US 2986521 A
Description (OCR text may contain errors)
United States Patent REVERSAL TYPE ELECTROSCOPIC DEVELOPER POWDER Henry Wielicki, Philadelphia, Pa., assignor to Radio Corpol-anon of America, a corporation of Delaware No Drawing. Filed Mar. 28,1958, Ser. No. 724,522
11 Claims. (Cl. 252-62.1)
This invention relates to electrostatic printing and particularly, but not exclusively, to improved reversal type electroscopic developer powders for electrostatic printing and to improved methods of electrostatic printing employing such developer powders. 7
An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an electrically-insulating layer. The process usually includes the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original, except that it may be a different size.
A typical electrostatic printing process may include producing an over-all, negative, electrostatic charge on the surface of a photoconductive material such as selenium, anthracene or zinc oxide dispersed in an insulating binder. A light image is focused on the charged surface, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition, to thus form an electrostatic image. The electrostatic image is rendered visible by applying a developer power which is held electrostatically to the charged areas of the surface. The powder image thus formed may be fixed directly to the photoconductive material or it may be transferred to another surface upon which the reproduced image may be desired and then fixed thereon. The fixing step commonly comprises fusing the developer powder to the photoconductive material by the application thereto of heat. For a more detailed description of electrostatic printing reference is made to Electrofax- Direct Electrophotographic Printing on Paper, by C. J. Young and H. G. Greig, RCA Review, volume 15, No. 4.
It is often desirable in any type of printing to produce a reverse copy of an original. By this is meant to prodduce a negative copy from a positive original or, on the other hand, a positive copy from a negative original. In electrostatic printing, image reversal can be accomplished by applying to the image a developer powder which is repelled by the charged areas of the image and adheres to the discharged areas. Developer powders of this type have heretofore possessed many undesirable characteristics. When an electrostatic image is developed with many of these developer powders spurious deposits in charged areas of the image result. Also, when such powders are sufficiently fine for proper image development, they generally tend to form into balls or clots which result in smears or streaks in the developed image. When fairly large discharged areas exist in an electrostatic image, most such powders tend to adhere more strongly at the edges of the discharged areas than near the center thereof resulting in an undesirable line effect in the developed image.
Accordingly, it is a general object of this invention to provide improved reversal type developer powders for electrostatic printing.
Another object is to provide improved reversal type developer powders which will not deposit in unwanted areas of an electrostatic image.
Yet another object of this invention is to provide improved reversal type developer powders which are sufficiently fine for proper image development but which 7 will not tend to form into balls or clots.
' improved methods of electrostatic printing for producing a reverse visible image from an electrostatic charge image.
The foregoing objects and other advantages are accomplished in accordance with this invention which provides improved reversal type developer powders comprising particles of electroscopic material coated with finelydivided colloidal silica. For proper development of electrostatic images on negatively charged surfaces, it is important that the electroscopic material have a positive triboelectric relationship with respect to the colloidal silica. It is also important that the developer powder be made up of electroscopic material and colloidal silica in proper proportions. Generally, a ratio of about 1.5 to 9 parts by weight of colloidal silica and 100 parts by weight of electroscopic material is satisfactory. The exact proportions will depend to a large extent on the particle size of both the electroscopic material and the colloidal silica. When desired a minor proportion of a suitable coloring agent may be incorporated in the developer powder to impart thereto a desired color.
The improved methods of this invention relate to electrostatic printing processes wherein an electrostatic image is produced consisting of negative electrostatic charge areas on an insulating surface. The improved developer powder of this invention is applied to the insulating surface to produce a visible image. The powder is repelled from the negatively charged areas and deposited in the other areas on the insulating surface to produce a visible reverse image of the electrostatic image. Application of the developer powder may be accomplished in many ways such as, for example, cascade development or magnetic brush development both of which are described in the Young and Greig publication, op. cit.
In a preferred embodiment of the invention the particles of electroscopic material are comprised of certain natural or synthetic resins, waxes or other low melting materials or mixtures thereof. A material is normally selected which has a melting point less than the temperature at which paper will char. A preferred temperature range is between 90 C. and 250 C. Such a material can be readily fused to a photoconductive surface by the application thereto of heat. When particles of such a material are coated with finely divided colloidal silica, a developer powder is provided which adheres to the discharged areas of a negatively charged electrostatic image and does not adhere to the charged areas thereof.
