US3623867A

US3623867A - Photographic method for producing a cathode ray tube screen structure

Info

Publication number: US3623867A
Application number: US864197A
Authority: US
Inventors: Theodore Alexander Saulnier
Original assignee: RCA Corp
Current assignee: RCA Licensing Corp
Priority date: 1969-10-06
Filing date: 1969-10-06
Publication date: 1971-11-30
Anticipated expiration: 1988-11-30
Also published as: DK127083B; FR2065041A5; DE2049064A1; GB1302425A; BE757125A; NL7014591A; JPS4913427B1; DE2049064B2; ES384065A1; AT303138B

Abstract

A PROCESS FOR PREPARING A LUMINESCENT SCREEN STRUCTURE OF A CATHODE RAY TUBE COMPRISING (1) COATING A SUPPOTING SURFACE SUCH AS THE INNER SURFACE OF THE FACEPLATE OF THE TUBE, WITH A PHOTOSENSITIVE POLYMERIC FILM CONTAINING 0.06 TO 0.50 WEIGHT PART, LIGHT SCATTERING PARTICLES PER PART POLYMERIC MATERIAL, (2) EXPOSING THE FILM TO A LIGHT IMAGE, (3) REMOVING THE STILL SOLUBLE PORTIONS OF THE FILM, (4) OVERCOATING THE RETAINED FILM PORTIONS WITH A LAYER CONTAINING PARTICLES OF SCREEN STRUCTURE MATERIAL (5) AND THEN REMOVING THE RETAINED FILM PORTION AND OVERCOATING THEREON.

Description

United States Patent O 3,623,867 PHOTOGRAPHIC METHOD FOR PRODUCING A CATHODE RAY TUBE SCREEN STRUCTURE Theodore Alexander Saulnier, Lancaster, Pa., assignor to RCA Corporation No Drawing. Filed Oct. 6, 1969, Ser. No. 864,197 Int. Cl. G03c /00, 7/06 US. Cl. 96-361 Claims ABSTRACT OF THE DISCLOSURE A process for preparing a luminescent screen structure of a cathode ray tube comprising (1) coating a supporting surface such as the inner surface of the faceplate of the tube, with a photosensitive polymeric film containing 0.06 to 0.50 weight part, light scattering particles per part polymeric material,

(2) exposing the film to a light image,

(3) removing the still soluble portions of the film,

(4) overcoating the retained film portions with a layer containing particles of screen structure material (5) and then removing the retained film portion and overcoating thereon.

BACKGROUND OF THE INVENTION This invention relates to a novel method for preparing a screen structure for a cathode ray tube and particularly, but not exclusively, to a novel method for preparing a light-absorbing matrix for a color television picture tube.

Color television picture tubes which include a light-absorbing matrix as a structural part of the luminescent screen have been described previously, for example, in US. Pat. No. 2,842,697 to F. J. Bingley and No. 3,146,368 to J. P. Fiore et al. These patents describe color television picture tubes of the aperture mask type (also called shadow mask type) in which a light-absorbing matrix is located on the inner surface of the faceplate of the tube. In this structure, the matrix has a multiplicity of holes therein, each phosphor dot being concentric with one hole in the matrix.

A reverse printing method for preparing a light-absorbing matrix for a cathode ray tube is described in patent application Ser. No. 626,752 filed Mar. 29, 1967 by Edith E. Mayaud, now Pat. No. 3,358,310, Jan. 26, 1971. In a preferred embodiment of that method, the inner surface of the face plate of a cathode ray tube is coated with a film of clear water-based photosensitive material (typically a dichromate-sensitized polyvinyl alcohol). A light image is projected on the film to insolubilize selected regions of the film. The film is flushed with water to remove the still soluble regions of the film while retaining the insolubilized regions in place. Then, the developed film is overcoated with a layer containing particles of light-absorbing material, such as graphite. Finally, the retained film regions are removed together with the overlying overcoating, while retaining overcoating in the regions previously occupied by removed portions of the film. Such a process produces a satisfactory light-absorbing matrix, although further improvements in the process are desirable, particularly by requiring a shorter exposure time of the photobinder and better definition of the matrix.

