DE69400780T2 - Planographic printing process and aqueous composition therefor - Google Patents

Description

The present invention relates to a planographic or offset printing process and an aqueous composition intended therefor.

Traditionally, offset printing (or lithography) currently uses a wetting water containing isopropyl alcohol, hereinafter referred to as IPA, in order to improve the wetting of hydrophilic zones of the offset plate on the one hand and to promote the emulsification of the water in the printing ink or varnish on the other.

However, the use of IPA has numerous disadvantages due to its volatility, its flammability and its toxicity; it has been proposed to replace it with various glycols, optionally bound to glycol ethers or water-soluble polymers such as carboxymethylcellulose, hydroxypropylmethylcellulose, polyvinylpyrrolidone (see in particular US-A-4 865 646, 4 641 579, 4 548 645 and 4 278 467). However, the wetting fluids obtained by using these products are not entirely satisfactory for the printer. The applicant has therefore surprisingly found a flat or offset printing process which does not require the use of IPA and which also improves the printing process at the working level in the ink or varnish machine as well as in terms of printing qualities: gloss, contrast, mechanical resistance, color overlay.

The planographic or offset printing process according to the invention is characterized in that it is carried out in the presence either in the wetting water, in the printing ink used or in the varnish of an ionic, hydrophilic, cross-linked, water-insoluble but water-swellable copolymer, referred to below as P, in the form of water-swollen particles with an average diameter of less than 1 µm based on the following acrylamide referred to as AAM, a copolymerizable diethylenic crosslinking agent and 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, potassium atom or an ammonium group of one or more cationic monomers of the 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.

The term "ionic copolymer" means an anionic or cationic copolymer.

Monomers of formula (I) may be mentioned as: acrylic acid and its sodium, potassium or ammonium salts, methacrylic acid and its sodium, potassium or ammonium salts.

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

Polymerizable diethylenic crosslinking agents can be methylenebisacrylamide, hereinafter referred to as MBA, and bisacrylamidoacetic acid, hereinafter referred to as ABAA.

As copolymers P there may be mentioned: anionic copolymers based on acrylamide and acrylic acid partially salified with ammonia and cross-linked with MBA or ABAA and in particular ABAA, and cationic copolymers based on acrylamide and 2-(N,N-dimethylamino)ethyl acrylate quaternized with methyl chloride (hereinafter referred to as CMA) and cross-linked with MBA or ABAA and in particular ABAA.

Of these latter copolymers, the following can be mentioned in particular:

- the copolymers AAM - AA - AANH₄ - ABAA in molar proportions of 29.6 - 65.1 - 5.1 - 0.2,

- the copolymers AAM - CMA - ABAA in molar proportions of 79.15

- 20.83 - 0.02.

The abbreviations used have the following meaning:

AAM Acrylamide

AA Acrylic acid

AANH4 Ammonium acrylate

ABAA Bisacrylamidoacetic acid

CMA 2-acryloyloxyethyltrimethylammonium chloride.

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

The anionic copolymers P have been proposed in particular as thickeners in pigment printing (cf. EP-A-0 325 065) or in paper coating processes (EP-A-0 501 846).

The cationic copolymers P are particularly proposed as thickeners for the production of household sweeteners (see EP-A-0 494 554).

The P copolymers are hydrophilic copolymers that can absorb up to about 1000 times their weight in water. Therefore, when the water-swollen microparticles of P copolymer with a diameter of less than 1 micrometer are brought into contact with water, they absorb up to about 1000 times their weight in water and their diameter increases to about 10 times their initial diameter; nevertheless, their water absorption capacity is a function of their degree of cross-linking. The water absorbed by the P copolymer microparticles is readily available. As soon as these water-swollen particles are subjected to a tension, they release a portion of the absorbed water, which they reabsorb as soon as the tension is released.

Under the preferred conditions for carrying out the process according to the invention, a self-reversible water-in-oil dispersion, referred to below as D, is used, which contains the copolymer P in the state of water-filled microparticles with a diameter of less than 1 µm in the dispersed aqueous phase.

A self-reversible water-in-oil dispersion is a dispersion whose continuous phase consists of oil and whose dispersed phase consists of water and which spontaneously and quickly inverts into an oil-in-water dispersion as soon as it is diluted with water. To this end, it is sufficient that it additionally contains one or more oil-soluble emulsifiers with an HLB value of less than 5, one or more water-soluble emulsifiers with an HLB value of more than 8, preferably more than 10, with the total of the emulsifiers having a global HLB value of more than 8, preferably more than 10.

