrolyzed in potassium hydroxide solution will be in the potassium ion form, that is. potassium will be the counterion. When precursor ionic polymer is hydrolyzed in water alone, hydrogen ion, or the proton, will be the counterion. The counterion can be changed, in a process called "ion exchange", by contacting the coated wire with a bath of a 0.01N (0.01 normal) to about 2N or 3N salt solution containing the desired ion, rinsing, and drying. The rate of exchange will vary with the concentration of the salt solution, so for practical reasons a preferred concentration is at least about 0.05N. Heating will accelerate the rate of ion exchange. Multiple exposures to fresh solutions can be used to promote faster and more complete ion exchange. When the precursor ionic polymer has functional groups that can be hydrolyzed in water alone, as in the case of the esters of carboxylic acids described above, hydrolysis and ion exchange can be combined in a single step by adding the counterion in the form a soluble salt, to the water. When the counterion characteristic of the conversion is monovalent, such as sodium or potassium among the cations, or chloride or nitrate among the anions, the polymer shows good physical properties, such as improved cut- through resistance, and good resistance to flow, especially as the temperature is increased. It is believed that this is at least partially attributable to the interactions among the ions in the polymer and to the increase in these interactions as the temperature rises and any residual moisture is driven off. Properties improve further if some of the monovalent counterions are replaced by multivalent counterions. For fluoropolymer having ionic sites for use in the present invention, counterions can be ions having valences of 1, 2, 3 or 4. Counterion valence is preferably 2 or greater, more preferably 3 or 4, and most preferably 3. Examples of cationic bivalent counterions include Ba, Zn, Mg, Ca, Sr, Cu, Sn, TiO and ZrO cations, examples of cationic trivalent counterions include Al, Bi, Cr and Fe cations, and examples of tetravalent counterions include Sn and Ti cations. A single cation, or a mixture of cations may be used. Examples of anionic bivalent counterions include sulfate and silicate. An example of an anionic trivalent counterions is phosphate. A single anion, or a mixture of anions may be used. Because the fluoropolymer with ionic sites respond differently to different multivalent counterion and because of variations among such fluoropolymers themselves, it is advisable to conduct preliminary tests to determine the optimum ion exchange conditions for cut-through resistance. It has also been found that cations of the same valence show different effectiveness in increasing cut-through resistance. The higher the valence of the counterion, the more effective at increasing cut-through resistance, although there is a tendency, as the cation valence increases, for the range of acceptable ion exchange to become narrower. This can lead to requirements that ion exchange conditions be more closely monitored and controlled. For example, bivalent counterion can have a broader acceptable range of ion exchange than trivalent. For this reason, it may be desirable to use both bivalent and trivalent counterion in ion exchange.

After the fluoropolymer having ionic sites is in the desired ionic form, the degree of drying of the insulation can affect the cut-through resistance of the insulation. Drying can be done at room temperature, but will be more rapid and complete if heat is applied. Temperatures in the range of 30°-250°C are preferable, if other polymers in the insulation can tolerate the upper temperature limit. More preferable are temperatures in the range of 75°-225°C, and most preferable are temperatures of 125°-200°C. The time necessary for drying will depend upon the temperature and upon the construction of the insulation, including the thickness of the layer of fluoropolymer having ionic sites, and the presence or absence of other polymer layers covering the fluoropolymer having ionic sites. Those skilled in the art will recognize that simple experiments to determine the time necessary to dry a length of insulated wire to constant weight can be used to find the drying time at the desired temperature. As a rule, the more completely the insulation is dried, and the higher the drying temperature, as long as the maximum stability temperatures of the component polymers are not exceeded, the greater will be the cut-through resistance of the insulation.

The more completely the insulation is dried and the higher the drying temperature, the less will be the loss of cut-through resistance on subsequent exposure to humidity or moisture. Nevertheless, the drier the environment, the better will the insulation retain cut-through resistance. For this reason, it is desirable to keep the environment around the insulated wire dry. One way to do this is to have an outer layer over the layer of the fluoropolymer having ionic sites. This outer layer should be a barrier to moisture and humidity and need not be thicker than necessary to accomplish this. The outer layer may be applied by extrusion, overwrapping. or by coating. This outer layer may be of non-ionic fluoropolymer such as described below, or may be of another material such as a fluorine-free polymer having properties suitable for the intended use.

