US3053688A

US3053688A - Electrostatic printing

Info

Publication number: US3053688A
Application number: US805740A
Authority: US
Inventors: Harold G Greig
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1959-04-13
Filing date: 1959-04-13
Publication date: 1962-09-11
Anticipated expiration: 1979-09-11
Also published as: GB951642A; CH399173A; NL250444A; DE1185922B

Description

United States Patent 3,053,688 Patented Sept. 11, 1962 3,053,688 ELECTROSTATIC PRINTING Harold G. Greig, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed Apr. 13, 1959, Ser. No. 805,740 19 Claims. (Cl. 117-37) This invention relates generally to electrostatic printing. More particularly, it relates to improved materials and methods for developing electrostatic images.

In the art of electrostatic printing, electrostatic images are produced on the surface of an insulating material. Such images comprise a pattern of electrostatic charges on the surface. Visible images are commonly produced therefrom by cascading across the surface a dry mixture of finely-divided developer particles and substantially larger carrier particles. When the developer particles are tiiboelectrically-charged in the opposite polarity to the electrostatic charges, they deposit in charged areas to produce a visible image in substantial configuration with the pattern of charges. When the developer particles have the same polarity as the electrostatic charges a visible image is produced in reverse configuration with respect to the pattern of charges.

The foregoing method of developing electrostatic images is described in Electrofax Direct Electrophotographic Printing on Paper, by C. J. Young and H. G. Greig, RCA Review, December 1954, vol. XV, No. 4. Also described in that publication are other methods of development such as: powder cloud, liquid mist and magnetic brush types.

The recording element may comprise almost any insulating surface but, preferably, the recording surface is also photoconductive to enable the recording of light images. Recording elements comprising photoconductive selenium coated plates are described in US. Patent 2,297,691, issued October 6, 1942 to C. F. Carlson. Recording elements comprising photoconductive coatings on paper are described in the Young and Greig publication, op. cit.

Recently, a so-called liquid process for developing electrostatic images has been proposed in which the solid developer particles are suspended in an insulating carrier liquid. Liquid development methods provide many distinct advantages over the use of dry developer mixtures and other methods of developing electrostatic images, for some applications. Basically, the liquid developer previously described consists of finely-divided developer particles dispersed in a hydrocarbon liquid. This developer can be flowed over a surface bearing an electrostatic image, or the surface can be immersed in a tray of liquid developer. It can also be sprayed or rolled on to the surface. When appropriate developer particles are dispersed in a properly selected liquid, they acquire an electrophoretic or triboelectric charge enabling them to be attracted to an electrostatic charge pattern of appropriate polarity. Deposition of the developer particles on the charge image is an example of the phenomenon known as electrophoresis or cataphoresis. A liquid developer process for charge images is described in greater detail by K. A. Metcalf and R. J. Wright in a paper entitled Xerography, published in the Journal of the Oil and Colour Chemists Association, November 1956, volume 39, No. 11, London, England and in another paper entitled Liquid Developers for Xerography published in the Journal of Scientific Instruments, February 1955, vol. 32.

Although the above-mentioned liquid developer compositions are suitable for many purposes, they do possess undesirable properties. Most hydrocarbon liquids are solvents for developer particles which include resins and waxes or organic pigments. When resinous particles are dispersed in such a liquid, they dissolve at least to some extent so that they become tacky and tend to agglomerate. Thus, such dispersions must be freshly made a short time prior to use. If the dispersions stand for any extended period of time, the developer particles will ball up or cake. The tackiness of the developer particles caused by the hydrocarbon liquid can also make them adhere in unwanted image areas which they may contact during development. Unless some fixative spray is applied to an image developed with such a dispersion, the tacky developer particles will tend to smear during handling. To improve the quality of the developed image and to accelerate drying of the surface on which the image rests, it is frequently desirable to heat that surface to drive off the liquid and diffuse the developed image into the base material. When hydrocarbon carrier liquids are employed, such a practice would be extremely dangerous in view of the fire hazard involved. Also, whether heated or not, most hydrocarbon liquids have a very objectionable odor and the vapors thereof are toxic. Thus, the use of such liquids calls for employing expensive auxiliary equipment such as exhaust hoods in order to remove fumes. For the above reason it can be readily seen that such liquids are unsuitable for many applications such as, for example, office copiers.

