ELASTIC FIBRE

Abstract: Elastic fibre of a composition containing a thermoplastic polymer and a chemically cross-linked rubber, in which the fibre has a permanent elongation after 50 % stretching of at most 5 % and process for producing an elastic fibre comprising the melt spinning of a composition comprising a thermoplastic polymer and a rubber, wherein the rubber is completely or almost completely cross-linked at the moment the fibre is formed and the formed fibre is drawn after cooling down.

Description: ELASTIC FIBRE The invention relates to an elastic fibre of a composition containing a thermoplastic polymer and a chemically crosslinked rubber. A fibre of this kind is disclosed in JP-A- 68/26348, which describes a fibre consisting of a mixture of polyethylene and a chemically crosslinked ethylene-propylene-1, -hexadiene rubber. A drawback of this known fibre is the high permanent elongation which occurs on stretching of the fibre. The aforementioned application indicates that, after the fibre is stretched to 50% more than its original length, it recovers from this stretch for not more than 85%. Thus, the length of the fibre has permanently increased by at least 7.5% of its original length. This significantly limits the possibilities of application of the known fibre as a component imparting elastic properties to a yarn or fabric. The invention now provides for a fibre with a permanent elongation of at most 5% after the fibre is stretched 50%. Indeed, a fibre has been found with a permanent elongation of at most 3 and even at most 1% after the fibre is stretched 50%. The permanent elongation is determined in relation to the fibre's length before it is stretched. Thus, the elastic fibre of the invention is characterized by an exceptionally high rate of elastic recovery and, besides that, it also has a high elongation at break. Many applications of elastic fibres involve elongations substantially higher than 50% of the original length. Even at these substantially higher elongations the fibre of the invention has been found to exhibit excellent elastic recovery. Even on being stretched 100% of the original length, the fibre of the invention exhibits a permanent elongation of at most 15% and in many cases of at most 10% or even 5% or 2% of the fibre's length before stretching. The permanent elongation after stretching, henceforth referred to as 'tension set', is measured at room temperature by gripping a fibre of given length in the jaws of a tensile testing machine and moving the jaws apart at a speed of 200 mm/min until the desired stretch is reached. To this end, markings are provided on the fibre at a distance of 50 mm, 1 0 . The fibre is kept in the stretched condition for 10 seconds, whereupon the tensile force acting on the fibre is removed and the fibre is removed from the jaws. After allowing the fibre to relax at room temperature for 1 hour, the tension set in % is determined by dividing the difference in distance between the markings, 1, on the fibre that has been allowed to relax on being stretched and the original distance, 1 0 , between the markings by that original length 1 0 and multiplying the quotient by 100. Suitable thermoplastic polymers in the fibre of the invention are linear or branched polymers having a processing temperature, in particular a melt temperature, which is below the temperature at which appreciable thermal degradation occurs in the polymer. Examples hereof are polyolefins, in particular polyethylene and polypropylene, polyamides, in particular nylon-6, nylon-6,6 and nylon-4,6, polyesters, in particular polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), pol (meth)acrylates, polycarbonate, polyvinyl polymers and aromatic vinyl polymers. It is preferred for the fibre to contain semicrystalline thermoplastic polymers because such fibres have been found to have a lower tension set. Examples hereof are polyolefins, polyamides, polyesters and polyvinyl polymers. The best tension set is exhibited by fibres in which semicrystalline polymers are present having a glass transition temperature of the amorphous phase which is below 80°C and preferably below 50°C. Examples hereof are polyolefins, polyamides and polyvinyl polymers; the latter should preferably be mixed with a plasticizer. Mixtures of different thermoplastic polymers may also occur in the fibres. It is preferred for the thermoplastic polymer to be a polyolefin. By a chemically crosslinked rubber is meant a rubber which through chemical reactions has been formed into an insoluble and unmeltable polymer, the molecule chains in which are interlinked to form a three- dimensional network structure. Examples of the said