MXPA01007682A - Tetrafluoro ethylene / hexafluoro propylene copolymers with better ductility - Google Patents

Abstract

The invention relates to a fluorinated ethylene propylene copolymer which is processed from the melt by fusion granulation and which consists essentially of monomer units of 78 to 95 wt.%tetrafluoroethylene, 5 to 22 wt.%hexafluoropropene and not more than 3 mol%fluorinated monomers that can be copolymerised with a mixture of tetrafluoroethylene and hexafluoropropene. Said copolymer has a molar ratio of weight average to number average of less than 2, has less than 80 unstable terminal groups per 1x106 carbon atoms and is produced by aqueous emulsion polymerisation. The copolymer is coagulated after polymerisation and then agglomerated. The agglomerate is isolated and dried to form a free flowing product, partial sintering being prevented. Said product is brought into contact with an effective amount of fluorine at a temperature between 60°C and the preliminary sintering temperature, whereby unstable terminal groups are converted into stable terminal groups. The fusion granulate is advantageously treated with ammonia or a compound which liberates ammonia, in water. The product can be used for coating wires and cables.

Description

TETRAFLUOROETHYLENE / HEXAFLUOROPROPYLENE COPOL1MERS WITH HIGHEST STRETCH CAPACITY FIELD OF THE INVENTION The invention relates to cast melt processable tetrafluoroethylene (TFE) / hexafluoropropylene copolymer (HFP) pellets having improved processability for wire and cable applications and to a process for using this polymer to coat wire and cable conductors.

BACKGROUND OF THE INVENTION The melt-processable copolymers with TFE and HFP are better known under the name FEP. As perfluorinated thermoplastics, said copolymers have unique end-use properties such as chemical resistance, weather resistance, low flammability, thermal stability and outstanding electrical properties. Like other thermoplastics, FEP is easily molded into wires, tubes, pipes, sheets and coated films. Because it has excellent thermal stability and is virtually non-flammable, FEP is frequently used in the design of multi-occupancy rooms and meeting rooms, to meet strict fire protection requirements. FEP is also the natural selection in data transmission cables due to its excellent dielectric properties (EP-A-423 995). The high processing speeds are desired when the wires and cables are coated by extrusion. However, the melt fracture limits these high extrusion rates in the case of many thermoplastics. The melt fracture results in surface roughness and / or non-uniform wall thicknesses. To increase the extrusion rate it is therefore assumed that the molecular weight distribution of the copolymer to be used should be very broad, as described, for example, for the FEPs in US-A-4,552,925. For substantial expansion of the molecular weight distribution, it is mainly used a mixture of at least two FEPs with markedly different molecular weights. Molecular weights are usually characterized by melt viscosity index or melt flow rate (MFI value). The mixtures that are often desired are produced by polymerising the components separately and mixing them in the form of networks, spheres or unconsolidated products prior to pelletizing by melting. In this way the manufacture of these mixtures is a problematic and expensive procedure. Other mixtures of FEP are described in DE 26 13 642 and DE 26 13 795.

