Patent Publication Number: US-2021187357-A1

Title: Method for forming high molecular weight thermoplastic polyurethane covers for golf balls

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to golf balls having covers made of thermoplastic polyurethane compositions. The golf ball includes an inner core and surrounding thermoplastic polyurethane outer cover. Multi-piece golf balls having outer cores, inner covers, and intermediate layers can be made. The invention includes methods for increasing the molecular weight of the thermoplastic polyurethane composition used to make the cover. The invention also encompasses the resulting balls. The finished balls with thermoplastic polyurethane covers have many advantageous physical and playing performance properties. 
     Brief Review of the Related Art 
     Both professional and amateur golfer use multi-piece, solid golf balls today. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene. The cover surrounds the inner core and may be made of a variety of materials including ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas. 
     Three-piece, four-piece, and even five-piece balls have become more popular over the years. More golfers are playing with these multi-piece balls for several reasons including new manufacturing technologies, lower material costs, and desirable ball playing performance properties. Many golf balls used today have multi-layered cores comprising an inner core and at least one surrounding outer core layer. For example, the inner core may be made of a relatively soft and resilient material, while the outer core may be made of a harder and more rigid material. The “dual-core” sub-assembly is encapsulated by a single or multi-layered cover to provide a final ball assembly. Different materials are used in these golf ball constructions to impart specific properties and playing features to the ball. 
     For instance, in recent years, there has been high interest in using polyurethane compositions to make golf ball covers. Basically, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with hydroxyl-terminated and amine curing agents. 
     Both thermoplastic and thermosetting polyurethanes are used to form golf ball covers. Thermoplastic polyurethanes have minimal cross-linking; any bonding in the polymer network is primarily through hydrogen bonding or other physical mechanism. Because of their lower level of cross-linking, thermoplastic polyurethanes are relatively flexible. The cross-linking bonds in thermoplastic polyurethanes can be reversibly broken by increasing temperature such as during molding or extrusion. That is, the thermoplastic material softens when exposed to heat and returns to its original condition when cooled. On the other hand, thermoset polyurethanes become irreversibly set when they are cured. The cross-linking bonds are irreversibly set and are not broken when exposed to heat. Thus, thermoset polyurethanes, which typically have a high level of cross-linking, are relatively rigid. 
     One advantage with using thermoplastic polyurethane compositions to form golf ball covers is that they have good processability. The thermoplastic polyurethanes generally have good melt-flow properties and different molding methods may be used to form the covers. Although thermoplastic polyurethane covers for golf balls have been used over the years, there are drawbacks with using some thermoplastic polyurethanes materials. For example, one drawback with some thermoplastic polyurethanes is they may not be as durable and tough as other polymers. For example, the resulting thermoplastic polyurethane cover may not have high mechanical strength, impact durability, and cut and scuff-resistance and shear-resistance. 
     Thus, it would be desirable to have method for improving the durability and strength of the thermoplastic polyurethane polymer. The present invention provides such a method. In one embodiment, the molecular weight of the thermoplastic polyurethane is increased. The resulting thermoplastic polyurethane composition can be used to form outer covers for golf balls having improved shear-resistance. 
     The present invention provides new methods for making thermoplastic polyurethane covers for golf balls having many advantageous features and benefits. The invention also includes the resulting golf balls having good physical and playing performance properties. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to golf balls having covers made of thermoplastic polyurethane compositions. The invention includes cast molding methods for increasing the molecular weight of the thermoplastic polyurethane composition used to make the cover ultimately a golf ball cover having high shear and cut-resistance. In one embodiment, a method for forming a cover layer for a golf ball is provided, wherein the method comprises the steps of: i) providing a golf ball sub-assembly comprising at least one core layer; ii) providing a lower and upper mold cavity, each mold cavity having an arcuate inner surface defining an inverted dimple pattern; iii) dispensing a liquid mixture comprising a reactive thermoplastic polyurethane prepolymer and chain-extender into the lower and upper mold cavities; iv) placing the core into the lower or upper mold cavity containing the liquid mixture; v) bringing the lower and upper mold cavities together under sufficient pressure so the liquid mixture reacts and forms a thermoplastic polyurethane outer cover layer, wherein the molecular weight of the thermoplastic polyurethane is sufficient to form a cover layer having a shear-durability rating of at least 3.0; and vi) detaching the mold cavities and removing the golf ball from the mold. 
     The thermoplastic polyurethane prepolymer can be prepared by mixing a reactive composition comprising polyisocyanate and polyol. A preferred chain extender used to form the polyurethane prepolymer is 1,4-butanediol. In one embodiment, the reactive composition further comprises a catalyst. In the present invention, the reactive composition preferably contains no catalyst, or a minimal amount of catalyst. For example, the reactive composition can contain 0.01 to about 0.05% by weight catalyst, preferably about 0.01 to about 0.025% catalyst. Suitable catalysts include those selected from the group consisting of dibutyl tin dilaurate, dibutyl tin acetylacetonate, dibutyl tin dibutoxide, dibutyl tin sulphide, dibutyl tin di-2-ethylhexanoate, dibutyl tin (IV) diacetate, dialkyltin (IV) oxide, tributyl tin laurylmercaptate, dibutyl tin dichloride, organo lead, tetrabutyl titanate, tertiary amines, mercaptides, stannous octoate, potassium octoate, zinc octoate, diazo compounds, and potassium acetate, and mixtures thereof. In another embodiment, the polyurethane prepolymer/chain extender liquid mixture further comprises a catalyst. In the present invention, the liquid mixture preferably contains no catalyst, or a minimal amount of catalyst. For example, the liquid mixture can contain 0.01 to about 0.05% by weight catalyst, preferably about 0.01 to about 0.025%. The liquid mixture can also contain additives such as an ultraviolet (UV) light stabilizer. 
     Different polyisocyanates, for example, aliphatic and aromatic diisocyanates, can be used. Suitable aliphatic diisocyanates include those selected from the group consisting of isophorone diisocyanate; 1,6-hexamethylene diisocyanate; 4,4′-dicyclohexylmethane diisocyanate; meta-tetramethylxylyene diisocyanate; trans-cyclohexane diisocyanate; and homopolymers and copolymers and blends thereof. Suitable aromatic diisocyanates include those selected from the group consisting of 4,4′-methylene diphenyl diisocyanate; 2,4′-methylene diphenyl diisocyanate; toluene 2,4-diisocyanate; toluene 2,6-diisocyanate; p-phenylene diisocyanate; and homopolymers and copolymers and blends thereof. 
     In one preferred embodiment, the ball sub-assembly comprises at least one core layer and a surrounding intermediate layer. For example, intermediate layer can be formed from a composition comprising an acid copolymer of ethylene and an α,β-unsaturated carboxylic acid, optionally including a softening monomer selected from the group consisting of alkyl acrylates and methacrylates. In one preferred embodiment, the intermediate layer has a Shore D midpoint hardness in the range of about 55 to about 75; and the outer cover layer has a Shore D outer surface hardness in the range of about 15 to about 60, and wherein the outer surface hardness of the outer cover layer is less than the midpoint hardness of the intermediate layer. The present invention also encompasses golf balls produced by the above-described cast-molding methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features that are characteristic of the present invention are set forth in the appended claims. However, the preferred embodiments of the invention, together with further objects and attendant advantages, are best understood by reference to the following detailed description in connection with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of upper and lower mold cavities that can be used to make the golf ball covers in accordance with the present invention; 
