CA1198148A - Electrostatic printing process - Google Patents

Abstract

TITLE
Electrostatic Printing Process ABSTRACT
An electrostatic printing process is disclosed wherein a printing roll is formed by forming a latent magnetic image in a magnetic imaging member, decorating the latent magnetic image with a non-conductive mag-netic toner to form a toner image, transferring the toner image to a conductive member and temporarily fix-ing the toner image to the conductive member. The toner image on the conductive member is electrostatically charged, while the charge is dissipated from the remain-ing area of the conductive member. The charged image of magnetic toner is decorated with an electrostatic toner comprising a colorant and a binder resin to form an electrostatic image which is transferred to a substrate.
Pattern change is accomplished by washing off the fused toner from the conductive print roll and forming a new fused toner image on it.

Description

TIT.~E
Electrostatic Printing Process Background of the Invention Field of the Invention The present invention relates to electrostatic printin~ rolls, and their preparation, b~ ma~netically forming an image of non-conductive toner on a conductive image bearing magnetic roll followe~ by transfer of the ton~r to a conductive substra~e to form the electrostatic printing roll. The areas of the electrostatic printing roll bearing the non--conductive toner are electrically charged while the charge is dissipated from the con ductive non-image areas. The charged ~on-conductive areas are decorated with an oppositely charged toner con-taining a colorant such as a dye or pi~ment, which toneris then ~ransferred to a substrate and Permanently fixed thereto.
Description of the Prior Art ~agnetic printing processes, particulaxly useful 20 in overco~ing ~he problem in ~lectrostatic copying proce-~ses of unsatis~actory copying of ~arge dark areas, are knDwn in the art. Such pr~ce~ses are described, for instance, i~ U.S. Pa~e~t ~os. 4,0g9,18~ and 4,117,498.
~he particular processes described in U~SO Patent 25 Nos. 4,099,1B6 and 4,117,498 relate to processes wherein a dye and/or other c~emical treating agent contain~d in a e~ro~agn~kic to~er is tr~nserred directly to a ~u~stra~e e.g~, such as a tPxtlle material, or is tra~s r ferred to a first substrate such as paper or subse~uent 3r trans~er to ~h~ ultimate su~stra e. However, all these techniques relied on removal of the resin and magnekic components of the toner rom the substrate after dyeing, hence, eliminating the US2 of this techni~u~ in the ~ig-ment printing of textiles.
More recently magnetic prin~ing has been used OR 5794 to form ~he resist when preparing printed circuits or printing plates by etching or ~lating, or to produce lithographic pxinting plates dir~ctly. Such processes are described in U.S. Patent Mos. ~,292,120 and 4,338,391 issued July 6, 1982.
Summary of The Invention The pr~cess of the present invention involves making electrostatic printing rolls by magnetography.
First a latent magnetic image is form~d on a conductive magnetic imaging memher. The latent magnetic image is decorated with a non-c~ndu~tlve magnetic toner and the toner transferred to a conductive roll. ~hen the toner is fused to the conductive roll. The fused non-conduc-tivP toner is then electrostatically charged with a suitable means such as a DC corona while the charge is removed from ~he conductive areas of the roll which are grounded. Then the electrostatically charged areas of the printing roll are decorated with electrostatic toner which is transferred ~o a subs~rate and permanen~ly f xed thereto. When a new image is to be printed, the toner image is removed from the conductive roll by washing it with a suitable resir~-dissolvirlg solvent drying and rePeating the above~de6cribed proces s .
~r:Lef Description of the Dra~ings Figure 1 is a srhematic view of the device used to form an image of magnetic toner on a. conductlve printing roll.
Figure 2 is a schema~ic view o~ a printer using three of the printing rolls prepared irl Figure 1.
Detailed Description o~ The Drawings Re~erriny now to Figure 1, a roll 11, surfaced with a conductive layer 12, which in turn is covered with a magnetic member 13, is rotated past a mag~etic decorator roll 14 fitted with mag~etic toner hopper lS~
After the magnetic toner has been applied to magnetlc ~ayer 13, by decorator roll 14, AC corona 16 serves to neutralize any elec~rostatlc charges which may he .3 attracting magnetic toner particle~ -to ma~netic imayiny member 13. Magne~ic toner part.icles which are on non image areas of magnetic imaging member 13 are removed by vacuum knife 17. The magnetic toner imaye is then transferred to conductive roll 18 by means of pressure and heat.supplied by lamp 19.
The conductive roll with the magnetic toner image is removed from the systemO If desired the magnetic toner image may be rurther treated such as with solvent vapors or heat to further coalesce the ~agnetic toner particles. Referriny now to Figure 2, a plurality of conductive rolls 18, 18', 18" with a ~on conductive magnetic toner image are mounted in a multi-stage printer. The magnetic toner areas of rolls 18, 18', 18"
are electrostatically charged by means o DC coronas 21, 22~ 23. Electrostatic toner is then cascaded over rolls 18, 18'~ 18" by decorators 24, 25, 26 to decorate the fused magnetic toner image thereon with electrostatic toner. A substrate 27 is unwound from roll 28 and passed onto endless belt 29 supported by rollers 31 and 32. As substrate 27 passes under rolls 18, 18' and 18", DC coronas 33, 34, 35 cause the tonex on rolls 18, 18', 18" to transfer to sub~trate 27. Toner which did not transfer to substrate 27 is neutralized hy AC coronas 36, 37, 38 and removed by vacuum brushes 39, 40 and 41.
After substrate 27 passes the la~t printing station the toner is fused to substrate 27 by heater 36. Finally sub~trate 27 is taken up on roll 42.
Detailed Description The magnetic imaging member used in the magnetlc printing step may be first magnetically structured and then selectively demagnetized in the background area~
by heating such background areas above the Curie point of the magnetic material in the magnetic imaging member to leave a latent magne~ic imàge. Alternatiwely the latent magnetic image may be formed in the mag~e~ic imaging me~ber by means of a magnetic write head.

