US3766072A

US3766072A - Edge and latitude developer

Info

Publication number: US3766072A
Application number: US00209858A
Authority: US
Inventors: K Metcalfe; I Smith
Original assignee: COMMW OF AU
Current assignee: COMMONWEALTH OF AU AU; COMMW OF AU
Priority date: 1968-12-30
Filing date: 1971-12-20
Publication date: 1973-10-16
Anticipated expiration: 1990-10-16
Also published as: AU4848268A; BE743883A; DE1965362B2; CA918985A; DE1965362A1; FR2027343A1; DE1965362C3; AU425595B2; GB1296999A; NL6919431A; SE359174B

Abstract

A DEVELOPER FOR DEVELOPING ELECTROSTATIC IMAGES ON SURFACES. THE DEVELOPER CONTAINS AT LEAST TWO TYPES OF PARTICLES, ONE OF WHICH IS AN INSULATOR AND IS PREFERABLY OF FINER SIZE AND THE OTHER OF WHICH IS A MORE CONDUCTIVE PARTICULATE DEVELOPER AND PREFERABLY OF COARSER SIZE WITH POSSIBLY A FURTHER PARTICULATE MATERIAL WHICH HAS ONE END INSULATING AND THE OTHER RELATIVELY MORE CONDUCTIVE OR POLAR WHEREBY THE MORE CONDUCTIVE PARTICLES TEND TO DEVELOP THE EDGES OF THE IMAGE AND THE INSULATING PARTICLES THE INTERMEDIATE PORTIONS BETWEEN THE EDGES BUT WHEREIN THE INSULATOR PARTICLES ENABLE THE RELATIVELY MORE CONDUCTIVE PARTICLES TO BE RETAINED ON A SURFACE BY A LIMITING CHARGE TRANSFER OF THE RELATIVELY MORE CONDUCTIVE PARTICLES TO THE SURFACE CONTAINING THE THE ELECTROSTATIC IMAGE, AND THE THIRD TYPE OF PARTICLE CAN ROTATE AS A STIRRER.

Description

Ct. 16, 19Y3 L K A METCALFE ET AL 3,766,972

LDGH AND LA'IL'IUDE DEVELOPER Original Filed Dec. 20, 1969 U nitcd States Patent U.S. Cl. 252-621 6 Claims ABSTRACT OF THE DISCLOSURE A developer for developing electrostatic images on surfaces. The developer contains at least two types of particles, one of which is an insulator and is preferably of finer size and the other of which is a more conductive particulate developer and preferably of coarser size with possibly a further particulate material which has one end insulating and the other relatively more conductive or polar whereby the more conductive particles tend to develop the edges of the image and the insulating particles the intermediate portions between the edges but wherein the insulator particles enable the relatively more conductive particles to be retained on a surface by a limiting charge transfer of the relatively more conductive particles to the surface containing the electrostatic image, and the third type of particle can rotate as a stirrer.

CROSS-RELATED APPLICATION This application is a continuation of copending application Ser. No. 889,138 filed Dec. 30, 1969, and now abandoned.

SUMMARY OF THE INVENTION This invention relates to electrostatic printing and in particular to the development of electrostatic images by the deposition of particles from gases or liquid vehicles. It is now well known to develop electrostatic images by subjecting them to a body of gas or liquid containing suspended particles and to cause the particles to be attracted to the surface of the receiving sheet in image form.

Electrostatic images produced by X-ray or gamma ray means require continuous tone development while at the same time they require high contrast and sharp delineation of line images. These requirements are conflicting and require special development.

One object of the present invention therefore is to produce a developer which operates so as to give high contrast images where there are sharp changes of X-ray intensity such as occur for a sudden change of thickness, and also simultaneously give a very low contrast for slow changes of X-ray intensity. These features enable details to be shown with a high contrast over a wider range of thicknesses in the object being examined than can be done with normal radiography.

Normal electrostatic images however require a more uniform development and edge effects require to be avoided where for instance a uniform voltage requires to be developed with a uniform deposition.

