Monofilament string

Abstract

 This invention relates to a monofilament string for use in tennis rackets and so on, which string comprises at least two phases, a first phase comprising a core containing an amount of a first polymer and a second phase comprising a matrix enclosing the core, in which the second phase contains an amount of a second polymer. The core and/or the matrix contains an amount of respectively at least a third and/or fourth polymer that can be the same or different. The first and second polymer can be the same or different, in which the first and third polymer are different and the second and fourth polymer are different.

Description

[0001] The present invention relates to a monofilament wire or string for use in tennis rackets and so on, which wire or string comprises at least two phases, a first phase comprising a core containing an amount of a first polymer and a second phase comprising a matrix enclosing the core, in which the second phase contains an amount of a second polymer, as described in the preamble of the first claim.

[0002] Strings for use in recreational equipments, such as for example as sport rackets for ball games such as tennis, badminton or squash, or fishing equipments, used to be made of natural materials. Although natural materials provide an excellent elasticity and ball control, they are, at present, because of their limited moisture proofness, more and more replaced by strings made of plastic material. For protection of the structural elements, a string made of plastic material is often coated with for instance nylon, polyurethane and polytetrafluoroethylene. However, because of the limited adhesion of the coating to the material and because the strings rub against each other and against the ball surface, the coating looses rather quickly and the coated strings are strongly subject to weathering. In order to enhance the resistance against rubbing, a multifilament string was developed, in which the filaments are caught in a composite matrix of for instance nylon or polyurethane wherein PTFE particles are dispersed.

[0003] A similar string is for instance described in US2001/0035002. US2005/0160714 describes a multifilament string in which a number of polyester filaments are woven to form a cord. Adhesion of the filaments is obtained through coating with a polyester thermoplastic elastomer. In order to have a perfectly cylindrical string, the string is coated with an elastomer, through which the resistance against weathering is enhanced. However, the production of a multifilament string is time consuming and expensive, because the filaments first have to be woven to form a string and then have to be imbedded into a matrix.

[0004] In order to solve the problem of the bad adhesion, monofilament strings were developed. These strings are mostly made in extruded polyester, particularly polyethyleneterephtalate (PET) or polytrimethyleneterephtalate (PTT). For amelioration of the elasticity and the elastic recovery, tensile strength, rub resistance, ball control and flatness after being tightened in a frame, JP2004068217 suggests monofilament strings, the core of which is made in polypropylene terephtalate and a shaft enclosing the core is made in polypropylene terephtalate and polyethyleneterephtalate. In the polyethyleneterephtalate a part of the phtalic acid can be replaced by another dicarboxylic acid, for instance by isophtalic acid or adipinic acid, and for instance propyleneglycol, tetramethyleneglycol, diethyleneglycol can be used as glycol component. The core as well as the shaft can have different forms.

[0005] These well known monofilament strings still show a stiffness which is too large and unsatisfying ball control.

[0006] This invention therefore aims at providing a monofilament string for use in recreational equipments, such as for example sport rackets for ball games such as tennis, badminton or squash, of which the stiffness can be varied within broad limits and therefore rendering the string suitable for players with a broadly varying way of playing, experience and quality. This invention also aims at providing a monofilament string with enhanced playing properties, that particularly provides a ball control tuned to the way of playing, experience and quality.

[0007] According to the invention this is achieved by a string showing the technical features of the characterizing part of the first claim.

[0008] To this end the string according to the invention is characterized in that the core and/or the matrix contains an amount of respectively a third and/or fourth polymer that can be the same or different, and in that the first and the second polymer can be the same or different. Within the scope of this invention the core may contain an amount of a third polymer or the matrix may contain an amount of a fourth polymer. However, it is also possible that the core contains an amount of a third polymer and that the matrix contains an amount of a fourth polymer.

[0009] Because the core is basically responsible for providing the strength and the tensile strength of the string or wire, the core is preferably mainly made of a polymeric material that provides the appropriate tensile strength, preferably a polyester. In order to provide an optimal strength and tensile strength, a person skilled in the art may choose using a mixture of two or more different polymers. The second polymer used for the matrix is initially chosen in such a way it shows an optimal compatibility, in particular shows an optimal adhesion to the first polymer used for the core. The first and the second polymer are preferably a polyester.

