US 3507647 A
Description (OCR text may contain errors)
United States Patent US. Cl. 96-1.5 12 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to method of rendering surfaces suitable for lithographic printing by causing the surface to be more hydrophilic without affecting the ability of such surfaces to receive and retain images having oleophilic characteristics. Said effects are achieved by applying to said surfaces a long chain film forming homopolymer of itaconic acid, or copolymer of itaconic acid with up to 50%, by weight, of acrylic acid, said homopolymers and copolymers having molecular weights ranging from 15,000 to about 200,000, and in aqueous solution at 33% solids content a viscosity ranging from about 100 to about 4000 cps. Such a solution can be added to a lithographic surface containing an alkaline reacting substance, or an alkaline-reacting substance such as an inorganic acid salt may be included in the aqueous solution being added to the lithographic surface, or the polymer or copolymer may constitute the binder for the lithographic coating.
This is a continuation-in-part of US. Ser. No. 459,703 filed May 28, 1965, and now abandoned.
This invention relates to lithographic surfaces for use in printing.
Heretofore, in the manufacture of lithographic surfaces on a suitable carrier which includes paper as well as other materials, a coating of suitable material was applied to the carrier in order to function as a water receptive hydrophilic surface. Since the advent of electro photography, lithographic surfaces suitable for imaging by such processes have likewise been prepared by applying to a suitable carrier a coating intended to function as a photoconductive insulating surface. In the case of such lithographic surfaces intended for use in connection with electrophotography and related processes, the nature of such coating surfaces is basically oleophilic as contrasted to hydrophilic. As defined herein, oleophilic surfaces are those which will pick up oily or greasy ink in preference to water solutions normally used as fountain solutions in printing operations. As defined herein, hydrophilic surfaces are those which Will maintain a water film sufficient to repel the oily or greasy inks so as to give little or no pick up of the oily or greasy material during printing operations.
As defined and referred to herein, a lithographic plate is a plate comprising a suitable carrier having a lithographic surface with such characteristics that it is usable for reproduction of copies of oleophilic images applied thereto; and is further characterized by the fact that the image to be reproduced is essentially at the same level as non-image bearing portions of the plate.
Also as defined and referred to herein, a lithographic surface may or may not include a photoconductive insulating material and thus may or may not be a coating to which images can be applied by electrophotographic or related processes.
In lithographic printing operations, a lithographic plate to which an image has been applied is first flooded with a water solution and thereafter ink is applied to the plate. The image areas are oleophilic and receptive to ink and, at the same time, tend to reject the water while the nonimage areas are hydrophilic, that is, water receptive, and, hence, tend to reject ink from the non-image or background areas in order to avoid background discoloration of the printed sheet.
The foregoing description is generally true concerning all types of lithographic printing operations and is equally applicable irrespective of the manner of applying the image to the lithographic plate. However, particular problems arise in connection with lithographic printing operations in those cases where the image is applied to the lithographic plate by electrophotographic or related processes.
The electrophotographic process may be described generally as one wherein a base plate of relatively low electrical resistance such as metal, paper, etc. having a photoconductive insulating surface thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak off rapidly to the base plate in proportion to the intensity of light to which any given area is exposed. After such exposure the coating is contacted with electrostatic marking particles in the dark. These particles adhere to the areas where the electrostatic charges remain, forming a powder image corresponding to the electrostatic image. In the case of xerography, the powder image is generally transferred to a sheet of transfer material, such as a lithographic plate, resulting in a positive or negative print on the plate. Thus, such plate does not need a photoconductive surface coating.
Alternatively, where the base plate is relatively inexpensive, as of paper, it may be desirable to fix the powder image directly to the plate itself. In the latter case, this process is generally known in the trade as Electrofax whereby direct electrophotographic printing is effected on a photoconductive paper which may then be used as a lithographic master. In the latter process, the photoconductive insulating coating is an important feature of the process and generally such coatings include photoconductive zinc oxides in a resin binder. Images applied by the Electrofax process are reproduced at essentially the same level as the rest of the plate when intended for use in connection with lithographic printing operations. As applied by the Electrofax process, the image areas must be oleophilic and the non-image or background areas must be hydrophilic for reasons noted heretofore.
