Developer Power Of Thermoplastic Special Particles Having Conductive Particles Radially Dispersed Therein

Abstract

Flowable, heat fusible, dry powder suitable for use as a developer powder in electrographic recording which comprises thermoplastic, essentially spherical particles, the thermoplastic material of which has a conductivity of at most 10.sup..sup.-12 mho/cm., in which are essentially completely embedded electrically conductive particles forming a radially disposed zone, said essentially spherical particles having: A. an electronic conductivity ranging monatonically without decreasing from between about 10.sup..sup.-11 and 10.sup..sup.-4 mho/cm. in a 100 v./cm. DC electrical field to between about 10.sup..sup.-8 and 10.sup..sup.-3 mho/cm. in a 10,000 v./cm. DC electrical field, B. a number average particle diameter below 15 microns, and C. a volume ratio of said electrically conductive particles to said total particle volume of between 0.01/100 and 4/100.

Description

This invention relates to a dry ink powder suitable for use in electrographic recording and a process for making such powder. In one aspect this invention relates to a developer powder having a good electrical conductivity in the presence of a relatively large impressed electric field, and low electrical conductivity (and hence good charge retention characteristics for the charge remaining on it) in the absence of this high impressed field. In still another aspect this invention relates to dry developer particles for electrophotography which are magnetizable. In still another aspect, this invention relates to a developer powder which has a pressure dependent conductivity, being more conductive under the influence of an impressed magnetic field during development, and less conductive (and hence having better individual charge retention characteristics) in the absence of this impressed magnetic field.

Electrostatic electrophotography originally employed two component dry ink powders, often called "triboelectric mixtures," for charge development of the electrostatic image. Recently dry powders in which all of the particles are of the same composition have been described. The relatively conductive dry inks of U.S. Pat. No. 3,116,510 (Jan. 19, 1965 ; Charles P. West and Jacques Benveniste) contain thermoplastic resin particles in which about 35 to 55 percent of the total particle weight is carbon black dispersed throughout the resin particles. In U.S. Pat. No. 3,196,032 (July 20, 1965 ; David W. Seymour) an electrostatic printing ink having carbon powder partially embedded in or adhered to the surface of resin particles is prepared in a fluid bed reactor.

In a new electrographic process, described in French Pat. No. 1,456,993, an exposed photoconductive sheet is contacted with conductive developer powder applied from a conductive surface, to which it is adhered, while creating a differential electrical field between the photoconductive sheet (i.e., field electrode) and the conductive surface containing the developer powder. The developer powder is transferred selectively to the photoconductive sheet in the nonexposed areas. Separation of the photoconductive sheet from the source of supply of developer powder is made while still maintaining the influence of the electrical field, and provision can be made for continuing the attraction of the developer powder to the surface of the photoconductive sheet after such separation. The developer powder in this process is electronically conductive, usually having a conductivity of at least 10.sup.-.sup.10 mho per centimeter (ohm.sup.-.sup.1 cm..sup.-.sup.1 ), preferably 10.sup.-.sup.2 to 10.sup.-.sup.7 mho per centimeter, at the applied electrical field (preferably at least 1,000 DC volts per centimeter). Conductivity measurements are made with the developer powder compressed into a 1 -centimeter cube between brass electrodes fitted in a rigid chamber, a pressure of 86 pounds per square inch (6.05 kg. per cm..sup.2) being applied across the sample before and during the measurement of conductance. If the developer powder is subsequently to be transferred from the photoconductive sheet to a receptor surface, it should also have electrical charge retention capability, to retain the electrical charge imparted to the developer particles by the applied electrical field during the development of the pattern on the field electrode. This may be accomplished by providing the developer particles with a highly resistive interior or core and a highly conductive surface or shell. However, the high conductivity of the developer particles desired to minimize voltage drop across them when they are in the electrical field, and the ability of the developer particles to retain the electrical charge, which characterizes high resistivity particles, are difficult to achieve satisfactorily, since one desirable characteristic is generally sacrificed to obtain the other.

