EP0824078A1 - Lithographic layer for a printing blanket and blanket provided with such a layer - Google Patents

Abstract

New lithographic layer (I) for printing blankets comprising a polyurethane (II) based thermoplastic material (III) incorporating a copolymer of ethylene vinyl acetate (IV) to confer a polar quality guaranteeing an optimum transfer of ink from (I) to the paper. Also claimed is a printing blanket or sleeve comprising (I). Preferably (II) is an ethylene-propylene copolymer. (III) incorporates bulk, such as magnesium silicates, aluminium silicates, or metal oxides, and plasticisers (for example an ester or a polymer). For 100 parts of (II), (I) contains 0-20 parts by weight of (IV), 0-30 parts by weight of bulk and 0-10 parts by weight of plasticiser. (I) contains at least one pigment (0-2 parts by weight for 100 parts of (III)). (II) has a polar component in water, formamide and dimethyl sulphoxide of 0-20 mJ/m<2> (5-12 mJ/m<2> in water and 0-10 mJ/m<2> in formamide preferred). (I) is used in printing. Because (I) has a significant polar component, virtually all the ink is transferred from (I) to the paper. This ensures a good quality print and avoids the need to clean or change the printing blanket too frequently.

Description

The main subject of the present invention is a lithographic layer for a printing blanket of any structure.

It also targets a blanket fitted with this layer.

In general, we know that the processes offset printing machines use a cylinder coated with offset plate receiving water and ink to form a latent image which is transferred to a blanket comprising a lithographic outer layer capable of transfer the image to a paper medium for example.

Water and ink transfers from the offset plate on the lithographic layer, then on the layer lithographic on paper are governed by a certain number of affinity parameters for water and ink the offset plate, from the lithographic layer of the blanket, and paper.

These parameters can be summarized by what we calls a surface energy which breaks down into a dispersive component and into polar components.

In this regard, reference may be made to the publication of R.J. Good, J. Adhesion Sci. Technol, Vol. 6, No. 12, 1269 (1992).

• la composante polaire à l'eau qui permet d'évaluer la mouillabilité par l'eau et la mouillabilité par l'émulsion eau-encre,
• la composante polaire au formamide qui permet d'évaluer le caractère basique de la surface, et donc l'affinité avec les solutions de mouillage acide, et
• la composante polaire au "DMSO" (diméthyl sulfoxyde) qui permet d'évaluer le caractère acide de la surface, et donc l'affinité avec les encres ayant une légère composition polaire basique.

Briefly speaking, the surface energies of a polar nature, expressed in millijoules per m2, and which are used to characterize the ability to transfer ink and water, are the following three components:

• the polar component with water which makes it possible to evaluate the wettability by water and the wettability by the water-ink emulsion,
• the polar component with formamide which makes it possible to evaluate the basic character of the surface, and therefore the affinity with the acid wetting solutions, and
• the polar component with "DMSO" (dimethyl sulfoxide) which makes it possible to evaluate the acid character of the surface, and therefore the affinity with inks having a slight basic polar composition.

In addition, surface energy of a dispersive nature is defined by its dispersive component.

Therefore, if we want a good transfer of ink on the paper you have to find a good compromise for values of the above components and this, again, so ensure good ink-water transfer from the plate offset on the lithographic layer of the blanket and since the blanket on paper.

Most of the lithographic layers known for printing blankets are made of rubber nitrile.

Such a layer constitutes a non-polar surface or very weakly polar, so that it is weakly polar wetted by polar water, so the ink has tendency to accumulate on said surface. So the blanket surface gets dirty easily. In addition the ink on paper is far from being transferred ideally so that printing on paper can be unsatisfactory.

Because, as we understand, if the dispersive component of the lithographic layer is weak, little ink in provenance of the offset plate will be retained by said layer and the printing process will be faulty.

If, on the other hand, the dispersive component of the layer lithographic is high, a large amount of ink will be retained on the blanket, but its release on the paper will be difficult, and said blanket will become dirty.

It will therefore frequently be washed or even replaced, not to mention the fact that printing with such a blanket nitrile rubber lithographic layer will consume lots of ink.

The object of the present invention is to remedy all of these problems and disadvantages in proposing a layer lithographic having polar components significant, so that substantially all of the ink retained on the lithographic layer of the blanket will transferred to paper.