The Electrofax process may be described generally as including the following steps for making a direct print. The paper is first made sensitive to light by giving it a blanket negative electrostatic charge on the coating side in the dark. One way of doing this is by ion transfer from a corona discharge. The sheet, now sensitive to light, is exposed by any of the conventional photographic procedures. The electrostatic charge is lost or reduced in the exposed areas and retained in the masked areas to form a latent electrostatic charge image on the surface of the paper. The latent image is then developed by applying a pigmented resin powder carrying a positive electrostatic charge. The powder is attracted and held by the negatively charged image areas. Finally, the powder image is fixed by melting the resin powder so it fuses to the paper surface to produce a durable light-fast image. The nature of the resin binders employed in the Electrofax process is such that the photoconductive insulating coating surface is more oleophilic in nature than is the case with other lithographic coatings, thus making more difficult the problem of rendering certain of the areas hydrophilic.
Prior to this invention, lithographic surfaces were in some instances basically hydrophilic in nature and a greasy or oily image was applied directly thereto by various means including commercial pens, pencils and typewriter ribbons. After applying an image to such a surface, etching solutions were employed to render the nonimaged areas more hydrophilic. Where the image is ap plied by Electrofax means so that the lithographic master contains a photoconductive insulating coating which is basically more oleophilic than other types of lithographic plates, the problem of rendering the non-imaged portions hydrophilic while maintaining the image portions oleophilic is much more difficult.
In preparation of the improved lithographic surface in accordance with the present invention, the lithographic surface is treated with an aqueous solution comprising a material selected from homopolymers of itaconic acid, copolymers of itaconic acid with up to 50%, by weight, of acrylic acid and derivatives of said homopolymers and copolymers, to render the surfaces more hydrophilic without destroying the oleophilic nature of an image already applied thereto or, alternatively, to render such surface more hydrophilic without affecting the ability of such surface to receive and retain an image having oleophilic characteristics. Such treatment is effected by any convenient means of distributing a thin uniform film of aqueous solution over the surface to be treated either prior to or subsequent to application of the image to such surface, thereby adapting the surface for use as a lithographic plate in offset or other lithographic printing operations.
Unlike prior art etching solutions and methods, the solutions and methods employed in accordance with the present invention provide the desired hydrophilic characteristics without necessity of including in the treatment complex inorganic salts to enhance Wettability. In accordance with the present invention, it is sometimes desirable to include a buffering agent which will at least partially neutralize the acid component of the etching solution. This is achieved through use of certain inorganic salts.
As noted above, in prior art treatment of lithographic plate surfaces with an aqueous etch solution, it is important that the etching solution remain on the surface of the plate and not work into the body of the plate since this may result in incomplete wetting of the plate surface during printing operations with the result that the background areas tend to lose their hydrophilic characteristics and pick up unwanted ink in these areas. In the prior art attempts have been made to overcome such difficulties by interposing a resinous water barrier between the carrier and the surface coating.
An object of the invention is to produce and to provide a method for producing a paper base lithographic plate which is capable of tenaciously holding an ink receptive image and has been rendered adaptable for use by such method so that non-image portions thereof are hydrophilic in nature without adversely effecting the tenacious or ink-receptive characteristics of the image portions.
A further object of the invention is to produce and to provide a method for rendering hydrophilic the background or non-image areas of a previously imaged electrophotographic coating of a lithographic plate without affecting the oleophilic nature of image areas thereon whereby said plate is rendered adaptable for use in lithographic printing operations.
Still another object of the invention is to provide a composition having as a component thereof a material selected from the group consisting of homopolymers of itaconic acid, copolymers of itaconic acid with up to 50%, by weight, of acrylic acid, and derivatives of such homopolymers and copolymers for treating a surface to adapt said surface for use in printing operations by rendering it more hydrophilic in nature without adversely affecting the oleophilic nature of other portions of said surface.
Other objects and advantages will become apparent from the following description.
For a better understanding of the present invention, description will now be made of the preparation of lithographic plates embodying features of the invention, but it is to be understood that the following examples are given by way of illustration and not of limitation.