Accordingly, it is a general object of this invention to provide improved liquid developer mixes for electro static printing.

It is a further object of this invention to provide an improved liquid developer composition which may, without danger of fire, be heated on a surface to fuse thermoplastic particles contained in the liquid, to the surface.

It is a further object of this invention to provide improved liquid developer compositions which are relatively odorless and non-toxic.

The foregoing objects and other advantages are accomplished in accordance with this invention which provides improved liquid compositions for developing electrostatic images. The compositions comprise dispersions of finelydivided developer particles in a liquid consisting essentially of a dimethyl polysiloxane, the developer particles being insoluble in the liquid. Also contemplated is the development of electrostatic images by applying thereto such a liquid dispersion wherein thermoplastic developer particles are employed. Subsequent to development, the developed image is heated to remove liquid on the surface on which it rests and to fuse thereon the developed image.

Specific examples and additional advantages of the liquid developers and of the improved methods of developing electrostatic images in accordance with this invention are included in the detailed description which follows:

Dimethyl Polysiloxanes An important feature of this invention is the provision of a carrier liquid consisting essentially of at least one dimethyl polysiloxane. Such compounds have a structural formulas where It may vary from 0 to 2000 and even higher. The higher the value of n, the higher the viscosity of the liquid at a given temperature. At room temperature, viscosity may vary from as low as 0.65 centistoke to as high as 1,000,000 centistokes, but for the purpose of the present invention, it is preferred to use only those members of the family or mixtures thereof having a viscosity up to about 3 centistokes at room temperature.

It has now been found that these liquids are extremely useful for electrostatic printing for the following reasons. The liquids are very poor solvents for organic plastics. The members of this family of compounds have relatively high flash points. For example, a dimethyl polysiloxane having a viscosity of 2 centistokes, has a flash point of 175 F. and one of 3 centistokes a flash point of 215 F. This compares with toluene at 40 F., turpentine at 90 F. and high-flash naphtha at 112 F. The dimethyl polysiloxanes are practically odorless and non-toxic making them feasible for use in ofi'ice copiers. The insulating organic liquids such as benzene, toluene, turpentine, petroleum fractions, carbon tetrachloride, cyclohexane, etc. which have previously been described as being suitable for electrophoretic development in electrophotography, are all deficient with respect to one or more of the properties mentioned above. The dimethyl polysiloxanes are also extremely hydrophobic and have excellent dielectric properties. This is of particular advantage in the present invention since most organic liquids lose their insulating properties in atmospheres of high relative humidity, whereupon they become too conductive for effective operation.

Developer Mixes To prepare a suitable developer composition, finelydivided particles of an electroscopic developer material, are dispersed in dimethyl polysiloxane in a proportion of 20 parts by weight of developer material to 80 parts by weight of a dimethyl polysiloxane having a viscosity of about 0.6 to about 3 centistokes. The ratio of developer material to liquid in this example is generally too high for most applications. However, this ratio is a convenient one for preparing a composition which is to be stored for an extended period or which is to be provided to the ultimate user. Prior to use, the mixture is diluted with additional dimethyl polysiloxane to provide a concentration of developer material in the composition of about 0.2% to about 6% by weight. An important property of dimethyl polysiloxane evidences itself when developer compositions are stored for extended periods. Developer particles comprising thermoplastic materials, examples of which are provided hereinafter, may be dispersed in such liquids and stored practically indefinitely without agglomerating.

A specific example of a suitable toner or developer material to be dispersed in the dimethyl polysiloxane comprises the following:

EXAMPLE I 200 parts by weight of Piccolastic Resin 4358A (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs of the Pennsylvania Industrial Chemical Corp, Clairton, Pennsylvania) 12 parts by weight Carbon Black 12 parts by weight Nigrosine SSB-Color Index No.