To this end, the present invention relates to a lithographic layer for printing blanket, characterized in that said layer is a layer of thermoplastic material ensuring maximum transfer blanket printing ink on paper.

According to another characteristic of the invention, the aforementioned thermoplastic material is based on polyurethane or ethylene-propylene copolymer.

According to an exemplary embodiment, the material thermoplastic is polyurethane including at least mineral and / or organic fillers such as for example magnesium silicate, aluminosilicates or metal oxides used separately or as a mixture, and plasticizers such as for example those of the ester type or polymeric.

According to yet another characteristic, the layer lithographic of the invention is characterized in that the thermoplastic material includes a copolymer ethylene vinyl acetate (EVA).

According to yet another characteristic, the layer lithographic of the invention consists of polyurethane comprising, per 100 parts by weight of polyurethane, about 0 to 30 parts by weight of fillers and about 0 to 10 parts by weight of plasticizer.

The lithographic layer according to this invention is still characterized by the fact that the EVA copolymer represents approximately 0 to 20 parts by weight per 100 parts by weight of polyurethane.

According to a preferred embodiment, the layer lithographic includes, per 100 parts by weight of polyurethane, 0 to 20 parts by weight of copolymer of EVA, 0 to 30 parts by weight of mineral fillers and 0 to 10 parts by weight of plasticizer.

The lithographic layer of the invention can also contain at least one pigment which can constitute up to about 2 parts by weight per 100 parts by weight of polyurethane.

According to yet another characteristic, the lithographic layer of this invention is characterized in that its surface has a polar character and has a polar component with water of between about 0 and 20 mJ / m ² , a polar component with formamide between approximately 0 and 20 mJ / m ² , and a polar component with dimethyl sulfoxide substantially equal to the polar component with formamide.

Preferably, the polar water component is between 5 and 15 mJ / m ² and the polar formamide component is between 0 and 10 mJ / m ² .

But other features and advantages of the invention will appear better in the description detailed which follows from the lithographic layer for printing blanket, conforms to the principle of the invention.

A following polar lithographic layer invention and having excellent capabilities printing and washing, is constituted, according to a example of embodiment, with polyurethane thermoplastic to which it is given a character polar thanks to the incorporation of elements or following ingredients: ethylene vinyl copolymer acetate, mineral fillers, plasticizer and possibly pigment (s).

As a thermoplastic polyurethane with good chemical resistance one can use the one known as trade name Laripur 7025, Uceflex PS 4075, Resamine P1078, or Estane 58206.

The ethylene vinyl acetate copolymer can be example a copolymer of the type known under the name Levapren commercial which has the advantage not only to have a polar character but also to play the role of a polymer plasticizer. We could also, without departing from the scope of the invention, use at the instead of ethylene vinyl acetate copolymer (EVA) different plastomers known in the art and giving adequate surface properties to the layer lithographic, such as for example polyethylene chlorosulfonated, polyetheramides of the type known as name Pebax, polyamide powder of the type known as Orgasol, and the like.

As mineral fillers, silicate of magnesium of the type known as "Mistron Vapor ", an aluminosilicate of the Sillitin type, and metal oxides.

The plasticizer is, as explained above, provided by the special characteristics of EVA Levapren, it being understood that we could use other specific pastifiers without departing from the the invention. The role of the plasticizer is essentially to adjust the elastic modulus of the lithographic layer in order to allow it to comply mechanically to the roughness of the paper and provide the flexibility required for the printing process.

The pigment (s) incorporated into the polyurethane thermoplastic can or can be any mineral pigment or organic appropriate.

According to a preferred embodiment, the constitution of the lithographic layer according to this invention comprises all of the above elements with the proportions by weight indicated: Thermoplastic polyurethane 100 parts by weight EVA copolymer 0 to 20 parts by weight Mineral fillers 0 to 30 parts by weight Plasticizer 0 to 10 parts by weight Pigment 0 to 2 parts by weight

With certain precise values within the proportions indicated above, the surface of the lithographic layer according to this invention advantageously has a polar character. More specifically, it has a polar water component of between about 0 and 20 mJ / m ² , a polar formamide component of between about 0 and 20 mJ / m ² , and a polar component of dimethyl sulfoxide substantially equal to the polar component with formamide.

