COATED ABRASIVE SUITABLE FOR USE AS A LAPPING MATERIAL

Abstract: This invention relates to novel coated abrasives, and, in particular, to a lapping material in sheet form. The earliest methods of lapping employed particulate abrasive materials suspended in a liquid vehicle to form a slurry. The suspension was worked against the surface of the workpiece until the desired finish was attained. More recently, lapping materials in pad or sheet form have been developed. U.S. Patent 4,288,233 describes lap­ ping pads for ophthalmic lenses. While the lapping material of this invention is useful, the components thereof, i.e. dia­ monds, and the method of making it, i.e. metal plating, are expensive. Furthermore the lapping materials described therein are not flexible. This invention involves a coated abrasive comprising a backing and abrasive grain adhered to said backing by means of a binder. The binder is prepared from a composition that is curable by free-radical polymerization. The composition con­ tains abrasive grain suspended therein at the time the com­ position is coated onto the backing. The abrasive exhibits high endurance for extended periods of use. The coated abrasive is prepared by a method comprising the steps of: (a) providing a coatable composition comprising a binder curable by free-radical polymerization, said composition having lapping size abrasive grains sus­ pended therein, (b) coating said composition on a backing, and (c) curing said composition by means of free-radical polymerization.

Description: Background of the Invention This invention relates to novel coated abrasives, and, in particular, to a lapping material in sheet form. "Lapping", as the term is used in this specification, means working a particulate abrasive material against the surface of a workpiece until an exceedingly fine, mirror-like finish is imparted thereto. The objective sought is the attainment of a very smooth surface finish, while retaining a high degree of dimensional control, so that the resulting product will conform to very precise size standards. The lapping of surfaces from their original state to the final finish is a progressive operation, involving the use of a series of abrasives ranging from relatively coarse at the beginning through successively finer sizes to the end. The results secured depend upon a number of factors, such as the properties of the abrasive employed, the pressure with which the abrasive is forced against the workpiece, the pattern of movement preserved in the contact of the workpiece with the abrasive particles and other considerations. The earliest methods of lapping employed particulate abrasive materials suspended in a liquid vehicle to form a slurry. The suspension was worked against the surface of the workpiece until the desired finish was attained. Examples of lapping methods employing abrasive slurries are described in U.S. Patents 2,655,775; 4,059,929; 4,046,524; 4,246,003. More recently, lapping materials in pad or sheet form have been developed. U.S. Patent 4,288,233 describes lapping pads for ophthalmic lenses. While the lapping material of this invention is useful, the components thereof, i.e. diamonds, and the method of making it, i.e. metal plating, are expensive. Furthermore the lapping materials described therein are not flexible. U.S. Patent 4,255,164 discloses a glass fining sheet suited for finishing rough ground vitreous surfaces to provide a surface finish which may be readily polished comprising a flexible, conformable backing sheet bearing a microcellular abrasive granule-resin matrix which, under use conditions and in the presence of an aqueous flow, generates a fining slurry. The fining sheet is prepared by coating a flexible conformable backing sheet with a foamed liquid abrasive granule-resin coating composition comprised of liquid curable binder material, abrasive fining granules, and sufficient compatible solvent to provide a coatable composition. The coating provides a cellular layer which releases the fining abrasive granules at a controlled rate under use conditions. The released granules perform the actual abrading. The sheet of this patent appears to be another means of preparing a fining slurry. GB-A-2,087,263 (Caul) discloses resin systems for high energy electron curable resin coated webs, the resin systems of which comprise an oligomer, a diluent, fillers and minor amounts of other additives such as surfactants, pigments and suspending agents but which do not contain coupling agents. Summary of the Invention In one aspect, the invention involves a method