WO2021009052A1

WO2021009052A1 - Monofilament string for a racket

Info

Publication number: WO2021009052A1
Application number: PCT/EP2020/069572
Authority: WO
Inventors: Sébastien Duval
Original assignee: Speed France Sas
Priority date: 2019-07-12
Filing date: 2020-07-10
Publication date: 2021-01-21
Also published as: TW202116380A; EP3997261A1; EP3997261B1; TWI836115B; EP3997261C0

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 A RACKET

TECHNICAL FIELD OF THE INVENTION

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

TECHNICAL BACKGROUNG

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

The evolution of the technology in this domain has pushed towards rackets being more and more competitive, involving great improvement in the structure and manufacture of the strings, in particular with regard to the materials constitutive of the strings.

From a general point of view, what is sought is to have a racket whose strings show good, or at least average, power, control, comfort, and durability properties. Power properties refer to the ability of the strings to increase the speed of the ball getting out of the strings when the player hits the ball. Control properties refer to the ability of the strings to influence the behavior of the ball, thus resulting in the possibility for the player to hit the ball towards a predetermined position with accuracy, to slow down the ball, and to influence the spin of the ball. Comfort properties refer to the ability of the strings to reduce the vibrations of the racket resulting from the strings undergoing the impact of the ball when the player hits the ball. And finally, durability properties refer to strings having a reduced degradation of their structure over time and use, which results in particular in a reduced tension loss, thus allowing them to keep their power, control, and/or comfort properties.

Among the different types of strings, strings made of natural guts have a low stiffness, which allows the player to accelerate the ball with no need of a high physical strength. However, they provide a poor control of the ball. Same goes for multifilament strings usually made of polyamide.

Monofilament and multifilament strings are usually made of polyethylene, polyester, or polyamide. Strings made of polyethylene and polyester have a high stiffness, which allow the player to be precise and to have a good control of the ball. However, the player needs to have a high physical strength in order to accelerate the ball. Strings made of polyamide show these characteristics while providing a great ability to dissipate the vibrations of the racket as well, but tend to degrade and to lose tension fast.

Therefore, there is a need for monofilament strings that show a good balance between power and control properties, while having also good comfort and durability properties. In particular, there is a need for monofilament strings that show high power properties, so that the player can easily increase the speed of the ball with no need of a high physical strength, while allowing the player to have a good control of the ball, and that maintain a substantially constant tension over time for a reasonable amount of time (preferably the time of a match, which is several hours, notably 2 to 4 hours, for an experienced player).

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

The intermediate reinforcing layer increases the durability of the strings by increasing their rigidity at the expense of their elasticity, but this causes the strings to have reduced power properties as their ability to bend at the impact of a ball is reduced.

The document WO2018234376 discloses monofilament strings that improve the balance between power and control properties. However, the durability of the strings over time and use needs to be further improved.

Carbon nanotubes have been reported in documents US 2008/0206559 and US 2012/0237767 as an additive in a wear-resistant coating wrapped around a multi-filament string. However, such carbon nanotubes substantially rigidify the material in which they are included and thus render it difficult to process.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a monofilament string that overcomes the above-mentioned drawbacks.

The invention especially aims to provide a monofilament string that show a good balance between power and control properties, while having also good comfort and durability properties, and enhanced durability over time and use compared to the existing strings, while allowing its manufacturing by a co-extrusion process.

To this end, one object of the 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 a polyamide, the sheath is made of a second material comprising at least a polyamide, wherein the second material is a polyamide matrix comprising 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.

By“polyamide matrix” is meant in the present text 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, 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 to the material used to form the sheath, the characteristics of the string remains substantially the same in terms of comfort, control and power. However, durability the string over time and use, i.e. their ability not to deteriorate, improves significantly.

Besides, the graphene or graphane content is chosen sufficiently low to allow manufacturing the monofilament by a co-extrusion process. In particular, during co extrusion, the monofilament has to be stretched to present the required mechanical properties. An excessive graphene or graphane content provides an excessive rigidity of the material during the co-extrusion process, which impedes a suitable stretching, resulting in poor mechanical properties of the monofilament. However, thanks to the low graphene or graphane content used in the present invention, the monofilament can be stretched during the co-extrusion process to a sufficient extent and thus achieve the required mechanical properties.

