US 2887379 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
PHOTOGRAPHIC ELEMENTS' Filed May 31. 1955 "Eis-vl, f
LSUPEORT Y, ssu'PPoRTA Vle ANTIABRAsloN LAYER I4 EMuLsloN' LAYER l2 PRosPHoR LAYER H suPPoRT ISYPRosPHoR vLAYER I5 EMuLssoN LAYER ,I7 ANTlAaRAsloN LAYER V 20 suPPoRT I8 PHosPHER LAYER 22 ADHESIVE LAYER NQ\\\\\\\\\\N-\Q\-A\x`\\\\ 27 ANTIABRAsloN LAYER ,f ,f f 25 EMuLsloN LAYER l ,f f f 24 suPPoRT g\\\\\\\x\\\.\\\\\\ 26 EMuLs|oN LAYER Y 2av ArmAaRAsloN LAYER 25` ADHESIVE LAYER .,f f I9 PHosPHoR LAYER f 2| SUPPORT INVENTORS RALPH KINGSLEY BLAKE FRANCIS PETER ALLES ATTORNEY United States Patent O PHOTOGRAPHIC ELEMENTS Ralph Kingsley Blake, Highland Park, and Francis Peter Alles, Westfield, NJ., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., 'a corporation of Delaware Application May 31, 1955, Serial No. 512,281 e 12 Claims. (ci. sis-8 2) Thisr invention relates to photographic elements comprlsing a sheet support, a layer of finely divided phosphor. particles dispersed in a chlorosulfonated olefin and a contiguous light-sensitive silver halide layer. More particularly it relates to such elements wherein the support and contiguous layer have a high degree of flexibility.
An object of this invention is to provide improved photographic elements having a layer of fluorescent ma-k terial. Another object is to provide such elements which have improved flexibility characteristics. Yet another object is to provide iexible photographic elements having an integral screen which are suitable for the production of radiographs. A further object is to provide such lms which can be bent about a small radius without dama-ge. Still other objects will be apparent from the following description of the invention.
It has been found that radiographic elements having a phosphor layer possessing a high degree of liexibility can be made by dispersing iinely divided phosphor particles in a chlorosulfonated olen addition polymer from a monoor diolen of not more than carbonatoms and coating the resulting layer on a flexible sheet support, e.g a hydrophobic lilm, a thin sheet of metal such as aluminum, or paper, or on a light-sensitive colloid silver halide layer coated on the support. In the case where the phosphor layer is coated directly on the surface of the support or on a suitable anchor layer on such surface, a colloid silver halide emulsion layer is coated on the phosphor layer, either directly or through an intervening adhesive layer. In one aspect of the invention the phosphor layer is placedbetween two colloid silver halide emulsion layers.
As will be apparent from the above general statement, there are many different specific types of photographic elements having the novel flexible phosphor layer which come within the scope of the present invention. `The phosphor layer may be a permanent part of the element or may be provided with its own support and detachably joined through an adhesive layer to a colloid silver halide layer on a permanent support. A flexible iilm base may have a colloid silver halide emulsion layer on each sur- In all cases where the colloid thelight-sensitive silver halide layer provided with a protective, `i.e., an antiabrasion layer.
Several representative embodiments of the present invention are illustrated in the accompanying drawing, wherein each of the iigures shows a cross-section view -of a photographic element of this invention. `The, elements and thickness of the layers are shown in exaggerated enlargement for purposes of clarity and are not `drawn to` exact scale.
In the embodiment illustrated by Fig. 1, a `support 1 'bears on its surface ilexible phosphor layer 2, which in 2,887,379 .PatentedMay 19, 1959 14 and 15each of which in turn maybe overcoatedl with antiabrasion layers 16 and 17.
In the embodiment illustrated by 1.9 coated on supports 20 and 2 1 are joined `adjacent the phosphor layers by means of adhesive layers 22 and 23, to opposite sides of an element of a support 24 coated on each side with emulsion layers 25 and 26, each ofwhich in turn bear an overcoating of antiabrasion layers 27 and 28.
