US 3166510 A
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United States Patent .Ofifice.
3,166,510 Patented Jan. 19,1965
3,166,510 ELECTROGRAPHIC PRENTING INK, PROCESS FGR THE MANUFACTURE THEREOF, AND METHOD CF ELECTRGSTATIC PRTNTING THEREWITH Charles P. West, Metuchen, and Jacques Benveniste,
Union, N.J., assignors to Burroughs Corporation, Detroit, Mich, a corporation of Michigan No Drawing. Filed Aug. 7, 1957, Ser. No. 676,716 12 Ciairns. (Cl.25Z--62.1)
This invention relates generally to electrographic printing apparatus and particularly to improvements in the dry powdered ink through the agency of which the latent electrostatic images produced in the operation of such apparatus are developed or made visible.
Certain features of the electrographic printing process have been disclosed in earlier filed patent applications, including application Serial No. 443,646, filed July 15, 1954, now US. Patent No. 3,012,839, assigned to the present assignee.
It is an object of the present invention to provide a new composition of matter adapted in specified powder form to serve as an ink in electrographic printing.
It is another object to provide an improved powdered electrographic ink which is particularly suitable for use in the form of a continuous mass thereof to develop latent electrostatic images on a record medium in contact therewith or transported therethrough.
It is another object to provide an uncharged electrographic inking powder adapted reliably to adhere to electrostatically charged areas of a record medium in contact with a continuous mass thereof and to form well defined visible images corresponding to said areas.
It is another object to provide a powdered electrographic printing ink of the aforesaid character which has thermoplastic properties.
A further object is to provide a method of preparation of such inks. a
The general function in the electrographic process of the pigmented powdered material commonly known as the ink is to adhere by electrostatic attraction to the charged areas of the printing medium which constitutethe initial invisible or latent printed image and thus render this image visible. More permanent adherence of the powder to the medium, to produce the final printed record, is attained by any of various fixing processes, one of which, when the ink is thermoplastic, combines heat with pressure to obtain intimate contact of the ink in a tacky state with the medium.
Earlier developed duplicating and printing processes in which latent electrostatic images are initially produced have employed a two-component powder to render these images visible, only one component of which is deposited on the charged areas of, the printing medium, the other component serving to impart, by friction within the mixture, a charge to the active or deposited powder particles that is opposite in sign to that of the charges on the medium. Such two-component powders areknown as triboelectric mixtures, from the manner in which the powder particles-receive their charges.
The present invention is for an inking powder in which the particles are all of the same composition and are uncharged, electrostatically, in their normal unused condition. Since no charging or carrier particles are employed, the bulk of the developing agent is reduced and, of greater importance, the electrostatic force necessary to attract the particles to the charged areas of the medium is reduced, because the particles do not have to be torn loose from oppositely charged particles of a triboelectric mixture. However, once an uncharged particle approaches a charged area of the medium, charges of opposite sign appear on diflferent portions thereof and in order to develop a high attractive force on the particle the charge having the same sign as that of the charged areas of the medium must be dissipated. This is accomplished according to the principles of the present invention by providing an ink of relatively high electrical conductivity, as measured in a continuous mass thereof, so that if the charged printing medium (in sheet form, for example) is drawn through or otherwise is brought in contact with such a mass for inking purposes, the undesired charges on the attracted particles can leak off through the conductive mass, the desired particle charges of opposite sign to that of the charged areas of the medium being bound by the attraction of these areas. It can also be shown that the attractive force on the uncharged ink particles is higher with higher conductivity particles, thereby further promoting reliable inking, and that a redistribution of these undesired charges on the medium is facilitated by a conductive powder. Thus some of the advantages of an electrically conductive, single powder, uncharged electrographic ink have been indicated, and there are disclosed hereinbelow specific improved compositions and methods of preparing such inks. In addition to electrical conductivity, color, particle size, specific gravity and the ability to flow freely are im portant properties of an electrographic ink, as used in certain type of apparatus, as well as thermoplastic properties it heat fixing is employed.
