FIXED OPHTHALMIC LENS POLISHING PAD

Abstract: A pad for polishing optical quality surfaces, such as lenses, comprises a flexible support (23, 23′) and a flexible layer (21, 21′) of polishing material secured to the support, the polishing material layer comprising a matrix of a latex material and a water-soluble polymer, the matrix containing polishing particles such as cerium oxide or the like. The pad is such that, in use with water, the polishing particles are gradually released by slow dissolution of the polymer. One form of polishing layer is produced by mixing a water-­ soluble polyalkylene oxide/phenolic complex with an acrylic latex and an alcoholic slurry of polishing particles.

Description: This invention relates to a pad for polishing optical quality surfaces, e.g. lenses. The principal steps in producing polished optical surfaces, such as for example polished surfaces on glass lenses, comprise three successive operations - namely, a rough generating step using a tool containing a coarse, hard abrasive such as diamond particles, or the like; a grinding or fining step using finer abrasive particles to remove deep scratches and to compensate for slight generating errors and to produce the desired curve on the lens itself; and a final polishing step using a compound of extremely fine particle size for removing small scratches and to provide a smooth lens surface of optical quality. The conventional method of polishing lenses has been to employ a liquid slurry comprising, for example, very fine polishing particles in an aqueous solution. The slurry is applied to the interface between the surface of the lens and the associated polishing pad or lap. Because of the obvious inconvenience of having to employ a slurry which contains polishing particles, efforts have been made over the years to provide a satisfactory polishing pad which can be secured over a polishing lap, and which itself contains the necessary polishing particles. In this way, only water need be applied to the pad during polishing. U.S. Patent no. 4,255,164 discloses a flexible, glass fining sheet or pad in which abrasive particles or granules are secured in a water-insoluble resinous binder, such as for example a thermosetting polymer modified by a small amount of thermoplastic polymeric latex. During the grinding (fining) operation, it is only necessary to apply water to the interface between the fining pad and the lens surface in order to create the necessary fining slurry. However, as pointed out in column 1 of this patent, a fining sheet or pad of this type is only for grinding (fining) the lens surface. The abrasive granules employed for such purpose, therefore, have a Knoop hardness of at least about 1,000 and an average particle size of about 10 to 80 µm, and these size ranges are not satisfactory for polishing purposes. Moreover, these abrasive particles are released from the water-insoluble binder during grinding as the result of the gradual mechanical erosion of the binder due to the effects of loading and surface friction. For both fining and polishing operations, experience has shown that best results are achieved when the abrasive or polishing particles are free to roll or move in the slurry generated between the lens surface and the fining or polishing pad surface. In the case of the above-noted U.S. patent no. 4,255,164, this release of abrasive particles depends solely upon the mechanically induced failure of the binder matrix, rather than upon the binder solubility. We have now found, however, that it is not only possible, but is actually more desirable, to have a polishing pad containing a water-soluble binder which, during use, dissolves at a rate that permits the controlled release of the polishing particles at a pre-determined rate, thus considerably increasing the quality, convenience and efficiency of the polishing operation. Although attempts heretofore have been made to produce a polishing or grinding matrix comprising a water-soluble binder composition, the efforts have proved to be unsatisfactory because of the uncontrolled, rapid disintegration of the matrix. U.S. patent no. 3,042,509, for example, proposes a water-soluble matrix of abrasive particles in admixture with polyethylene glycol (20-80%). Such a matrix is solid at room temperature, and has good lubricating properties during use. The problem with this type of matrix is, however, that it dissolves far too rapidly during use, and if used for polishing purposes is incapable of approaching a stock removal rate common to conventional slurry polishing techniques. By way of example, laboratory tests conducted on a conventional Coburn 505 polisher indicate that it is commonplace to achieve stock removal rates of approximately 120 mg of glass per twelve minutes of polishing a glass lens of 55.5 mm diameter using a conventional "Pellon" pad under 30 psig (207 kPag), and a slurry comprising a 5% concentration of a cerium oxide polishing