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

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

Such graphene or graphane nanoparticles allow for stabilizing the structure of the string, and the nanometric size of the carbon particles does not severely stiffen the string, contrary to certain other additives based on fluorine, molybdenum disulfide or Kevlar fiber in particular.

The graphene nanoparticles also improve the temperature resistance as well as the slipping of one string onto another, which is also directed towards the improvement of the stability over time by avoiding premature degradation of the string.

The term“rigidity” used herein refers to the tensile modulus (also called“Young’s modulus” or modulus of elasticity”) of a material. A material with a high rigidity presents a high tensile modulus and thus a low elasticity.

The term“geometric stiffness”, or simply“stiffness”, used herein is similar to the term “rigidity” but relates to a structure. The stiffness of the structure depends on the rigidity of the material it is made of and on its dimensional characteristics.

According to other optional features of the monofilament string:

- the monofilament string is obtained by co-extrusion of the core and the sheath; the second material comprises at least one of: polyamide 6, polyamide 6.6, polyamide 11 , polyamide 12, polyamide 66, and their mixtures; 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 first material has a greater tensile modulus than the second material the second material further comprises at least one additive selected from the group consisting of: slip agents and hydrophobic agents.

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

Another object of the invention is a process for manufacturing a monofilament as described above. In said process, the core and the sheath are formed by a co-extrusion process. Said process further includes stretching the monofilament.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the detailed description to follow, with reference to the appended drawings, in which:

- figure 1 is a cross-sectional view of a second embodiment of the monofilament string of the invention, wherein the monofilament string comprises a core and sheath, and the sheath includes graphene or graphane nanoparticles;

- figure 2 is a schematic view of a racket comprising a set of monofilament strings according to the invention;

- figure 3 is a graph representing the influence of graphene nanoparticles in the tensile strength for different monofilament strings;

- figure 4 is a graph representing the influence of graphene nanoparticles in the module of the tensile stress for different monofilament strings;

- figure 5 is a graph representing the influence of graphene nanoparticles in the mechanical resistance in tension for different monofilament strings;

- figure 6 is a graph representing the influence of graphene nanoparticles in the elongation for different monofilament strings;

- figure 7 is a graph representing the influence of graphene nanoparticles in the plastic deformation for different monofilament strings.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention proposes a monofilament string for a racket.

The monofilament string comprises a core and a sheath coaxial with the core and which is made of a polyamide matrix including graphene or graphane nanoparticles.

The polyamide matrix comprises at least one polyamide homopolymer and/or copolymer, which are preferably selected among the following: polyamide 6, polyamide 6.6, polyamide 1 1 , polyamide 12, polyamide 66, and their mixtures. The matrix may include other polymer(s) that polyamide polymers. However, the total weight percent of polyamide is always greater than the total weight percent of the other polymer(s). The polyamide matrix may also include fillers or additives.

The polyamide matrix provides the monofilament string with high stiffness, which allows the player to be precise and to have a good control of the ball, and a great ability to dissipate the vibrations of the racket which improves the comfort.

The presence of graphene or graphane nanoparticles in the polyamide matrix maintains the good properties provided by the polyamide matrix, while strongly increasing the durability of the string over time and use. As such, the set of strings of a racket maintain their good characteristics in terms of comfort, control and power for an improved period of time, and the breakage of the strings is strongly reduced, if not prevented, even when the strings are highly stressed. In other terms, the ability of the strings not to deteriorate improves significantly.

A quantity of 20% or less in weight of graphene or graphane nanoparticles relative to the weight of the portion including said nanoparticles is optimal to confer the string enhanced durability without increasing the stiffness too much. Preferably, the weight percentage of the graphene or graphane nanoparticles ranges from 0.1 % to 5%, more preferably from 0.1 % to 2%, and even more preferably from 0.1% to 1%.

Graphene and graphane are perfectly adapted to be incorporated within the polyamide matrix and all of them provide the string with the enhanced durability property. Graphene and graphane particles are considered as bi-dimensional (2D) particles since they have a sheet-like structure extending in a plane. By contrast, carbon nanotubes, that are other types of carbon nanoparticles, are three-dimensional (3D) particles since they can be considered as carbon sheets wound on themselves to form cylinders.

