EP0049196B1 - Strings of synthetic materials for tennis rackets - Google Patents

Abstract

1. A synthetic cord, in particular for a tennis racquet, comprising fibers of a first synthetic material and a binder of a second synthetic material interconnecting the said fibers, the fibers constituting an assembly of turns which are inclined relative to the longitudinal axis of the cord and which are interconnected by the binder, the cord being characterized in that the fibers (1) have a breaking strain in the range 10% to 25%, the binder (2) being constituted by a thermoplastic material having a melting point lower than the melting point of the fibers (1) and having a breaking strain greater than 200%, and the binder (2) having a Shore D hardness number not less than 55.

Description

The present invention relates to a synthetic rope intended in particular, but not exclusively, for the production of tennis racket strings and its manufacturing methods.

We know that strings made by twisting strips cut from animal casings, in particular sheep, have been used for a very long time and give excellent results when they are stretched over tennis rackets, only the "casing" being practically used in competition. The preference of high level players results essentially from the elastic return characteristics of such strings, that is to say from the rapid and integral recovery of their initial length after deformation due to rapid and repeated shocks. The elongation / applied traction curves of these cords are substantially rectilinear and vary very little from one traction cycle to the next, which indicates the absence of permanent deformation. Each of said curves, however, has stages each corresponding to the breaking of certain elementary filaments or to the detachment of the turns of the twisted rope. This phenomenon naturally limits the life of such a rope.

On the other hand, the casings are only a by-product of the breeding of animals for slaughter. The economic conditions of breeding lead to the slaughter of young animals often fed with synthetic feeds which, if they are of nutritional interest, do not develop a resistance of the intestines comparable to that developed by a natural food. This results in a degradation of the average characteristics of the casings which can only be remedied by a strengthening of the selection, which further increases the cost of said strings and limits the production of quality. In addition, the characteristics of the casing ropes very closely depend on the storage conditions and the hygrometric conditions during use.

Given the current development of tennis, the need for synthetic strings has increased and it is desirable to be able to produce on a large scale strings of constant and reproducible qualities, independent of sheep farming and having similar characteristics in play and even higher than those made in animal casings.

Various attempts have been made in this way for many years. Thus, ropes have already been proposed which consist of a monofilament extruded from thermoplastic material and, for example, from polyamide, modified polyvinyl, polyurethane or thermoplastic polyester. Unfortunately, if the return of these ropes to their initial position after a slight deformation remains elastic, although only relatively slow, the elongation becomes permanent as soon as the rope is subjected to a high tensile force. In practice, such cords can only be worked at 10% of their breaking load in continuous and repeated use. For the diameter of the strings mounted on the rackets, of the order of 1.5 mm, the breaking load of a monofilament is of the order of 50 to 60 kg. Since the strings are stretched over the frame with a variable tension, but which is commonly around 20 kg, the slightest shock causes deformation.

Ropes have also been proposed consisting of bundles of multifilaments, each bundle being surrounded by a thermoplastic sheath, possibly reinforced with fibers. In US-A-3,050,431, a tennis cord is obtained by surrounding a core of parallel thermoplastic multifilaments, for example nylon, with a thermoplastic braid, then passing the assembly through a heating die after impregnation in a bath with a nylon solution.

All the above-mentioned strings have a linear or unidirectional structure, sometimes reinforced by a winding. Unfortunately, they all work at a rate of traction which exceeds the elastic limits and it is thus that the curves of allocation according to the applied tension all indicate an irreversible deformation or non-recoverable elongation and, after a series of successive stresses, permanent elongation.

A tennis string of another design is known from FR-A-2 393 102. In this patent, the structure of the string is no longer linear, that is to say that it no longer has any core of parallel, mono or multifilament yarns. A bundle of multifilaments is formed, parallel to each other and is impregnated with a thermosetting adhesive, having after curing an appropriate elasticity. Then, the adhesive is pre-polymerized to a selected degree of partial prepolymerization and the bundle is then twisted, avoiding the dewatering of the impregnated multifilaments by virtue of the partial polymerization, the total polymerization being obtained subsequently.

It is, of course, very difficult to control the degree of partial polymerization and it emerges from the examples given in the aforementioned patent that the process described requires relatively long times (35 to 90 minutes) for the polymerization to take place. complete up to eight days. It is obvious that this process cannot allow continuous manufacturing.

The object of the present invention is to overcome the drawbacks of known synthetic ropes and to allow the continuous production of a new rope, the characteristics of which can be compared to those of natural gut ropes.

According to the present invention, the synthetic string for tennis rackets, comprising fibers of a first material and a binder of a second material joining said fibers, the fibers constituting an assembly of turns inclined relative to the longitudinal axis of the rope is characterized in that the fibers have a breaking elongation between 10 and 25%, the binder being constituted by a thermoplastic material whose melting point is lower than the melting point of the fibers and whose elongation at break is greater than 200%, the binder having a Shore D hardness at least equal to 55.