8 parts by weight Iosol Black-Color Index Solvent Black 13 This developer material is prepared by melting the resin and mixing in the other materials. When a uniform mix is obtained, it is cooled, ground to a fine powder and classified to obtain a desired particle size. A convenient particle size is one obtained by screening through a 200 mesh which provides a maximum particle diameter of about 74 microns. This developer material may be dispersed in liquid by any of the commonly known techniques.

EXAMPLE II A low-melting point (120 C.) developer material suitable for dispersion in a dimethyl polysiloxane may be comprised as follows:

60 parts by weight Piccolastic D 100 40 parts by weight Piccolastic C 125 9 parts by weight Carbon Black These materials are mixed together in powder form, then melted and mixed again to obtain a homogeneous dispersion. "The melt is then cooled, ground and classified to obtain the desired particle size. It has been found that even with a low-melting toner of this character, which has a tendency to cake with storage, a stable noncaking dispersion is obtained in a dimethyl polysiloxane having a viscosity of about 2 centistokes.

In the foregoing examples many organic resins and waxes may be substituted for those described. Some of these are the following:

Acrawax C (a synthetic waxoctadecenamide) The Glycol Products Co., Brooklyn, N.Y.--melting point between 133 and 140 C.

Carnauba Wax-melting point about C.

Polymekon Wax (a commercially modified microcrystalline wax of the Warwick Wax Co., N.Y.)melting point about 93 to 127 C.

Ultracera Amber Wax-a microcrystalline petroleum wax of the Bareco Wax Co., Barnsdall, Oklahomamelting point between about 108 and 112 C.

Be Square Wax White-a microcrystalline petroleum wax of the Bareco Wax Co., Barnsdall, Oklahomamelting point between about and 109 C.

Petronauba D Wax-a microcrystalline petroleum wax of the Bareco Wax Co.melting point about 103 C.

Piccolyte S-a thermoplastic hydrocarbon of the Pennsylvania Industrial Chemical Co., Clairton, Pa.-- melting point about 135 C.

Various coloring agents may be employed, singly or in combination, in the foregoing compositions in place of the black pigments or dyes specifically set forth in Examples 1 and II. Colored developer particles will generally include from .2 to 12 parts by weight of a coloring agent for each 100 parts by weight of developer particles. Suitable coloring agents include the following:

(1) Cyan Blue Toner GT (described in US. Patent 2,486,351 to R. H. Wiswall, Jr.)

(2) Benzidine Yellow (3) Brilliant Oil Blue BMA, Color Index No. 61555 (4) Sudan III Red, Color Index No. 26100 (5) Oil Yellow 2G, Color Index No. 11020 (6) Hansa Yellow G, Color Index No. 11680 Coated Particles Various thermoplastic developer materials which comprise coated particles may also be conveniently employed in accordance with this invention. It is preferred in such cases to incorporate in the particles a core material made up of zinc oxide. One type of zinc oxide, when fused onto a surface by means of the thermoplastic coating is incapable of retaining an electrostatic charge. Such a zinc oxide has a value of surface photoconductivity less than 10* ohm- /square/watt/cm'. when subjected to light of a wavelength of about 3900 A. When a fused visible image is produced with this zinc oxide it cannot be overprinted in subsequent operations.

Another suitable type of Zinc oxide comprises one having a value of surface photoconductivity of at least 10- ohm-Vsquare/watt/cm. when subjected to light of a wavelength of about 3900 A. These developer materials are convenient for use in color processes wherein one color is overprinted over another to provide for color mixing. When particles of too large diameter and which are insulating in character are deposited on an electro static image to produce a first color, such particles will prohibit over-printing thereon with another color. By providing a photoconductive zinc oxide core coated with a low-melting thermoplastic coating, developer particles are produced which, when fused to a surface, permit overprinting of a color with another and therefore provide for color mixing.

The process by which one type of coated zinc oxide particles provide for overprinting and by which another type prohibits overprinting is unique. When particles of either type are fused to a surface, the coating material melts to form a continuous layer adhering to the surface. After fusing, at least the topmost particles of zinc oxide are left protruding above the layer. When photoconductive Zinc oxide particles are employed, an image surface is produced which can be charged, exposed and overprinted as easily as an original photoconductive surface. When non-photoconductive zinc oxides are employed, developed image areas are incapable of retaining a charge and, hence, cannot be overprinted in subsequent procedures.