         FIG. 2  is a planar view of the lower mold cavity shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a four-piece golf ball having a dual-layered core; intermediate layer; and surrounding cover made in accordance with the present invention; 
         FIG. 4  is a cross-sectional view of a three-piece golf ball having a dual-layered core and surrounding cover made in accordance with the present invention; and 
         FIG. 5  is a perspective view of a finished golf ball having a dimpled cover made in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates generally to new methods for producing golf balls having a cover layer, particularly thermoplastic polyurethane cover layers. These methods help improve the durability and strength of the thermoplastic polyurethane polymer. In one embodiment, the molecular weight of the thermoplastic polyurethane is increased. The resulting thermoplastic polyurethane composition can be used to form outer covers for golf balls having improved shear-resistance. The resulting golf balls also have high resiliency and a soft feel. 
     Referring to the Figures, where like reference numerals are used to designate like elements, and particularly  FIG. 1 , a golf ball mold ( 10 ) used to form a traditional cover layer over a core (or ball subassembly) is generally shown. The mold ( 10 ) includes hemispherical mold cavities ( 12 ) and ( 14 ) having interior dimple patterns ( 12   a ) and ( 14   a ). When the mold cavities ( 12 ,  14 ) are mated, they define an interior spherical cavity ( 16 ) to form the cover for the ball. The mold cavities ( 12 ,  14 ) are mated together along a parting line ( 17 ) that creates an equator or seam for the finished ball. In recent years, mold cavities with non-planar mating surfaces have been used to create a golf balls having a staggered parting line. For example, Nardacci et al., U.S. Pat. No. 7,618,333 discloses a method for making golf balls having a staggered parting line. The upper and lower mold cavities have non-planar mating surfaces. When the cavities are mated, the parting line follows the dimple outline pattern and allows the dimple outline pattern of one mold cavity to interdigitate with the dimple outline pattern of the mating mold cavity, thereby forming a golf ball without an obvious parting line. In  FIG. 2 , the mold cavity ( 14 ) is shown in further detail. The mold cavity ( 14 ) includes a dimple pattern ( 14   a ) and locator slot ( 18 ) that fits over a locator pin on a mold frame (not shown) when the mold cavity ( 14 ) is inserted into the frame. 
     Polyurethane Composition 
     The golf balls of this invention include an outer cover layer preferably made of a thermoplastic polyurethane composition. In general, polyurethanes contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). The polyurethanes are produced by the reaction of a multi-functional isocyanate (NCO—R—NCO) with a long-chain polyol having terminal hydroxyl groups (OH—OH) in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with short-chain diols (OH—R′—OH). The resulting polyurethane has elastomeric properties because of its “hard” and “soft” segments, which are covalently bonded together. This phase separation occurs because the mainly non-polar, low melting soft segments are incompatible with the polar, high melting hard segments. The hard segments, which are formed by the reaction of the diisocyanate and low molecular weight chain-extending diol, are relatively stiff and immobile. The soft segments, which are formed by the reaction of the diisocyanate and long chain diol, are relatively flexible and mobile. Because the hard segments are covalently coupled to the soft segments, they inhibit plastic flow of the polymer chains, thus creating elastomeric resiliency. 
     By the term, “isocyanate compound” as used herein, it is meant any aliphatic or aromatic isocyanate containing two or more isocyanate functional groups. The isocyanate compounds can be monomers or monomeric units, because they can be polymerized to produce polymeric isocyanates containing two or more monomeric isocyanate repeat units. The isocyanate compound may have any suitable backbone chain structure including saturated or unsaturated, and linear, branched, or cyclic. By the term, “polyamine” as used herein, it is meant any aliphatic or aromatic compound containing two or more primary or secondary amine functional groups. The polyamine compound may have any suitable backbone chain structure including saturated or unsaturated, and linear, branched, or cyclic. The term “polyamine” may be used interchangeably with amine-terminated component. By the term, “polyol” as used herein, it is meant any aliphatic or aromatic compound containing two or more hydroxyl functional groups. The term “polyol” may be used interchangeably with hydroxy-terminated component. 
     Thermoplastic polyurethanes have minimal cross-linking; any bonding in the polymer network is primarily through hydrogen bonding or other physical mechanism. Because of their lower level of cross-linking, thermoplastic polyurethanes are relatively flexible. The cross-linking bonds in thermoplastic polyurethanes can be reversibly broken by increasing temperature such as during molding or extrusion. That is, the theremoplastic material softens when exposed to heat and returns to its original condition when cooled. On the other hand, thermoset polyurethanes become irreversibly set when they are cured. The cross-linking bonds are irreversibly set and are not broken when exposed to heat. Thus, thermoset polyurethanes, which typically have a high level of cross-linking, are relatively rigid. 
     Aromatic polyurethanes can be prepared in accordance with this invention and these materials are preferably formed by reacting an aromatic diisocyanate with a polyol. Suitable aromatic diisocyanates that may be used in accordance with this invention include, for example, toluene 2,4-diisocyanate (TDI), toluene 2,6-diisocyanate (TDI), 4,4′-methylene diphenyl diisocyanate (MDI), 2,4′-methylene diphenyl diisocyanate (MDI), polymeric methylene diphenyl diisocyanate (PMDI), p-phenylene diisocyanate (PPDI), m-phenylene diisocyanate (PDI), naphthalene 1,5-diisocynate (NDI), naphthalene 2,4-diisocyanate (NDI), p-xylene diisocyanate (XDI), and homopolymers and copolymers and blends thereof. The aromatic isocyanates are able to react with the hydroxyl or amine compounds and form a durable and tough polymer having a high melting point. The resulting polyurethane generally has good mechanical strength and cut/shear-resistance. 
     Aliphatic polyurethanes also can be prepared in accordance with this invention and these materials are preferably formed by reacting an aliphatic diisocyanate with a polyol. Suitable aliphatic diisocyanates that may be used in accordance with this invention include, for example, isophorone diisocyanate (IPDI), 1,6-hexamethylene diisocyanate (HDI), 4,4′-dicyclohexylmethane diisocyanate (“H 12  MDI”), meta-tetramethylxylyene diisocyanate (TMXDI), trans-cyclohexane diisocyanate (CHDI), and homopolymers and copolymers and blends thereof. Particularly suitable multi-functional isocyanates include trimers of HDI or H 12  MDI, oligomers, or other derivatives thereof. The resulting polyurethane generally has good light and thermal stability. 
     Any polyol available to one of ordinary skill in the art is suitable for use according to the invention. Exemplary polyols include, but are not limited to, polyether polyols, hydroxy-terminated polybutadiene (including partially/fully hydrogenated derivatives), polyester polyols, polycaprolactone polyols, and polycarbonate polyols. In one preferred embodiment, the polyol includes polyether polyol. Examples include, but are not limited to, polytetramethylene ether glycol (PTMEG) which is particularly preferred, polyethylene propylene glycol, polyoxypropylene glycol, and mixtures thereof. The hydrocarbon chain can have saturated or unsaturated bonds and substituted or unsubstituted aromatic and cyclic groups. 
