US 3038799 A
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J ne 12, 1 6 K. A. METCALFE ET AL 3,038,799
METHOD OF REVERSING THE IMAGE IN XEROGRAPHY Filed Dec. 50, 1958 3,033,799 METHOD OF REVERSlNG THE TMAGE IN XEROGRAPHY Kenneth Archibald Metcaife, Graymore, South Australia, and Robert John Wright, Hector-ville, South Australia, Australia, assignors to The Commonwealth of Australia, The Secretary, Department of Supply, Melbourne, Victoria, Australia Filed Dec. 30, 1958, Ser. No. 783,981
Claims priority, application Australia Jan. 13, 1958 6 Claims. (Cl. 961) This invention relates to a method of reversing images in xerography.
In silver halide photography it is customary when producing films or the like to reverse the image so that it is not necessary to make a positive from a negative film, the one film being all that is necessary.
At the present time there is no corresponding process available in xerographic work which will efficiently and effectively allow reversal of the image, and while it is possible to develop a film by xerographic methods in any particular color, there is nevertheless the disadvantage that color reversal on a given type of base is not effectively possible.
It is therefore the object of this invention to provide for xerographic methods, a system of reversal which will allow the image to be effectively produced in reverse colors and moreover which will allow this to be carried out in a simple and relatively cheap manner.
Briefly our invention consists in charging a xerographic base on a film or paper or the like in the normal manner and to modify the charge by exposure to light or the like after which the image is developed with a liquid developer which forms a modifying layer to subsequent charging, the developer preferably being non-permanent so that subsequent removal of this developed image can be carried out with little difliculty.
The developed image is now given a blanket charge without being subjected to light and is then re-developed with a permanent developer, the original developed image being subsequently removed if required.
The invention depends for its action on the phenomena that when an image has been developed by xerographic methods, the developer, if suitable, can act as an electrical modifying layer with respect to the re-charging of the developed areas, acting to cut off the charging either completely or partly according to the thickness of developer, and it then follows that, provided the developer can be subsequently removed, charging takes place after development in a selective manner depending strictly on the amount of coverage of the underlying photoconductor surface by the developer, and in this way the image appears in a charged form which is the reverse of the original charged areas.
Having obtained this second charge on the photoconductor base, it is then a simple matter to redevelop the image with a permanent developer and to remove the first developed image from the base.
The advantages of such an invention will at once be realized when it is pointed out that reversal of an image can take place on a film or any other base having a photoconductor surface thereon or which is otherwise charged to be receptive of developer means according to the applied pattern.
Once the image has been developed it then forms a modifying layer which prevents charging of the base material on the surface below and this image then forms the effective controlling medium when recharging to ensure that those areas which were previously charged will not now be charged but previously uncharged areas will be charged to take the second developer, the usual continuous tone being possible because the proportion of 3,938,799 Patented June 12, 1962 developer deposited in the first development will control the proportion of the charge which can be accommodated on the base in the second.process.
To enable the invention to be fully understood, embodiments will now be described with reference to the accompanying schematic drawings, in which:
FIG. 1 shows a complete process in a series of steps, the development being shown as for line reproduction, and
FIG. 2 is a view corresponding to E" of FIG. 1 but showing a continuous tone reproduction.
A base 1 has on it a photoconductor surface 2 of any usual or approved type such as Zinc oxide particles suspended in an electrically insulating base.
As shown in A of FIG. 1, the surface 2; is first charged by placing it beneath a charging device comprising a series of points 3 on a back 4 to which the necessary charging potential is applied.
In B is shown how an image may be projected on to the phctoconductor surface 2 by means of a projector 5, the light pattern bleeding away differentially the charge which was previously applied to the surface 2 in step A.
In C is shown how the developer may be applied by rolling over the surface with a roller 6 carrying a supply of the developer, this developer being differentially applied under influence of the electrostatic pattern on the surface 2. The modifying layer developer is designated 7.
Step D is now to recharge the surface 2 by again placing it beneath the charging points 3 of the back 4, the developer 7 now modifying the charge acceptable by the surface 2 by acting as an electrical blanket.
Step E depicts redevelopment of the surface 2 with a developer roller 8 carrying the pigment required for the final development, the developer being designated 9. The areas where this developer 9 is applied lie between the areas of the developer 7 where the photoconductor surface was exposed by the absence of the modifying layer developer, and the surface was therefore able to accept the second charge.
In step F the developer 7 has been removed, and a reverse image, consisting of the image developer 9, remains.