Examples of suitable materials include the following:

EXAMPLE III White Developer Powder 1 part by weight carnauba wax 2 parts by weight photoconductive zinc oxide The wax is melted and particles of zinc oxide having a particle size from 0.25 to .5 micron mean diameter are added to the belt. Particle size and shape of the zinc oxide determine to some extent the ratio of the Zinc oxide to the coating material. Continuous stirring of the melt from to minutes is sufficient to disperse the zinc oxide in the wax when the batch weighs about 100 grams. The mixture is allowed to cool, after which it is reduced to a fine powder and classified as to particle size.

EXAMPLE IV Blue Developer Powder 20 parts by weight Acrawax C (a synthetic wax-octadecenamide, of the Glyco Products Co., Brooklyn, New York) 30 parts by weight photoconductive zinc oxide 0.3 part by weight calcium stearate (pigment wetting agent) 1.5 parts by weight Cyan Blue Toner GT This composition is prepared the same as in Example III except that the calcium stearate is added to the melt before the Zinc oxide and the coloring agent after the zinc oxide.

EXAMPLE V Red Developer Powder 36 parts by weight Acrawax C 5 parts by weight of a solid silicone resin (such as Dow Corning R-5071) 80 parts by weight photoconductive zinc oxide 4 parts by weight Sudan 3 Red, Color Index No. 26100 2 parts by weight Oil Yellow 2 G Color Index No.

Preparation same as in Example IV.

Pigments It is possible to provide developer compositions which consist of organic pigments dispersed in dimethyl polysiloxane liquid. Preferably the dispersions comprise up to about 20 parts by weight of pigment the remainder being liquid. The term pigment as employed herein and in the claims is intended to include coloring agents which are sometimes referred to as dyes but which nevertheless are insoluble in the polysiloxane. When used as taught herein these so-called dyes have all the properties and attributes of pigments. Suitable pogments for such purposes include the following:

(1) Cyan Blue Toner GT (described in US. Patent 2,486,351 to Richard H. Wiswall, Jr.) (2) Benzidine Yellow (Color Index No. 21090) (3) Brilliant Oil Blue BMA-Color Index No. 61555 (4) Sudan 3 Red-Color Index No. 26100 (5) 'Oil Yellow 2 GC.I. No. 11020 (6) Pyrazalone pigment. (Such as C.I. 21080 C.I. Pigment Red 39) (7) Hansa Yellow GC.I. No. 11680 In many of the foregoing dispersions it is convenient to provide a surfactant (surface active agent) to enhance the electrical properties of a selected pigment. A surfactant solution may be prepared by dispersing 10 grams of Nalcamine Gl4 in 20 grams of toluene and, while mixing, heating the dispersion to dissolve the Nalcamine G14 in the toluene. Nalcamine G14 is a chemical of the type 1-(2-hydroxyethyl)-2-hydrogenated tallow-2-imidazoline (National Aluminate Corp, Chicago, Illinois). The Nalcamine G14 solution is added to pigment dispersions before they are ball milled in the dimethyl polysiloxane. Such a surfactant when applied, for example, to a red pyrazalone pigment substantially enhances the electropositive nature thereof.

Reversal Type Powders This invention also provides a developer composition which is capable of producing reverse images. By this is meant that when the composition is applied to an electrostatic image consisting of a pattern of negative electrostatic charges, the developer material will adhere in.

non-charged areas of the image rather than in the charged areas thereof. Such a developer composition may be prepared by dispersing a pigment in a binder material such as one which is predominantly comprised of polyvinyl chloride.

