TWI836115B - Monofilament string for a racket, racket comprising the same and manufacturing process thereof - Google Patents

Abstract

The present invention relates to a monofilament string (1) for a racket (5), comprising a core (2) consisting of a single filament and a sheath (3) extending around the core (2) and in contact with the core (2), wherein: - the core (2) is made of a first material comprising at least a polyamide, - the sheath (3) is made of a second material comprising at least a polyamide, wherein the second material comprises graphene or graphane nanoparticles with a concentration ranging from 0.1% to 5% in weight, preferably from 0.1% to 2% in weight, and more preferably from 0.1% to 1% in weight of the weight of the sheath.

Description

Monofilament string for racket, racket containing the same and manufacturing method thereof

Field of invention

The present invention relates to a monofilament string for use in a racket, such as a tennis racket, a squash racket, a badminton racket or the like, and a set of such strings.

Background of the invention

In the field of racket sports, a racket is made of a handle and a hoop, across which a set of strings extend in two orthogonal directions and are intended to withstand the impact of a ball, shuttlecock or the like.

The technological evolution in this area, which is pushing towards more and more competitive rackets, involves significant improvements in the construction and manufacture of the strings, especially in the materials that make up the strings.

From a general point of view, what is sought is a racket whose strings exhibit good or at least average energy, control, comfort and durability characteristics. Energy properties refer to the ability of a string to increase the speed of the ball leaving the string when a player hits the ball. Control characteristics refer to the ability of the string to influence the behavior of the ball, thereby making it possible for the player to accurately hit the ball to a predetermined location, slow down the ball, and influence the spin of the ball. Comfort characteristics refer to the ability of the string to reduce the vibration of the racket caused by the string experiencing the impact of the ball when the player hits the ball. And finally, durability properties refer to less deterioration of the thread's structure over time and use, which in particular results in less loss of tension, thereby allowing the thread to maintain its energy, control and/or comfort properties.

Among different types of strings, strings made of natural gut have low stiffness, which allows players to accelerate the ball without requiring high physical strength. However, it provides poor ball control. The same is true for multifilament threads, which are usually made of polyamide.

Monofilament and multifilament strings are usually made of polyethylene, polyester or polyamide. Strings made of polyethylene and polyester have high strength, which enables the player to be precise and have good ball control. However, the player needs to have high physical strength in order to accelerate the ball. Strings made of polyamide show these characteristics while also providing a strong ability to dissipate racket vibrations, but tend to deteriorate quickly and lose tension.

Therefore, a monofilament line showing a good balance between energy and control characteristics, while also having good comfort and durability characteristics, is required.

Specifically, there is a need for monofilament threads that exhibit high energy properties that allow the player to easily increase the speed of the ball without requiring high physical strength, while allowing the player to have good ball control over a reasonable amount of time (preferably game time, This is a period of several hours (especially 2 to 4 hours) for an experienced athlete to maintain a substantially constant tension.

Document FR 2 934 958 aims to enhance the durability of the racket string and discloses a monofilament string comprising a central core, a peripheral protective layer and an intermediate reinforcing layer made of composite material positioned between the central core and the peripheral protective layer.

This intermediate reinforcement layer increases the durability of the string by increasing its rigidity at the expense of its elasticity, but this results in a reduction in the energy characteristics of the string as the string is less able to bend upon ball impact.

Document WO2018234376 discloses a monofilament line that improves the balance between energy characteristics and control characteristics. However, the durability of the line over time and use needs to be further improved.

Carbon nanotubes have been reported in US 2008/0206559 and US 2012/0237767 as additives in wear-resistant coatings wrapped around multifilaments. However, such carbon nanotubes significantly stiffen the material in which they are included and thus make the material difficult to process.

Summary of the invention

One object of the present invention is to provide a monofilament wire that overcomes the above-mentioned disadvantages.

