FIELD OF THE INVENTION
The present invention relates to an ultraviolet (UV) curable coating. In particular, though not exclusively, the present invention relates to a high solids UV curable coating useful in a golf ball application.
BACKGROUND OF THE INVENTION
Golf balls can be classified as one-piece, two-piece, and three-piece balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded therein. Two-piece balls are comprised of a cover molded over a solid core. The core of a two-piece ball is typically formed of rubber and can be solid, semi-solid or have a liquid center. Three-piece balls, traditionally, include a rubber inner core, elastic wrappings and a balata or SURLYN® ionomer cover. The more recent trend in the golf ball art is towards the development of multi-component golf balls such as balls having two or more cover layers, two or more core layers or both multiple core and multiple cover layers.
Golf ball covers are presently formed from a variety of materials, such as balata, SURLYN® ionomer resin, IOTEK® resin and polyurethane, depending upon the performance characteristics desired for the golf ball. One of the softest materials conventionally used to form golf ball covers is balata, which is the trans form of the 1,4-chain polymer of isoprene. For many years, balata was the standard cover stock material used in forming most golf balls. Balata covered balls are favored among professionals and more advanced amateur players because the softness of the cover allows the player to achieve spin rates sufficient to precisely control ball direction and distance, particularly on shorter approach shots.
However, because of its softness, balata is susceptible to cuts or other damage to the cover resulting from a “mis-hit” shot. Accordingly, harder, more durable cover materials, e.g., SURLYN® ionomer resin, have been developed which provide higher durability, but less spin and feel, than the balata balls. SURLYN® resins are generally ionic copolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid. Metal ions, such as lithium, zinc or sodium are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer for use as a golf ball cover.
Additionally, various softening comonomers such as n-butyl acrylate may be added during the ionomer manufacturing process to improve golf ball performance characteristics such as spin and feel. In the early 1980s, low modulus SURLYN® resin were introduced and subsequently utilized to impart more spin and an improved, balata-like feel to golf balls.
Primer coat and topcoat layers are commonly applied to the golf ball cover to provide a high gloss and an overall enhanced appearance to the ball. Solvent-borne coatings are currently used as topcoats for golf balls or similar game balls. These coatings provide good adhesion to the surface of the balls, and good abrasion resistance. However, the solvent type systems have serious environmental pollution problems, and the disadvantage of long cure times, or high curing temperatures. U.S. Pat. No. 4,871,589 (hereinafter '589 patent) discloses a method of treating golf balls by applying solvent-borne coating onto a thermoplastic ionomer covered golf ball. The solvent-borne coating of '589 patent contains 50% solvents and needs to be cured for 10 minutes at 50° C.
In light of the increasing regulation of volatile organic compounds (VOC), the use of traditional solvent-borne adhesive is becoming problematic. To eliminate the problems posed by solvents, water-borne replacements have been developed. Current water-borne coatings suffer from drawback of long curing time. U.S. Pat. No. 4,459,326 discloses a water-based composition for coating synthetic plastic surfaces. The water-based coating need to dry for ½ to 4 hours.
U.S. Pat. No 4,278,578 (hereinafter '578 patent) teaches an aqueous coating containing polyfunctional aziridines. The '578 patent indicates that the use of more than 3% aziridine is not beneficial to coating properties.
U.S. Pat. No. 5,300,325 discloses a use of an aziridine in a water-borne primer to promote adhesion between the solvent-borne topcoat and the cover of a golf ball. The primer and topcoat are co-cured and crosslinked at 54° C. for six hours.
U.S. Pat. No. 6,146,288 discloses a UV-curable coating and method for coating golf balls and other substrates with a material that includes one or more low viscosity polyether acrylates, a functional carbodiimide resin, one or more low viscosity aliphatic urethane polyacrylate oligomer, and a photoinitiator selected from one or more of S mono-aryl ketones, trimethylbenzoyldiphenyl phosphinates, and/or phosphine oxides. In addition, a method of curing a UV-curable coating is disclosed. The method includes the steps of spraying the formulation onto the exterior of a substrate, surrounding the substrate in an inert gas environment, and irradiating the substrate with ultraviolet radiation from a doped medium pressure mercury vapor lamp.
