US 3673746 A
A glass workpiece is polished to obtain a surface having a high luster and a substantial absence of polishing defects by applying to the surface a slurry of a glass powder of substantially the same hardness and composition as the workpiece and a liquid carrier. The slurry also contains a buffering agent which stabilizes its pH between 8 and 12 and preferably between 10 and 11. Its specific gravity preferably lies between 1.14 and 1.19. A rotating polishing pad made of a synthetic fiber is then pressed against the workpiece to polish the piece.
Description (OCR text may contain errors)
United States Patent [1 3 673 746 Highberg July 4, 1972 54] METHOD OF POLISHING GLASS 3,404,488 10/1968 Cox ..s1/2s4  Inventor: Cafle w. Highberg Sylvania Ohio 3,587,196 6/1971 Dunn ..5l/283 X  Assignee: Engelhard Hanovia, lnc., Newark, NJ. i y mmi rD0nfl|d K lly Feb 3 1971 AttorneyBrumbaugh, Graves, Donohue & Raymond 21 Appl. No.: 112,428 [5 1 B RAC A glass workpiece is polished to obtain a surface having a high 52 us. Cl ..51/283 luster and a Substantial absence of Wishing defects by apply-  Int. Cl ..B24b 1/00 ing to the Surface a Slurry ofa glass Powder of Substamiany the 581 Field of Search ..5 l/283 284 3023 same hardness and compsifin as and a quid carrier. The slurry also contains a buffering agent which stabil- 5 f izes its pH between 8 and 12 and preferably between 10 and l 6] Re erences Cited 11. Its specific gravity'preferably lies between 1.14 and 1.19. UNITED STATES PATENTS A rotating polishing pad made of a synthetic fiber is then 2 000 240 /1 5 L 51/283 pressed against the workpiece to polish the piece.
Touvay ..5l/283 14 Claims, No Drawings METHOD OF POLISHING GLASS BACKGROUND OF THE INVENTION The present invention relates to methods of polishing glass, and more particularly to a novel and highly effective method of polishing glass in which glass powder is used as an abrasive.
According to conventional methods of producing a glass article having a highly polished surface, such as those desired on mirrors and television faceplates, a glass workpiece is first subjected to a grinding process. Grinding is accomplished using a relatively large grained loose abrasive such as sand or a diamond or cubic boron nitride grinding wheel where the abrasive particles are fixed in a rigid matrix.
After grinding, the workpiece is conventionally subjected to fine polishing with a relatively fine grained abrasive such as cerium oxide, zirconium oxide or iron oxide. The fine grained abrasive forms part of a slurry that is applied to the coarse ground surface. The workpiece is then conventionally held on a polishing table which reciprocates or rotates as it is fed past a rotating polishing pad formed of natural felt. This type of operation is often carried out using a vertical-spindle reciprocating or rotary lapping machine.
It has also been proposed that more than one step fine grinding or smoothing be carried out, employing mixtures of abrasives including substances such as quartz, sand, glass and porcelain. It has been further proposed in US. Pat. No. 2,000,240 that particles of ground glass could be utilized for an intermediate fine grinding step of the overall process.
SUMMARY OF THE INVENTION The process of this invention is an improvement of considerable significance in the art of polishing glass. By this process, a higher quality product can be obtained more simply, economically and at a high rate of productivity. A surface may be produced which is substantially free of polishing defects and has an unusually high luster.
The process uses as an abrasive a glass powder of substantially the same hardness as the workpiece. The powder is suspended in a liquid carrier to form a slurry. The carrier may be ordinary tap water. The pH of the slurry is maintained at relatively high alkaline level by the inclusion of a bufiering agent. If the abrasive particles are of a relatively large size, a suspending agent may be incorporated in the slurry.
The glass powder may be, and preferably is, of the same composition as well as the same hardness as the workpiece. The need to periodically replenish the abrasive supply is therefore eliminated to some extent because the glass removed during the polishing operation can be allowed to become part of the slurry and thereby replenish the abrasive supply. It is thus always possible to use the same slurry for several successive polishing operations. In some cases more abrasive material is formed than is used up during successive polishing operations, and in these cases successive polishing operations can be carried out indefinitely without further abrasive replenishment. Accordingly, the expense of added abrasive is reduced or eliminated and it is not necessary to frequently interrupt the polishing operation while the abrasive supply is replenished.
The abrasive required to initiate the polishing operation is readily and economically available as a byproduct of the grinding operation which precedes the polishing only when fixed abrasive (such as diamond) grinding is employed.
