EP0109319A1 - Magnetic printing method and transfer sheet for documents - Google Patents

Abstract

The invention relates to a method and a paper for magnetographic printing in a humid environment. Paper having a surface resistivity of at least 10¹ ° square ohms and a smoothness of at least 20 Bekk is used. Application to magnetographic printing in atmospheres of relative humidity which can exceed 75%.

Description

The present invention relates to magnetography, that is to say printing using a magnetic drum on which a magnetic image of the impression to be produced is formed, then particles are adhered to this magnetic image. solid magnet ink generally called "toner" which reveals the recorded image. By pressing the drum on a sheet of paper, the ink is transferred to the paper where it is fixed by heating. For this, we use magnetizable, fusible inks.

When it is desired to carry out magnetography printing in environments with high relative humidity, from around 50%, gray results are generally obtained due to poor transfer of the ink to the paper.

The problem is therefore to obtain clear copies in highly humid atmospheres, but nevertheless usual in living quarters in temperate countries.

Printing by magnetography is more particularly advantageous for printing at high speed documents originating from computers.

Research has apparently so far focused mainly on the nature and qualities of ink, but plain paper or papers specially studied for printing by electrography have been used which have proved disappointing.

When the paper has been exposed to a relative air humidity above 50%, the print is very pale, that is, its optical density is low.

With the inks hitherto used, magnetographic printing requires a low humidity of paper and a maintenance of this humidity by a sealed packaging protecting it from ambient humidity until the moment of printing. However, the printer must be able to operate in unconditioned atmospheres.

The object of the invention is to overcome the drawbacks of magnetography in highly humid atmospheres and to make it possible to obtain correct impressions without special precautions, even at high ambient humidities.

The subject of the invention is a process for printing by magnetography under ambient atmospheric conditions of relative humidity which can exceed 65%, in which a fusible pulverulent magnetizable ink is transferred onto a sheet of paper, distributed according to the configuration to be reproduced on a temporary magnetizable support, then the ink is fixed by fusion on the sheet of paper, characterized in that a sheet of paper is used whose surface resistivity in the conditions of relative humidity of the transfer is at least 10 ohms square , and whose smoothness is at least 20 Bekk.

A subject of the invention is also a sheet of paper for printing by magnetography under ambient atmospheric conditions of relative humidity which may exceed 75%, characterized in that that it has a surface resistivity of at least ₁₀ ¹⁰ ohms square measured with the Keithley device after exposure to 20 ° C in an atmosphere of relative humidity of 30% for one hour, then to 23 ° C in an environment of relative humidity of 50% for at least 10 hours, and in that it exhibits a smoothness of at least 20 Bekk.

It is advantageous that a sheet of paper according to the invention contains practically no fluorescent whitening agent or electrolyte.

According to one embodiment, a sheet of paper for magnetography according to the invention has a surface layer of mineral pigments in a non-hydrophilic binder on its face which is to receive the impression. Such an inorganic pigment advantageously consists mainly of powdered calcium carbonate with an average particle diameter of the order of a micron and of which approximately 80% of the particles have a diameter of less than approximately 2 microns.

The binder is advantageously constituted at least in part by a substance from the group relating to acrylate and styrene copolymers, polyvinyl alcohol, urea-formaldehyde resins, without plasticizer or solvent.

According to another embodiment, a sheet of paper having a porosity of the order of 200 to 300, measured according to the Bendtsen method and a pH below 7, is used. This corresponds to a sheet of paper without any particular coating produced. from paper pulps selected according to their chemical cleanliness, that is to say a practical absence of foreign ions, in particular sodium, potassium, chlorine, sulfur, and of a high intrinsic surface resistivity.

The refining of the pulp should be as low as possible, taking into account the mechanical strength of the paper that is desired.

The rate of mineral fillers should be as high as possible within the limit of the mechanical strength of the desired paper.

Finally, the production of paper must be carried out in a rather acid medium.

The optical density of printing produced on a magnetographic printer depends on the surface electrical resistivity at the time of printing, this factor being essential, but also on the surface smoothness which must be greater than 20 Bekk, this factor however being of second order.

The surface resistivity values indicated here are those measured with the Keithley device with the appropriate electrodes for measurements in "square ohms".

