Wrapping yarn for artificial turf

Abstract

A texturized multifilament artificial turf wrapping yarn (507, 508) made of polyethylene or polyethylene-polyamide polymer mixture with a compatibilizer. The wrapping yarn is adapted for wrapping around a plurality of turf fiber monofilaments (501) to form a bundle of the turf fiber monofilaments and protect their integrity during securing the bundle of the turf fiber monofilaments to a carrier (308) of the artificial turf.

Description

[0001] Europäisches Patentamt 80469 München Internal Ref.: POLY.223.18WOPolytex Sportbeläge Produktions-GmbH Vinkrather Str.43 D-47929 GrefrathWRAPPING YARN FOR ARTIFICIAL TURF- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -D e s c r i p t i o n -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Field of the inventionThe invention relates generally to artificial turf, more particularly, to an artificial turf wrapping yarn, to an artificial turf using said wrapping yarn, and a method of making said artificial turf. Background Artificial turf use for the surface of sport fields such as hockey, football, rugby and the like is increasing rather rapidly due to the convenience and economic efficiency of the artificial turfcompared to natural turf. Typically, making artificial turf involves integrating artificial turf fibers into a carrier by a tufting process which includes stitching a continuous fiber bundle ofmonofilaments multiple times through the carrier to form fiber bundle loops, and cutting thefiber bundle loops to form loose-end fibers. The fibers are not tufted individually, but typically in a bundle of multiple monofilaments perbundle. For holding the turf fiber bundle together before and during the tufting process, apolyester wrapping yarn is wound around the turf fiber bundle to ensure that the turf fiberbundle can pass through the carrier several times without being damaged. Once the fiber bundlehas successfully been tufted into the carrier, the fiber bundle loops are cut to create fiberbundles with loose fiber ends. From this point on, the wrapping yarn is not needed any more,but it remains in the artificial turf and makes recycling of the artificial turf difficult because thepolyester wrapping yarn cannot be recycled together with polyethylene fiber turf.Summary of the Invention The present invention provides for an improved wrapping yarn and method of manufacturingas specified in the independent claims.Specifically, the invention relates to a wrapping yarn made of polyethylene or polyethylene- polyamide polymer mixture, an artificial turf using said wrapping yarn, and methods of using and manufacturing the artificial turf fiber with the wrapping yarn as specified in the independent claims. Embodiments are given in the dependent claims. Embodiments and examples disclosed herein can freely be combined with each other if they are not mutually exclusive. According to embodiments, the wrapping yarn is made of a polyethylene-based polymer material. A polyethylene-based material as this term is used here means a polymer mixture that comprises or consists of polyethylene (PE) or a polyethylene-polyamide polymer with a compatibilizer, and optionally one or more additives, wherein the polyethylene is at least 65% by weight, or at least 70%, or at least 80% or at least 95% of the polymer mixture. The wrapping yarn made from the polyethylene-based polymer material may be referred to hereinafter also as a polyethylene-based wrapping yarn or simply a polyethylene wrapping yarn. Using a PE-based wrapping yarn may have the benefit of facilitating the recycling of artificialturf. State-of-the art wrapping yarns were often made of polyester as polyester ischaracterized by a particularly high tensile strength in longitudinal direction, which is ofparticular importance for wrapping yarn whose function is to provide mechanical stabilization for a fiber bundle before and during the tufting process. However, recycling mixtures of polyethylene (PE) and polyester presents several challenges: PE and polyester have differentmelting points, making them difficult to process together. Furthermore, thesematerials are chemically incompatible, leading to poor mechanical properties in recycled products. Furthermore, it is difficult to separate the two materials from each other during the recycling process, which makes the recycling process more complex, mor expensive and / or which may result in a degradation of the quality of the recycled material, limiting its applications. Using a PE-based wrapping yarn for wrapping PE-based artificial turf fiber bundles avoids these problems and may result an artificial turf which is more easily to recycleand hence more environmentally friendly.According to embodiments, the wrapping yarn comprises a plurality of texturized filaments. In particular, the texturized wrapping yarn filaments may be texturized such that they shrinkwhen cut together with the turf fiber bundle after tufting is complete and the formed turf fiber loops are cut to form loose end turf fibers. For example, the wrapping yarn may have been texturized such that upon being cut and temperature-treated – starting from amaximum expanded state – the wrapping yarn automatically shrinks to less than 60% of itsoriginal length when wrapping the fiber bundle, preferably in particular to less than 50 % ofits original length, for example to less than 30 % or less than 20% of its original length. Temperature-treated means heating the wrapping yarn at least once to minimum temperature, e.g., for at least 1 minute. The minimum temperature may be, for example, atleast 60°C, or at least at least 80°C or at least 100°C. Typically, a latex or polyurethane backingis applied to the backside of a carrier after having tufted the fiber bundles into the carrier. Inorder to accelerate the hardening process, the artificial turf fiber, including the backing and the tufted fibers, are heated. For example, the artificial turf may be moved through an oven.The heat will typically induce a shrinkage of the fiber. In some examples, the texturized fibersare heat-treated at least twice, e.g., before and after the tufting. The “original length whenwrapping the fiber bundle” is the length of the wrapping yarn before the cutting and beforeany heat-treatment. The texturization may have the advantage that the wrapping yarn, whose optical or other properties may not be the same as the desired properties of the artificial turf fibers, is not visible because the remains of the cut, shrunk wrapping yarn are covered by infill or are muchshorter than the artificial turf fiber. According to some embodiments, the manufacturing ofthe wrapping yarn comprises a step of texturizing the wrapping yarn.According to some embodiments, the manufacturing of the artificial turf comprises a step ofapplying a liquid backing onto the lower side of the carrier and letting the liquid backingsolidify to form a solid, secondary backing. The lower side is the side opposed to the side fromwhich the artificial turf fiber loops, created by the tufting process and to be cut later to formthe artificial turf fiber bundles, emerge. For example, the liquid backing may be latex or apolyurethane reaction mixture. The step of letting the liquid backing solidify may compriseheating the artificial turf with the applied backing in an oven to a temperature of above 60°C, inparticular above 80°C or in particular above 100°C. The heating step may have the advantage of further reducing the length of the cut wrapping yarn, because the heat may cause the material of the wrapping yarn to contract. The liquid backing is applied after the fiber bundleshave been tufted into the carrier.According to embodiments, less than 5 parts per weight, e.g., 2-4 parts per weight, of thewrapping yarn is used for wrapping 100 parts per weight of the artificial turf fibers into abundle. For example, 30 g of the wrapping yarn may be used for bundling 1000 g of multipleartificial turf fibers into a single bundle.This may have the advantage that the mechanical and chemical properties of the artificial turf may basically be determined by the respective properties of the artificial turf fibers, no the properties of the wrapping yarn. A small share of the wrapping yarn may further facilitate recycling. According to embodiments, the wrapping yarn comprises a plurality of wrapping yarn monofilaments respectively having a cross section with a diameter of less than 100micrometers, in particular less than 60 micrometers, or less than 50 micrometers, or less than40 micrometers, or a diameter of 20 to 100 micrometers, of 20 to 60 micrometers, or of 20 to 50micrometers, or of 20 to 40 micrometers, or of 20 to 30 micrometers. This may have theadvantage that the mass ratio of the wrapping yarn to synthetic turf fiber is relatively small,so that any remaining physical and chemical differences between the two types of fiber do not play a role in the recycling process, or at least play a lesserrole. According to embodiments, the wrapping yarn is made of PE or a PE-based polymer having an MFI (Melt flow index / melt flow rate) of at least 1.0 g / min, in particular from 1.0 g / min to 200 g / min, in particular from 3.0 g / min to 50 g / min, in particular from 5.0 g / min to 30 g / min,in particular from 12 g / min to 32 g / min, in particular from 15.0 g / min to 30 g / min measuredaccording to the ISO standard 1133-1 at 190 °C and a weight of 2.16 kg. According toembodiments, the wrapping yarn (507) is made of HDPE having an MFI of from 12 g / min to32 g / min measured according to the ISO standard 1133-1 at 190 °C and a weight of 2.16 kg, and a density below 0.960 g / cm3 measured according to ISO standard 1183-1 at 23 °C.The Melt Flow Index (MFI) of a polyethylene (PE) polymer is inversely related to its viscosityduring extrusion. A higher MFI indicates a lower viscosity, meaning the polymer flows more easily under heat and pressure. Conversely, a lower MFI corresponds to a higher viscosity, indicating a thicker, less easily flowing polymer. This relationship is crucial in extrusion processes, as the viscosity affects the ease of