It is therefore an object of this invention to provide new particles suitable for use as electrographic developers, particularly in the process of French Pat. No. 1,456,993, also referred to as the "Electropowder process." Still another object of this invention is to provide powder particles having both high conductivity and good electrical charge retention. Yet another object is to provide a process for the manufacture of such developer particles.

The FIGURE is a plot of electrical conductivity vs. DC applied electrical field for developer particles of this invention.

The developer powders of this invention comprise thermoplastic, essentially spherical particles (i.e., spherules), the thermoplastic material of which has a conductivity of at most 10.sup.-.sup.12 mho/cm., preferably at most 10.sup.-.sup.13 mho/cm., in which are essentially completely embedded electrically conductive particles forming a radially disposed layer or "zone," said essentially spherical particles having an electronic conductivity which ranges monatonically without decreasing from between about 10.sup.-.sup.11 and about 10.sup.-.sup.4 mho/cm. (preferably between 10.sup.-.sup.9 and 10.sup.-.sup.5 mho/cm.) in a 100 v./cm. DC electrical field to between 10.sup.-.sup.8 and about 10.sup.-.sup.3 mho/cm. (preferably between 10.sup.-.sup.7 and 10.sup.-.sup.4 mho/cm.) in a 10,000 v./cm. DC electrical field, and having a number average diameter below 15, preferably below 10, microns. Preferably, the average particle size range is such that at least about 95 number percent of the particles have a diameter greater than about 2 microns, while no more than 5 number percent have a diameter greater than about 15 microns. These dry ink powders are flowable to such an extent that they have a flowability angle of repose ranging from about 80.degree. to 125.degree. and preferably from 110.degree. to 125.degree.. For purposes of this invention, flowability is measured by feeding a thin stream of powder to the upper flat surface of a 3-inch diameter circular pedestal from a vibrating funnel, thereby creating a conical deposit of powder on the pedestal. The angle of repose is defined by the angle measured between opposite sides of the conical deposit, i.e., the apex angle of the cone, at 25.degree. C.

The dry ink powders of this invention and the thermoplastic materials used therein are preferably heat fusible in the range of 80.degree. to 115.degree. C., preferably from 90.degree. to 105.degree. C. For determining fusion temperatures the Durrans' Mercury method, as reported in SMS 114, is employed. Any heat fusible thermoplastic material having a conductivity of at most 10.sup.-.sup.12 mho/cm. may be used to form the spherules, although thermoplastic organic polymers are preferred. Examples of suitable resins include B-stage (i.e., partially cured) phenol aldehyde polymers, polyvinyl acetate, epoxy resins, etc.

In general, any highly electrically conductive material (i.e., a material having a conductivity of at least 10.sup.-.sup.2 mho/cm., such as conductive carbon, metal, etc.) may be used in powdered form as the electrically conductive particles forming the conductive zone of the dry ink particles, provided the resulting electrically conductive particles have an average diameter below 100 millimicrons, preferably under 40 millimicrons. Conductive carbon particles (e.g., those available under the trade name Vulcan XC-72 R, sold by Cabot Corporation) are preferred.

