EP0049672B1 - Coated papers and cardboards, and process for manufacturing them - Google Patents

Abstract

Coated papers and cardboards comprising at least one compressible layer of coating containing hollow microspheres filled with a gas that is inert with respect to the microsphere-forming material. The coated papers and cardboards of this invention are used particularly for rotogravure printing. A process for manufacturing these coated papers and cardboards.

Description

The present invention relates to paper or cardboard based on cellulosic fibers. It relates more particularly to a new paper or cardboard comprising at least one layer allowing or facilitating gravure printing. It also relates to a process for manufacturing these papers or cartons.

Paper and cardboard are generally coated (or coated) in order to improve their mechanical properties in particular and / or allow printing.

Known coated paper or cardboard has at least one coating layer containing pigments and binders. The most commonly used pigment is a clay, such as kaolin. Possibly other pigments are associated with it, such as calcium carbonate, titanium oxide, hydrargilite, talc, barium sulphate. The pigments are fixed to the paper using binders, for example a styrene-butadiene copolymer, an acrylic polymer, a vinyl acetate polymer, natural binders, starch, proteins, caseins, polyvinyl alcohols, or a mixture of these bodies.

The coating layer of known coated paper or cardboard can also comprise dispersing agents which have been used for its manufacture from aqueous compositions. Such agents are for example tetrasodium pyrophosphate, sodium hexametaphosphate, low molecular weight polyacrylates, at levels of between 0.2 and 0.5% by weight relative to the pigments.

The coating layer may also include agents capable of improving the water retention properties such as carboxymethylcellulose, alginates.

The properties that coated paper or cardboard must have different depending on the printing method for which they are intended: rotogravure, offset, letterpress printing, flexographic printing, screen printing, etc.

In rotogravure, the ink is contained in small cells and its transfer by capillarity on the coated paper or cardboard can only be done if the periphery of each cell is in perfect contact with the latter. It was therefore important to use paper or cardboard coated with excellent smoothness. However, the smoothness obtained by calendering is higher the higher the pressure of the calender. It is however necessary to limit the calendering pressure in order to maintain a layer porosity sufficient for the printability of the coated paper or, in the case of cardboard, so as not to weaken certain physical characteristics or mechanical properties such as the thickness and / or stiffness. Or you have to use a paper or cardboard with a higher grammage of the order of 10 to 20%.

This is why the known coated papers or cardboards, that is to say coated with compositions containing mineral pigments, generally have a poor aptitude for printing in gravure printing.

The invention proposes a new coated paper or cardboard having an excellent aptitude for gravure printing, at the same time as good mechanical properties, without however being of excessive grammage. The paper or cardboard according to the invention comprises at least one coating layer containing hollow, expanded microspheres, filled with an inert gas that is to say which is not capable of reacting chemically with the walls of the microspheres .

Document US-A-3 816 169 discloses a substrate, for example paper made opaque with the aid of a coating layer comprising opacifying microspheres of small diameter, less than 2 μm, containing pigments such Ti0 ₂ .

The expanded microspheres according to the invention have a diameter of about 25 μm approximately. These hollow microspheres provide the coating layer with great flexibility or compressibility, much greater than that of conventional layers containing mineral pigments or that described in document US-A-3,816,169. This flexibility allows continuous contact between the coated paper or cardboard and the cells containing the ink during gravure printing, without the need for excellent smoothness.

The coating layer can be an outer layer. In this case, however, the surface of the coated paper or cardboard has a high coefficient of friction which can be troublesome during the processing operations by preventing the sliding of the coated sheets. In addition, the absorption of inks and varnishes is important, which can alter the freshness and gloss of the print.

This is why, advantageously, the coating layer containing the microspheres is a sub-layer which is covered with an external film improving the surface properties of the paper or cardboard, for example sliding, gloss. This outer film is a known layer, for example a conventional coating layer based exclusively on mineral pigments, a film of an organic polymer loaded with pigments or not, or a film of an extruded organic polymer.

Preferably, this outer layer is very thin, so as not to alter the flexibility due to the coating layer containing the microspheres.