processing, the quality of the extruded product,and the energy required for extrusion. Using a PE for the wrapping yarn with a very high MFIensures that the PE polymer mass can be extruded through the very thin nozzles of the extruder. According to embodiments, the wrapping yarn is made of PE or a PE-based polymer having alow density, e.g., a density below 0.925 g / cm3, e.g., 0.918-0.925 g / cm3, e.g., 0.918. Forexample, the PE may basically consist of LDPE, e.g., comprise at least 95% by weight LDPE.According to one example, the PE of the wrapping yarn has an MFI of 25.0 g / min to 30 g / minand a density of 0.918-0.925 g / cm3. Providing a PE-based wrapping yarn whose PE has theabove-mentioned MFI range, in particular a PE having in addition the above-mentioned lowdensity, may have the advantage that performing an extrusion process through very tiny nozzleopenings as to create very tiny wrapping yarn fibers is possible. Using a PE whose density islower and whose MFI is higher than the density and MFI of standard PE used for forming a(comparatively thick) artificial turf fiber may have the advantage that both a low density and ahigh MFI ease the extrusion of the PE through the tiny openings of the extrusion plate.Applicant has observed that a wrapping yarn of the above-mentioned MFI value range maystill have a high tensile strength, e.g., a tensile strength of at least N / m2 , e.g., 1.0 N / m2 to1.2 N / m2, in particular about 1.1. N / m2. This may prevent that the wrapping yarn monofilaments break or get damaged during the extrusion process through the tiny extrusionnozzle openings. Hence, a good compromise between tensile strength on the on hand (whichis per se lower for PE than for PP or polyester) and the extrudability through particularly tiny extrusion nozzles has been found. Applicant has observed that the PE-types conventionally used for artificial turf fibermanufacturing are not suited for generating wrapping yarns, in particularly not forgenerating very thin, extruded wrapping yarns, because they are too viscous or have a too lowtensile strength as to allow for an extrusion process through small nozzle openings (smallenough to generate wrapping yarn fibers having a very thin diameter). However, applicant has observed that PE types having an MFI and density in the above-mentioned value range allowfor the production of very thin but at the same time very strong wrapping yarn fibers.By using polyethylene with the above MFI and density ranges, it is possible to produce very thin wrapping yarns in an extrusion process where the fibers have sufficient tensilestrength towithstand the high shear forces during extrusion and to reliably wrap a bundle of artificial turffibers into a bundle before and during the tufting process.According to some embodiment, the PE used for manufacturing the wrapping yarn is orcomprises radically crosslinked polyethylene. Radically crosslinked polyethylene (PE) is a type of PE where polymer chains are bonded together through a chemical reaction initiated by free radicals, forming a three-dimensional network. This process enhances the material'smechanical and thermal properties. It can be created by exposing PE to high-energy radiation(like gamma rays or electron beams) or using chemical crosslinking agents (like peroxides).This crosslinking improves the material's resistance to heat, chemicals, and improves tensile strength. For example, the cross-linking may be performed as described by S M Tamboli et al. in “Crosslinked polyethylene”, Indian Journal of Chemical Technology, Vol. 11, November 2004, pp.853-864, 12 June 2003, which is incorporated herewith in its entirety.This may increase the tensile strength, as covalent bonds between different PE-moleculesare formed, thereby forming high-molecular weight PE-molecules. As a result of the cross-linking reaction, a small portion of the PE, preferably less than 3%, in particular less than 1%,may have a somewhat duroplast-like nature, thereby significantly increasing the tensile strength without significantly affecting processability. For example, peroxides may be given asadditives to the PE- mixture for creating radicals which again cause PE-molecules to formcovalent bonds, e.g., while the PE-melt is in the extruder and / or when the PE mixture is extruded. Using a radical-induced crosslinking reaction may in particular be beneficial if the PE subjected to the crosslinking reaction has a very high MFI in the above-mentioned range,because the crosslinking will provide small portions within the PE-polymer mass having a highmolecular weight and a high tensile strength, while the large majority of the PE-mass remainshighly non-viscous and processible. An alternative to adding peroxide additives to the PE- mass in the extruder may be to use PE which has been pre-treated with ionizing radiation,e.g., UV radiation, beta-radiation or gamma-radiation. The pre-treated PE will thereforealready be (partially) crosslinked when it is fed into the extruder.According to embodiments, the PE used for manufacturing the wrapping yarn comprises HDPE and / or Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW), also known ashigh-modulus polyethylene (HMPE). HDPE is a PE with a high strength-to-density ratio, anMFI of 5.0 to 30 g / min, or 12 to 32 g / min, or 15 to 30 g / min, and a density of less than 0.970 g / cm3, or less than 0.960 g / cm3. The density of HDPE ranges from 0.930 to 0.970 g / cm3,(from 930 to 970 kg / m3) or from 0.940 to 0.960 g / cm3.Although the density of HDPE is only marginally higher than that of low-density polyethylene,HDPE has little branching, giving it stronger intermolecular forces and tensile strength (38MPa versus 21 MPa) than LDPE. HMPE is a type of polyethylene characterized by extremelyhigh tensile strength and modulus, resulting in exceptional stiffness and durability. This material is produced by drawing or stretching polyethylene fibers, aligning the polymerchains to enhance their strength. UHMWPE is a subset of the thermoplastic polyethylene. Ithas extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu.The longer chains serve to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. This results in a very tough material, with thehighest impact strength of any thermoplastic presently made.For example, the PE of the wrapping yarn may comprise 0.5-20.0%, in particular 8.0%-16.0%, by its weight the HDPE or HMPE or a mixture thereof. This may further increase the tensile strength without significantly lowering the processability and extrudability of the polymer mass. According to some embodiments, the polymer mixture used for creating the wrapping yarn fibers, and hence, also the individual wrapping yarn fibers, comprise a nucleating agent. For example, the nucleating agent may be an inorganic nucleating agent, an organic nucleating agent or a mixture thereof. The inorganic nucleating agent may comprise one or more of: talcum, kaolin (also known as “china clay”), calcium carbonate, magnesium carbonate, silicate, aluminium silicate, e.g., sodium aluminosilicate, amorphous and partiallyamorphous silica and mixed morphologies hereof, e.g., fumed silica, silicic acid and silicic acidesters (e.g., tetraalkyl orthosilicate, also known as orthosilicic acid ester), aluminiumtrihydrate, magnesium hydroxide, meta- and / or polyphosphates, and coal fly ash (CFA). Theorganic nucleating agent may comprise one or more of: 1,2-cyclohexane dicarbonic acid salts(also known as main component of “Hyperform ®”), in particular calcium salts of the 1,2-cyclohexane dicarbonic acid, benzoic acid, benzoic acid salt, sorbic acid, and sorbic acid salt.The benzoic acid salt may be, in particular, an alkaline metal salt of the benzoic acid (e.g.,sodium and potassium salts of the benzoic acid), and an alkaline earth metal salt of thebenzoic acid (e.g., magnesium and calcium salts of the benzoic acid. The sorbic acid salt maybe, in particular, an alkaline metal salt of the sorbic acid (e.g.,. sodium and potassiumsalts of the sorbic acid); and an alkaline earth metal salt of the sorbic acid (e.g., magnesiumand calcium salts of the sorbic acid). According to preferred embodiments, the nucleatingagent is sorbic acid.For example, the wrapping yarn may comprise 0.01- 3.0 % by its weight an inorganic nucleatingagent or 0.01 to 8.0% by its weight an organic nucleating agent. For example, it may compriseat least 0.5%, e.g., 0.5% to 4.0% , sorbic acid.The addition of a nucleating agent to extruded polyethylene may be beneficial as it promotesuniform crystallization throughout the polymer, resulting in a more consistent and uniform material structure. Applicant has also observed that the nucleating agent helps in forming smaller, more evenly distributed crystals, which can enhance the mechanical properties and in particular the tensile strength of the PE-based wrapping yarn. In addition, the uniform and fine crystal structure may result in increased impact resistance to all kinds of mechanical forces and increased rigidity of the wrapping yarn. Therefore, the addition of a nucleating agent cancompensate for the lower tensile strength of PE compared to polyester and can help toproduce a mechanically robust wrapping yarn even from a low density, high MFI PE polymeror polymer blend. In general, lower density and higher MFI often correlate with weakermaterial properties, which are undesirable but often unavoidable to ensure that the PE melt can be extruded through the thin extrusion dies. The nucleating agent can compensate for these disadvantages, resulting in PE-based wrapping yarns that are recyclable (along with the PE-based artificial turf fibers), very thin (and therefore cheap to produce), have high tensile strength (and therefore effectively keep the fiber bundle intact during tufting) and good processability (in particular low viscosity to facilitate extrusion).According to some embodiments, the PE of the wrapping yarn has a melting temperature of120°C to 140°C, and a tensile strength of 1.0 N / m2 to 1.2 N / m2, in particular about 1.1N / m2. According to embodiments, the wrapping yarn consists of a plurality of wrapping