US3827905A

US3827905A - Development enhacement of electrostatic images

Info

Publication number: US3827905A
Application number: US00161227A
Authority: US
Inventors: W Roth
Original assignee: DIAGNOSTIC INST Inc
Current assignee: DIAGNOSTIC INST Inc; DIAGNOSTIC INST INC US
Priority date: 1971-07-09
Filing date: 1971-07-09
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06

Abstract

A PROCESS OF ENHANCING THE DEVELOPMENT OF LATENT ELECTROSTATIC IMAGES ON A SUBSTRATE WHICH COMPRISES CONTACTING THE LATENT IMAGE WITH A POWDERED PIGMENTED, FUSIBLE AND POLARIZABLE DEVELOPER MATERIAL HAVING AN INDUCED ELECTROSTATIC CHARGE SUCH THAT IT WILL BE ATTRACTED TO THE SURFACE OF THE SUBSTRATE. AFTER THE POWDER PARTICLES ARE PREFERABLY FIXED OR FUSED ON THE SUBSTRATE, IT IS HEATED TO A TEMPERATURE IN EXCESS OF THE GLASS TRASITION TEMPERATURE OF THE FUSED POWDER PARTICLES THEREON AND SUBJECTED TO AN INDUCED ELECTRIC FIELD TO POLARLIZE THE PARTICLES INTO ELECTRETS. THE ELECTRETS ARE THEN CONTACTED BY ADDITIONAL ELECTROSTATICALLY CHARGED POWDER PARTICLES IN THE SAME MANNNER AS WAS THE ORIGINAL LATENT ELECTROSTATIC IMAGE SO THAT THE DEVELOPED IMAGE DENSITY IS ENHANCED DUE TO THE ADDITIONAL POWDER PARTICLES ATTRACTED TO THE SUBSTRATE.

Description

Aug. 6, 1974 N w, H 4 3,827,905

DEVELOPMENT ENHANCEMENT OF ELECTROSTATIC IMAGES Filed July9. 1971 l3 I5 l3 9 l3 l 1 H II H FIG. IA FIG. IB FIG. IC

I 28 f 29 l9a 1 iti II 3 /PI lg 23 v T OVEN T T919 FIG. 2

9 e e 9 as FIG. 3A FIG. 3B

INVENTOR.

WALTER ROTH SOKOLSKI a WOHLGEMUTH ATTORNEYS United States Patent US. Cl. 117-175 17 Claims ABSTRACT OF THE DISCLOSURE A process of enhancing the development of latent electrostatic images on a substrate which comprises contacting the latent image with a powdered pigmented, fusible and polarizable developer material having an induced electrostatic charge such that it will be attracted to the surface of the substrate. After the powder particles are preferably fixed or fused on the substrate, it is heated to a temperature in excess of the glass transition temperature of the fused powder particles thereon and subjected to an induced electric field to polarize the particles into electrets. The electrets are then contacted by additional electrostatically charged powder particles in the same manner as was the original latent electrostatic image so that the developed image density is enhanced due to the additional powder particles attracted to the substrate.

BACKGROUND OF THE INVENTION This invention relates to the development of latent electrostatic images which have been formed on electrically insulative substrates. More particularly, the invention relates to enhancing the optical density of developed electrostatic images.

The general process known as ionography is a means for making X-ray images without the utilization of silver halide film. The basic process was disclosed by E. L. Criscuolo in NAVORD Report 4033 of July 6, 1955, in US. Pat. 2,900,515, in an article by R. A. Youshaw and J. A. Holloway in Nondestructive Testing, September- October 1959, and by K. H. Reiss in Z. Angew. Physik, Vol. 19, p. 1 (1965). This process comprises the utilization of two parallel plate electrodes. A DC. voltage is applied across the electrodes such that one is a positive electrode and the other a negative one. When the positive electrode is nearest the X-ray beam it must not absorb much of the X-ray beam. It has affixed to it an image receiving sheet which may be transparent or opaque but must be an electrical insulator such as a thin sheet of a plastic film or the like. The negative electrode has a thin film or layer of a material which is an efficient absorber of X-rays applied to it. In the aforementioned Reiss reference a heavy metal, such as lead, tungsten or molybdenum was utilized as an absorber of the X-rays, and was in eiiect a photoemitter. The image receiving insulator on the anode and the photoemitter layer on the cathode face each other across the gap between the electrodes with the object being examined disposed on either the outer side of the anode or cathode, preferably on the outer side of the anode. A quenching gas is flowed, or in cases may be stationary, in the gap between the electrodes.