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

In more details, in graphite, which corresponds to a stack of graphene layers, the carbon planes are weakly related to each other. On the contrary, in each plane of graphene, the carbon atoms are connected by extremely solid bonds. Same goes for graphane nanoparticles. As a result, the material is mechanically very stiff in the plane (hundred times more than steel of the same thickness), while being deformable. Combined with a material that absorbs shocks, carbon nanoparticles can form very strong flexible materials. For example, Young’s modulus of graphene is about 1.0 TPa (TeraPascal), which makes it an extremely stiff material.

Graphene and graphane nanoparticles are also very light. For example, the density of graphene is about 2.25 g/cm³, which is very low.

As such, graphene and graphene nanoparticles, used in a suitable concentration as mentioned above, show high rigidity and lightness, thereby stabilizing the structure of the string without severely stiffening the string, contrary to certain other additives based on fluorine, molybdenum disulfide or Kevlar fiber in particular.

An important aspect having an impact on power properties of racket strings is the slipping of the strings on each other and the friction generated by the contact of the strings when slipping. In more details, when a player hits a ball, the ball engages the strings, causing them to bend and thus to slip on each other in a first direction while being pressed against each other. After hitting the ball, the ball comes out of the strings, causing them to get back to their initial rest position and to slip on each other in a second direction opposite to the first direction.

In addition to rigidity and lightness, graphene and graphane nanoparticles provide the string with good slipping properties, which reduces the friction between the strings when slipping.

Graphene and graphane nanoparticles show high thermal conductivity and thermal stability. The value of the thermal conductivity of graphene is about 5000 W.nr¹.K ¹, which is 10 times higher than copper, 20 times higher than aluminum, and 2 times higher than graphite. As a result, graphene and graphane nanoparticles provide the string with an increased ability to regulate and distribute heat in the set of strings of the racket.

An embodiment of the monofilament string of the invention is illustrated in figure 1.

The monofilament string 1 comprises a core 2 consisting of a single filament, and a sheath 3 extending around the core and in contact with the core. The core 2 has a round cross section and the sheath 3 has an annular cross section, the sheath being 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 second material comprising a second copolymer of polyamide 6 and polyamide 6.6 (second copolymer PA 6/6.6, which may 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 quite rigid polymers although polyamide 6 has a higher tensile modulus than polyamide 6.6.

As an example, the tensile modulus of the polyamide 6 generally ranges between 700

MPa (Mega Pascal) and 800 MPa, whereas the tensile modulus of the copolymer PA 6/6.6 generally ranges between 500 MPa and 600 MPa.

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

According to an embodiment (not illustrated) the polyamide matrix including the graphene or graphane nanoparticles is in both the core and the sheath. According to a preferred embodiment, the graphene or graphane nanoparticles are only in the sheath, and represent 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.

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

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

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

To this end, the first material comprises, in addition to the first copolymer PA 6/6.6, polyamide 6. Polyamide 6 provides the first material with a high rigidity, as well as a strong ability to dissipate the mechanical efforts (energy) when deformed elastically.

The core 2 thus provides the monofilament string 1 with a high geometric stiffness and the ability to strongly absorb/dissipate the mechanical efforts applied to it that occur when the string undergoes the impact of a ball or the like, which results in a better control of the ball as well as a reduction of the vibrations that propagate through the sieve 8 and the handle 9 of the racket 5 represented in figure 2.

One result is that the racket 5 allows the player to slow down the ball after receiving and hitting the ball for a better control of the ball. Another result is that the player receives fewer vibrations and shocks when hitting the ball for a better comfort thus 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 has to be understood that the second material can possibly comprise polyamide 6, but in a significantly lower amount compared to the first material. In this situation, the percentage by weight of polyamide 6 in the second material (relative to the second material) is significantly lower than the percentage by weight of polyamide 6 in the first material (relative to the first material).

Similarly, the amount of polyamide 6 in the copolymers 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 percentage by weight of polyamide 6 in the copolymer PA 6/6.6 of the second material is lower than the percentage by weight of polyamide 6 in the copolymer PA 6/6.6 of the first material. As a consequence, the second material (sheath) has a lower tensile modulus than the first material (core). Hence, the second material is more elastic, absorbs less energy when deformed elastically and releases more energy than the first material.

The sheath 3 thus provides the monofilament string 1 with the ability to strongly release the mechanical efforts applied to said string when the string undergoes the impact of a ball or the like.

