BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to golf balls, and especially, to the structure of a ball composed of several parts. 
     2. Description of Background and Other Information 
     Prior art can be divided into two main types of golf balls. On the one hand, there are the so-called &#34;two-piece&#34; solid balls. Their properties are derived from the combination of a rubber mono-piece spherical core and a hard ionomer resin thermoplastic envelope. The main advantage of these balls is their very high performance during long shots by virtue of their considerable initial speed. However, they feel hard on impact, the main reason for this being their substantial rigidity. Thus, during short shots or approach shots, the initial high speed and small contact surface hinders control of the ball. Also, lack of rotational speed adversely affects the behavior of the ball when it falls. 
     On the other hand, &#34;three piece&#34; rolled balls are known. These are constituted of a solid or liquid central core, of a rubber thread winding and of a thermoplastic envelope either made of ionomer resin or of balata type rubber of a thickness of 1 to 2 mm. The high deformability of the external layers of the rolled core results in a feeling of softness at impact, due to a smaller rate of compression and consequently a greater contact surface. However, this type of ball adversely affects performance during long shots, as opposed to the &#34;two piece&#34; balls. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to propose a new ball, whose performance will be satisfactory in every aspect of the game, that is, a ball with considerable initial speed, close to that of faster balls, and one having good feel and good control during approach shots. The golf ball of the present invention will also enable shots to be reproduced, and it will have excellent durability. 
     Another object of the invention is to enable a wide range of performances to be covered, responding to the precise requirements of users, and at the same time, requiring only very minimal variation in the parameters. 
     Another object of the invention is to enable the use of a reliable and automatic manufacturing method, enabling high productivity by virtue of its structure and the choice of materials used. 
     The desired properties are obtained according to the present invention by a structure made of three parts. The golf ball is constituted of an elastomer core, an intermediate layer and a thermoplastic cover, characterized mainly by the fact that the intermediate layer is made of a thermoplastic material containing at least 10%, and preferably at least 35% of ether block copolymer. 
     By ether block copolymer sequenced copolymers formed from blocks that have- certain chain lengths for their diverse components are meant. 
     They have two phases: one flexible, the ether block, and the other rigid. By playing with the nature and proportion of these phases, one can obtain a rather wide variety of products that range from being extremely flexible to relatively rigid. 
     Among these copolymers, those that are especially preferred are amide block copolymers on the one hand and ester block copolyethers on the other hand. The amide block copolymers (PEBA) are well known under the trademark PEBAX® commercialized by ATOCHEM. The use of GRILAMID® commercialized by the company EMS or VESTAMID® by HULS are noted. 
     As for ester block polyethers (PEBE), these are products that have a rigid phase of the terephtalate polybutadiene type (PBT). These are known under the trademark HYTREL® by the company DUPONT DE NEMOURS or ARNITEL® by the company AKZO. 
     The compositions of the invention may be used in a pure form to make an intermediate layer. One can thus obtain balls that are appreciated for their excellent feel, and for their initial high speeds. However, due to their weak elastic modulus, they tend to deform excessively at impact. 
     This problem may be solved advantageously by forming a compound of ether block copolymer and of ionomer(s). The ionomer(s) rate may vary from 0 to 90% by weight with respect to the total weight of the compound and is preferably comprised between 20 and 65%. The preferred ionomers have a shore D hardness comprised between 55 and 65, and a modulus of elasticity comprised between 250 and 350 N/mm 2 . The preferred amide block copolyethers have a shore D hardness of 30-40. In this case, the proportion of amide block polyether in the composition is of 35-80%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be better understood and its advantages will become more apparent from the following description of the embodiments. 
     FIG. 1 is a partial sectional view of a golf ball according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The ball of the invention is a ball of &#34;three-piece&#34; type, that is, it is constituted of three distinct layers. 
     As can be seen in FIG. 1, core 1 is constituted of a thermoplastic or thermohardenable or vulcanizable elastomer whose diameters are comprised between 34 and 38 mm. Its density is comprised between 1 and 1.3 g/cm 3  and its shore D hardness is located between 40 and 50. The compression of this core measured under a load of 150 kg is comprised between approximately 2.8 and 4.5 mm for a fixed diameter of 36 mm. 