These mixtures are claimed to be advantageous in those documents by suppressing foaming during the stabilization of FEP. This procedure is carried out by treating the resin at high temperatures (up to 400 ° C), preferably with water vapor. This procedure removes the thermally unstable end groups, mainly COOH and CONH2 groups. These end groups can be easily detected by IR spectroscopy. These mixtures have a very wide molecular weight distribution, and this is generally understood by the skill worker to give improved extrusion capability. The removal of thermally unstable end groups is required for the processing of FEP, in particular for wire coatings. The decomposition reaction of the unstable end groups described in Modern Fluoropolymers, editor John Scheirs, Wiley & Sons, 1997, page 228, leads to bubbles and holes in the final products. The formation of pellets by casting non-stabilized polymer resins results in corrosion of the equipment being used and in metal contamination of the function pellets produced. However, the stabilization procedures of DE 26 13 642 and DE 26 13 795 are very difficult to carry out, because they give rise to corrosion problems of the equipment used, due to the use of steam. Metal contaminants are difficult to control and can result in degradation and decomposition of the polymer at high processing temperatures. This decomposition generally leads to discoloration and degradation, and an accumulation of dice deposits. The die deposits are accumulations of molecular fractions of the polymer on the die orifice surface, and adversely affect the coating process. The phenomenon known as cone fracture can also occur. During the process of coating a wire, the molten polymer is extruded as a pipe or cover and is drawn under vacuum onto the wire. Cone fracture is discontinuity or fracture that occurs during this procedure. Each time this type of cone fracture occurs, the coating procedure must be restarted and there is a waiting time for the system to reach equilibrium again. In this way, long operating times are difficult to achieve. Productivity is also reduced. Additionally, the extrusion temperatures must be kept as low as possible to inhibit the decomposition reactions and the resulting evolution of toxic gases, the speed of which increases substantially as the temperature rises. On the other hand, lower extrusion temperatures result in higher melt viscosities and thus in the earlier emergence of melt fracture. Decreasing the intrinsic foundry viscosity by decreasing the molecular weight results in poorer mechanical properties.

To return to the more thermally stable material, therefore, it is necessary not only to remove the thermally unstable end groups but also to avoid contamination of metal and PM fractions which are relatively prone to shear degradation and / or thermal degradation. Another way to remove unstable end groups is post-fluorination, for example, as described in GB-A-1 210 794, US-A-4 743 658 and EP-B-457 255. This procedure generally uses dilute elemental fluorine. with nitrogen at elevated temperatures up to the melting scale of the polymer. When subjected to fluorination the polymer here may be in the form of melting pellets, agglomerates or unconsolidated material. Here, too, excessive metal contamination should be avoided. EP-B-222 945 describes the fluorination of hardened agglomerates, called granules there. Fluorination leads to perfluorinated end groups while the wet heat treatment described above can not mechanically result in a fully fluorinated polymer resin. It is believed that inserted double bonds are present here in the polymer backbone and lead to inherent thermal instability. These joints can lead to the discoloration that is observed with prolonged exposure to high temperatures.

Another FEP degradation reaction is described US-A-4 626 587. The emergence of this reaction is assumed to occur first by cutting the HFP dyads in the middle of the chain at temperatures above the melting point. These dyads are formed in the polymerization reaction of the free radical by recombination of the corresponding polymer radicals in a terminating step. The destruction of the dyads under processing conditions leads to the division of the molecular weight of these polymer chains, which adversely affects the mechanical properties of the polymer, and the formation of end groups that are more unstable. As taught in US-A-4 626 587, these dyads are destroyed by subjecting the material to very high shear rates at a temperature markedly above the melting point. This procedure is also very expensive. Another method to reduce the instability of the main chain is described in EP-A-789 038. The process uses relatively large amounts of a chain transfer agent to suppress the termination of polymer radicals.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a material that can be used for wire and cable coatings and that can be processed at higher speeds and at higher temperatures, giving longer operating times for the machinery. The invention further provides a manufacturing process that is more economical and more controllable for quality consistency. Additionally, the invention provides a method for reducing die deposits and frequency of cone fracture during extrusion coating of wires or cables.