A satisfactory ink in accordance with the invention has been made by spray drying a ball-milled slurry comprisin a carbon black, a synthetic resin, a solvent and a wetting agent and selecting a preferred distribution of sizes of the resulting particles, either by regulation of the process itself or by subsequent screening if necessary. The powder thus obtained is dry, black, electrically conductive, of relatively low specific gravity and flows freely. The following is an example of one formulation of such a slurry:
Carbon black: Statex F-12 (Columbian Carbon Corp.)
41% Resin: Picco450I-I (Pennsylvania Industrial Chemical Corp.) 59% The above percentages by weight in a dry mixture of the two components. Solvent: Toluene, industrial, 63% Wetting agent: Soya Lecithin Yelkin TTS (Ross & Rowe,
i Inc.) 1.6%
The last two percentages are by weight in the wet mixture. Yelkin TTS is an oil soluble commercial lechithin (62% with soybean oil) of fluid consistency, which has a brilliant amber color, according to the Handbook of Material Trade Names by O. T. Zimmerman and I. Lavine, published by Industrial Research Service, Dover, New Hampshire, 1953 Edition, which describes this product on page '63 1.
This slurry is ball-milled to form a dispersion of the carbon black in the dissolved resin. For example, in preparing a laboratory sample (2000 grams) of such a dispersion the mixture is loaded into an Abbe Assay Ball Q Mill with a 22 lb. load of one-half inch steel balls and the mill rotated at 36 r.p.m. for 8 hours. The better the dispersion of the slurry the less viscous and less thixotropic it is and in general the better the conductivity of a mass of the dry ink obtained therefrom.
When suitable dispersion has been accomplished, the slurry is dried in a spray drierj As manufactured by the Bowen Engineering Co., for example, this comprises an atomizer, a drying chamber and a size-selective collector. The slurry is fed into the atomizer Where it is broken up into droplets which are then carried through a cocurrent vortex of hot air in the drying chamber to remove the solvent. The resulting dry particles of the resin, containlng the finely divided carbon black dispersed throughout and bonded within the resin, are directed by the air stream into one or more collectors of the cyclone type. The adjustment of these collectors may serve to supply particles restricted to the desired size range directly or subsequent screening may be resorted to. In the laboratory type Bowen drier a centrifugal-type atomizer is used, with a Wheel speed of approximately 12,000 r.p.m. to give an optimum yield of particles in the desired size range.
T he properties of the components and their proportions in the slurry from which the powdered ink is formed are not extremely critical, although there are certain preferred ranges of values. Carbon blacks chosen from the class known in the trade as conducting furnace blacks, of which the various grades of Statex are examples, are most suitable to supply conductivity to the ink. Graphite, as a source of carbon in general does not give as high electrical conductivity as is desired.
As to the resin, many synthetic and some natural resins have properties suitable for the described purposes. Certain thermoplastic hydrocarbon resins and combinations of resins manufactured by the Pennsylvania Industrial Chemical Co. which have the prefix Picco as a part of their proprietary names have proved satisfactory for this use, a specific example, Picco 450H, being given above. In addition Panarez 6-210 (Pan American Chemical Co.) and certain of the phenolic Bakelite resins have proved suitable.
