EP1834041A1

EP1834041A1 - Method of producing paper or cardboard

Info

Publication number: EP1834041A1
Application number: EP05823380A
Authority: EP
Inventors: Lars Gädda; Ari Kiviranta; Olli Suontausta
Original assignee: M Real Oyj
Current assignee: Valmet Technologies Oy
Priority date: 2004-12-31
Filing date: 2005-12-30
Publication date: 2007-09-19
Also published as: WO2006070064A1; EP1834041A4; FI20041706L; FI20041706A0; FI121086B

Abstract

Method of coating paper or cardboard webs. The method comprises the steps of applying a coating colour onto a smooth surface to form a first layer, drying the first layer at least partially, applying at least one second coating colour with different coating mineral particles on top of the first layer to form at least one second layer, which contains an amount of binding polymer, partially drying the second layer on the first layer to form a combined layer, and transferring the combined layer onto the paper or card-board web. By means of the invention it is possible to transfer the coating curtain onto the web without interference from any unevenness or air streams associated with the moving web.

Description

Method of producing paper or cardboard

The present invention concerns a method according to the preamble of claim 1 for producing coated paper or cardboard.

According to a method of this kind, a base paper or cardboard web is provided with a coating layer, which at least partially contains pigments and binder.

Coated papers are big volume paper products on the markets nowadays. These kinds of papers normally serve as high quality printing bases. Printing papers are expected to simultaneously exhibit properties of high brightness, gloss and smoothness.

Paper coatings are made in different ways. Typically, a coating colour comprises mineral pigments, water and a binding polymer. The mineral pigment can be kaolin clay, gypsum, ground or precipitated calcium carbonate, titanium dioxide etc. The best opacity forming pigments are titanium dioxide (TiO₂) and Precipitated Calcium Carbonate (PCC). The best pigments are also the most expensive ones.

If the surface of the paper were completely flat, smooth, and was to remain so during the coating process, only a small amount of the coating colour pigment were needed. For this reason, the base paper is normally calendered before coating. However, unfortunately, 40 - 60 % of the smoothness gain during calendaring will be lost when the surface is rewet during coating because of the water contained in the aqueous coating colour.

For the above reasons, normally several coating layers are needed for obtaining the best coating result. Each layer contains particles having different medium size and possibly different mineral particle qualities. Small mineral particle size will retain more "pixels" at printing. This is the reason why particles having particle sizes in the range of 0.5 to 1.0 μm are usually employed in the topmost layer, forming the surface of the coated paper.

It should be pointed out that by coating it is possible to achieve, not only a desired degree of brightness but also a predetermined, high level of gloss. Conventionally, the coating colour is applied on the surface of a base paper or cardboard web with a number of different coating methods. As specific examples, blade coating, rod coating, spray coating and transfer coating can be mentioned. Of these conventional methods, blade and rod coating are the ones most frequently used. Transfer of the coating film from a precoated roll onto the paper web in a roll nip (also known as film transfer coating) is also a widely used method. Here the film is in wet condition when transfer takes place.

A rather new coating method is spray coating, where tiny coating colour droplets are sprayed to the webs surface. Dry coating is also known, and in a method of this kind, dry particles of mineral pigments are sprayed to the web with electrical forces and fixed with glue, which is already present on the surface.

None of the above methods will produce a really smooth surface, because of the nature of coating method and because of swelling of the paper beneath the coated layer.

There are known in the art some methods of transferring ready-made, dry coating layers onto the surface of a paper web from a smooth surface. Thus, US Patent No. 6,645,335 discloses a paper web coating using a method, where "barrier and printing properties" are improved by casting a mixture of mineral pigments and water on a smooth surface formed by a polished steel or other metal belt or roll or a smooth polymer belt, drying the mineral layer on the belt and then transferring the dried mineral layer with a glue or adhesive to the paper web. The known method gives rise to a smooth surface of the topmost coating layer because the surface of the coating layer abutting against the metal belt will be smoothened when the surface particles - usually finely divided pigments - become densely packed against the metal band. A polymer adhesive is used for picking up the coating layer from the smooth surface.