yarn fibers. Each of the wrapping yarn fibers is created in an extrusion process.According to embodiments, the wrapping yarn fibers are created in an extrusion process. Thecreation comprises: forming wrapping yarn fibers (wrapping fiber monofilaments) by directextrusion of a polymer mass through a plurality of small openings of a nozzle outlet of a firstextrusion apparatus, wherein the extrusion is performed at a temperature of at least 190°C,in particular at least 230°C, in particular 240°C to 260°C.Performing an extrusion process using the above-mentioned high temperature ranges may have the advantage that the viscosity of the extruded PE mass is reduced, thereby allowing togenerate very thin, extruded wrapping yarn monofilaments also from PE types with a MFI < 20g / min. The above mentioned features (MFI range, density range, HDPE and / or HMPE portion, high extrusion temperature, radical-induced cross-linking of PE molecules, nucleating agent or a combination of two or more of these features) may provide for a PE-based fiber with very hightensile strength even though the wrapping fiber may be very thin. For example, a singleextruded wrapping fiber monofilament may have a weight of less than 40 g / 10.000 m, e.g., about 32 g / 10.000 m. According to embodiments, the wrapping yarn (and the polymer mass used for manufacturing the same) is free of UV-stabilizing agents and HALS (Hindered Amine Light Stabilizers) and / or is free of pigments. This may have the advantage of decreasing the viscosity and allowing toincrease the temperature during extrusion to a comparatively high temperature range of atleast 190°C, preferably at least 230°C or higher. Light stabilizers and pigments may not benecessary, because thanks to the small mass ratio of the wrapping yarn relative to the artificialturf fiber and thanks to the fact that the wrapping yarn may be textured (and hence very short after being cut) and may be completely covered by infill, the wrapping yarn will neither bevisible nor exposed to sunlight.According to embodiments, the PE used for manufacturing the wrapping yarn and / or the artificial turf fibers is a bio-based PE. This may reduce the CO2 footprint of the artificial turf. According to embodiments, each bundle of artificial turf monofilaments consists of at least 3,e.g., 6 to 8, artificial turf fiber monofilaments.According to embodiments, the artificial turf fiber monofilaments are also created in an extrusion process. The creation comprises: -forming turf fiber monofilaments by direct extrusion through a plurality of smallopenings of a nozzle outlet of a second extrusion apparatus, or- first extruding a film which is then split into the plurality of the turf fibermonofilaments. The extrusion of the artificial turf monofilaments may be performed, according to embodiments, at temperatures below 150°C, e.g., at about 120°C to 140°C. Using thiscomparatively low extrusion temperature range may have the advantage that heat- sensitive additivescomprised in the extruded polymer mass to form the artificial turf fiber monofilaments such as UV-stabilizing agents and HALS (Hindered Amine Light Stabilizers) are not inactivated or degraded by the heat. According to embodiments, the wrapping yarn is made of the same type of material, e.g., PE,or a combination of PE, PA and a compatibilizer, like the type of material of the artificial turf fibers. Hence, the polyethylene wrapping yarn may have similar or identical properties to theproperties of the material (e.g., polyethylene) of the artificial turf fiber.This may have the advantage that the wrapping yarn can be readily recycled together with aturf fiber made of polyethylene or a polyethylene-polyamide polymer mixture with a compatibilizer.The terms polyethylene turf fiber or polyethylene turf fiber monofilaments as used here referto turf fiber or turf fiber monofilaments which are also made of a polyethylene-based material, i.e., polyethylene or a polyethylene-polyamide polymer mixture with acompatibilizer. Making the wrapping yarn and the turf fiber of a polyethylene-based materialfacilitates the recycling of the turf fiber and the wrapping yarn and reduces turf fiber waste.Turf fiber waste may be generated both during the manufacturing of the tuft fiber bundlesor at the end of the useful life of the artificial turf when the artificial turf is replaced. Using a polyethylene wrapping yarn facilitates the recycling process of a polyethylene artificial turf.A first aspect of the present invention is directed to an artificial turf wrapping yarn comprisingpolyethylene or polyethylene-polyamide polymer mixture with a compatibilizer. In anembodiment, the artificial wrapping yarn may be made of a material comprising more than 90% polyethylene, or more than 95% of polyethylene, or more than 99% of polyethylene by weight. The remainder of the polyethylene wrapping yarn material may comprise one or more additives and / or polyamide (PA). By adding PA in the PE may result in significant increase in the tensile strength of the polyethylene wrapping yarn monofilament, of at least 10%, or least 15%, or at least 20%, or at least 25%, or at least 30%. The polyethylene (PE) may include polyethylene homopolymer, or polyethylene random copolymer, including high density PE (HDPE), ultra-low density PE (ULDPE), low density PE(LDPE), linear low density PE (LLDPE), and medium density PE (MDPE). In particular, the PEmay be HDPE, LLDPE or a combination thereof. The polyamide may be a nylon such as, for example, nylon 6, nylon 66, nylon 11 and the like, or a high performance polyamide including aramid fibers such as, for example, KEVLAR and NOMEX. In an embodiment, the wrapping yarn may be made by extruding a polymer material comprising polyethylene (PE), polyamide (PA) and a compatibilizing agent. The PA may be less than 30%, in particular less than 10 % by weight of the polymer mixture. The compatibilizing agent may be in an amount of 0.1% to 2% by weight. The balance of the polymer mixture may be polyethylene and optionally one or more additives. In an embodiment, the polymer mixture comprises 70% to 95%, or 80% to 90% PE by weight, 0.1% to 2% by weight of the compatibilizing agent, and the balance of the polymer mixture may be PA and optionally one or more additives. In an embodiment the polymer mixture comprises between 1 and 20% by weight of the PA and the balance of the weight of the polymer mixture may be made up by the PE, thecompatibilizing agent, and optionally one or more additives.In an embodiment the polymer mixture comprises between 5% and 10% by weight of the PA and the balance of the weight of the polymer mixture may be made up by the PE, thecompatibilizing agent, and optionally one or more additives.In an embodiment the polymer mixture comprises between 1 % and 30% by weight of the PA and the balance of the weight may be made up by the PE, the compatibilizing agent, andoptionally one or more additives.In another embodiment the polymer mixture comprises between 80-90% by weight of the polyethylene, and the balance of the weight may be made up by the PA, the compatibilizer, and optionally one or more additives added to the polymer mixture.The compatiblizer may be any one of the following: a maleic acid grafted on polyethylene orpolyamide; a maleic anhydride grafted on free radical initiated graft copolymer of polyethylene, SEBS, EVA, EPD, or polypropylene with an unsaturated acid or its anhydride such as maleic acid, glycidyl methacrylate, ricinoloxazoline maleinate; a graft copolymer of SEBS with glycidyl methacrylate, a graft copolymer of EVA with mercaptoacetic acid andmaleic anhydride; a graft copolymer of EPDM with maleic anhydride; a graft copolymer ofpolypropylene with maleic anhydride; a polyolefin-graft-polyamide, polyethylene orpolyamide; and a polyacrylic acid type compatibilizing agent.In an embodiment the compatibilizer is a maleic anhydride grafted on polyethylene orpolyamide. The PE / PA polymer mixture for making the polyethylene wrapping yarncomprises a mixture of PE with PA and a compatibilizer and optionally one or more additivesadded to the polymer mixture. The term 'polymer mixture' may also be replaced with theterm 'master batch’ or 'compound batch'. The polymer mixture may be at least a three-phasesystem. A three-phase system as used herein encompasses a mixture that separates out intoat least three distinct phases. The PE, the PA and the compatibilizing agent form the phases of the three-phase system. The PE and the PA are immiscible and as a result the PA may form polymer beads surrounded by the compatibilizing agent within the PE phase. Any PE / PA polymer mixture described herein for making the polyethylene wrapping yarnmay likewise be used for manufacturing the artificial turf fibers and vice versa.According to some embodiments, the method of making the polyethylene wrapping yarn may comprise the step of extruding the polymer mixture into a monofilament. To perform this extrusion the polymer mixture may for instance be heated. The method furthercomprises the step of quenching the monofilament. In this step the monofilament is cooled.The method further comprises the step of reheating the monofilament. The method furthercomprises the step of stretching the reheated filament to deform the PA polymer beads intothread-like regions and to form the polyethylene wrapping yarn monofilament. In this step the monofilament is stretched. This causes the monofilament to become longer and in theprocess the PA polymer beads are stretched and elongated. Depending upon the amount ofstretching the PA polymer beads are elongated more. The PA polymer is believed to form thread-like regions which are embedded within the PE phase strengthening the polyethylene wrapping yarn and making practically impossible to separate the two polymers. The addition of the PA in the PE enables the properties of thepolyethylene wrapping yarn monofilament to be tailored. For instance, because PA is a morerigid plastic than the PE, the resulting wrapping yarn monofilament exhibits enhanced resilience and enhanced spring back after texturizing. A further advantage may