The so-called Picco resins, manufactured since about 1940 or earlier by Pennsylvania Industrial Chemical Corporation of Clairton, Pennsylvania, are coumaroneindene resin products having melting points ranging from well below 100 C. up to about 120 C., as disclosed in U.S. Patent No. 2,287,513, F. W. Corkery and R. H. Bailey, issued June 23, 1942, to that corporation, and also in a brocure headed with the name Picco and the title Piccoumaron Para-Coumarone Indene Resins which has been distributed for many years by that corporation. Reference to these sources, and particularly to the Patent No. 2,287,513, discloses that the Picco 450H resin is a catalytically synthesized coumaroneindene polymeric resin of moderate molecular weight having a degree of polymerization corresponding to a melting point of about 112 C. by the ball and ring method and having a color in the lightly colored range equivalent to a color figure no greater than about 3 on the para-coumarone resin color scale. Referring to Panarez resins, sold by Pan American Chemicals Corporation, New York, these are hydrocarbon resins derived from petroleum sources, produced by the polymerization of olefins and diolefins, and available in color grades ranging from pale lemon to dark brown with a normal softening point of ZOO-220 F., or 93-104 0., according to The Condensed Chemical Dictionary, Fifth Edition, revised by Arthur and Elizabeth Rose, Reinhold Publishing Corporation, New York, 1956, at page 819. Reference to the product information distributed by Pan American Chemicals Corporation discloses that the resin in this group identified more specifically as Panarez 6-210 has a softening point just below 104 C. and a color of 2 /2 on the Gardner scale. Alternatively, one of the aforementioned phenolic resins or other resinous materials with similar thermoplastic properties, involving, for example, similar softening points or melting points and thus suitable for heat fixing during brief exposure to a temperature around 300 F. or C. as mentioned below but without unwanted softening under usual conditions, can of course be selected by those familiar with such products available in the market.
The specific gravity of the resin particles should be chosen to avoid resins having especially high specific gravity values in order to reduce the effect of gravity thereon. For heat fixing, thermoplastic resins having suitable softening temperatures (relative to the printing medium employed) are selected. The upper limit of this temperature for use with paper media is around 300 F.; in other words, resins having a softening temperature below 150 C. should be chosen for this purpose.
In the finished ink the resin forms a bond between the carbon particles which retains the conductivity of the carbon to a large degree in a mass of the dry ink particles and prevents smearing of the inked image as a result of the presence of free carbon, provided the percentage of carbon is not too great. A satisfactory range of the ratio of resin to carbon (by weight) has been found to be about 0.8 to 1.6. Simple calculation shows that, for the usual mixture consisting of resin and carbon only, this range corresponds to a carbon content of about 55.5 percent to 38.5 percent. Thus, to give the desired conductivity to the dry ink, according to the invention, the carbon should be present as at least 35-40 percent of the mixture of resin and carbon, and the carbon black thus preferably constitutes between about 35 and 55 percent of the total weight of resin and carbon black. In some earlier triboelectric ink mixtures the toner or active powder has comprised a few percent of a carbon material, solely for coloration purposes and to facilitate heat-fixing of the image by accelerating the absorption of radiant heat. This small amount of carbon is insufficient to appreciably lower the high resistance of the material used as the principal constituent of such powders.
The resin solvent is selected primarily to be suitable for the type of resin used, but toxicity and other properties also have weight. In addition to toluene, benzene and xylene are examples of solvents for hydrocarbon resins while alcohol is suitable for some phenolic resins. The amount of solvent used is that required to give a dispersion viscosity suitable for efiicient milling and for feeding to the atomizer. Most solvents are used in the range of 55-70 percent by weight in the wet mixture. A small amount of a wetting agent is beneficial in processing the slurry. As high as about 2.6% has been satisfactorily used. An excessive amount produces a sticky, poorly flowing powder.
Particle size in the dry powdered ink is important. A determination of the optimum distribution of sizes has been made in connection with the present invention. In order to define such a distribution it is necessary to specify at least one parameter in addition to the mean particle size. A standard expression is the following log normal distribution equation:
Zh tl s L0 Zn log a As sources of information relating to particle size measurement and methods of specifying particle size distribution the following books are cited: J. M. Dalla Valle: Micromeritics, 2nd ed., Pittman, 1948, particularly p. 54; also, G. Herdan: Small Particle Statistics, Elsevier, 1953, particularly-pp. 113-115.