reactions are described in the Encyclopedia of Polymer Science and Engineering, Second Edition, John Wiley and Sons, Volume 4, page 350 et seq. and page 666 et seq. Suitable rubbers for the fibre of the invention are acryl rubbers, butyl rubbers, halogenated rubbers, for example brominated and chlorinated isobutylene-isoprene, (styrene-)butadiene rubbers, butadiene-styrene-vinylpyridine rubber, nitrile rubbers, natural rubber, urethane rubbers, silicone rubbers, polysulphide rubbers, fluorocarbon rubbers, ethylene-propylene-(diene-)rubbers (generally referred to as EP(D)M rubbers), polyisoprene, polyepichloro- hydrin, chlorinated polyethylene, polychloroprene, chlorosulphonated polyethylene. Preferably, the fibre contains the economically attractive and commonly used acryl rubbers, (styrene-)butadiene rubbers, butyl rubbers, chlorinated polyethylene, polychloroprene, chlorosulphonated polyethylene, polyepichlorohydrin, ethylene-propylene-(diene-)rubbers, nitrile rubbers, natural rubber, polyisoprene or silicone rubbers. EP(D)M rubbers are highly suitable. The fibre may also contain mixtures of different rubbers, at least one of which is chemically crosslinked. The rubber in the fibres may be crosslinked by any known technique, the most suitable technique being chosen for each rubber. Crosslinking is usually effected under the influence of crosslinking agents, familiar examples of which are sulphur, peroxides, metal oxides, epoxy resins, quinone dioximes, phenol resins, alkylphenol formaldehyde resins, diurethanes, bismaleimides and amines. Halogenated butyl rubber, for example, can be crosslinked with zinc oxide but also by using resins, for example (brominated) phenol resin and urethane resin. These resins are also suitable crosslinking agents for, for instance, EPDM rubber. Organic peroxides and sulphur are also known and suitable crosslinking agents. Crosslinking may optionally be effected in the presence of accelerators and/or activators. It is preferred for the composition to be a thermoplastic vulcanizate. A thermoplastic vulcanizate, known per se and usually abbreviated to TPV, is obtained by static or dynamic vulcanization or crosslinking of the rubber in the presence of the thermoplastic polymer. Dynamic vulcanization means a process by which in a composition containing an uncrosslinked rubber and a thermoplastic polymer the rubber is crosslinked under shear. Such dynamic vulcanization can take place in the known mixing devices, for instance roll mills, Banbury mixers, continuous mixers, kneaders and mixing extruders, of which twin-screw extruders are preferred. A summary of the known dynamic vulcanization techniques is given in Paper No. 41 of the Meeting of the Rubber Division of the American Chemical Society, November 4, 1992, in Nashville, Tennessee, USA. The choice of crosslinking agent is determined in the first instance by its ability to crosslink the rubber. In addition, the crosslinking agent should be so chosen that the crosslinking agent has no undesirable effect on the thermoplastic polymer. Peroxides, for instance, are known to cause crosslinking of polyethylene. Hence, peroxides are less suitable as crosslinking agent for the rubber when polyethylene is used as thermoplastic polymer. In any case, where this is not known in the art, those skilled in the art can establish through simple experiment whether the envisaged crosslinking agent and the envisaged thermoplastic polymer are compatible with each other. The rubber may contain the usual additives. Examples hereof are hardening agents, accelerators, retarders, activators, fillers, extenders, plasticizers, other polymers, colour modifiers, antidegradants such as antioxidants, antiozonants, compatibilizers, thermal stabilizers and UV stabilizers. In choosing and determining the number of rubber parts by weight in the fibre, the rubber exclusive of the additives present in it is used as a basis. The fibre may further contain substances that can have an effect on the appearance, the processability and the properties in use. Examples hereof are matting agents, paints, pigments and light, UV and heat stabilizers. The fibres of the invention have a titre of 1-1000 dtex, preferably between 2 and 500 dtex, more preferably between 5 and 250 dtex. The elongation at break practically coincides with that of the rubber itself when crosslinked and amounts to at least 100% and may be as high as 400% or even 600%. The fibres are particularly suitable for imparting elastic properties to textile materials, fabrics and knittings. Examples hereof are bathing suits, underwear, sports clothes, leisure clothes, stockings, tights, socks, elastic bands in clothes, diapers