be that the polyethylene wrapping yarn monofilament may have improved long term elasticity which also may enhance shrinking after it has been textured.In the PE / PA polymer mixture the PA polymer beads may comprise crystalline portions andamorphous portions. Stretching the PA polymer beads into thread-like regions may increasethe size of the crystalline portions and the rigidity of the PA polymer. The stretching of the monofilament may also increase the crystalline phase of the PE polymer. In an embodiment, creating the PE / PA mixture may comprise forming a first mixture by mixing the PA polymer with the compatibilizing agent. The first mixture may then beheated and extruded followed by granulating the extruded first mixture. The granulated first mixture may then be mixed with the PE polymer by heating the granulated first mixture with the PEto form the PE / PA mixture. This method of creating the PE / PA mixture may be advantageousbecause it allows precise control over how the PA and compatibilizer are distributed withinthe PA. For instance, the size or shape of the extruded first mixture may determine the sizeof the PA beads in the PE / PA mixture. In the aforementioned method of creating the PE / PA mixture a one-screw extrusion method may be used.According to a variation of this embodiment, the PE / PA mixture may be formed by mixing allof the components together at once. For instance, the PA, the PE, and the compatibilizeragent may be added together at the same time. Other ingredients such one or more additivesmay also be added at the same time. The mixing intensity of the PE / PA mixture may be increased, for instance by using a two-screw feed for the extrusion. In this case the desired distribution of the polymer beads can be achieved by using proper mixing intensity. The polymer material may further optionally comprise any one of the following: a wax, a dulling agent, a UV stabilizer, a flame retardant, an anti-oxidant, a pigment, or anycombination thereof, typically in small amounts.Importantly, the co-extruded PE / PA yarn is recyclable. See, for example, the BASF studyentitled “Coextruded PE / PA multilayer films are recyclable!” by Dr. Rolf-Egbert Grützner, Dr.Roland Both, BASF SE, D-67056 Ludwigshafen / Rhein; Institut cyclos-HTP GmbH, D-52076 Aachen. It has been found that the polyethylene wrapping yarn is effective for wrapping a plurality of polyethylene turf fiber monofilaments to form a bundle of the polyethylene turf fibermonofilaments and protect their integrity during the tufting process for forming the artificialturf.In an embodiment the polyethylene wrapping yarn comprises or consists of HDPE. In anembodiment, the polyethylene wrapping yarn may comprise a bundle of several individual PE or PE / PA yarn monofilaments, preferably from about 20 to 50 individual PE or PE / PAwrapping yarn monofilaments, and more preferably from about 30 to 40 individual PE orPE / PA wrapping yarn monofilaments. Preferably, each of the individual PE, or PE / PA wrapping yarn monofilaments may have a cross section with a diameter of 20 to 100 micrometers, of 20 to 60 micrometers, or of 20 to 50 micrometers, or of 20 to 40 micrometers, or of 20 to 30 micrometers. The cross section may have the shape of a circle, or ellipse.The polyethylene wrapping yarn may be textured so that it shrinks upon being cut and heat-treated – starting from a maximum expanded state – to less than 60% of its original length,preferably to less than 50 % of its original length, and more preferably to less than 30% of itsoriginal length. The original length of the polyethylene wrapping yarn as this term used hererefers to the length of the polyethylene wrapping yarn at the state when it is wrapped aroundthe turf fiber bundle.The polyethylene wrapping yarn comprises a bundle of several wrapping yarn monofilaments. The polyethylene wrapping yarn monofilaments may be formed by extruding the polymer material through a plurality of extrusion nozzle openings of anextruder apparatus plate and bundling the extruded individual wrapping yarn monofilamentsto form the polyethylene wrapping yarn.Another aspect of the present invention is directed to an artificial turf fiber bundle, comprising a plurality of turf fiber monofilaments, and the polyethylene wrapping yarn wrapped around the plurality of the turf fiber monofilaments such that a bundle of artificial turf fiber is formed. Yet another aspect of the present invention is directed to an artificial turf comprising aplurality of artificial turf fibers which are integrated in bundles at puncture points in a carrierof the artificial turf structure, and the polyethylene wrapping yarn, wherein the polyethylene wrapping yarn is integrated into the carrier at the puncture points. Yet another aspect of the present invention is directed to a method for manufacturing a wrapping yarn for artificial turf, the method comprising:- extruding polyethylene, or a PE / PA polymer mixture with compatibilizer, into a plurality ofmonofilaments; and- bundling the plurality of monofilaments together to form the polyethylene wrapping yarn.The method may employ a wrapping yarn made of preferably from about 20 to 50 individual wrapping yarn monofilaments, more preferably 30 to 40 wrapping yarn monofilaments, wherein each one of the wrapping yarn monofilaments has a cross section with a diameter of 20 to 30 micrometers. The bundling may include twisting the plurality of monofilaments and / or subjecting the plurality monofilaments to a jet-knot process. In this process, an air stream (jet) is directed onto one of the many wrapping yarn monofilaments formed during extrusion in such a way that it wraps around the other extruded monofilaments, thereby creating a yarn thread (wrapping yarn, wrapping yarn fiber). Applicant has observed that a wrapping yarn being itselfa wrapped bundle of very thin monofilaments shows a particularly high tensile strength, and isin addition very flexible.In the final artificial turf, the length of the wrapping yarn between the puncture points and the ends of the wrapping yarn (created by the cutting) have a length of less than 60% of thelength of the artificial turf fibers, in particular of less than 30% of the length of the artificial turffibers, preferably to less than 20 % of the length of the artificial turf fibers, and in someexamples less than 10% of the length of the artificial turf fibers.The length of the artificial turf fibers is measured from the carrier surface to the ends of the artificial turf fibers protruding therefrom. In case the turf comprises multiple fibers of differentlength, the “length of the artificial turf fibers” means the length of the longest type of artificialturf fibers, which is also referred to as pile height of the artificial turf. The pile height of artificialturf refers to the length of the synthetic grass blades from the carrier to the tip of thesynthetic grass fibers. It affects the turf's appearance and performance. In yet another embodiment, a method of manufacturing artificial turf is provided which employs the polyethylene wrapping yarn as described above and the artificial turf fiber bundle as described above, the method comprising: providing a plurality of turf fiber monofilaments; using the polyethylene wrapping yarn to wrap around a plurality of turf fibermonofilaments to form a continuous turf fiber bundle of preferably at least 3, morepreferably 5 to 10, or most preferably 6 to 8 turf fiber monofilaments wrapped withthe polyethylene wrapping yarn;tufting the continuous turf fiber bundle through a carrier to form loops of the turf fiber bundle on a front side of the carrier; andcutting the loops to form a plurality of free end fibers per loop pointing out from the carrier to form a grass-like artificial turf surface,wherein the polyethylene wrapping yarn that is wrapped around each bundle upon being cut shrinks to less than 30% of its original length when wrapping the fiber bundle, preferably to less than 20 % of its original length, and more preferably to less than 10% ofits original length.In the method the plurality of the turf fiber monofilaments may comprise preferably at least 3,more preferably 5 to 10, and most preferably 6 to 8 turf fiber monofilaments, and the plurality of the turf fiber monofilaments may be formed either by directly extruding the polymer material through a plurality of small openings of a nozzle outlet of an extrusion apparatus, or first forming a film which is then split into the plurality of the turf fiber monofilaments. Theturf fiber monofilaments may be made from polyethylene, or a polyethylene-polyamidepolymer mixture with a compatibilizer as the one described for the polyethylene wrappingyarn monofilaments.Adding polyamide in the polyethylene with a compatibilizer allows formation of PA thread- like regions in a central region of the turf fiber monofilament during the extrusion process.This leads to a concentration of the more rigid material in the center of the monofilamentand a larger amount of softer plastic on the exterior or outer region of the monofilament. This may further lead to an artificial turf fiber with more grass-like properties. A furtheradvantage may be that the artificial turf fibers have improved long term elasticity. This may require reduced maintenance of the artificial turf and require less brushing of the turf fibers because they more naturally regain their shape and stand up after use or being trampled. The plurality of the turf fiber monofilaments which are formed by directly extruding the polymer material through a plurality of small openings of a nozzle outlet of an extrusionapparatus may be quenched, e.g., by air and / or passing them through a water bath, and maythen be stretched to obtain the desired size and property characteristics, before bundling them together with the polyethylene wrapping yarn. The method may further comprise applying a polyurethane backing on the back side of the carrier to secure the turf fibers on the back side of the carrier, and heating the formed artificial turf in an oven to dry any residual liquid from the forming of the polyurethane