WO2004091926A1

WO2004091926A1 - Recording paper and method for manufacturing recording paper

Info

Publication number: WO2004091926A1
Application number: PCT/FI2004/000227
Authority: WO
Inventors: Heikki Korpela; Annukka Vuorinen; Tiina Gäddnäs; Marina Koskela; Maarit MÄENPÄÄ
Original assignee: Walki Wisa Oy
Priority date: 2003-04-16
Filing date: 2004-04-14
Publication date: 2004-10-28
Also published as: FI20030577A; FI20030577A0; FI114977B

Abstract

The invention relates to recording paper, particularly for ink-jet printing, which comprises a carrier (1) and on top of it an absorption layer (2) spread by an extrusion coating method, which absorption layer contains polyvinyl alcohol as melt-processable polymer and pigment distributed in the polymer. The polyvinyl alcohol has the following properties - hydrolysis rate between 60 and 80% - molar mass between 10,000 and 30,000 - polymerisation rate between 300 and 1,000 - melt index between 1 and 20 g/10 min (190°C; 2.16 kg). The pigment is either talc or barium sulphate or a mixture of these, and inorganic pigment, of which the aforementioned ingredients form at least 90 wt %, makes up between 20 and 50 wt % of the absorption layer. The absorption layer (2) has been spread on top of the carrier in coextrusion with the intermediate layer (2a), by means of which the absorption layer is attached to the carrier (1).

Description

Recording paper and method for manufacturing recording paper

The invention relates to recording paper, particularly to ink-jet recording paper. The invention also relates to a method for manufacturing recording paper, particularly ink-jet recording paper (so-called ink-jet paper).

With the increasing use of personal computers and digital cameras, the demand for high-quality printouts has increased considerably, and will continue to increase. Such print carriers comprise either paper or plastic film as a substrate on top of which an ink-receiving layer has been applied, which layer can also be called an absoφtion layer.

Ink-jet printers are commonly used for printing out images that have been created with the aforementioned devices. With the increasing use of ink-jet printers, there has been an increasing demand for recording papers with even better properties. Such recording papers generally have a substrate made of paper pulp, a so-called base paper, with an absoφtion layer on top. The purpose of the absoφtion layer is to receive the colour from the ink jet of the ink-jet printer and to absorb the liquid in the printing ink, whereby the colorants in the printing ink remain on the surface of the recording paper, forming a shaφ image. Due to the demands made on the absoφtion layer, the work of developing recording paper has focused in particular on developing absorption layer materials and spreading methods. Although the layer on top of the carrier can be called the absorption layer, it can often consist of two or more layers, which may differ from each other as to their composition and/or the spreading phase, and which have different functions in the recording paper.

At its best, the absoφtion layer is such, that it rapidly absorbs the solvent of the printing ink into the layer in the vertical direction (Z), but does not spread printing ink at all in the directions X and Y, i.e. in the direction of the plane of the image to be printed. Furthermore, the layer can be such that it chemically binds the colorants in the ink permanently to the ink- receiving layer without changing the nature of the colorant or the tone of the colour.

A long known method for manufacturing recording paper is to spread on the carrier, using a known coating method, an aqueous solution or an aqueous dispersion of the absoφtion material, which after having dried, forms an absoφtion layer on top of the carrier. A recording paper manufactured by this method is presented, for example, in publication EP- 650850.

Attempts have been made to avoid drying by choosing the material for the absorption layer so that it can be applied by an extrusion coating technique, in which case, instead of removing water, it suffices that the layer, which has been spread in the molten state, is left to cool down. The basic raw material of the absoφtion layer in this case is a polymer, which is spread in the molten state, and which forms an absoφtion layer after having solidified. Publications EP-1018438 and WO 01/62510, for example, present the formation of an absorption layer using an extrusion coating technique, for example from extrudable polyvinyl alcohol. With a polymer that can be spread by using the extrusion technique, it is possible to create a hydrophilic absoφtion layer into which pigments may also have been blended. It is mentioned in the same publications that a thinner surface layer containing pigment and binder can be spread on top of the layer that has been spread during the extrusion stage, which surface layer first receives the ink jet of the printer and on which surface layer the image produced by the colour pigments is formed. By applying a separately spread surface layer, suitable surface properties (smoothness, gloss) of the recording paper can be obtained. It has been presented in publication EP-1018438 that, in order to maintain gloss, the amount of pigments in the surface layer to be spread in the aqueous phase may not be more than approx. 10 wt % of pigments calculated from dry weight. According to the publication, the amount of pigments may only be greater when using pigments with an organic coating in the nanometre range.