Another preferred embodiment, particularly suited to color electrostatic printing, contemplates a developer powder including a particulate zinc oxide the particles of which have a first coating thereon of a thermoplastic, film-forming material and a second coating over the first of colloidal silica. In this embodiment the film-forming material is selected to have a positive triboelectric relationship with respect to the colloidal silica. The filmforming material has a melting point substantially within a range of from 90 C. to 250 C. and a viscosity within a range of from 45 to 10,000 centipoises. The zinc oxide and the film-forming coating thereon are combined in a ratio of about 50 to parts by weight of zinc oixde to 15 to 50 parts by weight of the film-forming, coating material. The zinc oxide and the thermoplastic coating together comprise the electroscopic material in this embodiment.
When particles of zinc oxide are included in a reversal type developer powder they are characterized by one of two properties. These particles may be capable of holding an electrostatic charge in darkness or they may be relatively conductive. The first property is found in a class of zinc oxides known as French process zinc oxides. The second property is found in the class known as American process zinc oxides. When French process zinc oxides are employed, a developed powder image is obtained which in subsequent printing procedures may be overprinted with another powder image. When American process zinc oxides are employed, a powder image is obtained which is incapable of being overprinted in subsequent printing procedures. When French process zinc oxides are used, dye sensitizing agents may be added to the developer powder to change the spectral response of the zinc oxide.
Specific examples and additional advantages of the reversal type developer powders and of the improved methods of electrostatic printing of this invention are included in the detailed description which follows.
ELECTROSCOPIC MATERIALS Many natural or synthetic resins and waxes can be provided in particulate form and coated with colloidal silica to provide reversal type developer powders in accordance with this invention. Examples of suitable materials which are triboelectrically positive with respect to colloidal silica include the following:
(1) Piccolastic 4358A (a thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs, made by the Pennsylvania Industrial Chemical Corporation, Clairton, Pennsylvania).
(2) Carnauba wax.
(3) Polymekon Wax (a chemically modified microcrystalline wax of the Warwick Wax Co., New York, N.Y.).
(4) Ultracera Amber Wax (a microcrystalline petroleum wax of the Bareco Oil Co., Barnsdall, Oklahoma).
(5) BE Square Wax White (a microcrystalline petroleum wax of the Bareco Oil Co.).
(6) Petronauba D Wax (a. microcrystalline petroleum wax of the Bareco Oil Co.).
(7) Piccolyte S-135 (a thermoplastic hydrocarbon terpene resin of the Pennsylvania Industrial Chemical Corp, Clairton, Pennsylvania).
(8) A mixture of Polymekon Wax and Piccolyte 8-1 15.
(9) A mixture of Acrawax C (a synthetic wax-octadecenamide, of the Glyco Products Co., Brooklyn, New York) and calcium stearate.
(10) A mixture of Acrawax C and a solid silicone resin (such as Dow Corning Silicone No. R-5071).
(11) A mixture of Piccolastic D-100 and Piccolastic 6-125.
COLLOIDAL SILICA Colloidal silicas which satisfy the requirements of this invention consists of at least 90% pure S10 with the remaining 10% made up of such impurities as Na SO A1 and Fe O These colloidal silicas generally have a particle size ranging from .015 to microns mean diameter. Two specific examples of colloidal silicas include Santocel and Cab-O-Sil. Santocel is a colloidal silica manufactured by the Monsanto Chemical Co., Inorganic Division, St. Louis, Missouri, and comprises from 90 to 93% SiO 1 to 5% volatile substances, 2 to 3% Na SO and about 1% A1 0 plus Fe O It has an average particle size Within a range of from 0.5 to 5 microns in diameter. Cab-O-Sil is a. colloidal silica manufactured by the Godfrey L. Cabot Co., Cambridge, Massachusetts, comprising between 99.0 and 99.7% SiO 0.2 to 2.0% free moisture and about .004% Fe O It 4 has a particle size range of from .015 to .020 micron mean diameter.
In general, the reversal type toner powder is prepared by thoroughly mixing 1.5 to 9 parts by weight of colloidal silica with 100 parts by weight of electroscopic particles. The optimum ratio of colloidal silica to electroscopic particles appears to be about 5 parts by weight of colloidal silica to 100 parts by weight of particles. It is preferred to add the colloidal silica in small quantities and to follow each addition by ball milling the mixture for approximately 30 minutes. The optimum ball milling time is from 3 to 5 hours.