Another aspect of this invention is the imparting of characteristic colors to the insulation for color coding or for the imprinting of text or symbols for identification purposes. The color can be controlled by the selection of counterions. For example, when iron is the counterion, the insulation layer containing fluoropolymer having ionic sites is yellow to orange colored. When copper is the counterion, the layer is blue. Where a greater variety or more intense coloration is desired, in addition to the metal counterions, a small percentage of the counterions can be chosen from the class of ionic dyes. There are a large number of these in a wide range of colors. Examples of cationic dyes include malachite green, brilliant green, methylene blue, crystal violet, and rhodamine B. The chemistry of cationic dyes is described in Chapter 8 of Colour Chemistry by R.L.M. Allen, Appleton-Century-Crofts, New York, 1971. Methods of employing cationic dyes are described in Dyeing Primer Part 3 published by the American Association of Textile Chemists and Colorists, 1981. The dye may be applied so as to penetrate the entire thickness of the layer of fluoropolymer having ionic sites, or only on the surface. It may be applied by dipping, spraying, or other coating means from aqueous or alcoholic solutions. It may also be printed on the layer of fluoropolymer having ionic sites by conventional means for printing on wire insulation. This dyeing or printing may be done before or after the drying of the insulated wire. If the dye chosen is temperature sensitive, post-drying application is preferred.

The fluoropolymers having ionic sites for use in accordance with this invention may be any number of polymers having ionic sites, including polymers with anionic sites groups that are preferably selected from the group consisting of sulfonate, carboxylate, phosphonate, imide, sulfonimide and sulfonamide groups. Various known polymers with ionic sites can be used including polymers and copolymers of trifluoroethylene, tetrafluoroethylene, etc., in which ionic groups have been introduced. As explained above, these polymers may be made and applied in the form of precursor ion exchange polymer, or as solutions. Polymers having ionic sites for use in accordance with the present invention can be highly fluorinated polymers having sulfonate ionic groups. "Highly fluorinated" means that at least 90% of the total number of univalent atoms in the polymer are fluorine atoms. Among such highly fluorinated polymers, perfluorinated polymers are preferred.

Preferably, the polymer comprises a polymer backbone with recurring side chains attached to the backbone with the side chains carrying the anionic groups. Possible polymers include homopolymers or copolymers of two or more monomers. Copolymers are typically formed from one monomer which is a nonfunctional monomer and which provides carbon atoms for the polymer backbone. A second monomer provides both carbon atoms for the polymer backbone and also contributes the side chain carrying the anionic group or its precursor, e.g., a sulfonyl halide group such a sulfonyl fluoride (-SO₂F), which can be subsequently hydrolyzed to a sulfonate group. For example, copolymers of a first fluorinated vinyl monomer together with a second fluorinated vinyl monomer having a sulfonyl fluoride group (-SO₂F) can be used. Possible first monomers include tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), and mixtures thereof. Possible second monomers include a variety of fluorinated vinyl ethers with sulfonate groups or precursor groups that can provide the desired side chain in the polymer. The first monomer may also have a side chain that does not interfere with the sulfonate group. Additional monomers can also be incorporated into these polymers if desired.

The copolymers may be of the type described as "random", by which is meant that the distribution of monomers in the polymer chain is determined in large part by monomer reactivity ratios, monomer concentrations, and the polymerization temperature range, said concentrations and range being kept reasonably uniform for the greater part of the polymerization. The copolymers may also be of the type described as "block" wherein oligomers or polymers more or less rich in one of the principal monomers are joined with oligomers or polymer more or less rich in another of the principal monomers, the resulting polymer therefore having segments that differ from one another in composition. An example of such a block copolymer is described in International Application WO 9843952.

A class of preferred polymers for use in the present invention includes a highly fluorinated, most preferably perfluorinated, carbon backbone and the side chain is represented by the formula -(O-CF₂CFRΛ_a-O-CF2CFR Sθ3X, wherein R -and RVare independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a = 0, 1 or 2, and X is H, Li, Na, K or N(Rl)(R2)(R3)(R4) and R¹, R², R³, and R⁴ are the same or different and are H, CH3 or C₂H₅, or multivalent cations such as Ba, Zn, Mg, Ca, Sr, Cu, Sn, TiO and ZrO, Al, Bi, Cr and Fe, Sn and Ti. Preferably X is Li, Na, or K, more preferably X is selected from as Ba, Zn, Mg, Ca, Sr, Cu, Sn. TiO and ZrO, Al, Bi, Cr, Fe, Sn and Ti, and most preferably selected from Al, Bi, Cr, Fe, Sn, Ti and ZrO. The preferred polymers include, for example, polymers disclosed in U.S. Patent 3,282,875 and in U.S. Patents 4,358,545 and 4,940,525. One preferred polymer comprises a perfluorocarbon backbone and the side chain is represented by the formula -O-CF₂CF(CF₃)-O-CF₂CF₂SO₃X, wherein X is as defined above. Polymers of this type are disclosed in U.S. Patent 3,282,875 and can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF₂=CF-O-CF₂CF(CF₃)-O-CF₂CF₂SO₂F, perfluoro(3,6-dioxa-

4-methyl-7-octenesulfonyl fluoride) (PDMOF), followed by conversion to sulfonate groups by hydrolysis of the sulfonyl halide groups and ion exchanging as necessary to convert to the desired form. One preferred polymer of the type disclosed in U.S. Patents 4,358,545 and 4,940,525 has the side chain -O- CF₂CF₂SO₃X, wherein X is as defined above. This polymer can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF₂=CF-O-CF₂CF₂SO₂F, perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed by hydrolysis and ion exchange as necessary.