In preparing the phosphor dispersions for coating to form a phosphor screen layer, the phosphor particles are dispersed in a suitable organic solvent including hydrocarbons, e.g., benzene, toluene, tetrahydronaphthalene, decahydronaphthalene; chlorinated hydrocarbons, eg., chloroform, carbon tetrachloride, trichloroethylene and tetrachloroethylene; or a ketone, e.g. methyl ethyl ketone and cyclohexanone. If desired, with the aid of a dispersing agent, e.g., tetraisopropyl titanate, dioctyl ester of sodium sulfosuccinic acid and mixtures of the latter with stearic acid, the dispersion is then admixed with the chlorosulfonated olefin addition polymer, the resulting solution is coated onto the support, or appropriate layer `on the support, e.g., colloid silver halide layer,
antiabrasion layer or adhesive layer, and the solvent or solvents are removed, e.g., by evaporation at room temperature or at elevated temperature. The chlorosulfonated oleiin addition polymer preferably is added from solvent solution to the dispersion of phosphor. `For convenience, the solvent can be the same as that used in the original phosphor dispersion. In order that the phosphor particles will be of line particle size and free from agglomerates and slugs` of polymer binding agent, the dispersion is milled, e.g., by means of a colloid mill or ball mill and then passed through a ne-mesh screen, lter cloth, or felt. Bythis procedure, the phosphors will have a particle size approximately 3 to i6 microns in average diameter. thickness from 0.001 to 0.015 inch.
Various adjuvants may be admixed with the phosphor dispersion in the olefin polymer. Among such adjuvants are vulcanizing agents, eg., magnesium oxide, 'lead oxide, and organic amines; vulcanizing accelerators, e.g., zinc dibutyldithiocarbamate and Z-mercaptoimidazoline; plasticizers, e.g., dioctylphthalate, and resins, e.g., coumaroneindene resins and the Epon resins having the formula:
e patents, McQueen, 2,212,786; McAlevy, 2,416,060 yand 2,416,061. They contain chlorine in the form of -SOaCl Fig. 4, phosphor coated supports composed of phosphorev layers 18A vand The phosphor layers may varyin` asevera groups and as chlorine atoms attached directly to carbon atoms of the polymer chain, and the chlorine on a percentage basis will vary from 20 to 45% by weight, at least 0.4% of which will be in -SOZCl groups and the amountk of sulfur in the formv of suchl groups will vary from 0.4% to 3.0% by weight.
The ratio of phosphor particles to the chlorosulfonated olefin addition polymer may vary from to. 30 parts of the former per part by weight of the latter. The amount will vary with the particular polymer, but, in general, best results are obtained with 15 to` 20 parts of the phosphor per part of polymer.
AsV stated above, the supports may be composed of various polymers, Among the useful polymers are cellulose derivatives, e.g., cellulose acetate, cellulose propionatc, cellulose acetate propionate, cellulose acetate butyrate, ethyl cellulose; polyvinyl chloride, poly(vinyl chloride co vinyl acetate) vinylidene chloride copolymers with vinyl chloride, vinyl acetate, acrylonitrile, styrene and isobutylene; polystyrene and polyesters, e.g., polyethylene terephtha'lates andl homologous polyesters obtainable by the processes described in Whineld et al.
' U.S. Patent 2,465,319. These lm bases may contain or be coated with dyes or finely divided pigments, eg., Tartrazine (Colour Index No. 640), Victoria Green WB Base (Cl. No. 657), Chinoline Yellow D Sol. in Spirits (CI. No. 800), Nubian Resin Black (Cl. No. 864), TiO2, lithopone, magnesium carbonate, aluminum oxide, carbon black or colored pigments as opacifying or lightabsorbing agents.
In a preferred aspect of the invention, a flexible support such as paper, waterproof paper or a hydrophobic lm having a thickness from 0.00025 to` 0.015 inch is provided with a layer of a barium sulfate/lead sulfate mixed crystal phosphor dispersed in a chlorosulfonated polyethylene, wherein the phosphor is applied at a coating weight of 0.5 to 15.0 grams per square decimeter and 5 to 20 parts of phosphor per part by weight of chlorosulfonated polyethylene over which is provided a lightsensitive colloid silver halide emulsion layer having a silver halide coating weight of 5 to l5() milligrams per square decimeter.
The invention will be further illustrated but is not intended to be limited by the following examples, wherein the silver halide coatings and subsequent treatment and use of the composite elements, prior to controlled exposure to actinic radiations, were protected from such radiations.
