RADIATION CURING OF PERFLUOROELASTOMERS

Abstract: Perfluoroelastomers cured by radiation and characterized by an absence of curing agent residues in the cured product. The elastomers are terpolymers derived from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether with alkyl of 1 to 5 carbon atoms, and up to 2 mole percent of a cure site monomer of vinylidene fluoride, 4-bromo-3,3,4,4-tetrafluorobutene-1, or 8-cyano-5-methyl-3,6-dioxa-1-octene. The polymers can be formed into films or a variety of shaped articles.

Description: TITLE RADIATION CURING OF PERFLUOROELASTOMERS Background of the Invention Perfluoroelastomers have long been used in a variety of applications that require excellent resistance to high temperatures, oxidative and chemical attack. One particularly outstanding fluoropolymer that has been used in elastomeric applications is that prepared from tetrafluoroethylene (TFE) and perfluoro (methyl vinyl) ether (PMVE) . To permit the crosslinking in these copolymers that is essential to good elastomeric properties,, a small percentage of termonomer is incorporated, as described, for example, in Finlay, U.S. Patent 4,529,784, Apotheker et al., U.S. Patent 4,035,565, and Breazeale, U.S. Patent 4,281,092. In the past, such perfluoroelastomers have been cured by compounding with the dipotassium salt of bis-phenol AF (K2AF) or peroxide curing agents, followed by treatment with elevated heat and/or pressure to cure the polymer. While these techniques have produced excellent cured products, it would be desireable to have a similar cured product without the presence of the curing agent residues, which can exude during subsequent processing or use. Fluoropolymers have, in the past, been treated with ionizing radiation to promote curing, as described, for example in U.S. Patents 3,223,689, 3,513,043, 4,256,856, and 4,565,614. However, such techniques have previously been limited to those compositions which were not perfluorinated. It was previously concluded that radiation would tend to degrade a perfluoropolymer without effective curing. Summary of the Invention The present invention is based on the discovery that certain perfluoroelastomers can. be cured with radiation alone, without the compounding with curing agents that was previously thought necessary. Specifically, the present invention provides a process for curing a terpolymer derived from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether wherein the alkyl group contains 1 to 5 carbon atoms, and up to about 2 mole percent of cure site monomer, which process comprises exposing the terpolymer to about from 2 to 20 megarads of radiation, in the substantial absence of curing agent. The present invention further provides cured perfluoroelastomers resulting from the above process, the perfluoroelastomers being substantially free from curing agent residue. Detailed Description of the Invention The present invention relates to fully fluorinated polymers, such as those prepared from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether in the which the alkyl group contains from 1 to 5 carbon atoms, and up to about two mole percent of cure site or crosslinking units. A wide variety of such crosslinking units can be used, including, for example, cyano-substituted perfluorovinyl ethers as described in Breazeale, U.S. Patent 4,328,092; bromine-containing olefins as described in Apotheker et al., U.S. Patent 4,035,565; and the non-perfluorinated cure-site monomers described in Finlay, U.S. Patent 4,529,784, all of which are hereby incorporated by reference. In general, the cure-site monomers are present in an amount sufficient to provide at least about 0.1 mole percent of each of hydrogen or perfluorophenyl and bromine or iodine in the resulting terpolymer. The hydrogen or perfluorophenyl and bromine or iodine functionalities can be part of the same ethylenically unsaturated monomer or can be provided by different ethylenically unsaturated monomers. Accordingly, the cure site monomer is generally selected from ethylenically unsaturated compounds having (i) hydrogen, nitrile or perfluorophenyl and (ii) bromo- or iodo- substituents, the units being present in an amount sufficient to provide at least about 0.1 mole percent each of hydrogen, perfluorophenyl or nitrile and bromine or iodine in the resulting terpolymer, ~~ Blends of perfluoroelastomers can also be used in the present invention, including, for example, those described in Aufdermarsh, U.S—Patent 4,413,094, also hereby incorporated by reference. The present invention is based on the discovery that these perfluoropolymers can be cured without the curing agents that have been used in the past, but by exposure to radiation in an amount of about from 2 to 20 megarads. Exposure to less than about 2 megarads does not provide reliable elastomer performance, while little additional benefit is obtained with more than about 20 megarads. Dosages of about from 6 to 10 megarads have been found to be particularly satisfactory. The radiation can be applied in a single dose or in multiple applications. Multiple applications can minimize heat buildup in the polymer being treated. The radiation can be applied to the perfluoroelastomers by a radiation source which has the following characteristics: 1. A high voltage power supply that generates DC power" and creates the acceleration potential for the electrons. 