of preparing a coated abrasive which is especially useful as a lapping material. The method comprises the steps of: (a) providing a coatable composition comprising a binder and a coupling agent having abrasive granules having an average particle size of from 0.3 to 35 micrometers suspended therein (b) coating said composition on a backing (c) curing said composition by free radical polymerization. The curable binder is preferably curable by radiation, and it may also be curable by thermal energy. The backing is preferably primed to promote adhesion between it and the cured binder. The coatable composition does not require a solvent, although a solvent may be used, if desired, for some purposes. The method of this invention is low in cost due to the savings derived from eliminating the solvent and solvent removal steps. In another aspect, this invention provides a coated abrasive suitable for use as a lapping material, said coated abrasive comprising (a) a backing, (b) an abrasive coating adhered to said backing, said abrasive coating being formed by coating a suspension comprising abrasive grains which have an average size of from 0.3 to 35 micrometers and a binder which includes a coupling agent and which is curable by free radical polymerization on to said backing and curing said binder by free radical polymerization. The coated abrasive of this invention exhibits high endurance for extended periods of use. The coated abrasive can be die cut to shape, if desired, e.g., for specialty microfinishing apparatus. Detailed Description As used herein, the term "curable binder" means the flowable or coatable composition from which the binder is prepared by means by free-radical polymerization; the term "binder" or "cured binder" means the dry, polymerized coating which adheres the abrasive grains to the backing. The backing should be sufficiently strong to support the binder and abrasive grains therein under contemplated use conditions. It should be sufficiently flexible to allow mounting thereof on the surfaces of lapping tools. Because most lapping operations require the use of water or oil or both, it is preferred that the backing be water-resistant and oil-resistant. it is also preferred that the backing be smooth and of uniform caliper so the lapping film can be used successfully for finishing high precision articles. Materials suitable for the backing include water-resistant paper and polymeric films. If polymeric film is to be used as a backing, it should preferably have a primed surface to promote adhesion between the backing and the binder. The preferred primer for the purposes of this invention is ethylene acrylic acid as described in U.S. Patent 3,188,265. Alternatively polymeric film having a roughened surface can provide excellent adhesion between the backing and binder. The preferred backing material is polyester, e.g. polyethylene terephthalate, which has been oriented and heat set and primed with ethylene acrylic acid. The backing should be sufficiently thick to provide sufficient strength to bear the coating, but not so thick as to adversely affect flexibility. Typically, the backing should have a thickness of less than 254 µm (10 mils), and a thickness of 50 to 75 µm (2 to 3 mils) is preferred. The abrasive grains can be any abrasive grain conventionally used for lapping processes. Abrasives suitable for the lapping film of this invention include iron oxide, silicon carbide, silicon nitride, silicon boride, diamond, aluminum oxide, chromic oxide, and alumina and magnesia spinel ceramic prepared in accordance with U.S. Patent 4,314,827, referred to by the trademark Cubitron®, commercially available from Minnesota Mining and Manufacturing Company, St. Paul, MN. The size range of the grains can range from 0.3 to 35 micrometers, the preferred range being from 1 to 20 micrometers. It is preferred that for a given lapping film, the grain size range be as narrow as possible, because a small number of excessively large grains can result in scratches on the workpiece surface. An excessive number of fine grains, however, will not result in this problem. The curable binder is curable by free-radical polymerization, preferably by radiation-initiation or generation of free-radicals. Sources of radiation that are useful for the process of the present invention include ultraviolet, visible, γ-radiation, X-rays, and electron beam, with electron beam being most preferred. The curable binder can also be cured by thermal energy in the presence of an appropriate catalyst. Suitable curable binders for use in this invention