The present invention is particularly directed to providing a monofilament cord which exhibits a good balance between energy and control properties, while also having good comfort and durability properties, and enhanced durability over time and use compared to existing cords, while enabling the cord to be manufactured by a co-extrusion process.

For this purpose, one object of the present invention is a monofilament string for a racket, comprising a core consisting of a single filament and a sheath extending around the core and in contact with the core, wherein: - the core is made of a first material comprising at least polyamide, - the sheath is made of a second material comprising at least polyamide, wherein the second material is a polyamide matrix, the polyamide matrix comprising graphene or graphane nanoparticles in a concentration ranging from 0.1% to 5% by weight, preferably from 0.1% to 2% by weight, and more preferably from 0.1% to 1% by weight, based on the weight of the sheath.

"Polyamide matrix" as used herein means a matrix comprising at least one polyamide homopolymer and/or copolymer. In particular, the matrix may include PA6, PA6.6, PA11, PA12, PA4.6 and/or copolymers thereof. The matrix may include other polymers, with the total weight percent of polyamide being greater than the total weight percent of the other polymer(s). The matrix may also include fillers or additives.

By incorporating graphene or graphane into the material used to form the sheath, the characteristics of the string remain essentially the same in terms of comfort, control and energy. However, the durability of the string over time and use, i.e. its ability not to deteriorate, is significantly improved.

Furthermore, the graphene or graphane content is chosen to be low enough to allow the production of monofilaments via a coextrusion process. Specifically, during coextrusion, the monofilaments must be stretched to exhibit the desired mechanical properties. Excessive graphene or graphane content provides excessive stiffness of the material during the coextrusion process, which prevents proper stretching, resulting in poor mechanical properties of the monofilament. However, thanks to the low graphene or graphane content used in the present invention, the monofilaments can be stretched to a sufficient extent during the coextrusion process and thereby achieve the desired mechanical properties.

Graphene is an allotrope of carbon in the form of a two-dimensional, atomic-scale hexagonal lattice, with one atom forming each vertex.

Graphane is a form of hydrogenated graphene. More precisely, graphane is a two-dimensional polymer of carbon and hydrogen with the chemical formula unit (CH) _n , where n is a large integer.

Such graphene or graphane nanoparticles make it possible to stabilize the wire structure, and the nanosize of the carbon particles does not seriously stiffen the wire, unlike some based on fluorine, molybdenum disulfide or especially Kevlar fibers. Other additives do the opposite.

Graphene nanoparticles also improve temperature resistance and the sliding of one wire over another, which also improves stability over time by preventing premature wire degradation.

The term "rigidity" as used herein refers to the tensile modulus (also called "Young's modulus" or "elastic modulus") of a material. Materials with high rigidity exhibit high tensile modulus and therefore low elasticity.

The term "geometric stiffness" or simply "stiffness" as used herein is similar to the term "stiffness" but refers to structures. The stiffness of a structure depends on the rigidity of the material from which it is made and its spatial characteristics.

Depending on other optional characteristics of the monofilament line:

- Monofilament wire is obtained by co-extrusion of core and sheath;

- The second material comprises at least one of the following: polyamide 6, polyamide 6.6, polyamide 11, polyamide 12, polyamide 66 and a mixture thereof;

- The first material comprises polyamide 6 and a first copolymer of polyamide 6 and polyamide 6.6, and the second material comprises a second copolymer of polyamide 6 and polyamide 6.6;

- The tensile modulus of the first material is greater than the tensile modulus of the second material.

- The second material further contains at least one additive selected from the group consisting of: slip agent and hydrophobic agent.

Another object of the invention is a racket comprising a set of monofilament strings as previously described.

Another object of the invention is a process for manufacturing monofilaments as described above. In this process, the core and sheath are formed through a co-extrusion process. The process further includes drawing the monofilament.