In the present invention, highly fluorinated carboxylate polymer, i.e., having carboxylate groups in the resulting wire insulation, can also be employed as will be discussed in more detail hereinafter. The term "carboxylate groups" is intended to refer to either carboxylic acid groups or salts of carboxylic acid groups, preferably alkali metal or ammonium salts. The ionic groups are represented by the formula -CO2X wherein X is defined as above with the same orders of preference. Preferably, the polymer comprises a polymer backbone with recurring side chains attached to the backbone with the side chains carrying the carboxylate groups. Polymers of this type are disclosed in U.S. Patent 4,552,631 and most preferably have the side chain -O-CF₂CF(CF₃)-O-CF₂CF₂CO₂X. This polymer can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF₂=CF-O-CF₂CF(CF₃)-O-CF₂CF₂CO₂CH3, methyl ester of perfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid) (PDMNM), followed by conversion to carboxylate groups by hydrolysis of the methyl carboxylate groups and ion exchanging if needed to convert to the desired form. While other esters can be used for insulation fabrication, the methyl ester is the preferred since it is sufficiently stable during normal extrusion conditions.

In this application, "ion exchange ratio" or "IXR" is defined as the number of carbon atoms in the polymer backbone in relation to the number of ionic groups. A wide range of IXR values for the polymer is possible. Typically, however, the IXR range used for layers of the wire insulation is usually from about 5 to about 100, preferably from 7 to 50, and more preferably from 7 to 33. For perfluorinated polymers of the type described above, the ion exchange capacity of a polymer is often expressed in terms of equivalent weight (EW). For the purposes of this application, equivalent weight (EW) is defined to be the weight of the polymer in acid form required to neutralize one equivalent of NaOH. In the case of a sulfonate polymer where the polymer comprises a perfluorocarbon backbone and the side chain is -O-CF₂-CF(CF₃)-O-CF₂-CF₂-SO₃H (or a salt thereof), the equivalent weight range which corresponds to an IXR of about 5 to about 100 corresponds to about 500 EW to about 5400 EW, while IXR of 7-33 corresponds to 700 EW to 2000 EW. IXR for this polymer can be related to equivalent weight using the following formula: 50 IXR + 344 = EW. While generally the same IXR range is used for sulfonate polymers disclosed in U.S. Patents 4,358,545 and 4,940,525, the equivalent weight is somewhat lower because of the lower molecular weight of the monomer unit containing an anionic group. For the IXR range of about 7 to about 33, the corresponding equivalent weight range is about 500 EW to about 1800 EW. IXR for this polymer can be related to equivalent weight using the following formula: 50 IXR + 178 = EW. For carboxylate polymers having the side chain -O-CF₂CF(CF₃)-O-CF₂CF₂CO₂X, a useful IXR range is about 5 to about 100, preferably about 7 to about 33. This corresponds to about 550 EW to about 5300 EW, preferably about 650 EW to about 2000 EW. IXR for this polymer can be related to equivalent weight using the following formula: 50 IXR + 308 = EW. IXR is used in this application to describe either hydrolyzed polymer that contains ionic groups or unhydrolyzed polymer that contains precursor groups that will subsequently be converted to the ionic groups during the manufacture of the membranes.

The concentration of ionic sites in the fluoropolymer resin component layer having such sites, i.e., in fluoropolymer having ionic sites or in a blend of fluoropolymer having such sites and fluoropolymer having no ionic sites, if fluoropolymer having no such sites is present, is surprisingly effective to improve the cut-through resistance of the wire insulation compared to similar insulation having no ionic sites. The concentration of ionic sites that is effective to improve cut-through resistance can vary at least with the type of ionic site, with the counterion, and with the nature of the counterion or counterions. The concentration of ionic sites present can be expressed relative to the number of main chain carbon atoms in the fluoropolymer resin. The desired concentration of ionic sites or precursors thereto can be achieved with a single fluoropolymer having ionic sites or precursors, or a mixture of such fluoropolymers. The desired concentration of ionic sites can also be achieved by blending one or more fluoropolymers having precursor ionic sites with one or more fluoropolymers having essentially no ionic sites. Such blending is carried out using precursor ionic polymer. In this embodiment, fluoropolymer having ionic site precursors acts as an "ionic site concentrate" that can be let down or diluted with fluoropolymer having no ionic sites. This approach has the advantage of permitting one to achieve a variety of ionic site (or precursor) concentrations with a single fluoropolymer having ionic sites by varying the blending ratio with fluoropolymer having no such sites, and is an alternative embodiment of the invention. The amount of fluoropolymer having ionic sites or precursors thereto in fluoropolymer blends used in tubular structures of the present invention will depend on the IXR of the blend component having ionic sites or precursors thereto and the IXR desired for the blend. Generally, the amount of fluoropolymer having ionic sites is at least 20%) by weight, based on total weight of fluoropolymer in the blend.