The inner surface of the faceplate is usually stippled or otherwise textured to conceal surface irregularities introduced during the manufacture of the faceplate. A practical problem of practicing the process described in the Mayaud application is that the pattern of clear resist on the surface of the faceplate is difficult to inspect visually, especially on a stipped surface. As a result, many faceplates with defective photobinder patterns or no pattern 3,623,867 Patented Nov. 30, 1971 at all are passed to the overcoating step, and must later be returned for reprocessing.

SUMMARY In the novel process, a support surface is coated with a film of a photosensitive polymeric material containing 0.06 to 0.50 weight part inert, light-scattering particles per part polymeric material. The particles have an average size of about 150 to 5,000 angstroms. The film is then exposed and developed to produce a polymeric image; the image is overcoated with a composition containing particles of light-absorbing or other screen structure material; and the polymeric image and overlying coating are removed as described in the above-identified Mayaud patent application. The process may be used to fabricate a lightabsorbing matrix, or a phosphor pattern, or other screen structure.

Several advantages result from the inclusion of a relatively low percentage of inert, light-scattering particles, such as titanium dioxide, zirconium dioxide, aluminum oxide or silicon oxide, in the coating composition. The film, both before and after development, has a milky to translucent appearance and thereby may be easily inspected visually for defects. It is noteworthy that the loading of particles in the film is much less than that used for pigmentation or opacification of film coatings. It is also noteworthy that the presence of these particles does not interfere with the exposure tep by dissipating the light energy. Instead, the preseric e of these particles enhances the effects of the light by requiring a shorter exposure time and by producing a polymeric image with better definition and more uniform hardening throughout.

DESCRIPTION OF THE PREFERRED EMBODI- MENTS.EXAMPLE l A novel method for preparing a light-absorbing matrix on the inner surface of a faceplate of an aperture mask type color television picture tube will now be described. First, the inner surface of a faceplate is cleaned in the usual way as with water, ammonium bifluoride, hydrofluoric acid, detergent, caustic, etc. to remove any foreign matter. Then, the surface is coated with a film of photosensitive material containing an amount of titanium dioxide. In this example, the film is a sensitized polymeric material, particularly polyvinyl alcohol containing soluble dichromate ions. The film may be produced by depositing on the surface a quantity of an aqueous photobinder solution containing about:

Wt. percent Polyvinyl alcohol 3.42 Titanium dioxide 0.43 Ammonium dichromate 0.34

Water Balance to The faceplate is rotated and tilted so that the solution spreads evenly over the surface. During the latter steps of the rotation, infrared heat is applied so that the water in the solution evaporates and a dry film is formed on the surface.

An aperture mask for the faceplate is positioned above the film and the assembly is placed in a lighthouse, which is an apparatus designed to expose the film on the faceplate by projection of light through the mask. A suitable light house is described in US. Pat. No. 2,885,935 to D. W. Epstein et al. In this example, the mask has circular apertures with a diameter of about 13 mils and a center-to-center spacing between apertures of about 28 mils near the center of the mask. The film is exposed for about 8 minutes to light from a 1,000 Watt quartz lamp positioned about 14 inches from the aperture mask.

During the exposure, light from the lamp is passed through a light pipe or collimator and projected through the mask causing beamlets of light to pass through the apertures in the mask incident upon the film. The irradiated regions of the film harden; that is, become insoluble in water. There is a slight enlargement of the exposed areas above the size of the beamlet (to about 16 mils) and a graded hardening at the margins of the exposed areas. The exposure through the mask is repeated three I times, each time with the light incident at a slightly different angle so that the beamlets harden the film in groups of three, as in the usual shadow mask screen manufacture. Substantially less exposure time is required with the novel process as compared with the prior process in which no titanium dioxide was present in the film. In some tests, reductions in exposure time of the order of 35% have been achieved.