comprise radiation-curable monomers, coupling agents and, optionally, reactive diluents. The curable binder may also contain conventional additives, for example, wetting agents, lubricants, dispersing agents and fillers. Radiation-curable monomers that are useful in this invention contain at least two ethylenically unsaturated moieties therein, e.g. acrylic (such as hexane diol diacrylate), methacrylic (such as hexane diol dimethacrylate). Radiation-curable monomers that are preferred include oligomers selected from urethane acrylates and methacrylates, isocyanurate acrylates and methacrylates, polyester-urethane acrylates and methacrylates, and epoxy acrylates and methacrylates. One class of oligomers that are preferred for the compositions for preparing the binders can be represented by the general formula: where n represents 2 or 3, x represents 1 to 3 inclusive, R represents an aliphatic group having, for example, from 1 to 20 carbon atoms, a cycloaliphatic group having, for example, from 5 to 6 ring carbon atoms, and up to 36 carbon atoms in total, or an aromatic group, for example benzyl, R' represents an aliphatic group having, for example, from 2 to 10 carbon atoms, R" represents hydrogen or a methyl group. Exemplary reaction schemes for preparing the oligomers for the radiation-curable compositions for preparing the binders are shown below: SCHEME 1 Trimer of hexamethylenediisocyanate SCHEME 2 2,2,4-Trimethylhexamethylenediisocyanate In Scheme 1 and Scheme 2, ZOH represents an aliphatic group having at least one acrylate functional group therein. Z can be represented by the structural formula wherein R', R" and x are as defined above. DBTDL represents dibutyl tin dilaurate, a catalyst The cyclic trimer of hexamethylene diisocyanate is commercially available from Mobay Chemical Corp. and 2,2,4-trimethylhexamethylene isocyanate is commercially available from Thorson Chemical. Representative examples of commercially available starting materials that can be characterized by the formula ZOH, supra , include pentaerythritoltriacrylate (available from Sartomer), hydroxyethyl methacrylate (available from Rohm and Haas Co.), and hydroxyethyl acrylate (available from Dow Chemical Co.). The coupling agent is included with the monomer in order to promote adhesion between the abrasive grains and the cured binder. Improved adhesion between cured binder and abrasive grains inhibits the grains from being loosened and removed from the binder during lapping operations, thus enhancing the durability of the lapping film. A preferred coupling agent is ν-methacryloxypropyl trimethoxy silane commercially available from Dow Corning Corp. under the trade designation Z6030 and Union Carbide Corp. under the trade designation A-174. Preferably, the amount of coupling agent ranges from about 0.1 to about 10% by weight, preferably from about 1.5 to about 5% by weight, based on weight of abrasive grains. It is preferred to include a reactive diluent with the monomer. Reactive diluents suitable for the present invention include mono- or multifunctional acrylates and methacrylates such as hexane diol diacrylate (HDDA), pentaerythritol triacrylate (PET₃A), pentaerythritol tetraacrylate (PET₄A), trimethylolpropanetriacrylate (TMPTA), β-hydroxyethylmethacrylate (HEMA), tetrahydrofurfural acrylate (THF-A) the preferred specie. The reactive diluent serves to reduce the viscosity of the composition for preparing the binder, thus improving the coatability of the composition, and to modify the hardness of the cured binder. Preferably, the ratio of monomer to reactive diluent is 85:15 to 25:75, more preferably, the ratio is 75:25 to 35:65, and most preferably, the ratio is 65:35 to 45:55. The cured binder can have a Knoop hardness, as measured on a Tukon® indentation tester, from 1 to 50. The Knoop hardness preferably ranges from 7 to 25. The cured binder should be compatible with the backing and primer thereon, i.e. the binder should be free of fisheyes, craters, voids, and orange-peels when coated and cured. The coated abrasive of this invention can vary with respect to product requirements. Depending upon the function of the coated abrasive, the user can specify hardness of cured binder and size of abrasive grains. For example, ophthalmic pads are characterized by a very hard resin combined with a relatively coarse mineral. Disc burnishing films are characterized by softer resin and finer mineral. The coated abrasive of this invention can be prepared by applying the curable binder and abrasive grains suspended therein onto the backing to