DE4131746A1 - Melt spun fibre giving chemically resistant textiles, etc. - composed of copolymer contg. tetra:fluoroethylene, ethylene] and opt. alpha olefin(s) - Google Patents

Abstract

The fibres (I) contain a thermoplastic, fibre-forming copolymer (II) based on tetrafluoroethylene (TFE), ethylene and opt. other alpha-olefins (III). (II) has MFI of at least 50 g/10 mins., measured at 297 deg. C with a nozzle of dia. 2.1 mm and length 8 mm, under a 2-kg load. More specifically, (I) have individual fibre dia. of 5-50 microns and uniformity of titre with coefft. of variation of 1.1-1.7. (II) has m.pt. above 160 deg. C and MFI of 50-70 g/10 mins., and contains 60 mol.% TFE units, 40-60 mol.% ethylene units and 0-20 mol. units of (III). Pref. (II) is a terpolymer of TFE, ethylene and hexafluoropropene (HFP) or perfluoropropyl vinyl ether (PPVE). (I) have max. tensile strength above 8 (pref. 10-15) cN/tex and max. elongation 15-60 (pref. 20-40)%. (I) can be produced as multifilaments or staple fibre, esp. multifilaments with a round cross-section or textured (esp. false-twist or blow textured) fibres or extrafine monofilaments with single fibre dia. 10-100 microns. USE/ADVANTAGE - (I) are useful for the prodn. of textile sheets, esp. woven, knitted or nonwoven fabric, and for the prodn. of filters, demisters, bandages, sewing thread, protective clothing, packing for seals and geotextiles (claimed). The invention enables the prodn. of fine, highly uniform fibres from PTFE polymers on conventional melt-spinning equipment, without the addn. of a melt-spinnable matrix polymer and without a subsequent cleavage or peeling process. The fibres can be textured and/or processed as above to give textiles with high resistance to UV, chemicals, hydrolysis, etc

Description

The present invention relates to new fibers, in particular continuous filaments selected copolymers based on tetrafluoroethylene, a process for their manufacture and the use of these new fibers.

Polymers based on tetrafluoroethylene (PTFE) are known. These polymers are characterized by a number of advantageous properties, for example through good UV permeability and thus good UV resistance, through good Weather resistance, good dielectric properties and high Resistance to chemicals, especially due to good resistance to hydrolysis. The strong The hydrophobic surface of molded parts made from these polymers leads to a correspondingly low adhesion behavior, which can be found, for example, in a pronounced dirt repellency.

Monofilaments made from such polymers are known and commercially available. So z. B. ®Hostaflon monofilaments with diameters of about 0.2 to 0.6 mm used as weaving wires for demisters.

For a number of technical applications, for example for use in the Filter technology or for the production of water-repellent and chemical-resistant Tissue, would be fine-titer fibers instead of the coarse-titled monofilaments he wishes.

Also for textile use, for example for the production of UV-permeable Bathing, sports or leisure clothing, such fibers would be desirable.  

So far, however, it has proven difficult to make fibers from PTFE according to the to obtain conventional fiber production methods. Melt spinning polyfluoro Although lefins have already been described, for example in EP-B-47 091, but with this previously known method, a specially designed Spinneret can be used.

An overview of manufacturing processes previously introduced in technology of fibers from polyfluoroolefins is used in chemical fibers / textile industry 40/92, 1990, M 113 ff. After that, such fibers were previously based on the matrix Spinning processes, by paste extrusion or by splitting or peeling processes produced. In the matrix spinning process or in paste extrusion, this is done Polyfluoroolefin in powder form and with the help of a fiber-forming matrix Component spun or with the help of a lubricant under Strand formation deformed, the matrix component or the lubricant in one subsequent step, for example by oxidation.

Paste extrusion and the splitting or peeling process usually only lead to relatively coarse titre fibers. Also, the uniformity of the titers leaves with these Processes obtained fibers often leave something to be desired. In contrast let it produce extremely fine-titre fibers using the matrix spinning process. this will e.g. B. described in DE-C-34 11 662.

DE-A-21 30 039 describes a process for the production of fibers and threads known from PTFE with reduced electrical resistance. The one described there Paste extrusion does not lead to uniform titers.

The spinnability of PTFE polymers using conventional melt spinning processes to fine-titer filaments is possible with the thermoplastic PTFE Unreachable polymers.