[0010] The presence of the fourth polymer, preferably an elastomer, in the matrix of the string reduces the stiffness and enhances the elasticity of the part of the string enclosing the core, without adversely influencing the recovery of the string after elastic deformation, in order to guarantee a constant quality of the string over a long period of use. The improved elasticity involves that the incorporation of the string of this invention in stringing, is simplified and that during the game a higher energy transfer of the string towards the ball may be achieved, resulting in a better ball control. The presence of the fourth polymer improves the water repellent character of the material, resulting in a longer period of use of the string.

[0011] As a consequence, this invention provides a monofilament string of which the strength, the stiffness and playing properties can be adjusted independently of each other, through an adequate choice of the polymers of which the core and the matrix are made.

[0012] When spinning the string of this invention, starting from the molten polymer, the person skilled in the art has to keep in mind that the first, second, third and fourth polymer may have a different melting point, and has to make sure that the processing temperature is chosen in a way that at the melting point of the first polymer the other polymers have not yet decomposed. Although a person skilled in the art would expect that the elastomer decomposes when heated at a too high temperature, the inventor has found that this effect is negligible in the string of this invention and that a string with a good strength, tensile strength and lifetime is obtained.

[0013] This invention presents the advantage that the end properties of the string as described above are achieved in one step, when producing the string, this contrary to the prior art where more steps were needed to gradually build up the desired properties.

[0014] A preferred embodiment of the invention is characterized in that at least one third and fourth polymer may be the same or different and are chosen from the group of elastomers, particularly polyolefines particularly thermoplastic olefines; polyethylene and thermoplastic polyethylene; polypropylene; ethylene-propylene copolymers, particularly EPDM, polyisoprene and copolymers thereof, polybutadiene and copolymers thereof, polyisobutylene and copolymers thereof; polyamides, for instance nylon 6, nylon 66, nylon 11, nylon 12 or nylon 46 or copolymers thereof; polyesters for instance polyethyleneterephtalate (PET), polytrimethyleneterephtalate (PTT), polybutyleneterephtalate (PBT) or copolymers thereof; polyphenylenesulfide, polyurethane and thermoplastic polyurethane (TPU) and copolymers thereof, styrene-butadiene-styrene copolymers and block copolymers, polyether-polyamide block copolymers, polyethers, polyethersulfones, polysulfones, polyesters, polyetheresters, polyphenyleneoxide, polyetheretherketone (PEEK), PEEKK (polyetheretherketonketone), ABS, polymethylacrylate and polymethylmethacrylate, ethylenetetrafluorethylene, ethylenechloor-tetrafluorethylene, poly-vinylidenefluoride (PVDF), teflon based polymers such as FEP, MFA en PFA and mixtures of two or more of these polymers or blends that contain one or more of afore-mentioned polymers.

[0015] Preferably the at least one third and fourth polymer is chosen from the group of thermoplastic elastomers, polyethers or polyether-polyamide block copolymers.

[0016] In a preferred embodiment of this invention the matrix contains an amount of a second polymer varying from 50 to 100 wt. %, preferably from 70 to 99,5 wt. %, more preferably from 75 to 98 wt. % and most preferably from 85 to 95 wt. %. The fourth polymer is preferably present in an amount of 0,5 to 50 wt. %, preferably from 0,5 to 30 wt. %, more preferably from 2 to 25 wt. % and most preferably from 5 to 25 wt. %.

[0017] In another preferred embodiment of this invention, the core contains an amount of a first polymer varying from 70 to 100 wt. %, preferably from 85 to 95 wt. %, and an amount of a third polymer varying from 0 to 30 wt. %, preferably varying from 5 to 15 wt. %.

[0018] The first and second polymer may be the same or different. Suitable first and second polymers for use in the string of this invention are chosen from from the group of polyamides, particularly nylon 6, nylon 66, nylon 11, nylon 12 or nylon 46 or copolymers thereof; polyesters, particularly polyethylenetheraphtalate (PET), polytrimethyleneterephtalate (PTT), polybutyleneterephtalate (PBT), polypolyetherketones, particularly polyethyletherketone (PEEK), PEEKK, polyethersulfones, polysylfones, polyoxymethylene (POM), polyvinylidenefluoride (PVDF), copolymers from polyesters or mixtures of two or more of these polymers.

[0019] Preferably the polyesters have a melting temperature between 250 and 275 °C.

[0020] The third polymer is preferably chosen different from the first polymer. The fourth polymer is preferably chosen different from the second polymer.