compound of the type distributed by Transelco Division of Ferro Corporation under the name "Ce-Rite" Rx 419. More recently, U.S. patent no. 4,138,228 has suggested incorporating a polishing abrasive having an average particle size of less than 10 µm (microns) in a micro-porous polymeric structure, which exists in the form of tiny platelets, rather than in the form of a monolithic film. The alleged advantage of this invention is that the abrasive particles are adhered on the surfaces of the platelets, or at the most are only slightly embedded in the platelets, so that when the abrasive surface of the pad is rubbed against the surface of a glass lens in the presence of water or the like, the combined action of the rubbing and the absorption of the liquid into the micro-porous or sponge-like polymer matrix effects controlled release of the polishing abrasive from the surface of the platelets. The essence of this type of pad is the fact that the particles are substantially entirely unencapsulated by the binder, so that during the polishing process they are released as the result of the mechanical activity generated during polishing. We have now devised an improved polishing pad which utilizes a water-soluble matrix that is specifically designed to provide controlled, gradual release of polishing particles during a polishing operation. According to the present invention there is provided a pad for polishing optical quality surfaces, including a flexible support (23,23'), and a flexible layer (21,21') of polishing material secured to one surface of said support, said layer of polishing material comprising a flexible matrix secured to said one surface of said support and containing a plurality of polishing particles (31), characterised in that said matrix comprises in combination (32) a latex material, a complex of a water-soluble polyalkylene oxide and a water-insoluble phenolic resin, in which said polyalkylene oxide is present in an amount of at least 30% of the total dry weight of the polyalkylene oxide and phenolic resin complex; and in that, in use of the pad in the presence of water, dissolution of the polyalkylene oxide causes gradual release of the polishing particles; the quantity of latex being no more than the quantity of polyalkylene oxide on a dry weight basis. In one preferred pad of the invention, the flexible polishing material layer is prepared by mixing a water-soluble polyalkylene oxide-phenolic resin complex with an acrylic latex and a cerium oxide-alcohol slurry. More specifically, the polyalkylene oxide preferably has an alkylene carbon chain of 5 or less such as, for example, polyethylene oxide. In one example, polyethylene oxide is combined with a phenolic resin to form a complex which is then mixed with an acrylic latex and a cerium oxide/alcohol slurry in amounts by weight which may be, by way of example, approximately 16%, 8%, and 76% respectively. The rate at which the cerium oxide polishing particles are released from the polyalkylene oxide/phenolic resin acrylic binder or matrix is a function of the rate at which the water-soluble binder dissolves when water is applied to the pad during a polishing operation. This dissolution rate is also a function of the weight ratio of the polyalkylene oxide to the phenolic resin component, and for purposes of this invention, this ratio is preferably in the range of 30-70% polyalkylene oxide to 70-30% phenolic resin. In order that the invention may be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a plan view of a typical polishing pad made according to one embodiment of this invention; Figure 2 is a greatly enlarged, fragmentary, diagrammatic view of this pad as it would appear in section taken along the line 2-2 in Figure 1; Figure 2A is a fragmentary sectional view similar to Figure 2 but showing a modified form of this pad; Figure 3 is a photomicrograph showing enlarged by 1000x a plan view of a portion of the polishing surface of an unused pad of the type shown in Figure 1; Figure 4 is a photomicrograph showing enlarged by 50x an upside down cross sectional view of a fractured portion of the pad shown in Figure 3; and Figure 5 is a photomicrograph of the same cross sectional view shown in Figure 4, but enlarged by 10,000x. Referring now to the drawings, reference numeral 10 denotes generally a rosette shaped polishing pad having therein a first set of four, equi-angularly spaced radial slots 12, which divide the pad into four, similarly shaped leaf or petal shaped sections 13. Each of the leaf sections 13 is in turn subdivided into two sections by a second set of four, radially extending slots 14, which are formed in the pad in equi-angularly spaced relation to one another and to the slots 12. Slots 14, it will be noted, do not extend radially inwardly as far as the slots 12, and each of the slots 12 and 14 is rounded at its inner end. The rosette or flower-like configuration of pad 10 serves the dual purpose