     In another embodiment, polyester polyols are included in the polyurethane material. Suitable polyester polyols include, but are not limited to, polyethylene adipate glycol; polybutylene adipate glycol; polyethylene propylene adipate glycol; o-phthalate-1,6-hexanediol; poly(hexamethylene adipate) glycol; and mixtures thereof. The hydrocarbon chain can have saturated or unsaturated bonds, or substituted or unsubstituted aromatic and cyclic groups. In still another embodiment, polycaprolactone polyols are included in the materials of the invention. Suitable polycaprolactone polyols include, but are not limited to: 1,6-hexanediol-initiated polycaprolactone, diethylene glycol initiated polycaprolactone, trimethylol propane initiated polycaprolactone, neopentyl glycol initiated polycaprolactone, 1,4-butanediol-initiated polycaprolactone, and mixtures thereof. The hydrocarbon chain can have saturated or unsaturated bonds, or substituted or unsubstituted aromatic and cyclic groups. In yet another embodiment, polycarbonate polyols are included in the polyurethane material of the invention. Suitable polycarbonates include, but are not limited to, polyphthalate carbonate and poly(hexamethylene carbonate) glycol. The hydrocarbon chain can have saturated or unsaturated bonds, or substituted or unsubstituted aromatic and cyclic groups. In one embodiment, the molecular weight of the polyol is from about 200 to about 4000. 
     There are two basic techniques that can be used to make the polyurethanes: a) one-shot technique, and b) prepolymer technique. In the one-shot technique, the diisocyanate, polyol, and hydroxyl-terminated chain-extender (curing agent) are reacted in one step. On the other hand, the prepolymer technique involves a first reaction between the diisocyanate and polyol compounds to produce a polyurethane prepolymer, and a subsequent reaction between the prepolymer and hydroxyl-terminated chain-extender. As a result of the reaction between the isocyanate and polyol compounds, there will be some unreacted NCO groups in the polyurethane prepolymer. The prepolymer should have less than 14% unreacted NCO groups. Preferably, the prepolymer has no greater than 8.5% unreacted NCO groups, more preferably from 2.5% to 8%, and most preferably from 5.0% to 8.0% unreacted NCO groups. As the weight percent of unreacted isocyanate groups increases, the hardness of the composition also generally increases. 
     Either the one-shot or prepolymer method may be employed to produce the polyurethane compositions of the invention. In one embodiment, the one-shot method is used, wherein the isocyanate compound is added to a reaction vessel and then a curative mixture comprising the polyol and curing agent is added to the reaction vessel. The components are mixed together so that the molar ratio of isocyanate groups to hydroxyl groups is preferably in the range of about 1.00:1.00 to about 1.10:1.00. In a second embodiment, the prepolymer method is used. In general, the prepolymer technique is preferred because it provides better control of the chemical reaction. The prepolymer method provides a more homogeneous mixture resulting in a more consistent polymer composition. The one-shot method results in a mixture that is inhomogeneous (more random) and affords the manufacturer less control over the molecular structure of the resultant composition. 
     The polyurethane compositions can be formed by chain-extending the polyurethane prepolymer with a single chain-extender or blend of chain-extenders as described further below. As discussed above, the polyurethane prepolymer can be chain-extended by reacting it with a single chain-extender or blend of chain-extenders. In general, the prepolymer can be reacted with hydroxyl-terminated curing agents, amine-terminated curing agents, and mixtures thereof. The curing agents extend the chain length of the prepolymer and build-up its molecular weight. In general, thermoplastic polyurethane compositions are typically formed by reacting the isocyanate blend and polyols at a 1:1 stoichiometric ratio. Thermoset compositions, on the other hand, are cross-linked polymers and are typically produced from the reaction of the isocyanate blend and polyols at normally a 1.05:1 stoichiometric ratio 
     A catalyst may be employed to promote the reaction between the isocyanate and polyol compounds for producing the prepolymer or between prepolymer and chain-extender during the chain-extending step. Preferably, the catalyst is added to the reactants before producing the prepolymer. Suitable catalysts include, but are not limited to, bismuth catalyst; zinc octoate; stannous octoate; tin catalysts such as bis-butyltin dilaurate, bis-butyltin diacetate, stannous octoate; tin (II) chloride, tin (IV) chloride, bis-butyltin dimethoxide, dimethyl-bis[1-oxonedecyl)oxy]stannane, di-n-octyltin bis-isooctyl mercaptoacetate; amine catalysts such as triethylenediamine, triethylamine, and tributylamine; organic acids such as oleic acid and acetic acid; delayed catalysts; and mixtures thereof. The catalyst is preferably added in an amount sufficient to catalyze the reaction of the components in the reactive mixture. In one embodiment, the catalyst is present in an amount from about 0.001 percent to about 1 percent, and preferably 0.1 to 0.5 percent, by weight of the composition. 
     The hydroxyl chain-extending (curing) agents are preferably selected from the group consisting of ethylene glycol; diethylene glycol; polyethylene glycol; propylene glycol; 2-methyl-1,3-propanediol; 2-methyl-1,4-butanediol; monoethanolamine; diethanolamine; triethanolamine; monoisopropanolamine; diisopropanolamine; dipropylene glycol; polypropylene glycol; 1,2-butanediol; 1,3-butanediol; 1,4-butanediol; 2,3-butanediol; 2,3-dimethyl-2,3-butanediol; trimethylolpropane; cyclohexyldimethylol; triisopropanolamine; N,N,N′,N′-tetra-(2-hydroxypropyl)-ethylene diamine; diethylene glycol bis-(aminopropyl) ether; 1,5-pentanediol; 1,6-hexanediol; 1,3-bis-(2-hydroxyethoxy) cyclohexane; 1,4-cyclohexyldimethylol; 1,3-bis -[2-(2-hydroxyethoxy) ethoxy] cyclohexane; 1,3-bis-{2-[2-(2-hydroxyethoxy) ethoxy]ethoxy}cyclohexane; trimethylolpropane; polytetramethylene ether glycol (PTMEG), preferably having a molecular weight from about 250 to about 3900; and mixtures thereof. 
     Suitable amine chain-extending (curing) agents that can be used in chain-extending the polyurethane prepolymer include, but are not limited to, unsaturated diamines such as 4,4′-diamino-diphenylmethane (i.e., 4,4′-methylene-dianiline or “MDA”), m-phenylenediamine, p-phenylenediamine, 1,2- or 1,4-bis(sec-butylamino)benzene, 3,5-diethyl-(2,4- or 2,6-) toluenediamine or “DETDA”, 3,5-dimethylthio-(2,4- or 2,6-)toluenediamine, 3,5-diethylthio-(2,4- or 2,6-)toluenediamine, 3,3′-dimethyl-4,4′-diamino-diphenylmethane, 3,3′-diethyl-5,5′-dimethyl4,4′-diamino-diphenylmethane (i.e., 4,4′-methylene-bis(2-ethyl-6-methyl-benezeneamine)), 3,3′-dichloro-4,4′-diamino-diphenylmethane (i.e., 4,4′-methylene-bis(2-chloroaniline) or “MOCA”), 3,3′,5,5′-tetraethyl-4,4′-diamino-diphenylmethane (i.e., 4,4′-methylene-bis(2,6-diethylaniline), 2,2′-dichloro-3,3′,5,5′-tetraethyl-4,4′-diamino-diphenylmethane (i.e., 4,4′-methylene-bis(3-chloro-2,6-diethyleneaniline) or “MCDEA”), 3,3′-diethyl-5,5′-dichloro-4,4′-diamino-diphenylmethane, or “MDEA”), 3,3′-dichloro-2,2′,6,6′-tetraethyl-4,4′-diamino-diphenylmethane, 3,3′-dichloro-4,4′-diamino-diphenylmethane, 4,4′-methylene-bis(2,3-dichloroaniline) (i.e., 2,2′,3,3′-tetrachloro-4,4′-diamino-diphenylmethane or “MDCA”); and mixtures thereof. One particularly suitable amine-terminated chain-extending agent is Ethacure 300™ (dimethylthiotoluenediamine or a mixture of 2,6-diamino-3,5-dimethylthiotoluene and 2,4-diamino-3,5-dimethylthiotoluene.) The amine curing agents used as chain extenders normally have a cyclic structure and a low molecular weight (250 or less). 
     When the polyurethane prepolymer is reacted with hydroxyl-terminated curing agents during the chain-extending step, as described above, the resulting polyurethane composition contains urethane linkages. On the other hand, when the polyurethane prepolymer is reacted with amine-terminated curing agents during the chain-extending step, any excess isocyanate groups in the prepolymer will react with the amine groups in the curing agent. The resulting polyurethane composition contains urethane and urea linkages and may be referred to as a polyurethane/urea hybrid. The concentration of urethane and urea linkages in the hybrid composition may vary. In general, the hybrid composition may contain a mixture of about 10 to 90% urethane and about 90 to 10% urea linkages. 
     More particularly, when the polyurethane prepolymer is reacted with hydroxyl-terminated curing agents during the chain-extending step, as described above, the resulting composition is essentially a pure polyurethane composition containing urethane linkages having the following general structure: 
     