The microspheres used in the composition of the compressible layer are for example hollow microcapsules, the flexible wall of which consists of a polymer capable of softening by heating, such as polyvinylidene chloride, polystyrene, a copolymer based on vinylidene chloride. and acrylonitrile, containing inside an inert gas, for example isobutane, chlorofluorinated hydrocarbons, chlorinated derivatives, carbon dioxide.

According to a characteristic of the invention, the compressible coating layer containing microspheres can have a calculated average thickness of the order of 2 to 50 μm and more.

In one aspect of the invention, the compressible coating layer comprises pigments of which 1 to 100% and preferably 25 to 100% by weight consist of hollow microspheres filled with an inert gas, the other conventional pigments being able to be mineral pigments such as clays such as kaolin, calcium carbonate, titanium oxide, hydrargilite, talc, barium sulphate.

The compressible coating layer also comprises at least one binder and possibly other ingredients such as dispersing agents, agents capable of improving the water retention properties, etc.

Generally, the compressible coating layer has a mass of 1 to 10 g and preferably 2 to 5 g per m ² of surface.

To increase the flexibility of the paper, it can be coated on one of its faces with two or more coating layers containing microspheres.

According to a characteristic of the invention, the paper or cardboard comprises a coating layer or sublayer containing microspheres on each of its faces.

The invention also relates to a process for the production of coated paper or cardboard.

According to the method, the paper or cardboard is coated on one or both sides with an aqueous composition containing synthetic polymers in the form of hollow microspheres containing an inert gas, expandable with heat, at least one binder, if necessary other pigments and other conventional ingredients as mentioned above. The coated paper or cardboard is then dried at the softening temperature of the hollow microspheres, generally between 80 ° C and 120 ° C approximately. This drying lasts a few seconds, or more if necessary, and has the effect of causing the microspheres to expand.

In a variant, an aqueous composition is used containing microspheres already expanded beforehand after having undergone a heat treatment.

A perfectly uniform and homogeneous layer is obtained which has a much greater flexibility than that of conventional layers based on mineral pigments.

After drying, the coated paper or cardboard is advantageously covered with an external film and can also undergo a finishing operation such as calendering, brushing or superglazing.

For the implementation of the coating process, it is possible to use, for example, the air knife, the size press, the Champion coater, the Massey coater, the trailing scraper.

Other characteristics and advantages will appear in the following description of examples according to the invention.

Examples 1 to 11

Aqueous coating compositions are prepared with the components and in the proportions indicated in Table 1. The amounts are expressed per 100 parts by weight by weight of pigment (microspheres plus mineral pigments). The microspheres consist of hollow capsules made of polyvinylidene chloride or a copolymer of vinylidene chloride and acrylonitrile containing isobutane. For example, the microspheres sold under the name SARAN by the company DOW CHEMICAL or the PVDC microspheres of the Swedish company KEMANORD are used. The expandable microspheres have an average diameter of 5 wm.

The compositions contain a binder which is a styrene-butadiene latex. They contain a thickener such as the carboxymethyl cellulose designated by the abbreviation CMC in the table. They also contain a lubricating agent such as a stearate. Finally, they contain a dispersant such as a polyacrylate.

The dry extract of the aqueous compositions is generally between 10 and 30% by weight.

A 150 g / cm ² paper, consisting of two jets of pulp, namely 100 g of bleached pulp and 50 g of unbleached pulp, is coated on the side of the bleached pulp using the aqueous compositions according to Table 1, prepared beforehand.

The coated paper is then dried by a battery of driers at a temperature of 90 ° C for approximately 15 seconds, which has the effect of softening the microspheres and allowing their expansion to an average diameter of the order of 25 μm approximately then it undergoes a calendering by passage between two cylinders with a diameter of 25 cm exerting a force of 20 kg (196 N) per linear cm.

When the layer containing microspheres is advantageously coated with an external film, it is possible to equip the radiator grille with a water doctor who regularly distributes, at the time of calendering, an aqueous polyethylene emulsion at a rate of 2 g / m ² .