2. An accelerator to raise the electrons to their full potential energy. 3. A scanning system which uniformly distributes the accelerated beams of electrons over the required area. The exposure to the radiation can be for a period of a few seconds to several minutes, depending on the intensity of the radiation source. A wide variety of commercially available radiation sources can be used, including, for example, high energy Van de Graaf electron beam accelerators, such as that commercially available from High Voltage Engineering as Model K-S. The curing can be carried out at ambient pressure and temperatures of about from 20 to 40 °C. The polymers used in the present invention should be formed into their desired final configuration prior to curing, as with typical elastomers and curing processes. If, for example, the polymer is formed into a film, the film will vary in thickness considerably, depending on the intended use. However, in general, the perfluoroelastomer films will exhibit a thickness of about from 0.05 to 2.5 mm. In the alternative, the polymers can be formed into a variety of shaped articles, as will be evident to those skilled in the art. The cured perfluoroelastomers exhibit excellent tensile properties, and are substantially free from curing agent residue; that is, the cured products prefereably have less than 500 parts per million, and especially less than 100 parts per million, of such residue. Accordingly, the cured compositions provide substantially chemical free perfluoroelasto eric parts, which have heretofore been unavailable using previously known curing techniques. The polymers can also contain one or more of the additives known to be useful in fluoropolymer compositions, such as pigments, fillers, pore-forming agents and plasticizers. The present invention is further illustrated by the following specific examples, in which parts and percentages are by weight unless otherwise indicated. Examples 1-18 In Examples 1-18, perfluoroelastomer compositions were prepared from 54 to 58 weight % tetrafluoroethylene, 40 to 44 weight % perfluoro (methylvinyl ether) and a cure site monomer. The cure site monomers used and their concentrations were as follows: Examples 1-3 - 1.1 weight % 4-bromo-3,3,4,4-tetrafluorobutene Examples 7-9 - 0.34 weight % vinylidene fluoride Examples 13-15 - 2.2 weight_% perfluoro-(8-cyano-5-methyl-3,6-dioxa-l- octene) In Examples 4-6, a blend of the perfluoroelastomer of Examples 1-3 and 15% of a melt processable TFE copolymer ("Teflon" PFA fluoropolymer) was used. In Examples 10-12, a blend of the perfluoroelastomer of Examples 7-9 and 15% "Teflon" PFA fluoropolymer was used. In Examples 16-18, a blend of the ------ perfluoroelastomer of Examples 13-15 and 15% "Teflon" PFA fluoropolymer was used. In each example, the perfluoroelastomer was formed into an 0-ring and then cured by exposure to radiation, using a 3 MEV electron beam Van de Graaf accelerator. The radiation dosages used varied, as summarized in Table I. The resulting cured O-rings were evaluated for their tensile properties, including Stress at 100% Strain, Tensile Streng t h at Break and Elongation at Break, using ASTM Method D412, and the results are reported in Table I. The O-rings were post cured in an oven at 232°C for a period of 16 hours. Control samples that had not been irradiated did not survive the post cure treatment without distortion. Each sample, if tested, would exhibit less than 100 ppm of curing agent residue. 5. These results indicate that cured perfluoroelastomers can be obtained using radiation alone, providing finished products without the curing agent residue that previously was present when using conventional curing agents. 0 TABLE I Example 1-3 2 - 4-6 2 7-92 10-122 13-15 16-18 8 MRADS - 5 M100 310 540 230(235) 385(380) 295(245) 490(455) TB 540 850 500(425) 430(675) 650(590) 990(950) EB 205 220 295(470) 150(320) 210(265) 230(280) 12 MRADS M100 310 550 230(240) 420(375) 300(240) 555(480) TB 470 830 440(510) 540(700) 605(595) 950(715) 0 EB 174 202 225(492) 145(360) 184(262) 185(195) 16 MRADS M100 335 550 250(215) 450(270) 300(270) 580(485) TB _ 455 710 485(440) 900(640) 605(560) 910(820) EB 153 150 200(420) 210(325) 173(235) 168(220) 5 _ ■ The values in parentheses are post-cured at 232C/16 hrs under nitrogen. 2. Did not survive post-cure. Borderline in surviving post-cure. Examples 19-42 and Control Examples A-G 0 In Examples 19-42 and Control Examples A-G, perfluoroelastomer compositions were prepared from the same weight percentages of tetrafluoroethylene and perfluoro (methylvinyl ether) as in Examples 1-18 along with a cure site monomer. The cure site monomers used 35 and their concentrations were as follows: Examples 19-24 and Controls A-B - 2.2 wt % perfluoro-(8-cyano-5-methyl-3,6-dioxa-l- octene) Examples 25-30 and Control C - 0.35 weight % vinylidene fluoride Examples- 31-36 and Controls D-E - 1.1 wt % 4-bromo-3,3,4,4-tetrafluorobutene These fluoropolymers were made in an aqueous emulsion using ammonium persulfate as an initiator. In Examples 37-42 and Controls F-G, a blend of 92% of the perfluoropolymer of Examples 27-30 and 8% of the melt processable TFE copolymer was used. In some of the compositions, as