Following exposure, the assembly is removed from the lighthouse and the mask separated from the faceplate. The exposed coating is subjected to flushing with a forced spray of water for about 30 seconds, after which the faceplate is rinsed with Water and dried. At this point in the process, the faceplate surface carries an adherent stencil comprised of open areas and of dots of hardened polymeric film coated on the surface. The stencil may be inspected visually, and defects may be detected more easily due to the presence of the small quantity of titanium dioxide in the film. With no titanium dioxide present, the film is so thin and so transparent that visual inspection is difficult.

The stencil is now overcoated with a composition comprised of light-absorbing particles. In this example, the overcoating is produced by applying to the stencil a slurry containing about 5.0 weight percent of colloidal graphite in water and then drying the overcoating. It is desirable to include a trace of a wetting agent in the slurry in order to facilitate the spreading of the graphite slurry over the stencil. The overcoating is dried thoroughly for about 1.5 minutes with the aid of infrared heat. After cooling, the overcoating is well adhered both to the polymeric dots of the stencil and to the bare faceplate surface, which is the open area of the stencil.

Next, a chemically digestive agent for the polymeric dots is applied to the overcoating. In this example, the digestive agent is an aqueous solution containing about 4 weight percent hydrogen peroxide. This solution may be applied to the overcoating as a wash or as a spray under pressure. The hydrogen peroxide solution penetrates the overcoating and the dots, causing the hardened polyvinyl alcohol of the dots to swell and soften. Subsequent flushing with water removes the softened dots together with the overlying portions of the overcoating, but leaves behind that portion of the overcoating which is adhered directly to the surface in the open areas of the stencil. At this point, the faceplate carries a black light-absorbing matrix having a multiplicity of circular holes therethrough about 16 mils in diameter. The edges of the pattern are relatively smooth as compared with a similar pattern prepared with a film that is free of titanium dioxide. Generally, the definition of the pattern prepared by the novel method is better than similar patterns prepared with films that do not have added particulate material.

The black, light-absorbing matrix is now rinsed with water and dried for about 4 minutes with the aid of infrared heat. Then, the faceplate is processed in the usual way to deposit red-emitting phosphor dots, greenemitting phosphor dots and blue-emitting phosphor dots about 17 mils in diameter over the holes in the matrix which are about 16 mils in diameter by the usual photographic technique using the same aperture mask as the photographic master in the process. The slight enlargement of the phosphor dots over the holes is achieved by the spreading of light during projection, which may be enhanced by increasing the exposure time of the hardened areas. The completed screen has a matrix with holes therein and phosphor dots substantially concentric therewith. A suitable process for depositing phosphor dots is described in an article entitled, Color-Television Screen- 4 ing by the Slurry Process, by T. A. Saulnier, Jr., in Electrochemical Technology, 4, 2731 (1966).

The luminescent screen may now be processed in the usual way to apply a reflective metal layer on top of the phosphor dots and the black matrix. The screen is baked and assembled with the aperture mask into a cathode ray tube in the usual way. A suitable process for filming and aluminizing is described in an article entitled, Emulsion Filming for Color Television Screens, by T. A. Saulnier, Jr., in Electrochemical Technology, 4, 31-34 (1966).

General considerations The particular steps described above in Example 1 may be varied within limits and still fall within the scope of the invention. Obviously, the novel method may be used to produce screen structures of different materials on other support surfaces, and for preparing other screen structures than that described in Example 1. By screen structure is meant a structure, luminescent or nonluminescent, which is a part of the screen or target of a cathode ray tube. Some of the variations in the novel process are described below.

The photosensitive fiIm.The photosensitive film is produced by coating a support surface, as by dipping, spraying, flow-coating or spin-coating, with a solution of a polymeric binder, a photosensitizer therefor and inert light-scattering particles. The preferred binder is a polyvinyl alcohol which may be sensitized with a small amount of ammonium dichromate, sodium dichromate, potassium dichromate; or a soluble salt of a metal, such as iron or mercury; or with an organic photosensitizer for a waterbased photosensitive material, such as a diazo compound.