When an object disposed adjacent the anode is irradiated by X-rays or gamma rays, this electromagnetic radiation is differentially absorbed by the object and passes through the transmissive anode and insulator layer afiixed thereto and across the gap to strike the photoemitter which, as a consequence, ejects electrons having energies up to many kilo-electron volts. The number of electrons emitted from any area or portion of the photoemitter is 3,827,905 Patented Aug. 6, 1974 dependent upon the number of X-ray photons absorbed in that portion, the depth of the absorption, and the photon energy. On leaving the photoemitter surface, the electrons find themselves in a DC field between the electrodes and travel toward the positive electrode. The quenching gas serves to slow down the electrons so that they will not scatter when reaching the insulator and to increase their number by secondary ionization. Upon arriving at the insulator surface, the electrons, and any negative ions which may have been formed by attachment to components of the quenching gas, are collected in an image configuration forming a latent electrostatic image consisting of negative charges corresponding to elements or portions of the object which are relatively transparent to X-rays and no charge or fewer charges corresponding to portions or elements of the object which are opaque or relatively opaque to X-rays. This latent image is then made visible by development or by cathode ray tube display techniques.

The development of the latent electrostatic images is accomplished generally by contacting the latent images with powder particles normally utilized for developing such images, not only in ionographic processes but other processes where latent electrostatic images are formed, such as xerography and the like. Typical developer powder particles include charcoal, carbon black and various carbonaceous type pigments. Additionally, finely divided material, such as powdered resins, having pigments or dyes added thereto can be used. The use of such resins is desired where the formed image is to be ultimately fused by heat or other means. The particle size of the powder material is relatively small in order to maintain a good resolution of the developed image. For example, particles having average diameters on the order of 1 to 10 microns are normally utilized. The latent electrostatic image can either have a positive or negative charge, depending upon the polarity of the adjacent electrode utilized during the process of forming the image. Similarly the powder particles have a charge thereon which can be induced by means of a corona discharge or in other ways.

If the powder particles have a charge opposite to that of the latent image, they will then be attracted to the latent image charges forming a positive image. Alternatively, if the powder particles have a charge of the same polarity as that of the latent image, they will be repelled by the latent image and cover the substrate in the ateas not occupied by the latent image, thus in efiect forming a negative image. Thus the powder particles are given a charge, either opposite to or the same as the latent electrostatic charges on the substrate, depending upon the type of developed image desired.

The density of the developed image is basically affected by the amount of latent electrostatic charge produced on the substrate material. Further, the density can be controlled by the amount of developer powder utilized and there is a generally optimum preferred density for a developed image. Further, there is a maximum density that can be achieved and this is determined principally by the strength of the electrostatic charge of the latent image. Thus, an excess of developer powder cannot make a latent image any denser than the limitations set by the latent image. On the other hand, a lesser amount of powder can decrease the density regardless of the amount of the latent electrostatic charges on the substrate. As has been indicated, ionography involves the utilization of X-rays. The ionographic process, which produces images of portions of the human body in a manner similar to normal X-rays, is particularly useful in mammography. It can be appreciated that the normal X-ray procedure involves a production of a photographic image whereas in ionography, a like image is produced on an insulative substrate by latent electrostatic charges which are subsequently developed.

A particular advantage of ionography, as compared to a normal X-ray technique, is the ability to produce clear images at lower X-ray doses. As indicated above, the strength of the electrostatic field of the latent image affects the density of the resulting image, but does not affect to any great degree the resolution or other image characteristics. As a result, it is desirable to produce optically denser images with latent electrostatic images of a given field or level of charge. This would mean, in effect, that a successful developed image could be produced with a shorter X-ray exposure.