The elongation at break of the binder is greater than 200%. To fix the ideas only, it can be specified that the values indicated correspond respectively to a polyamide of the nylon type and to a polyurethane elastomer.

Thus, the rope according to the invention is formed of turns joined by a binder. When the rope is subjected to a shock, the turns undergo a relative lateral displacement which tends to move them away from each other. This relative displacement is limited on the one hand, by the resistance to elongation of the fibers constituting the turns and, on the other hand by the reaction of the binder both inside the turns and between them. Given the limitation of relative displacement between the turns due to the resistance of the fibers, the elongation of the binder is limited to approximately 10% of its elongation at break which is greater than 200%. It follows that the deformations of the binder always remain elastic and that the latter always brings the turns against each other, that is to say that the rope regains after an impact of the ball very quickly its initial length, result that only natural casings could give during their lifespan, which, as already mentioned, is relatively short.

Of course, if certain elastomers have satisfactory elongation-rupture characteristics having regard to the need for elastic return, they do not, taken in themselves, have sufficient strength, which leads to the composite structure according to the invention.

Whereas in the cords of the prior art the response of the cord under impact was provided almost exclusively by an extension of parallel fibers, according to the invention this response comes from a controlled deformation of the spiral and the binder.

A cord according to the invention can be obtained by various methods which will be described below, all using a milling, wiring, braiding, stranding, wrapping or twisting carried out while the binder is in a plastic state. It is known that the operations mentioned above make it possible to apply wires against each other and help to give the rope a substantially circular section.

Other characteristics and advantages of the invention will appear during the following description of particular embodiments, given solely by way of nonlimiting examples, with reference to the figures which represent:

• La Fig.2, un schéma d'une installation d'enduction;
• La Fig. 3, un schéma d'une installation de retordage;
• La Fig. 4, une bande de fils après retordage;
• La Fig. 5, une autre installation de retordage;
• La Fig.6, une variante du procédé selon l'invention utilisant des fils d'élastomère thermofusible.

Fig. 1, a piece of rope according to the invention;

• Fig.2, a diagram of a coating installation;
• Fig. 3, a diagram of a twisting installation;
• Fig. 4, a strip of yarn after twisting;
• Fig. 5, another twisting installation;
• Fig.6, a variant of the method according to the invention using hot melt elastomer yarns.

As shown in Fig. 1, a cord 12 is in the form of an assembly of turns 10 adhering to one another, not only because of the treatment undergone, but also thanks to the presence of the binder 2. In fact, it is the choice characteristics of the bonding product 2 which makes it possible to obtain the elastic return of the rope after an impact. The disappearance of the permanent deformation results from the fact that the elongation of the wires constituting the turns limits the elongation of the binder, the elongation-rupture of which is at least 200%. The binder 2 and the wires or strands 1 work in an area which preserves all their elastic properties.

According to one of the characteristics of the invention, the binder 2 is chosen from the range of thermoplastic products whose melting or softening point is lower than that of the material of the threads. As will be described later, it is possible to use a thermoplastic binder which becomes thermosetting above a certain temperature.

A step in a first manufacturing process is shown diagrammatically in FIG. 2. A multifilament yarn 1, for example made of polyamide, is unwound from a spool 3 and passes through a tank 4 inside which is an elastomer in solution. It is known that thermoplastic products are generally soluble whereas it is not the same for thermosets after polymerization, even partial. The wire 1 is thus coated with an elastomer solution and calibrated at the outlet of the tank 4 by a not shown die which aims to remove the excess solution. Then, the coated wire 11 passes through a heating chamber 5 inside which the solvents evaporate and there remains only on the surface of the wire a coating which, by fusion will give the rope a structure homogeneous thereafter. It will be noted that when passing through the bath of dissolved or molten elastomer, the multifilaments are also impregnated. Preferably, the enclosure 5 is brought to a temperature of approximately 100 ° C. which ensures a rate of evaporation of the solvents compatible with the speed of threading of the wire without causing the melting of any of the components. At the exit of tunnel 5, the wire 11 is dry and can be wound on a take-up reel 6. The wire 11 can also be directly routed, as soon as it leaves tunnel 5 on a torsion machine where the second phase of the process is carried out . Any other coating process could be used at this stage and in particular pultrusion or extrusion which, however, given the thin coating thickness and the fineness of the wire 1, seem difficult to implement given the characteristics of machines which are currently on the market.