[0021] The polymer composition of core and/or matrix may further contain the usual additives, such as functional additives, for instance nanoparticles of the first, respectively the second polymer, fillers, for instance titanium dioxide, silica, aluminum oxide, calcium carbonate, clay, silicon nitride, zircon oxide, anti oxidants, fire decelerator, surface active materials, plasticizers,... The presence of nanoparticles of the second polymer in the material of the matrix affects of the matrix surface en influences the playing properties of the string. The presence of nanoparticles of the first polymer in the material of the core has the advantage that one can save on more expensive fillers and additives, particularly in case of use of nanoparticles of recycled first polymer.

[0022] The invention is further explained in the attached figures and the description of the figures.

[0023] Figure 1 shows a cross section of the monofilament string of this invention, with a coaxial core and matrix.

[0024] Figure 2 shows a cross section of the monofilament string of this invention in which the matrix contains more than one core.

[0025] Figure 3a, 3b and 3c, show respectively a cross section of the monofilament string of this invention, in which the cores extend along the edge of the string, are located more centrally in the string, and in which the string is surrounded with an additional mantle.

[0026] The core 1 and matrix 2 of the monofilament string 5 of this invention may take any form, found suitable by a person skilled in the art. The core may for instance take the form of a circular core, central or not central in an enclosing matrix, such as the co extruded string shown in figure 1. The core may also consist of a set of islands spread out over the enclosing matrix as shown in figure 2. The number of islands may be varied within broad limits, taking into account the desired properties of the string, and may for instance vary from 2, 3 to 6 or 9, 10, 37 or more. The islands may be spread out regularly or irregularly over the material of the matrix, and may or may not be arranged into predefined groups of islands. The islands may mainly extend along the edge of the string or they may be positioned more into the interior, or a combination of these two possibilities. It is for instance possible that 5 islands are shifted more towards the edge and 5 islands are shifted to the middle of the string. The string has in principal a circular cross section, but can also have an oval cross section or any other form found suitable by the person skilled in the art, for instance pentagonal, star-shaped. The island wires are preferably round, but can take whatever form. The string is preferably composed of an edge formed by the matrix (figure 3a). It is however possible that the islands are located primarily at the outside as shown in figure 3b. It is also possible that the matrix is enclosed by a mantle 3, that can be applied externally or can be extruded with the rest of the polymers (figure 3c). It is also possible to subject the string to a plasma treatment in order to primarily enhance the coating.

[0027] The string as described above is suitable for use in a number of applications, for example in ball games for the stringing of tennis rackets, racket ball, badminton, or squash rackets, for use as fishing line, and other application areas in which use is made of a tightened string with a high tensile strength.

[0028] This invention also relates to a process for the production of the string described above.

[0029] Thereto the polymer composition of the core, comprising the first and third polymer, is heated and melted in a first endless screw, and the polymer composition for the matrix comprising the second and fourth polymer, is heated and melted in a second endless screw. The temperature in the screw is in most cases between 250-275°C, but can vary from 160°C to 400°C, depending on the polymers that are used. The polymer composition of the matrix preferably comprises an amount of elastomer, that can be the same or different from the elastomer of the core. Next the first polymer composition is extruded by a first series of openings in a mould, and the second polymer composition is at the same time extruded through two openings of the mould at increased temperature, in such a way that the adhesion of the first and second polymer composition is obtained. The pressure performed during the extrusion varies in most cases from 60-90 bar. The wire obtained in this way is primarily cooled with air and next sent through a water bath for further cooling. However, in the course of this invention it is also possible to primarily melt the first and third polymer in separate endless screws and then to mix, before being extruded through a mould. In analogy, the second and fourth polymer can be melted in separate endless screws and then mixed, before being extruded in de mould as a matrix.
For the stretching of the wire, in order to obtain a string with the desired diameter of 0,1 mm to 1 mm, preferably 0,1 to 0,5 mm, the wire is in succession send over more than one roll, at increased temperature, temperature through ovens, infrared, microwaves, steam, warm water,... A next roll always has a higher rate of circulation than the former roll, with the exception of the last roll that has a lower rate of circulation, in order to give the material of the wire his shrinkage properties. Next the string is winded up a roll. In that way the rate of circulation of the third roll can be for instance 4 to 12 times higher than the rate of circulation of the first roll. However, the person skilled in the art is able to set the desired relationship taking into account the desired stretching degree of the wire. The rate of circulation of the rolls can be varied between broad limits, for instance from 5 m/min for the first roll to 200 m/min for a third and/or fourth roll.