backing. The heating may also increase the shrinking of the polyethylene wrapping yarn. The polyethylene wrapping yarn according to the invention can be more heavily texturized than polyester wrapping yarn and shrinks upon being cut much more than polyester wrapping yarn. The polyethylene wrapping yarn is also very thin. For example, 10000 m of the polyethylene wrapping yarn may have a weight of 4.8 g or less, or preferably 3.0 g or lessor 2 g or less. In an embodiment the polyethylene wrapping yarn has a deci-tex (“dtex”) valueof 2 (i.e., 2 g per 10000 meters).Although polyethylene has a lower tensile strength than polyester, it has been found that bundling a plurality of polyethylene monofilaments together provide adequate tensile strength and at the same time results in a wrapping yarn thar has higher shrinkage than polyester. In those embodiments employing a PE / PA polymer mixture with a compatibilizer for forming the polyethylene wrapping yarn, the addition of the PA increases the tensile strength of the polyethylene wrapping yarn.Also, because the polyethylene wrapping yarn is made of polyethylene or comprises at least65% polyethylene which has a much lower melting temperature than polyester, a heatingstep may be employed for enhancing the shrinkage effect of the polyethylene wrapping yarnfollowing the tufting of the artificial turf fibers on the carrier. For example, the artificial turf may be heated in an oven, for example, for 5 min at 85°C to increase the shrinkage effect.This is not possible with polyester wrapping yarn which has a melting point temperature of about 250°C. Polyethylene has a melting point temperature of about 130°C and may soften at much lower temperature. For some artificial turfs that employ a polyurethane backing, a heating step is typically employed for removing liquid leftover after the application of the polyurethane backing on the back side of the carrier. Hence, for such artificial turfs that employ polyurethane backing, shrinkage of the polyethylene wrapping yarn may be increased further during the post heating step that is typically employed for removing liquid leftover after the application of the polyurethane backing on the back side of the carrier.The polymer material used for making the artificial turf fibers may also be referred to as abase polymer material to distinguish it from the polyethylene wrapping yarn polymermaterial that is used for the polyethylene wrapping yarn. Hence, In an embodiment the basepolymer material used for the artificial turf fibers may comprise polyethylene or polyethylene copolymer or polyethylene alloy. Polyethylene alloy refers to a material thatis a blend or combination of different types of polyethylene. For example, a polyethylenealloy may involve combining high-density polyethylene (HDPE) with linear low-densitypolyethylene (LLDPE) to achieve a balance between strength and flexibility.Making the polyethylene wrapping yarn from polyethylene may also facilitate recyclability ofthe artificial turf.In an embodiment, both the polyethylene wrapping yarn and the artificial turf fibers may bemade of polyethylene.In an embodiment the base polymer may comprise low density polyethylene (LDPE), LLDPE,and / or HDPE and mixtures thereof. The LDPE may have a density from 0.905 g / cm3 to 0.920g / cm3, the LLDPE may have a density from 0.914 g / cm3to 0.928 g / cm3, and the HDPE mayhave a density from 0.92 to 0.98 g / cm3.In an embodiment, the base polymer may be a mixture of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE).In an embodiment, the polymer backbone for the fiber is high density polyethylene (HDPE). In an embodiment, the base polymer may bemixture of LDPE and HDPE wherein the LLDPE is preferably from 10 to 70 wt% of the polymermaterial, more preferably from 15 to 50% wt%, and more preferably from 25 to 45 % of the total amount of LDPE and HDPE. In an embodiment, the base polymer may be a mixture ofLDPE and LLDPE wherein the LLDPE is preferably from 10 to 70 wt% of the polymer material,more preferably from 15 to 50% wt%, and more preferably from 25 to 45 % of the totalamount of LDPE and LLDPE. In an embodiment, the LLDPE may be a random copolymer of polyethylene with at least one of butene, hexene, and octene. In an embodiment, the base polymer may be a mixture of LLDPE and HDPE, with the LLDPEbeing hexene or octene comonomer based and having a density from 0.916 and 0.920 g / cm³,while the HDPE has a density of 0.970 g / cm3 or less, or 0.960 g / cm³ or less, or from 0.930to 0.970 g / cm³, or from 0.930 to 0,960 g / cm3, or from 0.952 to 0.957 g / cm³ and is in anamount of from 5-18 wt% of the holistic polymer mixture, more preferably about 15 wt%.The base polymer for the artificial turf fibers may comprise various additives such as forexample a slip agent for reducing the friction coefficient of the turf fibers, an antimicrobial agent for inhibiting microbial growth, color pigments and the like. Such additives may optionally be added also in the yarn polymer. In an embodiment the “base polymer” may be a mixture of one or more polymers ofdifferent types, e.g., a mixture of polyethylene and polyamide, or a mixture of HDPE andLDPE, which may optionally comprise one or more additives. The polymer mixture may be a single-phase system such as a HDPE and LDPE mixture, or a multi-phase system, such as a liquid PE polymer comprising and embedding beads of an immiscible polyamide (PA)polymer. The polyamide polymer may be, for example, nylon.The term “Low-density polyethylene” (LDPE) as used herein refers to a thermoplastic made from the monomer ethylene having a density in the range of 0.910–0.940 g / cm3. According to embodiments, the base polymer may be or comprise LDPE whose density range is within the above specified sub-range. The term “Linear low-density polyethylene” (LLDPE) as used herein refers to a substantially linear polymer (polyethylene), with significant numbers of short branches. LLDPE differs structurally from conventional LDPE because of the absence of long chain branching. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE. In general, LLDPE is produced at lower temperatures and pressures by copolymerization of ethylene and alpha-olefins.The turf fiber may also include a reflective agent such as reflective particles, and / or reflectivepigments for preventing overheating of the fiber and thus further enhances the performancecharacteristics of the turf even in hot weather conditions.In an embodiment the artificial turf may include a backing. The backing may be made of a thermoset polymer material. The thermoset material may include, for example, a polyurethane resin.An advantage of the polyethylene wrapping yarn is that both in the manufacturing process(when the polyethylene wrapping yarn is wrapped around the monofilament bundles) and in the recycling or disposal of artificial grass, any waste produced consists of wrapping yarnon the one hand and polyethylene fibers on the other which are made of similar types ofplastic. These similar types of plastic have similar melting points, and therefore can bereadily recycled. This is good for the environment and may also significantly reduce theartificial turf cost because valuable fiber material can be reclaimed.These and other features and advantages of the present invention will become better understood from the following detailed description of the invention in conjunction with the accompanying drawings. Brief Description of the Drawings In the following, embodiments of the invention are explained in greater detail, by way ofexample only, making reference to the drawings in which:Figure 1A is a flowchart of a method of manufacturing a wrapping yarn; Figure 1B is a flowchart illustrating a method of manufacturing a wrapping yarn made from a PE / PA polymer;Figure 1C is a flowchart illustrating a method of manufacturing a PE / PA polymer mixture;Figure 1D is a simplified cross-sectional view of a PE / PA polymer mixture;Figure 1E illustrates the extrusion of the polymer mixture into a monofilament; Figure 1F shows a cross-section of a small segment of the monofilament; Figure 1G illustrates the effect of stretching the monofilament; Figure 2 is a flowchart of a method of manufacturing an artificial turf; Figure 3A is a simplified schematic illustration of an artificial turf with the artificial turf fiberloops and the polyethylene wrapping yarn in its extended form wrapped around the turf fiberbundle; Figure 3B is a simplified schematic illustration of the artificial turf of figure 3A after the loops are cut and the polyethylene wrapping yarn has shrunk;Figure 4 is an image of a wrapping yarn made of polyethylene;Figure 5 is an image of a wrapping yarn made of polyester; Figures 6, and 7 are DSC analysis graphs of the polyethylene wrapping yarn of figure 4, and the polyester wrapping yarn of figure 5, respectively. Detailed Description Like numbered elements in these figures are either equivalent elements or perform thesame function. Elements which have been discussed previously will not necessarily bediscussed in later figures if their function is equivalent.The present invention provides a polyethylene wrapping yarn for an artificial turf forwrapping around a plurality of polyethylene turf fiber monofilaments to form a turf fiber thatis a bundle of the polyethylene turf fiber monofilaments and protect the integrity of the turf fiber bundle during the tufting process of stitching the turf fiber bundle through a carrier of the artificial turf. The polyethylene wrapping yarn comprises a bundle of individual polyethylene yarnmonofilaments, preferably from about 20 to 50 individual polyethylene yarn monofilaments,more preferably 30 to 40 individual polyethylene yarn monofilaments.Each of the individual polyethylene yarn monofilaments may have a very thin cross-section, for example, a circular cross section with a diameter of 20 to 30 micrometers. The polyethylene wrapping yarn is heavily texturized so that it shrinks upon being cut to less than30% of its original length when it is extended and wrapping the turf fiber bundle, preferablyto less than 