DETAILED DESCRIPTION OF THE INVENTION The polymer according to the invention is a copolymer of TFE and HFP. It has an HFP content in the range of 5 to 22% by weight, preferably 10 to 18% by weight, a TFE content of 95 to 78% by weight, preferably 90 to 82% by weight, and optionally contains up to 3 mol% of a fluorinated monomer copolymerizable with HFP and TFE. The optional comonomer is preferably a perfluoro alkyl vinyl ether as described in EP-A-789 038 and DE-C-27 10 501. The monomer content can be measured by IR spectroscopy as described in US-A-4 552 925. The polymers of the invention typically have a melting point of 240 to 275 ° C, preferably of 245 to 265 ° C. The polymer of the invention is essentially free of thermally unstable end groups, these being removed by postflourination of the agglomerates. "Essentially free of end groups" means less than 80 end groups per million carbon atoms, preferably less than 40 end groups and particularly preferably less than 30 end groups per million carbon atoms. The material is essentially of high purity with respect to metals, that is, the total amount of iron, chromium and nickel is less than 200 parts per billion (ppb), preferably less than 100 ppb. The polymer of the invention that is used to coat wire and cable conductors has a very narrow molecular weight distribution, ie a ratio of PMp to PMn of less than 2 (PMp = weight average molecular weight, PMn = average molecular weight) in number). This ratio can be as low as 1.5. This is in contrast to recommended FEP grades for wire coating with high extrusion rates, a broad molecular weight distribution being recommended for those grades. The amplitude of the molecular weight distribution is measured according to the method of W.H. Tuminello in Polym. Eng. Sci 26, 1339 (1986). For high-speed wire extrusion the polymer MFl is >;fifteen. Lower MFIs are useful for other applications, such as coaxial foam cable. This polymer is essentially free of unstable end groups. It is particularly preferably the polymer of the invention. A copolymer in casting pellets according to the invention with an MFl value of 24 and 15% HFP can be produced as described below. This polymer can be extruded with a wire-coating extruder at, for example, 390 ° C at a rate of 454 m / min over a 6-hour machine operating time without discoloration and without producing any substantial amount of die deposit and with less cone fracture than commercial FEP grades. This amazing good performance is not completely understood. Despite the narrow molecular weight distribution, high processing speeds can be achieved. As discussed above, thermal teaches that the broad molecular weight distribution is necessary to achieve such high processing speeds. It has now been discovered that the narrow molecular weight distribution is better, thus overcoming a well-established bias. Additionally, discoloration does not occur during processing. This is an indication of the absence of any decomposition reaction. The MFl value of the extruded material is practically unchanged. The amount of end groups detectable by IR does not increase. Both findings indicate that there is no significant chain degradation. This observation indicates that the material has no weak links in its main chain, for example HFP dyads (US-A-4 626 587). The non-occurrence of discoloration, the almost unchanged MFl value and the almost unchanged number of end groups are evidence of the absence of any significant decomposition even at relatively high processing temperatures. It is believed that this results in reduced die deposits and the remarkably reduced frequency of cone fracture. Hence, the copolymer according to the invention exhibits surprisingly high thermal stability even under shear stress. The polymer of the invention can therefore be used advantageously in other applications. The evidence of the absence of decomposition reactions is surprising and not fully understood. It is believed that metal contaminants, in particular heavy metals, such as iron, nickel or chromium, can induce a decomposition reaction. In fact, the neutron activation analyzes showed that the amount of iron, nickel and chromium ions in the material used was very low: below 50 ppb. In this way the copolymer according to the invention can be classified as high purity. The polymer of the invention can be produced by the method described below. The polymerization can be carried out as a free radical aqueous emulsion polymerization of the prior art (see US-A-2 946 763). The ammonium or potassium peroxydisulfates can be used as initiators. As emulsifiers, standard emulsifiers, such as the ammonium salt of perfluorooctanoic acid, can be used. PH regulators such as NH3 (NH) 2CO3 or NaHCO3 can be added to the formulation. Typical chain transfer agents are used, such as H2, lower alkanes, methylene fluoride or methylene chloride. Chain transfer agents containing chlorine or bromine should be avoided. These components can cause marked corrosion during fluorination. The polymerization temperature may be in the range of 40 to 120 ° C, preferably 50 to 80 ° C; the polymerization pressure may be in the range from 8 to 25 bar, preferably from 10 to 20 bar. HFP forms an initial charge and is fed into the reactor in accordance with the copolymerization rules (see, for example "Modem Fluoropolymers", editor John Scheirs, Wiley & amp;; Sons, 1997, page 241). The preferred polymerization formulation is free of alkali metal salts. Additionally, it is preferable to carry out the copolymerization without the use of any chain transfer agent, in contrast to EPA-789 038. Chain transfer agents in an intrinsic manner extend the molecular weight distribution. The curing speed / time of polymerization should have the form as published in "Modern Fluoropolymers", editor Johns Scheirs, Wiley & Sons, 1997, page 226. As stated in that publication, the ratio of PMp / PMn can be easily calculated from the velocity / time curves in the absence of any chain transfer agent by means of equation (6) , page 230 and assuming that the termination occurs only by means of recombination. Recombination leads to a PMp / PMn ratio of 1.5 for small conversions. The termination mainly by chain transfer leads to a PMp / PMn ratio of 2. The free radical polymerization can also be carried out in non-aqueous medium, such as R1 13, as described in US-A-3. 