US3406418A - Lens-cleaning paper - Google Patents

Description

United States Patent Ofice 3,406,418 Patented Oct. 22, 1968 3,406,418 LENS-CLEANIN G PAPER Harry J. Hurley, Jr., 4119 Echo Valley Lane, Newtown Square, Pa. 19073, and Walter B. Shelley, 505 County Line Road, Radnor, Pa. 19087 No Drawing. Filed Sept. 27, 1967, Ser. No. 671,102 9 Claims. (Cl. -10493) ABSTRACT OF THE DISCLOSURE Herein are disclosed improved paper or cloth materials for cleaning lenses and the like. The improved cleaning materials contain adhering and dispersed particles of a finely-divided, Waxy, fluorocarbon telomer, such as tetrafluoroethylene telomer. Certain tissue papers, particularly those known in the art as Wrapping tissue paper, Type I lens tissue paper, Type II lens tissue paper, and cigarette paper, preferably are employed as the base cellulosic sheet material matrix for preparing the improved lens cleaning materials of the invention. Fluoi'ocarbon telomers avail-able commercially under the designation, Vydax fluorotelomers, and particularly that designated Vydax 201G, conveniently are employed. The improved lens cleaning materials are prepared by briefly contacting, such as by spraying or by immersion, the paper or cloth matrix with a dispersion of the solid, particulate fluorocarbon telomer in a liquid medium, and then substantially removing the liquid medium. The improved lens cleaning materials are substantially superior in cleaning effectiveness to conventional and currently available lens cleaning materials such as untreated lens tissue papers and a lens tissue paper treated with silicones.

Background of the invention The removal of grease and dirt films from finelyground and polished glass, plastic and ceramic surfaces is a requisite task of many vocations and avocations. The need to accomplish this as quickly and easily as possible and without injury to the ground surfaces is widely recognized, especially by those who wear eyeglasses, contact lenses or protective goggles, and also by those engaged in photography, microscopy, astronomy and certain laboratory and industrial work.

Surfactant or soap solutions, and solvents or combinations thereof, are commonly used in such cleaning, usually employing a soft, non-abrasive cloth or paper wetted with the cleaning fluid. The surface then must be wiped dry or air-dried. The attendant risks of damage by solvent to certain of the articles to be cleaned, such as for example to lenses mounted in solvent-sensitive cement, are appreciable. Another conventional method involves use of a silicone-treated, cleaning paper. The silicone is said to displace or loosen the grease, and also to deposit a silicone film which temporarily protects the cleaned surface from subsequent dirtying. Other specialized cleaning papers, such as photographic or microscope lens cleaning paper, and of special thickness and fiber construction but without any incorporated chemical cleanser or degreaser, are recommended for cleaning camera and microscope lenses, filters, eyepieces and the like. A solvent also may be applied to the surface by dipping or spraying. Possibly the technique most commonly in use for some articles, such as eyeglasses, is that involving simply breathing moisture onto the glass surface and rubbing vigorously with a soft cloth or tissue paper, repeatedly inspecting the surface and, if necessary, exhaling again onto the glass and rewiping in an effort to remove completely the grease and grime.

All of the conventional methods and cleaning materials involve laborious and time-consuming effort and not infrequently yield an unsatisfactory resulLSuch methods depend as much for their success on the vigor and persistence of the individual doing the cleaning as on the characteristics of technique or tools and materials being used. The ideal, and heretofore unavailable, technique thus would seem to combine a soft, non-abrasive and grease-absorbing paper or cloth, impregnated with a substance which would accomplish the removal of the grease film and any dirt particles with the very minimum of rubbing. Moreover, the impregnating material ideally should .be non-toxic and non-irritating, systemically as well as to skin and eyes, and also should be odorless and inexpensive.

Summary of the invention The present invention resides in the discovery that suitable cellulosic sheet materials, when treated with a special treating agent, exhibit all of the characteristics recited above for an ideal lens cleaning material, and accomplish the desired cleaning result in a far superior fashion. Thus, the newly-developed cleaning materials are markedly superior to conventional len s cleaning paper or tissue, both treated and untreated, and to other known cleaning materials 'and techinques. In general, the new cleaning material consists essentially of a suitable cellulosic sheet material, such as a selected paper or cloth, which contain adhering and dispersed particles of a finely-divided solid substance, in particular a suitable fluorocarbon telomer (or fluorotelomer) such as a solid, waxy tetrafluoroethylene telomer and the like.

