EP0252884A2 - Process for the resolution of printed images - Google Patents

Abstract

A process is described with the help of which high resolution diagrams can be achieved by means of printing processes in which individual dots are transferred. The process is characterised by a pre-treatment of the surface to be printed with an oil- and water-repellent coating of a fluoroaliphatic material. <IMAGE>

Description

The present invention relates to a method for achieving high resolution in printing methods in which the imaging takes place by means of individual dots. In particular, the invention relates to a method for preventing the lateral diffusion of the punctiform printing media on the substrates to be printed.

With the exception of thermal or laser printing or photographic printing processes in which the images are produced in heat- or light-sensitive layers and similar printing processes in which homogeneous media and substrate elements are used, the printing medium in the other printing processes is in the form of droplets or drop-like elements applied to the substrate. Inks, dyes, inks and similar media are brought into contact with the substrates to be printed in order to produce an image. The different media can be black or colored. They can also be based on oil, water or solvents.

The substrate can either be coherent films or nonwovens made of woven or non-woven fibrous material or sheets of caked, ground hollow balls or scales. The coherent films can be, for example, polymer films or sheets with a skin-like structure, such as parchment. The nonwovens usually consist of woven or non-woven textile fibers or are felt-like materials such as paper.

The printed image is created by applying the print medium either by direct contact transfer, such as in lithography, offset printing, screen printing, electrophotography or high pressure; or by absorption transfer, such as in gravure printing; or by spray transfer, the finely divided medium being sprayed onto the substrate. The control takes place, for example, by electrostatic charging, such as in the continuous jet process or by targeted application of pressure surges on the medium, such as in the pulse jet process (drop-on-demand process) or by applying pressure to the medium, such as in the case of Spraying colors during the production of billboards or large-scale advertising images.

When using colors in lithography, the image resolution can also depend on the pigments and their grain size distribution in the respective color or the drawing medium, which is to be transferred from the printing form to the paper to be printed.

In gravure printing, the resolution of the halftone image is limited by the resolution of the rotogravure form, which is used to resolve the image into its individual halftone dots.

With high-resolution processes of this type, 8 to 16 individual points per millimeter are generally applied.

Such a standard is sought in ink jet printing processes. Newer and improved inkjet printers are also able to achieve such a resolution.

Currently, the diffusion of the ink onto and / or into the substrate is the limiting factor in the engraving and / or ink jet printing processes, although a high standard of resolution could be achieved with the gravure form and / or the ink jet.

So far, it has been found that the drop diameter increased by 100 to 300%, based on the diameter of the drop originally applied, due to diffusion of the ink.

It is relatively easy to measure the droplets created in the ink jet and on the printed substrate; a comparison of droplet size on the printing plate with the drop size on the substrate shows that the diffusion of the drop is approximately in the same order of magnitude of the drop size, or about + _100% to + 300 _^0/0 by weight.

It would therefore be desirable to control and reduce this diffusion in order to achieve greater resolution in said printing processes.

So far, the problem of printing ink diffusion has been addressed by modification thereof, i.e. For example, viscosity-regulating additives are used or fast-drying solvents have been added to the ink.

Various surfactants are also added to the ink; this is done primarily to lower the surface tension of the printing ink in order to bring about a wetting of the substrate so that the ink is quickly absorbed by the paper and thus dries quickly.

However, a rapid absorption of the printing ink into the substrate surface is associated with a vertical and lateral spreading of the ink; this is usually undesirable if, for example, a high color density is desired.

The lateral spreading of the ink dot also leads to a reduction in the image resolution, since the dot is enlarged and thus the separation between a single dot and its neighbors is reduced.

Lateral spreading is also known in the art as feathering or dot-gain.

The current state of the art of ink jet printing with colored printing inks is described by Bjorkengreen et al. in Journal of Imaging Technology, 11, 1-3 (1985). In it are ink jets in the order of 5 - 10 j.Lm and systems that drop this size from larger Generate jet openings, described.