A further object of the invention therefore is to allow control of development to allow this uniform development to take place if this is required.

Another object of the present invention is to prevent deposition of developer in background non-image areas.

These objects are achieved by incorporating in the developer at least two main groups of developer materials, the first of which are relative conductors, or semicon- 3,766,072 Patented Oct. 16, 1973 ductors for edge development, the other being insulating particles which will migrate in the electric field and be attracted to areas to be retained at such area.

Thus chain forming particles may be used which comprise pigment-resin aggregates made by precipitating resins or the like in solvents of low solvent power for these resins and comprising particles of a range of particle sizes. The aggregate size is controlled by the addition of a dispersant or solnbilizing substance which is more soluble than the first resin in said solvent of low solvent power and is designed to increase or reduce the size of the aggregates of the first resin because of its solubility for the first resin and its miscibility with the solvent or because of its wettability for the particles of the first resin.

BRIEF DESCRIPTION OF THE DRAWING For better understanding of the invention reference should be made to the accompanying drawings in which:

FIG. 1 illustrates a series of tests made with both insulating and conductive materials and compound insulatorconductor materials when suspended in an insulating fluid and subjected to a unidirectional field,

FIG. 2 shows diagrammatically how a sheet of photoconductive material bearing a latent electrostatic image can be developed to give for instance an edge effect, and

FIG. 3 shows a particle of developer showing how it can be coated to control the required action. The particle could be conductive and the coating insulating, or vice versa.

DETAILED DESCRIPTION Referring first to observations we have made of the behavior of pieces of insulating material, relatively conductive material, and compound pieces, suspended in insulating liquids through which an electric field is applied as indicated in FIG. 1.

In the example tested, an insulating liquid 1 was prepared which comprised a mixture of Freon 113 and Isopar H. In this liquid a piece of insulating material 2 was suspended between a pair of electrodes 3 and 4 so that it floated in the liquid. An electric field in the range 1000 volts per inch to 10,000 volts per inch was applied and it was noted that the insulating piece migrated to one electrode and remained there as shown in A of FIG. 1.

In the second test we used a conductive piece 6 or a semiconductor piece and also we used insulating material coated over its whole surface with a layer of conductive material such as graphite, or wetted with a relatively conductive material such as water or glycerol triacetate. Such a conductive piece will migrate to the electrode nearest to it at the time of application of the field and then dissipate its charge and then acquire the same polarity of charge as the electrode and be repelled by this electrode and will oscillate between the electrodes. This is shown in B of FIG. 1.

In a third situation as shown in C of FIG. 1, the insulating piece 7 is coated with conductive material on one side only and it is found that the piece will migrate to the nearest electrode with the conductive of high dielectric constant side turning to a foremost position. Contact of the conductive end with the electrode results in repulsion of the piece which then migrates to the op posite electrode, again turning over, and the process is repeated.

It is important to note at this stage that the period of oscillation in the case of pieces B and C of FIG. 1 can be varied by either insulating the particle or the electrode, the thickness of the insulation controlling the time it takes to reverse the charge on the insulator. Thus a thin insulator increases the speed of reversal of direction of migration of the particle and thus where normally a conductive piece would oscillate, it is found that by inserting a thin insulating film between it and the electrode which it is approaching, results in delay in sharing of charge with the electrode and consequently, delay in repulsion. A similar insulating piece coated with a layer of semiconductive material migrates backwards and forwards in a similar way to the conductive piece but is characterized in that it pauses in oscillation after arrival at each electrode as if an insulating barrier existed on the electrode. Thus it follows, and this is an important effect of this invention, that while insulator particles can be readily built up on a latent image, conductive particles can first be moved down and then repulsed, so that the degree of insulation or conductivity now allows a selection of the type of effect desired. By mixing the particles agitation effects on the insulators can also be achieved.

Thus insulator particles can be more readily retained on a charged surface and will continue to build up according to charge level. Conductive particles will however migrate onto and then off the surface but the rate can be controlled by an insulating film.