indicated in Table II, 2.0 parts per hundred of the resin of tri ethyl allylisocyanurate radical trap (TMAIC) were added to the perfluoroelastomer. The perfluoropolymers of Examples 19-24 and Control Examples A-B contained 12 phr SAF reinforcing agent and 0.25 phr 18-Crown-6 as a processing aid. The perfluorpolymers of Examples 25-42 and Control Examples C-G contained 10 phr SAF black reinforcing agent and 0.25 phr 18-Crown-& as a processing aid. The perfluoropolymers of Examples 31-36 and Control Examples D-E also contained 3.0 phr of lead oxide. in each example, the perfluoropolymer was compounded into a dumbbell and then cured by exposure to radiation as described above. The radiation dosages used for the Control Examples A-G were 4 megarads, and varied for Examples 19-42 from 8 to 16 megarads, as summarized in Table II. The compounds were all post cured at 232°C for 16 hours under Nitrogen. The resulting elastomers were evaluated for tensile properties as in Examples 1-18, and the results are reported in Table II. TABLE II Example A and B and C and D and E and F and G and 19-21 22-24 25-27 28-30 31 ^ 33 34-36 37-39 40-42 TMAIC,phr — 2.0 2.0 2.0 2.0 4 MRADS M100 694 730 365 406 701 785 763 TB 2094 1900 433 437 1306 1156 1270 EB 242 254 694 523 323 592 514 8 MRADS M100 934 946 490 414 399 779 765 812 TB 2027 1746 841 1246 509 1630 1397 1758 EB 164 163 624 390 471 217 449 312 119 1295 515 496 428 870 739 941 809 1666 1189 1126 823 1198 1483 1748 133 119 401 217 364 134 316 204 M100 1209 498 649 448 1107 744 1237 TB 1585 1659 1115 1335 990 1291 1375 1783 EB 118 91 306 170 280 112 245 140 Examples 43-60 and Control Examples H-M In Examples 43-60 and Control Examples H-M, the general procedure of Examples 1-18 was repeated, using the unblended polymers. Accordingly, the perfluoropolymer of Examples 1-3 was used in Examples 43-48 and Control Examples H and I; the perfluoropolymer of Examples 7-9 was used in Examples 49-54 and Control Examples J and K; and the perfluoropolymer of Examples 13-15 was used in Examples 55-60 and Control Examples L and M. 2 phr TMAIC were added as indicated in Table III. All compounds contained 10 phr SAF black reinforcing agent and 0.25 phr 18-Crown-6 as a processing aid. In each Example and Control Example, the perfluoroelastomer was formed into an O-ring and then cured by exposure to radiation, followed by post-cure at 232°C for 16 hours. The cured perfluoroelastomers were evaluated for compression set resistance, and the " results summarized in Table III. TABLE III Example Cntrl H Cntrl I Cntrl J Cntrl K Cntrl L Cntrl N & 43-45 & 46-48 & 49-51 & 52-54 & 55-57 & 58-60 TMAIC,2PHR None Yes None Yes None Yes Compression Set Resistance 200C/70hrs 4 Mrads 100 8 Mrads 96 12 Mrads 86 16 Mrads 60

Claims: I Claim: 1. A process for curing a perfluoropolymer derived from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether wherein the alkyl group contains 1 to 5 carbon atoms, and up to about 2 mole percent of cure site monomer, which process comprises exposing the perfluoropolymer to about from 2 to 20 megarads of radiation, in the substantial absence of curing agent. 2. A process of Claim 1 wherein the perfluoropolymer is exposed to about from 6 to 10 megarads of radiation. 3. A process of Claim 1 wherein the cure site monomer units are derived from ethylenically unsaturated compounds having (i) hydrogen, nitrile or perfluorophenyl and (ii) bromo- or iodo- substituents, the units being present in an amount sufficient to provide at least about 0.1 mole percent each of hydrogen, perfluorophenyl or nitrile and bromine or iodine in the resulting terpolymer. 4. A process of Claim 1 wherein the terpolymer is prepared from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether and 4-bromo-3,3,4,4-tetrafluorobutene-1. 5. A process of Claim 1 wherein the terpolymer is prepared from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether and perfluoro-(8-cyano-5-methyl-3,6-dioxa-l-octene) . 6. A process of Claim 1 wherein the terpolymer is prepared from tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether and vinylidene fluoride. 7. A process of Claim 1 wherein the radiation is applied in a single dose. 8. A process of Claim 1 wherein the radiation is applied in two or more dosages. 9. Cured perfluoroelastomers resulting from the process of Claim 1, fluoroelastomer being substantially free from curing agent ^ residue. 10. A cured perfluoroelastomer of Claim 9, consisting essentially of tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether and 4-bromo-3,3 ,4,4-tetrafluorobutene-1. 11. A cured perfluoroelastomer of Claim 9, consisting essentially of tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether and perfluoro-(8-cyano-5-methyl-3,6-dioxa-l-octene) . 12. A cured perfluoroelastomer of Claim 9, consisting essentially of tetrafluoroethylene, a perfluoroalkyl perfluorovinyl ether and vinylidene fluoride. 13. A cured perfluoroelastomer of Claim 9 having less than 500 parts per million of curing agent residue. 14. A cured perfluoroelastomer of Claim 13 having less than 100 parts per million of curing agent residue.

Inventors: LOGOTHETIS ANESTIS LEONIDAS, LOGOTHETIS, ANESTIS, LEONIDAS
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: C08F2
IPC codes: B29C71, C08F2, C08F8, C08F14, C08J3, C08L27