EXAMPLE 1 Lithographic coatings of known compositions and characterized by inclusion as an ingredient thereof of zinc oxide were bonded to a suitable carrier with a variety of binders, some normally considered too oleophilic for use as offset masters because of the tendency of such surface coatings to pick up ink. The ratio of zinc oxide to binder ranged between about 3 to 1 to 10 to 1. Binder materials included styrene-butadiene copolymers and melamine resin, acrylic ester polymers, alkyd-styrene resins, vinyl acetate-crotonic acid copolymer, copolymerized vinyl acetate-crotonic acid and melamine crosslinking resins, melamine-alkyd resins, casein, and casein and styrene-butadiene mixtures. Except for the abnormally oleophilic characteristics referred to above, some of such surfaces were ordinary lithographic or planographic surfaces and others contained a photoconductive insulating coating of the type used in connection with the Electrofax process.
Oleophilic images were applied to such surfaces with commercial pens, pencils and typewriter ribbons and by xerographic means in the manner normally employed to apply such images to lithographic offset plates. Those surfaces which contained photoconductive insulating coatings had images applied thereto with a fusible powder and the powder was fused to the surface in accordance with methods employed in the Electrofax process using both powder and liquid toners as employed in such process.
Solutions of homopolymers of itaconic acid and of copolymers of itaconic acid with up to 50%, by weight, of acrylic acid, ranging in molecular Weight from 15,000 up to 200,000, and ranging in viscosity from about cps. to about 400 cps. for aqueous solutions at 33% solids content were prepared. The solutions were applied in concentrations ranging from about /z% through 38% as an etch to such surfaces to which the oleophilic images had previously been applied as described above. After etching, the treated surfaces were run on a commercial lithographic offset press using commercial lithographic offset ink. Such surfaces were found to produce useful offset copies of excellent quality. It was further found that if a short run of copies was made and the run discontinued, additional copies could be made merely by re-etching with the same solution and then continuing production of copies on the lithographic offset press.
Both the monomer and the short chain polymers of itaconic acid were found to be friable and not suitable for use in accordance with the present invention. The longchain homopolymers and copolymers of itaconic acid with up to 50%, by weight, of acrylic acid of the present invention are film-forming materials. Such materials can be readily produced by conventional means and are available commercially from Chas. Pfizer & Co. under the trade name Policon and range in molecular weights and viscosities set out above. Several of the materials from this source investigated and found to give excellent results in accordance with applicants claimed invention are as follows:
Average molecular weight Policon A homo-polymer of itaconic acid 15,000 Policon B copolymer of itaconic acid with acrylic acid 30,000 Policon C copolymer of itaconic acid with acrylic acid 100,000 Policon D copolymer of itaconic acid with acrylic acid 200,000
In each case, treatment of the lithographic coating with aqueous solutions of the above materials gave greatly improved reproductions.
EXAMPLE 2 In accordance with the technique set forth in Example 1, an oleophilic image was applied to a sheet of aluminum such as is normally used as a lithographic printing plate and the aluminum sheet was thereafter etched in accordance with the method set forth in Example 1, using the homopolymers and copolymers of itaconic acid set out therein, with excellent printing results.
EXAMPLE 3 A sheet of normal bond paper was surface treated with a solution of polyvinyl alcohol and 50% zinc oxide suspension which was then allowed to air dry. An oleophilic image was applied to the dried surface with pen, pencil and typewriter ribbon as descr bed in Example 1 and thereafter etched in accordance with techniques set forth in said Example 1 with a by weight, solution of itaconic acid homopolymer having a molecular weight of about 15,000. The thus treated paper surface waas employed as a lithographic master and was run on an offset press with good quality copies resulting therefrom.
EXAMPLE 4 The procedure of Example 1 Was repeated with the exception that the content of the etching solution was modified to include salt solutions including aluminum acid phosphate (Alkaphos C, manufactured by Monsanto Chemical Co.), mono sodium phosphate, mono potassium phosphate, diammonium phosphate, cobalt phosphate, zinc phosphate, nickel phosphate, copper phosphate, cadmium phosphate, aluminum sulfate, with the result that an improvement was observed over the results obtained in Example 1. Other inorganic acid salt neutralizing solutions compatible with the homopolymers of itaconic acid and copolymers of itaconic acid with up to 50%, by weight, of acrylic acid may likewise be satisfactorily used. Also, voltaile alkalies such as ammonia and diethylamine have been satisfactorily used.
