USRE28628E - Radiation treated poly(ethylene/chlorotrifluoroethylene) and poly(ethylene/tetrafluoroethylene) having improved high temperature properties - Google Patents

Abstract

The subjecting of poly(ethylene/tetrafluoroethylene) and poly(ethylene/chlorotrifluoroethylene) to a moderate amount of ionizing radiation has the effect of improving the tensile properties, especially ultimate elongation, of the copolymers at elevated temperatures. The amount of radiation required to obtain this improvement is minimized by following the radiation treatment with heat treatment of the copolymer.

Description

United States Patent 1191 Carlson et al.

111 E Re. 28,628

[ 51 Reissued Nov. 25, 1975 RADIATION TREATED POLY(ETHYLENE/CHLOROTRIFLUORO- ETHYLENE) AND POLY(ETI'IYLENE/TETRAFLUOROETHY- LENE) HAVING IMPROVED HIGH TEMPERATURE PROPERTIES [75] Inventors: Dana Peter Carlson, Parkersburg;

Norman Eugene West, Vienna, both of Va.

E. I. Du Pont de Nemours & Co., Wilmington, Del.

[22] Filed: Mar. 29, 1974 [211 App]. No.: 456,158

Related U.S. Patent Documents [73] Assignee:

Reissue of:

[64] Patent No.: 3,738,923

Issued: June 12, 1973 Appl. No.: 119,814 Filed: Mar. 1, 1971 U.S. Applications:

[63] Continuation-in-part of Ser. No. 4,395, Jan. 20, 1970, abandoned, which is a continuation-in-part of Ser. No. 777,172, Nov., 1968, abandoned.

(52] U.S. Cl. 204/1592; 204/159.l4; 260/634 A; 260/8076; 260/8077; 260/8078; 260/875 [51] Int. Cl. C08F 2/46; C08F 8/00 [58] Field of Search 204/1592; 260/875, 80.6;

OTHER PUBLICATIONS Paramagnetic Resonance Absorption by Radicals formed in Ethylene Tetrafluoroethylene copolymer, A Collection of Preparatory Manuscripts for Lectures 1, Physical Chemistry, Radiochemistry (March 1963), Ishii et al., p. 539.

Primary ExaminerRichard B. Turer [5 7] ABSTRACT The subjecting of poly(ethylene/tetrafluoroethylene) and poly(ethylene/chlorotrifluoroethylene) to a moderate amount of ionizing radiation has the effect of improving the tensile properties, especially ultimate elongation, of the copolymers at elevated temperatures. The amount of radiation required to obtain this improvement is minimized by following the radiation treatment with heat treatment of the copolymer.

14 Claims, No Drawings RADIATION TREATED POLY(ETHYLENE/CHLOROTRIFLUORO- ETIIYLENE) AND P()LY(ETIIYLENE/TETRAFLUOROETHYLENE) HAVING IMPROVED HIGH TEMPERATURE PROPERTIES Matter enclosed in heavy brackets 1 appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This application is a continuation-in-part of application Ser. No. 4,395, filed Jan, 20, i970, which is in turn a continuation-in-part of application Ser. No. 777,172, filed Nov. 19, 1968, both now abandoned, by the same inventors.

This invention relates to poly(ethylene/tetrafluoroethylene) and poly(ethylene/chlorotrifluoroethylene) and more particularly to such copolymer having improved solder-iron resistance and mechanical properties at high temperature.

Polytetrafluoroethylene has achieved notable success as a high temperature resistant wire coating. Apart from the high melting point of the polymer, one reason for this success is the high solder-iron resistance of the polymer. When a solder iron is used to connect terminals of the wire and the iron contacts the polymer coating, the coating does not flow away from the solder iron to leave the wire exposed. The use of polytetrafluoroethylene in this application, however, has the drawback of the polymer not being melt fabricable, which means the polymer has to be sintered after application to the wire. A high temperature-resistant dielectric copolymer which has adequate solder-iron resistance and which is also melt fabricable has not been found. For example, commercially available tetrafluoroethylene/- hexafluoropropylene copolymer, while it possesses all the other qualities desired for a high temperatureresistant wire coating, it melts and flows away from a solder iron to leave the wire exposed.