The incorporation of proper amounts of colloidal silica in the developer powders of this invention imparts additional advantageous properties thereto. Taekiness is substantially eliminated and the free flowing property of the developer powders substantially improved. Also substantially eliminated is any tendency for the powders to form into balls or clots.
COLORING AGENTS Various coloring agents may be added singly or in combination to the foregoing toner powders to provide any desired color. Color developer powders will generally include from .2 to 12 parts by weight of a coloring agent for each 100 parts by weight of electroscopic particles. Suitable coloring agents include the following:
(1) Carbon black (2) Fluorescein Sodium (Color Index No. 766) (3) Eosin Y (Color Index No. 768) (4) Rose Bengal (Color Index No. 779) (5) Brilliant Green (Color Index No. 662) (6) Patent Blue (Color Index No. 672) (7) Thioflavin TG (Color Index No. 49005 A specific example of a developer powder in accordance with this invention includes the following.
This developer powder is prepared in the manner described above.
ZINC OXIDES In accordance with the second embodiment of this invention reversal type developer powders are provides with comprise electroscopic particles consisting of particulate zinc oxide coated with low melting thermoplastic electroscopic material and overcoated with colloidal silica. The zinc oxides contemplated herein fall into two classes, the French process zinc oxides which permit overpn'nting in subsequent electrostatic printing procedures and the American process zinc oxides which are incapable of such overprinting. The determination as to which specific class a zinc oxide belongs is important. Various methods have been devised to make this determination.
Method 1.A mixture is prepared comprising about 10 milligrams of dry zinc oxide powder and a few drops of an solution of silicone resin in xylene (G.E. SR82, marketed by the General Electric Co., Silicone Products Division, Waterford, N.Y) diluted with tolacne in the ratio 60 grams solution to 105 grams toluene. The mixture is coated on filter paper and dried to produce a dry coating over an area about 0.25 inch in diameter. The dry coating is cooled to about -l C. and examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides (French process) which produce printable coatings vproduce a lavender or orange luminescence. Other zinc oxides (American process) exhibit a green or yellow luminescence.
Method 2.--About 0.25 gram of dry zinc oxide powder is placed in a silica boat. The boat is inserted into a silica tube and the system flushed with hydrogen gas. The tube and boat are fired for about 5 minutes at about 1000 C. in a stagnant hydrogen atmosphefe. The boat is cooled in hydrogen to room temperature. The fired zinc oxide is examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides (French process) which produce printable coatings luminesce brightly. Other zinc oxides (American process) luminesce weakly or not at all.
A preferred overprinting zinc oxide is one which pro: duces a lavender color in Method 1 and luminesces brightly in Method 2. A preferred non-overprinting zinc oxide is one which exhibits a green luminescence in Method 1 and which does not luminesce at all in Method 2.
-It has been found that the useful overprinting zinc oxides selected in accordance with the above procedures have a surface photoconductivity of at least about ohm- /square/watt/cm. when subjected to a light of a wavelength of about 3900 A. The surface photoconductivity of a zinc oxide can be determined by applying thereto a third method as follows:
Method 3.--A small quantity of zinc oxide is reduced to a powder and compressed under high pressure (about 15,000 lbs. per square inch) to form a pellet. Electrodes, as of silver paste, are applied on the surface of the pellet leaving a square area of surface uncoated. The pellet is then placed in a monochromator with the aforementioned uncoated surface area facing the light source and successive wavelengths of light throughout the spectrum are projected on this surface. The light beam projected onto the surface is chopped at about 23.5 c.p.s. by a constant speed rotating disc, pierced to produce equal intervals of light and darkness. A DC. potential is placed across the electrodes and the current flowing between the electrodes is measured as a function of wavelength with the intensity of radiation being held constant.
The zinc oxides which are suitable are those which are substantially electrically non-conductive in the dark. When exposed to light, they should exhibit a surface photoconductivity of a certain level in order to be of practical use for the purposes of this invention. In testing zinc oxides to determine their suitability and utilizing a pellet form, it is convenient to express the results of the measurements of the test as surface photoconductivity because substantially all of the light is absorbed in a thin layer at the surface of the pellet. It has been found that, to be useful for overprinting in this invention, the zinc oxide selected shrould have a surface photoconductivity of at least 10- ohmsquare/ watt/cm. when exposed to a wavelength of about 3900 A. In addition to the foregoing it has also been found that those zinc oxides which are suitable for use in non-overprinting developer powder possess another determinable property. Such zinc oxides must have a volume resistivity such that when combined with a suitable film-forming coating and fused, the fused mixture of zinc oxide and coating material will have a volume resistivity of 10 ohm-cm. or less.