528 954. However, this non-aqueous process is not preferred, because it is believed that it also generates relatively small amounts of high molecular weight products due to the gel effect arising in this "suspension polymerization". It is more likely that the gel effect gives rise to weak links in the main chain (HFP diads). A gel effect is less likely to occur in the aqueous emulsion polymerization because chain propagation and chain termination take place on the surface of the latex particles. The dispersion obtained from the polymerization is mechanically coagulated using a homogenizer (see EP-B-591 888) and agglomerated using an organic liquid not miscible in water, such as gasoline, a well-known technique in the medium (cf. "Modern Fluoropolymers", editor John Scheirs, Wiley &Sons, 1997, page 227). The agglomerates are spheres of free flow with a diameter of 0.5 to 2 mm. Free flowability is preferred for technical reliability when carrying out the subsequent handling steps. The agglomerate is dried by rinsing with nitrogen and then under moderate vacuum at temperatures up to 180 ° C. The chemical coagulation of the agglomerate can also be used. However, this is usually done using acids. This is not preferred, because it results in very high levels of metal contaminants in all subsequent handling steps. The agglomerate can then be fluorinated at temperatures of 60 to 150 ° C, preferably 100 to 140 ° C with a mixture of fluorine and nitrogen. The mixture generally comprises 10% by weight of fluorine. The fluorination continues until at least 90 to 95% of the end groups of the final agglomerate have been removed. The higher fluorination temperatures can lead to a change in the MFl value which can be up to 30% and is difficult to control. This can lead to the expansion of the molecular weight distribution and adversely affect the yield. The result is a lack of reproducibility with an adverse effect on the quality and consistency of wires and cables coated with the polymer. Reaction times are not substantially shortened by higher temperatures, and higher fluorination temperatures are therefore not considered to be advantageous. Moreover, higher temperatures can lead to pre-concretion or even concretion of the agglomerate, and adhesion of the material to the walls of the equipment. The fluorination is carried out in a rotary dryer that keeps the material moving. This gives more homogeneous reaction conditions. The freely flowable agglomerate should be as free as possible of fine powders and mechanically stable enough for the substantial non-production of fine powders during post-treatment. Fine powders can impair the reliability of the operation of the procedure. The agglomerate does not require the hardening described in EP-B-222 945. The fluorination of the agglomerate has two advantages. It is not controlled by diffusion, because the end groups reside on the surface of the latex particles. Therefore the reaction times are relatively short. The unhardened agglomerate is soft enough not to abrade the metal contaminants from the wall of the rotary dryer. In this way the level of metal contaminants is reduced. No characteristic prevents the fluorination of the casting pellets. In this case the fluorination process requires higher temperatures and much longer reaction times to allow diffusion control of the reaction. Additionally, sharp, sharp cast pellets wear a considerable amount of metal from the wall of the rotary dryer. Increasing the reaction time results in higher levels of metal contamination. This contamination is difficult to eliminate. The level of metal contamination is increased by up to two orders of magnitude when the pellet process is used. The fluorinated agglomerate is subsequently formed into cast pellets. Some crushing of the agglomerate takes place during drying and fluorination. This produces fine powders, which inhibit the free flow of the material. It is advantageous to compact the fluorinated agglomerate prior to the formation of melting pellets. This gives a more reliable constant feeding speed. The formation of melt pellets of fluorinated agglomerates provides many advantages over the formation of melt pellets of non-fluorinated agglomerates. The formation of melting pellets proceeds practically without decomposition. The MFl value remains almost unchanged. This discovery suggests that there is no substantial presence of weak links in the main chain. The corrosion of the equipment used is substantially reduced. The amount of metal contamination collected is therefore negligible. The emission of gaseous decomposition products in the die orifice is significantly reduced (for example by four orders of magnitude). In this way the whole procedure becomes substantially more reliable. The dice deposits are substantially reduced. Therefore the procedure needs less attention. The cast pellets do not exhibit any discoloration, in contrast to casting pellets that originate from non-fluorinated agglomerates, which are typically brown when they leave the extruder. The MF1 value of the melt pellets produced by the method described above is increased only slightly by about 10%, compared to the MF1 value of the copolymer from the polymerization. It is therefore to achieve a uniform quality. As described in DE-A-195 47 909, the melt pellets are subsequently subjected to an aqueous treatment to remove volatile materials and COF groups. Here too, the near absence of gaseous decomposition products and acid end groups considerably reduce the corrosion of the stainless steel water treatment vessel. There is a reduction in additional heavy metal contamination. Additionally, the water-soluble salts originating from the production processes are eliminated. The amount of extractable fluoride is reduced to less than 1 ppm.