It has been found that the amount of conductive material in the embedded zone of the dry ink particle, the type of conductive material used, the particle size of the embedded conductive particles, and the location of the embedded zone can influence the conductivity of the dry ink powder. Generally the volume ratio of electrically conductive material to the total particle volume in the ink powder can be in the range of 0.01/100 to 4.0/100, although 0.1/100 to 1.5/100 is preferred. The embedded zone of conductive particles is normally quite close to the surface of the ink particle and is preferably not thicker than one-tenth the radius of the essentially spherical developer particle. Although essentially all of the conductive particles are embedded, an occasional particle may protrude from the surface. The conductivity of these developer particles is "field dependent," i.e., the conductivity under high electrical fields differs from the conductivity under low electrical fields. In fact, as mentioned earlier, the electrical conductivity of the developer particles is a monatonically, nondecreasing function of the applied DC electrical field. It is preferred that the slope of the conductivity vs. applied electrical field curve also increases monatonically with the applied electrical field. This has been found to be extremely valuable for developer powders used in the process of French Pat. No. 1,456,993, since the developer particles display high conductivity under the high electrical field conditions of particle deposition on the field electrode and display lower conductivity (and hence better electrical charge retention) after they are removed from the high electrical field. As mentioned earlier, charge retention is particularly important when one desires to transfer the imagewise pattern of developer particles from the field electrode to a receptor sheet without loss of particles. Although the mechanism is not completely understood, the field dependent conductivity of these particles is believed to be attributable to their being essentially completely immersed or embedded in the relatively insulative, thermoplastic material. At the higher electrical fields the electrical current is believed to "tunnel" or pass through the thermoplastic material on the particle surface to reach the embedded zone or layer of conductive material. At the lower electrical fields the thermoplastic surface layer serves as an effective insulative barrier to current flow, resulting in a lower particle conductivity and a higher electrical charge retention capability.

Various other materials may be usefully incorporated in or on the developer particles of this invention, e.g., plasticizers, dyestuffs, pigments, magnetically permeable particles, etc. Magnetically permeable particles having an average diameter of 1 micron or less are particularly preferred, including magnetite, barium ferrite, nickel zinc ferrite, chromium oxide, nickel oxide, etc. A magnetically permeable core may also be used. Powdered flow agents may also be added to the dry particles to improve their flow characteristics.

The conductivity of these dry ink powders is related to the applied electric field across the powder particles, and measurement of conductivity is therefore made under standard conditions of sample size, sample compression and applied electric field. The following test procedure is used for the conductivity measurements presented herein.

The sample of ink is placed in a test cell between two brass electrodes of circular cross section, each with a cross-sectional area of about 0.073 cm..sup.2. An insulating cylindrical sleeve of polytetrafluoroethylene surrounds the ink and electrodes such that the ink sample is constrained to the shape of a small pill box. At least one of the electrodes is free to move like a piston in the insulating sleeve to provide a predetermined compression on the sample. The compression is obtained by placing a known weight on the movable electrode, and typically one uses a 100 gram weight to give a pressure of 1,370 g./cm..sup.2 on the sample. One places enough ink into the cell such that the final electrode spacing under the above pressure is about 0.05 cm. to about 0.1 cm., and preferably as close to 0.05 cm. as possible. The final spacing is measured carefully using a cathetometer. A voltage is applied in a series circuit arrangement consisting of the ink sample, an electrical current meter (such as a Keithley Model 601 Electrometer), and the voltage source. The ink conductivity is calculated from the voltage which appears across the sample electrodes and the current which flows through it in the usual manner. The voltage is varied and the resulting conductivity is calculated for various electric fields from about 10 v./cm. to about 1,000 to 4,000 v./cm. For fields higher than about 4,000 v./cm., the voltage cannot be applied to the sample for longer than a fraction of a second or so, before considerable heat develops in the sample, changing its characteristics, or causing it to "break down" entirely. To measure the electrical conductivity at high fields, therefore, the applied voltage is rapidly increased from about 0 to 2,000 v. or more (corresponding to fields of about 0 v./cm. to about 40,000 v./cm.) in about 10 milliseconds, and is then immediately returned to about 0 v. again before appreciable heating or breakdown occurs in the sample. This voltage "sweep" is accomplished by using a special, high voltage ramp (or sweep) generator. To measure the current through the sample, when using the voltage sweep, the current meter described earlier is replaced by a current-sampling resistor, typically of about 10,000 ohms. The voltage across this sampling resistor, as monitored by an oscilloscope, is proportional to the current flowing through the sample. The voltage across the sample is also monitored on an oscilloscope, using high voltage probes. Typically, the voltage across the current-sampling resistor is applied to the horizontal input to the oscilloscope, while the voltage across the ink sample itself is applied to the vertical input to the same oscilloscope, giving a direct plot proportional to the current (abscissa) vs. voltage (ordinate) characteristics of the ink sample on the oscilloscope screen, which is then photographed. From this, the conductivity vs. field characteristics of the ink sample at very high fields can be calculated. The electrical conductivity data given in Table I was obtained in the above manner. ##SPC1##