20 % of its original length, and more preferably to less than 10% of its original length. It has been found that compared to existing wrapping yarn made from polyester,the polyethylene wrapping yarn according to the present invention can be readily texturized to a greater degree than the polyester wrapping yarn. This is advantageous because it reduces the footprint of the polyethylene wrapping yarn that remains in thefinal artificial turfstructure and makes it less visible to the user of the artificial turf. This is particularly useful for artificial turf structures that do not use infill material.In addition, the higher shrinkage may allow wider opening of the turf fiber monofilaments that make up each turf fiber bundle once the turf fiber loops are cut which results in a more homogeneous and more dense turf fiber surface for the artificial turf. Making the polyethylene wrapping yarnReferring to figure 1A, a method of making the polyethylene wrapping yarn includes formingpolyethylene, preferably high density polyethylene HDPE, into a plurality of wrapping yarn monofilaments according to step 102. For example, forming the plurality of the turf fiber monofilaments may include extruding the individual wrapping yarn monofilaments from an extruder through a plurality of nozzle openings of an extrusion apparatus. An example ofextruding a monofilament from a polymer mixture is shown in figure 1E for a PE / PA polymermixture, however, the same extrusion apparatus may also be used for extruding apolyethylene monofilament.Alternatively, the turf fiber monofilaments may be formed by first forming a film shapefilament, e.g., also by extrusion but using a different nozzle opening, and then splitting thefilm into a plurality of polyethylene wrapping yarn monofilaments.The method for making the polyethylene wrapping yarn may further include bundling the plurality of the polyethylene wrapping yarn monofilaments together to form the polyethylene wrapping yarn according to step 104. The extrusion nozzle or plate of the extrusion apparatus may preferably have from 20 to50 openings (or holes), preferably from 30 to 40 openings. In figure 1E a one hole plate isshown as an example. In a more specific example, the plate of the extrusion nozzle mayhave 36 openings allowing extruding 36 monofilaments at the same time. Hence, duringthe extrusion, melted polyethylene exits in the form of a plurality of wrapping yarnmonofilaments, each monofilament having a cross-sectional diameter of preferably from 20 to 30 micrometers. The extrusion apparatus used for the making of the very thin monofilaments that form the polyethylene wrapping yarn by bundling them together needs to be fitted with an outlet plate having a plurality of very small openings, i.e.,preferably 20 to 50 openings, or more preferably 30 to 40 openings. In a specificembodiment the plate may have 36 openings.Figure 1B is a flowchart illustrating a method of manufacturing wrapping yarnmonofilaments from a PE / PA polymer mixture. Adding PA in the PE polymer mixture increases the rigidity and tensile strength of the produced wrapping yarn monofilaments which forms after the bundling of the monofilaments a more rigid, higher tensile wrappingyarn. The method comprises forming a polymer mixture comprising a three-phase system of PA, PE and a compatibilizer (also referred to as a compatibilizing agent) according to step 105. The PA and PE polymers are immiscible and the PA forms polymer beads surrounded by the compatibilizer within the second polymer. In the next step 106, the PE / PA polymer mixture is extruded into a monofilament. Next in step 107 the monofilament is quenchedor rapidly cooled down. Next in step 108 the monofilament is reheated and, according to step 109 the reheated monofilament is stretched to deform the PA polymer beads intothread-like regions and to form the monofilament into a wrapping yarn monofilament witha desired diameter, crystallinity, and tensile strength.According to step 110 a plurality of wrapping yarn monofilaments are bundled together and texturized to form the polyethylene wrapping yarn.Figure 1C is a flowchart illustrating a method of creating the PE / PA polymer mixture. The PE / PA polymer mixture is a three-phase system comprising PA polymer, PE polymer and the compatibilizer. The PE / PA polymer mixture may also comprise other additives such as color or flow agents for improving the flowing properties of the polymer mixture duringextrusion. Accordingly, a method for forming the PE / PA polymer mixture may comprise first, in step 120 forming a PA polymer mixture by mixing the PA polymer with the compatibilizer. Additional additives may also be added during this step. Next in step 121 the first mixture is heated. Next in step 122 the PA mixture is extruded. Then in step 123 the extruded PA mixture is granulated or chopped into small pieces. Next in step 124 the granulated PA mixture is mixed with the PE polymer. Additional additives may also be added to the polymer mixture at this time. Finally in step 125 the granulated PA mixture is heated with the PE polymer to form the PE / PA polymer mixture. The heating and mixing may occur atthe same time.Figure 1D shows a diagram which illustrates a cross-section of a PE / PA polymermixture 400.The PA / PE polymer mixture 400 comprises a PA polymer 402, a PE polymer 404, and a compatibilizer 406. The PA polymer 402 and the PE polymer 404 are immiscible. The PA polymer 402 is less abundant than the PE polymer 404. The PA polymer 402 is shown surrounded by the compatibilizer 406 and dispersed within the PE polymer 404.ThePA polymer 402 surrounded by the compatibilizer 406 forms several polymer beads 408.The polymer beads 408 may be spherical or oval in shape or they may also be irregularly- shaped depending up on how well the polymer mixture is mixed and the extrusion conditions such as the temperature of the heated polymer mixture. The PE / PA polymermixture 400 is a three-phase system. The first phase is made of the regions of the PApolymer 402, the second phase is the compatibilizer 406 and the third phase region is thePE polymer 404. The compatibilizer 406 separates the PA polymer 402 from the PE polymer 406.Figure 1E illustrates the extrusion of the PE / PA polymer mixture into a monofilament.Shown is an amount of a PE / PA polymer mixture 600. Within the PE / PA polymer mixture 600 there are many PA polymer beads 408 separated from the PE polymer 404 by the compatibilizer. A screw, piston or other device is used to force the polymer mixture 600 through a hole 604 in a plate 602. This causes the PE / PA polymer mixture 600 to be extruded into a monofilament 606. The monofilament 606 is shown as containing the PApolymer beads 408. The PE polymer 404 and the PA polymer beads 408 are extrudedtogether. In some examples the PE polymer 404 will be less viscous than the PA polymerbeads 408 and the PA polymer beads 408 will tend to concentrate in the center of themonofilament 606. This may lead to desirable properties for the final artificial turf fiber as this may lead to a concentration of the thread-like regions in the core region of the monofilament 606 endowing the monofilament with a softer outside surface and a more rigid core.Figure 1F shows a cross-section of a small segment of a wrapping yarn monofilament 606comprising the PA polymer beads 408 mixed inside the PE polymer and separated from the second polymer 404 by the compatibilizer 406 (not shown). To form the threadlike structures a section of the monofilament 606 is heated and then stretched along the length of the monofilament 606. This is illustrated by the arrows 700 which show thedirectionof the stretching.Figure 1G illustrates the effect of stretching the monofilament 606. In figure 1G an exampleof a cross-section of a stretched monofilament 606 is shown. The PA beads 408 in figure 1G have been stretched into thread-like structures. The amount of deformation of the PA polymer beads 408 depends upon how much the monofilament 606 has been stretched.Bundling of the polyethylene wrapping yarn monofilamentsBundling the individual wrapping yarn monofilaments together to form the polyethylene wrapping yarn may be done using any suitable method. For example, bundling may be performed via twisting or via a jet-knot process. The bundling of the individual wrapping yarn monofilaments together allows creating a polyethylene yarn filament that has adequate tensile strength to withstand the tuftingprocess forces and protect the turf fiber bundle during tufting.According to some embodiments, a twisting method may be used for forming the bundle of the polyethylene wrapping yarn monofilaments. Accordingly, wrapping yarn monofilaments after being formed via the extrusion process (or via the film and splitting process) may be fed into a twisting machine that twists the monofilaments together to form the polyethylene wrapping yarn bundle. By selecting the size and number of the individual wrapping yarn monofilaments the twisting provides a bundle that has an adequate tensile strength. Multiple wrapping yarn monofilaments may be fed in the twisting machine to create an adequately strong wrapping yarn bundle. In the twisting machine, the polyethylene wrapping yarn monofilaments are subjected to a twisting action which involves rotating the polyethylene wrapping yarn monofilaments in the opposite direction to their natural twist (known as S-twist or Z-twist) to create a balanced wrapping yarn. Tension is carefully controlled during the twisting process to ensure uniformity and prevent yarn breakage. Too much tension can weaken the yarn, while too little tension can result in uneven twisting. The direction of twist (S-twist or Z-twist) can vary. S-twist involves twisting to the left, while Z-twist involves twisting to the right. As the polyethylene wrapUping yarn monofilamentsare twisted together, the resulting twisted wrapping yarn is wound onto a bobbin or spoolto form a yarn bundle.According to some embodiments, a jet knot process (also known as air jet knotting or air knotting) may also be used for bunding the polyethylene wrapping