FILM FORMING MATERIALS as a binder holding the zinc oxide on the insulating sur-' face. The viscosity of the coating material comprises another important criterion. The viscosity must be low enough so that, when melted, the coating will flow off the zinc oxide particles leaving them partially exposed with only a thin film remaining on the exposed or protruding portions of the zinc oxide particles. After the film-forming material has been melted and fused to an insulating surface, it is important that the protruding particles of zinc oxide shall present a matte surface, i.e., the film of the coating material remaining on the protruding portions of zinc oxide must not be thick enough to provide a gloss finish. Were the film of coating material to have such a thickness, the particles of zinc oxide might be insulated to such a degree as to cause the coating material to retain an electrostatic charge and thereby impair subsequent printing operations. It is extremely difiicult to measure the thickness of such a film, however, if a physical appearance substantially like that described is achieved, the coated electroscopic particles will have the characteristics and properties contemplated in this invention. It is also preferred that the film-forming material should not be so free flowing as to allow it to migrate into unwanted areas of the surface when melted. A preferred viscosity range is from 45 to 10,000 centipoises as measured with a direct reading Brookfield viscosimeter with a spindle speed of 60 rpm. at a temperature just slightly above the melting point of the material.
Film-forming materials having the foregoing properties may comprise certain natural or synthetic resins, waxes, or other low melting materials or mixtures thereof. For example any of the following materials or combinations of materials may be used:
Film-forming materials such as those specified may also include modifying agents such as plasticizers, toughening agents, hardening agents, or dispersing agents, which are added to obtain desired physical and electrical properties.
A developer powder, in accordance with this embodiment of the invention, includes a ratio of zinc oxide to film-forming material Within a range of from 1 to 7 parts by weight of zinc oxide to one part by weight of film-forming material. Generally, the film-forming material is melted and finely-divided zinc oxide dispersed into the melt. The melt is then allowed to cool and harden, after which it is broken up and reduced to a desired powder form. Finally, the colloidal silica is mixed into the above powder in the manner described heretofore. The ratio of zinc oxide to film-forming material specified above is important in a given developer powder formula. The exact ratio depends to a large extent on the particle size and the dispersion of the zinc oxide chosen.
Coloring agents such as dyes, stains or pigments can be added to the foregoing powders to produce a desired color. The coloring agents specified heretofore may be employed singly or in any combination and in the specified proportions.
In addition to the foregoing, various sensitizing agents may be employed to vary the spectral response of the photoconductive French process zinc oxides. For example, photoconductive white zinc oxide has a spectral sensitivity having a peak in the near ultraviolet range. By adding various sensitizing agents a white photocon- Carnauba wax Polymekon Wax Ultracera Amber Wax BE Square Wax White Petronauba D Wax Piccolyte 8-135 A mixture of Polymekon Wax and Piccolyte S1l5 A mixture of Acrawax C and calcium stearate A mixture of Acrawax C and a solid silicone resin ductive zinc oxide can be provided having an additional sensitivity peak in other portions of the spectrum. Satisfactory sensitizing agents include the following. 7
(l) Fluorescein Sodium (Color Index No. 766) (2) Eosin Y (Color Index No. 768) (3) Rose Bengal (Color Index No. 779) (4) Brilliant Green (Color Index No. 662) (5) Patent Blue (Color Index No. 672) (6) Thioflavin TG (Color Index No. 49005) 'OVERPRINTING DEVELOPER POWDERS The following group of examples provide developer powders suitable for overprinting procedures. These powders when fused to an insulating surface present a matte finish which is easily overprinted by other developer powders in subsequent electrostatic printing steps.
Example 11 WHITE DEVELOPER POWDER Parts by weight Carnauba wax 1 French process zinc oxide 2 Colloidal silica 0.15
This is the simplest type of overprinting developer powder. The wax is melted and particles of the zinc oxide having a particle size of from .025 to .5 micron mean diameter are added to the melt. Particle size and shape of the zinc oxide determine to some extent the ratio of zinc oxide to coating material. Due to the bulking characteristic of zinc oxide finer particles usually require more coating material since there is more total surface to be covered. Continuous stirring from 15 to 30 minutes is sufiicient to thoroughly disperse the zinc oxide in the wax when the batch weighs about 100 grams. The mixture is then allowed to cool and harden after which it is reduced to a fine powder. This is accomplished by ball milling the mixture for about 3 hours and then classifying it as to particle size. For most purposes, the fract-ion below 200 mesh (74 microns) is suitable for use as an electroscopic developer powder. The classified particles are then intimately mixed with the colloidal silica.
