EP0627327B1 - Offset printing process and aqueous composition therefor - Google Patents

Description

The present invention relates to an offset printing process and an aqueous composition for offset printing.

Traditionally, in offset printing (or lithography), a dampening water containing isopropyl alcohol, commonly referred to as IPA, is commonly used, in order to improve on the one hand, the dampening of the hydrophilic zones of the offset plate and of favor on the other hand, the emulsification of water in ink or varnish.

The use of IPA, however, has many drawbacks due to the volatility, its flammability and its toxicity, so it has been proposed to replace it with various glycols possibly associated with glycol ethers or with water-soluble polymers. such as carboxymethylcellulose, hydroxypropylmethylcellulose, polyvinylpyrrolidone (cf. in particular US-A-4,865,646, 4,641,579, 4,548,645 and 4,278,467). However, the dampening water obtained by the use of these products does not entirely satisfy the printer. However, the Applicant has surprisingly discovered an offset printing process which does not require the use of IPA and which, moreover, improves the printing process both in terms of machine work with ink or varnish, which '' in terms of the qualities of the print: gloss, contrast, mechanical resistance, superposition of colors.

dans laquelle R représente un atome d'hydrogène ou un radical méthyle et M représente un atome d'hydrogène, de sodium, de potassium, ou un groupement ammonium, soit d'un ou plusieurs monomères cationiques de formule (II) :

dans laquelle R a la signification donnée précédemment, Z représente un atome d'oxygène ou un groupement NH, R₁ représente un radical méthyle ou éthyle, R₂ représente un atome d'hydrogène, ou un radical méthyle, éthyle ou benzyle, X représente un atome de chlore ou un groupement OSO₃R₂ dans lequel R₂ a la signification donnée précédemment et n représente 2 ou 3.The offset printing process according to the present invention is characterized in that it is carried out in the presence, either in fountain water, or in the ink or varnish used, or in water wetting and in the ink or varnish used, of a copolymer, hereinafter designated P, ionic, hydrophilic, crosslinked, insoluble in water but swellable with water, in the form of particles swollen with water '' an average diameter of less than 1 μm, based on acrylamide, hereinafter designated AAM, of a copolymerizable diethylenic crosslinking agent and either of one or more anionic monomers of formula (I).

in which R represents a hydrogen atom or a methyl radical and M represents a hydrogen, sodium or potassium atom, or an ammonium group, or one or more cationic monomers of formula (II):

in which R has the meaning given above, Z represents an oxygen atom or an NH group, R ₁ represents a methyl or ethyl radical, R ₂ represents a hydrogen atom, or a methyl, ethyl or benzyl radical, X represents a chlorine atom or an OSO ₃ R _{2 group} in which R ₂ has the meaning given above and n represents 2 or 3.

The expression “ionic copolymer” denotes a copolymer which is either anionic or cationic.

Among the monomers of formula (I), there may be mentioned: acrylic acid and its sodium, potassium or ammonium salts, methacrylic acid and its sodium, potassium or ammonium salts.

Among the monomers of formula (II), there may be mentioned: N, N-2-dimethylaminoethyl acrylate salified with hydrochloric acid or quaternized with methyl chloride or ethyl chloride, sodium acrylate N, N-2-diethylamino-ethyl salified by hydrochloric acid or quaternized with methyl chloride or ethyl chloride, N, N-2-dimethylamino-ethyl methacrylate salified by hydrochloric acid or quaternized with methyl chloride or ethyl chloride, chloride of (meth) acrylamidopropyltrimethylammonium.

Among the polymerizable diethylenic crosslinking agents, mention may be made of methylenebisacrylamide, hereinafter designated MBA and bisacrylamidoacetic acid, hereinafter designated ABAA.

Among the copolymers P, there may be mentioned those anionic, based on acrylamide and acrylic acid partially salified with ammonia and crosslinked with MBA or ABAA and in particular ABAA and those, cationic, with base of acrylamide and acrylate of N, N-2-dimethylaminoethyl quaternized with methyl chloride, hereinafter referred to as CMA, and crosslinked with MBA or ABAA and in particular ABAA.

• les copolymères AAM - AA - AANH₄- ABAA, 29,6 - 65,1 - 5,1 - 0,2 en proportions molaires,
• les copolymères AAM - CMA - ABAA, 79,15 - 20,83 - 0,02 en proportions molaires.