The pigments in the surface layer improve the absorbency of the surface layer in comparison with binders alone, which, on the other hand, make it possible to obtain higher gloss. As regards the needs of ink-jet printing, a recording paper whose surface has good water absorbency and high gloss and smoothness would be an optimal solution.

In addition to the end properties of the recording paper, attention should be paid to the process of manufacturing. By using extrusion of a molten polymer instead of water-assisted spreading of the absorption layer, certain process-related advantages have been achieved. However, optimisation of the raw materials of the absoφtion layer and the mixing methods used have received less attention.

The purpose of the invention is to present a recording paper that has been manufactured by using an extrusion coating technique and wherein the properties of the absoφtion layer are optimal. The purpose of the invention is also to present a method that allows the manufacture of recording paper in better process conditions than before as regards the quality of the end product. In order to fulfil this puφose, a recording paper in accordance with the invention is mainly characterised by that which is presented in the characterising part of the accompanying Patent Claim 1.

The absorption layer has as a pigment either talc or barium sulphate or a mixture of these, in which they may be in a suitable ratio. These pigments can increase the absorbency and drying speed of the recording paper, especially talc. Barium sulphate, on the other hand, has the effect of increasing the brightness. By using a mixture of the aforementioned pigments, different sizes of particles are formed in the absoφtion layer. When the pigment is formed from two different fractions with different average particle sizes, a high fill factor is obtained for the polymer matrix.

Polyvinyl alcohol, which has a hydrolysis rate between 60 and 80%, a molar mass of between 10,000 and 30,000, preferably between 15,000 and 25,000, and a polymerisation rate between 300 and 1 ,000, preferably between 500 and 700, is used as the absoφtion layer's extrudable matrix polymer, into which the aforementioned pigments are distributed. The melt index of the polymer is between 1 and 20 g/10 min, preferably between 2 and 7 g/10 min (190°C; 2.16 kg). The relatively low hydrolysis rate of the polyvinyl alcohol increases the absoφtion capacity.

The proportion of inorganic pigment in the absoφtion layer is preferably between 20 and 50 wt %, more preferably between 25 and 40 wt %. The pigment may contain other pigments than talc and/or barium sulphate, but the aforementioned pigments preferably make up at least 90 wt % of the total amount of inorganic pigments.

The absoφtion layer has been spread on top of the carrier in coextrusion with the intermediate layer, by means of which the absoφtion layer is attached to the carrier. With this method, it is possible to run the layers in one extrusion stage onto the carrier, of which layers one (absoφtion layer) may be chosen according to the absoφtion properties and the other (intermediate layer) according to the adhesion properties (to both carrier and absoφtion layer). In addition to this, the melt-processable plastic of the intermediate layer can be chosen for suitable barrier properties.

The grammage of the absoφtion layer is preferably between 10 and 40 g/m², more preferably between 20 and 30 g/m². The thickness of the layer and the size and quantity of inorganic pigment particles are preferably chosen so that there are at least 3 layers on average in the Z (thickness) direction. On top of the absoφtion layer there may be a surface layer for the puφose of finishing the surface of the recording paper, which surface layer has been formed by an aqueous solution of binder containing pigment spread on the surface.

When large quantities of fine-grained pigment, for example colloidal silica, are used in the top layer of the absoφtion layer that has been applied by extrusion, high gloss, smoothness and absorbency are obtained in the surface.

The method for manufacturing recording paper according to the invention is characterised in that the absoφtion layer is formed in an extrusion process from a compound, in which the aforementioned polyvinyl alcohol and the aforementioned pigments are in a solid granulate formed by melt mixing. The proportions of the ingredients in the granulate are equivalent to the proportion of the polymer and pigment in the absoφtion layer. By using a compound in which the ingredients are pre-mixed, it is possible to obtain a product where the absoφtion layer is more homogeneous. Such a compound is spread in coextrusion simultaneously with the intermediate layer, e.g. an intermediate layer acting as a barrier layer, on the surface of the carrier so that the intermediate layer remains as a uniform layer between the carrier and the layer formed from the compound, attaching the latter at the same time to the carrier.