Example l A mixture of the following materials was ball milled for 24 hours and strained through nainsook:
Toluene ml 435.0 Tetraisopropyl titanate grams 3.6 BaSO4/PbSO4 phosphor do- 2250.0 Stearic acid s ,-do 3.0
A 25% by weight solution in toluene of a chlorosulfonated polyethylene containing approximately 27.5% chlorine and 1.5% sulfur, wherein most of the chlorine is substituted along the hydrocarbon chain and the sulfur is combined with chlorine and attached to the carbon chain as sulfonyl chloride (SO2Cl), was stirred into the above mixture to give a barium sulfate/lead sulfate mixed crystal phosphor to chlorosulfonated polyethylene binder ratio of 15 to l on a weight basis. The resulting phosphor suspension was applied to waterproof paper (made by coating a conventional clay coated paper on both sides with a cellulose nitrate lacquer) by means of a doctor blade to give an air dried thickness of 0.005 inch and a phosphor coating weight of about 3.5 grams/dm?. An X-ray type gelatino-silver bromoiodide emulsion containing 1.5 mol percent silver iodide and 98.5 mol percent silver bromide was coated at 96 F. on the phosphor layer to a weight of about 24 mg. of silver salt per square decimeter (calculated as silver bromide) and the resulting element was dried. A gelatin antiabrasion layer was coated at 100 F. over the emulsion layer to a weight of about l0 mg. gelatin/ dm.2 and the layer was dried.
A sample sheet of the resulting photographic film having an integral intensifying screen, illustrated by Fig. l, was given an 0.8-second unscreened X-ray exposure at kvp., 30 ma. and 36-inch distance through a 1% inch thick aluminum machine part. The X-ray exposed material was substituted for the paper negative 1in a. Polaroid X-ray packet and then processed for l minute in a Pickerlolaroid processing unit as described in J. A. Reynolds article, A Preliminary Report on the Picker-Polaroid Process in Industrial Radiography, Non-Destructive Testing, Summer Number, 1,952, pages 24 to 27. The resulting positive paper radiograph showed similar but more sharply `deiined detail as compared with a radiograph obtained with a standard Polaroid X-ray packet exposed under the same conditions for 0.6 second in a Picker-Polaroid cassette containing a fluorescent intensifying screen and processed for 1 minute in the Picker- Polaroid processing unit. Exposure times were adjusted in the usual manner to give comparable positive prints.
In another test a second sample sheet of the photographic paper screen product was given an 0.8-second X-ray exposure as described above, developed at 68 F. for 45 seconds in a conventional aqueous metol-hydroquinone developer xed in a conventional aqueous xiug composition containing sodium thiosulfate, washed and dried to give a low contrast sharp negative radiograph, when viewed by either reiected or transmitted light.
The effective speed and contrast of a conventionally processed integral emulsion-screen product exposed for 0.3 second as described above was increased by intensification in the iron toner formula given in U.S. Patent 2,699,994, Example IX, except that the Wool Orange dye was omitted. Processing was carried out at 68 F. as follows: a 45-second development and ll/z minutes xing in the solutions described above, a 3-minute wash in water, a Z-minute toning treatment in the toner just described and a l-minute wash in water. After drying, the resulting negative radiograph was satisfactory for4 viewing by either reflected or transmitted light alone, or with the aid of colored lters.
An X-ray sensitive element made as described above but without the BaSO4/PbSO4 chlorosulfonated polyethylene phosphor layer required a 25-second exposure to give a negative radiograph of the same quality as that obtained by the 0.8-second exposure of the photographic paper having the integral BaSO4/PbSO4 layer, using the same processing conditions.
Example 2 A mixture of the following materials was ball milled for 24 hours and strained through nainsook:
A 25% by weight solution in toluene of the chlorosulfonated polyethylene described in Example 1 was stirred into the above mixture to give a CaWO4 phosphor to chlorosulfonated polyethylene binder ratio on a weight basis of 15 to l. The resulting phosphor suspension was applied to a conventional baryta coated photographic paper by means of a doctor blade to give an air dried thickness of 0.005 inch and a phosphor coating weight of about 5.4 grams per square decimeter. An' X-ray type gelatine-silver bromoiodide emulsion, containing 1.5 mol percent silver iodide and 98.5 mol percent silver bromoiodide, at F. was coated over the phosphor layer to a weight of about 30 mg. of silver salt per square decimeter (calculated as silver bromide per square decimeter) and dried. A gelatin antiabrasion layer was coated at :asevera lar to that of Example 1. The emulsion surface of the i exposed element was placed in contact with a special image-receptive coating on 0.005 inch triacetate made as directed in Example 2 of U.S. Patent 2,352,014. The two elements were then inserted into the slots of a twoslot Apeco Auto-Stat processing machine for copies up to 14 x 17 inches, made by the American Photocopy Equipment Company, 55 E. 34th Street, New York 16, N.Y. In this machine, the two elements were pulled through a developing solution of the following composition by electrically driven rubber rollers in about 8 seconds. vDuring passage through the machine, the sheets were held apart by metal guides to assure uniform wetting of each sheet by the developing solution, which follows:
Water to make 1 liter.