US Pat. No. 4,419,388 describes a method which prevents the ink dots from fusing after printing. The image is coated with a water-repellent composition after the ink jets have been applied to the paper and after the ink has dried; this composition contains certain alum known as water-repellent paper coating agents. However, this method cannot prevent dot enlargement or ink flow during image formation. Guenthner and Putnam et al. in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, 10, 897 (1980) and 16, 803-825 (1980) describe further approaches for controlling the drying or absorption of the ink. These articles describe additives for papermaking and in particular paper sizes that are added to the "varnish" during papermaking.

Such techniques are also described in Schwartz's recently published U.S. Patent 4,426,466, which describes paper sizes that impart both oil and water repellency. These glues are added to the varnish or applied to the finished paper; such quantities and media are used so that said glues can penetrate the cellulose fibers. In U.S. Patent 4,442,172 to Oshima et al. a surface coating for inkjet printing is proposed. These are essentially mixtures of synthetic zeolites and glass fibers, the zeolite particles leading to good ink absorption and the glass fibers reducing the lateral spread of the ink.

However, it remains to be noted that the size of the ink jet drops applied to the treated paper differs by ± 2.7-5.5% from the size of the drops on correspondingly untreated paper.

Finally, JP-OS 60-260,376 describes a recording material which contains an outer and an inner ink absorption layer, the latter containing water and a fluorine-containing oil-repellent.

The recording material allows an improved reproduction of the droplet density, the color tone and the ability to absorb the ink.

In the previous approaches to prevent the ink flow and the spreading of the transferred ink droplets, water-repellent paper glues or coatings or water-repellent additives are used in the printing inks.

Such processes are partially effective when using water-based printing inks; however, they are of limited value when printing inks based on oil or based on oil and other organic solvents are used.

When used in ink jet printers, the dot size that can be achieved with water-based inks is even about five times as large as the opening of the jet mouth. The size of this mouth is limited by the diffusion characteristics on the paper and by the equipment available, and the resolution in prior art methods is limited by the size of the ink dot sprayed on.

The present invention relates to a printing method for printing paper, the size of the drops to be transferred by the printing method being reduced and the independence of the individual transferred drops of reduced size also being retained after the image has been produced.

Furthermore, it can be determined in the fadeometer test that the color density of color images and the lifespan of such color images are generally increased by up to 200% by the method of the present invention.

Such and other objects and advantages can be achieved by the present invention, which is a method for improving the resolution and image quality of printing methods in which ink drops are transferred or in which contact pressure is applied.

The method comprises the step of contacting the ink, which contains dyes, pigments or dye precursors customary in such methods, with a surface to be printed; said surface is pretreated with a coherent coating of a composition before printing, so that said surface has oil and water-repellent properties to such an extent that the interfacial tension between the coating and ink also forms spherical droplets adhering to said coated substrate limited contact area leads to said substrate.

As a rule, these ink drops are hemispherical and dry on the coated substrate within the dimension specified by the interfacial tension.

Another result of the method according to the invention is a reduction in the size of the printed dots by more than half compared to a control experiment (same ink and same ink jet recorder). There is also a relationship between the diameter of an ink dot and the life of the color density. If the dimension of the ink drop is reduced, the lifespan of the color density increases logarithmically.

The combination of reduced dot size and the associated color density leads to a higher resolution in conventional inkjet printing processes.

The invention therefore relates to a method for improving the quality of images which can be obtained by printing methods in which individual dots are transmitted, comprising the steps.

• i) applying a coating of an oil and water repellent composition to the surface to be printed and
• ii) contacting the printing ink in the form of discrete areas or in the form of drops with the coated surface.

• Q eine mehrwertige Brückengruppe ist, die wenigstens eine Kohlenstoff-Kohlenstoff Doppelbindung oder eine cyclische oder nicht cyclische Gruppe mit wenigstens einem Heteroatom aufweist oder ein Rest -C_nH zn- ist, worin n 1 bis 20 bedeutet, oder ein mehrwertiger cycloaliphatischer oder polycycloaliphatischer Rest ist, und
• X eine löslichkeits- oder emulgierbarkeitsvermittelnde Gruppe ist, die wenigstens eine ionisierbare Funktion im Molekül bereitstellt.

