EP1316432A1 - Method of flexographic printing of a sheet of cellulose wadding - Google Patents

Abstract

In a method for flexographic printing of a sheet of cellulose wadding of specific weight 15-35 g/m<2>, preferably with density below 130 kg/m<3>, that includes at least one application of ink from a print roller, the new feature is the preliminary application of an underlayer (A) that limits absorption of ink, beneath those parts of the wadding that are to be printed. <??>An Independent claim is also included for cellulose wadding of specific weight 15-35 g/m<2> and density below 130 kg/m<3> that has a printed decoration and includes, under the printed regions, a layer of titanium dioxide pigment or varnish.

Description

The invention relates to the field of absorbent papers intended for a sanitary or domestic use, for example as a household paper towel, napkin tablecloth.

In this area, papers of low grammage ranging from 15 to 35 g / m ² are generally used, mainly consisting of wood fibers. Depending on the use for which they are intended, chemical additives can be added, for example, to improve one or other of the properties: softness, absorption, etc.

A papermaking process involves depositing the fibers in suspension in water on a canvas to form a sheet. We drain the sheet, then we transfer it to a felt which will allow to apply it with a press against a drying cylinder. The sheet is peeled off and is creped using a doctor blade. She is finally put in reel awaiting transformation into finished product. Such a technique, presented in summary, is said to be conventional. We have it designates in the field by CWP. There are of course variations.

Another technique is to dry the sheet after draining without apply pressure, at least in part, until it is dry enough to freeze the fibers within the leaf. If necessary, drying ends by applying the sheet to a heated cylinder. Thanks to this first drying, you can press the sheet on the cylinder without destroying its structure. It retains part of its volume. This cylinder allows besides its crêpe. Drying is carried out without pressure by blowing hot air through the foil after draining. This technique, which is designated in the field under the acronym TAD, makes it possible to obtain a thicker sheet, larger mass volume than the technique conventional. It is characterized by a more open structure and a greater permeability.

During further processing into a finished product, one can be led to printing patterns in one or more colors on the sheet.

The flexographic printing technique is generally used. This process uses relief shapes such as flexible plates, made by sunstroke or with a laser-etched polymer coating, which will apply a liquid ink on the support. These forms are mounted on a plate cylinder. Flexographic printers are well known and can be of different types: central drum, against individual back pressures or independent groups.

A printer includes an inking system that collects the ink from a tank such as a doctor blade chamber, a screened cylinder or anilox on which microscopic cells have been etched, and which measures the quantity of ink and transfers it to a plate cylinder. The sheet to print is entrained between the latter and a back pressure cylinder.

This process is widely used in the field for its robustness and the print quality it allows.

However, it can be seen that the print medium has a great influence on the quality of the print. In particular, impressions of cellulose wadding sheets, CWP on the one hand and TAD on the other, performed under the same operating conditions, differ significantly. We observe on the seconds less intense, less vivid colors. The color palette is reduced. Finally the finesse is less.

The Applicant has set itself the objective of improving the quality printing of cellulose wadding.

In particular, it has set itself the objective of improving the printing quality of the cellulose wadding produced according to the through air blowing technique whose density is less than 130 kg / m ³ , in particular 70 kg / m ³ .

The method according to the invention comprising at least one unit printing with a plate holder with which an ink is applied, is characterized by the fact that an undercoat is applied beforehand limiting the absorption of said ink by the cellulose wadding sheet, under the areas to be printed.

In fact, the colored ink is deposited by the plate directly on the surface of the sheet. It partially penetrates the substrate by diffusion along fibers in proportion the greater the greater the material absorbency. The coloring part or the pigment which has penetrated in depth in the support no longer fulfills its role, being masked by surface fibers. The ink also tends to diffuse outside of limits of the print pattern.

By applying an undercoat limiting the absorption of ink by the fibrous substrate we note, with surprise, that the quality of the printing is significantly improved.