Among these latter copolymers, there may be mentioned more particularly:

• the AAM - AA - AANH ₄ - ABAA copolymers, 29.6 - 65.1 - 5.1 - 0.2 in molar proportions,
• the AAM - CMA - ABAA copolymers, 79.15 - 20.83 - 0.02 in molar proportions.

• AAM : acrylamide
• AA : acide acrylique
• AANH₄ : acrylate d'ammonium
• ABAA : acide bisacrylamidoacétique
• CMA : chlorure d'acryloyloxy-2 éthyltriméthylammonium.

The abbreviations used have the following meaning:

• AAM: acrylamide
• AA: acrylic acid
• AANH ₄ : ammonium acrylate
• ABAA: bisacrylamidoacetic acid
• CMA: 2-acryloyloxy ethyltrimethylammonium chloride.

The copolymers P in the form of micro-particles swollen with water, with a diameter of less than 1 micrometer are known products.

The anionic copolymers are in particular proposed either as thickening agents in pigment printing (cf. EP-A-0 325 065) or in processes for coating papers (EP-A-0 501 846).

The cationic P copolymers are in particular proposed as thickening agents for the preparation of household softeners (cf. EP-A-0 494 554).

The copolymers P are hydrophilic copolymers which can absorb up to about 1,000 times their weight in water. As a result, when the microparticles of copolymer P swollen with water, with a diameter of less than 1 micrometer are brought into contact with water, they absorb up to approximately 1,000 times their weight of water and their diameter increases. up to about 10 times their initial diameter, however their water absorption capacity is a function of their crosslinking rate. The water absorbed by the microparticles of the copolymer P is readily available. In fact, as soon as these swollen particles of water are subjected to a stress, they release part of the absorbed water which they reabsorb as soon as the stress ceases.

Under preferential conditions for implementing the process according to the invention, a dispersion, designated below D, water in oil, self-reversible, containing in the dispersed aqueous phase the copolymer P in the form of swollen microparticles, is used. water, with a diameter of less than 1 µm.

By water-in-oil dispersion, self-reversible, is meant a dispersion in which the continuous phase is oil and the phase dispersed in water, reversing spontaneously and rapidly into an oil-in-water dispersion as soon as it is diluted with water. To do this, it suffices that it contains in addition to one or more oil-soluble emulsifiers, of HLB value less than 5, one or more water-soluble emulsifiers, of HLB value greater than 8, advantageously greater than 10, all of the emulsifiers having an overall HLB value greater than 8, advantageously greater than 10.

HLB denotes the hydrophilic-lipophilic balance of an emulsifier (cf. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, Vol. 8, pages 910-918).

The water-in-oil dispersions, self-reversible D, which can be used in the process according to the present invention, consist of a dispersed aqueous phase containing the copolymer P in the form of microparticles swollen with water, with a diameter of less than 1 µm, an oil continuous phase and an emulsifier system, designated E, containing on the one hand, an appropriate quantity of one or more oil-soluble emulsifiers, of HLB value less than 5, so that the dispersion is stable at room temperature, and on the other hand, one or more water-soluble emulsifiers, of HLB value greater than 8, advantageously greater than 10, in an amount such that all the emulsifiers constituting system E have an overall HLB value greater than 8, advantageously greater than 10.

The dispersed aqueous phase of dispersion D contains, in addition to the copolymer P, conventional water-soluble additives such as chelating agents to remove, if necessary, the polymerization inhibitor metals, buffering agents, transfer agents, polymerization initiators. However, the total weight of these various additives is less than or equal to about 5% of the total weight of the aqueous phase.

The oil phase consists of one or more hydrocarbons which are liquid at room temperature and have a boiling point above 100 ° C. Among these hydrocarbons, there may be mentioned straight or branched chain paraffins as well as commercial cycloparaffins, liquid at room temperature and having a boiling point of between 150 and 350 ° C. In addition, it can contain, in a proportion less than or equal to 5% of its weight, various conventional additives such as polymerization initiators or transfer agents.

Among the oil-soluble emulsifiers with an HLB value of less than 5, there may be mentioned the esters of fatty acids and of sorbitan such as sorbitan monoleate, sorbitan sesquioleate. Among the water-soluble emulsifiers with an HLB value greater than 8, nonylphenols ethoxylated with ethylene oxide can be mentioned, such as nonylphenol ethoxylated with 8 moles of ethylene oxide, nonylphenol ethoxylated with 10 moles ethylene oxide, sorbitan hexaoleate ethoxylated with 40 moles of ethylene oxide, sorbitan hexaoleate ethoxylated with 50 moles of ethylene oxide.