Test methods The MFl value is measured according to ASTM D 1238 (DIN 53725) at 372 ° C with a load of 5 kg. The value of MFl can be converted to the melt viscosity value in 0.1 Pas (Poise) by dividing 53150 by the value of MFl (g / 10 min). The content of HFP is measured by means of FTIR spectroscopy as described in US-A-4 552 925. The absorbances to sling numbers of 980 cm "1 and 2350 cm" 1, respectively, are measured on a film of 0.05. + 0.01 mm thick, produced at 350 ° C, with a FTIR-Nicolet Magna 560 FTIR spectrometer. The content of HFP is calculated according to the following equation: HFP content (% by weight) x 3.2. The end groups (-COOH, -COF, -CONH2) are determined by means of FTIR spectroscopy as described in EP-B-226 668 and -US-A-3 085 083. A film of 0.1 mm thickness produced at 350 ° C it is used together with a reference film of a material that does not contain the end groups to be analyzed. The Nicolet Magna 560 FTIR spectrometer was used, with software in interactive subtraction mode. When the number of end groups is established, this is the sum of associated COOH, CONH2 and COF groups. The melting points of the copolymers were determined by DSC by the method of ASTM D 4591-87 at a heating rate of 10 K min. The melting point set forth herein is the maximum temperature of the endotherm during the second casting process. The amplitude of the molecular weight distribution, characterized by the ratio of PMp / PMn was measured by means of rheological spectroscopy with an advanced rheometer expansion system (ARES) supplied by Rheometric Scientific. The measurements were carried out at 372 ° C and were evaluated by the method of W.H. Tuminello, Polym. Eng. Sci., 26, 1339 (1989). The metal contents were measured by extracting the samples with HNO3 of 3% strength for 72 hours at room temperature and subjecting the extracts to atomic absorption spectroscopy. The content of extractable fluorine ions from the metal pellets was measured by the method given in EP-B-220 910. However, the extraction was carried out only with water.