yarn monofilaments to form a yarn bundle during the winding or stretching of the polyethylene wrapping yarn monofilaments following the extrusion of the polyethylene wrapping yarn monofilamentsfrom the extrusion.For example, the jet knot process may, for example, use compressed air to bundle the individual yarn monofilaments. As the yarn monofilaments exit the extrusion openings a high-pressure air nozzle is used to blow a jet of air onto the plurality of the yarn monofilamentscausing them to come into contact temporarily. The yarn monofilaments are then twisted together, and the air jet helps to secure them in place temporarily. While the yarn monofilaments are twisted together, knots may be formed due to the twisting action at predetermined intervals. The number of knots may differ. In an embodiment a knot may beformed every some centimeters, for example every 2 to 3 centimeters. Inan embodiment theknots may be formed temporarily and may be later undone during further processing. Texturizing the polyethylene wrapping yarn Texturizing the polyethylene wrapping yarn may be performed using any suitable method. For example, the polyethylene wrapping yarn may first be heated to soften beforeintroducing crimps, or waves in the yarn by passing the polyethylene wrapping yarn througha series of rollers, plates, or gears that introduce tension, twists, or random undulations into the polyethylene wrapping yarn. In an embodiment, air jet texturing may be used. Accordingly, high-speed air jets may be used to create turbulence and cause the monofilaments to crimp or curl as they cool down. Another method that may be used is twist texturing by twisting in one direction and then untwisting in the opposite direction. After texturing, the polyethylene wrapping yarn is rapidly cooled to set the new textured structure. This ensures that the polyethylene wrapping yarn maintains its desired shape and characteristics. The textured wrapping yarn is cut into adesired length and can be used for wrapping the polyethylene turf fiber bundles to protectthe polyethylene turf fibers as they are tufted through the carrier of the artificial turf.It has been found that the polyethylene wrapping yarn can be readily texturized at a greater degree than polyester wrapping yarn that is currently used for wrapping polyethylene turf fiber bundles. Texturizing may be performed on the individual filaments before they are bundled or may be performed to the polyethylene wrapping yarn bundle. According to some embodiments, texturizing includes air texturizing. For example, the polyethylene wrapping yarn may pass through a texturizing jet of compressed air which disrupts the smooth surface of thepolyethylene wrapping yarn and creates crimps, loops, or other desired textures. The air jetparameters such as air pressure, temperature, and nozzle design may be adjusted to achieve different textures and degree of texturization. The textured wrapping yarn may then be cooled to set the new texture in place, for example, by passing the polyethylene wrapping yarn through a cooling chamber or over cooling rollers. The final textured (or texturized)wrapping yarn is wound onto bobbins or spools before use.Making the artificial turfReferring now to figure 2, a method for making an artificial turf comprises providing aplurality of turf fiber monofilaments according to step 202, using a polyethylene wrapping yarn to form a turf fiber bundle by wrapping the polyethylene wrapping yarn around the plurality of the turf fiber monofilaments according to step 204, tufting the turf fiber bundle through a carrier to form loops on a front side of the carrier according to step 206, and cutting the loops to form free end fibers according to step 208. The cutting of the loops also cuts the polyethylene wrapping yarn which retracts and shrinks to its textured shape. The polyethylene wrapping yarn may shrink to less than 30%, preferably less than 20%, and more preferably less than 10% of its original length when it is wrapped around the turf fiber loop according to step 210. Once the loops are cut, each loop forms two sets of free end turf fiber bundles and the polyethylene wrapping yarn retracts in its shrank, texturedstate.Figures 3A and 3B show how a bundle of a plurality of artificial turf fibers can be secured to acarrier 308, e.g., a textile plane, by means of tufting. Accordingly, the artificial turf fiberbundle 503 wrapped with the polyethylene wrapping yarn 507 can be pierced through thecarrier 308 multiple times to form loops of the fiber on a front side of the carrier according to a tufting method. After the tufting is competed, as depicted in Figure 3A, there is a pluralityof short U-shaped loops of the fiber pointing outside of the carrier’s surface. Then, one ormore blades cut 502 through the loops. As a result of the cutting step, two artificial turf fiber bundles per loop are created with each turf fiber bundle having a plurality of free end fibers 501 pointing out from the carrier and a grass-like artificial turf surface is generated. As a result of the cutting step, the polyethylene wrapping yarn 507 is also cut and shrinks in one or more wrapping yarn filaments in their textured shrank state 508.The tufting process forms, first parts 506 of the artificial turf fiber bundle exposed to a bottom side of the carrier 308, second parts 302 exposed to a top side of the carrier 308 and intermediate parts 504being inside the carrier 308.In an embodiment the tufting process may include loading the carrier material in a machine equipped with rows of needles or tufting heads. The turf fiber bundle wrapped with the polyethylene wrapping yarn is fed into the tufting machine and as the carrier material moves through the tufting machine, the needles push the turf fiber bundle through the carrier creating loops of the turf fiber bundle on the surface of the carrier. For example, the portionsof the continuous turf fiber bundle that pierces through the carrier may include 180 stitchesper meter of tuft row and form 52 tuft rows per meter. Each tuft bundle / stitch after loopcutting may correspond to 6 open fiber ends, resulting in 180 x 52 x 6 open fiber ends per square meter. During this stitching process the wrapper yarn helps to keep the monofilaments making the turf fiber bundle together. See figure 3A. After the loops are cut the polyethylene wrapping yarn (which is highly texturized) shrinks to its textured state as itgets also cut when the loops are cut. The shrinkage may be further increased by eating the artificial turf after loop cutting and, in some instances, after applying a backing on the back of the carrier. For example, some artificial turfs, following the tufting process, may have an artificial turf backing applied on the back side of the carrier 308 to cover the parts 506 of the turf fiber bundle. The backing may be, for example, a polyurethane backing and may be applied by adding a viscous polyurethane reaction mixture on the back side of the carrier 308 to cover the parts 506. The carrier may be a textile mesh or may comprise perforations that allow the fluid polyurethane mixture at the bottom side of the carrier to flow in the carrier and maybeeven slightly above the front side of the carrier. Thus, the carrier and parts of the fibersinserted in the carrier may become embedded in the polyurethane backing.For example, the polyurethane fluid mixture may be a mixture of polyols and polyisocyanates that solidifies into a polyurethane backing, however, any other type of backing may also be used. Also, the present invention is not limited to artificial turfs with a backing. Many artificial turf structures may not have a backing and may be securelyplaced on a flat substrate structure other than a backing.EXAMPLES In example 1, a polyethylene (PE) wrapping yarn comprising a bundle of 18 filaments (see FIGURE 4) was formed and analyzed in comparison to comparative example 1, i.e., a wrapping yarn made from polyester (PES) comprising a bundle of 35 PES filaments (seeFIGURE 5). The PE wrapping yarn was made of 18 filaments and had a thickness of 12 dtexand a combined linear density of 214 dtex.(12dtex, bobbin 214dtex18 filaments).The PES wrapping yarn was made of 35 filaments had a thickness of 5 dtex and a combined linear density of 180 dtex (5dtex, bobbin 180dtex / 35Filaments). Shrinkage and tensile strength analysis data for example 1 and comparative example 1 are presented in Table 1. Table 1: EXAMPLE C. EXAMPLE 1 (PES) 1 (PE) Shrinkage 10% 5% Tensile strength 1,1N / m2 3,4N / m2DSC analysis for the PE and PES wrapping yarns are shown in figures 6 and 7 respectively. Importantly, the wrapping yarn made of polyethylene of example 1 exhibits adequate tensile strength, and shrinkage and provides excellent protection of polyethylene turf fiber bundle (made of polyethylene, or a PE / PA mixture with compatibilizer) during tufting to a variety of carriers. The artificial turf backing may for instance be a textile or other flatstructure which is able to have fibers tufted into it.Although the invention has been described in reference to specific embodiments, it should be understood that the invention is not limited to these examples only and that many variations of these embodiments may be readily envisioned by the skilled person after having read the present disclosure which do not fall outside the scope of the invention asdefined by the claims.For example, although the description of the composition and formation of the polyethylene-polyamide mixture with the compatibilizer in reference to the figures refers primarily to the use of the polyethylene-polyamide mixture in making the wrapping yarn, it should be understood that the same apply equally to the formation of turf fiber monofilaments from a polyethylene-polyamide polymer mixture with a compatibilizer such as, for example, a maleic anhydride grafted on polyethylene or polyamide. Use of thepolyethylene-polyamide in making artificial turf fiber monofilaments is described inWO2015 / 144223A1 to Sick et al. entitled artificial turf and production method and which is incorporated herein by reference.

