Technical field

The present invention relates to a sports racket with string bed of filaments, to a method of stringing a sports racket by preparing a string bed, to a use of a polyethylene based filament, to a filament comprising polyethylene (PE) material, to a method of preparing a PE based filament, and to a kit of parts comprising a PE based filament.

Background to the invention

Sports rackets in the art comprise string beds made of polymer filaments or natural gut.

EP 1964949 proposes sports rackets with strings having a first polymer core and a surrounding matrix with a second polymer. A third polymer may be present in the core and a fourth polymer in the matrix. The document refers to long lists of possible polymer selections and combinations and shows multiple cross-sectional string designs. According to EP 1964949, string stiffness and elasticity can be adjusted and playing properties enhanced, particularly providing a ball control tuned to the way of playing, experience and quality. The document refers to polyester as preferred material for the first and second polymer, but does not disclose any specific string, string combination, or strung sports racket while there is also no hint as to how to select the four polymers to improve stiffness, elasticity or play properties.

High modulus polyethylene (HMPE) fibers based on ultrahigh molecular weight polyethylene (UHMWPE) are usually made by gel (solution) spinning, because the material (UHMWPE) does not melt sufficiently for conventional extrusion and large draw ratios. The end result is a thin fiber with diameters of at most 50 microns, usually around 10 microns. These gel spun filaments also require a high draw ratio of more than 50 times, otherwise showing high creep. Filaments based on HMPE fibers are multifilaments, the individual fibers being too thin for most practical purposes. US20140106104 relates to creep-optimized UHMWPE fibers consisting of branched polyethylene (PE) molecules. These fibers have high modulus (HMPE), for instance 1100 cN/dtex (100 GPa) and 76 Gpa (in table 1). The processing method produces fibers of micron dimensions. CA 2795363 suggests the use of high modulus UHMWPE for pre-meshed tennis stringing.

US5399308 discloses extrusion of polyethylene (PE) materials using a lubricant. Subsequently, the polymer is melt-drawn (at temperatures above the melting point) resulting in a high modulus filament. An example of a polymer is Mitsui Hi-Zex Million 145M which has a Mw of 1 million (M) g/mol.

US 5573850 claims a quasi-monofilament of high modulus (>44 GPa) UHMWPE multifilament yarn surrounded by polyethylene sheath.

US 5505900 describes the use of UV light during the PE filament production process. The Mw used in this document is about 1M. This requires additional UV cross-linking against creep. The filament's breaking strength (calculated from the tensile strength and diameters) is too small for tennis strings.

EP 401942 describes the use of particular PE based filaments as clips in orchards.

We have noticed that filaments, particular when used in sports racket stringing beds, notch, break, lose tension and/or limit other playing characteristics such as control, power and spin.

Summary of the invention

Through our research efforts, we have found that use of filaments of the art in sports rackets may lead to tension loss, low strength, string notching, string fraying and poor playing properties. However, we have now surprisingly found that the present invention overcomes one or more of the above mentioned problems.

Accordingly, the present invention is directed to a sports racket with string bed of filaments wherein at least one filament is a polyethylene (PE) based filament comprising PE material with an average molecular weight (Mw) of from 0.1M to 40M g/mol and wherein the filament has a modulus of from 1 to 40 GPa and a cross sectional area of from 0.1 to 1.8 mm ² .

In one embodiment, the sports racket is a tennis or racquet ball racket and at least one filament has a cross sectional area of between 0.3 and 0.95 mm ² . In another embodiment, the sports racket is a squash racket and at least one filament has a cross sectional area of between 0.2 and 0.6mm ² . In another embodiment, the sports racket is a badminton racket and at least one filament has a cross sectional area of between 0.1 and 0.3mm ² . Preferably, the at least one PE based filament has a tensile strength of from 0.4 to 4 GPa, a room temperature elongation to break of from 10 to 90% and a draw ratio of from 50% to 98% of the maximum draw ratio. Preferably, the at least one PE based filament is strung in the transverse direction of the string bed and wherein an at least second filament is strung in the longitudinal direction of the string bed, wherein the at least second filament comprises a material selected from polyamide, polyester, polyaryletherketone, polyurethane, polypropylene, polyethylene terephthalate, polybutylene terephthalate, natural gut, fluoropolymer and mixtures thereof.

Further, the invention is directed to a method of stringing a sports racket by preparing a string bed using at least one PE based filament comprising PE material with average molecular weight (Mw) of from 0.1M to 40M g/mol and having a modulus of from 1 to 40 GPa and a cross sectional area of from 0.1 to 1.8 mm ² .

