IE42823B1 - Process for the manufacture of suspension polymers of tetrafluoroethylene - Google Patents

Abstract

Tetrafluoroethylene is polymerised by the suspension process in the presence of modifiers. The modifiers employed are from 0.0004 to 0.0029 mol %, based on the tetrafluoroethylene, of a compound of formula I and/or II or of the formula III. Rf and n are as defined in Claim 1 or 2. The polymers prepared according to the invention can be used to produce press-sintered blocks without cracking.

Description

This invention relates to a process for the manufacture of polymers of tetrafluoroethylene by suspension polymerization .

U.S. Patent No. 3,142,665 describes a process for making dispersion polymers of tetrafluoroethylene having an average particle size of approximately 0.05 to 0.5 pm by polymerizing tetrafluoroethylene in emulsion in the presence of sinall amounts of perfluoroalkenes or perfluoro-alkoxytrifluoroethylenes. The modified dispersion polymers obtained are subsequently coagulated and dried. The resulting powders are mixed with the usual lubricants and can then be processed by the so called paste extrusion technique. Owing to their reduced molecular weight (SSS values in the range -of from 3 2.23 to 2.170 g/cm ; SSG = specific standard gravity accord5 ing to ASTM D 1457-62 T) and the resulting strong tendency towards crack formation, the polymers are unsuitable for the manufacture of thick-walled moulded and sintered objects.

It has also been proposed (U.S. Patent No, 3,655,611) to produce polytetrafluoroethylene powders by suspension ) polymerization, which are modified by adding from 0.05 to 0.5 mol% of hexafluoropropylene to reduce the so-called cold flow. British Patent Specification No. 1,116,210 proposes to polymerize tetrafluoroethylene by the suspension process together with 0.003 to 1.5 mol% of a perfluorinated ether - 2 42823 or perfluoro-alkoxyethylene in the presence of precipitating agents.

Suspension polymers of tetrafluoroethylene are processed, inter alia, into moulded and sintered blocks from which sliced sheets are made. Recent progress in the manufacture of sliced sheets has caused increased demand for sintered blocks of large diameter. Even under very mild cooling conditions cracks are formed in the interior of such blocks due to the increased contraction strain as a result of the great thickness. This phenomenon causes the entire block or at least the interior thereof to become unsuitable for processing into sliced sheets so that the manufacturer can only produce blocks below certain dimensions from the usual polymer s.

To obtain sliced sheets having the desired high dielectric strength, high densification of the blank produced from the polytetrafluoroethylene powder by moulding prior to sintering is necessary. To ensure the necessary densification the crude polymer from suspension polymerization must be subjected first to a wet grinding process and, after drying, to a fine grinding process in a hammer or air jet mill until an average particle size of about 30 nm is obtained. Hence, the grinding properties of such products are of decisive importance. In British Patent Specification No. 1,116,210 direct moulding of the dried crude polymers without previous grinding is said to be possible. However, the sliced sheets produced from the sintered blocks are very porous and, as a result, they have poor dielectric properties. When the crude polymer obtained - 3 42823 by the process of British Patent Specification No. 1,116,210 is subjected first to wet grinding and then to dry grinding, the grinding properties are noticeably deteriorated, as compared to non-modified tetrafluoroethylene suspension polymers. The ground material acquires a fibrous structure and contains an undesired proportion of coarse particles above 100 pm leading to the formation of so-called rish eyes in the sliced sheets made therefrom.

All products produced according to the teachings of the art have a high tendency towards crack formation which cannot be overcome even by fine grinding. It is, therefore, desirable to provide an improved tetrafluoroethylene polymer which, when processed into sintered blocks and other thick-walled objects, is completely or substantially free from the tendency to crack formation without the other properties, especially those which are important to the manufacture of sliced sheets, being affected.

The present invention provides a process for the suspension polymerization of tetrafluoroethylene in the presence of a catalyst and optionally a buffer, a precipitating agent, an emulsifier and a heavy metal salt, which comprises polymerizing the tetrafluoroethylene in the presence of, as modification agent, from 0.0004 to 0.0029 mol %, calculated on the tetrafluoroethylene, of a p-er5 fluorinated ether of the formula CF2=CF-0-Rf (I) in which R^ is a perfluoralkyl radical having from 1 to 10 carbon atoms, or of the formula - 4 42823 P ..

CP, / 3 ^F CF, -O-CF-CP.I 2 CF_ O-CF=CF_ (II) in which n is zero or an integer from 1 to 4, or of the formula cf3-cp2-cf2fP3 O-CF — CF.

