EP0040128B1 - Method for electrographic reproduction on any base by means of a magnetic monocomponent development toner - Google Patents

Abstract

This invention provides a process of electrographic reproduction onto an arbitary support, whereby an image of a magnetic, monocomponent developing powder is transferred, under the influence of electrical means, onto a support covered with a thin layer of volatile dielectric liquid having a volume resistivity greater than 103 ohm-cm2/cm.

Description

The present invention relates to a method of electrographic reproduction on any support using a single-component magnetic developing powder with a volume resistivity less than or equal to 1015 n cm ² / cm and a device for implementing said method. It relates more particularly to a method of electrographic reproduction in which an image of electrostatic charges formed on a temporary support, such as a photoconductor or any other surface capable of retaining an image of electrostatic charges, is developed using a single-component magnetic development powder with a volume resistivity less than or equal to 10 ¹⁵ Ω cm ² / cm to form a powder image which is transferred under the action of electrical means (electric field, corona discharge device, etc.) on any image support. It also relates more particularly to a device for implementing said method.

The processes for the electrographic reproduction of documents have developed considerably over the past ten years, and in particular the processes for electrophotographic reproduction on plain paper.

In its processes, a uniform charge of a photoconductor is generally carried out using a corona discharge device (hereinafter called "corona" device) by selective exposure from an original, a image of fillers which is then developed using a developing powder.

• - les poudres de développement à deux constituants appelées ci-après »bi-composants« qui utilisent deux types de particules: le véhicule (ou »carrier«) et le développateur (ou »toner«).

There are several types of developing powders among which one generally distinguishes:

• - two-component development powders hereinafter called "two-component" which use two types of particles: the vehicle (or "carrier") and the developer (or "toner").

• - les poudres de développement monocomposant qui n'utilisent qu'un seul type de particules qui sont généralement constituées de particules magnétiques enrobées de résines appropriées, ces particules ayant, suivant le cas, un caractère plus ou moins conducteur.

The vehicle generally consists of glass beads or other large dimensions compared to that of the developer particles, retained on its surface by triboelectricity, and which are based on carbon black coated with resin,

• - single-component developing powders which use only one type of particles which generally consist of magnetic particles coated with suitable resins, these particles having, depending on the case, a more or less conductive nature.

• - le procédé dit »en cascade« tel que décrit dans le brevet américain n° 2 618 552, dans lequel les particules de développateur se déposent sur les charges de l'image latente ayant un signe opposé à celui des charges portées par lesdites particules, par attraction électrostatique. Ces images de poudre ainsi développées sont généralement facilement transférables sous l'action d'un champ électrique extérieur ou sous l'action d'un corona,
• - le procédé de développement à la brosse magnétique tel qui décrit dans le brevet américain n° 2 874063, dans lequel les particules de véhicule sont constituées de limaille de fer doux, revêtues ou non d'une résine triboélectrique. Les particules de développateur sont généralement retenues par les particules de véhicule par triboélectricité.

The development of an image of electrostatic charges using a two-component developing powder can be carried out according to various methods, the most common of which are:

• the so-called "cascade" process as described in US Patent No. 2,618,552, in which the developer particles are deposited on the charges of the latent image having a sign opposite to that of the charges carried by said particles, by electrostatic attraction. These powder images thus developed are generally easily transferable under the action of an external electric field or under the action of a corona,
• - The magnetic brush development process as described in US Patent No. 2 874063, in which the vehicle particles consist of soft iron filings, whether or not coated with a triboelectric resin. Developer particles are generally retained by vehicle particles by triboelectricity.

In these two variants, using a two-component developing powder, it is essential that the developer particles are very insulating in order to maintain their charge necessary for the good development of the image. This development method using two-component development powders has, however, a certain number of drawbacks: the developer particles being retained electrostatically on the surface of the vehicle, it is essential to correctly dose the respective amounts of these two elements. When there is an excess of developer particles, these are no longer sufficiently retained by vehicle particles and disperse in the machine, which causes significant pollution of the latter, in particular at the level of the optical system. . It is therefore necessary to carry out frequent cleaning of the machine, which involves significant maintenance costs for this type of machine.

Some automatic metering devices for the amount of toner to be added after each copy have been developed, but cannot be entirely satisfactory since the amount of developer used for the development of a copy depends essentially on the nature of the original. , that is to say, the importance of the black parts on it.

Another drawback associated with the use of this type of developing powder is the need to mechanically clean the photoconductor after each copy. This cleaning requires a complex system of mechanical brushes and suction which are also causes of pollution of the device (unlike cleaning systems in the case of using single-component magnetic toner where a magnetic brush is used for the cleaning).