The composition for achieving the oil and water repellent properties on the surface to be printed contains an effective amount of a fluoroaliphatic oil and water repellent wetting agent or derivative of the general formula R _f -QX, where R _{f is} a fluoroaliphatic radical,

• Q is a polyvalent bridging group which has at least one carbon-carbon double bond or a cyclic or non-cyclic group with at least one heteroatom or is a radical -C _n H zn-, in which n is 1 to 20, or a polyvalent cycloaliphatic or polycycloaliphatic radical is and
• X is a solubilizing or emulsifying group that provides at least one ionizable function in the molecule.

There are preferably one to three groups R _f , which can either be identical or different from one another.

R _f is usually a mono- or divalent group containing at least three fully fluorinated carbon atoms.

The aliphatic chain generally has a length of 3 to 20 carbon atoms, preferably 6 to 16 carbon atoms and particularly preferably 8 carbon atoms.

The main chain of the radical R _f can be straight-chain or branched or branching can occur so that alicyclic groups are formed, each of which is fluorinated.

Q is preferably a multivalent bridging group with at least one carbon-carbon double bond or a cyclic or non-cyclic group with at least one heteroatom.

, -CO-, -NR'-, -CONR'-, -CON

, -S0₂ N

, -SO_z NR'-, -SOz -, -C_nH _2n-,- CH=CH-, -O-C_z H₄ -, -C₆ H₄ -, -C₆ H3

,

C₆ H₂

, -C₃ H₃ Cl-, -C₆ CI4 -, heteroaromatische Reste, cycloaliphatische Reste und polycyclische aliphatische Reste, worin R' Wasserstoff oder C₁ -C₄ -Alkyl ist und n 1 - 20 bedeutet.The polyvalent bridge group Q includes, for example, the radicals, -O-, -S-, -N

, -CO-, -NR'-, -CONR'-, -CON

, -S0 ₂ N

, -SO _z NR'-, -SOz -, -C _n H _2n -, - CH = CH-, -OC _z H ₄ -, -C ₆ H ₄ -, -C ₆ H3

,

C ₆ H ₂

, -C ₃ H ₃ Cl-, -C ₆ CI4 -, heteroaromatic radicals, cycloaliphatic radicals and polycyclic aliphatic radicals, wherein R 'is hydrogen or C ₁ -C ₄ alkyl and n is 1-20.

Preferred bridging groups are the sulfur-containing units and -O- mentioned above.

As group X, there is at least one solubility- or emulsifiability-imparting group which provides an ionizable unit, so that the entire molecule is soluble or emulsifiable in the application medium.

The preferred application medium is water.

Solubility-imparting groups which meet the criteria indicated above for water are, for example, carboxylic acids, their salts, esters or anhydrides; Amic acids; Urethanes; Amines; Amides; Carbamates; soluble metal salts, quaternary ammonium salts or ammonium phosphates.

, worin m 1, 2 oder 3 ist, o 1 oder 2 bedeutet und Rm⊕ ein ein- bis dreiwertiges Metal ist oder, falls m = 1, zusätzlich NH⊕₄ oder hydroxyalkyl-substituiertes Ammonium sein kann.Ammonium phosphates and carboxylic acid salts are particularly preferred in the context of this invention. Very particularly preferred groups X are - (COOo) m Rm⊕ and

, wherein m is 1, 2 or 3, o is 1 or 2 and Rm⊕ is a mono- to trivalent metal or, if m = 1, can additionally be NH⊕ ₄ or hydroxyalkyl-substituted ammonium.

The fluorine compounds identified above can be produced by methods known per se. These include, for example, electrochemical fluorination and telomerization, with intermediate products being formed which can be converted into the desired fluorine compounds by means of known processes. Such processes are, for example, hydrolysis, condensation reactions or addition reactions. Suitable preparative processes for the preparation of such fluorine compounds and intermediates are described, for example, in the above-mentioned article by Guenthner et al. described. Representative examples of fluoroaliphatic compounds are mentioned, for example, in U.S. Patents 3,989,725, 4,239,915, 4,267,087, 4,419,298, 4,426,666 and 4,515,640.