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Preferably the magnetic imaging member .is magnetically structured to have from about 40 to 1200 magnetic lines per cm. As used herein, a magn~tic line contains cne north pole and one south pole. Preferably the magnetic imaging member is formed o a layer of acicular chromium dioixde in a binder on an electrically conductiv~
support. The acicular chromium dioxide layer generally is from 1,3 to 50 micrometers in thickness, and preferably is ~rom 4 to 13 micrometers in thickness.
1~. The magnetic imaging memb~r can be used either mounted in the form of an endless b~lt supported by a plurality of rolls or mounted on a cylindrical pri~tin~
roll. The imaging and toning steps are separate entities which do not need to be dvne consecutively in predeter-mined sequential ashion. For insta~ce, it may be desired to mount a preimaged magnetic imaging member on a printing roll.
ThP imaging member containiny the latent magnetic image is then brouyht into superimposed rela-tionship wi~h the conductive member to which the tonerimage is to be transferxed. At this point a DC corona, situated on the side of the conductive member away from the imaging member bearing the toner, causes the toner to tra~sfer to the conductive member. At this point the conductive member must be insulated from ground.
After being transerred to the conductive ~ember the toner is temporarily fixed to the conductive member. Generally this is most readily achieved by the application o~ heat which causes the toner particles ~o coalesce and become fused to each other as well as to the conductive member. Generally the application of pressure is unnecessary; but ~f pressure is to be applied the pressure applying means should be covered with a material to which the toner will not adhere, such as poly(tetrafluoroethylene).

If desired the magnetic toner can be transferred ~rom the magnetlc imaging member to an inkermediate ~ransfer member and then permanently applied ~o the con-ductive nember, such as described in U.S. 4,292,120.
The conductive member is then mounted in a suitable electrostatic printing apparatus. Generally the conductive member is mounted on a roller which in turn is part of an electrostatic printing machine.
Then the toner image on the conductive member is ~o electrostatically charged. This is most readily achieved by exposing ~he toner image to a DC corona, while electrically grounding the conductive member.
Alternatively the conductive member can be electrically charged and then discharged leaving the toner image electrically charged~
The charged toner image is then decorated with an electrostatic toner. This can be done with a magnetic brush where the ton~r particles are charged triboelec-trically or by charging the toner particles ill a cascade type decorator-The electrostatic toner is then transferred to a sub~
strate such as cotton, wool, polyester/cotton or their blends 7 paper or a ilm. This ca~ be done either electro~
statically or by application of pressure or heat and 25 pressure.
The magnetic toner particles fused to ~he con ductive roll preferabl~ are magnetic pigments encapsul~ted in a suitable binder. Generally the toner particles have an average size ranging from 10 to 30 microns with a 30 preferred average si~e ranging from 15 to ?0 microns.
Spherical particles such as prepared by spray drying are preferred because o~ their superior flow ~roperties which can be enhanced by the addition of minute amounts of a flow additive such as fumed silica. A ~her ~ crip~on o~ ~e preparation of toner par~icles may be found in U.S.
Pat. No. 3,627,682. When using ~he apparatus disclosad herein the toner particles should have a low electrical conductivity. If the particles have high conductivity, they will be passed back and ~orth between the drum and the paper causing a dif~use image and low transfer efficiency~ Generally the toner powder electrical con-ductivity is less than 1 x 10 13 mho/cm. The ferro-~magnetic component can consist o~ hard magnetic particles or a binary mixture of hard and sot magnetic particles.
The magnetically soft partisles can be iron or another high-permeable, low-remanence material, such as certain ferrites, for example, (Zn, Mn~Fe2O4, or p~rmalloysO
The magnetically hard particles can be an iron oxide, :l5 preferably ~e3O4, y-Fe2O3, other ferrite~, for example, BaFel2019, chi-iron carbide, chromium dioxide or alloys of Fe3O4 and nickel or cobaltO A magnetically hard sub-stanoe has a high-intrinsic coercivity, ranging generally from about 40 to about 40,000 oersteds and a high remanence (20 percenk or more of the satuxation magne-tization) when removed rom the magnetic field.
Such suhstances are of low permeability and require high flelds or magnetic saturation. A magnetically ~o~t substance has low coercivity, for example, one oersted or less~ high permeability, permittin~ saturation to be obtained with a small applied field, and exhibits a remanence of less than 5 percent of the saturation magnetization~ A particularly preferred toner has an average particle size of 20 microns and contains 40 weight percent thermoplastic binder 30 weight percent Fe3O4 (magnetite) and 30 w~ight percent soft iron (carbonyl iron).
The electrostatic toner particle~ used in decorat-ing the electrostatic printing roll are a colorant en~
capsulatad in a suitable binder. ~enerally the electro-static toner will have an a~exage par~icle size of from 15 to 20 microns . Spherical par ticles such as prepared by spray drying are preferred because of their superior ~L~4~