Certain conventional tissue papers, such as wrapping tissue, cigarette paper and untreated lens paper, are suitably employed as the matrix for the impregnating or adhering substance or fluorotelomer. Many such papers are commercially available. A suitable cloth material also can be employed as the matrix. Suitable solid fluorotelomers are commercially available in the form of dispersions in various liquid vehicles such as trichlorotrifluoroethane (Freon TF), trichloromonofluoromethane (Freon 11), tetrachloroethylene (perchlor-oethylene) and the like. Such dispersions conveniently may be applied to the cellulosic sheet material matrix by spray or immersion techniques. Evaporation of the liquid vehicle or me dium leaves a soft, non-tacky, odorless and substantially colorless and imperceptible coating or impregnation of the dispersed, particulate fluorotelomer on the matrix. In order to prepare a satisfactory treated cleaning paper or cloth, the concentration of the fluorotelomer in the liquid medium should be within a certain limited range, which is somewhat dependent upon the manner in which the dsipersion is applied, among other things.

The superiority of the fluorotelomer-treated lens cleaning materials, and particularly of suitable tissue papers which have been so treated, over commercially-available, conventional lens cleaning tissue papers, and particularly over silicone-treated paper which is specially-designed for the cleaning of eyeglasses, has been clearly demonstrated.

Detailed description of the invention While cloth may be treated in accordance with the present invention to prepare superior lens cleaning materials, a suitable paper matrix is preferred for a number of reasons including economy and convenience. Papers suitable for use in practicing the present invention must satisfy a number of requirements. One essential requirement is that such paper must be lint-free, at least after treatment with the fluorotelomer. Thus, soft facial tissue and toilet tissue usually are unsatisfactory, because they typically are relatively friable and tend to leave dust or finelydivided paper particles or lint on a surface after cleaning; Another requirement of the paper matrix is that it must be thin, lightweight and flexible, yet sturdy enough to tolerate the manipulation attendant to normal use in lens cleaning. Thus, stiff, heavily-sized or glossy papers, and any papers that would be abrasive, are unsatisfactory. A further and essential requirement is that the paper exhibit a capacity properly to hold 'the fluorotelomer dispersion. In general, closely-knit or relatively nonporous papers give superior results and are preferred. Papers found generally to be satisfactory include: wrapping tissue paper; certain types of lens tissue paper; and cigarette paper, all as described hereinafter or defined in the Paper Yearbook, 1967 edition, published by Ojibway Press of Duluth, Minn.

Wrapping tissue paper, as above referred to, is a thin, lightweight paper, typically weighing less than about 18 pounds (lbs.) per ream (24" x 36480 sheets). This paper may be produced from one or more of several types of fibres on various types of paper-making machines. It may be machine finished or machine glazed, bleached or unbleached, and also may be white or colored and decorated or ribbed. For the purposes of the present invention, it should not be of the type described as waxed or anti-tarnish. A paper of this type found particularly suitable for the purposes of the present invention is that prepared from a 100% bleached chemical wood fiber. Only a small number of small holes or spaces should be visible on holding the paper up to the light.

Lens tissue, as referred to hereinabove, is a thin, lightweight paper for cleaning, wiping and polishing optical lenses or other delicate instruments. Its typical weight specification ranges from about /2 to about 11 /2 lbs. per ream (24" x 36"--500 sheets). Within the general specification designated UU-t-P-l 3 as appears in Paper Yearbrook, two specific types of such paper have been found to be generally suitable, viz.: Type Ia lightweight (about 5 /2 lbs. per ream), long-fibred paper made from hemp or rayon, and yielding an open formulation; and Type IIa short-fibred, heavier (about 8 /2 lbs. per ream (24" x 36"-500 sheets)) paper made from linen or cotton, and resulting in a densely formulated tissue. While the lens tissue designated Type I gives satisfactory results and may be used with the fluorotelomer treatment, that type designated Type II gives superior results and therefore is the prefererd paper type of this group of lens tissues.

Two other types of lens tissue falling Within the general specification cited above, but not recommended for use in practicing the present invention are: Type IIIa heavy cut (11 /2 lbs. per ream), fully-bleached chemical fibre treated with silicone, and yielding a. dense formulation; and Type IV-- a heavy cut /2 lbs. per ream), high wet-strength, open-formulation tissue made from hemp or rayon and useful for wet cleansing primarily.