form a coating and then curing the thus-applied coating by free-radical polymerization. The following method has been found to be useful in preparing the coated abrasive. It is preferred that the dry mineral grains first be treated with coupling agent. After the mineral grains are treated with coupling agent, they are then heat set in an oven. Typically, heat setting is conducted at 60°C-100°C for 1-1/2 to 2-1/2 hours. Alternatively, the coupling agent can be mixed in the curable binder along with the dry mineral grains. After the monomers and the reactive diluents, if any, are blended to form the curable binder, the grains of abrasive mineral are added to the curable binder under conditions of high-shear mixing. The composition is then applied, preferably by means of conventional coating equipment, to the backing. The viscosity of the composition determines the ease of coating. The viscosity of the composition can range from about 200 to about 5,000,000 centipoise at 25°C, preferably about 2000 to about 500,000 (1 cps = 1m Pas). At higher temperatures, more viscous compositions can be used. For example, a composition having a viscosity of 30,000 centipoise at 25°C can be coated fairly easily at 50°C. Rotogravure coating is preferred for the reason that the rotogravure coater can impart a uniform pattern of ridges and valleys to the binder composition, which, after the composition is cured, can serve as channels for flow of lubricants and for removal of abraded material. The coating is then cured by means of free-radical polymerization. Preferably, the free-radical polymerization is initiated by actinic radiation, preferably electron beam. However, other sources of radiation, such as ultraviolet, visible, and gamma, are also suitable with appropriate catalyst. Thermal initiation is also suitable with an appropriate catalyst. It should be noted that solvents are not required to be added to the curable binder to facilitate coating thereof onto the backing. This has the advantage of avoiding troublesome solvent removal problems. If desired, an inert solvent can be employed along with the monomers, reactive diluents, and coupling agent of the curable binder. In order to demonstrate the performance characteristics of the coated abrasive of the present invention, ophthalmic pads were prepared and tested. The testing procedures were designated as (a) first fine, (b) second fine, and (c) single fine. These testing procedures are designed to measure the amount of material removed and the character of finish imparted to a plastic CR-39 lens. First fine samples were prepared and tested according to the following procedure: The backside of the material to be tested was laminated with a pressure-sensitive adhesive. An ophthalmic test daisy was die cut with a standard die. The test daisy was mounted on a 2.12 diopter spherical lapping block. The lapping block was mounted on a Coburn Rocket lapping machine. The initial thickness of the lens was measured, and then the lens was clamped over the lapping block, air pressure was set at 137.8 kPa (20 psi). The lens and lapping blocks were flooded with water. A one minute test was run at settings of "medium" and "20 mm top stroke". The lens was removed and final thickness measured. Finish was measured with a Surtronic 3 instrument. Second fine samples were prepared and tested according to the same procedure, the only difference being that the test was run for two minutes, instead of one minute, and a 6.25/8.25 diopter lapping block was used. Single fine were prepared and tested according to same procedure as the first fine, the only difference being that the test was run for three minutes, instead of one minute. In the examples which follow, the following abbreviations will be used: HMDIT-A5: wherein HMDIT-A9: wherein Y is as defined above. TMDI-A4: wherein Z is as defined above. TMDI-A6: wherein Z is as defined above. n-BUMA: n-butyl(2-methacryloxy) ethyl carbamate THF-A: tetrahydrofurfuralacrylate HDDA: hexane diol diacrylate TMPTA: trimethylolpropanetriacrylate PET₃A: pentaerythritol triacrylate PET₄A: pentaerythritol tetraacrylate Preparation of HMDIT-A5 To one-gallon reaction vessel equipped with a thermometer, mechanical stirrer, and a dry air sparge was charged 6.5 equivalent of the trimer of hexamethylene diisocyanate ("Desmondur-N-3390"). In a second vessel, 2.5 g tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)-propionate]methane ("Irganox 1010") was dissolved with heat and stirring into 4.6 equivalent of hydroxyethylmethacrylate (HEMA). Dibutyltindilaurate (8 to 10 