EXAMPLE 5 A glass plate was cleaned by washing with sodium stearate and then rinsed with distilled water containing carbon dioxide and adjusted to a pH of about 5.8 until the surface was free of excess soap. The surface of the plate was then dried and exposed to ambient conditions until the surface could no longer be uniformly wet by water. The surface was then etched with a solution containing methyl alcohol 10 parts, aluminum acid phosphate 1% parts, water 20 parts and 6 parts, by weight, of a copolymer of itaconic acid with acrylic acid having a molecular weight of 200,000. Following the etching treatment, the surface was again rinsed with distilled water containing carbon dioxide and adjusted to a pH of about 5.8 and thereafter dried, The surface was then found to be rendered sufficiently hydrophilic as to be wet smoothly and uniformly when water was applied thereto and the tendency for the water to gather in drops on the surface was no longer observed.
6 EXAMPLE 6 The procedure set forth in Example 1 was repeated with the modification that an olephilic image was applied to a commercially available calcium carbonate coated paper by means of conventional lithographic reproducing pen, pencil and typewriter ribbon and the coating was thereafter etched with a solution containing 15% of a copolymer of itaconic acid with acrylic acid having a molecular weight of about 200,000 and a viscosity of about 4000 cps. at about 33% solids and also containing 2% aluminum acid phosphate (Alkophos C, manufactured by Monsanto Chemical Co.). This treated surface was employed as a lithographic master to produce reproductions of said images by offset printing means. It was found that the non-imaged areas of the surface had been rendered hydrophilic without altering the olephilic characteristics of the image areas so that good quality copies were made from such sheet. Additionally, it was observed that background areas of such surface that were initially smeared with ink during installation of the master on the offset printing machine and which at first produced dark background areas, soon were self-cleaning to produce good quality copies.
EXAMPLE 7 The techniques set forth in Examples 1 and 2 were repeated except that a modified etching solution containing as the modifier 120%, by weight, of butyrolactone or its hydrolysis products or glycollic acid was employed. The observed results were similar to those observed in connection with Examples 1 and 2 in that the treated surface was rendered hydrophilic without disturbing the oleophilic character of images applied thereto and, in general, the results obtained here were comparable with those obtained in connection with Examples 1 and 2. Additionally, the etching solution set forth in this example was used to treat the surface of a paper coating of the type set forth in Example 6 with the result that, following such treatment, the paper ran well and produced good copies when used as a lithographic master.
EXAMPLE 8 An electrophotographic surface containing about 10 parts zinc oxide and 1 part binder (crotonic acid-vinyl acetate copolymer) on a paper base carrier was surface treated in accordance with the technique of Example 1 with an etching solution comprising a /2% distilled water solution of a copolymer of itaconic acid and acrylic acid having a molecular weight of 200,000 and a viscosity of 4000 cps. at 33% solids content, and then dried. After drying, the thustreated surface was imaged by the Electrofax process, again treated with the above described etching solution or one of the other etching solutions of Example 1 and run on a commercial lithographic oifset press to produce copies. It was found that the surface treatment according to this example rendered the background areas of such surface, which was originally quite oleophilic in nature, sufliciently hydrophilic as to produce exceptionally good copies,
Thus, it has been found and demonstrated that the materials and techniques of the present invention provide improved results not only for the treatment of electrophotographic surfaces containing photoconductive insulating materials but in addition are generally useful for treatment of lithographic surfaces as well as other surfaces wherein there is a need to render portions of the sunface hydrophilic without destroying the oleophilic characteristics of other portions of such surface.
Generally, it has been found desirable that the acid component of the etch solution be at least partially neutralized by the presence of a compatible alkaline material capable of effecting such neutralization. However, where the surface to be treated contains alkaline react ng materials, such as calcium carbonate or zinc oxide,
capable of at least partial neutralization of such acid, it is not necessary to include such a material in the etch. Where inclusion of such a material is required, examples of materials suitable for this purpose are shown in Example 4.