When melt-fabricable fluoropolymer blends are used to prepare wire constructions of this invention, such blends can be melt-mixed compositions, compounded using melt processing equipment of conventional design and suitably equipped for handling fluoropolymers at melt temperatures, such as twin rotor mixers and extruders with good mixing capability. As one skilled in the art will recognize, melt-mixing can be carried out prior to the operation in which the blend is applied to the conductor, e.g., by extrusion, or can be carried out as part of the wire coating operation by feeding a powder or cube blend to the wire line extruder.

Fluoropolymers that can be used in the structures of the present invention can be amorphous or partially-crystalline, and include elastomeric and plastic fluoropolymers. Melt-fabricable fluoropolymer resins that can be used in the wire constructions of the present invention, either as discrete layers of insulation or in blends with fluoropolymer having ionic sites or precursors thereto, include copolymers of TFE with one or more copolymerizable monomers chosen from perfluoroolefins having 3-8 carbon atoms and perfluoro(alkyl vinyl ethers) (PAVE) in which the linear or branched alkyl group contains 1-5 carbon atoms, with comonomer present in sufficient amount to reduce the melting point substantially below that of TFE homopolymer. e.g.. to a melting point no greater than 315°C. Preferred perfluoropolymers include copolymers of TFE with at least one of hexafluoropropylene (HFP) and PAVE. Preferred comonomers include PAVE in which the alkyl group contains 1-3 carbon atoms, especially 2-3 carbon atoms, i.e. perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE). Additional fluoropolymers that can be used include copolymers (ETFE) of ethylene with TFE, optionally including minor amounts of one or more modifying comonomer such as perfluorobutylethylene (PFBE), and are also among preferred fluoropolymers. Other hydrogen-containing fluoropolymers that can be used include copolymers (ECTFE) of ethylene and chlorotrifluoroethylene (CTFE), and vinylidene fluoride homopolymers and copolymers. Generally, melt-fabricable resins have melt viscosity (MV) in the range of 0.5-50 x 10³ Pa-s though viscosities outside this range can be used. MV is measured according to ASTM D-1238 at the temperature appropriate for the particular fluoropolymer. Preferably, MV is in the range of 1-25 x 10³ Pa-s. Representative melt-fabricable fluoropolymers are described, for example, in ASTM Standard Specifications D-2116, D-3159, and D-3307. Such fluoropolymers are "non-ionic" fluoropolymers if they have essentially no ionic sites.

Fluoropolymers having ionic sites and the precursors thereof include fluoropolymers such as those described in the foregoing paragraph and additionally containing copolymerized units derived from monomers having ionic sites, such as PDMOF as described above.

Non-melt-fabricable fluoropolymer resins having melt viscosity so high, e.g., at least 10⁸ Pa-s, that they cannot readily be shaped by normal melt processing techniques can also be used for component layers of structures of the present invention. Such resins include poly tetrafluoroethylene (PTFE) and TFE polymers (modified PTFE) containing such small concentrations of copolymerizable modifying monomers that the melting point of the resultant polymer is not substantially reduced below that of PTFE, e.g., not lower than 320°C. The modifying monomer can be, for example, HFP, PPVE, PEVE, chlorotrifluoroethylene (CTFE), or other monomer that introduces bulky side groups into the molecule. The concentration of such modifiers is usually less than 1 wt%, more commonly less than 0.5 wt%. Fibrillatible PTFE and modified PTFE resins produced by dispersion polymerization are described, for example, in ASTM Standard Specification D-4895. Such resins can be converted to components of the structures of the present invention by a lubricated extrusion (paste extrusion) process in which the resin is blended with a lubricant, the lubricated resin (paste) is shaped by a low-temperature extrusion process, the lubricant is removed, and the resultant shape, which is in what is known as "the green state", is fused (sintered) at temperature above the melting point of the PTFE. The paste extrusion process can be used to form a tubular component directly, or can be used to form a tape that, in the green state, can be wrapped into a tubular shape.

Tubular structures within the scope of this invention that are not wire constructions (collectively, "tubing") can be made of materials and can have constructions similar to those described above for wire insulation systems.

Another aspect of the present invention is a blend of fluoropolymer having ionic sites and fluoropolymer having no ionic sites. In this embodiment of the invention, the ionic sites are preferably associated with one or more counterions, at least some of which have a valence of at least 2, more preferably having a valence of 3 or 4, and most preferably having a valence of 3. Preferably, fluoropolymer blends of the invention are melt-mixed blends, and the fluoropolymer having no ionic sites (non-ionic fluoropolymer) preferably is partially crystalline. By "partially crystalline" is meant that the fluoropolymer has a crystalline melting point and exhibits a melting endotherm with heat of fusion of at least 3 J/g by differential scanning calorimetry. Preferably, the non-ionic fluoropolymer has heat of fusion of at least 10 J/g.