The polishing operation of the invention is most effectively carried out when the pH of the polishing slurry lies between approximately 8 and 12. The best results are generally obtained when the pH lies between 10 and ll. The pH is stabilized at the desired alkaline level by incorporating a bufiering agent in the slurry. Any of a large number of known bufiering agents, such as a mixture of sodium hydroxide and boric acid could be used. It is highly preferable to use polishing pads made of synthetic fibers, such as nylon, rather than natural fibers, such as wool, because synthetic fibers are better able to withstand the relatively high pH of the slurry.
The specific gravity of the slurry preferably lies between approximately 1.14 and 1.19 and the preferred size of the glass particles lies between approximately 3 and 50 microns.
DESCRIPTION OF THE PREFERRED EMBODIMENT According to the invention, a highly polished glas surface is obtained by first subjecting a glass workpiece to a conventional grinding operation, such as grinding with a fixed abrasive such as, but not limited to, a diamond wheel. The final fine polishing of the glass surface is then begun. An example of a suitable workpiece which is ready to be fine polished and from which the ground glass powder may be obtained during the grinding process is a soda-lime glass plate of the type used for color television faceplates having a surface roughness of approximately 3.5 to 5.0 microinches (root-meansquare) as measured by a profilometer.
A slurry consisting of such glass powder and a carrier comprising ordinary tap water is applied to the ground surface. It is not necessary to add any other abrasive to the slurry insofar as glass powder of the same hardness and composition as the workpiece is preferred in the instant invention. It is, therefore, possible to utilize the glass powder removed during the grinding operation as an abrasive to initiate the polishing operation. Glass particles removed from the workpiece during the polishing process become part of the abrasive component of the slurry and permit the process to be continued without replenishment of the abrasive supply from another source.
The pH of the slurry has a very significant effect on the glass removal rate and the quality of the surface. Substantially higher pH values than those present during conventional polishing operations are preferred. Slurries having pH values ranging from approximately 8.0 to 12.0 yield good results, with the optimum pH range lying between approximately 10.0 and 11.0.
The pH of the slurry is stabilized at the desired high level by the addition of a conventional buffering agent.'The presence of the buffering agent causes the slurry to resist changes in its ion concentration upon the addition of an acid or an alkali. If it were not for the buflering agent the pH of the slurry would tend to be much lower and would become slightly more alkaline as the process continued. This would be undesirable because the effectiveness of the process is diminished if the pH of the slurry departs from the preferred range and because the continuous change which would otherwise occur in the pH would cause the process and the product to take on a nonuniform character.
An example of a suitable buffering agent is a mixture of sodium hydroxide and boric acid. A slurry containing this buffering agent can be prepared by dissolving 4 gms of sodium hydroxide in 1,000 ml of water to form a first solution and dissolving 6.2 gms of boric acid in 1000 ml of water to form a second solution. A quantity of 440 ml of the first solution is then combined in 500 ml of the second solution and 60 ml of plain water. The resulting slurry has a pH of 10.5 which is highly desirable.
The size of the glass particles when a batch is first prepared preferably ranges from about 3 to 50 microns with the optimum glass powder size range being approximately 5 15 microns. When the glass particle size is less than approximately 15 microns, an adequate suspension can readily be obtained 7 without using a suspending agent. When larger particles are used, however, it is generally preferable to include a suspending agent, such as plaster of paris, in the slurry. Larger glass particles (25 to 50 microns) have the advantage of providing a higher rate of glass removal without significantly affecting the quality of the surface produced.
The thickness of the slurry also affects the glass removal rate and the surface quality. The best results can be obtained using a slurry having a specific gravity which lies between approximately 1.14 and 1.19. This is substantially thicker than a conventional abrasive slurry.
It has been found that polishing pads of the type conventionally used to polish glass are unsuitable for use in the process of the invention. Conventional pads result in an inferior surface, a low rate of glass removal, and have a short pad life. This is because conventional pads are made of natural fibers which are adversely affected by the relatively high pH solutions that are used in this process. The present invention is practiced using pads made of synthetic fibers. Good results are obtained using pads made of polypropylene fibers. An even better pad material is nylon which can be used in the form of a felt or a carpet-type material. Felt yields somewhat more desirable results and is advantageous because it is commercially available in thicknesses of up to one-half of an inch. The thickness of the pad is important because it contributes to pad life, thereby affecting the down time required for pad replacement.
Under equal operating pressures applied to the polishing pad, glass powder develops relatively little drag in comparison to conventional polishing abrasives such as cerium oxide. Because of this low drag, it is possible to increase the pressure on the polishing pad to approximately two or three times that used with conventional abrasives without increasing the horsepower required. Moreover, it is possible to greatly increase the pressure applied to the pad without developing vibrations in the mechanical polishing apparatus. Thus, because of the low drag, operating pressure can be increased to pounds per square inch, for example, at which pressure the glass removal rate obtained using glass powder exceeds that which can be obtained using conventional batches of polishing abrasives.