1: Single filament wire

2: Core

3: sheath

4: Interface

5: Racket

6: Group

8: Screen

9: handle

With reference to the accompanying drawings, other features and advantages of the present invention will become apparent from the following embodiments, in which: - FIG. 1 is a cross-sectional view of a second embodiment of a single-filament thread of the present invention, wherein the single-filament thread comprises a core and a sheath, and the sheath comprises graphene or graphane nanoparticles; - FIG. 2 is a schematic diagram of a racket comprising a set of single-filament threads according to the present invention; - FIG. 3 is a graph showing the effect of graphene nanoparticles on the tensile strength of different single-filament threads; - FIG. 4 is a graph showing the effect of graphene nanoparticles on the tensile stress (tensile strength) of different single-filament threads. - Figure 5 is a graph showing the effect of graphene nanoparticles on the tensile mechanical resistance of different single filaments; - Figure 6 is a graph showing the effect of graphene nanoparticles on the elongation of different single filaments; - Figure 7 is a graph showing the effect of graphene nanoparticles on the plastic deformation of different single filaments.

Detailed description of the preferred embodiment

The present invention proposes a monofilament string for a racket.

The monofilament thread includes a core and a sheath coaxial with the core and made of a polyamide matrix including graphene or graphane nanoparticles.

The polyamide matrix comprises at least one polyamide homopolymer and/or copolymer, which is preferably selected from the following: polyamide 6, polyamide 6.6, polyamide 11, polyamide 12, polyamide 66 and mixtures thereof.

The matrix may include other polymer(s) other than the polyamide polymer. However, the total weight percent of polyamide is always greater than the total weight percent of the other polymer(s). Polyamide matrix can also be used Includes fillers or additives.

The polyamide matrix provides the monofilament string with high strength, which enables the player to be precise and have good ball control; and provides the monofilament string with a strong ability to dissipate racket vibrations, which improves comfort.

The presence of graphene or graphane nanoparticles in the polyamide matrix maintains the good properties provided by the polyamide matrix while strongly improving the durability of the string over time and use. As a result, the string set of the racket maintains its good characteristics in terms of comfort, control and energy for a continuously improved period of time and the string breaks are strongly reduced, if not prevented, even when the string is under high stress. In other words, the ability of the string not to deteriorate is significantly improved.

An amount of 20% by weight or less of graphene or graphane nanoparticles relative to the weight of the part including the nanoparticles is optimal for imparting enhanced durability to the wire without excessively increasing the stiffness. of. Preferably, the weight percentage of graphene or graphane nanoparticles is in the range of 0.1% to 5%, more preferably 0.1% to 2%, and even more preferably 0.1% to 1%.

Graphene and graphane are excellently suited for incorporation into polyamide matrices, and both provide enhanced durability properties to the threads. Graphene and graphane particles are considered two-dimensional (2D) particles due to their lamellar structures extending in a plane. In contrast, carbon nanotubes, which are other types of carbon nanoparticles, are three-dimensional (3D) particles because they can be viewed as carbon sheets that wrap around themselves to form a cylinder.

The enhanced durability comes from the specific mechanical and thermal properties of carbon nanoparticles.

In more detail, in graphite, which corresponds to a stack of graphene layers, the carbon planes are weakly correlated with each other. In contrast, in each graphene plane, the carbon atoms are connected by extremely strong bonds. This is also true for graphane nanoparticles. As a result, the material is extremely mechanically strong in the plane (more than a hundred times stronger than steel of the same thickness) and at the same time deformable. In combination with shock-absorbing materials, carbon nanoparticles can form extremely strong and flexible materials. For example, the Young's modulus of graphene is about 1.0 TPa (TeraPascal), which makes graphene an extremely strong material.

Graphene and graphane nanoparticles are also extremely light. For example, the density of graphene is about 2.25g/cm ³ , which is extremely low.

Thus, graphene and graphene nanoparticles used in suitable concentrations as mentioned above show high stiffness and lightness, thereby stabilizing the wire structure without severely stiffening the wire, contrary to some other additives based on fluorine, molybdenum disulfide or especially kevlar fibers.