— O-CP=CP.

(XIX) in which n is an integer from 1 to 4, or a mixture of any two or more such perfluorinated ethers.

In the perfluoro(alkyl vinyl) ethers of the formula (I), Rf is a perfluorinated alkyl radical, preferably a linear perfluorinated alkyl radical, having from 1 to 10 and preferably from 1 to 4 carbon atoms. Preferred are perfluoro-(methyl vinyl), -(ethyl vinyl), -(butyl vinyl) and especially perfluoro-(propyl vinyl) ether.

In the ethers of formula (II), n is preferably zero or 1, more preferably zero, and in the ethers of formula (III) n is preferably 1 or 2.

The ethers of formulae (I), (II), and (III) can be used alone in the form of mixtures of ethers having different radicals Rf or having different values for n. Alternatively, mixtures of any two or all three of the ethers of the above formulae can be used.

The amount of modification agent is from 0.0004 to 0.0029 mol %, preferably from 0.001 to 0.0025 and especially 43833 LO .5 O.OOI to 0.002 mol %, calculated on the tetrafluoroethylene It has proved advantageous to add the total amount of modification agent in dosed quantities as soon as the required polymerization pressure is reached. It is also possible, however, to introduce a partial amount into the reaction vessel prior to polymerization and to meter in the balance, or to meter in the total amount when up to 70%, preferably up to 30%, Of the tetrafluoroethylene has polymerized. Either continuous or discontinuous addition is possible.

Ethers of formula (I) can be prepared, for example, by thermolysis as described in U.S. Patents Nos. 3,180,895 and 3,250,808 and ethers of formula (II) are obtainable by the processes described in German Offenlegungsschriften Nos 2,434,992 and 2,517,357.

Ethers of formula (III) can be prepared according to German Patent No. 1,263,749 by thermolysis of a compound of the formula cf3-cp2-ce2CF_ I 3 O-CF—CF cf3 — O-CF—Y n in which n is an integer from 1 to 4 and Y is -COF or -COOM (M being a monovalent metal, preferably an alkali metal). These derivatives of perfluorinated ether-carboxylic acids can he prepared by oligomerization of hexafluoropropene oxide, for example, as described in British Patent Specific· > ations Nos. 928,315 and 1,033,574.

The suspension polymerization in accordance with the present invention is carried out under conventional 43823 conditions. Catalysts which can be used are persulfates, percarbonates, perborates, peroxides, per-acids, azocompounds, and permanganates (cf. U.S. Patents Nos. 2,393,967? 2,394,243; 2,471,959; 2,510,783; 2,515,628; 2,520,338; 2,534,058; 2,565,573; 2,599,299? 3,632,847). There may also be used redox catalyst systems comprising one of the above peroxidic compounds (cf. U.S. Patent No. 2,393,967, left hand column, page 3, lines 22 et seq.), especially persulfate, with a reducing component such as a bisulfite, hydrazine, a dithionite, or a water-soluble nitrogen compound liberating a diimine, for example, azodicarboxylic acid and the salts thereof, and azodicarbonamide.

The polymerization is suitably carried out at a pressure of from 0.5 to 20, preferably from 3 to 10, atmospheres gauge, and at temperatures of from 5 to 80°C, preferably from 10 to 35°C. The pH of the polymerization medium is not critical; it can be acid, neutral or alkaline. When permanganates or azo-compounds are used, an acid medium is preferably used, with all other catalysts an alkaline medium is preferred.

If desired, the polymerization can be carried out in the presence of small amounts of buffer substances, especially ammonium salts, for example, ammonium carbaminate, ammonium carbonate, ammonium chloride, or ammonium oxalate.

Known precipitating agents may also be added, for example borax and inorganic water-soluble phosphates. To facilitate catalyst decomposition, small amounts of heavy metal salts -5 -5 may be added in a concentration of from 1 x 10 to 40 x 10 , -5 -5 preferably from 3 x 10 to 20 x 10 % by weight, calculated on the total aqueous mixture, for example, the salts of bivalent copper, bi- or trivalent iron, trivalent chromium, and monovalent mercury.

In certain cases it has proved advantageous to add to the polymerization mixture small amounts of telogenically inactive emulsifiers, for example, salts of perfluorinated carboxylic acids. The concentration of such emulsifiers should, however, be below the amount necessary for emulsion formation, suitably below 30 ppm, preferably below 20 ppm.