Further, the invention is directed to a use of a PE based filament with an average molecular weight (Mw) of from 0.1M to 40M g/mol and having a modulus of from 1 to 40 GPa and a cross sectional area of from 0.1 to 1.8 mm ² for stringing of a sports racket.

Further, the invention is directed to a filament comprising PE material with an average molecular weight (Mw) of from 0.1M to 40M g/mol, wherein the filament has a modulus of from 1 to 40GPa and a cross sectional area of from 0.1 to 1.8 mm ² . Preferably, the filament has a tensile strength of from 0.4 to 4 GPa. Preferably, the filament has a tensile strength of from 0.4 to 4 GPa, a room temperature elongation to break of from 10 to 90% and a draw ratio of from 50% to 98% of the maximum draw ratio.

Further, the invention is directed to a method of preparing a PE based filament comprising PE material of an average molecular weight (Mw) of from 0.1M to 40M g/mol and having a modulus of from 1 to 40 GPa and a cross sectional area of from 0.1 to 1.8 mm ² , by extruding and drawing the PE material up to an elongation to break of from 10 to 90% measured at room temperature. Preferably, the PE material is ram-extruded and subsequently drawn. Further, the invention is directed to a kit of parts comprising a PE based filament comprising PE material with an average molecular weight (Mw) of from 0.1M to 40M g/mol, wherein the PE based filament has a modulus of from 1 to 40 GPa and a cross sectional area of from 0.1 to 1.8 mm ² and at least a second filament of higher surface hardness. Preferably, the at least second filament comprises a material selected from polyamide, polyester, polyaryletherketone, polyurethane, polypropylene, polyethylene terephthalate, polybutylene terephthalate, natural gut, fluoropolymer and mixtures thereof. We have surprisingly found that sports rackets of the invention show no or only limited string notching, string fraying and/or tension loss. Further, the invention allows for the use of significantly thinner filaments. The sports rackets have excellent playing properties while the stringing lasts a long time. Preferably, the PE based filament is drawn to near its maximum draw ratio while still providing a low modulus and with some degree of long chain molecular entanglement for low fraying, low creep and low wear. We have further found that larger molecular weight polymers give better strength, lead to lower creep and reduced wear. Also, we found that the use of a PE based filament in the string bed allows use of thinner filaments than used in the art, for the PE based filament and/or other filaments. Higher molecular weight PE material is preferably and beneficially ram-extruded and subsequently drawn to appropriate racket string diameter. The filament of the invention is beneficially combined with another filament of higher surface hardness in a kit for improved play and filament durability.

Details of the present invention

For the purpose of the invention, polyethylene (PE) material is defined as a polymer built up from ethylene monomer units to form a polymer chain, a polyethylene (PE). The PE chain may be branched but is preferably linear.

For the purpose of the invention, polyethylene with a molecular weight between 0.1M g/mol and 1M g/mol is defined as high molecular weight polyethylene or HMWPE and polyethylene with a molecular weight of higher than 1M g/mol is defined as ultra high molecular weight polyethylene or UHMWPE. For the purpose of the invention, a monofilament is defined as single, untwisted, string that is preferably synthetic. The monofilament may have one or multiple material components. A string may comprise one or more fibers, and/or filaments. A fiber is a string with small diameter (diameter below 50 micrometer). A monofilament has a larger diameter. A filament can be a monofilament or multifilament. A multifilament always comprises more than one fiber and/or filament. The string designation is often used for racket sports.

For the purpose of this invention, "PE based filament" is understood to refer to a filament comprising PE material. From this definition, it will be understood that the PE material may be on the outside of the filament and/or on the inside of the filament, as long as the filament comprises PE material.

For the purpose of the invention, modulus is defined as the Young's modulus which may also be called tensile modulus. Modulus represents a measure of the stiffness of the PE material. The modulus is determined by dividing the tensile stress by the extensional strain along the axis of the filament, which is, in a formula, as follows: E= σ/ε = (F/Ao) / (AL/Lo) wherein E is Young's modulus, σ is the tensile stress, ε is extensional strain, F is the force applied on the filament, Ao is the original cross-section of the filament, AL is the change in length and Lo is the original length of the filament. Due to the non-linearity of the strain-stress curve of lower modulus filaments, the modulus depends on the static force applied. For the purpose of this invention, reference is made to the modulus measured at 200 MPa applied static tension, which is similar to the stringing tension in the sports racket.