The copolymer of ethylene and tetrafluoroethylene has been known for some time (US. Pat. No. 2,468,664 to Hanford and Roland), but has not received any continued use in commerce because of certain ofits disadvantageous attributes. While the copolymer is known to have a melting point as high as 300C, it could not be used at high temperatures even far below this melting point in applications requiring strength, because of the deterioration of the mechani cal properties of the copolymer. For example, when used as a wire coating, the copolymer becomes brittle at 200 C., and cracks under low stress to leave the wire exposed. In a more quantitative sense, a copolymer (lzl mole ratio of monomers) which may have an ultimate elongation of over 300 percent at room temperature may have an ultimate elongation of less than 18 percent at 200 C.

Copolymers of ethylenelchlorotrifluoroethylene are disclosed in US. Patv No. 2,392,378 to Hanford, but such copolymers melt at temperatures below 200C. A procedure for preparing higher melting copolymers of these monomers is disclosed in European Polymer Journal, vol. 3, pages 129-144 (1967), but even these higher melting copolymers cannot be used at high temperature application since they suffer from the same disadvantage as the ethylene/tetrafluoroethylcnc co polymers. For example, a lzl mole ratio copolymer of ethylene/chlorotrifluoroethylcne melting at 235 C. has an ultimate elongation of greater than percent at room temperature but has an ultimate elongation of less than 32 percent at 200 C., making such copolymers useless as a wire coating intended for service at 200 C.

The present invention provides poly(ethylene/tetrafluoroethylene) and poly(ethylene/chlorotrifluoroethylene) which have both good mechanical, especially tensile, properties at high temperatures and high solderiron resistance, the latter being comparable to that of polytetrafluoroethylene, so as to make the copolymer especially suited for wire-coating applications at high temperatures. In terms of process, the present inven tion involves subjecting either copolymer to an effective amount of ionizing radiation conveniently at moderate temperatures. Another aspect of the process of this invention is to follow the ionizing radiation with heat-treatment of either copolymer, which improves the result obtained by the radiation or enables similar improvement to be obtained at a lower radiation dosage.

With respect to the ethylene and tetrafluoroethylene or chlorotrifluoroethylene content of each class of copolymer, from 40 to 60 mole percent of ethylene is present and, complementary to total [00 mole percent of ethylene plus either tetrafluoroethylene or chlorotrif'luoroethylene, from 40 to 60 mole percent of tetrafluoroethylene or chlorotrifluoroethylene is present. When either more or less tetrafluoroethylene or chlorotrifluoroethylene is present, the tensile properties and cut-through resistance of the copolymer become undesirably low. Description of the composition of the copolymers herein in terms of monomer content is intended to refer to the units making up the copolymer derived by copolymerization'of the monomers.

The effect of the radiation on the poly(ethylene/tetrafluoroethylene) and poly(ethylene/chlorotrifluoroethylene) is twofold, namely, to improve the tensile properties of the copolymer at high temperatures such as 200 C. and to increase the solder-iron resistance of the copolymer. A small amount, e.g. up to 10 mole percent but usually 0.1 to 10 mole percent based on ethylene plus tetrafluoroethylene or chlorotrifluoroethylene, of other copolymerizable monomer which is free of telogenic activity can be present in the copolymers irradiated according to the present invention, such monomer being of the type or in the amount present that does not significantly improve the high tempera ture tensile properties of the copolymer. The lack of significant improvement in such properties manifests itself by the resultant terpolymer having an ultimate elongation of less than 150 percent at 200 C.

By "copolymerizable is meant that the monomer must be able to form an integral part of the main copolymer chain and must not act as an inhibitor to prevent the copolymerization reaction from occurring. By free of telogenic activity" is meant that the monomer does not act as a chain transfer agent to an extent which undesirably limits the molecular weight of the copolymer. Examples of third monomers for terpoly mers in which the twofold improvement is obtained are the vinyl monomers having no more than one carbon atom in a side chain, such as hexafluoropropylene, isobutylene, and perfluoro(methyl vinyl ether). These monomers are to be distinguished from other copoly merizable monomers which in the proper amount improve the high temperature mechanical properties of the copolymer without irradiating. The latter monomers including the polyfluoroketones and the vinyl monomers having a substitutent containing at least two carbon atoms so as to provide a side chain of corresponding bulk in the terpolymer. Further description of the effect of irradiating the terpolymcrs containing these monomers is given in patent application Ser. No. 119,8 filed on the same data as the present application by the same inventors now abandoned but which subject matter is described in part in Ser. No. 459,288, filed April 9, 1974 by the same inventors.