Besides polyvinyl alcohol, other photosensitive or photosensitizable polymeric materials may be used. Some suitable water-soluble materials which can be made photosensitive are proteins such as gelatin, albumin and fish glue; carbohydrates such as gum arabic and starch; and synthetic materials such as polyvinyl pyrollidone, and certain acrylic acid derivatives. In general, multifunctional water-soluble polymers containing reactive groups, such as OH, COOH, NH CO, singly or in combination may be used. Mixtures of these materials may also be used. Some suitable solvent-soluble photosensitive materials are polyvinyl methyl ketone, KPR and KMER (available from Kodak, Rochester, N.Y.) aminated polystyrene, and hydroxy esters of polyacrylates. Water-soluble materials are preferred, at least because there are a large number of aqueous solutions that can be used in the subsequent step of graphic image development. Solvent-soluble photosensitive materials are not as readily attacked by aqueous solutions. Suitable reagents for the graphic image development of solvent-based photosensitive materials are acids, bases, and commercial strippers.

It is desirable that the photosensitive material form a smooth, unbroken and uncrazed layer or film since this will produce the sharpest, cleanest graphic images. To this end, it is preferred that the photobinder by film-forming either directly upon deposition or during a heating step subsequent to deposition. In many systems, the film forming temperature can be tailored by adjusting the relative proportions of ingredients constituting the photosensitive material.

The inert, light-scattering particles may be of any chemical composition. Titanium dioxide, zirconium dioxide, silicon dioxide and aluminum oxide are preferred examples, although other materials may be used for this purpose. The particles should have an average size of about to 5,000 angstrons. There is a preferred size range for each material. Titanium dioxide is particularly effective in overcoming image definition problems with a coating applied over the irregular strippled surface of a typical faceplate. Titanium dioxide with an average particle size of about 300 angstroms can be used to produce a coating with the desirable optical properties. However,

the optimum particle size and agglomerate size have not been determined at the present time. The very fine particle material mentioned is more easily kept in suspension at the low viscosities used in coating the film. Aluminum oxide having an average particle size in the range of 150 to 550 angstroms, such as Alon marketed by Cabot Corporation, Boston, Mass, may be used. Silicon dioxide having an average particle size in about the same range, such as Cabosil marketed by Cabot Corporation, may be used. Combination of two or more particulate materials may be used.

The particle sizes suggested here cover a wide range of commercial materials for pigment and give a broad choice. However, agglomerate size has a definite effect on the light-scattering properties of the particles. This can be demonstrated with a normal paint pigment grade of titanium dioxide where the particle size distribution is nearly ideal. A commercial pigment sold by Cabot Corporation, Boston, Mass. as RF-30 titanium dioxide performs poorly when simply added With stirring to a solution of polyvinyl alcohol. However, the preparation steps for the photobinder solution disclosed below in Example 3 have given excellent results.

The mechanisms which produce the improvement in the noxel method are not entirely understood, but they are believed to be related to fine-particle light-scattering within the coating by the particles or aggregates of the added particulate materials. This scattering tends to reduce lateral travel of light through the coating and to enhance the utilization of light in the exposed area where the light is incident. The effect is to produce a more uniform hardening and a better defined image of the illuminated areas of the film. The incident light is believed to be more uniformly diffused and absorbed in the localized regions of the film.

Where the photobinder is principally a Water-soluble polymer, the resistance of the stencil to the erosive action of the chemically digestive agent used in the graphic image development step may be adjusted, if desired, by including in the film a small amount of a less watersoluble polymer, which is referred to herein as a filler resin. For example, Where the photobinder is principally polyvinyl alcohol, it is preferred to include a small amount of an acrylate polymer or copolymer as a filler resin. One suitable acrylate formulation that may be used is Rhoplex C-72 (marketed by Rohm and Haas, Philadelphia, Pa.), which is an aqueous suspension containing alkyl and aryl acrylates and methacrylates. Other Rhoplex acrylate formulations may be used. The proportion of acrylate polymer may be up to about 100% of the proportion of polyvinyl alcohol present in the coating. One suitable photobinder solution which includes a small amount of acrylate polymer is set forth below in Example 2.