       
         
         
             
             
         
       
     
     where x is the chain length, i.e., about 1 or greater, and R and R 1  are straight chain or branched hydrocarbon chain having about 1 to about 20 carbons. 
     However, when the polyurethane prepolymer is reacted with an amine-terminated curing agent during the chain-extending step, any excess isocyanate groups in the prepolymer will react with the amine groups in the curing agent and create urea linkages having the following general structure: 
     
       
         
         
             
             
         
       
     
     where x is the chain length, i.e., about 1 or greater, and R and R 1  are straight chain or branched hydrocarbon chain having about 1 to about 20 carbons. 
     The polyurethane compositions used to form the cover layer may contain other polymer materials including, for example: aliphatic or aromatic polyurethanes, aliphatic or aromatic polyureas, aliphatic or aromatic polyurethane/urea hybrids, olefin-based copolymer ionomer compositions, polyethylene, including, for example, low density polyethylene, linear low density polyethylene, and high density polyethylene; polypropylene; rubber-toughened olefin polymers; acid copolymers, for example, poly(meth)acrylic acid, which do not become part of an ionomeric copolymer; plastomers; flexomers; styrene/butadiene/styrene block copolymers; styrene/ethylene-butylene/styrene block copolymers; dynamically vulcanized elastomers; copolymers of ethylene and vinyl acetates; copolymers of ethylene and methyl acrylates; polyvinyl chloride resins; polyamides, poly(amide-ester) elastomers, and graft copolymers of ionomer and polyamide including, for example, Pebax® thermoplastic polyether block amides, available from Arkema Inc; cross-linked trans-polyisoprene and blends thereof; polyester-based thermoplastic elastomers, such as Hytrel®, available from DuPont; polyurethane-based thermoplastic elastomers, such as Elastollan®, available from BASF; polycarbonate/polyester blends such as Xylex®, available from SABIC Innovative Plastics; maleic anhydride-grafted polymers such as Fusabone , available from DuPont; and mixtures of the foregoing materials. 
     In addition, the polyurethane compositions may contain fillers, additives, and other ingredients that do not detract from the properties of the final composition. These additional materials include, but are not limited to, catalysts, wetting agents, coloring agents, optical brighteners, cross-linking agents, whitening agents such as titanium dioxide and zinc oxide, ultraviolet (UV) light absorbers, hindered amine light stabilizers, defoaming agents, processing aids, surfactants, and other conventional additives. Other suitable additives include antioxidants, stabilizers, softening agents, plasticizers, including internal and external plasticizers, impact modifiers, foaming agents, density-adjusting fillers, reinforcing materials, compatibilizers, and the like. Some examples of useful fillers include zinc oxide, zinc sulfate, barium carbonate, barium sulfate, calcium oxide, calcium carbonate, clay, tungsten, tungsten carbide, silica, and mixtures thereof. Rubber regrind (recycled core material) and polymeric, ceramic, metal, and glass microspheres also may be used. Generally, the additives will be present in the composition in an amount between about 1 and about 70 weight percent based on total weight of the composition depending upon the desired properties. 
     Molding Method 
     The cover may be formed around the golf ball sub-assembly by dispensing polymeric material into the mold cavities and mating them together under sufficient heat and pressure. By the term, “sub-assembly” as used herein, it is meant the inner ball, that is the core and any intermediate layer(s) disposed between the core and outer cover layer. The core and intermediate layers are described in further detail below. 
     As discussed above, in the present invention, preferably a polyurethane composition is used to form the outer cover of the golf ball. The polyurethane composition is in generally liquid form so that it can be dispensed into the mold cavities and molded over the golf ball sub-assembly. The molding process of this invention is suitable for making thin outer cover layers. Particularly, covers having a thickness of less than 0.05 inches can be made, more preferably in the range of 0.015 to 0.045 inches. Castable polyurethanes, polyureas, and copolymers, hybrids, and mixtures of polyurethanes-polyureas are of particular interest, because these materials can be used to make a golf ball having high resiliency and a soft feel. 
     However, it is recognized that materials, other than polyurethanes, can be used to form the cover layer in accordance with the present invention. For example, olefin-based copolymer ionomer resins (for example, Surlyn® ionomer resins and DuPont HPF® 1000 and HPF® 2000, commercially available from E. I. du Pont de Nemours and Company; lotek® ionomers, commercially available from ExxonMobil Chemical Company; Amplify® IO ionomers of ethylene acrylic acid copolymers, commercially available from The Dow Chemical Company; and Clarix® ionomer resins, commercially available from A. Schulman Inc.); polyethylene, including, for example, low density polyethylene, linear low density polyethylene, and high density polyethylene; polypropylene; rubber-toughened olefin polymers; acid copolymers, for example, poly(meth)acrylic acid, which do not become part of an ionomeric copolymer; plastomers; flexomers; styrene/butadiene/styrene block copolymers; styrene/ethylene-butylene/styrene block copolymers; dynamically vulcanized elastomers; copolymers of ethylene and vinyl acetates; copolymers of ethylene and methyl acrylates; polyvinyl chloride resins; polyamides, poly(amide-ester) elastomers, and graft copolymers of ionomer and polyamide including, for example, Pebax® thermoplastic polyether block amides, commercially available from Arkema Inc; cross-linked trans-polyisoprene and blends thereof; polyester-based thermoplastic elastomers, such as Hytrel®, commercially available from E. I. du Pont de Nemours and Company; polyurethane-based thermoplastic elastomers, such as Elastollan®, commercially available from BASF; synthetic or natural vulcanized rubber; and combinations thereof. 
     In one embodiment, a polyurethane prepolymer and curing agent can be mixed in a motorized mixer inside a mixing head by metering amounts of the curative and prepolymer through the feed lines. The preheated lower mold cavities can be filled with the reactive polyurethane and curing agent mixture. Likewise, the preheated upper mold cavities can be filled with the reactive mixture. The lower and upper mold cavities are filled with substantially the same amount of reactive mixture. After the reactive mixture has resided in the lower mold cavities for a sufficient time period, typically about 40 to about 100 seconds, the golf ball subassembly can be lowered at a controlled speed into the reacting mixture. Ball cups can hold the subassemblies by applying reduced pressure (or partial vacuum). After sufficient gelling (typically about 4 to about 12 seconds), the vacuum can be removed and the subassembly can be released. Then, the upper half-molds can be mated with the lower half-molds. An exothermic reaction occurs when the polyurethane prepolymer and curing agent are mixed and this continues until the material solidifies around the subassembly. The molded balls can then be cooled in the mold and removed when the molded cover layer is hard enough to be handled without deforming. This molding technique is described in the above-mentioned Hebert et al., U.S. Pat. No. 6,132,324 along with Wu, U.S. Pat. No. 5,334,673 and Brown et al., U.S. Pat. No. 5,006,297, the disclosures of which are hereby incorporated by reference. 
     Prior to forming the cover layer, the ball subassembly may be surface-treated to increase the adhesion between its outer surface and cover material. Examples of such surface-treatment may include mechanically or chemically abrading the outer surface of the subassembly. Additionally, the subassembly may be subjected to corona discharge, plasma treatment, silane dipping, or other chemical treatment methods known to those of ordinary skill in the art prior to forming the cover around it. Other layers of the ball, for example, the core and cover layers, also may be surface-treated. Examples of these and other surface-treatment techniques can be found in U.S. Pat. No. 6,315,915, the disclosure of which is hereby incorporated by reference. 
     A dispensing process as described in US. Pat. Nos. 7,655,171; 7,490,975; and 7,246,937, the disclosures of which are hereby incorporated by reference, can be used in accordance with the present invention. This process involves pumping the reactive polyurethane components into a mixer body and mixing them together with a dynamic mixer element. The components are heated to a temperature in the range of about 150° F. to about 350° F. as the components flow through a dispensing port, which dispenses the components into the lower and upper half-molds. The dispensing port moves into and out of the mold cavity by pneumatic pressure so the components are deposited uniformly into the half-molds. 