The `Sterox CD of the above table is a polyoxyethylene ester of a tall oil acid made by Monsanto Chemical Company.
The two sheets of material emerged from the machine adhered together face to face and in a semi-dry condition due to the squeegeeing action of the rubber rollers. After about two minutes of surface contact, the sheets were peeled apart to reveal a positive transparency on the receptive sheet and a negative image in the emulsion layer of the photographic film having the integral screen. The negative image was formed by conventional development of the exposed silver salt image. The positive image was formed by silver transfer development. The unexposed silver halide of the negative emulsion dissolved in the developer and diffused to the surface of the receptive layer where the silver was precipitated by the catalytic action of the specially treated surface to give a positive image.
Similar results can be obtained by substituting for the processing machine described in the preceding example the processing machine described in Canadian Patent 487,883.
Example 3 Example 4 In a manner analogous to Examples 1 and 3, a flexible phosphor intensifying screen layer and support were made consisting of a 0.005 inch thick coating of BaSO4/PbSO4 in a chlorosulfonated polyethylene binder on one side of a 0.0025 inch polyethylene terephthalate film of the type described in Example 3. An X-ray type gelatine-silver positive images in the photo- A bromoiodide emulsion layer containing 1.5 mol percent` silver iodide and 98.5 mol percent silver bromide overcoated with a gelatin antiabrasion solution were applied to each side of the phosphorcoated film base to weights of about mg. silver salt (calculated as silver bromide) and 10 mg. gelatin, respectively, per square decimeter and the resulting composite element was dried.
The resulting flexible photographic film with the integral intensifying screens,'illustrated by Fig. 2, was bent to conform to an inside cylindrical surface of the 1% inch thick aluminum machine part referred tot in Example l, and exposed through a 1% inch thickness of aluminum at 70 kvp., 30 ma., X-radiation at a distance of 28 inches` for 0.3 second and then developed for 5 minutes in a solution made by admixing the following components:
N-methyl-paminophenol sulfate gran-rsi-, 2.5
Sodium suliite (anhydrous) do Hydroquinone a do.. Borax do"- Water to make 1.0 liter.
and fixed at 68 F. for 5 minutes in a solution admixing the following components:
Sodium thiosulfate grams- 240 Sodium sulte ..do- 6 Boric acid dn 6 Potassium alum do l2 Acetic acid (28%) ml-.. 35 Water to make 1 liter.
The finished radiograph, after washing, contained an image disclosing the internal structure of the part, when viewed by either reflected or transmitted light.
Example 5 BaSO4/PbSO4 phosphor layer and the emulsion layer was in turn coated with an aqueous gelatin solution to form an antiabrasion layer of weights of about 17 mg. calculated as silver bromide for the emulsion layer and 10 mg. Igelatin for the antiabrasion layer per square decimeter, respectively, and the coated element was dried. A high speed negative non-optically sensitized gelatino-silver bromoiodide emulsion, 3.4 mol percent silver iodide and 96.6 mol percent silver bromide, was coated on the CaWO4 layer and an aqueous gelatin solution was coated on the emulsion layer to form an antiabrasion layer, of weights of about 22 mg., calculated as silver bromide for the emulsion layer and 10 mg. gelatin for the antiabrasion layer, respectively, per square decimeter and the coatings were dried.
A sample sheet of the resulting highly flexible photomade by graphic lm having integral screens, illustrated by Fig. 3,`
was given a 0.2-seeond unscreened X-ray exposure as described in Example 1 and then developed and fixed at` 68 F. for 5 minutes each in the developer `and xing formulas described in Example 4. The nished radiograph after washing when viewed by reflected light showed images of different contrast on opposite sidesl of the radiot image was also adequate, when viewed by had been coated with the special irnage-rc-.cepti've4 layer as described in U.S. Patent 2,352,014, Example 2. The assembly of the three elements in juxtaposition was then inverse transfer developed in a manner similar to Example 2 to yield two positive prints inthe image-receptive layers of the papers and two negative images in the emulsion layers of the processed integral photographic emulsionscreen element.
