Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-10-22
2002-01-15
Buttner, David J. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S358000, C524S360000, C524S347000, C524S343000, C525S264000, C525S274000, C525S257000, C473S372000, C473S373000, C473S374000, C473S377000
Reexamination Certificate
active
06339119
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to golf balls, and, in particular, is directed to a composition used for the manufacture of golf ball cores, as well as a method for the manufacture of golf ball cores using the subject composition.
BACKGROUND OF THE INVENTION
Today, a wide variety of golf balls are available to meet the needs and desires of a wide range of golfers. Golf balls are generally available as one-piece (i.e., unitary), two-piece, and three-piece (i.e., wound or solid multi-component) balls. One-piece balls lack a separate cover, and are typically formed with a dimpled surface from a molded polybutadiene based compound. Since these balls typically spin at a high rate, and have a low velocity, they travel a relatively short distance when struck and are generally used as practice or driving range balls.
In contrast, two-piece golf balls, used by the typical amateur golfer, provide maximum durability and distance. These balls usually have a core formed of a single solid sphere, which is typically formed of a polybutadiene based compound, and a cover of SURLYN® or other similar ethylene-based ionomer that encloses the core.
Three-piece balls, which are preferred by professionals and low handicap amateur golfers for their spin characteristics and feel, include either a solid rubber core or a liquid center core that may be wound with many meters of elastic windings. Such cores are thereafter encased in a cover formed of SURLYN®, polyurethane or balata rubber. The winding provides three-piece balls with a higher spin rate and more control for better golfers.
Regardless of the form of the ball, for obvious reasons players generally seek a golf ball that has good durability. All golf balls, whether the covers are formed from ionomers, balata or some other cover composition, typically exhibit failures such as cuts, cracks or other fractures which appear in the outer surface of the cover of a golf ball after it is repeatedly struck with a club. Failures may appear anywhere in the cover and are either the result of a defect or occur towards the end of the useful life of the golf ball. Although the durability of ionomer resin covered golf balls varies depending upon the particular composition of the cover blend, conventional golf balls having ethylene-based ionomer resin covers (with a typical cover hardness of 65-70 Shore D) are generally expected to have a long useful life before the golf ball fails. Therefore, golf ball manufacturers seek to discover compositions that provide durable golf balls that deliver the maximum performance for golfers of all skill levels.
A number of elastomeric polymers, such as polybutadiene, natural rubber, styrene butadiene rubber (hereafter “SBR”) and polyisoprene, have been used in fabricating golf ball cores. Today, golf ball cores are predominantly made of polybutadiene. Moreover, in order to obtain the desired physical properties for golf balls, manufacturers have added a cross-linking agent, also known as a coagent, such as a metallic salt of an unsaturated carboxylic acid. The amount of cross-linking agent or coagent added is typically about 8 to about 60 parts per hundred parts of elastomeric polymer by weight (parts per hundred, hereafter “pph”). Most commonly, an acrylate neutralized with a metal ion, such as zinc diacrylate or zinc dimethacrylate, is used for this purpose.
In commercially available golf balls, a conventional peroxide is generally used to cross-link the elastomeric polymer during, e.g., the core molding process. The peroxide decomposes to form radicals which initiate cross-linking, as is well known to those of ordinary skill in this art. For example, typical peroxide compounds taught to initiate cross-linking of the elastomeric polymer/cross-linking agent include dicumyl peroxide (available as PEROXIMON DC 400KEP® from Elf Atochem N.A. and ESPERAL 115RG® from Witco), 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane (VAROX 231XL® from R.T. Vanderbilt, LUPERCO 231KE® from Elf Atochem N.A.), &agr;,&agr;′-bis(t-butylperoxy)-diisopropylbenzene (RETILOX F40KEP® from Elf Atochem N.A.), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (VAROX DBPH-50® from R.T. Vanderbilt, LUPERCO 101-XL® from Elf Atochem N.A.) and di(t-butyl) peroxide (from Witco).
Typically, about 5 to 50 pph of a particulate material, such as zinc oxide (ZnO), tin oxide (SnO), calcium oxide (CaO), or mixtures thereof, is also added to the composition. This particulate material may serve as both a filler and an activation agent for the cross-linker/peroxide cure system. For example, if comparable compositions with and without zinc oxide are compared, there is a reduction in cure enhancement for the composition lacking zinc oxide which results in less cross-linking and a corresponding reduction in compression and velocity. The high specific gravity of the particulate material, e.g., 5.57 for ZnO, can serve the dual purposes of adjusting the weight of the golf ball in addition to acting as an activation agent.
U.S. Pat. No. 4,546,980 to Gendreau et al. discloses a more recent development in cross-linking golf ball core compositions—the use of two free radical initiators, each with a substantially different reactivity or half-life at the same temperature, to yield golf balls with excellent durability and high initial velocity. This reference discloses the use of two initiators to cure the core composition, the half-life of one initiator being preferably three times longer than the other initiator and, more preferably, six times longer.
However, a difficulty encountered in all of the prior art cross-linking of elastomeric polymers with peroxides or mixed peroxides is the rapid increase in viscosity caused by the increase in polymer molecular weight which occurs during cross-linking. When the viscosity becomes too high, the partially cross-linked polymer cannot flow rapidly and does not completely fill the mold containing it. This phenomenon, known as scorch, results from premature cross-linking which may occur during compounding or manufacturing.
Scorch is accentuated by processing conditions that include high temperatures and/or high shear rates. For example, in an injection molding process, the elastomeric polymer and peroxide composition must be exposed to high shear rates as the composition flows rapidly through the injection nozzle, runners and gates on its pathway to the mold and to high temperatures, which are required to keep the composition fluid until it reaches the mold. If scorch occurs, the surface of the resulting molded object, e.g., a golf ball core, will be irregular and the composition may solidify in the runners leading to the mold, thereby unfavorably impacting the efficiency, scrap rate and safety of the process. High shear rates combined with high temperatures also occur in other common golf ball composition processing methods, such as in roll milling and extrusion.
One way to minimize scorch, i.e., increase the time to the onset of scorch, or scorch time, is to increase the half-life, i.e., decrease the rate of decomposition, of the peroxide initiator by lowering the temperature or by choosing a peroxide with a different chemical structure. However, this approach is generally unsatisfactory since longer half-lives result in a slower rate of cross-linking and unsatisfactory long cure times in the mold.
Moreover, a short scorch time increases the occurrence of backrinding. Backrinding describes the undesirable torn or gouged appearance of cross-linked articles at a mold parting line. Backrinding is caused by the continuing thermal expansion of an elastomeric polymer at the parting line in a mold after cross-linking occurs. This expansion forces cross-linked polymer into the opening at the parting line and causes the cross-linked polymer to rupture. Additionally, when a material that is being cross-linked is compressed under high pressures and forced to elongate and flow, backrinding is evidenced as the ripping and breaking occurring along molding seam.
Both molded part geometry and elastomeric polymer composition are known t
Gosetti Steven M.
Ladd Derek A.
Acushnet Company
Buttner David J.
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