Dispersions D are known products and some of them are widely used in the textile industry in pigment printing. They are miscible in inks or varnishes used in offset printing, giving colloidal solutions containing the copolymer P swollen with water. They are also miscible in all proportions with water, giving an oil-in-water dispersion in which the particles of copolymer P are dispersed in the continuous aqueous phase.

The dispersions D can be obtained by known methods such as the water-in-oil reverse suspension polymerization methods widely described in the literature.

• (i) On prépare une émulsion eau dans huile en introduisant sous agitation une solution aqueuse contenant les monomères et éventuellement les additifs choisis, à un hydrocarbure liquide contenant un ou plusieurs émulsifiants appropriées pour obtenir une émulsion inverse constituée par de fines particules aqueuses contenant les monomères, d'un diamètre inférieur à 1 micron, dispersées dans la phase huile continue, le rapport volumique, Rv, volume de la phase aqueuse sur volume total de l'émulsion étant inférieur à 74 %, avantageusement inférieur à 71 %,
• (ii) on soumet ensuite cette émulsion, parfaitement désoxygénée, à une réaction de polymérisation radicalaire, en introduisant sous agitation un ou plusieurs amorceurs générateurs de radicaux libres tels que les dérivés azoïques comme l'azobisisobutyronitrile, AIBN, le dichlorate d'azobisamidinopropane, ABAH, les peroxydes comme le persulfate de sodium ou d'ammonium, l'hydroperoxyde de cumène, les systèmes rédox,
• (iii) puis lorsque la réaction de polymérisation est terminée, on introduit dans le milieu réactionnel refroidi, la quantité appropriée d'un ou plusieurs émulsifiants solubles dans l'eau, de manière à ce que la valeur HLB globale de l'ensemble des émulsifiants soit supérieure à 8, avantageusement supérieure à 10. Eventuellement, si désiré, le copolymère P peut être isolé du milieu réactionnel avant l'introduction du ou des émulsifiants solubles dans l'eau par l'addition d'un solvant miscible à l'eau et à l'huile qui précipite le copolymère tel que l'acétone, suivie d'une filtration pour isoler le copolymère qui est séché puis redispersé dans de l'eau.

To do this, for example

• (i) A water-in-oil emulsion is prepared by introducing, with stirring, an aqueous solution containing the monomers and optionally the selected additives, to a liquid hydrocarbon containing one or more emulsifiers suitable for obtaining a reverse emulsion consisting of fine aqueous particles containing the monomers , with a diameter of less than 1 micron, dispersed in the continuous oil phase, the volume ratio, Rv, volume of the aqueous phase over total volume of the emulsion being less than 74%, advantageously less than 71%,
• (ii) this perfectly deoxygenated emulsion is then subjected to a radical polymerization reaction, by introducing, with stirring, one or more initiators generating free radicals such as the azo derivatives such as azobisisobutyronitrile, AIBN, azobisamidinopropane dichlorate, ABAH , peroxides such as sodium or ammonium persulfate, cumene hydroperoxide, redox systems,
• (iii) then, when the polymerization reaction is complete, the reaction medium is introduced into the cooled reaction medium. appropriate quantity of one or more water-soluble emulsifiers, so that the overall HLB value of all the emulsifiers is greater than 8, advantageously greater than 10. Optionally, if desired, the copolymer P can be isolated of the reaction medium before the introduction of the water-soluble emulsifier or emulsifiers by the addition of a solvent miscible with water and with the oil which precipitates the copolymer such as acetone, followed by filtration to isolating the copolymer which is dried and then redispersed in water.

• environ 71 ± 3 % d'une phase aqueuse contenant pondéralement environ 42 ± 3 % de copolymère P sous forme de microparticules gonflées d'eau, d'un diamètre inférieur à 1 µm.
• environ 25 ± 2,5 % d'une phase huile,
• environ 4 ± 1 % d'un mélange d'émulsifiants présentant une valeur HLB globale supérieure à 8 et constituée par plus de 50 % en poids d'un ou plusieurs émulsifiants de valeur HLB inférieure à 4.