An important aspect that has an influence on the energy characteristics of the racket strings is the sliding of the strings on each other and the friction caused by the contact of the strings as they slide. In more detail, when the player hits the ball, the balls engage the strings, causing the strings to bend and thereby slide on each other in a first direction while being pressed against each other. After the ball is struck, the ball leaves the strings, causing the strings to return to their original rest position and slide over each other in a second direction opposite to the first direction.

In addition to rigidity and lightness, graphene and graphane nanoparticles provide good sliding properties for the wires, which reduce friction between the wires when sliding.

Graphene and graphane nanoparticles show high thermal conductivity and thermal stability. The thermal conductivity value of graphene is about 5000 W.m ^-1 .K ^-1 , which is 10 times higher than copper, 20 times higher than aluminum, and 2 times higher than graphite. Therefore, graphene and graphane nanoparticles provide strings with improved ability to regulate and disperse heat in the string set of a racket.

An embodiment of the monofilament thread of the present invention is shown in FIG1 .

The monofilament cable 1 includes a core 2 composed of a single filament, and a sheath 3 extending around and contacting the core. The core 2 has a circular cross section and the sheath 3 has an annular cross section, and the sheath is coaxial with the core.

The core 2 is made of a first material comprising a first copolymer of polyamide 6 and polyamide 6.6 (first copolymer PA 6/6.6), and the sheath is made of a first material comprising polyamide 6 and polyamide 6.6. The second material is made of a second copolymer (second copolymer PA 6/6.6, which can be the same as the first copolymer).

Polyamide 6 and polyamide 6.6 are thermoplastic semi-crystalline polymers that exhibit good mechanical properties. They are both fairly rigid polymers, but the tensile modulus of polyamide 6 is higher than that of polyamide 6.6.

For example, the tensile modulus of polyamide 6 is generally 700MPa (Mega Pascal). Pascal)) and 800MPa, while the tensile modulus of copolymer PA 6/6.6 is generally between 500MPa and 600MPa.

The polyamide matrix including graphene or graphene nanoparticles is in the sheath.

According to one embodiment (not shown), a polyamide matrix including graphene or graphane nanoparticles is present in both the core and the sheath.

According to a preferred embodiment, the graphene or graphane nanoparticles are only in the sheath, and based on the weight of the sheath, it represents 0.1 wt% to 5 wt%, preferably 0.1 wt% to 2 wt%, and more preferably 0.1 wt% to 1 wt%.

The mechanical properties of copolymer PA 6/6.6 are generally somewhere between the mechanical properties of polyamide 6 and the mechanical properties of polyamide 6.6. The block copolymer PA 6/6.6 is preferred because the properties of the latter may be very close to the better properties of polyamide 6 and polyamide 6.6 without suffering a corresponding loss of other desired properties, depending on the copolymer PA 6/6.6 It depends on the structure, the respective proportions of polyamide 6 and polyamide 6.6 in the copolymer PA 6/6.6, and the manufacturing process of the copolymer PA 6/6.6.

Therefore, copolymer PA 6/6.6 has a tensile strength that is between the tensile strength of polyamide 6 and that of polyamide 6.6, or is substantially equal to the tensile strength of polyamide 6.6.

The first material is preferably selected so as to have a greater tensile modulus than the tensile modulus of the second material.

For this purpose, the first material comprises polyamide 6 in addition to the first copolymer PA 6/6.6. Polyamide 6 provides the first material with high rigidity and a high ability to dissipate mechanical effects (energy) during elastic deformation.

The core 2 thereby provides the monofilament string 1 with a high geometric stiffness, and the ability to strongly absorb/dissipate the mechanical forces exerted on the string when it is subjected to impact by a ball or the like, which results in better ball control, and Reduction of vibrations transmitted through the screen 8 and handle 9 of the racket 5 represented in Figure 2.