A method of transferring a two-component developing powder image in the presence of dielectric liquid has been proposed. Ansi patent FR 70/30 396 describes a process for transferring a developed electrophotographic image using a developing powder usually two-component on a metal plate in the presence of a dielectric liquid. According to this process, a potential difference is applied between the support carrying the powder image and the lithographic plate, they are brought into contact over their entire surface, the lithographic plate being previously coated with a layer of dielectric liquid, the support and the lithographic plate and the dielectric liquid is evaporated. The presence of the layer of dielectric liquid prevents any deterioration of the support carrying the image of powder due to the particles of the developing powder, during the transfer.

In recent years, electrostatic reproduction machines have been developed using single-component magnetic development powders, of a more or less conductive nature. To develop an image of electrostatic charges, a magnetic brush is generally used composed of a metal cylinder in which magnets are rotated, coated with a layer of single-component magnetic development powder. These powders generally have a more or less conductive nature and are charged by induction when approaching the image of charges to be developed, the development being able to be facilitated by the presence of an external electric field.

The one-component magnetic development powders have the advantage of not polluting the machine in which they are used, since they are kept permanently on the magnetic brush. These powders are currently widely used in so-called "direct" processes, that is to say for the development of zinc oxide photoconductive papers with which they are entirely satisfactory. However, this direct process is only used on machines with a small print run, since it is more economical to use ordinary paper machines when a large number of print runs are to be carried out, that is to say generally at -from 3000 to 5000 copies per month. In addition, a copy on ordinary media is generally more appreciated by users.

However, so far, and despite much research, it has been impossible to obtain good quality copies with images of single-component developer powder transferred to any medium, in particular to plain paper. In fact, the machines currently commercially available using one-component type developing powders to form a powder image on a support known as "plain paper", not using really ordinary paper, but treated paper having a low surface conductivity. Indeed, the use of really ordinary paper in these machines only allows to obtain mediocre quality images lacking in clarity and definition, this mediocrity being accentuated when the ambient humidity increases. The Applicant believes that electrical micro-discharges occur during the transfer of the powder image, this phenomenon being amplified when the surface conductivity of the support increases, in particular because of excessive humidity. In addition, this phenomenon is all the more important as the resistivity of the developing powder is low. This is why the machines currently using these one-component developing powders use treated papers coated with a layer of resin giving them a low surface conductivity. Among these treatments, it may be noted in particular, the process described by US Pat. No. 4,199,356, in which the paper intended to receive the final image is coated with a viscous liquid, and in particular silicone oil. Such a process, in addition to the fact that it only applies to a final receiving sheet, and not to intermediate organs, leaves an oily insulating deposit on the paper. These papers are much more expensive than ordinary papers and / or have an unpleasant appearance. They require a special supply and the user cannot therefore use the medium of his choice in one of these cases. For variations in conductivity linked to atmospheric conditions, it has also been proposed to use ordinary paper that has been previously dried. This requires special equipment in the machine and does not completely solve the problem. In fact, we note that, although the quality of the copy is improved, we still observe a phenomenon of "explosion" of the image which results in an absence of sharpness on the contours, sometimes rendering it unreadable when a great finesse of the line is necessary.

Other solutions have also been sought at the level of the one-component developing powder. For example, attempts have been made to reduce the conductivity thereof by using a high level of suitable coating resins. The quality of the transferred image is thus appreciably increased, but the development speed decreases unacceptably quickly. In fact, an electric charge being imparted to the developing powder by induction, due to the presence of the charges in the charge image, the charging speed depends on the time constant RC of the powder particles. When the resistivity of the powder increases, the induction polarization time of the powder particles when approaching the charges of the charge image increases and quickly becomes excessive compared to the development time of the charge image. Consequently, the speed of development of the charge image must be reduced and, correspondingly, the speed of movement of the paper must also be reduced. Consequently, it is no longer possible to achieve in this way a sufficient number of copies per unit of time which can make a machine using this type of developing powder competitive. None of the solutions proposed to date have really allowed to use single-component development powders in powder image transfer machines on a plain paper support and to obtain a good quality image (good contrast, without bursting, without edge effect, ... ) without limiting the speed of image development and whatever the atmospheric conditions.

The present invention provides a solution to this problem and makes it possible to avoid the drawbacks associated with the use of one-component developing powder. An electrophotographic reproduction method has now been found in which an image of charges is produced on a temporary support, said image then being developed using a single-component magnetic developing powder with a volume resistivity less than or equal to 10 ¹⁵ Ω cm ² / cm, to form a powder image which is transferred to an image medium under the action of electrical means, characterized in that the image medium is coated before the transfer of the powder image a thin layer of a volatile dielectric liquid with a volume resistivity greater than 10 ³ Ω cm ² / cm, with a volatility index of between 0.01 and 0.4, in an amount of between 0.1 and 16 g / m ² , said liquid remaining present on the image support for at least the time necessary for the transfer of the powder image on said image support, said dielectric liquid having a low solvent power with respect to the powder resins monocom magnetic development laying and / or the photoconductive surface of the temporary support.