In the following, the invention is described in more detail by referring to the appended drawing, which shows a cross-section of the recording paper.

The drawing presents a recording paper ready for printing, which recording paper comprises a carrier 1, which acts as the supporting structure of the paper. On top of the carrier an absoφtion layer 2 of molten polymer material has been spread by extrusion coating. In between the absorption layer 2 and the carrier 1 there is a special intermediate layer 2a that has also been formed from plastic in a molten state from plastic, which intermediate layer has been spread in the same extrusion coating stage. This intermediate layer 2a attaches the absoφtion layer 2 to the carrier 1 , and its significance is explained below.

The puφose of the carrier 1 is to form the supporting and cohesive structure of the recording paper, and it is most commonly of paper or paperboard. Even though paper or paperboard made from various fibre raw materials are widely used materials, other materials besides actual paper products can also be used, for example polymer films. When paper or paperboard is used, it can be coated by a known coating method for paper and paperboard. Paper or paperboard may also be coated with a layer of plastic on the side that comes into contact with the absoφtion layer 2. Such a plastic layer may also be pigmented. When choosing the plastic layer, attention should be paid to sufficient adhesion to the material of intermediate layer 2a. The plastic layer on top of the paper or paperboard may also act as a barrier layer, thus preventing the absoφtion of water from the absoφtion layer into the paper, in which case, the intermediate layer 2a spread simultaneously with the absoφtion layer 2, does not necessarily need to have barrier properties. According to a preferable embodiment, the absoφtion layer 2 and intermediate layer 2a are spread on top of the bare (non-plastic coated) paper in a coextrusion process, whereby a coating acting as a barrier layer can also be obtained at the same time.

The material of the absoφtion layer 2 should be a suitable extrudable polymer. When choosing the absorption layer, it must be taken into consideration that, during printing, the inks mainly remain on the surface layer 3 which is on top of the absoφtion layer, to be explained below, and the water acting as a solvent penetrates into the absoφtion layer underneath it, and as the ink is drying the water may flow back to the surface. The raw material of the absoφtion layer 2 is a thermoplastic hydrophilic compound, containing polyvinyl alcohol and fine-grained inorganic pigments for the puφose of improving the drying speed and increasing absorbency.

The ingredient (polyvinyl alcohol) that forms the matrix polymer of the absoφtion layer has a particular melt processing temperature at which it can be spread onto the carrier by extrusion. The temperature profile (the temperatures in different parts of the extruder) of the polyvinyl alcohol compound in the extruder is typically between 140 and 230°C.

The compound contains pigments as fillers in order to regulate the absoφtion properties of the absoφtion layer 2. In the compound, which is in the form of a solid granulate, the extrudable polymer has been mixed with pigments in the desired proportions. The absoφtion layer contains between 20 and 50 wt % of pigments, preferably between 25 and 40 wt %. The pigment is either talc or barium sulphate or a mixture of these two in order to improve the drying speed and to increase absorbency during printing.

The hydrolysis rate of the polyvinyl alcohol in the compound is between 60 and 80%, its molar mass between 10,000 and 30,000, preferably between 15,000 and 25,000, and the polymerisation rate between 300 and 1,000, preferably between 500 and 700. The melt index of the compound is between 1 and 20 g/10 min, preferably between 2 and 7 g/10 min (190°C; 2.16 kg).

The compound (particles containing polymer and filler) is brought to a molten state in the extruder and is spread by extrusion onto the surface of the carrier 1 in order to form an absorption layer. The absoφtion layer 2 is spread in coextrusion simultaneously with a polyolefin-based plastic, which forms the lower intermediate layer 2a, which acts as a barrier layer, preventing the solvents of the ink from penetrating into the carrier 1. When choosing the polymer material for the barrier layer 2a, it must be taken into consideration that it attaches the absoφtion layer 2 to the carrier 1. The total amount of the absoφtion layer 2, which is formed by extrusion on top of the carrier 1 , may be between 10 and 40 g/m², preferably between 20 and 30 g/m², and the amount of the intermediate layer i.e. barrier layer 2a may be between 2 and 30 g/m², preferably between 8 and 20 g/m². The grammage of the barrier layer 2a is normally lower than the grammage of the absorption layer 2.