For the purpose of the invention, tensile strength is defined as the maximum force to break, often called the stress to break. The tensile strength refers to the longitudinal tensile strength.

According to the invention, the tensile strength is determined by applying an increasing force to the filament starting from zero to the breaking force at room temperature. Preferably, the time to break is 20 seconds plus or minus three seconds to provide a consistent tensioning speed. For the purpose of the invention, transverse strength is used for the tensile strength of a filament in transverse direction. Transverse strength is sometimes called peel strength and is for instance indicative for the ability to withstand fraying. For the purpose of the invention, elongation to break of a filament is defined as the % of lengthening of the filament when an increasing force is applied on the filament. According to the invention, the elongation to break is determined to test a filament after manufacturing. According to the test an increasing force is applied to the filament at room temperature (RT) starting from zero to the breaking force. To provide a consistent tensioning speed, preferably a time to break is used of 20 seconds plus or minus three seconds. Preferably, the elongation to break is determined in the same test as the tensile strength test.

For the purpose of the invention, the draw ratio (DR) is defined as the ratio of the length of the filament after and before drawing.

For the purpose of the invention, maximum draw ratio of a particular filament is defined as ratio between the length of the filament and the filament when fully drawn and nearly breaking and is expressed in %. The maximum draw ratio is generally determined at increased temperature, preferably 120°C.

For the purpose of the invention, tension loss is defined as the viscoelastic deformation of the material that is caused by creep which is sometimes called cold flow. Tension loss is determined by tensioning the filament with a force of half the breaking strength of the filament and fixing the outer ends of a filament to keep the filament at a constant length. The tension of the filament is measured after 24 hours. The tension loss ratio is calculated by determining the ratio between the initial filament tension and the measured filament tension.

For the purpose of the invention, annealing is defined as applying an increased temperature in a certain atmosphere under a certain pressure during a certain time.

The invention relates to particular filaments made from PE (polyethylene) material. Preferably, the filaments are made from high and ultrahigh molecular weight polyethylene (often called HMWPE and UHMWPE) material. Preferably, the PE material of the invention has a weight average molecular weight (Mw) of from 0.1M (HMWPE), more preferably at least 0.5M and most preferably at least 1M (UHMWPE), and particularly preferred at least 3M g/mol and preferably up to 40M, more preferably up to 20M, most preferably up to 15M and particularly preferred up to 12M g/mol. An example of PE material for use in the invention is UHMWPE powder resin Ticona Celanse GU 4022 (Mw of 5M). Preferably, the filament of the invention is a monofilament.

Without wishing to be bound by any theory, we have found that PE based filaments with an average molecular weight (Mw) of below 100K have relatively short molecular chains which leads to low entanglement friction. We believe that this means that the molecular chains in these filaments slide more easily and thus such filaments show large levels of creep. In addition, their tensile strength at lower modulus would be lower and their wear would be higher in comparison with higher molecular weight filaments. For these reasons, we found that regular HDPE with Mw of lower than 0.1M, is not preferred for racket strings according to the invention.

Preferably, the PE material of the invention has a density of at least 0.93 and preferably up to 0.96, more preferably up to 0.95 g/cm3. In the art, this is often called high density PE. The density is 50% lighter than PET (1.40 g/cc) and 25% lighter than nylon (1.14 g/cc). The high density and the preferred smaller diameter string reduce string weight. The lower string weight beneficially reduces the racket vibrations, increase the racket response and reduce the racket head weight.

HMWPE AND UHMWPE has been used in the prior art to manufacture fibers. These have diameters of smaller than 50 micrometer, typically only 10 micrometer and have high to very high modulus, i.e. are stiff, making them distinct from filaments of the invention. They are formed by combining many micrometer diameter fibers and are used for purposes like textile yarn, rope, or ballistic armor. These fibers have high modulus and we have found that this makes these strings very sensitive to creep: only a slight amount of creep in these high modulus strings already generates a large tension loss. This is a further reason why we have found that these prior art fibers are unsuitable for use in sports rackets. Examples include Dyneema SK25 (DSM) which has a modulus of 52 GPa (the lowest modulus available), a tensile strength of 2.2 GPa, and an elongation to break of 4%; Dyneema SK78 (DSM) has a modulus of 132 GPa, a tensile strength of 3.9GPa and an elongation to break of 3%; and commercial Spectra UHMWPE fiber (Honeywell) varies in modulus between 66 and 135 GPa.

Preferably, the PE based filament of the invention has a tensile strength of from 0.4 and preferably up to 4GPa. This applies to all embodiments of the present invention, including the sports racket, the method of stringing a sports racket, the use the filament, the filament, the method of preparing the filament, and the kit of parts comprising the filament.