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

The string 1 is preferably obtained by co-extrusion of the core 2 and the sheath 3. Coextruding the core 2 and the sheath 3 forms an interface 4 at the contact zone between the core and the sheath where said core and sheath are intimately linked.

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

The strong mechanical and chemical cohesion of the core 2 and the sheath 3 at the interface 4 allows said core and sheath to act in synergy when the string is requested mechanically, thus further improving the overall mechanical properties of the string, in particular its durability as well as its ability to influence the spin of the ball.

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

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

In more details, the thickness of the sheath ranges preferably from 20 and 50 micrometers while the thickness (which corresponds to the diameter) of the core ranges from 1200 and 1500 micrometers.

Such high weight proportion of the core relative to the sheath allows, along with the composition of the first and second materials of the core and the sheath, having a string with high control properties.

Surprisingly, despite its resulting low weight proportion, the sheath is however sufficient to provide the string with high power properties, in particular by imparting to the string explosive properties. By“explosive” is meant in the present text that the racket returns the ball with a great speed. The combination of the core and sheath thus provides a good balance between control properties and power properties.

Of course, depending on the intended way of playing of the user, the compositions and proportions of the core and the sheath may be adjusted to provide an optimal trade-off between control and power properties.

In order to further reduce the friction between the strings when slipping, the sheath advantageously comprises one or more additive(s) that facilitate the slipping of the strings relative to each other thus providing the strings with enhanced dynamic and bouncing capacities, and in general, enhanced power properties.

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

Among slip agents, the preferred additives are selected from: erucamide, such as stearyl erucamide, ethylene bis stearamide, polyamide-based polydimethylsiloxane, polyamide-based siloxane with ultra-high molecular mass, fluorine-based polymer, polymer loaded with molybdenum disulfide.

Among hydrophobic agents, the preferred additives are selected from: siloxane-based polymer with ultra-high molecular mass, polydimethylsiloxane-based polymer, silicon dioxide-based compounds, ceramic nanoparticles-based compounds.

For the purpose of reducing the friction between the strings when slipping, a coating of such additives or other substances can also be applied on the peripheral surface of the sheath, in particular during the manufacture of the strings.

According to an embodiment, in addition to or as an alternative to the presence of slip agents or hydrophobic agents in the sheath, a coating may be applied onto the outer surface of the sheath. Said coating may have non-slip and/or water repelling properties.

The monofilament string according to the invention has the following properties: a shock-absorbing capacity provided by the core 2, due to its low elasticity;

a dynamic and bouncing capacity provided by the sheath 3, due to its high elasticity and low friction;

high durability properties with a reduced degradation of its structure and tension over time and use, due to the relatively high tensile module of polyamide 6 and copolymer PA 6/6.6,

the previous properties, as well as the overall mechanical properties of the string, being further improved with the co-extrusion of the core and sheath, and the formation of the interface 4 in between.

As a result, the monofilament string shows a good balance between power and control properties, while also having good comfort and durability properties.

The monofilament string according to the invention also have the following properties due to the presence of graphene or graphane nanoparticles in the sheath: increased rigidity and durability with acceptable lightness, due to the ability of the graphene or graphane nanoparticles to reinforce the structure of the string without severely stiffening said string (due to the nanometric size of the graphene or graphane particles) nor severely weighing said string (due to the low density of the graphene or graphane particles);

further increased dynamic and bouncing capacity, due to a reduction of friction between the strings when slipping.

EXPERIMENTAL RESULTS

Experimental testing and measures have been carried out on three different monofilament strings in order to determine their mechanical behavior and to note the impact of graphene onto mechanical properties of the strings.

The tested monofilament strings are the same in each of the five following examples. They have the same polyamide structure, but differ from the amount of graphene in the sheath.

The graphene nanoparticles have a thickness comprised between 1 nm and 2 nm and a lateral dimension comprised between 0.5 and 5 pm.

The graphene nanoparticles have been provided as a powder which was mixed with the plastic granules fed into the extrusion machine.

Alternatively, the powder of graphene nanoparticles may be mixed with the polyamide polymer to obtain a compound, and granules made from this compound may then be fed into the extrusion machine.