     In another embodiment, a conveyor belt system can be used for transporting the mold frames as described in co-assigned, co-pending, U.S. patent application Ser. No. 12/614,814, the disclosure of which is hereby incorporated by reference. In this system, the lower and upper frame plates containing the mold cavities are pre-heated to a temperature in the range of about 140° to about 165° F. Dispensing ports are used to inject the polyurethane mixture into the mold cavities. The upper mold frame plates containing the upper mold cavities are fed to a golf ball sub-assembly supply station, where the ball sub-assemblies are introduced into the cavities. The lower mold frame plates containing the lower mold cavities continue moving forward on the main conveyor belt line. At the next station, the upper and lower mold frame plates are fastened together. 
     At the assembly head station, the upper and lower frame plates are clamped together, preferably by bolts which are threaded through bores under pressure normally between about 400 to about 600 psi. After the mold frame is assembled, the frame is fed back to the main conveyor belt and carried to a curing tunnel. 
     Automated flippers grab the mold frames and reorient them so they stand in a vertical position prior to being introduced into the curing tunnel. This allows the system to maximize conveyor space and also achieve a higher degree of curing thermodynamics. The mold frames are then carried into the curing tunnel. Upon exiting the curing tunnel, the mold frames are pre-cooled on a meshed conveyor belt to allow directed air flow evenly over both upper and lower frame plates simultaneously. Then, the molds are fed into a tipping station, wherein they are reoriented to a horizontal position. The mold frames then are carried through a high efficiency chiller equipped with fans operated by zone control. After the mold frames have been chilled, the balls are de-molded by an automated in-line disassembly machine and then moved to a ball removal machine which automatically picks the golf balls out of the mold halves for further processing. 
     After the golf balls have been removed from the mold, they may be subjected to finishing steps such as flash trimming, surface-treatment, marking, coating, and the like using techniques known in the art. 
     Core and Intermediate Layers 
     As discussed above, the core and intermediate layer(s), if any are present, constitute the sub-assembly of the ball or inner ball which is encapsulated by the cover material. The core and intermediate layers may be made of a wide variety of thermoset and thermoplastic materials. 
     Preferably, the core is made of a thermoset rubber composition. Suitable thermoset rubber materials that may be used to form the inner core include, but are not limited to, polybutadiene, polyisoprene, ethylene propylene rubber (“EPR”), ethylene-propylene-diene (“EPDM”) rubber, styrene-butadiene rubber, styrenic block copolymer rubbers (such as “SI”, “SIS”, “SB”, “SBS”, “SIBS”, and the like, where “S” is styrene, “I” is isobutylene, and “B” is butadiene), polyalkenamers such as, for example, polyoctenamer, butyl rubber, halobutyl rubber, polystyrene elastomers, polyethylene elastomers, polyurethane elastomers, polyurea elastomers, metallocene-catalyzed elastomers and plastomers, copolymers of isobutylene and p-alkylstyrene, halogenated copolymers of isobutylene and p-alkylstyrene, copolymers of butadiene with acrylonitrile, polychloroprene, alkyl acrylate rubber, chlorinated isoprene rubber, acrylonitrile chlorinated isoprene rubber, and blends of two or more thereof. More preferably, the inner core is formed from a polybutadiene rubber composition. 
     The thermoset rubber composition may be cured using conventional curing processes. Suitable curing processes include, for example, peroxide-curing, sulfur-curing, high-energy radiation, and combinations thereof. Radical scavengers such as a halogenated organosulfur, organic disulfide, or inorganic disulfide compounds may be added to the rubber composition. These compounds also may function as “soft and fast agents.” The rubber composition also may include filler(s) such as materials selected from carbon black, clay and nanoclay particles, talc (e.g., Luzenac HAR® high aspect ratio talcs, commercially available from Luzenac America, Inc.), glass (e.g., glass flake, milled glass, and microglass), mica and mica-based pigments (e.g., Iriodin® pearl luster pigments, commercially available from The Merck Group), and combinations thereof. In addition, the rubber compositions may include antioxidants. Also, processing aids such as high molecular weight organic acids and salts thereof may be added to the composition. In another embodiment, foaming (blowing) agents are added to the rubber composition and the rubber composition is foamed. 
     One or more intermediate layers can be molded over the inner core. These intermediate layers also can be referred to as outer core, casing, or inner cover layers. In one embodiment, as described above, the intermediate layer is made of a second thermoset rubber composition. Thus, a dual-layered core having a first layer made of a thermoset rubber and a second layer made of a thermoset rubber can be made. A cover composition can be molded over this ball sub-assembly in accordance with this invention. In another embodiment, a thermoplastic composition is used to form the intermediate layer. Thus, in this embodiment, a dual-layered core having a first layer made of a thermoset rubber and a second layer made of a thermoplastic composition is made. 
     Referring to  FIG. 3 , one version of a four-piece golf ball that can be made in accordance with this invention is generally indicated at ( 20 ). The ball ( 20 ) contains an inner core (center) ( 22 ) and surrounding intermediate layers ( 24 ) and ( 26 ), which also can be referred to as the outer core and inner cover layers, respectively. This ball sub-assembly is encapsulated by an outer cover ( 28 ) made in accordance with the molding methods of this invention. Referring to  FIG. 5 , in another version, a three-piece golf ball ( 30 ) contains an inner core (center) ( 32 ) and outer core layer ( 34 ). Thus, the core is dual-layered. This core sub-assembly is surrounded by a single-layered cover ( 36 ) made in accordance with the molding methods of this invention. 
     For example, the intermediate layer may be made from an ethylene acid copolymer ionomer composition. Suitable ionomer compositions include partially-neutralized ionomers and highly-neutralized ionomers (HNPs), including ionomers formed from blends of two or more partially-neutralized ionomers, blends of two or more highly-neutralized ionomers, and blends of one or more partially-neutralized ionomers with one or more highly-neutralized ionomers. For purposes of the present disclosure, “HNP” refers to an acid copolymer after at least 70% of all acid groups present in the composition are neutralized. The composition used to make the intermediate layer can include additives, for example, fillers, cross-linking agents, chain extenders, surfactants, dyes and pigments, coloring agents, fluorescent agents, adsorbents, stabilizers, softening agents, impact modifiers, antioxidants, antiozonants, and the like. In another embodiment, foaming (blowing) agents are added to the thermoplastic or thermoset composition used to make the inner cover layer, and the composition is foamed. 
     Preferred ionomers are salts of O/X- and O/X/Y-type acid copolymers, wherein O is an α-olefin, X is a C 3 -C 8  α,β-ethylenically unsaturated carboxylic acid, and Y is a softening monomer. O is preferably selected from ethylene and propylene. X is preferably selected from methacrylic acid, acrylic acid, ethacrylic acid, crotonic acid, and itaconic acid. Methacrylic acid and acrylic acid are particularly preferred. Y is preferably selected from (meth) acrylate and alkyl (meth) acrylates wherein the alkyl groups have from 1 to 8 carbon atoms, including, but not limited to, n-butyl (meth) acrylate, isobutyl (meth) acrylate, methyl (meth) acrylate, and ethyl (meth) acrylate. 
     Preferred O/X and O/X/Y-type copolymers include, without limitation, ethylene acid copolymers, such as ethylene/(meth)acrylic acid, ethylene/(meth)acrylic acid/maleic anhydride, ethylene/(meth)acrylic acid/maleic acid mono-ester, ethylene/maleic acid, ethylene/maleic acid mono-ester, ethylene/(meth)acrylic acid/n-butyl (meth)acrylate, ethylene/(meth)acrylic acid/iso-butyl (meth)acrylate, ethylene/(meth)acrylic acid/methyl (meth)acrylate, ethylene/(meth)acrylic acid/ethyl (meth)acrylate terpolymers, and the like. The term, “copolymer,” as used herein, includes polymers having two types of monomers, those having three types of monomers, and those having more than three types of monomers. Preferred α, β-ethylenically unsaturated mono- or dicarboxylic acids are (meth) acrylic acid, ethacrylic acid, maleic acid, crotonic acid, fumaric acid, itaconic acid. (Meth) acrylic acid is most preferred. As used herein, “(meth) acrylic acid” means methacrylic acid and/or acrylic acid. Likewise, “(meth) acrylate” means methacrylate and/or acrylate. 