The tetrafluoroethylenacontaining copolymers hereinbefore described can be prepared by the non-aqueous polymerization Ser. No. 679,l62, filed Oct. 30, 1967 by Carlson (now U.S. Pat. No. 3,528,954) which comprises bringing the two or more monomers being copolymerized together in a hydrochlorofluorocarbon solvent, commonly available as a "Freon" at a temperature from 30 to 85 C. and in the presence of poly merization initiator active at such temperature and thereafter recovering the copolymer.

The chlorotrifluoroethylene-containing polymers hereinbefore described are preferably prepared in a nonaqueous polymerization system by a process described in the aforementioned artile in the European Polymer Journal. Instead of the initiators disclosed therein, one can use a low temperature initiator such as trichloroacetyl peroxide. For the copolymer to have a melting point above 200 C., the polymerization temperature should be less than 20 C., and preferably less than l0 C. A good balance of properties (except for high temperature mechanical properties) is obtained at polymerization temperatures from -l0 to +l0 C.

Generally, both the tetrafluoroethylene-containing copolymers and the chlorotrifluoroethylene-comaining copolymers are composed essentially of ethylene units alternating with either tetrafluoroethylene or chlorotrifluoroethylene units on a 1:1 basis.

The ionizing radiation used in the present invention is of sufficiently high energy to penetrate the thickness of the copolymer being treated and produce ionization thereinv The ionizing radiation can consist of X-rays, gamma rays, or a beam of electrons, protons, deuterons, alpha-particles, beta-particles or the like, or combinations thereof. This radiation and suitable sources for its generation are disclosed in U.S. Pat. No. 3,l l6,226 to Bowersv Generally. the energy level ofthe radiation is at least 500,000 electon volts, and preferably from l to 2 mev; although any energy level can be used which penetrates the thickness of the polymer being irradiated under the atmospheric conditions employed.

For the tetrafluoroethylene-containing copolymers, the amount of radiation to which the copolymer is sub jected to be effective to obtain improved high tempera' ture tensile properties or solderiron resistance is generally from 2 to 80 megarads. Below the lower amount the improvement is not appreciable and above the upper amount, copolymer properties become adversely affected to an undesirable extent. Preferably. the copolymer is subjected to from to 30 megarads. For the chlorotrifluoroethylene-containing copolymers, the general range of radiation is from 12 to 50 megarads and preferably from 25 to 50 megarads.

The temperature of the copolymer being irradiated is not important, but is generally less than 60 C., with 4 ambient temperature, 2025 C. being most convenicnt. Usually, the irradiation is conducted with the copolymer contained in an inert atmosphere, however, the irradiation can be conducted in air with some sacrifice in efficiency in the effect of the radiation on the copolymer.

The copolymer can be irradiated by conventional methods, i.e. the copolymer is irradiated after fabrication into its final form, such as film, fiber, tube, or coating such as on a wire. The irradiation can be carried out by passing the fabricated copolymer at a constant rate through the field of radiation. For example, the copoly mer can be extruded onto a wire, cooled, and the resul tant coated wire subjected to irradiation. This wire is useful at temperatures as high as 240250 C. (for coatings in which the copolymer has a higher melting temperature than 250 C.) for short periods oftime and is useful for continuous service at 180 C. Twenty gauge l9 strand) wire coated with 8 to [2 mils of the copolymer and irradiated according to the present invention does not crack under the stress caused by wrapping the wire 180 around a inch mandrel and having 2-lb. weights attached to each downwardly extending end of the wire, this mandrel bend test being conducted for hours at 200 C. and even as high as 240-250 C. In contrast, a similarly coated, but not irradiated, wirc exhibits numerous cracks and separations ofthe coating from the wire when subjected to the same mandrel bend test at temperatures as low as C.

Irradiation has been used heretofore to improve mechanical properties of such polymers as polyethylene and polyvinylidene fluoride above the melting point of the polymer so as to increase the use temperature of the polymer. In contrast, the melting points of poly(ethylene/tetrafluoroethylene) and of poly(ethylene/- chlorotrifluoroethylene), in the compositional ranges given hereinbefore, are sufficiently high before irradiation, and radiation is given the unusual job of improving mechanical properties well below their respective melting point. The lzl poly(ethylene/tetrafluoroethylene) has a melting point of about 275 C. and the l:] poly(ethylene/chlorotrifluoroethylene) can be made to have a melting point up to about 265 C. However, compositions of these copolymers in which melting points are as low as 220 C. have useful mechanical properties at 200 C. when irradiated according to the present invention.