Cigarette paper, as hereinabove referred to, is a strong tissue paper, free of pin holes and with great strength, which also may be treated and used in the novel manner described herein. Such paper usually is prepared from flax, and typically has a weight of about to about 22 grams per square meter.

Of all of the types of paper described above as generally suitable for use in accordance with the present invention, the type found to give the best results when impregnated or coated with the particulate fluorotelomer is the wrapping tissue paper. Such paper possesses a combination of most if not all of the characteristics which appear to be ideal, e.g., strength, lightness, minimal porosity and low cost.

The substance with which the paper or other ,cellulosic sheet material is treated to produce the superior lens cleaning materials of the present invention is a suitable finely-divided, solid fluorocarbon telomer which, for ease in application, conveniently is dispersed in a liquid medium. Suitable fluorotelomer dispersions are available commercially, for example, from E. I. du Pont de Nemours & Company under its generic trademark designation, Vyd-ax? fluoro telorners. All Vydax fluorotelomers currently avialable were explored for possible use in preparing the improved lens cleaning materials disclosed herein, and a number were found to be generally suitable, including Vydax 1000, Vydax 525, Vydax 550,? *Vydax AR, -Vydax BR and Vydax 201G.

-.Vydax 1000 is described (Du Pont Company Industrial Chemicals Information Bulletin 9b) as a dispersion of a' white, waxy fluorocarbon telomer in Freon TF solvent, and as exhibiting the following typical properties:

msrnn srou Appearance Grayptranslucent fluid. Solids content 7.5% wt. Particle size None greater than 30 microns. Visocsity (Brookfield at 77 F., 20 rpm.) 1800 cps. Density at 77 F 1.59 gms./ml.

.TELOMER Density gms./ml 2.1 Softening point F 612 Melting point (Crystalline) F 618 Molecular weight range 20,000

VEHICLE Name Freon TF solvent. Composition Trichlorotrifluoroethane CCl FCClF Boiling point 118 F. (48 C.).

Freezing point 31 F. (34 (3.). Density at 77 F. 1.56 gms./ml.

Dispersion Vydax 525 Vydax' B50" Solids content, percent 2. 5 5.0 Viscosity (Brookfield) at 77 F. and 20 r .m.,c s 12 Density at 77 1. 58 1. 59 Appearance.-. White, translucent fluid Particle size fimicrons (approximate average) TELOMER SOLIDS Density gms./ml 2.1 Softening point F..- 510 Melting point (crystalline) F 572 Molecular weight range 3,500-4,50O

- VEHICLE Name Freon TF solvent.

Composition Trichlorotrifiuoroethane CCizF-CCiFz. Boiling point 118 F. (48 C.). Freezing point -31 F. (-34 0.). Density at 77 F. 1.56 gms./ml.

Vydax 525 and Vydax 550 are described (Du Pont Company Industrial Chemicals Information Bulletin 10b) as dispersions of a while, waxy fluorocarbon telomer solid in Freon TF solvent, and as exhibiting the following typical properties:

Vydax AR is described (Du Point Company Industrial Chemicals Information Bulletin lg) as a dispersion of a white, waxy tetrazfluoroethylene telomer in Freon TF solvent, and as exhibiting the following typical properties:

Appearance White translucent fluid. Solids 20%.

Particle size 5 microns, approximately. Viscosity (Brookfield) --2S C. 1,600 cps. (at 20 r.p.m.). Density 25 C 1.65 gms/mi.

v TELOMER SOLIDS DATA Density 2.16 gms./ml. Softening point (ASTM E28 58T) 510 F. (265 C.).

Melting point (crystalline) 572 F. (300 C.). Hardness (ASTM D-l321-57T) (Penetrometer needle, penetration 25 C., 5 secs., 400 gm.

weight) 0.5 mm. Molecular weight 3,700 approximately.

SOLVENT DATA Name Freon TF solvent. Composition Trichlorotrifluoroethane CC1 F-CClF Boiling point 118 F. (48 C.). Freezing point 31 F. (34" C.). Density 25 C 1.46 gms./ml.

Vydax BR is described (Du Pont Company Industrial Chemicals Information Bulletin 5b) as a white, waxy tetrafluoroethylene telomer dispersed in Freon TF solvent, and as exhibiting the following typical properties:

Appearance White translucent liquid which slowly forms a clear top layer on standing. Solids 20%. Particle size 5 microns (approximate). Viscosity (25 C.) 400 cps. (Brookfield, 20

r.p.m.). Density (25 C.) 1.65 g./ml.