The preparation and identification of these compounds is described in these US patents and the content of these patents is also the subject of the present description.

In general, the preferred compounds used for coating impart sufficient surface energy to the substrate such that the dimensions of the ink droplet or spot on the surface are restricted and minimized as the ink dries or rapidly into the substrate or the coating is absorbed.

The magnitude of surface tension required in individual cases is determined by the specific velocity of the droplets in the ink jet and by parameters of the ink, such as viscosity, size and content of ink additives. The type and amount of the surface-active component selected in the individual case should therefore ensure that the finished coated surface, after drying, is both oil- and water-repellent, and that liquid drops form spheroids of minimal diameter upon contact and dry quickly.

As a result of the reducing effects, the ink drop does not diffuse laterally and can therefore only penetrate into the coating and the underlying substrate within the contact area. The wetting agent-containing compositions which are used for the coating may also contain an application medium, for example water, organic solvents or mixtures thereof. Conventional additives and auxiliaries can also be contained in these coating compositions. Examples of these are leveling agents, pigments, UV absorbers, light stabilizers, fillers, roughening agents, plasticizers, extenders, water-repellent additives and other additives which are usually used for the production of surface coatings for paper or for films to be printed.

In the case of inkjet printing, the coating agent should be applied to the substrate in an amount of 0.05-1.0 g / m ² , preferably 0.05-0.10 g / m ² , and the coating agent should be 0.005-10 wt. 0 / o, preferably 0.005 - 2% by weight, based on the composition, of surface-active component, so that the desired control of the point density and the point diameter can be achieved.

Any conventional coating method can be used to coat the substrate.

For the purposes of the present invention, any substrate can be used as a printing surface which can be coated with a composition which, after drying, gives a coherent film of coating agent which has the surface tension defined above and oil and water repellent properties.

For example, sized and unsized fine papers are suitable for coating in the manner described above and produce high-resolution images in accordance with the present invention. The papers can contain cellulose fibers or other fibers or mixtures of such fibers.

In addition to the conventionally used felt-like papers, the method according to the invention can also be carried out on recently used papers which are produced from polymer flakes or on printing surfaces made of polymer films. All of these substrates can be provided with oil and water repellent coatings and used in the process of this invention.

Printing surfaces which are preferably used are surfaces of substrates selected from the group consisting of coherent films and layers of woven or non-woven synthetic or polymeric fibers or particles or of cellulose fibers or particles.

The invention identified above is suitable for use in a variety of printing processes. However, it is explained in the present description and in the examples given with the aid of the ink jet printing method, since the individual and separate ink jet drops are accessible for direct measurement when they leave the pen openings, move through the air gap, hit the surface to be printed and on the surface Dry the surface. This method can therefore be used to examine the invention in more detail, and to explain its effect and the reasons for the results obtained. Inkjet printers are usually small table devices with which generally comparable results can be achieved as with a large offset printing press. They are particularly suitable for the reproduction of texts, drawings or color image photographs. Such printers are commonly referred to as "non-contact" because no printing plate or roll is required that comes into contact with the material to be printed.

An inkjet printer usually sprays a series of microscopic droplets of ink onto the paper to create an image.

With these printing processes, there are still some problems with regard to the image resolution and the light stability of the images.

The present invention leads to an improvement in these image properties.

Water-based printing inks for inkjet printing generally contain humectants in order to prevent evaporation from the ink supply vessel and thus clogging of the pen opening. However, if the ink has left the pen opening and has hit the paper, the drop must be dried as quickly as possible.

Manufacturers of printing papers for ink jet printing have attempted to solve this problem by producing relatively absorbent papers (with PVA and silicate content). The absorbency of the paper partially prevents the ink from running. However, when it hits the paper surface, the drop of ink increases by a multiple of the size of the original drop.

In order to improve the image quality, it is therefore useful if one can control the size of the ink drops on the paper and increase the resolution of color ink jet printing images by adding oil- and water-repellent wetting agents to coatings of ink jet paper.

• a) Verbesserte Auflösung der Tintenstrahlabbildungen,
• b) vergrösserte Lebensdauer der Farbdichte und
• c) erhöhte Farbdichte.