Example 6 An element similar to that described in Example 3 was made byy substituting a non-optically sensitized high speed negative gelatino-silver halide emulsion coated to a Weight of 36 mg. silver halide per square decimeter for the X-ray type emulsion of that example. The resulting integral emulsion-screen element was then exposed and inverse transfer developed as describedl in Example 2 to give a sharp positive transparency inthe image-receptive film element.
Example 7 The chlorosulfonated polyethylene/BaSO4/PbSO4` phosphor suspension of Example 1 was applied to a conventional baryta photographic paper base by means of a doctor blade to give an air dried thickness of 0.005 inch and a phosphor coating weight of about 3.5 grams per square decimeter. The phosphor layer was coated by means of a doctor blade with a mixture of 8 parts by weight of an epoxidized soya oil of molecular weight of about 1000 (made by Rohm and Haas) and 10 partsof a 12% (by weight) solution of the chlorosulfonated polyethylene of Example 1 dissolved in toluene to give a dry, pressuresensitive adhesive coating of about 0.001 inch thickness. Under safe lights, a sheet of the adhesive coated phosphor screen was squeegeed rrnly onto opposite surfaces of a piece of conventional X-ray lilm, containing 1.5 mol percent silver iodide and 98.5 mol percent silver bromide, to form the flexible integral emulsion-screen product, illustrated by Fig. 4. The product was given a 0.2-second X-ray exposure similar to Example 1, the screens removed, and the X-ray film processed in conventional fashion as described in Example 4 to give a clear, sharp, contrasting radiograph. To obtain similar results on unscreened X-ray lm the exposure had to be increased from 0.2 second to 6 seconds.
Still other image-receptive elements useful with the photographic elements having an integral phosphor screen of this invention are described in the following patents: 2,352,014; 2,698,236; 2,698,237; 2,698,245 and 2,705,676.
While in the above examples a specific chlorosulfonated polyethylene was used as the binding agent for the phosphor particles, the invention is not limited to this particular polymer. Various other chlorosulfonated polyolefin elastomers or mixtures of elastomers having the constitution and properties outlined above can be used in like manner.
In place of the gelatin which is used as a binding `agent for the light-sensitive silver halide, there may be substituted other water-permeable or hydrophilic colloids, which are insoluble in aqueous photographic developing, fixing, etc. solutions. Suitable colloids include agar-agar, polyglycuronic acid, hydrophilic polymers, e.g., polyvinyl alcohol and acetals thereof; hydrophilic cellulose derivatives, eg., ethyl cellulose, cellulose acetate containing to of solubilizing groups, such as acid phthalate groups, etc.
The colloid silver halide emulsion layers may contain in addition to the light-sensitive silver halide, eg., silver chlorides, silver-bromide-chloride, silver bromoiodide, etc., various sensitizing dyes and other materials which are common in emulsion layers. Such additional materials include fog inhibiting agents, emulsion hardeners, emulsion preservatives, etc.
In place of the specific uorescent pigments of the above examples, there can be substituted other such mater-ials which exhibit fluorescence under the influence of glow can be added if desired softhatthe intensilication will Y not be prolonged after X-ray exposure has ceased.
Flexible integral ennilsion-screenl products may be substituted for the inilexible X-ray photographic film-intensifying screen combinationscurrently used in medical and industrial radiography. The, new products may also prove useful for autoradiography and. high resolution. cine-radiography-of small objects. The new products may be modied to make screened X-ray exposures in White light without the use of a cassette or cardboard X-ray lilm holder of the type shown in Reuter U.S. Patent 2,590,892. This mayI be accomplished by substituting black-backed optically opaque paper for the paper support of the strippable iexible intensifying screens of the integral emulsionscreerl product described in Example 4. The phosphor layers are highly exible and do not crack or peel when bent about a small radius, and, while they are extremely thiny have an adequate amountA of phosphor to produce the desired image-intensification upon exposure to X-rays of a contiguous silver halide emulsion layer.