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

The self-reversible water-in-oil dispersions D that can be used in the process according to the invention consist of a dispersed aqueous phase that contains the copolymer P in the form of water-swollen microparticles with a diameter of less than 1 µm, a continuous oil phase and an emulsifier system referred to as E, which on the one hand contains a suitable amount of one or more oil-soluble emulsifiers with an HLB value of less than 5, so that the dispersion is stable at ambient temperature, and on the other hand contains one or more water-soluble emulsifiers with an HLB value of more than 8, preferably more than 10, in such an amount that the total of the emulsifiers forming the system E has a global HLB value of more than 8, preferably more than 10.

The dispersed aqueous phase of Dispersion D contains, in addition to the copolymer P, the conventional water-soluble additives such as chelating agents to eliminate the polymerization-inhibiting metals as required, buffering agents, transfer agents and polymerization initiators. Nevertheless, the total weight of these various additives is less than or equal to about 5% by weight of the aqueous phase in total.

The oily phase consists of one or more hydrocarbons which are liquid at ambient temperature and have a boiling point of more than 100ºC. Examples of such hydrocarbons include commercial linear or branched chain paraffins which are liquid at ambient temperature and cycloparaffins which have a boiling point of between 150 and 350ºC. It may also contain a proportion of less than or equal to 5% by weight of various conventional additives such as polymerisation initiators or transfer agents.

Emulsifiers soluble in oils with an HLB value of less than 5 include esters of fatty acids and sorbitan, such as sorbitan monooleate and sorbitan sesquioleate. Emulsifiers with an HLB value of more than 8 include nonylphenols ethoxylated with ethylene oxide, such as nonylphenol ethoxylated with 8 moles of ethylene oxide, nonylphenol ethoxylated with 10 moles of ethylene oxide, sorbitan hexaoleate ethoxylated with 40 moles of ethylene oxide and sorbitan hexaoleate ethoxylated with 50 moles of ethylene oxide.

The dispersions D are known products and certain of them are used extensively in the textile industry in pigment printing. They are miscible in the printing inks or varnishes used in offset printing and provide colloidal solutions containing the water-swollen copolymer P. They are also miscible in water in all proportions and provide 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 processes, e.g. the water-in-oil inverse suspension polymerization processes, which are extensively described in the literature.

For example,

(i) a water-in-oil emulsion is prepared by introducing, with stirring, an aqueous solution containing the monomers and, if appropriate, the selected additives into a liquid hydrocarbon containing one or more suitable emulsifiers to obtain an inverse emulsion consisting of fine aqueous particles containing the monomers and having a diameter of less than 1 µm, dispersed in the continuous oil phase, the volume ratio Rv of the volume of the aqueous phase to the total volume of the emulsion being less than 74%, preferably less than 71%;

(ii) this emulsion, which has been completely deoxygenated, is subjected to a radical polymerization reaction by introducing, with stirring, one or more free radical-forming initiators, such as azo derivatives, e.g. azobisisobutyronitrile, AIBN, azobisamidinopropane dichlorate, ABAH, peroxides, such as sodium or ammonium persulfate, cumene hydroperoxide and redox systems;

(iii) after completion of the polymerization reaction, the appropriate amount of one or more water-soluble emulsifiers is introduced into the cooled reaction medium in such a way that the overall HLB value of all the emulsifiers is greater than 8, preferably greater than 10. If desired, the copolymer P can optionally be isolated from the reaction medium by adding a water-oil miscible solvent which precipitates the copolymer, such as acetone, before introducing one or more water-soluble emulsifiers, followed by filtration to isolate the copolymer, which is dried and then redispersed in water.

Among the dispersions D usable in the process according to the invention, mention may be made in particular of self-reversible water-in-oil dispersions containing by weight:

about 71 ± 3% of an aqueous phase containing about 42 ± 3% by weight of copolymer P in the form of water-swollen microparticles with a diameter of less than 1 µm,

- about 25 ± 2.5 % of an oil phase,

- about 4 ± 1% of an emulsifier mixture with a global HLB value of more than 8, which consists of more than 50% by weight of one or more emulsifiers with an HLB value of less than 4.

Of these latter dispersions, one can mention in particular those which contain a copolymer

- AAM - AA - AANH4; - ABAA in molar proportions of 29.6 - 65.1 - 5.1 - 0.2;

- AAM - CMA - ABAA contained in molar proportions of 79.15 - 20.83 - 0.02.