TELOMER SOLIDS DATA Softening point (ASTM E-28- 58T) 424 F. (218 C.). Melting point (crystalline) 534 F. (279 C.). Density 2.09 g./rnl.

Hardness (ASTM D-l32157T) (Penetrometer needle, penetration 25 C., 5 secs, 400 gm. weight) 2.6 mm. Molecular weight 2,000 approximately.

FREON 'IF SOLVENT DATA Composition Trichlorotrifluoroethane CClgF-CClFg- Boiling point 118 F. (48 C.).

Freezing point --31 F. (34 C.).

Vydax 2016 is described (Du Pont Company Organic Chemicals Department New Product Information Bulletin 7b) as a dispersion of a white, waxy fluorocarbon telomer in Perclene perchloroethylene solvent, and as exhibiting the following typical properties:

6 PERCLENE" SOLVENT Name Perclene. Composition Perchloroethylene (tetrachloroethylene) Formula Cl C--CCl Boiling point 250 F. (121 C.). Freezing point 8 F. -22 C.).

Pounds per gallon (20 C.) 13.55 lbs/gal. Specific gravity (20/4 C.) 1.623 g./m1. Flash point None.

Vydax Br, while effectively facilitating lens cleaning and therefore suitable for such use, was found to convey an inherent offensive odor. Vydax 1000 was of somewhat lesser effectiveness than the other Vydax fluorotelomers tested. Vydax AR in 5% to 20% concentrations in the dispersion was unsatisfactory, and, while satisfactory in lower concentrations (i.e., from about 0.5 to about 1.5% diluted with Freon TF or trichloroethane), also was somewhat less effective than most of the other Vydax fiuorotelomers. Based on extensive experimentation, Vydax 201G fluorotelomer dispersion, in an appropriate concentration, appears to provide the most satisfactory results and therefore is preferred for use in the practice of the present invention.

The fluorotelomer treating substance must be applied to the sheet material matrix in a dispersed, particulate form. Unfortunately, a satisfactory product has not resulted from efforts to use a dry fluorotelomer powder. Furthermore, the fluorotelomer treating substance is, for practical purposes, substantially insoluble in most classes of solvents. Consequently, in preparing the improved lens cleaning materials of the present invention, it has been found desirable to apply the solid fluorotelomer in the form of a dispersion of finely-divided particles of suitable size and concentration in a liquid medium or vehicle permitting of a reasonably stable and homogeneous dispersion.

In general, fluorotelomer particle sizes of the order of magnitude of about 5 microns, or less, have been found to give excellent results and are preferred, although somewhat larger particles also can be employed with satisfactory results.

For techniques involving application of the fluorotelomer dispersion to the paper matrix by immersion or spraying, suitable concentrations of the fluorotelomer in the liquid dispersant medium are generally in the range of from about 0.01% to about 1.5% by weight, and preferably from about 0.10% to about 1.0% by weight. The use of somewhat higher concentrations commonly produces cleaning materials which, While effective, do not clean the surfaces as satisfactorily as those involving concentrations in the above-recited range. Moreover, substantially higher concentrations in fact are deleterious, possibly due to a deposition of a cloudy or opaque film of the fluorotelomer on the surface being cleaned.

It has been found that a very satisfactory suspension or dispersion of the solid, particulate fluorotelomer can be prepared with a liquid medium such as trichlorotrifluoroethane or trichloromonofiuoromethane, and that such a dispersion provides a proper impregnation or coating of the fluorotelomer in the paper or other sheet material. Other suitable but less satisfactory liquid vehicles or suspending agents include trichloroethylene, acetone, methanol and isopropanol. It is, of course, essential that the liquid suspending agent produce no significant agglomeration of the fluorotelomer particles.

Various other possible treating substances have been tested and found to produce a lens cleaning paper very much less effective than that produced with the use of the fluorotelomer. Thus, specimens of lens tissue paper and other papers treated, respectively, with paraffin wax, microcrystalline wax, polyethylene synthetic wax, blends of natural and synthetic waxes, and white petrolatum or mineral oil, not only failed to clean as effectively as the same paper similarly treated with a suitable fluorotelomer,

but in some cases actually resulted in' the deposition of more dirt on the surface being cleaned. Paper dipped in acetone'or alcohol also failed to produce adequate cleaning. Although paper wetted with xylene was somewhat superior to the foregoing substances (other than the fluorotelomers), it too at times left an additional dirt film unless repeated cleaning attempts were made. The fiuorotelomer-treated papers are stable over a wide range of storage temperatures, are non-irritating and non-toxic. Moreover, the fluorotclomers may be used on tinted or colored paper.