In accordance with a first embodiment of the method, we apply an undercoat varnish. In particular, a varnish suitable for this application is based on acrylic resin. Advantageously, the varnish has the composition of an ink without pigment. It consists of approximately 60% of solvent, such as water, of about 35% acrylic resin and 5% adjuvants, such as surfactants, drying retardants and pH stabilizers.

In accordance with another embodiment of the invention, an ink comprising white pigments, in particular TiO ₂ , is applied as an undercoat. Such an ink comprises for example around 60% of dry extract. In accordance with the invention, the dry extract comprises from 40 to 80% of TiO ₂ , the remainder preferably being an acrylic resin.

It was surprisingly found that such an application was suitable particularly well in the case of a TAD cellulose wadding sheet. This particularly favorable result is explained by the fact that this type of paper has a whiteness lower than that of CWP paper. The measurements give on average a whiteness 10% lower than that of CWP paper.

According to another characteristic, the viscosity of the ink of the undercoat is 20 to 35 seconds. The viscosity is measured according to the ISO standard with section 3.

la figure 1 représente une installation d'impression à laquelle s'applique le procédé de l'invention ;

la figure 2 représente une unité d'impression flexographique connue en soi ; et

la figure 3 est un graphique représentant les courbes d'engraissement entre une impression avec sous-couche et une impression sans sous-couche.

Other characteristics and advantages will appear on reading the following description with reference to the accompanying drawings in which

FIG. 1 represents a printing installation to which the method of the invention applies;

FIG. 2 represents a flexographic printing unit known per se; and

Figure 3 is a graph showing the fattening curves between a printing with an undercoat and a printing without an undercoat.

The installation 1, here with a rotary central drum 2 comprises a plurality of ink units 4, 6, 8, 10 arranged in series at its periphery. The whole is mounted on a frame 20. The printing sheet F comes in this example of a mother coil. It is applied to drum 2 and passes successively under the different ink units. These contribute each for a share in achieving the desired pattern. Means of drying are also planned. At the end of the printing installation, sheet F is rewound or dragged to other processing stations not shown. Of course this is only an embodiment; other arrangements are possible. For example, instead of a drum central the machine can include individual back pressure cylinders group by group.

A printing unit 30 is shown in FIG. 2. It comprises three cylinders. A first cylinder 32 plunges into a tank containing the ink. Instead of such a bubbling cylinder, a cylinder with doctor blade chambers. The cylinder 32 by turning transfers this ink on an anilox cylinder 34 whose function is to dose the ink. The cylinder 34 has cells calibrated on the surface. Their volume total and the rotation speed determine the ink flow transferred on a plate holder 36. The plates were fixed on this cylinder suitably engraved to reproduce the pattern with the assigned color to the printing unit. Drum 2 forms a smooth surface counter-cylinder and presses the sheet F against the plate mounted on the cylinder 36.

In accordance with the invention, apply in 4, that is to say with the first printing group an undercoat which will block diffusion ink which is applied to the sheet by the following groups 6, 8, 10. The extent of this sub-layer is preferably limited to the surface of the pattern, that is to say to the zones which receive ink by the groups which follow.

Various measurements were made to verify the results of the invention.

We first compared the printing qualities between a substrate made of a CWP sheet and a substrate made of a TAD sheet.

The same pattern was printed on a sample of CWP paper with a grammage of 21 g / m ² and on a sample of TAD paper with the same grammage of 21 g / m ² . It was a motif comprising a decoration associated with elements of print quality control.

The optical density D.O. was measured with a densitometer for three different dilutions of ink 100%, 300% and 500%.

The table below summarizes the results: DO difference 100% dilution CWP 0.62 -32% TAD 0.42 Dilution 300% CWP 0.45 -29% TAD 0.32 500% dilution CWP 0.34 -26% TAD 0.25

It can be seen that the TAD substrate, because of its structure, offers a low optical density compared to a substrate manufactured according to the technique CWP, whatever the dilution. The colors are less intense, the rendering is paler. The color palette is therefore reduced.