Among the dispersions D which can be used in the process according to the present invention, there may be mentioned in particular the self-reversible water-in-oil dispersions containing by weight:

• approximately 71 ± 3% of an aqueous phase containing by weight approximately 42 ± 3% of copolymer P in the form of microparticles swollen with water, with a diameter of less than 1 μm.
• about 25 ± 2.5% of an oil phase,
• approximately 4 ± 1% of a mixture of emulsifiers having an overall HLB value greater than 8 and consisting of more than 50% by weight of one or more emulsifiers with an HLB value less than 4.

• AAM - AA - AANH₄ - ABAA, 29,6 - 65,1 - 5,1 - 0,2 en proportions molaires,
• AAM - CMA - ABAA, 79,15 - 20,83 - 0,02 en proportions molaires.

Among these latter dispersions, there may be mentioned more particularly those which contain a copolymer:

• AAM - AA - AANH ₄ - ABAA, 29.6 - 65.1 - 5.1 - 0.2 in molar proportions,
• AAM - CMA - ABAA, 79.15 - 20.83 - 0.02 in molar proportions.

Under more preferred conditions for carrying out the process of the invention, an aqueous composition is used, designated hereinafter C, containing in the microparticles swollen with water a copolymer P as defined above and of preferably about 0.1 to 10% by weight of the dispersion D defined above. This aqueous composition C, miscible with water, can be used directly to obtain fountain water for offset printing. This aqueous composition C, is obtained by simple mixing in water of the desired amount of the dispersion D.

Among these aqueous compositions C, mention may be made of those which contain 0.5% by weight of a dispersion D as defined above.

The process according to the invention is carried out very simply by introducing, with stirring, either into the fountain water, or into the ink or the varnish, or into the fountain water and into the ink or the varnish, used in a printing or offset process, from 0.01 to 3% by weight of the copolymer P as defined above.

According to preferential conditions for implementing the invention, the process described above is carried out, either by introducing, with stirring, into the wetting water and / or into the ink or varnish used, from 0.01 to 3% by weight of copolymer P present in a self-reversible water-in-oil dispersion D, as defined above, or by introducing into wetting water from 0.01% to 3% by weight of copolymer P present in an aqueous composition vs.

• il évite l'utilisation d'alcool isopropylique ou d'un composé hydroxylé tels qu'un polyol, un monoéther d'un glycol,
• il modifie l'émulsification de l'eau dans l'encre ou le vernis en améliorant leurs propriétés "offset",
• il modifie le tack de l'encre ou du vernis, à savoir le report de l'encre d'une feuille sur la suivante, et sur le rendu de l'imprimé :
• il améliore l'uni visuel des aplats,
• il favorise la superposition des encres et/ou des vernis,
• il augmente le brillant, particulièrement dans le cas des vernis,
• il permet une meilleure définition de l'impression du fait d'une meilleure séparation des zones imprimantes et non-imprimantes,
• il améliore les contrastes d'impression en permettant d'imprimer à une plus forte puissance de couleur dans les aplats.

The use of the copolymer P as defined above, either in the fountain water, or in the ink or the varnish, or simultaneously in the fountain water and in the ink or the varnish of a process of Printing by lithography and in particular in offset printing has the following advantages, in particular during printing:

• it avoids the use of isopropyl alcohol or of a hydroxylated compound such as a polyol, a monoether of a glycol,
• it modifies the emulsification of water in ink or varnish by improving their "offset" properties,
• it modifies the tack of the ink or varnish, namely the transfer of the ink from one sheet to the next, and on the rendering of the print:
• it improves the solid color,
• it favors the superposition of inks and / or varnishes,
• it increases the gloss, particularly in the case of varnishes,
• it allows a better definition of the printing due to a better separation of the printing and non-printing areas,
• it improves the printing contrasts by making it possible to print at a higher color power in solid colors.

The same results are obtained when, either in the fountain water and / or in the ink or varnish, the polymer P dispersed in the dispersion D as defined above is introduced, or in the fountain water the polymer P dispersed in the aqueous composition C as defined above.

The tests were carried out with an offset printing machine using the "standard test offset + tolerances system Brünner" marketed by the Company known as: System Brünner SA, CH - 6600 Locarno, Switzerland.