In the case of paper, neither pigmented on the surface, nor coated, that is to say with papers of substantially homogeneous constitution in their thickness, a direct relationship can be established between the optical density of magnetographic prints and the surface resistivity. This relationship is materialized by the curve shown in the single figure of the accompanying drawing. This curve shows a very rapid increase in the optical density of the print between the surface resistivity levels of ₁₀ ¹⁰ and ₁₀ ¹¹ ohms square. The optical density of the prints can be considered good from the value 1.0, and poor below 0.8. The surface resistivity values between 10 ¹⁰ and 5.10 square ohms can thus be considered as representing a resistivity threshold above which the surface resistivity of the surface must lie. paper at the time of printing. Experience shows that most papers have a resistivity below this threshold when they are subjected to atmospheres with a humidity of at least 55 to 65% relative humidity. They are therefore unsuitable for magnetographic printing in climates wetter than 55 to 65% relative humidity.

Surface resistivity is a rapidly decreasing function of the relative equilibrium humidity of the paper.

In general, any process which makes it possible to reduce the hygroscopicity of paper, that is to say the quantity of water which it is capable of fixing when it is subjected to a given climate, will be favorable to magnetographic printing. A lower water content in fact leads to an increase in resistivity and it has been found that the base logarithm of the surface resistivity is a decreasing linear function of logarithm of the water content.

The invention also relates to documents printed by magnetography on papers according to the invention.

The invention will be better understood on reading the detailed description which follows of some nonlimiting examples of embodiments according to the invention.

EXAMPLE 1

• - pâte chimique blanchie de bois feuillus 52 %
• - pâte chimique blanchie de bois résineux 35 %
• - mélange en parts égales de talc et kaolin 13 %
• - produits de collage et agents de rétention en quantités habituelles.

A paper pulp is prepared by introducing the materials constituting the pulp suspension upstream of the paper machine:

• - bleached chemical pulp of hardwood 52%
• - bleached chemical pulp of softwood 35%
• - mixture in equal parts of talc and kaolin 13%
• - bonding products and retention agents in usual quantities.

• - 420 kg de craie pulvérulente, d'un diamètre moyen de particules de 1 micron, et dont 80 % des particules ont un diamètre inférieur à 2 microns, telle celle vendue sous la dénomination commerciale d'Omyalite par la Société dite OMYA,
• - 150 kg d'une suspension aqueuse à 50 % d'un copolymère d'acrylate et de styrène, exemple de plastifiants et de solvants, telle celle vendue sous la dénomination commerciale de latex S 320 D par la Société dite BASF,
• - 10 kg d'une suspension aqueuse anionique à 40 % d'un copolymère d'acrylates telle celle vendue sous la dénomination commerciale de Acrosol par la Société dite BASF,
• - 5 kg d'un alcool polyvinylique de taux d'hydrolyse supérieur à 98 et de viscosité faible, soit 7 à 11 centipoises, cette viscosité étant mesurée en solution aqueuse à 4 % et à 20°C,
• - 20 kg d'une résine urée-formol anionique en solution dans l'eau à 63 % telle celle vendue sous la dénomination commerciale de résine 41-22 par la Société dite ROUSSELOT,
• - 1 kg de dispersant tel celui à base de polyacryl- amine vendu sous la dénomination commerciale de Polysel par la Société dite BASF.

On the sheet of paper thus produced, a preparation comprising the following products is coated on the surface using a surfacing press of the so-called "size-press" type:

• - 420 kg of powdered chalk, with an average particle diameter of 1 micron, and of which 80% of the particles have a diameter of less than 2 microns, such as that sold under the trade name of Omyalite by the company known as OMYA,
• 150 kg of an aqueous suspension at 50% of a copolymer of acrylate and styrene, example of plasticizers and solvents, such as that sold under the trade name of latex S 320 D by the company known as BASF,
• - 10 kg of an anionic aqueous suspension at 40% of an acrylate copolymer such as that sold under the trade name of Acrosol by the company known as BASF,
• 5 kg of a polyvinyl alcohol with a hydrolysis rate greater than 98 and a low viscosity, ie 7 to 11 centipoise, this viscosity being measured in aqueous solution at 4% and at 20 ° C.,
• - 20 kg of an anionic urea-formaldehyde resin in solution in water at 63% such as that sold under the trade name of resin 41-22 by the company known as ROUSSELOT,
• - 1 kg of dispersant such as that based on polyacrylamine sold under the trade name of Polysel by the company known as BASF.