In practice, the values of'the quantities appearing in the above equations are determined by microscopic examination andmeasurement. According to the present invention preferred ink particle sizes should be within the region bounded by two curves having d and n as co-ordinates andrepresenting the first of the above equations when d =12 microns and 40 microns, respectively, with a =1.24 (log a =0.2l4) in both cases. These conditions are approximately fulfilled whend the geometric mean of the particle diameters, is within the range of 12-40 microns and 98 percent by weight of the particles have diameters within the range of 5-80 microns.
Particle sizes are specified herein as particle diameters. This is justifiable both to simplify statistical treatment and on the ground that an equivalent diameter (Stokes diameter) can be specified.
In addition to having the proper particle size distribution the ink must flow freely when applied to the surface of the printing medium. This property is measured by the angle of repose of the powder. It is found that the angle of repose of a mass of powder should not exceed 40 degrees with respect to the horizontal. The measurement of fluidity is an overall measurement which takes into account particle shape, stickiness, agglomeration and other properties which are difficult to specify individually.
An arbitrary method of measuring bulk or volume electrical resistivity (or conductivity) of the ink powder is useful in specifying limiting values. This makes use of a powder specimen 1 inch long in a 1 inch inside diameter cylindrical tube of insulating material and comprises measuring the resistance between conducting end plates when the powder is subjected to a force of 100 grams in the direction of the axis of the container. In connection with the limiting values of properties other than resistivity specified herein, an upper limit of 10,000 ohms is preferably placed on the value of resistance thus measured. This corresponds approximately to a volume resistivity of 20,000 ohm-centimeters (ohms per centimeter cube). Lower values are desirable for operation at high printing speeds and as far as present experience has indicated in most cases there is no lower limit for the value of ink resistivity.
The disclosure herein of certain electrographic ink formulations and methods of manufacture is by way of illustration and not by way of limitation of the invention as specifically defined in the appended claims.
What is claimed is:
1. As a new article of manufacture, a free-flowing electrically conductive ink powder for developing latent electrostatic images, 98 percent of the powder particles having diameters comprised between the limits of 5 and 80 microns, and the geometric mean diameter of all the particles being between the limits of 12 and 40 microns, said particles having a substantially uniform composition comprising at least 35 percent by weight of a furnace carbon black dispersed in a solidified thermoplastic resin, and a mass of said particles having an angle of repose of not more than 40 relative to the horizontal.
2. An electrically conductive ink as defined in claim 1 wherein the powder particles soften at a temperature not in excess of 300 F.
3. A free-flowing dry ink for developing latent electrostatic images, comprising: a thermoplastic resin in particulate form containing, dispersed throughout and bonded by said resin, at least 35 percent by weight of an electrically conductive grade of carbon black to impart to a mass of such particles a volume electrical resistivity not exceeding 20,000 ohm-centimeters as measured between the end surfaces of a cylindrical sample one inch in diameter and one inch in length when subjected to an end force of 100 grams, and wherein substantially all of the carbon-containing resin particles have diameters within the range between 5 and microns. 4. An electrically conductive powdered ink for develop ing latent electrostatic images, comprising: solution-spraydried particles of a thermoplastic resin with a softening temperature below 150 C., and at least 35 percent by weight of conducting furnace carbon black dispersed and bonded within the resin to impart to a mass of such particles volume electrical resistivity not exceeding 20,000 ohm-centimeters as measured between the end surfaces of a cylindrical sample one inch in diameter and one inch in length when subjected to an. end force of grams, the spray-dried carbon-containing resin particles being restricted to particles substantially all of which have diameters within the range between 5 and 80 microns with the geometric mean of the particle diameters falling between 12 and 40 microns, whereby said carbon-containing resin ink particles constitute a free-flowing powder having an angle of repose which does not exceed 40 degrees relative to the horizontal.