One result is that the racket 5 allows the player to decelerate the ball after receiving and hitting it for better control of the ball. Another result is that the player is subjected to less vibration and shock when hitting the ball for better comfort, thereby preventing injuries such as tennis elbow, for example in the case of a tennis racket.

Preferably, the sheath does not contain polyamide 6. However, it must be understood that the second material may contain polyamide 6, but in a significantly lower amount compared to the first material. In this case, the weight percentage of polyamide 6 in the second material (relative to the second material) is significantly lower than the weight percentage of polyamide 6 in the first material (relative to the first material).

Similarly, the amount of polyamide 6 in the copolymer PA 6/6.6 of the first and second materials is also adjusted so that the tensile modulus of the first material is greater than the tensile modulus of the second material. Advantageously, the weight percentage of polyamide 6 in the copolymer PA 6/6.6 of the second material is lower than the weight percentage of polyamide 6 in the copolymer PA 6/6.6 of the first material.

Therefore, the tensile modulus of the second material (sheath) is lower than the tensile modulus of the first material (core). Therefore, the second material is more elastic than the first material, absorbing less energy and releasing more energy when elastically deformed.

The sheath 3 thus provides the monofilament line 1 with the ability to strongly release the mechanical action applied to the line when the line is hit by a ball or the like.

One result is that the racket allows the player to powerfully accelerate the ball when hitting the ball.

The wire 1 is preferably obtained by co-extruding the core 2 and the sheath 3.

The co-extruded core 2 and sheath 3 form an interface 4 at the contact area between the core and the sheath, in which the core and the sheath are tightly connected.

As described previously, the core 2 and the sheath 3 of the wire 1 are similar in terms of chemical structure. Both the core and the sheath are actually made of a polyamide-based material, namely the copolymer PA 6/6.6.

The strong mechanical and chemical cohesion of core 2 and sheath 3 at interface 4 allows the core and sheath to work together when the string is mechanically required, thereby further improving the overall mechanical properties of the string, especially its durability and its ability to affect ball spin. .

In the line, the weight proportion of sheath 3 is small compared to the weight proportion of core 2. Specifically, the sheath preferably represents 5% to 20% by weight of the total weight of the thread 1, more preferably 8% to 16% by weight. The core preferably represents 80% to 95% by weight of the total weight of the thread, more preferably 84% to 92% by weight.

In terms of thickness, the thickness of sheath 3 represents 2% to 7%, preferably 3% to 6%, of the total thickness of wire 1, and the thickness of core 2 represents 93% to 98%, preferably 94%, of the total thickness of wire 1 to 97%.

In more detail, the thickness of the sheath is preferably in the range of 20 microns and 50 microns, while the thickness of the core (which corresponds to the diameter) is in the range of 1200 microns and 1500 microns.

Together with the composition of the first and second materials of the core and sheath, such a high weight ratio of the core to the sheath makes it possible to have a wire with high control properties.

Surprisingly, despite the low weight ratio of the sheath, the sheath is sufficient to provide high energy properties to the string, in particular by giving the string explosive properties. "Explosive" in the context of the present invention means that the racket returns the ball with great speed.

The combination of core and sheath thus provides a good balance between control and energy properties.

Of course, depending on how the user intends to play, the composition and proportions of the core and sheath can be adjusted to provide the best trade-off between control characteristics and energy characteristics.

To further reduce friction between the wires when sliding, the sheath advantageously contains one or more additives that facilitate sliding of the wires relative to each other, thereby providing the wires with enhanced power and rebound capabilities, and generally, enhanced energy properties.

The additives are preferably selected from the group consisting of: slip agents and hydrophobic agents.

Among slip agents, preferred additives are selected from: erucamide, such as stearyl erucamide; ethylidene distearylamide; polydimethylsiloxane based on polyamides Alkanes; polyamide-based siloxanes with ultra-high molecular weight; fluorine-based polymers; polymers loaded with molybdenum disulfide.