     Other suitable thermoplastic polymers that may be used to form the intermediate layer include, but are not limited to, the following polymers (including homopolymers, copolymers, and derivatives thereof); (a) polyesters, particularly those modified with a compatibilizing group such as sulfonate or phosphonate, including modified poly(ethylene terephthalate), modified poly(butylene terephthalate), modified poly(propylene terephthalate), modified poly(trimethylene terephthalate), modified poly(ethylene naphthenate), and those disclosed in U.S. Pat. Nos. 6,353,050, 6,274,298, and 6,001,930, the entire disclosures of which are hereby incorporated herein by reference, and blends of two or more thereof; (b) polyamides, polyamide-ethers, and polyamide-esters, and those disclosed in U.S. Pat. Nos. 6,187,864, 6,001,930, and 5,981,654, the entire disclosures of which are hereby incorporated herein by reference, and blends of two or more thereof; (c) polyurethanes, polyureas, polyurethane-polyurea hybrids, and blends of two or more thereof; (d) fluoropolymers, such as those disclosed in U.S. Pat. Nos. 5,691,066, 6,747,110 and 7,009,002, the entire disclosures of which are hereby incorporated herein by reference, and blends of two or more thereof; (e) polystyrenes, such as poly(styrene-co-maleic anhydride), acrylonitrile-butadiene-styrene, poly(styrene sulfonate), polyethylene styrene, and blends of two or more thereof; (f) polyvinyl chlorides and grafted polyvinyl chlorides, and blends of two or more thereof; (g) polycarbonates, blends of polycarbonate/acrylonitrile-butadiene-styrene, blends of polycarbonate/polyurethane, blends of polycarbonate/polyester, and blends of two or more thereof; (h) polyethers, such as polyarylene ethers, polyphenylene oxides, block copolymers of alkenyl aromatics with vinyl aromatics and polyamicesters, and blends of two or more thereof; (i) polyimides, polyetherketones, polyamideimides, and blends of two or more thereof; and (j) polycarbonate/polyester copolymers and blends. 
     In another embodiment, the intermediate layer is disposed about the inner core, wherein at least one of the inner core and intermediate layer comprises a foamed composition. For example, the inner core can comprise a foamed composition. In another example, the intermediate layer can comprise a foamed composition. In yet another example, both the inner core and intermediate layers can comprise a foamed composition. 
     Foamed thermoset and thermoplastic compositions can be used to form the inner core and/or intermediate layer. For example, thermoset compositions such as, for example, polybutadiene rubber, can be used. Also, thermoplastic polymers can be used, for example those selected from the group consisting of partially-neutralized ethylene acid copolymer ionomers; highly-neutralized ethylene acid copolymer ionomers; polyesters; polyamides; polyamide-ethers, polyamide-esters; polyurethanes, polyureas; fluoropolymers; polystyrenes; polypropylenes; polyethylenes; polyvinyl chlorides; polyvinyl acetates; polycarbonates; polyvinyl alcohols; polyester-ethers; polyethers; polyimides, polyetherketones, polyamideimides; and mixtures thereof. 
     The intermediate layer also may be referred to as a casing, mantle, or inner cover layer. In one example, a golf ball having inner and outer cover layers may be made. The multi-layered cover of the golf balls of this invention provide the ball with good impact durability, toughness, and wear-resistance. In general, the hardness and thickness of the different cover layers may vary depending upon the desired ball construction. 
     In one example, the inner cover layer hardness is about 15 Shore D or greater, more preferably about 25 Shore D or greater, and most preferably about 35 Shore D or greater. For example, the inner cover layer hardness may be in the range of about 15 to about 60 Shore D, and more preferably about 27 to about 48 Shore D. In another version, the inner cover layer hardness is about 50 Shore D or greater, preferably about 55 Shore D or greater, and most preferably about 60 Shore D or greater. For example, in one version, the inner cover has a Shore D hardness of about 55 to about 90 Shore D. In another embodiment, the inner cover has a Shore D hardness of about 60 to about 78 Shore D, and in yet another version, the inner cover has a Shore D hardness of about 64 to about 72 Shore D. More particularly, in one example, the inner cover has a hardness of about 65 Shore D or greater. The hardness of the inner cover layer is measured per the methods described further below. In addition, the thickness of the inner cover layer is preferably about 0.015 inches to about 0.100 inches, more preferably about 0.020 inches to about 0.080 inches, and most preferably about 0.030 inches to about 0.050 inches. Typically, the thickness of the inner cover is about 0.035 or 0.040 or 0.045 inches. 
     Concerning the outer cover layer, this layer may be relatively thin. The outer cover preferably has a thickness within a range having a lower limit of 0.004 or 0.006 or 0.008 and an upper limit of 0.010 or 0.020 or 0.030 or 0.040 inches. Preferably, the thickness of the outer cover is about 0.016 inches or less, more preferably 0.008 inches or less. The outer cover preferably has a material hardness of 80 Shore D or less, or 70 Shore D or less, or 60 Shore D or less, or 55 Shore D or less, or 50 Shore D or less, or 45 Shore D or less. In one example, the outer cover preferably has a Shore D hardness in the range of about 50 to about 80, more preferably about 55 to about 75. In another example, the outer cover preferably has a Shore D hardness in the range of about 10 to about 70, more preferably about 15 to about 60. The hardness of the inner and outer cover layers may be measured per the methods described below. 
     The hardness of a cover layer may be measured on the surface or midpoint of the given layer in a manner similar to measuring the hardness of a core layer as described further below. For example, the hardness of the inner cover layer may be measured at the surface or midpoint of the layer. A midpoint hardness measurement is preferably made for the inner and intermediate cover layers. The midpoint hardness of a cover layer is taken at a point equidistant from the inner surface and outer surface of the layer to be measured. Once one or more cover or other ball layers surround a layer of interest, the exact midpoint may be difficult to determine, therefore, for the purposes of the present invention, the measurement of “midpoint” hardness of a layer is taken within plus or minus 1 mm of the measured midpoint of the layer. A surface hardness measurement is preferably made for the outer cover layer. In these instances, the hardness is measured on the outer surface (cover) of the ball. Methods for measuring the hardness are described in detail below under “Test Methods.” 
     The different hardness and thickness levels of the cover layers provide the ball with high impact durability and cut-, shear- and tear-resistance levels. In addition, the multi-layered cover, in combination with the core layer, helps impart high resiliency to the golf balls. Preferably, the golf ball has a Coefficient of Restitution (CoR) of at least 0.750 and more preferably at least 0.800. The core of the golf ball generally has a compression in the range of about 20 to about 120 and more preferably in the range of about 50 to about 100. These properties allow players to generate greater ball velocity off the tee and achieve greater distance with their drives. At the same time, the cover layers provide a player with a more comfortable and natural feeling when striking the ball with a club. The ball is more playable and the ball&#39;s flight path can be controlled more easily. 
     The specific gravity (density) of the respective golf ball layers is an important property, because they affect the Moment of Inertia (MOI) of the ball. In one embodiment, the outer cover may have a relatively low specific gravity. For example, the outer cover layer may have a specific gravity (“SG outer cover”) within a range of about  0.30 to about 2.50. In another embodiment, the outer cover may have a relatively high specific gravity (for example, greater than 2.50). 
     Meanwhile, the core layer may have a relatively high specific gravity (SG outer ). Thus, in one embodiment, the specific gravity of the inner core (SG core ) is greater than the specific gravity of the outer cover layer (SG outer cover layer ). For example, the outer cover layer may have a specific gravity within a range of about 0.50 to about 4.00. In one embodiment, the specific gravity is in the range of about 0.80 to about 3.00. In another embodiment, the specific gravity is in the range of about 1.20 to about 2.60. In yet another embodiment, the specific gravity is in the range of about 1.40 to about 2.10. In another example, the specific gravity of the inner core (SG core ) is less than the specific gravity of the outer cover layer (SG outer cover layer ). 
     In  FIG. 5 , a finished golf ball ( 38 ) having a dimpled outer cover ( 40 ) made in accordance with the present invention is shown. As discussed above, various patterns and geometric shapes of the dimples ( 40 ) can be used to modify the aerodynamic properties of the golf ball. 
     As discussed above, the lower and upper mold cavities ( 48 ,  60 ) have interior dimple cavity details ( 48   a ) and ( 60   a ). When the mold cavities are mated together, they define an interior spherical cavity that forms the cover for the ball. The cover material encapsulates the inner ball subassembly to form a unitary, one-piece cover structure. Furthermore, the cover material conforms to the interior geometry of the mold cavities to form a dimple pattern on the surface of the ball. The mold cavities may have any suitable dimple arrangement such as, for example, icosahedral, octahedral, cube-octahedral, dipyramid, and the like. In addition, the dimples may be circular, oval, triangular, square, pentagonal, hexagonal, heptagonal, octagonal, and the like. Possible cross-sectional shapes include, but are not limited to, circular arc, truncated cone, flattened trapezoid, and profiles defined by a parabolic curve, ellipse, semi-spherical curve, saucer-shaped curve, sine or catenary curve, or conical curve. Other possible dimple designs include dimples within dimples, constant depth dimples, or multi-lobe dimples, as disclosed in Aoyama, U.S. Pat. No. 6,749,525. It also should be understood that more than one shape or type of dimple may be used on a single ball, if desired. 