drops) was then charged to the vessel containing HEMA. The contents of the second vessel were charged to the reaction vessel over a 30 minute period in such a manner, with cooling, that the reaction temperature is maintained at about 70°C. At the end of the 30-minute period, 2.3 equivalents of pentaerythritoltriacrylate ("Sartomer") was added to the reaction vessel over a 15 minute period. Heat was applied to maintain a reaction temperature of 70-80°C until the reaction was complete as determined by % NCO. Preparation of TMDI-A4 To a one gallon reaction vessel equipped with a thermometer, mechanical stirrer, and a dry air sparge was charged 10 equivalents of 2,2,4-trimethylhexamethylenediisocyanate. In a second vessel, 3.0 g of tetrakis-[methylene-3-(3',5'-di-t-butyl-4'hydroxyphenyl)propionate]methane ("Irganox 1010") was dissolved with heat and stirring into 5.35 equivalents of hydroxyethylmethacrylate (HEMA). Dibutyltindilaurate (8 to 10 drops) was then charged to the vessel containing HEMA. The contents of the second vessel were charged to the reaction vessel over a 30 minute period in such a manner, with cooling, that the reaction temperature was maintained at about 70°C. At the end of the 30-minute period, 5.0 equivalents of pentaerythritoltriacrylate ("Sartomer") was added to the reaction vessel over a 15 minute period. Heat was applied to maintain a reaction temperature of 70-80°C until the reaction was complete as determined by % NCO. Preparation of TMDI-A6 To a one gallon reaction vessel equipped with a thermometer, mechanical stirrer, and a dry air sparge was charged 7.0 equivalents of 2,2,4-trimethylhexamethylene diisocyanate. In a second vessel, 3.0g of tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)-propionate]methane ("Irganox 1010") was dissolved with heat and stirring, into 7.3 equivalents of pentaerythritoltriacrylate ("Sartomer"). Dibutyltindilaurate (8 to 10 drops) was then charged to the vessel containing the PET₃A. The contents of the second vessel were charged to the first over a 30 min. period, with heating and cooling, to maintain a temperature of 70°C. The reaction mixture was heated to maintain a temperature of 70-80°C until the raction is complete by % NCO. Preparation of n-BUMA To a one gallon reaction vessel equipped with a thermometer, mechanical stirrer, and a dry air sparge was charged 10 equivalents of n-butylisocyanate. In a second vessel, 2.5 g tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)-propionate]methane ("Irganox 1010") was dissolved with heat and stirring into 11 equivalents of hydroxyethylmethacrylate (HEMA). Dibutyltindilaurate (8 to 10 drops) was then charged to the vessel containing HEMA. The contents of the second vessel were charged to the reaction vessel over a 30 minute period in such a manner, with cooling, that the reaction temperature was maintained at about 70°C until the reaction was complete as determined by % NCO. Preparation of HMDIT-A9 To a one gallon reaction vessel equipped with a thermometer, mechanical stirrer, and a dry air sparge was charged 5.0 equivalents of the trimer of hexamethylenediisocyanate ("Desmondur-N-3390"). In a second vessel, 3.0 g tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)-propionate]methane ("Irganox 1010") was dissolved with heat and stirring into 5.25 equivalents of pentaerythritoltriacrylate (PET₃A). Dibutyltindilaurate (8 to 10 drops) was then charged to the vessel containing PET₃A. The contents of the second vessel were charged to the reaction vessel over a 30 minute period in such a manner, with cooling, and then heating, that the reaction temperature was maintained at about 70°C until the reaction was complete as determined by % NCO. The following examples, which are illustrative rather than limiting or delineative of the scope of the invention, serve to describe the novel compounds, their method of preparation, and their properties. EXAMPLE 1 The following runs demonstrate the effect of reactive diluent and the percentage thereof in the binder of the coated abrasive of the present invention. In the following runs, to a mixture containing 100 g of monomer and reactive diluent in the ratios as shown in Table I below was added 250 g Al₂O₃ (20 micrometers) which had been pretreated with 2% gamma-methacryloxy propyl trimethoxysilane (Z-6030), based on the weight of Al₂O₃. The resulting composition was coated at 25.4 µm (1 mil) thickness on ethylene acrylic acid primed polyethylene terephthalate film. The coated film was subjected to electron beam radiation at 250 kev, 80 joules per gram (8 Mrad). The