SUMMARY OF THE INVENTION Thus, it is an object of this invention to provide developed images of enhanced optical density from latent electrostatic images, and to provide high quality developed ionographic images with shorter X-ray exposures. Other objects and features will be in part apparent and in part pointed out hereinafter.

The above and other objects are accomplished by the novel methods and products of this invention in which latent electrostatic images are produced on insulative substrates, such as by the process of ionography. Such latent electrostatic images are then contacted with pigmented fusible polarizable particles. However, in accord with the present invention, a shorter exposure period of X-ray is required and thus the latent images formed on the substrate may have a weaker electrostatic charge so that a lesser amount of powder particles will be attracted thereto. Such images when developed may not have the desired density preferred for practical applications. But in accordance with this invention, the substrate and the powder particles, after fusion thereon to form an initial visible image (by a conventional technique such as using heat or other energy sources), are then subjected to an electric field, e.g., one established between two electrodes. The electrode adjacent the side of the substrate on which the fused image exists will have a polarity opposite to the polarity that is induced on the surface of the fused image. The substrate and fused polarizable material are then heated to a temperature above the glass transition point of the powder material while being subjected to the electric field which polarizes the fused material into electrets. This creates an exposed plurality of charges on the surface of the fused initial visible image which have the same polarity as the electrode on which the substrate resides. The electric field is maintained for a period sufficient to form such electrets and the material is then allowed to cool to room temperature while the field remains. The substrate with the electret image is then ex posed to a cloud of additional charged powder particles which, as indicated, will have a charge opposite to the charge of the surface of the electrets so as to be attracted thereto and intensify the optical density of the initial visible image. This process may be again repeated after the additional powder has been fused and converted again into electrets thereby to obtain eventually as dark or as dense a final visible image as desired. It can thus be seen that in accordance with this invention the density of the image is enhanced not by an increased duration of X-ray exposure or level of X-ray exposure, but rather by the formation of the aforementioned electrets from the developing powder particles and subsequent development of the electret image with additional toner particles.

As used herein an electret is a dielectric body possessing separated electric poles of opposite sign and of a permanent or semipermanent nature; it is the electrical analog of a permanent magnet.

The expression glass transition temperature is that temperature at which an amorphous polymer changes from a viscous or rubbery condition to a hard or brittle one, or vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. lA-lC schematically illustrate the development and fusing of an electrostatic image;

FIG. 2 illustrates a method of this invention for converting the applied developing powder to electrets; and

FIGS. 3A and 3B schematically illustrate the further development and fusing of the electrets formed in FIG. 2.

Corresponding reference characters indicate corresponding elements throughout the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS Turning now to FIGS. 1A--lC, there is schematically shown an insulative substrate 11 which can, for example, be of a polyester film, such as sold under the trade designation Mylar, or other suitable electrically nonconductive material. On the surface 13 of substrate 11 there are a plurality of positive charges 15 which form or constitute a latent electrostatic image thereon. Though positive charges 1-5 are shown, the latent electrostatic image can be formed of negative charges as well. If the electrostatic image is formed in an ionographic process, for example, the polarity of the charges will be dependent upon the polarity of the spaced-apart conductive plates utilized in the process, with the charges being of a polarity opposite to that of the conductive plate on which the substrate is disposed. Substrate 11 with the electrostatic image 15 formed thereon is then ready for a development process which normally comprises contacting the electrostatic image 15 with a cloud of fine particles 17 which have been charged to have a polarity opposite to that of the latent image. Thus, as shown in FIG. 1B, a cloud of negatively charged particles 17 are illustrated. Once again, this is a conventional technique in the development of electrostatic images. In the present invention, powder particles 17 are formed from fusible, polarizable resinous material which has a pigment therein, such as charcoal, lamp black, or the like. Typical such polarizable materials utilized include polymethyl methacrylate, polyethylene terephthalate, polytetrafiuoroethylene, polystyrene, and polyethylene. Additionally, materials such as paraflin wax, carnauba wax and other waxes are materials which may be formed or converted into electrets. Generally, it is preferred to use lower melting point resins or material which readily can be converted into electrets without deleteriously affecting the substrate material.