The tests, the application results of which are mentioned in Table I, were carried out by introducing into the fountain water, either 0.26% by weight of a dispersion prepared according to Example 1, or 1% by weight of a dispersion prepared according to Example 2, in comparison with a test carried out with a fountain water containing IPA, so as to obtain a surface tension of the order of 40 dynes / cm (40 mN / m or 40 mJ / m ² ). The tests mentioned were carried out on a 2-color offset machine, equipped with a UV dryer, with double-layer coated paper, "maine" marketed by the company Arjomari-Wetaggins, with "T4725 blue and red inks from the Plast range. "with an optical density, designated DO, of 1.5.

• un test d'abrasion des encres,
• un classement visuel sur les aplats,
• un contrôle qualité : DO (densité optique), trapping, brillance, élargissement de points (TF-TG),
• une évaluation de la qualité en analyse d'images :
  • PC % sur aplat,
  • spectre niveau de gris sur aplat,
  • circularité des points de trame (à 25 % de couverture) désignée pq.

In all these tests, we then produced on printed sheets:

• an ink abrasion test,
• a visual classification on the solid areas,
• quality control: DO (optical density), trapping, brightness, widening of points (TF-TG),
• a quality assessment in image analysis:
  • PC% on solid,
  • gray level spectrum on solid,
  • circularity of the weft dots (at 25% coverage) designated pq.

The ink abrasion test is carried out with a Wallace device, at 50 rpm, under a load of 2.5 kg, after 6 days.

TF-TG denotes the widening of the halftone dots.

Trapping denotes the superposition of one ink on another.

The gloss is determined with a gloss meter.

Image analysis allowing precise observation of the outline of the halftone dots is carried out after microscopic magnification; the minimum surface observed is 27 µm ² .

The percentage coverage of solid areas, referred to above as PC%, was measured at two levels of magnification. The quality of continuity of the solid areas was determined from the grain size curves at the different working resolutions. The results found are in agreement with the visual classification of the solid areas.

The gray level spectrum on the solid color was determined from microdensitometric curves at different magnifications on the solid colors. An increase in roughness is observed when the resolution power of the apparatus is increased. Whatever the magnification chosen, we find the results obtained by the visual classification.

The quality of the weft point contours is studied on wefts at 25% coverage, looking for the best circularity factor by determining the perimeter, designated pr, then from the area of 10 halftone dots. From the surface observed, we calculate the theoretical perimeter, designated pt, of a perfect disk of the same surface. The actual measured perimeter ratio, pr, minus the calculated theoretical perimeter, pt, over the theoretical perimeter leads to the circularity factor designated pq: $$\mathit{\text{pq}}\text{=}\frac{\mathit{\text{pr}}\text{-}\mathit{\text{pt}}}{\mathit{\text{pt}}}$$

EXAMPLES Preparation of a dispersion D

• 92 % d'un mélange d'hydrocarbures liquides, présentant un point d'ébullition de 310-342°C et contenant environ 80 % en poids d'hydrocarbures paraffiniques et 20 % en poids d'hydrocarbures naphténiques,
• 8 % de sesquioléate de sorbitane, HLB = 3,7, on introduit en 15 minutes, en atmosphère inerte et sous une agitation de 90 tr/mn, une phase aqueuse parfaitement désoxygénée, préparée extemporanément, d'un poids total de 140 000 g et contenant pondéralement :
• 42,9 % d'un mélange de monomères, M, défini dans le tableau I,
• n % d'acide citrique pur anhydre,
• 500 ppm de sel pentasodique de l'acide diéthylènetriaminepentaacétique, soit 7 g,
• 15 ppm, exprimé en proportions molaires par rapport à l'ensemble des monomères, d'un dérivé azoïque hydrosoluble Az, défini dans le tableau I.

In an oil phase, cooled to 10 ° C, with a total weight of 52,400 g, consisting by weight of:

• 92% of a mixture of liquid hydrocarbons, having a boiling point of 310-342 ° C and containing approximately 80% by weight of paraffinic hydrocarbons and 20% by weight of naphthenic hydrocarbons,
• 8% of sorbitan sesquioleate, HLB = 3.7, a perfectly deoxygenated aqueous phase, prepared extemporaneously, with a total weight of 140,000 g, is introduced over 15 minutes, in an inert atmosphere and with stirring at 90 rpm. and containing by weight:
• 42.9% of a mixture of monomers, M, defined in Table I,
• n% anhydrous pure citric acid,
• 500 ppm of pentasodium salt of diethylenetriaminepentaacetic acid, i.e. 7 g,
• 15 ppm, expressed in molar proportions relative to all of the monomers, of a water-soluble azo derivative Az, defined in Table I.