Example III BLUE DEVELOPER POWDER Preparation the same as in Example II except that the calcium stearate is added to the melt before the zinc oxide, and the coloring agent after the zinc oxide.
Example IV RED DEVELOPER POWDER Parts by weight Acrawax C 36 Silicone resin (solid) 5 French process zinc oxide 80 Sudan III Red 4 Oil Yellow 26 2 Colloidal silica 5 Preparation the same as in Example II except the Acrawax C and silicone resin are melted together before adding the zinc oxide and coloring agent.
Example V GREEN DEVELOPER POWDER Parts by weight Piccolyte S-l35 French process zinc oxide 30 Benzidine Yellow 1 Brilliant Oil Blue B.M.A. 0.23 Colloidal silica 2.5
Preparation as in Example II except add the coloring agents after adding the zinc oxide to the molten Piccolyte.
SENSITIZING AGENTS The following sensitizing agents may be employed, in the proportions indicated, with any of Examples II to V. These sensitizing agents are added to the melt after the zinc oxide and coloring agents have been added thereto.
Example VI Add Brilliant Green to any of Examples II to V in the ratio of about 0.1 part Brilliant Green to 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak of approximately 6230 A. in the orange-red portion of the spectrum.
Example VII Add Patent Blue to Examples 11 to V in a ratio of about 0.05 part Patent Blue to 70 parts by weight of zinc oxide. This provides a photocond-uctive developer powder having a second sensitivity peak of about 6380 A. in the orange portion of the spectrum.
Example VIII Add Thioflavin T6 to Examples II to V in a ratio of about 0.1 part of Thioflavin to about 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 4080 A. in the blue portion of the spectrum.
Example IX Add Rose Bengal to Examples II to V in a ratio of about 0.05 part of Rose Bengal to about 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 5500 A. in the green portion of the spectrum.
Example X Add Fluorescein Sodium to Examples II to V in a ratio of about 0.05 part of Fluorescein to about 70 parts by weight of zinc oxide. This provides a photoconductive developer powder having a second sensitivity peak at about 4770 A. in the blue-green portion of the spectrum.
Example XI added thereto the powder has a single sensitivity peak at about 37-50 A. in the ultraviolet portion of the spectrum. When one of the dyes of Examples VI to XI is added, the developer powder still exhibits a peak at about 3750 A. as well as an additional peak at the wavelength cited for each example. Thus, there is provided means for obtaining a spectral response at almost any portion of the spectrum from ultraviolet to red.
9 NON-OVERPRINTING DEVELOPER POWDERS Example XII WHITE DEVELOPER POWDER Parts by weight Ultracera Wax 20 American process zinc oxide 25 Colloidal silica 2.5
Prepared as in Example II.
Example XIII YELLOW DEVELOPER POWDER Parts by weight Petronauba D Wax 20 American process zinc oxide 50 Benzidine Yellow 2.5 Colloidal silica 3.5
Prepared as in Example V.
Example XIV RED DEVELOPER POWDER Parts by weight Piccolyte S-135 20 American process zinc oxide 30 Oil Red N-1700 3 Oil Yellow 2G 1.2 Colloidal silica 2.5
Prepared as in Example V.
Example XV BLUE DEVELOPER POWDER Parts by weight Polymekon Wax 15 Piccolyte 8-115 American process zinc oxide 50 Condensation Blue 1 Colloidal silica 3.5
Prepared as in Example IV.