In the most preferred conditions for carrying out the process according to the invention, an aqueous composition, hereinafter referred to as C, is used which contains, as water-swollen microparticles, a copolymer P of the type defined above and preferably about 0.1 to 10% by weight of the dispersion D defined above. This water-miscible aqueous composition C can be used directly to obtain the wetting water for offset printing. This aqueous composition C is obtained by simply mixing the desired amount of dispersion D in water.

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

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

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

The use of the copolymer P as defined above in the wetting water or the ink or the varnish or simultaneously in the wetting water and the ink or the varnish of a printing process by lithography and in particular in offset printing offers the following advantages, in particular in printing:

- it avoids the use of isopropyl alcohol or a hydroxylated compound such as a polyol or a glycol monoether,

- it modifies the emulsification of water in the ink or varnish by improving their "offset" properties;

- it modifies the stickiness of the ink or varnish, namely the transfer of the ink from one side to the next;

and regarding the print reproduction:

- it improves the visible uniformity of the paint application;

- it facilitates the overlay of printing inks and/or varnishes,

- it increases the shine, especially in the case of varnishes,

- it allows better definition of the print due to a better separation of printed and non-printed areas,

- it improves print contrasts by allowing printing with stronger color intensity in the color applications.

The same results are obtained by introducing the polymer P dispersed in the dispersion D defined above into the wetting water and/or the ink or varnish or by introducing the polymer P dispersed in the aqueous composition C defined above into the wetting water.

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

The tests, the application results of which are given in Table I, were carried out by introducing into the wetting water 0.26% by weight of a dispersion prepared according to Example 1 or 1% by weight of a dispersion prepared according to Example 2, compared with a test carried out with a wetting water containing IPA in such a way as to obtain a surface tension of the order of 40 dynes/cm (40 mN/m or 40 mJ/m²). The tests mentioned were carried out on a 2-colour offset press equipped with a UV dryer, with a double-coated paper "maine" commercially available from Société Arjomari-Wetaggins, with the printing inks "blue T4725 and rouge from the gamme Plast" with an optical density of 1.5, referred to as DO.

In all of these experiments, the printed pages were then:

- a printing ink abrasion test,

- a visual assessment of the paint application,

- a quality control: DO (optical density), trapping, brilliance, dot magnification (TF - TG),

- an assessment of quality using image analysis:

- % PC of paint application,

- Grey value spectrum of the ink application,

- Roundness of the grid points (at 25% coverage), referred to as pg.

through.

The abrasion test of the printing inks is carried out using a Wallace device at 50 rpm under a load of 2.5 kg after 6 days.

TF-TG refers to the magnification of the grid points.

Trapping is the process of overlaying one printing color onto another.

The brilliance is determined with a brilliance meter.

The image analysis, which allows the precise observation of the contour of the grid points, is carried out after microscopic magnification; the minimum surface observed is 27 µm².

The percentage of coverage of the paint applications, referred to above as % PC, was measured at two magnification levels. The quality of the continuity of the paint applications is determined based on granulometry curves at different working resolutions. The results found correspond to the visible evaluation of the paint applications.

The grey value spectrum of the ink application is calculated from microdensitometry curves at different magnifications on the It is evident that the roughness increases when the resolution of the device is increased. Whatever the magnification chosen, the results obtained by visual evaluation are confirmed.

The quality of the contours of the grid points is examined on the grids at 25% coverage by looking for the best roundness factor by determining the circumference, called pr and then the surface area of 10 grid points. Starting from the observed surface, the theoretical circumference, called pt, of a perfect disk of the same surface is calculated. The measured actual circumference value pr minus the calculated theoretical circumference pt, referred to the theoretical circumference, leads to the roundness factor, called pq

pq = pr - pt / pt

EXAMPLES Preparation of a dispersion D

In an oil phase cooled to 10ºC with a total weight of 52 400 g, which consists of:

- 92% by weight of a mixture of liquid hydrocarbons with a boiling point of 310-342ºC, containing about 80% by weight of paraffinic hydrocarbons and 20% by weight of naphthenic hydrocarbons,

- 8% by weight of sorbitan sesquioleate, HLB = 3.7 a previously prepared, completely oxygen-free aqueous phase with a total weight of 140,000 g is introduced within 15 minutes under an inert atmosphere and while stirring at 90 rpm, which consists of:

- 42.9% by weight of a monomer mixture M as defined in Table I,

- n % pure, anhydrous citric acid,

-500 ppm pentasodium salt of diethylenetriaminepentaacetic acid, namely 7 g,

- 15 ppm, expressed as molar proportions, based on the total monomers, of a water-soluble azo derivative Az as defined in Table 1.