and medical bandages. The fibres of the invention may be applied as they are, but it is also possible for other fibres, particularly polyamide and cotton, to envelop them or to be wound or spun round them, or the fibres may be processed together with other fibres by known techniques to form elastic yarns. "Fibre" as used earlier and later herein should be taken to include a tape or film and in general any object measuring at most 1000 μm, preferably at most 500 μm, more preferably at most 250 μm, and most preferably at most 100 or even 50 μm, in at least one direction. The cross-section of the fibre may be round, oval or multi-lobed, for instance tri- lobed. Examples of such shapes are to be found in Introductory Textile Science, Fifth Edition, by Marjory L. Joseph, published by Kolt, Rinehart and Winston Inc. , page 40. The invention also relates to a process for the manufacture of an elastic fibre as defined above, comprising the melt spinning of a mixture of a thermoplastic polymer and a rubber. A process of this kind is also known from JP- A-68/26348. In the said application, a fibre is produced by melt spinning a mixture of an uncrosslinked EPDM rubber and polyethylene. The fibre obtained is subjected to a crosslinking operation. A drawback of this known process is that the fibre produced by it exhibits very moderate elastic recovery. This manifests itself in the fibre, on being stretched 50%, recovering from that elongation for only 85%, so that the tension set is at least 7.5%. This significantly limits the possibilities of application of the fibre produced by the known process as a component imparting elastic properties to a yarn or fabric. The object of the invention is to provide a process that does not have this drawback or that has this drawback to a significantly lesser degree. This object is achieved by the invention in that the rubber is completely or almost completely crosslinked at the moment the fibre is formed and in that the formed fibre is drawn after cooling down. Surprisingly, the process of the invention has proved capable of producing elastic fibres exhibiting significantly better elastic recovery than is possible with the known process. It has proved to be possible thus to produce fibres with a tension set of at most 5% and even at most 3% and even 1% after stretching by 50% or even 100% of the original length. A further advantage of the process is that it allows significantly higher production rates to be achieved than the known process. This is achieved on the one hand by the high spinning speeds that have proved to be possible and is also due in large part to the process of the invention starting from already crosslinked rubber, so obviating the need for the time- consuming crosslinking of the spun fibre. Another advantage of the process of the invention is that the rubber in the fibre obtained is homogeneously crosslinked, whereas the degree of crosslinking in the fibre obtained by the known process may be expected to vary across the cross-section of the fibre. This results from the circumstance that crosslinking is effected by causing a crosslinking agent to act from outside only after the fibre has been formed. The good spinnability at a high spinning speed of a composition containing a completely or almost completely crosslinked rubber is surprising in itself, because the dynamic viscosity of such compositions at the required spinning temperature, which is 150-350°C depending on the thermoplastic polymer applied, is between 1,000,000 and 1000 Pa.s at shear rates of 0.1 and 200 /s, respectively. According to the textbook Plastic Extrusion Technology, ed. Friedhelm Hensen, Hansen Publishers, Munich, page 566, usual values for the viscosity of a spinnable composition are in the range from 80 to 300 Pa.s if an acceptable spinning speed is to be achieved. Considering the high viscosity, a person skilled in the art would expect the highest attainable spinning speed to be 10 m/min. However, much higher spinning speeds of from 100 to 200 m/min and higher have been found to be possible. Constraints, if any, are imposed only by the limited capabilities of the spinning equipment, not by the spinnability of the mixture. This being so, one may assume that significantly higher spinning speeds up to 1000 or even 2000 m/min are within the realm of possibility. Another advantage of the process of the invention is the possibility of producing very thin fibres in a simple manner. Thus, fibres with a titre of 2 and even 1 dtex can be produced. Generally, and also in the process of the invention, the production of thicker fibres entails fewer problems, such as yarn breakage, than the spinning