of permitting the pad better to conform to polishing or lapping tools, when the pad is used for polishing curved lenses, and at the same time permits rapid penetration and dispersement of the water supply which is used during the polishing operation. Pad 10 comprises a polishing layer or face 21, which has been deposited on the upper surface of a flexible, fabric substrate 23 or 23', which is designed to provide a cushioning and reinforcing support for the polishing layer. The upper surface of the reinforcing substrate may be substantially planar, as with layer 23 (Figure 2) in which case it would be completely covered by the polishing layer 21; or alternatively, the upper surface could contain spaced recesses or corrugations as in the case of layer 23' (Figure 2A). In the embodiment shown in Figure 2A, the polishing layer could be applied either completely to cover the surface of layer 23' or partially to fill its recesses as at 21' in Figure 2A, so that portions of layer 23' will project above the polishing layer 21'. In either embodiment portions of layer 21 or 21' may actually penetrate into the reinforcing substrate. Secured to the back or underside of substrate 23 by a layer 25 of adhesive is a thin film or layer 26 of plastic, such as polyester. Coated on the back or lower side of this plastic film is a layer or coating 27 of a pressure sensitive adhesive material, the underside or lower face of which is covered in a conventional manner with a removable layer 28 of release paper, which shields the pressure sensitive adhesive until the pad 10 is placed in use. The fabric substrate 23 may be made, for example, from a spun bonded polyester such as is sold by E.I. du Pont de Nemours & Co. under the trademark "Reemay". Alternatively, of course, non-woven nylon or woven polyester, polyester/cotton blends, cotton and similar fabrics could be employed for this purpose. The reinforcing film 26 cooperates with the substrate 23 to enable the pad 10 to be removed or peeled from a polishing lap when the pad requires replacement. In this connection the adhesive layer 27, which is used to adhere the pad 10 to a lap, should have good wet shear strength to prevent pad movement during polishing and moderate peel strength so that it can be peeled from the lap without leaving objectionable traces of the adhesive on the lap surface. The polishing layer 21 comprises two basic components: a plurality of fine polishing particles which are denoted by way of example at 31 in Fig. 2, and a resinous matrix or binder 32 in which the particles 31 are dispersed. The polishing particles 31 may comprise cerium oxide particles, or any suitable polishing compound having an average particle size in the range of from less than 0.5 µm (microns) to less than 10 µm (microns), and with a typical range of from 1.0 to 8.0 µm (microns). It has been found that a particularly suitable water-soluble binder or matrix 32 can be produced by combining an acrylic latex or the like, with a complex of polyalkylene oxide and phenol formaldehyde resin, wherein the polyalkylene oxide has a molecular weight in the range of 100,000 to 600,000. Various ways of associating or complexing a phenolic resin component with a polymeric oxygen ether component are disclosed in U.S. patent no. 3,125,544. In the case of the present invention, the complexing of these components might be visualized as a loose network haying hydrogen bonding between phenolic hydroxyl groups in the phenol-formaldehyde resin and oxygen in the polyalkylene oxide polymer: As will be apparent hereinafter, the solubility of this particular complex is, for the most part, dependent upon the ratio of the phenolic component to the polyalkylene oxide component. The higher the phenolic content,the more insoluble is the binder; while on the other hand the higher the content of the polyalkylene oxide, the more soluble is the binder. The best ratios to obtain the desired solubility of the binder depend, among other factors, upon the reactivity of the phenolic component. Another important factor which affects the solubility of this complex is the inclusion of alcohol, which as noted hereinafter, is employed during preparation of the polyalkylene oxide/phenolic complex and, preferably, in the slurry. Water miscible alcohols appear to have some solvating effect on the hydroxyl groups in the phenol formaldehyde resin, and conceivably delay the rapid complexing of the system by stabilizing the phenolformaldehyde resin. To determine the most desirable composition for the water soluble binder 32 (Fig. 2 and 2A),a series of tests was conducted on polishing pads for which the matrix material was made in accordance with the following Examples. The percentages are all by weight and, except for water and isopropanol, are on a dry basis. EXAMPLE NO. 1 Polyox (polyethylene oxide), for example as sold by Union carbide under the designation "WSRN-80", alcohol (isopropanol) and water were mixed