Among hydrophobic agents, preferred additives are selected from: siloxane-based polymers with ultra-high molecular weight, polydimethylsiloxane-based polymers, silica-based compounds, ceramic nano-based Particle compound.

For the purpose of reducing friction between the wires during sliding, coatings of such additives or other substances may also be applied to the peripheral surface of the sheath, especially during the manufacture of the wires.

According to one embodiment, the coating may be applied to the exterior surface of the sheath in addition to or as an alternative to the presence of a slip agent or hydrophobic agent in the sheath. The coating may have anti-slip and/or water-repellent properties.

The monofilament thread according to the invention has the following properties: - shock-absorbing capacity provided by the core 2, due to the low elasticity of the core; - power and rebound capacity provided by the sheath 3, due to the high elasticity and low friction of the sheath; - high Durability properties, with less deterioration of structure and tension over time and use. This is due to the relatively high tensile modulus of polyamide 6 and copolymer PA 6/6.6, the aforementioned properties of the thread, and the overall mechanical properties. This is further improved by co-extruded the core and sheath and forming an interface 4 therebetween.

Monofilament threads therefore display a good balance between energy and control properties, while also possessing good comfort and durability properties.

Due to the presence of graphene or graphane nanoparticles in the sheath, the monofilament thread according to the invention also has the following properties: - increased rigidity and durability and acceptable lightness, due to the ability of the graphene or graphane nanoparticles to reinforce the structure of the thread without seriously stiffening it (due to the nanosize of the graphene or graphane particles) or seriously weighing it (due to the low density of the graphene or graphane particles); - further increased power and rebound capabilities, due to the reduced friction between the threads during sliding.

Experimental results

Experimental testing and measurements were performed on three different monofilament threads to determine their mechanical properties and to document the effect of graphene on the mechanical properties of the threads.

The monofilament tested in each of the five examples below is identical. It has the same polyamide structure but with different amounts of graphene in the sheath.

Graphene nanoparticles have a thickness between 1nm and 2nm and a lateral dimension between 0.5μm and 5μm.

Graphene nanoparticles are provided in powder form, which is mixed with plastic pellets that are fed into an extruder.

Alternatively, graphene nanoparticle powder can be mixed with a polyamide polymer to obtain a compound, and particles made from this compound can then be fed into an extruder.

The tested monofilaments are as follows:

- Wire A (1% graphene) - single filament wire, comprising:

●A core containing polyamide 6 and a first copolymer of polyamide 6 and polyamide 6.6,

●A sheath containing a second copolymer of polyamide 6 and polyamide 6.6

●1% graphene in the sheath;

- Line B (3% graphene) - monofilament line, which contains:

● A core comprising polyamide 6 and a first copolymer of polyamide 6 and polyamide 6.6,

●Sheath comprising a second copolymer of polyamide 6 and polyamide 6.6

●3% graphene in the sheath

- Line C (without graphene) - single filament line, comprising:

● A core comprising polyamide 6 and a first copolymer of polyamide 6 and polyamide 6.6,

● A sheath comprising a second copolymer of polyamide 6 and polyamide 6.6.

Each monofilament thread sample was subjected to one hundred cycles of 250 Newton (Newton; N) tensile stress for 10 minutes: the sample was stretched and relaxed one hundred times. For each cycle, the tensile strength, Young's modulus, tension maintenance, elongation, and plastic deformation of the thread were measured, and the average of one hundred cycles was calculated.

Example 1: Effect of graphene on tensile strength

The graph in Figure 3 shows the evolution of the tensile strength (TS) of the wire as a function of the amount of graphene in the sheath.

Adding 1% graphene to the sheath increases the tensile strength of the wire from 564N to 600N.

In the case of 3% graphene in the sheath, the tensile strength of the wire is 587N, which is slightly lower than the case of 1% graphene but higher than the case without graphene.

Therefore, adding graphene to the wire increases tensile strength.