It will be observed that the polar component with water should preferably be between 5 and 15 mJ / m ² and the polar component with formamide will be between 0 and 10 mJ / m ² .

To demonstrate the benefits of the layer polar lithographic according to the invention, we carried out comparative tests with two lithographic layers belonging to the known state of the art, namely a layer A based on acrylonitrile, part of the blanket according to document US-4303721 and sold by the applicant under the name POLYCELL, as well that a layer B conforming to the composition of Example 3 from document US-5294481.

The proportions by weight of the elements constituting the lithographic layer of the invention used for the tests are as follows: Thermoplastic polyurethane Resamine P-1078 100 parts by weight EVA Levapren 700HV copolymer 10 parts by weight Mineral fillers 20 parts by weight Plasticizers 2 parts by weight Pigment 1 part by weight

In table 1 which follows and in which the surface energies were calculated from drop angle measurements obtained on the KRÜSS G10 device, it can be seen that the comparative tests with layer A and layer B consisted of measure the polar component with water (a), the polar component with formamide (b), and the polar component with dimethyl sulfoxide (c). Polyurethane the invention from Layer A Layer B a 8.3 0.1 0.1 b 6.7 0.1 2.3 c 5.3 4.8 0.1

We immediately see on this table that the values of a, b and c are significantly larger for the polyurethane according to the invention only for the layers previous lithographs A and B.

Thus, the polar character of the lithographic layer according to the invention is much more marked compared to the lithographic layers of the prior art, and will provide good washing ability as well as better transfer of ink to paper.

This is because the polarity of the surface facilitates the water-ink balance which is essential for the process offset printing, and also because the polarity of the surface makes ink adhesion to the blanket more easily reversible which, as we understand, facilitates ink transfer and washing.

Furthermore, ink transfer tests were carried out, by which the quantity of ink X (g / m ² ) necessary to transfer Y (g / m ² ) of ink was measured on the paper.

These tests were carried out as follows.

They were produced using a laboratory press of the IGT A2 type, an IGT AE inking device and a precision balance (to within 10 ^-4 g).

The lithographic layer is glued to the disc of the IGT device using a double-sided cloth adhesive tape. The printer layer and the printing medium must be regular in thickness (maximum deviations of 0.05 mm) and the total thickness must be less than 2.5 mm. The surface of the sample S _b must be determined (blanket sample dimensions: 20x210 chain direction, for 20 mm IGT discs).

The printing disc fitted with its printer layer is weighed before (m _o ) and after (m ₁ ) inking on the IGT device provided for this purpose. The amount of ink X (g / m ² ) deposited on the sample is: x = (m 1 - m 0 ) / S b

Skinnex magenta ink, reference 2X76, K + E has been selected. According to professionals, the latter is difficult to print because "pulling".

A strip of paper is placed on the rotating sector of the IGT device (paper sample dimensions 25x290 mm). Two types of paper, with different capillarities, were chosen: an uncoated, matt appearance, with a grammage of 87 g / m ² and a coated, glossy appearance, of 91 g / m ² . The first has a porous and absorbent structure (similar to a sponge), while the second is smoother and "closed". The papers, very sensitive to humidity, are stored in the test room and must not be touched too much with your fingers. On the other hand, the strips are always printed in the same direction and on the same side, chosen arbitrarily, in order to overcome the orientation effect of the paper fibers.

The printing conditions are kept constant in regarding pressure (250 N / cm) and speed (3.5 m / s) and estimated to be approximately constant with regard to the temperature (22 ° C) and the hygrometry (50% RH) of the local.

The disc is then reweighed (m ₂ )

The thickness of the test sample depends on the perimeter of the printing disc, therefore the sample / paper contact surface; the printing surface may be different from the printed surface, the development of the rotating sector being constant. The quantity of ink Y transferred is preferably determined from the difference in mass of the paper after (m ₄ ) and before (m ₃ ) printing and from the printed surface S _p . The amount of ink Y (g / m ² ) transferred to the paper is: Y = (m 1 - m 2 ) / S b Y = (m 4 - m 3 ) / S p

It is necessary to carry out several transfer tests by incrementing the quantity X of ink on the printing disc, from 1 to approximately 5 g / m ² , by leaving it longer on the inking device, or by increasing the quantity ink on it. The use of the tests consists in drawing a straight line Y = f (X) whose slope and the ordinate at the origin are determined by linear regression. This straight line makes it possible to determine the inking X necessary on the blanket to obtain a coverage Y of 1.0 and 1.5 g / m ² on the paper. These values of Y are representative of the ink coverage in offset printing and make it possible to achieve the optical densities required with this process.