To verify the advantage of the process of the invention, we then proceeded to a series of tests on a pilot machine with a printing unit at 4-color central drum, supplied by Cobden.

The characteristics of the process are as follows. For the printing of the underlay, a plate having a line size of 60 lines / inch was used. The tests were carried out with three different anilox cylinders whose volumes were respectively 4, 10 and 16 cm ³ / m ² for a line size of 100 lines / cm. The cylinder pressure was set to its minimum.

The table below summarizes the results of the tests carried out on a TAD type substrate which has been printed with blue ink. We tested 3 different dilutions of the ink: 100%, 300% and 500%. For each dilution of the ink, three tests were carried out, each with an undercoat of different thickness, corresponding to the volume of the anilox, and a test without undercoat. The ink of the sublayer was based on TiO ₂ . The annotation 0 in the column "White dilution" means that there is no undercoat

We see that the Optical Density increases with the quantity of ink applied as an undercoat.

Generally the Optical Density is higher by 20 to 50%

This is reflected, in the visual impact of the decorated product, by an increased intensity and freshness of the colors. We also observe a greater finesse of the printing. The rendering is considered flattering and attractive. White dilution Blue dilution White volume cm ³ / m ² Optical density difference 0 100% 0.42 100% 100% 4 0.53 26% 10 0.57 36% 16 0.61 45% 0 300% 0.32 100% 300% 4 0.33 3% 10 0.38 19% 16 0.41 28% 0 500% 0.25 100% 500% 4 0.31 24% 10 0.35 40% 16 0.38 52%

We then carried out a series of tests to verify the behavior of the substrate with regard to the fattening, this emanating mainly deformation of the printed halftone dots. This drift is appreciated by the difference in surface coverage of a printed pattern compared to to a theoretical cover corresponding to this same motif engraved on the cliche. Improving the quality of printing results in decrease in fattening.

Tests were carried out with the same ink, on the same substrate, first without an undercoat and then with a TiO ₂ undercoat, the volume of the anilox of which was 10 cm ³ / m ² .

The tests consist in printing a weft strip, also called range, with a line size of 60 lines / inch, in order to quantify precisely the fattening of the weft points for blankets of 3% to 10%.

The results were reported in a benchmark with the theoretical percentage of surface of the pattern on the abscissa and the percentage printed on the ordinate. We has drawn the trend curves, called fattening curves, A for the substrate with sublayer and S for the substrate without sublayer.

We observe in Figure 3 a higher fattening for the substrate S without underlay. This deformation is due to the diffusion of the ink which is superior. At the densitometer we measured a higher fattening from 5% to 20% for the TAD substrate.

Claims (8)

Flexographic printing process for a cellulose wadding sheet with a grammage of between 15 and 35 g / m ² , in particular with a density less than 130 kg / m ³ , comprising at least one printing unit with a holder plate by means of which an ink is applied to the sheet, characterized by the fact that a sub-layer is applied beforehand limiting the absorption of said ink by the cellulose wadding sheet, under the areas to be printed. Method according to the preceding claim, characterized in that a varnish is applied as an undercoat. Method according to the preceding claim, characterized in that the varnish is based on acrylic resin. Method according to claim 1, characterized in that an ink based on TiO ₂ is applied as an undercoat. Method according to the preceding claim, characterized in that 40% to 80% of the dry extract of the ink is TiO ₂ , for a dry extract of approximately 60%. Method according to one of claims 4 and 5, characterized in that the viscosity of the undercoat ink is between 20 s and 35 s, measured according to ISO standard with section 3. Method according to one of the preceding claims, characterized in that the substrate is a sheet of cellulose wadding with a density less than 130 kg / m ³ , in particular 70 kg / m ³ . Product made of cellulose wadding with a grammage between 15 and 35 g / m ² , with a density less than 130 kg / m ³ and comprising a printed decoration, characterized in that it comprises a layer either of TiO ₂ pigments or of varnish in undercoat of the printed areas.

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