LO It has surprisingly been found that extremely small quantities of the modification agents used according to the invention so drastically reduce the tendency towards crack formation of moulded and sintered blocks that sintered blocks having a weight of up to 80 kg (corresponding to a .5 diameter of up to 40 cm) can be produced which are free from crack formation right through to the core as proved by optical examination of sliced sheets made therefrom.

The following Table.·I shows that in this respect the products obtained by the process of the present invention are far superior to. the products produced according to the state of the art. - 8 43833 Crack formation in sintered blocks of 14 kg made from modified polytetrafluoroethylene fi 0 •rl P Φ ε P 0 lp + P Φ P Φ •rt cracked Ό Φ fi υ Φ Ρ ϋ Ό Φ fi ϋ Φ Ρ 0 Ό Φ fi ϋ Φ Ρ ϋ cracked cracked n Ό >1 >, >1 >1 >. >1 Γ*· o ε irt rrt r4 rrt rrt Φ o r4 rrt irt rrt ρ p β β β β ,Ρ β u 44 44 44 44 <Ρ 44 44 10 fi in •rt P Q Φ o o 0¾ p • • & o o r4 irt 0 0 fc ft w »0 ft Φ P CM •0 CM Q 0* «α β CM Ο Q cn φ 0 Γ> ο Ο Ο • o » ο ο ο ο o o' u trt r-4 0 Φ ε U ω P CM CM β ft • ft Φ □ ω ρ □ tn II β & II Φ ft 0 ft υ «. u β I Ρ Η 1 0 ο •Η Η o CM Ύ τι CM 1 P ft «*-» fi Φ ft CM Φ υ CM 0 rrt U U II ft u Λ ll CM Ή ft υ φ ft ft ΨΙ υ Ρ Ε-» u a ι 1 φ 1 Ό cn ω ,Γΐ • cn cn 0 ft ft Ρ <Ρ ft ft ε υ Ο 0 ϋ a u co fi CQ <υ irt Ρ > • fi •rl * 0 φ tP in Ϊ25 ε m •rt φ Φ Ρ Η tP Η Φ ft Φ Φ Η H ft φ Pt Φ φ X Φ Φ ε ω irt r4 ni Λ β >3 Λ χ Φ ΐ η Φ fl Α 0 β &< Eh ιη trt O «Ρ irt irt νθ 10 CM * «, CM m Φ m rrt Ρ irt \o rd β Η * * φ cn cn irt Ρ ω ω ra ft ΰ £ o CQ P β Λ ρ 4-1 P ft X □ CM Φ P Φ Ρ Ρ Φ Ό Φ Μ « < Φ υ Λ 0 Φ 04 Ρ Φ 44 Φ Ρ Φ Ρ Ρ η) Ο CM Η Φ Ρ Φ Ό § ® •rl 0 >0 0 ο ί S 5 Ή φ fi ϋ Ο Η Λ r-4 Φ ϋ •Η Μ •S υ >ι °Ο □ CO cn φ μ β Ρ φ Φ £ φ Ρ Ρ CQ φ Μ diameter is sliced crack formation can be observed from the indicated diameter on ο CM Ο Αϊ ο ο ι-4 Φ χί Ρ β Φ The products produced by the process of the present invention have surprising advantages over pure non-modified polytetrafluoroethylene and surprisingly they also have advantages over products produced with higher amounts of the modification agents used according to the present invention (over 0.0029 mol %).

As compared to unmodified polytetrafluoroethylene, the products of the present invention exhibit the following advantages: (1) Large blocks which are free from cracks and suitable for making sliced sheets can be produced by moulding and sintering. The sheets have a good quality throughout the whole diameter of large blocks. (2) There is a surprising improvement in the trans15 parency of the sheets in spite of the extremely small amount of modification agent used. (3) There is an improved thermostability of the sheets as a result of the variation of density after thermal treatment.