A special product useful for making phototemplates andk reproducing engineering drawings can be made using the elements of the present invention wherein a phosphor layer is coated on a thin, flexible metal sheet, eg., aluminum and then overcoated with an emulsion layer. When the emulsion layer is exposed to light through an engineering drawing or X-rays or gamma radiation through a metal pattern, and developed and fixed, an element is formed having a negative silver image over the fluorescent layer. This element in turncan be copied-on a photographic paper or metal surface bearing a photographic emulsion by exposing to X-rays in. amanner known to the art.v The great flexibility of the product makes it possible to conform it to the'surface ofcylindrical objects to be reproduced and also facilitatesstorage in rolls, in addition the use of a metal support results in greater dimensional control and reproducibility.
The integral screen/ photographic emulsion products of the invention wherein the emulsion is coated over the iiuorescent layer which is in turn coated on an opaque base material, such as heavy cardboard or metal are also useful in the preparation of relief images for, planographic or letterpress printing. By using metal stencils of type matter, line drawings, etc., and X-ray exposure, it is possible to expose the elements in a manner such that the strongest exposure takes place next to the base. When elements exposed in. this way are processed in a hardening developer, eg., one containing pyrogallol and the non-image areas washed'ot in hot water in the manner known :to the art, a relief image firmly anchored to the base is obtained. Because of their flexibility, such elements can be readily bent around printing cylinders, etc. for a variety of printing processes.
The integral photographic emulsion-screen elements of this invention have the following advantages: They are versatile; they may be exposed to gamma or X-radiation, while bent into uniform intimate contact on cylindrical or conical surfaces; they may be bent or flexed prior to, during or following processing; they may be processed conventionally to give sharp negative images of dilerent contrast on diierent sides ofk a single support, when viewed by reiiection or still diterent contrast whenviewed by transmission; or they may be processedv by inverse transfer development to provide positive prints or: transparencies.
A major advantage of the new elements of, this invention is that they eliminate the need forA separate; screens and yield gamma and X-ray sensitive materials, which are useful and easily handled for industrial and medical radiographic Work.
The invention claimed is:
l. A photographic element comprising a sheet support bearing at least one layer of light-sensitive silver halide dispersed in a Water-permeable colloid bindingagent and` coactive flexible screen layer comprising iinely divided particles of a phosphor dispersed in a chlorosulfonated addition polymer of an olefin containing 1 to 2 olefinic double bonds and not more than carbon atoms.
2. An element as set forth in claim 1 wherein said support is flexible and said olen is ethylene.
3. An element as set forth in claim 2 wherein said phosphor consists of BaSO4/PbSO4 mixed crystals.
4. An element as set forth in claim 2 wherein said support is very thin and is composed of a polyethylene terephthalate.
5. An element as set forth in claim 2 wherein said support is paper.
6. An element as set forth in claim 2 wherein said conctive layer is adjacent the support.
7. An element as set forth in claim 6 wherein an antiabrasion layer is superposed on the silver halide layer.
8. An element as set forth in claim 2 wherein said coactive layer is adjacent the support and the layer of silver halide can be stripped from said coactive layer.
9. An element as set forth in claim 8 wherein the layer of silver halide is carried by a separate support.
10. A photographic element comprising a flexible hydrophobic lm base bearing on each surface a flexible screen layer comprising nely divided particles of a phosphor dispersed in a chlorosulfonated ethylene addition l0 polymer and a coactive layer of light-sensitive silver halide on at least one of said screen layers.
11. An element as set forth in claim 10 wherein a coactive layer having an antiabrasion layer is on each screen layer.
12. A photographic element comprising a sheet support bearing at least one layer of light-sensitive silver halide dispersed in a water-permeable colloid, an antiabrasion layer on each layer of silver halide, an adhesive layer on each antiabrasion layer, a llexible screen layer comprising finely divided particles of a phosphor dispersed in a chlorosulfonated ethylene addition polymer on each adhesive layer and a paper support for each screen layer.
References Cited in the le of this patent UNITED STATES PATENTS 1,448,456 Levy et a1 Mar. 13, 1923 2,316,595 Kallmann Apr. 13, 1943 2,319,102 Albers et al. May 11, 1943 2,322,082 Wynd et al June 15, 1943 2,409,162 Stand Oct. 8, 1946 2,416,060 McAlevy et al Feb. 18, 1947 2,585,596 Stanton Feb. 12, 1952 2,633,423 Bower et al Mar. 31, 1953 2,716,082 Smith Aug. 23, 1955