Preferably, the filament of the invention has a modulus of from 1 GPa and preferably up to 40GPa, more preferably up to 30GPa, most preferably up to 20GPa. According to the invention, we have surprisingly found that filaments that are made with such low modulus PE materials provide benefits according to the invention as indicated herein.

Preferably, the filament of the invention has an elongation to break at room temperature ( T elongation to break as defined above) of at least 6%, more preferably at least 10%, most preferably at least 15% and preferably up to 90%, more preferably up to 70%, most preferably up to 50% and particularly preferred up to 30%. The prior art describes stiff UHMWPE fibers with an elongation to break of <4%. Preferably, the filaments of the invention have a length of from 3 meter and preferably up to 15 meter.

Preferably, the filaments of the invention has a diameter of from 0.4, more preferably from 0.5 and preferably up to 1.5mm, more preferably up to 1.3, most preferably up to 1.2mm, and in particular up to 1.1mm.

Preferably, the filament of the invention has a tension loss (due to creep) between 1 and 2. The low tension loss filaments of the invention allow playing sports with a racket for a longer time without losing significant string bed tension.

We have surprisingly found that the PE material according to the invention provides multiple benefits. First, the invention provides for a large structural strength. Also, the filaments now have excellent modulus for sports rackets. Further, the filaments have beneficial sliding properties, for instance when used as cross filament with respect to many different types of longitudinal filaments. Furthermore, the PE material allows for significantly thinner strings for sports racket.

Preferably, the PE based filament comprises from 50 to 100% by weight of PE material. Preferably, the PE material is present at least on the outside of the filament. Preferably, the PE based filament comprises at least 50% and preferably up to 100% by weight of PE material in a radial section of the filament. The radial section may be a sheath around a core and in that case the radial section preferably has a thickness of at least 0.1mm and for instance up to 0.3mm. Such radial section may provide the benefits of low friction and/or low wear sliding surface. An example of such a radial section would be a 0.2 mm coating on the outside, or a 0.1 mm coating covered by a wax or other sacrificial layer of 0.2 mm.

According to a preferred embodiment of the invention, it is however preferred that is PE material is present both in the core and on the surface of the filament. Preferably, the filament comprises at least 90%, more preferably at least 95%, and most preferably at least 99% by weight of PE material. In a particularly preferred embodiment the filament is substantially entirely made of PE material.

The weight % of PE material is based on the weight of the parts of the filament that provide structure to the filament, more preferably on the total amount of polymer of the filament. It will be understood that minor amounts of different PE material, other polymers, lubricants, pigments, solvents, or other additives may of course additionally be present. Non-substantial fillers such as air or oil in hollow cores are not considered structural parts of the filament. The same applies to wax or other sacrificial layer on the outside that would eventually expose the PE material of the invention, allowing the PE material to be the sliding surface for the opposing filaments in the string bed. Preferably, any other supporting or filler material (for instance additional filament material made of other materials or material in the core) does not deteriorate the sliding properties when the PE material is incorporated on the surface of the filament. The filament may comprise additives, for instance 10% or less or preferably 5% or less.

We have found that PE material in the transverse filament contributes to a longer lifetime of the longitudinal filament and decreases notching thereof. We have also found that, after a run-in period, the roundness of the PE based filament may be flattened (possibly due to abrasion and compression) and abrasion of the PE material of the filaments in the crosses may slow down significantly. To allow some abrasion of the transverse PE based filament, at least 0.1 mm, more preferably 0.2 mm radial width of PE material is present. Even more preferably, the filament is substantially entirely made of PE material. The present invention is directed to sports rackets. Preferably, the sports racket of the invention comprises of a handle and a racket head. The racket head preferably has a string bed. The string bed consists of two sets of filaments that cross each other and that are located in the plane of the racket head. One set of filaments is strung in longitudinal direction (i.e. in line with the handle) and are called the mains. The other filaments are strung in transverse direction and are called crosses. These filaments cross the mains in the plane of the racket head. Upon impact with an object (e.g. ball or shuttle), the filaments in the string bed move in perpendicular direction away from the plane. The mains also move laterally in the plane. The filaments subsequently bounce the object (e.g. ball or shuttle) back. The lateral movement of the mains over the crosses usually causes notching in the mains.