SURLYN® resin is the most widely used material to make golf ball cover layers. SURLYN® resin is an ionomer from the copolymer of ethylene and methacrylic acid. It is difficult for an UV curable coating to adhere to SURLYN® surface, in part because of the high shrinkage during the polymerization process. Urethane cured coatings are not easily adhered to ionomers. The present invention resulted from solving an adhesion problem of a UV curable coating which can be effectively utilized in coating ionomer substrates.
The problem addressed by the present invention was to provide an UV curable coating which would be capable of developing excellent adhesion, even on substrates which are traditionally difficult to adhere to with a UV coating. In addition, the UV curable coating of the present invention provides a high gloss finish, does not require the golf ball to be surrounded by an inert gas, is low in yellowing, and also offers good stain resistance and durability.
SUMMARY OF THE INVENTION
The present invention relates to an UV curable coating comprising (a) at least one UV-reactive component, (b) a polyfunctional aziridine, and (c) a photo-initiator. The UV curable coating of this invention is substantially free of solvents, and therefore, provides a nearly 100% solid system. The UV curable coating of the present invention is particularly suitable for coating ionomer substrates. Unexpectedly, the UV curable coating of the invention shows improved whiteness index (WI) compared to typical UV coatings. Specifically, the present invention discloses a high solids UV curable coating for golf balls or a similar game ball application.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The UV curable coating of this invention comprises the reaction product of (a) at least one UV-reactive component, (b) a polyfunctional aziridines, and (c) a photo-initiator.
Polyfunctional aziridines (“polyaziridine”) suitable for the purposes of the present invention correspond to the formula:
in which R is an organic aliphatic radical or a hydrogen atom, X is an alkylene group which may contain an ester group, an ether group, an amide group or a similar inert group, and m is a number of 2 to 4.
Preferred polyaziridines are those in which
where n=1 to 3, 1=1 to 3, and m=3 and R is a CH.sub.3—CH.sub.2—C radical.
Another preferred polyfunctional aziridines are those in which
where n=1, 1=2, and m=3 and R is an OH —CH.2—C radical.
The polyfunctional aziridine useful herein is preferably tri- or more highly functional compounds. The preferred materials include: pentaerythritol-tris-(beta-(N-aziridinyl)propionate); trimethylol-propane-tris-(.beta.-(N-aziridinyl)propionate); and mixtures thereof. Ethylenimine Technologies (EIT) sell representative polyfunctional aziridines under the trade name XAMA®-7 and XAMA®-2, which are particularly preferred in the present invention.
The polyfunctional aziridine is typically utilized in an amount of at least 0.1% by weight percent of the UV curable coating in the present invention. The polyfunctional aziridine is preferably utilized in an amount from about 0.1% to 15%, with 0.5% to 8% specially preferred, by weight percent of the UV curable coating in the present invention. The polyfunctional aziridine contributes to the adhesion of the UV curable coating to the golf ball cover. However, the use of a high amount of polyfunctional aziridine might cause the UV coating to undergo yellowing after cure. Surface treatments, such as vibratory abrasion and corona or plasma treatment, or other methods that result in surface modifications to the golf ball cover, may be used to enhance adhesion of the topcoat to the golf ball cover.