The present invention is therefore based on the object of fibers To provide PTFE polymers by means of conventional  Melt spinning devices can be manufactured; the manufacture of the Fibers according to the invention should therefore already be possible during primary shaping, d. H. without the aid of a spinnable matrix polymer or without implementation a subsequent splitting or peeling process.

In addition, the present invention is intended to provide a method the one with which it is possible in a simple manner, fibers higher Produce uniformity and fine titers from PTFE polymers.

The present invention is also intended to provide new fibers made of PTFE polymers are provided that have small fiber diameters (low fiber titer) or which can be textured and which by means of known textile techniques textile fabrics can be processed further.

These tasks are achieved by the fibers defined in claims 1 and 2 and solved by the method defined in claim 11.

Preferred embodiments of the fibers according to the invention are in the Claims 3 to 10 described.

Virtually any can be used to produce the fibers of the invention thermoplastic and thread-forming copolymers based on tetrafluoroethylene, Ethylene and optionally at least one further copolymerizable therewith α-olefin can be used, provided that it has the value of Has melt index. These copolymers are new and also a Subject of the present invention.

These copolymers preferably have a crystalline melting point of more than 160 ° C, in particular a crystallite melting point of 250 to 270 ° C.

Copolymers used with particular preference have a melt index of 50 up to 70 g / 10 min.  

The melt index (MFI value) is according to the method given in claim 1 determined wisely. This provision corresponds to DIN standard 53 735.

Thermoplastic and fiber-forming copolymers are preferably used on the basis of tetrafluoroethylene and ethylene.

In addition to the two monomers mentioned above, they can be used according to the invention copolymers to be added are other copolymers which can be copolymerized therewith α-olefins.

Examples of such further copolymerizable monomers are

• a₁) perfluorinated olefins of the formula CF₂ = CF-Rfl, wherein Rfl is a perfluoroalkyl radical 1 to 10, preferably 1 to 5 carbon atoms, is preferred above all Hexafluoropropylene (HEP);
• a₂) perfluorinated vinyl ethers of the formula CF₂ = CF-O-Rf2, wherein Rf2 Perfluoroalkyl radical having 1 to 10, preferably 1 to 4 carbon atoms. To call are the perfluoroethyl, perfluoro-n-butyl and especially the perfluoro-n- propyl residue (PPVE);
• a₃) perfluorinated vinyl ethers of the formula wherein n = 1 to 4, preferably 1 or 2;
• a₄) perfluorinated vinyl ethers of the formula wherein n = 0 to 1, preferably 0;
• a₅) perfluoro-2-methyl-4-methyl-1,3-dioxolane;
• a₆) perfluorinated vinyl ethers of the general formula CF₂ = CF-O- (CF ₂ ) _n -COX₁, wherein X₁ is OH, OR₁ or NR₂R₃, and wherein R₁ is an alkyl group with 1 to 3 carbon atoms and R₂ and R₃ each for represent a hydrogen atom or for R₁ and n is a number from 1 to 10;
• a₇) fluorinated vinyl ethers of the formula wherein X₂ is COOR₄, COOH or CN, R₄ is an alkyl group having 1 to 3 carbon atoms and n is an integer from 1 to 4;
• a₈) perfluoroalkyl-substituted vinyl compounds of the formula CH₂ = CH-RF3, wherein Rf3 is a perfluoroalkyl radical with 2 to 10, preferably 2 to 6, carbon atoms;
• a₉) fluorine-containing olefins of the formula CH₂ = CRf4-Rf3, wherein Rf4 = F or CF₃ and Rf3 is a perfluoroalkyl radical with 1 to 10, preferably 1 to 6, carbon atoms, in particular 3,3,3-trifluoro-2-trifluoromethylpropylene;
• a₁₀) 1,1,1-trifluoro-2- (trifluoromethyl) -4-penten-2-ol
• a₁₁) allyl-1-hydroxy-hexafluoroisopropyl ether
• a₁₂) compounds of the general formula CH₂-CH- (CH ₂ ) _n -O-CF₂-CFX₃H, wherein X₃ = F, Cl or trifluoromethyl, preferably F, and n is zero or 1;
• a₁₃) allyl or methallyl esters of the formula CH₂-CR₅-CH₂-O-CO-R₆, wherein R₅ = H or CH₃ and R₆ is an alkyl radical having 1 to 3 carbon atoms, preferably a Is methyl;
• a₁₄) vinyl esters of the general formula CH₂ = CH-O-CO-R₇, wherein R₇ is an alkyl ester with 1 to 3 carbon atoms, preferably a methyl radical;
• a₁₅) α-olefins having 3 to 4 carbon atoms, preferably isobutylene;
• a₁₆) acrylic acid esters and methacrylic acid esters, preferably their methyl bis Butyl ester;
• a₁₇) vinylidene fluoride and
• a₁₈) trifluorochloroethylene.