Although the known method allows for the production of a coating layer having a smooth surface, the apparatus described is complicated and it does not allow for the production of multilayered coatings, which means that expensive coating pigments comprising small particle sizes have to be used throughout the whole coating layer. It is an aim of the present invention to eliminate the problems of the prior art and to provide an improved method of forming a precast coating layer, which can be transferred onto a paper or cardboard surface in an economically and technically advantageous way.

The present invention is based on the idea of forming multilayer coating layers from at least two overlapping coating layers, which are first transferred from a non-contacting coater onto the smooth surface, where they are combined into a multilayer coating layer. The smooth surface is preferably heated up to aid in the drying of the coating layer against the surface.

The precast coating layer is transferred onto a paper or cardboard surface in a coating nip. By building up the coating nip from at least one pair of smooth-surfaced cylinders, both having polished surfaces, it is possible to use the cylinders of the coating nip both for smoothening and polishing the surface of the precast coating layer and for transferring the layer onto the paper or cardboard surface.

More specifically, the present invention is mainly characterized by what is stated in the characterizing part of claim 1.

Considerable advantages are obtained by the invention. Thus, in the present invention, a method partially similar to the know process described in US Patent No. 6,645,335 is used for achieving a precast coating layer. In the present invention, however, multiple layers of mineral coatings are applied to a polished hot belt or cylinder. According to the invention, finely divided particles need only be used in the coating layer abutting the polished cylinder or belt, whereas the second or second and further layers can be made up by more inexpensive pigments. It is also possible to produce the first layer essentially without polymeric binders, to achieve an extremely smooth surface. The second layer can comprise normal amounts of polymeric binders, such as latexes.

Conventionally, a problem related to curtain coating is that the curtain "lives" due to the unevenness of the paper or board web and because of air, which accompanies the web. A smooth roller or steel belt is much more stable than a conventional web and with a roller or belt as an intermediate coating surface, it becomes much -easier to prevent the generation of air flows during the transfer of the coating film from the application nozzles to the substrate and to make the curtain more stable during the curtain application stage. The ready-made coating layer can then easily be transferred from the roller or belt onto the web without interference.

Another problem associated with curtain coating is the difficulties in achieving proper coating of the area close to the edge of the web. Often, that edge is either over- or undercoated in the sense that the coating layer either leaves an uncoated area along the edge of the web, or the coating curtain extends past the edge. With the use of an intermediate roller or belt of the above-discussed kind it is possible to lever out the edge of the coating layer with a blade (such as a conventional coating blade) to obtain perfect match of the coating with the web when the coating layer is transferred onto the web.

Next, the invention will be examiner more closely with the aid of a detailed description with reference to the attached drawings.

Figure 1 shows in a schematic fashion the sideview of a first embodiment of the invention, involving the use of two opposing metal belts which run horizontally over two cylinders which form a coating nip, and Figure 2 shows in a schematic fashion the sideview of a simplified second embodiment of the invention, in which the coating layers are cast directly onto the surfaces of two cylinders forming a coating nip.

According to the present invention, the method of coating paper or cardboard webs, comprises the steps of

- applying a coating colour onto a polished, preferably heated, metal surface to form a first layer,

- drying the first layer at least partially,

- applying at least one second coating colour with different coating mineral particles on top of the first layer to form at least one second layer, which contains an amount of binding polymer,

- partially drying the second layer on the first layer to form a combined layer, and

- transferring the combined layer onto the paper or cardboard web. The coating colour is preferably applied onto the heated smooth surface by curtain coating, which ensures high-speed operation.

According to the present invention, the mineral layer(s) (i.e. layers of mineral pigments) overlapping the first one need not be completely dried. On the contrary, it is advantageous to dry it (them) only partially - this concerns in particular the last mineral layer, which abuts with the paper or cardboard surface in the coated product - because then the polymer binder present in the coating layer will automatically transfer the mineral layers to the paper web from the polished casting surface. When the remaining water is present in sufficiently small amounts in the last cast layer, the swelling of the body stock paper or board is insignificant and keeps the surface smooth. Typically, depending on the coating speed, the moisture content of the last cast layer, i.e. the layer abutting with the web, is in the range of 5 - 40 weight-%, preferably about 10 to 30 weight-%.