In FIG. 2 the developer 7 is applied as a continuous tone developer, that is the developer varies in thickness in proportion to the intensity of the electrostatic charge and therefore when applying the image developer 9 this also will be applied in various thicknesses. It is to be noted that where the developer 7 is thin such as the area designated 10 the image developer 9 is thick, gradual gradation being possible.
The developers for the modifying layers. generally must be substances which are sufficiently electrically conductive to dissipate a charge under normal operating conditions and therefore such substances, after they are deposited and at the stage where recharging is effected, should have an electrical resistance of less than 10 ohmcentimetre.
The following examples show various developers as well as solvents and generally detail how the invention is carried out.
METHOD 1 Reversal by Use of Metal Powder The photoconductor surface is charged and exposed and developed in a metal powder suspension, the powder of which is attracted to the charged areas of the paper. That is the image obtained is a facsimile of the original.
The surface is then recharged, having been kept in the dark in the meantime to ensure holding of the powder, and because of the conductive nature of the discrete metal particles of the image just developed, only takes a developable charge in those areas which are not covered by the metal powder.
The paper is then developed in a developer of the required color, which develops as a reversal of the original.
The metal powder of the first development may then be removed by rubbing or adhesive transfer or solution by, for example nitric acid solution in water).
The metal powder may be aluminum or bronze of the desired grain size, such as 1 micron average, dispersed in an aliphatic hydrocarbon solvent such as shellite," or chloro-fluorinated hydrocarbons such as those sold under the trade name of Freon, for example Freon 11 or Freon 113 or mixtures of these. Normally 1 gram of aluminum powder dispersed in 100 grams of carrier liquid will form a developer of suitable concentration. The metal powder should be dried before dispersion.
The second developers should preferably be dark in color to obtain the maximum of benefit from the process, and will be generally referred to herein as permanent developers.
Suitable developers are as follows.
(a) Blue developer:
Phthalocyanine blue 100 Resin Rhodene L9/5O Resin Pentarol 15 Kerosene Resin Rhodene L9/50 is the trademark of a linseed This acts as the control agent as it is sparingly soluble in the carrier liquid and this remains on the particles.
Resin Pentarol 20 is the trademark of a phenol modified penta-erythritol ester of rosin.
Acid value, 7-15 Specific gravity at 20 C., 1.09 Melting range, 1l0l20 C.
This acts as the fixing agent as it is more soluble and thus acts as the fixer when the liquid is evaporated.
The Pentarol 20 is digested in the kerosene, and when solution is complete, the Rhodene L9/5O is added. This developer base is then milled with the phthalocyanine blue pigment preferably in a triple roll mill. The resultant paste forms a developer concentrate which for use is diluted with a carrier liquid of volume resistivity greater than 10 ohm-centimetre and a dielectric constant of less than about 3, such as the aliphatic hydrocarbon solvents, cyclohexane, and the chlorofluorinated hydrocarbon solvents, carbon tetrachloride and the like, in the proportion of 0.5 gram paste to 100 mls. carrier liquid.
These pastes are prepared as previously described and in use 0.5 gram of black paste and 0.1 gram of red paste are dispersed together in 100 ml. of carrier liquid.
(0) Yellow developer: Grams Lead chromat 100 Rhodene L9/50 l5 Pentarol 20 15 Kerosene 25 4 Paste prepared as before. In use 0.5 gram of paste is dispersed in ml. of carrier liquid.
Intermediate colors can be obtained by the admixing of these developers in any desired proportions.
METHOD 2 Reversal by Use of Water Soluble Dye In this instance the paper or plate is charged and exposed as in Method 1, but the first development is carried out by use of a suspension of a water soluble dye in the carrier liquid. The powdered dye is preferably ground into the carrier liquid. Suitable dyes for this purpose include the so called white dyes" used in the textile and other industries such as Tinopal 2B (Tinopal is registered trademark of I. R. Geigy, SA.) and dyes used in the photographic printing industry such as that known under the trade name Tartrazine N-200 (a water soluble yellow dye sold in powder form by 1.0.1.).
This development gives a facsimile of the original and may be used in the proportion of 5 grams to 100 grams of the solvent naphtha.
After this first development the paper is once again recharged, again having been kept in the dark, and redeveloped in one of the permanent developers previously described for this purpose.
Once again development only occurs in those areas not covered by the first development, due to conductivity of the dye particles.
The end result is once again a reversal of the image.
The dye particles may be removed if desired by immersing the paper in Water.