     The elastomer especially preferred is a cross linked diene elastomer of the polybutadiene cis-1,4 type containing a reactive product formed of zinc oxide and zinc diacrylate. The composition also contains a cross linking agent such as dicumyl peroxide, for example. 
     The intermediate part 2 is an injectable and extrudable thermoplastic copolymer of amide block polyether and preferably of polyetheresteramide of the type whose obtention procedures is described in the French patent 2,273,021 by ATOCHEM. 
     As an example, the balls have been manufactured with an intermediate layer of pure PEBAX. The characteristics and properties are mentioned in the annexed Tables 1 and 2. 
     In a preferred embodiment, the intermediate part 2 is constituted of a compound of amide block polyether and of ionomer(s). Tables 3 and 4 indicate the characteristics and properties of balls whose intermediate layer is manufactured of a composition based on PEBAX 3533® and ESCOR®. 
     Generally, layer 2 formed around core 1 has a thickness of approximately 1 to 3 mm, and a modulus of elasticity of between 15 and 250 N/mm 2 . According to the type of copolymer used, the shore D hardness may vary between approximately 20-50. The influence of these parameters is important and affects the performance characteristics of the ball. Balls whose intermediate layer has a lesser shore hardness D of approximately 30-37 lesser modulus of elasticity for a given thickness, will be appreciated for their feel and their control because of their high rotational speed or &#34;spin&#34; speed. Conversely, balls whose intermediate layer has a shore D hardness of between 40 and 50 (greater flexion module) will be appreciated for their optimal initial speed and their durability. 
     A comparison of Tables 1 and 3 shows that if one added an ionomer to the ether block copolymer composition, the hardness values would not change. However, the modulus of elasticity in measured traction would increase considerably. This effect is especially interesting because the intermediate layer contributes to the mechanical resistance of the core at impact, and advantageously limits its deformation. The intermediate layer behaves dynamically like the elastic filamentary winding layer of the so-called &#34;winding&#34; force. 
     The assembly of the two parts, 1 and 2, forms the internal structure of the ball and has a compression of 2.5 to 4 under 150 Kgs for a total given diameter of 40 mm. 
     The external layer 3 forms the envelope or cover of the ball. It is made of a thermoplastic material and has a thickness comprised between 0.9 and 3 mm. 
     The choice of materials is relatively wide in view of the fact that the basic qualities required for the cover are resistance to impact and durability. Also, in order to conserve these characteristics for the ball according to the invention, it is important that the hardness of the cover be greater than that of the intermediate layer. 
     The range of shore D hardness allowed varies between 40-55 and preferably between 43 and 48, for a density of 0.8 to 1.2 grams/cm 3  and a modulus of elasticity of 30-280 N/mm 3 . 
     Among the prefferred materials, one would preferably choose ionomers of the SURLYN® type, commercialized by the company DUPONT DE NEMOURS or ionomers of the IOTEK® type by EXXON, amide block copolymers of the type used for the intermediate layer but with greater hardness, ionomer and amide block copolymer compounds, thermoplastic polyurethanes as well as compounds of these materials. 
     The association of the three layers 1, 2 and 3 give rise to a finished ball with a diameter of between 42.7 and 42.8 mm, its compression measured under 150 Kg of load is comprised between 2.5 and 4 mm. 