The results of the tests, namely the inking X necessary to obtain the result, are given for the two types of paper and the two levels of inking Y on the paper, in table 2 which follows.

It can be seen that these tests were carried out, as in the preceding table, on polar polyurethane according to the invention, on the known lithographic layer A and on the other known layer B. In reality, the tests were carried out on blankets provided at their periphery with the above layers, that is to say with polyurethane according to the invention, with layer A and layer B. These tests were carried out respectively on coated paper and on uncoated paper on which wishes to transfer an amount of ink corresponding to a coverage of 1 and 1.5 grams per m ² on the paper, as explained above.

We immediately see in this table that the values for the lithographic layer containing polyurethane polar according to the invention are less than all others, which indicates that the lithographic layer according to the invention turns out to require less ink on the blanket for a determined result on paper.

In other words, the ink consumption by the blanket is advantageously reduced and soiling on the lithographic layer will be very reduced since the blanket will require a relatively small proportion ink on the lithographic layer.

By the way, water consumption is, any proportion kept, less important, and the deformations of the paper by water will be reduced, since, due to the polar nature of water, it has a strong tendency to wet the lithographic surface or layer of the blanket.

A lithographic layer according to the invention can be integrated into an offset blanket intended to be mounted on an offset machine, or a sleeve that can be mounted removable on the offset machine.

Furthermore, it will be observed that, due to the nature thermoplastic of the lithographic layer, this can be regenerated or restored by action of heat for example, i.e. local application heat on the deteriorated part of the blanket or muff.

It will also be observed that the lithographic layer according to the invention having an advantageous polar character, it follows that the dispersive component is not critical with regard to improving the transfer and the cleanliness of said layer, unlike blankets with lithographic layer according to art prior.

We therefore produced according to the invention a layer lithographic with polar character presenting excellent qualities in terms of consumption ink, cleanliness and transfer to the paper, and which can be integrated into a blanket or removable sleeve which will require very frequent replacements infrequent.

Of course, the invention is by no means limited to described embodiments which have been given only as an example.

This is how instead of polyurethane for the layer lithographic, we can use a copolymer ethylene-propylene, or other elastomers thermoplastics without departing from the scope of the invention.

Therefore, this invention includes all equivalents techniques of the means described as well as their combinations if these are performed according to its mind.

Claims (8)

Lithographic layer for printing blanket comprising a thermoplastic material based on polyurethane, characterized in that in this material is incorporated an ethylene vinyl acetate (EVA) copolymer to give it a polar character ensuring a maximum transfer of printing ink from this layer On paper. Litographic layer for printing blanket comprising a thermoplastic material, characterized in that said material is based on ethylene-propylene copolymer in which a copolymer is incorporated ethylene vinyl acetate (EVA) to give it a polar character ensuring maximum ink transfer print this layer on the paper. Layer according to claim 1 or 2, characterized in what in said thermoplastic material are included in less mineral and / or organic fillers such as for example magnesium silicate, aluminosilicates or metal oxides used separately or as a mixture and plasticizers such as for example those of the ester or polymer type. Layer according to claims 1 and 3, characterized in that it comprises per 100 parts by weight of polyurethane, 0 to 20 parts by weight of copolymer of EVA, 0 to 30 parts by weight of fillers and 0 to 10 parts by weight of plasticizer. Layer according to one of the preceding claims, characterized in that it comprises at least one pigment up to about 2 parts by weight per 100 parts by weight of thermoplastic material. Layer according to claim 4, characterized in that its surface has a water polar component of between approximately 0 and 20 mJ / m ² , a polar formamide component comprised between approximately 0 and 20 mJ / m ² , and a component polar with dimethyl sulfoxide substantially equal to the polar component with formamide. Layer according to claim 6, characterized in that the polar component with water is between 5 and 15 mJ / m ² and the polar component with formamide is between 0 and 10 mJ / m ² . Printing blanket or sleeve incorporating at least a lithographic layer according to one of claims 1 to 7.