The improved results obtained by treating lithographic surfaces with solutions of the type described in Examples 1-8 may also be obtained by incorporating in the lithographic coating the said homopolymers of itaconic acid, coplymers of itaconic acid with up to 50%, by weight, of acrylic acid, and derivatives of said homopolymers and copolymers, said polymers, copolymers and derivatives having molecular weights ranging from 15,000 to 200,000 and in aqueous solution at 33% solids content viscosities ranging from 100 cps. to 4000 cps. It has previously been noted that electrophotographic papers comprise a photoconductive pigment and an insulating resin binder therefor. The binders commonly used are basically oleophilic in nature. While efforts have been directed to production of such coatings that are less oleophilic and more hydrophilic in their characteristics, such efforts have not been generally successful because when the composition is rendered sufiiciently hydrophilic this has usually been accompanied by adverse effects on the photoconductive insulating characteristics of the electrophotographic surface. These difficulties have been overcome in accordance with the present invention wherein homopolymers and copolymers of itaconic acid, and derivatives of said homopolymers and copolymers of the type defined herein are employed as a component of the electrophotographic coating surface with the result that the surface is rendered more hydrophilic and less oleophilic without adversely effecting the necessary characteristics of the photoconductive insulating components of said surface. The said itaconic acid polymer or copolymer may be substituted for part, or preferably, for all of the binder commonly used in such photoconductive coatings.
Description 'will now be made of preparation of electrophotographic lithographic plates embodying the features of the invention.
EXAMPLE 9 60 grams of photoconductive zinc oxide of the type normally used in preparation of electrophotographic plates was slurried in 30 grams of water to produce a thick mud-like consistency. A copolymer of itaconic acid with acrylic acid, having a molecular weight of about 200,000 and a viscosity in aqueous 33% solids content of about 4000 cps. (Policon D, manufactured by Chas. Pfizer & Co.), was adjusted with ammonium hydroxide to a pH value of about 8.7 and diluted to about 19%, by weight, polymer content.
One gram of the diluted copolymer solution was then added and mixed with the zinc oxide slurry and upon stirring the mud-like consistency of the slurry became almost water thin. After thorough working of the slurry to assure good dispersion, 51 grams of the diluted copolymer solution was stirred into the mix to form a resulting mixture.
The resulting mixture was applied as a coating to a suitable paper base stock in accordance with known techniques and then dried. The resulting electrophotographic master was then sensitized for use in an electro-- photographic process by exposure to a corona discharge, exposed to light through a contact negative, developed and the image fixed, all in accordance with known electrophotographic means. The master was then etched in accordance with prior art etching techniques and employing prior art etching materials to render all non-image bearing areas of the master surface hydrophilic without adversely effecting the oleophilic characteristics of the image bearing areas. After drying, the master was employed as such on a commercial lithographic offset press with the result that excellent copies were produced having high quality image reproduction and clarity of background characteristics.
The ratio of itaconic acid composition to pigment has not been found to be particularly critical for desired results, so long as customary binder-pigment ratios are employed. It has been found also that the compositions and techniques described in Example 9 are equally applicable to preparation of direct image lithographic plates where similar good results were obtained as shown in the following example.
EXAMPLE 10 A master was produced employing the materials and technique as set forth in Example 9 except that instead of imaging by Electrofax means, the image was applied to such master by means of typewriter ribbon, pencil and ball point pens. The lithographic results obtained in the example were equal to those obtained in the preceding example.
Investigations have further shown that various materials may be added to the compositions set forth in the preceding two examples for modifying the flexibility and binder strength of the coating if desired without adversely affecting the intended characteristics of the coating. Such materials include, for example, mineral pigments, starch, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl acetate, carboxyl containing styrene-butadiene copolymers, acrylic resin emulsions containing carboxyl groups and ammonia complexed metal salts (such as copper, cobalt, nickel and zinc) and ferric ammonium citrate.
It has also been found that derivative of itaconic acid polymers and copolymers may also be satisfactorily used in coatings of the type employed in Examples 9 and 10, as well as homopolymers and copolymers of itaconic acid with acrylic acid shown therein. Use of such materials is illustrated in the examples which follow.
EXAMPLE 11 The procedure set forth in Example 9 was carried out and equally good results obtained by substituting a neutral amide solution of a homopolymer of itaconic acid or copolymer of itaconic acid with acrylic acid for the copolymer shown therein. The amide solution was prepared by converting the itaconic acid copolymer referred to in Example 9 to the polyamide by raising the pH value to about 10.4 with ammonium hydroxide and then drying until substantially no free ammonia was found on dissolving and boiling with caustic. Best results were obtained when the amide solution was used with about twice as much zinc oxide as amide solids.