Sometimes it is convenient to express the ingredients of the photobinder solution as a weight ratio. The weight ratio of inert light-scattering particles to polymeric binder should be in the range of 0.06 to 0.50. This is considerably lower than proportions normally used for pigmentation or opacification where the weight ratios are generally 2.0 and higher. The Weight ratio of photosensitizer to polymeric binder should be in the range of about 0.01 to 0.30. The weight ratio of filler resin to polymeric binder should be in the range of about 0.0 to 1.0.

The photosensitive film may be of the type which is insolubilized when exposed to energy in the form of rays of either visible or invisible light or electrons. Such photosensitive materials are referred to herein as negativeacting. Instead one may use a photosensitive material of the type which is solubilized when exposed to radiant energy. This latter type of photosensitive material is referred to herein as positive-acting.

The photographic master.Any pattern form may be used as a photographic master for exposing the photosensitive film. Thus, conventional silver halide images may be used either by projection or contact printing. In the case of preparing luminescent screens for cathode ray tubes, one may also use an electron beam exposure whereby the electron beam traces out the pattern by conventional scanning techniques without the use of a photographic master. In other applications, a mask may be used between the electron gun and the screen. In preparing screen structures for color television picture tubes of the shadow mask type, it is preferred to use the aperture mask of the tube as a photographic master for exposing the photosensitive coating. In that case, the light source is placed at three separate locations in order to produce three sepa rate exposures on the coating, each at a different location. Thus, three holes are produced in the graphic image for each aperture in the mask. Of course, the aperture mask or other master may be used to produce only one, or two, or more than three exposures for the same coating. The shape and size of the apertures in the aperture mask is not critical.

The ph0toexposure.The photosensitive material is exposed to a pattern of energy rays in the range and of the type to which the photosensitive material is sensitive. Where dichromated polyvinyl alcohol is the photosensitive material, radiant energy in the form of electrom beams or as light in the blue and ultraviolet range of the spectrum may be used. Where a contact master is used, one may use a flood exposure. Where the exposure is by projection of an image, a small diameter source is preferred.

Stencil devel0pment.Where a photoresist technique has been used for producing the stencil, the exposed photosensitive film is developed in the manner of the use for that material. In the case of dichromated polyvinyl alcohol, the development is carried out by flushing the surface of the film with water or with other suitable solvent for the unexposed, still-soluble photosensitive material. With other films, the same or other solvents may be used. The development should leave the minimum residue on the bared support surface so as not to interfere with the subsequent overcoating step.

The 0verc0a!ing.The overcoating may be of any material which is adherent to the support surface. The overcoating may include a pigment or phosphor. Where it is desired to produce a light-absorbing matrix for a cathode ray tube, it is preferred to include in the overcoating a relatively high loading of a dark pigment. The pigment is preferably elemental carbon in the form of carbon black, acetylene black, or graphite. Other black pigments that may be used are silver sulfide, iron oxide, lead sulfide, ferrites, and manganese dioxide. The pigment may be black, white or colored. Where it is desired to produce a luminescent structure, it is preferred to include a relative high loading of phosphor particles in the overcoatmg. A process of this type for preparing patterns of phosphor particles is disclosed in U.S. Pat. No. 2,840,470 to A. K. Levine.

The overcoating must make a bond to the support surface that will endure the subsequent processing, such as removing the image stencil and depositing the phosphor dots. With some materials, such as some commerciallyavailable dispersions of graphite in water, the graphite upon drying makes a bond to a glass faceplate which s adequate. With other materials, it may be necessary to 1nclude a small amount of a binder in the overcoating such that the dry overcoating develops a bond to the support surface through the use of the binder. Colloidal sihca is a satisfactory binder for lamp black and acetylene black. For example, about 10% of a colloidal silica with respect to the percent pigment present produces a strong bond to the glass faceplate, especially where a small amount of ammonium dichromate is also present. Besides colloidal silica, alkali silicates may also be used as the binder.