EXAMPLE VI 4 grams carbon black 30 grams dimethyl polysiloxane, viscosity about 2 centistokes The carbon black is dispersed in the polysiloxane and the dispersion ball milled in a 2 ounce glass jar with steel balls for about 40 hours. The reversal type developer composition is then made up as follows:

3 grams carbon black dispersion in polysiloxane 5 grams of Vinylite VYNV (a copolymer, 96% vinyl chloride and 4% vinyl acetate) 30 grams dimethyl polysiloxane employing most of the pigments discussed heretofore.v Some of these are set forth below along with the proportions of the constituents therein.

EXAMPLE VII Red Reversal Powder 11 grams Vinylite V YNV 2 grams red pyrazalone pigment 30 grams dimethyl polysiloxane, viscosity about 2 centistokes This mixture is ball milled for about 32 hours and may be diluted with additional polysiloxane.

EXAMPLE VIII Yellow Reversal Powder 14 grams Vinylite V YNV 2 grams Hansa Yellow G 40 grams dimethyl polysiloxane, viscosity about 2 centistokes Preparation the same as for Example VI I.

7 EXAMPLE IX Blue Reversal Powder 12 grams Vinylite VYNV 2 grams Patent Blue 30 grams dimethyl polysiloxane, viscosity about 2 centistokes Preparation the same as in Example VII.

Use of any of the abovedescribed developer compositions in electrostatic printing processes as contemplated in this invention provides for new and substantially improved results. In accordance with this invention, the methods call for applying the developer composition to the electrostatic image by such means as, for example, flowing across the image, spraying, application with a roller or by immersing the image in a tray containing the liquid composition. When an electrostatic image is developed in this manner, the improved results are immediately evidenced in that there is far less deposition of developer particles in unwanted areas of the image than was hitherto deemed possible. By the simple step of heating the surface on which the developed image resides to a temperature above the melting point of the developer particles, excess carrier liquid is driven from the surface and the developer particles are fused thereto. By this method a durable image is formed which can withstand repeated handlings without smearing and which when applied to a flexible surface will flex with that surface rather than to peel or chip therefrom. During the heating step another unusual property of the dimethyl polysiloxane manifests itself. Although it may have a flash point of only about 175 F. it can, without danger of fire, be heated to a temperature of 425 F. or even more.

What is claimed is:

1. A composition of matter comprising a dispersion of electroscopic particles having a diameter not larger than about 74 microns in a liquid consisting essentially of a dimethyl polysiloxane having a viscosity of between about 0.6 and about 3 centistokes, said particles being substantially insoluble in said liquid, the concentration of said particles in said composition being no greater than about 20% by weight.

2. The composition of claim 1 wherein the concentration of said particles in said composition is no greater than about 6% by weight.

3. A composition of matter comprising a dispersion of electroscopic particles having a diameter not in excess of about 74 microns, said particles comprising a material selected from the class consisting of natural and synthetic waxes and resins, in a liquid consisting essentially of a dimethyl polysiloxane having a viscosity between about 0.6 and about 3 centistokes, the concentration of said particles in said composition being no greater than about 20% by weight.

4. The composition of claim 3 wherein the concentration of said particles in said composition is no greater than about 6% by weight.

5. The composition of claim 4 wherein said particles comprise a thermoplastic material having a melting point less than about 200 C.

6. The composition of claim 5 wherein said thermoplastic material is substantially colorless and has coloring matter incorporated therein.

7. A composition of matter comprising a dispersion of particles of zinc oxide coated with an electroscopic material selected from the class consisting of natural and synthetic waxes and resins having a melting point of from about 90 C. to about 200 C., in a liquid consisting es- 7 sentially of a dimethyl polysiloxane having a viscosity of about 0.6 to about 3 centistokes, the concentration of said particles in said composition being no greater than about 20% by weight.

8. The composition of claim 7 wherein the concentration of said coated particles in said composition is no greater than about 6% by weight.

9. The composition of claim 8 wherein said electroscopic material is substantially colorless and has coloring matter incorporated therein.

10. A composition of matter comprising a dispersion in a liquid of particles of photoconductive zinc oxide having a diameter not larger than about 74 microns, said particles having a coating thereon of a thermoplastic electroscopic material having a melting point within a range of from about C. to about 200 C., said liquid consisting essentially of a dimethyl polysiloxane having a viscosity of from about 0.6 to about 3 centistokes, said thermoplastic material being substantially insoluble in said liquid, the concentration of said particles in said composition being no greater than about 20% by weight.