A terpolymer based on is particularly preferably used Tetrafluoroethylene, ethylene and hexafluoropropene or based on Tetrafluoroethylene, ethylene and perfluoropropyl vinyl ether or a quater polymer based on tetrafluoroethylene, ethylene, hexafluoropropene and Perfluoropropyl vinyl ether.

The quantitative ratios of the individual monomers in the invention Copolymers to be used can vary within wide limits, provided that the required MFI value can be set.

For example, the tetrafluoroethylene can range up to 60 mol%, in particular from about 30 to 60 mol%, the ethylene in the range from about 60 to 40 mol% and the third comonomer in the range from 0 to 20 mol%, preferably 0 to 15 mol% be present in the copolymer. Here are the Molar data on the total amount of the monomers present in the polymer based.

Copolymers used with preference consist of up to 60 mol% copolymerized units of tetrafluoroethylene, from 40 to 60 mol% copolymerized units of ethylene, from 0.05 to 15 mol% copolymerized Units of hexafluoropropene or perfluoropropyl vinyl ether.  

Further preferably used copolymers consist of 30 to 55 mol% copolymerized units of tetrafluoroethylene, from 40 to 60 mol% copolymerized units of ethylene, from 1.5 to 10 mol% copolymerized Units of hexafluoropropene and from 0.05 to 2.5 mol% of copolymerized Units of perfluoropropyl vinyl ether.

The thermoplastic copolymers suitable according to the invention can be obtained by known copolymerization processes can be produced. examples for Such manufacturing processes can be found in IT-PS-8 75 765 and in DE-AS- 22 33 288, DE-A-23 55 469, DE-A-24 44 516 and DE-A-37 44 392.

The copolymers described in these documents must, however have melt index listed above to according to the present Invention can be used. In individual cases, this may require to lower the melt index of the copolymer using known methods and set to the required value.

Examples of such processes are the targeted adjustment of the molecular weight in the preparation of the copolymer by adding regulators or the targeted molecular weight reduction of the copolymer, for example by controlled thermal degradation of the copolymer in the extruder.

It is particularly preferred to use the preparation according to the invention Fibers terpolymers based on tetrafluoroethylene, ethylene and hexafluoropropene, which in particular has a melt index value of about 50 to 70 g / 10 min exhibit.

The fibers of the present invention can be analogous to others Thermoplastics, e.g. B. in polyesters, known melt spinning processes will. For this purpose, the melted copolymer is at a temperature of at least 30 ° C above the crystallite melting point of this copolymer extruding a spinneret into a spinning shaft, the extrudate in said  Allow the spin chill to cool and the continuous filaments formed with a Speed of at least 800 m / min, preferably more than 1000 m / min deducted.

The continuous filaments can be cooled in the spinning shaft by blowing on them a fluid, preferably with air. One such measure is but not absolutely necessary; you can also use the endless filaments without Allow the supply of a fluid to cool in the spinning shaft.

The spinning temperature is, among other things, dependent on the respective Copolymers, its melt index and process parameters, for example the desired take-off speed. For the most preferred Copolymers with a crystallite melting point of 250 to 270 ° C and a Melt index of 50 to 70 g / 10 min, the spinning temperature is preferably in Range between 280 and 310 ° C, especially selected at about 300 ° C.

The spinning is usually done by spinnerets, as in the case of conventional melt spinning processes can be used. It is about for example around spinnerets with 50 to 100 nozzle holes and diameters the nozzle holes from 0.05 to 0.3 mm.

The nozzle holes can have different shapes; thus can on fiber cross-sections can be varied in a simple manner. The fibers of the invention can have any cross section; for example barbell or kidney-shaped, triangular, tri- or multilobal or especially round Cross sections. Hollow fibers can also be produced.