We have found that in sports rackets with stringing of the prior art especially the longitudinal string notch within only a few hours of play. The notching causes a reduced cross sectional string area and proportionally reduced tensile strength, after several hours play even by 50%! This requires an initial tensile strength twice that of the string that would not notch, or twice the original cross sectional area. Commercial transverse strings (crosses) also need to be of larger diameter than required based on breaking strength, because when crosses are too thin, they even accelerate notching. To minimize notching, strings of the prior art often have filament wraps that do not contribute significantly to the tensile strength but make the strings thicker than they would need to be for tensile strength purposes alone. We have actually found that even such strings notch significantly.

In a preferred embodiment, the sports racket of the invention comprises the filament of the invention strung in the transverse direction of the string bed. The strings of the invention allow for the use of various types of monofilaments as mains without causing notching herein.

Preferably, the mains are made of a different material than the material of the surface of the crosses. Preferably, the mains are of higher surface hardness than the surface of the crosses. In case of a non-uniform surface, the average surface hardness of the filaments may be used in such a comparison. Preferred examples of polymers for the mains include polyamide, polyester, polyaryletherketone based materials (PEEK, PEK, PEKK and PEKEKK), polyurethane, polyethylene terephthalate, polybutylene terephthalate, natural gut, polypropylene, fluoropolymer and mixtures thereof. A preferred polymer is polyamide. These strings may be monofilaments and/or multi-filaments. Accordingly, we have surprisingly found that notching of the longitudinal filament is lowered or even prevented, leading to less longitudinal filament breakage.

Without wishing to be bound by any theory, it is believed that the PE based filaments of the invention facilitate low friction and low wear movement of the mains in lateral direction.

Although the filament of the invention improves lateral movement of the mains, it surprisingly also reduces notching of the mains. The lateral movement of the mains allows for enhanced power and spin on the ball. The filament shows little or no notching and fraying. The filament maintains tension well (low level of tension loss or creep) so that the sports racket does not immediately need restringing. The filament is strong and, most importantly, sufficiently elastic to allow for a perpendicular movement of the string bed caused by the ball. The low modulus of the high molecular weight PE material of the invention provides for the benefits as indicated above.

The filament of the invention has a cross sectional area of preferably more than 0.1 mm ² , more preferably more than 0.2mm ² and preferably less than 1.8 mm ² , more preferably less than

0.95mm ² . Preferably, the transverse and longitudinal strings do not have filament wraps. More preferably, the strings comprise monofilament without coating.

In one embodiment, the present invention relates to longitudinal and/or transverse strings (mains and crosses) that are significantly thinner. The diameter is preferably from 10% and preferably up to 40% thinner in comparison with filaments of the prior art, and for instance at least 0.1 mm thinner. We have found that the PE based filaments of the invention result in no or low notching and reduced wear. According to the present invention, use of the PE material allows reduction of the cross sectional area of at least one of the filaments while maintaining the filament's strength, improving play properties.

In one aspect, the sports racket is a tennis racket or a racquet ball racket. Preferably, the PE based filaments are circular and have a preferred diameter of larger than 0.7 mm, more preferably larger than 0.9 mm and preferably lower than 1.2 mm, more preferably lower than 1.0 mm. In a preferred embodiment, the cross sectional area of at least one of the filaments for tennis and/or racquet ball is from 0.6 mm ² and preferably below 0.95mm ² . Surprisingly, we have found that tennis or racquet ball rackets strung with these thin filaments are sufficiently strong and provide for better aerodynamics. These rackets are preferably strung with monofilaments using a weight between 20 and 30 kg. For tennis, the preferred modulus for the filaments is from 3 to 10 GPa. The filament of the tennis racket preferably has a required breaking strength of at least 50 kg. The length of the filament for the tennis racket is preferably at least 5 meter and preferably at most 15 meter, for instance around 6 meter when only used for the crosses and for instance around 12 meters when used for stringing the entire string bed.

In another embodiment, the sports racket is a racquet ball racket. As indicated, the racquet ball racket preferably has the same features as the above mentioned tennis racket, including string shape, diameter, stringing weight, modulus, breaking strength and length.

In another embodiment, the sports racket is a squash racket. Preferably, the PE based filaments are circular and have a preferred diameter of larger than 0.6 mm, more preferably larger than 0.7 mm and preferably lower than 1.0 mm, more preferably lower than 0.9 mm.

In a preferred embodiment, the cross sectional area of at least one of the filaments is from 0.4 to preferably up to 0.6 mm ² . Surprisingly, squash rackets with monofilaments of the invention with these cross sectional areas are sufficiently strong and do not notch and/or break.