COLOR PRINTING The reversal type developer powders described in Examples II through XV find particular utility in the electrostatic reproduction of color images. The overprinting powders of Examples II through XI are adapted for use in a printing method wherein an image of one color is superimposed upon an image of another color. One such method comprises the steps of: (1) uniformly charging a photoconductive surface in a negative polarity; (2) exposing the photoconductive surface to a light image to produce thereon an electrostatic image; (3) developing the latent electrostatic image by applying thereto a developer powder such as those of Examples II through XI to produce a visible powder image in the discharged areas on the photoconductive surface; (3) applying heat to the developer powder to cause the film-forming coating thereon to flow toward the photoconductive surface thereby leaving particles of zinc oxide protruding above the film-forming material and leaving only a very thin film of that material on the protruding portions of the zinc oxide particles; (3) repeating steps (1) and (2) to produce a second electrostatic image on the photoconductive surface having thereon the first developed image; (4) applying a different colored developer powder to the discharged areas of the second electrostatic image to produce a second visible image. When a French process zinc oxide is employed to produce the first visible image, the second visible image will overlap the first in those areas upon which light impinged during the second exposure. The steps of this procedure may be carried out as many times as desired to produce a composite image in substantially natural colors. When an American process zinc oxide, such as those included in Examples XII to XV, is employed, deposition of the second developer powder will only occur in areas not covered by the first powder image. Again the steps of the procedure may be carried out as many times as desired to produce a composite color image in which the separate colors are deposited in discrete contiguous areas on the photoconductive surface.
There have been described new and improved reversal type elecroscopic developer powders which make possible direct printing of reverse images in black and White or in any desired colors.
What is claimed is:
1. A reversal type developer powder for electrostatic printing comprising particles of electroscopic material coated with a finely-divided colloidal silica, said electroscopic material comprising a low melting point organic solid having a positive triboelectric relationship with respect to said colloidal silica, said developer powder comprising about 100 parts by weight of said electroscopic material and about 1 /2 parts to 9 parts by weight of said colloidal silica.
2. The developer powder of claim 1 wherein said electroscopic material comprises a thermoplastic, film-forming resin having a melting point substantially within a range of from 90 C. to 250 C.
3. The developer powder of claim 2 including therein a minor proportion of a coloring agent.
4. A reversal type developer powder for electrostatic printing comprising particles of zinc oxide having an electroscopic coating thereon of a thermoplastic material having a melting point substantially within a range of from 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises, and an additional coating on said particles consisting essentially of finely-divided colloidal silica, said developer powder comprising 50 to parts by weight of said zinc oxide, 15 to 50 parts by weight of said thermoplastic material, and 1% to 9 parts by weight of said colloidal silica.
5. The developer powder of claim 4 including a minor proportion therein of a coloring agent.
6. A reversal type developer powder for electrostatic printing comprising 50 to 85 parts by Weight of a particulate French process zinc o'xide having a surface photoconductivity of about 10' ohm- /square/watt/cm. at a wavelength of about 3900 A., 15 to 50 part by weight of an electroscopic coating on the particles of said zinc oxide consisting of a thermoplastic insulating material having a melting point substantially within a range of from C. to 250 C. and a viscosity substantially Within a range of from 45 to 10,000 centipoises at a temperature slightly above said melting point, and an additional coating on said particles consisting essentially of 1 /2 to 9 parts by weight of a finely-divided colloidal silica, said thermoplastic, insulating material having a positive triboelectric relationship with respect to said colloidal silica.
7. The developer powder of claim 6 including 0.2 to 12 parts by weight of a coloring agent.
8. The developer powder of claim 6 including 002 to 1.4 parts by weight of a dye sensitizing agent for said zinc oxide.
9. The developer powder of claim 8 including 0.2 to 12 parts by Weight of a coloring agent.
10. A reversal type developer powder for electrostatic printing comprising: 50 to 85 parts by weight of a particulate American process zinc oxide; 15 to 50 parts by weight of a fusible coating on the particles of said zinc oxide consisting of a thermoplastic, electroscopic material having a melting point substantially Within a range of from 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature slightly above said melting point; said coating and said particles of zinc oxide, when fused, having a volume resistivity not in excess of 10 ohm-cm; and an additional coating on said particles consisting essentially of 1 /2 to 9 parts by weight of a finely-divided colloidal silica; said thermoplastic material having a positive triboelectric relationship with respect to said colloidal silica.
1 1. The developer powder of claim 10 including 0.2 to 12 parts by weight of a coloring agent.
References Cited in the file of this patent 12 Walkup et a1. May 12, 1953 Grieg et a1. Feb. 21, 1956 Landrigan July 3, 1956 Sugarman Aug. 14, 1956 Coulter Mar. 19, 1957 Moncriefi-Yeates June 17, 1958 Mayer Mar. 10, 1959 Insalaco June 30, 1959 Steppe Aug. 25, 1959 OTHER REFERENCES Young et al.: Electrofax Direct Electrophotographic Printing on Paper, RCA Review, December 1954, vol.
XV, N0. 4.