After spinning in a spinning shaft, the extrudate is in said Cool the spinning shaft by contact with a fluid, preferably with air calmly. The cooling process is preferably accelerated by blowing. The blowing can be directly below the spinneret or further below it Spinning nozzle take place.  

The spun threads can pass through the Spinneret are reheated to recrystallize the filaments obtained to effect. In this variant of the method, a blowing with a fluid is intended take place, this is usually below the device for reheating carried out.

The spun threads are at the bottom of the spinning shaft in the generally provided with a spin finish and then with usual Deduction devices, for example with godets. The Take-off speed is usually more than 800 m / min, preferably between 1000 and 3000 m / min. It is also possible to add the filaments Subtract speeds of more than 3000 m / min. Such fast spun filaments are usually sufficiently pre-oriented so that an additional stretching step can be omitted.

After spinning, the multifilaments formed in the generally wound up and then, if desired, treated, for example by stretching, if necessary relaxing and texturing. The Fibers according to the invention can be textured, both according to the known ones False wire process as well as after the air blowing process.

The stretching can take place on conventional drafting systems, for example by two moving at different speeds Roller pairs, the speed ratio of which defines the degree of stretching. The multifilament yarn according to the invention can be located between these pairs of rollers by treatment with a heat transfer medium to the stretching temperature are heated, for example by superheated steam, by saturated steam or by glycerin. The heat necessary for the stretching process can, however can also be supplied by other means known per se, for example by Heating of the rolls by heat treatment before entering the first A pair of rollers of the drafting system, by a drafting pin ("hotpin") or by a Iron.  

The texturing step can be carried out using all of the usual methods and devices be carried out, for example in a false wire texturing apparatus or by means of a blow nozzle.

Those obtained by the melt spinning process according to the invention Continuous filaments can then be used as such for the production of textiles Sheets, such as woven, laid, knitted or knitted, are used be cut or cut into staple fibers. These staple fibers can then in turn, possibly in combination with staple fibers from other fibers, processed into yarns using conventional secondary spinning processes will. Those obtained by the melt spinning process according to the invention Continuous filaments can also be used for nonwovens, for example spunbonded fabrics can be processed by e.g. B. the spun fibers immediately then stretched, e.g. B. using a fluid, and the fibers obtained in Spreading texture on a conveyor belt and this primary fleece in itself mechanically stabilized in a known manner and then solidified, for. B. by Applying a binder.

The fibers of the invention can be dyed using methods known per se be, for example, by spin dyeing. The dyeing step can also first at the level of the textile fabric, for example by Printing.

The hiding ratio and the drawing temperature are usually in Dependence on the selected spinning delay and on the copolymer used choose.

The draw ratio is, for example, 1: 1.1 to 1: 5. The stretching takes place, for example, at temperatures between 120 and 260 ° C.

The titer of the multifilament yarns obtained in this way is usually about 50 to 500 dtex.  

The diameter of the individual fibers obtained can be within wide limits vary. So can both Manufacture monofilaments that e.g. B. have a diameter of 0.1 to 0.3 mm can; but it can also produce multifilaments, the z. B. diameter can have less than 100 microns.

Multifilaments with a single fiber diameter of are particularly preferred less than 50 µm, in particular less than 20 µm and very particularly of 5 to 15 µm. Of these multifilaments, those are particularly preferred have a practically round cross-section.

The single fiber titer of the fibers according to the invention is characterized by a high Uniformity. The thread uniformity or the titre uniformity of Multifilament yarns are expressed by the USTER value (DIN 53 817). For To determine this value, fiber bundles are passed through a measuring device, which converts the mass fluctuations of the fiber structure into a proportional one converts electrical signal. As a measure of the fluctuations in mass, the Use the coefficient of variation (cV value). This is defined as the quotient of the Standard deviation s of the mass fluctuations and the mean value X of the mass of the fiber structure (cV = s / X). The cV value is ideally round Cross section at 1.0. The fibers according to the invention generally have cV Values between 1.1 and 1.7 (measured using the ®USTER 2-C tester Zellweger-Uster AG, Uster, Switzerland). This titre uniformity is in front known fibers based on polymers of fluoroolefins so far not reached.

Multifilaments made of polyfluoroolefins with a single fiber diameter from 5 to 50 µm and a cV value of 1.1 to 1.7 are not yet known have become and are also the subject of the present invention.