Normally, radiation has the effect of decreasing the high temperature elongation of the polymer being irradialed (U.S. Pat. No. 3,142,629 to Timmerman). Unexpectedly, the elongations of poly(ethylene/tetrafluoroethylene) and poly(ethylene/chlorotrifluoroethylene) at 200 C. are greatly increased (the same is true for the terpolymers wherein the vinyl monomer has a side chain of no more than one carbon atom). Irradiation of polytetrafluoroethylene and polychlorotrifluoroethylene is known to degrade the polymer and sharply reduce its melting point (British Patent 768,554 and Nature, 172,76-77 (1953), respectively). An exception to this effect on fluorocarbon polymers is disclosed in US. Patent 3,l l6,226 to Bowers, wherein degradation is reported to be avoided by irradiation of fluorocarbon copolymers at temperatures above the glass transition temperature of the copolymer, eg, at least I50 C. Unexpectedly, beneficial results are obtained by radiation well below the glass transition temperature of the copolymers being treated. Even though these copolymers contain as much as about 85 percent by weight of tctraf'luoroethylene or chlorotrifluoroethylene. little effect of the radiation on melting point is obtained. In addition. the improvement obtained by radiation in the present invention is temperature stable. i.e., the improvements in tensile properties and solderiron resistance are retained even after prolonged exposure of the copolymer to high temperatures.

With respect to the heat treatment aspect of the present invention, the copolymer can be heated after being subjected to radiation to either improve the improvement obtained by irradiation, i.e. in mechanical properties and/or solder iron resistance, or lower the amount or dosage of radiation required to obtain a certain level of improvement. The heat treatment is generally practiccd by heating the copolymer for 30 seconds to 20 minutes or more depending on the improvement desired, at a temperature of at least 150 C. Somewhat lower heat treatment temperatures can be used but the time required to obtain a significant effect is undesirably prolonged. The copolymer is not heated to so high a temperature that the copolymer will flow for the particular amount of time of heating being used. Generally, the compolymer will not be heated above 250 C. The heat treatment is conducted in the substantial absenee of oxygen such as present in the atmosphere. This is accomplished by conducting the heating in an inert atmosphere or by having the heat time in air short enough that oxygen penetration into the copolymer is negligible. If heat treatment is omitted, it may occur in later high-temperature service of the copolymer in the presence of oxygen for an extended period of time, which tends to diminish the improvement obtained by the radiation. The preferred radiation dosage when heat treatment is used is from 5 to megarads. The radiation dosage required for the degree of improvement desired can also be reduced by incorporating a small amount of a crosslinking promoter such as triallylcyanurate into the copolymer prior to radiation.

The following examples are illustrative of the present invention and are not intended as a limitation on the scope thereof. Parts and percents are by weight unless otherwise indicated.

EXAMPLE I A tetrafluoroethylene/ethylene copolymer which contained 48.9 mole percent tetrafluoroethylene and had a melt viscosity at 300 C. of 5.55X10 poises was pressed into 4-inch X 4-inch films of 10 mils and 40 mils thickness by compression molding at 310 C. and quenching in ice water. One film of each thickness was placed on the water cooled table below the window of the electron beam unit. The films were enclosed in a small box covered by thin aluminum foil and kept under a purge of argon gas.

The electron source was a General Electric 2000-kvp resonant transformer operating at I ma. beam current. The dose rate from this source at 30 cm. distance was 0.115 megarad per second for the 10 mil films and 0.172 megarad per second for the 40 mil films.

The films were given exposures of 5, 25, 50, 250 and 500 seconds under the beam at room temperature. The 40-mil films were cut up into small pieces and their melt viscosity determined at 300 C. The IO-mil films were cut into strips k inch X 3 inches and the MIT flex life determined using 1.5 kg. load and flexing at the rate of about 10,000 flexes per hour. The results are shown below:

The polymers were cross-linked as indicated by the melt viscosity data. The flex life decreased after extensive exposure. but the polymer had usable properties even after 500 seconds exposure (about megarads).