In general, the coating colour used according to the present invention for forming a precast coating layer contains from 10 to 100 parts by weight of at least one pigment or mixture of pigments, from 0.1 to 30 parts by weight of at least one binder and from 1 to 10 parts by weight of other additives known per se.

In the first layer, there can be a single finely divided mineral pigment, or a mixture of several finely divided pigments as follows:

Coating pigment I 10 - 90 parts by weight (for example, fine carbonate)

Coating pigment II 10 - 90 parts by weight

(for example fine kaolin)

Pigment total 100 parts by weight

Binder 1 - 20 parts by weight Additives and auxiliary agents 0.1 - 10 parts by weight

Water balance

Water is added to this kind of a coating colour so that the dry solids content is typically from 50 to 75%.

The typical composition of the coating colour used for forming the second layer is as follows:

Coating pigment 100 parts by weight

(for example, coarse calcium carbonate)

Binder 1 - 20 % of the weight of the pigments

Additives and auxiliary agents 0.1 - 10 % of the weight of the pigments Water balance

Water is added to the precoating mix so that the solids content is generally from 40 to 70%.

Basically, the present invention is applicable to any pigment. Precipitated calcium carbonate, ground calcium carbonate, calcium sulphate, calcium oxalate, aluminium silicate, kaolin (hydrous aluminium silicate), aluminium hydroxide, magnesium silicate, talc (hydrous magnesium silicate), titanium dioxide and barium sulphate, and mixtures thereof can be mentioned as examples of the pigments. Synthetic pigments can also be used. Of the pigments mentioned above, the main pigments are kaolin, calcium carbonate, precipitated calcium carbonate and gypsum, which in general constitute over 50 % of the dry solids in the coating mix. Calcined kaolin, titanium dioxide, satin white, aluminium hydroxide, sodium silicoaluminate and plastics pigments are additional pigments, and their amounts are in general less than 25 % of the dry solids in the mix. Of the special pigments, special-quality kaolins and calcium carbonates, as well as barium sulphate and zinc oxide, should be mentioned.

The present invention is applied especially preferably to calcium carbonate, calcium sulphate, aluminium silicate and aluminium hydroxide, magnesium silicate, titanium dioxide and/or barium sulphate, as well as mixtures thereof, in which case, especially preferably, the principal pigment in the pre-coat mixes is calcium carbonate or gypsum and in surface-coat mixes and single-coat mixes the principal pigment consists of mixtures of calcium carbonate or gypsum and kaolin. The first layer, forming the surface of the coated paper or cardboard, can be very thin, containing high quality and small mineral particles, such as TiO₂ or PCC or mixtures thereof, in order to give the best possible printing and print ink holding properties. The other layers can comprise coarse materials, such as ground calcium carbonate or china clay. Generally, the particles of the first layer can have an average particle size, which is 1/3 to 1/50, preferably 1/5 to 1/20, in particular about 1/6 to 1/15, of the average particle size of the second layer(s). Thus, the particles of the first layer can have an average particle size in the range of 0.2.to 1.0 μm, and the thickness of the first layer is preferably about 0.5 to 5 μm, preferably about 1 to 3 μm. The thickness of the second layer is typically about 4 to 15 μm.

It is possible to use any known binders generally employed in paper production as binders in the coating colours. Besides the individual binders, it is also possible to use mixtures of binders. Examples of typical binders include synthetic latexes made up of polymers or copolymers of ethylenically unsaturated compounds, e.g. copolymers of the butadiene- styrene type, which possibly also have a comonomer containing a carboxyl group, such as acrylic acid, itaconic acid or maleic acid, and polyvinyl acetate having comonomers that contain carboxyl groups. Together with the materials cited above, it is also possible to use, for example, the water-soluble polymers, starch, CMC, hydroxyethyl cellulose and polyvinyl alcohol as binders.