in the weight preparations 20%, 40% and 40%, respectively. (Preferably the alcohol and water are mixed first, and the Polyox is then added.) This Polyox solution was then combined with phenol formaldehyde resin (e.g. Union Carbide "BRL-1302") in a one to one ratio (50% of the Polyox, dry wt.,and 50% phenol formaldehyde resin). A cerium oxide slurry was then prepared by mixing a commercially available polishing compound containing fine cerium oxide particles (e.g. "Ce-Rite" 403) with a water-miscible alcohol, again such as isopropanol. An acrylic latex and the Polyox/phenolic resin complex were then added to the cerium oxide slurry in the amounts of 8% latex, 16% Polyox/phenolic resin complex, and 76% cerium oxide slurry. EXAMPLE No. 2 The same procedures were followed as in Example 1, except that the ratio of polyethylene oxide to phenol formaldehyde resin during preparation of the Polyox/phenolic resin complex was 40% Polyox to 60% phenolic resin. EXAMPLE No. 3 The same as Example 1, except that the Polyox to phenolic ratio was 60% to 40%. EXAMPLE No. 4 The same as Example 1, except that the Polyox to phenolic ratio was 30% to 70%. EXAMPLE NO. 5 A cerium oxide slurry was prepared by mixing cerium oxide particles (75 wt. %) with a solution (25 wt. %) of equal parts of water and isopropanol. Polyox (WSRN-750) was mixed with water in ratios of 10% Polyox to 90% water. The cerium oxide slurry and polyox-water solution were mixed together in the dry weight ratios of 90% cerium oxide and 2.4% Polyox, and then combined with 5.2 dry wt. % of phenol formaldehyde resin (Union Carbide BRL1100). An acrylic latex (e.g. Union Carbide's Ucar 189) was then added in an amount of 2.4 wt. % to complete the polishing layer formulation. EXAMPLE NO. 6 The same procedures were followed as in Example 5, except that the Polyox material was of the WSRN-80 variety and was mixed with water in the ratio of 20% Polyox to 80% water. This solution was mixed with a cerium oxide slurry of the type noted in Example 5, but in dry weight ratios of 93% cerium oxide and 2.6% Polyox. Phenol formaldehyde resin (Union Carbide BKUA 2370) was then added in the amount of 2.9 wt. % followed by 1.5 wt. % of Ucar 189 acrylic latex. EXAMPLE NO. 7 (comparison) Polyethylene oxide (Union Carbide WSRN-80) was mixed with a quantity of a non-ionic surfactant ("Tergitol NP-13") in an amount sufficient to prevent flocculation of the Polyox when subsequently mixed with a cerium oxide polishing compound. This Polyox/surfactant composition was mixed in an amount of approximately 15% with an acrylic latex (8%) and a cerium oxide polishing compound in water (77%). In use, pads 10 having thereon a polishing layer matrix 32 made in accordance with Examples 1, 5 and 6 (above) were found to be most effective in exhibiting controlled release of the polishing particles during polishing. Tests have indicated that these results are attributable to the gradual dissolving of the thermoplastic matrix or binder system during polishing of glass lenses using only water. A matrix or binder made from this material results in a polishing layer which is thermoplastic and embosses rather easily. This is a desirable property in connection with a polishing pad of the type described in Figures 2 and 2A, since the embossing allows water to seep in and around the embossed portions of the pattern, thus enhancing polishing and also preventing undesirable suction between the pad and the lens which is being polished. As shown perhaps more clearly in Figures 3 - 5, wherein the cross sectional views of Figures 4 and 5 were prepared by lowering the pad temperature below its brittle transition temperature with liquid nitrogen and then fracturing the pad, it will be apparent that the binder material in the polishing layer 21 forms a relatively homogeneous, monolithic film in which the cerium oxide particles are bonded with the polyalkylene oxide polymer. Tests have indicated that it is the polyalkylene oxide binder which goes into solution during polishing with a water slurry, and in so doing slowly releases the cerium oxide particles for rolling movement between the polishing pad and the surface that is being polished. As used in connection with the polishing pads of the present invention, the water slurry refers to the water which is applied to the interface between a polishing pad and, for example, a lens during the polishing of the latter. The alcohol and water slurries referred to in Examples 1 to 6 exist in slurry form only for the purpose of enabling the cerium oxide particles and the complex polymer matrix material to be coated in a thin layer on the substrate 23 23', after which the liquids in these slurries evaporate, leaving the flexible polishing layer 21 or 21' on the associated substrate. As noted above, Examples 1, 5 and 6 provide the most desirable binders and the best glass removal rates during polishing, ranging from 120 to 144 mg. per twelve minutes. The