WO2003059638A1 - A recording medium and a method for manufacturing the same - Google Patents

Abstract

The invention relates to a recording medium, particularly for ink-jet printing. A substrate, such as paper, paperboard, a polymer film, or paper or paperboard coated with a polymer film or a pigmented polymer film, by extrusion or coextrusion, is coated with a an ink-receiving layer that absorbs ink and its solvent, in which layer there is fine-grained pigment in a polymer matrix, in such a way that the pigment particles are on top of one another in the said layer, at least 3 particles in the Z direction The weight percentage of the particles in the receiving layer is as least 35%, preferably 50-65% and the thickness of the said layer is in the range of 10-40 g/m2 .

Description

A recording medium and a method for manufacturing the same

The invention relates to a recording medium. The invention further relates to printing and particularly to printing with ink-jet printers, and quite particularly to the composition of recording media intended for printing out digitally recorded images, and a method for manufacturing these recording media. The recording medium can also be called printing paper, although the substrate on which the layer that receives the ink during printing is spread when manufacturing the recording medium, is not necessarily made from fibre pulp.

With the increasing use of personal computers and digital cameras, the need for high quality printouts has increased greatly and will continue to increase. In recording media of this kind, the substrate is either paper or a plastic film to which is applied an ink-receiving layer, which can also be called an absorption layer. Optimally this layer would be such that it rapidly absorbs the solvent of the ink into the layer in the vertical direction (Z), but does not spread ink at all in the X and Y directions, i.e. in the direction of the plane of the image to be printed. Furthermore, the layer may be such that it chemically binds the colorants in the ink permanently in the receiving layer, and does not change the characteristics of the colorant or the tone of the colour.

Printing papers or recording media known in the art are manufactured by spreading on their surface an aqueous solution or dispersion, whereby once the water has evaporated, binder and ink-receiving pigment remain on the surface.

US patent 6210808 shows how to obtain a good ink-jet recording medium when using as the ink-receiving layer a polymer mixture in which there is a water- soluble polymer and a non water-soluble polymer in the form of a micelle, and which both are cross-linked by UV radiation.

US patent 6228475 presents an ink-jet recording medium in which there are two layers on top of one another a) a solvent-absorbing, porous layer of a polyolefin material and b) a colloidal silica, which is bonded with the binder polymer by means of silanes.

US patent 6290814 presents a recording medium that consists of pigmented paper, on the surface of which has been spread a gelatin in the form of an aqueous solution together with a hydroxyethyl starch.

US patent 6248432 describes an ink-jet recording medium in which the ink- receiving layer comprises of a binder and a non water-soluble organic pigment containing amino groups, and in the said pigment there is at least one epoxy group per molecule. In the described arrangement, typical inks are retained well by the recording medium and their light fastness is excellent.

EP-A-650850 presents a printing paper in which the paper is coated with a polyolefin, and on top of this an absorption layer has been spread in the form of an aqueous solution, the absorption layer containing a synthetic hydrophilic polymer.

The purpose of the invention is to present a printing paper or recording medium that has improved water absorption properties and is able to effectively take up the liquid in the ink jet used in printing. In order to achieve this purpose, the invention is characterised by the features presented the Patent Claims.

In our laboratory experiments, it has been found that when a polymer matrix has been filled with pigment so that the matrix is filled to a degree that approaches the upper limit of workability, micron- or nanometer scale pores begin to form between the pigment and the polymer. According to the invention, these pores serve to carry the ink in the ink-jet droplet and its solvent water rapidly away from the surface and to make it absorb a) into the hydrophilic polymer and b) especially into the hydrophilic porous pigment particles and/or spaces between the said pigments, which spaces are not completely filled by the polymer. Through these spaces, the ink solvent travels rapidly from one pigment particle to another, either via the pores and/or the thin ridges of polymer binder. The upper limit of workability is reached when the film to be extruded no longer forms but is forced into the large gaps and/or the extruder's torque resistance rises so high that the polymer overheats and begins to burn and changes colour, or the polymer's molecular size begins to fall. In different pigments the said upper limit is different. Talc (2-20 μm), on the other hand, may still function at 65 wt% of the total mass, but silica prepared by combustion in the gaseous phase and having a nanometer-scale particle size, may be used as a filler up to 30 wt% at maximum. When a talc-silica mixture is used as a filler, its total amount may be up to 50 wt%. According to our studies, the amount of pigment is preferably such that it is about 5% below the upper limit of workability. The upper limit of workability depends on the specific surface area of the pigment or pigments.

It is widely known (e.g. in concrete engineering) that if fractions of several different particle sizes are used in a mixture (so-called grading curve), the highest possible packing density is obtained and binder matrix is correspondingly saved. However, in a perfectly proportioned mass in which a smaller particle size always fills the space between larger particles, the result is a mass that is extremely difficult to work, and whose viscosity is subject to various sudden changes.

It is also known that if a suitable fraction is left out of the said grading curve, a high degree of filling can be achieved, but the workability of the mass is not easily subject to changes (gap grading).

According to a preferred embodiment, the adhesion and barrier layer between the recording medium and the absorption layer is coextruded at the same time.

A pigment-polymer composition adapts to temperature in such a way that the pigments neither shrink nor expand much with a change in temperature, but the thermal expansion coefficients of all polymers are substantially higher than those of the mineral particles that usually constitute pigments. If a pigment- polymer mixture with a high degree of filling is cooled rapidly, more pores are obtained, as well as breaks and shrinkage of the polymers, i.e. the polymer becomes detached from the surface of the pigment particles.

While, for example, the thermal expansion of calcite is (4-7) x 10^"6/°C depending on the type and form of the crystal and on the degree of crystallisation, and for silica (11-13) x 10^"6/°C, for corresponding types of polymers it is in the range of (3-5) x 10^"3/°C, i.e. the difference is a thousand fold.

When the pigment density in the polymer matrix is high, which means that the ridge between the pigments in the polymer matrix is short, breaking takes place three-dimensionally, because the ridge cannot only become thinner in the Z direction when the polymer shrinks with cooling. This presupposes that there are more than one pigment layers on top of one another in the pigment-polymer layer in the Z direction, preferably at least 3 layers on average. The smaller the pigment particles are the better the result achieved.

If a polymer-pigment layer is typically e.g. 20 g/m² and it contains 60 wt% of pigment, the average size of the pigment particles is 2.4 μm and the density of particles is 2000 kg/m³, we obtain the result that the layer is about 13 μm thick (when the polymer density is 950 kg / m³) and an average of 5 particles are on top of one another.

The substances that can be used as pigments are precipitated calcium carbonate, potassium aluminium silicate, sodium aluminium silicate, aluminium oxide, amorphous silicic acid, amorphous silicon oxide in different forms and talc or kaolin or mixtures of these and superabsorbents known in the art, such as cross-linked polyacryl salts or corresponding starch acryl salts or cross- linked carboxymethyl cellulose salts and corresponding pigments or mixtures of these.

The polymers used can preferably be polyvinyl alcohol (PVOH) at different degrees of saponification and polymerisation, polyethyl oxaline / polyethyl oxazoline, polyethylene oxide, polyacrylate, polyacrylamides, polyethylene imines, polyvinyl pyrrolidine and polymethacrylates. Furthermore these can advantageously be supplemented with dry but water-soluble cellulose derivatives, natural polymers and/or fine-grained starches or gelatin.

The formation of micro- and nano-pores is further improved if the above- described pigment-polymer mixture applied from the extruder onto the substrate web is cooled as rapidly as possible. Since, for example, the most advantageous temperature for extruding typical PVOH polymers is approx. 225°C, it is preferable to cool them rapidly to a temperature even 200°C lower. In the case of e.g. polyethylene oxide, whose most advantageous extrusion temperature is approx. 170°C, a rapid reduction in its temperature to 30°C will correspondingly lower the polymer temperature by 140°C.

The above-mentioned absorption layer can further be lacquered with another thin layer, a surface layer, which forms the layer of the product that first receives the ink jet. This surface layer typically has a grammage of 0.5-5 g/m² and it can be spread as an aqueous solution. The polymers used in the surface layer are preferably water-soluble polymers, such as polyvinyl alcohols, polyvinyl pyrrolidine, carboxymethyl cellulose, polyethylene oxide, acrylates, styrene acrγlate copolymers, gelatin and/or mixtures of these or their pigmented forms, in which the pigments used are preferably fine-grained silicates, aluminium oxide, aluminium silicates or organic polymethyl metacrylate pigments and every layer may contain optical brighteners. The layer may also contain cross- linking agents, such as glyoxal, zirkonates, and/or boric compounds. Furthermore the layer may preferably contain cationic compounds, which are additives that fix the colour to the layer. These may, for example, be polyacrylamide, polyallyl dimethyl ammonium chloride and/or quaternary ammonium compounds, however without restricting the list to these.

The invention is described in the following by way of examples, which do not however limit the scope of the invention.

Examples: A) The paper used, on which the product was formed had a grammage of 150 g/m², onto which was coextruded 20g/m² of HD- PE and ethylene methacrylate, with a methacrylate content of 20%, melt index 7.5 g/10 min, 5 g/m² and simultaneously polyvinyl alcohol, with a cold density of 1250 kg/m³, together with amorphous silicon dioxide, which made up 40% of the total mass of the layer and which silicon dioxide had a particle size of 3.5 μm and its porosity was 700 m²/g, measured by the nitrogen- adsorption method. The extrusion temperature was 285°C for molten ethylene methacrylate and for molten PVOH, 225°C. The molten material was cooled rapidly during < 1.5 seconds to a temperature of 40°C.

A product was obtained which absorbed ink well, the black density averaging 2.30 (three different printers), while for the reference recording media the figure was 1.93 with the same printers. Three commercially sold products were used as the reference papers.

B)

Bleached 120 g/m² paper was coated with 15 g/m² HD-PE and ethylene methacrylate (15 g/m²) with a melt index of 7.5 g/10 min, and PVOH (cold density 1250 kg/ m³) containing 50 wt% aluminium oxide. The specific surface area of the aluminium oxide 120 m²/g and its average particle size 2.4 μm. The extrusion temperature for molten ethylene methacrylate was 280°C and for molten PVOH it was 215°C. The coating obtained was cooled rapidly during < 1.5 s to a temperature of approximately 30°C.

C)

Paper of 100 g/m² was coated with 20 g/m² HD-PE and the product obtained thereafter was coated by coextrusion in such as way that on top of the PE layer, 15 g/m² ethylene methacrylate (same as above) was extruded and at the same time on top of the last-mentioned was extruded polyethylene oxide (p = 1150 kg/m³), melting point 62°C + a sodium-acrylate based copolymer (heat resistance > 200°C, density 450 kg/m³), into which had been mixed molten sodium aluminate silicate 20 wt%, with the following properties: Pore size 1.0 nm, water absorbency 27 wt%. The amount of the above-mentioned mixture in the coating layer was 15 g/m². The extrusion temperature for the molten methacrylate was 280°C and for the molten polyethylene oxide 175°C. The molten coating was cooled rapidly during < 1.5 s to a temperature of approximately 30°C.

In example B similar good results were obtained in ink-jet printing, while in example C the result was clearly poorer.

Claims

Patent Claims:

1. A recording medium, particularly for ink-jet printing, characterised in that a substrate, such as paper, paperboard, a polymer film or paper or paperboard coated with a polymer film or a pigmented polymer film, is coated by extrusion or coextrusion with an ink-receiving layer that absorbs ink and its solvent, in which layer there is fine-grained pigment in a polymer matrix, in such a way that pigment particles are on top of one another in the said layer, at least 3 particles in the Z direction, that the weight percentage of the particles in the receiving layer is as least 35%, preferably 50-65% and that the thickness of the said layer is in the range of 10-40 g/m².

2. A recording medium as claimed in Patent Claim 1 , characterised in that the ink-receiving layer has, after extrusion on a recording medium, been cooled to a temperature at least 130°C lower, in at most 1.5 seconds from the time when the polymer matrix-pigment material has become attached to the substrate.

3. A recording medium as claimed in Patent Claims 1 or 2, characterised in that between the polymer matrix and the pigment particles, necks have formed and breaking of contact between polymers and particles has taken place.

4. A recording medium as claimed in any of the above Patent Claims 1-3, characterised in that the pigment is precipitated calcium carbonate, aluminium oxide, potassium-sodium aluminates, potassium-sodium aluminate silicates, amorphous silicic acid or various fine silicas, talc, kaolin or mixtures of the above-listed or their agglomerates.

5. A recording medium as claimed in any of the above-mentioned Patent Claims, characterised in that the binder polymers used are hydrophilic polymers, applied by hot melt extrusion, which polymers are polyvinyl alcohol at different degrees of saponification and at different molecular weights, acrylates and/or acryl copolymers or methacrylates or neutralised forms of polyacrylic acid or polyethylene oxides, polyethyl oxaline or polyethylene oxazoline or mixtures of these, to which may have been added water-soluble cellulose derivatives or gelatin or typical natural starches, which have been introduced into the polymer mixture either in soluble or non-soluble form.

6. A product as claimed in any of the Patent Claims 1-5, characterised in that the said rapid cooling takes place in contact with a cooling surface on the side where the ink-receiving layer is located.

7. A product as claimed in any of the Patent Claims 1-6, characterised in that the pigments in the ink-receiving layer are such that the ratio between the diameters of large and small particles is such that the small particles do not fit between three or five large particles, or that they do not fill the spaces between the said larger particles, i.e. the particle-to-particle contacts between different- sized particles are kept to a minimum (gap grading).

8. A product as claimed in any of the Patent Claims 1-7, characterised in that the pigments used are in the diameter range of 0.2-4μm.

9. A product as claimed in any of the Patent Claims 1-8, characterised in that the surface of the product is finished by lacquering with the same polymer or polymers as mentioned in Patent Claim 5, and furthermore that the lacquer layer may contain some of the pigments mentioned in Patent Claim 4 and that the lacquer layer may be in the thickness range of 0.5-5 g/m².

10. A product as claimed in Patent Claim 9, characterised in that the surface lacquer contains water-soluble polymers, such as polyvinyl alcohol, polyvinyl pyrroline, polethylene oxide, carboxymethyl cellulose, acrylates, styrene-acryl- copolymers, gelatin and/or pigmented forms of these, in which the pigments may be fine-grained silicates or silicas or aluminium silicates or organic polymethyl methacrylate pigments and furthermore, that the layer may contain optical brighteners known in the art and glyoxal, zirconates and/or boric compounds acting as cross-linking agents.

11. A product as claimed in either of the Patent Claims 9 or 10, characterised in that the gloss of the lacquer layer is regulated by mixing into it the required amount of colloidal silica, with a particle size of < 300 nm.