EXAMPLE 12 The procedure as set forth in Example 9 was repeated with equally good results except that in lieu of itaconic acid copolymer, use was made of the product obtained by adjusting polymonomethyl itaconic acid to a pH of about 10 with ammonium hydroxide. In preparing the coating, one part of this product was used with 2-10 parts by weight of photoconductive zinc oxide. The results obtained in both this and Example 11 were similar to those described in the preceding examples.
While particular embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various modifications and/or equivalents may be employed without departing from the invention, and it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the invention.
What is claimed is:
1. Method of treating lithographic surfaces to render them more hydrophilic without affecting the ability of said surfaces to receive and retain images having oleophilic characteristics which comprises applying to said surfaces a uniform film of an aqueous solution containing a long chain film-forming material selected from homopolymers of itaconic acid and copolymers of itaconic acid with up to 50%, by weight, of acrylic acid, said homopolymers and copolymers having molecular weights ranging from 15,000 to 200,000 and in aqueous solution at 33%, by weight, solids content, viscosities ranging from 100 cps. to 4,000 cps.
2. Method in accordance with claim 1 wherein the concentration of said solution is from about /2% through 38%, by weight, total solids. V
3. Method according to claim 1 wherein the lithographic surface to which said solution is added comprises an alkaline reacting material.
4. Method in accordance with claim 1 wherein the aqueous solution of said polymer is at least partially neutralized with an alkaline reacting material prior to addition to said lithographic surface.
5. Method according to claim 1 wherein the aqueous solution of said polymer is at least partially neutralized with an inorganic salt neutralizing agent.
6. Method according to claim 1 wherein the aqueous solution of said polymer is at least partially neutralized with a material selected from the group consisting of aluminum acid phosphate, monosodium phosphate, monopotassium phosphate, diammonium phosphate, cobalt phosphate, zinc phosphate, nickel phosphate, copper phosphate, cadmium phosphate and aluminum sulphate.
7. Method according to claim 1 wherein an oleophilic image is applied to said lithographic surface prior to application of said polymer solution to said surface.
8. Method according to claim 1 wherein an oleophilic image is applied to said lithographic surface subsequent to the application of said polymer solution to said surface.
9. Method according to claim 1 wherein an oleophilic image is applied to said lithographic surface subsequent to the application thereto of said polymer solution at least partially neutralized with a volatile alkali.
10. An etching composition for lithographic surfaces comprising an aqueous solution of a long-chain film-forming material selected from homopolymers of itaconic acid and copolymers of itaconic acid with up to 50%, by weight, of acrylic acid, said homopolymers and said copolymers having molecular weights ranging from 15,000 to 200,000 and in aqueous solution at 33% solids content,
viscosities ranging from 100 cps. to 4000 cps, wherein said composition additionally contains a material selected from the group consisting of aluminum acid phosphate, monosodium phosphate, monopotassium phosphate, diammonium phosphate, cobalt phosphate, zinc phosphate, nickel phosphate, copper phosphate, cadmium phosphate and aluminum sulphate, in an amount sufiicient to at least partially neutralize said film-forming material.
11. Lithographic plates suitable for direct electrostatic imaging comprising a substrate coated with a composition comprising an inorganic photoconductive pigment and a binder containing a material selected from the group consisting of homopolymers of itaconic acid, copolymers of itaconic acid with up to by weight, of acrylic acid, amides of said homopolymers and copolymers and polymonomethyl itaconic acid, said homopolymers, said copolyers and said derivatives being long-chain, film-forming materials having molecular weights ranging from 15,000 to 200,000 and in aqueous solution at 33% solids content, viscosities ranging from about to about 4000 cps.
12. Lithographic plates comprising a substrate coated with a thin layer of a film-forming material selected from the group consisting of a neutral amide of polyitaconic acid, polymonomethyl itaconic acid, homopolymers of itaconic acid, copolymers of itaconic acid with up to 50%, by weight, of acrylic acid, said film-forming material having molecular weights ranging from 15,000 to 200,000 and in aqueous solution at 33% solids content, viscosities ranging from 100 cps. to 4000 cps.
References Cited UNITED STATES PATENTS 3,238,042 3/1966 Uhlig 96-33 3,306,743 2/1967 Sanders 96I.8 3,338,164 8/1967 Webers 101-457 WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R.