The dry ink powder conductivity should be such that at high applied electric fields, it permits a relatively large current flow from the development electrode to the intermediate photoconductive imageable surface during the development step, which is carried out with a relatively large series voltage impressed. However, the powder should not be so conductive that after one layer is deposited on the intermediate photoconductive imageable surface it thereafter electrically "shields" subsequent layers of powder from the intermediate surface, accepting their charge but preventing their deposition as would happen with a highly conductive powder. Additionally, at low or zero applied electric field, the conductivity should be considerably smaller so the powder which was deposited on the intermediate photoconductive imageable surface retains its charge for a time period sufficient to permit transfer of the powder from the intermediate surface to a receptor sheet. After development is completed, the electric field holding the powder to the intermediate surface in areas where it is deposited is still relatively strong, but the nature of the interface in these areas is insulating enough to prevent the charge to flow from the powder into the intermediate itself. At the same time, the lateral electric field from particle to particle is very small or zero, so the charge on the deposited particles does not "leak" laterally to the more conductive areas on the intermediate surrounding the deposited powder. Furthermore, the electric field from layer to layer of deposited powder is small after development, so the charge does not readily leak from layers more remote from the intermediate surface to the layers more adjacent to said surface. Thus all deposited particles remain strongly bound to the intermediate and retain their charge for a time.

In preparing the developer powders a dry-powdered blend of appropriate composition is first obtained by any of several standard means, for example, by melting a resin, stirring in the solid filler, if any, allowing the mixture to cool, then grinding and classifying to the appropriate particle size range of approximately 1 to 15 microns diameter. This powder, which is pseudocubical in shape is then "spheroidized" by the following method: the powder is aspirated into a moving gas stream, preferably air, thus creating an aerosol. This aerosol is directed at about 90.degree. (.+-.5.degree.) through a stream of hot air, which has been heated to about 900.degree.-1,100.degree. F., into a cooling chamber, where the powder is then allowed to settle by gravity while it cools. The resulting powder is now made up of substantially spherical particles. It is then dry blended with conductive powder, such as conductive carbon black, and the mixture is directed at about 90.degree. (.+-.5.degree.) through a stream of gas, preferably air, heated to a temperature (e.g., 700.degree.-800.degree. F.) which can at least soften and desirably melt the thermoplastic resin in the particles and maintain that softened or melted condition for a period of time sufficient to permit the conductive powder to become essentially completely embedded, due to the effects of surface tension. The particles are then collected, such as by cyclone separation, and are preferably blended with a flow agent, such as "CAB-O-SIL" (finely divided silica, a trademarked product of Cabot Corporation) to insure that it will be free flowing.

In an alternative preparation of the developer powders of this invention the conductive material may be deposited, as a powder or as a continuous film, on the surface of the essentially spherical particles, and a thin film of insulative material, e.g., a resin, may be superimposed or deposited thereon to effectively embed the conductive material as a zone in the particles.

The following procedure represents a preferred method for manufacturing the dry ink powder.

EXAMPLE A

Four parts by weight of "Epon 1004" (epichlorohydrin/bisphenol A solid epoxy resin, melting point 95.degree.-105.degree. C., epoxide equivalent of 875-1,025, molecular weight of 1,400, a trademarked product of Shell Chemical Company) and 6 parts by weight of magnetite were blended thoroughly on a conventional heated-roll rubber mill. The resulting material was pulverized in an attrition-type grinder and was then classified in a standard air-centrifugal-type machine, the yield from which was about 20 percent by weight in the desired particle size distribution range. Particle size analysis of the product showed it to be about 95%> 1.3.mu., 50%> 4.1.mu., 5%> 12.6.mu. (by number).