This type of control has several advantages:

• a) improved resolution of the inkjet images,
• b) increased life of the color density and
• c) increased color density.

The following examples illustrate the invention.

Example 1: A number of oil- and water-repellent coating additives are examined for their usability, the droplet density and the trench diameter of images produced by inkjet printing.

Test coatings are produced on unsized paper (Schoeller C-15 paper) and an application amount of 0.1 g / m ² (based on the amount of non-volatile constituents) is used. The basic formulation for the coating compositions consists of: 13.01 g of polyvinyl alcohol chips (MW 22,000), 1.00 g wetting agent (Nical BX, (8% w / v) and up to a total of 100.00 g deionized water.

• A. 0,73 g des jeweiligen Additivs
• 15,00 g deionisiertes Wasser
• B. 12,60 g der oben definierten Basisformulierung
• 2,00 g kollodialer Kieselsäure.

The ready-to-use oil and water repellent coating agents are made by mixing suspensions A and B:

• A. 0.73 g of the respective additive
• 15.00 g of deionized water
• B. 12.60 g of the basic formulation defined above
• 2.00 g colloidal silica.

Suspensions A and B are mixed and then dispersed in an ultrasonic bath. The pH is adjusted to 7.0 with sodium hydroxide solution. The unsized paper is treated with the coating solution obtained. A doctor blade (Meyer # 2) is used and coated at a speed of approximately 11.4 cm / min. The amount applied is about 0.1 g / m ^2. After coating, the paper is dried at 50 ° C. for two minutes.

Ink jet images are formed on this coated paper. Similar paper is used in the comparison test, but is not coated.

The density of individually selected drops in the test images is determined with a microdensitometer, the gap length of which is 63 µm and the gap width of which is 0.8 µm. When recording, 1 millimeter corresponds to 1.99 µm on the pen.

Table I lists the various oil and water repellant additives used in the coating compositions. This table also contains the micro density of the printed ink dots and the diameter of these dots.

The numerical data are mean values which are obtained from representative ink dots of the sample from different runs of the microdensitometer.

Compounds A and E: R _f is a mixture of -C ₆ F ₁₃ , -C ₈ F ₁₇ and - C ₁₀ F ₂₁ Compounds F, G and H: R _f is -C ₈ H ₁₇ Compounds B and D: R _f is a mixture of -C ₆ F ₁₃ , -C ₈ F ₁₇ , -C ₁₀ F ₂₁ and -C ₁₂ F ₂₅ . Compound C: R _f is a mixture of -C ₈ F ₁₇ , -C ₁₀ F ₂₁ and -C ₁₂ F ₂₅ .

It can be seen from the values in Table I that the method according to the invention generally leads to a drop size drop of more than half of the control test.

Example 2: The samples produced according to Example 1 are loaded in a xenon fadeometer for 18.4, 39.7, 55.4 and 77.2 hours. After each interruption, the values of the macroscopic density (aperture 3 mm) of dark blue (cyan), magenta, yellow and black dots are determined. Tables II, III and IV list the densities of the individual colors according to the fadeometer load.

It can be seen from Tables II, III and IV that the service life of the color density is increased in a large number of cases by the use of the additives.

Furthermore, it can be shown by statistical analysis that there is an 820% correlation between the diameter of the ink drop and the lifetime of the color density.

This relationship is shown in Figure 1, which shows this correlation for the magenta dye.

This dye is selected because it has the lowest light fastness of the dyes used in color inks for inkjet printing.

The color density after exposure is an important property in the field of the inkjet printing process, since further areas of application are to be opened up for this method. For example, engraving processes in the manufacture of large-scale advertising billboards are to be replaced by such processes.

Such billboards are exposed to extreme weather conditions. Fadeometer tests are useful tools for estimating the service life under long-term exposure.

It can thus be seen that the coatings which are used in the printing process of the present invention allow a higher image resolution since the independence of the individual printed dots is preserved. The dots do not run into one another due to smearing and lateral diffusion of the ink and thus the blurred appearance is avoided, which can often be seen on printed products of ink jet printing, letterpress printing or gravure printing, in which uncoated paper or size or coated papers are used, which have only water-repellent or only oil-repellent properties.