As compared to products produced with large amounts (over 0.0029 mol %) of the perfluorinated modification agents (for example, according to British Patent Specification No. 1,116,210), the products of the present invention exhibit the following advantages: :5 (1) Larger blocks free from cracks can be produced by moulding and sintering. It is surprising that with the use of higher amounts of modification agent, increased crack formation is observed, so that for crack formation the optimum concentration of modification agent is below - 10 4*823 0.0029 mol %, preferably below 0.0029 mol %, calculated on tetrafluoroethylene. This is illustrated in Table 2. (2) The products obtained have improved mechanical properties. It has been observed that the tensile strength of the sintered material is distinctly diminished when the amount of modification agents exceeds the above value. (3) Sheets sliced from the outer zones of the sintered blocks have better electrical properties. In this case also, a higher amount of modification agent detrimentally affects the dielectric strength in the border zones of the blocks. (4) Better grinding properties. It has been observed that products modified with a higher amount of perfluorinated ether have poorer grinding properties on conventional air jet and hammer mills. As a result, the powders obtained have a lower apparent density than unmodified polytetrafluoroethylene powders. The sliced sheets made therefrom have areas of different transparency (specks and white spots). These disadvantages are not observed when using the small amounts of modification agent according to the present invention. 72JT5 TABLE 2 01 •P Φ Φ Λ to elongation at break % I 450 I 420 410 tj Φ Λ Φ Φ fi •H ip cn in •rl fiCM • to 02 φ ε r-l O fi P 5 cn tn cn Φ +>s •fi 0) B •P CO • •Pen cn r- in X 0 > ε 10 sf ω Φ flj .0 t-l i—l »P •rl Oi • » (0 & tn CM CM CM 02 g cn o o o 10 O •P s r* Φ > Φ 0 £ to •rl 0} p Λ CM P- rp in rd CM TJ «Ρ 0 u CO r- CO CO 4) Φ fi «Ρ ϋ (12 fi Φ p o •T in cn 10 rp •rl •P 0 co r> r*« 10 02 •a tc !i cn to 4) o •r| P I—l Ip fli rp P κ V 10 tp CM •P fi • r-i •P to 0 > o <1 0 P •P N ft 0 nJ W ω tn A4 fi ti r4 fi -P □ .fi 10 10 tp o o 0 »4 0 ti) • co ε ?t to u sintered bio processing | range °C 360-390 350-380 to n u ε is. ti 0 _ o O o o in fi ρ ε r* o in u 0 *ρ ε »P ip 10 cn in P 10 in 0 fi 0 S - 0 Q o • o •fi n •fi 0 H •fi O μ ip Λ xi • H to H -P &CN CM • +) fi • -fi Ο H • •fi υ H tn • Φ * tn •rl fi tji-rl tn •P fi κ Φ 10 fl ID 0 s Si. 0 £ II >H 0 s II tp fi 0 φ ω r-l fl XJ ip tn tP 0 tn 0 •rl •P XI ft ti fl iti in ti ti in tn n ~ ti in rP Ό -fi a • flj * EH « fi tp fi ip fi fi A Ε» H ip ti ti ti 10 0 Φ m s • * • TJ *—· · • Η II •se* • Η II ti > 0 CQ h m κ 0 X O 0 κ in 0 _ 0 c H 0 H 0 W 0 w ε m « rl S fi o w tp ε c ti •P TABLE 2 (Continued) 01 P Φ Φ Χί Ui elongation j at break % j 430 430 400 430 430 Ό Φ x; 0 Φ P •rl r-l O' in in fics » • ra ra Φ £ co CO CM CM c P J ro m m cn ro +1 ra a >1 P • «HCO ra in co o iri oo X U > £ Tf in in Tf ra 41 iti 6 iri iri rri rri •rl ft M\ • • • • * CM 01 Ol O' CM CM CM CM n S ro 10 in O' m O' 4) X O' co co 00 Φ > Q) 0 λ: w p XJ Ω co co o -n P n ft O' O' oo co O' 0) Q) C P u o O' Φ CM co O' •rl P 0 CO co GO r* 00 ra w ra ft ! CO n Φ o •rl Ρ iri rri «· rd ρ κ id o P β · iri ra 0 > ϋ >1 P Ρ *3 A T ϋ ιβ K m fcp AS P (0 •tf rri HOP V0 10 10 10 10 o ja o υ υ O o tw ω cn rri iri iri iri iri 0 tri ·Η « • • • • 0 Φ O' O' O' O' O' ra6> u 5 ttt Λ fi •ρ a tO wo o o o O o φ ra iri rri iri rri iri Ρ Φ Φ Φ 0 O' ri1 1 1 <* I tf* 1 7 +) 0 fi o o o o o β ρ «{ <0 «0 10 10 »0 ri ft p 01 co CO CO CO co ra AJ 0 β 18. Φ Φ Φ Φ W a o β c β β β Ρ Ρ β 0 0 0 0 0 a fi β c β β mple m CO •tf in <0 ra a iri rri iri Η Referring to Table 2 1) Processing range: the heating and cooling periods were the same as specified at the bottom of Table 1. The indicated temperature range includes the possible rest temperatures within which the sliced sheets did not exhibit oversintering (mottles) and had a tensile strength above 30 kg/cm 2) Molecular weight: the molecular weight was measured by the traction-creeping method according to C. Airoldi (ef.