One of the UV reactive components in the present invention is an UV curable oligomer. Proper selection of the oligomer is important to obtaining the desired physical properties of the resulting coating. The suitable UV curable oligomers in the present invention are those oligomers containing acrylate functionality, namely an acrylate double bond. The double bond can open to form a crosslinked polymer network when irradiated with UV light in the presence of photo-initiators. The suitable UV reactive components in the present invention include urethane acrylate, epoxy acrylate, polyether acrylate or polyester acrylate, and are known in the art. However the polyether and polyester acrylates must be low in acid content, such as an acid number of less than 10, preferably less than 5, to avoid premature reaction with the polyaziridine component. In general, urethane acrylate oligomers impart toughness and abrasion resistance to the final coatings, while epoxy acrylate oligomers and polyester acrylate oligomers impart hardness and chemical resistance. The UV curable acrylate component will consist of between 30 to 80 weight percentage of the total composition weight of the UV-curable coating. The polyether acrylates advantageously have a low viscosity, preferably in the range of about 75 to about 250 centipoise. Examples of commercially available oligoether acrylates include LR 8967, and 8863, which are all available from BASF.
The preferred UV curable component of the invention is urethane acrylate oligomers formed by the reaction of an isocyanate group, and an acrylic monomer having at least one hydroxyl group. Preferred acrylic monomers include hydroxyethyl acrylate, hydroxypropyl acrylate and caprolactone acrylate. Suitable isocyanates include aliphatic isocyanates, cycloaliphatic isocyanates, alkaryl isocyanates, arylalkyl heterocyclic isocyanates and aryl isocyanates. Suitable isocyanates of the invention could be monoisocyanates, diisocyanates, or triisocyanates. The diisocyanates are the preferred isocyanates of the invention.
Examples of diisocyanates include tetramethylene diisocyanate,pentamethylene diisocyanate, octomethylene diisocyanate, dodecylmethylenediisocyanate, 3,3-diisocyanatodipropyl ether, xylylene diisocyanate, meta-phenylenediisocyanate, 1-methylphenylene-2,4-diisocyanate, 2,6-toluenediisocyanate, 2,6-toluene diisocyanate, cyclohexane-1,4-diisocyanate,methane-bis(cyclohexy-4-isocyanate), and isophorone diisocyanate.
Preferred diisocyanates include Tolylene diisocyanate (TDI), Hexamethylene diisocyanates (HDI), ), Norbornane diisocyanates (NBDI), Isophorone diisocyanates (IPDI), Diphenylmethane diisocyanates (MDI), bis(4-isocyanatocyclohexyl) methane (Desmodur® W) and Polyisocyanates from HDI (Desmodur® N-100, Desmodur® N-3200, Desmodur® N3300). Aliphatic diisocyanates are the most preferred diisocyanates of the invention because of their non-yellowing characteristics.
In one preferred embodiment, urethane acrylate oligomers are formed by reacting an aliphatic diisocyanate with hydroxy monomer in stoichiometric proportions and further reacting this with a long chain polyol to form a flexible urethane acrylate oligomer.
Oligomers of molecular weight greater than 8000 can produce a coating with too much flexibility, while oligomers of molecular weight below 500 produce a coating that has low impact resistance. Preferably, the oligomers used in the present invention have molecular weights ranging from 500 to 8000, with preferred range from 500 to 5000. The urethane acrylate oligomer should be utilized in an amount ranging from about 30% to 80% by weight percent of the total coating, and more preferably, between 40% to 70% by weight percent of the total coating.
The urethane polyacrylate oligomer UV curable component has a relatively low viscosity, preferably in the range of about 8,000 to about 40,000 centipoise. The aliphatic urethane polyacrylate oligomer component provides the abrasion resistance and the resiliency of the polyurethane coating. As illustrative and non-limiting examples, the aliphatic urethane polyacrylate oligomer component are sold by companies such as Sartomer Company Inc., or under the BOMAR® designation by Bomar Specialties, Winsted, Conn.
In the preparation of an UV curable coating, the UV-curable oligomer is typically utilized in combination with a reactive diluent system. Broadly, suitable reactive diluent systems comprise at least one unsaturated addition polymerizable monomer, which is copolymerizable with the UV-curable oligomer upon exposure to radiation. The reactive diluent can be monofunctional, difunctional or polyfunctional. A single polyfunctional diluent can be used; or a combination of one or more monofunctional reactive diluents, one or more difunctional reactive diluents, and one or more polyfunctional reactive diluents can be used.
