US3107464A - Method of polishing methyl methacrylate sheets - Google Patents

Description

United States Patent @ffice 3,107,464 Patented Oct. 22, 1963 3,107,464 METHOD OF POLISHING METHYL METHACRYLATE SHEETS Richard L. Cook, Phoenix, Ariz., assignor to Goodyear Aircraft Corporation, Akron, Ohio, a corporation of Delaware No Drawing. Filed Dec. 4, 1961, Ser. No. 156,998

6 Claims. (Cl. 51-323) This invention relates to improved methods for polishing methyl methacrylate sheets, and, more particularly, is concerned with the elimination of haze, cracks, and surface crazing, in plastic sheets of the indicated type.

Heretofore, it has been the usual practice in removing haze, and the like, from methyl methacrylate sheets to grind and polish the surface of the sheet for a sufiicient period of time, and to the required depth, to remove the haze, but this often becomes such a laborious and timeconsuming operation that the operation is so very expensive as to be impractical.

It is the general object of the invention to avoid and overcome the foregoing and other difficulties of and objections to prior art practices by the provision of a relatively rapid, inexpensive, and improved method for elim inating haze, and other surface blemishes, upon methyl methacrylate sheets.

Another object of the invention is the provision of an improved process for resurfacing methyl methacrylate sheets, and particularly stretched sheets of cross linked methyl methacrylate, such as sold under the trademark Plexiglas 55.

The foregoing objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by that method of resurfacing a stretched methyl methacrylate sheet which includes the steps of subjecting the surface of the sheet to the action of a non-solvent wetting agent mixed with unpolymerized methyl methacrylate monomer for a selected period of time, the monomer acting to soften and fill the surface to the desired depth, and the wetting agent acting to slow the evaporation rate of the monomer and to slow its solvent action, gradually terminating the action of the monomer by the application of additional wetting agent and fine abrasive powder to the surface, and grinding the softened surface to the polish desired.

The principles of the invention are particularly applicable to the resurfacing of methyl methacrylate sheets wherein the molecules of the sheets are cross linked, as in Plexiglas 55, and to sheets of this type which have been stretched to flat or contoured form. In one particular application of the invention stretched airplane canopies of transparent Plexiglas 55 have been resurfaced to an optically satisfactory condition in a period of time equal to about ten percent of the time that would have been required to resurface the sheets with the conventional grinding and polishing operations of the prior art.

In the practice of the invention the methyl methacrylate sheet to be resurfaced is first treated with a solvent A, this constituting a mixture of a non-solvent wetting agent, such as kerosene, and an acrylic monomer, which is a thin, clear, volatile liquid constituting the basic material in unpolymerized form from which the methyl methac rylate sheet is made. In one typical example solvent A constituted:

Percent MMA monomer 50 Kerosene 50 the methyl methacrylate sheet to the depth of the haze or surface cracks, for example, up to ninety percent of the depth of such haze. Solvent A can be applied to only one surface of the sheet by laying the sheet horizontally and applying solvent A thereto in a layer, or by coating the surface to be refinished of a contoured sheet of methyl methacrylate, or by dipping the sheet to be resurfaced into a container containing solvent A.

Usually solvent A is applied to the surface of a severely hazed methyl methacrylate sheet to be refinished for a period of about sixty minutes. However, for less severe conditions of surface hazing, the time that the sheet to be resurfaced is subjected to solvent A can be reduced, or if the surface haze is particularly deep, the time of application of solvent A can be increased.

The monomer in solvent A acts to soften or swell the methyl methacrylate sheet with the kerosene acting as a wetting agent and acting to retard the solvent action of the monomer.

After the desired softening of the methyl methacrylate sheet has been achieved, and without regard to the residual solvent A on the sheet, additional kerosene and fine abrasive powder, such as grit classified A, is added to the sheet and polishing is begun.

Thus, there is a gradual to rapid reduction in the solvent action of the monomer upon the methyl methacrylate sheet concurrently with the polishing operation, the kerosene acting still as a Wetting agent.

Upon completion of the polishing operation the sheet is washed with naphtha to remove all oily residue.

Several theories have been advanced concerning the operation and the explanation of the operation of the process of the invention. In accord with the most favored theory of operation, solvent A is believed to act as a softening and swelling agent, but with the unpolymerized monomers of the acrylic material present in solvent A acting to fill down into all cracks and voids in the surface to be refinished and being polymerized therein whereby the optical transparency of the surface is materially improved. In accord with this theory, the swollen and softened surface being refinished, after being filled as described, is more readily ground and polished than the surface of the methyl methacrylate sheet which has not been swollen and softened, and with a faster and smoother grinding action being achieved because the surface to be refinished is still wet with the wetting agent in solvent A which has been added to during polishing.