of thin fibres. Thicker fibres of up to for instance 25, 50, 100 or even 250 dtex can be produced simply by using larger spinneret holes. Still thicker fibres of up to 500, 1000 or more dtex are possible although at such thicknesses one should really speak of a thread or a tape. Even at such thicknesses, the good spinnability of the starting composition affords the aforementioned process advantages whilst even then the favourable material properties are present in the products produced. In the process of the invention a composition comprising a thermoplastic polymer and a rubber is spun, with the rubber being completely or almost completely crosslinked the moment the fibre is formed. The composition usually contains 10-90 parts by weight of rubber against 90-10 parts by weight of thermoplastic polymer and preferably 30-75 parts by weight rubber against 70-25 parts by weight of polymer. Most preferably, the composition contains 60-70 parts by weight of rubber against 40-30 parts by weight of thermoplastic polymer. In determining the rubber content, the rubber should be considered exclusive of any additives present in it. Suitable and , preferred rubbers and thermoplastic polymers are those described in the foregoing as being suitable and preferred for the elastic fibre of the invention. The usual and known additives mentioned there may be added to the composition to be spun. The process can be carried out using any composition that has the required characteristics. From a process engineering point of view it is advantageous for the composition to be prepared and spun in a single continuous process operation. It is preferred for the composition comprising the crosslinked rubber and the thermoplastic polymer to be prepared from a mixture of non-crosslinked rubber and the thermoplastic polymer in the presence of a crosslinking agent. It is acceptable for the rubber to be already somewhat crosslinked when it is mixed with the thermoplastic polymer. It is essential, however, that at that point the rubber should be non-crosslinked to the extent that it still behaves as a thermoplastic and is miscible with the thermoplastic polymer in the melt. Suitable methods of preparing the composition have been described in the foregoing. Preferably, the composition is a TPV produced by dynamic vulcanization as described in the foregoing. In general, the mixing and kneading applied herein is continued until the rubber is completely or almost completely crosslinked. By this is meant that the rubber is crosslinked far enough for it to have such elastomeric properties as are commonly associated with a rubber that has been vulcanized in the usual manner, that is, as such and not dynamically in the presence of a thermoplastic polymer. The extent to which crosslinking has progressed can be characterized by the rubber fraction that is extractable from the dynamically vulcanized composition at elevated temperature by a solvent for the rubber. Preferably, this fraction is at most 40 wt.%, more preferably at most 25 wt.% or even at most 10 wt.% but most preferably at most 5 wt.% referred to the amount of rubber in the mixture. The tension set decreases as the extractable fraction decreases. The determination of the extractable rubber fraction is a technique known per se in the art. As solvent a solvent is used which is known to be good for the rubber in question. In general, for instance, boiling xylene is used for determining the extractable fraction in EP(D)M. Part of the crosslinking operation may also take place during the spinning step. In this spinning step the composition is melted, homogenized and conveyed to the spinning head, where the actual formation of the fibre takes place. As a rule, the said operations take place at an elevated temperature and under the exertion of shear stresses and so under conditions conducive to dynamic vulcanization. The wholly or, as described above, possibly only partially crosslinked composition may be fed to a spinning apparatus. The mixing equipment may be integrated with the spinning apparatus, which in that case is composed of for instance an extruder in which the rubber and the thermoplastic polymer are mixed with simultaneous crosslinking of the rubber. The composition may be heated in that process to a temperature higher than the melting point of the thermoplastic polymer, where it becomes melt-processable. The composition may be supplied in that form to a spinneret which closes the extruder, the spinneret having spinning holes of the desired shape and size and in the desired quantity. The molten composition may also be supplied to a spinning pump and from there to a spinneret. In that case, the