We extend with water up to 1000 kg.

No fluorescent whitening agent or other optical dyes are introduced into this preparation.

This preparation is placed on the sheet to about 16 g / m ² , or 8 g / m ² on each side.

Finally, the sheet is subjected to calendering to give it a smoothness greater than 20 Bekk.

EXAMPLE II

A dough is prepared with the same materials as in Example I.

The sheet of paper thus produced is subjected to a surface coating by means of a trailing blade coater which is more efficient than the surfacing press of Example I and allows coating on one side with a more uniform deposit.

• - 530 kg de craie pulvérulente, d'un diamètre moyen de particules de 1 micron, et dont 80 % des particules ont un diamètre inférieur à 2 microns, telle celle vendue sous la dénomination commerciale de Omyalite par la Société dite OMYA,
• - 140 kg d'une dispersion aqueuse à 50 % d'un copolymère d'acrylate et de styrène, exempte de plastifiants et de solvants, telle celle vendue sous la dénomination commerciale de S 320 D par la Société dite BASF,
• - 6,5 kg d'une suspension aqueuse anionique à 40 % d'un copolymère d'acrylates, exempte de plastifiants et de solvants telle celle vendue sous la dénomination commerciale de Acrosol par la Société dite BASF,
• - 4,5 kg d'un alcool polyvinylique de taux d'hydrolyse supérieur à 98 et de viscosité faible, soit 7 à 11 centipoises, cette viscosité étant mesurée en solution aqueuse à 4 % et à 20°C, tel celui vendu sous la dénomination commerciale de Rhodoviol 8.20 par la Société dite RHONE-POULENC,
• - 15 kg d'une résine urée formol anionique en solution dans l'eau à 63 % telle celle vendue sous la dénomination commerciale de résine 41-22 par la Société dite ROUSSELOT,
• - 1 kg de dispersant, tel celui vendu sous la dénomination commerciale de Polysel par la Société dite BASF.

The coating is carried out with a basic preparation comprising the following products:

• - 530 kg of powdery chalk, with an average particle diameter of 1 micron, and of which 80% of the particles have a diameter of less than 2 microns, such as that sold under the trade name of Omyalite by the company known as OMYA,
• 140 kg of a 50% aqueous dispersion of an acrylate and styrene copolymer, free of plasticizers and solvents, such as that sold under the trade name of S 320 D by the company known as BASF,
• 6.5 kg of an anionic aqueous suspension at 40% of an acrylate copolymer, free of plasticizers and of solvents such as that sold under the trade name of Acrosol by the company known as BASF,
• 4.5 kg of a polyvinyl alcohol with a hydrolysis rate greater than 98 and a low viscosity, ie 7 to 11 centipoise, this viscosity being measured in aqueous solution at 4% and at 20 ° C., such as that sold under the trade name of Rhodoviol 8.20 by the company known as RHONE-POULENC,
• - 15 kg of an anionic formalin urea resin in solution in water at 63% such as that sold under the trade name of resin 41-22 by the company known as ROUSSELOT,
• - 1 kg of dispersant, such as that sold under the trade name of Polysel by the company known as BASF.

The preparation is extended to 1000 kg by adding water and any introduction of bleaching agents or other optical agents is avoided.

A deposit of the order of 12 to 15 g / m is carried out on the side to be printed.

Finally, we calender the sheet to give it a smoothness greater than 20 Bekk.

EXAMPLE III

• - pâte chimique blanchie de bois feuillus contenant moins de 30 impuretés visibles au kg 62 %
• - pâte chimique blanchie de bois résineux contenant moins de 30 impuretés visibles au kg 20 %
• - Talc 18 %
• - Produits de collage acide conventionnels en quantité minimum,
• - agent de rétention cationique (amidon cationisé, tel celui vendu sous la dénomination commerciale de Cato par la Société dite ROQUETTE).

A pulp is produced by introducing the following materials upstream into the paper machine:

• - bleached chemical pulp of hardwood containing less than 30 visible impurities per kg 62%
• - bleached chemical pulp of softwood containing less than 30 visible impurities per kg 20%
• - Talc 18%
• - Conventional acid bonding products in minimum quantity,
• - cationic retention agent (cationized starch, such as that sold under the trade name Cato by the company known as ROQUETTE).