EXAMPLE 2 The same copolymer as described in Example 1 was pressed into 4-inch 4-inch films of 7-8 mils thickness by compression molding at 310 C. and cooling under pressure to 200 C. before quenching in ice water. The films were irradiated under the conditions described in Example l for exposures of 50, 100, 250, 500 and 1000 seconds. These exposures corresponded to radiation doses of 7.8, 15.5, 39.0, 78 and 155 megarads. respectively. The tensile properties of the films were determined at 200 C. and are shown below:

Tensile Ultimate strength elongation, Exposure(megarads) (p.s.i.) percent The tensile properties were considerably improved by the irradiation, as indicated by the above data. There appeared to be an optimum dose of radiation (between 15 and 39 megarads) to achieve the maximum improvement in properties of the copolymer. However, even samples which received much higher doses of radiation (e.g., 155 megarads) had better tensile properties at 200 C. than the unirradiated material.

EXAMPLE 3 A tetrafluoroethylene/ethylene copolymer which contained 50.6 moles percent TFE and had a melt viscosity of 0.89Xl0 poises at 300 C. was extruded onto a 7-strand, AWG 22 silver-coated copper wire to form a coating about 10 mils thick. The wire coating was carried out with a one-inch Killion extruder operating at a scrw speed of about 15 rpm. and with a heated barrel to give a melt temperature of about 315 C. A spidertype drawdown die with 0.188 inch OD and 0.101 inch ID was used. The wire was drawn through the die at about ftjmin. Immediately after coating the wire with the melt. it was passed through a water quench bath.

A sample of the coated wire was twisted around itself several times and then placed in a 200 C. air oven. The wire insulation cracked in several places after exposure to the air oven overnight.

Several lengths of the coated wire were exposed to the electron beam of the resonant transformer. The samples were exposed under the conditions described in Example 1 for 100 seconds. The total dose of radiation was about 15 megarads. Samples of the irradiated wire were twisted on themselves and placed in the air oven at 200 C. The irradiated samples did not crack after exposure to the oven overnight but rather survived over 3000 hours under these conditions without cracking.

EXAMPLE 4 In contrast to the two-component polymers of the foregoing examples and to illustrate the effect ofirradiation on terpolymers in which the third monomer was a vinyl monomer having a side chain of at least two carbons. a tetrafluoroethylene/ethylene/perfluoropropyl perfluorovinyl ether terpolymer was prepared which contained about 52 mole percent TFE, 46.5 mole percent ethylene and the remainder vinyl ether, and had a melt viscosity at 300 C of 32 l0 poises and melting point of 254 C. was pressed into 4-inch 1-inch films of 7-8 mil thickness by the conditions of Example 2. The films were irradiated by the same procedure cited in Example 1 for exposure of 50 to 500 seconds. This corresponded to doses in the range of 7.8 to 78 megarads.

A pencil-type, constant temperature solder iron rated at 25 watts was heated up for 15 minutes. Samples of nonirradiated and irradiated terpolymer films were placed on a piece of aluminum foil. The solder iron was pressed down on each sample of film. The nonirradiated sample readily flowed out and away from the solder iron. The irradiated samples were dented and deformed by the solder iron but did not flow away from Films of the copolymer of this example exhibited the following tensile properties at 200 C.

This example shows that even without irradiation, the terpolymer has good tensile properties at high temperature. but that the improvement in solder-iron resistance is still obtained with the irradiation.

EXAMPLE Several 4-inch X 4-inch films. 8 to 10 mils in thickness. were prepared from the same copolymer and under the same conditions as described in Example 2. Two films each were exposed for 25. 50 and 100 sec onds. respectively. in the resonant transformer as in Example l. One set of exposed films was stored in air was done in the other examples. The other set of films was stored under an N atmosphere in a glass tube. The glass tube containing the films was placed in a 200 C. oven for [5 minutes. At the end of this time. the films were removed from the tube and the tensile properties were compared to those of the untreated films. The test results are shown in the following table:

Tensile datu.200C.

Treated at 200C.

The tensile properties of the post-treated samples were improved over the irradiated, but not post-treated samples.

EXAMPLE 6 The solder-iron resistance of insulated wires was determined by measuring the time it takes the solder iron, supported at a 45 angle to the wire. to make electrical contact to the wire. The tip-temperature of the solder iron was controlled at either 357 C. or 419 C. for these tests. Weights were attached to the tip to put pressure on the wire. These weights ranged from k to 1 pound (tip weighed 1% 1b.).

A copolymer of ethylene and tetrafluoroethylene which contained 53 mole percent of TFE and which had a melt viscosity of 4.3X10" poises at 300 C. was melt extruded onto a l9-strand AWG 20 tin plated copper wire to a thickness of 9.8 mils. The coated wire was then irradiated at doses ranging from about 3 to 12 megarads. The wire was then heated at C. for about 45 seconds under N after the irradiation.