As shown in FIG. 1B, by utilizing toner or developer powder particles 17 which are charged to have a polarity opposite to that of the latent image, the powder will be directly attracted to the latent image and adhere thereto. As an alternate method of developing, the powder particles 17 are charged so as to have the same polarity as that of the latent electrostatic image. In that instance, the powder particles will be repelled by the latent electrostatic image and cover the surface '13 of the substrate 11 in the areas not carrying the electrostatic charges, thus in effect forming a negative image. I

Powder particles 17 on substrate surface 13- (as produced in FIG. 1B) are then heated to a temperature sufficient to fuse the resin particles together, forming a continuous mound 19 on the substrate surface 13 in the area of the latent electrostatic image as illustrated in FIG. 1C. This, thus, provides for a continuous line or a continuous dark area where the electrostatic image previously existed.

The sequence of steps in FIGS. lA-lC is old and wellknown in the art of forming and developing latent electrostatic images, and is used herein to comparatively illustrate the initial steps utilized in practicing the present invention.

The fused image formed in FIG. 1C, together with the substrate 11 to which it is 'fused, is then, in accordance with this invention, placed in an oven 21 as seen in FIG. 2. Substrate 11 is disposed between two

parallel plate electrodes

23 and 25, respectively, resting, for example, on the bottommost electrode 23. A power source is connected to these electrodes to provide a D0. electric field of about 30,000 v./cm., across a gap 28 between the plates. As shown in FIG. 2, the uppermost plate 25 is of negative polarity, while the bottommost plate 23 on which the substrate 11 rests, is positive. The polarity of the spaced-apart plates is established so as to produce or induce an apparent preferred polarity within fused resin material 19a such that the outer surface 29 of the fused material has a positive charge 31. This results from the distortion by the electric fiield of the electron distribution around the positive nuclei of the polymer molecules so that there is a net positive charge in the direction of the negative electrode. Materials which behave in this manner are referred to as being polarizable.

Similarly, fused mass 19a will have a negative polarity adjacent the substrate 11 due to the fact that the plate 23 is positive. In the embodiment shown in FIG. 2, the positive polarity of the fused mass on its surface is shown since the developing powder 17 as shown in FIG. 1B is negatively charged. If positively charged developing powder is to be utilized then the electric field would be reversed to cause the polarity of the fused mass at its surface to be negative. In other words, one purpose of this invention as indicated by FIG. 2, is to provide the fused mass with a plurality of charges or poles at its surface having a polarity opposite to that of the charged developing powder particles so that the additional developing particles will be attracted thereto for subsequent development. In order to accomplish this, and as indicated in FIG. 2, the fused mass is heated to a temperature greater than the glass transition temperature of the resin toner materials Tg utilized, but less than the glass transition temperature of the substrate 11 (Tg The temperature relationship is thus shown in FIG. 2. The thusheated material is subjected to the electric field and cooled thereby forming electrets of the fused resin material. Thus, in accordance with this invention, electrets are formed'from the fused resin particles 19a and thus the entire developed image becomes an electret image with an apparent polarity at its surface and charge sutficinet to attract additional developing powder.

The required temperature level is maintained in the oven while the field is applied for a period of time sufficient to accomplish the electret formation. This period can range from sec. to 30 min. and depends upon the viscosity of the material and the mobility of the molecules of polymer or other electret forming material. The proper length of time for subjecting the fused material to the electric field and its magnitude may be conveniently determined by running a series of tests at differing time periods.

Substrate 11 on which is formed the resulting electret image is then removed from the oven and further developed as illustrated in FIG. 3A where additional powder particles 17 contact the electret image 19a, being particularly attracted by the apparent positive charges 31 on the surface thereof. The additional toner particles 17 attracted to the surface are then subjected to a fusing temperature thereby forming an additional layer 33 on top of the previously fused electret image layer 19a. This serves to enhance the optical density of the resulting image since there is more developing powder at the side of the image on the substrate 11.