     In one preferred embodiment. the golf ball of this invention has a plurality of dimples on the spherical outer surface thereof, wherein the plurality of dimples is arranged in eight triangular dimple sections that are defined by projecting the eight faces of a square dipyramid onto the spherical outer surface of the ball, the eight triangular dimple sections being substantially identical in size and dimple arrangement. 
     It should be understood the terms, “first”, “second”, “top”, “bottom”, “upper”, “lower”, and the like are arbitrary terms used to refer to one position of an element based on one perspective and should not be construed as limiting the scope of the invention. 
     When numerical lower limits and numerical upper limits are set forth herein, it is contemplated that any combination of these values may be used. Other than in the operating examples, or unless otherwise expressly specified, all of the numerical ranges, amounts, values and percentages such as those for amounts of materials and others in the specification may be read as if prefaced by the word “about” even though the term “about” may not expressly appear with the value, amount or range. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. 
     Test Methods 
     Hardness: The center hardness of a core is obtained according to the following procedure. The core is gently pressed into a hemispherical holder having an internal diameter approximately slightly smaller than the diameter of the core, such that the core is held in place in the hemispherical portion of the holder while concurrently leaving the geometric central plane of the core exposed. The core is secured in the holder by friction, such that it will not move during the cutting and grinding steps, but the friction is not so excessive that distortion of the natural shape of the core would result. The core is secured such that the parting line of the core is roughly parallel to the top of the holder. The diameter of the core is measured 90 degrees to this orientation prior to securing. A measurement is also made from the bottom of the holder to the top of the core to provide a reference point for future calculations. A rough cut is made slightly above the exposed geometric center of the core using a band saw or other appropriate cutting tool, making sure that the core does not move in the holder during this step. The remainder of the core, still in the holder, is secured to the base plate of a surface grinding machine. The exposed ‘rough’ surface is ground to a smooth, flat surface, revealing the geometric center of the core, which can be verified by measuring the height from the bottom of the holder to the exposed surface of the core, making sure that exactly half of the original height of the core, as measured above, has been removed to within 0.004 inches. Leaving the core in the holder, the center of the core is found with a center square and carefully marked and the hardness is measured at the center mark according to ASTM D-2240. Additional hardness measurements at any distance from the center of the core can then be made by drawing a line radially outward from the center mark, and measuring the hardness at any given distance along the line, typically in 2 mm increments from the center. The hardness at a particular distance from the center should be measured along at least two, preferably four, radial arms located 180° apart, or 90° apart, respectively, and then averaged. All hardness measurements performed on a plane passing through the geometric center are performed while the core is still in the holder and without having disturbed its orientation, such that the test surface is constantly parallel to the bottom of the holder, and thus also parallel to the properly aligned foot of the durometer. 
     The outer surface hardness of a golf ball layer is measured on the actual outer surface of the layer and is obtained from the average of a number of measurements taken from opposing hemispheres, taking care to avoid making measurements on the parting line of the core or on surface defects, such as holes or protrusions. Hardness measurements are made pursuant to ASTM D-2240 “Indentation Hardness of Rubber and Plastic by Means of a Durometer.” Because of the curved surface, care must be taken to ensure that the golf ball or golf ball sub-assembly is centered under the durometer indenter before a surface hardness reading is obtained. A calibrated, digital durometer, capable of reading to 0.1 hardness units is used for the hardness measurements. The digital durometer must be attached to, and its foot made parallel to, the base of an automatic stand. The weight on the durometer and attack rate conforms to ASTM D-2240. 
     In certain embodiments, a point or plurality of points measured along the “positive” or “negative” gradients may be above or below a line fit through the gradient and its outermost and innermost hardness values. In an alternative preferred embodiment, the hardest point along a particular steep “positive” or “negative” gradient may be higher than the value at the innermost portion of the inner core (the geometric center) or outer core layer (the inner surface)—as long as the outermost point (i.e., the outer surface of the inner core) is greater than (for “positive”) or lower than (for “negative”) the innermost point (i.e., the geometric center of the inner core or the inner surface of the outer core layer), such that the “positive” and “negative” gradients remain intact. 
     As discussed above, the direction of the hardness gradient of a golf ball layer is defined by the difference in hardness measurements taken at the outer and inner surfaces of a particular layer. The center hardness of an inner core and hardness of the outer surface of an inner core in a single-core ball or outer core layer are readily determined according to the test procedures provided above. The outer surface of the inner core layer (or other optional intermediate core layers) in a dual-core ball are also readily determined according to the procedures given herein for measuring the outer surface hardness of a golf ball layer, if the measurement is made prior to surrounding the layer with an additional core layer. Once an additional core layer surrounds a layer of interest, the hardness of the inner and outer surfaces of any inner or intermediate layers can be difficult to determine. Therefore, for purposes of the present invention, when the hardness of the inner or outer surface of a core layer is needed after the inner layer has been surrounded with another core layer, the test procedure described above for measuring a point located 1 mm from an interface is used. Likewise, the midpoint of a core layer is taken at a point equidistant from the inner surface and outer surface of the layer to be measured, most typically an outer core layer. Once again, once one or more core layers surround a layer of interest, the exact midpoint may be difficult to determine, therefore, for the purposes of the present invention, the measurement of “midpoint” hardness of a layer is taken within plus or minus 1 mm of the measured midpoint of the layer. 
     Also, it should be understood that there is a fundamental difference between “material hardness” and “hardness as measured directly on a golf ball.” For purposes of the present invention, material hardness is measured according to ASTM D2240 and generally involves measuring the hardness of a flat “slab” or “button” formed of the material. Surface hardness as measured directly on a golf ball (or other spherical surface) typically results in a different hardness value. The difference in “surface hardness” and “material hardness” values is due to several factors including, but not limited to, ball construction (that is, core type, number of cores and/or cover layers, and the like); ball (or sphere) diameter; and the material composition of adjacent layers. It also should be understood that the two measurement techniques are not linearly related and, therefore, one hardness value cannot easily be correlated to the other. Shore hardness (for example, Shore C or Shore D hardness) was measured according to the test method ASTM D-2240. 
     The present invention is illustrated further by the following Examples, but these Examples should not be construed as limiting the scope of the invention. 
     EXAMPLES 
     The following examples describe thermoplastic polyurethane covers for golf balls. The core and intermediate layers of the golf ball can be made from any suitable thermoset or thermoplastic composition. For example, a polybutadiene rubber composition can be used to make the inner core; and an ethylene acid copolymer ionomer composition can be used to make the intermediate layer. The thermoplastic polyurethane cover can be made from any suitable composition as described above. One Example of a method (cast-molding parameters) that can be used to make thermoplastic polyurethane cover is described below in Table 1. A Comparative Example of a method (cast-molding parameters) that can be used to make a comparative cover also is described below in Table 1. The ingredients used to make the thermoplastic polyurethane covers are described in the footnotes to Table 1 are in weight percent, based on total weight of the composition, unless otherwise indicated. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Method for Making Sample Thermoplastic Polyurethane Covers 
               