Such an insulator film could be built up on the image by fine insulator particles in the developer to then allow large conductive or semiconductive particles to be retained due to these particles then being prevented from rapidly transferring charges they have acquired.

Conductive particles are more strongly attracted to a field of sutficient intensity but have a quicker charge transfer and leave the area quicker if the field strength is increased.

Non-conductive particles move slower in the field but when they deposit, provided the field is strong enough, they are held in place.

It must be appreciated that conductive particles when deposited on a surface allow lateral spread of the field, and thus by short development the condition of FIG. 2, namely edge development is achieved.

If development time is extended the conductive particles held in the weaker field areas extend the field and this field straightening efiect then allows relatively uniform development to occur.

if ions or migrating particles of substantially different size and polarizability are injected into the system, for example by adding iodine, metal naphthenates or other chain forming compounds which constrict the field, there is a signicant change in the type and velocity of migration of the particles or pieces, conductive pieces tend to remain stationary, insulating pieces tend to migrate to one electrode and stay there, pieces with regularly arranged surface spots of conductive coating also tend to remain stationary whereas pieces with conductive coatings predominantly on one end rotate in a stationary position. This is shown in D in FIG. 1.

In either system particularly where the pieces or particles have a high dielectric constant a situation readily arises in which fine articles will coat on to coarse particles even should the basic material be of the same composition. Mechanical agitation also can effect deposition and coating. Thus it is seen that the migration of pieces depends on their composition, conductivity, dielectric constant, anddistribution of surface properties as well as the magnitude and direction of the electric field.

All of the effects observed in these experiments show that there is increased activity in the cell as the electric field is increased. These experiments demonstrate that particle behavior as induced by high electric field intensity is not predictable by known particle transport mechanisms such as an electrophoresis or dielectro phoresis.

We have applied these phenomena to enable deposition to be made in addition to the conventional technique which provides continuous tone development. Thus the conductive particle which is shown to oscillate between the conductive electrodes and also to adhere to an in- 4 sulated'electrode is used in the developer to provide edge accentuation of the developed image.

The third experiment in which an insulating piece is used having a coating of a conductive material at one end is analogous to an aggregated resin which has been initially dissolved in a polar or relatively conductive solvent. In the case of development of radiographic images the abrupt changes in section of an object being radiographed produces an electrostatic image characterized by a high potential difference across the section and consequently a localized high field intensity.

From these observations we have learned that in order to carry out the present invention it is necessary to produce aggregates of particles which will be attracted initially only to a relatively strong electric field and therefore tend to exaggerate edges of images when these images are developed face up, but in addition the developers must contain particles of sufiiciently small size, or particles of non-polar type, which will develop weak image fields.

In one form of this invention the aggregate is obtained by precipitating a resin from solution in a good solvent by putting this solution into a poor solvent. For example if the resin is dissolved in an aromatic hydrocarbon, precipitation is obtained by injecting the solution into an aliphatic hydrocarbon containing various percentages of aromatic hydrocarbon for example from zero to 50% and it is found that the aggregate size increases with lower aromatic content until total precipitation occurs, depending on the type of resin. If it is desired to produce a final developer in a hydrocarbon solvent containing zero aromatics, the solubility of the solvent for the resin and hence the aggregate size may be refined by means of other resins and solubilizing agents which have an aflinity for both the final resin and also the solvent.

Pigmentation or coloration of the aggregates is achieved by incorporating a pigment or dye in the original resin solution prior to precipitation.

The fine material necessary for development of small charges or fields, is obtained either by parting of particles from aggregates or clusters or by separate additions to the concentrated developer of fine particle size material.