EXAMPLE 1 A 1500 L stainless steel reactor was charged with 1000 L deionized water with 3 kg of the ammonium salt of perfluorooctanoic acid. The air was removed by evacuation and rinsed with nitrogen. The reactor was heated to 70 ° C and the temperature remained constant. 2 kg of aqueous ammonia solution of 25% strength were added.

The reactor was pressurized with TFE and HFP at 17 bar of total pressure, the partial pressure of HFP being 12.5 bar. Polymerization was initiated within 10 minutes by adding 1600 g of ammonium persulfate in solution in 5 L of deionized water. The pressure was kept constant by feeding a gaseous mixture of TFE / HFP into the reactor. The weight ratio of TFE / HFP was 0.14. After 6 hours the reaction was stopped by interrupting the monomer feed. The monomers were vented. The reactor was cooled to room temperature and then the contents were discharged. The solids content of the polymer dispersion was 29%. The dispersion was virtually free of clots. The MF1 value was 20 g / 10 min. The HFP content of the copolymer was 13% by weight. The melting point was 255 ° C. The copolymer had 660 end groups COOH per 106 carbon atoms. The PMp / PMn was measured as 1.7, while a PMp / PMn value of 1.6 was calculated from the polymerization speed / time curve. The dispersion was coagulated using a homogenizer and agglomerated using gasoline. The agglomerate was washed three times with deionized water and dried for 6 hours at 180 ° C in a rotary dryer, purging first with nitrogen and then under vacuum. The resulting agglomerate was divided into two parts. A part was then formed into cast iron pellets, washed with water and dried to give a dark brown color. It was fluorinated and again treated with water to remove residual end COF groups, with which the discoloration disappeared. This sample was called AO. The material had 43 end groups per million carbon atoms. The other part of the agglomerate was fiuorinated first, then formed into cast pellets, treated with water and dried. This sample was called A1 and had only 18 end groups per million carbon atoms. In each processing step the content of iron, nickel and chromium was measured using the extraction method. Table 1 shows the results together with the number of end groups.

TABLE 1 Metal contaminations for AO and A1 samples after the various handling steps. The agglomerate had 660 end groups. Sample AO: fluorination of the melting pellets (comparison) *) 43 end groups per million carbon atoms Sample A1: Fluorination of the agglomerate (invention) *) 18 end groups per million carbon atoms. The fluorination was carried out in a 300 L stainless steel rotary dryer using a mixture of 10% fluorine in nitrogen at 140 ° C (sample A0) and 100 to 140 ° C (sample A1). The details are listed in table 2. The fluorine mixture had to be replaced several times (refill). At the end of the fluorination, the excess fluorine was removed by rinsing air through the reactor. The excess fluorine was absorbed by passing the air stream through a bed of AI2O3 granules and through a scrubber comprising an aqueous suspension of CaCO3.

TABLE 2: Fluorination conditions for samples A0 and A1 ^ refilled every half hour, except for the last hour ".} end groups are the total of COOH, COF and CONH2 per million carbon atoms Water treatment of the melting pellets (see DE-A-195 47 909) was carried out in a 1000 L stainless steel reactor, 200 kg of melting pellets and 400 L of deionized water with 1 L of 25% ammonia solution were charged to the reactor. ° C and kept at this temperature for 4 hours for the non-fluorinated cast pellets and for 1 hour for the fluorinated cast pellets This reaction time is required to bring the content of end-COF groups below 5 ppm. The reactor was cooled by replacing the water twice.The product was dried by injecting hot air into the reactor.The melting pellets had an extractable fluorine ion content of 0.1 ppm.

EXAMPLE 2 Sample A1 1 was run through a wire coating extruder under two different groups of conditions together with a commercial product designated C1. The production of sample A1 1 was similar to that of A1, but the product had an MFl value of 24 g / 10 min. The polymerization and manipulation of A1 1 and A1 were identical. A1 1 had 28 end groups and an iron content of 18 ppb. The PMp / PMn ratio was 1.6. The calculated value was 1.7. The content of extractable fluorine ions was 0.2 ppm. The coating conditions are listed in Table 3.

TABLE 3 Coating performance of the material according to the invention The temperature profiles, which are not given in the table, were adjusted slightly to maximize the line output while maintaining the deviation of the eccentricity of insulation between 0.00076 and 0.0018 cm. In runs 1 and 2 there were no marked die deposits and there were no cone fractures during run time. In run 3 there was a considerable level of die deposits and cone fracture during an identical run time. When C1 was aged above its melting point (ie, 250 ° C), it showed noticeable brown discoloration.

EXAMPLE 3 Samples A11, A12 and commercial products were run through a slightly different wire coating extruder. The coating conditions are listed in table 4.