Another embodiment of the present invention is a process for improving the cut-through resistance of a tubular structure comprising installing a tubular structure comprising fluoropolymer having ionic sites and their accompanying counterions in an enclosure, and maintaining a low humidity environment in said enclosure. In this embodiment, at least some of the ionic sites are preferably associated with a counterion having a valence of 2 or greater, more preferably having a valence of 3 or 4, and most preferably having valence of 3. By "low humidity environment" is meant that no condensed water is present, preferably that the relative humidity is less than 50%, more preferably that it is less than 25%), and most preferably that it is less than 10%. Examples of low humidity environments include instrument cabinets, the interior volume of tools, and wiring in airframes, and similar structures, machinery, and equipment. Further steps taken to maintain and improve the low humidity of environments of the type described will maintain and improve the cut-through resistance of articles made according to the present invention. Such steps include dehumidification, the sealing of joints, seams, and openings, and drying agents.

TEST METHODS The melt flow rate (MFR) of fluoropolymer resins is measured according to ASTM D-21 16. Cut-through resistance and dielectric strength of insulated wire are measured according to ASTM D-3032. Dielectric constant and dissipation factor are measured according to ASTM D-150. The zero strength temperature (ZST) is measured on a strip of test film, 5-6 mm wide and 1-3 mil (0.025-0.076 mm) thick. The film strip is placed over a horizontal segment of an immersion heater mounted in air, with one end anchored to a support and with an 8.8 g weight attached to the other end of the film. The power to the heater is regulated by a variable voltage source (Variac) and the temperature at the surface of the heater is measured with a thermocouple. By increasing the voltage the temperature of the heater is raised from room temperature at a programmed rate until the film fails. When the film fails, the weight drops into a pan, making a noise that signals the operator to record the temperature, i.e., the ZST. In general, the test is terminated after temperature exceeds 300°C if there is no sign of impending failure. The ZST test determines the temperature at which the polymer has essentially no strength. The test is also a measure of the resistance of the polymer to flow as a function of temperature.

EXAMPLES The fluoropolymers having ionic sites in the following examples are copolymers of TFE and PDMOF, identified above. Unless otherwise specified, the copolymer has an IXR of 14.5 (EW of 1070). The melt flow rate (MFR) measured at 270°C with a 1200 g weight falls within the range 12.5 ± 7.5 g/10 minutes. This resin is hereinafter called "Ionic- 1". The non-ionic fluoropolymer used in the following examples is a commercial ETFE resin modified with perfluorobutylethylene (Tefzel ETFE fluoropolymer resin grade 200, available from DuPont). This fluoropolymer resin is hereinafter called "ETFE". Melt blends of Ionic- 1 with ETFE are prepared by first mixing the polymer pellets together in desired proportion by hand, and then extruding the pellet blends once through a Wemer & Pfleiderer 28-mm twin- screw extruder having a vacuum port on zone 4, with a nitrogen blanket on the pellet feed to the extruder. The extruder is equipped with a 0.188-inch (4.76-mm) single-hole die, and the extruded strand is water-quenched and chopped into pellets. A vacuum is pulled on the vacuum port and the melt temperature is 309°-325°C.

Example 1

Insulation nominally 0.006 inch (0.15 mm) thick is extruded onto stranded AWG 20 nickel-plated copper conductor (diameter 0.037 inch or 0.94 mm, 19 strands of AWG 32 wire) from fluoropolymer resins, or resin blends prepared as described above, using a 1.25-inch (32-mm) Entwistle wire extrusion line equipped with a standard metering screw having a mixing torpedo (U.S. Patent 3,006,029) to provide a uniform melt, and using a melt draw technique. For ETFE resin or blends, the melt temperature is 610°F (321 °C). For extrusion of Ionic- 1, the melt temperature is 520°F (271°C). Resins or blends are identified in Table 1.

Wire constructions having conductor surrounded by Ionic- 1 and blends of Ionic- 1 are cut in 30-ft (9.1-m) lengths and wrapped in 3-4 inch (7.6-10.2 cm) diameter coils taking care not to twist or crimp the wire. These are placed flat in 500 ml wide-mouth jars containing hydrolysis solution consisting of 14% KOH, 30% DMSO and 56% water, and sealed. The jars are heated for 16 hr on a steam bath, yielding a temperature in the hydrolysis solution of 95°C. The wires are rinsed and soaked in deionized water for four days, and given a final rinse. Ionic- 1 is present as the potassium salt at this point. While still wet, the wire constructions are cut into 10-ft (3-m) lengths. One length is reserved for testing in the potassium salt form. A second length is placed in an aqueous 0.1N solution of the barium chloride and the third length is placed in an aqueous 0.1N solution of aluminum sulfate to exchange the counterions in Ionic- 1. Solutions and wire are placed in jars, which are sealed and heated in a 90°C oven for 3 hr. Then, the wires are removed from the solutions, washed with deionized water, and air dried. Surprisingly, no tarnishing of the conductor after any of the chemical treatment steps is observed. The wires are conditioned at 23 °C and 50%) RH. Cut-through measurements are made at 200°C following heating at 200°C for at least 15 min. Cut-through values in Table 1 show significant increase in high-temperature cut- through resistance for wire constructions having insulation comprising higher proportions of fluoropolymer having ionic sites.