• - des moyens pour réaliser une image de charges sur un support temporaire comportant une surface photoconductrice,
• - des moyens pour développer ladite image de charges à l'aide d'une poudre de développement magnétique monocomposant de résistivité volumique inférieure ou égale à 10¹⁵ Ω cm²/cm pour former une image de poudre,
• - des moyens électriques pour transférer l'image de poudre sur un support d'image,

caractérisé en ce que:It has also been found, to implement said method, a device which comprises:

• means for producing an image of charges on a temporary support comprising a photoconductive surface,
• means for developing said charge image using a single-component magnetic development powder with a volume resistivity less than or equal to 10 ¹⁵ Ω cm ² / cm to form a powder image,
• - electrical means for transferring the powder image onto an image support,

characterized in that:

the temporary support and the image support being in contact along a straight line, it comprises means for coating the image support with a volatile dielectric liquid with a volume resistivity greater than 10 ³ S2 cm ² / cm, with an index of volatility between 0.01 and 0.4 in an amount between 0.1 and 16 g / m ² , said coating means being in contact with the image medium upstream from the contact line of the temporary medium and of the image carrier with respect to the direction of movement of the image carrier.

Preferably, a liquid with a volume resistivity greater than 10 ⁷ Ω cm ² / cm will be used and even better results are obtained when it is greater than 10 ¹⁰ Ω cm ² / cm. However, it is around 10 ¹⁵ Ω cm2 / cm of volume resistivity that the best results are observed.

Such a method makes it possible, surprisingly, to obtain images of clearly improved quality compared to those obtained, all other things being equal, in the absence of dielectric liquid. Thus, such a method makes it possible to use any support and even very conductive as a metal as will be seen later, while obtaining an image of excellent quality, having a good density and excellent sharpness (no phenomenon explosion).

The Applicant thinks, without wishing to be bound by a theory, that the particles of relatively conductive developing powder, when they are transferred into an electric field on a surface of dielectric nature cannot instantly exchange their induced charge when they come into contact with the receiver. As a result, they remain attracted to this surface, which increases the amount of developer powder transferred.

In addition, the particles of developing powder are themselves wetted by the dielectric liquid, which contributes to limiting the exchange of charges between them and the copy medium: the parasitic discharges which generate the explosion of the image are deleted. In fact, when a developing particle comes into contact with a relatively conductive receiving surface, the charge carried by the particle can be canceled: the particle is no longer retained, it is pushed out of the image area or towards the photoconductor , which generates the deteriorations noted above.

In the context of the present invention, the term “one-component relatively conductive developing powder” means an electrographic image developing powder in which only one type of particle is present and having a volume resistivity less than or at most equal to 1015 n cm ² / cm. Indeed, beyond this value, there is no longer any notable improvement in the quality of the image transferred in the presence of the volatile dielectric liquid. The invention also extends to mixtures of powders as defined above, of various resistivity and particle size.

The improvement of the transferred image is entirely satisfactory for a resistivity between 10 ⁷ and 10 ¹⁵ Ω cm ² / cm. Preferably, development powders having a resistivity of between 10 ⁸ and 10 ¹³ Ω cm ² / cm will be used. The resistivity of the developing powder is measured in a cylindrical cell of section 0.07 cm ² on a 2 mm thick sample under a pressure of 750 g / cm ² and under a continuous electric field of 1000 V / cm.

The image support according to the present invention can be any, that is to say having a surface resistivity less than 10 ¹³ fl cm ² / cm. Low resistivity supports such as metal supports are also suitable in the context of the present invention. Depending on the type of support used, different products can be produced by applying the method according to the invention. In the case of hydrophilic supports (treated polyester, metal, coated paper, etc.), lithographic printing plates are produced directly using ink-absorbing toners. On transparent polyester films, projectable »transparencies« or negatives can be produced directly.

Dielectric liquids with volume resistivity as defined above must not be too volatile so that they are always present on the copy medium at the time of the transfer of the powder image, but also sufficiently volatile to evaporate enough quickly so that the copy comes out dry from the machine. Preferably, a liquid having a volatility index of between 0.01 and 0.4 will be used. By liquid volatility index is meant the quotient of the evaporation time on n-butyl acetate filter papers by the evaporation time of the selected dielectric liquid. For more details concerning the operating conditions and the equipment used, reference is made to French standard NF T 30-301 (August 1969).

According to a variant of the invention, means are provided for drying the support before or after fixing the image. These means, known per se, can for example be combined with the fixing means when using hot fixing rollers. More simply, one will use infrared fixing means performing fixing and drying functions. These means can also be separated when cold fixing is carried out: for example when using pressure rollers, infrared tubes or hot air will be used for drying. It will generally be desirable to provide ventilation to vent the vapors given off. In many cases, however, these drying means are not necessary when the liquid used has suitable volatility.

According to the specifications imposed, the person skilled in the art will determine the nature of the dielectric liquid with the indications mentioned above. In all cases, this dielectric liquid must properly wet the copy medium on which the transfer takes place so that a thin layer of liquid is effectively present at all points of the medium during the transfer.