The table shows the effect of the irradiation on the solder-iron resistance.

A chlorotrifluoroethylene/ethylene copolymer which contained 49 mole percent chlorotrifluoroethylene and had a melting point (DTA peak) of 235 C. was extruded onto a 7-strand. AWG 22 silver-coated copper wire to form a coating about 0.025 cm. thick. Short lengths of this coated wire were placed on the watercooled table below the window of the electron beam unit. The insulated wire samples were enclosed in a small box covered by thin aluminum foil and kept under a purge of nitrogen gas.

The electron source was a General Electric 2000-kvp resonant transformer operating at 0.5 ma. beam current. The dose rate from this source at 30 cm. distance was 0.078 magarad per second for the 0.025 cm. wire insulation.

The samples were given exposure of 38, 115, 154. 192. 320 and 640 seconds under the beam at room temperature. These exposures corresponded to radiation dosages of 3. 9, 12. 15,25, and 50 Mrads. respectively. The samples were heat treated for 20 minutes at 160 C. under a nitrogen atmosphere.

Solder-iron Exposure temperature (megarads) ("C.) Time to failure 3 400 Less than 3 sec. 9 400 Do.

l2 400 [3.8 sec. 15 400 L32 sec. 25 400 Greater than l mini 50 400 Do.

EXAMPLE 8 Extruded film (0.0l50.0l8 cm. in thickness) of the same copolymer described in Example 7 was irradiated under the conditions described in Example 7. The film was irradiated for exposures of 159 and 327 seconds. These exposures correspond to radiation doses of 12 and 25 Mrads, respectively. The ultimate elongations were determined at 200 C. and are shown in the following table:

Tensile properties at 200C.

Ultimate Ultimate Exposure strength, elongation, (megarads) kg./cm. percent 0 2i 1 23 25 I [.7 5 B0 The ultimate elongation at 200 C. was considerably improved by irradiation of 25 Mrads. The more than 4-fold increase in ultimate elongation more than counterbalances the decrease in ultimate strength in terms of value in use, especially as a wire coating.

EXAMPLE 9 A sample of the copolymer powder described in Example 7 was allowed to soak in a 1% solution of triallyl cyanurate in Fre0n 113 for l6 hours. The polymer slurry was placed in an air circulating oven at l25 C. for one hour. The dried mixture was compression molded at 250 C. An untreated sample of polymer was also compression molded under the same conditions.

The 0.0l 3 cm. films which were obtained were irradi ated as described in Example 7. The film samples were given exposures of 79 and 159 seconds. These expo sures corresponded to radiation dosages of 6 and l2 Mrads. respectively. The samples were heat treated for mins. at 160 C. The tensile properties of the films were determined at 200 C. and are shown below:

Tensile properties at 200C Ultimate Ultimate Exposure strength. elongation. Sample [megarads) kgjcm. percent E/CTFE 0 16.5 33 EICTFE+TAC 6 9.l 84 E/CTFE-i-TAC 12 8.8

NOTE. E/CTFE ethylene/chlormrifluoroethylene copolymer TAC triallyl cyanurate.

The addition of trially cyanurate improves the efficiency of irradiation as evidenced by improved ultimate elongation a low radiation dosage.

EXAMPLE l0 The copolymer used in this example was the same as the poly(tetrafluoroethylene/ethylene) used in Example 1 except that the copolymer was composed of about 52 mole percent tetrafluoroethylene and the remainder ethylene, and the copolymer had a melt viscosity of 4.9Xl0 poises. Film samples were prepared following the procedure of Example 2 and these samples were exposed to radiation using the resonant transformer described in Example l to obtain the results described in the following table.

Tensile properties at Ultimate Irradiation Yield Ultimate elonga Exposure temperature strength strength tion.

(megarads) (C.) [p.si.) (p.s.i.) percent 0 347 347 12 7 Room temperature 54l 840 545 7 ISO-I98" 541 8l3 42l 7 -600 436 816 392 l0 220-245 471 70l 340 These samples were irradiated on a water cooled plate which maintained the sample temperature at room temperature.

After irradiation, the samples were heat-treated at I62 C. in a nitrogen atmosphere for 20 minutes.

"These samples were irradiated on a hot plate which heated the samples to the lowest temperature ofthe ranges. subsequent irradiation caused the temperature to rise and the highest temperature of the range was the temperature at the time irradiation was stopped.