[0002] List of reference numerals 102 to 210 process steps 302 tuft fibers on top side of carrier 308 carrier 400 PE / PA polymer mixture 402 PA polymer404 PE polymer406compatibilizer 408 PA polymer beads501 free end turf fibers 502 blades503 turf fiber bundle 504 turf fiber inside the carrier 506 turf fiber loops on lower side of carrier 507 wrapping yarn wrapped around turf fiber bundle 508 wrapping yarn in shrank state600 PE / PA polymer mixture602 extrusion plate 604 hole of extrusion plate 606 monofilament700 direction of stretching

Claims

What is claimed is:

1. An artificial turf wrapping yarn (507) comprising a plurality of wrapping yarn monofilamentsmade of polyethylene or a polyethylene-polyamide polymer mixture with a compatibilizer, wherein each of the wrapping yarn monofilaments has a cross section with a diameter of lessthan 100 micrometers, in particular less than 60 micrometers, and is texturized.

2. Use of the wrapping yarn of claim 1 for making an artificial turf fiber bundle (503) comprising:- a plurality of turf fiber monofilaments (501), preferably comprising at least 3, morepreferably 5 to 10, and most preferably 6 to 8 turf fiber monofilaments (501), and- the wrapping yarn (507) of claim 1, wherein the wrapping yarn is wrapped around theplurality of the turf fiber monofilaments (501) to protect their integrity during securing theartificial turf fiber bundle to a carrier (308) of an artificial turf.

3. The use of claim 2, wherein the plurality of the turf fiber monofilaments (501) are made ofthe same type of material like the wrapping yarn, in particular polyethylene, or a polyethylene- polyamide polymer mixture with a compatibilizer.

4. The polyethylene wrapping yarn of claim 1,wherein the polyethylene in the polyethylene wrapping yarn (507) comprises HDPE or HMPE or a combination thereof, wherein in particular the polyethylene-polyamide polymer mixture in the wrapping yarnor in the turf fiber monofilaments comprises 70% to 95%, or 80% to 90% polyethylene byweight, 0.1% to 2% by weight of a compatibilizing agent, and the balance of the polymer mixtureis polyamide.

5. The wrapping yarn of the claims 1 or 4 wherein the wrapping yarn (507) comprises from about20 to 50 individual wrapping yarn monofilaments, more preferably 30 to 40 individual wrappingyarn monofilaments.

6. The wrapping yarn of any of the claims 1, 4, or 5, wherein each of the individual wrappingyarn monofilaments has a cross section of 20 to 100 micrometers, or 20 to 60 micrometers, or20 to 30 micrometers.

7. The wrapping yarn of any of the claims 1, 4, 5, or 6, , wherein in particular the wrapping yarnis texturized so that it shrinks upon being cut and heat treated – starting from a maximumexpanded state – to less than 60% of its original length when wrapping the fiber bundle, inparticular to less than 50 % of its original length, for example to less than 30 % or less than 20% of its original length.

8. The wrapping yarn of any of the claims 1, 4, 5, 6, or 7, wherein the plurality of the wrapping yarn monofilaments are bundled to form the wrapping yarn.

9. The wrapping yarn of any of the claims 1, 4, 5, 6, 7, or 8, wherein the wrapping yarn has a weightof less than 50 g, preferably less than 40 g per 10.000 m of the wrapping yarn.

10. The wrapping yarn of any of the claims 1, 4, 5, 6, 7, 8, or 9, wherein the wrapping yarn (507)is made of polyethylene which has an MFI of at least 1.0 g / min, in particular from 1.0 g / min to 200 g / min, in particular from 3.0 g / min to 50 g / min, in particular from 5.0 g / min to 30 g / min, in particular from 12 g / min to32 g / min, in particular from 15.0 g / min to 30 g / min measured according to the ISO standard1133-1 at 190 °C and a weight of 2.16 kg, and / or has a density below 0.960 g / cm3, in particular below 0.925 g / cm3measured according to ISO standard 1183-1 at 23 °C.

11. The wrapping yarn of any of the claims 1, 4, 5, 6, 7, 8, 9 or 10, wherein the wrapping yarn(507) is made of HDPE having an MFI of from 12 g / min to 32 g / min measured according to theISO standard 1133-1 at 190 °C and a weight of 2.16 kg, and a density below 0.960 g / cm3measured according to ISO standard 1183-1 at 23 °C.

12. The wrapping yarn of any of the claims 1, 4, 5, 6, 7, 8, 9, 10, or 11, wherein the wrappingyarn (507) comprises a nucleating agent, in particular sorbic acid.

13. The wrapping yarn of the claims 1, 4, 5, 6, 7, 8, 9, 10, 11, or 12, wherein the wrapping yarn (507)comprises a radically crosslinked polyethylene.

14. An artificial turf fiber bundle (503), comprising:- a plurality of artificial turf fiber monofilaments (501), the plurality of artificial turf fibermonofilaments (501) preferably comprising at least 3, more preferably 5 to 10, and mostpreferably 6 to 8 artificial turf fiber monofilaments, and- the wrapping yarn (507) of any of the claims 1, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13 wrapped aroundthe plurality of the artificial turf fiber monofilaments, wherein the artificial turf fiber monofilaments comprise polyethylene or a polyethylene-polyamide mixture with a compatibilizer.

15. An artificial turf comprising:- a carrier,- a plurality of artificial turf fiber monofilaments (501) which are integrated in bundles atpuncture points in the carrier, and- the wrapping yarn (508) according to any one of the previous claims 1, 4, 5, 6, 7, 8, 9, 10, 11,12, or 13,wherein the wrapping yarn is integrated into the carrier at the puncture points.

16. The artificial turf of claim 15, wherein each of the wrapping yarn monofilaments of the wrapping yarn has a length of less than 60% of the length of the artificial turf fiber monofilaments, in particular of less than 30% of the length of the artificial turf fiber monofilaments , preferably less than 20 % of the length of the artificial turf fiber monofilaments, and more preferably less than 10% of the length of the artificial turf fiber monofilaments.

17. A method for manufacturing a wrapping yarn for artificial turf, the method comprising: -extruding polyethylene or a polyethylene-polyamide mixture with acompatibilizer, into a plurality of monofilaments (102); and -bundling the plurality of monofilaments together to form the wrapping yarn(104).

18. The method of claim 17, wherein the extrusion of the wrapping yarn monofilaments is performed at a temperature of at least 190°C, in particular at least 230°C, or more in particular from 240°C to 260°C.

19. A method of manufacturing artificial turf by using the wrapping yarn of any of the claims1 or 4-13, the method comprising:- providing a plurality of turf fiber monofilaments (202);- using the wrapping yarn to wrap around the plurality of turf fiber monofilaments to forma continuous turf fiber bundle of turf fiber monofilaments wrapped with the wrapping yarn (204);- tufting the continuous turf fiber bundle through a carrier to form loops of the turf fiberbundle on a front side of the carrier (206); and- cutting the loops to form a plurality of free end fibers per loop pointing out from thecarrier to form a grass-like artificial turf surface (208),- wherein in particular the wrapping yarn that is wrapped around each bundle upon beingcut is heated and shrinks to less than 30% of its original length when wrapping the fiber bundle (210), preferably to less than 20 % of its original length, and more preferably to less than 10% of its original length.

20. The method of claim 19, further comprising:- after the tufting, applying a liquid backing onto a lower side of the carrier for fixingportions of the artificial turf fibers and portions of the wrapping yarn in the carrier, the lower side being the side opposed to the side to which the cut artificial turf fibers emerge; and- applying heat for letting the applied liquid backing solidify.

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