Where a pigment is used for the purpose of making a light-absorbing matrix for a picture tube of the shadow mask type, the pigment must be deposited in sulficient density to develop the necessary opacity for this purpose. In the case of acetylene black and lamp black, the pigment should be deposited in a weight of about 0.2 to 2.0 mg./cm. of surface area and, preferably, about 1.0 mg./cm. or more in order that suflicient thickness remains after tube processing. Where graphite or other pigments are used, slightly lower weights are required for achieving the same opacity in the final graphic image.

The overcoating should also be permeable to and substantially unaffected by the graphic image developer, which must swell or erode or dissolve at least apart of the image stencil. Where the overcoating is entirely particles, it is necessarily permeable. Where the overcoating contains a binder, the overcoating may be permeable by nature or may be made permeable by crazing the overcoating. The bond between the supporting surface and the overcoating is preferably not substantially attacked by the graphic image developer. When the overcoating-support surface bond is both inert to the attack of the graphic image developer, and is adherent to the surface, it is possible to develop the graphic image after softening with a high-pressure spray of water, without any alteration of the pattern due to localized overdevelopment. If desired, appreciable amounts of organic material may be incorporated in the overcoating.

Graphic image development.-Any substance that dissolves or degrades the polymeric material of the stencil into soluble, partially-soluble, or volatile fragments and leaves the overcoating substantially unattacked may be used for developing the graphic image.

The preferred method for graphic image development is to apply to the overcoating an aqueous solution of an oxidizing agent in a concentration sufliciently high such that rapid penetration of the overcoating and softening of the stencil occur. In the case of stencils of hardened polyvinyl alcohol, the stencil softens rapidly with aqueous solutions containing between 1 and 35 weight percent hydrogen peroxide. Higher concentrations may also be used. Instead of hydrogen peroxide solutions, aqueous solutions of the following may also be used: nitric acid, sodium peroxide, or other alkali peroxides, perchloric acid or alkali perchlorates, hydroflorous acid, alkali hypochlorites, peracetic acid, alkali borates, alkali perborates, sodium hydroxides and certain enzymes. The graphic image developer solution is chosen so that it will not substantially decrease the adherence of the matrix overcoating to the substrate.

The time and temperature for carrying out the graphic image development are not critical, especially in view of the fact that they depend only on the removal of the polymeric material of the stencil. However, too fast a development may result in disruption of the overcoating, and too slow a development may result in the weakening of the bond between the overcoating and the support surface. Hence, in each case, the optimum time and temperature for image development are empirically determined.

Image development may also be carried out with nonaqueous reagents and mixtures of solvents and waterbased reagents.

EXAMPLE 2 Follow the procedure of Example 1 except substitute the following photobinder solution:

This photobinder solution produces a film which is capable 9f printing patterns with superior resolution, sharp- 8 ness and with a shorter exposure time than an identical formulation which does not contain titanium dioxide. The presence of Rhoplex increases the resistance of the stencil to the erosive action of a hydrogen peroxide solution over the formulation of Example 1.

EXAMPLE 3 Follow the procedure of Example 1 except substitute the following photobinder solution:

This solution is preferably prepared by first preparing a 10 weight percent dispersion of titanium dioxide containing 0.1 weight percent dispersant. The dispersant is preferably Tamol 731 marketed by Rohm and Haas Co., Philadelphia, Pa. or Noposant K marketed by Nopco Chemical Co., Newark, NJ. The dispersion is agitated for extended period (30 minutes or more) in a high speed blender. A quantity of 10 weight percent polyvinyl alcohol stock solution is diluted with the remaining water of the formulation. Then, the required quantity of 10% dispersion is added slowly to the diluted polyvinyl alcohol solution with rapid stirring. Now, the required quantity of a 22.5 weight percent solids emulsion of Rhoplex C-72 is added slowly with rapid stirring. Finally, the required quantity of a 5 to 10% aqueous ammonium dichromate solution (adjusted to pH 7.0 with ammonium hydroxide) is slowly added with rapid stirring.

This formulation is designed for application by spin coating process over a stippled concave inner surface of a faceplate for a television picture tube. For this purpose, the viscosity is adjusted to about 26 centipoises at about 24 C. to provide the desired coating thickness. Other coating techniques and other viscosities may be used.