The following examples will serve to illustrate the invention:

EXAMPLE 1 A developer concentrate is prepared by first dissolving a relatively soluble resin such as Solprene 1205 in an aromatic hydrocarbon solvent such as Esso and also a relatively insoluble resin such as V.T.A.C. (vinyl toluene acrylic copolymer) in a similar solvent and milling these together with a colorant such as Microlith Black CT and a small proportion of Microlith Blue. The proportions of the Solprene 1205 in relation to the V.T.A.C. may for example be 3 parts of Solprene to 2 parts of V.T.A.C. by weight, and the proportions of pigment to those of the total resin may for example be 1 part of pigment to 1 part of resin by weight. A typical formulation is as follows:

Grams Solprene 1205 resin (styrene-butadiene resin of Phillips Corp., USA.) Pliolite (V.T.A.C. resin, Goodyear Corp., USA.) 60 Esso 100 solvent 200 Microlith Black pigment (C.I.B.A.) Microlith Blue pigment (C.I.B.A.) 25

The developer concentrate made in this way is then dispersed in an aliphatic hydrocarbon solvent such as Isopar G with vigorous stirring to form aggregates of particles in the proportions of 1 gram in 100 milliliters. It will be clear that as the amount of concentrate is increased in initial proportion to the carrier liquid that changes will occur in aggregate size because of increasing solvent power of the total liquid. The preferred range is 0.5 to 2 grams per 100 milliliters.

In the above example it should be noted that the styrene butadiene resin is a solvent for the V.T.A.C. resin and these-co-deposit in the Esso solvent as a fine developer of insulating particles.

The Microlith Black pigment has relatively conductive particles and the Microlith Blue somewhat poorer conductive particles.

The insulating resin particles do not Wet the conductive particles but co-exist in the insulating carrier liquid so that the conditions illustrated in MG. 2 are present and short term development of a latent image tends to accentuate edgeeifects as the larger conductive particles tend to build up quicker in the stronger edge fields but as development continues the conductive particles deposited on the insulator particles at the originally weaker field give lateral spread to develop more on the basis of the voltage gradient shown. In FIG. 2 a vessel 8 has in it the insulating carrier liquid 9 and a photoconductive sheet is shown on which is a latent image indicated by the voltage gradient line 11. The fieldgradient is shown by the dotted line 12.

The particle of FIG. 3 is designated 13, and has on it a film 14.

EXAMPLE 2 A typical formulation is as follows:

. Grams Solprene 1205 resin l6 Mineral turpentine solvent 40 Added to:

Elvacite 2010 resin 4 (The Solprene resin is the solvent for the Elvacite resin.)

T richlorethylene 4 And then added to:

Microlith Black CT (C.I.B.A.) 17

Microlith Green (C.I.B.A.) 3

Both of these are relatively conductive but are lightly coated and thus are partly insulated.

EXAMPLE 3 A developer concentrate is prepared as in Example 1 but the pigment is replaced by Coates Hydrocarbon dispersible flake black. This pigment is more conductive than the pigments of Example 1.

EXAMPLE 4 A developer concentrate is prepared as in Example 2 but the Elvacite (methyl methacrylate) resin is replaced by a Melamine resin. This is even more insoluble than the Elvacite resin and thus results in coarser insulating particles.

. EXAMPLE 5 A developer concentrate is prepared as in Example 1 but the Solprene 1205 resin is replaced'by Pliolite SSD (Goodyear Corp., U.S.A.). This again results in grain coarseningbecause it is a poorer solvent for the other resin.

EXAMPLE 6 i A developer concentrate suitable for use in Isopar G or E carrier liquidsis prepared from the following materials:

Grams Brillfast Black pigment 24 Solprene 1205 resin 18 Ethyl hydroxy ethyl cellulose resin 6 Solvesso solvent 48 These materials are blended in a bar mill at 40 psi. The Brillfast pigment is more conductive than the coated black pigment.

EXAMPLE 7 To Example 1 is added:

.005 by weight of solids of zirconium octoate or .001% of cobalt octoate or .001% of iodine, or

.001% of lead naphthenate to provide rotating particles.

IDENTIFICATION OF TRADEMARKS AND TRADE NAMES Solprene 1205 is a block copolymer of butadiene and styrene in the ratio of 75/25 manufactured by a solution of polymerization process by Phillips Corp., U.S.A., A.S.T.M. No. 1205, characterized in that the majority of the styrene molecules are added as polystyrene at the end of a long chain of butadiene units.