The optical density of the images formed is dependent on the amount or thickness of powder on the substrate surface since it is the powder particles that carry the pigment. In turn, the number of powder particles attracted is a function of the magnitude of latent electrostatic charges existing on the substrate. The magnitude of electrostatic charges that form the latent image is dependent on the degree of exposure used to produce them. In ionography, as indicated above, the process involves X-rays, or gamma rays striking a photoemittive surface. By reducing the radiation exposure, either as to the radiation intensity or the time of time of exposure, there will be fewer electrostatic charges formed on the substrate. In turn, the resulting optical image, though well-defined, may not be as dense as desired, nor as optimized, when the exposure is so cut down. But in accordance with this invention, developing powder is attracted to the latent electrostatic image on the substrate in an amount which corresponds to an optimum optical or visual image density. As a reduced radiation exposure is employed to produce the latent image and the resulting first fused layer 19 of toner particles, the optical density thereof is less than desirable. The density is then enhanced by the formation of an electret image and further deposition of a layer 33 of additional toner particles as seen in FIG. 3B.

The invention will be further illustrated by the following example:

EXAMPLE An ionographic image of an aluminum ste wedge was made and developed with toner, such as is available under the trade designation Xerox type 22 in a powder cloud apparatus and was then fused by exposure to the light from a xenon flashlamp such that the image received an energy of 8 watt sec/in. for about 1.5 msec. The image was then placed between two parallel aluminum electrodes. The image was formed on a .007 inch Mylar substrate. One-half of the image was backed with an additional strip of seven mil Mylar. A voltage of about 2 kv. was applied across the electrodes. The entire assembly was brought to a temperature of C. which is above the glass transition point of the toner, yet below the glass transition of the Mylar. The temperature conditions and voltage conditions were maintained for about twenty minutes. During this period, the electrode facing the toner image was negative. Then the assembly was cooled to room temperature while the field was maintained. The substrate and fused image were then subjected to additional development with the same Xerox toner and again flash-fixed. The portion of the image which was not backed by the strip of Mylar, and which was thus subjected to a higher field, was observed to have an increased optical density over the portion backed by the additional Mylar strip. Densitometry was performed on the resulting image. A set of neutral density filters were used to match transmission of both portions or halves of the image. The half of the image which received a reduced effect from the electrical field had a transmission of .23, while the other half appeared more intense and had a transmission of .17. These correspond to optical densities of .64 and .77, respectively. Thus, the additional development of the half of the image which resulted from the conversion to electrets gave a 20% increase in optical density.

Though the above invention has been described relative to use of a powder cloud of toner particles for development of the latent image, any other technique using charged developer particles would be applicable to the process of this invention. Thus, for example, liquid developers could be used where the charged toner particles are dispersed in a liquid medium.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process of claim 7, wherein said electrets comprise dielectric thermoplastic material adapted to be thermally fused to the substrate and are formed by:

heating said polarizable material as arranged in a pattern on said substrate to a temperature above the glass transition temperature of the polarizable material; and,

subjecting said heated polarizable material to an electrical field and cooling said material below its glass transition temperature thereby to form said electrets.

2. The process of claim 1 wherein said insulative substrate is a resin film having a glass transition temperature above that of said polarizable material and which further comprises heating said polarizable material to a temperature below the glass transition temperature of said substrate but above the glass transition temperature of said polarizable material.

3. The process of claim 1 further comprising placing said heated polarizable material between two parallel plate electrodes having a DC. voltage applied thereacross.

4. The process of claim 3 which further comprises applying the DC. voltage across the electrodes so that the polarity of the plate electrode adjacent the softened polarizable material will have the same polarity as that of said electrostatically charged developer powder particles to be subsequently used.

5. A process for improving the developing of a humanreadable image representation on a substrate comprising:

forming an image comprised of electrets adherently dispersed on a suitable dielectric substrate,

and contacting said electret image with electrostatically charged developer powder particles to adhere said particles to said electret image.

6. The process of claim wherein the electrets are thermally fused to a surface of the substrate.

7. A process for enhancing the development of a latent electrostatic image on an electrically insulative substrate comprising the steps of:

contacting said electrically insulative substrate bearing said electrostatic image with developer material comprising electrostatically charged powder particles of a softenable polarizable material to develop said latent electrostatic image,

softening said powder particles on said substrate;

forming said softened particles into electrets by subjecting them to an electrical field while in the softened state;

contacting said electrets with developer powder particles electrostatically charged and arranged to adhere to the so-formed electrets; and

fixing the last said powder particles to form an enhanced visible, usable image on this substrate. 8. The process of claim 7 wherein said developer material is the same as said developer powder.