actual formation of the fibres takes place in the spinneret. In that location the composition is present in melt-processable form and the rubber is completely or almost completely crosslinked. If desired, the preparation and the spinning of the composition may take place at separate times and places. The composition, which may or may not be completely crosslinked, may, optionally after cooling, be reduced in size and the granulate obtained or the original lumps may be supplied later and/or elsewhere to a spinning apparatus where the rubber is crosslinked further if necessary and the composition, together with the crosslinked rubber, is remelted and supplied to the spinneret as a melt. As spinning apparatus any known apparatus may be used which is optionally capable of preparing the composition with or without simultaneous crosslinking of the rubber but which is in any case capable of melting the composition and forcing the molten composition through a spinneret having holes of the desired shape and size at the desired speed. If necessary, it should also be possible for the conditions required for complete or partial crosslinking of the rubber to be established in the spinning apparatus. The fibre is spun in the air or in a space in which an inert gas or liquid is present. Depending on the composition used, the gas, air or liquid may be kept at ambient temperature or at an elevated temperature, the latter in any case being below the melting point of the thermoplastic polymer. The fibre will cool in the process, obtain a stable form and may be wound onto a bobbin. The fibre can be spun and wound onto a bobbin as a monofilament but also as a multifilament. The fibre may be subjected to a draw¬ down operation during or immediately after spinning, when the fibre still is in wholly or partially molten condition. In this way, fibres with a lower titre can be obtained. The fibre may further be subjected to post- treatments that are usual for fibres, such as drawing, heat treatment, shrinking, crimping and dyeing. Other fibres or yarns of for example polyamide, cotton and polyester may be spun round the fibre, or the fibre may be co-spun with other fibres or yarns or be knit or woven. Drawing of the fibre after it has been cooled down preferably to room temperature has appeared to improve the tension set considerably. Preferably the fibre is drawn to at least twice its original length and more preferably to at least 3 times its original length. The invention is illustrated by the following examples without being limited thereto. The fibres were spun with a a Fourne Spintester having a spinning pump of 1.2 cc. or with a Gδttfert Viscotester 1500 with a spinneret having a length L of 10 mm, a diameter D of 0.5 mm (L/D = 20). The diameter of the barrel was 12 mm and the plunger speed was 1 mm/s. The mechanical properties were investigated using a Zwick 1435 tensile testing machine at a crosshead speed of 20 cm/min and with the grips 5 cm apart. Example I A mixture of 39.8 parts by wt. Nylon-6 (Akulon® K120), 59.4 parts by wt. g of a nitrile butylrubber (Perbunan® N2807) and 0.2 parts by wt. Flectol® H/DS as stabilizer was melted in a ZSK53 twin- screw extruder, which was adjusted to 240°C. Through a side feeder 0.6 parts by wt. Perkadox® was added as crosslinking agent to the extruder. The throughput of the extruder was 25 kg/h. The rubber at the outlet of the extruder was not yet completely crosslinked due to the relatively short residence time of the mixture in the extruder. Therefore, the extrudate was immediately rapidly cooled to prevent further crosslinking outside the extruder. The partially vulcanized composition was fed to a ZSK30 twin-screw extruder for further crosslinking. The almost completely crosslinked extrudate so obtained was granulated. A yarn having the following properties was spun from the TPV granulate in the Viscotester 1500: Titre 1535 dtex Tensile strength 0.18 N/tex Elongation at break 230% Tension set after 50% elongation 3% Tension set after 100% elongation 11% Example II Sarlink® 4175 natural, a TPV consisting of polypropylene as thermoplastic component and oil- extended EPDM crosslinked with phenol resin as crosslinked elastomeric component was spun into a fibre at 200°C using a Viscotester 1500. The fibre had the following properties: Titre 1687 dtex Tensile strength 0.07 N/tex Elongation at break 550% Tension set after 50% elongation 2% Tension set after 100% elongation 10% A portion of the fibre was drawn to 5 x the original length, kept in drawn condition for 30 sec and then relaxed for 1 hour. The drawn fibre had a titre of 945 dtex. The tension set of the drawn fibre after 50% elongation