            
           
           
               
               
               
            
               
                   
                 Comparative Example A - 
                 Example 1 - 
               
               
                   
                 Thermoplastic 
                 Thermoplastic 
               
               
                 Example 
                 Polyurethane Cover** 
                 Polyurethane Cover** 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Mixer Capacity 
                 15-20 
                 grams 
                 50-100 
                 grams 
               
            
           
           
               
               
               
            
               
                 Catalyst Level 
                 0.1 to 0.5% 
                 0.01 to 0.02% 
               
            
           
           
               
               
               
               
               
            
               
                 Dwell Time in 
                 10-20 
                 seconds 
                 50-100 
                 seconds 
               
               
                 Mixer 
               
               
                 Mold Temperature 
                 130° 
                 F. 
                 130° 
                 F. 
               
               
                 Gel Time 
                 60 
                 seconds 
                 60 
                 seconds 
               
               
                 Mixer Temperature 
                 140° 
                 F. 
                 140° 
                 F. 
               
               
                   
               
               
                 **6% MDI Prepolymer (reaction product of pure MDI and polytetramethylene glycol (PTMEG 2000) with a post addition of Mondur MLQ to increase the free NCO groups to 12%. MDI or MMDI (monomeric MDI) is a standard abbreviation for “pure” diphenylmethane diisocyanate, methylene bisphenyl isocyanate, methylene diphenyl diisocyanate or methylene bis (p-phenyl isocyanate). Other synonyms for MDI are isocyanic acid: p,p′-methylene diphenyl diester; isocyanic acid: methylene dip-phenylene ester; and 1,1′-methylene bis (isocyanato benzene). Mondur MLQ is a mixture of 4,4′ and 2,4′ MDI (available from Covestro). 
               
               
                 **1,4-Butanediol Curing Agent (Mixed with the Prepolymer at a 1:1 stoichiometric ratio to form a thermoplastic polyurethane composition). 
               
               
                 **K-Kat XK639 Catalyst - available from King Industries (Zinc-based catalyst 50% active in Isopropanol.) 
               
            
           
         
       
     
     The above ingredients were mixed according to the above-described methods to form a castable liquid material that was used to fill mold cavities and form a thermoplastic polyurethane cover (Comparative A and Example 1). With minimal or no catalyst used in the liquid mixture of above Example 1 (Table 1), it was necessary to impart more mechanical energy into the polymer. By increasing the dwell time in the mixer, the thermoplastic polyurethane covers of Example 1 were successfully molded. In particular, a cast-molding method, wherein the dwell time in the mixer was set so that it fell within the range of about 50 to about 100 seconds with a mixer capacity of about 50 to about 100 grams, a mixer temperature of about 140° F., with a mold temperature of 130° F. and a gel time of about 60 seconds could be used with significantly reduced or no catalyst (0.01 to about 0.02% by weight base on total weight of composition) to produce a thermoplastic polyurethane cover. Preferably, fast mixing times (for example, mixing times in the range of 100 to 5000 rpm or 300 to 2500 rpm or 500 to 1000 rpm) are used. This process provides a thermoplastic polyurethane composition having increased molecular weight and ultimately a golf ball cover having high shear and cut-resistance. 
     One advantage of the casting methods of the present invention is that they help to increase the molecular weight of the thermoplastic polyurethane. This leads to an improvement in shear-durability. It is anticipated that the golf ball covers of this invention will provide the ball with good impact durability, toughness, and wear and tear-resistance. The balls will have good shear-resistance so there should be less cracks, fissures, splits, nicks, scuff marks, and/or other damage on their cover surface. 
     It is difficult to cast a liquid thermoplastic polyurethane composition for several reasons including, for example: a) the golf ball sub-assembly (for example, core structure or core and intermediate layer structure) needs to be centered in the mold cavities in a time and manner feasible for manufacturing; b) the thermoplastic composition needs to have sufficient green strength so that the mold cavities can be detached and the ball removed from the mold (demolding); and c) the thermoplastic polyurethane cover needs to have a sufficiently high molecular weight so that is has high shear durability. 
     Current methods of using a cast liquid process to obtain a durable thermoplastic polyurethane (TPU) golf ball cover have proven difficult for several reasons. First, many of the TPU formulations are based on MDI prepolymers cured with a diol. In many cases, this is a very slow, finicky reaction. Upon dispensing the liquid mixture of MDI prepolymer and diol into the mold, it often displays a non-uniform cure. That is, it may cure from the center of the mold outwards, outside to inward, or it may develop an “icing” effect, where the top of the liquid is a semi-solid while the underneath is still liquid. During golf ball molding, the ball sub-assembly is held in a vacuum cup above the liquid filled cavity. When the appropriate viscosity is reached, the ball sub-assembly is plunged into the liquid. The material viscosity needs to be uniform throughout the cavity to ensure the core is positioned correctly ultimately, because this will provide a concentrically placed core within the cover. To achieve a centering time conducive to golf ball manufacturing, between 30 and 75 seconds, the use of catalyst is required. The proper catalyst package can decrease this icing phenomenon helping to produce a blemish-free TPU cover using a cast process. Although the use of catalyst can modify the chemical formulations cure profile to achieve a centering time conducive to golf ball manufacturing, it has proven detrimental to the overall material properties of the ball. In particular, the shear resistance has suffered for some balls. The shear-resistance often decreases to a lesser value than what is adequate for a premium golf ball. In the current form, the needed catalyst type and loading level often does not allow the polymer to achieve a high molecular weight. Typically, there is a strong correlation with a higher molecular weight and increased shear and tear resistance. As discussed above, one problem is that high catalysis levels do not allow the polymer to achieve a high enough molecular weight. Thus, another method is needed to produce a TPU cover using the cast process. The present invention provides such a novel method. 
     The new process of the present invention involves using a dynamic mixing head with significantly increased volume. This increased volume allows for each shot to receive more mixing time, hence the shot when dispensed is further along in the cure process. The combination of increased dwell time, faster mixer speeds, and more aggressive rotor geometry, allows the formulator to produce a TPU golf ball cover using the cast process with significantly reduced or no catalyst. This process allows the polymer to achieve increased molecular weight over current methods, and ultimately superior shear and cut-resistance. The cast-molding methods of the present invention overcome these drawbacks and it is anticipated that these methods can produce a golf ball thermoplastic polyurethane cover having sufficiently high molecular weight so that the cover has a shear-durability rating of at least 3.0 as described further below in the prophetic examples. 
     The following prophetic examples describe the shear-durability values of sample thermoplastic polyurethane covers for golf balls that can be made in accordance with this invention and comparative thermoplastic polyurethane covers for golf balls. 
     Shear-Durability: A Vokey SM7™ golf club was used to strike a set of sample golf balls at a club head speed of about 100 mph. The balls were struck so they made impact with an angled steel plate—the balls then rebounded off the plate. Each set contained twelve (12) golf balls. Each given set of balls were then visually inspected with the naked eye to determine the wear and tear on the balls. The balls were visually inspected to determine if there were any cracks, fissures, splits, nicks, scuff marks, and/or other damage on their cover surface. The set of balls were then assigned an average shear-durability rating based on the visual damage to the balls. The following Shear-Durability scale was used: 
     Shear-Durability Rating 
     
         
         5—Excellent shear-durability 
         4—Good shear-durability 
         3—Fair shear-durability 
         2—Poor shear-durability 
         1—Deficient shear-durability 
       
    
     It is anticipated that the above-described Comparative Example A (Table 1) will have a Shear-Durability Rating of 2 or less. It is anticipated that the above-described Example 1 (Table 1) will have a Shear-Durability Rating of 3 or greater. 
     It should be understood that the methods and golf ball products described and illustrated herein represent only presently preferred embodiments of the invention. It is appreciated by those skilled in the art that various changes and additions can be made to such methods and products without departing from the spirit and scope of this invention. It is intended that all such embodiments be covered by the appended claims.