It is also possible to extrude a film of an organic polymer, for example polyethylene, then apply it to the layer containing the microspheres.

The paper is then conditioned at a temperature of 23 ° C in an atmosphere at 50% relative humidity.

Examples 12 to 14

These examples relate to papers coated with conventional aqueous compositions containing only mineral pigments and other ingredients as indicated in Table 2.

The coated papers according to Examples 1 to 14 are examined from the point of view of their aptitude for heliographic printing.

To carry out these tests, an apparatus using the principle of the heliographic printing process is used. A suitable device is the IGT device of the “Voor Graphische Technick tno Amsterdam • institute. After having scraped the excess ink from a hollow engraved wheel, we print dynamically the coated paper to be tested under given pressure (300 N / 3 cm and 600 N / 3 cm) and speed (100 cm / s) conditions.

Three types of engraving are combined on the same wheel: a solid area with conventional weft, a degraded area with weft, lines of dots.

We examine the papers on which the printing was carried out.

Printing in plain area allows only a qualitative judgment that is difficult to express. On the other hand, the prints in dotted lines and in degraded range show the good printability of the papers according to the invention.

Printing in dot lines determines the printability by the number of missing dots; the lower the number of missing dots, the better the impression. On the papers according to Examples 1 to 11, zero missing points are observed. On papers 12, 13 and 14, there are respectively 6, 11 and 12 missing points.

Printing in degraded area determines the aptitude for printing by the distance between the start of printing, starting from the strong tone, and the first point of imitation unprinted; the greater the distance, the better the uniformity of the impressions. On the papers according to examples 1 to 11, there is never an unprinted imitation dot. For the papers produced according to Examples 12 to 14, 24, 45 and 35 mm are noted respectively.

Claims (12)

1. Paper or cardboard based on cellulosic fibres, the paper or cardboard being covered by at least one coating layer containing pigments, suitable for printing such as by heliogravure, characterised in that the coating layer contains hollow, expanded microspheres filled with a gaz which is chemically inert to their walls, having a diameter of the order of about 25 ¡.Lm, rendering the layer flexible and compressible.

2. Paper or cardboard according to claim 1, characterised in that the microspheres are present in an amount from 25 % to 100 % of the weight of the pigments.

3. Paper or cardboard according to one of claims 1 and 2, characterised in that the coating layer containing the microspheres has a mass from 1 to 10 g/m² and preferably 2 to 5 g/m².

4. Paper or cardboard according to one of claims 1 to 3, characterised in that the polymer forming the walls of the microspheres is selected from polyvinylidene chloride, polystyrene, a copolymer based on vinylidene chloride and acrylonitrile, and in that the inert gas contained in the capsules is selected from isobutane, chlorofluoro hydrocarbons, chlorinated derivatives and carbon dioxide.

5. Paper or cardboard according to claim 4, characterised in that the microspheres are of polyvinylidene chloride and they contain isobutane.

6. Paper or cardboard according to claim 4, characterised in that the microspheres are of a copolymer based on vinylidene chloride and acrylonitrile and in that they contain isobutane.

7. Paper or cardboard according to one of claims 1 to 6, characterised in that the coating layer containing the microspheres contains other ingredients such as binders, catalysts and mineral pigments.

8. Paper or cardboard according to one of claims 1 to 7, characterised in that the coating layer comprising the hollow microspheres filled with inert gas is an under-layer covered with an exterior envelope improving the surface properties of the paper or cardboard.

.9. Paper or cardboard according to claim 8, characterised in that the exterior envelope is selected from a coating layer of known type based exclusively on mineral pigments, a film of organic polymer which may or may not be filled with pigments, and an extruded film.

10. Method of making paper or cardboard covered with at least one coating layer according to one of claims 1 to 9, characterised in that the paper or cardboard is coated by an aqueous composition containing expansible microspheres and the coated layer is heated to expand the microspheres.

11. Method according to claim 10, characterised in that the coating layer is covered by an exterior envelope.

12. Method according to claim 11, characterised in that after drying the coated paper or cardboard is subjected to a finishing operation such as calandering, brushing or spupercalandering.