Table 1. Insulations and Cut-Through Resistance for Example 1 Resin/Blend (parts) Cut-Through Res, (lb)

ETFE Ionic- 1 K Ba Al

100 7.4 25.0 33.9

100 — 0.9 0.9 0.9 95 5 1.2 1.1 1.2 50 50 1.7 1.6 3.4

Example 2

The wire construction of Example 1 having 100%) of Ionic- 1 surrounding the conductor is used. The Ionic- 1 is hydrolyzed in KOH DMSO solution as in Example 1. Then, lengths of the wire are placed in 0.1N solutions of Al(NO₃)₃, Al₂(SO₄) ₃, Fe(NO₃) ₃, or Cr(NO₃) ₃ at 90°C for 3 hr to replace potassium ions with the various trivalent metal ions, flushed with deionized water, and dried at

250°-260°C for 7 min. Potassium is replaced with hydrogen ion by soaking a film in 20% aqueous nitric acid at room temperature for 5 min, then rinsing and air drying. Surprisingly, no tarnishing of the conductor after any of the chemical treatment steps is observed. The exchanged wire constructions are conditioned, heat treated, and subjected to cut-through measurement as in Example 1, along with a sample of the wire having ionic sites in the potassium form and a sample of the wire that has not been hydrolyzed. Cut-through resistance values (at 200°C) in Table 2 shows the surprisingly high cut-through

Table 2. Cations, Anions, and Properties for Example 2

Cut-Through

Cation Anion Res. (lb)

Al NO₃ 9.3

Al SO₄ 8.4

Fe NO₃ 14.1

Cr NO₃ Cracks

H — 4.5

Not hydrolyzed Flows resistance for the wire constructions with fluoropolymer having ionic sites and trivalent metal cations as counterions. The cracking observed with chromium is symptomatic of excessive ion exchange. Good cut-through resistance with this cation can be achieved by reducing the extent of ion exchange.

Example 3

The wire construction of Example 1 that has 100%) of Ionic- 1 surrounding the conductor is used. The Ionic- 1 is hydrolyzed in KOH/DMSO solution as in Example 1. Then, different lengths of the wire are placed in 0. IN solutions of Al(NO₃) ₃ or Al₂(SO ) ₃ at 90°C for 2 hr, then rinsed and soaked in deionized water at 90°C for 1 hr, and then dried at 210°C for 20 min. Cut-through values (at 200°C) in Table 3 indicate that exchange is more effective when the anion of the exchange salt is nitrate.

Table 3. Exchange Anion and Exchange Temperature for Example 3

Cut-Through Anion Res, (lb)

NO₃ 19.0

SO₄ 7.4 Example 4

Composite wire constructions having total insulation thickness of approximately 0.006 inch (0.15 mm) are prepared using the conductor and equipment described in Example 1. and using Ionic- 1 in conjunction with two different fluoropolymer resins having no ionic sites. The two non-ionic resins are an ETFE resin (Tefzel ETFE fluoropolymer resin grade HT-2183, available from DuPont) and a TFE/PPVE copolymer resin (DuPont PFA fluoropolymer resin grade 340). In each construction, the non-ionic resin is in contact with the conductor and Ionic- 1 surrounds the layer of non-ionic resin. Thicknesses of the component layers are given in Table 4. The wires are hydrolyzed by a procedure similar to that in Example 1, except that the concentration of DMSO is 36%, the temperature is 80°C, and the time is 10 min. Potassium ion is replaced with aluminum ion by exchange in a 0.1N aluminum nitrate solution for the various times and temperatures shown in Table 4, or is replaced with hydrogen as in Example 2. Surprisingly, no tarnishing of the conductor after the chemical treatment steps is observed. The resultant wire constructions are then tested for cut-through resistance as in Example 1 along with samples with the layer of fluoropolymer having ionic sites in the potassium salt form. Cut-through values (at 200°C) also shown in Table 4 are good for the constructions having ionic sites in the K⁺ or H" form and are exceptionally good for the constructions having ionic sites in the aluminum ion form.