Squash rackets are preferably strung with monofilaments using a weight between 5 and 14 kg, preferably between 7 and 12 kg. For squash, the preferred modulus is from 2 to 4 GPa. The filament of the squash racket preferably has a required breaking strength of 30 kg. The length of the filament for the squash racket is preferably at least 4 meter and preferably at most 15 meters, for instance around 5 meter when only used for the crosses and for instance around 12 meters when used for stringing the entire string bed.

In another embodiment, the sports racket is a badminton racket. Preferably, the PE based filaments are circular and have a preferred diameter of larger than 0.4 mm, more preferably larger than 0.5 mm and preferably lower than 0.7 mm, more preferably lower than 0.6 mm. In a preferred embodiment, the cross sectional area of at least one of the filaments is from 0.2 and preferably up to 0.3mm ² .

Badminton rackets are preferably strung with monofilaments using a weight 7 and 12 kg. For badminton, the preferred modulus is from 1 to 3 GPa. The filament of the badminton racket preferably has a required breaking strength of 25 kg. The length of the filament for the badminton racket is preferably at least 3 meter and preferably at most 10 meter, for instance around 5 meter when only used for the crosses and for instance around 10 meter when used for stringing the entire string bed.

The method of preparation of the filaments of the invention is directed to providing a PE based filament of high molecular weight which is nearly maximally drawn and has a tensile strength and modulus as defined above.

UHMWPE material does not properly liquefy and remains in a rubbery state even at high temperatures. The mass flow index is too high for conventional melt extrusion. Accordingly, the prior art uses gel spinning to produce HMPE fibers based on UHMWPE material. Gel spinning involves dissolving UHMWPE powder in a solvent, spinning, drying and ultra-drawing the PE material. However, due to the high required solvent levels (only about 2% polymer), only fibers with very fine diameters (maximum 50 microns, but typically 10 microns) can be prepared.

Filaments based on HMPE fibers are multifilaments, the individual fibers being too thin for most practical purposes. Also, to prevent creep, the fibers need to be drawn with draw ratios of more than 50 times. This process yields high modulus, high tensile strength fibers. Without wishing to be bound be any theory, we have found that more than 90% of the molecular chains in those fibers are aligned along the fiber axis (fully oriented) and that crystallinity is >90%. These characteristics mean that these fibers have very high stiffness and low elongation to break of <4%. We have further found that, due to the near complete longitudinal orientation of the molecular chains, the transverse strength of HMPE fibers (even when fused together) is very low leading to rapid fraying when used as racket strings, making these fibers unsuitable for this purpose. Thus, the preparation processes of the prior art are unsuitable for preparation of strings of the present invention. We have further found that the filaments of the prior art cannot be beneficially used according to the invention as defined herein.

According to the present invention, we have however surprisingly found that PE based filament with low modulus are suitable for sports rackets and can be beneficially prepared. Without wishing to be bound by any theory, we believe that the process of the invention leads -in comparison to the prior art fibers- to a higher unoriented (amorphous) entanglement of the PE material which subsequently leads to improved filaments with the benefits of the present invention including low fraying, low creep, low modulus, low wear, yet sufficiently high strength. Preferably, our preparation method is directed at preserving (or restoring) randomly oriented (amorphous) molecular entanglement regions of the PE material. The molecular chains are more randomly oriented (not fully oriented) and are less crystalline. Preferably, the post-draw crystallinity is higher than 40%, more preferably higher than 60% and preferably up to 80%, more preferably up to 70%. Our preparation process leads to a much higher percentage of amorphous regions. We believe that this allows for the low modulus or longitudinal flexibility. The molecular chains are more randomly oriented than in HMPE fibers and provides for a larger transverse strength. Because we do not require ultra-high draw ratios (of say >50 times) of the prior art, we do not require full dissolution of UHMWPE material in solvent and we have found that we can produce monofilaments with diameters directly suitable for racket sports, of say larger than 0.4 mm and up to 1.5 mm.

Accordingly, the invention relates to a method of preparing a PE based filament with a tensile strength of from 0.4 GPa and a modulus of from 1 to 40 GPa, by extruding and drawing PE material of average molecular weight of from 0.1M to 40M g/mol up to from 2 to 30 times.