These results show improved high temperature me chanical properties at both low and high temperatures of irradiation. The best results, however, occurred using room temperature irradiation followed by heat treatment.

The high temperatures of irradiation were above the glass transition temperature of the copolymer which had a glass transition temperature of about 1 10 C. The glass transition temperature of the copolymer was measured by increasing the temperature of the copolymer and at the same time measuring its internal friction by means of a torsion pendulum operating at a frequency of one cycle per second. The glass transition temperature is taken as the highest temperature of any transition below the melting point which can also be called the alpha-relaxation temperature. Further description of glass transition temperature and procedure for de termining same is disclosed in N. G. McCrum, B. E. Read, and G. Williams, Analastic and Dielectric Effects in Polymeric Solids," Wiley and Sons, New York ([957) pages 192-195.

EXAMPLE 11 Tensile properties at Ultimate Ultimate Exposure Irradiation strength elongation, (megarads) temperuturetC.) (psi) percent 330 54 25 125-155 331 200 25 Room temperature 212 539 Irradiation was followed by heat treatment at 160C for minutes The melting points given in this specification are determined by differential thermal analysis using a heating rate of C. per minute and the minimum point (DTA peak) on the curve as the melting point.

The values for ultimate elongation disclosed herein are determined as follows:

Four bars are cut from film 10 mils thick (unless oth erwisc specified) of the polymer being tested with microtensile die as described in ASTM D 1708. The films are prepared by compression molding at 310 C. for the tetrafluoroethyletie-containing polymer and 260 C. for the chlorotrifluoroethylene-containing polymer. followed by quenching in ice water. The tensile test ma chine conforms to specifications in ASTM D 638 and is fitted with an insulated test chamber which is main tained at 200 11 C. with heated air. The ultimate elongation is determined by the procedure described in ASTM D 638. except that the test specimens are obtained from the film described above. The initial jaw separation of the test machine is 22.21103 mm. and the crosshead speed is 5.1 cm. per minute. Elongation at break (ultimate elongation) is determined from the recording chart by dropping a perpendicular line from the break point ofthe curve on the chart. The distance in cm. from the perpendicular line from the beginning of the load time curve is read from the chart. and this distance times 18.1 is the ultimate elongation in per cent.

The melt viscosities disclosed herein unless otherwise specified are determined in the same manner as disclosed in U.S. Pat. No. 2.946.763 except that the conversion factor is 32,000 instead of 53.150 and for the tetrafluoroethylene-containing copolymer (and ter polymer). the melt temperature is 300 C. and for the chlorotrif'luoroethylene-containing copolymer (including terpolymer) the melt temperature is 260 C.

Freon" 113 is 1.1.2trichloro 1.2.2-triflu0roethane.

As many apparently widely different embodiments of this invention maybe made without departing from the spirit and scope thereof. it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

What is claimed is:

l. A process for [improving the high temperature tensile properties] increasing the ultimate elongation of poly(ethylene/tctrafluoroethylene) comprising subjecting the copolymer I: to an effective amount of ionizing radiation, after meltfabrication to an amount of ionizing radiation effective to increase the ultimate elon' gation of the copolymer when measured at 200C. the ethylene content of said copolymer being from 40 to 60 mole percent and the tetrafluoroethylene content of said copolymer being from 40 to 60 mole percent, based on the ethylene plus tetrafluoroethylene content of the copolymer.

2. The process of claim 1 and additionally heat-treating the copolymer. following the subjecting step.

3. The process of claim 1 wherein the subjecting step is carried out at a temperature less than 60 C.

4. The process of claim 1 wherein the content of ethylene and tetrafluoroethylene is from 45 to 55 mole percent for each.

5. The process of claim 1 wherein said radiation is at a dosage of from 2 to megarads and at a temperature below the glass transition temperature of said copolymer.

6. The irradiated [copolymer] product of claim 1.

7. A process for [improving the high temperature tensile properties] increasing the ultimate elongation of a copolymer selected from the group consisting of poly(ethyleneltetrafluoroethylene) or poly(ethylene/- chlorotrifluoroethylene). comprising subjecting the co polymer [to an effective amount of ionizing radiation,] after melt fabrication to an amount of ionizing radiation effective to increase the ultimate elongation of the copolymer when measured at 200 C. the ethylene content of said copolymer being from 40 to 60 mole percent and the tetrafluoroethylene or chlorotrifluoro' ethylene content of said copolymer being from 40 to 60 mole percent, based on the ethylene plus tetrafluoroethylene 0r chlorotrifluoroethylene content of the copolymer.