Spirit soluble dyes may be used instead of the water dyes, providing these are chosen to be attracted to the charged areas of the paper during the first development to give a facsimile of the original on the first development.
METHOD 3 Reversal by the Use of a White Resinous Developer for the First Developer Steps are the same as before in that paper is charged, exposed and developed, followed by recharging and redevelopment. In this case the first development is carried out using a white developer, which may be a triggered developer, the characteristic of this being its ability to form a modifying layer-of varying resistance to the recharging and redevelopment cycle so that continuous tone images may be simply reversed. In this case the first image produced is a white reproduction of the original on a white background, however the developed image is such that it allows either selective recharging, or redevelopment, or adherence of the redeveloped image.
The second or permanent development may be carried out using any of the developers already listed for this purpose.
The first development can be carried out in a white developer, the ones detailed below being typical preferred formulations.
(1) White triggered developer:
Paste (a)-- Grams Zinc white 75 Linseed oil 25 Paste (b)- Barium white 70 Linnseed oil 30 These pastes are separately milled in a triple roll mill and in use 0.4 gram of each paste is dispersed in 100 ml. of the carrier liquid of suitable electrical characteristic as already described, but in this case for maximum effect it is desirable to use either the aliphatic hydrocarbon solvent shellite or gasoline as the carrier liquid in which the linseed oil fixer substance is soluble.
(2) Alternative white developer (not triggered):
Grams Titanium dioxide 50 Rhodene L42/70 18 Power kerosene 12 These are milled together in a ball mill for 24 hours. After milling the following are added:
The resultant cream is used in the proportion 1 gram of cream to 100 ml. carrier liquid, such as the aliphatic hydrocarbon solvent shellite, of suitable electrical characteristic.
Rhodene L42/ 70 is a safflower oil modified pentaerythritol alkyd resin of the following specification:
Type of oil Safilower. Oi'l length 64%.
Acid value 6-10.
Sp. gr., 20 C 0965-0975.
This is a slower drying resin permitting the first developer to be removed within a reasonable time such as 10 minutes, but as a fixer is used in both of these developers, it is not so essential to keep the surface in the dark between first and second development.
The second permanent development can be, as said, carried out after use of either of the above while developers and any of the more permanent redevelopers previously mentioned may not be used.
In some cases it may be found that the second development covers more or less completely the image of the first development. However, should this occur the excess can be removed either by vigorous washing in clean carrier liquid, or by first drying this print and then rubbing off the excess material with cotton wool or the like.
1. The method of reversing the image in xerography comprising the steps of placing a first charge on a photoconductor surface in the absence of light, exposing the said photoconductor surface to a light image to produce a selective electrostatic pattern wherein the magnitude of the latent electrostatic image is proportional to the image density, developing the said electrostatic pattern in the absence of light with a developer comprising a substance which has a volume resistivity of less than 10 ohm centimetre and which is suspended in an electrically insulating carrier liquid having a volume resistivity of more than 10 ohm centimetre whereby deposition takes place from the said carrier liquid of a modifying layer in thickness depending on the magnitude of the said latent image which thereby proportionally shields the photoconductor against recharging, maintaining the said modifying layer in place while recharging the said photoconductor surface in the absence of light whereby the recharging produces an electrostatic latent image opposite to that first produced, and developing the second said image in the absence of light.
2. The method of reversing the image in xerography according to claim 1 characterized by the further step of removing the modifying layer after the last developing step.
3. The method of reversing the image. in xerography according to claim 1 characterized in that the modifying layer developer is a metal powder.
4. The method of reversing the image in xerography according to claim 1 characterized in that the modifying layer has an oil base.
5. The method of reversing the image in xerography according to claim 1 characterized in that the modifying layer has a resin base.
6. The method of reversing the image in xerography according to claim 1 characterized in that the modifying layer substance is water soluble.
References Cited in the file of this patent UNITED STATES PATENTS 992,898 Payne May 23, 1911 2,647,464 Ebert Aug. 4, 1953 2,758,525 Moncrieff-Yeates Aug. 14, 1956 2,817,598 Hayford Dec. 24, 1957 2,817,765 Hayford et al Dec. 24, 1957 2,868,642 Hayford et al. Jan. 13, 1959 2,907,674 Metcalfe et al. Oct. 6, 1959 2,914,403 Sugarman Nov. 24, 1959 2,939,787 Giaimo June 7, 1960 2,940,847 Kaprelian June 14, 1960 FOREIGN PATENTS 203,907 Australia Nov. 1, 1956