The surface layer 3, the so-called top-coating layer, has been spread on top of the absoφtion layer 2. The surface layer may be spread as an aqueous composition, which is described in more detail below. The surface layer 3 can also be spread in several consecutive stages. The part layers of the surface layers, which have been formed during these consecutive spreading stages, are referred to by the numbers 3a, 3b and 3c in the drawing. The compositions of the different part layers of the surface layer may differ, and particularly the topmost layer 3c, which forms the recording paper's finished outer surface with which the printing ink first comes into contact in ink-jet printing and on which the image is formed, can be made in such a way as to have a different composition. The surface layer 3 is usually thinner calculated in terms of grammage than the absoφtion layer 2 spread by extrusion, for example between 1 and 7 g/m², preferably between 2.5 and 4.5 g/m². The possible methods for spreading the surface layer are explained below.

The surface layer 3 acts as an ink-receiving layer (Tintenaufnahmeschicht) in the finished recording paper. Such a layer having high gloss and smoothness reproduces from the printing ink that has remained on it, for example, in colour ink-jet printing, an image with high resolution and high colour density. The surface layer contains pigment for the puφose of absorbency and binder for the puφose of fixing the pigment. The surface properties can be influenced by the proportions of the pigment and binder and by the selection of the aforementioned ingredients.

In the drawing the surface layer 3 comprises three layers, the undermost layer 3a, which has been spread on the surface of the absoφtion layer 2, the middle layer 3b, and the topmost layer 3c, which forms the outer surface of the finished recording paper. The surface layer 3 contains pigment in such a proportion that it makes up at least 40 wt %, preferably between 40 and 80 wt % of the total dry weight of the surface layer 3. Particularly good ink-jet printing results have been obtained with recording paper in which the ratio of pigment to binder is close to 1:1, for example between 45:55 and 55:45. Of the other ingredients in the surface layer, binder normally makes up over 90 wt %. Other possible agents are cationic additives (e.g. polyDADMAC), optical brighteners, anti-foaming agents, cross-linking agents (e.g. glyoxal and zirconates) and wetting agents. The different layers of the surface layer 3 are spread in different stages and their compositions may differ from one another, or at least the topmost layer forming the outer surface layer 3c may differ from one or more of the lower layers 3b, 3a, of which there is normally one layer or two layers of identical composition.

The pigment used is colloidal silica. Due to the large amount of pigment and the quality of the pigment, high gloss and good absorbency of printing ink water are obtained, despite the fact that underneath the surface layer 3 there is a non-glossy absoφtion layer 2 formed by extrusion coating as a base. The colloidal silica may be anionic or cationic.

The surface layer 3 may be applied on top of the absoφtion layer in one stage or in several consecutive stages, whereby in the latter case, two or more part layers are formed. The composition that forms the layer can be spread by the aqueous solution method or dispersion method. The spreading can be done by gravure, flexographic printing, roll coating, reverse gravure, blade coating, air brush coating or by a combination of the aforementioned methods. The surface layer can be applied either online in the same line in which the absoφtion layer 2 is formed by extrusion on top of the carrier, or off-line on top of a carrier that has previously been extrusion-coated and temporarily wound on a roll.

The surface layer 3 may be spread as an aqueous solution to which the pigment has been added by dispersion and in which the binder is a polymer, such as polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, polyethylene oxide or polyoxyalkylene. In a finished layer the minimum proportion of pigment is 40 wt % of the dry weight, preferably between 40 and 80 wt %. The total thickness of the surface layer 3 may be between 1 and 7 g/m², preferably between 2.5 and 4.5 g/m². The amount of water to be evaporated is low due to the thinness of the layer.

The surface layer 3 can be formed as follows (parts expressed as dry weight): The undermost layer 3a and the middle layer 3b: between 45 and 55 parts colloidal silica and between 45 and 55 parts binder, such as polyvinyl alcohol, forming 100 parts altogether, the grammage of both layers being between 1 and 3 g/m². The topmost layer 3c: between 45 and 55 parts colloidal silica and between 45 and 55 parts binder, such as polyvinyl alcohol, forming 100 parts altogether. In addition, between 1 and 4 parts cross-linking agent, such as glyoxal or zirconate. Grammage between 1 and 3 g/m².

The layers are dried before the next application.