Productivity is increased and a superior polished surface can be obtained using the glass powder polishing process described herein, particularly at higher operating pressures. Possibly'this surprising observation is due to a healing effect on the surface of the workpiece, i.e., the glass near a surface defect is welded or fused tending to smooth and fill the defect. This healing efiect then might reduce the quantity of glass which must be removed to obtain the highly polished surface desired. This observation is demonstrated by the following tests carried out on 2-inch diameter circular discs of soda-lime glass with equivalent diamond grinding history. In Example I, cerium oxide, a conventional polishing abrasive, was used. In Example ll, glass powder was used as an abrasive. Both processes carried out until grinding damage could no longer be observed using a microscope having a magnification power of 100.
EXAMPLE 1 Cerium Oxide Polishing Pad Bausch 8t Lomb Optical Polishing Pad Cerium Oxide Content of Slurry 1% by Wt. Pressure 23.5 lbs. Polishing Time 15 minutes Glass removal required to eliminate diamond grinding damage l 100 Microinches.
EXAMPLE ll Glass Powder Polishing Pad Huyck 9000 C-2 Felt Glass Powder Size 13.0 Microns Average Spec. Grav. of Slurry l. l7 gnns./c.c. pH of Glass Powder Slurry 10.5 suspending Agent 6.5% by Wt. Plate Glass Stucco Pressure 34.5 lbs. Polishing Time l3 minutes Glass removal required to completely eliminate diamond grinding damage 775 Microinches.
When cerium oxide was used it was necessary to remove 1,100 microinches of glass to eliminate the scratches produced by diamond grinding. When glass powder was used 775 mieroinches of removal was required. Both workpieces originally contained diamond grinding scratches having a depth of l rnicroinches.
The examples described above show that when the glass powder process is used, the same diamond grinding damage can be eliminated with the removal of 30 percent less glass from the workpiece. Thus, even if the glassremoval rates are the same, and they can be higher when using glass powder, productivity can be increased 30 percent using the process of the invention.
Because of the high polishing effect of the process, and because of the other advantages described above, a smoother surface of significantly higher quality can be obtained at a higher production rate as compared to conventional processing. The superiority of this process is, in part, attributable to the higher rate of glass removal obtainable and, in part,
due to the fact that not as much glass removal is required. The superiority of this process is also partly attributable to the elimination of the need to replenish the abrasive supply because the glass removed from the workpiece becomes part of the slurry and replenishes the abrasive component of the slurry.
It will be obvious to those skilled in the art that the abovedescribed embodiment is meant to be merely exemplary and that it is susceptible of modification and variation without departing from the spirit and scope of the invention. Therefore, the invention is not deemed to be limited except as defined by the appended claims.
1. A method of polishing a glass workpiece comprising ap-- plying a slurry to the surface of the workpiece and pressing a moving polishing pad made of a synthetic fiber against the surface to polish the workpiece, said slurry comprising an abrasive component consisting essentially of glass powder of substantially the same hardness as the workpiece, a liquid carrier and a buffering agent, the pH of said slurry being maintained by said buffering agent between about 8 and 12.
2. The method of claim 1 wherein the pH of said slurry is maintained by said buffering agent between about 10 and l l.
3. The method of claim 1 wherein the glass powder is of the same composition as the workpiece.
4. The method of claim 3 wherein the glass workpiece is first subjected to a grinding process and wherein the glass removed from the workpiece during the grinding of the workpiece forms at least part of the abrasive component of the slurry whereby the need for replenishment of the abrasive from another source is diminished.
5. The method of claim 1 wherein the liquid carrier is tap water.
6. The method of claim 1 wherein the particles forming the glass powder have a size range between approximately 3 and 50 microns.
7. The method of claim 1 wherein the particles forming the glass powder have a particle size range of between approximately 15 and 50 microns and the slurry includes a suspending agent.
8. The method of claim 1 wherein the specific gravity of the slurry lies between approximately 1. l4 and 1.19.
9. The method of claim 1 wherein the workpiece is ground with a fixed abrasive wheel before the slurry is applied.
10. The method of claim 9 wherein the fixed abrasive wheel is a diamond wheel.
11. The method of claim 1 wherein the polishing pad is made of nylon.
12. The method of claim 1 wherein the polishing pad is made of nylon felt.
13. The method of claim 1 wherein the workpiece is made of soda-lime glass.
14. A method of polishing the surface of a diamond ground glass workpiece following the grinding of said workpiece comprising applying thereto a slurry having a specific gravity ing polishing pad made of nylon felt to the workpiece, the glass removed from the workpiece during the coarse grinding thereof forming at least a part of the slurry whereby the polishing operation can be carried out without continuously replenishing the abrasive from another source.