According to the invention, the PE material is first subjected to an extrusion step, a well-known process well described in the art. Preferably, the invention is directed to ram-extrusion, particularly for PE material of higher molecular weight, preferably higher than 1M and more preferably higher than 2M. Ram extrusion is a process in which powder, preferably a dry powder, is fed into an extruder with a plunger, which compresses the powder under very high pressure and high temperature. The UHMWPE material is extruded at a suitable pre-draw diameter that is preferably of from 0.5 to 10mm for instance 3 mm. Ram-extrusion has a low speed (mm to cm per minute) and is therefore not used for filament production of large quantities such as used in fishing nets or carpeting. However the quantity used per racket stringing (transverse strings) is only about 4 grams, which makes this method viable. Careful machine design and parameter adjustment minimize irregular outer diameter of the semi-solid extrudate which may be due to wall friction, so-called surge lines. Preferably, the ram extrusion step is followed by quenching in water. Preferably, the ram-extruded PE material is dry-drawn at a temperature between 95 ^° C to 130°C.

As part of the extrusion step, the PE material preferably subjected to a quenching step. The quenching step involves passing the PE material through a water bath. The water preferably has a temperature of from 5 to 20 °C. The passage time can for instance be from 1 second to 1 minute. After the extrusion step, the PE material is subjected to a drawing step. The preferred drawing temperature is below the crystal melting point of the PE material. This preserves low modulus amorphous fraction of the PE material. Preferably, the drawing temperature is from 95 °C and preferably up to 130 °C, for instance at 120 ^° C or in two step for instance at 100°C and subsequently at 120 °C.

In an embodiment of the invention, the extrusion step is followed by an anneal step which is subsequently followed by the drawing step. The anneal step preferably takes place above the melt temperature of the PE material, for instance at 150 °C for a period of 10 minutes to 2 hours, for instance around 30 minutes, and preferably at atmospheric pressure. As part of the anneal step, the PE material preferably subsequently subjected to a quenching step. The quenching step involves passing the PE material through a water bath. The water preferably has a temperature of from 5 to 20 °C. The passage time can for instance be from 1 second to 1 minute. The anneal step and the quench step as well as their timing are preferably used to affect the pre- draw crystallinity. The pre-draw crystallinity of the PE material contributes to the draw ratio (D ) characteristics of the PE material. The appropriate draw ratio (DR) characteristics lead to the desired low modulus filament of the invention. For instance, a low temperature ice-water quench immediately after annealing reduces pre-draw crystallinity and increases the maximum draw ratio. On the other hand, a warmer quench or longer time between anneal and quench results in higher pre-draw crystallinity, lower maximum draw ratio and lower modulus at maximum draw ratio. The best draw temperature is sufficiently high to enable setting of a drawn molecular structure. Preferably, the draw ratio (DR) is at least 3, more preferably at least 4, most preferably at least 5 and preferably at most 28, more preferably at most 15, most preferably at most 12, for instance from 6 to 10 times. Preferably, the filament of the invention is drawn up from 50%, more preferably from 80% and preferably up to 98%, more preferably up to 90% of the maximum draw ratio at an increased temperature, preferably of 120 °C. Thus, the PE based filament of the invention preferably has a draw ratio of from 50%, more preferably from 80% and preferably up to 98%, more preferably up to 90% of the maximum draw ratio, as measured at an increased temperature, preferably of 120 °C. We have surprisingly found that, in this manner, the PE based filament can be used in a sports racket, can elongate during the sports game without breaking and has little tension loss. Filaments not drawn to our preferred percentage of the maximum draw ratio may show very large creep during use, likely due to the continued cold draw when experiencing permanent tension when strung in a racket. As discussed above, the PE material is preferably drawn to arrive at a PE based filament, which is for the purpose of this invention called the "post-drawn" PE based filament. This post-drawn PE based filament can be beneficially used according to the invention because it does not show high creep like the prior art PE material. In fact, our post-drawn filament is preferably further characterized by a maximum draw ratio that is preferably measured at 120 °C and that is called the 120 ^° C maximum draw ratio of the post-drawn filament. This 120 ^° C maximum draw ratio of the post-drawn filament is determined at a temperature of 120 ^° C by determining the length of the post-drawn filament that is (further) drawn until the point that it nearly breaks and dividing that by the length of the post-drawn filament at 120 ^° C. This 120 ^° C maximum draw ratio of the post-drawn filament according to the invention is preferably at least 1.02, more preferably at least 1.05 and preferably at most 1.7, more preferably at most 1.3.