When pigment and binder are used in the surface layer 3 in the aforementioned proportions, the result is an optimal paper for recording. When a cross-linking agent is used in the topmost layer (it need not be used in the layers underneath), the result is also a universal recording paper, which does not distort black ink colour on any printer. This means that the "copper" phenomenon (colour distortion), which is a problem particulariy with Epson's black ink, can be avoided. A glyoxal cross-linking agent is particularly good for preventing the copper phenomenon.

The polyolefin-based plastic to be used as a material for the barrier layer 2a may be a copolymer, teφolymer or ionomer. This layer, which is formed simultaneously in coextrusion with the absoφtion layer, serves to prevent the penetration of the ink's solvent or the ink into the carrier 1 , which is made of paper or paperboard, whereby the "cockling" phenomenon due to moisture in the paper or paperboard can be avoided. The barrier layer may contain between 0.1 and 20% of colour pigments in order to regulate the colour tone, e.g. Ti0₂. The thickness of the layer can be between 2 and 30 g/m², typically between 8 and 20 g/m². The barrier layer 2a also reduces the roughness due to a paper-based or paperboard- based canier. The barrier layer also improves runnability in extrusion since the runnability of polyvinyl alcohol as such is poor, as is its ahdesion to paper or paperboard, and to improve the adhesion, water or an aqueous dispersion would have to be used, which in turn increases the probability of the aforementioned cockling. It is essential to achieve good adhesion of the barrier layer 2a to both paper or paperboard and to the thermoplastic hydrophilic polymer (PVOH) of the absoφtion layer 2.

The temperature profile (temperatures in different parts of the extruder) of the polyolefin-based plastic in the extruder is typically between 180 and 300°C.

Maleic anhydride/ethylene/acrylic ester teφolymer is one example of a barrier layer material that can be used, particularly when extruding the absorption layer in a coextrusion process on top of carrier 1 made of paper or paperboard. This random copolymer of ethylene, alkyl acrylate and maleic anhydride is suitable for use in coextrusion with, for example, polyvinyl alcohol, due to its good adhesion, and it also gives good adhesion to the carrier 1.

The barrier layer 2a can also be formed from several plastic layers, whose compositions may be different. These layers are formed in coextrusion simultaneously with the absoφtion layer.

A compound that has been created by melt mixing can also be used as the source material for the barrier layer 2a, but in mixing its ingredients before extrusion in the proportions required for layer 2a, a normal dry mixing technique may also be used.

The surface of the cooling cylinder used in running the absoφtion layer and barrier layer in the coextrusion process is highly polished (R_max-value between 0.1 and 0.9 μm).

The extrusion or coextrusion process may be a known process, i.e. a process, in which a molten polymer film is spread from the extruder's die on top of a carrier in a form of a continuous moving web, which polymer film after having solidified forms the desired layers on top of the carrier.

On the back side (the side opposite the printing side) of the carrier 1 made of paper or paperboard, a plastic protective layer may be used, preventing water from being absorbed from the back side and improving the mechanical properties of the paper in printing. This layer has been indicated by the reference number 1 a in the drawing. The back side can be coated, for example, by extrusion with a polyethylene layer which can be spread in a coextrusion process as a combination of two different grades, for example, LDPE against the surface of the carrier 1 and HDPE as the outer layer. Even though recording paper has been used as the name for the invention, the carrier 1 may be other than a paper product, and the name describes a product that is used in the manner of paper.

In the following, the invention is described by way of examples, which do not limit the scope of the invention. The backing plastic used in all the products was 15 gsm PE-HD (density 940 kg/m³). The base paper used in all examples was 100 gsm (g/m²) woodfree supercalendered. The barrier layer used in all examples was 10 gsm Lotader 3210 (an acrylic ester of ethylene and a teφolymer of maleic anhydride), with a comonomer content of 9%, a melt index of 5 g/10 min (190°C/2.16 kg). Parts or percentages refer to proportions based on weight unless otherwise stated.

Example 1

Absoφtion layer 25 gsm: Vinex 2144 70% (PVOH, density 1250 kg/m³) + Blanc fixe super F 30% (barium sulphate, average particle size 1 μm, density 4.4 kg/dm³).

Surface layer as an aqueous dispersion 4 gsm: Mowiol 8-88 (PVOH) 60 parts + Ludox SK-I (colloidal silica) 40 parts + Cartabond TSI (Glyoxal) 2 parts

Example 2

Absoφtion layer 25 gsm: Vinex 2025 70% (PVOH, density 1250 kg/m³) + Finntalc C10 30% (talc, average particle size approximately 2 μm, density 2750 kg/m³).

Surface layer as an aqueous dispersion 3 gsm: (Alcotex 72.5 (PVOH) 90 parts + Gasil 35M (synthetic amoφhous silica) 10 parts + Cartabond TSI (glyoxal) 5 parts Example 3

Absoφtion layer 25 gsm: Ecomaty AX-2000 (PVOH, density 1270kg/nrf 3\) 60% + Blanc fixe super F 10% (barium sulphate, average particle size 1 μm, density 4.4 kg/dm³) + Finntalc C10 30% (talc, average particle size approximately 2 μm, density 2750 kg/m³).

Surface layer as an aqueous dispersion 4 gsm: Imagel iVIA (gelatine) 80 parts + PVP K90 (polyvinyl pyrrolidone) 20 parts + Zonyl FSN (fluorosurfactant) 3 parts

Example 4

Absoφtion layer 25 gsm: Ecomaty AX-2000 (PVOH, density 1270 kg/nr 3\ ) 60% + Blanc fixe super F 35% (barium sulphate, average particle size

1 μm, density 4.4 kg/dm³) + Finntalc C10 5% (talc, average particle size approximately 2 μm, density 2750 kg/m³).

Surface layer as an aqueous dispersion 3 gsm: IJ-2 (polyurethane) 50 parts + IJ-50 (polyurethane) 50 parts + Gasil 35M (synthetic amoφhous silica) 10 parts + CR-5L (aliphatic epoxy, functional cross-linking agent) 2 parts.

Claims

Patent Claims

1. Recording paper, particularly for ink-jet printing, comprising a carrier (1 ) and on top of it an absoφtion layer (2) spread by an extrusion coating technique, which absoφtion layer comprises polyvinyl alcohol (PVOH) as a melt-processable polymer and pigment distributed in polymer, characterised in that the polyvinyl alcohol has the following properties

hydrolysis rate between 60 and 80% - molar mass between 10,000 and 30,000 polymerisation rate between 300 and 1,000 melt index between 1 and 20 g/10 min (190°C; 2.16 kg),

and that the pigment is either talc or barium sulphate or a mixture of these, and inorganic pigment, of which the aforementioned ingredients form at least 90 wt %, makes up between 20 and 50 wt % of the absoφtion layer, and that the absoφtion layer (2) has been spread on top of the carrier in coextrusion with the intermediate layer (2a), by means of which the absoφtion layer is attached to the carrier (1).

2. Recording paper according to Claim 1 , characterised in that the absorbency of the intermediate layer (2a) is poorer than that of the absorption layer, whereby it acts as a barrier layer between the absoφtion layer (2) and the carrier(1).

3. Recording paper according to Claim 2, characterised in that the basic raw material of the barrier layer (2a) is polyolefin-based plastic.

4. Recording paper according to any of the above Claims, characterised in that the molar mass of the polyvinyl alcohol is between 15,000 and 25,000, the polymerisation rate is between 500 and 700 and the melt index between 2 and 7 g/10 min.

5. Recording paper according to any of the above Claims, characterised in that the pigment is a mixture of barium sulphate and talc and the average particle sizes of the two aforementioned pigments are different.

6. Recording paper according to any of the above Claims, characterised in that the grammage of the absoφtion layer (2) is between 10 and 40 g/m², preferably between 20 and 30 g/m².

7. Recording paper according to any of the above Claims, characterised in that on top of the absoφtion layer (2) there is a surface layer (3) of lower grammage, which contains binder and pigment.

8. Method for manufacturing recording paper, particularly for ink-jet printing, in which method an absoφtion layer (2) containing pigment and polyvinyl alcohol is spread on top of a carrier (1) by an extrusion coating technique, which method is characterised in that the absoφtion layer (2) is formed from a compound in an extrusion process, in which a polymer according to any of the Claims 1 to 3 and a pigment are present as a mixture in the form of solid particles, which have been formed by melt mixing the components together, and that the absoφtion layer (2) is spread in coextrusion with the intermediate layer (2a), by means of which the absorption layer is attached to the carrier (1 ).

9. Method according to Claim 8, characterised in that the compound contains polymer and pigment according to the proportions in Claim 1.