The next phase of this first process is shown diagrammatically in FIG. 3. It consists in the molding of a plurality of coated wires 11. The molding is an operation practiced for a very long time. It can be carried out on a turntable 7 on which pins are arranged. These pins can be animated relative movements with respect to each other, be animated by a proper movement of rotation in the opposite direction to the direction of rotation of the plate or have no own movement. Said pins carry coils 6 of coated wire during the first step of the process. The wires 11 unwound from the coils 6 converge in a ring 9. Between the ring 9 and the plate 7 is arranged a heating tunnel 8 whose purpose is to cause the softening or at least partial melting of the binder 2 which spreads thus between the turns during their formation. When the coating consists of a thermoplastic elastomer of the polyurethane type and the strands are made of polyamide, the temperature may be greater than 120 ° C. and must be adjusted so as to cause the melting of the binder and not that of the strands. The wires and the binder cool beyond the ring 9 and the finished cord 12 can be wound on a take-up reel. Before use, the rope is subjected to thermofixing as will be described later. It will be noted that the wires are stranded while the binder is more or less melted. This results in an impregnation to the hearts of the cord 12, impregnation which gives it the necessary homogeneity. It is obvious that the first two phases of the process which has just been described can be carried out on suitable equipment allowing continuity of manufacture which, moreover, results in a saving of the energy consumed. Indeed, at the exit of the tunnel 5, the coated wire is already hot and consequently a lower number of calories is necessary to raise the cord to the melting point of the binder 2. The same applies with regard to the third phase of this first process which consists of a heat setting which can be carried out immediately after formation of the cord in the ring 9, while the cord is still hot. The twist rate during molding is preferably between 100 and 200 turns per meter of length after molding. It is obvious that the rate of twist conditions the angle of the turns and accentuates the difference with synthetics with linear structure. In practice, we choose the maximum torsion rate compatible with the surface quality of the rope.

A second method consists in coating, by passing through a tank containing an elastomer in solution, a set of parallel wires kept spaced apart during the impregnation. We then obtain a strip of wires embedded in and joined by the binder. This strip is then hot-twisted and a cord 12 is obtained such as that which is shown in FIG. 4. The parallel wires can also be assembled in "daisy", that is to say around a central wire.

A third method is shown in FIG. 5 on which there is a turntable 7 on which are mounted coils 6 of uncoated wire 1. In the middle of the plate 7 is provided an orifice 15 allowing the passage of a fine wire 14 (2 / 10th of mm) unwound from a coil 13. The wires 1 are grouped in a ring 9. According to a characteristic of this process, the wire 14 is previously coated with an excess of thermoplastic elastomer in the dissolved state. During the twisting, the threads 1 wring out the thread 14 so that the elastomer spreads inside and between the turns in formation. A heating tunnel 16 disposed above the ring 9 causes on the one hand the evaporation of the solvents and, on the other hand the at least partial melting of the thermoplastic elastomer. After drying and cooling the rope 12 obtained can be wound on a reel. Of course, the material of the central wire is chosen so that the elongation of the latter is at least equal to that of the completed cord so that it participates in the response of the cord when it is stressed in tension , but without showing any permanent deformation. One can also use a hot-melt wire 14 which disappears when passing through the tunnel 16. In the case of a cord constituted by polyamide wires and a polyurethane elastomer, the temperature of the components inside the tunnel 16 is order of 14CTC.

In the previous embodiment, there may remain a central wire core. In any event, the proportion of central wire in the whole cannot exceed 15% by weight of the total structure, so as to preserve the spiral structure of the cord which allows the abovementioned advantages to be obtained. This proportion clearly defines the strings according to the invention with respect to the cords of the prior art in which the linear structure represents at least 90% of the total structure.

A fourth method is shown diagrammatically in FIG. 6. This process again uses a turntable 7, intended to ensure the formation of turns by molding. As in the case shown diagrammatically in FIG. 5, wires 1, not coated with elastomer, are wound on coils 6 carried by pins. Other coils 17 do not carry polyamide thread, for example, but a textile thread 18 made of hot-melt elastomer such as hot-melt polyurethane. All the wires 1 and 17 are joined by a ring 9 which forms the turns, the ring 9 being followed, in the direction of progression of the rope by a heating tunnel 16 which, as previously causes the elastomer to melt. distributes in and around the turns of fibers. The polyurethane threads are used after melting of the binder. Unlike previous processes, the binder is not applied in solution on the fibers.

The strings obtained according to one of the above methods, however, appear, when used directly, to have response curves greater than those of the known synthetic strings mentioned above, but do not reach the quality of the best natural casings. According to a characteristic common to all the processes, the strings obtained are subjected in the terminal phase to a thermofixing, operation which consists in heating the strings while they are under tension. It makes it possible to reduce the elongations by sometimes increasing the resistance, to regularize the structure and to avoid twists. In the case of tennis strings according to the invention, the applied voltage is between 3 and 10 kg, the temperature being above 140 "C and the duration of one to two minutes. This phase has not been shown on the drawings.