Example 2: The impact of graphene on Young's modulus

The graph in Figure 4 shows the evolution of the Young's modulus (Y) of the wire as the amount of graphene in the sheath changes.

Adding 1% graphene to the sheath increased the Young's modulus of the wire from 1629N/mm ² to 1726N/mm ² .

Further adding graphene to 3% in the sheath further increased the Young's modulus of the wire to 1774N/mm ² .

Example 3: Effect of graphene on tension maintenance

Each wire sample was subjected to a tensile stress (TSS) of 250N initially for 10 minutes. The tensile stress of the wire sample naturally decreased over time. After 10 minutes, the residual tensile stress applied to each wire sample was measured, and the residual tensile stress remained constant with the wire tension in Newtons (N). The results are plotted on the graph in Figure 5.

Adding 1% graphene to the sheath increases the line tension from 223.7N to 225.8N.

Further adding graphene to the sheath to 3% further increases the line tension to 226.5N.

Tension maintenance affects the durability of the line and allows the mechanical properties of the line to be maintained at the same level while the line is in use.

Example 4: Effect of graphene on elongation

Figure 6 is a graph plotting the evolution of the elongation (E) at a stress of 25 kg, corresponding to a standard tension tennis racket, as a function of the amount of graphene in the sheath.

Adding 1% graphene to the sheath reduced the wire elongation from 10.1% to 9.6%.

Further addition of graphene to 3% in the sheath did not change the elongation, which remained at 9.6%.

Example 5: Effect of graphene on plastic deformation

The graph in Figure 7 shows the evolution of the plastic deformation (P) of the wire as a function of the amount of graphene in the sheath.

Adding 1% graphene to the sheath reduces the plastic deformation of the wire from 0.94% to 0.87%.

Further adding graphene to the sheath to 3% further reduces the plastic deformation of the wire to 0.80%.

It was also observed that the deformation was much lower than that of natural gut string D (1.36%). This highlights the better stability of monofilament strings over time, as the less plastic deformation of the string, the more stable the string set of the racket is during deformation.

In summary, adding graphene to the string makes it possible to improve the mechanical properties (tensile strength, Young's modulus, tension maintenance, elongation and plastic deformation of the string), thereby improving the durability of the string set of the racket (better tension maintenance, less deformation over time) while maintaining good playing characteristics (comfort, control and energy).

1: Monofilament line

2: Core

3: Sheath

4:Interface

Claims (8)

A monofilament string for a racket, the monofilament string comprising a core composed of a single filament and a sheath extending around the core and in contact with the core, wherein: - the core is made of a first material comprising at least one polyamide, - the sheath is made of a second material comprising at least one polyamide, wherein the second material comprises graphene or graphane nanoparticles at a concentration in the range of 0.1 wt% to 5 wt%, preferably 0.1 wt% to 2 wt%, and more preferably 0.1 wt% to 1 wt%, based on the weight of the sheath. The monofilament thread of claim 1, wherein the monofilament thread is obtained by co-extruding the core and the sheath. The monofilament thread of claim 1 or claim 2, wherein the second material includes at least one of the following: polyamide 6, polyamide 6.6, polyamide 11, polyamide 12, polyamide 66, and its mixture. A monofilament as claimed in claim 1 or claim 2, wherein: - the first material comprises polyamide 6 and a first copolymer of polyamide 6 and polyamide 6.6, - the second material comprises a second copolymer of polyamide 6 and polyamide 6.6. The monofilament thread of claim 1 or claim 2, wherein the first material has a larger tensile modulus compared to the second material. The monofilament thread of claim 1 or claim 2, wherein the second material further includes at least one additive selected from the group consisting of: slip agent and hydrophobic agent. A racket comprising a set of monofilament strings as claimed in any one of claims 1 to 6. A method for manufacturing a monofilament thread as claimed in any one of claims 1 to 6, wherein the core and the sheath are formed by a co-extrusion process, the method comprising stretching the monofilament thread.