The improved lens cleaning materials of the present invention find application principally in cleaning such articles as eyeglasses, contact lenses, camera lenses, microscope lenses, various types of projector lenses, telescope lenses, hand magnifying lenses, microscope slides and cover slips, window and windshield glass, and special transparent or translucent bulbs for lighting or projection. Moreover, quartz or plastic materials which are transparent or translucent may be similarly cleaned. In fact, almost any finely-ground or polished surface such as used for magnification, reflection or as a transparent or translucent background support for light transmission or treating agent consisting essentially of a suspension or dispersion of particulate fluorocarbon telomers in a liquid dispersant or medium. The fluorotelomer employed was procured commercially from theDu Pont Company under its designation, Vydax 201G, and comprised approximately 5% by weight of the solid fluorotelomers dispersed in tetrachloroethylene (perchloroethylene). Three volumes of this commercially-available dispersion were further diluted with volumes of commercial trichlorocthylene in order to produce the actual treating agent. The paper was air-dried following removal from the liquid fluorotelomer dispersion, to allow removal of the liquid by evaporation. Small strips, each approximately 5.5 inches by 2.5 inches, then were cut from the larger, treated sheets to produce the improved lens cleaning tissues.

The efficacy of the above fluorotelomer-treated paper as lens cleaning tissues was demonstrated and compared with the same paper, untreated, and with a conventional silicone-treated lens cleaning paper (Type III lens tissue paper) obtainable commercially from DowCorning Corporation under its trademark designation, Sight Savers. The results of such testing are set forth in the table below:

Number of rubbing strokes required to completely clean Papers lens of a standard film of grease 1 Test 1 Test 2 Test 3 Test 5 Test 6 Average Type III lens tissue paper (silicone-treated) 90 70 70 60 60 70. 0 Wrapping tissue (untreated) 20 20 18 16 20 18.8 Wrapping tissue treated with Du Pont Vydax 201G (3 volumes dispersed in 20 volumes triehloroethylene) 9 9 10 8 7 8. ll

1 Single thumb-print layer of sebum on typical eyeglass lens.

visualization of objects may be cleaned using the fluorotelomer-treated cloth or paper of this invention.

While particularly useful in cleaning lenses and the like of oils or greases (the commonest type of lens obscuring films), the fluorotelomer-trcated cleaning materials effectively remove dust and other contaminants as Well. They are remarkably effective in removing sebum, the product of the cutaneous oil glands of humans, and the commonest form of fouling deposit on eyeglasses and other lenses handled by humans. However, they are not particularly efi'cctive for the removal of moisture. While the fluorotelomer-treated papers and other sheet materials are not impermeable to the passage of water and can absorb it, there are indications that water may displace or render ineffective some portion of the deposited fiuorotelomer from the paper.

The use of the treated lens cleaning materials of this invention in cleaning lenses requires simply the rubbing with the treated paper or other material held by the fingers, in rotational or radially-linear strokes, over the surface to be cleaned. Thin films of grease usually require only a few strokes, and no water, water vapor, or other chemicals are necessary. After cleaning with the fluorotelomer-treated material, the cleaned lens surfaces normally regain their ultimate intrinsic optical clarity. Moreover, it is believed possible that an ultra-fine and invisible film or layer of the particulate fluorotelomer is left, on the lens surface, thus facilitating subsequent cleaning. Finally, no macroscopic or microscopic damage to ground or polished surfaces is caused by cleaning with th fluorotelorner-treated lens cleaning papers or other materials of this invention.

Specific embodiment of the invention In order to illustrate a specific embodiment of the in- .vention, an improved lens cleaning material was prepared in the following manner. A wrapping tissue paper meeting the description and specifications hereinabove set forth, and available commercially under the designation, No. 1 White Wrapping Tissue Paper, from Crystal Tissue Company, Middletown, Ohio, was employed as the sheet material matrix. This paper was immersed briefly in a As is shown in the above table, 7 to 10 times more rubbing of a surface to be cleaned was necessary with the conventional papers as compared with the fluorotelomertreated paper. Even then, a generally less-satisfactory cleaning of the glass surface usually was evident in the case of these other papers.