When the introduction is complete, the reaction medium is carefully oxygenated by bubbling nitrogen through and then subjected, while maintaining the temperature at 10 ° C., in an inert atmosphere, to vigorous stirring so as to obtain a Brookfield viscosity of 3.6 ± 0.2 Pa.s determined at 20 ° C, at a speed of 20 rpm with axis 4.

• 200 ppm en proportions molaires par rapport aux monomères d'hydroperoxyde de cumène en solution dans 850 g du mélange d'hydrocarbures utilisé dans la préparation de la phase huile, puis 5 minutes après la fin de l'introduction, on introduit en 45 minutes, sous agitation et en atmosphère inerte en laissant se développer la réaction de polymérisation exothermique :
• 112,5 ppm en proportions molaires par rapport aux monomères de disulfite de sodium en solution dans 3 000 g d'eau.

In the emulsion thus obtained having a temperature of 10 ° C and maintained under stirring in an inert atmosphere, the following are introduced in 5 minutes:

• 200 ppm in molar proportions relative to the cumene hydroperoxide monomers dissolved in 850 g of the mixture of hydrocarbons used in the preparation of the oil phase, then 5 minutes after the end of the introduction, the following are introduced in 45 minutes, with stirring and in an inert atmosphere, allowing the exothermic polymerization reaction to develop:
• 112.5 ppm in molar proportions relative to the sodium disulfite monomers in solution in 3000 g of water.

When the introduction is complete, the temperature of the reaction medium is approximately 80 ° C. It is kept at this temperature for one hour with stirring and then cooled to room temperature.

• 2 000 g de nonylphénol éthoxylé avec 8 moles d'oxyde d'éthylène, HLB = 12,3. puis
• 2 000 g de nonylphénol éthoxylé avec 10 moles d'oxyde d'éthylène, HLB = 13,3.

Then introduced with stirring:

• 2,000 g of ethoxylated nonylphenol with 8 moles of ethylene oxide, HLB = 12.3. then
• 2,000 g of ethoxylated nonylphenol with 10 moles of ethylene oxide, HLB = 13.3.

• 71,41 % d'une phase aqueuse contenant pondéralement 42 % de polymère P,
• 24,50 % d'une phase huile,
• 4,09 % d'un système émulsifiant constitué pondéralement par 51,17 % d'un émulsifiant de HLB = 3,7 et 48,83 % d'un mélange 1/1 de deux émulsifiants de HLB respectifs de 12,3 et 13,3.

About 200 250 g of a self-reversible water-in-oil dispersion are thus obtained, containing by weight:

• 71.41% of an aqueous phase containing by weight 42% of polymer P,
• 24.50% of an oil phase,
• 4.09% of an emulsifier system consisting by weight of 51.17% of an HLB emulsifier = 3.7 and 48.83% of a 1/1 mixture of two HLB emulsifiers of 12.3 and 13 respectively , 3.

• Ve : viscosité Brookfield de la dispersion déterminée à 20°C à la vitesse de 20 tr/mn avec l'axe 2,
• Vs : Viscosité Brookfield de la solution aqueuse contenant 3,33 % de la dispersion,
• Vn : viscosité Brookfield de la solution aqueuse contenant 3,33 % de la dispersion et 0,1 % de chlorure de sodium.

TABLEAU I EXEMPLE 1 EXEMPLE 2 Préparation des dispersions D M (en proportions molaires) AAM 29,58 79,15 AA 5,12 0,00 AANH₄ 65,14 20,83 CMA 0,00 20,83 ABAA 0,16 0,02 $\overline{\text{100,00}}$

$\overline{\text{100,00}}$

n (en pourcentage pondéral) 0,00 0,857 Az ACVA ABAH Ve (mPa.s) axe 2, vitesse 20 tr/min, 20°C 2000 ± 500 1500± 500 Vs (Pa.s) axe 6, vitesse 20 tr/min, 20°C (*axe 4) 55 ± 15 25 ± 5* Vn (mPa.s) axe 4, vitesse 20 tr/min, 20°C (*axe 2) 2000 ± 50 1500 ± 500* Tests applicatifs Dose en pourcentage pondéral dans l'eau de mouillage 0,26 % 1 % Test d'abrasion des encres 5 5 Classement visuel des aplats 5 3 TF-TG 5 5 Trapping 5 N.R PC % 5 3 Spectres de niveaux de gris 5 3 (pq) 5 5 5 très bon 3 moyen 1 mauvais 4 bon 2 passable 0 très mauvais This dispersion has the characteristics given in Table I in which:

• Ve: Brookfield viscosity of the dispersion determined at 20 ° C at a speed of 20 rpm with axis 2,
• Vs: Brookfield viscosity of the aqueous solution containing 3.33% of the dispersion,
• Vn: Brookfield viscosity of the aqueous solution containing 3.33% of the dispersion and 0.1% of sodium chloride.

TABLE I EXAMPLE 1 EXAMPLE 2 Preparation of dispersions D M (in molar proportions) AAM 29.58 79.15 AA 5.12 0.00 AANH ₄ 65.14 20.83 CMA 0.00 20.83 ABAA 0.16 0.02 $\overline{\text{100.00}}$

$\overline{\text{100.00}}$

n (in percentage by weight) 0.00 0.857 Az ACVA ABAH Ve (mPa.s) axis 2, speed 20 rpm, 20 ° C 2000 ± 500 1500 ± 500 Vs (Pa.s) axis 6, speed 20 rpm, 20 ° C (* axis 4) 55 ± 15 25 ± 5 * Vn (mPa.s) axis 4, speed 20 rpm, 20 ° C (* axis 2) 2000 ± 50 1500 ± 500 * Application tests Dose as a percentage by weight in the fountain water 0.26% 1% Ink abrasion test 5 5 Visual classification of solid areas 5 3 TF-TG 5 5 Trapping 5 NR PC% 5 3 Grayscale spectra 5 3 (pq) 5 5 5 very good 3 medium 1 bad 4 good 2 fair 0 very bad

Claims (7)

1. A method for offset printing characterised in that it is performed in the presence, either in the wetting water or in the ink or lacquer used, or in the wetting water and in the ink or lacquer used, of a copolymer P, which is ionic, hydrophilic, cross-linked and insoluble in but capable of swelling in water, in the form of water-swollen microparticles of diameter less than approximately 1 µm, based on acrylamide, a copolymerisable diethylenic cross-linking agent and one or more anionic monomers of formula (I)

   

   in which R represents a hydrogen atom or a methyl radical and M represents an atom of hydrogen, sodium or potassium or an ammonium group, or one or more cationic monomers of formula (II):

   

   in which R has the meaning given above, Z represents an oxygen atom or an NH group, R₁ represents a methyl or ethyl radical, R₂ represents a hydrogen atom or a methyl, ethyl or benzyl radical, X represents a chlorine atom or an OSO₃R₂ group in which R₂ has the meaning given above and n represents 2 or 3.
2. A method according to claim 1, characterised in that copolymer P is an anionic copolymer based on acrylamide and acrylic acid partly converted into a salt with ammonia and cross-linked with a diethylene monomer selected from the group comprising methylene bisacrylamide and bisacrylamidoacetic acid.
3. A method according to claim 1, characterised in that copolymer P is a cationic copolymer based on acrylamide and N,N-dimethylaminoethyl acrylate converted into a salt with hydrochloric acid or quaternised with methyl chloride and cross-linked with a diethylene monomer selected from the group comprising methylenebisacrylamide and bisacrylamidoacetic acid.
4. A method according to claim 2, characterised in that the anionic copolymer is a copolymer based on acrylamide, acrylic acid, ammonium acrylate and bisacrylamidoacetic acid, 29.6 - 65.1 - 5.1 - 0.2 in molar proportions.
5. A method according to claim 3, characterised in that the cationic copolymer is an acrylamide-based copolymer of acryloyloxy-2-ethyltrimethylammonium chloride and bisacrylamidoacetic acid 79.15 - 20.83 - 0.02 in molar proportions.
6. A method according to claim 1, characterised in that copolymer P is dispersed in the state of water-swollen microparticles in a self-reversible water-in-oil dispersion.
7. An aqueous composition which can be used in an offset printing process in either the wetting water or the ink or lacquer or even in both the wetting water and the ink or lacquer, characterised in that it contains a copolymer (P) as defined in claim 1 in the form of water-swollen microparticles.