Pliolite VT Resin is a styrene/butadiene type copolymer rubber made by the Goodyear Corp., U.S.A. and prepared by the G.R.S. method in which the butadiene polymerizes in the main by a 1,4-addition. Pliolite VT is a vinyl toluene/butadiene random copolymer rubber, soluble in mineral spirits.

Pliolite SSD is a styrene/butadiene copolymer, KB value 60, manufactured by Goodyear Corp., U.S.A.

Pliolite V.T.A.C. is a vinyl toluene/acrylate copolymer, KB value 36.

Esso 100 Solvent is a hydrocarbon solvent supplied by Esso Chemicals Australia Limited, having an aromatic content of 98%, flash point of 108 F., and distillation range 159182 C.

Microlith Pigments comprise a pigment and a resinous carrier. Microlith Black pigment contains pure neutral carbon black together with a toluene soluble carrier resin such as Stabilite Ester 10 of the Hercules Powder C0., U.S.A.

Microlith Blue 4GT comprises a stable phthalocyanine blue pigment with a greenish cast together with Stabilite Ester 10 resin.

Microlith Green GT comprises a medium shade of phthalocyanine green together with Stabilite Ester l0 resin, the microlith pigments are manufactured by Ciba C0., Switzerland.

Color Index of the pigments:

Microlith Blue-Color Index No. 74160 Microlith Green-Color Index No. 72455 Elvacite Resin referred to in Examples 2 and 4 is an acrylic resin manufactured by Du Pont, Delaware, U.S.A.

Coates hydrocarbon dispersible flake black comprises pure carbon black together with ethyl hydroxy cellulose resin.

What is claimed is:

1. A developer for developing electrostatic images on a surface comprising a carrier liquid medium having an electrical resistivity sutficiently high to prevent destruction of a latent electrostatic image during development, developer particles suspended in said carrier liquid medium and including at least two types of particles, the first of said two types being insulator particles and having an inherent charge but being electrical insulators unable to exchange charges with the surface during development and the second of said type types being relatively more conductive polar particles whereby charges can be exchanged with a latent image bearing surface, the insulator particles adhering to a surface having an appropriate image field polarity without repulsion because of the inability to transfer charges, the relatively more conductive polar particles acquiring the image field polarity and exchanging charges with the surface to accept the surface image charged to be repulsed thereby by acquired similar charge characteristics unless insulated from the surface by attracted insulator particles, said insulator particles being smaller than the more conductive polar particles whereby the insulator particles tend to deposit on lower image charge areas, the more conductive polar particles tending to deposit with some of said insulator particles on higher image charge areas, the more conductive polar particles being chain forming and comprising pigment-resin aggregates which are precipitation products of resins in combination with pigments in solvents of low solvent power for the resins.

2. A developer according to claim 1 including ions or migrating particles of substantially different size and polarizability selected from the group consisting of iodine, metal naphthanates and metal octoates in the developer to reduce the velocity of migration of the developer particles.

3. A developer according to claim 1 including added insulating particles, having one side conductive comprising aggregated resin which has been initially dissolved in a polar or relatively conductive solvent, and including ions or migrating particles of substantially dilferent size and polarizability selected from the group consisting of iodine, metal naphthanates and metal octoates in the de- 8 veloper to cause the added insulating particles to rotate within the developer carrier fluid.

4. A developer according to claim 1 wherein the insulator particles comprise pigment particles coated with a block copolymer of butadiene and styrene in the ratio of about percent butadiene and 25 percent styrene, the styrene molecules being in the form of polystyrene at the end of long chain butadiene units.

5. A developer according to claim 1 wherein the insulator particles comprise pigment particles coated with a styrene/butadiene copolymer.

6. A developer according to claim 1 wherein the insulator particles comprise pigment particles coated with a vinyl toluene/acrylate copolymer.

References Cited v UNITED STATES PATENTS Ricker 252-62.1X

ROLAND E. MARTIN, Primary Examiner