C. Airoldi, C. Carbuglio and M. Ragazzini, J. appl. Polym. So. volume 14, pages 79 to 88 (1970)). 3) Crystallisation speed: in a precision dilatometer (H. Wilski, Makromol. Chemie 150 (1971) page 209) test specimens having a weight of 1.5 g and taken from sintered plates were heated at a constant rate of 30°C/hr from 25°C to 35O°C and then cooled at the same rate. The indicated values relate to the reduction of the specific volume (Av. spec.) on Cooling from 320°C to 25O°C. 4) Dielectric strength: the block having a weight of 14 kg (dimensions: about 20 cm high and about 20 cm in diameter) was sliced from the outside towards the centre into a sheet having a thickness of 200 μ,. The dielectric strength was tested with a dielectric strength tester. Point 1 corresponds to the slicing range of from 190 to 173 mm, point 2 to the slicing range of from 149 to 130 mm and point 3 to 77 to mm of the block diameter.

The dielectric strength tester used as a device of Messrs. MeBWandler Bau GmbH, Bamberg, type JPG 30/0.5 (electrodes: on top ball electrode diameter 20 mm, at bottom plate electrodes diameter 50 mm; according to VDE 0303/T2 or DIN 53 481. The average value was calculated from at least individual values, values below 75% of the second highest value were discarded. The number of individual values used to calculate the average value must amount to at least 70% of all individual values.

) Specific gravity: it was measured with disks having a diameter of 25 mm which had been compressed at 352 bars and sintered for 1 hour at 38O°C by the buoyancy method at 23°C. 6) Tensile strength and elongation at break: were measured according to ASTM D 1457-62 T with test bars according to ASTM D 1708. The indicated values are average values of 10 measurements on sliced sheets of different zones of the 14 kg block.

The following examples illustrate the invention.

EXAMPLES 1 to 16 (cf. Table 3) a) Suspension polymerization The polymerization autoclave used had a capacity of 180 1 and an enamel inside lining and was provided with a baffle. It was charged with 120 1 of desalted water and the additives specified in Table 3. The stirrer was switched on at a speed of 100 revolutions per minute, the autoclave was flushed 15 times with nitrogen and two times with tetrafluoroethylene (2 atmospheres gauge) and tetrafluoroethylene was forced in up to the desired pressure (as indicated in Table 3). After increase of the stirrer speed to 180 revolutions per minute, the additives indicated in column 3 were metered in and it was rinsed with 900 cc of water. Polymerization was continued under the specified pressures and temperatures until the solids content given in % by weight, calculated on the mixture, was reached. b) Processing The pressure of the polymerization vessel was released and the vessel was flushed three times with nitrogen (4 to 5 atmospheres gauge). The aqueous medium was discharged and in the polymerization autoclave the polymer was washed three times, each time with 100 litres of water at a stirring speed of 110 revolutions per minute. Next, about 1/3 of the .0 polymer was ground two times in a 100 litre glass vessel, each time with 50 litres of water for 10 minutes with water replacement, to an average particle size of 200 to 400 pm.

The water in excess was removed in a straining bowl and the product obtained was dried in a layer of from 5 to 7 cm for 6 hours at 240°C in a drying cabinet with circulating air.