5. An electrically conductive powdered ink for developing latent electrostatic images, comprising: spray-dried particles of a dispersion of conducting furnace carbon black particles in a dissolved thermoplastic resin, substantially all of the carbon-containing resin particles having diameters within the range between 5 and 80 microns, and said carbon black constituting between 35 and 55 percent of the total weightt of resin and carbon black, whereby a quantity of said carbon-containing resin ink particles has an angle of repose which does not exceed 40 degrees relative to the horizontal and has a volume resistivity which does not exceed 20,000 ohm-centimeters as measured between the end surfaces of a cylindrical sample one inch in diameter and one inch in length when subjected to an end force of 100 grams.
6. An electrically conductive powdered ink for developing latent electrostatic images, the particles of said ink comprising grains of carbon black bonded by a thermoplastic resin, the ratio of resin to carbon black in a mass of the ink being within the range of 0.8 to 1.6, by weight, and wherein the geometric mean diameter of the ink particles is within the range of 12-40 microns and 98 percent by weight of the particles have diameters within the range of 5-80 microns.
7. An electrically conductive powdered ink as defined in claim 6, wherein the thermoplastic bonding resin comprised in the particles of said ink soften at a temperature which does not exceed 300 F.
8. An electrically conductive powdered ink as defined in claim 6 wherein the specific gravity of the bonding resin does not exceed 4.0.
9. An electrically conductive powdered i-nk as defined in claim 6 wherein said carbon black comprises as a major portion thereof a grade of furnace black.
10. A process for the manufacture of electrically conductive ink powders suitable for developing latent electrostatic images, comprising the steps of ball-milling a slurry made up of a furnace carbon black, a thermoplastic hydrocarbon resin dissolved in a solvent chosen from the group consisting of toluene, xylene and benzene, and a wetting agent, thereby to obtain a uniform dispersion of the furnace carbon black in the resin solution, the ratio by weight of said resin to said furnace black being within the range of 0.8 to 1.6 and said wetting agent being less than 2.6 percent by weight of the slurry, spray drying the slurry to extract the solvent and selecting from the resultant dry powder a mass of particles having a geometric mean diameter of 12-40 microns and of which 98 percent by weight have a diameter within the range of 5-80 microns.
11. The process defined in claim 10 wherein said ratio of resin to carbon black is further restricted to values .4 giving-to the dry powdered ink 21 volume electrical resistivity not exceeding 20,000 ohm-centimeters as measured between the end surfaces of a one inch diameter cylindrical sample one inch long when subjected to an end force of 100 grams.
12. In a method of electrostatic printing in which local electrostatic charges are applied to a printing medium to form thereon a latent electrostatic image to be printed, the improvement comprising bringing into contact with the record medium carrying said latent electrostatic image a dry electrographic printing ink comprising free-flowing particles between about 5 and 80 microns in diameter, said particles comprising from about 35% to about 56% by weight of grains of carbon black bonded by a thermo- O i a plastic resin, whereby portions of said mass of ink adhere by electrostatic attraction to said latent image.
References Cited in the file of this patent UNITED STATES PATENTS 2,068,113 Schellenger et a1 Jan. 19, 1937 2,336,095 Edgar et al. Oct. 2, 1945 2,446,387 Peterson Aug. 3, 1948 2,649,428 Moore et a1 Aug. 18, 1953 2,688,576 Ryan et a1 Sept. 7, 1954 2,753,308 Landrigan July 3, 1956 2,795,680 Peck June 11, 1957 FOREIGN PATENTS 532,665 Great Britain Ian. 29, 1941 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N00 3,166,510 v January 19, 1965 Charles Pig West et al Q It is hereby certified that error appears in'the above numbered patent reqliring correction and that the said Letters Patent should read as corrected, below v 1 Column 2, line 32, for "type" read types line 47 after "above" insert are line. 55,- for "lechithin" read' lecithin column 3, line 50, for "brocure" read -brochur column 8, after line 10, insert 2 ,713 ,563 Kuhn- July 19, 1955' Signed and sealed this 24th day of August 1965:,
ERNEST w. SWIDER v EDWARD J. BRENNER Allcsting Officer Commissioner of Patents I