Preferably, as the dielectric liquid, pure or mixed aliphatic hydrocarbons, branched or not, the boiling points of which range between 60 ° C. and 230 ° C. and preferably between 100 ° C. and 200 ° C.

Similarly, other compounds having these properties can be used such as saturated cyclic hydrocarbons, polyisobutenes, polyfluoroethylenes or a mixture of these products. Mention may in particular be made of hexane, heptane, octane, isododecane or the products sold under the names "ISOPAR - C, ISOPAR - E, ISOPAR - G, ISOPAR - K, ISOPAR - L, ISOPAR - M , SHELL SOL 70, SHELL SOL 71, SHELL SOL 72, SHELL SOL T, SHELL SOL TD, SHELL SOLTP, SOL PAR 195 - 230 ".

Preferably, the dielectric liquids used will not be solvents for the photoconductive layer in order to avoid damaging the latter. Preferably, also, these liquids will not be solvents for the resins used for the production of the developing powder so as not to cause even partial softening of the toner which would then be liable to be fixed on the photoconductive layer in a detrimental manner.

The quantity of liquid deposited on the copy medium depends in particular on the running speed of the copy medium, the nature of the latter (porosity, etc.) as well as the nature of the dielectric liquid (evaporation rate , etc ...). It also depends on the distance between the means for coating the liquid on the support and the place of transfer of the powder image. In general, it has been found that an amount of liquid between 0.1 g / m ² and 16 g / m ^{2 makes it} possible to achieve the desired result. However, in most cases, it is found that an amount of dielectric liquid of between 2 g / m ² and 5 g / m ² gave excellent results, in particular when fixing the powder image by pressure and cold. The transfer of the image of developing powder onto the copy medium is carried out according to the devices used and the nature of the copy medium under the action of an electric field or a corona effect device. The parameters which require the use of one or the other transfer means as well as the voltages used are well known to those skilled in the art.

However, as will be seen below, the invention presents a preferred variant with a device with three superimposed rollers, the upper roller being the photoconductor on which the powder image is produced, the two lower rollers being rollers metallic, the adjacent or photoconductive roller receiving the powder image, while the transfer of the latter onto any support is effected by pressure between the two rollers. In the case of the use of such a device, it has been found that it is preferable to partially discharge, in particular by light radiation, the photoconductor before transferring the powder image, under the action of a field. electric, on the adjacent metal roller coated with dielectric liquid. Indeed, we note that if we do not partially discharge the photoconductor, whatever the transfer voltage, we have either a slightly "exploded" image (high transfer voltage) or, an image which lacks contrast (voltage low transfer). Therefore, the transfer voltage and the photo voltage conductor before transfer (which results from the presence of surface loads) must be adjusted relative to each other. These considerations are only valid when we are in the presence of a direct image, that is to say when the developing powder covers the charges of the charge image. In the case of reverse development, it is not necessary to partially discharge the photoconductor, but simply to adjust the transfer voltage.

• la figure 1, une illustration du phénomène se produisant au moment du développement de l'image à l'aide d'une poudre de développement monocomposant,
• les figures 2a, 2b et 2c, un rappel des principales variantes du transfert de poudre de développement sur un support de copie à l'aide de moyens électriques,
• la figure 3, un exemple de réalisation de l'invention,
• la figure 4, une variante préférée de la figure précédente,
• la figure 5, une variante de la figure 4, spécialement adaptée à la microcopie,
• la figure 6, un schéma complet d'une machine utilisant l'invention représentée sommairement dans la figure 4.

The invention will be better understood with the aid of the following exemplary embodiments, given without limitation, together with the figures which represent:

• FIG. 1, an illustration of the phenomenon occurring at the time of image development using a one-component developing powder,
• FIGS. 2a, 2b and 2c, a reminder of the main variants of the transfer of developing powder onto a copy medium using electrical means,
• FIG. 3, an exemplary embodiment of the invention,
• FIG. 4, a preferred variant of the previous figure,
• FIG. 5, a variant of FIG. 4, specially adapted for microcopying,
• FIG. 6, a complete diagram of a machine using the invention briefly shown in FIG. 4.

In these figures, the same elements have the same references.

In FIG. 1, the magnetic brush 15 magnetically retains on its surface, the single-component magnetic toner which forms chains in grains such as 22. When the charges 23 of the charge image, formed on the photoconductor 9, approach, grains of developing powder become polarized by influence. Thus, the part of the grain, opposite the negative charge 23, takes an equal positive charge. The electrostatic force between these two charges is then sufficient, due to the presence of the electric field created by the image of charges, to attract the grains of powder to the photoconductor 9, thus forming the image of powder 20. In the figures 2a, 2b and 2c are shown the three best known variants for transferring a loaded developer powder onto a support. In these three variants, the polarities of the voltages are relative to positively charged particles, as explained in FIG. 1. It is obvious that the polarities of the voltages must be reversed in the case where the developing powder charges negatively.