flow propexties. Generally the electrostatic toner will contain ~rom 1.0 to 20.0 wt.~ pigment and Erom 80.0 to 99.0 wt.% of a thermoplastic binder. Suitable pigments include copper phthalocyanine, halogenerated copper phthalocyanines, quinacridon~, quinacridone~
quinona, etc.
Example ~ A magne~ic imagin~ member formed of a 350~
(8.9~ meters~ thick layer of acicular chromium dioxide in a binder on an electrically grounded silver coated rubber roll which is 12 inches (0.3 meter) wide. The magnetic imaging member is magneticially structured to 460 pole reversals/inch 118 pole reversals/mm) or 230 cycles/inch (9 cycles/mm) or 55 microns per pole reversal b~ recording a square wave with a magnetic write head at 35 m Amps and 6 to 8 volts. A fllm positive of th~ image to be pri~ed i.s placed in contact with the magnetic roll and stepwise uniformly illuminated by a Xenon flash at 3.3 KV with a 15 turn per flash passing through the film positive, corresponding to the areas to be printed, absorb the energy o the Xenon flash; whereas the claar areas transmit the light and heat the aciculax chromium dioxide beyond its Curie point of about 116C thereby demagnetizing the exposed magnatiæed-lines of acicular chromium dioxide. A ~on~
conductive toner is fed ~rom a slot in a hopper to decorate the latent magnetic image by means of a decorator. The decorator comprises a rotating mag~etic cylinder inside a non-magnetic sleeve. As the magnetic imaging member rotates after being deeoxated with toner it passes an AC corona which serves to neutralize any electrostatic charges which may cause ~oner to adhere to the magnetic imaging member. Then a vacuum knife removes stray toner from the non-image areasO The tonex is then negatlvely charged with a DC corona. The toner is ~hen ~ransferred to a positively chaxged copper sheet having a polyethylene terephthalate ilm backing.

The toner is then Eused to the copper sheet. ~he copper sheet is grounded and th~ toner ~used there~o is posi~ively charged with a DC corona. ~n electrostatic toner is negatively charged and then poured over the side of ~he copper sheet to which ~he charges fused toner is adhe~ed. The negatively charged toner adheres to the charged fused toner and not to the yrounded background copper areas. ~ sheet of paper is laid over the toner and positively charged with a DC corona to effect transfer of the negatively charged toner to the paper. The toner is then ~used to the paper by heating.

Claims (6)

1. A process comprising forming a latent mag-netic image in a magnetic imaging member, decorating the latent magnetic image with non-conductive magnetic toner, transferring the magnetic toner to a conductive member to form a non-conductive toner image fused to said con-ductive member, electrostatically charging the non-conductive toner image, decorating the charged non-conductive toner image with electrostatic toner com-prising a resin and a colorant,transferring the resulting electrostatic toner image to a substrate.

2. The process of claim 1 wherein the colorant in the electrostatic toner is a pigment.

3. The process of claim 2 wherein the conduc-tive member is a metal printing roll.

4. The process of Claim 3 wherein the substrate being printed is a textile material.

5. The process of Claim 4 wherein the textile material is cotton, wool, polyester or blends thereof.

6. The process of Claim 3 wherein the sub-strate is paper or a film.