Claims

1. A monofilament string for use in tennis rackets and so on, comprising at least two phases, a first phase comprising a core containing an amount of a first polymer and a second phase comprising a matrix enclosing the core, in which the second phase contains an amount of a second polymer, characterized in that, the core and/or the matrix contains an amount of respectively at least a third and/or a fourth polymer that can be the same or different, and because the first and second polymer can be the same or different, the first and third polymer are different and the second and the fourth polymer are different.

2. A monofilament string according to claim 1, characterized in that, at least a third and fourth polymer can be the same or different and chosen from the group of elastomers, particularly polyolefines particularly thermoplastic olefines; polyethylene and thermoplastic polyethylene; polypropylene; ethylene-propylene copolymers, particularly EPDM, polyisoprene and copolymers thereof, polybutadiene and copolymers thereof, polyisobutylene and copolymers thereof; polyamides, for instance nylon 6, nylon 66, nylon 11, nylon 12 or nylon 46 or copolymers thereof; polyesters for instance polyethyleneterephtalate (PET), polytrimethyleneterephtalate (PTT), polybutyleneterephtalate (PBT) or copolymers thereof; polyphenylenesulfide, polyurethane and thermoplastic polyurethane (TPU) and copolymers thereof, styrene-butadiene-styrene copolymers and block copolymers, polyether-polyamide block copolymers, polyethers, polyethersulfones, polysulfones, polyesters, polyetheresters, polyphenyleneoxide, polyetheretherketone (PEEK), PEEKK (polyetheretherketonketone), ABS, polymethylacrylate and polymethylmethacrylate, ethylenetetrafluorethylene, ethylenechloor-tetrafluorethylene, poly-vinylidenefluoride (PVDF), teflon based polymers such as FEP, MFA en PFA and mixtures of two or more of these polymers or blends that contain one or more of afore-mentioned polymers.

3. A monofilament string according to claim 2, characterized in that at least one third and fourth polymer is chosen from the group of thermoplastic elastomers, polyethers or polyether-polyamide block copolymers.

4. A monofilament string according to anyone of claims 1-3, characterized in that the matrix contains an amount of a second polymer varying from 50 to 100 wt. % and in that the fourth polymer is present in an amount from 0,5 to 50 wt. %, preferably an amount of a second polymer varying from 85 to 95 wt. % and a fourth polymer from 5 to 15 wt. %.

5. A monofilament string according to anyone of claims 1-4, characterized in that the core contains an amount of a first polymer varying from 70 to 100 wt. %, preferably from 85 and 95 wt. % and an amount of the third polymer from 0 to 30 wt. %, preferably from 5 to 15 wt. %.

6. A monofilament string according to anyone of claims 1-5, characterized in that the first and the second polymer is chosen from the group of polyamides, particularly nylon 6, nylon 66, nylon 11, nylon 12 or nylon 46 or copolymers thereof; polyesters, particularly polyethylenetheraphtalate (PET), polytrimethyleneterephtalate (PTT), polybutyleneterephtalate (PBT), polypolyetherketones, particularly polyethyletherketone (PEEK), PEEKK, polyethersulfones, polysylfones, polyoxymethylene (POM), polyvinylidenefluoride (PVDF), copolymers from polyesters or mixtures of two or more of these polymers.

7. A monofilament string according to anyone of claims 1-6, characterized in that the first and second polymer are a polyester containing a dicarboxylic acid from the group of phtalic acid, isophtalic acid, adipinic acid, sebacinic acid, 2, 6-naphtalenedicarbonic acid, and a diol chosen from the group of polyalkyleneglycol, diethyleneglycol, propyleneglycol, tetramethyleneglycol, 1,4-cyclohexanediol, or copolymers from these polyesters or blends of two or more of these polyesters or copolymers thereof.

8. A monofilament string according to anyone of claims 1-7, characterized in that the string has a rounded shape, because co-axial with a wall of the string, on a first and second distance of an axis of the string, a first and second series of cores is applied made from the same or a different material.

9. A monofilament string according to anyone of claims 1-8, characterized in that the matrix and/or core contain an amount of nanoparticles of the first, respectively second polymer.

Drawing