In FIG. 2a, the cylinder 10 connected to a positive voltage source is covered with a photoconductor 9 on which there is a powder image 20 which must be transferred to the copy medium 5 which moves on the guide medium 4 grounded. The particles of developing powder are transferred to the copy medium under the action of the electric field existing between the photoconductive drum and the medium 4 and directed towards it. In FIG. 2b, the same elements as those in FIG. 2a bear the same references. In this figure, the copy medium has the form of a conductive cylinder connected to ground, the transfer being improved by the pressure contact existing between the photoconductive drum and the cylinder 4.

In FIG. 2c, the transfer takes place under the action of a corona effect device 4, the photoconductive roller 9 being grounded.

In the rest of the description of the invention, any of the means described in these figures 2a, 2b or 2c or any equivalent means will be designated by electrical transfer means.

In FIG. 3, the copy supports 2 stored in a cassette 1 are engaged on the support 4 when the user wishes to make a copy. The copy support 5, already engaged, is coated with a layer of dielectric liquid 8, contained in the reservoir 7, using the coating device 6. Simultaneously, the powder image is formed on the photoconductor 9 which covers the metal roller 10 connected to a positive voltage source, in the case where the toner is positively charged by influence. A corona effect device 13 deposits a uniform charge on the photoconductor9.

After selective illumination according to the original through the optical system 14, the charge image is developed using the magnetic brush 15 and a powder image 20 is formed.

This is transferred at 21 to the support 5 under the action of the positive voltage existing between the photoconductive roller 10 and the support 4 grounded. The image is then fixed in the infrared oven 11 and the copy recovered in the tray 12.

FIG. 4 illustrates a particularly interesting embodiment of the invention and with which certain examples given below have been produced. The same means have the same references as in the previous figures. The powder image 20 is formed with the same means and in the same way as in FIG. 3.

The image of powder 20 is then transferred to 2 l on the metal roller 16 put the mass. This roller 16 is previously coated using the coating device 17 with dielectric liquid as defined above. The image of powder thus transferred is then transferred again by pressure, onto the copy medium 5 which advances thanks to the rotation of the two rollers 16 and 18, the pressure of which also fixes the powder image 20. In this In this case, the pressure between the rollers 16 and 18 is approximately 30 kg / linear cm. The lighting device 25 ensures partial discharge of the photoconductor before transferring the image, when this is necessary. The lighting device 24 makes it possible to completely discharge the photoconductor 9 before cleaning with the magnetic brush 29.

FIG. 5 represents an alternative embodiment of the invention specially intended for the reproduction of microfilmed images. In this case, a photoconductive strip 9 will be used on which the image will be projected completely. the charge image is then developed using the brush 15 and transferred as in FIG. 4, onto a metal roller 16 coated with dielectric liquid using the coating system 17. To ensure good transfer, there is a counter-roller 50 on the other side of the photoconductive strip. The other elements have the same meaning as those in Figure 3.

FIG. 6 represents a prototype of a device according to the invention, in which the same elements as those of the preceding figures bear the same references. The image of charges on the photoconductor 9 (under which a layer of foam 49 can be placed but which is preferably rigid on the surface) is produced using the image block 30 and the optics 14 after uniform loading using corona 13.

The image is developed using the developing powder 31 uniformly deposited on the brush 15 using the doctor blade 32. The photoconductor is then partially discharged using the lighting means 33 whose intensity is adjustable using a flap 35 articulated around the axis 34 using a cam 36. The image is then transferred to the roller 16 previously coated with dielectric liquid, contained in the reservoir 37, using the brush 17. After transfer to 21, the powder image is dried by hot air sent by the fan 44 into the duct 46, the photoconductor also being dried by the hot air arriving in the conduit 45. The image is then transferred and fixed by pressure using the two rollers 16 and 18 on the copy support 5. The latter is detached from the cylinder 16 using the doctor blade 39, said cylinder being then cleaned using the brush 40. The cylinder 18 is cleaned by the brush 41. The photoconductor is discharged using the lighting means 24, after transfer of the image, then cleaned using the magnetic brush 47. The excess powder on this brush is recovered in the tank 48.

The following exemplary embodiments are given without limitation:

Example 1

This example is carried out using a SHARPFAX SF 730 machine using a zinc oxide photoconductive surface, a single-component magnetic developer and a cold pressure fixing between two metal rollers. The machine is used at 20 ° C and 65% relative humidity, the copy medium being ordinary paper commercially available under the name VOIRON VELIN SH. A HMT 824/4 reference single-component magnetic developer sold by the company HITACHI METALS Ltd is used, with a volume resistivity, measured according to the above-mentioned method, equal to 3 × 10 ¹⁰ cm cm 2 / cm.

The original to be reproduced is a screen having different ranges varying from 1 line per mm to 6.3 lines per mm and also having uniform parts enabling the optical density of the image to be measured.

The quality of the image obtained is poor: there is a tendency for the line to burst and a fairly average contrast of the image. There is a definition of 2.8 lines per mm and an image density of 1.09 (density measured with the Macbeth TR 524 densitometer used in reflection with the green filter).