9. The process of claim 7 wherein the charged powder particles of polarizable material are pigmented.

10. A process for improving the image quality of an image pattern of polarizable material disposed upon a relatively electrically insulative surface comprising:

subjecting the polarizable material to an electric field to form polarized electret material which presents an attracting field of a prescribed first polarity at the surface of the image pattern, this material being conditioned, while so subjected to this electric field, so as to accommodate this polarization; providing a quantity of developer powder particles, sufficient at least to contact a substantial portion of the polarized pattern and charged to a prescribed second polarity, opposite to said first polarity; and

further developing the polarized electret image pattern by dusting it with said developer particles so that these particles are electrostatically attracted to the surface of the pattern andretained thereon for view- 11. The process of claim 10 wherein the insulative surface comprises a dielectric material having a softening characteristic which is relatively unaffected by the forming of the polarized material and wherein the formation of the polarized material includes softening it while simultaneously subjecting it to said electric field.

12. The process of claim 11 wherein said developer particles are polarizable and further comprising the additional steps of softening said developer particles in contact with said electret image while simultaneously subjecting said developer particles to an electric field to thus form an additional, superposed electret image pattern adhered to the underlying electret pattern and then,

contacting the final uppermost electret pattern layer so formed with a further layer of electrostatically charged toner particles adapted to be electrostatically retained thereon and adhered thereto for viewing. 13. The process of claim 11 wherein said developer particles applied as the final particle layer are heatfused to the underlying electret particles to be relatively permanently retained thereon.

14. A method for enhancing the optical density of an image pattern formed with electrostatic toner particles on an electrically insulative substrate comprising the steps of:

selecting said toner particles to comprise polarizable, dielectric material, at least on the surface thereof;

forming at least a portion of said toner particles, in situ, as disposed in the said image pattern on the substrate, in at least the initial layer, into electrets, so as to exhibit sufiicient electrostatic charge to attract and retain a further layer of toner particles thereon, and, then,

applying at least one further layer of electrostatically charged toner particles onto this electret pattern, said particles being electrostatically charged and adapted to be attracted and retained on the electret pattern.

15. A product comprising an electrically insulative substate, a plurality of electrets fused to a surface of said substrate in a raised discrete image defining pattern, and a layer of fused developer powder particles covering said fused electrets.

16. An image rendition comprising an electrically insulative substrate; at least one layer of electret patriculate material disposed on this substrate and adhered thereto in a prescribed image pattern;

and a layer of dielectric particulate developer material covering at least a substantial portion of this electret pattern and adhered thereon to render a visible image.

17. An image pattern for viewing comprising an arrangement of material distributed and adhered upon an electrically insulative substrate in the pattern configuration, with at least the surface portion of this material being in a prescribed electret condition; and electrostatically charged developer material disposed and charged so as to be retained on at least a substantial portion of the material in this electret condiiton.

References Cited UNITED STATES PATENTS 3,576,624 4/1971 Matkan 96-1.3 3,364,020 1/1968 Fehlberg 96-1 R 3,005,707 10/1961 Kallmann et al. 96-1 R 3,597,073 8/1971 Grier 961.3 3,598,485 8/1971 Grier 96-1.3 3,666,365 5/1972 Tanaka et al. 96-1 R 3,519,461 7/1970 Stowell 117-175 OTHER REFERENCES Pillai et al.: Review article on Thermoelectrets and Their Application, Phys. Stat. 801., vol. 13a, pp. 341-51 (1972).

MICHAEL SOFOCLEOUS, Primary Examiner US. Cl. X.R.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION P'atent 3,827,905 Dated August 6, 197

Inventor 3 Walter Roth It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The tabulation of the claims, showing the patent claim number, 'the order in which the claims should have issued and the original application claim number is shown below:

4 Q r -iginal application Signed and sealed this 17th day of December 1974.

(SEAL) .Attest: I a a McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 U.S. GPVERNMENT PRINTING OFFICE! 9. 9 0

FORM PO-IOSO (O-69)-