Particularly preferred reactive diluents are unsaturated addition-polymerizable monofunctional, difunctional and polyfunctional acrylic monomers. Acrylate monomers useful as a reactive diluent system are well known and examples of such monomers include isobornyl acrylate, phenoxyethyl acrylate, isodecyl acrylate, hexyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl acrylate, stearyl acrylate, 2-phenoxy acrylate, 2-methoxyethyl acrylate, lactone modified esters of acrylic and methacrylic acid, methyl methacrylate, butyl acrylate, isobutyl acrylate, methacrylamide, allyl acrylate, tetrahydrofuryl acrylate, n-hexyl methacrylate, 2-(2-ethoxyethoxy)ethyl acrylate, n-lauryl acrylate, 2-phenoxyethyl acrylate, glycidyl methacrylate, glycidyl acrylate, acrylated methylolmelamine, 2-(N,N-diethylamino)-ethyl acrylate, neopentyl glycol diacrylate, alkoxylated neopentyl glycol diacrylate, ethylene glycol diacrylate, hexylene glycol diacrylate, diethylene glycol diacrylate, tripropylene glycol diacrylate, tetraethylene glycol diacrylate, pentaerythritol di-, tri-, tetra-, or penta-acrylate, trimethylolpropane triacrylate, alkoxylated trimethylol-propane triacrylate which contains from 2 to 14 moles of either ethylene or propylene oxide, triethylene glycol diacrylate, tetraethylene glycol diacrylate, polyethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, polyethylene glycol diacrylate, combinations thereof, and any corresponding methacrylates thereof.
The reactive diluent system typically comprises from about 10 to 60, preferably from about 20 to 50 percent by weight of the total UV curable coating. The photo-initiator can be any of the known photo-initiators. Specific examples include benzophenone, benzoin, acetophenone, benzoin methyl ether, Michler's ketone, benzoin butyl ether, xanthone, thioxanthone, propiophenone, fluorenone, carbozole, diethyoxyacetophenone, the 2-, 3- and 4- methylacetophenones and methoxyacetophenones, the 2- and 3-chloroxanthones and chlorothioxanthones, 2-acetyl-4-methylphenyl acetate, 2,2′-dimethyoxy-2-phenyl acetophenone, benzaldehyde, fluorene, anthraquinone, triphenylamine, 3- and 4-allyl-acetophenone, p-diacetylbenzene, 3-chloro-2-nonylxanthone, 2-chlorobenzophenone, 4-methoxybenzophenone, 2,2′,4,4′-tetrachlorobenzophenone, 2-chloro-4′-methylbenzophenone, 4-chloro-4′-methylbenzophenone, 3-methylbenzophenone, 4-tert-butyl-benzophenone, isobutyl ether, benzoic acetate, benzil, benzilic acid, amino benzoate, methylene blue, 2,2-diethoxyacetophenone, 9,10-phenanthrenequinone, 2-methyl anthraquinone, 2-ethyl anthraquinone, 1-tert-butyl-anthraquinone, 1,4-naphthoquinone, isopropylthioxanthone, 2-chlorothioxanthone, 2-iso-propylthioxanthone, 2methylthioxanthone, 2-decylthioxanthone, 2-dodecyl-thioxanthone, 2-methyl- 1-[4-(methyl thio)phenyl)]-2-morpholinopropanone-1, combinations thereof and the like. The photo-initiator or combination of photo-initiators is typically utilized in an amount ranging from about 0.5 to 15, preferably from about 1 to 8 percent by weight of the UV curable coating in the present invention. The peak absorbance is typically in the range of about 240 nm to about 390 nm.