In the printing process according to the invention, specifically coated papers are used, as a result of which the ink droplets are arranged in a pearl-like manner and thus result in smaller droplets on the paper, which limits the area within which the dyes are absorbed. Furthermore, with the decreasing drop diameter, the lifespan of the color density increases logarithmically (cf. Figure 1 and the image resolution obtainable by the method according to the invention will also be present after long exposure of the image.

The present invention thus represents a new approach for improving images obtainable by single-dot printing processes. The surface to be printed is coated in the manner characterized above.

Variations of the process steps, proportions and materials are of course possible and are also encompassed by the invention defined in the following claims.

Claims

• 1. Procedures for improving the quality of images that are obtainable by printing processes in which individual dots are transferred include the steps.
  • i) applying a coating of an oil- and water-repellent wetting agent of the general formula R _f -QX to the surface to be printed, in which R _{f is} a fluoroaliphatic radical, Q is a polyvalent bridging group which has at least one carbon-carbon double bond or a cyclic or not cyclic group having at least one hetero atom, or a radical -C _n H _{2 n} -, where n is 1 to 20, or a polyvalent cycloaliphatic or polycycloaliphatic radical, and X is a solubility or dispersibility-imparting group which has at least one ionizable Provides function in the molecule, and
  • ii) contacting the printing ink in the form of discrete areas or in the form of drops with the coated surface.
• 2. The method according to claim 1, wherein one to three radicals R _f are present and R _{f is} a mono- or divalent fluoroaliphatic group which has at least three completely fluorinated C atoms and a straight-chain, branched or cycloaliphatic C ₃ -C ₂₀ group.
• 3. The method according to claim 2, wherein said aliphatic group has 8 carbon atoms.
• 4. The method according to claim 1, wherein Q is a polyvalent bridging group which has at least one carbon-carbon double bond or a cyclic or non-cyclic group having at least one heteroatom.
• 5. The method according to claim 1, wherein Q is selected from the group consisting of
  
  
  , heteroaromatic radicals, cycloalipatic radicals and polycycloaliphatic radicals, wherein R 'is hydrogen or C ₁ -C ₄ alkyl and n is 1 to 20.
• 6. The method according to claim 5, wherein Q is selected from the group consisting of
  
• 7. The method according to claim 1, wherein X is selected from the group consisting of carboxylic acids, salts of carboxylic acids, esters and anhydrides, amic acids, urethanes, amines, amides, carbamates, soluble metal salts, quaternary ammonium salts or ammonium phosphates.
• 8. The method according to claim 7, wherein X is selected from the group consisting of - (COO) Rrne and
  
  wherein m is 1, 2 or 3, o is 1 or 2 and Rm⊕ is a mono- to trivalent metal or, if m = 1, can additionally be NHe or a hydroxyalkyl-substituted ammonium.
• 9. The method according to claim 1, wherein said wetting agent is applied in an aqueous medium.
• 10. The method according to claim 9, wherein said aqueous composition contains 0.005 to 10% by weight, based on the composition, of the wetting agent and is applied in an amount of 0.05 to 1.0 g / m ² .
• 11. The method according to claim 1, wherein said surface is the surface of a substrate which is selected from the group consisting of coherent films and layers of woven or non-woven synthetic or polymeric fibers or particles or of cellulose fibers or particles.
• 12. The method according to claim 1, wherein a water-based ink is used.
• 13. The method according to claim 1, wherein an oil-based ink is used.

Claims (1)

14. The method according to claim 1, wherein said wetting agent is selected from the group consisting of

and

where R

is a mixture of radicals of the formulas -C ₆ F ₁₃ , -C ₈ F ₁₇ and -C ₁₀ F ₂₁ , R is a mixture of radicals of the formulas -C ₈ F ₁₇ , - C ₁₀ F ₂₁ and -C ₁₂ F _{25 1} and R

, is a mixture of residues-C ₆ F ₁₃ , -C ₈ F ₁₇ , -C ₁₀ F ₂₁ and-C ₁₂ F ₂₅ .

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