performance characteristics of the coated abrasives thus prepared are shown in Table I. The coated abrasive of the present invention exhibited improved results according to the second fine test procedure, as the cut was dramatically increased while finish and abrasive wear continued to be acceptable. EXAMPLE 2 This example demonstrates the effect of coupling agent on the coated abrasive of the present invention. In the following runs, to a mixture containing 100 g of monomer and reactive diluent in the ratio 1:1 (by weight) was added 150 g Al₂O₃ (12 micrometers). In the control run, no coupling agent was employed. In the second run 1% by weight of coupling agent, based on weight of abrasive grains, was used to pretreat the Al₂O₃ abrasive. The compositions were knife coated at 2.54 µm (1 mil) thickness on ethylene acrylic acid primed polyethylene terephthalate film. The coated film was subjected to electron beam radiation at 240 Kev, 90 joules per gram (9 Mrad). The performance characteristics of the coated abrasive thus prepared are shown in Table II. The addition of a coupling agent resulted in an improvement in the durability, and, consequently, the performance of the coated abrasive as well as a reduction in the viscosity of the coating composition. EXAMPLE 3 This example demonstrates the preferred combination of monomer, reactive diluent, coupling agent, and abrasive grain. In the following runs, the monomers, and reactive diluents were blended, and abrasive grains pretreated with gamma-methacryloxypropyltrimethoxysilane coupling agent (Z6030) was introduced into the mixture. Dyes were also utilized in the mixtures so that the size of the abrasive grains in each run could be identified. The ratios of ingredients are set forth in Table III. The compositions were knife coated at 2.54 µm (1 mil) thickness on ethylene acrylic acid primed polyethylene terephthalate film. The coated film was subjected to electron beam radiation at 250 Kev, 80 joules per gram (8 Mrad). The performance characteristics of the coated abrasives thus prepared are shown in Table IV.

Claims: A coated abrasive suitable for use as a lapping material, said coated abrasive comprising (a) a backing, (b) an abrasive coating adhered to said backing, said abrasive coating being formed by coating a suspension comprising abrasive grains which have an average size of from 0.3 to 35 micrometers and a binder which includes a coupling agent and which is curable by free radical polymerization on to said backing and curing said binder by free radical polymerization. A coated abrasive according to claim 1 wherein said binder comprises a monomer having at least two ethylenically unsaturated moieties. A coated abrasive according to claim 2 wherein said monomer is selected from the group consisting of urethane acrylates, urethane methacrylates, isocyanurate acrylates, isocyanurate methacrylates, polyester-urethane acrylates, polyester-urethane methacrylates, epoxy acrylates, and epoxy methacrylates. A coated abrasive according to any preceding claim wherein said coupling agent is γ-methacryloxypropyl trimethoxy silane. A coated abrasive according to any preceding claim wherein said binder includes a reactive diluent. A coated abrasive according to claim 5 wherein said reactive diluent is selected from the group consisting of hexane diol diacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, β-hydroxyethylmethacrylate, and tetrahydrofurfuralacrylate. Method of preparing a coated abrasive according to any preceding claim comprising the steps of (a) providing a coatable composition comprising a binder and a coupling agent having abrasive granules having an average particle size of from 0.3 to 35 micrometers suspended therein (b) coating said composition on a backing (c) curing said composition by free radical polymerization. The method of claim 7 wherein said composition is cured by means of actinic radiation. The method of claim 7 wherein said composition is cured by means of thermal energy. The method of any one of claims 7, 8 and 9 wherein said curable binder is a monomer having at least two ethylenically unsaturated moieties. The method of any one of claims 7, 8, 9 and 10 wherein said composition includes a reactive diluent.

Inventors: CHASMAN, JONATHAN N., MINNESOTA MINING AND, HEGEL, RAMON F., MINNESOTA MINING AND, KENDALL, PHILIP E., MINNESOTA MINING AND, POSTMA, NATHAN B., MINNESOTA MINING AND, SPENCER, DOUGLAS S., MINNESOTA MINING AND
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1986
Main classification: B24D3
IPC codes: B24D3, B24D11, B24B37