The 120 ^° C maximum draw ratio is actually also a reflection of the level of that the PE material was initially drawn (the initial drawing process of the invention) as a percentage of the maximum draw ratio. This percentage has been described above. For instance, if the -initial- draw ratio is carried out until 90% of the maximum draw ratio at 120°C, then the 120 °C maximum draw ratio of the post drawn filament (after again heating up the filament to 120 °C, measuring its length and determining the length that this post-drawn filament breaks upon further drawing) will be 1.11 (or 11% in percentages) because 100% / 90% = 1.11. In this manner, the 120 °C maximum draw ratio of the post-drawn filament reflects (and allows for determination of) the % of the maximum draw ratio that the PE based filament has been -initially- drawn during preparation of the PE based filament.

In order to arrive at the maximum tensile strength with the desired low modulus, it is preferred according to the invention that the PE material has the preferred draw ratio characteristics. To obtain the optimal draw ratio, the PE material is preferably prepared as follows:

For PE material with Mw of from 0.1M up to 2M, the PE material is preferably processed by using an annealing step after the extrusion step and before the drawing step, as described above. For PE material with Mw of higher than 2M up to 40M, the PE material is preferably processed by using ram extrusion and drawing, as described above.

These preferred preparation methods lead to PE material that can be drawn by the preferred draw ratios as mentioned above, leading to PE based filaments with the low modulus and high tensile strength of the invention.

Preferably, the variation in draw ratio along the PE based filament of the invention is less than 20% over a distance of 5 meter. Preferably, the variation in the modulus along the PE based filament of the invention is less than 20% over a distance of 5 meter. Preferably, the variation in the diameter of the PE based filament of the invention is less than 20% over a distance of 5 meter.

The process of the invention preferably results in an ultra high molecular weight PE monofilament with a modulus between 1 and 40 GPa, tensile strength of from 0.4 to 4 GPa and with low creep. An added benefit of PE monofilament is that we have surprisingly found that its properties are insensitive to moisture/humidity.

Examples

Example 1 - Tennis or racquet ball filament

GU 4022 UHMWPE powder with a molecular weight (Mw) of 5 million is ram-extruded to a diameter of 3.16 mm and immediately quenched in water. The extrudate is now of relative low crystallinity. Hereafter, the extrudate is drawn to a draw ratio of 4 times at lOOC, and

subsequently drawn at 120 ^° C to a total draw ratio of 10 times, followed by a relaxation to 95% length at 120°C. The diameter is now 1.0 mm, the tensile strength 550MPa, the elongation to break 20% and the modulus 8 GPa. The tension loss ratio at 50% break strength after 24 hours is 1.4.

Example 2 - Squash filament

GUR 4022 UHMWPE powder with a molecular weight (Mw) of 5 million (M) is ram-extruded to a diameter of 2.7 mm., and immediately quenched in water. The extrudate is now of relative low crystallinity and fully relaxed (low stress). Hereafter, the extrudate is drawn at 120 ^° C to a draw ratio of 9 times. The diameter is now 0.9 mm, the tensile strength 500MPa, the elongation to break 24% and the modulus 3 GPa. The tension loss ratio at 50% break strength after 24 hours is 1.4.

Example 3 - Badminton filament

GUR 4022 UHMWPE powder with a molecular weight (Mw) of 5 million is ram extruded to a diameter of 1.8 mm. and immediately quenched in water. The extrudate is now of relative low crystallinity and fully relaxed (low stress). Hereafter, the extrudate is drawn at 120°C to a draw ratio of 9 times. The diameter is now 0.6 mm, the tensile strength 500MPa, the elongation to break 24% and the modulus 3 GPa. The tension loss ratio at 50% break strength after 24 hours is 1.4.

The above examples show that we surprisingly obtain filaments based on PE material that are particularly suitable for tennis, racquet ball, squash, and badminton by properly choosing processing conditions.

Example 4 - filament evaluation

We evaluated notching and monofilament breakage after playing with a tennis racket that is strung with the following filament combinations under 25kg.

Test Crosses mains Result

Comparative Polyester (PET) Polyamide 6 Severe notching of polyamide

filaments after 15 minutes of play. After notching, friction is enhanced.

Comparative Polyamide 6 Polyester Less notching and somewhat longer service life. Polyester mains are too stiff for prior art rackets for, squash, racquetball and badminton and/or show significant tensile loss

Comparative Polyamide 6 Polyamide 6 Severe notching of longitudinal filaments after 15 minutes of play

Invention * UHMWPE Polyamide 6 No notching of longitudinal

monofilament filaments. Long lifetime (200-300 hours), good playing characteristics * the string produced in Example 1.

It is concluded that the monofilaments of the invention provide for fewer notches, less filament breakage, less wear, no fraying, and low tension loss. Most importantly, use of the filament gave a great feel to the tennis racket when hitting the ball, with less effort.