Conventional golf balls can be divided into two general types or groups: solid balls and wound balls. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Balls having a solid construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. Solid balls are generally made with a single solid core, usually made of cross-linked rubber, which is encased by a cover material. Typically the solid core is made of polybutadiene which is chemically cross-linked with zinc diacrylate and/or similar cross-linking agents and is covered by a tough, cut-proof blended cover. The cover is generally a material such as SURLYN®, which is a trademark for an ionomer resin produced by DuPont. The combining of the core and cover materials provides a ball that is virtually indestructible by golfers. Further, such a combination imparts a high initial velocity to the ball that results in improved distance. Because these materials are very rigid, two-piece balls have a hard “feel” when struck with a club. Likewise, due to their hardness, these balls have a relatively low spin rate, which provides greater distance.
Wound balls typically have either a solid rubber or liquid center core around which many yards of a stretched elastic thread or yarn are wound. The wound core is then covered with a durable cover material such as ionomer or polyurethane. Wound balls are generally softer and provide more spin, which enables a skilled golfer to have control over the ball's flight and final position. Particularly, with approach shots into the green, the high spin rate of soft, wound balls enables the golfer to stop the ball very near its landing position.
The design and technology of golf balls has advanced to the point whereby the United States Golf Association has now instituted a rule that prohibits the use, in a USGA sanctioned event, of any golf ball which can achieve an initial velocity of 255 ft/s when struck by an implement having a velocity of 143 ft/s. (Herein referred to as the USGA test.)
Manufacturers place a great deal of emphasis on producing golf balls that consistently achieve the highest possible velocity in the USGA test without exceeding the limit, which are available with a range of different properties and characteristics, such as velocity spin and compression. Thus, a variety of different balls are available to meet the needs and desires of a wide range of golfers.
Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with high initial velocities.
As a result, golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance for golfers at all skill levels, and seek to discover compositions that provide the performance of a high compression ball with lower compression.
The physical characteristics of a golf ball are determined by the combined properties of the core, any intermediate layers, and the cover. These, in turn, are determined by the chemical compositions of each. The composition of some balls will provide for increased distance. Other compositions provide for improved spin. Manufacturers are constantly looking to develop the ideal materials, silicone elastomers for example, have been examined for their innate ability to provide material having fairly high ultimate elongation, which is a very desired property in the make-up of a golf ball. However, they also have only low-moderate tensile strengths. One of the least attractive properties of silicone elastomers in the manufacture of golf balls is that the materials require covalent cross-linking to develop useful properties. This is because linear or branched silicone (polydimethylsiloxane) (PSX) homopolymers are viscous liquids or millable gums at room temperature. Fabrication of these materials must include, or be followed by, cross-linking to form chemical bonds among adjacent polymer chains. The infinite network thus formed gives the polymer its rubber elasticity and characteristic physical-mechanical properties. Cross-linking of extrudable and moldable silicone stock is usually done via peroxide-generated free radicals adding to vinyl groups incorporated along the polymer backbone, or increasingly, by the platinum-catalyzed addition of silane (—Si—H) terminal vinyl groups. Certain low-strength (RTV) silicone adhesives vulcanize at room temperature by condensation reactions, eliminating an acid or alcohol to generate —Si—OH or silanols, followed by the elimination of water as silanols condense to form —Si—O—Si— (siloxane) bonds and create a three-dimensional network.
Regardless of how the cross-linking or vulcanization is effected, the resulting thermoset silicone cannot be re-dissolved or re-melted. This severely reduces the number of post-fabrication operations that could be used in the fabrication process of golf balls. Thermal forming, radio frequency welding, heat sealing and solvent bonding are all essentially unavailable when working with conventional silicone elastomers. In contrast to cross-linked silicone rubbers, many polyurethane elastomers are thermoplastic in nature. That is, they can be processed by methods that involve melting or dissolving the polymer to reshape it. The molecular structure of a typical thermoplastic urethane (TPU) consists of alternating high-melting “hard” urethane segments and liquid-like “soft” segments.
Hard segments are almost always the reaction product of an aromatic or aliphatic diisocyanate and a low molecular weight, chain-extending dialcohol or diol. The diisocyanates may be selected from the group consisting of alkyl diisocyanates, arylalkyl diisocyanates, cycloalkylalkyl diisocyanates, alkylaryl diisocyanates, cycoalkyl diisicyanates, arly diisocyanates, cycloalkylaryl diisocyanates, all of which may be further substituted with oxygen, and mixtures thereof. The chain extender of the hard segment used in the preparation of the copolymers may be an aliphatic polyol or an aliphatic or aromatic polyamine such as known for preparing polyurethanes and polyureas. The polyol for the hard segment may be preferably selected from the group consisting of alkylene, cycloalkylene, arylene diols, triols, tetraalcohols and pentaalcohols, and mixtures thereof. The polyamine of the hard segment may be selected from the group consisting of alkyl, cycloalkyl, and aryl amines that may be further substituted with nitrogen, oxygen, halogen, complexes thereof with alkali metal salts and mixtures thereof.
Soft segments may be built from polyols with terminal hydroxyl (—OH) groups. The hydroxyl creates a urethane group, while the reaction between isocyanates and existing urethane groups will form allophanate groups that can produce minor amounts of covalent cross-linking in TPUs. When a TPU is heated, the hydrogen-bonded hard segments and any allophanate cross-links, both of which hold the polymer together at its use temperature, dissociate to allow the polymer to melt and flow. Dissolution in a polar solvent can also disrupt the hydrogen bonds that hold together the hard segments on adjacent chains. Once these virtual cross-links are broken, the polymer can be fabricated into golf balls. Upon cooling or solvent evaporation, the hard segments de-mix from the soft segments to re-associate by hydrogen bonding. This restores the original mechanical properties of the polyurethane elastomer. Polyether and polycarbonate TPUs generally have excellent physical properties, combining high elongation and high tensile strength, albeit having fairly high-modulus. Varying the hard segment of a TPU during synthesis can produce a whole family of polymers of related chemistry but with a wide range of hardness, modulus, tensile-strength properties and elongation. In the fabrication of golf balls, the use of TPUs of different hardness values within a single family provides considerable versatility in manufacturing.
Therefore, there exists a need for a golf ball comprising a thermoplastic silicone-urethane having improved golf ball performance.