Furthermore, it is possible to use conventional additives and auxiliary agents, such as dispersants (e. g. sodium salt of polyacrylic acid), agents affecting the viscosity and water retention of the mix (e. g. CMC, hydroxyethyl cellulose, polyacrylates, alginates, benzoate), so-called lubricants, hardeners used for improving water-resistance, optical auxiliary agents, anti-foaming agents, pH control agents, and preservatives in the coating composition. Examples of lubricants include sulpfonate oils, esters, amines, calcium or ammonium stearates; of agents improving water resistance, glyoxal; of optical auxiliary agents, diaminostilbene disulfonic acid derivatives; of anti-foaming agents, phosphate esters, silicones, alcohols, ethers, vegetable oils; of pH control agents, sodium hydroxide, ammonia; and finally of preservatives, formaldehyde, phenol, quaternary ammonium salts. The present invention can be characterized as a semi-dry transfer coating method, which differs from film transfer coating in that the coating layer is, as a whole, substantially dry. The conditions at which this happens are, according to a preferred embodiment, such that the first layer is totally or essentially dry, the second and any third and further layers are partially dry so that the layer closest to the paper or cardboard web is still suitable tacky for gluing the layers to the web by transfer from the polished belt.

According to a preferred embodiment, no separate polymeric binder layer is needed for transferring the precast coating layer onto the paper or cardboard web.

The transfer can be carried out in a pressure nip or pressure belt. In this method the coating can be made at high speed and the coatings do not need any kind of drying after the transfer.

In the embodiment where the first layer, comprising finely divided particles, is produced essentially without any conventional binder normally used in coating colours, the first layer is advantageously provided with a small amount of hydrophilic polymer for preventing surface dusting of the ready product. This hydrophilic polymer can be carboxyl-methyl cellulose, gelatin, dextrin, starch or a similar polymer, which is used in small amounts of about 1 to 3 % calculated by the weight of the layers.

As far as the invention is concerned it is very advantageous that in the first layer, if expensive titanium dioxide is used, it is mixed before with fine PCC particles which partially agglomerate on the surface of the titanium dioxide particles. This will improve the optical properties, when the TiO₂ particles do not come into direct contact with each other.

If some glue in addition to the polymer binder is used for transferring the coating layers to the web, it is advantageous that this layer contains as little as possible water. Ideally, the glue is selected from the group of "hot melt" glues, and it can be applied to the uppermost surface layer immediately before transfer. This prevents paper surface swelling at transfer.

As mentioned above, non-contact application of the layers onto the smooth, polished metal surface can be carried out by curtain coating. For that purpose it is preferred to have a minimum volumetric flow rate/width of more than 1.0 cm²/s, in particular at least 1.5 cm²/s, to obtain a stable curtain from the applicator to the transfer surface. Further, the dynamic surface tension of the coating colour should be about 20 to 60 mN/m, in particular about 35 - 55 mN/m, typically less than about 40 mN/m.

The coating curtain is applied onto the transfer surface vertically from above. Typically, the clearance between the nozzle of the curtain coater and the transfer surface is about 1 - 1000 mm, preferably about 1 - 500 mm, in particular about 10 - 500 mm. The height depends on the amount of coating colour and the coating speed.

At the nozzle, the thickness of the film is usually about 0.1 - 10 mm, in particular about 1 - 5 mm, during free falling the film is thinned to below 100 micrones.

Turning now to the drawings, it can be noted that in Figure 1 there is shown a sideview of an embodiment of the invention, where coating colours are precast on polished metal belts. Thus, as shown in Figure 1, a paper web 1, which is adapted to move vertically from an upper roll 2 to a lower roll 3, is coated in a coating nip 4 formed between two cylinders 5 and 6. The cylinders both support a polished metal belt 7 and 8, which run in opposite directions, such that belt 7 runs clockwise in the drawing and belt 8 counter-clockwise. The cylinders 5 and 6 can have a surface layer of, e.g. rubber or other thermoelastic material for increasing friction between the cylinder surface and the metal belt.

As indicated in Figure 1, the coating colours are spread onto the casting surfaces 7, 8 by non-contact application. Such application can be achieved with a curtain coating unit 9, 10. These coaters allow for the formation of two or more overlapping layers which form an integral film 11, 12, which is free-falling from the applicator nozzle to the surface of the metal belt.

Figure 2 shows a simplified working example of the previous embodiment, where the coating colours are precast directly on the cylinders 25, 26 forming the coating nip 24.