11. The composition of claim 10 wherein the concen tration of said particles in said composition is no greater than about 6% by weight.

12. The composition of claim 10 wherein said electroscopic material is selected from the class consisting of substantially colorless natural and synthetic waxes and resins and has coloring matter included therein.

13. A composition of matter comprising a dispersion in a liquid of colored electroscopic particles having a diameter not larger than about 74 microns, said particles comprising a pigmented resinous material the major proportion of which is polyvinyl chloride, said liquid consisting essentially of a dimethyl polysiloxane having a viscosity of from about 0.6 to 3 centistokes, the concentration of said particles in said composition being no greater than about 20% by weight.

14. The composition of claim 13 wherein the concentration of said particles in said composition is no greater than about 6% by weight.

15. The composition of claim 13 wherein said resinous material comprises at least 90% by weight of polyvinyl chloride.

16. The composition of claim 13 wherein said resinous material is a copolymer consisting essentially of about 96% by weight of vinyl chloride and 4% by weight of vinyl acetate.

17. In a method of developing an electrostatic image on an insulating surface, the improvement comprising applying to said surface a developer composition consisting essentially of finely-divided electroscopic thermo plastic particles dispersed as a phase in a carrier liquid consisting essentially of a dimethyl polysiloxane having a viscosity of between about 0.6 and 3 centistokes, depositing said thermoplastic particles on said surface by electrostatic attraction thereto, and heating said surface to fuse said particles thereto and remove said dimethyl polysiloxane therefrom.

18. In a method of developing an electrostatic image on an insulating surface, said image comprising a pattern of negative electrostatic charges, the improvement comprising: applying to said surface a developer composition consisting essentially of pigmented thermoplastic developer particles dispersed as a phase in a dimethyl polysiloxane liquid having a viscosity of between about 0.6 and 3 centistokes; depositing said developer particles on said pattern of negative electrostatic charges; and, heating said surface to fuse said developer particles thereto and to remove said dimethyl polysiloxane therefrom.

19. In a method of developing an electrostatic image on an insulating surface, said image including areas hearing negative electrostatic charges, the improvement comprising: applying to said surface a developer composition comprising pigmented developer particles, the major proportion of which is polyvinyl chloride, dispersed as a phase in a dimethyl polysiloxane liquid having a viscosity of about from 0.6 to 3 centistokes; depositing said developer particles in areas on said surface other than said areas bearing negative electrostatic charges; and, heating said surface to fuse said developer particles thereto and to remove said dimethyl polysiloxane therefrom.

References Cited in the file of this patent UNITED STATES PATENTS Carlson Oct. 6, 1942 Greig et a1. Feb. 21, 1956 Walkup Mar. 5, 1957 Mayer Mar. 10, 1959 Mayer June 9, 1959 Mayer June 30, 1959 Straughan Aug. 11, 1959 FOREIGN PATENTS France Nov. 9, 1955 Great Britain Aug. 22, 1956 OTHER REFERENCES Dow Corning Silicone Notebook, Fluid Series No. 3,

Claims

17. IN A METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE ON AN INSULATING SURFACE, THE IMPROVEMENT COMPRISING APPLYING TO SAID SURFACE A DEVELOPER COMPOSITION CONSISTING ESSENTIALLY OF FINE-LY ELECTROSCOPIC THERMOPLASTIC PARTICLES DISPERSED AS A PHASE IN A CARRIER LIQUID CONSISTING ESSENTIALLY OF A DIMETHYL POLYSILOXANE HAVING A VISCOSITY OF BETWEEN ABOUT 0.6 AND 3 CENTISTOKES, DEPOSITING SAID THERMOPLASTIC PARTICLES ON SAID SURFACE BY ELECTROSTATIC ATTRACTION THERETO, AND HEATING SAID SURFACE TO FUSE SAID PARTICLES THERETO AND REMOVE SAID DIMETHYL POLYSILOXANE THEREFROM.