The fibers produced by the method according to the invention have generally a fineness-related maximum tensile force above 8 cN / tex, preferably  over 10 cN / tex, and their maximum tensile elongation is generally 15 and 60%, especially 20 to 40%.

Multifilament yarns from the fibers according to the invention are characterized by a good processability to textile fabrics, such as fabrics, scrims, Knitted knitted or nonwovens. They are particularly noteworthy Uniformity and fineness of the titer of the fibers and yarns produced, what are the reasons for the good processability in these processes.

The fibers according to the invention and the textile fabrics produced therefrom are characterized by good resistance to UV and visible electromagnetic radiation, due to high chemical resistance, in particular due to good hydrolysis resistance, due to pronounced dirt repellency, through a high translucency especially for UV radiation and good electrical Isolator effects. They also have fibers and those made from them textile fabrics have low sliding friction.

The continuous filaments or staple fibers of the present invention Continuous filaments can also be used in blended yarns or in blends with others Yarns are used, for example in combination with continuous filaments or staple fibers made of polyester or polyaramid yarns or in a mixture with Yarns made from such polymers.

Areas of use for yarns containing the continuous filaments or Staple fibers made from these continuous filaments are, for example, filters, demisters, Wound covers, sewing threads, protective clothing, sealing packs or Geotextiles.

The following examples illustrate the invention without limiting it.

example 1

A copolymer is spun that was made by Copolymerization of 48 mol% tetrafluoroethylene, 48 mol% ethylene and 4 mol% Hexafluoropropene, where the percentages refer to the sum of the monomers Respectively. The copolymer used has a melt index (MFI value) of 50 to 70 g / 10 min (measured at 300 ° C with a nozzle of 2.1 mm diameter and a length of 8 mm under a load with a mass of 2 kg). The MFI value is obtained by adding a known controller to the Copolymerization stopped.

The copolymer is dried for 10 hours at 11 ° C and then in one spun conventional melt spinning apparatus. It will have a spinneret 64 holes and a nozzle hole diameter of 0.2 mm are used. The Spinning temperature is 298 ° C and the take-off speed is 1000 m / min. The extruded multifilaments become one with air in the spinning shaft Blown sideways at a speed of 0.1 m / s. The thread tension in the Spinning shaft is 14 cN. The spun multifilament yarn is at the exit of the spinning shaft with a spinning preparation and then wound up.

The multifilament yarn obtained is then in a conventional Stretching apparatus stretched 1: 1.2 at a temperature of 180 ° C and then textured in the usual way using a friction swirl device. Man receives a multifilament yarn with a yarn count of 200 dtex and one Single filament titre of 3.1 dtex. The fineness-related maximum tensile force of this Yarn is 8.7 cN / tex and the maximum tensile strength is 52%. The yarn can easily be used in knitting, weaving and knitting.

Example 2

One works with the same copolymer and after the same Procedure as described in Example 1, with the modification that the  Drawing at a temperature of 130 ° C and at a drawing ratio of 1: 1.72 is carried out.

A multifilament yarn with a yarn count of 111 dtex is obtained. The The maximum tenacity of this yarn is 14.1 cN / tex and the Maximum tensile elongation is 7.1%. The yarn can be easily in of knitting, weaving and knitting.

Example 3

One works with the same copolymer and after the same Procedure as described in Example 1, with the modification that the Drawing at a temperature of 130 ° C and at a drawing ratio of 1: 1.615 is carried out.

A multifilament yarn with a yarn count of 119 dtex is obtained. The The maximum tenacity of this yarn is 13.4 cN / tex and the Maximum tensile elongation is 7.5%. The yarn can be easily in of knitting, weaving and knitting.

Example 4

One works with the same copolymer and after the same Procedure as described in Example 1, with the modification that the Drawing at a temperature of 93 ° C and with a drawing ratio of 1: 1.209 is carried out.

A multifilament yarn with a yarn count of 159.5 dtex is obtained. The The maximum tenacity of this yarn is 12.2 cN / tex and the Maximum tensile elongation is 2.9%. The yarn can be easily in of knitting, weaving and knitting.  