Similarly to Figure 1, the paper web 21 is adapted for vertical movement from an upper roll 22 to a lower roll 23, and it is double-sided coated in the coating nip 24. The cylinders 25 and 26 both have polished metal surfaces.

In both of the above embodiments, the metal or polymeric surfaces of the belts 7, 8 and cylinders 25, 26, respectively, are smooth. Generally, a roughness of about 0.1 to 1 um is acceptable for the present invention.

As mentioned above, the smooth surfaces are preferably heated, in particular the first layers are dried against heated surfaces having temperatures higher than the boiling point of water, typically at least 110 °C, preferably about 120 to 300 ⁰C, in particular at about 180 to 250 °C, in order to evaporate the water and/or solvent of the coating colour. The smaller the pigments, the higher temperatures should be used for drying.

The polished metal belt or drum is then preferably cooled down at point just before the coating layer is transferred to the paper or cardboard surface in order to promote separation from the metal surface.

As mention above, the width of the combined layer can be adjusted to match the actual width of the paper or cardboard web by, e.g. cutting any overlapping parts of the curtain with a blade before the precast layer is transferred to the web. Thus, according to a preferred embodiment of the invention, the precast layer is cast to a width that is at least somewhat greater, preferably about 1 to 20 %, in particular about 1.5 to 10 % greater, than the width of the web, which is to be coated.

The present invention can be employed for coating any desired paper or board or polymer industrial web or sheet-formed product. As a practical matter, the term "paper" or "paper web" is herein used to designate both "paper" and "paperboard" and "paper web" and "paperboard web", respectively. The terms "paper" and "paperboard" refer to sheet-formed products containing cellulosic or lignocellulosic fibres. "Paperboard" is synonymous with "cardboard" or simply "board". The grammage of the paper or board can vary within broad ranges from about 30 to about 500 g/m².

Claims

Claims:

1. Method of coating paper or cardboard webs, comprising the steps of

- applying a coating colour onto a smooth surface to form a first layer, — drying the first layer at least partially,

- partially drying the second layer on the first layer to form a combined layer, and - transferring the combined layer onto the paper or cardboard web.

2. The method according to claim 1, wherein the first and the second coating colours are simultaneously applied in the form of a multilayer coating layer on the smooth surface.

3. The method according to claim 1 or 2, wherein the coating colours are transferred from a non-contacting coater onto the smooth surface.

4. The method according to any of claims 1 to 3, wherein the smooth surface is heated in order to promote drying of the coating layer against the surface.

5. The method according to any of the preceding claims, wherein the particles of the first layer have an average particle size, which is 1/5 to 1/20 of the average particle size of the second layer.

6. The method according to any of claims 1 to 5, wherein the first layer contains 0.5 to 3 % of a hydrophilic polymer in order to prevent dusting at the final product.

7. The method according to any of claims 1 to 6, wherein the polished metal belt or drum is cooled down at point just before the coating layer is transferred to the paper or cardboard surface in order to promote separation from the metal surface.

8. The method according to any of claims 1 to 7, wherein the particles of the first layer have an average particle size in the range of 0.2.to 1.0 μm, and the thickness of the first layer is 1 to 3 μm.

9. The method according to any of claims 1 to 8, wherein the thickness of the second layer is 4 to 15 μm.

10. The method according to any of claims 1 to 9, wherein the first layer, which is cast against the polished surface, comprises titanium dioxide, precipitated calcium carbonate or a mixture thereof.

11. The method according to claim 10, wherein the first layer comprises titanium dioxide particles precoated with precipitated calcium carbonate in order to keep titanium dioxide particles partially or totally separated form each other.

12. The method according to any of claims 1 to 11, wherein hot-melt glue is used for assisting the transfer of coating to the web, said glue is applied just before transfer to the web.

13. The method according to any of claims 1 to 12, wherein the width of the combined layer is adjusted to match the actual width of the paper or cardboard web.

14. The method according to claim 13, wherein the combined layer is cast to a width that is at least somewhat greater, preferably about 1 to 20 %, in particular about 1.5 to 10 % greater, than the width of the web, which is to be coated.

15. The method according to any of claims 1 to 14, wherein the moisture content of the layer abutting with the web is in the range of 5 - 40 weight-%, preferably about 10 to 30 weight-%.