After the introduction is complete, the reaction medium is carefully deoxygenated by bubbling with nitrogen, then it is subjected to vigorous agitation under an inert atmosphere while maintaining the temperature at 10°C, 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 spindle 4.

Into the emulsion thus obtained at a temperature of 10ºC, which is kept under agitation in an inert atmosphere, introduce over a period of 5 minutes:

- 200 ppm, in molar proportions relative to the monomers, of cumene hydroperoxide in solution in 850 g of the hydrocarbon mixture used in the preparation of the oil phase;

5 minutes after the end of the introduction, introduce in 45 minutes while stirring and under an inert atmosphere, during which an exothermic polymerization reaction may develop:

- 112.5 ppm in molar proportions, based on the monomers, of sodium bisulfite in solution in 3000 g of water.

Once the introduction is complete, the temperature of the reaction medium is about 80ºC. It is kept at this temperature for one hour while stirring, then it is cooled to ambient temperature.

Then add while stirring:

- 2000 g nonylphenol ethoxylated with 8 moles of ethylene oxide, HLB = 12.3,

then

- 2000 g of nonylphenol ethoxylated with 10 moles of ethylene oxide, HLB = 13.3.

This gives about 200 250 g of a self-reversible water-in- Oil dispersion containing:

- 71.41% by weight of an aqueous phase containing 42% by weight of polymer P,

- 24.50 wt.% of an oil phase,

- 4.09% by weight of an emulsifier system consisting of 51.17% by weight of an emulsifier with HLB = 3.7 and 48.83% by weight of a 1:1 mixture of two emulsifiers with an HLB value of 12.3 and 13.3 respectively.

This dispersion has the properties given in Table I, in which:

Ve = Brookfield viscosity of the dispersion, determined at 20ºC at a speed of 20 rpm with spindle 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% sodium chloride. TABLE 1

Claims (7)

1. Process for planographic or offset printing, characterized in that it is carried out in the presence in the wetting water, in the printing ink or varnish used or in the wetting water and in the printing ink or varnish used of an ionic, hydrophilic, cross-linked, water-insoluble but water-swellable copolymer P in the form of water-swollen microparticles with a diameter of less than about 1 µm based on acrylamide, a copolymerizable diethylenic cross-linking agent and one or more anionic monomers of the formula (I): CH₂ = - COOM (I) in which R represents a hydrogen atom or a methyl radical and M represents a hydrogen, sodium, potassium atom or an ammonium group of one or more cationic monomers of the 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, R 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. Process according to claim 1, characterized in that the copolymer P is an anionic copolymer based on acrylamide and acrylic acid partially converted into a salt with ammonia, crosslinked with a diethylenic monomer selected from the group consisting of methylenebisacrylamide and bisacrylamidoacetic acid. 3. Process according to claim 1, characterized in that the copolymer P is a cationic copolymer based on acrylamide and 2-(N,N-dimethylamino)ethyl acrylate, which is converted into a salt with hydrochloric acid or quaternized with methyl chloride, crosslinked with a diethylenic monomer, selected from the group consisting of methylenebisacrylamide and bisacrylamidoacetic acid. 4. Process according to claim 2, characterized in that the anionic copolymer is a copolymer based on acrylamide, acrylic acid, ammonium acrylate and bisacrylamidoacetic acid in molar proportions of 29.6 - 65.1 - 5.1 - 0.2. 5. Process according to claim 3, characterized in that the cationic copolymer is a copolymer based on acrylamide, 2-acryloyloxyethyltrimethylammonium chloride and bisacrylamidoacetic acid in molar proportions of 79.15 - 20.83 - 0.02. 6. Process according to claim 1, characterized in that the copolymer P is dispersed in the state of water-swollen microparticles in a self-reversible water-in-oil dispersion. 7. Aqueous composition which can be used in a planographic or offset printing process in the wetting water, in the printing ink or the varnish or in the wetting water and in the printing ink or the varnish, characterized in that it contains as water-swollen microparticles a copolymer P as defined in claim 1.