Examples 5 to 7

You work with the same polymers and the same procedure as described in Example 1, with the modification that the spin titer by Variation of the spinning take-off speed is changed and that the stretching is carried out at 93 ° C. The following results were obtained:

Example 8

Production of fabrics from the multifilaments according to the invention. Woven fabrics were made from the multifilaments according to the invention and from commercially available polyester fibers. The following table shows characteristic parameters of these fabrics. As polyester fibers in the Warp threads of 50 f 20 dtex each used. As the invention Fibers are used in the weft threads of the titre 90 to 190 f 64 dtex.

Example 9

Production of knitted fabrics from the multifilaments according to the invention The fibers according to the invention with a titer of 90 to 120 f became 64 dtex Knitted fabrics made with the following parameters

Claims (15)

1. melt-spun fibers containing a thermoplastic and fiber-forming copolymer based on tetrafluoroethylene, ethylene and optionally at least one further copolymerizable therewith α-olefin, characterized in that the copolymer has a melt index, measured at 297 ° C with a nozzle of 2.1 mm in diameter and a length of 8 mm under a load with a mass of 2 kg, of at least 50 g / 10 min. 2. Melt-spun fibers in the form of multifilaments containing a thermoplastic and fiber-forming copolymer based on Tetrafluoroethylene, ethylene and optionally at least one other α-olefin copolymerizable therewith, characterized in that the fibers a single fiber diameter of 5 to 50 microns and the multifilaments one expressed by a coefficient of variation (cV value) of 1.1-1.7 Show titer uniformity. 3. Fibers according to claim 1 or 2, characterized in that the thermoplastic copolymers have a crystalline melting point of more than 160 ° C. 4. Fibers according to claim 1 or 2, characterized in that it is the thermoplastic copolymer is a copolymer, containing up to 60 mol% recurring structural units derived from Tetrafluoroethylene, 40 to 60 mol% of repeating structural units derived from ethylene and 0 to 20 mol% of recurring Structural units derived from copolymerizable therewith α-olefinic comonomers. 5. Fibers according to claim 4, characterized in that it is in the thermoplastic copolymers around a terpolymer based on  Tetrafluoroethylene, ethylene and hexafluoropropene or based on Tetrafluoroethylene, ethylene and perfluoropropyl vinyl ether. 6. Fibers according to claim 1, characterized in that the melt index of the thermoplastic copolymer is 50 to 709/10 min. 7. Fibers according to one of claims 1 to 6, characterized in that this a fineness-related maximum tensile force above 8 cN / tex, preferably 10 to 15 cN / tex and a maximum tensile strength extension of 15 and 60%, in particular 20 to 40%. 8. Fibers according to one of claims 1 to 7, characterized in that it are multifilaments or staple fibers, in particular Multifilaments with a round cross-section. 9. Fibers according to one of claims 1 to 8, characterized in that it textured, especially false-wire textured or blast textured fibers. 10. Fibers according to claim 1, characterized in that it is Fine wire monofilaments with a single fiber diameter of 10 to 100 µm. 11. A process for making the fibers of claim 1 comprising the Extruding melted thermoplastic and fiber-forming Copolymers based on tetrafluoroethylene, ethylene and optionally at least one further α-olefin copolymerizable therewith at one Temperature of at least 30 ° C above the crystallite melting point this copolymer through a spinneret into a spinning shaft and Cooling the extrudate in said spinning shaft, preferably by Blowing with a fluid, especially with air, and pulling off the continuous filaments formed at a speed of at least  800 m / min, characterized in that the thermoplastic used Copolymers have a melt index, measured at 297 ° C with a nozzle of 2.1 mm diameter and a length of 8 mm under a load with has a mass of 2 kg, of at least 50 g / 10 min. 12. Use of the fibers according to one of claims 1 to 10 for the manufacture Elution of textile fabrics, especially of fabrics, knitted fabrics, Knitted or nonwovens. 13. Use of the fibers according to one of claims 1 to 10 for the manufacture filter, demister, wound cover, sewing thread, Protective clothing, sealing packs or geotextiles. 14. Nonwoven fabric, characterized in that these fibers according to one of the Claims 1 to 8 contains. 15. Thermoplastic and fiber-forming copolymer based on Tetrafluoroethylene, ethylene and optionally at least one other α-olefin copolymerizable therewith, which has a melt index measured at 297 ° C with a nozzle of 2.1 mm diameter and a length of 8 mm under a load with a mass of 2 kg of at least 50 g / 10 min having.