was 1%, after 100% elongation 2% and after 200% elongation 3%. Example III Sarlink® 2160 natural, a TPV consisting of polypropylene as thermoplastic component and oil- extended isoprene-isobutylene rubber crosslinked with peroxide as crosslinked elastomeric component (Sarlink® 2160 natural) was spun into a fibre at 200°C using a Viscotester 1500. The fibre had the following properties: Titre 1593 dtex Tensile strength 0.055 N/tex Elongation at break 325% Tension set after 50% elongation 4% Tension set after 100% elongation 12% Example IV The following materials were successively added to a Haake 50 cc Banbury mixer: at t=0 11.6 g of polypropylene (Stamylan® 13E10), 24.7 g of EPDM (Keltan® 578), 2.8 g of phenol resin (Schenectady® SP1045) and 0.12 g of Irganox® 1076 as stabilizer; at t=4 min. 0.28 g of SnCl 2 and 0.56 g of ZnO. The temperature of the mixer was 180°C and the speed was 80 r.p.m. At t=8 min. the composition obtained was discharged and pelletized. A portion of the composition was spun into a fibre using a Viscotester 1500. The fibre had the following properties: Titre 1698 dtex Tensile strength 0.074 N/tex Elongation at break 408% Tension set after 50% elongation 3% Tension set after 100% elongation 10% A similar fibre was drawn to 5 x its original length. The properties of the fibre were measured after allowing the fibre to relax for 1 hour. The results were as follows: Titre 978 dtex Tension set after 50% elongation 1% Tension set after 100% elongation 2% Example V The following materials were successively added to a Haake 50 cc Banbury mixer: at t=0 11.9 g of LDPE (Stamylan® 2004TC00), 25.2 g of EPDM (Keltan® 578) and 0.12 g of Irganox® 1076 as stabilizer; at t=4 min. 0.12 g of phenol resin (Schenectady® SP1045). The temperature of the mixer was 220°C and the speed was 80 r.p.m. At t=8 min. the composition obtained was discharged. A portion of the composition was spun into a fibre using a Viscotester 1500. The fibre had the following properties: Titre 1611 dtex Tensile strength 0.063 N/tex Elongation at break 311% Tension set after 50% elongation 1% Tension set after 100% elongation 5% Example VI The TPV of Example II was melt spun into a monofilament elastic fibre using a Fourne Spintester under the following conditions: Melt temperature 220°C Spinneret opening 1 x 0.5 mm L/D-ratio 2 Wind-up speed 150 mm/min. The spun fibre had the following properties: Titre 1100 dtex Tensile strength 0.08 N/tex Elongation to break 360% Tension set after 50 % elongation 2% Tension set after 100 % elongation 8% A portion of the fibre was drawn to 3 x the original length, kept in drawn condition for 30 sec and then relaxed for 1 hour. The drawn fibre had a titre of 740 dtex. The tension set of the drawn fibre after 50% elongation was 1%, after 100% elongation 2% and after 200% elongation 3 %.

Claims: C L A I S 1. Elastic fibre of a composition containing a thermoplastic polymer and a chemically crosslinked rubber, characterized in that the fibre has a permanent elongation after 50% stretching of at most 5%. 2. Fibre according to claim 1 with a permanent elongation after 50% stretching of at most 3%. 3. Fibre according to claim 1 with a permanent elongation after 50% stretching of at most 1%. 4. Fibre according to any one of claims 1-3, in which the thermoplastic polymer is a polyolefin. 5. Fibre according to any one of claims 1-4, in which the rubber is an EP(D)M rubber. 6. Fibre according to any one of claims 1-5, in which the composition is a thermoplastic vulcanizate. 7. Fibre substantially consisting a thermoplastic vulcanizate. 8. Process for producing an elastic fibre comprising the melt spinning of a composition comprising a thermoplastic polymer and a rubber, characterized in that the rubber is completely or almost completely crosslinked at the moment the fibre is formed and in that the formed fibre is drawn after cooling down. 9. Process according to claim 8, in which the rubber in the composition is an EP(D)M rubber. 10. Process according to claim 8 or 9, in which the thermoplastic polymer in the composition is a polyolefin. 11. Process according to any one of claims 8-10, in which the composition is a thermoplastic vulcanizate. 12. Fibre and process as substantially described and illustrated by the examples.

Inventors: BASTIAANSEN CORNELIS WILHELMUS, GELISSEN FRANCISCUS WILHELMUS, BASTIAANSEN, CORNELIS, WILHELMUS, MARIA, GELISSEN, FRANCISCUS, WILHELMUS, MARIA
Applicants: BASTIAANSEN CORNELIS WILHELMUS, DSM NV, GELISSEN FRANCISCUS WILHELMUS, BASTIAANSEN, CORNELIS, WILHELMUS, MARIA, DSM N.V., GELISSEN, FRANCISCUS, WILHELMUS, MARIA
Application year: 1996
Main classification: D01F6
IPC codes: C08L21, D01F6