These particles, which are sharp edged and pseudocubical in shape, were then "spheroidized" such that most of the particles were transformed into spherelike shapes or round-edged particles by the following process. The powder was fed to an air aspirator in a uniform stream of about 800 grams per hour. The aspirator sucks the particles into the airstream and disperses them, forming an aerosol. This aerosol was directed at 90.degree. into a heated airstream, the temperature of which was about 950.degree.-1,000.degree. F. The powder was then allowed to settle and was collected by filtration.

At this point the majority of the particles had been transformed into spherelike shapes and were ready for the next step in the process which was to mix the powder with the appropriate quantity of conductive carbon black which in this case was 1.33 parts conductive carbon black, approximate diameter 30 millimicrons, per 100 parts powder by weight. After the two components were thoroughly mixed, the carbon was embedded into the resin by the spheroidization process, exactly as it was described above, except that the temperature of the hot airstream was adjusted to about 740.degree. F. and the product was collected in a cyclone-type separator.

The final step in the process was to blend 0.1 percent by weight of a small particle size SiO.sub.2 flow agent to cause the powder to become sufficiently free flowing for use in the electropowder process. This ink was coded A, and the conductivity vs. applied electrical field curve is shown in the FIGURE.

Table I shows the properties obtained when several other formulations (B-F) were prepared by the method given in the above example, and the conductivity vs. applied electrical field curves are presented in the FIGURE. The two dotted lines in the FIGURE represented the upper and lower limits of conductivity over the range of applied DC electrical fields, as mentioned earlier.

Claims

What is claimed is:

1. Flowable, heat fusible, dry powder suitable for use as a developer powder in electrographic recording which comprises thermoplastic, essentially spherical particles, the thermoplastic material of which has a conductivity of at most 10.sup.-.sup.12 mho/cm., in which are essentially completely embedded electrically conductive particles having a conductivity of at least 10.sup.-.sup.2 mho/cm. and an average diameter below about 100 millimicrons forming a radially disposed zone, said essentially spherical particles having:

a. an electronic conductivity ranging monatonically without decreasing from between about 10.sup.-.sup.11 and 10.sup.-.sup.4 mho/cm. in a 100 v./cm. DC electrical field to between about 10.sup.-.sup.8 and 10.sup.-.sup.3 mho/cm. in a 10,000 v./cm. DC electrical field,

b. a number average particle diameter below 15 microns, and

c. a volume ratio of said electrically conductive particles to said total particle volume of between 0.01/100 and 4/100.

2. The dry powder of claim 1 in which said essentially spherical particles contain therein magnetizable particles.

3. The dry powder of claim 1 in which said electrically conductive particles are particles of highly conductive carbon having a conductivity of at least 10.sup.-.sup.2 mho/cm.

4. The dry powder of claim 1 in which the particle size range of said spherical particles is such that at least about 95 number percent of the particles have a diameter greater than about 2 microns and no more than 5 number percent have a diameter greater than 13 microns.

5. The dry powder of claim 1 in which said spherical particles have a flowability angle of repose between 80.degree. and 125.degree. .

6. The dry powder of claim 1 in which said spherical particles have an electronic conductivity ranging monatonically without decreasing from between 10.sup.-.sup.9 and 10.sup.-.sup.5 mho/cm. in a 100 v./cm. DC electrical field to between 10.sup.-.sup.7 and 10.sup.-.sup.4 mho/cm. in a 10,000 v./cm. DC electrical field.

7. The dry powder of claim 1 in which said thermoplastic material is an organic resin.

8. The dry powder of claim 1 which is heat fusible in the range of from about 80.degree. to 115.degree. C.