In general, exemplary monomers, oligomers, and photoinitiators can be found in “Chemistry and Technology of UV and EB Formulations for Coatings, Paints, and Inks”, P. K. T. Oldring, ed., (1991), ISBN 0 947798 10.2.
An UV curable coating, comprising at least one UV reactive component, a polyfunctional aziridine, and a photo-initiator of the present invention, may also contain other optional ingredients known to those skilled in the art of UV curable compositions. An optional ingredient is a whitener that improves the overall appearance of the UV curable coating. Suitable whiteners include 2,5-Bis (5 tert-butyl-2-benzoxazolyl) thiophene (Uvitex® OB from Ciba Specialty Chemical Co.), 7-(2h-napthol (1,2-d)-triazol-2-yl)-3-phenyl-coumarin (Leucopure® EGM from Clariant).
Other optional components of the present UV curable coating include surfactants to modify the flow and wetting characteristics of the coating
The UV curable coating as described above of the present invention contains more than 90%, preferably greater than 95% solids, more preferably more than 98% solids, and most preferably essentially 100% solids. The UV curable outer coating is substantially solvent free (<5%) by weight, preferably less than 2% of volatile organic solvent. Most preferably the coating is essentially solvent-free which avoids the problems of energy consumption in evaporating the solvent, solvent pollution, and the cost of solvent. However, the present UV curable coating can also be diluted with solvent for improved application to various substrates if desired. The choice of solvent employed for dilution is not critical. Preferred solvents are those that are classified as exempt from (volatile organic component) VOC classification such as acetone. However VOC containing solvents can also be used. For instance, 10% solvent may be incorporated to reduce the viscosity of the UV curable coating in order to carry out the desired performance of spraying.
UV curable coatings can be applied to a substrate to be coated or bonded by various techniques well known in the art e.g. roll coater, curtain coater, vacuum coater, and spray to form a coating thickness in the range of 0.1 mils to 3.0 mils, preferably 0.2 to 1.0 mils. The coating is particularly advantageous for polymeric substrates including polyvinyl chloride, either rigid or flexible PVC, polycarbonates as the hard to adhere polymers such as polypropylene and polyethylene, as well as the thermoplastic vulcanizates, and thermoplastic olefins (TPO).
Solvents can be added to aid in application if necessary. After application of the coating to a substrate, polymerization of the coating is initiated. The polymerization may be initiated by any method or means known in the art for initiating radiation-curable materials. At the present invention, it is preferred to initiate polymerization of the coating by exposing the coating to any source of actinic radiation at a wavelength within the ultraviolet or visible spectral regions. Suitable sources of radiation include mercury, xenon, carbon arc and tungsten filament lamps, sunlight, etc. Exposures may be from less than about 0.1 second to 10 minutes or more depending upon the amounts of particular polymerizable materials and photo-initiators being utilized and depending upon the radiation source and distance from the source. The compositions may also be polymerized by exposure to electron beam irradiation in a dosage typically ranging from less than about 1 megarad to 100 megarad or more. The use of thermal energy during or after exposure to a radiation source will also generally accelerate the curing reaction, and even a moderate increase in temperature may greatly accelerate cure rate.
An UV curable coating of the present invention can be used in essentially any type of coating or adhesive application known in the art. In particular, though not exclusively, the UV curable coating is particularly suitable for golf ball application, wherein the golf ball comprises an ionomer resin layer.
Typical golf ball cover materials and compositions include thermoplastic ionomer resins such as those sold under the trademarks SURLYN®, available from E.I. Du Pont de Nemours and Co., Wilmington, Del.; or IOTEK®, available from Exxon Chemical Co., Polymers Group, Baytown, Tex.
The UV curable coating in the invention has enhanced adhesion to the ionomer substrates, such as SURLYN® resin. Adhesion was tested by using Cross-hatch test method. Adhesion of an UV coating without polyaziridine varied from 4% to 97% depending on the surface treatment. Adhesion was improved to 100% after polyaziridine incorporated.