Table 4. Constructions, Exchange Conditions, and Properties for Example 4

Thickness (mil) Exchange Cond. Cut-Through

ETFE PFA Ionic- 1 T (°C) t (min) Cation Res. (lb) 3.0 — 3.0 — none K 1.5

RT 5 H 1.4

60 1 Al 5.1

5 Al 4.9

100 1 Al 4.8

5 Al 5.0

4.5 1.2 — none K 2.3

RT 5 H 2.0

60 1 Al 7.1

5 Al 7.5

100 1 Al 7.8

5 Al 5.3

3.0 3.0 — none K 3.2

RT 5 H 1.3

60 1 Al 4.6

5 Al 5.3

100 1 Al 4.6

5 Al 6.2

5.0 1.0 none K 2.4

RT 5 H 1.2

60 1 Al 3.9

5 Al 4.0

100 1 Al 4.3

5 Al 6.2

Exampli e 5

The wire construction of Example 1 that has 100% of Ionic- 1 surrounding the conductor is used. The Ionic- 1 is hydrolyzed in KOH/DMSO solution as in Example 1. Then, different lengths of the wire are placed in 0. IN solutions of Al(NO₃) ₃ at the temperatures and for the times given in Table 5. One wire sample is exchanged in 0.1N Cr(NO₃)₃ solution for 6 hr at room temperature (23°C). The samples were dried and heated at 200°C for 15 minutes. Cut-through resistance was measured at 23 °C and at 200°C. Table 5 summarizes the results. Longer exchange times and higher exchange temperatures result in higher cut through resistance. Cut-through resistance is generally better at 200°C than at 23°C.

Table 5. Exchange Conditions and Cut-Through for Example 5

Exchange Cond. Cut-Through Resistance (lb)

Cation T (°C) t (min) 23°C 200°C

Al 23 10 13.3 26.2

60 18.3 30.1

360 18.0 37.7

Cr 23 360 21.6 29.0

Al 60 1 13.6 11.8

5 14.4 30.0

Al 100 1 17.0 33.3

5 15.9 40.0

Example 6

Two additional fluoropolymers having precursor ionic sites are used, both copolymers of TFE and PDMOF. "Ionic-2" has IXR of 13 (EW of 997) and "Ionic-3" has.IXR of 11.7 (EW of 930). Melt blends of Ionic- 1 or Ionic-3 with FEP in the proportions shown in Table 6 are prepared essentially by the procedure described above for blends of Ionic- 1, except that melt temperatures are 316°-347°C. The blend pellets are then extruded as film 5-6 mm wide and 0.001-0.0025 inch (0.025-0.064 mm) thick using the same extruder equipped with a film die and with melt temperature of 294°C. The extruded film is quenched on a casting drum heated internally with hot oil. The films are essentially clear with little or no light scattering, and so consist of a single phase. Films of unblended Ionic-2 are also prepared.

Sections of films are placed in large trays with a hydrolysis solution consisting of 20% KOH, 30% DMSO, and 50% water. Bubbles trapped under the films are removed by pressing on the middle of the films so that the air bubbles escape at the edge. After 3 days at room temperature, more hydrolysis solution is added and the films are soaked for 7 more days. The solution is drained off and the films are rinsed with water. Ionic sites in the films are in the potassium ion form at this point, and some specimens are air dried for use in this form. Other specimens are treated with 10% nitric acid for 3 days, and then the films are rinsed and soaked in water until the water is neutral. Ionic sites in the films are in the acid (hydrogen ion) form at this point, and some specimens are air dried for use in this form. Other specimens are converted to the barium ion (Ba^**) or the aluminum ion (Al^**^*) form by soaking for 3 min in 0.2N barium chloride solution or 0.2N aluminum nitrate solution, then washing and air drying.

ZST is measured as described above for films of fluoropolymer having ionic sites in various cation forms using heating rates in the range 7°-10°C/min except for two heating rates outside of this range. ZST (Table 6), is higher when the counterions are metal ions and increases with the valence of the counterion. This indicates that the tubular structure of the present invention comprising fluoropolymer having ionic sites will have improved mechanical properties, including strength and resistance to flow, when at least some of the counterions have a valence of 2 or greater.

Table 6. Compositions and ZST for Example 6

Blend Composition Film

Ionic Resin FEP Thick, ZST (°C) for Named Cation

Type (wt%) (wt%) (mm) H* K* Ba** Al***

1 81 19 0.025 216 >285 >302 —

2 100 — 0.051 170 — >302 >327

3 90 10 0.025 189 308 >300 —

3 75 25 0.064 179 — >320 >327

3 50 50 0.051 214 290 >300 >302

Exai tnple 7

To demonstrate the how preliminary testing may be done to determine the degree of ion exchange needed to obtain optimum cut-through resistance with the multivalent counterion under consideration, wire constructions were treated under various conditions with aluminum ion as summarized in Table 7. The samples are wire constructions similar to those of Example 1. Polymer thickness is 6 mils

(0.15 mm). It is seen that excessive ion exchange leads to a decline in cut-through resistance. That the low cut-through resistance is a result of excessive rather than insufficient multivalent ion exchange is determined by inspection of the cut material. Excessive ion exchange leads to cracking and shattering of the polymer at the cut. Conditions should be chosen to avoid excessive ion exchange. Table 7. Optimizing Exchange Conditions