The strings obtained according to the invention have been found to have qualities at least equivalent to those of natural casings, both with regard to the "feeling" or impression of the player, and the speed of return after impact. In addition, their lifespan, under identical working conditions, is approximately ten times that of casings.

Example

Firstly, a 6-6 multifilament polyamide yarn titrating 940 decitex is used. The thread drawn by a take-up reel passes through an impregnation bath filled with a solution of thermoplastic polyether in a mixture of cyclohexanone and tetrahydrofuran. The dry extract equal to 15% determines the final weight ratio of the wire and the binder. The solvents are evaporated in an oven brought to 150 ° C. The passage time required for evaporation is 4 minutes. The impregnated and dry wire is stored on the take-up spool.

Secondly, nine take-up coils are mounted on a turntable. The nine impregnated and dry wires are twisted under a tension of 3 kg, at the rate of 150 turns per meter. The strand then passes through an oven two meters long at a speed of 30 cm / min. The temperature of the oven is brought to 230 ° C and ensures the welding between the wires and the binder, then the thermofixing. The rope thus produced is cut to length and wound.

The polyamide yarn has a tensile strength of approximately 7 kg and an elongation at break of 22% the polyether has a Shore D hardness greater than 55 and an elongation at break greater than 300%. The rope obtained has a tensile strength of 70 kg and an elongation at break of 25%. It offers no possibility of untwisting and has a homogeneous and transparent appearance. The traction curves show an almost absolute similarity with those of the usual natural casings.

In order to avoid excessively rapid wear of the rope on its fixing points, the binder is chosen from bodies having a Shore D hardness at least equal to 55.

Of course, the strings according to the invention can be used in any other ball sport than tennis and in particular for squash or badmonton, whenever it is desirable to have a permanent elastic return.

It goes without saying that many variants can be introduced, in particular by substitution of equivalent means or products, without departing from the scope of the present invention.

Claims (8)

1. A synthetic cord, in particular for a tennis racquet, comprising fibers of a first synthetic material and a binder of a second synthetic material interconnecting the said fibers, the fibers constituting an assemly of turns which are inclined relative to the longitudinal axis of the cord and which are interconnected by the binder, the cord being characterized in that the fibers (1) have a breaking strain in the range 10% to 25%, the binder (2) being constituted by a thermoplastic material having a melting point lower than the melting point of the fibers (1) and having a breaking strain greater than 200%, and the binder (2) having a Shore D hardness number not less than 55.

2. A method of manufacturing a cord according to claim 1, characterized in that the method consists in:

covering each fiber by passing it through a tank containing a solution of thermoplastic elastomer;

evaporating the solvents;

reheating the set of fibers to melt the elastomer, and then twisting the said set to a pitch of 100 to 200 turns per meter; and

thermofixing under tension.

3. A method of manufacturing a cord according to claim 1, characterized in that the method consists in:

impregnating a set of adjacent multi-filament threads with a thermoplastic elastomer in solution;

passing the set through an oven to evaporate the solvents;

reheating the set to at least partially melt the elastomer;

twisting the set while the elastomer is at least partially in the melted state; and

thermofixing.

4. A method of manufacturing a cord according to claim 1, characterized in that the method consists in:

mounting a first set of bobbins carrying multi-filament threads on a rotary plate, and mounting an interspersed second set of bobbins carrying threads of thermoplastic elastomer on the plate;

heating and twisting the two sets of threads to melt the elastomer and form turns; and

thermofixing.

5. A method of manufacturing a cord according to claim 3 or 4, characterized in that, before or after twisting, the elastomer is melted at a temperature greater than 140°C, the twisting pitch being in the range 100 to 200 turns per meter of final length, and the thermofixing taking place at a temperature greater than 150°C at a tension of 3 to 10 kg for a period of one to two minutes.

6. A method of manufacturing a synthetic cord comprising fibers of a first material and a binder of a second material, characterized in that the method consists in:

mounting a set of bobbins carrying multi-filament fiber threads on a rotary plate;

bringing these threads together via a ring which receives a central thread impregnated with an excess of elastomer in solution;

twisting the set of threads so as to wring out the central thread;

heating the set to evaporate the solvents and melt the thermoplastic elastomer; and

thermofixing.

7. A cord obtained by the method of claim 6, characterized in that the percentage of fibers disposed in the direction of the longitudinal axis of the cord relative to the total number of fibers is less than 15%, the other fibers being wound in a spiral around the longitudinal axis.

8. A cord according to claim 1 or 7, characterized in that the multi-filament fibers are made of polyamide, and the second material is a heat- meltable polyurethane elastomer.

Images