The same experiment is repeated, but by previously coating the same copy medium with an isoparaffinic hydrocarbon sold under the trade name ISOPAR G by the company ESSO. This dielectric liquid has a resistivity of 5 x 10 ¹⁴ Ω cm ² / cm at 20 ° C and a volatility index of 0.18.

The quantity of liquid deposited on the paper is approximately 3.2 g / m ² . All the test parameters are the same as before. The sheet of paper is then immediately introduced into the paper tray of the SHARPFAX SF 730 machine, and a printout is performed as before. The comparison of the two images obtained with and without dielectric liquid shows a marked improvement in the quality of the image when the dielectric liquid is used. The definition obtained in this case is 4.5 lines per mm and the image density is 1.61.

Example 2

The same test is carried out as above under the same conditions, replacing the previous paper with paper sold under the trade name AUSSEDAT REY UNIMAT 80 g. The same improvement in the results is observed: 3.2 lines per mm without dielectric liquid and 5 lines per mm with the dielectric liquid, the image density also increasing.

Example 3

The same test is carried out as in Example 1 using a matte polyester copy medium intended for the graphic arts and sold under the trade name REGMA FM by the company RHONE-POULENC SYSTEMES. In this case, HMT 808 developer powder from HITACHI METALS Ltd. is used. The copy made without dielectric liquid is unusable in practice, because the image is blurred, blurred, heterogeneous. On the other hand, by depositing ISOPAR G dielectric liquid on this support before copying, one obtains an astonishing quality image by its homogeneity and its sharpness.

The improvement obtained is a function of the quantity of dielectric liquid deposited. This example also makes it possible to measure the influence of the quantity of liquid deposited, since the absorption of this type of support is zero.

The results obtained are as follows:

These results show that the quality of the image obtained increases with the quantity of liquid deposited per unit of surface. However, in practice, it is sometimes desirable not to increase too much the quantity of liquid deposited because the drying of the copy is then necessary.

Example 4

The procedure is as in Example 1 using the developing powder supplied with the SHARPFAX SF 730 machine, with a resistivity of 8.3 × 10 ⁹ Ω cm ² / cm by treating the VOIRON SH paper with isodecane. with a resistivity equal to 1 × 10 ¹⁵ Ω cm ² / cm and whose volatility coefficient is 0.22. This draw is compared to that obtained, all other things being equal, in the absence of dielectric liquid. Here too, there is an improvement in the sharpness of the line and the density of the image.

Example 5

The device in FIG. 2b is used to transfer the image 20 to the copy medium 5.

Preferably, in the case of this figure, the cylinder 10 will be coated with a layer of flexible foam of a few millimeters before having the photoconductive strip 9 on the outside. In this case, the quality of the transfer of the image.

The charge image formed on photoconductor is developed using HITACHI HMT403 toner with a resistivity close to 10 ¹² Ω cm ² / cm. The powder image is transferred at 400 V, after partial discharge of the photoconductor, onto a grained aluminum sheet for CRAO reference lithography (from the company AGFA-GEVAERT).

When this support is coated, before transfer, with a layer of ISOPAR G, practically all the powder is transferred to the support, the quality of the image obtained is excellent. In the absence of dielectric liquid, all other things being equal, the transfer of the powder image does not exceed 40%. The powder image is then fixed hot in an oven at 140 ° C., then treated with a phosphoric acid solution to regenerate the hydrophilicity of the areas not covered by the developing powder.

Several hundred prints were then made without observing any lubrication phenomenon with very good image quality (11 lines per mm).

Example 6

Using the same device as that of Example 5 and under the same conditions, an image of HITACHI HMT 824/4 single-component toner powder with a resistivity equal to 3 × 1010 n cm ² / cm, is transferable. on a polyester sheet coated with a matting layer for the drawing (Applicant's FM reference). The sheet coated with 5 g / m ² of ISOPAR G makes it possible to obtain a good quality copy and a transfer of 95% toner, the transfer being carried out at 300 V and partial discharge of the photoconductor. The same experiment carried out without the dielectric liquid makes it possible to note a bursting of the image, just after the transfer of this one, at the time of the separation of the photoconductor and the support. In this case, the image obtained is so blurred that it is unusable.

Example 7

The experiment of Example 5 is repeated using a heat-fixable magnetic toner HMT403, from the company Hitachi with a resistivity close to 10 ¹² n cm ² / cm. The transfer is carried out at a voltage of 400 V, the photoconductor being partially discharged before transfer. The image is transferred to VOIRON VELIN SH paper previously described. In the absence of dielectric liquid, the image is transferred at 80%, but it is blurred. Using a mixture of isoparaffin with the trade name SHELL SOL T (from the company SHELL OIL Cy) with a resistivity equal to 3 × 10 ¹³ Ω cm ² / cm, a clear image transferred to 90% is obtained.