TABLE 4 Coating performance of the material according to the invention compared to two commercial products The temperature profiles were adjusted slightly to maximize line output while maintaining the eccentricity deviation of insulation between 0.00076 and 0.0018 cm. Run No. 1 did not show any notable die deposit, and only two cone fractures, during a period of 29 hours of wire extrusion which were blue, green, orange, brown and white.

Run No. 2 showed a considerable level of die deposits and averaged 6 to 8 cone fractures during a run time of 24 hours.

Claims (15)

NOVELTY OF THE INVENTION CLAIMS

1. - A melt processable cast-iron copolymer comprising essentially monomer units of from 78 to 95% by weight of tetrafluoroethylene, from 5 to 22% by weight of hexafluoropropene and at most 3 mol% of fluorinated monomers copolymerizable with a mixture of tetrafluoroethylene and hexafluoropropene, and having a weight average molecular weight ratio to number average molecular weight of less than 2, and having less than 80 unstable end groups per 1-106 carbon atoms, and is obtained by polymerization of aqueous emulsion.

2. The copolymer according to claim 1, further characterized in that it comprises less than 200 ppb of heavy metals.

3. The copolymer according to claim 1 or 2, further characterized in that it contains less than 40 unstable end groups per 1-106 carbon atoms.

4. The copolymer according to claim 1 or 2, further characterized in that it essentially does not contain extractable fluorine.

5. A process for producing a copolymer comprising essentially units of 78 to 95% by weight of tetrafluoroethylene, 5 to 22% by weight of hexafluoropropene and at most 3 mol% of fluorinated monomers copolymerizable with a mixture of tetrafluoroethylene and hexafluoropropene, and having a weight average molecular weight ratio to number average molecular weight of less than 2, and having less than 80 unstable end groups per 1-106 carbon atoms, polymerizing the monomers by aqueous emulsion polymerization in a medium aqueous, coagulating the copolymer essentially by mechanical means, agglomerating the coagulated copolymer by contacting it with an organic liquid that is essentially non-miscible with water, isolating the agglomerate, drying the agglomerate, without partial concretion, to give a free-flowing product, putting the free-flowing agglomerate in contact with an effective amount of fluorine at a temperature of from 6 0 ° C at the preconcretion temperature for a sufficient time to essentially convert unstable end groups into stable end groups, pelletizing the fluorinated agglomerate into pellets, and bringing the melt pellets into contact with water at a temperature of 60 ° C. 130 ° C.

6. The process according to claim 5, further characterized in that the aqueous polymerization medium that is used is essentially free of chain transfer agents.

7. The method according to claim 5, further characterized in that the polymerization medium used is water.

8. - The process according to claim 5, further characterized in that the polymerization medium that is used is essentially free of alkali metal ions.

9. The process according to claim 5, further characterized in that in the agglomeration step, the organic liquid that is used is free of halogen atoms.

10. The method according to claim 5, further characterized in that the fluorination temperature is from 60 to 150 ° C.

11. The process according to claim 5, further characterized in that the cast pellets are contacted with water comprising 0.01 to 1% by weight of ammonia or of a compound that releases ammonia under the conditions under which the materials are put in contact.

12. A method for reducing the frequency of cone fracture during wire extrusion coating, comprising the steps of: a) preparing a copolymer obtained from 78 to 95% by weight of tetrafluoroethylene, from 5 to 22 % by weight of hexafluoropropene and at most 3 mol% of fluorinated monomers copolymerizable with a mixture of tetrafluoroethylene and hexafluoropropene, the ratio of molecular weight by weight to number average molecular weight being less than 2; b) prepare a wire or cable conductor; c) extruding the copolymer around the conductor at a temperature sufficient to provide uniform flow of the polymer.

13. - The method according to claim 12, further characterized in that the polymer has less than 80 unstable end groups per 1-106 carbon atoms.

14. The process according to claim 12 or 13, further characterized in that the polymer comprises less than 200 ppb of heavy metals.

15. A coated wire produced by a method according to any of claims 12 to 14.