What is claimed is:

1. In a process for preparing a luminescent screen structure of a cathode ray tube, the steps comprising,

(a) coating a support surface with a film of a polymeric material whose solubility is altered when it is exposed to radiant energy, said film containing 0.06 to 0.50 weight part inert, light-scrattering particles per part polymeric material, said particles having an average size ofabout to 5,000 angstroms,

(b) exposing said film to an image in the form of radiant energy until the solubility of the irradiated regions thereof are selectively altered, thereby producing in said film regions of greater solubility and regions of lesser solubility,

(c) removing those regions of said film with greater solubility thereby baring the areas of the support surface underlying said regions of greater solubility, While retaining those regions of said film of lesser solubility,

(d) coating said support surface and said retained film regions with a composition containing particles of screen structure material,

(e) and then removing at least a portion of said retained coated film regions and the overcoating composition thereon, while retaining the overcoating composition adhering to said support surface.

2. The process defined in claim 1 wherein said screen structure material is luminescent.

3. The process defined in claim 1 wherein said screen structure material is nonluminescent and light-absorbing.

4. The process defined in claim 3 including the subsequent step of depositing luminescent material on areas of the supporting surface previously occupied by said retained film regions of lesser solubility.

5. A method for producing a luminescent screen structure upon the inner surface of the glass faceplate of a cathode ray tube comprising:

(a) coating said surface with a film of a water-soluble polymeric composition whose solubility in water is lowered when it is exposed to radiant energy, said film containing 0.06 to 0.50 weight part inert, lightscattering particles per part polymeric material, said particles having an average size of about 150 to 5,000 angstroms,

(b) exposing said film to an image in the form of said radiant energy until the solubility of the irradiated regions thereof is selectively lowered, thereby producing in said film regions with greater solubility and regions with lesser solubility,

(c) flushing said exposed film with water to remove said regions of greater ulobdwilityonotit) said regions of greater solubility down to said surface while retaining those regions of lesser solubility,

(d) overcoating said surface and said retained regions of lesser solubility with a composition containing particles of screen structure material, said overcoatting being permeable to a solution of chemically digestive agent for said retained film regions,

(e) applying to said overcoating said solution of chemically digestive agent for said retained film regions whereby said agent pentrates said overcoating and softens at least a portion of the retained film regions thereunder,

(f) and then flushing away said solubilized film regions 10 and the overlying overcoating composition, while retaining the overcoating composition on said surface.

6. The method defined in claim 5 wherein said screen structure material is a black, inorganic, particulate material.

7. The method defined in claim 5 wherein said water soluble polymeric composition is a dichromate-sensitized polyvinyl alcohol.

8. The method defined in claim 7 wherein said inert light-scattering particles are of titanium dioxide and said particles of screen structure material are of graphite.

9. The process defined in claim 5 wherein said inert, light-scattering particles are titanium dioxide, zirconium dioxide, aluminum oxide, silicon oxide, or combinations thereof.

10. The process defined in claim 9 wherein said inert, light-scattering particles are of titanium dioxide.

References Cited UNITED STATES PATENTS 5/1967 Mayaud 9636.1 1/1968 Fiore et al. 96-36.1

US. Cl. X.R.

9638.3; ll733.5C, 33.5CM, 33.5CP

Patent No. 3 a 623 867 Dated November 30, 1971 Inventor(s) Theodore Alexander Saulnier Column 1, line 47 Column 1, line 71 Column 4, line 58 Column 8, line 25 Column 9, line 17 (SEAL) Attest:

EDWARD FLFLETCHER, JR. Attesti'ng Officer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

After "3,358,310" insert -issued- After "a" change "stipped" to --stippled-- After "photobinder" change "by" to After "for" insert an-- Remove "said regions of greater s ulobdwilityonotiO" Signed and sealed this 2nd day of May 1972.

ROBERT GOTTSCHALK Commissioner of Patents FORM PO-IOSO (10-69) USCOMM-DC 60376-F'69 a u.s GOVERNMENT PRINTING orrlc: 1969 0-30-354