binders of Examples 2 and 3 also provide a gradual release of the polish particles during use of the pad with water, but result in a somewhat less desirable binder than that produced by Examples 1,5 and 6. Example No. 4 was not satisfactory because the pad matrix was nearly insoluble in water during use, and was extremely difficult to emboss. Example 7 was not in accordance with the invention. It utilized a mixture of latex, Polyox and a cerium oxide slurry in water, and whilst being capable of good glass removal during polishing, it proved to be too soluble in water during use. Also its tendency to flocculate produced inconsistent test results. In addition to the above-described Examples, an all latex binder system was tested but proved to be very insoluble in water and did not satisfactorily release the polishing particles during use. Tests were also conducted using water-soluble polymers as the sole binder (exclusive of latex), but these binder systems proved to be too soluble and released the polishing particles too rapidly with consequent poor polishing results. HIn all the tests which were conducted, the effectiveness of a given pad was not dependent upon the rate at which water was supplied to the interface between the pad and the lens being polished. This contrasts with some types of pads which require careful control of the rate of application of the water to the polishing interface. From the foregoing, it will be apparent that the invention provides polishing pads having a polishing layer binder made from a water-soluble polyalkylene oxide polymer, a latex, and a phenolic resin component which tends to reduce the solubility of the polymer during polishing operations of the type described herein. Such a polyalkylene oxide/phenolic resin latex binder system also functions most effectively when prepared in the presence of alcohol and water. It will be apparent to one skilled in the art, that instead of cerium oxide particles, the slurry may contain other polishing particles, such as for example iron oxide or zirconium oxide, particularly in connection with the polishing of glass lenses. For polishing other types of vitreous surfaces, or for plastics lenses, still other known types of polishing particles can be employed. Moreover, it will be obvious also that the various components of the matrix 32 do not have to be mixed in the precise order disclosed by the above-noted Examples. For example, when isopropanol is used, it matters not if it is mixed with either the cerium particles, the polyethylene oxide, or both.

Claims: A pad for polishing optical quality surfaces, including a flexible support (23,23'), and a flexible layer (21,21') of polishing material secured to one surface of said support, said layer of polishing material comprising a flexible matrix secured to said one surface of said support and containing a plurality of polishing particles (31), characterised in that said matrix comprises in combination (32) a latex material, a complex of a water-soluble polyalkylene oxide and a water-insoluble phenolic resin, in which said polyalkylene oxide is present in an amount of at least 30% of the total dry weight of the polyalkylene oxide and phenolic resin complex; and in that, in use of the pad in the presence of water, dissolution of the polyalkylene oxide causes gradual release of the polishing particles; the quantity of latex being no more than the quantity of polyalkylene oxide on a dry weight basis. A pad according to claim 1, wherein the ratio by dry weight of said polyalkylene oxide to said phenolic resin is in the range of 30-70% polyalkylene oxide to 70-30% phenolic resin. A pad according to claim 1 or 2, wherein said polyalkylene oxide has an alkylene carbon chain of 5 or less. A pad according to claim 1, 2 or 3, wherein said polyalkylene oxide is mixed with a non-ionic surfactant, and is present in a weight percentage approximately twice that of the latex. A pad according to any of claims 1 to 4, wherein said support comprises a substrate (23,23') having said layer (21,21') of polishing material secured to one side thereof, a thin, tough film (26) of plastic secured to the opposite side of said substrate, and adhesive means (27) on the side of said film opposite said substrate for removably attaching said pad to a tool. A pad according to claim 5, wherein said one side of said substrate has therein a plurality of spaced recesses (12,14) at least partially filled with said polishing material. A pad according to any of claims 1 to 6, wherein said flexible support is made of a spun bonded polyester. A pad according to any of claims 1 to 7, wherein the latex is an acrylic latex. A pad according to any of claims 1 to 8, wherein the polishing particles are of cerium oxide.

Inventors: DE PIERNE, OTTO S., SHUKLA, JAYENDRA G., SOHN, KI GAP, TWICKLER, CARL
Applicants: FERRO CORPORATION
Application year: 1985
Main classification: B24D11
IPC codes: B24D3, B24B13, B24D11