Aluminum ion Exposure Temperature Cut-through

(normality) (seconds) °C Resistance (lbs)

R.T. 200°C

0.1 10 R.T. - 5.9

0.1 25 R.T. - 4.8

0.1 75 R.T. - 2.6

0.1 10 19 18.1 12.1

0.1 10 50 18.1 10.1

0.1 5 92 20.8 11.3

0.1 5 20 9.4 8.9

0.1 2 20 9.4 10.2

0.02 10 20 7.2 9.1

0.1 1800 20 1

Notes: Aluminum nitrate is the source of aluminum ion. R.T. = room temperature. Ionic-4 is similar in melt flow rate and IXR to Ionic- 1.

These examples show the superior cut-through resistance of tubular structures of this invention and the improvement of cut-through resistance when temperature is increased above room temperature, contrary to the usual experience with polymers. Furthermore, the beneficial effect of counterions of valence 2 or greater is shown.

Claims

WHAT IS CLAIMED IS:

1. A tubular structure comprising fluoropolymer having ionic sites and their accompanying counterions, or precursors thereto.

2. The tubular structure of claim 1, wherein said structure is tubing or electrical insulation surrounding an electrical conductor.

3. The tubular structure of claim 2, wherein said structure is said tubing and said ionic sites or precursors thereto are ionic sites and the counterions have a valence of 2 or greater.

4. The tubular structure of claim 3, wherein said valence is 2 or 3.

5. The tubular structure of claim 3, wherein said valence is 3 or 4.

6. The tubular structure of claim 2, wherein said structure is said electrical insulation surrounding an electrical conductor.

7. The tubular structure of claim 6, wherein said ionic sites or precursors thereto are said precursors.

8. The tubular structure of claim 6, wherein said ionic sites or precursors thereto are said ionic sites having counterions having valences equal to or greater than l.

9. The tubular structure of claim 8, wherein said valence of at least some of the counterions is 2, 3 or 4.

10. The tubular structure of claim 9, wherein said valence of at least some of the counterions is 3 or 4.

11. The tubular structure of claim 8, wherein said valence of at least some of the counterions is 2 or 3.

12. The tubular structure of claim 1, wherein said ionic sites or precursors thereto are ionic sites and at least some of the counterions are ionic dyes.

13. The tubular structure of claim 1, wherein said structure has at least two component layers, at least one of said component layers containing no ionic sites or precursors thereto, and at least another of said component layers comprising said fluoropolymer.

14. The tubular structure of claim 13, wherein at least one of said component layers is comprised of fluoropolymer.

15. The tubular structure of claim 1 , wherein said fluoropolymer has an ion exchange ratio of no more than 100.

16. The tubular structure of claim 1, comprising a blend of said fluoropolymer with fluoropolymer having no ionic sites or precursors thereto.

17. The tubular structure of claim 1 wherein said ionic sites are anionic sites.

18. The tubular structure of claim 17 wherein said ionic sites are carboxylate or sulfonate sites.

19. The tubular structure of claim 18 wherein said ionic sites are sulfonate sites.

20. The tubular structure of claim 16, wherein the amount of said fluoropolymer having ionic sites or precursors thereto in said blend is at least 20% by weight based on total weight of fluoropolymer in said blend.

21. A fluoropolymer blend, comprising a) fluoropolymer, and b) at least 20%) by weight, based on total fluoropolymer in said blend, of fluoropolymer having ionic sites and with at least some counterions having a valence of 2 or greater.

22. The fluoropolymer blend of claim 21, wherein said valence is 3 or 4.

23. The fluoropolymer blend of claim 22. wherein said fluoropolymer is tetrafluoroethylene copolymer.

24. The fluoropolymer blend of claim 23, wherein said tetrafluoroethylene copolymer contains units derived from at least one comonomer selected from ethylene, hexafluoropropylene. and perfluoro(alkyl vinyl ether) wherein said alkyl contains 1-3 carbon atoms.

25. A process for improving the cut-through resistance of a tubular structure comprising installing a tubular structure comprising fluoropolymer having ionic sites and their accompanying counterions in an enclosure, and maintaining a low humidity environment in said enclosure.

26. The process of claim 25 wherein said environment is an instrument cabinet, the interior volume of a tool, or an aircraft airframe.

27. A process comprising fabricating an article from fluoropolymer having ionic site precursors in an amount effective to increase the cut-through resistance of said fluoropolymer, converting said precursors to ionic sites with at least some counterions having valences of 2 or greater, and maintaining said article in a low humidity environment, thereby increasing the cut-through resistance of said article.

28. The process of claim 27 wherein the maintaining of the article in a low humidity environment is accomplished by dehumidification, the sealing of joints, seams, and openings, or by drying agents.

29. The process of claim 28, wherein said article is a wire construction having insulation comprising said fluoropolymer.