The tested monofilament strings are the following:

String A (1 % graphene) - monofilament string 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

• 1 % graphene in the sheath;

String B (3% graphene) - monofilament string comprising:

• a sheath comprising a second copolymer of polyamide 6 and polyamide 6.6

• 3% graphene in the sheath

String C (No graphene) - monofilament string comprising:

• a sheath comprising a second copolymer of polyamide 6 and polyamide 6.6. Each monofilament string sample have undergone a hundred cycles of tensile stress of 250 Newton (N) for a duration of 10 minutes: the sample was stretched and relaxed a hundred times. For each cycle, tensile strength, Young’s modulus, tension maintenance, elongation, and plastic deformation of the string have been measured and the mean value over the hundred cycles has been calculated.

Example 1 : influence of graphene on the tensile strength

The evolution of the tensile strength (TS) as a function of the amount of graphene in the sheath of the string is illustrated in the graph of figure 3.

The addition of 1 % graphene in the sheath increases the tensile strength of the string, from 564 N to 600 N.

With 3% graphene in the sheath, the tensile strength of the string is 587 N, which is slightly lower than with 1 % graphene but higher than without graphene.

As such, the addition of graphene in the string increases the tensile strength.

Example 2: influence of graphene on the Young’s modulus

The evolution of the Young’s modulus (Y) as a function of the amount of graphene in the sheath of the string is illustrated in the graph of figure 4.

The addition of 1 % graphene in the sheath increases the Young’s modulus of the string, from 1629 N/mm² to 1726 N/mm²

Further addition of graphene to 3% in the sheath further increases the Young’s modulus of the string to 1774 N/mm².

Example 3: influence of graphene on the tension maintenance

Each string sample has undegone a tensile stress (TSS), of an initial value of 250 N, for a duration of 10 minutes. The tensile stress of the string samples naturally decreased as the time passed. After 10 minutes, the remaining tensile stress applied to each string sample was measured, and corresponds to the tension maintenance of the string, in Newton (N). The results are illustrated on the graph of figure 5.

The addition of 1 % graphene in the sheath increases the tension maintenance of the string, from 223.7 N to 225.8 N.

Further addition of graphene to 3% in the sheath further increases the tension maintenance of the string to 226.5 N.

The tension maintenance influences the durability of the strings and allows for maintaining the mechanical properties of the strings at the same level as they are used. Example 4: influence of graphene on the elongation

The evolution of the elongation (E) at a stress of 25kg, which corresponds to a standard tension tennis racket, as a function of the amount of graphene in the sheath of the string is illustrated in the graph of figure 6.

The addition of 1 % graphene in the sheath decreases the elongation of the string, from 10.1 % to 9.6%.

Further addition of graphene to 3% in the sheath does not change the elongation which remains at 9.6%.

Example 5: influence of graphene on the plastic deformation

The evolution of the plastic deformation (P) as a function of the amount of graphene in the sheath of the string is illustrated in the graph of figure 7.

The addition of 1 % graphene in the sheath decreases the plastic deformation of the string, from 0.94% to 0.87%.

Further addition of graphene to 3% in the sheath further decreases the plastic deformation of the string to 0.80%.

It is also observed that the deformation is much lower than the deformation of a natural gut D (1.36%). This highlights a better stability of the monofilament string over time, since the less the string plastically deforms, the more the string set of the racket is stable in deformation.

In conclusion, the addition of graphene in the strings 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 in time) while maintaining good playing properties (comfort, control and power).

Claims

1. 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.

2. Monofilament string according to claim 1 , wherein said monofilament string (1) is obtained by co-extrusion of the core (2) and the sheath (3).

3. Monofilament string according to claim 1 or claim 2, wherein the second material comprises at least one of: polyamide 6, polyamide 6.6, polyamide 11 , polyamide 12, polyamide 66, and their mixtures.

4. Monofilament string (1) according to any one of claims 1 to 3, 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.

5. Monofilament string (1) according to any of claims 1 to 4, wherein the first material has a greater tensile modulus than the second material.

6. Monofilament string (1) according to any of claims 1 to 4, wherein the second material further comprises at least one additive selected from the group consisting of: slip agents and hydrophobic agents.

7. Racket (5) comprising a set (6) of monofilaments strings (1) according to any of claims 1 to 6.

8. Process for manufacturing a monofilament string according to any of claims 1 to 6, wherein the core and the sheath are formed by a co-extrusion process, said process including stretching the monofilament.