Example 8

The device shown diagrammatically in FIG. 4 and shown in more detail in FIG. 6 is used, with a zinc oxide photoconductor (dual-charge paper REGMA M 100 BC) developed with the HMT 824/4 toner mentioned above. The receiving support is VOIRON VELIN SH paper. The transfer takes place at a voltage of 300 V by partially discharging the photoconductor.

In the absence of dielectric liquid, only 5% of the powder image is transferred to the paper: the image is therefore barely perceptible.

When the cylinder is wetted with isopropyl alcohol with a resistivity equal to 10 ⁷ Ω cm ² / cm, about 90% of the powder image is transferred. However, we see that it is slightly blurred. When the cylinder is wet with ISOPAR G of resistivity equal to 5 x 10 ¹⁴ Ω cm ² / cm, the transfer is approximately 90% and the image is very clear (in both cases, approximately 4 g / m ² of dielectric liquid). This example corresponds to a preferred embodiment of the invention.

Example 9

• - qu'en l'absence de liquide diélectrique, on ne transfère que 10% de l'image sur le papier diélectrique, ladite image étant de plus éclatée,
• - qu'en présence d'ISOPAR G, le transfert est de 80% et l'image est nette.

The procedure is as in the previous example with the REGMA 720 D reference dielectric paper as receiving medium having a surface resistivity close to 10 ¹⁵ Ω cm ² / cm and provided with a conductive layer on the back. Under the same operating conditions as in Example 8, under a voltage of 400 V, it is found:

• - that in the absence of dielectric liquid, only 10% of the image is transferred to the dielectric paper, said image being further exploded,
• - in the presence of ISOPAR G, the transfer is 80% and the image is clear.

Example 10

• - en l'absence de liquide diélectrique, le transfert n'excède pas 5%, l'image étant de plus éclatée,
• - en présence d'ISOPAR G, le transfert s'effectue à 90%, l'image étant nette.

The same device is used as in the previous example, the image being developed with a HITACHI HMT 824/4 toner, the support to which the image is transferred being a REGMA FM polyester film, at 300 V of voltage and after partial discharge of the photoconductor. We aknowledge:

• - in the absence of dielectric liquid, the transfer does not exceed 5%, the image being further exploded,
• - in the presence of ISOPAR G, the transfer takes place at 90%, the image being clear.

Example 11

The device of FIG. 3 is used in which the photoconductive strip is of the REGMA M 100 BC type. This photoconductive surface has the distinction of being dual charge, that is to say, to accept both positive charges and negative charges. It is therefore possible, with this type of photoconductor, to carry out an inverted development of the image, that is to say, deposit the monocomponent powder on the discharged areas of the photoconductor, by applying a positive voltage on the magnetic brush. 300 V with respect to the support of the photoconductive surface, the photoconductor having an image of positive charges.

Unloaded areas are thus developed using HITACHI HMT 824/4 toner.

• - en l'absence de liquide diélectrique, la quantité de toner transférée est de 5% et l'image est floue,
• - en présence de liquide diélectrique (ISOPAR G), la quantité de poudre transférée est de 95% et l'image est nette.

The copy support is AUSSEDAT-REY UNIMAT 80 g reference paper. The image transfer voltage is 200 V. We then see:

• - in the absence of dielectric liquid, the amount of toner transferred is 5% and the image is blurred,
• - in the presence of dielectric liquid (ISOPAR G), the quantity of powder transferred is 95% and the image is clear.

Example 12

This comparative example summarizes a certain number of tests carried out with the device of FIG. 3 with different toners of variable resistivity. The dielectric liquid used is ISOPAR G. For a given resistivity of the toner, the voltage between the photoconductive roller and the roller 16, on the one hand, and the illumination of the photoconductive, on the other hand, were varied. view to get the best picture possible. The same experiments were carried out, in the absence of dielectric liquid. In the presence of dielectric liquid, the results obtained are as follows:

The resistivity of the toner is measured according to the method mentioned above. Illumination is the operation which consists in discharging more or less the photoconductor after development of the image, but before transfer of this one. In the absence of dielectric liquid, the results obtained are as follows:

These two tables clearly show that whatever the resistivity of the toner, the image obtained is always better in the presence of dielectric liquid. This improvement is significant when the resistivity of the toner is less than 10 ¹⁵ Ω cm2 / C _M. Beyond that, the improvement is weaker, but is still noticeable, in particular in terms of the amount of powder transferred.

Example 13

This example shows the improvement obtained in the image quality for a given toner, a given transfer voltage, for a given partial discharge of the photoconductor before image transfer, as a function of the resistivity of the dielectric liquid used (experiment performed on the device of Figure 3).

Toner HMT 824/4 - Transfer voltage 300 V - Discharge at 30% of the photoconductor - Receiver support VOIRON VELIN SH.

Example 14 Realization of a printed circuit

A sheet of REGMA R 220 reference electrophotographic paper is loaded uniformly using a negative sign corona device. The drawing of the printed circuit made on a transparent original is brought into contact with said sheet of paper, the assembly then being exposed to light. The charge image thus formed is then developed using a HMT403 one-component developer powder.