8. The process of claim 7 wherein said copolymer is poly(ethylenelchlorotrifluoroethylene).

9. The process of claim 8 wherein the content of ethylene and chlorotrifluoroethylene is from 45 to 55 mole percent for each.

10. The process of claim 8 and additionally heat treating the copolymer. following the subjecting step.

11. The irradiated I: copolymer] product of claim 8.

[12. A process for increasing the ultimate elongation at 200 C. of a copolymer selected from the group consisting of po1y(ethylene/tetrafluoroethylene) and poly(ethylene/ch]orotrifluoroethylene). comprising subjecting the copolymer to an ultimate elongation increasing amount of ionizing radiation. the ethylene content of said copolymer being from 40 to 60 mole percent and the tetrafluoroethylene or chlorotrifluoroethylene content of said copolymer being from 40 to 60 mole percent. based on the ethylene plus tetrafluoroethylene or chlorotrifluoroethylene content of the copolymer] 13. The process of claim I: 12] 7 wherein each said copolymer has a melting point of at least 220 C.

14. The process ofclaim 8 wherein said irradiation is at a dosage offrom 12 to 50 megarads.

15. The process ofclaim 8 wherein said irradiation is at a dosage offrom 25 to 50 megarads.

Claims (14)

1. A process for (improving the high temperature tensile properties) increasing the ultimate elongation of poly(ethylene/tetrafluoroethylene) comprising subjecting the copolymer (to an effective amount of ionizing radiation,) , after melt fabrication to an amount of ionizing radiation effective to increase the ultimate elongation of the copolymer when measured at 200*C. the ethylene content of said copolymer being from 40 to 60 mole percent and the tetrafluoroethylene content of said copolymer being from 40 to 60 mole percent, based on the ethylene plus tetrafluoroethylene content of the copolymer.

2. The process of claim 1 and additionally heat-treating the copolymer, following the subjecting step.

3. The process of claim 1 wherein the subjecting step is carried out at a temperature less than 60* C.

4. The process of claim 1 wherein the content of ethylene and tetrafluoroethylene is from 45 to 55 mole percent for each.

5. The process of claim 1 wherein said radiation is at a dosage of from 2 to 80 megarads and at a temperature below the glass transition temperature of said copolymer.

6. The irradiated (copolymer) product of claim 1.

7. A PROCESS FOR (IMPROVING THE HIGH TEMPERATURE TENSILE PROPERTIES) INCREASING THE ULTIMATE ELONGATION OF A COPOLYMER SELCTED FROM THE GROUP CONSISTING OF POLY(ETHYLENE/TETRAFLUOROETHYLENE) OR POLY(ETHYLENE/CHLOROTRIFLUOROETHYLENE), COMPRISING SUBJECTING THE COPOLYMER (TO AN EFFECT AMOUNT OF IONIZING RADIATION) AFTER MELT FABRICATION TO AN AMOUNT OF IONIZING RADIATION EFFECTIVE TO INCREASE THE ULTIMATE ELONGATION OF THE COPOLYMER WHEN MEASURED AR 200*C THE ETHYLENE CONTENT OF SAID COPOLYMER BEING FROM 40 TO 60 MOLE PERCENT AND THE TETRAFLUOROETHYLENE OR CHLOROTRIFLUOROETHYLENE CONTENT OF SAID COPOLYMER BEING FROM 40 TO 60 MOLE PERCENT BASED ON THE ETHYLENE PLUS TETRAFLUOROETHYLENE OR CHLOROTRIFLUOROETHYLENE CONTENT OF THE COPOLYMER.

8. The process of claim 7 wherein said copolymer is poly(ethylene/chlorotrifluoroethylene).

9. The process of claim 8 wherein the content of ethylene and chlorotrifluoroethylene is from 45 to 55 mole percent for each.

10. The process of claim 8 and additionally heat treating the copolymer, following the subjecting step.

11. The irradiated (copolymer) product of claim 8.

13. The process of claim (12) 7 wherein each said copolymer has a melting point of at least 220* C.

14. The process of claim 8 wherein said irradiation is at a dosage of from 12 to 50 megarads.

15. The process of claim 8 wherein said irradiation is at a dosage of from 25 to 50 megarads.