     As an example, the physical characteristics, the behavior and the properties of several balls according to the invention have been compared to earlier balls that were commonly used commercially. The results are presented in Tables 1-4 that follow: 
     
                                           TABLE 1__________________________________________________________________________                         PRIOR ART      INVENTION                 3 Piece Balls      Ball 1           Ball 2                Ball 3                     Ball 4                         2 Piece Balls                                A      B__________________________________________________________________________COMPOSITION      POLYBUTADIENE CIS-1,4                         SOLID  ROLLED ROLLEDDIAMETER (mm)      36.4 36.4 36.4 36.4                         RUBBER CORE   COREDENSITY    1.18 1.18 1.18 1.18                         CORE(g/cm.sup.3)COMPRESSION      3.8  3.8  3.8  3.8in mm under150 KG ofconstant loadCOMPOSITION      INTERMEDIATE AMIDE BLOCK      POLYETHER      (PEBAX)SHORE      37   37   40   43HARDNESS(D)THICKNESS (mm)      1.8  1.8  1.8  1.8DENSITY    1.01 1.01 1.01 1.01(g/cm.sup.3)ELASTICITY 40   40   55   80MODULUSIN TRACTIONCOMPOSITION      THERMOPLASTIC      SURLYN ®                                SURLYN ®                                       BALATA      ENVELOPE           ENVELOPE                                ENVELOPE                                       ENVELOPESHORE HARDNESS      43   45   45   47(D)THICKNESS (mm)      1.35 1.35 1.35 1.35DENSITY    1    1    1    1(g/cm.sup.3)ELASTICITY 80   100  100  120MODULUSIN TRACTION__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________                         PRIOR ART         INVENTION              3 Piece Ball         Ball 1             Ball 2                 Ball 3                     Ball 4                         2 Piece Ball                                A   B__________________________________________________________________________DIAMETER (mm) 42.7             42.7                 42.7                     42.7                         42.7   42.7                                    42.7WEIGHT (G)    45.5             45.4                 45.5                     45.4                         45.5   45.5                                    45.3COMPRESSION (mm)         3.7 3.6 3.4 3.1 2.8    3.3 3.2DRIVER:INITIAL SPEED VI (m/s)         64.2             64.1                 64.2                     64.4                         64.7   64.3                                    64.2ROTATION (SPIN) (t/mn)         3800             3650                 3600                     3300                         2900   3300                                    3700IRON 5:ROTATION (SPIN) (t/mn)         7500             7300                 7300                     6800                         5600   6500                                    7300PROPERTIES:CONTROL       B   B   E   E   M      B   EFEEL AT IMPACT         B   B   E   B   M      B   EDURABILITY    B   B   B   B   E      B   MREPRODUCTABILITY         E   E   E   B   B      M   M__________________________________________________________________________ E = EXCELLENT B = GOOD M = MEDIOCRE 
    
     
                       TABLE 3______________________________________         Ball 5  Ball 6    Ball 7______________________________________COMPOSITION      ELASTOMER CORE            POLYBUTADIENE            CIS-1,4DIAMETER (mm)    36.4     36.4      36.4DENSITY (g/cm3)  1.18     1.18      1.18COMPRESSION in mm            3.8      3.8       3.8UNDER 150 KGCOMPOSITION      INTERMEDIATE LAYER            (PEBA/IONOMER)SHORE HARDNESS (D)            37       40        43THICKNESS (mm)   1.8      1.8       1.8DENSITY (g/mm3)  1        1         1ELASTICITY MODULUS            90       120       150IN TRACTION (N/mm2)COMPOSITION      THERMOPLASTIC            ENVELOPESHORE HARDNESS (D)            43       45        47THICKNESS (mm)   1.35     1.35      1.35DENSITY (g/cm3)  1        1         1ELASTICITY MODULUS            90       100       120IN TRACTION(N/mm2)______________________________________ 
    
     
                       TABLE 4______________________________________           Ball 05                  Ball 06  Ball 07______________________________________DIAMETER (mm)     42.7     42.7     42.7WEIGHT (G)        45.5     45.4     45.5DRIVER: (46M/S)INITIAL SPEED (m/s)             64.2     64.3     64.2ROTATION (SPIN) (T/mn)             3800     3600     3400IRON 5:ROTATION (SPIN) (T/MN)             7500     7300     6900PROPERTIES:CONTROL           E        E        BFEEL AT IMPACT    B        E        BDURABILITY        B        B        BREPRODUCTABILITY  E        E        E______________________________________ E = EXCELLENT B = GOOD M = MEDIOCRE 
    
     The instant application is based upon French patent application 90.10696 of Aug. 22, 1990, the disclosure of which is hereby expressly incorporated by reference thereto, and the priority of which is hereby claimed.
The invention concerns a golf ball constituted of a three part structure, constituted of an elastomer core, an intermediate layer, and a thermoplastic envelope. The intermediate layer is made of a thermoplastic material containing at least 10%, and preferably at least 35% of ether block copolymer. The material may also be advantageously constituted of ether block copolymer to which is added 0 to 90% of ionomer(s) with respect to the total weight of the compound.