Absolutely avoid any fluorescent dye or other bleaching agents and limit refining to a minimum.

Using a surfacing press, a coating is carried out with a starch preparation based on starch degraded by enzymation at a concentration of 10%, free of any fluorescent dye and any electrolyte.

We calender the paper to give it a smoothness greater than 30 Bekk.

The porosity of the finished paper, which must be between 200 and 500, is measured, measured according to the Bendtsen method. This porosity is the index of a weak refining, according to the known law linking refining to porosity, the latter being a decreasing function of refining.

Furthermore, this paper produced in an acid medium has a pH of less than 7 measured by the method of "determining the pH of the aqueous extracts of the papers" as described in standard AFNOR Q 03 005 or standard ISO / DIS 6588.

The table below brings together the analytical results of the papers according to the invention produced according to examples I, II and III above.

Note that the surface resistivity of the paper in Example I at 75% relative humidity (RH) is only 0.98 x 10 ¹⁰ although it is suitable _- not perfectly for printing by magnetography.

However, because the spacing of the surface resistivity measurement electrodes is much greater than the thickness of the paper, Keithley's apparatus seems incapable of measuring only the resistivity of the surface without simultaneously measuring that of the interior paper, unless the surface and interior are very different electrically. However, in the case of pigmented paper like that of Example I, actually having an insulating surface reaching the resistivity threshold at 75% relative humidity required according to the invention, the desired object is obtained, although the apparatus de Keithley does not allow this resistivity threshold to be measured.

The use of calcium carbonate (chalk) as a filler is favorable for obtaining a high resistivity. However, it is difficult to use in an acid medium due to its decomposition.

Under the aforementioned conditions of exposure, successively at 20% and 50% relative humidity, a paper according to Example III must have a surface resistivity greater than ₃ . ₁ 0 ¹²

This paper being homogeneous in its thickness, the density-surface resistivity relationship applies well in this case despite the limits of the Keithley apparatus unlike the coated and pigmented papers of Examples I and II.

Of course, the invention is in no way limited to the examples described, it is capable of numerous variants accessible to those skilled in the art: according to the applications envisaged and without departing from the scope of the invention.

Claims (8)

1. Magnetography printing process under ambient atmospheric conditions of relative humidity which can exceed 65%, in which a fusible pulverulent powdery magnetized ink distributed according to the configuration to be reproduced is transferred to a temporary magnetizable support, then fixes the ink by fusion on the sheet of paper, characterized in that a sheet of paper is used whose surface resistivity under the conditions of relative humidity of transfer is at least 10 ¹⁰ ohms square, and whose smoothness is at least 20 Bekk. 2. Sheet of paper for printing by magnetography in ambient atmospheric conditions of relative humidity which may exceed 75%, for the implementation of the method of claim 1, characterized in that it has a surface resistivity of at least minus 10 ¹⁰ ohms square, measured with the Keithley device after exposure to 20 ° C in a 30% relative humidity atmosphere for one hour, then to 23 ° C in a 50% relative humidity environment for at least 10 o'clock, and in that it has a smoothness of at least 20 Bekk. 3. Sheet of paper according to claim 2, characterized in that it practically does not contain no fluorescent bleach or electrolyte. 4. A sheet of paper according to claim 2 or 3, characterized in that it has a surface layer of mineral pigments in a non-hydrophilic binder. 5. Sheet of paper according to claim 4, characterized in that the mineral pigment consists mainly of powdered calcium carbonate with an average diameter of particles of the order of a micron and of which about 80% of the particles have a diameter less than about 2 microns. 6. Sheet of paper according to claim 4 or 5, characterized in that the binder consists at least in part of a substance from the group comprising copolymers of acrylate and styrene, polyvinyl alcohol, urea-formaldehyde resins, without plasticizer or solvent. 7. Sheet of paper according to claim 2 or 3, characterized in that it has a porosity of the order of 200 to 300 measured according to the Bendtsen method and a pH of less than 7. 8. Document printed by magnetography, characterized in that it comprises a paper sheet support according to any one of claims 2 to 7.

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