The product was ground in a helical jet mill or hammer mill to an average particle size of 20 to 70 pm. 43823 Suspension polymerization of tetrafluoroethylene in the presence of small amounts of perfluorovinyl ethers φ g Ή ίί Ο υ ω β φ Λ Ρ Φ r-J & •Η > Ο Μ Ο β rd m »4 φ ft <Η ο ω •Ρ β β ο r—I ϊ-ί m ο φ υ β φ (Ο φ β ft φ Λ +» β cn Ή φ β φ S Ρ Φ Ο Μ Ο β «Η Ψ» Φ Μ 4J Φ μ Μ-ί Ο β Ο Ή •Ρ Φ Ν Ή Μ Φ I r—J Ο ft β ο •Η ω α φ ft ω β W 48623 TABLE 3 (Continued) Suspension polymerization of tetrafluoroethylene in the presence of small amounts of perfluorovinyl ethers Suspension polymerization of tetrafluoroethylene in the presence of small amounts of perfluorovinyl Suspension polymerization of tetrafluoroethylene in the presence of small amounts of perfluorovinyl k Φ x: •P QJ Suspension polymerization of tetrafluorqethylene in the presence of small amounts of perfluorovinyl χ» 03 β Ό QJ •id P rd β • 0 O dd tt CJ s QJ Cp β oj β O k rt Η β Cp -P 03 rt » · n qj g •id k -P k ft rt QJ rd · O ft I u P o 0) k tji OJ β k •id β > tt rt tt xi QJ k k ft QJ p β Qd 0 rt •id β P rt •H N 03 •H k 0) QJ k QJ dd $, rd 0) O g ft Λ +1 •id js •fi +1 0) Cn g Ό Ό Q) OJ g ϋ β Ό Ο k Ρ β •Η m m m CM CM CM in in in • • • O' tf tf in in in i—1 rd rd Cp Cp Cp Cp rd g CM cn vo CM • « 00 in h- CM in rd *-* «χ-» x_> ω cn cn rd rd rd • « . κ W K ft w ft β β β •id •id •id 03 rd 03 rd 01 s’''. CO o rd rt rt rt O II II o CM II II dd P rd W tt β tt β tt CM β β QJ a> QJ «—X k k k E tf 1« «X o ΦΧ •χ QJ H Q) H QJ rt g. Η Η x h χ H XJ $3 Η Η P W dd H 4J CM Ο CM VO Ε υ χλ 8^ CM cn cn cn rd rd rd β Ή tt rt X X w M β fi •id •id tt tt rt rt CM CM CM si1 in vo H rd rd P XJ CP •id QJ & >1 Λ QJ k rt tt β o Π3 P •id 0 O k dd tt ft QJ rt P k rt P II fl £ β QJ Ό O CM 0) β P O rt W rd O fc cn rt o O 0) 0) o cn QJ XJ u rd dd ft CM C- Md χ 53 U O tt Q> P rt β ‘id rt o QJ •id β •2 rt u •rl Tf O N rt QJ k -P •id rd 'ΰ QJ CP k rt χί u rd QJ QJ β O QJ g, rt o P k QJ Qd QJ k tt QJ CP rt P β QJ •tf CM o — CM CM k □ cm a ffl P k O CM 2 •id QJ ·« cm O rd ft β £ tf U 0 VO E X—’ O QJ •id W 2 CM If) XJ P U £ CM P rt

Claims (11)

1. CLAIMS:1. A process for the suspension polymerization of tetrafluoroethylene which comprises polymerizing tetrafluoroethylene in the presence of a catalyst and in the presence of, 5 as modification agent, from 0.0004 to 0.0029 mol %, calculated on the tetrafluoroethylene, of a perfluorinated ether of the formula CF 2 =CF— 0— R f (I) in which R^ is a perfluoroalkyl radical having from 1 to 10 10 carbon atoms, or of the formula 0-CF=CF_ (II) in which n is zero or an integer from 1 to 4, or of the formula ““ CF. cf 3 -cf 2 -cf 2 O-CF—CF_ O-CF=CF„ (III) 15 in which n is an integer from 1 to 4, or of a mixture of any two or more such ethers.

2. A process according to claim 1, wherein the modification agent is a perfluorinated ether of the formula (I) or of the formula (II) specified in claim 1, or a 20 mixture of any two or more such ethers.

3. A process according to claim 1 or claim 2, wherein R f is a straight chain radical having from 1 to 4 carbon atoms.

4. A process accdrding to claim 3, wherein R f is an n-perfluoropropyl radical.

5. A process according to any one of claims 1 to 4, wherein in formula (II) nis zero or 1.

6. A process according to claim 1, wherein the modification agent is a perfluorinated ether of the formula (III) specified in claim 1, or a mixture of any two or more such ethers.

7. A process according to claim 6, wherein in formula 0 (III) n is 1 or 2.

8. A process according to any one of claims 1 to 7, wherein polymerization is carried out in the presence of from 0.001 to 0.002 mol % of the modification agent, calculated on the tetrafluoroethylene. >

9. A process for the suspension polymerization of tetrafluoroethylene carried out substantially as described in any one of Examples 1 to 16 herein.

10. A tetrafluoroethylene polymer whenever prepared by a process according to any one of claims 1 to 9.

11. A moulded or sintered shaped article manufactured from a polymer according to claim 10.