In another specific embodiment, lint-free wrapping tissue paper is impregnated with a fluorotelomer wax by dip immersion of sheets of the paper in a liquid treating bath consisting of 1 volume of commercially-available Du Pont Vydax 525 (2.5% fluorotelomer solids in trichlorotrifluoroethane) diluted with 24 volumes of trichloromonofluoromethane. After removal, the sheets are air-dried in a closed chamber, allowing solvent evaporation and permitting a subsequent recovery of the solvent by condensation. The impregnated sheets then may be cut into appropriate sizes for use as cleaning papers for a wide variety of ground surfaces. For eyeglasses, a suggested size is 5.5 inches by 2.5 inches, and such sheets conveniently can be dispersed in a stapled package of sheets.

Other embodiments of the invention provide similarly beneficial results. i

The invention claimed is:

1. Lens cleaning material consisting essentially'of a thin, fleigible, cellulosic sheet material selected from the group consisting of cloth and tissue paper, and dispersed particles of a solid-finely-divided fluorocarbon telomer.

2. Lens cleaning material consisting essentially of tissue paper and dispersed particles of a finely-divided solid fluorocarbon telomer.

3. Lens cleaning material consisting essentially of: a tissue paper selected from the group consisting of wrapping tissue paper, Type I lens tissue paper, Type II lens tissue paper and cigarette paper; and dispersed particles of a finely-divided, solid fluorocarbon telomer.

4. Lens cleaning material in accordance with claim 3, wherein the particle size of said fluorocarbon telomer is not more than about 5 microns in diameter.

5. Lens cleaning material consisting essentially of la tissue paper and dispersed particles of a finely-divided, solid fluorocarbon telomer, said material being prepared by immersing said tissue paper briefly in a dispersion of said fluorocarbon telomer in a liquid medium, and removing said medium.

6. Lens cleaning material consisting essentially of: a tissue paper selected from the group consisting of wrapping tissue paper, Type I lens tissue paper, Type II lens tissue paper and cigarette paper; and dispersed particles of a finely-divided, solid fluorocarbon telomer having a particle size of approximately 5 microns in diameter; said lens cleaning material being produced by contacting said tissue paper briefly with a treating agent comprising from about 0.01 to about 1.5% by weight of said fluorocarbon telomer dispersed in a liquid medium, and removing said liquid medium.

7. Lens cleaning material consisting essentially of: a tissue paper selected from the group consisting of wrapping tissue paper, flype I lens tissue paper, Type II lens tissue paper and cigarette paper; and dispersed particles of a finely-divided, solid fluorocarbon telomer having a particle size of 1 to 5 microns in diameter, approximately, a density at 25 C. of about 2.12 grams per milliliter and a melting range of from about 608 to about 615 F.

8. Lens cleaning material consisting essentially of: a tissue paper selected from the group consisting of Wrapping tissue paper, Type I lens tissue paper, Type II lens 25 tissue paper and cigarette paper; and dispersed particles of a finely-divided, solid fluorocarbon telomer; said lens cleaning material being produced by contacting said tissue' paper briefly with a dispersion in liquid medium of from about 0.01 to about 1.5% by weight of a fluorocarbon telomer having a particle size of 1 to 5 microns in diameter, approximately, a density at 25 C. of about 2.12 grams per milliliter and a melting range of from about 608 to about 615 F., and subsequently removing said liquid medium.

9. Lens cleaning material according to claim 8 wherein said liquid medium is selected from the group consisting of trichlorotrifluoroethane, trichloromonofluoromethane, tetrachloroethylene, trichloroethylene and mixtures thereof.

References Cited UNITED STATES PATENTS 1,966,633 7/1934 Lee 15104.93 XR 2,288,714 7/1942 Jones 15104.93 2,333,794 11/1943 Jones 15104.93 2,353,978 7/1944 Weber 15-104.93 2,633,593 4/1953 Wright et al 15-10493 2,947,016 8/1960 Thompson 15-104.93 3,075,228 1/1963 Elias l5104.93

CHARLES A. WILLMUTH, Primary Examiner.

R. I. SMITH, Assistant Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.0. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,406,418 October 22, 1968 Harry J. Hurley, Jr., et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 33, "P-13"' should read P-313 Column 4, line 39, "while" should read white Column 5, line 19, "1.46" should read 1.56

Signed and sealed this 5rd day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer