WO2002022953A1

WO2002022953A1 - Process for the production of overlay paper

Info

Publication number: WO2002022953A1
Application number: PCT/EP2001/009829
Authority: WO
Inventors: Klaus Alsbach; Detlef Stoltefaut; Wilhelm Westerteiger
Original assignee: Fibermark Lahnstein Gmbh & Co. Ohg
Priority date: 2000-09-12
Filing date: 2001-08-27
Publication date: 2002-03-21
Also published as: EP1186708A1; AU2001291798A1

Abstract

The present invention relates to a process for the production of an abrasion-resistant paper, a composition containing hard material particles having a medium particle size in the range of 10 to 80 νm and a binder being applied to a surface of the paper by means of a screen-printing drum, and also to a paper which can be obtained in accordance with this process.

Description

PROCESS FOR THE PRODUCTION OF OVERLAY PAPER

The present invention relates to a process for the production of abrasion resistant paper especially overlay paper or decorative paper coated with abrasion-resistant hard material particles, using the screen printing process, the abrasion resistant paper which can be obtained in this way and the laminates produced therewith.

Overlay paper is normal y used to protect a paper surface which is exposed to mechanical stressing as a result of friction, scratching or the like. These are, for example, decorative surfaces of, for example, table tops and cabinet tops, kitchen worktops and laminate floors and daily-use articles configured in a decorative way, such as trays. Overlay papers normally consist of α-cellulose, which is impregnated with a melamine resin and subsequently pressed with the underlayer to be protected. During this compression, overlay papers become transparent, so that the decorative layer located underneath is not visually impaired by the overlay paper. The overlay papers applied in this way offer the paper underneath increased abrasion resistance, which may be recorded by internationally recognized measurement methods (e.g. the Taber Abraser test). The initial abrasion of a decorative paper covered with an overlay paper is increased, for example, to about 150 to 250 revolutions (measured with the standardized "Taber Abraser" apparatus). The abrasion protection can be improved still further by abrasion-resistant hard material particles being applied together with the overlay paper.

Overlay papers which contain such hard material particles or are coated with these are usually produced for this purpose. Also described are processes which permit the direct application of hard material particles to the decorative layers to be protected.

The hard material particles used are various materials which are distinguished by good abrasion resistance. The materials used most frequently in this case are primarily aluminium oxide (in particular corundum), but also silicon dioxide, silicon carbide and further hard minerals. Corundum offers the advantage that its refractive index lies very close to that of cellulose, so that when this material is used, no clouding of the overlay paper occurs. The particle sizes used are usually in the range from 10 - 80 μm, preferably 30 - 60 μm. When smaller particles are used, the abrasion resistance is no longer adequately provided; larger particles give the surface a rough structure, which has a detrimental effect on the use and on the visual properties.

One such overlay paper with incorporated corundum is disclosed by DE- A 2107091. This describes a paper into which corundum is introduced during the production operation, so that after the paper has been finished, the corundum is firmly incorporated in the interior, that is to say between the cellulose fibres. The drawback with this type of overlay paper resides primarily in the moderate abrasion resistance of the uppermost layer of the paper, which consists only of cellulose fibres, while the corundum is deposited in the centre of the paper. After stressing, this leads to a non-uniform surface appearance. Since, in this process, the corundum is already contained in the fibrous stock used for the paper production during the production of the paper, all the apparatus used in this case is not only contaminated by the filler particles but also highly stressed because of the friction.

The European patent specification EP-B 329154 describes a process for the application of hard material particles to a paper web (e.g. during the production of overlay paper), the particles here being scattered with the aid of a metering roll onto a paper web drawn past underneath the roll. The paper web bears, as the topmost layer, a thermosetting melamine resin, which takes up the particles scattered on. However, the corundum layer applied in accordance with this method is not completely uniformly distributed on the paper surface and in the resin layer, since irregularities in the area distribution occur because of the scattering process. For this reason, non-uniform wear of the surface is to be expected, which manifests itself as a qualitative disadvantage of a paper produced in this way - above all in the case of very glossy overlay surfaces. In addition, in this case, the smoothness of the surface often cannot be reached to the desired extent, since the particles are scattered onto the resin shortly before it is dried and do not necessarily penetrate uniformly into the resin.

DE-A 19604907 relates to a process for the production of an overlay paper in which a stock, which contains corundum and possibly a binder, is applied to the web on the paper machine at the wet end, using a slot- opening coating head.

DE-C 19508797, DE-C 2858182 and EP-A 186257 disclose processes in accordance with which decorative papers are covered directly with an abrasion-resistant layer, without the application of an overlay paper. In the processes described, in each case a relatively small proportion of corundum is mixed with cellulose fibres or microcrystalline cellulose and applied to the decorative sheets either together with the melamine resin or in two successive operations (first coating, then impregnation). The application of the mixture is in this case carried out by means of a wire doctor, a reverse-coating system or an engraved roll. The decorative sheets produced in accordance with this process therefore bear an abrasion-resistant corundum layer directly, the latter being distinguished by the fact that relatively little corundum is contained and it is not bound to the fibres of the decorative papers but to the cellulose located in the applied layer. The drawback with this process is that either a relatively thick resin layer is necessary in order to avoid too high a loss of corundum, or else the bonding of the corundum layer to the decorative paper is very difficult, and therefore increased loss of corundum has to be expected. However, a thicker resin layer is not so flexible and more easily becomes brittle, which makes the processing of the decorative sheets more difficult.

In all the processes described, reference is made again and again to the problem of the uniform distribution of the abrasion-resistant particles onto the paper sheets. In order to obtain a uniform degree of wear over the entire area to be protected, the particles must be distributed uniformly in this area. However, because of their density, the particles tend to sink in the application mixture, so that non-uniform distribution over the area of the paper easily occurs. This effect can be counteracted firstly by intensive stirring of the mixture during the application, secondly by improved application methods.

DE-A 3718561 discloses a process for the preparation of an article comprising an abrasive surface e.g. an abrasive paper, wherein a composition comprising abrasive grain and a binder is applied on a substrate by means of screen printing. Since surface roughness should be avoided in the preparation of abrasion resistant overlay papers or decorative papers, as described above, this reference is irrelevant. Furthermore no hint can be found in DE-A 3718561 that by using screen printing uniform distribution of abrasive grain can be achieved.

The object of the present invention is to provide a process for a paper which provides uniform protection, high over the entire area, for sensitive surfaces with, at the same time, good processability and low outlay in terms of cost. This object is achieved by a process for the production of an abrasion-resistant paper, a composition containing hard material particles having a medium particle size in the range of 10 - 80 μm and a binder being applied to a surface of the paper by means of a screen-printing drum, and also by an abrasion-resistant paper which can be obtained by this process. The application of a mixture containing hard material particles is carried out, according to the invention, on one side by means of the rotational screen-printing process. The rotational screen-printing process has the particular advantage that the mixture to be applied continues to be mixed continually during the application as a result of the rotation of the screen-printing roll, and therefore the mixture continually remains in its original percentage composition. This makes additional mixing elements immediately upstream of the application element, such as are known from the prior art, e.g. DE-A 19604907, superfluous and, as compared with this prior art, effects improved homogeneity of the composition at the time at which it meets the paper web. In addition, application by means of a micro-perforated screen- printing stencil has the advantage that the mixture is distributed extremely uniformly on the paper web. The paper which can be obtained in accordance with the process of the invention therefore differs from the papers from the prior art.

For the production of the overlay paper according to the invention, use is preferably made of a high-quality paper, which is distinguished by the ability to withstand thermal loading and by high wet strength. This may be a paper made of cellulose, preferably α-cellulose, produced from 100# coniferous wood. Particular preference is given to a paper made of coniferous sulphate pulp with a proportion of up to 8% wet strengthening agent, based on the cellulose. The gramage of the paper may be 15- 60 g/m², preferably 25-40 g/m², quite particularly preferably at least 30 g/m². The bulk density of the paper is preferably around 0.25- 0.60 g/cm³, particularly preferably in a range from 0.32-0.45 g/cm³.

In a preferred embodiment, the composition used for the production of the abrasion resistant paper according to the invention contains: a) hard material particles, preferably in an amount of 72 - 90* by weight, b) binders, preferably in an amount of 9.5 - 25* by weight, c) wet strengthening agent, preferably in an amount of 0.5 - 3* by weight, and d) water.

The composition particularly preferably contains 77 - 86* by weight of hard material particles, 13.5 - 21* by weight of binders and 0.5 - 2* by weight of wet strengthening agent. The stated percentages in each case relate to the total weight of the solid materials in the composition, and the solids content is 30-60* by weight, based on the total composition.

In the composition used in the process according to the invention, the usual binders used in papermaking can be employed. However, it has been shown that, in the process according to the invention, it is possible to dispense with the use of melamine-formaldehyde resins as binders, which are normally used in the prior art in the production of overlay paper. This results in considerable advantages in terms of health and safety at work and in complying with environmental targets.

Preferred binders are selected from branched or unbranched, short-chain or long-chain polysaccharides, polyurethanes, polacrylates, polyacetals, polyvinyl acetate, polyvinyl alcohol, polymers and copolymers of styrene, acrylate esters, acrylic and methacrylic esters, acrylonitr le, acrylate salts, acrylamide, gelatine, guar, viscose, or a mixture of two or more of these substances. The polysaccharides are particularly advantageous with regard to the binding force for the hard material particles and desired transparency after lamination, and can comprise modified or unmodified starch or cellulose, including one in crystalline and microcrystalline form, such as CMC. A preferred binder for use in accordance with the invention is starch, it being possible for any starch that can be employed in papermaking to be used. Use is preferably made of a cold-water soluble starch, particularly preferably an acetylated starch of medium to high viscosity (viscosity from 1000 to 1800 BU; 25* [HB], T = 30°C, according to Brabender). A particularly preferred binder is a cold-soluble, substituted, weakly anionic potato starch. The use of such a carrier material has the advantage that the hard material particles, in particular corundum, are deposited on it and may be dispersed particularly well in the composition.

In order to increase the abrasion resistance of a surface, hard material particles are applied to a paper substrate according to the present invention. The medium particle size of the hard material used is usually in the range of 10 - 80 μ , preferably 20 - 70 μm and most preferably 30 - 60 μm. When smaller particles are used, the abrasion resistance is no longer adequately provided; larger particles give the surface a rough structure, which has a detrimental effect on the use and on the visual properties.

Use is preferably made in this invention of those hard material particles which, when applied to a surface to be protected, do not manifest themselves in a visually disruptive manner. In this case, corundum (aluminium oxide, A1₂0₃) has been shown to be particularly suitable, since this material becomes transparent when compressed together with the surface to be protected, for example a decorative paper, during the production of a laminate. However, use can also be made of any other material which exhibits the desired properties, such as silicon carbide, tantalum carbide, tungsten carbide, silicon nitride, silicon dioxide, titanium nitride or boron nitride.

For the production of the overlay paper according to the invention, use is preferably made of corundum with an average grain size of 10-80 μm, preferably with a grain size of 20-70 μm. Particular preference is given to a corundum with an average grain size of 35-55 μ .

The corundum applied can also contain traces of other minerals, such as Si0₂, Na₂0, Ti0₂, FeO, CaO, SiO, K₂0, Fe₂0₃, MgO and the like, but preferably in a proportion of less than 2*, particularly preferably less than 1*.

Particularly suitable wet strengthening agents for use in the process according to the invention are polyamide amine epichlorohydrin resins, el amin-formaldehyde resins and epoxy resins. Melamin-formaldehyde resins are particularly preferred when high transparency of the overlay paper, after it has been compressed to form a laminate, is desired. The highest preference is given to modified mel amine-formaldehyde resins, in particular methyl -etherefied melamine-formaldehyde resins, which are low in free formaldehyde. Additional additives which are considered for the mixture according to the invention are all additives known in papermaking, in particular wetting agents, dispersants, retention aids, defoamers, colorants and surfactants.

The viscosity of the mixture according to the invention when applied is preferably in a range from 250-600 mPa*s at 25°C, particularly preferably in the range from 300-500 mPa*s. The solids content in the mixture according to the invention is set as a function of the application stencil and rate used, but is preferably in a range from 30- 60*, based on the total weight of the mixture, particularly preferably between 38-46*.

The mixture according to the invention is produced with intensive stirring and continues to be mixed continually during application by the rotation of the screen-printing drum. During screen printing, the formulation is forced through a fine-mesh screen (stencil) by means of a squeegee. The amount applied is influenced by the mesh size of the stencil, the thickness of the stencil, the thickness (diameter) of the squeegee and by the formulation.

In order to ensure a defined and uniform application of hard material, the mixture to be applied and the screen-printing stencil used are optimized to each other.

For the purpose of applying the mixture according to the invention described here, it is possible in particular for the mesh size, the hole diameter and the thickness of the stencil to be varied.

The preferred amount of the above-described mixture applied is 14- 23 g/m², particularly preferably 15-21 g/m², based on solids. In this case, it is preferable to select the composition of the mixture and the total amount of the mixture applied to the paper in such a way that the quantity of binder applied is 2.0 - 4.1 g/m², preferably 2.6 - 3.5 g/m², particularly preferably 2.8 - 3.2 g/m², the quantity of hard material particles applied is 12 - 18 g/m², preferably 13 - 17 g/m², particularly preferably 14 - 16 g/m², and the quantity of wet strengthening agent applied is 0.12 - 0.5 g/m², preferably 0.12 - 0.3 g/m², particularly preferably 0.15 - 0.23 g/m², in order to achieve the optimum ratio between good transparency and good abrasion properties.

As a result of the application process according to the invention for the hard material composition for producing an abrasion-resistant paper, fi stly an extremely uniform distribution of the hard material over the surface, secondly high incorporation of the hard material particles is achieved. As a result, high abrasion resistance is achieved with an extremely low loss of hard material particles. In addition, applying the hard material to one side has the particular advantage that, during the further processing of the overlay paper, for example during the production of the laminate, the apparatus used during the processing is protected against more intensive wear from the hard material particles.

The grammage of the overlay paper finally produced is around 20-100 g/m², preferably 25-80 g/m².

The present invention has been described above with reference to the production of overlay paper. The process of the present invention can likewise be employed for making abrasion resistant decorative paper, whereby the composition comprising hard material and binder is directly applied to a decorative paper substrate by means of screen printing. The same advantages as described above with respect to overlay paper are achieved and the disadvantages described for the prior art can be avoided.

The abrasion resistant decorative paper can by directly laminated as a top layer onto a substrate. Alternatively in order to protect the machinery in subsequent processing of the abrasion resistant decorative paper for example in the lamination process an overlay paper containing no or only a small amount of hard material particles or an overlay paper produced according to the present invention with the hard material containing side contacting the abrasion resistant decorative paper can be applied as top layer in the lamination process.

Fig. 1 shows a schematic illustration of an apparatus with which the process according to the invention can be carried out.

The composition containing hard material particles according to the invention is fed continuously to a screen-printing drum 1, which is equipped with a microperforated screen-printing stencil, so that a pond of material 2 is always maintained within the screen-printing drum 1. The paper web 3 is guided through between the rotating screen-printing drum 1 and a supporting roll 4, likewise rotating. The composition containing hard material particles is applied to the paper web 3 through the openings in the screen-printing drum 1, with the assistance of a roller doctor 5. The application thickness of the composition according to the invention to the paper web 3 may be regulated during this process via the viscosity of the composition, the mesh size and thickness of the screen-printing stencil used and the diameter of the doctor rod 5 used.

Exemplary embodiment:

A corundu -containing mixture is produced by combining:

19.5* by weight of a commercially available cold-water soluble acetylated and oxidized potato starch as binder, 79.5* by weight of corundum with an average particle size of 44.5 ±

2 μm, and 1.0* by weight of a commercially available wet strengthening agent based on an extremely low-formaldehyde melamine- formaldehyde resin, the percentage statements referring to the total weight of the solids in the composition, and water in an amount such that a viscosity of 300-500 mPa*s is obtained at 25°C, with a total solids content of about 41* in the mixture.

Using a base paper made of 100* coniferous sulphate pulp with a proportion of 5* wet strengthening agent (polyamide amine epichlorohydrin resin), based on the cellulose, with a grammage of about 30 g/m², the corundum-containing mixture described above is applied with an application weight of 15-18 gm/m², based on solids. For this purpose, the mixture, initially produced with intense stirring and set to the desired viscosity, is put into a screen-printing drum, which is fitted with a screen-printing stencil which has a mesh size of 40, a hole diameter of 161 μ and a thickness of 100 μm and, according to Fig. 1, is applied to the paper. In this case, an application rate of 40- 60 /min can be achieved.

Using such a stencil, the desired application quantity can be achieved with an application which is extremely uniform over an area. The overlay paper obtained in this way is dried in a conventional drier, wound up in the conventional manner and then cut to size to the desired format. According to a visual assessment, the corundum is distributed completely uniformly on the overlay paper produced.

This corundum-containing overlay paper is treated with a quantity of about 200* by weight of resin, based on the weight of the paper. This resin-coated overlay paper is laid over a decorative paper and, using an MDF board, is compressed for 35 seconds at 180°C and under a pressure of 200 bar. The product obtained in this way has abrasion values in Class AC 3 (> 2500 cycles in the Taber Abraser test).

List of reference symbols:

Screen-printing drum Pond of material Paper web Supporting roll Roller doctor

Claims

PATENT CLAIMS

1. Process for producing an abrasion-resistant paper, a composition containing hard material particles having a medium particle size in the range of 10 to 80 μm and a binder being applied to a surface of the paper by means of a screen-printing drum.

2. Process according to Claim 1, the composition being applied to the paper web al eady formed.

3. Process according to one of the preceding claims, the composition having a viscosity of 250-600 mPas at 25°C.

4. Process according to one of the preceding claims, the composition containi g: a) hard material particles, b) binder, c) wet strengthening agent, and d) water.

5. Process according to one of the preceding claims, the composition containing: 72 - 90* by weight of hard material particles, 9.5 - 25* by weight binder,

0.5 - 3* by weight wet strengthening agent, in each case based on the total weight of the solids in the composition, and has a solids content of 30-60* by weight, based on the total composition.

6. Process according to one of the preceding claims, the binder being selected from branched or unbranched, short-chain or long-chain polysaccharides, polyurethanes, pol acryl ates, polyacetals, polyvinyl acetate, polyvinyl alcohol, polymers and copolymers of styrene, acrylate esters, acrylic and methacrylic esters, acrylonitrile, acrylate salts, acrylamide, gelatine, guar, viscose, or a mixture of two or more of these substances.

7. Process according to Claim 6, the binder being a starch, in particular a cold-water soluble starch.

8. Process according to one of the preceding claims, the hard material being selected from corundum, silicon carbide, tantalum carbide, tungsten carbide, silicon nitride, silicon dioxide, titanium nitride and boron nitride.

9. Process according to one of the preceding claims, the composition being applied in a quantity which corresponds to 14 - 23 g/m² solids.

10. Abrasion-resistant paper obtainable in accordance with one of Claims 1-9.

11. Abrasion-resistant paper, the paper having a grammage of 20- 100 g/m².

12. Abrasion-resistant paper according to any of claims 10 and 11 selected from overlay paper and abrasion-resistant decorative paper.

13. Laminate having, as a covering layer, an abrasion-resistant overlay paper according to claim 12, with a decorative layer located underneath.

14. Laminate according to claim 13, wherein.

15. Laminate having an abrasion-resistant decorative paper according to claim 12 laminated as a top layer onto a substrate.

16. Laminate according to claim 15, wherein an overlay paper containing no hard material particles is laminated on top the decorative paper.

17. Laminate according to claim 15, wherein an overlay paper according to claim 12 with the side of the overlay paper the hard material particles are applied on in contact with the decorative paper is laminated on top of the decorative paper.