A sheet of copper-coated epoxy glass is then treated with nitric acid in order to give good wettability to the copper-colored face which is then coated on one half with a thin film of ISOPAR G, the other half on the copper-colored face. not being treated.

The copper plate is then placed on a grounded metal plate, the copper side up. An electrical contact is established between the metal plate and the copper-colored face which is thus earthed.

• - que la moitié traitée à l'aide d'ISOPAR G comporte une image représentant un transfert d'environ 90% de l'image de poudre, la qualité de l'image étant très bonne, tandis que la gravure est d'une excellente définition d'au moins 3,5 traits au millimètre,
• - que la moitié non traitée comporte une image représentant un transfert d'environ 40% de l'image de poudre, l'image étant éclatée.

Electrophotographic paper is placed on the copper side with its powder image down. A corona device is then moved uniformly over the entire width of the assembly (one round trip). This is done in the light. The electrophotographic paper is then removed and the transferred image is heat fixed at 150 ° C. This copper-colored support is then etched in iron perchloride, then the remaining toner is removed using trichlorethylene. We then observe:

• - that half treated with ISOPAR G has an image representing a transfer of approximately 90% of the powder image, the image quality being very good, while the engraving is of excellent definition of at least 3.5 lines per millimeter,
• - that the untreated half contains an image representing a transfer of approximately 40% of the powder image, the image being exploded.

Claims (15)

1. Electrophotographic reproduction process in which a charge image is produced on a temporary base, the said image then being developed by means of a single-component magnetic development powder with a volume resistivity lower than or equal to 10¹⁵Ω cm²/cm to form a powder image which is transferred to an image base under the action of electrical means, characterised in that the image base is coated before the transfer of the powder image with a thin layer of a volatile dielectric liquid with a volume resistivity greater than 10³ Ω cm²/cm, a volatility index of between 0.01 and 0.4, in a quantity of between 0.1 and 16 g/m², the said liquid remaining present on the image base for at least the time required for the transfer of the powder image onto the said image base, the said dielectric liquid having a low dissolving capacity for the resins in the single-component magnetic development powder and/or in the photoconductive surface of the temporary base.

2. Process according to Claim 1, characterised in that the dielectric liquid has a volume resistivity greater than 10¹⁰ Ω cm²/cm.

3. Process according to one of Claims 1 or 2, characterised in that the dielectric liquid has a boiling point of between 60 and 230° C.

4. Process according to any one of Claims 1 to 3, characterised in that the single-component magnetic development powder has a volume resistivity of between 10⁷ and 10¹⁵ Ω cm²/cm.

5. Process according to any one of Claims 1 to 4, characterised in that the volatile dielectric liquid is chosen from straight-chain, branched or cyclic saturated aliphatic hydrocarbons, polyisobutenes, polyfluoroethylenes, employed alone or in mixtures.

6. Process according to any one of Claims 1 to 5, characterised in that the powder image is fixed on the image base.

7. Process according to any one of Claims 1 to 5, characterised in that the powder image which has first been transferred onto the image base is then transferred onto a copy base and then fixed.

8. Process according to any one of Claims 1 to 5, characterised in that the powder image which has first been transferred onto the image base is simultaneously transferred and fixed on a copy base by cold pressure.

9. Process according to one of Claims 7 or 8, characterised in that the volatile dielectric liquid is evaporated off from the image base before the transfer of the powder image onto the copy base.

10. Process according to any one of the previous claims, characterised in that the powder image is transferred onto an image base which is a metal roll.

11. Process according to Claim 1, characterised in that the powder image is discharged before its transfer onto the image base.

12. Device for making use of the process according to Claim 1, which incorporates:

- means (14) for producing a charge image on a temporary base (9) incorporating a photoconductive surface,

- means (15) for developing the said charge image by means of a single-component magnetic development powder with a volume resistivity lower than or equal to 10¹⁵ 92 cm²/cm to form a powder image (20),

- electrical means for transferring the powder image (20) onto an image base (16),

characterised in that:

the temporary base (9) and the image base (16) being in contact along a straight line (21), it incorporates means for coating (17) the image base (16) with a volatile dielectric liquid with a volume resistivity greater than 10³ Ω cm²/cm, a volatility index of between 0.01 and 0.4, in a quantity of between 0.1 and 16 g/m², the said coating means (17) being in contact with the image base (16) upstream of the straight line (21) of contact between the temporary base (9) and the image base (16) relative to the direction of motion of the image base (16).

13. Device according to Claim 12, characterised in that it additionally incorporates means (33) for discharging the powder image (20) before its transfer onto the image base (16).

14. Device according to Claim 13, characterised in that the said means (33) for discharging the powder image (20) are lighting means.

15. Device according to any one of Claims 12 to 14, characterised in that the temporary base is a photoconductive cylinder and the image base is a metal roll.

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