In any event, regardless of the exact technical explanation of what happens in every step of the process of the invention, it has been found that the process of the invention is much more rapid and is much less expensive than prior known grinding and polishing operations, and with the process of the invention resulting in optically transparent resurfaced sheets of methyl methacrylate of commercially satisfactory character. Exposure tests to ultraviolet light and sunlight have demonstrated that the process of the invention produces refinished surfaces on methyl methacrylate sheets which are just as satisfactory weatherwise and exposure-wise as are refinished surfaces made with prior art grinding and polishing operations.

While a certain representative embodiment and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is: Y

1. That method of removing haze and the like from a surface of a transparent cross-linked methyl methacrylate sheet which includes the steps of applying to the surface of the sheet a solution containing about 50% of a nonsolvent wetting agent and containing about 50% acrylic monomers, leaving the solution in contact with the sur face for a period of time sufficient to swell the sheet a distance substantially equal to the thickness of the haze, progressively reducing and then terminating the action of the solution by the application of additional quantities of the wetting agent to the surface, together with abrasive powder, abrasive grinding the surface to the polish desired while continuing to add the wetting agent and abrasive powder to the surface, and washing the polished sheet to remove the wetting agent and abrasive powder therefrom.

2. That method of removing haze and the like from a surface of a transparent cross-linked methyl methacrylate sheet which includes the steps of applying to the surface of the sheet a solution containing about 50% of kerosene and containing about 50% acrylic monomers, leaving the solution in contact with the surface for a period of time sufiicient to swell the sheet a distance substantially equal to the thickness of the haze, progressively reducing and then terminating the action of the solution by the application of additional quantities of kerosene to the surface together with abrasive powder, abrasive grinding the surface to the polish desired while continuing to add the kerosene and abrasive powder to the surface, and washing the polished sheet with naphtha to remove the kerosene and abrasive powder therefrom.

3. That method of removing haze and the like from a surface of a transparent cross-linked methyl methacrylate sheet which includes the steps of applying to the surface of the sheet a solution comprising a non-solvent wetting agent and acrylic monomers, leaving the solution in contact with the surface for a period of time sufficient to swell the sheet a distance substantially equal to the thickness of the haze, progressively reducing and then terminating the action of the solution by the application of additional quantities of the wetting agent to the surface, together with abrasive powder, and abrasive grinding the surface to the polish desired while continuing to add the wetting agent and abrasive powder to the surface.

4. That method of removing haze and the like from a surface of a transparent cross-linked methyl methacrylate sheet which includes the steps of applying to the surface of the sheet a solution containing a wetting agent and an acrylic monomer, leaving the solution in contact with the surface for a period of time sufiicient to swell the sheet a distance approaching the thickness of the haze, and abrasive grinding the surface to the polish desired in the presence of additional wetting agent and fine abrasive powder.

5. That method of removing haze and the like from a surface of a transparent cross-linked methyl methacrylate sheet which includes the steps of applying to the surface of the sheet a solution containing kerosene and an acrylic monomer, leaving the solution in contact with the surface for a period of time suflicient to swell the sheet a distance approaching the thickness of the haze, progressively reducing the solvent action of the solution by the application of additional quantities of kerosene to the surface, together with abrasive powder, and abrasive grinding the surface to the polish desired in the presence of additional kerosene and abrasive powder.

6. That method of removing haze and the like from a surface of a transparent cross-linked methyl methacrylate sheet which includes the steps of applying to the surface of the sheet a solution containing a wetting agent and an acrylic monomer, and abrasive grinding the surface to the polish desired while progressively reducing the solvent action of the acrylic monomer by the addition of the wetting agent only to the sheet plus the addition of fine abrasive.

References Cited in the file of this patent UNITED STATES PATENTS 1,621,780 Hirsh Mar. 22, 1927 2,210,728 Orfald Aug. 6, 1940 2,327,495 Budner Aug. 24, 1943 FOREIGN PATENTS 281,751 Great Britain Nov. 28, 1927

Claims (1)

1. 6. THAT METHOD OF REMOVING HAZE AND THE LIKE FROM A SURFACE OF A TRANSPARENT CROSS-LINKED METHYL METHACRYLATE SHEET WHICH INCLUDES THE STEPS OF APPLYING TO THE SURFACE OF THE SHEET A SOLUTION CONTAINING A WETTING AGENT AND AN ACRYLIC MONOMER, AND ABRASIVE GRINDING THE SURFACE TO THE POLISH DESIRED WHILE PROGRESSIVELY REDUCING THE SOLVENT ACTION OF THE ACRYLIC MONOMER BY THE ADDITION OF THE WETTING AGENT ONLY TO THE SHEET PLUS THE ADDITION OF FINE ABRASIVE.